Measuring Scaling Effects in Small Two-Stroke Internal Combustion Engines

: As internal combustion (IC) engines decrease in displacement, their cylinder surface area to swept volume ratio increases. Examining power output of IC engines with respect to cylinder surface area to swept volume ratio shows that there is a dramatic change in power scaling trends at approximately 1.5 cm-1. At this size, thermal quenching and friction losses are expected to dominate , so power production and efficiency characteristics suffer. Furthermore, small IC engines (100cc displacement) have limited technical performance data compared to IC engines in larger size classes. Therefore, it is critical to establish accurate performance figures for a family of geometrically similar engines in the size class of approximately 1.5 cm-1 in order to better predict the phenomena that contribute to lower efficiencies in small ICEs. A series of three two stroke, single cylinder, spark ignited, air cooled, and carbureted IC engines were tested in this study. They had a displacement of 28cc, 55cc and 85cc corresponding to cylinder surface area to swept volume ratios of 1.81 cm-1, 1.46 cm-1, and 1.28 cm-1 respectively. The engines share design features like compression ratio, gas exchange port design, and scavenging method.