RESEARCH ON THERMO-OXIDATIVE STABILITY AS ONE OF THE KEY INDICATORS OF ENGINE OIL LIFESPAN

Abstract

This article examines the topical issue of evaluating the performance characteristics of motor oils based on their thermal-oxidative stability under real-world operating conditions. It is demonstrated that the reliability and durability of internal combustion engines depend to a significant extent on the quality of the motor oil, which performs the functions of lubrication, cooling, cleaning friction surfaces of wear products, and protecting parts from corrosion. It is shown that during engine operation, the oil is exposed to high temperatures, atmospheric oxygen, fuel combustion products, metal contaminants, and mechanical loads, resulting in oxidation processes accompanied by the formation of organic acids, resinous substances, varnishes, sludge, and other aging products. This leads to a deterioration in the physical, chemical, and operational properties of the lubricant, a reduction in the effectiveness of additives, the formation of deposits on engine components, and accelerated wear. The aim of the study is to evaluate the operational properties of motor oil based on indicators of thermal-oxidative stability. To achieve this objective, the oxidation processes of engine oil during vehicle operation were investigated, the thermal-oxidative stability coefficient was determined, its dependence on vehicle mileage was established, and the oil’s performance under real-world operating conditions was evaluated. The subject of the study was SAE 5W40 ZIC SK X9 synthetic motor oil, which was used in Renault Logan passenger cars.

Samples were taken every 3,000 km of mileage. The tests were conducted using a laboratory apparatus that simulates the oxidation processes of engine oil: a 250-ml sample was heated to 180 °C while being stirred at a speed of 330 rpm for 3 hours. The study determined the thermal-oxidative stability coefficient and the evaporation coefficient. It was found that as the vehicle’s mileage increased from 0 to 12,000 km, the thermal-oxidative stability coefficient rose from 0.462 to 0.988, and the evaporation coefficient from 0.029 to 0.157. The most significant changes were recorded in the 6,000–9,000 km range, indicating the gradual aging of the engine oil and a decrease in the effectiveness of antioxidant additives. The results obtained are of practical significance for assessing the quality of motor oils, predicting the duration of their effective use, and justifying optimal replacement intervals during vehicle operation.