Some methods have been developed with a view to obtaining a quasi-isothermal compression or expansion process, according to which, in order to obtain the theoretical condition of an isothermal transformation, i.e., the maintenance of equality between the mechanic work received during the compression phase or yielded during the expansion phase and the heat evacuated during the compression phase or the heat absorbed during the expansion phase respectively, the work space of variable size of an engine has been connected to a cooled heat exchanger, consisting of one or more heat exchange units, in series, during the compression phase and a heated heat exchanger during the expansion phase (U.S. Pat. No. 3,867,815). This method has the disadvantage that the volume of the heat exchangers adds to the volume of the dead space, detemined by the constructive parameters of the work space of variable size, thus preventing high compression ratios from being reached. In addition, owing to the fact that only one heat-exchanger is used, the equality between the received or transferred machanic work and the evacuated or absorbed heat respectively, cannot be ensured at any instant, consequently, the transformation curve moves significantly away from the theoretical isothermal curve, thereby damaging the efficiency of the cycle on the whole. Then there are also Stirling external combustion engines built according to different principles, in which, after the compression phase, the working agent is cooled inside a heat exchanger, afterwords run through a regenerator and finally introduced into a heated expansion space (Stirling engine, by G. Walker). This type of external combustion engines has the disadvantage of not being able to reach higher compression values, thereby affecting the general output of the engine.