New combustion chambers tame pollutant emissions - Rolls-Royce
The demands placed on modern aircraft engines are high. They need to be powerful. And efficient. And, of course, as low in emissions as possible. To master this balancing act, Rolls-Royce is driving forward the Pearl programme in Dahlewitz near Berlin. The combustion chamber, in which the energy of the fuel is converted, is the centrepiece - and the field of activity of Dr Thomas Dörr's team.
"Our aim was to further reduce the emission of nitrogen oxides and particles from the combustion chamber," says the aeronautical engineer. "On the one hand, this was necessary because the process temperatures and pressures continue to rise with the efficiency of new engines. And this in turn leads to the increased formation of nitrogen oxides and soot," he continues, adding: "On the other hand, the statutory emission limits are also being constantly reduced, meaning that engines have to operate with ever lower emissions per se."
Rolls-Royce
The secret is stoichiometry
The solution lies in the air - in the truest sense of the word. In addition to the fuel, the air that is sucked into an engine plays a key role. And in two respects. Firstly, it is necessary in the core engine so that the fuel can burn at all. Secondly, it cools the combustion chamber to a manageable level. Both are relevant not only for engine performance, but also for pollutant emissions. This is because, as with all chemical reactions, the quantity of the individual end products in the combustion of fuel depends on the temperature on the one hand and on the quantity of the starting materials on the other. Chemists refer to the latter as stoichiometry. If you adjust both of these factors in the right way, you can get closer to the dream of a powerful but low-emission engine.
"With the Pearl family, we have achieved a significant reduction in cooling air, which is now available to us to control the stoichiometry," says Dr Thomas Dörr. A novel and radical approach was taken to the mixture within the combustion chamber to control this combustion stoichiometry, he says, which in turn has led to new challenges. "When it gets hotter and hotter in the engine, but at the same time we have less cooling air available, the combustion chamber is pushed to the limit." The team had to come up with something. This approach was also radical. The inside of the combustion chamber is lined with ceramic-coated shingles, through whose tiny channels the cooling air flows and removes the engine heat from the component. Normally, such shingles are moulded and then cylindrical channels are drilled into them with a laser. It would be ideal to set the cooling holes at an angle so that the path of the cooling air is as long as possible and a lot of heat can be dissipated. However, only straight holes can be drilled with a laser. "That's why we opted for 3D printing," says the aerospace engineer. "This makes it possible to create even complicated, fan-shaped and curved cooling holes, so that heat dissipation and the cooling film effect can be optimised." The first engine with a combustion chamber with these shingles will be the Pearl 10X, which has just completed its flight tests.
Rolls-Royce
Revolution instead of evolution
The development steps towards new engines are often evolutionary. On the one hand, this concerns the geometry of a component or changes to the ratio of fuel to air. But with the Pearl engine family, Rolls-Royce and Dr Thomas Dörr were planning a revolution. In the project, he and his team took on almost everything to do with the combustion chamber. And LuFo helped them. "The Pearl combustion chamber was developed over several LuFo projects," he says. "It started with LuFo IV and LuFo V programmes for the technology development of new combustion chambers for the Pearl engine family. In the current LuFo VI programme, we are already working on the next-generation technology, and we would also like to tackle future developments with LuFo support." The funding programme helped to develop the various technologies to such an extent that they were able to prove themselves in various combustion chamber component tests on the test benches. But that was by no means the end of the story for Rolls-Royce. Dr Thomas Dörr and his team continued to drive the development as part of the engine development programmes through to series production, with the result that the first two stages of the Pearl family, the Pearl 15 and the Pearl 700, are now certified and in use. This step will soon be taken for the latest member, the Pearl 10X. "Such a revolutionary step, in which we are rethinking and developing technologies from scratch, would hardly have been possible without LuFo," believes Dr Thomas Dörr. "For me, it's a big plus point that LuFo also allows us to take fundamentally different paths without the pressure to deliver a finished product in three or four years."
Of course, financial support is not everything. The network of partners that can be established and maintained through LuFo projects is also invaluable. And, of course, the expertise that can be built up over the years. "Back then, we worked on the first LuFo programme with four or five people in the P&T department," he recalls. "We now have a combustion chamber team here in Dahlewitz that is highly regarded throughout the Group. This development of expertise was significantly supported by LuFo funding programmes."
Text: Kai Dürfeld
Projektträger Luftfahrtforschung
Contact:
Rolls-Royce Deutschland Ltd & Co Kg
E-Mail: rrdinfo@rolls-royce.com
www.rolls-royce.de