Fuel efficient engine oils, additive interactions, boundary friction, and wear

In order to encourage and accelerate development of advanced engine oils which would further contribute to improvement of engine fuel efficiency, Ford developed and made available to the oil industry a new engine test for determination of fuel efficiency of engine oils. This test, called Sequence VIE, was incorporated into the ILSAC GF-3 engine oil standard to be introduced around the year 2000. The main features of this test are increased emphasis on benefits derived under boundary/mixed lubrication conditions and improved retention of fuel efficiency during engine oil use. Friction reducing capabilities under boundary lubrication conditions can be improved through application of effective friction reducing additives, such as molybdenum dialkyldithiocarbamates (MoDTC), which, in combination with zinc dialkyldithiophosphates (ZnDTP) and other antioxidants, must provide good retention of friction reducing capabilities and also adequate antiwear properties. Formulation of such additive systems requires better understanding of various factors affecting performance of MoDTC and their interactions with other additives. From the results of our studies it is clear that ligand exchange reactions between MoDTC and ZnDTP and oxidation and antioxidant reactions involving base oil components, additives, and intermediates derived from them are all important in optimizing the performance and maximizing the benefits derived from these systems. These reactions, although occurring first in the bulk lubricant, also play a very important role in tribochemical conversions in boundary contacts where they are enhanced by more severe conditions. Thus, fundamental understanding of mechanisms and kinetics of these reactions is essential in the process of designing optimized lubrication systems that provide efficient and lasting friction reduction. Along these lines, this paper is intended to review available information, present the most recent data, explain some of the observations, draw some general conclusions, and outline future needs.