Measuring Scaling Effects in Small Two-Stroke Internal Combustion Engines 2014-32-0010

As 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 change in power scaling trends at approximately 1.5 cm⁻¹. At this size, it is suggested that heat transfer from the cylinder becomes the dominant thermal loss mechanism and performance and efficiency characteristics suffer. Furthermore, small IC engines (>1 cm⁻¹) 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 understand the thermal losses that contribute to lower efficiencies in small IC engines.

The engines considered in this scaling study were manufactured by 3W Modellmotoren, GmbH. In particular, they are the 3W-28i, 3W-55i, and 3W-85Xi engines, which have cylinder surface area to swept volume ratios of 1.83 cm⁻¹, 1.46 cm⁻¹, and 1.27 cm⁻¹ respectively. All three engines are two-stroke cycle, single cylinder, spark ignited, air cooled, and air carbureted. The engines have a similar gas exchange port design and utilize crank case compression and Schnuerle method scavenging. They have the same 10:1 geometric compression ratio. These engines were chosen due to their similarity in design while falling within the relevant size regime for evaluating thermal losses.