GB2391264A

GB2391264A - Spark plug

Info

Publication number: GB2391264A
Application number: GB0217694A
Authority: GB
Inventors: Paul Tinwell; Simon Alexander Freeman; John Russell Askew
Original assignee: Federal Mogul Ignition Co
Current assignee: Federal Mogul Ignition LLC
Priority date: 2002-07-31
Filing date: 2002-07-31
Publication date: 2004-02-04
Anticipated expiration: 2022-07-31
Also published as: GB0217694D0; AU2003251341A1; WO2004012311A1; GB2391264B

Abstract

A spark plug (50) comprises an electrically-insulating sleeve (12), a first electrode (16) mounted within the sleeve and having a tip (16a), an electrically-conducting shell (20) surrounding said sleeve, and a second electrode (22) mounted on and electrically-connected to said shell, the second electrode having a tip (22a) positioned so that with the tip (16a) of said first electrode (16) defines a spark gap (G1) of the plug. Said second electrode (22) is joined to an end surface (20b) of the shell (20) by a joining member (51) which makes an arcuate junction (53) with said shell. The joining member (51) is shaped so that, between the shell and the electrode, it reduces in transverse cross-sectional area.

Description

( 1 2391264

SPARK PLUG

This invention is concerned with a spark plug for use in providing an ignition spark to ignite the fuel of an internal combustion engine.

A typical conventional spark plug is shown in Figure 1 and is described in detail below. This plug comprises an electrically-insulating sleeve which extends along a central axis of the plug. Such sleeves are made of ceramic material, usually alumina. The plug also comprises a first electrode mounted within the sleeve and having a tip projecting axially beyond an end portion of the sleeve. The electrode extends centrally within the sleeve and is electrically connected to a terminal projecting from the other end of the sleeve. The connection between the terminal and the first electrode may include a resistor also contained within the sleeve which serves to control the peak current. In the operation of the plug, a high tension lead is applied to the terminal so that a high voltage can be applied to the first electrode. The plug also comprises an electrically-conducting shell surrounding said sleeve. The shell is fixed, normally by a screw thread, into the head of an engine so that the tip of the first electrode projects into the combustion chamber of a cylinder of the engine.

The plug also comprises a second electrode mounted on the shell, and electrically-connected to the shell. The second electrode is in the form of a square bar of constant transverse cross-section which is secured to point on the end surface of the shell by welding. The second electrode has a tip which is positioned within the combustion chamber so that a spark gap is formed between the tips of the two electrodes.

In the typical conventional spark plug described above, it is found that, under certain operating conditions the temperature of the insulating sleeve becomes greater than is desirable. This leads to the requirement to improve the cooling of this sleeve. One known measure is to shorten the shell so that the insulating sleeve projects further beyond the end of the shell so that a greater proportion of the sleeve is exposed to the cooling effect of incoming fuel. This, however, means that the second electrode has to be longer since it is further from the shell to the spark gap. It is found that such longer electrodes are subject to cracking in service and it is an object of the present invention to reduce this problem.

The invention provides a spark plug comprising an electrically-insulating sleeve extending along a central axis of the plug, a first electrode mounted within the sleeve and having a tip projecting axially beyond an end portion of said sleeve, an electrically-conducting shell surrounding said sleeve, and a second electrode mounted on and electrically-connected to said shell, the second electrode having a tip positioned so that with the tip of said first electrode it defines a spark gap of the plug, the shell having a generally hollow cylindrical end portion terrrinating at a generally annular end surface of the shell, wherein said second electrode is joined to said end surface of the shell by a joining member which makes an arcuate junction with said shell and, between the shell and the electrode, reduces in transverse cross-sectional area.

In a spark plug according to the invention it is possible to achieve improved cooling of the electrically insulated sleeve without increased risk of cracking of the second electrode. The presence of the joining member results in less possibility of the electrode resonating during service and hence in a reduced possibility of cracking.

In a spark plug according to the invention, the joining member may be integral with the second electrode and may be secured to the shell by welding.

