Patent Publication Number: US-8113175-B2

Title: Fuel injection system

Description:
TECHNICAL FIELD 
     The present invention relates to a fuel injection system for an internal combustion engine. In particular, the invention relates to a fuel injection system including an accumulator volume in the form of a common rail. 
     BACKGROUND OF THE INVENTION 
     In known fuel injector designs, a nozzle control valve is provided to control movement of a fuel injector valve needle relative to a seating and, thus, the delivery of fuel from the injector. A so-called Electronic Unit Injector (EUI) is an example of such an injector. An EUI includes a dedicated pump having a cam-driven plunger for raising fuel pressure within a pump chamber, and an injection nozzle through which fuel is injected into an associated engine cylinder. A metering valve is operable to control the pressure of the fuel within the pump chamber. When the metering valve is in an open position, the pump chamber communicates with a low pressure fuel reservoir so that fuel pressure within the pump chamber is not substantially affected by movement of the plunger and fuel is simply drawn into and displaced from the pump chamber as the plunger reciprocates. Closure of the metering valve causes pressure in the pump chamber to rise as the plunger is driven to reduce the volume of the pump chamber. Each EUI also has an electronically controlled nozzle control valve that is arranged to control the timing of commencement and termination of the injection of fuel into an associated engine cylinder. Typically, the engine is provided with a plurality of EUIs, one for each cylinder of the engine. 
     Although the use of a nozzle control valve in an EUI provides a capability for controlling the injection timing, and such units are capable of achieving high injection pressures, both injection pressure and injection timing are limited to some extent by the nature of the associated cam drive. 
     In common rail fuel injection systems, a single pump is arranged to charge an accumulator volume, or common rail, with high pressure fuel for supply to a plurality of injectors of the fuel system. As in an EUI, the timing of injection is controlled by means of a nozzle control valve associated with each injector. One advantage of the common rail system is that the timing of injection of fuel at high pressure is not dependent upon a cam drive, and so the flexibility of injection timing is good. However, achieving very high injection pressure within a common rail system is problematic and requires a dedicated high pressure pump of significant cost. 
     Recognising that both EUI and common rail systems have certain disadvantages, in granted U.S. Pat. No. 7,047,941 (Delphi Technologies Inc.) the Applicants have previously proposed a hybrid fuel injection system which combines the functionality and benefits of both types of system, whilst avoiding several of the drawbacks of each of them. 
       FIG. 1  shows a hybrid system of the aforementioned type including a common rail fuel pump  10  which supplies fuel at a moderately high and injectable pressure level (e.g. 300 bar) to a common rail  12 . This is referred to as the first pressure level. The common rail  12  supplies pressurised fuel to a first supply passage  14  which communicates with a pump chamber  16  under the control of a rail control valve  18 . The pump chamber  16  forms part of a pump arrangement including a pumping plunger  20  that is driven by means of a driven cam  22 , typically a roller and rocker mechanism. The pump chamber  16  supplies fuel to a dedicated fuel injector  24  which is separated from the pump chamber  16  by a second supply passage  26 . The fuel injector  24  is arranged to inject fuel into the engine when a valve needle  28  of the injector is caused to lift under the control of an injector control valve  30 . 
     The fuel injection system of  FIG. 1  has two key modes of operation. If the rail control valve  18  is closed, movement of the plunger  20  under the influence of the cam  22  causes fuel within the pump chamber  16 , which is initially at a relatively low level, to be increased to a variable, higher pressure level. Fuel at this second, variable higher pressure level is then delivered through the second supply passage  26  to the injector  24  and is delivered to the engine under the control of the injector control valve  30 . If, however, the rail control valve  18  is open, the action of the plunger  20  has no pressurizing effect on fuel within the pump chamber  16  and so fuel is delivered to the injector  24  at the first pressure level. By controlling the status of the rail control valve  18  it is therefore possible to vary the injection pressure between the first and second pressure levels. 
     Whilst the hybrid system in  FIG. 1  provides many advantages over the more conventional systems, it is not compatible with many existing engine installations. Where manufacturers have invested heavily in production line facilities for one type of engine installation, the cost of re-tooling can be prohibitive to manufacturing different types of engine. 
     It is with a view to addressing this problem in particular that the invention provides an improved fuel injection system which is compatible with many existing assembly line facilities. 
