Abstract: Presented is an internal combustion engine that utilizes horizontally opposed pistons operating in direct opposition to one another in axially aligned open ended cylinders suspended below a base plate and arranged generally symmetrically on opposite sides of a vertical plane coincident with the rotational axis of a longitudinally extending main power crankshaft also suspended for rotation below the base plate. A pair of auxiliary crankshafts are provided spaced on opposite sides of the vertical plane and having rotational axes that lie in a common plane below and parallel with the plane of the base plate and coincident with the longitudinal axis of the cylinders. A cam shaft is provided to be driven by at least one of the auxiliary crankshafts in timed sequence with linear displacement of the pistons and operation of poppet-type intake and exhaust valves actuated by the cam shaft to admit a fuel mixture into the combustion chamber defined by the juxtaposed heads of the pistons.