Ignition coils. Design and mode of operation

Structure of ignition coil

Ignition coils work on the transformer principle. They basically consist of a primary winding, a secondary winding, the iron core and a housing with isolation material, nowadays two-component epoxy resin. On the iron core of individual thin steel sheets two coil elements are applied e.g.:

• The primary winding is made of thick copper wire with approx. 200 windings (diameter approx. 0.75 mm2)
• The secondary winding is made of thin copper wire with approx. 20,000 windings (diameter approx. 0.063 mm2)

As soon as the primary coil circuit closes, a magnetic field is generated in the coil. Induced voltage is generated in the coil by self induction. At the time of ignition, the coil current is switched off by the ignition output stage. The instantaneously collapsing magnetic field generates a high induction voltage in the primary winding. This is transformed on the secondary side of the coil and converted in the ratio of "number of secondary windings to primary windings". A high voltage flashover occurs at the spark plug, which in turn leads to ionization of the sparking distance and thus to a flow of current.

This continues until the saved energy has been discharged. As it jumps, the spark in turn ignites the fuel / air mixture.

The maximum voltage depends on:

• The ratio of the number of windings from the secondary winding to primary winding
• The quality of the iron core
• The magnetic field