DE69900064T2 - Spark plug and ignition arrangement for use in an internal combustion engine - Google Patents

Description

The present invention relates to a spark plug for use in an internal combustion engine, and further relates to an ignition system for use in an internal combustion engine equipped with spark plugs.

As shown in Fig. 11 of the accompanying drawings, a distributor has been conventionally used in an ignition system for an automotive internal combustion engine having spark plugs. In an ignition system 249 shown in Fig. 11, an ignition coil 251 includes a primary coil 252 which receives electricity from a battery 256 through an ignition switch 257 and is connected to an ignition device, and a secondary coil 253 which is connected to a distributor 250. When an electronic control unit 255 gives an interruption command signal to the ignition device 254 at a predetermined ignition timing, the ignition device 254 causes a contactless switching arrangement to operate so that the flow of current through the primary coil 252 is interrupted. As a result, a high-voltage current is induced in the secondary coil 253. The distributor 250 distributes the induced current via high-voltage cable C to spark plugs 100.

In recent times, the distributor ignition system described above has been replaced by a fully transistorized coil plug ignition system (hereinafter abbreviated to "DLI system", derived from the term Distributor-Less Ignition). The DLI system enables simple ignition timing control and does not require any contact maintenance. In the DLI system, an ignition coil is arranged directly on each spark plug. A control unit interrupts the flow of current in the primary coil of the ignition coil of each spark plug at a predetermined time so that the spark plug generates a spark. Since Since ignition coils are mounted directly on the individual spark plugs, no high-voltage cables are required.

In order to improve the spark consumption resistance of a spark plug, a Pt (platinum) chip acting as an ignition portion is usually formed at one end of an electrode of the spark plug. However, Pt is expensive and its melting point is about 1769ºC, indicating that the spark consumption resistance of Pt is insufficient, so Ir (iridium) has been proposed as the chip material because Ir has a melting point of about 2454ºC. However, an ignition portion made of Ir generates a volatile oxide at a temperature of 900ºC to 1000ºC, indicating the tendency of the material to be consumed within this temperature range.

In a spark plug having a chip made of Ir-based material as an ignition section, the use of the DLI system explained above may have a significant adverse effect on the durability of the ignition section. The spark discharge of a spark plug is generally classified into a glow discharge and an arc discharge depending on the shape. For example, a glow discharge occurs when the impedance of a power supply (hereinafter referred to as "power supply impedance") is relatively high. Since a discharge current is relatively weak, the glow discharge causes a less significant temperature increase and less consumption of the ignition section. On the other hand, an arc discharge often occurs when a power supply impedance is relatively low. Consequently, a large discharge current tends to flow, resulting in a considerable increase in the temperature of the ignition section, thereby promoting the consumption of the ignition section. From the standpoint of suppressing the consumption of the ignition section, therefore, the glow discharge is preferably dominant in the ignition discharge.

In a distributor ignition system, the power supply impedance is large due to the electrical resistances of a contact gap and a high-voltage cable. Accordingly, the glow discharge predominates in an ignition discharge. However, in the DLI system, the power supply impedance is low because the electrical resistances of a contact gap and a high-voltage cable are absent. Consequently, depending on the material used for an electrode, the transition rate from the glow discharge to the arc discharge increases in an ignition discharge, which may cause the electrode consumption. According to a study conducted by the inventors of the present invention, an ignition portion made of an Ir-based material exhibits a particularly high transition rate from the glow discharge to the arc discharge, which may shorten the life of the spark plug. This tendency is further accelerated by the consumption of the ignition portion caused by volatilization by oxidation.

Japanese Patent Laid-Open No. 7-50192 (US Patent 5,514,929), which is believed to be the closest prior art, discloses that when a spark plug having a tip composed mainly of Ir is used in a gas engine, the energy of induced discharge can be reduced by using a resistor having a resistance value of not less than 50 kΩ and not more than 200 kΩ. However, although a gas engine does not have the problem of ignitability even if the discharge energy decreases, a gasoline engine has a problem of ignitability when the discharge energy decreases.

A first object of the invention is to provide a spark plug in which arc discharge is unlikely to occur, despite the fact that a spark portion is formed of an Ir-based metal, to thereby suppress consumption of an electrode and deterioration of ignitability.

