Dynamo-meters are used to measure torque and power over the engine operating ranges of speed and load. They do this by using various methods to absorb the energy output of the engine, all of which eventually ends up as heat.
Some dynamometers absorb energy in a mechanical friction brake (prony brake). These are the simplest dynamometers but are not as flexible and accurate as others at higher energy levels.
- Fluid or hydraulic dynamometers: absorb engine energy in water or oil pumped through orifices or dissipated with viscous losses in a rotor-stator combination. Large amounts of energy can be absorbed in this manner, making this an attractive type of dynamometer for the largest of engines.
- Eddy current dynamo-meters: use a disk, driven by the engine being tested, rotating in a magnetic field of controlled strength. The rotating disk acts as an elec- trical conductor cutting the lines of magnetic flux and producing eddy currents in the disk. With no external circuit, the energy from the induced currents is absorbed in the disk.
One of the best types of dynamometers is the electric dynamometer, which absorbs energy with electrical output from a connected generator. In addition to having an accurate way of measuring the energy absorbed, the load is easily varied by changing the amount of resistance in the circuit connected to the generator output. Many electric dynamo meters can also be operated in reverse, with the gen- erator used as a motor to drive (or motor) an unfired engine. This allows the engine to be tested for mechanical friction losses and air pumping losses, quantities that are hard to measure on a running fired engine