The text bellow was posted before on this forum:
The basic thrust setting parameters for modern high bypass engines are Engine Pressure Ratio EPR used by Pratt & Whitney and Rolls Royce, and Fan speed N1, used by GE and CFM. The engine manufacturers have documented the reasons for their choice but in fact neither of the 2 parameters gives a proper indication of engine thrust. Direct thrust management is a continuing research topic.
Early turbojet engines used rpm as the operating parameter to establish thrust, while more modern turbofan low bypass engines like the JT8 use EPR. EPR is the ratio of the total pressure at the front of the compressor to the total pressure at the rear of the turbine, which may be thought as thr product of pressure and cross sectional area. On a hot day, compressor rpm for given thrust will be higher than on a cool day. Furthermore, a dirty or damaged compressor will reduce thrust for a given rpm. Using EPR as the thrust indicator means that on a hot day it is quite possible for the engine rpm to exceed 100%.
On newer high-bypass ratio engines, such as GE CF6, the fan speed N1 is used as the primary method of setting power because most of the total thrust is generated by the fan. Engine control is acomplished by keeping N1 constant for a selected throttle lever position, regardless of the ambient conditions, and (with Autothrust disconneced)let the pilot adapt to the non-linear relationship between rpm and thrust. The pilot is selecting not thrust but a proportion of the maximum thrust that can be developed by the engine at a particular altitude and Mach No.
The advantages of using N1 parameter for control are:
-rotational speed can be measured easily with good accuracy
-measurement is independent of aircraft altitude
The problem with monitoring thrust through rpm is that thrust changes greatly for the last few percent of engine speed before 100% rpm is reached. The system must be very sensitive to small changes in rotational speed.
Although EPR is also a reliable indicator of thrust the main disadvantage is that pressures cannot be measured as exactly as rotational speed. In a high bypass-ratio engine the EPR method would be more complex, because the thrust of both the fan and the core engine streams would have to be calculated separately. On the Rolls-Royce RB211 engine the parameter used to indicate and manage thrust is the (IEPR) integrated engine pressure ratio. This parameter is the integrated average of the fan and core exhaust pressures divided by the inlet total pressure. RR feels that because IEPR is based on both fan and core presssure ratios, provides the most accurate indication of engine thrust compared to other parameters.
On modern FADEC engines thrust control is enhanced. On the PW4000 series engine power is controled by both EPR and N1 rpm. The EPR is the rated or normal mode, while N1 is the alternate or fault mode.
Regards