US 3890519 A
A spark discharge plug comprises an outer annular electrode and an inner electrode, the two electrodes in use being connected to a source of electric supply, an annular air gap intermediate the electrodes and a dielectric material having a high permittivity interposed between one of the electrodes and the air gap.
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Description (OCR text may contain errors)
United States Patent Chafer et al.
[ June 17, 1975 SPARK DISCHARGE PLUGS Inventors: Henry James Chafer, Hemel Hempstead; Stuart Graham, Coventry, both of England Assignee: Joseph Lucas (Industries) Limited,
Birmingham, England Filed: May 30, 1973 Appl. No.: 365,154
Foreign Application Priority Data June 8, 1972 United Kingdom 26692/72 U.S.Cl ..313/l30; 313/131 R;3l3/l37; 313/138; 313/143 Int. Cl. HOlt 13/22 Field of Search 313/130, 131 R, 137, 138, 313/131 A  References Cited UNITED STATES PATENTS 2,025,914 12/1935 Tetlow 313/137 2,833,950 5/1958 Hastings 313/137 2,926,275 2/1960 Peras 313/131 R 3,488,556 1/1970 Burley .4 313/131 R Primary Examiner-R. V. Rolinec Assistant Examiner-Darwin R. Hostetter Attorney, Agent, or Firm-Holman & Stern 5 7 ABSTRACT A spark discharge plug comprises an outer annular electrode and an inner electrode, the two electrodes in use being connected to a source of electric supply, an annular air gap intermediate the electrodes and a dielectric material having a high permittivity interposed between one of the electrodes and the air gap.
8 Claims, 1 Drawing Figure SPARK DISCHARGE PLUGS This invention relates to spark discharge plugs of the kind used in internal combustion engines for igniting an air/fuel mixture.
A known form of spark discharge plug comprises an outer annular electrode and an inner electrode, the two electrodes in use being connected to a source of electric supply, and air gap surrounding the inner electrode and an annular body of semi-conductive material interposed between the air gap and the outer electrode. With such plugs when the eletrodes are connected to a source of supply such that the air gap breaks down, a spark occurs which transfers to the outer electrode so that the main discharge takes place between the electrodes. This form of plug has been known for many years but it has the disadvantage that the semiconductive material is eroded by the high temperature to which it is subjected with the result that the plug has to be replaced at fairly frequent intervals.
The object of the invention is to provide a spark discharge plug in a simple and convenient form.
According to the invention a spark discharge plug of the kind specified comprises an annular outer electrode, an inner electrode, an annular air gap intermediate the electrodes and a dielectric material having a permittivity interposed between one of the electrodes and the air gap, said dielectric material having a high resistance to corrosion.
One example of a spark plug in accordance with the invention will now be described with reference to the accompanying drawing which shows a sectional side elevation of the plug for use in the combustion chamber of a gas turbine engine.
With reference to the drawing, there is provided a hollow body formed from metal and having a hollow cylindrical extension 11 at the end of which is mounted an annular electrode 12, the electrode having a tapered internal surface. The body 10 defines an inclined step 13 against which is located in sealing relationship, a complementary surface formed on a flange 14 on a hollow cylindrical insulator 15 having a skirt 16 extending within the body 10. The insulator is retained in position by a sleeve member 17 and this in turn is retained by means ofa rolled over portion of the body 10. The insulator is formed for example, from alumina and a further skirt is provided which extends within the sleeve member. This skirt accommodates, in use, a high voltage lead the core of which is connected to a central electrode 18 of rod like form and having a head 19 which is brazed to the insulator. Moreover, interposed between the sleeve member 17 and the flange is a compression seal which can accommodate differences in the thermal expansions of the individual materials.
The central electrode 18 has a tip 20 which extends flush with the electrode 12 and is formed from silicided tungsten. Surrounding the central electrode is a further insulator 21 this is formed from material having a high permittivity, at least 50, and which is also resistant to corrosion by the hot gases resulting from the combustion of the air/fuel mixture within the combustion chamber. Thus insulator 21 has an end portion having tapered surface which co-operates with the tapered surface of the electrode 12, the insulator being actuated to maintain the tapered surfaces in contact with each other. Furthermore, the insulator 21 has an enlargement which cooperates as a sliding fit, with the bore in the body 10. The electrode 18 extends with a small annular clearance within a bore formed in the insulator 21. This is particularly the case at the end of the plug. Moreover, by virtue of the tapered surfaces the thickness of the insulator 21 at the end of the plug is less than at any other position.
In operation, the fact that the insulator is formed from material having a high permittivity means that the potential across the air gap between the electrode tip 20 and the insulator 21 is greater than the potential across the insulator. As a result when the electrodes are connected to a source of supply a substantial proportion of the voltage of the supply appears across the air gap and when the supply voltage is sufficient, ionisation of the air gap takes place and a spark occurs. The voltage across the air gap in this form plug is therefore determined not in a resistive manner as is the case with the known form of plug but in a capacitive manner. The voltage required to cause breakdown is higher than in the known form of plug but the insulator is better able to withstand the effect of the hot gases in the combustion chamber of the engine. One material which is particularly suited for the insulator 21 is calcium titanate this material having a value of permittivity of the order of 100. One such material is that supplied by Stealite and Porcelain Products Limited as TEMPRADES NISOO. This permittivity of at l Mc/s and 20c.
1. A spark discharge plug for igniting and air/fuel mixture in an internal combustion engine and comprising an annular outer electrode, and inner electrode, an annular air gap intermediate the electrodes and an a nular body of dielectric material having a permittvity of at least 50 interposed between one of the electrodes and the air gap, said body acting to ensure that when the electrodes are connected to a source of electric supply a substantial proportion of the voltage of the electric supply appears across the air gap.
2. A spark discharge plug as claimed in claim 1 in which the air gap is positioned adjacent the inner electrode.
3. A spark discharge plug as claimed in claim 2 in which said dielectric material is calcium titanate.
4. A spark discharge plug as claimed in claim 1 comprising a hollow metallic body having at one end said outer electrode, the internal surface of said outer electrode tapering inwardly towards said end of the body, a hollow insulator within the body and having an outer end surface complementary to said tapered portion of the body, means urging said surface of said insulator into engagement with the tapered portion of the body and an electrode extending with clearance through said insulator, said insulator being formed from said dielectric material and said electrode defining said inner electrode.
5. A spark discharge plug as claimed in claim 4 in which said inner electrode is mounted on a further insulator carried within the body, said further insulator having a skirt portion surrounding said first mentioned insulator and a flange having a surface in sealing engagement with the body.
6. A spark discharge plug as claimed in claim 5 including a sleeve member for retaining said further insulator in position within the body said sleeve member being retained in the body by a rolled over portion of the body.
7. A spark discharge plug as claimed in claim 6 in which said means comprises a coiled compression spring.
8. A spark discharge device as claimed in claim 7 in which the material is calcium titanate.