The Scaling of Loss Pathways and Heat Transfer in Small Scale Internal Combustion Engines

Understanding the performance parameters of small remotely piloted aircraft powerplants.

Air Force Institute of Technology, Wright-Patterson Air Force Base, Ohio

The rapid expansion of the remotely piloted aircraft market includes an interest in 10 kg to 25 kg vehicles (Group 2) for monitoring, surveillance, and reconnaissance. Power plant options for those aircraft are often 10 cm³ to 100 cm³ displacement internal combustion engines. Both power and fuel conversion efficiency decrease increasingly rapidly in the aforementioned size range, with fuel conversion efficiency falling from approximately 30% for automotive and larger scale engines (greater than 100 cm³ displacement) to less than 5% for micro glow fuel engines (less than 10 cm³ displacement).

Based on the literature, it was unclear which loss mechanisms were responsible for the increasing rate of decreasing performance. Moreover, predictive models for losses such as friction, heat transfer, and short-circuiting (scavenging) were unavailable for ICEs in the stated size range. Previous research also indicated that these losses could cause an inherent relaxation in an engine's fuel octane requirement, possibly allowing small ICEs below a certain size to be converted from gasoline to JP-8 or diesel with little to no modification. To investigate these issues, three research objectives were proposed addressing the scaling of loss pathways; the modelling of heat transfer, friction losses, and gas exchange; and the determination of fuel anti-knock requirements for engines in the 10 cm³ to 100 cm³ displacement size range.