In order to make clear how the efflux angle changes in a small crankcase scavenged engine cylinder and in a large loop scavenged engine cylinder, the inlet flow pattern in the single cycle model cylinder has been observed by a high speed motion camera. In general, the scavenging air stream is not effluent in the designed direction of the scavenging port at comparatively slight opening, and the efflux angle changes in proportion to port opening advance. In a small crankcase scavenged engine cylinder, to keep the scavenging air stream in the direction of the scavenging port it is effective to incline the scavenging air passage between the crankcase and the cylinder, and to make thicker the cylinder wall where located scavenging port. In a large loop scavenged engine cylinder to coincide the scavenging air stream with the direction of the scavenging port in the early stage of the scavenging, it has an effect to set the guide plate the position of 1/2 at port height.