Power explained for the people

By Barry Lake

TWO of the most often used terms relative to motor cars are two of the most misunderstood: Power and torque.

Perhaps some of the confusion comes from common usage of the word power. Think of a powerful man and you think of Arnold Schwarzenegger; lots of brute strength, but probably not the man you'd put your money on in a speed event.

With a car engine, the equivalent of muscle is actually the engine's torque output. Power is a combination of the torque, multiplied by the rate at which the work is done.

In a car engine, this rate is the engine speed, or revolutions per minute (rpm).

Torque is what is commonly known as "grunt", the sort of push you get from a big six-cylinder or V8 engine at low engine speeds.

Power can be the result of a large torque figure multiplied by relatively few rpm, as is common in road-going big-sixes and V8s.

It also can be developed from a much lower torque figure, produced by a small engine running at very high rpm, as is common with modern small, four-valves-per-cylinder engines.

Torque is a measure of force which, in a car engine, is a twisting action measured at the rear of the crankshaft (or at the flywheel). This occurs as the pressure of the burning gases above the piston pushes the piston downwards and this force is transferred via the connecting rod and a crank on the crankshaft.

So, torque is a measure of force. Power, however, is a term used for an amount of work done in a given period of time. Another way to demonstrate it is by looking at someone riding a bicycle. Torque is the twisting force produced by the cyclist pushing the pedals of a bicycle. The power produced by that cyclist is this same torque multiplied by the speed at which the cyclist is pedaling.

If the cyclist is pushing very hard but slowly on the pedals, then a lot of torque is being produced, but the cyclist might be moving very slowly up a steep hill, so the power output is quite low.

Changing to a lower gear and pushing less hard but doing it more often (as when pedaling more quickly), can produce the same final power output. That is, the cyclist can arrive at the top of the hill in the same period of time, either by pushing hard but slowly, or pushing less hard but faster.

The end result is that a given load (the bike and rider) were carried to the top of the hill in the same period of time. So, the same amount of work was completed in the same time, but in two different ways.

This is why a large engine can produce a lot more torque than a smaller engine, but not necessarily more power. A big engine, restricted in its ability to run at high revolutions by the weight of its components, usually produces a very high torque figure at low rpm.

The same end result can be achieved by a small engine with very efficient high-speed breathing capability (four valves per cylinder combined with good port and combustion chamber design) producing lower torque but doing it at much higher rpm.

A key factor in which is the best way to go is usually cost. A big, "lazy" engine is generally less costly to produce than a small, "high tech" engine. But since the latter type now combines good performance with outstanding fuel economy, the larger engines have been unable to match; this increasingly is becoming the norm.

Engine power is often referred to as "horsepower". The term was originally used to describe exactly what it states: the power of a horse. James Watt set the standard in 1783 when, after conducting tests with horses used in Cornish mines, he stated that, on average, a horse could lift a weight of 33 pounds (15 kg) through a distance of one foot (30.5 cm) in one minute.

When cars began to become common-place just over a century later, it was natural for people to compare their power to that of a horse, or team of horses, using this same formula.

Since the switch to metric measurement, however, we now measure power in Watts and, in the case of engine output, in kilowatts or kW (thousands of Watts). Thus we honor James Watt and his mathematical formula even though the new measurement is no longer directly related to horses.