The thermodynamic system consisting of two high thermal capacity bodies and of a Carnot heat engine operating between them is studied. These two bodies, by finite thermal conductances, can exchange heat with a very high thermal capacity external fluid whose temperature periodically varies in time with given trend, imposed from the outside. The possibility to optimize the energy efficiency of these devices both by choosing opportunely the operating temperature of the two bodies, and by distributing the available total thermal conductance between input and output heat transfers is discussed. Finally, the following subject is investigated: which is, for energy aims, the optimal profile of the external fluid thermal oscillation.