JP2013251193A - Spark plug for internal combustion engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a spark plug for internal combustion engine which can achieve high ignitability and erosion resistance, in an internal combustion engine including super charging means and producing a high flow rate environment.SOLUTION: By setting the ratio B/A of the diameter A of a center electrode discharger 11 and the diameter B of a ground electrode discharger 41 to be B/A≥1.4, and setting the diameter B of a ground electrode discharger 41 to be 0.9 (mm)≤B, the re-discharge frequency can be reduced at the time of blow-off, heat dissipation from the ground electrode discharger 41 to the outside can be promoted, and erosion of the discharger can be reduced. Furthermore, by setting the diameter B of a ground electrode discharger 41 to be B≤1.4 (mm), crack initiation probability due to thermal stress of the ground electrode discharger 41 is reduced, and damage, e.g., separation, of the ground electrode discharger 41, can be suppressed.

Description

  The present invention relates to a spark plug for an internal combustion engine that achieves both high ignitability and suppression of consumption by spark discharge in an internal combustion engine provided with supercharging means.

  2. Description of the Related Art Conventionally, spark plugs that define the dimensions of various elements related to a center electrode and a ground electrode and are intended to improve ignitability and durability are known. For example, in Patent Document 1, the ratio of the ground electrode discharge part is 0.8 mm or less, the ratio of the ground electrode discharge part divided by the diameter of the center electrode discharge part is 1.4 or more, and the ground electrode discharge part is grounded. A spark plug intended to obtain higher ignitability and durability by setting the protruding length from the electrode surface to 0.5 mm to 1.2 mm is shown.

  Conventionally, the diameter of the discharge part of the ground electrode of the spark plug has a small size because the generated spark may absorb the amount of heat by the discharge part and the flame extinguishing action may be generated. There has been a need to be.

  By the way, in recent years, there has been a demand for improvement in fuel efficiency of vehicles due to exhaust gas regulations and the like, and a high supercharged internal combustion engine is used to meet this demand. The spark plug used in such an internal combustion engine is placed in a high flow velocity environment in the combustion chamber. That is, an air-fuel mixture in which air compressed by supercharging means provided in an intake pipe of an internal combustion engine and the like is mixed flows into a combustion chamber of the internal combustion engine in a higher pressure state than a normal internal combustion engine, The spark plug protrudes into the combustion chamber and flows in a space (discharge gap) between the ground electrode discharge portion and the center electrode discharge portion at a speed higher than that of a normal internal combustion engine. Under such a high flow velocity environment, the discharge current locus, which is the locus of the discharge current generated by applying a high voltage to the discharge gap of the spark plug, is easily blown by the high flow velocity mixture.

JP-A-2005-123181

  When a spark plug is placed in a high flow rate environment of an internal combustion engine equipped with a supercharging means, the discharge current locus generated between the discharge gap between the ground electrode discharge part and the center electrode discharge part is blown by the mixture having a high flow rate. There is a possibility that a phenomenon in which discharge is interrupted (hereinafter referred to as blow-off phenomenon) is generated. In particular, this phenomenon is likely to occur in an internal combustion engine having a small diameter of the ground electrode discharge portion and provided with supercharging means. Due to the occurrence of blow-off phenomenon, the combustion becomes unstable, or discharge occurs again after the blow-off phenomenon occurs, so that the number of discharges increases, and the temperature of the ground electrode discharge part increases accordingly. There is a possibility that the consumption of the electrode discharge part is accelerated.

  The present invention has been made in view of the above-mentioned problems, and its purpose is to reduce the frequency of discharge blow-off in an internal combustion engine that generates a high flow velocity environment equipped with a supercharging means, and the ground electrode discharge portion By providing a spark plug for an internal combustion engine that promotes dissipating the amount of heat to the outside, suppresses a decrease in bonding strength due to the thermal stress of the ground electrode discharge part, and can obtain high ignitability and wear resistance is there.

  In order to solve the above problems, the spark plug (1) for an internal combustion engine according to the present invention is used in an internal combustion engine provided with a supercharging means (70) for supercharging intake air sucked into a combustion chamber (3). The diameter of the portion of the ground electrode discharge portion (41) facing the center electrode discharge portion (11) is B (mm), and the diameter of the portion of the center electrode discharge portion (11) facing the ground electrode discharge portion (41) is When A (mm), 0.9 (mm) ≦ B ≦ 1.4 (mm) and B / A ≧ 1.4 are satisfied.

