Patent Publication Number: US-6982619-B2

Title: Solenoid stator assembly having a reinforcement structure

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The invention relates to a solenoid stator assembly for an electromechanically actuated fuel injector and, more particularly, to a solenoid stator assembly with a reinforcement structure. 
   2. Background Art 
   Conventional solenoid stator assemblies for electromechanically actuated fuel injectors include a stator core with a stator coil for developing a magnetic force upon an armature of a fuel injector. The armature is typically part of a valve assembly for regulating the flow of fuel to an injector nozzle. The solenoid stator assembly commonly includes a housing formed of an electrically insulating material for enclosing the stator core and the stator coil. Electrical terminals, which extend into the housing, are connected to an input lead and an output lead for the stator coil. 
   Electrical current under the control of an electronic engine controller is distributed to the stator coil for controlling injection timing and fuel metering by the valve assembly. Fuel passing through the valve assembly during a fuel injection pulse is pressurized at a high injection nozzle pressure. Fuel passing through the valve assembly between injection pulses, which is referred to as spill fuel flow, is substantially lower than nozzle injection pressure. The stator assembly, particularly the stator housing, is in contact with the lower pressure spill flow, but the spill flow pressure still is sufficiently high to cause undesirable pressure loading. The pressurized fuel may seep between the core and the housing, thus pressurizing and deforming the housing. Continued pressure applied to the stator assembly may cause the housing to fatigue, fracture, or separate from the core. 
   Since the solenoid stator assembly is used in fuel injectors for motor vehicles, it may experience also large changes in temperature. Due to differing rates of thermal expansion of the materials used in injectors, the solenoid stator assembly may experience thermal loading, which may exacerbate separation of the housing from the stator core. Further, the solenoid stator assembly may undergo cavitation erosion caused by fluid dynamics associated with the reciprocating armature. 
   Prior art solenoid stator assemblies have attempted to overcome these difficulties with various degrees of success. For example, U.S. Pat. No. 5,155,461, which is assigned to assignee of the present invention, discloses a preloaded solenoid stator assembly to overcome the loads encountered during use. The &#39;461 patent also discloses a stator core having a plurality of external configurations for bonding with an over-molded polymer housing. 
   Attempts have been made using other prior art solenoid stator assemblies to improve robustness by providing an external housing or band, typically metallic, about an insulated housing. An example of a design of this type is disclosed in U.S. Pat. No. 5,339,063 issued to Pham. Another prior art reference, U.S. Pat. No. 5,926,082, issued to Coleman et al., discloses a reinforcement band disposed about the lower end of a stator housing. 
   Although the prior art references disclose various solenoid stator assemblies that are structurally enhanced to overcome mechanical and hydraulic loads, they generally are costly due to complex manufacturing processes required and the special materials needed. 
   SUMMARY OF THE INVENTION 
   The present invention comprises a solenoid stator assembly for a control valve actuator assembly of an electro-mechanically actuated fuel injector characterized by enhanced robustness. The assembly includes a permeable stator core having a central pole piece and an outer pole piece, each terminating at a pole face. A stator coil is wound about the central pole piece for developing a magnetic flux flow path. A housing formed of an electrically insulating material, such as a moldable polymer, encloses the stator core and stator coil such that the pole face is oriented proximate to an armature with a calibrated air gap therebetween. A reinforcement structure disposed within the housing is oriented generally about the stator core for structurally enhancing the housing. A pair of electrical terminals extends through the housing for completing an electrical circuit through the stator coil. 
   The present invention further comprises a method for forming a robust, structurally-enhanced solenoid stator assembly described above. The method includes the step of orienting a stator coil about a central pole piece for a stator core. Then the stator core and a reinforcement structure are inserted into a mold, the reinforcement structure being spaced from the stator core throughout the stator core periphery. An electrically insulating material, such as a moldable polymer, then is injected between the reinforcement structure and the stator core using an injection molding technique, thereby forming a housing about the stator core that encapsulates the reinforcement structure. 
   The reinforcement structure supports compression loads of attachment bolts that secure the actuator assembly of which the stator assembly is a part to an injector body. The design of the stator assembly further provides stiffness in a radial direction as well as in the direction of the axis of the armature. 
   By encapsulating the reinforcement structure with a molded polymer, there is no need to use a pressing operation for assembling the reinforcement structure in place. Press fits that would be required in such a pressing operation would require close dimensional control to avoid stress failure due to mechanical forces associated with press fitting. 
