Abstract:
A fuel injector for an internal combustion engine includes a nozzle body, a needle, a control piston and a force disperser. The nozzle body has a valve seat for starting and stopping fuel injection. The needle is slidably disposed in the nozzle body and has a valve head for seating on the valve seat. The control piston is for transmitting a drive force to the needle. The force disperser disperses the drive force when the needle displaces a predetermined distance relative to the nozzle body. The force disperser provides for an elastic deformation of at least one of the nozzle body and the needle to prevent concentration of the drive force on the valve seat and the valve head.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]     This application is based upon and claims the benefit of priority of Japanese Patent Application No. 2004-037224 filed on Feb. 13, 2004, the content of which is incorporated herein by reference.  
       FIELD OF THE INVENTION  
       [0002]     The present invention relates to a fuel injector for injecting fuel into a cylinder of an internal combustion engine and, more particularly, to a fuel injector having a force disperser capable of dispersing a drive force away from a valve seat.  
       BACKGROUND OF THE INVENTION  
       [0003]     U.S. Patent Application No. U.S. 2003/0052202A1 (JP2003-166457A) discloses a conventional fuel injector for injecting fuel into cylinders of a common rail-type fuel injection system for an internal combustion engine such as a diesel engine. This type of fuel injector includes a nozzle body having injection holes for injecting fuel, a needle disposed in the nozzle body for longitudinal reciprocation to open and close the injection holes, a nozzle holder retaining the nozzle body, and a control piston disposed in the nozzle holder for longitudinal reciprocation to directly or indirectly actuate the needle (refer to  FIG. 4 ).  
         [0004]     This above-described fuel injector has a fuel seal structure for providing a valve for tightly closing the injection holes by applying a force of the control piston to the needle, thereby forcing the needle onto a seat of the valve. A high-pressure fuel in a pressure control chamber generates the driving force of the control piston. The pressure of the high-pressure fuel is increased and decreased by opening and closing an electromagnetic valve.  
         [0005]     The conventional fuel injector is effective for providing a tight seal to the valve for closing the injection hole. However, the high-pressure fuel may wear the seat of the valve via an excessive force applied by the needle.  
       SUMMARY OF THE INVENTION  
       [0006]     An object of the present invention is to provide a fuel injector for an internal combustion engine that controls a control piston disposed in a nozzle holder for longitudinal reciprocation to directly or indirectly actuate a needle for opening and closing a valve located just upstream of injection holes without applying an excessive valve-seating pressure.  
         [0007]     Another object of the present invention is to provide a fuel injector to control a control piston disposed in a nozzle holder for longitudinal reciprocation to directly or indirectly actuate a needle for opening and closing a valve located just upstream of injection holes without applying an excessive valve-seating pressure such that any change in a fuel injection amount caused by wear of the valve is limited.  
         [0008]     To achieve the above objects, the fuel injector according to the present invention includes a nozzle body, a needle, a control piston and a force disperser. The nozzle body has a valve seat for starting and stopping fuel injection. The needle is slidably disposed in the nozzle body and has a valve head for engaging the valve seat. The control piston transmits a drive force to the needle. The force disperser disperses the drive force when the needle displaces a predetermined distance relative to the nozzle body. The force disperser provides for an elastic deformation of at least one of the nozzle body and the needle to reduce the amount of the drive force concentrated on the valve seat and the valve head. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]     Other features and advantages of the present invention will be appreciated, as well as methods of operation and the function of the related parts, from a study of the following detailed description, the appended claims, and the drawings, all of which form a part of this application. In the drawings:  
         [0010]      FIG. 1  is a partial cross-sectional side view of a nozzle portion of a fuel injector according to a first embodiment of the present invention;  
         [0011]      FIG. 2  is a cross-sectional view of the fuel injector according to the first embodiment of the present invention;  
         [0012]      FIG. 3  is a partial cross-sectional side view of a nozzle portion of a fuel injector according to a second embodiment of the present invention; and  
         [0013]      FIG. 4  is a partial cross-sectional side view of a nozzle portion of a conventional fuel injector. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0014]     Each of the fuel injectors described in the embodiments below are applied to a common rail-type fuel injection system for a diesel engine.  
