Spark Plugs & Glow Plugs

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. more...

Spark plugs have an insulated center electrode which is connected by a heavily insulated wire to an ignition 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). Karl Benz is also credited with the invention.

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.

Spark plugs may also be used in other applications such as furnaces where a combustible mixture should be ignited. In this case, they are sometimes referred to as flame igniters.

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, similar to lightning and thunder.

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