Abstract:
A plunger assembly for a high pressure fuel pump head, wherein an axial internal drilling provided in a first plunger section, and a clearance between the first and a second plunger section, are exposed to a pumping fuel pressure, and wherein a pressure gradient along an outside wall of the first section and the bore, and pressure within the drilling and clearance, results in an increasing deformation of the first section and thereby a decreasing clearance between the first section and the bore, moving towards the second section, thereby providing sealing from one end of the plunger to the opposite end.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a national stage application under 35 USC 371 of PCT Application No. PCT/EP2015/078413 having an international filing date of Dec. 2, 2015, which is designated in the United States and which claimed the benefit of GB Patent Application No. 1501282.6 filed on Jan. 27, 2015, the entire disclosures of each are hereby incorporated by reference in their entirety. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates to a fuel pump for an internal combustion engine, and in particular to a plunger assembly for a pump head for a high pressure fuel injection system such as a diesel fuel injection system. 
       BACKGROUND OF THE INVENTION 
       [0003]    Known fuel pumps for high pressure fuel injection systems, such as a common rail diesel application, typically comprise a pump head and a plunger which is reciprocally moveable within a bore thereby to establish a pressure difference. To ensure the necessary pressure difference is achieved, a sealing length is provided, comprising a minimal clearance between the plunger and bore. The sealing length and the clearance between the plunger and bore are two factors which define the volumetric efficiency (VE) of the pump. 
         [0004]    To ensure that OEMs can meet CO2 efficiency targets, fuel injection systems are running at higher pressures. A known problem of pump heads running at enhanced pressures is a reduction in the VE of the pump head. 
         [0005]    An example of a known hydraulic pump head is illustrated in  FIG. 1 . The pump head  2  comprises a pump housing  4  including a turret portion  10 , an inlet valve arrangement  70 , and an outlet valve arrangement  80 . A plunger  6  arranged for reciprocal movement within a bore  8  provided in the housing  4 . 
         [0006]    The pump head  2  illustrated in  FIG. 1  represents a current pump head design suitable for high pressure applications, such as 2000 bar or higher. 
         [0007]    Due to the configuration of the outlet port  80  in the prior art pump  2 , a chamber or annulus  42  is provided which opens above the plunger guiding length. The annulus  42  ensures that the fuel pumped by the plunger is not restricted. 
         [0008]    Current hydraulic pump head design has been constrained by several factors, such as:
       external packaging constraints, such as restriction of leakage length due to restriction of pump height;   internal pump packaging constraints, such as restriction of turret stiffness due to restriction of turret outer diameter;   plunger bore clearance, defined by manufacturing capability of the plunger and the bore.       
 
         [0012]    The prior art pump head design as discussed above exhibits a reduction in VE at higher pressures. An increase in clearance between the plunger and the bore while operating at elevated pressures results in an increased leakage. 
         [0013]    A further known pump head design, suitable for use in heavy duty applications, is partially illustrated in  FIG. 2 . This embodiment of head plunger assembly is not reliant on a long leakage length having a specific clearance to reduce leakage. Rather it utilises a short length of very tight clearance. 
         [0014]    The plunger  206 , which is reciprocally movable within a barrel  266 , is provided with an internal central drilling  248 . A recess  264  is provided in the plunger  206 , leaving a full diameter section  268  at the top end of the plunger  206 , which has a tight clearance with the bore  208 . Sealing is provided by this tight clearance; sealing may be over a relatively short length, such as  2 mm. A relatively short sealing length reduces the necessary depth of the drilling  248 , thereby reducing dead volume. 
         [0015]    The known pump head of  FIG. 2  operates as follows:
       fuel pressure is increased by the movement of the plunger  206 ;   central drilling  248  is exposed to high pressure, whereas the clearance between the full diameter section  268  and the bore  208  is subjected to a pressure gradient, decreasing from a highest pressure at the top (in the orientation of  FIG. 2 ), to a cambox or return line pressure at recess  264 ;   a tip of the plunger  206  expands due to the effect of a resulting pressure difference;   clearance between the sealing land and the barrel  266  is reduced;   flow of high pressure fuel through the clearance is minimised.       
 
       SUMMARY OF THE INVENTION 
       [0021]    It is an object of the present invention to provide an improved plunger assembly for a high pressure pump head. 
         [0022]    Accordingly the present invention provides, in a first aspect, a plunger assembly according to claim  1 . 
         [0023]    The present invention enables an efficient plunger design to be integrated into different hydraulic pump head designs, for example with different configurations of outlet valve, without incurring a significant dead volume penalty. 
         [0024]    The first section of the plunger assembly may comprise an expanding zone wherein, during use of the pump head, the expanding zone is caused to expand by fuel pressure within the clearance and thereby form a zone of minimum clearance between the first section and the bore. 
         [0025]    The internal portion of the plunger assembly which is subjected to a pumping fuel pressure may comprise an internal drilling, and a clearance between the first section and the second section, wherein the internal drilling opens at a first end remote from the second section of the plunger assembly onto the chamber, and opens at a second end onto the clearance. 