  According to the present invention, in the spark plug (1) of the internal combustion engine provided with the supercharging means, the blow-off phenomenon can be suppressed, and the amount of heat of the ground electrode discharge part (41) can be dissipated to the outside. In addition to being able to promote, it is possible to suppress a decrease in bonding strength due to thermal stress, and as a result, it is possible to achieve high ignitability and wear resistance.

  That is, as a result of providing the supercharging means, the inventors have a phenomenon that the discharge current locus (2) is blown in a high flow velocity environment (for example, a flow velocity of 20 m / s or more) in the discharge gap (50). However, attention was paid to the point that ignition occurs at a location away from the discharge part and heat is not easily transmitted to the discharge part. The diameter B of the ground electrode discharge part (41) is set to be relatively large, unlike the conventional way of thinking, and the diameter B of the ground electrode discharge part (41) is set larger than the diameter A of the center electrode discharge part (11). Is set to be larger than a predetermined ratio, so that the discharge current locus (2) to be blown can move in the direction in which the ground electrode discharge part end face (42) is blown to its edge part (43), and the center electrode It has been found that the discharge part (11) can be maintained without increasing the electric field concentration, the blow-off phenomenon can be suppressed, and high ignitability and wear resistance can be realized.

  Based on these findings, the ratio B / A of the diameter A of the central electrode discharge part (11) to the diameter B of the ground electrode discharge part (41) is set to B / A ≧ 1.4, and the ground electrode discharge part ( By setting the diameter B of 41) to 0.9 (mm) ≦ B, the frequency of re-discharge at the time of blow-off can be reduced, and the dissipation of heat from the ground electrode discharge part (41) to the outside can be promoted, It is possible to keep the consumption amount of the discharge part low. Further, by setting the diameter B of the ground electrode discharge part (41) to B ≦ 1.4 (mm), the probability of cracking due to thermal stress of the ground electrode discharge part (41) is reduced, and the ground electrode discharge part ( 41) It is possible to suppress breakage such as separation.

  In addition, the code | symbol in the parenthesis described in the claim and the said means section shows the correspondence with the specific means as described in embodiment mentioned later, and limits the technical scope of this invention. It is not a thing.

1 is a half sectional view showing a first embodiment of the present invention. The enlarged view of the spark plug discharge part of this invention seen from the X direction of FIG. The schematic block diagram of the internal combustion engine provided with the supercharging means in which the spark plug of 1st Embodiment is used. (A) is explanatory drawing of the discharge state of the spark plug arrange | positioned at the internal combustion engine of a low flow velocity environment, (B) is explanatory drawing of the discharge state of the spark plug of a high flow velocity environment. The characteristic view which shows the result of the visualization spark discharge bench test which evaluated the relationship between the re-discharge frequency in a spark plug, and B / A. The characteristic view which shows the result of the blowing-out suppression effect C / D with respect to the flow rate of the visualization spark discharge bench test in FIG. The characteristic view which shows the relationship between the lean limit A / F for every flow velocity in a spark plug, and the diameter B of a ground electrode discharge part. The characteristic view which shows the relationship between the wear resistance in a spark plug, and the diameter B of the ground electrode discharge part. (A) is a graph which shows the result of the endurance test for wear resistance evaluation in a naturally aspirated engine, (B) is a graph which shows the same result in a supercharged engine. The characteristic view which shows the relationship between the generated stress which arose in the ground electrode discharge part of the spark plug in the thermal cycle test, and the diameter B of the ground electrode discharge part. The characteristic view which shows the result of an engine durability test about the relationship between the gap expansion ratio in a spark plug, and the length L which the ground electrode discharge part protrudes. The characteristic view which shows the result of an engine durability test about the relationship between the lean limit A / F in a spark plug and the length L which the ground electrode discharge part protrudes.

