Fuel system

A fuel system for supplying pressurised fuel to a plurality of fuel injectors (14a-14f) comprises an accumulator assembly (16) having first and second accumulator volumes (18, 20) defined within a common accumulator housing (22), supply means (42) for supplying fuel at a supply pressure level to the first accumulator volume (18) and a plurality of unit pumps (10a-10f). Each unit pump is arranged to receive fuel at the supply pressure level from the first accumulator volume (18) and pressurises said fuel to an injectable pressure level for supply to the second accumulator volume (20). Each unit pump (10a-10f) includes a pumping plunger (50) for pressuring fuel within an associated pump chamber (52) and being integrated with the accumulator housing (22) so as to permit communication between the first accumulator volume (18) and the pump chamber (52) internally within the accumulator housing (22).

BACKGROUND AND SUMMARY OF THE INVENTION

The present invention relates to a fuel system for an internal combustion engine and, in particular, to a fuel system including an accumulator volume in the form of a common rail for supplying fuel to a plurality of injectors.

In conventional common rail fuel injection systems, it is common to provide a single pump for charging an accumulator volume, or common rail, with high pressure fuel for supply to a plurality of injectors of the fuel system. 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 mechanism, and so fast and accurate control of the timing of injection can be achieved with the nozzle control valves. However, achieving very high injection pressure within a common rail system is problematic and the high levels to which fuel must be pressurised can cause high stresses within the pump and within the rail. The rail must therefore be provided with a relatively thick wall for pressure containment, making it heavy and bulky. Parasitic fuel losses can also be high.

It has been recognised that significant improvements in combustion quality and efficiency may be achieved by rapidly varying the injection pressure level and injection rate within an injection event. Such variations in the injection characteristics can be difficult to achieve rapidly with common rail systems, and the efficiency of the system can be limited. For example, in a common rail system designed to achieve injection at a high rail pressure, it is also possible to achieve a lower injection pressure by relieving some of the high pressure fuel to a low pressure reservoir. This, however, is an inefficient use of pumping energy.

By way of background to the present invention it is acknowledged that Electronic Unit Pumps (EUPs) provide a different fuel system concept to that of the common rail system. In an EUP fuel system, one EUP is provided for each cylinder of the engine and has a dedicated injector to which pressurised fuel is supplied by the EUP for injection purposes. The EUP includes a dedicated pump having a cam-driven plunger for raising fuel pressure within a pump chamber, from where pressurised fuel is supplied to the associated injector. In an EUP system, however, the constraints of the cam drive mechanism can limit the range of injection timing that can be achieved. It is also acknowledged that Electronic Injectors (EUIs) are known, in which the associated injector is incorporated within the same unit as its dedicated plunger and injection is controlled by means of a nozzle control valve of the unit.

It is one aim of the present invention to provide a common rail fuel system which provides improvements over known common rail fuel systems and which addresses, in particular, the issue of variable injection characteristics and of parasitic fuel losses so as to provide enhanced system efficiency.

According to a first aspect of the present invention there is provided a fuel system for supplying fuel to a plurality of injectors, the fuel system comprising;

an accumulator assembly having first and second accumulator volumes defined within a common accumulator housing,

supply means for supplying fuel at a supply pressure level to the first accumulator volume,

a plurality of unit pumps, each for receiving fuel at the supply pressure level from the first accumulator volume and for pressurising said fuel to an injectable pressure level for supply to the second accumulator volume,

each unit pump including a pumping plunger for pressuring fuel within an associated pump chamber, wherein the unit pump is integrated with the accumulator housing so as to permit communication between the first accumulator volume and the pump chamber internally within the accumulator housing.

Preferably, each unit pump is integrated with the accumulator housing by mounting within an opening or cross bore provided in the accumulator housing, so that the unit pumps pass through the accumulator housing.

The accumulator assembly is preferably a rail assembly comprising a first rail volume (the first accumulator volume) and a second rail volume (the second accumulator volume) housed within a rail housing (the accumulator housing).

It is a particular benefit of the fuel system of the present invention that a first rail volume for lower pressure fuel may be arranged adjacent to, side by side or in parallel with a second rail volume for higher pressure fuel, within a common rail housing, and thus a cooling effect is provided for high pressure fuel within the second rail volume.

