An accumulator type fuel injector system for an internal combustion engine and a method of operating said system

Abstract

 The invention provides an accumulator type fuel injector system (10) for an i.c. engine comprising an accumulator chamber (12) for containing fuel to be injected via an injector assembly (14). The accumulator chamber (12) is charged with fuel at an elevated pressure via a non-return valve (16) means under control of a solenoid valve means (20) also controlling the injector assembly (14). The arrangement is such that, when the accumulator chamber (12) is being vented via the injector assembly (14), means (30) associated with the chamber (12) subject it to additional venting when the pressure in said chamber (12) has decayed to a predetermined level. Thus, closure of a needle valve (24) and consequently termination of a fuel injection event is determined by fuel pressure which can be varied rather than constructional features of the injector.

Description

[0001] The present invention relates to an accumulator type fuel injector system for an internal combustion engine and a method of operating said system. The system of the present invention is intended particularly, but not exclusively, for use in a diesel fuelled internal combustion engine.

[0002] A prior art accumulator type fuel injector system for an internal combustion (i.c.) engine is disclosed in reissued United States patent specification No. Re 33,270. This system includes a solenoid valve means for controlling the admission of fuel supplied at an elevated pressure into an accumulator chamber via a non-return valve means, said non-return valve means opening in response to the opening of the solenoid valve means when the pressure of the supplied fuel exceeds the residual pressure in the accumulator chamber so that fuel at the elevated pressure is admitted into the chamber, and closing in response to the closing of the solenoid valve means relieving pressure on the non-return valve means, and an injector assembly responsive to the closing of the non-return valve means for causing an injection of a fuel charge, said injection being powered by the pressure of fuel in the accumulator chamber. In this system, injection is terminated by a needle valve of the injector assembly when the pressure of the fuel in the accumulator chamber decays to the level of a threshold pressure, i.e. a needle valve closing pressure, of the injector assembly.

[0003] A number of problems are encountered with the prior art injector system. For example, since the injection of a fuel charge is powered by the pressure of fuel in the accumulator chamber, and during injection, the pressure of the fuel in said chamber decays, the final part of this decay can result in poorly atomised low pressure fuel being injected via an injector nozzle of the injector assembly into a combustion chamber of the engine. Poor atomisation of injected fuel can compromise the complete and controlled combustion process needed for maintaining low levels of pollutants in gas exhausted from the engine and efficient fuel consumption.

[0004] In the prior art system, the needle valve closing pressure is normally determined by the loading of a spring which acts to close the needle valve against an opposing force exerted on it by fuel in the accumulator chamber. Despite the fact that injector assemblies in an engine are constructed to very precise tolerances, some variability in their construction exists such that the needle valve closing pressures for the assemblies may have different levels, albeit that the differences between the levels are relatively small. The needle valve closing pressure for each injector assembly is therefore dependent on factors relating to the construction of that assembly including the loading of the needle valve closure spring. Variability in construction of injector assemblies can therefore result in variability in fuel delivery for a given fuel supply pressure. It is desirable to have a system in which needle valve closure for any injector assembly is determined by means which are independent of the construction of the assembly.

[0005] In the prior art system, for fuel injection to occur, the pressure of fuel in the accumulator chamber must exceed an opening pressure of the needle valve. However, since the accumulator chamber must have a volume sufficiently large to contain a fuel charge for maximum power engine operation, the amount of fuel which must be admitted to the chamber to cause injection when the engine is to operate at an idle speed is greater than the amount actually required to attain engine idle speed. Consequently, the engine idle speed is higher than desired and this constitutes inefficient fuel consumption under these conditions.

[0006] It is an object of the present invention to obviate and mitigate the aforesaid problems of the prior art accumulator type fuel injector system.

[0007] According to a first aspect of the present invention, there is provided an accumulator type fuel injector system for an i.c. engine comprising an accumulator chamber for containing fuel to be injected via an injector assembly, the accumulator chamber being charged with fuel at an elevated pressure via a non-return valve means under control of a solenoid valve means also controlling the injector assembly, the arrangement being such that when the accumulator chamber is being vented via the injector assembly, means associated with the chamber subject it to additional venting when the pressure in said chamber has decayed to a predetermined level.

