Patent Application
20070080618
The present invention relates to a spark plug which is to be mounted on and used in an internal combustion engine, and more particularly to a spark plug which realizes an excellent flame developing property.
A spark plug is used as an ignition device which performs a spark discharge while using a gap between a center electrode and a ground electrode that are exposed in a combustion chamber of an internal combustion engine, as a spark discharge gap. Conventionally, in order to realize a prolonged lifetime of a spark plug, a tip of a noble metal alloy (hereinafter, also referred to merely as noble metal tip) which is excellent in spark wear resistance and resistance to oxidation at high temperatures (hereinafter, the spark wear resistance and the resistance to oxidation at high temperatures are often referred in combination as durability) is joined to a tip end portion(s) of one or both of the electrodes which will function as the starting point of a spark discharge (for example, see JP-A-8-339880). When a noble metal tip which is excellent in durability is used, it is possible to reduce the diameter of the noble metal tip which will function as the starting point of a spark discharge. Therefore, a configuration where the lifetime can be prolonged, and, when the tip is made very thin, spread of flame (hereinafter, also referred to merely as flame developing property) which is an ignited air-fuel mixture is not impeded can be realized (for example, see JP-A-2002-313524).
Before a very thin noble metal tip is used as means for improving the flame developing property, various improvements have been made because of the following reason. A spark plug is used while a male thread formed on a metal shell is fastened to a female thread formed in an internal combustion engine. In the mounting process, it is very difficult to mount a spark plug in consideration of the direction of the ground electrode. When an air-fuel mixture proceeds from the periphery of the spark plug to a place where a spark discharge occurs, and when the air-fuel mixture is ignited and then a flame develops, the movement of the air-fuel mixture or the development of the flame is blocked by the presence of the ground electrode, and hence there arises a problem in that the combustion efficiency is lowered.
In order to solve such a problem, a configuration which is disclosed in, for example, JP-A-11-121142, and in which a ground electrode has a circular section shape (hereinafter, such a ground electrode is referred to merely as round ground electrode) has been proposed. When this configuration is employed, a phenomenon that a flow of an air-fuel mixture is blocked by the ground electrode and a gas flow is separated from the ground electrode can be prevented from occurring, or another phenomenon that a flame in a growth process is contacted with the ground electrode, and hence heat is drawn from the flame, thereby impairing the development of the flame can be prevented from occurring.
In the field of an automobile in which a spark plug is used, miniaturization and efficiency increase of an engine have been advanced. For example, the diameters of intake/exhaust ports are increased, and an injector is placed in the vicinity of a combustion chamber as a result of direct injection of fuel. Therefore, it is requested to reduce the diameter of a spark plug. In order to meet the request, the thread diameter of a spark plug is reduced to, for example, M12 or smaller. However, an insulator which holds a center electrode must have a thickness which is sufficient for, when a high voltage for generating a spark discharge, preventing penetration breakage from occurring, and a predetermined gap is formed in order to prevent a spark discharge between the insulator and the metal shell, i.e., a so-called side spark from occurring. In consideration of these structures, the tip end face of the metal shell to which the ground electrode is to be joined is allowed only to have a very limited thickness.
A configuration where such a small-diameter metal shell is used to reduce the diameter of a spark plug, and a very thin noble metal tip is joined to a round ground electrode so as to provide a prolonged lifetime and an excellent flame developing property will be considered. As described above, however, it is not easy to sufficiently ensure the thickness of the tip end face of the metal shell to which the ground electrode is to be joined. In the case of a prismatic ground electrode which has been conventionally used, and which is substantially rectangular, the ground electrode can be joined to the tip end face of the metal shell while forming a width which is larger than the thickness the tip end face of the metal shell, and hence it is possible to avoid the possibility that the heat dissipation property of the ground electrode (heat conductance from the ground electrode to the metal shell) is insufficient. In the case of a round ground electrode, however, the electrode has a circular section shape, and hence its diameter is allowed only to have a value corresponding to the thickness of the tip end face of the metal shell, with result that it is difficult to ensure a sufficient heat dissipation property. It may be contemplated that a round ground electrode is configured by using a metal having an excellent thermal conductivity, such as Cu as a core member. According to this configuration, however, the number of production man-hours is inevitably increased. If these problems are solved and a very thin noble metal tip is joined, it is possible to realize a spark plug which is excellent in lifetime and flame developing property.
A configuration where a very thin noble metal tip is used in order to prolong the lifetime and improve the ignitability, and the noble metal tip is joined to a round ground electrode so as to realize a further excellent flame developing property will be considered. Since the outline of a round ground electrode is arcuately bent, it is not easy to firmly join a noble metal tip which is formed into a substantially columnar shape having a flat face at the upper and lower ends, to the electrode. For the purpose of improving the joining property between the noble metal tip and the round ground electrode, the joining face of the noble metal tip may be previously processed to form a recess or an arcuate portion so as to enable the joining face to be engaged with the round ground electrode. However, there is a problem in that this configuration is not practical because of increases of the production cost and man-hours, and difficulty in facing of the noble metal tip.
