Spark plug with nickel alloy electrode base material

Details Spark plug with nickel alloy electrode base material Spark plug with nickel alloy electrode base material

2001-09-17

2004-04-20

Patel, Vip (Department: 2879)

Electric lamp and discharge devices

Spark plugs

Shaped electrode chamber, insulator end, shell skirt, baffle...

C313S141000, C313S142000

Reexamination Certificate

active

06724133

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a spark plug for an internal combustion engine.
2. Description of the Related Art
Recently, with improvement of engine performance, spark plugs are required to have further extended service life and further improved resistance to contamination. For example, a so-called creeping discharge spark plug is a spark plug for an internal combustion engine having improved contamination resistance. The creeping discharge spark plug is configured such that a spark generated at a spark discharge gap propagates along a surface of an insulator; i.e., in the form of creeping discharge, at all times or depending on particular conditions. A semi-creeping discharge spark plug, which is one type of the creeping discharge spark plug, includes a center electrode, an insulator surrounding the center electrode, and a ground electrode having at its end a discharge surface, which is disposed to face a side surface of the center electrode. The tip end portion of the insulator is disposed to have a positional relationship with the center electrode and the ground electrode such that the end portion of the insulator is located between the center electrode and the discharge surface of the ground electrode (i.e., located in the spark discharge gap). In such a semi-creeping discharge spark plug, when a spark travels along the tip end surface of the insulator, aerial discharge occurs between the surface of the insulator and the discharge surface at the tip end of the ground electrode.
When a spark plug is used for a long period of time at a low temperature not higher than 450° C.; for example, during predelivery, the spark plug becomes “sooted” or “covered with fuel.” In such a state, the insulator surface is covered with a conductive contaminant, such as carbon, which causes defective operation. However, in the case of the above-described creeping discharge spark plug, while spark discharge creeps across the surface of the insulator, an adhering contaminant is burned off at all times, and thus the creeping discharge spark plug exhibits improved resistance to contamination as compared with a parallel-electrode-type spark plug.
Meanwhile, such a creeping discharge spark plug involves frequent occurrence of a spark which creeps across the surface of an insulator, and thus tends to suffer so-called channeling, or a phenomenon whereby the surface of an insulator is abraded and grooves are formed on the surface. Progress of channeling is apt to impair heat resistance or reliability of a spark plug, and channeling is particularly apt to occur during high-speed or heavy-load operation. With the recent trend toward high engine output, there has been demand for spark plugs of excellent durability, and a requirement for prevention or suppression of channeling is becoming stricter.
In some cases, the center electrode of a spark plug is formed of an Ni-base heat-resistant alloy in order to improve heat resistance. However, since the Ni-base heat-resistant alloy contains a relatively large amount of a secondary component such as Cr or Fe, thermal conductivity decreases considerably, depending on the composition. As a result, the heat-transfer performance of the electrode is lowered with resultant acceleration of consumption of the electrode or consumption of a noble-metal discharge portion formed on the electrode. Thus, when the spark plug is used in an environment in which the electrode temperature is prone to rise; i.e., during high-speed, heavy-load operation, the service life of the plug is shortened.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a spark plug whose center electrode has improved heat-transfer performance, which has improved durability against electrode consumption and excellent contamination resistance, and which hardly causes channeling.
In order to achieve the above object, the present invention provides a spark plug of a first structure comprising:
a center electrode:
an insulator surrounding the center electrode; and
a ground electrode positioned relative to a tip end portion of the insulator and a tip end portion of the center electrode such that a spark discharge gap is formed between the ground electrode and the tip end portion of the center electrode, and creeping spark discharge along a surface of the tip end portion of the insulator can occur at the spark discharge gap, wherein
an electrode base material which forms at least a surface layer portion of the center electrode is made of an Ni alloy having a coefficient of thermal conductivity of 17 to 30 W/m·K, the Ni alloy containing Ni as a predominant component and an element (hereinafter referred to as an “NTC element”), as a secondary component, which element can form an oxide semiconductor having a resistivity of negative temperature coefficient (hereinafter also referred to as an “NTC oxide semiconductor”).
When the center electrode is formed of an Ni alloy containing an NTC element as a secondary component and having a coefficient of thermal conductivity falling within the above-described range, a layer containing an NTC oxide semiconductor and serving as a corrosion suppression layer is easily formed on the surface of the tip end portion of the insulator. Thus, corrosion of the surface of the tip end portion of the insulator due to creeping spark discharge can be suppressed effectively, and the electrode can have improved heat transfer property, so that durability in terms of electrode consumption can be greatly improved.
The above-described corrosion suppression layer decreases the discharge voltage at the spark discharge gap. When this effect is utilized, suppression of consumption of the electrode (or a noble-metal consumption-resistant portion formed on the electrode) and further reduction of channeling can be attained. Moreover, in order to enable creeping spark discharge, the shortest distance between the insulator and the ground electrode is preferably made shorter than the shortest distance between the center electrode and the ground electrode.
In the first structure of the present invention, two or more ground electrodes can be disposed around the center electrode. This configuration enables sparks to be generated at positions distributed along the circumference of the insulator, and therefore is advantageous in suppressing formation of deep channels.
The spark plug having the first structure according to the present invention may be embodied as follows. That is, a plurality of ground electrodes are disposed around the center electrode; and at least one ground electrode among them is a semi-creeping ground electrode which is disposed such that its end surface faces a side surface of the center electrode, while at least a portion of the tip end portion of the insulator is interposed therebetween to thereby form a semi-creeping discharge gap between the end surface of the semi-creeping ground electrode and the side surface of the center electrode. In this structure, since the end surface of the ground electrode and the side surface of the center electrode face each other, while sandwiching at least a portion of the tip end portion of the insulator, creeping spark discharge along the surface of the insulator occurs more frequently, so that the spark plug can have excellent contamination resistance. In conventional spark plugs, the above-described structure is not necessarily desirable from the viewpoint of suppression of channeling of the insulator. However, in the present invention, since the center electrode is made of an Ni alloy containing the above-described NTC element as a secondary component as described above, a spark plug can be realized which exhibits excellent channeling resistance even when creeping spark discharge frequently occurs. Further, the distance E between the tip end surface of the insulator and the rear-side edge of the end surface of the ground electrode; i.e., the distance of overlap between the tip end surface of the ground electrode (semi-creeping ground electrode) and the side

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