The high voltage generated in the secondary winding of the ignition coil produces a spark between the center and ground electrodes of the spark plug in order to ignite the air-fuel mixture that is compressed in the cylinder.
The explosion of the air-fuel mixture by a spark from the spark plug is generally called combustion. Combustion, however, does not occur in an instant, but proceeds as described below. The spark travels through the air-fuel mixture from the center electrode to the ground electrode. As a result, the air-fuel mixture is activated along the path of the spark, reacts chemically (through oxidation), and generates heat to form a so-called flame nucleus. The flame nucleus activates the surrounding air-fuel mixture, which further activates the surrounding air-fuel mixture. Thus, the heat of the flame nucleus expands outward in a process known as flame propagation, to burn the air-fuel mixture. If the temperature of the electrodes is too low or the spark plug gap is too small, the electrodes will absorb the heat that was generated by the spark. As a result, the flame nucleus is extinguished, causing a misfire. This phenomenon is called electrode quenching. If the quenching effect of the electrodes is great due to the heat generated by the flame nucleus, the flame nucleus will be extinguished. The smaller the electrode is, the lesser the quenching function will be. And the squarer the electrode is, the easier the discharge will be. Some spark plugs have a U-shaped groove in the ground electrode or a V-shaped groove in the center electrode in order to improve ignitability. Those spark plugs provide a smaller quenching effect than the spark plugs without grooved electrodes, which allows the flame to form a large nucleus. Also, there are some spark plugs that reduce the quenching effect by providing thinner electrodes.