  Below, the spark plug 1 for internal combustion engines which is the 1st Embodiment of this invention is demonstrated using FIGS. 1-3. FIG. 1 is a half sectional view of a spark plug 1 according to the present invention. The spark plug 1 includes a center electrode 10 provided at the tip of the center electrode discharge part 11 projecting into the combustion chamber (hereinafter referred to as the tip side), and the center electrode 10 disposed on the outer periphery of the center electrode 10. The insulator 30 to be held, the housing 20 disposed on the outer periphery of the insulator 30 and holding the insulator 30, and protrudes from the end portion 22 on the distal end side of the housing 20, and bent so as to face the center electrode discharge portion 11. And a ground electrode 40 having a ground electrode discharge portion 41 on the opposite surface. As will be described later, the spark plug 1 is attached to a combustion chamber 3 of an internal combustion engine provided with supercharging means.

  The center electrode 10 has a terminal portion 14 exposed from the insulator 30 on the side opposite to the distal end side (hereinafter referred to as a proximal end side), and a diameter from the proximal end side toward the distal end side at the end portion on the combustion chamber side. A center electrode tip portion 13 formed in a substantially truncated cone shape and a center electrode discharge portion 11 formed in a substantially cylindrical shape by a conductor on the tip side of the center electrode tip portion 13 are provided. A substantially cylindrical insulator 30 is disposed on the outer periphery of the center electrode 10, and a housing 20 formed of a metal conductor so as to cover a part of the insulator 30 is disposed on the outer periphery of the insulator 30. The The housing 20 is provided with a screw portion 21, and the spark plug 1 is fixed to the internal combustion engine by the screw portion 21 being fastened to the internal combustion engine.

  A ground electrode 40 formed of a metal conductor is joined to the housing front end 22. The ground electrode 40 has an arm portion protruding from the end 22 on the front end side of the housing, and a part of the arm portion is bent so as to face the center electrode discharge portion 11, and the ground electrode discharge portion 41 is discharged from the bent portion to the center electrode discharge. It protrudes so as to face the part 11.

  FIG. 2 shows a schematic enlarged view of a spark plug discharge portion constituted by the center electrode tip portion 13, the center electrode discharge portion 11, the ground electrode discharge portion 41, and the ground electrode 40. The center electrode discharge part 11 arranged at the center electrode tip 13 is made of, for example, an iridium alloy to which rhodium or the like is added, and the ground electrode discharge part 41 protruding from the ground electrode 40 toward the center electrode 10 is added with, for example, rhodium or the like. Both are formed in a substantially cylindrical shape.

  The center electrode discharge part 11 and the ground electrode discharge part 41 have a center electrode discharge part end face 12 and a ground electrode discharge part end face 42 facing each other, and the diameter B of the ground electrode discharge part 41 is 0 as described later. The diameter A of the center electrode discharge portion 11 is formed such that the relationship B / A with the diameter B of the ground electrode discharge portion 41 is in the range of 1.4 or more. .

  The ground electrode discharge portion 41 is welded to the ground electrode 40 and protrudes from the ground electrode 40 in the axial direction of the center electrode 10 by a length L. The length L is 0.5 mm or more and 1. The range of 2 mm or less is preferable. The welding is performed by, for example, arc welding or the like, and is performed on the outer periphery of the ground electrode discharge portion 41 having a substantially cylindrical shape on the ground electrode side.

  FIG. 3 shows an internal combustion engine provided with an exhaust turbine supercharger 70 as supercharging means. The exhaust turbine supercharger 70 has turbines on the exhaust side and the intake side, respectively, and both turbines are structurally connected. Hereinafter, a method of supercharging by the exhaust turbine supercharger 70 will be described.

  The exhaust turbine supercharger 70 includes an exhaust side turbine 71 that receives power on the exhaust side, and an intake side turbine 72 that functions as a centrifugal compressor that receives power and compresses intake air. The air-fuel mixture 63 combusted by the internal combustion engine is discharged from the combustion chamber 3 as exhaust 62. The exhaust side turbine 71 receives a part of kinetic energy and heat energy from the exhaust 62, and rotates at high speed using the energy. The intake-side turbine 72 connected to the exhaust-side turbine 71 rotates as the exhaust-side turbine 71 rotates at a high speed. With this rotational force, the intake-side turbine 72 supercharges the intake air 60 and is compressed after being supercharged. The intake air 61 is sucked into the combustion chamber 3.

