A spark plug is an electrical device that fits in the head of some internal combustion thermal engines and ignites compressed gasoline by means of an electric spark. It is used, therefore, in petrol engines with Otto cycle.
The spark plugs have an isolated central electrode which is connected by a strongly insulated wire to a circuit of the ignition coil on the outside, forming, with a terminal connected to ground at the base of the plug, a spark gap inside the cylinder.
How does a spark plug work? How is the spark generated?
As the electrons flow from the coil, a voltage difference develops between the central electrode and the lateral electrode. No current can flow because the fuel and air in space is an insulator, but as the voltage increases, the structure of the gases between the electrodes begins to change.
Once the voltage exceeds the dielectric resistance of the gases, the gases are ionized. The ionized gas becomes a conductor and allows electrons to flow through space. Spark plugs generally require a voltage greater than 20,000 volts to 'shoot' properly.
Because the spark plug is inside the engine and is the only easily removable part, it can be used as an indicator of the state of tuning and condition of the engine. The spark plug also says if you need to change something in the mechanics.