The ignition coils are transformers that are able to provide high voltage pulses.

The purpose of these pulses is to generate a spark (arc) between the electrodes of the spark plugs on the internal combustion engine that is of sufficient intensity and duration to start the internal combustion of the air-fuel mixture successfully.

The principle of operation of these coils consists of a primary winding which is controlled by the electronic switch so that it reaches the required current. At the moment when the current is interrupted, the high voltage generated at the secondary output typically ranges between 20 and 30 Kv, which guarantees the triggering of the spark.

Coil types vary in complexity, in line with the evolution of engines over the years:

  - Oil bath coils: this is the most out-dated technology. Facet has decided not to produce them as they are highly polluting and have limited yield. We would suggest replacing them with coils of a more recent generation (see below).

  - Electro-mechanical distribution coils  (first generation): these are single coils and the high voltage is "distributed" to the spark plugs through special cables, from a rotary distributor mounted on the distributor, within the "distributor cap".

  - Wasted spark coils (second generation): this type of coil is called wasted spark because, while during the compression phase in the cylinder one has the normal ignition spark, during the exhaust phase in the cylinder there is a low intensity spark. The ignition systems that use these coils are called “static distribution" or "distributorless" systems, because with them the distributor, cap and high tension cables are eliminated, making the ignition system more reliable. The electronic control systems of these coils are more accurate and can manage the intensity of the primary current by controlling the pulse duration, so that the current reaches the set value.
This type of coil also includes some multiple plug-top coils.

- "Multiple coils": a grouping of coils connected to the spark plugs via high-voltage cables.

  - Direct coils (third generation): these coils are used in totally electronic ignition systems.
In these coils, the negative output of the secondary circuit is directly connected to a single spark plug, while the other end is connected to ground with a low voltage connection, or to the positive terminal of the primary circuit inside the coil. We therefore have a coil for each spark plug.
The power dissipated by each direct coil is half of that dissipated by a wasted spark coil: this advantage is very important, since the working temperature of coils mounted directly on the spark plugs is greater. The direct coils can be equipped with built-in electronic controls

Direct coils different types:

- ”Pencil": they are equipped with axial windings to the spark plug, which give it the characteristic "pencil" shape.

- "Plug top": the coils are assembled in a single block mounted on the engine head.