Editing Spark plug (section)

===Parts of the plug===

====Terminal====
The top of the spark plug contains a terminal to connect to the [[ignition system]].  Over of 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.<ref>Notice the back inside page of the 2015-2016 Champion catalog from Europe.</ref>

====Insulator====
The main part of the insulator is typically made from [[sintered]] [[alumina]] (Al<sub>2</sub>O<sub>3</sub>),<ref>{{cite web|url=http://www.globaldenso.com/en/products/aftermarket/plug/basic_knowledge/construction/index.html |title=Denso's "Basic Knowledge" page |publisher=Globaldenso.com |access-date=2011-09-17}}</ref><ref>The Bosch Automotive Handbook, 8th Edition, Bentley Publishers, copyright May 2011, {{ISBN|978-0-8376-1686-5}}, pp 581–585.</ref> a very [[hardness|hard]] ceramic material with high [[dielectric strength]], printed with the manufacturer's name and identifying marks, then [[Ceramic glaze|glazed]] to improve resistance to surface spark tracking. Its major functions are to provide mechanical support and electrical insulation for the central electrode, while also providing an extended spark path for flashover protection. This extended portion, particularly in engines with deeply recessed plugs, helps extend the terminal above the cylinder head so as to make it more readily accessible.
[[File:Spark plug insulator.jpeg|thumb|right|Dissected modern spark plug showing the one-piece [[sintered]] [[alumina]] insulator. The lower portion is unglazed.]]
A further feature of sintered alumina is its good heat conduction – reducing the tendency for the insulator to glow with heat and so light the mixture prematurely.

====Ribs====
By lengthening the surface between the high voltage terminal and the grounded metal case of the spark plug, the physical shape of the ribs functions to improve the electrical insulation and prevent electrical energy from leaking along the insulator surface from the terminal to the metal case. The disrupted and longer path makes the electricity encounter more resistance along the surface of the spark plug even in the presence of dirt and moisture. Some spark plugs are manufactured without ribs; improvements in the dielectric strength of the insulator make them less important.{{Citation needed|date=December 2011}}

====Insulator tip====
[[File:Candela rotta.jpg|thumb|right| Two spark plugs in comparison views in multiple angles, one of which is consumed regularly, while the other has the insulating ceramic broken and the central electrode shortened, due to manufacturing defects and / or temperature swing]]
On modern (post 1930s) spark plugs, the tip of the insulator protruding into the combustion chamber is the same sintered aluminium oxide (alumina) [[ceramic]] as the upper portion, merely unglazed. It is designed to withstand {{convert|650|°C|-1|abbr=on}} and 60&nbsp;kV.

Older spark plugs, particularly in aircraft, used an insulator made of stacked layers of [[mica]], compressed by tension in the centre electrode.

With the development of [[Tetraethyllead|leaded petrol]] in the 1930s, lead deposits on the mica became a problem and reduced the interval between needing to clean the spark plug. Sintered alumina was developed by [[Siemens]] in Germany to counteract this.<ref name="Banks" >
{{cite book
  |title=I Kept No Diary
  |author=Air Commodore F. R. Banks
  |year=1978
  |page=113
  |publisher=Airlife
  |isbn=0-9504543-9-7
}}</ref>  Sintered alumina is a superior material to mica or porcelain because it is a relatively good thermal conductor for a ceramic, it maintains good mechanical strength and (thermal) shock resistance at higher temperatures, and this ability to run hot allows it to be run at "self cleaning" temperatures without rapid degradation.  It also allows a simple single piece construction at low cost but high mechanical reliability. The dimensions of the insulator and the metal conductor core determine the [[#Heat range|heat range]] of the plug. Short insulators are usually "cooler" plugs, while "hotter" plugs are made with a lengthened path to the metal body, though this also depends on the thermally conductive metal core.

