Radial piston pump for providing high pressure in fuel injection systems of internal combustion engines

Abstract

The invention relates to a radial piston pump (1) for high-pressure fuel generation in fuel injection systems of internal combustion engines, in particular in a common rail injection system, having a drive shaft (4) which is mounted in a pump casing (2) and has an eccentric shaft section (6) on which a running roller (8) is mounted, and having preferably a plurality of pistons (16), which are arranged in a respective cylinder (14) radially with respect to the drive shaft (4) and each have a piston footplate (18), which makes contact with the circumferential surface (10, 12) of the running roller (8), at their ends facing the running roller (8). The invention provides that at least that surface (28) of the piston footplate (18) which is in contact with the circumferential surface (10, 12) of the running roller (8) consists of a wear-resistant material, namely of hard metal, a ceramic material, a cast carbide material, or cermet.

Description

The invention is based on a radial piston pump for high-pressure fuel generation in fuel injection systems of internal combustion engines, in particular in a common rail injection system, having a drive shaft which is mounted in a pump casing and has an eccentric shaft section on which a running roller is mounted, and having preferably a plurality of pistons, which are arranged in a respective cylinder radially with respect to the drive shaft and each have a piston footplate, which makes contact with the circumferential surface of the running roller, at their ends facing the running roller, in accordance with the preamble of claim 1.

A radial piston pump of this type is known, for example, from DE 198 09 315 A1. The piston footplate and the running roller of the known radial piston pump generally consist of case-hardened steel or of heat-treated steel. Over the course of time, however, sliding wear to these components can occur as a result of adhesion, abrasion or surface spalling. This undesirable wear can lead to failure of the radial piston pump and therefore also to failure of the internal combustion engine.

By contrast, the present invention is based on the object of further developing a radial piston pump of the type described in the introduction in such a manner as to increase its reliability.

This object is achieved according to the invention by the characterizing features of claim 1.

The susceptibility of the piston footplate/running roller sliding pairing to wear is significantly reduced by virtue of the fact that, for the first time, at least that surface of the piston footplate which is in contact with the circumferential surface of the running roller consists of a wear-resistant material, namely of hard metal, a ceramic material, a cast carbide material or cermet. The materials listed have a significantly higher modulus of elasticity compared to the steel materials used hitherto, which results in reduced deformation under load and consequently also in a more uniform surface pressure without significant stress peaks. If ceramic materials are used, in particular their lower weight plays an advantageous role, since the piston footplate together with the piston is accelerated and decelerated at a high frequency, and consequently the mass inertia is significantly reduced.

The piston footplate may be made entirely from the wear-resistant material, or else it consists, as hitherto, of case-hardened steel or heat-treated steel but bears at least one insert made from the wear-resistant material on its surface facing the running roller. The use of inserts brings the advantage of a modular structure, i.e. a standardized piston footplate can be provided with inserts made from different material, so that numerous variants can be produced.

If a ceramic material is used, this material preferably contains silicon nitride Si₃N₄ and has a surface roughness R_z of between 0.15 μm and 0.5 μm. Hard metals may consist, for example, of G20, GC37 or GC20 and may have a surface roughness R_z of between 0.3 μm and 1.0 μm, while the cast carbide material is formed by a chilled cast iron material, in particular by GGH or SoGGH, which has a surface roughness R_z of between 0.5 μm and 2.0 μm.

It is particularly preferable for the piston footplate, on its surface facing the running roller, to have at least two grooves which cross one another. This eliminates the overlap region of piston foot disk and running roller without a supply of lubricant. Fuel can accumulate in the grooves, which act as build-up gaps, and this fuel, on account of the sliding velocity between the running roller and the piston footplate, promotes the formation of a hydrodynamic sliding film, which further reduces the wear to the sliding surfaces.

Exemplary embodiments of the invention are illustrated in the drawings and explained in more detail in the description which follows. In the drawings:

FIG. 1 shows a cross-sectional illustration of a radial piston pump with a piston footplate and a drive shaft in accordance with a first embodiment of the invention;

FIG. 2 shows a large cross-sectional illustration of a piston and piston footplate in accordance with a further embodiment.

