Do electric cars have gearboxes?
If you’re getting into a fully electric car for the first time, you may be surprised to find that there isn’t a manual gearstick to change ratios. Drive one for the first time, and you might be even more surprised to find that, as the car accelerates and decelerates, it doesn’t automatically shift through gears. That’s because there are none.
Electric vehicles are unique in that they do not feature a multi-speed gearbox like conventional diesel or petrol vehicles. Instead, they get away with just a single gear. This is especially impressive considering how quick some electric vehicles can accelerate. The Tesla Model S P100D, for example, can do 0-60mph in just 2.4 seconds, making the family saloon faster than most supercars.
Why do electric vehicles have only one gear?
There are several reasons why fully electric cars have only a single gear to drive the wheels. The first is that the electric motors on board electric vehicles rev to a significantly higher rate than conventional diesel or petrol engines. A typical electric vehicle motor can rev up to 20,000rpm, far higher than the usual 4,000-6,000rpm limit found in conventional cars.
It’s also important to note that electric motors are power efficient throughout this rev range, meaning they do not have to be sitting in a small, narrow rev band to attain optimum performance.
This also means electric vehicles produce almost instantaneous torque from zero revs, again meaning they do not sit in a specific rpm range to be used during low-speed driving or under acceleration.
The reason conventional diesel and petrol cars require a gearbox with multiple gears is because the engine is capable of generating useable torque and power in a very narrow band of engine speeds.
The varying gear ratios help the conventional engine keep within the narrow power band at different speeds. That’s why a petrol car will easily accelerate to 20mph in first gear, but won’t go much faster without reaching the engine’s rpm limit, or red line. With the same logic, drivers will struggle to pull away from a set of lights in sixth gear, as this ratio is designed for faster driving.
This all means an electric vehicle can reach its top speed in a single gear, with little compromise in low-speed driving or usability. What engineers effectively do is pick a gear ratio that contains a good balance between acceleration and top speed. Pick a ratio too low, and the engine will accelerate very quickly but may be limited to only a low top speed. Vice versa, if engineers pick an extremely high ratio, the gearing may be optimal for top-speed runs but acceleration will suffer.
Can electric vehicles have more than one gear?
Yes; in fact, there are some applications in which electric vehicles come with a multi-stage gearbox. Formula E, the electric single-seater race series, has some teams that opt to run their vehicles with a three-speed gearbox.
Although the cars are limited to a maximum power of 170kW, the speed at which they reach this is crucial. Some teams, like Audi ABT Schaeffler, ran their 2017 driver’s championship-winning car with a three-speed gearbox. The addition of a gearbox helped the car reach maximum power slightly quicker. However, for the 2018 season, the team has opted to run a high-efficiency transmission system with a single gear.
For road cars, engineering firm GKN Driveline recently introduced a new, two-geared eTwisterX electric driveline. A significantly smaller system than is found elsewhere, the eTwisterX is designed to fit any vehicle from small city cars such as the VW e-up! to large plug-in hybrid SUVs like the Mitsubishi Outlander PHEV.
In simple terms, the addition of a second gear allows for better acceleration and top speed than single-speed electric drive units. For low-speed acceleration, the electric vehicle sits in first gear, helping it launch quicker. When on the motorway, the car will automatically shift to the second gear for better high-speed driving and a higher top speed.
A similar two-stage automatic gearbox is found in the BMW i8 plug-in hybrid sports car that GKN has helped develop.
Does that mean plug-ins and hybrids have an electric motor with more than one gear?
Because the BMW i8 is a sports car, BMW specifically designed the two-stage system with GKN Driveline to improve the car’s driving characteristics. However, most hybrids and PHEVs feature a standard electric motor with a single gear, as this is cheaper and less complicated to manufacture. And as discussed above, single-geared units are giving away very little in terms of performance or efficiency, mitigating the need for a dual or three stage gearbox.
However, there is always the potential that as more mainstream manufacturers begin converting their sports and supercar ranges into electric vehicles, they will consider the performance improvements that a multiple-stage electric vehicle transmission might bring.
What are the benefits of having just a single gear?
From a manufacturing point of view, the biggest benefit of having just a single gear to operate the electric motor regards cost and a lack of complexity. Installing a driveline system with multiple gears immediately adds to cost and complexity, which then brings up vehicle price.
Given that electric vehicles already command a premium over their diesel and petrol counterparts, its unlikely manufacturers will want to add unnecessarily to the price. This is especially true when the single gear set-up is already optimal for most electric vehicles, and delivers equal or even better performance than conventional powertrains can offer.