In the standard of spark plugs, the thermal value is usually used to characterize the thermal characteristics of the spark plug. The thermal value of the spark plug indicates the balance of the heat absorption and heat dissipation of the spark plug insulator skirt, and the higher the calorific value. The stronger the balance between heat absorption and heat dissipation, the lower the heat value of the hot spark plug and the higher the heat value of the cold spark plug. Generally, engines with high power and large compression ratio use cold spark plugs with high calorific value; on the contrary, engines with low power and small compression ratio select hot spark plugs with low calorific value. The selection of general spark plugs is determined by the factory through product stereotypes experiments, and should not be replaced at will.