Spark plug is a spare parts mounted on the internal combustion engine with the tip of the electrode in the fuel. Spark plugs installed to burn the gasoline that has been compressed by the piston. Spark plugs in the form of an electric spark. At the center of the spark plug electrodes are connected by wires to the ignition coil (ignition coil) outside the spark plugs, and with the ground at the bottom of the spark plug, forming a spark gap inside the cylinder. The patents for spark plugs provided separately to Nikola Tesla, Richard Simms, and Robert Bosch. Karl Benz is also one which is considered as the architect of the spark plug.

Internal combustion engine can be divided into the engine with spark, which require spark plugs to sprinkle a mixture of gasoline and air, and compression engine (Diesel engine), which no spark, compressing a mixture of gasoline and air until there is spark by itself (so it does not require spark plugs).

Spark plug connected to the voltage of the magnitude of thousands of volts generated by the ignition coil (ignition coil). Voltage from the ignition coil produces voltage difference between electrodes in the middle of the spark plug with an on the side. Currents can not flow because of gasoline and air in the gap are insulators, but the greater the voltage difference, the structure of gas between the electrodes is changed. When the voltage exceeds the dielectric strength than the existing gas, these gases have ionization process and that had to be insulators, turned into a conductor.

Once this happens, the flow of electrons can flow, and with the flow of electrons, the temperature at the spark plug gap rises dramatically, and 60,000 K. Very high temperatures makes the ionized gas to expand rapidly, like a small explosion. This is the spark plugs, which in principle is similar to thunder or lightning.