EP0701311A1

EP0701311A1 - Spark plug with radial spark gap

Info

Publication number: EP0701311A1
Application number: EP95202208A
Authority: EP
Inventors: Randolph Kwok-Kin Chiu; Keith Allen Penney; John Franklin Mckeon Jr.
Original assignee: Motors Liquidation Co
Current assignee: Motors Liquidation Co
Priority date: 1994-09-06
Filing date: 1995-08-14
Publication date: 1996-03-13

Abstract

A spark plug is provided which is suitable for use in an internal combustion engine. The spark plug is configured to have a radial spark gap to promote flame propagation within the combustion chamber, and is also equipped with a ground electrode which is specially configured to enhance the performance and service life of the spark plug, and to enable the radial spark gap to be readily and accurately set in mass production. In particular, the ground electrode is radially spaced apart from the longitudinal axis of the center electrode, and is equipped with a firing tip attached to a radial surface of the ground electrode adjacent the distal end of the ground electrode. The ground electrode firing tip projects approximately radially from the ground electrode toward the firing tip of the center electrode, and is axially spaced from the insulator approximately the same distance as the firing tip of the center electrode, such that the firing tips are disposed proximate each other and define the radial spark gap of the spark plug. As a result, the length of the ground electrode is equal to or only slightly greater than the electrode projection of the spark plug. The benefits of the ground electrode configuration include a lower operating temperature which results in a longer service life for the ground electrode firing tip, a reduced tendency for quenching the flame kernel, and a firing location placed a maximum depth in the combustion chamber for a given electrode projection so as to improve combustion without promoting pre-ignition.

Description

The present invention generally relates to a spark plug of the type used in an electric spark ignition system of an internal combustion engine. In particular, this invention relates to such a spark plug having a ground electrode which extends axially from the body of the spark plug and has a firing tip extending radially therefrom which forms a radial spark gap with a center electrode of the spark plug. As a result, the ground electrode operates at a cooler temperature, while thermal fatigue of the firing tip is reduced, and also the manufacture and gap setting of the spark plug is facilitated.

