GB2282640A - Glow plug - Google Patents

Abstract

A plug for assisting cold starting of diesel engines includes a screw-in housing 10, and a heater body 26 having a central electrode 18 therein electrically insulated from the body except at the tip 28. The body (26) is formed from electrically conductive fused ceramic material formed of a sintered mix of ceramic and metal powders. The metal powder may be chromium, nickel, iron or aluminium or an alloy or mixture of the metals. The exterior of the body 26 may be coated with a platinum catalyst on a layer of alumina. <IMAGE>

Description

GLOW PLUG This invention relates to electrically operated glow or hot spot devices, commonly called "glow plugs" as used in vehicle diesel and other compression ignition engines to facilitate speedy engine start-up particularly from cold. However, devices embodying the invention may also have applications in assisting start-up or ignition of other forms of heat engine or appliances operated by burning gas or vapourised liquid fuels.

Commonly known forms of glow plug include a housing adapted to mount the plug in the wall of a cylinder or other combustion chamber e.g. of a vehicle engine, in use, a tubular metal sheath carried by the housing to project into the combustion chamber in use, the distal extremity or tip of the sheath containing a totally enclosed wire filament heating element whereby the tip of the sheath speedily attains a high temperature when electric current is applied thereto by way of an electrode carried in the housing, and a resistance element, also usually a coiled wire filament, and also totally enclosed in the sheath electrically interconnecting the heating element with the electrode and formed of a material having a high positive temperature coefficient of resistance.The latter feature provides a glow plug which is self-limiting in that as the plug heats the resistance of the resistance element rises to reduce the current reaching the heating element with a view to prolonging its working life and preventing it burning out even if the current is applied for longer than the normal duration.

Even with these precautions the conventional forms of glow plug have a limited service life, the wire filaments being particularly susceptible to damage, once broken the plug is useless and has to be replaced.

Moreover the manufacture and assembly of conventional glow plugs is costly in terms of the materials needed, the number of components which have to be assembled and connected electrically, e.g. the filament or filaments have to be spot welded or otherwise secured to the sheath tip and electrode, needing close quality control to provide an acceptable product.

Also known is a glow plug in which the tabular sheath is formed of a ceramic material again heated by a metal electrical resistance element within the sheath.

This latter construction still suffers from the problem of limited service life due to the vulnerability of the wire filament elements, particularly if high temperature operation is necessary for effective ignition.

Attempts have been made to improve the ignition properties of a glow plug so that it will operate at lower temperatures by providing the exposed part of the sheath with a coating of a catalytic material, e.g. a catalyst consisting of platinum compound, which reacts with the fuel/air mixture.

The object of the present invention is to provide a glow plug or other glow or hot spot device which is of particularly simple construction, easy to manufacture to high tolerances, and which is particularly reliable and durable in use even under the most adverse operating conditions.

According to the invention there is provided an electrically operated glow or hot spot device for preheating or aiding ignition of a charge of fuel mix in the combustion chamber of a compression ignition or other heat engine or appliance comprising a mounting for securing the device in its position of use; a body carried by the mounting to project into or otherwise be exposed to said charge within the chamber in use, said body or at least that part thereof which is operatively exposed as aforesaid being formed of an electrically conductive fused ceramic material; and electrical connections in or on the mounting disposed to pass current through said body in use whereby at least said exposed part thereof is heated by the electrical resistance of said material.

Conveniently the device will take the general shape of a conventional compression ignition engine glow plug, the mounting being an externally screw threaded metal plug mounting or housing operatively received in and forming the earth electrical contact with a threaded port in the combustion chamber wall; and said ceramic material being shaped into a generally cylindrical pin projecting from the operatively inner end of the housing; an insulated central electrode feeding the current to the distal tip portion of the pin to flow back therethrough to the housing. The pin or other body may be shaped so that the maximum heating is concentrated at the tip.

The ceramic material is preferably a compound of a ceramic substance with an admixture of a metallic powder e.g. of nickel/iron alloy rendering it electrically conductive but with sufficient resistance to cause heating at the voltages commonly available from vehicle electrical systems.

