Performance and operating modes of a thermal-lag Stirling engine with a flywheel

•Theoretical model of a thermal-lag Stirling engine with a flywheel is proposed.•Different operating modes of a thermal-lag Stirling engine are studied.•The performance of a thermal-lag Stirling engine with a flywheel is studied.•The comparison with simulation and experimental results is conducted. The thermal-lag Stirling engine is a type of external combustion engine that differs from traditional Stirling engines in that it has only one cylinder and one piston. In the cylinder, a porous medium with a temperature gradient is installed to separate high- and low-temperature regions. Although its construction is simpler than that of the traditional Stirling engine, the thermal-lag Stirling engine has numerous operating modes at different heating temperatures and initial speeds. In this study, a theoretical model for analyzing the performance and operating modes of a thermal-lag Stirling engine is proposed. The model was solved by the method of multiple scales. The results indicate that the engine is operated by the thermal-lag effect caused by imperfect heat transfer in working spaces. Three operating modes were predicted: the decay, swinging, and rotating modes. Seven operating regions for the different modes were illustrated in the temperature-ratio and frequency-ratio domain. The transitions between modes were also predicted using the proposed theory. The performance of the proposed engine under different loadings in a stable operating state was evaluated. The dependence of the indicated power on engine speed was also determined. The results reveal that an optimal loading exists for achieving maximum power. A minimum value of the operating engine speed was also predicted by the proposed model.