An ignition system is a system for igniting a fuel-air mixture. The earliest internal combustion engines used a flame, or a heated tube, for ignition but these were quickly replaced by systems using an electric spark. The two most common ignition systems are battery/coil and magneto. Battery/coil ignition systems are standard on most cars and many motorcycles. Battery/coil ignition systems are logically divided into primary and secondary sides. The primary side consists of the components that carry battery level voltage. The secondary side consists of those components that are part of the circuit once the electrical voltage is boosted to 30,000 volts. For our purposes, we will discuss the ignition system in terms of functionality and parts, ignoring the primary and secondary classifications.
The ignition system is critical to the operation of the motorcycle. Every motorcycle must have some version of the ignition system, although some have highly abbreviated versions. Motorcycles of any size and complexity have some version of a charging system, unless the bike is highly specialized and the charging system has been stripped away to save weight.
A basic battery ignition system consists of a battery, coil, spark timing mechanism (either distributor or electronic ignition ie ECU and pickup coil) and a spark plug. The battery provides the initial current to the coil when the engine is started. This current is directed to the primary windings of the ignition coil by the electronic ignition, or by the distributor when the points are closed. At the direction of the electronic ignition (or when the points open), the electricity stops flowing. What happens next is one of those incredible, hard to understand electrical events. The coil transforms the six (or 12) volts into 400 volts and then into 30,000 volts which rip out of the coil to the spark plug. Viola, spark. This process happens once for each cylinder during every power stroke, and takes only a fraction of a second to complete.
The major types of battery ignition systems are either electronic or mechanical. Mechanical systems use some form of distributor (consisting of points, capacitor and cam) to direct electricity to the coil. Electronic systems use an electronically controlled switch to perform the same function. Electronic ignition is higher tech, providing more dependable timing control and better spark. The disadvantage of electronic ignition is that when the system fails, it fails completely. Additionally electronic ignitions can be very hard to diagnose for the layman and often require substituting a working part to be 100% sure. Check your shop manual for more information on how to diagnose an electronic ignition that doesn't seem to be working.
The magneto, invented in 1903 by German engineer Robert Bosch, combines most of the features of a battery/coil ignition system into a single unit. Driven directly from the engine, it performs the same functions as the coil and distributor of a battery/coil system. The magneto ignition has the advantage of generating its own power, eliminating the need for a battery. Therefore, a magneto is used on motorcycles where reducing weight is important, such as trail bikes.
Magnetos operate on a very simple principle. An armature wound with a primary and secondary coil rotates between poles of a stationary magnet. As the armature rotates, current is "induced" in the primary winding when the contact breaker points are closed. When the breakers open, the magnetic field associated with the current collapses, and very high voltage is induced in the secondary winding. This voltage is discharged through the spark plug.