Patent Publication Number: US-10316807-B2

Title: Piston fuel pump for an internal combustion engine

Description:
This application is a 35 U.S.C. § 371 National Stage Application of PCT/EP2014/079055, filed on Dec. 22, 2014, which claims the benefit of priority to Serial No. DE 10 2014 202 794.2, filed on Feb. 17, 2014 in Germany, the disclosures of which are incorporated herein by reference in their entireties. 
     BACKGROUND 
     The disclosure relates to a piston fuel pump in accordance with the following description. 
     Fuel systems of internal combustion engines are known from the marketplace, in which fuel systems the fuel is delivered from a fuel tank under high pressure by means of a mechanically driven piston fuel pump into a fuel rail and passes from there via injectors into combustion chambers of an internal combustion engine. 
     In the piston fuel pump which is known from DE 10 2004 063 074 A1, for example, a displaceably mounted pump piston is provided which compresses fuel in a delivery space of the piston fuel pump. The pump piston is guided in a piston bushing with a small sealing gap and such that it slides by way of a close fit. In order to support and seal via a gap seal, the piston bushing has to have a certain length and possibly absorb great transverse forces. Therefore, the piston bushing is often produced from steel. On account of high tolerance requirements, moreover, what is known as a “piston pairing” is used, that is to say each pump housing is assigned a defined piston. Furthermore, the cylinder in the pump housing has to be honed in a complicated manner. There is a requirement for a fuel piston pump which is less complicated to produce. 
     SUMMARY 
     The problem on which the present disclosure is based is solved by way of a fuel piston pump having the features described herein. Advantageous developments of the disclosure are mentioned in the following description. Moreover, further features which are important for the disclosure are found in the following description and in the drawing. 
     The fuel piston pump according to the disclosure has the advantage that a piston bushing and the corresponding highly accurate fit of the piston in the piston bushing are no longer absolutely necessary and therefore considerable costs can be saved. Instead, the piston fuel pump has a seal which is arranged on the circumference of the pump piston and for its part has at least one bump which points in an axial direction. 
     In the present case, a bump is understood to mean a protuberance-like elevation, for example with a round base area. For example, conical and spherical shapes are possible. Other shapes are also possible. The bump points with its end region which lies opposite the base area in an axial direction, that is to say, in particular, in or counter to the movement direction of the pump piston in the pump cylinder. 
     The at least one bump is particularly preferably configured as a locally convexly configured region in an otherwise concavely configured surrounding of the seal. The at least one bump is particularly preferably formed integrally on the seal. 
     The fuel piston pump is, in particular, a pump which has a pump housing, in which a working space which is delimited by the pump piston is formed. The compression of the fuel takes place, in particular, in said working space, in particular by way of an axial movement of the pump piston which reduces the size of the working space. In particular, a compression of the fuel in the working space takes place to a high pressure level, for example to from 100 bar to 600 bar. 
     The seal according to the disclosure is configured, in particular, between the working space and a low pressure region of the pump. The pressure in the low pressure region is lower than the high pressure level which is generated in the working space of the pump. The pressure level in the low pressure region can lie, for example, at from 3 bar to 10 bar and can be generated by way of a separate forepump. 
     The working space is connected, in particular, via an outlet valve to a pump outlet and is connected, in particular, via an electrically actuable inlet valve to a pump inlet. The electrically actuable inlet valve can be configured, in particular, as a quantity control valve. As an option, furthermore, a damping device for damping pulses in the low pressure region of the pump can additionally be provided between the pump inlet and the working space. 
     The damping device for damping pulses in the low pressure region can comprise, for example, a gas volume which is enclosed between two diaphragms: details with regard to the damping device can be configured as shown in DE10327408A1. 
     A further valve which is arranged between the pump outlet and the working space and is arranged in an antiparallel manner with respect to the outlet valve can be provided and can act, in particular, as a pressure limiting valve for a high pressure accumulator which can be connected to the pump. 
     The outlet valve and/or the inlet valve and/or the pressure limiting valve are/is preferably fixed in a stationary manner with respect to the pump housing and to this extent also in a stationary manner with respect to the pump cylinder. Fixing of said components on the pump piston is ruled out in this regard, in particular. 
     The advantage arises that the mass of the pump piston is low and therefore the dynamics and/or ease of movement of the pump is/are improved. 
