Common rail fuel pump

A common rail fuel pump for an internal combustion engine includes a pumping plunger (10) that is reciprocable within a plunger bore (14) provided in a pump housing (16) under the influence of a drive arrangement (18, 20) to cause fuel pressurisation within a pump chamber (12). An inlet metering valve (46) is operable to meter the quantity of fuel supplied to the pump chamber (12) during the return stroke of the plunger (10). An outlet valve (58) controls the supply of pressurised fuel from the pumping chamber (12) through an outlet passage (30) to the common rail fuel volume in circumstances in which the inlet metering valve is closed. The outlet passage (30) has a pump outlet (38), from where fuel is supplied to the common rail fuel volume. The inlet metering valve (46), the plunger (10) and the pump outlet (38) are arranged in axial alignment, providing a compact and lightweight pump for installation in existing and purpose-built engines.

The present invention relates to common rail fuel pump suitable for use, in particular, in a fuel injection system of a compression ignition internal combustion engine. The invention also relates to a common rail fuel supply system for supplying fuel to a plurality of injectors of the engine.

In known common rail fuel injection systems for diesel engines, it is common to provide a single high pressure pump for supplying fuel at an injectable pressure level to a plurality of associated injectors. The high pressure fuel pump supplies pressurised fuel to an accumulator volume or common rail, which is arranged to supply fuel to all of the injectors of the system. Typically, each injector is provided with an electronically controlled nozzle control valve to control movement of a fuel injector valve needle and, thus, to control the timing of delivery of fuel from the injector. The high pressure pump is commonly of radial pump design and requires a “rotary” drive. Radial fuel pumps also occupy a relatively large accommodation space.

Other types of diesel fuel injection system are known in which a plurality of unit pumps are provided, each of which delivers fuel at high pressure to a separate high pressure fuel line and, from here, to a dedicated injector. Each unit pump typically includes a tappet that is driven by means of a cam to impart drive to a plunger, thereby causing the plunger to reciprocate and resulting in pressurisation of fuel within a pumping chamber of the unit. In such systems it is necessary to provide each engine cylinder with a set of separate pump components, the set consisting of a cam, a tappet, a unit pump, a high pressure line and an injector, with the cams for each set of pump components being formed on a common drive shaft.

The unit pumps are arranged “in a line” along the axis of the cam shaft, with a drive end of each unit pump co-operating with a lobe 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 cam shaft has three lobes associated with each engine cylinder; one for driving the associated pumping plunger and the other two for controlling engine valve timing.

It has been recognised that unit pump injection systems of the aforementioned type have their disadvantages. For example, each unit pump typically functions by pressurising a substantially fixed volume of fuel during a pumping cycle, and then spilling pressurised fuel that is not required for an injection event to low pressure. This introduces system inefficiency. Additionally, the system has a high part count, and therefore is of relatively high cost, particularly as it requires one unit pump to be provided for each fuel injector.

Despite the drawbacks of unit pump injection systems, the machining and assembly line facilities for the manufacture of engine installations of this type are well established, and engine installations designed to accommodate this type of system are widely used.

The problem addressed by the present invention is to provide a common rail fuel pump which avoids or obviates the aforementioned disadvantages, whilst permitting continued use of production line facilities and engine installations that are already in existence.

According to a first aspect of the present invention there is provided a common rail fuel pump for supplying fuel to a common rail fuel volume of an internal combustion engine, the fuel pump comprising a pumping plunger that is reciprocable within a plunger bore provided in a pump housing under the influence of a cam drive arrangement to cause fuel pressurisation within a pump chamber, wherein the drive arrangement includes a cam driven drive member coupled to the plunger to impart drive thereto, in use, so that the plunger performs a pumping cycle including a pumping stroke and a return stroke. An inlet metering valve is operable to control the quantity of fuel supplied to the pump chamber during the return stroke of the plunger, and an outlet valve controls the supply of pressurised fuel from the pump chamber, through an outlet passage to the common rail fuel volume in circumstances in which the inlet metering valve is closed. The outlet passage communicates with a pump outlet which is substantially co-axially aligned with both the inlet metering valve and the plunger. Closure of the inlet metering valve part way through the plunger return stroke controls the quantity of fuel that is supplied to the pump chamber for pressurisation during a subsequent plunger pumping stroke and, hence, controls the quantity of high pressure fuel that is delivered to the common rail.

