Patent Publication Number: US-2006010683-A1

Title: Method of manufacturing ignition coil filled with resin

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
      This application is based upon and claims benefit of priority of Japanese Patent Application No. 2004-209722 filed on Jul. 16, 2004, the content of which is incorporated herein by reference.  
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to a method of manufacturing an ignition coil for igniting an internal combustion engine.  
      2. Description of Related Art  
      An ignition coil having a primary coil, a secondary coil and a magnetic core contained in a casing is conventionally manufactured by supplying thermoplastic resin into the casing in which the components are already contained, as exemplified in JP-A-2003-318056. On the other hand, an ignition coil directly connected to a spark plug and ignition coil integrally formed with a spark plug have been proposed. Further, a space available for an ignition coil is becoming smaller due to requirements for downsizing an engine or for installing two plugs to each cylinder. Therefore, it has been strongly required to make an ignition coil small in size.  
      In a small-sized ignition coil, insulating resin has to be supplied into the casing through a small opening. Further, it has been proposed to cover either one of a primary coil or a secondary coil with resin (potting with resin) to reduce stress imposed on components of a small-sized ignition coil. In this case, the resin has to be supplied only to an outer circumference of one of the coils through a small opening. A considerably long time is required to supply the resin through a small opening. Sometimes, it has been difficult to supply the resin only to limited portions through a small opening.  
     SUMMARY OF THE INVENTION  
      The present invention has been made in view of the above-mentioned problems, and an object of the present invention is to provide a method of manufacturing an ignition coil, in which insulating resin is easily supplied even when resin-potting is required only for limited portions.  
      The ignition coil includes a cylindrical casing, an outer coil functioning as a primary coil, and an inner coil functioning as a secondary coil. The outer coil is wound around a cylindrical outer spool, and the inner coil is wound around cylindrical inner spool. The outer coil wound around the outer spool and the inner coil wound around the inner spool are coaxially disposed in the cylindrical casing. An inner space of the cylindrical casing including a first space between the inner coil and the outer spool and a second space between the outer coil and the cylindrical casing is filled with thermosetting insulating resin. Alternatively, either the first space or the second space may be filled with the resin if such is advantageous to avoid additional stress imposed on the components of the ignition coil.  
      The ignition coil is manufactured in the following process. First, a plug cap and other components are connected to the cylindrical casing, forming a case unit, and the inner coil wound around the inner spool is inserted into the outer spool around which the outer coil is wound, forming a coil unit. Then, a predetermined amount of liquid state resin is supplied into the cylindrical casing. Then, the coil unit is inserted into the cylindrical casing containing the resin therein. The liquid state resin contained in the cylindrical casing is pushed up, filling the inner space of the cylindrical casing including the first space and the second space. Finally, the coil assembly is heated to harden the resin filling the inner space of the cylindrical casing.  
      Preferably, the steps of supplying the resin into the cylindrical casing and inserting the coil unit are performed under a vacuum atmosphere to avoid generation of voids in the resin. An additional resin may be added after the coil unit is inserted into the cylindrical casing to supply sufficient amount of the resin to the inner space. A viscosity of the liquid state resin is set in an adequate range, e.g., 0.3-1.35 Pa·S, and a diameter of a nozzle for supplying the resin is made larger than 1.0 mm. The inner spool and the outer spool are formed so that the resin is supplied only to desired spaces, i.e. both of the first and the second spaces, or either one of the spaces.  
      According to the present invention, an opening for supplying the resin can be made sufficiently large because the coil unit is inserted into the cylindrical casing after the resin is supplied to the casing. Therefore, the resin-supplying step can be easily and quickly performed. Further, only the desired portions in the cylindrical casing can be filled with the resin by forming the spools in a proper shape. The desired spaces are surely filled with the resin even the spaces are small, because the coil unit is inserted into the cylindrical casing in which the liquid state resin is already contained. The present invention is similarly applicable to an ignition device having a spark plug integrally connected to an ignition coil.  