  The supercharged intake air 61 supercharged by the above method becomes an air-fuel mixture 63 by mixing with fuel. The air-fuel mixture 63 flows into the combustion chamber 3 at a pressure higher than the pressure of the air-fuel mixture 63 in the internal combustion engine using natural intake air (NA), and is grounded to the center electrode discharge portion 11 of the spark plug 1 protruding into the combustion chamber 3. It flows between the discharge gaps 50 between the electrode discharge portions 41. The flow rate of the air-fuel mixture 63 flowing through the discharge gap 50 in an internal combustion engine having no supercharging means is smaller than an average flow velocity of 10 m / s (meter / second), whereas the air-fuel mixture 63 of the internal combustion engine provided with supercharging means. The average flow velocity is in the range of 20 m / s to 40 m / s. In the present invention, the high flow velocity environment refers to an environment in which, for example, the above-described supercharging means is used, and a flow velocity higher than that of the air-fuel mixture 63 in an internal combustion engine such as a naturally aspirated engine that does not use the supercharging means is generated.

  Further, the supercharging means is not limited to the above-described exhaust turbine supercharger 70. For example, air is forcibly sent into the combustion chamber using the power of an internal combustion engine such as a mechanical supercharger or an electric motor. Supercharging means such as a mechanism or a method of supercharging using the pressure of air obtained by running depending on the shape or structure of the intake pipe may be used.

  The state of the spark plug discharge part in the high flow velocity environment and the low flow velocity environment in which the air-fuel mixture flows at a lower speed than the environment will be described with reference to FIG. 4A shows the state of discharge in the discharge part in the low flow velocity environment, and FIG. 4B shows the state of discharge in the discharge part in the high flow rate environment, and this (B) is blown away. It shows the situation when the occurrence of.

  Discharge between the center electrode discharge portion 11 and the ground electrode discharge portion 41 occurs at the opposed center electrode discharge portion end face 12 and ground electrode discharge portion end face 42. That is, the high voltage applied to the spark plug 1 causes a dielectric breakdown in the discharge gap 50, and a discharge starts when a current flows in this state, and the path appears as a discharge current locus 2. This discharge ignites the air-fuel mixture and generates flame nuclei. As the generated flame kernel expands, it reaches the ground electrode discharge portion 41, and the discharge portion absorbs the heat generated by the flame kernel. That is, the cooling effect by the ground electrode discharge part 41 occurs. As a result, a phenomenon in which the flame kernel loses its heat amount in the discharge gap 50, that is, a flame extinguishing action may occur.

  In the discharge in which the flow rate of the air-fuel mixture 63 is in a low flow rate environment as shown in FIG. 4A, flame nuclei are formed in the discharge gap 50, so that the diameter of the ground electrode discharge portion 41 is increased to cool by the electrodes. There is a possibility that the ignition will become unstable due to the increased effect and the extinguishing action. Therefore, in general, the ground electrode discharge part 41 tends to suppress the extinguishing action by reducing the diameter.

  Compared with the contents described above, in the discharge under a high flow velocity environment as shown in FIG. 4B, the discharge current locus 2 generated on the opposite surface by the flow of the air-fuel mixture 63 flowing through the discharge gap 50 is Flowed in the flow direction 80. At this time, the end of the discharge current locus 2 in the ground electrode discharge portion 41 moves on the end surface 42 of the ground electrode discharge portion, reaches the edge portion 43 and is outside the discharge gap 50 so that the discharge current locus 2 draws a substantially arc. Reach up to. That is, the discharge current locus 2 is blown by the rapid flow of the air-fuel mixture 63. The air-fuel mixture 63 is ignited by the blown-out discharge, and a flame nucleus is formed outside the discharge gap 50 in the discharge portion radial direction.

  Further, when the discharge current locus 2 is blown and the discharge is interrupted, a blow-off phenomenon occurs. At this time, the remaining energy lost by the high-voltage blow-off applied to the spark plug 1 between the discharge gaps. The re-discharge is started. Therefore, when flame nuclei are blown off or when a blow-off phenomenon occurs, the flame nuclei are formed outside the discharge gap, so the influence of the cooling effect is reduced, and the ground electrode discharge part 41 is ignited even if the diameter is large. The effect on sex is reduced.

  Hereinafter, an experiment conducted in order to conceive the configuration of the present invention and confirm the effects thereof and the result thereof will be described.

