Dual polarity type ignition system for a spark plug group

In a dual polarity type ignition system for a spark plug group, a cylindrical metal shell is provided in which an insulator is provided. The insulator has an axial bore in which a center electrode is provided whose front end has a first noble metal tip. A ground electrode extends from a front end of the metal shell and having a second noble metal tip to form a spark discharge gap between the first noble metal tip and the second noble metal tip. The group of the spark plugs is divided into two groups, one is a positive polarity spark plug group in which a positive high voltage is applied to the center electrode, and the other group is a negative polarity spark plug group in which a negative high voltage is applied to the center electrode. The first noble metal tip of the center electrode of the positive polarity spark plug group is dimensionally smaller than the first noble metal tip of the center electrode of the negative polarity spark plug group. The second noble metal tip of the ground electrode of the negative polarity spark plug group is dimensionally smaller than the second noble metal tip of the ground electrode of the positive polarity spark plug group.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to a dual polarity type ignition system for a spark 
plug group in which a noble metal tip is secured to an electrode to 
advantageously improve a spark erosion resistance property, and 
particularly relates to a spark plug which is energized with a dual 
polarity type ignition device as a power source. 
2. Description of Prior Art 
In a dual polarity type distributorless ignition device (DLI), a row of 
spark plugs are categorically divided into two groups, one is a group in 
which a terminal is connected to a positive high voltage terminal of a 
secondary coil in an ignition coil, while the other is a group in which 
the terminal is connected to a negative high voltage terminal of the 
secondary coil in the ignition coil. In each of the groups of the spark 
plugs, spark plugs having the same structure have been incorporated into 
the ignition device. 
On the other hand, a Pt-related noble metal tip has been used on a firing 
portion of a center and ground electrode to exhibit a spark erosion 
resistant property. In a platinum spark plug in which the Pt-related metal 
tip is provided, it is possible to prevent a spark gap from inadvertently 
increasing due to the spattering action in which the firing portion would 
be spark eroded so that a part of the firing portion is gradually 
dissipated. An experimental test result showed that the durability in 
terms of the spark erosion had been improved from approx. 30000 km to 
100000 km. However, the Pt-related noble metal is generally very 
expensive. 
Upon incoporating the platinum spark plug into the dual polarity type 
distributorless ignition device (DLI), the same dimensional Pt-related 
metal tip has been used indiscriminately regardless of whether the spark 
plug is connected to a negative or positive polarity side. 
In the platinum spark plug incoporated into distributorless ignition device 
(DLI), a ground electrode of the spark plug group in which the positive 
high voltage is applied to the center electrode, is spark eroded faster 
than that in which the negative high voltage is applied to a center 
electrode. The center electrode of the spark plug group in which the 
negative high voltage is applied to a center electrode, is spark eroded 
faster than that in which the positive high voltage is applied to the 
center electrode. 
Despite the fact that the Pt-related metal tip is unacceptably eroded at an 
end of the serviceable period in the ground electrode of the spark plug 
group in which the positive high voltage is applied to the center 
electrode, the Pt-related metal tip is only slightly eroded in the ground 
electrode of the other spark plug group in which the negative high voltage 
is applied to the center electrode. 
The same is true in spite of the Pt-related metal tip being unacceptably 
eroded in the center electrode of the spark plug group in which the 
negative high voltage is applied to the center electrode, the Pt-related 
metal tip is only slightly eroded in the center electrode of the other 
spark plug group in which the positive high voltage is applied to the 
center electrode. 
For this reason, the platinum spark plug is wastefully replaced with a new 
one although the expensive Pt-related metal tip sufficiently remains on 
the center or ground electrode in the specified spark plug group. 
Therefore it is a main object of the present invention to provide a dual 
polarity type ignition system for a spark plug group which is capable of 
leveling off the spark erosion of a noble metal tip irrespective of 
whether a negative or positive high voltage is applied to a center 
electrode, thereby insuring an economical use of the expensive noble metal 
without sacrificing a good spark erosion resistant property. 
SUMMARY OF THE INVENTION 
According to the invention of a dual polarity type ignition system, a first 
noble metal tip of a center electrode of a spark plug (positive polarity 
group) to which a positive high voltage is applied, is spark eroded slower 
than that of a spark plug (negative polarity group) to which a negative 
high voltage is applied. On the other hand, a second noble metal tip of a 
ground electrode of the spark plug (negative polarity group) in which the 
negative high voltage is applied to the center electrode, is spark eroded 
slower than that of a spark plug (positive polarity group) in which the 
positive high voltage is applied to the center electrode. 