      Other objects and features of the present invention will become more readily apparent from a better understanding of the preferred embodiments described below with reference to the following drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a cross-sectional view showing an ignition coil as a first embodiment of the present invention;  
       FIG. 2  is a cross-sectional view showing a process of supplying resin into a cylindrical casing;  
       FIG. 3A  is a cross-sectional view showing a coil unit to be inserted into an inner bore of the cylindrical casing;  
       FIG. 3B  is a cross-sectional view showing the casing in which a predetermined amount of resin is contained;  
       FIG. 4  is a cross-sectional view showing an ignition coil as a second embodiment of the present invention;  
       FIG. 5  is a cross-sectional view showing an ignition coil integrally formed with a spark plug as a third embodiment of the present invention; and  
       FIG. 6  is a cross-sectional view showing an ignition coil as a fourth embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      A first embodiment of the present invention will be described with reference to  FIGS. 1-3B . An ignition coil shown in  FIG. 1  is used for supplying a high voltage (e.g., 30 kV) to a spark plug installed in a cylinder of an internal combustion engine. The ignition coil is formed in a rod-like shape and directly connected to an insulator of a spark plug.  
      As shown in  FIG. 1 , an elongate center core  110  made of silicon steel is positioned in the center of the ignition coil. Permanent magnets  111 ,  112 , each having a polarity opposite to a polarity induced by an outer coil  120  (functioning as a primary coil), are positioned at both longitudinal ends of the center core  110 . An inner coil  130  functioning as a secondary coil is disposed outside the center core  110 . The inner coil  130  is wound around a cylindrical inner spool  131  made of an insulating resin material such as PPE. The bottom end of the cylindrical inner spool  131  is closed and the open top end thereof is closed with a cap  132 . The center core  110  and the permanent magnets  111 ,  112  are disposed in the cylindrical inner spool  131 .  
      A high voltage of the inner coil  130  is electrically connected to a spark plug (not shown), which is coupled to a plug cap  170 , through a high tension terminal  133  and a spring  134  made of a conductive material. A low voltage side of the inner coil  130  is electrically connected to an on-board direct current power source or the ground. An outer coil  120  functioning as a primary coil is disposed outside the inner coil  130 . The outer coil  120  is wound around a cylindrical outer spool  121  made of an insulating resin material such as PPE. A primary current is intermittently supplied to the outer coil  120  based on signals from an ignitor (not shown), and a high voltage is induced in the inner coil  130 .  
      A cylindrical outer core  140  made of silicon steel or the like is disposed outside the outer coil  120 . A cylindrical casing  150  made of a resin material such as PPS is disposed outside the outer core  140 . A high tension tower  160  made of a resin material in a stepped cylindrical shape is fixedly connected to the bottom end of the cylindrical casing  150 . A high tension terminal is fixedly inserted into a center hole of the high tension tower  160 . The bottom end of the cylindrical casing  150  is closed with the high tension tower  160  and the high tension terminal  133 . A cylindrical plug cap  170  made of a resin material is fixedly connected to a high tension tower  160  sticking out of the casing  150 .  
      A cylindrical connector  180  made of resin is connected to the top end of the casing  150 , thereby closing the top end of the casing  150 . A terminal  181  for supplying a primary current to the outer coil  120  is positioned in the connector  180 . The inner space of the casing  150 , including a first space  100  between the inner coil  130  and the outer spool  121  and a second space  200  between the outer coil  120  and the outer core  140 , is filled with insulating resin  190 . Thermosetting epoxy resin is used as the insulating resin  190 .  
      Now, a process of manufacturing the ignition coil will be described. As shown in  FIG. 2 , the high tension terminal  133 , the spring  134 , the outer core  140 , the casing  150 , the high tension tower  160  and the plug cap  170  are connected to one another, forming a case unit  400 . On the other hand, the center core  110 , the inner coil  130  wound on the inner spool  131 , the outer coil  120  wound on the outer spool  121 , permanent magnets  111 ,  112 , the cap  132  and the terminal  181  are all assembled as shown in  FIG. 3A , forming a unitary coil unit  500 .  
      Then, as shown in  FIG. 2 , a predetermined amount of the insulating resin in a liquid state is supplied into the case unit  400  from a nozzle  201  of a resin-supplying device  202 . The diameter of the nozzle  201  is made larger than 1.0 mm, and the liquid state resin is kept at a temperature of 70° C. A viscosity of the resin at 70° C. is in a range of 0.30-1.35 Pa·S.  
