Patent Publication Number: US-8991371-B2

Title: Ignition coil

Description:
TECHNICAL FIELD OF INVENTION 
     The present invention relates to an ignition coil for developing a spark firing voltage that is applied to one or more spark plugs of an internal combustion engine. 
     BACKGROUND OF INVENTION 
     Ignition coils are known for use in connection with an internal combustion engine such as an automobile engine. Ignition coils typically include a primary winding, a secondary winding, and a magnetic circuit. The magnetic circuit conventionally may include a central core extending along an axis and located radially inward of the primary and secondary windings and magnetically coupled thereto. In one arrangement, a C-shaped high permeance structure is included to provide a high permeance magnetic return path. The high permeance structure may include a base section from which a pair of legs extends. The central core is placed between the legs such that the axis of the core extends through the legs of the high permeance structure and such that at least one end of the core is spaced apart from the leg to which it is adjacent to define an air gap. The primary winding, secondary winding, core and high permeance structure are contained in a case formed of an electrical insulating material. The case is filled with an insulating resin or the like for insulating purposes. In this configuration, insulating resin that fills the air gap may be subject to stress from the core during operation of the ignition coil. This stress may lead to undesired performance of the ignition coil. 
     What is needed is an ignition coil which minimizes or eliminates one or more of the shortcomings as set forth above. 
     SUMMARY OF THE INVENTION 
     Briefly described, an ignition coil for an internal combustion engine includes a magnetically-permeable core extending along a core longitudinal axis, the core having a pair of end surfaces on axially-opposite ends thereof. The ignition coil also includes a primary winding disposed outward of the core, a secondary winding disposed outward of the primary winding, and a structure comprising magnetically-permeable steel laminations having a base and a pair of legs, the structure defining a magnetic return path. The core is disposed between the pair of legs such that the core longitudinal axis extends through the legs and the end surfaces face toward the legs and at least one of the end surfaces of the core is spaced apart from a respective one of the legs to define an air gap. The structure is over-molded with an over-molding material such that the over-molding material fills at least a portion of the air gap. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       This invention will be further described with reference to the accompanying drawings in which: 
         FIG. 1  is a simplified cross-section view of an ignition coil in accordance with the present invention; 
         FIG. 2  is a radial cross-section view of a core of the ignition coil of  FIG. 1 ; 
         FIG. 3  is and isometric view of a high permeance structure and core of the ignition coil of  FIG. 1 ; 
         FIGS. 4 and 5  are isometric views of the high permeance structure of  FIG. 3  with an over-molding material over-molded thereto; 
         FIG. 6  is an isometric view of a second embodiment of a high permeance structure with an over-molding material; 
         FIG. 7A  is an elevation view of a portion of the high permeance structure and core of  FIG. 3  in the direction of arrow  7 A; 
         FIG. 7B  is a radial cross-section view of the core of  FIG. 7A ; 
         FIG. 8A  is a cross-section view similar to the cross-section view of  FIG. 7A  except with a core having a circular cross-sectional shape; and 
         FIG. 8B  is a cross-section view of the core of  FIG. 8A . 
     
    
    
     DETAILED DESCRIPTION OF INVENTION 
     Referring now to the drawings wherein like reference numerals are used to identify identical components in the various views,  FIG. 1  is a simplified cross-section view of an ignition coil  10 . Ignition coil  10  may be controlled by a control unit  12  or the like. Ignition coil  10  is configured for connection to a spark plug  14  that is in threaded engagement with a spark plug opening (not shown) in an internal combustion engine (also not shown). Ignition coil  10  is configured to output a high-voltage (HV) output to spark plug  14 , as shown. Generally, overall spark timing (dwell control) and the like is provided by control unit  12 . One ignition coil  10  may be provided per spark plug  14 . 