     SUMMARY OF THE INVENTION 
     According to the present invention there is provided a fuel injection system comprising a fuel injector which is arranged within a housing unit; an accumulator volume for supplying fuel to the fuel injector; and a fuel pump arrangement comprising a pumping plunger which is movable within the housing unit to cause pressurisation of fuel within a pump chamber. A metering (or spill) valve is operable to control fuel flow into and out of the pump chamber. A fuel passage provides communication between the pump chamber and the accumulator volume. A non-return valve is located in the fuel passage between the pump chamber and the accumulator volume and the injector communicates with the fuel passage downstream of the non-return valve. The communication between the first fuel injector and the accumulator volume is uninterrupted. 
     By “uninterrupted” in the sense of an uninterrupted communication, it is meant that the fluid flow path between the accumulator volume and the fuel injector is free from physical barriers to the movement of fuel. In particular, the passage (or passages) that connect(s) the fuel injector to the accumulator volume does not contain any hydraulic devices, such as valves, membranes or pistons, which may act to control (or prevent) the movement of fuel from the accumulator volume to the fuel injector. In this way, the fuel injector is not prevented or substantially hindered from receiving a substantial flow of fuel, suitable for a main injection event, directly from the accumulator volume. 
     The invention provides the advantage that it is compatible with existing engine installations designed for EUIs as the injector and the pumping plunger are both accommodated within the same housing unit, together with the fuel passage between them. It is therefore possible for engine manufacturers to use existing production line facilities designed for engines with EUI systems without the need to re-tool, whilst at the same time providing an engine to the end user which has the benefits of a common rail system also. For example, injection timing is not dependent on the nature of the cam drive, but can be independent of this due to the presence of the common rail accumulator volume. The timing of injection is therefore much more flexible. 
     It is a further advantage of the invention that as the injector component of the system is in close proximity to the pumping element of the system (i.e. the two are located within the same housing, or are located in immediately adjacent housing parts forming a common housing unit), pressure wave effects, which can otherwise adversely affect performance, are minimised. 
     Moreover, the system does not require a separate and dedicated high pressure pump to supply pressurised fuel to the common rail as the pumping arrangement of the system provides this function itself. 
     It is a further advantage over known hybrid common rail-EUI systems that the pumping chamber and the metering valve are isolated from the high pressure fuel source (the common rail) for most of the pumping stroke, and so high pressure fuel leakage (e.g. through the plunger bore and the metering valve) is reduced. 
     In one embodiment, in use, the fuel injector receives fuel primarily from the accumulator volume and not from the pump chamber. Thus, during a fuel injection event the non-return valve is typically closed, essentially isolating the accumulator volume and fuel injector on one side from the pump chamber on the other. Thus, a further advantage of the invention is that the fuel injection system may suitably comprise more than one injector for each fuel pump; each of the fuel injectors receiving fuel from a high pressure reservoir (or accumulator volume), rather than from a dedicated fuel pump. 
     The injector is preferably an electronically controlled injector and may include a three way control valve for controlling movement of an injector valve needle so as to control fuel injection into the engine. In other versions of the system the injector includes a two way control valve. 
     In a further preferred embodiment, the fuel injection system includes at least one restriction between the non-return valve and the accumulator volume. This provides a degree of variation of the injector pressure with engine speed. By way of example, the restriction may be located approximately at the outlet of the accumulator volume. 
     By way of further example, the system may comprise a supply passage to the injector for receiving fuel from the fuel passage and through which fuel is delivered to the injector, wherein the restriction is located immediately upstream of an interconnection between the supply passage and the fuel passage. 
     In one mode of operation, a control method for a fuel injection system of the invention includes driving the pumping plunger by means of a cam having a cam profile with a rising flank and a falling flank, wherein the rising flank corresponds to a pumping stroke of the plunger pumping cycle during which the pumping plunger is driven to reduce the volume of the pump chamber and the falling flank corresponds to a return stroke of the plunger pumping cycle during which the pumping plunger is retracted from the pump chamber to increase the volume of the pump chamber, operating the metering valve so that it is open during at least a portion of the return stroke so as to allow fuel to flow into the pump chamber for pressurisation and operating the metering valve so that it is closed during at least a portion of the pumping stroke so as to cause pressurisation of fuel within the pump chamber. The metering valve may, advantageously, be held closed at the end of the pumping stroke until the plunger has ridden over the cam nose. This mode of operation provides an advantage in terms of energy conservation and also in terms of audible noise level. 
