<Li> (R) Connecting rod </Li> <Li> (C) Crankshaft </Li> <Li> (W) Water jacket for coolant flow </Li> <P> The plug is connected to the high voltage generated by an ignition coil or magneto . As current flows from the coil, a voltage develops between the central and side electrodes . Initially no current can flow because the fuel and air in the gap is an insulator, but as the voltage rises further it begins to change the structure of the gases between the electrodes . Once the voltage exceeds the dielectric strength of the gases, the gases become ionized . The ionized gas becomes a conductor and allows current to flow across the gap . Spark plugs usually require voltage of 12,000--25,000 volts or more to "fire" properly, although it can go up to 45,000 volts . They supply higher current during the discharge process, resulting in a hotter and longer - duration spark . </P>

How does a spark plug work in a car