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Technology + Science Next-generation engines – quieter and thriftier By Martina Vollmuth Commercial aviation is riding a fast growth track: industry experts see air traffic doubling by 2020. To contain the environmental toll this stellar growth will take, the industry is working on thriftier, quieter and cleaner aircraft. The Advisory Committee for Aeronautics Research in Europe (ACARE) wants airliners by 2020 to burn 50 percent less fuel per passenger mile. A daunting challenge indeed, and companies like MTU Aero Engines have for years been honing new technologies to meet it. The target is ambitious: ACARE wants commercial jets not just to consume less fuel, it moreover mandates noise to be halved and emissions slashed 80 percent, from present levels. “Engines will have to bear the brunt of it,” explains Dr. Günter Wilfert, a technology strategist at MTU. Their fuel consumption will have to come down 20 percent, noise levels six decibels and oxides of nitrogen emissions 80 percent. Engine builders have two alternative options to achieve improvements. “The one is to optimize existing propulsion concepts, and the other is to develop entirely new technologies,” explains Prof. Dr. Klaus Broichhausen, chairman of the newly founded Bauhaus Luftfahrt think tank. Optimizing means that technologists and researchers dissect components—compressors, combustors and turbines—to see where they might be able to squeeze out improvements. As a rule of thumb, if you raise the efficiency of a component by one percent, you reduce fuel consumption by up to one percent. In Broichhausen’s estimation, tweaking the core components—high-pressure compressor, combustor and high-pressure turbine— gets you a fuel reduction margin of totally two to three percent, while optimizing a fan or low-pressure compressor and low-pressure turbine will net you three to four percent. Improvements can be achieved also by the way engines are operated. Today’s aircraft engines are not just delivering thrust but electrical power as well, providing the airframe with current to energize the air conditioning system, onboard electronics and hydraulic systems. Says Broichhausen: “If you relieve the engine of these auxiliary tasks, you could trim it down to the basics, making it thriftier.” The power supply for the airframe could then be provided by a standalone power generator, and the engine could be electrified. Substituting hydraulic components with electrically actuated equipment, too, would save five to seven percent fuel. The turbine center frame has a major impact on the engine’s overall efficiency. Its design, therefore, demands painstaking care. 10 REPORT REPORT 11
Technology + Science High-speed low-pressure turbines are among MTU Aero Engines’ chief domains. They are key components of every geared turbofan. Yet all potential refinements taken together would not be sufficient to achieve the ambitious ACARE targets; new technologies are needed. Broichhausen cautions: “The new engine concepts will work only provided the various components are operating at their very best.” To make them do that, research The ATFI geared turbofan technology demonstrator is a joint MTU, Pratt & Whitney Canada and Avio program. has been underway for years. Jointly with other engine manufacturers, scientists and research institutes, MTU is working on advanced technologies, with the most likely solutions promised by the geared turbofan, by heat exchanger technologies and by the ‘active’ engine. Geared turbofan One of the most intriguing next-generation programs is the Advanced Technology Fan Integrator (ATFI), a joint effort by MTU, Pratt & Whitney, Pratt & Whitney Canada and Avio. This demonstrator features a reduction gear intervening between the fan and low-pressure turbine, whereas on conventional engines the two components are rigidly connected by a common shaft. The gearbox decouples the fan from the turbine, enabling the large-diameter fan to run much slower than on conventional engines, and the turbine much faster. In this fashion, both components can operate at their respective optimum speed, lending the geared turbofan engine outstanding efficiency and quietness. Heat exchanger The geared turbofan can be made still fuelthriftier if it is recuperated using a heat exchanger. MTU has demonstrated the utility of recuperated geared turbofans under a European technology program dubbed Clean (Component validator for environmentally friendly aero engine). In this arrangement, the heat exchanger picks up residual heat in the exhaust gas stream and dumps it in the air issuing from the high-pressure compressor, before it reaches the combustor. Potential fuel savings run as high as 20 percent. The idea itself is not new, but heat exchangers have not caught on, owing to their bulky size and poor efficiency. MTU has succeeded in Diagrammatic sketch of a recuperated geared turbofan. This novel propulsion concept holds promise of drastic fuel savings and significant noise reductions. developing new almond-shaped recuperator tubes that can be packed very densely while providing a sufficiently large heat transfer area. After five years in the making, the Clean demonstrator in February 2005 excelled at the Stuttgart altitude test facility, hitting all targets. The intercooler is a crucial component of the fuel-thrifty recuperated engine of the future. It is being developed in partnership with Rolls-Royce under a new joint project. The NEWAC (New Aero Engine Core Concepts) program will probably be launched before year-end. Under the program, with MTU as the lead company, the British engine maker and other European manufacturers, research institutes and universities will be exploring new core engine technologies. Active engine MTU’s role under NEWAC is to develop a socalled ‘active’ core engine. “An engine of that kind will have, among other novel features, a smart compressor that adjusts to the respective flight regime,” explains MTU’s NEWAC program manager Dr. Günter Wilfert. This is achieved, for instance, by active clearance control (ACC), where the width of the gap between the blade tips and inner compressor casing wall is actively controlled. Early surge detection and surge control, too, will allow the compressor to operate closer to the surge limit. The active engine also boasts cooling air control: because cooling air extracts energy from the engine, the cold air influx should be metered judiciously in accordance with engine power Highly efficient heat exchangers like MTU’s can massively help reduce fuel consumption. Next-generation compressors will be clearly more efficient if actively controlled throughout. First tests to that effect are already underway. output—enhancing engine performance and efficiency. Despite these extensive research endeavors, the engine to satisfy the whole litany of ACARE requirements is still very much in the future. It will take time to mature the various technologies, and experts doubt a prototype will be forthcoming before 2015. From there, series production would still be ten or more years away. For additional information, contact Dr. Günter Wilfert +49 89 1489-4347 Further information is available on the Internet at: www.mtu.de/report 12 REPORT REPORT 13
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