Eco-friendly engines

In an effort to improve performance, engine designers are focusing on reducing three things: noise, CO2 and Nox.

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By  Colin Baker Published  April 11, 2006

|~||~||~|The aviation industry is under huge pressure to improve its environmental performance, and while airframe design, land planning and operational procedures all have a role to play, improvements in engine technology will be crucial. That is not to say that great strides haven’t already been made. As Francis Couillard, general manager for environmental affairs at Snecma, notes: “Since the 1960s, engine noise has been reduced by 75%, or 20db, which is a significant amount. Fuel burn, which produces CO2, is down by 60% to 70%. And Nox [nitrogen oxide] has also been reduced significantly.” He notes, however, that “technological progress is now more and more difficult. You have to deal with trade-offs and maintenance costs.” Engine manufacturers have to deal with three key issues when it comes to the environment: climate change, noise, and air quality around airports. As Couillard points out, it is becoming increasingly difficult to deal with each of these areas without having an adverse knock-on affect on one, or even both of the other factors involved. In Europe, where the demands on aviation from environmentalists are greatest, a joint government/industry Advisory Council for Aeronautics Research in Europe (ACARE) has set goals that manufacturers are using as a benchmark for technological development. These include a 50% reduction in fuel burn by 2020, compared with 2000, an 80% reduction in Nox over the same time period, and a 50% reduction in perceived noise. Engines are expected to account for 20% of the reduction in fuel burn, with 20% coming from improved airframe design, and 10% from operational efficiencies. The halving of noise will see a 30dba cumulative reduction across three measuring points: approach, landing, and sideline. Of these, approach is seen as the toughest nut to crack from an engine manufacturer’s point of view. Couillard estimates that noise on approach is split roughly 50:50 between engine and airframe. Colin Beesley, head of environmental strategy at Rolls-Royce, says that the company has been working with airframe manufacturers on this issue. “One idea is to use part of the airframe as a shield to stop some of the noise reaching the ground,” he says. Rolls-Royce has been leading the pan-European affordable near-term low emissions (ANTLE) research programme. Last year, Rolls-Royce used a highly-modified Trent 500 as a technology demonstrator, putting the engine through a 30-hour test programme. Targeted at three-shaft engines above 40,000lb thrust, and with all-new components aft of the Trent 500 fan and low-pressure compressor, the programme aims to demonstrate a 12% cut in fuel burn compared with 1990s technology, a 12% cut in carbon dioxide emissions and 60% lower nitrous oxides. Beesley explains that the manufacturer has been trying out a new combustor. “The tricky thing with low-emissions is that in order to get increased efficiency you have to push up temperatures,” he says. “This leads to higher thermal efficiency, but unfortunately, at those temperatures, the nitrogen in the air burns and gives off nitrogen monoxide.” Rolls-Royce has been trying to get around this problem by pushing the air through the engine as quickly as possible, with a stable flame. Beesley said the ANTLE programme has proved to be “broadly in line with its targets.” The manufacturer is now hoping to get funding for its environmentally friendly engine (EFE) programme, which will pick up from where ANTLE left off. Rolls-Royce’s latest engine, the Trent 1000, has been chosen as one of the power plants for the Boeing 787. The Trent 1000 is aerodynamically based on the Trent 900, but the major difference in the 787 design is that significantly higher electrical power is needed for the aircraft’s pressurisation system, which will be supplied by electrically powered compressors instead of using air bled directly from the HP compressor. This has proved to be to Rolls-Royce’s advantage. Tests have shown that taking the generator drive from the intermediate instead of the HP shaft improved compressor aerodynamics and resulted in significantly lower idle speeds, reducing noise while taxi-ing, as well as delivering a 50% reduction in fuel burn during approach. Beesley says that the three shaft design gives more flexibility, enabling power to be taken off the intermediate shaft to meet the electrical requirements of the 787. The other engine specified for the 787 is GE’s GEnx, which will also power the Airbus A350. This will make use of materials such as powdered metal alloys and carbon fiber composites, and will have a high bypass composite fan design. As a result there will be fewer smog-causing emissions than the maximum allowed by 2008 international standards (94% fewer hydrocarbon emissions and 57% Nox emissions) while fuel consumption is 15% lower, while noise will be 30% less than current GE-powered aircraft. GE says the GEnx aircraft engines projected to be sold in the next 20 years will emit an estimated 77 million fewer tons of greenhouse gases than would have been produced by older comparable engines. The manufacturer adds that if today’s fleet of 200-300 passenger aircraft with comparable power plants had GEnx engines, annual carbon dioxide emissions would be reduced by an amount equal to: removing more than 800,000 cars from the road for a year, adding more than 1.2 million acres of forest, 4% of the annual carbon dioxide emissions produced by all U.S. commercial aircraft, and savings of nearly 90 million gallons of fuel. GEnx engines include GE’s combustor called the Twin