     Ignition coil  10  may include a magnetically-permeable core  16 , a magnetically-permeable structure  18  configured to provide a high permeance magnetic return path which has a base section  20  and a pair of legs  22  and  24 , a primary winding spool  26 , a primary winding  28 , a quantity of encapsulant  30  such as an epoxy potting material, a secondary winding spool  32 , a secondary winding  34 , a case  36 , a low-voltage (LV) connector body  38  having primary terminals  40  (only one primary terminal  40  is visible in the figures due to being hidden behind primary terminal  40  shown in  FIG. 1 ), and a high-voltage (HV) tower  42  having a high-voltage (HV) terminal  44 . 
     Now referring to  FIGS. 1 and 2 , core  16  extends along a core longitudinal axis A and is generally oval in overall shape in radial cross-section as shown in  FIG. 2 , which is a radial cross-section view of core  16 . Core  16  includes an upper end surface  46  at one axial end and a lower end surface  48  at the other axial end which is opposite of upper end surface  46 . Core  16  may comprise laminated steel plates  50   1 ,  50   2  . . .  50   n  as shown in  FIG. 2 . Alternatively but not shown, core  16  may comprise compression molded insulated iron particles rather than laminated steel plates  50 . Core  16  will be described in more detail later. 
     Now referring again to  FIG. 1 , primary winding spool  26  is configured to receive and retain primary winding  28 . Primary winding spool  26  is disposed adjacent to and radially outward of core  16  and is preferably in coaxial relationship therewith. Primary winding spool  26  may comprise any one of a number of conventional spool configurations known to those of ordinary skill in the art. In the illustrated embodiment, primary winding spool  26  is configured to receive one continuous primary winding. Primary winding spool  26  may be formed generally of electrical insulating material having properties suitable for use in a relatively high temperature environment. For example, primary winding spool  26  may comprise plastic material such as PPO/PS (e.g., NORYL® available from General Electric) or polybutylene terephthalate (PBT) thermoplastic polyester. It should be understood that there are a variety of alternative materials that may be used for primary winding spool  26 . 
     Primary winding  28 , as described above, is wound onto primary winding spool  26 . Primary winding  28  includes first and second ends that are connected to the primary terminals  40  in LV connector body  38 . Primary winding  28  is configured to carry a primary current I P  for charging ignition coil  10  upon control of control unit  12 . Primary winding  28  may comprise copper, insulated magnet wire, with a size typically between about 20-23 AWG. 
     Secondary winding spool  32  is configured to receive and retain secondary winding  34 . Secondary winding spool  32  is disposed adjacent to and radially outward of the central components comprising core  16 , primary winding spool  26  and primary winding  28  and, preferably, is in coaxial relationship therewith. Secondary winding spool  32  may comprise any one of a number of conventional spool configurations known to those of ordinary skill in the art. In the illustrated embodiment, secondary winding spool  32  is configured for use with a segmented winding strategy where a plurality of axially spaced ribs forms a plurality of channels therebetween for accepting the windings. However, it should be understood that other known configurations may be employed, such as, for example only, a configuration adapted to receive one continuous secondary winding (e.g., progressive winding). Secondary winding spool  32  may be formed generally of electrical insulating material having properties suitable for use in a relatively high temperature environment. For example, secondary winding spool  32  may comprise plastic material such as PPO/PS (e.g., NORYL available from General Electric) or polybutylene terephthalate (PBT) thermoplastic polyester. It should be understood that there are a variety of alternative materials that may be used for secondary winding spool  32 . 
     Encapsulant  30  may be suitable for providing electrical insulation within ignition coil  10 . In a preferred embodiment, encapsulant  30  may comprise an epoxy potting material. Sufficient encapsulant  30  is introduced in ignition coil  10 , in the illustrated embodiment, to substantially fill the interior of case  36 . Encapsulant  30  also provides protection from environmental factors which may be encountered during the service life of ignition coil  10 . There are a number of encapsulant materials known in the art. 
     Secondary winding  34  includes a low-voltage (LV) end and a high-voltage (HV) end. The LV end may be connected to ground by way of a ground connection through LV connector body  38  or in other ways known in the art. The HV end is connected to HV terminal  44 , a metal post or the like that may be formed in secondary winding spool  32  or elsewhere. Secondary winding  34  may be implemented using conventional approaches and material (e.g. copper, insulate magnet wire) known to those of ordinary skill in the art. 
