EP0537156B1

EP0537156B1 - Spark plug for internal combustion engine

Info

Publication number: EP0537156B1
Application number: EP91907630A
Authority: EP
Inventors: Michael Lenk
Original assignee: Cooper Industries LLC
Current assignee: Cooper Industries LLC
Priority date: 1990-04-04
Filing date: 1991-04-02
Publication date: 1996-07-03
Anticipated expiration: 2011-04-02
Also published as: DE69120677D1; WO1991015887A1; DE69120677T2; ATE140107T1; GB9007659D0; EP0537156A4; AU7682291A; GB2242703A; EP0537156A1; GB2242703B

Abstract

The spark plug comprises an outer shell (1), a central electrode (2), an insulator (3) surrounding the central electrode (2) and at least one ground electrode (4B) connected at at least one point to the outer shell (1). At least one (4B) of the electrodes consists of an outer layer made of a corrosion resistant material (e.g. nickel or a nickel alloy) and of an inner core made of a material having good thermal conductivity characteristics and a good corrosion and erosion resistance (e.g. silver or a silver alloy). In front of the bottom end of the other electrode (2), i.e. at the spark gap (5) the inner core projects through the outer layer perpendicularly to the longitudinal axis of said one electrode (4B). Several embodiments of the spark plug as well as several methods for making the electrodes of the spark plug are described.

Description

The present invention relates to a spark plug comprising an outer shell, a central electrode, an insulator surrounding the central electrode, said central electrode having a top end portion and a bottom end portion, and further comprising at least one ground electrode connected at at least one point to said outer shell and forming a spark gap with the bottom end portion of the central electrode. At least one of the electrodes of the spark plug consists of an outer layer made of a corrosion resistant material and of an inner core made of a material having good thermal conductivity characteristics and a good corrosion and erosion resistance. The invention further relates to several methods for forming the electrodes of said spark plug.
Spark plugs as defined above, i. e. in particular spark plugs having at least one electrode made of two different materials (composite electodes), are well known in prior art. The materials most commonly used in such electrodes are nickel or a nickel alloy for the outer layer, and copper or a copper alloy, respectively silver or a silver alloy for the inner core.
The reason for making spark plug electrodes provided with an inner core of copper or of silver is to obtain a better performance of the plug, as compared to spark plug electrodes consisting of only one single material (such as for example nickel or a nickel alloy). The better performance is due to the higher thermal conductivity characteristics of copper and of silver, i. e. the inner core made of copper or of silver evacuates more rapidly the heat produced by the combustion of the air/fuel mixture in the combustion chamber of the engine, so that the electrodes of the spark plug will remain cooler when the engine is running. This cooling action has a positive effect on the performance and on the useful life of the spark plug because it reduces the corrosion and the erosion of the electrodes.
To the contrary of copper, silver has a verv good corrosion and erosion resistance, but its bad mechanical properties at high temperature (in particular as far as the ground electrode is concerned) and its relatively high price prevent the manufacture and use of spark plug electrodes completely made of pure silver, except possibly for very particular applications.
German patent application DE 24 044 54 shows a spark plug provided with a central electrode completely made of silver. As already said above such an electrode is very expensive and can only be manufactured for very particular applications. In other words, a spark plug provided with a central electrode made completely of pure silver cannot be manufactured with the best possible cost/benefit ratio.
German patent DE 26 14 274 discloses a spark plug wherein at least one of the two electrodes is made of an outer layer of a corrosion resistant material, such as for example a nickel alloy, and of an inner core made of silver. In the two embodiments shown in this patent the spark surface of each nickel/silver electrode, i.e. the spark surfaces forming the spark gap between the central electrode and the ground electrode consist of an inner core made of silver and of an outer ring made of nickel or of a nickel alloy, i.e. of two materials having different resistances to corrosion and erosion. This type of construction is in no way the best solution for guaranteeing the best performance and the longest possible useful life of composite electrodes. Indeed the silver core should ideally form alone the spark surface of a composite electrode of this type.
FR-A-2451648 describes a spark plug in which the central electrode comprises an outer layer of a material which has good thermal conductivity, for example copper or a copper alloy, and a central core of a material which is resistant to corrosion and erosion, e.g. nickel or a nickel alloy.
A first object of the present invention is to provide a novel spark plug having an improved performance and wherein the electrodes have a substantially prolonged useful life period.
A second object of the present invention is to provide a novel spark plug having the best possible cost/benefit ratio when compared with spark plugs provided with precious metal electrodes, or provided with copper cored nickel electrodes such as known in prior art.
According to the present invention there is provided a spark plug comprising an outer shell, a central electrode, an insulator surrounding the central electrode, and at least one ground electrode connected at at least one point to the outer shell, the central electrode having a spark surface at one end thereof which forms a spark gap with an opposed spark surface of the ground electrode, at least one of the electrodes comprising an outer layer of a first material and an inner core of one or more other materials, characterised in that the spark surface of said at least one electrode is provided by an exposed surface of said core which surface is substantially parallel to the longitudinal axis of said at least one electrode.
A spark plug according to the invention, wherein the inner core of at least one of the electrodes consists of one single materials, can either have the feature that the lateral surface or surfaces of said projection are completely surrounded by the outer lever of said one electrode or that only a portion of the lateral surface or surfaces or only some of the later surfaces of said projection are surrounded by the outer layer of said one electrode. Said one ground electrode has further the feature that the surface or surfaces of said projection, which are not surrounded bv the outer laver of said one electrode, are in alignment with the surface or surfaces of said electrode
In the same manner a spark plug according to the invention, wherein the inner core of at least one electrode consists of two materials, a first material nearest to the spark gap and a second material away from the spark gap, can either have the feature that the lateral surface or surfaces of said projection (first material) are completely surrounded by the outer layer of said one electrode or that only a portion of the lateral surface or surfaces or only some of the later surfaces of said projection (first material) are surrounded by the outer layer of said one electrode. Said one ground electrode has further the feature that the surface or surfaces of said projection (first material), which are not surrounded by the outer layer of said one electrode, are in alignment with the surface or surfaces of said electrode.
Another object of the invention is to provide a novel spark plug comprising electrodes wherein not only the good thermal conductivity, but also the good corrosion and erosion resistance of silver is fully used without prohibitive costs.
The materials preferably used in the electrodes of the spark plug according to the invention are (a) for the outer layer : a corrosion resistant material such as for example nickel or a nickel alloy, (b) for the single material inner core : a material having good thermal conductivity characteristics and a good corrosion/erosion resistance such as for example silver or a silver alloy, and (c) for the two material inner core : a first material (nearest to the spark gap) having good thermal conductivity characteristics and a good corrosion/erosion resistance such as for example silver or a silver alloy and a second material (away from the spark gap) having good thermal conductivity characteristics such as for example copper or a copper alloy. It is to be noted that other materials could be used and in particular the silver could principally be replaced by other precious metals.
It is a further object of the invention to provide several methods for forming several embodiments of an electrode of a spark plug such as disclosed above. These methods will be explained in conjunction with the detailed description of the appended drawings. Two methods will be described directly hereunder as examples.
One method comprises the steps of :

