GB2235493A

GB2235493A - Spark plug electrodes

Info

Publication number: GB2235493A
Application number: GB8919361A
Authority: GB
Inventors: Michael Lenk
Original assignee: Champion Spark Plug Europe SA
Current assignee: Champion Spark Plug Europe SA
Priority date: 1989-08-25
Filing date: 1989-08-25
Publication date: 1991-03-06
Also published as: GB8919361D0

Abstract

The centre and/or earth electrode comprises an outer layer of nickel or nickel alloy and an inner core of one or two materials, e.g. copper or copper alloy or silver or silver alloy. The or one of the core materials projects beyond the adjacent end of the outer layer which is exposed or covered by core material (Fig. 1). The electrode may be produced by die extrusion (Figs 31, 32, 37, 38, 44 to 46) of a composite billet and the earth electrode may be U-shaped (Figs. 19 to 22). <IMAGE>

Description

SPARK PLUG FOR INTERNAL COMBUSTION ENGINE 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 conductjvitv characteristics and a good corrosion and erosion resistance. The Invention further re!ates to several methods for forming the electrodes of said spark plug.

5park plugs as defined above. 1. e in particular spark plugs having at least one electrode made of two different materials (composite electrodes), are well known in prior art. The materials most commonly used tn such electrodes are nickel or a nickel allow for the outer lever, and copper or a copper aliov, respectivelv silver or a silver allow for the inner core.

The reason for making spark plug electrodes provided with an inner chore of copper or of silver is to obtaln better performances as compared to spark plug electrodes consisting of only ane single material 'sucn as for example nickel or a nickel alloy). the better performances are due to the higher thermal conductivity characteristics of zipper and of silver. i. e. the inner core made of copper or of si?ver evacuates more rapidly the heat pruced bv the combustion of the air/tuel 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 non negligible positive effect on the performance and on the useful life of the spark plug because it reduces the corrosion and the erosion the electrodes are subiected to.

To the contrarv of copper. silver has a very good corrosion and erosion resistance. but its bad mechanical propertles at hlgh 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 possiblv for very particular applications.

5erm3n patent application DE 24 04 454 shows a spark plug prow with a central electrode N pletelv made of silver As already s3ld above such an electrode is very expensive ana can only be manufactured for very particular applications where the price of the spark plug is secondary. In other words, a spark plug provided with a central electrode made completely of pure silver cannot be mass produced and sold with a reasonable profit.

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 allay, and of an inner core made of silver. In one embodiment it can be seen that at the spark surface (cross-sectional surface) of the central electrode the nickel alloy of the outer layer and the silver of the inner core are both visible and project bv the same amount out of the insulator of the spark plug. In other words, the spark surface of the central electrode consists of an outer surface made of a nickel alloy and of an inner surface made of silver.

The spark plug according to this German patent thus not only uses the good thermal conductivity of silver for evacuating the heat produced, but also, to a certain extend, its good resistance to corrosion and erosion sine at the spark surface the silver is not covered bv the nickel alloy The main disadvantage of a spark plug designed according to the teachings of this German patent is the fact that the spark surface consists of two materials having different resistances to corrosion and erosion.

This indeed is not the best solution for guaranteeing the longest possible useful life of the electrode electrodes.

A first object of the invention is to provide a novel spark plug .'iavtng improved performances and wherein the electrodes have a substantlallv prolonged useful life period A second oblect of the invention is to Drovlde a novel spark plug having the best possible cost/benefit ratio when compared with prior art spark plugs provided with precious metal and/or copper cored nickel electrodes.

For reaching these objects the spark plug according to the invention is substantially characterized by the fact that at least one of the electrodes consists, over most of its length, of an outer layer of one material and of an inner core of one or more other materials and consists, nearest to the spark gap, only oi one of said other materials A spark plug according to the Invention, wherein the inner core of at least one of the electrodes consists oi one single material, can either have the feature that in the area of the spark gap said single material extends over the whole cross sectlonal surface of the electrode or have the feature that in the area of the spark gap said single material extends over onlv a portion of the cross-sectional surface of the 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 in the area of the spark gap said first material extends over the whole cross-sectional surface of the electrode or have the feature that in the area of the spark gap said first material extends over only a portion of the cross-sectlonal surface of the electrode Another ob!ect 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 iryertion are (a) for the outer layer a corrosion resistant material such as for example nickel or a nickel allov, (b) for the single material inner core . a material having good thermal conductlvitv characteristics and a good corroslon/eroslon 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 sorrosior/erssion resistance such as for example silver or a silver alloy and a second material (away from the spark gap) hashing good thermal conductivity cnaracteristics 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 principaily 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.

