Spark plug structure

A spark plug structure comprising; a cylindrical metallic shell; a joint type insulator having a center bore, and including a front half piece and a rear half piece, each made of a tubular aluminum nitride (AlN), and the front and rear half pieces being joined at their respective end by means of a glass sealant, and encased into the metallic shell; a center electrode placed into the center bore of the insulator; an elongated terminal placed into the rear half piece of the insulator; an electrically conductive glass provided to seal respective spaces appeared between the center electrode, insulator and the terminal; the front half piece having an elongated projection, the length of which is more than 2.0 mm, and the rear half piece having a recess the depth of which is more than 2.0 mm, the front and rear half pieces being jointed at the projection and the recess by means of an annular glass sealant which has thickness of less than 2.0 mm and length of more than 2.0 mm.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to a spark plug structure in use for internal 
combustion engine in which an insulator includes rear and front half 
pieces which join at their respective ends, and particularly concerns to a 
spark plug structure in which the front half piece is made from aluminum 
nitride of good thermal conductivity. 
2. Description of Prior Art 
In a spark plug generally used for internal combustion engine, and 
insulator of the spark plug has been mainly made of alumina (Al.sub.2 
O.sub.3). Due to low thermal conductivity of alumina, the insulator is 
unable to release sufficient quantity of heat in a combustion chamber when 
applied to high efficient engine of these days. The heat-laden insulator 
causes unfavorable preignition 
According to Japanese Patent Publication No. 55-46634, it is suggested that 
the insulator is made from aluminum nitride (AlN) of good thermal 
conductivity so as to release the heat in a combustion chamber. 
In order to save cost, it is proposed that the insulator is divided into 
two pieces of rear and front half pieces. The front half piece is made 
from aluminum nitride (AlN) of good thermal conductivity, and the rear 
half piece is made from alumina (Al.sub.2 O.sub.3). Two pieces are joined 
at their respective ends by means of glass sealant. 
Due to relatively poor strength at the joined portions, there holds risk of 
cracks occurring on the glass sealant so as to loosen the jointed portions 
at the time of providing the glass sealant. 
Further, cracks may occur on the insulator at the time of caulking a 
metallic shell which encases the insulator. 
Therefore, it is an object of this invention to provide a spark plug 
structure which is capable of avoiding cracks from occurring on an 
insulator at the time of providing glass sealant, and at the time of 
caulking a metallic shell. 
It is another object of this invention to provide a spark plug structure 
which has improved insulator to prevent preignition, and thermal shock 
from occurring even when applied to high efficient engine in which the 
insulator is exposed to rapid cooling and heating cycle with huge 
difference of temperature and pressure. 
It is further object of this invention to provide a spark plug structure 
which is capable of contributing to cost-saving, high yield and mass 
production. 
According to the present invention, there is provided a spark plug 
structure comprising; a cylindrical metallic shell having a ground 
electrode; a joint type insulator having a center bore, and including a 
front half piece and a rear half piece, each made of a tubular aluminum 
nitride (AlN), and the front and rear half pieces being joined at their 
respective end by means of a glass sealant, and concentrically encased 
into the metallic shell; a center electrode concentrically placed into the 
center bore of the insulator with a front end of the electrode somewhat 
extended outside that of the insulator to form a spark gap with the ground 
electrode; an elongated terminal placed into the rear half piece of the 
insulator with a rear end of the terminal somewhat extended outside that 
of the rear half piece; an electrically conductive glass provided to seal 
respective spaces appeared between the center electrode, the insulator and 
the terminal; the front half piece having an elongated projection, the 
length of which is more than 2.0 mm, and the rear half piece having a 
recess, the depth of which is more than 2.0 mm, the front and rear half 
pieces being jointed at the projection and the recess by means of an 
annular glass sealant which has thickness of less than 2.0 mm and length 
of more than 2.0 mm. 
The annular glass sealant is determined to have thickness of less than 2.0 
mm and length of more than 2.0 mm. 
Enough strength is imparted to the glass sealant to sufficiently resist to 
a load of 200 Kg required when the glass sealant is provided. 
The recess is surrounded by an annular periphery, thickness of which is 
determined to be more than 1.5 mm. This enables to prevent cracks from 
occurring on the insulator at the time of caulking the metallic shell. 
