Short Answer:

Viscosity index is a measure that shows how much the viscosity of lubricating oil changes with temperature. When the temperature increases, the oil becomes thinner, and when the temperature decreases, it becomes thicker. The viscosity index helps in understanding this behavior.

A lubricating oil having a high viscosity index maintains its thickness almost constant at different temperatures, while an oil with a low viscosity index becomes too thick in cold and too thin in heat. Therefore, high viscosity index oils are preferred for engines and machines working under variable temperature conditions.

Detailed Explanation :

Viscosity Index

Viscosity index (VI) is one of the most important properties of lubricating oil. It is a numerical measure that shows how much the oil’s viscosity changes with temperature. The viscosity of any liquid decreases as temperature increases, but in the case of lubricating oils, the rate of change varies from one oil to another. The viscosity index helps to compare and classify oils based on their resistance to viscosity change due to temperature variations.

In simple terms, a high viscosity index means that the oil’s viscosity changes very little when the temperature changes, while a low viscosity index means that the oil’s viscosity changes a lot with temperature. This property is especially important for engines and machines that operate under a wide range of temperatures.

Need for Viscosity Index

Machines and engines often operate under different temperature conditions. For example, during a cold start, the temperature of engine oil is low, and it becomes thicker. During high-speed operation, the engine temperature rises, and the oil becomes thinner. If the viscosity of the oil changes too much, it cannot provide proper lubrication under all conditions.

Therefore, it is essential to use lubricating oils whose viscosity remains relatively stable with temperature changes. The viscosity index helps in selecting such oils and ensures smooth operation of machines without friction or wear.

Definition of Viscosity Index

The viscosity index can be defined as:

“A numerical measure that indicates the change in viscosity of lubricating oil with respect to change in temperature.”

An oil with a high viscosity index undergoes a smaller change in viscosity when the temperature changes, whereas an oil with a low viscosity index undergoes a larger change.

Scale of Viscosity Index

The concept of viscosity index was developed by the Society of Automotive Engineers (SAE) in the United States. The VI scale is based on two reference oils:

• Oil A: Poor quality oil with high viscosity variation (assigned a VI of 0).
• Oil B: Excellent quality oil with very little viscosity variation (assigned a VI of 100).

All other oils are compared with these reference oils, and their viscosity index is calculated accordingly. Oils having a VI less than 0 are considered very poor, while those with a VI above 100 are considered excellent. Modern synthetic oils can even have VI values greater than 150 or 200.

Formula for Calculating Viscosity Index

The viscosity index is calculated using standard test data at 40°C and 100°C. The calculation is done as per ASTM standards (American Society for Testing and Materials). The formula is:

Where:

• U = Kinematic viscosity of the test oil at 40°C
• L = Kinematic viscosity of low VI reference oil at 40°C
• H = Kinematic viscosity of high VI reference oil at 40°C

This formula gives a numerical value for the viscosity index.

Factors Affecting Viscosity Index

1. Type of Base Oil:
   Mineral oils generally have a lower viscosity index, while synthetic oils have a higher viscosity index because they are more thermally stable.
2. Additives:
   Viscosity index improvers are polymer additives mixed with oil to improve its viscosity-temperature behavior. These additives make the oil thicker at high temperatures and thinner at low temperatures.
3. Refining Process:
   Highly refined oils with fewer impurities show better viscosity index because impurities often cause rapid viscosity changes with temperature.
4. Operating Conditions:
   Continuous exposure to very high temperatures can degrade the oil, reducing its viscosity index over time.

Importance of Viscosity Index

• Ensures consistent lubrication under varying temperatures.
• Reduces engine wear during cold start and high-speed operations.
• Improves fuel efficiency because oil maintains an ideal thickness.
• Helps in selection of oil suitable for specific climate conditions.
• Extends oil life by maintaining stability under heat and pressure.

Desirable Viscosity Index Values

• For ordinary lubricating oils: VI = 0 – 100
• For good quality mineral oils: VI = 100 – 150
• For synthetic or high-grade oils: VI = above 150

Oils with a higher viscosity index are mostly used in aircraft engines, racing vehicles, and modern high-performance engines where temperature changes rapidly.

Improvement of Viscosity Index

To improve the viscosity index of lubricating oil, special viscosity index improvers are added. These are high-molecular-weight polymers such as polymethacrylates, polyisobutylenes, and olefin copolymers.
At low temperatures, these additives remain coiled and do not affect the oil’s flow much. But at high temperatures, they expand, increasing the oil’s resistance to thinning. This helps in keeping the viscosity relatively stable across temperatures.

Conclusion :

The viscosity index is a key indicator of a lubricating oil’s performance under temperature changes. Oils with high viscosity index are more stable and provide better protection to machine parts, ensuring smooth operation and long service life. Therefore, while selecting a lubricating oil, the viscosity index should always be considered along with other properties like flash point, pour point, and oxidation stability.