In view the above, it is found that the spark erosion resistant property is 
not affected substantially by making the first noble metal tip 
dimensionally smaller in which the positive high voltage is applied to the 
center electrode of the spark plug (positive polarity group) than that of 
a spark plug (negative polarity group) in which the negative high voltage 
is applied to the center electrode. 
The same is true when making the second noble metal tip of the ground 
electrode of that of a spark plug (negative polarity group) dimensionally 
smaller in which the negative high voltage is applied to the center 
electrode than that of a spark plug (positive polarity group) in which the 
positive high voltage is applied to the center electrode. 
This makes it possible to reduce an amount of the noble metal in which the 
electrode is slowly eroded, and thus decreases an entire amount of the 
noble metal used for the spark plug so as to contribute to cost reduction 
of the spark plug and the ignition system without losing a good spark 
erosion resistance. 
This holds true when devoid of the second noble metal tip of the ground 
electrode of the spark plug in which the negative high voltage is applied 
to the center electrode. 
According to another aspect of the invention of a dual polarity type 
ignition system, a first noble metal alloy tip of a center electrode of a 
spark plug (positive polarity group) to which a positive high voltage is 
applied, is spark eroded slower than that of a spark plug (negative 
polarity group) to which a negative high voltage is applied. On the other 
hand, a second noble alloy metal tip of a spark plug in which the negative 
high voltage is applied to the center electrode, is spark eroded slower 
than that of a spark plug to which the positive high voltage is applied to 
the center electrode. 
In view the above, it is found that the spark erosion resistant property is 
not affected substantially by making a noble metal component of the first 
noble metal alloy tip smaller in which the positive high voltage is 
applied to the center electrode than that of a spark plug in which the 
negative high voltage is applied to the center electrode. 
The same is true when making a noble metal component of the second noble 
metal alloy tip of the ground electrode smaller in which the negative high 
voltage is applied to the center electrode than that of a spark plug in 
which the positive high voltage is applied to the center electrode. 
This makes it possible to reduce the noble metal component of the noble 
metal alloy tip in which the electrode is eroded slower, and thus 
decreases an entire amount of the noble metal used for the spark plug so 
as to contribute to cost reduction of the spark plug and the ignition 
system without losing a good spark erosion resistance. 
According to another aspect of the invention of a dual polarity type 
ignition system, when used by combining a mono-gap type spark plug and a 
multi-gap type spark plug, a negative high voltage is applied to the 
center electrode in the mono-gap type spark plug, while a positive high 
voltage is applied to the center electrode in the multi-gap type spark 
plug. 
In the multi-gap type spark plug, a noble metal tip or a noble metal alloy 
tip is secured to an elevational side of a front end of the center 
electrode to which the positive high voltage is applied. In this instance, 
the noble metal tip provided on the ground electrodes can be omitted 
because the spark erosion is shared by the pluralistic ground electrodes. 
In the mono-gap type spark plug, a noble metal tip is provided preferably 
on both the center electrode and the parallel type ground electrode to 
reduce the spark erosion of the center and ground electrode to which a 
negative and positive high voltage is in turn applied. 
This makes it possible to decrease an entire amount of the noble metal used 
for the spark plug so as to contribute to cost reduction without losing a 
good spark erosion resistance. 
According to still another aspect of the invention of a dual polarity type 
ignition system, a first noble metal tip of a center electrode of a spark 
plug to which a positive high voltage is applied, is spark eroded slower 
than a second noble metal tip of a ground electrode, and a second noble 
metal tip of a spark plug to which a positive high voltage is applied to a 
center electrode, is spark eroded slower than a first noble metal tip of a 
center electrode. 
This makes it possible to decrease an amount of the noble metal tip of the 
center electrode of the spark plug to which the high positive voltage is 
applied more than that of the ground electrode without losing a good spark 
erosion resistant property. This holds true when no noble metal tip is 
provided on the center electrode of the spark plug in which the high 
positive voltage is applied to the center electrode. 
This also makes it possible to decrease an amount of the noble metal tip of 
the ground electrode of the spark plug in which the high negative voltage 
is applied to the center electrode more than that of the center electrode 
without losing a good spark erosion resistant property. This holds true 
when no noble metal tip is provided on the ground electrode of the spark 
plug in which the high negative voltage is applied to the center 
electrode. 