Various other objects and advantages to be obtained by the present 
invention will appear in the following description and in the accompanying 
drawings.

DETAILED DESCRIPTION OF THE EMBODIMENTS 
Referring to FIGS. 1 and 2, a spark plug 100 according to the present 
invention, has a metallic shell 90 having a ground electrode 50a 
integrally. Into the metallic shell 90, a tubular insulator 30 is 
concentrically placed. 
The insulator 30 is joint type comprising rear and front half pieces 20 and 
10. The front half piece 10 is made from aluminum nitride (AlN) of high 
thermal conductivity, while the rear half piece 20 is made of alumina 
(Al.sub.2 O.sub.3) for the purpose of cost-saving. The front half piece 10 
has an elongated projection 11, and the rear half piece 20 has a recess 
21. The rear and front half pieces 20 and 10 are joined at their recess 21 
and the projection 11 by means of an annular glass sealant 40. 
In the meanwhile, the recess 21 is surrounded by an annular periphery, 
thickness dimension (W) of which is determined to be more than 1.5 mm as 
described in detail hereinafter. 
The common length in which the rear and front half pieces 20 and 10 are 
joined, corresponds to the length (l) of the glass sealant 40. The glass 
sealant 40 is made from CaO, BaO, Al.sub.2 O.sub.3 or SiO.sub.2 -based 
vitreous material, and determined at its length (l) and thickness (t) to 
be 4.0 mm and 1.0 mm respectively. It is noted that minimum limit of the 
length (l) is 2.0 mm, while the maximum limit of the thickness (t) is 2.0 
mm to sufficiently resist the maximum load of 200 Kg applied to the glass 
sealant 40 when providing it. 
The recess 21 is, as mentioned before, surrounded by an annular periphery, 
the thickness dimension (W) of which is determined to be 3.0 mm by way of 
illustration. The thickness dimension (W) is required at least 1.5 mm to 
resist to maximum load of around 5 tons applied when the metallic shell 90 
is squelched of an annular end 91 by means of caulking. 
On the other hand, the front half piece 10 of the insulator 30 has axial 
bores 13 and 14 of different diameter. The rear half piece 20 of the 
insulator 30 has an axial bore 22 communicated with the bores 13 and 14 so 
as to constitute a central bore as a whole. Into the axial bores 13 and 
14, a center electrode 50 is placed with the front end somewhat extended 
outside from that of the front half piece 10 to form a spark gap (Sp) with 
the ground electrode 50a. 
The center electrode 50 has a flanged head 51 at its rear end, and made 
from a copper-based core clad by a nickelbased alloy. At the time of 
assemble, the center electrode 50 is inserted through the rear ends of the 
axial bores 13, 14 and 22, and received at its flanged head 51 by a 
shoulder 14a of the diameter-increased bore 14. In this instance, the 
center electrode 50 may be adhered to an inner surface of the bore 13 by 
means of a heat-resistant adhesive 52. 
At the space in which the two bores 15 and 22 meet, a resistor 61 is placed 
with its upper head and bottom sandwiched by electrically conductive 
layers 60 and 60a for the purpose of noise reduction. Into the axial bore 
22, an elongated terminal 80 is air-tightly inserted in a manner to 
sandwich the conductive layer 60 with the resistor 61. 
Now, FIGS. 3 and 4 show the result of strength test carried out by changing 
the thickness (t) and length (l) of the glass sealant 40 which has joined 
the rear and front half pieces 20 and 10. 
FIG. 3 shows the result of tensile test in which the joint type insulator 
30 has undergone under the ambient temperature of around 1000 degrees 
Celsius depending on the thickness dimension (t) of the glass sealant 40 
with the length (l) as constant 4.0 mm. 
FIG. 4 shows the result of tensile test in which the joint type insulator 
30 has undergone under the ambient temperature of around 1000 degrees 
Celsius depending on the length dimension (l) of the glass sealant 40 with 
the thickness dimension (t) as constant 1.0 mm. 
As a result, it has found that the requirements of l .gtoreq.2.0 mm, t 
.ltoreq.2.0 mm are apparently obtained to resist the maximum load of 200 
Kg. 
FIG. 5 shows the result of the strength test carried out by changing the 
thickness dimension (W) of the annular periphery 21a in the recess 21. 
