GB2044849A - A spark plug - Google Patents

Abstract

At least one electrode, preferably the central electrode 10, is protected at its surface by an electrically insulating refractory material provided by a constituent in the electrode. The refractory material may consist of alumina resulting from oxidation of aluminium at the surface of an aluminium-containing ferritic alloy which may contain up to 22% chromium, 0.5 to 12% aluminium and 0.05 to 3% yttrium. Both electrodes 9, 10 of the plug may be protected by such an electrically insulating refractory material. The invention has one application in spark plugs for use in stratified charge engines. <IMAGE>

Description

SPECIFICATION A spark plug This invention relates to a plug for providing a spark of an electric ignition system and more particularly, but not exclusively, to such a plug for use in various forms of stratified charge internal combustion engines.

The continual development of the petrolfuelled internal combustion engine by improvements in combustion chamber design, higher engine revolutions, and higher compression ratios has increased the thermal loading of combustion chamber components, such as the sparking plug of the electric ignition system. Prolonged driving of vehicles on motorways has led to engine components being subjected to long periods of high speed and high temperature operation.

In addition, projected engine developments are likely to increase sparking plug electrode temperature since the future is likely to see a significant trend in the use of various forms of so-called stratified charge engines in order to comply with engine exhaust emission control legislation. In stratified charge engines, the fuel/air charge is prepared so that burning starts in a fuel-rich zone and spreads through the rest of the charge which is fuel lean.

Basically, carbon monoxide and hydrocarbon emission in the exhaust decreases as the amount of oxygen available for combustion increases.

Two current types of stratified charge engines, are the indirect fuel injection Hondo CVCC engine which uses a pre-chamber, and the Ford PROCO and Texaco TCCS engines which use direct fuel injection.

It is anticipated that the continuing develop ment of stratified charge engines will demand the use of improved sparking plugs having an extended central core, larger gaps between the electrodes, and higher energy ignition than those currently in common use. Such sparking plugs would have to operate at higher temperatures with associated problems of chemical corrosion and electrical erosion of their electrodes.

According to one aspect, the invention com prises a plug for providing a spark of an electric ignition system, the plug having at least one electrode protected at the surface of the electrode by an electrically insulating re fractory material comprising a constituent of said electrode.

Preferably, said one electrode comprises a central electrode of the plug.

The refractory material may comprise a refractory oxide.

According to a further aspect of the present invention, there is provided a plug for provid ing a spark of an electric ignition system, the plug having at least one electrode comprising an aluminium-containing ferritic alloy.

Desirably, said one electrode comprises a central electrode of the plug.

The ferritic alloy might contain chromium and yttrium, for example: up to 22% chromium, 0.5 to 12% aluminium, 0.05 to 3% yttrium, and the balance iron, the percentages being expressed by weight.

Many known sparking plugs use a central electrode made from a material, such as nickel or a nickel alloy, which in use forms an oxide film as a result of the high temperature oxidising conditions in the combustion chamber of an engine. Such oxide films are electrically conductive and do not impede the electrical operation of the sparking plug to any serious extent. However, nickel-containing materials in the presence of hydrocarbon vapours are liable to become fouled by carbonaceous deposition.

Use of a plug according to the invention results in the formation of an electrically insulating refractory material (e.g. alumina) at the surface of the electrode, and the presence of the electrically insulating refractory material might be expected to inhibit the electrical sparking mechanism between the central electrode and a side electrode of the plug. Surprisingly, a plug having an electrode protected by such a material has been found to function satisfactorily.

The electrically insulating refractory material of the invention (e.g. alumina) may be more resistant to carbon deposition than the aforementioned nickel or nickel alloy electrodes.

The invention will now be further described by way of example only with reference to the accompanying drawing, the single figure of which shows a side view in part section of a sparking plug.

Referring to the drawing, the sparking plug shown is generally of conventional shape (e.g.

Champion N9Y) and has a metal body 1, an aluminium insulator 2, and a terminal 3 for connection to a high-tension electric lead (not shown). The body 1 has a hollow threaded shank 6 from which a side electrode 9 extends and is angled therefrom so as to lie parallel to the end face of the sparking plug.

A central electrode 10 protruding from an alumina insulating screen 11 defines a gap 14 between the side electrode 9, this gap 14 being adjustable by displacement of the side electrode 9. The other end of the central electrode 10 locates inside and is welded to the terminal 3.

The central electrode 10 is joined by a lead oxide-based glass frit 1 2 to be insulating screen 11, and alumina cement 1 3 is used at the end face of the insulating screen 11 to protect the glass frit 1 2.

The central electrode 10 is made of a FECRALLOY (Registered Trade Mark) alumin ium-containing Ferritic alloy, which has an alumina film at its surface as a result of oxidation of the aluminium at the surface of the alloy, and this alumina at the surface provides the required electrically insulating refractory material. The alumina film grows within the electrode 10 with prolonged oxidation, is adherent and self-healing in that it quickly reforms in the event of it cracking or spalling.

The side electrode 9 may also be made from the aluminium-containing Ferritic alloy as well as the central electrode 1 0.

Example Several of the aforedescribed sparking plugs of the drawing were made by modifying conventional sparking plugs, using as a central electrode 10 a 2.3 mm diameter wire of FECRALLOY aluminium-containing Ferritic alloy having an approximate composition 19.5% Cr, 5.47% Al, 0.21% Y, and the balance largely Fe, percentages being expressed by weight. A gap of 0.025" was set between the central electrode 10 and the side electrode 9 of the plugs.

The modified sparking plugs were found to function satisfactorily in tests with equivalent but conventional sparking plugs, with no discernable misfiring or loss of power over the equivalent of 3500 miles of intermittent use.

The conventional sparking plugs before modification conformed generally in shape to the following manufacturer's equivalent range: N9Y Champion AG 32/32A Motorcraft HLNY Lodge BP6ES NGK FE65P KLG W175M30 Bosch [thread diameter-14 mm : reach-1 9 mm] It will be understood that although a plug according to the invention has advantages in stratified charge engines, such a plug may also be used where appropriate in other applications, such as premium life sparking plugs, and in other ignition systems.

Claims (9)

1. A plug for providing a spark of an electric ignition system, the plug having at least one electrode protected at the surface of the electrode by an electrical insulating refractory material comprising a constituent of said electrode.

2. A plug as claimed in Claim 1, wherein the refractory material comprises a refractory oxide.

3. A plug as claimed in Claim 2, wherein the refractory oxide comprises alumina.

4. A plug as claimed in any one of the preceding Claims, wherein the one electrode comprises an aluminium-containing ferritic alloy.

5. A plug for providing a spark of an electric ignition system, the plug having at least one electrode comprising an aluminiumcontaining ferritic alloy.

6. A plug as claimed in Claim 4 or Claim 5, wherein the ferritic alloy comprises: up to 22% Cr, 0.5 to 12% Al, 0.05 to 3% Y.

7. A plug as claimed in any one of the preceding Claims, wherein the one electrode comprises a central electrode of the plug.

8. A plug as claimed in Claim 7, including a ceramic screen around the central electrode such as to expose a tip portion of the central electrode, glass sealing means joining the central electrode to the screen, and alumina sealing means between the screen and the central electrode to protect the exposed end of the glass sealing means.

9. A plug as claimed in Claim 5, and substantially as hereinbefore described with reference to the accompanying drawing.