Originally Posted by Mr.Vortex
I'm heard some old engine like JT8D use the EGT as the factor to determine the maximum thrust output
All turbine engines have limits on spool speeds, gas temperature, and possibly compressor delivery pressure to protect engine integrity. Gas temperature depends on where it measured: EGT aft of the last turbine stage, and TGT is measured between turbine stages. EGT/TGT and rpm limits together serve to limit turbine entry temperature TET which is too hot to be measured directly.
EDIT: The maximum thrust output is defined in the rating structure selected (somewhat arbitrarily) by the engine manufacturer, and must comply with the limits described above.
-The higher we go, the more ram air pressure [for a typical flight], and hence
the compressor can produce more pressure to turn the turbine
Ram air pressure ratio is a function of Mach, which may vary with altitude.
Or the Fuel Control Unit always work to keep the Compressor ratio constant?
As the name implies, the FCU controls the fuel flow in response to throttle setting and other parameters.
AFAIK it doesn't control compressor (pressure) ratio directly.
regards,
HN39