Turbofan

Most jet aircraft use turbofan jet engines, which employ a gas turbine to drive a ducted fan, which accelerates air around the turbine to provide thrust in addition to that which is accelerated through the turbine. The ratio of air passing around the turbine to that passing through is called the by-pass ratio.

They represent a compromise between turbojet (with no bypass) and turboprop forms of aircraft propulsion (primarily powered with bypass air).

Subsonic aircraft, such as airliners, employ high by-pass jet engines for fuel efficiency. Supersonic aircraft, such as jet fighters, use low-bypass turbofans. However at supersonic speeds, the air entering the engine must be decelerated to a subsonic speed and then re-accelerated back to supersonic speeds after combustion. An afterburner may be used on combat aircraft to increase power for short periods of time by injecting fuel directly into the hot exhaust gases. Many jet aircraft also use thrust reversers to slow down after landing.