The MOTOR THE SPARK JUMP

Evening Post, Volume CXVIII, Issue 36, 11 August 1934, Page 26

The MOTOR

THE SPARK JUMP

situation developing in the oversea markets.: "It was primarily to help our export trade that the Budget concession was made," states one writer, "the British manufacturer's supremacy in the Homo market being supposedly impregnable. In the past eighteen months or two years British cars sold extremely well abroad, and won a much larger share than at any time since the war of the temporarily limited demand. .Last year the Americans had almost dropped out of, Australia, by iar the most important of the Empire markets. British exports.are continuing to do well. Our. chief firms report that sales for tho first three months of this year are 50 , per cent, above those for last year. . But ,thereis no disguising'the fact. that they. aro < now faced with i a very hard fight to retain the ground recently, won. , , . ■ ■ ,

"The Americans. arc coming back. In New South. Wales, for instance, whei-e the demand for new-cars is. steadily reviving,. Americans, according to the 'Motor Trader,' -wore selling in May five cars, to-.every two British. To make -matters worse, there were ample of .British,cars, but hundreds of orders for. Americans that could not be filled. ..'./.;', -.....-•• . .".- '

. f'Two • thing's , are, helping them and hindering.the .British agent. Australia demands only 25 per cent 'Empire content' to qualify for "preferential rates of duty, and air the Canadian-exported Americans which' qualify under this generous definition of Empire-made are actually advertised in Australia as 'British' cars. \

. .."In New Zealand, where British imports far outnumber American and Canadia'n-Anieriean,' 75 per cerit.' Empire content is^demarided to qualify for Imperial -preference." ' : In Britain, American cars-which are 50 per cent. Canadian in labour 'and material,' rank: as Canadian, and pay 25 per cent, duty instead of 33 l-3p'er cent. '•■.■■;■. . ' ■■ ■ •"■ '- It is stated: that1- the appeals are to be made to the British Government to establish with Canada a basis of 75 per cent, if or imports'in Britain, and tci make representations to Australia: of the-desirability of her'following suit. ■ : . '

NEW THEORY ADVANCED

According to the author-of a paper read before the American Society of Automotive Engineers, it is easier for a. spark of given .voltage to jump tho gap of a sparking plug uiider normal working 'conditions than to: jump the same gap in. air at atmospheric conditions. If that be: correct, it is contrary to the idea current on tho subject since the early 'days of motoring.

It has always beou said, and till now has gone unquestionet that resistance, to spark jump was increased under engine compression, and that on this account it was'not .much use testing a plug in tho open air, since then it might spark regularly, but fail to fire in the engine. . ,

If the author of tho pape< is right (and ho ought to know, as.he is an official of a famous firm manufacturing ignition equipment) resistance- to the jumping of the spark in'a compressed mixture of petrol vapour and air is less than, in air alone at atmosphericpressure, so, long as tho mixture is not compressed much beyond sto 1. But on some of the new engines, with compression ratios running from 5.5 to C.5 to 1, it .has been found, lie said, that tho voltage required to jump the .spark gaps is ten to ■ fifteen times greater than that required with engines of 5 to 1 compression.

.' Why this great difference should o.ecur ba.s ,not . been determined definitely, apparently, but ho says,it is believed to be.due not so much to the increased . compression .as to cylinder head designs'by which the'higher compression is. £a|ncd» .