LO 10-2 Describe the purpose, properties, function, composition, types, and applications of diesel engine lubricating oils

Viscosity Change and Viscosity Index Improver
Looking at the viscosity numbers of multigrade oil 15W-40, it appears that the oil is thicker when hot and thinner when cold. The explanation for this is that multigrade oils are formulated with an additive to control viscosity change. The additive not only prevents the oil from thinning out when hot but also can actually thicken the oil. Viscosity additives are necessary because of the nature of petroleum oil. Because petroleum oil is made of a variety of hydrocarbon molecules, some of which are light and thin while others are thick and tar-like, changes to viscosity with temperature can occur more dramatically with some oils compared to others. This change is measured by the viscosity index (VI), which refers to the change in oil viscosity with temperature Figure 19. Oil with a low VI will change viscosity faster than oil with a high VI. Oil with a low VI thickens and thins too much with temperature change, giving it an undesirable, uneven lubrication performance. An additive ingredient called viscosity index (VI) improver can be used to control the VI properties of lubrication oil. The VI improver is a special hydrocarbon polymer or, more simply, a chain of oil molecules that changes shape with temperature. The additive minimizes the change in oil viscosity by thickening oil when it is hot and thinning oil viscosity when it is cold.

FIGURE 10-19: The viscosity index of oil refers to how much its viscosity changes with temperature. A high viscosity index changes little while a low viscosity index changes considerably. Ideal oils have a high viscosity index.

FIGURE 10-20: The volume of a viscosity index improver molecule expands with temperature to thicken engine oil as its temperature increases.

The unique property of the VI improver polymer molecule is that when cold, it coils into a smaller ball-like shape. This means that when cold, the oil tends to be less viscous compared to oil without the VI improver additive Figure 20. When heated, the polymer molecule changes shape again, stretching into a bigger ball or a long rod, which thickens the oil. When added to oil, VI improver will counteract the oil’s natural tendency to thin when hot and thicken when cold. Unfortunately, one problem with VI improver additives is that they wear out as the polymer chains are broken through the mechanical action of shearing. Shearing takes place when moving metal edges and lubricated parts actually cut or shear the VI improver molecules, making them much smaller and limiting their ability to control temperature-related changes to oil viscosity.