Ignition coil for internal combustion engine

In an ignition coil for an internal combustion engine, a spark plug is incorporated which has an insulator including an insulator head that has no corrugation. The ignition coil has a coil body part generating high voltage and a joint part, which holds therein a conducting member electrically connecting the coil body part and a spark plug. The joint part has a plug cap into which the insulator head of the spark plug is inserted and which is formed of an elastic member having a cylindrical shape. The plug cap has a close-contact part whose inner peripheral surface is closely brought into contact with an outer peripheral surface of the insulator. The close-contact part has a tip end-side close-contact part positioned at a tip end side with respect to a middle position in an axial direction, and a base end-side close-contact part positioned at a base end side with respect to the middle position. At least part of the base end-side close-contact part is provided with a strong strained force part that has strained force for fastening the insulator head, the strained force being stronger than strained force of all portions of the tip end-side close-contact part.

This application is the U.S. national phase of International Application No. PCT/JP2016/061349 filed Apr. 7, 2016 which designated the U.S. and claims priority to JP Patent Application No. 2015-079346 filed Apr. 8, 2015, the entire contents of each of which are hereby incorporated by reference.

TECHNICAL FIELD

The present invention relates to an ignition coil for an internal combustion engine, for applying high voltage to spark plugs arranged in the internal combustion engine to ignite the spark plugs.

BACKGROUND ART

An ignition coil for an internal combustion engine has a coil body part generating high voltage and a joint part, which holds therein a conducting member electrically connecting the coil body part and a spark plug. The joint part of the spark plug is inserted in a plug hole of an engine head. Then, the conducting member of the ignition coil is electrically connected with the spark plug at a terminal fitting. In addition, the spark plug is attached to the engine head at a housing.

Here, a joint part of the ignition coil has a cylindrical plug cap into which an insulator of the spark plug is fitted. Patent literature 1 discloses a structure in which a plug cap is formed so that the inner peripheral surface thereof has a convexo-concave shape in the axial direction to attach a spark plug to the plug cap and detach the spark plug from the plug cap.

CITATION LIST

Patent Literature

SUMMARY OF THE INVENTION

Technical Problem

However, the ignition coil disclosed in patent literature 1 has the following problems.

At a concave portion of the inner peripheral surface of the plug cap, the strained force for fastening an insulator is easily weakened, and a minute gap (air space) is easily formed between the portion and the insulator. If the gap is formed, discharge may be caused in the gap. Furthermore, if discharge is repeatedly caused in the gap between the plug cap and the insulator, the inner surface of the plug cap deteriorates (carbonizes). Furthermore, if the deterioration extends to the whole close-contact surface between the plug cap and the insulator in the axial direction, the insulation properties between a terminal fitting and a housing may be lowered.

To solve the above problems, it can be considered that the strained force for fastening the insulator is increased for the whole plug cap to prevent a gap between the plug cap and the insulator from being formed, thereby ensuring the insulation properties. However, a new problem arises that when the spark plug is fitted into the plug cap, or when the spark plug is extracted from the plug cap, strong force is required.

Solution to Problem

An embodiment provides an ignition coil for an internal combustion engine, the ignition coil being able to ensure insulation properties without difficulties in attaching or detaching a spark plug to or from a plug cap.

In the ignition coil for an internal combustion engine of an embodiment, a spark plug is incorporated which has an insulator including an insulator head that has no corrugation. The ignition coil has a coil body part generating high voltage and a joint part, which holds therein a conducting member electrically connecting the coil body part and a spark plug. The joint part has a plug cap into which the insulator head of the spark plug is inserted and which is formed of an elastic member having a cylindrical shape. The plug cap has a close-contact part whose inner peripheral surface is closely brought into contact with an outer peripheral surface of the insulator. The close-contact part has a tip end-side close-contact part positioned at a tip end side with respect to a middle position in an axial direction, and a base end-side close-contact part positioned at a base end side with respect to the middle position. At least part of the base end-side close-contact part is provided with a strong strained force part that has strained force for fastening the insulator head, the strained force being stronger than strained force of all portions of the tip end-side close-contact part.

DESCRIPTION OF THE EMBODIMENTS

Hereinafter, in the description of an ignition coil for an internal combustion engine, the side at which a spark plug is inserted is defined as a tip end side, and the opposite side of the tip end side is defined as a base end side.

First Embodiment

An embodiment of an ignition coil for an internal combustion engine will be described with reference toFIG. 1toFIG. 4.

As shown inFIG. 3, into an ignition coil1for an internal combustion engine, a spark plug8is incorporated which has an insulator81including an insulator head82that has no corrugation.

