GB2300968A - Method of manufacturing a spark plug insulator - Google Patents

Abstract

A spark plug insulator which includes a thermocouple 16 having a junction 24 near the tip of the nose of the insulator is manufactured by securing the thermocouple 16 to a green ceramic core or blank and isostatically moulding more green ceramic material around the thermocouple before firing the insulator. The wires of the thermocouple 16 each have a sleeve (20, Fig. 2) made of a material which is thermally decomposable and/or volatilisable. During firing the wires are held under tension, and the sleeves (20) burn off. The wires do not buckle or get snagged by the shrinking ceramic during firing because of the extra free space produced by the disappearing sleeves (20) and the tension under which the wires are held. Where sleeving was present, the external profile of the finished insulator is not distorted by adhesion of the wires to the insulator. In a variation of the technique, dummy wires are used to produce an insulator with bores to receive the thermocouple wires, and the assembly is again fired after enclosing the junction 24 in the nose region.

Description

METHOD OF MANUFACTURING A SPARK PLUG INSULATOR The present invention relates to a method of manufacturing a spark plug insulator, and particularly an insulator which has a thermocouple for measuring the insulator nose temperature.

A spark plug for an internal combustion engine comprises an insulator, a centre electrode which is disposed longitudinally through a bore in the centre of the insulator, and a metal collar which is secured to the insulator and which has a screw thread for securing the spark plug in a cylinder of the engine. The metal collar is connected to, or integral with, a side or ground electrode, the tip of which is in close proximity to the tip of the centre electrode for generating a spark therebetween.

The insulator comprises a shank portion and a nose portion. The shank portion may taper gradually to merge with the nose portion of the spark plug, or the shank portion may be provided with one or more shoulders which reduce its width.

When the spark plug is secured in an engine cylinder the shank portion projects outside the engine cylinder and the nose portion projects into the engine cylinder. The position of the nose portion, through which the centre electrode is disposed, determines where in the cylinder the spark will be generated.

Typically a spark plug insulator is manufactured by compacting particles of a ceramic material and an organic binder into a blank by a process of "isostatic moulding" in which the particles are pressurised in a mould to a point where they bind together to form a shape retaining structure known as a "green" ceramic. A typical ceramic material is alumina, with fluxing agents. The organic material is selected to be thermally degradable and/or volatilisable, for example a wax or cellulosic ether. The blank is then machined to a desired shape and fired at high temperature, for example around 15000C. The firing process hardens the insulator, and it shrinks by about 16%. The organic binder is driven off in the firing process, by volatilisation and/or degradation to leave a ceramic insulator.

It is well known to provide the nose portion of the insulator with a thermocouple to measure the temperature at the tip of the nose. See for example US patent number 3 940 987, or European Patent application number 0 225 713.

The temperature measurement may be used to determine if the plug is properly matched to the engine cylinder, or operating within the preferred temperature range for optimum engine performance. If the insulator nose temperature is too cold, carbon deposits will be produced, leading to fouling of the spark plug. If the insulator nose temperature is too hot, pre-ignition will occur, reducing engine performance, and possibly causing engine damage.

In European patent application 0 225 713, it was proposed to bed a thermocouple in the insulator by forming a green ceramic core (also called a blank) having a nose-forming portion which is slightly smaller in diameter than the nose of the finished insulator; securing a small thermocouple to the surface of the nose-forming portion adjacent its tip; providing the thermocouple with electrical leads which extend along the surface of the core to locations remote from the tip of the nose for ready access from the outside of the engine; forming a green ceramic shell over the undersized portion of the core, the thermocouple and the leads; machining the shell to a pre-firing shape conforming essentially to that of the finished insulator and such as to keep the thermocouple junction and leads submerged slightly below the finished surface of the nose of the insulator for connection to appropriate circuitry responsive to temperature changes monitored by the thermocouple; and finally firing the core and shell together into an integral mass embedding the thermocouple leads therein.

The thermocouple is retained in the insulator after firing because of the shrinkage of the ceramic during firing which causes the ceramic material firmly to bind the wires of the thermocouple.

A problem with this method of manufacture is that the shrinkage and contraction of the ceramic material during firing causes the wires of the thermocouple to bend, causing bulges on the insulator surface. Bending of the insulator may also result because of the shrinkage of the ceramic material whilst it is bound to the wires.

