Ignition coil assembly for internal combustion engine

Abstract: An ignition coil assembly for internal combus­ tion engine has a voltage withstanding air gap (57) of a predetermined shape and an insulating resin cover (58) for covering the external iron core (4) provided in addition to resin mold (6) and partitioning wall (53) in order to electrically insulate from the secondary coil (2) the external iron core (4) constituting a magnetic path together with the internal iron core (3).

Description: BACKGROUND OF THE INVENTION This invention relates to a resin-molded type ignition coil assembly for internal combustion engines (e.g., U.S. Patent No. 4,763,094 issued on August 9, 1988, (and assigned to the same assignee of this invention), and particularly to one which uses a case molded integrally with an external iron core. Fig. 6 shows a partial cross-sectional front view of a conventional ignition coil assembly for internal combustion engines, and Fig. 7 shows a partial cross-sectional plan view taken along a line B-B of Fig. 6. This ignition coil assembly is a resin-molded type ignition assembly which uses a case molded integrally with an external iron core. This ignition coil assembly comprises a primary coil 1a, a secondary coil 2a coaxially disposed on the outside of the primary coil 1a, an internal iron core 3a inserted through the primary and secondary coils 1a and 2a, an external iron core 4a combined with the internal iron core 3a to constitute a closed magnetic path, a resin case 5a which is made of a resin material to be integral with the external iron core 4a and to have a mold space portion 50a, and a molded resin portion 6a which is formed by injecting a resin material into the mold space portion 50a in which the primary and secondary coils 1a and 2a and the internal iron core 3a are placed. In this ignition coil assembly, a central partitioning wall portion 51a of the case 5a, which separates a central portion 40a of the external iron core 4a (which is parallel to the internal iron core 3a) from the mold space 50a, is formed to be thick and to be in intimate contact with the central portion 40a of the external iron core 4a. This thick wall of the central partitioning wall portion 51a assures the insulation between the low-potential external iron core 4a and the high-potential secondary coil 2a. When the central partitioning portion 51a of the case 5a is molded thick enough to increase the dielectric strength, however, internal defects 100 such as nests and voids may be caused in the central partitioning wall portion 51a, so that a corona discharge may occur within the internal defects 100 upon operation, which will lead to a reduction of the insulation ability of the case 5a and, finally, to an insulation breakdown. On the contrary, when the central partitioning wall portion 51a is made thin in order to avoid the above mentioned problem, the distance between the external iron core 4a and the secondary coil 2a becomes short and, thus, the strength of the electric field across the central partitioning wall portion 51a increases. As a result, a corona discharge may occur across the very small gap (not shown) of the boundary between the corners 48a, 49a of the external iron core 4a, where the electric field strength reaches its highest value, and the central partitioning wall portion 51a. When a corona discharge is caused thereat, the insulation ability of the case 5a is deteriorated as described above and, finally, the insulation may be broken down. The very small gap is created depending on the difference between the thermal expansion coefficients of the external iron core 4a and the case 5a. Furthermore, document EP-A-072 716 discloses an ignition coil for an internal combustion engine, wherein a primary and a secondary coil which are spaced from each other and which form part of a closed magnetic path are provided within an ignition coil case. The space between the primary and the secondary coil is isolated by completely molding the case with isolating material. SUMMARY OF THE INVENTION It is an object of the invention to provide an ignition coil assembly for an internal combustion engine which does not easily cause any corona discharge and which has a high reliability. According to the present invention, there is provided an ignition coil assembly for an internal combustion engine comprising a primary coil, a secondary coil coaxially disposed around the outside of said primary coil, an internal iron core inserted through said primary and secondary coils to make part of a closed magnetic path, an external iron core combined with said internal iron core to constitute said magnetic path and having its central portion extended along said internal iron core with a distance kept therebetween, a case made of a resin being formed integrally with said external iron core and having a mold space wherein said primary coil, said secondary coil and said internal iron core are enclosed, and a molded resin portion being formed by injecting a resin material into said mold space so as to surround and to fix said primary coil, said secondary coil and said internal iron core, said ignition coil assembly being characterized in that said case made of resin is provided with a withstand voltage air gap of a predetermined shape between said mold space and said central portion of said external iron core, and a central partitioning wall portion between said withstand voltage air gap and said mold space. The thickness of the central partitioning wall portion is selected to be preferably, for example, in a range from about 0,5 mm from the molding point of view to about 3,5 mm from the viewpoint of nests, voids and so on, which begin to occur thereat, though the values depend on the conditions of the case molding process. The thickness of the withstand voltage air gap is