Abstract:
A pump for fluid, comprising a pump body defining a bore, a pumping plunger having an end region which is slidably received within the bore, a spill port defined by the pump body in a wall of the bore which port is coverable by the end region of the pumping plunger as the plunger is driven on a pumping stroke, in use, so as to initiate fluid pressurization within a pumping chamber; and a spill feature defined by the pumping plunger being arranged to uncover the spill port during a pumping stroke so as to cause depressurization of the pumping chamber. The pumping plunger further defines a speed sensitive feature configured to permit a controlled leak of fluid from the pumping chamber to the spill port towards the end of a pumping stoke. The invention also encompasses a pumping plunger particularly suited for use in the above described pump.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a fluid pump. More specifically the invention relates to a ‘unit pump’ type fuel injection pump for use in a compression ignition internal combustion engine in which a pumping plunger reciprocates within a plunger bore in order to pressurize fuel within a pumping chamber. In addition, the invention relates to a pumping plunger for use within such a pump. 
       BACKGROUND ART 
       [0002]      FIG. 1  shows a sectional view of a known fuel injection pump, indicated generally as  2 , which is suitable for use as a means of supplying pressurized fuel to a fuel injector of an internal combustion engine. Such a pump is generally known in the art as a mechanical ‘unit pump’. The fuel pump  2  includes a generally tubular pump housing  4  having an axially disposed bore  6  within which a pumping plunger  8  is slidable. The plunger  8  has a lower end  10  (in the orientation shown in  FIG. 1 ) that is coupled to a lower spring plate  12 . Although not shown in  FIG. 1 , in practice the lower spring plate  12  may couple to a tappet member against which a driving means (also not shown) acts, a cam for example, to transmit reciprocating motion to the plunger  8 . 
         [0003]    A biasing means in the form of a helical spring  17  is received over the plunger  8  such that the spring  17  is disposed between the pump housing  4  and the lower spring plate  12 . An upper end  18  of the spring  17  abuts a spring plate  20  that is attached to a lower end of the pump housing  4 , and a lower end  22  of the spring  17  abuts the lower spring plate  12 , the spring  17  thus serving to bias the plunger  8  downwards in the orientation shown. 
         [0004]    An upper end of the pump housing  4  defines a cup-shaped recess  26  into which a lower end of an outlet valve arrangement  28  is received. The lower end of the outlet valve arrangement  28  closes off the plunger bore  6  and defines a pumping chamber  30  between it and the upper end of the plunger  8 . The other end of the outlet valve arrangement  28  remote from the recess  26  defines an outlet connection  31 . 
         [0005]    The inner surface of the bore  6  in the region of the pumping chamber  30  defines two opposed ports: a fill port  32  and a spill port  33 . The fill port  32  is shown to the left in  FIG. 1  and is connectable to a fuel supply circuit such that fuel is permitted to fill the pumping chamber  30  through the fill port  32 . The spill port  33  is shown on the right in  FIG. 1  and is connectable to a drain connection such that high pressure fuel from the pumping chamber  30  can escape to low pressure through the spill port  33 . 
         [0006]    In use, the plunger  8  is driven on a pumping stroke during which fuel within the chamber  30  is pressurized. When the pressure of fuel within the chamber  30  reaches a predetermined pressure, the outlet valve arrangement  28  opens to permit pressurized fuel to flow through the outlet connection  31 . Although not shown in  FIG. 1 , a fuel conduit may be attached to the outlet connection  31  to convey fuel to a fuel injector, for example. 
         [0007]    In order to terminate fuel delivery, the plunger  8  is provided with a spill groove  41 . As the plunger  8  is driven further on its pumping stroke, a point will be reached at which the spill groove  41  overlaps the spill port  33  so that pressurized fuel can escape from the pumping chamber  30  along the spill groove  41  to the spill port  33  which terminates pressurization and, thus, fuel delivery from the pump  2 . 
         [0008]    Following termination of pumping, the plunger  8  is driven further such that it passes a top dead centre position and thus commences a return stroke under the force of the spring  17 . During the return stroke, fuel is permitted to fill the chamber  30  through the fill port  32  which is connected to a source of fuel at a relatively low pressure. 
