Abstract:
A fuel injector of the outwardly opening type comprising a nozzle body provided with a first bore, a valve needle slidable within the bore and engageable with a seating to control the supply of fuel from the bore, first and second control chambers for receiving fuel under pressure and a control valve arrangement for controlling the fuel pressure within the first and second control chambers. The valve needle is moveable in response to a change in fuel pressure in at least one of the first and second control chambers.

Description:
TECHNICAL FIELD 
     The 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 outwardly opening type. 
     BACKGROUND OF THE INVENTION 
     Known fuel injectors of the outwardly opening type include a valve needle, slidable within a bore and engageable with a seating to control the supply of fuel from the bore. The valve needle is moved outwardly of the bore to move the needle away from its seating under the control of a piezoelectric actuator. The distance through which the valve needle is moved is typically controlled by controlling the energization level, and hence the axial length, of a piezoelectric stack. Such an actuation technique is thought to be undesirable as piezoelectric stacks of dimensions suitable for use in such applications are relatively expensive and can be difficult to control. 
     It is an object of the present invention to provide a fuel injector of the outwardly opening type in which the distance moved by the valve needle can be controlled by alternative means. 
     SUMMARY OF THE INVENTION 
     According to the present invention, there is provided a fuel injector of the outwardly opening type comprising a nozzle body provided with a first bore, a valve needle slidable within the bore and engageable with a seating to control the supply of fuel from the bore, first and second control chambers for receiving fuel under pressure and control valve means for controlling the fuel pressure within the first and second control chambers, the valve needle being moveable in response to a change in fuel pressure in at least one of the first and second control chambers. 
     The fuel injector of the present invention therefore uses hydraulic means to control movement of the valve needle. The control valve means can therefore be operated conveniently by means of an electromagnetic actuator arrangement. The cost of the fuel injector is therefore reduced compared to fuel injectors in which valve needle movement is controlled by means of a piezoelectric actuator. Furthermore, it is easier to control movement of the valve needle with greater accuracy. 
     Alternatively, the control valve member may be operated by means of a piezoelectric actuator. 
     In one embodiment of the invention, the control valve means may include a single control valve member having first and second valve seatings, whereby movement of the control valve member away from the first valve seating only causes movement of the valve needle into a first fuel injecting position and movement of the control valve member away from both the first and second valve seatings causes movement of the valve needle into a second fuel injecting position. 
     The valve needle may include first and second fuel outlet passages axially spaced on the valve needle such that, when the valve needle is in the first fuel injecting position, fuel is only discharged through the first outlet passage and, when the valve needle is in the second fuel injecting position, fuel is also discharged through the second outlet passage. In this way, the rate of fuel injection into the engine can be carefully controlled. 
     Conveniently, the fuel injector includes a thrust member, moveable in response to a change in fuel pressure in at least one of the first and second control chambers, the thrust member acting on the valve needle to control valve needle movement. 
     The valve needle may be provided with further outlet passages and the control valve means may be arranged to control movement of the valve needle between first, second and further fuel injecting positions. 
     In an alternative embodiment, the control valve means may include two control valve members for controlling the fuel pressure within the first and second control chambers independently. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be described with reference to the following drawings, in which: 
     FIG. 1 is a sectional view of a fuel injector in accordance with an embodiment of the present invention; and 
     FIG. 2 is an enlarged sectional view of a part of the fuel injector shown in FIG.  1 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The injector illustrated in FIGS. 1 and 2 comprises a nozzle body  10  having a through bore  12  formed therein. The bore  12  is shaped, adjacent its lower end, to define a seating  14 . A valve needle  16  is located within the bore  12 , the needle  16  including, at its lower end, a region  18  of enlarged diameter which is engageable with the seating  14  to control the supply of fuel from the fuel injector. The valve needle  16  is provided with a central bore  22  (shown in dash lines) communicating, through a drilling  22   a , with the bore  12  and with first and second outlet passages  24 , 26  (also shown in dash lines), the first and second outlet passages being axially spaced on the valve needle  16 . Only two outlet passages are shown at each axial position, but additional outlet passage may also be provided at each axial position. 