rhese methods will be explained In coniunction with the detailed description of the appended drawings two methods will be described directly hereunder as examples According to one method a tubular cup having a closed and an open end Is formed from a corrosion resistant material such as for example nickel or a nickel alloy. A billet having a cylindrical form with a T-shaped axial cross section and made of a material having good thermal conductivitv charac'eristics and a good corrosion and erosion res'st3nce. such as for example silver or a silver allov. ls positioned within the cup so as to contact the inner closed end surface, the inner lateral surface and the outside top surface of the cup.The resulting composite billet is then inserted, open end of the cup first, into a close fitting bore of an extrusion die having a reduced diameter extrusion orifice. A plunger is advanced into said bore to force most of the composite billet through the extrusion orifice1 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 material having good thermal conductivity characteristics and a good corros-ion/erosion resistance (I. e. the silver or the silver alloy) extends, at the bottom end portion of the electrode, over the whole cross-sectional surface of said electrode.

According to another method a tubular cup having a closed and an open end is also formed from a corrosion resistant material such as for example nickel or a nickel alloy A billet having a cylindrical form and made of a material having good thermal conductivity characteristics and a good corrosion and erosion resistance, such as for example silver or a silver alloy, is partially positioned within the cup so as to contact the inner closed end surface and the inner lateral surface of the cuD and so as to partially project out of the cup The resulting composite billet is then inserted, open end of cup first, into a close fitting bore of an extrusion die having a cylindrical extrusion orifice with a diameter smaller than the outer diameter of the cup, but larger than the diameter of the cvlindrical billet A plunger is advanced 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 material having good thermal conductivity characteristics and a good corrosion/eroslon resistance (i. e. the silver or the silver alloy) extends, at the the bottom end por-tion of the electrode, over only a portion of the crass-sectional surface 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, i. e. the ground electrode can for example be bent so as to have an elbow-shaped or so as to have a U-shaped form. The ground electrode having an elbow-shaped form is connected to the outer shell of the spark plug by one of its ends and the around electrode having a U-shaped form is connected to the outer snell of the spark plug bv its two ends 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 oi a spark plug according to a sixth embodiment oi 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 eiahth embodiment of the invention; Figs 9 through 12 are sectional views of the centra' electrodes of respectively the embodiments of Figs 1, 3, 5 and 7, F'.gs 13 and 14 are sectional views of ground electrodes according to the invention.

which can be used in combination with the central electrodes of Figs 9 to 12; Figs 15 through 1 8 are sectional views of prior art ground electrodes which can be used in combination with the central electrodes of Figs 9 through !2, Fig. 19 is a sectional view of a around electrode according to the invention. which can oe used in combination with the central electrodes of Figs 9 through 12; Figs 20 through 22 are sectional view of prior art ground electrodes which can be used in combination with the central electrodes of Figs 9 through 12, Figs 23 through 26 are sectional views of the ground electrodes of respectively the embodiments of Figs 2, 4, 6 and 8;; Figs 27 through 29 are sectional views of prior art central electrodes which can be used in comb,natlon with the ground electrodes of Figs 23 through 26 Fig 30 is an axial sectional view of a cvindr1cal composite billet used for manu facturing an electrode of a spark plug according a first method of the invention; Figs 31 and 32 are sectional views of a die, of a composite billet and of a plun per and show how an electrode of a spark plug is formed according to the first method of the invention; Figs 33 through 35 are sectional views of electrodes formed according to the first method of the invention;; Fig. 36 is an axial sectional view of a cylindrical composite billet used for manufacturing an electrode of a spark plug according to a second method of the invention; Figs 37 and 38 are sectional views of a die, of a composite billet and of a plun ger and show how an electrode of a spark plug is formed according to the second method of the invention; Figs 39 through 41 are sectional views of electrodes formed according to the second method of the invention; Fig. 42 is an axial sectional view of a cylindrical composite billet used for manufactu ring an electrode of a spark plug according to a third method of the invention;; Fig 43 is an axial sectional view of a cylindrical composite billet used for manufacturing an electrode according to a fourth method of the 'Invention, Fig. 44 is a sectional view of a die, of a composite billet and of a plunger and shows how the composite billet is inserted into the die for formlng an electrode of a spark plug according to a fifth method of the invention; and Figs 45 and 46 are sectional views of a die, of a composite billet and of two plungers and show how the electrode of a spark plug is formed according to the fifth 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 oniv examples because, as will be explained here!nder, many other embodiments are possible and it is one of the major advantages of the invention to allow for a very large number of embodiments, so that an adequate high performance spark plug can be provided for manv different applications.

Figs 1,3,5 and 7 relate to spark plugs provided with different central electrodes according to the invention and Figs 2,4,6 and 8 relate to spark plug provided with different ground 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,4C4D. Between the central electrode and the ground electrode of each spark olug there is a spark gap 5,5' 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 central and of the ground electrodes such as respectively shown in Figs 1, 3, 5, 7 and Figs 2, 4, 6, 8 Four possible structures of the central electrode according to the invention are represented separately In Figs 9 through 12.

