Germany's two engine manufacturersstill have one programme in common.
Andrzej Jeziorski/MUNICH
DAIMLER-BENZ Aerospace problem-child MTU Munchen and its upstart rival BMW Rolls-Royce, along with the German Aerospace Research Establishment (DLR), are participating in a Government-sponsored research programme to develop technology for the next generation of civil powerplants. Dubbed Engine 3E 2010 - the Es standing for environment, efficiency and economy - the project is a first step towards a ducted-propfan demonstrator engine which could be developed into a production engine early in the next century.
Engine 3E is funded from the German Government's four-year, DM600 million, civil-aerospace research-support package, which has been divided among fixed-wing-aircraft, helicopter and powerplant research between 1995 and 1998. In all, some DM144 million ($100 million) has been dedicated to Engine 3E, of which MTU has the lion's share, with a little over 48%, while BMW R-R gets 29%. Under the terms of the Government programme, both companies have to match the state's contribution from their own coffers.
MTU and BMW R-R agreed to focus on a geared ducted-fan engine, with a bypass ratio of about 15. The goals of the programme are to cut noise and pollutant emission, fuel consumption and production and maintenance costs while increasing operational life and flexibility.
MTU's share of the work focuses on compressor and turbine technology, while BMW R-R is concentrating on the combustion chamber. The development of technology for the fan is led by the DLR, building on experience gained with MTU in the counter-rotating integrated shrouded-propfan research project.
According to MTU's Engine 3E programme chief, Dr J"rg Sieber, 90% of the engine components have already been defined, and the programme has moved into the hardware stage. The various elements of the programme for which MTU is responsible build on work which the company did in earlier advanced-ducted-propfan (ADP) investigations with its strategic partner, Pratt & Whitney, and MTU's own Compressor 2000 investigation.
CUTTING COMPONENT STAGES
The company's main goal in developing the compressor is to cut down the number of stages. Most of the current effort is going into the high-pressure (HP) compressor, which was also one of the key components built by MTU for the ADP demonstrator engine which was tested during 1993 and 1994.
MTU now aims (with a six-stage compressor) for the same overall pressure ratio as Pratt & Whitney's PW2000 engine reaches in 12 compressor stages, says Sieber. This means that the mean stage-pressure ratio must be about 1.5.
"This is the highest stage-pressure ratio for any HP compressor we know," says Sieber. To achieve this, the compressor must be run at a higher speed than in a conventional engine, introducing mechanical and aerodynamic problems as compressibility effects come into play.
The main mechanical problem, according to Sieber, is the connection between the blades and the disks. Blisk (integrally bladed disk) technology, which MTU is already using in the fan and compressor of the Eurojet EJ200 engine for the Eurofighter EF2000, is to be incorporated into the design, but Sieber says that the company is now looking one stage further.
Sieber's team is investigating the use of an integrally bladed ring (bling) in the first stage of the compressor - essentially a blisk with the central portion removed, making it much lighter and allowing engine bearings to be located in the newly available space.
To make the bling strong enough, however, requires new-materials technology. Sieber believes that the solution lies in metal-matrix composites. "You have to use titanium metal with ceramic fibre in it to strengthen the whole construction, so you reduce weight and increase circumferential speed," he says. The cost of running the compressor faster, however, is a drop in efficiency as the airflow over the blades becomes transonic and shock waves form.
MTU still believes that it can maintain current levels of efficiency even with transonic flow, by careful aerodynamic design. The flow field has to be precisely calculated in three dimensions, using Navier-Stokes codes, to allow the blades to be designed for maximum efficiency.
In the low-pressure (LP) compressor, the disks and blades can potentially be produced from epoxy/carbonfibre composite (CFC). High temperatures do not allow these materials in the HP compressor - Sieber expects temperatures of about 1,000K at the compressor outlet. Here, the problems faced are mostly in production technology, as MTU effectively wants to make CFC blisks, requiring a complex fibre-weave.
The danger of birdstrike complicates the issue even more. The CFC blades, running at high speed, could suffer delamination from the impact of a bird. One safeguard which could be incorporated is to have fibres running through the thickness of the blades, binding the layers of CFC together. This would make production even more complex, however, so an alternative solution of using hollow titanium blades is still being considered.
The use of composites, however, would allow blade sweep to be introduced in the first stage of the compressor for improved aerodynamic efficiency. This could not be done with heavier metal blades.
Variable geometry is another new technology which MTU hopes to introduce into its compressor. "This is important for the LP compressor because we have a fan with variable-blade pitch, and that means the flow through the core varies a great deal," says Sieber.