  FIG. 5 shows the relationship between the re-discharge frequency at the time of blow-off in the spark plug and the ratio B / A of the diameter A of the center electrode discharge portion 11 and the diameter B of the ground electrode discharge portion 41 of the air-fuel mixture 63 in the discharge gap. The result of the visualization spark discharge bench test which evaluated flow velocity as a parameter is shown. That is, a test on the relationship between the ratio B / A and the number of redischarges per spark when the flow rate of the air-fuel mixture 63 in the discharge gap 50 of the spark plug 1 and the diameter B of the ground electrode discharge part 41 is changed. It is what I did.

  The center electrode discharge part 11 in the used spark plug has a length protruding from the center electrode tip 13 of 0.8 mm, and the diameter A of the center electrode discharge part 11 is 0.55 mm, 0.7 mm, and 1.0 mm. Various types are prepared, and the ground electrode discharge part 41 has a protruding length L from the ground electrode 40 of 0.8 mm, and the diameter B of the ground electrode discharge part 41 is 0.55 mm, 0.8 mm, 1.0 mm, 1.2 mm. Various types having a thickness of 1.4 mm were prepared.

  In the above-described spark plug, the flow rate of the air-fuel mixture 63 can be changed, and the combination of the ratio B / A is changed for each flow rate in a visualized high-flow spark discharge bench that can visualize the state of the spark discharge. The number of re-discharges was investigated.

  According to FIG. 5, it can be seen that the discharge is stably performed without re-discharge in an environment where the flow velocity is 5 m / s. However, when the flow rate is 10 m / s or more, a phenomenon that causes re-discharge is observed. This phenomenon becomes more noticeable and the re-discharge frequency increases as the flow rate becomes higher.

  It can also be seen that when B / A is small, the re-discharge frequency is high, whereas when B / A is large, the re-discharge frequency is reduced and re-discharge is suppressed. In particular, when B / A is larger than 1.4, the slope of the approximate line of the experimental value at the same flow rate becomes small, and the re-discharge frequency is suppressed.

  Here, the discharge when the approximate decrease slope of the redischarge frequency in the range where B / A is less than 1.4 is C, and the approximate decrease slope of the redischarge frequency in the range where 1.4 is greater than 1.4 is D. FIG. 6 shows the relationship between the flow rate of the gap 50 and the blowout suppression effect C / D indicating the level of blowout suppression at the flow rate.

  From FIG. 6, as the flow velocity increases, the blowout suppression effect C / D at the flow velocity increases, and the C / D increases greatly at a flow velocity of 15 m / s or more. That is, when the flow velocity is 15 m / s or more, there is a remarkable blow-off suppressing effect, and when the flow velocity is 20 m / s or more, it shows that a larger suppressing effect is obtained.

  This is because the diameter B of the ground electrode discharge part 41 is made larger than the diameter A of the center electrode discharge part 11 so that B / A is 1.4 or more, so that the high voltage applied to the center electrode discharge part 11 is increased. The electric field concentration is kept relatively large, and in addition, the surface area that receives the discharge in the ground electrode discharge portion 41 is relatively large, so that even if a discharge is caused by the flow of the air-fuel mixture 63, the blowout phenomenon is less likely to occur. is there. By suppressing the blow-off phenomenon, the temperature rise of the ground electrode discharge part 41 due to re-discharge is suppressed, and it becomes possible to suppress the consumption of the ground electrode discharge part 41 due to this.

  Next, in order to verify an effective range for the diameter B of the ground electrode discharge part 41 in a state where the flow velocity of the discharge gap 50 is changed, the protrusion length from the center electrode tip part 13 is 0.8 mm and the diameter is 0. Various spark plugs including a center electrode discharge portion 11 having a length of 0.7 mm and a ground electrode discharge portion 41 having a protruding length L from the arm portion of 0.8 mm and a diameter of 0.7 mm to 1.6 mm. Was installed in a 6-cylinder gasoline engine with a displacement of 2000 cc, and an engine durability test was performed in an idling state at an engine speed of 600 rpm, and the lean limit A / F was examined.

  FIG. 7 is a characteristic diagram showing the relationship between the lean limit A / F and the diameter B of the ground electrode discharge portion 41 for each flow velocity. The lean limit A / F is the thinnest fuel for satisfying the combustion fluctuation rate PmiCOV (dispersion / average value of average effective pressure, which is set to 15% in this experiment) such that combustion is established without misfiring. It is A / F (mixing ratio of air and fuel).

