[0001] The present invention relates to a spark plug for use with an internal combustion
engine and to an ignition system for use with an internal combustion engine having
the spark plugs.
[0002] As shown in FIG. 11 of the accompanying drawings, an ignition system for use with
an automotive internal combustion engine having spark plugs has conventionally employed
a distributor. In an ignition system 249 of FIG. 11, an ignition coil 251 includes
a primary coil 252, which receives electricity from a battery 256 via an ignition
switch 257 and is connected to an igniter 254, and a secondary coil 253, which is
connected to a distributor 250. When an electronic control unit 255 issues a break
instruction signal to the igniter 254 at a predetermined firing timing, the igniter
254 causes a contactless switch unit to operate so as to interrupt current flowing
to the primary coil 252. As a result, a high-voltage current is induced in the secondary
coil 253. The distributor 250 distributes the induced current to spark plugs 100 through
high-tension cables C.
[0003] However, recently, the above-described distributor ignition system has been replaced
by a full-transistor type coil-on-plug ignition system (hereinafter referred to as
a "DLI" (Distributor-Less Ignition) system). The DLI system features easy control
of ignition timing and does not require maintenance of contacts. In the DLI system,
an ignition coil is mounted directly on each spark plug. A control unit interrupts
current flowing to the primary coil of the ignition coil of each spark plug at a predetermined
timing to thereby fire the spark plug. Since ignition coils are mounted directly on
the respective spark plugs, high-tension cables are not required.
[0004] Conventionally, in order to improve resistance to spark consumption of a spark plug,
a chip of Pt (platinum) serving as a spark portion is formed at one end of an electrode
of the spark plug. However, since Pt is expensive and the melting point thereof is
approximately 1769°C indicating that resistance to spark consumption of Pt is insufficient,
use of Ir (iridium), which has a melting point of approximately 2454°C, as material
for the chip has been proposed. However, a spark portion of Ir produces a volatile
oxide at a temperature of 900°C to 1000°C, indicating a tendency to be consumed within
this temperature range.
[0005] In a spark plug having a chip of an Ir-based material as a spark portion, employment
of the above-mentioned DLI system may have a significantly adverse effect on durability
of the spark portion. Specifically, spark discharge of a spark plug is generally classified,
according to form, into glow discharge and arc discharge. A glow discharge occurs,
for example, when the impedance of a power source (hereinafter referred to as a "power-source
impedance") is relatively high. Since a discharge current is relatively weak, the
glow discharge causes a less severe temperature increase and less consumption of the
spark portion. By contrast, an arc discharge often occurs when a power-source impedance
is relatively low. Accordingly, a strong discharge current tends to flow, causing
a considerable temperature increase in the spark portion with a resultant advancement
of consumption of the spark portion. Therefore, from the viewpoint of suppression
of consumption of the spark portion, glow discharge is desirably dominant in a spark
discharge.
[0006] In the distributor ignition system, the power-source impedance is high because of
the electric resistances of a contact gap and a high-tension cable. Accordingly, glow
discharge is dominant in a spark discharge. However, in the DLI system, the power-source
impedance is low, since the electric resistances of a contact gap and a high-tension
cable are not present. Accordingly, depending on the material used for an electrode,
the rate of transition from glow discharge to arc discharge increases in a spark discharge,
potentially causing consumption of the electrode. According to a study conducted by
the inventors of the present invention, a spark portion of an Ir-based material exhibits
a particularly high rate of transition from glow discharge to arc discharge, potentially
shortening spark plug life. This tendency is further accelerated by consumption of
the spark portion caused by volatilization through oxidation.
[0007] Further, Japanese Patent Application Laid-Open No. 7-50192 (USP 5,514,929) describes
that when a spark plug with a tip mainly formed of Ir is used in a gas engine, the
energy of induced discharge can be decreased by use of a resister having a resistance
not less than 50 kΩ but not greater than 200 kΩ. However, although such a gas engine
would not have a problem in relation to ignitability even when the discharge energy
decreases, a gasoline engine would have a problem in relation to ignitability when
the discharge energy decreases.
[0008] A first object of the present invention is to provide a spark plug in which an arc
discharge becomes unlikely to occur in spite of a spark portion being formed from
an Ir-based metal, to thereby suppress consumption of an electrode and deterioration
of ignitability.
[0009] A second object of the present invention is to provide an ignition system for use
with an internal combustion engine having the spark plugs.
[0010] To achieve the first object, the present invention provides a spark plug comprising:
a center electrode; an insulator which surrounds the center electrode; a metallic
shell which surrounds the insulator; a ground electrode which faces the center electrode;
and a spark portion which is fixedly attached to at least either one of the center
electrode and the ground electrode to thereby define a spark discharge gap. The spark
portion is formed from a metal which contains not less than 60% by weight Ir. The
spark plug further comprises a metallic terminal fixedly attached into one end portion
of a through-hole formed axially in the insulator, the center electrode being fixedly
attached into the other end portion of the through-hole; and a resistor disposed within
the through-hole and between the metallic terminal and the center electrode so as
to establish an electric resistance of not less than 10 kΩ but not greater than 25
kΩ between the metallic terminal and the center electrode.
