FLUG REVUE January 2004: Technologies for the next generation of engines

FUTURE TECHNOLOGIES FOR CIVIL AERO ENGINES

By Patrick Hoeveler

The requirements are clear. New aircraft like the Boeing 7E7 should achieve significant improvements in operating and manufacturing costs. Naturally, engines account for a large proportion of these costs. Thus, for example, GE Aircraft Engines is aiming for 20 percent lower fuel consumption with future engine configurations, an 85 percent reduction in emissions of nitrogen oxide compared with the ICAO's 1996 limit, and 55 percent less noise than on present aircraft. To achieve these ambitious objectives, several technology solutions are available, but the precise way forward is still undecided, as the identification and selection of new key technologies is a major challenge for the designers. An engine is a highly complex machine and a technology employed on one part of the system can sometimes have effects on other elements which are difficult to determine.

The time horizon is also relevant, as Professor Klaus Broichhausen, Vice-Preisdent New Products and Technologies at MTU Aero Engines in Munich, explains. “New technology invariably invites the question, 'When will it be used?'” In his opinion, evolutionary changes such as advances in the area of materials should be the first to be implemented. In the case of a blade made from a new material, for example, the basic architecture of the engine has not altered, but new technology has been used to improve it.

The titanium and nickel alloys generally used today are well advanced and have relatively little further potential. On the other hand, the intermetallic material titanium aluminium (TiAl) promises weight savings of up to 50 percent. Moreover, it is proving more temperature-resistant than normal titanium alloys and could therefore be used on a turbine bladed ring (“bling”). The advantages of TiAl and blings are generally known, but the problem of how to manufacture blades effectively, for example, has not yet been resolved. Induction pressure welding could replace the linear friction welding used up to now in classic bladed disk or “blisk” production. Under the new process, an induction loop is based around the material and a short current input makes the material “doughy” (close to the melting point). The parts are then joined together with a lower expenditure of force and hence more accurately. According to Professor Broichhausen, the process permits the manufacture and repair of blade elements of all kinds, including blisks and blings.

The new metal matrix composite (MMC) material, which consists of both ceramic material (silicon fibre) and metal (titanium) and is up to 20 percent lighter than conventional titanium alloys, would be suitable for fan blings. The concept has been proven, but it still has to be tried in military applications, as Professor Broichhausen explains. Product verification would require some 30 rotors in the workshop. “Without government support that isn't possible. We have a major challenge here in Europe.” The military demonstration could then be used later on in the civil domain, just as the blisk was developed from the EJ200. Ceramic matrix composites (CMC) based on silicon and aluminium are not yet employed on the Eurofighter engine as they are not suitable for sustained high temperatures above 1000ºC.

Equally light and high-strength are polymer matrix composites (PMC). These fibre-reinforced synthetic materials have already been tested in a military application at MTU on a hybrid stator. But at present they can only be used at temperatures up to 200ºC and their relatively high manufacturing costs are also a problem, which makes Broichhausen cautious about possible applications. “We have the technologies, but the key is to master the entire process chain from casting through to repair.” Many developments have failed due to the fact that they were not considered in their entirety, he says. “If one adopts a more broad-minded approach, one could well find other efficiency models for the life cycle costs as well.” Thus, he argues, one needs to consider the question of whether a service life of up to 50 years designed using ultra-high technology is actually sensible, or whether a manufacturer's guarantee that the parts will last at least ten years and will then be replaced would not be better. This would mean that customers always had a “new” engine on their aircraft.

Other potential improvements to today's engines lie in computer-aided development. “In the old days you designed a gas turbine engine around a gas path analysis, but nowadays three-dimensional computer simulations consider all the cavities as well.” They include not just the aerodynamics of these “virtual engines” but also the mechanics, both steady-state and transient. Thus the entire HDV12 high pressure compressor for the Pratt & Whitney PW6000 was modelled on the computer and computer forecasts were developed as to how it would behave in the engine. To the surprise of the engineers on the test rig, “the results were entirely to the point.”

On the other hand further major advances can only be achieved through fundamental changes in the design of future engine models. Here the foremost goal of the engineers is a high bypass ratio. “Good engines currently have a bypass ratio of 7. With existing methods we are already squeezing bypass ratios of up to 10 out of the new designs.” A geared turbofan, consisting essentially of a slowly rotating fan which is driven by a high-speed low-pressure turbine via a reduction gear, offers further scope for performance improvement. Such a geared fan, as is being studied by MTU and P&W Canada in the Advanced Technology Fan Integrator (ATFI) programme, is supposed to reduce fuel consumption by seven percent and offer 30 percent less stages and blades. However, one drawback is that the design is still too heavy.

Another possible solution currently undergoing a revival in European research is the principle of a counter-rotating fan, which can push through 10 to 20 percent more air mass for the same nacelle diameter. MTU had previously investigated such a concept up to the early 1990s in the Counter-Rotating Integrated Shrouded Propfan (CRISP) programme. This was a core engine plus low pressure part with a two-stage, shrouded propfan, each of which had between ten and twelve swept blades made of fibre-composite material. The shroud was attached to the core with stiffening ribs. At that time bypass ratios of up to 26 were thought possible. But the German Federal Ministry for Research and Technology withdrew its financial support in 1991. The noise level was a particular problem. Similar projects from the USA such as GE's Unducted Fan (UDF) also came to nothing. An additional safety issue with the UDF was the possibility of a fan blade coming loose.