As shown inFIG. 1toFIG. 3, the ignition coil1for the internal combustion engine has a coil body part2that generates high voltage and a joint part4that holds therein a conducting member3electrically connecting the coil body part2and a spark plug8. As shown inFIG. 3, the joint part4has a plug cap5into which the insulator head82of the spark plug8is inserted and which is formed of an elastic member having a cylindrical shape. As shown inFIG. 2, the plug cap5has a close-contact part6whose inner peripheral surface is closely brought into contact with the outer peripheral surface of the insulator81. The close-contact part6has a tip end-side close-contact part61positioned at the tip end side with respect to the middle position in the axial direction X, and a base end-side close-contact part62positioned at the base end side with respect to the middle position. At least part of the base end-side close-contact part62is provided with a strong strained force part7that has strained force for fastening the insulator head82(hereinafter, simply referred to as strained force), the strained force of the strong strained force part7being stronger than that of all portions of the tip end-side close-contact part61.

The coil body part2has a primary coil and a secondary coil that are magnetically coupled to each other. As shown inFIG. 1, the coil body part2has a high voltage tower part20formed so as to project in the axial direction X. The high voltage tower part20holds therein a high voltage output terminal21that outputs high voltage generated from the coil body part2, and a resistive element22. The end of the resistive element22comes into contact with the conducting member3in the joint part4.

The joint part4has a cylindrical pole joint41and the plug cap5fitted to the pole joint41at the end part of the pole joint41. Between the high voltage tower part20and the pole joint41, a connection seal member11is provided which connects the high voltage tower part20and the pole joint41and seals therebetween. For example, the pole joint41is formed of resin having insulation properties, and the plug cap5is formed of rubber.

As shown inFIG. 1andFIG. 2, the plug cap5has a cylindrical shape. The base end part of the plug cap5is fitted to the pole joint41. As shown inFIG. 3, the insulator head82of the spark plug8is fitted into the inside of the plug cap5from the tip end side thereof.

As shown inFIG. 3andFIG. 4, the spark plug8, into which the plug cap5is fitted, has the insulator81, a housing83holding the insulator81, a terminal fitting84held inside the insulator81so that the base end part thereof projects, a center electrode85and an earth electrode86for generating spark discharge. The terminal fitting84is connected to the conducting member3in the joint part4, thereby being electrically connected to the ignition coil1. The insulator81is held by the housing83in the state where the insulator head82is exposed to the base end side from the housing83. The insulator head82has an external diameter constant along the axial direction X. That is, the outer peripheral surface of the insulator head82is formed just along the axial direction X, and has a shape having no corrugation. As shown inFIG. 3, in the spark plug8, the insulator head82is fitted into the plug cap5so that the outer peripheral surface of the insulator head82is closely brought into contact with the inner peripheral surface of the close-contact part6.

As shown inFIG. 2, the close-contact part6has the tip end-side close-contact part61and the base end-side close-contact part62. The base end-side close-contact part62is provided with the strong strained force part7. The strong strained force part7is formed by making the thickness thereof in the radial direction larger than that of the tip end-side close-contact part61. In the present embodiment, the strong strained force part7is formed so that the thickness thereof in the radial direction becomes larger than that of the tip end-side close-contact part61by swelling the inner peripheral surface of the plug cap5inward. It is noted that although the close-contact part6is divided into the tip end-side close-contact part61and the base end-side close-contact part62, this configuration is made for the sake of convenience to describe the close-contact part6by dividing it into a tip end-side portion and a base end-side portion. In addition, for the sake of convenience, inFIG. 2, the end of the close-contact part6is denoted by a sign T, and the base end of the close-contact part6is denoted by a sign B.

As shown inFIG. 2, the inner peripheral surface of the strong strained force part7is formed so as to gradually swell inward in the radial direction. That is, the strong strained force part7is formed so that the amount of inward projection in the radial direction gradually increases along the axial direction X and toward the center. In the axial direction X, the strong strained force part7has a length equal to or more than half of the length of the base end-side close-contact part62. The strong strained force part7is formed on the whole circumference of the inner peripheral surface of the base end-side close-contact part62.

The tip end-side close-contact part61has an internal diameter constant in the axial direction X. The strong strained force part7of the base end-side close-contact part62has an internal diameter smaller than that of the tip end-side close-contact part61. In addition, the internal diameter of the close-contact part6is smaller than the external diameter of the insulator head82. Hence, in the state where the spark plug8is fitted into the plug cap5, the close-contact part6closely contacts the insulator head82and strains the insulator head82. In addition, the strong strained force part7has strained force stronger than that of the tip end-side close-contact part61.

It is noted that, as shown inFIG. 2, the strong strained force part7has a shape swelling to the inner peripheral side in a so-called free state in which the spark plug8is not fitted into the plug cap5. However, as shown inFIG. 3, in the state where the spark plug8is fitted, the swelling is not present.

