Patent Publication Number: US-6668810-B1

Title: Ignition coil assembly with spark plug connector

Description:
FIELD OF THE INVENTION 
     This invention relates to an ignition coil assembly for an internal combustion engine and, in particular, to such a device mounted directly onto its associated spark plug. 
     BACKGROUND OF THE INVENTION 
     In order to provide enhanced ignition performance, modern internal combustion engines used in automotive applications employ an ignition coil mounted directly on each of the spark plugs. Various styles of directly mounted ignition coils are used in which an ignition coil is mounted to a spark plug or adjacent to an associated spark plug without the need for a long flexible high voltage lead. Variations include so-called “pencil”, “stick”, “cigar”, “plug hole” or. “coil-on-plug” type ignition coils. Modern ignition systems generate extremely high voltages necessary to precisely and reliably initiate the combustion process within the combustion chamber. Directly mounted ignition coils provide numerous advantages over other types including: elimination of high voltage leads, elimination of “waste spark” mode, and packaging benefits. Typically, a soft rubber boot is used to encase the spark plug, which is an extension of the coil-on-plug assembly. 
     The electrical connection between the high voltage terminal of the ignition coil and the spark plug terminal is very important. A reliable and secure connection must be made and it has been found that conventional coil spring type connectors can lead to ignition system failures when the cut end portions of the spring ends cause electromagnetic field concentrations. This can lead to dielectric failure of the ignition coil (i.e., arcing between the spring to a conductive surface outside the rubber boot). The ignition coil assembly is inserted substantially inside the spark plug insertion hole in the cylinder head of the internal combustion engine and, therefore, it is surrounded by an electrical ground. If a point of electro-magnetic field concentration is present at a location where the gap between it and a ground surface is small, dielectric failure can occur. 
     In addition to the design consideration mentioned previously, the ignition coil assembly must provide a reliable electrical connection, enable servicing, and minimize the number of separate components, and especially loose parts, which tend to lead to assembly related defects. 
     Other types of ignition coil connectors are also presently used. For example, metal springs or clips can be rigidly attached to the high voltage terminal on the coil-on-plug ignition coil assembly. In order to prevent the spring or clip from falling out of the ignition coil assembly spark plug boot, this design approach requires the spring or clip to be pushed onto and over the high voltage terminal. These configurations are not ideal for certain installation and packaging design approaches. 
     SUMMARY OF THE INVENTION 
     In accordance with this invention, a directly mounted type ignition coil assembly is provided having a spark plug boot section formed of a highly resilient material. A coil type high voltage connection spring is installed in the boot section and at opposite ends, electrically contacts the coil and spark plug high voltage terminals. Preferably, the end segments of the spring have a smaller bending radius than the spring center section to avoid problems associated with the electro-magnetic field concentrations mentioned previously since the cut ends are positioned to create a large gap to a ground surface. The internal passageway of the coil boot features a generally uniform cylindrical diameter. However, a number of ribs are provided which engage the center section of the spring so that the spring can be retained in the boot before the ignition coil is installed on the spark plug. The ribs are formed in a manner that does not interfere with the molding of the boot section. 
     The high voltage connection spring is inserted into the orifice of the spark plug boot section and is pressed into position such that it engages the boot passageway ribs. When the coil assembly is installed on the spark plug, the spring is compressed and pushed past the area where the ribs are present. During servicing, when the ignition coil assembly is removed, the spring is allowed to expand, but again engages with the ribs, preventing the spring from falling out of the ignition coil assembly. 
     The ignition coil assembly in accordance with this invention provides the advantages of a connector which is rigidly connected in that the parts are retained, yet avoids the shortcomings of such designs which can lead to failures mentioned previously. 
    