The central electrode (2A) according to the invention shown In Figs 1 and 9 comprises a seat 2A' (extrusion butt) at the top end of the electrode, an outer laver 2A", an inner core 6A and an enlarged portion 6A" of the inner core protecting out of the outer layer 2A" at the bottom end of the electrode The seat 2A' and the outer layer 2A" are made of a corrosion resistant material such as for example nickel or a nickel alloy.The inner core 6A and the enlarged projecting portion 6A" are made of a material having good thermal conductivity characteristics and a good corrosion and erosion resistance, such as for example silver or a silver alloy It Is to be noted that for an easv reading of the appended drawings the seat and the outer layer of the electrodes according to the invention or of prior art electrodes have not been hatched in ahy of the sectional views of said drawings.

the central electrode (2B) according to the invention shown in Figs 3 and 10 comprises a seat 2B' (extrusion butt) at the top end of the electrode, an outer laver 2B", an inner core 6B and a portion 6B" of the inner core projecting out of the outer aver 2B" at the bottom end of the electrode The seat 2B' and the outer laver 2B" are made of a corrosion resistant material such as for example nickel or a nickel alloy The inner core 6B and the projecting portion 6B" are made of a material having good thermal conductivity characteristics and a good corrosion and erosion resistance, such as for example silver or a silver alloy.

The central electrode (2C) according to the invention shown in Figs 5 and 11 comprises a seat 2C' (extrusion butt) at the top end of the electrode, an outer layer 2C", an inner core made of a first material 6C' and of a second material 6C, and an enlarged portion 6C" of the inner core projecting out of the outer layer 2C" at the bottom end of the electrode. The seat 2C' and the outer laver 2C" are made of a corrosion resistant material such as for example nickel or a nickel alloy The portion 6C of the Inner core is made of a material having good thermal conductivity characteristics such as for example copper or a copper alloy.The portion 6C' of the inner core and the enlarged projecting portion 6C" are made of a material having good thermal conductivitv characteristics and a good corrosion and erosion resistance. such as for example silver or a silver alloy The central electrode (2D) according to the invention shown in Figs 7 and 12 comprises a seat 2D' (extrusion butt) at the top end of the electrode, an outer layer 2D", an inner core made of a first material 6D' and of a second material 6D, and a portion 6D" of the inner core projecting out of the outer layer 2D" at the bottom end of the electrode. The seat 2D' and the outer laver 2D" are made of a corrosion resistant material such as for example nickel or a nickel alloy.The portion 6D of the inner core is made of a material having good thermal conductivity characteristics such as for example copper or a copper alloy.. The portion 6D' of the inner core and the projecting portion 6D" are made of a material having good thermal conductivitv characteristics and a good corrosion and erosion resistance, such as for example silver or a silver alloy It is to be noted that in Figs 1,3,5,7 the ground electrode 4 which is shown as a single material electrode can also be a composite electrode and in Figs 2,4,6,8 the central electrode which is shown as a single material electrode can also be a composite electrode.

Figs 13 through 26 show examples of elbow-shaped and U-shaped ground electrodes according to the invention and prior art ground electrodes which all can be combined with one of the central electrodes according to the invention of Figs 9 through 1 2 It is to be noted that if the elbow-shaped ground electrodes of Figs 1 3 through 1 8 or if the U-shaped ground electrodes of Figs 19 through 22 are used the spark gap 5 of the spark plug according to the invention is located beneath the lower portion of the central electrode . whereas if the elbow-snaped ground electrodes of Figs 23 through 26 are used said spark gap 5' is located on the side of the lower portion of the central electrode.

In Figs 13 and 14 two elbow-shaped ground electrodes ( 4A ,4B' ) according to the invention are represented They have a single material core, but could also have a two material core. The ground electrode 4A' consists of an outer layer 7.8 and Ot an inner core 9 having an enlarged prolecting portion 10.The ground electrode 4B' consists of a outer layer 11,12 and of an inner core 1 3 having a projecting portion 14. in these two electrodes 4A',4B' the outer layer 7,8,11,12 is made of a corrosion resistant material such as for example nickel or a nickel alloy and the inner core 9,13 as well as the projecting portions 10,14 are made of a material having good thermal conductivity characteristics and a good corrosion and erosion resistance such as for example silver or a silver alloy.

Figs 15 through | 18 show prior art elbow-shaped ground electrodes 4',4",4"',4. The ground electrode of Fig. 15 consists of an outer laver 15,16 and of an inner core 17; the ground electrode of fig. 16 consists of an outer layer 18 and of an inner core 19; the ground electrode of Fig. 17 consIsts of an outer layer 20,21; and of an inner core 22 and the ground electrode of Fig. 18 is a single material electrode.