  That is, as the lean limit A / F is larger, combustion is possible with less fuel. From FIG. 7, when the flow velocity is 0 m / s, the value of the lean limit A / F decreases as the diameter B of the ground electrode discharge portion 41 is increased. When the flow velocity is large, the flow velocity is particularly 20 m / s. In some cases, even if the diameter B of the ground electrode discharge portion 41 is increased, the lean limit value is only gradually decreased, and it can be seen that there is little fluctuation. That is, it can be seen that, under a high flow velocity environment, the lean limit A / F is not easily attenuated even if the diameter B of the ground electrode discharge portion 41 is relatively large. This is because the flame nuclei are likely to be generated outside the discharge gap 50 by being blown in a high flow velocity environment, and the amount of heat of the flame nuclei is not easily absorbed by the ground electrode discharge part 41. This is because the effect of the flame extinguishing effect by taking B is small and there is no deterioration in ignitability.

  In addition, the diameter B of the ground electrode discharge portion 41 can be made relatively large from the viewpoint of ignitability in a high flow velocity environment, while the diameter B of the ground electrode discharge portion 41 is depleted of the ground electrode discharge portion 41. It has been found that there is an optimum range in terms of volume and bonding strength. Next, FIG. 8 to FIG. 10 show test results of evaluating the ground electrode discharge portion 41 from the viewpoint of the consumed volume and the bonding strength.

  FIG. 8 shows an evaluation of the wear resistance of the ground electrode discharge part 41 in a 6-cylinder gasoline engine with a displacement of 2000 cc. ) Prepare an engine and attach various spark plugs such that the ratio B / A of the diameter A of the center electrode discharge part 11 to the diameter B of the ground electrode discharge part 41 is 1.5, and 5600 rpm-WOT (wide open) The engine endurance test was performed continuously for 200 hours at the throttle), and the consumption volume of the ground electrode discharge part 41 was measured. In addition, rpm-WOT is a unit which shows the rotation speed in a throttle fully open state.

  The specific dimensions of the various spark plugs are such that the diameters A and B of the center electrode discharge portion 11 and the ground electrode discharge portion 41 are 0.4 mm and 0.6 mm, 0.6 mm and 0.9 mm, and 0.8 mm, respectively. And 1.2 mm, and B / A is 1.5 respectively.

  In FIG. 8, in the naturally aspirated engine, the consumption volume of the ground electrode discharge part 41 is hardly changed without depending on the diameter B of the ground electrode discharge part 41. However, in the engine equipped with the supercharging means, the ground electrode discharge part 41 It can be seen that as the diameter B decreases, the consumption volume increases and increases rapidly at 0.9 mm or less. In other words, it can be seen that the consumption volume is small and stable when the diameter B of the ground electrode discharge portion 41 is 0.9 mm or more.

  This indicates that it is effective to have a certain surface area or more in order for the ground electrode discharge part 41 to dissipate the amount of heat obtained by absorbing from the flame kernel or combustion to the air-fuel mixture 63. Unlike a naturally aspirated engine in which a flame kernel is formed in the discharge gap 50 and the ground electrode discharge part 41 absorbs and dissipates heat, the supercharged engine in which a flame kernel is formed outside the discharge gap 50 In this case, it is useful to make the ground electrode discharge part 41 large so that it is easy to dissipate, and to reduce the consumption volume without sacrificing the ignitability.

  Further, as shown in FIGS. 9A and 9B, in a naturally aspirated engine and an engine equipped with a supercharging means, the electrode consumption of the conventional spark plug used as a comparative example and the spark plug of the present invention are compared. In order to evaluate, an engine durability test (a 6-cylinder gasoline engine with a displacement of 2000 cc, 5600 rpm-WOT, 200 hours) under the same conditions as the test of FIG. 8 was performed. That is, the conventional spark plug and the spark plug 1 of the present invention were attached to a naturally aspirated engine and an engine equipped with a supercharging means, respectively, and the consumption volume was measured. FIG. 9A shows the test result of the naturally aspirated engine, and FIG. 9B shows the test result of the engine equipped with the supercharging means. In FIG. 9, the shaded portion is the consumed volume of the ground electrode discharge portion 41, and the portion without the shaded portion is the consumed volume of the center electrode discharge portion 11.

