Title of Invention

GEAR-WHEEL PUMP, IN PARTICULAR FOR A HIGH-PRESSURE FUEL PUMP

Abstract

In a gear pump having a housing (18), two gear wheels (14, 16) which are arranged in the housing and engage with one another, and at least one groove (22) which is foffi1ed in the housing on the delivery side of the gear pump, cavitation damage at high rotational speed is to be avoided. For this purpose, it is provided that the groove has a first section (24) which extends starting from the delivery side and in which the base' of the groove (22) is at a small distance from the tips of the teeth (20) of the gear wheel, and a second section (26) which adjoins the first section and in which the base of the groove (22) 1S at a maximum distance from the tips of the teeth, which distance is greater than the distance in the first section, the first section extending over a smaller angular range (α) than the second section and the groove extending in total over an angular range (α,β)which is somewhat greater than the angular distance between two teeth (20).

Full Text

Prior art The invention relates to a gear-wheel pump, in particular for a high-pressure fuel pump. Such a gear pump can serve, in particular, as a prefeed pump for a high-pressure fuel pump, said pump being supplied with the fuel with a pressure of approximately 6 bar by the prefeed pump. The high-pressure fuel pump then generates a pressure which can be in the order of magnitude of 1800 bar, such as is used in what is referred to as a common rail injection system. The gear pump is driven with the same rotational speed as the high-pressure fuel pump and must already feed a sufficient quantity of fuel at the engine starting speed. For this reason it is necessary for the gear wheels to run with as little play as possible with respect to the housing and also for the wrap-round length of the two gear wheels, that is to say the angular range over which the intermediate spaces, filled with the fuel to be fed, between the intake side and the delivery side of the gear pump are sealed by the housing, to be as large as possible. However, given a maximum engine speed, the gear pump must not feed too large a quantity of fuel. Instead of a complex valve control for regulating the quantity, a throttle, which limits this delivery quantity, is usually used on the intake side. This leads to the intermediate spaces between the teeth no longer being completely filled with fuel when a specific delivery quantity is reached. If such an intermediate space between the teeth, which is not completely filled with fuel, emerges from the housing on the delivery side of the pump and enters the pressure space, there is the risk of cavitation damage to the edges of the gear teeth or to the housing. For this purpose the groove is provided, which is intended to permit an as far as possible continuous pressure rise in the intermediate space between the teeth which is not completely filled with fuel. The groove acts here as a throttle which permits controlled flowing back of the fuel from the delivery side of the pump into the intermediate space between the teeth which is located in the region of the groove. The disadvantage of the previously known fuel pumps is that the groove which extends over a comparatively large angular range was necessary in order to prevent cavitation damage even at high rotational speeds. The large angular extent of the groove has the effect, however, of reducing the wrap-around angle between housing and gear wheel, resulting in a smaller delivery quantity at lower rotational speeds. The object of the invention is to develop a gear pump of the type mentioned at the beginning to the effect that a large delivery quantity is achieved even at low rotational speeds, while at the same time cavitation damage is to be avoided at high rotational speeds. Advantages of the invention In the gear pump according to the invention having the features, the groove forms a type of prechamber which is connected to the delivery side by the comparatively narrow gap which is formed in the first section between the base of the groove and the tips of the teeth of the gear wheel. At high rotational speeds, the narrow gap leads, in conjunction with the overflow passage which is formed in the region of the second section of the groove, to a continuous rise in pressure of the intermediate space between the teeth which actually opens up to the groove in each case. The groove has overall an extent over a comparatively small angular range so that a large wrap¬around angle is produced between gear wheel and housing, which is advantageous for the delivery quantity at high rotational speeds. Accordingly, the present invention provides a gear-wheel pump, in particular for a high-pressure fuel pump, comprising a housing, two gear wheels which are arranged in the housing and engage with one another, and at least one groove which is formed in the housing on the delivery side of the gear pump, characterized in that the groove has a first section which extends starting from the delivery side and in which the base of the groove is at a small distance from the tips of the teeth of the gear wheel, and a second section which adjoins the first section and in which the base of the groove is at a maximum distance