The Past, Present, and Future of Electric Vehicle Transmissions

Single speed, no gear changes. This is the driving setup 99.99% of electric vehicle drivers are used to. It’s the standard.

But it might not be the standard forever.

The Porsche Taycan is the first production electric vehicle to launch with a multi-speed gearbox since the original Tesla Roadster, which doesn’t really count for reasons to be covered later in this article. ZF, an automotive parts company that a wide variety of automakers purchase from, has designed a two-speed gearbox of its own.

There are potential benefits to be gained from using a multi-speed gearbox in electric vehicles, and it’s likely a few more automakers will continue to experiment with them in their cars.

Why did single speed gearboxes become the norm for electric vehicles?

Internal combustion engines and electric motors are both tasked with the same job—driving the wheels—and they both accomplish this by turning stored energy into rotational motion. However, they go about that job in drastically different manners and their performance characteristics are dramatically different.

Internal combustion engines work by detonating fossil fuel in a controlled environment—the cylinders—and using the energy released by the explosion to move pistons, which in turn rotate the crankshaft.

A big drawback of internal combustion engines (besides pollution, use of nonrenewable resources, reliability issues, noise, vibrations, inefficiency, high cost of operation, etc.) is that they can only operate within a very limited band of rotation speeds, measured in rotations per minute (RPM). If the engine turns too slowly, it stalls out and shuts off. If it turns too fast, the increased speeds wear out all the moving parts and the engine damages itself. Even within the available range of engine speeds between “too slow” and “too fast,” there is a limited range of RPMs that the engine actually produces enough power to drive the wheels. This ideal range is called the powerband. This varies from engine to engine, but for most road cars it’s between 800 and 7,000 RPMs.

Hypothetically, you could connect an engine directly to the wheels of the car, but the RPM of an engine does not line up nicely with the RPM of wheels.

This is where gearboxes come in. The gears act as an intermediary between the engine and the wheels, allowing them to spin at different speeds. The difference in rotational speed between the engine and the wheels can be measured as a ratio, which is how gears are sized. For example, a gear ratio of 2.07:1 means the engine turns 2.07 times for every one turn the wheels make. Each gear ratio can only cover a certain range of road speeds for an internal combustion car. The 2.07:1 gear ratio might only cover 20 to 50 miles per hour. Slower than 20, and the engine falls to the bottom of its powerband and stalls. Faster than 50, and the engine will rise above its powerband, likely bouncing off of a manufacturer-installed rev limiter to prevent damage caused by over-revving.

Therefore, traditional cars need multiple gears because the limited powerband of an internal combustion engine needs several different gear ratios to cover the entire range of speeds driving demands. First gear might only cover from 1 to 25 miles per hour, second gear might only cover from 20 to 50 miles per hour, third gear from 45 to 70 miles per hour, and so on.

Horsepower/Torque Curve

Example Horsepower/Torque Curves. Brown = Internal Combustion. Blue = Electric.

Electric motors, on the other hand, can comfortably and safely operate over a much, much wider range of RPMs. They produce peak torque at 0 RPM and can operate in speeds up to 20,000 RPM or higher. The motor in the Tesla Roadster, for example, maxed out at 14,000 RPM. While all electric vehicles still have a gear to allow the motor to turn at a different speed from the wheels, they only need the single gear to cover the entire range of driving speeds they could encounter.

To use the original Tesla Roadster as an example again, its single gear had a ratio of 8.27:1—and that gear and electric motor allowed the car to operate from zero all the way up to 125 miles per hour.

The single speed gearbox wasn’t part of the Roadster’s original design, though, and it wasn’t even the first transmission to ship with the car.

The original Tesla Roadster launched with two-speed manual transmission

When the Tesla Roadster was originally being developed, the automaker tried to implement a multispeed transmission.  Most pre-production prototypes carried a two-speed direct shift manual transmission—there was no clutch, but there was a gear lever the driver used to shift from neutral, first, second, and reverse.