In this strength test, various loads are measured when the cracks occurred 
on the annular periphery 21a at the time of caulking the metallic shell 90 
as designated by (x). 
As the result of this test, it has found that it is necessary to arrange as 
W .gtoreq.1.5 mm to cope with the maximum load of around 5 tons. 
As understood from the foregoing description, it is necessary to arrange 
dimensions (t), (l) and (W) as follows: 
That is, t .ltoreq.2.0 mm, l .gtoreq.2.0 mm and W .gtoreq.1.5 mm. 
These dimensional arrangement enables to prevent cracks from occurring on 
the joint type insulator 30. 
As a modified form of this invention, the front half piece 10 is made of 
sintered aluminum nitride (AlN) of more than 60 w/mk in thermal 
conductivity. On an outer surface of the front half piece 10, a 
non-crystallized alumina layer of 1-30 microns is coated by means of CVD 
or the like. The rear and front half pieces 20 and 10 are bonded by a 
vitreous adhesive of high melting point. 
The front half piece 10 is coated with fine-structured alumina, so that the 
alumina layer is prevented from transforming into Trigonal corundum by 
oxidation, at the same time, prevented from being separated, thus 
contributing to long service life. 
EXAMPLE 1 
The alumina (Al.sub.2 O.sub.3) layer is made by previously oxidizing the 
aluminum nitride piece 10 of 20 mm in length. The experiment is carried 
out under 5500 rpm X 4/4 of six-cylinder engine with displacement of 2000 
cc for 100 hours. 
After the experiment, oxidation degree is measured by EPMA, it is found 
from Table 1 that the thicker the alumina layer is, the lesser the 
formation of Al.sub.2 O.sub.3 is as seen from sample A to sample E. The 
alumina layer of 1 micron is sufficient to protect the aluminum nitride 
from being oxidized into Al.sub.2 O.sub.3 more than necessary. However, 
the upper limit of the thickness of the alumina layer is around 30 
microns, because too much alumina causes to separation. 
TABLE 1 
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previous thickness 
thickness of Al.sub.2 O.sub.3 
oxidation of Al.sub.2 O.sub.3 
after 100 hours 
______________________________________ 
sample A no oxidation 
0 .mu.m 40 .mu.m 
sample B oxidation 0.8 .mu.m 
35 .mu.m 
sample C oxidation 1 .mu.m 25 .mu.m 
sample D oxidation 3 .mu.m 20 .mu.m 
sample E oxidation 10 .mu.m 18 .mu.m 
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EXAMPLE 2 
The samples A to E as used in the experiment 1, are undergone the 
anti-preignition test under four-cylinder engine with displacement of 1600 
cc. As seen in Table 2, the thickness of Al.sub.2 O.sub.3 substantially 
has no affect on the anti-preignition. The samples C, D and A have figures 
similar to those of sample F which has no layer of Al.sub.2 O.sub.3, and 
representing high heat-resistant characteristics compared to the prior and 
BPR6EY plug. 
Now, various kinds of Vitreous materials is listed in Table 3 to be applied 
to the annular glass sealant 40. These vitreous materials are of high 
melting point of more than 500 degrees Celsius, and of 32-80 X 10.sup.-1 
in thermal expansion which falls between that of AlN and that of Al.sub.2 
O.sub.3. 
TABLE 2 
__________________________________________________________________________ 
(mm)combustion chamberlength exposed to 
of Al.sub.2 O.sub.3thickness 
##STR1## 
__________________________________________________________________________ 
sample A 20 40 
##STR2## 
sample B 20 35 
##STR3## 
sample C 20 25 
##STR4## 
sample D 20 20 
##STR5## 
sample E 20 18 
##STR6## 
sample F 20 0 
##STR7## 
Al.sub.2 O.sub.3 BPR4EY 
20 -- 
##STR8## 
Al.sub.2 O.sub.3 BPR6EY 
14 -- 
##STR9## 
__________________________________________________________________________ 
TABLE 3 
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thermal volume 
expansion melting sintered 
resistance 
vitreous (.times.10.sup.-7 / 
point temp. Log.rho.(.OMEGA..m) 
material .degree.C.) 
(.degree.C.) 
(.degree.C.) 
at 150.degree. C. 
