Torque Shop

How can electric vehicles (EVs), in particular electric passenger cars, increase their top speed?

You will find that while the 0-100kmh acceleration figures for electric cars are generally above average, their top speeds are considerably more modest.

Take, for example, the BMW i3, which will sprint from rest to 100kmh in just 7.3 seconds, but tops out at 150kmh.

The maximum speed limit in Singapore is 90kmh.

A similarly sized petrol car might take one or two seconds more to complete the drag, but will peak at 180kmh or more.

This is because most manufacturers of electric cars limit the top speed to preserve battery charge. Aerodynamic drag increases significantly as speed rises and hence battery power is consumed more rapidly when a high cruising speed is maintained.

The other reason has to do with an electric car's drivetrain. Since maximum torque from a typical motor is available the moment it starts to spin, there is no need for multiple gearing.

This is why most EVs are designed with just one fixed ratio for the purpose of reducing the electric motor speed, which is usually between 8,000 and 10,000rpm, before it is transmitted to the wheels.

The gearbox ratio is sized to provide, first and foremost, sufficient torque at the wheels for brisk urban driving.

If a high top speed is an objective, like for Porsche's upcoming Taycan, a two-speed gearbox would need to be part of the design. A second gear will allow such a car to reach or exceed 250kmh.

Being low-slung helps too.

Shreejit Changaroth

A version of this article appeared in the print edition of The Straits Times on January 19, 2019, with the headline 'Torque Shop'. Print Edition | Subscribe