FIG. 2a shows an enlarged excerpt from FIG. 2;

FIG. 2b shows a further enlarged excerpt from FIG. 2;

FIG. 3 shows a view of the piston footplate from FIG. 2 from below;

FIG. 4 shows a cross-sectional illustration of a piston with piston footplate and a drive shaft in accordance with a further embodiment;

FIG. 5 shows a cross-sectional illustration of a drive shaft in accordance with a further embodiment;

FIG. 6 shows a view on line VI-VI from FIG. 5;

FIG. 7 shows a view on line VII-VII from FIG. 6.

The radial piston pump 1 shown in FIG. 1 is preferably used to generate the system pressure for the high-pressure reservoir (rail) of a common rail injection system of a compression-ignition internal combustion engine. It comprises a drive shaft 4 mounted in a pump casing 2 with an eccentric shaft section 6, on which a polygonal running roller 8, which can rotate with respect to the shaft section 6, is mounted. The polygonal running roller 8 has planar flat sections 12 arranged at a circumferential distance from one another along its circumferential surface 10.

The piston footplate 18 of a piston 16 guided radially with respect to the drive shaft 4 in a cylinder 14 is supported on each of the flat sections 12 of the running roller 8. The piston footplate 18 is preferably pivotably connected, by means of a spherical bearing 20, to the end of the piston 16 which faces towards the drive shaft 4. The spherical bearing 20 is realized, for example, by the end of the piston being designed as a partial ball 22 which engages in a spherical recess 24 of complementary design in the piston footplate 18. Furthermore, the piston footplate 18, together with the piston 16, is prestressed by a spring 26 onto the associated flat section 12 of the running roller 8. The way in which a radial piston pump 1 of this type functions is described, for example, in DE 198 02 475 A1 and therefore will not be dealt with in any further detail here.

At least that surface 28 of the piston footplate 18 which is in contact with the circumferential surface 10 of the running roller 8 consists of a wear-resistant material, namely of hard metal, a ceramic material, a cast carbide material or cermet. This is preferably realized by virtue of the fact that the piston footplate 18, on its surface 28 facing towards the running roller 8, has at least one, for example disk-like, insert 30 made from the wear-resistant material. The insert 30 may be positively and/or cohesively connected to the remaining piston footplate 18, for example by adhesive bonding or soldering. The insert 30 may, as shown in FIG. 1, extend over the entire contact surface 28 of the piston footplate 18 with the running roller 8 or only over part of this contact surface. Alternatively, it is also possible for the entire piston footplate 18 to be made from the wear-resistant material, so that there is no need for an additional insert 30.

If a ceramic material is used for the piston footplate 18, it preferably contains silicon nitride Si₃N₄. Hard metals may, for example, consists of G20, GC37 or GC20, while the cast carbide material may contain a chilled cast iron material, in particular GGH or SoGGH.

Furthermore, the piston 16 itself may be made from wear-resistant material, for example from an Si₃N₄ ceramic or a ZrO₂ ceramic. The piston 16 may be produced by extrusion and have a porosity of less than 5%, in which case the surface is infiltrated with MOS₂. Alternatively, the piston 16 may also be isostatically pressed and sintered.

Not least, it is also the case that at least part of the running roller 8, in particular the flat sections 12, consists of a wear-resistant material, namely of hard metal, a precision-cast material, a cast carbide material, a sintered tool steel or an alloyed nitriding steel.

As in the case of the piston footplate 18, this is preferably realized by virtue of the fact that the flat sections 12 are each provided with an insert 32 of the wear-resistant material, as shown in FIG. 1. An insert 32 of this type is in each case held positively and/or cohesively in a recess 34 of complementary shape in the flat section 12, for example by adhesive bonding or soldering. Alternatively, the entire running roller 8 may consist of the wear-resistant material.

If hard metal is used for the inserts 32 or for the running roller 8 itself, examples of suitable materials include G20, GC37 and GC20. A suitable precision-cast material is formed, for example, by GX-210WCr13H, while a suitable cast carbide material is locally remelted, carbide SoGGH (gradient material). A suitable sintered tool steel is ASP23. A nitriding steel which has been specially alloyed with Cr and/or Mo and/or V and/or C by nitriding or gas-nitriding is used for a variant with a gradient material. The basic elements and the process parameters used in the nitriding lead to deep diffusion with hardnesses of HV 750 to 850 combined, at the same time, with a higher strength of the base material. The compound layer which is formed is removed by grinding for functional reasons.

