Multi-speed Transmissions Mean Better Efficiency for Electric Cars

By · August 23, 2011

Could you imagine comparing a 1955 Chevrolet with a brand new Tesla Roadster, and finding something technically better in the old Chevy? Here's one thing: The old Chevrolet will likely have a 2-speed automatic Powerglide transmission. Lo and Hi. The Tesla Roadster has only one gear, just like the Nissan LEAF, and most other EVs. That sounds strange when you can buy a gas car with an 8-speed transmission from BMW or Lexus. Is there an unwritten law stating that all gas cars must have a transmission, and that electric cars should never have one?

A Morgan Aero Supersport

A Morgan Aero Supersport

There's no such law, but there are horsepower and torque characteristics that reduce the need for a choice of gears on an electric motor. Controllers have improved over the years. On old EVs, with DC motors, there was a need of a transmission to allow reverse driving, but there's no need for that anymore. On today's AC motors, the controller can just command the motor to turn the other way around. So that mechanically, the car could drive in reverse at the same speed as it drives forward. Of course, the manufacturer will prevent this with some software tricks applied to the controller, and that means that a car like a Nissan LEAF doesn't have anything that a mechanic would call a transmission. All it has is a reduction gear.

The Furtive eGT from Exagon Motors

The Furtive eGT from Exagon Motors

Electric motors have a wider power range than an internal combustion engine. Their torque curve is flatter, but their efficiency curve is far from flat. There would be a lot to gain from a choice of gears. Imagine a Nissan LEAF with a 5-liter V8 engine and a 5-speed manual blocked in third gear. Performance would be the same as with the standard electric motor, but the efficiency would be terrible. It would be much better with a 2-liter and a choice of gears, and the same reasoning also applies to an electric drive, though to a significantly lesser degree. With a transmission offering several speeds, the Nissan LEAF could do with a smaller motor, and it would be more efficient because it would be possible to keep that motor within the most efficient RPM range.

Do EV drivers want better performance and more range? Then just add gears. I'm pleased to report that several manufacturers are looking seriously into this.

A 3-speed transmission specifically designed for an electric motor, from Antonov Transmission

A 3-speed transmission specifically designed for an electric motor, from Antonov Transmission

The latest is Morgan, which is building an electric Aero Supersport with the help of Zytek, a British specialist in electric drives. Morgan will use a modified 5-speed manual. Then there is the Furtive eGT from Exagon Motors, which will arrive with an innovative 4-speed. That transmission will be totally new, and designed from scratch to work with an electric motor. Fisker Automotive also recently said it was looking into transmissions as a way to dramatically improve performance on EVs.

Last but not the least, a car company can already buy a gearbox specifically designed for EVs from a transmission manufacturer. That's Antonov, which is proposing a 3-speed transmission. The company already won several awards for it, and I bet there are more to come. Electric motors don't readily accept sudden rev changes the way an ICE does. Some fine tuning software will be required, but it's on the way.

Comments

· · 2 years ago

It's an interesting argument. I would like to see the efficiency gains with a transmission.

I am skeptical though: Replace the transmission every so often (~80k miles) for $1800-$3500, it'll make the car less efficient due to the added weight, and the extra energy needed to move the gears and power the transmission controller.

At this point I can't wait to get away from a transmission and the smooooth ride that entails (no gear switching!), in fact I look forward to the day when small motors are used to power the wheels directly - be it from the axil or in-wheel - and completely remove gears from the equation altogether.

So I guess I am in support of no transmission, but I'll try to keep an open mind ;)

· · 2 years ago

Transmissions should last the whole lifetime of the car. It's true that electric motors can run at very high speeds, but with gears, it should possible to dramatically reduce motor speed.

· · 2 years ago

Why have EV manufactures not looked at continually variable transmissions? They are by far the most efficient as the engine or motor is always kept running at the most efficient speed, and i would have thought they would be perfectly suited to an EV. One of the main reasons they never took off in ICE vehicles was that customers didnt like the sensation of the engine note remaining constant while the car changed speed.

· · 2 years ago

I know transmissions 'should' last the life of the car, but I can't think of a case where a transmission didn't need repairs.

My '95 Explorer's transmission went out like clock work every 50k miles - IIRC that was $5k every time!

· Les Moss (not verified) · 2 years ago

OK, here is my idea: Use two motors, one front, one rear. Optimize the front for higher speed efficiency. Optimize the rear for lower speeds. Where 4 wheel traction or max power output are not needed, use the optimum motor to maximize efficiency.