[0008] According to a second aspect of the present invention, an accumulator type fuel injector system for an i.c. engine in accordance with the present invention is characterised over the prior art system in that it includes means arranged to vent fuel from said accumulator chamber when the pressure of fuel in said chamber has decayed to a predetermined level during injection of the fuel charge.

[0009] The system may include a pressure intensifying means to elevate the pressure of fuel supplied to the accumulator chamber by several times the pressure of fuel supplied to the intensifying means from a fuel source.

[0010] The means for venting fuel from the accumulator chamber during injection of the fuel charge may include means for returning vented fuel to the fuel source.

[0011] Alternatively, said means may carry the vented fuel to the injector assembly to assist in closing a needle valve of the injector assembly which, on closing, ends injection of the fuel charge.

[0012] The fuel venting means may comprise a non-return valve means which seals a port of the accumulator chamber and a push rod means which is actuated to cause a push rod member to engage the non-return valve means to open it in order to allow fuel to be vented from said accumulator chamber via said port when the pressure of fuel in said chamber has decayed to a predetermined level during injection of the fuel charge.

[0013] Said push rod means may comprise the armature of a second solenoid valve means which can be controlled to cause the push rod member to disengage the non-return valve means of the fuel venting means.

[0014] Alternatively, the push rod means may comprise a piston means which is actuated to cause the push rod member to engage the non-return valve means to open it in order to allow fuel to be vented from the accumulator chamber.

[0015] The non-return valve means of the fuel venting means is preferably the non-return valve means of the accumulator chamber for admitting fuel at an elevated pressure to said chamber.

[0016] Alternatively, said fuel venting means may comprise a poppet-type valve means or any other suitable valve means.

[0017] The fuel venting means may be spring actuated, wherein the loading of a spring determines the pressure level in the accumulator chamber at which said means is actuated to vent fuel from said chamber.

[0018] The loading of the spring may be adjustable. Alternatively, said fuel venting means may be actuated by fluid supplied at a pressure from a fluid pressure means, whereby the predetermined pressure in the accumulator chamber at which said venting means is actuated is dependent on the supplied pressure of the fluid or is dependent on the sum of the supplied pressure of said fluid and the loading of a spring also acting to actuate the fuel venting means.

[0019] The pressure of the supplied fluid may be variable.

[0020] The supplied fluid may be fuel which may be supplied to said fluid pressure means from an engine fuel source.

[0021] The injector system may include an electronic control system for controlling the pressure level of fluid supplied to actuate the fuel venting means, wherein, for each successive injection of a fuel charge, the control system predetermines a pressure level of fuel in the accumulator chamber at which the fuel venting means is to be actuated for a respective injection event.

[0022] According to a third aspect of the present invention, there is provided an i.c. engine including an accumulator type fuel injector system according to the next sixteen preceding paragraphs.

[0023] According to a fourth aspect of the present invention, there is provided a method of operating an accumulator type fuel injector system for an i.c. engine comprising supplying fuel at an elevated pressure to an accumulator chamber, controlling a solenoid valve means to open and close to admit fuel supplied at the elevated pressure into the accumulator chamber via a non-return valve means, said non-return valve means opening in response to the opening of the solenoid valve means when the pressure of the supplied fuel exceeds the residual pressure in the accumulator chamber, and closing in response to the closing of the solenoid valve means relieving pressure on the non-return valve means, wherein an injector assembly injects a fuel charge in response to the closing of the non-return valve means, said injection being powered by the pressure of fuel in the accumulator chamber, and means are arranged to vent fuel from the accumulator chamber when the pressure of fuel in said chamber has decayed to a predetermined level during the injection of the fuel charge.

[0024] Further features of the method of the invention relate to dependent features of the system of the invention.

[0025] The foregoing and further features of the present invention will be more readily understood from the following description of preferred embodiments, by way of example thereof, with reference to the accompanying drawings, of which:

Figure 1 is a diagrammatic representation of a prior art accumulator type fuel injector system for an internal combustion (i.c.) engine;

Figure 2 is a diagrammatic representation of an accumulator type fuel injector system in accordance with a first embodiment of the present invention;

Figure 3 is a graph illustrating decay of fuel pressure in an accumulator chamber during fuel injection;

Figure 4 is a graph illustrating the flow rate of fuel through an injector assembly;

Figure 5 is a diagrammatic representation of an accumulator type fuel injector system in accordance with a second embodiment of the present invention; and

Figure 6 is a diagrammatic representation of an accumulator type fuel injector system in accordance with a third embodiment of the present invention.