Since the noble metal tip is formed into a columnar shape having a flat face at the upper and lower ends, it is not easy to realize firm joining with the round ground electrode having the arcuate face. The case where the joining process is conducted by, for example, resistance welding will be considered. In the lower face (the end face on the side to be joined to the ground electrode) of the columnar tip, only a portion which is near the center is contacted with the ground electrode. Therefore, a current does not flow through a portion which is remote from the center, with the result that welding is performed in so-called “floating state” in which only the portion in the vicinity of the center is joined to the ground electrode. By contrast, the case where the joining process is conducted by laser welding will be considered. In contrast to the lower face of the noble metal tip which is a flat face, the side face of the round ground electrode has a predetermined radius curvature. Therefore, only a portion of the noble metal tip which is near the center is contacted with the side face of the ground electrode, and a portion which is remote from the center is separated also from the side face. As a result, even when irradiation with a laser beam is conducted, the degree of penetration between the round ground electrode and the noble metal tip is low. Also in this case, consequently, firm joining is hardly realized.
The invention has been conducted under the above-discussed various circumstances. It is an object of the invention to provide a small-diameter spark plug in which a noble metal tip is firmly joined to a round ground electrode to enhance the reliability, which comprises a ground electrode that has a flame developing property equivalent to that of the round ground electrode, that is enabled to have a prolonged lifetime, and that has an excellent heat dissipation property. It is another object of the invention to provide a method which enables the production of such a spark plug to be performed without unnecessarily increasing the production cost and the man-hours.
In order to solve the problems, a spark plug of a first configuration of the invention is a spark plug comprising:
a center electrode (which may be a rod-like shape) extending in a direction of an axis of the plug;
a cylindrical insulator which has a shaft hole, and which holds the center electrode in the shaft hole in a state where a tip end of the center electrode is projected from a tip end face of the insulator;
a metal shell which holds the insulator; and
a ground electrode in which one end is joined to a tip end face of the metal shell, and which extends from the one end toward another end, wherein
the ground electrode has an inner side face which faces the center electrode, and an outer side face which is a back face with respect to the inner side face,
the outer side face is formed as an arcuately curved face when viewed from a side of the another end in an extending direction of the ground electrode, and a columnar noble metal tip is joined to the inner side face.
According to the configuration, in the case where the spark plug is mounted on an internal combustion engine, even when the ground electrode is not ideally directed, the combustion efficiency can be maximized. In the case where a spark plug is configured by using a conventional round ground electrode, as compared with a configuration where a rectangular columnar ground electrode is used, an air-fuel mixture more easily enters a spark discharge gap, and therefore it can be the that the combustion efficiency is improved. It has been checked that the combustion efficiency is further improved in the configuration of the invention. Although the reason is not clear, this is caused by the following phenomenon. Since the noble metal tip which protrudes from the ground electrode toward the center electrode exists, the distance from the ground electrode to a middle portion of the spark discharge gap is prolonged, and the air resistance (including resistances due to the air-fuel mixture and the combustion gas) and their flows (swirl and tumble flows, and the like) in the combustion chamber exert influences during a period when the air-fuel mixture reaches the middle portion of the spark discharge gap, thereby eliminating a phenomenon that a Karman vortex caused by the ground electrode impedes the flow of the air-fuel mixture. Moreover, also the following phenomenon affects the improvement. An ignited air-fuel mixture, i.e., a combustion gas spreads over the periphery of the noble metal tip which has a smaller volume, when its flame kernel is small. After the flame kernel sufficiently grows, the combustion gas spreads over the periphery of the ground electrode which is larger in volume than the noble metal tip. Therefore, the growth of the flame kernel immediately after ignition is hardly impeded.
In order to further improve the combustion efficiency, preferably, a direction in which the noble metal tip protrudes from a portion that is joined to the ground electrode is set as a tip axis, and a protrusion length h in the direction of the tip axis is larger than a width d in a direction which is perpendicular to the direction of the tip axis. In a noble metal tip having a so-called vertically elongated shape (h>d), the size of h is desired to be 16% or more of the external dimension of the round ground electrode.
As described above, it is not easy to join a columnar noble metal tip to a round ground electrode to obtain sufficient joining strength. As a third configuration of the invention, therefore, it is preferable to form a configuration where, when the ground electrode is viewed from the side of the another end in the extending direction of the ground electrode, the ground electrode has:
a flat face which is in the inner side face, and in which an outline of the ground electrode is formed as a linear portion; and
two first arcuate portions which begin at end portions of the linear portion, and which are positioned on sides of ends of the linear portion, respectively, and
the tip end face of the noble metal tip protrudes more than an intersection of two virtual extended arcs on a side of the inner side face, the virtual extended arcs having a radius curvature R which is equal to radius curvatures of the first arcuate portions, and being formed be extending the first arcuate portions.
Usually, a noble metal tip is produced by drawing an ingot of a noble metal to form a very thin wire, and conducting a step of cutting the wire to be formed into a columnar shape having a flat face at the upper and lower ends. In order to join a noble metal tip to a round ground electrode to obtain firm joining strength, with respect to the flat face of the noble metal tip, a flat face is disposed on the side of the round ground electrode to which the noble metal tip is to be joined, and which faces the center electrode.
