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MTU Aero Engines plans to reduce CO2 emissions by 30 percent
By Patrick Hoeveler
A three-phase strategy should improve engine environmental acceptability significantly by 2035. The culmination of these efforts could be a propfan with heat exchanger.
What does the future hold in store for engines? In its CLAIRE (Clean Air Engine) programme, MTU Aero Engines is setting the direction it wishes to follow in-house. As Dr. Jörg Henne, Head of Development and Technology at MTU, explained in an interview with FLUG REVUE, “It is a matter of a long-term view of technologies that are suitable for further reducing emissions. The path which we are looking at includes not just fuel consumption and carbon dioxide emissions, but noise emissions as well. We think that the issue of noise has receded too much into the background.”
In his opinion, future powerplants will have to consider all three aspects. “Five to eight years ago, nitrogen oxides were still the central concern. Now suddenly not everything can have been wrong.”
CLAIRE envisages a total of three phases aimed at reducing carbon dioxide emissions by 30 percent by the year 2035. The first phase is the geared turbofan developed jointly with Pratt & Whitney (see page 56). The second phase specifies an even more radical change in the engine design. A shrouded, counter-rotating fan will produce 20 percent less CO2 emissions by 2025. MTU had in fact pursued such a concept back in the 1980s in its Counter-Rotating Integrated Shrouded Propfan (CRISP) programme. However, due to the relatively high development requirement and the fall in the price of oil, the Munich-based company called off the project after a few years.
“But the idea is still right, and convincing too,” says Dr Henne. It offers the advantage of a much bigger bypass ratio of up to 25:1. The one-metre diameter wind tunnel model is still at the German Aerospace Research Establishment, where it has been used for trials at using active noise control to reduce noise emissions. However, the problems that emerged before are still there today: “The rotors are very close together. Due to the very high relative speed, a lot of noise is produced. With all counter-rotating rotors, the key problem is noise.”
One remedy here could be shrouding, combined with active measures and aerodynamic improvements. Moreover, by increasing the spacing between the rotors it should be possible to reduce the noise level. Nevertheless, noise remains a challenge. “We are not expecting any improvement compared with the geared turbofan, but for things to remain stagnant in this area.”
The blades, which will very likely resemble propeller blades, pose quite a few mechanical challenges for the engineers. They have to be adjustable and also deliver reverse thrust. At the same time they need to be as light as possible, as a double fan naturally carries a weight penalty. For example, hybrid moving blades were used on CRISP, as enormous forces occur at the root. “It is very difficult to make an adjustment here with composites. So at that time, a titanium root was coupled with a composite blade.”
Moreover, the contrarotation has to be generated. This requires a special gearbox. “Other companies (such as GE with its then UDF) go for counter-rotating turbines which do not have any guide segments. This approach involves considerable mechanical challenges and departs more radically from today's configurations,” says the technology expert. MTU is therefore considering a low-pressure shaft which drives the counter-rotating propellers through a gearbox.
The open propfans have even greater bypass ratios. “On the other hand this is at the cost of additional noise. For that reason, as far as we are concerned, propfans should also always be shrouded. Theoretically, the “open rotor” concept has a fuel consumption advantage. But in practice, it causes significant negative accompanying effects on the aircraft. The tail installation is heavier overall, as there has to be a balance.”
“One significant advantage of the shrouded propfan is the fact that it still allows high air speeds of up to Mach 0.85.” The “open rotor” cannot match this as it allows only Mach 0.75 or Mach 0.80 at the most. As Dr Henne points out, “On a long-haul flight that makes quite a difference.”
Whether the propfan technology is already suitable for the future generation of medium-range commercial aircraft remains to be seen. “We are working on the assumption that the new aircraft will enter into service in 2014 or 2015. The geared turbofan would then be suitable for them. But if it were to be 2020, counter-rotating rotors could also be used.
However, Dr Henne does not believe that the first trials will take place in this decade. “There is still a lot of study work ahead of us. But collaboration with airframe and engine nacelle manufacturers is also important. Without good integration into the aircraft, such new concepts are not possible.” A concrete demonstrator could be running by 2012/13 at the earliest.
The third phase envisaged under the initiative is projected for the year 2035 and will then offer a reduction in carbon dioxide emissions of no less than 30 percent. It combines the propfan with a heat exchanger system that draws heat energy out of the exhaust jet and conveys it back to the compressed air in front of the intake to the combustor. The result is an optimised circular process.
Meanwhile the challenge lies in the integration, as Dr Henne explains. “Thermodynamically the concept is not so difficult, and there are already static applications of heat exchangers. In the CLEAN programme we investigated what losses occur in the heat exchanger circuit. However, we were not attempting a complete integration at that time.”
As well as safety aspects, the designers also have to include weight in their deliberations. “Such engines are getting more complex and probably heavier as well. And this will not entirely be compensated by the use of new, light materials either.” Such heat exchanger propfans would achieve their maximum effectiveness on long-range aircraft and in this way would help to reduce the environmental impact.
From FLUG REVUE 10/2007
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