     In another mode of operation, the control method includes operating the metering valve so that it is closed during at least a portion of the pumping stroke so as to cause pressurisation of fuel within the pump chamber, and re-opening the metering valve at the end of the pumping stroke prior to the plunger riding over the cam nose. It has been found that this mode of operation provides a benefit as it reduces Hertz stresses on some cam profiles. 
     The background to the invention has already been described, by way of example only, with reference to  FIG. 1  which shows a schematic diagram of a known fuel injection system having a common rail, which is supplied with fuel from a high pressure pump, in combination with an additional pumping element. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will now be described, by way of example only, with reference to the following figures in which: 
         FIG. 1  shows a hybrid system that supplies fuel to a common rail; 
         FIG. 2  which shows a schematic diagram of one cylinder of a fuel injection system of a first embodiment of the invention; 
         FIG. 3  shows a schematic diagram of a fuel injection system of a second embodiment of the invention comprising a plurality of fuel injectors and pump arrangements; 
         FIG. 4  is a schematic diagram of a fuel injection system of a third embodiment of the invention comprising a single injector and a single pump arrangement; 
         FIG. 5  is a schematic diagram of a fuel injection system of a fourth embodiment of the invention comprising a plurality of fuel injectors and pump arrangements; 
         FIG. 6  shows an alternative configuration of the fuel injection system of the fourth embodiment of the invention; and 
         FIG. 7  is a schematic diagram of a fuel injection system according to a fifth embodiment of the invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The fuel injection system in  FIG. 2  includes an injector  40  which is supplied with fuel via an injector inlet passage  42 . The injector inlet passage  42  receives fuel from a fuel passage  44  which communicates, at one end  44   a , with an accumulator volume in the form of a common rail  46 . As would be understood by a person skilled in the art, the phrase common rail is not intended to be limiting and is used to describe any volume for storing fuel, whether it is elongate (i.e. a length of pipe), cylindrical, spherical or any other shape. 
     The injector  40  is arranged within (or at least in part defined by) an injector housing unit  38 , indicated generally by the dashed line. The injector  40  is not shown in detail, but typically includes an injector valve needle (not shown) which is movable towards and away from an injector valve seat to control the delivery of fuel from the injector  40  into the associated engine cylinder. To control opening and closing of the injector valve needle, an injector control valve (also not shown) is provided which controls the pressure of fuel supplied to the back of the valve needle (i.e. the end remote from the injector valve seat). The injector control valve may be a two-way valve or a three-way valve, the design and operation of which would be familiar to a person skilled in the art. A description of an injector having a three-way injector control valve can be found, for example, in EP 1359316 (Delphi Technologies Inc.). 
     The fuel injection system further includes a pump arrangement  68  which is arranged within the injector housing  38 , together with the injector  40 . The pump arrangement  68  includes a pumping plunger  48  which is movable within a bore  50  provided in the injector housing  38  so as to cause, in certain circumstances, pressurisation of fuel within a pump chamber  52  formed at one end of the bore  50 . The pumping plunger  48  is driven by means of a drive arrangement (not shown) which includes an engine driven cam having one or more cam lobes. The pumping plunger  48  is typically driven by the cam via a roller and rocker mechanism (not shown) in a known manner. Alternatively, the pumping plunger may be driven by the cam via a guided tappet. 
     The pump chamber  52  communicates with a second end  44   b  of the fuel passage  44  remote from the common rail  46 . A non-return valve  54  is located within the fuel passage  44  so that the common rail  46  communicates with the pump chamber  52  via the non-return valve  54 . The non-return valve  54  includes a ball  56  which is biased against a valve seat  58  by means of a spring  60 . The biasing force of the spring  60  sets an opening pressure for the valve at which the ball  56  is caused to lift from its seat  58  to allow the pump chamber  52  to communicate with the common rail  46 , and ensures the valve remains on its seat when there is no pressure in the system. 
