One of the key functions of lubricating oil additives in diesel engines is to control oil thickening caused by soot accumulation. Over the last several years, it has become apparent that the composition of the base oil used within the lubricant plays an extremely important role in the oil thickening phenomenon. In particular, oil thickening observed in the Mack T-8 test is significantly affected by the aromatic content of the base oil.
We have found that the Mack T-8 thickening phenomenon is associated with high electrical activity, i.e., engine drain oils which exhibit high levels of viscosity increase show significantly higher conductivities. These findings suggest that electrical interactions are involved in soot-induced oil thickening.
In order to understand this correlation we have studied the electrical properties for model systems composed of carbon black as a model for diesel soot, neat solvents simulating the composition of various base stock components, and a commercially available poly isobutylene succinimide dispersant. Using mixtures of the dispersant added to the solvents containing carbon black, we have measured the charging and conductivity of the mixture. The aromatic solvent had the highest solution conductivity and caused the highest level of particle charging.
We have used these findings to develop a preliminary model which shows how those electrical effects can lead to the type of thickening observed in the Mack T-8 engine test.