Abstract: A hydraulic device for a reciprocating piston engine to convert the piston motion into rotation of a drive shaft. A hydraulically driven rotor is mounted for rotation on the drive shaft and has a close machine fit with the rotor housing. The cylinder in which the piston reciprocates is mounted on the rotor housing and communicates with the peripheral surface of the rotor through specially located openings formed in a plate. The rotor contains hydraulic fluid and its periphery has a portion provided with open passages to direct fluid into the base end of the cylinder during the compression stroke. During the power stroke of the piston, the fluid in the base end of the cylinder is expelled against vanes which are formed on the rotor periphery, thereby turning the rotor. A cam and associated linkage mechanically force the piston to the bottom dead center position at the end of each power stroke.