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Thursday, May 1, 2008

Green Engines

Charlotte Adams

Spurred by environmental regulations and ever rising fuel costs, jet engine manufacturers are developing products that reduce emissions and noise pollution. Researchers in Europe and America are focusing on next-generation concepts such as open rotor and embedded engines to increase efficiency and reduce noise. But today’s new engines such as the Rolls-Royce Trent 1000, the General Electric GEnx and the Pratt & Whitney geared turbofan (GTF) already claim advances in environmental-friendly technologies.

Rolls-Royce has signed up to goals set by the Advisory Council for Aeronautics Research in Europe (ACARE) an organization with about 40 members, including government agencies and private companies. ACARE calls for the following achievements by 2020, compared with the baseline year of 2000:

  • 50 percent reduction in fuel burn and carbon dioxide (CO2) emissions per passenger kilometer;

  • 80 percent reduction in nitrous oxide (NOx); and

  • 50 percent reduction in the perceived external noise level.

Of the 50 percent reduction in fuel burn, however, only 20 percent is expected from engines, 20 percent from airframes and 10 percent from efficiencies in air traffic control, such as flying more direct routes. European engine maker MTU, by contrast, has proposed its own CLAIRE (Clean Air Engine) strategy, targeting a cumulative 30 percent reduction in fuel burn from the powerplant alone by 2035.

CLAIRE includes three phases. Phase 1, based on the P&W GTF, targets a 15 percent improvement in fuel burn by 2015, compared with a typical year 2000 engine, such as the CFM56 or the V2500. In phase 2, at roughly 2025, the use of counter-rotating integrated shrouded propfan technology (CRISP) could help reduce fuel burn another 5 percent — to 20 percent — compared with the 2000 baseline. The addition of a recuperated propfan could get fuel burn to the minus 30 percent goal by 2035.

European programs like Clean Sky also are putting billions of dollars into environmental research. NASA’s smaller Subsonic Fixed Wing (SFW) program, looking out to 2035, focuses on aggressive noise and emissions goals through integrated airframe and propulsion systems.

Engine designers focus on pollutants such as NOx — a major component of smog — carbon monoxide (CO), unburned hydrocarbons, smoke and CO2. Although CO2 emissions are not regulated under ICAO’s Committee on Aviation and Environmental Protection (CAEP), improving specific fuel consumption (SFC) is equivalent to reducing CO2. To reduce NOx, engine companies use techniques to control burn temperature and burn time.

SFC typically improves by about 1 percent a year, according to Robert Nuttall, Rolls-Royce’s vice president of strategic marketing. The accelerating price of fuel, which can account for as much as half of an airline’s operating costs, also drives this research.

Propulsive Efficiency

Propulsive efficiency is a major focus in turbofan development. Bypass ratios — simplistically understood as the ratio of air flowing through the fan outside of the core versus air flowing through the core — have increased from 5-6 to 10-11 in the past decade, Nuttall said. But increasing the bypass ratio entails increasing the size and weight of the engine. The point of diminishing returns for current engine technology is a ratio of "about 10," according to Rolls-Royce. But NASA and European Union-backed research aims at bypass ratios of up to 15, which could be accomplished, in part, by reducing system weight.

Up to a point, fan efficiency increases with size. The Trent 1000 engine has a bypass ratio of 10 and a fan diameter of 112 inches, compared to the predecessor Trent 700, which has diameter of 97 inches and a bypass ratio of 5. The Trent 1000 increases fuel consumption efficiency by 13 to 14 percent, compared to the Trent 700.

The GEnx improves SFC by about 15 percent, compared to the CF6, an older GE engine in the same thrust class. And the new engine is about 7 percent more fuel-efficient than the GE90, a much higher thrust powerplant. The GEnx has a 111-inch fan diameter, compared with 93 inches for the CF6-80C2. The GEnx’s bypass ratio is around 9.5, compared with about 5 for the CF6. GE also focuses on reducing the fan pressure ratio, the ratio of the air pressure going out of the fan nozzle versus the air pressure coming into the fan. The lower fan pressure ratio, and the resulting lower exhaust velocity, improve propulsive efficiency and SFC, said Steve Csonka, program leader for advanced products for GE Aviation. The fan pressure ratio of the GEnx is 1.5-1.6 versus 1.7-1.8 for the CF6.

