Design and Implementation of a Free Piston Compressor

Abstract

This thesis presents the design, dynamic characterization, and implementation of a free piston compressor (FPC). 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. 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. An analytical model of the proposed device is developed, and two separate FPC prototypes are designed and built and their yielded experimental results are compared with theoretical.