The present invention provides a convenient, small and relatively lightweight common rail fuel pump, particularly by virtue of the inlet metering valve itself forming an ‘integral’ part of the pump and being arranged in axial alignment with the plunger and the pump outlet.

The provision of the inlet metering valve provides a facility for inlet metering the quantity of fuel to be pressurised, if desired, and therefore avoids the requirement for a separate inlet metering valve to be provided for the pump assembly. A further benefit of the system is that it is compatible with existing engine installations and production line technology designed for unit pump injection systems, therefore providing cost benefits. The fuel system has particular application in relatively small industrial and agricultural engines, although it may also be used in larger engines.

It will be appreciated that the fuel pump pressurises fuel for supply to the injectors of the fuel injection system with which it is used, but does so via the common rail fuel volume. The pump outlet may be in direct communication with the common rail fuel volume, or optionally the pump outlet may communicate with the common rail fuel volume through additional pipework.

The drive arrangement typically includes a cam for driving the drive member and the plunger. If the fuel pump is intended for use in smaller engines (for example one, two or three cylinders) a single unit pump of the invention may be sufficient, with several lobes being provided on one cam if necessary. For larger engine applications (for example four, five or six cylinders), it may be necessary to provide a plurality of such unit pumps.

Preferably, the inlet metering valve of the pump includes an elongate, and preferably generally cylindrical, inlet valve member that is co-axially aligned with the plunger.

The outlet valve is preferably arranged within an outlet passage and the inlet metering valve is preferably housed within a valve housing which is received within a recess or opening provided in an end region of the pump housing so that respective drillings provided in the valve housing and the pump housing align to define, at least in part, the outlet passage.

Preferably, the outlet valve of the pump is a hydraulically operable non-return valve located within the outlet passage.

The fuel pump of the present invention is particularly versatile and has several optional modes of operation. In particular, the inlet metering valve may be operable in one of several ways so as to control the quantity of fuel that is pressurised within the pump chamber for delivery to the rail.

The inlet metering valve may be further operable so as to allow the pump chamber to be filled through the open valve during the plunger return stroke, with the inlet metering valve being maintained open for an initial period of the pumping stroke so that some of the fuel within the pump chamber is dispelled back to low pressure. The inlet metering valve is closed when it is required to initiate pressurisation of fuel within the pump chamber and is preferably opened again prior to a final period of the pumping stroke (i.e. prior to top-dead-centre). This method is advantageous as only a short period of valve actuation is required part way through the pumping stroke, providing an efficiency benefit and accurate control of the timing of fuel delivery to the rail. By opening the inlet metering valve again prior to the final period of the pumping stroke, Hertz stresses on the cam of the drive arrangement are also reduced.

The plunger bore of the pump assembly may also be provided with a filling port that is co-operable with the plunger to provide a filling function for the pump chamber, whereby when the plunger covers the filling port fuel is unable to flow into the pump chamber through the filling port and when the plunger uncovers the filling port fuel is able to flow into the pump chamber through the filling port.

The filling port may be defined at one end of a filling passage provided in the pump housing, wherein said filling passage communicates with a low pressure fuel reservoir.

The provision of the filling port and the filling passage provides a supplementary filling means for the pump chamber. This may be particularly advantageous if the supply pump for the system provides a supply pressure that is too low for filling through the inlet metering valve.

It will be appreciated that the common rail fuel pump may be manufactured and sold independently of the common rail fuel volume and other parts of the common rail fuel injection system or, alternatively, may be manufactured and sold as part of a complete fuel system.

Alternatively, therefore, according to a second aspect of the invention, there is provided a common rail fuel supply system for use in an internal combustion engine, the system including a common rail fuel volume for supplying fuel to a plurality of injectors of the engine, and a plurality of common rail fuel pumps of the first aspect of the invention, each pump being arranged to supply fuel through a respective pump outlet to the common rail fuel volume.

