Abstract: This invention relates to supercharged four-stroke Diesel engines.The operating conditions of a supercharged four-stroke Diesel engine are improved, especially at starting and during low-load running, by keeping the exhaust valves open, both at starting and during all working conditions of the engine, at the end of the exhaust stroke and during the major portion of the induction stroke; the difference in natural pressure existing on low loads between the intake to and the exhaust from said engine is selectively increased to control the quantity of exhaust gases sucked back during the induction stroke.