The engine comprising this invention comprises a rotatable power takeoff drum which encloses a cylindrical cylinder block rotatable with said drum about an axis which is eccentric to that of said drum. A shaft for said block extends through a bore in a main shaft and contains longitudinal fluid inlet and exhaust passages for operating fluid which communicate at the inner ends with the radially innermost ends of cylinder bores which extend radially within said block and said bores have aligned bore sections which have progressively larger diameters outward from the inner ends thereof to provide multi-stage cylinder sections. Connecting rods connect the outer ends of pistons in said bores to the inner surface of the rim of said drum to rotate it as said cylinders communicate successively with said fluid inlet and exhaust passages.