     A pump supply passage  62  branches from the fuel passage  44 , on the pump chamber side of the non-return valve  54 , and allows fuel from a low pressure fuel reservoir  64 , located external to the injector housing  38 , to flow into the pump chamber  52 . In contrast, the injector supply passage  42  branches from the fuel passage  44  on the common rail side of the non-return valve  54  (i.e. on the other side of the non-return valve  54  to the pump supply passage  62 ). The pump supply passage  62  is provided with an electronically controlled valve  66 , also referred to as a “metering valve” (or sometimes a “spill valve”), which is operable between an open state, in which fuel is able to flow into the pump chamber  52  from the low pressure fuel reservoir  64 , and a closed state in which communication between the low pressure fuel reservoir  64  and the pump chamber  52  is broken. 
     As the injector  40  and the pump arrangement form part of a shared housing unit  38 , they can be positioned in an engine in the same way as a conventional EUI with the tip of the injector  40  (referred to as the nozzle) protruding into the associated engine cylinder in a conventional way. In practice, in this embodiment the fuel injection system may include a plurality of fuel injectors, each of which may be provided with its own dedicated pump arrangement  68  in a shared housing, as illustrated in  FIG. 3  in which like reference numerals are used to denote similar parts. Advantageously, the system is provided with only one common rail  46  so that the common rail delivers fuel to each of the injectors  40  of the engine via respective supply passages (such as passage  44 ). 
     Operation of the fuel injection systems of  FIGS. 2 and 3  will now be described in further detail. 
     In use, the pumping plunger  48  is driven by the main engine camshaft  72  to perform a pumping cycle consisting of a return stroke, in which the pumping plunger  48  is withdrawn from the bore  50  to expand the volume of the pump chamber  52 , and a pumping stroke, in which the pumping plunger  48  is driven into the bore  50  so as to reduce the volume of the pump chamber  52 . At the start of the plunger return stroke (i.e. just after the end of the pumping stroke), the pumping plunger  48  is said to be at the top of its stroke and the pump chamber volume is a minimum, and at the end of its return stroke (i.e. just before the start of the pumping stroke) the pumping plunger  48  is said to be at the bottom of its stroke and the pump chamber volume is a maximum. 
     In a first mode of operation, initially the metering valve  66  is open so that as the pumping plunger  48  performs its return stroke fuel is drawn through the pump supply passage  62 , through the open metering valve  66  and into the pump chamber  52 . Once the pumping plunger  48  has completed its return stroke, so that the pump chamber  52  is filled with low pressure fuel, it commences its pumping stroke. At an appropriate point in the pumping stroke, the metering valve  66  is closed to prevent further communication between the low pressure fuel reservoir  64  and the pump chamber  52 . By holding the metering valve  66  open for an initial period of the plunger pumping stroke, a proportion of fuel that is drawn into the pump chamber  52  during the return stroke is dispelled through the open metering valve  66 , back to the low pressure reservoir  64 , before pressurisation within the pump chamber  52  commences. Continued motion of the pumping plunger  48  through its pumping stroke results in fuel within the pump chamber  52  being pressurised to a high level. 
     Once fuel pressure in the pump chamber  52  exceeds that within the common rail  46 , the non-return valve  54  is caused to open, against the spring force, to allow pressurised fuel in the pump chamber  52  to flow to the common rail  46 . The flow of fuel continues until the pump chamber  52  is at its minimum volume, i.e. when the pumping plunger  48  is at the top of its stroke. The pumping plunger  48  rides over the nose of the cam to start its return stroke, and the pressure in the pump chamber  52  gradually starts to reduce. Eventually, part way through the return stroke, the pressure in the common rail  46  will exceed that in the pump chamber  52  and the non-return valve  54  will be caused to close under the force of the biasing spring  60  so that communication between the pump chamber  52  and the common rail  46  is broken. High pressure fuel then remains trapped in the common rail  46 . 
     Once fuel pressure in the pump chamber  52  has reduced to a low level and the non-return valve  54  has closed to trap high pressure fuel in the common rail  46 , the metering valve  66  is opened once more to allow communication between the low pressure fuel reservoir  64  and the pump chamber  52  and, hence, the next filling cycle commences. The metering valve  66  is suitably opened just after the pumping plunger  48  has started its return stroke. By operating in this mode it has been found to provide a benefit in terms of energy conservation and audible noise levels. 