Next, effects of the above embodiment will be described.

In the ignition coil1for an internal combustion engine, at least part of the base-end side close-contact part62is provided with the strong strained force part7. Hence, sliding frictional force is easily reduced when the spark plug8is attached to or detached from the plug cap5. That is, the spark plug8can be fitted into the plug cap5with relatively weak force at least to the middle position of the close-contact part6in the axial direction X. In addition, the spark plug8can be removed from the plug cap5with relatively weak force at least from the middle position of the close-contact part6in the axial direction X.

In addition, at least part of the base end-side close-contact part62is provided with the strong strained force part7. Hence, a gap can reliably be prevented from being generated at least between the strong strained force part7and the insulator head82, whereby discharge can reliably be prevented from being caused between the strong strained force part7and the insulator head82. As a result, the insulation properties can reliably be ensured between the tip end side and the base end side of the close-contact part6.

In addition, the strong strained force part7is formed by making the thickness thereof in the radial direction larger than the thickness of the tip end-side close-contact part61. Hence the strong strained force part7can be easily formed.

In addition, the strong strained force part7is formed by swelling the inner peripheral surface of the plug cap5inward so that thickness thereof in the radial direction becomes larger than the thickness of the tip end-side close-contact part61. Hence, the strained force of the strong strained force part7is easily ensured.

As described above, according to the present embodiment, an ignition coil for an internal combustion engine can be provided, the ignition coil being able to ensure insulation properties without difficulties in attaching or detaching a spark plug to or from a plug cap.

The present example evaluates insulation properties between the tip end side and the base end side of the close-contact part6obtained when the position where the strong strained force part of the close-contact part6is formed is variously changed in the axial direction X.

In the present example, while the basic configuration is similar to that of the first embodiment, six plug caps, in which the position where the strong strained force part is formed was variously changed in the axial direction X, and a plug cap that has no strong strained force part were prepared. The respective six plug caps had the strong strained force parts whose middle positions in the axial direction X are distanced from the end of the close-contact part6by 5 mm, 10 mm, 15 mm, 20 mm, 25 mm, and 30 mm. The length of the close-contact part6in the axial direction X of each of the plug caps was 32 mm. In addition, each of the plug caps was left for 120 hours at temperature of 180 C°, thereby being deteriorated.

Then, the spark plugs8described in the first embodiment were fitted into the ignition coils1including the respective plug caps5, and the plug caps5and the spark plugs8were immersed in an aqueous solution including 5% by weight of salt. In this condition, a predetermined voltage was applied between the center electrode85and the earth electrode86at a frequency of 50 Hz for 20 hours. During that time, it was observed whether creeping discharge was caused between the tip end side and the base end side of the close-contact part6. The presence or absence of the creeping discharge was determined by confirming a voltage waveform between the ignition coil and the spark plug.

The above test was performed with the applied voltage variously changed. Then, regarding the configurations using the plug cap having the strong strained force part, the minimum voltages (flashover generated voltage) at which creeping discharge was caused are plotted on a graph shown inFIG. 5to illustrate an approximate curve. In addition, the flashover generated voltage obtained when a plug cap having no strong strained force part was used was 42 kV. In the graph shown inFIG. 5, the horizontal axis indicates the length between the end of the close-contact part6and the end of the strong strained force part. The vertical axis indicates flashover generated voltage.

As can be understood fromFIG. 5, as the position of the strong strained force part7is closer to the base end part from the end of the close-contact part6, the flashover generated voltage tends to increase. That is, as the position of the strong strained force part7is closer to the base end part from the end of the close-contact part6, the insulation properties between the end part and the base end part of the close-contact part6tends to improve. In addition, the configuration, in which the distance from the end of the close-contact part to the position of the strong strained force part7at the base end side is 10 mm or more, has a high flashover generated voltage compared with the configuration having no strained force part. In addition, the configuration, in which the distance from the end of the close-contact part6to the position of the strong strained force part7at the base end side is 15 mm or more, has a sufficiently high flashover generated voltage. Hence, if the strong strained force part7is positioned at the base end side with respect to the middle position of the close-contact part6(the position distanced from the end of the close-contact part6by 16 mm) in the axial direction X, the flashover generated voltage can be sufficiently high.

From the above results, it can be understood that the ignition coil1having the plug cap5, which is provided with the strong strained force part7at at least part of the base end-side close-contact part62, has good insulation properties between the tip end side and the base end side of the close-contact part6.

