Running on air

Hybrid engines that use compressed air to help power vehicles could dramatically improve fuel economy, particularly in built-up areas, according to research for Ford by the University of California.

Researchers at the university investigated the use of air hybrid engines as part of a study on behalf of the car maker into alternative methods of combining combustion systems. They found that an air hybrid engine, where braking energy is stored as compressed air and reused to help power the vehicle, could improve fuel efficiency by up to 64 per cent within towns and cities, and by 12 per cent on motorways.

Air hybrids could be a promising technology, said Tom Watson, powertrain manager for Ford’s Escape petrol-electric hybrid engine. ‘It is very similar to an electric hybrid, in that it recovers regenerative energy. It would use a cylinder to store compressed air.’

Start-and-stop driving within cities means a large amount of fuel is needed to accelerate, and much of this is converted into heat in brake friction during deceleration. Capturing, storing and reusing this braking energy to give additional power can improve fuel efficiency.

In a standard electric hybrid when you brake it takes the momentum of the vehicle and turns it into kinetic energy and then electricity, which is stored in the battery, said Watson. ‘With an air hybrid engine the system takes energy from the momentum of the vehicle, and uses a pump to compress air into a cylinder. The compressed air can be stored and used to help propel the vehicle.’

The air hybrid is simpler than the electric version as it does not require a second propulsion system in the form of an electric motor, as the compressed air can be injected into the engine and expanded in a chamber, where it pushes down on the pistons to move the crankshaft. Further weight reduction is not needed to compensate for heavy equipment.

Ford believes more research is needed into the durability of air pumps, as they would have to last for 10 years and around 150,000 miles. ‘Air pumps with the capacity needed to store this kind of energy have not been used in an automotive application, so a lot more work is needed to prove the technology for this environment,’ said Watson.

Prof Garel Rhys, director of the Centre for Automotive Industry Research at Cardiff Business School, said car makers are looking at all the alternatives. ‘This is another option in the wardrobe of approaches. It is interesting in that the duel-fuel option can either mean two engines or one engine with two fuel sources.’

Note: The car industry should be focusing on synthetic fuels rather than hydrogen fuel cells to reduce carbon dioxide emissions, said the chief executive of Volkswagen Bernd Pischetsrieder. The firm is developing a synthetic fuel from biomass made from plants and waste products, which it claims can be stored and distributed using existing infrastructure and is less costly to produce than hydrogen.

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