Originally Posted by Mad (Flt) Scientist
EPR, being Engine Pressure ratio, is measured by measuring various air pressures inside (and outside) the engine and calculating their ratio.
To get the EPR, you only really need to measure intake pressure and exhaust pressure, EPR is their quotient. The various intermediate pressures are also sensed to monitor correct operation of the engine and possibly adjust parameters such as variable stator vanes and/or bleed valves.
Nasa has a nice
picture on engine pressures.
The general layout is similar to that in the GE engines, an Ad on which system was dissected here at some length a while ago. The RR system is analagous, but of course different in detail.
The major differences between the GE90 and the Trent 800 engines is that the GE is a 2-shaft design, and uses N1 (low pressure rotor rpm) as its primary control parameter, whereas the Trent is a 3-shaft design and uses EPR as its primary control parameter.
Bernd