From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
Article by A.E. Bennett discussing the development and performance of magneto ignition systems, with diagrams and test results.
| Identifier | ExFiles\Box 4\5\ 05-page046 | |
| Date | 21th February 1914 | |
| FEBRUARY 21, 1914 MAGNETO IGNITION.* By A.{Mr Adams} E.{Mr Elliott - Chief Engineer} BENNETT. AFTER the author had traced the development of the magneto from its earliest form, and had demonstrated the superiority of this form of ignition over the coil and accumulator system by the aid of photographs showing the sparks obtained with the two systems, he gave the results of several series of tests made on a motor cycle engine having a bore of 82 mm. and a stroke of 75 mm. The engine was coupled up to an electric generator, the plugs were arranged as shown in Fig. 1, and the magneto was specially designed for two-point ignition. Figs. 2 and 3 indicate the effect of varying the spark position upon the power output at speeds of 1,500 and 2,500 revolutions per minute. These tests, which were confirmed by subsequent experiments on 4 and 6-cylinder engines, show that over 6 per cent. increase in power may be obtained by the use of two-point ignition above that when plug B (Fig. 1) alone is used, and over 16 per cent. increase over that developed when plug A is used at the higher speed of revolution. The author emphasised the importance of placing the plugs in such a position that an equal volume of gas is dealt with by each plug, and that the depth to which they penetrate into the combustion chamber should be correct (see Fig. 4). He said that in one two-stroke motor cycle petrol engine, of which he was not at liberty to disclose the name, over 20 per cent. decrease in petrol consumption had been obtained by the use of two-point ignition. Attention was directed to the form of coupling used for driving the magneto, which should not only compensate for any slight variation in the position of the magneto, but also relieve the armature of shocks, and suggested the form shown in Fig. 5, where a laminated spring is employed. He mentioned, as an interesting feature, that experience had shown that by far the greatest stresses are put upon the armature spindle by the rapid deceleration of the engine, more particularly in the present-day high-speed engine. Troubles of the internal-combustion engine, he observed, were largely attributable to the use of improperly insulated H.T. cables. They should be covered by a material having a high insulating resistance, and the material should not become impoverished with frequent rise and fall of temperature. It was a great mistake to use cheap cables. Mr. Bennett condemned the practice of detaching the high-tension cable from the sparking-plug while the engine was running, and illustrated the simple and cheap form of terminal shown in Fig. 6. The lecturer concluded by referring to the difficulties associated with sparking-plugs, because of the small diameter of the hole in which it was screwed, as well as the low heat conductivity of substances used for the insulation of the central electrode. *Extract from a paper read before the Institution of Automobile Engineers on 11th February. Fig. 1. Fig. 2.—At 1,500 revs. per min. Fig. 3.—At 2,500 revs. per min. Fig. 4. Incorrect. Correct. Fig. 5. Fig. 6.—Left: Cable with copper strands bared. Centre: Brass tube passed over wire and into insulation. Right: Tube bent over to form terminal. Graph 2 Labels: Y-Axis: WATTS X-Axis: ADVANCE in DEGREES Curves: PLUGS A & B SWITCHED ON, PLUG B ONLY SWITCHED ON, PLUG A ONLY SWITCHED ON Graph 3 Labels: Y-Axis: WATTS X-Axis: ADVANCE in DEGREES Curves: PLUGS A & B SWITCHED ON, PLUG A ONLY SWITCHED ON, PLUG B ONLY SWITCHED ON Handwritten Note (left margin): X2394 | ||