The relationship between the oil degradation state and operation conditions (temperature and gaseous components) was determined by clarifying characteristics of oil and blow by gas collected from piston second land region by link mechanism during actual operation of an engine. Obtained results showed that the total base number was lower and the total acid number was larger in oil at the land region than in oil in an oil pan and the formation of products of deterioration and an intensive consumption of additives were found. These phenomena were explained by temperature at the land region that was higher due to combustion heat that in the oil pan region and by the effect of blow by gases (hydrocarbon, NO{sub}x and oxygen) on the oil degradation. The level of oil degradation at land region was higher at lower speeds of rotation and higher loading conditions and it was established that the degree of degradation was strongly correlated to an amount of oil at the land region.
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