EP0171153A1

EP0171153A1 - Spark plug

Info

Publication number: EP0171153A1
Application number: EP85304243A
Authority: EP
Inventors: William Edward Voldrich
Original assignee: General Motors Corp
Current assignee: Motors Liquidation Co
Priority date: 1984-07-09
Filing date: 1985-06-14
Publication date: 1986-02-12
Also published as: JPS6134877A

Abstract

A cool operating spark plug (10) is disclosed comprising a fused seal (34) between a spark plug centre electrode (24) and a surrounding insulator body (16). The seal (34) comprises a glassy matrix containing metal particles and provides a heat transfer path from the electrode (24) to the insulator (16).

Description

This invention relates to a spark plug construction capable of relatively cool operation. More specifically, this invention relates to a spark plug design in which the centre electrode is bonded to the insulator.

Background

It has long been recognized that the design of the spark plug centre electrode and the surrounding insulator can have an effect on the temperature at which the plug operates most efficiently. When the plug operates at too high a temperature, it induces premature ignition of the combustible mixture in the cylinder of the engine (sometimes called engine knocking). Several different approaches have been employed to achieve cooler spark plug operation. Composite centre wire electrodes have been employed having a copper core portion for high thermal conductivity and a durable nickel alloy sheath. It is also possible to achieve cooler spark plug operation by having a thicker profile insulator tip and/or thicker centre wire, or by reducing the distance that the centre electrode extends beyond the insulator tip, or decreasing the length of the insulator tip relative to the spark plug shell.
It has also been a practice to cast a high thermal conductivity metal such as silver into the space between the centre electrode wire and the ceramic insulator. This may improve the thermal performance of the spark plug but adds an expensive ingredient to the plug. Further, the casting operation is not readily accomplished at high production rates. Another practice is to fill the space between the centre wire and the insulator with a cement that sets at ambient temperatures or with other unfused compositions such as talc and silicon. Such filler materials may serve as a seal and may produce moderate improvement in thermal conductivity, but they do not provide a significantly cooler running plug and the practice does not lend itself to high speed production.
Accordingly, it is an object of the present invention to provide a thermally-conductive cermet composition that is easily introduced between the centre electrode wire and the spark plug insulator and wherein the glass portion thereof is fused during the normal spark plug manufacturing operations.
It is another object of the present invention to provide a cool operating spark plug having a thermally-conductive, fused metal particle-filled glassy seal between the centre electrode and the insulator.
In accordance with a preferred embodiment of the present invention, these and other objects and advantages are accomplished as follows.

Brief Summary

Spark plugs typically include a fired ceramic insulator body with a centre bore. A terminal post is fitted in the upper part of the centre bore. Of course, the terminal post is adapted to be connected to a spark plug lead from an ignition distributor. A centre electrode is positioned in the lower part of the insulator bore. The centre electrode is substantially co-axial with the terminal post, but their facing ends do not touch. The terminal post is connected to the centre electrode through a suitable resistive or conductive material depending upon the environment in which the spark plug is to operate. For example, the terminal post may be electrically connected to the centre electrode through a fused electrically resistive glass seal material when the spark plug is intended to operate in an automobile where radio interference may be a concern.
Typically there is a small annular space, of the order of a few hundredths of a millimetre, between the centre electrode and the insulator body. The centre electrode extends slightly beyond the tip of the insulator. In accordance with the present invention, the space between the outside diameter of the centre electrode and the insulator bore wall is sealed with a fused glassy material filled with iron powder and sintered or fused particles of copper or aluminium. The seal is formed by introducing a slurry of iron powder, aluminium powder (or copper powder) and a mixture of oxides fusible to a glass onto the portion of the insulator bore intended to receive the centre electrode. The centre electrode is then dropped into the coated bore. A fusible resistive and/or conductive mixture is introduced on top of the centre electrode, a terminal post is inserted, and the assembly is fired at a suitable temperature, e.g., up to 927° - 954°C (1700°-1750°F). The resistive mixture fuses to form an impervious seal interconnecting the terminal post and the centre electrode. At the same time, the cermet composition is fused to form an effective thermally-conductive seal between the confined surface of the centre electrode and the surrounding insulator bore. It has been found that by employing such a seal composition in a spark plug, the plug operates as a much more efficient heat sink, resulting in a cooler spark plug.
The invention and how it may be performed are hereinafter particularly described with reference to the accompanying drawings, in which:

Figure 1 is an elevational view partly in section of a spark plug embodying the present invention;
Figure 2 is an enlarged illustration of the centre electrode and insulator tip portion of the plug shown in Figure 1, illustrating the fused cermet seal according to the invention; and
Figure 3 is a graph illustrating the indicated mean effective pressure (I.M.E.P.) (in psi) rating of several different types of plugs, some of which employ the present invention.

