Overall pressure ratio
From Wikipedia, the free encyclopedia
Overall Pressure Ratio is an engine cycle term used in gas turbine engineering and is defined as the ratio of the stagnation pressure at combustor entry, to that at compression entry. The term should not be confused with the more familiar term compression ratio (which is a volume ratio) used on reciprocating engines, such as a Diesel.
Modern civil turbofans have very high levels of overall pressure ratio; the next generation engines that will enter service with the new Boeing 787 will have overall pressure ratios of the order of 52:1 at the Top-of-Climb. Modern in-service engines exceed 40:1.
Military combat turbofans have significantly lower overall pressure ratios than civil turbofans, because they are required to fly at high flight speeds (e.g. Mach 1.1) at Sea Level, before invoking a mechanical limit on HP compressor delivery temperature, 
. A typical overall pressure ratios for a combat turbofan is 25:1, although some exceed 32:1. Trainer turbofans are usually far more modest in overall pressure ratio.
