Difference Between Viscosity and Viscosity Index

Difference Between Viscosity and Viscosity index

Viscosity is a property that tends to decrease as the temperature increases. The viscosity index indicates the resistance of an oil to the changes in viscosity with temperature. The smaller the change in viscosity with temperature, the higher the viscosity index of the oil.

Viscosity rating is a measure of the oil's resistance to flow or thickness and is the single most important characteristic of hydraulic oil. The viscosity index number indicates the extent of thickness, or resistance to flow, with temperature changes.


The viscosity of a fluid is a measure of its resistance to gradual deformation by shear stress or tensile stress. For liquids, it corresponds to the informal notion of "thickness". For example, honey has a higher viscosity than water.

Newton's law of viscosity defines the relationship between the shear stress and shear rate of a fluid subjected to mechanical stress. The ratio of shear stress to shear rate is a constant, for a given temperature and pressure, and is defined as the viscosity or coefficient of viscosity.

Viscosity is due to friction between neighboring parcels of the fluid that are moving at different velocities. When fluid is forced through a tube, the fluid generally moves faster near the axis and very slowly near the walls, therefore some stress (such as a pressure difference between the two ends of the tube) is needed to overcome the friction between layers and keep the fluid moving. For the same velocity pattern, the stress required is proportional to the fluid's viscosity. A liquid's viscosity depends on the size and shape of its particles and the attraction between the particles.

A fluid that has no resistance to shear stress is known as an ideal fluid or inviscid fluid. Zero viscosity is observed only at very low temperatures, in a superfluid. The unit of viscosity is newton-second per square meter, which is usually expressed as pascal-second in SI units.

The two common types of viscosity are kinematic viscosity and dynamic viscosity. Dynamic (or absolute) viscosity gives information about the force needed to make the lubricant flow, while kinematic viscosity tells how fast the lubricant flows when force is applied. Formally, viscosity (represented by the symbol η "eta") is the ratio of the shearing stress (F/A) to the velocity gradient (∆vx/∆y or dvx/dy) in a fluid.

Viscosity Index

Viscosity index (VI) is an arbitrary measure for the change of viscosity with variations in temperature. It is used to characterize lubricating oil in the automotive industry. An arbitrary number is assigned as a measure of the constancy of the viscosity of lubricating oil with the change of temperature with higher numbers indicating viscosities that change little with temperature.

The viscosity of liquids decreases as temperature increases. The viscosity of a lubricant is closely related to its ability to reduce friction. Generally, the least viscous lubricant which still forces the two moving surfaces apart is desired. If the lubricant is too viscous, it will require a large amount of energy to move (as in honey); if it is too thin, the surfaces will come in contact and friction will increase.

Many lubricant applications require the lubricant to perform across a wide range of conditions, for example, automotive lubricants are required to reduce friction between engine components when the engine is started from cold (relative to the engine's operating temperatures) up to 200 °C or 392 °F when it is running. The best oils with the highest VI will remain stable and not vary much in viscosity over the temperature range. This allows for consistent engine performance within the normal working conditions.

Lubricants fulfill several purposes. First and foremost they form a protective film between two moving parts to prevent them from wear and tear, while at the same time allowing the parts to move without requiring too much energy. Additionally, lubricants are also needed to transmit forces and transfer heat. In order to fit the wide variety of applications, lubricants are available in many different formulations. When it comes to characterizing lubricants and assessing their quality, the most important parameter is the viscosity index (VI). This dimensionless number describes the behavior of a lubricant’s viscosity with changing temperature.

• An oil with a high viscosity index will show only small changes in viscosity when the temperature changes

• An oil with a low viscosity index will have significantly different viscosities at different temperatures

Why is it important to know the viscosity index?

This is due to the fact that not every lubricant oil can be utilized for every purpose. For instance, while 10W-40 grade engine oil might be perfectly fine for the typical Central European climate, using this type of oil in colder regions might result in a lack of lubrication and eventually lead to engine damage. Employing 5W or 0W engine oil instead would be the better choice. However, the importance of the viscosity index is not just limited to engine oils. In fact, VI is applicable for lubricants across all sectors of modern life, be it in food manufacturing or bottling plants, inside transformers or hydraulic machinery


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