METHOD FOR CONTROLLING THE TEMPERATURE PARAMETERS AFFECTING THE OPERATIONAL CAPACITY OF MOTOR OIL MIXTURES

. The results of a study of the temperature performance parameters of a Toyota Castle 10W-30SL mineral motor oil having 20% admixture of the Kixx Gold 10W-40SJ partially-synthetic motor oil are presented. For the testing process, the following items were used: a device for the temperature control of oils, a photometric device and electronic scales. The research method involved testing in two stages: at the first stage, the mineral oil was subjected to thermostatting, while at the second stage its admixture with partially-synthetic oil at temperatures of 160, 170 and 180 °С was tested. Graphs of the dependency of optical density, evaporation and thermal-oxidative stability on the test time were constructed to the test results of the commercially-produced oil and its admixtures. For each temperature, the regression equations of these dependencies were determined, on the basis of which the following were calculated: the onset temperature of oxidation and evaporation processes of oils during temperature control, as well as the critical temperatures of these processes. In the course of the analysis of the research results, we established the influence of the synthetic additive to the mineral motor oil on the service life of the mixed product in terms of the onset temperature of oxidation processes, the critical oxidation temperature, onset temperature of evaporation critical evaporation


INTRODUCTION
The main factor affecting the service life of motor oils is the temperature at the friction surfaces, which accelerates the processes of oxidation, destruction and chemical reactions with metals [1][2][3][4][5]. Therefore, in order to make an informed choice of oils for engines at various load levels, it is necessary to know their working temperature range, i.e. process onset temperatures and critical temperatures. In addition, it is necessary to develop technologies for increasing the working temperature ranges of motor oils [6], especially mineral oils [7][8][9][10].
The purpose of this study was to study the effect of synthetic additives to mineral oil on its service life and working temperature range.

EXPERIMENTAL PART
For the study, the all-season mineral motor oil Toyota Castle 10W-30SL was selected; for its 20% admixture, the Kixx Gold 10W-40SJ partially-synthetic motor oil, intended for petrol engines, was used.
For the testing process, the following items were used: a device for the temperature control of oils, a photometric device and electronic scales.
The research method involved testing in two stages: at the first stage, the mineral oil was subjected to thermostatting, while at the second its admixture with partially-synthetic oil was tested at temperatures of 160, 170 and 180 °С 1,2 . A sample of oil weighing 100 ± 0,1 g was poured into the glass beaker of the temperature control device and tested while stirring with a glass stirrer at a rotational speed of 300 rpm for 8 hours, with the temperature and rotational speed of the stirrer being maintained automatically. Then the beaker with the oxidised sample was weighed to determine the mass of the evaporated oil, a part of the sample (2 g) was taken for direct photometry and an optical density calculation performed D where 300 is the photometer reading in the absence of oil in the cuvette, µA; P is the photometer reading when the cuvette is filled with oil, µA. After measuring the optical density, the oil from the cuvette was poured into a glass beaker of a temperature control device, which was reweighed; the tests were then continued for 8 hours. The tests were stopped after the optical density reached values greater than 0.6. According to the values of optical density D and evaporation G of oil obtained during the test, the thermooxidative stability coefficient P tos was determined: where K G is the coefficient of evaporation, calculated in turn as Here m and M are the mass of the evaporated oil and the mass of the remaining sample after oxidation, respectively (in grams).
According to the test results, dependency graphs were constructed -D = f (t) ; G = f (t); P tos = f (t) , and regression equations for each temperature were determined, from which the time was calculated for which the optical density and thermal oxidative stability reached a value of 0.1. According to the dependency G = f (t), the time taken to reach evaporability of 3 g was determined for each temperature.In addition, the above dependencies were used to determine the values of optical density, thermal oxidative stability and evaporation after 8 hours of testing. According to the data obtained, graphs were plotted based on the time to reach the specified values D, G and P tos , with the corresponding values obtained after 8 hours of testing, the test temperature, which determined the temperatures of the onset of oxidation, evaporation and changes indicator P tos , as well as the critical temperatures of the studied oils.

