Intake Stroke - describe whts happening.
- Begins at TDC w/ exhaust valve CLOSED and the intake valve OPEN
- Downward motion of the piston creates a low pressure in the cylinder
- Atmospheric pressure outside of the cylinder forces air into the cylinder
- Piston travels from TDC to BDC
Compression Stroke - describe whts happening
- Begins at BDC w/ intake and exhaust valves BOTH closed
- Compresses the air/fuel mixture in the cylinder - causing increase in pressure and temperature Piston travels from BDC to TDC
Power Stroke - describe whts happening
- Near TDC on the compression stroke, spark plug fires, causing air/fuel mixture to start burning
- Ignition of the air/fuel mixture CAUSES the power stroke to begin at TDC w/ valves closed
- Piston travels from TDC to BDC
- Begins at BDC w/ intake valve CLOSED and exhaust valve OPEN
- Upward motion of the piston - forces the burned gases out of the cylinder.
- Piston travels from BDC to TDC
Every stroke occurs through ___ degree of crankshaft rotation, resulting in a total of ___ degrees or ___ complete revolutions, to compete ___ cycle. (Four Stroke Cycle)
180, 720, 2, 1
is a comparison between the volume of the cylinders and the amount of air that actually enters the engine during operation, expressed as a %
Volumetric efficiency deceases as engine speed increases.
What are the 2 Fuel Types ?
Spark Ignited fuel or Fuel Ignited (by the heat of compression)
Most common Spark Ignited Fuel is ______ ?
What does "LPG" stand for ?
Liquefied Propane Gas
What does "CNG" stand for ?
Compressed Natural Gas
What does "M85" stand for ?
What is the most common "Compression-Ignited Fuel" ?
Naturally Aspirated Engines ?
Engines use only atmospheric pressure to force air into the engine. A low pressure area is created by the pistons as they travel downward on to the intake strokes.
Intake Stoke (Diesel Engine)
- Diesel Engine does not hav a throttle plate to restrict the air flow, therefore draws in a full charge of air on every intake stroke
- no fuel is introduced into the intake air stream.
- piston moves down from TDC to BDC
- intake valve is OPEN Exhaust Valve CLOSED
Compression Stroke (Diesel Engine)
- piston moves from BDC to TDC
- both intake & exhaust valves are CLOSED
- Compression Ratio can range from 15:1 up to 27:1
- Diesel fuel auto ignites when it is injected into the hot compressed air, injection occurs just before the piston reaches TDC.
Power Stroke (Diesel Engine)
Ignition of the air/fuel mixture CAUSES the power stroke to begin at TDC w/ valves closed Piston travels from TDC to BDC
Exhaust Stroke (Diesel Engine)
Upward motion of the piston - forces the burned gases out of the cylinder. Piston travels from BDC to TDC
The angle of the V in the various V- Configured engines may be:
- 90 degrees
- 60 degrees
- 15 degress
Octane Rating is ?
an indication of the fuel's ability to resist self-ignition
Cylinder Leakage Test
- helps to pinpoint the exact location of compression loss.
- pressurizes the cylinders w/ a metered flow of air & measures how well the cylinder retains that air pressure.
- the cylinder being tested must be EXACTLY at TDC on the Compression Stroke.
What is considered acceptable in a Cylinder Leakage test result.
20% is considered acceptable.
Cylinder Leakage test thts considered excessive ? (%)
A compression test is performed to ?
determine if an excess amount of compression pressure is leaking out of the cylinder.
A vacuum gauge - describe unit, temp. when vehicle when tested & readings
- measures intake manifold vacuum.
- measurement of vacuum is in inches of mercury (Hg) instead of PSI
- engine should be at be at operating temperature for a good vacuum test.
Piston Slap - when is audible (also during driving conditions) & how does it sound ?
- Audible Piston Slap occurs when there is too much clearance between the piston skirts and the cylinder wall.
- knocking noise that be described as a hollow clatter.
- most audible when the engine is first started cold and diminishes as the engine warms up.
Piston Pin - when is it most audible, sound made by loose piston pin.
- audible at idle
- noise is often referred to as rattle or double knock .
Wet Compression Test
squirt a little bit of motor oil into each spark plug hole and crank the engine over to distribute the oil evenly.
must be done on a warm engine and helps to identify whether compression leakage in the dry test is past the valves or the piston rings.
if any compression readings increased significantly on the cylinders that were low on the dry test, leakage is likely past the piston rings.
If the compression readings did not increase or increased only slightly in the wet test, the leakage is likely past the valves.
Excess Valve Clearance (cause)
- Noise caused by excess valve clearance
- often called tappet noise
- tappet noise is at camshaft speed, which is half the crankshaft speed.