spark plugs
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A spark plug (sometimes, in
British English British English (BrE, en-GB, or BE) is, according to Oxford Dictionaries, " English as used in Great Britain, as distinct from that used elsewhere". More narrowly, it can refer specifically to the English language in England, or, more broa ...
, a sparking plug, and, colloquially, a plug) is a device for delivering electric current from an
ignition system An ignition system generates a spark or heats an electrode to a high temperature to ignite a fuel-air mixture in spark ignition internal combustion engines, oil-fired and gas-fired boilers, Rocket engine#Ignition, rocket engines, etc. The widest ...
to the
combustion chamber A combustion chamber is part of an internal combustion engine An internal combustion engine (ICE or IC engine) is a heat engine in which the combustion of a fuel occurs with an oxidizer (usually air) in a combustion chamber that is an integra ...

combustion chamber
of a
spark-ignition engine A spark-ignition engine (SI engine) is an internal combustion engine An internal combustion engine (ICE or IC engine) is a heat engine in which the combustion of a fuel occurs with an oxidizer (usually air) in a combustion chamber that is an ...
to ignite the compressed fuel/air mixture by an
electric spark An electric spark is an abrupt electrical discharge that occurs when a sufficiently high electric field creates an Ionization, ionized, Electric current, electrically conductive channel through a normally-insulating medium, often air or other ga ...
, while containing combustion pressure within the engine. A spark plug has a metal threaded shell, electrically isolated from a central
electrode An electrode is an electrical conductor used to make contact with a nonmetallic part of a Electronic circuit, circuit (e.g. a semiconductor, an electrolyte, a vacuum or air). Electrodes are essential parts of Electric battery, batteries that can ...

electrode
by a
ceramic A ceramic is any of the various hard, brittle, heat-resistant and corrosion-resistant materials made by shaping and then firing an inorganic, nonmetallic material, such as clay, at a high temperature. Common examples are earthenware, porcelain, ...

ceramic
insulator. The central electrode, which may contain a
resistor A resistor is a passivity (engineering), passive terminal (electronics), two-terminal electronic component, electrical component that implements electrical resistance as a circuit element. In electronic circuits, resistors are used to reduce c ...

resistor
, is connected by a heavily insulated wire to the output terminal of an
ignition coil An ignition coil (also called a spark coil) is an induction coil An induction coil or "spark coil" (archaism, archaically known as an inductorium or Ruhmkorff coil after Heinrich Rühmkorff) is a type of electrical transformer used to pro ...

ignition coil
or
magneto A magneto is an electrical generator that uses magnet, permanent magnets to produce periodic pulses of alternating current. Unlike a dynamo, a magneto does not contain a Commutator (electric), commutator to produce direct current. It is catego ...
. The spark plug's metal shell is screwed into the engine's
cylinder head In an internal combustion engine, the cylinder head (often abbreviated to simply "head") sits above the cylinder (engine), cylinders and forms the roof of the combustion chamber. In sidevalve engines, the head is a simple sheet of metal; whereas ...
and thus electrically grounded. The central electrode protrudes through the porcelain insulator into the
combustion chamber A combustion chamber is part of an internal combustion engine An internal combustion engine (ICE or IC engine) is a heat engine in which the combustion of a fuel occurs with an oxidizer (usually air) in a combustion chamber that is an integra ...

combustion chamber
, forming one or more
spark gap A spark gap consists of an arrangement of two Conductor (material), conducting electrodes separated by a gap usually filled with a gas such as air, designed to allow an electric spark to pass between the conductors. When the potential difference ...
s 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 Saab Direct Ignition is a capacitor discharge ignition developed by Saab Automobile, then known as Saab-Scania, and ''Mecel, Mecel AB'' during the 1980s. It was first shown in 1985 and put into series production in the Saab 9000 in 1988. One of ...
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 A gas engine is an internal combustion engine An internal combustion engine (ICE or IC engine) is a heat engine in which the combustion of a fuel occurs with an oxidizer (usually air) in a combustion chamber that is an integral part of the ...
, 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 Nikola Tesla ( ; ,"Tesla"
''Random House Webster's Unabridged Dictionary''.
; 1856 – 7 January 1943 ...

