Abstract:
An ignition coil has concentrically arranged components including a core, a secondary winding spool adjacent to the core, a secondary winding wound on the spool, a case made of electrical insulating material, a primary winding wound on an outer surface of the case, and a shield disposed radially outwardly of the primary winding. The case is configured so as to allow the primary winding to be wound thereon and to space the shield a predetermined space from the primary winding. A radially inner surface of the case and a radially outer portion of the secondary winding define a single potting channel for receiving an epoxy potting material. The configuration eliminates the primary winding spool, which reduces material cost, and further eliminates the effective dielectric contribution of the primary winding spool for reduced capacitance to thereby provide improved secondary winding output.

Description:
BACKGROUND OF THE INVENTION 
     1. Technical Field 
     This invention relates generally to an ignition coil for use in producing a spark across a gap of a spark plug in an internal combustion engine, and more particularly, to an ignition coil having a primary winding disposed outwardly of a secondary winding. 
     2. Discussion of the Related Art 
     Known ignition coils utilize primary and secondary windings and a magnetic circuit. The magnetic circuit portion has taken a wide variety of configurations, and is typically constructed using magnetically-permeable material, such as steel laminations or compression molded insulated iron particles. One configuration known in the art comprises an elongated, generally cylindrical core. 
     This slender core is known to be used for so-called “pencil coils,” useful in space limited engine configurations. In such a “pencil” coil, a number of components are typically concentrically arranged and include, from inside to outside, a core, a primary winding wound around the core, a secondary spool made of insulating material spaced from the primary winding, a secondary winding wound on the spool, a case formed of insulating material, and a shield formed of electrically conductive material and generally grounded. It is further known in the foregoing configuration to fill a first potting channel defined between the primary winding and an inside diameter of the secondary winding spool with epoxy potting material (i.e., dielectric material), and further, to fill a second potting channel defined between the secondary winding and an inside diameter of the case with epoxy potting material. The potting process for this configuration therefore involves filling two potting channels with potting material. 
     Another known configuration includes the following components, also generally concentrically arranged, in order from inside to outside, a core, a secondary winding spool made of insulating material, a secondary winding wound on the secondary winding spool, a primary winding spool, a primary winding wound on the primary winding spool, a case made from electrical insulating material, and a shield (grounded). This known configuration also includes two potting channels, namely, the annular space between the secondary winding and an inside diameter of the primary winding spool, and, the annular space between the primary winding and an inside diameter of the case. The potting process for this configuration thus also involves the filling of two potting channels. In addition, three principal dielectric barriers exist between the shield and the core, namely, the case, the wall of the primary winding spool, and the wall of the secondary winding spool (in addition to the epoxy potting material which makes some dielectric contribution). The increased capacitance resulting from the dielectric barriers impacts the secondary voltage performance (i.e., the voltage characteristics delivered to the spark plug). Since the form factor of the above-described configurations (i.e., slender form factor) is useful, it would be desirable to improve upon the foregoing. 
     Thus, there is a need to provide an improved ignition coil that minimizes or eliminates one or more of the shortcomings as set forth above. 
     SUMMARY OF THE INVENTION 
     An ignition coil in accordance with the present invention is configured to allow improved manufacture thereof by having only one potting channel (i.e., eliminating the second potting channel). In addition, the inventive ignition coil configuration reduces manufacturing costs due to the elimination of a primary winding spool. Moreover, an ignition coil according to the invention minimizes capacitance by eliminating a principal dielectric barrier (i.e., the primary winding spool), which yields improved secondary voltage performance. 
     As to the particular configuration, an ignition coil in accordance with the present invention includes a core, a secondary winding spool, a secondary winding wound on the spool, a case, a primary winding wound on the case, and a shield. The core may be generally elongated, comprise magnetically-permeable material, and have a main, longitudinal axis associated therewith. The secondary winding spool is disposed adjacent to and radially outwardly from the core. The case is formed of electrical insulating material and is spaced radially outwardly from the spool. The case has a configuration suitable for receiving a primary winding wound therearound. In particular, the case has an inner surface, an outer surface, an upper annular shoulder, and a lower annular shoulder. The primary winding, advantageously, is wound on a portion of the outer surface between the upper and lower annular shoulders, thereby eliminating the need for a primary winding spool. The shield is disposed radially outwardly from the primary winding, is electrically conductive, and is preferably grounded. 
     By disposing the primary winding on the outer surface of the case, the case may be configured so that epoxy potting material need only be disposed in one potting channel namely, between the inner surface of the case and the secondary winding. Elimination of the second potting channel found in conventional configurations improves the potting process that is required for manufacture of the ignition coil. In addition, the case and the primary winding spool found in conventional configurations have been integrated into the case alone in the present invention, thereby eliminating the need for a separate primary winding spool. This results in an reduced cost ignition coil. 
     Other objects, features, and advantages of the present invention will become apparent to one skilled in the art from the following detailed description and accompanying drawings illustrating features of this invention by way of example, but not by way of limitation. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a simplified cross-section view of an ignition coil in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 is a simplified, cross-section view of an ignition coil  10  in accordance with the present invention. As is generally known, ignition coil  10  may be coupled to, for example, an ignition system  12 , which contains primary energization circuitry for controlling the charging and discharging of ignition coil  10 . 