selected to be preferably, for example, in a range from about 0,75 to 6 mm. In other words, when the thickness of the withstand voltage air gap is less than about 0.75 mm, the distance between the external iron core and the secondary coil becomes short, and, thus, the field strength in the withstand voltage air gap is increased. This may cause a corona discharge. Although it is good that the withstand voltage air gap can resist a higher voltage, excessive increase of the dielectric strength will uselessly make the assembly large-sized. As a result, the preferable thickness of the withstand voltage air gap should be within the above range in which at least a corona discharge can be fully prevented. Since the electric field which is of interest here is most concentrated around the corner of the external iron core, it is desirable to provide the withstand voltage air gap at least between the corner of the external iron core and the secondary coil. In order to prevent the peculiar concentration of the field within the withstand voltage air gap, the thickness of the withstand voltage air gap should be made substantially constant, and the inner surface of the withstand voltage air gap should be smooth. The internal defects such as nests and voids which occur easily in the central partitioning wall portion 51a of a thick wall of the prior art assemblies can be considered as a kind of air gap. These internal defects are irregular in shape and size, thus, causing an unpredictable field concentration, and, hence, a corona discharge. In the ignition coil assembly for internal combustion engines according to the present invention, a withstand voltage air gap of a predetermined shape and a central partitioning wall portion, which makes part of the case and separates the withstand voltage air gap from the mold space, are provided between the mold space in which the secondary coil is placed and a mold resin is filled, and the external iron core. Therefore, part of the thickness of the central partitioning wall portion is, in a sense, replaced by the withstand voltage air gap so that the central partitioning wall portion is made thin enough not to cause within itself nests and voids. Thus, the electric field strength between the secondary coil and the external iron core is divided by the mold resin portion within the mold space, the central partitioning wall portion and the withstand voltage air gap. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 shows a partially cross-sectional plan view of a first embodiment of an ignition coil assembly for an internal combustion engine according to the present invention; Fig. 2 shows a partially cross-sectional front view taken along a line A-A' in Fig. 1; Figs. 3, 4 and 5 show partially cross-sectional front views of other embodiments of an ignition coil assembly for an internal combustion engine according to the present invention; Fig. 6 shows a partially cross-sectional plan view of a conventional ignition coil assembly for an internal combustion engine; and Fig. 7 shows a partially cross-sectional front view of the ignition coil assembly of Fig. 6. DESCRIPTION OF THE PREFERRED EMBODIMENTS EMBODIMENT 1 One embodiment of an ignition coil assembly for an internal combustion engine according to the present invention will be described with reference to Figs. 1 and 2. Fig. 1 shows a partial cross-sectional plan view of the one embodiment, and Fig. 2 shows a partial cross-sectional front view taken along a line A-A' in Fig. 1. This ignition coil assembly comprises a primary coil 1, a secondary coil 2 coaxially disposed around the outside of the primary coil 1, an internal iron core 3 which is inserted through the primary and secondary coils 1 and 2, an external iron core 4 which is combined with the internal iron core 3 to constitute a closed magnetic path, a case 5 which is molded integrally with the external iron core 4 and has a mold space 50, and a molded resin portion 6 which is formed by injecting a resin material into the mold space 50. The internal iron core 3 is an I-shaped core of a cylindrical shape and of a rectangular shape in cross-section which is produced by stacking a large number of silicon steel laminas of the same shape. Around the outside of the internal iron core 3 is provided a primary spool 10 of a prism shape and of a predetermined wall thickness which is integrally molded with a thermoplastic resin. The internal iron core 3 has flux penetration surfaces 31, 32 at the opposite ends of the internal iron core 3, which coincide with the ends 11, 12 of the primary spool 10, respectively. Ribs 10a are projected from near the ends 11, 12 of the primary spool 10. The primary coil 1 is wound around the outside of the primary spool 10. Further, the primary spool 10 is coaxially inserted in a secondary spool 20 being of a substantially prism shape, and the opposite ends of the inner peripheral surface of the secondary spool 20 are supported by the ribs 10a of the primary spool 10. The secondary spool 20 has a great number of ribs 20a with a predetermined spacing provided to project from its outer peripheral surface in the direction perpendicular to the axis of the internal iron core 3. Moreover, a large rib 20b is projected from each of the opposite ends of the outer peripheral surface of the secondary spool 20, and the secondary coil 2 is wound on the secondary spool 20. The external iron core 4 is formed of a great number of silicon steel laminas of the same shape to be substantially U-shaped with its cross-section being rectangular. Moreover, this U-shaped external iron core 4 has a central portion 40, and arms 42, 43 extending from both ends of the central portion 40 at right angles to the central portion 40. The arms 42, 43 have provided at their ends opposite magnetic flux penetration surfaces 44, 45 of a square