<p>Consideration has been given to stationary irreversible thermodynamic systems containing a thermomechanical converter. It is shown for the simplest systems of this kind — a heat engine — that no secondary processes in a working medium can make its efficiency higher than the Carnot efficiency and also what can be used for such processes to increase the real efficiency. We have obtained conditions that distinguish realized temperature fields and conditions that divide these fields into two classes. For one of them, mechanical energy is supplied (consuming ones), while for the other one it is extracted from the system (generating ones). In both cases, we obtained relations determining the minimum supplied power and the maximum outgoing power. It is shown that for Newtonian heat transfer at a maximum power regime, the ratios of absolute temperatures of the working medium’s contact with reservoirs are equal to the square root of the ratios of the reservoirs’ temperatures. Consideration has been given to the problem of thermostatting for a system of interconnected chambers of arbitrary configuration and conditions have been found for an optimal selection of free temperatures and distribution of energy fluxes.</p>