A spark plug (sometimes in British English, a sparking plug) is an electrical device that fits into the cylinder head of some internal combustion engines and ignites compressed aerosol gasoline by means of an electric spark. Spark plugs have an insulated center electrode which is connected by a heavily insulated wire to an induction coil or magneto circuit on the outside, forming, with a grounded terminal on the base of the plug, a spark gap inside the cylinder. Early patents for spark plugs included those by Nikola Tesla ( in US patent 609,250 for an ignition timing system), (1898), Richard Simms (GB 24859/1898, 1898), and Robert Bosch (GB 26907/1898).
Internal combustion engines can be divided into spark-ignition engines, which require spark plugs to begin combustion, and compression-ignition engines (diesel engines), which compress the fuel/air mixture until it spontaneously ignites. Compression-ignition engines may use glow plugs to improve cold start characteristics.
Operation
The spark plug is connected to thousands of volts generated by the ignition coil. As the electrons are pushed in from the coil, a voltage difference appears between the center electrode and side electrode. No current can flow because the fuel and air in the gap is an insulator, but as the voltage rises further, it begins to change the structure of the gases between the electrodes. Once the voltage exceeds the dielectric strength of the gases, the gases become ionized. An ionized gas becomes a conductor and an ionized gas can pass electrons.
As the current of electrons surges across the gap, it raises the temperature of the spark channel to 60,000 K. The intense heat in the spark channel causes the ionized gas to expand very quickly, like a small explosion. This is the "click" you hear when watching a spark.
The heat and pressure force the gasses to react with each other and at the end of the spark event there should be a small ball of fire in the spark gap as the gases burn on their own. The size of this fireball or kernel depends on the exact composition of the mixture between the electrodes and the level of combustion chamber turbulence at the time of the spark. A small kernel will make the engine run as though the ignition timing was retarded and a large one like the timing was advanced for that individual cycle.
Spark plug construction
A spark plug is composed of a shell, insulator and the conductor. It pierces the wall of the combustion chamber and therefore must also seal the combustion chamber against high pressures and temperatures, without deteriorating over long periods of time and extended use.
Overview
The spark plug consists of a plated metal shell with a ceramic insulator and an electrode extending through the center of the insulator. Threads on the metal shell allow it to be screwed into the cylinder head and a short earth electrode attached to one side is bent in towards the center electrode.
The electrodes are of special alloy wire with a set recommended gap between them. The spark bridges this gap to ignite the air-fuel mixture in the combustion chamber.
Spark plugs are identified by three different features.
These are:
Source: CDX Global & Wikipedia
