A spark plug for motor vehicles

Abstract

 A spark plug for motor vehicles comprises a hollow metal body (2) in which a central insulated electrode (4) is disposed for connection to a high-tension electrical supply. A bush (16) is fixed to the metal body (2) and at least one earth electrode (18) which cooperates with the central electrode (4) is formed thereon.

Description

[0001] The present invention relates to a spark plug for motor vehicles of the type comprising a hollow metal body in which a central insulated electrode is disposed for connection to a high-tension electrical supply, and at least one earth electrode cooperating with the central electrode and carried by the metal body.

[0002] In known plugs, the earth electrode (or electrodes) is constituted by a metal bar welded (usually by arc welding) to one end of the metal body.

[0003] The welding of the earth electrode is a critical operation since it introduces great variability in the heat rating of the plug.

[0004] In order to resolve this problem, the subject of the present invention is a spark plug for motor vehicles of the type specified above, characterised in that the earth electrode is formed on a bush fixed to the metal body.

[0005] As will become clear in the course of the detailed description which follows, in addition to the fact that it does not need weldingl the plug according to the present invention also enables the earth electrode to be made of any material, particularly corrosion-resistant materials which by their nature are unsuitable for welding. Moreover, the bush can be shaped so as to provide any number of earth electrodes and these electrodes can have many different shapes.

[0006] Further characteristics and advantages of the present 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 an axial section of a plug according to the present invention,

Figure 2 is an axial section of a part of the plug according to the present invention,

Figure 3 is an axial section of the element indicated III in Figure 2,

Figure 4 is a view taken on the arrow IV of Figure 3,

Figure 5 is a section corresponding to Figure 1, showing a second embodiment of the plug according to the present invention, and

Figure 6 is a view of the part indicated by the arrow VI in Figure 1, on an enlarged scale.

[0007] With reference to Figure 1, a spark plug for motor vehicles, indicated 1, comprises a hollow metal body 2 in which is disposed a central electrode 4 surrounded by an insulating cover 6, for example, of ceramic material. The central electrode 4 has an end 8 by means of which it is connected in known manner to a high-tension electrical supply usually constituted by the ignition coil of the vehicle (not shown).

[0008] In known manner, the metal body 2 has a threaded portion 10 for fixing the plug 1 to the engine head and a hexagonal portion 12 which can be engaged by a socket spanner (not shown).

[0009] A metal bush 16, shown in greater detail in Figures 3 and 4, is fixed to one end 14 of the metal body 2, in the manner which will be explained further below, and surrounds an end portion of the central electrode 4. The bush 16 has three appendages 18 which constitute the earth electrodes of the plug 1 and are spaced at 120° from each other, their ends being bent so as to assume the configuration shown in Figure 1 after the bush 16 has been fixed to the body 2.

[0010] As can be seen in Figures 1, 2 and 6, the body 2 has an annular projection 20 which projects from the cylindrical wall 22 of its cavity. The projection 20 has a trapezoidal outline in cross-section, the inclined sides of which are inclined to a radial line A at an angle B of substantially 30° (Figure 2).

[0011] The end 14 of the metal body 2 is heated (for example, by induction) to cause it to expand so that the bush 16 can be inserted in the body 2. The cooling of the body 2 causes radial interference which retains the bush 16. The edge of the bush 16 which faces inwardly of the body 2, indicated 24, is then upset against a first bearing surface 26 of the projection 20 by means of a punch. The outer surface of the bush 16 has a chamfer 28 which is urged against a second bearing surface 30 of the projection 20.

[0012] The insulating cover 6 of the central electrode 4 also has a chamfer 32 which is urged axially against the edge 24 of the bush 16 as a result of the fixing of the cover 6 in the body 2 by the upsetting of the edge 34 of the body 2 against the cover 6. The contact between the surface of the chamfer 32 and the edge 24 of the bush 16 creates a gas-tight zone which avoids the need for the sealing ring which is usually interposed between the insulating cover 6 and the metal body 2.

[0013] Figure 5 shows a second embodiment of the plug according to the present invention. The parts which correspond to those described above are indicated by the same reference numerals. In this second embodiment, the bush 16 has a single electrode 18 which is bent after the bush 16 has been fixed to the body 2.

[0014] By virtue of the bush 16, it is possible to produce plugs with any number of earth electrodes and these electrodes can be formed in many different shapes. Moreover, the earth electrodes 18 can be made of a corrosion-resistant material, even though such a material is not suitable for welding.

Claims

1. A spark plug for motor vehicles, comprising a hollow metal body (2) in which a central insulated electrode (4) is disposed for connection to a high-tension electrical supply, and at least one earth electrode (18) cooperating with the central electrode (4), characterised in that the earth electrode (18) is formed on a bush (16) fixed to the metal body (2).

2. A plug according to Claim 1, characterised in that the bush (16) is inserted with radial interference in one end of the metal body (2) and surrounds an end portion of the central electrode (4).

3. A plug according to Claim 2, characterised in that an annular projection (20) is formed on the inner cylindrical surface of the metal body (2) and has a first bearing surface (26) against which an edge (24) of the bush (16) facing inwardly of the metal body (2) is upset.

4. A plug according to Claim 3, characterised in that the insulating cover (6) of the central electrode (4) has a chamfer (32) which is urged axially against the bent edge (24) of the bush (16) so as to constitute a gas-tight zone.

5. A plug according to Claim 3, characterised in that a chamfer (28) is formed on the outer surface of the bush (16) and is urged against a second bearing surface (30) of the projection (20).

6. A plug according to Claim 3, characterised in that the annular projection (20) of the annular body (2) has a trapezoidal outline in cross-section, the inclined sides of which constitute the first and second bearing surfaces (26, 30).

7. A plug according to Claim 6, characterised in that the first and second bearing surfaces (26, 30) of the annular projection (20) are inclined to a radial line (A) at an angle (B) of between 15° and 45°.

8. A plug according to Claim 7, characterised in that the angle (B) of inclination of the first and second bearing surfaces (26, 30) to the radial line (A) is substantially 30°.

9. A plug according to any one of the preceding claims, characterised in that the bush (16) has three earth electrodes (18) spaced at 120° from each other.

Drawing