Mcgyver,
There are two basic methods to generate a spark for ignition. Both involve storing electrical energy and then releasing it to generate the spark. There are also many variations of these basics. The first stores the energy in the magnetic field of the ignition coil and the second stores the energy external to the coil.
The conventional Kettering ignition stores the energy in the magnetic field of the ignition coil. When the points close, the current builds slowly in the ignition coil due to the inductance of the coil until it reaches a maximum value limited by the resistance of the coil. When the points open, the magnetic field collapses and a high voltage of several hundred volts is generated across the primary. The condenser slows this high voltage rise to allow the points to open more before the voltage gets high and this reduces the arcing. Further the condenser charges to the high voltage and then discharges through the primary of the coil. This voltage is typically in the range of 300 volts. The turns ratio of most ignition coils is in the range of 75:1 up to 100:1. Some are outside of these ratios. It is this 300 or so volts times the turns ratio of the coil that produces the 20,000 or so volts necessary for the spark plug operation. It is also the reason the capacitor across the points has to have a high voltage rating. Replacing the points with a semiconductor allows the removal of the capacitor, but the semiconductor must be capable of both the maximum current for the coil and the high voltage generated then the current is cut off. When using a hall effect instead of points, it is sometimes necessary to use several magnets to keep the semiconductor on long enough to allow the current to rise in the coil to store the energy. This is more common on higher speed engines, and would probably not be necessary on your hit-miss engine as it will be operating at low speed. This dwell time, or points closed time, must be long enough to allow the current to rise in the primary of the coil at the highest speed the engine will run at. When operating at lower speeds the current in the coil, after it reaches the maximum value, maintains the magnetic field and is converted into heat in the coil.
If a simple 12 volt AC voltage is applied to the primary of a coil with a 100:1 turns ration, the secondary would only have 1200 volts output. This is not enough to operate a spark plug so this is not an option. Special coils can be wound, but coil inductance and other things start creating other problems.
The second method involves applying a high voltage pulse to the coil from an external source. The most common of these is the capacitor discharge ignition system (CDI). On a CDI, a high voltage of 300 or so volts is generated externally and used to charge a capacitor to store the energy. Then when the points close, or open depending on the circuit, this stored energy is applied to the coil to create the spark. One advantage to this is the coil does not have the excess heat caused by the maintenance current of long dwell times at low speeds. The biggest disadvantage is the extra circuit to supply the high voltage. This is most often a simple inverter that charges the capacitor. Once the capacitor is charged, the inverter input current is low so there is little excess current used at low speeds.
Although there are other methods also used, such as piezo ignition, most ignition system are some variant of the basic two. A magneto is a form to the Kettering where the coil charging current is generated by a rotating magnet. An ignitor system just uses a simple inductor and battery in series with the points. The points are in the cylinder and there is no capacitor across the points. When the points open, it is the arc across the points that ignites the fuel.
I hope this helps you make up you mind as what method you want to use.
Gail in NM, USA