The common rail fuel pump of this second aspect of the invention may have any of the preferred and/or optional features of the first aspect of the invention.

According to a third aspect of the invention, there is provided a control method for a common rail fuel pump as set out in the accompanying claims, the method including holding the inlet metering valve open during the return stroke to permit fuel to be supplied to the pump chamber from a low pressure source, closing the inlet metering valve to permit pressurisation of fuel within the pump chamber during the subsequent pumping stroke, and opening the inlet metering valve prior to a final period of the pumping stroke so as to terminate pressurisation of fuel within the pump chamber and to ensure Hertz stresses on the cam are minimised.

In one embodiment, the invention includes closing the inlet metering valve after an initial period of the pumping stroke so as to dispel a proportion of fuel that is supplied to the pump chamber during the return stroke back to low pressure, so as to control the quantity of fuel which is pressurised within the pump chamber during the pumping cycle.

Referring toFIG. 1, a common rail fuel pump assembly, or unit fuel pump, referred to generally as8, includes a pumping plunger10which is driven, in use, to pressurise fuel within a pump chamber12defined at the end of a plunger bore14provided in a main or unit pump housing16. The unit pump housing16includes an upper region16aof enlarged diameter compared to a lower reduced diameter region16b.The plunger10is movable within the bore14under the influence of a cam drive arrangement, including an engine driven cam (not shown), which is mounted upon or forms part of an engine driven shaft and co-operates with a roller and a tappet arrangement,18,20respectively. The plunger bore14is provided with a groove15to enlarge its diameter part way along its axial length. The groove15communicates with a drain passage17so as to permit leakage fuel from the pump chamber12through the plunger bore14to escape to low pressure.

A roller18of the drive arrangement co-operates with the surface of the cam as it rotates, in use. A lower end of the plunger10(in the orientation shown) projects from the plunger bore14and is coupled at its end to the tappet20through a spring plate22. The plate22defines an abutment surface for one end of a plunger return spring24, the other end of which engages with a step in the outer surface of the pump housing16between the enlarged16aand reduced16bdiameter regions. At its lower end, the plunger10extends through, and is coaxial with, the return spring24. The return spring24acts to provide a return spring force to the plunger10to effect a plunger return stroke, as will be described in further detail below, and is located within a return spring chamber25. The return spring chamber25is vented to low pressure.

As the roller18rides over the cam surface it co-operates with the tappet20so as to impart a drive force to the tappet20and, hence, to the plunger10. Tappet motion is guided by means of a guide bore26provided in an outer pump housing or sleeve28which is secured, at its upper end, to the unit pump housing16. In an alternative embodiment (not shown) the sleeve may be removed, and instead the guide bore26may be provided directly within the engine block of the associated engine.

The pump chamber12communicates with one end of a first drilling provided in the upper region16aof the unit pump housing16. The first drilling defines a part of an outlet passage30, or delivery passage, of the unit pump8through which high pressure fuel is supplied to a downstream common rail fuel volume of the fuel system. The common rail is not shown inFIG. 1, but it will be appreciated that it may take the form of any accumulator volume for receiving high pressure fuel and for supplying fuel to a plurality of injectors of the fuel system. For example, the common rail may be of the linear rail type, in which the accumulator volume takes the form of an elongate pipe, or may be of radial type, in which the accumulator volume has a central hub delivering fuel to a plurality of supply passages, each for supplying fuel to a different one of the injectors.

The outlet passage30of the pump assembly8is also defined by drillings provided in a valve housing32, an insert34and a pump outlet housing36, respectively. The pump outlet housing36is provided with a pump outlet38in communication with the common rail. The pump outlet38may communicate with the rail directly, or optionally through additional pipework, but with no further means for pumping fuel between the pump outlet38and the rail.

The pump outlet housing36is of generally U-shaped cross section, defining a downwardly extending annular wall and an internal end surface43. The annular wall of the outlet housing36extends into a recess40provided at the upper end16aof the unit pump housing16. The recess40and the internal surface of the annular wall together define an internal chamber or housing space42within which the valve housing32and the insert34are received so that the valve housing32abuts the unit pump housing16, at its uppermost end, and the insert34separates the valve housing32from the internal end surface43.