With the smaller amount or no amount of the noble metal of the electrode 
which is spark eroded slowly, this makes it possible to decrease an entire 
amount of the noble metal so as to contribute to cost reduction of the 
spark plug and the ignition system without losing a good spark erosion 
resistance. 
According to another aspect of the invention of a dual polarity type 
ignition system, a first noble metal alloy tip of a center electrode of a 
spark plug to which a positive high voltage is applied is spark eroded 
slower than a second noble metal alloy tip of a ground electrode, and a 
second noble alloy metal tip of a spark plug to which a positive high 
voltage is applied to a center electrode is spark eroded slower than a 
first noble metal tip of a center electrode. 
This makes it possible to decrease a noble metal component of the noble 
metal alloy tip of the center electrode of the spark plug in which the 
high positive voltage is applied to the center electrode more than that of 
the ground electrode without losing a good spark erosion resistant 
property. This holds true when no noble metal alloy tip is provided on the 
center electrode of the spark plug to which the high positive voltage is 
applied. 
This also makes it possible to decrease an amount of the noble metal 
component of the ground electrode of the spark plug in which the high 
negative voltage is applied to the center electrode more than that of the 
center electrode without losing a good spark erosion resistant property. 
This holds true when no noble metal alloy tip is provided on the ground 
electrode of the spark plug in which the high negative voltage is applied 
to the center electrode. 
With the smaller amount or no amount of noble metal of the electrode which 
is spark eroded slowly, it is possible to decrease an entire amount of the 
noble metal so as to contribute to cost reduction of the spark plug and 
the ignition system without losing a good spark erosion resistance. 
According to still another aspect of the invention of a dual polarity type 
ignition system, it is possible to eliminate or reduce an amount of a 
noble metal tip of a center electrode of a spark plug to which a positive 
high voltage is applied more than that of the ground electrode without 
losing a good spark erosion resistant property. 
On the other hand, it is possible to eliminate or reduce an amount of a 
noble metal tip of a ground electrode of a spark plug to which a negative 
high voltage is applied to the center electrode more than that of the 
ground electrode of the spark plug to which a high positive voltage is 
applied to the center electrode without losing a good spark erosion 
resistant property. 
With the smaller amount or no amount of noble metal of the electrode which 
is spark eroded slowly, it is possible to decrease an entire amount of the 
noble metal so as to contribute to cost reduction of the spark plug and 
the ignition system without losing a good spark erosion resistance. 
According to still another aspect of the invention of a dual polarity type 
ignition system, it is possible to eliminate or reduce a noble metal 
component of the noble metal alloy tip of a center electrode of a spark 
plug to which a positive high voltage is applied more than that of the 
center electrode of a spark plug to which a high negative voltage is 
applied without losing a good spark erosion resistant property. 
On the other hand, it is possible to eliminate or reduce a noble metal 
component of the noble metal alloy tip of a ground electrode of a spark 
plug to which a negative high voltage is applied to the center electrode 
more than that of the ground electrode of a spark plug in which a high 
positive voltage is applied to the center electrode without losing a good 
spark erosion resistant property. 
With the smaller amount or no amount of noble metal of the electrode, it is 
possible to decrease an entire amount of the noble metal so as to 
contribute to cost reduction of the spark plug and the ignition system 
without losing a good spark erosion resistance. 
According to another aspect of the invention of a dual polarity type 
ignition system, it is possible to eliminate or reduce a noble metal 
component of the noble metal alloy tip (or an amount of the noble metal 
tip) of a center electrode of a spark plug to which a positive high 
voltage is applied more than that of the center electrode to which a 
negative high voltage is applied without losing a good spark erosion 
resistant property. 
On the other hand, it is possible to eliminate or reduce a noble metal 
component of the noble metal alloy tip (or an amount of the noble metal 
tip) of a ground electrode of a spark plug in which a negative high 
voltage is applied to the center electrode more than that of the ground 
electrode of a spark plug in which a positive high voltage is applied to 
the center electrode without losing a good spark erosion resistant 
property. 
With the smaller amount or no amount of noble metal tip of the electrode, 
it is possible to decrease an entire amount of the noble metal so as to 
contribute to cost reduction of the spark plug and the ignition system 
without losing a good spark erosion resistance. 
According to another aspect of the invention of a dual polarity type 
ignition system, since the center electrode of a multi-gap type spark plug 
to which a negative high voltage is applied, is spark eroded faster than 
that to which a positive high voltage is applied, a noble metal tip is 
provided on the center electrode to which the negative high voltage is 
applied. In this instance, it is possible to obviate the noble metal tip 
provided on the ground electrodes because the spark erosion is shared by 
each of the pluralistic ground electrodes. 
