Schematic turbofan engines; the high-bypass engine (lower) has a large fan that routes much air around the turbine, the low-bypass engine (upper) has a smaller fan routing more air into the turbine.
The bypass air is shown in pink, whilst the core gases are shown in red.
The term bypass ratio (BPR) relates to the design of turbofan engines, commonly used in aviation. It is defined as the ratio between the mass flow rate of air drawn in by the fan but bypassing the engine core to the mass flow rate passing through the engine core.
A high bypass ratio gives a lower (actual) exhaust speed. This reduces the specific fuel consumption, but reduces the top speed and gives a heavier engine.
A lower bypass ratio gives a higher exhaust speed, which is needed to sustain higher, usually supersonic, airspeeds. This increases the specific fuel consumption.
In spite of this, it turns out that for jet engines in general, at optimum bypass ratios, the fuel burnt to travel any particular distance is largely independent of airspeed, but with supersonic jet engines being slightly more efficient in practice, at their design point.
Jet engines are generally able to create considerably more energy than they can use in moving air through the engine core. This is because the limiting factor is the temperature at the turbine face, and that is a function of the total amount of fuel burned. Increasing airflow, and thus thrust, would imply burning more fuel and generating higher temperatures. It is possible to increase the airflow by burning "too much" fuel or adding water in front of the turbine to cool it, but both methods lead to incomplete combustion and very poor fuel efficiency. This was nevertheless commonly practiced in early jet engines because of a need to produce added thrust on takeoff. This is also why the exhaust plumes of older aircraft produce so much visible smoke (which is nothing more than unburned carbon from wasted jet fuel).
Rolls–Royce came up with a better use of the extra energy in their Conway turbofan engine, developed in the early 1950s. In the Conway, an otherwise normal axial-flow turbojet was equipped with an oversized first compressor stage (the one closest to the front of the engine), and centered inside a tubular nacelle. While the inner portions of the compressor worked "as normal" and provided air into the core of the engine, the outer portion blew air around the engine to provide extra thrust. The Conway had a very small bypass ratio of only 0.3, but the improvement in fuel economy was notable; as a result, it and its derivatives like the Spey became some of the most popular jet engines in the world.
If the fan of a turbofan engine drives two kilograms of air around the engine for every kilogram that passes through the engine's core, the engine is said to have a bypass ratio of 2 (or 2 to 1). Higher bypass ratios generally give better specific fuel consumption as an increasing amount of thrust is being generated without burning more fuel. This is achieved since the engine propels a larger amount of air rearwards at slower speed, rather than a smaller amount of air at higher speed- because thrust is the momentum given to the air per second the thrust is the same. However energy is a square law on speed, and so it takes less energy to generate the same thrust; and hence less fuel is needed, the specific fuel consumption reduces.
Thus, with the example, for engines with the same thrust, the fuel efficiency would be improved by something less than 50%.
High bypass ratios are also correlated with lower noise, since the large flow of air surrounding the jet exhaust from the engine core helps to buffer the noise produced by the latter.
Through the 1960s the bypass ratios grew, making jetliners competitive in fuel terms with piston-powered planes for the first time. Most of the very-large engines in this class were pioneered in the United States by both Pratt & Whitney and General Electric, which for the first time was out-competing the United Kingdom in engine design. Rolls-Royce also started the development of the high-bypass turbofan, and although it caused considerable trouble at the time, the RB.211 would go on to become one of their most successful products.
Turbofans are typically broken into one of two categories: low–bypass and high–bypass ratio. In a low–bypass turbofan, only a small amount of air passes through the fan ducts and the fan is of very small diameter. The fan in a high–bypass turbofan is much larger to force a large volume of air through the ducts. The low–bypass turbofan is more compact, but the high-bypass turbofan can produce much greater thrust, is more fuel efficient, and is much quieter.
Today, almost all jet engines include some amount of bypass. Lower bypass ratios are appropriate at high speeds because the exhaust velocity must exceed the airspeed to give forward net thrust. For lower speed operations, such as airliners, modern engines use bypass ratios up to 17, while for higher speed operations such as fighter aircraft the ratios are much lower, around 1.5; and around 0.5 for sustained speeds around Mach 2 and somewhat above.
At transonic and supersonic speeds, very high bypass ratios still present engineering challenges.