The valve housing32forms part of an inlet metering valve arrangement, the housing32being provided with a valve bore44within which an elongate and generally cylindrical inlet valve member46is movable under the influence of an electromagnetic actuator arrangement. The electromagnetic actuator arrangement includes a winding48and an armature50that is coupled to the valve member46. The armature50is provided with a through drilling51through which a part of the valve housing32extends. The part of the valve housing32which extends through the drilling51defines a portion of the outlet passage30for high pressure fuel. The valve housing32is mounted relative to the unit pump housing16so that the inlet valve member46is generally axially aligned with the plunger10. It is a particular feature of the invention that the pump outlet38is aligned along a common axis with both the inlet valve member46and the plunger10, so that all three components are generally co-axially aligned.

The inlet valve takes the form of a single seat, two position valve that is operable to control communication between the outlet from the pump chamber12(via the outlet passage30) and a low pressure passage52defined within the valve housing32. The passage52communicates with the housing space42which vents to low pressure.

Whether or not the outlet passage30communicates with the low pressure passage52is determined by the position of the valve member46, which is movable between a first open state in which it is spaced from a valve seat (not identified) and a second closed state in which it seats against the valve seat. The inlet valve member46is biased towards its open state by means of a valve spring54. In order to close the valve member46, the winding48is energised so as to attract the armature50(i.e. movement of the armature in a downward direction in the illustration shown), thereby causing the valve member46to move against the spring force into engagement with the valve seat. If the winding48is de-energised, the valve spring54serves to urge the valve member46away from the valve seat and, hence, the valve member46is opened.

Mounted upon one side of the unit pump housing16is an electrical connector arrangement56for providing a current to the winding48to control energisation and de-energisation thereof to open and close the inlet metering valve46, as required. A controller (not shown) for the pump8is arranged to provide the necessary control signals, via the connector56, to operate the valve46.

The region of the outlet passage30within the pump outlet housing36is provided with an outlet valve in the form of a hydraulically operable non-return valve58having a light non-return valve spring60. The provision of the non-return valve58ensures high pressure fuel remains trapped within the common rail and cannot return to the outlet passage30. Should fuel pressure within the outlet passage30exceed an amount that is sufficient to overcome fuel pressure in the rail (acting in combination with the spring force), the non return valve58is caused to open to permit high pressure fuel delivery through the pump outlet38and, hence, to the common rail.

The fuel pump8shown inFIG. 1has several modes of operation. In each mode, as the cam is driven to rotate the roller18is caused to ride or roll over the cam surface, thereby imparting a drive force to the tappet20, and hence to the plunger10, resulting in reciprocating motion of these parts10,20. The plunger10performs a pumping cycle during which it is driven inwardly within its bore14to perform a pumping stroke and urged outwardly from its bore14, under the force of the return spring24, to perform a return stroke.

One mode of operation of the pump assembly ofFIG. 1will now be described. The winding48of the actuator is in a de-energised state at the start of the return stroke, so that the inlet valve member46is spaced away from the valve seat under the force of the valve spring54. With the inlet metering valve open, continued movement of the plunger10through the return stroke causes fuel to be drawn into the pump chamber12, filling the chamber12ready for the subsequent pumping stroke. Part way through the plunger return stroke, when the valve member46would otherwise be biased away from the valve seat due to the force of the spring54, the winding48of the actuator is energised to cause the valve member46to seat. Closing the inlet metering valve part way through the return stroke provides a means for metering the quantity of fuel that is supplied to the pump chamber12and, thus, a means for metering the quantity of fuel that is pressurised during a subsequent pumping cycle; further movement of the plunger through the return stroke with the inlet metering valve46closed prevents any further fuel to be drawn into the pump chamber12. The pump chamber12is therefore only filled for that period of the return stroke for which the inlet metering valve is open.

Throughout the plunger return stroke it will be appreciated that the non return valve58is held closed as the force due to high fuel pressure within the rail, acting in combination with the spring60, overcomes the force due to fuel pressure within the outlet passage30(in practice the non return valve spring force is relatively low and provides a much less significant force than rail20pressure).

