A method of and apparatus for starting an air-compressing four-stroke cycle internal combustion engine, in which the air for the combustion process is preheated in the intake system or in chambers communicating with the intake system. The air conveying engine parts are, prior to the self-sustaining operation of the engine, heated by means of the preheated intake air, while the engine during the preheating operation is driven by means of separate power and the inlet and outlet valves are controlled in conformity with the two-stroke cycle so that each upward stroke of a piston provides an exhaust stroke and every downward stroke of a piston provides an intake stroke. The shift-over of the operation of the inlet and outlet valves from two-stroke cycle operation to four-stroke cycle operation and the admission of fuel into the cylinders are effected only when the air conveying engine parts and the air charge have reached a temperature sufficient for the start of the engine.
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