     In addition or as an alternative, the pump piston is preferably configured as a solid body, with the result that it can withstand the high pressures which act during the fuel injection, in particular in the case of direct gasoline injection, without deformation. A capability of flow to pass through the pump piston in the longitudinal direction is ruled out in this regard. Further details of the arrangement of the working space, outlet valve and pressure limiting valve with respect to one another and in the pump body can be configured, for example, as shown in DE102004013307A1. 
     The pump cylinder can be configured in a bushing which is fixed in the pump body. As an alternative, the pump cylinder can also be provided directly in the pump body. 
     The pump body, the pump piston, the pump cylinder and/or all pump parts which come into contact with the fuel preferably consist only of steels and of plastics, such that there is a high resistance even to ethanol-containing fuels and/or other aggressive fuels as a result. 
     It is provided, in particular, that the seal bears against the pump piston in the axial direction. It is to be noted here, however, that component tolerances and assembly tolerances cannot be avoided in practice, with the result that the seal and the components which are arranged adjacently with respect to it possibly end up being arranged in a manner which differs somewhat from their target position. In particular, if material with high strength and low elasticity is used for the seal and piston, this has the consequence that the forces which occur in the region of the seal can differ from the actually provided forces. 
     The provision according to the disclosure of a bump which points in an axial direction reduces the bearing area of the seal in comparison with a seal without a bump, in particular if the seal comes into axial contact via the bump with the pump piston or a component which is fixed on the pump piston or is formed integrally on the pump piston. Component tolerances and assembly tolerances can be compensated for in this way by way of a corresponding deformation of the bump in the axial direction, without excessively great forces occurring in the region of the seal. 
     Developments of the disclosure provide that the seal is subjected axially even to a prestress by way of and/or on the piston. The seal can be arranged on the circumference of the piston such that it is compressed, in particular, in the axial direction. By way of the measures which are mentioned, a desired prestress can be maintained relatively precisely even in the case of component tolerances and assembly tolerances which occur. 
     Developments of the disclosure provide that the at least one bump comprises a plurality of bumps. The plurality of bumps can all point in the same axial direction. It is also possible to provide a plurality of bumps, of which some, for example half of the bumps, point in the one axial direction, and the remaining bumps, for example the other half of the bumps, point in the other axial direction. For example, at least three bumps can be provided which point in the same axial direction. 
     Component tolerances and assembly tolerances can be compensated for in a particularly effective manner by way of the provision of a plurality n of bumps which point in the same axial direction. This applies to a particular extent if the bumps on the seal are arranged homogeneously in the circumferential direction, for example on an imaginary circular line. Here, the n bumps can be spaced apart from one another by a circumferential angle of 360°/n. 
     The dimensioning of the bump/bumps is carried out in accordance with the function thereof. It is preferred here for bumps with an extent in the axial direction which lies in the range of from 3% to 20% of the axial extent of the seal and/or with an extent in the radial direction which lies in the range of from 3% to 10% of the radial extent of the seal that from three to nine bumps which point in an axial direction are provided. 
     One development of the disclosure provides that the seal bears axially via the at least one bump against a cap which is pushed onto the pump piston, in particular bears axially against it under stress. 
     It is preferred that, although the cap bears axially against the bump or the bumps, it deforms the latter only to a smaller extent in the axial direction than corresponds to the axial extent of the bump or the bumps. The cap then bears against the seal merely at the bump or at the bumps, but not at a region which is situated away from the bump or between the bumps as viewed in the circumferential direction. 
     The cap can be, for example, of cup-shaped or sleeve-shaped configuration. It preferably consists of steel and/or is a deep drawn part. 
     As an alternative or in addition, the seal can bear axially, in particular via the at least one bump, against a step which is configured on the pump piston, in particular can bear axially against it under stress. 
     As an alternative or in addition, the seal can also bear, in particular via the at least one bump, against a component which is fixed on the pump piston, in particular can bear against it under stress, for example against a circlip which is fixed in a groove of the piston. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the following text, examples of the present disclosure will be explained in greater detail with reference to the appended drawings, in which: 
         FIG. 1  shows a diagrammatic illustration of a fuel system of an internal combustion engine with a detail of a piston fuel pump according to the disclosure, 
         FIG. 2  shows an enlarged sectional illustration of the detail of the piston fuel pump according to  FIG. 1 , 
         FIG. 3  shows an alternative embodiment of the piston fuel pump, 
         FIG. 4  shows a further alternative embodiment of the piston fuel pump, 
         FIGS. 5 and 6  show the seal axially from above and below, and 
         FIG. 7  shows an axial end region of the seal on an enlarged scale. 
