PURPOSE: To obtain a 2-4 strokes switching engine which performs scavenging through a scavenging port during 4-stroke operation and thus improves heat efficiency.

CONSTITUTION: An intake valve 4 arranged on a cylinder head 1 is held in a blocked condition during 2-stroke operation, and opened/closed during 4-stroke operation, for performing intake. New air is sent into a cylinder from an exhaust port 12 on a lower side of the cylinder for scavenging in an initial period of an exhaust process of the 4-stroke operation. Supercharge air from a turbocharger 5 is compressed again in a crank case 30 for supplying to the scavenging port 12. The space inside the cylinder is forcibly scavenged in an exhaust process of the 4-stroke operation, so that an amount of the residual gas in the cylinder gets extremely small. The new air receives heat from a wall surface at the scavenging time in the 4-stroke operation, and evergy is recovered by the turbocharger 5 and the like, for improving heat efficiency. The intake air is flowed from the scavenging port 12 into the cylinder again near the termination of the intake process, and thus the intake efficiency is improved.