Technical Requirements For Spark Plugs

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 is weak, and it is easy to cause carbon deposition and leakage; The gap is too large, the required breakdown voltage is increased, the engine is not easy to start, and the "misfire" phenomenon is easy to occur at high speed, so the spark plug gap should be appropriate. Generally, the spark plug gap used in the battery ignition system is 0.7~0.8mm, and the individual spark plug gap can reach more than 1.0mm.
Temperature requirements:
The insulator skirt of the spark plug (refers to the conical part of the insulator outside the central electrode of the spark plug) directly contacts with the high-temperature gas in the combustion chamber and absorbs a large amount of heat. The absorbed heat is transferred to the cylinder head and the 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 maintain 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 deposit will be formed on the insulator skirt, causing electric leakage between the electrodes and affecting the spark plug spark. If the temperature of the insulator is too high (more than 900 ℃), when the mixture contacts with such a hot insulator, a hot ignition will occur, resulting in early ignition of the engine. The normal working temperature of the spark plug is between 450 and 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.
Performance requirements:
The main requirements of spark plug are thermal characteristics, tensile strength, heat resistance, electrical conductivity, insulation strength, sealing and corrosion resistance.
The spark plug must be heat-resistant. Put it into a crucible electric furnace or muffle electric furnace that has been heated to 700 ℃ for 10 minutes, and then cool it in the air. There should be no cracks and seed cracks on the insulator surface.
Requirements for sealing performance of spark plug: after heat resistance test, when the air pressure difference is 1MPa, the air leakage shall not exceed 40cm3 per minute (if the central electrode adhesive is made of conductor sealing material, the air leakage shall not exceed 5cm2 per minute).
Spark plug insulator assembly shall be able to withstand power frequency (50Hz) voltage of 2200vV (effective value) for 30s.
Material: The conductivity of the central electrode and the 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 material requires high heat resistance. Therefore, many spark plugs use Ni-Mn alloy. In order to improve heat resistance, some spark plugs are made of nickel-clad copper. In addition to thermal conductivity, copper gaskets also play a sealing role. The insulator is made of high alumina ceramics.