The surfaces of the piston footplate 18 and of the running roller 8 preferably have a surface roughness R_z of between 0.15 μm and 2 μm, depending on the materials used, on the sliding surfaces. The lower limit applies to ceramic, in particular a range from 0.15 μm to 0.5 μm, while the upper limit applies to metals such as SoGGH or ASP23. A surface roughness R_z of between 0.3 μm and 1 μm is provided for hard metal.

The table below lists preferred material pairings for the piston footplate 18, on the one hand, and the running roller 8, on the other hand. If inserts are used both in the running roller 8 and in the piston footplate 18, any desired combinations of material pairings are possible with the support bodies in each case unchanged. In particular, with the pairings in the table in which the running roller 8 preferably consists entirely of the wear-resistant material (“solid material”), it is alternatively also possible to use inserts 32 made from the corresponding material in the region of the flat sections 12, as has already been demonstrated in FIG. 1. The running roller 8 as support body for the inserts 32 may then consist of a different material, for example 50Cr4, 42CrV4 or 16MnCr5.

The exemplary embodiment in the third line of the table plays a particular role. In this case, a carbide zone is in each case formed in the region of the flat sections 12 of the running roller 8 consisting of a cast steel material and illustrated separately in FIG. 5. This carbide zone is produced either by a targeted solidification rate during casting of the running roller 8 or by remelting and then preferably forms the gradient material SoGGH. Consequently, the result is a running roller 8 in which a carbide zone 33 has been formed in the region of the surface sections 12, while the remaining zones and regions of the running roller 8 consist of cast steel with unchanged properties.

TABLE
Preferred material pairings
Running roller         Piston foot disk
Inserts of hard metal, Solid material or inserts
e.g. G20, GC37, GC20   comprising
                       a) ceramic, e.g. Si3N4 ceramic
                       b) chilled cast iron, e.g.
                       SoGSH
                       c) Cermet
Solid precision-cast   Solid material or inserts
material, e.g. GX-     comprising
210WCr13 H             a) ceramic, e.g. Si3N4 ceramic
                       b) hard metal, e.g. G20
                       c) Cermet
Solid cast carbide     Solid material or inserts
material, e.g. chilled comprising
cast iron SoGGH        a) ceramic, e.g. Si3N4 ceramic
                       b) hard metal, e.g. G20
                       c) Cermet
Solid material         Solid material or inserts
comprising sintered    comprising
tool steel, e.g.       a) ceramic, e.g. Si3N4 ceramic
ASP23,                 b) hard metal, e.g. G20
comprising C, Cr, Mo,  c) Cermet
V-alloyed nitriding    d) cast carbide material, e.g.
steel                  SoGGH

In each case one or more transverse grooves 36 may be formed in the region of the flat sections 12 of the running roller 8, as can be seen most clearly from FIG. 6. As can be seen from FIG. 7, the transverse groove 36 is arranged in the center of a depression 29, forming a groove run-out, in the flat section 12. The depression 29 is formed by two planes arranged at an angle with respect to the flat section 12, with the transverse groove 36 at their intersection line. The depression angle γ of the depression 29 is, for example, less than 15 degrees. The transition from the depression 29 to the flat section 12 is rounded with a radius R₄ of preferably less than or equal to 1 mm. The radius R₄ is produced for example by grinding. Fuel can accumulate in this transverse groove 36 or depression 29, which acts as a build-up gap, which fuel, on account of the sliding velocity between the flat sections 12 of the running roller 8 and the piston footplate 18, promotes the formation of a hydrodynamic sliding film, thereby reducing the wear to the sliding surfaces.

