Ignition coil and manufacturing method and apparatus thereof

An axial end portion of a dielectric sheet, which is wound around a central core of an ignition coil, includes an axially projecting portion, which projects from an axial end surface of the central core positioned radially inward of primary and secondary coils in a case. An inner peripheral side axial end corner and an outer peripheral side axial end corner in the axially projecting portion are respectively formed into a blunt smooth round shape. Dielectric resin is filled in spaces in the case. Alternatively, the axially projecting portion may have a folded end portion, which includes a plurality of generally arcuate segments folded one after another on one circumferential side thereof.

CROSS REFERENCE TO RELATED APPLICATION

This application is based on and incorporates herein by reference Japanese Patent Application No. 2005-238870 filed on Aug. 19, 2005 and Japanese Patent Application No. 2006-130655 filed on May 9, 2006.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an ignition coil, which is used to generate a spark from a spark plug of an internal combustion engine, and a manufacturing method and a manufacturing apparatus for manufacturing such an ignition coil.

2. Description of Related Art

An ignition coil, which is used to generate a spark from a spark plug in an internal combustion engine of, for example, a vehicle, includes a primary coil and a secondary coil arranged in a coil case. Furthermore, a central core, which is made of a magnetic steel plate(s), is arranged radially inward of the primary coil and the secondary coil. A dielectric sheet, which serves as a thermal stress relief member, is wound around an outer peripheral surface of the central core. The dielectric sheet protects the dielectric resin, which is filled in spaces in the coil case, from thermal stress that is applied by the heating and cooling cycle of the engine.

The above described type of ignition coil, which includes the dielectric sheet wound around the central core, is disclosed in, for example, Japanese Unexamined Patent Publication No. H10-92670, Japanese Unexamined Patent Publication No. 2004-14548 (corresponding to U.S. Pat. No. 6,980,073) and Japanese Unexamined Patent Publication Number 2004-111714.

In Japanese Unexamined Patent Publication No. H10-92670, the dielectric sheet is interposed between a central iron core (i.e., the central core) and a secondary winding core (i.e., a spool around which a secondary coil is wound). Here, even if an air bubble is generated in the secondary winding core, which is made of thermoplastic resin, the electric insulation between the central iron core and the secondary coil is maintained by the dielectric sheet.

In Japanese Unexamined Patent Publication No. 2004-14548, the thickness of the dielectric sheet, which is wound around the central core and serves as the thermal stress relief member, is set to an appropriate thickness. In this way, formation of a crack in the dielectric resin, which is filled in the space between the central core and the cylindrical spool, is limited.

In Japanese Unexamined Patent publication No. 2004-111714, a cover member (gel) is applied to an entire length of a winding end of the dielectric sheet, which is wound around the outer peripheral surface of the central core and serves as the thermal stress relief member, so that unintentional removal and radially outward protrusion of the winding end of the dielectric sheet from the central core are limited. When the winding end of the dielectric sheet projects from the central core, stress concentration occurs in the winding end of the dielectric sheet, and a crack is generated due to thermal stress in the dielectric resin filled in the space between the central core and the coil. However, the unintentional projection of the winding end of the dielectric sheet from the central core is advantageously limited by the cover member.

Furthermore, in some previously propose cases, as shown inFIGS. 28-30, a dielectric sheet94is wound around a central core923in such a manner that an axial end portion941of the dielectric sheet94projects from an axial end surface924of the central core923to limit generation of a crack in dielectric resin911, which contacts the axial end surface924of the central core923.

However, when the dielectric sheet94is wound multiple times around the central core923, axial end portions941of layered dielectric sheet constituent sections940of the dielectric sheet94may possibly be displaced from one another in the axial direction, so that steps942are formed at the axial end portions941of the layered dielectric sheet constituent sections940of the dielectric sheet94. In such a case, as shown inFIG. 30, edged grooves (sharp grooves)912are formed in the dielectric resin911that is filled in the coil case in contact with the axial end portions941of the layered dielectric sheet constituent sections940, which form the steps942. The stress concentration may occur at the edged grooves (the sharp grooves)912. The edged grooves912may possibly become an origin of the crack in the dielectric resin911.

Furthermore, it is difficult to align the axial end portions941of the layered dielectric sheet constituent sections940in the axial direction. In view of the above disadvantage, as shown inFIG. 31, it is conceivable to make a projecting length of each axial end portion941of the dielectric sheet94, which projects from the axial end surface924of the central core923, longer than a required length to provide an excess region. After the winding of the dielectric sheet94around the central core923, it is conceivable to mechanically cut the excess region by a cutting blade.

However, when the excess region of the dielectric sheet94is mechanically cut by the cutting blade, the axial end portions941of the layered dielectric sheet constituent sections940at the cutting end may possibly be spread radially outward and thereby separated from one another, as shown inFIG. 31. Furthermore, as shown inFIG. 32, corners943of the cut end parts of the axial end portions941may possibly form sharp edges. As a result, edged grooves912, which correspond to the corners943having the sharp edges, are formed in the dielectric resin911filled in the coil case. Each of the edged grooves912may possibly serve as an origin of a crack in the dielectric resin911.

Furthermore, as shown inFIG. 33, the above described disadvantage of the mechanically cutting the excess region of the dielectric sheet94with the cutting blade may also occur in a case of cutting the dielectric sheet94, which is wound only once around the central core (see the corner943having the sharp edge). The above disadvantage of the mechanically cutting the dielectric sheet94with the cutting blade may also occur in a case of cutting the dielectric sheet94along the axial end surface924of the central core923, as shown inFIG. 34.

