Torque Shop

Why is it that electric cars such as the BMW i3, Hyundai Ioniq Electric and Renault Zoe do not have a six-speed or eight-speed transmission like normal petrol or diesel cars?

Internal combustion engines fuelled by petrol or diesel depend on the induction of air before the air-fuel mixture is compressed and ignited to produce power.

In an internal combustion engine, pistons moving downward in the cylinders give rise to air induction, which is never consistent throughout an engine's speed range.

Torque output rises as the engine speed increases and eventually reaches a peak before tailing off again. The low starting torque that is transmitted to the wheels must therefore be multiplied to get the car moving and to build up speed.

A multi-speed gearbox or automatic transmission serves to multiply engine torque at various road speeds.

However, an electric motor is different. There is no air or fuel to ignite. Instead, just a supply of electrical energy is needed to produce the mechanical power.

The reason a gearbox can be eliminated in an electric car is that an electric motor's maximum torque is developed the moment the motor is energised.

Because an electric motor typically has a high rated speed often exceeding 10,000 revolutions per minute, a single-stage fixed reduction gear is incorporated to attain conventional tyre and road speeds.

Shreejit Changaroth

A version of this article appeared in the print edition of The Straits Times on December 22, 2018, with the headline 'Torque Shop'. Print Edition | Subscribe