     With the common rail  46  charged with high pressure fuel, the injector  40  can then be operated so as to inject fuel into the engine cylinder. Injection is initiated by operating the injector control valve so as to cause the valve needle of the injector to lift away from the injector valve seat. Fuel in the common rail  46  is delivered through the fuel passage  44  to the injector inlet passage  42 , and hence to the injector, but is unable to return to the pump chamber  52  due to the closed non-return valve  54 . 
     In a second mode of operation, rather than holding the metering valve  66  closed as the plunger rides over the cam nose, the metering valve  66  may be re-opened at the end of the pumping stroke so as to reduce Hertz stresses on the cam. In this mode of operation, the metering valve  66  may be closed relatively early in the pumping stroke, just after bottom-dead-centre and earlier on the accelerating part of the cam. 
     In a third mode of operation, the metering valve  66  may be closed part way through the return stroke of the pumping plunger  48  so as to control the quantity of fuel that is actually drawn into the pump chamber  52  (i.e. part-filling of the pump chamber  52 ) for pressurisation. In this mode of operation the metering valve therefore provides an inlet metering function. 
     A third embodiment of the invention is shown in  FIG. 4 , in which like reference numerals are used to denote similar parts. Here, the fuel passage  44  between the non-return valve  54  and the common rail  46  is provided with first and second orifices or restrictions  70   a ,  70   b . The first orifice  70   a  is located at the outlet of the common rail  46  and the second orifice  70   b  is located just upstream of the point at which the fuel passage  44  feeds the injector inlet passage  42 . The presence of the orifices  70   a ,  70   b  has two effects. Firstly, pressure wave effects arising within the fuel passage  44  as a result of the pumping action of the pumping plunger  48  are damped. Secondly, the orifices  70   a ,  70   b  provide a tuning mechanism to facilitate variation in injection pressure due to the variable pressure drop across the orifices  70   a ,  70   b  with engine speed (i.e. plunger speed). 
     A further advantage of providing an orifice in the fuel passage  44 , between the point of interconnection between the fuel passage  44  and the injector inlet passage  42  and the common rail  46 , is that for particularly high engine speeds (i.e. higher plunger speeds) the pressure supplied to the injector through the inlet passage  42  will be higher than fuel pressure within the common rail  46 . It is possible to locate both the pumping element of the system (i.e. the plunger  48 ) and the injector  40  upstream of the orifices  70   a ,  70   b  as the plunger  48  and the injector  40  are housed within a common housing. 
     In practice only one orifice may be provided, either at the location of orifice  70   a  or that of orifice  70   b , or two orifices may be provided as discussed above. It will be appreciated that where a fuel injection system of the invention comprises more than one fuel injector and fuel pump (as depicted in  FIG. 3 ), either or both of the orifices  70   a  and  70   b  may be employed in one or more, and suitably each, of the fuel supply passages  44 . 
     A further modification to the embodiments of the invention previously described is illustrated by way of non-limiting example in  FIG. 5 , in which the fuel injection system comprises a plurality of fuel injectors. 
     In this embodiment the fuel injection system includes a plurality of (six) fuel injectors  40  which are supplied with fuel via respective injector inlet passages  42 . The injector inlet passages  42  receive fuel from fuel passages  44  (as before) which communicate, at one end  44   a , with a single accumulator volume in the form of a common rail  46 . 
     The fuel injection system further includes three pump arrangements  68  (as previously described), which are arranged to supply pressurised fuel via the respective fuel passages  44  to the common rail  46 . As before, the pumping plunger  48  of each pump arrangement  68  is driven by means of a drive arrangement (not shown), which includes an engine driven cam having one or more cam lobes and a main engine camshaft  72 . The pumping plunger  48  is typically driven by the cam via a roller and rocker mechanism (not shown) in a known manner. Alternatively, the pumping plunger may be driven by the cam via a guided tappet. Each pump chamber  52  of the pump arrangements  68  communicates with a second end  44   b  of its respective fuel passage  44  remote from the common rail  46 . 