Furthermore, as shown inFIGS. 35 and 36, in some previously proposed ignition coils, an axial end portion951of a sheet95is folded against an axial end surface925of a central core923to limit generation of a crack in dielectric resin filled in spaces in a coil case. At the time of folding the axial end portion951of the sheet95, a punch97is used to press and weld the axial end portion951of the sheet95.

However, as shown inFIG. 37, when the axial end portion951of the sheet95is pressed by the punch97, buckling or radially outward partial deformation of the axial end portion951of the sheet95may possibly occur. This occurs due to a rigidity of the sheet95, which limits smooth folding movement of the axial end portion951of the sheet95toward a center of the central core923. When the buckling or deformation of the sheet95occurs, a crack may possibly be generated in the dielectric resin filled in the spaces in the coil case.

Particularly, when a thermoplastic resin film, such as polyethylene terephthalate (PET) film, is used to form the sheet95, the above disadvantage may possibly occur due to a relatively high rigidity of such a film.

The present invention addresses one or more of the above disadvantages. According to one aspect of the present invention, there is provided an ignition coil, which includes a central core, primary and secondary coils, a case, a dielectric sheet and dielectric resin. The central core is produced from at least one magnetic steel plate. Primary and secondary coils are produced by concentrically winding primary and secondary electric wires, respectively, around the central core. The case receives the primary and secondary coils. The dielectric sheet is wound around an outer peripheral surface of the central core. At least one of axial end portions of the dielectric sheet includes an axially projecting portion, which projects from an axial end surface of the central core. An inner peripheral side axial end corner and an outer peripheral side axial end corner in the axially projecting portion are respectively formed into a blunt smooth round shape. The dielectric resin is filled in spaces in the case.

According to another aspect of the present invention, there is provided, an ignition coil, which includes a central core, primary and secondary coils, a case, a dielectric sheet and dielectric resin. The central core is produced from at least one magnetic steel plate. The primary and secondary coils are produced by concentrically winding primary and secondary electric wires, respectively, around the central core. The case receives the primary and secondary coils. The dielectric sheet is wound around an outer peripheral surface of the central core. An outer peripheral side axial end corner of at least one of axial end portions of the dielectric sheet is formed into a blunt smooth round shape. The dielectric resin is filled in spaces in the case.

According to another aspect of the present invention, there is provided an ignition coil, which includes primary and secondary coils, a case, a central core, a sheet and thermoset resin. The case receives the primary and secondary coils. The central core is made of a magnetic material and is positioned radially inward of the primary and secondary coils. The sheet relieves a thermal stress and is wound around the central core. At least one of axial end portions of the sheet forms a folded end portion, which is folded against a corresponding axial end surface of the central core or an axially outer end surface of a connecting member connected to the corresponding axial end surface of the central core. The folded end portion includes a plurality of generally arcuate segments, which are folded one after another on one circumferential side thereof. A first circumferential end and a second circumferential end of each generally arcuate segment are folded in such a manner that the first circumferential end of the generally arcuate segment is folded over a second circumferential end of an adjacent one of the plurality of generally arcuate segments located on a first circumferential end side of the generally arcuate segment, and the second circumferential end of the generally arcuate segment is folded beneath a first circumferential end of another adjacent one of the plurality of generally arcuate segments located on a second circumferential end side of the generally arcuate segment. The thermoset resin is filled in spaces in the case.

According to another aspect of the present invention, there is provided an ignition coil, which includes primary and secondary coils, a case, a central core, a sheet and thermoset resin. The case receives the primary and secondary coils. The central core is made of a magnetic material and is positioned radially inward of the primary and secondary coils. The sheet relieves a thermal stress and is wound around the central core. At least one of axial end portions of the sheet forms a folded end portion, which is folded against a corresponding axial end surface of the central core or an axially outer end surface of a connecting member connected to the corresponding axial end surface of the central core. The folded end portion includes a plurality of generally arcuate bottom side segments and a plurality of generally arcuate top side segments, which are alternately arranged in a circumferential direction. A first circumferential end and a second circumferential end of each generally arcuate top side segment are folded in such a manner that the first circumferential end of the generally arcuate top side segment is folded over an adjacent one of the plurality of generally arcuate bottom side segments located on a first circumferential end side of the generally arcuate top side segment, and the second circumferential end of the generally arcuate top side segment is folded over another adjacent one of the plurality of generally arcuate bottom side segments located on a second circumferential end side of the generally arcuate top side segment. The thermoset resin is filled in spaces in the case.

According to another aspect of the present invention, there is also provided a manufacturing method of an ignition coil, which includes a central core that is produced from at least one magnetic steel plate; primary and secondary coils that are produced by concentrically winding primary and secondary electric wires, respectively, around the central core; a case that receives the primary and secondary coils; a dielectric sheet that is wound around an outer peripheral surface of the central core; and dielectric resin that is filled in spaces in the case. According to the manufacturing method, the dielectric sheet is wound around the outer peripheral surface of the central core in such a manner that at least one of axial end portions of the dielectric sheet projects from a corresponding axial end surface of the central core to form an axially projecting portion. An excess region of the axially projecting portion is melted and is cut by one of a hot wire and a laser beam in such a manner that an inner peripheral side axial end corner and an outer peripheral side axial end corner in the axially projecting portion are respectively formed into a blunt smooth round shape.