[0011] To achieve the second object, the present invention provides an ignition system for
use with an internal combustion engine comprising a spark plug and a coil unit.
[0012] The spark plug comprises a center electrode; an insulator which surrounds the center
electrode; a metallic shell which surrounds the insulator; a ground electrode which
faces the center electrode; and a spark portion which is fixedly attached to at least
either one of the center electrode and the ground electrode to thereby define a spark
discharge gap. The spark portion is formed from a metal which contains not less than
60% by weight Ir. The spark plug further comprises a metallic terminal fixedly attached
into one end portion of a through-hole formed axially in the insulator, the center
electrode being fixedly attached into the other end portion of the through-hole.
[0013] The coil unit comprises a casing attached to the spark plug; and an ignition coil
accommodated within the casing and connected to the metallic terminal of the spark
plug in order to apply a high voltage to the spark plug for effecting an electrical
discharge.
[0014] The ignition system further comprises a resistance portion disposed between the ignition
coil and the center electrode so as to establish an electric resistance of not less
than 10 kΩ but not greater than 25 kΩ between the ignition coil and the center electrode.
[0015] When the spark portion is formed from an Ir-based metal, the metal must contain Ir
in an amount of not less than 60% by weight; otherwise, the high melting point of
Ir fails to lead to sufficient improvement in resistance to spark consumption of the
spark portion. However, as described previously, in the DLI system, a high Ir content
of the spark portion tends to cause transition to a strong-current discharge, such
as an arc discharge. As a result, the temperature of the spark portion increases to
such a level that an Ir component volatilizes through oxidation, so that the spark
portion is consumed accordingly.
[0016] The present inventors conducted extensive studies and, as a result, found that even
in the DLI system a spark plug whose spark portion is of the above-described Ir-based
metal (hereinafter may be referred to as an "Ir-type plug") stably maintains an electrical
discharge with a relatively weak current, such as a glow discharge, through establishment
of an electric resistance of not less than 10 kΩ (corresponding to a power-source
impedance) between the ignition coil and the center electrode. On the basis of this
finding, the present invention has been achieved. Through establishment of such an
electric resistance, even when the Ir-type plugs are employed in the DLI system, transition
to a strong-current discharge, such as an arc discharge, becomes unlikely to occur.
Thus, even at high-speed or heavy-load operation, consumption of the spark portion
caused by volatilization of Ir through oxidation can be suppressed, thereby extending
spark plug life. Notably, electric resistance as measured between the ignition coil
and the center electrode is preferably not less than 15 kΩ. However, if the electric
resistance is in excess of 25 kΩ, ignitability may be impaired.
[0017] In order to establish an electric resistance of not less than 10 kQ but not greater
than 25 kΩ between the ignition coil and the center electrode, there may be utilized
a resistor incorporated in a spark plug and adapted to reduce radio noise. In this
case, the electric resistance of the resistor may be increased such that an electric
resistance of not less than 10 kΩ (preferably not less than 15 kΩ) but not greater
than 25 kΩ is established between the metallic terminal and the center electrode.
When the resistor is not incorporated as in the case of an inexpensive, popular spark
plug, a resistance portion, such as a resistor, may be provided in the coil unit such
that an electric resistance of the above-mentioned range is established between the
ignition coil and the center electrode.
[0018] In a spark plug, as the diameter of an end portion of the center electrode decreases,
the volume of the end portion decreases. As a result, the end portion of the center
electrode absorbs less heat from ignited flame, thereby improving ignitability. In
the spark plug or ignition system of the present invention, in which the spark portion
of the above-described Ir-based metal is formed at an end portion of the center electrode,
the diameter of the end portion is preferably adjusted to not greater than 1.1 mm.
By rendering the diameter of the end portion not greater than 1.1 mm, ignitability
is improved significantly. More preferably, the diameter of the end portion is adjusted
to 0.3 mm to 0.8 mm. By rendering the diameter of the end portion not greater than
0.8 mm, ignitability is further improved. When the diameter of the end portion becomes
less than 0.3 mm, the temperature of the spark portion tends to increase due to spark
concentration. As a result, the spark portion tends to be consumed due to volatilization
of Ir through oxidation.
[0019] Generally, in a spark plug, the metallic shell surrounds the insulator. When the
surface of the insulator becomes contaminated due to, for example, soot or fuel adhesion,
a spark occurs between the inner surface of the metallic shell and the outer surface
of the insulator, potentially hindering a normal generation of electrical discharge
across a spark discharge gap. Decreasing the spark discharge gap is an effective way
to maintain normal electrical discharge across the gap when the surface of the insulator
becomes contaminated. In order to maintain resistance to contamination of the spark
plug, the spark discharge gap is preferably set to not greater than 1.2 mm, more preferably
not greater than 0.8 mm. In order to prevent the occurrence of a short circuit across
the gap, the spark discharge gap is preferably set to not less than 0.3 mm.