     A non-return valve  54  is located within each fuel passage  44  that connects the common rail  46  to a pump chamber  52  of a pump arrangement  68 . In this way, the common rail  46  can only communicate with the pump chambers  52  via the non-return valves  54 . The non-return valves  54  are arranged to ensure that the valves are closed when there is no pressure in the system, and when the fuel pressure in the respective fuel passages  44  exceeds that of the respective pump chambers  52 . In one mode of operation, the non-return valves  54  also remain closed during fuel injection events, such that fuel to the injectors  40  is supplied from the common rail  46  and not from the pump chambers  52  of the fuel pumps. In this way, the fuel injection system provides the advantage that injection events may be completely independent of the fuel pump and cam rotation cycle, as described elsewhere herein. 
     For each fuel passage  44  that communicates with a fuel pump, a pump supply passage  62  branches from that fuel passage  44 , on the pump chamber side of the non-return valve  54 , and allows fuel from a low pressure fuel reservoir  64  to flow into the pump chamber  52 . The pump supply passage  62  is again provided with an electronically controlled valve  66 , in the form of a “metering valve” (but it may also be a “spill valve”), which is operable between an open state, in which fuel is able to flow into the pump chamber  52  from the low pressure fuel reservoir  64 , and a closed state in which communication between the low pressure fuel reservoir  64  and the pump chamber  52  is broken. 
     For each of the fuel passages  44  that also communicates with a fuel pump, the injector supply passage  42  branches from the fuel passage  44  on the common rail side of the non-return valve  54  (i.e. on the other side of the non-return valve  54  to the pump supply passage  62 ). In contrast, for the fuel passages  44  that are not in communication with a pump chamber  52  of a pump arrangement  68 , the fuel passage  44  communicates at the end  44   a  with the common rail  46  (as previously described), and at the other end with an injector supply passage  42  and an injector  40 . 
       FIG. 6  shows an alternative configuration of the fuel injection system of  FIG. 5 . Again, three fuel pump arrangements  68  supply a single common rail  46 , which provides high pressure fuel to six fuel injectors  40 . In this embodiment, in contrast to that of  FIG. 5 , the fuel pump arrangements  68  are spaced apart, such that the fuel passages  44  that communicate with pump chambers  52  also communicate via injector supply passages  42  to non-adjacent fuel injectors  40 . 
     A further embodiment of the fuel injection system of the invention is illustrated in  FIG. 7 . In this embodiment, two fuel pump arrangements  68  supply pressurised fuel to a single common rail  46 , which feeds high pressure fuel for injection to six fuel injectors  40 . As in all previous embodiments, a non-return value  54  is located in the two fuel passages  44  that communicate with pump chambers  52 , to allow the benefits of the invention to be achieved. 
     In another mode of operation, the non-return valve  54  may be open during a fuel injection event, such that fuel for that injection event appears to come directly from the fuel pump. However, this circumstance is merely a reflection of the coincidence between the fuel injection event and the pumping stroke of the fuel pump (i.e. at the time of the fuel injection event the fuel pressure in the pumping chamber  52  exceeds that in the common rail  46 ): it is not a dependent relationship. Advantageously, even in this mode of operation, over a period of use, fuel for injection is principally derived directly from the accumulator volume rather than from the fuel pump. The uninterrupted communication between the fuel injector(s) in the fuel injection system of the invention helps ensure that a substantial flow of fuel, suitable for a main injection event, can be obtained from the accumulator volume, as required. 
     Accordingly, the fuel injection system of the invention provides the distinct advantage over structurally similar prior art systems, in that the fuel injection cycles/events are independent of the pumping cycle of the fuel pump(s). In more detail engine operation with a fuel injection system of the invention can be considered to involve a number of inter-related “cycles” or processes, for example: (i) the engine camshaft rotation cycle, which in turn causes rotation of one or more cams; (ii) the fuel pump pumping plunger, which is driven through pumping and return strokes by the changing profile of the rotating cam; (iii) the opening and closing of a non-return valve situated between the pump chamber and the accumulator volume (or common rail), which is dependent on the balance of the fuel pressure in the pump chamber and the accumulator volume; and (iv) the operation of the fuel injector to allow a fuel injection event, which is inter alia dependent on the power demand of the engine. The system of the invention further operates to allow the fuel pressure within the accumulator volume to be maintained at a relatively constant, high pressure, which is suitable for injection into the one or more engine cylinders. Accordingly, cycle (iv) above (the fuel injection event), which includes the primary (or main) fuel injection event in circumstances where pre- or post-injections are also used, can occur at any point in time and/or at whichever frequency is required to meet the engine demand. This is a distinct and important advantage in comparison to prior art fuel injection systems, in which the high fuel pressure required for a primary (or main) fuel injection event (as opposed to a pre- or post-injection event) is dependent on the pumping cycle (e.g. the timing of the pumping stroke) of the pumping plunger (step (ii) above), which is in turn dependent on the camshaft cycle (step (i) above). 
