Next to contact breaker, spark plug is probably the weakest part of the ignition system. It is mounted in the combustion chamber of the engine, where working conditions are- severe. During peak combustion conditions the temperature in the combustion chamber in a modern car engine may be around 2500 ºc and the pressure about 7 Mpa. Moreover a spark plug is also exposed to thermal and load cycling fatigue due to sudden changes in temperature and pressure-from the high temperature of burnt gas to the relatively low temperature of the air/fuel mixture and from the high pressure at the time of explosion of the air/fuel mixture to low pressure during induction. In addition, the spark plug has to endure high voltage, mechanical vibration and the corrosive atmosphere of combustion gages. A modern spark plug has an economical life of about 10,000-16,000 km.
Requirements
The requirements of a good sparking plug are:
1. Very high resistance to current leakage.
2. Continued maintenance of the proper gap under all conditions.
3. Gas tightness-Any leakage of hot gas will upset the normal steady state of the plug, causing such high temperatures to be reached that the insulator and the electrode disintegrate und full into the cylinder.
4. Resistance to corrosion-corrosion is detrimental because it increases gap length and thus raises the sparking potential required.
5. sufficient 'reach' into the combustion chamber-Reach is defined as the length of the thread portion that enters the cylinder. If a long-reach plug is fitted into a short-reach hole, combustion chamber space is reduced which increases the compression ratio, more heat is produced and the exposed edges of the threaded portion may be heated to incandescence' resulting in preignition. Moreover the protruding end of a long reach plug in a short reach hole may also damage the piston or valve. If a short-reach plug, on the other hand, is fitted into-a long reach hole the combustion-chamber space is increased, compression ratio is reduced and carbon is deposited, in the threads of the hole which makes difficult the fitting of a correct reach plug at a later date.
6. Electrode temperature must be maintained between certain limits. At higher temperature (850ºC or above) there is danger of preignition and also corrosion of electrodes. At temperatures lower than 300ºC, however, carbon or even oil deposits are formed causing the leakage of electricity to earth instead of sparking across the plug electrodes. The optimum temperature range is 500-800ºC.
Construction
The plug has three main Parts, the centre electrode, the ground electrode and the insulation separating them. Besides these, these are the body shell, the sealing ring and the gasket washer. The upper end of the centre electrode is connected to the spark plug terminal, where H.T. cable from the ignition coil in case of single cylinder engines (or from distributor in case of multicylinder engines) is connected. The lower end of the centre electrode projects beyond the insulator to form a gap with the ground electrode. The insulator is meant to fulfill the following functions :
(i) to insulate the centre electrode from body shell, thereby preventing the leakage of high voltage surge from leaking to earth within the shell.
(ii) to control the working temperature of the centre electrode by suitably adopting the thermal conductivity of the insulating material, its shape and the length of the heat path, while designing the spark plug for a given engine. The body shell serves to house the electrodes and the insulator. Gas-tight seal is necessary to prevent the hot gas from leaking between the insulator and the body shell and between the insulator and the centre electrode. Such seals may be of different types, e.g. solid ring, dry powder, metal powder fused into glass, etc. Besides above the hot gas from the combustion chamber may also leak between the plug and the cylinder head. To seal this flat ring gasket washer is commonly used. In some modern spark plugs, the centre electrode is made in two pieces. By doing so the designer can use different metals to suit best the different requirements of the upper piece which has to the connected to the h.t. cable and the lower piece which has to go into (he combustion chamber. In some spark plugs, centre electrode contains a built-in resistance in series with the electrode. This resistance reduces the number of high voltage surges accompanying the full voltage surge for the spark, thereby reducing the radio and television interference due to ignition system. The resistance also increases the plug life by cutting down peak current that would burn the electrodes. Such plugs are called resistor spark plugs and their life is about double the life of ordinary plugs.
Materials
The body shell is generally made of low carbon steel. The body is formed by impact extrusion. The threads are cold rolled to provide a low-friction profile. Porcelaim was used initially for insulation, but this has the disadvantages of brittleness and low resistance to thermal shock. Porcelain was therefore replaced by mica. The use of mica, too, was restricted by leaded fuels which attacked it. Present day insulators are almost exclusively of sintered alumina. These possess some distinct advantages :
(i) They are much stronger than porcelain.
