Multi-Speed Gearboxes Are Coming to EVs

By · December 26, 2013

Audi A3 e-tron

Audi A3 e-tron

Gasoline cars currently have something electrics don't: a gearbox. This may be about to change though, as the advantage of having a choice of gears is simply too great for electric carmakers to ignore. Two multi-speed plug-ins have recently been introduced though, and this article will take a closer look at both of them—perhaps precursors to a time when all EVs have gearboxes.

Even Tesla understood the attractiveness of multi-speed transmissions in electric cars. It notoriously failed to develop a two-speed gearbox to fit its Roadster, leading to several production delays and eventually a one-speed solution. Volkswagen was more successful, though not with a pure battery-electric car. The electric Golf will get a one-speed transmission, but the Audi A3 e-tron plug-in hybrid has no less than six gears.

Audi A3 e-tron drive unit

Audi A3 e-tron drive unit.

Just like the standard gas A3—but with an electric motor sandwiched between the gas engine and the gearbox—the plug-in hybrid A3 will use all of its six gears in EV mode. Some people may wonder why? How could today's EVs benefit from more than one speed? The answer lies in improved performance, economy and fun.

It's very true that an electric motor can rev higher than a gas one, but its efficiency changes at every speed. An electric motor has a torque curve, a power curve, and an efficiency curve. A gearbox will help keep it at maximum efficiency at all times.

A choice of gears will also give plug-ins a huge boost in performance. Is it acceptable that the average EV has a top speed one-third slower than a comparable gas car? Plug-ins would be better off having several gears to get onto the highway, and it's no fun that when driving at 80 mph an EV's range falls like leaves from a tree in autumn.

Engineers have had a hard time finding a compromise for the perfect ratio on one-speed EVs, and the results are always flawed. Only a gearbox would solve this problem and also make these cars more fun to drive. Changing gears may not be entertaining to everyone, but feeling the torque going up and down should be.

Exagon Furtiv eGT

Exagon Furtiv eGT

A Unique Advantage to Multi-Speed EVs

Electric motors also help to solve one of the biggest issues with gearboxes—that torque flow is briefly interrupted when the driver changes gears. Modern transmissions have gotten much better at this so the trouble has been much reduced, but the French company Exagon Motors has found an even better solution for its supercar.

The Exagon Furtiv e-GT's approach to gear switching is absolutely unique. It has two motors, each with its own two-speed gearbox. When starting, both motors are in their first gear. When the car accelerates, one motor changes to second gear. As the driver keeps his right foot down, the car's second motor switches into second gear. The result is a choice of three speeds—without any torque flow interruption during gear changes because one motor is always in gear.

This approach is definitely extreme—and expensive. The world is still waiting for an affordable electric car with a gearbox, but have no doubt that it will come eventually. Perhaps 2014 will bring big news about future multi-speed EVs.

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