Abstract: A rotary power plant that uses high-pressure, low volume hydraulic fluid has a housing journalling a rotor formed by a main shaft portion in the circumference of which one or more grooves are formed partially therearound; if more than one they are phased for partial overlap. Forming an expansion chamber with each groove, in addition to the sides of the groove and a radial end of the groove and the housing is a valve plunger spring biased against the groove, in which an end of the valve plunger rises as an automatically retractable barrier. An alternative embodiment provision of different-diameter shaft portions is disclosed.