EP0480886A1

EP0480886A1 - A spark plug

Info

Publication number: EP0480886A1
Application number: EP91830415A
Authority: EP
Inventors: Pietro De Filippis; Roberto Rossi
Original assignee: Industrie Magneti Marelli SRL; Magneti Marelli SpA
Current assignee: Industrie Magneti Marelli SRL; Marelli Europe SpA
Priority date: 1990-10-12
Filing date: 1991-10-08
Publication date: 1992-04-15
Also published as: IT9067778A0; IT1241643B; IT9067778A1

Abstract

A ring (10) of roble metal or an alloy thereof is disposed around that portion of the central electrode (4) which projects from the insulator (34, 3), in electrical contact therewith, so that the spark or sparks jump between the ring (10) and the earth electrode or electrodes (9) in operation.

Description

The present invention relates to spark plugs for internal combustion engines with controlled ignition.
More specifically, the invention relates to a spark plug of the type comprising:
a tubular, outer, metal body,
a tubular insulator disposed in the body,
a central electrode which extends through the insulator and projects axially from its end which faces the combustion chamber in use, and
at least one earth electrode which extends from the outer metal body towards the side surface of that portion of the central electrode which projects from the insulator.
An object of the invention is to provide a spark plug of this type which is suitable for greatly extended operation (a "long-life" plug).
A known "long-life" plug has an earth electrode which, according to well-known geometry, extends from the metal outer body of the plug to face the end surface or face of the central electrode. Coatings of noble metal, typically platinum or alloys thereof, are welded to the end surface of the central electrode and the facing portion of the earth electrode between which the spark is struck in use.
It has been found, however, that the use of such "long-life" plugs has some disadvantages. The vibrations to which these plugs are subject in operation, particularly when they are used in engines with four valves per cylinder, may result in the plugs losing their entire earth electrodes which, typically, are welded to the metal outer bodies of the plugs.
A further disadvantage of these known "long-life" plugs is their high cost.
The object of the present invention is to provide a "long-life" plug which does not have the disadvantages of those of the prior art.
According to the invention, this object is achieved by a spark plug of the type specified above, whose main characteristic lies in the fact that a ring of noble metal or an alloy thereof is disposed around the projecting portion of the central electrode, in electrical contact therewith, so that the spark or sparks jump between the ring and the at least one earth electrode in operation.
Further characteristics and advantages of the invention will become clear from the detailed description which follows with reference to the appended drawings, provided purely by way of non-limiting example, in which:

Figure 1 is a perspective view of a spark plug according to the invention,
Figure 2 is a partial section taken on the line II-II of Figure 1,
Figure 3 is a section similar to that of Figure 2, showing a variant of the spark plug according to the invention, and
Figures 4 to 6 are partial axial sections of further variants of the spark plug according to the invention.