However, other shapes and forms of the device are contemplated, for example a dome or button shaped ceramic body carried in or on a screw-in or other mounting, or possibly an elongated rod or bar having longitudinally spaced connecting electrodes, e.g. at or near each end and exposed or partly exposed within the combustion chamber.

An example of the invention is now more particularly described with reference to the accompanying diagrammatic drawing being a side elevation, part longitudinally sectioned, of a glow plug for a vehicle diesel engine.

The plug comprises a metal mounting or housing 10 of generally conventional construction having an externally threaded inner end portion 12 which is operatively screw engaged in a bore through the wall of a combustion chamber of the engine and a hexagon head 14 at its outer end.

A central through bore of housing 10 locates a tubular electrically insulating bush 16 in its outer end part, which in turn locates a metal electrode rod 18 the outer end of which connects with a screw connecting terminal 20.

Rod 18 is formed of a heat resistant metal, e.g. a nickel alloy, and extends substantially beyond the inner end of housing 10 so that its distal end part projects into the combustion chamber in the position of use. The extreme tip portion 22 of this distal part is shaped into a knob or head of slightly greater diameter than the remainder of the rod.

The periphery of rod 18 which is outside insulation bush 16 is provided with a heat resistant electrically insulating coating 24, e.g. of glass, except for said extreme end portion 22 which is left uncoated.

The active part of the plug is an elongated resistance pin or body 26 formed to totally enclose the distal part of rod 18 back into the inner end part of housing 10. Body 26 is generally cylindrical but its distal tip 28 is reduced in diameter so that the thickness around the extreme end portion 22 of rod 18 is substantially less than the thickness around the remainder of the rod.

Body 26 is formed from an electrically conductive ceramic material, typically a compound of a ceramic powder with a proportion, e.g. 5-10S by volume of a powdered metal such as chromium, iron or aluminium or some alloy or mixture thereof. The body is shaped by moulding and pressing the material as a wet paste followed by sintering or other heat treatment to fuse it into a hard solid.

The formed body 26 containing the electrode rod 18 is finally secured in housing 10 by swaging an annular rib 30 indicated in broken lines on the drawing radially inwardly until it is flush with the adjoining parts of the housing periphery, so gripping and sealing the part of body 26 within the housing bore.

In use the plug is activated, e.g. to assist starting the engine from cold, by applying electrical current through terminal 20 and hence to electrode rod 18, the earth return being by way of the metal housing 10 in conventional manner. The current flows from the uninsulated end portion 22 of rod 18 into the reduced thickness tip 28 of the conductive body 26, the resistance of the material causing speedy localised heating at said tip. The increased thickness main part of body 26 will not heat so rapidly and serves as a selflimiting resistance cutting the flow of current as the plug heats in similar fashion to known types of selflimiting plug using wire filament heating and resistance elements.

The self-limiting property has hitherto been an essential requirement in most of the glow plugs commonly used hitherto in an effort to prolong their service life by protecting the filaments, in particular the heating filament, from burning out even if the plug should be left switched on for a long period. The plug of the invention does not utilise any vulnerable wire heating or resistance elements, both the electrode rod 18 which can be of almost any convenient thickness and the hard heat resistant ceramic body 26 are extremely durable and are expected to have an extremely long and trouble free service life even if the plug should be abused by overheating due to excess current or being left switched on for over-long periods.Thus it is contemplated that the self-limiting feature is not essential, it will not matter in practice whether the whole of the exposed part of body 26 attains red heat for substantial periods, and the reduced diameter shaping of tip 28 could be dispensed with. There is a residual and remote risk that extreme heating might cause localised melting of the plug components but this could be guarded against in other ways.

Not only is the plug resistant to damage during current carrying operation, it is also extremely resistant to corrosion during its continuous exposure to the heat, pressure, and chemical reactions within the combustion chamber while the engine is running, there are no exposed metal surfaces on the active part of the plug such as the metal outer sheath of conventional glow plugs to be attacked by the sulphur and other corrosive products of combustion.