Figure 1 illustrates a representative spark plug 10 employing the present invention. Spark plug 10 comprises an outer metal shell 12 having a ground electrode 14 welded to the lower end, the spark discharge end. Positioned and secured within the metal shell 12 is a fired ceramic insulator 16. As seen, the upper lip 13 of shell 12 is rolled against the insulator body 16, and an outer shoulder 15 on the body bears against gasket 17 which in turn bears on an internal shoulder 19 of the shell 12. Ceramic insulator 16 is preferably of a high alumina base material containing 85 percent or more aluminium oxide. Such an insulator has good mechanical strength and heat-shock resistance as well as the ability to form a good bond with glass, which is useful in forming the thermally conductive seal of this invention.
Insulator 16 is formed with a centre bore having a lower portion 18 of relatively small diameter and an upper portion 20 of larger diameter. The upper and lower portions merge at a shoulder 22. Positioned in the lower bore portion 18 of the insulator is a centre wire electrode 24. The centre electrode 24 has an enlarged head -26 which rests on shoulder 22 and a lower end 28 which projects beyond the lower tip of insulator 16.
Positioned in the upper centre bore portion 20 of the insulator is a terminal screw 30. Terminal screw 30 and centre electrode 24 are connected by an electrically resistive and/or conductive metal-glass (cennet) seal 32 which is bonded to both members and to the wall of the upper centre bore portion 20.
As best seen in Figure 2, the space between the centre electrode 24 and the surrounding insulator 16 is sealed with a seal 34 comprising a dense fused glass matrix filled with iron particles and sintered or melted aluminium particles (or copper particles). The seal 34 suitably contains, by weight, 29 to 50 percent iron, 49 to 70 percent glass and 1 to 6 percent aluminium. An important characteristic of this fused seal composition is that it is thermally-conductive. It provides a larger heat flow path between the centre electrode 24 and the insulator 16 than is found in known spark plugs, causing the spark plug to operate at lower temperatures than it would operate without such seal material. Of course, heat can flow from the insulator 16 through several paths including through gasket 17 to metal shell 12.
The practice of the present invention is not dependent upon the specific composition of the centre electrode 24. Frequently centre electrodes are fabricated wholly of a nickel-base alloy such as Inconel®. Nickel-base centre electrodes are employed because they are durable and resistant to deterioration from spark discharges. Composite centre electrodes are also employed in spark plugs and may be used in accordance with the present invention. An example of such a composite electrode is one having a copper core and a nickel alloy sheath. The copper core is employed for improved thermal conductivity, to convey heat away from the tip of the electrode.
The seal of the present invention employs a high proportion of iron powder because it is relatively inexpensive and has suitable thermal conductivity for the purpose of the seal. Preferably the iron powder is not fused in the formation of the seal. A relatively small amount of lower melting metal such as aluminium or copper is employed in combination with the iron. This metal is intended to sinter or melt during the fusion of the glass precursor materials to provide heat transfer paths between iron particles throughout the cermet seal. The seal composition also comprises a combination of oxides that can be softened and fused to form glass at temperatures between, for example, 843°C and 954°C (1550°F and 1750°F). The specific composition of such a glass precursor mixture is not critical to the practice of this invention provided it can be fused at such a temperature range. An example of a suitable glass is a borosilicate type glass having a composition by weight of 65 percent silica, Si0₂, ₂₃ percenc boron oxide, B₂O₃, 5 percent alumina, Al₂O₃ and 7 percent sodium oxide, Na₂0. Borosilicate glasses are preferred for use in the invention because they have relatively low coefficients of thermal expansion -- typically lower than the insulator body 16 material. The cermet precursor mixture may also contain about 1 percent lithium carbonate. A preferred precursor seal composition (that is, prior to firing) comprises by weight 50 percent iron powder (-325 mesh), 3.3 percent aluminium powder (-325 mesh), 45 percent glass frit of the above composition (-325 mesh) and 1.1 percent lithium carbonate.
A spark plug incorporating the seal of the present invention is assembled and prepared as follows. A slurry of the above seal precursor composition in water, alcohol or other relatively volatile medium is prepared. It is flowed onto or painted onto the centre bore portion 18 of the insulator intended to receive the centre electrode 24. The centre electrode 24 is then dropped into the insulator through the wide bore portion 20 until it is received within lower bore portion 18. Any excess seal precursor material carried out by the portion of the electrode extending beyond the insulator tip is wiped off or washed off and the medium is evaporated. A suitable glass/metal composition is then introduced into centre bore portion 20 on top of and around the head 26 of the centre electrode 24. This composition may be relatively electrically conductive or resistive depending on the intended purpose of the spark plug. The specific composition of this seal composition constitutes no part of the present invention.