This also makes it possible to decrease an entire amount of the noble metal 
used to the multi-gap type spark plug so as to contribute to cost 
reduction without losing a good spark erosion resistant property.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION 
Referring to FIGS. 1 and 2 which shows a mono-gap type spark plug B (C) 
having a dual polarity ignition source according to a first embodiment of 
the invention, the spark plug B (C) has a cylindrical metal shell 1 and an 
insulator 2 provided in the metal shell 1 in the a front end 20 of the 
insulator 2 extends from a front end 11 of the metal shell 1. The 
insulator 2 has an axial bore 21 in which a center electrode 4 is fixedly 
place so that its front end 41 extends from a front end 201 of the 
insulator 2. On a front end surface of the center electrode 4, a noble 
metal tip 3 is fixedly placed as described In detail hereinafter. To the 
front end 11 of the metal shell 1, a parallel type ground electrode 6 is 
welded whose front inner side 61 has a noble metal tip 5 to form a spark 
discharge gap Gp with the noble metal tip 3 of the center electrode 4. 
The spark plug B (C)thus structured is to be mounted on a cylinder head of 
an internal combustion engine (each not shown) via a gasket 131. Numeral 
71 shows a terminal electrode, numerals 72, 73 a glass sealant, numeral 74 
a resistor element. 
For the purpose of convenience, the denotation B represents the mono-gap 
type spark plug in which a negative high voltage is applied to the center 
electrode 4 (negative polarity side group), while the denotation C 
represents the mono-gap type spark plug in which a positive high voltage 
is applied to the center electrode 4 (positive polarity side group) 
according to the present and subsequent embodiments of the invention. When 
using the denotation A, it represents a comparable mono-gap type spark 
plug which has been used with no consideration taken with respect to 
adjusting an amount of the noble metal depending on whether the center 
electrode 4 is in the negative or positive polarity side. 
The metal shell 1 is made of a low carbon steel whose outer surface 120 has 
a threaded portion 12. The metal shell 1 further has a barrel portion 13 
and a hexagon 14. The barrel portion has the gasket 131 at the boundary 
with the threaded portion 12. The hexagon 14 is used to mount the metal 
shell 1 on the cylinder head by means of a wrench. 
The insulator 2 is made from a ceramic material with alumina as a main 
constituent, and having an insulator nose 22, a diameter-increased portion 
23 and a tubular head 24 whose outer surface forms a corrugation portion 
241. The insulator nose 22 is surrounded by the threaded portion of the 
metal shell 1, and the diameter-increased portion 23 is surrounded by the 
barrel portion 13. The axial bore 21 runs through an entire length of the 
insulator 2. 
The insulator 2 is firmly placed in the metal shell 1 by resting a seat 
portion 221 on a tapered shoulder portion 123 of a ledge portion 122 of 
the metal shell 1 via a metallic packing 121. The insulator 2 is 
stabilized by caulking a rear end fin 141 tightly against the insulator 2 
to hermetically seal the insulator 2 by means of O-rings 142, 143 and a 
talc sealant 144. 
The center electrode 4 is made of a nickel-based alloy in which a copper or 
silver metal core 40 is embedded. The center electrode 4 has a flange 
portion 42 and an elongation portion 43 and a frusto-cone shaped portion 
44 which extends forward from the elongation portion 43. On a front end 
surface of the frusto-cone shaped portion 44, the noble metal tip 3 is 
fixedly placed. When referring to the front end of the center electrode 4, 
it includes a part of the elongation portion 43, the frusto-cone shaped 
portion 44, and the noble metal tip 3. An extension of the center 
electrode 4 from the front end 201 of the insulator 2 is 1.5 mm in length, 
and the spark discharge gap Gp is 1.0 mm in width. 
The noble metal tip 3 is made of a Pt-based alloy containing 20% Ir by 
weight, and having the following dimension. 
The noble metal tip 3 measures 0.8 mm in diameter and 0.5 mm in thickess 
when referring to the mono-gap type spark plug B. 
The noble metal tip 3 measures 0.6 mm in diameter and 0.2 mm in thickess 
when referring to the mono-gap type spark plug C. 
The noble metal tip 3 measures 0.8 mm in diameter and 0.5 mm in thickess 
when referring to the mono-gap type spark plug A. 
In the meanwhile, the parallel type ground electrode 6 is formed into 
L-shaped configuration whose front inner side 61 has the noble metal tip 5 
facing a front end surface 31 of the noble metal tip 3 of the center 
electrode 4. 
