Electric lamp and discharge devices – Spark plugs – Particular electrode structure or spacing
Reexamination Certificate
1998-08-26
2001-02-20
Day, Michael H. (Department: 2879)
Electric lamp and discharge devices
Spark plugs
Particular electrode structure or spacing
C313S143000, C313S144000, C313S145000, C123S1690EL
Reexamination Certificate
active
06191525
ABSTRACT:
BACKGROUND OF THE INVENTION
This application claims the priority of Japanese Patent Applications No. H9-231282 filed on Aug. 27, 1997, No. H9-364921 filed on Dec. 18, 1997 and No. H9-364922 filed on Dec. 18, 1997, which are incorporated herein by reference.
The present invention relates to spark plugs to be used for internal combustion engines and, more particularly, to a spark plug into which a resistor for prevention of occurrence of radio frequency noise is incorporated.
As this type of spark plug, there has conventionally been known one having a structure that a terminal is inserted and fixed into a through hole formed along the axial direction of an insulator from one end side of the through hole while a center electrode is similarly inserted and fixed thereinto from the other end side of the through hole, where a resistor is placed between the terminal and the center electrode within the through hole. Between the resistor and the terminal or between the resistor and the center electrode, generally, an electrically conductive glass seal portion for joining together the two members is placed. This spark plug is manufactured, for example, by the following process.
That is, into the through hole of the insulator, after the center electrode is inserted, conductive glass powder is filled, then material powder of the resistor composition is filled, and further conductive glass powder is filled again. As a result, in the through hole, a conductive glass powder layer, a resistor composition powder layer and another conductive glass powder layer are formed in this order from the center electrode side. Then, in this state, the insulator is heated above the glass softening point, and further a terminal is press fitted into the hole from the side opposite to the center electrode, in which arrangement the stacked layers are pressed axially so as to be compressed and sintered, thus forming a conductive glass seal portion, a resistor and another conductive glass seal portion, respectively.
In the above spark plug manufacturing method, it has been practiced that after the conductive glass powder layer, the resistor composition powder layer and another conductive glass powder layer are formed one after another and heated with the center electrode downside, the individual layers are compressed in one axial direction from the side opposite to the center electrode, by which the glass seal portions and the resistor are formed. In this case, there are some cases where enough pressing force does not act on the conductive glass powder layer located at the lowest side because of friction between the upper filler material and the through hole wall surface, so that the compression or fluidization after the glass melting and moreover the sintering of the conductive glass powder layer do not proceed enough. If the spark plug is used for a long time in such a state, it may occur that carbon in the conductive glass seal portion burns out or the metal component oxidizes so that the conducting state between the resistor and the center electrode becomes incomplete, causing the conduction resistance to increase, which may obstruct the normal ignition.
Also, when the outside dimensions of the insulator are specified by specifications of the spark plug or the like, increase in the length of the resistor is limited. In this case, one available method would be that the place of the protruding-portion receiving surface is moved toward the front end of the center electrode, so that the axial length of the second portion of the through hole is extended, by which the length of the resistor is increased by the portion. However, this method has a disadvantage that the insulator is thinned in wall thickness at the position of the protruding-portion receiving surface so that the strength of this portion is likely to lack. In this case, the crossing portion between the protruding-portion receiving surface and the second portion may serve as a kind of notch, which often causes problems in terms of strength.
Next, there is a further problem other than the above. That is, in the conventional spark plug, as shown in
FIG. 16
, the taper angle of a center electrode receiving portion
104
formed in a through hole
103
of an insulator
102
(where the taper angle is an angle as viewed from a reference surface that crosses the center axis line of the through hole
103
) is set to 20°-40°. Then, the present inventors have found out that the conventional spark plug (in which the taper angle of the center electrode receiving portion
104
is 20°-40°) has the following disadvantage. That is, because a clearance (seal portion)
109
between an electrode-fixing protruding portion
105
and a through hole inner wall
107
of a proximity
106
of a center electrode receiving portion is narrow and deep, a conductive glass powder layer
108
in the glass seal is not fluidized enough, so that the conductive glass seal portion
109
is liable to become nonuniform in compactness. In particular, the conductive glass powder layer
108
of the proximity
106
of the center-electrode receiving portion is liable to lower in vitrifiability, which may cause a problem of deterioration in thermal conductivity. Further, poor vitrifiability of the conductive glass seal portion
109
would lead to deterioration in heat performance (anti-preignition performance) of the spark plug, while occurrence of nonuniformity in compactness of the conductive glass seal portion
109
would lead to variations in the heat performance from product to product, as further disadvantages.
SUMMARY OF THE INVENTION
A first object of the present invention is to provide a spark plug which can be improved in the conducting state between the resistor and the center electrode with the glass seal portion sandwiched therebetween. A second object of the present invention is to provide a spark plug which can be increased in the length of the resistor even when outside dimensions of the insulator are limited, and which is superior in the radio frequency noise prevention effect. Further, a third object of the present invention is to provide a spark plug which has been designed for improvement in the vitrifiability as well as stabilization in compactness of the clearance (seal portion) between the electrode-fixing protruding portion and the through hole inner wall of the center-electrode receiving portion, and which has realized improvement and stabilization of heat performance.
The spark plug of the present invention has a constitution that, to a through hole formed along an axial direction of an insulator, a terminal is inserted and fixed on one end side of the through hole while a center electrode is inserted and fixed on the other end side of the through hole, and a resistor is placed between the terminal and the center electrode within the through hole, and with a view to achieving the above first object, being characterized in that:
a stem cross-section diameter of the center electrode is set smaller than a stem cross-section diameter of the resistor, and with one side toward a tip end of the center electrode taken as a front side, the through hole of the insulator has a first portion which allows the center electrode to be inserted therethrough, and a second portion which is formed on a rear side of the first portion so as to be larger in diameter than the first portion and which accommodates the resistor therein, where the second portion is connected to the first portion via a connecting portion including a two- or more-stepped reduced-diameter portion; and that an electrically conductive glass seal portion is placed
at a position corresponding to the connecting portion between the resistor and the center electrode.
In this spark plug, a connecting portion including a two- or more-stopped reduced-diameter portion is formed between the second portion of a larger diameter at which the resistor is placed in the through hole of the insulator, and the first portion of a smaller diameter into which the center electrode is inserted, and an electrically conductive glass seal portion
Sugimoto Makoto
Tanaka Yutaka
Day Michael H.
Haynes Mack
NGK Spark Plug Co. Ltd.
Snider Ronald R.
Snider & Associates
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