Approaching efficiency bounds at maximum power in Carnot cycles

Accounting for the finite cycle time set by the piston speed, we analyze low-dissipation Carnot cycles with classical, quantum, and photon gases, determining the efficiency at maximum power, which is bounded by universal limits. These bounds can be closely approached at physically realizable source–sink temperature ratios, and the rate of approach can be tuned by the working substance and the difference in dissipation between the hot and cold branches. In the slow-piston limit, the maximum power is set by the temperature ratio and the isothermal expansion ratio, while the efficiency approaches the Carnot value.