Editing Spark plug (section)

==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&nbsp;mm (16&nbsp;mm), 14&nbsp;mm (21&nbsp;mm; sometimes, 16&nbsp;mm), and 18&nbsp;mm (24&nbsp;mm, sometimes, 21&nbsp;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).<ref>2015 Champion Master Spark Plug Applications Catalog, p. VI</ref>

===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>

===Spark plug gap===

[[File:Spark plug gauge.jpg|right|thumb|''Gap gauge:'' A disk with a tapered edge; the edge is thicker going counter-clockwise, and a spark plug will be hooked along the edge to check the gap.]]

Spark plugs are typically designed to have a spark gap which can be adjusted by the technician installing the spark plug, by bending the ground electrode slightly. The same plug may be specified for several different engines, requiring a different gap for each. Spark plugs in automobiles generally have a gap between {{convert|0.6|and|1.8|mm|in|abbr=on}}. The gap may require adjustment from the out-of-the-box gap.

A ''spark plug gap [[Gauge (engineering)|gauge]]'' is a disc with a sloping edge, or with round wires of precise diameters, and is used to measure the gap. Use of a [[feeler gauge]] with flat blades instead of round wires, as is used on [[distributor]] points or [[poppet valve|valve]] lash, will give erroneous results, due to the shape of spark plug electrodes.{{citation needed|date=May 2012}} The simplest gauges are a collection of keys of various thicknesses which match the desired gaps and the gap is adjusted until the key fits snugly. With current engine technology, universally incorporating solid state ignition systems and computerized [[fuel injection]], the gaps used are larger on average than in the era of [[carburetor]]s and breaker point distributors, to the extent that spark plug gauges from that era cannot always measure the required gaps of current cars.{{citation needed|date=April 2012}}<ref>For example, in the 1967 Champion spark plug catalog, the "Deluxe Gap Tool & Gauges" on p. 38 is designed to handle gaps from {{convert|0.015|to|0.040|in|mm|abbr=on|order=flip}}, which is less than the gap required by many modern cars.  As for older cars before c. 1960, notice the vintage vehicle section of the 1997 AC Delco Spark Plug Catalog, page 250 to 264.  Gaps in the 1920s for many makes were often {{convert|0.025|in|mm|abbr=on|order=flip}}.  However, many modern cars have gaps not much larger, such as those made by Volvo from 1967 to 2014 normally had gaps of {{convert|0.028|to|0.030|in|mm|abbr=on|order=flip}}.  See this make's listings in the 2015 Champion Master Spark Plug Application Catalog, pp. 333 to 339, for which the only exception were some 4.4 liter engines.</ref>  Vehicles using compressed natural gas generally require narrower gaps than vehicles using gasoline.<ref>For example, the Ford Crown Victoria's 4.6 liter engine required a {{convert|0.044|in|mm|abbr=on|order=flip}} gap when using CNG, but requires a {{convert|0.054|in|mm|abbr=on|order=flip}} gap when using gas.  See the 2015 Champion Master Spark Plug Application Catalog, p. 124; a technical explanation is found on p. 825.</ref>

The gap adjustment (also called "spark plug gapping") can be crucial to proper engine operation. A narrow gap may give too small and weak a spark to effectively ignite the fuel-air mixture, but the plug will almost always fire on each cycle. A gap that is too wide might prevent a spark from firing at all or may misfire at high speeds, but will usually have a spark that is strong for a clean burn. A spark which intermittently fails to ignite the fuel-air mixture may not be noticeable directly, but will show up as a reduction in the engine's power and [[fuel efficiency]]. Gap adjustment is not recommended for iridium and platinum spark plugs, because there is a risk of damaging a metal disk welded to the electrode.<ref>{{Cite web |date=2022-01-27 |title=How to Choose Proper Spark Plugs for Your Engine |url=https://vinsonar.com/help/spark-plug-guide/ |access-date=2022-05-12 |website=VIN Sonar {{!}} help - Automotive Guides and Online Tools |language=en-US}}</ref> 