Fig. 19 represents a U-shaped ground electrode (40A) according to the invention It consists of an outer layer 23,24,25 and of an inner core 26 having an enlarged central portion 27. The inner core is made of one single material, but it could be made of two different materials.In this electrode 40A the outer layer 23,24,25 consists of a corrosion resistant material such as for example nickel or a nickel alloy and the inner core 26 as well as the central portion 27 consist of a material having good thermal conductivity characteristics and a good corrosion and erosion resistance such as for example silver or a silver alloy Figs 20 through 22 show prior art U-shaped ground electrodes 40',40",40 The ground electrode of Fig 20 consists of an outer layer 28,29 and of an inner core 30, the ground electrode of Fig. 21'consists of an outer layer 31 and of an inner core 32 and the ground electrode of Fig. 22 is a single material electrode, Figs 23 through 26 show ground electrodes (4A,4e,4C,4D) according to the invention.The ground electrodes 4A and 4B can also be seen in Figs 2 and 4 and thev have the same basic structure as the ground electrodes 4A and 4B of respectvelv Figs 13 and 14 The ;round electrode 4C consists of an outer layer 33,34, of an inner core made of a first material 35 and of a second material 36, and of an enlarged projecting portion 37. The ground electrode 4D consists of an outer layer 38,39, of an inner core made of a first material 42 and of a second material 431 and of a protecting portion 44 In the electrodes 4C,4D the outer layer 33,34,38,39 is made of a corrosion resistant material such as for example nickel or a nickel alloy, the portions 35,42 of the inner core are made of material having good thermal conductivity characteristics such as for example copper or a copper alloy, and the portions 36,43 of the inner core as well as the projecting portions 37,44 are made of a material having good thermal conductivity characteristics and a good corrosion and erosion resistance such as for example silver or a silver alloy.

As alre said above all the ground electrodes shown In fias 13 through 26 are examples of around electrodes which can be combined with anv of the central electrodes according to the invention shown in Figs 1,3,5,7 as well in Figs 9 through 1 2. Moreover the ground electrodes according to the invention of Figs 13,14 and 23 through 26 can be combined with any of the prior art central electrodes shown as examples in Figs 27 through 29.

The prior art central electrode 2' of Fig 27 consists of an outer laver 45 and of an inner core 46; the prior art central electrode 2" of Fig 28 consists of an outer layer 47,48 and of an inner core 49; and the prior art central electrode 2 of Fig. 29 is a single material electrode.

Figs 30 through 35 relate to the first method for manufacturing an electrode of a spark plug according to the invention. fig. 30 represents a composite billet 50,51 for forming said electrode. It comprises a tubular cup 50 having a closed and an open end and a cvlindrical billet 51 having a T-shaped axial cross section. Fig.

31 shows how the composite billet 50,51 is inserted into an extrusion die 52; Fig 32 shows how the electrode 2A is formed by extrusion; and Figs 33 through 35 show three electrodes which can be obtained bv said first method.

According to this first method a tubular cup 50 hoing a closed and an open end is formed from a corrosion resistant material such as for example nickel or a nickel allay. A billet S1 having a cylindrical form with a T-shaped axial cross section and made of a material having good thermal conductivity characteristics and a good corrosion and erosion resistance, such as for example silver or a silver alloy, is positioned within the cup 50 so as to contact the inner closed end surface. the inner lateral surface and the outside top surface of the cuD SO. The resulting composite billet 50,51 is then inserted, open end of the cup 50 first, into a close fitting bore 54 of an extrusion die 52 having a reduced diameter extrusion orifice 52A.A plunger 53 is advanced into said bore 54 to force most of the composite billet 50,51 through the extrusion orifice 52A leaving above the extrusion orifice 52A only an extrusion butt 2A' at the top end of the thus formed composite electrode 2A In the electrode formed bv this method the material having good thermal conductivity characteristics and a good corrosion/erosion resistance ti. e the silver or the silver alloy) extends, at the bottom end portion of the electrode 2A, over the whole cross-sectional surface of said electrode.

the electrode 2A obtained bv the first method is shown in Fig. 33. The electrode 2A can be given the form of the electrode 54 of Fig 34 or the form of the electrode 56 of Fig. 35 by cutting off, in the desired way (55,57), the extrusion butt 3A' shown in Fig 33. The electrodes 54,56 can further be given an elbow-shaped form, so that they can be used as ground electrodes.

