Ignition Systems; historical & modern types, with an emphasis on BMW motorcycles.
Vastly more information is available on the Author's website, https://bmwmotorcycletech.info/index.html .....there are numerous articles on the BMW airhead electrical systems and several are on ignition systems for single spark plug per cylinder, dual spark plug per cylinder, timing units, points ignitions, and electronics ignitions.
There is CONSIDERABLY MORE technical information, including repairs, etc., in the author's website, particularly in these two articles:
https://bmwmotorcycletech.info/ignitiontheory.htm (Expands upon the information in the below article.
The following article is a SIMPLIFIED explanation of ignition systems, although lengthy.
A common type of ignition in very early cars typically consisted of wooden boxes containing various items: A vibrating electrical contact pair sent battery energy into a coil of a modest number of turns of thing solid copper wire wound on an iron core of some sort. There was another coil of wire wound on that iron core, but with thousands of turns of a much thinner wire, and these turns 'transformed' the lower voltage of the 'primary winding' fed by the battery to a few thousand volts, and this was applied to the spark plugs. In the earliest engines the spark was continuous due to the vibrating contacts, and thus was applied continuously to the spark plugs. Somewhat later a rotating switch was used for the high voltage. It had a contact or close by contact, one for each cylinder, and this item was called a 'distributor'. The only problem with the vibrating contact system is that the spark output can not be set to 'fire' the spark plug at a very specific piston position, although the distributor method helped some. With the very low compression ratio and very low power output of these engines per cylinder volume, together with the very long stroke and large diameter pistons, these ignition methods worked OK. There were many variations of these internal combustion engines, including types with very large heavy flywheels that were arranged to only fire the mixture after a number of revolutions...these were typically used by farmers for pumping water.
Magneto's, which were popular on both antique cars and early motorcycles...even into fairly recent motorcycles (particularly single cylinder types), are simply a permanent magnet method of using mechanical rotational energy (instead of a battery) to produce the high voltage needed to 'fire' a spark plug. The spark (that FIRE thing) is simply a very hot means of igniting the air-gasoline mixture. An engineer would not look at it that way, the engineer would be nerdy and describe the ion/plasma as the electricity was about to cause the spark (magneto or regular coil ignition). But, no need for such nerdiness here. Except for the earliest magneto's which produced an untimed spark, something like the vibrating points wooden box type driven by a battery, Magneto's are equipped with a set of points (contact points plate), which are nothing more than a mechanically coupled ON-OFF switch...and the energy transfer from the small number of turns coil in that contact circuit is done at the moment the points open, which is set via a cam located in the magneto which drives the points, so as to precisely coincide with a particular high energy alignment of the iron core with the wire in relationship to the magnet. The relatively small number of turns in the 'primary winding' coil acts in somewhat the same way as in the vibrating points method and the later points-condenser-coil method....there is a 'transforming action'. This SAME transforming action is used in all coil ignitions. The basic method is called the Kettering Ignition, usually meant to be a coil, points, and condenser (capacitor) across the points....and used with a battery; but the magneto type works somewhat the same. More on all this later herein.
BMW motorcycles used magneto's in the early models, prior to late 1969 when the /5 series was introduced. Magneto's are difficult to keep down in size when you need quite high energy sparks. Lean burning engines demand high energy sparks; although engines were not overly lean back then in the /5/6/7 era. Magneto's one big advantage is that no battery is needed. Magnetos produce more spark voltage at higher and higher rotational speeds, which is another reason why magnetos were popular, particularly on some race engines a long time ago....besides the weight of a battery not being needed (race engines were not generally started by 'starter motors' and did not need lights way back then). Unfortunately, magnetos need to be fairly large to produce a good spark at cranking rotation speeds. Magnetos were, and still are, used on some 'stationary' engines; and versions of magneto ignition are still used on some small motorcycles, lawnmowers, snowblowers, and many types of similar engine'd devices. Many small engine magnetos are built so that the magnets are part of the engine flywheel, and located near the edge, so the speed of the magnets going by a stationary coil, is fairly high, which can create a reasonably decent spark.
From the the time of introduction of the BMW /5 series in December of 1969, until 1981 when BMW introduced the Bosch electronic ignition, BMW's had coil ignition with the points contacts being driven by the engine camshaft, that same camshaft that operates the valves. The contacts, nothing more than a switch, have a capacitor, often called a condenser, electrically connected to them. This system of contact points, capacitor, and coil, all being driven by a battery, was invented by an engineer from GM, and even now, this system is still called the 'Kettering' ignition. In our Airheads, as in all common 4-stroke engines, the camshaft rotates at precisely half the crankshaft speed.
At the forward nose area of the camshaft is a bob-weight type of simple mechanism called the Automatic Advance Unit (ATU), whose purpose is