Potentiels d'augmentation des performances des moteurs

Boosting Powerplant Performance

Nov 2, 2009

By Paul Seidenman and David J. Spanovich

As airframe manufacturers continue to develop the next generation of commercial and business jets, engine OEMs have been tasked with incorporating performance enhancements to make their products tighter on fuel burn, more environmentally sensitive, quieter and less maintenance intensive than those in service.

Improvements in engine performance are expected to be far more than incremental. For example, the "2008 Addendum To The Strategic Research Agenda, Advisory Council for Aeronautics Research In Europe" (ACARE) sets a goal for European airlines to reduce carbon dioxide emissions by 50%, nitrogen oxide by 80% and perceived external noise by 50% by 2020. At the same time, the industry must make what the council calls "substantial progress towards green manufacturing, maintenance and disposal" by 2020.

ACARE's objectives have not been lost on the engine makers. Alan Newby, Rolls-Royce chief engineer of future programs, says the OEM is trying to decrease fuel burn through two parallel research tracks--propulsive efficiency and thermal efficiency.

Pratt & Whitney's PurePower geared turbofan engine's turbine stages are made of a super nickel alloy and the compressor stages are titanium, while the gear is steel. Credit: PRATT & WHITNEY Propulsive efficiency targets increases in the engine's bypass ratio, because the higher the ratio, the greater the fuel efficiency.

"The current generation of engines normally has a bypass ratio of 10 to 11, compared to a ratio of 4 to 5" for earlier engines, Newby notes. "At Rolls-Royce, some of our research and development effort is concentrated on the open rotor design concept, which could deliver a bypass ratio of 50--or five times greater than it is today." (See sidebar p. 30 for more on open-rotor technology.)

This technology, says Newby, has the potential to improve engine fuel efficiency by as much as 30% relative to today's engines. "At the same time, we'd gain about a 30% reduction in CO2 emissions," he adds.

Thermal efficiency, Newby reports, will improve with increased engine temperatures and greater overall pressure ratios. To achieve that, Rolls-Royce is developing improved materials technology that targets the metal alloys and thermal barrier coatings to be used in turbine blade fabrication. The ceramic-based coatings would shield the turbine blades from the high operating temperatures required to increase thermal efficiency, which Newby estimates at 1,800 to 1,900 degrees Kelvin (2,780.33 to 2, 960.33 degrees F).

"The high temperature materials technologies we are developing will be applicable to all future engines," says Newby. "That includes large-engine developments, such as the Trent XWB, as well as a V2500 replacement engine for 150-seat aircraft. This technology can be applied to either improve performance by reducing cooling air requirements at a given life or to increase on-wing life and reduce maintenance costs for a given level of cooling air. Today, the airframe OEMs and their customers want to see a big reduction in operating costs," from fuel to maintenance.

Pratt & Whitney believes that technologies in its geared turbofan engine will revolutionize engine performance. It formally launched the new product family in 2008, after some 20 years of research and development. The 21,000-23,000-lb. thrust PW1500G and the 15,000-17,000-lb. thrust PW1200G geared turbofan initially will be applied to short-range narrowbody jets. Bombardier's 100-149-seat CSeries jet is the PW1500G launch customer, and Mitsubishi Aircraft's 70-100 seat MRJ is the first application for the PW1200G. Service-entry dates for the two aircraft are 2013 and 2014, respectively.

According to Alan Epstein, Pratt & Whitney's VP technology and environment, the PW1500G is projected to burn 15% less fuel than currently produced engines in its size and thrust range because of the engine's gear, which interfaces with the fan and core to allows a higher bypass ratio.

"The geared engine will have a bypass ratio of 12 compared to a ratio of 5 for the direct drive engines used on some current production 70-90 passenger aircraft," Epstein remarks. "The high bypass ratio will also make the engine 20 decibels quieter than Stage 4 noise standards."

Epstein points out that the geared turbofan represents the first fundamental shift in engine architecture since the first high bypass direct drive turbofan engines. "What's really important is that, with the incorporation of a gear, the spools spin faster without speeding up the fan," he says. "Conversely, the fan can be slowed down to speed up the rest of the engine."

Along with this, Epstein says that the gear allows for considerable engine weight savings.

