Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
Aviation and the Global Atmosphere
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7.4.1.2. Propulsive and Overall Efficiency
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The power produced by engine thrust is the product of thrust and flight velocity (V). The ratio of this useful power to the increment in kinetic energy given to the flow in
passing through the engine is the propulsive efficiency (h p ); a good approximation of this variable may written in the form h p = 2V/(V + V j ), where V j is the jet velocity.
High levels of thermal efficiency require the T 4 /T 2 ratio to be as high as possible with appropriate pressure ratio and component efficiencies. In the case of a simple
turbojet gas turbine engine, this requirement would mean that the jet velocities are also relatively high. For example, for a ratio of T 4 /T 2 = 5.6 together with an engine
pressure ratio of 40, the jet velocity V j would be about 817 ms -1 , (Cumpsty, 1997). At Mach 0.85 at 10.7 km, when the flight velocity V is 252 ms -1 , the relationship
shown above gives a propulsive efficiency of only about 47%. Thermal efficiency at this condition is about 48%, so h o = h p x h therm = 0.48 x 0.47 = 23% for the
assumptions of this simplified example. For typical aircraft, overall efficiency ranges between 20 and 40%.
The most practical method of raising overall efficiency is to lower the jet velocity and thereby increase propulsive efficiency; this approach has been adopted in the
bypass engine used so widely today. In this engine design, hot gases leaving the core turbine expand through further turbine stages, which drive the fan mounted in
front of the core compressor. In most modern engines, the pressure ratio across the fan at cruise condition is about 1.6, giving a bypass ratio of about 6 and a jet
velocity of about 400 ms -1 . At this jet velocity, propulsive efficiency is about 77% at a cruise speed of Mach 0.85 at 10.7 km. Unfortunately, losses associated with the
inefficiency of the fan and the turbine driving it inevitably reduce these benefits somewhat, so a typical value for the overall efficiency of such an engine is currently
about 30 to 37% at cruise. Increasing the bypass ratio clearly offers the prospect of further increases in propulsive efficiency, but this approach has to be weighed
against the penalties of increased size and weight of the installed engine and associated changes in drag (see Section 7.2 for further discussion of airframe efficiency
and installation effects). Other prospects for increasing propulsive efficiency (e.g., propellers and unducted fans) are discussed in Section 7.4.3.
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