[0020] Embodiments of the invention will be further described by way of example only with
reference to the accompanying drawings, in which:
FIG. 1 is a longitudinal, partially sectional view of a spark plug according to an
embodiment of the present invention;
FIG. 2 is an enlarged sectional view of portions of the spark plug of FIG. 1 located
in the vicinity of a spark discharge gap;
FIG. 3 is a circuit diagram showing an ignition system example employing the spark
plugs of FIG. 1;
FIG. 4 is a schematic front view showing the ignition system of FIG. 3 mounted on
an engine;
FIG. 5 is a graph showing the results of a test for a gap-increasing behavior conducted
on the spark plugs of FIG. 1;
FIG. 6A is a graph showing the waveform of one electrical discharge;
FIG. 6B is a graph showing the waveform of another electrical discharge;
FIG. 7 is a graph showing the effects of spark gap and electric resistance on the
frequency of transition from glow discharge to arc discharge;
FIG. 8 is a graph showing the frequency of transition from glow discharge to arc discharge
as measured with respect to the ignition system of FIG. 3 and an ignition system of
FIG. 11;
FIG. 9 is a graph showing the relationship between a consumed volume of an electrodc
and an electrode diameter;
FIG. 10 is a graph showing the behavior of gap increase with operating hours;
FIG. 11 is a circuit diagram showing a distributor ignition system;
FIG. 12 is a graph showing the relationship between the number of durability cycles
and a spark gap; and
FIG. 13 is a graph showing the relationship between resistance and ignitable limit.
[0021] Embodiments of the present invention will next be described in detail with reference
to the drawings.
[0022] FIG. 1 shows a spark plug 100, into which a resistor is incorporated, according to
an embodiment of the present invention. The spark plug 100 includes a cylindrical
metallic shell 1; an insulator 2, which is fitted into the metallic shell such that
a tip end portion is projected from the metallic shell 1; a center electrode 3, which
is provided within the insulator 2 such that a tip end is projected from the insulator
2; and a ground electrode 4, which is disposed such that one end is connected to the
metallic shell 1, and the other end faces the tip end of the center electrode 3. As
shown in FIG. 2, a spark portion 32 is formed on the ground electrode 4 in such a
manner as to face a spark portion 31 of the center electrode 3. The facing spark portions
31 and 32 define a spark discharge gap g therebetween.
[0023] The insulator 2 is formed from a ceramic sintered body, such as alumina or aluminum
nitride. The metallic shell 1 is formed from, for example, low-carbon steel and serves
as the housing of the spark plug 100. A screw portion 7 is formed on the outer surface
of the metallic shell 1 and is adapted to attach the spark plug 100 to an unillustrated
engine block. The designation of the screw portion 7 is, for example, M14S. Length
L
1 between an open end from which the center electrode 3 is projected and the rear end
of the insulator 2 (the term "rear" refers to the upper side of FIG. 1) is, for example,
58.5 mm.
[0024] Body portions 3a and 4a (FIG. 2) of the center electrode 3 and the ground electrode
4, respectively, are formed from an Ni alloy (e.g., Inconel, Trademark). The spark
portions 31 and 32 are formed from a metal that contains Ir in an amount of not less
than 60% by weight.
[0025] As shown in FIG. 2, the body portion 3a of the center electrode 3 is tapered such
that the diameter is decreased toward the tip end, and the face of the tip end is
finished to a flat surface. A disk chip of an alloy, serving as the spark portion
31, is fixedly attached onto the end face of the body portion 3a through circumferential
welding along the boundary between the disk chip and the body portion 3a. As a result
of this welding, a weld zone W is formed along the boundary. Specific examples of
this welding include laser welding, electron beam welding, and resistance welding.
The spark portion 32 is formed in the following manner. A disk chip is positioned
on the ground electrode 4 so as to be aligned with the facing spark portion 31. A
weld zone W is formed along the boundary between the disk chip and the ground electrode
4 through welding as in the case of the spark portion 31, thereby fixedly attaching
the disk chip onto the ground electrode 4. These chips may be formed from, for example,
a fused material obtained by mixing components of an alloy in predetermined proportions
and melting the resultant mixture, or a sintered material obtained by compacting and
sintering an alloy powder or a mixture of powders of metal components of predetermined
proportions.
[0026] Examples of alloy to be used as material for the above-mentioned chips are as follows:
(1) An alloy which contains Ir as a main component and Rh in an amount of 3% by weight
to 40% by weight. Through use of the alloy, consumption of the spark portion, which
would otherwise result from volatilization of Ir through oxidation at high temperature,
is effectively suppressed, thereby realizing a spark plug having excellent durability.
