Magnetos are simple, self-contained ignition systems. Reliable and compact, they actually work pretty well. Almost all motorcycles up to the early '70s used magnetos and they continue to fly around quite happily in some light aircraft. They do, however, suffer from low power output at high rpm and offer poor dynamic timing control. Magnetos create electrical power from a magnet on a flywheel passing by a coil of wire – a little like a dynamo. At the required moment of ignition, this electrical power, or current, is turned off by a set of internal contacts. Abruptly turning off the current in this way creates a large magnetic field around the coil of wire.
Coil ignition systems take the significant step of using an external power source to provide the spark energy. A basic coil ignition system comprises of two circuits; a low-voltage primary circuit and a high-voltage secondary circuit. In the primary circuit is the power source, (usually 12v DC) ignition trigger and primary ignition coil. The secondary circuit contains the secondary ignition coil, (which is combined with the primary coil in one unit) HT lead and spark plug. Power is fed to the primary coil through the ignition trigger – early systems used a set of mechanical contacts for this, called points, these were opened and closed by a cam on the end of the crankshaft – the primary coil charges with electricity while the contacts are closed, referred to as the dwell time (or dwell angle if you prefer). At the required moment of ignition, the contacts open, cutting off the supply to the primary coil, this creates a magnetic field around the primary coil, inducing a current in the secondary coil which is tightly wound around the primary. The current in the secondary is roughly 10 to 20 times lower than in the primary, but the voltage is correspondingly higher, peaking at around 30,000 volts, this electrical energy is fed along the HT lead to the spark plug where it jumps across the gap producing a spark.
What's CDI Ignition?CDI ignition was first introduced in 1968 on the Kawasaki KH500, Capacitor Discharge Ignition (CDI) uses an electrical storage device (capacitor) to deliver a sudden surge of electrical energy to the primary side of an ignition coil at the required point of ignition – rather than switching power off like coil ignition. CDI systems are great for engines prone to plug fouling, such as two-strokes, because they can deliver very high voltages at the spark plug. The downside is that the spark duration is relatively short compared to modern electronic coil ignition systems.
Magneto and Coil Ignition sound like similar systems, why only use coil ignition?The beauty of coil ignition is its ability to feed the ignition coil with large amounts of electrical energy. Early points-triggered systems still suffered from reducing coil charging times at high rpm, but not as severely as magneto ignition systems. The mechanical points-triggered ignition systems had several drawbacks, they wore out, didn't work that great at high rpm, were fiddly to set up and didn't offer great flexibility for delivering different spark timing at different engine loads and speeds.
As engine rpm increases, less time is available for the combustion process to take place. As such, the moment of ignition (spark timing) must take place progressively earlier – known as timing advance. Alternatively, as engine load increases (wider throttle opening) spark timing should occur later because high load means that the combustion chamber has more charge packed in it waiting to be ignited. The charge is at higher pressure and temperature than when at low load, causing it to burn faster. If the ignition timing is not retarded, engine damage can result. The next advance from points-triggered ignition was transistorized ignition, where the current feeding the primary coil was switched by electronic switches. These were more reliable, faster and could handle more power than points.
By the early '90s electronic ignition became commonplace. The ignition control unit receives signals from reliable electronic sensors detecting engine speed and position. They could also account for other factors like air and coolant temperature, allowing further timing optimization.
Electronic ignition was the pivotal step towards the combining of ignition and fuel injection systems to give us electronic engine management. Modern engine management systems determine the optimum ignition point based on a stored map of several variables such as engine load, rpm, coolant temperature, air temperature, air-fuel ratio and even gear position. A few motorcycles even use sensors to detect if the engine is knocking, meaning that the ignition timing can safely run as close as possible to optimum at all times, accounting for different fuel grades and engine wear.