zutonofgoth t1_irpbsnh wrote
Reply to comment by Aviator506 in TIL that passenger jet engines produce most of their thrust from fan at the front, not from the jet exhaust, and that this is called a high-bypass engine. by Rilot
It's important to note there is more accelerate than squeeze. It's almost an order of magnitude less than a car engine. The back end of the jet is open with only some blades blocking the exit of the heated gas.
Some people get the idea the pressure is massive but it's more like bike tyre pressure, so jet engine shrouds don't need to be very strong.
straighttoplaid t1_irpgao3 wrote
Modern engines can run pretty high compression ratios. The GE9X is advertised as having a 61:1 overall pressure ratio. I don't think you're going to see ~900 psi in a bike tire.
zutonofgoth t1_irpquv6 wrote
The compression ratio is the difference between the static air pressure at the front of the engine and the dynamic pressure at the rear of the engine. The dynamic pressure includes velocity in the calculation. It's not really comparing apples with apples. So you can not just multiply the values out.
900psi is more like rocket engines?
julie78787 t1_irqujss wrote
A lot of rocket engines have maximum pressures in the combustion chamber of 200-300 bar, so 3,000 to 4,500 PSI.
Aviator506 t1_irpfbmr wrote
Yeah, most of the squeeze is done by the forward momentum of the engine through the air. And at a certain speed (~mach 3) you don't even need compressors, it will act as a ram jet, which is what the SR-71 would do as well as the fictional DarkStar in TopGun Maverick. It's certainly MUCH more complicated than how I explained, I just did the basic principles of it.
straighttoplaid t1_irpggj7 wrote
> Yeah, most of the squeeze is done by the forward momentum of the engine through the air.
That is not true for commercial engines.
Aviator506 t1_irpx2ly wrote
High bypass turbo fans like airliners do get a significant bit of the compression from the forward momentum. They will never be able to function like ram jets, but the momentum makes a big difference especially at high altitude where the air is less dense and there isn't as much to suck in from the fans/compressors alone. The high bypass engines of the airliners will function much more efficiently at high altitude because of this than that of a low bypass military fighter, however it won't be able to achieve as high of a top speed as that same fighter under the same conditions. Basically the low bypass gets the advantage of speed while the high bypass gets the advantage of fuel efficiency, but both greatly benefit from momentum based compression.
KGandtheVividGirls t1_iruxjqu wrote
Think what you’re saying here is engine pressure ratio, or EPR. Ratio across the entire engine, not at a particular location, or station. The speed of the air doesn’t change that much after hitting the inlet and first stage of the compressor. A series of blades do accelerate the air then the air passes through a set of stators which are divergent ducts which slow the air converting that energy into pressure. This happens across multiple stages. At the end of the compressor is a diffuser which acts like an extra special stator. Air is at the highest pressure here. On a large modern engine it could be 800psig and over 1000F. This is simplified, there are things that need done with a compressor to get it working across the operating range.
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