  In FIG. 9, the spark plug used as a comparative example has a center electrode discharge portion 11 made of an iridium-based alloy whose diameter A of the center electrode discharge portion 11 is 0.55 mm and the protruding length is 0.8 mm; A spark plug including a ground electrode discharge portion 41 made of an alloy mainly composed of platinum having a diameter B of 0.7 mm and a protruding length L of 0.8 mm of the ground electrode discharge portion 41 is used in this embodiment. In the spark plug, the diameter B of the center electrode discharge part 11 made of an alloy mainly composed of iridium having a diameter A of 0.7 mm and a protruding length of 0.8 mm, and the ground electrode discharge part 41 is 1. A spark plug including a ground electrode discharge portion 41 made of an alloy mainly composed of platinum having a length of 0.0 mm and a protrusion length L of 0.8 mm.

  As shown in FIG. 9A, in the naturally aspirated engine, the ground electrode discharge part 41 and the center electrode discharge part 11 have almost no difference in consumption volume between the comparative example and the present example. As can be seen, in the supercharged engine, the consumption volume of the ground electrode discharge part 41 is considerably suppressed in the present embodiment from the comparative example. As described above, it is effective to make the ground electrode discharge part 41 large so that it is difficult to absorb heat from the flame kernel and to easily dissipate the heat possessed by the ground electrode discharge part 41, and a supercharging means is provided. In the spark plug of the comparative example attached to the engine, re-discharge is easily performed due to the occurrence of the blow-off phenomenon, and the ground electrode discharge part 41 becomes hot and the volume consumption of the ground electrode discharge part 41 is promoted. This is because, in the spark plug of the example, re-discharge is suppressed and the temperature does not rise easily.

  Next, in order to evaluate the bonding strength between the ground electrode discharge part 41 and the ground electrode 40, the ground electrode discharge part 41 having a diameter B of 1.0 to 1.8 mm was welded. A grounding electrode 40 was prepared, and a thermal cycle test was performed in which the thermal stress in the engine was simulated. As the evaluation conditions, the ground electrode 40 was exposed for 6 minutes each at 150 ° C. and 900 ° C., and this was repeated 200 cycles. The number of samples was 10. FIG. 10 shows the relationship between the diameter B of the ground electrode discharge part 41 and the generated thermal stress, and Table 1 as the evaluation result is shown below.

上記表１において示す発生応力とは熱応力を指し、判定は接地電極放電部４１が接地電極４０から分離しなかったものを合格（○）、分離したものを不合格（×）として表現している。この表１から、接地電極放電部４１の直径Ｂが１．４ｍｍ以下においては接地電極放電部４１の分離を回避することが可能であることが分かる。

The generated stress shown in Table 1 above refers to thermal stress, and the judgment is expressed as a pass (◯) when the ground electrode discharge part 41 is not separated from the ground electrode 40, and a separated one as a reject (x). Yes. From Table 1, it can be seen that separation of the ground electrode discharge part 41 can be avoided when the diameter B of the ground electrode discharge part 41 is 1.4 mm or less.

  As a reason why the ground electrode discharge part 41 is separated from the ground electrode 40 and is damaged, the influence of thermal stress can be considered. That is, the thermal stress is generated at the melted portion interface between the ground electrode discharge portion 41 and the ground electrode 40 by being repeatedly placed in the low temperature state and the high temperature state, and this thermal stress exceeds the crack initiation stress at the melted portion interface, so that cracks occur. Will occur. Once the crack has occurred, it is considered that the bonding strength is significantly reduced because oxygen enters and the crack progresses at once. This reduction in the bonding strength may cause the ground electrode discharge portion 41 to separate from the ground electrode 40. By avoiding this in the spark plug 1 of the present invention, the reliability is improved.

  Next, in order to investigate the optimal protrusion length of the ground electrode discharge part 41, the protrusion length L (mm) of the ground electrode discharge part 41 and each element were tested. The results are shown in FIGS.

  FIG. 11 shows a spark plug in which the diameter A of the central electrode discharge portion 11 is 0.7 mm and the protrusion length is 0.8 mm. The diameter B of the ground electrode discharge portion 41 is 1.0 mm and the protrusion length L is 0. Engine endurance test under the same conditions as the test of FIG. 8 (5 cc, 6-cylinder gasoline engine, 5600 rpm-WOT, 200 hours) The gap enlargement ratio was examined. The gap expansion ratio is a ratio obtained by dividing the distance between the discharge gaps 50 after the test by the initial distance between the discharge gaps.