However, simple disposition of such a flat face fails to always attain the effect. When an excessively large flat face is formed, there is a possibility that the effect of the round ground electrode, i.e., an excellent flame developing property cannot be attained. In order to effectively obtain the effect of the round ground electrode, a configuration may be formed in which virtual extended arcs are formed by virtually extending two first arcuate portions that are continuous to the both ends of the linear portion constituting the flat face, toward the center electrode, and the tip end face of the noble metal tip which is close to the center electrode protrudes more than an intersection of the virtual extended arcs. According to the configuration, flame which is ignited between the tip end face of the noble metal tip and that of the center electrode is not contacted with the flat face of the ground electrode until the flame develops to some extent, and hence high ignitability can be realized. Thereafter, the flame spreads along the side face of the ground electrode which is formed into an arcuate shape, and hence it is possible to obtain also a high flame developing property. Preferably, the flat face to which the noble metal tip is to be joined has an area which is equal to or larger than a butting face of the noble metal tip, so as to obtain more firm joining strength.
As a fourth configuration of the invention, preferably, when the ground electrode is viewed from the side of the another end in the extending direction of the ground electrode, in one of the two first arcuate portions, a center of an arc forming the one first arcuate portion (a center of a circle including the one first arcuate portion) is on a centerline dividing the linear portion or on a side of another one of the two first arcuate portions with respect to the centerline (is on a side of another one of the two first arcuate portions, the side including a centerline dividing the linear portion). The centerline means a line which is drawn in a radial direction of the plug axis to divide the linear portion when the ground electrode is viewed after the ground electrode is unbent linearly as shown in FIGS. 2 to 5. When the first to third configurations are employed, the ignitability, the flame developing property, and the lifetime can be respectively improved. From the viewpoint of improvement of the flame developing property, particularly, the fourth configuration may be employed.
When the first arcuate portions have a fixed radius curvature, particularly, it is preferable to, as a fifth configuration of the invention, employ a configuration where, when the ground electrode is viewed from the side of the another end in the extending direction of the ground electrode, the ground electrode has: a flat face which is in the inner side face, and in which an outline of the ground electrode is formed as a linear portion; and two first arcuate portions which begin at end portions of the linear portion, and which are positioned on sides of ends of the linear portion, respectively, and the flat face is positioned between a midpoint of (h+1) and the tip end face of the noble metal tip, (h+1) being defined by: a protrusion length h by which, when a direction in which the noble metal tip protrudes from a portion that is joined to the ground electrode is set as a tip axis, the noble metal tip protrudes in the direction of the tip axis; and a thickness l of the ground electrode in a radial direction of the plug axis.
When paying attention to the noble metal tip, it is preferable to, as a sixth configuration of the invention, employ a configuration where a spark discharge gap G which is formed by the tip end of the center electrode and the noble metal tip joined to the ground electrode, and a gap H which is a minimum gap between the tip end of the center electrode and the ground electrode satisfy a relationship of 0.5 mm≦H−G≦1.0 mm.
A ground electrode which is employed in a usual spark plug has a substantially rectangular columnar shape. Therefore, a spark discharge is often conducted in a place where the spark discharge gap is narrowest, or at an edge where the electric field is steep. By contrast, as compared with a substantially rectangular ground electrode, a ground electrode having an arcuate portion as in the invention has a reduced number of portions where the electric field is steep, and hence a spark discharge sometimes occurs on a side which is approximately opposite to the center electrode while proceeding around the round ground electrode. By contrast, a molten portion between the noble metal tip and the base material of the round ground electrode has a low work function, and hence a discharge easily occurs in the portion. Therefore, a spark discharge sometimes occurs in the portion. When the latter spark discharge repeatedly occurs, particularly, the portion to which the noble metal tip is joined is consumed to be decreased, and there is a possibility that the joining strength is lowered.
When the above sixth configuration is employed for such a problem, it is possible to maintain a spark plug in which the frequencies of the spreading of a spark discharge around the ground electrode, and a discharge to a molten portion are lowered, and which is excellent in ignitability and flame developing property for a long term.
Now, attention is paid particularly to the improvement of the heat dissipation property of the ground electrode. A seventh configuration of the invention may be employed where, when the ground electrode is viewed from the side of the another end in the extending direction of the ground electrode, the ground electrode is formed by: a flat face which is in the inner side face, and in which an outline of the ground electrode is formed as a linear portion; two first arcuate portions which have a radius curvature r, and which begin at end portions of the linear portion, which extend toward other ends of the first arcuate portions, and which are positioned on sides of ends of the linear portion; and a second arcuate portion through which the other ends of the first arcuate portions are coupled together, and which has a radius curvature R.
As in the seventh configuration, the shape as viewed from the side of the another end in the extending direction of the ground electrode, i.e., the outline of a section of the ground electrode is formed into a substantially semicircular shape by connecting the both ends of the linear portion with the arcuate portions (first arcuate portions) having a radius curvature which is relatively small, and end points of the arcuate portions through the arcuate portion (second arcuate portion) having a radius curvature which is relatively large, whereby the ground electrode is enabled to be joined to the tip end face of the metal shell through a joining area which is relatively larger as compared with the case of a round ground electrode (however, the both ends of the linear portion of the substantially semicircular shape are configured by the first arcuate portions, and have a form which is R-chamfered). Therefore, it is possible to avoid the phenomenon that the heat dissipation property of the ground electrode is insufficient, and the ground electrode can be provided with an excellent heat dissipation property which is a property of a round ground electrode. When this configuration is provided and the first and second arcuate portions having different radius curvatures are formed, it is possible to prevent the phenomenon that a gas flow such as an air-fuel mixture is separated from the ground electrode and the combustion efficiency is lowered, from occurring. In a more preferable mode, the length of the linear portion is larger than the thickness of the tip end face of the metal shell, and joining is performed so that the linear portion does not protrude from the tip end face of the metal shell. An eighth configuration of the invention is a more specific configuration of the more preferable mode.
