US3517247A - Spark plug with transparent insulating core - Google Patents

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June 23; 3517247 SPARK PLUG WITH TRANSPARENT INSULATING com 4 Sheets-Sheet 1 Original Filed Jan. 9, 1968 Inventor VAN 52/4/96 8 ML 4 4 A tlorneys I. SZILAGYI 3,517,247

SPARK PLUG WITH TRANSPARENT INSULATING CORE June 23,, 1970 Original Filed Jan. 9, 1968 4 Sheets-Sheet 2 Inventor .5 Tm/y 52 mas 1 SZILAGYI 3,517,247

SPARK PLUG WITH TRANSPARENT INSULATING CORE June 23, 1970 Original Filed Jan. 9, 1968 4 sheets-sheet s Inventor [firm 521096 7 By 6144/1444 9444 wmfg 1. SZILAGYI 3,517,247

SPARK PLUG WITH TRANSPARENT INSULATING CORE June 23, 1970 4 Sheets-Sheet A Original Filed Jan. 9, 1968 w W M M. w 5 4/ MQD 7 v. A M

3,517,247 SPARK PLUG WITH TRANSPARENT INSULATING CORE l stvan Szilagyi, Budapest, Hungary, assignor, by mesne assignments, to Messrs. Centropa, Handelsgesellschaft m.b.I-I., Vienna, Austria, a body corporate of Austria Continuation of application Ser. No. 696,602, Jan. 9, 1968. This application May 16, 1969, Ser. No. 825,309 Claims priority, application Great Britain, May 20, 1968, 23,975/ 68 Int. Cl. H01t 13/20, 13/46, 13/48 US. Cl. 313-123 30 Claims ABSTRACT OF THE DISCLOSURE This application is a continuation-in-part of application Ser. No. 696,602, filed Jan. 9, 1968, now abandoned.

This invention relates to spark plugs.

It has previously been proposed to make the insulating body of a spark plug in the form of a hollow glass body surrounding the central electrode to enable the combustion gases to penetrate in operation into the space between the central electrode and the glass body and thus to enable an observer to make deductions about the operational efficiency of e.g. the carburetor by observing the ignition flame colour and size.

It has also been found that the heat resistance of these glass insulating bodies is a function of the thickness of these bodies; in other words, for a given diameter of glass tube the resistance against thermal shocks increases with a decrease of wall thickness. At the same time the resistance against high pressures increases with increasing wall thickness. These contradictory requirements have previously been met by an obviously not fully satisfactory compromise. Moreover, since the dimensions of the threads by means of which the spark plug may be inserted into the cylinder head of an internal combustion engine have been internationally determined, this has meant a maximum limit on the outer diameter of the glass insulating body. The inner diameter of the insulating body, on the other hand, has been limited by the amount of space available to provide the thermal value for which the spark plug has been designed. It has thus been found that known spark plugs made with glass insulating bodies have been prone to break readily when operational disturbances are set up.

This invention is based on the recognition that the above defects may be substantially overcome if, in order to observe the ignition flame, not the transparency but the light conduction of the insulating body wall is utilised.

According to the present invention, there is provided a spark plug having a central electrode and at least one ground electrode to form a spark gap with said central electrode, a metallic housing, said central electrode being mounted in a hollow elongated annular insulating body of light-conducting material which is secured in said metallic housing, said insulating body having at least partly unobstructed end surfaces and being at least in part of frusto-conical shape converging in a direction nited States Patent 3" 1 Patented June 23, 1970 away from said spark gap, the said insulating body being formed or treated so that, in operation, substantially all the light entering thereinto at one of said end surfaces emerges at the other of said end surfaces only.

Said material may consist of substantially axially extending light-conducting, e.g. glass, fibres which may be bonded together by a heat resistant synthetic resin material.

The said insulating body may be made of solid glass the said end surfaces of which have beenpolished.

In this case said end surfaces are preferably plane, parallel with each other, and extend substantially normally to the axis of the spark plug.

Alternatively, said end surfaces are plane, the end surface nearer the spark gap extending substantially normally to the axis of the spark plug, while the other end surface may extend at an angle thereto.

