Which statement correctly describes compressor inlet pressure, compressor discharge pressure, and their relation to Engine Pressure Ratio (EPR)?

• A. Inlet pressure is ambient/intake; discharge is after compression; their ratio defines EPR.
• B. Inlet pressure is after compression; discharge is ambient; EPR is the ratio of discharge to inlet pressures.
• C. Inlet pressure and discharge pressure are the same; EPR is the ratio of ambient to exhaust.
• D. Inlet pressure is ambient; discharge pressure is below ambient; EPR is the product of pressures.

Answer: (A)

EPR measures how much the compressor increases air pressure. The pressure at the compressor inlet is the ambient air pressure entering the engine. The pressure after the compressor is the discharge pressure, the higher pressure produced by compression. Engine Pressure Ratio is the ratio of that discharge (post-compression) pressure to the inlet (ambient) pressure. This is why the described relationship—ambient/inlet at the intake, discharge after compression, and EPR being the ratio of discharge to inlet pressures—is correct. The other statements mix up where the pressures come from or how EPR is formed (it’s a ratio, not a product, and it uses inlet/ambient pressure rather than exhaust or ambient in the wrong place).