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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All heat engines (so turbines, piston engines, rockets, etc.) are governed by a rule called Carnot’s Law, which basically states that the maximum efficiency of any heat engine cycle is determined by;
eta = 1 – (Tc/Th)
where Tc and Th are the temperatures of the cold and hot side of the reaction. The cold side is the ambient environment, and the hot side is essentially a stand-in for the pressure and temperature of the chamber where combustion is happening. Thus, raising the pressure of the combustion chamber *before* combustion actually starts will increase your overall efficiency, because it’ll push the combustion temperatures higher as well.
As for why Carnot’s Law holds; it’s a consequence of entropy and the 2nd Law of Thermodynamics. Heat always seeks to flow from high energy to low energy (i.e. from hot to cold). The “forcefulness” of this flow (for lack of a better word) is tied directly to the difference in energy between the hot and cold sides of the reaction, thus the bigger the difference is, the more energy you can extract as work.
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