         [0015]      FIG. 2  depicts an entire structure of a fuel injector according to a first embodiment of the present invention. In general, the fuel injector includes a nozzle portion  10 , a nozzle holder  50 , a control piston  60 , a pressure control chamber  71 , an electromagnetic valve  80  and a limiter  91 . A retaining nut  19  fastens the nozzle portion  10  to a head-side portion of the nozzle holder  50 . The fuel injector is for injecting high-pressure fuel, which is supplied from a common rail (not shown), into a combustion chamber of a diesel engine.  
         [0016]      FIG. 1  depicts a nozzle portion  10  of the fuel injector of  FIG. 2 . As shown in  FIGS. 1 and 2 , the nozzle portion  10  has a nozzle body  11  containing a needle  31 . The needle  31  is slidably disposed in the nozzle body  11  for longitudinal reciprocation.  
         [0017]     The nozzle body  11  is generally cylindrical and defines a guide bore  12 , a valve seat  13 , injection holes  41  and a sack  15 . The guide bore  12  extends in the longitudinal direction of the nozzle body  11  so that one end thereof communicates with the valve seat  13  and the opposite end opens on a tail-side end face of the nozzle body  11 . The guide bore  12  has a generally uniform internal diameter.  
         [0018]     The valve seat  13  has a generally conical surface. An upper end of the valve seat  13  has a larger internal diameter than a lower end and communicates with the guide bore  12 . The lower end with the smaller internal diameter communicates with the sack  15 . The needle  31  has a contact portion  36  at a head-side end thereof that is arranged for engaging and disengaging the valve seat  13 , thereby serving as a valve head. The contact portion  36  is desirably substantially circular.  
         [0019]     The sack  15  is located at the head-side end portion of the nozzle body  11 . The sack  15  is a chamber having a relatively small volume for accumulating high-pressure fuel to be injected into a combustion chamber. The injection holes  41  are small-diameter passages communicating between the sack  15  and space outside of the nozzle body  11  such as a combustion chamber of an internal combustion engine.  
         [0020]     The nozzle body  11  also defines a fuel accumulation chamber  16  located midway along the longitudinal dimension of the guide bore  12 . The fuel accumulation chamber  16  includes an annular concavity surrounding and communicating with the guide bore  12 . The fuel accumulation chamber  16  also communicates with a first fuel passage  17  defined by the nozzle body  11 , through which high-pressure fuel is supplied.  
         [0021]     The nozzle body  11  is segmented into a stem portion  11   a  and an attachment portion  11   b . The stem portion  11   a  has an outer diameter smaller than that of the attachment portion  11   b , as shown in  FIG. 1 . The needle  31  has a thick portion  32  at a tail-side thereof and a thin portion  34  at a head-side thereof, as shown in  FIG. 1 . The stem portion  11   a  of the nozzle body  11  contains the thin portion  34  of the needle  31  and defines the valve seat  13 , the sack  15 , and the injection holes  41 . The attachment portion  11   b  of the nozzle body  11  contains the thick portion  32  of the needle  31  and defines the fuel accumulation chamber  16  and the first fuel passage  17 . The retaining nut  19  fastens the nozzle body  11  to the nozzle holder  50 .  
         [0022]     The needle further has a pin  33 , a head base  35  and a conical portion  37 . The thick portion  32  of the needle  32  has a generally uniform outer diameter and is slidably supported in the guide bore  12  defining a slight clearance therebetween. The clearance enables a smooth sliding motion of the needle  31  in the guide bore  12 . The thin portion  34  of the needle  31  and the guide bore  12  also define a clearance therebetween in which high-pressure fuel flows from the fuel accumulation chamber  16  to the sack  15 .  
         [0023]     The head base  35  of the needle  31  is disposed between the head-side end of the thin portion  34  of the needle  31  and the conical portion  37  of the needle  31 . A circumferential face on which the head base  35  and the conical portion  37  contact each other constitutes the contact portion  36  of the needle  31 .  
