Abstract:
A fuel injection system for an internal combustion engine comprises a plurality of pumps arranged to supply respective flows of pressurized fuel to a common accumulator volume that supplies the pressurized fuel in turn to a plurality of fuel injectors. An engine control unit controls the flow rate of pressurized fuel into the accumulator volume in response to engine load. The flow rate of pressurized fuel from at least one pump of the plurality is dependent upon engine speed; whereas at least one other pump of the plurality comprises a fuel output control responsive to the engine control unit enabling the flow rate of pressurized fuel from that pump to be varied independently of engine speed. In this way, the engine control unit controls the aggregate flow rate of pressurized fuel from the pumps into the accumulator volume, while controlling only one of the pumps. This reduces the cost of control apparatus and allows greater freedom of pump selection and flow circuit design.

Description:
TECHNICAL FIELD 
       [0001]    This invention relates to a fuel injection system for a compression ignition internal combustion engine. In particular, the invention relates to a fuel injection system that includes a plurality of pumps, preferably a plurality of unit pumps. The invention also relates to an engine installation incorporating such a fuel injection system. 
       BACKGROUND OF THE INVENTION 
       [0002]    A known fuel injection system may include a plurality of unit pumps, each delivering fuel at high pressure to a respective, separate high pressure fuel line. Each unit pump typically includes a tappet that is driven by a cam to impart drive to a plunger, thereby causing the plunger to reciprocate, in turn, pressurizing fuel within a pumping chamber of the unit. Each unit pump is arranged to supply fuel to an injection nozzle of a respective dedicated injector so as to facilitate delivery of fuel to an associated cylinder of the engine. In such fuel injection systems, it is, therefore, necessary to provide each engine cylinder with a set of separate pump components, each consisting of a cam, a tappet, a unit pump, a high pressure line and an injector, wherein the cams for each set of pump components, typically, are carried on a common camshaft. 
         [0003]    The cam of each unit pump is suitably mounted upon and driven by a camshaft that also carries the cams that control engine valve timing. In that case, the unit pumps are spaced in line along the axis of the camshaft, with a drive end of each unit pump co-operating with a lobe or lobes of its associated cam and the injection nozzle end of each unit pump being arranged to deliver fuel to the associated engine cylinder. Typically, the camshaft has at least three lobes associated with each engine cylinder; one for driving the associated pumping plunger and the other two for controlling engine valve timing. The camshaft extends through the crankcase of the engine, which is provided with pockets or bores for accommodating the unit pumps. The unit pumps are all therefore effectively housed within a common engine housing. For the purpose of this specification, any reference to the camshaft “carrying” a cam is intended to include carrying or mounting a separate cam upon the camshaft, or integrally forming the cam with the camshaft. 
         [0004]    Fuel injection pumps are known wherein a plurality of pumping elements or plungers are incorporated within a unitary housing. Such arrangements are commonly referred to as ‘in-line’ pump arrangements, as the pumping elements are mounted in a line parallel to the axis of a camshaft that drives the plungers. Such systems require a set of tappets and a set of pumping plungers, one tappet and one plunger for each engine cylinder, with each tappet and its associated plunger being arranged within the associated unitary housing. As in unit pump arrangements, each pumping element has an associated pumping chamber that is connected to its associated injector through a separate high pressure fuel line. As a separate pumping element is provided for each engine cylinder, again, the costs of such systems are relatively high. 
         [0005]    Common rail fuel injection systems are also known and typically include a common rail fuel pump having a plurality of pumping plungers driven by a common eccentric cam surface. The cam surface is rotatable by means of a drive shaft, and such pumps may include three or more plungers radially spaced around the drive shaft. The cam surface of the pump co-operates with all of the plungers to cause phased, cyclical movement of the plungers and, hence, pressurization of fuel within their associated pumping chambers. That pressurized fuel is fed to a common rail accumulator volume that in turn supplies fuel to all of the injectors of the system. Whilst common rail systems such as this avoid the need for one pumping element per engine cylinder, such radial pump arrangements are incompatible with existing in-line cam drive arrangements such as that described previously and hence a totally different engine layout is required to accommodate the system. 
         [0006]    The machining and assembly line facilities for the manufacture of engine installations having unit pump fuel injection are well established, and engine installations that can accommodate unit pump fuel injection systems are widely used. It is therefore desirable to permit continued use of such existing production facilities and engine installations. However, it is also desirable to avoid or at least to mitigate several disadvantages associated with fuel injection systems having a plurality of unit pumps. 
