WO2009127868A1

WO2009127868A1 - Fuel injector

Info

Publication number: WO2009127868A1
Application number: PCT/GB2009/050369
Authority: WO
Inventors: Samuel Smith; Mark Graham; Andrew Knight; Robert D'aubyn
Original assignee: Delphi Technologies, Inc.
Priority date: 2008-04-17
Filing date: 2009-04-15
Publication date: 2009-10-22
Also published as: GB0807014D0

Abstract

A fuel injector for an internal combustion engine operated by a liquefied gas fuel includes an isolation valve (60) that interrupts the fuel supply passage (2) supplying pressurised fuel, such as liquefied dimethyl ether, to the injector nozzle (22). A further isolation valve (60') may be included in a fuel outlet passage (50) of the injector. The isolation valves (60,60') are maintained in an open position by the fuel pressure in the injector when the pressure reaches a predetermined level, such as occurs when the engine is switched on, but close when the engine is switched off and the fuelpressure decays. The injector nozzle may therefore be isolated from the remainder of the fuel system when the engine is not running. To control injection timing, the injector may also include a statically leakless valve (102) in fluid communication with afluid outlet line (130) upstream of the further isolation valve (60') and isolated from the fuel supply passage, thereby reducing the risk of stem leakage.

Description

FUEL INJECTOR

FIELD OF THE INVENTION

The invention relates to a fuel injector for use in the delivery of fuel under high pressure to a combustion chamber of an internal combustion engine. In particular, the invention relates to a fuel injector for use in delivery of a liquefied gas fuel.

BACKGROUND OF THE INVENTION

In recent years, concerns over noxious emissions from internal combustion engines have motivated engine manufacturers not only to develop more efficient engines but also to consider the use of alternative, less polluting fuels. In this regard, dimethyl ether (DME) has been identified as a promising alternative fuel for compression-ignition (diesel) engines. Specifically, DME is able to deliver energy efficiency comparable to conventional diesel engines and exhaust gas emissions also compare favourably.

It will be appreciated however that DME, in common with other liquefied gas fuels, such as liquefied petroleum gas (LPG), has different physical properties than conventional fuels, such as diesel fuel, and hence engines must be specifically adapted to operate successfully with these fuels.

Under ambient conditions, DME must be pressurized to keep it in liquid state. Accordingly, common rail fuel injection systems which have become well known in diesel engine technology and which involve storing fuel at high pressure have potential application for use with liquefied gas fuels, such as DME.

Benefits of common rail fuel injectors include minimal engine warm-up time, lower engine noise and lower emissions, as compared to other known systems. Typically, a common rail fuel system includes a common pressure accumulator, called the "rail", which is mounted along the engine block and fed by a high pressure pump. The pressure level of the rail is electronically regulated by a combination of metering on the supply pump and fuel discharge by a high-pressure regulator (when fitted). The pressure accumulator operates independently of engine speed or load, so that high injection pressure can be produced at low speeds if required. A series of injectors are connected to the rail, and each is opened and closed, such as by means of a solenoid valve or piezoelectric actuator, as directed by the engine control unit (ECU) which opens each injector electronically.

One form of conventional common rail fuel injector is shown by way of example in Figure 1. It comprises a nozzle needle (26) slidable within a bore formed in a nozzle body (22) and engageable with a seating (21 ) at the free end of the bore to control delivery of fuel from the bore to a combustion chamber via one or more outlets (24) adjacent the free end. The bore includes a region towards the nozzle end of a diameter similar to the diameter of the needle (26) so that the bore acts as guide for the sliding movement of the needle. The bore also includes a region of enlarged diameter defining a gallery (30) for receiving fuel under pressure from a fuel supply passage (2).

The nozzle body (22) abuts a piston housing (40) that includes a bore (46) for receiving and guiding a projection that cooperates with, or extends from, the nozzle needle (26). A piston spring (29) acts upon a spring abutment surface formed by an extended diameter region (27) to urge the needle (26) towards the seating (21 ).

In the described example, the fuel supply passage (2) extends through a main body (140) of the injector and the piston housing (40) for conveying pressurised fuel to the fuel gallery (30) in the bore of the nozzle body (22). The fuel gallery (30) also communicates, continuously, with a restricted outlet passage (50), that allows fuel to be in communication with the supply passage (2) when the pin (104) is in the de-energised state. In the energised state of the pin (104), fuel is allowed to return to a relatively low pressure fuel reservoir. The outlet fuel passage (50) is generally shaped to restrict the rate at which fuel can flow from the fuel gallery (30).

