Abstract: It is known, that thermo-oxidative stability, which characterizes the resistance of a lubricant to oxidation, is one of the most important operational properties of engine oil. In addition to the temperature and time modes of operation of the combustion engine, oxygen concentration and catalytic effect of the metal of the parts, the intensity of thermo-oxidative processes is also affected by the new oil filling-up to the crankcase, the need for which is explained by the oil leakage and burning during operation. However, there is no consensus on the effect on a thermal oxidative stability of a particular concentration of refilled oil in the general engine lubrication system. However, there is no consensus on how a different concentration of new oil in the general lubrication system affects thermo-oxidative stability. Thus, the purpose of this research was to determine the quantitative effect of filling up on oil thermo-oxidative stability. Semi-synthetic engine oil G-Energy 10w-40 SL / CF was chosen as the object of research. The research methodology included testing a sample of motor oil on a device for thermal oxidation in two stages: without filling up and with filling up of new oil. Every 8 hours of testing, the mass of the evaporated oil was recorded, and photometric device readings were taken to determine the optical properties of the oxidized oil. In the case of the filling test, the sample was filling up with new oil to its original weight before each subsequent eight-hour test. The article presents the dependences of the coefficient of thermal-oxidative stability on the test time for oil samples tested without filling and with filling. A quantitative evaluation of the effect of oil filling up on thermo-oxidative stability was carried out by establishing the dependence of the increment of the coefficient of thermo-oxidative stability on the mass of refilled new oil. It was found that the direction and effect of the filling up on the thermo-oxidative stability depend on the temperature and time conditions of the test. A quantitative evaluation of the effect of filling up on thermo-oxidative stability showed that in the case of tests at temperatures of 160 ° C, 170 ° C and 180 ° C, the maximum increase in thermal-oxidative stability is observed at the different mass of new oil. However, in the general case, the optimum filling up volume is 10–13 % of the total oil volume in the engine lubrication system.
Index terms: semi-synthetic engine oil, filling-up, thermo-oxidative stability, coefficient of thermo-oxidative stability, increment of the coefficient of thermo-oxidative stability.