Fat sparks need high voltages – much greater than the modest voltages flowing from a battery or source coil. Something has to help create the 12,000 volts-plus that eventually jump the sparkplug's gap. That something is the ignition coil.
The ignition coil takes the low voltage or low tension created by the battery or source coil and turns it into a high voltage or high tension that can provide a spark to jump the sparkplug's gap. Air is a poor conductor of electricity and that resistance in the combustion chamber can be increased by bad wiring, incorrect fuel/air mixture and fouled plugs. It is the task of the ignition coil to supply sufficient voltage to jump the sparkplug gap against the factors conspiring against it. Given that the voltage required can be as high as 45,000 volts to meet the requirement of newer high-revving, high-compression engines, it's a pretty big ask.
As high as the voltages needed for efficient sparks are, the levels of current involved are actually quite low. The coil meets its function through electromagnetic induction. If an electrical current is passed through a wire, a small electromagnetic field forms around it. If you then passed another wire through that field, a small voltage would in turn induced in that second wire. The same phenomenon can be observed in stationary wires if the current is the first wire is interrupted by switching it off and on. The electricity flowing in such a set-up would, of course, be insufficient to create a spark of the magnitude required in an ignition system.
An ignition coil uses the transformer effect to convert small current pulses into high voltage sparks. An ignition coil in fact consists of two coils wound concentrically. The primary coil, through which the battery or source coil current flows, is typically few hundred turns (usually around 300) of fairly heavy gauge wire, while the secondary might be 20,000 turns of much finer wire. Both coils have a laminated iron core, the purpose of which is the concentrate the electromagnetic field or 'flux density' created in the coil.
When the contact breaker or CDI interrupts the flow in the primary coil, the change in magnetic flux has an effect in all turns of the coil; the secondary voltage created in the secondary coil, which is connected to the HT lead, is sufficient to create a spark across the plug. An energy transfer system, such as the flywheel magnetos you might find on small two-stroke and some competition and off-road four-stroke, uses a coil of similar construction. In this case, though, the points are arranged in parallel with the primary coil and open/close to the point of maximum current flow, and the energy is suddenly applied to the primary coil. By contrast, in conventional coil ignition, the points are in series with the primary coil and interrupt the current flowing through the primary to induce the larger voltage in the secondary.
You will find one of three types of coil on motorcycles in Thailand. The first and oldest of these is the canister style, which has an iron core around which the secondary coil is wound. The primary winding is on the outside of this and the two coils are not connected. The whole assembly is housed in a metal can which is often filled with oil the deal with the heat generated byt the business of creating high voltages.
The second type is the one that's encountered most often, and that is the molded coil. These have a central laminated coil around which the primary coil is wound with its secondary outboard. The whole assembly is encapsulated in resin to counteract the potentially damaging effect of vibration on the delicate windings. Multi-cylinder motorcycles typically use one coil to feed two plugs through a pair of HT leads coming off the secondary winding. This gives rise to the phenomenon known as the 'wasted spark' as while one cylinder is firing, the other will be on its exhaust stroke when the plugs fire.
The technical disadvantage here is that the current is flowing in reverse in one plug during this process, and more voltage is required to do this. This means that the plugs have to be kept in good condition for things to function satisfactorily. A four-cylinder machine will use a pair of such coils firing two pots apiece with the supply metered by an electronic control unit made to operate two coils. All of this allows motorcycle manufacturers to do away with the need for a car-style distributor and the weight, space and expense this would require.
More recently, motorcycle manufacturers have turned to this stick coil, which combines the coil with the plug cap and does away with the need for an HT lead. As well as being compact and reliable, they're also cheaper to manufacture. Tag: IgnitionIgnition-CoilsEngineTechnicalPartsHow-To