Control of the fuel pressure within the injector is achieved using an electromagnetic actuator (100) that acts by way of a coil winding (108) which pulls an armature (1 14) against the force of a spring (106) to lift a pin (104) off its associated valve seat (1 18). This allows the fuel to "spill" across the valve seat (118) and hence causes the fuel pressure within the outlet passage (51 ) immediately above the needle (26) to fall to an intermediate pressure, below rail pressure but above back leak pressure. The resultant force imbalance on the nozzle needle (26) due to the difference between rail and intermediate pressures causes the nozzle needle (26) to rise thereby initiating an injection. It will be understood that the above described fuel injector is but one of many variations that are possible, and the present invention described below is not limited to one having all of the features mentioned.

Although common rail type fuel injectors such as the one exemplified above have the potential for use with fuels that are required to be kept under pressure, they have been designed for fuels having higher viscosities than the likes of DME. Accordingly, owing to the fact that the viscosity of DME is in the region of 10% that of typical diesel fuel, it has a propensity to leak through the system. Moreover, because DME exists as a gas at room temperature and pressure, and in the gaseous state is heavier than air, when the engine is off it has the potential to leak from the fuel injector's nozzle into the combustion chamber and the engine crankcase. When fuel has leaked in this way it can lead to detonation in the cylinder and a consequentially noisy re-start, or more hazardously can result in a build up of explosive mixture in the crankcase.

Hence, it is desirable to provide a fuel injector that is adapted for use with liquefied gas fuels, particularly a fuel injector that is able to overcome or at least alleviate the aforementioned problem of fuel leakage.

SUMMARY OF THE INVENTION

In broad terms, and from a first aspect, the invention provides a fuel injector for an internal combustion engine operated by liquefied gas fuel, the injector comprising a nozzle assembly including a nozzle body with a nozzle outlet adjacent its free end for allowing fuel to be injected into a combustion chamber, a nozzle needle axially moveable along a longitudinal recess of the nozzle body to open or close the nozzle outlet, a fuel supply passage for supplying pressurised fuel to the nozzle body, and an injector body including an actuator for initiating axial movement of the nozzle needle to allow an injection of fuel, characterised in that the fuel supply passage is interrupted by an isolation valve that is maintained in an open position by the fuel pressure when it reaches a predetermined pressure level but closes when the pressure decays below the predetermined pressure level.

In a preferred fuel injection system, the predetermined pressure level required to maintain the isolation valve in its open position is generated by the engine when the engine is switched on; when the engine is switched off, the pressure level decays below the predetermined level whereupon the valve closes.

By means of the invention, any fuel that is present in the fuel supply line upstream of the isolation valve when the pressure falls, such as when engine is switched off, is prevented from leaking through to the nozzle assembly. In this way, the potential for unwanted cylinder combustion/detonation or explosive mixture build up is contained.

Preferably the isolation valve is located at or close to the nozzle end of the injector thereby to minimise the volume of fuel downstream of the isolation valve that is potentially at risk of leaking from the injector.

The isolation valve may conveniently take the form of a linear movement valve, such as one comprising an isolation valve pin with a preloaded spring and an associated valve seat against which the pin is urged by the spring when the fuel pressure within the injector falls, for example as the engine is switched off. When the engine is switched on, fuel pressure within the injector increases causing the isolation valve pin to rise up against the force of the spring to open the valve. The isolation valve may include a top seat to restrict upward movement of the pin once the valve is fully open.

The value of the opening pressure for the isolation valve may be dictated by its valve seat diameter or the preload of its spring, or a combination of both.

It will be appreciated that fuel will be present in other parts of the injector as well as in the fuel supply passage and nozzle assembly; for example, the nozzle assembly may communicate with a fuel outlet passage which may in turn communicate with further backleak passages or galleries where fuel may pass or reside. In one injector arrangement, a fuel outlet passage, or drain, may lead from the nozzle assembly to the actuator to serve as a control line, so that for example a change in fuel pressure in the control line caused by the actuator initiates an injection; in another arrangement a fuel outlet passage may lead back to the fuel supply passage, thereby to recirculate fuel within the injector.

Accordingly, to further reduce the volume of fuel that has the potential to leak from the injector via the nozzle, the injector preferably includes in a fuel outlet passage thereof a further isolation valve that is maintained in an open position by the fuel pressure when it reaches a predetermined pressure level but closes when the pressure decays below the predetermined pressure level.