shape, between which the internal iron core 3 is fitted so that the flux penetration surfaces 44, 45 of the external iron core 4, respectively oppose the flux penetration surfaces 31, 32 of the internal iron core 3 through a slight gap. The central portion 40 of the external iron core 4 has apertures 47a, 47b for insertion of clamping bolts bored at its opposite ends in the direction of stacking the silicon steel laminas. The resin case 5 has a thin wall of a cup-like shape with its upper end opened and forms the mold space 50 of a substantially rectangular parallel-piped shape. In other words, the case 5 has a flat bottom portion 52 of a substantially rectangular shape, from the respective sides of which are integrally extended upwards a first side wall portion 53, a second side wall portion 54, a third side wall portion 55 and a fourth side wall portion (not shown). The second and third side wall portions 54, 55 are parallel to each other, and the first and fourth side wall portions 53 are parallel to each other. Moreover, slightly below the outer surface of the first side wall portion 53 is formed an external iron core cover 58 which is made of a resin (see Fig. 2). The wall thickness of the cover, which covers the surface of the external iron core 4, is substantially constant. In this embodiment, the lower surface 46 of the external iron core 4 and the inner peripheral surface of the apertures 47a, 47b are not covered by this external iron core cover 58, but exposed to the external space. The ends of the arms 42, 43 of the external iron core 4 reach the outsides of the second and third side wall portions 54, 55 of the case 5, and the flux penetration surfaces 44, 45 of the external iron core 4 are exposed to the mold space 50. In the mold space 50 are enclosed the primary coil 1, the secondary coil 2, the internal iron core 3, the primary spool 10 and the secondary spool 20. As described above, the flux penetration surfaces 44, 45 of the external iron core 4 are respectively opposed to the flux penetration surfaces 31, 32 of the internal iron core 3 through a slight gap. The inside portion 58a of the cover 58 of the case 5 which covers the central portion 40 of the external iron core 4 is opposed to the first side wall portion 53 of the case 5 through an about 2.5 mm wide withstand voltage air gap 57. The upper and lower ends of the withstand voltage air gap 57, as shown in Fig. 2, have openings 57a, 57b for communicating with the external space. The molded resin portion 6 is formed by injecting a thermoplastic resin such as epoxy resin into the mold space 50 and fixes and protects the primary coil 1, the secondary coil 2 and the internal iron core 3 enclosed in the mold space 50. As described above, between the first side wall portion 53 of the case 5 and the external iron core 4 are located the inner side portion 58a of the external iron core cover 58 of which the wall thickness is about 1.5 mm, and the withstand voltage air gap 57 is about 2.5 mm thick. Upon operation of the ignition coil assembly for internal combustion engines, the external iron core 4 is grounded and the secondary coil 2 is at a high potential. The potential difference between the external iron core 4 and the secondary coil 2 is divided by the inner side portion 58a of the external iron core cover 58, the withstand voltage air gap 57, the first side wall portion 53 and the mold resin 6 in the mold space 50. Thus, the strength of electric field between the external iron core 4 and the secondary coil 2 is decreased by the withstand voltage air gap 57. Therefore, even if the first side wall portion 53 of the case 5 and the inside portion 58a of the external iron core cover 58 thereof are decreased in the wall thickness, no corona discharge occurs between the external iron core 4 and the secondary coil 2. Particularly, the electric field flux between the external iron core 4 and the secondary coil 2 is normally collected at the corners 48, 49 of the external iron core 4, but in this embodiment, since the withstand voltage air gap 57 is larger than the total height h (see Fig. 2) of the external iron core 4, the electric field flux is fully prevented from being collected at around the corners 48, 49 of the external iron core 4. Moreover, since the first side wall portion 53 of the case 5 and the wall of the external iron core cover 58 thereof are thin, internal defects such as nests and voids are difficult to occur. EMBODIMENT 2 Fig. 3 is a partial cross-sectional front view of a second embodiment of the ignition coil assembly for an internal combustion engine according to the present invention. In this figure, like elements corresponding to those of the first embodiment are identified by the same reference numerals. This ignition coil assembly comprises the first coil 1, the second coil 2, the internal iron core 3, the external iron core 4, the case 5 and the molded resin portion 6. The second embodiment is different from the first embodiment only in the shape of part of the case 5. In other words, in the case 5, the external iron core cover 58 and the first side wall portion 53 are connected at the upper end of the withstand voltage air gap 57, and, as a result, the withstand voltage air gap 57 and the external space are not communicated with each other at the upper side. In this embodiment, since the withstand voltage air gap 57 is extended above the corner 48 of the external iron core 4, the electric field flux is fully prevented from being collected at near the corner 48 of the external iron core 4. EMBODIMENT 3 Fig. 4 is a partial cross-sectional front diagram of a third embodiment of the ignition coil assembly for an internal combustion engine according to the present invention. In Fig. 4, like elements corresponding to those of the second embodiment are identified by the same