A second object of the invention is to provide an ignition system for use in an internal combustion engine with spark plugs.

To achieve the first object, the present invention provides a spark plug comprising: a center electrode; an insulator surrounding the center electrode; a metal shell surrounding the insulator; a ground electrode facing the center electrode; a spark portion made of metal containing not less than 60 wt% of Ir and fixedly attached to the center electrode and/or the ground electrode to thereby define a spark discharge gap, a metallic terminal being fixedly attached to one end portion of a through hole formed axially in the insulator, the center electrode being fixedly attached to the other end portion of the through hole; and a resistor connected in the through hole between the metallic terminal and the center electrode, characterized in that the resistor has an electrical resistance value of not less than 10 kΩ but not more than 25 kΩ.

To achieve the second object, the present invention provides an ignition system for use in an internal combustion engine comprising a spark plug and a coil unit according to the features of claim 7.

The spark plug includes a center electrode; an insulator surrounding the center electrode; a metal shell surrounding the insulator; a ground electrode facing the center electrode, and a spark portion fixedly attached to either the center electrode or the ground electrode to thereby define a spark discharge gap. The spark portion is formed of a metal containing not less than 60 wt% Ir. The spark plug also includes a metallic terminal fixedly attached in one end portion of a through hole formed axially in the insulator, while the center electrode is fixedly attached in the other end portion of the through hole.

The coil unit includes a housing attached to the spark plug and an ignition coil housed in the housing and connected to the metal terminal of the spark plug for applying a high voltage to the spark plug to cause an electrical discharge.

The ignition system further includes a resistance portion disposed between the ignition coil and the center electrode to provide an electrical resistance of not less than 10 kΩ and not more than 25 kΩ between the ignition coil and the center electrode.

When the spark portion is formed of an Ir-based metal, the metal must contain Ir in a proportion of not less than 60 wt%, otherwise the high melting point of Ir cannot result in a sufficient improvement in the resistance of the spark portion to consumption caused by sparks. However, as described above, in a DLI system, a high Ir content of the spark portion tends to cause transition to a high 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 evaporates by oxidation, with the result that the spark portion is consumed accordingly.

The inventors have made extensive studies and, as a result, found that even in the DLI system, a spark plug having a spark portion made of the Ir-based metal described above (hereinafter, this will be occasionally referred to as "Ir plug") stably maintains an electric discharge at a relatively low current, for example, in the form of a glow discharge, by providing an electric resistance of not less than 10 kΩ (corresponding to a power supply impedance) between the ignition coil and the center electrode. Based on this finding, the present invention has been completed. By providing such an electric resistance, even when using Ir plugs in the DLI system, system, transition to a high-current discharge such as an arc discharge is unlikely. Thus, even during high-speed or heavy-load operation, consumption of the spark portion, which would otherwise occur due to volatilization of Ir by oxidation, can be suppressed, thus extending the life of the spark plug. In particular, the electrical resistance measured between the ignition coil and the center electrode is preferably not less than 15 kΩ. However, if the electrical resistance is more than 25 kΩ, the ignitability may be impaired.

In order to provide an electrical resistance of not less than 10 kΩ and not more than 25 kΩ between the ignition coil and the center electrode, a resistor built into a spark plug and used to reduce high frequency noise may be used. In this case, the electrical resistance of the resistor may be increased so that an electrical resistance of not less than 10 kΩ (preferably not less than 15 kΩ) and not more than 25 kΩ is provided between the metallic terminal and the center electrode. When the resistor is not built in, as is the case with an inexpensive ordinary spark plug, a resistance portion may be housed as a resistor in the coil unit so that an electrical resistance in the above range is provided between the ignition coil and the center electrode.

In a spark plug, when 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 the ignited flame, thereby improving ignitability. In the spark plug or the ignition system according to the invention in which the igniting portion is formed of the above-mentioned Ir-based metal at an end portion of the center electrode, the diameter of the end portion is preferably set to be not more than 1.1 mm. By setting the diameter of the end portion to be not more than 1.1 mm, ignitability is significantly improved. Even more Preferably, the diameter of the end portion is set to 0.3 mm to 0.8 mm. By limiting the diameter of the end portion to not more than 0.8 mm, the ignitability is further enhanced. When the diameter of the end portion is less than 0.3 mm, the temperature of the ignition portion tends to increase due to spark concentration, and as a result, the ignition portion tends to be consumed by volatilization of Ir by oxidation.

