The function of spark plugs
The spark plug is an important component of the ignition system in a gasoline engine. It can introduce high voltage into the combustion chamber and make it jump over the electrode gap to generate a spark, thereby igniting the combustible mixture in the cylinder. It is mainly composed of a connection nut, an insulator, a connection screw, a central electrode, side electrodes and a housing. The side electrodes are welded to the housing.
The spark plug, commonly known as the "fire nozzle", functions to release the pulsed 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 an electric spark to ignite the mixed gas in the cylinder. The main types include: quasi-type spark plugs, edge body protruding type spark plugs, electrode type spark plugs, seat type spark plugs, electrode type spark plugs, face jumping type spark plugs, etc.
Spark plugs are installed on the side or top of the engine. In the early days, spark plugs were connected to the distributor through cylinder wires. In the past decade or so, most engines in small cars have been modified to have the ignition coil directly connected to the spark plug. The working voltage of a spark plug is at least 10,000V. The high voltage is generated by the ignition coil from 12V electricity and then transmitted to the spark plug.
Under the effect of high voltage, the air between the central electrode and the side electrode of the spark plug will rapidly undergo ionization, forming positively charged ions and negatively charged free electrons. When the voltage between the electrodes reaches a certain value, the number of ions and electrons in the gas increases like an avalanche, causing the air to lose its insulating property. A discharge channel is formed in the gap, and a "breakdown" phenomenon occurs. At this point, the gas forms a luminous body, which is called a "spark". As it expands due to heat, a "pop pop" sound also occurs. The temperature of this electric spark can reach as high as 2000 to 3000 degrees Celsius, which is sufficient to ignite the mixture in the combustion chamber of the cylinder.
According to the calorific value, there are cold type and hot type. According to the electrode materials, there are nickel alloys, silver alloys and platinum alloys, etc. If we are more professional, the types of spark plugs are roughly as follows:
Quasi-type spark plug: Its insulator skirt is slightly retracted into the end face of the housing, and the side electrode is outside the end face of the housing. It is the most widely used type.
Edge body protruding spark plug: The insulator skirt is relatively long and protrudes beyond the end face of the housing. It has the advantages of large heat absorption and good anti-fouling ability. Moreover, it can be directly cooled by the intake air to lower the temperature, and thus is less likely to cause hot ignition. Therefore, it has a wide range of thermal adaptability.
Electrode-type spark plugs: Their electrodes are very fine. They are characterized by strong sparks, good ignition capacity, and can ensure the engine starts quickly and reliably even in cold seasons. They have a wide thermal range and can meet various uses.
Seat spark plug: Its housing and the screw thread are made in a conical shape, so it can maintain a good seal without a gasket, thereby reducing the volume of the spark plug and being more beneficial to the design of the engine.
Polar spark plugs: The side electrodes are generally two or more. Their advantages are reliable ignition and the gap does not need to be adjusted frequently. Therefore, they are often used in some gasoline engines where the electrodes are prone to erosion and the spark plug gap cannot be adjusted frequently.
Face spark plug: Also known as face gap type, it is the coldest type of spark plug, and the gap between the central electrode and the end face of the housing is concentric.
Standard type and protruding type spark plugs
The standard spark plug is a single-sided electrode spark plug with the insulator skirt end slightly lower than the threaded end face of the housing. It adopts the traditional ignition end structure that is most widely used in side-mounted valve engines. To distinguish it from the "protruding type" that emerged later, this structure is called the "standard type".
The protruding spark plug was originally designed for overhead valve engines. Its insulator skirt protrudes from the threaded end face of the shell and extends into the combustion chamber. It absorbs a considerable amount of heat in the combustion mixture, has a relatively high working temperature at the combustion speed, and avoids contamination. At high speeds, due to the valve being placed at the top, the inhaled airflow is directed towards the skirt of the insulator, cooling it. As a result, the maximum temperature does not increase much, and thus the thermal range is relatively large. Protruding spark plugs are not suitable for side-mounted valve engines because they have many turns in the intake passage and the airflow has little cooling effect on the insulator skirt.
Single-pole and multi-pole spark plugs
The traditional single-pole spark plug has a distinct drawback, that is, the side electrode covers the central electrode. When high-voltage discharge occurs between the two poles, the mixture gas in the spark gap will absorb the heat of the spark and be activated due to ionization to form a "spark core". The location where the spark core forms is generally near the side electrode. During this period, more heat will be absorbed by the side electrode, which is known as the "flame suppression effect" of the electrode. This reduces the spark energy and lowers the flameout performance.
So, in the 1920s, three-pole spark plugs emerged. Compared with the single-side electrode, the spark gap of the multi-side electrode 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 electrodes covering the central electrode, increases the "accessibility" of the spark, has greater spark energy, and is easier to penetrate into the interior of the cylinder, which helps to improve the combustion condition of the mixture and reduce exhaust emissions. Due to the multi-sided poles providing multiple spark channels, the service life is prolonged and the reliability of ignition is enhanced. It must be pointed out here that at the moment of discharge, only one channel can spark, and it is impossible for multiple poles to spark simultaneously. The discharge process of high-speed photography proves this point.
The suffix letters (the letters following the calorific value) D, J, and Q in domestic spark plug models respectively represent double-pole, triple-pole, and four-pole.
Nickel-based alloy and copper core electrode spark plugs
The most fundamental requirements for electrodes extending into the combustion chamber are resistance to ablation (both electrical and chemical corrosion) and good thermal conductivity. With the development of materials science and process technology, electrode materials have undergone an evolution process from iron, nickel, nickel-based alloys, nickel-copper composite materials to precious metals. The most commonly used alloy nowadays is nickel-based alloy. Generally, pure metals have better thermal conductivity than alloys, but pure metals (such as nickel) are more sensitive to the chemical corrosion reaction of combustion gases and the solid deposits they form than alloys. Therefore, the electrode material adopts nickel-based materials with the addition of elements such as chromium, manganese, and silicon. Chromium enhances the resistance to electrical erosion, while manganese and silicon improve the resistance to chemical corrosion, especially the resistance to the highly hazardous sulfur oxide.
Common type and resistance type spark plugs
The spark plug, as a spark discharge generator, is a broadband continuous electromagnetic radiation interference source. Since the 1960s, countries around the world have accelerated the development of resistive spark plugs to suppress the strong interference of electromagnetic radiation caused by sparks to the radio field, protect radio communication and prevent malfunctions of on-board electronic devices. China has also issued a series of mandatory national standards for electromagnetic compatibility, imposing strict restrictions on the radio interference characteristics of vehicle devices driven by spark plug ignition engines. As a result, the demand for resistive spark plugs has significantly increased. Resistive spark plugs have no significant structural difference from the common type; the only difference is that the conductor sealant inside the insulating body is changed to resistive sealant.
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