Abstract: An internal combustion engine (12) having a rotating crankplate (33) mounted underneath the cylinders (45) and connected to the pistons (54) by connecting rods (72) that are constrained to move in a straight line up and down according to the movement of the pistons (54). The crankcase (15) has a cylindrical inner wall (18) and a cylindrical outer wall (21) that each have opposing guide rails (108) defined therein. The guide rails (108) constrain the motion of the connecting rods (72) to motion in a single plane. The cylinders (45) are stationary and the connecting rods (72) can only move up and down. The force of the connecting rods (72) on an inclined cam surface on the cam lobe (90) during the power stroke causes the crankplate (33) to rotate.