Reciprocating Engines

There is less rolling friction when ball bearings are used than when roller bearing are employed.

The smaller contact area of a ball bearing causes it to produce less rolling friction.

Ball bearings are used in high-powered reciprocating engines, where keeping friction to a minimum is important.

Ball bearings can be designed and installed in such a way that they reduce friction in axial loads as well as in radial loads.

Bearings are required to take radial loads, thrust loads, or a combination of the two.

An example of a radial load would be a rotating shaft being held or contained in one position on a radial plane.

Thrust load would be the rotating shaft being contained from moving axially along the shafts axis.

There are two ways in which bearing surfaces move in relation to each other.

One is by the sliding movement of one metal against the other (sliding friction), and the second is for one surface to roll over the other (rolling friction).

The three different types of bearings in general use are plain, roller, and ball.

Plain Bearings

Plain bearings are generally used for the crankshaft, cam ring, camshaft, connecting rods, and the accessory drive shaft bearings.

Such bearings are usually subjected to radial loads only, although some have been designed to take thrust loads.

Plain bearings are usually made of nonferrous (having no iron) metals, such as silver, bronze, aluminum, and various alloys of copper, tin, or lead.

Master rod or crankpin bearings in some engines are thin shells of steel, plated with silver on both the inside and the outside surfaces and with lead-tin plated over the silver on the inside surface only.

Smaller bearings, such as those used to support various shafts in the accessory section, are called bushings.

Porous Oilite bushings are widely used in this instance.

They are impregnated with oil so that the heat of friction brings the oil to the bearing surface during engine operation.

Ball Bearings

A ball bearing assembly consists of grooved inner and outer races, one or more sets of balls, in bearings designed for disassembly, and a bearing retainer.

They are used for shaft bearings and rocker arm bearings in some reciprocating engines.

Special deep-groove ball bearings are used to transmit propeller thrust and radial loads to the engine nose section of radial engines.

Since this type of bearing can accept both radial and thrust loads, it is used in gas turbine engines to support one end of a shaft (radial loads)and to keep the shaft from moving axially (thrust loads).

Roller Bearings

Roller bearings are made in many types and shapes, but the two types generally used in the aircraft engine are the straight roller and the tapered roller bearings.

Straight roller bearings are used where the bearing is subjected to radial loads only.

In tapered roller bearings, the inner- and outer-race bearing surfaces are cone-shaped.

Such bearings withstand both radial and thrust loads.

Straight roller bearings are used in high power reciprocating aircraft engines for the crankshaft main bearings.

They are also used in gas turbine applications where radial loads are high.

Generally, a rotating shaft in a gas turbine engine is supported by a deep-groove ball bearing radial and thrust loads) on one end and a straight roller bearing (radial loads only) on the other end.

hydraulic lock

Radial and inverted engines have some cylinders below the crankcase, and when the engine is idle, oil will leak from the crankcase, past the piston rings, and fill the combustion chamber.

This condition is called a hydraulic lock.

If this oil is not removed before the engine is started, the piston will move against the noncompressible oil and cause serious damage.

Whenever a radial engine remains shut down for any length of time beyond a few minutes, oil or fuel may drain into the combustion chambers of the lower cylinders or accumulate in the lower intake pipes ready to be drawn into the cylinders when the engine starts.



As the piston approaches top center of the compression stroke (both valves closed), this liquid being incompressible, stops piston movement. If the crankshaft continues to rotate, something must give. Therefore, starting or attempting to start an engine with a hydraulic lock of this nature may cause the affected cylinder to blow out or, more likely, may result in a bent or broken connecting rod.

To eliminate a lock, remove either the front or rear spark plug of the lower cylinders and pull the propeller through in the direction of rotation.

The piston expels any liquid that may be present.

If the hydraulic lock occurs as a result of overpriming prior to initial engine start, eliminate the lock in the same manner (i.e., remove one of the spark plugs from the cylinder and rotate the crankshaft through two turns).

Never attempt to clear the hydraulic lock by pulling the propeller through in the direction opposite to normal rotation.

This tends to inject the liquid from the cylinder into the intake pipe with the possibility of a complete or partial lock occurring on the subsequent start.

Low oil temperatures

All of the alternatives except low oil temperature would likely be caused by failed or failing engine bearings in a reciprocating engine.

Low oil temperature would be the least likely of these alternatives.



Before the new engine is flight tested, it must undergo a thorough ground check. Before this ground check can be made, several operations are usually performed on the engine.

To prevent failure of the engine bearings during the initial start, the engine should be pre-oiled.

When an engine has been idle for an extended period of time, its internal bearing surfaces are likely to become dry at points where the corrosion-preventive mixture has dried out or drained away from the bearings.

Hence, it is necessary to force oil throughout the entire engine oil system.

If the bearings are dry when the engine is started, the friction at high rpm destroys the bearings before lubricating oil from the engine-driven oil pump can reach them.

If the bearings have failed or are failing, the result is friction.

This friction can cause high temperatures (as friction causes heat) and excessive oil consumption.

• To enable the engine RPM to be increased with an 
•     accompanying in power and allow the propeller to         remain at lower, more efficient RPM.

The horsepower produced by a reciprocating engine is determined by its RPM.

The higher the RPM, the greater the power.

But the efficiency of a propeller decreases as the blade tip speed approaches the speed of sound.

In order to get the best of both conditions, many of the more powerful aircraft engines drive the propeller through a set of reduction gears.

Reduction gears allow the engine to turn fast enough to develop the required power.

At the same time, the propeller tip speed is kept low enough that the tips do not approach the speed of sound.

The increased brake horsepower delivered by a high horsepower engine results partly from increased crankshaft rpm.

It is therefore necessary to provide reduction gears to limit the propeller rotation speed to a value at which efficient operation is obtained.

Whenever the speed of the blade tips approaches the speed of sound, the efficiency of the propeller decreases rapidly.

Reduction gearing for engines allows the engine to operate at a higher rpm, developing more power while slowing down the propeller rpm.

This prevents the propeller efficiency from decreasing.

Since reduction gearing must withstand extremely high stresses, the gears are machined from steel forgings.

Many types of reduction gearing systems are in use.

The three types most commonly used are:

spur planetary

bevel planetary

spur and pinion

Improper valve timing, sharp bends in the induction          system, and high carburetor air temperatures.

• 1. Full throttle operation.
• 2. Low cylinder head temperatures.
• 3. Improper valve timing.
• 4. Sharp bends in the induction system.
• 5. High carburetor air temperatures.

The volumetric efficiency of a reciprocating engine is the ratio of the weight of the fuel/air charge taken into the cylinder, to the weight of a charge that would completely fill the entire volume of the cylinder at the same pressure.

Anything that decreases the weight of the air entering the cylinder decreases the volumetric efficiency.

Improper valve timing, sharp bends in the induction system, and high carburetor air temperature will all decrease the volumetric efficiency.



Volumetric efficiency is a ratio expressed in terms of percentages.

It is a comparison of the volume of fuel/air charge (corrected for temperature and pressure) inducted into the cylinders to the total piston displacement of the engine.

Various factors cause departure from a 100 percent volumetric efficiency.

The pistons of an naturally aspirated engine displace the same volume each time they travel from top center to bottom center of the cylinders.

The amount of charge that fills this volume on the intake stroke depends on the existing pressure and temperature of the surrounding atmosphere.

Therefore, to find the volumetric efficiency of an engine, standards for atmospheric pressure and temperature had to be established.

The U.S. standard atmosphere was established in 1958 and provides the necessary pressure and temperature values to calculate volumetric efficiency.

The standard sea level temperature is 59 °F, or 15 °C.

At this temperature, the pressure of one atmosphere is 14.69 lb/ in2, and this pressure supports a column of mercury (Hg) 29.92 inches high, or 29.92"Hg.

These standard sea level conditions determine a standard density, and if the engine draws in a volume of charge of this density exactly equal to its piston displacement, it is said to be operating at 100 percent volumetric efficiency.

An engine drawing in less volume than this has a volumetric efficiency lower than 100 percent.

An engine equipped with true supercharging (boost above 30.00 "Hg) may have a volumetric efficiency greater than 100 percent.

Deep-groove ball

Deep-groove ball bearings are used as the thrust bearing in most radial engines.

This type of bearing is the best of those listed for reducing friction while carrying both thrust and radial loads.

A ball bearing assembly consists of grooved inner and outer races, one or more sets of balls, in bearings designed for disassembly, and a bearing retainer.

They are used for shaft bearings and rocker arm bearings in some reciprocating engines. 

Special deep-groove ball bearings are used to transmit propeller thrust and radial loads to the engine nose section of radial engines. 

Since this type of bearing can accept both radial and thrust loads, it is used in gas turbine engines to support one end of a shaft (radial loads)and to keep the shaft from moving axially (thrust loads).

master rod bearing (radial engine)

The master rod bearing in a radial engine is always a plain bearing.

Rocker arm bearings may be either ball, roller, or plain type and the crankshaft main bearings for radial engines are usually ball bearings.



The master rod bearing in a radial engine is always a plain bearing.

Plain bearings are generally used for the crankshaft, cam ring, camshaft, connecting rods, and the accessory drive shaft bearings.

Such bearings are usually subjected to radial loads only, although some have been designed to take thrust loads.

Plain bearings are usually made of nonferrous (having no iron) metals, such as silver, bronze, aluminum, and various alloys of copper, tin, or lead.

Master rod or crankpin bearings in some engines are thin shells of steel, plated with silver on both the inside and the outside surfaces and with lead-tin plated over the silver on the inside surface only.

Smaller bearings, such as those used to support various shafts in the accessory section, are called bushings.

Porous Oilite bushings are widely used in this instance.

They are impregnated with oil so that the heat of friction brings the oil to the bearing surface during engine operation.

Master-and-Articulated Rod Assembly

The master-and-articulated rod assembly is commonly used in radial engines.

In a radial engine, the piston in one cylinder in each row is connected to the crankshaft by a master rod.

All other pistons in the row are connected to the master rod by articulated rods.

In an 18-cylinder engine, which has two rows of cylinders, there are two master rods and 16 articulated rods.

The articulated rods are constructed of forged steel alloy in either the I- or H-shape, denoting the cross-sectional shape.

Bronze bushings are pressed into the bores in each end of the articulated rod to provide knucklepin and piston-pin bearings.

The master rod serves as the connecting link between the piston pin and the crankpin.

The crankpin end, or the big end, contains the crankpin or master rod bearing.

Flanges around the big end provide for the attachment of the articulated rods.

The articulated rods are attached to the master rod by knuckle pins, which are pressed into holes in the master rod flanges during assembly. 

A plain bearing, usually called a piston-pin bushing, is installed in the piston end of the master rod to receive the piston pin.

When a crankshaft of the split-spline or split-clamp type is employed, a one-piece master rod is used.

The master and articulated rods are assembled and then installed on the crankpin; the crankshaft sections are then joined together.

In engines that use the one-piece type of crankshaft, the big end of the master rod is split, as is the master rod bearing.

The main part of the master rod is installed on the crankpin; then the bearing cap is set in place and bolted to the master rod.

The centers of the knuckle pins do not coincide with the center of the crankpin.

Thus, while the crankpin center describes a true circle for each revolution of the crankshaft, the centers of the knuckle pins describe an elliptical path.

The elliptical paths are symmetrical about a center line through the master rod cylinder.

It can be seen that the major diameters of the ellipses are not the same.

Thus, the link rods have varying degrees of angularity relative to the center of the crank throw.

greater

When a radial engine is operating, the cast aluminum alloy cylinder head expands far more than the steel push rod.

As the cylinder head expands, the rocker arm moves away from the cam ring and the hot, or running, valve clearance becomes much greater than the cold clearance.

1,283 cubic inches

The piston displacement of a reciprocating engine is the total volume swept by the pistons in one revolution of the crankshaft.

The piston displacement of one cylinder may be obtained by multiplying the area of the cross section of the cylinder by the total distance the piston moves in the cylinder in one stroke.

For multicylinder engines this product is multiplied by the number of cylinders to get the total piston displacement of the engine.

Since the volume (V) of a geometric cylinder equals the area (A) of the base multiplied by the altitude (H), it is expressed mathematically as:

V = A x H, multiply this by the number of cylinders to get the total piston displacement.

The area of the cylinder is found by: A = 3.1416 x R2

• So, for this problem:
• V = A x H
• V = 3.1416 x (2.75 x 2.75) x 6
• V = 3.1416 x 7.5625 x 6
• V = 23.7584 x 6
• V = 142.5504

Multiply the volume of a single cylinder by 9 to get the total piston displacement of 1,283 cubic inches.

intake, compression, ignition, power, and exhaust

The five events that take place in a reciprocating engine during each cycle of its operation are:

Intake -- The fuel/air mixture is taken into the cylinder.

Compression -- The fuel/air mixture is compressed as the piston moves upward (outward) in the cylinder.

Ignition -- As the piston nears the top of its stroke, an electrical spark ignites the mixture so it burns and releases its energy.

Power -- As the fuel/air mixture burns, it forces the piston downward. This movement of the piston rotates the crankshaft and performs useful work.

Exhaust -- After the piston has reached the bottom of its stroke and done the most of its useful work, the piston pushes upward, forcing the burned gases out of the cylinder.



Four-Stroke Cycle

The vast majority of certified aircraft reciprocating engines operate on the four-stroke cycle, sometimes called the Otto cycle after its originator, a German physicist.

The four-stroke cycle engine has many advantages for use in aircraft.

One advantage is that it lends itself readily to high performance through supercharging.

In this type of engine, four strokes are required to complete the required series of events or operating cycle of each cylinder.

Two complete revolutions of the crankshaft (720°) are required for the four strokes; thus, each cylinder in an engine of this type fires once in every two revolutions of the crankshaft.

In the following discussion of the four-stroke cycle engine operation, note that the timing of the ignition and the valve events vary considerably in different engines.

Many factors influence the timing of a specific engine, and it is most important that the engine manufacturer’s recommendations in this respect be followed in maintenance and overhaul.

The top or head of the cylinder opens to allow the burned gases to escape, and the momentum of the crankshaft and the propeller forces the piston back up in the cylinder where it is ready for the next event in the cycle.

Another valve in the cylinder head then opens to let in a fresh charge of the fuel/air mixture.

The valve allowing for the escape of the burning exhaust gases is called the exhaust valve, and the valve which lets in the fresh charge of the fuel/air mixture is called the intake valve.

These valves are opened and closed mechanically at the proper times by the valve-operating mechanism. 

The bore of a cylinder is its inside diameter.

The stroke is the distance the piston moves from one end of the cylinder to the other, specifically from top dead center (TDC) to bottom dead center (BDC), or vice versa

provide for balance and eliminate vibration to the greatest  extent possible.

The firing order of an engine is the sequence in which the power event occurs in the different cylinders. 

The firing order is designed to provide for balance and to eliminate vibration to the greatest extent possible.

• In radial engines, the firing order must follow a special pattern since the firing impulses must follow the motion of the crank throw during its rotation.
• In inline engines, the firing orders may vary somewhat, yet most orders are arranged so that the firing of cylinders is evenly distributed along the crankshaft.

• Six-cylinder inline engines generally have a firing order of 1-5-3-6-2-4.
• Cylinder firing order in opposed engines can usually be listed in pairs of cylinders, as each pair fires across the center main bearing.

The firing order of six-cylinder opposed engines is 1-4-5-2- 3-6.

The firing order of one model four-cylinder opposed engine is 1-4-2-3, but on another model it is 1-3-2-4.

• Opposed Engine
• https://en.wikipedia.org/wiki/Opposed-piston_engine

just after top center at the beginning of the power stroke. 

The ignition of the fuel/air mixture in the cylinder of a reciprocating engine is timed so it occurs when the piston is about 20 to 30 degrees of crankshaft rotation before reaching top center on the compression stroke.

If the mixture ratio and ignition timing are both correct, the fuel/air mixture will be all burned shortly after the piston passes over top dead center.

Top dead center

The position of a piston in a reciprocating engine when the piston is at the top of its stroke and the wrist pin, crankpin, and center of the crankshaft are all in line.

The expanding gases caused by absorbing heat from the burning mixture will exert the maximum amount of push on the descending piston during the power stroke

• 
• 

preignition and burned valves.

If a valve is ground with a feather edge (a thin edge) the heat in the cylinder will cause the thin area to glow red hot and this will ignite the fuel/air mixture before the correct time for ignition.

This will result in preignition and burned valves.

An important precaution in valve grinding, as in any kind of grinding, is to make light cuts only. Heavy cuts cause chattering, that may make the valve surface so rough that much time is lost in obtaining the desired finish.

After grinding, check the valve margin to be sure that the valve edge has not been ground too thin.

A thin edge is called a feather edge and can lead to preignition; the valve edge would burn away in a short period of time, and the cylinder would have to be overhauled again.

Moveable counterweights serve to reduce the torsional vibrations in an aircraft reciprocating engine. 

Torsional vibration caused by firing impulses of the engine are minimized by the installation of moveable counterweights suspended from certain crank cheeks.

• These moveable counterweights, called dynamic dampers, rock back and forth and act as pendulums, changing the resonant frequency of the rotating elements, thus reducing the torsional vibration.

• A crankshaft is dynamically balanced when all the forces created by crankshaft rotation and power impulses are balanced within themselves so that little or no vibration is produced when the engine is operating. 
•  
• To reduce vibration to a minimum during engine operation, dynamic dampers are incorporated on the crankshaft. 
•  
• A dynamic damper is merely a pendulum that is fastened to the crankshaft so that it is free to move in a small arc. 
•  
• It is incorporated in the counterweight assembly. 
•  
• Some crankshafts incorporate two or more of these assemblies, each being attached to a different crank cheek. 
•  
• The distance the pendulum moves and, thus, its vibrating frequency corresponds to the frequency of the power impulses of the engine. 
•  
• When the vibration frequency of the crankshaft occurs, the pendulum oscillates out of time with the crankshaft vibration, thus reducing vibration to a minimum.

The construction of the dynamic damper used in one engine consists of a movable slotted-steel counterweight attached to the crank cheek. 

Two spool-shaped steel pins extend into the slot and pass through oversized holes in the counterweight and crank cheek.

The difference in the diameter between the pins and the holes provides a pendulum effect.

An analogy of the functioning of a dynamic damper is shown below:

Exhaust and intake. 

Both the intake and exhaust valve are open at the same time only during the period of valve overlap.

Valve overlap occurs at the end of the exhaust stroke and the beginning of the intake stroke.

The intake valve opens a few degrees of crankshaft rotation before the piston reaches the top of the exhaust stroke.

The exhaust valve remains open until the piston has moved down a few degrees of crankshaft rotation on the intake stroke.

• 
• 

 Plain.

• Master rods used in radial engines have plain bearings in both their big end that fits around the throw of the crankshaft and
• the small end that fits around the wrist pin in the piston.

A bearing is any surface which supports, or is supported by, another surface.

A good bearing must be composed of material that is strong enough to withstand the pressure imposed on it and should permit the other surface to move with a minimum of friction and wear.

The parts must be held in position within very close tolerances to provide efficient and quiet operation, and yet allow freedom of motion.

To accomplish this, and at the same time reduce friction of moving parts so that power loss is not excessive, lubricated bearings of many types are used.

Bearings are required to take radial loads, thrust loads, or a combination of the two.

An example of a radial load would be a rotating shaft being held or contained in one position on a radial plane.

Thrust load would be the rotating shaft being contained from moving axially along the shafts axis.

These radial and thrust loads are illustrated in the figure below:

• 
• There are two ways in which bearing surfaces move in relation to each other.

One is by the sliding movement of one metal against the other (sliding friction), and the second is for one surface to roll over the other (rolling friction).

The three different types of bearings in general use are plain, roller, and ball.

• 
• Plain bearings are generally used for the crankshaft, cam ring, camshaft, connecting rods, and the accessory drive shaft bearings.
• Such bearings are usually subjected to radial loads only, although some have been designed to take thrust loads.

Plain bearings are usually made of nonferrous (having no iron) metals, such as silver, bronze, aluminum, and various alloys of copper, tin, or lead.

Master rod or crankpin bearings in some engines are thin shells of steel, plated with silver on both the inside and the outside surfaces and with lead-tin plated over the silver on the inside surface only.

Smaller bearings, such as those used to support various shafts in the accessory section, are called bushings.

Porous Oilite bushings are widely used in this instance.

They are impregnated with oil so that the heat of friction brings the oil to the bearing surface during engine operation.

• 
• A ball bearing assembly consists of grooved inner and outer races, one or more sets of balls, in bearings designed for disassembly, and a bearing retainer.

They are used for shaft bearings and rocker arm bearings in some reciprocating engines.

Special deep-groove ball bearings are used to transmit propeller thrust and radial loads to the engine nose section of radial engines.

Since this type of bearing can accept both radial and thrust loads, it is used in gas turbine engines to support one end of a shaft (radial loads) and to keep the shaft from moving axially (thrust loads).

• 
• Roller bearings are made in many types and shapes, but the two types generally used in the aircraft engine are the straight roller and the tapered roller bearings.

Straight roller bearings are used where the bearing is subjected to radial loads only.

In tapered roller bearings, the inner- and outer-race bearing surfaces are cone-shaped.

Such bearings withstand both radial and thrust loads.

Straight roller bearings are used in high power reciprocating aircraft engines for the crankshaft main bearings.

They are also used in gas turbine applications where radial loads are high.

Generally, a rotating shaft in a gas turbine engine is supported by a deep-groove ball bearing (radial and thrust loads) on one end and a straight roller bearing (radial loads only) on the other end.

 brake horsepower.

The actual horsepower delivered to the propeller of an aircraft engine is called brake horsepower.

This name is used because brake horsepower was originally measured with a prony brake loading the engine with mechanical friction.

Modern measurements of brake horsepower are made with a dynamometer which loads the engine with electrical or fluid flow opposition.

Indicated horsepower is the theoretical power of a frictionless engine.

The total horsepower lost in overcoming friction must be subtracted from the indicated horsepower to arrive at the actual horsepower delivered to the propeller.

• The power delivered to the propeller for useful work is known as brake horsepower (bhp).
• The difference between indicated and brake horsepower is known as friction horsepower
• which is the horsepower required to overcome mechanical losses, such as the pumping action of the pistons, the friction of the pistons, and the friction of all other moving parts.

provide a better fit at operating temperatures. 

• A cam-ground piston is one whose diameter is a few thousandths of an inch greater in a plane perpendicular to the wrist pin boss than it is parallel to the boss.
• When the piston reaches its operating temperature, the large mass of metal in the piston pin boss expands enough that the piston becomes round.
• Since the piston is round at its operating temperature, it provides a better seal than it would if it were round while cold and expanded to an out-of-round condition when hot.

245°.

The intake valve closes 45° of crankshaft rotation after the piston passes bottom dead center, moving upward on the compression stroke.

Both valves are closed at this point, and they both remain closed until the piston passes over top center and comes down to 70° before bottom dead center on the power stroke.

At this time the exhaust valve opens.

• Both valves are on their seats for 45° + 180° + 20°, or 245°.

provide a straight cylinder bore at operating temperatures.


Some aircraft engine cylinders are ground with the diameter at the top of the barrel, where it screws into the head, slightly smaller than the diameter in the center of the barrel.

This is called choke grinding.

The large mass of the cylinder head expands more when heated than the smaller mass of the cylinder barrel, so the diameter of a choke-ground cylinder becomes uniform when the engine is at its operating temperature.

When a piston engine is running, the top of the cylinder (near the head) is at a higher temperature than the bottom (near the crankcase).

The higher temperature at the top causes the cylinder to expand and loose compression.

To compensate for this, some aircraft engine manufacturers equip their product with choked or taper-ground cylinders in order to provide a straight cylinder bore at operating temperatures.

Cylinder grinding is accomplished by a firmly mounted stone that revolves around the cylinder bore, as well as up and down the length of the cylinder barrel.

The cylinder, the stone, or both may move to get this relative movement.

The size of the grind is determined by the distance the stone is set away from the centerline of the cylinder.

Some cylinder bore grinding machines produce a perfectly straight bore, while others are designed to grind a choked bore. 

A choked bore grind refers to the manufacturing process in which the cylinder walls arc ground to produce a smaller internal diameter at the top than at the bottom.

The purpose of this type grind or taper is to maintain a straight cylinder wall during operation.

As a cylinder heats up during operation, the head and top of the cylinder are subjected to more heat than the bottom.

This causes greater expansion at the top than at the bottom, thereby maintaining the desired straight wall.

After grinding a cylinder, it may be necessary to hone the cylinder bore to produce the desired finish.

In this case, specify the cylinder regrind size to allow for some metal removal during honing.

The usual allowance for honing is 0.001 inch.

If a final cylinder bore size of 3.890 inches is desired, specify the regrind size of 3.889 inches, and then hone to 3.890 inches.

During normal operation. 

There is no clearance in the valve operating mechanism when an engine equipped with hydraulic valve lifters is operating normally and the minimum oil and cylinder-head temperatures for takeoff have been reached.

Hydraulic valve lifters are used because they remove all of the clearance between the rocker arm and the tip of the valve stem.

By keeping all of this clearance removed, the valves operate with less noise and less wear.