       SUMMARY OF THE INVENTION 
       [0007]    In EP 1336752, for example, the Applicant recognized that systems comprising one unit pump per fuel injector suffer from a high part count and therefore high cost. To solve this problem while retaining the basic unit pump engine architecture, EP 1336752 proposed a fuel injection system comprising two or more unit pumps and a greater plurality of fuel injectors. Typically for engines with four to six cylinders, two or three unit pumps may be used whereas engines with six or eight cylinders may use three or four unit pumps, for example. Pressurized fuel from the pumping chambers of the unit pumps is fed directly to an accumulator volume, such as a common rail, through respective high pressure fuel lines; the accumulator volume in turn supplies pressurized fuel to all of the injectors of the system. 
         [0008]    Previously, unit pumps were only known in fuel injection systems wherein they supply fuel directly to a dedicated fuel injector. In contrast, the unit pumps in EP 1336752 deliver fuel to the injectors indirectly, with each unit pump delivering fuel through its associated high-pressure fuel line to a separate, intermediate fuel volume (in the form of the common rail) from where fuel is delivered to the injectors. 
         [0009]    The fuel injection system of EP 1336752 can be incorporated readily into existing engine installations that were originally intended for use with separate unit fuel injection pumps delivering fuel to dedicated fuel injectors, while preserving the existing engine layout. In particular, there is no need to modify the existing pump mounting, camshaft location or cam drives. Production costs associated with re-tooling an engine production line can therefore be reduced or avoided. 
         [0010]    Moreover the unit pumps of EP 1336752 have inlet metering arranged to control the rate of flow of fuel into the pumping chamber, thereby to control the quantity of fuel to be pressurized within the pumping chamber during a pumping cycle. This improves efficiency as only the quantity of fuel that is required for an injection event is pumped during a pumping cycle of each of the unit pumps. In previous fuel injection systems associated with this type of engine installation, an excess quantity of fuel is pumped on each pumping stroke, with the excess being spilled to low pressure before delivery to the injectors. The fuel injection system of EP 1336752 improves efficiency because the quantity of fuel pumped during each pumping cycle is controlled by the inlet metering valve. 
         [0011]    FIG. 6 of EP 1336752 discloses an arrangement of two unit pumps supplied through a common inlet metering system, with one of the unit pumps having an inlet metering valve and the other of the unit pumps being supplied with fuel from the inlet metering valve of the first unit pump. This is more expensive than simpler systems using uncontrolled unit pumps and requires careful matching of the performance of the two pumps and flow circuit design to achieve optimum output characteristics, especially as only one control means is used. 
         [0012]    The present invention seeks to solve these problems of prior injection systems by reducing the cost of control apparatus and by allowing greater freedom of pump selection and flow circuit design. 
         [0013]    The invention resides in a fuel injection system for an internal combustion engine, the system comprising:
       a plurality of pumps arranged to supply respective flows of pressurized fuel to a common accumulator volume that supplies the pressurized fuel in turn to a plurality of fuel injectors;   an engine control unit for controlling the flow rate of pressurized fuel into the accumulator volume in response to engine load; and   an engine load data input for inputting engine load data to the engine control unit;   wherein the flow rate of pressurized fuel from at least one first pump of the plurality is dependent upon engine speed; and   at least one second pump of the plurality comprises a fuel output control responsive to the engine control unit enabling the flow rate of pressurized fuel from that pump to be varied over a range of settings at a given engine speed, whereby the engine control unit controls the aggregate flow rate of pressurized fuel from the pumps into the accumulator volume.       
 
         [0019]    In the system of the invention, the first pump preferably has substantially constant delivery at a given engine speed and the second pump has variable delivery over a range of settings at that engine speed. Thus, control of just the second pump is sufficient to control the aggregate flow rate of pressurized fuel into the accumulator volume: the first pump needs no control system. 
         [0020]    The fuel output control may comprise an inlet metering valve arranged to control the rate of flow of fuel into the second pump. Alternative arrangements may comprise a solenoid-controlled spill valve acting on the second pump, or a mechanical control that alters the effective stroke of a plunger of the second pump. Such a mechanical control may comprise a rack that acts on the plunger of the second pump and that may be actuated by a stepper motor. 
         [0021]    The pumps may be unit pumps that may have a cam-driven tappet drive arrangement or a shoe and roller drive arrangement, for example. It is also possible for the pumps to be pumping units of a rotary drive pump wherein the pumping units of the rotary drive pump may be disposed within a common housing. 
         [0022]    The accumulator volume is suitably a common rail, and the number of fuel injectors is preferably greater than the number of unit pumps. 
