Comparative Physicochemical Study of the Thermally and Thermo-oxidatively Decomposed Automotive Crankcase Oils

Abstract

The performance characteristics of the automotive crankcase oil, be it of synthetic or natural origin is basically a function of its stability at the engine operation temperature. The performance characteristics usually drop with aging occasioned by a combination of thermal and thermo-oxidative decomposition. The two modes oil degradation have been investigated by simulating those conditions encountered in the automotive crankcase engines under normal operation. Two commercially available multigrade crankcase lubricants (synthetic oil and mineral blend) were subjected to thermal and thermo-oxidative decomposition using a modified version of the turbine oil oxidation test apparatus. Thermal decomposition was carried out in the absence of air while thermo-oxidative degradation was achieved with an air flow rate of 1.0 litre per hour for 15hours. The physicochemical changes in oil composition were evaluated by measuring the Total Acid Number (TAN), Total Oxidation Products (TOP) and the infra-red spectral changes of the decomposition products at various temperatures ranging from 100°C to as high as 380°C. The results indicated that the mineral oil showed a higher aging characteristics in both the thermal and thermo-oxidative decomposition than its synthetic multigrade counterpart. The infra-red analysis revealed that the physicochemical changes associated with aging of the oil samples at high temperature were mainly due to the breakdown of additives and hydrocarbon matrix at the onset of aging into monomeric fragments of peroxides, organic acids and carbonyl compounds which polymerize at a much higher temperature into dark resinous materials. The physicochemical changes with temperature may therefore be used in evaluating the temperature-related aging hence the thermal and thermooxidative stabilities as well as the performance characteristics of crankcase lubricants.