MAGNETO BASICS

With the principals as described in the chapters "ignition basics" and "electromagnets", the magneto is a lot easier to understand. It is in fact the same as battery ignition, but the low tension circuit gets its energy from a simple dynamo, not the battery. A big magnet turns inside the iron core of a coil. Again a changing magnetic field gives a changing (alternating) voltage on the ends of the coil. For simplicities sake, Bosch has used the low tension coil for this purpose. If the contact points would always be closed, then an AC-current would flow through the low tension coil and the points, swapping direction twice per rotation of the magnet. The strength of this current depends on the strength of the magnet and how fast the magnet turns around. The AC-current will be transformed to the high tension circuit, but that is not enough to get a spark at the sparkplug. So the magneto is designed so we can open the points when the current is maximal. The resultant sudden collapse of the current causes again a selfinduction spike. This spike transformed to the high tension coil gives the spark. Because the strenght of the current depends on how quick the magnet turns around, the resultant spark will also be at its weakest when turning the engine round on the kickstarter, and the strongest at maximum RPM's. That is opposed to the battery ignition. There you always have a supply voltage of 6 Volt, but as rpm's rise, the voltage across the coil doesn't get the chance to rise fully in the short time that the points are closed. That was the main reason why magneto's were used on racing motorcycles. Magneto's were also used on military machines, tractors, stationary engines, etc, because the engine could run without a battery. Similar systems are still used in the two stroke engines of mopeds, chainsaws, lawn movers etc. Here you will find several small magnets on the flywheel. The coil is installed inside the flywheel. They don't call it a magneto anymore, but flywheel ignition.