According to another aspect of the present invention, there is provided a manufacturing method of an ignition coil, which includes a central core that is produced from at least one magnetic steel plate; primary and secondary coils that are produced by concentrically winding primary and secondary electric wires, respectively, around the central core; a case that receives the primary and secondary coils; a dielectric sheet that is wound around an outer peripheral surface of the central core; and dielectric resin that is filled in spaces in the case. The dielectric sheet is wound around the outer peripheral surface of the central core in such a manner that at least one of axial end portions of the dielectric sheet projects from a corresponding axial end surface of the central core to form an axially projecting portion. The axially projecting portion is heated in such a manner that an inner peripheral side axial end corner and an outer peripheral side axial end corner in the axially projecting portion are respectively formed into a blunt smooth round shape.

According to another aspect of the present invention, there is provided a manufacturing method of an ignition coil, which includes: primary and secondary coils; a case that receives the primary and secondary coils; a central core that is made of a magnetic material and is positioned radially inward of the primary and secondary coils; and thermoset resin filled in spaces in the case. According to the manufacturing method, a sheet, which relieves a thermal stress, is wound around the central core. At least one of axial end portions of the sheet is folded against a corresponding axial end surface of the central core or an axially outer end surface of a connecting member connected to the corresponding axial end surface of the central core, in such a manner that a plurality of points of the axial end portion, which are arranged one after another in a circumferential direction of the axial end portion, are partially folded first, so that a folded end portion, which includes a plurality of generally arcuate segments folded one after another in the circumferential direction, is formed.

According to another aspect of the present invention, there is provided a manufacturing apparatus for manufacturing an ignition coil, which includes: primary and secondary coils; a case that receives the primary and secondary coils; a central core that is made of a magnetic material and is positioned radially inward of the primary and secondary coils; and thermoset resin filled in spaces in the case. The manufacturing apparatus includes a folding jig that folds an axially projecting portion of a sheet, which relieves a thermal stress and is wound around the central core, against a corresponding axial end surface of the central core or an axially outer end surface of a connecting member connected to the corresponding axial end surface of the central core. An axial end of the folding jig includes a recessed processing portion that engages a plurality of points of the axially projecting portion, which are arranged one after another in a circumferential direction of the axially projecting portion, to form creases in the plurality of points, respectively.

DETAILED DESCRIPTION OF THE INVENTION

First Embodiment

As shown inFIG. 1, an ignition coil1of the present embodiment includes a central core23, a primary coil21, a secondary coil22and a coil case20. The central core23is made of magnetic steel plates. The primary coil21and the secondary coil22are produced by concentrically winding a primary electric wire and a secondary electric wire around the central core23. The coil case20receives the primary coil21and the secondary coil22. In the ignition coil1, a dielectric sheet4, which reduces an applied stress, is wound around an outer peripheral surface of the central core23, and dielectric resin11is filled in gaps inside the coil case20.

As shown inFIG. 3, at least one axial end portion of the dielectric sheet4, which extends in an axial direction L, projects from a corresponding axial end surface232of the central core23to form an axially projecting portion43. As shown inFIGS. 4 and 5, an inner peripheral side axial end corner44and an outer peripheral side axial end corner45of the axially projecting portion43are respectively formed into a blunt smooth round shape.

The ignition coil1and a manufacturing method thereof will be described with reference toFIGS. 1 to 14.

As shown inFIG. 1, the ignition coil1of the present embodiment is a stick type ignition coil. Specifically, a cylindrical portion2of the ignition coil1, which includes the primary coil21, the secondary coil22, the central core23and the coil case20, is fitted into a corresponding plughole of the engine.

The primary coil21is formed by winding the insulation-coated primary electric wire multiple times around an outer peripheral surface of a cylindrical primary spool211made of resin. The secondary coil22is formed by winding the insulation-coated secondary electric wire multiple times around a cylindrical secondary spool221made of resin. Here, the number of turns of the secondary electric wire is larger than that of the primary electric wire. The secondary coil22is placed radially inward of the primary coil21, and the central core23, which is made of the magnetic steel plates, is placed radially inward of the secondary coil22. The primary coil21is received in the cylindrical coil case20made of resin. An outer peripheral core24, which is made of magnetic steel plates and has a cylindrical cross section, is positioned radially outward of the coil case20.

The magnetic steel plates, which constitute the central core23, are planar silicon steel plates, each of which has a dielectric insulation coating. The planar silicon steel plates are stacked one after another in a diametric direction, which is perpendicular to the axial direction L of the ignition coil1. The magnetic steel plates of the outer peripheral core24are cylindrical silicon steel plates, each of which has a slit (a gap) that extends in the axial direction L. These cylindrical steel plates are stacked one after another in a radial direction in such a manner that bonding agent is radially interposed between each adjacent two cylindrical steel plates to bond therebetween. A magnetic flux, which is generated upon supplying electric current to the primary coil21, can be amplified by passing the magnetic flux through the central core23and the outer peripheral core24.

FIG. 2is an enlarged view around one end of the central core23at one axial side D1, andFIG. 3is an enlarged view showing the other end of the central core23at the other axial side D2.

As shown inFIG. 2, one axial end portion of the dielectric sheet4at the one axial side D1is folded radially inward along an end surface231of the central core23at the one axial side D1. A resilient body (a cushioning member)29is positioned at the end surface231of the central core23at the one axial side D1to clamp a portion of the dielectric sheet4in corporation with the end surface231of the central core23.