This welding is along an arc rather than at a point as is conventional. Such welding may be continuous or may be provided by a series of spot welds.

Alternatively, the joining member may be integral with the shell, ie the joining member may be formed by a projection of the shell. In this case, the electrode may be welded to the projection.

The joining member, preferably, has a generally triangular shape (when viewed in a longitudinal plane of the plug normal to the plane occupied by the electrode). The joining member, if integral with the electrode, preferably has a thickness (in the plane occupied by the electrode) which is equal to the thickness of the electrode. For example, the second electrode and the joining member may be formed as a bar of substantially constant transverse cross-

section provided with one or more transverse projections which extend from the arcuate junction with the shell to which the electrode is secured. Where the joining member is integral with the shell, it, preferably, has the same thickness as the shell.

In order to increase the area of the electrodes which define the spark gap, preferably, the tips of the electrodes are both inclined at an angle to the longitudinal axis of the plug. Such an angle may be between 30 and 60 degrees. The angles may be arranged to be different so that a spark gap reduces in width in a direction towards the free end of said second electrode, ie the end thereof which is not secured to the shell. This encourages formation of sparks at the free end of the second electrode which is the best position for ensuring ignition of the fuel.

In the typical conventional spark plug described above, under normal working conditions, the spark jumps across the spark gap defined by the tips of the first and the second electrodes and goes to ground through the shell and

the engine head. However, in operation, the spark plug often becomes fouled by carbon which is deposited on the portion of the insulating sleeve which is exposed to the combustion chamber. This makes the surface of the insulating sleeve conductive, creating a potential alternative path to ground avoiding the spark gap. Eventually, the resistance of the alternative path to ground becomes comparable with or less than that across the spark gap. If this occurs, the electricity ceases jumping the spark gap and "runs" along the surface of the insulating sleeve. In this case, the electricity may jump across the clearance between the end portions of the insulating sleeve and the shell, is the spark occurs "within the plug". The spark within the plug may cause ignition of the fuel but ignition may not occur because the spark is less favourably positioned than at the spark gap as it is to some extent masked by the end portions of the shed and the sleeve. The further that the spark occurs from the end surface of the shell, the greater the masking is. A spark jumping said clearance does have the beneficial effect of burning the carbon depose off the surface of the sleeve, thereby increasing the electrical resistance and increasing the chance of the next spark occurring at the spark gap. Thus, H a spark occurs away from the spark gap, it is not desirable but may result in ignition and tends to return the plug to normal operation. However, if the electricity runs along the surface of the sleeve all the way to the sleeve's junction with the shell, no spark occurs and ignition is impossible. Furthermore, this situation is likely to be sustained.

These factors lead make it desirable that a plug has a long length of exposed insulating sleeve and that the firing end of the plug operates at high temperature in order to assist in cleaning carbon off the insulating sleeve.

These considerations limit the range of applications for a particular plug, complicate manufacture, and reduce the durability of the plug.

In the known spark plugs described as being of "the closed bore type", examples of which are disclosed in US Patent Nos. 4209990 and 5244188, in order to ensure that the electricity does jump to form a spark, the shell of the plug is provided with an internal flange in the region of its end surface. This

( flange reduces the clearance between the shell and the sleeve (typically to about 0.5 mm). The flange acts to shield the end portion of the sleeve within the shell against receiving carbon deposits and also provides a small gap (a "secondary spark gap") for the electricity to jump to the flange where it is only masked from the fuel mixture to a limited extent. In some cases' the length of the path along the surface of the sleeve is increased by providing undulations in the external surface of the insulating sleeve on the portion thereof which is within the shell (see Figure 3 of US 4289990). However, the provision of a flange on the shell does not eliminate the masking of the spark since the spark may jump to any point of the flange, including the inner face thereof.

Furthermore, the provision of a flange on the shell shortens the distance from the tip of the first electrode to the shell, and, by reducing the clearance, reduces the electrical resistance in the region of the flange. Both these factors increase the chance that sparking away from the intended spark gap will occur. In addition, the flange greatly restricts access to a volume within the plug in which unburned hydrocarbons and fuel droplets may collect and possibly bridge the secondary spark gap which would render the plug inoperative.

