Turbofan Aircraft Engine

A turbofan engine is much the same as a turbojet, but with an enlarged fan at the front which provides thrust in much the same way as a propeller. A turbofan has extra turbine stages to turn the fan. Thus, more power is extracted from the exhaust gases before they leave the engine. This operation is a more efficient way to provide thrust than the jet nozzle alone, resulting in improved fuel-efficiency. Turbofans were the first engines to use multiple spools; concentric shafts which are free to rotate at their own speed; in order to allow the engine to react more quickly to changing power requirements. Although the fan creates thrust like a propeller, the surrounding duct frees it from many of the restrictions which limit propeller performance. Turbofans are more efficient than propellers in the trans-sonic range of aircraft speeds, and can operate in the supersonic realm. Turbofans are coarsely split into low-bypass and high-bypass categories. Bypass air flows through the fan, but around the jet core, not mixing with fuel and burning. The ratio of this air to the amount of air flowing through the engine core is the bypass ratio. Low-bypass engines are preferred for military applications such as fighters due to high thrust-to-weight ratio, while high-bypass engines are preferred for civil use for good fuel efficiency and low noise. High-bypass turbofans are usually most efficient when the aircraft is traveling at 500 to 550 miles per hour (800 to 885 km/h), the cruise speed of most large airliners. Low-bypass turbofans can reach supersonic speeds, though normally only when fitted with afterburners.

Source: Wikipedia