JP2936730B2 - Spark plug for internal combustion engine - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a spark plug for an internal combustion engine capable of reducing a spark voltage discharged between a center electrode and a ground electrode.

[0002]

2. Description of the Related Art In recent years, in internal combustion engines for automobiles, there has been a strong demand for reducing fuel consumption. In response to this demand, higher compression and lean burn of internal combustion engines are being promoted. Therefore, in the spark plug, it is required that the spark voltage for discharging to the gap between the center electrode and the ground electrode be as low as possible. Therefore, as a countermeasure for reducing the spark voltage, it has been conventionally proposed to join a noble metal tip such as platinum to the discharge surface of the spark plug to suppress the consumption of the spark gap. That is, for example, in Japanese Patent Application Laid-Open No. 57-145288, a noble metal tip provided with an uneven groove on the surface is disposed on the discharge surface of the spark plug, and the spark voltage is determined by a plurality of edges formed by the uneven groove. It is shown that the reduction of the number is achieved. In this technology, the discharge energy generates fine particles on the surface of the noble metal tip, which prevents the gap from increasing and causes the uneven shape effect of the fine particles, which suppresses the rise in spark voltage. To use. Then, in order to promote this effect, the above-mentioned concave and convex grooves are provided on the surface of the noble metal tip.

[0003]

However, in order to always maintain the edge effect due to the uneven shape of the discharge surface portion during use of the spark plug, an appropriate concave groove width must be processed in advance. As a result of studying this point, the inventors have found that in order to make full use of the above-mentioned edge effect, it is necessary to increase the width of the concave groove width and to make each convex part an independent element. Knew. However, in this shape, the noble metal tip must have a large diameter, and a large amount of expensive noble metal such as platinum is used, resulting in an increase in cost. SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned conventional problems, and has as its object to provide a spark plug for an internal combustion engine that can maintain a discharge surface portion always in an uneven state even during use and can suppress an increase in spark voltage.

[0004]

The present invention provides an insulator, a center electrode held by the insulator, a housing fixed to the outer periphery of the insulator, and a gap provided between the center electrode provided in the housing. In a spark plug for an internal combustion engine comprising an opposed ground electrode, an electrode material comprising a metal and an inorganic material powder not alloyed with the metal is disposed on at least one of the discharge surfaces of the center electrode and the ground electrode. And the content V (volume%) of the inorganic material powder in the electrode material is determined by the average particle size D (μm) of the inorganic material powder.
m) which satisfies the following relational expression: logV ≧ 1.1 × logD−2.3 The most remarkable point in the present invention is that the discharge surface portion is constituted by an electrode material composed of a metal and an inorganic material powder, and the content and the average particle size of the inorganic material powder And the relationship above. In the present invention, the electrode material is disposed on the discharge surface. The electrode material is disposed by bonding a separately prepared chip to one or both of the discharge surface of the center electrode and the discharge surface of the ground electrode. Alternatively, the electrode material can be directly and integrally formed on one or both of the discharge surfaces of the center electrode and the ground electrode. Further, the above-mentioned electrode material is a material in which an inorganic material powder is mixed and dispersed in a metal matrix. Such metals include Ni alloys such as Ni-Cr alloys mainly composed of Ni and Ni-Cr-Fe alloys. Further, there are alloys of white metal elements such as pure Pt, Pt-based Pt-Ir alloys, Pt-Ni alloys, Pt-Ir-Ni alloys, and Pt-Ph alloys. In addition, W, Co, or Y
There is a platinum alloy to which a rare earth element such as is added. These alloys have both wear resistance to discharge, material strength, and high hardness, and are all preferred metals. On the other hand, as the inorganic material powder to be dispersed in the metal, Al ₂ O ₃ (alumina), SiO ₂ (silica), SiC (silicon carbide),
There are ceramic powders such as Si ₃ N ₄ (silicon nitride) and glass powders such as quartz glass and borosilicate glass. The content (% by volume) of the inorganic material powder in the electrode material is determined according to the average particle size (μm) of the inorganic material powder, and it is necessary to satisfy the above-mentioned relational expression (log is logarithmic). (See FIGS. 3 and 4).

[0005]

[Operation and Effect] In the spark plug of the present invention,
The electrode material on the discharge surface is composed of a metal as a conductor and an inorganic material powder mixed therein. Therefore, the metal is melted by the discharge energy, but the molten metal is formed as fine particles and fixed to the inorganic material powder. Therefore, the discharge surface portion forms an uneven surface by the metal fine particles fixed to the surface of the dispersed inorganic material powder, and a large number of discharge starting points are formed. Then, in principle, for the discharge, a place where the spark is most likely to be selected is selected. However, when observing this, the discharge starting point repeatedly moves for each discharge, and the spark voltage varies. To cope with this phenomenon, forming a large number of irregularities on the discharge surface means that a large number of discharge spots that can ignite at the minimum voltage value are arranged in parallel, suppressing voltage variations and suppressing the maximum value. Can be done. Therefore, in the spark plug of the present invention, even when the discharge surface portion is consumed by the discharge, an uneven surface based on the adhesion of the metal fine particles to the inorganic material powder is constantly formed. Therefore, the discharge surface portion is always maintained in a shape capable of suppressing the spark voltage, and the spark voltage reduction effect is maintained until all the metal in the electrode material is consumed. The spark voltage reduction effect is particularly remarkable when the above relational expression is satisfied, as is known from FIGS. Therefore, according to the present invention, it is possible to provide a spark plug for an internal combustion engine capable of always maintaining the discharge surface in an uneven state and suppressing an increase in spark voltage.

