Pratt & Whitney’s geared turbofan program is set to take to the skies with innovations ranging from reduction gearing to low-emission combustion cycles.
Living in an airport flight path? Your skies may soon be quieter and cleaner if aircraft-engine maker Pratt & Whitney has anything to say about it. The company recently finished ground-based testing of its geared turbofan (GTF) concept. The next step is flying the 30,000-lb-thrust engines on a P&W-owned Boeing 747SP testbed. Airbus has also agreed to flight-test the engines on one of its own A340s later this year, using the same test program.
The prototype engines are said to be around 50% quieter than those they could replace. In addition, P&W expects the engine will cost 30% less to maintain and will be greener than competitors’. The company claims it uses 12% less fuel and cuts emissions by 20%.
Japan’s All Nippon Airways has already gambled on the design, inking an agreement to purchase 15 Mitsubishi Regional Jets carrying the new powerplant, with the option to buy 10 more. Mitsubishi will use a version of the GTF that puts out 14,000 to 17,000 lb of thrust for its smaller planes. Bombardier also plans to power its CSeries jets, scheduled for sale later this year, with a 23,000-lb version. The Bombardier engine is similar to those needed by medium- duty Boeing 737-300s. Both the Mitsubishi and Bombardier jets should enter service in 2013.
The test flights and future orders are the culmination of a 20-year-long development program. Working with MTU Aero Engines, Avio, Volvo Aero, Goodrich Aerostructures, and NASA, P&W has rethought many of the major systems that make up commercial-jet engines starting with the intake fan and moving aft through the compressor stages, combustor, and turbine stages. Bypass ratio, fan speed, and combustion efficiency are all getting an overhaul under the GTF program.
So what is it about the geared turbofan that has P&W’s Bob Saia, vice president of the Next Generation Product Family, calling it a “game-changing technology”?
Turbofans, in which a fan powered by a turbine in the jet-exhaust stream directs the incoming air into and around the combustion chamber, were tested as early as World War II and implemented commercially in the 1960s. Originally, the mass of air flowing around the combustor was about a third of that flowing through it. Since then, this bypass ratio has steadily risen. The engines currently powering most 737s and Airbus A320 variants have bypass ratios between 5:1 and 7:1.
P&W’s Advanced Technology Fan Integrator (ATFI), the PW308-based precursor to the GTF, boasted a bypass ratio of 11:1. As the GTF approached fruition, its diameter has had to be scaled back to fit under existing aircraft wings. The new goal is a 10:1 bypass ratio.
The bypass flow exits the engine at a lower velocity than the combustor exhaust. Its exact velocity depends on the intake-fan pressure and the flight conditions. For fuel economy, bigger and slower air bypass is better. Thrust-specific fuel consumption (TSFC) in particular is lower when bypass air mass is greater and bypass air speed is lower.
The trade-off is that slower bypass air means less airspeed overall. While low-bypass jets can fly at supersonic speeds, their high-bypass cousins are restricted to Mach 0.85 or lower. Since commercial airplanes aren’t designed for transonic flight, airlines are happy to take the higher fuel efficiency associated with high-bypass turbofans.
To push the large mass of air into the Goodrich-designed inlet and fan cowl and around the combustor, the engine needs a big fan with a lot of power. That power comes from a turbine turning in combustion exhaust. The high-velocity, high-pressure exhaust spins the turbine blades at high rpms, turning a shaft that drives both the forward fan and the low-pressure compressor stages.
“Turbines naturally like to run fast. They are most efficient at high speeds,” Saia said. The same is true for compressor stages forward of the point of combustion. But spinning the big fan quickly makes more noise, puts more stress on the fan blades, and ups torque requirements.
To generate enough torque to drive the large fan, designers would have to add turbine stages and beef up the driveshaft. The narrow combustion chamber common in high-bypass engines can’t accommodate much growth in driveshaft diameter. Additional turbine stages mean more moving parts and greater up-front and maintenance costs. These two dictates combined lead to longer, heavier engines that can erode fuel savings.
Finally, instabilities encountered by the fan-blade tips as they approach the speed of sound add to the substantial stresses on the fan blades.
Noise may seem like a small concern compared to the difficulty of adding torque or the possibility of the fan flying apart midflight, but airlines beg to differ. Fast-spinning fans broadcast a distinctive high-pitched whine. And the noise level goes up with every additional turbine stage.
For two-engine planes, like 737s, FAA regs previously set ground noise limits of 98 dB on the approach, 94 dB at takeoff once airborne, and 89 dB for sideline noise created when applying reverse thrust after landing, when taxiing, and when accelerating for takeoff. Larger planes get larger noise allowances, indexed logarithmically to their weight and maxing out at 103 dB, 105 dB, and 106 dB for sideline, approach, and takeoff measurements, respectively.
The newest criteria, called Stage 4, shaves 10 dB off the sum of approach, sideline, and takeoff noise allowed under the previous regulations. The Stage 4 rules only apply to new aircraft designs submitted to the FAA and International Civil Aviation Organization (ICAO) after January 2006. Although existing aircraft and pre-2006 designs still in production are exempt, and the majority of commercial jets are already 2 to 4 dB under these criteria, quieter operation still has its advantages.