Once the plunger10has reached bottom-dead-centre and commences the subsequent pumping stroke, the inlet metering valve is maintained closed and fuel pressure within the pump chamber12starts to increase. During the initial part of the pumping stroke the non-return valve58will remain closed due to the pressure differential across it and the non return valve spring force holding it closed. At some point during the pumping stroke, fuel pressure within the pump chamber12will increase to a pressure level that is sufficient to cause the non-return valve58to open against the force of rail pressure (and the non-return valve spring60). Pressurised fuel within the pump chamber12is therefore able to flow through the outlet passage30, through the pump outlet38and into the common rail. The common rail communicates with the injectors of the fuel system, so as to permit fuel that is pressurised within the pump chamber12and supplied to the rail to be delivered to the injectors for injection. It will be appreciated that the quantity of fuel delivered through the pump outlet38to the rail during a pumping cycle is determined by that quantity of fuel supplied to the pump chamber12through the open inlet metering valve during the previous return stroke.

Once the plunger10has reached top-dead-centre and commences the subsequent return stroke, the winding48is de-energised to open the inlet metering valve once again, allowing the pump chamber12to re-fill during the return stroke, but only during an initial period of the return stroke, before closing the inlet valve again to ensure only the desired quantity of fuel is delivered to the pump chamber12for subsequent pressurisation. The inlet metering valve is preferably opened at or just after top-dead-centre.

The sequence of events may be continued, as described previously, for the subsequent pumping cycles.

The inlet metering valve46of the pump assembly is further operable to allow an alternative mode of pump operation, if desired, in which its metering function is modified. The pump controller may optionally control the inlet metering valve during the pumping stroke. In a second mode of operation, therefore, at the start of the return stroke the winding48is de-energised so that the inlet valve member46is spaced away from the valve seat under the force of the inlet valve spring54. With the inlet metering valve46open, continued movement of the plunger10through the return stroke causes fuel to be drawn into the pump chamber12, filling the chamber12ready for the subsequent pumping stroke. At bottom-dead-centre, the plunger is at its outermost position within the bore14. The pump chamber12is filled with fuel at relatively low pressure and the winding48of the actuator is de-energised so that the inlet metering valve46is in its open state in which it is spaced from its valve seat. As described previously, during the plunger return stroke the non return valve58is held closed as the force due to high pressure fuel within the rail, acting in combination with the spring60, overcomes the force due to fuel pressure within the outlet passage30.

For an initial period of the pumping stroke (i.e. with the plunger10moving between bottom-dead-centre and top-dead-centre) the inlet metering valve46is maintained in its open state so that some of the fuel that has been supplied to the pump chamber12is dispelled back through the open inlet valve to low pressure. At this stage of the pumping stroke the non-return valve58will remain closed due to the pressure differential across it and the non return valve spring force holding it closed.

Following this initial period of the pumping stroke (i.e. to a point part way through the pumping stroke on the accelerating part of the cam) the winding48of the actuator is energised to move the inlet valve member46into engagement with the valve seat. Communication between the outlet passage30and the low pressure passage52is then broken. With the inlet metering valve46closed, the pumping plunger10continuing through the pumping stroke and the volume of the pump chamber12reducing, fuel pressure within the pump chamber12increases until a pressure level is reached that is sufficient to cause the non-return valve58to open against the force of rail pressure (and the non-return valve spring60). Pressurised fuel within the pump chamber12is therefore able to flow through the outlet passage30, through the pump outlet38and into the common rail. The common rail communicates with the injectors of the fuel system so as to permit fuel that is pressurised within the pump chamber12and supplied to the rail to be delivered to the injectors for injection.

Prior to the final period of the pumping stroke, and so before the plunger10reaches top-dead-centre, the inlet metering valve46is opened by de-energising the winding48. When the inlet metering valve46is opened fuel pressure within the pump chamber12starts to reduce as communication is established between the outlet passage30and the low pressure passage52. A point will be reached during the remainder of the plunger pumping stroke when the non return valve58is caused to close under the force of rail pressure and the non return valve spring60, thus terminating the supply of fuel through the pump outlet38to the common rail. The inlet metering valve46is maintained in its open state during the subsequent plunger return stroke, to allow filling of the pump chamber12through the open valve46, as described previously.