Background of the Invention

Spark ignition of an internal combustion engine generally involves igniting an air/fuel mixture with an electric spark generated between a center electrode and a ground electrode of a spark plug. The facing surfaces of the center and ground electrodes are typically flat, and serve as arcing or firing surfaces between which the electric spark is generated. Typically, the electrodes are formed from a nickel-base alloy which is resistant to the harsh electrical, thermal, chemical and mechanical environment of an engine's combustion chamber. The nickel-base alloy is often applied over a copper core which improves the thermal conductivity of the electrodes, such that excessive electrode temperatures are avoided that might otherwise cause auto-ignition.
It is known in the art to substitute a noble metal for the more conventional nickel alloys for the purpose of extending the life of the electrode. The use of a noble metal electrode is particularly advantageous when attempting to minimize the size of the electrode. As is known, minimizing the size of an electrode reduces the potential for a phenomenon known as flame quenching or extinguishing, which occurs when an excessive amount of thermal energy in the flame kernel produced within the spark gap is absorbed by the electrode. As is also known in the prior art, a smaller electrode also serves to lower the demand voltage of its spark plug.
To minimize the amount of noble metal required, a noble metal firing tip, such as a thin platinum alloy disk, may be attached to the firing surface of an otherwise conventional nickel alloy electrode for the purpose of minimizing the amount of noble metal required. U.S. Patent No. 4,700,103 to Yamaguchi et al. teaches a variation of this, in which firing tips having minimal mass are welded to and project from the spark plug's ground and center electrodes. As a result, the firing tips are specifically configured to benefit from the advantages noted above with smaller electrodes. Several ground electrode firing tips taught by Yamaguchi et al. have their longitudinal axes oriented transverse to the axis of the center electrode. Other ground electrode firing tips are shown as being offset from the center electrode, such that only an edge of the firing tip is proximate to the center electrode.
Notably, Yamaguchi et al. also disclose various spark plug configurations which employ a radial spark gap, as opposed to the more conventional axial spark gap. An advantage to the use of a radial spark gap is that the spark flame is more able to freely propagate from the spark gap into the combustion chamber. In addition, use of a radial spark gap allows the firing location of the electrodes to project deeper into the combustion chamber for a given electrode projection into the chamber. As is known in the art, the distance that the electrodes of a spark plug are allowed to project into the combustion chamber is limited by the stroke of the piston within the combustion chamber.
Advantageously, the use of a radial spark gap enables the firing location to be placed deeper within the combustion chamber for a given electrode projection, in that the firing tip of the ground electrode is not disposed on the surface of the ground electrode facing the center electrode. Therefore, radial spark gaps are particularly advantageous for certain engine applications which require or benefit from long electrode projections.
However, Yamaguchi et al. primarily utilize a conventional J-shaped ground electrode, which results in a relatively long heat path from the ground electrode firing tip to the spark plug's shell, to which the ground electrode is attached. Specifically, the length of the heat path is significantly longer than the electrode projection of the spark plug, often by a factor of about two. As a result, the operating temperature of the ground electrode is higher, which promotes erosion of the ground electrode during operation. Higher thermal stresses are also created within the noble metal firing tips as a result of the higher operating temperature and different coefficients of thermal expansion between the electrode material and the firing tip material.
As a result, the durability and reliability of the noble metal firing tips are reduced. In addition, the relatively large mass of a J-shaped ground electrode increases the tendency for the electrode to serve as a heat sink and quench the flame kernel, thereby limiting the propagation of the flame front from the flame kernel. Finally, a spark plug configured to have a relatively long electrode projection and a J-shaped ground electrode is more susceptible to pre-ignition due to the electrode's relatively high operating temperature.
While Figure 19 of Yamaguchi et al. depicts a ground electrode which is not J-shaped, but rather bent and inclined toward the center electrode, the firing tip is imbedded in the ground electrode so as to provide an edge of the firing tip as the firing location. The firing tip projects from the distal end of the ground electrode and is flush with the lateral surface of the ground electrode closest to the center electrode. As a result, the firing tip is attached to the ground electrode prior to assembly, necessitating that the required length of the ground electrode be accurately predetermined in order to precisely locate the firing tip relative to the center electrode so as to form an appropriate spark gap. Accordingly, it is difficult to manufacture a spark plug with this ground electrode configuration to have a repeatable spark plug gap due to inherent variations in ground electrode length which result from typical manufacturing tolerances.
From the above, it can be appreciated that it would be desirable to provide a spark plug whose electrode configuration offers the advantages of noble metal firing tips and a radial spark gap, but with improved performance and service life. In particular, such a spark plug would have a ground electrode with a relatively low operating temperature so as to promote the service life of the ground electrode and its firing tip, while being configured to minimize the potential for flame quenching, maximize the distance which the firing location can extend into the combustion chamber without promoting pre-ignition, and enable accurate gapping of the firing tips in mass production.