The noble metal tip 5 is made of a Pt-based alloy containing 20% Ir by 
weight in the same component of the noble metal tip 3, and having the 
following particulars. 
The noble metal tip 5 measures 0.5 mm in diameter and 0.2 mm in thickess 
when referring to the mono-gap type spark plug B. 
The noble metal tip 5 measures 0.9 mm in diameter and 0.4 mm in thickess 
when referring to the mono-gap type spark plug C. 
The noble metal tip 5 measures 0.9 mm in diameter and 0.4 mm in thickess 
when referring to the mono-gap type spark plug A. 
A spark erosion resistance experimental test was carried out to compare the 
mono-gap type spark plug B (C) to a mono-gap type spark plug H in which no 
noble metal tip (3, 5) was provided. 
The spark discharge gap and the raw material of the mono-gap type spark 
plug H is the same as those of mono-gap type spark plug B (C) except that 
no noble metal tip is provided with the mono-gap type spark plug H which 
has a straight type center electrode (2.5 mm in dia.). 
Upon carrying out the experimental test, the spark plugs A, B, C, H were 
mounted on a 3000 cc, V-type six-cylinder engine with the use of dual 
polarity type ignition device (DLI) in a dual polarity type ignition 
system to run the engine at 5500 rpm.times.W.O.T. (full throttle 
condition) in the following combination. 
As shown in FIG. 3, no significant difference was found in the spark 
discharge gap increase in terms of the spark erosion speed between of the 
mono-gap type spark plug B (negative polarity group) in which the center 
electrode is in the negative polarity side and the mono-gap type spark 
plug A which belongs to the dual polarity group. 
As shown in FIG. 4, likewise, no significant difference was found in the 
spark erosion rate between the mono-gap type spark plug C (positive 
polarity group) in which the center electrode is in the positive polarity 
side and the mono-gap type spark plug A which belongs to the dual polarity 
group. 
FIG. 5 shows an amount of the noble metal used for each of the spark plugs 
A. B, C, When the amount of the noble metal used to the spark plug A is 
converted to 1.0, the amount of the noble metal used for the spark plugs B 
and C in turn comes approximately to 0.57 and 0.61. 
With the adoption of the mono-gap type spark plug B in which the center 
electrode is in the negative side and the mono-gap type spark plug C 
(positive polarity group) in which the center electrode is in the positive 
side, it is found that the noble metal tip needs only 59.5% of the noble 
metal used when the mono-gap type spark plug A is uniformly adopted. This 
makes it possible to reduce the price of the product without losing a good 
spark erosion resistant property which is insured substantially when the 
mono-gap type spark plug A is uniformly used. 
It is to be noted that it is possible to obviate the noble metal tip 
provided on the center electrode 4 of the mono-gap type spark plug C 
(positive polarity group) without losing the good spark erosion resistant 
property as obtained in the first embodiment of the invention. 
In reference to FIGS. 1 and 2 which also depict mono-gap type spark plug B2 
(C2) according to a second embodiment of the invention, the center 
electrode 4 in the mono-gap type spark plug B2 (negative polarity group) 
is in the negative polarity side, and the center electrode 4 in the 
mono-gap type spark plug C2 (positive polarity group) is in the positive 
polarity side. 
With respect to particular dimensions, the spark discharge gap and the raw 
material of the mono-gap type spark plug B2 (C2) are the same as those of 
mono-gap type spark plug B (C). 
The noble metal tip 3 measures 0.8 mm in diameter and 0.5 mm in thickness. 
The noble metal tip 3 is made of Pt-based alloy having the following 
constituents. 
The Pt-based alloy contains 5% nickel by weight when referring to the 
mono-gap type spark plug B2. 
The Pt-based alloy contains 20% nickel by weight when referring to the 
mono-gap type spark plug C2. 
The Pt-based alloy contains 5% nickel by weight when referring to the 
mono-gap type spark plug A2. 
The noble metal tip 5 has the same material as the noble metal tip 3, and 
measures 0.9 mm in diameter and 0.4 mm in thickness. 
The noble metal tip 5 is made of Pt-based alloy having the following 
constituents. 
The Pt-based alloy contains 30% nickel by weight when referring to the 
mono-gap type spark plug B2. 
The Pt-based alloy contains 10% nickel by weight when referring to the 
mono-gap type spark plug C2. 
The Pt-based alloy contains 10% nickel by weight when referring to the 
mono-gap type spark plug A2. 
