Dynamic Characteristics of a Free Piston Compressor

Abstract

The design and dynamic characterization of a free piston compressor is presented in this paper. The free piston compressor is a proposed device that utilizes combustion to compress air into a high-pressure supply tank. 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 small-scale 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. The combined factors of a high-energy density fuel, the efficiency of the device, the compactness and low weight of the device, and the use of the device to drive lightweight linear pneumatic actuators (lightweight as compared with power comparable electric motors) is projected to provide at least an order of magnitude greater total system energy density (power supply and actuation) than state of the art power supply (batteries) and actuators (electric motors) appropriate for human-scale power output. A thermodynamic analysis reveals relationships between key design variables with regard to efficiency. A dynamic model of the proposed device is developed and simulation results are discussed. This model shows the potential of the proposed device.