Summary of the Invention

It is an object of this invention to provide a spark plug having a radial spark gap so as to promote flame propagation within a combustion chamber.
It is another object of this invention that such a spark plug have a ground electrode of minimal mass so as to reduce the tendency for the ground electrode to quench the flame kernel.
It is yet another object of this invention that the ground electrode have a minimal length to reduce the heat path from the firing location on the ground electrode, so as to reduce the operating temperature of the ground electrode and thereby extend the service life of the spark plug.
It is a further object of this invention that the ground electrode optimize the depth which the firing location can be placed in the combustion chamber for a given electrode projection so as to improve combustion characteristics, while simultaneously reducing the tendency for pre-ignition.
It is yet a further object of this invention that the spark plug be equipped with firing tips formed from a noble metal so as to further extend the service life of the spark plug.
It is still another object of this invention that the electrodes of the spark plug be configured to allow the radial spark gap to be readily and accurately set in mass production.
In accordance with a preferred embodiment of this invention, these and other objects and advantages are accomplished as follows.
According to the present invention, there is provided a spark plug which is suitable for use in a spark ignition system of an internal combustion engine. The spark plug is configured to have a radial spark gap to promote flame propagation within the combustion chamber, and is also equipped with a ground electrode which is specially configured to enhance the performance and service life of the spark plug, and enable the radial spark gap to be readily and accurately set in mass production. In addition, the spark plug is equipped with a noble metal firing tip on each of its electrodes for the purpose of extending the life of the spark plug.
The spark plug of this invention generally includes center and ground electrodes which define a radial spark gap across which an electric spark can be generated for igniting a fuel mixture within the combustion chamber of an internal combustion engine. Mounted to each of the electrodes is a firing tip, such that the firing tips are aligned to be approximately coaxial. The pair of firing tips are preferably formed to be substantially smaller than the electrodes for the purpose of reducing heat loss to the electrodes, while also serving to reduce demand voltage as a result of their smaller diameters. As such, it is preferable to form the firing tips from a noble metal alloy to promote the ability of the firing tips to resist erosion during operation. The electrodes are preferably formed from an electrically conductive material having a coefficient of thermal expansion approximately equal to that of the noble metal alloy from which the firing tips are formed, so as to promote the integrity of the bond between the firing tips and their electrodes.
The spark plug includes an electrically conductive shell which defines the longitudinal axis of the plug, and a electrically nonconductive body, or insulator, disposed along the longitudinal axis. The center electrode projects from the nonconductive body substantially along the longitudinal axis, while the ground electrode projects from the shell, terminating at a distal end. A firing tip is attached to the center electrode so as to be axially spaced from the insulator, such that the firing tip provides a firing location axially spaced from the insulator.
Contrary to the prior art, such as the J-shaped ground electrodes whose distal ends intersect the longitudinal axis of the spark plug, the ground electrode of this invention is radially spaced apart from the longitudinal axis along its entire length. Furthermore, the ground electrode is equipped with a firing tip attached to a radial surface of the ground electrode which is disposed axially from the proximate to the distal end of the ground electrode. The ground electrode firing tip projects approximately radially from the ground electrode toward the firing tip of the center electrode, and is axially spaced from the shell approximately the same distance as the firing tip of the center electrode, such that the firing tips are disposed proximate each other and define the radial spark gap of the spark plug.
As a result of the above configuration, the length of the ground electrode is approximately equal to the electrode projection of the spark plug - i.e., the axial distance between the outermost electrode and the end of the shell. Therefore, depending on the location of the firing tip on the ground electrode, the ground electrode provides a heat path whose length is approximately equal to or less than the electrode projection of the spark plug. Such a result is contrary to conventional J-shaped ground electrodes, which create a heat path that is significantly longer than the electrode projection of the spark plug, often by a factor of about two.
In accordance with this invention, several preferred configurations for the ground electrode exist. One such configuration is to form the ground electrode to be substantially parallel to the longitudinal axis of the spark plug. An alternative configuration is to form the ground electrode to be substantially straight but disposed at an angle to the longitudinal axis of plug. Yet another suitable configuration is to form an arc in the ground electrode, such that a distal portion of the electrode is substantially parallel to the longitudinal axis of the plug, with the firing tip being attached to the distal portion of the ground electrode.
The present invention allows for alternative configurations for the center electrode, in which the firing tip of the center electrode is secured to a distal surface of the electrode such that the firing tip projects axially from the electrode, or is secured to a radial surface of the electrode such that the firing tip projects radially from the electrode.
From the above, it can be seen that ground electrodes formed in accordance with this invention are specially configured to have a minimal heat path length so as to reduce its operating temperature, and also are able to be manufactured to have minimal ground electrode shrouding so as to reduce the tendency for quenching of the flame kernel, as in the case of a conventional spark plug equipped with a J-shaped ground electrode.
In addition, ground electrodes of this invention are configured to optimize the distance the firing location extends into the combustion chamber for a given electrode projection so as to improve combustion, yet simultaneously reduces the tendency for pre-ignition caused by an excessively large and hot ground electrode extending deeply into the combustion chamber.
Finally, because the radial spark gap between the ground and center electrodes is not dependent on the length of the ground electrode, the spark gap can be readily and accurately set during the manufacture of the spark plug in mass production.
Other objects and advantages of this invention will be better appreciated from the following detailed description.