As with the fuel supply side isolation valve, the predetermined pressure level required to maintain the further isolation valve in its open position is preferably that generated by the engine when the engine is switched on.

By means of this further isolation valve, any fuel that is present downstream in the fuel outlet line when the engine is turned off is also prevented from leaking back to the nozzle assembly. In this way, the volume of fuel contained in the complete fuel system is substantially prevented from draining through the injector and into the combustion chamber.

As with the feed line isolation valve, the further isolation valve is also preferably located at or close to the nozzle end of the injector. With such an arrangement, it is possible effectively to isolate the injector nozzle from the remainder of the fuel system. Thus the only fuel that is at risk of leakage when the engine is switched off is that volume present in the nozzle body.

The aforementioned further isolation valve may be of the same or different type to the isolation valve associated with the fuel supply. Moreover, the respective opening pressures for each isolation valve may have different values, for example, by having different valve seat diameters or spring preloads, or both.

In a preferred arrangement, both the isolation valve (the feed side valve) and the further isolation valve (the control side valve) are provided in an isolation valve housing intermediate the nozzle assembly and the injector body. A particularly convenient arrangement includes an isolation valve housing having first and second valve bores, ideally parallel bores, extending therethrough from an upper housing surface to a lower housing surface and along which the respective valve pins of the first and further isolation valve pins slide for opening and closing. The top seat and valve seat for each isolation valve are preferably located at or towards opposite ends of their respective valve bores.

For example, the internal diameter of each valve bore may be reduced towards its upper end thereby creating a shoulder portion which can serve as a top seat for its associated isolation valve pin. Preferably, each valve pin includes a collar portion, where the external diameter of the pin is increased, which is urged against the shoulder portion (the top seat) of the valve bore when the valve is in its fully open position. In other words, the bore shoulder and pin collar co-operate to limit upward movement of the pin in the valve bore when the valve is open.

In an alternative arrangement, the top seat may incorporate an O-ring, retained in a groove beneath the shoulder portion, against which the collar portion of the valve pin is urged in the valve open position.

The isolation valve pins may include a hollow portion to accommodate at least partially a respective spring member that urges the pin to close the valve when the pressure within the injector falls below an intermediate level, such as when the engine is switched off. These hollow portions are generally at lower pressure and are preferably in communication with a backleak gallery of the injector, that is the space that exists between the cap nut and the external walls of any components that are retained together by the cap nut.

The first, feed side, valve bore is in fluid communication with (and indeed forms part of) the fuel supply line that ultimately directs pressurised fuel to the injector nozzle. Advantageously, the isolation valve housing includes a secondary, fuel supply bore extending at one end from the upper housing surface where it is in fluid communication with the fuel supply passage of the injector body to the other end where it opens into the first, feed side valve bore upstream of the associated isolation valve seat.

A particularly convenient arrangement is one in which the secondary, fuel supply bore runs substantially parallel to the first, feed side valve bore, with an angled portion at its lower end where it opens into the said valve bore above the valve seat.

The second, further valve bore that is in fluid communication with the fuel outlet passage, preferably forms part of the injector's control line in which a change in fuel pressure in the control line caused by the actuator initiates an injection. More preferably, the isolation valve housing includes a secondary, fuel outlet bore extending at one end from the upper housing surface where it is in fluid communication with the actuator of the injector body to the other end where it opens into the second, fuel outlet valve bore downstream of the associated isolation valve seat. Again, a particularly convenient arrangement is one in which the secondary, fuel outlet bore runs substantially parallel to the further, control side valve bore, with an angled portion at its lower end where it opens into the said valve bore above the valve seat.

In order to assist in controlling the flow of the liquefied gas fuel in the injector, and the pressure changes that effect an injection event, the secondary fuel outlet bore (otherwise referred to as the actuator-side control passage) in the isolation valve housing may include a restriction, typically a length of narrowed diameter. Most preferably, the narrowed diameter portion does not extend as far as the upper isolation valve housing surface in order to help reduce differential fuel pressure at the interface of the isolation valve housing and an adjacent component of the injector.

At the lower surface of the isolation valve housing, the first and second valve bores communicate respectively with a part of the fuel supply line that leads into the nozzle and a part of the fuel outlet that leads from the nozzle. In a particularly preferred injector arrangement, the lower isolation valve housing surface abuts the upper surface of a piston housing having first and second passages serving respectively as the fuel supply line to, and fuel outlet from, the nozzle.