In another embodiment of the invention, the end sur-= face of the insulating body remote from said spark gap may be convex. The end surface of the insulating body nearer the said spark gap may then be concave.

The central electrode may be in the shape of a hollow annular body the interior of which is at least partly filled with an insert made of a material treated or formed so that it conducts light substantially axially only.

Preferably the end surfaces of the light-conducting material filling said central electrode and the end surfaces of the insulating body are of the same curvature and are arranged to form respective continuous surfaces so as to constitute a lens.

Preferably the hollow annular insulating body defines between itself and the central electrode a space into which in operation combustion gases may penetrate, the said space being wholly within the metallic housing.

Only a portion of said insulating body may be frustoconical, the remainder of the insulating body being cylindrical, whereby the said one end surface is substantially larger than the said other end surface, the insulating body thus acting as a lens to intensify the light emerging from the said other end surface.

Preferably said cylindrical part of the insulating body is closed off against entry of combustion gases. A glass bead may be provided for preventing entry of combustion gases into said cylindrical part, said bead being formed integrally with an annular body attached to and surrounding part of the central electrode to increase the leakage path.

The central electrode may be secured in said annular insulating body by means of a resilient binding layer extending therebetween, and said binding layer may be made of an electrically conducting material.

The central electrode is'preferably made of a metallic material, but it may be made of semi-conducting material.

In one preferred embodiment, said metallic housing has a portion which extends axially beyond the said insulating body in the direction of entry ;of combustion gases, said portion having a venturi-shaped tube readily removably secured thereto, the said tube having external threads for screwing into the cylinder head of an internal combustion engine.

The internal surface of said tube may at least partly be formed with a keyhole for receiving a key whereby to enable removal of the tube from said cylinder head.

The major portion of the central electrode may be covered by an annular glass sheath.

Preferably, said at least one ground electrode is embedded in said insulating body and forms a spark gap both with the central electrode and with the metallic housing. Alternatively, or additionally, said at least one ground electrode is provided with an insulating sheath.

There may be a readily detachable electric lead attachment for said central electrode which is provided with means for deflecting the light emerging from said insulating body to a predetermined direction.

In one preferred embodiment, the said means includes a hollow housing, and a mirror which is secured in said housing at an oblique angle to the axis of said insulating body, said housing being provided with an aperture to allow deflected light to pass out therefrom.

In another preferred embodiment, the said means includes a hollow housing and a mirror which is hingeably secured to said housing so as to permit the mirror to be set in a plane which extends at any desired acute angle with respect to the longitudinal axis of the said insulating body.

Preferably, said housing and said mirror are integrally formed, said mirror being constituted by a polished metallic plate.

In a further preferred embodiment, the said means include a sleeve surrounding said central electrode, the sleeve being adapted to form a continuation of said insulating body and being made of the same material, the said face of said sleeve remote from the outer end of said insulating body being bevelled to extend at an angle to the axis of the insulating body.

Said sleeve may be rotatable on said central electrode and may be provided with a resilient metallic insert to permit it to be secured in any desired angular orientation.

The metallic housing is preferably provided with a flange of substantially hexagonal cross-section, and there is an integral collar above the said flange for providing additional support for the insulating body, there being a layer of binding material between the collar and the insulating body, and the collar having a wall thickness equal to, or less than, that of the rest of the said housing.

The invention will be illustrated, merely by way of example, in the accompanying drawings, in which:

FIG. 1 is a longitudinal sectional view of a spark plug according to one embodiment of the present invention,

FIG. 2 is a detailed view of a part of FIG. 1 shown on an enlarged scale,

FIG. 3 is a longitudinal section of another embodiment of a spark plug according to the present invention, but with the metallic housing omitted for the sake of clarity,

FIG. 4 is a bottom end view of FIG. 3,

FIG. 5 is a side view of the lower part of the spark plug shown in FIG. 3,

FIG. 6 is a longitudinal section of yet another embodiment of a spark plug according to the present invention,

FIG. 7 is a top plan view of the spark plug of FIG. 6,

FIG. 8 is a bottom plan view of the spark plug of FIG. 6,

FIG. 9 is a longitudinal section of a further embodiment of a spark plug according to the present invention,

FIG. 10 is a bottom plan view of the spark plug of FIG. 9,

FIG. 11 is an axial section of the spark plug of the present invention provided with a light-reflecting attachment,

FIG. 12 is an axial section of an alternative attachment,

FIG. 13 is an axial section of a further alternative attachment, and

FIG. 14 is a bottom plan view of the attachment of FIG. 13.