The spark erosion resistance experimental test was carried out in the same 
manner as described in the first embodiment of the invention. 
Upon carrying out the experimental test, the spark plugs B2, C2, A2 were 
mounted on a 3000 cc, V-type six-cylinder engine with the use of a dual 
polarity type device in a dual polarity type ignition system to run the 
engine at 5500 rpm.times.W.O.T. (full throttle condition) in the following 
combination. 
The result shows that no significant difference was found in the spark 
discharge gap increase in terms of the spark erosion rate between the 
mono-gap type spark plug B2 in which the center electrode is in the 
negative polarity side and the mono-gap type spark plug A2 in which the 
center electrode is in the negative polarity. 
The result further shows that no significant difference was found in the 
spark erosion rate between the mono-gap type spark plug C2 in which the 
center electrode is in the positive polarity side and the mono-gap type 
spark plug A2 in which the center electrode is in the positive polarity. 
With the combinatorial use of the mono-gap type spark plug B2 (negative 
polarity group) in which the center electrode is in the negative side with 
greater amount of the noble metal component and the mono-gap type spark 
plug C2 (positive polarity group) in which the center electrode is in the 
positive side with smaller amount of the noble metal component, it is 
possible to reduce the amount of the noble metal compared to that required 
when the mono-gap type spark plug A2 is uniformly adopted. This makes it 
possible to reduce the price of the product without losing a good spark 
erosion resistant property which is obtained substantially when the 
mono-gap type spark plug A2 is uniformly used. 
It should be noted that it is possible to omit the noble metal tip provided 
on the center electrode 4 of the mono-gap type spark plug C,C2 (positive 
polarity group) without losing the good spark erosion resistant property 
as achieved by the second embodiment of the invention. 
In reference to FIGS. 1, 2 and 8 which show a third embodiment of the 
invention, the mono-gap type spark plug B (negative polarity group) is 
adopted in which the center electrode 4 is in the negative side, and a 
multi-gap type spark plug F (positive polarity group) is adopted in which 
the center electrode 4 is in the positive side. 
The multi-gap type spark plug F has the metal shell 1, the insulator 2 and 
the center electrode 4 whose front end 41 extends from the front end 201 
of the insulator 2. As designated by numeral 62, three ground electrodes 
extend from the front end 11 of the metal shell 1. Each of the front end 
surfaces 621 of the ground electrodes 62 has the noble metal tip 622 which 
faces an elevational side 411 of the front end 41 of the center electrode 
4. 
The noble petal tip 622 is made of Pt-based alloy containing 20% Ir by 
weight, and measures 0.9 mm in diameter and 0.4 in thickness. In this 
Instance, the noble metal tip may be provided around the front end 41 of 
the center electrode 4 along its entire circumferential length. 
The spark erosion resistance experimental test was carried out in the same 
manner as described in the first embodiment of the invention. 
Upon carrying out the experimental test, the spark plugs B, F were mounted 
on a 3000 cc, V-type six-cylinder engine with the use of a dual polarity 
type device (DLI) in a dual polarity type ignition system to run the 
engine at 5500 rpm.times.W.O.T. (full throttle condition) in the following 
combination. 
The result shows that no significant difference was found in the spark 
discharge gap increase in terms of the spark erosion rate between the 
mono-gap type spark plug B (negative polarity group) in which the center 
electrode is in the negative polarity side and the mono-gap type spark 
plug A (dual polarity group) in which the center electrode is in the 
negative polarity. 
The result also shows that no significant difference was found in the spark 
erosion rate between the multi-gap type spark plug F (positive polarity 
group) in which the center electrode is in the positive polarity side and 
the mono-gap type spark plug A (dual polarity group) in which the center 
electrode is in the positive polarity. 
With the combinatorial use of the mono-gap type spark plug B in which the 
center electrode is in the negative side and the multi-gap type spark plug 
F in which the center electrode is in the positive side, it is possible to 
reduce the amount of the noble metal compared to that required when the 
mono-gap type spark plug A is uniformly adopted. This makes it possible to 
reduce the price of the product without losing a good spark erosion 
resistance property which is achieved substantially when the mono-gap type 
spark plug A is uniformly used. 
It should be observed that it is possible to omit the noble metal tip 
provided on the center electrode 4 of the multi-gap type spark plug F 
(positive polarity group). It is also possible to omit the noble metal tip 
provided on the ground electrode of the mono-gap type spark plug B 
(negative polarity group). It is possible to combinatorially use these two 
spark plugs without losing the good spark erosion resistant property as 
insured by the third embodiment of the invention. 
