Volumetric expander for a closed cycle plant, particularly a Rankine cycle, said volumetric expander comprising: ➢ at least one casing exhibiting at least one general inlet configured for enabling to introduce a working fluid inside the casing and at least one general outlet configured for enabling the expulsion of the working fluid from said casing, ➢ a piston housed in said casing and adapted to define a variable volume expansion chamber, ➢ a main shaft cinematically connected to the piston and configured for rotatively moving around a main axis, ➢ at least one valve associated with the casing configured for selectively opening and closing an inlet and outlet (8, 9) of said expansion chamber enabling at least: ∘ a condition in which the working fluid is introduced into the expansion chamber, ∘ a condition in which the working fluid expands in the expansion chamber, and ∘ a condition in which the working fluid is discharged from said expansion chamber, wherein the casing defines inside a discharge chamber in direct fluid communication with the general outlet, the discharge chamber being further configured for being put in direct fluid communication with the outlet of the expansion chamber during the condition of discharging the working fluid from the expansion chamber itself, characterised in that the casing comprises at least one auxiliary inlet which is only directly in communication with the discharge chamber of the casing and therefore, through this latter, with the general outlet, the auxiliary inlet being configured for enabling the working fluid to directly enter the casing. ➢ at least one casing exhibiting at least one general inlet configured for enabling to introduce a working fluid inside the casing and at least one general outlet configured for enabling the expulsion of the working fluid from said casing, ➢ a piston housed in said casing and adapted to define a variable volume expansion chamber, ➢ a main shaft cinematically connected to the piston and configured for rotatively moving around a main axis, ➢ at least one valve associated with the casing configured for selectively opening and closing an inlet and outlet (8, 9) of said expansion chamber enabling at least: ∘ a condition in which the working fluid is introduced into the expansion chamber, ∘ a condition in which the working fluid expands in the expansion chamber, and ∘ a condition in which the working fluid is discharged from said expansion chamber, ∘ a condition in which the working fluid is introduced into the expansion chamber, ∘ a condition in which the working fluid expands in the expansion chamber, and ∘ a condition in which the working fluid is discharged from said expansion chamber, wherein the casing defines inside a discharge chamber in direct fluid communication with the general outlet, the discharge chamber being further configured for being put in direct fluid communication with the outlet of the expansion chamber during the condition of discharging the working fluid from the expansion chamber itself, characterised in that the casing comprises at least one auxiliary inlet which is only directly in communication with the discharge chamber of the casing and therefore, through this latter, with the general outlet, the auxiliary inlet being configured for enabling the working fluid to directly enter the casing.