Spark Plugs for Internal Combustion Engines

This invention relates to spark plugs for use in internal combustion engines, and is particularly concerned with the construction of the centre electrode of such spark plugs.

The electrodes of spark plugs operate under very unfavourable conditions and call for compromises as regards the various properties required in the materials used therefor. On the one hand good electrical and thermal conductivity is required, while on the other hand the material should have adequate corrosion resistance, particularly at high temperatures, to the reaction products resulting from combustion of the fuel, and a very slow burning rate is necessary in order to achieve a satisfactory service life. Furthermore the workability of the material must be such as to facilitate mass production, and a sufficiently high mechanical strength at elevated temperatures is required.

A consideration of these requirements will indicate, that pure metals or alloys are generally not capable of providing a favourable combination of the required properties. Centre electrodes made up of several different metallic materials have therefore been developed. However, the known arrangements suffer from various technical disadvantages and in many cases production costs are high. The present invention seeks to provide a centre electrode made up of several different metals which while providing the best combination of the desired electrode properties is inexpensive to produce.

According to the invention there is provided a centre electrode for use in the spark plugs of internal combustion engines, which electrode comprises a length of composite material made up of at least two metals, one forming a matrix and the other being constituted by a plurality of wires or fibres embedded in the matrix, and wherein at least one of the metals possesses the properties of high electrical and thermal conductivity, the composite material being produced by the joint plastic shaping of the metals, which initially are in the form of wires, to produce a solid metallic bond therebetween.

Investigations have shown that the method used for producing the composite material decisively influences the properties of the material when in use. The composite material is therefore produced by the joint plastic shaping of the initial metals to form a solid metallic bond therebetween.

In one embodiment of the invention, the other metal takes the form of wires each having a diameter in the order of millimetres. (See Example V hereinafter).

In an advantageous embodiment, the composite material may take the form of a composite fibre reinforced material in which a large number of fibres, each having a diameter in the order of micro-meters, are embedded in the matrix metal. (See Examples I to IV)

In one preferred embodiment, the fibres consist of a metal having high electrical and thermal conductivity, which fibres are embedded in a matrix metal having high corrosion resistance. In one favourable combination, the composite material comprises a matrix metal of nickel, or a nickel alloy containing at least 50% by weight of nickel, in which fibres of fine silver are embedded. In a further embodiment, the composite material includes a further initial metal of tubular shape, which forms a metal jacket around the matrix and embedded wires or fibres. Preferably, the jacket is made of corrosion resisting metal. It may also be advantageous to use thermally stable metals such as tungsten and molybdenum as the material of the fibres and/or matrix.

Advantageously, the constituents of the composite material are uniformly distributed over the cross-section of the electrode. For certain applications however, the distribution of the constituents of the composite material and particularly the disposition of the fibres in the matrix, may be other than uniform; for example a symmetrical arrangement in the form of a ring, symmetrical about a centre point, or a star-shaped distribution may be preferred.

A particular advantage may be achieved if the initial metals are solid wires. The effectiveness of the plastic shaping of the initial metals in producing a composite material in which there is a solid metallic bond between these initial metals can be confirmed by means of micrographs of the finished product, i.e. of the fully shaped centre electrode, or by any other known method.

Alternatively, the initial metals may be sheathed wires, the metal of the wire itself being one of the initial metals and the metal of the sheath the other.

By using the above-described features, it is possible to produce a centre electrode for the spark plugs of internal combustion engines that has good electrical and thermal properties combined with a slow burning rate and low production costs.

Other features of the invention will become apparent from the following discussion, taken with the accompanying drawings, wherein:

FIG. 1 is a diagramatic transverse cross-section, by way of example only, through a spark plug incorporating a centre electrode in accordance with the invention;

FIG. 2 is an enlarged diagramatic transverse cross-section through the sparking end of the electrode of FIG. 1 illustrating one example of electrode construction in accordance with the invention; and

FIG. 3 is an enlarged diagramatic transverse cross-section through an electrode which additionally has therearound a surrounding jacket, preferably of corrosion-resistant material.

Various examples of electrode construction in accordance with the invention will now be given:

With reference to the drawings, a centre electrode 1 is produced from a composite fibre reinforced wire, 2 mm in diameter, contained 150 embedded fibres 3 each having a diameter of 100 .mu.m. The composite wire comprises approximately 60% by weight of nickel as the matrix metal, 2 and 40% by weight of fine silver as the fibre metal.

It will be understood that FIGS 2 and 3, being diagramatic only, do not actually illustrate all 150 fibres.

A composite fibre reinforced wire, 2mm in diameter, for producing the centre electrodes of spark plugs, contained 50 embedded fibres each having a diameter of approximately 170 .mu.m. The wire comprises up to 50% by weight of silver as the matrix metal, and approximately 50% by weight of a nickel alloy (made up of Ni 95%, Mn 3% and Si 2%) as the fibre metal.

As shown in FIG. 3, a composite fibre reinforced wire, 2mm in diameter, produced in the same way as in Example II, also had a jacket 4 made of a nickel alloy as specified in Example II, the jacket having a thickness of approximately 100 .mu.m.

A composite fibre reinforced wire, 2 mm in diameter, for producing the centre electrodes of spark plugs, contained 5000 embedded fibres each having a diameter of approximately 15 .mu.m. The wire comprised 50% by weight of a nickel alloy, made up as in Example II, as the matrix metal, and approximately 50% by weight of fine silver as the fibre metal.

A composite fibre reinforced wire formed by a plurality of solid wires comprised, in its initial condition, i.e. before joint plastic shaping was carried out, 19 pure nickel wires, 2mm in diameter, and 12 wires of fine silver; 1mm in diameter, which were distributed symmetrically over the cross-section that they formed and were put into a tubular sleeve of pure nickel having a wall-thickness of 1mm. The bunch of wires, together with the tubular sleeve, were subjected to plastic shaping by drawing, and this resulted in the wires becoming welded together in the cold state. The material was heat-treated at approximately 800.degree. C. between each two consecutive plastic shaping passes, and, by a process of diffusion, the embedded wires were bonded together with a solid metallic bond. As deformation proceeded there was formed a composite body which, at a diameter of 2mm, was used as the centre electrode of a spark plug.