     In use, fuel is supplied to the bore  12  from a suitable source of fuel under pressure, for example the common rail of a common rail fuel supply system. As the needle  16  moves downwardly away from the seating  14  by an initial, relatively small amount, the first outlet passages  24  are exposed and fuel ejects therefrom. As the needle moves downwardly by a further amount, the second outlet passages  26  become exposed and fuel also ejects therefrom. In this way, the rate of delivery of fuel can be controlled by controlling the extent of movement of the valve needle  16 . 
     The upper end of the valve needle  16  is provided with a screw-thread formation (not shown) which engages a corresponding formation provided on the interior of a spring abutment member  34 . The spring abutment member  34  takes the form of a cylindrical sleeve having an outer diameter slightly smaller than the diameter of the adjacent part of the bore  12 . It will be appreciated that the engagement of the spring abutment member  34  with the corresponding adjacent part of the bore  12  and the engagement between the part of the needle  16 , having the central bore  22 , and the corresponding adjacent part of the bore  12  serves to guide the valve needle  16  for movement along the axis of the bore  12 . 
     The bore  12  defines a step  30  with which a second spring abutment member  28  engages. A compression spring  32  is located between the spring abutment member  34  and the second spring abutment member  28  to bias the valve needle  16  in an upward direction and therefore to bias the enlarged part  18  of the valve needle  16  into engagement with the seating  14 . The spring  32  is housed within a spring chamber  36  defined by an upper part of the bore  12 . Fuel injector housing parts  40   a , 40   b , 40   c  and the nozzle body are provided with drillings to provide a supply passage  38  for fuel. The supply passage  38  provides fluid communication between a suitable source of fuel under pressure (not shown) to the spring chamber  36 . The spring chamber  36  communicates with the bore  12  such that, in use, fuel under pressure can be supplied to the bore  12 . 
     The upper end of the valve needle  16  engages a lower end of a thrust member  42 , the other end of the thrust member  42  engaging a piston  44 . The piston  44  is slidable within a bore  46  provided in the housing part  40   b . The thrust member  42  extends centrally through a chamber  48  defined in the housing part  40   a  and is slidable within a bore  12   a  which is coaxial with the bore  12 . An annular stop member  50  is housed within the chamber  48 , the inner diameter of the stop member  50  being slightly larger than the diameter of the thrust member  42  such that the stop member  50  forms a close fit around the thrust member  42 . A compression spring  47  is also housed within the chamber  48  and serves to bias the stop member  50  in an upwards direction against a seating  51  defined by a part of the lower end-face of the housing part  40   b . When the stop member  50  is in its seated position, there is a substantially fluid tight seal between the housing part  40   b  and the stop member  50 . The lower surface of the stop member  50  and the housing part  40   a  define a first clearance gap  49 . The chamber  48  forms a first control chamber to which fuel is supplied from supply passage  38  through a drilling  60 . 
     The diameter of the thrust member  42  is slightly smaller than the diameter of the adjacent part of the bore  12   a  such that the thrust member  42  fits closely within the bore  12   a . As can be seen most clearly in FIG. 2, as the seal is formed between the stop member  50  and the housing part  40   b , the stop member  50  need not be a close fit with the outer wall of the chamber  48 , thereby simplifying manufacture. 
     A second control chamber  52  is defined by the housing part  40   b , part of the thrust member  42 , the lower end of the piston  44  and the upper surface of the stop member  50 , the thrust member  42  extending centrally through the second control chamber  52  and engaging the piston  44 . As can be seen most clearly in FIG. 2, the upper surface of the stop member  50  and the lowermost end of the piston  44  define a second clearance gap  53  within the second control chamber  52 . 
     A chamber  54  is formed within the piston  44 , the chamber  54  housing a compression spring  56  which serves to bias the piston  44  in a downwards direction. The chamber  54  communicates, via a drilling  58 , with the supply passage  38 . The chamber  54  also communicates with the second control chamber  52  by means of a narrow passage  55  provided by a drilling in the piston  44 . 