a) forming a tubular cup having a closed and an open end from a corrosion resistant material, said cup having an opening in its lateral wall near its closed end,
b) positioning a billet having a cylindrical form and made of a material having good thermal conductivity characteristics and a good corrosion and erosion resistance within the cup so as to contact the inner closed end surface of the cup and so as to contact the inner lateral surface of the cup except for a portion at its open end and bending said open end portion of the cup over the billet inside the cup, and
c) inserting the resulting composite billet, closed end first, into a close fitting bore of an extrusion die having a reduced diameter extrusion orifice and advancing a plunger into said bore to force most of the composite billet through the extrusion orifice, leaving above the extrusion orifice only an extrusion butt at the top end of the thus formed composite electrode.

In the electrode formed by this method the inner core consisting of the material having good thermal conductivity characteristics and a good corrosion/erosion resistance (i.e. the silver or the silver alloy) projects through the outer layer perpendicularly to the longitudinal axis of said one electrode and the lateral surface or surfaces of said projection are completely surrounded by the outer layer of said electrode.
Another method comprises the following steps :

a) forming a tubular cup having a closed and an open end from a corrosion resistant material in such a way that for one circumferential portion of the cup the length of the cup is different than for its other circumferential portion,
b) positioning a billet having a cylindrical form and made of a material having good thermal conductivity characteristics and a good corrosion and erosion resistance within the cup so as to contact the inner closed end surface and the inner lateral surface of the cup, and
c) inserting the resultant composite billet, open end first, into a close fitting bore of an extrusion die having a reduced diameter extrusion orifice and advancing a plunger into said bore to force most of the composite billet through the extrusion orifice, leaving above the extrusion orifice only an extrusion butt at the top end of the thus formed composite electrode.

