An engine is provided comprising a cylinder, a piston, a connecting rod, and a crank. A cylinder combustion occurs in the cylinder to induce movement of the piston in a lateral direction. The cylinder combustion comprises a first stage during which a first portion of a fuel and air mixture is burned over a substantially constant volume, and a second stage during which a second portion of the fuel and air mixture is burned over a substantially constant pressure. The piston comprises a piston head and a rigid piston rod mounted to the piston head. The connecting rod is pivotally mounted to the rigid piston rod and capable of receiving a pulling force applied by the piston. The crank is pivotally mounted to the connecting rod such that the connecting rod applies a torque capable of rotating the crank about a fixed axis as the piston moves within the cylinder.