5-4 Turbojet and Turbofan Engines

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DESCRIBE a turbojet engine
- Simplest form of gas turbine engine.
- 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.

Author

dundane

320830

Card Set

5-4 Turbojet and Turbofan Engines

Description

Enabling Objectives

Updated

2016-06-08T18:39:18Z

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