      Then, as shown in  FIGS. 3A and 3B , the coil unit  500  is inserted into the inner bore of the case unit  400  in which the liquid state resin is contained at its bottom end portion. By inserting the coil unit  500 , the resin located at the bottom portion of the case unit  400  is pushed up, and the inner space of the casing  150  including the first space  100  and the second space  200  is filled with the resin. The amount of the resin supplied to the case unit  400  is determined so that the first space  100  and the second space  200  are fully filled with the resin. In order to avoid generation of voids in the resin, the process of supplying resin into the case unit  400  and the process of inserting the coil unit  500  are performed under a vacuum atmosphere.  
      After the coil unit  500  is inserted into the case unit  400 , the ignition coil is heated to harden the thermosetting resin  190  filling the inner space of the casing  150 . Thus, the process of manufacturing the ignition coil shown in  FIG. 1  is completed.  
      The opening of the case unit  400  for supplying the resin can be made sufficiently large since the resin is supplied into the casing  150  before the coil unit  500  is inserted into the casing  150 . Therefore, the liquid state resin can be quickly supplied into the casing  150 . The inner spool  131 , both longitudinal ends of which are closed, is inserted into the case unit  400 . Therefore, the resin does not enter into an inner bore of the inner spool  131  even when the resin is pushed up above the top end of the inner spool  131 . This is advantageous when supply of the resin into the inner bore of the inner spool  131  is not desired.  
      A second embodiment of the present invention will be described with reference to  FIG. 4 . The second embodiment is similar to the first embodiment described above, except that only the first space  100  between the inner coil  130  and the outer spool  121  is filled with the resin  190  while keeping the second space  200  unfilled. In other words, in the second embodiment, resin-potting is applied only to the inner coil  130 .  
      As shown in  FIG. 4 , a circular flange  122  contacting the outer core  140  is formed on a bottom portion of the outer spool  121 . Further, a cylindrical wall  123  extending above the cap  132  is additionally formed at the top end of the inner spool  121 . The second embodiment is manufactured in the same manner as the first embodiment. In the process of inserting the coil unit  500  into the case unit  400  containing the liquid state resin, the resin is pushed up in the same manner as in the first embodiment. However, the resin  190  is prevented by the circular flange  122  from further flowing up beyond the circular flange  122 . Further, the resin overflowing from the top end of the first space  100  is prevented from entering into the second space  200  by the extended cylindrical wall  123 . Therefore, the resin  190  is not supplied to the second space  200 , while filling the first space  100  with the resin  190  in the same manner as in the first embodiment.  
      A third embodiment of the present invention will be described with reference to  FIG. 5 . An ignition device shown in  FIG. 5  includes a spark plug  320  and an ignition coil  330 , both being integrally formed. The spark plug  320  and the ignition coil  330  are contained in a common cylindrical casing  310 . The ignition device is installed in a cylinder head of an internal combustion engine, so that the spark plug  320  exposes to a combustion chamber of the engine.  
      The casing  310  is made of a metallic material that is conductive and magnetic, such as S45C or SUS430. A cylindrical ceramic insulator  340  integrally forming a plug insulator  341  and an outer spool  342  is inserted in the casing  310 . An outer coil  331  functioning as a primary coil is wound on the outer spool  342 . The spark plug  320  includes a metallic stem  321 , a center electrode  322  made of a conductive material and a ground electrode  323  made of a conductive material. The metallic stem  321  and the center electrode  322  are inserted into a center hole of the plug insulator  341 , and a tip portion of the center electrode  322  exposes to the combustion chamber. The ground electrode  323  is connected to the casing  310  by welding or the like to face the center electrode  322  with a certain spark gap apart therefrom.  
      The ignition coil  330  includes an outer coil  331  functioning as a primary coil wound on an outer spool  342 , an inner coil  332  functioning as a secondary coil wound on an inner spool  334 , and a center core  333  made of a magnetic material in a pillar shape. The inner spool  334  is made of insulating resin in a cylindrical shape having a closed bottom end. The outer coil  331  is electrically connected to terminals  351  positioned in a connector  350 . A primary current is supplied to the outer coil  331  from the terminals  351  based on signals from an ignitor (not shown).  
