A spark plug (sometimes, in British English, a sparking plug, and, colloquially, a plug) is a device for delivering electric current from an ignition system to the combustion chamber of a spark-ignition engine to ignite the compressed fuel/air mixture by an electric spark, while containing combustion pressure within the engine. A spark plug has a metal threaded shell, electrically isolated from a central electrode by a ceramic insulator. The central electrode, which may contain a resistor, is connected by a heavily insulated wire to the output terminal of an ignition coil or magneto. The spark plug's metal shell is screwed into the engine's cylinder head and thus electrically grounded. The central electrode protrudes through the porcelain insulator into the combustion chamber, forming one or more spark gaps between the inner end of the central electrode and usually one or more protuberances or structures attached to the inner end of the threaded shell and designated the side, earth, or ground electrode(s). Spark plugs may also be used for other purposes; in Saab Direct Ignition when they are not firing, spark plugs are used to measure ionization in the cylinders – this ionic current measurement is used to replace the ordinary cam phase sensor, knock sensor and misfire measurement function. Spark plugs may also be used in other applications such as furnaces wherein a combustible fuel/air mixture must be ignited. In this case, they are sometimes referred to as flame igniters.
History In 1860 Étienne Lenoir used an electric spark plug in his gas engine, the first internal combustion piston engine. Lenoir is generally credited with the invention of the spark plug.
Early patents for spark plugs included those by Nikola Tesla (in U.S. patent 609,250 for an ignition timing system, 1898), Frederick Richard Simms (GB 24859/1898, 1898) and Robert Bosch (GB 26907/1898). In 1902, Robert Bosch's engineer Gottlob Honold invented the first commercially viable high-voltage spark plug as part of a magneto-based ignition system. This finally made possible the development of the spark-ignition engine. Subsequent manufacturing improvements can be credited to Albert Champion, to the Lodge brothers, sons of Sir Oliver Lodge, who developed and manufactured their father's idea and also to Kenelm Lee Guinness, of the Guinness brewing family, who developed the KLG brand. Helen Blair Bartlett played a vital role in making the insulator in 1930.
Operation
The function of a spark plug is to produce a spark at the required time to ignite the combustible mixture. The plug is connected to the high voltage generated by an ignition coil or magneto. As current flows from the coil, a voltage develops between the central and side electrodes. Initially 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. The ionized gas becomes a conductor and allows current to flow across the gap. Spark plugs usually require voltage of 12,000–25,000 volts or more to "fire" properly, although it can go up to 45,000 volts. They supply higher current during the discharge process, resulting in a hotter and longer-duration spark. 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" heard when observing a spark, similar to lightning and thunder. The heat and pressure force the gases 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 delayed, and a large one as though the timing was advanced.
Spark plug construction A spark plug is composed of a shell, insulator and the central conductor. It passes through 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. Spark plugs are specified by size, either thread or nut (often referred to as Euro), sealing type (taper or crush washer), and spark gap. Common thread (nut) sizes in Europe are 10 mm (16 mm), 14 mm (21 mm; sometimes, 16 mm), and 18 mm (24 mm, sometimes, 21 mm). In the United States, common thread (nut) sizes are 10mm (16mm), 12mm (14mm, 16mm or 17.5mm), 14mm (16mm, 20.63mm) and 18mm (20.63mm).
Parts of the plug
Terminal The top of the spark plug contains a terminal to connect to the ignition system. Over the years, variations in the terminal configuration have been introduced by manufacturers. The exact terminal construction varies depending on the use of the spark plug. Most passenger car spark plug wires snap onto the terminal of the plug, but some wires have eyelet connectors which are fastened onto the plug under a nut. The standard solid non-removable nut SAE configuration is common for many cars and trucks. Plugs which are used for these applications often have the end of the terminal serve a double purpose as the nut on a thin threaded shaft so that they can be used for either type of connection. This type of spark plug has a removable nut or knurl, which enables its users to attach them to two different kinds of spark plug boots. Some spark plugs have a bare thread, which is a common type for motorcycles and ATVs. Finally, in very recent years, a cup-style terminal has been introduced, which allows for a longer ceramic insulator in the same confined space.
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