SCAVENGE LOSS MECHANISMS AND THEIR DRIVING FORCES IN LOOP-SCAVENGED HIGH-PERFORMANCE TWO-STROKE ENGINES

To fulfill the forthcoming EPA and CARB emission legislations for handheld outdoor power equipment, strong efforts are undertaken to reduce the high hydrocarbon emissions of two-stroke engines. This is due to the fact that the two-stroke engine has a much better power-to-weight ratio than a mini-four-stroke engine. The research focuses on the one hand into the development of new two-stroke technologies such as charge stratification or compression wave injection for example. On the other hand there is still a significant potential in improving the scavenging process of the conventional Schnürle-scavenged two-stroke engine.

This paper focuses into the analysis of the internal flow behavior of the Schnürle-typ scavenge flow in a 64 cm³ high-performance two-stroke engine for hendheld products. The flow analysis is conducted by means of transient 3D-CFD calculations that are based on experimental data. A newly developed postprocessing approach allows for a detailed analysis of different loss paths during scavenging. The results show that four distinguished loss mechanisms exist: Direct mixture short-circuiting, late loop losses, central mixing losses and near-wall secondary flow losses. These four loss mechanisms are quantified in their contribution to the overall losses and the driving mechanisms are outlined. The newly identified secondary flow path carries a quantity of hydrocarbon losses as significant as direct mixture short-circuiting.