By HANK HOGAN, Engine Air Magazine
The sky’s the limit, the old saying goes. Unfortunately, that applies as much to fuel prices as to aircraft these days. The benchmark price for a barrel of oil climbed from just under $60 in mid-2007 to over $140 a year later. Predicting where it will head next is difficult, but the trading of futures indicates that the price will not come back down any time soon. For engine manufacturers, that situation presents a problem. They have to cut fuel consumption, while still delivering the performance demanded by airframe makers. For East Hartford, Connecticut–based Pratt & Whitney, the solution is to use a geared turbofan engine, an approach the company says will allow both the turbine and the fan in such an engine to turn at an optimum speed. The gear that does this is not immediately obvious when looking at the engine, notes Pratt & Whitney Vice President of Technology and the Environment, Alan Epstein. “When I show pictures of it, I have to draw arrows to point out where the gear is. It’s buried inside the engine.”
But what may seem like a relatively small change, when added together with other improvements that it enables, adds up to a big boost in efficiency.The company forecasts a 12 to 15 percent reduction in fuel burn from the engine, which can translate to as much as a 25 percent reduction in fuel burn in a next-generation aircraft. In addition to the reduced fuel burn, Pratt & Whitney predicts other significant benefits to this new engine design. One will be a similarly sized decrease in carbon dioxide emissions, a direct consequence of the higher fuel efficiency. The new design also trims smog-producing nitrogen oxide output by 55 percent and cuts the engine noise in half. Operating costs savings are expected to be in the double digits, with more than $1.5 million saved per aircraft per year. Some of those savings are from fuel efficiency; others are from lowered emissions fees, noise fees, and maintenance costs.
RUNNING AT THE RIGHT SPEED
According to Epstein, there are three key technologies behind the geared turbofan’s performance. The first is the gear itself. The second and third technologies are very light, efficient fans and a larger, more aerodynamic nacelle that encloses the engine. What makes everything possible is the use of the reduction gearbox that sits between the fan and the turbine. In a conventional engine approach, both the fan and the turbine are directly connected to each other by a shaft that runs through the center of the engine.
A larger fan increases the engine bypass ratio, thereby increasing fuel efficiency. But a direct connection to the turbine means that, as the fan is made larger, the turbine has to expand as well, which makes the turbine heavier and less efficient. At some point, all things being equal, the gain from the larger fan will be canceled out by the loss in efficiency due to the corresponding increase in the turbine size. Pratt & Whitney’s solution puts a gear between the fan and turbine, enabling the two components to turn at different speeds.
The fan can move slowly, upping its efficiency while cutting the noise produced. At the same time, the turbine can rotate very rapidly, boosting its efficiency. Epstein notes that this approach provides other benefits as well. “You save a lot of weight by putting the gear in. Also the gear is conventional gear steel and replaces nickel titanium alloy, so it’s much cheaper,” he says. He adds that the use of the gear cuts six stages and 1,500 airfoils out of the turbo compressor, the part of the engine that produces shaft power.
Because of the removed parts, the engine on the whole is 10 percent lighter than previous models. That reduction takes into account the amount that the gear itself adds back in — an estimated 5 to 6 percent of the weight of the engine. A portion of the weight savings also is used to enhance other parts of the engine. Thus, there is a net savings and overall improvement in efficiency.
THE OTHER TWO LEGS
While the addition of a gear leads to some improvement in engine performance, Epstein notes that the full increase of efficiency in Pratt & Whitney’s geared turbofan engine arises because of synergies with other improved technologies. He points to the fans as one area where this is evident. The decrease in weight in the power train of the engine means that the fans can be made larger.
That alone would provide some improvement, but the company coupled the larger size fan with material and design advances to wring out even more of a benefit. “We now know how to make very light fans, with hollow metal or composite blading. We now can, compared to 10 or 20 years ago, make much more efficient fans,” says Epstein. He characterizes the result of the larger, improved fan design and geared power train as an ultra-high bypass ratio engine, one where the ratio of the amount of air pushed through the fan to the air mixed with the fuel to drive the fan is very high. That ups the efficiency of the engine, making it gulp fuel more slowly.
The final element behind the overall increase in engine performance is the nacelle, which houses the blades and powerplant. Because the nacelle now can be made out of improved composites, the weight of this housing also comes down, and fuel efficiency goes up. Another contributing factor to the positive effect on performance is the advent of more computer power for design analysis and better aerodynamic modeling. Together, these have improved the design of the nacelle with regard to airflow, cutting drag. This improvement in airflow actually extends beyond the engine itself. Epstein notes that the engine will be mounted under the wing and that has an effect on the wing’s aerodynamics. “The airframe manufacturers are more sophisticated now in designing the wing not only not to be adversely impacted by the nacelle but, in fact, to take advantage of the nacelle being there,” he says.
That is one reason why the reduction in fuel burn at the engine level is projected to be 12 to 15 percent but may be as much as 20 to 25 percent when the engine is installed on a nextgeneration aircraft. Overall, it contributes to the 15 percent cash operating cost advantage that Bombardier is projecting for its CSeries family, which is the aircraft that will use the Pratt & Whitney engine.
BUCKING THE TREND
Pratt & Whitney recognized what could be done with a geared turbofan 15 years ago. Since then, the company has spent something in the vicinity of a billion dollars in research and development perfecting the technology. Along the way, Pratt & Whitney built a number of demonstration engines to showcase the technology and win over customers. A key part of the R&D effort that produced the latest engine generation was intended to overcome customer qualms about geared turbofans.
The gear, after all, is a new piece of machinery, and the addition of such components has not been the direction the industry has been moving — instead, the emphasis has been on parts reduction. David Stewart, principal of the aeronautics industry analyst firm AeroStrategy, points out that the Pratt & Whitney geared turbofan means fewer stages in the compressor and turbine sections, so the parts count ultimately may not be that much higher.
He does note another issue, though: “There’s some concern because, with the hard-working gears, heat dissipation is a challenge, especially as the engine gets scaled up.” In order to sell the engine, the company had to show that the addition of a gear would not undermine engine reliability. This was done, explains Pratt & Whitney’s Epstein, by carefully designing the gear and then fully testing it. Built out of standard material and manufactured on conventional machinery, the gear is nonetheless rugged enough that the company guarantees customers nothing will need to be done to the gear for 20,000 takeoffs and landings.
In making that promise, it helps that Pratt & Whitney claims to have made more geared engines than any other firm, with most of these being turboprops made by the company’s Canadian division. It also helps that the gear design in the latest engine actually is simpler, with only seven parts, than the gears used in the engines made in Canada. It should be noted that, despite the advantages Pratt & Whitney cites for its approach, it is the only big engine maker that is pursuing geared turbofans. The reasons for this, however, might have as much to do with the familiarity of various companies with the technology as with the merits of geared turbofans themselves.
Pratt & Whitney’s Epstein points to the amount of money and time that must be spent on research and development as one explanation why his company stands alone. Others would have had to start their investment decades ago to be able to offer a comparable geared turbofan engine today. In May 2008, the first demonstrator geared turbofan engine completed a 250-hour ground test program and began flight testing on Pratt & Whitney’s 747 flying test bed in July.
The engine will accumulate approximately 50 hours of flight testing before being transferred to Toulouse, France, to begin a joint flight test program with Airbus on an A340 flight test aircraft. Epstein has no doubt that the production geared turbofan will fly fairly soon and be certified shortly thereafter. Talking about the engines intended for the Bombardier and Mitsubishi aircraft, he says, “By 2011, we’ll be flight testing these engines, the actual production design.”
Images Courtesy of Pratt & Whitney Copyright 2013.