         [0009]    In order to vary the delivery volume of the pump  2 , the plunger  8  is provided with a control arm  40  which extends radially away from the approximate mid point of the plunger  8 . Angular movement of the control arm  40  varies the angular position of the plunger  8 . 
         [0010]    In use, the control arm  40  engages a fuel delivery rack (not shown) via a control pin  42  that depends downwardly from a radially outer end of the control arm  40 . The position of the fuel delivery rack is determined by the engine governor and the rack, in turn, acts on the control arm  40  to cause radial movement of the plunger  8  about its longitudinal axis. The radial position of the plunger  8  determines the point of the pumping stoke that the spill groove  41  registers with the low pressure spill port  33 , thus terminating fuel pressurization earlier, or later, in the pumping stroke depending on the direction and extent of angular movement of the plunger  8 . The variation of the effective stroke between the upper surface of the plunger  8  and the spill groove  41  varies the fuel delivery to the associated engine. 
         [0011]    In circumstances where mechanical unit pumps are used in low emission engines, for example in relatively small cubic-capacity road vehicles, some unit pumps are fitted with constant-pressure unloading delivery valves. 
         [0012]    The general configuration and function of such delivery valves is known in the art and an example of which is shown in  FIG. 1  as the outlet valve arrangement  28 . Their purpose is to trap fuel in a fuel passageway downstream of the outlet connection  31  at a predetermined pressure at the end of the pumping stroke. By way of explanation, when pressure in the pumping chamber  30  collapses at the end of a pumping stroke, fuel pressure in the fuel passageway is at an injectable pressure level. It would be undesirable for the pressure of fuel to remain at this level since there would be a risk of secondary injection events. Conversely, it would be undesirable to permit pressure uncontrolled collapse in the fuel passageway. Therefore, the constant pressure unloading valve  28  allows a sufficient amount of fuel to pass back from the high pressure fuel passageway through to the pumping chamber  30 , and thus to the spill port  33 , so as to reduce the pressure in the fuel passageway by a predetermined amount. The controlled pressure reduction suppresses pressure waves emanating from the pump in the direction of the associated injector so as to avoid undesirable secondary injection events. In addition, cavitation is guarded against since fuel pressure downstream of the outlet connection is not permitted to collapse completely and in an uncontrolled manner. 
         [0013]    Although such valves have associated benefits, the characteristic fuel delivery curve shape produced by mechanical unit pumps incorporating the aforementioned outlet valve arrangements usually exhibits considerably higher fuel delivery at rated load when the pumps are running at low speed compared to when the pumps are running at high speed. This characteristic tends to cause excessive smoking from the associated engine which can limit the use of such pumps for low emission engines despite the advantage they provide in eliminating the risk of secondary injections. 
       DISCLOSURE OF THE INVENTION 
       [0014]    It is against this background that the invention provides, in a first aspect, a pump for fluid, comprising a pump body defining a bore, and a pumping plunger having an end region which is slidably received within the bore, a spill port defined by the pump body in a wall of the bore which port is coverable by the end region of the pumping plunger as the plunger is driven on a pumping stroke, in use, so as to initiate fluid pressurization within a pumping chamber, and a spill groove defined by the pumping plunger and being arranged to uncover the spill port during a pumping stroke so as to cause depressurization of the pumping chamber. The pumping plunger further defines a speed sensitive feature configured to permit a controlled leak of fluid from the pumping chamber to the spill port towards the end of a pumping stoke, the leak rate being dependent on the speed of operation of the pump. 
         [0015]    The speed sensitive feature is adapted preferably to be effective during a rated load operating condition of the pump and has the effect of reducing the delivery of the pump at lower pump speeds compared to a pump having plungers without the speed sensitive feature. However, at higher pump speeds delivery remains substantially unaffected. The invention has particular utility in internal combustion engines in which it is important to minimize exhaust emissions, for example small capacity road vehicles. Moreover, the speed sensitive feature is configured to be effective towards the end of a pumping stroke which enables the relationship between fuel delivery and injection timing for high speed operation to be preserved. Although the speed sensitive feature may be effective over a broad range of angular movement of the plunger, it is preferred that it is effective during approximately the last 20 percent of the range of angular movement of the plunger which corresponds approximately to the rated load operating condition of the pump. 
