Pratt & Whitney Geared Turbofan

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The Pratt & Whitney Geared Turbofan (GTF) is a high-bypass geared turbofan engine currently selected as the exclusive engine for both the Mitsubishi MRJ and Bombardier CSeries regional jets. The project was originally known as the Advanced Technology Fan Integrator (ATFI).

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[edit] Geared fan

In order to extract thrust efficiently from a jet engine, the exhaust speed has to be tuned to the flying speed of the aircraft. In general terms, a pure jet engine's exhaust is much faster and hotter than it should be for efficient thrust in the range of speeds at which most commercial aircraft fly. The solution to this problem is to use additional turbine stages to extract power from the exhaust and use that to produce some other form of thrust. The most obvious example of this is the turboprop, which extracts as much power as possible from the jet and uses that to power a propeller. Typical designs use a "low pressure turbine" connected through a shaft to the propeller, although they almost always include a gearbox to slow the high speeds at which the turbines want to operate.

Between the pure-jet for high-speed use and the turboprop for low-speed use lies the turbofan, which is tuned to operate at speeds around 400 to 600 mph. As the quality of the fans has improved over time, the turbofan has evolved to use more "bypass", using the fan to blow more of the air around the engine than through the engine core. Early examples like the Rolls-Royce Conway had bypass ratios on the order of 0.5:1, meaning that they blew 50% as much air around the engine as through it. Modern engines typically use ratios of 5:1 or more.

As bypass ratio increases, the size of the turbine in comparison to the fan decreases. Larger and lower-pressure turbines generally want to operate at lower speeds, so in this case if the fan is to rotate at its optimum blade speed the LPT blades will be running too slow (because they are on the same shaft), so additional LPT stages will be required to extract sufficient energy to drive the fan. Introducing an epicyclic reduction gearbox, with a suitable gear ratio, between the LP shaft and the fan, enables both the fan and LP turbine to operate at their optimum speeds. This increases the engine's complexity, however, which is why it is not a common solution. Only the Honeywell TFE731 has seen widespread use.

[edit] History

Pratt & Whitney first attempted to build a geared turbofan starting around 1998, known as the PW8000.[1] This essentially was an upgrade of the existing Pratt & Whitney PW6000 that replaced the fan section with a gearing system and new two-stage fan. After several years of development the PW8000 essentially disappeared.[2] Soon after the ATFI project appeared, still using the PW6000 turbomachinery. This has developed into the GTF project. The primary difference between the PW8000 and current GTF is a new gearbox and the use of a single-stage fan.

Pratt & Whitney claims the GTF will be 10% to 15% more fuel efficient than current engines used on regional jets and single-aisle jets, as well as being substantially quieter.

[edit] Applications

[edit] Notes

  1. ^ Pratt & Whitney's surprise leap.
  2. ^ The Short Life and Untimely Demise of the PW8000

[edit] See also

[edit] External links

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