Main Page | See live article | Alphabetical index 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 or magneto 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. See Also ignition system This article is from Wikipedia. All text is available under the terms of the GNU Free Documentation License.