Hydraulic valve lifters are normally adjusted at the time of overhaul.

They are assembled dry (no lubrication), clearances checked, and adjustments are usually made by use of pushrods having different lengths.

A minimum and maximum valve clearance is established.

Any measurement between these extremes is acceptable but approximately half way between the extremes is desired.

Hydraulic valve lifters require less maintenance, are better lubricated, and operate more quietly than the screw adjustment type.

• Hydraulic valve lifters have no clearance in their valve-operating mechanism after the minimum inlet oil and cylinder head temperatures have been reached

 Timing disk. 

A top dead center indicator is used to show when the piston in cylinder number one is on top dead center.

A timing disk is clamped to the propeller shaft and positioned so the pointer, which is held straight up by a weight on one end, points to zero degrees.

As the crankshaft is rotated, the pointer indicates on the scale of the timing disk the number of degrees the crankshaft has rotated.

The timing disk is a more accurate crankshaft positioning device than the timing reference marks.

This device consists of a disk and a pointer mechanism mounted on an engine driven accessory or its mounting pad.

The pointer, which is indirectly connected to the accessory drive, indicates the number of degrees of crankshaft travel on the disk.

The disk is marked off in degrees of crankshaft travel.

By applying a slight torque to the accessory drive gear in a direction opposite that of the normal rotation, the backlash in the accessory gear train can be removed to the extent that a specific crankshaft position can be obtained with accuracy time after time.

Timing Disks

Most timing disk devices are mounted to the crankshaft flange and use a timing plate.

The markings vary according to the specifications of the engine.

This plate is temporarily installed on the crankshaft flange with a scale numbered in crankshaft degrees and the pointer attached to the timing disk.

 at maximum velocity 90° after TDC. 

The piston in a reciprocating engine is not moving when it is at the top and bottom of its stroke.

As it leaves top dead center, it accelerates from zero velocity to a maximum velocity, which is reached when it is 90° beyond top dead center.

It then decelerates to zero velocity at bottom dead center.

backfiring into the induction system. 

The intake valve opens when the piston is moving upward at the end of the exhaust stroke.

Opening at this point allows the low pressure caused by the inertia of the exiting exhaust gases to assist in starting the fuel/air mixture flowing into the cylinder.

If the intake valve opens too early, some of the burning exhaust gases could flow into the intake manifold and ignite the mixture.

This would cause a backfire in the induction system.

If the intake valve opens too soon, some of the hot exhaust gases could flow in to the intake manifold and ignite the fuel-air mixture.

The result would be a backfire.

There are specific instructions concerning mixture ratios for each type of engine under various operating conditions.

Failure to follow these instructions results in poor performance and often in damage to the engine.

Excessively rich mixtures result in loss of power and waste of fuel.

With the engine operating near its maximum output, very lean mixtures cause a loss of power and, under certain conditions, serious overheating.

When the engine is operated on a lean mixture, the cylinder head temperature gauge should be watched closely.

If the mixture is excessively lean, the engine may backfire through the induction system or stop completely.

Backfire results from slow burning of the lean mixture. If the charge is still burning when the intake valve opens, it ignites the fresh mixture and the flame travels back through the combustible mixture in the induction system.

The intake valve is opened considerably before the piston reaches TDC on the exhaust stroke, in order to induce a greater quantity of the fuel/air charge into the cylinder and thus increase the horsepower.

• The distance the valve may be opened before TDC, however, is limited by several factors, such as the possibility that hot gases remaining in the cylinder from the previous cycle may flash back into the intake pipe and the induction system.
• 
• Afterfiring vs Backfiring

You may have heard people on the street talk about hearing a "car's exhaust backfire."

While we all understand what they mean by this, this is also technically wrong.

What they are describing is AFTERFIRING through the exhaust. "Backfiring" involves combustion coming "back" through the induction system.

Backfiring through the intake is normally associated with the mixture being too lean.

Any FAA answer choices that says something like "backfiring through the exhaust" is INCORRECT.

The FAA often uses this as a distractor.

Backfiring

When a fuel/air mixture does not contain enough fuel to consume all the oxygen, it is called a lean mixture.

Conversely, a charge that contains more fuel than required is called a rich mixture.

An extremely lean mixture either does not burn at all or burns so slowly that combustion is not complete at the end of the exhaust stroke.

The flame lingers in the cylinder and then ignites the contents in the intake manifold or the induction system when the intake valve opens.

This causes an explosion known as backfiring, which can damage the carburetor and other parts of the induction system.

Incorrect ignition timing, or faulty ignition wires, can cause the cylinder to fire at the wrong time, allowing the cylinder to fire when the intake valve is open, which can cause backfiring.

A point worth stressing is that backfiring rarely involves the whole engine.

Therefore, it is seldom the fault of the carburetor.

In practically all cases, backfiring is limited to one or two cylinders.

Usually, it is the result of faulty valve clearance setting, defective fuel injector nozzles, or other conditions that cause these cylinders to operate leaner than the engine as a whole.

There can be no permanent cure until these defects are discovered and corrected. Because these backfiring cylinders fire intermittently and, therefore, run cool, they can be detected by the cold cylinder check.

In some instances, an engine backfires in the idle range but operates satisfactorily at medium and high power settings.

The most likely cause, in this case, is an excessively lean idle mixture.

Proper adjustment of the idle fuel/air mixture usually corrects this difficulty.

Afterfiring

Afterfiring, sometimes called afterburning (when this won't be confused with military-jet-type afterburning), often results when the fuel/air mixture is too rich.

Overly rich mixtures are also slow burning, therefore, charges of unburned fuel are present in the exhausted gases.

Air from outside the exhaust stacks mixes with this unburned fuel that ignites.

This causes an explosion in the exhaust system.

Afterfiring is perhaps more common where long exhaust ducting retains greater amounts of unburned charges.

As in the case of backfiring, the correction for afterfiring is the proper adjustment of the fuel/air mixture.

Afterfiring can also be caused by cylinders that are not firing because of faulty spark plugs, defective fuel-injection nozzles. or incorrect valve clearance.

The unburned mixture from these dead cylinders passes into the exhaust system, where it ignites and burns.

Unfortunately, the resultant torching or afterburning can easily be mistaken for evidence of a rich carburetor.

Cylinders that are firing intermittently can cause a similar effect.

Again, the malfunction can be remedied only by discovering the real cause and correcting the defect.

Dead or intermittent cylinders can be located by the cold cylinder check.

nitriding. 

The walls of an aircraft engine cylinder are subjected to a great deal of wear as the iron piston rings rub against them.

The walls of some cylinders are treated to increase their hardness and resistance to wear.

There are two methods of hardening these surfaces: hard chrome plating and nitriding.

Nitriding is a process in which the surface of the steel cylinder wall is changed into a hard nitride by an infusion of nitrogen from the ammonia gas used in the nitriding heat treatment process.



Cylinder Barrels:

The cylinder barrel in which the piston operates must be made of a high-strength material, usually steel.

It must be as light as possible, yet have the proper characteristics for operating under high temperatures.

It must be made of a good bearing material and have high tensile strength.

The cylinder barrel is made of a steel alloy forging with the inner surface hardened to resist wear of the piston and the piston rings which bear against it. 

This hardening is usually done by exposing the steel to ammonia or cyanide gas while the steel is very hot.

The steel soaks up nitrogen from the gas, which forms iron nitrides on the exposed surface.

As a result of this process, the metal is said to be nitrided.

This nitriding only penetrates into the barrel surface a few thousands of an inch. 

As the cylinder barrels wear due to use, they can be repaired by chroming.

This is a process that plates chromium on the surface of the cylinder barrel and brings it back to new standard dimensions.

Chromium-plated cylinders should use cast iron rings. 

Honing the cylinder walls is a process that brings it to the correct dimensions and provides crosshatch pattern for seating the piston rings during engine break-in.

Some engine cylinder barrels are choked at the top, or they are smaller in diameter to allow for heat expansion and wear.

In some instances, the barrel has threads on the outside surface at one end so that it can be screwed into the cylinder head.

The cooling fins are machined as an integral part of the barrel and have limits on repair and service.

Nitriding is unlike other casehardening processes in that, before nitriding, the part is heat treated to produce definite physical properties.

Thus, parts are hardened and tempered before being nitrided.

Most steels can be nitrided, but special alloys are required for best results.

These special alloys contain aluminum as one of the alloying elements and are called "nitralloys."

In nitriding, the part is placed in a special nitriding furnace and heated to a temperature of approximately 1,000°F. With the part at this temperature, ammonia gas is circulated within the specially constructed furnace chamber.

The high temperature cracks the ammonia gas into nitrogen and hydrogen.

The ammonia which does not break down is caught in a water trap below the regions of the other two gases.

The nitrogen reacts with the iron to form nitride.

The iron nitride is dispersed in minute particles at the surface and works inward.

The depth of penetration depends on the length of the treatment.

In nitriding, soaking periods as long as 72 hours are frequently required to produce the desired thickness of case.

Nitriding can be accomplished with a minimum of distortion, because of the low temperature at which parts are casehardened and because no quenching is required after exposure to the ammonia gas.

The intake valve closes on the intake stroke. 

The intake valve in a four-stroke cycle aircraft engine closes somewhere around 60° after bottom center on the compression stroke.

The exhaust valve opens about 70° before bottom center on the power stroke.

The intake valve opens about 20° before top center on the exhaust stroke.

The exhaust valve closes about 15° after top center on the intake stroke.

In a four-stroke engine the conversion of chemical energy into mechanical energy occurs over a four stroke operating cycle.

The intake, compression, power, and exhaust processes occur in four separate strokes of the piston.

The intake stroke begins as the piston starts its downward travel.

When this happens, the intake valve opens and the fuel/air mixture is drawn into the cylinder.

The compression stroke begins when the intake valve closes and the piston starts moving back to the top of the cylinder.

This phase of the cycle is used to obtain a much greater power output from the fuel/air mixture once it is ignited.

The power stroke begins when the fuel/air mixture is ignited.

This causes a tremendous pressure increase in the cylinder, and forces the piston downward away from the cylinder head, creating the power that turns the crankshaft.

The exhaust stroke is used to purge the cylinder of burned gases.

• It begins when the exhaust valve opens and the piston starts to move toward the cylinder head once again
• 
• Intake Stroke
• During the intake stroke, the piston is pulled downward in the cylinder by the rotation of the crankshaft.

This reduces the pressure in the cylinder and causes air under atmospheric pressure to flow through the carburetor, which meters the correct amount of fuel.

The fuel/air mixture passes through the intake pipes and intake valves into the cylinders.

The quantity or weight of the fuel/air charge depends upon the degree of throttle opening.

The intake valve is opened considerably before the piston reaches TDC on the exhaust stroke, in order to induce a greater quantity of the fuel/air charge into the cylinder and thus increase the horsepower.

The distance the valve may be opened before TDC, however, is limited by several factors, such as the possibility that hot gases remaining in the cylinder from the previous cycle may flash back into the intake pipe and the induction system.

In all high-power aircraft engines, both the intake and the exhaust valves are off the valve seats at TDC at the start of the intake stroke.

As mentioned above, the intake valve opens before TDC on the exhaust stroke (valve lead), and the closing of the exhaust valve is delayed considerably after the piston has passed TDC and has started the intake stroke (valve lag).

This timing is called valve overlap and is designed to aid in cooling the cylinder internally by circulating the cool incoming fuel/air mixture, to increase the amount of the fuel/ air mixture induced into the cylinder, and to aid in scavenging the byproducts of combustion from the cylinder.

The intake valve is timed to close about 50° to 75° past BDC on the compression stroke, depending upon the specific engine, to allow the momentum of the incoming gases to charge the cylinder more completely.

Because of the comparatively large volume of the cylinder above the piston when the piston is near BDC, the slight upward travel of the piston during this time does not have a great effect on the incoming flow of gases.

This late timing can be carried too far because the gases may be forced back through the intake valve and defeat the purpose of the late closing.

• Compression Stroke
• After the intake valve is closed, the continued upward travel of the piston compresses the fuel/air mixture to obtain the desired burning and expansion characteristics.

The charge is fired by means of an electric spark as the piston approaches TDC.

The time of ignition varies from 20° to 35° before TDC, depending upon the requirements of the specific engine to ensure complete combustion of the charge by the time the piston is slightly past the TDC position.

Many factors affect ignition timing, and the engine manufacturer has expended considerable time in research and testing to determine the best setting.

All engines incorporate devices for adjusting the ignition timing, and it is most important that the ignition system be timed according to the engine manufacturer’s recommendations.

• Power Stroke
• As the piston moves through the TDC position at the end of the compression stroke and starts down on the power stroke, it is pushed downward by the rapid expansion of the burning gases within the cylinder head with a force that can be greater than 15 tons (30,000 psi) at maximum power output of the engine.

The temperature of these burning gases may be between 3,000° and 4,000 °F.

As the piston is forced downward during the power stroke by the pressure of the burning gases exerted upon it, the downward movement of the connecting rod is changed to rotary movement by the crankshaft.

Then, the rotary movement is transmitted to the propeller shaft to drive the propeller.

As the burning gases are expanded, the temperature drops to within safe limits before the exhaust gases flow out through the exhaust port.

The timing of the exhaust valve opening is determined by, among other considerations, the desirability of using as much of the expansive force as possible and of scavenging the cylinder as completely and rapidly as possible. 

The exhaust valve is opened considerably before BDC on the power stroke (on some engines at 50° and 75° before BDC) while there is still some pressure in the cylinder. 

This timing is used so that the pressure can force the gases out of the exhaust port as soon as possible.

This process frees the cylinder of waste heat after the desired expansion has been obtained and avoids overheating the cylinder and the piston.

Thorough scavenging is very important, because any exhaust products remaining in the cylinder dilute the incoming fuel/air charge at the start of the next cycle.

• Exhaust Stroke
• As the piston travels through BDC at the completion of the power stroke and starts upward on the exhaust stroke, it begins to push the burned exhaust gases out the exhaust port.

The speed of the exhaust gases leaving the cylinder creates a low pressure in the cylinder.

This low or reduced pressure speeds the flow of the fresh fuel/air charge into the cylinder as the intake valve is beginning to open.

The intake valve opening is timed to occur at 8° to 55° before TDC on the exhaust stroke on various engines.

 Near the top of the cylinder. 

In normal operation, an aircraft engine cylinder wears more at the top than in the center or at the bottom.

This greater wear is caused by the heat of combustion decreasing the efficiency of the lubrication at the top of the cylinder.

• Inspect the cylinder barrel for wear, using a cylinder bore gauge, a telescopic gauge, and micrometer or an inside micrometer.
• 

Dimensional inspection of the barrel consists of the following measurements:

• Maximum taper of cylinder walls
• Maximum out-of-roundness
• Bore diameter
• Step
• Fit between piston skirt and cylinder

All measurements involving cylinder barrel diameters must be taken at a minimum of two positions 90° apart in the particular plane being measured.

It may be necessary to take more than two measurements to determine the maximum wear.

Taper of the cylinder walls is the difference between the diameter of the cylinder barrel at the bottom and the diameter at the top. 

The cylinder is usually worn larger at the top than at the bottom.

This taper is caused by the natural wear pattern.

At the top of the stroke, the piston is subjected to greater heat and pressure and more erosive environment than at the bottom of the stroke.

Also, there is greater freedom of movement at the top of the stroke. 

Under these conditions, the piston wears the cylinder wall more at the top of the cylinder.

In most cases, the taper ends with a ridge, that must be removed during overhaul.

with a contour or radius gauge. 

One recommended way of checking exhaust valves for stretch is by measuring the diameter of the valve stem with a vernier outside micrometer caliper at a point specified by the engine manufacturer.

If the valve has stretched, the stem diameter will be smaller than it should be.

Another way of determining if a valve has been stretched is by using a valve radius gauge to see if the radius between the valve stem and head is the same radius the valve had when it was manufactured.




If a micrometer is used, stretch is found as a smaller diameter of the valve stem near the neck of the valve.

Measure the diameter of the valve stem, and check the fit of the valve in its guide.

Shortly before the piston reaches the top of the compression stroke. 

Ignition occurs in a reciprocating engine somewhere around 30° of crankshaft rotation before the piston reaches top center on the compression stroke.

By timing the ignition to occur when the piston is in this position, the maximum pressure inside the cylinder is reached just after the piston passes over top center and starts down on the power stroke.

The fuel/air mixture is ignited by the spark in the combustion chamber and commences burning as the piston travels toward top dead center on the compression stroke.

The ignited charge is rapidly expanding at this time, and pressure is increasing so that as the piston travels through the top dead center position, it is driven downward on the power stroke.

• The intake and exhaust valve ports are located in the cylinder head along with the spark plugs and the intake and exhaust valve actuating mechanisms.
• 
• 

373° 

The crankshaft rotates 28° on the compression stroke after the ignition occurs.

The crankshaft rotates 180° on the power stroke.

The crankshaft rotates 165° on the exhaust stroke before the intake valve opens.

• The total crankshaft rotation between the time ignition occurs and the time the intake valve opens is:
• 28° + 180° + 165° = 373°.

• The total crankshaft rotation between ignition and intake valve opening is:
• 28° on the compression stroke after ignition, plus

180° on the power stroke, plus

165° on the exhaust stroke before the exhaust valve opens

• 28° + 180° + 165° = 373°
• 

To prevent valves from falling into the combustion chamber. 

Some aircraft engine poppet valves have a groove cut in their stem that is fitted with a safety circlet, a small snap ring that grips the valve stem in this groove.

If the tip of the valve stem should ever break off in operation, this safety circlet will contact the top of the valve guide and prevent the valve from dropping into the cylinder.

The valve head has a ground face that forms a seal against the ground valve seat in the cylinder head when the valve is closed.

The face of the valve is usually ground to an angle of either 30° or 45°.

In some engines, the intake-valve face is ground to an angle of 30°, and the exhaust-valve face is ground to a 45° angle.

Valve faces are often made more durable by the application of a material called stellite.

About 1⁄16 inch of this alloy is welded to the valve face and ground to the correct angle.

Stellite is resistant to high-temperature corrosion and also withstands the shock and wear associated with valve operation.

Some engine manufacturers use a nichrome facing on the valves.

This serves the same purpose as the stellite material.

• The valve stem acts as a pilot for the valve head and rides in the valve guide installed in the cylinder head for this purpose
• 
• The valve stem is surface hardened to resist wear.

The neck is the part that forms the junction between the head and the stem.

The tip of the valve is hardened to withstand the hammering of the valve rocker arm as it opens the valve.

A machined groove on the stem near the tip receives the split-ring stem keys. 

• These stem keys form a lock ring to hold the valve spring retaining washer in place.
• 

during which both valves are off their seats. 

• Valve overlap is the number of degrees of crankshaft rotation that both the intake and exhaust valves are off their seat at the end of the exhaust stroke and the beginning of the intake stroke.
• Valve overlap allows a greater charge of fuel/air mixture to be inducted into the cylinder.

For a reciprocating engine to operate properly, each valve must open at the proper time, stay open for the required length of time, and close at the proper time.

Intake valves are opened just before the piston reaches top dead center, and exhaust valves remain open after top dead center.

At a particular instant, therefore, both valves are open at the same time (end of the exhaust stroke and beginning of the intake stroke).

This valve overlap permits better volumetric efficiency and lowers the cylinder operating temperature.

• This timing of the valves is controlled by the valve operating mechanism.
• 

a specified amount above zero 

Hydraulic valve lifters are used to keep all of the clearance out of the valve system when the engine is operating and the lifters are pumped up.

When the lifters are completely flat, there will be clearance in the system of a specified amount above zero.

Hydraulic Valve Tappets/Lifters

Some aircraft engines incorporate hydraulic tappets that automatically keep the valve clearance at zero, eliminating the necessity for any valve clearance adjustment mechanism.

• A typical hydraulic tappet (zero-lash valve lifter) is shown below.
• 
• When the engine valve is closed, the face of the tappet body (cam follower) is on the base circle or back of the cam.

The light plunger spring lifts the hydraulic plunger so that its outer end contacts the push rod socket, exerting a light pressure against it, thus eliminating any clearance in the valve linkage.

As the plunger moves outward, the ball check valve moves off its seat.

Oil from the supply chamber, which is directly connected with the engine lubrication system, flows in and fills the pressure chamber.

As the camshaft rotates, the cam pushes the tappet body and the hydraulic lifter cylinder outward.

This action forces the ball check valve onto its seat; thus, the body of oil trapped in the pressure chamber acts as a cushion.

During the interval when the engine valve is off its seat, a predetermined leakage occurs between plunger and cylinder bore, which compensates for any expansion or contraction in the valve train. 

Immediately after the engine valve closes, the amount of oil required to fill the pressure chamber flows in from the supply chamber, preparing for another cycle of operation.

Hydraulic valve lifters are normally adjusted at the time of overhaul.

They are assembled dry (no lubrication), clearances checked, and adjustments are usually made by using push rods of different lengths.

A minimum and maximum valve clearance is established.

Any measurement between these extremes is acceptable, but approximately half way between the extremes is desired.

• Hydraulic valve lifters require less maintenance, are better lubricated, and operate more quietly than the screw adjustment type.

compression stroke 

• The intake valve closes when the piston is moving upward on the compression stroke.
• At this time, the exhaust valve is already closed.

In a four-stroke engine the conversion of chemical energy into mechanical energy occurs over a four stroke operating cycle.

The intake, compression, power, and exhaust processes occur in four separate strokes of the piston.

• The intake stroke begins as the piston starts its downward travel.
• When this happens, the intake valve opens and the fuel/air mixture is drawn into the cylinder.

The compression stroke begins when the intake valve closes and the piston starts moving back to the top of the cylinder.

This phase of the cycle is used to obtain a much greater power output from the fuel/air mixture once it is ignited.

The power stroke begins when the fuel/air mixture is ignited.

This causes a tremendous pressure increase in the cylinder, and forces the piston downward away from the cylinder head, creating the power that turns the crankshaft.

The exhaust stroke is used to purge the cylinder of burned gases.

• It begins when the exhaust valve opens and the piston starts to move toward the cylinder head once again.
• 

 Three 

The maximum compression ratio of an engine is limited by the ability of the engine to withstand detonation in its cylinders.

The detonation characteristics of the fuel used is a limiting factor.

Fuels having a low critical pressure and temperature must not be used with high compression engines.

The design limitations of the engine are important, because engines that are not designed strong enough to withstand high cylinder pressures, must not have a high compression ratio.

The degree of supercharging is extremely important, because the cylinder pressures are a function of both the initial pressure in the cylinder (the pressure caused by the supercharger) and the compression ratio.

• The only alternative that does not limit the compression ratio is the spark plug reach.
• Three of the factors below establish the maximum compression ratio limitations of an aircraft engine.

• YES: Detonation characteristics of the fuel used.
• YES: Design limitations of the engine.
• YES: Degree of supercharging.
• NO: Spark plug reach.

Compression Ratio

All internal combustion engines must compress the fuel/air mixture to receive a reasonable amount of work from each power stroke.

The fuel/air charge in the cylinder can be compared to a coil spring in that the more it is compressed, the more work it is potentially capable of doing.

• The compression ratio of an engine is a comparison of the volume of space in a cylinder when the piston is at the bottom of the stroke to the volume of space when the piston is at the top of the stroke.
• 
• This comparison is expressed as a ratio, hence the term compression ratio. 

Compression ratio is a controlling factor in the maximum horsepower developed by an engine, but it is limited by present day fuel grades and the high engine speeds and manifold pressures required for takeoff. 

For example, if there are 140 cubic inches of space in the cylinder when the piston is at the bottom and there are 20 cubic inches of space when the piston is at the top of the stroke, the compression ratio would be 140 to 20.

If this ratio is expressed in fraction form, it would be 140/20 or 7 to 1, usually represented as 7:1.

The limitations placed on compression ratios, manifold pressure, and the manifold pressure’s effect on compression pressures has a major effect on engine operation. 

Manifold pressure is the average absolute pressure of the air or fuel/ air charge in the intake manifold and is measured in units of inches of mercury ("Hg).

Manifold pressure is dependent on engine speed (throttle setting) and the degree supercharging.

The operation of the supercharger increases the weight of the charge entering the cylinder.

When a true supercharger is used with the aircraft engine, the manifold pressure may be considerably higher than the pressure of the outside atmosphere.

The advantage of this condition is that a greater amount of charge is forced into a given cylinder volume, and a greater output of horsepower results.

Compression ratio and manifold pressure determine the pressure in the cylinder in that portion of the operating cycle when both valves are closed.

The pressure of the charge before compression is determined by manifold pressure, while the pressure at the height of compression (just prior to ignition) is determined by manifold pressure times the compression ratio.

For example, if an engine were operating at a manifold pressure of 30 "Hg with a compression ratio of 7:1, the pressure at the instant before ignition would be approximately 210 "Hg.

However, at a manifold pressure of 60 "Hg, the pressure would be 420 "Hg.

Without going into great detail, it has been shown that the compression event magnifies the effect of varying the manifold pressure, and the magnitude of both affects the pressure of the fuel charge just before the instant of ignition.

If the pressure at this time becomes too high, pre-ignition or detonation occur and produce overheating.

Pre-ignition is when the fuel air charge starts to burn before the spark plug fires.

