How does an ignition coil work?
If you’ve ever driven a car, chances are it has spark plugs. You may have wondered, how do these fantabulous little devices work, and what do they do? The spark plugs are directly responsible for igniting the fuel/air mixture in a gasoline engine (diesel engines ignite the fuel using compression, not spark) and they do this by way of a high-voltage pulse generated by the ignition coil.
To most, these coils can seem like a magical little object that takes in electricity and spits it back out at a very high voltage. However, they use a very simple principle to do this: inductance!
The way that an ignition coil works is that a voltage is applied across the two terminals of the primary windings, which will cause current to flow through the primary winding. This causes a magnetic field to form inside of the ignition coil, and it encompasses both the primary and secondary windings. When you interrupt the current flowing through the primary, the magnetic field begins to collapse while forcing current through the coils (this is inductance!) Since this current has to go somewhere, the voltage in both coils will rise relative to how many windings are in each coil (think Ohm’s law, at a constant current but higher resistance, voltage rises). Because of this, the voltage in the secondary winding will rise far past the voltage across the primary winding. Eventually, the voltage in the secondary coil becomes so high that it reaches the breakdown voltage of the air between the electrodes of the spark plug, causing an arc to jump as the air begins to ionize and conduct current. Now that the air is conductive, the current will flow through in the form of a high-temperature plasma which is able to ignite the fuel in the engine.
Modern cars typically use coils that are located directly on top of the spark plugs, and they have igniters built in. An igniter is a small digitally controlled circuit that sends high-current power through the primary winding using transistors and then interrupts it at a precise time to generate the voltage necessary. Older cars and motorcycles will sometimes use CDI (capacitive discharge ignition) boxes which use electronic sensors and control circuits that charge a capacitor to drive the current through the coil. Even older vehicles will use something called points-based ignition, in which a rotor that spins at the same speed as the engine will actually physically touch contact points in order to charge the coil up. When the rotor has rotated past the contact, the circuit breaks, and the coil will fire.
Note: High voltages can be really dangerous! Ignition coils typically output over 20 kV. Even though the pulse is incredibly short, this can still seriously hurt you. M5 has a policy prohibiting projects involving over 40V unless specifically approved and supervised.
