Patent Publication Number: US-6337617-B1

Title: Ignition coil device having spool including glass fiber and silica

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application relates to and incorporates herein by reference Japanese Patent Application No. 11-41651 filed on Feb. 19, 1999. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a stick-type ignition coil device which is directly mountable in a plug hole of an internal combustion engine. 
     2. Related Art 
     Stick-type ignition coil devices as proposed in JP-A10-289831 (U.S. patent application Ser. No. 09/023,613 filed Feb. 13, 1998) must be sized under the limitation that it is fitted in a narrow plug hole of an internal combustion engine. A resinous insulating material fills in the ignition coil device to ensure electrical insulation between various members closely disposed in the ignition coil device. Spools for windings are shaped in an elongated cylindrical form and disposed coaxially around a stick-shaped central core. Each spool is preferably as thin as possible not to enlarge the outer diameter of the ignition coil device. Glass fibers are admixed in a resin base material as a reinforcing material to restrict plastic deformation of thinned spools. Further, a rubber material may be admixed in the resin base material to increase toughness of the spool. 
     However, micro voids tend to occur around the glass fibers due to difference in the linear thermal expansion coefficients between the resinous base material and the glass fibers, when the spool is molded from an admixture of the resinous base material and the glass fibers. Further, the rubber material which has a lower thermal decomposition temperature tends to sublimate due to electrical discharges to cause voids, if the rubber material is admixed in the resin base material. These voids will enable the discharges to occur from the surface of the spool to the voids, thus causing treeing which is a kind of dielectric breakdown. If treeing grows to cause the dielectric breakdown in the spool, the spool will lose its insulating function. If treeing further passes through the resinous insulating material and grows to bridge a high voltage part and a low voltage part in the ignition coil device, a secondary coil of the ignition coil device will be unable to generate a required high voltage. 
     Further, because the resinous insulating material not only ensures electrical insulation but also cements the various members to one another, the members having different linear thermal expansion coefficients are subjected to restraining forces when expanding and contracting in accordance with changes in surrounding temperature. Thus, the spool tend to distort and tend to crack in the end. Cracks in the spool will cause electrical discharges between adjacent coil wires. 
     It has therefore been proposed to wind a thin film around the outer periphery of the spool, or to coat the coil wires for enabling a separation between the thin film and the resinous insulating material cementing the coil or for enabling a separation between the coated coil and the resinous insulating material. Thus, the inner peripheral side and the outer peripheral side of the ignition coil can expand and contract independently of each other thereby restricting spool cracking. 
     However, the electrical discharge concentrates in the voids caused by the separation, thus causing erosion locally on the surface of the spool. The local erosion will enable treeing to grow, resulting in the dielectric breakdown of the spool. Although the continuous part of the thin film is less likely to be eroded by the electrical discharge, the spool is still possibly eroded by the electrical discharge passing through connection parts of the thin film. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide an ignition coil device that is capable of restricting treeing caused by electrical discharges in a spool from growing. 
     According to the present invention, an ignition coil device for internal combustion engines includes a stick-type core, a primary spool disposed coaxially with the core, a primary coil wound around the primary spool, a secondary spool disposed coaxially with the core, a secondary coil wound around the secondary spool, and a resinous insulating material filling a space in those parts. At least one of the spools located between the primary coil and the secondary coil is made of a resin base material admixed with glass fibers and silica. The glass fibers restricts plastic deformation of the spool and silica restrict a growth of treeing in the spool caused by electrical discharges. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description made with reference to the accompanying drawings. In the drawings: 
     FIG. 1 is a sectional view showing an ignition coil device according to an embodiment of the present invention; 
     FIG. 2 is a schematic sectional view showing a mode of separation between a primary coil and a primary spool in the embodiment of FIG. 1; 
     FIG. 3 is a table showing a result of experiments conducted on the embodiment shown in FIG.  1  and comparative examples; and 
     FIG. 4 is a sectional view showing an ignition coil device according to a modification of the embodiment of FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring first to FIG. 1, an ignition coil device  10  is constructed as a stick-type for mounting in a plug hole in an internal combustion engine (not shown) and is electrically connectable to a spark plug (not shown) at its lower side. 