     Referring now to  FIGS. 1 and 3 , high permeance structure  18  is configured to provide a high permeance magnetic return path for the magnetic flux produced in core  16  during operation of ignition coil  10 . High permeance structure  18  may be formed, for example, from a lamination stack that includes a plurality of silicon steel laminations  52   1 ,  52   2 , . . .  52   m  or other adequate magnetic material (i.e., magnetically-permeable material), roughly in the form of a C-shape. As described previously, high permeance structure  18  includes base section  20  and a pair of legs  22  and  24 . Leg  22  may extend substantially perpendicular from an end of base section  20  that is proximal to upper end surface  46  of core  16  while leg  24  may extend substantially perpendicular from an end of base section  20  that is proximal to lower end surface  48  of core  16 . As shown in  FIGS. 1 and 3 , a face  22   a  of leg  22  that faces the concave portion (faces core  16 ) of high permeance structure  18  may be tapered from a thicker section that is proximal to base section  20  to a thinner section that is distal from base section  20 . Upper end surface  46  of core  16  is tapered to be substantially parallel to face  22   a  of leg  22 . Similarly, a face  24   a  of leg  24  that faces the concave portion of high permeance structure  18  may be tapered from a thicker section that is proximal to base section  20  to a thinner section that is distal from base section  20 . Lower end surface  48  of core  16  is tapered to be substantially parallel to face  24   a  of leg  24 . Alternatively, but not shown, only one of face  22   a  and face  24   a  may be tapered while the other of face  22   a  and face  24   a  may be substantially perpendicular to base section  20 . Also alternatively, but not shown, face  22   a  and face  24   a  may both be substantially perpendicular to base section  20 . 
     In the illustrated embodiment, lower end surface  48  of core  16  mates with face  24   a  of leg  24  of high permeance structure  18 . Upper end surface  46  of core  16 , on the other hand, is spaced apart from the leg  24  by a predetermined distance defining an air gap  54 . Core  16 , in combination with high permeance structure  18 , in view air gap  54 , forms a magnetic circuit having a high magnetic permeability. The typical range for air gap  54  is 0.5 mm to 2 mm. To maximize energy stored, air gap  54  should be large enough to keep core  16  from saturating to the normal operating current, or level of ampere-turns (primary current×primary turns). 
     Now referring to  FIGS. 1 ,  4 , and  5 , high permeance structure  18  may be over-molded with an over-molding material  56  which may be an elastomeric polymer, for example, Hytrel®. While the majority of high permeance structure  18  is covered with over-molding material  56 , the portion of face  24   a  of leg  24  which mates with lower end surface  48  of core  16  is not covered with over-molding material  56  because intimate contact between face  24   a  of leg  24  which mates with lower end surface  48  of core  16  is needed. Over-molding material  56  may reduce the stress concentrations in encapsulant  30  at upper end surface  46  of core  16 . It should be noted that for clarity, high permeance structure  18  is shown in  FIG. 3  without over-molding material  56 . 
     Over-molding material  56  may be formed with lip  58  to aid in holding core  16  in place during assembly. Lip  58  may be shaped to be substantially similar to a portion of the perimeter of upper end surface  46  of core  16  and defines recessed region  60  within which upper end surface  46  of core  16  is received. As shown in  FIG. 4 , lip  58  is arranged to prevent movement of core  16  (not shown in  FIG. 4 ) in three directions during manufacture as indicated by arrows A 1 , A 2 , A 3 . As shown, the three directions indicated by arrows A 1 , A 2 , A 3  lie in a plane defined by recessed region  60 . Arrows A 1 , A 2  are in opposing directions to each other and parallel to the direction in which silicon steel laminations  52  are stacked while arrow A 3  points toward base section  20  and is in a direction perpendicular to arrows A 1 , A 2 . Recessed region  60  may include air gap setting window  62  therethrough which exposes a portion of face  22   a  of high permeance structure  18 . Air gap setting window  62  is formed with a part of the mold (not shown) which is used to form over-molding material  56  on high permeance structure  18 . This allows for a precise thickness of over-molding material  56  on face  22   a  of high permeance structure  18  which is needed for a maintaining air gap  54  at a desired thickness. Air gap setting window  62  may preferably be spaced away from lip  58  and may preferably be substantially centered within recessed region  60  so that core  16  may be supported by recessed region  60  around the perimeter of core  16 . While lip  58  has been described to be shaped to be substantially similar to a portion of the perimeter of upper end surface  46  of core  16  and defines recessed region  60  within which upper end surface  46  of core  16  is received, it should now be understood that the shape of lip  58  need not be substantially similar to a portion of the perimeter of upper end surface  46  of core  16 , but rather may be shaped substantially different, but sized to substantially prevent movement of core  16  in the direction of arrows A 1 , A 2 , A 3 . For example only, while core  16  is substantially oval in cross-sectional shape, lip  58  may be substantially rectangular in shape. 
