Patent Publication Number: US-6336595-B1

Title: Fuel injector

Description:
TECHNICAL FIELD 
     This invention relates to a fuel injector for use in delivering fuel under pressure to a combustion space of an internal combustion engine. In particular the invention relates to a fuel injector of the electromagnetically actuable type suitable for use in a common rail type fuel system arranged to deliver diesel fuel to a compression ignition internal combustion engine. 
     BACKGROUND OF THE INVENTION 
     A known fuel injector for use in such a fuel system is illustrated in FIG.  1  and comprises a valve needle  10  slidable within a bore  11 . The needle  10  includes a surface exposed to the fuel pressure within a control chamber  12 . The control chamber  12  is supplied with fuel from a supply passage  13  through a restriction  14 , thus fuel is only permitted to flow to the control chamber  12  at a restricted rate. An electromagnetically actuable control valve  15  controls communication between the control chamber  12  and a chamber  16  which communicates with a low pressure drain reservoir through a backleak passage which is common to several injectors. The injector of FIG. 1 will be described in greater detail below. 
     Reliable, consistent operation of the control valve  15  is important to achieve as inconsistencies in the operation of the control valve may result in undesirable variations in the quantity of fuel injected and in the timing of fuel injection. Pressure waves may be transmitted to the control chamber  16  from other identical injectors via the common backleak passage. It will be appreciated that the application of pressure waves to the chamber  16  may impair the performance of the valve. It is an object of the invention to provide an injector in which this disadvantage is obviated or mitigated. 
     SUMMARY OF THE INVENTION 
     According to the present invention there is provided a fuel injector comprising a valve needle slidable within a bore, a surface associated with the needle being exposed to the fuel pressure within a control chamber, an electromagnetically actuable control valve controlling communication between the control chamber and a low pressure chamber, and a damping arrangement arranged to damp pressure waves applied to the low pressure chamber. 
     The damping arrangement conveniently comprises a volume which communicates with the low pressure chamber, the volume containing gas, in use. The gas, for example air, fuel vapour or a mixture thereof, is readily compressible and acts to damp pressure waves applied to the low pressure chamber. 
     The volume is conveniently defined by a blind drilling orientated, in use, with its blind end uppermost to retain the gas therein. The drilling conveniently extends adjacent part of the actuator for the control valve. 
    