     
    
    
     DETAILED DESCRIPTION 
     In  FIG. 1 , a fuel system of an internal combustion engine is given the designation  10  overall. It comprises a fuel container  12 , from which an electric forepump  14  delivers the fuel into a low pressure line  16 . The latter leads to a high pressure pump in the form of a piston fuel pump  18 . From the latter, a high pressure line  20  leads to a fuel rail  22 . A plurality of injectors  24  which inject the fuel directly into combustion chambers (not shown) which are assigned in each case to them are connected to said fuel rail  22 . 
     The piston fuel pump  18  comprises a pump housing  26  which is indicated only in regions and in which a pump piston  28  is guided or mounted displaceably. Said pump piston  28  can be set into a to and fro movement by a cam drive (not shown), which is indicated by way of a laterally illustrated double arrow  30 . The pump piston  28  is loaded by a helical spring  32  into a bottom dead center in  FIG. 1 . The pump piston  28  and the pump housing  26  delimit a delivery space  34 . Said delivery space  34  can be connected via an inlet valve  36  to the low pressure line  16 . Furthermore, the delivery space  34  can be connected via an outlet valve  38  to the high pressure line  20 . 
     Both the inlet valve  36  and the outlet valve  38  are configured as check valves. Here, an embodiment of the inlet valve  36  as a quantity control valve is not shown but is possible. In an embodiment of this type, the inlet valve  36  can be normally open during a delivery stroke of the pump piston  28 , with the result that the fuel is delivered not into the fuel rail  22 , but rather back into the low pressure line  16 . As a result, the fuel quantity which is delivered into the fuel rail  22  by the piston fuel pump  18  can be set. 
     The pump piston  28  is guided in a pump cylinder  40  which to this extent is part of the pump housing  26 . At an end which faces the delivery space  34 , the pump piston  28  has an upper end section  42  in  FIG. 1 . In the vicinity of said upper end section  42 , furthermore, the pump piston  28  has a circularly annular step  44  in the manner of a radially projecting circumferential collar. A seal  46  comes into contact with the pump piston  28  or with the step  44 . 
     At its end which faces away from the delivery space  34 , furthermore, the pump piston  28  has a lower end section  52  in  FIG. 1 . In the vicinity of said lower end section  52 , a guide sleeve  54  is arranged fixedly on the pump housing  26 . An O-ring seal  56  is provided in a groove  58  between the guide sleeve  54  and the pump housing  26 . The guide sleeve  54  has a cylindrical section  60  which extends coaxially with respect to the pump piston  28  and by way of which the helical spring  32  is guided. Along a piston longitudinal axis  62 , the helical spring  32  dips at least in sections into a spring receiving groove  64  of the guide sleeve  54 , where it is supported axially against the guide sleeve  54 . 
     Furthermore, the guide sleeve  54  has a circularly cylindrical receiving section  66  in the interior, which receiving section  66  is formed substantially by way of the inner circumferential wall of the cylindrical section  60 . An annular sealing element  68  is arranged in a stationary manner relative to the pump housing  26  in said receiving section  66 , the sealing element  68  having an H-shaped cross section. Furthermore, a guide element  72  is likewise arranged in a stationary manner relative to the pump housing  26  in a collar section  70  which extends radially inward at the projecting end of the cylindrical section. Together with the seal  46 , said guide element  72  which is therefore spaced apart clearly from the seal  46  as viewed in the axial direction of the pump piston  28  provides the guidance or two-point mounting of the pump piston  28 . 
     The configuration of the region of the seal  46  and its mounting is of particular importance in the present case. Said aspects will therefore be described in detail with reference to the following  FIGS. 2-7 . 