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Na.sub.2 O.sub.3.B.sub.2 O.sub.3.SiO.sub.2 - 
75.5 697 990 11.2 
based glass I 
Na.sub.2 O.sub.3.B.sub.2 O.sub.3.SiO.sub.2 - 
57.0 705 1050 11.4 
based glass II 
Na.sub.2 O.sub.3.B.sub.2 O.sub.3.SiO.sub.2 - 
45.5 698 1050 11.5 
based glass III 
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EXAMPLE 3 
FIG. 6 of (a), (b) shows that Al.sub.2 O.sub.3 -coated (layed of 10 um) 
front half piece 10 is stronger than non Al.sub.2 O.sub.3 -coated front 
half piece when bonding strength between the rear and front half pieces 20 
and 10 is compared. As seen in FIG. 6 of (a), the bonding strength rapidly 
increases with the increase of the bonding area compared to that of (b). 
As further modified form of the present invention, the annular glass 
sealant 40 is made of vitreous material which has a melting point of more 
than 500 degrees Celsius, and has a temperature of 800-1400 degrees 
Celsius required when the sealant 40 is provided. The thermal expansion of 
the vitreous material falls within the range from 32 X 10.sup.-7 to 80 X 
10.sup.-7. 
Maximum temperature which causes from the combustion chamber of the engine, 
corresponds to the temperature in which preignition occurs. At this time, 
the glass sealant rises its temperature as high as around 500 degrees 
Celsius. 
Accordingly, it is required for the glass sealant 40 to have a melting 
point of more than 500 degrees Celsius so as to properly function. A glass 
use for resistor has a melting temperature of 800-1000 degrees Celsius 
that the glass sealant 40 is desired to have a temperature of more than 
800-1000 degrees Celsius which is required at the time of providing it. 
But, the temperature is preferably below 1400 degrees Celsius so as not to 
facilitate oxidation toward the aluminum nitride (AlN). The thermal 
expansion of the aluminum nitride (AlN) is 32-48 X 10.sup.-7 /.degree.C., 
while that of alumina (Al.sub.2 O.sub.3) is 69-80 X 10.sup.-7 /.degree.C. 
Therefore, it is necessary that the thermal expansion of the glass sealant 
40 falls on the range between 32-48 X 10.sup.-7 /.degree.C. and 69-80 X 
10.sup.-7 /.degree.C. to prevent cracks from occurring on the glass 
sealant 40. A power supply is normally 40 KV, so that it is necessary for 
the glass sealant 40 to have enough length (l) to withstand 40 KV at the 
temperature of 500 degrees Celsius. Vitreous examples which meet those 
requirements are shown at Table 4. 
TABLE 4 
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yield thermal withstand 
vitreous point expansion voltage at 500.degree. C. 
material (.degree.C.) 
(10.sup.-7 /.degree.C.) 
(KV/mm) 
______________________________________ 
B.sub.2 O.sub.3 SiO.sub.2 - 
550 45 18.0 
based glass A 
B.sub.2 O.sub.3 SiO.sub.2 - 
715 67 22.5 
based glass B 
BaO- 670 67 22.0 
based glass A 
BaO- 710 68.5 23.5 
based glass B 
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The temperature of specified portion (A) of FIG. 7 in the glass sealant 40 
is measured with the use of spark plugs each corresponding to BPR4EY and 
BPR7EY. The engine used in this experiment is four series-cylinder, DOHC 
four-valve with the displacement of 1600 cc under the condition of 6000 
rpm X 4/4. The ignition timing is represented by advance angles which is 
needed to cause preignition. The result is shown at Table 5 which teaches 
that the temperature of the glass sealant 40 reaches up to 500 degrees 
Celsius. From this result, it is apparently necessary to use vitreous 
material having a melting point of more than 500 degrees Celsius so as to 
ensure strength and electrical conditions of the glass sealant 40. 
TABLE 5 
______________________________________ 
spark plug 
ignition timing BTDC 
temperature at (A) 
______________________________________ 
BPR4EY 30.degree. 485.degree. C. 
BPR7EY 57.5.degree. 460.degree. C. 
