Engine builders take ‘Vital’ step on environment
European engine manufacturers are joining forces on Vital, a research program co-funded by the European Commission that aims to reduce carbon dioxide (CO2) and noise emissions by commercial aircraft.
Snecma Moteurs and the EC officially launched the program in January. Focusing on the low-pressure spool, the ?90 million ($110.7 million) effort accompanies a research program called CLEAN, which concentrates on the high-pressure spool. Since 2000 CLEAN has demonstrated several new technologies to reduce CO2 and nitrous oxide emissions.
Fifty-three partners work under the Vital banner. They include major European engine manufacturers such as Rolls-Royce, Volvo Aero, MTU, ITP, Avio, Techspace Aero and Rolls-Royce Deutschland. Airbus too has joined. The project accounts for part of the EC’s Sixth Framework research program, scheduled to last four years into late in 2008.
The team does not plan to build a full engine demonstrator–only partial test rigs. It expects trials to begin during the first half of next year. “We’ll validate test results by transposing them at the engine level,” Jean-Jacques Korsia, Snecma’s co-coordinator for Vital, told Aviation International News. “We’ll determine which kind of aircraft contra-rotating fans are better for short-, medium- or long-haul airplanes.”
The Vital partners target a 7-percent reduction in CO2 emissions [the main contributor to the greenhouse effect] for a given thrust. CO2 emissions occur in proportion to fuel burn. In this area researchers hope to see significant improvement from a higher bypass ratio–in other words, an increased fan diameter. A highly loaded low-pressure turbine will also contribute to the effort. MTU, Avio and ITP lead the turbine project, which centers on reducing the number of blades and lowering noise levels.
The trio hopes to reduce noise by five to eight decibels. “Contra-rotating fans break down the aerodynamic load on two blades. Hence a lower rotation speed for a given thrust,” Korsia said. On conventional fans, blade tips running at transonic speeds are a major source of noise. Snecma and Rolls-Royce will endeavor to determine whether contrafans are worth the trouble.
Again on noise, researchers will study nozzles with “fluid” chevrons. Chevron (or sawtooth) nozzles better mix the cold and hot gas flows at the engine exhaust, thereby reducing noise. But an advantage at takeoff becomes a drawback at cruise, as the device creates significant drag. Also, noise does not present a huge problem at altitude. Instead, injecting air at the trailing edge of the nozzle can generate fluidic chevrons that the operator can turn off during cruise.
Both the contrafans and the increased diameter reduce noise levels. However, “they are challenging in terms of weight,” Korsia acknowledged. Therefore, the researchers need to find ways of lightening other parts of the engine. For example, Snecma is studying how it can make thrust reversers lighter. “A bigger engine drags more and therefore needs less reverse thrust,” Korsia said. Volvo Aero will work on lighter casings.
In addition, Snecma and MTU will study new materials for engine shafts, such as silicon carbide fiber-reinforced titanium, a composite material. “A bigger fan implies more torque,” Serge Eury, Snecma Moteurs’ head of research and technology, pointed out. This could in turn mean a bigger shaft, which would increase the engine’s diameter and make size and weight spiral. Hence the search for materials that can handle more torque at a given shaft diameter.
The CLEAN research program, launched under the EC’s earlier Fifth Framework, has pioneered at least two engine technologies. First, the lean prevaporized premixed (LPP) combustor. “Combustion of a gaseous fuel is colder than that of droplets of fuel, efficiency being equal,” Eury told Aviation International News. Lower temperatures mean less nitrous oxide. Tests showed that the LPP concept met the ACARE (Advisory Council for Aeronautics Research in Europe) goal of 80 percent less nitrous oxide than the CAEP/2 standard [the Committee on Aviation Environmental Protection works inside the International Civil Aviation Organization].
However, one obstacle remains before CLEAN finds its way to an application. The LPP combustion is unsteady at low engine speed. “We solved the problem with a double annular architecture–a first ring of injectors specializes in idle, whereas a second one is used at full throttle,” Eury explained.
No one knows for sure whether the improved performance is worth the additional engine complexity. In the longer term, anyway, researchers hope further work will stabilize the LPP combustion. The engine would then get rid of one ring of injectors.
Active surge control might prove to be a second pioneering technology. In the high-pressure compressor, sensors detect surge precursors–vibrations and pressure fluctuation. The digitally controlled system can then open relief valves just before the compressor stalls. It can therefore run with less surge margin and therefore improve efficiency by “a few percent,” Eury said. This translates into lower fuel burn and, in turn, less CO2 emissions.
On the CLEAN demonstrator, the active surge control prevented surging in several instances. “We still need to improve sensor measurement reliability,” Eury admitted. Valves should also be made simpler and lighter.
MTU, which leads the project with Snecma, wants to introduce a heat exchanger. Under the “intercooled recuperative aeroengine” concept, exhaust nozzle heat would increase the temperature of the air entering the combustor, while air gets cooled between the low-pressure and high-pressure compressors. But CLEAN tests have confirmed the exchanger is too heavy for an aeronautic application. Instead, it might find a use on ground power stations or marine turbomachines.
Beside Snecma and MTU, Volvo Aero and Avio have participated in CLEAN.