  We discuss the non-equilibrium properties of a thermally driven micromachine
consisting of three spheres which are in equilibrium with independent heat
baths characterized by different temperatures. Within the framework of a linear
stochastic Langevin description, we calculate the time-dependent average
irreversibility that takes a maximum value for a finite time. This time scale
is roughly set by the spring relaxation time. The steady-state average entropy
production rate is obtained in terms of the temperatures and the friction
coefficients of the spheres. The average entropy production rate depends on
thermal and/or mechanical asymmetry of a three-sphere micromachine. We also
obtain the center of mass diffusion coefficient of a thermally driven
three-sphere micromachine as a function of different temperatures and friction
coefficients. With the results of the total entropy production rate and the
diffusion coefficient, we finally discuss the efficiency of a thermally driven
micromachine.
