A spark plug is an electrical device that fits into the cylinder head of some internal combustion engines and ignites compressed fuels such as aerosol, gasoline, ethanol, and liquefied petroleum gas by means of an electric spark. Spark plugs have an insulated central electrode which is connected by a heavily insulated wire to an ignition coil or magneto circuit on the outside, forming, with a grounded terminal on the base of the plug, a spark gap inside the cylinder. Reciprocating internal combustion engines can be divided into spark-ignition engines, which require spark plugs to initiate combustion, and compression-ignition engines, which compress the air and then inject diesel fuel into the heated compressed air mixture where it autoignites...