     Further, also as is well known, the relatively high voltage produced by ignition coil  10  is provided to a spark plug  14  (shown in phantom-line format) for producing a spark across a spark gap thereof, which may be employed to initiate combustion in a combustion chamber of an engine. Ignition system  12 , and spark plug  14  perform conventional functions well known to those of ordinary skill in the art. 
     Ignition coil  10  is adapted for installation to a conventional internal combustion engine through a spark plug well onto a high-voltage terminal of spark plug  14 , which is retained by a threaded engagement with a spark plug opening into the above-described combustion cylinder. Ignition coil  10  comprises a substantially slender high voltage transformer including substantially, coaxially arranged primary and secondary windings and a high permeability magnetic core. 
     Referring to FIG. 1, in accordance with the invention, ignition coil  10  includes a core  16 , a secondary winding spool  18 , a secondary winding  20 , a case  22 , potting material  24 , a primary winding  26 , a shield  28 , and a low voltage connector body  30 . 
     Core  16  may be elongated, having a main, longitudinal axis “A” associated therewith. Core  16  includes an upper, first end  31 , and a lower, second end  32 . Core  16  comprises magnetically permeable material, for example, steel laminations, or, a composite material of ferromagnetic particles, for example, iron, in a binder of electrical insulating material. Core  16  may be a conventional core known to those of ordinary skill in the art, for example, as described and illustrated in U.S. Pat. No. 5,706,792 entitled “INTEGRATED IGNITION COIL AND SPARK PLUG”, issued Jan. 13, 1998, hereby incorporated by reference in its entirety. As illustrated, core  16 , in the preferred embodiment, takes a generally cylindrical shape. 
     Secondary winding spool  18  is configured to receive and retain secondary winding  20 . Spool  18  is disposed adjacent to and radially outwardly of core  16 , and, preferably, in coaxial relationship with core  16 . Spool  18  may comprise any one of a number of conventional spool configurations known to those of ordinary skill in the art. In the illustrated embodiment, spool  18  is adapted to be wound in a segmented style configuration. However, it should be understood that spool  18  need not be so configured, and may alternatively be configured to receive one continuous secondary winding, as known. 
     As illustrated in FIG. 1, spool  18  may include a plurality of axially spaced ribs  34  forming a plurality of channels  36  therebetween. The depth of the respective channels  36  in this embodiment, may decrease from the top of spool  18  (i.e., near the upper end  31  of core  16 ), to the other end of spool  18 , by way of a progressive gradual flare of the spool body. The result of the flare or taper is to increase the distance between secondary winding  20  and primary winding  26 , progressively, from the connector-body-end (top) to the spark-plug-end (bottom). As is known in the art, the voltage gradient in the axial direction, which increases toward the spark plug end of the secondary winding, may require increased dielectric insulation between the secondary and primary windings, and, may be provided for by way of the progressively increased separation between the secondary and primary windings. In addition, as will be described in detail hereinafter, the increased separation distance is, in the illustrated embodiment, substantially occupied by epoxy potting material  24 , which is a dielectric material that serves the above-described dielectric insulation function. 
     In addition, the body portion of spool  18  tapers on a lower end thereof to a spark plug connector feature  38 . Connector feature  38  includes a generally cylindrical outer surface sized to provide an interference fit with respect to a corresponding through-aperture at the lower end of case  22 . In addition, connector feature  38  includes a blind bore or well configured in size and shape to accommodate the size and shape of the high-voltage connector terminal of spark plug  14 . Although not shown, spool  18 , in the illustrated embodiment, may further include an electrically conductive, high voltage terminal connected between the high voltage end of secondary winding  20  and a contact spring  40  disposed in the blind bore. Contact spring  40  is configured to engage the high-voltage connector terminal of spark plug  14 . This arrangement for coupling the high voltage developed by secondary winding  20  to plug  14  is exemplary only; a number of alternative connector arrangements, particularly spring-biased arrangements, are known in the art. 
     Spool  18  is formed generally of electrical insulating material having properties suitable for use in a relatively high temperature environment. For example, spool  18  may comprise plastic material such as polybutylene terephthalate (PBT) thermoplastic polyester. It should be understood that there are a variety of alternative materials which may be used for spool  18  known to those of ordinary skill in the ignition art, the foregoing being exemplary only and not limiting in nature. 
     Spool  18  may further include a pair of annular features  44  at axially opposite ends thereof. Features  44  may be configured, although not shown in FIG. 1, so as to engage an inner surface of case  22  to center the core  16 /spool  18  assembly in the cavity of case  22 . 
     In a preferred embodiment, a high voltage terminal which connects the high voltage end of secondary winding  20  to contact spring  40 , and core  16  are both insert molded into secondary winding spool  18 . Technology for insert molding is well known and need not be described in any further detail. 
