Constructed by the addition of an inlet and an exhaust section to the basic gas generator.
Gas generator uses 75% of total heat energy for compressor and accessories.
25% used for thrust.
DESCRIBE the characteristics of a turbojet engine
Advantages
Lightest specific weight (weight per pound of thrust produced).
Higher and faster than any other engine.
Disadvantages
Low propulsive efficiency at low forward speeds
Relatively high TSFC at low altitude and low airspeeds
Long takeoff roll required
Best suited for high speed and/or high-altitude flights.
DESCRIBE a turbofan engine
Combines propulsive thrust of exhaust with additional thrust that is generated by a duct-enclosed fan.
Fan accelerates a fairly large mass of air around the gas generator (bypassed or ducted air).
Fan is larger than the compressor front area.
Bypassed air can provide 30-60% of thrust.
Fan is driven by a free or power turbine, which is aft of the gas generator turbines and not connected, using part of the 25% of air used for thrust to power the fan.
DESCRIBE the characteristics of a turbofan engine
Advantages
Higher thrust at low airspeeds
Lower TSFC
Shorter takeoff distance
Considerable noise reduction, 10-20% over the turbojet
Disadvantages
Higher specific weight
Larger frontal area
Inefficient at higher altitudes
DEFINE thrust specific fuel consumption
the amount of fuel required to produce one pound of thrust.
COMPARE the thrust specific fuel consumption of turbojet engines
Relatively high TSFC at low altitude and low airspeed.
More air requires more fuel.
COMPARE the thrust specific fuel consumption of turbofan engines
Lower than turbojet.
Higher bypass ratio yields lower TSFC.
COMPARE the propulsive efficiency of airplane engines
Propulsive efficiency is determined by the efficient conversion of kinetic energy to propulsive energy.
Turbojet
Low speed - more wasteful than turboprop.
High speed - if the speed equaled the exhaust velocity, 100% Efficiency is reached.
Turbofan
More efficient than turbojet by converting more of the fuel energy into pressure energy.
It takes a relatively small amount of energy to turn the turbine to drive the fans.
DESCRIBE the effect of bypass ratio on turbofan engine performance
Bypass ratio is the comparison of bypass air to the amount of air that passes through the gas generator.
The higher the bypass ratio, the more fuel-efficient.