      The center core  333 , the inner coil  332  wound on the inner spool  334  and the outer coil  331  wound on the outer spool  342  are coaxially disposed in the cylindrical casing  310 . A space between the inner coil  332  and the outer spool  342  is filled with resin  360 . The resin  360  is insulating resin such as thermosetting epoxy resin. A high voltage end of the inner coil  332  is electrically connected to the center electrode  322  through the metallic stem  321 , and its low voltage end is electrically connected to the casing  310  through a terminal (not shown). The casing  310  is grounded to a vehicle body through the cylinder head. A high voltage generated in the inner coil  332  is discharged through the spark gap between the center electrode  322  and the ground electrode  323 , and thereby mixture in the combustion chamber is ignited.  
      Now, a process of manufacturing the ignition device shown in  FIG. 5  will be described. The ceramic insulator  340 , on which the outer coil  331  is wound and to which the metallic stem  321  and the center electrode  322  are connected, is inserted into the cylindrical casing  310  to which the ground electrode  323  is connected. Thus, a case unit as an integral body is formed. On the other hand, an inner spool unit is formed by winding the inner coil  334  around the inner spool  334  and inserting the center core into the inner spool  334 .  
      Then, liquid state resin is supplied into the case unit, i.e., into the inner bore of the ceramic insulator  340  from a resin-supplying device. A predetermined amount of the liquid state resin is contained in a bottom portion of the case unit. Then, the inner spool unit is inserted into the case unit. By inserting the inner spool unit, the resin contained in the bottom portion of the case unit is pushed up and flows into the space between the inner coil  332  and the outer spool  342 . The amount of resin supplied to the case unit is determined so that the resin does not overflow the outer spool  342 , while fully filling the space between the inner coil  332  and the outer spool  342 . Thus, only the inner coil  332  is potted with the resin, and the outer coil  331  is kept free from the resin. Then, the connector  350  is fixedly connected to the casing  310 , closing the upper opening of the casing  310 . In order to avoid generation of voids in the resin  360 , the process of supplying resin into the case unit and the process of inserting the inner spool unit are performed under a vacuum atmosphere.  
      Then, the resin  360  filling the space between the inner coil  332  and the outer spool  342  is hardened by heating the assembled ignition device. Thus, the process of manufacturing the ignition device shown in  FIG. 5  is completed.  
      Since the liquid state resin is supplied into the case unit from an open upper end thereof, the resin-supplying process can be performed in a short period of time. Since the inner spool unit having a closed bottom is inserted into the resin contained in the case unit, the resin does not enter into the inner bore of the inner spool  334 . This is advantageous when potting of the inner bore of the inner spool  334  is not desired.  
      A fourth embodiment of the present invention will be described with reference to  FIG. 6 . The fourth embodiment is similar to the first embodiment, except that only the second space  200  between the outer coil  120  and the outer core  140  is filled with the resin while leaving the first space  100  unfilled.  
      The process of inserting the coil unit into the case unit is performed in the same manner as in the first embodiment. To prevent the resin  190  from flowing into the first space  100 , a circular projection  1311  contacting the outer spool  121  is formed around the outer surface of the inner spool  131 . The resin pushed up by inserting the coil unit into the case unit is stopped at the circular projection  1311 . The first space  100  is kept free from the resin while the second space  200  is sufficiently filled with the resin. Other process of manufacturing the fourth embodiment is the same as that of the first embodiment.  
      The present invention is not limited to the embodiments described above, but it may be variously modified. For example, though the secondary coil is disposed inside and the primary coil is disposed outside in the foregoing embodiments, their relative positions in the cylindrical casing may be reversed. The amount of resin to be supplied in the casing is predetermined in the foregoing embodiments. However, a certain amount of resin may be supplied into the casing in the resin-supplying process, and an additional amount of resin may be supplied after the coil unit is inserted so that a total amount of resin becomes adequate. It is also possible to supply the resin in the following manner: inserting the outer spool  121  on which the outer coil  120  is wound into the case unit  400  containing the resin; then supplying some more resin into the outer spool  121 ; and then inserting the inner spool  131  on which the inner coil  130  is wound into the outer spool  121 .  
      While the present invention has been shown and described with reference to the foregoing preferred embodiments, it will be apparent to those skilled in the art that changes in form and detail may be made therein without departing from the scope of the invention as defined in the appended claims.