Ideally, the upper, open end of each of the first and second piston housing passages overlap respectively with the lower, open end of the first and second valve bores of the isolation valve housing. Advantageously, the diameter of each of the passages, at least at their upper ends, is less than the diameter of the corresponding valve bore such that the upper surface of the piston housing surrounding each of the passages may support the valve seat of its associated isolation valve. Of course, it will be understood that the seat of each isolation valve may alternatively be located below the isolation valve housing, such as on the piston housing.

The first piston housing passage may run parallel to the longitudinal axis of the injector body and at its lower end open directly into the fuel gallery of the nozzle body. The second housing passage on the other hand, preferably extends not directly from the open part of the fuel gallery, but from the upper end of the piston bore. In this way, fuel exiting the fuel gallery is forced across nozzle needle restrictions provided on the nozzle needle projection. The nozzle needle restrictions, along with any restrictions in the secondary fuel outlet bore of the isolation valve housing, serving to control the fall in pressure when the actuator is activated.

To further assist in reducing fuel leakage, the actuator advantageously comprises an actuator valve for controlling injection timing wherein the valve is isolated from the fuel supply line. Rather, the valve is in fluid communication only with the fuel outlet line and hence comprises a statically leakless valve (SLV). Such a valve offers improved performance over conventional control valves hitherto used in fuel injectors in that it is able to eliminate or at least significantly reduce stem leakage, that is, leakage around the pin of the actuator during injection.

Preferably the SLV is housed in an SLV guide housing interposed between the aforementioned isolation valve housing and the injector body. The guide housing comprises a recess in its upper surface for accommodating the armature of the actuator in its activated and deactivated states and a passage extending from the recess through the body of the housing for accommodating and guiding the actuator pin that extends from the armature.

The pin passage preferably communicates at its lower, open end with the upper, open end of the secondary fuel outlet bore in the isolation valve housing, thereby completing a fuel outlet pathway from the nozzle body to the actuator. The pin passage ideally includes a shoulder portion, or other such partial obstruction, that acts as a valve seat for the pin. When the armature is at rest (that is, when it is in its deactivated state), the pin rests on the valve seat and hence closes the valve.

To permit fuel to exit the injector, a fuel escape passage preferably extends from the pin passage downstream of the valve seat to an outlet in the SLV housing, such as an outlet on the housing upper surface. The fuel escape passage may also communicate with the backleak gallery of the injector, being the space that exists between the exterior of the nozzle body, piston guide housing, isolation valve housing and SLV housing, and the cap nut that typically retains these components together.

The guide housing may comprise a fuel supply passage extending between upper and lower SLV housing surfaces, the fuel supply passage at its lower, open end, preferably communicating with the upper, open end of the secondary fuel supply bore in the isolation valve housing. The injector overall may include further measures to improve its ability to prevent or at least substantially reduce the risk of fuel leakage. For example, the isolation valve seat is preferably made from a polyimide material, such as "Vespel"^® (Du Pont), known for its durability in demanding physical environments. Moreover, the injector may include one or more additional seals not usually required when operating with diesel fuels.

Advantageously, an isolation valve seal may be incorporated beneath the isolation valve seat to inhibit leakage across the face between the isolation valve housing and the piston guide housing. Alternatively, or preferably in addition, seals may be provided above and below the nozzle body where the nozzle body abuts the piston housing and the cap nut respectively. Although some face leakage may occur between adjacent components where no additional seal is provided, where the components are above the isolation valve assembly any such leakage will not reach the nozzle.

From a further aspect, the invention resides in a common rail fuel injection system comprising a fuel injector in accordance with the first aspect.

It will be appreciated by the person skilled in the art that all relevant features of the components of the first aspect of the invention may be incorporated within the second aspect of the invention, where appropriate.

The invention also relates to an internal combustion engine having a fuel injector in accordance with the first aspect of the invention therein.

These and other aspects, objects and the benefits of this invention will become clear and apparent on studying the details of this invention and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will further be described, by way of example, with reference to the accompanying drawings, in which:

Figure 2 is a schematic view of a liquefied gas fuel injector in accordance with a first embodiment of the invention; Figure 3 is a schematic view of part of a liquefied gas fuel injector showing an alternative arrangement to that of Figure 2.

