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In physics, the definition of torque is a force that acts on a body through a lever arm.
Applied to internal combustion engines or electric motors, torque indicates the force to which the drive shaft is subjected.
Torque is expressed in pound-feet (lb-ft) or newton-meters (Nm).
- The interaction of torque and engine speed (rpm) determines the engine power.
If you’ve never hit the road in an electric car (➜ Read more: Electric cars and plug-in hybrids explained), the time is ripe. Not only is it good for your green conscience, but driving pleasure (➜ Read more: Busting electric car myths) certainly hasn’t been neglected either. One of the reasons for this is the torque. Or more specifically: the immediacy with which this applies when you press on the accelerator. But what role does engine power play in that, i.e. the kW or hp figure?
To answer that question, you have to go back a step, as BMW expert Michael Griese explains: “What you as a driver expect from your vehicle is important. Is it a high level of engine power or of torque?” In the end it comes down to whether you really want a high top speed or quick accelerator response.
Let’s start with the theory. In physics, the definition of torque is the force that acts on a pivot point with the aid of a lever arm. The formula for torque, the law of the lever is: torque = force (N) x lever arm (m). The units for torque are newton-meters (Nm) or pound-feet (lb-ft). Newton (or pound) stands for the force that is acting, while meters (or feet) stand for the length of the lever arm. Another name for torque that perhaps gives a clearer indication of its meaning is rotational force. As this name suggests, rotational force or torque ensures that an object rotates. It thus indicates the force that acts on the vehicle’s drive shaft when it rotates. Force (N), on the other hand, accelerates objects linearly. An engine’s power is the product of force and the speed with which this force acts. Engine power and torque are both indicators that depend on the engine speed.
A high torque enables efficient and energy-saving driving. A high engine power enables a car to accelerate quickly and reach a high top speed.
Project Manager for Electrical Drive Systems, BMW Group
So, that was the theory – now for the practice. In combustion engine design, the aim is to provide the driver with high torque even at low engine (rotation) speeds. At the same time, the torque should be applied over the largest possible engine speed range. In terms of driving experience, high torque means the shortest possible delay between the driver pressing the gas pedal and the engine responding. The driver perceives the high torque as great traction when setting off or overtaking.
So, powerful torque applied early means a high degree of certainty (when overtaking), great driving pleasure and efficient propulsion for the driver. Nevertheless, with all combustion engines there’s always a momentary delay (albeit a small one, depending on the design) before the torque kicks in – unlike with an electric car engine, as we’ll see below. In short, high torque available early only offers advantages for drivers.
Torque and its importance to drivers is best illustrated by its distinction from the term engine power or output. Here, too, it’s best to provide a brief theoretical backdrop as a basis for the explanation from the BMW expert: with engine power, the time factor comes into play. Put simply, it indicates the energy converted in a given period of time. The physics formula is: power = force x velocity. It is quoted in kilowatts (kW), formerly in horsepower (hp). One watt (W) stands for one newton-meter per second, while 1 horsepower is the power required to lift 33,000 pounds exactly one foot in one minute. An engine achieves high power either through high torque or high engine (rotation) speeds. The maximum engine power that the manufacturers advertise and that is also specified in the vehicle registration document is usually available at high engine speeds.
The measurement of standard acceleration (➜ Read more: From 0 to 60: Get up to speed on acceleration) fits well here: the gears are fully extended, the engine is operating at high (rotation) speed ranges, the maximum rated power can be called upon. To understand engine torque and engine power, and by extension everyday use of the car, it’s important to know that an internal combustion engine achieves its maximum torque at engine speeds lower than the maximum power output. So when we talk about good power from the lower engine speed range, we mean that there is high torque even at low engine speeds. This is useful knowledge for limiting gear changes when driving manual transmission vehicles. Or for pulling trailers in your car and for driving uphill.
An example illustrates the difference between torque and engine power. If, as a driver of an internal combustion engine car, you keep an eye on the dials in the display, you’ll see that, as the engine speed (rpm) increases, you’ll reach the point at which the torque is at its maximum. At the same time, engine power increases with increasing engine speed. When the driver feels that the engine power, the engine performance, is decreasing in relation to the engine speed, they shift up a gear (an automatic does this itself, of course). The engine speed then drops, and the game starts all over again.
Contrary to popular belief (i.e. what your friends think), in everyday life with a car, the torque of an engine plays a more important role than the (maximum) engine power. This is because, in order to conserve resources – the environment, people’s wallets and their nerves – engineers want drivers to drive with good traction in the low engine speed range. Which brings us to the electric car engine. What’s the big plus with electric cars? They provide full torque from the word go. With no delay whatsoever. That’s why electric cars’ acceleration (➜ Read more: Electric car facts from around the world) is so memorable for both drivers and passengers. Not only that, but jolt-free and without gear changes, as many electric cars come with a single-speed automatic transmission.
At low engine speeds it’s the torque that matters, at high speeds it’s the engine power.
Project Manager for Electrical Drive Systems, BMW Group
BMW expert Griese explains what’s special about electric cars. “The high torque of an electric motor enables vehicles with such motors to get around efficiently and in an energy-saving manner.” The highest possible speed, which is achieved through high engine power, is not the only focus for electric cars. Electric cars offer great traction that enables comfortable and efficient driving, but they don’t neglect driving pleasure either!
In other words, maximum torque is actually reached at a low rpm in an electric motor. “And it stays close to constant right up until you reach maximum engine power,” as engineer Griese explains further. From there, the torque drops and the engine power remains constant. As soon as the point of maximum torque is reached, the vehicle no longer accelerates faster. BMW expert Griese sums up as follows: “At low engine speeds it’s the torque that matters, at high speeds it’s the engine power.”
So what’s more important, a high engine power or a high torque? It all comes down to what you as a driver need. If, for example, you prefer a high top speed, you’ll find this in a vehicle with a high rated power output. If, on the other hand, you prefer an efficient and comfortable driving style with a fast start at the lights, you’re relying on the torque factor – as is the case with electric cars.
What is torque in a car?
Torque is a physical quantity that indicates the traction in an engine design. The physical unit for torque is the newton-meter (Nm). For cars with an internal combustion engine, the aim is to achieve a high torque as early as possible at low engine speeds. In electric cars torque is available immediately, and that’s an advantage.
Author: Nils Arnold; Illustrations: Michael Bloss; Photos/Video: BMW