from the tips of the teeth, which distance is greater than the distance in the first section, the first section extending over a smaller angular range than the second section and the groove extending in total over an angular range, which is greater than the angular distance between two teeth. Advantageous embodiments are described herein. Drawings The invention is described below with reference to a preferred embodiment which is illustrated in the appended drawings, in which, Figure 1 is a schematic sectional view of a gear pump in conjunction with a high-pressure fuel pump; Figure 2 shows a gear pump according to the prior art in a schematic, broken sectional view; and Figure 3 shows a gear pump according to the invention in a view corresponding to that in Figure 2. Description of the exemplary embodiment Figure 1 shows a high-pressure fuel pump 5 which can compress fuel to a high pressure in the order of magnitude of up to 1800 bar by means of a pump element 7. The pump element is supplied with the fuel via a gear pump 10 which is connected to a drive shaft 12 for the pump element 7. The gear pump 10 has two gear wheels 14, 16 (see Figure 2) which engage with one another and are arranged in a housing 18. By rotating in the direction of the arrow, the gear wheels 14, 16 feed the fuel, supplied on the intake side ND to the delivery side HD by means of the intermediate space between two adjacent gear wheel teeth 20. Figure 2 shows a groove 22 which is arranged in the housing starting from the delivery side. The groove 22 has the purpose of permitting an as far as possible uniform, controlled rise in pressure in the intermediate spaces between two adjacent gear wheel teeth if a lower pressure is present in the intermediate spaces between the teeth at the outlet from the housing 18 and the passage to the dehvery side than on the delivery side, and is not completely filled with the fuel. If an abrupt rise in pressure were to take place in this state, the bubbles of vapor in the fuel would implode into the intermediate spaces between the teeth and cavitation damage could occur to the housing and to the edges of the gear wheel teeth 20. In particular the material which is sensitive to cavitation damage would be affected by this. In the conventional embodiment of the groove 22 shown in Figure 2, the pressure equalization in the intermediate spaces between the teeth takes place very rapidly at a high rotational speed so that a pressure wave is produced which, on the one hand, brings about strong pressure vibrations and, on the other hand, causes the cavitation bubbles in the intermediate space between the teeth to implode at high speed. Figure 3 shows the embodiment of the groove 22 according to the invention. The groove is composed here of a first section 24 which extends over an angular range a. and of a second section 26 which extends over an angular range (3, the angular range a being much smaller than the angular range p. In the angular range a, the distance s between the tips of the gear wheel teeth and the base of the groove 22 is comparatively small, for example in the order of magnitude of 0.2mm, while the maximum distance t between the tips of the teeth and the base of the groove 22 in the second section is significantly greater, for example of the order of magnitude of 0.7 mm. In the first section, the base of the groove 22 extends approximately concentrically with the axis of rotation of the gear wheel 14, while the base of the groove 22 in the second section extends approximately in the shape of a parabola starting from the first section. The contour of the groove in the second section is selected such that it merges, in an approximately radial direction at its end facing away from the first section, with the region of the housing which is directly adjacent to the tips of the gear wheel. In the embodiment shown, the angular region a is approximately 5°, while the angular region (3 is approximately 36°. The angular regions are matched to the distance between the teeth 20 of the gear wheel in such a way that the groove 22 extends in total over an angular range which is slightly greater than the angular distance between two gear wheel teeth. This results in a large wrap-round angle y, that is to say a large angular range over which the intermediate spaces between the teeth are covered by the housing 18 between the intake side and the delivery side. This large wrap-round angle y is advantageous with respect to low overflow losses at low rotational speeds, that is to say with respect to a large delivery quantity. The specific embodiment of the groove 22 leads to a continuous rise in pressure in the region of the intermediate spaces between the teeth when an intermediate space between the teeth leaves the region of the wrap-around and passes through the housing and into the region of the delivery side. At the start of the rise in pressure, when the gear wheel 14 is therefore in the position shown in Figure 3 in which a gear wheel tooth 20 located in front of the intermediate space 28 between the teeth which is under consideration enters the second section 26 of the groove 22, a comparatively narrow gap is produced between the housing and the corresponding gear wheel tooth so that the fuel flows out