DETAILED DESCRIPTION OF THE DRAWINGS

There is shown in Figure 2 a schematic view of an embodiment of the invention, namely a fuel injector that is suitable for incorporation in a common rail fuel injection system of an internal combustion engine that uses a liquefied gas fuel.

The injector has an injector body (140) accommodating an injector control unit that acts on coil winding (108) to control lift of armature (1 14) and hence actuator pin (104) to initiate an injection of fuel.

Liquefied gas fuel is supplied for injection through injector body (140) via fuel supply passage (132) in the valve guide housing (1 10) and then through secondary fuel supply bore (90) in the isolation valve housing (70) where it feeds into first, valve bore (75).

Valve bore (75) communicates with fuel supply passage (42) in piston housing (40) via a first isolation valve (60) composed of an isolation valve pin (62) and a spring (64) that is at least partially housed in a hollow portion of the pin and which urges the pin against isolation valve seat (76). The hollow portion of the pin is in fluid communication with backleak gallery (170).

The fuel supply passage (42) in the piston housing conveys fuel to the fuel gallery (30) of the nozzle body (22) from where it can be injected into a combustion chamber of the engine via nozzle outlet(s) (24).

The nozzle needle is urged by nozzle spring (29) to close the nozzle outlets (24) in the same way as described above with reference to Figure 1. In Figure 2, a second passage in the form of fuel outlet (44) leads fuel from fuel gallery (30) through the piston bore (46) to the second valve bore (71 ) via second isolation valve (60'). The second valve bore (71 ) communicates via angled portion (96) with secondary fuel outlet bore (94) containing a restriction (98) to control flow of fuel. The secondary fuel outlet bore (94) communicates with pin passage (1 16) in the guide valve housing (110) below valve seat (1 18). Above valve seat (118), the pin passage (116) has a wider diameter to receive actuator pin (104) which at its upper end cooperates with armature (114) sitting in recess (1 12). The pin passage (1 16) also communicates with fuel escape passage (130) which in turn communicates with backleak gallery (170) and backleak passage (172).

The valve guide housing (1 10), isolation valve housing (70), piston guide housing (40) and nozzle body (22) are surrounded and held together by means of cap nut (150) having an aperture at its lower end through which the nozzle outlet (24) is exposed and which, at its upper end, fits to the injector body (140).

The injector operates as follows:

Upon starting the engine (not shown), the fuel rail pressure increases and pressure begins to build under the feed-side isolation valve pin (62), thereby partially opening it. The fuel then flows across the restrictions (28) on the nozzle needle (26) and causes pressure to build under the control-side isolation valve pin (62'). Once sufficient pressure is reached, the isolation valve pins (62, 62') rise fully and locate on their top seats (80, 80'). The respective opening pressures for each isolation valve (60, 60') are determined by a combination of the diameters of their valve seats (76, 76') and the preload of their springs (64, 64'). Fuel feed pressure is maintained in the high pressure areas of the injector whilst the SLV valve seat (118) remains closed by the force of the spring (106) above it .

When an injection of fuel is required, the coil winding (108) is activated and the armature (1 14) is pulled against the force of the spring (106) to lift the actuator pin (104) off the valve seat (19). The fuel is then permitted to flow across the valve seat (1 18) and 'spill' through the SLV guide housing (110) via fuel escape passage (130) and out of the injector through backleak passage (172).

The pressure in the control lines formed by secondary valve bore (71 ) and fuel outlet passage (44) then falls to an intermediate pressure, determined by the size of the nozzle needle restrictions (28) and the restriction (98) in the secondary valve bore (71 ) (the "control port restriction"). This intermediate pressure is below rail pressure and above backleak pressure. The resultant force imbalance on the nozzle needle (26) due to the difference between the rail and intermediate pressures causes it to rise, initiating injection. To end the injection, the coil activating current acting on coil winding (108) is removed, causing the actuator pin (104) to locate once again on the valve seat (118). This causes the intermediate pressure in the control lines (71 , 44) to rise back up to rail pressure and the nozzle needle (26) to close. Throughout the injecting and non-injecting phases the isolation valves (60, 60') remain open; the intermediate pressure being sufficient to keep the fuel supply-side valve pin (62) lifted off the seat (76) and the control-side valve pin (62') lifted off the seat (76').

