Engine pressure ratio (EPR) is defined as what?

• A. The ratio of turbine discharge pressure to compressor inlet pressure
• B. The ratio of compressor discharge pressure to turbine inlet pressure
• C. The ratio of ambient pressure to compressor inlet pressure
• D. The ratio of exhaust pressure to ambient pressure

Answer: (A)

Engine pressure ratio is the ratio of the pressure at the turbine discharge to the pressure at the compressor inlet. This tells you how much pressure the engine maintains in the hot gas path compared with the air entering the compressor, and it correlates with the thrust the engine can produce for a given inlet condition. A higher EPR indicates greater thrust potential, which is why pilots monitor it during takeoff and climb. Other pressure relationships described in the choices don’t reflect the core comparison used to gauge thrust: they mix up which pressures are being compared or use ambient or exhaust pressures that aren’t the defining ratio for EPR.