Local governments impose noise taxes or restricted operating hours to minimize the impact of noise on residents around airports. Airlines can save money and gain schedule flexibility with quieter planes. They can also avoid the steep, fuel-churning, maximumthrust takeoffs that louder jets can be forced to make.
P&W says the GTF’s extra bypass air flowing between the cowling and the combustor acts as an acoustic insulator. The wide-chord swept fan blade design has cut the number of blades from 22 to 18. With fewer blades in the fan, a reduction in turbine stages, slower fan tip speed, and the extra insulation, you get planes that operate about 20 dB quieter than Stage 4 regs.
The recently completed ground tests used 32 microphones and 16 sound-pressure transducers to confirm the engine is on track to meet its low-noise goals. The GTF’s performance was “right on target” according to Saia.
“The ground test noise data will be compiled with flight test measurements taken later this year to validate the GTF’s noise signature,” he said.
But how do you get the desired low fan speed and turbine efficiency? P&W’s fix is a reduction gear system that gets the turbine and fan closer to their optimum speeds while letting them use the same driveshaft.
The GTF’s 3:1 planetary reduction gearing, developed with Avio, accepts 31,000 shp (shaft horsepower) at around 9,000 rpm from the turbine. That input shaft ends in a sun gear, which turns five star gears within a ring gear. The ring gear drives the fan at 3,200 rpm. The assembly is positioned just aft of the fan and forward of the low pressure compressor stages.
Temperature, lubrication, and wear are still significant concerns, however. P&W engineers modeled the heat generation in similar gearboxes and found that 80% of the heat is caused by churning excess oil. The new design calls for spray application of oil only to trouble spots and for rapid recirculation to keep heat from building up. The lubrication redesign has cut peak oil pressure to 57% of the design limit.
P&W engineers say GTF’s reduction gearing is within design limits established on previous generation gearboxes. They estimate gear tooth bending stress and gear contact stress are at 83 and 81% of their respective design limits. They’ve been able to get there, in part, with specially designed bellows that isolate the gears from engine flexing during operation. The unit load on the gearbox’s journal bearings is 81% of the limit, and the bearing temperature hovers at 89% of the limit.
Developmental programs in the 1990s took the gearbox up to 40,000 shp, and more recent rig testing at 31,000 shp featured worst-case scenarios like windmilling, oil-flow interruption, and speeds, torques, and loads outside the design limits. Upcoming flights will also run the journal bearings without oil.
Downstream of the reduction gearing, about 10% of the air funnels toward the low-pressure compressor (LPC). Because LPC speed is no longer a compromise between optimum turbine and fan speeds, it can spin three times faster than in most conventional engines.
High speed means the compressor can do more work with fewer stages and less weight. However, the blades may have to be specially designed not to fly apart at the higher speeds. Another concern is whether the seals can take the higher pressures.
After the LPC, the inlet air reaches the high-pressure compressor (HPC) stages. Here, speed jumps even more as the HPC is driven by the high-pressure turbine (HPT) on a driveshaft concentric with that of the low-pressure stages.
By the time it has passed the HPC, the air pressure is about 20 higher than at the intake, with a corresponding air temperature, rise to 1,300°F, before combustion begins.
Once in the combustor, the explosive burning of jet fuel in the air can more than double the temperature. The walls of the combustor are composed of thermally isolated segments for better heat control. P&W uses materials like cobalt alloys that resist oxidation to prolong combustor life.
But there is another problem with cranking up the heat: nitrogen- oxide compounds, also known as NOx. Higher temperatures and stoichiometric fuel-air mixtures promote the formation of these pollutants.
To combat this, P&W along with NASA has developed a richquench- lean cycle in the Technology for Advanced Low Nitrogen Oxide (Talon) program. Enough atomized fuel goes into the forward part of the combustor to create a fuel-rich mixture. Although this seems counterintuitive in a fuelefficient design, the excess fuel means there is little free oxygen left after combustion to form NOx.
The mostly combusted fuel continuously moves aft where it is flooded with cooling bypass air in the quench step. Whatever fuel is left is now part of a lean mixture that can burn more completely at lower temperatures. In addition, the bypass air can oxygenate carbon monoxide and corrosive hydrogen radicals in the combustion gases.
Spinning blades Powered by the combustion, the exhaust leaves the combustor and encounters the high-pressure turbine (HPT), which takes some energy out of the exhaust stream to power the HPC. Since the high-pressure spool (HPC, HPT, and driveshaft) is usually decoupled from the lowpressure spool and inlet fan, its components are designed to withstand high speeds, temperatures, and pressures.
However, the low-pressure turbine (LPT), which comes next, is running far faster than in conventional turbofan engines. MTU Aero used some high-pressure spool know-how when redesigning the LPT stages for P&W. They changed the blades’ outer shroud design to cut centrifugal forces, beefed up the cross-sectional areas for lower airfoil stress levels, and tightened up the disk design to prevent hoop overstress.
Volvo Aero designed the turbine exhaust case while Goodrich is responsible for the exhaust system.
All Nippon Airways, ana.co.jp
MTU Aero Engines, mtu.de/en
Pratt & Whitney, pw.utc.com
Volvo Aero, volvo.com/volvoaero/global/en-gb