A device that screws into the combustion chamber of an internal combustion engine to provide a pair of electrodes between which an electrical discharge is passed to ignite the combustible mixture. The spark plug consists of an outer steel shell that is electrically grounded to the engine and a ceramic insulator, sealed into the shell, through which a center electrode passes (see illustration). The high-voltage current jumps the gap between the center electrode and the ground electrode fixed to the outer shell.
The electrodes are made of nickel and chrome alloys that resist electrical and chemical corrosion. Some center electrodes have a copper core, while others have a platinum tip. Many spark plugs have a resistor in the center electrode to help prevent radio-frequency interference. The parts exposed to the combustion gases are designed to operate at temperatures hot enough to prevent electrically conducting deposits but cool enough to avoid ignition of the mixture before the spark occurs. See Ignition system
a device used to ignite the air-fuel mixture in the cylinders of internal-combustion engines equipped with carburetors. The mixture is ignited by a spark passing between the spark plug’s electrodes. Spark plugs are threaded into the engine’s cylinder head and consist of a steel shell with a side electrode and an insulator with a center electrode. A terminal is mounted on the top part of the center electrode. A high voltage is applied periodically across the gap between the center electrode and the side electrode, causing a spark to jump across the gap. The length of the skirt of the insulator determines the thermal characteristic of the spark plug. A short skirt provides a high heat transfer from the insulator to the shell. Spark plugs with short skirts are called cold plugs; those with long skirts are called hot plugs. Cold plugs are used for continuous engine operation under heavy loads or at elevated temperatures. [23–208–]