1. Field of the Invention
The present invention relates to a spark plug used for ignition in an internal combustion engine.
2. Description of the Related Art
FIG. 9 shows a conventional spark plug 300 used for ignition in an internal combustion engine, such as an automobile gasoline engine. The spark plug 300 is mounted on a cylinder head SH of an engine by a male-threaded portion 301a formed on an outer circumferential surface of a metallic shell 301. When the spark plug 300 is mounted on the cylinder head SH, a spark discharge gap g defined by a ground electrode 304 and a center electrode 303 is positioned within a combustion chamber BR and is adapted to ignite a fuel-air mixture. A hexagonal portion 305 (tool engagement portion) is formed on an outer circumferential surface of the metallic shell 301 and is adapted to tighten the male-threaded portion 301a by a tightening tool. The metallic shell 301 assumes a substantially cylindrical shape having a bore 306 for receiving an insulator 302 and is conventionally manufactured by cold plastic processing and machining. In many spark plugs, in order to improve manufacturing efficiency, a schematic profile and the bore 306 are formed by die forging, and a final profile including the male-threaded portion 301a is finished by machining. Since the metallic shell 301 has a thin-walled portion, the dimensions of the metallic shell 301 must be designed in consideration of a material flow during die forging; otherwise, a working defect is likely to occur.
With a recent tendency toward complication of engine head structure, space allocated around a valve for installation of the spark plug 300 is decreasing. Thus, the hexagonal portion 305 needs to be reduced to increase space for use on the head side. However, reducing the size of the hexagonal portion 305 causes the following problems.
(1) To prevent an excessive reduction in the wall thickness of the hexagonal portion 305 in association with a reduction in the size of the hexagonal portion 305, a diameter D1 of a portion (hereinafter referred to as xe2x80x9ca major-bore portionxe2x80x9d) 306a of the shell bore 306 must be reduced. Also, the outside diameter of the insulator 302 must be reduced accordingly. However, when a diameter D2 of a portion (hereinafter referred to as xe2x80x9can intermediate-bore portionxe2x80x9d) of the shell bore 306 corresponding to the male-threaded portion 301a is reduced, a forging punch becomes excessively thin, when the intermediate-bore portion 306b is formed by forging and thus may be damaged or may cause a working defect when a large working load is applied thereon. This problem arises particularly in the case when the male-threaded portion 301a has a long screw reach.
(2) A portion of the insulator 302 positioned within the major-bore portion 306a is formed into a flange portion 302e. When the metallic shell 306 is swaged onto the insulator 302, the flange portion 302e bears a swaging force. A metallic terminal 313 and a center electrode 303 are connected by a glass seal portion 315. In the step of the glass seal portion 315, the flange portion 302e bears a pressing force. In particular, the center electrode 303, a material powder of the glass seal portion 315, and the metallic terminal 313 are disposed within a through-hole formed in the insulator 302. Then the insulator 302 is inserted into a bore formed in a seat die such that the flange portion 302e rests on an inner seat portion formed on the wall of the bore. In this state, the entire insulator 302 is heated to a temperature equal to or higher than a glass softening point, and the metallic terminal 313 is pressed inwardly in the axial direction to press the material powder with the center electrode 303, thereby forming the glass seal portion 315. During this pressing process, the flange portion 302e bears a pressing force.
If the outside diameter of the insulator 302 is too small to meet the demand described above in (1), manufacturing the insulator 302 becomes very difficult. Therefore, there is a certain limit to a reduction in the outside diameter of the insulator 302. As the size of the hexagonal portion 305 is reduced, the diameter D1 of the major-bore portion 306a is reduced accordingly. Thus, the diameter of the flange portion 302e, which is accommodated within the major-bore portion 306a, is also reduced. Because of a reduction in the size of the hexagonal portion 305, the diameter of the flange portion 302e must be reduced because there is a certain limit to a reduction in the diameter of a portion of the insulator 302 other than the flange portion 302e (for example, a portion of the insulator 302 positioned within the intermediate-bore portion 306b; hereinafter referred to as xe2x80x9can intermediate-trunk portion 302axe2x80x9d). As a result, the amount of a projection of the flange portion 302e decreases, causing, for example, a decrease in the area of contact between the flange portion 302e and the seat portion of the seat die used in the step of forming the glass seal portion 315. Consequently, a load concentration causes breakage of the seat die or galling of the insulator 302 and the seat die.
(3) If the diameter of the intermediate-trunk portion 302a of the insulator 302 is reduced to meet the demand described above in (2), and also the diameter D2 of the intermediate-bore portion 306b of the metallic shell 306 is set to a rather large value to attain favorable workability during the process in (1), a gap is likely to be formed between the intermediate-bore portion 306b and the intermediate-trunk portion 302a of the insulator 302. The presence of this gap tends to cause an eccentric disposition of the insulator 302 within the metallic shell 301, potentially causing an impairment in spark plug performance (for example, lateral sparking).
An object of the present invention is to provide a spark plug capable of increasing the degree of freedom with respect to space around a cylinder head on which the spark plug is mounted, through reduction in the size of a tool engagement portion, such as a hexagonal portion, and capable of implementing the following:
(1) in spite of a reduction in the size of the tool engagement portion, a metallic shell can be manufactured efficiently and at high yield;
(2) during formation of a conductive glass seal layer or a resistor by use of a seat die, breakage or galling of the seat die is less likely to occur; and
(3) during incorporation of an insulator into the metallic shell, an eccentric disposition of the insulator within the metallic shell is less likely to occur.
