Design analysis of a mesoscale free piston engine for powering miniature unmanned aerial vehicles

Abstract

There is a need for a smaller, more efficient power solution for modern autonomous vehicles. In this work, current power solutions are discussed alongside modern manufacturing techniques to better leverage the potential in mesoscale engines (less than 50 cc engine size) for use in miniature Unmanned Aerial Vehicles (UAVs) (Raven RQ-11B). Through additive manufacturing, it is possible to produce complex geometry at small-scale, this allows for the fabrication of a mesoscale Free Piston Engine (FPE) which can outperform current power solutions (lithium-ion batteries and fuel cells) by using energy dense fuels such as methane and hydrogen. A zero-dimensional, closed Otto cycle, mathematical model is developed where the effects of variable fluid properties $(c_p and c_p)$ on engine performance are evaluated and found to be negligible (< 5%). Using constant fluid properties, a mesoscale FPE is analyzed, and a parametric study is performed to determine the optimal engine design and operating parameters. To benchmark the FPE, a comparison study is carried out where the FPE is compared to an Internal Combustion Engine (ICE), of the same length scale, using both methane and hydrogen fuels. From this study, it is determined that the FPE yielded higher power densities and thermal efficiencies than that of the ICE. Furthermore, it is found that the FPE can produce 400 W of power which is more than what is required to operate an Aveox 250 W DC motor, currently used in the Raven RQ-11B miniature UAV.