Prediction on onset conditions of alpha, beta, and gamma type free piston Stirling generators

The study of the onset characteristics of the free piston Stirling generator (FPSG) is essential for its development. In this study, the onset characteristics of the FPSG were discussed based on a thermodynamic model using the Routh stability criterion. First, the onset conditions of three types of FPSG, alpha, beta, and gamma, were analyzed. Then an experimental and numerical study of a 100 W beta-type FPSG prototype was conducted. The impacts on the onset temperatures of spring stiffness, charging pressure, and external load were further explored. The results demonstrate that no matter how the parameters are altered, alpha-type FPSG cannot start up. Whether beta and gamma type FPSG can start up depends on the design and operating parameters. For the prototype, as the displacer spring stiffness increases, the onset temperature rises first and then drops. When the displacer spring stiffness is 19.2 N/mm, the corresponding onset temperature is the lowest of 489 K. The FPSG is easier to start up at a higher external load and higher charging pressure. The experimental results of the impacts of the spring stiffness, charging pressure, and external load on the onset temperature agree well with the calculated results, with errors ranging from 1.7% to 9.12%. The proposed approach can play a guiding significance in studying FPSG onset conditions.