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An eddy current dynamometer (EC dyno) is the most popular type of dynamometer used in chassis dyno construction. Typically air-cooled, there are water-cooled versions of the dynamometer used primarily for very high horsepower or high-torque applications. An eddy current dynamometer uses an electric core moving across a magnetic field to produce the required resistance needed to test an engine for horsepower. Iron cores are the most common, however, aluminum, copper and other conductive materials are also used in the manufacture of the dynamometer. Most of the EC dynos use a series of large cast-iron disks, similar to disk brakes used on a vehicle, to place the required resistance against an engine or chassis when conducting a power test.
While some types of dynamometer use water to generate resistance against rotational power and torque, an eddy current dynamometer uses an electrical-magnetic charge to resist the rotational power of an engine via a chassis dyno. The chassis dyno measures the actual amount of horsepower and torque that an engine makes as measured at the drive tires. This type of power monitoring and measuring is important because it measures the real power of an engine. Components such as the transmission, drive gears and engine accessories are power robbers in the form of parasitic loss. The eddy current dynamometer measures the engine's power output at the wheels after all of these components have drained power from the engine, providing a true or real power figure.
Prior to the use of an eddy current dynamometer and a chassis dynamometer, power levels of any given engine were calculated using a water brake type dyno. This gave an accurate horsepower figure for the engine as taken at the flywheel. It was later understood that parasitic loss claimed many horsepower from an engine and subsequent power figures were taken at the tire for a more accurate depiction of the amount of horsepower a vehicle was producing. This type of dynamometer uses a computer to adjust the amount of electro-magnetic braking force required to measure the power of a vehicle on the chassis dyno.
Braking the dyno against a high horsepower engine creates heat. The most common style of eddy current dynamometer is air-cooled. In some of the larger designs, a water-cooled version of the dynamometer is used to better manage the heat generated from attempting to stop the crankshaft from turning in a high-powered engine. Maintenance determines when the cast-iron braking disks require changing to maintain proper power measurements.