The process of admitting air and expelling exhaust gas is known as scavenging. The three main types of scavenging for two stroke engines are cross flow, uniflow and loop flow. Notice on the below image that the cross flow and loop flow scavenging designs use ports only, whilst the uniflow design uses valves (for exhaust) and ports (for inlet).

In the normal operation of a two-stroke cycle engine, when the engine fires every cycle, the cylinder is full of combustion products at the begin- ning of the scavenging process. At the end of the scavenging process, only a fraction of the combustion products remains in the cylinder.