Detonation occurs when the fuel air charge is ignited by the spark plug, but instead of burning at a controlled rate, it explodes causing cylinder temperatures and pressures to spike very quickly.

If this condition exists for very long, the engine can be damaged or destroyed.

One of the reasons for using engines with high compression ratios is to obtain long-range fuel economy, to convert more heat energy into useful work than is done in engines of low compression ratio.

Since more heat of the charge is converted into useful work, less heat is absorbed by the cylinder walls.

This factor promotes cooler engine operation, which in turn increases the thermal efficiency.

Here again, a compromise is needed between the demand for fuel economy and the demand for maximum horsepower without detonation.

Some manufacturers of high compression engines suppress detonation at high manifold pressures by using high octane fuel and limiting maximum manifold pressure.

The choice of spark plugs to be used in a specific aircraft engine is determined by the engine manufacturer after extensive tests.

When an engine is certificated to use hot or cold spark plugs, the plug used is determined by the compression ratio, the degree of supercharging, and how the engine is to be operated.

• High-compression engines tend to use colder range plugs while low-compression engines tend to use hot range plugs.
• 
• A spark plug with the proper reach ensures that the electrode end inside the cylinder is in the best position to achieve ignition.

The spark plug reach is the length of the threaded portion that is inserted in the spark plug bushing of the cylinder.

Spark plug seizure and/or improper combustion within the cylinder can occur if a plug with the wrong reach is used.

In extreme cases, if the reach is too long, the plug may contact a piston or valve and damage the engine.

If the plug threads are too long, they extend into the combustion chamber and carbon adheres to the threads making it almost impossible to remove the plug.

This can also be a source of preignition.

Heat of combustion can make some of the carbon a source for ignition, which can ignite the fuel-air mixture prematurely.

It is very important to select the approved spark plugs for the engine.

both the piston and the small end of the connecting rod 

A full-floating piston pin is free to rotate in both the piston and the small end of the connecting rod.

Full-floating piston pins are usually a push fit in the piston.

They are kept from damaging the cylinder walls as they move up and down by soft aluminum or brass plugs in the ends of the pin.

The piston pin joins the piston to the connecting rod.

It is machined in the form of a tube from a nickel steel alloy forging, casehardened and ground.

The piston pin is sometimes called a wrist pin because of the similarity between the relative motions of the piston and the articulated rod and that of the human arm.

The piston pin used in modern aircraft engines is the full-floating type, so called because the pin is free to rotate in both the piston and in the connecting rod piston-pin bearing. 

The piston pin must be held in place to prevent the pin ends from scoring the cylinder walls.

• A plug of relatively soft aluminum in the pin end provides a good bearing surface against the cylinder wall.

obtain the best volumetric efficiency and lower cylinder operating temperatures 

Valve overlap is the angular travel of the crankshaft during the time both the intake and exhaust valves are off their seats, and is used to increase the volumetric efficiency of the engine.

The exhaust valve remains open until after the piston has started down on the intake stroke to allow the maximum amount of burned exhaust gases to leave the cylinder.

The intake valve opens shortly before the piston reaches the top of its travel on the exhaust stroke.

The inertia of the exhaust gases leaving the cylinder when the intake valve opens, helps start the fresh fuel/air charge to flow into the cylinder

For a reciprocating engine to operate properly, each valve must open at the proper time, stay open for the required length of time, and close at the proper time.

Intake valves are opened just before the piston reaches top dead center, and exhaust valves remain open after top dead center.

At a particular instant, therefore, both valves are open at the same time (end of the exhaust stroke and beginning of the intake stroke).

This valve overlap permits better volumetric efficiency and lowers the cylinder operating temperature.

• This timing of the valves is controlled by the valve operating mechanism.

The valves will open late and close early 

The cylinder head of an air-cooled engine expands much more than the pushrod.

Because of this, air-cooled engines equipped with solid valve lifters (this applies primarily to radial engines) have a much larger valve clearance when the engine is hot than when it is cold.

If the valves are adjusted to the hot (running) clearance when the cylinder is cold, the clearance in the valve train will be too great when the engine is at its normal operating temperature.

The valves will open late and close early.

The cam will have to turn farther to open the valve and the valve will close before the cam has turned to the normal valve-closing position.

In order for a valve to seat, the valve must be in good condition, with no significant pressure being exerted against the end of the valve by the rocker arm.

If the expansion of all parts of the engine including the valve train were the same, the problem of ensuring valve seating would be very easy to solve.

Practically no free space would be necessary in the valve system.

However, since there is a great difference in the amount of expansion of various parts of the engine, there is no way of providing a constant operating clearance in the valve train.

The clearance in the valve actuating system is very small when the engine is cold but is much greater when the engine is operating at normal temperature.

The difference is caused by differences in the expansion characteristics of the various metals and by the differences in temperature of various engine parts.

There are many reasons why proper valve clearances are of vital importance to satisfactory and stable engine operation.

For example, when the engine is operating, valve clearances establish valve timing.

Since all cylinders receive their fuel/air mixture (or air) from a common supply, valve clearance affects both the amount and the richness or leanness of the fuel/air mixture.

• Therefore, it is essential that valve clearances be correct and uniform between each cylinder.

• 8039. To eliminate valve spring vibration or surging
•                    
• 8039a. eliminate valve spring surge 

Every mechanical device has a resonant frequency.

If the valve is operating at the resonant frequency of the valve spring, the spring will lose its effectiveness and will surge, allowing the valve to float.

By using two or more valve springs wound with a different pitch and a different size wire, the resonant frequency of the springs will be different and there will be no engine RPM at which point, the valves will float.

Each valve is closed by two or three helical coiled springs.

If a single spring were used, it would vibrate or surge at certain speeds.

To eliminate this difficulty, two or more springs (one inside the other) are installed on each valve.

Each spring will therefore vibrate at a different engine speed, and rapid damping out of all spring surge vibrations during engine operation will result.

• Two or more springs also reduce danger of weakness and possible failure by breakage due to heat and metal fatigue.
• 
• 

water-mixed degreaser residues may cause engine oil contamination in the overhauled engine

Extreme care must be used if any water-mixed degreasing solutions containing caustic compounds of soap are used for cleaning engine parts.

Such compounds, in addition to being potentially corrosive to aluminum and magnesium, may become impregnated in the pores of the metal and cause oil foaming when the engine is returned to service.

Degreasing can be done by immersing or spraying the part in a suitable commercial solvent.

Extreme care must be used if any water-mixed degreasing solutions containing caustic compounds or soap are used.

Such compounds, in addition to being potentially corrosive to aluminum and magnesium, may become impregnated in the pores of the metal and cause oil foaming when the engine is returned to service. 

Therefore, when using water-mixed solutions, it is imperative that the parts be rinsed thoroughly and completely in clear boiling water after degreasing.

• Regardless of the method and type of solution used, coat or spray all parts with lubricating oil immediately after cleaning to prevent corrosion.
• 
• FAA test taking note: This question is problematic and the FAA has changed their answer on this one at least once. Even though the current FAA powerplant handbook clearly states that water-mixed degreasers can cause corrosion in aluminum and magnesium parts, the FAA at this time seems to want to make you aware of the danger of engine oil contamination, leading to foaming, which could result.

• In a table in AMT Handbook - Powerplant - Lubrication and Cooling Systems section reproduced below the FAA makes a positive link between "foaming" and "contamination":
• 

The power impulses are spaced closer together 

One of the main factors that affect the smoothness of operation of a reciprocating engine is the closeness with which the power impulses are spaced.

The greater the number of cylinders, the closer the power impulses are together and the smoother the engine will operate.

Engines with a greater number of cylinders are smoother because the power impulses are spaced closer together.

To imagine this, coinsider an engine with a very large number of cylinders set to fire sequentially in very rapid succession.

The engine would be super smooth (though loud!) as the firings would amount to a steady humm as any opposing firing forces quickly counterbalanced.

A crankshaft is dynamically balanced when all the forces created by crankshaft rotation and power impulses are balanced within themselves so that little or no vibration is produced when the engine is operating.

On the other hand, on engines equipped with a large number of cylinders, the uniform distribution of the mixture becomes a greater problem, especially at high engine speeds when full advantage is taken of large air capacity.

• Additionally, cylinder cooling becomes an issue as the placement of front cylinders can get cool air from reaching back cylinders.
•  

cylinder volume with piston at bottom dead center and at top dead center 

The compression ratio of a reciprocating engine is the ratio of the volume of the cylinder with the piston at the bottom of its stroke to the volume of the cylinder with the piston at the top of its stroke.

Compression Ratio

All internal combustion engines must compress the fuel/air mixture to receive a reasonable amount of work from each power stroke.

The fuel/air charge in the cylinder can be compared to a coil spring in that the more it is compressed, the more work it is potentially capable of doing.

• The compression ratio of an engine is a comparison of the volume of space in a cylinder when the piston is at the bottom of the stroke to the volume of space when the piston is at the top of the stroke.
• 
• This comparison is expressed as a ratio, hence the term compression ratio.

Compression ratio is a controlling factor in the maximum horsepower developed by an engine, but it is limited by present day fuel grades and the high engine speeds and manifold pressures required for takeoff.

For example, if there are 140 cubic inches of space in the cylinder when the piston is at the bottom and there are 20 cubic inches of space when the piston is at the top of the stroke, the compression ratio would be 140 to 20.

If this ratio is expressed in fraction form, it would be 140/20 or 7 to 1, usually represented as 7:1.

The limitations placed on compression ratios, manifold pressure, and the manifold pressure’s effect on compression pressures has a major effect on engine operation.

Manifold pressure is the average absolute pressure of the air or fuel/ air charge in the intake manifold and is measured in units of inches of mercury ("Hg).

Manifold pressure is dependent on engine speed (throttle setting) and the degree supercharging.

The operation of the supercharger increases the weight of the charge entering the cylinder.

When a true supercharger is used with the aircraft engine, the manifold pressure may be considerably higher than the pressure of the outside atmosphere.

The advantage of this condition is that a greater amount of charge is forced into a given cylinder volume, and a greater output of horsepower results.

Compression ratio and manifold pressure determine the pressure in the cylinder in that portion of the operating cycle when both valves are closed.

The pressure of the charge before compression is determined by manifold pressure, while the pressure at the height of compression (just prior to ignition) is determined by manifold pressure times the compression ratio.

For example, if an engine were operating at a manifold pressure of 30 "Hg with a compression ratio of 7:1, the pressure at the instant before ignition would be approximately 210 "Hg.

However, at a manifold pressure of 60 "Hg, the pressure would be 420 "Hg.

Without going into great detail, it has been shown that the compression event magnifies the effect of varying the manifold pressure, and the magnitude of both affects the pressure of the fuel charge just before the instant of ignition.

If the pressure at this time becomes too high, pre-ignition or detonation occur and produce overheating.

Pre-ignition is when the fuel air charge starts to burn before the spark plug fires.

Detonation occurs when the fuel air charge is ignited by the spark plug, but instead of burning at a controlled rate, it explodes causing cylinder temperatures and pressures to spike very quickly.

If this condition exists for very long, the engine can be damaged or destroyed.

One of the reasons for using engines with high compression ratios is to obtain long-range fuel economy, to convert more heat energy into useful work than is done in engines of low compression ratio.

Since more heat of the charge is converted into useful work, less heat is absorbed by the cylinder walls.

This factor promotes cooler engine operation, which in turn increases the thermal efficiency.

Here again, a compromise is needed between the demand for fuel economy and the demand for maximum horsepower without detonation.

• Some manufacturers of high compression engines suppress detonation at high manifold pressures by using high octane fuel and limiting maximum manifold pressure.

005 inch 

Crankshaft runout is measured by clamping a dial indicator to a solid part of the engine and placing the arm of the indicator against the part of the crankshaft where the runout reading is to be measured.

Place the indicator at zero with the arm against the crankshaft.

Rotate the crankshaft for a complete revolution.

The total runout is the difference between the negative and the positive readings.

If the positive reading is +0.002 and the negative reading is -0.003, the total runout is five thousandths of an inch (0.005 inch).

Crankshaft runout is measured by clamping a dial indicator to a solid part of the engine and placing the arm of the indicator against the part of the crankshaft where the runout reading is to be measured.

Place the indicator at zero with the arm against the crankshaft.

Rotate the crankshaft for a complete revolution.

The total runout is the difference between the negative and the positive readings.

• If the positive reading is +0.002 and the negative reading is -0.003, the total runout is five thousandths of an inch (0.005 inch).
• 
• Using a surface plate and a dial indicator, measure the shaft runout. 

If the total indicator reading exceeds the dimensions given in the manufacturer‘s table of limits, the shaft must not be re-used. 

A bent crankshaft should not be straightened.

Any attempt to do so results in rupture of the nitrided surface of the bearing journals, a condition that causes eventual failure of the crankshaft.

• Measure the outside diameter of the crankshaft main and rod bearing journals using a micrometer.
• 
• Internal measurements can be made by using telescoping gauges, and then measuring the telescoping gauge with a micrometer.
• 
• Runout is a measure of the amount a shaft, flange, or disk is bent or fails to run true.

Runout is normally measured with a dial indicator.

• Since runout is the total amount of distortion, we understand this FAA question to imply that as the readings are .002 inches one way and .003 inches the other way for a total runout of .005 inches.

both No. 1 and No.2 are true 

• Statement 1 is true. Only cast iron piston rings can be used in nitrided or chrome-plated cylinders.
• Statement 2 is also true. Chrome plated rings can be used in plain steel cylinders.

• The piston rings prevent leakage of gas pressure from the combustion chamber and reduce to a minimum the seepage of oil into the combustion chamber.
• The rings fit into the piston grooves but spring out to press against the cylinder walls; when properly lubricated, the rings form an effective gas seal.

Most piston rings are made of high-grade cast iron.

• After the rings are made, they are ground to the cross-section desired.
• Then they are split so that they can be slipped over the outside of the piston and into the ring grooves that are machined in the piston wall.

Since their purpose is to seal the clearance between the piston and the cylinder wall, they must fit the cylinder wall snugly enough to provide a gastight fit.

They must exert equal pressure at all points on the cylinder wall, and must make a gastight fit against the sides of the ring grooves.

Gray cast iron is most often used in making piston rings.

In some engines, chrome-plated mild steel piston rings are used in the top compression ring groove because these rings can better withstand the high temperatures present at this point. 

Chrome rings must be used with steel cylinder walls. 

Never use chrome rings on chrome cylinders.

The cylinder barrel in which the piston operates must be made of a high-strength material, usually steel.

It must be as light as possible, yet have the proper characteristics for operating under high temperatures.

It must be made of a good bearing material and have high tensile strength.

The cylinder barrel is made of a steel alloy forging with the inner surface hardened to resist wear of the piston and the piston rings which bear against it.

This hardening is usually done by exposing the steel to ammonia or cyanide gas while the steel is very hot.

The steel soaks up nitrogen from the gas, which forms iron nitrides on the exposed surface.

As a result of this process, the metal is said to be nitrided.

This nitriding only penetrates into the barrel surface a few thousands of an inch.

As the cylinder barrels wear due to use, they can be repaired by chroming.

This is a process that plates chromium on the surface of the cylinder barrel and brings it back to new standard dimensions. 

Chromium-plated cylinders should use cast iron rings. 

Honing the cylinder walls is a process that brings it to the correct dimensions and provides crosshatch pattern for seating the piston rings during engine break-in.

Some engine cylinder barrels are choked at the top, or they are smaller in diameter to allow for heat expansion and wear.

In some instances, the barrel has threads on the outside surface at one end so that it can be screwed into the cylinder head.

The cooling fins are machined as an integral part of the barrel and have limits on repair and service.

• 
• 

By placing the rings in the cylinder and measuring the end-gap with a feeler gauge

The end gap in piston rings is measured by placing the piston ring inside the cylinder and pushing it up with the top of the piston so that it is square in the cylinder bore and in line with the cylinder flange.

With the ring in this position, measure the distance between the two ends of the ring with a feeler gauge.


If it is necessary to replace the rings on one or more of the pistons, check the side clearance against the manufacturer's specification, using a thickness gauge. The ring end gap must also be checked.

If it is necessary to remove material to obtain the correct side clearance, it can be done either by turning the piston grooves a slight amount on each side or by lapping the ring on a surface plate.

If the end gap is too close, the excess metal can be removed by clamping a mill file in a vise, holding the ring in proper alignment, and dressing off the ends. In all cases the engine manufacturer's procedures must be followed.

Where cylinders are built with an intentional choke, measurement of taper becomes more complicated.

Cylinder choke is where the top of the cylinder has been made with the very top diameter of the cylinder smaller, to compensate for wear and expansion during operation.

It is necessary to know exactly how the size indicates wear or taper.

Taper can be measured in any cylinder by a cylinder dial gauge as long as there is not a sharp step.

The dial gauge tends to ride up on the step and causes inaccurate readings at the top of the cylinder.

The measurement for out-of-roundness is usually taken at the top of the cylinder.

However, a reading should also be taken at the skirt of the cylinder to detect dents or bends caused by careless handling.

A step, or ridge, is formed in the cylinder by the wearing action of the piston rings.

The greatest wear is at the top of the ring travel limit.

The ridge that results is likely to cause damage to the rings or piston.

If the step exceeds tolerances, it should be removed by grinding the cylinder oversize, or it should be blended by hand-stoning to break the sharp edge.

A step also may be found where the bottom ring reaches the lowest travel.

This step is rarely found to be excessive, but it should be checked.

Check the cylinder flange for warpage by placing the cylinder on a suitable jig.

Check to see that the flange contacts the jig all the way around.

The amount of warpage can be checked by using a thickness gauge.

• A cylinder whose flange is warped beyond the limits should be rejected.
• 
• 

7:1 

The compression ratio of a reciprocating engine is the ratio of the volume of a cylinder with the piston at the bottom of its stroke to the volume of the cylinder with the piston at the top of its stroke.

• If the cylinder has a volume of 70 cubic inches with the piston at the bottom of its stroke and 10 cubic inches with the piston at the top of its stroke, the compression ratio is 7:1.
• 
• Compression Ratio

All internal combustion engines must compress the fuel/air mixture to receive a reasonable amount of work from each power stroke.

The fuel/air charge in the cylinder can be compared to a coil spring in that the more it is compressed, the more work it is potentially capable of doing.

The compression ratio of an engine is a comparison of the volume of space in a cylinder when the piston is at the bottom of the stroke to the volume of space when the piston is at the top of the stroke.

This comparison is expressed as a ratio, hence the term compression ratio.

Compression ratio is a controlling factor in the maximum horsepower developed by an engine, but it is limited by present day fuel grades and the high engine speeds and manifold pressures required for takeoff.

For example, if there are 140 cubic inches of space in the cylinder when the piston is at the bottom and there are 20 cubic inches of space when the piston is at the top of the stroke, the compression ratio would be 140 to 20. If this ratio is expressed in fraction form, it would be 140/20 or 7 to 1, usually represented as 7:1.

The limitations placed on compression ratios, manifold pressure, and the manifold pressure’s effect on compression pressures has a major effect on engine operation.

Manifold pressure is the average absolute pressure of the air or fuel/ air charge in the intake manifold and is measured in units of inches of mercury ("Hg).

Manifold pressure is dependent on engine speed (throttle setting) and the degree supercharging.

The operation of the supercharger increases the weight of the charge entering the cylinder.

When a true supercharger is used with the aircraft engine, the manifold pressure may be considerably higher than the pressure of the outside atmosphere.

The advantage of this condition is that a greater amount of charge is forced into a given cylinder volume, and a greater output of horsepower results.

Compression ratio and manifold pressure determine the pressure in the cylinder in that portion of the operating cycle when both valves are closed.

The pressure of the charge before compression is determined by manifold pressure, while the pressure at the height of compression (just prior to ignition) is determined by manifold pressure times the compression ratio.

For example, if an engine were operating at a manifold pressure of 30 "Hg with a compression ratio of 7:1, the pressure at the instant before ignition would be approximately 210 "Hg. However, at a manifold pressure of 60 "Hg, the pressure would be 420 "Hg.

Without going into great detail, it has been shown that the compression event magnifies the effect of varying the manifold pressure, and the magnitude of both affects the pressure of the fuel charge just before the instant of ignition.

If the pressure at this time becomes too high, pre-ignition or detonation occur and produce overheating.

Pre-ignition is when the fuel air charge starts to burn before the spark plug fires.

Detonation occurs when the fuel air charge is ignited by the spark plug, but instead of burning at a controlled rate, it explodes causing cylinder temperatures and pressures to spike very quickly.

If this condition exists for very long, the engine can be damaged or destroyed.

One of the reasons for using engines with high compression ratios is to obtain long-range fuel economy, to convert more heat energy into useful work than is done in engines of low compression ratio.

Since more heat of the charge is converted into useful work, less heat is absorbed by the cylinder walls.

This factor promotes cooler engine operation, which in turn increases the thermal efficiency.

Here again, a compromise is needed between the demand for fuel economy and the demand for maximum horsepower without detonation.

Some manufacturers of high compression engines suppress detonation at high manifold pressures by using high octane fuel and limiting maximum manifold pressure.

A ratio is the comparison of two numbers or quantities.

• A ratio may be expressed in three ways: as a fraction, with a colon, or with the word "to." For example, a gear ratio of 5:7 can be expressed as any of the following:
• 5/7
• 5:7
• 5 to 7

Aviation Applications

Ratios have widespread application in the field of aviation.

Example: Compression ratio on a reciprocating engine is the ratio of the volume of a cylinder with the piston at the bottom of its stroke to the volume of the cylinder with the piston at the top of its stroke.

For example, a typical compression ratio might be 10:1 (or 10 to 1).

Aspect ratio is the ratio of the length (or span) of an airfoil to its width (or chord).

A typical aspect ratio for a commercial airliner might be 7:1 (or 7 to 1).

Air-fuel ratio is the ratio of the weight of the air to the weight of fuel in the mixture being fed into the cylinders of a reciprocating engine.

For example, a typical air-fuel ratio might be 14.3:1 (or 14.3 to 1).

Glide ratio is the ratio of the forward distance traveled to the vertical distance descended when an aircraft is operating without power.

For example, if an aircraft descends 1,000 feet while it travels through the air for two linear miles (10,560 feet), it has a glide ratio of 10,560:1,000 which can be reduced to 10.56: 1 (or 10.56 to 1).

Gear ratio is the number of teeth each gear represents when two gears are used in an aircraft component.

In Figure 3-7, the pinion gear has 8 teeth and a spur gear has 28 teeth.

The gear ratio is 8:28.

Using 7 as the LCD, 8:28 becomes 2:7.

Speed ratio is when two gears are used in an aircraft component; the rotational speed of each gear is represented as a speed ratio.

As the number of teeth in a gear decreases, the rotational speed of that gear increases, and vice-versa.

Therefore, the speed ratio of two gears is the inverse (or opposite) of the gear ratio.

• If two gears have a gear ratio of 2:9, then their speed ratio is 9:2.

some of the soap will become impregnated in the surface of the material and subsequently cause engine oil contamination and foaming 

When cleaning aluminum and magnesium parts during engine overhaul, solutions containing soap should not be used, as it is very difficult to remove all traces of the soap.

When the engine is assembled and operating, heat will bring out any soap remaining on the surface or in the pores of the metal.

This soap will contaminate the engine oil and cause severe foaming.

When cleaning aluminum and magnesium engine parts with solutions containing soap, some of the soap will become impregnated in the surface of the material.

When heated, the soap will come out of the pores of the metal and it will cause engine oil contamination and foaming.

Engine parts can be degreased by using the emulsion-type cleaners or chlorinated solvents.

The emulsion-type cleaners are safe for all metals, since they are neutral and noncorrosive.

Cleaning parts by the chlorinated solvent method leaves the parts absolutely dry.

If they are not to be subjected to further cleaning operations, they should be sprayed with a corrosion-preventive solution to protect them against rust or corrosion.

The hot section, which generally includes the combustion section and turbine sections, normally require inspections at regular intervals.

The extent of disassembly of the engine to accomplish this inspection varies from different engine types.

Most engines require that the combustion case be open for the inspection of the hot section.

However, in performing this disassembly, numerous associated parts are readily accessible for inspection.

The importance of properly supporting the engine and the parts being removed cannot be overstressed.

The alignment of components being removed and installed is also of the utmost importance.

After all the inspections and repairs are made, the manufacturer’s detailed assembly instructions should be followed.

These instructions are important in efficient engine maintenance, and the ultimate life and performance of the engine.

• Extreme care must be taken during assembly to prevent dirt, dust, cotter pins, lock wire, nuts, washers, or other foreign material from entering the engine

To determine satisfactory performance 

A power check of a reciprocating engine is a check to determine that the engine is developing the correct static RPM and manifold pressure.

The purpose of this check is to determine that the engine is performing satisfactorily.

The purpose of a power check on a reciprocating engine is to determine satisfactory performance.

Specific rpm and manifold pressure relationship should be checked during each ground check.

This can be done at the time the engine is run-up to make the magneto check.

The purpose of this check is to measure the performance of the engine against an established standard. 

Calibration tests have determined that the engine is capable of delivering a given power at a given rpm and manifold pressure.

The original calibration, or measurement of power, is made by means of a dynamometer in a test cell.