"On the PW1500G, six stages of turbine machinery--three low pressure stages out of the back of the engine and three compressor stages from the front--were eliminated," he says. "Also, the gear itself weighs about as much as one low pressure turbine stage."

While the turbine stages are made of a super nickel alloy and the compressor stages are titanium, the gear is steel, a less costly material, Epstein notes. About the size of an automobile tire at 28 in. in diameter, the gear weighs under 300 lb. and can generate the equivalent of 28,000 hp. The fan case and strut will be graphite epoxy, making the PW1500G the first commercial aircraft engine with a composite fan case, according to Epstein.

"From a maintenance perspective, testing of the gear has indicated that a major inspection would not be required for about 20,000 cycles, or once every two overhaul cycles, predicated on seven to 10 years of use, on average," Epstein says. "It is very clear to us at Pratt & Whitney that a geared engine is the future. If we were in the process right now of developing a new generation of engines for widebodied aircraft, it would be a geared model."

Dr. Klaus-Peter Rued, director of advanced product design at MTU Aero Engines, reports that the company is working with Pratt & Whitney as a primary partner on the compressor and low pressure turbine for the geared turbofan engine. He explains some of the performance enhancements of the new engine.

"Compared to the IAE V2500 and the CFM56 families, the geared turbofan will deliver about a 20% reduction in fuel burn, 20% less CO2 and NOx emissions, and reduce engine maintenance costs by 30%, by using more durable metal alloys and more sophisticated design concepts for longer, on-wing life," Rued says. "The airline industry has indicated that it wants to achieve about a 40% reduction in total engine maintenance costs."

MTU, he adds, designed its share of the engine components to achieve a weight savings of 20%, also in comparison to the equivalent components on the V2500 and CFM56. That has been achieved using lighter weight metal alloys, he says.

Rued believes that the geared turbofan concept will have application to the A320 and 737 replacements, which MTU estimates will have a 2018 entry into service.

Leaping Into The Future

CFM International, the joint company of GE and Snecma, also is going after the next-generation narrowbodied transport market through the development of a 25,000-35,000 lb. thrust engine, currently referred to as LEAP-X. LEAP (Leading Edge Aviation Propulsion Program) is CFM's overall technology development program, which succeeded Project TECH56, a similar program that ended in 2005. According to LEAP Program Director Ron Klapproth, a LEAP-X engine is slated for roll-out in 2012 as a technology demonstrator.

"Our goal is to reduce fuel burn by 16% for the propulsion system, compared to the CFM56-5B and -7B models, and to reduce noise output by 15 decibels relative to today's regulations describing Stage 4," Klapproth says. "This will be the equivalent of reducing the noise level by 50% over current airplane measurements."

He points out that in addition to a 16% cut in carbon dioxide emissions, the new engine will reduce nitrogen oxide output by 50-60% relative to current certification requirements under ICAO's Committee on Aviation Environmental Protection (CAEP). "All of these improvements will use the most advanced technology under development by both Snecma and its GE partner without sacrificing the reliability record of today's products."

As Klapproth explains, LEAP-X is being designed as "a clean-sheet engine" that incorporates new materials, aerodynamic designs and cooling technology to accomplish the objectives put forward by LEAP. Among the examples of new materials he cites is the fan itself.

"In the CFM56 family, the fan blades are solid titanium, but for the LEAP-X, we will use graphite epoxy composite blades that incorporate a three-dimensional woven resin transfer molding. This will be the first time a composite has been applied to the construction of the fan blades of a small diameter (70-75 in.) fan. This is one of the key technologies in the engine," he says. "The fan case will also be composite. Together this will achieve a 1,000-lb. weight savings at the airplane level, compared to a fan of similar size made of solid metal."

State-of-the-art technology will enable CFM to realize the 50-60% NOx emissions reduction, which Klapproth calls "an unprecedented achievement," if successful. "It would be accomplished by going from a rich burn to a lean burn combustion, which uses less fuel and more air in the combustion process," he says. "To do that, the combustor will incorporate a twin annular premix swirler (TAPS), which was developed under Project TECH56."

TAP was certified by GE for its GEnx, the 70,000-75,000 lb. thrust engine selected by Boeing to power the 787 and the 747-8. "For LEAP-X, this will be the second generation of TAPS, which will deliver a lower nitrogen oxide output than the first generation TAP technology," he notes.

