NONEQUILIBRIUM DIRECTED DIFFUSION AND INHERENTLY IRREVERSIBLE HEAT ENGINES

We consider models which are symmetric under time-reversal and which p roduce net currents under parametrical, dichotomous, thermal excitatio n. The simplest is based on a three-level system, which is the basic u nit of a 'minimal' thermally driven ratchet We analyse the system's be haviour under periodic, dichotomous temperature changes and calculate the current, work and efficiency of the engine as functions of the upp er and lower temperatures and of the modulation period. The system's b ehaviour differs greatly from a quasistatically working heat engine (s uch as based on a Carnot cycle). We discuss how this behaviour arises due to the inherently irreversible nature of the underlying process.