How a spark-plug works

A spark-plug produces an electric arc, referred to as a “spark”, which ignites the fuel/air mixture in an engine’s combustion chambers.
Spark-plugs are made up of a metal rod, called an electrode, passing through the centre of a ceramic insulator. On the lower half of the ceramic section is a metal casing with a screw thread. The thread is screwed into the engine’s cylinder head. This threaded part also features another electrode which by contact with the cylinder head becomes electrically earthed. The two electrodes are separated by a very small gap.
A high-tension (HT) current is provided either by the distributor, or in most modern cars an electronic “distributor-less” ignition system. This current flows down the spark-plug’s central electrode and jumps across the gap between the electrodes forming a spark.

Different types of spark-plugs

Spark-plugs are classified according to their heat range. This is the ability to transfer the heat of the tip of the central electrode and dissipate it the engine’s cooling system.
This is why there are “hot” and “cold” type spark-plugs. The “cold” plugs have short insulators, so there is a shorter distance for the heat to travel. It therefore transfers heat more quickly and so is generally used for high performance engines and constant high speed running. In a standard, cooler-running engine, they tend to foul up which dulls their performance considerably.
The “hot” plugs have longer insulators and take longer to transfer heat from the tip, thus functioning at higher temperatures in order to compensate for the cooler running of the engine. This type of plug is unsuitable for a tuned or high performance engine, as it would over-heat and cause pre-ignition.

Maintaining optimum performance

In order to obtain good engine performance, the spark must be large enough to ignite the fuel/air mixture efficiently. This requires the gap between the electrodes to be fairly wide. If the gap is wider, it will require a higher voltage to produce a spark. On many modern cars, plugs usually have a recommended gap ranging from 0.5mm to 1mm. This clearance must be checked on a regular basis, as the electrodes slowly become eroded and in many cases, coated with deposits.
Carbon deposits from combustion can form around the ceramic surrounding the central electrode. They divert some of the ignition energy towards the earth because they provide conductivity and thus, a lower resistance to the flow of electrons than the gap between the electrodes. Electrons will always take the easiest path…
A loose spark-plug will not provide a gas-tight seal for the combustion chamber, and will impair engine performance drastically.
Spark-plugs should not be over-tightened when fitted.

Variations of design

It is possible to buy spark-plugs with multiple electrodes, usually three on the external threaded section of the plug. A plug like this will last longer than the traditional design but is usually more expensive to buy.
Many modern plugs feature exotic materials such as gold, silver and platinum! Silver is a better thermal conductor and platinum resists corrosion.