====Seals====
Because the spark plug also [[seal (mechanical)|seals]] the combustion chamber of the engine when installed, seals are required to ensure there is no leakage from the combustion chamber. The internal seals of modern plugs are made of compressed glass/metal powder, but old style seals were typically made by the use of a multi-layer [[brazing|braze]]. The external seal is usually a [[crush washer]], but some manufacturers use the cheaper method of a taper interface and simple compression to attempt sealing.

====Metal case/shell====
The metal case/shell (or the ''jacket'', as many people call it) of the spark plug withstands the torque of tightening the plug, serves to remove heat from the insulator and pass it on to the cylinder head, and acts as the ground for the sparks passing through the central electrode to the side electrode. Spark plug threads are cold rolled to prevent thermal cycle fatigue.  It's important to install spark plugs with the correct "reach," or thread length.  Spark plugs can vary in reach from {{convert|0.0375|to|1.043|in|cm|abbr=on|order=flip}}, such for automotive and small engine applications.<ref>For examples, see the listing in the plug type chart of the 2015 Champion master spark plug application catalog, pp. VI</ref> Also, a marine spark plug's shell is double-dipped, zinc-chromate coated metal.<ref>{{cite web|title=Marine Spark Plug Savvy|url=http://marineenginedigest.com/diy/sparkplugsavvy.htm|publisher=MarineEngineDigest.com|date=29 April 2012|access-date=1 December 2012}}</ref>

====Central electrode====
[[File:Bougie3.jpg|thumb|right|Central and lateral electrodes]]
The central electrode is connected to the terminal through an internal wire and commonly a ceramic series resistance to reduce emission of [[Radio frequency|RF]]  noise from the sparking.  Non-resistor spark plugs, commonly sold without an "R" in the plug type part number, lack this element to reduce electro-magnetic interference with radios and other sensitive equipment. The tip can be made of a combination of [[copper]], [[nickel]]-[[iron]], [[chromium]], or [[noble metal]]s.  

In the late 1970s, the development of engines reached a stage where the heat range of conventional spark plugs with solid nickel alloy centre electrodes was unable to cope with their demands.  A plug that was cold enough to cope with the demands of high speed driving would not be able to burn off the carbon deposits caused by stop–start urban conditions, and would foul in these conditions, making the engine misfire.  Similarly, a plug that was hot enough to run smoothly in town could melt when called upon to cope with extended high speed running on motorways.  The answer to this problem, devised by the spark plug manufacturers, was to use a different material and design for the centre electrode that would be able to carry the heat of combustion away from the tip more effectively than a solid nickel alloy could.  Copper was the material chosen for the task and a method for manufacturing the copper-cored centre electrode was created by [[Floform]].

The central electrode is usually the one designed to eject the electrons (the [[cathode]], i.e. negative polarity<ref name="Hillier, Spark plug" >{{Cite book
  |title=Fundamentals of Motor Vehicle Technology
  |last=V.A.W.  |first=Hillier
  |edition=4th
  |year=1991
  |publisher=Stanley Thornes
  |isbn=0-7487-05317
  |ref={{harvid|Hillier|1991}}
  |chapter=74: The ignition system
  |page=450
}}</ref> relative to the engine block) because it is normally the hottest part of the plug; it is easier to emit electrons from a hot surface, because of the same physical laws that increase emissions of vapor from hot surfaces (see [[thermionic emission]]).<ref name=ih>International Harvester, [http://www.liberatedmanuals.com/TM-5-4210-230-14-and-P-1.pdf Truck Service Manual TM 5-4210-230-14&P-1] - Electrical - Ignition Coils and Condensers, CTS-2013-E p. 5 (PDF page 545)</ref>  In addition, electrons are emitted where the electrical field strength is greatest; this is from wherever the radius of curvature of the surface is smallest, from a sharp point or edge rather than a flat surface (see [[corona discharge]]).<ref name=ih />  Using the colder, blunter side electrode as negative requires up to 45 percent higher voltage,<ref name=ih /> so few ignition systems aside from [[wasted spark]] are designed this way.<ref name=wastedspark>NGK, [https://www.ngk.com/glossary/8/spark-plug/W Wasted Spark Ignition]</ref>  Waste spark systems place a greater strain upon spark plugs since they alternately fire electrons in both directions (from the ground electrode to the central electrode, not just from the central electrode to the ground electrode).  As a result, vehicles with such a system should have precious metals on both electrodes, not just on the central electrode, in order to increase service replacement intervals since they wear down the metal more quickly in both directions, not just one.<ref>See p. 824 of the 2015 Champion Master Catalog.  http://www.fme-cat.com/catalogs.aspx {{Webarchive|url=https://web.archive.org/web/20180601144828/http://fme-cat.com/catalogs.aspx |date=2018-06-01 }}</ref>