     The ignition coil device  10  comprises a coil casing  11  and a high voltage tower  12  both of which are made of a resin material and in a cylindrical shape. The coil casing  11  accommodates therein a central core  15 , permanent magnets  16 ,  17 , a secondary spool  20 , a secondary coil  21 , a primary spool  23 , a primary coil  24  an outer core  25  and the like. An epoxy resin  26  fills spaces in the coil casing  11  and the high voltage tower  12  to electrically insulate the component parts accommodated therein. 
     The central core  15  is made of thin silicon steel plates stacked in the radial direction to provide a stick-type cylindrical shape. The permanent magnets  16 ,  17  are positioned at the top side and the bottom side of the central core  15 . The permanent magnets  16 ,  17  are magnetized in the polarities which are opposite to the direction of magnetic flux generated upon energization of the primary coil  23 , so that the output voltage generated by the secondary coil  21  increases. A cylindrical rubber member  18  surrounds the outer peripheral surface of the central core  15 . 
     The primary spool  23  is made of a resin material and disposed outside the secondary spool  21 . Specifically, as shown in FIG. 2, the resin material for the primary spool  23  includes a base material  50  such as PBT (polybutylene terephthalate resin) which has a low melting viscosity and a high flowability for molding. The PBT is added with an olefin rubber (not shown) in 5 wt. %, glass fibers  51  in 12.5 wt. % and granular silica (not shown) in 12.5 wt. %. The glass fibers  51  are admixed to restrict plastic deformation of the primary spool  23 . The olefin rubber is admixed to increase toughness of the primary spool  23 . The silica is admixed to restrict growth of treeing in the primary spool  23 . Acrylic rubber or any other rubber may alternatively be used in place of the olefin rubber to increase the toughness of the primary spool  23 . 
     The primary coil  24  is constructed by winding an electrical coil wire  60  which comprises a wire body  61  and a separating material  62  coated around the wire body  61  around the outer periphery of the primary spool  23 . The separating material  62  may be PET (polyethylene terepthalate), silicone, wax or the like which has an electrical insulating property. 
     The secondary spool  20  is disposed outside the rubber member  18  and made of a resin material. The secondary spool  20  may be molded from the same composition as the primary spool  23  or from the similar composition which does not include silica as opposed to the primary spool  23 . The secondary coil  21  is wound around the secondary spool  20 . A dummy coil  22  is wound some turns at the high voltage side of the secondary coil  21 . The dummy coil  22  connects the secondary coil  21  to a terminal plate  40 . Because the secondary coil  21  and the terminal plate  40  are electrically connected not via a single straight wire but via the dummy coil  22 , the surface area of contact between the secondary coil  21  and the terminal plate  40  increases thereby to reduce the concentration of the electric field on the electrical connection part. 
     The outer core  25  is disposed outside the primary coil  24 . The outer core  25  is made of a thin silicon steel plate wound cylindrically. The winding start part and the winding end part of the steel plate are not connected, so that the outer core  25  had an inner spacing in the axial direction. The outer core  25  extends axially from a position adjacent to the outer periphery of the permanent magnet  16  to a position adjacent to the outer periphery of the permanent magnet  17 . 
     An electrical connector  30  is fitted with the coil casing  11  and protrudes outwardly in a manner that it is connectable at the outside of the plug hole. The connector  30  includes a plurality of insert-molded terminal pins which are connected to a built-in igniter circuit  27  and to the ground sides of the primary coil  24  and the secondary coil  21 . The igniter  27  is disposed atop the coil casing  11  for switching on or off the primary current supplied to the primary coil  24 . The terminal pins  31 , the igniter  27 , the primary coil  24  and the secondary coil  21  are connected electrically through electrical lead wires. 