         [0024]     The conical portion  37  of the needle  31  is steeper than the face of the valve seat  13 . This allows the contact portion  36  to securely and sealingly engage the valve seat  13 . The head-side end of the conical portion  37  faces the sack  15  when the contact portion  36  engages the valve seat  13 . The valve seat  13  and the contact portion  36  provide a nozzle seal for sealing high-pressure fuel.  
         [0025]     The thick portion  32  of the needle  31  includes a first slide portion, which longitudinally reciprocates within the guide bore  12  of the nozzle body  11 . The thin portion  34 , the head base  35 , and the conical portion  37  of the needle  31  define a first insert portion. The first insert portion has a diameter that is smaller than a diameter of the first slide portion. The thick portion  32  and the thin portion  34  are connected by a tapered portion. The tapered portion has a circumference that inclines along the longitudinal direction of the nozzle body  11 . The tapered portion receives the pressure of fuel in the fuel accumulation chamber  16 . High-pressure fuel in the fuel accumulation chamber  16  urges the tapered portion toward the tail-side end of the nozzle body  11  (upward in the FIGS.), so as to move the needle  31  and lift the contact portion  36  off of the valve seat  13 . The first insert portion  34 ,  35  and  37  is surrounded by the fuel accumulation chamber  16 .  
         [0026]     As shown in  FIG. 2 , the nozzle holder  50  defines a second fuel passage  51 , a cylindrical bore  52 , a first discharge passage  53 , and a third fuel passage  61 . High-pressure fuel supplied from the common rail flows via the second fuel passage  51  to an orifice plate  70  and via the third fuel passage  61  to the first fuel passage  17  in the nozzle portion  10 . The first discharge passage  53  returns high-pressure fuel toward a low-pressure side of the injector such as to a fuel tank. The cylindrical bore  52  contains a second insert portion  64  forming a head-side portion of the control piston  60 .  
         [0027]     A room  56  defined between the cylindrical bore  52  and the second insert portion  64  of the control piston  60  constitutes a back-pressure chamber for the needle  31 . This room  56  communicates with fuel discharge passages  54  and  55 , which are connected to the first discharge passage  53  and returns the high-pressure fuel to the low-pressure side.  
         [0028]     The cylindrical bore  52  slidably contains the control piston  60  and the limiter  91 . The limiter  91  connects the needle  31  to the control piston  60 . The control piston  60  has a second slide portion  62  disposed at the tail-side portion thereof. The second insert portion  64  has a smaller diameter than that of the second slide portion  62 . The second slide portion  62  and the second insert portion  64  longitudinally reciprocate within the cylindrical bore  52 .  
         [0029]     The pin  33  at the end of the needle  31  and the control piston  60  contain the limiter  91  therebetween. A spring  69  located in the room  56  around the control piston  60  biases the needle  31  toward the head-side of the nozzle portion  10  to bias the contact portion  36  into engagement with the valve seat  13 . The limiter  91  is generally cylindrical and has a concavity  91   a  on a head-side end face thereof, into which the pin  33  provided at the tail-side end of the needle  31  is disposed. A tail-side end face of the limiter  91  contacts the head-side face of the second insert portion  64  of the control piston  60 .  
         [0030]     The limiter  91  is arranged to slide together with the needle  31  and the control piston  60 . A clearance L separates the head-side end face of the limiter  91  and a tail-side end face of the stem portion  11   a  of the nozzle body  11 . The clearance L is set to a predetermined length L 0 , which can decrease by the assembly of the nozzle body  11  and the needle  31 .  
         [0031]     During operation of the fuel injector, the pressure control chamber  71  accumulates fuel at a predetermined pressure. The pressurized fuel in the pressure control chamber  71  actuates the control piston  60  to push the needle  31  into engagement with the valve seat  13  of the nozzle body  11 . This eliminates the clearance L between the limiter  91  and the stem portion  11   a  of the body  11 .  
