Definition
In a turbojet or turbofan engine, the ratio of the turbine discharge pressure to the compressor inlet pressure. EPR is displayed in the cockpit and is used as the primary indication of thrust being produced by the engine.
Plain English
A number that compares the air pressure leaving the back of the engine to the air pressure entering the front. The bigger the difference, the more thrust the engine is producing. Pilots read this number on a cockpit gauge to know how hard the engine is working.
Context Anchor
Seen on turbine-engine cockpit instruments, performance charts, and power-setting procedures for some jet aircraft.
Derivation
The word 'ratio' comes from the Latin 'ratio,' meaning a calculation or proportion — one number compared to another. Here, it's the proportion between two pressures inside the engine, which directly reflects how much work the engine is doing on the air passing through it.
Why Pilots Care
It directly indicates available thrust and helps detect engine problems such as compressor stalls or performance loss.
Grounding Statement
If the pressure leaving the engine is 15 units and the pressure entering it is 10 units, the Engine Pressure Ratio is 1.5.
Intuition Check
Engine Pressure Ratio is not the same as oil pressure or fuel pressure. It is a comparison between two air-pressure readings inside a turbine engine.
Example Sentence 1
The captain advanced the throttles until EPR reached the takeoff setting on both engines.
Example Sentence 2
A sudden drop in engine pressure ratio during climb prompted the crew to reduce power and investigate.