That two-speed transmission proved problematic for Tesla at the time. They couldn’t build one that allowed Tesla to meet its performance goal of a four-second zero-to-60 time while also having the durability expected of a road-going production car. The transmission simply couldn’t handle the incredible torque output of the motor.

The transmission woes led to delays of the original Roadster while Tesla worked on a new design. Tesla finally settled on the now-standard single-speed design and tasked BorgWarner with building the part. Tesla still decided to ship the first Roadsters with the multi-speed gearbox to make sure they could start delivering cars, edging the part toward durability rather than performance.

“To help speed delivery of cars, we will begin production in 2008 with an interim transmission design,” said Ze’ev Drori, former President and CEO of Tesla motors in a 2007 blog post. “These transmissions will meet high standards for reliability and durability, but the car will not meet the original performance spec for acceleration, reaching 60 mph in 5.7 seconds instead of the promised 4 seconds. When the final transmission is ready, we will retrofit all cars, at Tesla’s expense, to meet the promised performance specifications.”

The new gearbox came as part of a more comprehensive “Powertrain 1.5” upgrade, which also included smaller updates to the electric motor and Power Electronics Module. All Roadsters with the original gearbox and 1.0 powertrain received this upgrade free of charge.

Tesla dual motor all-wheel drive unlocks multiple gear ratios with just one gear per motor

Ever since that Powertrain 1.5 upgrade, every single one of Tesla’s electric motors has operated with a single-speed gearbox—but most of Tesla’s cars come with multiple electric motors, one on each axle.

The motor on the front and rear axles can be different sizes from each other, and both can have their own gear ratio for their single speed. The car’s computer can divert more power to the front or the rear based on the demands of the driver and road conditions, essentially allowing the Tesla to have multiple gear ratios despite having just one fixed gear ratio at each motor.

For example, most Tesla vehicles have a smaller front motor, and that motor and gear ratio is better for efficiency than the larger, more powerful rear motor. When accelerating, the car may send more power to the rear to get up to speed faster. Then, when cruising on the highway, the car may send more power to the front to increase efficiency. Tesla vehicles can actually put one of the two motors to sleep while driving.

“The dual motor Model S will quickly torque sleep a drive unit when torque is not needed and instantly wake it up as the accelerator is pressed to command more torque,” said JB Straubel, Tesla’s former Chief Technical Officer, in a 2014 blog post. “It continues spinning while asleep and the digital torque wake up is so fast that the driver can’t perceive it.”

Model S Dual Motor

Note the Model S’s rear motor is larger than the front. Image via Tesla.

This increases both the efficiency and performance of the vehicle, sometimes even allowing a dual-motor car to achieve more range than a single motor car. The all-wheel drive 2015 Tesla Model S 85D was rated at 270 miles of range while the rear-wheel drive Model S 85, with the same battery pack, was rated at just 265.

Despite the different functional gear ratios available, Tesla drivers never feel a gear change because there are no gears changing. The single-speed system works really well for electric vehicles and it compliments their smooth operation nicely—gear changing can induce a jerking motion in the car as the gears physically move from place-to-place and the motor suddenly changes speeds. No gear changes means smooth operation 100 percent of the time.

However, the electric future of driving doesn’t mean that multi-speed gearboxes are entirely a thing of the past. Tesla may have given up on multi-speed gearboxes in favor of the single-speed solution, but other automakers might decide differently.

Porsche already has.

Porsche Taycan

Porsche Taycan 4S

The Porsche Taycan carries the first multi-speed EV gearbox since the original Tesla Roadster

The Porsche Taycan is the first production electric vehicle to use a multi-speed gearbox. Like dual motor Tesla vehicles, the Taycan has one motor on each axle, and its front motor has just one gear. However, the rear motor has a transmission with two gears that are shifted automatically.

The Taycan uses the two gears in its automatic transmission to bring out different performance purposes. The first gear has an approximate 15:1 gear ratio and is used to increase acceleration. The second gear has an approximate 8:1 gear ratio and allows the car to cruise more efficiently and reach higher top speeds.