Figs 36 through 41 relate to the second method for manufacturing an electrode of a spark plug according to the invention. Fig. 36 represents a composite billet 60,61 for forming said electrode. It comprises a tubular cup 60 having a closed and an open end and a cylindrical billet 61. Fig 37 shows how the composite billet 60,61 is inserted into an extrusion die 52; Fig 38 shows how the electrode 2B is formed bv extrusion; and Figs 39 through 41 show three electrodes which can be obtained bv said second method.

According the second method a tubular cup 60 having a closed and an open end is formed from a corrosion resistant material such as for example nickel or a nickel alloy. A billet 61 having a cylindrical form and made of a material having good thermal conductivity characteristics and a good corrosion and erosion resistance, such as for example silver or a silver alloy, is partially positioned within the cup 60 so as to contact the inner closed end surface and the inner lateral surface of the cup b0 and so as to partially project out of the cup 60.The resulting composite billet 60,61 is then inserted, open end of cup 60 first, into a close fitting bore 54 of an extrusion die 52 having a cylindrical extrusion orifice 52A with a diameter smaller than the outer diameter of the cup 60. but larger than the diameter of the cvl:ndrical billet 61. A plunger 53 is 3dvanced into said bore 54 to force most of the composite billet 60,61 through the extrusion orifice 52A, leaving above the extrusion orifice 52A only an extrusion butt 2B' at the top end of the thus formed composite electrode 25.

In the electrode formed by this method the material having good thermal conductivity characteristics and a good corrosion/erosion resistance (i. e. the silver or the silver allov) extends. at the the bottom end portion of the electrode 25, over only a portion of the cross-sectional surface of said electrode The electrode 2B obtained bv the second method is shown In Fig. 39.

The electrode 25 can be given the form of the electrode 64 of Fig 40 or the form of the electrode 66 of Fig. 41 by cutting off, in the desired way (65,67), the extrusion butt 3B' shown in Fig 39. The electrodes 64,66 can further be given an elbow-shaped form, so that they can be used as ground electrodes.

Fig. 42 shows a composite billet 70,71,72 which is used in a third methoa for manulacturing an electrode (2C) for a spark plug according to the invention.

This third method comprises the steps of forming a tubular cup 70 having a closed and an open end from a corrosion resistant material, - positioning a first billet 71 having a cylindrical form and made of a material having good thermal conductivity characteristics within the cup 70 so as to contact the inner closed end surface and the inner lateral surface of the cup 70, - positioning a second billet 72 having a cylindrical form with a T-shaped axial cross section and made of a material having good thermal conductivity characteristics and a good corrosion and erosion resistance within the cup 70 so as to contact the upper surface of the first billet 71 and so as to contact the inner lateral surface and the outer top surface of the cup 70, - Inserting the resulting composite billet 70,71,72, open end first, into a close fitting bore 54 of an extrusion die 52 having a reduced diameter extrusion orifice 52A and - advancing a plunger 53 into said bore 54 to force most of the composite billet 70,71,72 through the extrusion orifice 52A, leaving above the extrusion orifice 52A onlv an extrusion butt 2C' at the top end of the thus formed composite electrode 2C.

Fig 43 shows a composite billet 80.81,82 which is used in a fourth method for manufacturing an electrode (2D) for a spark plug acording to the invention.

This method comprises the steps of - forming a tubular cup 80 having a closed and an open end from a corrosion resistant material, - positioning a first billet 81 having a cylindrical form and made of a material having good thermal conductivity characteristics within the cup 80 so as to contact the inner closed end surface and the inner lateral surface of the cup 80, - partially positioning a second billet 81 having a cylindrical form and made of a material having good thermal conductivity characteristics and a good corrosion and erosion resistance within the cup 80 so as to contact the upper surface of the first billet 81 and the inner lateral surface of the cup 80 and so as to partially project out of the cup 80, - Inserting the resulting composite billet 80.81.82, open end first. into a close fitting bore 54 of an extrusion die 52 having a cylindrical extrusion orifice 52A with a diameter smaller than the outer diameter of the cup 80 but larger than the diameter cf the two billets 81,82 and - advancing a plunger 53 into said bore 54 to force most of the composite billet 80,81 82 through the extrusion orifice 52A, leaving above the extrusion orifice 52A only an extrusion butt 2D' at the top end of the thus formed composite electrode 2D Figs 44 through 46 show how an electrode according to the invention is formed by a fifth method.The type of electrode obtained Is the same as the one obtained by the above mentloned first method, 1. e. the electrode 2A of Fig. 9. This fifth method comprises the steps of.

