The absence of the separate induction and exhaust strokes in the two-stroke engine is the fundamental difference from four-stroke engines. In two-stroke engines, the gas exchange or scavenging process can have the induction and exhaust processes occurring simultaneously. Consequently, the gas exchange processes in two-stroke engines are much more complex than in four-stroke engines, and the gas exchange process is probably the most important factor controlling the efficiency and performance of two-stroke engines.