In the electrode formed by this method the portion of the inner core consisting of the material having good thermal conductivity characteristics and a good corrosion/erosion resistance (i.e. the silver or the silver alloy) projects through the outer layer perpendicularly to the longitudinal axis of said one electrode and only a portion of the lateral surface or only some of the lateral surfaces of said projection are surrounded by the outer layer of said electrode
It is to be noted that if the composite electrode formed by the two above mentioned methods is a central electrode the extrusion butt at its top end can serve as a seat in the insulator of the spark plug. If said electrode is a ground electrode the extrusion butt is cut off and the electrode is then bent as required by the type of spark plug to be manufactured.
Other objects and advantages of the invention will become apparent and the different embodiments of the spark plug according to the invention as well as the different methods for manufacturing the electrodes of said spark plug will be better understood when reading the following portions of the description in conjunction with the appended drawings, wherein :

Fig. 1: is an axial sectional view of a spark plug according to a first embodiment of the invention;
Fig. 2: is an axial sectional view of a spark plug according to a second embodiment of the invention ;
Fig. 3: is an axial sectional view of a spark plug according to a third embodiment of the invention ;
Fig. 4: is an axial sectional view of a spark plug according to a fourth embodiment of the invention;
Fig. 5: is an axial sectional view of a spark plug according to a fifth embodiment of the invention;
Fig. 6: is an axial sectional view of a spark plug according to a sixth embodiment of the invention;
Fig 7: is an axial sectional view of a spark plug according to a seventh embodiment of the invention;
Fig. 8: is an axial sectional view of a spark plug according to an eighth embodiment of the invention;
Figs 9 through 12: are sectional views of the ground electrodes of respectively the embodiments of Figs 1,2,5 and 6;
Figs 13 through 16: are sectional views of the central electrodes of respectively the embodiments of Figs. 3,4,7 and 8;
Fig. 17: is a perspective view of a tubular cup used in a composite billet for manufacturing an electrode of a spark plug according to a first method of the invention;
Fig. 18 and 19: show, in axial sectional views, two successive stages of the composite billet comprising the tubular cup of Fig. 17;
Fig. 20: is an axial sectional view of a composite billet used for manufacturing an electrode of a spark plug according to a second method of the invention;
Figs 21 and 22: are sectional views of a die, of a composite billet, and of a plunger and show how an electrode of a spark plug is formed according to the first method of the invention;
Fig. 23: is a perspective view of a tubular cup used in a composite billet for manufacturing an electrode of a spark plug according to a third method of the invention;
Fig. 24: is an axial sectional view of the composite billet comprising the tubular cup of Fig. 23;
Fig. 25: is an axial sectional view of a composite billet used for manufacturing an electrode of a spark plug according to a fourth method of the invention; and
Figs 26 and 27: are sectional views of a die, of a composite billet, and of a plunger and show how an electrode of a spark plug is formed according to the third method of the invention.