Another approach is to provide one or more additional electrodes on the shell of the plug. The additional electrodes provide secondary spark gaps at relatively unmasked locations. However, such electrodes are difficult and expensive to provide, only cause cleaning of the shell opposite the additional electrode so that carbon deposits can build up elsewhere, and limit the width of the intended spark gap, as it is necessary to avoid the additional electrode becoming the primary path for the electricity which may occur especially as the plug wears.

In a spark plug according to the invention, therefore, the plug preferably also comprises at least one projection formed from insulating material, the projection being positioned on the sleeve and projecting from the sleeve so that it reduces said clearance in the region of the end surface of the shell thereby

( forming a secondary spark gap. This achieves a spark plug in which the chance of the spark occurring away from the intended spark gap is reduced 1 without reducing the chance of no spark occurring. The projection may be integral with the sleeve or may be secured to the sleeve, being either a coating deposited on the sleeve or a separate piece attached to the sleeve. Where the 3 projection is not integral with the sleeve, it may be formed from a different insulating material to the sleeve.

Where the sleeve has a projection, it may extend as an annular rib I around the sleeve. This gives the advantages of increasing the length of the path to ground along the surface of the sleeve and of partially shielding the portion of the end portion of the sleeve which is within the shell from carbon deposits. Alternatively, there may be a series of projections distributed around the external surface of the sleeve. Each projection may have a domed shape in cross-section axially of the plug or may have a pointed crest. Preferably, each projection is aligned so that its crest is at least approximately co-planar with the end surface of the shell. The portion of the end portion of the sleeve which is within the shell may be provided with path-lengthening undulations.

There now follow detailed descriptions to be read with the accompanying

drawings of two known spark plugs and of a spark plug which is illustrative of the invention In the drawings: Figure 1 is a longitudinal cross-sectional view taken through a known spark plug; Figure 2 is a detailed view of the spark gap region of a second known spark plug;: Figure 3 is a side elevational view of the illustrative plug according to the invention, including a view in detail of the spark gap region of the plug; and

i ( Figure 4 is a view similar to Figure 3 but taken at 90 degrees thereto.

The known spark plug 10 shown in Figure 1 comprises an electrically-

insulating sleeve 12 extending along a central axis 14 of the plug 10. The sleeve 12, which is formed from alumina, defines a space 12a extending along the axis 14. The plug 10 also comprises a first electrode 16 mounted within the space 12a and having a tip 16a projecting beyond said sleeve 12. The plug 10 also comprises a temninal 18 mounted within the space 12a and projecting beyond the sleeve 12 at the other end thereof to the tip 16a of the electrode 16.

Within the space 12a, the electrode 16 and the terminal 18 are in electrically-

conductive contact with one another.

The plug 10 also comprises an electrically-conducting shell 20 surrounding a portion of said sleeve 12, the sleeve 12 being mounted in the shell 20. The shell 20 has an external threaded portion 20a by which the plug 10 can be mounted on an engine head. The shell 20 has an end surface 20b, i formed on an end portion 20c of the shell 20, which is generally annular. The end surface 20b extends radially of the axis 14. The end portion 20c is generally in the form of a hollow cylinder, having an internal surface 20d. The end portion 20c extends over an axial distance designated "d2" from the surface 20b to an inwardly-projecting flange 20e of the shell 20. The flange 20e projects into close proximity to the external surface 12b of an end portion 12c of the sleeve 12. A seal 21 seals the gap between the flange 20e and the sleeve 12. Between the flange 20e and the end surface 20b of the shell 20, there is a clearance between the internal surface 20d of the end portion 20c of the shell 20 and the external surface12b of the end portion 12c of the sleeve 12. The external surface 12b is generally frusto-conical so that the clearance increases in width in the direction towards the tip 16a. The end portion 12c passes through the end portion 20c of the shell 20 with a clearance therebetween, and extends through the surface 20b. The end portion 12c projects beyond the surface 20b by a distance designated "d1".

