The GKN Hybrid Power Gyrodrive is an electric flywheel that captures energy as the car brakes, then uses it to drive an electric motor that boosts power and cuts fuel consumption.

Gyrodrive-equipped buses saw a whopping 20 percent bump in fuel efficiency, more than enough to convince GKN and Go-Ahead to install the system on 500 buses over the next two years, starting in London and Oxford. The system, which weighs 130 pounds and is roughly the size of a passenger car wheel, can be retrofitted onto a bus in a few days.

Gyrodrive can be installed on other big vehicles, such as garbage trucks, which are well-suited to the technology because they make frequent stops. Day says GKN is also looking at rail vehicles, possibly using bigger or multiple flywheels to account for the jump in size.

via F1 Tech Is About to Make Buses Way More Efficient | Autopia | WIRED.

I wonder if there are additional applications outside of motor vehicles.