Meso-scale power systems (10 W to 1000 W) are needed to power untethered mobile robots and assisting devices such as powered exoskeletons. Air-breathing combustion driven actuators, used in a direct acting manner, can be used for such applications and take advantage of the high power density of fluidic actuators and the high energy density of chemical fuels. However, fuel-to-mechanical energy conversion efficiency is critical to make such chemical systems viable over electrical systems. This paper presents the efficiency-based design and experimental characterization of two combustion driven actuators intended to reach high specific power and specific energy. First, efficiency oriented design principles are derived from internal combustion engine theory: (1) an ideal-cycle thermodynamic model of a generic constant volume combustion system suggests that compression ratio and the expansion/compression ratio should both be maximized, and (2) the practical effects of heat, mass and friction losses as well as fuel choice in a small scale combustion chamber context are discussed. Second, two simplified prototypes are built and tested. The first prototype uses a rolling diaphragm seal to limit the effect of mass and friction losses. The second prototype consists of a standard air cylinder that minimizes heat losses by reducing the surface-to-volume ratio of the combustion chamber. Hydrogen is selected as fuel because it allows lean combustion which limits the effect of heat loss with low combustion temperatures. Compression ratio and equivalence ratio are varied experimentally to evaluate their effect on efficiency. Experimental results demonstrate an energy conversion efficiency of 15.3% at a compression ratio of 4.15 and a low equivalence ratio of 0.3. Ragone analysis of relevant meso-scale power systems for mobile robotic suggest that, with proper optimization and system integration, combustion driven power systems can become a viable solution for lightweight and long range meso-scale robotic applications.

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