Brief Description of the Drawings

The above and other advantages of this invention will become more apparent from the following description taken in conjunction with the accompanying drawings, in which:

Figure 1 shows a side view of a prior art spark plug having a conventional J-shaped ground electrode; and
Figures 2 through 5 are side views of ground and center electrodes in accordance with preferred embodiments of this invention.

Detailed Description of the Invention

The present invention provides a spark plug which is adapted for use in a spark ignition system of an internal combustion engine. For illustrative purposes, a spark plug 10 of a type known in the prior art is represented in Figure 1. As shown, the spark plug 10 is generally conventional in its construction, in that it includes a steel shell 14 which houses an insulator body 12. The insulator body 12 electrically isolates a center electrode 20 from a ground electrode 18. The center electrode 20 is disposed within a passage in the insulator body 12, while the ground electrode 18 is L-shaped and welded to the shell 14.
As shown, the pair of electrodes 18 and 20 form an axial spark gap across which an electric spark is generated for igniting an air/fuel mixture within a combustion chamber of an engine. The distal end portion 16 of the ground electrode 18 intersects the longitudinal axis of the center electrode 20. As a result, the length of the ground electrode 18 is approximately twice the electrode projection of the spark plug 10. As used herein, the electrode projection is defined as the axial distance between the outermost surface of the furthest projecting electrode (the ground electrode 18 as shown here) and the near end of the shell 14.
Several notable disadvantages ensue as a result of the J-shaped ground electrode configuration. First, a relatively long heat path is present between the firing surface of the ground electrode 18 and the shell 14, such that the operating temperature of the ground electrode 18 is relatively high. As a result, the ground electrode will erode at a higher rate during operation. Another adverse consequence arises if noble metal firing tips are welded to the surfaces of the electrodes 18 and 20 in that thermal stresses, created within the noble metal firing tips due to differing coefficients of thermal expansion in the materials for the electrode 18 and the firing tip, increase as the operating temperature of the ground electrode 18 increases. As a result, the durability and reliability of the noble metal firing tips are reduced.
An additional potential shortcoming of the spark plug 10 is the shrouding of the flame kernel by the J-shaped ground electrode 18, which increases the tendency for the electrode 18 to serve as a heat sink capable of quenching the flame kernel, and thereby limiting the propagation of the flame front from the flame kernel. Finally, if the spark plug 10 is configured to have a relatively long electrode projection, the J-shaped ground electrode 18 increases the likelihood of pre-ignition due to the electrode's relatively high operating temperature.
In contrast to the prior art spark plug 10 of Figure 1, a spark plug configured in accordance with this invention is equipped with a ground electrode adapted to achieve a relatively low operating temperature so as to promote the service life of the ground electrode and its firing tip, while also being configured to minimize the potential for flame quenching and maximize the distance which the firing location can extend into the combustion chamber without promoting pre-ignition.
With reference to Figure 2, a first embodiment of a spark plug is shown to have a generally conventional construction to the extent that it includes a steel shell 114 which houses an insulator body 112. The insulator body 112 electrically isolates a center electrode 120 from a ground electrode 118. The center electrode 120 is disposed within a passage in the insulator body 112, while the ground electrode 118 is welded to the shell 114.
The electrodes 118 and 120 are preferably equipped with noble metal firing tips 122 and 124, respectively, which define the firing surfaces 126 and 128, respectively, for the electrodes 118 and 120. As shown, the firing tips 122 and 124 are resistance welded to facing radial surfaces 130 and 132, respectively, of the electrodes 118 and 120. The noble metal from which the firing tips 122 and 124 are formed is preferably a lead resistant platinum alloy, such as Pt-10Pd-6Ru, although other suitable alloys such as Pt-Ir, Pt-Ni, Pt-Ru and Pt-Pd could also be used, as well as others. The electrodes 118 and 120 are preferably formed from a suitable alloy having a thermal coefficient of expansion which is approximately equal to that of the noble metal from which the firing tips 122 and 124 are formed. For example, the center electrode 120 is preferably formed from a copper-cored Inconel 600 material, while the ground electrode 118 is preferably formed from Inconel 600, an Fe-15Cr-4Al alloy, or other suitable material.
