Understanding the Challenges Associated with Soot-in-Oil from Diesel Engines: A Review Paper 2021-01-0568

The major drivers in the development of the latest generation of engines are environmental. For diesel engines, mitigating the effects of soot contamination remains a significant factor in meeting these challenges. There is general consensus of soot impacting oil performance. Considerable efforts have been made towards a greater understanding of soot-lubricant interaction and its effects on engine performance. However, with evolution of engine designs resulting in changes to soot composition/ properties, the mechanisms of soot-lubricant interaction in the internal combustion engine continue to evolve. A variety of mechanisms have been proposed to explain soot-induced wear in engine components. Furthermore, wear is not the only topic among researchers. Studies have shown that soot contributes to oil degradation by increasing its viscosity leading to pumpability and lubricant breakdown issues. It has also been noted that soot affects friction, but conflicting results have been presented in the literature. This paper aims to critically review and discuss the current understanding of soot-induced mechanisms in heavy-duty diesel engines, as reported in the literature. Emphasis will be given to the aspects of wear, friction, and viscosity, critically highlighting the main pathways for future research. Different hypotheses on wear and the potential mechanisms behind the soot-lubricant interaction are also discussed, showing potential issues related to soot contamination as well as the strong relationship with oil formulation. Multiple factors are responsible for wear and its impact seems to depend on the boundary lubrication conditions achieved during the test. Moreover, different wear rates have been found among the studies, ranging from some 10³μm³/Nm to 200 10³μm³/Nm. Although the particulate matter is the cause of poorer lubrication performances in many investigations, lower viscosity reductions with temperatures have been found in presence of soot contamination. Finally, variations in the friction coefficient up to 144% suggest a significant impact of particulate matter on the frictional behaviour of a lubricant.