The thermal characteristics of a spark plug are usually characterized by the calorific value of the spark plug, which indicates the balance between heat absorption and heat dissipation of the skirt of the spark plug insulator, and the higher the calorific value. The stronger the balance of heat absorption and heat dissipation, the lower the calorific value of the hot spark plug and the higher the calorific value of the cold spark plug. Generally, the engine with high power and large compression ratio uses a cold spark plug with high calorific value, on the contrary, the engine with low power and small compression ratio chooses a hot spark plug with low calorific value. Generally, the selection of spark plugs is determined by the factory through product finalization experiments, and should not be replaced at will.