         [0016]    Preferably the spill feature may be a groove extending around a portion of the outer surface of the pumping plunger at an angle to the longitudinal axis of the pumping plunger. Similarly, the speed sensitive feature may also be a groove disposed in the surface of the plunger, and preferably may be in advance of the spill groove. 
         [0017]    Furthermore, the speed sensitive groove may communicate with the spill groove and, preferably, is configured to extend away from the spill groove in a direction substantially perpendicular to the longitudinal axis of the plunger. 
         [0018]    Although the pumping plunger may be manufactured so to have various cross sections, for example oval, elliptical, or square, for convenience it is preferred that the pumping plunger is substantially circular in cross section. As a result of this, the spill groove is substantially helical in shape since it is machined in the surface of the plunger at an angle to its longitudinal axis. 
         [0019]    The precise dimensions of the speed sensitive groove are to a large extent dependent on the size of the pump, the associated plunger and the desired effect on the pump delivery. However, for a plunger that is typically approximately 7 to 10 millimetres in diameter it is preferred that the depth of the speed sensitive groove is approximately 30 to 60 micrometres and, more preferably between 40 and 50 micrometres. It should be appreciated, however, that some pumping plungers may have greater diameters than this in certain applications, large capacity marine engines for example. 
         [0020]    In a second aspect the invention provides a pumping plunger comprising an elongate rod defining an end face and a side surface, and including a spill groove provided in the side surface of the plunger defining an angle with a longitudinal axis of the plunger, and a speed sensitive feature in communication with the spill groove and configured to permit a controlled flow of fluid therethrough, in use, at an end of a pumping stroke of the plunger. 
         [0021]    The plunger is particularly suited to use in a mechanical unit-type fuel injection pump, for example the pump of the first aspect, the spill groove and speed sensitive feature being arranged to communicate with a spill port of the pump during at least a part of the pumping stroke. 
         [0022]    The plunger is preferably circular in cross section, chiefly for convenience of design and manufacture, and to minimize leakage past the plunger, in use. However, it should be appreciated that the inventive concept also encompasses plungers that have other cross sectional shapes, for example elliptical or square. 
         [0023]    Preferably, the spill groove winds around the circumference of a portion of the plunger. In other words, the spill groove is approximately helical in shape and, preferably, subtends a central angle of approximately 30 to 70 degrees. 
         [0024]    The speed sensitive groove may be in direct communication with the spill groove and arranged to extend away from the spill groove to define an angle therewith. In this embodiment it is preferred that the speed sensitive groove defines an angle of approximately 30 to 70 degrees with the spill groove. 
         [0025]    Preferably, the speed sensitive groove extends away from the spill groove in a direction substantially perpendicular to the longitudinal axis of the pumping plunger, although an angle in the range of about 70 to 110 degrees is also applicable. 
         [0026]    It should be appreciated that preferred and/or optional features of the first aspect of the invention may be combined with the second aspect of the invention, and vice versa. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0027]    Reference has already been made to  FIG. 1  which shows a known fuel pump. In order that the invention may be more readily understood, reference will now be made, by way of example only, to the accompanying drawings in which: 
           [0028]      FIG. 2  is a sectional view of a fuel pump in accordance with a first embodiment of the invention; 
           [0029]      FIG. 3  is a perspective view of a pumping plunger of the fuel pump in  FIG. 2 ; 
           [0030]      FIG. 4  is a side view of the pumping plunger in  FIG. 3 ; and 
           [0031]      FIG. 5  is a graph of delivery against pump speed comparing the delivery characteristics, at rated load, for the fuel pumps in  FIG. 1  and  FIG. 2 . 
       
    
    
     DETAILED DESCRIPTION 
       [0032]    Referring to  FIG. 2 , a mechanical fuel injection pump  100  (hereinafter ‘fuel pump’) is shown that is similar to the fuel pump  2  described above with reference to  FIG. 1 . 