The firing tips 122 and 124 are preferably resistance welded directly to the radial surfaces 130 and 132 of the electrodes 118 and 120, respectively. The firing tips 122 and 124 have a diameter of about 0.5 to about 1.4 millimeters. If the firing tips 122 and 124 have a round cross-section, a preferred diameter is about 0.9 millimeter, and if formed to have a square cross-section, a preferred width is about 0.8 millimeter, though these dimensions may vary greatly depending on the particular application.
The radial surfaces 130 and 132 of the electrodes 118 and 120 to which the firing tips 122 and 124 are welded are both preferably planar, so as to facilitate welding the tips 122 and 124 to the electrodes 118 and 120. To provide the radial surface 130 of the ground electrode 118, the ground electrode 118 may be formed to have a rectangular cross-section or a approximately circular or elliptical cross-section with at least one flat formed on one side, though other cross-sectional shapes could be used.
Preferably, the cross-sectional dimensions of the ground electrode 118 provide a cross-sectional area on the order of about 1.6 to about 4.7 square millimeters. In contrast, it is preferred to provide the center electrode 120 with a circular cross-section whose diameter is on the order of about 2 to about 2.9 millimeters, and then machine a flat on the center electrode 120 in order to form the radial surface 132. Although it is noted that these dimensions may foreseeably vary depending on the particular application.
In accordance with a preferred aspect of this invention, the ground electrode 118 of Figure 2 is oriented to be essentially parallel with the longitudinal axis of the spark plug and, therefore, substantially parallel to the center electrode 120. As shown, the firing tips 122 and 124 are coaxially aligned between the center and ground electrodes 120 and 118, and their firing surfaces 126 and 128 are substantially parallel to each other. Ideally, the firing tips 122 and 124 are located near the distal ends 116a and 116b of the electrodes 118 and 120, respectively, as shown. To provide an appropriate radial spark gap on the order of about 1.5 millimeters, each firing tip 122 and 124 radially projects approximately about 1 millimeter from its respective radial surface 130 or 132.
The ground electrode 118 of Figure 2 is advantageously characterized by a heat path whose length is slightly shorter than the axial distance which the ground electrode 118 projects from the shell 114. As such, the ground electrode 118 defines a heat path which is about one half that of a conventional J-shaped ground electrode, such that the operating temperature of the ground electrode 118 is relatively low. The ground electrode 118 also has minimal shrouding so as to reduce the tendency for quenching of the flame kernel.
In addition, because the firing tips 122 and 124 are located close to the distal ends 116a and 116b of the electrodes 118 and 120, the distance which the firing surfaces 126 and 128 can project into a combustion chamber for a given electrode projection is maximized, so as to further improve combustion characteristics over prior art spark plugs having radial spark gaps, while simultaneously reducing the tendency for pre-ignition because of the reduced length and operating temperature of the ground electrode 118. Finally, because the radial spark gap between the ground and center electrodes 118 and 120 is not dependent on the length of the ground electrode 118, the spark gap can be readily and accurately set during the manufacture of the spark plug in mass production.
To illustrate the above, a spark plug configured as that shown in Figure 2 to have a center electrode 120 which projects axially about 5.0 millimeters from the shell 114 and a spark gap of about 1.5 millimeters can be produced to have a maximum electrode projection of about 5.77 millimeters and a heat path of about 4.77 millimeters - i.e., the axial distance between the distal end 116a of the ground electrode 118 and the near end of the shell 114 is about 5.77 millimeters, and the distance which the center of the firing surfaces 126 and 128 projects into the combustion chamber from the shell 114 is about 4.77 millimeters. In contrast, the conventional spark plug 10 of Figure 1 with an identical electrode projection and an identical spark gap would require a ground electrode length of about 10.30 millimeters, resulting in a heat path of about 10 millimeters, yet only allow the midpoint between the firing surfaces to project about 3.75 millimeters into the combustion chamber - 1.02 millimeters less than that of the spark plug of Figure 2 for the same electrode projection.
