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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The simplest way to explain it is that by compressing the air, adding fuel to it, and igniting the mixture makes it expand a great deal more when released. This provides more thrust from the engine than using uncompressed air. This does two things. Primarily, it provides the thrust that moves the plane forward. But it also passes through another set of turbines that then power the compressor stages of the engine. Thus making the system self sustaining. Once it gets started, the only way to stop it is to stop providing fuel or air. (or catastrophic destruction)
In regards to your question about the PE and KE causing drag: The compressor stage is the front third to half of the length of the engine. It’s a dozen or two alternating rows of turbines and such that pull air from the front of the engine and compress it a very great deal before it enters the combustion chamber. Jet engines have an enormous amount of suction and you’ll never want to be within 50-100 ft of the intake of one that’s running. So in essence you could say that the engine is partly using thrust to propel the plane forward while also using suction to aid the forward momentum.
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