Central to the project is the use of a compact high-speed flywheel as an energy store, recycling the kinetic energy that would otherwise be wasted whenever the vehicle brakes. Compared to a typical battery-based electric hybrid system, the mechanical system offers comparable gains in fuel economy in a package that is a fraction of the size, weight and cost.

The Flybus system is being installed in an Optare Solo midibus fitted with an Allison automatic transmission. The flywheel-hybrid unit attaches to an unused power take-off shaft with Torotrak’s traction drive managing the flow of energy in and out of Ricardo’s high-speed carbon composite “Kinergy” flywheel.

Given the long service life of buses, there is a clear need for a simple hybrid system that can be retrofitted cost-effectively to existing vehicles, radically reducing fuel costs and CO₂ emission levels. “We believe this breakthrough will be welcomed equally by bus companies, commercial fleet operators and regional authorities.

—Torotrak’s Engineering Director, Roger Stone

Successful development of a mechanical hybrid system for commercial vehicles could provide the consortium partners with an opportunity to manufacture and sell the hardware for both newly-built vehicles and existing bus and truck fleets across the world. The system could be equally effective on commercial vehicles, such as delivery vans and trucks operating a stop-start schedule. It is also easily scalable, meaning smaller units could be developed for city cars.

Flywheel hybrids, just like electrical hybrids, recycle the kinetic energy that would otherwise be wasted when the vehicle brakes. As the bus slows, instead of converting its kinetic energy into heat in the brakes, the Torotrak continuously variable transmission (CVT) transfers energy to the flywheel, spinning it up to speeds of around 60,000rpm.

As the vehicle pulls away from rest, the CVT returns energy from the flywheel to the bus, meaning there is less work for the engine to do and reducing fuel consumption. In the process, the flywheel gives up energy and slows down until re-energized during the next vehicle deceleration.

A flywheel’s high power density helps to make the system lighter, easier to package and more cost-effective than battery systems.

The mechanical flywheel unit connects via the Torotrak traction drive to the Allison automatic transmission, standard equipment on the Optare Solo, via one of its available Power Take-Off (PTO) drive provisions. As the flywheel speed is independent of both vehicle and engine speed, to provide the correct amount of torque at all times the flywheel must connect to the driveline via a stepless transmission providing a continuously variable speed ratio—a CVT.

Based on the torque requirement, Torotrak’s CVT manages energy delivery by applying the appropriate hydraulic pressure to the discs and rollers at the heart of its traction drive. The rollers then self-steer to the appropriate ratio. It is the combination of torque-controlled operation with continuously variable speed ratios that makes the Torotrak traction drive so effective.