What Is a Bypass Relationship? Definition, Meaning and Concept

Bypass ratio (BPR) is a term used to express the ratio of the amount of air flowing through the bypass fan and around the core of a modern jet engine to that passing through the core. In early jet engines, most of the air that passed into the engine inlet was used in the combustion process and passed through the engine core to exit through the engine exhaust. Although these early aircraft engines produced enough thrust, they burned a lot of fuel, produced excessive emissions, and were very noisy. Advances in turbine propulsion technology and the constant push to produce quieter, cleaner and more fuel efficient aviation plants have led to the development of engines with much higher bypass ratios.

In very basic terms, the average turbine power plant, or jet engine as they are more commonly called, consists of two main sections or stages, interconnected by a central hub. These two sections are housed within a closed tube and consist of a set of compressor blades at the front of the engine and a set of turbine blades at the rear. The area between the two sections is used as the combustion chamber. Both ends of the tube are open to the outside atmosphere, with the front or forward end serving as the inlet and the rear opening as the exhaust.

When the engine is running, the air entering the intake is compressed by the compressor stage and forced into the combustion chamber. There, compressed air mixes with atomized fuel and ignites. The rapidly expanding gas then passes and rotates the turbine stage before exiting through the exhaust. This hot gas provides a percentage of the engine's thrust and, because the turbine and compressor are interconnected, it maintains the entire cycle. In older jet engines a large proportion of the air entering the engine was used in this process with most of the total engine thrust developed by the exhaust gases.

Although this system worked well, it had several drawbacks, such as high fuel consumption, large amounts of emissions produced by the engines, and excessive noise. Spiraling fuel costs and increasing environmental awareness, coupled with pressure to lower noise levels around airports, eventually led to the development of what is now known as the high-bypass engine. These engines still feature the same basic structure as the older varieties, but have a very large first stage fan enclosed in a nacelle surrounding the core. When these motors run, most of the air going into the intake bypasses the core.

This has a number of significant benefits. The first is fuel consumption with the large increase in bypass thrust reducing the amount of thrust required from the central core combustion process. The second is noise reduction caused by the lower exhaust pressure and the damping effect of the bypass air passing through the exhaust. Bypass air also cools the engine, allowing more complete fuel combustion with commensurate emission reductions.

As of 2011, modern high-bypass-ratio engines feature ratios up to 10 times higher than earlier types. A Pratt and Whitney JT 8D on an old Boeing 737–200 had a bypass ratio of 0.96 to one. A Rolls Royce Trent 900 in the new Airbus A380 or Boeing 777 has a ratio of 8.7 to one. This means that almost nine times more air flows around the motor than through the core. However, the only time low bypass ratio motors are superior is in supersonic flight applications. A good example is the Concorde engines, which featured a zero-to-one bypass ratio with all intake air going straight down the red lane.