Interesting question, but some of the answers have only confused me.
I have no experience with jet engines at all, so please, no abuse.
Might I ask the following question given this scenario?
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.
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.
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?
Am I completely off base?
Regards,
W.B.