         [0033]    The fuel pump  100  includes a block-like pump body, or housing  102  having an axially disposed bore  104  within which a pumping plunger  108  is slidable. The plunger  108  has a lower end  110  (in the orientation shown in  FIG. 2 ) that is coupled to a lower spring plate  112  for transmitting reciprocating motion to the plunger  108 . Although not shown in the diagram, a tappet and cam member (not shown) may be arranged with respect to the lower spring plate  112  such that rotation of the cam member causes reciprocation of the plunger  108 . This form of plunger drive arrangement is known in the art. 
         [0034]    A biasing means in the form of a helical spring  114  is received over the plunger  108  such that the spring  114  is disposed between the pump housing  102  and the lower spring plate  112 . An upper end  116  of the spring  114  abuts a spring plate  120  attached to a lower end of the pump housing  102  and a lower end  122  of the spring  114  abuts the lower spring  112 , the spring  114  thus serving to bias the plunger  108  downwards in the orientation shown. 
         [0035]    An upper end of the pump housing  102  defines a cup-shaped recess  130  into which an outlet valve arrangement  132  is received. A lower end  133  of the outlet valve arrangement  130  closes off the plunger bore  104  and defines a pumping chamber  134  between it and an upper end face  140  of the plunger  108 . The outlet valve arrangement  132  is of the type referred to in the art as a ‘constant pressure unloading valve’ and the general configuration thereof is known in the art. 
         [0036]    The inner surface of the bore  104  in the region of the pumping chamber  134  defines two opposed ports: a fill port  142  and a spill port  154 . The fill port is shown to the left in  FIG. 2  and is connectable to a fuel supply circuit (not shown) such that fuel is permitted to fill the pumping chamber  134  through the fill port  142 . The spill port  154  is shown on the right in  FIG. 2  and is connectable to a drain connection (not shown) such that high pressure fuel from the pumping chamber  134  can escape to low pressure through the spill port  154 . 
         [0037]    In use, the plunger  108  is driven from its bottom dead centre position on a pumping stroke during which pressurization of fuel within the pumping chamber  134  is started when the upper end face  140  of the plunger  108  travels past both the fill port  142  and the spill port  154  so the ports are covered by an end region of the plunger  108 . 
         [0038]    When the pressure of fuel within the pumping chamber  134  reaches a predetermined pressure, the outlet valve arrangement  132  opens to permit pressurized fuel to flow through an associated outlet connection  135 . Although not shown in  FIG. 1 , a fuel conduit may be attached to the outlet connection  135  to convey fuel to a fuel injector, for example. 
         [0039]    In order to terminate fuel delivery, the plunger  108  is provided with a groove formation, shown generally as  156 . As the plunger  108  is driven further on its pumping stroke a point will be reached at which the groove formation  156  overlaps the spill port  154  so that pressurized fuel can escape from the pumping chamber  134  along the groove formation  156  to the spill port  154 , which terminates pressurization and, thus, fuel delivery from the pump  100 . 
         [0040]    Following termination of pumping, the plunger  108  is driven further such that it passes a top dead centre position and thus commences a return stroke under the force of the spring  114 . During the return stroke, the upper end face  140  of the plunger  108  travels past the fill port  142  so the port  142  is uncovered by the plunger  108  and fuel is therefore permitted to fill the pumping chamber  134 . 
         [0041]    In order to vary the delivery volume of the fuel pump  100 , the plunger  108  is provided with a control arm  150  which extends radially away from the approximate mid point of the plunger  108 . Angular movement of the control arm  150  varies the angular position of the plunger  108 . 
         [0042]    In use, the control arm  150  engages a fuel delivery rack. The position of the fuel delivery rack is determined by the engine governor and the rack, in turn, acts on the control arm  150  to cause radial movement of the plunger  108  about its longitudinal axis. The radial position of the plunger  108  determines the point of the pumping stoke that the groove formation  156  registers with the spill port  154 , thus terminating fuel pressurization earlier, or later, in the pumping stroke depending on the direction and extent of rotation of the plunger  108 . The variation of the effective stroke between the upper end face of the plunger  108  and the groove formation  156  varies the fuel delivery by an associated injector to an engine. 