In a further preferred embodiment the assembled unit pump and rail assembly, forming an integrated pump/rail assembly, is configured such that each unit pump is received within the accumulator housing so as to permit communication between the second accumulator volume and its pump chamber internally within the pump/rail assembly, with the communication path conveniently traversing an interface between unit pump and rail housings.

Preferably, a plurality of unit pumps are provided, equal in number to the number of injectors to which fuel is to be supplied.

The first rail volume may be communicable with the pump chamber of each unit pump within the actuator housing via first valve means, typically in the form of a non-return valve. The first valve means has an open position, in which the pump chamber communicates with the first rail volume, and a closed position in which said communication is broken.

It is a particular benefit of being able to inject fuel at two pressure levels, that a sequence of a main injection of fuel having a second (higher) pressure level followed by a post injection of fuel having a first (moderate) pressure level can be achieved and this can have benefits for after-treatment purposes. It is also desirable to inject a pilot injection of fuel at a first pressure level followed by a main injection of fuel at a second pressure level, or to provide a boot-shaped injection characteristic, which also provides benefits in terms of engine noise and emissions levels.

The fuel system therefore preferably includes second valve means, wherein the second rail volume is communicable with the pump chamber of each unit pump through the second valve means.

Preferably, the second valve means is a rail control valve which is operable between an open position in which a supply of fuel at the injectable pressure level, being the first injectable pressure level, is supplied from the second rail volume to the injectors of the system and a closed position in which communication between the pump chamber and the second rail volume is broken so that the unit pump is operable to increase fuel to a second injectable pressure level.

Conveniently the rail control valve and/or the non-return valve form an integral part of the unit pump, being contained within a common pump housing.

In a preferred embodiment, the plunger of each unit pump is movable within the plunger bore to perform a pumping cycle having a pumping stroke and a return stroke. During the plunger pumping stroke, pressurisation of fuel occurs within the pump chamber. During the plunger return stroke, the pumping chamber is filled with fuel to be pressurised during the following pumping stroke.

Each unit pump is preferably driven by means of a cam arrangement, with the plunger co-operating with a drive member, such as a tappet, to effect plunger motion. A cam follower such as a roller may be provided for driving the drive member in response to rotation of an engine driven cam, so as to drive plunger movement.

It will be appreciated that the fuel system may, but need not, include the fuel injectors and may, but need not, include respective high pressure supply passages for supplying fuel from the pump chamber of each unit pump to an associated one of the injectors.

In one particular embodiment, each unit pump forms an EUI-type unit, in which the unit pump is incorporated with an associated injector (electronically controlled) within a common pump/injector unit. The requirement for a high pressure supply passage between the unit pump and its associated injector is avoided in this embodiment.

The system may include control valve means operable to control the timing of commencement of injection at a first and/or second injectable pressure level. The control valve means may, in a first embodiment, include a nozzle control valve that is operable to control fuel pressure within an injector control chamber so as to permit control of injection timing at the first and/or second injectable pressure level.

The injector may include a valve needle that itself has a surface exposed to fuel pressure within the control chamber, so that by controlling fuel pressure within the control chamber by means of the nozzle control valve, opening and closure of the valve needle can be controlled.

The supply means may take the form of a transfer pump for supplying fuel at the supply pressure level. It will be appreciated, however, that whilst the supply means of the system may include the transfer pump, the system need not include the pump, and may be manufactured without it, in which case the supply means may simply take the form of an inlet to the first rail volume.

According to a second aspect of the present invention, there is provided an accumulator assembly for a common rail fuel system having a plurality of unit pumps, the accumulator assembly including an accumulator housing within which is defined a first accumulator volume for fuel at a supply pressure level and a second accumulator volume for fuel at an injectable pressure level, wherein the accumulator housing is provided with a plurality of openings, each for receiving one of the unit pumps, in use, so as to permit communication between respective pump chambers of the unit pumps and the first accumulator volume internally within the accumulator housing.

It will be appreciated that any one or more of the preferred and/or optional features described previously for the first aspect of the invention may be included as preferred or optional features of the second aspect of the invention also.

DETAILED DESCRIPTION OF THE DRAWINGS

Referring toFIG. 1, there is shown a fuel system for supplying fuel to a plurality of fuel injectors14a-14f(six of which are shown), each of which is supplied with fuel at an injectable pressure through respective high pressure supply lines or passages12a-12f.The fuel system includes pump means in the form of a plurality of unit pumps10a-10f,each of which is dedicated to a respective one of the injectors14a-14f.Each unit pump10a-10fis of a generally similar type to the known Electronic Unit Pump (EUP), as described previously, although the significant modifications to the construction and operation will be described in further detail later.

