THE next generation of hybrid cars could get a boost from an old technology - the humble flywheel. By replacing hefty batteries in hybrid electric vehicles with a lightweight flywheel that uses a novel form of magnetic gearing, a British engineering company claims that the same fuel-efficiency savings can be achieved at a much lower cost.

The system, called Kinergy, is to be tested initially in airport buses, starting this week. It uses a carbon-fibre flywheel spinning at up to 60,000 revolutions per minute to store energy recovered from the engine and braking, which it then delivers back when needed. Engineers led by Andy Atkins of Ricardo, the Shoreham-on-Sea firm that developed the system, hope it will increase buses' energy efficiency by 13 per cent in urban driving conditions.

Storing vehicles' energy in flywheels has been tried in trains and buses for decades, but the devices have typically proven too large and heavy to be practical. Ricardo's design is just 23 centimetres in diameter and the wheel weighs only 4.5 kilograms, but can deliver 30 kilowatts of power to a vehicle's transmission.