Under high voltage, the air between the center electrode and the side electrode of the spark plug will rapidly undergo ionization, forming positively charged ions and negatively charged free electrons. When the voltage between the electrodes reaches a certain value, the number of ions and electrons in the gas increases like an avalanche, causing the air to lose its insulation, and the gap forms a discharge channel, resulting in "breakdown" phenomenon. At this point, the gas forms a luminous body, known as a "spark".

Spark plugs are key components of engines, consisting of a steel shell, central electrode, insulator, and side electrode. The lower part of the steel shell has a fine thread, which is used to fit with the spark plug hole in the cylinder head. The upper part has an outer hexagonal nut, which is used to fit the spark plug sleeve to tighten or unscrew the spark plug. The central electrode is a metal rod, with the lower end in contact with the upper end of the central electrode through a conductor glass body. The upper end of the metal rod is equipped with a wiring nut. There is a high alumina ceramic insulator between the steel shell and the central electrode, which insulates the central electrode when energized. The bottom part of the steel shell is equipped with curved side electrodes. There is a copper washer in the middle of the outer part of the steel shell.