Spark plug is a device used to produce electric spark to ignite the compressed air fuel mixture inside the cylinder. The spark plug is screwed in the top of the cylinder so that it electrode project in the combustion chamber.
A spark plug consist of mainly three parts:
1. Center electrode or insulated electrode.
2. Ground electrode or outer electrode.
3. Insulation separating the two electrodes.
The upper end of the center electrode is connected to the spark plug terminal, where cable from the ignition coil is connected. It is surrounded by insulator. The lower half portion of the insulator is fastened with a metal shell. The lower portion of the shell has a short electrode attached to one side and bent in towards the centre electrode, so that there is a gap between the two electrodes. The two electrodes are thus separated by the insulator. The sealing gaskets are provided between the insulator and the shell to prevent the escape of gas under various temperature and pressure conditions. The lower part of the shell has screw threads and the upper part is made in hexagonal shape like a nut, so that the spark plug may be screwed in or unscrewed from the cylinder head.
Cleaning the Spark Plug
Due to the combustion of fuel in the cylinder, carbon particles deposit on and around the electrode which not only reduce the plug gap but also prevent the spark to occur. If the spark is still occurring, it is too weak that it cannot ignite the fuel. Hence the spark plug is to be cleaned. Carbon particles can deposit due to any reason like, nature of fuel, mixture strength, lubricating oil, etc. The spark plug can be cleaned by a sand paper.