The plug 10 also comprises a second electrode 22 mounted on and electrically-connected to said shell 20. Specifically, the second electrode 22 is! welded to the surface 20b and projects in a "J" shape to a tip 22a thereof positioned in opposed relationship with the tip 16a so that the tips 16a and 22a together define a spark gap designated "G1". I The plug 10 is mounted in an engine head by means of the threaded portion 20a of the shell 20 so that the tip 16a of the first electrode 16, the end portion of the sleeve 12 adjacent to the tip 16a, and the second electrode 22 project into a combustion chamber of the engine. When a high voltage is applied to the terminal 18, in nominal operation of the plug 10, a spark is created which ignites fuel mixture in the combustion chamber. It is intended that the I spark will cross the gap G1. However, in service, fouling (depositing of carbon) occurs on the end portion 12c of the insulating sleeve 12. This leads to the possibility that, instead of jumping the gap G1, the electric ty may run along the surface 12b of the sleeve 12 into the clearance between the surfaces 12b and 20d. In this case, a spark may jump to the shell 20 anywhere along the distance d2. Unless the spark jumps near the surface 20b, the spark is at least: to some extent masked from the fuel mixture by the shell 20 and the sleeve 12, making ignition uncertain. Indeed, it is possible for the electricity to travel along the end portion 12c all the way to the flange 20e and no spark may occur. To reduce this possibility, it is desirable if the distances d1 and d2 are made relatively long, d1 to discourage sparking away from the gap G1, and both d1 and d2 to reduce the possibility of no spark occurring. However, in practice, d1: is usually determined by other considerations so that the clearance between the surfaces 12b and 20d may have to be increased instead. I The second known spark plug 30 shown in Figure 2 differs from the plug 10 only as described below and like parts are given the same reference numbers without further description. Specifically, the plug 30 differs in the

shape of its electrodes 16 and 22. Specifically, the first electrode 16 of the plug 30 has its tip 16a inclined at an angle to the longitudinal axis 14 of the plug and the second electrode 22 has its tip 22a also inclined to the longitudinal axis 14 but at a different angle. The angles of inclination are arranged so that the width of the spark gap reduces in a direction towards the free end of the second electrode 22. Another difference is that the shell 20 of the plug 30 has an unthreaded skirt portion 20f projecting downwardly around the sleeve 12 of the plug 3Q. The second electrode 22 is in the form of a bar of constant transverse cross-section which is welded to a point on the lower surface 20b of the shell 20. The illustrative spark plug 50 according to the invention is shown in Figures 3 and which include detail views of the spark gap area of the plug which can be compared with Figure 2. With the exceptions noted below the illustrative spark plug 50 is the same as the plug 30 and like parts are given the same reference numbers and not described further.

The spark plug 50 differs from the plug 30 in the form of the lower end of the shell 20 thereof, in the form of the second electrode 22 thereof, and in the way in which the second electrode 22 is secured to the shell 20. A further difference is that the electrically-insulating sleeve 12 has a projection 52 thereon. The shell 20 differs from that of the plug 30 in that the unthreaded skirt portion 20f is ommed, thereby causing the sleeve 12 to project further beyond the end surface 20b of the shell 20. The greater projection of the sleeve 12 exposes it to a greater extent to incoming fuel, thereby improving cooling. The second electrode 22 of the plug 50 is illustrated in Figures 3 and 4.

The electrode 22 is secured to the end surface 20b of the shell 20 by means of a joining member 51. The joining member 51 is integral with the electrode 22 and makes an actuate junction 53 with the end surface 20b of the shell 20. At

the arcuate junction 53, the joining member 51 is welded to the shell 20 by a continuous weld extending along said junction 53. Between the shell 20 and the electrode 22, the joining member 51 reduces in transverse cross-sectional area so that, as shown in Figure 4, it has a generally triangular shape with the base resting on the surface 20b and the apex merging with the electrode 22.

