The design and dynamic characterization of a free piston compressor (FPC) is presented in this paper. The FPC is a proposed device that utilizes combustion of a hydrocarbon fuel to compress air into a high-pressure supply tank, thus serving as a portable pneumatic power supply for mobile untethered robotic systems. The device is configured such that the transduction from thermal energy to stored energy, in the form of compressed gas, is efficient relative to other smallscale portable power supply systems. This efficiency is achieved by matching the dynamic load of the compressor to the ideal adiabatic expansion of the hot gas combustion products. It is shown that a load that is dominantly inertial provides a nearly ideally matched load for achieving high thermodynamic efficiency in a heat engine. The device proposed exploits this fact by converting thermal energy first into kinetic energy of the free piston, and then compressing air during a separate compressor phase. The proposed technology is intended to provide a compact pneumatic power supply source appropriate for human-scale robots. An analytical model of the proposed device is developed, and an FPC prototype is designed and built and its yielded experimental results are compared with theoretical.

Keywords:free-piston engine, over expansion engine, pneumatic compressor, portable power supply, untethered robots