In the eighth configuration of the invention, the radius curvature r of the first arcuate portions, and the radius curvature R of the second arcuate portion satisfy a relationship of r<R. As described above, when the ground electrode is joined so that the flat face (which is seen as a linear form when viewed in the extending direction) faces the center electrode, and the radius curvatures r, R satisfy the relationship of r<R, the joining area between the ground electrode and the metal shell can be obtained more efficiently. The noble metal tip is joined to a portion constituting the flat face, in the side face of the ground electrode. Therefore, the joining of the noble metal tip to the ground electrode can be made more stable.
It is preferable to set the radius curvature r of the first arcuate portions of the eighth configuration in the following manner. Namely, a ninth configuration is employed where, when a distance between two intersections at which a virtual arc formed by extending an outer peripheral edge of the second arcuate portion intersects with a virtual extension line of the linear portion is D1, the radius curvature r of the first arcuate portions is D1/9≦r≦D1/5.
When the radius curvature r of the first arcuate portions is set to be D1/9 or larger, it is possible to prevent the phenomenon that a gas flow flowing from the outside of the spark plug to the center is separated from the ground electrode at inflection points where the second arcuate portion is transferred to the first arcuate portions and the combustion efficiency is lowered, from occurring. When the radius curvature r is suppressed to D1/5 or smaller, a flat face which is sufficient for facilitating the joining of the noble metal tip can be ensured.
When paying attention to the noble metal tip, it is preferable to set an outer diameter d of the noble metal tip to 0.4 mm≦d≦0.8 mm, and the ground electrode satisfies a relationship of D1−2r≧1.5d (tenth configuration). When the outer diameter d of the noble metal tip is smaller than 0.4 mm, it is difficult to realize a long lifetime even in the case where a noble metal which is excellent in spark wear resistance and resistance to oxidation at high temperatures is used. By contrast, when d is larger than 0.8 mm, the flame developing property is impeded, and this configuration is out of the spirit of the invention. In the case where such a noble metal tip is used, when D1−2r, i.e., the length of the flat face of the ground electrode is equal to or larger than 1.5d, the noble metal tip can be joined to the ground electrode while the noble metal tip is provided with sufficient strength.
By contrast, from the viewpoint of the joining property between the ground electrode and the noble metal tip, it is preferable to employ the following eleventh configuration. In the configuration, when the ground electrode is viewed from the side of the another end in the extending direction of the ground electrode,
the inner side face of the ground electrode is divided by two straight lines which pass through a portion of an outer peripheral face of the noble metal tip, the portion being closest to the ground electrode, and which are parallel to the tip axis, and, when a section of the divided inner side face which is between the two straight lines is set as a section A, and a section of the divided inner side face which excludes the section A is set as a section B, a radius curvature RA of an inner side face in the section A, and a radius curvature RB of an inner side face in the section B satisfy a relationship of RA≧RB.
As in the eleventh configuration, when, in the side face of the round ground electrode, the radius curvature RA of the inner side face of the portion to which the noble metal tip is to be joined (corresponding to the section A) is set to be equal to or larger than the radius curvature RB of the inner side face of the portion to which the noble metal tip is not to be joined (corresponding to the section B), the following effects can be expected. When the noble metal tip is pressed, the butting area of the near-center portion where the lower face of the noble metal tip and the side face of the round ground electrode are contacted with each other is increased. In resistance welding, for example, this causes the portion which melts in an initial stage of the resistance welding, to become larger than that in the conventional art. Therefore, the welding property is improved, and the defect that joining is performed in “floating state” can be reduced. Also when laser welding is conducted, the distance by which the side face of the round ground electrode is separated from the lower face of the noble metal tip can be shortened, and hence more stable joining is enabled. It is a matter of course that it is more preferable to set RA>RB while excluding RA=RB.
The risk of dropping off of the noble metal tip can be considered as follows.
Unlike a ground electrode which has a substantially rectangular section, and which is conventionally used, the round ground electrode does not have an edge in which concentration of an electric field easily occurs. In the periphery of the round ground electrode, consequently, there are few portions where the electric field gradient is steep. Originally, a spark discharge is performed between the tip end of the center electrode and the noble metal tip joined to the round ground electrode. Unlike a substantially rectangular ground electrode, however, there is no edge, and hence a spark discharge sometimes proceeds around the round ground electrode to occur on a side which is approximately opposite to the center electrode. The molten portion between the noble metal tip and the base material of the round ground electrode has a low work function, and hence a discharge easily occurs in the portion. Therefore, a spark discharge sometimes occurs in the portion. When the latter spark discharge repeatedly occurs, particularly, the portion to which the noble metal tip is joined is consumed to be decreased, and there is a possibility that the joining strength is lowered. The heat generated by combustion in an engine is not sufficiently dissipated from the noble metal tip. Consequently, even a possibility that the noble metal tip is peeled off from the ground electrode by vibration of the engine and drops off into the combustion chamber is caused.