Typically the glass seal resistor/conductor composition is added on top of the centre electrode and tamped. The terminal screw 30 is then partially inserted into the bore so that its lower end rests on top of the resistive seal powder. The loosely assembled spark plug is then fired in a kiln maintained at about 927⁰C (1700°F). The total time of passage of assembly through the kiln may be about 30 minutes. During this time both the resistive seal material 32 and the thermally-conductive seal material 34 are fused. When the assembly is transported out of the kiln, the terminal screw 30 is pushed down into the fused resistive and/or conductive mass 32 to finally position the terminal 30 therein. The assembly is then very rapidly cooled to harden both the resistive/conductive seal 32 and the thermally-conductive seal 34. Subsequently, the metal shell 12 is formed on the insulator 16 to complete the manufacture of the spark plug 10.
The operating characteristics of spark plugs can be evaluated by running them in a single cylinder engine under controlled operating conditions. The load on the engine is increased until pre-ignition or knocking caused by an overly hot spark plug tip occurs. This load condition expressed in terms of I.M.E.P. (indicated mean effective pressure) in psi is a measure of whether the plug runs hot or cool. A cooler plug can be operated to a higher I.M.E.P.
For purposes of comparison, several spark plugs embodying the thermally-conductive seal of the present invention were thus evaluated and compared with some commercially available spark plugs. The results of this evaluation are summarized in Figure 3. Figure 3 is a graph where the spark plug sample (4 or 5 similar plugs) is identified by letter on the X axis and the corresponding average I.M.E.P. rating (in psi) of the sample is stated on the Y axis. The Sample A spark plugs were standard nickel-base alloy centre wire spark plugs employing no seal between the centre wire and the insulator. As seen in Figure 3, the average I.M.E.P. of these plugs was about 229 psi. Sample B was a group of the same basic spark plugs (that is, a standard nickel-base alloy centre wire) made containing the seal composition of the invention specifically described above. As indicated in Figure 3, the average rating of these plugs was considerably higher -- about 244 psi. Sample C was a group of commercial spark plugs incorporating a copper core, nickel sheath centre electrode and a cement sealing material between the electrode and the insulator body. This cement seal did not contain metal particles and was not fused. As illustrated in Figure 3, the rating of this sample is also only slightly higher than Sample B, a nickel centre wire electrode spark plug incorporating a seal in accordance with the present invention.
Sample D is a commercially available copper core nickel sheath centre wire electrode spark plug designed specifically for cool operation but not containing the thermally-conductive seal according to the invention between the electrode and the insulator core. Sample E is the spark plug of sample D except manufactured to incorporate the seal composition of the invention specifically outlined above. It is seen that the I.M.E.P. rating of Sample E is significantly higher than that of Sample D.
Several spark plugs of each of sample types A, B, D and E were respectively subjected to a 300 hour durability test. The spark plugs were installed in a 3.8 litre displacement V-6 gasoline engine. The fuel contained 0.4 gm/L lead. The plugs were rotated among cylinders every 25 hours of engine operation. They were inspected and gap-checked each 100 hours of operation. At the end of 300 hours, the plugs were rated by I.M.E.P. as described above.
The average I.M.E.P. of each of the sample types (A, B, D and E) decreased compared to new plugs of the same type. However, the ratings of the plugs containing the cermet seal compositions of the invention still markedly exceeded the rating of similar plugs that did not incorporate the cermet seal.
Thus, the cermet seal composition of the invention, when employed to seal the space between a centre wire electrode and the surrounding insulator body of a spark plug, provides an excellent heat path from the centre electrode to the insulator body, bringing about cooler operation of the spark plug. Furthermore, the composition is readily applied during normal spark plug assembly and can be fused in place without any additional firing operation. The seal composition of the invention is fused in place when the conventional conductive or resistive seal is formed and fired between the terminal post and the centre wire. Finally, the seal composition of the invention does not require a separate metal casting operation or the employment of an expensive ingredient such as silver.

Claims

1. A spark plug (10) for an internal combustion engine comprising an insulator body (16) having a longitudinal central bore (18,20) adapted to receive a terminal electrode (30) at one end and a centre wire electrode (24) at a spark-discharge end of the insulator body (16), and a centre wire electrode (24) generally centred in and sealed within said insulator bore (18,20) with a portion of the electrode (28) extending beyond the spark discharge end of the insulator body (16), characterised in that the spark plug (10) includes a fused glass-metal particle seal (34) filling the space between the insulator bore (18) and the outer surface of the electrode (24) along substantially the length of the portion of the electrode (24) within the bore (18), the seal material prior to fusion comprising iron particles, particles of a metal taken from the group consisting of aluminium and copper, and a mixture of oxides fusible to a glass at 954⁰C (1750°F) or lower, said seal (34) providing a thermally-conductive path between the centre electrode (24) and the insulator body (16).

2. A spark plug according to Claim 1, characterised in that the glass formed by fusing the mixture of oxides is a borosilicate glass.

3. A spark plug according to Claim 1 characterised in that the seal (34) comprises, by weight, 29 to 50 percent iron, 1 to 6 percent aluminium and copper, and the remaining percent glass.