The turbofan or fanjet is a type of airbreathing jet engine that is widely used for aircraft propulsion. The word "turbofan" is a portmanteau of "turbine" and "fan", the turbo portion refers to a gas turbine engine which takes mechanical energy from combustion,[1] and the fan, a ducted fan which uses the mechanical energy from the gas turbine to accelerate air rearwards. The ratio of the mass-flow of air bypassing the engine core compared to the mass-flow of air passing through the core is referred to as the bypass ratio. The engine produces thrust through a combination of these two portions working in concert; engines that use more jet thrust relative to fan thrust are known as low bypass turbofans, while those that have considerably more fan thrust than jet are known as high bypass. Most commercial aviation jet engines in use today are of the high-bypass type, and most modern military fighter engines are low-bypass.[citation needed]

Since most of the air flow through a high-bypass turbofan is low-velocity bypass flow, even when combined with the much higher velocity engine exhaust, the net average exhaust velocity is considerably lower than in a pure turbojet. Engine noise is largely a function of exhaust velocity, therefore turbofan engines are significantly quieter than a pure-jet of the same thrust. Other factors include turbine blade and exhaust outlet geometries, such as noise-reducing "chevrons" seen on the Rolls-Royce Trent 1000 and General Electric GEnx engines used on the Boeing 787.

Since the efficiency of propulsion is a function of the relative airspeed of the exhaust to the surrounding air, propellers are most efficient for low speed, pure jets for high speeds, and ducted fans in the middle. Turbofans are thus the most efficient engines in the range of speeds from about 500 to 1000 km/h (310 to 620 mph), the speed at which most commercial aircraft operate.[2][3] Turbofans retain an efficiency edge over pure jets at low supersonic speeds up to roughly Mach 1.6, but have also been found to be efficient when used with continuous afterburner at Mach 3 and above.

The vast majority of turbofans follow the same basic design, with a large fan at the front of the engine and a relatively small jet engine behind it. There have been a number of variations on this design, however, including rear-mounted fans which can easily be added to an existing pure-jet design, or designs that combine a low-pressure turbine and a fan stage in a single rear-mounted unit.

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