Engine Oils in the Field: A Comprehensive Tribological Assessment of Engine Oil Degradation in a Passenger Car

In this study, the deterioration of the friction and wear properties of a SAE 5W-30 engine oil during its service life as well as the differences in tribofilms formed by different oil conditions are investigated. A field test was conducted with a passenger car equipped with a modern, turbocharged gasoline engine under real driving conditions, where several used oil aliquots were collected. The collected used oil samples underwent tribological model tests, which revealed a 9% increase in friction and an over 420% increase in wear compared to the fresh oil during the 20,000 km service life. Furthermore, the composition of the tribofilms was correlated with previously published data on additive degradation in the used engine oils. The observed tribofilms changed significantly depending on additive degradation. In detail, the degradation of zinc dialkyl dithiophosphate (ZDDP) antiwear additive was analysed. The results showed a significant decrease in phosphorus and zinc as well as an increase in iron content in the tribofilms once ZDDP is depleted, which correlates well with the increase in the observed wear rates. Additionally, comparison with high-resolution mass spectrometry data showed that depletion of the original dialkyl dithiophosphate additive and successive formation of dialkyl thiophosphates has only minor influence on the tribofilm composition, the mentioned changes occur once the dialkyl thiophosphates are completely depleted and only dialkyl phosphates remain in the engine oil. This offers some insight on the mechanism of tribofilm formation by in-service, partially degraded ZDDP, which is prevalent under real operating conditions. Graphical Abstract