Technology explained

What about the transmission in electric cars?

(Bild: Stephan Schätzl)

Seven gears for a manual transmission, ten for an automatic - for a long time, car manufacturers have concentrated on increasing the number of gear ratios in passenger cars. With electric cars, however, a single gear is usually sufficient. This is how it works.

The drive technology of electric cars is much simpler than that of cars with a combustion engine. A petrol engine has a good ten times as many moving parts as a typical electric motor. The latter has a total of 50 to 100 components, while the combustion engine has several hundred.

The situation is similar for the gearbox. Some petrol and diesel engines use highly complex multi-speed transmissions, which have been continuously refined over the past decades. Dual-clutch transmissions and automatic gearboxes with 9, 10 or even 12 gears, such as those used by Volvo in the FH series, are particularly complex.

Electric cars cannot do without a gearbox either, but they usually have a fixed ratio. This is referred to as a single-speed transmission, which can do without a shift mechanism, synchronizer rings, shift lever, clutch or torque converter and much more. It therefore only has one gear.

That's why combustion engines need several gears
There are reasons for this. Classic petrol or diesel engines provide a large part of their power in a relatively small speed range. In order to make sensible and, above all, efficient use of this range, a transmission with several gears is required to make optimum use of the engine's limited speed range. Electric motors are different: they provide their power over a much wider speed range and develop high torque at both low and high speeds.

Theoretically, electric cars could therefore manage with a rigid connection between the motor and drive shaft. In practice, however, an input gearbox is usually used as a so-called reduction gearbox, which reduces the engine speed by a fixed factor, usually between 1:8 and 1:10, and transmits it to the wheels. For example, an electric motor can run at up to 15,000 rpm, while the wheels typically rotate at 1000 rpm or less.

Ford installs a ten-speed automatic transmission in vehicles such as the Mustang or Bronco.

(Bild: Stephan Schätzl)

This type of transmission is also typically used in electric vehicles with multiple drive motors (e.g. one motor per axle or one motor per wheel). In these cases, each motor usually has its own input gearbox. The motors work independently of each other and drive the respective axle or wheels directly. There is no mechanical connection between the motors; synchronization takes place via the vehicle software (torque vectoring).

It does not matter how many motors the e-drive consists of: As with an automatic transmission, all the driver has to do after activating the drive system is shift to "D" (for Drive) and press the accelerator pedal.

Transmission supports recuperation
The transmission in the electric car has another important function: it also supports recuperation, in which the electric motor works as a generator. When braking, energy is fed back into the battery. The gearbox ensures that the motor also operates in the optimum range during recuperation.

One gear is enough to whizz off in an electric car. One gear is also enough to drive an electric car at higher speeds on the highway. However, many electric vehicles are driven at moderate speeds on the highway. This is because power consumption increases disproportionately, primarily due to air resistance, which is particularly noticeable at speeds above 130 or 140 km/h.

Two-speed gearbox for electric cars
Some manufacturers are trying to counter this problem with a two-speed gearbox. Porsche, for example, is installing it in the Taycan, while Mercedes will also equip the new electric CLA with a two-speed gearbox in 2025. This will then even have a multi-plate and dog clutch as well as two planetary sets. The first gear with a short ratio of 11:1 enables rapid acceleration from a standstill and ensures high efficiency in city traffic. The second gear (5:1 ratio), on the other hand, is designed for good power delivery at high speeds and high efficiency at cruising speed. The CLA is capable of 210 km/h in second gear.

However, the savings potential of two-speed transmissions is in the single-digit percentage range. This is offset by higher costs for the more complex transmission. In order not to make the currently still comparatively expensive e-cars even more expensive, most manufacturers are sticking with the simpler solution with just one gear. However, the two-speed gearbox has another advantage, especially in the Porsche Taycan: the electric sports car from Zuffenhausen would not reach 260 km/h with just one gear ratio.