Abstract: When converting thermal into mechanical energy by a working medium containing a mixture of at least two materials having different boiling and condensation points, which is fed to a condenser, and is condensed therein, the condenser condensation pressure may increase and the efficiency for generating the mechanical energy thus decreases because the mixture of materials is separated into a liquid phase and a vapor phase upstream of the condenser. To prevent this, the liquid phase of the working medium is mixed with the vapor phase of the working medium before or while the working medium is condensed, thus once again creating a homogeneous mixture of materials which condenses at a lower pressure than the separated working medium, thereby preventing loss of efficiency. This can be applied to the use of thermal energy from low-temperature sources such as geothermal fluids, industrial waste heat, or waste heat from internal combustion engines.