Patent Publication Number: US-2006016916-A1

Title: Fuel injector provided with a high flexibility plunger

Description:
The present invention relates to a fuel injector.  
      The following description will make explicit reference, without consequently losing its general nature, to an electromagnetic injector for a direct fuel injection system.  
     BACKGROUND OF THE INVENTION  
      An electromagnetic fuel injector normally comprises a cylindrical tubular body with a central channel which performs the function of a fuel duct and ends with an injection jet controlled by an injection valve operated by an electromagnetic actuator; in particular, the injection valve is provided with a plunger, which is rigidly connected to a mobile armature of the electromagnetic actuator so as to be displaced by the action of the electromagnetic actuator between a closed position and an open position of the injection jet against the action of a spring which tends to hold the plunger in the closed position.  
      One example of an electromagnetic fuel injector of the above-described type is given in U.S. Pat. No. 6,027,050-A1, which relates to a fuel injector provided with a plunger which at one end cooperates with a valve seat and at the opposite end is integral with a mobile armature of an electromagnetic actuator; the plunger is guided at the top by the armature and is guided at the bottom by sliding of the end portion of the plunger in a guide portion of the valve seat.  
      When the plunger is guided at the bottom by the valve seat, the dimensions and positioning of the plunger, of the valve seat and of the armature must be very accurate. Indeed, if structural tolerances are relatively large, when the armature strikes against a fixed armature of the electromagnet, transverse forces may arise which are transmitted to the plunger and are in part dissipated at the level of the coupling between the end portion of the plunger and the guide portion of the valve seat; it has been observed experimentally that if such forces exceed a certain value, localised wear phenomena may occur on the plunger and/or the guide portion of the valve seat with a consequent reduction in the service life of the injector.  
      As stated above, in order to keep such transverse forces at acceptable levels, the plunger and the guide parts of the plunger must be manufactured to very fine tolerances which accordingly involves complex and costly processing.  
     SUMMARY OF THE INVENTION  
      The object of the present invention is to provide a fuel injector which does not exhibit the above-stated disadvantages and, in particular, is simple and economic to produce.  
      The present invention provides a fuel injector as specified in the attached claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The present invention will now be described with reference to the attached drawings, which illustrate some non-limiting embodiments of the invention, in which:  
       FIG. 1  is a diagrammatic, partially sectional, side view of a fuel injector produced according to the present invention;  
       FIG. 2  shows an enlarged view of an injection valve of the injector of  FIG. 1 ;  
       FIG. 3  shows an enlarged view of a mobile armature of the injector of  FIG. 1 ;  
       FIG. 4  shows another embodiment of the mobile armature of  FIG. 3 ;  
       FIG. 5  shows an enlarged view of a plunger of the injector of  FIG. 1 ; and  
       FIG. 6  shows another embodiment of the plunger of  FIG. 5 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      In  FIG. 1, 1  denotes the overall fuel injector, which exhibits a substantially cylindrical symmetry around a longitudinal axis  2  and is capable of being operated to inject fuel from an injection jet  3  which opens directly into an explosion chamber (not shown) of a cylinder. The injector  1  comprises a supporting body  4 , which has a tubular cylindrical shape of variable cross-section along the longitudinal axis  2  and comprises a supply channel  5  extending along the entire length of said supporting body  4  to supply the pressurised fuel to the injection jet  3 . The supporting body  4  accommodates an electromagnetic actuator  6  at the level of an upper portion thereof and an injection valve  7  at the level of a lower portion thereof; in service, the injection valve  7  is actuated by the electromagnetic actuator  6  to control the flow of fuel through the injection jet  3 , which is provided at the level of said injection valve  7 .  
      The electromagnetic actuator  6  comprises an electromagnet  8 , which is accommodated in fixed position within the supporting body  4  and which, when energised, is capable of displacing a mobile armature  9  of ferromagnetic material along the axis  2  from a closed position to an open position of the injection valve  7  against the action of a spring  10  which tends to hold the mobile armature  9  in the closed position of the injection valve  7 . In particular, the electromagnet  8  comprises a coil  11 , which is supplied with electricity by an electronic control unit (not shown) and is accommodated outside the supporting body  4 , and a fixed magnetic armature  12 , which is accommodated inside the supporting body  4  and has a central hole  13  to allow the fuel to flow towards the injection jet  3 .  
      Inside the central hole  13  of the fixed magnetic armature  12 , an abutment member  14  is driven into a fixed position, which abutment member is of a tubular cylindrical shape (optionally open along a generating line) to allow the fuel to flow towards the injection jet  3  and is capable of holding the spring  10  in a compressed state against the mobile armature  9 .  
      The mobile armature  9  is part of a mobile assembly which moreover comprises a poppet or plunger  15  having an upper portion integral with the mobile armature  9  and a lower portion which cooperates with a valve seat  16  (shown in  FIG. 2 ) of the injection valve  7  to control the flow of fuel through the injection jet  3  in known manner.  
      As shown in  FIG. 2 , the valve seat  16  is defined by a sealing member  17 , which is disc-shaped, seals the bottom of the supply channel  5  of the supporting body  4 , and is passed through by the injection jet  3 . A guide member  18  rises up from the discoid sealing member  17 , which guide member is tubular in shape, receives within it the plunger  15  to define a lower guide for said plunger  15  and has an external diameter smaller than the internal diameter of the supply channel  5  of the supporting body  4 , so as to define an external annular channel  19  through which the pressurised fuel can flow. According to an alternative which is not shown, the guide member  18  has an external diameter which is equal to the internal diameter of the supply channel  5  and has flattened portions on the outside so as to create passages for the fuel.  
      In the lower part of the guide member  18 , there are provided four through-holes  20  (only two of which are shown in  FIG. 2 ), which are arranged perpendicularly to the longitudinal axis  2  and open into the valve seat  16  to allow the pressurised fuel to flow towards said valve seat  16 . The through-holes  20  may be arranged offset relative to the longitudinal axis  2  such that they do not converge towards said longitudinal axis  2  and, in service, they impart a swirling flow to the respective streams of fuel.  