Nikola Tesla
(in for an ignition timing system, 1898),
Frederick Richard Simms Frederick Richard Simms (12 August 1863 – 22 April 1944)M.I.M.E., M.I.A.E., M.I.Ae.E., M.S.E.; Member of the Institution of Mechanical Engineers, Member of the Motor Industry Research Association#History, Institution of Automobile Enginee ...
(GB 24859/1898, 1898) and
Robert Bosch Robert Bosch (23 September 1861 – 12 March 1942) was a German industrialist, engineer and inventor, founder of Robert Bosch GmbH. Biography Bosch was born in Langenau, Albeck, a village to the northeast of Ulm in southern Germany as the elev ...
(GB 26907/1898). Only the invention of the first commercially viable high-voltage spark plug as part of a
magneto A magneto is an electrical generator that uses magnet, permanent magnets to produce periodic pulses of alternating current. Unlike a dynamo, a magneto does not contain a Commutator (electric), commutator to produce direct current. It is catego ...
-based
ignition system An ignition system generates a spark or heats an electrode to a high temperature to ignite a fuel-air mixture in spark ignition internal combustion engines, oil-fired and gas-fired boilers, Rocket engine#Ignition, rocket engines, etc. The widest ...
by Robert Bosch's engineer
Gottlob Honold image:Honold2.jpg, 220px, Gottlob Honold 1910 Gottlob Honold (26 August 1876 – 17 March 1923) was the leading engineer in the workshop of Robert Bosch and, with Bosch, was the inventor of the spark plug and the modern internal combustion engine, ...
in 1902 made possible the development of the
spark-ignition engine A spark-ignition engine (SI engine) is an internal combustion engine An internal combustion engine (ICE or IC engine) is a heat engine in which the combustion of a fuel occurs with an oxidizer (usually air) in a combustion chamber that is an ...
. Subsequent manufacturing improvements can be credited to Albert Champion, to the Lodge brothers, sons of Sir
Oliver Lodge Sir Oliver Joseph Lodge, (12 June 1851 – 22 August 1940) was a British physicist and writer involved in the development of, and holder of key patents for, radio. He identified electromagnetic radiation independent of Heinrich Rudolf Hertz, H ...
, who developed and manufactured their father's idea and also to
Kenelm Lee Guinness Kenelm Edward Lee Guinness Member of the Most Excellent Order of the British Empire, MBE (14 August 1887 – 10 April 1937) was a London-born racing driver of the 1910s and 1920s mostly associated with Sunbeam Motor Car Company, Sunbeam racin ...

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 An ignition coil (also called a spark coil) is an induction coil An induction coil or "spark coil" (archaism, archaically known as an inductorium or Ruhmkorff coil after Heinrich Rühmkorff) is a type of electrical transformer used to pro ...

ignition coil
or
magneto A magneto is an electrical generator that uses magnet, permanent magnets to produce periodic pulses of alternating current. Unlike a dynamo, a magneto does not contain a Commutator (electric), commutator to produce direct current. It is catego ...
. 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 Ionization, or Ionisation is the process by which an atom or a molecule acquires a negative or positive Electric charge, charge by gaining or losing electrons, often in conjunction with other chemical changes. The resulting electrically charged a ...
. 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 Lightning is a naturally occurring electrostatic discharge during which two electric charge, electrically charged regions, both in the atmosphere or with one on the land, ground, temporarily neutralize themselves, causing the instantaneous ...
and
thunder Thunder is the sound caused by lightning. Depending upon the distance from and nature of the lightning, it can range from a long, low rumble to a sudden, loud crack. The sudden increase in temperature and hence pressure caused by the lightning pr ...
. 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 A spark gap consists of an arrangement of two Conductor (material), conducting electrodes separated by a gap usually filled with a gas such as air, designed to allow an electric spark to pass between the conductors. When the potential difference ...
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 In a spark ignition internal combustion engine, ignition timing is the timing, relative to the current piston position and crankshaft angle, of the release of a Electric spark, spark in the combustion chamber near the end of the compression stroke ...
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 A combustion chamber is part of an internal combustion engine An internal combustion engine (ICE or IC engine) is a heat engine in which the combustion of a fuel occurs with an oxidizer (usually air) in a combustion chamber that is an integra ...

