Gas/electric hybrid vehicles tend to be pricier than their conventional counterparts, and many people still worry about the limited range of all-electrics. If you want to move away from purely petrol-powered vehicles, though, is there any alternative? The four-company Flybus consortium would definitely say there is. It recently rigged up a bus with a prototype flywheel-based energy recover system, that stores the energy that would be wasted when the vehicle brakes, then returns that energy to the drivetrain when the bus accelerates. The researchers claim that it could deliver hybrid-like fuel economy, at a fraction of the price.

The flywheel assembly is mounted on the side of the bus's transmission, which is a product of consortium member Allison Transmission. The setup could reportedly be easily added to existing conventional transmissions, however.

When the bus driver wishes to stop or slow down, a variable drive unit created by Torotrak transfers kinetic energy from the drivetrain to a carbon composite flywheel, made by engineering group Ricardo. This energy, which would ordinarily just be wasted by the brakes, instead sets the flywheel spinning at speeds of up to 60,000 RPM. When it's time to regain speed, the energy from the flywheel is fed back into the drivetrain, allowing the engine to do less work.

The system has been installed in an Optare Solo Midibus.

"The recovery and reuse of kinetic energy during stop-start drive cycles is a priority for bus operators, not just because of the positive impact on emissions but also because it reduces fuel costs and brake wear," said Torotrak's John Fuller. "Electric hybrid systems are expensive, often doubling the transaction cost of a bus. Initial cost estimates suggest that the Flybus system could be available at a fraction of the cost of an electric hybrid, whilst simulation results indicate fuel savings comfortably in excess of 10 percent. With the completion of the mechanical design and installation phase of the programme, we are now ready to start evaluating the fuel economy benefits on the vehicle itself."

This certainly isn't the first time that flywheels have been used to power buses. The gyroscopic effect of early systems such as those used in the Gyrobus, however, reportedly made the vehicles difficult to handle.

Flybus's prototype is currently on display at the Low Carbon Vehicle event in the UK.