WO1999031775A1

WO1999031775A1 - Spark plug

Info

Publication number: WO1999031775A1
Application number: PCT/GB1998/003649
Authority: WO
Inventors: Joseph Gibson Dawson
Original assignee: Dawson Royalties Ltd
Current assignee: Dawson Royalties Ltd
Priority date: 1997-12-17
Filing date: 1998-12-07
Publication date: 1999-06-24
Anticipated expiration: 2000-06-17
Also published as: GB2346933A; GB0012762D0; AU1494599A

Abstract

A spark plug for an internal combustion engine is described, the spark plug including two electrodes, the electrodes comprising a central electrode and a ground electrode, there being a gap between the two electrodes and wherein the gap between the central and ground electrodes is bridged by a bridging member of a monolithic, electrically insulating material.

Description

SPARK PLUG

The present invention relates to spark plugs for internal combustion engines.

Conventional spark plugs in use at the present time typically have a life expectancy of about 12,000 to 18,000 miles before they need to be replaced.

Major contributing factors to the gradual deterioration of a spark plug include: the direction and intensity of the spark leading to electrode erosion; temperature differential between the hotter central electrode and the cooler ground electrode due to poor heat transfer from the central electrode; limits on the life of the electrode materials; the size of the spark gap; and, corrosion due to acids formed during combustion.

The centre electrode is constantly under the greatest stress. The centre electrode works harder than the ground electrode and functions as the cathode emitter to form a spark. The central electrode is unable to dissipate heat efficiently due to being surrounded along its length by a ceramic insulator and being connected to a plastics material insulated high tension (HT) lead having a carbon trace core conductor. Typically, a new spark plug starts with a sharp edged flat surface on the centre electrode and a corresponding flat surface on the ground electrode, there being a gap in the region of 0.028" to about 0.060" depending upon the engine builders specification and application. During the first 500 to 1000 miles of the spark plug's life, heat and spark erosion cause the initial sharp edges of the centre electrode to become rounded and, at 10,000 miles the centre electrode is domed or virtually ball-ended which also leads to a higher breakover voltage which further accelerates erosion of the centre electrode.

Heat and spark erosion are the two factors which most affect spark plug longevity. Heat is produced in two forms: initially the heat and intensity of the spark itself as it crosses the gap; and, the heat of combustion itself. The formation of the spark can raise the temperature of the centre electrode far higher than that of the ground electrode which is on the "cool side" of the spark and has the benefit of being firmly attached to the metal shell of the spark plug body which itself is in good thermal contact with the cooled engine cylinder head so that heat may be readily dissipated. As stated above, the central electrode is surrounded by a ceramic insulator, is carrying 50 millijoules, transmits from 5 to 25 KV, has an air gap at one end and is connected at its outer end to a poorly conducting HT lead. Thus, if the centre electrode becomes too hot, erosion due to the spark and increasing breakover voltage will be rapid.

US-A-4 939 409 describes a spark plug having a surface discharge member bridging the gap to the ground electrode. The surface discharge member comprises an alumina body having conductive surface coatings of a sintered mixture of alumina and from 60 to 85 wt% platinum at each end where the body is in contact with the centre and ground electrodes respectively and, a non- conductive surface film of alumina and from 10 to 40 wt% platinum in between the two conductive surface coatings. The surface discharge member itself is expensive to manufacture and the remaining structure of the spark plug and the manufacturing process to make the spark plug appear to place this spark plug outside the scope of an economic device which could be used in ordinary vehicles.

It is an object of the present invention to provide a spark plug for an internal combustion engine having a longer life than conventional spark plugs and which may be produced at an economic cost.

According to the present invention, there is provided a spark plug for an internal combustion engine, the spark plug including two electrodes, the electrodes comprising a central electrode and a ground electrode, there being a gap between the two electrodes and wherein the gap between the central and ground electrodes is bridged by a member made of a monolithic, electrically insulating material .

In the spark plug according to the present invention the gap between the centre electrode and the ground electrode is bridged by a bridging member made of a monolithic material which is electrically insulating under "normal" ambient conditions but is preferably a good thermal conductor. The term "bridging member" is used to denote any configuration or cross-section of member which bridges or connects the two electrodes.

The material should be electrically insulating to 25 Mega Volts per metre. Examples of materials fulfilling these conditions are aluminium nitride and silicon nitride. Aluminium nitride is a ceramic and is used in high temperature resistors, has heat conduction properties more akin to metals than ceramics yet is a strong engineering ceramic having excellent electrical insulating properties at high temperatures. Silicon nitride has good thermal shock resistance and is also a strong engineering ceramic.