Figure 3 shows that, in a plane normal to that shown in Figure 4, the joining member 51 has a thickness which is the same as the electrode 22.

From its junction with the joining member 51, the electrode 22 bends so that its tip 22a is opposite the tip 16a of the first electrode 16 of the plug, thereby defining a spark gap. As in the plug 30, the tips 16a and 22a are angled at different angles with respect to the central axis 14 of the plug. The electrode 22 is in the shape of a bar of substantially constant transverse cross-

section. Another way of looking at the shape of the piece of metal which forms the electrode 22 and the joining member 51 is that it is a bar of constant transverse cross-sectional area with two buttresses 51a projecting therefrom in opposite directions. These buttresses 51a are each in the fomm of a right-

angled triangle with the base welded to the surface 20b and the hypotenuse providing a tapering edge of the member 51. Each of the buttresses 51a is curved so that the base of the member 51 matches the curvature of the surface 20b. In an alternative arrangement (not illustrated) the joining member 51 is integral with the shell 20 and the electrode 22 is welded thereto. In this case the member 51 can be formed by providing the shell 20 with a skirt portion such as the skirt portion 20f which is machined away to leave the required projection to form the member 51.

As mentioned above, the plug 50 also differs from the plug 30 in the form of its insulating sleeve 12 in that it is provided with the projection 52. This projection 52 is integral with the sleeve 12 and projects from the end portion 12c so that, in the region of the end surface 20b of the shell 20, it reduces the clearance between the end portion 12b and the end portion 20c of the shell 20 to a gap which is designated "G2" since it provides a secondary spark gap where sparks are most likely to jump if they do not jump at the intended gap G1. The projection 52 extends as an annular rib around the sleeve 12 so that the clearance between the projection 52 and the surface 20d is substantially equal around the sleeve 12, being 0.8mm.

In cross-section, axially of the plug 30, the projection 52 has a domed sinusoidal shape with the crest of the dome substantially co-planar with the surface 20b. In other words, the projection 52 is aligned with its centre line co-

planar with the end surface 20b of the sleeve 20.

Claims

1 A spark plug comprising an electrically-insulating sleeve extending along a central axis of the plug, a first electrode mounted within the sleeve and having a tip projecting axially beyond an end portion of said sleeve, an electrically-conducting shell surrounding said sleeve, and a second electrode mounted on and electrically-connected to said shell, the second electrode having a tip positioned so that with the tip of said first electrode it defines a spark gap of the plug, the shell having a generally hollow cylindrical end portion terminating at a generally annular end surface of the shell, wherein said second electrode is joined to said end surface of the shell by a joining member which makes an arcuate junction with said shell and, between the shell and the electrode, reduces in transverse cross-sectional area.

2 A spark plug according to claim 1, wherein said joining member is integral with the second electrode.

3 A spark plug according to claim 1, wherein said joining member is integral with the shell.

4 A spark plug according to any one of claims 1 to 3, wherein the joining member has a generally triangular shape.

5 A spark plug according to any one of claims 1 or 4, wherein the tips of the two electrodes are both inclined at an angle to the longitudinal axis of the plug.

(

6 A spark plug according to any one of claims 1 to 5, wherein said spark gap reduces in width in a direction towards the end of said second electrode which is not secured to the shell.

7 A spark plug according to any one of claims 1 to 6, wherein the plug also comprises at least one projection formed from insulating material, the projection being positioned on the sleeve and projecting from the sleeve so that it reduces said clearance in the region of the end surface of the shell thereby forming a secondary spark gap.

8 A spark plug according to claim 7, wherein the projection is integral with the sleeve.

9 A spark plug according to either one of claims 7 and 8, wherein the projection extends as an annular rib around the sleeve.

10 A spark plug according to any one of claims 7 to 9, wherein the projection has a domed shape in cross-section axially of the plug.

11 A spark plug according to any one of claims 7 to 10, wherein the projection is aligned so that a centre line of the projection, circumferentially of the sleeve, is co-planar with the end surface of the shell.