The coil operates according to Faraday's Law of electro-magnetic induction.
As the high tension voltage in the secondary winding builds up, a particular voltage level is reached when the spark plug gap suddenly becomes conductive, allowing the spark to occur. This is known as the “plug firing voltage”.
The level of plug firing voltage depends on many factors, such as:
When the spark reaches the point where a high intensity current is flowing in the secondary circuit, the voltage drops to a much lower spark voltage level, sufficient to sustain the spark. This gives the mixture the opportunity to ignite during the duration of the spark.
Spark Plug Firing Voltages
While the perception that the spark plugs fire instantaneous, this is not technically correct. On the average, they can take milliseconds to fire. An oscilloscope pattern will show much more than a single instantaneous firing sequence or event.
When the breaker points or solid state ignition unit (switching device) interrupts current flow in the primary ignition circuit and induces current flow into the secondary windings of the coil, there is an instantaneous voltage surge or spike. This represents the voltage required to overcome the air gaps at the spark plug and distributor rotor gaps including the clearance between the rotor and the distributing poles inside the distributor cap. Once the induced voltage bridges the spark gap, the secondary voltage required to sustain the spark across the gap is much less and drops. The spark continues to fire across the gap at a fairly constant voltage until the induced voltage in the form of an arc is extinguished. The induced voltage within the coil energy drops it can not sustain the spark duration any longer. During this spark duration, the spark plug actually fires several times. This is caused by high frequency oscillations in the primary and secondary windings of the coil, which continues to induce voltage surges or spikes. All of this takes place in roughly one thousandth of a second. Any time you have a flow of electric current you will have a magnetic field or a generated magnetic field. Components such as, ignition coils, circuit relays, switches, solenoids, generators, servomotors all can have an affect on audio/communication equipment, electronic circuits and computers. The higher the voltages, the more critical this becomes. Anytime you have the spark jump a gap or a contact, you can induce a radio interference signal.
Source: CDX Global & Wikipedia - en.wikipedia.org
