  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).