Figs 1 through 8 show eight different embodiments of a spark plug according to the invention. It is however to be noted that these embodiments are only examples because many other combinations are possible between (a) the ground electrodes according to the invention, (b) various prior art central electrodes, (c) the central electrodes according to the invention, (d) various prior art ground electrodes, and it is one of the major advantages of the invention to allow for a very large number of combinations, so that an adequate high performance spark plug can be provided for many different applications.
Figs 1, 2, 5 and 6 relate to spark plugs provided with different ground electrodes according to the invention and Figs 3, 4, 7 and 8 relate to spark plug provided with different central electrodes according to the invention. Each of the spark plugs comprises an outer shell 1, a central electrode 2, 2A, 2B, 2C, 2D, an insulator 3 and a ground electrode 4, 4A, 4B, 4C, 4D. Between the central electrode and the ground electrode of each spark plug there is a spark gap 5, 5A which can be located either beneath or on the side of the lower portion of the corresponding central electrode.
The invention relates in particular to the structure of the ground and of the central electrodes such as respectively shown in Figs 1, 2, 5, 6 and Figs 3, 4, 7, 8. Four possible structures of the ground electrode according to the invention are represented separately in Figs 9 through 12 and four possible structures of the central electrode according to the invention are represented separately in Figs 13 through 16.
Fig. 9 shows the first embodiment (see Fig. 1) of an electrode according to the invention, i. e. an elbow-shaped ground electrode 4A comprising an outer layer 6 and a single material inner core 7, said inner core 7 projecting (8) through the outer layer 6 perpendicularly to the longitudinal axis of the electrode near its bottom end surface 9. The inner core 7 does not extend up to the two end surfaces 9,10 of the electrode.
Fig. 10 shows the second embodiment (see Fig. 2) of an electrode according to the invention, i. e. an elbow-shaped ground electrode 4B comprising an outer layer 16 and a single material inner core 17, said inner core 17 projecting (11) through the outer layer 16 perpendicularly to the longitudinal axis of the electrode at its bottom end surface 12. The inner core 17 does not extend up to the top end surface 13 of the electrode, but it is in alignment with the bottom end surface 12 of the electrode.
The materials used in the ground electrodes of Figs 9 and 10 are : (a) a corrosion resistant material such as for example nickel or a nickel alloy for the outer layers 6, 16 and (b) a material having good thermal conductivity characteristics and a good corrosion and erosion resistance such as for example silver or a silver alloy for the inner cores 7,17.
It is to be noted that for an easy reading of the appended drawings the the outer layer of the ground electrodes 4A, 4B, 4C, 4D and of the central electrodes 2A, 2B, 2C, 2D according to the invention have not been hatched in any of the sectional views of said drawings.
Fig. 11 shows the fifth embodiment (see Fig. 5) of an electrode according to the invention, i. e. an elbow-shaped ground electrode 4C comprising an outer layer 26 and a two material inner core 27A (first material) 27 (second material), the portion 27A of the inner core projecting (14) through the outer layer 26 perpendicularly to the longitudinal axis of the electrode near its bottom end surface 15. The inner core 27A, 27 does not extend up to the two end surfaces 15, 18 of the electrode.
Fig. 12 shows the sixth embodiment (see Fig. 6) of an electrode according to the invention , i. e. an elbow-shaped ground electrode 4D comprising an outer layer 36 and a two material inner core 37A (first material) 27 (second material), the portion 37A of the inner core projecting (19) through the outer layer 36 perpendicularly to the longitudinal axis of the electrode at its bottom end surface 20. The portion 37 of the Inner core does not extend up to the top end surface 21 of the electrode, but the portion 37A of the inner core is in alignment with the bottom end surface 20 of the electrode
The materials used in the ground electrodes of Figs 11 and 12 are : (a) a corrosion resistant material such as for example nickel or a nickel alloy for the outer layers 26, 36, (b) a material having good thermal conductivity characteristics and a good corrosion and erosion resistance such as for example silver or a silver alloy for the first material (27A, 37A) of the inner cores and (c) a material having good thermal conductivity characteristics such as for example copper or a copper alloy for the second material ( 27, 37) of the inner cores.
The central electrode 2A of Fig. 13, i. e. the third embodiment of an electrode according to the invention (see Fig. 3) has basically the same structure as the ground electrode 4A of Fig. 9. It comprises an outer layer 6A, a single material inner core 7A and a seat 10A (extrusion butt). The inner core 7A projects (8A) through the outer layer 6A perpendicularly to the longitudinal axis of the electrode near its bottom end surface 9A.
The central electrode 28 of Fig. 14, 1. e. the fourth embodiment of an electrode according to the invention (see Fig. 4) has basically the same structure as the ground electrode 48 of Fig. 10. It comprises an outer layer 16A, a single material inner core 17A end a seat 13A (extrusion butt). The inner core 17A projects (11A) through the outer layer 16A perpendicularly to the longitudinal axis of the electrode at its bottom end surface 12A.
The materials used in the central electrodes of Figs 13 and 14 are (a) a corrosion resistant material such as for example nickel or a nickel alloy for the outer layers 6A, 16A and (b) a material having good thermal conductivity characteristics and a good corrosion and erosion resistance such as for example silver or a silver alloy for the inner cores 7A, 17A.
The central electrode 2C of Fig. 15, i. e. the seventh embodiment of an electrode according to the invention (see Fig. 7) has basically the same structure as the ground electrode 4C of Fig. 11. It comprises an outer layer 26A, a two material inner core 27B, 27C and a seat 18A (extrusion butt). The portion 27C of the inner core projects (14A) through the outer layer 26A perpendicularly to the longitudinal axis of the electrode near its bottom end surface 15A.
The central electrode 2D of Fig. 16 i. e. the eighth embodiment of an electrode according to the invention (see Fig. 8) has basically the same structure as the ground electrode 4D of Fig. 12. It comprises an outer layer 36A, a two material inner core 37B, 37C and a seat 21A (extrusion butt). The portion 37C of the inner core projects (19A) through the outer layer 36A perpendicularly to the longitudinal axis of the electrode at its bottom end surface 20A.
The materials used in the central electrodes of Figs 15 and 16 are : (a) a corrosion resistant material such as for example nickel or a nickel alloy for the outer layers 26A,36A, (b) a material having good thermal conductivity characteristics and a good corrosion and erosion resistance such as for example silver or a silver alloy for the first material (27C, 37C) of the Inner cores and (c) a material having good thermal conductivity characteristics such as for example copper or a copper alloy for the second material (27B, 37B) of the inner cores.
As already said above spark plugs of a very large number of types, with a choice of performances, can be manufactured by using each time one of the four ground electrodes 4A, 4B, 4C, 4D, (Figs 9 through 12) in combination with various prior art central electrodes or by using each time one of the four central electrodes 2A, 2B,2C,2D, (Figs 13 through 16) in combination with various prior art ground electrodes. For example : the central electrodes of Figs 1, 2, 5, 6 and the ground electrodes of Figs 3, 4, 7, 8 which are shown as single metal electrodes could be replaced by composite electrodes of a type or types different than the corresponding composite electrodes according to the invention.
Figs 17,18,19 and Figs 21,22 relate to the first method for making an electrode of a spark plug according to the invention. Fig. 17 represents a tubular cup 22 having a closed end 24 (Fig. 18) and an open end 28. In its lateral wall it is provided with an opening 23 near its closed end 24. The opening 23 can have various shapes such as for example : square, rectangular, circular, oval, etc. and it can have other dimensions than those shown. In Fig. 18 a cylindrical billet 25 has been inserted into the cup 22 and in Fig. 19 the open end portion 29 of the cup has been bent over the billet 25 inside the cup so as to form a composite billet 22A, 25. Fig. 21 shows how the composite billet 22A,25 is inserted into an extrusion die 40 and Fig. 22 shows how the electrode 2A is formed by extrusion.
This first method for making an electrode of a spark according to the invention comprises the steps of :

