The performance of a two-stroke engine is strongly dependent on how well the burnt gases are scavenged from the cylinder volume and replaced with fresh charge: that is, on the efficiency of the scavenging process and on the amount of fresh charge trapped inside the cylinder at the end of the process. The characteristics of the scavenging process, that is, the scavenging flow details, its overall efficiency, and the distribution of the charge retained in the cylinder at the end of the process, depend very much on the operating conditions of the engine. With motored engine tests it is presumed that the scavenging characteristics are only weakly dependent on the combustion process. Static or steady flow model tests are mainly used to study the flow direction and distribution of the entering fresh charge, without introducing the effects of the moving piston and the unsteady nature of the process on the scavenging characteristics.