[0026] Referring firstly to figure 1, it can be seen that a prior art accumulator type fuel injector system 10 comprises an accumulator chamber 12 for containing fuel to be injected into a combustion chamber (not shown) of an engine (not shown) via an injector assembly 14. The accumulator chamber 12 is charged with fuel at an intensified (elevated) pressure via a non-return valve 16. The fuel is supplied from a variable pressure fuel source 18 under control of a solenoid valve 20. The pressure of the fuel supplied by the fuel source 18 is intensified by a pressure intensifier 22 which elevates the pressure of fuel supplied to the accumulator chamber 12 by several times the pressure of fuel supplied from the fuel source 18.

[0027] This prior art system is an electronically controlled fuel injector system 10 in which the timing of the injection of a fuel charge into an engine combustion chamber is not fixed relative to the rotational position of the camshaft or any other engine component. Thus, this system provides an advantage in that fuel can be injected into an engine combustion chamber at any time during the engine cycle.

[0028] In operation, an electrical control system (not shown) supplies an electrical signal to the solenoid valve 20 a portion of the engine cycle in advance of the time when injection of a fuel charge is to be commenced. In response to the electrical signal, the solenoid valve admits fuel from the fuel source 18 to the pressure intensifier 22. The intensifier 22 supplies fuel at an intensified pressure to the accumulator chamber 12. Fuel flows through the non-return valve 16 into the accumulator chamber 12. Fuel continues to flow into the accumulator chamber 12 until a pressure balance is reached in the intensifier 22 or the solenoid valve 20 shuts, whichever is earlier. Under control of the electrical control system, the solenoid valve 20 shuts off the flow of fuel to the intensifier 22. The presence of fuel at an intensified pressure on an intensifier side of the non-return valve 16 of the accumulator chamber 12 is discontinued. The pressure of fuel on the intensifier side of the non-return valve 16 reduces to the pressure of a fuel rail to which the intensifier vents on closing of the solenoid valve. The pressure in the accumulator chamber 12 now exceeds the pressure on the intensifier side of the non-return valve 16 which shuts to seal the accumulator chamber 12. Whilst the solenoid valve 20 is open, fuel at the intensified pressure is applied to a needle valve 24 of the injector assembly 14 in order to maintain the needle valve 24 in a closed position to prevent fuel injection when the accumulator chamber 12 is being charged with fuel. Due to the release of intensified fuel pressure on the intensifier side of the non-return valve 16 and thus on the needle valve 24, simultaneously with the closing of the non-return valve 16 injection of a fuel charge into the engine combustion chamber via the injector assembly commences. The injection of the fuel charge is powered by the pressure of fuel within the accumulator chamber 12 and injection will continue until such time as the pressure in the chamber 12 decays to a level equal to a needle valve closing pressure. Thus, in this case, end of injection is generally determined by the loading of an injector assembly spring 26 which acts to close the needle valve 24.

[0029] The mass of a fuel charge admitted to the accumulator chamber 12 is dependent on the volume of the chamber 12 and the level of intensified pressure at which said fuel is admitted. Consequently, since the volume of the chamber is fixed, the masses of successive fuel charges admitted to the chamber 12 is controlled by varying the level to which fuel from the fuel source 18 is intensified by the intensifier 22.

[0030] The volume of the accumulator chamber 12 must be sufficiently large to contain a fuel charge necessary for maximum power engine operation. A consequence of this is that the minimum mass of fuel required to be admitted to the accumulator chamber to cause injection is greater than an amount required for idle engine operation. Thus the engine operates at a higher than desired engine idle speed. This problem could be overcome by reducing the needle valve closing pressure, but a problem already exists with a system of this type in that, for a latter portion of the injection period when the pressure of fuel in the accumulator chamber has substantially decayed, fuel being injected is poorly atomised resulting in poor combustion of fuel. The needle valve closing pressure is set at a value which is a compromise between ensuring good atomisation of fuel toward the end of injection of a fuel charge and achieving a large turn down ratio, i.e. the ratio of the quantity (mass) of fuel injected per fuel charge at maximum engine power over the quantity of fuel per charge injected at lowest engine power, i.e. idle engine operation.

