Patent Publication Number: US-6209805-B1

Title: Fuel injector

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a fuel injector for use in supplying fuel under pressure to a combustion space of a compression ignition internal combustion engine. In particular, the invention relates to a fuel injector of the type in which the commencement of injection is controlled using an electromagnetic actuator. The invention is particularly suitable for use in a pump/injector arrangement, but it will be appreciated that the invention may be used in other applications. 
     In a known pump/injector arrangement, the commencement of injection is controlled by controlling the fuel pressure within a control chamber, the fuel pressure within the control chamber applying a force to a valve needle urging the needle towards its seating. The fuel pressure within the control chamber is controlled using an appropriate electromagnetically actuated valve. Such an arrangement is relatively complex and difficult to control accurately. 
     SUMMARY OF THE INVENTION 
     According to the present invention there is provided a fuel injector comprising a valve needle biased by a spring towards a seating, and an electromagnetic actuator arrangement arranged to vary the magnitude of the biasing force applied to the needle by the spring. 
     In such an arrangement, the spring is conveniently arranged to apply a sufficiently large biasing force to the needle to ensure that injection does not occur when the actuator is energised to a first energization level. Upon energizing the actuator to a second energization level, the actuator acts against the spring to reduce the magnitude of the biasing force applied to the needle by the spring to a level sufficient to allow movement of the injector needle thus allowing injection to commence. 
     Preferably, the actuator includes an armature carried by a control member, the spring load being transmitted to the needle through the control member. 
     As the fuel injector does not rely upon the operation of a valve to control injection, the number of drillings, bores and other features which must be provided in the injector can be reduced thereby simplifying construction. The fuel injector is further relatively easy to control, thus permitting accurate control of the timing of injection. 
     The invention is particularly suitable for use in a pump/injector arrangement in which the timing of fuel injection relative to the timing of closing a drain valve controls the injection pressure. Clearly, in such an arrangement, the invention permits improved control of the injection pressure. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will further be described, by way of example, with reference to the accompanying drawings, in which: 
     FIG. 1 is a sectional view of a fuel injector in accordance with an embodiment; and 
     FIG. 2 is a view of part of the injector of FIG. 1 to an enlarged scale. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The fuel injector illustrated in the accompanying drawings comprises a nozzle body  10  which is provided with a blind bore  12 . A valve needle  14  is slidable within the bore and is engageable with a seating defined adjacent the blind end of the bore to control communication between the bore  12  and one or more outlet openings which communicate with the bore  12  downstream of the seating. The bore  12  is shaped to define an upper region of diameter substantially equal to the diameter of the adjacent part of the needle  14  which guides the needle  14  for sliding movement in the bore  12 . This part of the bore  12  is shaped to define an annular gallery  16  which communicates with a supply passage  18 . The bore  12  further defines a lower region of enlarged diameter which houses a reduced diameter portion of the needle  14  and defines with the adjacent part of the needle  14 , a chamber from which fuel is supplied, in use, past the seating to the outlet openings. The valve needle  14  is shaped to include a plurality of flutes which define flow paths between the annular gallery  16  and the chamber defined between the lower part of the bore  12  and the adjacent part of the needle  14 . At the intersection between the upper, relatively large diameter part of the needle  14  and the reduced diameter part thereof, a thrust surface is defined which is exposed to the fuel pressure within the chamber. 
     The upper surface of the nozzle body  10  abuts a distance piece  20  which is provided with a through bore into which an end part of the needle  14  extends. A load transmitting member  22  engages the upper part of the needle  14  and is located in a part of the bore of the distance piece  20  of enlarged diameter. Drillings  24  are provided in the distance piece  20 , the drillings  24  communicating with the supply passage  18 . 
     The upper surface of the distance piece  20  abuts the lower end surface of a second distance piece  26  which is provided with drillings  28  communicating with the drillings  24  of the first distance piece  20 . The second distance piece  26  is further provided with a through bore which includes a region of relatively large diameter defining a spring chamber  30 . A control member  32  extends into the spring chamber, the lower end of the control member  32  including an outwardly extending flange  34 , the upper surface of which carries a shim  36 , a helical compression spring being engaged between a step defined at an end of the spring chamber  30  and the upper surface of the shim  36 . The spring  38  biases the member  32  in a downward direction in the orientation illustrated, biasing the lower end surface of the member  32  into engagement with the load transmitting member  22 , hence biasing the valve needle  14  into engagement with the seating. 
     The upper end of the control member  32  defines a step with which a shim  40  engages, the shim acting to locate an armature  42 , a screw-threaded member  44  securing the armature  42  and shim  40  to the member  32 . The armature  42  is moveable under the influence of a magnetic field generated, in use, by a first winding  46  forming part of an actuator arrangement  48  located within an actuator housing  50 . A passage  52  extends through the actuator housing  50 , the passage  52  communicating with the drillings  28 . 
     The upper surface of the actuator housing  50  abuts a valve housing  54  which includes a drilling  56  communicating with the passage  52 . The valve housing includes a through bore  58  within which a valve member  60  is slidable, the valve member  60  including a region which is dimensioned to engage a seating defined by part of the through bore  58 . The through bore  58  and valve member  60  together define an annular chamber  62  located upstream of the seating which communicates through a drilling  64  and a recess  66  formed in the upper surface of the valve housing  54  with the drilling  56 . The lower end of the valve member  60  is secured to an armature  68  by means of a screw-threaded member  70  which engages a screw-threaded part of the valve member  60 . The armature  68  is moveable under the influence of a magnetic field generated, in use, by a second winding  72  forming part of the actuator  48 . 
