This has really got me confused lately. I’ve read online that it increases efficiency, the potential energy of the air and helps with combustion, but my question is:
If the same amount of air is passing through the engine regardless, why pressurise it? Wouldn’t the gain in PE of the air just be equal to the loss of KE of the aircraft due to the air hitting/dragging the aircraft back as it’s compressed? I’m almost certain I’m wrong here, but any explanation which could clear this up for me would be great thanks.
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It depends on the type of jet engine, not all jet engines need to compress the air to function. A pulsejet engine comes to mind, like what was seen on the V-1 flying bomb produced by Germany, the closely related American Republic-Ford JB-2, and the American Helicopter XH-26 Jet Jeep. The way that a pulsejet operates is simply through a combustion chamber and a one way valve on the intake. Fuel is injected before the combustion chamber. The valves open allowing the fuel/air mixture to flow into the combustion chamber. A spark plug ignites this fuel/air mixture and the expanding gasses are pushed out of the back of the chamber down to a nozzle. As the gasses escape down this nozzle, the shape of the chamber combined with the expelled exhaust creates a negative pressure that draws more air and fuel into the chamber and the process repeats.
Anyways, the reason to pressurize the air in a turbojet engine is to achieve higher combustion efficiencies through higher temperature combustion without detonation. If you look at the formula for an ideal Carnot Engine, it makes this relationship quite clear Efficiency = (T^(hot) – T^(cold))/T^(hot) (yes, a jet engine is not an ideal carnot engine). Now as for the energy, compressing the air does produce some minor losses as the air is heated up when it is compressed and that heat is transferred to the engine, but the higher combustion efficiency more than makes up for it. With regards to slowing the aircraft down, it has little effects as the energy used for compression is regained during the expansion of the gasses on the exhaust side.
There are also ramjet engines and scramjet engines that utilize the forward momentum of the airplane to compress the air and have no turbine blades, however they are incapable of a standing start or getting up to speed without an alternative propulsion method. An example of this is the SR-71 that had a hybrid turbojet engine and a ramjet engine. At low speeds the engine would operate entirely as a turbojet engine, as it went faster it would operate in a hybrid cycle with some of the air being diverted around the turbojet engine.
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