Ignition coil

An ignition coil (also called a spark coil) is an induction coil in an automobile's ignition system which transforms a storage battery's 12 volts to the thousands of volts needed to spark the spark plugs.

This specific form of the autotransformer, together with the contact breaker, converts low voltage from a battery into the high voltage required by spark plugs in an internal combustion engine.

In older vehicles a single (large) coil would serve all the spark plugs via the ignition distributor.

In modern systems, the distributor is omitted and ignition is instead electronically controlled. Much smaller coils are used with one coil for each spark plug or one coil serving two spark plugs (so two coils in a four-cylinder car). These coils may be remote-mounted or they may be placed on top of the spark plug (coil-on-plug or Direct Ignition). Where one coil serves two spark plugs (in two cylinders), it is through the "wasted spark" system. In this arrangement the coil generates two sparks per cycle to both the cylinders. The fuel in the cylinder that is nearing the end of its compression stroke is ignited, whereas the spark in its companion that is nearing the end of its exhaust stroke has no effect. The wasted spark system is more reliable than a single coil system with a distributor and cheaper than coil-on-plug.

Where the coils are remote mounted they may all be contained in a single moulded block with multiple high-tension terminals. This is commonly called a coil-pack.