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.

Comments

· · 15 weeks ago

Well, plugins cars such as Volt already have "variable speed" gearbox since the second motor is working to modulate the drive/torque ratio of the main traction motor at higher speed.

· · 15 weeks ago

Good article to point out the compromise currently used in EVs. I propose the use of a CVT such as used by the Nissan Note. The beauty of the CVT is you can keep the motor at or near it's best efficiency point which is based on rpm and the load on the electric motor. Acceleration can be improved also by modulating the ratios of the pulleys to take advantage of the low rpm torque curve of the motor.

· · 15 weeks ago

An electric motor's torque curve is incredibly flat; there is little gain to be had in terms of performance. There is much more to be had in terms of efficiency; electric motors' efficiency drops at higher RPMs. Having an "overdrive" could certainly help with range at highway speeds.

· · 15 weeks ago

I would never own a car with the VW/Audi DSG transmission shown in the top picture since they have very expensive fluid change requirements every 40k and failure prone dual mass flywheels. Give me the simplicity and somewhat lower efficiency of a single gear reduction any day.

· · 15 weeks ago

JIminy I agree. A 6 speed dsg is stupid in an EV, overengineered for the application. All an EV needs is a 2 speed for better efficiency at fast highway speeds, and the ability of the Model S to go faster than 130MPH.
The Volt as ModernMarvelFan said, the smaller motor/generator next to the motot somehow helps via the e-CVT to have the main motor turn slower at ~70MPH and quicker, so it's a pseudo 2-speed transmission for the electric motors and a CVT for the engine.
I don't know if the Ford Fusion/C-max, Prius with the e-CVTs have the same behavior with their 2motors. My guess is they haven't, simple because I haven't heard of it with those cars even though mechanically it may be possible.

· · 15 weeks ago

What are the specs on the e-tron's gas engine and electric motor? (HP & Torque). Just cann't help wondering given how large the engine and transmission are compared to the electric motor pictured above. Reminds me of the power density of the Tesla Model S motor (~400 HP) in a similar small size!

"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."
Please show us the curves?

A electric motors torque curve is very flat across it's normal operating range. Power curve can vary depending on short sprints, or continuous operation; but liquid cooling (which doesn't appear to exist on e-tron) allows higher higher across full power band. Efficiency of an electric motor drive-train is more a function of the inverter and driver style than the motor. (electric motor operate at 80-95% efficiency vs. 15-25% for an internal combustion engine, with 3-10% additional loss in the transmission)

· · 15 weeks ago

"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."

Like your range doesn't do the same thing in a gas car? Oh yeah, you don't have a digital measurement of your range in a gas car, so who can tell?

It's all about the cube law of air resistance.

· · 15 weeks ago

"Like your range doesn't do the same thing in a gas car? Oh yeah, you don't have a digital measurement of your range in a gas car, so who can tell?"

Actually you do in many of the modern cars. My old SUV had one. It doesn't move nearly as fast as my EV range predicator.

However, the other info that display is "instant MPG". And it can drop to single digit when the car is floored. My old SUV does about 17mpg at 80mph.

· · 15 weeks ago

I own a Brammo with a gearbox, and while I thought I would like it since it would make the bike feel more like an ICE, I am pretty convinced that my next will not have a transmission. Sure, keeping the motor at the ideal speed is more efficient, but is the added efficiency worth the extra weight/cost of the transmission? For a highway cruiser, maybe. For a city commuter, definitely not. It is far more fun/quick to accelerate in 1st and bang through the gears than to just stick it in 4th and go, but that benefit will go away once batteries can accommodate higher power motors (that is not a problem as much with cars that have less space limitations). Plus driveline lash is much more apparent in a vehicle with no engine vibration. Lash is acceptable if it's just me in the vehicle, but much less so with a passenger.

· · 15 weeks ago

A gear box is not required in an EV because if the motor is well done it can handle the tork over the entire range of speed, exactly what an engine is not able to do.
Actually putting gears in an EV is a proof of your unability to make a proper motor. If the builder can't make a proper motor chance are the rest of his EV will be bad quality as well.

· · 15 weeks ago

I really want to see the authors torque curve he talks about.

· · 15 weeks ago

Priusmaniac - torque at the wheels is the result of torque at the motor multiplied through any reduction in the drive line. Having a large reduction in the drive line will result in increased torque at the wheels, while a small reduction will result in less torque at the wheels. Power output is (sort of) a function of torque. The end result is more torque at the wheels gives faster acceleration independent of the torque of the motor. Manufacturers are playing drivability (acceleration) against efficiency (keeping the motor at the most efficient RPM while cruising). My Brammo will accelerate from a stop in 6th...but it is painfully slow (not to mention that "efficient" cruising doesn't happen until about 85mph). The article is arguing having a gear box is the only way to maximize both factors. While my money went to the concept argued, my experience makes me hope that manufacturers will be able to balance the two without the need of multiple gears.

· · 14 weeks ago

From what I understand gears can get you a whopping 16% gain in efficiency, which isn't much of a benefit at all. This author also makes broad generalizations without providing data to back it up. He's basically saying "Geared EV's are better, trust me. Even though no manufacturer has used them in mass produced EV's yet."

One of the BEST attributes of electric vehicles is the ability to ditch gearboxes because they are not nearly as helpful as they are a nuisance to maintain. Adding the complexity unnecessarily doesn't make sense. (FWIW, I drive an electric car and have been riding electric motorcycles since 2007.)

· · 14 weeks ago

Totally agree with benswing - improving the efficiency of EVs is a game of diminishing returns. Going from ICE to EV brings you from ~25% to ~85% efficient. Multi speed transmissions will help you chip away at that last 15%, but there's only so much improvement you can get out of it. We have much bigger fish to fry with EV technology: cost, range, and charging infrastructure. Multi speed transmissions could improve range, but would also increase cost significantly. Not worth it at this stage, in my opinion. At least for passenger vehicles anyways, definitely worth it for heavier duty vehicles.

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