Originally Posted by White Bear
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Like any internal combustion engine that does not have a throttled air intake, the power produced is controlled only by the amount of fuel introduced into the combustion chamber.
So at idle, a jet engine has much more air passing through it than can be used by the fuel introduced, so it is in effect running lean.
Not on most engines. The efficiencies of the compressor and turbine are optimized for high power, and thus off-design at idle. Thus the steady-state fuel/air ratio is probably higher than you'd expect. In fact - if you make a very slow accel on the throttle, stop every 5% or so and let it stabilize, note the EGT and then move on to the next point, you may well find that idle is NOT the coolest EGT. There may be a dip or "bucket" at 75% or so.
Increasing the amount of fuel will allow the engine to make use of more of that air and turn it into useful work, by increasing the rpm of the rotors, which brings in more air etc.
As the amount of fuel introduced brings the air/fuel ratio more towards adiabatic, the exhaust temp will rise, because combustion temp is rising.
Then the fuel supply is held constant, but the induction continues to increase (as the engine is still spooling up, and from increased ram intake pressure) once again leaning the mixture and tail pipe temps drop.
Agreed.
[EDIT: Ooops - I just reread this - On modern engines there's never a "fuel supply is held constant" condition; it's always being modulated to hold some RPM or other parameter to a constant value. Once the RPM stabilizes the FF may LOOK pretty constant, but it's constantly being trimmed because of changing externals (Temp & Pressure) and internals (component efficiencies). Think closed loop control.]
It would seem to me that rpm is controlled only by the amount of fuel introduced, because the amount of air available for combustion always exceeds the amount of fuel available.
If fuel were not restricted then would the engine continue to accelerate until it destroys itself?
Yes - and for this reason the governor is probably double- or triple- redundant.
Am I completely off base?
Regards,
W.B.
No, you have a good grasp of gas turbines, except for the (admittedly esoteric) idle case.
