Camshaft adjuster

1. A camshaft adjuster for a camshaft, comprising

a pressure chamber (AB) and an positioning element arranged in the pressure chamber (AB),

the positioning element divides the pressure chamber (AB) into a first working chamber (A) and a second working chamber (B),

hydraulic medium can be supplied to and discharged from the first working chamber (A) through a first hydraulic medium duct (AA) and supplied to and discharged from the second working chamber (B) through a second hydraulic medium duct (BB),

such that the positioning element can be moved, resulting in a rotation of the camshaft, by a pressure difference between the first working chamber (A) and the second working chamber (B),

the supply and discharge of the hydraulic medium is controlled by a control device which has a valve housing, a valve sleeve arranged in and rotatable relative to the valve housing, and a control piston axially displaceable within the valve sleeve,

the valve housing has orifices which communicate with the first working chamber (A) and the second working chamber (B),

the valve housing rotates synchronously with the camshaft and, in the process, the orifices of the valve housing are closed off and opened up by an orifice arrangement formed on the valve sleeve,

the orifice arrangement of the valve sleeve communicates with orifices of the control piston as a function of an axial position of the control piston, and

the camshaft adjuster has a third hydraulic medium duct (CC) which positions the positioning element in an intermediate position, and the third hydraulic medium duct (CC) has an opening which communicates with one of the working chambers (A, B).

2. The camshaft adjuster as claimed in claim 1 , wherein the third hydraulic medium duct (CC) opens into one of the working chambers (A, B) depending on a position of the positioning element during operation of the camshaft adjuster.

3. The camshaft adjuster as claimed in claim 2 , wherein the opening of the third hydraulic medium duct (CC) is closed off to the one of the working chambers (A, B) when the positioning element is in the intermediate position.

4. The camshaft adjuster as claimed in claim 1 , wherein the third hydraulic medium duct (CC) is arranged in the camshaft adjuster such that, when the positioning element is in a certain position, the opening of the hydraulic medium duct (CC) is covered by the positioning element itself.

5. The camshaft adjuster as claimed in claim 1 , wherein a spacing (x) between the opening of the third hydraulic medium duct (CC) and a stop surface has a same size as a spacing (y) between a further opening of the third hydraulic medium duct (CC) and a stop, so that the positioning element assumes a central position.

6. The camshaft adjuster as claimed in claim 2 , wherein the hydraulic medium ducts (AA, BB) are pressurized and the third hydraulic medium duct (CC) is open to a tank (T) in order to position the positioning element in the intermediate position.

7. The camshaft adjuster as claimed in claim 1 , wherein axial positions of the control piston correspond, with increasing energization of a central magnet proceeding from a deenergized position, to a sequence of modes as follows: intermediate or central position, OPA, CTA, regulated position, CTA, OPA.

8. The camshaft adjuster as claimed in claim 1 , wherein axial positions of the control piston correspond, with decreasing energization of a central magnet proceeding from a fully energized position, to a sequence of modes as follows: intermediate or central position, OPA, CTA, regulated position, CTA, OPA.

9. The camshaft adjuster as claimed in claim 1 , wherein the camshaft adjuster has a locking mechanism which is activated by axial positions of the control piston which correspond to CTA and central position modes.

10. The camshaft adjuster as claimed in claim 1 , wherein the camshaft adjuster has a restoring spring, a torque of which acts on the positioning element counter to a drag torque of the camshaft.