[0031] Figure 2 is a diagrammatic representation of a first embodiment of an accumulator type fuel injector system in accordance with the present invention. In the following description of this embodiment like numerals will be used to designate like parts as in the prior art system of figure 1.

[0032] This embodiment of the present invention is characterised over the prior art system in that it includes means 30 for venting fuel from the accumulator chamber 12 when the pressure of fuel in the chamber has decayed to a predetermined level during injection of a fuel charge. The means 30 for venting fuel comprises a non-return valve 32 which seals a port 34 of the accumulator chamber 12 and a push rod means 36. The push rod means comprises the armature 38 of a second solenoid valve 40.

[0033] In operation, fuel is admitted to the accumulator chamber 12 under control of the (first) solenoid valve 20 in a same manner to that of the prior art system. Fuel supplied to the accumulator chamber 12 has its pressure intensified by the intensifier 22 and, during admission of fuel into the accumulator chamber 12, fuel at the intensified pressure is applied to the needle valve 24 of the injector assembly 14 to maintain it in its closed position. Similarly to the prior art system, closing of the first solenoid valve 20 releases the intensified fuel pressure from the non-return valve 16 and the needle closing valve 24, simultaneously causing closing of the non-return valve 16 to seal the accumulator chamber 12 and opening of the needle closing valve 24 of the injector assembly 14 to commence injection of a fuel charge. Unlike the prior art system, injection of the fuel charge is brought to an end by actuation of the fuel venting means 30 at an accumulator fuel pressure level greater than the needle valve closing pressure (i.e. threshold pressure) of the injector assembly 14.

[0034] A push rod member 42 of the armature member 38 of the fuel venting means 30 engages the non-return valve 32 of said means 30 to cause it to open and allow fuel to be vented from the accumulator chamber 12. The pressure level at which this occurs is determined by the loading of a spring 44 which acts on said armature member 38 to urge it in a direction towards the non-return valve 32. During a first portion of injection of a fuel charge, the non-return valve 32 of the fuel venting means 30 is maintained closed by the pressure of fuel within the accumulator chamber 12 which exerts a force on the push rod member 42 greater than the force applied to said push rod member 42 by the spring 44. However, when the pressure of fuel in the chamber 12 decays to a level where it exerts a force less than that of the spring 44, the spring 44 causes the push rod member 42 to open the non-return valve 32. Fuel vented from the accumulator chamber 12 causes a very rapid decrease in pressure within the chamber 12 which decreases rapidly to the threshold pressure of the injector assembly 14 at which point the needle valve 24 closes to terminate the injection of the fuel charge.

[0035] The loading of the spring 44 of the fuel venting means 30 is dependent on the characteristics of the spring 44 but may be adjusted by the inclusion of pre-load shims 46. Alternatively, the loading of the spring 44 may be adjusted by means of an adjusting screw 48 accessible from the exterior of the accumulator system. Thus, in an i.c. engine employing a number of injector systems in accordance with the invention, the accumulator pressure at which each fuel venting means 30 will actuate can be adjusted to be identical for all systems and thus eliminate variabilities resulting from construction factors.

[0036] Fuel vented from the accumulator chamber 12 by the fuel venting means 30 is returned to the fuel source 18 by a fuel passageway 50. Alternatively, vented fuel may be carried by a passageway 52 (shown in broken outline in figure 2) to the injector assembly 14 such that said vented fuel acts on the needle valve 24 to more rapidly bring to an end the injection of a fuel charge.

[0037] It will be recognised that, since the pressure in the accumulator chamber 12 will drop below the pressure at which the fuel venting means actuates, the non-return valve 32 of the fuel venting means 30 will remain open on the subsequent admission of fuel to the accumulator chamber 12.

