I seem to remember something about buses having a great big flywheel attached to their motors that spun up to speed whilst the bus was moving and effectively stored up energy to be used when it needed to accelerate from a stop. A much more elegant and efficient system has been developed by Vincent Carman. It uses hydraulic pressure to store up energy and transfer it to the wheels. The pressurisation means the engine can always be running at its most efficient without losing power through a geared transmission. Carman reckons his system, currently being tested in two cars, doubles fuel economy with no other changes.