      The plunger  15  ends in a sealing head  21 , substantially spherical in shape, which is capable of resting in sealing manner against the valve seat  16 . Furthermore, the sealing head  21  rests so as to slide on a cylindrical internal surface  22  of the guide member  18 , so that it will be guided as it moves along the longitudinal axis  2 .  
      As shown in  FIG. 3 , the mobile armature  9  is a monolithic body and comprises an annular member  23  and a discoid member  24 , which closes the bottom of the annular member  23  and has a central through-hole  25  capable of receiving an upper portion of the plunger  15  and a plurality of peripheral through-holes  26  (only two of which are shown in  FIG. 3 ) capable of allowing the fuel to flow towards the injection jet  3 . A central portion of the discoid member  24  is suitably shaped to receive a lower end of the spring  10  and hold it in position. The plunger  15  is preferably made integral with the discoid member  24  of the mobile armature  9  by means of an annular weld  27 .  
       FIG. 4  shows an alternative embodiment of the mobile armature  9 ; as shown in  FIG. 4 , the annular member  23  is distinct from the discoid member  24  and is connected rigidly to said discoid member  24  by means of an annular weld  28 .  
      The annular member  23  of the mobile armature  9  has an external diameter substantially identical to the internal diameter of the corresponding portion of the supply channel  5  of the supporting body  4 ; in this manner, the mobile armature  9  can slide relative to the supporting body  4  along the longitudinal axis  2 , but cannot make any movement transverse to the longitudinal axis  2 , relative to the supporting body  4 . Since the plunger  15  is rigidly connected to the mobile armature  9 , it is clear that the mobile armature  9  also acts as an upper guide for the plunger  15 ; as a result, the plunger  15  is guided at the top by the mobile armature  9  and at the bottom by the guide member  18 .  
      According to an alternative embodiment which is not shown, an antirebound device is attached to the lower face of the discoid member  24  of the mobile armature  9 , which antirebound device is capable of damping the rebound of the sealing head  21  of the plunger  15  against the valve seat  16  when the plunger  15  moves from the open position to the closed position of the injection valve  7 .  
       FIG. 5  shows the plunger  15 ; it can be seen that the plunger  15  has an upper rod  29  with cylindrical symmetry, to which is connected the substantially spherical sealing head  21  by means of an annular weld  30 . As shown in  FIG. 5 , the rod  29  of the plunger  15  is of different diameters along its length; in particular, the end portions of the rod  29  are of a larger diameter relative to the central portion of the rod  29 .  
      According to another embodiment shown in  FIG. 6 , the rod  29  of the plunger  15  is of a perfectly cylindrical shape with a constant diameter along its entire length. In service, when the electromagnet  8  is de-energised, the mobile armature  9  is not attracted by the fixed magnetic armature  12  and the resilient force of the spring  10  thrusts the mobile armature  9  downwards together with the plunger  15 ; in this situation, the sealing head  21  of the plunger  15  is pressed against the valve seat  16  of the injection valve  7 , so isolating the injection jet  3  from the pressurised fuel. When the electromagnet  8  is energised, the mobile armature  9  is magnetically attracted by the fixed magnetic armature  12  against the resilient force of the spring  10  and the mobile armature  9  moves upwards together with the plunger  15  until it comes into contact with said fixed magnetic armature  12 ; in this situation, the sealing head  21  of the plunger  15  is lifted relative to the valve seat  16  of the injection valve  7  and the pressurised fuel can flow through the injection jet  3 .  
      When the mobile armature  9  comes to a standstill against the fixed magnetic armature  12 , direct longitudinal stresses parallel to the longitudinal axis  2  obviously appear on the mobile armature  9 . Due to the inevitable structural tolerances of the various components, the upper surface of the mobile armature  9  may not be perfectly plane and perfectly parallel to the lower surface of the fixed magnetic armature  12  and the plunger  15  may not be perfectly perpendicular relative to the mobile armature  9 ; consequently, when the mobile armature  9  comes to a standstill against the fixed magnetic armature  12 , direct transverse stresses perpendicular to the longitudinal axis  2  may appear on the mobile armature  9 . A proportion of such transverse stresses is also transmitted to the plunger  15  and is dissipated at the level of the coupling between the sealing head  21  of the plunger  15  and the guide member  18 .  
      It is necessary to limit the intensity of the stresses which dissipate at the level of the coupling between the sealing head  21  of the plunger  15  and the guide member  18 , so as to avoid excessive localised wear phenomena of the sealing head  21 . The approach to limiting the intensity of such negative stresses has always been to limit the transverse stresses generated at the level of the mobile armature  9  by means of precision machining of the components in order to obtain very tight structural tolerances. However, it has been observed that it is also possible to use a different approach in order to limit the intensity of such negative stresses, namely instead of limiting the transverse stresses generated at the level of the mobile armature  9 , it is possible to limit the transmission of the transverse stresses from the mobile armature  9  to the sealing head  21  of the plunger  15 . To this end, it is possible to make the rod  29  of the plunger  15  in such a manner as to impart relatively high flexibility to said rod  29  (or in other words relatively low flexural rigidity) which flexibility is certainly greater than that normally present in known, currently commercially available injectors; it has in fact been observed that increasing the flexibility of the rod  29  reduces the transmission of transverse stresses from the mobile armature  9  to the sealing head  21 . In other words, if the rod  29  of the plunger  15  is sufficiently flexible, the transmission of transverse stresses from the mobile armature  9  to the sealing head  21  is reduced and it is then no longer necessary to precision machine the components with the aim of achieving very tight structural tolerances.  
      It is important to note that the rod  29  of plunger  15  must not be too flexible, because if it were too flexible it would not be capable of ensuring rapid and precise operation of the injection valve  7 .  
      Theoretical analyses and experimental testing have led to the definition of a flexibility parameter P f , which is a reliable indicator of the flexibility of the rod  29  and has the dimensions of a pressure (N/mm 2 ). It is important to note that, since the flexibility parameter P f  has the dimensions of a pressure (N/mm 2 ), said flexibility parameter P f  may be traced back to the phenomenon of contact/impact pressure wear between the sealing head  21  and the internal surface of the guide member  18 .  
      The flexibility parameter P f  is calculated using the following equation: 
 
