Performance requirements for spark plugs

Clearance requirements
The gap between the electrodes of a spark plug has a significant impact on its operation. If the gap is too small, the spark is weak and is prone to leakage due to carbon deposition; If the gap is too large, the required breakdown voltage increases, making it difficult for the engine to start and prone to "misfire" at high speeds, the spark plug gap should be appropriate. Generally, the spark plug gap used in battery ignition systems is 0.7-0.8mm, and some spark plug gaps can reach 1.0mm or more.
Temperature requirements
The insulation skirt of the spark plug (referring to the conical part of the insulation outside the central electrode of the spark plug) directly contacts 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 atmosphere through the casing. Experiments have shown that to ensure the normal operation of automotive engines, the insulation skirt of spark plugs should maintain a temperature of 500-600 ℃ (this temperature is called the self-cleaning temperature of spark plugs). If the temperature is lower than this value, carbon deposits will form on the insulation skirt, causing electrode leakage and affecting spark plug ignition. If the insulation temperature is too high (over 900 ℃), when the mixture comes into contact with such a hot insulation, a hot ignition will occur, leading to early ignition of the engine. The normal working temperature of the spark plug is between 450 and 870 ℃. At this point, the spark plug turns yellow brown. If the working temperature of the spark plug is below 450 ℃ for a long time, there will be a lot of carbon deposition around the spark plug, and the spark plug will be black [9].
performance requirement
The main requirements for spark plugs are thermal characteristics, tensile strength, heat resistance, conductivity, insulation strength, sealing performance, and corrosion resistance.
The spark plug must be heat-resistant, and the part that it is screwed into should be placed in a crucible electric furnace or muffle electric furnace that has been heated to a temperature of 700 ℃ for 10 minutes, and then cooled in air. There should be no cracking or seed cracking on the surface of the insulator.
Spark plug sealing performance requirements: After conducting a heat resistance test, the air leakage rate shall not exceed 40cm3 per minute at a pressure difference of 1MPa (if the central electrode adhesive is made of conductor sealing material, the air leakage rate shall not exceed 5cm2 per minute).
The spark plug insulator assembly should be able to withstand a power frequency (50Hz) voltage of 2200vV (effective value) for a test period of 30 seconds.
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 extremely high requirements 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 nickel manganese alloy. To improve heat resistance, some spark plugs also use nickel clad copper material. Copper gaskets not only have thermal conductivity, but also serve as seals. The insulator is made of high alumina ceramics.