Zone of fouling with soft deposits - vehicle engine run at idling speed or its low load can result in the setting of soft (dry) combustion deposits on the insulator tip, even if the fuel does not burn in liquid state. Cold starts and frequent moving off from rest in cold weather will accelerate the fouling of the insulator tip. The decreasing insulation resistance of the insulator tip results in an occasional failure of ignition. In addition, the insulator tip is wet from the non-evaporated fuel. Level of creation of combustion deposits is significant. Diffusion (atomization) of fuel is low and the fuel burns in its liquid state. The mixing ratio of fuel and air is very low in this case (rich mixture). Zone of fouling with non-evaporated fuel - this is the zone of the highest degree of fouling for spark plugs. Zone of fouling with dry combustion deposits.Zone of fouling with non-evaporated fuel.Zones of spark plug fouling and self-cleaning zone depending on the air/fuel ratio and on the spark plug insulator tip temperature. On the other hand, the combustion deposits previously set will burn if the insulator tip temperature rises above 475 ☌. The setting of combustion deposits on the insulator tip is caused by an imperfect combustion due to a "rich" air/fuel mixture. But even a spark plug featuring a properly selected heat range, is influenced by the processes of fouling and self-cleaning of the insulator tip. The choice of a proper heat range is very important. "Cold" spark plugs feature a relatively short insulator tip and they remove heat from the combustion area quite fast, in order to avoid advanced ignitions. They have a longer insulator tip and they achieve a temperature higher than the deposition zone relatively fast. "Hot" spark plugs remove heat from the combustion area relatively slowly. The range of thermal values for BRISK spark plugs extend from the warmest to the coldest, namely 19, 18, 17, 15, 14, 12, 10 and 08. In order to achieve the correct temperature of the insulator tip for a given engine, the spark plugs are produced in various thermal values. High temperature results in pre-ignitions of the air-fuel mixture and further compression of the mixture already ignited leads to high temperature, which can cause serious damage to the engine. Too high temperature of the insulator tip is undesirable. The spark plug operates in an optimum manner. Providing a higher temperature of the insulator tip, no further combustion deposits are formed, but those already existing will not be burnt until the insulator tip temperature rises above 475 ☌ - the so-called self-cleaning zone. In this temperature range, no new deposits are formed and those existing will be burnt. A consequence of these combustion deposits on the insulator tip is reduction in electrical insulation resistance accompanied by failing ignitions and after a certain period of time even by a failure of the spark plug performance. If the insulator tip temperature drops into the so-called deposit zone, combustion deposits (carbon, non-combusted fuel, lubrication oil, impurities from the atmosphere) start to form on the insulator tip surface. One heat range changes the spark plug insulator tip temperature by about 75 - 100 degree Celsius.Ī. In order to ensure sufficient insulation between center and ground electrodes it is necessary, to keep the insulator tip (the part of the insulator projecting into the engine area) within an optimum temperature range. Insulator tip temperature is influenced by the proper choice of spark plug heat range. Two basic conditions for proper performance of a spark plug are given by sufficient electrical insulation between the center and ground electrodes, and heat transfer from the parts of the spark plug projected into the combustion chamber.
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