Spark plug for internal combustion engine

In a spark plug having a cylindrical metallic shell a front end of which is extended into a combustion chamber of an internal combustion engine, an insulator is provided within the metallic shell so as to provide an annular clearance between a front end of the metallic shell and that of the insulator, a width of the annular clearance being within a range of 0.65 mm .+-.0.25 mm. An extension skirt which the metallic shell is extended into the combustion chamber has a length within a range from 1.0 mm to 3.0 mm. A tapered surface is provided by planing off an inner edge of a front end of the metallic shell, and the surface angularly falls within angles from 20 to 40 degrees. One end of an outer electrode is welded to the tapered surface while other end of the outer electrode is bent to oppose a front end of a center electrode.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a spark plug which has a metallic shell extended 
into a combustion chamber of an internal combustion engine. 
2. Description of Prior Art 
In a spark plug which is usually employed in an internal combustion engine, 
it has been suggested that a firing tip is protracted into a combustion 
chamber of the internal combustion engine to improve ignition efficiency. 
With the protraction of the firing tip, a front end of the metallic shell 
is extended into a combustion chamber to protect an outer electrode 
against excessive heat. 
In order to prevent the outer electrode from being accidentally broken when 
the outer electrode is bent, a tapered surface is provided with a front 
end of the metallic shell to reduce a bending degree of the outer 
electrode when the outer electrode is welded to the tapered end surface of 
the metallic shell. 
Further, it has been taught to decrease an annular clearance between a 
front end of the metallic shell and that of an insulator located within 
the metallic shell. This is a countermeasure against fouling of a front 
end of the insulator, an option of which is to remove an accumulation of 
carbon deposit on the insulator by spark discharge occurring through the 
annular clearance. 
With a recent high-output performance of the internal combustion engine, it 
is required to prevent the outer electrode from being accidentally broken, 
and at the same time, protecting the insulator against fouling when 
operating the engine with a low load. 
To cope with this requirement, it is considered to adopt the individual 
countermeasures in combination. 
It, however, is found that only combining the individual countermeasures 
leads to interfering the individual advantages so as to bear no good 
results. 
Therefore, it is an object of the invention to provide a spark plug which 
is capable of maintaining good ignition without misfire, and preventing an 
outer electrode from being accidentally broken, and at the same time, 
protecting the insulator against fouling when operating the engine with a 
low load. 
Then, the invention contributes to an extended period of service life, and 
achieving these effects with a relatively simple construction. 
SUMMARY OF THE INVENTION 
According to the invention, in a spark plug including a cylindrical 
metallic shell a front end of which is extended into a combustion chamber 
of an internal combustion engine, the spark plug comprising: a tubular 
insulator concentrically located within the metallic shell so as to 
provide an annular clearance between an inner wall of a front end of the 
metallic shell and an outer wall of a front end of the insulator, a width 
of the annular clearance being within a range of 0.65 mm.+-.0.25 mm; a 
center electrode concentrically located within the insulator; an extension 
skirt which the metallic shell is extended into the combustion chamber, a 
length of the extension skirt falling within a range from 1.0 mm to 3.0 
mm; a tapered surface provided by planing off an inner edge of a front end 
of the metallic shell, the tapered surface angularly falling within angles 
from 20 degrees to 40 degrees; and an outer electrode, one end of which is 
securely welded to the tapered surface of the metallic shell, while other 
end of the outer electrode is bent to oppose a front end of the center 
electrode to form a spark gap therebetween. 
Further, a plurality of axial slits are circumferentially provided with the 
extension skirt of the metallic shell. 
With the extension of a front end of the metallic shell into the combustion 
chamber, it becomes possible to maintain good ignition without incidence 
of misfires. The extension exceeding to 3 mm could cause to oxidize the 
electrodes by high temperature. By decreasing the annular clearance 
between a front end of the metallic shell and that of an insulator, it is 
prevented from introducing excessive heat into the spark plug to provide 
heat-resistant property. 
Carbon deposit on the insulator works to decrease its electrical 
resistance, so that the spark discharge runs through the clearance to 
remove the carbon deposit. 
Optimum width of the clearance is found to be 0.65 mm.+-.0.25 mm with the 
length of the extension skirt as 1 mm to 3 mm. 
In order to prevent the outer electrode from being accidentally broken when 
the outer electrode is bent, a tapered surface is provided with a front 
end of the metallic shell to reduce a bending degree of the outer 
electrode when the outer electrode is welded to the tapered end surface of 
the metallic shell. 
An increased taper of the end surface of the metallic shell, however, 
serves to eliminate the necessity of bending the outer electrode too much, 
but it often establishes abnormal spark between an inner wall of the 
metallic shell and a front end of the insulator unless the tapered surface 
falls within angles from 20 to 40 degrees. 
Moreover, the axial slits circumferentially provided with the extension 
skirt, works to cool the insulator so as to further add heat-resistant 
property to the spark plug when air-fuel mixture is taken into the 
combustion chamber to flow along the extension skirt. 
