Low‐ and zero‐sulphated ash, phosphorus and sulphur anti‐wear additives for engine oils

Modern engine lubricant specifications include compositional constraints in terms of the permitted level of sulphated ash, phosphorus and sulphur (SAPS). This necessitates a reduction in the concentration of the additive zinc dialkyldithiophosphate (ZDDP) used in engine oils, and there is currently great interest in identifying anti‐wear additives that contain low or zero SAPS to partially or wholly replace ZDDP. This paper reviews the main chemical classes that are reported in the literature as potential low‐ or zero‐SAPS anti‐wear agents. There are many such possible additives, although none appears to be quite as versatile as ZDDP. Instead, combinations of anti‐wear and extreme pressure additives are likely to be employed in the future. The literature reveals a strong imbalance in the amount and depth of research carried out on the various additive types, with a great deal of research on a few classes of additives and very little recent work on most others.