Other potential materials may include electrically insulating reaction bonded silicon carbide and beryllium oxide for example.

The bridging member may be allowed to 'float' in and be located and retained by recesses in the centre electrode and possibly also in the ground electrode.

In one embodiment of a spark plug according to the present invention, the bridging member in the form of a rod of silicon nitride or aluminium nitride is fixed between the two electrodes across the gap therebetween. The rod may be fixed at one end into the centre electrode and at the other into the ground electrode.

In another embodiment of the present invention, the recess in the centre electrode may have a depth substantially the same or slightly greater than the axial length of the bridging member. Thus, when the spark plug is inverted, the bridging member slides completely inside the centre electrode and the gap between the end thereof and the ground electrode may be set with standard feeler gauges without fear of damaging the bridging member. When the spark plug is then held in the generally upright position adopted in most engine designs, the bridging member slides out of its recess and contacts or approaches the ground electrode.

In a further modification of the spark plug according to the present invention, the end of the bridging member in contact with the ground electrode may have a substantially conical or pointed end, the spark formed in this embodiment being analogous with the conical, focused spark formed in the spark plug described in our co- pending International patent application number PCT/GB98/01434.

It has been found that where the original "gap" is about 0.7mm for example, the breakover voltage to establish a spark between the electrodes is effectively that which would be needed if the "gap" were an air gap of about 0.125mm ie about 7.5 KV. We have also found that doubling the gap' to 1.4mm with a bridging member in place only increases the breakover voltage to about 9 KV, ie the relationship between 'gap' and breakover voltage is non-linear. Furthermore, we have found that to restore a breakover voltage of 15 KV which is commonly employed in prior art spark plugs having an air gap, an electrode face 'gap' of about 4.25mm is reguired with the bridging member in place, ie a ^λgap' increase of a factor of about X6. The spark plug industry is seeking to provide spark plugs with gaps of about 2mm but is unable to do so with ignition systems commonly and economically available. Thus, spark plugs of the present invention are easily able to provide spark gaps of 2mm at breakover voltages much less than are currently employed with conventional spark plugs which is a very considerable advantage of the present invention.

The rod is able to expand or contract as temperature dictates due to the relatively little resistance thereto by the hot ground electrode which is able to deform slightly.

Alternatively, the rod may be fixed to the centre or ground electrode only, as described above, and the remote end left to move within a bore provided in the other electrode.

It has been found that the rod conducts heat away from the centre electrode to the ground electrode preventing excessive heating thereof. The nature of the spark has been altered due to the rod providing a path therefor which has resulted in greatly reduced erosion of the electrodes. Furthermore, breakover voltage is reduced due to the presence of the bridging rod thus reducing stress on the coil and allowing the use of a larger "gap" if desired and which is beneficial for fuel combustion.

Tests have confirmed that a spark plug life approaching 100,000 miles may be obtainable with spark plugs according to the present invention.

In order that the present invention may be more fully understood, examples will now be described by way of illustration only with reference to the accompanying drawings, of which:

Figure 1 shows a part-sectioned side elevation of part of a spark plug according to a first embodiment of the present invention;

Figure 2 shows a monolithic bridging member as used in the embodiment of Figure 1;

Figure 3 shows a partially sectioned view similar to Figure 1 of a second embodiment of a spark plug according to the present invention; and

Figure 4 which shows a perspective view of a bridging member for the embodiment shown in Figure 3. Referring now to the drawings and where the same features are denoted by common reference numerals.

Figure 1 shows a part-sectioned portion of a spark plug 10 according to the present invention. The spark plug comprises a metal body 12 having a screw threaded portion 14 for fixing the spark plug into the cylinder head (not shown) for example of an associated internal combustion engine (not shown) ; a ceramic insulator 16; a centre electrode 18 within the insulator 16; a ground electrode 20 fixed to the body 12; and, a gap bridging member 22 in the form of a cylindrical rod (also shown alone in Figure 2). The centre electrode 18 extends through the spark plug 10 surrounded by the insulator 16 and has at one end a recess 24 to accept one end 26 of the bridging member 22 and a terminal (not shown) at its remote end for connecting to a high voltage source. The ground electrode 20 is made of metal, welded at one end 30 to the threaded portion 14 and has a recess 32 for the other end 34 of the bridging member 22. The bridging member 22 is made of silicon nitride (e.g. Nitrasil- trade name). The gap 40 between the centre 18 and ground 20 electrodes is bridged by the member 22. The member 22 is of high electrical resistance but has good thermal conductivity and enables a spark to form by initially tracking across the surface of the bridging member which then allows a spark ^λbody' to spread out over the opposing faces of the centre and ground electrodes to establish a substantial spark volume to ignite the fuel charge. A "gap" 40 of about 0.7mm with the bridging member 22 bridging thereacross has substantially the same breakover voltage as an air gap of about 0.125mm therefore, as mentioned above larger electrode separation gaps and consequently larger sparks may be employed at one third to one half of the breakover voltage required by conventional spark plugs. The member 22 conducts heat away from the centre electrode 18 to the ground electrode 20 and thence to the cylinder head (not shown) to be dissipated thus reducing the thermal stress on the electrode 18. The bridging member 22 may be a 'floating' or interference fit in the recesses 24 and 32 and not fixed.

