A jet engine is different from a conventional piston engine, as thrust is the direct force output from the engines and is roughly proportional to the fuel flow. On modern commercial jets, the primary source of information regarding the engine’s power is the N1 and the EPR.
The N1, displayed to the pilots as a percentage, is nothing else than the compressor’s first stage (=fan) rotation speed. The EPR is the ratio between the low-pressure turbine exhaust pressure (the engine’s last stage) and the engine inlet pressure (the fan’s pressure).
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