     Movement of the piston  44  and the thrust member  42  is controlled by means of a control valve arrangement. The control valve arrangement includes a control valve member  62  slidably mounted within a bore  64  formed in the housing part  40   c . The control valve member  62  is engageable with a first valve seating  66  defined by the bore  64 . The second control chamber  52  communicates, via passages  71  and  70 , with an annular chamber  72  defined by the bore  64  and a reduced diameter region of the control valve member  62 . When the control valve member  62  is moved away from the first seating  66 , fuel can flow from the second control chamber  52 , through passages  71 , 70 , into the annular chamber  72  and past the first seating  66  into chamber  79 . Chamber  79  is connected to a low pressure fuel reservoir (not shown). 
     The control valve member  62  has a region of reduced diameter towards its lowermost end upon which a sleeve  63  is mounted defining a step  65 . Upward movement of the control valve member  62  by a sufficient amount results in the step  65  engaging an annular collar member  67  surrounding the control valve member  62 . The annular collar member  67  is arranged such that it seats against a second seating  68 , defined by seating member  68   a  when the control valve member  62  is in its lowermost position. If the control valve member  62  is moved upwardly by only a small amount, the control valve member  62  lifts away from the first seating  66  but the annular collar member  67  remains seated against the second seating  68 . A spring is provided to bias the annular collar member  67  towards the second seating  68 . Further movement of the control valve member  62  in an upwards direction causes the step  65  to move into engagement with a lower surface of the annular collar member  67 , thereby causing the annular collar member  67  to lift away from the second seating  68 . 
     The first control chamber  48  communicates, via a narrow passage  76  provided in housing part  40   a , with a passage  74  provided in housing parts  40   b ,  40   c . Thus, when the annular collar member  67  is moved away from the second seating  68 , fuel within the first control chamber  48  can flow through passages  76  and  74 , past the second seating  68  and into a chamber  78 . The chamber  78  is in communication with a low pressure fuel reservoir (not shown). The control valve arrangement is preferably actuated by means of an electromagnetic actuator arrangement, only the armature of which is shown. In use, fuel under pressure is supplied through the supply passage  38  to the bore  12 . Prior to the commencement of fuel injection, the control valve member  62  is positioned such that it is seated against the first seating  66  and the annular collar member  67  is seated against the second seating  68 . Fuel supplied through the supply passage  38  also flows into the chamber  54  and, thus, also into the second control chamber  52  via the inlet passage  55 . Fuel also flows into the first control chamber  48  through the inlet passage  60 . The nozzle body  10  and the valve needle  16  are appropriately dimensioned to ensure that, in these circumstances, fuel pressure within the bore  12  acts on the valve needle  16  in such a way that the valve needle  16  is biased in an upwards direction. The valve needle  16  is also biased in an upwards direction by means of the spring  32  in the spring chamber  36 . The upward biasing of the valve needle  16  is countered by the force due to fuel pressure within the chamber  54 . 
     In order to commence fuel injection, the control valve member  62  is operated, by the electromagnetic actuator, such that it moves in an upwards direction away from the first valve seat  66  by a small distance insufficient to move the annular collar member  67 . Fuel within the second control chamber  52  therefore flows through passages  71 , 70 , past the first valve seat  66  to low pressure. Fuel pressure within the second control chamber  52  drops, the passage  55  restricting the rate at which fuel can enter the second control chamber  52 , and as a result the piston  44  moves in a downwards direction due to the force applied by fuel pressure in the chamber  54 . The rate at which fuel flows from the second control chamber  52  is determined by the dimensions of the narrow passage  71 . The movement of the piston  44  is transmitted through the thrust member  42  to the valve needle  16 . 
     When the piston  44  has moved in a downwards direction by an amount equal to the clearance gap  53  it abuts the stop member  50 . The fuel pressure within the first control chamber  48  is still high as the annular collar member  67  is seated against the second valve seat  68 . Thus, although the piston  44  abuts the stop member  50  it does not provide sufficient force to overcome fuel pressure in the first control chamber  48  and to move the stop member  50  away from the seating  51 . The movement of the valve needle  16  results in the enlarged region  18  thereof moving away from the seating  14  and the first outlet passages  24 , but not the second outlet passages  26 , are exposed causing fuel to be ejected from the first outlet passages  24  only. It will therefore be appreciated that fuel injection occurs at a relatively low rate. 