As shown inFIG. 3, the other axial end portion of the dielectric sheet4at the other axial side D2protrudes in the axial direction L from the end surface232of the central core23and thereby forms the axially projecting portion43. The axially projecting portion43of the dielectric sheet4is received in the other axial end portion of the secondary spool221at the other axial side D2.

As shown inFIG. 5, the dielectric sheet4includes a sheet base41, which is made of synthetic resin, and a bonding layer42made of bonding agent is coated over a back surface of the sheet base41. In the present instance, the sheet base41is a polyethylene terephthalate (PET) film, and the bonding layer42is made of an acrylic bonding agent. A thickness of the dielectric sheet4(a sum of a thickness of the sheet base41and a thickness of the bonding layer42) is in a range of 0.025 to 0.1 mm.

As shown inFIG. 1, the dielectric resin11is filled in a space between the dielectric sheet4placed over the outer peripheral surface of the central core23and the secondary coil22, a space between the secondary coil22and the primary coil21and a space between the primary coil21and the coil case20. In the present instance, the dielectric resin11is epoxy resin.

As shown inFIG. 1, in the ignition coil1, an igniter arrangement3, which supplies electric power to the primary coil21, is provided at the one axial side D1of the coil case20, and a plug connector arrangement25, to which a spark plug is connected, is provided at the other axial side D2of the coil case20.

The igniter arrangement3includes an igniter32, which supplies the electric power to the primary coil21and is received in an igniter case31. After installation of the igniter32in the igniter case31, the dielectric resin11is filled in the igniter case31. The dielectric resin11is continuously filled in the respective spaces in the coil case20and the respective spaces in the igniter case31.

The igniter32includes an electric power control circuit and an ionic current sensing circuit. The electric power control circuit includes a switching element, which is operated by a signal supplied from an engine control unit (ECU). The ionic current sensing circuit senses ionic current.

In the ignition coil1, when pulsed spark generation signals are outputted from the ECU to the igniter32, the electric power control circuit of the igniter32is operated, so that electric current flows instantaneously through the primary coil21, and thereby a magnetic field, which passes through the central core23and the outer peripheral core24, is generated. Then, an induction magnetic filed, which passes the central core23and the outer peripheral core24in a direction opposite from the above magnetic field, is generated. Due to the generation of the induction magnetic field, an induced electromotive force (a back electromotive force) is generated in the secondary coil22, and thereby a spark is generated from the spark plug connected to the ignition coil1.

Furthermore, as shown inFIG. 1, the plug connector arrangement25includes a plug cap26made of rubber. The plug cap26is provided to an extended portion201, which extends from the coil case20. A plug installation opening261, into which the spark plug is installed, is formed in the plug cap26. Furthermore, a coil spring28is arranged in the plug installation opening261to engage with the spark plug. The coil spring28is electrically connected to a high voltage side end of the secondary coil22through a high voltage terminal27.

FIG. 4is a diagram showing the axially projecting portion43of the dielectric sheet4, andFIG. 5is a diagram showing the axial end part431of the axially projecting portion43in an enlarged scale.

In the present instance, as indicated inFIG. 4, the dielectric sheet4is wound multiple times around the outer peripheral surface of the central core23, so that multiple dielectric sheet constituent sections401are formed as layers. Each adjacent two dielectric sheet constituent sections401are bonded together by the corresponding bonding layer42provided to the back surface of the sheet base41.

Furthermore, as shown inFIG. 5, at the axial end part431of the axially projection portion43, the adjacent wound parts of the adjacent sheet base41are thermally welded to each other. The bonding layer42at the axial end part431of the axially projecting portion43at each dielectric sheet constituent section401is melted and is dissolved into the sheet base41when the wound parts of the sheet base41are thermally fused together, i.e., thermally welded together.

Furthermore, as shown inFIG. 5, the inner peripheral side axial end corner44, which has the blunt smooth round shape, is formed as an inner peripheral side axial end corner44of the radially innermost dielectric sheet constituent section401. In addition, the outer peripheral side axial end corner45, which has the blunt smooth round shape, is formed as an outer peripheral side axial end corner45of the radially outermost dielectric sheet constituent section401.

Although not illustrated, a permanent magnet, which increases a magnetic flux density, may be provided to each of the opposed end surfaces of the central core23, which are opposed to each other in the axial direction L. In such a case, at least one of the end portions of the dielectric sheet4, which is wound around the outer peripheral surface of the central core23, may have the axially projection portion43, which projects in the axial direction L from an end surface of the corresponding permanent magnet.

Next, the manufacturing method and advantages of the ignition coil1will be described.

In the manufacturing method of the present embodiment, the dielectric sheet4is wound around the central core23through a sheet winding step and a melting and cutting step described below.

Specifically, in the present embodiment, the dielectric sheet4is wound around the central core23in the sheet winding step. The dielectric sheet4is wound multiple times around the outer peripheral surface of the central core23in such a manner that the bonding layer42, which is provided to the back surface of the dielectric sheet4, is bonded to the radially adjacent part of the sheet base41. At this time, the dielectric sheet4is wound around the central core23in a state where the axial end portion of the dielectric sheet4projects from the end surface232of the central core23at the other axial end side D2.

Then, as shown inFIG. 6, an axial end portion of each dielectric sheet constituent section401wound around the central core23at the other axial side D2projects in the axial direction L from the end surface232of the central core23at the other axial side D2to form an axially projecting portion43A. Furthermore, an end portion of each layered dielectric sheet constituent section401at the one axial side D1projects in the axial direction L from the end surface231of the central core23at the one axial side D1to form a foldable projecting portion46.