[0006]

Embodiment 1 A spark plug for an internal combustion engine according to an embodiment of the present invention will be described with reference to FIGS. The spark plug 2 of this embodiment includes an insulator 20, a center electrode 4 held by the insulator 20, a housing 25 fixed to the outer periphery of the insulator 20, and the center electrode 4 provided on the housing 25. And a ground electrode 3 opposed to the gap 30. The discharge surface 4 of the center electrode 4
At 0, an electrode material 1 composed of a metal and an inorganic material powder that does not alloy with the metal is provided. The content of the inorganic material powder in the electrode material 1 satisfies the above relational expression. The electrode material 1 is welded to the discharge surface 40 of the center electrode 4. The material of the electrode material 1 is
For example, it is composed of platinum as a metal and ceramics as an inorganic material powder. The insulator 20 is made of high-purity alumina, and is caulked and fixed to an inner hole of a housing 25 via a ring-shaped hermetic packing 24 and a ring 23. The center electrode 4 is made of a Ni—Cr alloy or Ni—Cr alloy whose inner material is copper and whose outer material is excellent in heat resistance and corrosion resistance.
-Made of Fe alloy. And the center electrode 4
Is held at the insulator 20 with its lower end protruding below the insulator 20. In addition, the insulator 20
A central shaft 42 having a terminal 421 at the top is inserted into the shaft hole 22 and is heat-sealed integrally with the center electrode 4 by a conductive glass sealing material 44. The ground electrode 3 made of the same material as the outer material of the center electrode 4 is connected to the housing 25.
Is joined to the annular end surface 251.

Example 2 Next, with respect to the spark plug shown in Example 1, the type of the electrode material 1 was changed, and the effect of reducing the spark voltage was measured. The results are shown in FIGS. That is, first, as an electrode material, a sintered body composed of a Pt (platinum) -20% by weight Ir (iridium) alloy as a metal and an alumina powder as an inorganic material powder was used. As the alumina powder, various powders having an average particle size of 1, 5, 10, and 50 μm were used, and the content (volume%) in the electrode material was variously changed to be mixed in the electrode material. Then, the sintered body of the electrode material was welded to the discharge surface portion 40 of the center electrode. The discharge tip of the electrode material was reduced to a diameter of 1 mm, and the spark gap was set to 1.1 mm. The spark plug configured as described above was discharged for 1000 hours in compressed air at a pressure of 4 kg / cm ² gauge, and the effect of reducing the spark voltage at this time was measured. The result is shown in FIG.
The vertical axis in FIG. 3 is an index indicating the voltage reduction ratio, and indicates the ratio of the spark voltage at the time of 1000 hours discharge to a spark plug using a conventional platinum alloy (Pt-20 wt% Ir) as an electrode material. The abscissa indicates the content volume% of the alumina powder in the electrode material. FIG. 5 shows the measured values when the alumina powders having the above-mentioned various average particle diameters were used. The voltage reduction ratio is indicated by the maximum value of the voltage variation. FIG. 3 shows that the voltage reduction rate increases as the alumina powder content increases and as the average particle diameter increases. Next, when the voltage reduction ratio was 5% at which a clear reduction effect was recognized, the content in the electrode material was determined for each particle size of the alumina powder from FIG. 3, and this is shown in FIG. From FIG. 4, the content V (volume%) of the alumina powder, which is essential for obtaining a reliable voltage reduction effect, is expressed by the average particle diameter D (μ
m), it can be seen that it is represented by the following logarithmic (log) equation. logV ≧ 1.1 × logD−2.3
In this example, the case where an electrode material composed of the above-mentioned Pt-Ir alloy as a metal and alumina powder as an inorganic material powder is used is shown. The same is true.

[Brief description of the drawings]

FIG. 1 is a perspective view of a main part of a spark plug according to a first embodiment.

FIG. 2 is a partial cross-sectional view of the spark plug according to the first embodiment.

FIG. 3 is a diagram showing the relationship between the alumina powder content and the voltage reduction rate in Example 2.

FIG. 4 is a diagram showing the relationship between the average particle size and the content of alumina powder when the voltage reduction rate is 5% in Example 2.

[Explanation of symbols]

 1. . . 1. Electrode material . . Spark plug 20. . . Insulator 3. . . Ground electrode 4. . . Center electrode

Claims (1)

(57) [Claims] An insulator, a center electrode held by the insulator, a housing fixed to an outer periphery of the insulator, and a ground electrode provided in the housing with a gap provided between the center electrode and the ground electrode. And an electrode material comprising a metal and an inorganic material powder not alloying with the metal is disposed on at least one of the discharge surfaces of the center electrode and the ground electrode. Content V of inorganic material powder in electrode material (volume%)
A spark plug for an internal combustion engine, characterized by satisfying the following relational expression in relation to the average particle diameter D (μm) of the inorganic material powder. logV ≧ 1.1 × logD−2.3