Patent GB-711778-A (GB)
Title: Low-tension sparking plug for internal combustion engines
IPC: H01T13/52
Filed: 1950-06-08
Abstract: 711,778. Sparking plugs. BENDIX AVIATION CORPORATION. June 8, 1950 [May 8, 1950], No. 14381/50. Class 7(4) [Also in Group V] In a low-tension sparking plug comprising a metallic shell 10, Fig. 1, with an annular electrode 12 and an assembly including a metallic rod 14 having an enlarged head 17, the latter co-operating with electrode 11 to form an annular spark gap 18 which is bridged by one surface of a semi-conductor, such as a ring 19, that is spaced or insulated from the shell 10 and rod 14, the opposite surface of the ring is disposed in contact with an insulator sleeve 21, the peripheral surfaces of which engage the rod and shell; the centre electrode is also of annular form, the sleeve 21, ring 19 and rod 14 being secured together, with the ring pressed into firm engagement with the electrodes by a tubular nut 26 threaded on to the shell. Tungsten or nickel-alloy ring electrodes 12 and 17 are located on flanges 11 and 16 respectively, the earth electrode 12 being silver soldered to the shell and the centre electrode 17 similarly secured to the rod 14; the rod is recessed and filled with copper 15 or other similar metal having a high thermal conductivity. Beyond the sleeve 21 is a member 23 of copper or similar metal that has a counterbore to receive the head of the rod 14. The electrode 17, ring 19, insulator 21, member 23 and rod 14 comprise a central-electrode assembly. Between member 23 and shell 10 is an insulator 29 formed of such material as powdered aluminium oxide, clay or magnesium oxide, whilst in contact with the top of both the member 23-and insulator 29 is a further insulator 27, the whole being held in position by the nut 26. .The latter, which is externally threaded to receive the female part of a plug and socket connector 1, has a contact terminal 31 mounted therein by a glass seal 32, the inner end of the terminal making electrical contact with the prongs of a socket 33 that is seated in a metal sleeve 34. The ring 19 comprises fused or compressed mixtures of such insulating materials as sodium silicate, clay, titanium dioxide, zirconium oxide, urea formaldehyde resins and/or aluminium oxide with finely divided electrical conductors such as silicon carbide, titanium carbide, nickel, boron carbide, graphite or iron; fused mixtures of silicon carbide, alumina and sodium silicate and of iron powder and alumina may also be used. The conductive material may be concentrated near the surface of the ring 19 and an insulating compound utilized to fill the void spaces within the plug. In another embodiment, the electrode rings are dispensed with and the annular spark gap is formed between the flanges on the main end of the body and the centre electrode rod; the spark gap is bridged by a ring of ceramic or other insulating material, which ring has a coating of conductive material fused thereon or applied thereto by electrolysis. The coating may comprise a mixture of iron powder and alumina made into a paste with water, oil or alcohol, which paste is fired on the ring at temperatures between 2,000‹-3,000‹F. A colcothar clay-mixture may also be used to coat the ring. In a further embodiment, the ring has one or more recesses in its active face filled with semi-conductive material which is either pre-formed or compressed in the recess in powder form. In both the foregoing embodiments the insulator in contact with the semi-conductive ring may be combined with the latter. In a further embodiment, the centre electrode assembly comprises a tubular ceramic or aluminium oxide insulator 119, Fig. 6, within the shell 110, the centre electrode rod 121 with an enlarged outer end 122 passing through the insulator and having a partly tapered semi-conductive member 120 bridgting the spark gap around its lower part although insulated therefrom by an insulating glaze, which member 120 is of smaller outer diameter than the insulator 119 to ensure concentricity of the assembled parts; the outer diameter of member 120 may also be coated with insulating glaze. The rod 121 is connected to a connection terminal 124 by a wire conductor 125 brazed or soldered to transverse holes in the terminal and electrode rod; the wire, which may be of molybdenum, is encased in a mixture of material known under the Registered Trade Mark of "Pyrex" and lead oxide. After insertion of the centre electrode assembly a soft metallic bushing 127, such as copper. is pressed between the insulator 119 and the copper tapered part 115 of the shell 110, the bushing being first coated with soldering or brazing flux, for example borax, the whole is then heated to a temperature of 1,100‹-1,300‹F. whilst applying an axial pressure; grooves may be provided in the insulator and tapered body part for locking the parts in position. The member 120 has a tapered lower portion which seats on a corresponding part 117 of the inner end of the shell, and the inner surface of the enlarged end 122 of the rod 121 may be parallel to the outer surface thereof, but either or both of these surfaces may be conical and converge towards the member 120. The tapered portion of member 120 actually bridges the spark gap 123 and the enlarged end part of rod 121 serves to deflect combustible gases into the gap 123. The earthed electrode is made of stainless steel and the rod 121 either of stainless steel or nickel-chrome alloy, whilst member 120 may comprise a composition of silicon carbide, cobalt oxide, aluminium oxide and urea formaldehyde resin and the glaze coating a composition of frit, ball clay and zircon. The spark gap width along the tapered surface of member 120 is about 0.02 inches. The plug may be screwed into the opening in an engine cylinder or secured to a jet type engine by a mounting flange.