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 An ignition system generates a spark or heats an electrode to a high temperature to ignite a fuel-air mixture in spark ignition internal combustion engines, oil-fired and gas-fired boilers, Rocket engine#Ignition, rocket engines, etc. The widest ...
. 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.


Insulator

The main part of the insulator is typically made from
sintered image:LDClinkerScaled.jpg, Clinker (cement), Clinker nodules produced by sintering Sintering or frittage is the process of compacting and forming a solid mass of material by pressure or heat without melting it to the point of liquefaction. Si ...
alumina (Al2O3), a very hard ceramic material with high dielectric strength, printed with the manufacturer's name and identifying marks, then 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. 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.


Insulator tip

On modern (post 1930s) spark plugs, the tip of the insulator protruding into the combustion chamber is the same sintered aluminium oxide (alumina)
ceramic A ceramic is any of the various hard, brittle, heat-resistant and corrosion-resistant materials made by shaping and then firing an inorganic, nonmetallic material, such as clay, at a high temperature. Common examples are earthenware, porcelain, ...

ceramic
as the upper portion, merely unglazed. It is designed to withstand and 60 kV. Older spark plugs, particularly in aircraft, used an insulator made of stacked layers of
mica Micas ( ) are a group of silicate minerals whose outstanding physical characteristic is that individual mica crystals can easily be split into extremely thin elastic plates. This characteristic is described as perfect Cleavage (crystal), basa ...
, compressed by tension in the centre electrode. With the development of 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 Siemens AG ( ) is a German Multinational corporation, multinational Conglomerate (company), conglomerate corporation and the largest industrial manufacturing company in Europe headquartered in Munich with branch offices abroad. The principal ...
in Germany to counteract this. 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 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
seals Seals may refer to: * Pinniped Pinnipeds (pronounced ), commonly known as seals, are a widely range (biology), distributed and diverse clade of carnivorous, fin-footed, semiaquatic, mostly marine mammal, marine mammals. They comprise the ext ...
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
braze Brazing is a metal A metal (from ancient Greek, Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and conducts electrical resist ...
. 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 , such for automotive and small engine applications. Also, a marine spark plug's shell is double-dipped, zinc-chromate coated metal.


