Why does air have to be compressed in a jet engine?

Question

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.

In: 12

Answer 1

In a piston engine the work done is through expansion of the gasses in the cylinders. In a turbine engine the work is ALSO done through gas expansion, but its captured by turbine blades in the hot combustion part of the engine – those spin with the hot explody gas and drive the compressor.

Note that in a piston cylinder there is still compression going on – the piston itself does that during its cycle and just before the spark plug ignites it. This allows a small amount of fuel and a BIG amount of air to combine in just the right ratio to get the most bang for the buck.
So you still need some compression. If you didn’t compress your air at all you’d get a very weak little _pfumpf_ instead of a bang (or a continuous roar in a turbine).. unless you dumped a LOT more fuel in there – which would make the engine very fuel inefficient.

So if you throw some compression turbines on there – driven by the hot exhaust turbines, you can obtain the same air to fuel ratios that make the biggest bang for the buck.

Also there’s the problem that without an incoming cold air stream that is of equal or higher air pressure to the hot explody exhaust air, the explody exhaust would want to go back out the wrong way or hangout in the combustion chamber if the airflow dropped below a certain speed. So having a compressor on there keeps the airflow going the right way – doesn’t entirely solve the problem, you can still get an engine fire under some circumstances, but it minimizes the occurrence. In a pinch you can restart an engine through using bleed air or even a motor or something to spin the compressor up to speed and getting that airflow going again. If you didn’t have the compressor up there you’d have to dive and get the air flowing thru the engine.