   During manufacture, the stator core face is finish-ground in a post-encapsulation step. The presence of the encapsulating polymer will allow any burrs developed during grinding to be flushed away by coolant fluid. There is not a cavity surrounding the core where burrs can accumulate. 
   The stator, which is defined by steel laminations, does not need to be contoured to reduce fuel seepage or to secure the polymer encapsulation to the stator. Because of this, there is no reduction in magnetic force on the armature for a given actuating current, and injector response is improved. 
   The single, one-piece reinforcement structure has a further manufacturing advantage because it can be formed from a flat steel workpiece using a series of punching and forming steps. The seam that is created then can be welded or crimped. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a partial sectional view of a fuel injector that includes the solenoid stator assembly of the present invention; 
       FIG. 1   a  is a side elevation view of the injector of  FIG. 1 ; 
       FIG. 2  is an enlarged cross-sectional view of the stator assembly of the injector of  FIG. 1 ; 
       FIG. 2   a  is a side elevation of the stator assembly of  FIG. 2 , seen from the right side of the stator assembly of  FIG. 2 ; 
       FIG. 3  is a side elevation view of the stator core and housing of  FIG. 2 , seen from the left side of the stator assembly of  FIG. 2 , with parts shown by phantom lines; 
       FIG. 4  is a perspective view of a first embodiment of a reinforcement structure; 
       FIG. 5  is a view similar to  FIG. 3 , with parts shown by phantom lines, of an alternate embodiment of the invention; 
       FIG. 5   a  is a detail isometric view of a reinforcement element of the alternate embodiment of the invention shown in  FIG. 5 ; 
       FIG. 5   b  is an isometric assembly view of reinforcement elements of the alternate embodiment of  FIG. 5 ; and 
       FIG. 6  is a plan view of another alternate embodiment of a reinforcement structure embodying features of the invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1  shows a unit pump for a fuel injector assembly. It comprises a pump body  10 , which is formed with a central cavity or bore  12  in which a piston plunger  14  is situated. The plunger  14  and the bore  12  define a high-pressure pumping chamber  16 , which is in communication with a high-pressure fuel delivery passage  18 . 
   A control valve chamber  20  is formed in the upper portion of the body  10 . It intersects the high-pressure fuel delivery passage  18  as shown. A control valve element  22  is positioned in the valve chamber  20 . A valve seat  24  formed in the pump body at the left end of the valve opening  20  is engaged by a valve land on the end of valve element  22 , as shown at  26 . 
   A valve stop opening  28  receives a valve stop  30  situated in close proximity to the valve land  26 . When the valve element  22  is shifted in a left-hand direction, the valve land  26  becomes unseated, thereby establishing communication between valve stop chamber  28  and passage  18  through the valve space defined by annular valve opening  25  surrounding the valve element  22 . When the valve element  22  is shifted in the right-hand direction to close the valve land  26  against the valve seat  24 , a high injection pressure is developed in passage  18  as the plunger  14  is driven into the pumping chamber  16 . 
   Plunger  14  is connected to a spring shoulder element  32 , which engages plunger spring  34 . Spring  34  is seated on spring body seat  36  on the pump body  10 . 
   The plunger  14  and the spring seat element  32  are driven with a pumping stroke by engine camshaft-operated cam follower assembly  38 . A spring sleeve  40 , surrounding spring  34 , is carried by the follower assembly  38 . 
   A low-pressure spill passage  42  communicates with the valve stop space  28  and returns fuel from passage  18  to a flow return port in communication with annular groove  44  in the pump body  10 . A fuel supply groove  46 , which is connected to a fuel supply pump, communicates with a valve spring chamber  48 . A valve spring  50  in the valve spring chamber  48  is seated on spring seat  52  and is engageable with a spring shoulder  54  carried by valve element  22 . The spring  50  normally urges the valve element  22  to an open position, the limit of the valve travel being determined by valve stop  30 . The spacing between valve element  22  and the stop  30  is shown at  29 . 
   The valve element  22  is connected to an armature  56 , which forms a part of the actuator assembly. This will be described in detail with reference to  FIGS. 2–4 . The injector assembly includes a fluid fitting  58 , which is connected to a fuel injection nozzle (not shown). 
   Reference may be made to U.S. Pat. No. 6,276,610, issued to Gregg R. Spoolstra, for an understanding of the mode of operation of the valve and valve actuator for developing a fuel injection pressure pulse in passage  18 . The actuator assembly is generally designated in  FIGS. 1–4 , as well as in  FIG. 1   a , by reference numeral  60 . 