Generally, in a spark plug, the insulator is enclosed by the metal shell. When the surface of the insulator becomes contaminated, for example, by soot or fuel adhering thereto, sparks occur between the inner surface of the metal shell and the outer surface of the insulator, possibly hindering normal formation of an electric discharge across a spark discharge gap. Reducing the spark discharge gap is an effective means of maintaining normal electric discharge across the gap when the surface of the insulator becomes contaminated. In order to maintain the contamination resistance of the spark plug, the spark discharge gap is preferably set to not more than 1.2 mm, more preferably not more than 0.8 mm. In order to prevent the occurrence of a short circuit at the gap, the spark discharge gap is set to not less than 0.3 mm.

In the following, embodiments of the invention are explained by way of example with reference to the accompanying drawings. They show:

Fig. 1 is a longitudinal partial sectional view of an embodiment of a spark plug according to the invention;

Fig. 2 is an enlarged sectional view of parts of the spark plug according to Fig. 1, which are located near a spark discharge gap;

Fig. 3 is a circuit diagram of an example of an ignition system using spark plugs according to Fig. 1;

Fig. 4 is a schematic front view of the ignition system according to Fig. 3, installed in an engine;

Fig. 5 is a graphical representation of the results of a test concerning the behavior of spark gap enlargement, carried out for the spark plugs according to Fig. 1;

Fig. 6A is a graphical representation of the waveform of an electrical discharge;

Fig. 6B is a graphical representation of the waveform of another electrical discharge;

Fig. 7 is a graphical representation of the effects of the spark discharge gap and the electrical resistance on the frequency of the transition from glow discharge to arc discharge;

Fig. 8 is a graphical representation of the frequency of transition from glow discharge to arc discharge, measured for the ignition system according to Fig. 3 and an ignition system according to Fig. 11;

Fig. 9 is a graphical representation of the relationship between a consumed volume of an electrode and an electrode diameter;

Fig. 10 is a graph showing the behavior of the spark gap increase as a function of operating hours;

Fig. 11 is a circuit diagram of a distributor ignition system;

Fig. 12 is a graphical representation of the relationship between the number of durability cycles and a spark gap; and

Fig. 13 is a graphical representation of the relationship between resistance and ignition limit.

Embodiments of the invention are described below with reference to the drawings.

Fig. 1 shows a spark plug 100 in which a resistor is built, according to an embodiment of the invention. The spark plug 100 includes a cylindrical metal shell 1, an insulator 2 fitted into the metal shell 1 such that a tip end portion protrudes from the metal shell 1, and a ground electrode 4 arranged such that one end is connected to the metal shell 1 and the other end faces the tip of the center electrode 3. As shown in Fig. 2, a spark portion 32 is formed on the ground electrode 4 in such a way that it faces a spark portion 31 of the center electrode 3. The opposing spark portions 31 and 32 define a spark discharge gap γ therebetween.

The insulator 2 is formed of a ceramic sintered body, for example, of alumina or aluminum nitride. The metal shell 1 is made of, for example, low carbon steel and serves as a housing for the spark plug 100. A threaded portion 7 is formed on the outer surface of the metal shell 1 and serves to fasten the spark plug 100 to an engine block (not shown). The dimension of the threaded portion 7 corresponds to, for example, M14S. The length L₁ between an open end from which the center electrode 3 protrudes outward and the rear part of the insulator 2 ("rear" refers to the top in Fig. 1) is, for example, 58.5 mm.

Main ground portions 3a and 4a (Fig. 2) of the center electrode 3 and the ground electrode 4, respectively, are formed of a Ni alloy (for example, Iconel, trade name). The spark portions 31 and 32 are formed of a metal containing Ir in an amount of not less than 60 wt%.

