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Spark plugs are typically designed to have a variable spark gap. The spark gap may be adjusted by the technician installing the spark plug. A spark plug gap gauge is used to set the gap. The simplest gauges are a collection of keys of various thicknesses which match the desired gaps and the gap is adjusted until the key fits snugly.
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 cold start characteristics.