[0038] Accordingly, the second solenoid valve 40 is controlled to withdraw the armature member 38 such that the push rod member 42 disengages the non-return valve 32 prior to or on the subsequent admission of fuel to the accumulator chamber 12. Thus, the return valve 32 of the fuel venting means 30 will reset to seal the port 34 of the accumulator chamber 12. The second solenoid valve 40 is controlled to release the armature member 38 at some point after the pressure of fuel in the accumulator chamber 12 has risen above the actuation pressure of the fuel venting means 30 but, of course, at a point in time before it is desired to terminate the injection of a fuel charge.

[0039] Figure 3 illustrates the fuel pressure in the accumulator chamber 12 against time for the prior art system versus a system in accordance with the first embodiment of the present invention.

[0040] The solid line curve represents fuel pressure in the accumulator chamber 12 for a maximum fuel charge admitted to the chamber 12. It can be seen that, on commencement of injection, the pressure of fuel in the chamber 12 drops rapidly and, in the prior art system, is terminated (point A) when said pressure reaches the threshold pressure P_T, i.e. the needle valve closing pressure of the injector assembly 14. It will be appreciated that, during a latter portion (B) of the injection of a fuel charge, the pressure driving injection is at a generally low level and this can result in poor atomisation of injected fuel.

[0041] The broken line curve in figure 3 represents fuel pressure in the accumulator chamber 12 for a minimum admitted fuel charge. For the prior art system, the amount of fuel injected (represented by the area (X+Y) under the curve) is greater than the amount required for idle speed engine operation. In addition, the pressure at which said fuel is admitted is relatively low over a greater portion of the period of injection thus resulting in poor atomisation of injected fuel during said portion of the period of injection.

[0042] The present invention enables the pressure at which injection is ended to be predetermined by a means other than the injector assembly 14. It will be seen from the graph that, for a maximum fuel charge, injection is ended (point C) when the pressure in the accumulator chamber 12 decays to an actuation pressure (P_A) of the fuel venting means 30. While the mass of fuel injected is less than for the equivalent prior art system, the average pressure over the injection period is substantially greater providing better atomisation of fuel. The mass of a maximum fuel charge to be injected can, of course, be easily rectified by providing a system with an increased size accumulator chamber 12.

[0043] At idle speed, the amount of fuel (X) injected in a system according to the present invention can be set to be exactly that required for a desired engine idle speed. Similarly to the case of the maximum fuel charge injection, the average pressure over the period of injection is greater in this system than in the prior art system.

[0044] Figure 4 is a graph illustrating the flow rate of fuel through an injector assembly for the situations described with reference to figure 3.

[0045] Figure 5 is a diagrammatic representation of a second embodiment of an accumulator type fuel injector system in accordance with the present invention. Once again, like numerals designate like parts. This is similar to the first embodiment insofar that it includes a spring actuated means 30 for venting fuel from the accumulator chamber 12 when the fuel pressure in said chamber 12 decays to a level determined by a spring 44 during injection of a fuel charge. However, this embodiment differs from the first embodiment insofar that the return valve of the fuel venting means 30 is the return valve 16 of the accumulator chamber 12 for admitting fuel at an intensified pressure to said chamber 12. Thus, this embodiment has a much simpler construction to that of the first embodiment.

[0046] In operation, fuel at an intensified pressure is admitted to the accumulator chamber 12 via the non-return valve 16 in a manner the same as that in the prior art system and the system of the first embodiment of the present invention. Prior to or on actuation of the first solenoid valve 20, the second solenoid valve 40 is actuated to disengage the push rod member 42 from the non-return valve 16. Thus, the push rod member 42 will not interfere with fuel flow into the accumulator chamber 12 prior to injection of a fuel charge. The second solenoid valve 40 is controlled to release the push rod member 42 to engage the non-return valve 16 in time for it to operate to terminate injection of a fuel charge at the fuel venting means actuation pressure. In this embodiment, vented fuel is directed to the injector assembly 14 to assist in a more rapid closing of the needle valve member 24.

[0047] Figure 6 is a diagrammatic illustration of a third embodiment of the present invention. In this embodiment, the return valve of the fuel venting means 60 is the return valve 16 for admitting fuel at an intensified pressure to the accumulator chamber 12. This embodiment differs, however, from the first and second embodiments insofar that the fuel venting means 60 comprises a piston member 62 reciprocally mounted within a housing 64. The piston member 62 can be actuated by a spring 44 and/or by a pressurised fluid acting on an inner end face 66 thereof.