GE also emphasizes lightweight materials. New in the GEnx is the use of a composite fan case, which reduces weight and improves corrosion control. The composite fan case alone trims off 350 pounds, compared with a metal version.

Thermal Efficiency

Thermal efficiency can increase by reducing aerodynamic losses in the engine components and increasing the overall pressure ratio (and resulting temperatures) in the core. The higher the pressure, the better the efficiency. But since NOx emissions increase as pressures and temperatures rise, combustor technologies need to adjust. Rolls-Royce cites as critical technologies those that minimize the need for cooling air, improve cooling configurations for blades and improve materials and thermal barrier coatings.

Rolls-Royce has increased the overall compression ratio from the Trent 700 to the Trent 1000 from 33 to 50, which is the same pressure as that experienced half a kilometer beneath the surface of the ocean, Nuttall said. The company also uses computers to improve the design of compressor blades, individually and in relation to each other.

GE likewise asserts high compression ratios. The compression ratio for the GEnx core, alone, is 23:1, Csonka said. The overall pressure ratio — from the free stream of air in front of the fan to the end of the high-pressure compressor, including both the high-pressure and low-pressure spools — is "in the 45 class," he said.

GE focuses on improving the aerodynamics of compressor blades to design out "loss mechanisms." The company has introduced "blisks" or bladed disks, with airfoils that have been machined out of a solid piece of material or have been joined to the disk with friction welding. This approach increases strength and durability while it decreases weight and aerodynamic loss, the company said. "There’s no path for the air to go underneath or around that airfoil any more," Csonka said. A drawback is that operators must use new repair techniques to repair or replace airfoils in the event that they are damaged in operation, GE said.

After weighing the benefits versus the costs, GE decided to use blisks on three of the GEnx’s 10 compressor stages. Even at that moderate rate of use, the blisks end up increasing the overall efficiency of the engine, Csonka maintained. The compressor accounts for most of the 30 percent decrease in the number of components from the CF6 to the GEnx.

TAPS Combustor

The GEnx uses TAPS (twin annular pre-mixing swirlers) fuel nozzles that tailor the mixture of fuel and air entering the combustion zone. There are 22 TAPS fuel nozzles arranged circumferentially around the combustor. The swirling process, whose details are proprietary, "creates a stable, leaner mix of fuel and air which, when burned, maintains a lower temperature," compared to conventional combustors, GE said.

The TAPS swirlers generate tiny vortices around the fuel nozzles in the combustor, manipulating the air from the compressor in such a way that it "burns lean," Csonka said. Minimizing the amount of fuel allows the mixture to burn at a lower temperature. The result: GEnx NOx emissions will be more than 30 percent below those of GE’s CF6 and "about 50 percent" lower than international standards require, the company claimed.

The Trent 1000’s use of heat-resistant tiles to line the combustor also reduces NOx emissions, Nuttall said. "The tiles mean you need less cooling air to cool the combustor," he explained. With less cooling air, which takes up space, the same amount of fuel burns in a larger volume, lowering peak temperature.

Turbines

Farther aft, engine designers focus on air flow through the turbines. The Trent 1000 continues the use of counter-rotating turbines, initiated on the Trent 900, to reduce energy loss. Nuttall compared the advantages of this approach to the operation of two consecutive rotating doors. If the two doors rotate in opposite directions, a person passing from one to the other "doesn’t have to turn at as big an angle to get back in again," he said.

In the GEnx the high-pressure and low-pressure spools counter-rotate. The benefit is gained through the interaction of the two spools: as Csonka put it, the counter-rotation requires less manipulation of the air flow between the exit of the high-pressure turbine and the entrance of the low-pressure turbine. Designers can use fewer vanes to direct the air or use vanes that "don’t have to do as much."

GE also uses computers to analyze the air flow through the turbines. The company has done extensive "end-wall contouring," for example, designing the contours at the base of blades in the turbines to "almost exactly match the flow fields" through the blades, Csonka said. This type of minute tailoring helps to reduce aerodynamic loss, increasing component efficiency.

The Trent 1000 and the GEnx use "crenellations" or "chevrons" on the trailing edge of the nacelles in order to reduce noise. These chevrons help to "premix" the core air and bypass air flows before they exit the aircraft. The GEnx is expected to reduce noise footprint by 50 percent, compared with the CF6.