The unit pumps10a-10fare integrated with a rail assembly, referred to generally as16, including first and second accumulator or rail volumes defined by first and second rails18,20. The first rail18receives fuel at relatively low pressure from a fuel supply means (not shown inFIG. 1) including a rail inlet. Typically the fuel supply means also includes a transfer pump feeding fuel to the rail inlet. The second rail20receives fuel which has been pressurised to an injectable pressure level by the unit pumps10a-10f.The first and second rails18,20are arranged adjacent to and in parallel with one another and are defined or integrated within a common accumulator housing in the form of a rail housing22. When assembled with the rail assembly16, the unit pumps10a,10bdefine an “integrated rail/pump assembly”.

The rail assembly16will now be described in further detail with reference toFIGS. 2 to 5. InFIG. 5, only a first one of the unit pumps10a is visible and this illustrates the location of the unit pump10arelative to the rail housing22. The rail housing22is provided with a plurality of cross bores or openings24a-24f,each of which extends through the housing22so as to intersect, and interrupt, the first rail volume18. Each unit pump10a-10fis mounted within the rail assembly16so that its pump chamber (not visible) is able to communicate with the first rail18at points internally within the rail housing22, therefore avoiding the need for external pipe connections and external seals between the unit pump10a-10fand the rails18,20. It is a further feature of the mounting of the unit pumps10a-10fwithin the assembly that the pump chamber of each unit communicates with the second rail20internally within the pump/rail assembly via internal interface seals between faces of the unit pump housing and the rail housing22, as described in further detail later.

The total number of openings24a-24fprovided in the rail housing22is equal to the number of unit pumps10a-10fof the system (i.e. six in the example shown), so that each opening receives a respective one of the unit pumps10a-10fwhen the system is assembled. Appropriate fixing points26, for example bolt holes, are provided for each opening24a-24f(indicated for the first opening24aonly) to provide a means of fixing or clamping each unit pump10a-10fto an engine housing (not identified), typically the engine cylinder block, when it is received within its opening24a-24f.Each unit pump10a-10fsits within the opening24a-24fso that its longitudinal axis intersects the longitudinal axis of the first rail18.

One end of the second rail20is provided with a pressure sensor28which senses the pressure of fuel within the second rail20and provides an output signal to an Engine Control Unit (ECU) (not shown). A pressure relief valve30is provided at the opposite end of the second rail20. The pressure relief valve30is electronically controllable by the ECU, which controls the pressure relief valve in response to the rail pressure sensor output signal so as to prevent over-pressurisation of fuel within the second rail20. A return drilling32is provided in the rail housing16, as shown inFIG. 3, to provide a flow path for fuel that is relieved through the valve30into the first rail18at lower pressure.

FIG. 6shows the hydraulic arrangement of the unit pumps10a-10f,the first and second rails18,20and the injectors14a-14f.For simplicity only a single injector14aand its dedicated unit pump10aare shown inFIG. 6in relation to the first and second rails18,20although, as will be apparent from the foregoing description, the first and second rails18,20are common to all injectors14a-14fand all unit pumps10a-10fof the system. Only a single one of the unit pumps10aand a single one of the injectors14awill be described in detail, as all unit pumps and all injectors are substantially identical.

The injector14aincludes an injection nozzle34and a control valve means in the form of a nozzle control valve36(alternatively referred to as a needle control valve), which is arranged to control movement of a valve needle38so as to control the delivery of fuel from the injection nozzle34. The valve needle38is engageable with a valve needle seating and movement of the valve needle38away from the seating permits fuel to flow through one or more injection nozzle outlets (not indicated) into the associated engine cylinder or other combustion space.

The nozzle control valve36is arranged within a flow path40between fuel supply means42, which may be within the cylinder block, and an injector control chamber44arranged at the back end of the valve needle. A surface of the valve needle38is exposed to fuel pressure within the control chamber44, so that fuel within the control chamber44applies a force to the valve needle38which serves to urge the valve needle38against its seating. The valve needle38is provided with a needle spring46, housed within the control chamber44, which also serves to urge the needle38towards its closed or seated position. The fuel supply means takes the form of a transfer pump42for supplying fuel at relatively low pressure, typically between 3 and 7 bar, to the flow path40.

