Spark plug

spark plug: see ignition.

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spark plug

Device that fits into the cylinder head of an internal-combustion engine and carries two electrodes separated by an air gap, across which current from a high-tension ignition system discharges, creating a spark and igniting the fuel. The electrodes and the insulator separating them must withstand high temperatures, as well as an electric stress of up to several thousand volts. Spark-gap length affects the energy of the spark, and the shape of the insulator affects the temperature of operation.

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Spark plug

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