Scaling of Miniature Piston-Engine Performance, Part 1: Overall Engine Performance

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Scaling of Miniature Piston-Engine Performance, Part 1: Overall Engine Performance

Menon, S. / Cadou, Christopher P.

While considerable effort has been devoted in recent years to building miniature heat engines, relatively little attention has been paid to understanding how their performance changes with engine size. This paper presents scaling rules for the performance of miniature two-stroke piston engines that are candidates for battery replacements in miniature unmanned vehicles and man-portable electronic systems. In so doing, it quantifies the current state of the art in small engine performance and provides a means to estimate a minimum thermodynamically sensible length scale for a two-stroke engine. The scaling rules are derived from comprehensive dynamometer investigations of nine of the smallest commercially available internal combustion engines. Engine mass ranges from 15 to 500 g, displacement ranges from 0.16 to $7.5 {cm}^{3}$ , power outputs range from 8 to 650 W, and overall efficiency ranges from 3 to 12%. Peak torque, power, and normalized power follow power law scaling relationships with engine displacement. Efficiency and brake mean effective pressure follow similar scalings until approximately $10 {cm}^{3}$ , where they begin to drop precipitously. The data show that the minimum length scale of a thermodynamically viable piston engine based on present technology is approximately 5 mm (displacement $\approx 0.1 {cm}^{3}$ ). A companion paper (Menon, S., and Cadou, C. P., “Scaling of Miniature Piston-Engine Performance, Part 2: Energy Losses,” Journal of Propulsion and Power, doi:10.2514/1.B34639) quantifies the losses that drive the scaling results.