Time for engine design to lead the way
Just how cost-effective and low-emission can the passenger aircraft of the future be? The answer depends to a large extent on the next evolutionary steps in the development of engines. Everywhere you look, developers are working on new projects and design concepts – and aerotec sheds a light on what might be about to revolutionize engine design and construction.
For more than twenty years, engineers have been working hard to make aircraft lighter, more frugal and quieter. The average kerosene consumption of aircraft has been reduced by nearly thirty percent during this time. “The airlines wanted to fly their planes as inexpensively as possible, and they wanted to avoid trouble with legislators and residents suffering from noise emission”, explains Odilo Mühling, spokesperson of Munich engine manufacturer MTU Aero Engines. Of course, the engines are a major contributing factor towards fuel consumption. For example, reducing the weight of an engine by 250 kg will save 1.2 tons of CO2 und 400 kg of kerosene on a flight from Frankfurt to New York. MTU is aiming to develop even more frugal engines during the next few years – and has set up the Clean Air Engine (CLAIRE) program to do just that.
The program promises major benefits, as MTU is the largest engine manufacturer in Germany and the sixth-largest in the world, with an annual turnover of around Euro 2.7 billion. So where to start? The basic principle of generating thrust has found its optimum design so far in the form of the dual-circuit engine with an additional cold bypass flow. This cold bypass creates the bulk of the thrust. In recent years, all manufacturers have attempted to reduce the jet speed and increase the air mass throughput in the engine. Overall, this leads to an increase in propulsion efficiency. A key parameter here is the bypass ratio (BPR). This figure describes the ratio of cold to hot mass air flow in the dual-circuit engine. One example: if eight times more cold air flows over the engine core than flows through it at the same time, then the bypass ratio is equal to eight. As a rule of thumb, the higher the bypass ratio, the more efficiently the engine runs.
Of course, it is not only engine manufacturers who have to deliver in terms of the environment. In 2001, representatives of the entire aviation industry came together and committed themselves under the banner of “ACARE Vision 2020″ (Advisory Council for Aeronautical Research in Europe) to a list of highly ambitious, self-imposed targets which are to be met by 2020: a halving of aircraft noise, and a reduction of NOx emissions by 80% and CO2 emissions by 50%. These targets for improvement are based on the state of the art in 2000. The CLAIRE program is intended to make a significant contribution to this in three stages. CLAIRE I focuses on the gearbox turbofan (GTF). This is being developed in conjunction with P&W and will be ready for use by 2013. CLAIRE II will see development of the Counter-Rotating Shrouded Integrated Propfan (CRISP) to series maturity by around 2025, after which the roadmap has scheduled the third stage: CLAIRE III with IRA, the Intercooled Recuperated Aeroengine. This will then be a CRISP engine with intercooler and exhaust gas heat exchanger, which is due to be flight-ready by 2035. You might ask: why go to such lengths? At this point, it might be helpful to look at some of the physics involved. To date, the main approach for engine optimization has been to keep increasing the bypass ratio, i.e. to steadily increase the amount of cold air fed over the hot engine core. If no other aspect of the design is modified, the engines will eventually keep increasing in terms of diameter. The air resistance of such large engines would in turn hugely increase fuel consumption.
This is because an engine fulfils its role as an integral component of the overall system. However, the weight and aerodynamic resistance of the aircraft play a key role in terms of energy consumption. This means that – in terms of only increasing the bypass flow ratio – there are limits to the available scope and therefore to the size of the front surface of the engine. This is why MTU believes that the gearbox turbofan is the best solution. In order to increase the propulsion efficiency of engines with a bypass ratio set to be greater than ten, it is necessary to uncouple the fan from the shaft of the low-pressure turbine with the aid of a reduction gearbox. This is because – while the fan offers very high efficiency at low rotational speeds – the low-pressure turbine only reaches its best efficiency at very high rotational speeds. By uncoupling these components, they can both be operated within their ideal rotational speed range. Thanks to the increased efficiency of the low-pressure turbine that is achieved in this way, the number of required turbine stages can be reduced.
The advantages include a shorter design, reduced weight and – last but not least – reduced production and servicing costs for the low-pressure system. A transmission ratio of approximately three permits a fast-running turbine that drives a large, slowly rotating fan and thereby enables bypass ratios of more than ten to be achieved. A slowly rotating fan in conjunction with a low fan pressure ratio (FPR) helps to further optimize the propulsion efficiency. The gear fan has the potential to solve the problems of fan noise on the one hand and the efficiency demands which are placed on the low-pressure turbine on the other hand.
