Ignition coil with locating projection in aperture for tower-side terminal

A high voltage post terminal is accurately located within a support aperture during manufacturing processes by at least one inwardly directed projection to avoid assembly problems caused by sizing the entire aperture itself to locate the terminal with sufficient accuracy. By thus improving the concentricity of the high voltage coil components it is possible to achieve more reliable insulation between components at even relatively small-sized coils used for individual engine plug holes. An especially advantageous range of critical parameters has been discovered as achievable by using this more reliable manufacturing process.

CROSS REFERENCE TO RELATED APPLICATION
 This application is based on and incorporates herein by reference Japanese
 Patent Application Nos. Hei 9-213073 filed on Aug. 7, 1997, Hei 9-356425
 filed on Dec. 25, 1997, and Hei 9-357144 filed on Dec. 25, 1997.
 BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates to an ignition coil for installation inside
 an engine plug-hole.
 2. Related Art
 A conventional stick-type ignition coil includes primary and secondary
 coils rolled around two spools having respective different diameters and a
 bar-like center core. The primary and secondary coils and the center core
 are concentrically installed inside a cylindrical coil casing. A
 high-voltage terminal connectable to an ignition plug is attached to the
 lower end of the coil casing by adhesive. Filler such as epoxy-based
 thermosetting resin fills the upper otherwise open end of the coil casing.
 In this conventional ignition coil, as shown in FIGS. 9A and 9B, a
 high-voltage terminal 1 is formed at one coil and by bending copper wire
 into a U-shape, for reducing manufacturing cost. This high voltage
 terminal 1 is vertically press-inserted into a terminal support 2
 integrally formed at the lower end of the secondary spool 5. Further, a
 pin-shaped central high-voltage terminal 3 protruding upwardly from a
 high-voltage tower portion (not illustrated) is upwardly inserted into and
 connected to the U-shaped high-voltage terminal 1. Here, the terminal
 support 2 has a circularly-shaped terminal insertion hole 4 into which the
 high-voltage terminal 3 is upwardly inserted.
 However, as shown in FIG. 9B, when the high-voltage terminal 3 is pinched
 by the U-shaped high-voltage terminal 1, both sides of the high-voltage
 terminal 3 are expanded to a certain degree to attain sufficient
 contacting pressure therebetween. In this condition, the resilient forces
 of both sides of the high-voltage terminal 1 push the high-voltage
 terminal 3 toward the opening side of the U-shaped high-voltage terminal 1
 as denoted by arrow A. When the high-voltage terminal 3 is inserted into
 the high-voltage terminal 1, the coil casing is not yet filled and the
 lower end of the secondary spool 5 (terminal support 2) is not fixed.
 Thus, the lower end of the secondary spool 5 slides toward the opposite
 side of arrow A to an eccentric position due to the reaction force acting
 on high-voltage terminal 3 opposite arrow A. Therefore, the desired
 concentricity between each component inside the coil casing is reduced and
 the electrical insulating distance therebetween varies, thereby reducing
 the degree of insulation between components.
 In this case, as the maximum offset of the secondary coil 5 is defined by
 clearance B between the terminal insertion hole 4 and the high-voltage
 terminal 3, the offset of spool 5 can be reduced by making clearance B
 small. However, when clearance B is made small, it becomes more difficult
 to insert high-voltage terminal 3 into hole 4, and the assembling process
 becomes less desirable. That is, in the above-described conventional
 high-voltage terminal connection structure, it is difficult to
 simultaneously attain both high accuracy distances between assembled parts
 (insulating performance) and efficient, relatively easy assembly
 processes.
 JP-A-8-213259 discloses another conventional stick type ignition coil. This
 ignition coil includes, as shown in FIG. 10, a bar-like center core 102, a
 secondary coil 104 rolled around a secondary spool 103 disposed at the
 outer side of center core 102, a primary coil 106 rolled around a primary
 spool 105 disposed at the outer side of secondary coil 104, and an outer
 core 107 disposed at the outer side of primary coil 106. A thermosetting
 resin fills the gaps between these components to attain electrical
 insulation and mechanical strength inside housing 101.