a) forming a tubular cup 22 having a closed and an open end 24, 28 from a corrosion resistant material, said cup 22 having an opening 23 in its lateral wall near its closed end 24,
b) positioning a billet 25 having a cylindrical form and made of a material having good thermal conductivity characteristics and a good corrosion and erosion resistance within the cup 22 so as to contact the inner closed end (24) surface ot the cup 22 and so as to contact the inner lateral surface of the cup 22 except for a portion at its open end 28 and bending (29) said open end portion of the cup 22 over the billet 25 inside the cup 22, and
c) inserting the resulting composite billet 22A, 25, closed end 24 first, into a close fitting bore 40A of an extrusion die 40 having a reduced diameter extrusion orifice 40B and advancing a plunger 41 into said bore 40A to force most of the composite billet 22A, 25 through the extrusion orifice 40B, leaving above the extrusion orifice 40B only an extrusion butt 10A (Fig. 13) at the top end of the thus formed composite electrode 2A.

The corrosion resistant material used in this first method is preferably nickel or a nickel alloy and the material having good thermal conductivity characteristics and a good corrosion and erosion resistance is preferably silver or a silver alloy.
A can be seen on Fig. 22 the electrode obtained by this first method is the central electrode 2A of Figs 3,13 which corresponds to the ground electrode 4A of Figs 1,9, the ground electrode 4A requiring the further steps of cutting off the extrusion butt 10A (Fig. 13) and of bending the body of the electrode.
Fig. 20 shows the composite billet 22A, 25B, 25A which is used in the second method for making an electrode according to the invention. It is substantially similar to the composite billet of Fig. 19, but comprises two cylindrical billets 258, 25A of different materials.
The second method for making an electrode of a spark plug according to the invention comprises the steps of :

a) forming a tubular cup 22 having a closed and an open end 24, 28 from a corrosion resistant material, said cup 22 having an opening 23 in its lateral wall near its closed end 24,
b) positioning a first billet 25B having a cylindrical form and made of a material having good thermal conductivity characteristics and a good corrosion and erosion resistance within the cup 22 so as to contact the inner closed end (24) surface of the cup 22 and so as to contact the inner lateral surface of the cup 22 up to somewhat beyond said opening 23,
c) positioning a second billet 25A having a cylindrical form and made of a material having good thermal conductivity characteristics within the cup 22 so as to contact the upper surface of the first billet and so as to contact the inner lateral surface of the cup 22 except for a portion at its open end 28 and bending (29) said open end portion of the cup 22 over the billets 25B, 25A inside the the cup 22, and
d) inserting the resulting composite billet 22A,25B,25A, closed end 24 first, into a close fitting bore 40A of an extrusion die 40 having a reduced diameter extrusion orifice 40B and advancing a plunger 41 into said bore 40A to force most of the composite billet 22A, 25B,25A through the extrusion orifice 40B, leaving above the extrusion orifice 40B only an extrusion butt 18A (Fig. 15) at the top end of the thus formed composite electrode 2C.