     In order to terminate fuel injection the control valve member  62  is moved back into a position where it is seated against the first valve seating  66 . High fuel pressure is then re-established in the second control chamber  52  by fuel entering through the inlet passage  55 , until the fuel pressure applied to the chamber  54  balances the fuel pressure within the second control chamber  52 . The forces on the valve needle  16  then cause the valve needle  16  to return to the position illustrated, causing the enlarged region  18  to move back against the seating  14 . 
     Alternatively, instead of terminating fuel injection, the control valve member  62  may be operated such that it moves in an upwards direction by a further amount sufficient to lift the annular collar member  67  from the second seating  68 . As described previously, the pressure in the second control chamber  52  reduces as fuel flows through passages  71  and  70  and past the first seating  66 . Additionally, fuel pressure in the first control chamber  48  is reduced as fuel flows through passages  76  and  74  past the second seating  68 , the passage  60  limiting the rate at which fuel can enter the first control chamber  48 . The rate at which fuel flows from the first control chamber  48  is determined by the dimensions of the narrow passage  76 . In such circumstances, the piston  44  moves in a downward direction under the force applied by fuel pressure within the chamber  54 . As the fuel pressure in the first control chamber  48  is reduced, when the piston  44  abuts the stop member  50  it is caused to move away from the seating  51  by an amount equal to the clearance gap  49 . Thus, the thrust member  42  is moved by a further amount in a downwards direction, thereby moving the enlarged region  18  of the valve needle  16  a further distance away from the seating  14 . Movement of the enlarged region  18  away from the seating  14  by this further amount exposes the second outlet passages  26  and therefore fuel is also ejected from the second outlet passages  26 . It will therefore be appreciated that the rate of fuel injection is increased. 
     In order to terminate injection, the control valve member  62  is moved downwardly such that it seats against the first valve seat  66  and the annular collar member  67  seats against the second valve seat  68 . The pressure in the first and second control chambers  48 , 52 , therefore equalises as fuel can no longer pass through the passages  71 , 70  and passage  76 , 74  respectively to low pressure. As the fuel pressures equalise in the first and second control chambers  48 , 52 , the thrust member  42  moves in an upwards direction allowing the enlarged region  18  of the valve needle  16  to move into the seating  14 . Fuel injection is therefore terminated. 
     It will be appreciated that by moving the control valve member  62  upwardly such that the annular collar member  67  moves away from the second seating  68  with the control valve member  62  already lifted away from the first seating  66 , it is possible to move from a first fuel injection rate to a second fuel injection rate at a pre-selected time. The rate at which fuel is injected can therefore be controlled with greater accuracy than is possible with conventional piezoelectric actuators. As illustrated, appropriate shims  44   a , 50   a  or spacers may be used to set the distances through which the valve needle  16  is moved, in use. 
     In an alternative embodiment, a piezoelectric actuator may be used to control the control valve member  62  instead of an electromagnetic solenoid arrangement. The piezoelectric actuator may act directly on the control valve member  62  or may act on the control valve member  62  by means of a hydraulic control arrangement. The movements and forces required to move the control valve member  62  are relatively small compared to known fuel injectors using piezoelectric actuators. Therefore, although the use of a piezoelectric actuator increases the cost of the fuel injector, some advantage is still obtained. In the embodiment of the invention hereinbefore described, the fuel pressure within the first and second control chambers  48 , 52 , is controlled by means of a common control valve arrangement. However, in an alternative embodiment of the invention, the fuel pressure in the second control chamber  52  may be controlled independently using a second control valve arrangement operated by a second electromagnetic actuator. Alternatively, fuel pressure in the second control chamber  52  may be controlled by an external pressure source, for example as described in UK patent application GB 9907565.7. 
     It will be appreciated that the valve needle may be provided with third and further outlet passages occupying different axial positions on the valve needle, with the fuel injector being adapted such that valve needle movement between third and further axial positions can be controlled . It will also be appreciated that the valve needle may take a different form. For example, fuel may be discharged from the fuel injector by passing through a narrow clearance defined between the bore  12  and the valve needle  16 , the extent of movement of the valve needle  16  away from the seating  14  controlling the delivery rate of fuel or the fuel injection characteristics.