 In general, an ignition coil needs to be installed in a restricted space
 like an engine plug-hole in which the coil portion outer diameter is less
 than 24 mm. Thus, permanent magnets 109, 110 need to be disposed at both
 ends of center core 102 for generating required ignition coil voltage.
 Here, the excitation poles of permanent magnets 109, 110 are opposite to
 the polarity of center core 102.
 A rare-earth magnet such as neodymium is used for permanent magnets 109,
 110, so as to generate sufficiently high magnetic force in the restricted
 small space. The need for permanent magnets 109, 110 increases
 manufacturing cost for the ignition coil.
 SUMMARY OF THE INVENTION
 The present invention provides an ignition coil with improved assembling
 accuracy and process for connecting together a coil-side high-voltage
 terminal and a tower-side high-voltage terminal.
 The invention also provides required ignition coil performance without a
 permanent magnet.
 In one exemplary embodiment, a convex portion is formed at the inner
 peripheral surface of a terminal insertion hole to improve assembling
 accuracy of a tower-side high-voltage terminal with respect to the
 terminal insertion hole. That is, the lower portion of a secondary spool
 (terminal support portion) is very accurately set in place with respect to
 the tower side high-voltage terminal. In this way, the convex portion can
 adjust the center of the secondary spool lower end, thereby improving
 concentricity and insulating performance between components inside a coil
 casing.
 As the convex portion improves concentricity, the inner diameter of the
 terminal insertion hole does not need to be made so small. Even when the
 tower-side high-voltage terminal contacts the convex portion while the
 tower-side high-voltage terminal is inserted into the terminal insertion
 hole, because the contacting area is small, the resistant force is not so
 large. Therefore, the tower-side high-voltage terminal can be easily
 inserted into the terminal insertion hole. In general, the terminal
 support portion and the convex portion are made of insulating resin (as is
 the spool). Thus, when the tower-side high voltage terminal contacts the
 convex portion, the convex portion easily moves relative to the outer
 shape of the tower-side high-voltage terminal. As a result, the resisting
 force is made small, and assembling performance is improved.
 If the diameter x mm of a center core and the thickness y mm of an outer
 core satisfy certain specified relationships, the size of an ignition coil
 having no permanent magnet need not be substantially increased with
 respect to a coil having permanent magnets.

DETAILED DESCRIPTION OF PREFERRED EXEMPLARY EMBODIMENTS
 (First Embodiment)
 A first embodiment will be described with reference to FIGS. 1-5. FIG. 1
 shows the entire structure of an ignition coil 10.
 A cylindrical coil casing 11 is made of insulating resin, and has a head
 casing 12 integrally formed at its upper end. A connector housing 14, into
 which connector pin 13 is insert-formed, is press-inserted to head casing
 12. The connector housing 14 includes an integral base plate 15. An
 igniter 16 is installed on base plate 15. An igniting signal from an
 engine-control computer (not illustrated) is input into igniter 16 through
 connector pin 13.
 Inside coil casing 11, a bar-like center core 18 and a cylindrical outer
 core 19 are concentrically installed. A primary coil 20 rolled around a
 primary spool 45 made of insulating resin is installed inside cylindrical
 outer core 19. A secondary coil 22 rolled around a secondary spool 21 made
 of insulating resin is installed inside primary coil 20. The center core
 18 is installed inside secondary spool 21. Cushion members 23 are provided
 at upper and lower ends of center core 18. The cushion members 23 are made
 of heat resistant elastic material such as anti-magnetic strain sponge, an
 elastomer or the like. Inside coil casing 11 and head casing 12, an
 insulating resin such as an epoxy-based thermosetting resin is vacuum
 filled as filler 30.
