A type of jet engine which uses two different airflow streams (one passing through the core and another blown past it by the fan) for obtaining thrust.

The turbofan is an extremely common type of , derived from the earlier, more primitive . Although the concept of a turbofan dates back almost as far as that of a turbojet, the first turbofan engine (the Rolls-Royce Conway, iteration RCo.2) did not run until July 1952, as jet engine development during World War II had concentrated on turbojets in order to get a jet into the air as soon as possible.

The core of a turbofan is very similar to a turbojet; however, a turbofan's core has one or more additional turbine stages after those driving the compressor. These additional turbine stages drive a large fan or fans, which blow additional air (known as bypass air) around the engine core, providing additional thrust. The fan(s) are usually located at the very front of the engine, ahead of the compressors, although a few turbofans (such as General Electric's CJ805-23 and CF700) instead used fans located at the back of the engine.

One important measure of a turbofan is its , the ratio between the amount of bypass air passing through the engine and the amount of air that enters the engine's core (for instance, a turbofan that blows 10 tonnes of air through its bypass duct in 1 second and draws only half a tonne through the engine core in that same time period would have a bypass ratio of 20:1). Turbofans are classified into two main categories, based on their bypass ratio:

  1. Low-bypass turbofans have a bypass ratio of less than 5:1, and usually generate most of their thrust from the core air. All early turbofans were low-bypass, and some smaller narrowbody (single-aisle) jetliners continued to use low-bypass turbofans (principally the popular Pratt & Whitney JT8D) well into the 1990s; they remain common on combat aircraft.
  2. High-bypass turbofans have a bypass ratio of 5:1 or greater, and usually generate most of their thrust from their bypass air. The earliest high-bypass turbofan (the General Electric TF39, bypass ratio 8:1) first ran in 1965, and entered service in 1968 aboard 's C-5 Galaxy military transport. Large high-bypass turbofans (being far more powerful and efficient than their low-bypass cousins) were almost immediately appropriated for use on the widebody (twin-aisle) jetliners then under development; the , which first flew in February 1969 and entered service the following January, was not only the first widebody airliner, but also the first airliner powered by a high-bypass turbofan (the Pratt & Whitney JT9D). All newer jetliners use high-bypass turbofans, as do most large military transports (midsize and smaller transports instead use engines).

As the bypass air of a turbofan is colder and slower-moving than the core air, it is more efficient at generating thrust at relatively low speeds than the core air (or the exhaust of a turbojet) is; this effect is stronger for high-bypass turbofans, due to their greater amount of bypass air. As such, high-bypass turbofans are the most efficient aircraft engines at speeds between approximately mach 0.65 (below this speed, turboprops and piston engines are more efficient) and mach 1, while low-bypass turbofans are the best choice for the mach 1 - mach 2 range (above which they are, in turn, surpassed by turbojets); civilian jetliners and large military transports, which generally cruise in the former speed range, use high-bypass turbofans, while combat aircraft (which place more emphasis on the ability to go if need be, although not as much as they used to) are powered by low-bypass turbofans. At speeds, a turbofan's efficiency can be increased essentially without limit by increasing its bypass ratio (as long as the engine core is powerful enough to drive such a large fan).

The large amount of (relatively) cold, slow-moving air shrouding the core exhaust also reduces the amount of noise generated by turbofans, with the result that even low-bypass turbofans are many times quieter than turbojets, and high-bypass turbofans are quieter still. A turbofan's noise level can be decreased even further by equipping it with a nozzle that mixes bypass air into the core exhaust stream; early turbofan jetliners commonly used highly-distinctive scalloped nozzles which forced the core and bypass air into intertwined streams. When the first high-bypass turbofans came about, they were seen (or, rather, heard) as enough quieter on their own to not need exhaust-mixing nozzles. However, since then, aircraft noise restrictions have gotten much stricter, and many jetliners now use various methods of exhaust mixing; some (such as the 's GE/SNECMA CFM56s) simply enclose the entire engine with a long shared cowling, allowing the turbulence at the boundary between the core and bypass flows to mix them to some extent, while others (for example, the 's GE GEnxes) give the core tailpipe a sawtoothed rear edge which creates turbulence at the boundary between the core and bypass flows and encourages mixing.

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