         [0032]     The clearance L is eliminated due to an elastic elongation of the stem portion  11   a  of the nozzle body  11  that is caused by a compressive stress between the contact portion  36  of the needle  31  and the valve seat  13 . It should be appreciated that the attachment portion  11   b  of the nozzle body  11  experiences no elastic deformation.  
         [0033]     The orifice plate  70  is located at the upper end portion of the nozzle holder  50  where the cylindrical bore  52  opens. The orifice plate  70  defines the pressure control chamber  71  that communicates with the cylindrical bore  52 .  
         [0034]     The orifice plate  70  has an entry orifice (not shown) and an exit orifice  72  located upstream and downstream of the pressure control chamber  71 , respectively. The exit orifice  72  has an internal diameter larger than that of the entry orifice.  
         [0035]     The entry orifice is located between the second fuel passage  51  and the pressure control chamber  71 . The exit of the entry orifice opens on a tapered side face of the pressure control chamber  71 . The exit orifice  72  is located at the tail-side of the pressure control chamber  71  and in communication with the first discharge passage  53  via the electromagnetic valve  80 . The pressure control chamber  71  is supplied with high-pressure fuel via the second fuel passage  51  located in the nozzle holder  50 .  
         [0036]     The electromagnetic valve  80  has an armature  81 , a spring  82  and a solenoid  83 . The armature  81  connects and interrupts a communication between the exit orifice  72  and the first discharge passage  53 . The spring  82  urges the armature  81  in a direction to close the electromagnetic valve  80  (downward in  FIGS. 1 and 2 ). The solenoid  83  actuates the armature  81  in a direction to open the electromagnetic valve  80 . The electromagnetic valve  80  is installed at the tail-side of the nozzle holder  50  and sandwiches the orifice plate  70  therebetween. A retaining nut  84  fastens the electromagnetic valve  80  to the nozzle holder  50 . A current flowing into the solenoid  83  opens the exit orifice  72  by lifting up the armature  81  against the restitutive force of the spring  82 . Interrupting the current flowing into the solenoid  83  closes the exit orifice  72  by pushing the armature  81  with the restitutive force of the spring  82 .  
         [0037]     The pressure control chamber  71  and the electromagnetic valve  80  actuate the control piston  60  to directly or indirectly move the needle  31  to cause the contact portion  36  to engage the valve seat  13 .  
         [0038]     The operation of the fuel injector is as follows.  
         [0039]     High-pressure fuel supplied from the common rail to the fuel injector flows via the third fuel passage  61  into the first fuel passage  17  in the nozzle portion  10  and via the second fuel passage  51  into the pressure control chamber  71 . When the electromagnetic valve  80  is closed, that is, when the armature  81  closes the exit orifice  72 , pressurized fuel in the pressure control chamber  71  acts on the needle  31  via the control piston  60  and the pin  31  in a direction to seat the contact portion  36  onto the valve seat  13  together with the restitutive force of the spring  69 .  
         [0040]     High-pressure fuel in the first fuel passage  17  flows into the fuel accumulation chamber  16  and acts on the tapered portion of the needle  31  in a direction to lift the contact portion  36  off of the valve seat  13 .  
         [0041]     When the electromagnetic valve  80  is closed, the force to seat the contact portion  36  exceeds that to lift the contact portion  36  and the contact portion  36  of the needle  31  maintains engaged with the valve seat  13  of the nozzle body  11 . Thus, fuel is not injected through the injection holes  41 .  
         [0042]     When the electromagnetic valve  80  opens, that is, when the armature  81  opens the exit orifice  72 , the exit orifice  72  communicates with the first discharge passage  53  in the nozzle holder  50 . Thus, high-pressure fuel in the pressure control chamber  71  flows via the exit orifice  72  and is discharged by the first discharge passage  53 . Furthermore, high-pressure fuel continues to flow via the entry orifice into the pressure control chamber  71 . However, the diameter of the entry orifice is smaller than that of the exit orifice  72 , thus, the pressure in the pressure control chamber  71  acting on the command piston  60  gradually decreases. Thus, balancing the pressure of fuel in the pressure control chamber  71 , the force to lift the contact portion  36  of the needle  31  off the valve seat  13 , and the restitutive force of the spring  69 . When the force generated to lift the needle  31  exceeds that to seat the needle  31 , the contact portion  36  of the needle  31  disengages the valve seat  13  to open the injection holes  41 . Thus, fuel is injected through the injection holes  41 . The lift height of the needle  31  corresponds to the clearance between the valve seat  13  and the contact portion  36 .  