Referring first to the embodiment shown in FIGS. 1 and 2, there is shown a spark plug having a central electrode 21, a ground electrode 22 and an annular metallic housing 23 which partly surrounds the central electrode 21.

The central electrode 21 is secured within an insulating body 24 made of a light-conducting material which may consist of a plurality of light-conducting fibres bonded together, e.g. glass fibres. Alternatively, and as illustrated, the insulating body 24 may be made of solid glass. As can be seen, the insulating body 24 comprises an upper substantially cylindrical portion 25 and a lower frusto-conical portion 26 which is open at the end adjacent the spark gap 27 defined between the central electrode 21 and the radially inner end of the ground electrode 22 and tapers away therefrom. Thus a space 28 is defined within the frusto-conical portion 26 of the insulating body into which, in operation, combustion gases may penetrate. The space 28 is limited, in the plane where the frusto-conical portion 26 joins the cylindrical portion 25, by a frusto-conical cap 30 which closes off the space 28. It will be observed that the part of the central electrode 21 which extends through the space 28 is of reduced diameter as compared with the portion thereof extending axially through the cylindrical portion 25.

It will be noted that the ground electrode 22 is embedded in the lower end of the insulating body 24 and extends therethrough both radially inwardly and outwardly. It will therefore be appreciated that the part of the ground electrode 22 which extends radially outwardly of the insulating body 24 forms a second spark gap 31 with the lower end of the metallic housing 23. It has been found that the illustrated method of mounting the ground electrode 22 will have an ionising effect, since even before the appearance of the spark, the increasing voltage on the central electrode 21 causes the ground electrode 22 to be charged capacitatively and thereby helps to initiate sparking.

The central electrode 21 is mounted within the instilating body 24 by means of a resilient binding layer 33 which is an important feature of the present invention. It has been found that the provision of this layer 33 which extends throughout at least the major portion of the length of the cylindrical portion 25 of the insulating body 24 helps to equalise the differential thermal expansion of the insulating body 24 and the central electrode 21 and moreover, helps to damp the vibrations of the central electrode 21 caused by the pressure variations which are set up in operation and which are transmitted to the central electrode 21. A suitable binding layer 33 may be made of a mixture of a metallic powder and a synthetic resin.

A binding layer 34 similar to the layer 33 is provided between the insulating body 24 and the metallic housing 23.

In order to enable observation of the ignition flame from the outside of the spark plug 20 in this embodiment of the invention, the annular end surfaces 35, 36 of the insulating body 24 have been flame polished and have been made plane parallel. Moreover, it will be observed that the bottom end surface 35 is completely unobstructed by the remainder of the spark plug 20. From this it will follow that light rays indicated by arrows will enter the insulating body 24 and will be conducted axially therealong to exit at the top end surface 36. No light is scattered out sideways since total internal reflection conditions pre-- vail within the insulating body 24 when it is made of glass and similarly such conditions will prevail when it is made of glass fibres. Thus an observer looking down from above on the spark plug 20 will see an annular beam of light the colour of which will impart information as to the conditions prevailing in the region of the spark gap. Moreover, the intensity of this beam will be relatively large by virtue of the geometry of the body 24. Clearly the area of the surface 35 is greater than the area of the surface 36, and since substantially no light is lost sideways, the body 24 will act as a light-amplifying or focus ing lens.

While the end surfaces 35, 36 are illustrated as being plane parallel and extending at substantially right angles to the axis of the plug 20, in practice their edges may be radiused or rounded and this is especially relevant to the top end surface 36 where it meets the central electrode 21. It has been found that if radiusing is provided on the inner diameter of the annular insulating body 24, this will help to reduce the likelihood of cracks starting there and propagating across the insulating body 24. Radiusing of the edges of the end surfaces 35, 36 is equally applicable to the embodiments described below. Moreover,

although the end surfaces 35, 36 have been described as being plane parallel, this has been done merely to make manufacturing simpler and of course it is also advantageous for transmission of light. However, if desired, the top end surface 36 could lie at an angle to the axis of the plug, e.g. by bevelling, if it is desired to inspect the colour of the ignition from a different angle.

