EPR is the ratio of which two pressures?

• A. Compressor Inlet Pressure To Engine Jet Pipe Pressure
• B. Engine Jet Pipe Pressure To Compressor Inlet Pressure
• C. Jet Pipe Pressure To Ambient Pressure
• D. Ambient Pressure To Compressor Inlet Pressure

Answer: (B)

EPR is a pressure ratio that indicates thrust by comparing the pressure in the engine’s exhaust path to the pressure at the compressor inlet. Specifically, it uses the engine jet pipe (exhaust) pressure divided by the compressor inlet pressure. The jet pipe pressure is the pressure found in the exhaust duct after the turbine and before the nozzle, while the compressor inlet pressure is the air pressure as it enters the compressor. The higher this ratio, the more thrust the engine is producing for a given operating condition, because it reflects how effectively the engine converts intake air pressure into high-pressure exhaust.

This definition matches the idea of comparing the jet pipe pressure to the compressor inlet pressure, not to ambient pressure or in the reverse order. An option that uses the jet pipe pressure relative to ambient would describe something else, and taking the inverse or using ambient pressure with the inlet would not represent EPR.