During the ground check, power is measured with the propeller.

With constant conditions of air density, the propeller, at any fixed-pitch position, always requires the same rpm to absorb the same horsepower from the engine.

This characteristic is used in determining the condition of the engine.

With the governor control set for full low pitch, the propeller operates as a fixed-pitch propeller, because the engine is static. Under these conditions, the manifold pressure for any specific engine, with the mixture control in rich, indicates whether all the cylinders are operating properly. With one or more dead or intermittently firing cylinders, the operating cylinders must provide more power for a given rpm. Consequently, the carburetor throttle must be opened further, resulting in higher manifold pressure. Different engines of the same model using the same propeller installation, and at the same barometer and temperature readings, should require the same manifold pressure to within 1 "Hg. A higher than normal manifold pressure usually indicates a dead cylinder or late ignition timing. An excessively low manifold pressure for a particular rpm usually indicates that the ignition timing is early. Early ignition can cause detonation and loss of power at takeoff power settings.

• The accuracy of the power check may be affected by the following variables:
• Wind—any appreciable air movement (5 mph or more) changes the air load on the propeller blade when it is in the fixed-pitch position. A head wind increases the rpm obtainable with a given manifold pressure. A tail wind decreases the rpm.
• Atmospheric temperatures—the effects of variations in atmospheric temperature tend to cancel each other. Higher carburetor intake and cylinder temperatures tend to lower the rpm, but the propeller load is lightened because of the less dense air.
• Engine and induction system temperature—if the cylinder and carburetor temperatures are high because of factors other than atmospheric temperature, a low rpm results since the power is lowered without a compensating lowering of the propeller load.
• Oil temperature—cold oil tends to hold down the rpm, since the higher viscosity results in increased friction horsepower losses.

A worn or defective ring(s) indication

• If the gaps are aligned, they will allow more blow-by and give a false indication of worn or defective ring(s).

• Cylinder Installation Instructions:
• See that all preservative oil accumulation on the cylinder and piston assembly is washed off with solvent and thoroughly dried with compressed air.

Install the piston and ring assembly on the connecting rod.

Be sure that the piston faces in the right direction.

The piston number stamped on the bottom of the piston head should face toward the front of the engine.

Lubricate the piston pin before inserting it.

It should fit with a push fit.

If a drift must be used, follow the same precaution that was taken during pin removal.

Oil the exterior of the piston assembly generously, forcing oil around the piston rings and in the space between the rings and grooves. 

Stagger the ring gaps around the piston and check to see that rings are in the correct grooves, and whether they are positioned correctly, as some are used as oil scrapers, others as pumper rings.

The number, type, and arrangement of the compression and oil-control rings vary with the make and model of engine.

The example instruction excerpt below calls for piston ring gaps to be staggered at 90 degree intervals, roughly.

• If the piston ring gaps were all in line, oil and gases would blow through the gap much more readily
• 

Reduced valve overlap period

Engine, airplane, and equipment manufacturers provide a powerplant installation that gives satisfactory performance. 

Cams are designed to give best valve operation and correct overlap.

But valve operation is correct only if valve clearances are set and remain at the value recommended by the engine manufacturer.

If valve clearances are set wrong, the valve overlap period is longer or shorter than the manufacturer intended.

The same is true if clearances get out of adjustment during operation.

Where there is too much valve clearance, the valves do not open as wide or remain open as long as they should. 

This reduces the overlap period.

At idling speed, it affects the fuel/air mixture, since a less-than-normal amount of air or exhaust gases is drawn back into the cylinder during the shortened overlap period.

As a result, the idle mixture tends to be too rich.

When valve clearance is less than it should be, the valve overlap period is lengthened.

A greater than normal amount of air, or exhaust gases, is drawn back into the cylinder at idling speeds.

As a result, the idle mixture is leaned out at the cylinder.

The carburetor is adjusted with the expectation that a certain amount of air or exhaust gases is drawn back into the cylinder at idling.

If more or less air, or exhaust gases, are drawn into the cylinder during the valve overlap period, the mixture is too lean or too rich.

When valve clearances are wrong, it is unlikely that they are all wrong in the same direction.

Instead, there is too much clearance on some cylinders and too little on others.

Naturally, this gives a variation in valve overlap between cylinders.

This results in a variation in fuel/air ratio at idling and lower-power settings, since the carburetor delivers the same mixture to all cylinders.

The carburetor cannot tailor the mixture to each cylinder to compensate for variation in valve overlap

The effect of variation in valve clearance and valve overlap on the fuel/air mixture between cylinders is illustrated above.

Note how the cylinders with too little clearance run rich, and those with too much clearance run lean.

Note also the extreme mixture variation between cylinders.

Valve clearance also effects volumetric efficiency.

Any variations in fuel/air into, and exhaust gases out of, the cylinder affects the volumetric efficiency of the cylinder.

With the use of hydraulic valve lifters that set the valve clearance automatically engine operation has been greatly improved.

Hydraulic lifters do have a limited range in which they can control the valve clearance, or they can become stuck in one position that can cause them to be a source of engine trouble.

• Normally engines equipped with hydraulic lifters require little to no maintenance.
• 

 controlling air flow through the oil cooler 

The floating control thermostat controls the oil cooler air exit door.

It varies the air flow through the cooler to control oil temperature.

One of the most widely used automatic oil temperature control devices is the floating control thermostat that provides manual and automatic control of the oil inlet temperatures.

With this type of control, the oil cooler air-exit door is opened and closed automatically by an electrically operated actuator.

Automatic operation of the actuator is determined by electrical impulses received from a controlling thermostat inserted in the oil pipe leading from the oil cooler to the oil supply tank.

The actuator may be operated manually by an oil cooler air-exit door control switch.

Placing this switch in the "open" or "closed" position produces a corresponding movement of the cooler door.

• Placing the switch in the "auto" position puts the actuator under the automatic control of the floating control thermostat
• 
• The thermostat shown in above is adjusted to maintain a normal oil temperature so that it does not vary more than approximately 5° to 8 °C, depending on the installation.

During operation, the temperature of the engine oil flowing over the bimetal element causes it to wind or unwind slightly.

This movement rotates the shaft (A) and the grounded center contact arm (C). As the grounded contact arm is rotated, it is moved toward either the open or closed floating contact arm (G).

The two floating contact arms are oscillated by the cam (F), which is continuously rotated by an electric motor (D) through a gear train (E).

When the grounded center contact arm is positioned by the bimetal element so that it touches one of the floating contact arms, an electric circuit to the oil cooler exit-flap actuator motor is completed, causing the actuator to operate and position the oil cooler air-exit flap.

• Newer systems use electronic control systems, but the function or the overall operation is basically the same regarding control of the oil temperature through control of the air flow through the cooler.

Lower than normal static RPM, full throttle operation 

A test club is used to test and break in reciprocating engines.

They are made to provide the correct amount of load on the engine during the test break-in period.

The multi-blade design also provides extra cooling air flow during testing.

Static RPM is the number of revolutions per minute an aircraft engine can produce when the aircraft is not moving.

Static RPM at a given temperature and altitude is a direct indication of engine power production.

Correct Answer:

If the club propeller is spinning at a lower RPM than expected ("lowr than normal static RPM") at full power, this directly indicates that the engine is producing less power than expected and hence the engine is in a generally weak condition.

• Incorrect Answers
• "Lower than normal manifold pressure..." is incorrect.

If this situation occurred, the engine would be developing good RPM for a LOWER manifold pressure than expected.

Hence, the engine would actually be performing better, not worse, than expected.

"Manifold pressure lower at idle RPM than static RPM" doesn't tell you very much.

You expect ANY engine to have a lower MP at idle than at full power static RPM.

• It would be very weird if this were not the case on any engine!

Cylinder compression check 

Checking the cylinder compression is required by 14 CFR Part 43 Appendix D when performing an annual or a 100-hour inspection on a reciprocating engine aircraft.

• One-hundred-hour inspection
• An inspection required by 14 CFR part 91, section 91.409 for FAA-certificated aircraft operated for hire or used for flight instruction for hire.

A 100- hour inspection is identical in content to an annual inspection, but can be conducted by an aviation maintenance technician who holds an Airframe and Powerplant rating, but does not have an Inspection Authorization. 

See 14 CFR part 43, Appendix D for list of the items that must be included in an annual or 100-hour inspection.

Appendix D to Part 43—Scope and Detail of Items (as Applicable to the Particular Aircraft) To Be Included in Annual and 100-Hour Inspections

Each person performing an annual or 100-hour inspection shall, before that inspection, remove or open all necessary inspection plates, access doors, fairing, and cowling.

He shall thoroughly clean the aircraft and aircraft engine.

Each person performing an annual or 100-hour inspection shall inspect (where applicable) the following components of the fuselage and hull group:

Fabric and skin—for deterioration, distortion, other evidence of failure, and defective or insecure attachment of fittings.

• Systems and components—for improper installation, apparent defects, and unsatisfactory operation.
• Envelope, gas bags, ballast tanks, and related parts—for poor condition.

Each person performing an annual or 100-hour inspection shall inspect (where applicable) the following components of the cabin and flight deck group:

Generally—for uncleanliness and loose equipment that might foul the controls.

Seats and safety belts—for poor condition and apparent defects.

Windows and windshields—for deterioration and breakage.

Instruments—for poor condition, mounting, marking, and (where practicable) improper operation.

Flight and engine controls—for improper installation and improper operation.

Batteries—for improper installation and improper charge.

All systems—for improper installation, poor general condition, apparent and obvious defects, and insecurity of attachment.

Each person performing an annual or 100-hour inspection shall inspect (where applicable) components of the engine and nacelle group as follows:

Engine section—for visual evidence of excessive oil, fuel, or hydraulic leaks, and sources of such leaks.

Studs and nuts—for improper torquing and obvious defects.

Internal engine—for cylinder compression and for metal particles or foreign matter on screens and sump drain plugs.

If there is weak cylinder compression, for improper internal condition and improper internal tolerances.

Engine mount—for cracks, looseness of mounting, and looseness of engine to mount.

Flexible vibration dampeners—for poor condition and deterioration.

Engine controls—for defects, improper travel, and improper safetying.

Lines, hoses, and clamps—for leaks, improper condition and looseness.

Exhaust stacks—for cracks, defects, and improper attachment.

Accessories—for apparent defects in security of mounting.

All systems—for improper installation, poor general condition, defects, and insecure attachment.

Cowling—for cracks, and defects.

Each person performing an annual or 100-hour inspection shall inspect (where applicable) the following components of the landing gear group:

All units—for poor condition and insecurity of attachment.

Shock absorbing devices—for improper oleo fluid level.

Linkages, trusses, and members—for undue or excessive wear fatigue, and distortion.

Retracting and locking mechanism—for improper operation.

Hydraulic lines—for leakage.

Electrical system—for chafing and improper operation of switches.

Wheels—for cracks, defects, and condition of bearings.

Tires—for wear and cuts.

Brakes—for improper adjustment.

Floats and skis—for insecure attachment and obvious or apparent defects.

Each person performing an annual or 100-hour inspection shall inspect (where applicable) all components of the wing and center section assembly for poor general condition, fabric or skin deterioration, distortion, evidence of failure, and insecurity of attachment.

Each person performing an annual or 100-hour inspection shall inspect (where applicable) all components and systems that make up the complete empennage assembly for poor general condition, fabric or skin deterioration, distortion, evidence of failure, insecure attachment, improper component installation, and improper component operation.

Each person performing an annual or 100-hour inspection shall inspect (where applicable) the following components of the propeller group:

Propeller assembly—for cracks, nicks, binds, and oil leakage.

Bolts—for improper torquing and lack of safetying.

Anti-icing devices—for improper operations and obvious defects.

Control mechanisms—for improper operation, insecure mounting, and restricted travel.

Each person performing an annual or 100-hour inspection shall inspect (where applicable) the following components of the radio group:

Radio and electronic equipment—for improper installation and insecure mounting.

Wiring and conduits—for improper routing, insecure mounting, and obvious defects.

Bonding and shielding—for improper installation and poor condition.

Antenna including trailing antenna—for poor condition, insecure mounting, and improper operation.

Each person performing an annual or 100-hour inspection shall inspect (where applicable) each installed miscellaneous item that is not otherwise covered by this listing for improper installation and improper operation.

Next in firing order to the one from which they were removed and swapped bottom to top 

Spark plugs should be reinstalled next in firing order to the one from which they were removed and swapped bottom to top.

This rotation pattern does two things.

First, it rotates bottom and top plugs.

• This helps even out "deposit" problems caused by gravity such as lead or oil buildup. Second, it swaps plugs in polarity.
• 

The piston was positioned past top dead center 

• If the air pressure causes a movement of the propeller in the direction of engine rotation, the piston was positioned past top dead center.

• Differential Pressure Tester
• The differential pressure tester checks the compression of aircraft engines by measuring the leakage through the cylinders.

The design of this compression tester is such that minute valve leakages can be detected, making possible the replacement of cylinders where valve burning is starting.

The operation of the compression tester is based on the principle that, for any given airflow through a fixed orifice, a constant pressure drop across the orifice results.

As the airflow and pressure changes, pressure varies accordingly in the same direction.

If air is supplied under pressure to the cylinder with both intake and exhaust valves closed, the amount of air that leaks by the valves or piston rings indicates their condition; the perfect cylinder would have no leakage. 

• The differential pressure tester requires the application of air pressure to the cylinder being tested with the piston at top-center compression stroke
• 
• Guidelines for performing a differential compression test are:

Perform the compression test as soon as possible after engine shutdown to provide uniform lubrication of cylinder walls and rings.

Remove the most accessible spark plug from the cylinder, or cylinders, and install a spark plug adapter in the spark plug insert.

Connect the compression tester assembly to a 100 to 150 psi compressed air supply.

With the shutoff valve on the compression tester closed, adjust the regulator of the regulated pressure gauge compression tester to obtain 80 psi.

Open the shutoff valve and attach the air hose quick-connect fitting to the spark plug adapter.

The shutoff valve, when open, automatically maintains a pressure in the cylinder of 15 to 20 psi when both the intake and exhaust valves are closed.

By hand, turn the engine over in the direction of rotation until the piston in the cylinder being tested comes up on the compression stroke against the 15 psi. 

Continue turning the propeller slowly in the direction of rotation until the piston reaches top dead center.

Top dead center can be detected by a decrease in force required to move the propeller.

If the engine is rotated past top dead center, the 15 to 20 psi tends to move the propeller in the direction of rotation.

If this occurs, back the propeller up at least one blade prior to turning the propeller again in the direction of rotation.

This backing up is necessary to eliminate the effect of backlash in the valve-operating mechanism and to keep the piston rings seated on the lower ring lands.

Close the shutoff valve in the compression tester and re-check the regulated pressure to see that it is 80 psi with air flowing into the cylinder.

If the regulated pressure is more or less than 80 psi, readjust the regulator in the test unit to obtain 80 psi.

When closing the shutoff valve, make sure that the propeller path is clear of all objects.

There is sufficient air pressure in the combustion chamber to rotate the propeller if the piston is not on top dead center.

With regulated pressure adjusted to 80 psi, if the cylinder pressure reading indicated on the cylinder pressure gauge is below the minimum specified for the engine being tested, move the propeller in the direction of rotation to seat the piston rings in the grooves.

• Check all the cylinders and record the readings.
• 
• If low compression is obtained on any cylinder, turn the engine through with the starter, or re-start, and run the engine to takeoff power and re-check the cylinder, or cylinders, having low compression.

If the low compression is not corrected, remove the rocker-box cover and check the valve clearance to determine if the difficulty is caused by inadequate valve clearance.

If the low compression is not caused by inadequate valve clearance, place a fiber drift on the rocker arm immediately over the valve stem and tap the drift several times with a 1 to 2 pound hammer to dislodge any foreign material that may be lodged between the valve and valve seat.

After staking the valve in this manner, rotate the engine with the starter and re-check the compression.

Do not make a compression check after staking a valve until the crankshaft has been rotated either with the starter or by hand to re-seat the valve in normal manner.

The higher seating velocity obtained when staking the valve will indicate valve seating, even though valve seats are slightly egged or eccentric.

This procedure should only be performed if approved by the manufacturer.

Cylinders having compression below the minimum specified should be further checked to determine whether leakage is past the exhaust valve, intake valve, or piston.

Excessive leakage can be detected (during the compression check):

At the exhaust valve by listening for air leakage at the exhaust outlet;

• At the intake valve by escaping air at the air intake; and
• Past the piston rings by escaping air at the engine breather outlets.

Next to valve blow-by, the most frequent cause of compression leakage is excessive leakage past the piston.

This leakage may occur because of lack of oil.

To check this possibility, apply engine oil into the cylinder and around the piston.

Then, re-check the compression.

If this procedure raises compression to or above the minimum required, continue the cylinder in service.

If the cylinder pressure readings still do not meet the minimum requirement, replace the cylinder.

• When it is necessary to replace a cylinder as a result of low compression, record the cylinder number and the compression value of the newly installed cylinder on the compression check sheet.

 late and closing early 

Engine, airplane, and equipment manufacturers provide a powerplant installation that gives satisfactory performance. 

Cams are designed to give best valve operation and correct overlap.

But valve operation is correct only if valve clearances are set and remain at the value recommended by the engine manufacturer.

If valve clearances are set wrong, the valve overlap period is longer or shorter than the manufacturer intended.

The same is true if clearances get out of adjustment during operation.

Where there is too much valve clearance, the valves do not open as wide or remain open as long as they should. 

This reduces the overlap period.

At idling speed, it affects the fuel/air mixture, since a less-than-normal amount of air or exhaust gases is drawn back into the cylinder during the shortened overlap period.

As a result, the idle mixture tends to be too rich.

When valve clearance is less than it should be, the valve overlap period is lengthened.

A greater than normal amount of air, or exhaust gases, is drawn back into the cylinder at idling speeds.

As a result, the idle mixture is leaned out at the cylinder.

The carburetor is adjusted with the expectation that a certain amount of air or exhaust gases is drawn back into the cylinder at idling.

If more or less air, or exhaust gases, are drawn into the cylinder during the valve overlap period, the mixture is too lean or too rich.

When valve clearances are wrong, it is unlikely that they are all wrong in the same direction.

Instead, there is too much clearance on some cylinders and too little on others.

Naturally, this gives a variation in valve overlap between cylinders.

This results in a variation in fuel/air ratio at idling and lower-power settings, since the carburetor delivers the same mixture to all cylinders.

The carburetor cannot tailor the mixture to each cylinder to compensate for variation in valve overlap

The effect of variation in valve clearance and valve overlap on the fuel/air mixture between cylinders is illustrated below.

Note how the cylinders with too little clearance run rich, and those with too much clearance run lean.

Note also the extreme mixture variation between cylinders.

Valve clearance also effects volumetric efficiency.

Any variations in fuel/air into, and exhaust gases out of, the cylinder affects the volumetric efficiency of the cylinder.

With the use of hydraulic valve lifters that set the valve clearance automatically engine operation has been greatly improved.

Hydraulic lifters do have a limited range in which they can control the valve clearance, or they can become stuck in one position that can cause them to be a source of engine trouble.

• Normally engines equipped with hydraulic lifters require little to no maintenance.
• 

may be a result of abnormal plain type bearing wear and is cause for further investigation 

Any metal particles found in oil filters requires further investigation.

if the particles are non ferrous, they are either from the pistons or plain bearings.

Oil Filter/Screen Content Inspection Check for premature or excessive engine component wear that is indicated by the presence of metal particles, shavings, or flakes in the oil filter element or screens.

The oil filter can be inspected by opening the filter paper element.

Check the condition of the oil from the filter for signs of metal contamination.

Then, remove the paper element from the filter and carefully unfold the paper element; examine the material trapped in the filter.

If the engine employs a pressure screen system, check the screen for metal particles.

After draining the oil, remove the suction screen from the oil sump and check for metal particles

If examination of the used oil filter or pressure screen and the oil sump suction screen indicates abnormal metal content, additional service may be required to determine the source and possible need for corrective maintenance.

To inspect the spin on filter the can must be cut open to remove the filter element for inspection.

Using the special filter cutting tool, slightly tighten the cutter blade against filter and rotate 360º until the mounting plate separates from the can.

Using a clean plastic bucket containing varsol, move the filter to remove contaminants.

Use a clean magnet and check for any ferrous metal particles in the filter or varsol solution.

Then, take the remaining varsol and pour it through a clean filter or shop towel.

• Using a bright light, inspect for any nonferrous metals.
• 
• 

shock mounts point toward the engine's center of gravity 

Dynafocal mounts point toward the engine-propeller center of gravity.

They absorb and isolate the vibrations from the aircraft structure.

The section of an engine mount where the engine is attached is known as the engine mount ring.

It is usually constructed of steel tubing having a larger diameter than the rest of the mount structure.

It is circular in shape so that it can surround the engine, which is near the point of balance for the engine.

The engine is usually attached to the mount by dynafocal mounts, attached to the engine at the point of balance forward of the mount ring.

Other types of mounting devices are also used to secure the different engines to their mount rings.

As aircraft engines became larger and produced more power, some method was needed to absorb their vibration.

This demand led to the development of the rubber and steel engine-suspension units called shock mounts.

This combination permits restricted engine movement in all directions.

These vibration isolators are commonly known as flexible, or elastic, shock mounts.

An interesting feature common to most shock mounts is that the rubber and metal parts are arranged so that, under normal conditions, rubber alone supports the engine.

Of course, if the engine is subjected to abnormal shocks or loads, the metal snubbers limit excessive movement of the engine. 

Dynafocal engine mounts, or vibration isolators, are units that give directional support to the engines.

• Dynafocal engine mounts have the mounting pad angled to point to the CG of the engines mass
• 

the cause should be identified and corrected before the aircraft is released for flight 

The cause of metallic particles in the oil filter should be identified and corrected before the aircraft is released for flight.

• Oil Filter/Screen Content Inspection:
• Check for premature or excessive engine component wear that is indicated by the presence of metal particles, shavings, or flakes in the oil filter element or screens.

The oil filter can be inspected by opening the filter paper element.

Check the condition of the oil from the filter for signs of metal contamination.

Then, remove the paper element from the filter and carefully unfold the paper element; examine the material trapped in the filter.

If the engine employs a pressure screen system, check the screen for metal particles.

After draining the oil, remove the suction screen from the oil sump and check for metal particles.

If examination of the used oil filter or pressure screen and the oil sump suction screen indicates abnormal metal content, additional service may be required to determine the source and possible need for corrective maintenance.

To inspect the spin on filter the can must be cut open to remove the filter element for inspection.

Using the special filter cutting tool, slightly tighten the cutter blade against filter and rotate 360º until the mounting plate separates from the can.

Using a clean plastic bucket containing varsol, move the filter to remove contaminants.

Use a clean magnet and check for any ferrous metal particles in the filter or varsol solution.

• Then, take the remaining varsol and pour it through a clean filter or shop towel.
• Using a bright light, inspect for any nonferrous metals.
• 
• 

 low oil supply 

A low oil supply will often cause the oil pressure gauge fluctuates over a wide range from zero to normal operating pressure.

• It reads normal when the pump delivers oil, but it reads zero when the pump only draws air

Eliminate cam bearing clearance when making valve adjustment 

Some large radial engines have floating cam rings.

A floating cam ring is held centered over its bearing by the forces exerted by the valve springs.

When checking the valve clearance on an engine equipped with a floating cam, the bearing clearance must be eliminated by depressing two valves on the opposite side of the engine from the valves being checked.

• Depressing the valves removes the pressure of their valve spring from the cam allowing the cam ring to move tight against its bearing on the side where the valves are being checked.

The valve will not seat positively during start and engine warmup 

Overhead valves have their smallest clearance when the engine is cold.

The clearance increases as the engine temperature comes up to normal.

• If the clearance is too small, the valves will not seat positively when the engine is below normal temperature during start and engine warm up

decrease for both intake and exhaust valves 

Engine, airplane, and equipment manufacturers provide a powerplant installation that gives satisfactory performance. 

Cams are designed to give best valve operation and correct overlap.

But valve operation is correct only if valve clearances are set and remain at the value recommended by the engine manufacturer.

If valve clearances are set wrong, the valve overlap period is longer or shorter than the manufacturer intended.

The same is true if clearances get out of adjustment during operation.

Where there is too much valve clearance, the valves do not open as wide or remain open as long as they should. 

This reduces the overlap period.

At idling speed, it affects the fuel/air mixture, since a less-than-normal amount of air or exhaust gases is drawn back into the cylinder during the shortened overlap period.

As a result, the idle mixture tends to be too rich.

When valve clearance is less than it should be, the valve overlap period is lengthened.

A greater than normal amount of air, or exhaust gases, is drawn back into the cylinder at idling speeds.

As a result, the idle mixture is leaned out at the cylinder.

The carburetor is adjusted with the expectation that a certain amount of air or exhaust gases is drawn back into the cylinder at idling.

If more or less air, or exhaust gases, are drawn into the cylinder during the valve overlap period, the mixture is too lean or too rich.

When valve clearances are wrong, it is unlikely that they are all wrong in the same direction.

Instead, there is too much clearance on some cylinders and too little on others.

Naturally, this gives a variation in valve overlap between cylinders.