In further reference to FIGS. 6, 7 which show a fourth embodiment of the 
invention, the multi-gap type spark plug D is adopted in which the center 
electrode 4 is in the negative side, and a multi-gap type spark plug E is 
used in which the center electrode 4 is in the positive side. 
The multi-gap type spark plug E has the metal shell 1, the insulator 2 and 
the center electrode 4 whose front end 41 extends from the front end 201 
of the insulator 2. As designated by numeral 62, three ground electrodes 
extend from the front end 11 of the metal shell 1 to make their front end 
surface 621 face an elevational side 411 of the front end 41 of the center 
electrode 4. 
The spark erosion resistance experimental test was carried out in the same 
manner as described in the first embodiment of the invention. 
Upon carrying out the experimental test, the multi-polarity type spark 
plugs D, E were mounted on a 3000 cc, six-cylinder engine with the use of 
a dual polarity type device (DLI) in a dual polarity type ignition system 
to run the engine at 5500 rpm.times.W.O.T. (full throttle condition) in 
the following combination. 
The result shows that no significant difference was found in spark 
discharge gap increase in terms of the spark erosion rate between the 
multi-gap type spark plug D (negative polarity group) in which the center 
electrode is in the negative polarity side and a multi-gap type spark plug 
(not shown) in which the noble metal tip is provided on both the 
electrodes, and the center electrode is in the negative polarity. 
The result also shows that no significant difference was found in the spark 
erosion rate between the multi-gap type spark plug E (positive polarity 
group) in which the center electrode is in the positive polarity side and 
the multi-gap type spark plug (not shown) in which the noble metal tip is 
provided on both the electrodes, and the center electrode is in the 
positive polarity. 
With the combinatorial adoption of the multi-gap type spark plug D 
(negative polarity group) and multi-gap type spark plug E (positive 
polarity group), it is possible to reduce the amount of the noble metal 
compared to that required when the multi-gap type spark plug D is 
uniformly used to each of the cylinders of the internal combustion engine. 
This makes it also possible to reduce the price of the product without 
losing a good spark erosion resistant property which is substantially 
insured when the multi-gap type spark plug D is uniformly used to each of 
the cylinders of the internal combustion engine. 
In reference to FIGS. 6, 8 which also show a fifth embodiment of the 
invention, the multi-gap type spark plug D (negative polarity group) is 
adopted in which the center electrode 4 is in the negative side, and a 
multi-gap type spark plug F (positive polarity group) is used in which the 
center electrode 4 is in the positive side. In the multi-gap type spark 
plug F, the noble metal tips 622 are provided on the ground electrodes 
instead of the center electrode of the multi-gap type spark plug D. 
Reverting to FIG. 6, the multi-gap type spark plug D has the metal shell 1, 
the insulator 2 and the center electrode 4 whose front end 41 extends from 
the front end 201 of the insulator 2. The three ground electrodes 62, 
which extends from the front end 11 of the metal shell 1, have the front 
end surface 621 which faces the noble metal tip 51 provided on the 
elevational side 411 of the front end 41 of the center electrode 4. 
The noble metal tip 622 is made of Pt-based alloy containing 20% Ir by 
weight, and measures 0.9 mm in diameter and 0.4 in thickness. 
The spark erosion resistance experimental test was carried out in the same 
manner as described in the first embodiment of the invention. 
Upon carrying out the experimental test, the spark plugs D, F were mounted 
on a 3000 cc, V-type six-cylinder engine with the use of a dual polarity 
type device (DLI) in a dual polarity type ignition system to run the 
engine at 5500 rpm.times.W.O.T. (full throttle condition) in the following 
combination. 
The result shows that no significant difference was found in the spark 
discharge gap increase in terms of the spark erosion rate between the 
multi-gap type spark plug D, F (negative polarity group, positive polarity 
group) in which the center electrode is in the negative polarity side and 
a multi-gap type spark plug (not shown) in which the noble metal tip is 
provided on both the center electrode and the ground electrode. The same 
is true between the multi-gap type spark plug F (positive polarity group) 
and the multi-gap type spark plug (not shown) in which the noble metal tip 
is provided on both the center electrode and the ground electrode. 