===Variations on the basic design===
[[File:sparkplug3.jpg|thumb|upright=0.50|Spark plug with two side (ground) electrodes]]
Over the years variations on the basic spark plug design have attempted to provide either better ignition, longer life, or both. Such variations include the use of two, three, or four equally spaced ground electrodes surrounding the central electrode. Other variations include using a recessed central electrode surrounded by the spark plug thread, which effectively becomes the ground electrode (see "surface-discharge spark plug", below). Also there is the use of a V-shaped notch in the tip of the ground electrode. Multiple ground electrodes generally provide longer life, as when the spark gap widens due to electric discharge wear, the spark moves to another closer ground electrode. The disadvantage of multiple ground electrodes is that a shielding effect can occur in the engine combustion chamber inhibiting the flame face as the fuel air mixture burns. This can result in a less efficient burn and increased fuel consumption.  They also are difficult or nearly impossible to adjust to another uniform gap size.

===Surface-discharge spark plug===
A piston engine has a part of the combustion chamber that is always out of reach of the piston; and this zone is where the conventional spark plug is located.  A [[Wankel engine]] has a permanently varying combustion area; and the spark plug is inevitably swept by the rotor's apex seals. If a  spark plug were to protrude into the Wankel's combustion chamber it would be hit by the passing apex seal, but if the plug were recessed to avoid this, mixture access to the spark would be reduced, leading to misfire or incomplete combustion. So a new type of "surface discharge" plug was developed, presenting an almost flat face to the combustion chamber.  A stubby centre electrode projects only very slightly, and the entire earthed body of the plug acts as the side electrode. The electrodes thus sit just beyond the reach of the passing apex seal, while the spark is accessible to the fuel/air mixture. The arc gap remains constant throughout the entire service life of a surface-gap spark plug, {{citation needed span|and the spark path will continually vary (instead of darting from the centre to the side electrode as in a conventional plug).|date=March 2021}} A further advantage of the surface-gap design is that the side electrode cannot break off and potentially cause engine damage, though this also doesn't often happen with conventional spark plugs.{{CN|date=March 2021}}

===Sealing to the cylinder head===
[[File:Spark plugs old and replacement 7058.JPG|thumb|Old spark plug removed from a car, new one ready to install.]]
Most spark plugs seal to the cylinder head with a single-use hollow or folded metal washer which is crushed slightly between the flat surface of the head and that of the plug, just above the threads. Some spark plugs have a tapered seat that uses no washer. The torque for installing these plugs is supposed to be lower than a washer-sealed plug.<ref>For example, notice the torque recommendations on p. 823 of the 2015 Champion Master Spark Plug Application Catalog.</ref>  Spark plugs with tapered seats should never be installed in vehicles with heads requiring washers, and vice versa.  Otherwise, a poor seal or incorrect reach would result because of the threads not properly seating in the heads.

===Tip protrusion===
[[File:67d2scd.jpg|thumb|right|Different spark plug sizes.
The left and right plug are identical in threading, electrodes, tip protrusion, and heat range. The centre plug is a compact variant, with smaller hex and porcelain portions outside the head, to be used where space is limited. The rightmost plug has a longer threaded portion, to be used in a thicker [[cylinder head]].]] The length of the threaded portion of the plug should be closely matched to the thickness of the head. If a plug extends too far into the combustion chamber, it may be struck by the piston, damaging the engine internally. Less dramatically, if the threads of the plug extend into the combustion chamber, the sharp edges of the threads act as point sources of heat which may cause [[pre-ignition]]; in addition, deposits which form between the exposed threads may make it difficult to remove the plugs, even damaging the threads on aluminium heads in the process of removal. 

The protrusion of the tip into the chamber also affects plug performance, however; the more centrally located the spark gap is, generally the better the ignition of the air-fuel mixture will be, although experts believe the process is more complex and dependent on combustion chamber shape. On the other hand, if an engine is "burning oil", the excess oil leaking into the combustion chamber tends to foul the plug tip and inhibit the spark; in such cases, a plug with less protrusion than the engine would normally call for often collects less [[fouling]] and performs better, for a longer period. Special "anti-fouling" adapters are sold which fit between the plug and the head to reduce the protrusion of the plug for just this reason, on older engines with severe oil burning problems; this will cause the ignition of the fuel-air mixture to be less effective, but in such cases, this is of lesser significance.