With new noise reduction technologies such as advance calculation of the acoustics during development and new acoustic liners, Professor Broichhausen believes that shrouded fans have a chance. “From 2012 (in-service date) something could be happening the areas of geared fans or counter-rotating fans.” For the new generation up to the year 2010 (applications on 7E7, possible Airbus response, possible 100-seater from Bombardier), he predicts that conventional turbofans will continue to be used. From 2015/20 engines with intermediate cooling and heat exchangers are a possibility, as are currently being researched in the EU Component Validator for Environmentally Friendly Aero Engines (CLEAN) programme and the associated MTU heat exchanger.

MTU

“After that completely different installations are a possibility, for example, with a propulsor in or on the wing which is powered by a gas turbine in the fuselage. One has to consider the integrative advantages.” Further potential could come from eliminating system boundaries for individual components and looking at a complete system. In this area, MTU is already discussing new concepts with Airbus. But one must not neglect the installation aspect. Thus a mounting on the top of the wing would require a video inspection system for the crew and much more besides.

“The combination of nanotechnology and aerodynamics could produce another leap forward,” according to Broichhausen. Such adaptive control functions for “smart engines” should systematically monitor vibrations, airflow, temperatures or emissions on an engine via a sensor network and thus push back the operating limits.

Thus, for example, at certain frequencies an injection of air could influence surge behaviour in the compressor. This would reduce the safety margin (surge margin), resulting in better compressor performance. Fast-acting electronics would have to measure disturbances and respond accordingly. Such micro-electromechanical systems (MEMS) could even be worked into individual blades in the distant future and thus influence and control the aerodynamics. However, the valves required are still too expensive and too large today.

MEMS could possibly also be used in the area of noise reduction. In the context of active noise control, MEMS in the form of small nozzles could reduce the aero-acoustic vibrations. However, configurations such as the German Aerospace Research Establishment (DLR) is trying out in laboratory experiments with microphones to measure the sound and loudspeakers as a noise cancellation source, are too large and too heavy.

Other possibilities discussed recently at a symposium on micro systems engineering at MTU are a system for the detection of leaks, a miniaturised sensor for measuring blade vibration (active control of vibrations would spare expend the service life of blades and vanes) and a micro-heat exchanger. But the biggest challenge is still integration into the extreme operating conditions that prevail in aero engines. As well as high temperatures and pressures, the designers also need to allow for changes in components, such as structural weakening due to holes, cavities etc, and resilience against electromagnetic pulses, which together pose enormous challenges as regards the robustness, service life and maintainability of the MEMS.

Nevertheless, the concept of the “electric engine” features on the list of subjects requiring research in the research departments of the big engine manufacturers, not least since Boeing announced that it wants a bleedless engine for the 7E7. According to Professor Broichhausen, such “more electric engines” are part of an “ongoing trend aimed at eliminating the hydraulics”. A bleedless arrangement would have “an enormous impact on the design of the engine” and should reduce the present mix of electric, hydraulic and pneumatic energy to a predominantly electrical energy supply. The advantages mentioned by MTU include energy savings from the use of intelligent actuators, higher reliability through self-diagnosis and prognosis and of course improved engine efficiency due to the decision not to use bleed air from the compressor.

Instead, the energy would be created by a generator on the compressor shaft. The list of requirements which this has to satisfy is long: weight, size, possible back-coupling to the shaft dynamics, reliability, possible consequences in the event of failure, ease of maintenance and the tough operating conditions in an engine. Integration on the shaft is unlikely to be simple, nor is the extraction of power from the shaft, as the MTU engineer explained. “To take the power from the shaft, a powerful core engine is needed. That is a huge challenge for power management.” Other elements of such an “electric” powerplant include electronic add-ons such as fuel metering, oil pump, electrical actuators and magnetic bearings.

The researchers are also studying fuel cells, as currently used in submarines, as a possible replacement for the auxiliary power unit. In principle this reverse electrolysis converts hydrogen and oxygen into water while producing energy. Once again there are issues of weight and cost. However, the non-potable water on board an airliner could be enriched with the liquid. When one considers that the Airbus A380 will need between 1,875 and 2,500 litres per flight, depending on the customer's requirements, this sounds sensible.

All in all, in the opinion of Professor Broichhausen, safety and cost considerations alone rule out any abrupt, revolutionary developments. “Aviation must remain conservative.” For this reason it is extremely important to try the technology out in a military application first so as to then be able to apply evolutionary steps, such as a geared fan or a counter-rotating fan in civil applications. In this context, MTU is already studying a two-part fan for military use in collaboration with the German Aerospace Research Establishment. So an appropriately funded, future technology demonstrator based on the EJ200 would make sense. This fits in with MTU's view: “We want to continually demonstrate technologies on demonstrators.”

Thus, new materials are likely to be the first items to bring new dynamism in the future. According to Broichhausen, recuperative engines similar to the CLEAN programme constitute the upper end of progress. He believes that both improvements in existing technology and also the application of new concepts at the same time offer promise. Thus under CLEAN, the optimisation of other components could balance the higher weight of the intermediate cooler and heat exchanger.

However, pulse detonation engines, such as are being studied in the USA in the military area, as representatives of the next thermodynamic generation lie in the still more distant future. In GE's vision pulse detonations on a turbofan could replace the entire high-pressure system and combustor, thus offering fewer parts and greater power. At any rate, there are more than enough ideas for technological innovations. One can await their implementation with suspense.

From page 82 of FLUG REVUE 1/2004

Last updated 9 December 2003

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