As shown in Fig. 2, the main mass portion 3a of the center electrode 3 is tapered so that the diameter decreases toward the tip, the end face of the tip is machined as a flat plane. A circular disk of an alloy, which functions as a spark portion 31, is firmly attached to the end face of the main ground portion 3a by circumferential welding along the boundary between the disk and the main ground portion 3a. As a result of the welding, a weld zone W extends along the boundary. Specific examples of this welding include laser welding, electron beam welding, and resistance welding. The spark portion 32 is manufactured in the following manner: a circular disk is placed on the ground electrode 4 so as to be flush with the opposite spark portion 31. A weld zone W is formed along the boundary between the disk and the ground electrode 4 by welding, as in the spark portion 31, thereby firmly attaching the disk to the ground electrode 4. These discs can be made, for example, from a molten material obtained by mixing components of an alloy in predetermined proportions and melting the resulting mixture, or from a sintered material obtained by compacting and sintering an alloy powder or a mixture of powders of metal components in predetermined proportions.

Examples of the alloy used for the material of the above-mentioned discs are as follows:

(1) An alloy containing Ir as a main component and Rh in a proportion of 3 wt% to 40 wt%. By using the alloy, consumption of the ignition portion, which would otherwise occur due to volatilization of Ir due to oxidation at high temperature, is effectively suppressed, so that a spark plug with excellent durability is obtained.

If the Rh content of the alloy is less than 3 wt%, the effect of suppressing the volatilization by oxidation of Ir becomes insufficient. As a result, the spark section tends to shrink, which causes Deterioration of the durability of the spark plug. When the Rh content of the alloy is 40 wt% or more, the melting point of the alloy begins to decrease, with the result that in some cases the durability of the spark plug also begins to decrease. Accordingly, the Rh content of the alloy is set to 3 wt% to 50 wt% (exclusive), preferably 7 wt% to 30 wt%, more preferably 15 wt% to 25 wt%, most preferably 18 wt% to 22 wt%.

(2) An alloy containing Ir as a main component and Pt in an amount of 1 wt% to 20 wt%. By using the alloy, consumption of the spark portion which would otherwise occur due to volatilization of Ir due to oxidation at high temperature is effectively suppressed, so that a spark plug with excellent durability is obtained. In particular, when the Pt content of the alloy is less than 1 wt%, the effect of suppressing volatilization due to oxidation of Ir becomes insufficient. As a result, the spark portion tends to be consumed, which affects the durability of the spark plug. When the Pt content of the alloy is 20 wt% or more, the melting point decreases, which affects the durability of the spark plug.

A material for the wafer (spark portion) may contain an oxide or a composite oxide of a metallic element of Group 3A (so-called rare earth metals) or Group 4A (Ti, Zr and Hf) of the periodic table in an amount of 0.1 wt.% to 15 wt.%. By adding such an oxide, consumption of the ignition portion which would otherwise result from volatilization of Ir due to oxidation can be more effectively suppressed. Therefore, when such an oxide is added to the material of the wafer, a metal component of the material may be elemental Ir as well as the Ir alloy described in (1) or (2) above. If the oxide content of the material is less than 0.1 wt.%, the addition of such an oxide cannot sufficiently achieve the effect of suppressing volatilization by Oxidation of Ir. If the oxide content of the material is over 15 wt. %, the resistance of the wafer to thermal shock is impaired. As a result, for example, when the wafer is welded to the electrode, the wafer may break. In particular, Y₂O₃ is preferred as the above-mentioned oxide, and La₂O₃, ThO₂ or ZrO₂, for example, are also preferred.

The diameter δ of the spark portion 31, that is, the diameter δ of the end portion of the center electrode 3, is not more than 1.1 mm, preferably 0.3 mm to 0.8 mm. A dimension y of the spark discharge gap γ is not more 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 γ is formed by the spark portion 31 and the ground electrode 4 or by the spark portion 32 and the center electrode 3.

Returning to Fig. 1, in the spark plug 100, a through hole 6 is formed axially in the insulator 2. A metallic terminal 13 is tightly fitted into one end of the through hole 6, while the center electrode 3 is tightly fitted into the other end portion of the through hole 6. A resistor 15 is located within the through hole 6 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 sealing layers 16 and 17, respectively.

The metallic terminal 13 is formed of, for example, low carbon steel. A Ni plating layer (with a thickness of, for example, 5 µm) is formed on the surface of the metallic terminal 13 as an anti-corrosive agent. The metallic terminal 13 includes a sealing portion 13c (an end tip), a terminal portion 13a protruding from the rear end of the insulator 2, and a rod portion 13b located between the terminal portion 13a and the sealing portion 13c. The sealing portion 13c assumes an axially extending cylindrical shape and is inserted into the conductive glass sealing layer 17 so that the distance between the sealing portion 13c and the wall of the through hole 6 is sealed by the sealing layer 17.

