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Published on July 22nd, 2019 | by Jennifer Sensiba

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EV Transmissions Are Coming, And It’s A Good Thing

July 22nd, 2019 by  


ZF’s 2-speed drive unit. Image provided by ZF.

Recently, ZF announced a two-speed EV transaxle/drive unit, and around the same time, more details about the Porsche Taycan came out. For the Taycan, there was more confirmation that the vehicle will have a two-speed gearbox (at least in the rear). In response, I’ve seen a good number of EV fanatics on Twitter and Facebook panning one announcement or the other. 

“EVs don’t need transmissions,” seems to be the common criticism. Supposedly, the added complexity, maintenance, and expense aren’t worth it.

There’s just one problem: that’s wrong. Let’s take a look at why!

Why ICE Cars Have Multi-speed Transmissions

For those not very familiar with internal combustion technology, I’ll give a quick rundown of the “why” behind transmissions. I know that it can be a hard concept to learn at first, so I’m going to share this video from EngineeringExplained (one of my favorite Youtube channels):

For those who know how to drive manual, the reason for transmissions should be fairly obvious. ICE engines, with rare exception, only make good power and/or efficiency in relatively narrow RPM ranges. Depending on the goal (power or efficiency), the transmission needs to shift to the next gear at the right time to keep the engine in the right range of RPMs.

The ranges in which they can run at all are wider, but are still not wide enough for automotive applications. For instance, go much below 1,000 RPM, and a gas engine will stall. Go past redline (where the red is on a tachometer), and the engine will start damaging itself in various ways. For this reason, most modern cars have a “rev limiter” safety feature that cuts fuel and/or spark to stop the engine from going faster and self-destructing.

Finally, there’s the tradeoff between vehicle speed and torque multiplication. In “low” gears, the gearing has the same effect as having a longer lever: more torque. This allows a vehicle to have an acceptable amount of “oomph” when you first start. However, if you stay in that low gear, the engine will be going too fast before you can get up to an acceptable speed. To be able to go faster, you need to switch to a different gear that won’t turn the engine so fast, but at the cost of losing that initial torque. You’re already moving, so that doesn’t matter as much.

For the highway, there are even “overdrive” gears that allow the engine to turn more slowly than the wheels for better efficiency and less wear.

Why EVs Usually Don’t Have Transmissions … Or Do They?

There’s one big reason that EVs can get away with not having a multi-speed transmission: electric motors have a wide operating range. At the bottom end, they can go all the way to 0 RPM without stalling. There’s no need to idle, and when you hit the skinny pedal, full power is available. At the top end, most EV motors can go beyond 10,000 RPM without damage, with some approaching 20,000 RPM at top speed. With few moving parts, they don’t fly to pieces at those speeds.

There’s a problem, though. Electric motors do not generate the same torque from zero to maximum RPM. They all put out full power until a certain speed, and then their torque begins to drop off. Efficiency is also not consistent across the full range of speeds the motor is capable of going. The speeds at which they’re most efficient can vary, but the “sweet spot” is usually around ⅓ to ½ power at 30–40 MPH (50–65 km/h). 

While an EV will work at anywhere from 0 RPM to max motor speed, it will have lower power and/or less range at highway speeds if its single gear is optimized for city driving. Making this gear “taller” could help, but then the car would suffer from lower performance and efficiency in the city.

“But Teslas are the best and they don’t have a transmission!” people are screaming. The article isn’t even published as I write this, and I can already hear and see the incoming comments and tweets. The problem with screaming that at me is that it’s not really true. Tesla’s struggle with the gearing issue has been particularly interesting, and the company has come up with a couple of workarounds. 

You may or may not know that Tesla originally planned to put a 2-speed gearbox in the original Roadster. To achieve the best performance, they wanted to have a lower gear for initial acceleration and a higher gear for a high top speed. Contrast this with the other EVs in development at the time: the Nissan LEAF with its 94 MPH (151 km/h) top speed and the Chevrolet Volt, which ended up with a top speed of about 101 MPH (as tested by myself in Mexico).

Tesla’s quest to avoid these nasty compromises kept running into a brick wall, though. It simply couldn’t come up with a multi-speed gearbox that wouldn’t get totally shredded by the sudden onset of torque a big, mean electric motor can dish out. Already facing delays and cost overruns, they had to abandon the multi-speed transmission, but work on the Model S “Whitestar” prototype gave them another solution: better power electronics that could cram in more and more electrons to create an even longer torque curve (allowing for more power at speed).