Turning now to the embodiment shown in FIGS. 3 to 5 in which similar parts have been allotted the same reference numbers, for the sake of clarity the metallic housing 23 and the binding layer 34 have been omitted.

The central electrode 21 in this embodiment is formed with sharp threads 40 so as to facilitate the location of the inner binding layer 33 on said electrode 21 and thereby to ensure that this layer 33 will be effective in equalising the thermal expansion between the central electrode 21 and the insulating body 24 and also in damping the vibrations set up in the electrode 21.

Once again, the insulating body 24 has an upper cylindrical portion 25 and a lower frusto-conical portion 26, resulting in light focusing, as has been described above. The interior 28 of the frusto-conical portion 26 is open to the entrance thereinto of combusion gases but the cylindrical portion 25 is closed off by a substantially spherical glass bead 41. As can be seen, the glass bead is formed on the lower, thinner portion of the central electrode 21 and moreover, the bead 41 is integral with an annular glass mantle or sheath 42 closely surrounding this lower, thinner portion of the central electrode 21, thereby increasing the leakage path between the central electrode 21 and the ground electrode 22. The ground electrode 22 is also surrounded along the major part of its length with a glass mantle or envelope 43.

A spark gap 27 is formed between the lower end of the central electrode 21 and the radially inner end of the transversely extending ground electrode 22. The radially outer end of the ground electrode 22 extends radially outwardly of the outer surface of the frusto-conical portion 2'6 and thus once again two spark gaps may, if desired, be formed, the second spark gap being formed between the said radially outer end of the ground electrode 22 and the lower end of the metallic housing 23 (not shown).

The operation of the spark plug in accordance with the embodiment shown in FIGS. 3 to 5 is extremely similar to that shown in FIGS. 1 and 2. Thus light from the combustion will enter the bottom end surface and will exit at the top end surface 36 under conditions of total internal reflection. Once again, if desired, instead of having a glass insulating body 24, the latter may be made of light-conducting fibres, e.g. glass fibres.

Such axially directed light-conducting fibres are shown in the embodiment of a spark plug in accordance with the present invention as shown in FIGS. 6 to 8. In this embodiment the central electrode is formed by the annular binding layer 33 which is made electrically conductive. Within the annular binding layer 33, however, unlike in the previous embodiments, there is provided an insert 46 made of a light-conducting material, e.g. glass.

The fibres of the insulating body 24 are bonded together by heat resistant synthetic resin material.

As will be observed, both the top end surface 36 and the bottom end surface 35 are arcuate and the corresponding top and bottom surfaces of the binding layer 33 and the insert 46 are shaped so as to form a continuous arcuate surface -With the said end surfaces 35 and 36. The bottom surface 35 will thus be concave with respect tolight rays coming from the ignition of combustion gases in operation, whereas the top surface 36 will be convex. In this way, the light-conducting fibres 24, together with the light-conducting insert 46, will form an optical lens: so as to enhance the axial conduction of light and thereby facilitate the observation of the flame colour.

FIGURES 7 and 8 show somewhat schematically theindividual glass fibres which make up the insulating body 24, these light fibres having been allotted the reference:

numeral 47. The synthetic resin binding material holding the individual fibres together will substantially decrease the likelihood of the insulating body 24 breaking and will substantially enhance its heat resistance. It is well-known. that such composite materials have the advantage that their mechanical, electrical and thermal characteristics are, in effect, added together and in practice practically achieve the individual characteristics of their constituents. Moreover, in the case of an insulating body 24 made of glass fibres, cracks cannot propagate at all since the fibres 47 are isolated from each other. The spark gap 27 is formed between the lower end of the metallic housing 23 and the lower end of the conducting binding layer 33, the spark creeping across the bottom end surface 35.

