Abstract The rapid expansion of the market for remotely piloted aircraft (RPA) includes a particular interest in 10-25 kg vehicles for monitoring, surveillance, and reconnaissance. Power-plant options for these aircraft are often 10-100 cm3 internal combustion engines. Both power and fuel conversion efficiency decrease with increasing rapidity in the aforementioned size range. Fuel conversion efficiency decreases from ∼30% for conventional-scale engines (>100 cm3 displacement) to <5% for micro glow-fuel engines (<10 cm3 displacement), while brake mean effective pressure decreases from >10 bar (>100 cm3) to <4 bar (<10 cm3). Based on research documented in the literature, the losses responsible for the increase in the rate of decreasing performance cannot be clearly defined.
Abstract In this work, in-cylinder pressure was measured in a 55 cc single cylinder, 4.4 kW, two stroke, spark ignition engine. In cylinder pressure measurements were taken using two different pressure transducers to determine if the performance differences between the two transducers are discernible in a small, spark ignition engine. A Kistler brand measuring spark plug was compared to a Kistler brand flush mount high temperature pressure sensor. Both sensors employ piezo-electric pressure sensing elements and were designed to measure indicated mean effective pressure as well as to detect knock at high temperature engine conditions. The pressure sensors were installed and adjusted to ensure cylinder volume after sensor installation matched the engine's original configuration within reasonable manufacturing tolerances. A series of tests at four throttle settings ensued to determine if either device altered the combustion volume or the engine's performance.
Abstract 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.