[0048] In the case where the piston member 62 is spring actuated, the housing 64 on the inner end face side of the piston member 62 need only be exhausted to atmosphere. Thus, when the pressure of fuel in the accumulator chamber 12 decays to an actuation pressure of the fuel venting means 60 during injection of a fuel charge, the piston member 62 is advanced by the spring 44 to cause the push rod member 42 to engage the return valve 16 to open it to vent fuel from the chamber 12 in order to end the injection of the fuel charge. Vented fuel is directed out of the chamber 12 to the injector assembly 14 to assist the needle valve closure spring 26 to more rapidly close said needle valve 24. Once again, the loading of the spring 44 may be adjustable by the insertion of pre-load shims 46 or by means of a screw adjuster 48 accessible from an exterior of said means 60. During admission of fuel to the accumulator chamber 12, fuel at the intensified pressure acts on an outer end face 68 of the piston member 62 to cause it to compress the spring 44 to disengage the push rod member 42 from the return valve 16.

[0049] Pressurised fluid may be supplied to the housing 64 to act on the inner end face 66 of the piston member 62 in order to determine, in the absence of or in addition to the spring 44, the actuation pressure of the fuel venting means 60. In one arrangement, the housing 64 at an inner end face, side of the piston member 62 communicates with a passageway 70 linking it to the variable pressure fuel source 18 such that fuel at the fuel supply pressure is supplied to the housing 64 to act on the inner end face 66 of the piston member 62. Thus, when the solenoid valve 20 closes, the pressure of fuel in said line 70 counteracts the rail pressure of the fuel remaining in front of the piston member 62 between it and the return valve 16.

[0050] In broken outline in figure 6 is shown a pressure supply means 72 which allows fuel from an engine supply 74 to be supplied to the housing 64 on the inner end face side of the piston member 62 at an elevated pressure. The level of this pressure may be adjustable and may be under control of an engine electronic control system which allows the level to be predetermined for each injection event. Thus, when fuel has been admitted to the accumulator chamber 12 and the return valve 16 has closed said chamber 12 to commence injection, fuel at an elevated pressure can be supplied from the pressure supply means 72 to set the actuation pressure of the fuel venting means 60. The pressure supply means 72 is preferably a pump.

[0051] The advantage of this arrangement is that termination of injection of a fuel charge can be predetermined in advance in response to measured engine parameters. It also has the advantage that, by a combination of the control of the pressure at which fuel is admitted to the accumulator chamber 12 and the pressure preselected for actuation of the fuel venting means 60, the quantity of a fuel charge to be injected can be predetermined as can its rate of injection. For example, under some circumstances, it may be desired to have high speed fuel injection at low engine power operation. Thus, fuel at a relatively high intensified pressure level may be admitted to the accumulator chamber 12 where the quantity of fuel actually admitted would, under normal conditions, represent high engine power operation. However, by selecting a relatively high actuation pressure for the fuel venting means 60, after injection is started, it can rapidly be brought to an end and thus only a small portion of the admitted fuel is injected as a fuel charge. Consequently, a low fuel charge is injected at a rapid rate.

[0052] It will be understood that the described embodiments of the invention are representative of the scope of the invention and that other embodiments incorporating a combination of features from the described embodiments are readily realisable.

Claims

1. An accumulator type fuel injector system for an i.c. engine comprising an accumulator chamber for containing fuel to be injected via an injector assembly, the accumulator chamber being charged with fuel at an elevated pressure via a non-return valve means under control of a solenoid valve means also controlling the injector assembly, the arrangement being such that when the accumulator chamber is being vented via the injector assembly, means associated with the chamber subject it to additional venting when the pressure in said chamber has decayed to a predetermined level.