         [0043]    With reference now also to  FIGS. 3 and 4 , the plunger  108  is a generally cylindrical rod formed of a high strength material such as through-hardened alloy steel having a diameter of 8 millimetres, although it should be noted that this is not essential to the invention and such plungers in small automotive applications typically have diameters in the range of 7 to 10 millimetres. The pumping plunger  108  includes a lower end region  160  of substantially uniform diameter (only partially shown in  FIG. 3 ) and a upper end region  162  that bears the groove formation  156  in the form of a plurality of grooves, slots or recesses machined onto the outer surface of the plunger  108 . 
         [0044]    The upper end region  162  of the plunger  108  is provided with an annular recess  164  which divides the upper end region  162  into two approximately equal length portions: the annular recess  164  and a plunger head  166  that has a greater diameter than the recess  164 . In use fuel surrounds the annular recess  164  which serves to pressure balance the plunger  108  within the bore  104  in order to ensure smooth movement. The plunger head  166  has a diameter that is substantially equal to the lower end region  160  of the plunger  108 . 
         [0045]    The plunger head  166  is provided with a longitudinal groove  170  that cuts through and extends away from the upper end face  140  of the plunger  108  and communicates with the annular recess  162 . Hereinafter, the longitudinal groove  170  is referred to as the ‘stop groove’. 
         [0046]    The function of the stop groove  170  is to reduce delivery of the fuel pump substantially to zero when desired. To this end the plunger  108  is moved angularly, or rotated, so that the stop groove  170  is axially aligned with the spill port  154 . Therefore, the spill port  154  is never fully covered and so as the plunger  108  reciprocates within the bore  104  fuel can flow from the pumping chamber  134  along the stop groove  170  and to the spill port  154 . As a result, pressurization of fuel pressure in the pressurization chamber  134  does not begin. 
         [0047]    The plunger head  166  is also provided with a second groove  172  which is cut at an angle to the stop groove  170  so as to extend around the outer surface of the plunger  108  for approximately one quarter of its circumference—the groove  172  is thus approximately helical in shape. As shown in  FIGS. 3 and 4 , the groove  172  is cut into the plunger  108  so as to define an angle with the longitudinal axis of the plunger  108  of approximately 45 degrees. However, it should be appreciated that this need not be the case and that the angle the groove  172  defines depends on the pump application. Typically, however, in such pumps the groove is cut to define angles in the range of about 30 to 70 degrees with the plunger axis. 
         [0048]    The purpose of the groove  172  is to provide a means to vary the volume of pressurized fuel that is expelled or ‘spilled’ from the fuel pump  100  through angular movement of the plunger  108 . Hereafter, the groove  172  will be referred to as the ‘spill groove’. 
         [0049]    Depending on the angular position of the plunger  108 , in use, the upper edge of the spill groove  172  will uncover the spill port  154  earlier or later in the pumping stroke as the case may be, thus varying the point at which pressurized fuel is spilled from the pumping chamber  134 . It should be noted at this point that although the spill groove  172  has been described as extending around approximately a quarter of the circumference of the plunger  108 , this is only exemplary and the spill groove  172  could be configured to extend around more or less of the surface of the plunger  108  depending on the requirements of the pump application. Typically, however, such spill grooves extend around the surface of the associated plunger so as to subtend a central angle in the range of about 30 to 70 degrees. 
         [0050]    One end of the spill groove  172  communicates with the stop groove  170  and its other end communicates with a further groove  174 , hereinafter referred to as the ‘start fueling groove’. The start fueling groove  174  is, in effect, a continuation of the spill groove  172  but extends laterally around the surface of the plunger  108 , substantially perpendicular to the plunger axis. In use, prior to engine start, the plunger  108  is moved angularly such that the start fueling groove  174  is axially aligned with the spill port  154 . It should be appreciated that in this position, the plunger is at one extremity of the range of its angular movement. Due to the relatively large distance between the upper end face  140  of the plunger  108  and the start fueling groove  174 , the fuel pump  100  delivers a relatively large volume of fuel per pumping stroke when the plunger  108  is in this angular position. Such an operational mode is beneficial for engine starting, particularly during cold conditions. 
