Viscosity is a key characteristic of oil properties. Viscosity is not constant and depends on pressure and temperature. The higher the system pressure, the higher the viscosity. At low temperatures, the viscosity also increases.
All modern oils are a combination of base oil and additives. To create the base oil, fractions obtained from oil refining, artificial organic substances and their mixtures are used. Thus, technologically, there are 3 types of oils - mineral, synthetic and semi-synthetic. Viscosity is one of the main physicochemical parameters of the oil; the degree of viscosity is used to judge the strength of the resistance to the flow of the liquid.
Depending on the viscosity and how it changes with temperature changes, oils are divided into summer, winter and all-season. Winter oils have the lowest viscosity. This ensures that the engine starts at low outside temperatures. However, with an increase in outside temperatures, the low viscosity of winter oil excludes the formation of an oil film on rubbing surfaces, which can lead to engine seizure.
High viscosity summer oils provide effective lubrication of surfaces in summer, but excessively high viscosity of summer oil in winter can also cause problems with starting the engine. Thick, solidified oil may not reach the target nodes, resulting in dry friction. In this regard, today the most common multigrade oils, which change the viscosity in the desired way depending on the temperature. That is, in the summer season, such oil is more viscous, and in the winter it is more fluid. In practice, such oil is much more convenient, since it does not require seasonal replacement and reduces the risk of damage to the engine part to a minimum.
Influence on the car engine
One of the important characteristics of modern oils is the HTHS index (high temperature - high shear strength). To date, regulatory requirements have been determined for the dynamic viscosity (shear force) of the oil at a temperature of + 150 ° C. This is due to the task of forming a protective film on rubbing surfaces. Moreover, this task takes precedence over the problem of pumping oil into friction zones. That is, the higher the load on the assembly, the higher the temperature and the more viscous the oil should be. This approach prevents the destruction of the lubricating film and increases the service life of the parts.
Manufacturers have developed a system for the international classification of engine oils by viscosity, taking into account temperature conditions. It is recommended to use this classification when selecting oil by viscosity. If you use additional additives, then you must take into account the fact that they change the viscosity of the engine oil.