 An inner set-place cylindrical portion 35 is provided for establishing the
 fitted position of the upper end of secondary spool 21. An outer set-place
 cylindrical portion 36 is provided for establishing the position of the
 upper end of the primary spool 45. The set-place portions 35 and 36 are
 integrally formed at the under surface of base plate 15 of connector
 housing 14. The upper end of secondary spool 21 is fitted into a ring-like
 gap between both set-place cylindrical portions 35, 36. Here, the
 secondary spool 21 has an elastic attaching nail 37 integrally formed at
 its upper end. The elastic attaching nail 37 protrudes toward the outer
 peripheral side of secondary spool 21, and is attached into the step
 portion of the outer set-place cylindrical portion 36. In this way, the
 secondary spool 21 is connected to the base plate 15. The upper end of
 center core 18 is fitted into the inside of the inner set-place
 cylindrical portion 35. Thereby the upper end of center core 18 is
 established.
 A terminal support portion 25 is integrally formed at the lower end of
 secondary spool 21. A coil-side high-voltage terminal 26 connected to one
 end of secondary coil 22 is connected to terminal support portion 25. The
 coil-side high-voltage terminal 26 is, as shown in FIGS. 3-5, made by
 bending one end of a lead wire (such as copper wire) into substantially
 U-shape. The coil-side high-voltage terminal 26 also defines a connecting
 portion 27 at its other end connected to one end of secondary coil 22. The
 coil-side high-voltage terminal 26 is, as shown in FIGS. 2, 3, press
 inserted horizontally into an insertion hole 28 of the terminal support
 portion 25 and fixed thereto.
 At the center of terminal support portion 25, a circularly shaped terminal
 insertion hole 31 is formed into which a tower-side high-voltage terminal
 29 described below is inserted upwardly. The terminal insertion hole 31
 defines a tapered surface 34 at its lower end inner peripheral edge for
 guiding the insertion of the tower-side high-voltage terminal 29. Three
 locating convex portions 32, 33 are formed at the inside surface of
 terminal insertion hole 31 for establishing the location of tower-side
 high-voltage terminal 29. These locating convex portions 32, 33 are formed
 above the coil-side high-voltage terminal 26 to have substantially equal
 distances from each other. Out of these locating convex portions 32, 33,
 two convex portions 32 are formed at the opening side (right side in FIG.
 3) of the coil-side high-voltage terminal 26, and one remaining convex
 portion 33 is formed at the bottom side (left side in FIG. 3) of the
 coil-side high-voltage terminal 26. The lower portion of each convex
 portion 32, 33 has an inclined surface inclining diagonally upwardly with
 respect to the inner periphery of terminal support portion 25. The
 inclining surface guides the tower-side high-voltage terminal 29 as it is
 being inserted. Here, the convex portions 32, 33 may be press contacted,
 merely contacted, or only made to be adjacent to the tower-side
 high-voltage terminal 29 inserted into terminal insertion hole 31.
 As shown in FIG. 1, high-voltage tower portion 38 made of insulating resin
 is connected to the lower end of coil casing 11 by adhesive. In the upper
 central portion of this high-voltage tower portion 38, a terminal cup 39
 is insert formed to which the pin-shaped tower side high-voltage terminal
 29 is upwardly affixed. The coil-side high-voltage terminal 26 pinches
 tower side high-voltage terminal 29, and these are thus electrically
 connected with each other. When high-voltage tower portion 38 is inserted
 into an engine plug hole (not illustrated) and press-inserted onto the top
 of an ignition plug terminal (not illustrated), an electrically conductive
 spring 40 inside terminal cup 39 is press-contacted onto an ignition plug
 terminal. Thus, one end of secondary coil 22 is electrically connected to
 an ignition plug terminal through coil-side high-voltage terminal 26,
 tower-side high-voltage terminal 29, terminal cup 39 and spring 40.
