Every gasoline engine needs spark plugs, and each cylinder has one, specific engine has even two. The drivers know about this well.
When high-voltage electric current ( over 10000V) from ignition coil entrains into the cylinder, it creates spark between electrodes of spark plug and ignites mixed gas. The working condition of spark plug is very hard, such four-stroke engine as an example, on return stroke the temperature is 60C and the pressure is 90 kPa only, but on power stroke the temperature will raise up to 2000-3000C, and pressure will reach 4000 kPa in a twinkling. And the frequency of exchanging hot and cold is very high. Common material can't bear. Especially under this working condition the material must keep its good insulating performance still.

Spark plug is composed of insulator and metallic housing mainly. The key part is insulator. It should be providZd with good mechanical performance and high voltage-resisting, thermo-shockproof, and chemical corrosion-resisting. Usually the insulator is made of ceramic ( content of Al2O3 d®j—5%). The fundamental dimension of spark plugs is identical in the all over the world, and therefore they can be suitable for various vehicle. However, spark plugs are divided into two versions due to the difference of gasoline engines hot and cold versions. They express thermo-performance of spark plugs. Spark plugs can work well only under proper temperatY6ëjÀýthout carbon. According to practice when insulators of spark plugs keep 450-900²¢ the oil adhered to their sJrface can be burnt up without carbon. If the temperature is higher, it will ignite earlier than expected. Conversely, it will create carbon. Because of the different working temperature of engines designers change the length of insulator's skirt and the other factors to suit different requirement. Short skirt has smaller area to be heated, absorption ofheat is a little and ther9ä8Ò´the temperature of skirt is lower. This kind of spark plugs is called cold version. They are suitable to be used for engines |[«j—´ with high speed, high compression ratio and strong power. Conversely, they are called hot version. They are suitable to be used for engines with medium or low speed, low compression ratio and small pPwer. The users should select according to the requirement of engine manufacturers.