INTERNAL COMBUSTION ENGINE

PROBLEM TO BE SOLVED: To aim at improving both engine performance and quiteness while maintaining constant displacement. SOLUTION: An engine is provided with cylinders 2a, 2b for actual combustion and a cylinder 4 for torque balance rather than combustion. Each capacity of the cylinders 2a, 2b is expanded since the bore of the cylinder 4 for torque balance is reduced. As a result, combustion efficiency and gas pressure torque of those cylinders 2a, 2b can be improved. During running of the engine 1, only air cycling is executed within the cylinder 4. The expansion pressure torque of the compressed air produces maximum torque substantially equal to that of those cylinders 2a, 2b. Even when the number of cylinders for combustion is reduced to two, the resultant torque balance characteristics can be substantially the same level as that of the 3-cylinder engine.