Annular, Pre-mixing Swirler (TAPS). GE, which has been maturing TAPS combustor technologies for almost a decade, reports that rig tests this year on the GEnx TAPS combustor have demonstrated very promising results. The manufacturer says that, at comparative thrust levels, GEnx Nox emissions will be more than 30% lower than the Nox emissisons of GE’s CF6 engines powering commercial widebody aircraft. In addition to lowering ozone-depleting NOx emissions, GEnx’s TAPS combustor will produce low levels of carbon monoxide, and unburned hydrocarbons. TAPS also has the potential to significantly reduce soot and related exhaust particulates. Also, because the TAPS combustor burns at lower temperatures, it will improve the life of components further downstream in the GEnx engine. CFM International, the joint venture between General Electric and Snecma, has also been active in the research arena. At last year’s Paris Air Show, the manufacturer announced a new technology acquisition and development program, named LEAP56TM (Leading Edge Aviation Propulsion). “We believe that any new products entering the commercial market over the next decade will need to be substantially better than those they will replace,” said CFM president Pierre Fabre at the show. CFM says it is aiming to find a balance between an architecture that that will provide lower fuel burn while keeping simple designs that the manufacturer says will keep maintenance costs down. The company adds, however, that the greatest challenge will come from environmental requirements. CFM has developed a list of initial technologies to be evaluated, validated and demonstrated through the LEAP56 program. Basic engine design technologies being considered include lightweight structures; advanced composite fan blade technology; power generation; and advanced 3-D compressor and turbine technology. CFM has been facing the same issues as Rolls-Royce in terms of trade offs. “You need to have lower temperatures in the combustion process to reduce Nox, but if you want to improve fuel efficiency, you need higher temperatures,” notes Couillard at Snecma. “To reduce specific consumption, you need higher bypass ratios and a bigger fan, although this means more weight. It is not a question of sacrificing one for the other, however. It is a question of optimisation.” Snecma is heading the Eur 90 million (US $108.3 million) VITAL project, focusing on the development of technology to reduce engine noise and carbon dioxide levels. This is building on previous projects involving Snecma, including SILENCER and the Efficient and Environmentally-Friendly Aero Engine (EEFAE). The project is also aiming at the ACARE targets, and will concentrate on the engine low-pressure system, including counter-rotating and lightweight fans, and highly loaded and reduced blade-count turbines. The manufacturer explains that the resultant weight reductions will enable development of very-high bypass-ratio turbofans that reduce noise by 5-8dB while cutting CO2 emissions. Pratt & Whitney, meanwhile, recently conducted demonstrations of a geared turbofan (GTF) engine, in a project which also involves Boeing and NASA. This was carried out with the next generation of narrowbodies in mind, which are expected to have similar characteristics to the likes of the 787 in terms of electronics. Boeing itself is planning a third stage of its ongoing Quiet Technology Demonstrator (QTD) programme for around 2008. Unlike the first two QTD stages, which focused on the 300-plus seat market, the third stage will look at the 150 seat sector, potentially proving a nice fit for the Pratt & Whitney initiative. There have been concerns about the maintenance costs of the geared fan, although Pratt & Whitney is confident that these issues can be overcome. Pratt & Whitney is also due to work with NASA on the Quiet Aircraft Technology/Ultra-Efficient Engine Technology (UEET) programme, and says the geared fan will exceed the requirements of this programme. Meanwhile, Pratt & Whitney’s low emissions TALON II (Technology for Advanced Low NOx) combustor is also being certified for the PW4158/PW4168 engines. TALON is designed to reduce NOx, CO and HC emissions on PW4000 engines beyond that of previous generation TALON I combustors. On the PW4168, TALON II reduces NOx about 30% from 1995 engines. The design includes a combustor liner and an advanced quick quench zone designed to ensure the TALON II combustor is effective at controlling NOx emissions. The new liner is designed to tailor cooling to the varying heat loads inside the combustor. It directs cooling to the combustor’s hot regions and removes cooling where it’s not needed. Since less air is needed to cool the combustor, more is available to mix with fuel, thus improving emission levels. More efficient cooling also maximises the TALON II combustor’s life. Environmental concerns now play centre stage in every manufacturer’s research and development, although clearly it’s a question of gradual progress rather than quantum leaps. All manufacturers are at pains to point out that there is only so much they can do — others will have to play their part as well. Collaboration with airframe designers has a major role to play, particularly on the thorny issue of noise generated on approach. Manufacturers make clear, however, that in order to achieve the targets that are being set by the likes of ACARE, and which are likely to bet set on a legal basis by ICAO and, perhaps independently by the European Commission, areas such as air traffic management, land planning and operational procedures will have a vital role to play. In the meantime, the battle to develop engines that are capable of battling on all three fronts — noise, CO2 and Nox — goes on.||**||

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