The problem with that approach is that it isn’t terribly efficient. Sure, you can allow the motor to use up more power to partially solve the performance tradeoff, but battery technology isn’t such that you can get away with that for very long.

Ultimately, Tesla ended up finding another way to have two different gears: put one in the front and one in the back. Dual-motor Model S, Model X, and Model 3 vehicles all have different gear ratios in their front and rear drive units. At lower speeds, at least half of the power goes to the rear drive unit, which is optimized for lower speeds. When you get up to highway speeds and have “range mode” enabled, the car’s computers direct power to the front drive unit, which has a better highway gear ratio.

So, in the end, even Tesla uses multiple gear ratios. Sure, Tesla vehicles don’t have a multi-speed gearboxes like ZF is releasing, but they still show us how multiple ratios can help any EV.

The Proven Benefits of More Gear Ratios

ZF’s test driver in a modified VW e-Golf. Note the display where the motor’s efficiency “sweet spots” can be seen on BSFC charts. Photo provided by ZF.

In online discussions, I’ve shared the above with people (Tesla’s use of multiple gears), and they’re sometimes still skeptical. “Would multiple gears really help in the real world, though?”

For that, I’ll have to refer readers to the world of EV conversions. For decades before mass-produced EVs emerged on the scene, people in garages were taking their ICE (internal combustion engine) vehicles and turning them into electric cars. Without the ability to have a special gear reducer made, they often just bolted an electric motor to the ICE car’s transmission. We don’t have to wonder whether this would help, because it’s proven to have all of the theoretical benefits.

Here’s a really wild example: a Ferrari 308 that was converted to run on batteries after it was nearly destroyed by fire. They added three electric motors, all of which put power into a chain that feeds a Porsche 4-speed manual transmission. (Our own Nicolas Zart reviewed this electric Ferrari.)

The cool thing about using a manual transmission is that you avoid the problems Tesla faced with its failed 2-speed project: you can interrupt the power during shifts to keep from destroying the gears. In the case of a manual, the driver does this, but if you wanted to build a custom automatic EV, a torque converter could just as easily soften the blow a bit (though, you’d want it to have a TCC system to lock that converter up between shifts).

With a manual transmission EV, shifting is actually largely optional, which makes it much easier to drive than a manual ICE. Many owners of converted cars simply leave the car in 2nd gear most of the time. Electric motors don’t stall, so there’s no need to press the clutch or shift to neutral at stops. You could get away with highway driving in 2nd gear, but you get better range by putting the motor back in its more efficient range in 3rd or 4th.

For performance, though, you’ll want to make use of the gears. Jalopnik’s Andrew P. Collins described the Ferrari’s first gear as “borderline scary.” The electric motor’s torque, multiplied through a low gear, gives a lot of hard acceleration.

In ZF’s testing, it’s proven that the benefits are as real for mass-production EVs as they are for conversions. Not only did they achieve a 5% increase in vehicle range, but they also got better acceleration numbers compared to a single-speed vehicle. This allows a vehicle to sidestep the tradeoff between efficiency and performance by allowing for both. In ZF’s case, they have the unit shift gears at 70 km/h (just under 45 MPH). Since they sell mostly to manufacturers, they also allow for the car’s computer to command the drive unit and optimize shifting to better fit the manufacturer’s goals (performance, efficiency, towing, etc).


Final Thoughts

At the end of the day, it all comes down to cost. For a cheaper vehicle that largely gets driven in the city, one gear is fine. For a performance car, or one that is going to spend more time at higher speeds, it makes sense to find ways to get at least one more gear ratio. That’s what Tesla did.

At the end of the day, though, the era of “city-only” electric vehicles is coming to an end. People expect their vehicles to be broadly usable in many different environments. Customer demands are going to keep the pressure up.

To get the performance people expect, manufacturers are going to be putting in multi-speed gearboxes. And you’re going to love it!

Featured image provided by ZF

 
 





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About the Author

Jennifer Sensiba is a long time efficient vehicle enthusiast, writer, and photographer. She grew up around a transmission shop, and has been experimenting with vehicle efficiency since she was 16 and drove a Pontiac Fiero. She likes to explore the Southwest US with her partner, kids, and animals.



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