Engine oil performance issues span a broad range of criteria, with the most often addressed including thermal and oxidative degradation control, anti-wear efficacy, contaminant suspension / dispersion capability, acid neutralization, maintenance of low temperature properties, and fuel economy retention. Each of these performance characteristics has an impact on the life expectancy of a lubricant. An additional factor that has been an area of ongoing research in this laboratory is the effect of fuel and fuel additives on the quality and performance of engine oils.
It is recognized that fuel can reach the thin film of lubricant on the cylinder wall prior to combustion in an internal combustion engine. In this manner the base fuel constituents and fuel additives can enter the crankcase unburned. Laser induced fluorescence studies conducted previously demonstrated the delivery mechanism in an optically accessible gasoline powered engine (1). The extent of fuel and fuel additive build-up in the oil in turn is dependent upon engine type, and operating cycle. Extended drain intervals further magnify any fuel/lubricant compatibility constraints or benefits that may exist.
The effect of compliance fuel additives on passenger car engine oils containing Group I, Group III, and Group IV base stocks was examined as a function of drain interval in an extensive fleet test. Base stock influence on the lubricant's reaction to accumulating compliance fuel additives was significant in that the vehicle response ranged from no harm to catastrophic engine failure. The fleet results prompted the development of a proprietary in-house engine test to examine the no-harm nature of fuel additives as part of the fuel additive development program. Laboratory bench tests utilizing thermogravimetric analysis techniques, differential scanning calorimetry, and rheological instrumentation were also devised as part of the no-harm battery of tests. The field test data, engine dynamometer evaluations, and the bench methods used to examine fuel additive/lubricant compatibility are reviewed in light of base oil type and additive chemistry.