- forming a tubular cup 60 having a closed. and an open end from a corrosion resistant material, - partially positioning a billet 61 having a cylindrical form and made of a material having good thermal conductivity characteristics and 8 good corrosion and erosion resistance within the cup 60 so as to contact the inner closed end surface and the inner lateral surface of the cup 60 and so as to partially project out of the cup 60, - inserting the resultant composite billet 60,61, open end first, into a close fitting bore 94 of an extrusion die 92 having a cylindrical extrusion bore 92A with a diameter smaller than the outer diameter of the cup 60 but larger than the diameter of the cylindrical billet 61, - advancing a first plunger 53 downwards into said close fitting bore 94 to force most of the comoosite billet 60.61 through the extrusion bore 92A. leaving above the extrusion bore 92A onlv an extrusion butt 2B' at the top end of the thus formed composite electrode 2B and - advancing a second plunger 93 upwards into the extrusion bore 92A so as to flatten the portion of the cylindrical billet 61 which projects out of the extruded cup 28".

Starting from the composite billet of Fig. 43 a sixth method, similar to the above mentioned fifth method permits to obtain the electrode obtained bv the above -mentioned third method. 1 e. the electrode 2C of Fig. II. This sixth method comprises the steps of - forming 3 tubular cup 80 having a closed and an open end from a corrosion resistant material.

- positioning a first billet 81 having a cylindrical form and made of a material having good thermal conductivity characteristics within the cup 80 so as to contact the inner closed end surface and the inner lateral surface of the cup 80, - partially positioning a second billet 82 having a cylindrical form and made of a material having good thermal conductivity characteristics and a good corrosion an erosion resistance within the cup 80 so as to contact the upper surface of the first billet and the inner lateral surface of he cup 80 and so as to partially project out of the cup 80, - nserting the resulting composite billet 80.81.82. open end first. into a close fitting bore 94 of an extrusion die 92 having a cv!indrical extrusion bore 92A with a diameter smaller than the outer diameter of the cup 80 but larger than the diameter of the two cylindrical billets 81,82, - advancing a first plunger 53 downwards into said close fitting bore 94 to force most of the composite billet 80,81 ,82 through the extrusion bore 92A. leaving above the extrusion bore only an extrusion butt 2D' at the top end of the thus formed composite electrode 2D and - advancing a second plunger 93 upwards into the extrusion bore 92A so as to flatten the portion of the second cylindrical billet 82 which protects out of the extruded cup 80.

In the third, fourth. fifth and sixth method the corrosion resistant material 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, just as in the first and in the second method. whereas in the third. fourth and sixth method the material having good thermal conductivity characteristics is preferably copper or a copper alloy In the above described stx methods it is said that the composite billets 50,51; 60,61; 70,71,72; 80,81,82 are only partially forced through the extrusion orifices 52A or 92A and that extrusion butts 2A',2B',2C',2D' are left at the top ends of the extruded electrodes 2A,2B,2C,2D.This way to proceed has the advantage that said extrusion butts 2A'.2B',EC's2D' can serve as seats in the insulators 3 of the spark plugs if central electrodes are manufactured If ground electrodes are manufactured the dimensions of the composite billets and of the extrusion bores/orifices will generally be different than for central electrodes and the extruded electrode will be bent as needed after the extrusion butt has been cut off as desired.

Four basic novel structures for central and for ground electrodes have thus been described as well as six methods for their manufacture by extrusion. As already said above these novel structures permit a very large number of combinations central electrode/ground electrode by taking into consideration (a) the four types of central and ground electrodes according to the invention, (b) the various prior art central and ground electrodes and (c) the fact that the spark gap can either be located beneath or at the side of the lower end portion of the central electrode.

Claims

1.- A spark plug comprising an outer shell (1), a central electrode (2, 2A, 2B, 2C, 2D), an insulator (3) surrounding the central electrode, said central electrode (2,2A,2B,2C,2D) having a top end portion and a bottom end portion, and further comprising at least one ground electrode (4, 4A, 48, 4C, 4D, 40A)) connected at at least one point to said outer shell (1) and forming a spark gap (5,5') with the bottom end portion of the central electrode (2, 2A, 28,20, 2D), characterized In that at least one of the electrodes (2A, 28, 2C, 2D; 4A, 48, 4C. 4D; 40A)) consists, over most of its length, of an outer layer (2A",2B",2C",2D";8, 8,12,34,39; 25) of one material and of an inner core (6A, 6B, 6C, 6C', 6D, 6D'; 9, 13, 35, 36, 42, 43, 26) of one or more other materials and consists, nearest to the spark gap (5, 5'), only of one (6A, 68, 6C', 6D', 9,13,36,43,26) of said other materials.

2.- A spark plug according to claim 1 wherein the inner core (6A; 9? of at least one of the electrodes (2A; 4A,'40A)) consists of one single material, characterized in that in the area of the spark gap (5, 5') said single material extends over the whole cross-sectional surface (6A"; 10;27) of the electrode (2A, 4A; 40A)

3.- A spark plug according to claim '1 wherein the Inner core (6B; 13) of at least one of the electrodes (28; 48) consists of one single material.

characterized in that in the area of the spark gap (5 5') said single material extends over only a portion of the cross-sectional surface (6B", 14) of tre electrode (28,48).

