Simulation of Scavenging in a Small Two-Stroke Gasoline Engine Typical in South Asia

Popular and inexpensive, small carbureted two-stroke engines in developing countries dominate city air pollution, mostly with hydrocarbon and particulate/aerosol emissions that short-circuit the scavenge process. Retrofit of a direct fuel-injection system can provide significant emissions reduction, if optimum injection timing is used to promote mixing while avoiding premature fuel loss. This required detailed understanding of the scavenging flow. Computational study of scavenge flow in a typical small vehicle engine is accomplished using multi-dimensional moving-mesh unsteady and quasi-steady models. Both models indicate that typical multi-port inlet placement and timing allow significant mixture short-circuit, consistent with emissions observations. Residual gas is well scavenged from the hemi-spherical head cylinder. Insights are derived for future incorporation of direct fuel injection for similar engines.