Mechanical hybrid paper to challenge conventional thinking

at SAE Commercial Vehicle Engineering Congress (Oct 6th).
New approach based on F1 technology claims lower cost solution than electric hybrid powertrains, with smaller package and simplified integration

As manufacturers strive to reduce CO2 emissions from buses and urban commercial vehicles, numerous trials have been carried out using electric hybrid powertrains with regenerative braking. Such systems have a significant cost penalty; in some cases doubling the cost of the base vehicle. At the SAE Commercial Vehicle Congress on October 6-7, full-toroidal traction drive specialist, Torotrak, will present a paper advocating a more cost effective approach using flywheel hybrid technology based on the KERS (Kinetic Energy Recovery System) developed for F1 racing. As well as saving cost, the system is claimed to offer more than 30 percent fuel saving over the London bus test cycle and to package around the existing transmission.

"Our solution offers a much shorter pay back time on investment and does not reduce the number of passengers that can be carried," explains Chris Brockbank of Torotrak, co-author of the paper. "It is also a fundamentally more efficient approach as energy remains in the mechanical state; with electrical regeneration there is an efficiency loss at each state change from mechanical to electrical to chemical and back again."

Brockbank’s paper will highlight past successes with flywheel hybrid bus applications and the system designed for F1, for which Torotrak developed the full-toroidal traction drive that provides the vital link between the flywheel storage device and the remainder of the powertrain. "There is a surprisingly close match between the energy storage requirements of an urban bus and the current F1 regulations," Brockbank continues. "This also makes the packaging so much easier than an electrical solution; we can even retro-fit a system without impacting on the passenger space."

The basic architecture of the mechanical hybrid is a lightweight, high speed flywheel connected via a continuously variable transmission (CVT) to the base powertrain. The CVT comprises two major elements; a full-toroidal traction drive, or variator, and an epicyclic gear train. Together, these elements accommodate the large speed variations between the flywheel and the driveline while permitting the exchange of mechanical energy in either direction.

Chris Brockbank will be giving the paper at 1pm on October 6, room 10, Donald E Stevens Convention Center, Rosemont, Illinois. Torotrak will also be exhibiting at the event (booth 303) and can provide media interviews with senior staff during the Congress.