In order to avoid the possibility, it is preferable to employ the following configuration which is a thirteenth configuration of the invention. In the configuration, a spark discharge gap G which is formed by the tip end of the center electrode and the noble metal tip joined to the ground electrode, and a gap H which is a minimum gap between the tip end of the center electrode and the ground electrode satisfy a relationship of 0.5 mm≦H−G≦1.0 mm.
When the thirteenth configuration of the invention is employed, the possibility that a spark discharge occurs on a side opposite to the center electrode, or that a spark discharge is performed on the molten portion between the round ground electrode and the noble metal tip can be reduced. In order to attain the effect more remarkably, preferably, the amount of the protrusion of the noble metal tip toward the center electrode with respect the joining face with the round ground electrode is equal to or longer than 0.5 mm and equal to or shorter than 1.0 mm.
A method of producing a spark plug having a configuration where a noble metal tip is joined to a round ground electrode may comprises: a resistance welding step of temporarily fixing the noble metal tip to the one side face of the ground electrode by resistance welding; and a step of, after the temporary fixation, applying laser welding on a whole periphery of a joining face between the noble metal tip and the ground electrode, thereby performing fixation. When a spark plug is produced in this manner, the noble metal tip having a flat lower face can be stably fixed to a very unstable portion, i.e., the side face of the round ground electrode.
Another production method is a method of producing a spark plug in which a columnar noble metal tip is laser welded to one side face of a columnar ground electrode, wherein the method comprises: means for positioning the noble metal tip with respect to the side face of the ground electrode; and means for performing welding while changing an irradiation angle or irradiating position of a laser beam in accordance with a positioning state of the noble metal tip. Even when the laser irradiation angle and the laser irradiating position are fixed, welding may be performed while changing the power in accordance with the positioning state.
When a spark plug is produced by the production method, the noble metal tip can be satisfactory joined to the columnar ground electrode. The means for positioning the noble metal tip with respect to the round ground electrode may be the temporary fixation by means of resistance welding, or may use a positioning jig. Namely, any means may be used as far as the position of the noble metal tip with respect to the round ground electrode remains unchanged until the welding is completed.
Hereinafter, an embodiment of the invention will be described with reference to the accompanying drawings.
The center electrode 10 is formed into a substantially rod-like shape having a flange 11 in a rear end portion, with using a Ni-base alloy such as INCONEL 600 (registered trademark) as the base material. For the purpose of improving the thermal conductivity, a Cu alloy constitutes a core 12 in a center portion of the base material made of the Ni-base alloy. An inner tip 13 which is made of a noble metal alloy containing Pt, Ir, and the like, and which has a excellent durability is joined to the tip end of the center electrode 10. In the embodiment, the joining is performed by so-called laser welding in which laser beam irradiation and a pressing operation are conducted.
By contrast, the ground electrode 40 disposed on the metal shell 50 is configured by using a Ni-base alloy as the base material, formed into a rod-like shape, and joined to the tip end of the metal shell 50. The ground electrode 40 is bent at a substantially right angle so that one side face at the tip end cooperates with the inner tip 13 joined to the center electrode 10 to form a spark discharge gap G. For the purpose of improving the ignitability, the flame developing property, and the durability, an outer tip 43 made of a noble metal alloy in the same manner as the center electrode 10 is joined to one side face of the tip end portion of the ground electrode 40. The outer tip 43 corresponds to “noble metal tip” in the invention. The configuration of the ground electrode 40 which is the essential point of the invention will be described later in detail.
The metal shell 50 is first formed into a substantially cylindrical shape by applying a plastic working process on an iron-base metal member such as S15C or S25C, or a metal member such as stainless steel. After a finishing process such as cutting, an approximate shape of the metal shell is formed. A thread portion 51 for mounting the spark plug 100 on an internal combustion engine which is not shown is formed in the tip end side of the outer peripheral face of the metal shell 50. On the outer surface of the rear end side of the thread portion 51, formed is a flange 52 having a seating face which, when mounted on an internal combustion engine, hermetically seals the combustion chamber via a gasket 4. A tool engagement portion 53 which has, for example, a hexagonal section shape, and with which, when attached to an internal combustion engine, a tool such as a plug wrench is to be engaged is formed on the rear end side of the flange 52. In the metal shell, a portion between the tool engagement portion 53 and the flange 52 is formed as a thin portion so that the portion buckles when the insulator 30 is attached (
The rear end side of the tool engagement portion 53 is formed into a thin cylindrical portion so that a crimp cover 60 which is configured as a rearmost end portion of the metal shell 50 in completion of the spark plug 100 is formed. In an inner hole 57 of the metal shell 50, a small-diameter hole 54 is formed in the axial position where the thread portion 51 is formed, and a shelf 55 which protrudes in an inner radial direction is formed on the tip end side of the small-diameter hole 54. In the rear end side continuous to the small-diameter hole 54, a large-diameter hole 56 is formed so as to extend from the axial position where the flange 52 is formed, to the rear end. The ground electrode 40 is joined to the tip end of the thus formed metal shell 50. This joining is performed by the resistance welding. After a welding protrusion is removed away, a process of plating with zinc or the like is conducted on the metal shell 50 together with the ground electrode 40.