It would be easiest to pull electrons from a pointed electrode but a pointed electrode would erode after only a few seconds.  Instead, the electrons emit from the sharp edges of the end of the electrode; as these edges erode, the spark becomes weaker and less reliable.

At one time it was common to remove the spark plugs, clean deposits off the ends either manually or with specialized [[sandblasting]] equipment and file the end of the electrode to restore the sharp edges, but this practice has become less frequent for three reasons:
# cleaning with tools such as a wire brush leaves traces of metal on the insulator which can provide a weak conduction path and thus weaken the spark (increasing emissions).
# plugs are so cheap relative to labor cost, economics dictate replacement, particularly with modern long-life plugs.
# iridium and platinum plugs that have longer lifetimes than copper have become more common.

The development of noble metal high temperature electrodes (using metals such as [[yttrium]], [[iridium]], [[tungsten]], [[palladium]], or [[ruthenium]], as well as the relatively high value [[platinum]],  [[silver]] or [[gold]]) allows the use of a smaller center wire, which has sharper edges but will not melt or corrode away.  These materials are used because of their high melting points and durability, not because of their electrical conductivity (which is irrelevant in series with the plug resistor or wires).  The smaller electrode also absorbs less heat from the spark and initial flame energy.

[[Polonium]] spark plugs were marketed by [[Firestone Tire and Rubber Company|Firestone]] from 1940 to 1953. While the amount of radiation from the plugs was minuscule and not a threat to the consumer, the benefits of such plugs quickly diminished after approximately a month because of polonium's short half-life, and because buildup on the conductors would block the radiation that improved engine performance. The premise behind the polonium spark plug, as well as [[Alfred Matthew Hubbard]]'s prototype [[radium]] plug that preceded it, was that the radiation would improve ionization of the fuel in the cylinder and thus allow the plug to fire more quickly and efficiently.<ref>{{cite web|url=https://www.orau.org/health-physics-museum/collection/consumer/miscellaneous/spark-plugs.html|title=Radioactive spark plugs|publisher=Oak Ridge Associated Universities|date=January 20, 1999|access-date=October 7, 2021}}</ref><ref>{{cite web|url=http://www.utoledo.edu/nsm/ic/elements/polonium.html|first=Cassandra|last=Pittman|title=Polonium|work=The Instrumentation Center|publisher=University of Toledo|date=February 3, 2017|access-date=August 23, 2018}}</ref>

====Side (ground, earth) electrode====
The side electrode (also known as the "ground strap") is made from high nickel [[steel]] and is welded or hot forged to the side of the metal shell. The side electrode also runs very hot, especially on projected nose plugs. Some designs have provided a copper core to this electrode, so as to increase heat conduction. Multiple side electrodes may also be used, so that they don't overlap the central electrode.  The ground electrode can also have small pads of platinum or even iridium added to them in order to increase service life.<ref>For example, notice the 2015-2016 Champion master catalog from Europe's type chart, which flips out from the back.  In many cases, depending on the design, "platinum" is listed as the metal type.</ref>