Other new technologies being incorporated into LEAP-X include the application to the low pressure turbine of titanium aluminide, an alloy used in the high pressure GEnx turbine. For the LEAP-X high pressure turbine, a next generation of super alloys will be employed. CFM also is evaluating a type of ceramic matrix composite for both the low and high pressure turbine airfoils and shrouds.

New powdered nickel alloy technology for the compressor and the high pressure turbine will permit the engine to run at higher heat cycles, yet deliver faster rotor speeds. "This will help us to achieve our fuel savings goal and give the engine longer life on-wing," says Klapproth.

Darin DiTommaso, manager of preliminary design and systems technology for GE Aviation, reports that the company's engine performance research and development efforts are focused on multiple platforms, from small business jets to widebodied commercial transports.

"A big area of industry focus is on the next generation of narrow-bodied airplanes, since there will be a number of those being produced," DiTommaso says. He says GE hopes to cut fuel burn by 16% over current production engines.

"To achieve that, one of the technologies we're looking at is silicon carbide-based ceramic matrix composites (CMC), which would replace super metal alloys, in such areas as the high pressure turbine. CMCs are about one-third the density of the metal alloys currently used. In addition, they are durable and more heat resistant, and therefore require less cooling air," he says.

Weight reductions and increased durability also are being achieved with the use of carbon fiber composites of bonded plies to be used in the fan blades and case. Along with lighter weight, those composites have no corrosion issues and show higher impact resistance. Carbon fiber bonded plies are used in both the fan blades and fan case on the GEnx, and for the fan blades, only, on the GE90.

"We can also get further weight reductions with the use of metal alloys such as a titanium aluminide alloy in the low pressure turbine blades," DiTommaso says. And, because the blades weigh less, the turbine disc can be made of lighter weight materials.

GE is evaluating technologies to address the issue of noise reduction, including "looking at the wake that is created by the fan blades and the interaction of that wake with the outlet guide vane," he explains.

Maintenance cost reductions also are one of GE's goals, and in that regard DiTommaso points to TAPS. "Along with reducing nitrogen oxide emissions, TAPS provides a more uniform temperature distribution which improves the lifespan of the hardware downstream of the combustor, including such high pressure turbine hardware as the nozzles, blades and shrouds. At the same time, the TAPS design eliminates the dilution holes--used to mix out the combustion gases--in the combustor liner, which should extend the liner's life. "

Honeywell Materials

Ron J. Rich, director of advanced technology for Honeywell Aerospace, says that one of the small-engine OEM's primary areas of focus is on green technology, with the objective of a 5-10% improvement in fuel burn and a 50% reduction in nitrogen oxide.

"In addition, we want to reach a noise level of Stage 4 minus 20 decibels, improved time on-wing, and more on-condition maintenance."

Among the approaches Honeywell is pursuing, says Rich, is the use of materials and technologies that will permit higher turbine temperatures and extend engine life.

"This includes the use of Alloy 10, a nickel-based, powder metal super alloy for the turbine disk, along with the next generation of ceramic thermal barrier coatings, and better effusion cooling techniques and improvements to the liner and dome of the combustor," Rich explains. Because the ceramic coatings should improve cooling of the vanes, blades and surrounding components, he says Honeywell will have more options for optimizing the combustion system. That should translate to better durability, longer time on the wing and lower emissions.

Another goal is a 40% improvement in engine power (thrust) to weight ratio. That, says Rich, means reducing the absolute weight of the engine without a power tradeoff.

Rich reports that these technological developments will be incorporated into Honeywell's new-production engines. Two near-term examples are the HTF7250G, and HTF7500E. Gulfstream has selected the HTF7250G for its G250, which is expected to enter service in 2011. The 7,400-lb. thrust engine is slated for certification by year-end 2010.

The HTF7500E, to be certified in late 2011, will be rated at 6,500 lb. thrust, and will power the Embraer 450 and 500 model business jets, projected to enter service in 2013 and 2012, respectively. Both engines are rated in the 7,000 lb. thrust range.

"The new engines will come with the ability to generate significant amounts of data for prognostics and diagnostics," says Rich, "giving the operator a better understanding of how the engines are performing and when to schedule maintenance on the various components."