4 - A spark plug according to claim 1 wherein the inner core '60, 6C'; 35, 36) of at least one of the electrodes (2C;4C) consists of two materials, a first material nearest to the spark gap (5, 5') and a second material away from the spark gap (5,5'), characterized In that in the area of the spark gap (5.5') said first material extends over the whole cross-sectional surface (6C"; 37) of the electrode (2C; 4C)

5.- A spark plug according to claim I wherein the Inner core (6D, 60';42,43) of at least one of the electrodes (20; 4D) consists of two materials, a first material nearest to the spark gap (S, 5') and a second material aw7y from the spark gap (5, 5'), characterized in that in the area of the spark gap (5,5') said first material extends over only a portion of the cross-sectional surface (6D", 44) of the electrode (2D;4D).

6.- A spark plug according to claim 2 or 3, characterized In that the material of said outer layer (2A",28";8, 12;25) Is a corrosion resistant material and that the single material of the inner core (6A,68; 9, 13;26) is a material having good thermal conductivity characteristics and a good corrosion and erosion resistance.

7- A spark plug according to claim 6, characterized in that the material of the outer layer (2A", 28"; 8, 12; 25) is nickel or a nickel alloy and that the single material of the inner core (6A,68;9, 13;26) is silver or a silver alloy.

8.- A spark plug according to claim 4 or 5, characterized In that the material of said outer laver (2C". 2D", 34, 39) is a corrosion resistant material, that the first material of the inner core (6C',6D';36.43)1s a material having good thermal conductivity characteristics and a good corrosion and erosion resistance, and that the second material of the inner core (6C, 6D; 35, 42) is a material having good thermal conductivity characteristics.

9.- A spark plug according to claim 8, characterized in that the material of the outer layer (2C", 2D", 34, 39) is nickel or a nickel alloy, that the first material of the inner core (6C', 6lid;' 36.43) is silver or a silver alloy, and that the second material of the inner core (6C,6D; 35, 42) is copper or a copper alloy 10- A spark plug according to claims 1 through 5, characterized in that the ground electrode (4A, 48,4C, 40) is elbow-shaped and is connected to the outer shell (1) by one of its ends.

11 - A spark plug according to claims 1 through 5, characterized in that the ground electrode (40A) is U-shaped and is connected to the outer shell (1) by its two ends.

12.- A method for making an electrode (2A) of a spark plug according to claim 6, characterized in that it comprises the steps of forming a tubular cup (50) having a closed and an open end from a corrosion resistant material, positioning a billet (51) having a cylindrical form with a T-shaped axial cross section and made of a material having good thermal conductivity characteristics and a good corrosion and erosion resistance within the cup (50) so as to contact the inner closed end surface, the Inner lateral surface and the outer top surface of the cup (50), inserting the resulting composite billet (50,51), open end first, into a close fitting bore (54) of an extrusion die (52) having a reduced diameter extrusion orifice (52A) and advancing a plunger t53) into said bore (54) to force most of the composite billet (50,51) through the extrusion orifice (52A), leaving above the extrusion orifice (52A) only an extrusion butt (2A') at the top end of the thus formed composite electrode (2A).

13.- A method for making an electrode (28) of a spark plug according to claim 6, characterIzed in that it comprises the steps of forming a tubular cup (60) having a closed and an open end from a corrosion resistant material, partially positioning a billet (61) having a cylindrical form and made of a material having good thermal conductivity characteristics and a good corrosion and erosion resistance within the cup (60) so as to contact the inner closed end surface and the inner lateral surface of the cup (60) and so as to partially project out of the cup (60), inserting the resulting composite billet (60.61), open end first, into a close fitting bore (54) of an extrusion die (52) having a cylindrical extrusion orifice (52A) with a diameter smaller than the outer diameter of the cup (60) but larger than the diameter of the cylindrical billet (61) and advancing a plunger (53) into said bore (54)- to force most of the composite billet (60,61) through the extrusion orifice (52A), leaving above the extrusion orifice (52A) only an extrusion butt (28') at the top end of the thus formed composite electrode (28)..

14.- A method for making an electrode (2C) of a spark plug according to claim 8, characterized in that it comprises the steps of forming a tubular cup (70) having a closed and an open end from a corrosion resistant material, positioning a first billet (71) having a cylindrical form and made of a material having good thermal conductivity cnarxteristics within the cup (70) so as to contact the inner closed end surface and the inner lateral surface of the cup (70), positioning a second billet (72) having a cylindrical form with a T-shaped axial cross section aric made of a material having good thermal conductivity characteristics and a good corrosion and erosion resistance within the cup (70) so as to contact the upper surface of the first billet (71) and so as to contact the inner lateral surface and the outer top surface of the cup (70), inserting the resulting composite billet (70,71,72), open end first. into a close fitting bore (54) of an extrusion die (52) having a reduced diameter extrusion orifice (52A) and advancing a plunger (53) into said bore (54) to force most of the composite billet (70,71,72) through the extrusion orifice (52A), leaving above the extrusion orifice (52A) only an extrusion butt (2C') at the top end of the Thus formed composite electrode (2CJ.