   Fuel is supplied to spring chamber  48  through passage  62 , which in turn communicates with the valve stop chamber  28  through crossover passage  64 . The spring chamber communicates also with the valve stop chamber  28  through an internal passage (not shown) formed in the valve element  22 . 
   As seen in  FIGS. 2 ,  2   a  and  3 , the actuator assembly  60  includes a solenoid stator assembly  62  and the previously described armature  56 . The solenoid stator assembly includes a stator core  64 , which is comprised of laminations of permeable magnetic material, such as carbon steel. The laminations can be seen best in the end view of  FIG. 3 . The cross-section of the stator core, when viewed in  FIG. 2 , has a generally E-shaped profile with a central pole piece  66  sorrounded by outer pole piece  68 . Each of these pole pieces terminates at a pole face oriented proximate to mounting end  63  of the solenoid stator assembly  62 . The outer pole piece  68  and the central pole piece  66  are integrally formed in the embodiment illustrated. However, the outer pole piece  68  may be formed instead by a flux guide that is separate from the central pole piece  66 . 
   A stator coil  70  is oriented about the stator core central pole piece  66 . The stator coil  70  comprises conductor windings wound about a bobbin or spool positioned about central pole piece  66 . The windings of the stator coil  70  are insulated in known fashion to prevent a short circuit between individual windings and between the windings and the stator core  64 . 
   The stator coil  70  includes a pair of leads, not shown, for connecting it to a power source. The solenoid stator assembly  62  may include a pair of electrical terminals  88  and  90  extending from the assembly. Each of the terminals  88  and  90  is connected to one of the pair of leads emerging from the stator coil  70 . As the current flows through the stator coil  70 , a magnetic field is generated, providing a flux flow pattern at the central pole piece  66 . Selective control of current through the stator coil  70  provides timed actuation of the armature  56 . 
   The solenoid stator assembly  62  includes a housing  65  formed of an electrically insulating material, preferably a polymer, for enclosing the stator core  64  and stator coil  70 . The housing  65  is generally cup shaped with a closed end  75  and an open end at a mounting surface  76  of the solenoid stator assembly  62 , as seen in  FIG. 2 . The housing  65  has an outer wall  72  and an internal cavity  74  enclosing the stator core  64  and stator coil  70  such that the distal end of the stator core central pole piece  66  is oriented proximate to the mounting surface  76  of the housing  65 . The mounting surface  76  is formed about the periphery of the wall  72  and is attachable to the fuel injector body  10  for sealed engagement therewith. Accordingly, the housing  65  includes a hole pattern  78 , as best illustrated in  FIGS. 2   a  and  3 . The hole pattern  78  includes a plurality of apertures for receiving fasteners, such as bolts  80 , for attaching the solenoid stator assembly  62  to the fuel injector body  10 , as illustrated in  FIG. 1 . A spacer  82 , of the same general shape as the shape of housing  65 , is interposed between body  10  and surface  76 . O-ring seals  83  and  85  prevent leakage. The housing  65  encloses the inner components of the solenoid stator assembly  62 .  FIG. 3  is an end view of the stator assembly  62  with the inner components shown in phantom, including the laminations of stator core  64 . 
   The housing  65  is preferably formed by an injection molding process. Injection molding is a cost effective method for forming the housing  65  and for encapsulating the stator core  64 . Further, the injection molding process securely bonds the housing  65  to the stator core  64 . In order to improve bonding engagement between the stator core  64  and the housing  65 , the stator core  64  may include a plurality of external attachment slots  84  for mechanically interlocking the housing  65  to the external surfaces of the stator core  64 . This mechanical interlock enhances the attachment and helps prevent pressurized fuel from seeping between the core and the housing. 
   The solenoid stator assembly  62  further includes an insulator cap  86  for supporting the terminals  88  and  90  outside of the housing  65 . The leads for coil  70  are electrically connected to terminals  88  and  90 , preferably by soldering. The cap  86  is formed of a suitable electrically insulating material and rests atop the stator assembly  62  for properly orienting the terminals  88  and  90 , as shown, during the molding process. The insulator cap  86  also includes grooves  92  for mechanically retaining in place wire leads for stator coil  20  during the encapsulating step. The wire leads are routed through grooves  92  as they are extended to terminals  88  and  90 . 
   The coil  70  further includes a rigid, insulating seal  94  for preventing pressurized fuel from seeping within the stator core  62  about the stator coil  70 . The seal  94  may be integral with the spool or bobbin of which coil  70  is a part. The seal  94  may be integral also with the housing  65  and may be formed during the injection molding process of the housing  65 . 
