Abstract: An engine producing combustion of a fuel by compression rather than ignition comprises a conventional engine cylinder having a pair of opposed secondary cylinders mounted side-by-side on the upper portion of the cylinder chamber. The secondary cylinders are driven by a cam arrangement in a direction transverse to the action of the main engine cylinder and at predetermined synchronized time intervals. The secondary pistons operate in conjunction with the main piston entering the respective compression chambers on the 0.degree. top dead center system of the main piston to produce combustion by compression thereby eliminating the need for electric ignition and providing more power per cylinder. As further advantage, there is less pollution due to more efficient combustion in an arrangement which also eliminates the necessity for engine tuneups.