The insulator 30 is prepared in the following manner. A binder or the like is mixed with insulating ceramic powder such as alumina or aluminum nitride, and then a pressing process is performed on the mixture to form an approximate shape of the insulator. The approximate shape is ground by a grinding wheel, and then fired. The insulator 30 has a substantially cylindrical shape. A shaft hole 31 is formed in the insulator. A flange-like middle trunk portion 32 which protrudes radially outward is formed in a substantially middle of the outer surface in the direction of the axis O. A tip-end side trunk portion 34 having a step 33 directed to the tip end is formed on the tip end side of the middle trunk portion 32. By contrast, a rear-end side trunk portion 35 having an approximately identical outer diameter is formed on the rear end side of the middle trunk portion 32. When the spark plug 100 is completed, a portion which is on the tip end side of the step 33 formed in the tip-end side trunk portion 34 constitutes a leg portion 36 which is to be exposed to the combustion gas. A support step 37 which supports the flange 11 of the center electrode 10 is formed on the rear end side of the leg portion 36. The inner diameter of the shaft hole 31 is formed so that the tip end side of the support step 37 is smaller in inner diameter than the rear end side. The axis O corresponds to “plug axis” in the invention.
The manner of combining the center electrode 10 and the terminal electrode 20 with the insulator 30 will be described. The center electrode 10 is inserted into the shaft hole 31 of the insulator 30 so that the tip end is downward directed. The flange 11 of the center electrode 10 is engaged with the support step 37 of the insulator 30. As well known, a glass seal material which is prepared by mixing a glass powder and a metal powder, and a resistance material which is prepared by changing the mixing ratio of the raw-material powders are filled into the shaft hole. The terminal electrode 20 is inserted into the rear end of the insulator 30 so that a leg portion 21 of the terminal electrode 20 which is formed into a shaft-like shape is buried by the glass seal material which is filled in the hole. The insulator 30 into which the terminal electrode 20 is inserted is loaded into a heating oven to heat the insulator to a predetermined temperature, and the terminal electrode 20 is pressed to be positioned at a predetermined position. Thereafter, the insulator 30 is unloaded from the heating oven. As a result, the glass seal material and the resistance material are hardened to be formed as glass seals 5, 5 and a resistor 6, respectively, and the center electrode 10 and the terminal electrode 20 are fixed together in a state they are electrically conductive to each other through the materials. Usually, this step is called the glass sealing step. During the glass sealing step, firing of a glaze layer which is to be formed on the outer surface of the rear-end side trunk portion 35 may be simultaneously conducted.
The components are configured as described above. The insulator 30 having the center electrode 10 and the terminal electrode 20, and the metal shell 50 having the ground electrode 40 form the crimp cover 60 by a well-known crimping step while using plate packing 7, wire packing 8, 8, talc 9, and the like in order to improve the hermetical sealing property, thereby completing the spark plug 100.
Hereinafter, the ground electrode 40 which is the essential point of the invention will be described in detail with reference to
As shown in
The distance (indicated by 1 in
The ground electrode 40 is joined to the tip end face 58 of the metal shell 50. The heat which is received by the ground electrode 40 through the joining portion is transmitted to the metal shell 50. If it is assumed that the ground electrode 40 has a columnar shape of a substantially true circle, the width DA is equal to the thickness l and is about φ1.4 mm (in
The outer tip 43 is joined by laser welding to the ground electrode 40 which has the above-described shape. In the welding of the outer tip 43, the tip is joined to the flat face 401 of the ground electrode 40 which is formed on the side of the center electrode 10, thereby enabling the joining to be performed at satisfactory joining strength. The outer tip 43 has a length h of 0.8 mm, and is formed into a columnar shape of φ0.7 mm. When the spark plug 100 is completed, the length h of the outer tip 43 is handled as the protrusion length h of the noble metal tip. In the embodiment which is illustrated by the diagram, the lengths are synonymous with each other. Strictly speaking, a molten portion is produced in the welding process, and hence the protrusion length h of the noble metal tip is sometimes shorter than the length h of the noble metal tip.
An arc which is drawn by virtually extending toward the center electrode 10 at the same radius curvature R as the arcuate portion 402 which begin at the both ends of the linear portion 401′ is indicated as a virtual extended arc VX. In
The invention is not restricted to the first embodiment, and may be configured as the second embodiment shown in
Unlike the first embodiment, the second embodiment has a mode which is configured so that the portion corresponding to the arcuate portion 402 in the first embodiment is divided at inflection points P into first arcuate portions 4021R, 4021L and a second arcuate portion 4022. According to the configuration, even when the width DB of the ground electrode 40 is equal to the width DA in the first embodiment, the joining area between the tip end face 58 of the metal shell 50 and the ground electrode 40 can be increased, and the heat dissipation property of the ground electrode 40 can be improved. In order to more effectively attain the effect, it is preferable to make the radius curvature R′ of the second arcuate portion 4022 larger than the radius curvature R of the first arcuate portions 4021R, 4021L. When the difference is excessively large, there arises a possibility that the gas flow (which means ignited flame, an uncombusted air-fuel mixture) is separated from the ground electrode 40 at the inflection points P. In order to suppress the separation of the gas flow, the difference between the radius curvatures may be limited to a certain degree, or the radius curvature in ranges beginning at middles of the first arcuate portions 4021R, 4021L may be gradually changed. The radius curvature of the first arcuate portions 4021R, 4021L may be set as that of the arc between the flat face 401 and the portion from which the width DB of the ground electrode 40 begins. When the radius curvature of the arc in the zone is not constant, the portion on the side of the flat portion 401 among the portions adjacent to the width DB may be deemed as the reference of the radius curvature.
