Firing up the engines

Rolls-Royce is to develop new materials to allow its aircraft engines to operate at much higher temperatures, significantly reducing carbon dioxide emissions.

The £3.7m Materials for Arduous Cycle and Emissions (MACE) project will develop materials that are suitable for use in advanced-cycle engine designs, with higher operating temperatures and more efficient compressors.

Such engines will help meet the goal set by the Advisory Council for Aerospace Research in Europe to reduce CO2 emissions per passenger km by 20 per cent by 2020.

The three-year project, which is due to begin next month, will develop ultra-high temperature alloys for use in turbine discs, said Colin Small, Rolls-Royce’s MACE project leader and project manager for external materials research.

‘The whole engine cycle is determined by what the high-pressure turbine can do, so thatis where we are always lookingto push the temperature up,’ he said.

Existing materials lose their strength at high temperatures, and nickel alloy development is reaching the end of its life, he said. ‘We are unlikely to be able to add a new element and find a new wonder alloy,’ said Small.

But the MACE project team believes it can still improve theperformance of nickel alloys bytailoring the microstructure of the materials. By manipulating the way they heat and forge the alloy, they can control the size and shape of the grains making up the material.

This will allow them to create optimal grain sizes and distributions in different parts of the disc, to meet the differing mechanical properties required in each area of the component. But significantly increasing the turbine’s operating temperature will also make the discs more prone to corrosion and sulphur attack.

So the team hopes to produce sulphur-resistant coatings, capable of operating at up to 730ºC, to protect the disc material from damage.

Computer modelling will also be used to predict how the next generation of single-crystal alloys for turbine blades — already under development and likely to be introduced in 2008 — will operate at these higher temperatures.

Finally, the MACE researchers also plan to develop high-temperature abradables, the materials used to seal the gas path around the tip of the compressor rotor blades.

‘To get good efficiency in the compressor, you don’t want aerodynamic leakage round the end of the blades, because if you get the air going over the blade tip rather than round it, it runs inefficiently,’ said Small. ‘We seal that with a class of materials known as abradables, which are very difficult to operate with.’

The coatings are designed to release fine debris when machined by the blades, without causing blade wear. Existing abradables, such as aluminium polyester, aluminium silicon and nickel cubic boron nitride are unreliable and only suitable for use at up to 400ºC, reducing the efficiency of the compressor.

The team hopes to develop a robust abradable from a combination of materials capable of operating reliably up to 600ºC.

The MACE project, which is partly funded by the DTI, also includes Praxair Surface Technologies, coatings specialist Sermatech, Cambridge, Cranfield and Birmingham universities and the University of Wales at Swansea.