Newton-metres and kilowatts: the most confusing concepts that explain the magic of the kick in your back when your car accelerates.
From linguists to mechanics to motoring journalists, the ability to simplify the concepts of power and torque to simple and understandable English remains one of the most challenging tasks.
Yet a basic grasp of these metrics allows an enhanced view of how engines perform and why they behave the way they do.
Let’s start with torque.
In South Africa, an engine’s ability to deliver torque is expressed in Newton-metres, abbreviated to Nm, or foot-pounds in the UK and the USA. In its simplest form, torque is the measurement of rotational force (or to be slightly more scientific, the force of one newton being applied to the end of a moment arm that is one metre long).
An engine’s ability to deliver torque can be measured, whereas power is calculated. Power, expressed in kilowatts (kW) in South Africa (and horsepower in the UK and USA), is a unit of work performed divided per unit of time (one Watt = one joule per second).
To go full anorak on this definition, one horsepower is equivalent to 33 000 foot-pounds, or the power required to lift 550 pounds by one foot for one second, or about 746 watt (1 horsepower = 746 watt).
Kilowatt is a function of torque and revolutions per minute and is calculated as following: Power (kW) = torque (Nm) x speed (revolutions per minute, or RPM) / 9.5488.
So how does my car’s engine make power?
By rotating an output shaft at a certain number of revolutions per minute. The amount of torque made is affected by the RPM used at that given moment. The gearbox acts as a torque multiplier by changing the rotational rate at which the engine is turning. In low gears, torque is amplified at the expense of speed – such as when pulling away from standstill or hauling heavy loads. The converse occurs at high speeds, when engine torque is sacrificed for wheel torque.
The more torque an engine produces, the greater its ability to do work. Power is how rapidly that work is produced. The amount of torque produced is limited by amount of air flowing through the engine. Which is why large capacity engines, or artificially aspirated engines that are turbo or supercharged make more power than their naturally-aspirated counterparts.
So, which is the most desirable?
As stated before, power is a by-product of torque, so it’s not a case of either-or, but rather whichever is preferred is determined by the application in question. Different engines also make torque in different ways: if you have lots of weight to pull, torque is better, but to go fast you need power. Two extreme examples: the large-capacity, low-revving engine of a thirty-ton truck has 16 gears and thousands of Newton-metres but develops relatively few kilowatts for its engine capacity. By contrast a one-litre Japanese superbike has just six gears and a meagre torque count but a respectable kilowatt quota developed at stratospheric revs.
The ideal passenger car engine would deliver something in between – lots of torque spread over a wide as possible curve which allows for sprightly acceleration and good overtaking acceleration while minimising gear changes. Find all the power and torque you want on AutoTrader. AutoTrader South Africa has a multitude of new and used vehicles for every possible kind of preference. AutoTrader South Africa has been the leading media marketplace for the past 25 years, for the buying and selling of vehicles. Whether you are looking to buy a brand-new car or are in the market for a pre-loved model, AutoTrader has over 71 000 quality cars to choose from! Visit AutoTrader today to find your dream car.