How do jet engines work?

[996c2]

I understand that air is force in from the front, burnt in the middle and expel out the back but what prevent the burnt gas going out the front as well as the back.

Surely hot gas expand in all direction so how do they make it go all out the back?

[magpie215]

http://science.howstuffworks.com/transport/flight/...

[anonymous-user]

Fundamentally it's the understanding that the "energy" in a stream of gas, is both determined by its pressure AND its temperature. Hence, although the air entering the combustion chambers is at a higher total pressure (dynamic + static) than the air leaving the combustion chambers, the burning fuel has heated the air so that the final energy of the exhaust stream is much higher than that of the intake stream.


Look up "dynamic" and "static" pressure to understand more.

[MattYorke]

Conceptually, they work the same as piston engines. ie Suck, Squeeze, Bang, Blow. Except in this case, the suck & squeeze are done by the compressor, the bang occurs in the combustion chamber, and the "blow" occurs - well, out the back. There is also a turbine that draws some power off the "blow" to power the compressor.

[ColinM50]

This question has always fascinated me too and what's worse is I work in the aircraft industry so should really know but I can't get my head round how jet engines work. Have asked several Licensed Engine mechanics over the years and they all tell me the same. Magic or smoke and mirrors. Both seem the same to me.

[996c2]

Max_Torque said:

Fundamentally it's the understanding that the "energy" in a stream of gas, is both determined by its pressure AND its temperature. Hence, although the air entering the combustion chambers is at a higher total pressure (dynamic + static) than the air leaving the combustion chambers, the burning fuel has heated the air so that the final energy of the exhaust stream is much higher than that of the intake stream.


Look up "dynamic" and "static" pressure to understand more.

Thanks Max_Toque.

I should have realised that the combustion chamber in a jet engine is not a closed system where the pressure would have to rise with increasing temperature. From the equation pV=NkT, in a close system where V is constant, p will have to rise with T.

The big, low pressure, hole at the back of the jet engine means that the temperature in the combustion chamber can rise significantly without increasing the pressure at the same time. That is, in an open system, the Volume of the gas can rise with its Temperature without affecting the pressure.

If the pressure gradient from the front of the combustion chamber to the back of the jet engine is maintained then gas will flow from the chamber to the back.

Is this right?

[anonymous-user]

correct, you are turning a high static pressure into a high dynamic one, and the thrust is produced by the opposing reactive force. The trick with jet engines is then to expand the exhaust stream to as close to ambient conditions as linearly as possible, remembering that ambient at high altitudes is well below the std atmospheric pressure and temperature.


On something REALLY trick like an SR-71, just the intake system (used to deccelerate the incoming airstream from Mach 3.2) can produce a possitive pressure ratio in the forward direction, and actually provide "thrust"..... (hence why an SR-71 engine gets more efficient as the KEAS increases ;-)

[chuntington101]

Max_Torque said:

correct, you are turning a high static pressure into a high dynamic one, and the thrust is produced by the opposing reactive force. The trick with jet engines is then to expand the exhaust stream to as close to ambient conditions as linearly as possible, remembering that ambient at high altitudes is well below the std atmospheric pressure and temperature.


On something REALLY trick like an SR-71, just the intake system (used to deccelerate the incoming airstream from Mach 3.2) can produce a possitive pressure ratio in the forward direction, and actually provide "thrust"..... (hence why an SR-71 engine gets more efficient as the KEAS increases ;-)

Didn;t the SR-71nhave massive trouble with supersonic ways bouning off the first comp stage and basically destroying the engines/airframe? I think thye had to actually slow the air down to lower supersonics speeds to provent the problem (hence the pointy cones stuc in the intakes to the engines).

Chris.

[anonymous-user]

chuntington101 said:

Didn;t the SR-71nhave massive trouble with supersonic ways bouning off the first comp stage and basically destroying the engines/airframe? I think thye had to actually slow the air down to lower supersonics speeds to provent the problem (hence the pointy cones stuc in the intakes to the engines).

Chris.

Any compressor system that does not utilise "internal" compression but instead uses the dynamic pressure (i.e. velocity) must have a positive velocity gradient across it in order to function. Hence, because the tips of the compressor blades cannot be critical (> mach1), the intake airstream velocity must be subsonic. Effectively, the compressors "gulp" ability (that of the subsonic pressure wave to pull air towards the blades, and hence then be accelerated into the device) must be maintained, other wise the mass flow will "stall" (post compressor static pressure > pre compressor static pressure = reversed airflow direction).

So, on any supersonic aircraft, the intake air must be decelerated to subsonic before it enters the first stage compressor. On most planes with say a maximum mach of approx 2, this is faily easy, and a std passive intake system is sufficent to do this. On the SR-71, because of the extremely high speed potential (3.2+) and the requirement for the engine efficiency to be maintained at high speed (SR-71 is the only plane to be able to operate continously in an afterburning power setting!) a dynamic system that maintains the critical shock in the correct position in the intake tract was designed (in about 1960 without an CFD etc!!!!). It uses a movable cone, that produces the primary shock cone, and as the mach cone changes in included angle with velocity, this move bacwards as velocity increases to keep this primary shock impingeing in the intake nozzle (convergent-divergent nozzle). Multiple normal shocks are then captured and the position of final "critcal" shock is controlled by "relief" doors (which modify the nozzle pressure ratio to maintain the critical shock position)

When operating correctly, the cone itself provides a significant degree of thrust, because the total pressure behind it (and infront of the compressor) is higher than the total pressure immediately behind the primary shock. The J-58 engine uses a bypass system (after the 6th compressor stage) which at high speed allows a partial "ramjet" effect, with bypass air diverted into the exhaust nozzle infront of the afterburner. With the use of adjustable exit nozzle "feathers" the exhaust stream pressure recovery is maximised, allowing a very high thrust ratio at high speeds


The biggest issue was accurately controlling the cones and relief doors, to maintain a carefully optimised pre-compressor pressure (especially at high Alpha's). Inappropriately set, the aircrafts range was massively reduced as the bleed air exiting the forward relief doors caused a huge drag when meeting the supersonic airstream.

Interestingly, airframes maximum KEAS was actually limited by the maximum compressor intake temperature the engine could witstand (compressing air increases it's temp, and the intake system trades dynamic pressure for static pressure) so even when flying in ambient air at -50degC, the precompressor temps had to be below 427degC.



Interesting thread here about the effects on Concorde:

http://www.pprune.org/tech-log/426900-concorde-eng...

Edited by anonymous-user on Wednesday 2nd March 20:26

[thong]

I've built small turbines and run them, also flown them(wren turbine) in boomerang jets,
Don't forget what comes out the tail cone is waste it dose not push the aircraft forward, that's
Internal