A spark plug configuration as shown in Figure 2 can be achieved by utilizing generally conventional processing methods. Generally, a wire having an appropriate cross-sectional shape and formed of an appropriate material for the ground electrode 118 is welded to the end of the shell 114, and subsequently cut to a suitable length, for example, about 5.77 millimeters. A wire formed from a suitable noble metal alloy, such as the preferred Pt-10Pd-6Ru alloy, and having a diameter of about 0.9 millimeter if round or about 0.8 millimeter if square, is resistance welded to the radial surface 130 of the electrode 118. The wire is then cut and coined to form the flat firing surface 126. As noted previously, a suitable length for the firing tip 122 is about 1 millimeter, corresponding to a weight of about 20 milligrams.
A suitable wire is then assembled into the passage of the insulator body 112 to form the center electrode 120. The radial surface 132 is then machined or formed, to which suitable noble metal wire is resistance welded to form the firing tip 124 for the center electrode 120. The wire is then cut to the appropriate length, and coined to form the firing surface 128. The resulting insulator assembly is then inserted into the shell 114, and the firing tips 122 and 124 are aligned to form the desired radial spark gap. The spark gap is then set by inserting a gapping blade between the firing tips 122 and 124, and forcing the ground electrode 118 toward the center electrode 120 until contact is made between the firing tips 122 and 124 and the gapping blade.
Additional ground electrode configurations which achieve the above advantages of this invention are illustrated in Figures 3 through 5. Figure 3 illustrates a spark plug whose ground electrode 218 is substantially straight, or rectilinear, but disposed at an acute angle toward the center electrode 220 and the longitudinal axis of the spark plug. To compensate for the inclination of the ground electrode 218, the corresponding firing tip 222 is resistance welded to be substantially normal to the radial surface 230 of the ground electrode 218, but its firing surface 226 is formed to be substantially parallel to the center electrode 220.
The firing tip 224 of the center electrode 220 is also shortened to compensate for the inclination of the ground electrode 218 toward the center electrode 220. For this purpose, the firing tip 224 can be formed as a thin disk preferably having a diameter of about one millimeter and an axial length or thickness on the order of about 0.2 to about 0.4 millimeter, such that the entire firing tip 224 is recessed within the flat which forms the radial surface 232 of the center electrode 220. Alternatively, the flat which forms the radial surface 232 could be formed to extend radially deeper into the center electrode 220 so as to permit a firing tip 224 having a size on the order of that shown in Figure 2.
Manufacture of the spark plug represented by Figure 3 is essentially the same as that described for Figure 2, but requires that the ground electrode 218 be inclined toward the center electrode 220 after the insulator assembly has been inserted in the shell 214 and the firing tips 222 and 224 have been aligned.
Figure 4 illustrates a spark plug whose ground electrode 318 has a lower portion 318a which is substantially straight but disposed at an angle toward the center electrode 320, an arcuate intermediate portion 318b, and a distal portion 318c which is substantially straight and substantially parallel to the center electrode 320. To compensate for the inclination of the ground electrode 318, the firing tips 322 and 324 of both electrodes 318 and 320 are formed as thin disks having an axial length or thickness on the order of about 0.2 to about 0.4 millimeter and a diameter of about one millimeter. Again, the entire firing tip 324 is recessed within the flat which forms the radial surface 332 of the center electrode 320.
As before, the ground electrode 318 forms a heat path from its firing tip 322 to the shell 314. Though the heat path is slightly longer than those of the embodiments of Figures 2 and 3, the heat path length is still approximately equal to the spark plug's electrode projection - i.e., the axial distance between the outermost surface of the furthest projecting electrode (the distal end 316a of the ground electrode 318) and the near end of the shell 314. Furthermore, the heat path is substantially shorter than that for the prior art J-shaped ground electrode 18 of Figure 1, roughly by a factor of two.
