In recent years, as for a variable displacement oil pump, a two-stage discharge pressure characteristic is often required for supplying different apparatus and parts, whose required discharge pressures differ from each other, for example, moving engine parts and a variable valve actuation device configured to control engine-valve operating characteristics, with oil discharged from an oil pump. According to such a two-stage discharge pressure characteristic, the pump discharge pressure can be maintained at a first discharge pressure in a first pump speed range and also maintained at a second discharge pressure in a second pump speed range. One such variable displacement oil pump has been disclosed in Japanese Patent Provisional Publication No. 2008-52450 (hereinafter referred to as “JP2008-524500”), corresponding to International Publication No. WO 2006/066405 (A1).
To satisfy such a two-stage discharge pressure characteristic, the variable displacement oil pump, as disclosed in JP2008-524500, has a cam ring, which is moveable or pivotable against the spring force of a return spring. The variable displacement oil pump is configured to achieve the two-stage discharge pressure characteristic by supplying the discharge pressure (the pressurized working fluid) to a selected one of two pressure-receiving chambers defined on the outer peripheral surface of the cam ring and by changing an eccentricity of a geometric center of the cylinder bore of the cam ring with respect to the axis of rotation of a rotor (exactly, a vane rotor)