     The high voltage terminal  41  is press-fit into the high voltage tower  12 . The terminal plate  40  has a nail part at its central location to receive the high voltage terminal  41 . With the top end of the high voltage terminal  41  being inserted into the nail part of the terminal plate  40 , secondary coil  21  is electrically connected to the high voltage terminal  41  through the terminal plate  40 . The high voltage side of the dummy coil  22  is electrically connected to the terminal plate  40  by fusing or soldering. A spring  42  is accommodated within the high voltage tower  12  and electrically connected to the high voltage terminal  41  at its one end. The spring  42  is electrically connectable to a spark plug at its other end, when the ignition coil device  10  is fitted in the plug hole. A plug cap  19  made of a rubber is fitted around an open side of the high voltage tower  12 . The plug cap  19  is fitted around the spark plug. 
     The secondary coil  21  generates a high voltage when the primary current flowing in the primary coil  24  is switched off by the igniter circuit  27 . This high voltage is applied to the spark plug through the dummy coil  22 , the terminal plate  40 , the high voltage terminal  41  and the spring  42 . 
     In the above embodiment, the linear thermal expansion coefficients of the wire body  61  of the primary coil  24 , the epoxy resin  26  and the resin base material  50  of the primary spool  23  are different one another. Further, the epoxy resin  26  is cemented to the primary spool  23 , and the separating material  62  coated on the wire body  61  is easily separable from the epoxy resin  26 . Therefore, when those members repeat expansions and contractions in correspondence with changes in the surrounding temperature of the ignition coil device  10 , the coil wire  60  and the epoxy resin  26  tend to separate thus causing voids  70  therebetween as shown in FIG.  2 . 
     As a result, electrical discharges  71  tend to occur in the voids  70  due to the potential difference between the primary coil  24  which has a low potential and the secondary coil  21  which is located radially inside the primary coil  24  and has a high potential. When the electrical discharges  71  occur, the resin base material  50  of the primary spool  23  existing between the primary coil  24  and the secondary coil  21  sublimates at the side of the primary coil  24 . Thus, erosion  72  occurs causing the electrical discharge  71  to concentrate thereat. Although the glass fibers  51  are used to restrict the plastic deformation of the primary spool  23 , voids (not shown) occur around the glass fibers  51  due to the difference in the linear thermal expansion coefficients of the resin base material  50  and the glass fibers  51  when the spool  23  is molded. The electrical discharges  71  tend to be directed from the erosion  72  to the voids around the glass fibers  51 , thus promoting the growth of treeing. The rubber material added to the resin base material  50  to increase the toughness has a low thermal decomposition temperature, and hence it sublimates when the electrical discharges occur. This results in voids which promote the electrical discharges. That is, the glass fibers  51  and the rubber material promote the growth of treeings caused by the electrical discharges and shortens the life of the primary spool  23 . 
     The details of experiments conducted on the above embodiment and two comparative examples 1 and 2 are shown in FIG.  3 . 
     In the comparative example 1, the primary spool is made by adding olefin rubber in 5 wt. % and glass fibers in 25 wt. % to PBT. However, no silica is added. In the comparative example, the primary spool is made by adding olefin rubber in 5 wt. % and silica in 25 wt. % to PBT. However, no glass fibers are added. Both examples are constructed to have the same ignition coil device diameter (25 mm) as the above embodiment. 
     Because no silica is added in the comparative example 1, the speed of growth of treeing in the primary spool cannot be restricted and hence the primary spool is led to the dielectric breakdown in a short period of time. On the other hand, in the comparative example 2, because silica is added, the speed of growth of treeing in the primary spool is slowed down. However, because no glass fibers are added, the primary spool is likely to plastically deform. Cracks actually occurs. 