In the embodiments shown in FIG. 2 to FIG. 4, those parts which remain the same as and have the same action as in the example shown in FIG. 1 are denoted by the same reference designations. By contrast, in the example shown in FIG. 2, the piston footplate 18 is held on the associated piston 16 by a plate holder 38. The piston footplate 18, on its surface facing the piston 16, has a circular recess 40, in which the spherically shaped end 42 of the piston 16 engages, coming into contact with the base of the recess 40. The plate holder 38 is locked on the piston 16 by means of a circlip 46 engaging in a groove 44 in the piston 16. A circular insert 30 made from one of the wear-resistant materials described above is held in a recess 48 of complementary shape in the piston footplate 18, for example by cohesive bonding, in particular by soldering. As can be seen from FIG. 2a, the insert 30 is provided at the edge side, on its surface 31 facing the running roller 8, with an angled run-out 35, the run-out angle α amounting to approximately 15 degrees. Furthermore, the transition between this surface 31 and the run-out 35 is rounded with a radius R₂ of approx. 2 mm. The transition between the run-out 35 and the edge surface 37 of the insert 30 is also rounded by means of a radius R₁ of less than or equal to 1 mm.

Similarly to the flat sections 12 of the running roller 8, the inserts 30 of the piston footplate 18 preferably have at least two grooves 50 which cross one another, as can be seen most clearly from FIG. 3. On account of the grooves 50 being arranged so as to cross one another, there is a high probability that, with regard to the piston footplate 18 which can rotate with respect to the plate holder 38, one of the grooves 50 will be oriented transversely with respect to the direction of movement, in order to promote the formation of a hydrodynamic lubricating film. The grooves 50 are preferably produced by pressing. This results in a lower notch effect compared to chip-forming processes, since the material fibers are not severed. As can be seen from FIG. 2b, the grooves 50 are each arranged in the center of a depression 39, forming a groove run-out, in the surface 31. The depression is formed by two planes arranged at an angle with respect to the surface 31, with the respective groove 50 located at the intersection line of these planes. The depression angle β of the depression 39 is, for example 5 degrees. The transition between the depression 39 and the surface 31 is rounded with a radius R₃ of preferably less than or equal to 1 mm.

In the exemplary embodiment shown in FIG. 4, the piston footplate 18 consists entirely of one of the wear-resistant materials mentioned above and is fitted into the passage hole 52 in an annular bush 54 which consists of steel. The connection between the annular bush 54 and the piston footplate 18 is preferably produced by soldering. Of course, there are also other conceivable options for arranging wear-resistant material on the mutually associated sliding surfaces 12, 28 of the running roller 8 and piston footplate 18.

Claims

1-8. (canceled)

9. A radial piston pump (1) for high-pressure fuel generation in fuel injection systems of internal combustion engines, in particular in a common rail injection system, having a drive shaft (4) which is mounted in a pump casing (2) and has an eccentric shaft section (6) on which a running roller (8) is mounted, and having preferably a plurality of pistons (16), which are arranged in a respective cylinder (14) radially with respect to the drive shaft (4) and each have a piston footplate (18), which makes contact with the circumferential surface (10, 12) of the running roller (8), at their ends facing the running roller (8), wherein at least that surface (28, 31) of the piston footplate (18) which is in contact with the circumferential surface (10, 12) of the running roller (8) consists of hard metal, a cast carbide material, or cermet.

10. The radial piston pump as claimed in claim 9, wherein the piston footplate (18), on its surface (31) facing the running roller (8), bears at least one insert (30) made from hard metal, from a cast carbide material or from cermet.

11. The radial piston pump as claimed in claim 9, wherein the hard metal contains G20, GC37 or GC20 and has a surface roughness Rz of between 0.3 μm and 1.0 μm.

12. The radial piston pump as claimed in claim 9, wherein the cast carbide material contains a chilled cast iron material, in particular GGH or SoGGH, and has a surface roughness Rz of between 0.5 μm and 2.0 μm.

13. The radial piston pump as claimed in claim 9, wherein the piston footplate (18), on its surface (31) facing the running roller (8), has at least two grooves (50) which cross one another.

14. The radial piston pump as claimed in claim 13, wherein one such groove (50) is in each case arranged in the center of a depression (39), forming a groove run-out, in the surface (31).

15. The radial piston pump as claimed in claim 9, wherein the surface of the piston footplate (18) and/or of the running roller (8) has a surface roughness Rz of between 0.15 μm and 2 μm.