Patent GB-776332-A (GB)
Title: Improvements in igniters of the surface discharge type
IPC: H01T13/52
Filed: 1955-09-14
Abstract: 776,332. Sparking plugs. BENDIX AVIATION CORPORATION. Sept. 14, 1955 [Sept. 30, 1954], No. 26284/55. Class 7 (4). In a surface-discharge igniter having a central electrode 41, Fig. 1, and an annular outer-electrode 32, separated by a tubular insulator 35, the cross-section of which latter decreases linearly towards its extremity, the end surface of this extremity being semiconductive and extending across the spark-gap between the electrodes, wherein the centre electrode 41 in the form of a threaded ring having a conically - diverging outer - surface screwed on to a stud 43 supported within the insulator, the conical outer-surface of the central electrode is seated against a complementarily conical surface in the insulator. The plug is cooled by air from the engine air-inlet passage, not shown, passing into tangential ports 22 and flowing along a groove 27 in a middle shell-section 21, a radial slot in a ring 25, the bore of the latter, and slots 40 into the space around the insulator 35 and thence through the passage formed by a split in the electrode 32. (a) Terminal end An upper shell-section 10 houses the insulator 11 which seats on a gasket 12 and is retained in position by a nut 14; the head of a central conductor 15 which rests on an insulator shoulder, is connected to the terminal 16 by a wire 17 in a lead oxide and glass insulating- mass 18. The terminal 16 may be connected to the power supply by a plug 19 and pin 20 located in an elbow. The middle shell-section 21, which is welded to the section 10, may pass through the engine air-inlet passage and is provided with one or more tangential ports 22; the section 21 accommodates a tubular insulator 23 seated against a flange 24 and secured in place by the ring 25 having a press fit in the section whilst the conductor 15 is supported against vibration by a metal guide 26 threaded in an insulator flange. (b) Sparking end A spacer shell section 29 upon the upper end of which sits the ring 25 is welded to the section 21 and to the lower shell section 30 having the outer end portion 31 of its bore tapering inwardly to form a seat for the earth electrode 32 which is in the form of a split ring incorporating a number of axial slots whilst the insulator 35 rests on the internal tapered wall of the electrode 32; the lower ends of the insulator 35 and electrode 32 are flush with or extend beyond the adjacent end face of the section 30. The electrode 32 may be secured by pins 36 that pass through holes in the shell and engage notches in the electrode ; the pins may be secured by staking. The insulator 35 and electrode 32 are also secured in the section 30 by a ring 39 press fitted or threaded into the latter which ring has one or more radial slots 40 in its lower edge. The ring 41 engages a tapered part of the insulator 35 and is threaded and welded or soldered to a tubular member 43 having a part spherical head which is supported by a tapered insulator shoulder 44, the conductor 15 extending through and being welded or soldered to the member 43. The active end of electrode 41 is flush with the end faces of the electrode 32 and insulator 35 which latter is coated with carbon to render it semi-conductive by placing it in an airtight furnace at a temperature of 1375‹ F. and flowing therethrough a liquefied petroleum gas containing the propane hydrocarbon which is then cleaned from the surfaces on which it is not required by grinding or vapour blasting; the carbon may be left on surfaces engaged by the electrodes 32 and 41 and the member 43. If the active end of the insulator is burnt away the plug may be reconditioned by grinding the ends of the electrodes 32 and 41. In a modification, the shell 51, Fig. 6, which comprises the earth electrode, surrounds the insulator 50 and the centre electrode consists of a nut 53 which engages a stud 54 through which the conductor 15 extends ; the operative ends of members 51, 53 and 54 are ground flush and the parts 15, 53 and 54 are secured at their ends by welding. In other modifications the air cooling may be dispensed with and the electrode 32, Fig. 1, need not therefore be split, the use of a middle shell section 21 may be discontinued, and either or both electrodes 32 and 41 may embody internal tapers which differ from their external tapers.