Central electrode

The central electrode is connected to the terminal through an internal wire and commonly a ceramic series resistance to reduce emission of 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 Copper is a chemical element with the Symbol (chemistry), symbol Cu (from la, cuprum) and atomic number 29. It is a soft, malleable, and ductility, ductile metal with very high thermal conductivity, thermal and electrical conductivity. A fre ...
,
nickel Nickel is a chemical element with Chemical symbol, symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel is a hard and Ductility, ductile transition metal. Pure nickel is chemically reactive bu ...
-
iron Iron () is a chemical element with Symbol (chemistry), symbol Fe (from la, Wikt:ferrum, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 element, group 8 of the periodic table. It is, Abundance ...
,
chromium Chromium is a chemical element with the Symbol (chemistry), symbol Cr and atomic number 24. It is the first element in Group 6 element, group 6. It is a steely-grey, Luster (mineralogy), lustrous, hard, and brittle transition metal. Chromium me ...
, or
noble metal A noble metal is ordinarily regarded as a metallic chemical element that is generally resistant to corrosion and is usually found in nature in its native element, raw form. Gold, platinum, and the other platinum group metals (ruthenium, rhodium ...
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 A cathode is the electrode from which a conventional current leaves a polarized electrical device. This definition can be recalled by using the mnemonic ''CCD'' for ''Cathode Current Departs''. A conventional current describes the direction in ...
, i.e. negative polarity 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 Thermionic emission is the liberation of electrons from an electrode by virtue of its temperature (releasing of energy supplied by heat). This occurs because the thermal energy given to the charge carrier overcomes the work function of the mater ...
).International Harvester
Truck Service Manual TM 5-4210-230-14&P-1
- Electrical - Ignition Coils and Condensers, CTS-2013-E p. 5 (PDF page 545)
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 A corona discharge is an electrical discharge caused by the ionization of a fluid such as air surrounding a conductor (material), conductor carrying a high voltage. It represents a local region where the air (or other fluid) has undergone e ...
). Using the colder, blunter side electrode as negative requires up to 45 percent higher voltage, so few ignition systems aside from
wasted spark A wasted spark system is a type of ignition system used in some four-stroke cycle internal combustion engines. In a wasted spark system, the spark plugs fire in pairs, with one plug in a cylinder on its compression stroke and the other plug in a ...
are designed this way.NGK
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. 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 Sandblasting, sometimes known as abrasive blasting, is the operation of forcibly propelling a stream of abrasive material against a surface under high pressure to smooth a surface roughness, rough surface, roughen a smooth surface, shape a su ...
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 Yttrium is a chemical element with the Symbol (chemistry), symbol Y and atomic number 39. It is a silvery-metallic transition metal chemically similar to the lanthanides and has often been classified as a "rare-earth element". Yttrium is almost ...
,
iridium Iridium is a chemical element with the Symbol (chemistry), symbol Ir and atomic number 77. A very hard, brittle, silvery-white transition metal of the platinum group, it is considered the second-densest naturally occurring metal (after osmium) ...
,
tungsten Tungsten, or wolfram, is a chemical element with the Symbol (chemistry), symbol W and atomic number 74. Tungsten is a rare metal found naturally on Earth almost exclusively as compounds with other elements. It was identified as a new element in 1 ...
,
palladium Palladium is a chemical element with the Symbol (chemistry), symbol Pd and atomic number 46. It is a rare and lustrous silvery-white metal discovered in 1803 by the English chemist William Hyde Wollaston. He named it after the 2 Pallas, asteroid ...
, or
ruthenium Ruthenium is a chemical element with the Symbol (chemistry), symbol Ru and atomic number 44. It is a rare transition metal belonging to the platinum group of the periodic table. Like the other metals of the platinum group, ruthenium is inert to ...
, as well as the relatively high value
platinum Platinum is a chemical element with the Symbol (chemistry), symbol Pt and atomic number 78. It is a density, dense, malleable, ductility, ductile, highly unreactive, precious metal, precious, silverish-white transition metal. Its name origina ...
,
silver Silver is a chemical element with the Symbol (chemistry), symbol Ag (from the Latin ', derived from the Proto-Indo-European wikt:Reconstruction:Proto-Indo-European/h₂erǵ-, ''h₂erǵ'': "shiny" or "white") and atomic number 47. A soft, whi ...
or
gold Gold is a chemical element with the Symbol (chemistry), symbol Au (from la, aurum) and atomic number 79. This makes it one of the higher atomic number elements that occur naturally. It is a Brightness, bright, slightly orange-yellow, dense, s ...
) 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 Polonium is a chemical element A chemical element is a species of atoms that have a given number of protons in their atomic nucleus, nuclei, including the pure Chemical substance, substance consisting only of that species. Unlike chemical ...
spark plugs were marketed by 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 Radium is a chemical element with the Symbol (chemistry), symbol Ra and atomic number 88. It is the sixth element in alkaline earth metal, group 2 of the periodic table, also known as the alkaline earth metals. Pure radium is silvery-white, but ...
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.


Side (ground, earth) electrode

The side electrode (also known as the "ground strap") is made from high nickel
steel Steel is an alloy made up of iron with added carbon to improve its strength of materials, strength and fracture toughness, fracture resistance compared to other forms of iron. Many other elements may be present or added. Stainless steels that ...
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.


