D E S C R I P T I O N

Variable displacement lubricant pump

The present invention refers to a variable displacement lubricant pump for providing pressurized lubricant for an internal combustion engine.

The mechanical pump comprises a pump rotor with radially slidable vanes rotating in a shiftable control ring, whereby the control ring envelopes numerous pump chambers. The shifting of the control ring is not necessarily a linear movement but can be a pivoting movement as well. The pump chambers rotate through a charge and a discharge zone inside the control ring. Further, the pump comprises a pretensioning element which is pushing the control ring to a high pumping volume direction, A first control chamber is pushing the control ring to a low pumping volume direction, and a second control chamber is pushing the control ring to a high pumping volume direction if the lubricant is pressurized. The pump also comprises a pump outlet which is connected to the first control chamber. Both control chambers, i.e. the first and the second control chamber, are connected to each other via a throttle valve. Both control chambers can have a different circumferential extend around the control ring, so that the effective surfaces of the two control chambers and the respective moment arms are different. Both control chambers act against each other, i.e. act in different pumping volume directions.

Such pumps are known from WO 2006 066405 Al. The displacement of the lubricant is controlled by the eccentricity of the control ring. The eccentricity of the control ring is controlled by the equilibrium forces between the first control chamber, the second control chamber and the pretensioning element. When the pump rotor is rotating in the control ring, the pump rotor causes a compression of the lubricant in the discharge zone. The lubricant is compressed at maximum in the discharge pump chamber which is located at the end of the discharge zone, i.e. the pump chamber with the minimum volume in the discharge zone. This maximum compression of the lubricant can cause high local pressure peaks, especially at high rotating speeds and if the control ring is positioned at a high pumping volume position so that the equilibrium forces between the first control chamber, the second control chamber and the pretensioning element are temporarily disturbed. As a consequence, the lubricant flow rate can be temporarily incorrect and not adapted to the engine demand.

It is an object of the present invention to provide a variable displacement lubricant pump with an improved flow control quality at high rotating speed.

This object is solved with a variable displacement lubricant pump with the features of claim 1.

The control ring of the mechanical variable displacement lubricant pump is provided with a pressure relief valve which directly connects the discharge zone to the second control chamber.

The pressure relief valve forms a second connection of the discharge zone inside the control ring to the second control chamber. The pressure relief valve avoids effectively local differential pressure peaks in the discharge zone. The pressure relief valve aliows only a calibrated leakage of the lubricant from the discharge zone to the second control chamber so that the pressures between the second control chamber and the discharge zone remain stiil different but without high differential pressure peaks. As a consequence, the equilibrium forces between the first control chamber, the second control chamber and the pretensioning element remain undisturbed so that the control of the lubricant flow rate remains adapted to the engines demand.

Preferably, the pressure relief valve is a radial groove in the control ring. A radial groove in the control ring is simple to realize and cost-efficient. Further, the leakage of the lubricant from the discharge zone to the second control chamber can be calibrated by the area of the cross-section of the radial groove. The calibrated leakage allows only a minimal lubricant relief flow. The minimal lubricant relief flow avoids high local differential pressure peaks but maintains the operating pressures between the second control chamber and the discharge zone different.

Preferably, the pressure relief valve is arranged at a final sector of the discharge zone. In this sector a maximum compression of the lubricant appears in the pump chamber, especially when the control ring is positioned in a high pumping volume position. The final sector of the discharge is the region next to the vertex point where the discharge action turns to the charge action. The pressure relief valve, i.e. a radial groove in the control ring, allows a calibrated leakage so that high differential pressure peaks in this most susceptible sector can effectively be avoided.

According to a preferred embodiment, the shiftable control ring is pivotably supported by a fulcrum pin. The term "shiftable" is not restricted to a linear movement of the control ring. The shiftable control ring is pivotable in a defined radius. The fulcrum pin is arranged between the two control chambers, i.e. the first and the second control chamber.

In a preferred embodiment the first control chamber is defined between the fulcrum pin and a first sealing element and the second control chamber is defined between the fulcrum pin and a second sealing element. In a preferred embodiment, the throttle valve is positioned adjacent to the fulcrum pin and bypasses the fulcrum pin which is forming a sealing between the first and the second control chamber, According to a preferred embodiment, the pump outlet is directly connected with the first control chamber. The direct connection between the first control chamber and the pump outlet can be realized by an opening which avoids any pressure drop even at high lubricant flow rates through the opening.