         [0023]    The invention may also be expressed as a fuel injection system for an internal combustion engine, the system comprising:
       a plurality of pumps arranged to supply respective flows of pressurized fuel to a common accumulator volume that supplies the pressurized fuel in turn to a plurality of fuel injectors;   an engine control unit for controlling the flow rate of pressurized fuel into the accumulator volume in response to engine load; and   an engine load data input for inputting engine load data to the engine control unit;   wherein at least one pump of the plurality has an uncontrolled fuel output; and   at least one other pump of the plurality comprises a fuel output control responsive to the engine control unit to control the aggregate flow rate of pressurized fuel from the pumps into the accumulator volume wherein the flow rate of pressurized fuel from the at least one other pump is capable of being varied over a range of settings at a given engine speed.       
 
         [0029]    The inventive concept extends to a method of operating a fuel injection system of an internal combustion engine, the method comprising:
       driving a plurality of pumps to supply respective flows of pressurized fuel to a common accumulator volume; and   controlling the flow rate of pressurized fuel from at least one, but less than all, of the plurality of pumps in response to engine load to control the aggregate flow rate of pressurized fuel from the pumps into the accumulator volume wherein the flow rate of pressurized fuel from at least one of the plurality of pumps is varied over a range of settings at a given engine speed.       
 
         [0032]    The flow rate of pressurized fuel from at least one of the plurality of pumps may be dependent on engine speed, and the flow rate of pressurized fuel from at least one other of the plurality of pumps may be varied independently of engine speed, such that the flow rate of pressurized fuel may be varied over a range of settings at a given engine speed. 
         [0033]    The inventive concept also embraces an engine fitted with the fuel injection system of the invention or capable of operating in accordance with the method of the invention. 
         [0034]    The invention may therefore be embodied as a common rail fuel system with two different unit pumps as the high pressure supply source. The first pump has substantially constant delivery and does not require a control system; the second pump has variable delivery achieved by inlet metering or other means. The first pump may be configured to give a fuel delivery rate sufficient for idling and low load operation; the second pump then need only be activated when higher fuel flow rates are required, such as during medium- and high-load operation or engine starting. The system has the potential for low cost as the need to control two pumps is avoided. 
         [0035]    The second pump with variable delivery can be controlled by limiting the inlet flow to the pumping chamber—known as inlet metering—or by using a solenoid valve to allow excess fuel to flow back from the pumping chamber at low pressure when not required and hence control the effective stroke, as is done in an EUI (electronic unit injector) or an EUP (electronic unit pump). Theoretically mechanical control, as used in a mechanical unit pump, could also be used with rack actuation, for example, by a stepper motor. 
         [0036]    The system is especially suitable for use in engines for cost-sensitive markets where the resulting higher level of pressure fluctuation and drive torque can be accepted. The system gives better efficiency than one wherein completely uncontrolled pumps are used and wherein surplus high pressure fuel is discharged from the system by, for example, a high pressure discharge valve on the rail. 
         [0037]    The principle of the invention can also be used in a rotary drive pump containing two or more pumping units within a common housing. 
         [0038]    The invention resides in a fuel injection system for an internal combustion engine, the system comprising:
       a plurality of pumps arranged to supply respective flows of pressurized fuel to a common accumulator volume that supplies the pressurized fuel in turn to a plurality of fuel injectors;   an engine control unit for controlling the flow rate of pressurized fuel into the accumulator volume in response to engine load; and   an engine load data input for inputting engine load data to the engine control unit;   wherein the flow rate of pressurized fuel from at least one first pump of the plurality is dependent upon engine speed; and   at least one second pump of the plurality comprises a fuel output control responsive to the engine control unit enabling the flow rate of pressurized fuel from that pump to be varied independently of engine speed, whereby the engine control unit controls the aggregate flow rate of pressurized fuel from the pumps into the accumulator volume.       
 
         [0044]    The invention may also be expressed as a fuel injection system for an internal combustion engine, the system comprising:
       a plurality of pumps arranged to supply respective flows of pressurized fuel to a common accumulator volume that supplies the pressurized fuel in turn to a plurality of fuel injectors;   an engine control unit for controlling the flow rate of pressurized fuel into the accumulator volume in response to engine load; and   an engine load data input for inputting engine load data to the engine control unit;   wherein at least one pump of the plurality has an uncontrolled fuel output; and   at least one other pump of the plurality comprises a fuel output control responsive to the engine control unit to control the aggregate flow rate of pressurized fuel from the pumps into the accumulator volume.       
