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Bypass ratio (BPR) is a term used to express the ratio between the quantity of air flowing through the bypass fan and around the core of a modern jet engine and that which passes through the core. In early jet engines, the majority of the air passing into the inlet of the engine was used in the combustion process and passed through the core of the engine to exit at the engine exhaust. Although these early aircraft engines produced sufficient thrust, they burned a lot of fuel, produced excessive emissions, and were very noisy. Advancements in turbine propulsion technology and constant pressure to produce quieter, cleaner, and more fuel-efficient aviation power plants have led to the development of engines with far higher bypass ratios. The latest generation of jet engines as of 2011 return ratios as high as eight-to-one, making them quiet, clean, and far 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 shaft. These two sections are housed within a closed tube, and are made up 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 leading or front end serving as an inlet and the rear opening as an exhaust.
When the engine is running, air entering the inlet is compressed by the compressor stage and forced into the combustion chamber. There, the compressed air is mixed with atomized fuel and ignited. The rapidly expanding gas then passes over and rotates the turbine stage before exiting at the exhaust. This hot gas provides a percentage of the engine's thrust and, because the turbine and compressor are interconnected, sustains the entire cycle. In older jet engines, a high proportion of the air entering the engine was utilized in this process with the majority of the total engine thrust being developed by the exhaust gas.
Although this system worked well, it had several drawbacks, such as high fuel consumption, large quantities of emissions produced by the engines, and excess noise. Spiraling fuel costs and ever-increasing environmental awareness, along with pressure to decrease 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 run, the majority of the air that passes into the inlet bypasses the core completely.
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 muffling effect of the bypass air passing the exhaust. The bypass air also cools the engine, allowing for more complete combustion of the fuel with commensurate emission reductions.
As of 2011, modern high bypass ratio engines feature ratios up to 10 times higher than early 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 as much air flows around the engine than through the core. The only time that low bypass ratio engines are superior though is in supersonic flight applications. A good example are the engines on the Concorde, which featured a bypass ratio of zero to one with all of the intake air going straight down the red lane.