Spark plug 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 . The gap may require adjustment from the out-of-the-box gap. A ''spark plug gap
gauge Gauge ( or ) may refer to: Measurement * Gauge (instrument), any of a variety of measuring instruments * Gauge (firearms) * Wire gauge, a measure of the size of a wire ** American wire gauge, a common measure of nonferrous wire diameter, esp ...
'' 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 A feeler gauge is a tool used to measure gap widths. Feeler gauges are mostly used in engineering to measure the clearance between two parts. Description They consist of a number of small lengths of steel of different thicknesses with measureme ...
with flat blades instead of round wires, as is used on
distributor A distributor is an enclosed rotating switch used in Spark-ignition engine, spark-ignition internal combustion engines that have mechanically timed ignition system, ignition. The distributor's main function is to route high voltage Electric c ...
points or
valve A valve is a device or Heart valve, natural object that regulates, directs or controls the flow of a fluid (gases, liquids, fluidized solids, or slurries) by opening, closing, or partially obstructing various passageways. Valves are technical ...
lash, will give erroneous results, due to the shape of spark plug electrodes. 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 Fuel injection is the introduction of fuel in an internal combustion engine, most commonly automotive engines, by the means of an injector. This article focuses on fuel injection in Reciprocating engine, reciprocating piston and Wankel engin ...
, the gaps used are larger on average than in the era of
carburetor A carburetor (also spelled carburettor) is a device used by an internal combustion engine to control and mix air and fuel entering the engine. The primary method of adding fuel to the intake air is through the venturi tube in the main meterin ...
s and breaker point distributors, to the extent that spark plug gauges from that era cannot always measure the required gaps of current cars. Vehicles using compressed natural gas generally require narrower gaps than vehicles using gasoline. 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 Fuel efficiency is a form of thermal efficiency, meaning the ratio of effort to result of a process that converts chemical energy, chemical potential energy contained in a carrier (fuel) into kinetic energy or Mechanical work, work. Overall fuel ...
. 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.


Variations on the basic design

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 The Wankel engine (, ) is a type of internal combustion engine using an Eccentric (mechanism), eccentric rotary combustion engine, rotary design to convert pressure into rotating motion. It was invented by German engineer Felix Wankel, and desi ...
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, 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.


Sealing to the cylinder head

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. 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

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 Pre-ignition (or preignition) in a spark-ignition engine is a technically different phenomenon from engine knocking In spark ignition internal combustion engine An internal combustion engine (ICE or IC engine) is a heat engine in which th ...
; 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 Fouling is the accumulation of unwanted material on solid surfaces. The fouling materials can consist of either living organisms (biofouling) or a non-living substance (inorganic or organic). Fouling is usually distinguished from other surf ...
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.


