Within every combustion engine, a spark plug can be found, and most spark plugs are essentially constructed the same.
Within the centre of the spark plug is an electrode that carries the current; this electrode is insulated with a glass insulator from the rest of the body of the spark plug. At the top of the plug, the electrode is exposed for the plug to be connected to the spark plug lead.
The rest of the spark plug body is made up of ceramic materials and metal. The metal portion of a spark plug is threaded to allow it to be screwed into the cylinder head; doing this causes the plug to be grounded.
Between this metal portion and the bottom of the electrode, there is a gap. This is what is known as the gap of the spark plug. The size of the gap determines the size of the spark that will be fired into the combustion chamber.
As this electrical spark is fired into the chamber, liquid fuel and air mixture is injected in at the same time. A small explosion then occurs, which drives the piston forward, generating power for the engine. This process is then repeated within the next combustion chamber, and so on down the line.
In vehicles, the spark itself is originally generated from the battery. The charge starts out small, but is changed into a higher voltage through the coil. The timing of the spark is controlled by the distributor cap, which will open and close switches to allow the charge from the coil to go to the spark plugs in the correct sequence.
The secondary purpose of the spark plug is to draw heat away from the combustion chamber, after it has sparked the explosion within the chamber. When a spark plug is designed by the manufacturer, it is rated based upon its performance at removing this thermal energy, as well as the temperature that it needs to operate at.
If the temperature inside the chamber is too low, the spark plug will be fouled. If the temperature is too high, it will cause the fuel to ignite before it is time. This is known as pre-ignition.
The rating of the spark plug is a measure of its performance at removing the heat, and its optimal temperature. The heat range of the spark plugs is noted by their manufacturers by different numbers, depending on if the spark plug is designed to work within what is termed "hot" or "cold" temperature ranges.
The thickness of the ceramic body, the length and insulation of the electrical conductor and the gap between the grounding portion of the spark plug and the end of the conductor will determine whether the plug will be a hot or cold type.
Different manufacturers use a different scale when marking their spark plugs for sale, with some using ascending numbers for hotter temperatures, and some doing the opposite. It is recommended to refer back to your vehicles service manual when charging your plugs to make sure the right spark plugs are installed.