The corrosion resistant material used in this second method is preferably nickel or a nickel alloy, the material having good thermal conductivity characteristics and a good corrosion and erosion resistance is preferably silver or a silver alloy, and the material having good thermal conductivity characteristics is preferably copper or a copper alloy.
The electrode obtained by this second method is the central electrode 2C of Figs 7, 15 which corresponds to the ground electrode 4C of Figs 5, 11, the ground electrode 4C requiring the further steps of cutting off the extrusion butt 18A (Fig. 15) and of bending the body of the electrode
The opening 23 in the lateral wall of the tubular cup 22 used in the first and in the second method for making a spark plug according to the invention can, as already said above, have various shapes and dimensions. Consequently the projections 8, 14, 8A, 14A of the finished electrode 4A, 4C, 2A, 2C can also have various shapes and dimensions. In particular the dimensions of said projections 8, 14, 8A, 14A can be such that directly in front of the bottom end surface (spark surface) of the other electrode 2,4 of the spark plug only the outside surface of the inner core 7, 27A, 7A, 27C of said finished electrode 4A, 4C, 2A, 2C is visible. In other words, no portion of the outer layer 6, 26, 6A, 26A of said finished electrode 4A, 4C, 2A, 2C is located directly in front of the bottom end surface of the other electrode 2, 4 of the spark plug. Thus in this case only the metal (silver or a silver alloy) having the best corrosion and erosion resistance is located directly in front of the bottom end surface of the other electrode 2, 4.
Figs 23, 24 and Figs 26, 27 relate to the third method for making an electrode of a spark plug according to the invention. Fig 23 represents a tubular cup having a closed end 34 (Fig. 24) and an open end 38. In its lateral wall a recess 33 has been formed at Its open end 38. The recess 33 can have configurations and dimensions different from those shown. In Fig. 24 a cylindrical billet 35 has been inserted into the cup so as to form a composite billet 32, 35. Fig 26 shows how the composite billet 32, 35 is inserted into an extrusion die 40 and Fig. 27 shows how the electrode 28 is formed by extrusion.
The third method for making an electrode of a spark plug according to the invention comprises the steps of :

a) forming a tubular cup 32 having a closed and an open end 34, 38 from a corrosion resistant material in such a way that for one circumferential portion of the cup 32 the length A, B of the cup 32 is different than for its other circumferential portion,
b) positioning a billet 35 having a cylindrical form and made of a material having good thermal conductivity characteristics and a good corrosion and erosion resistance within the cup 32 so as to contact the inner closed end (34) surface and the inner lateral surface of the cup 32, and
c) inserting the resultant composite billet 32, 35, open end 38 first, into a close fitting bore 40A of an extrusion die 40 having a reduced diameter extrusion orifice 40B and advancing a plunger 41 into said bore 40A to force most of the composite billet 32, 35 through the extrusion orifice 40B, leaving above the extrusion orifice 40B only an extrusion butt 13A (Fig. 14) at the top end of the thus formed composite electrode 2B.

The corrosion resistant material used in this third method is preferably nickel or a nickel alloy and the material having good thermal conductivity characteristics and a good corrosion and erosion resistance is preferably silver or a silver alloy.
A can be seen on Fig. 27 the electrode obtained by this first method is the central electrode 2B of Figs 4,14 which corresponds to the ground electrode 4B of Figs 2,10, the ground electrode 4B requiring the further steps of cutting off the extrusion butt 13A (Fig. 14) and of bending the body of the electrode.
Fig. 25 shows the composite billet 32, 35B, 35A which is used in the fourth method for making an electrode according to the invention. It is substantially similar to the composite billet of Fig. 24, but comprises two cylindrical billets 35B, 35A of different materials.
The fourth method for making an electrode of a spark plug according to the invention comprises the steps of :

a) forming a tubular cup 32 having a closed and an open end 34, 38 from a corrosion resistant material in such a way that for one circumferential portion of the cup 32 the length A, B of the cup 32 is different than for its other circumferential portion,
b) positioning a first billet 35B having a cylindrical form and made of a material having good thermal conductivity characteristics within the cup 32 so as to contact the inner closed end (34) surface and a portion of the inner lateral surface of the cup 32,
c) positioning a second billet 35A having a cylindrical form and made of a material having good thermal conductivity characteristics and a good corrosion an erosion resistance within the cup 32 so as to contact the upper surface of the first billet and the remaining inner lateral surface of he cup 32, and
d) inserting the resulting composite billet 32, 35B, 35A, open end 38 first, into a close fitting bore 40A of an extrusion die 40 having a reduced diameter extrusion orifice 40B and advancing a plunger 41 into said bore 40A to force most of the composite billet through the extrusion orifice 40B, leaving above the extrusion orifice 40B only an extrusion butt 21A (Fig. 16) at the top end of the thus formed composite electrode 2D.

The corrosion resistant material used in this fourth method is preferably nickel or a nickel alloy, the material having good thermal conductivity characteristics and a good corrosion and erosion resistance is preferably silver or a silver alloy, and the material having good thermal conductivity characteristics is preferably copper or a copper alloy.
The electrode obtained by this fourth method is the central electrode 2D of Figs 8,16 which corresponds to the ground electrode 4D of Figs 6, 12, the ground electrode 4D requiring the further steps of cutting off the extrusion butt 21A (Fig. 16) and of bending the body of the electrode.
The recess 33 in the lateral wall of the tubular cup 32 used in the third and in the fourth method for making a spark plug according to the invention can have various shapes and dimensions. Consequently the projections 11, 19, 11A, 19A of the finished electrode 4B, 4D,2B,2D can also have various shapes and dimensions. In particular the dimensions of said projections 11,19,11A,19A can be such that directly in front of the bottom end surface (spark surface) of the other electrode 2, 4 of the spark plug only the outside surface of the inner core 17,37A,17A,37C, of said finished electrode 4B,4D,2B,2D is visible. In other words, no portion of the outer layer 16, 36,16A,36A of said finished electrode 4B, 4D, 2B, 2D is located directly in front of the bottom end surface of the other electrode 2, 4 of the spark plug. Thus in this case only the metal (silver or a silver alloy) having the best corrosion and erosion resistance is located directly in front of the bottom end surface of the other electrode 2, 4.
It is to be noted that the reduced diameter extrusion orifice 40B of the extrusion die 40 can principally have a cross section of any shape, such as for example : circular, oval, square rectangular, etc. Electrodes having various cross sections can thus be made by the four above described methods.