  As shown in FIG. 11, it can be seen that the gap increases as the protrusion length L increases. Furthermore, when the protrusion length L is 1.2 mm or more, the gap expansion ratio is significantly increased. This is because by increasing the protrusion length L, the temperature of the ground electrode discharge part 41 rises due to combustion and the volume consumption increases. The required discharge voltage rises due to the widening of the gap, and the discharge cannot be normally performed at the required discharge voltage at a normal distance, which may cause a problem in ignitability. Accordingly, it is desirable that the protruding length is 1.2 mm or less in order to suppress temperature rise and ensure wear resistance and prevent deterioration of ignitability.

  FIG. 12 shows a spark plug in which the diameter A of the central electrode discharge portion 11 is 0.7 mm and the protrusion length is 0.8 mm. The diameter B of the ground electrode discharge portion 41 is 1.0 mm and the protrusion length L is 0. Engine endurance test under the same conditions as the test in FIG. 8 (6 cc engine with a displacement of 2000 cc, 5600 rpm-WOT, 200 hours) prepared with 3mm, 0.5mm, 0.8mm and 1.2mm. The lean limit A / F was examined by

  According to FIG. 12, the lean limit A / F increases by increasing the protruding length L of the ground electrode discharge part 41, but significantly decreases below 0.5 mm. This is because the extinguishing length L of the ground electrode discharge portion 41 is made small so that the flame extinguishing action by the ground electrode 40 also acts in addition to the ground electrode discharge portion 41, and the ignitability is deteriorated because the lean limit is greatly lowered. May occur.

  Therefore, the protrusion length L is preferably 0.5 mm or more, and it is understood from the results of FIG. 11 that 0.5 mm or more and 1.2 mm or less is desirable. In this case, it is possible to provide the ground electrode discharge part 41 with good wear resistance while suppressing deterioration in ignitability.

DESCRIPTION OF SYMBOLS 1 Spark plug, 2 Discharge current locus, 3 Combustion chamber, 10 Center electrode, 11 Center electrode discharge part, 12 Center electrode discharge part end surface, 13 Center electrode tip part, 14 Terminal part, 20 Housing, 21 Screw part, 22 Housing tip Side end, 30 insulator, 40 ground electrode, 41 ground electrode discharge part, 42 end face of ground electrode discharge part, 43 edge end part, 50 discharge gap, 60 intake air, 61 intake air after supercharging, 62 exhaust gas, 63 mixture , 70 Exhaust turbine type supercharger, 80 Flow direction of mixture, A Diameter of central electrode discharge part, B Diameter of ground electrode discharge part, L Projection length of ground electrode discharge part

Claims (3)

A central electrode (10) having a central electrode discharge section (11) at the tip;
An insulator (30) disposed on an outer periphery of the center electrode (10) and holding the center electrode (10);
A housing (20) disposed on an outer periphery of the insulator (30) and holding the insulator (30);
A ground electrode (40) having a ground electrode discharge part (41) opposed to the central electrode discharge part (11) via a discharge gap (50);
A spark plug for an internal combustion engine used in an internal combustion engine provided with a supercharging means (70) for supercharging intake air sucked into a combustion chamber (3),
The diameter of the portion of the ground electrode discharge portion (41) facing the center electrode discharge portion (11) is B (mm), and the portion of the center electrode discharge portion (11) facing the ground electrode discharge portion (41) When the diameter of A is A (mm),
0.9 ≦ B ≦ 1.4 (mm), and
B / A ≧ 1.4,
A spark plug for an internal combustion engine characterized by satisfying the relationship:
The spark plug for an internal combustion engine according to claim 1,
When the length of the ground electrode discharge part (41) protruding from the ground electrode (40) toward the center electrode discharge part (11) is L (mm),
0.5 (mm) ≦ L ≦ 1.2 (mm),
A spark plug for an internal combustion engine characterized by satisfying the relationship:
The spark plug for an internal combustion engine according to claim 1 or 2,
A mixture of fuel and air flowing in the discharge gap (50) between the center electrode discharge part (11) and the ground electrode discharge part (41) by being supercharged by the supercharging means (70). The spark plug for an internal combustion engine is characterized by having a flow velocity of 20 m / s or more.

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