Also in the second embodiment of
Next, a comparative example is shown in
In the same manner as the first embodiment, the center C of radius curvature R of the arcuate portion 402 is on the center line O′ of the ground electrode 40, but the tip end face of the outer tip 43 does not protrude more than the intersection of the virtual extended arc VX. According to the configuration, flame which is ignited at a substantial middle between the inner tip 13 and the outer tip 43 reaches the flat face 401 of the ground electrode 40 before the flame sufficiently grows, and spreading of the flame is impaired. In order to avoid this situation, preferably, a configuration in which, as shown in the first and second embodiments, the tip end face of the outer tip 43 protrudes more than the intersection of the virtual extended arc VX toward the center electrode 10 is employed.
For further comparison, the second comparative example shown in
In the same manner as the second embodiment, the second comparative example is configured so that the portion corresponding to the arcuate portion 402 is divided at the inflection points P into the first arcuate portions 4021R, 4021L and the second arcuate portion 4022 (in
In both the examples of
In the example of
In the example of
In the configuration which is common in
In both of the first and second embodiments, the flat face 401 is positioned between the midpoint m of h+1 and the tip end face of the noble metal tip 43. By contrast, in the second comparative example of
Next, the third embodiment shown in
As shown in
The value which can be taken by the thickness l of the ground electrode 40 in the third embodiment is determined in the same manner as the first embodiment. Therefore, the description of the value is omitted.
Next, the width DD of the ground electrode 40 will be described. The end points P1, P2 of the outer arcuate portion 3403 are extended until intersecting with an extension line VE of the linear portion 3401′, thereby drawing virtual arcs VA1, VA2. The intersections are indicated by J1, J2, respectively, and the distance between J1 and J2 is indicated by D1 (hereinafter, referred to as the intersection distance D1). In the embodiment, the intersection distance D1 corresponds to two times the radius curvature of the outer arcuate portion 3403. In further consideration of the production tolerance, the distance can take a value which is slightly larger than 2R. It is a matter of course that margins are set and the distance may be smaller than 2R. The end points P1, P2 correspond to “other ends of said first arcuate portions” in the invention.
The radius curvature r of the inner arcuate portions 3402 will be described.
The radius curvature r can be set with reference to the intersection distance D1. Tests which were conducted in order to determine the radius curvature r will be described. Spark plugs 100 in which the intersection distance D1 is 2.7 mm, and the radius curvature r of the inner arcuate portions 3402 are changed in the step of 0.1 mm were prepared. The ignition limit air-fuel ratio was obtained for each of the spark plugs 100. The ignition limit is determined in the following manner. The air-fuel ratio is gradually increased. When the number of occurrences of misfiring reaches 1% (in 1,000 tests, the number of occurrences of misfiring is ten), it is determined that the ratio reaches the limit. Results are shown in Table 1 and
From these results, it is confirmed that, when the radius curvature r is 0.3 mm or more, i.e., when the radius curvature is equal to or larger than 1/9 of the intersection distance D1, the ignition limit air-fuel ratio is remarkably improved. When the radius curvature r is smaller than D1/9 (for example, r=0.2 mm), the inner arcuate portions are excessively small, and hence it may cause a phenomenon that a gas flow in a combustion chamber is disturbed at the inflection points (corresponding to the end points P1, P2 in
The outer diameter d of the outer tip 43 is set so as to be equal to or larger than 0.4 mm and equal to or smaller than 0.8 mm, on the basis of the relationships of the durability and the flame developing property. In further consideration of the outer diameter d, the intersection distance D1, the radius curvature r of the inner arcuate portions 3402, and the outer diameter d of the outer tip 43 satisfy a relationship of D1-2r≧1.5d. The dimension of D1-2r means the width which can be obtained in the flat face 3401 on the side of the center electrode 10. When the dimension is smaller than 1.5d, there arises a possibility that the joining strength of the outer tip 43 is lowered, thereby causing fear of a trouble such as a dropping off of the tip. Moreover, d means the outer diameter of the columnar noble metal tip. After the spark plug is completed, therefore, it corresponds to the outer diameter of the tip end (on the side of the center electrode 10) of the outer tip 43.
The intersection distance D1, and the thickness l, width DD, and radius curvatures r, R of the ground electrode 40 which are determined as described above have respectively the following values:
D1=2.7 mm,
1=1.3 mm,
DD=2.5 mm,
r=0.4 mm,
R=1.3 mm.
As described above, according to the invention, even in the case where the dimensions cannot be sufficiently ensured, such as the case where the thread diameter of the metal shell is M12 or less, or the thickness of the tip end face of the metal shell is, for example, 1.0 to 1.5 mm, provision of a flame developing property equivalent to that of a round ground electrode, and a prolonged lifetime can be realized.
Various modifications of the third embodiment may be formed. In the third embodiment, for example, the radius curvature R of the outer arcuate portion 3403 is set so that, when the virtual arcs VA1, VA2 are drawn, they intersect with the extension line VE of the linear portion 3401′ of the inner flat face 3401 to form a semicircle having a radius of R. Even when the ground electrode 40 is produced under this setting, a small flat face may be possibly formed at the midpoint M (the fourth embodiment, see
In the embodiment, a core member is not used in the ground electrode 40. When a spark plug is produced while being specialized in improvement of the performance, a well-known Cu core or the like may be used.