     The independent relationship between fuel injection events and the cycle of the fuel pump in the systems of the invention, therefore, may provide the further advantage that the number of fuel pumps does not need to be equivalent to the number of fuel injectors: i.e. it is not necessary to have a 1:1 relationship between the fuel pumps of the engine and the fuel injectors (and engine cylinders). Advantageously, there may be fewer fuel pumps than fuel injectors (for example, 2 fuel injectors for each 6 fuel injectors and engine cylinders), as described herein. Thus, the production, maintenance/servicing and replacement costs of the fuel injection systems of the invention can be significantly reduced compared to some prior art fuel injection systems. Moreover, the reduction in the production costs of the fuel injection system of the invention (compared to prior art systems) means that the beneficial systems of the invention may be used in smaller and/or cheaper vehicles (e.g. small cars) and still provide the advantages associated with the invention. 
     It will be appreciated that modifications to the embodiment of the invention depicted in  FIG. 5  may be made, without departing from the scope of the invention. In this regard, the fuel injection system may be configured using various combinations of fuel pumps and injectors with one or more distinct accumulator volumes: the important factor being that there is one or more, for example, 2, 3, 4, 5 or 6 fuel injectors for each fuel pump. 
     Thus, in accordance with another embodiment of the invention there is provided a fuel injection system for an internal combustion engine, the fuel injection system comprising: a first and a second fuel injector  40  which are arranged within (or at least in part defined by) first and second housing units  38 , an accumulator volume  46  for supplying fuel to the first and second fuel injectors  40 , a first pumping plunger  48  which is driven, in use, to cause pressurisation of fuel within a first pump chamber  52 , a first metering valve  66  which is operable to control fuel flow into the pump chamber  52 , a first fuel passage  44  providing communication between the first pump chamber  52  and the accumulator volume  46 , a first non-return valve  54  located in the first fuel passage  44  between the first pump chamber  52  and the accumulator volume  46 , wherein the first fuel injector  40  communicates with the first fuel passage  44  at a position between the first non-return valve  54  and the accumulator volume  46 ; and a second fuel passage  44  providing communication between the accumulator volume  46  and the second fuel injector  40 ; and wherein in use the first and second fuel injectors  40  receive fuel from the accumulator volume  46  and not from the first pump chamber  52 . 
     Thus, in some embodiments, the fuel injection system of the invention may comprise between 1 and 12 fuel injectors, for example, 10, 8, 6 or between 1 and 6 fuel injectors; an accumulator volume; and between 1 and 12 fuel pump arrangements, suitably less than 12, for example, 10, 8, 6 or between 1 and 6 fuel pump arrangements. Advantageously, in embodiments where the fuel injection system of the invention comprises a plurality of fuel injectors, there are less fuel pump arrangements than there are fuel injectors. By way of example, fuel injection systems of the invention may suitably comprise: six fuel injectors and five fuel pump arrangements; six fuel injectors and four fuel pump arrangements; six fuel injectors and three fuel pump arrangements; six fuel injectors and two fuel pump arrangements; or six fuel injectors and one fuel pump arrangements. Advantageously, in each case there is provided one accumulator volume, although it is possible that there may be more than one (e.g. 2) accumulator volumes. In each embodiment, the fuel injection system of the invention may further include the additional features of the fuel injection system of the invention as described herein. 
     It will be appreciated that for each of the plurality of fuel injectors in these embodiments there may be a corresponding housing unit, compatible with known EUI systems. Thus, typically, each of the plurality of fuel injectors is conveniently arranged within (or at least in part defined by) a separate housing unit, such that there is one housing unit for each fuel injector. 
     Hence, in comparison with common rail systems, the invention provides the benefit that no (or only minor/minimal) changes are required to engines designed for use with EUIs. Furthermore, as already described, where it may be necessary to make such minor adjustments to known engine designs, these changes are beneficial, for example, in that there may be less rockers/rollers required and so on, which may reduce manufacturing cost and increase engine reliability. A hydraulic benefit is also achieved in that the pump chamber of the system is located relatively near to the injector (i.e. the injector is between the common rail  46  and the pump chamber  52 ). 