Preferably, the pretensioning element is a mechanical metal spring.

One preferred embodiment of the invention is described with reference to the drawing, wherein figure 1 shows a cross-sectional view of a variable displacement lubricant pump.

In figure 1, a variable displacement lubricant pump 10 for an internal combustion engine is shown. The lubricant pump 10 is adapted to suppiy an internal combustion engine with a lubricant, and more particularly, with a lubricant discharge pressure which should not depend proportionally on the rotational pump speed. The variable displacement lubricant pump 10 comprises a metal housing 12 in which a shiftable control ring 14 is arranged axiaiiy between two side walls (not shown). The control ring 14 is provided with a pivot axis 16 at which the control ring 14 pivots, so that the control ring 14 is shifted between a low and a high pumping volume position. The pivot axis 16 is realized by a fulcrum pin 18. The metal housing 12 contains a pump rotor 20 with numerous radially slidabie vanes 22, whereby the slidable vanes 22 are rotating inside the shiftable control ring 14. The control ring 14 surrounds numerous rotating pump chambers 24 which are separated by the vanes 22. The pump chambers 24 with an increasing chamber volume define a charge zone 32 and the pump chambers 24 with a decreasing chamber volume define a discharge zone 34,

The pump rotor 20 is provided with a protrusion (not shown) which protrudes axialiy to the outside of one of the side walls. The protrusion of the pump rotor 20 can be rotated by a pump actuator which is not shown. Both, the pump rotor 20 and the control ring 14 are sitting on a supporting ring 21 which is mounted to one of the side walls (not shown). The supporting ring 21 is provided with a discharge opening 26 through which the lubricant is delivered from the pump chambers 24 to a pump outlet 48.

The position of the shiftable control ring 14 is determined by three elements, i.e. a pretensioning element 42 which is a mechanical preload metal spring, a first control chamber 28 and a second control chamber 30.

The two control chambers 28, 30 which are formed by the housing 12, the two side walls (not shown) and the control ring 14 have a different circumferential extend around the control ring 14 so that the effective surfaces of the two control chambers 28, 30 and the respective moment arms are different. Both control chambers 28, 30 are opposed to each other with respect to the pivot axis 16 or the fulcrum pin 18 of the control ring 14, respectively. The circumferential extend of the two control chambers 28, 30 is defined by two sealing elements 36, 38 which are form-fitted hold in respective axial slots 40, 41 of the control ring 14. Therefore, the two control chambers 28, 30 are separated from each other by the fulcrum pin 18 and are sealed at their circumferential ends by the two sealing elements 36, 38.

The pump 10 is provided with a pump inlet 46 and the pump outlet 48. The pump inlet 46 is leading into an inlet prechamber 50 which is separated by the control ring 14 from the pump chambers 24. The inlet prechamber 50 is circumferential!y restricted by the two sealing elements 36, 38. The connection between the inlet prechamber 50 and the pump chambers 24 can be realized, for instance, by radial recess-like openings (not shown) in the control ring 14. The pump outlet 48 of the pump 10 is directly connected with the first control chamber 28.

Adjacent to the fulcrum pin 18 a throttle valve 52 is provided. The throttle valve 52 connects the first control chamber 28 with the second control chamber 30 so that the lubricant bypasses the fuicrum pin 18 via the throttle valve 52. The throttle valve 52 allows a throttled lubricant flow from the first control chamber 28 to the second controi chamber 30.

The control ring 14 is provided with a pressure relief valve 54 which connects the discharge zone 34 with the second controi chamber 30, and more particularly, connects a final sector 56 of the discharge zone 34 with the second control chamber 30. The pressure relief valve 54 is defined by a radial groove which is provided at one axial side of the control ring 14. The final sector 56 of the discharge zone 34 is defined by one or maximally two of the pump chambers 24 which are located at the end of the discharge zone 34, i.e. the pump chambers 24 with the minimum pumping volume.

The connection between the final sector 56 of the discharge zone 34 and the second control chamber 30, realized by the pressure relief valve 54, avoids effectively high local differential pressure peaks in the discharge zone 34 so that the !ubricant flow rate remains adapted to the engines demand, especially at high rotating speeds.