The efficiency of passenger cars could be improved dramatically by research at Newcastle University aimed at developing "hybrid" vehicles powered by a flywheel in addition to a conventional engine.

The researchers, based at the faculty of engineering, explain that flywheels have been used for many years in vehicles for storing energy that would otherwise be wasted. Normally such flywheel systems need a permanently operating vacuum pump. But New-castle researchers are working on a design that avoids the need for such a pump to maintain the vacuum.

Project leader Paul Acarnley explained: "A big advantage of our device over conventional systems will be that the power will be transmitted to and from the device in electrical form. It will therefore be much easier to maintain the vacuum seal."

Conventional flywheel systems which rely on mechanical transmission of power need very complex vacuum seals that are expensive to purchase and maintain, he says.

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The device being designed and assembled by the researchers will be capable of storing large amounts of energy and then releasing it quickly, making it useful as an auxiliary energy storage device for cars or buses. The device operates at 90 per cent efficiency compared with about 40 per cent efficiency for an internal combustion engine.

Backed by the Engineering and Physical Sciences Research Council, Professor Acarnley's group is now assembling a prototype flywheel system capable of storing 400 kilojoules of energy at its maximum speed of 50,000 rpm. The power rating of the device is 25 kilowatts, which is similar to a small family-sized car.

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Professor Acarnley says that one of the big drawbacks of typical urban driving is that the energy the car sheds during braking is lost as heat. With the new hybrid vehicle his team are developing, the flywheel device can be used during braking to absorb that. He says: "We expect that putting this device in a vehicle would give a 30 per cent range or fuel efficiency improvement in urban driving."

While the device would be particularly useful in cars and buses, researchers believe it could be used in other applications for smoothing out the peaks and troughs of power demand. These include lift motors in buildings or for wind-powered electricity generation where the device could store energy when the blades are turning and releasing it to the grid when the wind drops.

Or it could be used in conjunction with solar power systems, releasing stored energy into the electricity supply when the sun has gone down.