Patent Publication Number: US-6220528-B1

Title: Fuel injector including an outer valve needle, and inner valve needle slidable within a bore formed in the outer valve needle

Description:
TECHNICAL FIELD 
     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. 
     BACKGROUND OF THE INVENTION 
     In order to reduce emissions levels, it is known to provide fuel injectors in which the total area of the openings through which fuel is delivered can be varied, in use. One technique for achieving this is to use two valve needles, one of which is slidable within a bore provided in the other of the needles to control the supply of fuel to some of the outlet openings independently of the supply of fuel to others of the outlet openings. 
     Such arrangements have the disadvantages that fuel may be able to flow between the inner and outer needles giving rise to substantially continuous delivery of fuel at a low rate. Further in order to control the movement of the inner and outer needles, separate actuators may be required resulting in the injector being of increased complexity. 
     SUMMARY OF THE INVENTION 
     According to a first aspect of the invention there is provided a fuel injector comprising an outer valve needle, an inner valve needle slidable within a bore formed in the outer valve needle, an inner end of the inner valve needle being located within the bore, the inner end of the inner valve needle being provided with a recess whereby the application of fuel under pressure to the bore deforms the inner valve needle to form a substantially fluid tight seal between the inner and outer valve needles. 
     Such an arrangement is advantageous in that leakage and fuel delivery at undesirable points in the engine operating cycle can be reduced or avoided. 
     According to another aspect of the invention there is provided an injector comprising an outer valve needle and an inner valve needle, the inner needle being slidable within a bore formed in the outer needle, the inner and outer needles being exposed to the fuel pressure within a control chamber, and a single actuator controlling the fuel pressure within the control chamber. 
     The actuator may take the form of an electromagnetically actuated valve, or alternatively may comprise a piston moveable by a piezoelectric actuator. 
     Such an arrangement permits independent control of the inner and outer valve needles using a single actuator, movement of the inner and outer needles being dependent upon the pressure differential between the upper and lower ends thereof, the effective cross sectional areas exposed to fuel under pressure and the effect of any spring biasing. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will further be described, by way of example, with reference to the accompanying drawings, in which like reference numerals are used to denote like parts, and in which: 
     FIG. 1 is a sectional view of part of an injector in accordance with an embodiment; 
     FIG. 2 is a view, to an enlarged scale, of part of the injector of FIG. 1; 
     FIGS. 3 and 4 are views similar to FIGS. 1 and 2 illustrating an alternative embodiment; and 
     FIG. 5 is a view similar to FIG. 2 illustrating a further embodiment. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The fuel injector illustrated in FIGS. 1 and 2 comprises a nozzle body  10  provided with a blind bore  12 . Adjacent the blind end of the bore, the bore  12  is shaped to define a seating of substantially frusto-conical shape. An outer valve needle  14  is slidable within the bore  12 , the outer valve needle  14  defining, adjacent its lower end, a region of substantially frusto-conical shape arranged to engage the frusto-conical seating to control the supply of fuel from the bore  12  to a first group of outlet openings  16 . 
     The upper end of the outer valve needle  14  is shaped to be of diameter substantially equal to the diameter of the adjacent parts of the bore  12  to form a substantially fluid tight seal therewith and to guide the outer valve needle  14  for sliding movement in the bore  12 . As illustrated in FIG. 1, the outer valve needle  14  further includes a lower region of smaller diameter, the relatively large diameter upper region and the lower, small diameter region together defining an angled thrust surface  18  which is exposed to the fuel pressure within a chamber  20  defined between the lower part of the outer valve needle  14  and the adjacent part of the bore  12 . A part of the lower, conical end surface of the outer valve needle  14  is also exposed to the fuel pressure within the chamber  20 . 
     The bore  12  defines an annular gallery  22  which communicates with a supply passage  24  which, in use, communicates with a source of fuel under pressure, for example a common rail charged with fuel by an appropriate fuel pump. 
