Patent GB-697258-A (GB)
Abstract: 697,258. Sparking plugs. BOEING AIRPLANE CO. May 16, 1951 [May 20. 1950], No. 11455/51. Class 7 (iv). In a sparking plug comprising a body 1, Fig. 1, having therein an antechamber including a part 15 of maximum diameter and a restricted opening 17 of minimum diameter through the inner end 16 of the body 1, the diameter of the antechamber is arranged to be at least three-fifths of the maximum diameter for a length of at least three-fifths of the distance between the inner body end 16 and the part 15 whilst the electrodes 3 and 21 are located remote from the restricted opening; the antechamber may also include a metal bushing 2 forming an inwardly facing shoulder, an insulator 30 extending through the bushing but spaced therefrom to form a narrow annular part of the antechamber and secured by means of threads to the body 1; the minimum area of opening 17 may also be between one-ninth and four-ninths of the area of part 15, the volume of the antechamber at the side of the inwardly facing shoulder away from the end 16. being a minor portion of the total volume of the antechamber, which latter, in cubic inches, is approximately double the area of opening 17 in square inches. The body 1 has an internal shoulder 12 against which bears a step 20 in the bushing 2 that fits loosely within the shell to impede the passage of heat therebetween; the shell also incorporates a further internal shoulder 14 against which the inner end of the bushing rests. The distance between the inner end of the bushing 2 and the inner end 16 of the plug body is not appreciably greater and is preferably less than the length of the threadedbody portion 10. The inner end of the bushing carries the earthed electrode 21 whilst the centre electrode 3 is mounted in the porcelain insulator 30 that has secured thereto a threaded metal band 31 which screws into the shell, a washer 4 being interposed between the outer end of the bushing 2 and the band 31 in order to establish the correct length of the spark gap. The outer end of the shell has a hollow cap 5 threaded therein which is lined with a porcelain or other insulator 51 and accommodates an electrical conductor, not shown, which connects to the plug terminal 33; the cap also has external threads 52'which receive either a protective cap or a high-frequency shielding device. The convergent antechamber walls are preferably concave and faired into the opening 17, the curvature of the walls being preferably of a circular-arcuate shape, the maximum diameter of the chamber coinciding with a radius of such circular are, which radius is approximately three times as great as the maximum diameter 15. The electrodes are located in the body of the bushing although they may be placed in the chamber at the part 15. The inner end of the insulator 30 is tapered and the bushing 2 has an enlarged portion 23 and a part 22 of reduced diameter these parts being so correlated that sufficient clearance is left between the bushing and insulator to accommodate some deposit of carbon and lead compounds from the fuel. The spacing between the point of minimum diameter and the shoulder 14 should be at least one and a half times the minimum diameter. In a modification, the bushing is dispensed with and the shoulder 14<SP>11</SP>, Fig. 4 and part 22' of reduced diameter are formed in the shell 1<SP>11</SP>; the insulator 30' is retained in the shell by a nut 36 and seats at the inner end on a gasket 35. In a further modification. the antechamber is formed as a separate member which is threaded onto a conventional sparking plug, the body of which replaces the bushing.