The gap between the spark plug electrodes has a great influence on the work of the spark plug. If the gap is too small, the spark is weak, and it is easy to leak due to carbon deposition; the gap is too large, the required breakdown voltage is increased, the engine is not easy to start, and at high speed The "missing fire" phenomenon is easy to occur, so the spark plug gap should be appropriate. The spark plug gap used in the general battery ignition system is 0.7-0.8mm, and the individual spark plug gap can reach more than 1.0mm.
The spark plug insulator skirt (referred to as the insulator tapered portion outside the spark plug center electrode) directly contacts the high-temperature gas in the combustion chamber to absorb a large amount of heat, and the absorbed heat is transmitted to the cylinder head and the atmosphere through the casing, respectively. The experiment shows that to ensure the normal operation of the car engine, the spark plug insulator skirt should maintain a temperature of 500 ~ 600 ℃ (this temperature is called the spark plug self-cleaning temperature), if the temperature is lower than this value, it will form a product in the insulator skirt Carbon causes leakage between the electrodes and affects spark plug sparking. If the temperature of the insulator is too high (more than 900°C), when the mixed gas comes into contact with such a hot insulator, a hot ignition will occur, resulting in premature engine ignition. The normal working temperature of the spark plug is between 450～870℃. At this time the spark plug was yellow-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 will be black.
The main requirements of spark plugs are thermal characteristics, tensile strength, heat resistance, electrical conductivity, insulation strength, sealing, corrosion resistance.
The spark plug must be heat-resistant. Put it into the crucible furnace or muffle furnace which has been heated to 700℃ for 10 minutes, and then cool it in the air. There should be no cracks or cracks on the surface of the insulator. .
Spark plug sealing performance requirements: after the heat resistance test, the air leakage is not allowed to exceed 40cm3 per minute when the pressure difference is 1MPa (if the center electrode glue is used with conductor sealing material, the air leakage is not allowed to exceed 5cm2 per minute ).
The insulation component of the spark wall should be able to withstand the power frequency (50Hz) voltage 2200vV (effective value) for a test of 30s.
Material: The electrical conductivity of the spark plug's center electrode and side electrode depends on the contact between the electrode and the gas mixture. Therefore, the electrode's resistance to corrosion is extremely high. At the same time, the temperature during combustion is extremely high, and the electrode material requires high heat resistance. Therefore, many spark plugs use nickel-manganese alloys. In order to improve the heat resistance, some spark plugs use nickel-coated copper material. In addition to its thermal conductivity, the copper gasket also acts as a seal. The insulator is made of high alumina ceramics.