2. An accumulator type fuel injector system for an i.c. engine comprising a solenoid valve means for controlling the admission of fuel supplied at an elevated pressure into an accumulator chamber via a non-return valve means, said non-return valve means opening in response to the opening of the solenoid valve means when the pressure of the supplied fuel exceeds the residual pressure in the accumulator chamber so that fuel at the elevated pressure is admitted into the chamber, and closing in response to the closing of the solenoid valve means relieving pressure on the non-return valve means, and an injector assembly responsive to the closing of the non-return valve means for causing an injection of a fuel charge, said injection being powered by the pressure of fuel in the accumulator chamber, wherein it includes means arranged to vent fuel from said accumulator chamber when the pressure of fuel in said chamber has decayed to a predetermined level during injection of the fuel charge.

3. A system as claimed in claim 2, wherein it includes a pressure intensifying means to elevate the pressure of fuel supplied to the accumulator chamber by several times the pressure of fuel supplied to the intensifying means from a fuel source.

4. A system a claimed in claim 3, wherein the means for venting fuel from the accumulator chamber during injection of the fuel charge includes means for returning vented fuel to the fuel source.

5. A system as claimed in claim 2 or claim 3, wherein the means for venting fuel carries the vented fuel to the injector assembly to assist in closing a needle valve of the injector assembly which, on closing, ends injection of the fuel charge.

6. A system as claimed in any one of claims 2 to 5, wherein the fuel venting means comprises a non-return valve means which seals a port of the accumulator chamber and a push rod means which is actuated to cause a push rod member to engage the non-return valve means to open it in order to allow fuel to be vented from said accumulator chamber via said port when the pressure of fuel in said chamber has decayed to a predetermined level during injection of the fuel charge.

7. A system as claimed in claim 6, wherein the push rod means comprises the armature of a second solenoid valve means which is controlled to cause the push rod member to disengage the non-return valve means of the fuel venting means.

8. A system as claimed in claim 6, wherein the push rod means comprises a piston means which is actuated to cause the push rod member to engage the non-return valve means to open it in order to allow fuel to be vented from the accumulator chamber.

9. A system as claimed in any one of claims 2 to 8, wherein the non-return valve means of the fuel venting means is the non-return valve means of the accumulator chamber for admitting fuel at an elevated pressure to said chamber.

10. A system as claimed in any one of claims 2 to 8, wherein the fuel venting means comprises a poppet-type valve means or any other suitable valve means.

11. A system as claimed in any one of claims 2 to 10, wherein the fuel venting means is spring actuated and the loading of a spring determines the pressure level in the accumulator chamber at which said means is actuated to vent fuel from said chamber.

12. A system as claimed in claim 11, wherein the loading of the spring is adjustable.

13. A system as claimed in any one of claims 2 to 10, wherein the fuel venting means is actuated by fluid supplied at a pressure from a fluid pressure means and the predetermined pressure in the accumulator chamber at which said venting means is actuated is dependent on the supplied pressure of the fluid or is dependent on the sum of the supplied pressure of said fluid and the loading of a spring also acting to actuate the fuel venting means.

14. A system as claimed in claim 13, wherein the pressure of the supplied fluid is variable.

15. A system as claimed in claim 13 or claim 14, wherein the supplied fluid is fuel which is supplied to said fluid pressure means from an engine fuel source.

16. A system as claimed in any one of claims 2 to 15, wherein the injector system includes an electronic control system for controlling the pressure level of fluid supplied to actuate the fuel venting means, wherein, for each successive injection of a fuel charge, the control system predetermines a pressure level of fuel in the accumulator chamber at which the fuel venting means is to be actuated for a respective injection event.

17. An i.c. engine including an accumulator type fuel injector system according to any one of claims 1 to 16.

18. A method of operating an accumulator type fuel injector system for an i.c. engine comprising supplying fuel at an elevated pressure to an accumulator chamber, controlling a solenoid valve means to open and close to admit fuel supplied at the elevated pressure into the accumulator chamber via a non-return valve means, said non-return valve means opening in response to the opening of the solenoid valve means when the pressure of the supplied fuel exceeds the residual pressure in the accumulator chamber, and closing in response to the closing of the solenoid valve means relieving pressure on the non-return valve means, wherein an injector assembly injects a fuel charge in response to the closing of the non-return valve means, said injection being powered by the pressure of fuel in the accumulator chamber, and means are arranged to vent fuel from the accumulator chamber when the pressure of fuel in said chamber has decayed to a predetermined level during the injection of the fuel charge.

Drawing