In summary, therefore, during this second mode of operation the inlet valve46is operable to control the quantity of fuel supplied to the rail by controlling how much low pressure fuel is displaced back through the open valve prior to commencement of pumping. With this in mind, it should be noted that reference to the ‘plunger pumping stroke’ is defined as the stroke of the plunger between bottom-dead-centre and top-dead-centre, and not just that period of the pumping cycle for which fuel pressurisation occurs.

It has been recognised that by using this mode of operation, with the inlet metering valve46being opened prior to the final period of the pumping stroke, an advantage is achieved in that Hertz stresses on the cam are minimised. This arises because the pump8is only in a “pumping mode” (i.e. when fuel pressure within the pump chamber12is increasing) during periods of the pumping cycle for which the roller18is co-operating with regions of the cam form having a large contact radius. In the aforementioned operation the inlet metering valve46is actuated to close only for a short period part way through the pumping stroke and, thus, the method provides an accurate means of controlling the timing of fuel delivery to the rail.

The step of opening the inlet metering valve46prior to the final period of the pumping stroke so as to reduce Hertz stresses on the cam may also be implemented, to provide the same advantage, when the valve is operated in its normal mode of metering the quantity of fuel supplied to the pump chamber12during the return stroke.

The net effect of the second mode of operation is the same as the first; control of the quantity of fuel that is pressurised and supplied to the common rail fuel volume during a pumping cycle. This is achievable through the normal metering operation of the inlet metering valve46, in which only the required quantity of fuel is allowed to flow into the pump chamber12through the open valve, or through modified operation of the inlet metering valve46in which the pump chamber12is fully filled and then a proportion of fuel is dispelled back to low pressure during an initial part of the pumping stroke.

In a modification to this second mode of operation, the inlet metering valve46may be closed at an earlier stage of the pumping stroke, just after bottom-dead-centre and earlier on the accelerating part of the cam. Again the inlet metering valve46is opened just before the end of the pumping stroke to provide the aforementioned advantage of reducing Hertz stresses on the cam.

In a still further modification, the inlet metering valve46may be held closed until after the plunger10has passed top-dead-centre and commenced its return stroke. As the plunger10starts the return stroke, moving towards bottom-dead-centre, the pressure of fuel within the pump chamber12starts to fall and a point will be reached during the plunger return stroke at which the non return valve58is urged to close as the force due to fuel pressure within the rail, acting in combination with the spring60, overcomes the force due to fuel pressure within the outlet passage30. When it is required to commence filling of the pump chamber12, ready for the next pumping stroke, the winding48of the actuator is de-energised causing the valve member46to move away from the valve seat under the force of the valve spring54. With the inlet metering valve46open, continued movement of the plunger10through the return stroke causes fuel to be drawn into the pump chamber12ready for the subsequent pumping stroke. As described previously, having reached bottom-dead-centre at the end of the return stroke (i.e. just prior to commencement of the next pumping stroke), the plunger10starts to move inwardly within the bore14causing some of the fuel that has filled the chamber12during the return stroke to be dispelled back to low pressure. The inlet metering valve46is then closed, in this case at a relatively late stage of the pumping stroke, and remains closed until just after top-dead-centre, as mentioned before.

It will be appreciated that in all modes of operation described previously, the inlet valve46controls the quantity of fuel that is pressurised within the pump chamber12during the pumping stroke. This may achieved by operating the inlet metering valve46during the return stroke to allow fuel supply to the pump chamber12during only a part of the return stroke or, optionally, by controlling the inlet metering valve46so as to allow fuel to flow into the pump chamber12throughout the return stroke and then dispelling a portion of fuel from the pump chamber12during an initial period of the subsequent pumping stroke.

The pump assembly is advantageous in that it can be readily incorporated into existing engine installations, for example unit pump type installations, where the available accommodation space is limited. The pump assembly of the system is also relatively compact, particularly due to the inlet valve and its actuator (i.e. the inlet valve member46, the armature50and the winding48) being located co-axially with the plunger10, and being mounted within a housing32adjacent to, and directly on top of, the unit pump housing16for the plunger10and its associated drive components18,20. The fuel system therefore provides size and weight benefits also. Pump efficiency is good as there is no necessity to spill pressurised fuel to low pressure to control the quantity of fuel supplied to the rail; the use of the inlet metering valve46in the manner described avoids this disadvantage.