Porsche Taycan Transmission

Exploded view of the Taycan’s two-speed gearbox. Image via Porsche.

According to a Porsche press release about the Taycan’s powertrain, first gear is mainly used in Sport or Sport Plus driving modes. Launch control is also available in these modes. The transmission remains in first gear for a relatively long time, then shifts into second gear with a shift overboost.

We’ve had a chance to drive the Taycan here at Current Automotive and the feeling of an electric vehicle shifting gears is something to get used to. It’s fun to feel the kick down and a burst of speed, but it does slightly detract from the absolute smoothness you’re used to experiencing from an electric vehicle.

That said, the transmission delivers performance. It has insane acceleration off the line, sustained acceleration into higher speeds, and a high top speed for an electric vehicle.

Rimac’s electric supercars and some Formula E cars also use multispeed transmissions to accomplish the same goals.

The benefits of a multispeed transmission may not be limited to luxury brands and race cars, either. Other automakers will likely be tempted to install multispeed gearboxes in their EVs in the future.

ZF develops two-speed automatic electric vehicle transmission

In July 2019, ZF, a German auto parts builder, announced it had developed a new electric powertrain concept consisting of an electric motor and a two-speed automatic gearbox in one unit. The company could potentially make this part available for any automaker to buy and plug in to an electric vehicle design, just like it has with its 8HF eight-speed automatic transmission that is used in everything from Volkswagens and BMWs to Jeeps, RAM trucks and Aston Martins.

Of course, ZF brags about bringing the same benefits Porsche’s transmission offers to other vehicles.

“Until now, with electric motors, vehicle manufacturers have had to choose between high initial torque and top speed,” said Bert Hellwig, a head in ZF’s E-Mobility program in a 2019 press release.

ZF's Two-Speed EV Transmission

Rendering of ZF’s Two-Speed EV transmission. | Photo via ZF.

The concept features a 140 kilowatt electric motor and gear shifts that take place at approximately 43 miles per hour, but ZF says more powerful motors could be used. The company doesn’t say as much, but the gear ratios would likely be tweakable for different applications as well.

The transmission also offers a significant range increase, which may be one of the biggest draws for automakers. ZF says its two-speed unit extends range by up to five percent compared to a one-speed transmission.

No vehicles are using this yet and it’s somewhat unclear if this is production-ready or just a prototype, but ZF clearly believes there will be a market for this kind of drive unit—otherwise the company wouldn’t be developing it.

The future of electric vehicle transmissions and other EV technologies

With electric’s future on the precipice of achieving critical mass, it’s easy to think that all of the answers to automobile powertrains have been solved: the EVs of the future will use electric motors with a single-speed gearbox and electricity will be stored by a Lithium-ion battery pack.

However, every part of the electric vehicle powertrain will be subject to constant research, development, and advancement, just like the combustion engine powertrain that has been the norm for more than 100 years now.

Several paradigm shifts occurred over the life of the internal combustion engine. Engines went from big and heavy with 8, 10, and 12 cylinders being the norm to small, lightweight, turbocharged engines reigning supreme.

Manual transmissions were overtaken by automatic transmissions, and the automatic transmissions of yesteryear are slowly being overtaken by new technology like continuously variable transmissions (CVTs) and dual-clutch transmissions (DCTs).

Electric vehicles will see the same kind of advancements as manufacturers begin putting the full weight of their R&D departments into the technology. A new battery chemistry might change the industry overnight and multispeed transmissions might become favored for performance and efficiency benefits. At the same time, multispeed transmissions in electric vehicles might never catch on—leaving the original Tesla Roadster and Porsche Taycan as interesting footnotes in EV history. There’s no way to predict exactly how EV tech will develop.

The electric vehicle future is important for helping people switch to sustainable transportation—but sustainability doesn’t have to mean boring. People have eagerly followed the development of ICE cars for decades, and there is just as much to be excited about in the realm of EV development—even in parts as small as some gears in a box.

Posted in Electric Vehicle Education October 12th, 2020 by