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Turbofan

From Academic Kids

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Cfm56-3-turbofan.jpeg
CFM56-3 turbofan, lower half, side view.
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Jet_engine_intake.JPG
Boeing 747 jet engine up close

The turbofan is a type of airplane engine which is evolved from the turbojet jet engine essentially by increasing the size of the first-stage compressor to the point where it acts as a ducted multiple thin propeller (or fan) blowing air past the "core" of the engine. This type of engine runs best from about 250 to 650 mph (400 to 1,000 km/h), which is why the turbofan is by far the most used type of engine for aviation use. The turbofan is also referred to as a high-bypass-ratio turbojet.

If the propeller is better at low speeds, and the turbojet is better at high speeds, it might be imagined that at some speed range in the middle a mixture of the two is best. Such an engine is the turbofan (originally termed bypass turbojet by the inventors at Rolls Royce). Turbofans essentially increase the size of the first-stage compressor to the point where they act as a ducted propeller (or fan) blowing air past the "core" of the engine. The difference between a ducted fan and a propeller is that the duct slows the air before it arrives at the fan. As both propeller and fan blades must operate subsonically to be efficient, ducted fans allow efficient operation at higher vehicle speeds.

The bypass ratio (the ratio of bypassed air mass to combustor air mass) is an important parameter for turbofans. Early turbofans (and most modern jet fighter engines) are low-bypass turbofans. The Rolls-Royce Conway, the first turbofan, had a bypass ratio of 0.3, similar to the modern General Electric F404 fighter engine. In the late 1960s, high-bypass turbofans, which generally have bypass ratios of about 5, began to appear. These engines not only have much lower fuel consumption than low-bypass engines, but also could provide much higher thrust, which made wide-body aircraft much more practical, albeit at the expense of a much larger frontal area. Today, almost all jet airliners are powered by high-bypass turbofans.

Turbofans (especially high bypass engines) are relatively quiet compared to turbojets. The noise of a jet engine is strongly related to the velocity of the air coming out the exhaust. A turbofan has a larger mass flow of air for a given thrust than a turbojet, so the exhaust velocity will be slower and hence the turbofan engine will be quieter than an equivalent turbojet. Jet aircraft are often considered loud, but a conventional piston engine or a turboprop engine delivering the same power would be much louder. (NASA has a web page (http://www-psao.grc.nasa.gov/Reengine/noise_primer.html) with details on jet noise.)

Turbofan engine manufacturers

The turbofan engine market is been dominated by General Electric, Rolls-Royce plc and Pratt & Whitney, in order of market share.

General Electric

GE Aircraft Engines, part of the General Electric Conglomerate, currently has the largest share of the turbofan engine market. Through joint ventures CFM International and Engine Alliance, they have created the very successful CFM56 series and the new GP7200.

Rolls-Royce

Rolls-Royce plc is the second largest manufacturer of turbofans and is most noted for their RB11 and Trent series.

Pratt & Whitney

Although Pratt & Whitney is behind GE and Roll-Royce, the JT9D has the proud distinction of being chosen by Boeing to power the original 747 "Jumbo jet".


Other meanings

The Unicode standard includes a turbofan character, #274B, in the dingbats range. Its official name is "HEAVY EIGHT TEARDROP-SPOKED PROPELLER ASTERISK = turbofan". In appropriately-configured browsers, it should appear in quotes here: "❋";


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