Abstract: In a multi-cylinder internal combustion engine having a plurality of cylinders which are supplied with air via one common throttle valve and a control unit for regulating the number of operating cylinders which suspends the operation of an arbitrary number of cylinders by intercepting the air supply to said cylinders, it is noted at a point the output of the engine under Z.sub.1 -cylinder operation coincides with the output under Z.sub.2 -cylinder operation at the same degree of opening of the throttle valve at a given constant rate of engine rotation. Based on such a fact, the multi-cylinder internal combustion engine is made capable of shifting to and from Z.sub.1 -cylinder operation and Z.sub.2 -cylinder operation in the proximity of said point where these outputs coincide. This way, there will be no fluctuation in the output nor shock at the time of switching, and the control unit for this purpose can be made simple in construction.

Abstract: Balancer shafts are provided on the right and left of the crank shaft of an engine and so as to be driven thereby. Each balancer shaft has a balance weight divided into two portions, one on either side of a bearing supporting the balancer shaft. This construction protects the bearings from undesirable wear.

Abstract: A four cylinder in-line engine is disclosed with a secondary balancer system, wherein at least one pair of balancer elements are arranged substantially at an identical horizontal distance from the longitudinal axes of the engine cylinders and on lines which are substantially parallel to the plane of the longitudinal axes. The distance taken between the axes of the paired balancer elements in the direction of the cylinder axes is determined at such desired values as to satisfy the engine usage and design. The paired balancer elements are rotated in the opposite directions to each other with a speed of rotation twice as high as that of the engine crackshaft, such that the lower balancer element is rotated in the opposite direction to that of the engine crackshaft.