     The outer valve needle  14  is provided with flutes  26  whereby fuel is able to flow from the annular gallery  22  to the chamber  20 . 
     The outer valve needle  14  is provided with an axially extending bore  28 , an inner valve needle  30  being slidable within the lower part of the bore  28 . The inner valve needle  30  is shaped, at its lower end, to define a frusto-conical region which is engageable with a part of seating located closer to the lower end of the nozzle body  10  than the first group of openings  16 . A second group of openings  32  communicate with the bore  12  downstream of the position at which the inner valve needle  30  engages the seating. It will be appreciated that the engagement between the inner valve needle  30  and the seating controls the supply of fuel under pressure to the second group of outlet openings  32 . 
     As shown most clearly in FIG. 2, the upper end surface of the inner valve needle  30  is provided with a recess  34 , the provision of the recess  34  resulting in the upper part of the inner valve needle  30  being of relatively small wall thickness. The recess  34  is conveniently formed using a low force machining technique, for example electric discharge or electrochemical machining. A load transmitting member  36  is received within the recess  34 , the upper end of the member  36  engaging a shim  38 , which in turn engages a helical compression spring  40 . The load transmitting member  36  is shaped to permit fuel to flow into the recess  34  and to be engageable with a step or shoulder defined by part of the bore  28  to limit movement of the inner valve needle  30  relative to the outer valve needle  14 . 
     At its upper end, the nozzle body  10  engages a distance piece  42 , the distance piece  42  being provided with a drilling  44  whereby fuel under pressure from the fuel source is supplied to the supply passage  24 . A flow restrictor is provided in the drilling  44 . 
     The distance piece  42  is further provided with a recess of annular shape defining a control chamber  46 , the upper part of the outer valve needle  14  being exposed to the fuel pressure within the control chamber  46 . A spring  48  is located within the control chamber  46 , the spring  48  engaging the upper surface of the outer valve needle  14  to bias the valve needle  14  into engagement with the seating. A small diameter drilling  50  provides a restricted flow path between the drilling  44  and the control chamber  46 . It will be appreciated that, in use, the provision of the restrictor in the drilling  44  permits the formation of a pressure differential across the valve needles  14 ,  30 . 
     Within the control chamber  46 , the distance piece  42  defines a projection  52  provided with an axially extending passage  54 . The spring  40  engages the lower end of the projection  52 . The passage  54  communicates through a restricted passage  56  with a recess  58  formed in the upper surface of the distance piece  42 , a further restricted passage  60  connecting the recess  58  to the drilling  44 . 
     The upper end of the distance piece  42  engages a valve housing  62  provided with a drilling  64  communicating with the drilling  44 . The valve housing  62  is further provided with a through bore  66  within which a valve member  68  is slidable, the valve member  68  including a region engageable with a seating to control communication between a passage  70  which communicates with the recess  58 , and a chamber  72  which communicates, in use, with a low pressure drain reservoir. The valve member  68  is spring biased into engagement with its seating, and movement of the valve member  68  away from its seating is controlled by an electromagnetic actuator (not shown) which, in conjunction with an armature  74  carried by the valve member  68  can apply a force to the valve member  68  to lift the valve member  68  from its seating. 
     In use, with the supply passage  24  communicating with the source of fuel under high pressure, and with the actuator de-energized so that the valve member  68  engages its seating, the fuel pressure within the chamber  20  is relatively high, thus a force is applied to the valve needle  14  urging the valve needle  14  away from the seating. This force is countered by the action of the fuel under pressure within the control chamber  46  and the action of the spring  48  with the result that the lower end of the outer valve needle  14  engages the seating. As a result, it will be appreciated that fuel under pressure is unable to flow from the chamber  20  to a position downstream of the engagement of the outer valve needle  14  with the seating. Fuel is therefore unable to flow to either of the first or second groups of outlet openings  16 ,  32 . 
