1,196,665. Rotary absorption dynamometers. CLAYTON MFG. CO. June 15, 1967 [June 22, 1966], No.27642/67. Heading G1W. [Also in Division G3] Apparatus for testing the underload performance characteristics of a prime mover comprises a friction-type rotary power absorber 2 having a shaft 4 driven by the prime mover and a load controller including a positive displacement pump P driven by the shaft 4 to generate fluid pressure for loading an actuator 176 operating brake shoes 68 in the absorber 2 so that the fluid pressure and thence the retarding force opposing the torque of the prime mover varies as a function of the speed of rotation of the prime mover. In the chassis dynamometer shown the shaft 4 is driven by rollers supporting the drive wheels W of a vehicle whose engine is under test. The torque absorbed is measured by the reaction between an arm 200 fixed to the stator 62 and a pressure transducer 192 and is indicated by a pressure gauge 198. The power absorber 2 includes a casing through which liquid coolant can be pumped. This casing and the rotor which surrounds the stator can be readily removed for the brake shoe linings to be inspected and replaced. The inlet 212 of the pump is connected with a reservoir 224 of liquid 225 and its outlet 214 is connected through a flow restrictor 234 with a conduit 232 and through a conduit 190 with the brake actuator 176. The fluid pressure supplied through the conduit 190 can be preset (or adjusted during testing) for any given speed of the power input shaft 4 by setting the variable orifice of the restrictor valve 234. For any given setting the pressure differential across the valve 234 will vary as the square of the flow rate therethrough. Thus as the pump is driven by the shaft 4 and the brake force applied is determined by the pressure supplied through conduit 190, the retarding force opposing the driving torque varies with the square of the speed of the vehicle engine, thus being zero at zero speed and increasing and decreasing faster than the engine to reduce the tendency of the engine to stall when it momentarily loses power.