Chassis dynamometer - how does it work?

Dynamometer (aka rolling-road, dyno or Leistungpruefstand) is a mechanical device for measuring power and torque of a car, bike, truck or even an agricultural machine. The torque produced by an engine is converted to traction force by transmission (gearbox, differential) at wheels. You may ask – where is the place for „power” then? Power is the torque of the engine multiplied by the rotation speed – so it is enough to measure traction force to be able to calculate the wheel power of a car, truck, motorbike. Of course our dynamometer must know noticeably more to show us the real power of the engine – power losses must be measured, also the dyno must know the transmission ratio (wheel speed to engine speed ratio) or directly the engine speed (for instance – from OBD port or CANBUS of the car).

Because the word „dynamometer” covers quite a wide area of various force measuring devices, not especially intended for passive measurement of torque of gasoline or diesel engine, we name dynamometers for cars „chassis dynamometers”. Dynamometers for engines, dismounted from cars (or even prototypes, never installed to any car) are just called engine dynamometers. These are passive dynos too (sink torque instead of generating it). Some other types of dynamometers that are capable to drive (deliver force) to measured machines to determine torque and power requrements (like pumps, brakes, fans, etc) are named with the prefix „active”.

Chassis dynamometers can be used not only for simple torque and power measurement. Very common usage is fuel consumption and emissions measurement according to governmental emissions testing cycles, or to determine real fuel consumption under load. Technical schools and universities use dynamometers for investigations in combustion physics, chemical comparisons of fuel, or just to simulate the road while testing the car for various purposes.

A very common usage of chassis dynamometers with electromagnetic brakes (eddy current brakes) is the calibration of ECU (engine control units like Bosch EDC17, MED17 Siemens Simos), various engine management controllers, not only as development at car manufacturers, but also for motorsport (racing cars & vehicles).

In modern chassis dynamometer we have a power absorption system, that is intended to simulate real loads for car’s engine. There are two main sources of this load. The first is own innertia of rotating car parts, wheels, dyno rollers, shafts, etc. Innertia, while accelerating with car at dyno, counterforces. So it is better when dyno rollers are heavy and have bigger diameter – as this increases innertia.

 The second source is optional – it is an electromagnetic brake (eddy-current brake). Such brake uses electromagnetic forces to apply resistance force to the shaft of the dyno. It gives an opportunity to measure higher power and torque outputs, especially in the case when dyno rollers are thin and light (so they do not deliver enough natural innertia). Eddy-current brake is not obligatory, and it has also a disadvantage – it needs a separate measurement of its counterforcing torque (kind of extensometer), and this reduces precision and accuracy of measurement. But if you need to tune standalone race ECUs – electromagnetic brake will help a lot, stabilizing rpms with full or any partial load, so tuning of AEM, Gems, EMU or other standalone engine controllers is easier.

DAC, or data acquisition system is an integral part of a chassis dynamometer. It commands the load (if the brake is installed), measures rotational speeds, acceleration of rollers, resistance force generated at the electromagnetic brake. Advanced dynos offer also a possibility of measuring other parameters that are important for a car tuner or a workshop – AFR (air to fuel ratio of mixture), EGT (exhaust gas temperature), MAP (manifold absolute pressure, aka boost), and many other parameters reachable via the OBD port – like ignition advance, various temperatures, etc. It is worth to remember that chassis dynamometer is not a simple device to determine power and torque – it can be like ECG (electrocardiography) for us humans – a powerful tool in hands of a „car’s doctor”.