A projecting length X1of the axially projecting portion43A is made longer than a required length X2, so that the axially projecting portion43A has an excess region430, which is melted and is cut in the following melting and cutting step. InFIG. 6, the axial end part431of the axially projecting portion43, which has the required length X2, is indicated by a dot-dot-dash line R Next, as shown inFIG. 7, a hot wire (a hot wire cutter)51is used to melt and cut the excess region430of the axially projecting portion43A in the melting and cutting step. The hot wire51is made of, for example, a Nichrome wire and is connected to a power source device52. When the hot wire51is energized by the power source device52, a Joule heat is generated from the hot wire51. The hot wire51is relatively moved in a direction perpendicular to the axial direction L of the central core23, so that the hot wire51melts and cuts the excess region430of the axially projecting portion43A of the dielectric sheet4wound around the central core23.

Even when the bonding agent42of the dielectric sheet4adheres to the hot wire51, the adhered bonding agent42can be removed by appropriately controlling the temperature of the hot wire51.

As shown inFIG. 8, in place of the hot wire51, a laser beam531may be used to melt and cut the excess region430of the axially projecting portion43A of the dielectric sheet4. In such a case, the laser beam531is outputted from a laser gun53in the direction perpendicular to the axial direction of the central core23. At that time, the central core23, around which the dielectric sheet4is wound, is rotated relative to the laser beam531, so that the laser beam531melts and cuts the excess region430of the axially projecting portion43A of the dielectric sheet4.

As shown inFIGS. 4 and 5, at the time of melting and cutting, a subject cutting segment of the axially projecting portion43to be cut is melted by the hot wire51. Then, the adjacent layered wound parts of the sheet base41at the subject cutting segment are thermally fused together. In this way, the axial end part431of the axially projecting portion43is strongly joined together, and the inner peripheral side axial end corner44and the outer peripheral side axial end corner45in the axial end part431of the axially projecting portion43are respectively formed into the blunt smooth round shape.

Thereby, the winding of the dielectric sheet4relative to the central core23is finished.

The axial end surface432of the axially projecting portion43may be formed into a wavy form like one shown inFIG. 5. Alternatively, the axial end surface432of the axially projecting portion43may be formed into a convex shape like one shown inFIG. 9. Further alternatively, the axial end surface432of the axially projecting portion43may be formed into a concave shape like one shown inFIG. 10.

Thereafter, the ignition coil1is assembled by placing the primary coil21, the secondary coil22and the central core23with the wound dielectric sheet4into the coil case20.

At the time of this assembly, the axially projecting portion43of the dielectric sheet4, which is wound around the central core23, is placed in the end portion of the secondary spool221at the other axial side D2, and the foldable projecting portion46of the dielectric sheet4is folded radially inward against the end surface231of the central core23at the one axial side D1. Then, the resilient body29, which is received in the igniter case31, is placed against the foldable projecting portion46, which is folded against the end surface231of the central core23at the one axial side D1.

Then, after the assembling of the ignition coil1, the dielectric resin11, which has been heated and melted, is filled in the respective spaces in the igniter case31and the respective spaces in the coil case20.

Thereafter, when the filled dielectric resin11is solidified, edged grooves (sharp grooves), which cause stress concentration, will not be formed in the dielectric resin11, which is filled in the coil case20and is located adjacent to the rounded inner peripheral side axial end corner44and the rounded outer peripheral side axial end corner45. Instead of the edged grooves (the sharp grooves), rounded smooth grooves are formed in the dielectric resin11.

In the ignition coil1of the present embodiment, the inner peripheral side axial end corner44and the outer peripheral side axial end corner45of the axially projecting portion43are respectively formed into the blunt smooth round shape. In this way, at the time of repeating the heating and cooling cycle, it is possible to limit formation of a crack in the dielectric resin11.

Specifically, at the time of repeating the combustion process and the exhaust process of the internal combustion engine, to which the above ignition coil1is installed, when the heating and cooling is repeated in the ignition coil1, a thermal stress is generated in the constituent components of the ignition coil1. A coefficient of linear expansion of the central core23and a coefficient of linear expansion of the dielectric resin11differ from each other, and the dielectric resin11experiences a larger thermal deformation in comparison to the central core23. At the time of the thermal deformation, when the dielectric resin11contracts due to the cooling of the ignition coil1, a contraction force of the dielectric resin11can be reduced by the dielectric sheet4, which is wound around the outer peripheral surface of the central core23.

The rounded smooth grooves are formed in the dielectric resin11, which is filled in the coil case20and is positioned adjacent to the inner peripheral side axial end corner44and the outer peripheral side axial end corner45at the axially projecting portion43of the dielectric sheet4. Therefore, at the time of contraction of the dielectric resin11due to the cooling of the ignition coil1, the stress concentration in the dielectric resin11can effectively limited, and thereby it is possible to limit the generation of a crack in the dielectric resin11.

In the above case, the dielectric sheet4is wound multiple times around the outer peripheral surface of the central core23to form the multilayered dielectric sheet constituent sections401of the dielectric sheet4. Alternatively, as shown inFIG. 11, the dielectric sheet4may be wound only once around the outer peripheral surface of the central core23to form a single layer of the dielectric sheet4.

Even in such a case, the axial end surface432of the axially projecting portion43of the dielectric sheet4may be formed into a wavy form like one shown inFIG. 12. Alternatively, the axial end surface432of the axially projecting portion43may be formed into a convex shape like one shown inFIG. 13. Further alternatively, the axial end surface432of the axially projecting portion43may be formed into a concave shape like one shown inFIG. 14.

