How Does a Dyno Work?

Physics

A dynamometer, or dyno for short, is a tool used to measure the amount of power being generated by an engine. The physics behind a dyno is what enables the measurement to be taken. It begins with the equation that states that force is equal to mass times acceleration. This means that the amount of force can be calculated by finding the weight of an object being moved and then multiplying that by the rate at which it is accelerated. With this number it is then possible to calculate how much work is being done and then how much power is being generated. These equations are integral to all dynos and make it possible for engine power to be measured. There are two main types of dynos that are used by car enthusiasts today.

The Chassis/Inertia Dyno

In this type of dynamometer, a car is driven up onto a platform until its wheels are on a set of large metal cylinders. The car is then strapped in and a computer is connected to the car's ignition system through a spark plug wire, in order to monitor RPMs. The test will begin and the car will be driven through its full RPM range, from idle to redline (the maximum RPM that the engine is designed to safely operate at).

Once this is done, and the test is complete, the computer will use the speed of spinning cylinders to calculate the acceleration, and then use this number, along with the weight of the cylinders which it already knows, to get an answer to the force equation. It will then use this calculation to give the tester accurate numbers for the horsepower and torque of the car. Because this test takes the readings at the wheels, and not at the engine, this is known as wheel horsepower and is significantly less than the actual engine horsepower, due to power lost through the drive train.

The Brake/Engine Dyno

In this type of dynamometer, the engine has to be removed from the car and directly connected to the testing mechanism. The testing mechanism then uses either hydraulic fluid or water to create resistance to the engine's spinning force. This resistance is continued until the engine's maximum turning force is measured at every RPM, giving the tester an accurate reading of the engine's torque. A computer or the tester can then use the same equations to derive a horsepower number for the engine.

Because the measurement is taken at the engine itself, and does not have to be concerned with power loss through the drive train, the measurement is referred to as engine horsepower, brake horsepower or horsepower at the crank.

References

• Engine Dyno Testing
• Bristol Dyno

Resources

• Speed Engineering