Claims

A spark plug comprising an outer shell (1), a central electrode (2, 2A, 2B, 2C, 2D), an insulator (3) surrounding the central electrode, and at least one ground electrode (4, 4A, 4B, 4C, 4D) connected at at least one point to the outer shell (1), the central electrode (2, 2A, 2B, 2C, 2D) having a spark surface at one end thereof which forms a spark gap with an opposed spark surface of the ground electrode (4, 4A, 4B, 4C, 4D), at least one of the electrodes comprising an outer layer (6, 16, 26, 36, 6A, 16A, 26A, 36A) of a first material and an inner core (7, 17, 27A, 37, 37A, 7A, 17A, 27B, 27C, 37B, 37C) of one or more other materials, characterised in that the spark surface of said at least one electrode is provided by an exposed surface of said core which surface is substantially parallel to the longitudinal axis of said at least one electrode.
A spark plug according to claim 1, wherein the inner core of said at least one electrode projects through the outer layer, perpendicularly to the longitudinal axis of said one electrode.
A spark plug according to claim 2, wherein the lateral surface or surfaces of said projection (8, 14, 8A, 14A) are completely surrounded by the outer layer (6, 26, 6A, 26A) of said one electrode (4A, 4C, 2A, 2C).
A spark plug according to claim 2, wherein only a portion of the lateral surface or only some of the lateral surfaces of said projection (11, 19, 11A, 19A) are surrounded by the outer layer (16, 36, 16A, 36A) of said one electrode (4B, 4D, 2B, 2D).
A spark plug according to claim 4, wherein the surface or surfaces of said projection (8, 11, 14, 19, 8A, 11A, 14A, 19A), which are not surrounded by the outer layer (6, 16, 26, 36, 6A, 16A, 26A, 36A) of said one electrode (4A, 4B, 4C, 4D, 2A, 2B, 2C, 2D), are in alignment with the surface or surfaces of said electrode (4A, 4B, 4C, 4D, 2A, 2B, 2C, 2D).
A spark plug according to any one of the preceding claims, wherein the inner core (7, 17, 7A, 17A) of at least one of the electrodes (4A, 4B, 2A, 2B) consists of one single material, characterised in that the material of the outer layer (6, 16, 6A, 16A) is a corrosion resistant material and the single material of the inner core (7, 17, 7A, 17A) is a material having good thermal conductivity characteristics and a good corrosion and erosion resistance.
A spark plug according to claim 6, wherein the material of the outer layer (6, 16, 6A, 16A) is nickel or a nickel alloy and the single material of the inner core (7, 17, 7A, 17A) is silver or a silver alloy.
A spark plug according to any one of claims 1 to 5, wherein the inner core (27, 27A, 37, 37A, 27B, 27C, 37B, 37C) of at least one of the electrodes (4C, 4D, 2C, 2D) consists of two materials, a first material nearest to the spark gap (5, 5A) and a second material away from the spark gap (5, 5A), characterised in that the material of the outer layer (26, 36, 26A, 36A) is a corrosion resistant material, the first material of the inner core (27A, 37A, 27C, 37C) is a material having good thermal conductivity characteristics and a good corrosion and erosion resistance, and the second material of the inner core (27, 37, 27B, 37B) is a material having good thermal conductivity characteristics.
A spark plug according to claim 8, wherein the material of the outer layer (26, 36, 26A, 36A) is nickel or a nickel alloy, the first material of the inner core (27A, 37A, 27C, 37C) is silver or a silver alloy, and the second material of the inner core (27, 37, 27B, 37B) is copper or a copper alloy.
A method of making an electrode (2A) of a spark plug according to claim 6 and comprising the steps of:
a) forming a tubular cup (22) having a closed and an open end (24, 28) from a corrosion resistant material, said cup (22) having an opening (23) in its lateral wall near its closed end (24);