Heat range

The
operating temperature An operating temperature is the allowable temperature range of the local ambient environment at which an electrical or mechanical device operates. The device will operate effectively within a specified temperature range which varies based on the de ...
of a spark plug is the actual physical
temperature Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measurement, measured with a thermometer. Thermometers are calibrated in various Conversion of units of temperature, temp ...
at the tip of the spark plug within the running engine, normally between . This is important because it determines the efficiency of plug self-cleaning and is determined by a number of factors, but primarily the actual temperature within the combustion chamber. There is no direct relationship between the actual operating temperature of the spark plug and spark voltage. However, the level of
torque In physics and mechanics, torque is the rotational equivalent of linear force. It is also referred to as the moment of force (also abbreviated to moment). It represents the capability of a force to produce change in the rotational motion of the ...
currently being produced by the engine will strongly influence spark plug operating temperature because the maximal temperature and pressure occur when the engine is operating near peak torque output (torque and rotational speed directly determine the
power Power most often refers to: * Power (physics) In physics, power is the amount of energy transferred or converted per unit time. In the International System of Units, the unit of power is the watt, equal to one joule per second. In older work ...
output). The temperature of the insulator responds to the thermal conditions it is exposed to in the combustion chamber, but not vice versa. If the tip of the spark plug is too hot, it can cause pre-ignition or sometimes detonation/knocking, and damage may occur. If it is too cold, electrically conductive deposits may form on the insulator, causing a loss of spark energy or the actual shorting-out of the spark current. A spark plug is said to be "hot" if it is a better heat insulator, keeping more heat in the tip of the spark plug. A spark plug is said to be "cold" if it can conduct more heat out of the spark plug tip and lower the tip's temperature. Whether a spark plug is "hot" or "cold" is known as the heat range of the spark plug. The heat range of a spark plug is typically specified as a number, with some manufacturers using ascending numbers for hotter plugs, and others doing the opposite – using ascending numbers for colder plugs. The heat range of a spark plug is affected by the construction of the spark plug: the types of materials used, the length of insulator and the
surface area The surface area of a solid object is a measure of the total area Area is the quantity that expresses the extent of a region on the plane or on a curved surface. The area of a plane region or ''plane area'' refers to the area of a s ...
of the plug exposed within the combustion chamber. For normal use, the selection of a spark plug heat range is a balance between keeping the tip hot enough at idle to prevent fouling and cold enough at maximal power to prevent pre-ignition or
engine knocking In spark ignition internal combustion engine An internal combustion engine (ICE or IC engine) is a heat engine in which the combustion of a fuel occurs with an oxidizer (usually air) in a combustion chamber that is an integral part of the ...
. By examining "hotter" and "cooler" spark plugs of the same manufacturer side by side, the principle involved can be very clearly seen; the cooler plugs have a more substantial ceramic insulator filling the gap between the center electrode and the shell, effectively allowing more heat to be carried off by the shell, while the hotter plugs have less ceramic material, so that the tip is more isolated from the body of the plug and retains heat better.
Heat In thermodynamics, heat is defined as the form of energy crossing the boundary of a thermodynamic system by virtue of a temperature difference across the boundary. A thermodynamic system does not ''contain'' heat. Nevertheless, the term is al ...
from the combustion chamber escapes through the exhaust gases, the side walls of the cylinder and the spark plug itself. The heat range of a spark plug has only a minute effect on combustion chamber and overall engine temperature. A cold plug will not materially cool down an engine's running temperature. (A too hot plug may, however, indirectly lead to a runaway pre-ignition condition that ''can'' increase engine temperature.) Rather, the main effect of a "hot" or "cold" plug is to affect the temperature of the tip of the spark plug. It was common before the modern era of computerized fuel injection to specify at least a couple of different heat ranges for plugs for an automobile engine; a hotter plug for cars that were mostly driven slowly around the city, and a colder plug for sustained high-speed highway use. This practice has, however, largely become obsolete now that cars' fuel/air mixtures and cylinder temperatures are maintained within a narrow range, for purposes of limiting emissions. Racing engines, however, still benefit from picking a proper plug heat range. Very old racing engines will sometimes have two sets of plugs, one just for starting and another to be installed for driving once the engine is warmed up. Spark plug manufacturers use different numbers to denote heat range of their spark plugs. Some manufacturers, such as Denso and NGK, have numbers that become higher as they get colder. By contrast, Champion, Bosch, BRISK, Beru, and ACDelco use a heat range system in which the numbers become bigger as the plugs get hotter. As a result, heat range numbers need to be translated between the different manufacturers. The same numbers have very different meanings between different manufacturers. In this case, plugs with the same heat range numbers can't be interchanged casually as being equal. To give an extreme case, NGK's BR2LM is equivalent to Champion's RJ19LM, which is a common spark plug for many lawnmowers.