______________________________________ 
An insulator is made by using materials as listed at Table 6. The insulator 
is applied to a spark plug corresponding to BPR4ES with the thermal 
expansion of the glass sealant varying as Table 7. The engine used in this 
experiment is water-cooling type of six series- cylinder, OHC with the 
displacement of 2000 cc under operating condition of 6000 rpm X 4/4 (one 
minute) and idling (one minute) for 200 hours. In this experiment, six 
test pieces are used at each case. The result of Table 7 shows that the 
thermal expansion of the glass sealant 40 is needed to fall between that 
of the aluminum nitride and that of alumina. 
TABLE 6 
______________________________________ 
material thermal expansion (10.sup.-7 /.degree.C.) 
______________________________________ 
AlN 34 
Al.sub.2 O.sub.3 
80 
______________________________________ 
TABLE 7 
______________________________________ 
vitreous thermal 
material expansion 
(yield point) 
(10.sup.-7 /.degree.C.) 
result 
______________________________________ 
C (600.degree. C.) 
24 two out of six . . . cracks at (a) 
D (550.degree. C.) 
45 all (6/6) . . . no cracks 
E (595.degree. C.) 
95 five out of six . . . cracks at (b) 
______________________________________ 
Then, relationship between withstand voltage (KV/mm) and the length (l) of 
the glass sealant is checked in regard to the vitreous materials listed at 
Table 4. The experiment is carried out with the use of a spark plug 
corresponding to BPR4ES. 
In this experiment, voltage of 40 KV is applied to the section designated 
at (Y) of FIG. 7 under the ambient temperature of 500 degrees Celsius to 
check whether the glass sealant 40 is perforated or not. The result is 
shown at Table 8 in which it is represented by criss-cross when the glass 
sealant 40 is perforated, while it is represented by circle when the glass 
sealant is not perforated. 
It is noted that the withstand voltage is simply expressed by the product 
of insulation withstand voltage and the length (l). 
TABLE 8 
______________________________________ 
withstand 
vitreous voltage 1: length (mm) 
material (KV/mm) 0.5 1.0 1.5 2.0 2.5 3.0 
______________________________________ 
B.sub.2 O.sub.3, SiO.sub.2 - 
18.0 X X X O O O 
based glass A 
B.sub.2 O.sub.3, SiO.sub.2 - 
22.5 X X X O O O 
based glass B 
BaO- 22.0 X X X O O O 
based glass A 
BaO- 23.5 X X X O O O 
based glass B 
______________________________________ 
Now, FIGS. 9 through 11 shows another embodiment of the invention. 
A spark plug 101 comprising a center electrode 104, a tubular insulator 
102, a metallic shell 103 and a spiral thread 105 cut at an outer surface 
of the metallic shell 103. The insulator 102 is joint type including rear 
and front half pieces 108 and 106. The front half piece 106 is made form 
ceramic material of good thermal conductivity such as beryllium oxide 
(BeO) and aluminum nitride (AlN), each of which has transparent property. 
The rear half piece 108 is made of alumina (Al.sub.2 O.sub.3) on the other 
hand. 
Such is the structure of the front half piece 106 that the front half piece 
106 permit to release the heat so as to prevent preignition even when the 
piece 106 is exposed to high temperature gas in combustion chamber. 
Expensive material of aluminum nitride (AlN) is used for the front half 
piece 106, thus contributing cost-saving as a whole. The rear and front 
half pieces 108 and 106 are bonded at 107 by means of oxidation soldering, 
alumina cement or glass sealant. At the portion 107, the length of 
projection 109 falls within the range from 0.5 mm to 8.0 mm to ensure high 
voltage insulation, and ready manufacturing as seen FIGS. 10 and 11. 
When the thermal expansion of the front half piece 106 is greater than that 
of the rear half piece 108, the two pieces 106 and 108 are joined as shown 
in FIG. 10. When the thermal expansion of the front half piece 106 is 
smaller than that of the rear half piece 108, the two pieces 106 and 108 
are joined as shown in FIG. 11. 
It is noted that a resistor 112 is placed at a center bore 112a of the rear 
half piece 108 with the resistor 112 sandwiched between a terminal 113 and 
a center electrode 104 by way of an electrically conductive glass 111 and 
111a. 
It will be understand that various changes and modifications may be made in 
the above described structures which provide the characteristics of this 
invention without departing from the spirit thereof.