Although it initiates the spark to create combustion, the spark plug doesn’t sustain it. It does help to transmit heat out of the combustion chamber into the water jacket of the cylinder head.
The ability of a spark plug to dissipate heat from the combustion chamber is defined by the spark plug “heat range.” The temperature of the firing end of the spark plug must be maintained at a level high enough to prevent fouling, but low enough to prevent pre-ignition. Spark plug manufacturers refer to this as “thermal performance.” Thermal performance, or heat range of the spark plug, has nothing to do with the amount of energy transferred from the ignition system through the spark plug. Spark plug heat range is the area in which the spark plug functions thermally.
The spark plug is a seemingly simple device, although it is tasked with a couple of different but critical jobs. First and foremost, it creates (literally) an artificial bolt of lightning within the combustion chamber (cylinder head) of the engine. The electrical energy (voltage) it transmits is extremely high in order to create a spark and to “light the fire” within the controlled chaos of the combustion chamber. Here, the voltage at the spark plug can be anywhere from 20,000 to more than 100,000 volts.
In terms of operation, the spark plug is connected to the high voltage generated by an ignition coil (by way of a conventional distributor or by way of an electronic means). As electricity flows from the coil, a voltage difference develops between the center electrode and ground electrode on the spark plug.
Because of the spark plug “gap,” coupled with the air/fuel mixture (which acts as an insulator) within the gap, the spark plug cannot immediately fire.
As the voltage rise increases to approximately 20,000 volts, the gap within the spark plug can be “breached” and it fires. With a spark plug removed from the cylinder head and properly grounded to fire, you can hear a definitive click. If conditions are dark enough, you can see the spark.
The click you hear is essentially a miniature clap of thunder, and the spark you observe is similar to a miniature form of lightning.
Within the combustion chamber, the intense heat created by the spark plug creates a small fireball within the gap. The fireball or combustion “kernel” expands and the cylinder experiences complete combustion.