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 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. In this temperature range, no new deposits are formed and those existing will be burnt. 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 500 ☌ - the so-called self-cleaning zone. 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. Insulator tip temperature influence on the proper choice of spark plug heat range In order to ensure sufficient insulation between center and ground electrodes it is necessary, during operation, to keep the insulator tip (the part of the insulator projecting into the engine area) within an optimum temperature 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. Adapt to specific engine characteristics and widely varying driving/load conditions.Run hot enough to burn off combustion deposits that would otherwise collect on the insulator tip and cause fouling that results in misfire.Stay cool enough to avoid pre-ignition and/or electrode destruction due to detonation.It must do this in precise and controlled manner so the spark plug will: Heat range is the measure of how fast the spark plug tip dissipates combustion heat.
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