Rolls-Royce is now exploring future engine technologies that, although challenging, are key to the ambitious Advisory Council for Aerospace Research in Europe (ACARE) goals for 2020 in terms of nitrous oxides (NOx) and carbon dioxide (CO2) emission cutting and noise reduction. These technologies range from heat exchangers to shape memory alloys and magnetic bearings. As a closer step toward greener engines, the UK-based manufacturer is pushing for an ANTLE 2 (affordable near-term low emissions) demonstration program, after the success of the current European research program.
The goals set by the Advisory Council include a 50-percent reduction of CO2 emissions per passenger mile, an 80-percent cut in NOx and halving the current perceived average noise levels. These targets are for new aircraft, relative to 2000 levels.
According to Ric Parker, director of research and technology, meeting the 50-percent fuel burn target needs changes in all areas–engines, airframe, as well as air traffic management and operations. Engine improvements may account for 20 percent, whereas airframe possible design solutions may yield a 30-percent contribution. ATM and operations may account for the remaining 10 percent. “The first two imply engine and aircraft people working more closely,” Parker said.
Optimizing ATM and operations may result in more direct routes. More radically, it may also mean avoiding ultra-long-haul flights (the quantity of fuel needed makes the aircraft heavier, which in turn makes fuel burn spiral). At the other end of the spectrum, too-short flights are not optimal either when ground transportation is available.
Ways of drastically cutting engine fuel burn (and therefore CO2 emissions) may be found in advanced cycles. In a recuperated cycle, for example, a heat exchanger takes some heat of the exhaust nozzle and brings it back to the air entering the combustor. This increases the efficiency of the combustion process. In a new-generation engine, it would reduce the optimum overall pressure ratio and, according to Parker, improve part load performance. It would improve the performance of small engines. Finally, it would reduce the number of stages in the low-pressure turbine and its noise.
However, the performance benefit for large engines would be small. In addition, higher temperatures hold the potential for higher NOx emissions, Parker insisted. Such a system also adds cost, weight and complexity.
Lowered NOx Gases
An intercooled engine, where compressed air is cooled between the low- and high-pressure compressors, would bring advantages like smaller core components and lower NOx emissions. Performance would be improved, too, and cooling air would be cooler. But it would need a higher optimum pressure ratio and would translate into more compressor variables. A novel engine configuration might be required.
Combining recuperation and intercooling would improve specific fuel consumption at all thrusts. It would also have the potential for NOx reduction. The parts count would be reduced and lower cost materials might be used. However, there would also be a significant weight penalty, Parker pointed out.
Exploring these advanced cycles is at a quite early stage for aero engines. Rolls-Royce’s WR21 marine gas turbine, which powers the Type 45 Frigate, uses inter-cooling and recuperation. But the latter systems triple the size of the base engine. Heat exchange technology is therefore far from aviation standards in terms of volume and weight. The needed ultra-efficient, lightweight heat exchangers are still some way off but seem a credible candidate for 2020 engines.
Another, more exotic cycle advance might be constant volume combustion. This process uses explosive waves. “It might result in an efficiency improvement but more work is required,” Parker said. Constant volume combustion is not currently attractive due to practical considerations, he emphasized.
In alternative power sources, hydrogen used through a fuel cell has been much talked about. But it appears unlikely to produce power on an aircraft (except as an auxiliary power unit) in the short- or mid-term. “The fuel cell is an efficient converter of air and hydrogen into electricity with minimum pollution, water vapor being the main by-product,” Parker noted. However, its weight is so far prohibitive. Even car makers, in spite of repeated promises, have not succeeded in bringing a fuel cell to market yet. Rolls-Royce has this year launched Rolls-Royce Fuel Cells Ltd. but is initial products are aimed at land-based electrical power production in the 1- to 10-MW range. “We have a long way to go to miniaturize,” Parker concluded.
Apart from novel cycles, new enginearchitectures may be studied such as geared fans for ultra-high bypass ratios. Fans could become aft fans or contra rotating fans. Last, but not least, a return to open rotors may yield higher efficiencies. It would be at the expense of noise, as propellers are less quiet than ducted fans.
Better noise control without drag penalty might be achieved thanks to shape memory alloys. These very particular metal alloys can change the shape of a part according to external conditions such as temperature. One could therefore adapt the shape of a noise-reducing device to flight conditions.
For example, a saw-tooth-shaped aerodynamic device at the rear of the nacelle can control the mix of the bypass stream and the ambient air. But the turbulence generated goes in the wrong direction for engine efficiency. With a shape memory alloy, serrations could be deployed at takeoff and climb, when noise is critical. They would then be withdrawn at cruise.
Shape memory alloys have already found applications for small actuators in other sectors. However, according to Parker, no aero application is to be expected before 10-15 years. “There is still a lot to understand, such as fatigue issues,” he said.
Simultaneously with exploring future technologies, Rolls-Royce is trying to define the outlines of its mid-term research work. ANTLE, a Rolls-Royce-led European program, completed testing last May at Spain's INTA (Instituto Nacional de Técnica Aeroespacial). “These 30 hours of trials will translate into many more hours of analysis,” Parker commented. The full-size engine demonstrator is based on the Trent 500 and is aimed at reducing CO2 and NOx emissions. It has notably demonstrated more efficient components and cleaner combustion, according to Parker.
Rolls-Royce engineers and research managers are pushing for an ANTLE 2 in the European seventh framework program (FP7). “ANTLE has been part of the FP5 and, in the current FP6, the Vital research program will rig-test several new components-ANTLE 2 could integrate these components,” Parker suggested. An ANTLE 2 engine could run in seven or eight years.
Another European research program called power-optimized aircraft, in which Rolls-Royce is participating, aims at “a step change in engine performance” by the application of “more electric” and “all electric” technologies. For example, magnetic bearings could reduce friction in the turbomachinery. Rolls-Royce’s U.S. research and development department in Indianapolis, Indiana, has already designed such devices, Parker said.