![]() The other penalty is that combustion is less efficient at lower speeds. This reduces the speed of the propelling jet while pushing more air, and thus more mass. A turbofan harvests that wasted velocity and uses it to power a ducted fan that blows air in bypass channels around the rest of the turbine. This wake contains kinetic energy that reflects the fuel used to produce it, rather than the fuel used to move the aircraft forwards. While the turbojet uses the gas from its thermodynamic cycle as its propelling jet, for aircraft speeds below 500 mph there are two penalties to this design which are addressed by the turbofan.įirstly, energy is wasted as the propelling jet is going much faster rearwards than the aircraft is going forwards, leaving a very fast wake. įrank Whittle envisioned flight speeds of 500 mph in his March 1936 UK patent 471,368 "Improvements relating to the propulsion of aircraft", in which he describes the principles behind the turbofan, although not called as such at that time. This is done mechanically by adding a ducted fan rather than using viscous forces by adding an ejector, as first envisaged by Whittle. It achieves this by pushing more air, thus increasing the mass and lowering the speed of the propelling jet compared to that of the turbojet. The turbofan was invented to improve the fuel consumption of the turbojet. The low-pressure spool is colored blue and the high-pressure one orange. Principles Schematic diagram illustrating a modern 2-spool turbofan engine installation in a nacelle. An early configuration combined a low-pressure turbine and fan in a single rear-mounted unit. Modern turbofans have either a large single-stage fan or a smaller fan with several stages. Afterburners are used on low-bypass turbofan engines with bypass and core mixing before the afterburner. Most commercial aviation jet engines in use today are of the high-bypass type, and most modern military fighter engines are low-bypass. The engine produces thrust through a combination of these two portions working together engines that use more jet thrust relative to fan thrust are known as low-bypass turbofans, conversely those that have considerably more fan thrust than jet thrust are known as high-bypass. The ratio of the mass-flow of air bypassing the engine core to the mass-flow of air passing through the core is referred to as the bypass ratio. A turbofan thus can be thought of as a turbojet being used to drive a ducted fan, with both of these contributing to the thrust. Thus, whereas all the air taken in by a turbojet passes through the combustion chamber and turbines, in a turbofan some of that air bypasses these components. The word "turbofan" is a portmanteau of "turbine" and "fan": the turbo portion refers to a gas turbine engine which achieves mechanical energy from combustion, and the fan, a ducted fan that uses the mechanical energy from the gas turbine to force air rearwards. The turbofan or fanjet is a type of airbreathing jet engine that is widely used in aircraft propulsion.
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