Seals (78, 78') are present adjacent their respective isolation valve seats (76, 76') to prevent fuel leakage across the face between the isolation valve housing (70) and the piston guide housing (40). Additional seals (180a,b,c,d) are located above and below the nozzle body (22). In this way, all leak paths from the fuel feed to the nozzle holes (24) are sealed.

When the engine is switched off, fuel pressure within the injector falls below intermediate pressure. This drop in pressure results in the isolation valves (60, 60') closing, their respective pins (62, 62') being urged against their associated valve seats (76, 76'). Hence only the volume of DME, or other such fuel, contained below the isolation valve pins, that is the fuel that remains within the piston guide housing and nozzle body, is at risk of leakage past the nozzle needle.

Figure 3 illustrates an alternative top seat arrangement to the isolation valve shown in the fuel injector of Figure 2. Specifically, instead of collar portion (82, 82') of the isolation valve pins (62, 62') respectively abutting shoulder portion (80, 80') of first and second valve bores (75, 71 ), the bores have grooves (86, 86') beneath their shoulder portions in which O-ring seals (84, 84') are located and against which the collar portions (82, 82') are urged when the isolation valves are in their open position.

While the invention has been described in terms of preferred embodiments thereof, it is not intended to limit the invention to the precise form disclosed. For example, in an alternative embodiment to that described, the seats for the isolation valves may be located in the injector component immediately below the isolation valve housing, typically the nozzle assembly, and the associated seals may be located above the seats in order to retain the face sealing function. Other variations are also possible. Accordingly, reference should be made to the appended claims and other conceptual statements herein rather than the foregoing specific description as indicating the scope of the invention.

Claims

CLAIMS:

1. A fuel injector for an internal combustion engine operating with a liquefied gas fuel comprising: a nozzle assembly (20) including a nozzle body (22) with a nozzle outlet (24) adjacent its free end for allowing fuel to be injected into a combustion chamber, a nozzle needle (26) axially moveable along a longitudinal recess of the nozzle body (22) to open or close the nozzle outlet (24); a fuel supply passage (2) for supplying pressurised fuel to the nozzle body (22); and an injector body (140) including an actuator (100) for initiating axial movement of the nozzle needle (26) to allow an injection of fuel, characterised in that the fuel supply passage (2) is interrupted by an isolation valve (60) that is maintained in an open position by the fuel pressure when it reaches a predetermined pressure level but closes when the pressure decays below the predetermined pressure level.

2. An injector according to claim 1 , wherein the predetermined pressure level is generated and maintained by the engine when the engine is switched on but decays when the engine is switched off.

3. An injector according to claim 1 , wherein the isolation valve (60) is located at or close to the nozzle end of the injector.

4. An injector according to any one of the preceding claims, wherein a fuel outlet passage (50) of the injector is interrupted by a further isolation valve (60') that is maintained in an open position by the fuel pressure when it reaches a predetermined pressure level but closes when the pressure decays below the predetermined pressure level.

5. An injector according to claim 4, wherein the further isolation valve (60') is located at or close to the nozzle end of the injector.

6. An injector according to claim 5, wherein the isolation valve (60) and further isolation valve (60') are provided in an isolation valve housing (70) intermediate the nozzle assembly (20) and injector body (140).

7. An injector according to claim 6, wherein the isolation valve housing (70) comprises first and second parallel bores (75,71 ) extending therethrough from an upper housing surface (72) to a lower housing surface (74) for accommodating respectively the isolation valve (60) and further isolation valve (60').

8. An injector according to any one of the preceding claims, wherein one or both of the isolation valves (60,60') comprises a linear movement valve.

9. An injector according to claim 8, wherein one or both of the isolation valves (60,60') comprises an isolation valve pin (62,62'), a preloaded spring (64,64') and a valve seat (76,76') against which the pin (62,62') is urged by the spring (64,64') when the fuel pressure within the injector falls below a predetermined intermediate pressure such as when the engine is switched off.

10. An injector according to any one of claims 7 to 9, wherein the first and second parallel bores (75,71 ) have a reduced internal diameter at or towards their upper end in use to provide a shoulder (80,80') serving as a top seat for its associated isolation valve (60,60').

1 1. An injector according to claim 10, wherein each isolation valve comprises an isolation valve pin (62,62') slidable within its respective valve bore (75,71 ) and having a collar portion (82,82') that is urged against shoulder (80,80') when the isolation valve (60,60') is in its fully open position.