Second Embodiment

A second embodiment of the present invention will be described with reference toFIGS. 15 and 16. In the second embodiment, in place of the melting and cutting step of the first embodiment, a heating step described below is performed at the time of winding the dielectric sheet4around the outer peripheral surface of the central core23.

Specifically, as shown inFIG. 15, in the sheet winding step of the present embodiment, the dielectric sheet4is wound around the outer peripheral surface of the central core23without providing the excess region430in the axially projecting portion43of the dielectric sheet4. In the present embodiment, the dielectric sheet4is also wound multiple times around the central core23. At the time of winding the dielectric sheet4, in the axially projecting portion43, which projects from the end surface232of the central core23at the other axial side D2, axial end parts431of the dielectric sheet constituent sections401at the axially projecting portion43are slightly displaced from one another in the axial direction L. Thus, steps433are formed at the axial end parts431of the dielectric sheet constituent sections401in the axially projecting portion43.

Next, as shown inFIG. 16, in the heating step, the axially projecting portion43is heated. In the present embodiment, a hot plate54, which is hated to the high temperature (the temperature that is sufficient to melt the dielectric sheet4), is opposed to the axially projecting portion43of the dielectric sheet4, which is wound around the central core23. At this time, the axial end parts431of the layered dielectric sheet constituent sections401are fused together by the radiant heat541of the hot plate54, and thereby the wound parts of the sheet base41in the axial end parts431of the layered dielectric sheet constituent sections401are thermally fused together.

In this way, the steps433, which are initially present at the axial end parts431of the axially projecting portion43, are substantially smoothened, and thereby the inner peripheral side axial end corner44and the outer peripheral side axial end corner45are respectively formed into the blunt smooth round shape.

In place of the hot plate54, as shown inFIG. 17, hot air551may be blown to melt and thermally fuse the axial end parts431of the layered dielectric sheet constituent sections401of the axially projecting portion43of the dielectric sheet4together. In this case, the hot air551is discharged from a hot air nozzle55toward the axially projecting portion43of the dielectric sheet4, which is wound around the central core23, so that the axial end parts431of the layered dielectric sheet constituent sections401in the axially projecting portion43are melted and fused together.

In the present embodiment, the structure and the manufacturing method of the ignition coil1are similar to those of the first embodiment and can achieve the advantages similar to those of the first embodiment.

In the above embodiments, the sheet base41is made of the PET. Alternatively, the sheet base41may be made of a polyester resin. Furthermore, in the above embodiments, the dielectric resin11is the epoxy resin. Alternatively, the dielectric resin11may be thermoset resin, such as phenolic resin.

Third Embodiment

In the following description of a third embodiment, components similar to those of the first embodiment will be indicated by the same numerals and will not be described further in detail.

As shown inFIGS. 18,19, in the ignition coil1according to the third embodiment, the primary coil21and the secondary coil22are received in the case20. Furthermore, the central core23, which is made of the magnetic body (magnetic steel plates), is positioned radially inward of the primary coil21and the secondary coil22. The dielectric resin (thermoset resin)11is filled in the spaces in the case20. A resilient body (a cushioning member)47, which serves as a connecting member, is connected to each of the axial end surfaces231,232of the central core23. A dielectric sheet (the dielectric tape)8, which relieves thermal stress, is wound around the central core23and the two resilient bodies47. Axial end portions of the sheet8, which are axially opposed to each other in the axial direction L, form folded end portions6, respectively. Each folded end portion6is folded against an axially outer end surface434of the corresponding resilient body47.

As sown inFIG. 18, the folded end portion6of the present embodiment has a plurality of generally arcuate segments61, which are folded one after another on one circumferential side C1thereof in a circumferential direction C of the folded end portion6. One circumferential end (first circumferential end)601of each arcuate segment61is folded over the other circumferential end (second circumferential end)602of the next adjacent arcuate segment61, and the other circumferential end602of each arcuate segment61is folded beneath the one circumferential end601the precedent adjacent arcuate segment61.

Each folded end portion6of the sheet8is heated by ultrasonic wave or heat, so that the arcuate segments61are thermally fused together, i.e., thermally welded together.

The structure of the ignition coil1, a manufacturing method of the ignition coil1and a manufacturing apparatus7of the ignition coil1will be described with reference toFIGS. 18 to 27.

The structures of the primary coil21, of the secondary coil22, of the central core23, and of the outer peripheral core24are substantially the same as those of the first embodiment and thereby will not be described further.

As shown inFIG. 19, an igniter311is provided to one axial end of the ignition coil1to supply electric power to the primary coil21. Furthermore, a plug cap331is provided to the other axial end of the ignition coil1to receive a spark plug.

The case20includes a plug base portion36, a coil case portion35and an igniter base portion34. The plug cap331, a secondary terminal (a high voltage terminal)332and a spring333are arranged in the plug base portion36. The primary coil21, the secondary coil22, the central core23and the outer peripheral core24are arranged in the coil case portion35. The igniter311is arranged in the igniter base portion34.

Although not depicted, the spark plug is received in the plug cap331in such a manner that a terminal portion of the spark plug engages the spring333. Furthermore, in the igniter base portion34, conductive pins313of the igniter311are insert molded to form a connector312. Furthermore, in the coil case portion35, a flange321, which is used to install the ignition coil1to the engine, is formed.

