| Summary: | Low-temperature-differential (LTD) Stirling engines are heat engines that can operate autonomously with a slight temperature difference between low-temperature heat reservoirs and are thus expected to contribute to a sustainable society. A minimal dynamical-system model with only two variables has been proposed to explain the principle of autonomous rotational motion caused by temperature differences, and the maximum efficiency of the engine was formulated [Y. Izumida, Europhys. Lett. 121, 50004 (2018)0295-507510.1209/0295-5075/121/50004; Phys. Rev. E 102, 012142 (2020)2470-004510.1103/PhysRevE.102.012142]. This paper aims to investigate the synchronous and asynchronous transitions and clarify the coupling effects on the power and thermal efficiency of a pair of weakly coupled LTD Stirling engines and formulate the maximum thermal efficiency of the coupled system. We show that the dependence relation between the effective frequency difference and the coupling strength is characterized by a hysteresis, which comes from different kinds of bifurcations in the process of increasing and decreasing the value of the coupling strength. Then, by generalizing thermodynamic fluxes and forces and their quasilinear relations for engines under weak coupling, we show that the coupling improves the power exerted against the load torques and the thermal efficiency. We further show that their maximum values are achieved when the engines are synchronized. Since the thermal efficiency depends on the effective frequency difference, the dependence of thermal efficiency on the coupling strength is also characterized by a hysteresis. Finally, the load torque that achieves the maximum thermal efficiency of the coupled system is formulated.
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