Manufacture of the spark plug represented by Figure 4 is essentially the same as that described for Figure 2, but requires that the ground electrode 318 undergo a forming operation to generate the lower, intermediate and distal portions 318a, 318b and 318c. Ideally, this operation can be performed with the coining tool used to coin the firing tip 326 on the ground electrode 318, and before the insulator assembly has been inserted in the shell 314 and the firing tips 322 and 324 have been aligned.
Finally, Figure 5 illustrates a spark plug whose ground electrode 418 is similar to that of Figure 3, but which utilizes a center electrode firing tip 424 which projects axially from the distal end 416b of the center electrode 420. The firing tip 424 has a preferred diameter on the order of about 0.9 millimeter, necessitating that the firing tip 422 of the ground electrode 418 be longer than that required for the firing tips 122 or 222 of Figures 2 or 3. Advantages associated with this configuration include elimination of a flat radial surface on the center electrode 420, the firing tip 424 can be welded to the center electrode 420 prior to its assembly with the insulator 412, only the ground electrode's firing tip 422 need be aligned to properly form the spark gap, and the firing location is more towards the central axis of the spark plug such that the spark is more fully exposed to the gas mixture.
Manufacture of the spark plug represented by Figure 5 is essentially the same as that described for Figure 3, though without the requirement to form a flat radial surface on the center electrode 420 since the firing tip 428 is welded to the distal end 416b of the center electrode 420.
With each embodiment illustrated in Figures 2 through 5, the firing tips have been described as having a particular length so as to form an appropriate spark gap. However, the shape of any one of the firing tips could be altered as long as appropriate compensation is provided by inclining or shaping the ground electrode or alternatively through appropriately lengthening or shortening of the opposing firing tip.
With each of the spark plugs described above, it can be seen that a primary feature of this invention is that, in addition to the advantages made possible with the use of a radial spark gap, the ground electrodes are specially configured to have a minimal length for a given electrode projection into a combustion chamber, while simultaneously maximizing the distance which the firing surfaces will project into the combustion chamber. The advantages of this feature are (1) a heat path whose length is approximately equal to the distance that the electrodes project from the shell, such that the operating temperature of the ground electrode is relatively low; (2) the service life of the firing tips is prolonged due to the lower operating temperature of the ground electrode; (3) a ground electrode having minimal shrouding so as to reduce the tendency for quenching of the flame kernel; (4) firing tips which project deeply into the combustion chamber so as to further improve combustion characteristics over previous spark plugs having radial spark gaps; and (5) a reduced tendency for pre-ignition because of the reduced mass and operating temperature of the ground electrode.
In addition, an important advantage of this invention is that the width of the radial spark gap between the ground and center electrodes is not dependent on the length of the ground electrode, the radial spark gap can be readily and accurately set during the manufacture of the spark plug in mass production. In contrast, the spark plug illustrated in Figure 19 of Yamaguchi et al. has a firing tip which is welded into a recess on the underside of the electrode, such that the spark gap is not a true radial gap and is more difficult to set. In addition, the firing surface of the ground electrode's firing tip is limited to an edge of the firing tip, resulting in accelerated gap wear which causes the spark gap to increase rapidly during the life of the spark plug.
While our invention has been described in terms of a preferred embodiment, it is apparent that other forms could be adopted by one skilled in the art, for example, by substituting appropriate materials, modifying the geometry or construction of the components, or modifying the processing and assembly steps. Accordingly, the scope of our invention is to be limited only by the following claims.

