This is how the ignition spark is created (Audi A4 B5)

The basic principle of ignition is that battery current first passes through the primary winding of the ignition coil.

This winding consists of several turns of thick wire. Under the influence of current, a powerful magnetic field is formed around the ferromagnetic core of the ignition coil - ignition energy.

In order for the ignition spark to appear, the battery voltage to the ignition coil is disconnected. This occurs on command from the control unit at the so-called "output stage".

When the current is turned off, the magnetic field in the ignition coil disappears. The following happens:

In the secondary winding, consisting of numerous turns of very thin wire, a high-voltage current pulse of several tens of thousands of volts occurs.

This voltage is sent to the spark plug that is next in line in the firing order. The fuel-air mixture ignites, the engine continues to run. The electrical circuit is closed again, and a new cycle begins.

(The original publication in its entirety is posted on the website: AUDImanual)