When the Rh content of the alloy becomes less than 3% by weight, the effect of suppressing
volatilization-through-oxidation of Ir becomes insufficient. As a result, the spark
portion tends to be consumed, causing impairment in spark plug durability. When the
Rh content of the alloy becomes 40% by weight or higher, the melting point of the
alloy starts to decrease, with the result that in some cases, the durability of the
spark plug starts to decrease. Thus, the Rh content of the alloy is 3% by weight to
50% by weight (excluded), preferably 7% by weight to 30% by weight, more preferably
15% by weight to 25% by weight, most preferably 18% by weight to 22% by weight.
(2) An alloy which contains Ir as a main component and Pt in an amount of 1% by weight
to 20% by weight. Through use of the alloy, consumption of the spark portion, which
would otherwise result from volatilization of Ir through oxidation at high temperature,
is effectively suppressed, thereby realizing a spark plug having excellent durability.
Notably, when the Pt content of the alloy becomes less than 1% by weight, the effect
of suppressing volatilization-through-oxidation of Ir becomes insufficient. As a result,
the spark portion tends to be consumed, causing impairment in spark plug durability.
When the Pt content of the alloy becomes 20% by weight or higher, the melting point
of the alloy lowers, causing impairment in spark plug durability.
[0027] A material for the chip (spark portion) may contain an oxide or composite oxide of
a metallic element belonging to group 3A (so-called rare-earth metals) or group 4A
(Ti, Zr, and Hf) of the periodic table in an amount of 0.1% by weight to 15% by weight.
Through addition of such an oxide, consumption of the spark portion, which would otherwise
result from volatilization of Ir through oxidation, is more effectively suppressed.
Accordingly, when such an oxide is added to the material for the chip, a metallic
component of the material may be elemental Ir, as well as the Ir alloy described above
in (1) or (2). When the oxide content of the material is less than 0.1% by weight,
the addition of such an oxide fails to sufficiently yield the effect of suppressing
volatilization-through-oxidation of Ir. When the oxide content of the material is
in excess of 15% by weight, resistance to thermal shock of the chip is impaired. As
a result, when, for example, the chip is welded to the electrode, the chip may crack.
Notably, Y
2O
3 is preferred as the above-mentioned oxide. Further, La
2O
3, ThO
2, or ZrO
2, for example, may also be preferred.
[0028] The diameter δ of the spark portion 31, i.e., the diameter δ of the end portion of
the center electrode 3, is not greater than 1.1 mm, preferably 0.3 mm to 0.8 mm. A
dimension γ of the spark discharge gap g is not greater than 1.2 mm, preferably 0.3
mm to 1.1 mm, more preferably 0.6 mm to 0.9 mm. Either the spark portion 31 or the
spark portion 32 may be omitted. In this case, the spark discharge gap g is defined
by the spark portion 31 and the ground electrode 4 or by the spark portion 32 and
the center electrode 3.
[0029] Referring back to FIG. 1, in the spark plug 100, a through-hole 6 is formed axially
in the insulator 2. A metallic terminal 13 is fixedly inserted into one end portion
of the through-hole 6, while the center electrode 3 is fixedly inserted into the other
end portion of the through-hole 6. A resistor 15 is disposed within the through-hole
6 and between the metallic terminal 13 and the center electrode 3. The opposite ends
of the resistor 15 are connected to the center electrode 3 and the metallic terminal
13 via conductive glass seal layers 16 and 17, respectively.
[0030] The metallic terminal 13 is formed from, for example, low-carbon steel. An Ni plating
layer (for example, 5 µm thick) is formed on the surface of the metallic terminal
13 against corrosion. The metallic terminal 13 includes a seal portion 13c (a tip
end portion), a terminal portion 13a projected from the rear end of the insulator
2, and a bar portion 13b extending between the terminal portion 13a and the seal portion
13c. The seal portion 13c assumes an axially extending cylindrical form and is inserted
into the conductive glass seal layer 17, so that the space between the seal portion
13c and the wall of the through-hole 6 is sealed by the seal layer 17.
[0031] The resistor 15 is fabricated by the steps of: mixing glass powder, ceramic powder,
metal powder (which contains, as a main component, a metal selected singly or in combination
from the group consisting of Zn, Sb, Sn, Ag, and Ni), nonmetallic conductive substance
powder (for example, amorphous carbon (carbon black) or graphite), and an organic
binder in predetermined proportions; and sintering the resulting mixture by a known
method, for example, by use of a hot press. The composition and dimensions of the
resistor 15 are adjusted so as to establish an electric resistance of not less than
10 kΩ (preferably not less than 15 kΩ) but not greater than 25 kΩ as measured between
the metallic terminal 13 and the center electrode 3.
