The spark plug is placed on the top of cylinder. It is used to ignite charge (air-fuel mixture) inside the cylinder. The air gap between the electrodes of spark plug provides high voltage electrode pulse to ignite the mixture at the desired point in the cycle.
A spark plug mainly consists of three parts
1. Centre electrode
2. Ground electrode (side electrode)
The centre electrode carries high voltage into the cylinder so it must be well insulated. The ceramic insulator is formed around the centre electrode. The ceramic insulator is joined to a metal shell. The shell has threads which allow the plug to be screwed into the combustion chamber. To the shell a side electrode is attached which is placed at a small distance from centre electrode. This distance is known as spark gap or air gap which varies from 0.9mm to 1.8mm but it depend upon the engine.
Sparks plugs are of two types according to heat range.
a). Hot spark plug: For long path of heat travel
b). Cold spark plug: For short heat rejecter path