Applicable fundamentals of aerodynamics are summarized as a help in designing optimum scavenging flow ducts. Certain patterns of “scavenging pictures” serve as criteria for stability and are derived from requirements for scavenging air flow in the cylinder. For a good performance of the engine, further requirements must be met with regard to exhaust and intake pressure fluctuations, antiknock properties, and the formation and distribution of the fuel-air mixture. Engine performance characteristics of a 3 cyl 2 stroke cycle, spark ignition engine of 61 cu in. displacement are shown. The excellent performance indicates good potentials of the 2 stroke cycle engine.