    
     Additional benefits and advantages of the present invention will become apparent to those skilled in the art to which the present invention relates from the subsequent description of the preferred embodiment and the appended claims, taken in conjunction with the accompanying drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a pictorial view of an ignition coil assembly in accordance with this invention with the spark plug boot section shown in cross-section; 
     FIG. 2 is an enlarged cross-sectional view taken from FIG. 1; 
     FIG. 3 illustrates the ignition coil assembly installed onto the spark plug; 
     FIG. 4 is a partial cross-sectional view of the spring in a compressed state, but with the spark plug removed, for illustrating the configuration of the boot section passageway; and 
     FIG. 5 is a cross-sectional view taken along line  5 — 5  from FIG.  4 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     An ignition coil assembly in accordance with this invention is shown in a completely assembled condition in FIG.  1  and is generally designated there by reference number  10 . Ignition coil assembly  10  includes body  11  having an enlarged diameter head  12 , a generally uniform diameter barrel section  14 , and a lower spark plug boot section  16 . 
     Coil assembly head  12  includes electrical connector  18  which provides a receptacle for installation for a snap-fitting type mating electrical connector which provides the electrical signal for the coil assembly  10  to initiate an ignition spark. Head  12  further forms retention flange  20  which mates with a correspondingly shaped part in the engine cylinder head for retaining the coil assembly  10  in its installed position. Barrel section  14  encloses internal components, such as the conductor coil windings of the coil assembly  10 , and has a generally uniform diameter, enabling the coil assembly  10  to be inserted into the associated passageway in the cylinder head (not shown). Spark plug boot section  16  features a hollow inside passageway  22  having a generally constant diameter smooth inside cylindrical surface which opens at the bottom of the coil assembly at orifice  24 . Boot section  16  is preferably formed of a highly elastic material, such as silicone rubber. 
     Now with reference to the sectioned portions of FIG.  1  and FIG. 2, internal features and elements of boot section  16  are shown in more detail. A high voltage coil terminal  26  protrudes from barrel section  14  into boot passageway  22  and is surrounded by a rigid hollow tube  28 . Tube  28  has exterior ribs  30  which are provided to enhance the insulation properties of the tube. 
     High voltage connection spring  32  is a coil type spring and is shown installed within passageway  22 . Spring  32  has a center section  33  having a number of turns of wire having a generally constant bending radius. The outside diameter of the center section  33  is slightly less than the inside diameter of passageway  22 . First and second spring end segments  34  and  36 , however, have a reduced radius as compared with center section  33 . In other words, end segments  34  and  36  are “turned down” to have a continuously decreasing radius. Cut ends  35  (not shown) and  37  of end segments  34  and  36  respectively are the terminal ends of the wire forming spring  32 . Preferably, cut ends  35  and  37  have the smallest radius of curvature of the turns forming spring  32 . Cut ends  35  and  37  are positioned near the longitudinal centerline  39  of spring  32  (and passageway  22 ) as shown in FIG.  2 . 
     Boot passageway  22  features a number of radially inwardly protruding ribs  38 . In the embodiment illustrated by the figures, four ribs  38  are provided, as best shown in FIG.  5 . Ribs  38  have a generally semi-circular configuration, smoothly blending with the inside wall surface of boot passageway  22  at their edges  40  and  42  (i.e., where they merge along the longitudinal direction). Ribs  38  are elongated in the longitudinal direction along passageway  22 . Ribs  38  and inside passageway  22  are configured so that boot section  16  can be formed by molding of the elastic material over a post and, following curing of the material of boot section  16 , allowing the boot section to be pulled off of the forming post, without damaging the inside passageway  22 . 
     Other configurations for ribs  38  may be provided in accordance with this invention. For example, a greater or fewer number of ribs  38  could be provided. It would be necessary, however in the case of a one-piece molded part, to ensure that the part may be pulled off of the forming post or tool. 
     After ignition coil assembly  10  is fully fabricated and assembled, spring . 32  is installed by pushing it through orifice  24  and into passageway  22  until it engages with ribs  38  which acts as a retainer means for retaining the spring in the passageway. The outside constant diameter of spring center section  33  in its relaxed state is slightly less than the inside diameter of passageway.  22 , except in the area of ribs  38 . Ribs  38  protrude radially inwardly and frictionally retain spring  32  in position, as shown in FIGS. 1 and 2. In this condition, coil assembly  10  may be handled and transported without causing spring  32  to fall from its installed position from within passageway  22 . FIG. 3 shows the coil assembly  10  installed onto a representative spark plug  44 . As shown, spark plug  44  includes high voltage terminal  46  and ceramic insulator  48 . As coil assembly  10  is installed over spark plug  44 , high voltage terminal  46  engages spring end segment  36  and compresses the spring, forcing it to move upwardly through passageway  22 , past the location of ribs  38 . As shown in FIG. 3, spring  32  becomes compressed between high voltage terminals  26  and  46 . In this condition, spring end segments  34  and  36  are maintained in contact with the terminals  26  and  46  solely due to the compression of spring  32  between the terminals. In the installed condition, ribs  38  no longer function to interact with spring  32 . Inside passageway  22  of the boot section conforms closely to spark plug insulator  48  to provide a sealed connection with spark plug  44 . The compression of spring  32  allows the installed position of coil assembly  10  to vary with respect to spark plug terminal  26  while providing a good electrical connection. 
     FIG. 4 shows the compressed condition of spring  32  when the spark plug  44  is in position. However, FIG. 4 deletes the spark plug for purposes of better illustrating the configuration of internal features, such as ribs  38 . FIG. 4 also illustrates that the spring end of segment  36  is turned to a continuously decreasing radius, terminating at cut end  37 . As mentioned previously, this configuration avoids failure due to electromagnetic field concentrations which tend to collect at points. Since the position of spring cut ends  35  and  37  is positioned near the longitudinal centerline of coil assembly  10 , the greatest distance exists between these points of field concentration and any electrical ground, such as that provided by the cast metal material of the associated cylinder head. In addition, cut ends  35  and  37  are firmly pressed against terminals  26  and  46  by compression of springs  32 , increasing the conduction at those points. 
     Preferably, spring end segments  34  and  36  are of identical configurations, enabling spring  32  to be installed with either of their cut ends  35  and  37  contacting coil terminal  26 . 
     When it is desired to service the associated internal combustion engine, coil assembly  10  may be removed from its connection with spark plug  44 . When this occurs, spring  32  expands back to the position shown in FIGS. 1 and 2. In this condition, ribs  38  again retain spring  32  in position and prevent it from becoming loose from the assembly. 
     While the above description constitutes the preferred embodiment of the present invention, it will be appreciated that the invention is susceptible to modification, variation, and change without departing from the proper scope and fair meaning of the accompanying claims.