This results in a variation in fuel/air ratio at idling and lower-power settings, since the carburetor delivers the same mixture to all cylinders.

The carburetor cannot tailor the mixture to each cylinder to compensate for variation in valve overlap

The effect of variation in valve clearance and valve overlap on the fuel/air mixture between cylinders is illustrated above.

Note how the cylinders with too little clearance run rich, and those with too much clearance run lean.

• Note also the extreme mixture variation between cylinders.
• 

• Valve clearance also effects volumetric efficiency. Any variations in fuel/air into, and exhaust gases out of, the cylinder affects the volumetric efficiency of the cylinder. With the use of hydraulic valve lifters that set the valve clearance automatically engine operation has been greatly improved. Hydraulic lifters do have a limited range in which they can control the valve clearance, or they can become stuck in one position that can cause them to be a source of engine trouble. Normally engines equipped with hydraulic lifters require little to no maintenance.

Better scavenging and cooling characteristics 

While considering the operational effect of valve clearance, keep in mind that all aircraft reciprocating engines of current design use valve overlap.

Valve overlap is when the intake and exhaust valves are open at the same time.

This takes advantage of the momentum of the entering and exiting gases to improve the efficiency of getting fuel/air in and exhaust gases out

The figure below shows the pressures at the intake and exhaust ports under two different sets of operating conditions.

In one case, the engine is operating at a manifold pressure of 35 "Hg. 

Barometric pressure (exhaust back pressure) is 29 "Hg.

This gives a pressure acting in the direction indicated by the arrow of differential of 6 "Hg (3 psi).

During the valve overlap period, this pressure differential forces the fuel/air mixture across the combustion chamber toward the open exhaust. 

This flow of fuel/air mixture forces ahead of it the exhaust gases remaining in the cylinder, resulting in complete scavenging of the combustion chamber.

This, in turn, permits complete filling of the cylinder with a fresh charge on the following intake event. This is the situation in which valve overlap gives increased power.

There is a pressure differential in the opposite direction of 9 "Hg (4.5 psi) when the manifold pressure is below atmospheric pressure, for example, 20 "Hg.

These cause air or exhaust gases to be drawn into the cylinder through the exhaust port during valve overlap.

In engines with collector rings, this inflow through the exhaust port at low power settings consists of burned exhaust gases.

These gases are pulled back into the cylinder and mix with the incoming fuel/air mixture.

However, these exhaust gases are inert; they do not contain oxygen.

Therefore, the fuel/air mixture ratio is not affected much.

With open exhaust stacks, the situation is entirely different.

Here, fresh air containing oxygen is pulled into the cylinders through the exhaust.

This leans out the mixture.

• Therefore, the carburetor must be set to deliver an excessively rich idle mixture so that, when this mixture is combined with the fresh air drawn in through the exhaust port, the effective mixture in the cylinder will be at the desired ratio
• 

400 RPM 

A a crankshaft must turn 400 RPM if each cylinder of a four stroke cycle engine is to be fired 200 times a minute.

In a 4-stroke engine, each cylinder fires every other revolution of the crankshaft.

• Four-Stroke Cycle
• The vast majority of certified aircraft reciprocating engines operate on the four-stroke cycle, sometimes called the Otto cycle after its originator, a German physicist.

The four-stroke cycle engine has many advantages for use in aircraft.

One advantage is that it lends itself readily to high performance through supercharging.

In this type of engine, four strokes are required to complete the required series of events or operating cycle of each cylinder.

Two complete revolutions of the crankshaft (720°) are required for the four strokes; thus, each cylinder in an engine of this type fires once in every two revolutions of the crankshaft.

In the following discussion of the four-stroke cycle engine operation, note that the timing of the ignition and the valve events vary considerably in different engines.

Many factors influence the timing of a specific engine, and it is most important that the engine manufacturer’s recommendations in this respect be followed in maintenance and overhaul.

The timing of the valve and ignition events is always specified in degrees of crankshaft travel.

• It should be remembered that a certain amount of crankshaft travel is required to open a valve fully; therefore, the specified timing represents the start of opening rather than the full-open position of the valve.
• 

1 and 3 

Engine crankshaft run-out is usually only checked during engine overhaul or after a "prop strike" or sudden engine stoppage.

• The following text appears in the Sudden Stoppage (ENGINE) section of the AMT Handbook - 
• Powerplant:
• Remove the propeller and check the crankshaft, or the propeller drive shaft on reduction-gear engines, for misalignment.

Clamp a test indicator to the nose section of the engine. Use the dial-indicator that has 1⁄1000-inch graduations.

Remove the spark plugs from all the cylinders.

Then, turn the crankshaft, and observe if the crankshaft, propeller shaft, or flange turns straight without any bending taking place. 

If there is an excessive runout (bend in the crankshaft or propeller flange) reading at the crankshaft or propeller-drive shaft at the front seat location, the engine should be removed. 

Consult the applicable manufacturer’s instructions for permissible limits.

If the crankshaft or propeller drive shaft runout does not exceed these limits, install a serviceable propeller.

Make an additional check by tracking the propeller at the tip in the same plane, perpendicular to the axis of rotation, to assure that blade track tolerance is within the prescribed limits.

• The following text is in the Engine Overhaul (General) section of the same document:
• General Overhaul Procedures -
• Because of the continued changes and the many different types of engines in use, it is not possible to treat the specific overhaul of each engine in this text.

However, there are various overhaul practices and instructions of a nonspecific nature that apply to all makes and models of engines. 

Any engine to be overhauled completely should receive a runout check of its crankshaft or propeller shaft as a first step. 

• Any question concerning crankshaft or propeller shaft replacement is resolved at this time, since a shaft whose runout is beyond limits must be replaced

turn the propeller by hand three to four revolutions in the normal direction of rotation to check for liquid lock 

The cylinders below the center line of a radial engine may accumulate oil causing a hydraulic lock.

We check for this condition by turning the propeller by hand three to four revolutions in the normal direction of rotation.

Do not stand within the arc of the propeller blades while turning the propeller

check for one or more cold cylinders

If an engine misses the same on either magneto, one or more cylinders is not firing.

A good way to find the cylinder is to run the engine briefly at the RPM that causes the 'miss' most consistently then check for a cold cylinder.

Incorrect ignition timing, or faulty ignition wires, can cause the cylinder to fire at the wrong time, allowing the cylinder to fire when the intake valve is open, which can cause backfiring.

A point worth stressing is that backfiring rarely involves the whole engine.

Therefore, it is seldom the fault of the carburetor.

In practically all cases, backfiring is limited to one or two cylinders.

Usually, it is the result of faulty valve clearance setting, defective fuel injector nozzles, or other conditions that cause these cylinders to operate leaner than the engine as a whole.

There can be no permanent cure until these defects are discovered and corrected.

Because these backfiring cylinders fire intermittently and, therefore, run cool, they can be detected by the cold cylinder check.

Cold Cylinder Check

The cold cylinder check determines the operating characteristics of each cylinder of an air-cooled engine.

The tendency for any cylinder, or cylinders, to be cold, or to be only slightly warm, indicates lack of combustion or incomplete combustion within the cylinder.

This must be corrected if best operation and power conditions are to be obtained.

The cold cylinder check is made with a cold cylinder indicator.

Engine difficulties that can be analyzed by use of the cold cylinder indicator are:

Rough engine operation

Excessive rpm drop during the ignition system check

High manifold pressure for a given engine rpm during the ground check when the propeller is in the full low-pitch position

Faulty mixture ratios caused by improper valve clearance

In preparation for the cold cylinder check, head the aircraft into the wind to minimize irregular cooling of the individual cylinders and to ensure even propeller loading during engine operation.

Operate the engine on its roughest magneto at a speed between 1,200 and 1,600 rpm until the cylinder head temperature reading is stabilized.

If engine roughness is encountered at more than one speed, or if there is an indication that a cylinder ceases operating at idle or higher speeds, run the engine at each of these speeds, and perform a cold cylinder check to pick out all the dead or intermittently operating cylinders.

When low power output or engine vibration is encountered at speeds above 1,600 rpm when operating with the ignition switch on both, run the engine at the speed where the difficulty is encountered until the cylinder head temperatures have stabilized.

When cylinder head temperatures have reached the stabilized values, stop the engine by moving the mixture control to the idle cutoff or full lean position.

When the engine ceases firing, turn off both ignition and master switches. 

Record the cylinder head temperature reading registered on the flight deck gauge.

As soon as the propeller has ceased rotating, apply the instrument to each cylinder head, and record the relative temperature of each cylinder.

Start with number one and proceed in numerical order around the engine, as rapidly as possible.

To obtain comparative temperature values, a firm contact must be made at the same relative location on each cylinder.

Note any outstandingly low (cold) values.

Compare the temperature readings to determine which cylinders are dead (cold cylinders) or are operating intermittently.

Difficulties that may cause a cylinder to be inoperative (dead) when isolated to one magneto, either the right or left positions, are:

• Defective spark plugs
• Incorrect valve clearances
• Leaking intake pipes
• Lack of compression
• Defective spark plug lead

Defective fuel-injection nozzleRepeat the cold cylinder test for the other magneto positions on the ignition switch, if necessary.

Cooling the engine between tests is unnecessary.

The airflow created by the propeller, and the cooling effect of the incoming fuel/air mixture is sufficient to cool any cylinders that are functioning on one test and not functioning on the next.

In interpreting the results of a cold cylinder check, remember that the temperatures are relative.

A cylinder temperature taken alone means little, but when compared with the temperatures of other cylinders on the same engine, it provides valuable diagnostic information.

• The readings shown here:
• illustrate this point.
• On this check, the cylinder head temperature gauge reading at the time the engine was shut down was 160 °C on both tests.

A review of these temperature readings reveals that, on the right magneto, cylinder number 3 runs cool and cylinders 5 and 6 run cold.

This indicates that cylinder 3 is firing intermittently, and cylinders 5 and 6 are dead during engine operation on the plugs fired by the right magneto.

Cylinders 4 and 6 are dead during operation on the plugs fired by the left magneto.

Cylinder 6 is completely dead.

An ignition system operational check would not disclose this dead cylinder, since the cylinder is inoperative on both right and left switch positions.

A dead cylinder can be detected during run-up, since an engine with a dead cylinder requires a higher than normal manifold pressure to produce any given rpm below the cut-in speed of the propeller governor.

A dead cylinder could also be detected by comparing power input and power output with the aid of a torquemeter.

Defects within the ignition system that can cause a cylinder to go completely dead are:

• Both spark plugs inoperative
• Both ignition leads grounded, leaking, or open
• A combination of inoperative spark plugs and defective ignition leads
• Faulty fuel-injection nozzles, incorrect valve clearances , and other defects outside the ignition system

In interpreting the readings obtained on a cold cylinder check, the amount the engine cools during the check must be considered.

To determine the extent to which this factor should be considered in evaluating the readings, re-check some of the first cylinders tested, and compare the final readings with those made at the start of the check.

• Another factor to be considered is the normal variation in temperature between cylinders and between rows. This variation results from those design features that affect the airflow past the cylinders.
• 
• 

exhaust valve blow-by 

Exhaust valve blow by is usually evidenced by a hissing sound from the exhaust stacks when the propeller is being pulled through manually.

This is because the valve is not seating fully.

Valve blow-by is indicated by a hissing or whistle when pulling the propeller through prior to starting the engine, when turning the engine with the starter, or when running and blow-by past the intake valve is audible through the carburetor.

Correct valve blow-by immediately to prevent valve failure and possible engine failure by taking the following steps:

Perform a cylinder compression test to locate the faulty cylinder.

Check the valve clearance on the affected cylinder.

If the valve clearance is incorrect, the valve may be sticking in the valve guide.

To release the sticking valve, place a fiber drift on the rocker arm immediately over the valve stem and strike the drift several times with a mallet.

Sufficient hand pressure should be exerted on the fiber drift to remove any space between the rocker arm and the valve stem prior to hitting the drift.

If the valve is not sticking and the valve clearance is incorrect, adjust it as necessary.

Determine whether blow-by has been eliminated by again pulling the engine through by hand or turning it with the starter.

• If blow-by is still present, it may be necessary to replace the cylinder.

engine's lubrication system is probably operating normally 

Many reciprocating aircraft engines incorporate a compensating oil pressure relive valve.

An oil pressure relief valve limits oil pressure to the value specified by the engine manufacturer.

Oil pressure settings vary from 35 psi to 90 psi, depending on the installation.

The oil pressure must be high enough to ensure adequate lubrication of the engine and accessories at high speeds and powers.

On the other hand, the pressure must not be too high, since leakage and damage to the oil system may result.

• When the temperature or the oil increases, a thermostatic valve opens and allows oil pressure to remove the force on 1 of the springs.

leaking oil dilution valve 

An oil dilution system injects gasoline into the oil when the engine is shut down.

Oil dilution is used in cold climates to make the engine easier to start.

When an oil dilution valve leaks during normal operation, too much gasoline in the oil makes it too thin.

The oil pressure will drop and the temperature will go up as a result of the low viscosity.

Oil dilution is A method of temporarily decreasing the viscosity of the lubricating oil to make it possible to start a reciprocating engine when the temperature is very low.

Before shutting the engine down, enough gasoline from the fuel system is mixed with the lubricating oil in the engine to dilute it so the starter can turn the engine over when the oil is cold and viscous.

• When the engine starts and the oil warms up, the gasoline evaporate

A mixture leaner than a manual lean mixture of .060 

Rich mixtures burn faster than lean mixtures. 

A mixture leaner than a manual lean mixture of .060 (or about 17:1) may still be burning as the gases exit through the exhaust valve.

• This can cause severe overheating o f the exhaust valve.

Top dead center 

Cylinder Removal

Assuming that all obstructing cowling and brackets have been removed, first remove the intake pipe and exhaust pipes.

Plug or cover openings in the diffuser section.

Then remove cylinder deflectors and any attaching brackets which would obstruct cylinder removal.

Loosen the spark plugs and remove the spark-plug lead clamps.

Do not remove the spark plugs until ready to pull the cylinder off.

Remove the rocker box covers.

First remove the nuts and then tap the cover lightly with a rawhide mallet or plastic hammer.

Never pry the cover off with a screwdriver or similar tool.

Loosen the pushrod packing gland nuts or hose clamp, top and bottom.

Pushrods are removed by depressing the rocker arms with a special tool or by removing the rocker arm. 

Before removing the pushrods  turn the crankshaft until the piston is at top dead center on the compression stroke.

This relieves the pressure on both intake and exhaust rocker arm. 

It is also wise to back off the adjusting nut as far as possible  because this allows maximum clearance for pushrod removal when the rocker arms are depressed


On some model engines. tappets and springs of lower cylinders can fall out. Provision must be made to catch them as the pushrod and housing are removed.

After removing the pushrods, examine them for markings or mark them so that they may be replaced in the same location as they were before removal.

The ball ends are usually worn to fit the sockets in which they have been operating.

Furthermore, on some engines pushrods are not all of the same length.

A good procedure is to mark the pushrods near the valve tappet ends "No. 1 IN," "No. 1EX." "No. 2 IN," "No. 2 EX." etc.

On fuel injection engines, disconnect the fuel injection line and remove the fuel injection nozzle and any line clamps which will interfere with cylinder removal.

If the cylinder to be removed is a master rod cylinder, special precautions, in addition to regular cylinder removal precautions must be observed.

Information designating which cylinder has the master rod is included on the engine data plate.

Arrangements must be made to hold the master rod in the mid-position of the crankcase cylinder hole (after the cylinder has been removed).

Templates or guides are usually provided by the manufacturer for this purpose  or they are manufactured locally.

Under no circumstances should the master rod be moved from side to side.

It must he kept centered until the guide is in place.

Do not turn the crankshaft while the master rod cylinder is removed and other cylinders in the row remain on the engine.

• These precautions are necessary to prevent bottom rings on some of the other pistons from coming out of the cylinders expanding and damaging rings and piston skirts.
• If several cylinders are to Ix removed one of which is the master rod cylinder, it should always be removed last and should be the first installed.

The nest step in removing the cylinder is to cut the lockwire or remove the cotter pin and pry off the locking device from the cylinder-attaching capscrews or nuts.

Remove all the screw or nuts except two located 180' apart.

Use the wench specified for this purpose in the special tools section of the applicable manual.

Finally: while supporting the cylinder, remove the two remaining screws or nuts and gently pull the cylinder away from the crankcase.

Two men must work together during this step as well as during the remaining procedure for cylinder replacement.

After the cylinder skirt has cleared the crankcase and before the piston protrudes from the skirt, provide some means (usually a shop cloth) for preventing pieces of broken rings from falling into the crankcase.

After the piston has been removed, remove the cloths and carefully check for piston ring pieces.

To make certain that no ring pieces have entered the crankcase, collect and arrange all the pieces to see that they form a complete ring.

Place a support on the cylinder mounting pad and secure it with two capscrews or nuts.

Then remove the piston and ring assembly from the connecting rod.

When varnish makes it hard to remove the pin. a pin pusher or puller tool must be used.

If the special tool is not available and a drift is used to remove the piston pin the connecting rod should be supported so that it will not have to take the shock of the blows.

If this is not done the rod may be damaged.

After the removal of a cylinder and piston  the connecting rod must be supported to prevent damage to the rod and crankcase.

This can be done by supporting each connecting rod with the removed cylinder base oil seal ring looped around the rod and cylinder base studs.

Using a wire brush clean the studs or capscrews and examine them for cracks damaged threads or any other visible defects.

If one capscrew is found loose or broken at the time of cylinder removal all the capsrews for the cylinder should he discarded since the remaining capscrews may have been seriously weakened.

A cylinder hold-down stud failure will place the adjacent studs under a greater operating pressure and they are likely to be stretched beyond their elastic limit.

The engine manufacturer's instructions must he followed for the number of studs that will have to he replaced after a stud failure.

• When removing a broken stud take proper precautions to prevent metal chips from entering the engine power section.
• In all cases both faces of the washers and the seating faces of stud nuts or contacts must be cleaned and and roughness or burrs removed.
• 

indicated horsepower 

The indicated horsepower produced by an engine is the horsepower calculated from the indicated mean effective pressure and the other factors which affect the power output of an engine. 

Indicated horsepower is the power developed in the combustion chambers without reference to friction losses within the engine.

This horsepower is calculated as a function of the actual cylinder pressure recorded during engine operation.

To facilitate the indicated horsepower calculations, a mechanical indicating device, such as is attached to the engine cylinder scribes the actual pressure existing in the cylinder during the complete operating cycle.

This pressure variation can be represented by the kind of graph shown below


Notice that the cylinder pressure rises on the compression stroke, reaches a peak after top center, and is developed in the cylinders without including friction losses.

• Both shaft horsepower and brake horsepower measure actual usable horsepower and they do include friction losses
• decreases as the piston moves down on the power stroke.

Since the cylinder pressure varies during the operating cycle, an average pressure (line AB) is computed.

This average pressure, if applied steadily during the time of the power stroke, would do the same amount of work as the varying pressure during the same period.

This average pressure is known as indicated mean effective pressure and is included in the indicated horsepower calculation with other engine specifications.

If the characteristics and the indicated mean effective pressure of an engine are known, it is possible to calculate the indicated horsepower rating.

The indicated horsepower for a four-stroke cycle engine can be calculated from the following formula, in which the letter symbols in the numerator are arranged to spell the word "PLANK" to assist in memorizing the formula:

Indicated Horsepower = PLANK / 33,000

• Where
• P = Indicated mean effective pressure, in psi
• L = Length of the stroke, in feet or in fractions of a foot
• A = Area of the piston head or cross-sectional area of the cylinder, in square inches
• N = Number of power strokes per minute: rpm / 2
• K = Number of cylindersIn the formula above, the area of the piston multiplied by the indicated mean effective pressure gives the force acting on the piston in pounds.

This force multiplied by the length of the stroke in feet gives the work performed in one power stroke, which, multiplied by the number of power strokes per minute, gives the number of ft-lb per minute of work produced by one cylinder.

Multiplying this result by the number of cylinders in the engine gives the amount of work performed, in ft-lb, by the engine.

• Since hp is defined as work done at the rate of 33,000 ft-lb per minute, the total number of ft-lb of work performed by the engine is divided by 33,000 to find the indicated horsepower.
• 
• 

run smoothly and give the desired performance at all speeds 

Operating flexibility is the ability of an engine to run smoothly and give desired performance at all speeds from idling to full power output.

• The aircraft engine must also function efficiently through all the variations in atmospheric conditions encountered in widespread operations.

 have relatively thin walls and may be nitrided 

Unlike automobile engines, aircraft engines have thin walls.

Some have zero allowance for overbore.

Nitriding is a method of case hardening steel.

Steel is placed in a retort (a sealed, high-temperature furnace), and heated to a specified temperature while surrounded by ammonia gas (NH3).

The ammonia breaks down into nitrogen and hydrogen, and the nitrogen unites with some of the alloying elements in the steel to form an extremely hard surface.

Nitriding hardens crankshaft bearing surfaces and cylinder walls in reciprocating engines.

• It takes place at a lower temperature than other forms of case hardening, and does not cause warping

• drop in torquemeter pressure indication
•                       or
•              slight drop in RPM

Ground Check

The ground check is performed to evaluate the functioning of the engine by comparing power input, as measured by manifold pressure, with power output, as measured by rpm or torque.

The engine may be capable of producing a prescribed power, even rated takeoff, and not be functioning properly.

Only by comparing the manifold pressure required during the check against a known standard is an unsuitable condition disclosed.

The magneto check can also fail to show shortcomings, since the allowable rpm dropoff is only a measure of an improperly functioning ignition system and is not necessarily affected by other factors.

Conversely, it is possible for the magneto check to prove satisfactory when an unsatisfactory condition is present elsewhere in the engine.

The ground check is made after the engine is thoroughly warm.

It consists of checking the operation of the powerplant and accessory equipment by ear, by visual inspection, and by proper interpretation of instrument readings, control movements, and switch reactions.

During the ground check, the aircraft should be headed into the wind, if possible, to take advantage of the cooling airflow.

• A ground check procedure is outlined below:
• Control position check
• Cowl flaps (if equipped): open
• Mixture: rich
• Propeller: high rpm
• Carburetor heat: cold
• Check propeller according to propeller manufacturer’s instruction.
• Open throttle to the run-up rpm setting as per manufacturer’s instructions (specified RPM and manifold pressure).
• Ignition system operational check.

In performing the ignition system operational check (magneto check), the power-absorbing characteristics of the propeller in the low fixed-pitch position are utilized.

In switching to individual magnetos, cutting out the opposite plugs results in a slower rate of combustion, which gives the same effect as retarding the spark advance.

The drop in engine speed is a measure of the power loss at this slower combustion rate.

When the magneto check is performed, a drop in torquemeter pressure indication is a good supplement to the variation in rpm.

In cases where the tachometer scale is graduated coarsely, the torquemeter variation may give more positive evidence of the power change when switching to the individual magneto condition. 

A loss in torquemeter pressure not to exceed 10 percent can be expected when operating on a single magneto.

• By comparing the rpm drop with a known standard, the following are determined:
• Proper timing of each magneto.
• General engine performance as evidenced by smooth operation.
• Additional check of the proper connection of the ignition leads.

Any unusual roughness on either magneto is an indication of faulty ignition caused by plug fouling or by malfunctioning of the ignition system.

The operator should be very sensitive to engine roughness during this check.

Lack of dropoff in rpm may be an indication of faulty grounding of one side of the ignition system.

Complete cutting out when switching to one magneto is definite evidence that its side of the ignition system is not functioning.

Excessive difference in rpm drop off between the left and right switch positions can indicate a difference in time between the left and right magnetos.

Sufficient time should be given to the check on each single switch position to permit complete stabilization of engine speed and manifold pressure.

There is a tendency to perform this check too rapidly with resultant wrong indications.

Operation as long as 1 minute on a single ignition system is not excessive.

Another point that must be emphasized is the danger of sticking tachometer.

The tachometer should be tapped lightly to make sure the indicator needle moves freely.

In some cases using older mechanical tachometers, sticking has caused errors in indication to the extent of 100 rpm.

Under such conditions, the ignition system could have had as much as a 200 rpm drop with only a 100 rpm drop indicated on the instrument.

In most cases, tapping the instrument eliminates the sticking and results in accurate readings.

In recording the results of time ignition system check, record the amount of the total rpm drop that occurs rapidly and the amount that occurs slowly.

This breakdown in rpm drop provides a means of pinpointing certain troubles in the ignition system.

This can reduce unnecessary work by In recording the results of time ignition system check, record the amount of the total rpm drop that occurs rapidly and the amount that occurs slowly.

This breakdown in rpm drop provides a means of pinpointing certain troubles in the ignition system.

This can reduce unnecessary work by confining maintenance to the specific part of the ignition system that is responsible for the trouble.

Fast rpm drop is usually the result of either faulty spark plugs or faulty ignition harness.

This is true because faulty plugs or leads, take effect at once.

The cylinder goes dead or starts firing intermittently the instant the switch is moved from "both" to the "right" or "left" position.

Slow rpm drop usually is caused by incorrect ignition timing or faulty valve adjustment.

With late ignition timing, the charge is fired too late (in relation to piston travel) for the combustion pressures to build up to the maximum at the proper time.