 The above described ignition coil 10 is assembled as explained hereinafter.
 At first, each component such as secondary coil 22, primary coil 20, center
 core 18 and the like is installed inside coil casing 11. Then,
 high-voltage tower portion 38 is connected to the lower end of coil casing
 11. At this time, tower-side high-voltage terminal 29 is inserted upwardly
 into terminal insertion hole 31 of terminal support member 25 and pinched
 into contact with the U-shaped coil-side high-voltage terminal 26.
 Here, before tower-side high-voltage terminal 29 is pinched by the
 coil-side high-voltage terminal 26, as shown in FIG. 4, the distance
 between both sides of the U-shaped portion of coil-side high-voltage
 terminal 26 is smaller than the diameter of tower-side high-voltage
 terminal 29. However, after tower-side high-voltage terminal 29 is pinched
 by the coil-side high-voltage terminal 26, as shown in FIGS. 3 and 4, both
 sides of the U-shaped portion of the coil-side high-voltage terminal 26
 are expanded. Under this condition, the resilient forces from both sides
 of coil-side high-voltage terminal 26 push the tower-side high-voltage
 terminal 29 toward the opening side of the coil-side high-voltage terminal
 29 as denoted by arrow A.
 Here, because the convex portions 32, 33 are formed at the inner periphery
 of terminal insertion hole 31, the lower end (terminal support portion 25)
 of secondary spool 21 is prevented by these convex portions 32, 33 from
 sliding toward the opposite direction of arrow A due to the resilient
 forces of the coil-side high-voltage terminal 26. Thus, the lower end
 (terminal support member 25) of secondary spool 21 can be accurately
 located with respect to the tower-side high-voltage terminal 29. That is,
 the convex portions 32, 33 adjust the center location of the lower end
 portion of secondary spool 21, thereby improving concentricity between
 components inside coil casing 11 and therefore improving the consistency
 of insulation performance therebetween.
 In general, in the past, when the inner diameter of terminal insertion hole
 31 is made small in an attempt to minimize clearance so as to improve
 concentricity, it becomes difficult to insert tower-side high-voltage
 terminal 29 into terminal insertion hole 31 and assembling efficiency is
 lessened. However, in the present embodiment, because the convex portions
 32, 33 together with resilient forces in the direction of arrow A improve
 concentricity, the inner diameter of the terminal insertion hole no longer
 needs to be made so small. Further, during insertion, even when tower-side
 high-voltage terminal 29 contacts or press-contacts convex portions 32,
 33, the resistant forces upon insertion are not so large because the
 contacting area of these portions is comparatively small. Thus, tower side
 high-voltage terminal 29 is comparatively more easily inserted into
 terminal insertion hole 31.
 In the present embodiment, terminal support member 25 and convex portions
 32, 33 are made of insulating resin as is secondary spool 21. Thus, when
 tower side high-voltage terminal 29 press-connects with convex portions
 32, 33, convex portions 32, 33 transform in accordance with the outer
 shape of tower-side high-voltage terminal 29. Therefore, the contact
 resistant forces caused by convex portions 32, 33 during the insertion
 operation are small. Thus, assembling efficiency is improved.
 Further, in the present embodiment, because three convex portions 32, 33
 are formed at the inner peripheral surface of terminal insertion hole 31,
 the tower side high-voltage terminal is accurately located at the center
 of terminal insertion hole 31. Thereby, concentricity between them is
 improved. This effect can also be attained when four or more convex
 portions are present.
 Here, the number of convex portions may be two or only one. In this case,
 one main object of the present invention still can be sufficiently
 achieved. Further, the shape of coil-side high-voltage terminal 26 is not
 restricted to a substantially U-shape, but may be a substantially V-shape
 instead.
 In general, the outer diameter of center core 18 of ignition coil 10 having
 no permanent magnet, as in the present embodiment, is larger than that in
 an ignition coil having permanent magnets. Therefore, the insulating
 distance between the secondary coil and the primary coil must be small.