In order to lead in the current to the conducting inner layer 33, the top end of the insulating body 24 is provided with an annular metallic sheath 48 to which the external current leads may be attached.

While the light-conducting light insert 46 has been described and illustrated as completely filling the entire interior of the annular binding layer 33, it could, if desired, extend only partly within said interior and may for instance extend only within the cylindrical portion 25 of the insulating body 24, leaving the conical portion 26 hollow to allow combustion gases to penetrate thereinto during operation.

If desired, only the bottom surface 35 is made arcuate, or alternatively, only the top surface 36 is made arcuate. It will further be appreciated that the bottom and top surfaces 35, 36 may be made arcuate in all the foregoing embodiments, as well as in the embodiments to be described below.

It will be appreciated that the area of the insert 46 at the light-entry or bottom end is substantially greater than that at the light-exit or top end, whereby the insert 46 will act as a lens to intensify the emerging light, and the light-amplifying effect of the body 24 will be enhanced.

The mode of operation of the spark plug of this embodiment is extremely similar to that of the previously described embodiments and will not be set out again; it is to be particularly noted that the annular body 24 of this embodiment also has the light-amplifying property of the body 24 of the embodiments of FIGS. l-S by virtue of its frusto-conical and cylindrical shape.

Referring now to the embodiment shown in FIGS. 9 and 10, there is shown a spark plug 20 having a central electrode 21, a ground electrode 22, a frusto-conical glass insulating body 24 which is mounted via binding layer 34 in a metallic housing 23. The central electrode 21 passes through the top surface 36 via a metallic sheath 60 em bedded in the insulating body 24.

As can be seen, the metallic housing 23 extends sub stantially beyond the bottom end surface 35 of the insulating body 24 and it also extends beyond the radial plane in which the spark gap 27 is located, in the direction of the cylinder head into which the spark plug 20 in operation would be inserted.

The internal surface of the metallic housing 23 is provided with an annular groove 65 into which is placed and secured an electrically conducting flexible annular ring 66. This ring 66 is cut in one place and one or both ends of the ring are bent radially inwards towards the central electrode 21 to form a spark gap 27 therebetween. In FIG. 12 one such bent-in portion 67 has been illustrated.

The metallic housing 23 has no exterior threads but in the portion which extends below the plane of the spark gap 27 it is provided with internal threads 61. A metallic tube 62 is provided with external screw threads adapted to mate with the screw threads 61 of the metallic housing 23. The tube 62 is substantially annular and hollow and its interior is for-med in the shape of a venturi tube having a throat at 64. As can be seen, the end of the tube 62 nearest to the spark gap 27 is sharply cut away so as to allow light from the ignition to enter the bottom end surface 35 of the insulating body 24.

At the throat 64 of the metallic tube 62 a substantially hexagonal keyhole 68 is provided.

It will be appreciated that when the spark plug in accordance with the embodiment shown in FIGS. 9 and 10 is used, the internal space within the venturi-shaped metallic tube 62 will act as a buffer chamber which will act in operation to protect the insulating body 24 and the central electrode 21. Moreover, since it is the tube 62 which is screwed into the cylinder head of an internal combustion engine, the dimensions of the operative parts of the spark plug itself are made independent from the prescribed dimensions of the screwed-in part and therefore the walls of the insulating body 24 may be made thicker than would otherwise be possible. Moreover, if it is desired to change the spark plug 20, e.g. for cleaning the glass insulating body 24, it will no longer be necessary to unscrew the entire plug but only the metallic housing 23 needs to be detached from the tube 62. In this way the expensively machined threads in the cylinder head of an internal combustion engine are spared from excessive wear.

The internal keyhole 68 is particularly expedient if the spark plug 20 is to be used for observation of ignition in. cylinder heads or other ignition spaces of varying screwthread diameter apertures since in this way only one actual spark plug is required which, together with a set of metallic tubes 62 of varying diameter, may be screwed into these various apertures.

The particular method of mounting the ground electrode 22 is also advantageous from the point of view of changing of parts since it is merely pressed into position in the annular groove 65 owing to the flexibility thereof.

