Relation between fluctuations and efficiency at maximum power for small heat engines

Abstract

We study the ratio between the variances of work output and heat input, ${\eta }^{(2)}$, for a class of four-stroke heat engines which covers various typical cycles. Recent studies on the upper and lower bounds of ${\eta }^{(2)}$ are based on the quasistatic limit and the linear response regime, respectively. We extend these relations to the finite-time regime within the endoreversible approximation. We consider the ratio ${\eta }_{\text{MP}}^{(2)}$ at maximum power and find that the square of the Curzon-Ahlborn efficiency, ${\eta }_{\text{CA}}^2$, gives a good estimate of ${\eta }_{\text{MP}}^{(2)}$ for the class of heat engines considered, i.e., ${\eta }_{\text{MP}}^{(2)}\simeq {\eta }_{\text{CA}}^2$. This resembles the situation where the Curzon-Ahlborn efficiency gives a good estimate of the efficiency at maximum power for various kinds of finite-time heat engines. Taking an overdamped Brownian particle in a harmonic potential as an example, we can realize such endoreversible small heat engines and give an expression of the cumulants of work output and heat input. The approximate relation ${\eta }_{\text{MP}}^{(2)}\simeq {\eta }_{\text{CA}}^2$ is verified by numerical simulations. This relation also suggests a trade-off between the efficiency and the stability of finite-time heat engines at maximum power.

14 pages, 7 figures

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