Insulators which are distorted beyond the permissible quality standard may therefore result, causing a high reject rate.

This problem was not recognised in EP 0 225 713, because the thermocouple was formed in situ from inks painted onto the surface of the core. The resulting thermocouple foil was therefore relatively thin and of low mass. A problem with using thermocouples of such construction is that a further painting or printing step is required in the manufacturing process, and the process may not provide a sufficiently reproducible conductive track for the thermocouple to provide reproducible temperature measurement across a batch of spark plug insulators.

It is an object of the present invention to provide a method of making a spark plug insulator which has a thermocouple for measuring the insulator nose temperature, and which may be used with different types of thermocouple wire alloys.

According to the present invention there is provided a method of manufacturing a ceramic insulator for a spark plug for use in an internal combustion engine, the insulator having a shank portion adapted to project outwards of the engine with which the plug is to be used, a nose portion including a tip and adapted to project into the engine, and a thermocouple comprising two dissimilar wires joined at a junction, having the junction buried within the tip, the method comprising the steps of:: a) forming a green ceramic blank about which the remainder of the insulator is to be formed, the blank having a shank-forming portion and a nose forming portion which is diametrally smaller than the nose portion of the finished insulator; b) securing the thermocouple to the surface of the ceramic blank so that the junction of the thermocouple is adjacent the tip and the wires of the thermocouple are disposed along the shank-forming portion for connection to external circuitry; c) forming a green ceramic shell about the blank so as to submerge the thermocouple junction and wires in ceramic material; d) machining the shell to a pre-firing profile which, after shrinkage, will approximate that of the finished insulator, and such as to keep the thermocouple junction and wires submerged in the ceramic material;; e) heating the product from d) sufficiently to fire the blank and shell together into an integral mass and to shrink the mass to the finished size of the insulator; characterised in that each wire of the thermocouple is held under tension during at least part of stage e), and in that each wire of the thermocouple is provided with a sleeve of material which is thermally decomposed and/or volatilised during stage e), the sleeve being disposed along the wire in a region where the wire is submerged in the ceramic material.

By keeping the wires taught during firing, they are prevented from snagging and buckling within the insulator, thereby causing bulges on the insulator surface and/or bending of the insulator. Burning off or volatilising of the sleeve leaves space in the insulator and removes the clamping force of the insulator on the wires, allowing the wires to move out of the rear of the insulator as longitudinal shrinkage occurs during firing which would otherwise cause the insulator to bend and/or the wire to buckle.

Preferably the sleeve does not cover the junction of the thermocouple. Direct contact between the thermocouple junction and the ceramic material prior to firing helps ensure good thermal contact between the insulator and the nose in the finished article. The thermocouple junction may therefore be embedded in the ceramic, whilst the wires remote from the junction are free to move radially within bores in the insulator.

The absence of sleeving in the insulator nose area ensures positive location of the junction position, and also maximises the ceramic section thickness and increases mechanical/electrical robustness.

The sleeve may comprise any suitable material which is thermally decomposable and/or volatilisable. For example the sleeve may comprise a stress relieved polymer sleeving known as Kynar (TM) insulation sleeving. If there is a gap between the sleeve and the wire, a wax, for example molten paraffin was, is preferably introduced into the gap, to ensure that the sleeve is glued in position and to exclude air between the wire and the sleeve. Too much air would allow the sleeve to be compressed during stage c), and the resultant recovery during the firing stage could cause cracking of the insulator. The wax may be introduced by any suitable technique, for example by vacuum impregnation. To simplify manufacture, it would also be possible to form the sleeve around the wire without significant air gaps, for example by forming the sleeve of wax.

The sleeve length is preferably selected so that the sleeve will protrude beyond the rear of the green ceramic in step c). This will allow the insulator to be produced having thermocouple wires which are free to move within bores in the insulator.

It is preferred that the sleeve is positioned on each wire so as to end 3 to 7 mm from the junction, and preferably 4 to 6 mm from the junction so as to reduce the possibility of voids forming between the junction and the insulator during firing.