     The fuel injection systems of the present invention differs from the known hybrid scheme in  FIG. 1  in that the common rail  46  in the invention does not supply fuel to the pump chamber  52 , but instead only receives fuel from the pump chamber  52  in circumstances in which the non-return valve  54  is open. The non-return valve  54  prevents fuel flowing from the common rail  46  to the pump chamber  52  and, providing a near perfect seal, ensures that the pumping chamber  52  and the metering (or spill) valve  66  are isolated from the common rail  46  for most of the stroke, thereby reducing fuel losses. A further significant difference between the two schemes is that the injector  40  in the system of the invention primarily receives fuel directly from the common rail  46 , whereas in  FIG. 1  the supply of fuel to the injector from the common rail is via the pump chamber. As previously described, this benefit is manifested in the fact that fuel injection is not dependent on the pumping cycle of the fuel pump. 
     In order to assemble the fuel injection system into an engine, the EUIs are clamped to the engine cylinder head in a conventional manner and then the common rail is clamped to the engine cylinder head. The necessary pipe connections are then assembled to connect the EUIs with the common rail. In another assembly, the EUIs are clamped into the engine manifold in a conventional manner and then the common rail is clamped directly to the EUIs, without the need for additional pipework. Alternatively the combined common rail-EUI system may be clamped to the engine cylinder head as a single unit. 
     In any of the embodiments of the invention, whether those described specifically or those envisaged within the scope of the accompanying claims, the injector housing unit  38  for the injector  40  and the pump arrangement  68  may comprise two or more housing parts arranged adjacent to one another, rather than being a single housing part. Whether one or more housing parts are provided to form the injector/pump unit, it is an important feature of the invention that there is no need for a separate fuel pipe or pipes to carry fuel between the injector and pump chamber of the system. 
     In a further modification to that described previously, the non-return valve  54  need not take the form of a ball but may take an alternative valve form (for example, a plate valve). 
     Since fuel injection events are independent of the fuel pump, it is possible to pump fuel several times per engine cycle. Therefore, in some embodiments the cam may be a multi-lobe cam so as to provide two or more pumping strokes per engine rotation. Suitably such a multi-lobe cam may comprise 2, 3 or 4 lobes; more suitably 2 or 3 lobes; and most suitably 2 lobes. 
     A multi-lobe cam may be employed to allow the same or greater fuel pressurisation capability to be achieved by its associated fuel pump but with a smaller plunger diameter. Advantageously, using a multi-lobe cam provides greater pumping capacity per cam revolution, which may also allow a reduction in the number of fuel pumps in a fuel injection system or engine. By way of example, by employing a 2-lobe cam, the pump chamber of a fuel pump may be filled and evacuated (through return and pumping strokes of the pumping plunger, respectively) twice during a single engine (camshaft) rotation. Thus, for instance, the same fuel pressurisation of an associated accumulator volume can be achieved with half as much pumping capacity per pumping stroke and, hence, the diameter of the pumping plunger can be reduced. A number of functional benefits may be achieved by reducing the pumping plunger diameter of a fuel pump. For example, smaller diameter pumping plungers are useful for reducing fuel leakage between the edges of the plunger and the inside of the bore in which the plunger reciprocates and, therefore, for improving engine efficiency. A further advantage is that by significantly reducing the plunger diameter (to less than is practical with traditional EUIs) enables significantly higher injection pressures to be generated in existing engine designs. This may enable an existing engine with, for example, a stress limit of approximately 2500 bar (EUI) to be upgraded to above 4000 bar using the system of the invention, with negligible other modifications (as previously discussed). Smaller diameter pumping plungers may also be beneficial in reducing the fluctuations in cam drive torque required; and it has been recognised, in particular, that a relatively small plunger diameter can be beneficial in reducing stresses (loadings) arising in other engine components, such as the cam and drivetrain. 
     In addition, it will be appreciated that it can be useful to size the pumping plungers in an engine against the power rating of the engine. Typically, it is advantageous to employ smaller diameter pumping plungers in lower performance engines, for example, so that cost savings can be achieved.