1,196,921. Dynamometers; speedometers. D. BASTOW. July 12, 1967 [July 12, 1966; Oct. 1, 1966], Nos. 31146/66 and 43972/66. Headings G1K; G1W. [Also in Division G3] An energy absorption dynamometer comprises a restricted hydraulic circuit around which oil is pumped by a main pump 2 and an auxiliary pump 6 both driven by a prime mover, the output power of which is to be measured, the circuit including a heat exchanger 4, the auxiliary pump maintaining a positive return pressure in the circuit at the input to the main pump and means being provided for measuring the input speed and torque of both pumps and determining the output power as a function thereof. As shown in Fig. 1 both pumps 2 and 6 are driven by the output shaft 1 of the prime mover and are of fixed displacement type. A relief valve 8 in the return leg 7 of the circuit maintains the pressure therein constant and this pressure is used as a power source for various control units. The input torque T to the dynamometer is measured by a balance 70, Fig. 7, connected between a piston rod 72 and arm 69 on the trunnion mounted pump 2, the circuit return pressure being used to maintain a constant moment arm by applying the pressure to one side or other of the piston 74 wheneve" the arm 69 deviates from a central horizontal position. A monitoring pressure representing the square of the speed of the output shaft 1 is produced by a gear type hydraulic governor pump 22 in the hydraulic circuit. The pump 22 is driven the shaft 1 and forces oil through a control orifice 23 for return to the circuit upstream of the pump 6 so long as temperature-sensitive valve immediately downstream of the orifice 23 keeps a bye-pass closed. The monitoring pressure is tapped just above the orifice 23. The hydraulic circuits has a localized restriction formed by an adjustable spring-loaded restrictor valve 3, Fig. 3, which is incorporated in a hydraulic servo governor having a spring loaded piston 42 slidable in a cylinder 39 under the influence of the speed-monitoring pressure. Upon any deviation from a predetermined speed, the piston 42 moves a piston valve member 49 to direct hydraulic fluid from the circuit return leg 7 to one side or other of a double-acting piston 52 to move a spring abutment 57 of a compression spring 58 loading a valve member 59 restricting the flow of hydraulic fluid through the valve 3 whose inlet 60 is supplied from the pump 2 and whose outlet 62 feeds the return leg 7 of the circuit. The consequential change in the load on the main pump 2 tends to restore the speed to the predetermined value, so that the pressure differential across the restrictor valve is proportional to the input torque. A rack 46 meshing with a pinion 45 with left and right hand threaded holes receiving piston rods 43, 47, is provided for setting the predetermined speed. In order to delay the application of load to the prime-mover when starting up, off-loading means 40 are provided comprising a piston 55 slidable in a cylinder 54 carried by the rod 53 of piston 52. Hydraulic fluid from the return leg 7 is admitted to the cylinder 54 so that when this pressure is small during starting from rest, the piston 55 floats with the abutment 57 carried by its piston rod 56 thus reducing the spring-loading on the restrictor valve 3 to zero. The return pressure in leg 7 of the hydraulic circuit is also used by hydraulic motors 11, 19 driving fans 10, 18 of heat exchangers 4, 17 in the hydraulic circuit and water-cooling circuit of the prime mover respectively. The fans may have temperature controlled shutters or the speeds of their motors may be temperature controlled. The hydraulic servo governor of Fig. 3 may be modified by using a cam in place of the speed setting mechanism to introduce any desired speedtorque relationship and the piston 42 and cylinder 39 may be replaced by a diaphragm assembly responsive to the speed monitoring pressure. The hydraulic governor pump may be replaced by a centrifugal governor. If automatic speed control is not required, a simple combined off-loading means 40 and restrictor valve 3 may be manually controlled. Variable delivery type pumps 2 and 6 may be used with their controls spring loaded to zero and maximum delivery positions respectively and the off-loading means 40 can then be dispensed with. Pressure representing the output torque obtained from the present dynamometer may be applied to the variable speed device of Specification 1, 196, 922 Cognate with which when driven at the output speed of the prime mover will indicate its output power.