The result is a power loss greater than normal for single ignition because of the lower peak pressures obtained in the cylinder.

However, this power loss does not occur as rapidly as that which accompanies a dead spark plug.

This explains the slow rpm drop as compared to the instantaneous drop with a dead plug or defective lead.

Incorrect valve clearances, through their effect on valve overlap, can cause the mixture to be too rich or too lean.

The too rich or too lean mixture may affect one plug more than another, because of the plug location and show up as a slow rpm drop on the ignition check.

Switch from "both" to "right" and return to "both." Switch from "both" to "left" and return to "both."

Observe the rpm drop while operating on the right and left positions.

The maximum drop should not exceed that specified by the engine manufacturer

a dead cylinder 

With a dead cylinder, the engine will run rough and the throttle will have to be opened farther to achieve a given RPM.

This will cause a high MP at any given RPM.

With the governor control set for full low pitch, the propeller operates as a fixed-pitch propeller, because the engine is static.

Under these conditions, the manifold pressure for any specific engine, with the mixture control in rich, indicates whether all the cylinders are operating properly.

With one or more dead or intermittently firing cylinders, the operating cylinders must provide more power for a given rpm.

Consequently, the carburetor throttle must be opened further, resulting in higher manifold pressure.

Different engines of the same model using the same propeller installation, and at the same barometer and temperature readings, should require the same manifold pressure to within 1 "Hg.

A higher than normal manifold pressure usually indicates a dead cylinder or late ignition timing.

An excessively low manifold pressure for a particular rpm usually indicates that the ignition timing is early.

• Early ignition can cause detonation and loss of power at takeoff power settings

High oil consumption 


High oil consumption often indicated worn valve guides allowing oil to flow from the rocker box into the cylinder.

Oil control rings are placed in the grooves immediately below the compression rings and above the piston pin bores.

There may be one or more oil control rings per piston; two rings may be installed in the same groove, or they may be installed in separate grooves.

Oil control rings regulate the thickness of the oil film on the cylinder wall.

If too much oil enters the combustion chamber, it burns and leaves a thick coating of carbon on the combustion chamber walls, the piston head, the spark plugs, and the valve heads.

This carbon can cause the valves and piston rings to stick if it enters the ring grooves or valve guides.

In addition, the carbon can cause spark plug misfiring as well as detonation, preignition, or excessive oil consumption.

• To allow the surplus oil to return to the crankcase, holes are drilled in the bottom of the oil control piston ring grooves or in the lands next to these grooves

 A normal indication 

In a nine-cylinder radial engine, each cylinder fires 80° of crankshaft rotation after the cylinder before it, in firing order.

When the crankshaft is rotated 260° after the piston in cylinder 1 is at top dead center on its compression stroke, the piston in cylinders 7 and 8 are near the top of their strokes.

The piston in cylinder 3 is near the bottom of its power stroke and its exhaust valve is open.

• It is normal for a cylinder not to hold air pressure when its piston is near the bottom of its power stroke and its exhaust valve is open.

At the end of the exhaust stroke and the beginning of the intake stroke 

Both the intake and exhaust valve are open at the same time, only during the period of valve overlap.

Valve overlap occurs at the end of the exhaust stroke and the beginning of the intake stroke.

The intake valve opens a few degrees of crankshaft rotation before the piston reaches the top of the exhaust stroke.

The exhaust valve remains open until the piston has moved down a few degrees of crankshaft rotation on the intake stroke.

Reduced valve operating temperatures 

Metallic-sodium filled valves reduce the valve operating temperature.

When the engine running, the sodium melts and as the valve opens and closes it sloshes back and forth within the valve.

• When it is in the head, it absorbs heat, and when it is in the stem, it transfers heat to the valve guide.
• 

push rod replacement 

Installing push rods of slightly different lengths lets us achieve the specified valve clearance on opposed type engines using hydraulic lifters.

Hydraulic Valve Tappets/Lifters

Some aircraft engines incorporate hydraulic tappets that automatically keep the valve clearance at zero, eliminating the necessity for any valve clearance adjustment mechanism.

• A typical hydraulic tappet (zero-lash valve lifter) is shown below.
• 
• When the engine valve is closed, the face of the tappet body (cam follower) is on the base circle or back of the cam.

The light plunger spring lifts the hydraulic plunger so that its outer end contacts the push rod socket, exerting a light pressure against it, thus eliminating any clearance in the valve linkage.

As the plunger moves outward, the ball check valve moves off its seat.

Oil from the supply chamber, which is directly connected with the engine lubrication system, flows in and fills the pressure chamber.

As the camshaft rotates, the cam pushes the tappet body and the hydraulic lifter cylinder outward.

This action forces the ball check valve onto its seat; thus, the body of oil trapped in the pressure chamber acts as a cushion.

During the interval when the engine valve is off its seat, a predetermined leakage occurs between plunger and cylinder bore, which compensates for any expansion or contraction in the valve train. 

Immediately after the engine valve closes, the amount of oil required to fill the pressure chamber flows in from the supply chamber, preparing for another cycle of operation.

Hydraulic valve lifters are normally adjusted at the time of overhaul.

They are assembled dry (no lubrication), clearances checked, and adjustments are usually made by using push rods of different lengths.

A minimum and maximum valve clearance is established.

Any measurement between these extremes is acceptable, but approximately half way between the extremes is desired.

Hydraulic valve lifters require less maintenance, are better lubricated, and operate more quietly than the screw adjustment type.

Oil starvation of bearings and other parts 

No engine should be run at high power settings until it is warmed up and the oil is warm enough to flow freely through all of the passages.

High power settings with cold oil can lead to oil starvation of the bearings

Proper engine warm-up is important, particularly when the condition of the engine is unknown.

Improperly adjusted idle mixture, intermittently firing spark plugs, and improperly adjusted engine valves all have an overlapping effect on engine stability.

Therefore, the warm-up should be made at the engine speed where maximum engine stability is obtained. Experience has shown that the optimum warm-up speed is from 1,000 to 1,600 rpm.

The actual speed selected should be the speed at which engine operation is the smoothest, since the smoothest operation is an indication that all phases of engine operation are the most stable

Some engines incorporate temperature-compensated oil pressure relief valves.

This type of relief valve results in high engine oil pressures immediately after the engine starts, if oil temperatures are very low.

Consequently, start the warmup of these engines at approximately 1,000 rpm and then move to the higher, more stable engine speed as soon as oil temperature reaches a warmer level.

During warm-up, watch the instruments associated with engine operation.

This aids in making sure that all phases of engine operation are normal.

For example, engine oil pressure should be indicated within 30 seconds after the start.

Furthermore, if the oil pressure is not up to or above normal within 1 minute after the engine starts, the engine should be shut down. 

Cylinder head or coolant temperatures should be observed continually to see that they do not exceed the maximum allowable limit.

A lean mixture should not be used to hasten the warm-up.

Actually, at the warm-up rpm, there is very little difference in the mixture supplied to the engine, whether the mixture is in a rich or lean position, since metering in this power range is governed by throttle position.

 increase 

The cylinder pressure applied to the crankshaft through the connecting rod bearings is determined by the compression ratio of the engine and the manifold pressure.

If the manifold pressure for a given RPM is increased, the bearing load imposed on the crankshaft will increase.

before the stop at the control lever is reached in both HOT and COLD positions 

• When rigging any engine control, the stop at the component must be reached before the stop on the flight deck is reached.
• 
• The figure above is a diagram of an induction system used in an engine equipped with a carburetor.

In this induction system, carburetor normal flow air is admitted at the lower front nose cowling below the propeller spinner and is passed through an air filter into air ducts leading to the carburetor.

• A carburetor heat air valve is located below the carburetor for selecting an alternate warm air source (carburetor heat) to prevent carburetor icing.
• 
• Carburetor icing occurs when the temperature is lowered in the throat of the carburetor and enough moisture is present to freeze and block the flow of air to the engine.

The carburetor heat valve admits air from the outside air scoop for normal operation, and it admits warm air from the engine compartment for operation during icing conditions.

The carburetor heat is operated by a push-pull control in the cockpit.

When the carburetor heat air door is closed, warm ducted air from around the exhaust is directed into the carburetor.

This raises the intake air temperature.

An alternate air door can be opened by engine suction if the normal route of airflow should be blocked by something.

The valve is spring loaded closed and is sucked open by the engine if needed.

Induction system ice can be prevented or eliminated by raising the temperature of the air that passes through the system, using a carburetor heat system located upstream near the induction system inlet and well ahead of the dangerous icing zones.

This air is collected by a duct surrounding the exhaust manifold. 

Heat is usually obtained through a control valve that opens the induction system to the warm air circulating in the engine compartment and around the exhaust manifold.

Improper or careless use of carburetor heat can be just as dangerous as the most advanced stage of induction system ice.

Increasing the temperature of the air causes it to expand and decrease in density.

This action reduces the weight of the charge delivered to the cylinder and causes a noticeable loss in power because of decreased volumetric efficiency.

In addition, high intake air temperature may cause detonation and engine failure, especially during takeoff and high power operation.

Therefore, during all phases of engine operation, the carburetor temperature must afford the greatest protection against icing and detonation.

When there is danger of induction system icing, the cockpit carburetor heat control is moved to the hot position. 

Throttle ice or any ice that restricts airflow or reduces manifold pressure can best be removed by using full carburetor heat.

If the heat from the engine compartment is sufficient and the application has not been delayed, it is only a matter of a few minutes until the ice is cleared.

excessively enriching the air/fuel mixture 

The power in a piston engine is produced by burning the fuel-air mixture.

This mixture burns most effectively at a specific ratio of fuel to air.

As altitude increases the air becomes less dense.

To maintain the correct ratio, we reduce the amount of fuel going into the cylinder.

With less fuel and air to burn, we develop less power.

Due to the drop in atmospheric pressure as altitude is increased, the density of the air also decreases.

A normally-aspirated engine has a fixed amount or volume of air that it can draw in during the intake stroke, therefore less air is drawn into the engine as altitude increases.

Less air tends to make carburetors run richer at altitude than at ground level, because of the decreased density of the airflow through the carburetor throat for a given volume of air.

Thus, it is necessary that a mixture control be provided to lean the mixture and compensate for this natural enrichment.

Some aircraft use carburetors in which the mixture control is operated manually.

• Other aircraft employ carburetors which automatically lean the carburetor mixture at altitude to maintain the proper fuel/air mixture.

Decreased engine power at a constant RPM and manifold pressure 

The amount of energy released by a burning fuel/air mixture is determined by the weight of both the fuel and the air in the mixture.

Water vapor weighs only about 5/8 as much as dry air, and when an engine takes in air with a high relative humidity, it produces less power at the same RPM and manifold pressure than it would produce if it were taking in dry air.

• neither No. 1 nor No. 2 is true 
•                     or
•         only No. 1 is true

This quotation, taken from the canonical FAA source material, is the reason why as per current FAA guidance, statement #1 is true.

• "When ignition in the cylinder occurs before the optimum crankshaft position is reached, the timing is said to be early. If ignition occurs too early, the piston rising in the cylinder is opposed by the full force of combustion. This condition results in a loss of engine power, overheating, and possible detonation and preignition."- Aviation Mechanic Handbook - Powerplant, 4-23
• Statement #1 is TRUE. 
• "Preignition can be caused by improper ignition timing" is true. 

Read the bold text below to verify this - it is a direct quotation from the current (at the time of this witing) FAA AMP Handbook and it establishes a direct link from improper ignition timing to preignition.

Statement #2 is FALSE. This description applies to preignition, not detonation.

An aircraft’s ignition system is the result of careful design and thorough testing.

The ignition system usually provides good, dependable service, provided it is maintained and inspected properly.

However, difficulties can occur with normal wear, which affects ignition system performance, especially with magneto systems.

Breakdown and deterioration of insulating materials, breaker point wear, corrosion, bearing and oil seal wear, and electrical connection problems are all possible defects that can be associated with magneto-ignition systems.

The ignition timing requires precise adjustment and painstaking care so that the following four conditions occur at the same instant:

The piston in the No. 1 cylinder must be in a position a prescribed number of degrees before top dead center on the compression stroke.

The rotating magnet of the magneto must be in the E-gap position.

The breaker points must be just opening on the No. 1 cam lobe.

The distributor finger must be aligned with the electrode serving the No. 1 cylinder.

If one of these conditions is out of synchronization with any of the others, the ignition system is out of time.

If the spark is out of time, it is not delivered to the cylinder at the correct time and engine performance decreases.

When ignition in the cylinder occurs before the optimum crankshaft position is reached, the timing is said to be early.

If ignition occurs too early, the piston rising in the cylinder is opposed by the full force of combustion.

This condition results in a loss of engine power, overheating, and possible detonation and preignition.

If ignition occurs at a time after the optimum crankshaft position is reached, the ignition timing is said to be late.

If it occurs too late, not enough time is allowed to consume the fuel-air charge, and combustion is incomplete.

As a result, the engine loses power and requires a greater throttle opening to carry a given propeller load.

• Preignition:
• Ignition of the fuel-air mixture inside the cylinder of an engine before the time for normal ignition.

• Preignition is often caused by incandescent objects inside the cylinder.
• Detonation
• An uncontrolled explosion inside the cylinder of a reciprocating engine.

Detonation occurs when the pressure and temperature of the fuel inside the cylinder exceeds the critical pressure and temperature of the fuel.

Detonation may be caused by using fuel that has a lower octane rating or performance number than is specified for the engine.

We have gotten reports that a different test prep source claims that statement #1 is false because because they claim that pre-ignition is caused by hot spots, not improper timing. 

The different test prep source is wrong.

The bolded text from the "Reciprocating Engine Ignition System Maintenance and Inspection" section of the Engine Ignition and Electrical Systems chapter of the FAA AMP handbook clearly establishes a direct link from improper ignition timing to preignition.

Detonation occurs when the unburned charge explodes instead of burning normally.

"Hot spots in the combustion chamber" cause pre-ignition, not detonation to occur.

Fouled spark plugs and shorted out wiring cause rough engine handling and other issues, but not detonation.

Detonation = Explosion.

• Explodes
• During normal combustion, the fuel-air mixture burns in a very controlled and predictable manner.

In a spark ignition engine, the process occurs in a fraction of a second.

The mixture actually begins to burn at the point where it is ignited by the spark plugs.

It then burns away from the plugs until it is completely consumed.

• This type of combustion causes a smooth build-up of temperature and pressure and ensures that the expanding gases deliver the maximum force to the piston at exactly the right time in the power stroke.
• 
• Detonation is an uncontrolled, explosive ignition of the fuel-air mixture within the cylinder’s combustion chamber.

It causes excessive temperatures and pressures which, if not corrected, can quickly lead to failure of the piston, cylinder, or valves.

In less severe cases, detonation causes engine overheating, roughness, or loss of power.

Detonation is characterized by high cylinder head temperatures and is most likely to occur when operating at high power settings.

Common operational causes of detonation are:

• Use of a lower fuel grade than that specified by the aircraft manufacturer
• Operation of the engine with extremely high manifold pressures in conjunction with low rpm
• Operation of the engine at high power settings with an excessively lean mixture

Maintaining extended ground operations or steep climbs in which cylinder cooling is reducedDetonation may be avoided by following these basic guidelines during the various phases of ground and flight operations:

Ensure that the proper grade of fuel is used.

Keep the cowl flaps (if available) in the full-open position while on the ground to provide the maximum airflow through the cowling.

Use an enriched fuel mixture, as well as a shallow climb angle, to increase cylinder cooling during takeoff and initial climb.

Avoid extended, high power, steep climbs.

Develop the habit of monitoring the engine instruments to verify proper operation according to procedures established by the manufacturer.

Preignition occurs when the fuel-air mixture ignites prior to the engine’s normal ignition event.

Premature burning is usually caused by a residual hot spot in the combustion chamber, often created by a small carbon deposit on a spark plug, a cracked spark plug insulator, or other damage in the cylinder that causes a part to heat sufficiently to ignite the fuel-air charge.

Preignition causes the engine to lose power and produces high operating temperature.

As with detonation, preignition may also cause severe engine damage because the expanding gases exert excessive pressure on the piston while still on its compression stroke.

Detonation and preignition often occur simultaneously and one may cause the other.

Since either condition causes high engine temperature accompanied by a decrease in engine performance, it is often difficult to distinguish between the two.

• Using the recommended grade of fuel and operating the engine within its proper temperature, pressure, and rpm ranges reduce the chance of detonation or preignition.

Engine installation 

A new engine has no oil in the various lines and passages to the bearings.

We must pre-oil the engine to ensure the bearings will be adequately lubricated after start until the oil pump delivers normal oil flow through the system.

Before a new engine is flight tested, it must undergo a thorough ground check.

Before this ground check can be made, several operations are usually performed on the engine.

To prevent failure of the engine bearings during the initial start, the engine should be pre-oiled.

When an engine has been idle for an extended period of time, its internal bearing surfaces are likely to become dry at points where the corrosion-preventive mixture has dried out or drained away from the bearings.

Hence, it is necessary to force oil throughout the entire engine oil system.

If the bearings are dry when the engine is started, the friction at high rpm destroys the bearings before lubricating oil from the engine-driven oil pump can reach them.

There are several methods of pre-oiling an engine.

The method selected should provide an expeditious and adequate pre-oiling service.

Before using any pre-oiling method, remove one spark plug from each cylinder to allow the engine to be turned over more easily with the starter.

Also, connect an external source of electrical power (auxiliary power unit) to the aircraft electrical system to prevent an excessive drain on the aircraft battery.

In using some types of pre-oilers, such as that shown below, the oil line from the inlet side of the engine-driven oil pump must be disconnected to permit the pre-oiler tank to be connected at this point.

Then, a line must be disconnected, or an opening made in the oil system at the nose of the engine, to allow oil to flow out of the engine.

Oil flowing out of the engine indicates the completion of the pre-oiling operation, since the oil has now passed through the entire system

In order to force oil from the pre-oiler tank through the engine, apply air pressure to the oil in the tank while the engine is being turned through with the starter.

When this action has forced oil through the disconnection at the nose of the engine, stop cranking the engine and disconnect the pre-oiler tank.

A motor-driven oil pump can also be used to pump oil through the engine during the pre-oiling operation.

When no external means of pre-oiling an engine are available, the engine oil pump may be used.

Fill the engine oil tank, or crankcase, to the proper level.

Then, with the mixture in the idle cutoff position (reciprocating engine), the fuel shutoff valve and ignition switches in the off position, and the throttles fully open, crank the engine with the starter until the oil pressure gauge mounted on the instrument panel indicates oil pressure.

After the engine has been pre-oiled, replace the spark plugs and connect the oil system.

• Generally, the engine should be operated within 4 hours of being pre-oiled; otherwise, the pre-oiling procedure normally must be repeated.
• 

be checked for the amount of thread engagement by means of the inspection holes 

The inspection holes provide a means to ensure we have the correct amount of thread engagement.

The following instructions cover some of the basic inspections and procedures for rigging and adjusting fuel controls, fuel selectors, and fuel shutoff valves.

Inspect all bellcranks for looseness, cracks, or corrosion.

Inspect rod ends for damaged threads and the number of threads remaining after final adjustment.

Inspect cable drums for wear and cableguards for proper position and tension.

While rigging the fuel selector, power controls, and shutoff valve linkages, follow the manufacturer’s step-by-step procedure for the particular aircraft model being rigged.

The cables should be rigged with the proper tension with the rigging pins installed.

The pins should be free to be removed without any binding; if they are hard to remove, the cables are not rigged properly and should be rechecked.

The power lever should have the proper cushion at the idle and full-power positions.

The pointers, or indicators, on the fuel control should be within limits.

The fuel selectors must be rigged so that they have the proper travel and do not restrict the fuel flow to the engines.

Many older conventional turbofan engines use various power lever control systems.

One of the common types is the cable and rod system.

This system uses bellcranks, push-pull rods, drums, fairleads, flexible cables, and pulleys.

All of these components make up the control system and must be adjusted or rigged from time to time.

On single-engine aircraft, the rigging of the power lever controls is not very difficult.

The basic requirement is to have the desired travel on the power lever and correct travel at the fuel control.

On multiengine turbojet aircraft, the power levers must be rigged so that they are aligned at all power settings.

On older style aircraft the power lever control cables and push-pull rods in the airframe system to the pylon and nacelle are not usually disturbed at engine change time and usually no rigging is required, except when some component has been changed.

Before adjusting the power controls at the engine, be sure that the power lever is free from binding and the controls have full throw on the console.

If they do not have full throw or are binding, the airframe system should be checked and the discrepancies repaired.

After all adjustments have been made, move the power levers through their complete range, carefully inspecting for adequate clearance between the various push-pull rods and tubes.

Secure all locknuts, cotter pins, and safety as required.

If there are inspection holes to be used during the checking, make sure to use them. 

• Ultimately the goal is to ensure that the rods are secure and properly adjusted.
• Push pull control rods are generally not secured with safety wire.

The number of visible threads, if any, should be as specified by the manufacturer.

There is no fixed "two threads but not four" rule.

• The following image shows the location of a push-pull rod in an engine control syste
• 

Use of fuel with too low an octane rating 

Detonation is characterized by high cylinder head temperatures and is most likely to occur when operating at high power settings.

Some common operational causes of detonation include:

Using a lower fuel grade than that specified by the aircraft manufacturer or operating the engine after it has been sitting for an extended period; after 3 weeks or as indicated by the POH, drain old fuel and replenish with fresh fuel.

Operating the engine at high power settings with an excessively lean mixture.

Extended ground operations.

Detonation may be avoided by following these basic guidelines during the various phases of ground and flight operations:

Make sure the proper grade of fuel is being used.

Drain and refuel if the fuel is old.

• Develop a habit of monitoring the engine instruments to verify proper operation according to procedures established by the manufacturer

lose power due to the reduced density of the air drawn into the cylinders 

The unsupercharged - or naturally aspirated - engine has no means of providing a manifold pressure any greater than the induction system inlet pressure.

As altitude is increased with full throttle and a governed RPM, the airflow through the engine is reduced and BHP decreases.

The first forms of supercharging were of relatively low pressure ratio and the added airflow and power could be handled at full throttle within detonation limits.

• Such a "ground boosted" engine would achieve higher output power at all altitudes but an increase in altitude would produce a decrease in manifold pressure, airflow, and power output.

 Lean mixture 

• There are specific instructions concerning mixture ratios for each type of engine under various operating conditions. 
•  
• Failure to follow these instructions will result in poor performance and often in damage to the engine. 
•  
• Excessively rich mixtures result in loss of power and waste of fuel. 
•  
• With the engine operating near its maximum output, very lean mixtures will cause a loss of power and under certain conditions, serious overheating. 
•  
• When the engine is operated on a lean mixture, the cylinder head temperature gauge should be watched closely. 
•  
• If the mixture is excessively lean, the engine may backfire through the induction system or stop completely. 
•  
• Backfire results from slow burning of the lean mixture. 
•  
• If the charge is still burning when the intake valve opens, it ignites the fresh mixture and the flame travels back through the combustible mixture in the induction system

• Afterfiring vs Backfiring
• You may have heard people on the street talk about hearing a "car's exhaust backfire." 
•  
• While we all understand what they mean by this, this is also technically wrong. 
•  
• What they are describing is AFTERFIRING through the exhaust. 
•  
• "Backfiring" involves combustion coming "back" through the induction system. 
•  
• Backfiring through the intake is normally associated with the mixture being too lean. 
•  
• Any FAA answer choices that says something like "backfiring through the exhaust" is INCORRECT. The FAA often uses this as a distractor.
•  
• BackfiringWhen a fuel/air mixture does not contain enough fuel to consume all the oxygen, it is called a lean mixture. 
•  
• Conversely, a charge that contains more fuel than required is called a rich mixture. 
•  
• An extremely lean mixture either does not burn at all or burns so slowly that combustion is not complete at the end of the exhaust stroke. 
•  
• The flame lingers in the cylinder and then ignites the contents in the intake manifold or the induction system when the intake valve opens. 
•  
• This causes an explosion known as backfiring, which can damage the carburetor and other parts of the induction system.

• Incorrect ignition timing, or faulty ignition wires, can cause the cylinder to fire at the wrong time, allowing the cylinder to fire when the intake valve is open, which can cause backfiring. 
•  
• A point worth stressing is that backfiring rarely involves the whole engine. 
•  
• Therefore, it is seldom the fault of the carburetor. 
•  
• In practically all cases, backfiring is limited to one or two cylinders. 
•  
• Usually, it is the result of faulty valve clearance setting, defective fuel injector nozzles, or other conditions that cause these cylinders to operate leaner than the engine as a whole. 
•  
• There can be no permanent cure until these defects are discovered and corrected. 
•  
• Because these backfiring cylinders fire intermittently and, therefore, run cool, they can be detected by the cold cylinder check.

• In some instances, an engine backfires in the idle range but operates satisfactorily at medium and high power settings. 
•  
• The most likely cause, in this case, is an excessively lean idle mixture. Proper adjustment of the idle fuel/air mixture usually corrects this difficulty.