In Figure 1, a spark plug according to the invention for an internal combustion engine is generally indicated 1. The plug includes, in known manner, a tubular, outer metal body 2 with an externally threaded portion.
A substantially tubular element 3 of electrically-insulating, typically ceramic, material is disposed in the body 2 in known manner (Figure 2). An end portion 3a of the insulator projects axially from the end of the metal body 2 which faces the combustion chamber in use.
A central electrode, indicated 4, extends through part of the insulator 3 and projects axially from its end 3a.
A conductive core 5 is also disposed in the insulator. One end of the core projects axially from the insulating element 3 and constitutes an electrical connecting terminal 6.
A conductive or resistive sealing mastic, indicated 7 in Figure 2, is interposed between the central electrode 4 and the conductive core 5 within the insulating element 3.
A metal ring, onto which an end edge 2a of the metal body 2 is upset, is indicated 8 (Figure 2). The ring 6 is made, for example, of a nickel- or copper-based alloy and has a plurality of integral appendages 9 which extend from its inner edge towards the end portion 3a of the insulator in equiangularly-spaced positions. The appendages constitute the earth electrodes of the plug.
As an alternative to the solution shown in Figure 2, the ring 8 with its integral appendages 9 which act as electrodes may be welded to the end of the metal outer body 2 of the plug, for example, by laser welding.
In the embodiment shown by way of example in Figures 1 and 2, the central electrode 4 has a substantially disc-shaped head 4a.
A ring, indicated 10, of noble metal (for example, platinum) or an alloy thereof, is disposed around the central electrode 4 between its head 4a and the end 3a of the insulator 3 (Figure 2). The noble metal ring may simply be fitted around the electrode and then clamped between its head 4a and the insulator 3 or may be placed around the stem of the central electrode 4 and then welded to its head 4a.
As can better be seen in Figure 2, the arrangement is such that, in operation, the sparks jump between the noble metal ring 10 and the earth electrodes 9.
The solution described above with reference to Figures 1 and 2 provides "long-life" plugs suitable for distances of 80,000 km or more and requires an extremely small and simple modification to an already known plug and of the corresponding existing production lines.
Conventionally, the central electrode 4 and the core 5 of the plug of Figures 1 and 2 are fitted as follows.
The end 4b of the central electrode opposite its head 4a has one or more transverse diametral slots, one of which, indicated 11, can be seen in Figure 2. In Figure 2, the portions of the end 4b adjacent the slots 11 are shown opened out. Before the central electrode 4 is inserted in the insulator 3, however, they form extensions of the cylindrical surface of the intermediate portion of the central electrode 4. The end 4b of this electrode can thus be inserted in the insulating element 3 through its end 3a until the end 4b projects into a larger-diameter central portion 3b of the duct defined in the insulator 3. At this point, the end 4b of the central electrode is opened out by a tool (inserted into the insulator from its end opposite the end 3a); the central electrode can thus no longer be removed and separated from the insulator 3.
A quantity of sealing mastic 7 is then introduced into the portion 3b of the axial duct in the insulator 3 and the conductive core 5 is then inserted in the insulator 3. The resistive sealing mastic 7 is squashed between the central electrode 4 and the conductive core 5 by the application of opposing forces in the directions of the arrows F in Figure 2. This operation involves a risk that the end portion 3a of the insulator 3 may be damaged by the formation of cracks or other flaws which are not readily detectable but which may compromise the efficiency and functionality of the plug in operation and, in particular, reduce its life.
These problems can be prevented by a spark plug according to the invention produced in the manner which will now be described with reference to Figure 3, in which parts and elements already described have again been designated by the same reference numerals.
In the variant of Figure 3, the central electrode 4 is generally cylindrical and does not have a head on its end portion which is intended to face the combustion chamber. Its opposite end which, in this embodiment, is again indicated 4b is shaped so that it can "mate" by form-coupling with an internal tapered shoulder 3c defined in the axial duct in the insulator 3.
In the embodiment of Figure 3, the central electrode 4 can thus be inserted in the insulator 3 from its end opposite the end 3a. In order to fix the central electrode to the insulator and to afford the plug a long operating life, a ring 10 of noble metal or an alloy thereof is welded around the end 4a of the electrode 4 which projects from the insulator 3.
A quantity of resistive sealing mastic 7 can then be inserted in the axial duct 3b of the insulator 3 and squashed under pressure exerted solely by the core 5, whilst the ceramic insulator 3 is restrained by its tapered lateral surface where it is less delicate, without the risk of damage to its tip 3a.
In the variant described above with reference to Figure 3, the arrangement of the noble metal ring 10 is also such that, in operation, the sparks jump between the ring and the earth electrodes 9.
Figure 4 shows a variant in which the plug 1 includes one (or more) substantially arcuate earth electrodes 19 connected (for example, by welding) to the annular end surface of the metal body 2 with their ends facing the noble metal ring 10 which is welded to the central electrode 4. In the variant of Figure 4, part of the end surface of each earth electrode 19 also faces the ceramic insulator 3 and this plug is therefore of the partial-surface-discharge type.
In the embodiments of Figures 1 to 4, the noble metal ring 10 has a smaller diameter than the end 3a of the ceramic insulator 3.
Figures 5 and 6 show two embodiments similar to those illustrated in Figures 3 and 4 respectively but with the noble metal ring 10 projecting radially from the insulator 3.
These solutions afford better thermal protection to the end or tip 3a of the insulator 3 and eliminate the formation of carbon deposits on the insulator during the cold starting of the engine.
Naturally, the principle of the invention remaining the same, the forms of embodiment and details of construction may be varied widely with respect to those described and illustrated purely by way of non-limiting example, without thereby departing from the scope of the present invention.

Claims

A spark plug comprising:
a tubular, outer metal body (2),
a tubular insulator (3) disposed in the body (2),
a central electrode (4) which extends through the insulator (3) and projects axially from its end (3a) which faces the combustion chamber in use, and
at least one earth electrode (9) which extends from the outer metal body (2) towards the side surface of the portion of the central electrode (4) which projects from the insulator (3),
characterised in that a ring (10) of noble metal or an alloy thereof is disposed around the projecting portion of the central electrode (4), in electrical contact therewith, so that the spark or sparks jump between the ring (10) and the at least one earth electrode (9) in operation.
A spark plug according to Claim 2, in which the projecting portion of the central electrode (4) has a head (4a), characterised in that the ring (10) is disposed around the central electrode (4) between the end (3a) of the insulator (3) and the head (4a) of the central electrode (4).
A spark plug according to Claim 2, characterised in that the ring (10) is welded to the head (4a) of the central electrode (4).
A spark plug according to Claim 2 or Claim 3, characterised in that the ring (10) projects radially beyond the head (4a) of the central electrode (4).
A spark plug according to Claim 2 or Claim 3, characterised in that the side surface of the ring (10) is substantially flush with that of the head (4a) of the central electrode (4).
A spark plug according to Claim 1, in which the end of the central electrode (4) which faces the combustion chamber in use is cylindrical or tapered, characterised in that the ring (10) is fitted onto the end of the central electrode (4) which projects from the insulator (3) and is then welded to the electrode (4).
A spark plug according to any one of the preceding claims, characterised in that the at least one earth electrode (19; Figures 4 and 6) is arcuate and at least part of its end surface faces the noble metal ring (10).
A spark plug according to Claim 7, characterised in that part of the end surface of the at least one earth electrode (19) faces the end portion (3a) of the insulator (3).
A plug according to any one of the preceding claims, characterised in that the ring (10) of noble metal or an alloy thereof projects radially beyond the end (3a) of the insulator (3).