The resistor 15 is manufactured by the following steps: mixing glass powder, ceramic powder, metal powder (containing as a main component a single or a combination of Zn, Sb, Sn, Ag and Ni), a non-metallic powder of a conductive substance (for example, amorphous carbon (soot) or graphite), and an organic binder in predetermined proportions; and sintering the resulting mixture according to a known method, for example, using a hot press. The composition and dimensions of the resistor 15 are adjusted so as to form an electrical resistance of not less than 10 kΩ (preferably not less than 15 kΩ), but not greater than 25 kΩ, measured between the metallic terminal 13 and the center electrode 3.

The conductive glass sealing layers 16 and 17 are mixed from glass mixed with metal powder, the metal powder containing as a main component a single or a combination of the metals Cu and Fe. The metal content of the resulting mixture is 35 wt.% to 70 wt.%, preferably the conductive glass sealing layers 16 and 17 may contain a semiconductive inorganic compound powder, for example TiO₂ in an appropriate proportion.

Fig. 3 shows an ignition system using the spark plugs 100. As shown in Fig. 3, an ignition system 150 does not use a distributor, but includes ignition coils 51 that can directly apply a voltage to associated spark plugs 100. Each of the ignition coils 150 includes a primary coil 52 for receiving electricity from a battery 156 connected to a breaker 154. The ignition coil 51 further includes a secondary coil 53 that is connected to the associated spark plug 100. The interrupter 154 contains contactless switches, such as transistors, associated with the ignition coils 51. Upon receipt of an interrupt command signal issued from the associated output of an electronic control unit 155, each of the contactless switches assumes a disconnected or open state. A diode 51a between each ignition coil 51 and an associated spark plug 100 prevents renewed energization of the spark plug 100, which would otherwise occur when the corresponding contactless switch changes from the open state to the conductive state.

As shown in Fig. 4, in an internal combustion engine 180 in the form of a multi-cylinder gasoline engine, the spark plug 100 is installed in each of the cylinders 181 by means of the threaded part 7 so that the spark discharge gap γ is located within a combustion chamber. Coil units 50 are attached to the spark plugs 100 in a one-to-one correspondence and are connected to the electronic control unit 155. The coil unit 50 includes a housing 60 which is inserted into the rear end portion of the spark plug 100. The housing accommodates the ignition coil 51 and the breaker 154. The ignition coil 51 is electrically connected to the metal terminal 13 of the spark plug 100 via a terminal part of the coil unit 50 (not shown).

In the spark plug 100, the resistor 15 may be omitted, and the metallic terminal 13 and the center electrode 3 may be connected, for example, through a single conductive glass sealing layer. In the spark plug 100 having the resistor 15 and the conductive glass sealing layer 16 between the resistor 15 and the center electrode 3, the conductive glass sealing layer 16 may be omitted. In this case, a resistor may be provided, for example, between the ignition coil 51 and the terminal part of the coil unit 50 to form an electrical resistance of not less than 10 kΩ (preferably not less than 15 kΩ), but not more than 25 kΩ between the ignition coil 51 and the center electrode 3 of the spark plug 100.

EXAMPLE

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), ZrO₂ powder (average grain size 3 µm; 60 parts by weight) as ceramic powder, Al powder (average grain size 20-50 µm; 1 part by weight) as metal powder, carbon black (2-9 parts by weight) as non-metallic conductive powder, and dextrin (3 parts by weight) as organic binder were mixed. The resulting mixture was wet-milled in a ball mill using water as a solvent. The resulting mixture was dried to obtain a precursor material. Coarse glass powder (average grain size 250 µm) was mixed with the precursor material in an amount of 400 parts by weight per 100 parts by weight of the precursor material, thereby obtaining a resistor composition in powder form. A material for the glass powder was borosilicate lithium glass, which was obtained through the steps of mixing 50 parts by weight of SiO₂, 29 parts by weight of B₂O₅, 4 parts by weight of Li₂O and 17 parts by weight of BaO and melting the resulting mixture, the softening temperature of the mixture being 585°C.

