Lubrication of internal combustion engines has been a practice for many decades, yet continual improvement in lubricant technology is ongoing as new engines and new standards have been developed. Formulations directed to spark ignition engines and compression ignition engines, for instance, must address limits placed on sulfated ash, phosphorus, and sulfur content (“SAPS”), and restrictions in these components often lead to upper limits on the amount of metal-containing additives that can be included in the lubricant. Reduction in metal containing additives is necessary to reduce the impact of metal ash on exhaust aftertreatment devices and to reduce the emission of particulate matter.
Chief among these metal-containing additives are zinc dialkyldithiophosphates (ZDDP) for wear and oxidation protection and overbased metal detergents for cleanliness and acid control. ZDDP has been the industry standard for reducing valve train wear, protecting against liner wear, and reducing oxidation leading to corrosive wear. However the zinc contributes to an increase in sulfated ash in the lubricating oil and the phosphorus causes inactivation of oxidation catalysts used in exhaust after-treatment devices.