Spark Plug

A Spark Plug is a device for delivering electric current from an ignition system to the combustion chamber to ignite the compressed fuel and air mixture by an electric spark while containing combustion pressure within the engine. A spark plug has a metal threaded shell, electrically isolated from a central electrode by a ceramic insulator. The central electrode, which may contain a resistor, is connected by a heavily insulated wire to the output terminal of an ignition coil or magneto.
Spark plugs may also be used for other purposes when they are not firing, spark plugs are used to measure ionization in the cylinders – this ionic current measurement is used to replace the ordinary cam phase sensor, knock sensor and misfire measurement function.