Claims

A spark plug comprising:
an electrically conductive shell having a longitudinal axis;
an electrically nonconductive body disposed along said longitudinal axis of said shell;
a center electrode projecting from said body substantially along said longitudinal axis;
a first firing tip attached to said center electrode, said first firing tip being axially spaced from said body;
a ground electrode projecting from said shell and terminating at a distal end, said ground electrode being radially spaced apart from said longitudinal axis along the entire length of said ground electrode, said ground electrode having a radial surface disposed axially from said distal end; and
a second firing tip attached to said radial surface of said ground electrode and projecting approximately radially from said ground electrode toward said first firing tip of said center electrode, said second firing tip being spaced axially from said shell so as to define a firing location spaced an axial distance from said shell, such that said first and second firing tips are disposed proximate each other so as to define a radial spark gap therebetween.
A spark plug as recited in claim 1 wherein each of said first and second firing tips is formed from a noble metal alloy.
A spark plug as recited in claim 2 wherein each of said first and second firing tips is formed from an electrically conductive material having a coefficient of thermal expansion approximately equal to that of said noble metal alloy.
A spark plug as recited in claim 2 wherein said noble metal alloy is a lead resistant platinum alloy containing palladium and ruthenium.
A spark plug as recited in claim 1 wherein said ground electrode has a length approximately equal to the electrode projection of the spark plug.
A spark plug as recited in claim 1 wherein said ground electrode is substantially parallel to said, longitudinal axis of said shell.
A spark plug as recited in claim 1 wherein said ground electrode is substantially straight but disposed at an angle to said longitudinal axis of said shell.
A spark plug as recited in claim 1 wherein said ground electrode has a first portion disposed at an angle to said longitudinal axis of said shell, and a distal portion which is substantially parallel to said longitudinal axis of said shell, and wherein said second firing tip is attached to said distal portion of said ground electrode.
A spark plug as recited in claim 1 wherein said center electrode has a radial surface, and wherein said first firing tip is attached to said radial surface of said center electrode such that said first firing tip projects radially from said center electrode.
A spark plug comprising:
an electrically conductive shell having a central opening formed therein and disposed along a longitudinal axis of said shell;
an electrically nonconductive body received in said central opening in said shell;
a center electrode projecting from said body substantially along said longitudinal axis;
a first noble metal alloy firing tip attached to said center electrode such that said first firing tip projects from said center electrode, said first firing tip being axially spaced from said body;
a ground electrode projecting from said shell and terminating at a distal end, said ground electrode being radially spaced apart from said longitudinal axis along the entire length of said ground electrode, said ground electrode having a radial surface disposed axially from said distal end; and
a second noble metal alloy firing tip attached to said radial surface of said ground electrode and projecting radially from said ground electrode toward said first firing tip of said center electrode, said second firing tip being axially spaced from said body so as to define a firing location spaced an axial distance from said shell, such that said first and second firing tips are disposed proximate each other so as to define a radial spark gap therebetween;
whereby said ground electrode forms a heat path from said second firing tip to said shell, wherein said heat path has a length which is approximately equal to said axial distance between said firing location and said shell.
A spark plug as recited in claim 10 wherein said ground electrode has a length approximately equal to the electrode projection of the spark plug.
A spark plug as recited in claim 10 wherein said first and second firing tips are welded to said center and ground electrodes, respectively.
A spark plug as recited in claim 10 wherein each of said center and ground electrodes is formed from an electrically conductive material having a coefficient of thermal expansion approximately equal to that of said first and second firing tips.
A spark plug as recited in claim 10 wherein said first and second firing tips are formed from a lead-resistant platinum alloy containing palladium and ruthenium.
A spark plug as recited in claim 10 wherein said radial surface of said ground electrode is a planar surface.
A spark plug as recited in claim 10 wherein said center electrode has a radial surface, and wherein said first firing tip is attached to said radial surface of said center electrode such that said first firing tip projects radially from said center electrode.
A spark plug as recited in claim 16 wherein said radial surface of said center electrode is a planar surface.
A spark plug as recited in claim 16 wherein said ground electrode is substantially parallel to said longitudinal axis of said shell.
A spark plug as recited in claim 16 wherein said ground electrode has a first portion disposed at an angle to said longitudinal axis of said shell, and a distal portion which is substantially parallel to said longitudinal axis of said shell, and wherein said second firing tip is attached to said distal portion of said ground electrode.
A spark plug as recited in claim 10 wherein said ground electrode is substantially straight but disposed at an angle to said longitudinal axis of said shell.