Effect of metallic lubricant additives on morphology, nanostructure, graphitization degree and oxidation reactivity of diesel particles

This paper investigates the impact of metallic lubricant additives on the morphology, nanostructure, graphitization degree, and oxidation reactivity of diesel exhaust particles. The experiments were conducted on a turbocharged heavy-duty diesel engine. Four typical lubricant oil additives, including Ca-based, Zn-based, Mo-based and ashless additives, were mixed into diesel at 0.5% and 1.0% by mass. Analytical characterization equipment used in this study includes a high resolution transmission electron microscopy (HRTEM), a Raman spectroscopy, and a Thermogravimetric analyzer (TGA). Results showed that the lubricant additives significantly changed the soot properties. Diesel fuels blended with ashless and Zn-based additives led to a more disordered nanostructure of diesel particles, thereby improving their oxidation reactivity. When Ca and Mo additives participated in combustion, the oxidation mass loss curve of soot particles shifted to a higher temperature range due to the combined effect of the physical and chemical characteristics of soot particles and the catalytic oxidation of metallic ash. Although Ca, Mo, and other metals in lubricant additives could promote the soot oxidation, the changes in the physicochemical properties of soot particles (including increased fringe length, reduced fringe tortuosity, and higher graphitization degree) rendered it more difficult to oxidize.