Enclosed positive displacement mechanism (10; 110; 210; 310), particularly for fluid machines, comprising: - a body (12, 312) defining a substantially cylindrical cavity with an internal side surface and having at least a first and a second inlet port (22, 122) and at least a first and a second outlet port (24, 124) intended to allow a fluid to inflow into said substantially cylindrical cavity and to allow said fluid to outflow from said substantially cylindrical cavity, respectively; - a rotor mounted in said body (12, 312) so as to be rotatable about a main or revolution axis (X - X); - an orbiting piston, which is located in said cylindrical cavity, is mounted on said rotor so as to be rotatable about a secondary or rotation axis (Y - Y) eccentric with respect to said main axis (X - X) and is arranged to roll on the internal side surface of said substantially cylindrical cavity; and said mechanism being characterized in that it comprises - a vane slidable in said orbiting piston, which vane is mounted in a peripheral portion of said body (12, 312) located between one of said inlet ports (22, 122) and one of said outlet ports (24, 124), contiguous to one of said inlet ports, so as to be able to oscillate, said vane being mounted in said body (12, 312) so that it passes through said orbiting piston and is guided with a free end thereof in contact with the internal side surface of said cavity during its entire oscillation; in that - a single vane is provided, and in that - an arc-shaped sector, which delimits the oscillation of said vane and on which the free end of said vane is arranged to tangentially slide during its entire oscillation, is hollowed out of the internal side surface of said cavity. - a body (12, 312) defining a substantially cylindrical cavity with an internal side surface and having at least a first and a second inlet port (22, 122) and at least a first and a second outlet port (24, 124) intended to allow a fluid to inflow into said substantially cylindrical cavity and to allow said fluid to outflow from said substantially cylindrical cavity, respectively; - a rotor mounted in said body (12, 312) so as to be rotatable about a main or revolution axis (X - X); - an orbiting piston, which is located in said cylindrical cavity, is mounted on said rotor so as to be rotatable about a secondary or rotation axis (Y - Y) eccentric with respect to said main axis (X - X) and is arranged to roll on the internal side surface of said substantially cylindrical cavity; and said mechanism being characterized in that it comprises - a vane slidable in said orbiting piston, which vane is mounted in a peripheral portion of said body (12, 312) located between one of said inlet ports (22, 122) and one of said outlet ports (24, 124), contiguous to one of said inlet ports, so as to be able to oscillate, said vane being mounted in said body (12, 312) so that it passes through said orbiting piston and is guided with a free end thereof in contact with the internal side surface of said cavity during its entire oscillation; in that - a single vane is provided, and in that - an arc-shaped sector, which delimits the oscillation of said vane and on which the free end of said vane is arranged to tangentially slide during its entire oscillation, is hollowed out of the internal side surface of said cavity.