Gregory Behrendt | Searcharxiv

arXiv2026

Ideal heat engine cycles at maximal efficiency -- the ideal gas and beyond

Given a particular heat engine cycle, what is the optimal working medium that results in the highest efficiency? While one might jump to the conclusion that it must surely be the ideal gas, the situation is actually more intricate. Starting with a general Hemlholtz potential that depends polynomially on molar volume and temperature we derive exact expressions for the ideal Stirling, Otto, and Brayton cycles. We find that quite universally the maximal efficiency is achieved for thermodynamics systems, whose working mediums are linear in temperature. This includes the ideal gas, but also classical harmonic oscillators and pheonmenological models of the rubber band.

cond-mat.stat-mech↗

arXiv2025

Endoreversible Stirling cycles: plasma engines at maximal power

Endoreversible engine cycles are a cornerstone of finite-time thermodynamics. We show that endoreversible Stirling engines operating with a one-component plasma as working medium run at maximal power output with the Curzon-Ahlborn efficiency. As a main result, we elucidate that this is actually a consequence of the fact that the caloric equation of state depends only linearly on temperature and only additively on volume. In particular, neither the exact form of the mechanical equation of state, nor the full fundamental relation are required. Thus, our findings immediately generalize to a larger class of working plasmas, far beyond simple ideal gases. In addition, we show that for plasmas described by the photonic equation of state the efficiency is significantly lower. This is in stark contrast to endoreversible Otto cycles, for which photonic engines have an efficiency larger than the Curzon-Ahlborn efficiency.

cond-mat.stat-mech↗