It reduces fan noise, which is produced approximately proportionately to the fourth power of the fan speed, and jet noise, which increases with the jet speed. On account of the high circumferential speeds and the resulting lesser torque of the low-pressure turbine, the diameter for the low-pressure shaft can be made smaller. This facilitates system integration and reduces the overall weight. This technology has been at work since 2007 in a joint demonstrator project of P&W, MTU and Avio which is being used to underline the operating characteristics of the design and represents a highly promising approach to reducing noise.
A GTF gearbox developed by Avio was able to successfully complete its ground testing on 13 November 2007, and the entire engine has already been mounted on a B747 and on an Airbus as flying engine test benches. The next step on the MTU roadmap is the CRISP, the Counter-Rotating Shrouded Integrated Propfan. These drives were already in the focus of manufacturers as far back as the end of the 1980s and the start of the 1990s. At the time, the intention was to counter the oil price explosion through significant improvements in terms of propulsion efficiency. Today, the propfan concepts have come once again into focus, both for reasons relating to fuel costs and for environmental reasons. Then as now, the unsolved noise problems associated with open propfan engines stood in the way of market introduction. Integration in the wing or fuselage also has its problems, as the reduction of the low-frequency sound components acting on the cabin and the necessity of a power transfer system in the form of a high-performance transmission ratio for increased transmission ratios are other challenges which still need to be solved.
MTU has remained unwavering in its pursuit of the CRISP concept, which combines various technological elements from a conventional turbofan design with new propfan elements. For CRISP, one major advantage – particularly in terms of noise – is found in the use of conventional shrouding. For MTU, it is also decisive that both of the design concepts GTF and CRISP are characterized by similar basic technological requirements for the associated core engines. In contrast to just one large fan, two counter-rotating fans have the potential to move a large air mass flow at high speed through the engine at low fan rotation speeds. As the fan pressure ratio is increased through two fan stages, fewer rotor blades are needed in each stage. Consequently, the surface ratio in the fan is improved, and at high axial flow speeds the aerodynamic impact effects are reduced. The engine runs more quietly and requires less servicing. The final step in the MTU roadmap is the IRA, the Intercooled Recuperated Aeroengine – an intercooled engine with exhaust gas heat exchanger. IRA combines a – from a thermodynamic point of view – highly efficient cyclic process, in which residual heat from the jetwash is fed back to the compressed air upstream of the combustion chamber, with a GTF gearbox.
With IRA, bypass ratios in the region of 20 are possible. In terms of its basic structure, the heat exchanger used in the process comprises a large number of coupled pipes, through which the compressor air flows. The much hotter exhaust gases move along these pipes. This allows energy to be transferred from the hotter air flow to the colder air flow. This type of heat exchanger is used in IRAs to feed the residual energy contained in the jetwash back to the compressed air upstream of the combustion chamber. This delivers a significant increase in efficiency. The method is already used intensively on gas turbines, for example in power stations. This cyclic process with intercooling between the low and high-pressure compressor and energy regeneration via a lancet heat exchanger in the jetwash, which is capable of withstanding the extreme thermal stresses of flight operations, will form the basic elements of the complete recuperative engine concept. The very layout of this conceptual design delivers its compactness, as the lancets have an elliptical cross-section and are very tightly packed. With a total surface area of around 200 square meters, around eight heat exchangers are deployed to absorb the heat from the hot exhaust gas and thus heat up the compressor air.Fuel savings of up to 10 percent.
The IRA engine concept is favored by MTU in order to achieve fuel reductions particularly on long-haul flights, as it delivers fuel savings of up to 10 percent. However, one new approach that Lufthansa is exploring dispenses without new technology altogether. This is because it is also possible to reduce the fuel consumption per passenger and kilometer flown by improving the capacity utilization of the aircraft. All that is required to do this is to use a plane which is appropriate in size for the number of passengers on the flight – i.e. avoid flights with aircraft which are too large and therefore half-empty.
However, this particular goal is one that the airlines have already been pursuing for a very long time, often without any success. Looking at the combined savings potential of all of these different options, it seems that a 50 percent reduction in fuel consumption by 2020 is not impossible – at least in theory. In order to make sure that it also becomes reality, the largest players in the industry – Airbus, Dassault, Saab and Rolls Royce – have embarked on a partnership with the EU called Clean Sky. A total of 1.6 billion Euros – around half of which will be contributed by the EU – will flow into the development of new technologies. Here again, the larger goals include a reduction of CO2 and nitrogen oxide emissions by 40 percent. Aircraft are to become 20 dB quieter. And all this with the current consumption of around four liters of Kerosene per 100 passenger kilometers being squeezed to two liters. It remains to be seen what can be achieved for the environment by 2020 with these large investments. The fact that these efforts are necessary goes without saying.
– Robert Wouters -
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