       FIG. 2  shows the region of the seal  46  of the piston fuel pump  18 . In a lower region in  FIG. 2 , the seal  46  is pushed over the step  44  of the pump piston  28  to such an extent that it comes into axial contact with the step  44  on a shoulder  469  which is configured on it. In particular, that material region of the seal  46  which is present radially outside the outer circumferential face of the step  44  forms a bearing or guide region  48 , by way of which the pump piston  28  is guided in a sliding manner in the pump cylinder  40  and is mounted radially. 
     From an inner circumferential wall of the pump cylinder  40 , the guide region  48  has a spacing of approximately 2/100 mm which cannot be seen in the figures. In the axial direction, that is to say along the piston longitudinal axis  62 , the sealing region  50  which is configured as a sealing lip  467  extends toward the delivery space  34  in a manner which adjoins the guide region  48 . Here, the sealing lip  467  extends substantially coaxially with respect to the pump piston  28  as a tubular section which is integrally formed on the guide region  48  and is prestressed elastically radially to the outside. The sealing lip  467  bears against the inner circumferential wall of the pump cylinder  40 . In said example, the guide region  48  and the sealing region  50  are configured in one piece. 
     A cap  101  is pushed axially onto the pump piston  28 , which cap  101  comes into contact with the seal  46  radially inward of the sealing lip  467  and on the working space side of the shoulder  469 . The cap  101  is seated fixedly on the pump piston  28  by way of radial pressure and exerts an axially acting force on the seal  46 . The seal  46  which is arranged between the cap  101  and the step  44  of the pump piston  28  is therefore under axial prestress. 
     In said example, the cap  101  is configured as a sleeve  101   a , that is to say it has the form of a ring or tubular section which is open on two sides. The sleeve  101   a  is pushed completely onto the pump piston  28  and terminates flush with the latter on the working space side. As an alternative, pushing the sleeve  101   a  yet further onto the pump piston  28  or a projection on the working space side of the sleeve  101   a  would likewise also be possible in principle and possibly practicable. 
       FIG. 3  shows one alternative to the configuration of the cap  101  as a sleeve  101   a . Here, the cap is configured as a cup  101   b . The cup  101   b  has a cup bottom and a cup wall and is pushed onto the pump piston  28  with its open end first. 
     In the example which is shown in  FIG. 3 , the cup  101   b  is pushed completely onto the pump piston  28 . To this extent, its bottom comes into contact with the end side of the piston. 
     In said example, the cup bottom has a small hole  300  in the sectional plane of  FIG. 3 , through which small hole  300  air can escape from the cup when the cup  101   b  is pushed onto the pump piston  28 . 
     In principle, in particular in the embodiments as a sleeve  101   a  or as a cup  101   b , the cap  101  can be produced as a deep drawn part, for example made from steel. The cap  101  preferably consists of a material, the coefficient of thermal expansion of which coincides or approximately coincides with that of the pump piston  28 . For example, the cap  101  can consist of the same material as the pump piston  28 . Furthermore, the cap  101  can be configured, for example, with a wall thickness of 1 mm. 
     In the preceding examples, the seal  46  is configured axially between the cap  101  and a step  44  which is configured in one piece on the pump piston  28 . In principle, the one-piece nature is not absolutely necessary. A simplification in terms of production technology is possible by virtue of the fact that the step  44  is realized by way of a circlip which is inserted into a groove of the pump piston  28 . 
       FIG. 4  shows yet another solution in this regard. Here, a hat-shaped holding element  102  is pushed with its opening first over the working space-side end of the pump piston  28 . Here, a bottom  102   a  of the holding element  102  comes axially into contact with the end side of the pump piston  28 , and a side wall  102   c  of the holding element  102  comes into contact radially with the pump piston  28 . A rim  102   b  of the holding element  102  which lies axially opposite the bottom  102   a  of the holding element  102  is spread radially and to this extent forms a step  44 . 
     As in the examples which are shown in  FIGS. 2 and 3 , the seal  46  is in contact with the step  44  which to this extent is provided in a functionally identical manner. 
     In said example, the pump piston  28  has a uniform diameter along its entire length. Particularly simple and inexpensive production, for example machining of the pump piston  28  by means of throughfeed grinding, that is to say with a stationary grinding disk, is possible in this way. 