   The solenoid stator assembly  62  includes an elongate reinforcement structure  96  disposed within the housing  65 . The reinforcement structure  96  is oriented generally about the stator core  64  for structurally enhancing the housing  65 . The reinforcement structure  96  has a length generally equal to that of the housing  65 . 
   One embodiment of the reinforcement structure  96  is best illustrated in  FIGS. 3 and 4 . It is generally rectangular and may be formed from a band of stamped sheet steel manufactured in a progressive die stamping operation. Accordingly, the band would be crimped or welded together to form the continuous tubular design. Alternatively, the tubular profile of the reinforcement structure  96  could be cut from an elongate tubular piece of steel, thus eliminating the crimping or welding operation. 
   The reinforcement structure  96  is preferably formed from low carbon steel for structurally enhancing the housing  65 . It supports compressive loads applied by the plurality of fasteners  80  that mount the actuator assembly  60  to the fuel injector body  10 , as illustrated in  FIG. 1 . A reinforcing plate  55 , seen in  FIG. 2 , can be positioned on the outer side of closed end  75 , the fasteners  80  extending through fastener openings in plate  55 . Plate  55  can be used also as a name plate if that is desired. 
   The reinforcement structure  96  also enhances the housing  65  by providing support for internal pressure loading applied by pressurized fuel in the fuel injector body  10 . Accordingly, the reinforcement structure  96  may experience hoop stress about its periphery. It may be oriented relative to the hole pattern  78  for enclosing the pressure loaded regions of the housing  65 . The reinforcement structure  96  is oriented within the wall  72  for preventing radial deformation of the insulating material of the housing  65 , thereby preventing fatigue failure. 
   Preferably, the reinforcement structure  96  is molded within the housing  65 , as is the stator core  64  and stator coil  70 . These components are inserted into a mold and then the polymer material forming the housing  65  is injection molded thereabout. To enhance the engagement of the housing  65  and the reinforcement structure  96 , the reinforcement structure may include a plurality of configurations, such as cutouts  98  and  98 ′, seen in  FIG. 4 , for mechanically interlocking the electrically insulating material of the housing  65  with the reinforcement structure  96 . One of the cutouts  98 ′ is used to provide clearance for the terminals  88  and  90 , which extend from the housing  65 . The reinforced housing  65  is effective for supporting compressive loads as well as hydraulic pressure loading. 
   The simplified solenoid stator assembly  62  eliminates several manufacturing steps needed in the manufacture of prior art designs, such as press fitting an external sleeve about the housing. Additionally, machining of the mounting surface  76  does not require a deburring operation because the reinforcement structure  96  is disposed within the wall  72 . The distal ends of the central pole piece  66  and the outer pole piece  68  are not covered by insulating material, which enhances the magnetic force and consequently the injector response. 
     FIGS. 5 and 5   a  show an alternative embodiment of a solenoid stator assembly in accordance with the present invention. Similar elements shown in these figures retain same reference numerals with prime notations, but new elements are assigned new reference numerals. The solenoid stator assembly  62 ′ includes a distinct pair of reinforcement elements  100  and  102 , which are oriented about the stator core  62 ′ and positioned within the wall  72 ′ of the housing  65 ′. Although the reinforcement structure provided by reinforcement members  100  and  102  reduces material costs, it is not as resistant to hydraulic pressure loading as a continuous design, as in the embodiment of  FIGS. 1–4 . Accordingly, the reinforcement elements  100  and  102  may be ideal in applications having lower pressure loading, thus reducing the cost of the solenoid stator assembly. Reinforcement elements  100  and  102  have rounded end openings  106  and  106 ′, which receive clamping bolts. 
     FIG. 6  shows another alternative embodiment of a reinforcement structure for a solenoid stator assembly. The reinforcement structure of  FIG. 6  is generally of square, tubular shape, as shown at  108 , and is disposed within the wall  72 ″ of the housing. Unlike the prior embodiments, the entire perimeter of the reinforcement structure  108  is oriented within the hole pattern  78 ″. This alternative design directs compressive loads applied in a region proximate to each individual fastener aperture in a direction that is opposite to that of the hydraulic pressure loading. Accordingly, this alternative design structurally enhances in an alternate fashion the structural integrity of the solenoid stator assembly. 
   While embodiments of the invention have been illustrated and described, it is not intended that these embodiments illustrate and describe all possible forms of the invention. Rather, the words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the invention.