Second Embodiment

In the present embodiment, as shown inFIG. 6, the strong strained force part7is formed so that the thickness thereof in the radial direction becomes larger than that of the tip end-side close-contact part61by swelling the outer peripheral surface of the plug cap5outward. Hence, the strained force of the strong strained force part7is stronger than the strained force of the tip end-side close-contact part61. It is noted that, inFIG. 6, the outer peripheral surface of the plug cap that does not have the strong strained force part7is indicated by broken lines.

The outer peripheral surface of the strong strained force part7is formed so as to gradually swell outward in the radial direction. That is, the strong strained force part7is formed so that the amount of outward projection in the radial direction gradually increases along the axial direction X and toward the center. The strong strained force part7is formed on the whole circumference of the outer peripheral surface of the base end-side close-contact part62. In the present embodiment, the internal diameter of the base end-side close-contact part62is the same as the internal diameter of the tip end-side close-contact part61. That is, in the present embodiment, the internal diameter of the close-contact part6is constant totally along the axial direction X.

Other configurations are similar to those of the first embodiment. It is noted that the same signs used in the present embodiment and the drawings concerning the present embodiment as the signs used in the first embodiment indicate components and the like similar to those of the first embodiment unless otherwise stated.

According to the present embodiment, since the thickness of the strong strained force part7in the radial direction can be increased, elastic force of the strong strained force part7in the radial direction can be strong. Hence, the strained force of the strong strained force part7can be strong, whereby the insulation properties can reliably be ensured between the tip end side and the base end side of the close-contact part6.

Additionally, the present embodiment has effects similar to those of the first embodiment.

Third Embodiment

In the present embodiment, as shown inFIG. 7andFIG. 8, an attachment structure10is formed in which the spark plug8is fitted into the ignition coil1of the second embodiment. In addition, in the spark plug8described in the first embodiment, the shape of the insulator head82is modified. That is, the insulator head82has an insulator swelling part821whose outer peripheral surface swells outward. The outer peripheral surface of the insulator swelling part821is formed so as to gradually swell outward. In the state where the spark plug8is fitted into the plug cap5of the ignition coil1, the insulator swelling part821is formed at the same position in the axial direction X as the position where the strong strained force part7of the plug cap5is formed. The insulator swelling part821is formed on the whole circumference of the insulator81.

Other configurations are similar to those of the second embodiment. It is noted that the same signs used in the present embodiment and the drawings concerning the present embodiment as the signs used in the second embodiment indicate components and the like similar to those of the second embodiment unless otherwise stated.

In the present embodiment, in the state where the spark plug8is fitted into the plug cap5of the ignition coil1, the insulator swelling part821is arranged at the same position in the axial direction X as that of the strong strained force part7. Hence, the strained force of the strong strained force part7can be stronger, whereby the insulation properties can be further ensured between the tip end side and the base end side of the close-contact part6.

Additionally, the present embodiment has effects similar to those of the first embodiment.

In the ignition coil (1) for an internal combustion engine of the above embodiment, a spark plug (8) is incorporated which has an insulator (81) including an insulator head (82) that has no corrugation. The ignition coil (1) has a coil body part (2) generating high voltage and a joint part (4), which holds therein a conducting member (3) electrically connecting the coil body part and a spark plug (8). The joint part (4) has a plug cap (5) into which the insulator head (82) of the spark plug (8) is inserted and which is formed of an elastic member having a cylindrical shape. The plug cap (5) has a close-contact part (6) whose inner peripheral surface is closely brought into contact with an outer peripheral surface of the insulator (81). The close-contact part (6) has a tip end-side close-contact part (61) positioned at a tip end side with respect to a middle position in an axial direction (X), and a base end-side close-contact part (62) positioned at a base end side with respect to the middle position. At least part of the base end-side close-contact part (62) is provided with a strong strained force part (7) that has strained force for fastening the insulator head (82), the strained force being stronger than strained force of all portions of the tip end-side close-contact part (61).

In the above ignition coil for an internal combustion engine, at least part of the base-end side close-contact part is provided with the strong strained force part. Hence, sliding frictional force is easily reduced when the spark plug is attached to or detached from the plug cap. That is, the spark plug can be fitted into the plug cap with relatively weak force at least to the middle position of the close-contact part in the axial direction. In addition, the spark plug can be removed from the plug cap with relatively weak force at least from the middle position of the close-contact part in the axial direction.

In addition, at least part of the base end-side close-contact part is provided with the strong strained force part. Hence, a gap can reliably be prevented from being generated at least between the strong strained force part and the insulator head, whereby discharge can reliably be prevented from being caused between the strong strained force part and the insulator head. As a result, the insulation properties can reliably be ensured between the tip end side and the base end side of the close-contact part.

As described above, the embodiment can provide an ignition coil for an internal combustion engine, the ignition coil being able to ensure insulation properties without difficulties in attaching or detaching a spark plug to or from a plug cap.

REFERENCE SIGNS LIST