In addition to classification as absorption, motoring, or universal, as described above, dynamometers can also be classified in other ways. A dyno that is coupled directly to an engine is known as an engine dyno. A dyno that can measure torque and power delivered by the power train of a vehicle directly from the drive wheel or wheels without removing the engine from the frame of the vehicle), is known as a chassis dyno. Dynamometers can also be classified by the type of absorption unit or absorber/driver that they use. Some units that are capable of absorption only can be combined with a motor to construct an absorber/driver or "universal" dynamometer. Types of absorption units Eddy current (absorption only) Magnetic powder brake (absorption only) Hysteresis brake (absorption only) Electric motor/generator (absorb or drive) Fan brake (absorption only) Hydraulic brake (absorption only) Force lubricated, oil shear friction brake (absorption only) Water brake (absorption only) Compound dyno (usually an absorption dyno in tandem with an electric/motoring dyno) Eddy current type absorber Eddy current (EC) dynamometers are currently the most common absorbers used in modern chassis dynos. The EC absorbers provide a quick load change rate for rapid load settling. Most are air cooled, but some are designed to require external water cooling systems. Eddy current dynamometers require an electrically conductive core, shaft, or disc moving across a magnetic field to produce resistance to movement. Iron is a common material, but copper, aluminum, and other conductive materials are also usable. In current (2009) applications, most EC brakes use cast iron discs similar to vehicle disc brake rotors, and use variable electromagnets to change the magnetic field strength to control the amount of braking. The electromagnet voltage is usually controlled by a computer, using changes in the magnetic field to match the power output being applied. Sophisticated EC systems allow steady state and controlled acceleration rate operation. Powder dynamometer A powder dynamometer is similar to an eddy current dynamometer, but a fine magnetic powder is placed in the air gap between the rotor and the coil. The resulting flux lines create "chains" of metal particulate that are constantly built and broken apart during rotation, creating great torque. Powder dynamometers are typically limited to lower RPM due to heat dissipation problems. Hysteresis dynamometers Hysteresis dynamometers use a magnetic rotor, sometimes of AlNiCo alloy, that is moved through flux lines generated between magnetic pole pieces. The