Disclosed is a rotary internal combustion engine having a generally circular geometry in vertical cross section. The engine's rotor is circular, and it is disposed in a housing in two portions, each portion being a section of a different circle, the circles being offset along the engine's vertical centerline. One of these circles is identical in center and radius as the rotor. The other, owing to the displacement between its center and the center of rotation of the rotor, has a gap between it and the rotor. The engine's crank shaft rotates about an axis passing through the center of this second circle. The rotor has a plurality of radially extending holes connected by connecting rods from the crank shaft, each connecting rod having attached to it a sealing means making the holes effective as combustion chambers. This improved engine is unique in the design of the individual combustion chambers and pistons which undergo reciprocal movement within said chambers. The cooperative relationship of these pistons and connecting rods attached thereto affords the ability of the rotor to transition from the compression cycle to the power cycle essentially free from the sealing problems commonly encountered in engines of this design.