[0032] The conductive glass seal layers 16 and 17 are formed from glass mixed with metal
powder, which contains, as a main component, metal selected singly or in combination
from among metals including Cu and Fe. The metal content of the resulting mixture
is 35% by weight to 70% by weight. Notably, the conductive glass seal layers 16 and
17 may contain semiconducting inorganic compound powder, such as TiO
2, in an appropriate amount.
[0033] FIG. 3 shows an ignition system employing the spark plugs 100. As shown in FIG. 3,
an ignition system 150 does not employ a distributor, but includes ignition coils
51 adapted to directly apply voltage to the corresponding spark plugs 100. Each of
the ignition coils 51 includes a primary coil 52 adapted to receive electricity from
a battery 156 and connected to an igniter 154. The ignition coil 51 further includes
a secondary coil 53 connected to the corresponding spark plug 100. The igniter 154
includes contactless switches, such as transistors, corresponding to the ignition
coils 51. Upon reception of a break instruction signal issued from the corresponding
output port of an electronic control unit 155, each of the contactless switches comes
into a broken or open state. A diode 51a is provided between each ignition coil 51
and each spark plug 100 in order to prevent re-electrification of the spark plug 100,
which would otherwise occur when the corresponding contactless switch returns to a
conducting state from the open state.
[0034] As shown in FIG. 4, when an internal combustion engine 180 assumes the form of a
multiple-cylinder gasoline engine, the spark plug 100 is mounted, by means of the
mounting screw portion 7, on each of cylinders 181 such that the spark discharge gap
g is located within a combustion chamber. Coil units 50 are attached to the spark
plugs 100 in one-to-one correspondence and are connected to the electronic control
unit 155. The coil unit 50 includes a casing 60 fitted to the rear end portion of
the spark plug 100. The casing accommodates the ignition coil 51 and the igniter 154.
The ignition coil 51 is electrically connected to the metallic terminal 13 of the
spark plug 100 by means of an unillustrated terminal portion of the coil unit 50.
[0035] In the spark plug 100, the resistor 15 may be omitted, and the metallic terminal
13 and the center electrode 3 may be connected by means of, for example, a single
conductive glass seal layer. In the spark plug 100 provided with the resistor 15 and
the conductive glass seal layer 16 disposed between the resistor 15 and the center
electrode 3, the conductive glass seal layer 16 may be omitted. In this case, a resistor
may be disposed, for example, between the ignition coil 51 and the terminal portion
of the coil unit 50 so as to establish an electric resistance of not less than 10
kΩ (preferably not less than 15 kΩ) but not greater than 25 kΩ between the ignition
coil 51 and the center electrode 3 of the spark plug 100.
EXAMPLE
[0036] In order to confirm the effect of the above-described spark plug 100 and ignition
system 150, the following experiments were conducted. Fine glass powder (average grain
size 80 µm; 30 parts by weight), ZrO
2 powder (average grain size 3 µm; 60 parts by weight) serving as ceramic powder, Al
powder (average grain size 20-50 µm; 1 part by weight) serving as metal powder, carbon
black (2-9 parts by weight) serving as nonmetallic conductive substance powder, and
dextrin (3 parts by weight) serving as an organic binder were mixed. The resulting
mixture was wet-milled in a ball mill while water was used as solvent. The resulting
mixture was dried, obtaining a preliminary material. Coarse glass powder (average
grain size 250 µm) was mixed with the preliminary material in an amount of 400 parts
by weight per 100 parts by weight of the preliminary material, obtaining a resistor
composition in the form of powder. A material for the glass powder was borosilicate
lithium glass, which was obtained by the steps of mixing 50 parts by weight SiO
2, 29 parts by weight B
2O
5, 4 parts by weight Li
2O, and 17 parts by weight BaO and melting the resulting mixture and whose softening
temperature was 585°C.
[0037] Next, the resistors 15 were formed from the resistor composition powder by use of
a hot press. Through use of the resistor 15, there were manufactured various samples
of the spark plug 100 of FIG. 1 into which the resistor 15 is incorporated. In the
samples, the center electrode 3 was formed from an Ni alloy (INCONEL 600) and had
an axial length of 20.7 mm and a cross-sectional diameter of 2.6 mm. The diameter
of the through-hole 6 formed in the insulator 2 (substantially identical to the cross-sectional
diameter of the resistor 15) was 4.0 mm. Hot pressing was performed at a heating temperature
of 900°C and an applied pressure of 100 kg/cm
2. Conductive glass powder employed was a mixture of conductive powders of, for example,
Cu, Fe, Sn, and TiO
2, and borosilicate calcium glass powder (the conductive powders are contained in an
amount of approximately 50% by weight). In the obtained spark plug samples, the length
L2 of the resistor 15 was 7.0 mm to 15.0 mm. The electric resistance R
k as measured between the center electrode 3 and the metallic terminal 13 was adjusted
to 5 kΩ to 30 kΩ through adjustment of the length L2 and composition of the resistor
15.
