5-4 Turbojet and Turbofan Engines

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  1. 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.
  2. 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.
  3. 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.
  4. 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
  5. DEFINE thrust specific fuel consumption
    the amount of fuel required to produce one pound of thrust.
  6. COMPARE the thrust specific fuel consumption of turbojet engines
    • Relatively high TSFC at low altitude and low airspeed.
    • More air requires more fuel.
  7. COMPARE the thrust specific fuel consumption of turbofan engines
    • Lower than turbojet.
    • Higher bypass ratio yields lower TSFC.
  8. 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.
  9. 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.
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320830
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5-4 Turbojet and Turbofan Engines
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