Spark Plug: A spark jumps between the electrodes of the spark plug when the voltage developed by the ignition system reaches the required value. This electrical spark occurs at a particular piston position and ignites compressed fuel-air mixture in the petrol engine cylinder. The spark plug is one of the most stressed parts on the engine. It is exposed to :

• temperatures ranging from 60^oC to 2500^oC in the combustion chamber, fluctuating constantly according to the engine operation,
• combustion chamber pressure fluctuates from a partial vacuum (during intake stroke) to 60 kg/cm² (850 psi) (during combustion),
• electrical loading as high as 30,000 volts., and
• corrosive action of the combustion gases.

More often than not, engine defects are attributed to spark plugs. However, in most cases, the spark plugs are not the cause of trouble but they may lie in the malfunctioning of other engine parts/accessories and adverse operating conditions.