12. An injector according to claim 10 or claim 11 , wherein a groove (86,86') is provided beneath the shoulder (80,80) in the first and second parallel bores (75,71 ) for retaining an O-ring (84,84') against which the collar portion (82,82') is urged when the isolation valve (60,60') is in its fully open position.

13. An injector according to any one of claims 9 to 12, wherein the or each isolation valve pin (62,62') is provided with a hollow portion (66,66') open at its upper end to accommodate at least partially a respective isolation valve spring member (64,64'), and wherein the hollow portion (66,66') is in fluid communication with a backleak gallery (170).

14. An injector according to any one of claims 9 to 13, wherein the diameter of the valve seat (76,76') or the spring preload, or both, are selected to dictate the pressure required to open the or each isolation valve (60,60').

15. An injector according to any one of claims 4 to 14, wherein the opening pressure for the isolation valve (60) is different from the opening pressure for the further isolation valve (60').

16. An injector according to any one of claims 7 to 15, wherein the isolation valve housing (70) includes a secondary, fuel supply bore (90) extending at one end from the upper housing surface (72) where it is in fluid communication with the fuel supply passage (2) of the injector body (140) to the other end where it opens into the feed side valve bore (75) upstream of its associated isolation valve seat (76).

17. An injector according to any one of claims 7 to 16, wherein the isolation valve housing (70) includes a secondary, fuel outlet bore (94) extending at one end from the upper housing surface (72) where it is in fluid communication with the actuator (100) of the injector body (140) to the other end where it opens into the fuel outlet valve bore (71 ) downstream of the associated isolation valve seat (76').

18. An injector according to claim 17, wherein the secondary, fuel outlet bore (94) includes a restriction (98) for controlling fuel flow.

19. An injector according to any one of claims 6 to 18, wherein the lower surface (74) of the isolation valve housing (70) abuts an upper surface (41 ) of a piston guide housing

(40) for the nozzle needle (26) and wherein the first and second valve bores (75,71 ) at their lower ends communicate respectively with a fuel supply passage (42) that leads through the piston guide housing (40) into the fuel gallery (30) of the nozzle body (22) and a fuel outlet passage (44) that leads from the fuel gallery (30).

20. An injector according to claim 19, wherein the fuel outlet passage (44) extends from the upper end of a piston bore (46) of the piston guide housing (40) to the lower end of the second valve bore (71 ).

21. An injector according to claim 19 or 20, wherein the diameter of the fuel supply passage (42) and the fuel outlet passage (44), at least at their upper ends, is less than the diameter of the corresponding valve bore (75,71 ) such that the upper surface of the piston guide housing (40) surrounding each of the passages (42,44) supports the valve seat of its associated isolation valve.

22. An injector according to any one of claims 4 to 21 , further comprising a statically leakless valve (SLV) (102) in fluid communication with a fluid outlet line (130) upstream of the further isolation valve (60') and isolated from the fuel supply passage (2).

23. An injector according to claim 22 when dependent on claim 6, wherein the SLV (102) is accommodated in an SLV guide housing (1 10) interposed between the isolation valve housing (70) and the injector body (140).

24. An injector according to claim 23, wherein the SLV guide housing (1 10) comprises a recess (1 12) in its upper surface (120) for accommodating an armature (1 14) of actuator (100) in its activated and deactivated states and a passage (1 16) extending from the recess (1 12) through the body of the housing (1 10) for accommodating and guiding an actuator pin (104) that extends from the armature (1 14).

25. An injector according to claim 24 when dependent on claim 16, wherein pin passage (1 14) communicates at its lower, open end with the upper, open end of the secondary, fuel outlet bore (90) in the isolation valve housing (70).

26. An injector according to claim 24 or claim 25, wherein a fuel escape passage (130) extends from the pin passage (1 14) upstream of an actuator pin valve seat (118) to an outlet in the SLV housing.

27. An injector according to claim 26, wherein the fuel escape passage (130) communicates with a backleak gallery (170) of the injector.

28. An injector according to any one of claims 23 to 28 when dependent on claim 16, wherein the SLV guide housing (1 10) comprises a fuel supply passage (132) extending between upper and lower SLV housing surfaces (120,122), and wherein the fuel supply passage (132) at its lower, open end, communicates with the upper, open end of the secondary, fuel supply bore (90) in the isolation valve housing (70).

29. A common rail fuel injection system comprising an injector as claimed in any one of claims 1 to 28.

30. An internal combustion engine comprising a fuel injector according to any one of claims 1 to 28.