         [0043]     By interrupting the current flow to the solenoid  83 , the armature  81  closes the exit orifice  72  so that the pressure of fuel in the pressure control chamber  71  increases and urges contact portion  36  of the needle  31  to engage the valve seat  13 . When the force to seat the contact portion  36  exceeds that to lift the contact portion  36 , the contact portion  36  of the needle  31  engages the valve seat  13  of the nozzle body  11  and fuel ceases to inject through the injection holes  41 .  
         [0044]     When the contact portion  36  of the needle  31  engages the valve seat  13 , the electromagnetic valve  80  is closed to interrupt the communication with the first discharge passage  53 . This causes the pressure of fuel in the pressure control chamber  71  to increase up to the same level of that of the high-pressure fuel supplied to the fuel accumulation chamber  16  in the nozzle portion  10 .  
         [0045]     When the needle  31  engages the valve seat  13 , each of the pressure control chamber  71  and the fuel accumulation chamber  16  are filled with high-pressure fuel supplied from the common rail. High-pressure fuel in the pressure control chamber  71  generates a driving force to push the control piston  60 . The driving force presses the contact portion  36  of the needle  31  via the control piston  60  into engagement with the valve seat  13  of the nozzle body  11 .  
         [0046]     Thus, the contact portion  36  is compressed onto the valve seat  13 , generating an elastic elongation of the stem portion  11   a  of the nozzle body  11 . Then, the predetermined level of fuel pressure in the pressure control chamber  71  continues to apply the driving force until the elastic elongation reaches the length L 0 . At this time, the clearance L between the head-side end face of the limiter  91  and a tail-side end face of the attachment portion  11   b  of the nozzle body  11  decreases to zero and the limiter  91  engages the attachment portion  11   b  of the nozzle body  11 . Thus, the driving force generated by the high-pressure fuel in the pressure control chamber  71  is dispersed by the head-side end face of the limiter  91  and the attachment portion  11   b  of the nozzle body  11 , thereby reducing the amount of concentration of the driving force on the valve seat  13  of the nozzle body  11  an the contact portion  36  of the needle  31 .  
         [0047]     The limiter  91  and the attachment portion  11   b  of the nozzle body  11  form a force disperser of the present invention. Together, the limiter  91  and the attachment portion  11   b  serve to disperse the driving force to other portions of the fuel injector other than the valve seat  13  and the contact portion  36  when the elastic elongation of the stem portion  11   a  reaches the predetermined length L 0 .  
         [0048]     The above-described driving force exceeds a minimum force required to force the contact portion  36  into engagement with the valve seat  13 . Thus, the present invention prevents the excessive force from damaging the contact portion  36  of the needle and/or the valve seat  13  of the nozzle body  11 .  
         [0049]     The advantages of the fuel injector according to the first embodiment are as follows.  
         [0050]     First, while the needle  31  is seated on the valve seat  13  of the nozzle body  11 , if the displacement of the needle  31  relative to the nozzle body  11  exceeds a predetermined length L 0 , the force disperser  11   91  disperses the driving force generated in the pressure control chamber  71  to prevent it from being concentrated on the valve seat  13  and the contact portion  36 . Thus, if the elastic deformation in accordance with the driving force of the predetermined strength occurs, the fuel injector disperses the driving force of excessive strength by the force disperser  11 ,  91  thereof not to concentrate on the valve seat  13  and the contact portion  36 .  