The injection nozzle34includes a delivery chamber48which receives fuel at an injectable pressure level through the supply passage12a,and from where fuel is supplied to the injection nozzle outlets when the valve needle38is unseated. It will be appreciated by comparingFIG. 1andFIG. 6that the high pressure supply passage12abetween the unit pump10aand its injector14ainFIG. 1is hydraulically equivalent to the identically numbered passage inFIG. 6. It will now also be appreciated that the flow path40, identified inFIG. 6, between the transfer pump42and the injector10ais not shown inFIG. 1but may be, for example, through a gallery or rail in the cylinder head or block of the engine.

The nozzle control valve36is movable between a first position (open) and a second position (closed). When the nozzle control valve36is opened, the supply passage12acommunicates with the control chamber44of the injector so that high fuel pressure within the chamber44acts on valve needle38, in combination with the needle spring46, to seat the valve needle38. When the nozzle control valve36is closed, the control chamber44communicates with the transfer pump42and communication between the supply passage12aand the control chamber44is broken, so that the pressure of fuel within the control chamber44acting on the valve needle is reduced. By closing the nozzle control valve36, the valve needle38is thus caused to lift due to high pressure fuel acting on valve needle thrust surfaces which are exposed to fuel pressure within the delivery chamber48. Operation of the nozzle control valve36to control fuel pressure within the control chamber44therefore provides a means of controlling valve needle movement towards and away from its seating to control injection.

FIG. 6also shows the second rail20and the rail pressure sensor28(as shown inFIG. 1). Communication between the second rail20and the pump chamber52is controlled by means of an electrically controllable valve in the form of a rail control valve58forming part of the unit pump10a.

Each unit pump (e.g.10a) has a pumping element or plunger50and a pump chamber52in communication with one end of the supply passage12a.The plunger50is movable within a plunger bore54provided in a unit pump housing (not identified) under the influence of a cam drive arrangement (not shown inFIG. 6) so as to pressurise fuel within the pump chamber52. The plunger bore54is provided with an internal groove55, or an enlarged diameter region, which serves to collect leakage fuel from the pump chamber52down the plunger bore54and drains to a low pressure drain, as described in further detail later.

The pump chamber52also communicates with the transfer pump42through a supply passage which is hydraulically equivalent to the first rail18. This supply passage, or first rail18, is provided with a hydraulically operable non-return valve56provided with a non-return valve spring57, and receives fuel at relatively low pressure from the transfer pump42, in use. If the non return valve56is in its open position, the transfer pump42is able to supply fuel to the pump chamber52at a relatively low pressure through the first rail18. If the non return valve56is in the closed position the communication path between the pump chamber52and the first rail18, and hence the transfer pump42, is closed off.

The construction of the unit pump10ais shown in further detail inFIGS. 7 and 8. The unit pump10aincludes a unit pump housing60which is provided with the bore54within which the plunger50moves and within which the pump chamber52is defined. The plunger50has an associated plunger return spring62and a tappet drive member64(also identified inFIG. 1as64a-64f), as is common in a known EUP. The tappet64co-operates with a roller66which rides over the surface of the cam so as to impart drive to the tappet64and, hence, to the plunger50so as to effect a plunger pumping stroke, during which the plunger50is driven inwardly within the bore54to reduce the volume of the pump chamber52. The plunger return spring62serves to drive a return stroke of the plunger50, during which the plunger50is urged outwardly from the bore54, increasing the volume of the pump chamber52.

The rail control valve58and the non-return valve56are housed, adjacent to one another, within a rail control valve housing59located at an upper end of the unit pump10a.The rail control valve58is operable by means of an electromagnetic actuator arrangement including an energisable winding62and an armature (not identified) coupled to a rail control valve member64so that energisation and de-energisation of the winding62causes movement of the rail control valve member64to open and close the rail control valve58.

The pump chamber52communicates with an outlet passage72defined by a drilling provided in the unit pump housing60which, in turn, communicates with the supply passage12athrough a high pressure circuit76provided in the various housing parts. The outlet passage72also communicates with a rail circuit74defined by drillings in various housing parts, depending on the position of the rail control valve58.

