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).
Magazine article explaining the function and adjustment of a magneto's contact breaker.
| Identifier | ExFiles\Box 4\5\ 05-page038 | |
| Date | 2nd November 1912 | |
| THE AUTOCAR, November 2nd, 1912. 793 How the Magneto Works. rule it is fitted to the special knives which the magneto makers supply. These knives are very hard. They are fitted with a good blade, a file suitable for trimming up the contact surfaces, and a gauge. Here is a gauge. You will see that it is merely a blade, like a knife blade, and you should be able to pass this between the contact blocks when they are separated by the cam without having to move them further apart (fig. 11). To adjust the contacts, the lock nut is unscrewed and the fixed platinum block moved further up or down by unscrewing the hexagon head in one direction or the other by means of the small spanner supplied with the magneto. 'I found in my tool kit some very small spanners, but did not know what they were for.' 'These were for the platinum contacts. Some of these small spanners are fitted with blades which serve as gauges for adjustment of the contact points (see fig. 12).' 'Do the contact points require frequent adjustment?' 'No, not at all. The adjustment is only necessary after the contact surfaces have been cleaned, or if the fibre block, which strikes the steel blocks, gets hammered down. You will gather that the fibre block moves at very high speed with the engine, so that it gets rather knocked about, with the result that it is hammered down slightly and the platinum points do not separate quite so far as they should.' 'Well, from what I suppose I need not examine the contact breaker until I have travelled some thousands of miles?' 'No, as long as your engine runs all right you may know that it is correct. Once every three or four thousand miles it is worth while examining the magneto to see if the contact surfaces are clean, and if they are properly adjusted. With this periodic examination your magneto will run perfectly and give you no trouble. At the back of the contact breaker base plate (see fig. 9) there is what is termed a carbon brush. It is merely a piece of carbon, somewhat resembling pencil lead, and sometimes coated with copper to improve it. At the back of this is a spring which presses the carbon brush against the cover plate on the magnets. When the contact breaker is removed at any time just see that the end of this carbon brush is clean.' 'When I was being taught to drive, the agent talked about retarding the ignition as the engine runs slowly and told me to move this lever.' 'That lever is connected to the adjustable ring (fig. 1) which carries the steel blocks or cams. You will see that this ring can be rocked backwards and forwards, with the result that the fibre block strikes the sloping steel blocks earlier or later in relation to the rotation of the contact breaker base plate.' 'Why is that?' 'You can easily imagine that when the explosion in the cylinders takes place it is not instantaneous. The measure of time is extremely small, but there is certainly an interval between the ignition and the explosion. The explosion should, of course, take place when the piston is just about to descend on the firing stroke. On account of the time it takes actually to ignite the gas the spark should occur earlier than this.' 'Then the spark occurs on the compression stroke?' 'That is so. Now when the engine is running slowly the piston does not move so far during the interval between the ignition and explosion as it does at high speeds, so that unless steps were taken to prevent it, the explosion at low speeds might take place before the piston has reached the top of the compression stroke. Consequently, when one can slightly alter the working of the contact breaker the time of actual explosion can be delayed when the engine is running slowly, with the consequence that it occurs after passing the top dead centre.' 'The lever on the adjustable ring is connected up to the advance spark lever on the steering column. By moving the advance spark lever you will understand that the adjustable ring on the contact breaker can be rocked backwards and forwards. When rocked to one extreme in the direction in which the armature revolves the ignition is retarded. If now it is moved to the other extreme it is obvious that the fibre block will strike the steel blocks on the adjustable ring earlier than when the ignition was fully retarded. Thus the spark takes place early on the compression stroke when the ignition is fully advanced and slightly down the firing stroke when it is retarded.' 'I understand that now, but when I start the engine I have to have the ignition advanced. Why is that?' 'The strength of the current which the magneto develops depends on its speed. When the magneto is stationary it obviously develops no current, and the faster it runs the more current and the stronger the spark. When the contact breaker is retarded, that is to say, turned round so that the steel blocks are struck by the fibre block comparatively late, it happens that the current is not as strong as when the contact breaker is advanced.' SPANNER FOR CONTACT BREAKER SCREWS FEELER GAUGE Fig. 12.—Compact tool for magneto adjustment. 'Why is that?' 'Well, if you really want to know why that is, I must explain to you about the 'magnetic field' and 'maximum points,' etc. As I told you, it is really not essential for you to go deeply into the theory of the magneto so long as you are willing to take it for granted that the spark is weakened when you retard the ignition.' 'I am willing to take it for granted, but I do not see what that has to do with the starting.' 'Well, I have told you that when the engine is running slowly the current is usually weak, and that when the contact breaker is retarded it is still further weakened. Now for starting you want a pretty good spark, so that the combination of retarded contact breaker and the slow rotation of the armature due to turning the engine by hand, will not produce a sufficiently powerful spark to ignite the gas. To start with the contact breaker retarded you must revolve the engine extremely fast, faster than can be done by hand. The alternative is to advance the ignition somewhat, and the spark will therefore be strong enough.' 'But won't that cause the explosion to take place before the end of the compression stroke?' 'No, because only a weak spark is generated on account of the slow rotation of the armature. The stronger the spark the quicker the actual explosion occurs. Owing to the slow speed the spark is weakened and the interval between the ignition and explosion is comparatively long, so that the dead centre is reached and passed before the actual explosion occurs.' | ||