Dynamometers: everyone talks horsepower, but how do you measure it?

Publish Date: 
Jun 19, 2014
By Tracy Martin

Few horses of even the largest breeds can pull that much weight for any length of time, and there was speculation that Watt had exaggerated the number to his advantage for the purpose of overvaluing his steam engine’s capabilities. Another view is that Watt was just applying good marketing techniques by comparing horses (a familiar form of power and effort at the time) to new technology -- the steam engine. With the proliferation of the steam engine, and Watt’s formula for horsepower, a way to measure power output was needed.

The Dynamometer

The first dynamometer was invented in 1821 by Gaspard de Prony. The de Prony brake, as it was called, was used to measure the performance of engines and other types of machines. Dynamometers have been widely used since the late 1800s to measure the torque of steam engines. The water brake type of dynamometer, sometimes mistakenly called a hydraulic dynamometer, is the oldest type of design and is still used today. These power absorption units can accommodate anything from a Briggs and Stratton lawnmower engine that makes two horsepower to marine diesel engines that can produce hundreds of thousands of horsepower. These early dynamometers basically consist of two half couplings -- a rotor and stator.

For measuring horsepower from powersports engines, there are two basic types of dynamometers: engine and chassis. Engine dynamometers are used to measure power directly at the engine’s crankshaft or flywheel. The engine is tested without its transmission or drivetrain connected -- in other words, it’s not installed in a motorcycle but rather on a test stand. For the majority of riders, removing their engine for this type of testing is too costly and impractical. The chassis dynamometer measures power at the motorcycle’s rear wheel and the bike simply has to be ridden on to the chassis dyno and strapped down.
Let’s take a look at the two most common dynamometers we use -- inertia and eddy current dynamometers.

Inertia Dynamometer

The most common design of dynamometer for powersports use is the inertia type. It doesn’t actually measure torque, but instead calculates it by measuring acceleration. The rear wheel of a motorcycle (or ATV) accelerates a 900-lb. steel drum. Force at the surface of the drum is measured indirectly by measuring its acceleration from one revolution to the next. Force is calculated using Newton’s 2nd law (mass times acceleration). Because the mass, or weight, of the drum is known, force (horsepower) can be calculated.

A typical dyno run begins with the engine running just over idle, in fourth or fifth gear, with the rear tire turning the drum. When the throttle is opened, the engine accelerates the dynamometer’s drum as engine speed increases to redline. Computer software used with inertia dynos can accurately measure acceleration of the drum over small increments of time and calculate a value for torque. Using torque and engine RPM, rear wheel horsepower can be calculated.

Eddy Current Dynamometer

The eddy current brake type dynamometer uses electricity to place a load on an engine by creating a magnetic field.

The engine under test is connected to the dyno’s input shaft that spins a metallic rotor creating a magnetic field. When current is increased to the dyno’s internal electromagnetic coils, the rotor shaft becomes harder to rotate and thus loads the engine. Torque load is measured using a strain gauge similar to those used on a water brake dynamometer. The rotor gets hot as the dyno resists the engine’s power and must be cooled.

Eddy current dynamometers that are used for testing motorcycle engines are usually air-cooled, employing what looks like an oversized automotive brake rotor with large cooling fins.

Eddy current dynos are accurate and offer the flexibility to perform steady-state load testing or acceleration sweep testing like the inertia dynamometer.