Driving on Air

The Engineer Online
August 21, 2006

Research is taking place on both sides of the Atlantic into innovative air-hybrid engines that promise to be lighter, cheaper and more environmentally-friendly than existing electric-hybrids.

A collaborative project between researchers at Brunel University -which has a patent on the design - and car manufacturers Lotus and Ford is to begin early next year to develop a prototype that could turn a conventional combustion engine into an air compressor.

The theory behind the design of an air-hybrid is not new but according to the project's leader, Prof Hua Zhao from the university's school of engineering and design, it has never been put into practice before.

An air-hybrid engine recovers the braking energy during deceleration by turning off the engine's combustion and using the valve timings to change the engine into an effective compressor that pumps highly-compressed air into a tank, where it is stored.

The engine compresses the air to a high temperature and pressure in the 20-30 litre air tank, which holds it until it can be used, according to Zhao.

Extra air

'It can be used to drive the engine, making the engine effectively an air motor,' he said. 'Another possibility could be providing something like a turbo-boosting effect by supplying extra air for a short period of time when accelerating.'

Zhao said that it could be used to crank the engine instead of using an electric starter motor. This would be particularly effective for urban driving. The engine can be stopped at traffic lights, and when the car needs to accelerate the compressed air can power the engine with the conventional combustion only kicking in at a certain point or speed.

In another application the air could be used in the after-treatment of exhausts - particularly when the car is starting from cold - and could help car manufacturers adhere to stringent EU legislation on emissions in the future. 'Unburnt hydrocarbons need to be controlled soon after you start the engine. In this case we could use air to burn the fuel in a catalyst with the tank providing the additional air through a separate air pump,' said Zhao.

One of the biggest advantages of air-hybrid engines over electric-hybrids is that no heavy batteries or generators are needed so the whole system is lighter and less expensive to operate and install.

Zhao and his team will begin the research by modelling the engine using simulation software. Lotus has provided the electro-hydraulic system to be used and Ford has given the team detailed information about its engines.

While Brunel's government-funded research is scheduled to be completed in 2010, engineering work is already under way on a potential air-hybrid rival in the US. Texas-based SouthWest Research Institute and its UK subcontractors - including Powertrain Technology based at Shoreham-by-Sea, West Sussex - are developing an innovative version of the air-hybrid engine which is claimed to be far more efficient than Brunel's design.

The work is sponsored and managed by the The Scuderi Group, which has recently received a $1.2m engine development grant from the US Department of Defence to develop the patented Scuderi Split-Cycle Engine.

Unique design

The Scuderi design dedicates one side of the engine to combustion and the other two pistons as a dedicated air compressor. Gas is compressed in the compression cylinder and transferred to the power cylinder through a gas passage. All this unique design needs to be converted into an air-hybrid is the addition of an air tank, according to Steve Scuderi, one of the firm's founders.

Scuderi is honest in his assessment of the drawbacks of an air-hybrid design but is certain that the technology's advantages make it an exciting alternative to electric-hybrids.

'While electric-hybrids can take energy and hold it for a longer period of time the problem with air-hybrids is that the energy is stored as heated, compressed air. This, of course, means that as the tank cools the energy dissipates,' he said.

However not only are the batteries in electric-hybrids big, heavy and difficult to dispose of, they also take a long time to store the energy, according to Scuderi.

In his view, air-hybrids have a big advantage because the energy can be pumped up quickly then discharged in massive amounts. He also believes that his firm's design would be far more effective than the work being undertaken by Lotus and Ford with Brunel university.

'They have to switch their pistons between combustion engine mode and compressor mode which costs efficiency - we don't have to do that,' he said.

The Scuderi Group's prototype engine is expected to be completed by early 2008.