What a Dynamometer Does

Here's my second installation of the Mechanical Mythology series.

Claim: A dynamometer measures torque, not horsepower.

Status: Multiple - depends on the type of dyno.

A dynamometer, or dyno for short, is used for measuring the performance of an engine. These come in several designs. Engine dynos attach directly to an engine on a stand, while chassis dynos measure power at the car's driving wheels. The machinery inside a dyno also comes in several different designs.

The earliest design was a brake dynamometer. Most brake dynos are engine dynos, although it's possible to build a brake-type chassis dyno too. A brake dyno simply consists of a brake mounted on a stand that uses a load cell to determine the torque the brake is applying to the engine. The dyno operator sets the brake to a load that keeps the engine steady, measures the torque and RPM, then moves on to the next RPM point and repeats the process. Horsepower is then calculated using this formula:

HP = Torque * RPM / 5252

An innertial dyno uses the engine or the car's drive wheels to spin a heavy drum. A computer measures the speed of the drum. On an engine dyno, you can then calculate both horsepower and torque from this directly. On a chassis dyno, however, you cannot measure the engine torque directly. On an innertial dyno, you can calculate the horsepower from the rate of acceleration of the drum, while brake type chassis dynos measure road speed and what's known as tractive effort, and calculate horsepower from that. To obtain a torque measurement requires taking the RPM readings and reversing the above formula to calculate torque.