This task is performed by an inductive ignition system, the main part of which is the ignition coil (high voltage transformer).
Ignition voltage must be supplied to each spark plug. For this purpose, ignition coils are mounted directly on the spark plugs in Audi A4 engines (with built-in output stage). Thus, ignition sparks are produced without the use of a distributor. Electronic components have replaced the mechanical runner (rotor) of the ignition distributor.
The exception is the 1.6L ALZ engine. This engine uses dual spark ignition with two ignition coils.
The principle of operation of the ignition coil
The ignition coil consists of a primary winding with a relatively smaller number of turns and a secondary winding with several thousand turns of thin wire. The typical ratio between the number of turns is 1:100. The windings encircle a laminated iron core: the secondary winding from the inside of the core, the primary winding from the outside. Current is supplied to the primary winding from the battery via terminals 15 and 1 on command from the control device (low voltage). This creates a magnetic field that is amplified by the iron core. If the control device interrupts the current circuit, the magnetic field disappears instantly for a few fractions of a second. At the same time, a voltage of up to 400 volts is generated. This voltage induces a high-voltage current pulse in the secondary winding, which is transmitted to the spark plug, and the ignition energy is discharged in the spark gap of the plug.
The original text is available on the website: audimanual