15,- A method for making an electrode (2D) of a spark plug according to claim 8, characterized in that it comprises the steps of forming a tubular cup (80) having a closed and an open end from a corrosion resistant material, positioning a first billet (81) having a cylindrical form and made of a material having good thermal conductivity characteristics within the cup (80) so as to contact the inner closed end surface and the inner lateral surface of the cup (80), partially positioning a second billet (81) having a cylindrical form and made of a material having good thermal conductivitv characteristics and a good corrosion and erosion resistance within the cup (80) so as to contact the upper surface of the first billet (81) and the inner lateral surface of the cup (80) and so as to partially project out of the cup (80), inserting the resulting composite billet (80,81,82), open end first, into a close fitting bore (54) of an extrusion die (52) having a cylindrical extrusion orifice (52A) with a diameter smaller than the outer diameter of the cup (80) but larger than the diameter of the two billets (81,82) and advancing a plunger (53) into said bore (54) to force most of the composite billet (80,81,82) through the extrusion orifice (52A), leaving above the extrusion orifice (52A) only an extrusion butt (2D') at the top end of the thus formed composite electrode (2D)

16.- A method for making an electrode (2A) of a spark plug according to claim 6, characterized in that it comprises the steps of forming a tubular cup (60) having a closed and an open end from a corrosion resistant material, partially positioning a billet (61) having a cylindrical form and made of a material having good thermal conductivity characteristics and a good corrosion and erosion resistance within the cup (60) so as to contact the inner closed end surface and the inner lateral surface of the cup (60) and so as to partially project out of the cup (60), inserting the resultant composite billet (60,61), open end first, into a close fitting bore (94) of an extrusion die (92) having a cylindrical extrusion bore (92A) with a diameter smaller than the outer diameter of the cup (60) but larger than the diameter of the cylindrical billet (61), advancing a first plunger (53) downwards into said close fitting bore (94) to force most of the composite billet (60.61) through the extrusion bore (92A), leaving above the extrusion bore (92A) only an extrusion butt (28') at the top end of the thus formed composite electrode (2B) and advancing a second plunger (93) upwards into the extrusion bore (92A) so as to flatten the portion of the cylindrical billet (61) which projects out of the extruded cup (2B").

17.- A method for making an electrode (2C) of a spark plug according to claim 8, characterized in that it comprises the steps of forming a tubular cup (80) having a closed and an open end from a corrosion resistant material, positioning a first billet (81) having a cylindrical form and made of a material having good thermal conductivity characteristics within the cup (80) so as to contact the inner closed end surface and the inner lateral surface of the cup (80), partially positioning a second billet (82) having a cvlindrical form and made of a material having good thermal conductivity characteristics and a good corrosion an erosion resistance within the cup (80) so as to contact the upper surface of the first billet and the inner lateral surface of he cup (80) and so as to partially project out of the cup (80), inserting the resulting composite billet (80,81,82), open end first, into a close fitting bore (94) of an extrusion die (92) having a cylindrical extrusion bore (92A) with a diameter smaller than the outer diameter of the cup (80) but larger than the diameter of the two cylindrical billets (81,82), advancing a first plunger (53) downwards Into said close fitting bore (94) to force most of the composite billet (80,81,82) through the extrusion bore (92A), leaving above the extrusion bore only an extrusion butt (2D') at the top end of the thus formed composite electrode (2D) and advancing a second plunger (93) upwards into the extrusion bore (92A) so as to flatten the portion of the second cylindrical billet (82) which prolects out of the extruded cup (80).

18.- A method according to claims 12, 13 and 16, characterized in that the corrosion resistant material Is nickel or a nickel allov and that the material having good thermal conductivity characteristics and a good corrosion and erosion resistance is silver or a silver alloy.

19 - A method according to claim 14, 15 and 17, characterized in that the corrosion resistant material is nickel or a nickel alloy, that the material having good thermal conductivity characteristics is copper or a copper alloy alloy, and that the material having good thermal conductivitv characteristics and a good corrosion and erosion resistance is silver or a silver alloy

20.- A method for making a ground electrode (4A,48,4C,4D; according to claims 12 through 17, characterized in that it comprises the further steps of cutting off the extrusion butt (2A',28',2C',2D') 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).