An axial piston internal combustion engine designed to improve the thermal efficiency of the engine by utilizing a sine wave/connecting rod assembly (5) to cause the piston (6) of the engine to travel vertically parallel to the engine's main shaft (13). The upper sine wave barrel (2) and lower sine wave barrel (3) have a channel in them that the cylindrical ends of the connecting rod (4) travel in, forcing the connecting rod (4) and piston (6) to travel vertically. The engine also utilizes slide valve (20) that has slots in it that when slid past the ports of the upper cylinder head (18) and lower cylinder head (19) controls the intake and exhaust timing events of the engine. The channel in the upper sine wave barrel (2) and lower sine wave barrel (3) is designed such that the vertical expansion/exhaust strokes of the engine are longer than the intake/compression strokes which allow the engine to make more efficient use of the energy produced by combusting the fuel, improving the thermal efficiency of the engine. The design also reduces the friction between the cylinder walls and piston (4) and the friction associated with typical engines valvetrain components, improving the overall efficiency of the engine. By reducing the frictional losses and improving the thermal efficiency the new engine should have decreased fuel consumption.