However, while that does increase thrust somewhat, the exhaust jet leaves the engine with even higher velocity, which at subsonic flight speeds, takes most of the extra energy with it, wasting fuel. To raise the efficiency of a turbojet, the obvious approach would be to increase the burner temperature, to give better Carnot efficiency and fit larger compressors and nozzles. Turbofans were invented to circumvent the undesirable characteristic of turbojets being inefficient for subsonic flight. The low-pressure spool is colored blue and the high-pressure one orange. 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 not used on high-bypass turbofan engines but may be used on either low-bypass turbofan or turbojet engines. 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.