Spark plug: Difference between revisions

A spark plug (sometimes, in British English, a sparking plug,^[1] 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.^[2] 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.^{[citation needed]}

Purpose

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The purpose of a spark plug is to convert the high-voltage electricity (generated by an ignition coil in modern engines) into a spark. The spark occurs inside the combustion chamber and ignites the air/fuel mixture shortly before the combustion stroke.

Design

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The main elements of a spark plug are the shell, insulator and central electrode. The main part of the insulator is typically made from sintered alumina (Al₂O₃), a hard ceramic material with high dielectric strength.^[3][4][5] In marine engines, the shell of the spark plug is often a double-dipped, zinc-chromate coated metal.^[6]

The central electrode is connected to the terminal through an internal wire. The central electrode is usually the one designed to eject the electrons (the cathode, i.e. negative polarity^[7] 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).^[8] 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).^[8] Using the colder, blunter side electrode as negative requires up to 45 percent higher voltage,^[8] so few ignition systems (aside from wasted spark systems) are designed this way.^[9]

A spark plug passes through the wall of the combustion chamber, therefore it must also form part of the seal for the high-pressure gasses within the combustion chamber.

Gap size

The distance between the tip of the spark plug and the central electrode is called the "spark plug gap" and is a key factor in the function of a spark plug. Spark plug gaps for car engines are typically 0.6 to 1.8 mm (0.024 to 0.071 in).^{[citation needed]} Modern engines (using solid-state ignition systems and electronic fuel injection) typically use larger gaps than older engines that use breaker point distributors and carburetors.

Smaller plug gap sizes usually are more reliable at producing a spark, however the spark may be too weak to ignite the fuel-air mixture. A larger plug gap size will produce a stronger spark, however the spark might not always be produced (such as at high RPM). 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.^[10]

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.^[11]

Thread sizes

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).^[12]

Wasted spark applications

Wasted 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.^[13]

Tip location

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; 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.

Indexing

"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. 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 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.^{[citation needed]}

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 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, and the spark path will continually vary (instead of darting from the centre to the side electrode as in a conventional plug).^{[citation needed]} 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.^{[citation needed]}

Heat range

An important factor for a spark plug is the temperature that the tip is designed to withstand, called the heat range. Typical heat ranges for passenger car engines are usually between 500 and 850 °C (932 and 1,562 °F).^[14][15] A hotter spark plug has more insulation between itself and the cylinder head, causing less heat to be dissipated from the spark plug and therefore the spark plug remaining hotter.^[16] Temperatures higher than 450 °C (842 °F) are needed to prevent carbon build-up on the spark plug, while temperatures over 800 °C (1,470 °F) can cause overheating of the plug.^[17]

Switching to a higher heat range is sometimes used to compensate for fuel delivery or oil consumption problems, however this increases the risk of pre-ignition.^[18]

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 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 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 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. 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 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.^[19]

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.^[why?]

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 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. 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.^{[citation needed]}

Spark plug reading viewers, which are simply combined flashlight/magnifiers, are available to improve the reading of the spark plugs.

History

Belgian-French engineer Étienne Lenoir is generally credited with the invention of the spark plug in 1860, due to its use in the early Lenoir gas engine.^[20][21] However, some historians claim that the spark plug was invented in 1839 by Edmond Berger, an immigrant from Togo.^{[22][23][better source needed]}

Several patents for spark plugs were filed in 1898, including from Serbian engineer Nikola Tesla,^[24] British engineer Frederick Richard Simms (British patent number 24859/1898)^{[citation needed]} and German engineer Robert Bosch (British patent number 26907/1898).^{[citation needed]} However, the use of high-voltage spark plugs in commercial viable engines was only made possible after the 1908 invention of magneto-based ignition systems by Bosch engineer Gottlob Honold in 1902. Early manufacturers of spark plugs included American company Champion,^[25] British company Lodge brothers^[26] and London-based KLG (who pioneed the use of mica as an insulator).

During the 1930s, American geologist Helen Blair Bartlett developed an alumina ceramic-based insulator for the spark plug.^[27]

Polonium 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 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.^[28][29]

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.^[30][31]
• Ignition system
• Wasted spark
• Ignitor
• Glowplug
• Glow plug (model engine)
• Ninja rocks
• Coso artifact

References

Wikibooks has a book on the topic of: Automobile Repair/Spark plugs

Wikimedia Commons has media related to Spark plugs.