 
         [0050]    The inventive concept extends to a method of operating a fuel injection system of an internal combustion engine, the method comprising:
       driving a plurality of pumps to supply respective flows of pressurized fuel to a common accumulator volume; and   controlling the flow rate of pressurized fuel from at least one, but less than all, of the plurality of pumps in response to engine load to control the aggregate flow rate of pressurized fuel from the pumps into the accumulator volume.       
 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0053]    In order that the invention may be readily understood, reference will now be made, by way of example only, to the accompanying drawings, in which: 
           [0054]      FIG. 1  is a schematic diagram of a fuel injection system in accordance with one embodiment of the present invention, comprising two unit pumps; 
           [0055]      FIG. 2  is a partial sectional view of one of the unit pumps of the fuel injection system in  FIG. 1 ; 
           [0056]      FIG. 3  is a sectional view of an inlet metering valve associated with the unit pump shown in  FIG. 2 ; 
           [0057]      FIG. 4  is a block diagram of the fuel injection system of  FIG. 1 , showing how the system is controlled by an ECU taking engine load data input from an engine load sensor; and 
           [0058]      FIGS. 5 and 6  are schematic diagrams of fuel injection systems that illustrate alternative embodiments of the present invention having different unit pump control means. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0059]    Referring firstly to  FIG. 1 , a common rail fuel injection system  8  for an internal combustion engine receives fuel drawn from a low pressure reservoir through a filter by a low pressure pump. Those components are well known and entirely routine in the art and so are omitted from the drawings for clarity. That fuel is supplied through respective first supply lines  10  to the inlets of first and second high-pressure unit pumps, referred to generally as  12  and  14  respectively. The first unit pump  12  has no control apparatus. The second unit pump  14  has control apparatus that, in this embodiment, comprises an inlet metering control valve shown schematically at  50 . Inlet metering limits the inlet flow to the pumping chamber and thereby controls the output of the second unit pump  14  to suit the varying load on the engine as will be described. 
         [0060]    Each unit pump  12 ,  14  pressurizes a quantity of fuel to a substantially higher pressure than the output of the low-pressure pump, and delivers that high-pressure fuel though respective second supply lines  20  to an accumulator volume in the form of a common rail  22 . Thus, each unit pump  12 ,  14  has a pump outlet that is spaced from a respective inlet to the accumulator volume or common rail. 
         [0061]    In conventional manner, the common rail  22  includes a pressure sensor  16 , a pressure relief valve  18  and a plurality of high-pressure fuel lines  24  that extend from and are spaced along the rail  22 . Each high-pressure fuel line  24  is arranged to supply fuel to a respective injector  26  of the fuel system, from which fuel is delivered to an associated engine cylinder or other combustion space. Six high-pressure fuel lines  24  and injectors  26  in the embodiment shown mean that the system of  FIG. 1  is suitable for a six-cylinder compression ignition engine. 
         [0062]    The injectors of the fuel system are therefore spaced apart from the unit pumps. 
         [0063]    The common rail  22  shown in  FIG. 1  is of axially-extending tubular configuration but the rail may alternatively be of generally spherical configuration, that is of the type having a central hub, from which radially-extending delivery flow paths extend to the injectors. 
         [0064]    The injectors  26  may be of any conventional type, the design and operation of which will be well known to those familiar with the art. For example, the injector may be of an electromagnetically- or piezoelectrically-actuable type, may be of the direct actuation type or may be of the type including a hydraulic amplifier arrangement for controlling injector valve needle movement. 
         [0065]    Whilst not shown in  FIG. 1 , the fuel injection system  8  may be incorporated within an engine installation that includes an engine housing, typically the engine crankcase. The engine housing may have a plurality of pockets that each receive a respective one of the unit pumps  12 ,  14 . For example, the engine housing may define an axially-extending opening, through a camshaft extends, in use, with the pockets being arranged to extend radially from the opening. The opening may be defined in an integral or unitary engine housing or, alternatively, may be defined by adjacently mounted engine housing parts. 
         [0066]    Two or more high-pressure unit pumps  12 ,  14  are provided in the system  8  but for clarity and simplicity only the second unit pump  14  will now be described in detail with reference to  FIG. 2 . The inlet metering control valve  50  of the second unit pump  14  will be described in detail thereafter with reference to  FIG. 3 . The description of the second unit pump  14  with reference to  FIG. 2  will also suffice to explain the operation of the first unit pump  12 , which operates in much the same way as the second unit pump  14 , but which omits its inlet metering control valve  50 . 
