Low temperature degradation of lubricant oil for gasoline engine

To clarify the degradation characteristic of the gasoline engine oil, the influence of the oil temperature and the gaseous components was examined. In the condition of the constant oil temperature, the decrease in base number of oil under the temperature lower or higher than 90 degrees C is larger than that under the temperature of 90 degrees C. The principal causes of degradation are different between cases of the lower and the higher temperature tests. It is thought to be caused mainly by the formation of the acid with SO2, NO2 and water in the lower temperature tests, and by the acceleration of the nitration and the oxidation with NO2 and oxygen in the higher temperature tests. In addition, the decrease in base number of oil was found to be promoted when the temperature rises from low ones to 90 degrees C. It was presumed that the acid that mixed to oil under the lower temperature reacted with oil at 90 degrees C test condition. From the results mentioned above, it has been found that the oil temperature history from the engine start is very important for the degradation of engine oil.