What is power? We explain in plain English!

We’ve already discussed the concept of an engine’s torque output, but there’s a mathematical relationship between the measurement of this twisting force and the engine’s power output as well. 

In simple terms, power refers to how much work (how much torque) the engine can deliver over time, and indicates how much fuel the engine can burn (converting chemical energy from the fuel into rotational energy) in a given time period.

This is relevant when evaluating on-road performance: a high-torque but low-power engine (like an old-fashioned diesel) won’t have the sustained high-speed stamina of a lower-torque but higher-power engine (like an old-fashioned petrol), leading to a lower top speed and slower acceleration as wind resistance increases.

​Related: What is torque? We explain in plain English!

How is power created?

As mentioned, power is simply of the product of the engine’s torque, multiplied by its rotational speed (multiplied by a constant correction factor, if you’re not working in SI units). Little torque combined with low engine speed results in a low power rating, but little torque combined with a high engine speed will result in a higher power output (think old-fashioned VTEC as an example).

The inverse is true as well, and is demonstrated by modern, forced-induction engines: High torque at low-to-moderate engine speeds can give a decent power output (as in a turbodiesel), and high torque at higher speeds will give a high power output as well (as in a turbocharged petrol engine).

Related: For a more-detailed and scientific explanation of an engine’s power, take a look at this useful reference article.

Units of measurement

Power is measured in watt (W), according to international (SI) standards, but because the order of magnitude of this rating is so high, we customarily divide the power rating by 1000 to obtain a kilowatt (kW) number instead. 

In certain countries, the metric measurement system has not been fully adopted, with the antiquated imperial system still serving as reference point. In these countries, the measurement unit for power is horsepower (hp), essentially an imperial measure that equals the SI measure multiplied by a constant correction factor. 

There’s another rating as well, which is used in some European countries, named PS. This unit is very close to the “traditional” horsepower (hp), and is essentially a “metricised” derivative of the traditional unit – this time, multiplied by a constant of 0.9863 to convert from hp to PS.

1 Horsepower equals 0.7457 kilowatt (kW), so converting from hp to kW entails multiplying the imperial value by 0.7457. Conversely, going from kW to hp thus means dividing the imperial value by 0.7457.

Related: For a visual explanation of the difference between power and torque (and a host of other cool technical content), watch this excellent video on the Driving 4 Answers YouTube channel.

 

Why is power important?

The more power an engine has, the better it will be at doing a lot of work in a short time. It’s easy to visualise this by imagining a car at speed, where the wind resistance does constant work against the car’s forward motion. The more power at the car’s disposal, the more wind resistance (work) it can counteract, keep accelerating up to a higher speed.