         [0051]    Approximately two thirds of the way along the spill groove  172 , the plunger  108  is provided with a third groove  176  that extends away from the spill groove  172  along a plane substantially perpendicular to the longitudinal axis of the pumping plunger  108 . 
         [0052]    The third groove  176  is configured as a speed sensitive feature that permits a controlled leak or ‘spill’ of fuel to flow from the pumping chamber  134  to the spill port  154  toward the end of a pumping stroke (preferably in the last 10% to 25% of the effective pumping stroke) and, thus, at the end of an injection period. Hereinafter, the third groove  176  will be referred to as the ‘speed sensitive groove’. 
         [0053]    The dimensions of the speed sensitive groove  176  are selected so that, at lower pump speeds, for example, below about 60 percent of rated speed, a comparatively large volume of fuel is permitted to flow through the speed sensitive groove  176  to the spill port  154 . Conversely, at higher engine speeds, for example between about 60 percent and 100 percent of rated speed, the dimensions of the speed sensitive groove  176  are such that the flow of fuel through it has a negligible effect on the delivery from the fuel pump  100 . Therefore, a greater amount of fuel flows through the speed sensitive groove  176  at relatively low pump speeds compared to when the pump is operating at a relatively high speed. 
         [0054]    The pumping plunger  108  is mounted in the bore  104  so as to be able to move angularly and has a range of angular movement during which the fuel delivery from the pump is caused to be varied. The speed sensitive groove  176  becomes effective during circumstances in which the angular position of the plunger  108  causes the speed sensitive groove  176  to be in axial alignment with the spill port  154 , which is approximately during the last 20 percent of its angular movement and, more preferably, during the last 15 percent of angular movement. Therefore, as the plunger  108  rises from a bottom dead centre position within the bore  104  during a pumping stroke, firstly the upper end face  140  of the plunger  108  will traverse across and eventually occlude the spill port  154 . When the upper end face  140  of the pumping plunger fully covers the spill port  154 , fuel is trapped within the pumping chamber  134  and is therefore pressurized with further upward movement of the plunger  108 . 
         [0055]    Continued upward movement of the plunger  108  (in the orientation shown in  FIGS. 2 ,  3  and  4 ) eventually causes the upper edge of the speed sensitive groove  176  to uncover the spill port  154 , since the groove  176  is in advance of the spill groove  172 , and fuel is permitted to flow at a predetermined rate from the pumping chamber  134  to the spill port  154  through the speed sensitive groove  176 . 
         [0056]    At low engine speeds, the movement of the plunger  108  is relatively slow so the flow area of the speed sensitive groove  176  is sufficient for it to act as the control edge of the spill groove  172  as the restriction to fuel flow is minimal. As a result, at lower engine speeds, at rated load, the volume of fuel delivered by the fuel pump  100  is reduced compared to a fuel pump using a similar plunger without the speed sensitive groove  176 . A skilled person will appreciate that the term ‘rated load’ relates to the maximum load for which the engine is designed to operate safely and continuously. 
         [0057]    Conversely, at higher engine speeds, and at rated load, the flow area of the speed sensitive groove  176  has a restrictive effect on the flow of fuel through it since the groove  176  registers with the spill port  154  for such a short period of time. Due to the viscosity of the fuel, when the speed sensitive groove  176  registers with the spill port  154  fuel cannot escape quickly though the groove  176  to reduce fuel delivery significantly. Instead, pressurization of the fuel within the pumping chamber  134  continues until the control edge defined by the spill groove  172 , having a larger flow area, registers with the spill port  154 . 
         [0058]    The effect of the speed sensitive groove  176  can be seen clearly in  FIG. 5  which is a graph showing the delivery of the fuel pumps  2 ,  100  of  FIGS. 1 and 2 , respectively, on the vertical axis and the speed of the fuel pumps  2 ,  100  on the horizontal axis. It should be appreciated that in  FIG. 5  the delivery of the fuel pumps  2 ,  100  is shown across a range of speeds with the pumps  2 ,  100  having a plunger angular position that provides the correct fuel delivery to an engine for running at a rated load and full speed condition. 
