Electric lamp and discharge devices – Spark plugs – Particular electrode structure or spacing
Reexamination Certificate
1999-09-20
2001-12-04
Patel, Vip (Department: 2879)
Electric lamp and discharge devices
Spark plugs
Particular electrode structure or spacing
C313S142000
Reexamination Certificate
active
06326720
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a spark plug for use with an internal combustion engine and to an ignition system for use with an internal combustion engine having the spark plugs.
2. Description of the Related Art
Ignition systems used with automotive internal combustion engines having spark plugs have conventionally employed a distributor. In such ignition systems, an ignition coil includes a primary coil, which receives electricity from a battery via an ignition switch and is connected to an igniter and a secondary coil which is connected to a distributor. When an electronic control unit issues a break instruction signal to the igniter at a predetermined firing timing, the igniter causes a contactless switch unit to operate so as to interrupt current flowing to the primary coil. As a result, a high-voltage current is induced in the secondary coil. The distributor distributes the induced current to spark plugs through high-tension cables.
However, recently, the above-described distributor ignition system has been replaced by a full transistor type coil-on-plug ignition system (hereinafter referred to as a “DLI” (Distributor-Less Ignition system). The DLI system features easy control of ignition timing and does not require maintenance of contacts. In the DLI system, an ignition coil is mounted directly on each spark plug. A control unit interrupts current flowing to the primary coil of the ignition coil of each spark plug at a predetermined timing to thereby fire the spark plug. Since ignition coils are mounted directly on the respective spark plugs, high-tension cables are not required.
Conventionally, in order to improve resistance to spark consumption of a spark plug, a chip of Pt (platinum) serving as a spark portion is formed at one end of an electrode of the spark plug. However, since Pt is expensive and the melting point thereof is approximately 1769° C. indicating that resistance to spark consumption of Pt is insufficient, use of Ir (iridium), which has a melting point of approximately 2454° C., as material for the chip has been proposed. However, a spark portion of Ir produces a volatile oxide at a temperature of 900° C. to 1000° C., indicating a tendency to be consumed within this temperature range.
In a spark plug having a chip of an Ir-based material as a spark portion, employment of the above-mentioned DLI system may have a significantly adverse effect on durability of the spark portion. Specifically, spark discharge of a spark plug is generally classified, according to form, into glow discharge and arc discharge. A glow discharge occurs, for example, when the impedance of a power source (hereinafter referred to as a “power source impedance”) is relatively high. Since a discharge current is relatively weak, the glow discharge causes a less severe temperature increase and less consumption of the spark portion. By contrast, an arc discharge often occurs when a power source impedance is relatively low. Accordingly, a strong discharge current tends to flow, causing a considerable temperature increase in the spark portion with a resultant advancement of consumption of the spark portion. Therefore, from the viewpoint of suppression of consumption of the spark portion, glow discharge is desirably dominant in a spark discharge.
In the distributor ignition system, the power source impedance is high because of the electric resistances of a contact gap and a high tension cable. Accordingly, glow discharge is dominant in a spark discharge. However, in the DLI system, the power source impedance is low, since the electric resistances of a contact gap and a high tension cable are not present. Accordingly, depending on the material used for an electrode, the rate of transition from glow discharge to arc discharge increases in a spark discharge, potentially causing consumption of the electrode. According to a study conducted by the inventors of the present invention, a spark portion of an Ir-based material exhibits a particularly high rate of transition from glow discharge to arc discharge, potentially shortening spark plug life. This tendency is further accelerated by consumption of the spark portion caused by volatilization through oxidation.
Further, Japanese Patent Application Laid-Open No. 7-50192 (U.S. Pat. No. 5,514,929) describes that when a spark plug with a tip mainly formed of Ir is used in a gas engine, the energy of induced discharge can be decreased by use of a resistor having a resistance not less than 50 k&OHgr; but not greater than 200 k&OHgr;. However, although such a gas engine would not have a problem in relation to ignitability even when the discharge energy decreases, a gasoline engine would have a problem in relation to ignitability when the discharge energy decreases.
SUMMARY OF THE INVENTION
A first object of the present invention is to provide a spark plug in which an arc discharge becomes unlikely to occur in spite of a spark portion being formed from an Ir-based metal, to thereby suppress consumption of an electrode and deterioration of ignitability.
A second object of the present invention is to provide an ignition system for use with an internal combustion engine having the spark plugs.
To achieve the first object, the present invention provides a spark plug comprising: a center electrode; an insulator which surrounds the center electrode; a metallic shell which surrounds the insulator; a ground electrode which faces the center electrode; and a spark portion which is fixedly attached to at least either one of the center electrode and the ground electrode to thereby define a spark discharge gap. The spark portion is formed from a metal which contains not less than 60% by weight Ir. The spark plug further comprises a metallic terminal fixedly attached into one end portion of a through-hole formed axially in the insulator, the center electrode being fixedly attached into the other end portion of the through-hole; and a resistor disposed within the through-hole and between the metallic terminal and the center electrode so as to establish an electric resistance of not less than 10 k&OHgr; but not greater than 25 k&OHgr; between the metallic terminal and the center electrode.
To achieve the second object, the present invention provides an ignition system for use with an internal combustion engine comprising a spark plug and a coil unit.
The spark plug comprises a center electrode; an insulator which surrounds the center electrode; a metallic shell which surrounds the insulator; a ground electrode which faces the center electrode; and a spark portion which is fixedly attached to at least either one of the center electrode and the ground electrode to thereby define a spark discharge gap. The spark portion is formed from a metal which contains not less than 60% by weight Ir. The spark plug further comprises a metallic terminal fixedly attached into one end portion of a through-hole formed axially in the insulator, the center electrode being fixedly attached into the other end portion of the through-hole.
The coil unit comprises a casing attached to the spark plug and an ignition coil accommodated within the casing and connected to the metallic terminal of the spark plug in order to apply a high voltage to the spark plug for effecting an electrical discharge.
The ignition system further comprises a resistance portion disposed between the ignition coil and the center electrode so as to establish an electric resistance of not less than 10 k&OHgr; but not greater than 25 k&OHgr; between the ignition coil and the center electrode.
When the spark portion is formed from an Ir-based metal, the metal must contain Ir in an amount of not less than 60% by weight. Otherwise, the high melting point of Ir fails to lead to sufficient improvement in resistance to spark consumption of the spark portion. However, as described previously, in the DLI system, a high Ir content of the spark portion tends to cause transition to a strong-current discharge, such as an arc discharge. As a result, the temperature of the spark porti
Ito Shoichiro
Matsubara Yoshihiro
Brinks Hofer Gilson & Lione
NGK Spark Plug Co. Ltd.
Patel Vip
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