The invention relates to a hydraulic
camshaft adjuster (1) for adjusting the control time of
gas exchange valves of an
internal combustion engine, comprising a
stator (2) which can be synchronously rotated by the
crankshaft of the
internal combustion engine, and a rotor (3) which is mounted so as to rotate with the
stator (2), and which can synchronously rotate with the
camshaft, several ribs (6)being provided on the
stator (2), the ribs dividing an annular chamber (7) between the stator (2) and the rotor (3) into a plurality of pressure chambers (8), wherein the rotor (3) comprises a rotor hub (4) and a plurality of vanes (5) extending radially outward from the rotor hub (3), the pressure chamber (8) being divided into two groups of working chambers (9, 10) having a different direction of work and being impinged upon by respectively a pressure medium (19) upstream or downstream in a pressure medium circuit, and comprising a central
locking mechanism (11) for locking the rotor (3) ina central position, wherein the hydraulic
camshaft adjuster (1) is connected at least partially to a front side (20, 21) by a cover (17, 18), wherein a reservoir (14) for storing the pressure medium (19) is formed in one of the covers (17, 18). The cover (17, 18) comprises an overflow opening (23) through which the pressure medium (19) can exit in the axial direction from the hydraulic camshaft adjuster (1), and wherein the reservoir (14) is dimensioned in relation to the overflow opening (23) such that when the hydraulic camshaft adjuster (1) is at a standstill, a pressure medium (25) remainsat the bottom in the reservoir (14), which ensures the pressure medium is supplied to the return valve (28) of the central
locking mechanism (11) when the hydraulic camshaft adjuster (1) is running.The invention also relates to a method for operating said type of camshaft adjuster (1).