An alternative embodiment of the pump assembly is shown inFIG. 2. Similar parts to those shown inFIG. 1are identified with like reference numerals and will not be described in further detail. InFIG. 2, the fuel pump8further includes a filling port64for the pump chamber12defined at one end of a filling passage62provided within the unit pump housing16. The filling passage62communicates, at its end remote from the filling port64, with a low pressure fuel supply or reservoir (not shown) so that as the plunger10reciprocates within the plunger bore14co-operation between its outer surface and the filling port64provides a supplementing fuel supply to the pump chamber12by controlling the supply of low pressure fuel through the filling passage62to the pump chamber12.

The filling port64is positioned along the plunger bore axis so as to be uncovered by the plunger10only during an end period of the return stroke, typically over a plunger travel distance of, for example, between 2 and 4 mm. Fuel supply to the pump chamber12through the port64therefore only occurs during the end period of the plunger return stroke.

Filling of the pump chamber12through the inlet metering valve46occurs during the return stroke when the valve member46is unseated and, additionally, through the filling port64when it is uncovered by the plunger10. Supplementary filling of the pump chamber12through the filling port64only occurs, however, if the pump chamber12is not already full at the point in the return stroke when the port64is uncovered, for example if supply pressure is too low to fill the pump chamber12completely through the inlet metering valve46.

In one mode of operation the valve member46is held open for an initial period of the pumping stroke and is only closed after the point at which the filling port64has been closed by the plunger10. During this initial period some of the fuel within the pump chamber12will be dispelled back to low pressure through the open inlet valve, and additionally through the filling passage62(until the port64is closed by the plunger10), as the plunger10continues through the pumping stroke. When it is required to supply pressurised fuel to the common rail, the winding48is energised to cause the inlet valve member46to seat against the inlet valve seat, thus closing the inlet valve. As the filling port64is already closed at this time, closure of the inlet valve causes pressure within the pump chamber12to increase. Subsequently, the non-return valve58will open and, hence, fuel at high pressure is delivered through the pump outlet38to the common rail.

In an alternative mode of operation, the winding48is energised to seat the inlet valve member46at an earlier stage of the pumping cycle, and before the plunger10has closed the port64. In such circumstances it is closure of the port64by the plunger10that causes pressurisation of fuel within the chamber12, subsequent opening of the non-return valve58and, hence, high pressure fuel delivery through the pump outlet38to the common rail.

In both the first and second modes of operation of the fuel pump8inFIG. 2, the winding48is de-energised before the final period of the plunger pumping stroke (i.e. prior to top-dead-centre), causing the non return valve58to close to trap high fuel pressure within the common rail. As mentioned before, the benefit of using this method is that Hertz stresses on the cam are minimised as the plunger10is only pumping during periods for which the roller18is co-operating with regions of the cam form having a large contact radius.

In a third alternative mode of operation of the fuel pump8inFIG. 2, the winding48is energised at a later stage of the pumping stroke, after the filling port64is closed, so as to seat the valve member46. Subsequently, the non-return valve58will open and, hence, fuel at high pressure is delivered through the pump outlet38to the common rail. The valve member46is held in this position for the remainder of the pumping stroke and so that fuel delivery to the rail continues until plunger top-dead-centre. Only after the plunger10has commenced the return stroke is the winding de-energised to unseat the inlet valve member46, thus permitting filling of the pump chamber12through the inlet valve ready for the subsequent pumping stroke. The non return valve58is caused to close to trap fuel pressure in the rail at just about plunger top-dead-centre.

It will be appreciated that the reference in this document to the plunger10, the inlet valve member46and the pump outlet38being generally co-axially aligned is intended to include arrangements where one component may be slightly off-axis, in particular due to manufacturing tolerances, but where nonetheless the plunger10, inlet metering valve46and the pump outlet38are arranged in an approximately linear and compact manner within a single unit pump assembly8.