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’s easier to see in a piston engine. You get power by burning the fuel air mixture when the piston is up. The high temperature increases the pressure and pushes the piston down. Now imagine if you decrease how far the piston can go down — you get less energy out of the hot gasses. If the piston can’t move at all, then you get no energy from the gasses. This is the the “no pressurization” condition. The piston doesn’t move, so you get no compression and no power.
Jet turbine engines operate on the same basic physics. And just like the piston engine, if you don’t compress the air before heating it in the burner you get zero power out. It can be explained graphically using a PV (pressure volume) diagram:
[https://en.wikipedia.org/wiki/Carnot_cycle](https://en.wikipedia.org/wiki/Carnot_cycle)
Note that “getting more air in” is NOT the answer. Tiny model jet engines, and huge gas turbines run with vastly different amounts of air coming in. But non of them will run without compression before burning the fuel. *
*Note: at very high speeds (like supersonic), special types of jet engines can run using just the compression from the “ram” air. They don’t need a mechanical compressor section, but the air is still compressed.
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