Several civilian vehicles in China operate in urban traffic conditions and have their motor oil changed every 5,000 km. This study investigates the variations in oil properties after servicing at 5,000 km, based on systematic road tests (including a repeated test, a parallel test, and a new vehicle test). The physicochemical properties, changes in components, oxidation stability, detergent-dispersant performance, and tribological properties of motor oils were analyzed. The results showed that the total acid number (TAN) of oils increased with the operation mileage, by up to 1.41 mgKOH/g. The total base number (TBN) decreased after the road tests were completed, and the decrease was less than 44.6%. The kinematic viscosity (KV) of most oils decreased initially and then stabilized in the middle stage, before starting to increase later in the experiment. The change in KV at 100 ℃ was less than 15.96%. The oxidation onset temperature (OOT) of the oils diminished gradually with the operation mileage. All OOT values of the used oils were higher than 210 ℃. A spot test indicated that the used oils retained their detergent-dispersant performance to an appropriate extent. The four-ball wear scar diameters and friction coefficient of the used oils did not increase significantly after the road tests were completed. This study can serve as a reference for end-users when changing motor oils.

The condition of the motor oil in civilian cars is difficult to monitor; hence, we propose a method to evaluate the degree of degradation of motor oil using an on-board diagnostic (OBD) system. Three civilian cars and four motor oils (containing mineral oils and synthetic oils) were subjected to five groups of road tests under urban traffic and high-way conditions. The operation information, oil service time, mileage, engine operation time, idle time of the engine, and number of start-ups of the engine were obtained using the proposed OBD system. Physiochemical properties and changes in the components of motor oils during road tests were analyzed in laboratory. The theoretical model of the comprehensive indicators of driving parameters and oil properties were established. The proposed method was successfully applied to different cars, motor oils, and operating conditions in road tests. All the theoretical models had high accuracy and precision. Herein, we provide a method to monitor the oil condition with real-time driving parameters and provide a reference for end users to change their motor oil reasonably.

The ester base oil of dioctyl adipate (DOA) was oxidized in an oven at 200 ℃ for 30 h, and variations in the physicochemical and tribological properties were studied. To investigate the thermal-oxidation mechanism, the thermal-oxidation products were analyzed by gas chromatography–mass spectrometry (GC−MS), and the thermal-oxidation process was simulated using visual reactive force field molecular dynamics (ReaxFF MD). The results indicated that the total acid number (TAN) increased significantly because of the presence of 14% carboxylic acids and low molecular weight monoesters. The tribological properties were improved by the formation of the strongly polar carboxylic acids. Additionally, the increase in kinematic viscosity was limited due to the formation of high molecular weight polymerization products and low molecular weight degradation products. Thermal-oxidative degradation and polymerization mechanisms were proposed by combining ReaxFF MD simulations and GC−MS results.