Spark plug for an internal combustion engine

Details Spark plug for an internal combustion engine Spark plug for an internal combustion engine

2002-03-14

2004-09-21

Patel, Ashok (Department: 2879)

Electric lamp and discharge devices

Spark plugs

Particular electrode structure or spacing

Reexamination Certificate

active

06794803

ABSTRACT:

BACKGROUND OF THE INVENTION
This invention relates to a spark plug employed in an internal combustion engine. More specifically, the present invention relates to an electrode material of a spark plug and its composition which satisfy required fundamental performances and can improve heat resistance, and therefore can be applied to a high-performance spark plug employed in a high-performance or high-advanced engine subjected to severe thermal load environment having not been experienced by conventional engines.
A spark plug conventionally employed in an internal combustion engine of an automotive vehicle, as understood with reference to
FIG. 1
, comprises a center electrode
30
fixed to an insulator
20
and a ground electrode
40
welded to
51
a metal housing
10
. The metal housing
10
firmly surrounds an outer surface of insulator
20
. The spark plug is securely fixed to an engine body via the metal housing
10
. A distal end surface
41
of ground electrode
40
is opposed to an apical surface
31
of center electrode
30
so as to form a discharge gap
50
. The discharge gap
50
causes a spark to ignite fuel gas mixture.
In this case, the material constituting an electrode material is required to satisfy sufficient high-temperature strength, anti-fusion property, high-temperature corrosion resistance, and spark exhaustion durability. For example, as preferable electrode material, U.S. Pat. No. 4,329,174 (corresponding to JP 60-43897) discloses a Ni-based alloy containing, in weight percentage (hereinafter, ‘%’ represents ‘weight %’), 0.2~3% Si, not larger than 0.5% Mn, and at least two kinds of additive components selected from the group consisting of 0.2~3% Cr, 0.2~3% Al, and 0.01~1% Y in addition to the main component Ni and unavoidable impurities.
However, recent internal combustion engines are required to operate at higher engine speeds. The recent combustion engines often use high-octane gasoline. These factors lead to the remarkable increase of combustion temperature in the combustion chamber. Accordingly, the spark plug electrode material, constituting the center electrode
30
and the ground electrode
40
, is inevitably subjected to such high-temperature combustion atmosphere. The above-described conventional Ni-base alloy shows appropriate anti-fusion property, high-temperature corrosion resistance, and spark exhaustion durability in the high-temperature atmosphere. However, the above-described conventional Ni-base alloy is dissatisfactory in high-temperature strength. Thus, the life of the above-described conventional Ni-base alloy is relatively short.
To satisfy such requirements, Japanese Patent No. 2587864 discloses a Ni-base alloy containing Si, Mn, Cr and Al in addition to Ni, unavoidable impurities, and appropriate amount of rare earth elements. Adding the rare earth elements is effective to improve the high-temperature strength. More specifically, adding a very few amount of Ce, Nd, or La to an electrode material leads to remarkable improvement of high-temperature strength in a combustion atmosphere of 800° C., according to the disclosure of this patent.
However, the recent lean-burn combustion technique realized by a direct fuel injection system or the like necessitates many of the automotive manufactures to develop a high output/high performance and clean engine which is excellent in fuel consumption due to reduction of idling speed and is also capable of reducing the amount of CO
2
or other harmful emission gases. To realize such advanced engines, the recent spark plugs are required to have excellent heat resistance in a severer high-temperature combustion atmosphere, e.g., 950° C. or above at the ground electrode constituting the spark plug.
However, the excellent anti-fusion property, high-temperature corrosion resistance, and spark exhaustion durability of the above-described conventional electrode material is limited to the temperature level of approximately 800° C. If the above-described conventional electrode material is exposed to a severe combustion atmosphere exceeding 950° C., the ground electrode material will cause a damage accompanied by abnormal oxidation in the grain boundaries. The discharge gap will increase to a greater value (e.g., 1.2 mm) from its initial value (e.g., 0.8 mm). An increased amount of voltage will be required to ignite the spark plug. The spark plug may cause misfire in the worst case. In this manner, the above-described conventional electrode material will cause various problems when it is employed in a spark plug for an advanced high output/high performance engine.
SUMMARY OF THE INVENTION
In view of the above-described problems encountered in the prior art, the present invention has an object to provide a spark plug for an internal combustion engine which satisfies required fundamental performances of the spark plug electrode material and assures excellent heat resistance in a severe combustion atmosphere exceeding 950° C. which was not experienced by the conventional engine.
To accomplish the above and other related objects, the inventors of this application have worked on the research and development focused into the electrode materials which satisfy the required fundamental performances of an engine spark plug and have excellent heat resistance in a severer high-temperature combustion environment. As a result of research and development conducted by the inventors of this application, the optimized composition and size for an electrode material are experimentally found out. The present invention is derived from the experimental result.
The present invention provides a first spark plug for an internal combustion engine comprising an insulator, a center electrode fixed to a leg portion of the insulator which is exposed to a combustion chamber of an internal combustion engine, a metal housing firmly surrounding an outer surface of the insulator, and a ground electrode fixed to an end of the metal housing so as to form a spark discharge gap between the center electrode and the ground electrode.
The first spark plug of the present invention is characterized in that at least one of the center electrode and the ground electrode is a Ni-base alloy containing, in weight percentage, 0.5~2.5% Si, 0.1~1.2% Mn, 3.2~5.0% Al, 0.9~2.8% Cr, 0.001~0.025% C in addition to Ni and unavoidable impurities, and a value S/V is in a range from 1.7 mm
−1
to 3.9 mm
−1
when ‘S’ represents a surface area of the ground electrode and ‘V’ represents a volume of the ground electrode.
When the electrode material has the above-described composition, it becomes possible to provide a spark plug which satisfies the fundamental performances required for an internal combustion engine spark plug and assures reliable heat resistance even in a severe combustion atmosphere exceeding 950° C. in electrode temperature.
Furthermore, when the ratio S/V of the surface area ‘S’ to the volume ‘V’ of the ground electrode is in the range from 1.7 mm
−1
to 3.9 mm
−1
, not only the heat resistance can be assured in the combustion atmosphere exceeding 950° C. in electrode temperature but also the bending work of the ground electrode can be facilitated. If the ratio S/V is less than 1.7 mm
−1
, the bending work of the ground electrode for adjusting an initial discharge gap will become difficult. If the ratio S/V is larger than 3.9 mm
−1
, the thermal conductivity of the ground electrode will be worsened and it will be difficult to obtain reliable heat resistance.
According to the present invention, it is preferable that at least one of the center electrode and the ground electrode is a Ni-base alloy containing, in weight percentage, 1.0~2.5% Si, 0.1~0.9% Mn, 3.5~5.0% Al, 1.3~2.5% Cr, 0.001~0.025% C in addition to Ni and unavoidable impurities.
When the electrode material has the above-described composition, it becomes possible to provide a spark plug which satisfies the fundamental performances required for an internal combustion engine spark plug and assures excellent heat resistance even in a severer combustion atmosphere

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