FLUG REVUE February 2006: Engine researchers seek fuel consumption reduction

February 2006

IN SEARCH OF BETTER FUEL EFFICIENCY

By Patrick Hoeveler

At this time of year the accounts department of the airlines are laboriously working on the annual report.. The exorbitant hike in the price of oil is ruining many financial statements. Against this background, the calls for further reductions in fuel consumption are getting louder and louder. But it is not only in the area of engine development that even modest improvements are gained only after a tough fight. Is there actually any potential left, given the huge technological advances that have already been achieved? Professor Reinhard Mönig, Director of the Institute of Propulsion Technology, part of the German Aerospace Centre (DLR), is in no doubt on this point. “For many years the trend has been towards engines with ever greater bypass ratios. But today the engineers are in a dilemma: should they concentrate on optimising efficiency so as to cut fuel consumption, or should noise reduction be the priority?” Thus the A380 was the first development programme on which Airbus yielded to its customers' wishes and accepted a slightly higher fuel consumption in order to comply with the stringent noise limits in force around London Heathrow airport.

This goes to show that fuel consumption is not the only criterion, a view shared by Mark Pearson, General Manager Advanced Technology and Preliminary Design at GE. “The customer looks at the overall costs. Low consumption is important, but it must not be at the expense of higher maintenance costs. The airlines expect a new engine to be just as reliable and to have just as long a service life as its predecessors. New technologies have to fulfil these criteria.”

So how much more performance can one coax out of an engine? “With conventional concepts, the potential compared with current engines is around six percent.” Such is the estimate of Dr. Andreas Döpelheuer, head of Engine Systems at the DLR's Institute of Propulsion Technology. After all, the bypass ratio, which is so critical, cannot go on being raised ad infinitum, and in his view with present concepts it cannot be expected to rise above 15:1. As well as the strength requirement for the extremely large fan blades, current aircraft design, in which the engines are mounted under the wings, also places constraints on the size of the fan. On the other hand new high temperature resistant ceramic composite materials which require less cooling air from the compressor in the combustor, for example, offer further opportunities for improving efficiency.

Again, the industry wants to make its products even more economical. According to Pearson, “Over the last 30 to 40 years, GE has reduced fuel consumption by an average of one percent per year, and we want to continue this trend.” An integrated approach is critical. With the GEnx, for example, the powerplant was viewed much more as a single entity during development than on any other GE engine. The technology specialist believes that further potential lies in the areas of materials, aerodynamics intelligent powerplants – “On the GEnx we have only scratched the surface of what is possible” – and optimised integration into the airframe.

Anything more would require new architectures, but, according to Professor Mönig, there is a potential of up to 12 percent less fuel consumption to be gained in this area. However, this technology would take at least another ten years to reach maturity. Thus, on the VITAL programme, in which the DLR is involved, the Europeans are working on a counter-rotating fan, amongst other things. One of the crunch points here is noise, as the turbulence caused by the first fan would be flowing straight into the second fan. Another objective on which engineers have their sights set is combustors with lean combustion. GE is also examining new concepts. However, Pearson is not overly enthusiastic about the geared fan. He would prefer an unducted propfan such as GE tried out some years ago. With present day resources, it might be possible to also solve the noise problem. A counter-rotating ducted fan is currently in the pipeline and, according to Pearson, “will be operational before VITAL.”

But drastic changes are required if even greater savings are to be achieved. These range from mounting the engines in a different place through to radical concepts such as a flying wing with distributed propulsion. In the case of the flying wing, the propulsion could be supplied by a number of small electric motors distributed over the trailing edge and powered by fuel cells. The German Aerospace Centre is currently working out possible approaches under a two-year programme. But we cannot expect any substitute for kerosene in the near future. In Professor Mönig's view, hydrogen will not be feasible as a means of propulsion before 2035, if at all, due to many unresolved problems. “It is more likely that there will be synthetically produced fuels. But we will continue to fly with kerosene for at least the next ten to fifteen years. There are still a lot of ideas as to how one can develop present-day engines still further. Research in this area is more important than ever.”

From FLUG REVUE 2/2006

Last updated 12 January 2005

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