The blank may be formed by known techniques using standard materials well known to those in the art; for example an alumina blank, comprising alumina particles, an organic binder, and water, may be isostatically pressed around a needle template. The blank may then be machined to a preferred wall thickness such that the thermocouple wire when placed on the surface of the blank will be about mid way between the outside and inside diameters of the finished insulator, particularly in the nose region. A groove or other indicating mark is preferably ground in the blank to identify the position where the thermocouple junction will be located, typically around 1.0 mm from the tip of the fired insulator.

Any suitable means may be used to secure the thermocouple to the blank, for example twine or tape. Preferably the securing means are removable once the thermocouple has been cemented in place by further ceramic material. We have found that twine, particularly PTFE twine, is suitable for securing the thermocouple to the blank.

The thermocouple wires may be maintained under tension by twisting them together and hanging the assembly in a furnace so that the insulator is suspended by means of the wires. Alternatively a load may be hung on the thermocouple wires with the insulator held substantially vertically so as not too bend the wires unduly.

Suitable thermocouples are well known to those skilled in the art, and these may be used in the present invention.

However it is particularly preferred to use a type B thermocouple comprising platinum - 6% rhodium as negative, and platinum - 30% rhodium as positive. Type B thermocouple wire has higher mechanical strength than Type R, and hence reduced probability of fracture, and it has negligible calibration drift during firing of the insulator. The output of a Type R thermocouple is depressed by 2.5%, primarily because of degradation of the platinum limb.

In a preferred embodiment the thermocouple wires are 0.2 mm in diameter, and the positive and negative wires are cut to different lengths or otherwise distinguished for identification purposes.

After firing, the insulator may optionally be machined to a preferred final shape. The thermocouple may be calibrated, for example by comparison with a mineral insulated calibrated thermocouple in a bath of molten tin.

The use of molten tin ensures that the insulator nose is at the tin bath temperature with as little as 12.5 mm immersion depth.

The position of the thermocouple junction in the insulator may be verified by radiography or other suitable well known analytical methods.

The insulator may optionally be glazed, taking care not to glaze the thermocouple wires. This may be achieved by masking the wires with a suitable length of silicone rubber tubing, blocked at one end. The tubing is then removed and the glazed assembly is fired at an appropriate temperature.

The insulator may then be fitted with a centre electrode and incorporated in a spark plug using standard methods of manufacture.

Although it is preferred that the spark plug be manufactured with the thermocouple embedded in the ceramic before firing, as set forth above, it would also be possible to manufacture the spark plug using one or more wires located substantially where the thermocouple is to be finally located. The wires could be of metal coated with a thermally decomposable or volatilisable sleeve as set forth above, or the wires could be entirely composed of thermally decomposable or volatilisable material, for example a length of fishing line.

The wires would be held under tension during the firing stage so as to produce straight bores. Removal of the wires by heat during firing, or subsequently by mechanical means, would allow the thermocouple to be retro-fitted to the insulator. This may be achieved, for example, by excavating the nose tip, pulling out the wire or wires, and inserting the thermocouple so that the junction is correctly located in the nose tip. The nose tip could then be re-built and the assembly fired again.

The invention will now be further described by way of example, with reference to the following drawings in which: Figure 1 is a view of a known spark plug; Figure 2 shows a thermocouple for use in the present invention; Figures 3 and 4 are sectional views of a blank for use in the present invention; Figure 5 shows the blank of Figures 3 and 4, with the thermocouple of Figure 1 secured to it; Figure 6 shows an insulator prepared from the blank shown in Figure 5, prior to firing; Figure 7 shows the insulator of Figure 6 during firing; Figure 8 is a part sectional view of the insulator of Figure 7, to which a glaze has been applied; and Figure 9 is a sectional view of a spark plug fitted with a thermocouple.

The conventional spark plug shown in Figure 1 has an insulator 4, to which is secured a metal body 2 by means of a hexagon nut 50. The insulator 4 has a shank portion 48 which projects outside the engine cylinder, and a nose portion (not shown) which projects into the engine cylinder when the spark plug is in use.

The body 2 is provided with a shell 8 which has threads by which the plug can be screwed into a tapped bore in the cylinder head of an engine. A side electrode 10 is attached to the shell. A centre electrode 12 is disposed in a longitudinal bore in the insulator 4 and connected to a high tension source by means of a top contact or cap 6 on the insulator 4. A spark gap 14 is provided between the two electrodes 10, 12, for generating a spark when the spark plug is mounted in the cylinder.

