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Efficiency at maximum power of a chemical engine

A cyclically operating chemical engine is considered that converts chemical energy into mechanical work. The working fluid is a gas of finite-sized spherical particles interacting through elastic hard collisions. For a generic transport law for particle uptake and release, the efficiency at maximum power $η$ takes the form 1/2+cΔμ+ O(Δμ^2), with 1/2 a universal constant and $Δμ$ the chemical potential difference between the particle reservoirs. The linear coefficient c is zero for engines featuring a so-called left/right symmetry or particle fluxes that are antisymmetric in the applied chemical potential difference. Remarkably, the leading constant in $η$ is non-universal with respect to an exceptional modification of the transport law. For a nonlinear transport model we obtain η= 1/(θ+1), with θ>0 the power of $Δμ$ in the transport equation