Electric, solar-powered and alternate fuel vehicles may be the wave of the future, but for now most automobiles run on gasoline, which they burn in an internal combustion engine to convert into motion. For combustion to take place, a spark is needed to ignite the fuel mixture in the engine. The vehicle’s ignition system is designed so that a 12-volt battery can generate the very high voltage required to create such a discharge. The heart of this system is a device called an ignition coil.
This coil is a kind of transformer. Transformers transfer voltage from one circuit to another, either as a higher voltage (as in a step-up transformer, of which the ignition coil is an example), or a lower voltage (a step-down transformer). The key principle that makes transformers work is electromagnetic induction: A moving magnetic field, or a change in a stationary magnetic field (the case in our ignition coil), can induce a current in a wire exposed to that field.
This ignition coil is a pulse-type transformer. Like other transformers, it consists, in part, of two coils of wire, as shown in the diagram at right. These are both wrapped around the same iron core. Because this is a step-up transformer, the secondary coil has far more turns of wire than the primary coil, which is wrapped around the secondary. In fact, the secondary coil has several thousand turns of thin wire, whereas the primary coil has just a few hundred. In your car, this allows some 40,000 volts of electricity to be generated by a modest battery, as the tutorial below illustrates.
Click the blue Turn On button to close a Knife Switch and create a circuit that allows current (depicted by a red glow in the circuit) to flow into one of the Primary Terminals to the primary coil. Alternating current (AC) is used in most types of transformers because the constantly changing magnetic field that it creates in a primary coil allows for a continuous induction in the secondary coil. In a car’s ignition system, however, direct current (DC) is used (provided by the Battery), because the idea is not to create steady, continuous induction, but one single, dramatic induction from a sudden collapse of a magnetic field.
As current flows to the primary coil, an increasingly large magnetic field builds up around it as well as around the secondary coil housed inside. If you click on the red Turn Off button to stop this current, the field suddenly collapses, and this rapid change induces a surge of current in the secondary coil, which streams out the high voltage Output Terminal and is enough to jump the Spark Gap in the circuit. This spark then ignites the fuel mixture and gets the motor running.
There is just one problem with this scenario. The collapsing field also induces a lesser surge (back EMF) in the primary coil, creating a second, unwanted surge of electricity traveling back through one of the primary terminals toward the switch. To keep that surge from reaching the switch (and creating a damaging spark across those points), a Capacitor is inserted in the circuit. This capacitor – called a condenser in auto ignition systems – safely absorbs the back emf.
Thanks to our scientific advisors on this page, James Andy Powell, head electronics engineer in the MagLab's Instrumentation & Operations division, and Russell Wood.