[0038] The spark portions 31 and 32 were fabricated in the following manner. Ir and Pt of
predetermined amounts were mixed and melted, thereby obtaining an alloy which contains
Pt in an amount of 5% by weight and Ir as the balance. The alloy was formed into disk
chips having a diameter of 0.2 mm to 1.6 mm and a thickness of 0.6 mm. By use of the
chips, the spark portions 31 and 32 of the spark plug 100 shown in FIGS. 1 and 2 were
formed (in other words, spark plug samples having spark portions of various sizes
ranging from 0.2 mm to 1.6 mm were fabricated). The spark discharge gap g was initially
set to various values ofy ranging from 0.4 mm to 1.4 mm.
[0039] The thus-obtained spark plug samples were mounted on a 6-cylinder gasoline engine
(engine capacity 1998 cc). The engine was continuously operated for up to 800 hours
at an engine speed of 5600 rpm (at a center electrode temperature of approximately
780°C) while throttles were completely opened. After engine operation was halted,
an increase in the spark discharge gap g was measured. The test employed the ignition
system shown in FIG. 3. The ignition system effected an electrical discharge under
the following conditions: the polarity of the center electrode was negative; the peak
value of the secondary current was 70 mA; and the discharge energy was 65 mJ. During
discharge, current and voltage waveforms were recorded by use of an oscilloscope.
For comparison, a similar test was conducted by use of the distributor ignition system
(DIS) shown in FIG. 11. In this case, the electric resistance as measured between
the ignition coil 251 and the far end of each high-tension cable C was set to 5 kΩ
to 10 kΩ.
[0040] FIG. 5 shows the results of a test for a gap-increasing behavior (i.e., electrode
consumption). The test was conducted under the following conditions: the electric
resistance Rk was 5 kΩ; the end diameter δ of the center electrode was 1.0 mm; and
the initial spark discharge gap γ was 0.5, 0.8, and 1.1 mm. As seen from FIG. 5, spark
plugs having an initial spark gap γ of 0.8 or 1.1 mm cause a large amount of electrode
consumption, so that the gap increases considerably. Since it was considered that
the form of an electrical discharge was responsible for a difference in gap increase,
waveforms of discharge were observed. FIG. 6A shows the waveform of an electrical
discharge at a γ value of 0.5 mm, and FIG. 6B shows the waveform of an electrical
discharge at a γ value of 0.8 mm. In FIG. 6A, current shows a relatively stable behavior,
implying that glow discharge is dominant. By contrast, in FIG. 6B, current frequently
shows an abruptly increasing behavior, implying the occurrence of arc discharge. Particularly,
it is conceivable that a strong current flows at the moment of transition from glow
discharge to arc discharge. Conceivably, in the case of FIG. 6B, the frequency of
transition from glow discharge to arc discharge within a single discharge cycle increases;
hence, an instantaneous flow of a strong current occurs frequently, resulting in a
significant consumption of the electrode.
[0041] In FIG. 6A, in a region where glow discharge conceivably occurs, while the variation
in the current falls within a range of 5 mA, the absolute value of current gradually
decreases toward the end of a discharge cycle; i.e., a background current level is
formed. In the present example, one discharge cycle is divided in 0.5 ms units, and
an average value in each division is calculated to thereby obtain the above-mentioned
background current level. When current which is at least 20 mA greater than the obtained
background current level flows, it is interpreted as transition from glow discharge
to arc discharge. The number (frequency) of transitions within a single discharge
cycle was counted to thereby evaluate transition susceptibility.
[0042] FIG. 7 shows result of a test in which the frequency of transition from glow discharge
to arc discharge was measured while the electronic resistance R
k and the initial spark discharge gap γ were changed. Specifically, a first group of
spark plugs in which the end diameter δ of the center electrode was set to 1.0 mm
and the electronic resistance R
k was set to 5 kΩ were manufactured, while the initial spark discharge gap γ was changed
in the range of 0.4 - 1.4 mm. A second group of spark plugs in which the end diameter
δ of the center electrode was set to 1.0 mm and the electronic resistance R
k was set to 10 kΩ were manufactured, while the initial spark discharge gap γ was changed
in the range of 0.4 - 1.4 mm. Similarly, a third group of spark plugs in which the
end diameter 6 of the center electrode was set to 1.0 mm and the electronic resistance
R
k was set to 15 kΩ were manufactured, while the initial spark discharge gap γ was changed
in the range of 0.4 - 1.4 mm. Subsequently, the frequency of transition from glow
discharge to arc discharge was measured for each of the thus-manufactured spark plugs.
In FIG. 7, the frequency of transition is represented in the form of index when the
frequency of transition as measured at a γ value of 0.8 mm and an R
k value of 5 kΩ is taken as 100. Table 1 shows measurements.
