Under high voltage, the air between the center electrode and the side electrode of the spark plug will rapidly undergo ionization, 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, causing the air to lose its insulation, and the gap forms a discharge channel, resulting in "breakdown" phenomenon. At this point, the gas forms a luminous body, known as a "spark".

The working principle of a spark plug is that the insulation skirt of a standard spark plug is slightly shrunk into the end face of the housing, and the side electrode is outside the end face of the housing, making it the most widely used type. The insulation skirt of the edge protruding spark plug is longer and protrudes beyond the end face of the housing. It has the advantages of large heat absorption and good anti fouling ability, and can be directly cooled by the intake air to lower the temperature, so it is not easy to cause hot ignition, so the thermal adaptation range is wide.