     At this point in the operating cycle of the injector, it will be appreciated that the fuel pressure within the bore  28  of the outer valve needle  14  is high, thus the upper end of the inner valve needle  30  is exposed to fuel under high pressure. The action of the fuel under pressure upon the upper end surface of the inner valve needle  30  in combination with the action of the spring  40  maintains the inner valve needle  30  in engagement with the seating. The action of the fuel under pressure on the upper part of the inner valve needle  30 , and in particular the action of the fuel under high pressure within the recess  34  acts to deform the upper part of the inner valve needle  30  to expand the outer diameter thereof, thus forming a substantially fluid tight seal between the inner and outer valve needles  30 ,  14 . 
     In order to commence injection, the actuator is energized, and as a result the valve member  68  is lifted from its seating. Fuel is able to escape from the control chamber  46  through the passages  54 ,  56 , the recess  58  and the passage  70  to the low pressure reservoir. The fuel pressure within the control chamber  46  applied to the upper surface of the outer valve needle  14  is therefore reduced, and a point will be reached beyond which the force urging the valve needle  14  away from its seating is sufficient to overcome the action of the spring  48  and the fuel pressure within the control chamber  46 , and the outer valve needle  14  will lift away from the seating, thus permitting fuel to flow to the first group of outlet openings  16 . The flow of fuel across the open end of the bore  28  maintains the fuel pressure within the bore  28  to which the upper end surface of the inner valve needle  30  is exposed at a relatively high pressure, thus although the outer valve needle  14  moves, the inner valve needle  30  remains in contact with the seating. As a result, it will be appreciated that fuel delivery occurs only through the first group of outlet openings  16 , fuel not being delivered through the second group of outlet openings  32  at this time. Additionally, as the inner valve needle does not move, it can assist in guiding the movement of the outer needle. 
     Once the outer valve needle  14  has lifted to its fully opened position, the upper end thereof engages the projection  52 , thus the flow of fuel from the control chamber  46  to the low pressure drain through the passage  54  is terminated. Fuel flows to the control chamber  46  through the restricted passage  50 , thus the fuel pressure within the control chamber  46  rises. However, as, at this point in the injection cycle, the effective area over which fuel under pressure acts to urge the needle away from the seating is large, the increase in fuel pressure within the control chamber  46  does not result in movement of the needle  14  to terminate injection. As the flow of fuel from the control chamber  46  to the low pressure drain is terminated, the fuel pressure within the bore  28  starts to fall, reducing the deformation of the inner valve needle  30 . Further, a point will be reached beyond which the fuel pressure acting upon the exposed part of the inner valve needle  30  is able to lift the inner valve needle  30  against the action of the spring  40  in combination with the remaining fuel pressure within the bore  28  to allow fuel injection through both the first group of outlet openings  16  and the second group of outlet openings  32 . Movement of the inner valve needle  30  is limited by engagement between the member  36  and the step defined by the bore  28 . 
     The fuel pressure within the control chamber  46  increases as the flow of fuel from the control chamber  46  to the low pressure drain is terminated. Increased fuel pressure within the control chamber  46  increases the downward force applied to the outer valve needle  14 , thereby serving to urge the needle  14  into engagement with the seating. In addition, fuel under pressure within the bore  28  further increases the downward force applied to the outer valve needle  14 . Fuel pressure within the control chamber  46  and within the bore  28  is increased to an extent sufficient to cause movement of the needle  14  into engagement with the seating to terminate injection through the first group of outlet openings  16 . 
     It will be appreciated that the embodiment of FIGS. 1 and 2 has the advantages that a single actuator is used to control movement of both the outer valve needle  14  and the inner valve needle  30 . Further, the escape of fuel between the inner and outer valve needles  30 ,  14  is reduced or avoided. 
     In the arrangement described hereinbefore, movement of the inner valve needle occurs only when the pressure of fuel applied to the injector exceeds a predetermined level and when the outer needle has reached its fully lifted position. By appropriate control of the injector, the total area of the outlet openings in use can be controlled to permit the duration of injection to be maintained at a relatively low level even under high engine speed or load conditions. 