     Alternatively, lip  58  may be modified as indicated by lip  58 ′ shown in  FIG. 6 . Lip  58 ′ differs from lip  58  in that lip  58 ′ completely surrounds core  16  (not shown in  FIG. 6 ) and is shaped to be substantially similar to the entire perimeter of upper end surface  46  of core  16 . In this way, lip  58 ′ not only prevents movement in the three directions indicated by arrows A 1 , A 2 , A 3 , but also a fourth direction A 4  which is in the opposite direction as arrow A 3 . While lip  58 ′ has been described to be shaped to be substantially similar to the entire perimeter of upper end surface  46  of core  16  and defines recessed region  60  within which upper end surface  46  of core  16  is received, it should now be understood that the shape of lip  58 ′ need not be substantially similar to a portion of the perimeter of upper end surface  46  of core  16 , but rather may be shaped substantially different, but sized to substantially prevent movement of core  16  in the direction of arrows A 1 , A 2 , A 3 , A 4 . For example only, while core  16  is substantially oval in cross-sectional shape, lip  58  may be substantially rectangular in shape. 
     As can be seen in  FIGS. 4 ,  5 , and  6 ; there are additional openings through over-molding material  56  that exposes other areas of high permeance structure  18  besides portions of face  22   a  and face  24   a . As oriented in  FIGS. 4 and 6 , silicon steel lamination  52   m  (numbered in  FIG. 3 ) is exposed through six circular shaped openings (not numbered) through over-molding material  56 . Similarly, as oriented in  FIG. 5 , silicon steel lamination  52   1  (numbered in  FIG. 3 ) is exposed through six circular shaped openings (not numbered) through over-molding material  56 .  FIGS. 4 ,  5 , and  6  also show that several silicon steel laminations  52  (numbered in  FIG. 3 ) are exposed at base section  20  through an elongated opening (not numbered) through over-molding material  56 . It should be noted that the circular openings exposing portions of silicon steel lamination  52   1  and silicon steel lamination  52   m  and the elongated opening exposing several silicon steel laminations  52  at base section  20  do not serve a function in completed ignition coil  10 , but are the result of the over-molding process used to apply over-molding material  56  to high permeance structure  18 . Over-molding material  56  is applied to high permeance structure  18  by a conventional over-molding process in which high permeance structure  18  is placed in a mold (not shown) and over-molding material  56  in liquid form is injected into the mold, thereby filling the void between the mold and high permeance structure  18 . In this case, the mold that is used includes features that contact high permeance structure  18  to keep high permeance structure precisely positioned in the mold to accurately apply over-molding material  56 . Over-molding material  56  is allowed to solidify and the mold is removed to reveal high permeance structure  18  that is substantially over-molded with over-molding material  56 . 