 The present invention is much more effective for this type of ignition
 coil, because the internal coil parts are suppressed from becoming
 eccentric and thus retain sufficient insulating distance therebetween via
 convex portions 32, 33.
 Further, the coil shape is not restricted to the above-described shapes.
 For example, a spool without a high-voltage side flange may be used. Here,
 because the spool without a flange is likely to become more eccentric than
 the spool provided with a flange, the advantages of the present invention
 are even more pronounced.
 (Second Embodiment)
 A second embodiment provides an ignition coil that can be downsized even
 when permanent magnets are eliminated from the center core as in the first
 embodiment.
 The second embodiment will be described with reference to FIGS. 6-8.
 As shown in FIG. 6, an ignition coil 51 is installed in a plug-hole formed
 in every cylinder of an engine, and electrically connected to an ignition
 plug (not illustrated). The outer diameter W of a coil portion, which is
 located in the plug-hole, is typically less than 24 mm.
 The ignition coil 51 includes cylindrical housing 52 made of resin. In the
 housing 52, a center core 53, a secondary spool 54, a secondary coil 55, a
 primary spool 56, a primary coil 57 and an outer core 58 are provided (in
 order from the center to the outside). A thermosetting insulating resin
 (for example, an epoxy-based resin) is filled in gaps between these
 internal elements.
 The center core 53 is formed into columnar shape and constructed by
 laminating thin silicon steel plates in an axial direction. The center
 core 53 is located in place by the inside wall of secondary spool 54, and
 there is no permanent magnet at either end.
 Secondary spool 54 forms the second coil 55. Secondary spool 54 is located
 in place by the inside wall of primary spool 56, and is made of resin.
 Secondary coil 55 is formed cylindrically by rolling an insulated thin coil
 wire around the outer periphery of secondary spool 54. Secondary coil 55
 is electrically connected to high-voltage terminal 62 as described below.
 Primary spool 56 forms primary coil 57. Primary spool 56 is located in
 place by housing 52 and the inside wall of outer core 58, and is made of
 resin.
 Primary coil 57 is formed cylindrically by rolling an insulated coil wire
 (thicker than the coil wire of secondary coil 55) around the outer
 periphery of primary spool 56. Primary coil 57 is electrically connected
 to input terminal 61 as described below.
 Outer core 58 contacts the inside wall of housing 52. The outer core is
 shaped cylindrically with a slit to insulate a roll-start point from a
 roll-end point of the thin silicon steel plate.
 A thermosetting insulating resin 59 fills gaps between each component
 assembled in housing 52, and firmly insulates these components from each
 other. Further, the thermosetting insulating resin 59 fixes and integrates
 these components to prevent them from being broken apart by vibration.
 Connector 60 is provided at the upper end of housing 52 in such a manner
 that it protrudes from the plug-hole. The input terminal 61 is
 insert-formed in connector 60, which supplies a control signal to primary
 coil 57. Here, a switching circuit (not illustrated) supplying a control
 signal to input terminal 61 is disposed outside ignition coil 51.
 The high-voltage terminal 62 is insert-formed or press-formed at the lower
 end of housing 52, and electrically connected to spring 63. The spring 63
 is electrically connected to ignition coil 51 when it is installed in the
 plug-hole. A high-voltage generated in secondary coil 55 is supplied to
 the ignition plug through high-voltage terminal 62 and spring 63.
 A high-voltage tower portion 64 made of insulating resin is connected to
 the lower end opening of housing 52.
 The above-described ignition coil 51 satisfies the following conditions.
 FIGS. 7 and 8 are magnetic simulation graphs showing simulation results of
 generated voltage in accordance with relations between the diameter x mm
 of center core 53 and the thickness y mm of outer core 58. Here, FIGS. 7
 and 8 show simulation results under a condition that primary coil 57 has
 230 turns and secondary coil 55 has 17,480 turns and the electric current
 supplied to primary coil 57 is 6.5 A. FIG. 7 shows the result in the case
 where an iron tube is rolled around the plug tube, and FIG. 8 shows the
 result in the case where an aluminum tube is rolled around the plug tube.