    
     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 illustrating a fuel injector; 
     FIG. 2 is a diagrammatic view illustrating part of the injector of FIG. 1; and 
     FIG. 3 is a view similar to FIG. 2 illustrating part of a fuel injector in accordance with an embodiment of the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     As described briefly hereinbefore, the fuel injector illustrated, in part, in FIGS. 1 and 2 comprises a valve needle  10  which is slidable within a bore  11 . The bore  11  takes the form of a blind bore formed in a nozzle body  17 . Adjacent the blind end of the bore  11 , a plurality of outlet openings (not shown) is provided. The bore  11  is shaped to define a seating with which the needle  10  is engageable to control communication between the region of the bore  11  upstream of the seating and the outlet openings. 
     The bore  11  is supplied with fuel under high pressure, in use, through the supply passage  13 , the supply passage  13  being connected to a fuel source in the form of a common rail which, in use, is charged with fuel to a high pressure by a suitable fuel pump. As illustrated in FIG. 1, the supply passage  13  is shaped to define a region  18  of reduced diameter, the region  18  restricting the rate at which fuel is able to flow along the supply passage  13  towards the bore  11 . 
     The nozzle body  17  abuts a distance piece  19  which is shaped to include a recess which defines the control chamber  12 , an upper end surface of the needle  10  being exposed to the fuel pressure within the control chamber  12 . As described hereinbefore, the control chamber  12  communicates with the supply passage  13  through a restriction  14 . A spring  20  is located within the control chamber  12 , the spring  20  acting to apply a biasing force to the needle  10  urging the needle  10  into engagement with the seating. The recess which defines the control chamber  12  is shaped to define an internal projection which serves as a lift stop, controlling the distance through which the needle  10  can lift away from its seating. A drilling  21  is provided within the projection, the drilling  21  communicating through a further drilling  22  with a surface of the distance piece  19  remote from the nozzle body  17 . The end of the distance piece  19  remote from the nozzle body  17  abuts a valve housing  23  which is provided with a through bore  24  within which a valve member  25  is moveable. The valve member  25  forms part of the electromagnetically actuated control valve  15 . The valve member  25  is shaped to include a region of diameter smaller than the adjacent part of the bore  24 , defining an annular chamber which communicates through passages  26  with the drilling  22 , and hence with the control chamber  12 . The valve member  25  includes a region of enlarged diameter which is engageable with a seating adjacent an end of the bore  24  to control communication between the passages  26  and the low pressure chamber  16 . As illustrated in FIG. 2, the low pressure chamber  16  communicates with a return passage  27  provided in an actuator housing  28  which abuts the surface of the valve housing  23  remote from the distance piece  19 . The return passage  27  communicates, in use, with a backleak passage (not shown), the backleak passage being common to all of the injectors associated with the engine, the backleak passage further communicating with a low pressure fuel reservoir, for example a fuel tank. 
     Within the chamber  16 , an armature  29  is provided, the armature  29  being secured to the valve member  25  and moveable under the influence of an electromagnetic actuator  30  located within a bore provided in the actuator housing  28 . The actuator  30  includes a return spring arranged to bias the valve member  25  into engagement with its seating. 
     The lower end of the valve member  25  extends into a chamber  31  defined between the valve housing  23  and the distance piece  19 , the chamber  31  communicating through a passage (not shown) with an appropriate low pressure fuel reservoir. 
     A cap nut  32  is used to secure the nozzle body  17 , the distance piece  19  and the valve housing  23  to the actuator housing  28  in the usual manner. 
     In use, as described hereinbefore, the supply passage  13  is arranged to receive fuel under high pressure, and it will be appreciated that provided the actuator  30  is not energized, and hence the valve member  25  engages its seating, then both the bore  11  and the control chamber  12  will have high pressure fuel applied thereto. The fuel pressure within the bore  11  applies a force to appropriately angled thrust surfaces  10   a  of the needle  10 , urging the needle  10  away from its seating. The action of the fuel upon the thrust surfaces  10   a  is countered by the action of the fuel under pressure within the control chamber  12  and the action of the spring  20 . The fuel pressure within the control chamber  12 , which acts over a relatively large effective area of the needle  12 , in combination with the action of the spring  20 , is sufficient to ensure that the valve needle  10  remains in engagement with its seating. 
     When injection is to commence, the actuator  30  is energized, urging the armature  29  and valve member  25  to move against the action of the spring  35  of the actuator  30 , lifting the valve member  25  away from its seating. As a result, fuel is able to escape from the control chamber  12  to the chamber  16  which, as described hereinbefore, is at relatively low pressure due to its connection with the low pressure reservoir by the return passage  27  and the backleak passage. As fuel is only permitted to flow towards the chamber  12  at a low rate through the restriction  14 , it will be appreciated that the fuel pressure within the control chamber  12  falls, and as a result, the force applied to the needle  10  urging the needle  10  towards its seating also falls. A point will be reached beyond which the fuel pressure acting upon the thrust surfaces  10 a is sufficient to lift the valve needle  10  away from its seating, thus permitting fuel from the bore  11  to flow past the seating to the outlet openings, and into the combustion space with which the injector is associated. 
     During injection of fuel, as fuel is permitted to flow towards the bore  11  at a restricted rate through the restriction  18 , and as fuel is able to escape from the bore  11  by being injected through the outlet openings, it will be appreciated that the fuel pressure within the bore  11  falls, and thus the magnitude of the force urging the valve needle  10  away from its seating is reduced. 
     In order to terminate injection, the actuator  30  is de-energized, the valve member  25  returning into engagement with its seating under the action of the spring of the actuator  30 . As a result, fuel is unable to escape from the control chamber  12  to the low pressure chamber  16 , and as fuel is permitted to flow to the chamber  12  through the restriction  14 , the fuel pressure within the control chamber  12  will rise and thus the force urging the needle  10  into engagement with its seating will rise. A point will be reached beyond which the needle  10  moves into engagement with its seating as a result of the fuel pressure within the control chamber  12  and the action of the spring  20  overcoming the action of the fuel under pressure upon the thrust surfaces  10   a . Once the needle  10  has moved into engagement with its seating, fuel injection terminates. As, during injection, the fuel pressure within the bore  11  falls, it will be appreciated that termination of injection occurs more rapidly than would otherwise be the case. Additionally, it will be appreciated that as the internal projection which serves as a lift stop reduces the volume of the control chamber  12 , repressurisation of the control chamber  12  can be achieved relatively quickly, thereby aiding rapid closure of the valve needle  10 . 
     As discussed hereinbefore, the low pressure chamber  16  is connected to a common backleak passage. As a result, there is risk that the operation of other injectors associated with the engine may result in pressure waves being transmitted along the backleak passage and along the return passage  27  to the low pressure chamber  16 , and the action of the pressure waves upon the valve member  25  and armature  29  may impair the operation of the control valve  15  such that the control valve  15  does not open immediately upon energization of the actuator  30  or in the control valve  15  opening prematurely. Similarly, movement of the valve member  25  into engagement with its seating may be impaired. 
     In accordance with the invention, a damping arrangement is provided in association with the low pressure chamber  16  to damp pressure waves, and hence reduce the risk of the operation of the control valve  15  being impaired. In the embodiment illustrated in FIG. 3, the damping arrangement comprises a volume  33  defined by a blind drilling  34  which extends adjacent the actuator  30  and which communicates with the low pressure chamber  16 . The orientation of the blind drilling  34  is such that, in use, air or fuel vapour, or a mixture thereof, will become trapped within the drilling  34 . The valve is operated in a vertical plane and the drilling is filled continuously by trapping fire bubbles which are separated during the violent fuel depressurisation which occurs within chamber  16 . As such gases are readily compressible, pressure waves transmitted to the chamber  16  along the return passage  27  will be damped to a large extent by the presence of the compressible gases within the volume  33 . The damping of the pressure waves applied to the low pressure chamber  16  reduces the risk of the performance of the control valve  15  being impaired, and as a result, consistent, reliable operation of the injector is more readily achievable. 
     Although in the description hereinbefore, the damping arrangement comprises a blind drilling which extends adjacent the actuator  30 , it will be appreciated that other techniques may be used to damp the application of pressure waves to the low pressure chamber  16 .