The automakers Volvo and Jaguar are testing the possibility of using flywheels instead of batteries in hybrid electric vehicles to aid acceleration and help engines operate more efficiently.
There are trade offs in such a system. It appears not to be useful for long range driving:
One advantage of flywheel systems over batteries is their compact size. "Most hybrids with batteries provide a 15- to 25-kilowatt boost of power. The flywheel can deliver 60 kilowatts in a way smaller package," says Andrew Atkins, chief engineer of technology at Ricardo. The trade-off is that flywheels can't supply energy for very long.