  We investigate the performance of a Stirling cycle with a working substance
(WS) modeled as the quantum Rabi model (QRM), exploring the impact of
criticality on its efficiency. Our findings indicate that the criticality of
the QRM has a positive effect on improving the efficiency of the Stirling
cycle. Furthermore, we observe that the Carnot efficiency is asymptotically
achievable as the WS parameter approaches the critical point, even when both
the temperatures of the cold and hot reservoirs are finite. Additionally, we
derive the critical behavior for the efficiency of the Stirling cycle,
demonstrating how the efficiency asymptotically approaches the Carnot
efficiency as the WS parameter approaches the critical point. Our work deepens
the understanding of the impact of criticality on the performance of a Stirling
heat engine.
