An engine is disclosed. In one implementation, the engine includes a camwheel mounted to an output shaft. Eighteen cylinder-enclosed pistons are positioned in a circular plane about the camwheel such that the radius of the circular plane and the camwheel are coplanar. Lobes integrally formed on the camwheel define a guiderail including a continuously, substantially sinusoidal surface biased for rotation in the direction of the rotary motion of the output shaft. Drive bearings associated with each piston engage and maintain contact with the guiderail. The rotary motion is obtained by transferring the displacement of the plurality of pistons from respective top dead center positions and bottom dead center positions to the guiderail, thereby rotating the camwheel and output shaft.