of a region with relatively high pressure into the intermediate space 28 between the teeth at a comparatively slow rate. The flow runs in the radial direction here so that it follows the edge of the gear wheel in the direction of the tooth base. This is ensured by the profile of the contour of the groove 22 in this region. As the fuel overflows into the intermediate space between the teeth which is to be filled up, the pressure in the preceding intermediate space between the teeth drops, which is in turn compensated by a subsequent flow of fuel through the narrow gap between the tooth tip and the base of the groove in its first section 24. If the gear wheel rotates further in the direction of the arrow, both the flow passage between the first section 24 of the groove 22 and the tooth tip lying opposite and the flow passage between the subsequent gear wheel tooth and the end of the groove 22 are enlarged. This permits a complete pressure equalization in the intermediate space 28 between the teeth before the outlet to the delivery side. In this way, cavitation damage both to the gear wheel teeth and to the housing of the gear pump is avoided. The groove 22 described can, of course, also be provided for the second gear wheel 16 in order to prevent cavitation damage there. For the cross-sectional configuration of the groove 22 the following rules apply: where Tf = Filling time for an intermediate space between the teeth by the groove N = Rotational speed of gear wheel Z = Number of teeth of the gear wheel Vd = Volume of vapor in the intermediate space between the teeth Vp = Volume flow of fuel through the groove to the intermediate space between the teeth w = Flow rate in the groove AN = Effective flow passage in the groove WE CLAIM : 1. Gear-wheel pump, in particular for a high-pressure fuel pump, comprising a housing (18), two gear wheels (14, 16) which are arranged in the housing and engage with one another, and at least one groove (22) which is formed in the housing on the delivery side of the gear pump, characterized in that the groove has a first section (24) which extends starting from the delivery side and in which the base of the groove (22) is at a small distance from the tips of the teeth (22) of the gear wheel, and a second section (26) which adjoins the first section and in which the base of the groove (22) is at a maximum distance from the tips of the teeth, which distance is greater than the distance in the first section, the first section extending over a smaller angular range (a) than the second section and the groove extending in total over an angular range (a, P), which is greater than the angular distance between two teeth (20). 2. Gear-wheel pump as claimed in claim 1, wherein the contour of the first section of the groove (22) has such a profile that a constant cross section is produced. 3. Gear-wheel pump as claimed in any one of claims 1 and 2, wherein the contour of the second section of the groove (22) has such a profile that a decreasing cross section is produced. 4. Gear-wheel pump as claimed in claim 3, wherein the contour of the second section of the groove (22) has a parabolic profile. 5. Gear-wheel pump as claimed in claim 4, wherein the contour of the second section (26) on the side facing away from the first section (24) extends radially with respect to the axis of rotation of the corresponding gear wheel. 6. Gear-wheel pump as claimed in any one of the preceding claims, wherein it is assigned to a high-pressure fuel pump (5), and the distance (t) between the tips of the teeth and the base of the groove in the second section is equal to the effective flow passage in the groove divided by the height of the gear wheel, while the distance (s) between the tips of the teeth of the gear wheel (14, 16) and the base of the groove (22) in the first section is equal to a third of the distance in the first section. 7. Gear-wheel pump as claimed in claim 6, wherein the distance (t) between the tips of the teeth and the base of the groove in the second section is equal to 0.7 mm. 8. Gear-wheel pump as claimed in claim 6, wherein the distance (s) between the tips of the teeth of the gear wheel (14, 16) and the base of the groove (20) in the first section is equal to 0.2 mm. 9. Gear-wheel pump as claimed in claim 6, wherein the first section (24) of the groove (22) extends over an angular range of 5°, while the second section (26) extends over an angular range of 36°. 10. Gear-wheel pump, in particular for a high-pressure fuel pump, substantially as hereinabove described and illustrated with reference to the accompanying drawings.

Documents:

in-pct-2001-1757-che abstract-duplicate.pdf

in-pct-2001-1757-che abstract.jpg

in-pct-2001-1757-che abstract.pdf

in-pct-2001-1757-che claims-duplicate.pdf

in-pct-2001-1757-che claims.pdf

in-pct-2001-1757-che correspondence-others.pdf

in-pct-2001-1757-che correspondence-po.pdf

in-pct-2001-1757-che description(complete)-duplicate.pdf

in-pct-2001-1757-che description(complete).pdf

in-pct-2001-1757-che drawings.pdf

in-pct-2001-1757-che form-1.pdf

in-pct-2001-1757-che form-18.pdf

in-pct-2001-1757-che form-26.pdf

in-pct-2001-1757-che form-3.pdf

in-pct-2001-1757-che form-5.pdf

in-pct-2001-1757-che pct.pdf

in-pct-2001-1757-che petition.pdf