Thus directs to contribute to an extended period of service life, and 
achieving above-mentioned effects with a relatively simple construction. 
These and other objects and advantages of the invention will be apparent 
upon reference to the following specification, attendant claims and 
drawings.

DETAILED DESCRIPTION OF THE EMBODIMENTS 
Referring to FIGS. 1 and 2 in which an embodiment of the invention is 
shown, numeral 1 designates a spark plug for use in an internal combustion 
engine. The spark plug 1 has a cylindrical metallic shell 2, an axial 
length (l) of which measures 20.5 mm somewhat longer than 19.0 mm of the 
usual spark plug. Within the metallic shell 2, is a tubular insulator 4 
concentrically located into which a center electrode 5 is concentrically 
placed. In this instance, a front end of the insulator 4 extends beyond 
that of the metallic shell 2 by a length of 0 mm to 0.5 mm as designated 
at (n) in FIG. 1. 
On the other hand, a front end portion of the metallic shell 2 has an 
extension skirt 6 which is extended into a combustion chamber (Ch) of the 
engine. The length (m) which the extension skirt 6 is extended into the 
combustion chamber (Ch) falls e.g. 1.5 mm, but the length (m) of the skirt 
6 is acceptable as long as it falls within a range from 1.0 mm to 3.0 mm 
inclusive. 
Meanwhile, the metallic shell 2 has a front end surface, an inner edge of 
which is planed off to form a tapered surface 7 which progressively 
decreases its diametrical dimension toward a rear end of the metallic 
shell 2. An angle (.theta.) of the tapered surface 7 falls e.g. 79 
degrees, but it may fall within angles from 20 to 40 degrees inclusive. 
Numeral 3 is an outer electrode which is made of e.g. nickel-based alloy 
to impart it with spark-corrosion resistant property. The outer electrode 
3 has one end securely welded to the tapered surface 7 of the metallic 
shell 2, while other end of the outer electrode 3 is bent to vertically 
oppose a front end (firing tip) of the center electrode 5 so as to form a 
spark gap (Gs) therebetween. In this instance, the tapered surface 7 
eliminates the necessity of bending the outer electrode 3 too much so as 
to prevent the outer electrode 3 from being accidentally broken. 
Further, the front end of the insulator 4 is located within the extension 
skirt 6 to provide an annular clearance 9 between an inner wall of a front 
end portion of the extension skirt 6 and an outer wall of a front end 
portion of the insulator 4. Width (p) of the annular clearance 9 falls 
e.g. 0.65 mm which is narrow enough to substantially avoid heat from being 
introduced into the spark plug 1. However, the width (p) of the annular 
clearance 9 may fall within 0.65 mm.+-.0.25 both inclusive. With the 
extension skirt 6, are a plurality of axial slits 8 circumferentially 
provided which works to cool the insulator 4 so as to further impart 
heat-resistant property to the insulator 4 when air-fuel mixture is taken 
into the combustion chamber (Ch) to flow along the extension skirt 6. In 
this instance, the number of the axial slits 8 is e.g. four, and each 
width (q) of the axial slits 8 falls 1.5 mm. 
Now, FIG. 3 is a schematic diagram showing how anti-fouling effect and 
incidence of misfire during idling operation of the engine change upon the 
width (p) of the annular clearance 9 between the extension skirt 6 and the 
metallic shell 2. In FIG. 3, how much the insulator 4 is fouled by carbon 
deposit accumulated on the insulator 4 is measured by its electrical 
resistance (M.OMEGA.). FIG. 3 apparently teaches how well the 
above-determined width (p) of the annular clearance 9 has improved the 
anti-fouling effect and the incidence of misfire during idling operation 
of the engine. 
As understood from the foregoing description, the extension skirt 6 is 
extended into the combustion chamber (Ch) to maintain good ignition 
without being oxidized by high temperature, and the tapered surface 7 
prevents the outer electrode 3 from being accidentally broken without 
sacrifying the good ignition. 
Moreover, the axial slits circumferentially provided with the extension 
skirt 6 works to cool the insulator 4 so as to further impart 
heat-resistant property to the insulator so as to cope with the carbon 
deposit which otherwise is accumulated on the insulator 4. 
According to the invention, the spark plug 1 is capable of coping with 
high-output, high-rpm engine, and maintaining good ignition without 
misfire, and preventing an outer electrode from being accidentally broken, 
and at the same time, protecting the insulator against the fouling when 
operating the engine with a low load. 
It is noted that the outer electrode may be secured to the extension skirt 
by means of brazing, spot welding or electrical resistant welding. 
While the invention has been described with reference to the specific 
embodiments, it is understood that this description is not to be construed 
in a limitting sense in as much as various modifications and additions to 
the specific embodiments may be made by skilled artisan without departing 
from the spirit and scope of the invention.