Spark plugs are installed on the side or top of the engine. In the early days, spark plugs were connected to the power distributor through cylinder wires. In the past decade, engines in small cars have mostly been changed to have ignition coils directly connected to spark plugs. The working voltage of the spark plug is at least 10000V, and the high-voltage electricity is generated by the ignition coil from 12V electricity, which is then transmitted to the spark plug.

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.