The dielectric resin11is filled continuously in the case20, which is surrounded by the plug base portion36, the coil case portion35and the igniter base portion34.

The igniter311includes an electric power control circuit and an ionic current sensing circuit. The electric power control circuit includes a switching element, which is operated by a signal supplied from an engine control unit (ECU). The ionic current sensing circuit senses ionic current.

As shown inFIG. 22, the sheet8includes a sheet base (a tape base)56, which is made of synthetic resin, and a bonding layer57, which is made of bonding agent, is applied to a back surface of the sheet base56. In the present instance, the sheet base56is the polyethylene terephthalate (PET) film, and the bonding layer57is made of an acrylic bonding agent. A thickness of the sheet8(a sum of a thickness of the sheet base56and a thickness of the bonding layer57) is in a range of 0.025 to 0.1 mm. Furthermore, in the present embodiment, the sheet8is wound multiple times over the outer peripheral surface of the central core23. The number of turns (layers) of the sheet8may be for example, 2 to 5 times (layers).

As shown inFIG. 19, the dielectric resin11is filled in the case20. More specifically, the dielectric resin11is filled in the space between the sheet8around the central core23and the secondary spool221, the respective space between turns of the secondary electric wire of the secondary coil22, the space between the secondary coil22and the primary spool211, the respective space between turns of the primary electric wires of the primary coil21, the space between the primary coil21and the outer peripheral core24and the space between the outer peripheral core24and the case20. In the present instance, the dielectric resin11is epoxy resin.

With reference toFIG. 19, as described above, in the ignition coil1, the resilient bodies47, which serve as connecting members, are provided to the axial end surfaces231,232, respectively, of the central core23. In the present instance, the resilient body47, which is arranged at the one axial end surface231of the central core23, is sponge47A, and the resilient body47, which is arranged at the other axial end surface232of the central core23is rubber47B.

One axial end portion of the sheet8is folded against an axially outer end surface434of the sponge47A, which is arranged at the one axial end surface231of the central core23. The other axial end portion of the sheet8is folded against an axially outer end surface434of the rubber47B, which is arranged at the other axial end surface232of the central core23.

Furthermore, the central core23and the rubber47B are received in the secondary spool221, and the sponge47A is received in an aligning part314, which is formed in the igniter base portion34.

In the ignition coil1, when pulsed spark generation signals are outputted from the ECU to the igniter311, the electric power control circuit of the igniter311is operated, so that electric current flows through the primary coil21, and thereby a magnetic field, which passes through the central core23and the outer peripheral core24, is generated. Then, when the electric current flowing in the primary coil21is stopped, an induction magnetic filed, which passes the central core23and the outer peripheral core24in a direction opposite from the above magnetic field, is generated. Due to the generation of the induction magnetic field, an induced electromotive force (a back electromotive force) is generated in the secondary coil22, and thereby a spark is generated from the spark plug connected to the ignition coil1.

A manufacturing apparatus7and a manufacturing method for manufacturing the ignition coil1will be described.

The manufacturing apparatus7for manufacturing the ignition coil1is used to assemble a central core assembly48, in which the sheet8is wound around the central core23.

As shown inFIG. 20, the manufacturing apparatus7includes a folding jig71, which is used to fold an axially projecting portion (a projecting end portion)58of the sheet8, which projects from the central core23and the corresponding resilient body47in the axial direction L, against the end surface434of the corresponding resilient body47provided to the corresponding end surface231,232of the central core23. The folding jig71includes a recessed processing portion711, which engages a plurality of points of the axially projecting portion58of the sheet8, which are arranged one after another in the circumferential direction C, so that creases611are formed at these points (seeFIG. 22)

As shown inFIG. 20, the recessed processing portion711includes a plurality of tilted wedge projections712that project from multiple points, respectively, of the recessed processing portion711, which are arranged one after another in the circumferential direction C. Each tilted wedge projection712has a tilted configuration for radially inwardly urging the axially projecting portion58of the sheet8. Furthermore, each tilted wedge projection712extends radially outward from the center of the recessed processing portion711and is tilted on the one circumferential side C1. Furthermore, each tilted wedge projection712has a crease forming blade (a crease forming edge)713at an apex of the tilted wedge projection712. The crease forming blade713is used to form the corresponding crease611in the axially projecting portion58of the sheet8.

Furthermore, the manufacturing apparatus7includes a holding jig (not shown) and a drive source (not shown). The holding jig holds the central core23, around which the sheet8is wound. The drive source drives the folding jig71to move toward the holding jig.

In the manufacturing method of the ignition coil1, a winding step, a folding step and a welding step described below are performed to produce the central core assembly48.

At the time of producing the central core assembly48, first, as shown inFIG. 21, the sheet8, which relives the thermal stress, is wound around the central core23in the winding step. At this time, each of the axial end portions of the sheet8is projected from the axially outer end surface434of the corresponding resilient body47connected to the corresponding end surface231,232of the central core23, so that the pair of axially projecting portions58are formed.

Next, in the folding step, each axially projecting portion58is bent against the axially outer end surface434of the corresponding resilient body47connected to the corresponding end surface231,232of the central core23, so that the pair of folded end portions6are formed.

As shown inFIG. 22, in the folding step, the central core23, around which the sheet8is wound, is held by the holding jig71, and the recessed processing portion711of the folding jig71is opposed to the corresponding axially projecting portion58of the sheet8, which projects from the axially outer end surface434of the corresponding resilient body47connected to the corresponding axial end surface231,232of the central core23. Then, the folding jig71is moved toward the axially projecting portion58of the sheet8.