Then, the resistors 15 were formed from the resistor composition powder by means of a hot press. Using the resistor 15, various samples of the spark plug 100 shown in Fig. 1 were formed, in which the resistor 15 was incorporated. In the samples, the center electrode 3 was made of a 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 in the insulator 2 (substantially identical to the cross-sectional diameter of the resistor 15) was 4.0 mm. The hot pressing was carried out at a heating temperature of 900°C and an applied pressure of 100 kg/cm². The conductive glass powder used was a mixture of conductive powders of for example, Cu, Fe, Sn and TiO₂ and borosilicate calcium glass powder (the conductive powders are contained in an amount of about 50 wt%). In the obtained spark plug samples, the length L2 of the resistor 15 was 7.0 to 15.0 mm. The electric resistance Rk measured between the center electrode 3 and the metallic terminal 13 was adjusted to 5 kΩ to 30 kΩ by adjusting the length L2 and the composition of the resistor 15.

The spark portions 31 and 32 were prepared as follows: Ir and Pt were mixed in predetermined amounts and melted to obtain an alloy containing Pt in an amount of 5 wt% with the balance Ir. The alloy was formed into circular disks with a diameter of 0.2 mm to 1.6 mm and a thickness of 0.6 mm. Using these disks, 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 with spark portions of different sizes from 0.2 mm to 1.6 mm were prepared).

The spark discharge gap γ was initially set to different values y from 0.4 mm to 1.4 mm.

The spark plug samples thus obtained were installed in a 6-cylinder gasoline engine (displacement 1998 cm³). The engine was operated continuously for up to 800 hours at an engine speed of 5600 rpm (at a center electrode temperature of about 780ºC) with the throttles fully open. After stopping the engine operation, an increase in the spark discharge gap γ was measured. The test made use of the ignition system shown in Fig. 3. This ignition system caused an electric charge 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 the discharge, current and voltage waveforms were recorded using an oscilloscope. For comparison, a similar test was carried out using the ignition system shown in Fig. 11. In this case, the electrical resistance between the ignition coil 251 and the opposite end of each high voltage cable C was measured to be 5 kΩ to 10 kΩ.

Fig. 5 shows the results of a test on the behavior of spark gap increase (i.e., electrode consumption). The test was carried out under the following conditions: the electric resistance Rk was 5 kΩ, the final 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 can be seen from Fig. 5, in spark plugs with an initial spark gap γ of 0.8 or 1.1 mm, there is a large electrode consumption, so that the electrode gap increases considerably. Since it was considered that the shape of the electric discharge was responsible for a difference in the electrode gap increase, waveforms of the discharge were observed. Fig. 6A shows the waveform of an electric discharge at a γ value of 0.5 mm, and Fig. 6B shows the waveform of an electric discharge at a γ value of 0.8 mm. According to Fig. 6A, the current shows a relatively stable behavior, which suggests that the glow discharge dominates. In contrast, in Fig. 6B, the current often shows an abruptly increasing behavior, which implies that arc discharges occur. In particular, it is understandable that a large current flows at the time of transition from glow discharge to arc discharge. It is apparent that in the case of Fig. 6B, the frequency of transition from glow discharge to arc discharge is increased within a single discharge cycle. Consequently, an instantaneous flow of a large current often occurs, resulting in considerable electrode consumption.

In Fig. 6A, in a zone where a significant glow discharge occurs, while the current fluctuation falls within a range of 5 mA, the absolute value of the current gradually decreases toward the end of the discharge cycle, that is, a background current level is formed. In the present example, a discharge cycle is measured in units of 0.5 ms and an average value is calculated at each division to determine the above-mentioned background current level. If the current is at least 20 mA greater than the determined background current level, this is interpreted as a transition from glow discharge to arc discharge. The number (frequency) of transitions within a single discharge cycle was counted to determine the transition sensitivity.