(ii) Their thermal conductivity is higher than in case of porcelain.
(iii) These possess high electrical resistivity at operating temperatures.
(iv) These can be moulded accurately and with ease, to the desired shape.
(v) These have a high resistance to abrasion, erosion and chemical attack from the products of combustion. Thus with these materials, the plug can be cleaned by sand blasting in contrast to mica and porcelain plugs which are eroded by sand blasting. The ground electrodes are usually formed of nickel or sometime and alloy of nickel and manganese. The addition of manganese (2.5-3.5 per cent) improves the tensile strength and the resistance to sulphur attack at high temperatures. For centre electrodes silicon-manganese-nickel alloys are used. These are particularly resistant to the high temperature effects of leaded fuels. Platinum alloys are still better, but then high cost limits their use in automotive spark plugs.
Types
The spark plugs may be long reach or short reach type depending upon the length of threaded portion and should be used only in the corresponding hole in the combustion chamber as described earlier. The spark plugs may also be classified as detachable type or of non-detachable type. Because of the application of sand blasting for cleaning spark plugs, there was no necessity to take out the plugs and thus non-detachable type was introduced. Sand blasting has the advantage of speed, but the electrodes and insulators may be severely damaged by excessive blasting and it is sometimes difficult to completely remove the sand particles after the cleaning. Detachable type plugs do not have these disadvantages, but then cleaning in that case is comparatively slow and there are chances of leakage through the joint. Because of these considerations both types have been used. However, in majority of the cases detachable type are employed From heat dissipation point of view also the spark plugs are divided into two classes, viz, ‘hot’ or ‘hard’ and ‘cold’ or ‘soft’. Hot plug runs hotter than the cold plug because the path of heat dissipation of the cooling water in the jackets is longer in the hot plug than in the cold plug. Some cold spark plugs have a copper core in the centre electrode to help carry heat from the tip of the electrode. Cold plugs are generally used in heavy duty, high speed engines where high temperatures are encountered. Lower speed, medium duty and colder operating conditions require a hot plug.
Air gap
The correct spark plug gap size is specified by the manufacturers but in general it varies from 0.6 mm to 1 mm. For the new plug gap is generally 0.6 mm, but with use it is increased, thus requiring higher voltage for the spark to jump across it. On the other hand the carbon deposits on the plug electrodes provide alternative paths for the current to pass thereby limiting the maximum voltage applied across the gap to a lower value ; the result is voltage surge of lower intensity which may not be sufficient to make the spark jump across the plug points, thus causing the plug to mis.
Surface discharge plug
This is a special type of spark plug where the space between the plug electrodes is filled with some semi-conductor material. Thus the spark has to jump across this material instead of air in the ordinary plugs .The advantages of this types of plug are the more intense and efficient spark which results in fuel economy, increased output and improved starting. The plug is claimed to be not effected by carbon deposits on the electrodes. The only disadvantage is its higher cost.
General
1. The spark plug sizes have been standardized. 14 mm diameter thread size is most commonly used. 18 mm diameter plugs are also used, however.
2. Higher temperatures are encountered in air cooled engines than the water cooled ones, due to which spark plugs for the motor cycles and scooters have to be of higher heat value than the comparable car engines.
3. The frequency of firing in two stroke engines is twice the value for four stroke engines. Therefore expected spark plug life in motor cycles with. Two stroke engines is only half the life in case of cars which always have four stroke engines. In actual practice it is even less due to other reasons, e.g. less time available for cooling by the fresh charge.
4. Spark plugs are designated in a prescribed manner. One such method (courtesy Mics, India)
SPARK PLUG DEFECTS
If insulator tip of the spark plug is greyish-yellow to light brown, it indicates normal condition of the spark plug, which means the engine running is correct and the correct spark plug has been used to give optimum engine performance.
12">
Erosion of electrodes
The spark plug gap increases as a result of erosion of electrodes, which is caused by the corrosive combustion gases. The increased gap size requires higher voltage to cause the spark to jump across it. If the gap size increases so much that the voltage surge produced is not sufficient for the spark to jump, the plug will misfire. The gap may be readjusted till it is found that electrodes are so much eroded away that the plug has to be replaced.
