The spark plug is a gasoline engineignition systemIt can lead high-voltage electricity into the combustion chamber and make it skip the electrode gap to generate sparks, thus igniting the combustible mixture in the cylinder.It is mainly composed of connection nut, insulator, connection screw, center electrode, side electrode and shell, and the side electrode is welded on the shell[1]。
Chinese name
spark plug
Foreign name
spark plug
Alias
Firemouth
component
It is composed of wiring nut, insulator, wiring screw, center electrode, side electrode and shell[1]
Type
Standard spark plug, flange protruding spark plug, etc
Interpretation
Important components of gasoline engine ignition system
Spark plug, commonly known as fire nozzle, is used to discharge the pulse high-voltage electricity sent by the high-voltage wire (fire nozzle wire), break through the air between the two electrodes of the spark plug, and generate electric spark to ignite the mixed gas in the cylinder.The main types are: quasi spark plug, rim protruding spark plug, electrode spark plug, seat spark plug, pole spark plug, surface spark plug, etc[2]。
spark plug
The spark plug is installed on the side or top of the engine. In the early days, the spark plug was connected to the distributor by the cylinder line. In the past ten years, the engines on cars have basically changed toIgnition coilConnect directly to the spark plug.The working voltage of the spark plug shall be at least 10000V, and the high-voltage electricity is generated by 12V electricity from the ignition coil and then transmitted to the spark plug[2]。
Structure introduction
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1. Steel housing: the lower part of the steel housing is fine thread, which is used for matching with the spark plug hole of the cylinder head. The upper part is equipped with an external hexagonal nut, which is used for installing the spark plug sleeve to tighten or unscrew the spark plug[3]。
2. The metal rod is the central electrode.The lower end of the metal rod contacts the upper end of the central electrode through the conductor glass body, and the upper end of the metal rod is equipped with a wiring nut[3]。
3. Between the steel shell and the central electrodeHigh alumina ceramicsInsulator to insulate the central electrode when it is energized[3]。
4. The lowest part of the steel shell is equipped with curved side electrodes[3]。
Spark plug mechanism diagram
5. Copper washer in the middle of the outer steel shell[3]。
Under the action of high voltage, the air between the center electrode and the side electrode of the spark plug will rapidly ionize, forming positive ions and negative free electrons.When the voltage between electrodes reaches a certain value, the number of ions and electrons in the gas increases like an avalanche, which makes the air lose its insulation, and the gap forms a discharge channel, causing "breakdown".At this time, the gas forms a luminous body, or "spark".With its thermal expansion, there is also a "popping" sound.The temperature of this spark can reach 2000~3000 ℃, which is enough to ignite the mixture in the combustion chamber of the cylinder[4]。
Common types
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According to the calorific value, there are cold type and hot type;According to electrode materials, there are nickel alloy, silver alloy and platinum alloy;If more professional, the types of spark plugs are generally as follows:
1. Quasi spark plug: its insulator skirt is slightly shrunk into the shell end face, and the side electrode is outside the shell end face, which is the most widely used one[5]。
2. Edge protruding spark plug: the insulator skirt is longer and protrudes beyond the shell end face.It has the advantages of large heat absorption, good anti fouling ability, and can be directly cooled by the intake air to reduce the temperature, so it is not easy to cause hot ignition, so it has a wide thermal range[5]。
3. Electrode type spark plug: its electrode is very thin, characterized by strong spark and good ignition ability. It can also ensure the engine to start quickly and reliably in severe cold seasons, with a wide thermal range, and can meet a variety of purposes[5]。
4. Seat type spark plug: its shell and screw thread are made into a cone, so it can keep a good seal without gasket, thus reducing the volume of spark plug, which is more beneficial to the design of engine[5]。
5. Pole type spark plug: generally, there are two or more side electrodes. The advantage is that the ignition is reliable and the gap does not need to be adjusted frequently, so the electrode is easy to be ablated and the gap between spark plugs cannot be adjusted frequentlygasoline engineOften used on[5]。
6. Surface spark plug: namely surface gap type, it is the coldest spark plug, and the gap between the central electrode and the housing end face is concentric[5]。
7. Standard and protruding spark plugs
The standard spark plug is a single side electrode spark plug whose insulator skirt end is slightly lower than the shell thread end face. It uses the traditional ignition end structure that is most widely used in side valve engines.In order to distinguish from the later "protruding type", this structure is called "standard type"[6]。
