Engine oils play a significant role in maintaining the performance and longevity of internal combustion engines. The use of proper additives in the production of engine oils maintains engine cleanliness and impedes the formation of impurities. The dissolution of additives in the base oils leads to interaction with the base oil components. The composition of the base oil, as well as the degree of its purification, significantly affects the effectiveness of the additives in it [1, 2].

It is important to evaluate the interactions of additives with the hydrocarbon classes, such as polyaromatic hydrocarbons and tar-asphaltene substances. These classes of hydrocarbons are involved in the process of polycondensation and formation of high-temperature deposits. On the other hand, they determine the colloidal stability of additives [3, 4]. Polyaromatic hydrocarbons and resinous-asphaltene substances in the lubricants are in the state of associates which complicates the assessment of the interactions of additives with them [5-7].

Experimental Part

The high-temperature catalytic oxidation method was used to get the most informative indicators of lubricants operating at high-temperatures and to study the changes in the physicochemical properties of the lubricating oils [8-10]. The method simulates the oxidation (thermochemical transformation) of lubricants in the most stressed temperature part of the engine cylinder, which allows to test lubricants in the most severe operating conditions and predict its operation in the engine [10, 11].

In order to evaluate the high-temperature properties of base oils, six samples (three petroleum fractions, as well as their mixture and two synthetic base oils) were subjected to high-temperature catalytic oxidation for 180 min. at 230°C (Table 1.).

Table 1. High-temperature deposit forming tendency of base oils

Results and Discussion

Synthetic base oils (PAO-4 and PAO-8) possessed the least tendency to form deposits and destruction of the base stock. Increasing the boiling point of petroleum fractions leads to the rising of the deposits. The formation of the small amount of sediment in synthetic oils is due to the presence of free double bonds in PAO. The presence of branched hydrocarbons, as well as methylene groups undesirably affects the high-temperature properties of base oils [3, 12].

During the exploitation process aging of engine oil occurs which is accompanied by a change in the structural group composition. The alteration in the composition of the base stock was evaluated after three-hour oxidation of the residue >500^⸰C (Table 2).

The initial fraction contains 81.58 % of “desirable” components – naphthenic-paraffinic and monocyclic aromatic hydrocarbons. Hydrocarbons which are prone to sedimentation account for 18.42%. The following ratio is observed after the three-hour oxidation process: the “desirable components”- 64.85 %, deposit formers – 35.15% which comprise of 9.24 % asphaltenes and copper naphthenate.

Table 2. Structural group composition of residue >500^⸰C

In order to assess the effect of various groups of hydrocarbons on the formation of deposits the correlation coefficient (R) was determined for various hydrocarbon groups of petroleum fractions (Table 2).

The correlation coefficient characterizes the static relationship between two random variables. Assuming that a strict order relation is given on the values of the variables, then a negative correlation – is a correlation in which an increase in one variable is associated with a decrease in another variable, while the correlation coefficient can be negative; a positive correlation under such conditions – is a correlation in which a rise in one variable is associated with an increase in another variable, however, the correlation coefficient can be positive [13].

In the case of naphthenic-paraffinic hydrocarbons the correlation coefficient is negative, which indicates that with an increase in the content of these hydrocarbons, the amount of deposit in the oil decreases, however in the case of other groups of hydrocarbons, the correlation coefficient is positive. The closer the value of the correlation coefficient is to 1 the greater the influence of the group hydrocarbons has on the formation of high-temperature deposits [14].

Bicyclic hydrocarbons, polycyclic hydrocarbons and resins have the greatest deposit forming tendency (Fig.1).

Fig.1. Influence of structural-group composition of residue on the formation of high-temperature deposits:
1 – bicyclic aromatic hydrocarbons; 2 – polycyclic aromatic hydrocarbons; 3 – resins

Conclusion

The assessment of the oxidation of lubricants by the high-temperature catalytic oxidation method showed that as the base oil oxidizes, the amount of bicyclic, polycyclic hydrocarbons and resin increases, that causes the rising of the high temperature deposit forming tendency of lubricating oils. The branching degree of hydrocarbons and the presence of unsaturated bonds influence the formation process of high-temperature deposits in the synthetic base oils.