By the end of next year, 500 buses in London could be using an electromechanical hybrid system originally developed by the Williams Formula 1 team.
Using a composite flywheel to store energy from the brakes and feed it back to an electric motor, it offers a simpler, cheaper and more durable solution than storing the energy in batteries.
The flywheel hybrid system is the first step in a planned revolution in the way buses are designed in a bid to reduce both pollution and noise.
It was first developed by Williams with an F1 application in mind, but the system instead ended up in sports cars competing in the World Endurance Championship.
The advantage of a flywheel system is that it is light, compact and efficient, but the disadvantage is that it can only store energy for short periods. It is suited to racing where cars are continually either accelerating or braking but is of limited use in a road car where there is more long-distance running.
It is, however, ideally suited to an urban bus, which is generally either accelerating or braking and whose substantial mass means a large amount of kinetic energyis produced.