The solution now being examined is to introduce variable-pitch guide vanes, as well as a variable-pitch first-stage (and possibly second- stage) stator. Trying to introduce variable pitch into the rotors would be a mechanical nightmare, and is not under consideration.
Tests are to begin in July on an LP compressor with variable-geometry stators, while HP compressor tests are expected to get under way in November. These tests will be carried out on hardware originally built for the ADP and Compressor 2000, but with the introduction of new blades and other components developed solely for Engine 3E.
Overall, with the new materials and bling technology, MTU expects to achieve a weight saving of about 30% over a conventional compressor with the same number of stages. The reduction in the number of stages then offers an even bigger weight advantage.
REDUCTIONS TARGET
By the end of this phase of the programme in 1998, MTU is aiming to have achieved a 20% reduction in the number of stages, a 10% cut in length, and a 20% cut in the number of blades in the HP compressor, compared with today's technology. In its LP-turbine work, the company wants to cut the number of stages by 40%, length by 20%, and blade count by 50%. It is also aiming for a 20% weight cut compared with conventional LP turbines.
MTU envisages a four-stage turbine with the performance of a conventional seven-stage unit. Again, the circumferential speed of the blades will be high, compared with today's engines.
If anything, blade design and flow-field modelling is more critical in the turbine than in the compressor. Sieber says that the high-rotary-speed HP compressor experiences Mach numbers comparable to those found in the LP compressors of today's military engines, while the turbine experiences even higher Mach numbers and must be even more carefully designed, to minimise the peak value, with the blades shaped for transonic flow and their profiles flattened where shock waves form.
Both blisk and bling technology are ruled out in the turbine for the time being, because of the need for a cooling system. If they were used, cooling holes would have to be drilled through the blade length and the disk, or ring, complicating an already challenging production task. For now, though, MTU has settled for conventional materials and construction, but is looking for ever-improved materials such as titanium aluminides, which are lighter and still offer sufficient high-temperature performance to be used in the rear stage of the LP turbine.
Improved single-crystal metals are also being considered for the blades, while the diffuser between the HP and LP turbine could be manufactured from new-technology ceramic-matrix composites. Sieber says that construction would be relatively simple. It experiences low mechanical loading, but high temperatures - a suitable trial for the new materials.
The Engine 3E LP turbine is being prepared for testing around mid-year. The company is using the existing component from the ADP demonstrator, and plans to test this thoroughly from the beginning of August in a high-altitude test site at the University of Stuttgart, before fitting new blades which are being designed under the Engine 3E programme.
BMW R-R COMBUSTER
Work on the combustor is primarily BMW R-R's province. The company has a total budget for Engine 3E work of DM93.1 million, about half of which is from Government funding, and DM60 million of which is dedicated to the low-nitrous-oxide (NOx) combustor project, which started in June 1994. BMW R-R is pursuing a two-stage combustor concept, which it says has proceeded to a component-test phase, yielding "very encouraging" results and proving the technical validity of the concept.
The company adds that it plans to test the staged combustor in a core engine in the first half of 1997, also at Stuttgart, where MTU's turbine tests are to be conducted. The combustor tests will evaluate the NOx-reduction potential of the combustor, as well as testing its controls and fuel system under realistic steady-state and transient-flight conditions.
BMW R-R is not alone in investigating combustors, however. MTU is doing its own work on a rival concept ,which it says is cheaper and more reliable than the two-stage combustor.
NOx-EMISSIONS GOAL
According to Sieber, the goal is to reduce NOx emissions by 60% by 1998, while aiming to improve by 25% the homogeneity of the temperature profile at the combustor exit.
A flat temperature distribution at the exit would be ideal because, although high mean temperatures are needed for high efficiency, any hot spots in the burned mixture coming out of the combustion chamber cause problems for the HP turbine.
To achieve this, a homogeneous mixture of air and fuel is needed, and this also leads to lower NOx production. MTU believes that significant improvements can still be achieved with a single-stage structure, which can be produced at lower cost and is more reliable than two-stage designs to date. While Sieber admits that BMW R-R could get lower NOx emissions with its design, he believes that customers will prefer to opt for lower cost, as long as the design is within ever-tightening international emissions-criteria.
The method being used to improve the injection system is largely that of trial and error. Today's analytical codes need substantial improvement to be able to deal adequately with the combination of aerodynamic and chemical phenomena in the combustion chamber.
Up until 1998, the research and test programmes being carried out will lay the groundwork for a follow-on demonstrator programme expected to run between 1999 and 2003. Whether both MTU and BMW R-R will still be involved at that stage is far from certain, but neither is likely to be able to deal alone with a programme of that magnitude.
Source: Flight International