         [0026]    The pressure reducing feature may comprise a radial recess which extends over portions of outer diameters of the first section and the second section of the plunger assembly and which, during use of the pump head is exposed to a fuel feed, cambox or return line fuel pressure. 
         [0027]    The pressure reducing feature may alternatively comprise axial grooves. 
         [0028]    In one embodiment, an extension section of the second section of the plunger assembly is retained within a recess provided in the first section by an interference fit along an interference zone between the extension section and an annular wall section which circumferentially defines the recess. 
         [0029]    The present invention further comprises a pump head for a high pressure fuel pump for a vehicle, comprising a plunger assembly as described above. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0030]    The present invention is now described by way of example with reference to the accompanying drawings in which: 
           [0031]      FIGS. 1 and 2  are cross-sectional views of a known hydraulic pump head and a portion of a known pump head design respectively; 
           [0032]      FIG. 3  is a cross-sectional view of a pump head in accordance with the present invention; 
           [0033]      FIG. 4  is a cross-sectional view of a plunger assembly in accordance with the present invention; and 
           [0034]      FIG. 5  is a cross-sectional partial view of the plunger assembly of  FIG. 4 . 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0035]    An embodiment of the present invention is described below in relation to the orientation of the figures. Terms such as upper, lower, above, below, top and bottom are not intended to be limiting. 
         [0036]    Referring to  FIGS. 3 to 5 , a pump head  102  in accordance with the present invention comprises a pump body or pump housing  104 , an inlet valve arrangement  170  (indicated on  FIG. 3 ) and an outlet valve arrangement  180  (indicated on  FIG. 3 ). The pump housing  104  is provided with a bore  108  extending along a longitudinal axis A. 
         [0037]    A plunger assembly  106  is located within the bore  108 . A chamber, or annulus  142  (indicated on  FIG. 3 ) is provided within the bore  108  above a first, upper end  160  of the plunger assembly  106 . 
         [0038]    The plunger assembly  106  is reciprocally movable within the bore  108  along the longitudinal axis A, by force transferred from a cam arrangement (not shown) to a second, lower end  162  of the plunger assembly  106 . 
         [0039]    The plunger assembly  106  is guided within the bore by guidance zones  136 ,  138  (both indicated on  FIG. 4 ). 
         [0040]    The plunger assembly  106  comprises two parts; a first, upper section,  110  and a second, lower section  120 . An extension section  122  of the second, lower section  120  extends into a recess  112  (indicated in  FIGS. 4 and 5 ) provided in the first, upper section  110  and defined circumferentially by an annular wall section  114 . As explained in greater detail below, the annular wall section  114  comprises an expanding zone  134  (indicated on  FIG. 5 ), and an interference zone  130  ( FIGS. 4 and 5 ). 
         [0041]    The first, upper section  110  and the second, lower section  120  are retained together by a retaining means, which in the embodiment illustrated in  FIGS. 3 and 4  comprises an interference fit along the interference zone  130 , between the extension section  122  and the annular wall section  114  which circumferentially defines the recess  112 . 
         [0042]    Between an end  124  (indicated in  FIG. 5 ) of the extension section  122  of the second, lower section  120  of the plunger assembly  106 , and a base  150  (indicated on  FIGS. 4 and 5 ) of the recess  112  of the first, upper section  110  of the plunger assembly  106 , is a clearance  140  (indicated on  FIG. 5 ). 
         [0043]    A central drilling  116  is provided in the first, upper section  110  of the plunger assembly  106 , axially along a longitudinal axis A of the plunger assembly  106 . The axial drilling  116  opens at a first, upper end  152  (remote from the second section  120  of plunger assembly  106 ), to an annulus  142  provided in the bore  108  of the housing  104 ; the annulus  142  ensures that the pumped fuel is not restricted. 
         [0044]    The central drilling  116  opens at a second, lower end  154  (remote from the first end  152 ), onto the clearance  140  between the base  150  of the recess  112  of the first section  110  and the end  124  of the extension section  122  of the second section  120 . 
         [0045]    A pressure reducing feature comprising, which in the present embodiment comprises a radial recess  126  (indicated on  FIG. 4 ) in an outer diameter of the plunger assembly  106 . The radial recess  126  has been exaggerated in  FIG. 4  for illustrative purpose. The radial recess  126  extends along part of the outer diameter of the first section  110  of the plunger assembly  106  and along part of the outer diameter of the second section  120  of the plunger assembly  106 , and is subject to a fuel feed, cambox or return line pressure during use of the pump head  102 . 
         [0046]    During use of the pump head  102 , the annulus  142  is exposed to pumping pressure, and therefore, an internal space of the plunger assembly  106 , comprising the internal drilling  116  in the first section  110 , and the clearance  140  between the first section  110  and the second section  120 , are also open to pumping pressure. 