The tip of the centre electrode 12 protrudes from a nose of the insulator 4. If it is desired to measure the temperature in the region of the tip of the nose, a thermocouple may be provided within the insulator, the junction of the thermocouple being close to the tip of the nose so as to reduce to a practical minimum the temperature difference between the junction and the nose tip. The wires of the thermocouple may conveniently be connected to appropriate circuitry via the cap 6. However the wires could also protrude from any convenient points on the shank portion 48 above the hexagon nut 50.

A thermocouple 16 for use in the present invention comprises a positive wire 18a and a negative wire 18b which are welded together at-a junction 24. Each wire is 0.2 mm in diameter and has a respective sleeve 20a, 20b which extends close to the junction 24 but which does not cover the junction. A gap 22 of about 5 mm is left on each side of the junction. The sleeve 20a, 20b, has a 0.5 mm outside diameter and a 0.25 mm inside diameter, obtained by stripping Kynar (TM) insulation from RS wire 357-261, annealed at 1100C for one hour. The 0.05 mm radial gap between the sleeve 20 and the wire 18 is filled with molten paraffin wax by vacuum impregnation. Any excess of wax is removed from the wires. Setting of the wax secures the sleeve 20 in the correct position on the wire 18.It would also be possible to provide the wires 18 with sleeves of a wax or other polymer by direct coating or moulding techniques well known to those skilled in the art.

A green ceramic blank 26a is prepared from a slurry of alumina particles which have been spray dried with an organic binder, and distilled water. The blank 26a is isostatically pressed around an appropriate needle 28, as shown in Figure 3.

The needle 28 with the blank 26a is transferred to a lathe, where the blank 26a is ground to provide a generally conical blank 26, as shown in Figure 4. The wall thickness of the blank 26 is such that the thermocouple wires 18 when placed on the surface will be mid way between the outside and inside diameters of the finished insulator 4, particularly in the nose region'30, A groove 32 is ground to identify the position where the junction 24 of the thermocouple 16 will be located. The groove is typically 1.0 mm from the tip of the fired insulator.

Referring now to Figure 5, the thermocouple 16 is placed on the blank 26 and secured in position by tying it with PTFE twine 34. The free ends of the wires 18a, 18b are wrapped around the rear of the blank 26 and rest against a collar portion of the needle 28. A ceramic slurry, made from the alumina used to form the blank 26a is then used to cement the thermocouple 16 to the blank 26, taking care not to cement the PTFE twine 34 to the blank.

The assembly is then left to dry, preferably in an air draught. The twine 34 is then removed and more slurry is applied to the thermocouple 16 so that the thermocouple 16 is completely covered with ceramic material. The assembly is allowed to dry.

The assembly is then placed in an isostatic press and more alumina is pressed around it. The green ceramic is then ground to a preferred pre-firing shape approximating that of the finished insulator, taking care to keep the thermocouple junction 24 and wires 18 submerged in the ceramic material.

The insulator is then carefully removed from the needle 28, and excess of ceramic material at the nose 30 is removed with a scalpel, to leave a longitudinal through bore 36 which will receive a centre electrode when the insulator is used to make a spark plug. The tail wires 18 are excavated as shown in Figure 6. An identification mark or number 38 may be scribed onto the insulator, if required, at this stage.

As shown in Figure 7, the ends of the thermocouple wires 18a, 18b are then twisted together and the insulator 4 is hung from a support 40 in a high temperature furnace (around 15000C). The polymer sleeve 20 and wax coating are burnt off during firing of the ceramic, leaving the wires 18 free to move within bores within the insulator 4 in those regions where the sleeve 20 was present.

The sintering and shrinkage of the insulator 4 during firing firmly grips and embeds the thermocouple 16 in the region of its junction 24, where there is no sleeve 20, so that the thermocouple 16 is fixed in the ceramic material of the insulator 4 at its junction 24, and free in regions remote from the junction 24, wherein the sleeve 20 had been disposed.

The tensioning of the wires 18 helps to ensure that they move out of the insulator without snagging during the firing process, as the insulator shrinks longitudinally and radially.

