Prof Pericles Pilidis of Cranfield’s gas turbine engineering group, which is leading the research, said the work is essential for reducing aviation pollution. ‘Radically new aircraft designs will be necessary because the present subsonic transport configuration is unlikely to meet demanding EC targets and other environment requirements,’ he said.
The researchers will test a range of concepts using advanced computer modelling, including the potential of hydrogen fuel, blended wing bodies, combustion at a constant volume, intelligent engines that maximise performance, and distributed propulsion with engine and propulsor separated.
The aircraft industry is watching hydrogen fuel research carefully (The Engineer, 16 April 2004) because the fuel is lighter and cleaner than kerosene. However, there are major safety and storage obstacles. Companies like Boeing have also backed work in the past on blended wing bodies, aircraft that are effectively one large wing with the potential to cut fuel consumption by up to 25 per cent.
The Cranfield team will examine how lightweight and compact constant volume combustion engines could be designed. Today’s aeroengines operate at constant pressure with a continuous, uninterrupted airflow, said Pilidis. The airflow is like a river feeding a hydroelectric dam.
However, constant volume devices with an intermittent airflow, such as piston engines, could offer better efficiency. Unfortunately they are currently unsuitable for aircraft because the additional high-temperature parts needed to create an intermittent airstream mean a vastly higher weight and size and so negate any efficiency advantages.
One concept to achieve constant volume combustion through safe detonations could be a pulsating flow of air or ‘shocks’, said Pilidis. The movement of the air in the combuster in this way could be compared to the subsonic air diffuser of Concorde where the air decelerates in a series of steps rather than continuously due to the design of the diffuser wall. ‘It’s not a simple thing to do,’ said Pilidis. ‘A huge amount of loss would take place around the shocks. The moment you introduce small penalties you lose all the advantages.’
The team will also look into distributed propulsion, where a few power sources embedded in the airframe could feed a large number of small propulsors, unlike today’s engines which have all of them in one unit. Distributed propulsion leads to greater aircraft efficiency because the power sources can be integrated into the airframe.
‘If you separate the power source from the propulsor, however, somehow you need to transmit the energy,’ said Pilidis. ‘This could be done by aerodynamic, electrical or mechanical systems with tomorrow’s technology.’ For example electrical transmission even via superconductors is today insufficient and too heavy.
Intelligent engine control could also improve aircraft performance, said Pilidis. ‘You have margins for coping with deterioration, crosswinds and distortion, but this results in penalties,’ he said. ‘An intelligent engine trades off these margins and improves performance without sacrificing safety.’ So, for example, the control system would know exactly how much fuel to inject for best performance.
Another concept the team will examine is ‘boundary later ingestion’. Re-energising the boundary layer, the air layer adjacent to the aircraft body, by feeding it back into the engine could mean less energy would be wasted and yield better aerodynamics. However, subjecting the engine to complicated flows can cause distortion and instability.
Pilidis expects most of the concepts not to be realised until the latter half of the century, and new ideas may emerge during the five-year EPSRC-funded work. ‘The current configuration of the aircraft engine is very light, very efficient and very reliable. That’s the nature of the problem — it is so hard to beat. Many of the concepts will be new and uncertain but by applying state-of-the-art analytical methods we hope to gain better insights,’ he said.