         [0067]    Referring to  FIG. 2 , it can be seen that the unit pump  14  includes a single pumping plunger  30  that is slideable within a plunger bore  32  provided in a pump housing  34  to pressurize fuel within a pumping chamber  36 . The pumping plunger  30  is driven, in use, by a drive arrangement referred to generally as  38 , including a generally cylindrical tappet member  40 , a roller member  42  and a cam carried by a drive shaft. 
         [0068]    The drive shaft is not shown in  FIG. 2  but is visible schematically in  FIG. 1 : in practice a single camshaft can drive both pumps  12  and  14  via respective cam lobes spaced along the camshaft to align with the pumps  12  and  14 . The camshaft may be of the type used in engine installations as described previously, that is, installations originally intended to include separate unit fuel injection pumps that each deliver fuel to a dedicated injector. In such existing engine installations, the camshaft carries a plurality of lobes or cam forms, each intended to drive a plunger of a respective one of the unit fuel injection pumps. 
         [0069]    In the system  8  of  FIG. 1 , the existing cam drive arrangement is used in a different manner, but nonetheless the requirement to redesign the engine installation can be substantially avoided. Specifically the unit pumps  12  and  14  are arranged in a line substantially parallel to the axis of the camshaft, and are accommodated within a common engine housing provided with a plurality of pockets or bores, each of the unit pumps  12 ,  14  being mounted within a respective one of the pockets or bores. Typically the engine housing may take the form of the engine crankcase, which is provided with an axially-extending opening, through which the camshaft extends. The pockets for receiving the unit pumps extend radially from this opening, and thus define the locations for the unit pumps within the installation. As the unit pumps  12 ,  14  of the fuel injection system  8  do not supply fuel directly to just one injector, the operating principle of the system contrasts to that of systems that pre-date EP 1336752. However by making the fuel injection system  8  compatible with those previous engine installations, the need to re-design existing engine installations and tooling equipment is advantageously avoided. 
         [0070]    As seen in  FIG. 2 , the roller  42  is arranged to co-operate with a surface  46  of the cam such that, as the drive shaft rotates, the cam is driven and the roller  42  is caused to ride over the cam surface  46 . The roller  42  and the tappet  40  are reciprocable within a guide bore  44  provided in an engine housing  39  that is secured to the pump housing  34 . An internal surface of the tappet  40  is provided with an annular groove, within which an abutment plate  47  for a return spring  48  is mounted. The return spring  48  is arranged to urge the tappet and roller arrangement  40 ,  42  outwardly from the guide bore  44  (downward in the orientation shown in  FIG. 2 ) into engagement with the cam surface and, hence, serves to allow the pumping plunger  30  to be urged outwardly from the plunger bore  32  to perform a return stroke of a pumping cycle, as described in further detail below. The tappet  40  and pumping plunger  30  are arranged such that they are able to move axially relative to one another. Thus, as the tappet  40  is urged inwardly within the guide bore  44  upon rotation of the cam surface, a point will be reached in its range of travel, at which it moves into engagement with the pumping plunger  30  to urge the pumping plunger inwardly within the plunger bore  32 . 
         [0071]    An efficiency advantage is achieved by virtue of an inlet metering valve arrangement, referred to generally as  50 , that is provided on the second unit pump  14 . The inlet metering valve arrangement  50  is located at the end of the pumping plunger  30  remote from the tappet  40 , and is located within a separate valve housing  52  secured to a face of the pump housing  34 . The inlet metering valve  50  is in communication with a pump inlet  54  that communicates with the first supply line  10  in  FIG. 1 , such that a supply of low-pressure fuel is delivered to the inlet metering valve  50  from a low pressure pump. The inlet metering valve  50  is arranged to control the rate of flow of fuel delivered to the pumping chamber  36  of the second unit pump  14  through an inlet check valve, referred to generally as  56 , under the control of an Engine Control Unit or ECU  74  shown in the system block diagram of  FIG. 4 . 
         [0072]    Whilst the inlet metering valve  50  shown here includes a valve housing that is adapted to be mounted to the unit pump housing, the inlet metering valve arrangement may instead be housed in a common housing with the pumping plunger and other components of the unit pump. The inlet metering valve arrangement may be of the type that is controlled by electrical, and preferably electronic, means. 