In the embodiment shown in Figure 1, the ground electrode recess 32 may be provided with a channel (not shown) extending to the side edge of the electrode to allow the ground electrode to be twisted so as to locate the member 22.

The material of the bridging member 22 may be loaded with conductive particulate material to vary and control the resistance thereof.

Figures 3 and 4 show a second embodiment of a spark plug according to the present invention wherein the central electrode 18 has a recess 50 of a depth sufficient to accommodate the entire length of the bridging member 52 (shown in more detail in Figure 4) . This enables the gap 40 between the flat face 56 of the centre electrode 18 and the opposing face 58 of the ground electrode 20 to be set accurately without danger of damaging the bridging member. The spark plug is merely inverted to allow the member 52 to slide inside the recess 50. Once the gap 40 is set, the plug is merely held in a generally upright position to enable the member 52 to slide out of the recess and contact the face 58 of the ground electrode. In this embodiment the end of the bridging member 52 in contact with the ground electrode 20 is in the form of a cone 60 having a pointed end 62. In this embodiment, the shape of the spark is at least partially conical as indicated by the dashed lines 64, the spark 64 being formed over the surface of that portion of the bridging member which projects from the recess 50. In this embodiment, when the spark gap is set at about o.7mm, the breakover voltage is reduced to about 16% of the normal voltage. In a test, it was found that two identical spark plugs sharing a common ground and a common HT supply, both gapped at 0.86mm, one a conventional open gap, the other bridged according to the present invention; that the conventional open gap must be reduced to 0.4mm or thereabouts before the bridged plug switches off. This applied to both pointed or flat based bridging members as described hereinabove.

The size of the bridging members 22, 52 in both the embodiments shown in Figures 2 and 4 is about 3mm in length and about 1mm in diameter.

It has also been found that spark duration is increased due to the breakover voltage being so dramatically reduced leading to improved combustion of the fuel charge. Spark duration increases by about 12 to 16%.

Furthermore, by comparison with air, a bridged discharge is silent.

Claims

1. A spark plug for an internal combustion engine, the spark plug including two electrodes, the electrodes comprising a central electrode and a ground electrode, there being a gap between the two electrodes and wherein the gap between the central and ground electrodes is bridged by a bridging member made of a monolithic, electrically insulating material.

2. A spark plug according to claim 1 wherein the electrically insulating material is silicon nitride.

3. A spark plug according to claim 1 wherein the electrically insulating material is selected from the group comprising: reaction bonded silicon carbide; alumina; aluminium nitride; and, beryllium oxide.

4. A spark plug according to any one preceding claim wherein the bridging member is in the form of a rod.

5. A spark plug according to any one preceding claim wherein the bridging member is located at one end in the centre electrode and at the other in the ground electrode.

6. A spark plug according to any one preceding claim from 1 to 4 wherein the bridging member is located by a recess in the centre electrode.

7. A spark plug according to claim 6 wherein the recess in the centre electrode is of sufficient depth to accommodate the entire length of the bridging member.

8. A spark plug according to either claim 6 or claim 7 wherein one end of the bridging member rests on the surface of the ground electrode.

9. A spark plug according to claim 8 wherein the end of the bridging member resting on the ground electrode has a generally conical end.

10. A spark plug according to any one preceding claim wherein the material of the bridging member is loaded with material to control the resistance thereof.

11. A spark plug according to claim 1 wherein the bridging member is located by the centre electrode only.

12. A spark plug according to claim 1 wherein the bridging member is located by the centre and ground electrodes .

13. A spark plug according to claim 12 wherein the bridging member is a 'floating' fit in recesses in the centre and ground electrodes.

14. A spark plug according to claim 12 wherein the bridging member is an interference fit in recesses in the centre and/or the ground electrodes.

15. A spark plug substantially as hereinbefore described with reference to the accompanying description and Figures 1 and 2; or Figures 3 and 4 of the drawings.