Maintainability

Although "greenness" is not directly related to maintainability, the new engines are designed to reduce maintenance expense. For example, they collect and report health data, which reduces upkeep costs. The Trent 1000 can report data to the ground while the aircraft is in flight, a concept that was proven on a transpacific flight using a prototype data collection system. Time on wing for the Trent 1000 is expected to be about 20,000 hours.

The Trent 1000’s "tiled combustor" also is designed to increase durability and reduce maintenance costs. The area exposed to high temperatures is lined with 2-by-6-inch, overlapping, heat-resistant tiles. This lining can grow and shrink with temperature cycles, shielding the metal rings of the combustor from the full effects of the heat and reducing cracking stress. When tiles are damaged, they can be quickly replaced, Nuttall maintained. That’s preferable to welding cracks in the combustor.

GE also claimed a number of maintenance advantages for its new engine. The company predicted the new system will have 20 percent longer time on wing than its predecessor, the CF6. The number of parts has been minimized. A cooler-running combustor and new heat-resistant thermal barrier coatings on turbine hardware will extend life. Prior experience with the GE90 fan has built up confidence in the longevity of the GEnx fan module. GE also stressed the modularity and ease of disassembly of the fan module from the propulsor (the engine minus the fan module), making it easier to change out the propulsors and keep an aircraft in service.

The GEnx also incorporates next-generation diagnostics, including a "reasoner" that can detect and report anomalies. This engine information system can, in urgent cases, automatically radio data down to the ground via the aircraft ACARS link. The GEnx’s enhanced diagnostic capability stems from expanded data processing and advanced analytical capability, for both onboard and ground-based systems, rather than from an increase in the number of sensors, GE stressed.

Geared Turbofan

Pratt & Whitney aims to drive engine efficiency higher and pollutant emissions and noise levels lower with its geared turbofan, a system that differs significantly from the Trent 1000 and the GEnx. A gear system, inserted between the fan and the low-pressure turbine "allows us to run the fan and the low turbine at different speeds to optimize the fan speed independently from the low-pressure turbine speed," explained Paul Adams, senior vice president of engineering. The company aims at bypass ratios of up to 16-18 and a noise figure of 20 dB below Chapter 4 of the ICAO standard.

Key partners in GTF development include: MTU Aero Engines for the low-pressure turbine, Avio for the fan drive gear system, Volvo Aero for the turbine exhaust case and Goodrich for the nacelle.

Typically, as fan size grows, the fuel efficiency of a turbofan engine eventually is "overwhelmed by the system impact of the drag of the nacelle and the weight of the engine," Adams said. But the gear system, by decoupling the fan and the low-pressure turbine, allows a much more efficient fan and a much lighter, faster low-pressure turbine. The fan runs about 30 percent slower than would be expected on comparably sized conventional engines, but the low-pressure compressor and low-pressure turbine run three times faster than these components would in comparable turbofans.

The version of the GTF in the 20,000-pound to 24,000-pound thrust class has a fan diameter of 72-74 inches, whereas a conventional engine P&W might design in the 23,000- to 30,000-pound thrust class, without the gear system, probably would have a fan diameter of 65 inches, Adams said.

But the larger the fan diameter, at the same optimized blade tip speed, the slower the rotational speed. So, in conventional turbofan engines, as the fan grows, the low-pressure turbine gets slower and slower. But decoupling the fan and the turbine on the low-pressure spool allows the low-pressure turbine and low-pressure compressor to run much faster than the fan. Letting the low-pressure turbine "go at the speed it wants to go" increases efficiency and reduces the number of turbine stages. Whereas a comparable conventional engine might require seven low turbine stages, the GTF is "down to three stages of low turbine," Adams said. The engine likewise has three, rather than five, stages to its low-pressure compressor. Although P&W adds a gear system, it subtracts major expenses associated with the low-pressure spool.

The company thinks it can get a "step change" in both fuel efficiency and noise reduction with this configuration. It claims the GTF is the only technical solution that can simultaneously improve fuel burn and reduce noise signature. The GTF will be 12 percent more fuel-efficient than any production engine in its thrust class, Adams said. The engine also uses a "rich lean quench system" in the combustor to reduce NOx emissions. The primary zone of the combustor runs rich and then the gas mixture is very quickly quenched with cooling air to minimize NOx generation. NOx emissions will be 50 percent below the current CAEP 6 standard, he predicted.