 As is understood from FIGS. 7 and 8, when one of center core 53 and outer
 core 58 magnetically saturates at a predetermined voltage, there arises a
 substantial L-shape characteristic. Near this characteristic bent point, a
 relation ratio can be discerned in which waste of center core diameter and
 outer core thickness is minimized. This characteristic bent point area
 generally exists in the vicinity of where
 (11/50)x-1.3.ltoreq.y.ltoreq.(11/50)x-0.6, in a generated voltage range of
 25 kV-40 kV.
 The inventors have carried out various experiments, and have concluded that
 generated voltages of 25 kV-40 kV can be attained when the center core
 diameter x mm and the outer core thickness y mm are chosen to satisfy the
 following relationships:
EQU (11/50)x-1.3.ltoreq.y.ltoreq.(11/50)x-0.6
EQU 6.0.ltoreq.x.ltoreq.11.0
EQU 0.5.ltoreq.y.ltoreq.1.5
 The inventors have concluded that generated voltages of more than 30 kV can
 be attained when the center core diameter x mm and the outer core
 thickness y mm are set to satisfy the following relationships:
EQU 6.0.ltoreq.x.ltoreq.11.0
EQU 0.5.ltoreq.y.ltoreq.1.5
 Further, the inventors have concluded that the above described
 characteristic is attained when the ampere-turns A.times.T are within
 700-2500, and preferably within 800-2000. Here, the ampere-turns A.times.T
 are defined as a product of the amperage A of the electric current
 supplied to primary coil 57 and the number of turns T of primary coil 57.
 In the present exemplary embodiment, as an optimum example, the electric
 current amperage is set to 6.5 A and the turn number of primary coil 57 is
 set to 230T. Therefore, the product ampere-turn A.times.T is about 1500.
 Here, cross-sectional area influences magnetic saturation of outer core 58.
 That is, magnetic saturation of outer core 58 is influenced by not only by
 its thickness but also by its diameter. Then, the inventors concluded that
 the above-described characteristic is attained when the outer diameter of
 the outer core 58 is set within 20-24 mm.
 In the present embodiment, there exists housing 52 of which its thickness
 is 0.5-1.0 mm at the outside of outer core 58, and the outer diameter W of
 the coil portion is set to about 22.0-23.5 mm, for example. According to
 these restrictions, the outer diameter of the outer core 58 of the present
 exemplary embodiment is set within 20.0-23.0 mm.
 By setting each element to satisfy the above-described relationships, the
 outer diameter W of the coil portion is kept under 24.0 mm even when a
 permanent magnet is not used in center core 53, and one is able to
 generate the required voltage. That is, ignition coil 51 does not have to
 be upsized.
 Further, as ignition coil 51 generates the r equired voltage without a
 permanent magnet, its manufacturing cost is reduced.
 In the above-described embodiments, housing 52 is provided at the outside
 of outer core 58. Alternatively, outer core 58 may finction as the
 housing, without using housing 52. In this case, seizing rubber to the
 slit of outer core 58 seals the inside of outer core 58.
 When housing 52 is not provided at the outside of outer core 58, the outer
 diameter of outer core 58 is set within 22.0-23.5 mm.
 The present invention is not restricted to be applied to a stick type
 ignition coil, and may be applied to an ignition coil having a connecting
 portion between a secondary coil-side terminal and a high-voltage
 tower-side terminal.
 While some exemplary embodiments of the invention have been described in
 detail, those skilled in the art will appreciate that many variations and
 modifications may be made in these exemplary embodiments while yet
 retaining some or all of the benefits and advantages of this invention.
 Thus all such variations and modifications are to be included within the
 scope of the following claims.