Spark plug is an important component of gasoline engine ignition system, it can introduce high voltage into the combustion chamber, and make it skip electrode gap and spark, so as to ignite the combustible mixture in the cylinder. It is mainly composed of a wiring nut, an insulator, a wiring screw, a center electrode, a side electrode and a shell, and the side electrode is welded on the shell. Under the action of high voltage, the air between the center electrode and the side electrode of the spark plug will rapidly ionize, forming positively charged ions and negatively charged free electrons. When the voltage between the electrodes reaches a certain value, the number of ions and electrons in the gas increases like an avalanche, so that the air loses its insulation, and the gap forms a discharge channel, resulting in a "breakdown" phenomenon. At this time, the gas forms a luminous body, that is, "spark". With its thermal expansion, there is also a "patting" sound. The temperature of this spark can be as high as 2000 ~ 3000℃, which is enough to ignite the mixture in the cylinder combustion chamber