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Consequently, net thrust increases, whilst specific fuel consumption (fuel flow/net thrust) decreases. A similar trend occurs with unmixed turbofans. Turbofan engines can be made more fuel efficient by raising overall pressure ratio and turbine rotor inlet temperature in unison.
All practical airbreathing jet engines heat the air by burning fuel. [1] Alternatively a heat exchanger may be used, as in a nuclear-powered jet engine. [6] Most modern jet engines are turbofans, which are more fuel efficient than turbojets because the thrust supplied by the gas turbine is augmented by bypass air passing through a ducted fan. [4]
Turbofans are usually more efficient than turbojets at subsonic speeds, but at high speeds their large frontal area generates more drag. [21] Therefore, in supersonic flight, and in military and other aircraft where other considerations have a higher priority than fuel efficiency, fans tend to be smaller or absent.
It aimed to cut fuel consumption by 6–7%, emissions by 15%, and generate less noise due to lower fan tip speed of 950 ft/s (290 m/s), down from 1,400 ft/s (430 m/s) in conventional 5:1 bypass turbofans. While the gearbox and larger fan weighed more, this was mitigated by using 40% composites by weight, up from 15%.
The fuel consumption per mile or per kilometre is a more appropriate comparison for aircraft that travel at very different speeds. [citation needed] There also exists power-specific fuel consumption, which equals the thrust-specific fuel consumption divided by speed. It can have units of pounds per hour per horsepower.
The type of jet engine used to explain the conversion of fuel into thrust is the ramjet.It is simpler than the turbojet which is, in turn, simpler than the turbofan.It is valid to use the ramjet example because the ramjet, turbojet and turbofan core all use the same principle to produce thrust which is to accelerate the air passing through them.
This operation is a more efficient way to provide thrust than simply using the jet nozzle alone, and turbofans are more efficient than propellers in the transsonic range of aircraft speeds and can operate in the supersonic realm. A turbofan typically has extra turbine stages to turn the fan.
A corollary of this is that, particularly in air breathing engines, it is more energy efficient to accelerate a large amount of air by a small amount, than it is to accelerate a small amount of air by a large amount, even though the thrust is the same. This is why turbofan engines are more efficient than simple jet engines at subsonic speeds.