What is engine pressure ratio (EPR) and how is it used?

• A. The ratio of turbine discharge pressure to inlet pressure.
• B. The ratio of compressor discharge pressure to inlet pressure; used as an indicator of thrust/power on some engines.
• C. The ratio of exhaust pressure to ambient pressure.
• D. The ratio of inlet stagnation pressure to ambient pressure.

Answer: (B)

Engine pressure ratio is the comparison between the pressure after the compressor and the pressure at the engine inlet. By looking at how much the compressor raises the air pressure, it reflects how much the engine is breathing and, in turn, how much thrust it can produce. Because higher pressure rise generally means more air energy available for combustion and jet acceleration, EPR serves as a practical indicator of thrust or power for many engines, especially certain older or fixed-geometry designs. It’s calculated from signals at the compressor discharge and the inlet, so pilots can monitor engine performance quickly and set targets (like a takeoff EPR) to ensure the desired thrust. Keep in mind that EPR can be affected by ambient conditions and nozzle settings, so it isn’t universal across all engines or flight conditions. The other pressure ratios mentioned involve different parts of the flow (turbine discharge, exhaust, or inlet stagnation) and don’t describe the compressor discharge to inlet ratio used for EPR.