Due to the manner of formation and operation of the glow plugs or other devices of the invention the active part, i.e. the exposed portion of body 26, can be formed in almost any shape, it need not have the probe-like cylindrical shape of conventional glow plug pins. It could, for example, be a hemispherical or domed body with an exposed surface forming the hot spot in an engine or other combustion chamber, or it could be a rod or the like mounted to lie along or in a combustion chamber wall surface with spaced electrical connections for passing current through an active part. This makes it possible to suit the shape of the glow or hot spot device to the combustion chamber geometry, and to enable its mounting at the most convenient or effective position therein rather than the combustion chamber having to be shaped to accommodates a glow plug of conventional design.

Positioning the latter may dictate a combustion chamber shape which is not the most efficient possible in theory, e.g. it may affect the positioning and size of valve seats or the shaping of a piston head or inlet or exhaust passages.

In a modified form of the above embodiment the exterior surface of body 26, e.g. over the area of the tip 28, is coated with a catalytic material, e.g. a platinum compound. To avoid effects caused by reaction between the coating and some types of ceramic material a protective layer, e.g. of alumina, may first be applied to form a barrier between the coating and body 26. The catalyst enables the plug to provide effective ignition at lower temperatures than would otherwise be the case, ensuring that the current which can be carried by the ceramic material will provide sufficient resistive heating thereof to cause fast starting from cold, reducing warm-up time and providing a glow plug which, it is believed, may last the full life of an engine without needing replacement. A ceramic catalytical glow plug may provide reliable and effective ignition at around 6500C compared with the 8500C or more needed for start-up with a non-catalytic version.

It is also contemplated that glow plugs made in accordance with the invention will reduce or contribute to reduction of unburnt hydrocarbons in the exhaust, particularly on start-up, thus reducing wastage and atmospheric pollution.

Claims (14)

1. An electrically operated glow or hot spot device for pre-heating or aiding ignition of a charge of fuel mix in the combustion chamber of a compression ignition or other heat engine or appliance co:aprising a mounting for securing the device in its position of use; a body carried by the mounting to project into or otherwise be exposed to said charge within the chamber in use, said body or at least that part thereof which is operatively exposed as aforesaid being formed of an electrically conductive fused ceramic material; and electrical connections in or on the mounting disposed to pass current through said body in use whereby at least said exposed part thereof is heated by the electrical resistance of said material.

2. A device as in Claim 1 wherein the mounting is an externally screw threaded metal plug housing operatively received in and forming an earth electrical contact with a threaded port in the combustion chamber wall; and said ceramic material is shaped into a generally cylindrical pin projecting from the operatively inner end of the housing; an insulated central electrode feeding the current to the distal tip portion of the pin to flow back therethrough to the housing.

3. A device as in Claim 2 wherein the pin is shaped so that the maximum heating is concentrated at said tip portion.

4. .t device as in Claim 1 in the for of a do or button shaped ceramic body carried in or on a screw-in or other oundin, or an elongated rod or bar having longitudinally spaced connecting electrodes, exposed or partly eposed within the combustion chamber in use.

5. A device as in any preceding claim wherein the ceramic material is a compound of a ceramic substance with an admixture of a metallic powder rendering it electrically conductive but with sufficient resistance to cause heating at the voltages commonly available from vehicle electrical systems.

6. A device as in Claim 5 wherein said metallic powder is chromium, nickel, iron, or aluminium, or an alloy or mixture including one or more of said metals.

7. A device as in Claim 6 wherein the ceramic material includes 5-102 by volume of the metallic powder.

8. A device as in Claim 5, 6 or 7 wherein the ceramic material is shaped as a wet paste to form the body followed by sintering or other heat treatment to fuse it into a hard solid.

9. A device as in Claim 8 wherein the hard solid body is secured in the mounting by staging.

10. A device as in any preceding claim wherein at least part of the operatively exposed exterior surface of the body is coated with a catalytic material.

11. A device as in Claim 10 wherein said catalytic material is a platinum compound.

12. A device as in Claim 10 or 11 including a protective layer of alumina or other material between said coating and said exterior surface of the body.

13. A glow or hot spot device substantially as hereinbefore described with reference to and as shown in the accompanying drawing.

14. A compression ignition internal combustion engine including a flow or hot spot device as in any preceding claim.