Mechanical bolt-on KERS hybrid for buses offers 30% fuel saving
It seems that the lessons learned in developing a mechanical KERS system for F1 may yet hold the key to a low-cost, high-efficiency hybrid system particularly suited for the stop-start patterns of buses, which are quite similar to the distances between capturing and delivering energy of those of a race car. Torotrak will deliver a paper at the SAE Commercial Vehicle Congress in Illinois next week showing how flywheel KERS for buses can offer more than 30 percent fuel saving over the London bus test cycle, yet package around an existing transmission.
We’ve written before about Torotrak and its mechanical KERS system, and it is far from the only company to have recognized that mechanically storing energy via a flywheel is a more efficient and cost-effective system than storing the energy harvested from the potential energy of a moving vehicle electrically.
When Williams F1 investigated alternatives for its F1 KERS system for 2009, it was so impressed with the mechanical KERS alternatives that it committed long term to the system and indeed, even set up a separate company to commercialize the technology not just for racing, but for the automotive market in general.
Torotrak’s mechanical hybrid uses a lightweight, high speed flywheel connected via a continuously variable transmission (CVT) to an existing powertrain. The CVT comprises 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.
Torotrak’s speciality is toroidal traction drive technology which enables it to create highly efficient Continuously Variable Transmissions (CVT) and Infinitely Variable Transmissions (IVT) for application in flywheel-based mechanical hybrid systems and for use as auxiliary drives.
The mechanical hybrid paper to be delivered at the SAE Commercial Vehicle Engineering Congress claims it to be a lower cost solution than electric hybrid powertrains, with a much smaller package and greatly simplified integration, particularly as a retrofit to existing vehicles.
“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.” Torotrak claims the system offers more than 30 percent fuel saving over the London bus test cycle yet can be packaged around an existing transmission.