Scaling of Miniature Piston Engine Performance Part 2: Energy Losses

Previous work based on measurements of the overall performance of small, (<10 cc displacement) two-stroke piston engines has shown that power and efficiency decrease more rapidly with size in miniature (<10 cc displacement) than in larger-scale engines. The present work seeks explanations for these trends by using energy balance analyses to identify and quantify the principal loss mechanisms in miniature engines. Energy losses in order of decreasing importance are found to be: incomplete combustion, heat transfer, sensible enthalpy in the exhaust, and friction. This ordering is almost opposite to that observed in conventional-scale engines. The results show that the scale-dependence of thermal and mechanical efficiency drive the drop in overall efficiency but that the biggest problem is incomplete combustion that consumes 50-60% of the fuel energy. Power output decreases somewhat less rapidly than efficiency because delivery ratio increases with decreasing scale in the set of engines investigated here. Taken together, the results indicate that improving the combustion process at a small scale is the first step toward building high performance miniature piston engines with practical levels of efficiency.