Spark plug

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A spark plug is a vital component in spark-ignition engines, providing an electric current path from the ignition system to the combustion chamber. It delivers this spark to ignite the fuel/air mixture within the engine while effectively containing combustion pressure. The spark plug consists of a metal shell with a porcelain insulator separating it from the central electrode, which may contain a resistor. This central electrode is connected to an ignition coil's output terminal via a heavily insulated wire. Upon installation into the engine's cylinder head, the spark plug's metal shell becomes electrically grounded. Inside the combustion chamber, the central electrode forms one or more spark gaps with the side, earth, or ground electrodes, located at the inner end of the threaded shell.