     Reference will now be made to  FIGS. 3 ,  7 A, and  7 B where  FIG. 7A  is a view in the direction of arrow  7 A of  FIG. 3  of a portion of core  16  and leg  22  of high permeance structure  18  and  FIG. 7B  is a radial cross-section view of core  16 . As described previously, core  16  is preferably generally oval in overall radial cross-sectional shape. Accordingly, core  16  includes major axis A major  and minor axis A minor . Major axis A major  extends in the direction across the radial cross-section of core  16  defined by each laminated steel plate  50   1 - 50   n  while minor axis A minor  extends in the direction across the radial cross-section of core  16  which is perpendicular to major axis A major . Major axis A major  also extends in the same direction as the width W (parallel to the direction in which silicon steel laminations  52  are stacked) of high permeance structure  18  which is the sum of the thicknesses of silicon steel laminations  52   1 - 52   m . The generally oval shape of core  16  is accomplished by varying the width of each laminated steel plate  50   1 - 50   n  in the direction of minor axis A minor . As shown in  FIG. 7B , a core middle section  64  may have laminated steel plates of common width in the direction of minor axis A minor  while a first core end section  66  and a second core end section  68  have laminated steel plates of decreasing width from core middle section  64  to laminated steel plates  50   1  and  50   n  respectively. This arrangement produces a generally oval or racetrack shape with straight sides  70   a ,  70   b  that are parallel to each other and connected at each end by arcuate ends  72   a ,  72   b  that oppose each other. 
     Reference will now be made to  FIGS. 8A and 8B  where  FIG. 8A  is a view similar to that of  FIG. 7A  except that core  16  is replaced with core  16 ′ which is generally circular in radial cross-sectional shape and  FIG. 8B  is a radial cross-section view of core  16 ′. Core  16 ′ includes laminated steel plates  50 ′ 1 ,  50 ′ 2 , . . .  50 ′ x . 
     In order to maintain the same overall packaging size of the ignition coil when using generally circular core  16 ′, the dimension of core  16 ′ in the same direction as width W of high permeance structure  18  must be decreased in comparison to core  16 . This may be most readily visible in  FIG. 3  which includes core  16 . If the dimension of core  16  along major axis A major  is held constant and the dimension of core  16  along minor axis A minor  is adjusted to produce substantially circular core  16 ′ as shown in  FIG. 8B , the core would extend beyond leg  22  and leg  24  of high permeance structure  18 , thereby increasing the overall packaging size of ignition coil  10 . Referring now to  FIGS. 7B and 8B , the overall packaging size of the ignition coil is maintained by having the dimension of core  16 ′ along axis A′ minor  the same as the dimension of core  16  along axis A minor . However, the dimension of core  16 ′ along axis A′ major  is decreased (in comparison to the dimension of core  16  along axis A major ) to be the same dimension as the dimension of core  16 ′ along axis A′ minor , thereby making core  16 ′ substantially circular in cross-section. 
     Now referring to  FIGS. 7A and 8A , the benefit of the radial cross-section shape of core  16  over core  16 ′ can be appreciated by a comparison of flux lines  74  shown in  FIG. 7A  and flux lines  74 ′ shown in  FIG. 8A . As can be seen in  FIG. 8A , flux lines  74 ′ that are near laminated steel plates  50 ′ 1 ,  50 ′ x  and silicon steel laminations  52   1 ,  52   m  are approaching being perpendicular to laminated steel plates  50 ′ and silicon steel laminations  52  which increases flux loss due to an increase of eddy currents. Also as can be seen in  FIG. 7A , flux lines  74  that are near laminated steel plates  50 ′ 1 ,  50 ′ n  and silicon steel laminations  52   1 ,  52   m  do not approach being perpendicular to laminated steel plates  50  and silicon steel laminations  52  to the same extent as in  FIG. 8A  which uses substantially circular core  16 ′. Flux lines  70  being more close to paralleling laminated steel plates  50  and silicon steel laminations  52  near laminated steel plates  50 ′ 1 ,  50 ′ n  and silicon steel laminations  52   1 ,  52   m  reduces flux loss due to a decrease in eddy currents. 
     While core  16  has been described as being generally oval in overall shape in radial cross-section, it should now be understood that core  16  may take the form of other non-circular shapes in radial cross-section. For example only, core  16  may be rectangular, hexagonal, or octagonal. Preferably, regardless of shape, the dimension of core  16  along axis A major  is greater than the dimension of core  16  along axis A minor . 
     While this invention has been described in terms of preferred embodiments thereof, it is not intended to be so limited, but rather only to the extent set forth in the claims that follow.