In an alternative construction, instead of screw threads, the tube 62 and the metallic housing 23 could be connected together by a bayonet-type lock.

It will also be appreciated that by virtue of the venturi tube construction of the tube 62, the spark plug in operation will act eificiently in assisting the gases to penetrate into and stream out of the observation space within the frusto-conical insulating body 24.

The actual mode of operation of the spark plug in accordance with the embodiment of FIGS. 9 and 10 does not differ from the previously described ones and of course here too, if desired, the insulating body 24 could be made of glass fibres.

It will further be appreciated that a tube 62 could be provided for the spark plugs in accordance with the other embodiments of the present invention.

Once again, the geometry of the body 24 gives rise, in operation, to a light focusing function of the body 24.

The central electrode 21 of any of the embodiments described above may be formed of a semi-conductor material such as a cermet.

In certain internal-combustion engine cylinder configurations, e.g. in certain so-called V-S engines, at least some of the spark plugs 20 may be so located that direct or axial observation of the surface 36 is not possible or is impeded. In order, nevertheless to permit such obser vation, a light-deflecting attachment is provided, pre ferred embodiments of which are illustrated in FIGS. 11 to 14, wherein FIG. 11, by way of example shows the spark plug of FIG. 1. However, it is to be understood that the attachments of FIGS. 11 to 14 are also usable with the spark plugs of any of the other figures.

The attachment 140 of FIG. 11 comprises a cylindrical housing 141 open at one end and split, as shown at 142, at two diametrically opposite portions. The housing 141 is dimensioned to be a tight fit over the exterior of the insulating body 24. The splits 142 give resilience to the insulating body-engaging end. The other end of the housing 141 is closed by a cap 143 through which extends a connection 144 for connecting to the high-tension supply of the spark plug 20. Dotted-line arrows indicate the assembled position of the spark plug 20 and the attachment 140.

The housing 141 is provided with an aperture 146 and mounted within the housing opposite said aperture 146 is a mirror 147, shown merely schematically. The mirror 147 is mounted obliquely-in the illustrated example, at to the axis of the spark plug 20, and is so arranged as to reflect rays of light emerging from the insulating body 24 at 36 through the aperture 146 outside of the housing 141 in the direction of an observers eye; one such reflected ray is shown.

In order to establish electrical contact between the outer end of the central electrode 21 and the connection 144, a resilient metal strip 150 is provided. The strip 150 is clamped to the cap 143 by the connection 144, is bent at right angles to pass into the interior of the housing 14] along the outside of the cap 143 and is bent twice more, as shown, on the one hand to help secure mirror 147 in position, and on the other hand to provide a resilient contact for the central electrode 21.

The mirror 147 may be secured in position by being of greater diameter than the internal dimension of the bolls ing 141 in the oblique plane in which it is to be mounted. In this case, appropriate oblique mirror receiving apertures may be cut in the housing 141. The latter is then preferably made of an epoxy resin material.

Alternatively, the housing 141 may be metallic and may be made integrally with the mirror 147. In that case, the mirror will be a highly polished metallic plate.

While the end surfaces 35, 36 are illustrated as being plane parallel and extending at substantially right angles to the axis of the plug 20, in practice their edges may be 'radiused or rounded and this is especially relevant to the top end surface 36 where it meets the central electrode 21. It has been found that if radiusing is provided on the inner diameter of the annular insulating body 24, this will help to reduce the likelihood of cracks starting there and propagating across the insulating body 24. Radiusing of the edges of the end surfaces 35, 36 is equally applicable to the embodiment described below.

Moreover, although the end surfaces 35, 36 have been described as being plane parallel, this has been done merely to make manufacturing more simple and of course it is also advantageous for transmission of light.

Reference was made hereinabove to the fact that, if desired, the top end surface 36 could lie at an angle to the axis of the plug, e.g. by bevelling, if it is desired to inspect the colour and intensity of the ignition from a different angle. While this is certainly feasible, a certain amount of diificulty would arise in practice in ensuring that, when the spark plug 20 is screwed home in the cylinder head, its bevelled end surface 36 lies in its desired orientation.

To overcome this difficulty, an attachment 155, shown in FIG. 12 has been devised. The attachment is made of the same material as the insulating body 24 and has substantially the same radial dimensions.

