A spark plug
is an electrical device that fits into the cylinder head
of some internal-combustion engines and ignites compressed aerosol gasoline
by means of an electric spark. Spark plugs have an insulated wire which is connected with a induction coil
circuit on the outside, and forms, with another terminal on the base of the plug, a spark gap
inside the cylinder.
Internal-combustion engines can be divided into spark-ignition engines, which require spark plugs to begin combustion, and compression-ignition engines (diesel engines), which compress the fuel/air mixture until it spontaneously ignites. Compression-ignition engines may use glow plugs to improve performance when the engine is cold.