The whole ignition works this way, a simple description
Points-condenser-coil ignition:
The coil is really a transformer with two windings which are called the primary and secondary. The turns ratio is around 100 or 150 to one, meaning that for every single turn in the primary winding there are 100-150 turns in the secondary. This means that for every volt put into the primary winding when the points open there will be 100 to 150 volts in the secondary, which is connected to the spark plug.
The ignition points are simply a switch that's opened and closed by a small cam on the end of the crankshaft (in a two-stroke). While the points are closed, current from the battery flows in the series circuit comprised of the coil primary and the points. Nothing happens when there is steady current flow in this circuit. But when the points are pushed open by the points cam, current flow abruptly stops and a very useful phenomenon comes into play: the sudden ending of current flow in the inductor that is the coil primary causes a collapse of the magnetic field that had been set up by the steady current flow. The sudden collapse of this field generates an "inductive kick" (properly called counter-emf) which is of much higher peak voltage than the battery voltage that had been causing the steady current flow. If the battery voltage is 12 volts, the peak voltage of the counter-emf will be around 200 volts across the primary of the coil. With a turns ratio of 100:1 the peak voltage in the secondary coil will be 20,000 volts which should be enough to fire the spark plug.
So what does the condenser do? First of all, it's properly called a capacitor, has been since the late 1940s. A capacitor can be thought of as a device that blocks direct current (DC) but allows alternating current (AC) to pass through it. The inductive kick we generated with the opening of the points starts in the primary of the coil, flows through the condenser which is connected across the open points, and passes on to ground (the chassis of the bike) to complete the circuit back to the battery.
Energy transfer system:
This scheme is similar to the points-condenser-coil system except that there's no battery. There is, however, an alternator being spun on the end of the crankshaft, similar to the one that would charge the battery in a conventional points system. The alternator winding is connected directly to the points and condenser that feed the coil, and the points and alternator winding are synchronized so that the points open just as the alternator coil is at the peak of its sine-wave output voltage. Sounds reasonable. The major problem is that when you're trying to kick the damn thing over, the peak voltage from the alternator winding is pretty weak, and consequently so is the spark.
Capacitor-discharge (CD) system:
Electronic ignition systems are usually CD systems. Here we still have the ignition coil, as in the other systems, but there is no steady current flow in it. Instead, there is a capacitor in the black box that's charged to several hundred volts by an electronic oscillator that steps up the 12 volts DC from the battery. Then, at the right time, a switch (usually optical or magnetic) that's actuated by a disk on the end of the crankshaft puts out a pulse that causes a switching transistor (also in the black box) to dump the capacitor's several hundred volts into the coil. The coil is now just acting as a transformer that takes the 300 volt pulse and transforms it to about 30,000 volts for the spark plug (remember the 100:1 turns ratio). There are three major advantages over coil-points-condenser systems:
• Higher voltage for a hotter spark.
• Faster "rise time" - that is, the pulse goes from zero volts to its maximum maybe five times faster than the inductive kick, so the quick spark can fire a fouled spark plug that would cause the slower voltage rise from the inductive kick to bleed off through the deposits on the insulator rather than jump the spark plug's gap.
• No moving parts except the disk, so no mechanical wear. Once the timing is set it should never change.
The ignition coil in an OEM (original equipment) CD ignition system has less inductance than a conventional ignition coil, allowing an even faster risetime, but aftermarket systems often use the original standard ignition coil. If the aftermarket CD system can handle the higher switching current of the CD-type coil it will produce a hotter spark.