Electric lamp and discharge devices – Spark plugs – Particular electrode structure or spacing
Reexamination Certificate
2002-06-13
2004-11-16
Glick, Edward J. (Department: 2882)
Electric lamp and discharge devices
Spark plugs
Particular electrode structure or spacing
C313S140000, C313S143000, C445S007000
Reexamination Certificate
active
06819032
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a spark plug for use in an internal combustion engine.
BACKGROUND ART
A certain conventional spark plug includes a center electrode which is disposed in an insulator in such a manner as to project from the front end face of the insulator, and a parallel ground electrode whose one end is disposed in parallel with the end face of the center electrode and whose other end is joined to a metallic shell, and the spark plug is adapted to ignite a fuel mixture gas through spark discharge across a gap between the center electrode and the parallel ground electrode.
In order to enhance an ignition property for igniting a fuel mixture gas, Japanese Patent Application Laid-Open (kokai) Nos. 5-326107 and 7-130454 propose a spark plug which includes, in addition to a ground electrode which faces the end face of a center electrode in parallel, auxiliary ground electrodes whose end faces face the circumferential side surface of the center electrode. An object of disposing the auxiliary ground electrodes is not to induce sparking across the gap between the end face of an auxiliary ground electrode and the circumferential side surface of the center electrode, but to improve distribution of electric field between the parallel ground electrode and the center electrode so as to induce sparking between the parallel ground electrode and the center electrode at a lower discharge voltage, thereby enhance ignition characteristics. The structural design of the proposed spark plugs is not intended to bring an edge of the end face of an auxiliary ground electrode in the vicinity of the front end face of an insulator.
Japanese Patent Application Laid-Open (kokai) No. 9-199260 proposes a spark plug that includes, in addition to a parallel ground electrode which faces the end face of a center electrode, auxiliary ground electrodes provided in the vicinity of the end face of an insulator.
However, the above-mentioned spark plugs described in Japanese Patent Application Laid-Open (kokai) Nos. 5-326107 and 7-130454 involve a problem in that spark discharge tends to fail to occur at a predetermined position, upon occurrence of so-called “carbon fouling.” At the time of regular operation, in which an internal combustion engine is operating at a predetermined temperature and at a predetermined rotational speed or higher, the temperature of a leg portion of an insulator of a spark plug increases to an appropriate level, and the surface temperature as measured in the vicinity of the end face of the insulator located within a combustion chamber increases to about 500° C. At such a temperature, carbon adhering to the surface of the insulator is burnt out, so that the insulator surface is held clean. Therefore, a problem associated with “carbon fouling” does not arise. By contrast, at the time of low-load operation, in which the internal combustion engine is operating at low rotational speed and at extremely low temperature, the surface temperature of the insulator does not increase, and thus carbon accumulatively adheres to the surface of the insulator; i.e., the “carbon fouling” state is established. When “carbon fouling” progresses, insulation between the center electrode and the ground electrode is impaired; as a result, spark discharge is disabled, leading to engine stall.
The above-mentioned Japanese Patent Application Laid-Open (kokai) No. 9-199260 does not specify the relationships among the distance between the parallel ground electrode and the center electrode (air gap), the distance between an auxiliary ground electrode and the center electrode (semi-creepage gap), and the distance between the end face of an auxiliary ground electrode and the circumferential side surface of the insulator (insulator gap).
Japanese Patent Application Laid-Open (kokai) No. 59-71279 discloses a semi-creepage discharge spark plug configured such that a ground electrode is disposed in opposition to the circumferential side surface of an insulator. In the semi-creepage discharge spark plug, a spark creeps along the surface of the insulator, and thus carbon adhering to the insulator surface is burnt out. Therefore, “carbon fouling” does not raise a serious problem. However, since a spark continuously creeps along the surface of the insulator, the insulator surface is damaged by sparks; i.e., a so-called “channeling” problem arises, shortening the life of the spark plug.
An object of the present invention is to provide a spark plug which is less affected by “carbon fouling,” exhibits excellent ignition characteristics, and reduces the amount of channeling.
DISCLOSURE OF THE INVENTION
To achieve the above object, a spark plug of the present invention assumes the following basic structure. Specifically, the spark plug of the present invention comprises an insulator having a center through-hole formed therein; a center electrode held in the center through-hole and disposed at an end portion of the insulator; a metallic shell for holding the insulator such that an end portion of the insulator projects from an end face thereof; a parallel ground electrode disposed such that one end thereof is joined to a front end face of the metallic shell, and a side face of the other end faces, in parallel, an end face of the center electrode; and a plurality of semi-creeping discharge ground electrodes disposed such that one end of each of the electrodes is joined to the metallic shell, and the other end of each of the electrodes faces a circumferential side surface of the center electrode and/or a circumferential side surface of the insulator. In the spark plug of the present invention, an air gap (&agr;) is formed between the parallel ground electrode and the end face of the center electrode; a semi-creepage gap (&bgr;) is formed between the end face of each of the semi-creeping discharge ground electrodes and the circumferential side surface of the center electrode; an insulator gap (&ggr;) is formed between the end face of each of the semi-creeping discharge ground electrodes and the circumferential side surface of the insulator; a distance &agr; across the air gap (&agr;) and a distance &bgr; across the semi-creepage gap (&bgr;) satisfy the relationship “&agr;<&bgr;;” and the distance &agr; across the air gap (&agr;) and a distance &ggr; across the insulator gap (&ggr;) satisfy the relationship “&agr;>&ggr;.” Notably, herein, reference symbols (&agr;), (&bgr;), and (&ggr;) for denoting gaps as structural elements of the invention may also be used to denote the sizes of gaps. In this connection, the following convention for usage of reference symbols may be adopted: reference symbols G
&agr;
, G
&bgr;
, and G
&ggr;
are used to denote gaps as structural elements, whereas reference symbols &agr;, &bgr;, and &ggr; are used to denote the sizes of gaps. However, herein, in order to avoid complication of description, the same reference symbols are used to denote gaps as structural elements and the sizes of the gaps.
Satisfaction of the above-described relationships yields the following advantages. Since the distance &agr; across the air gap (&agr;) is shorter than the distance &bgr; across the semi-creepage gap (&bgr;) (&agr;<&bgr;), in the normal state; i.e., when the “carbon fouling” state is not established, spark discharge occurs across the air gap (&agr;) associated with the parallel ground electrode. Since the distance &ggr; across the insulator gap (&ggr;) is shorter than the distance &agr; across the air gap (&agr;), when the front end face of the insulator is fouled with carbon to thereby enter the “carbon fouling” state, spark discharge called semi-creeping discharge occurs along the end face of the insulator between an edge of the end face of the semi-creeping discharge ground electrode and the circumferential side surface of the center electrode. A spark associated with semi-creeping discharge runs across the insulator gap (&ggr;) and then along the surface of the insulator (along the reverse route when the voltage polarity is inverted). When semi-creeping discharge is repeated sever
Glick Edward J.
Keaney Elizabeth
NGK Spark Plug Co. Ltd.
Sughrue & Mion, PLLC
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