Abstract: A rotary piston engine having a pear-shaped piston with convex sides is mounted for rotation within a housing whose internal cross-section presents a symmetrical oval shape which is slightly constricted in the middle. The annulus between the piston and the housing is subdivided into intake, compression, expansion and exhaust chambers by four spring loaded vanes. A compression vane and a counter-piston vane are mounted on opposite sides of an external combustion chamber and their movements are coordinated to seal the combustion chamber with respect to the exhaust chamber and the intake chamber during compression and combustion. An engine separation vane, mounted for reciprocal movement and sliding engagement against the piston, separates the intake and exhaust chambers. A vane carried by the piston bears against the housing bore and subdivides each chamber in turn as the piston rotates.