The technical answer
In petrol engines, the ignition coil is located in a vehicle’s ignition system in order to convert a car battery’s low voltage, of around 12 volts, to the thousands of volts needed to create an electric spark in the spark plugs and ignite the fuel.
An ignition coil supplies a high voltage that is made to jump from one electrode to the other at the spark plug, creating a spark. The voltage needed depends on a variety of factors.
The level of voltage required can change and vary from 5,000 volts up to 25,000 or more. The voltage can reach as high as a whopping 30,000 volts.
How do they work?
The ignition coil actually consists of two coils; a primary winding which has relatively few turns of heavy wire, and a secondary winding which has thousands of turns of very thin wire. Both windings are wound around each other.
The primary winding is connected to battery voltage and its earth can be switched on and off by the engine control unit (ECU). The secondary winding is connected to the spark plug. When the primary winding is switched on, the current flow creates a strong magnetic field around both windings. When it is switched off the magnetic field collapses, moving over both the primary and secondary windings. This induces a high voltage in the secondary winding which causes a spark at the spark plug.
All modern variations of ignition coils, including single coils and coil rails, work in a similar way – with the exception of capacitive discharge coils.
Ignition Coil Failures
Ignition coils have a relatively long service life, but they can fail for a variety of reasons. Heat and vibration can damage the coil’s insulation, but the biggest cause of ignition coil damage is voltage overload, which is caused by poor quality or faulty spark plugs and/or HT leads.
If the ignition coil’s output voltage rises too high, it burns through the coil’s internal insulation, causing a short – the coil’s insulation can be damaged if the output exceeds 35,000 volts. If this happens, the coil’s output voltage may drop causing ignition misfire. In worst case scenarios, the coil may cease to put out any voltage at all, preventing the engine from starting or running. febi recommends fitting new spark plugs (and HT leads where applicable) at the same time as replacing a faulty ignition coil.
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