Abstract: An internal combustion engine including: a cylinder, a combustion chamber in the cylinder, two opposed pistons within the cylinder and relatively movable towards and away from each other. The combustion chamber is defined between the two pistons. Each piston has an inner side defining a movable wall of the combustion chamber and also has an outer side. Two rotary output shafts extend generally transverse to the axial direction of movement of the pistons. The pistons are freely independently movable within the cylinder. An eccentric cam portion is provided on each output shaft so as to be rotatable therewith and each piston has a cam follower portion in the form of a rotary member on the outer side thereof. Each cam follower portion is arranged to bear against the associated eccentric cam portion of the respective output shaft so as to transmit outwardly directed gas forces arising within the combustion chamber and acting on the piston inner sides to the accentric cam portions so as to drive the output shafts.