Next, the third to fifth embodiments are examined.
Spark plugs 100 in which the intersection distance D1 is 2.7 mm, the thickness l of the ground electrode 40 is 1.3 mm, the width DD of the ground electrode 40 is 2.5 mm, the radius curvature r is 0.4 mm, the radius curvature R is 1.3 mm, the tip diameter d is 0.7 mm, and the gap G is 1.1 mm are prepared. In the spark plugs 100, the gap H between the tip end of the inner tip 13 joined to the center electrode 10 and a portion of the ground electrode 40 which is closest to the center electrode 10 is variously changed so that H−G has relationships listed in Table 2 below.
The produced spark plugs 100 are subjected to a desk spark test in which a spark discharge is produced 100 times under atmospheric pressure of 0.4 MPa. The number of spark discharges in which a discharge occurs not on the outer tip 43 joined to the ground electrode 40 but on the molten portion for joining the outer tip 43 with the base material of the ground electrode 40 is counted. The incidence rate is shown as the occurrence rate of side sparks.
By contrast, spark plugs 100 which were produced in the same manner as described above so that H−G has relationships listed in Table 3 below were subjected to a simulation test which corresponds to a travel distance of 100,000 km in an actual vehicle, and the increase of the gap was checked. A four-cylinder engine having 2,000 cc displacement was used in the test.
From Table 2 above, the result that, when H−G is 0.5 mm or more, the occurrence rate of side sparks is 0% was obtained. With respect to H−G, therefore, 0.5 mm is set as the lower limit. By contrast, the consumption of the noble metal tip 43 will be considered. When H−G exceeds 1 mm, specifically, when H−G is 1.2 mm, the gap increase exceeds 0.2 mm. In a high-durability spark plug, usually, it is requested that the gap increase after a travel distance of 100,000 km is 0.2 mm or less. With respect to H−G, therefore, 1.0 mm is set as the upper limit.
Next, a sixth embodiment will be described.
As shown in
Referring to
Two straight lines (indicated by long-short dash lines) which divide the section A from the section B, and which are parallel to the axis O′ of the outer tip 43 are requested to pass through a portion of the outer tip 43 which is closest to the ground electrode 40, and may perform the division in any manner. In this description, the division lines are coincident with the outer peripheral edge of the outer tip 43. When the radius curvatures RA, RB of the sections A, B which are formed as a result of the division are compared with each other, the relationship of RA>RB is obtained. Therefore, the joining face of the ground electrode 40 with respect to the outer tip 43 can be made substantially flat, and hence more firm joining strength can be obtained.
Next, a step of welding the noble metal tip 43 to the columnar ground electrode 40 will be described.
The welding step is roughly divided into a positioning step of positioning the noble metal tip 43 with respect to the ground electrode 40, and a welding step of joining the positioned noble metal tip 43 to the ground electrode 40.
First, the states of the ground electrode 40 and the noble metal tip 43 in the positioning step will be described.
The positioning step may be performed by a method in which a state where the noble metal tip 43 butts against the ground electrode 40 but not joined is held by a holding tool (not shown), i.e., means for positioning in a so-called non-joined state, or a method in which the noble metal tip 43 is joined to the ground electrode 40 in a simple manner, i.e., means for positioning in a temporary joined state (temporary fixation).
As shown in
As shown in
Next, the welding step of joining the noble metal tip 43 to the ground electrode 40 will be described.
In the diagram of
The irradiation with the laser beam LB is sequentially conducted in the circumferential direction of the noble metal tip 43. When the irradiation proceeds by about 90° from the start position 0° of
When the laser welding further proceeds by about 90°, i.e., at a position of 180° from the start position 0°, the irradiation angle returns to θ1. At a position where the laser welding further proceeds by about 90° (position of 270° from the start position 0°), the irradiation angle is again set to θ2.
In the above, the welding step has been described. Alternatively, the irradiation angle of the laser beam LB may be fixed, and the irradiation energy of the laser beam LB may be changed depending on the position to be irradiated with the laser beam LB.
The irradiation angle θ and the irradiation energy are not always changed in a sinusoidal manner as shown in
The thus produced spark plug 100 is realized as a spark plug in which a noble metal tip is firmly joined to a round ground electrode, and which is excellent in spark wear resistance and has a superior flame developing property.
As the fifth embodiment of the invention, the example in which the radius curvatures RA, RB of the sections A, B have the relationship of RA>RB has been described. As set forth in the appended claims, a configuration in which a relationship of RA=RB is established, or in which the outline of the ground electrode 40 forms a substantially true circle can be employed.
This application is based on Japanese Patent application JP 2005-297046, filed Oct. 11, 2005, Japanese Patent application JP 2005-297047, filed Oct. 11, 2005, and Japanese Patent application JP 2005-297048, filed Oct. 11, 2005, the entire contents of which are hereby incorporated by reference, the same as if set forth at length.
| Number | Date | Country | Kind |
|---|---|---|---|
| P2005-297046 | Oct 2005 | JP | national |
| P2005-297047 | Oct 2005 | JP | national |
| 2005-297048 | Oct 2005 | JP | national |