b) positioning a billet (25) having a cylindrical form and made of a material having good thermal conductivity characteristics and a good corrosion and erosion resistance within the cup (22) so as to contact the inner closed end (24) surface of the cup (22) and so as to contact the inner lateral surface of the cup (22) except for a portion at its open end (28) and bending (29) said open end portion of the cup (22) over the billet (25) inside the cup (22); and

c) inserting the resulting composite billet (22A, 25), closed end (24) first, into a close fitting bore (40A) of an extrusion die (40) having a reduced diameter extrusion orifice (40B) and advancing a plunger (41) into said bore (40A) to force most of the composite billet (22A, 25) through the extrusion orifice (40B), leaving above the extrusion orifice (40B) only an extrusion butt (10A) at the top end of the thus formed composite electrode (2A).
A method of making an electrode (2C) of a spark plug according to claim 8 and comprising the steps of:
a) forming a tubular cup (22) having a closed and an open end (24, 28) from a corrosion resistant material, said cup (22) having an opening (23) in its lateral wall near its closed end (24);

b) positioning a first billet (25B) having a cylindrical from and made of a material having good thermal conductivity characteristics and a good corrosion and erosion resistance within the cup (22) so as to contact the inner closed end (24) surface of the cup (22) and so as to contact the inner lateral surface of the cup (22) up to somewhat beyond said opening (23);

c) positioning a second billet (25A) having a cylindrical form and made of a material having good thermal conductivity characteristics within the cup (22) so as to contact the upper surface of the first billet (25B) and so as to contact the inner lateral surface of the cup (22) except for a portion at its open end (28) and bending (29) said open end portion of the cup (22) over the billets (25B, 25A) inside the cup (22); and

d) inserting the resulting composite billet (22A, 25B, 25A), closed end (24) first, into a close fitting bore (40A) of an extrusion die (40) having a reduced diameter extrusion orifice (40B) and advancing a plunger (41) into said bore (40A) to force most of the composite billet (22A, 25B, 25A) through the extrusion orifice (40B), leaving above the extrusion orifice (40B) only an extrusion butt (18A) at the top end of the thus formed composite electrode (2C).
A method of making an electrode (2B) of a spark plug according to claim 6 and comprising the steps of:
a) forming a tubular cup (32) having a closed and an open end (34, 38) from a corrosion resistant material in such a way that for one circumferential portion of the cup (32) the length (A, B) of the cup (32) is different from than for its other circumferential portion:

b) positioning a billet (35) having a cylindrical form and made of a material having a good thermal conductivity characteristics and a good corrosion and erosion resistance with the cup (32) so as to contact the inner closed end (34) surface and the inner lateral surface of the cup (32); and

c) inserting the resultant composite billet (32, 35), open end (38) first, into a close fitting bore (40A) of an extrusion die (40) having a reduced diameter extrusion orifice (40B) and advancing a plunger (41) into said bore (40A) to force most of the composite billet (32, 35) through the extrusion orifice (40B), leaving above the extrusion orifice (40B) only an extrusion butt (13A) at the top end of the thus formed composite electrode (2B).
A method of making an electrode (2D) of a spark plug according to claim 8 and comprising the steps of:
a) forming a tubular cup (32) having a closed and an open end (34, 38) from a corrosion resistant material in such a way that for one circumferential portion of the cup (32) the length (A, B) of the cup (32) is different from that for its other circumferential portion;

b) positioning a first billet (35B) having a cylindrical form and made of a material having good thermal conductivity characteristics within the cup (32) so as to contact the inner closed end (34) surface and a portion of the inner lateral surface of the cup (32);

c) positioning a second billet (35A) having a cylindrical form and made of a material having good thermal conductivity characteristics and a good corrosion and erosion resistance within the cup (32); and

d) inserting the resulting composite billet (32, 25B, 35A), open end (38) first, into a close fitting bore (40A) of an extrusion die (40) having a reduced diameter extrusion orifice (40B) and advancing a plunger (41) into said bore (40A) to force most of the composite billet through the extrusion orifice (40B), leaving above the extrusion orifice (40B) only an extrusion butt (21A) at the top end of the thus formed composite electrode (2D).
A method according to claim 10 or 12, wherein the corrosion resistant material is nickel or a nickel alloy and the material having good thermal conductivity characteristics and a good corrosion and erosion resistance is silver or a silver alloy.
A method according to claim 11 or 13, wherein the corrosion resistant material is nickel or a nickel alloy, the material having good thermal conductivity characteristics is copper or a copper alloy, and the material having good thermal conductivity characteristics and a good corrosion and erosion resistance is silver or a silver alloy.
A method of making a ground electrode (4A, 4B, 4C, 4D) according to claim 14 or 15 and comprising the steps of cutting off the extrusion butt (10A, 13A, 18A, 21A) at the top end of the electrode (2A, 2B, 2C, 2D) and of giving an elbow-shaped form to said electrode (4A, 4B 4C, 4D).