Protruding spark plugs were originally designed for overhead valve engines.Its insulator skirt protrudes from the threaded end face of the shell and extends into the combustion chamber.Absorb more heat in the burning mixture, and have a higher working temperature at the breathing rate to avoid fouling;At high speed, because the valve is overhead, the air flow inhaled is aligned with the insulator skirt to cool it, so that the maximum temperature does not increase much, so the thermal range is large.Protruding spark plugs are not suitable for side valve engines because the air flow has little cooling effect on the insulator skirt due to many bends in the air inlet[6]。
8. Single pole and multi pole spark plugs
One obvious defect of the traditional single pole spark plug is that the side electrode covers the center electrode.When the high voltage discharge occurs between the two poles, the mixture at the spark gap will absorb the spark heat and be activated due to ionization to form a "fire core".The place where the fire core is formed is generally near the side electrode, and the heat will be more absorbed by the side electrode, that is, the "flame suppression effect" of the electrode, which reduces the spark energy and reduces the flashover performance[6]。
As a result, in the 1920s, three pole spark plugs appeared.Compared with the single side electrode, the multi side electrode spark gap is composed of the cross sections of multiple side electrodes (punched into round holes) and the cylindrical surface of the central electrode. This side mounted spark gap eliminates the disadvantage of the side electrode covering the central electrode, and increases the "accessibility" of the spark. The spark energy is larger, and it is easier to penetrate into the cylinder,It helps to improve the combustion of mixture and reduce exhaust emissions.Since the multi side pole provides multiple fire jump channels, the service life is prolonged and the ignition reliability is improved.It must be pointed out here that at the moment of discharge, only one channel can ignite, and it is impossible to ignite multiple side poles at the same time.The discharge process of high-speed photography proves this[6]。
Suffix letters (letters after calorific value) D, J and Q in domestic spark plug models indicate double side pole, three side pole and four side pole respectively[6]。
The most basic requirements for the electrode extending into the combustion chamber are ablation resistance (electrical and chemical corrosion) and good thermal conductivity.With the development of material science and technology, electrode materials have experienced the evolution process of iron, nickel, nickel base alloy, nickel copper composite and precious metals.Nickel base alloys are now most commonly used.Generally, pure metals have better thermal conductivity than alloys, but pure metals (such as nickel) are more sensitive to chemical corrosion of combustion gases and solid deposits formed by combustion gases than alloys.Therefore, nickel base electrode material is used to add elements such as chromium, manganese and silicon. Chromium improves the resistance to electric corrosion, while manganese and silicon improve the resistance to chemical corrosion, especially the resistance to harmful sulfur oxide[6]。
10. Ordinary and resistance spark plugs
spark plug
As a spark discharge generator, spark plug is a broadband continuous electromagnetic radiation interference source.In order to suppress the strong interference of electromagnetic radiation caused by fire jumping on the radio field, protect radio communication and prevent the malfunction of on-board electronic devices, countries around the world have accelerated the development of resistance spark plugs since the 1960s.China has also issued a series of national standards for mandatory electromagnetic compatibility, which strictly limits the radio interference characteristics of vehicle devices driven by spark plug ignition engines. Therefore, the demand for resistive spark plugs has also increased significantly.The resistance type spark plug has no great difference from the common type in structure, just changing the conductor sealant in the insulation body to the resistance sealant[6]。
characteristic
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In the standard of spark plug, the heat value is usually used to characterize the thermal characteristics of spark plug. The heat value of spark plug indicates the balance ability of heat absorption and heat dissipation of the insulator skirt of spark plug. The higher the heat value.The balance between heat absorption and heat dissipation is stronger, so the calorific value of hot spark plug is lower and that of cold spark plug is higher.For engines with high power and compression ratio, cold spark plugs with high calorific value are selected;On the contrary, engines with low power and low compression ratio choose thermal spark plugs with low calorific value.The selection of general spark plugs is determined by the factory through the product approval test, and should not be replaced at will[7]。
choice
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The selection of spark plug can determine its calorific value according to the compression ratio, speed and air-fuel ratio of the engine[8]。
