Pratt & Whitney Begins Ground Testing on Geared Turbofan Demonstrator Engine Ahead of Schedule; Targeting 12%+ Improvements in Fuel Burn
15 November 2007
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| The geared turbofan uses a special drive gear system to allow the fan to operate at a different speed from that of the low-pressure compressor and turbine. Click to enlarge. |
Pratt & Whitney’s Geared Turbofan (GTF) demonstrator engine successfully completed its first ground test, ahead of schedule, at the company’s advanced test facility in West Palm Beach, Florida. The full-scale demonstrator engine successfully started and ran, marking the beginning of a ground test program that will run through May 2008.
The Geared Turbofan engine targets a more than 12% improvement in fuel burn with significant reductions in engine noise, environmental emissions and operating costs. In a Geared Turbofan engine, a state-of-the-art fan drive gear system allows the engine’s fan to operate at a speed different from that of the low-pressure compressor and turbine, resulting in greater fuel efficiency and a slower fan speed which results in less noise.
For aircraft of 70 to 150 passenger size, the Geared Turbofan engine reduces the fuel burned, and thus the CO2 produced, by more than 12% compared to today’s aircraft, while reducing cumulative noise levels about 20dB below the current Stage 4 regulations. This noise level, which is about half the level of today’s engines, is the equivalent difference between standing near a garbage disposal running and listening to the sound of my voice right now.
Our Geared Turbofan engine offers a balanced approach to engine noise, fuel efficiency and reduced emissions. We continue to work on more advanced technology that will offer still lower noise and fuel burn in the future. An advanced Geared Turbofan engine will deliver the low fuel burn and CO2 output of the giant supersonic propellers now being studied, without the inherent noise disadvantages. Indeed in the future, it will be possible to design aircraft in which the primary noise sources are not the engines but are instead the airframe itself.
—Alan H. Epstein, Vice President, Technology and Environment, Pratt & Whitney
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| The geared turbofan approach breaks the current linkage between lower noise and higher fuel burn. Click to enlarge. |
The Geared Turbofan engine is part of Pratt & Whitney’s technology readiness program for the next generation of commercial aircraft. The company is actively testing key components on 15 test rigs around the world; flight testing on Pratt & Whitney’s 747 flying test bed will begin in mid-2008.
On Oct. 9, 2007, Pratt & Whitney announced that the Geared Turbofan engine was selected by Mitsubishi Heavy Industries Ltd. to power the new proposed Mitsubishi Regional Jet. The sole-source agreement is the first airframe application for the Geared Turbofan engine, which is scheduled to enter service in 2013.
Pratt & Whitney has more than 17,000 aircraft engines installed with hundreds of airlines throughout the world. Additionally, Pratt & Whitney is a leading partner in two joint venture companies that manufacture commercial aircraft engines: International Aero Engines, which makes the V2500 engine for the Airbus A320 family of aircraft, and the Engine Alliance, whose GP7200 engine is FAR 33 certified for the new Airbus A380 aircraft.
(A hat-tip to Anthony!)
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It's long been known that reducing fan speed and increasing fan diameter will improve fuel economy and cut noise. Efficiency goes up when the velocity difference between ambient air and propelled air is reduced. Power can be kept constant by increasing the mass flow.
Gears and bearings are also well-understood disciplines, though for obvious reasons aircraft engines must meet exceptionally high reliability and longevity criteria. Jet engine manufacturers have to be very conservative in their designs, much like their counterparts in the large marine diesel market.
It is possible that additional complexity was the primary argument against the development of geared turbofans up to now. However, expensive fuel plus ever-stricter noise emissions regs make a lot of ideas once considered marginal look quite attractive now.
Counter-rotating fans plus heat recuperation present additional avenues for improving fuel economy at the expense of slightly higher noise levels (see MTU Aero slides). However, these measures are even harder to implement.
Posted by: Rafael Seidl | 15 November 2007 at 06:14 AM
"It's long been known that reducing fan speed and increasing fan diameter will improve fuel economy and cut noise."
although there comes a certain point when the increased drag of the increased fan diameter (and hence engine nacelle size) cancels out the fuel economy benefit, as well as there is a physical limit on how big an engine you can fit underneath the wing without hitting the tarmac. so is this a 12% improvement over the wide-chord, ultra-high bypass tbfans already in production?
Posted by: gavin walsh | 15 November 2007 at 06:42 AM
Speaking of advanced aircraft engines:
Years ago there was a lot of publicity in the aeronautical engineering world about using unducted fans (i.e., good old fashioned PROPELLERS!) whereby you have thin, multiple blades, swept back (like swept back wings) so as to enable the aircraft to fly at ~Mach 0.8 or so. Such engines were tested on a Boeing 727 test bed, and attained reduced fuel burn compared to classical turbofan engines like the JT8.
Unfortunately mother nature, as she so often does, threw a frustrating show-stopper at us: Noise. Unducted fans are noisy. And noise has been a big problem, forcing captains of jetliners to follow all kinds of procedures they'd rather not do, because of noise complaints from folks living near airports.
Perhaps reduction gearing might be an enabler for unducted fans to attain improved propulsion efficiency plus acceptable noise signature. I hope so.
