Abstract: Apparatus for four-cycle rotary engine operation includes a cylinder block rotatable about a first axis and having six cylinders uniformly spaced and radially arranged about the axis, a set of six pistons rotatable about a second axis which is offset from and parallel with the first mentioned axis, each piston being disposed to move in a different one of the cylinders, and a valve system for introducing fuel charges into and exhausting combustion products from the cylinders. A crankshaft extends within the cylinder block and is coupled to the pistons to rotate as the cylinder block rotates and as the pistons move in their respective cylinders. The fuel charges are successively fired in alternate pairs of adjacent cylinders to cause the cylinder unit and crankshaft to rotate in a smooth and efficient manner.