Low and zero SAPS antiwear additives for engine oils

Abstract

Almost all modern engine lubricants use the additive zinc dialkyldithiophosphate (ZDDP) to provide antiwear and extreme pressure protection. However existing and proposed emissions regulations include constraints in the concentration of ZDDP or other sulphated ash-, phosphorus- and sulphur- (SAPS) containing additives in engine oils, as well as limits to the permissible phosphorus loss from the oil in running engines. The deleterious effects of SAPS on exhaust aftertreatment systems from ZDDP decomposition has lead to a great interest in identifying alternative low and zero SAPS antiwear additives that can partially of fully replace ZDDP in the next generation of engine oils to extend the life of exhaust after-treatment systems.

The aim of the work described in this thesis is to explore under the same test conditions, the film-forming, friction and wear-reducing properties of a very wide range of low and zero SAPS antiwear additives as possible replacements for ZDDP in engine oils, and, where additive types are effective, to investigate their mechanism of action.

Some of the alternative low and zero SAPS antiwear additives investigated show wear-reducing performance comparable to the ZDDP used as a benchmark in this research. The most promising alternatives to be used as supplements/replacements for ZDDP are suggested and discussed.

One characteristic of ZDDP is that it can form quite thick films on surfaces during rubbing. It is found that many different types of additive can also form thick films, though not generally as thick as ZDDP. However it has been found that there is no significant correlation between the ability of an additive to form a thick boundary film and its ability to control wear. Thick boundary film formation does, however, correlate positively with an increase in friction in contacts operating at intermediate entrainment speeds, so thick film formation should not be taken as a necessary or desirable feature of antiwear additives. It is also found that antiwear additives can give a very wide range of boundary friction coefficient values, depending on the molecular structure of the additive. There thus appears to be scope for optimising the structure of antiwear additives to provide reduced boundary friction.