I’ve taken some heat for referring to the reduction gear assembly in battery electric vehicles (BEV) as a gearbox. While it is true that all production BEVs to date have used a single-speed reduction gear contained in a structure (i.e., a box with gears), the popular definition of gearbox generally seems to require gears of various ratios, able to be connected and disconnected by a shifting mechanism. A flurry of activity by transmission suppliers, however, suggests the single-speed gearbox may be about to go the way of the carburetor, inner tube and mechanical brake.

Multi-speed transmissions for BEVs are now available, and they offer substantial advantages. 

A reduction gear is required in the first place as modern electric motors for vehicles typically have top speeds around 20,000 rpm, while the wheels of a typical passenger car rotate at less than 1,000 rpm at 100 kph (62 mph). Since electric motors have poor efficiency at low speed, the reduction gear allows the engine to turn much faster than the wheels (typically 7-10 times faster) so the motor is running near peak efficiency while cruising, thus increasing range.

Since electric motors provide little torque at high speed, little power is available for acceleration or hill climbing. One way to remedy this situation is a multi-speed gearbox, which would allow the engine to turn slower when the vehicle is at high speed, thus increasing the power available.

There are downsides to a multi-speed transmission for a BEV, however. Most obvious are additional cost, weight and mechanical complexity. Transmission suppliers point to increases in efficiency, as much as 15%, as offsetting these considerations. One remaining problem, however, is primarily tribological.

The reduction gears in current BEVs are usually lubricated with conventional automatic transmission fluid (ATF), which is optimized to manage friction and wear on gears that operate at high load only at high engine speeds. The electric motor, as previously noted, generates maximum torque at zero speed. While research into an ideal BEV gear fluid is ongoing, adding additional speeds further complicates the issue. Now the gears must be able to mesh and unmesh smoothly and without excessive wear under high-load, low-speed conditions as well as low load, high speed. 

The automotive industry should recognize the need for intensive research into new ATFs for electric vehicles, perhaps modeled after the way engine oils are currently developed. Working on a common solution would greatly benefit automakers worldwide, and customers and service technicians would be able to avoid the confusion of possibly dozens of fluids, each specific to a particular manufacturer or model.