     According to the above embodiment, however, glass fibers  51 , rubber material and silica are added to the resin base material  50  to increase the toughness of the primary spool  23  by restricting its plastic deformation. As a result, the growth of trees arising from the erosion is restricted, and the life of the primary spool  23  is improved. With the secondary spool  20  being constructed in the same composition as the primary spool  23 , the growth of trees in the secondary spool  20  is also restricted, even if electrical discharges occurs between the secondary coil  21  and a low voltage part existing inside the secondary coil  21  and erosions occur in the secondary spool  20 . 
     In the above embodiment, the resin base material  50  for the primary spool  23  is not limited to PBT, but may be any resin as long as it is of the type which has a low melting viscosity and a high flowability. The diameter and the length of the glass fibers  51  added to the resin base material  50  to restrict plastic deformation are not limited. However, it is advantageous to add the glass fibers  51  in more than 10 wt. %, preferably 15 wt. %, so that the primary spool  23  has a mechanical rigidity sufficient to withstand the force applied during a coil winding operation. The rubber material is preferably added in more than 5 wt. % to ensure toughness. 
     Further, size of granules of silica added to restrict the growth of treeing is not limited. However, it is important to maintain a weight ratio between the weights of the added silica and the added glass fibers  51 , that is, added silica weight divided by added glass fiber weight. It is found that the growth of trees is restricted and the life of the primary spool  23  is increased, as the weight ratio increases closely to 1. The life of the primary spool  23  does not change so much, if the weight ratio exceeds 1. Therefore, it is preferred to add the glass fibers  51  and the silica in substantially the same weight amount. 
     The above embodiment may be modified as shown in FIG. 4 in which the same or similar reference numerals designate the same or similar parts. In this modification, no permanent magnets are disposed at the top and bottom axial ends of a central core  15   a.  Although the magnetic flux generated in the primary coil  24  is decreased, the decrease is compensated for by increasing the diameter of the central core  15   a  than in the first embodiment. Thus, the secondary coil  21  is enabled to generate a required high voltage. 
     Unless the diameter of the plug hole is not increased, the diameter of the ignition coil device  10  is not allowed to be increased in correspondence with the increase in the central core  15   a.  Thus, it is inevitable to decrease the diameter of either of the members or the thickness of the same. It is only possible to thin spools  20   a,    23   a  from the various constraints imposed on the ignition coil device  10  to satisfy the required performance and characteristics. 
     In the modification shown in FIG. 4, the primary spool  23   a  is made of the same materials as the primary spool  23  in the embodiment shown in FIGS. 1 and 2, but is more thinned than in the above embodiment. Because the thinned primary spool  23   a  is still capable of restricting the growth of trees, the life of the primary spool  23   a  and hence of the ignition coil device  10  is increased. A secondary spool  20   a  may be made of the same materials as the primary spool  23   a,  and may be more thinned than the secondary spool  20  in the above embodiment. 
     In the above embodiment and modification, the primary spool  23 ,  23   a  may be made without the rubber material. Further, the secondary spool  20 ,  20  may be made without silica, as long as the secondary spool  20 ,  20   a  is located inside the primary spool  23 ,  23   a.  If the secondary spool  20 ,  20   a  is located outside the primary spool  23 ,  23   a,  however, at least the secondary spool  20 ,  20   a  must include the glass fibers  51 , rubber material and silica in addition to the resin base material  50 . The primary spool  23 ,  23   a  may have the same composition as the secondary spool  20 ,  20   a,  or it need not include silica. That is, it is preferred that the spool include the glass fibers  51 , rubber material and silica in addition to the resin base material  50 , as long as it is disposed between the primary coil  24  and the secondary coil  21 . Further, it is necessary that at least one of the primary spool  23 ,  23   a  and the secondary spool  20 ,  20   a  includes the glass fibers  51 , rubber material and silica in addition to the resin base material  50 . 
     Further, the PET, silicone or wax used as the separating material  62  may be eliminated, and instead a thin film made of PET may be wound around the primary spool  23  as the separating material. Still further, no separating material may be used for the primary coil  24 . 
     The present invention should not be limited to the disclosed embodiment and its modifications, but may be implemented in many other ways without departing from the spirit of the invention.