         [0047]    A pressure gradient, indicated by arrows P on  FIGS. 4 and 5 , occurs along an outer wall  190  of the first section  110  of the plunger assembly  106 , i.e. the pressure is at its greatest value at the top of the outer wall  190  of the first section  110 , and decreases moving away from the annulus  142  (i.e. downwards in the orientation of the Figures), towards a first, upper edge  196  (indicated on  FIGS. 4 and 5 ) of the radial recess  126 , which is subject to fuel feed, cambox or return line pressure during use of the pump head  102 . 
         [0048]    The pressure gradient P causes a radial inward force to be applied to the first section  110  of the plunger assembly  106 , above the first, upper edge  196  of the radial recess  126 ; the inward force decreases with the pressure gradient moving towards the upper edge  196  of the radial recess  126 . 
         [0049]    High fuel pressure within the clearance  140  causes a radial outward force to be applied to part of the annular wall section  114  above the interference zone  130 . 
         [0050]    As a result of the radial inward and outward forces being applied to the first section  110 , deformation of the first section  110  varies along the outer wall  190 . Accordingly, clearance between the outer wall  190  of first section  110  and the bore  108  varies, generally decreasing moving downwardly in the orientation of the figures. In particular, the expanding zone  134  of the wall section  114  forms a minimum clearance zone  192  (indicated generally on  FIGS. 4 and 5 ), at which clearance between the first section  110  and the bore  108  is at a minimum value. 
         [0051]    Sealing across the plunger assembly  106  is provided by the reduced clearance between the first section  110  of the plunger assembly  106  and the bore  108 ; the sealing maintains the required pressure difference between the first, upper end  160  of the plunger assembly and the first, upper edge  196  of the radial plunger recess  126 . 
         [0052]    The stiffness of the expanding zone  134  of the first section of the plunger can be optimised to seal at different pump head pressures as required. 
         [0053]    Furthermore, the position of the expanding zone  134  can be selected to ensure that the guidance zones  136 ,  138 , are of sufficient length to support radial side loads encountered during use of the pump head  104 . 
         [0054]    If the internal drilling of the prior art plunger of  FIG. 2  were to be combined into a hydraulic pump head such as that illustrated in  FIG. 1 , a large dead volume penalty would be incurred, due to the necessary depth of the internal drilling in the plunger. The internal drilling would need to be deeper than the sealing length, to ensure a sufficiently large pressure difference across the plunger wall to cause it to expand and reduce the running clearance and therefore leakage, and would also need to be significantly deeper due to the configuration and location of the outlet port and annulus. The present invention enables the internal drilling of the prior art embodiment of  FIG. 2  to be integrated into a hydraulic pump head such as that illustrated in  FIG. 1 , without incurring a large dead volume penalty. 
         [0055]    The depth of the recess  112  in the first section  110 , and accordingly the length of the extension section  122  of the second section  120 , could be increased to suit manufacturing requirements, for example a deeper recess  112  would ease manufacture of the internal drilling  116 . 
         [0056]    Furthermore, the form of the extension section  122  of the second section  120 , and that of the recess  112  of the first section  110 , may be different to those illustrated in the Figures. 
         [0057]    Although in the embodiment described above, the pressure reducing feature comprises a radial recess  126 , in alternative embodiments, an alternative pressure reducing feature could be provided, such as axial grooves, or any other feature which reduces the pressure towards the lower end of the plunger assembly  106 , remote from the annulus  142 . 
       REFERENCES 
       [0058]    Prior art 
         [0059]    
       FIG. 1 
     
         [0060]    pump head  2   
         [0061]    pump housing  4   
         [0062]    plunger  6   
         [0063]    bore  8   
         [0064]    turret portion  10   
         [0065]    annulus  42   
         [0066]    inlet valve arrangement  70   
         [0067]    outlet valve arrangement  80   
         [0068]    
       FIG. 2 
     
         [0069]    plunger  206   
         [0070]    bore  208   
         [0071]    central drilling  248   
         [0072]    recess  264   
         [0073]    barrel  266   
         [0074]    full diameter section  268   
         [0075]    Invention 
         [0076]    pump head  102   
         [0077]    pump housing  104   
         [0078]    plunger assembly  106   
         [0079]    bore  108   
         [0080]    plunger first, upper section  110   
         [0081]    plunger first section recess  112   
         [0082]    first section annular wall section  114   
         [0083]    first section central drilling  116   
         [0084]    plunger second, lower section  120   
         [0085]    plunger second section extension section  122   
         [0086]    extension section end  124   
         [0087]    plunger radial recess  126   
         [0088]    interference zone  130   
         [0089]    flexible portion (of first section)  134   
         [0090]    guidance zones  136 ,  138   
         [0091]    clearance  140   
         [0092]    bore annulus/pumping chamber  142   
         [0093]    first section recess base  150   
         [0094]    axial drilling first, upper end  152   
         [0095]    axial drilling second, lower end  154   
         [0096]    plunger first, upper end  160   
         [0097]    plunger second, lower end  162   
         [0098]    inlet valve arrangement  170   
         [0099]    outlet valve arrangement  180   
         [0100]    first section outer wall  190   
         [0101]    minimum clearance zone  192   
         [0102]    first, upper edge of radial recess  196   
         [0103]    longitudinal axis A 
         [0104]    pressure gradient P