The calorific value of commercial spark plugs ranges from 5 to 13.The larger the value is, the colder the spark plug will be, that is, the so-called cold type spark plug is required for engines with high compression ratio.The smaller the value, the more "hot" the spark plug is, that is, the hot spark plug is used for engines with low compression ratio[8]。
Heating value of spark plug
The cooler spark plug is more sophisticated than ordinary products, so it can ensure the accuracy and quality of ignition when the engine rotates at high speed, thus ensuring the maximum power of the engine at its limit.In addition, its resistance is also controlled to be very small, and the number of misfires will not be missed due to the increase of rotating speed.Therefore, the use of spark plugs with higher values is beneficial to the habit of increasing engine speed[8]。
technical requirement
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Clearance requirements
The gap between the electrodes of the spark plug has a great impact on the work of the spark plug. If the gap is too small, the spark will be weak, and it is easy to leak electricity due to carbon deposition;The gap is too large, the required breakdown voltage increases, the engine is not easy to start, and "misfire" is easy to occur at high speed, soSpark plug gapIt should be appropriate. The spark plug gap used in general battery ignition system is 0.7~0.8mm, and the gap of individual spark plugs can be more than 1.0mm[9]。
Temperature requirements
The insulator skirt of spark plug (refers to the conical part of insulator outside the center electrode of spark plug) directly contacts with the high-temperature gas in the combustion chamber and absorbs a lot of heat. The absorbed heat is transferred to the cylinder head and atmosphere through the shell.The experiment shows that in order to ensure the normal operation of the automobile engine, the insulator skirt of the spark plug should be kept at a temperature of 500~600 ℃ (this temperature is called the self-cleaning temperature of the spark plug). If the temperature is lower than this value, carbon deposits will form on the insulator skirt, causing electric leakage between electrodes, which will affect the spark plug ignition.If the temperature of the insulator is too high (over 900 ℃), hot ignition will occur when the mixture contacts such a hot insulator, which will lead to early ignition of the engine.The normal working temperature of spark plug is between 450~870 ℃.At this time, the spark plug is yellowish brown.If the working temperature of the spark plug is lower than 450 ℃ for a long time, there will be a lot of carbon deposits around the spark plug, and the spark plug is black[9]。
performance requirement
The main requirements of spark plug are thermal characteristics, tensile strength, heat resistance, conductivity, insulation strength, sealing and corrosion resistance[3]。
The spark plug must be heat resistant. Put the screwed part into a crucible electric furnace or muffle electric furnace heated to 700 ℃ for 10min, and then cool it in air. There should be no cracks and seed cracks on the surface of the insulator[10]。
Sealing performance requirements of spark plug: after the heat resistance test, when the air pressure difference is 1MPa, the air leakage per minute is not allowed to exceed 40cmthree(If the center electrode is glued with conductorsealing material The air leakage rate shall not exceed 5cm per minutetwo)[10]。
The spark plug insulator assembly shall withstand power frequency (50Hz) voltage of 2200vV (effective value) for 30s[10]。
Spark plug carbon deposit
Material: The conductivity of the center electrode and side electrode of the spark plug depends on the contact between the electrode and the mixture, so the electrode has a high requirement for corrosion resistance.At the same time, the temperature during combustion is extremely high, and the electrode materials have high requirements for heat resistance.Therefore, many spark plugs are made of nickel manganese alloy.In order to improve the heat resistance, some spark plugs are made of nickel clad copper.In addition to thermal conductivity, copper gaskets also play a sealing role.For insulatorHigh alumina ceramicsmake[3]。
Development and improvement
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With the development of the automobile industry, the performance of spark plugs is constantly improving to improve the working quality of gasoline engines. For example, in order to improve the exhaust purification effect, wide gap spark plugs (1.0~1.2 mm gap) are used;In order to limit the noise of automobile radio waves, anti-interference spark plugs are developed.In the past, the service life of spark plugs was very short. Automobile manufacturers stipulated that the spark plugs must be checked or replaced after 3000 km (or 6 months).With the spark plug and relatedIgnition deviceThe service life of spark plugs has been greatly increased by the improvement of, and some measures for exhaust gas purification.Generally, the spark plug must be checked or replaced after 10000 km (or every year).The spark plug with platinum electrode has a longer service life, and generally does not need to be checked or replaced within 100000km[7]。
