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Pratt & Whitney's next leap in engine technology
An engine meant to set new standards for reliability, operating cost, fuel burn, and noise levels for the 21st century is under development by Pratt & Whitney (pratt-whitney.com). "The PW8000 geared turbofan is the next leap in engine technology," said Pratt & Whitney president Karl J. Krapek. "This engine, an extension of our PW6000 family, reduces operating costs as much as 10%, reduces fuel burn 9%, cuts noise levels significantly, and boosts reliability by eliminating more than half the airfoils in the compressor and turbine sections."
The gearbox for the PW8000 geared turbofan has run for 1000 h in Pratt & Whitney test stands and for 100 hours on a full-scale engine.
The geared turbofan represents more than 10 years of research and over $350 million in development costs by Pratt & Whitney. The engine's fan, which produces most of the thrust, is driven through a reduction gearbox, rather than being directly connected to the rest of the engine. "The gearbox between the fan and low-pressure compressor and turbine allows us to select the best possible operating speed for each engine section," said Krapek. "Each runs much more efficiently, allowing us to reduce the number of engine stages and parts."
The jet engine is divided into high and low spools, consisting of a compressor and a turbine. The 17-in., 32,000-shp gearbox enables the fan and low spool to operate at different speeds. The low spool drives the fan and gives an engine its propulsive power.
Fans operate best at slower speeds, while compressors and turbines run more efficiently at higher speeds. Therefore a gearbox allows the fan, compressors, and turbines to achieve their most efficient operating speeds, leading to a quieter engine with better fuel burn and fewer parts to maintain. The gearbox design also allows for a larger fan on the PW8000, which provides it with an efficient 11:1 bypass ratio.
The PW8000's fan diameter measures 76 in.about 11 in. larger than fan diameters of typical engines in its class. About 90% of the engine's propulsive power is provided by its large fan, which reduces the number of stages needed in the low-pressure compressor and low-pressure turbine. Fewer stages mean fewer parts to be repaired or replaced. The PW8000 features 13 stages compared with 20 for typical engines powering the same class of airplane, which represents a 40% reduction.
The PW8000 can produce 25,000-30,000 lb of thrust with a bypass ratio of 11:1.
With 40% fewer stages than conventional turbofans of the same size and 52% fewer compressor and turbine airfoils, the PW8000 has 10% lower operating costs and nearly 30% less maintenance costs. According to Pratt & Whitney, this means a typical 120-180 passenger aircraft will save $600,000 annually more due to lower operating and maintenance costs and improved productivity.
The new geared turbofan also features reduced emissions of at least 40% below the 1996 regulations. This is a result of the engine's use of an advanced burner that produces fewer emissions.
"We have known since the early 1980s that the theoretical argument for a geared fan engine was compelling in terms of cost, fuel burn, noise, and reliability," Krapek said. "We also knew that it would take careful technology development to create an engine with those new levels of performance and the ruggedness and reliability the world's airlines expect." During thousands of hours of development testing, Pratt & Whitney has incorporated many revolutionary advances in gearbox bearing design. When coupled with advances in gear and lubrication system design, these innovations have resulted in a gearbox with an efficiency of better than 99% and a heat load less than half that originally predicted. In addition, gear misalignment and stress have been all but eliminated using Pratt & Whitney's "self-centering" technology.
The PW8000 gear system has gone through more than 1000 h of component testing and another 1000 h of full-scale testing. In addition, a 40,000-shp gearbox, 25% larger than required for the PW8000, has been operated in a full test engine for about 100 h.
The PW8000 family will cover a range of 25,000-35,000 lb of thrust for aircraft carrying between 120 and 180 passengers. The engine, which is scheduled for certification within the next 2 ½ years, features common core technology developed for the PW6000. Together, both engine families will cover aircraft markets between 80 and 180 passengers.