With the combinatorial use of the multi-gap type spark plug D (negative 
polarity group) and the multi-gap type spark plug F (positive polarity 
group), it is possible to reduce the amount of the noble metal compared to 
that required in which the multi-gap type spark plug in which the noble 
metal tip is provided on both the center and ground electrode. This makes 
it possible to reduce the price of the product without losing a good spark 
erosion resistance property which is insured substantially when the 
multi-gap type spark plug is uniformly used in which the noble metal tip 
is provided on both the center electrode and the ground electrode. 
It is to be appreciated that the noble metal tip 622 used in the fifth 
embodiment of the invention may be made of Pt-Ni alloy metal as the same 
manner in the second embodiment of the invention. With this structure thus 
provided, it is possible to achieve the same effects as those mentioned in 
the fifth embodiment of the invention. 
In reference to FIGS. 1, 9 which show a sixth embodiment of the invention, 
a mono-gap type spark plug G (negative polarity group) is adopted in which 
the center electrode 4 is in the negative side, and the mono-gap type 
spark plug C (positive polarity group) is used in which the center 
electrode 4 is in the positive side. 
The mono-gap type spark plug G is structurally the same as the spark plug 
of FIGS. 1, 2 except that a noble metal tip 30 is provided only on the 
center electrode 4. The noble metal tip 30 is made of Pt-based alloy 
containing 20% Ir by weight, and measures 0.8 mm in diameter and 0.5 mm in 
thickness. 
The spark erosion resistance experimental test was carried out in the same 
manner as described in the first embodiment of the invention. 
Upon carrying out the experimental test, the spark plugs C, G were mounted 
on a 3000 cc, V-type six-cylinder engine with the use of a dual polarity 
type device (DLI) in a dual polarity type ignition system to run the 
engine at 5500 rpm.times.W.O.T. (full throttle condition) in the following 
combination. 
The result shows that no significant difference was found in the spark 
discharge gap increase in terms of the spark erosion rate between the 
mono-gap type spark plug G (negative polarity group) in which the center 
electrode is in the negative polarity side and a mono-gap type spark plug 
C (positive polarity group) in which the center electrode is in the 
positive polarity. 
With the combinatorial arrangement of the mono-gap type spark plug G and 
the mono-gap type spark plug C, it is possible to reduce the amount of the 
noble metal compared to that required when the mono-gap type spark plug C 
(alternatively A) is uniformly used to each of the cylinders of the 
internal combustion engine. This makes it also possible to reduce the 
price of the product without losing a good spark erosion resistant 
property which is substantially insured when the mono-gap type spark plug 
C is uniformly used to each of the cylinders of the internal combustion 
engine. 
It should be observed that it is possible to replaced the noble metal tip 
30 of the mono-gap type spark plug (negative polarity group) by the noble 
metal tip 3 provided on the center electrode of the mono-gap type spark 
plug B2 of the second embodiment of the invention, while at the same time, 
replacing the mono-gap type spark plug (positive polarity) by the mono-gap 
type spark plug C2 of the second embodiment of the invention. With the 
structure obtained above, it is possible to achieve the same effects as 
those mentioned in the sixth embodiment of the invention. 
FIG. 10 shows one example of the distributorless ignition device (DLI) in 
the dual polarity type ignition system for use in a V-type four-cylinder 
engine. In the ignition device (DLI) as designated at 100, each of 
ignition coils 101 has a primary coil 111 whose one end is connected via a 
power source VI, and whose other end connected to an interrupter member 
104 which includes a switching element 141 and a signal generator 142. 
From a secondary coil L2 of the ignition coil 101, a main line 112 leads 
through a diode 113 to the spark discharge gap Gp of the spark plug which 
is arranged with its polarity according to each of the aforementioned 
embodiments of the invention. For the purpose of convenience, the spark 
discharge gap is represented by the single denotation Gp regardless of 
whether the spark plug is mono-gap type or multi-gap type one in FIG. 10. 
It is to be noted that the noble metal tip may be made of not only Pt-Ir 
alloy and Pt-Ni alloy but Pt-Ir-Ni alloy, Ir-Ni, alloy Pt-Pd and the like 
as well. 
It is to be observed that in the dual polarity type ignition system, the 
dual polarity type DLI device can be used in which the number of the 
ignition coils is the same or half the number of the cylinders of the 
internal combustion engine. 
It is further to be observed that in the dual polarity type ignition 
system, the spark plugs used on a half side of the V-type engine have the 
same polarity to categorially unify the spark plugs used on half the 
number of the cylinder banks of the V-type engine so as to protect 
assembly workers from confusing them. 
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 limiting sense in as much as various modifications and additions to 
the specific embodiments may be made by skilled artisans without departing 
from the scope of the invention.