The spark plug insulator skirt (referring to the tapered portion of the insulator outside the center electrode of the spark plug) directly contacts the high temperature gas in the combustion chamber to absorb a large amount of heat, and the sucked heat is transmitted to the cylinder head and the atmosphere through the outer casing. Experiments have shown that to ensure the normal operation of the car engine, the spark plug insulator skirt should maintain a temperature of 500 ~ 600 ° C (this temperature is called the self-cleaning temperature of the spark plug), if the temperature is lower than this value, it will form a product in the skirt of the insulator Carbon causes leakage between electrodes, which affects spark plug flashover. If the temperature of the insulator is too high (over 900 ° C), hot ignition will occur when the mixture is in contact with such a hot insulator, resulting in premature combustion of the engine. The temperature at which the spark plug works normally is between 450 and 870 °C. At this time, the spark plug is yellowish brown. If the spark plug operating temperature is lower than 450 ° C for a long time, there will be a lot of carbon around the spark plug, and the spark plug will be black.