It is adapted to be fitted on the central electrode 21 to lie abuttingly against the surface 36. At its other end, it has a surface 56 bevelled to a desired angle. Light emerging from the surface 36 will thus be deflected by the sur-- face 156 in a desired direction, and the orientation of the surface 156 will be independent of the position in which the spark plug 20 has come to a stop on being screwed home in a cylinder head.

To secure the attachment 155 to the central electrode 21, a resilient metallic insert 158 is provided for resiliently engaging the electrode 21 so as to be rotatable but not readily displaceable.

Referring now to the embodiment of FIGS. 13 and 14, parts corresponding to those illustrated in FIG. 11 have had their reference numbers increased by 100.

The attachment is generally indicated at 240 and includes a substantially cylindrical hollow, open-ended housing 241 preferably made of a lightweight, but durable synthetic resin material. The bottom end 242 of housing 241 is in the form of a metallic ring to which the housing 241 is bonded, and the internal cross-section of the end 242 is substantially hexagonal, and of a dimension enabling it to be snap-fitted onto the hexagonal upper portion @f the metallic housing 23 of the spark plug 20 (not shown in FIGS. 13 and 14).

Substantially mid-way along the axial length of the housing 241, a resilient metallic contact strip 250, which is substantially U-shaped in'section, is secured to the interior of the housing by means of eg a simple nut-andbolt connection 244 extending through the housing wall. The free end of the strip 50 is engaged by the top end of the central electrode 21 (FIG. 11) when the attachment 240 is mounted on the spark plug 20. The external circuitry of the spark plug can then be secured to the connection 244.

The top end of the housing 241 is bonded to a metallic mirror mounting ring 245 which is integrally formed with a pair of substantially parallel, spaced apart, apertured flanges 245a. A pivot 246 in the form of a nut-and-bolt connection extends through the flanges 245a. Mounted on the pivot 246 is a mirror mounting arm 249 to the free end of which is secured, by means of a pin 249a, a discshaped mirror backing plate 248 carrying a circular plain mirror 247.

It will be understood that by means of this arrangement, the mirror 247 is securably settable at any desired angle to the axis of the housing 241 to deflect light passing therethrough in a desired direction.

I claim:

1. A spark plug having a central electrode, at least one ground electrode to form a spark gap with said central electrode, a metallic housing, said central electrode being mounted in a hollow elongated annular insulating body of light-conducting material which is secured in said metallic housing, said insulating body having at least partly unobstructed end surfaces and being at least in part of frusto-conical shape converging in a direction away from said spark gap, the said insulating body being treated so that, in operation, substantially all the light entering thereinto at one of said end surfaces emerges at the other of said end surfaces only.

2.'A spark plug as claimed in claim 1 wherein said material includes substantially lengthwise extending lightconducting fibres.

3. A spark plug as claimed in claim 2 wherein said fibres are glass fibres bonded together by a heat-resistant synthetic resin material."

4. A spark plug as claimed in claim 1 wherein said insulating body is madeof solid glass the said end surfaces of which have been polished.

5. A spark plug as claimed in claim 4 wherein said end surfaces are plane, parallel with each other, and extend substantially normally to the axis of the spark plug.

6. A spark plug as claimed in claim 4 wherein said end surfaces are plane, the end surface nearer the spark gap extending substantially normally to the axis of the spark plug, while the other end surface extends at an angle thereto.

7. A spark plug as'claimed in claim 1 wherein the end surface of the insulating body remote from said spark gap is convex.

8. A spark plug as claimed in claim 1 wherein the end surface of the insulating body nearer the said spark gap is concave.

9. A spark plug as claimed in claim 1 wherein the central' electrode is in the shape of a hollow annular body theinterior of which is at least partly filled with an in-= sert made of a material treated so that it conducts light substantially axially only.

10. A spark plug as claimed in claim 9 wherein respective end surfaces of the light-conducting material filling said central electrode and the end surfaces of the insulating body are of the same curvature and are arranged to form respective continuous surfaces so as to constitute a lens.

