SPARK PLUG
Spark plugs are more than 100 years old and, in spark-ignition engines (also referred to as petrol engines), they ignite the air-fuel mixture in the combustion chamber. High voltage is supplied to the terminal, which causes a spark to jump the spark-plug gap at the tip and so initiating combustion. The spark timing is crucial for optimum engine performance. Generally the advancement of ignition timing before top dead centre (TDC) increases power output until the onset of “knock”, or auto ignition, at higher engine loads. In this case, part of the air-fuel ratio elsewhere in the combustion chamber self-ignites and can cause severe engine damage.
The terms “hot plug” or “cold plug” refer to the amount of insulation surrounding the centre electrode because this determines its operating temperature. The ideal electrode temperature is around 600 °C and colder temperatures may lead to fouling and higher temperatures to electrode overheating, causing damage.