Multiphase Stirling Engines

Analysis, design, fabrication, and experimental assessment of a symmetric three-phase free-piston Stirling engine system is discussed in this paper. The system is designed to operate with moderate-temperature heat input that is consistent with solar-thermal collectors. Diaphragm pistons and nylon flexures are considered for this prototype to eliminate surface friction and to provide appropriate seals. In addition, low loss diaphragm pistons, etched and woven-wire screen heat exchangers, and plastic flexures, as the main components of the system, are outlined. The experimental results are presented and compared with design analysis. Experiments successfully confirm the design models for heat exchanger flow friction losses and gas spring hysteresis dissipation. Furthermore, it is revealed that gas spring hysteresis loss is an important dissipation phenomenon for low-power Stirling engines and should be carefully addressed in design. Analysis shows that the gas hysteresis dissipation is reduced drastically by increasing the number of phases in a multiphase Stirling engine system. It is further shown that for an even number of phases, half of the engine chambers could be eliminated by utilizing a reversing mechanism within the multiphase system. The mathematical formulation and modal analysis of multiphase Stirling engine system are then extended to a system that incorporates a reverser. By introducing a reverser to the fabricated prototype, the system successfully operates in engine mode. The system proves its self-starting capability and validates the computed start-up temperature.