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Heat range is the measure of how quickly the spark plug tip dissipates combustion heat. It must do this in precise and controlled manner so the spark plug will:
Stay cool enough to avoid pre-
Run hot enough to burn off combustion deposits that would otherwise collect on the insulator tip and cause fouling that results in misfire.
Adapt to specific engine characteristics and widely varying driving/load conditions.
Why is Heat Range Critical?
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. Both conditions are directly related.
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.
Insulator tip temperature influence on the proper choice of spark plug heat range
Too cold spark plug for a given engine
Suitable spark plug for a given engine
Too hot spark plug for a given engine
If the insulator tip temperature drops into the so-
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 °C -
Too high temperature of the insulator tip is undesirable. High temperature results in pre-
In order to achieve the correct temperature of the insulator tip for a given engine, the spark plugs are produced in various thermal values. 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.
"Hot" spark plugs remove heat from the combustion area relatively slowly. They have a longer insulator tip and they achieve a temperature higher than the deposition zone relatively quickly.
"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.
The choice of a proper heat range is very important. But even a spark plug featuring a properly selected heat range, is influenced by the processes of fouling and self-
Zones of fouling and self-
Zone of fouling with non-
Zone of fouling with soft deposits -
Inert zone -
Determination of thermal value of a spark plug
While the engine is running, the spark plug is being heated to a certain temperature. The highest temperature can be detected at the insulator tip end. Thermal balance between the input and output of heat from the spark plug is determined by the value known as the spark plug heat range. An important parameter of this heat range is given by the so-
Determination of engine equipment with spark plugs
The equipment test of a particular engine is carried out with the help of special equipment making it possible to detect self-