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".

Spark plugs are installed on the side or top of the engine. In the early days, spark plugs were connected to the power distributor through cylinder wires. In the past decade, engines in small cars have mostly been changed to have ignition coils directly connected to spark plugs. The working voltage of the spark plug is at least 10000V, and the high-voltage electricity is generated by the ignition coil from 12V electricity, which is then transmitted to the spark plug.