If you look at the specs sheets of most electric cars these days you’ll see that it might have large torque figures, but they’re also complemented by 0 to 100 km/h acceleration times that aren’t that fantastic as well as rather low top speeds. This is actually because most EVs run on a single speed transmission, so there’s only one gear that has to be balanced to do both the work of ‘pick up’ and ‘cruising’.
Tesla had originally planned for the Elise-based Tesla Roadster to have a two-speed transmission but apparently it was too challenging and it settled for a BorgWarner single-speed transmission. Having more than one transmission ratio will enable manufacturers to downsize electric motors and keep them running longer at the medium loads and speeds where peak efficiency of over 95 percent is possible, translating into potentially more performance, greater range and longer lasting batteries. The single speed transmission in current EVs force the motor to run outside its optimum efficiency range most of the time.
Zeroshift has been working on a multi-speed gearbox for electric vehicles (EVs) that needs no clutch – a damper inside the gear hubs and electronic control of the motor make ratio changes seamless without interruption. The synchromesh in a regular manual gearbox is replaced with paired interlocking rings that change ratios without interrupting the torque. To provide the required levels of shift refinement, Zeroshift’s concept uses electronic control of the electric motor or motors to match the shaft speeds and an integrated a passive damper system within the drive hub to isolate any vibrations.
“Our studies suggest that by using a compact, multi-speed transmission and a smaller electric motor, manufacturers can gain an operating efficiency of up to 10 percent. You can use that 10 percent to improve EVs’ range or reduce the size, weight and cost of battery packs,” says Bill Martin, Zeroshift’s managing director.