· · 2 years ago

I share Travisty's skepticism. Yes, the efficiency curve of a good electric motor isn't flat, but it's still much flatter than an ICE's curve. I suspect the best approach for EVs like the LEAF will be to continue to improve the efficiency characteristics of the electric motor, not add cost and complexity by dropping in a transmission.

On the other hand, a transmission might make sense for higher performance EVs, to enable them to effectively reach higher speeds. But in a "consumer-grade" EV, I couldn't care less if the top speed is limited to 80 or 90 mph.

· · 2 years ago

I am happy with my Leaf's performance whether driving 30 or 70. Why do I want a transmission? I love the simplicity of very few serviceable parts in my EV which should last many many times longer than my ICE car. Please keep it simple!

When I saw a Mitsubishi Miev at a show, it looked like it had a transmission. I took that as a compromise
for a whimsy motor. I will never buy one.

· · 2 years ago

All I see is (component) manufacturers making EV's more complex so they can make more money over the life time of the car.

How much efficiency gained equals simplicity lost?

· Anonymous (not verified) · 2 years ago

@Les Moss: BRILLIANT!

· Anonymous (not verified) · 2 years ago

It would seem that this is just another effort to make EV's more like ICE's. First it was noise, then creeping while idle, and now a jerky, heavy, and costly transmission. Will smoke be next?

· Lad (not verified) · 2 years ago

The only gear set in the Leaf is the differential. The more gears; the greater the friction losses, the less reliable the car, the heavier the car, and more expensive the product. The Leaf goes to 60 in about 10 secs has a 70 mile range and can exceed 90mph. For a commuter, it's great as it sits. And, it's on the road, a real EV, not vapor and a PR bate car like the Volt and the Focus.

All that need happen now is for Nissan to improve the Leaf s the technology becomes available and it really doesn't need a transmission unless you are "goin' racing."

· · 2 years ago

One thing no one has mentioned yet is are these transmissions automatic or manual? most people driving a manual car change gear not by looking at the rev counter but by listening to the engine note change, this i think would not work in an EV.

· 54mpg (not verified) · 2 years ago

"but their efficiency curve is far from flat."

If the electric motor efficiency curve is flat, how would a transmission help? What ever rpm the motor is running it will always have the same efficiency, right?

ICEs require a transmission, simply because their efficiency curve is not flat. A transmission is an attempt to flatten that efficiency curve.

I can understand the argument that a small + high rpm electric motor is more efficient than a big + low rpm electric motor. This is the reason LEAF uses a reduction gear.

· James Killick (not verified) · 2 years ago

I have converted my 1996 Ford Probe to electric and I still have the 5 speed manual transmission. Recently I locked it in 3rd gear and removed the gear lever as I never used it. That was a 2.5L v6 before i converted it. I still get 70 mph and it pulls away quicker than most cars. I believe hub motors are the ideal solution as each moto can be smaller, but no loss of traction through differentials in the wet or snow. Constant 4 wheel drive, why not? just use approx 1/4 power on each wheel. With the right sensors you could develop an amazing stability system that "knows" exactly how fast each wheel should be going to get the cornering and such like just right. That would be so much safer. Forget gearboxes, move on and develop more precise controls on hub motors. Fantastic dynamics are the future for enjoying driving, not noise and gear changes and ranting at the controls. This will then be more like the thrill of the motorbike. See more about my car at:

http://www.probatron.blogspot.com

· · 2 years ago

Nissan has published the motor efficiency graph ...

· JJ - Can (not verified) · 2 years ago

One more thing to break down - the dealerships will love it !
And it's added weight.

· rc toys (not verified) · 2 years ago

i love 1955 Chevrolet, and i also love electric cars, because of environment protection

· · 2 years ago

Transmissions and reduction gears lose some efficiency, because there is friction and other losses. Some of the best transmissions are ~98% efficient; or about 2% loss. And high torque electric motors are tough on gears.

The Illuminati Motor Works 7 had a 5-speed transmission, and they run it in 3rd for performance and in 4th for better efficiency. Fourth gear was about 2% more efficient than 3rd. And they chewed the heck out of the gears and the final drive differential.

So, direct drive would have higher mechanical efficiency, and a multi-speed transmission might not overcome the losses.