2. Fouling of plug
Spark plug fouling provides alternative paths for the current to pass, due to which the voltage surge is not able to attain its maximum value. This may lead to misfiring. This can be detected by any of the two methods :
(i) Stop the running engine and touch the spark plugs. The plug which was misfiring would be cool.
(ii) While the engine is running, disconnect the distributor lead of the plug which is suspected to misfire. If there is a change of sound, the plug in question was not missing, but if there is no change in the sound the plug was missing.
The fouling of spark plug may be due to oil or petrol. The oil fouling is visible in the form of shining black, oily deposits. The plug may be cleaned with a cloth and refitted. But the causes of the oil fouling have also to be removed. The possible causes may be worn out cylinder, piston rings or defective valve seals (most common).The carbon deposits due to burning of petrol on the spark plug are recognised by their black colour and dry, fluffy nature. The possible causes for the same are, defective fuel system delivering extra rich mixture, defective choke in the carburettor which does not open automatically when engine starts, clogged air cleaner, poor engine compression, continuous low speed/light load operation, insufficient high tension voltage or use of a plug colder than that which is specified by the manufacturer. The exact fault may be traced out and-remedied. The spark plug one grade hotter may be tried if necessary. However the foulded plug may be cleaned and reused. The lead deposits are caused by the use of tetraethyl lead in the high octane value fuels and by continuous running of the engine at high speeds. The deposits are easily recognized by the colour which may be red, white or yellow. The plug may be cleaned with usual methods.
Plug overheating
Metal particles on insulator tip, eroded electrodes, insulator burnt white with pearly deposits of metallic enamel beads indicate that the plug was running overheated.
This defect may arise on account of any of the following reasons:
(i) Ignition may be too much advanced.
(ii) The plug used may be “hotter” than the specified one.
(iii) Weak mixture.
(iv) Defective cooling system.
(v) Loose fitting causing the exhaust gases to leak.
(vi) Improper closing of valve.
The reason may be traced out and corrected.
SERVICING THE SPARK PLUG
1. First of all the spark plug is cleaned. This may be best done in a spark plug cleaning machine. One simply has to insert the firing end of the spark plug into the rubber adapter of the cleaner, lightly hold down with the finger tip and press the switch to blast the cleaning compounds on to the plug. The plug is rotated slowly with the finger so that it is cleaned uniformly on all sides. After cleaning, the air blast switch is pressed to remove the cleaning compounds from the firing end of the spark plug.
2. After cleaning in the cleaner :
(a) Inspect the plug for any cracks on the firing end of the insulator. If discovered, the plug should be discarded.
(b) Clean the metal shell and the thread with a wire bush and wipe the insulator with a dry and clean cloth.
(c) Inspect the centre electrode. If it is worn out, file its tip evenly to make a sharp edge. Again clean the plug.
(d) Adjust the spark plug gap according to the specifications. Replace the gasket in case the same has been damaged or is missing.