         [0059]    Line A represents the fuel delivery across a range of pump speeds for the pump  2  fitted with a known pumping plunger as described above with reference to  FIG. 1 . It is seen that at lower engine speeds, for example between approximately 400 and 1000 revolutions per minute (rpm), the delivery of the fuel pump  2  is above that obtained at higher engine speeds. This is disadvantageous since the increased delivery results in an increased level of unburned hydrocarbons (generally visible in the form of smoke emissions) being emitted from the engine. 
         [0060]    In contrast, line B represents the fuel pump  100  of the above described embodiment of the invention with a pumping plunger  108  bearing the speed sensitive groove  176 . As is seen, line B exhibits a more regularly shaped delivery curve with a comparatively reduced delivery at lower engine speeds which is a desirable characteristic of the curve in order to reduce smoke emissions from the engine with which the fuel pump  100  is used. More specifically, at an engine speed of approximately 200 rpm the speed sensitive groove reduces the delivery volume by about 50 percent. The percentage reduction in delivery volume reduces in magnitude gradually over the pump speed range between approximately 200 rpm and approximately 1300 rpm at which point the delivery volume is substantially equal for Line A and Line B. 
         [0061]    In order to provide the required characteristics it is preferred that the speed sensitive groove  176  has a depth in the range of 30 to 60 micrometres and a width in the range of 0.3 to 1 millimetres. The inventors have determined that particularly beneficial dimensions for the groove  176  on a plunger having a diameter of 8.5 millimetres are a depth of between 40 to 50 micrometres and a width of 0.5 millimetres. However, it should be appreciated that the stated dimensions are exemplary only and that the precise dimensions are specific to the size and shape of a given pump and the associated pumping plunger. The overriding aim is to dimension to slot so as to achieve an approximate reduction in the delivered fuel volume of between approximately 5 percent and 20 percent when the pumping plunger occupies an angular position of between about 80 and 100 percent of full load position, for engine speeds up to approximately 70 percent of rated speed. 
         [0062]    The length of the speed sensitive groove  176  is dependent on the other features that are machined into the surface of the plunger  108  and it is important that the length of the groove  176  is selected so that it does not interfere with the correct functioning of the other features. For example, on  FIGS. 3 and 4  it can be seen that the end of the groove  176  does not overlap with the start fueling groove  174 . 
         [0063]    It should be appreciated that various modifications and/or improvements may be made to the above described embodiment(s) without departing from the broad concept of the invention, as defined by the claims. 
         [0064]    For example, although the speed sensitive groove  176  is described above and shown in  FIG. 3 and 4  as extending away from the spill groove  172  in a direction perpendicular to the longitudinal axis of the plunger  108 , it should appreciated that this need not be the case and that, instead, the speed sensitive groove  176  could be arranged to form a range of angles with the plunger axis, for example in order fine tune the fuel delivery rate. In this respect, it is envisaged that the speed sensitive groove  176  could extend away from the spill groove  172  so as to define an angle between approximately 75 and 110 degrees with the longitudinal axis of the plunger  108 . 
         [0065]    Further, although the speed sensitive groove  176  has been described to have a specific length and depth, it should be appreciated that the above specified dimensions are not essential and, as such, may be modified in order to attain the desired effect on the torque curve of the engine in which the pump is installed. 
         [0066]    For purposes of manufacturing efficiency, it is preferred that the speed sensitive groove  176  is manufactured as a straight slot cut into the surface of the plunger  108 . However, such a feature could also be provided by other means. For example, instead of a slot, the functionality of the speed sensitive groove  176  could be provided by a shallow flat or a reduced diameter portion of the plunger at a longitudinal plunger position that corresponds to the end region of a pumping stroke. 
         [0067]    Also, it should be appreciated that the stop groove  170  is an optional feature of the plunger  108  and is not essential to the invention. For example, the spill groove  172  could be configured so that a part thereof extends so as to cut through the end face  140  of the plunger  108 .