         [0051]     Second, the clearance L between the limiter  91  and the attachment portion  11   b  of the nozzle body  11  is set to a predetermined length L 0 . During operation of the fuel injector, when the needle  31  is seated on the valve seat  13  of the nozzle body  11  and when the elastic displacement L 0  in accordance with the driving force of the predetermined strength occurs, the clearance L decreases to zero and the limiter  91  engages the attachment portion  11   b  of the nozzle body  11 . Thus, the driving force is not concentrated on the valve seat  13  and the contact portion  36 , and is dispersed by the contact faces of the limiter  91  and the attachment  11   b.    
         [0052]     Third, this embodiment is suitable for a fuel injector having a pressure control chamber  71 , an electromagnetic valve  80  for increasing and decreasing the pressure of fuel in the pressure control chamber  71 , and a control piston  60  that receives the pressure of fuel in the pressure control chamber  71  at a end thereof opposite to another end facing the needle  31 . Thus, the driving force generated by the pressure of fuel in the pressure control chamber  71  may exceed a predetermined strength causing elastic deformation of the predetermined length L 0 . That is, the pressure of fuel can be set to a high level to inject the fuel through injection hole  41  at a high pressure.  
         [0053]     Fourth, the fuel injector of the above-described embodiment can prevent an excessive driving force generated in the driving force generator such as the pressure control chamber  71  from being concentrated on the valve seat  13  of the nozzle body  11  and the contact portion  36  of the needle  31 . Thus, the fuel injector can reduce the wear of the valve seat  13  and the contact portion  36  so as to reduce variations in the fuel injection amount.  
         [0054]     With reference to  FIG. 3 , a fuel injector according to a second embodiment of the present invention includes a force disperser including a nozzle holder  50  and a control piston  160 .  
         [0055]     The control piston  160  has a slide portion slidably supported by a cylindrical bore  52  and an insert portion  164  having a diameter that is smaller than that of the slide portion. The insert portion  164  includes a thick portion  164   b  and a thin portion  164   a  having a diameter that is smaller than that of the thick portion  164   b . The cylindrical bore  52  has a narrow portion  57  having an internal diameter that is smaller than that of other portions of the bore  52  and larger than that of the thin portion  164   a  of the control piston  160 .  
         [0056]     A boundary of the narrow portion  57  of the cylindrical bore  52  forms a step  52   a . A boundary between the thick portion  164   b  and the thin portion  164   a  of the insert portion  164  of the control piston  160  forms a stopper. An axial clearance L exists between the step  52   a  and the stopper. During operation, pressurized fuel in a pressure control chamber  71  (similar to that described above in accordance with the first embodiment) applies a driving force to the control piston  160  for pushing a needle  31  onto a valve seat  13 . This causes the clearance L to decrease to zero due to an elastic deformation of a stem portion  11   a  of the nozzle body  11  and the needle  31 , similar to that described above in accordance with the first embodiment.  
         [0057]     When the clearance L decreases to zero, the stopper of the control piston  160  engages the step  52   a  of the nozzle holder  50  and disperses the driving force, thereby reducing an amount it concentrates on the valve seat  13  of the nozzle body  11  and the contact portion  36  of the needle  31 .  
         [0058]     It should therefore be appreciated that the fuel injector according to the second embodiment has advantages equivalent to those of the fuel injector according to the first embodiment.  
         [0059]     In the first and the second embodiments, the fuel pressure in the pressure control chamber  71  applies a driving force to the control piston  60 ,  160 . In the present invention, the power source for actuating the control piston is not limited to the above-described pressure control chamber  71 , which increases and decreases the pressure of fuel therein by opening and closing the electromagnetic valve  80 . It should be appreciated that the present invention may alternatively include a power source such as a pressure generator or a displacement generator, which magnifies a displacement of a piezoelectric stack by an action of lever to actuate the control piston.  
         [0060]     In the second embodiment, the step  52   a  of the nozzle holder  50  and the stopper in the slide portion  164  of the control piston  160  forms the force disperser. In an alternative embodiment, the force disperser may be formed by a different portion of the control piston  160  ( 60 ), for example, in a second slide portion  62  (refer to  FIG. 2 ).