         [0073]    The inlet metering valve  50  may typically be of the type shown in further detail in  FIG. 3  wherein a metering valve member  75  is movable under the influence of an electromagnetic actuator, referred to generally as  77 , to control the extent of opening of an orifice or restriction  79  in a flow path between the pump inlet  54  and the inlet check valve  56 , thereby to vary the rate of flow of fuel through the orifice  79  to the pumping chamber  36 . The metering valve member  75  is movable between a closed position, in which communication between the pump inlet  54  and the inlet check valve  56  through the orifice  79  is closed, and a fully open position, in which a maximum rate of flow of fuel through the orifice  79  is permitted. Movement of the metering valve member  75  is effected by energizing and de-energizing a winding  81  of the actuator  77  under the control of the ECU  74 . Further details of the operation of a metering valve of the type shown in  FIG. 3  will be familiar to those skilled in the art of engine fuel system design. 
         [0074]    Returning to  FIG. 2 , the inlet check valve  56  of the second unit pump  14  includes a valve abutment member  60  defining a valve seat  62 , with which a check valve member  58  is engageable to control the metered flow of fuel from the inlet metering valve  50  to the pumping chamber  36 . The valve abutment member  60  is provided with axially and radially extending passages that communicate with one another such that, when the check valve member  58  is caused to lift from the valve seat  62 , fuel delivered to the pump inlet  54  and passing through the inlet metering valve  50  is able to flow into the radially extending passage in the valve abutment member  60 , into the axially extending passage and past the valve seat  62  into the pumping chamber  36 . Although not shown in  FIG. 2 , in practice it may be desirable to provide the inlet check valve  56  with a relatively low spring pre-load to urge the check valve member  58  into a position, in which it engages the valve seat  62 . 
         [0075]    Whilst the flow into the pumping chamber  36  is controlled by means of the inlet metering valve  50  and the inlet check valve  56 , the flow of fuel out of the pumping chamber  36  is controlled by means of an outlet delivery valve arrangement, referred to generally as  64 . The outlet valve arrangement  64  takes the form of a ball valve having a ball  66  that is engageable with a further valve seat  68  to control fuel flow between the pumping chamber  36  and a high pressure supply line  70  forming part of or being in communication with the supply line  20 . The outlet valve arrangement  64  may be provided with an outlet valve spring (not shown) having a relatively low pre-load that serves to urge the ball  66  into engagement with the further valve seat  68 . 
         [0076]    The high pressure flow line  70  is defined by a passage provided in an insert member  72  located, in part, within a further bore  73  provided within the pump housing  34  and partially extending from the pump housing  34 . The high pressure flow line  70  is substantially coaxially aligned with the pumping plunger  30  and is arranged to communicate, at its end remote from the pump housing  34 , with an end of the second supply line  20  to the common rail  22 . Thus, in use, high pressure fuel delivered from the pumping chamber  36  to the high pressure flow line  70  is able to flow into the second supply line  20 , and into the common rail  22 , for delivery to the injectors  26 . 
         [0077]    In use, as the drive shaft is rotated and the roller  42  rides over the cam surface, the tappet  40  is caused to reciprocate within the guide bore  44 , thereby imparting axial movement to the pumping plunger  30  as the tappet  40  is moved into engagement with, and moves with, the pumping plunger  30 . A pumping cycle consists of two phases: a filling phase and a pumping phase. During the filling phase, the inlet check valve  56  is open to permit fuel delivery from the inlet metering valve  50  to the pumping chamber  36 , and the outlet valve arrangement  64  is held closed by means of high pressure fuel within the high pressure flow line  70  to the common rail. During the filling phase, the pumping plunger  30  is urged outwardly from the plunger bore  32  to perform a return stroke due to the pressure exerted on the plunger  30  by the flow of fuel from the inlet metering valve  50 , through the inlet check valve  56  and into the pumping chamber  36 . 
         [0078]    During a subsequent pumping phase of the pumping cycle, the inlet check valve  56  is caused to close due to increasing fuel pressure within the pumping chamber  36  as the plunger  30  starts to move inwardly under the drive of the tappet  40 , to prevent further flow of fuel into the pumping chamber  36  from the inlet metering valve  50 . Additionally, as fuel pressure within the pumping chamber  36  increases further, the outlet valve arrangement  64  is caused to open to permit pressurized fuel within the pumping chamber  36  to flow into the high pressure flow line  70 . During the pumping phase the pumping plunger  30  is urged inwardly within the plunger bore  32 , under the influence of the tappet  40  co-operating with the roller  42  and the driven cam surface, to cause fuel pressurization within the pumping chamber  36 . 
