An enclosed positive displacement mechanism including a body with an inlet and an outlet; a rotor mounted in the body and rotatable about a main axis; an orbiting piston located in the cavity, rotatable about an eccentric secondary axis and arranged to orbit around the main axis to roll on the internal side surface of the cavity; and a vane located in the cavity, slidable in the piston and mounted in the body between one inlet and one outlet so as to oscillate. The orbiting piston and the vane divide in cyclic manner the cavity into a first and a second chamber with variable volume, which are mutually complementary and communicate with the inlet and the outlet. During a portion of its oscillation, the vane passes through the piston and is in contact with the side surface of the cavity, thereby separating the chambers in a fluid-tight manner.