Bypass Ratio (BPR) is the ratio of air flowing through the bypass fan and around the core of a jet engine. High-bypass engines have a larger first stage fan that reduces fuel consumption, noise, and emissions. Modern engines have ratios up to 10 times higher than earlier types.
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 which passes through the core. In early jet engines, most of the air entering the engine intake 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 drive technology and the constant pressure to produce quieter, cleaner, and more fuel-efficient aircraft plants have led to the development of engines with much higher bypass ratios. The latest generation of jet engines as of 2011 have a return ratio of up to eight to one, making them quiet, clean, and much more efficient.
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 a combustion chamber. Both ends of the tube are open to the outside atmosphere, with the forward or forward end serving as the intake and the rear opening serving 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 is mixed with atomized fuel and ignited. The rapidly expanding gas then passes through 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 engine’s total thrust developed by 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 that surrounds the core. When these engines are running, 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 the 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 combustion of the fuel with commensurate emission reductions.
As of 2011, modern high-bypass-ratio engines boast ratios up to 10 times higher than earlier types. A Pratt & Whitney JT 8D on an old Boeing 737–200 had a bypass ratio of 0.96 to one. A Rolls Royce Trent 900 on 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 engine 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’s engines, which featured a zero-to-one bypass ratio with all intake air going straight down the red rail.
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