Table 1
Spark discharge gap |
Frequency of transition (index) |
(mm) |
Rk = 5 kΩ |
Rk = 10 kΩ |
Rk = 15 kΩ |
0.4 |
8 |
4 |
3 |
0.5 |
33 |
17 |
12 |
0.6 |
67 |
33 |
23 |
0.7 |
83 |
42 |
29 |
0.8 |
100 |
50 |
35 |
0.9 |
83 |
42 |
29 |
1 |
67 |
33 |
23 |
1.1 |
33 |
17 |
12 |
1.2 |
8 |
4 |
3 |
1.3 |
0 |
0 |
0 |
1.4 |
0 |
0 |
0 |
[0043] As seen from FIG. 7, as the electric resistance R
k increases, the frequency of transition decreases. Meanwhile, in order to examine
resistance to contamination of spark plugs, a predelivery durability test as specified
in JIS D1606 was conducted on three groups of spark plugs, in which the first group
of spark plugs were manufactured such that their electric resistances were set to
10 kΩ and their initial spark discharge gaps γ were set to 0.8 mm, the second group
of spark plugs were manufactured such that their electric resistances were set to
10 kΩ and their initial spark discharge gaps γ were set to 1.2 mm, and the third group
of spark plugs were manufactured such that their electric resistances were set to
10 kΩ and their initial spark discharge gaps γ were set to 1.3 mm. The spark plugs
were mounted on the engine of a test car, and the test car underwent a test run. While
a travelling pattern specified in JIS D1606 is taken as one cycle, there was counted
the number of cycles until a rough idle occurred or until the insulation resistance
of the spark plug sample decreased to 1 MΩ or less (the number of durability cycles).
Resistance to contamination was evaluated in terms of the number of durability cycles.
The test results are shown in FIG. 12. As seen from FIG. 12, when the value of γ exceeds
1.2 mm, the number of durability cycles begins to decrease, indicating impairment
in resistance to contamination.
[0044] FIG. 8 shows results of a test performed for each of the DLI system of FIG. 3 and
the DIS system of FIG. 11, in which the frequency of transition from glow discharge
to arc discharge was measured while the electric resistance R
k was changed. That is, for each of the DLI system of FIG. 3 and the DIS system of
FIG. 11, spark plugs having initial spark discharge gap γ of 0.8 mm were manufactured
such that the spark plugs had respective R
k values with in the range of 5 kΩ - 30 kΩ. The frequency of transition was measured
for each of the thus-manufactured spark plugs. Table 2 shows measurements.
Table 2
|
Frequency of transition (index) |
Electric resistance (kΩ) |
DIS |
DLI |
5.00 |
63.5 |
100 |
7.50 |
45.8 |
70 |
10.00 |
30.6 |
50 |
12.50 |
|
40 |
15.00 |
|
35 |
20.00 |
|
32 |
22.50 |
|
30 |
25 |
|
28 |
27.5 |
|
26 |
30 |
|
24 |
[0045] As seen from FIG. 8, even when the DLI system is employed, the frequency of transition
from glow discharge to arc discharge decreases with the electric resistance R
k. At an electric resistance R
k of not less than 10 kΩ, the frequency of transition is suppressed as low as that
in the case of the DIS system. Notably, at an electric resistance R
k of not less than 20 kΩ, a decrease in the frequency of transition becomes gradual.
[0046] FIG. 9 shows a consumed volume of a center electrode per spark as measured with respect
to spark plugs having various values of end diameter 6 of the center electrode after
a continuous test operation of 800 hours was completed. This test employed an initial
spark discharge gap γ of 1.1 mm and an electric resistance R
k of 5 kΩ. As seen from FIG. 9, an electrode of a smaller diameter is consumed more
per spark. Conceivably, this is because an electrode of a smaller diameter increases
in temperature more readily and is thus more susceptible to temperature increase effected
by glow-to-arc transition. FIG. 10 shows the behavior of gap increase with operating
hours (up to 800 hours) as measured with respect to an electric resistance R
k of 5 kΩ, 10 kΩ, and 15 kΩ. This test employed an initial spark discharge gap γ of
0.5 mm and an end diameter δ of 1.0 mm of the center electrode. As seen from FIG.
10, electrode consumption can be suppressed more effectively by increasing the electric
resistance R
k to 10 kΩ, and this effect is enhanced by increasing the electric resistance R
k to 15 kΩ.
[0047] FIG. 13 is a graph showing the results of a test performed in order to evaluate the
ignitabilty of spark plug samples each manufactured such a manner that the spark discharge
gap γ was set to 0.8 mm, the end diameter δ of the center electrode was set to 0.8
mm, and the electric resistance R
k was set to a value in the range of 10 kΩ to 30 kΩ (the values are shown in Table
3). The spark plug samples were mounted on a 6-cylinder gasoline engine of a DOHC
lean-burn type (engine capacity 1998 cc). The engine was operated at a boost pressure
of 350 mmHg and an engine speed of 2000 rpm (corresponding to a driving speed of 60
km/h), while the air-fuel ratio was changed. An air-fuel ratio at the time when misfire
reached 1% was measured as an ignitable limit.