reference numerals. This ignition coil assembly comprises the first coil 1, the secondary coil 2, the internal iron core 3, the external iron core 4, the case 5 and the molded resin portion 6. The third embodiment is different from the second embodiment only in the shape of part of the case 5. In other words, in this case 5, the inside portion 58a (see Fig. 3) of the external iron core cover 58 in the embodiment 2 is replaced by the withstand voltage air gap 57, so that the dielectric strength of the withstand voltage air gap 57 can be further increased. EMBODIMENT 4 Fig. 5 is a partially cross-sectional front view of a fourth embodiment of the ignition coil assembly for an internal combustion engine according to the present invention. In Fig. 5, like elements corresponding to those of the first embodiment are identified by the same reference numerals. This ignition coil assembly comprises the first coil 1, the secondary coil 2, the internal iron core 3, the external iron core 4, the case 5 and the molded resin portion 6. This embodiment is different from the embodiment 3 only in the shape of part of the case 5. In this embodiment, the external iron core cover 58 has large recesses 48m, 48n provided at positions close to the corners 48, 49 of the case 5, which prevent a discharge from being caused along the surface of the external iron core cover 58. In the ignition coil assembly for an internal combustion engine according to the present invention, since the withstand voltage air gap 57 of a predetermined shape is provided between the central partitioning wall portion of the case 5 having the mold space portion 50 in which the primary coil 1, the secondary coil 2 and the internal iron core 3 are enclosed, and the central portion of the external iron core 3 close to this central partitioning wall portion 53, a corona discharge is difficult to occur, and, thus, the insulation ability of the case 5 can be prevented from deteriorating even if the central partitioning wall portion 53 of the case 5 is decreased in its thickness. Moreover, since the central partitioning wall portion 53 of the case 5 can be decreased in its thickness, internal defects such as nests and voids of irregular shapes, through which a corona discharge is easy to occur, can be suppressed from being caused.

Claims: An ignition coil assembly for an internal combustion engine comprising a primary coil (1), a secondary coil (2) coaxially disposed around the outside of said primary coil (1), an internal iron core (3) inserted through said primary and secondary coils (1, 2) to make part of a closed magnetic path, an external iron core (4) combined with said internal iron core (3) to constitute said magnetic path and having its central portion extended along said internal iron core (3) with a distance kept therebetween, a case (5) made of a resin being formed integrally with said external iron core (4) and having a mold space (50) wherein said primary coil (1), said secondary coil (2) and said internal iron core (3) are enclosed, and a molded resin portion (6) being formed by injecting a resin material into said mold space (50) so as to surround and to fix said primary coil (1), said secondary coil (2) and said internal iron core (3), said ignition coil assembly being characterized in that said case (5) made of resin is provided with a withstand voltage air gap (57) of a predetermined shape between said mold space (50) and said central portion of said external iron core (4), and a central partitioning wall portion (53) between said withstand air gap (57) and said mold space (50). An ignition coil assembly for an internal combustion engine according to claim 1, characterized in that said case (5) has a side wall portion for making said mold space (50), said central partitioning wall portion (53) as part of said side wall portion being provided between said external iron core (4) and said secondary coil (2), and an insulating resin cover portion (58) for covering the surface of said external iron core (4) to form said withstand voltage air gap (57) between said central partitioning wall portion (53) and said resin cover portion (58). An ignition coil assembly for an internal combustion engine according to claim 1, characterized in that said withstand voltage air gap (57) has opposite openings (57a, 57b) exposed to ambient space outside the coil. An ignition coil assembly for an internal combustion engine according to claim 3, characterized in that one opening of said withstand voltage air gap (57) is closed by said side wall portion which is formed integrally with said external iron core resin cover portion (58). An ignition coil assembly for an internal combustion engine according to claim 1, characterized in that said external iron core resin cover portion (58) is extended beyond said end corners of said external iron core (4) to prevent the concentration of electric field in the proximity of said end corners. An ignition coil assembly for an internal combustion engine according to claim 1, characterized in that said case (5) has a side wall portion for forming said mold space (50) to be of a cup-like shape, with said withstand voltage air gap (57) being formed between part of said side wall portion and the coil-side surface of said external iron core (4), and an insulating resin cover portion (58) for covering the other surfaces of said external iron core (4). An ignition coil assembly for an internal combustion engine according to claim 6, characterized in that said external iron core resin cover portion (58) has recesses (48m, 48n) at the coil-side corners (48, 49) of the external iron core (4).

Inventors: ITO SHIGEMI, NARITA KATSUNORI, ITO, SHIGEMI, NARITA, KATSUNORI
Applicants: NIPPON DENSO CO, NIPPONDENSO CO., LTD.
Application year: 1989
Main classification: H01F31
IPC codes: H01F38, H01F27