The external profile of the insulator 4 is therefore substantially undistorted by bending of the wires 18.

In a final stage, shown in Figure 8, the insulator 4 may be glazed by immersing in liquid glaze up to a pre-set level 46. The wires 18 are masked by means of a silicone rubber tube 42 blocked off at one end by a bung 44.

Excess of glaze is shaken off, and the insulator 4 is left to dry. The silicone tube 42 is then removed and the glaze is fired onto the insulator 4 by a suitable firing cycle for the glaze, for example 950"C for one hour.

Suitable glazes and ceramic mixtures for use in the invention are well known to those skilled in the art, and it is to be understood that the appropriate firing conditions will be used to match the firing requirements of the glaze or ceramic material which is selected.

Figure 9 shows the location of the thermocouple 16 in the insulator 26 in a finished spark plug. The gap 22 between the sleeve and the junction 24 is shown for illustration.

Claims (9)

1. A method of manufacturing a ceramic insulator for a spark plug for use in an internal combustion engine, the insulator having a shank portion adapted to project outwards of the engine with which the plug is to be used, a nose portion including a tip and adapted to project into the engine, and a thermocouple comprising two dissimilar wires joined at a junction, having the junction buried within the tip, the method comprising the steps of:: a) forming a green ceramic blank about which the remainder of the insulator is to be formed, the blank having a shank-forming portion and a nose forming portion which is diametrally smaller than the nose portion of the finished insulator; b) securing the thermocouple to the surface of the ceramic blank so that the junction of the thermocouple is adjacent the tip and the wires of the thermocouple are disposed along the shank-forming portion for connection to external circuitry; c) forming a green ceramic shell about the blank so as to submerge the thermocouple junction and wires in ceramic material; d) machining the shell to a pre-firing profile which, after shrinkage, will approximate that of the finished insulator, and such as to keep the thermocouple junction and wires submerged in the ceramic material;; e) heating the product from d) sufficiently to fire the blank and shell together into an integral mass and to shrink the mass to the finished size of the insulator; characterised in that each wire of the thermocouple is held under tension during at least part of stage e), and in that each wire of the thermocouple is provided with a sleeve of material which is thermally decomposed and/or volatilised during stage e), the sleeve being disposed along the wire in a region where the wire is submerged in the ceramic material.

2. A method as claimed in Claim 1, wherein the sleeve comprises a stress relieved polymer which is secured in position on the wire by a wax which bridges a gap between the sleeve and the wire.

3. A method as claimed in claim 2, wherein the wax is introduced between the sleeve and the wire when molten, by vacuum impregnation.

4. A method as claimed in any one of the preceding claims, wherein the thermocouple is a Type B thermocouple.

5. A method as claimed in any one of the preceding claims, wherein the wires are held under tension by suspending the insulator from a support by the wires.

6. A method substantially as herein described with reference to any one of Figures 2 to 8.

7. A method of manufacturing a ceramic insulator for a spark plug for use in an internal combustion engine, the insulator having a shank portion adapted to project outwards of the engine with which the plug is to be used, a nose portion including a tip and adapted to project into the engine, and the insulator having a bore for a thermocouple, the method comprising the steps of:: a) forming a green ceramic blank about which the remainder of the insulator is to be formed, the blank having a shank-forming portion and a nose forming portion which is diametrally smaller than the nose portion of the finished insulator; b) securing a wire to the surface of the ceramic blank so that the wire is disposed along the shank-forming portion and extends to a point adjacent the tip; c) forming a green ceramic shell about the blank so as to submerge the wire in ceramic material; d) machining the shell to a pre-firing profile which, after shrinkage, will approximate that of the finished insulator, and such as to keep the wire submerged in the ceramic material; e) heating the product from d) sufficiently to fire the blank and shell together into an integral mass and to shrink the mass to the finished size of the insulator; ; characterised in that the wire is held under tension during at least part of stage e), and in that at least part of the external surface of the wire is thermally decomposed and/or volatilised during stage e), in a region where the wire is submerged in the ceramic material.

8. An insulator for a spark plug, obtainable by the method of any one of the preceding claims.

9. A spark plug including an insulator as claimed in Claim 8.