     Secondary winding  20 , as described above, is wound on spool  18 , and includes a low voltage end and a high voltage end. The low voltage end may be connected to ground in a manner to be described hereinafter. The high voltage end is connected to the above-described high voltage terminal disposed in spool  18  for electrically connecting the high voltage generated by secondary winding  20  to contact spring  40  for firing spark plug  14 . As known, an interruption of a primary current Ip through primary winding  26 , as controlled by ignition system  12 , is operative to produce a high voltage at the high voltage end of secondary winding  20 . Winding  20  may be implemented using conventional approaches and material known to those of ordinary skill in the art. 
     Case  22  includes an inner, generally cylindrical surface  46 , an outer surface  48 , a first annular shoulder  50 , a second annular shoulder  52 , a third annular shoulder  54 , a spacer feature  56 , a raceway  58 , a through-aperture  60 , and a through-bore  62 . 
     Inner surface  46  is configured in size to receive and retain the core  16 /spool  18  assembly. The inner surface  46  of case  22  may be slightly spaced from spool  18 , particularly the annular spacing features  44  thereof (as shown), or may engage the spacing features  44  of spool  18 . 
     Annular shoulders  50 , and  52  are located near upper, and lower ends of case  22 , respectively. The primary winding  26  is wound on a portion of outer surface  48  that is intermediate the upper and lower annular shoulders  50 , and  52 . Annular shoulders  50 , and  52 , in conjunction with annular shoulder  54 , and spacer feature  56 , are configured to space shield  28  a predetermined distance apart from primary winding  26 . 
     Raceway  58  is provided for carrying leads from the first and second ends of primary winding  26  to the low voltage connector body  30 . 
     Aperture  60  is configured in size and shape (i.e., generally cylindrical) to provide an interference fit with an outer surface of connector feature  38 , described above. When connector feature  38  is inserted in aperture  60 , therefore, a seal is made. 
     Bore  62  is provided to accept a conventional fastener to secure ignition coil  10  to an internal combustion engine (not shown). 
     Case  22  is formed of electrical insulating material, and may comprise conventional materials known to those of ordinary skill in the art (e.g., the PBT thermoplastic polyester material referred to above). 
     Potting material  24  preferably comprises epoxy potting material which is introduced into a potting channel defined between inner surface  46  of case  22 , and, among other things, the secondary winding  20 . The potting channel is filled with potting material  24 , in the illustrated embodiment, up to approximately the level indicated by the potting level  64 . The potting material performs the function of electrical insulation between the secondary winding  20  and other electrically conductive components (e.g., primary winding  26 ), and, provides protection from environmental factors which may be encountered during the service life of ignition coil  10 . There are a number of suitable epoxy potting materials well know to those of ordinary skill in the art. 
     Primary winding  26  includes first and second ends and is configured to carry a primary current IP for charging coil  10  upon control of ignition system  12 . Winding  26  may be implemented using known approaches and conventional materials. 
     Shield  28  is generally annular in shape and is disposed radially outwardly of primary winding  26 , and, preferably, is spaced a predetermined distance therefrom. The shield  28  is preferably electrically conductive, and more preferably metal, such as steel or other adequate magnetic material. Shield  28  provides not only a protective barrier for ignition coil  10  generally, and primary winding  26  in particular, but, further, provides a magnetic path for the magnetic circuit portion of ignition coil  10 . Shield  28  may be grounded. 
     Low voltage connector body  30  is configured to electrically connect the first and second ends of primary winding  26  to an energization source, such as, the energization circuitry included in ignition system  12 . Connector body  30  is generally formed of electrical insulating material, but also includes a plurality of electrically conductive output terminals (e.g., pins). As illustrated in diagrammatic form, leads  66  are coupled from the ends of primary winding  26  to connector body  30 , which are then internally routed through body  30  via electrical conductors to predetermined ones of the output terminals (e.g., pins). Ignition system  12  may then control energization of the primary winding  26 . In addition, also illustrated in diagrammatic form, connector body  30  includes a ground terminal  68 , which is also electrically routed through body  30  to a predetermined one of the output terminals thereof. From ground terminal  68  may be electrically connected to (i) the low voltage end of secondary winding  20 , and (ii) shield  28 , to provide the respective ground connections described above. 
     In accordance with the present invention, ignition coil  10  only includes one “potting” channel (i.e., between secondary winding  20  and inner surface  46  of case  22 ), thus eliminating the second potting channel found in conventional ignition coil designs. This elimination improves the potting process portion of the coil assembly. In addition, the inventive configuration of case  22  eliminates the need for a primary winding spool, thus reducing the cost of ignition coil  10  (i.e., no cost for the primary spool). Eliminating the spool for the primary winding also provides increased space in the same package size for more winding turns (or increased core material) for improved ignition coil performance. In addition, elimination of the primary spool also eliminates a principal dielectric barrier that is found in conventional configurations. As shown in FIG. 1, only the wall of case  22 , and the wall of spool  18  are principal dielectric barriers. The minimized capacitance yields improved secondary voltage performance. 
     It is to be understood that the above description is merely exemplary rather than limiting in nature, the invention being limited only by the appended claims. Various modifications and changes may be made thereto by one of ordinary skill in the art which embody the principles of the invention and fall within the spirit and scope thereof.