The fuel system is capable of providing injection at first and second injectable pressure levels, depending upon the operating state of the rail control valve58. In a first mode of operation, the system operates in a common rail-type mode in which plunger movement has minimal or no effect on the pressure level in the pump chamber52due to the rail control valve58being open, and fuel at the first, moderate rail pressure, which is stored in the second rail20, is delivered to the injector14a.In a second mode of operation the system operates in an EUP-type mode in which plunger movement increases the pressure level to a second higher level, due to the rail control valve58being closed, and fuel at this higher level is delivered to the injector14a.

To clarify, when the rail control valve58is opened, the pump chamber52of the unit pump10acommunicates with the second rail20through the rail circuit74and also with the supply passage12a.When the rail control valve58is closed the communication path (i.e. the rail circuit74) between the pump chamber52and the second rail20is broken, and instead the pump chamber52communicates only with the supply passage12a(through the high pressure circuit76). Actuation and de-actuation of the rail control valve58is controlled by means of control signals supplied by the ECU. The operating state of the nozzle control valve36determines whether injection takes place and, thus, provides a control means for the timing of commencement and termination of injection.

Various modes of operation of the fuel system will now be described in further detail, particularly with reference toFIGS. 6 to 8.

In use, during a return stroke of the plunger50, the volume of the pump chamber52is expanding and, with the rail control valve58closed, a point will be reached at which the non-return valve56opens to permit fuel to be supplied to the pump chamber52. At the start of the plunger pumping stroke the non-return valve56is still open. As the driven tappet64acts on the plunger50it is urged inwardly within the bore54, thereby reducing the volume of the pump chamber52. With the rail control valve58closed, movement of the plunger50through the pumping stroke causes fuel pressure within the pump chamber52to be increased. As the pressure differential across the non-return valve56increases, due to increasing fuel pressure within the pump chamber52acting in combination with the valve spring57, a point will be reached at which the non-return valve56is caused to close. Further movement of the plunger50through the pumping stroke causes fuel pressure within the pump chamber52to increase further, until such time as the rail control valve58is opened to permit pressurised fuel, at a first pressure level, to fill the second rail20.

During this first mode of operation the pressure level (referred to as the first pressure level) to which fuel within the pump chamber52is pressurised is higher than transfer pressure supplied by the pump42, but typically is less than the pressure that would be achieved by a high pressure common rail-type pump. Typically, for example, this first pressure level may be up to about 1000 bar. If the rail control valve58is opened during the period for which the non return valve56is closed, fuel at the first injectable pressure level is supplied, via the outlet passage72and the rail circuit74, to the second rail20. Fuel at this first injectable pressure level also fills the supply passage12athrough the drilling76and, hence, supplies fuel at the first injectable pressure level to the injection nozzle34.

Continued plunger movement through its pumping cycle causes fuel at the first pressure level to be supplied to and drawn out of the pump chamber52through the open rail control valve58, with the unit pumps10a-10bbeing operable in a phased cyclical manner so that fuel volume that is displaced from one pump chamber52of one unit pump and supplied to the second rail20during its pumping stroke coincides with fuel within the second rail20being supplied to the pump chamber52of another unit pump during its return stroke, so as to maintain rail fuel volume.

In order to inject fuel at the first injectable pressure level, the nozzle control valve36is actuated to move into its closed position so that fuel in the control chamber is able to return to the transfer pump42, therefore allowing the valve needle38to open. Injection may be terminated by actuating the nozzle control valve36to move into its open position so that high fuel pressure is re-established within the control chamber44to seat the needle38.

If the rail control valve58is closed during the plunger pumping stroke (i.e. with the non return valve56closed) the pressure of fuel within the pump chamber52, which during the start of the pumping stroke is held at about 1000 bar, will be increased during the pumping stroke of the plunger50to a second pressure level that is higher than the first as fuel can no longer flow into and out of the second rail20. Typically, this second injectable pressure level may be between 2000 and 3000 bar. With the rail control valve58closed, injection at the second injectable pressure level is initiated by actuating the nozzle control valve36to allow the control chamber to communicate with the transfer pump42, as described previously. In a similar manner termination of injection at the second injectable pressure level may be implemented by actuating the nozzle control valve36to re-establish high fuel pressure within the control chamber44.

In order to re-fill the second rail20following an injection event, the rail control valve58is closed during the plunger return stroke. As the plunger withdraws from the pump chamber52, increasing the pump chamber volume, the pressure drop across the non-return valve56causes it to open, permitting a supply of new fuel into the pump chamber52ready for the next pumping cycle.