     It is provided in the exemplary embodiments which are shown in  FIGS. 2, 3 and 4  that the seal  46  has at least one integrally formed bump  461  which points in an axial direction, and that the seal  46  bears axially via the at least one bump  461 . By way of example, in said examples, the seals  46  even have in each case a plurality of bumps  461  which point in the direction of the working space  34  and via which the seal  46  bears against the cap  101  and is prestressed axially as a result. In said example, the bumps  461  have a hemispherical design. As an alternative, they might also be conical or frustoconical. The bumps  461  have, for example, a diameter of approximately 0.6 mm, approximately 10% of the diameter of the seal  46 , and a height of approximately 0.3 mm, approximately 10% of the height of the seal  46 . 
     Even if the cap  101  bears against the bumps  461  under stress, the resulting deformation of the bumps  461  is so comparatively low that bearing of the cap  101  against a region of the seal  46  which lies between the bumps  461  in the circumferential direction is suppressed. 
       FIG. 5  shows the seal  46  in plan view, from above in relation to  FIGS. 2, 3 and 4 . It can be seen that a total of eight bumps  461  which are formed integrally on the seal  46  point in the direction of the working space  34 , which bumps  461  are arranged on an imaginary circular ring about the piston axis  62  and are spaced apart from one another in the circumferential direction in each case by 45°. 
     It goes without saying that the bumps  461  might also be configured in addition or as an alternative on the axially opposite side of the seal  46 , on the step  44 , in an otherwise unchanged manner. 
     In said examples, the seal  46  consists of the fiber-reinforced thermoplastic material PEEK 150CA30 or PA66CF20 and is produced by means of injection molding. Via the injection molding technique which is explained in the following text, and in particular the arrangement of the injection points  462  and the ventilating points  463 , it can be achieved that the orientation of the fibers is disordered. 
     Here, the injection molding takes place via injection points  462  which can be seen in  FIG. 5 , lie on a common imaginary circular line with the bumps  461  and are spaced apart from one another by 90° in the circumferential direction. The injection points  462  appear on the finished product, for example, as small annular or crescent-shaped burrs or as small warts. The injection points have a diameter of 0.9 mm or of not more than 0.9 mm. 
     Furthermore, the injection molding takes place via ventilating points  463  which are arranged on the axially opposite side of the seal  46 , on the bottom in  FIGS. 2, 3 and 4 . They can be seen in  FIG. 6 . In the present case, eight ventilating points  463  are provided which lie on an imaginary circular line and are spaced apart from one another by 45° in the circumferential direction. The ventilating points  463  have a diameter of 0.7 mm or of not more than 0.7 mm and appear on the finished product, for example, as small cavities. 
     It can be provided that injection points  462  and ventilating points  463  are always arranged offset in the circumferential direction with respect to one another. In this way, improved thorough mixing of the liquefied injection molding material takes place in the injection mold and a directed orientation of the fibers is avoided and anisotropic material properties of the seal  46  are avoided. 
     In the present case, an axial end region  464  of the seal  46  is configured on the sealing lip  467  on the working space side.  FIG. 7  shows a detail of the seal  46  which is once again enlarged accordingly. 
     It is provided that a radially outwardly lying surface of the seal  46  which lies opposite an inner surface of the pump cylinder  40  is inclined in an axial end region  464  of the seal  46  radially inward at an angle α of from 10° to 60° with respect to the inner wall of the pump cylinder  40 . This has the effect, or it is provided as an alternative, that a relative movement between the pump cylinder  40  and the pump piston  28  in the axial direction, in particular in the direction toward the working space  34 , aids raising up of the seal  46  from the pump cylinder  28  in a radially inwardly pointing direction. In this case, a liquid film which consists of fuel is formed between the seal  46  and the pump cylinder  40 , which liquid film considerably reduces the wear of the piston fuel pump  18  with a small leakage. 
     For this purpose, an outwardly pointing, circumferential web  468  is integrally formed at or on the sealing lip  467 , which web  468  has approximately the shape of an isosceles triangle in the longitudinal direction in cross section, of which isosceles triangle the two opposite acute corners point in axial directions and the third obtuse corner of which bears against the pump cylinder  40  (statically). It is provided that merely said web comes into contact (statically) with the pump cylinder  40 , whereas the seal  46  or the sealing lip  467  is otherwise spaced apart from the pump cylinder  40  by a gap. A width s of the gap is, for example, 20 μm. In the case of a relative movement, as depicted above, raising up of the web  468  from the pump cylinder  40  is also provided, furthermore.