PRECAUTIONS REGARDING FITTING OF SPARK PLUGS
1. Select the spark plug type as specified by the engine manufacturer. The plug gap is also to be kept as specified.
2. Ensure that the gasket on the plug is not damaged.
3. Make sure that the surfaces are clean on both the plug as well as the engine head.
4. Use only a suitable box spanner for tightening.
5. Never over tighten the spark plug. As a rule of thumb, first screw in the plug by hand as far as it can go. After that use the correct box spanner. If it is a new plug, tighten after first resistance by further 90º approximately, whereas in case of already used spark plug, tighten further with box spanner, by approximately 30º
Every type of ignition system uses spark plugs. The spark plugs provide the crucial air gap across which the high voltage from the coil flows in the form of an arc. The three main parts of a spark plug are the steel core, the ceramic core, or insulator which acts as a heat conductor; and a pair of electrodes, one insulated in the core and the other grounded on the shell. The shell holds the ceramic core and electrodes in a gas-tight assembly and has the threads needed for plug installation in the engine. An ignition cable connects the secondary to the top of the plug. Current flows through the center of the plug and arcs from the tip of the center (or side) electrode to the ground electrode. The resulting spark ignites the air/fuel mixture in the combustion chamber. Most automotive spark plugs also have a resistor between the top terminal and the center electrode. This resistor reduces radio frequency interference (RFI), which prevents noise on stereo equipment. Voltage peaks from RFI could also interfere with, or damage, on-board computers. Therefore, when resistor-type spark plugs are used as original equipment, replacement spark plugs must also be resistor-type. The resistor like all other resistances in the secondary, increases the voltage needed to jump the gap of the spark plug. Spark plugs come in many different sizes and designs to accommodate different engines. To fit properly, spark plugs must be of the proper size and reach. Another design factor that determines the usefulness of a spark plug for a specific application is its heat range. The desired heat range depends on the design of the engine and on the type of driving conditions to which the vehicle is subject. A terminal post on top of the center electrode is the point of contact for the spark plug cable. The center electrode, commonly made of a copper alloy, is surrounded by a ceramic insulator and a copper and glass seal is located between the electrode and the insulator. These seals prevent combustion gases from leaking out of the cylinder. Ribs on the insulator increase the distance between the terminal and the shell to help prevent electric arcing on the outside of the insulator. The steel spark plug shell is crimped over the insulation, and a ground electrode, on the lower end of the shell, is positioned directly below the center electrode. There is an air gap between these two electrodes, and the width of this air gap is specified by the auto manufacturer. Some spark plugs have platinum-tipped electrodes, that greatly extend the life of the plug.
A spark plug socket may be placed over a hex-shaped area near the top of the shell for plug removal and installation. Threads on the lower end of the shell allow the plug to be threaded into the engine's cylinder head. Size. Automotive spark plugs arc available in either 14-or 18-millimeter diameters. All 18-millimeter plugs feature tapered seats that match similar seats in the cylinder head and need no gaskets. The 14-millimeter variety can have either a flat seat that requires a gasket or a tapered seat that does not. The latter is the most commonly used. All spark plugs have a hex-shaped shell that accommodates a socket wrench for installation and removal. The l4-millimete4 tapered seat plugs have shells with a⅝- inch (47 .7 -mm) hex; l4-millimeter gasketed and 18-mil- limeter tapered seat plugs have shells with a 13/16-inch (20.67-mm) hex. Reach. One of the most important design characteristics of sparkplugs is the reach. This refers to the length of the shell from the contact surface at the seat to the bottom of the shell, including both threaded and non-threaded sections. Reach is crucial. The plug’s air gap must be properly placed in the combustion chamber so it can produce the correct amount of heat. Installing plugs with too short a reach means the electrodes are in a pocket and the arc is not able to adequately ignite the air/fuel mixture. If the reach is too long, the exposed plug threads can get so hot they will ignite the air/fuel mixture at the wrong time, causing preignition. Preignition is a term used to describe abnormal combustion, which is caused by something other than the heat of the spark.
HEAT RANGE
When the engine is running, most of the Plug’s heat is concentrated on the center electrode. Heat is quickly dissipated from the ground electrode because it is threaded into the cylinder head. The spark plug heat path runs from the center electrode through the insulator into the shell and to the cylinder head, where the heat is absorbed by engine coolant circulating through the cylinder head. Spark plug heat range is determined by the depth of the insulator before it contacts the shell. For example, in a cold-range spark plug, the depth of the insulator is short before it contacts the shell. This cold-type spark plug has a short heat path, which provides cooler electrode operation. In a hot spark plug, the insulator depth is increased before it makes contact with the shell. This provides a longer heat path and increases electrode temperature. A spark plug needs to retain enough heat to clean itself between firings but not be so hot that it damages itself or causes premature ignition of the air/fuel mixture in the cylinder. If an engine is driven continually at low speeds, the spark plugs may become carbon-fouled. Under this condition, a hotter-range spark plug may be required. Severe high-speed driving over an extended time period may require a colder-range spark plug to prevent electrode burning from excessive combustion chamber heat. The heat range is indicated by a code imprinted on the side of the plug, usually on the porcelain insulator.
SPARK PLUG AIR GAP
The correct spark plug air gap is essential to achieve optimum engine performance and long plug life. A gap that is too wide requires higher voltage to jump the gap. If the required voltage is greater than what is available, the result is misfiring. Misfiring results from the inability of the ignition to jump the gap or the inability to maintain the spark. On the other hand, a gap that is too narrow requires lower voltages, which leads to rough idle and prematurely burned electrodes, due to higher current flow. Always set the gap according to the manufacturer's specifications.
0 comments:
Post a Comment