     A shim  74  is located beneath the screw-threaded member  70 , a helical compression spring  76  being engaged between the shim  74  and the upper surface of the screw-threaded member  44 . 
     The upper surface of the valve housing  54  abuts the lower end of a pump housing  78  which includes a bore  80  within which a pumping plunger  82  reciprocal under the influence of a cam and tappet arrangement, against the action of a return spring  84 . 
     It will be appreciated that the shims  36 ,  40 ,  74  are selected depending upon the intended application of the injector, the shims setting the prestressing of the springs  38 ,  76  and the travel of the control member  32 . 
     In use, whilst the plunger  82  is being withdrawn from the plunger bore  80  under the action of the spring  84 , and with the first and second windings  46 ,  72  of the actuator  48  de-energized, the valve member  60  is biased by the spring  76  away from the seating, thus permitting communication between a source of fuel under low pressure which communicates with a chamber  86  located downstream of the seating and the plunger bore  80 . As a result, fuel flows to the plunger bore  80 , the flow of fuel continuing until the plunger  82  reaches its outermost position. It will be appreciated that during this stage of the operation of the injector, the fuel pressure applied to the valve needle  14 , and in particular to the angled thrust surfaces thereof exposed to the fuel pressure within the bore  12 , is relatively low. The force applied to the valve needle  14  by the application of fuel under pressure is therefore insufficient to lift the valve needle  14  away from its seating, the spring  38  acting to ensure that the valve needle  14  remains in engagement with the seating. 
     Once inward movement of the plunger  82  commences, whilst the actuator  48  remains de-energized, fuel is displaced from the plunger bore  80  past the valve member  60  and seating to the low pressure reservoir. When it is determined that pressurization of fuel should commence, the second winding  72  is energized resulting in movement of the armature  68  towards the winding  72  and bringing the valve member  60  into engagement with the seating. This movement breaks the communication between the plunger bore  80  and the low pressure fuel reservoir, and as fuel is no longer permitted to escape from the plunger bore  80 , continued inward movement of the plunger  82  pressurises the fuel in the plunger bore  80  and passages in communication therewith. During this stage of the operation of the injector, although the fuel pressure applied to the needle  14  increases, the fuel pressure is still insufficient to cause movement of the valve needle away from its seating against the action of the spring  38 . 
     When injection is to commence, the first winding  46  is energized attracting the armature  42  towards the winding  46 . This attractive force is transmitted through the control member  32  to the spring  38 , and it will be appreciated that as a result, the biasing force applied to the needle  14  by the spring  38  is reduced. The reduction in the biasing force applied to the needle  14  is sufficient to permit the valve needle  14  to lift from its seating under the action of the fuel pressure within the bore  12 . Such movement of the needle  14  allows fuel to flow past the seating to the outlet openings, thus commencing injection. 
     In order to terminate injection, the second winding  72  is de-energized, and as a result the valve member  60  lifts away from its seating under the action of the spring  76 . The movement of the valve member  60  permits fuel to escape to the low pressure fuel reservoir, thus permitting a rapid reduction in the fuel pressure within the plunger bore  80  and other passages within the injector. The fuel pressure applied to the needle  14  therefore falls, and as a result of the reduced pressure applied to the needle  14 , the needle  14  returns into engagement with its seating under the action of the spring  38  to terminate injection. If desired, the first winding  46  may also be de-energized when the second winding  72  is de-energized, thus increasing the magnitude of the biasing force applied to the valve needle  14  by the spring  38  at the termination of injection. 
     After termination of injection, continued inward movement of the plunger displaces further fuel to the low pressure reservoir. 
     By ensuring that the attractive force between the first winding  46  and armature  42  rises as rapidly as possible, the timing at which commencement of injection occurs can be controlled relatively accurately, even allowing for slight inaccuracies in the effective area of the valve needle  14  exposed to the fuel pressure within the bore  12  urging the needle  14  away from its seating. As the timing of commencement of injection can be controlled relatively accurately, the injection pressure can also be controlled accurately using the apparatus described hereinbefore. 
     In an alternative mode of operation, rather than energizing the first winding  46  separately for each injection, the first winding  46  may be continuously energized to ensure that injection commences as soon as a predetermined pressure is reached, the predetermined pressure being dependent upon the rate of the spring  38 , the magnitude of the attractive force between the actuator  48  and armature  42 , and the effective area of the valve needle  14  exposed to the fuel pressure within the bore  12 . In this mode of operation, the magnitude of the attractive force between the actuator  48  and the armature  42  can be varied, in use, to vary the pressure at which coramencement of injection occurs. 
     Although in the embodiments described hereinbefore, the invention is incorporated into a pump injector arrangement, it will be appreciated that the invention is also applicable to other types of fuel injector in which the commencement of injection is controlled electronically, the invention being applicable to arrangements both where the timing of commencement of injection is controlled and arrangements in which commencement of injection is to occur when a predetermined pressure is reached.