• Afterfiring
• Afterfiring, sometimes called afterburning (when this won't be confused with military-jet-type afterburning), often results when the fuel/air mixture is too rich. 
•  
• Overly rich mixtures are also slow burning, therefore, charges of unburned fuel are present in the exhausted gases. 
•  
• Air from outside the exhaust stacks mixes with this unburned fuel that ignites. 
•  
• This causes an explosion in the exhaust system. 
•  
• Afterfiring is perhaps more common where long exhaust ducting retains greater amounts of unburned charges. 
•  
• As in the case of backfiring, the correction for afterfiring is the proper adjustment of the fuel/air mixture.

• Afterfiring can also be caused by cylinders that are not firing because of faulty spark plugs, defective fuel-injection nozzles or incorrect valve clearance. 
•  
• The unburned mixture from these dead cylinders passes into the exhaust system, where it ignites and burns. 
•  
• Unfortunately, the resultant torching or afterburning can easily be mistaken for evidence of a rich carburetor. 
•  
• Cylinders that are firing intermittently can cause a similar effect. 
•  
• Again, the malfunction can be remedied only by discovering the real cause and correcting the defect. 
•  
• Dead or intermittent cylinders can be located by the cold cylinder check.

Oil will flow from the engine return line or indicator port 

When pre-oiling a dry sump reciprocating engine, you know there is oil in all the passages when oil flows from the engine return line or from the port to which the oil pressure gauge is connected.

 Reduce the manifold pressure, then the RPM 

• When both manifold pressure and rpm need to be changed, avoid engine overstress by making power adjustments in the proper order:
• When power settings are being decreased, reduce manifold pressure before reducing rpm. 

If rpm is reduced before manifold pressure, manifold pressure will automatically increase, possibly exceeding the manufacturer’s tolerances.

When power settings are being increased, reverse the order—increase rpm first, then manifold pressure.

• Remember: "Prop Up, Throttle Down.
• 
• Constant-Speed Propeller
• A constant-speed propeller keeps the blade angle adjusted for maximum efficiency during most flight conditions.

The pilot controls the engine rpm indirectly by means of a propeller control, which is connected to the propeller governor.

• For maximum takeoff power, the propeller control is moved all the way forward to the low pitch/high rpm position, and the throttle is moved forward to the maximum allowable manifold pressure position.
• 
• To reduce power for climb or cruise, the pilot reduces manifold pressure to the desired value with the throttle, and then reduces engine rpm by moving the propeller control back toward the high pitch/low rpm position.

The pilot sets the rpm accurately using the tachometer.

When an airplane engine runs at a constant governed speed, the torque (force) exerted by the engine at the propeller shaft equals the force resisting the moving blades.

The pilot uses the propeller control to change engine rpm by adjusting the propeller blade pitch, which increases or decreases the air resistance on the rotating propeller.

For example, pulling back on the propeller control moves the propeller blades to a higher pitch.

This increases the air resistance exerted on the spinning propeller and puts an additional load on the engine, which causes it to slow down until the forces reach equilibrium.

Advancing the propeller control reduces the propeller blade pitch.

This reduces the resistance of the air against the propeller.

In response, the engine rpm increases until the opposing forces balance.

In order for this system to function, a constant-speed propeller governor needs the means to sense engine rpm and a means to control the propeller AOA.

In most cases, the governor is geared to the engine crankshaft giving it a means to sense engine rpm.

The "Blade Angle Control" section of this chapter discusses the ways a propeller governor adjusts propeller blade angle.

Other factors affect constant-speed propeller blade pitch.

When an airplane is nosed up into a climb from level flight, the engine tends to slow down.

Since the governor is sensitive to small changes in engine rpm, it decreases the blade angle just enough to keep the engine speed constant.

If the airplane is nosed down into a dive, the governor increases the blade angle just enough to keep the engine speed constant.

This allows the engine to maintain a constant rpm and power output.

The pilot can also set engine power output by changing rpm at a constant manifold pressure; by changing the manifold pressure at a constant rpm; or by changing both rpm and manifold pressure.

The constant-speed propeller makes it possible to obtain an infinite number of power settings.

Takeoff, Climb, and Cruise During takeoff, when the forward motion of the airplane is at a low speed and when maximum power and thrust are required, the constant-speed propeller sets up a low propeller blade pitch.

• The low blade angle keeps the blade angle of attack, with respect to the relative wind, small and efficient at the low speed.
• 
• At the same time, low blade pitch allows the propeller to handle a smaller mass of air per revolution.

This light propeller load allows the engine to turn at maximum rpm and develop maximum engine power.

Although the mass of air per revolution is small, the number of rpm is high, and propeller thrust is maximized until brake release.

Thrust is maximum at the beginning of the takeoff roll and then decreases as the airplane gains speed.

As the airspeed increases after lift-off, the load on the engine is lightened because of the small blade angle.

The governor senses this and increases the blade angle slightly.

Again, the higher blade angle, with the higher speed, keeps the blade AOA with respect to the relative wind small and efficient.

For climb after takeoff, the power output of the engine is reduced to climb power by decreasing the manifold pressure and increasing the blade angle to lower engine rpm.

At the higher (climb) airspeed and the higher blade angle, the propeller is handling a greater mass of air per second at a lower slipstream velocity.

This reduction in power is offset by the increase in propeller efficiency.

The blade AOA is again kept small by the increase in the blade angle with an increase in airspeed.

At cruising altitude, when the airplane is in level flight, airspeed increases, and less power is required.

Consequently, the pilot uses the throttle to reduce manifold pressure and uses the propeller control to reduce engine rpm.

The higher airspeed and higher blade angle enable the propeller to handle a still greater mass of air per second at still smaller slipstream velocity.

At normal cruising speeds, propeller efficiency is at or near maximum efficiency.

• Blade Angle Control
• Once the rpm settings for the propeller are selected, the propeller governor automatically adjusts the blade angle to maintain the selected rpm.

It does this by using oil pressure.

Generally, the oil pressure used for pitch change comes directly from the engine lubricating system.

When a governor is employed, engine oil is used and the oil pressure is usually boosted by a pump that is integrated with the governor.

The higher pressure provides a quicker blade angle change.

The rpm at which the propeller is to operate is adjusted in the governor head.

The pilot changes this setting by changing the position of the governor rack through the flight deck propeller control.

On some constant-speed propellers, changes in pitch are obtained by the use of an inherent centrifugal twisting moment of the blades that tends to flatten the blades toward low pitch and oil pressure applied to a hydraulic piston connected to the propeller blades which moves them toward high pitch.

Another type of constant-speed propeller uses counterweights attached to the blade shanks in the hub.

Governor oil pressure and the blade twisting moment move the blades toward the low pitch position, and centrifugal force acting on the counterweights moves them (and the blades) toward the high pitch position.

In the first case above, governor oil pressure moves the blades towards high pitch and in the second case, governor oil pressure and the blade twisting moment move the blades toward low pitch.

A loss of governor oil pressure, therefore, affects each differently.

• Governing RangeThe blade angle range for constant-speed propellers varies from about 11.5° to 40°. The higher the speed of the airplane, the greater the blade angle range.
• 
• The range of possible blade angles between high and low blade angle pitch stops define the propeller’s governing range.

As long as the propeller's blades operate within the governing range and not against either pitch stop, a constant engine rpm is maintained.

However, once the propeller blades reach their pitch-stop limit, the engine rpm increases or decreases with changes in airspeed and propeller load similar to a fixed-pitch propeller.

For example, once a specific rpm is selected, if the airspeed decreases enough, the propeller blades reduce pitch in an attempt to maintain the selected rpm until they contact their low pitch stops.

From that point, any further reduction in airspeed causes the engine rpm to decrease.

Conversely, if the airspeed increases, the pitch angle of the propeller blades increase until the high pitch stop is reached.

The engine rpm then begins to increase.

• Constant-Speed Propeller Operation
• The engine is started with the propeller control in the low pitch/high rpm position.

This position reduces the load or drag of the propeller and the result is easier starting and warm-up of the engine.

During warm-up, the propeller blade changing mechanism is operated slowly and smoothly through a full cycle.

This is done by moving the propeller control (with the manifold pressure set to produce about 1,600 rpm) to the high pitch/low rpm position, allowing the rpm to stabilize, and then moving the propeller control back to the low pitch takeoff position.

• This is done for two reasons:
• to determine whether the system is operating correctly and to circulate fresh warm oil through the propeller governor system.

Remember the oil has been trapped in the propeller cylinder since the last time the engine was shut down.

There is a certain amount of leakage from the propeller cylinder, and the oil tends to congeal, especially if the outside air temperature is low.

Consequently, if the propeller is not exercised before takeoff, there is a possibility that the engine may over-speed on takeoff.

An airplane equipped with a constant-speed propeller has better takeoff performance than a similarly powered airplane equipped with a fixed-pitch propeller.

This is because with a constant-speed propeller, an airplane can develop its maximum rated horsepower (red line on the tachometer) while motionless.

An airplane with a fixed-pitch propeller, on the other hand, needs to accelerate down the runway to increase airspeed and aerodynamically unload the propeller so that rpm and horsepower can steadily build up to their maximum.

With a constant-speed propeller, the tachometer reading should come up to within 40 rpm of the red line as soon as full power is applied and remain there for the entire takeoff.

Excessive manifold pressure raises the cylinder combustion pressures, resulting in high stresses within the engine.

Excessive pressure also produces high-engine temperatures.

A combination of high manifold pressure and low rpm can induce damaging detonation.

In order to avoid these situations, the following sequence should be followed when making power changes.

When increasing power, increase the rpm first and then the manifold pressure

When decreasing power, decrease the manifold pressure first and then decrease the rpm

The cruise power charts in the AFM/POH should be consulted when selecting cruise power settings.

Whatever the combinations of rpm and manifold pressure listed in these charts—they have been flight tested and approved by engineers for the respective airframe and engine manufacturer.

Therefore, if there are power settings, such as 2,100 rpm and 24 inches manifold pressure in the power chart, they are approved for use.

With a constant-speed propeller, a power descent can be made without over-speeding the engine.

The system compensates for the increased airspeed of the descent by increasing the propeller blade angles.

If the descent is too rapid or is being made from a high altitude, the maximum blade angle limit of the blades is not sufficient to hold the rpm constant.

When this occurs, the rpm is responsive to any change in throttle setting.

Although the governor responds quickly to any change in throttle setting, a sudden and large increase in the throttle setting causes a momentary over-speeding of the engine until the blades become adjusted to absorb the increased power.

If an emergency demanding full power should arise during approach, the sudden advancing of the throttle causes momentary over-speeding of the engine beyond the rpm for which the governor is adjusted.

Some important points to remember concerning constant speed propeller operation are:

The red line on the tachometer not only indicates maximum allowable rpm; it also indicates the rpm required to obtain the engine’s rated horsepower.

A momentary propeller overspeed may occur when the throttle is advanced rapidly for takeoff.

This is usually not serious if the rated rpm is not exceeded by 10 percent for more than 3 seconds.

The green arc on the tachometer indicates the normal operating range.

When developing power in this range, the engine drives the propeller.

Below the green arc, however, it is usually the windmilling propeller that powers the engine.

Prolonged operation below the green arc can be detrimental to the engine.

On takeoffs from low elevation airports, the manifold pressure in inches of mercury may exceed the rpm.

This is normal in most cases, but the pilot should always consult the AFM/POH for limitations.

• All power changes should be made smoothly and slowly to avoid over-boosting and/or over-speeding.

The mixture ratio which gives the best power is richer than the mixture ratio which gives maximum economy 

If more fuel is added to the same quantity of air charge than the amount giving a chemically perfect mixture, changes of power and temperature will occur.

The combustion gas temperature will be lowered as the mixture is enriched, and the power will increase until the fuel/air ratio is approximately 0.0725.

From 0.0725 fuel/air ratio to 0.080 fuel/air ratio the power will remain essentially constant even though the combustion temperature continues downward.

Mixtures from 0.0725 fuel/air ratio to 0.080 fuel/air ratio are called best power mixtures, since their use results in the greatest power for a given airflow or manifold pressure.

In this fuel/air ratio range, there is no increase in the total heat released, but the weight of nitrogen and combustion products is augmented by the vapor formed with the excess fuel; thus, the working mass of the charge is increased.

In addition, the extra fuel in the charge (over the stoichiometric mixture) speeds up the combustion process, which provides a favorable time factor in converting fuel energy into power.

Enriching a fuel/air ratio above 0.080 results in the loss of power besides reduction of temperature, as the cooling effects of excess fuel overtake the favorable factor of increased mass.

The reduced temperature and slower rate of burning lead to an increasing loss of combustion efficiency.

If, with constant airflow, the mixture is leaned below 0.067 fuel/air ratio, power and temperature will decrease together.

This time, the loss of power is not a liability but an asset.

The purpose in leaning is to save fuel.

Air is free and available in limitless quantities.

The object is to obtain the required power with the least fuel flow and to let the air consumption take care of itself.

A measure of the economical use of fuel is called SFC (specific fuel consumption), which is the pounds of fuel per hour per hp.

Thus, SFC = pounds fuel/hour/hp. By using this ratio, the engine's use of fuel at various power settings can be compared.

When leaning below 0.067 fuel/air ratio with constant airflow, even though the power diminishes, the cost in fuel to support each horsepower hour (SFC) also is lowered for a while.

While the mixture charge is becoming weaker, this loss of strength occurs at a rate slower than that of the reduction of fuel flow.

This favorable tendency continues until a mixture strength known as best economy is reached.

With this fuel/air ratio, the required hp is developed with the least fuel flow, or, to put it another way, a given fuel flow produces the most power.

The best economy fuel/air ratio varies somewhat with rpm and other conditions, but, for cruise powers on most reciprocating engines, it is sufficiently accurate to define this range of operation as being from 0.060 to 0.065 fuel/air ratios with retard spark, and from 0.055 to 0.061 fuel/air ratios with advance spark.

These are the most commonly used fuel/air ratios on aircraft where manual leaning is practiced.

Below the best economical mixture strength, power and temperature continue to fall with constant airflow while the SFC increases.

As the fuel/air ratio is reduced further, combustion becomes so cool and slow that power for a given manifold pressure becomes so low as to be uneconomical.

The cooling effect of rich or lean mixtures results from the excess fuel or air over that needed for combustion.

Internal cylinder cooling is obtained from unused fuel when fuel/air ratios above 0.067 are used.

• The same function is performed by excess air when fuel/air ratios below 0.067 are used.

an excessively lean mixture 

When a fuel/air mixture does not contain enough fuel to consume all the oxygen, it is called a lean mixture.

Conversely, a charge that contains more fuel than required is called a rich mixture.

An extremely lean mixture either will not burn at all or will burn so slowly that combustion is not complete at the end of the exhaust stroke.

The flame lingers in the cylinder and then ignites the contents in the intake manifold or the induction system when the intake valve opens.

This causes an explosion known as backfiring, which can damage the carburetor and other parts of the induction system.

• Afterfiring vs Backfiring
• You may have heard people on the street talk about hearing a "car's exhaust backfire."

While we all understand what they mean by this, this is also technically wrong.

What they are describing is AFTERFIRING through the exhaust. "Backfiring" involves combustion coming "back" through the induction system.

Backfiring through the intake is normally associated with the mixture being too lean.

Any FAA answer choices that says something like "backfiring through the exhaust" is INCORRECT.

The FAA often uses this as a distractor.

• Backfiring
• When a fuel/air mixture does not contain enough fuel to consume all the oxygen, it is called a lean mixture.

Conversely, a charge that contains more fuel than required is called a rich mixture.

An extremely lean mixture either does not burn at all or burns so slowly that combustion is not complete at the end of the exhaust stroke.

The flame lingers in the cylinder and then ignites the contents in the intake manifold or the induction system when the intake valve opens.

This causes an explosion known as backfiring, which can damage the carburetor and other parts of the induction system.

Incorrect ignition timing, or faulty ignition wires, can cause the cylinder to fire at the wrong time, allowing the cylinder to fire when the intake valve is open, which can cause backfiring.

A point worth stressing is that backfiring rarely involves the whole engine.

Therefore, it is seldom the fault of the carburetor.

In practically all cases, backfiring is limited to one or two cylinders.

Usually, it is the result of faulty valve clearance setting, defective fuel injector nozzles, or other conditions that cause these cylinders to operate leaner than the engine as a whole.

There can be no permanent cure until these defects are discovered and corrected.

Because these backfiring cylinders fire intermittently and, therefore, run cool, they can be detected by the cold cylinder check.

In some instances, an engine backfires in the idle range but operates satisfactorily at medium and high power settings. The most likely cause, in this case, is an excessively lean idle mixture.

Proper adjustment of the idle fuel/air mixture usually corrects this difficulty.

• Afterfiring
• Afterfiring, sometimes called afterburning (when this won't be confused with military-jet-type afterburning), often results when the fuel/air mixture is too rich.

Overly rich mixtures are also slow burning, therefore, charges of unburned fuel are present in the exhausted gases.

Air from outside the exhaust stacks mixes with this unburned fuel that ignites.

This causes an explosion in the exhaust system.

Afterfiring is perhaps more common where long exhaust ducting retains greater amounts of unburned charges.

As in the case of backfiring, the correction for afterfiring is the proper adjustment of the fuel/air mixture.

Afterfiring can also be caused by cylinders that are not firing because of faulty spark plugs, defective fuel-injection nozzles. or incorrect valve clearance.

The unburned mixture from these dead cylinders passes into the exhaust system, where it ignites and burns.

Unfortunately, the resultant torching or afterburning can easily be mistaken for evidence of a rich carburetor.

Cylinders that are firing intermittently can cause a similar effect.

Again, the malfunction can be remedied only by discovering the real cause and correcting the defect.

Dead or intermittent cylinders can be located by the cold cylinder check.

1, 2, 3 

Detonation is an uncontrolled, explosive ignition of the fuel/air mixture within the cylinder’s combustion chamber.

It causes excessive temperatures and pressures which, if not corrected, can quickly lead to failure of the piston, cylinder, or valves.

In less severe cases, detonation causes engine overheating, roughness, or loss of power.

Detonation is characterized by high cylinder head temperatures and is most likely to occur when operating at high power settings.

Some common operational causes of detonation include:

Using a lower fuel grade than that specified by the aircraft manufacturer or operating the engine after it has been sitting for an extended period; after 3 weeks or as indicated by the POH, drain old fuel and replenish with fresh fuel.

Operating the engine at high power settings with an excessively lean mixture.

Extended ground operations.

Detonation may be avoided by following these basic guidelines during the various phases of ground and flight operations:

Make sure the proper grade of fuel is being used.

Drain and refuel if the fuel is old.

• Develop a habit of monitoring the engine instruments to verify proper operation according to procedures established by the manufacturer

 At low RPM 

Since there is a great volume of air moving at high RPM, a small induction air leak has minimal impact.

At low RPM, however, the air leak is a greater percentage of the total air moving through the system.

Induction System Inspection and Maintenance

The induction system should be checked for cracks and leaks during all regularly scheduled engine inspections.

The units of the system should be checked for security of mounting.

The system should be kept clean at all times, since pieces of rags or paper can restrict the airflow if allowed to enter the air intakes or ducts.

Loose bolts and nuts can cause serious damage if they pass into the engine.

On systems equipped with a carburetor air filter, the filter should be checked regularly.

If it is dirty or does not have the proper oil film, the filter element should be removed and cleaned.

After it has dried, it is usually immersed in a mixture of oil and rust-preventive compound.

The excess fluid should be allowed to drain off before the filter element is reinstalled.

• Paper-type filters should be inspected and replaced as needed

manifold pressure is reduced with the throttle control before the RPM is reduced with the propeller control 

When both manifold pressure and rpm need to be changed, avoid engine overstress by making power adjustments in the proper order:

When power settings are being decreased, reduce manifold pressure before reducing rpm. 

If rpm is reduced before manifold pressure, manifold pressure will automatically increase, possibly exceeding the manufacturer’s tolerances.

When power settings are being increased, reverse the order—increase rpm first, then manifold pressure.

• Remember "Prop Up, Throttle Down."
• 
• Constant-Speed Propeller
• A constant-speed propeller keeps the blade angle adjusted for maximum efficiency during most flight conditions.

The pilot controls the engine rpm indirectly by means of a propeller control, which is connected to the propeller governor.

• For maximum takeoff power, the propeller control is moved all the way forward to the low pitch/high rpm position, and the throttle is moved forward to the maximum allowable manifold pressure position.
• 
• To reduce power for climb or cruise, the pilot reduces manifold pressure to the desired value with the throttle, and then reduces engine rpm by moving the propeller control back toward the high pitch/low rpm position.

The pilot sets the rpm accurately using the tachometer.

When an airplane engine runs at a constant governed speed, the torque (force) exerted by the engine at the propeller shaft equals the force resisting the moving blades.

The pilot uses the propeller control to change engine rpm by adjusting the propeller blade pitch, which increases or decreases the air resistance on the rotating propeller.

For example, pulling back on the propeller control moves the propeller blades to a higher pitch.

This increases the air resistance exerted on the spinning propeller and puts an additional load on the engine, which causes it to slow down until the forces reach equilibrium.

Advancing the propeller control reduces the propeller blade pitch.

This reduces the resistance of the air against the propeller.

In response, the engine rpm increases until the opposing forces balance.

In order for this system to function, a constant-speed propeller governor needs the means to sense engine rpm and a means to control the propeller AOA.

In most cases, the governor is geared to the engine crankshaft giving it a means to sense engine rpm.

The "Blade Angle Control" section of this chapter discusses the ways a propeller governor adjusts propeller blade angle.

Other factors affect constant-speed propeller blade pitch.

When an airplane is nosed up into a climb from level flight, the engine tends to slow down.

Since the governor is sensitive to small changes in engine rpm, it decreases the blade angle just enough to keep the engine speed constant.

If the airplane is nosed down into a dive, the governor increases the blade angle just enough to keep the engine speed constant.

This allows the engine to maintain a constant rpm and power output.

The pilot can also set engine power output by changing rpm at a constant manifold pressure; by changing the manifold pressure at a constant rpm; or by changing both rpm and manifold pressure.

The constant-speed propeller makes it possible to obtain an infinite number of power settings.

Takeoff, Climb, and CruiseDuring takeoff, when the forward motion of the airplane is at a low speed and when maximum power and thrust are required, the constant-speed propeller sets up a low propeller blade pitch.

• The low blade angle keeps the blade angle of attack, with respect to the relative wind, small and efficient at the low speed.
• 
• At the same time, low blade pitch allows the propeller to handle a smaller mass of air per revolution.

This light propeller load allows the engine to turn at maximum rpm and develop maximum engine power. Although the mass of air per revolution is small, the number of rpm is high, and propeller thrust is maximized until brake release.

Thrust is maximum at the beginning of the takeoff roll and then decreases as the airplane gains speed.

As the airspeed increases after lift-off, the load on the engine is lightened because of the small blade angle.

The governor senses this and increases the blade angle slightly.

Again, the higher blade angle, with the higher speed, keeps the blade AOA with respect to the relative wind small and efficient.

For climb after takeoff, the power output of the engine is reduced to climb power by decreasing the manifold pressure and increasing the blade angle to lower engine rpm.

At the higher (climb) airspeed and the higher blade angle, the propeller is handling a greater mass of air per second at a lower slipstream velocity.

This reduction in power is offset by the increase in propeller efficiency.

The blade AOA is again kept small by the increase in the blade angle with an increase in airspeed.

At cruising altitude, when the airplane is in level flight, airspeed increases, and less power is required.

Consequently, the pilot uses the throttle to reduce manifold pressure and uses the propeller control to reduce engine rpm.

The higher airspeed and higher blade angle enable the propeller to handle a still greater mass of air per second at still smaller slipstream velocity.

At normal cruising speeds, propeller efficiency is at or near maximum efficiency.

Blade Angle ControlOnce the rpm settings for the propeller are selected, the propeller governor automatically adjusts the blade angle to maintain the selected rpm.

It does this by using oil pressure. Generally, the oil pressure used for pitch change comes directly from the engine lubricating system.

When a governor is employed, engine oil is used and the oil pressure is usually boosted by a pump that is integrated with the governor.

The higher pressure provides a quicker blade angle change.

The rpm at which the propeller is to operate is adjusted in the governor head.

The pilot changes this setting by changing the position of the governor rack through the flight deck propeller control.

On some constant-speed propellers, changes in pitch are obtained by the use of an inherent centrifugal twisting moment of the blades that tends to flatten the blades toward low pitch and oil pressure applied to a hydraulic piston connected to the propeller blades which moves them toward high pitch.

Another type of constant-speed propeller uses counterweights attached to the blade shanks in the hub.

Governor oil pressure and the blade twisting moment move the blades toward the low pitch position, and centrifugal force acting on the counterweights moves them (and the blades) toward the high pitch position.

In the first case above, governor oil pressure moves the blades towards high pitch and in the second case, governor oil pressure and the blade twisting moment move the blades toward low pitch.

A loss of governor oil pressure, therefore, affects each differently.

• Governing Range
• The blade angle range for constant-speed propellers varies from about 11.5° to 40°. The higher the speed of the airplane, the greater the blade angle range.
• 
• The range of possible blade angles between high and low blade angle pitch stops define the propeller’s governing range.

As long as the propeller's blades operate within the governing range and not against either pitch stop, a constant engine rpm is maintained.