Fig. 7 shows a result of a test in which the frequency of transition from glow discharge to arc discharge was measured while changing the electronic resistance Rk and the initial spark discharge gap γ. Specifically, a first group of spark plugs was manufactured in which the final diameter 5 of the center electrode was set to 1.0 mm and the electronic resistance Rk to 5 kΩ while the initial spark discharge gap γ was changed in the range of 0.4-1.4 mm. A second group of spark plugs was manufactured in which the final diameter 8 of the center electrode was set to 1.0 mm and the electronic resistance Rk to 10 kΩ 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 was manufactured in which the final diameter δ was set to 1.0 mm and the electronic resistance Rk to 10 kΩ while the initial spark discharge gap γ was changed in the range of 0.4-1.4 mm. of the center electrode was set to 1.0 mm and the electronic resistance Rk to 15 kΩ, while the initial spark discharge gap γ was varied in the range of 0.4-1.4 mm. Then, the frequency of transition from glow discharge to arc discharge was measured for each of the spark plugs thus manufactured. Fig. 7 shows the frequency of transition in index form, where the frequency of transition when measured at a γ value of 0.8 mm and a Rk value of 5 kΩ was taken as 100. Table 1 shows the measurement results. Table 1

As is clear from Fig. 7, the transition frequency decreases as the electrical resistance Rk increases. Now, in order to examine the contamination resistance of the spark plugs, a pre-shipment durability test was conducted in accordance with Japanese Industrial Standard (JIS) D1606 on the three groups of spark plugs, of which the first group of spark plugs was manufactured such that their electrical resistances were set to 10 kΩ and their initial spark discharge distances γ were set to 0.8 mm, the second group of spark plugs had their electrical resistances set to 10 kΩ and their initial spark discharge distances γ were set to 1.2 mm, and the third group of spark plugs had their electrical resistances set to 10 kΩ and their initial spark discharge distances γ were set to 1.3 mm. The spark plugs were mounted in the engine of a test vehicle, and the test vehicle was subjected to a test. While taking a driving pattern according to JIS D1606 as one cycle, the number of cycles until rough idling occurred or until the insulation resistance of the sample spark plug decreased to 1 MΩ or less was counted. had decreased (number of durability cycles). The contamination resistance was determined in terms of durability cycles. The test results are shown in Fig. 12. As can be seen from Fig. 12, when the γ value exceeds 1.2 mm, the number of durability cycles begins to decrease, indicating deterioration of the contamination resistance.

Fig. 8 shows results of a test conducted for both 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 electrical resistance Rk was changed. That is, for both the DLI system of Fig. 3 and the DIS system of Fig. 11, spark plugs with an initial spark discharge gap γ of 0.8 mm were manufactured so that the spark plugs had Rk values in the range of 5 kΩ to 30 kΩ. The frequency of transition was measured for each of the spark plugs thus manufactured. Table 2 shows the measurement results. Table 2

As can be seen from Fig. 8, even when the DLI system is used, the transition frequency from glow discharge to arc discharge decreases with the electric resistance Rk. When the electric resistance Rk is not less than 10 kΩ, the transition frequency is suppressed to a value as in the case of the DIS system. Note that when the electric resistance Rk is not less than 20 kΩ, the decrease in the transition frequency takes place gradually.

Fig. 9 shows a consumed volume of a center electrode per spark, measured for spark plugs with different values of the final diameter S of the center electrode after a duration test over 800 hours. This test used an initial spark discharge gap γ of 1.1 mm and an electrical resistance Rk of 5 kΩ. As can be seen from Fig. 9, an electrode with a smaller diameter is consumed more per spark. Obviously, this happens because an electrode with a smaller diameter increases its temperature more easily and is thus more susceptible to temperature increases caused by a glow-arc transition. Fig. 10 shows the behavior of the spark gap distance depending on operating hours (up to 800 hours), measured for an electrical resistance Rk of 5 kΩ, 10 kΩ and 15 kΩ. This test used an initial spark discharge gap γ of 0.5 mm and a final diameter δ of 1.0 mm of the center electrode. As can be seen from Fig. 10, the electrode consumption can be suppressed more effectively by increasing the electrical resistance Rk to 10 kΩ, and this effect is further enhanced by increasing the electrical resistance Rk to 15 kΩ.

Fig. 13 is a graph showing the results of a test conducted to determine the ignitability of sample spark plugs each manufactured so that the spark discharge gap γ was set at 0.8 mm, the final diameter δ of the center electrode was set at 0.8 mm and the electrical resistance Rk was set to a value in the range of 10 kΩ to 30 kΩ (the values are shown in Table 3). The sample spark plugs were mounted in a 6-cylinder double overhead camshaft lean-burn type gasoline engine (displacement 1998 cm). The engine was operated at a boost pressure of 350 mmHg and an engine speed of 2000 rpm (corresponding to a vehicle speed of 60 km/h) while the air-fuel ratio was changed. As an ignitability limit, an air-fuel ratio was measured at the time when misfire reached a value of 1%.