 P   f   =K   eq   /D   h  
 
 in which: 
 
      P f  [N/mm 2 ] is the flexibility parameter;  
      D h  [mm] is the diameter of the sealing head  21  of the plunger  15 ;  
      K eq  [N/mm] is the equivalent rigidity of the rod  29  of the plunger  15 .  
      The equivalent rigidity K eq  of the rod  29  of the plunger  15  is defined by assuming that the rod  29  is restrained at one end and subjected to a force F at the opposite end such as to inflect the rod  29  by a deflection f at its free end; in the above-stated situation, the equivalent rigidity K eq  of the rod  29  is calculated using the following equation: 
 
 K   eq   =F/f  
 
 in which: 
 
      K eq  [N/mm] is the equivalent rigidity of the rod  29  of the plunger  15 ;  
      F [N] is the force applied to the free end of the rod  29 ;  
      f [mm] is the deflection of the free end of the rod  29 .  
      In the case of a rod  29  of a constant circular cross-section made from a single material, the equivalent rigidity K eq  may be calculated using the following equation: 
 
 K   eq =( E*D   s   3 )/(6.8 *L   s   4 ) 
 
 in which: 
 
      K eq  [N/mm] is the equivalent rigidity of the rod  29  of the plunger  15 ;  
      D s  [mm] is the diameter of the circular cross-section of the rod  21 ;  
      L s  [mm] is the length of the rod  21 ;  
      E [N/mm 2 ] is the modulus of elasticity of the constituent material of the rod.  
      In the case of a rod  29  made from a single material and composed of two or more cylindrical sections of different diameters, the equivalent rigidity K eq  may be calculated using the following equation: 
 
1/ K   eq =Σ i  1 /K   i  
 
 in which: 
 
      K eq  [N/mm] is the equivalent rigidity of the rod  29  of the plunger  15 ;  
      K i  [N/mm] is the equivalent rigidity of the i-th cross-section of the rod  29  calculated using the above-stated formula.  
      In order to achieve the desired effect of limiting the transmission of the transverse stresses from the mobile armature  9  to the sealing head  21  without however prejudicing the performance of the injection valve  7 , the flexibility parameter P f  must be between 1 and 2 N/mm 2 . The flexibility parameter P f  is preferably between 1.3 and 1.5 N/mm 2  and is substantially equal to approx 1.4 N/mm 2 .  
      By way of example, in order to obtain a desired value of the flexibility parameter P f , it is possible to use several approaches which are alternatives and/or may be combined with one another in different ways: the transverse section of the rod  29  may be varied, a material of greater or lesser elasticity may be used to produce the rod  29 , the cross-sectional shape of the rod  29  may be varied.