Abstract: An engine is disclosed in which a piston is mounted to orbit in a circular path within a working chamber for use as a fluid pump or motor. The working chamber includes a pair of juxtaposed cylindrical chambers which are interconnected by an arcuate chamber, with inlet and outlet ports opening into the cylindrical chambers. The walls of the cylindrical chambers are formed with recesses for preventing hydraulic lock-up and vacuum formation. The piston includes a pair of spaced-apart crank ends which carry an arcuate mid-span as the principal displacement element. A drive train conjointly moves the crank ends in circular paths within the cylindrical chambers which in turn gyrates the mid-span in a circular orbit within the arcuate chamber. The working volumes within the engine successively expand and contract for inducting fluid through either port, depending upon the direction of gyration, and for exhausting the fluid through the other port.