Abstract: A rotor having the configuration of a spherical wedge is mounted in a hemispherically shaped chamber for rotary, nutating motion about the center of curvature of the chamber. The rotor divides the chamber into two separate sections, and revolves about a stub shaft carried on the inner end of a crankshaft, which projects into the chamber at an angle of approximately 18.degree. to the stub shaft. An apex formed by a pair of intersecting, plane surfaces on the rotor, is maintained in sliding engagement with the flat, base end of the hemispherical chamber. During each half revolution the rotor pivots about its apex to swing its intersecting, plane surfaces alternately into contiguous relation to the flat end of the chamber so that the two separate sections thereof are alternately compressed and expanded. Necessary ports are provided for supplying fluid to, and exhausting fluid from, said chamber sections.