Torque Shop

I understand that a car with more power is faster, but am not clear what torque is and what it means to a car's performance. What is the relationship between power and torque, and is it be the same for an electric vehicle (EV)?

Power is a derived value, calculated as a product of torque and the engine's speed.

Since engine speed is a function of time, the calculated value of power (Nm per second or Nm/s) is work done per unit time or the rate at which work is done. In metric units, 1 Nm/s is equal to 1 watt.

Hence, if an engine is producing 200Nm of torque at 3,000rpm, you can calculate its power at that speed as follows: 200Nm x 3,000rpm x 0.1047 = 62,820W or 62.82kW (or around 85hp).

An internal combustion engine's (ICE) torque output is not constant. It rises steadily with engine rpm up to a point, and then begins to diminish even though engine speed can continue to rise.

An ICE produces maximum power when the combination of engine speed and torque results in the highest mathematical product of torque and engine speed.

For example, even though maximum torque occurs at 3,000rpm and falls off after that, because engine speed continues to rise to 6,000rpm or more, power continues to increase.

Modern forced induction engines develop much higher torque across a range of rpm. Obviously then, within this window of constant torque, the rise in power is more linear. This is why most turbocharged cars today feel very punchy in the mid-range.

On the other hand, a normally aspirated Formula One V8 engine that has been used previously can be rated at 758hp despite producing just 300Nm of torque (less than a Volkswagen Golf GTI), because its engine speed reaches 18,000rpm.

The same principles apply to an electric motor. What is distinctly different is the way the motor develops torque. It does not have to spin at any particular speed for the torque to rise.

Instead, as soon as the motor begins to spin, it achieves its maximum torque within fractions of a second, continuing to maintain this figure up to its maximum rated speed (after which the torque begins to tail off).

It is this trait which makes EVs highly responsive when accelerating from low speeds.

Shreejit Changaroth

A version of this article appeared in the print edition of The Straits Times on October 17, 2020, with the headline 'Torque Shop'. Print Edition | Subscribe