Low-cost Flybus hybrid bus just around the corner02 November 2010

Construction of the first Flybus hybrid prototype – aimed at demonstrating the viability of a cost-effective alternative to battery-hybrid buses – has begun, with test results due next year.

The system is now being installed in an Optare Solo midibus fitted with an Allison automatic transmission. Its 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, compact Kinergy flywheel.

Torotrak engineering director Roger Stone says that, for a fraction of the cost of other systems, Flybus will deliver up to 20% savings in fuel consumption and CO2 emissions in stop-start operations.

"The system offers a potential alternative route to cleaner city centre air," comments Stone. "Besides saving cost, the technology offers easier installation than battery-electric systems. Its size and compatibility with the existing driveline should make it possible to convert existing fleets without reducing passenger capacity," he adds.

Stone explains that the key is its compact, high-speed flywheel, acting as the energy store and recycling the kinetic energy that would otherwise be wasted whenever the vehicle brakes. Compared to a battery-based electric hybrid, he insists, the system offers gains in fuel economy in a package that is a fraction of the size, weight and cost.

In brief detail, as the bus slows, the Torotrak continuously variable transmission (CVT) transfers energy to the flywheel, spinning it up to around 60,000rpm. Then, as the vehicle pulls away, the CVT returns energy from the flywheel to the bus, slowing the flywheel until re-energised during the next vehicle deceleration.

Among the keys to success is the flywheel's high power density, which 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, via one of its PTO drives. As the flywheel speed is independent of both vehicle and engine speed, to provide the correct amount of torque the flywheel must connect to the driveline via a stepless transmission providing a continuously variable speed ratio – hence the 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. Stone says that it is the combination of torque-controlled operation with continuously variable speed ratios that makes the Torotrak traction drive so effective.

"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," he says. "We believe this breakthrough will be welcomed equally by bus companies, commercial fleet operators and regional authorities."

Author
Brian Tinham

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