         [0079]    The sequence of events during a pumping cycle will now be described in further detail. At the start of the pumping cycle, the pumping plunger  30  adopts its innermost position within the plunger bore  32  (i.e. uppermost position in the orientation in  FIG. 2 ) and fuel pressure within the pumping chamber  36  is high due to the pressurization caused by the previous pumping stroke. The outlet valve arrangement  64  is closed due to the equalization of fuel pressures in the pumping chamber  36  and the high pressure flow line  70 . The tappet  40  is also at its innermost position in the guide bore  44 , and high fuel pressure within the pumping chamber  36  serves to urge the pumping plunger  30  into contact with the tappet  40 . 
         [0080]    Upon commencement of its return stroke, the plunger member  30  is initially allowed to retract from the plunger bore  32  due to decompression within the pumping chamber  36  and retraction of the tappet  40  under the force of the return spring  48  as the roller  42  rides over the cam surface. As the pumping chamber  36  is decompressed, a point will be reached, at which the pressure in the pumping chamber  36  falls below the pressure required to lift the check valve member  58  from the valve seat  62  due to the flow of fuel from the inlet metering valve  50 , and the next filling phase commences. 
         [0081]    Further movement of the pumping plunger  30  outwardly from the plunger bore  32  is effected by a force due to pressure within the pumping chamber  36  caused by the flow of fuel from the inlet metering valve  50 , through the radially and axially extending passages in the valve abutment member  60  and though the inlet check valve  56  into the pumping chamber  36 . Further retraction of the tappet  40  from the guide bore  44  (i.e. outward movement of the tappet  40  from the bore  44 ) occurs under the force of the return spring  48 , causing the roller  42  to ride over the cam surface. 
         [0082]    During the filling phase, the ball  66  of the outlet valve arrangement  64  remains seated against the further valve seating  68  due to high pressure fuel within the high pressure flow line  70  and due to the force of the outlet valve spring. 
         [0083]    After the tappet  40  reaches its outermost position within the guide bore  44 , the roller  42  is urged in an upward direction (in the illustration shown in  FIG. 2 ) as it follows the cam surface, and a point will be reached, at which the tappet  40  moves into engagement with the plunger member  30 , thereby causing the pumping plunger  30  to be driven inwardly within the plunger bore  32 . As the pumping plunger  30  is driven inwardly within the plunger bore  32 , fuel within the pumping chamber  36  is pressurized. 
         [0084]    As fuel pressure within the pumping chamber  36  starts to increase, a point will be reached part way through the pumping stroke, at which point, the check valve member  58  of the inlet check valve  56  is urged against its seating, due to increasing fuel pressure within the pumping chamber  36 , to prevent further flow of fuel into the pumping chamber  36  and return flow from the pumping chamber  36  towards the inlet metering valve  50 . 
         [0085]    As the plunger pumping stroke continues, fuel within the pumping chamber  36  is pressurized to a sufficiently high level to cause the ball  66  to lift from the further valve seating  68 , thereby permitting pressurized fuel to flow from the pumping chamber  36  into the high pressure flow line  70  and, hence, to the supply line  20  to the common rail  22 . At the end of the pumping stroke, when the pumping plunger  30  reaches the end of its range of travel, the ball  66  will be urged against the further valve seating  68  due to high pressure fuel within the high pressure flow line  70  and the force of the outlet valve spring, thereby holding high fuel pressure within the high pressure flow line  70 , the second supply line  20  and, hence, within the common rail  22 . 
         [0086]    The extent of plunger movement during the pumping stroke will be determined by the quantity of fuel delivered to the pumping chamber  36  during a filling phase, as this determines the extent to which the pumping plunger  30  is retracted from the plunger bore  32  during the return stroke. The quantity of fuel delivered to the pumping chamber  36  during the filling phase therefore determines the point in the range of travel of the tappet  40 , at which it engages the pumping plunger  30  to commence the plunger pumping stroke. 
         [0087]    The quantity of fuel delivered to the pumping chamber  36  during one pumping cycle is therefore determined by the rate of flow of fuel through the inlet metering valve  50 , and the time for which the inlet check valve  56  is held open to permit fuel flow into the pumping chamber  36 . The time, for which the inlet check valve  56  is held open, is determined by: (i) the spring rate of the inlet valve spring (if provided); (ii) the hydraulic force acting on the check valve member  58  as fuel is pressurized within the pumping chamber  36 ; (iii) and the speed of the associated engine, which determines the rate of movement of the tappet  40 . The quantity of fuel delivered to the pumping chamber  36  can therefore be varied by adjusting the inlet metering valve setting to vary the fuel flow rate through the inlet check valve  56 . 