     FIGS. 3 and 4 illustrate an arrangement which is similar to that of FIGS. 1 and 2, but in which the fuel pressure within the control chamber  46  is controlled using a piezoelectric actuator arrangement which controls the position of a piston  76 . The inner and outer valve needles  30 ,  14  are both exposed, throughout the range of movement of outer valve needle  14 , to the fuel pressure within the control chamber  46 , thus movement of both of the valve needles is dependent upon the pressure differential between the upper and lower surfaces thereof, the effective cross sectional areas exposed to the fuel under pressure and the effect of spring biasing. In the arrangement illustrated in FIGS. 3 and 4, the inner valve needle  30  is not spring biased, the only spring biasing being by way of a spring  78  which is engaged between the piston  76  and a shim  80  which engages a shoulder defined by the bore  28 . The spring  78  serves to maintain the outer valve needle  14  in engagement with the seating when fuel under pressure is not being supplied to the injector. 
     In use, initially the piston  76  is urged by the piezoelectric actuator towards a position in which the fuel pressure within the control chamber  46  is maintained at a high level. The application of high pressure to the control chamber  46  maintains the inner and outer valve needles  30 ,  14  in engagement with the seating against the action of fuel under pressure within the chamber  20 . In order to commence injection, the piezoelectric actuator is energized to permit movement of the piston  76  to reduce the fuel pressure within the control chamber  46 , and as a result the outer valve needle  14  moves to permit fuel delivery through the first group of outlet openings  16 . This movement occurs against the action of the spring  78 , and results from the pressure differential between the upper and lower surfaces of the valve needle  14  and the effective areas to which fuel under pressure is applied. 
     Once the outer valve needle  14  has lifted, fuel under pressure is applied to the inner valve needle  30 . If the piston  76  is moved to reduce the pressure within the control chamber  46  relative to that applied to the lower part of the needle  30 , the inner valve needle  30  is able to move against the action of the fuel pressure within the control chamber  46  to permit fuel delivery through both the first group of outlet openings  16  and the second group of outlet openings  32 . 
     Termination of injection occurs by energizing the piezoelectric actuator to move the piston  76  to increase the fuel pressure within the control chamber  46 . As a result, the fuel pressure applied to the inner and outer valve needles  30 ,  14  increases, and a point will be reached beyond which the fuel pressure within the control chamber  46  is sufficient to cause the valve needles  14 ,  30  to return into engagement with their respective seatings. 
     As described hereinbefore, the embodiment of FIGS. 3 and 4 requires the provision of only a single actuator to control movement of the inner and outer valve needles  30 ,  14  and leakage of fuel between the inner and outer valve needles  30 ,  14  is restricted by the application of fuel under pressure to the recess  34  provided in the upper part of the inner valve needle  30  deforming the inner valve needle  30  to form a substantially fluid tight seal with the outer valve needle  14 . 
     FIG. 5 illustrates an arrangement in which an inner needle  30  is slidable within a blind bore  28  formed in the outer needle  14 . The inner needle  30  and bore  28  together define a chamber  92  which communicates, through a restricted passage  94  with a part of the bore  12  upstream of the first group of outlet openings  16 . 
     In use, an appropriate actuator is used to control movement of the outer needle  14 . If the outer needle  14  moves slowly, the fuel is able to flow at a sufficiently high rate through the passage  94  to the chamber  92  to ensure that the inner needle  30  remains seated. However, if the outer needle  14  moves quickly, the fuel pressure within the chamber  92  will fall as fuel is unable to flow to the chamber  92  at a sufficient rate to maintain the fuel pressure within the chamber, and the inner needle  30  will lift away from its seating. During injection, as fuel can continue to flow, at a low rate, to the chamber  92 , the inner needle  30  will gradually move towards its seating. 
     As described hereinbefore, the inner needle  30  is provided with a recess  34  such that the application of fuel under pressure to the chamber  92  causes dilation of the inner needle  30  to improve the seal between the inner needle  30  and the bore  28 , thus reducing fuel leakage.