Reading spark plugs

The spark plug's firing end will be affected by the internal environment of the combustion chamber. As the spark plug can be removed for inspection, the effects of combustion on the plug can be examined. An examination, or "reading" of the characteristic markings on the firing end of the spark plug can indicate conditions within the running engine. The spark plug tip will bear the marks as evidence of what is happening inside the engine. Usually there is no other way to know what is going on inside an engine running at peak power. Engine and spark plug manufacturers will publish information about the characteristic markings in spark plug reading charts. Such charts are useful for general use but are of almost no use in reading racing engine spark plugs, which is an entirely different matter. A light brownish discoloration of the tip of the block indicates proper operation; other conditions may indicate malfunction. For example, a sandblasted look to the tip of the spark plug means persistent, light
detonation Detonation () is a type of combustion involving a supersonic exothermic front accelerating through a medium that eventually drives a shock front propagating directly in front of it. Detonations propagate supersonically through shock waves with s ...
is occurring, often unheard. The damage that is occurring to the tip of the spark plug is also occurring on the inside of the cylinder. Heavy detonation can cause outright breakage of the spark plug insulator and internal engine parts before appearing as sandblasted erosion but is easily heard. As another example, if the plug is too cold, there will be deposits on the nose of the plug. Conversely if the plug is too hot, the porcelain will be porous looking, almost like sugar. The material which seals the central electrode to the insulator will boil out. Sometimes the end of the plug will appear glazed, as the deposits have melted. An idling engine will have a different impact on the spark plugs than one running at full
throttle A throttle is the mechanism by which fluid flow is managed by constriction or obstruction. An engine's power can be increased or decreased by the restriction of inlet gases (by the use of a throttle), but usually decreased. The term ''throttle'' ...
. Spark plug readings are only valid for the most recent engine operating conditions and running the engine under different conditions may erase or obscure characteristic marks previously left on the spark plugs. Thus, the most valuable information is gathered by running the engine at high speed and full load, immediately cutting the ignition off and stopping without idling or low speed operation and removing the plugs for reading. Spark plug reading viewers, which are simply combined flashlight/magnifiers, are available to improve the reading of the spark plugs.


Indexing spark plugs

"Indexing" of plugs upon installation involves installing the spark plug so that the open area of its gap, not shrouded by the ground electrode, faces the center of the combustion chamber rather than one of its walls. The theory holds that this will maximize the exposure of the fuel-air mixture to the spark, also ensuring that every combustion chamber is even in layout and therefore resulting in better ignition. Indexing is accomplished by marking the location of the gap on the outside of the plug, installing it, and noting the direction in which the mark faces. Then the plug is removed and washers are added to change the orientation of the tightened plug. This must be done individually for each plug, as the orientation of the gap with respect to the threads of the shell is
random In common usage, randomness is the apparent or actual lack of pattern or predictability in events. A random sequence of events, symbols or steps often has no :wikt:order, order and does not follow an intelligible pattern or combination. Ind ...
. Some plugs are made with a non-random orientation of the gap and are usually marked as such by a suffix to the model number; typically these are specified by manufacturers of very small engines where the spark plug tip and electrodes form a significantly large part of the shape of the combustion chamber. The
Honda Insight The is a hybrid electric vehicle that is manufactured and marketed by Honda. Its first generation was a two-door, two passenger liftback (1999–2006) and in its second generation was a four-door, five passenger liftback (2009–2014). In it ...
has indexed spark plugs from factory, with four different part numbers available corresponding to the different degrees of indexing to achieve most efficient combustion and maximal fuel efficiency.


See also

* François Isaac de Rivaz – invented (around 1807) a hydrogen–oxygen internal combustion engine with electric ignition. * Étienne Lenoir – patented (1886), among internal combustion engine improvements, the first modernly recognizable spark plug. Figure 6. *
Ignition system An ignition system generates a spark or heats an electrode to a high temperature to ignite a fuel-air mixture in spark ignition internal combustion engines, oil-fired and gas-fired boilers, Rocket engine#Ignition, rocket engines, etc. The widest ...
*
Wasted spark A wasted spark system is a type of ignition system used in some four-stroke cycle internal combustion engines. In a wasted spark system, the spark plugs fire in pairs, with one plug in a cylinder on its compression stroke and the other plug in a ...
* Ignitor * Glowplug *
Glow plug (model engine) A glow plug engine, or glow engine, is a type of Model engine, small internal combustion engine typically used in model aircraft, model cars and similar applications. The ignition is accomplished by a combination of heating from compression, h ...
* Ninja rocks * Coso artifact


References


External links

{{DEFAULTSORT:Spark Plug Ignition systems Electric arcs Engine components