However, once the propeller blades reach their pitch-stop limit, the engine rpm increases or decreases with changes in airspeed and propeller load similar to a fixed-pitch propeller.

For example, once a specific rpm is selected, if the airspeed decreases enough, the propeller blades reduce pitch in an attempt to maintain the selected rpm until they contact their low pitch stops.

From that point, any further reduction in airspeed causes the engine rpm to decrease.

Conversely, if the airspeed increases, the pitch angle of the propeller blades increase until the high pitch stop is reached.

The engine rpm then begins to increase.

• Constant-Speed Propeller Operation
• The engine is started with the propeller control in the low pitch/high rpm position.

This position reduces the load or drag of the propeller and the result is easier starting and warm-up of the engine.

During warm-up, the propeller blade changing mechanism is operated slowly and smoothly through a full cycle.

This is done by moving the propeller control (with the manifold pressure set to produce about 1,600 rpm) to the high pitch/low rpm position, allowing the rpm to stabilize, and then moving the propeller control back to the low pitch takeoff position.

This is done for two reasons: to determine whether the system is operating correctly and to circulate fresh warm oil through the propeller governor system.

Remember the oil has been trapped in the propeller cylinder since the last time the engine was shut down.

There is a certain amount of leakage from the propeller cylinder, and the oil tends to congeal, especially if the outside air temperature is low.

Consequently, if the propeller is not exercised before takeoff, there is a possibility that the engine may over-speed on takeoff.

An airplane equipped with a constant-speed propeller has better takeoff performance than a similarly powered airplane equipped with a fixed-pitch propeller.

This is because with a constant-speed propeller, an airplane can develop its maximum rated horsepower (red line on the tachometer) while motionless.

An airplane with a fixed-pitch propeller, on the other hand, needs to accelerate down the runway to increase airspeed and aerodynamically unload the propeller so that rpm and horsepower can steadily build up to their maximum.

With a constant-speed propeller, the tachometer reading should come up to within 40 rpm of the red line as soon as full power is applied and remain there for the entire takeoff.

Excessive manifold pressure raises the cylinder combustion pressures, resulting in high stresses within the engine.

Excessive pressure also produces high-engine temperatures.

A combination of high manifold pressure and low rpm can induce damaging detonation.

In order to avoid these situations, the following sequence should be followed when making power changes.

When increasing power, increase the rpm first and then the manifold pressure

When decreasing power, decrease the manifold pressure first and then decrease the rpm

The cruise power charts in the AFM/POH should be consulted when selecting cruise power settings.

Whatever the combinations of rpm and manifold pressure listed in these charts—they have been flight tested and approved by engineers for the respective airframe and engine manufacturer.

Therefore, if there are power settings, such as 2,100 rpm and 24 inches manifold pressure in the power chart, they are approved for use.

With a constant-speed propeller, a power descent can be made without over-speeding the engine.

The system compensates for the increased airspeed of the descent by increasing the propeller blade angles.

If the descent is too rapid or is being made from a high altitude, the maximum blade angle limit of the blades is not sufficient to hold the rpm constant.

When this occurs, the rpm is responsive to any change in throttle setting.

Although the governor responds quickly to any change in throttle setting, a sudden and large increase in the throttle setting causes a momentary over-speeding of the engine until the blades become adjusted to absorb the increased power.

If an emergency demanding full power should arise during approach, the sudden advancing of the throttle causes momentary over-speeding of the engine beyond the rpm for which the governor is adjusted.

Some important points to remember concerning constant speed propeller operation are:

The red line on the tachometer not only indicates maximum allowable rpm; it also indicates the rpm required to obtain the engine’s rated horsepower.

A momentary propeller overspeed may occur when the throttle is advanced rapidly for takeoff.

This is usually not serious if the rated rpm is not exceeded by 10 percent for more than 3 seconds.

The green arc on the tachometer indicates the normal operating range.

When developing power in this range, the engine drives the propeller.

Below the green arc, however, it is usually the windmilling propeller that powers the engine.

Prolonged operation below the green arc can be detrimental to the engine.

On takeoffs from low elevation airports, the manifold pressure in inches of mercury may exceed the rpm.

This is normal in most cases, but the pilot should always consult the AFM/POH for limitations.

All power changes should be made smoothly and slowly to avoid over-boosting and/or over-speeding.

spark plug condition 

Whenever problems develop during engine operation, which appear to be caused by the ignition system, it is recommended that the spark plugs and ignition harnesses be checked first before working on the magnetos.

• The following are the more common spark plug malfunctions and are relatively easy to identify.
• Fouling
• Carbon fouling is identified by the dull black, sooty deposits on the electrode end of the plug.

Although the primary causes are excessive ground idling and rich idle mixtures, a cold heat range may also be a contributing factor.

Lead fouling is characterized by hard, dark, cinder-like globules which gradually fill up the electrode cavity and short out the plug.

The primary cause for this condition is poor fuel vaporization combined with a high tetraethyl-lead content fuel.

A cold heat range may also contribute to this condition.

Oil fouling is identified by a wet, black carbon deposit over the entire firing end of the plug.

This condition is fairly common on the lower plugs in horizontally-opposed engines, and both plugs in the lower cylinders of radial engines.

Oil fouling is normally caused by oil drainage past the piston rings after shutdown.

However, when both spark plugs removed from the same cylinder are badly fouled with oil and carbon, some form of engine damage should be suspected, and the cylinder more closely inspected.

Mild forms of oil fouling can usually be cleared up by slowly increasing power, while running the engine until the deposits are burned off and the misfiring stops.

• Fused Electrodes
• There are many different types of malfunctions which result in fused spark plug electrodes; however, most are associated with pre-ignition either as the cause or the effect.

For this reason, any time a spark plug is found with the following defects, further investigation of the cylinder and piston should be conducted.

Occasionally, the ceramic nose core will crack, break away, and remain trapped behind the ground electrode.

This piece of insulation material will then buildup heat to the point it will ignite the fuel/air mixture prematurely.

The high temperatures and pressures encountered during this condition can cause damage to the cylinder and piston and ultimately lead to fusing and shorting out of the plug.

Corrosive gases formed by combustion and the high voltage spark have eroded the electrodes.

Spark plugs in this condition require more voltage to fire — often more than the ignition system can produce.

• Bridged Electrodes
• Occasionally, free combustion chamber particles will settle on the electrodes of a spark plug and gradually bridge the electrode gap, resulting in a shorted plug.

Small particles may be dislodged by slowly cycling the engine as described for the oil-fouled condition; however, the only remedy for more advanced cases is removal and replacement of the spark plug.

• Metal Deposits
• Whenever metal spray is found on the electrodes of a spark plug, it is an indication that a failure of some part of the engine is in progress.

The location of the cylinder in which the spray is found is important in diagnosing the problem, as various types of failures will cause the metal spray to appear differently.

For example, if the metal spray is located evenly in every cylinder, the problem will be in the induction system, such as an impeller failure.

If the metal spray is found only on the spark plugs in one cylinder, the problem is isolated to that cylinder and will generally be a piston failure.

• In view of the secondary damage which occurs whenever an engine part fails, any preliminary indication such as metal spray should be thoroughly investigated to establish and correct the cause.
• Flashover
• It is important that spark plug terminal contact springs and moisture seals be checked regularly for condition and cleanliness to prevent "flashover" in the connector well.

Foreign matter or moisture in the terminal connector well can reduce the insulation value of the connector to the point the ignition system voltages at higher power settings may flash over the connector well surface to ground and cause the plug to misfire.

If moisture is the cause, hard starting can also result.

• Any spark plug found with a dirty connector well may have this condition, and should be reconditioned before reuse.

Plugged crankcase breather 

• The crankcase breather deserves special consideration when preparing for cold weather. 
•  
• Frozen breather lines have created numerous problems. 
•  
• Most of the water of combustion goes out of the exhaust; however, some water enters the crankcase and is vaporized. 
•  
• When the vapor cools, it condenses in the breather line subsequently freezing it closed. 
•  
• Special care is recommended during the preflight to assure that the breather system is free of ice. 
•  
• Reports are common of engine oil loss from blown crankshaft seals caused by pressure generated by frozen breather tubes

Breather tubes should be inspected to ensure that the inside surfaces are clear and unobstructed.

It is normal practice for the airframe manufacturer to provide some means of preventing freezeup of the crankcase breather tube.

The breather tube may be insulated, designed so the end is located in a hot area, equipped with an electric heater, or it may incorporate a hole, notch, or slot which is often called a "whistle slot."

• The operator of any aircraft should know which method is used and ensure that the configuration is maintained as specified by the airframe manufacturer.

decreases valve overlap 

Engine, airplane, and equipment manufacturers provide a powerplant installation that gives satisfactory performance. 

Cams are designed to give best valve operation and correct overlap.

But valve operation is correct only if valve clearances are set and remain at the value recommended by the engine manufacturer.

If valve clearances are set wrong, the valve overlap period is longer or shorter than the manufacturer intended.

The same is true if clearances get out of adjustment during operation.

Where there is too much valve clearance, the valves do not open as wide or remain open as long as they should. 

This reduces the overlap period.

At idling speed, it affects the fuel/air mixture, since a less-than-normal amount of air or exhaust gases is drawn back into the cylinder during the shortened overlap period.

As a result, the idle mixture tends to be too rich.

When valve clearance is less than it should be, the valve overlap period is lengthened.

A greater than normal amount of air, or exhaust gases, is drawn back into the cylinder at idling speeds.

As a result, the idle mixture is leaned out at the cylinder.

The carburetor is adjusted with the expectation that a certain amount of air or exhaust gases is drawn back into the cylinder at idling.

If more or less air, or exhaust gases, are drawn into the cylinder during the valve overlap period, the mixture is too lean or too rich.

When valve clearances are wrong, it is unlikely that they are all wrong in the same direction.

Instead, there is too much clearance on some cylinders and too little on others.

Naturally, this gives a variation in valve overlap between cylinders.

This results in a variation in fuel/air ratio at idling and lower-power settings, since the carburetor delivers the same mixture to all cylinders.

• The carburetor cannot tailor the mixture to each cylinder to compensate for variation in valve overlap
• 
• The effect of variation in valve clearance and valve overlap on the fuel/air mixture between cylinders is illustrated above.

Note how the cylinders with too little clearance run rich, and those with too much clearance run lean.

Note also the extreme mixture variation between cylinders.

Valve clearance also effects volumetric efficiency.

Any variations in fuel/air into, and exhaust gases out of, the cylinder affects the volumetric efficiency of the cylinder.

With the use of hydraulic valve lifters that set the valve clearance automatically engine operation has been greatly improved.

Hydraulic lifters do have a limited range in which they can control the valve clearance, or they can become stuck in one position that can cause them to be a source of engine trouble.

Normally engines equipped with hydraulic lifters require little to no maintenance.

Critical altitude 

Altitude turbocharging (sometimes called "normalizing") is accomplished by using a turbocharger that will maintain maximum allowable sea level manifold pressure (normally 29 – 30 inches Hg) up to a certain altitude.

This altitude is specified by the airplane manufacturer and is referred to as the airplane’s critical altitude.

Above the critical altitude, the manifold pressure decreases as additional altitude is gained. 

• Ground boosting
• on the other hand, is an application of turbocharging where more than the standard 29 inches of manifold pressure is used in flight.

• In various airplanes using ground boosting, takeoff manifold pressures may go as high as 45 inches of mercury.
• 
• The above is a schematic of a sea level booster turbosupercharger system.

• This system used widely is automatically regulated by three components:
• Exhaust bypass valve assembly
• Density controller
• Differential pressure controller

By regulating the waste gate position and the "fully open" and "closed" positions, a constant power output can be maintained.

When the waste gate is fully open, all the exhaust gases are directed overboard to the atmosphere, and no air is compressed and delivered to the engine air inlet.

Conversely, when the waste gate is fully closed, a maximum volume of exhaust gases flows into the turbocharger turbine, and maximum supercharging is accomplished.

Between these two extremes of waste gate position, constant power output can be achieved below the maximum altitude at which the system is designed to operate.

An engine with a critical altitude of 16,000 feet cannot produce 100 percent of its rated manifold pressure above 16,000 feet.

Critical altitude means the maximum altitude at which, in standard atmosphere, it is possible to maintain, at a specified rotational speed, a specified power or a specified manifold pressure.

A critical altitude exists for every possible power setting below the maximum operating ceiling.

If the aircraft is flown above this altitude without a corresponding change in the power setting, the waste gate is automatically driven to the fully closed position in an effort to maintain a constant power output.

Thus, the waste gate is almost fully open at sea level and continues to move toward the closed position as the aircraft climbs, in order to maintain the preselected manifold pressure setting.

When the waste gate is fully closed (leaving only a small clearance to prevent sticking), the manifold pressure begins to drop if the aircraft continues to climb.

If a higher power setting cannot be selected, the turbocharger’s critical altitude has been reached.

Beyond this altitude, the power output continues to decrease.

• Turbocharging
• The turbocharged engine allows the pilot to maintain sufficient cruise power at high altitudes where there is less drag, which means faster true airspeeds and increased range with fuel economy.

At the same time, the powerplant has flexibility and can be flown at a low altitude without the increased fuel consumption of a turbine engine.

When attached to the standard powerplant, the turbocharger does not take any horsepower from the engine to operate; it is relatively simple mechanically, and some models can pressurize the cabin as well.

The turbocharger is an exhaust-driven device that raises the pressure and density of the induction air delivered to the engine.

It consists of two separate components: a compressor and a turbine connected by a common shaft.

The compressor supplies pressurized air to the engine for high-altitude operation.

The compressor and its housing are between the ambient air intake and the induction air manifold.

• The turbine and its housing are part of the exhaust system and utilize the flow of exhaust gases to drive the compressor.
• 
• The turbine has the capability of producing manifold pressure in excess of the maximum allowable for the particular engine.

In order not to exceed the maximum allowable manifold pressure, a bypass or waste gate is used so that some of the exhaust is diverted overboard before it passes through the turbine.

The position of the waste gate regulates the output of the turbine and therefore, the compressed air available to the engine.

When the waste gate is closed, all of the exhaust gases pass through and drive the turbine.

As the waste gate opens, some of the exhaust gases are routed around the turbine through the exhaust bypass and overboard through the exhaust pipe.

The waste gate actuator is a spring-loaded piston operated by engine oil pressure.

The actuator, which adjusts the waste gate position, is connected to the waste gate by a mechanical linkage.

The control center of the turbocharger system is the pressure controller.

This device simplifies turbocharging to one control:

• the throttle
• Once the desired manifold pressure is set, virtually no throttle adjustment is required with changes in altitude.

The controller senses compressor discharge requirements for various altitudes and controls the oil pressure to the waste gate actuator, which adjusts the waste gate accordingly.

Thus the turbocharger will maintain the manifold pressure called for by the throttle setting

• Ground Boosting Versus Altitude Turbocharging
• Altitude turbocharging (sometimes called “normalizing”) is accomplished by using a turbocharger that maintains maximum allowable sea level manifold pressure (normally 29–30 "Hg) up to a certain altitude.

This altitude is specified by the airplane manufacturer and is referred to as the airplane’s critical altitude.

Above the critical altitude, the manifold pressure decreases as additional altitude is gained.

• Ground boosting 
• on the other hand, is an application of turbocharging where more than the standard 29 inches of manifold pressure is used in flight.

In various airplanes using ground boosting, takeoff manifold pressures may go as high as 45 "Hg.

Although a sea-level manifold pressure setting and maximum rpm can be maintained up to the critical altitude, the engine may not be developing sea-level power.

Because the turbocharged induction air is heated by compression, lower induction air density causes a loss of engine power.

Maintaining the equivalent horsepower output requires a somewhat higher manifold pressure at a given altitude than if the induction air were not compressed and heated by turbocharging.

If, on the other hand, the system incorporates an automatic density controller, which automatically positions the waste gate so as to maintain constant air density to the engine, a near equivalent to sea-level horsepower output results.

• Operating Characteristics
• First and foremost, all movements of the power controls on turbocharged engines should be slow and smooth.

Aggressive or abrupt throttle movements increase the possibility of over-boosting.

Carefully monitor engine indications when making power changes.

When the waste gate is open, the turbocharged engine reacts the same as a normally aspirated engine when the rpm is varied.

That is, when the rpm is increased, the manifold pressure decreases slightly.

When the engine rpm is decreased, the manifold pressure increases slightly.

However, when the waste gate is closed, manifold pressure variation with engine rpm is just the opposite of the normally aspirated engine.

An increase in engine rpm results in an increase in manifold pressure, and a decrease in engine rpm results in a decrease in manifold pressure.

• Above the critical altitude 
• where the waste gate is closed, any change in airspeed results in a corresponding change in manifold pressure.

This is true because the increase in ram air pressure with an increase in airspeed is magnified by the compressor resulting in an increase in manifold pressure.

The increase in manifold pressure creates a higher mass flow through the engine, causing higher turbine speeds and thus further increasing manifold pressure.

When running at high altitudes, aviation gasoline tends to vaporize prior to reaching the cylinder.

If this occurs in the portion of the fuel system between the fuel tank and the engine-driven fuel pump, an auxiliary positive pressure pump may be needed in the tank.

Since engine-driven pumps pull fuel, they are easily vapor locked.

A boost pump provides positive pressure, which pushes the fuel and reduces the tendency to vaporize.

• Heat ManagementTurbocharged engines
• should be thoughtfully and carefully operated with continuous monitoring of pressures and temperatures.

• There are two temperatures that are especially important—turbine inlet temperature (TIT)
• exhaust gas temperature (EGT) and
• cylinder head temperature.

TIT or EGT limits are set to protect the elements in the hot section of the turbocharger, while cylinder head temperature limits protect the engine’s internal parts.

Due to the heat of compression of the induction air, a turbocharged engine runs at higher operating temperatures than a non- turbocharged engine.

Because turbocharged engines operate at high altitudes, their environment is less efficient for cooling.

At altitude, the air is less dense and, therefore, cools less efficiently.

Also, the less dense air causes the compressor to work harder.

Compressor turbine speeds can reach 80,000–100,000 rpm, adding to the overall engine operating temperatures.

Turbocharged engines are also operated at higher power settings a greater portion of the time.

High heat is detrimental to piston engine operation.

Its cumulative effects can lead to piston, ring, and cylinder head failure and place thermal stress on other operating components.

Excessive cylinder head temperature can lead to detonation, which in turn can cause catastrophic engine failure.

Turbocharged engines are especially heat sensitive.

The key to turbocharger operation is effective heat management.

Monitor the condition of a turbocharged engine with manifold pressure gauge, tachometer, exhaust gas temperature/turbine inlet temperature gauge, and cylinder head temperature gauge.

Manage the “heat system” with the throttle, propeller rpm, mixture, and cowl flaps.

At any given cruise power, the mixture is the most influential control over the exhaust gas/TIT.

The throttle regulates total fuel flow, but the mixture governs the fuel-to-air ratio.

The mixture, therefore, controls temperature.

Exceeding temperature limits in an after-takeoff climb is usually not a problem since a full rich mixture cools with excess fuel.

At cruise, power is normally reduced and mixture adjusted accordingly.

Under cruise conditions, monitor temperature limits closely because that is when the temperatures are most likely to reach the maximum, even though the engine is producing less power.

Overheating in an en route climb, however, may require fully open cowl flaps and a higher airspeed.

Since turbocharged engines operate hotter at altitude than normally aspirated engines, they are more prone to damage from cooling stress.

Gradual reductions in power and careful monitoring of temperatures are essential in the descent phase.

Extending the landing gear during the descent may help control the airspeed while maintaining a higher engine power setting.

This allows the pilot to reduce power in small increments which allows the engine to cool slowly.

It may also be necessary to lean the mixture slightly to eliminate roughness at the lower power settings.

• Turbocharger Failure
• Because of the high temperatures and pressures produced in the turbine exhaust system, any malfunction of the turbocharger should be treated with extreme caution.

In all cases of turbocharger operation, the manufacturer’s recommended procedures should be followed.

This is especially so in the case of turbocharger malfunction.

However, in those instances where the manufacturer’s procedures do not adequately describe the actions to be taken in the event of a turbocharger failure, the following procedures should be used.

• Over-Boost Condition: 
• If an excessive rise in manifold pressure occurs during normal advancement of the throttle (possibly owing to faulty operation of the waste gate):

Immediately retard the throttle smoothly to limit the manifold pressure below the maximum for the rpm and mixture setting.

Operate the engine in such a manner as to avoid a further over-boost condition.

• Low Manifold Pressure: 
• Although this condition may be caused by a minor fault, it is quite possible that a serious exhaust leak has occurred creating a potentially hazardous situation:

Shut down the engine in accordance with the recommended engine failure procedures, unless a greater emergency exists that warrants continued engine operation.

If continuing to operate the engine, use the lowest power setting demanded by the situation and land as soon as practicable.

• It is very important to ensure that corrective maintenance is undertaken following any turbocharger malfunction.

Piston ring leakage 

Cylinders having compression below the minimum specified after staking should be further checked to determine whether leakage is past the exhaust valve, intake valve, or piston.

• Excessive leakage can be detected:
• at the exhaust valve by listening for air leakage at the

• exhaust outlet;
• at the intake valve by escaping air at the air intake; and
• past the piston rings by escaping air at the engine breather outlets

an excessively rich mixture 

• Afterfiring
• sometimes called afterburning, often results when the fuel/air mixture is too rich.

Overly rich mixtures are also slow burning.

Therefore, charges of unburned fuel are present in the exhausted gases.

Air from outside the exhaust stacks mixes with this unburned fuel which ignites.

This causes an explosion in the exhaust system.

Afterfiring is perhaps more common where long exhaust ducting retains greater amounts of unburned charges.

As in the case of backfiring, the correction for afterfiring is the proper adjustment of the fuel/air mixture.

Afterfiring can also be caused by cylinders which are not firing because of faulty spark plugs, defective fuel injection nozzles, or incorrect valve clearance.

The unburned mixture from these dead cylinders passes into the exhaust system, where it ignites and burns.

Unfortunately, the resultant torching or afterburning can easily be mistaken for evidence of a rich carburetor.

Cylinders which are firing intermittently can cause a similar effect.

Again, the malfunction can be remedied only by discovering the real cause and correcting the defect.

Either dead or intermittent cylinders can be located by the cold cylinder check.

• Afterfiring vs Backfiring
• You may have heard people on the street talk about hearing a "car's exhaust backfire."
• While we all understand what they mean by this, this is also technically wrong.

What they are describing is AFTERFIRING through the exhaust.

"Backfiring" involves combustion coming "back" through the induction system.

Backfiring through the intake is normally associated with the mixture being too lean.

Any FAA answer choices that says something like "backfiring through the exhaust" is INCORRECT.

The FAA often uses this as a distractor.

• Backfiring
• When a fuel/air mixture does not contain enough fuel to consume all the oxygen, it is called a lean mixture.

Conversely, a charge that contains more fuel than required is called a rich mixture.

An extremely lean mixture either does not burn at all or burns so slowly that combustion is not complete at the end of the exhaust stroke.

The flame lingers in the cylinder and then ignites the contents in the intake manifold or the induction system when the intake valve opens.

This causes an explosion known as backfiring, which can damage the carburetor and other parts of the induction system.

• Incorrect ignition timing
• or faulty ignition wires, can cause the cylinder to fire at the wrong time, allowing the cylinder to fire when the intake valve is open, which can cause backfiring.

A point worth stressing is that backfiring rarely involves the whole engine.

Therefore, it is seldom the fault of the carburetor.

In practically all cases, backfiring is limited to one or two cylinders.

Usually, it is the result of faulty valve clearance setting, defective fuel injector nozzles, or other conditions that cause these cylinders to operate leaner than the engine as a whole.

There can be no permanent cure until these defects are discovered and corrected.

Because these backfiring cylinders fire intermittently and, therefore, run cool, they can be detected by the cold cylinder check.

In some instances, an engine backfires in the idle range but operates satisfactorily at medium and high power settings.

The most likely cause, in this case, is an excessively lean idle mixture.

Proper adjustment of the idle fuel/air mixture usually corrects this difficulty.

• Afterfiring
• Afterfiring, sometimes called afterburning (when this won't be confused with military-jet-type afterburning), often results when the fuel/air mixture is too rich.

Overly rich mixtures are also slow burning, therefore, charges of unburned fuel are present in the exhausted gases.

Air from outside the exhaust stacks mixes with this unburned fuel that ignites.

This causes an explosion in the exhaust system.

Afterfiring is perhaps more common where long exhaust ducting retains greater amounts of unburned charges.

As in the case of backfiring, the correction for afterfiring is the proper adjustment of the fuel/air mixture.

Afterfiring can also be caused by cylinders that are not firing because of faulty spark plugs, defective fuel-injection nozzles. or incorrect valve clearance.

The unburned mixture from these dead cylinders passes into the exhaust system, where it ignites and burns.

Unfortunately, the resultant torching or afterburning can easily be mistaken for evidence of a rich carburetor.

Cylinders that are firing intermittently can cause a similar effect.

Again, the malfunction can be remedied only by discovering the real cause and correcting the defect.

• Dead or intermittent cylinders can be located by the cold cylinder check