At this time, the crease forming blades713formed at the tilted wedge projections712, respectively, of the recessed processing portion711are engaged with the multiple points of the axially projecting portion58of the sheet8, which are arranged one after another in the circumferential direction C. In this way, the creases611are formed at the multiple points of the axially projecting portion58of the sheet8, which are arranged one after another in the circumferential direction C and are engaged with the crease forming blades713, respectively, so that these multiple points of the axially projecting portion58, at which the creases611are formed, are partially folded first.

The creases611are formed toward the one circumferential side C1in the axially projecting portion58of the sheet8due to the fact that the tilted wedge projections712are tilted on the one circumferential side C1. In this way, the folding jig71folds the axially projecting portion58of the sheet8to a predetermined tilt angle relative to the axial direction L of the central core23.

Then, as shown inFIG. 23, in the welding step, a welding jig75, which has a planar press surface751, is used to fold the axially projecting portion58of the sheet8, which has been previously folded to the predetermined angle, against the axially outer end surface434of the corresponding resilient body47connected to the corresponding axial end surface231,232of the central core23.

Specifically, the planar press surface751of the welding jig75is opposed to the axially projecting portion58of the sheet8, which has been previously folded to the predetermined angle. Then, the welding jig75, which is heated to the welding temperature, is moved toward the axially projecting portion58of the sheet8.

At this time, the planar press surface751of the welding jig75engages the entire axially projecting portion58of the sheet8. In this way, the multiple arcuate segments61, which are folded one after another on the one circumferential side C1in the circumferential direction C, are formed to correspond with the multiple creases611in the axially projecting portion58of the sheet8. Furthermore, in the thus produced folded end portion6, the multiple arcuate segments61are thermally welded together at the overlapped points between the multiple arcuate segments61.

The above folding step and welding step are performed on the corresponding resilient body47connected to the corresponding axial end surface231,232of the central core23.

In this way, as shown inFIG. 18, in the present embodiment, there is produced the folded end portion6, in which the one circumferential end601of each arcuate segment61is folded over the other circumferential end602of the next adjacent arcuate segment61, and the other circumferential end602of each arcuate segment61is folded beneath the one circumferential end601of the precedent adjacent arcuate segment61.

In this folded end portion6, the multiple arcuate segments61are regularly folded to correspond with the multiple creases611. Therefore, it is possible to limit occurrence of bucking or radially outward partial deformation in the folded end portion6.

Thereafter, in the following assembling step, the primary coil21, the secondary coil22and the central core23are installed in the case20. Then, the dielectric resin11is filled in the case20. At this time, it is possible to limit occurrence of the filling failure of the dielectric resin11or formation of the edge grooves (the sharp grooves), which cause the stress concentration, in the dielectric resin11.

Therefore, through use of the manufacturing apparatus7and the manufacturing method for manufacturing the ignition coil1of the present embodiment, it is easy to produce the ignition coil1, which can effectively limit generation of the crack in the dielectric resin11.

Fourth Embodiment

In a fourth embodiment, various other types of folded end portion6of the sheet8will be described.

As shown inFIG. 24, in the folded end portion6of the present embodiment, generally arcuate bottom side segments61A and generally arcuate top side segments61B are alternately arranged in the circumferential direction. Each top side segment61B is folded over its adjacent bottom side segments61A. One circumferential end (a first circumferential end)601of each top side segment61B is folded over the next adjacent bottom side segment61A, and the other circumferential end (a second circumferential end)602of each arcuate segment61is folded over the precedent adjacent bottom side segment61A.

As shown inFIG. 25, the folding jig71of the manufacturing apparatus7for manufacturing the ignition coil1according to the present embodiment includes protruding tilted surfaces714, which are provided at multiple points, respectively, of the recessed processing portion711, which are arranged one after another in the circumferential direction C. Each protruding tilted surface714has a tilted configuration for radially inwardly urging the axially projecting portion58of the sheet8and is formed to extend linearly from the center of the recessed processing portion711in a radially outward direction. Furthermore, each protruding tilted surface714has two crease forming blades (crease forming edges)715at its circumferential edges to form creases611.

As shown inFIG. 26, in the folding step of the manufacturing method of the ignition coil1according to the present embodiment, the crease forming blades715proved to the circumferential edges of each protruding tilted surface714form the creases611at the axially projecting portion58of the sheet8.

Furthermore, as shown inFIG. 27, in the welding step of the present embodiment, it is possible to form the folded end portion6, in which the bottom side segments61A and the top side segments61B are alternately arranged in the circumferential direction, and each top side segment61B is folded over its adjacent bottom side segments61A. Furthermore, in the thus produced folded end portion6, the multiple arcuate segments61are thermally welded together at the overlapped points between the multiple arcuate segments61.

The fourth embodiment also provides other advantages, which are similar to those discussed above in the third embodiment.

Additional advantages and modifications will readily occur to those skilled in the art. The invention in its broader terms is therefore not limited to the specific details, representative apparatus, and illustrative examples shown and described. Furthermore, any one or more components of any one of the above embodiments can be combined with any one or more components of any other one of the above embodiments. For instance, the folded end portion of the dielectric sheet of the third or fourth embodiment can be implemented in the dielectric sheet of the first or second embodiment. Also, the manufacturing method and the manufacturing apparatus of the third or fourth embodiments may be applied to the first or second embodiment.