Heat Flow and Efficiency in a Microscopic Engine
  We study the energestics of a thermal motor driven by temperature differences, which consists of Brownian particles moving in a sawtooth potential with an external load where the viscous medium is alternately in contact with hot and cold heat reservoir. The motor can work as a heat engine or a refrigerator under different conditions. The heat flow via both potential and kinetic energy are considered. The former is reversible when the engine works quasistatically and the latter is always irreversible. The efficiency of the heat engine (Coefficient Of Performance (COP) of a refrigerator) can never approach Carnot efficiency (COP). 
Baoquan Ai, Liqiu Wang, Huizhang Xie, Lianggang Liu