Posted by: Alex Kovnat | 15 November 2007 at 06:45 AM
I think Pratt & Whitney is designing this technology so it gets incorporated into the successor to the International Aero Engines' V2500 engine. This means the successors to the Airbus A320 Family and the Boeing 737 could use this engine, with fuel savings of as high as 20% compared to today's high-bypass turbofans.
Posted by: Raymond | 15 November 2007 at 07:03 AM
Time to start mounting engines above the wing.
Posted by: richard schumacher | 15 November 2007 at 07:11 AM
I remember reading about the Geared Turbofan two decades ago! It disappeared from sight and then the UDF, UnDucted Fan, engine was going to be the Next Great Thing.
Posted by: DS | 15 November 2007 at 10:59 AM
This reminds me of the bicycle wheel. It seems 700C (about 27" in diameter) is the optimum wheel diameter. As you go past 27", the amount of energy to turn the wheel begins to exceed the practical limits of the wheel. Better idea: Re-invent the wheel. Making it lighter is the key to optimum efficiency. Increasing diameter leads to heavier wheels, or in this case, turbofans.
Posted by: Kerry | 15 November 2007 at 02:12 PM
@ Richard Schumacher -
... or, you could mount the wings at the top of the fuselage and use a short landing gear.
I've seen some civilian BWB concepts with jet engines mounted above the wing and others that had them mounted at the trailing edge. The conventional tube-and-wing design seems to work better with engines slung underneath or in the tail section.
@ DS -
... and then noise emissions regs were tightened and nixed the UDF. Besides, growing perfect single crystal vanes that could support supersonic speeds at the tip proved a tad harder than anticipated.
Posted by: Rafael Seidl | 15 November 2007 at 02:27 PM
@Rafael,
Ultimately the limiting criteria in turbine design is is tip speed speed. Any jet engine is in effect a perfect fatigue testing machine. It is tough enough to bear the bending and resonance frequency fatigue motions without accepting the shearing forces from supersonic tip speeds, irrespective of blade materials.
That was why propeller planes could not really achieve supersonic flight. Aside from lack of power and poor aerodynamic plane bodies, the fans just could not take the abuse.
The art of growing single crystal "buckets" has made tremendous leaps in capabilities. The technology is expensive but I don't think it is a limiting factor any longer, like it used to be, when I was in the blade engineering business.
As a matter of fact that single crystal technology, will serve to shorten the time frame to controlled fusion power plants by a decade or more. It is now practical to create single crystal facing materials for the "First Wall" facing the Plasma, that were impossible to envision as short as ten years ago. The great benefit is performance, facing the plasma, and not contaminating it, and furthermore retaining little residual radioactivity.
Posted by: Stan Peterson | 15 November 2007 at 05:54 PM
The P&W geared (turbo fan)? engine of the 1980's was called a 'Prop-fan' and was supposed to be used on the MD-94 passenger plane in the mid 1990's. However MD was being bought out by Boeing during that period of time and perhaps Boeing seeing it as competition to it's own 737 future sales perhaps forced the termination of the MD-94 program.
Posted by: gary | 15 November 2007 at 06:58 PM
P+W presented this engine today at SuperComputing 2007. Seems most of the efficiency gains in the geared turbofan come from allowing the high pressure axial turbine to spin at a higher higher rate than in a an ungeared engine.
Classic turbofans have an axial speed described as "low", the new geared engine was described as having "medium" axial speed HP stage, and further efficiency gains could be acheived with higher gear ratio.
BTW. The tips of the fan in the new geared engine run supersonic. The point at they start to get wider is the diameter at which supersonic transition occurs.
Posted by: pogo | 15 November 2007 at 07:16 PM
Definitely, geared P&W turbofan is not a new development.
As Stan suggested, when the by-pass ratio greatly increases, the fan's diameter become much larger than the turbine core. As such, if the fan and the second stage turbine driving the fan are to turn at the same speed, then the fan blade tips would be traveling at much faster speed than the second stage turbine blade tips. This would mean too much aerodynamic drag on the fan tips and the risk of exceeding the sound barrier, hence very high drag and stress level for the fan blades. At the same time, the second stage turbine blades will not be turning fast enough to deliver enough power to power the much larger fan disk. The aerodynamic drag increases at the square of the airspeed.
Geared fan is inevitable once the by-pass ratio exceeds certain number.
Posted by: Roger Pham | 15 November 2007 at 07:32 PM
The only experimental aircraft I know of that had a supersonic prop was the XF-84H "Thunderscreech".
"The propeller blades were supersonic even while the airplane was running up on the ground. Each blade shed a shock wave that spiraled outward. It was more than just sound but also the shock waves created that were detrimental... Ground crew reported that wearing standard ear protectors "made no difference" and a person standing some distance from the airplane was subjected to rapid-fire shock waves. The shock waves acted on the body, causing spasms, nausea and loosening of the bowels. Even epileptic seizures were reported."
Posted by: Cervus | 15 November 2007 at 08:57 PM
I think the russian Tupolev_Tu-95 "Bear" also had supersonic counter-rotating props that would clock 1.05 Mach at the tips.
Posted by: Ben | 18 November 2007 at 09:56 AM