To achieve the above object, the present invention provides a spark plug including an elongate center electrode, an insulator enclosing the center electrode, a metallic shell having open opposite ends and enclosing the insulator, the metallic shell having a male-threaded portion formed on a front-side outer circumferential surface of the metallic shell, and a tool engagement portion formed on the outer circumferential surface of the metallic shell at a rear side with respect to the male-threaded portion, the tool engagement portion projecting circumferentially outwardly, and a ground electrode connected to the metallic shell and defining a spark discharge gap in cooperation with the center electrode.
In the specification, the term xe2x80x9cfrontxe2x80x9d refers to a spark discharge gap side with respect to an axial direction of the center electrode, and the term xe2x80x9crearxe2x80x9d refers to a side opposite the front side.
The insulator has a stepped annular insulator-side engagement portion for engaging with an annular shell-side engagement portion projected inwardly from a portion of an inner surface of the metallic shell corresponding to the male-threaded portion, and |Axe2x88x92E|xe2x89xa61.5 mm, and 0.4xe2x89xa6(D2/E)2xe2x89xa60.6, where A is a dimension of the tool engagement portion represented by a diameter of an inscribed circle of a cross-sectional outline of the tool engagement portion, E is an effective diameter of the male-threaded portion, and D2 is an inner diameter of an intermediate-bore portion of the metallic shell located on a rear side with respect to the shell-side engagement portion.
According to the above-described structure, the dimension A of the tool engagement portion (for example, a hexagonal portion) is reduced with respect to the effective diameter E of the male-threaded portion such that |Axe2x88x92E| becomes not greater than 1.5 mm. Thus, the degree of freedom with respect to space around a cylinder head on which the spark plug is mounted can be increased. Even when available space around a valve for installation of the spark plug decreases due to complication of cylinder head structure, the spark plug can be easily mounted on the cylinder head. Although the outside diameter of the insulator decreases in association with a reduction in the size of the tool engagement portion, so long as 0.4xe2x89xa6(D2/E)2xe2x89xa60.6, the wall thickness of the male-threaded portion of the metallic shell falls within an appropriate range. Thus, during forging of the metallic shell, a forging punch is less susceptible to breakage and is less likely to cause a working defect. That is, the problem described previously in (1) is solved, and the metallic shell can be manufactured efficiently and at high yield.
More particularly, (D2/E)2 represents the ratio of the cross-sectional area of the intermediate-bore portion having the diameter D2 xe2x80x9cxcfx80(D2/2)2xe2x80x9d to the cross-sectional area of the male-threaded portion having the effective diameter E xe2x80x9cxcfx80(E/2)2.xe2x80x9d The smaller the value (D2/E)2 (i.e., the more the effective diameter E of the male-threaded portion increases with respect to the diameter D2 of the intermediate-bore portion), the greater the wall thickness of the male-threaded portion. When (D2/E)2 is less than 0.4, the wall thickness of the male-threaded portion becomes excessively large, causing an insufficient diameter of the intermediate-bore portion. As a result, when the intermediate-bore portion is to be formed through cold working, such as forging, a forging punch to be used becomes excessively thin and thus may be damaged or may cause a working defect when a large working load is imposed thereon. When (D2/E)2 is in excess of 0.6, the wall thickness of the male-threaded portion becomes excessively thin. As a result, formation of the male-threaded portion through cold working becomes difficult, and the formed male-threaded portion suffers insufficient strength. More preferably, (D2/E)2 ranges from 0.45 to 0.55.
A flange portion may be formed on the outer circumferential surface of the insulator on the rear side with respect to the stepped portion. In this case, preferably, d2/d1 is not greater than 0.75, where d1 is the outside diameter of the flange portion, and d2 is the outside diameter of an intermediate-trunk portion extending between the flange portion and the stepped portion. As mentioned previously in (2), in the case of reducing the outside dimension A of the tool engagement portion such that |Axe2x88x92E| is not greater than 1.5 mm, if the outside diameter of the intermediate-trunk portion becomes excessively small, manufacture of the insulator becomes very difficult. Also, a reduction in the size of the tool engagement portion unavoidably requires a reduction in the outside diameter of the flange portion. In other words, the diameter ratio d2/d1 between the intermediate-trunk portion and the flange portion tends to increase. As d2/d1 increases, the amount of projection of the flange portion from the outer circumferential surface of the intermediate-trunk portion decreases. As a result, as mentioned previously, the step of forming a glass seal portion is likely to involve breakage of a seat die or galling between the insulator and the seat die. Through employment of a d2/d1 of not greater than 0.7, the amount of projection of the flange portion becomes sufficiently large, thereby effectively preventing the above-mentioned problem associated with a reduction in the size of the tool engagement portion; i.e., solving the problem described previously in (2). More preferably, d2/d1 is not greater than 0.65. However, d2/d1 is excessively small, the intermediate-trunk portion becomes too thin for manufacture of the insulator. Therefore, in order to avoid such a problem, the value d2/d1 must be adjusted as adequate.
As mentioned previously in (3), if the diameter of the intermediate-bore portion is set to a rather large value in order to attain favorable workability of the metallic shell while the diameter of the intermediate-trunk portion of the insulator is decreased in association with a reduction in the size of the tool engagement portion, a gap is likely to be formed between the intermediate-bore portion of the metallic shell and the intermediate-trunk portion of the insulator. In the case of formation of such a gap, preferably, an eccentricity preventive portion is provided substantially concentrically with the intermediate-bore portion and the intermediate-trunk portion in such a manner as to partially fill the gap. In the step of incorporating the insulator into the metallic shell, the eccentricity preventive portion restricts lateral movement of the insulator; i.e., an eccentric disposition of the insulator within the metallic shell, thereby solving the problem described previously in (3).
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.