On the maintainability front, the GTF will be less susceptible to foreign object damage because the fan runs at slower speeds, Adams said. The engine is designed for onwing time of more than 30,000 hours. The GTF also will have 40 percent fewer blades because of the reduction in the low-pressure combustor and low-pressure turbine stages.

NASA

NASA is in the second year of a Subsonic Fixed-Wing (SFW) program, which emphasizes greenness. The nominally five-year project, budgeted at around $90 million in each of the first two years, looks at integrated air frame/powerplants with estimated entry into service dates of 2015 (N+1), 2020-2025 (N+2), and 2030-2035 (N+3). About half of the resources are devoted to propulsion and half to airframes. The agency expects to release a NASA Research Announcement (NRA) for N+3 subsonic and supersonic systems concepts in April of 2008, followed by contract awards in June of that year.

The N+1 work focuses on engines with bypass ratios of "15 or so," including GTF and direct drive concepts, according to Fay Collier, the principle investigator on the SFW program. NASA has collaborated with Pratt & Whitney on wind tunnel testing of its current geared turbofan technology. The agency plans to partner with P&W on an airframe integration and test exercise for its geared turbofan with an eye to minimizing drag.

NASA also plans to collaborate with GE in reviving the open rotor concept. Both parties have ideas about working the noise challenges, he said, and NASA has expertise in noise and performance prediction tools. Other obvious issues with open rotors are vibration and containment. "Our approach is working advanced concepts, generating data and bringing along the design methods and tools to implement the concepts," Collier said. The program plans to deliver design tools, databases, advanced concepts and prototypes.

The open rotor work will "start where we left off in the late 1980s and 1990s," Collier said. According to NASA studies, the concept, compared with a 1997 or 1998 single-aisle aircraft baseline, promises a 20-22 percent improvement in fuel burn, around 20 percent for the open rotor engine itself. With the addition of other technologies, such as laminar flow control, fuel burn could be decreased on the order of 30-35 percent.

N+1 environmental goals include a 70 percent reduction in landing/takeoff cycle (LTO) NOx emissions with reference to the CAEP 2 standard and a 33 percent improvement in aircraft fuel burn, relative to a configuration baselined on the B737 with the CFM56 powerplant. The noise goal is -42 dB "cum below Stage 3." This means that average noise is 42 dB below Stage 3, a classification referring to older platforms such as the B727 and MD80. These goals are set out in a recent NASA SFW program pre-bid document.

Some of the NOx work includes "active control of the combustion process," Collier said. "As the combustion process is occurring, you’d gather and analyze data and adjust the rates of fuel or air injection."

The N+2 strategy focuses on an integrated airframe propulsion concept, the blended wing body (BWB), which features a more smoothly contoured transition from the fuselage to the wing and the absence of a tail. Engines are embedded in the center aft section of the body to optimize reductions in noise, fuel burn and emissions. Because the entire nacelle is not exposed, there is less drag and noise. Ingesting the surface boundary layer into the engine could also improve propulsive efficiency in cruise by 3-5 percent, according to some reports.

But the challenges are legion. "Picture the back of the fuselage," said Rich Wahls, SFW project scientist. "Instead of an engine sitting up above it with a circular entry, now push down to where half the engine is inside the back of the wing body." The air doesn’t go straight to the fan. It flows through a curved, s-shaped duct, and "you have to maintain a certain flow uniformity through the duct." The trick is to minimize the aerodynamic losses and distortions when the air passes through the duct. The curved surfaces cause the air flow to separate, "so you get distorted pressure signals hitting the fan." The resultant uneven loading can be extremely detrimental to the engine.

According to the pre-bid document, N+2 emissions goals include NOx LTO of 80 percent against CAEP 2 and a 50 percent reduction in fuel burn, compared with a baseline of the B777 with the GE90 powerplant. The noise goal is -52 dB cum below Stage 3.

The N+3 goals were still in definition at the time of this report, but will continue the program’s emissions, fuel burn and noise themes, with more aggressive targets. The noise target is such that "you could be in your backyard outside the gate of the airport and carry on a normal conversation," Collier said. The NOx reduction goal is better than 80 percent LTO plus work to mitigate the formation of aviation-induced cirrus cloud cover, which is thought to contribute to global warming. N+3 also aims at a better than 70 percent reduction in fuel burn relative to a B737/CFM56 baseline and to explore non-fossil fuel energy sources.

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