Table 3

Resistance (kΩ) Air-fuel ratio (L/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

From the above test results, it is understood that when the resistance becomes equal to or greater than 20 kΩ, the ignitability limit gradually decreases (it becomes impossible to ignite fuel unless the air-fuel ratio is increased), and that the ignitability limit begins to decrease sharply when the resistance value exceeds 25 kΩ.

Obviously, numerous modifications and variations of the 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.

Claims (11)

1. Spark plug comprising a center electrode (3), an insulator (2) surrounding the center electrode (3), a metal shell (1) surrounding the insulator (2), a ground electrode (4) opposite the center electrode (3), an ignition portion (31, 32) formed of a metal containing not less than 60 wt.% Ir, fixedly attached to at least one of the center electrode (3) and the ground electrode (4) to define a spark discharge gap (g), a metallic terminal (13) fixedly attached in one end portion of a through hole (6) formed axially in the insulator (2), the center electrode (3) fixedly attached in the other end portion of the through hole (6), and a resistor arranged in the through hole (6) between the metallic terminal (13) and the center electrode (3), characterized in that the resistor (15) has an electrical resistance value of not less than 10 kΩ, but not more than 25 kΩ. 2. Spark plug according to claim 1, characterized in that the electrical resistance value between the metallic terminal (13) and the center electrode (3) is not less than 15 kΩ. 3. Spark plug according to claim 1 or 2, characterized in that the ignition portion (31, 32) is formed at an end portion of the center electrode (3), and the diameter of the end portion of the center electrode (3) is not larger than 1.1 mm. 4. Spark plug according to claim 3, characterized in that the diameter of the end portion of the center electrode (3) is set to 0.3 mm to 0.8 mm. 5. 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. Spark plug according to claim 5, characterized in that the spark discharge gap (g) is not greater than 0.8 mm. 7. Ignition system for use with an internal combustion engine, comprising: a spark plug (100) comprising: a center electrode (3), an insulator (2) surrounding the center electrode (3), a metal sleeve (1) surrounding the insulator (2), a ground electrode (4) opposite the center electrode (3), an ignition section (31, 32) formed of a metal containing not less than 60 wt.% Ir fixedly attached to at least one of the center electrode (3) and the ground electrode (4) to define a spark discharge gap, a metallic terminal (13) fixedly attached in one end portion of a through hole (6) formed axially in the insulator (2), the center electrode (3) fixedly attached in the other end portion of the through hole (6), and a resistor arranged in the through hole (6) between the metallic terminal (13) and the center electrode (3), characterized in that it further comprises: a coil unit (50) with a housing (60) attached to the spark plug (100), wherein an ignition coil (51) is accommodated in the housing, which is connected to the metal terminal (13) of the spark plug (100) in order to apply a high voltage to the spark plug (100) to cause an electrical discharge, wherein a resistance portion is arranged between the ignition coil (51) and the center electrode (3) to provide an electrical resistance value of not less than 10 kΩ and not more than 25 kΩ between the ignition coil (51) and the center electrode (3). 8. Ignition system according to claim 7, characterized in that the resistive section creates an electrical resistance of not less than 15 kΩ between the ignition coil and the center electrode (3). 9. Spark plug according to one of claims 1 to 6 or ignition system according to claim 7 or 8, characterized in that the ignition section (31, 32) is formed from a metal which contains Ir as the main component and also Rh in an amount of 3 wt.% to (exclusively) 50 wt.%. 10. Spark plug according to one of claims 1 to 6 or ignition system according to claim 7 or 8, characterized in that the ignition section (31, 32) is formed from a metal which contains Ir as the main component and Pt in an amount of 1 wt.% to 20 wt.%. 11. Spark plug according to one of claims 1 to 6 or ignition system according to claim 7 or 8, characterized in that the material of the ignition section (31, 32) is an oxide or composite oxide of a metallic element of group 3A or group 4A of the periodic table in an amount of 0.1 wt.% to 15 wt.%.