         [0088]    With reference the system block diagram of  FIG. 4 , the inlet metering valve  50  of the second unit pump  14  is operable by means of the ECU  74  between a fully open state, corresponding to maximum filling and a maximum pumping plunger stroke, and a fully closed state corresponding to zero filling and zero pumping plunger stroke, and has a range of settings between its fully open and closed states to vary the extent of filling of the pumping chamber  36  and, hence, the quantity of fuel delivered by the second unit pump  14  to the common rail  22  during any given pumping cycle. 
         [0089]    So, in this embodiment of the invention, low pressure fuel delivered to the inlet check valve  56  is regulated by means of the inlet metering valve  50  to control the quantity of fuel pumped within the pumping chamber  36  of the second unit pump  14  during a pumping cycle. The provision of the inlet metering valve  50  provides the advantage that only the quantity of fuel required for an injection event is pumped during a pumping cycle. This provides improved mechanical efficiency over pump designs wherein an excess quantity of fuel is pumped on each pumping stroke, with the excess being spilled to a drain port prior to delivery to the injectors. 
         [0090]    Although the flow rate of fuel required for an injection event may be greater than can be provided by a single unit pump  12 , fuel injection demand is satisfied throughout the engine load range because two or more unit pumps  12 ,  14  are used and can work in parallel when necessary. Specifically, by using the ECU  74  to control the inlet metering valve  50  in response to engine load data provided to the ECU  74  by, for example, a load sensor  76  as shown in  FIG. 4 , the second unit pump  14  can be activated when higher fuel flow rates are required, such as during medium- and high-load operation or during engine starting. Conversely, when the engine is idling or in other low-load operation, the second unit pump  14  can be, in effect, shut down; the first unit pump  12  is configured such that its fuel delivery rate alone is sufficient for those less demanding operating conditions. The ECU  74  can also respond to the pressure sensor  16  on the common rail  22  to control the second unit pump  14  to adjust the fuel pressure in the common rail  22  as necessary. 
         [0091]    The first unit pump  12  runs constantly as the engine is running, albeit at a speed that varies with engine speed, and its delivery is not controlled by the ECU  74  or otherwise. The fuel injection system of the invention therefore the potential for low cost as the need to control two pumps is avoided. The system is especially suitable for use in engines for cost-sensitive markets where a higher level of fuel pressure fluctuation and hence engine torque output can be accepted. The system gives better efficiency than a system that includes completely uncontrolled pumps wherein surplus high pressure fuel is simply discharged from the system by, for example, a high pressure discharge valve on the common rail. 
         [0092]    The invention has the advantage that it allows the use of two pumps where adequate capacity cannot be obtained with one pump without the necessity to balance the two pumps and their associated plumbing to give proper operation with a single inlet metering valve. This may be useful where the engine construction is such that a single inlet metering valve cannot be conveniently mounted in a way that feeds the two pumps equally. 
         [0093]    At low load, when only the first unit pump  12  is working, the working stroke of that single working pump will be greater than if two pumps were together pumping the same flow rate of fuel. This will give rise to less plunger leakage than would be the case for two pumps working with lesser strokes, hence improving pumping efficiency. 
         [0094]    Whilst variable delivery of the second unit pump  14  can be achieved by inlet metering as described above, it can also be achieved by other means. For example, the variable delivery of the second unit pump  14  can be controlled by using a solenoid valve  78  as shown in the system  80  of  FIG. 5 . This allows excess fuel to flow back from the pumping chamber at low pressure when not required and hence controls the effective stroke, as is done by the solenoid-controlled spill valve used in Delphi&#39;s currently-marketed EUI (electronic unit injector) and EUP (electronic unit pump) arrangements. 
         [0095]    Theoretically mechanical control, as used in a mechanical unit pump, could also be used with rack actuation, for example, by a stepper motor  82  and rack  84  as shown in the system  86  of  FIG. 6  to alter the effective stroke of the plunger of the second unit pump  14 . 
         [0096]    In the preferred embodiments shown, the fuel injection system of the invention includes a number of fuel injectors that is greater than the number of unit pumps. For example, if there are four engine cylinders, and hence four fuel injectors, there may only be two or three unit pumps. In that case, an existing camshaft of the engine, which was designed for use with four unit pumps (and hence four fuel injection system cams), will have at least one redundant cam. In general, therefore, the camshaft may be formed with or may carry a plurality of cams, at least one of which does not have an associated unit pump and, therefore, is redundant. 
         [0097]    It will be appreciated that although the fuel injection system of the present invention is shown to include unit pumps having a tappet drive arrangement that co-operates with its associated cam, other drive arrangements are also possible, for example shoe and roller arrangements. Also, the principle of this invention can be used in a rotary drive pump containing two or more pumping units within a common housing.