GB2024929A - Spark plug electrode - Google Patents

Abstract

A central electrode 7 for a spark plug comprises a core 9 of a refractory material such as a nickel-chromium alloy, and a sheath 12 of thermally conductive material, such as copper, said core having a tip 11 which projects from one end of the sheath and which has a diameter greater than the internal diameter of the sheath and preferably equal to the external diameter thereof. The electrode is installed in a spark plug with the tip only projecting from the end of an insulating body 3 so that the sheath is protected from the working environment by the tip and the insulator but is nevertheless capable of conducting heat away from the tip. <IMAGE>

Description

SPECIFICATION Spark plug electrode This invention relates to central electrodes for spark plugs.

It is desirable that the central electrodes of spark plugs should be thermally conducted so that heat is conducted away from the tip of the electrode as quickly as possible. Although several metals, especially copper, are excellent conductors of heat and electricity, they are often unsuitable for use as a central electrode because of their poor resistance to the corrosive conditions to which the central electrode of an internal combustion engine spark plug is subjected in use.

In order to utilise the high thermal conductivity of such metals, central electrodes have been produced in which a core of thermally conductive metal is encased in a sheath of a refractory metal such as nickel-chromium alloy or stainless steel.

Such electrodes are produced by inserting a strand of copper into a tube of the refractory metal and sealing the end of the tube around the end of the copper strand. The step of sealing the end of the refractory tube has however been difficult to effect in the large scale manufacture of the electrodes.

According to the present invention we provide a central electrode for a spark plug comprising a core of a refractory metal and a sheath of thermally conductive material, said core having a tip which projects from one end of the sheath and which has a greater diameter than the internal diameter of the sheath.

The invention also includes a spark plug comprising a body adapted for connection to an engine block and carrying an earth electrode, an insulator mounted in the body and a central electrode according to the invention mounted on the insulator with the tip of the central electrode ,projecting from the insulator and lying adjacent the earth electrode, and the sheath lying wholly within the insulator.

The tip of the core can be formed by a conventional forming process for example in a conventional die, thus simplifying manufacture of the electrode. The electrode is used in a spark plug comprising a body adapted for connection to an engine block and carrying an earth electrode, and an insulator mounted in the body, the central electrode being mounted in the insulator with the tip of the central electrode projecting therefrom and lying adjacent the earth electrode and the sheath lying within the insulator.

The sheath is therefore protected from the corrosive atmosphere surrounding the tip of the electrode by the insulator and the tip of the core, but is nevertheless able to conduct heat away from the tip of the electrode.

The diameter of the tip may be larger than the external diameter of the sheath; but preferably it is substantially equal to the external diameter of the sheath.

The sheath may be formed from any thermally conductive metal, such as copper, brass, silver or aluminium, and the core may be made from any refractory metal such as stainless steel, nickel, molybdenum, or nickel-chromium alloy. Other suitable materials will be known to those skilled in the art.

An embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a partial axial cross-section through a spark plug incorporating an electrode in accordance with the invention, Figure 2 is a side view of the central electrode of the spark plug of Figure 1, on an enlarged scale, Figure 3 is an axial cross-section through the central electrode of Figure 2, and Figure 4 is an end view of the electrode of Figures 2 or 3.

Referring to Figure 1, the spark plug comprises a hollow metal body 1 adapted to be screwed into an opening in an engine block and having an integrally formed earth electrode 2. The body 1 receives an insulator 3 of ceramic material which is itself hollow. A conductor 4 extends through the centre of the insulator 3 from an exterior terminal 5 to a central chamber 6, which is packed with a particulate conductive material such as graphite 6a. An axially elongated central electrode 7 is mounted on the opposite end of the insulator 3 from the terminal 5 in electrical contact with the material 6a in the central chamber 6. The tip of the central electrode projects from the end of the insulator 3 and its exterior end is spaced from the earth electrode 2 by a spark gap 8.

Referring to Figures 2 and 3, the central electrode 7 comprises a core 9 of a refractory metal such as a nickel-chromium alloy sold under the trade mark "Inconel". Such alloys typically have the following composition by weight:- Nickel: at least 75% Chromium: 14to 16% Iron: 6 to 8% Manganese: 0.1 to 0.5% Silicon: 0.1 to 0.5% Carbon: 0.04 to 0.1% The cone 9 includes a cylindrical body portion 10 and a cylindrical tip 11 of larger diameter than the body portion. Typically the body portion 10 has a diameter of about 1 mm and the tip has a diameter of about 2.8 mm. The body portion 10 is encased in a sheath of thermally conductive metal, such as copper.The tip 11 projects beyond the end of the sheath 12 and is equal in diameter to the external diameter of the sheath 12. The opposite ends of the sheath 12 and core 9 are formed into a head 14.

The electrode 7 is formed by inserting a strand of the refractory metal into a tube of thermally conductive metal. The tube will normally be about 25 mm in length, and the strand about 30 mm in length so that the strand projects from one end of the tube by about 5 mm. The exposed part of the strand of refractory metal is then formed into a cylindrical tip of 2 to 3 mm in length in a head die.

The opposite end of the sheath is also formed into a head die. This terminal part of the head is then crimped to form a flat projection 1 5 having indentations 1 6 in each of its face.

The electrode 7 is then installed in the spark plug using conventional assembly techniques. As seen in Figure 1, the electrode is positioned in the insulator so that the tip 11 projects from the insulator 3 whilst the sheath 12 lies wholly within the insulator 3. As a result, only the tip 11 is exposed to the working environment of the spark plug and the sheath, which is composed of material less resistant to corrosion and erosion than the material of the tip, is protected from the working environment by the insulator 3 and the tip 11.

The central electrode 7 is thus produced by a simple and easily performed method, involving only mechanical working of the electrode.

Claims (6)

1. A central electrode for a spark plug comprising a core of a refractory metal and a sheath of thermally conductive material, said core having a tip which projects from one end of the sheath and which has a greater diameter than the internal diameter of the sheath.

2. An electrode according to claim 1 wherein the diameter of the tip is substantially equal to the external diameter of the sheath.

3. An electrode according to claim 1 or claim 2 wherein the end of the electrode opposite the tip is formed into a head.

4. A central electrode for a spark plug substantially as hereinbefore described with reference to the drawing.

5. A spark plug comprising a body adapted for connection to an engine block and carrying an earth electrode, an insulator mounted in the body and a central electrode according to any one of claims 1 to 4 mounted in the insulator with the tip of the central electrode projecting from the insulator and lying adjacent the earth electrode, and the sheath lying wholly within the insulator.

6. A spark plug substantially as hereinbefore described with reference to the drawings.