If the rail control valve58is opened during the pumping stroke it will be appreciated that the non-return valve56stays closed due to pressure within the pump chamber52being higher than transfer pressure.

It will be appreciated that the timing of operation of the rail control valve58is important, so as to ensure that fuel is pressurised within the pump chamber52to the second pressure level at the required time (i.e. by closing the rail control valve58) and also to ensure fuel is supplied to the pump chamber52by the pump42following an injection event. In practice, for example, the duration for which the valve58is open, and the relative timing of its opening and closure, will be controlled by control signals provided by the engine controller in accordance with look-up tables or data maps containing pre-stored information. The implementation of look-up tables and data maps for control of engine fueling and timing would be familiar to a person skilled in this technical field.

It is a further feature of the fuel system ofFIGS. 1 to 7that should it be desirable to reduce the pressure of fuel that is stored within the second rail20, the pressure relief valve30can be opened to permit fuel within the second rail20to flow into the first rail18at lower pressure through the return drilling32(as shown inFIG. 3).

It is one advantage of the invention that an injection event comprising a pilot injection of fuel at a first, moderate pressure level followed by a main injection event at a second, higher pressure level can be achieved by switching the rail control valve58. It has been found that this combination of a pilot followed by a main injection of fuel provides a benefit for emission levels and noise. The fuel system can also be used to implement a main injection of fuel at a higher pressure level followed by a late, post injection of fuel at a lower pressure level. This can be useful for after-treatment purposes. A boot-shaped injection characteristic, comprising an initiate higher rate fuel injection immediately followed by a lower rate injection, can also be achieved through rapid switching of the rail control valve58and the nozzle control valve36, when appropriate.

It is a further advantage of the invention that the locality of the first rail18to the second rail20provides benefits for cooling of the second rail, as cooler lower pressure fuel (i.e. at transfer pressure) within the first rail18provides a cooling effect for higher pressure fuel within the second rail20. In an alternative embodiment (not illustrated), the rail/pump assembly may also be provided with means for feeding fuel pressure within the first rail18to a low pressure drain, thereby improving the cooling effect of the first rail further. For example, an additional feed drilling or passage may be provided in the rail housing22in communication, at one end, with the first rail18and communicating at the other end with the low pressure drain.

Another benefit is achieved in that the hydraulic connection between the first rail and the unit pump10ais internally within the rail housing22. The need for additional pipework, additional connections and additional seals is therefore avoided. It is also an advantage that the hydraulic connection between the second rail20and the unit pump10ais internally within the rail/pump assembly, at the interface between the unit pump housing60and the rail housing22(as can be seen inFIG. 8), and so the need for external high pressure connections and seals is avoided here also.

The rail assembly16is also simple and convenient to manufacture and assemble. Once the rail housing22has been machined to provide the openings24a-24f,each unit pump10a-10f,in its fully assembled state, is inserted into a respective one of the openings24a-24fso as to make the required communications between the pump chambers and the rails18,20. When the unit pumps10a-10fare inserted into the openings24a-24f,the appropriate fixing means are then inserted through the bolt holes to secure the unit pumps10a-10fin position.

In a further modification to that described previously, a third rail volume may be provided within the rail housing22. The third rail volume may be arranged adjacent to, or in side by side arrangement with the first and second rails18,20and may be arranged to communicate, through an additional drilling in the unit pump, with the plunger leakage groove55.

In an alternative embodiment of the invention to that described previously, the injector14a-14fassociated with each unit pump10a-10fmay itself form part of the unit pump10a-10f(i.e. within a common housing), in an EUI-type arrangement. The EUI has a first injector end, at which the injector is arranged, and an opposite pump end, at which the pumping elements are arranged. It will be appreciated that either the injector end of the EUI or the pump end of the EUI may be inserted into the respective opening24a-24fto mount the unit within the rail assembly16. As before, the unit pump incorporating the injector is mounted within its respective opening24a-24fso that the pump chamber of each EUI communicates with the rail internally within the accumulator housing22.

It will be appreciated that although the embodiment of the invention described previously includes a rail control valve58for permitting the system to switch between first and second injectable pressure levels, a rail control valve58that operates in this manner is not an essential element of the invention. The first and second rails18,20may be provided to give the aforementioned advantages in an EUP-accumulator type system, even if the system is configured to enable fuel injection at only one injectable pressure level.