Neil

· EvDriver (not verified) · 2 years ago

Continually variable transmissions (cvt) tend to be one of the less efficient because they have high internal friction but make up for it by allowing the gasoline engine to be tuned to run at a narrower range of speeds, with an electric motor you would not see the same kinds of gains,
With an electric motor having 2 or maybe even 3 speeds should be plenty and from what I've seen of transmissions an automatic shifting gear box is going to have fewer issues then a torque converter and is going to have less internal friction.
I drive an old single speed electric car and at times I wish it had a two speed gear box, but the rest of the time I just enjoy how smooth it drives! I would have a hard time justifying leaving the transmission in any electric vehicle that I build aside from the need for a differential that is often packaged together with the transmission.

· · 2 years ago

Electric motors are similar to ICEs in many respects in their efficiency characteristics. They are not as efficient at low loads, or at very low speeds. Efficiency can range from 95% to less than 85%. A transmission would keep it at the upper end of efficiency, and overall efficiency would be improved as long as there is not a large loss in the transmission.

The Leaf uses a three shaft gear reduction, so there are already gear losses there.

The automakers are going to be looking to extend EV range, and it may be cheaper to add a transmission than to add battery capacity.

· · 2 years ago

The Antonov transmission increases efficiency by 15%. Would you put in a 3-speed transmission for a 15% boost in range? Gizmag has an article that explains how it is done.

· SVL (not verified) · 2 years ago

@reeler
I'm not sure I would, but manufactures probably would, if it's cheaper than 15% more battery capacity. And my guess is: it is.

· · 2 years ago

I understand adding a transmission would make an EV heavier, more expensive and more complicated, but I think it's worth it. Maybe not on a Nissan Leaf, but certainly on bigger cars.

The upcoming Tesla Model S will have a GVWR of probably more than 6000 pounds, it will be really difficult to give the car the right balance of performance and efficiency with only one gear.

· JJ - Can (not verified) · 2 years ago

I wonder if they'll use transmissions on future electric heavy duty (pick ups) work trucks or on big 18 wheeler EV's in the future?

· · 2 years ago

This is ridiculous.
Laurent, I highly recommend that you provide some sort of data to show that "their efficiency curve is far from flat" in such a way that justifies how a transmission would really help.
It's late and I'm too busy to deal with this right now but you are very WRONG and I'm disappointed to see such a technically incorrect article on Plugincars.com.

· · 2 years ago

I can be wrong, but I'm not alone. I see many engineers from different companies all over the world being wrong with me.

· Bubonix (not verified) · 2 years ago

Hey ppl , are you so blind ? yes producer of electric motors wrote about 93% effeciency . But did you ever seen the effeciency map ? , low RPM and extreme torque ( like in direct drives motors ) have <60% or <70% effeciency. So with a nice hill climbing you will have big problem with cooling system and also your Current will go to the sky. I think simple gear with 2 max 3 gears will be enougth to rise performance and effeciency of motor and vehicle. Also from material point of view , bigger motor and bigger batteries cost a lot of rare material. Gear did not.
Electric motors are exccelent but they have also limited performance so each type can be used only in best suited aplication, with gear you can have much more aplications per one motor.
THKX :)

· alexis (not verified) · 1 year ago

I guess it will be expensive if you're looking for replacement transmission parts but certainly its all worth it. Electric cars will be more efficient... I'd like to see the advantages of multi-speed transmission.

· alexis (not verified) · 1 year ago

I guess it will be expensive if you're looking for replacement transmission parts but certainly its all worth it. Electric cars will be more efficient... I'd like to see the advantages of multi-speed transmission.

· · 1 year ago

Most discussions I've seen concerning multi-geared EVs are centered around power curves,efficiency, acceleration etc.
I'd like to ask a simpler question concerning a 2 speed transmission;
Would it give me more mileage on the highway?
EVs increase mileage in city traffic by using regenerative braking.
On the highway, it is pretty much straight consumption, and I wonder if reducing the electric motor's RPMs via a transmission would get me farther. Say 50+ mph would be where it switches gearing.
It might be an easier way to extend long distance travel without a bigger battery or frequent (if available) charging.
Just asking in terms I can understand - please don't beat me up over it.
Thanks!

· · 34 weeks ago

@Michael Graham

Dear Michael,

under same drag condition with same EV there will be no increase mileage at designed cruising speed on highway. The referred extramileage with multispeed transmission is just a result of optimized efficiency of the electricmotor under condition of higher forces to overcome like contrarywind, rising street and the force needed for acceleration.

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