RESULTS AND DISCUSSION
In the course of the experiment, it was established that with a decrease in the test temperature, the rate of the oxidation process slows down both in the commercial oil product (curves 1, 2 and 3) and in the oil mixture (curves 1´, 2´ and 3´). To assess the effect of temperature on the oxidation process, the concept of the potential resource P is introduced; this is determined by the time taken for the optical density to reach 0,6. Fig. 1 shows the dependency of optical density on the time and hermostatted temperature of the studied oils. Fig. 2 shows the dependency of the potential resource on the evaporation temperature of the commercial oil (Curve 1) and its mixture with 20% partially-synthetic oil (Curve 2). It was established that, at a temperature of 180 °С, the potential resource of oils is the same, while at a temperature of 160 °С it was: for commercial oil -87 hours; for the mixture -106 hours.
The regression equations for this relationship are: for the commercial oil product (curve 1): mixtures of oils (Curve 2): The correlation coefficient is ≈0,998. Using equations (5) and (6), the critical temperature at which the resource of the oils under study is minimal was determined: for the commercial product, it was 183,3 °С; for the mixture -184 °С. At these temperatures, the time to reach an optical density of 0,6 will be: for the commercial product -21,7 hours; for the mixture -23 hours.
The two critical characteristics of motor oils are the temperature of the beginning of the oxidation and evaporation processes, as well as the critical temperatures of these processes at which the continued operation of engines is not possible. To determine the critical oxidation temperature of the studied oils, graphs of the oxidation time versus test temperature were plotted at an optical density of 0,1 (Fig. 3, a); to determine the onset temperature of the oxidation processes, graphs were plotted showing the dependency of optical density on the test temperature for 8 hours (Fig. 3, b).
The regression equations for the dependencies presented above are: for the commercial oil product (Curve 1): mixture of oils (Curve 2): The correlation coefficient is ≈0,998. Solving these equations, we calculated the critical temperature T cr and the onset temperature of oxidation processes T n , which were: for the commercial oil product -T cr = 185 °С; T n = 155 °С; for the mixture of oils -T kr = 200 °С; T n = 160 °С. Thus, the mixture of oils gives an increase in the onset temperature of oxidation by 5 °С and an increase in the critical temperature by 15 °С.
The use of evaporability as a performance indicator is due to its causing a rupture of the oil film separating the friction surfaces and thus leading to the formation of microasperities; thus, a determination of the evaporation onset temperature along with the critical temperature permits an informed choice of oils for engines of varying degrees of loading. Fig. 4 shows the dependency of evaporability G on the test time and temperature of the studied oils, namely: evaporation at the test temperature for 8 hours and evaporation time of 3 g of oils at the test temperature.
The temperature of the beginning of the evaporation process (Fig. 5, a) was determined by the evaporation of the oils over 8 hours of testing, the dependency on testing temperature of which is described by a second-order polynomial (Formula (4)
The critical temperatures of this process were determined by the evaporation time for 3 g (Fig. 5, b).
The regression equations of dependency t = f (T) take the form: for the commercial oil product (Curve 1): for the oil mixture (Curve 2): Using equations (13) and (14), we calculated the critical evaporation temperature; for the commercial oil product, it was 188 °C; for the mixture -186,5 °C.
When using the index of thermo-oxidative stability P tos , which, when thermostatted, takes into account the change in optical properties and evaporation, the oxidation and evaporation onset temperatures, as well as the critical temperatures, their combined effect is taken into account. Fig. 6 shows the dependency of thermal oxidative stability on the time and temperature of the test.
According to the graphs presented in Fig. 6, the mixture of oils will reduce the rate of change of thermal-oxidative stability at test temperatures of 170 and 160 °С. Fig. 7 shows the dependence of thermal-oxidative stability on temperature over 8 hours of testing (Fig. 7, a) and on the temperature of the test when P tos = 0,1 (Fig. 7, b).
The critical temperature for the studied oils was determined from the time it takes the thermo-oxidative stability to reach 0,1 (see Fig. 7, a). The regression equations for dependency: t= f (Т) take the form: for the commercial oil product (Curve 1): mixture of oils (Curve 2): Having solved equations 15 and 16, we determined the critical temperature, which was: for the commercial oil product -188 °С, for the mixture -205 °С, which exceeds the critical temperatures of oxidation and evaporation processes.
The temperatures of the beginning of the change in the index P tos are determined by the dependency P tos = f (T) , described by regression equations: for the commercial oil product (Curve 1): Having solved these equations, we calculated the temperatures of the beginning of the change of the P tos index for the commercial oil product (151 °С) and for the oil mixture (159 °С), which is lower than the oxidation and evaporation onset temperatures.

CONCLUSIONS
On the basis of the conducted research, it was established that a 20% admixture of a partiallysyn-thetic product to the Toyota Castle 10W-30 SL mineral motor oil increases the service life of the resultant mixture, increasing the oxidation process onset temperature by 5 °C, and the critical temperatureby 15 °C. When it comes to evapora-tion, the onset temperature increases by 10 °С, while the critical temperature increases by 1,5 °С. Considering both oxidation and evaporation processes, the admixture of the synthetic additive, increases the temperature of the beginning of the processes occurring in the oil from 151 to 159 °С, while the critical temperature is increased from 188 to 205 °С.