Table 3
Resistance (kΩ) |
Air-fuel ratio (A/F) |
10 |
22.2 |
15 |
22.2 |
20 |
22.1 |
21 |
22.07 |
22 |
22.03 |
23 |
21.98 |
24 |
21.92 |
25 |
21.85 |
26 |
21.77 |
27 |
21.69 |
28 |
21.6 |
29 |
21.5 |
30 |
21.4 |
[0048] From the test results, it is understood that when the resistance becomes equal to
or greater than 20 kΩ, the ignitable limit gradually decreases (it becomes impossible
to ignite fuel unless the air-fuel ratio is increased) and that the ignitable limit
starts to sharply decrease when the resistance exceeds 25 kΩ.
[0049] Obviously, numerous modifications and variations of the present invention are possible
in light of the above teachings. It is therefore to be understood that within the
scope of the appended claims, the present invention may be practiced otherwise than
as specifically described herein.
1. A spark plug comprising a center electrode (3), an insulator (2) which surrounds said
center electrode (3), a metallic shell (1) which surrounds said insulator (2), a ground
electrode (4) which faces said center electrode (3), a spark portion (31, 32) fixedly
attached to at least either one of said center electrode (3) and said ground electrode
(4) to thereby define a spark discharge gap (g), a metallic terminal (13) fixedly
attached into one end portion of a through-hole (6) formed axially in said insulator
(2), said center electrode (3) being fixedly attached into the other end portion of
the through-hole (6), and a resistor (15) disposed within the through-hole (6) and
between said metallic terminal (13) and said center electrode (3), characterized in
that
said spark portion (31, 32) is formed from a metal which contains not less than
60% by weight Ir; and said resistor (15) has an electric resistance of not less than
10 kΩ but not greater than 25 kΩ.
2. A spark plug according to Claim 1, characterized in that the electric resistance between
said metallic terminal (13) and said center electrode (3) is not less than 15 kQ.
3. A spark plug according to Claim 1 or 2, characterized in that said spark portion (31,
32) is formed at an end portion of said center electrode (3), and the diameter of
the end portion of said center electrode (3) is not greater than 1.1 mm.
4. A spark plug according to Claim 3, characterized in that the diameter of the end portion
of said center electrode (3) is adjusted to 0.3 mm to 0.8 mm.
5. A spark plug according to Claim 1, 2, 3 or 4 characterized in that the spark discharge
gap (g) is not greater than 1.2 mm.
6. A spark plug according to Claim 5, characterized in that the spark discharge gap (g)
is not greater than 0.8 mm.
7. An ignition system for use with an internal combustion engine comprising:
a spark plug (100) having a center electrode (3), an insulator (2) which surrounds
the center electrode (3), a metallic shell (1) which surrounds the insulator (2),
a ground electrode (4) which faces the center electrode (3), a spark portion (31,
32) fixedly attached to at least either one of the center electrode (3) and the ground
electrode (4) to thereby define a spark discharge, and a metallic terminal (13) fixedly
attached into one end portion of a through-hole (6) formed axially in the insulator
(2), the center electrode (3) being fixedly attached into the other end portion of
the through-hole (6); and
a coil unit (50) having a casing (60) attached to said spark plug (100), an ignition
coil (51) accommodated within said casing (60) and connected to the metallic terminal
(13) of said spark plug (100) in order to apply a high voltage to said spark plug
(100) for effecting an electrical discharge, characterized in that
said spark portion is formed from a metal which contains not less than 60% by weight
Ir, and a resistance portion is disposed between the ignition coil (51) and the center
electrode (3) so as to establish an electric resistance of not less than 10 kΩ but
not greater than 25 kΩ between the ignition coil (51) and the center electrode (3).
8. An ignition system for use with an internal combustion engine according to Claim 7,
characterized in that said resistance portion establishes an electrical resistance
of not less than 15 kΩ between the ignition coil (51) and the center electrode (3).
9. A spark plug according to any one of claims 1 to 6 or an ignition system according
to Claim 7 or 8, characterized in that said spark portion (31, 32) is formed from
a metal which contains Ir as a main component and Rh in an amount of 3% by weight
to 50% by weight (excluded).
10. A spark plug according to any one of claims 1 to 6 or an ignition system according
to Claim 7 or 8, characterized in that said spark portion (31, 32) is formed from
a metal which contains Ir as a main component and Pt in an amount of 1% by weight
to 20% by weight.
11. A spark plug according to any one of claims 1 to 6 or an ignition system according
to any one of Claims 7 - 10, characterized in that the material of said spark portion
(31, 32) contains an oxide or composite oxide of a metallic element belonging to group
3A or group 4A of the periodic table in an amount of 0.1% by weight to 15% by weight.