Effect of degradation on tribological performance of engine lubricants at elevated temperatures

High operating temperatures in engines produce a thermal degradation promoting the oil oxidation that affects its tribological performance. In this paper, an alternative method is proposed to simulate engine oil degradation by mixing degraded oil with fresh oil to simulate a mileage. The simulated and real degraded engine oils were compared by infrared spectroscopy analysis. The dynamic viscosities of mixed oils were measured by a rheometer at 50 °C and 80 °C and a block-on-ring tribometer was used to evaluate their tribological performance under mixed lubrication regime at these two temperatures. The results showed that the simulated degradation is acceptable, the coefficient of friction decreased at higher temperature and reduced as the engine oil is close to its end of service life.