11. A spark plug as claimed in claim 1 wherein the hollow annular insulating body defines between itself and the central electrode a space into which in operation combustion gases may penetrate, the said space being wholly within the metallic housing.

12. A spark plug as claimed in claim 1 wherein only a portion of said insulating body is frusto-conical, the remainder of the insulating body being cylindrical, whereby the said one end surface is substantially larger than the said other end surface, the insulating body thus acting as a lens to intensify the light emerging from the said other end. surface.

13. A spark plug as claimed in claim 12 wherein said cylindrical part of the insulating body is closed olf against entry thereinto of combustion gases.

14. A spark plug as claimed in claim 13 wherein a glass bead is provided for preventing entry of combustion gases into said cylindrical part, said bead being formed integrally with an annular body attached to and sur rounding part of the central electrode to increase the leakage path.

15. A spark plug as claimed in claim 1 wherein the central electrode is secured in said annular insulating body by means of a resilient binding layer extending therebetween.

16. A spark plug as claimed in claim 15 wherein said binding layer is made of an electrically conducting material.

17. A spark plug as claimed in claim 1 wherein the central electrode is made of a semi-conducting material.

18. A spark plug as claimed in claim 1 wherein said metallic housing has a portion which extends axially be yond the said insulating body in the direction of entry of combustion gases, said portion having a venturi-shaped tube readily removably secured thereto, the said tube having external threads for screwing into the cylinder head of an internal combustion engine.

19. A spark plug as claimed in claim 18 wherein the internal surface of said tube is at least partly formed with a keyhole for receiving a key whereby to enable removal of the tube from said cylinder head.

20. A spark plug as claimed in claim 1 wherein the major portion of the central electrode is covered by an annular glass sheath.

21. A spark plug as claimed in claim 1 wherein said at least one ground electrode is embedded in said insulating body and forms a spark gap both with the central electrode and with the metallic housing.

22. A spark plug as claimed in claim 1 wherein said at least one ground electrode is provided with an insulating sheath.

23. A spark plug as claimed in claim 1 wherein there is provided a readily detachable electric lead attachment for said central electrode which is provided, with means for deflecting the light emerging from said insulating body to a predetermined direction.

24. A spark plug as claimed in claim 23 wherein said means includes a hollow housing, and a mirror which is secured in said housing at an oblique angle to the axis of said insulating body, said housing being provided with an aperture to allow deflected light to pass out therefrom.

25. A spark plug as claimed in claim 23 wherein said means includes a hollow housing and a mirror which is hingeably secured to said housing so as to permit the mirror to be set in a plane which extends at any desired acute angle with respect to the longitudinal axis of the said insulating body.

26. A spark plug as claimed in claim 24 wherein said housing and said mirror are integrally formed, said mirror being constituted by a polished metallic plate.

27. A spark plug as claimed in claim 23 wherein said means include a sleeve surrounding said central electrode, the sleeve being adapted to form a continuation of said insulating body and being made of the same material the said face of said sleeve remote from the outer end 11 of said insulating body being bevelled to extend at an angle to the axis of the insulating body.

28. A spark plug as claimed in claim 27 wherein said sleeve is rotatable on said central electrode and is provided with a resilient metallic insert to permit it to be secured in any desired angular orientation.

29. A spark plug as claimed in claim 1 wherein the metallic housing is provided with a flange of substantially hexagonal cross-section, and there is an integral collar above the said flange for providing additional support for the insulating body, there being a layer of binding material between the collar and the insulating body, and the collar having a wall thickness equal to, or less than, that of the rest of the said housing.

30. A spark plug having a central electrode, at least one ground electrode to form a spark gap with said central electrode, a metallic housing, said central electrode being mounted in a hollow elongated annular insulating body of light-conducting material which is secured in said metallic housing, said insulating body having at least partly unobstructed end surfaces and being of substan tially uniform wall thickness, the said insulating body be- 12 ing so treated that in operation, substantially all the light entering therein at one of said end surfaces emerges at the other of said end surfaces only.

References Cited UNITED STATES PATENTS JAMES W. LAWRENCE, Primary Examiner C. R. CAMPBELL, Assistant Examiner U.S. c1. X.R.

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