Viscosity Index: Definition & Formula

Instructor: Saran Narang

Saran has had significant consulting exposure with a Bachelors degree in Mechanical Engineering.

Selecting the right lubricant for protecting your car's engine depends largely on the fluid's viscosity index. In this lesson, learn more about the viscosity index, its equation and significance.

Defining Viscosity Index

Before understanding the viscosity index, we need to understand a physical property of fluids called viscosity. Matter is what everything is made of - from the tires of a car to the water in a pond. But they don't all look the same, right? That's because they are different states of matter with the most prevalent being solids, liquids and gases. Liquids & gases are defined by their ability to flow and take the shape of what they are contained in - like a glass of water. Gases & liquids are more free to flow as compared to solids and are known together as fluids.

Sticky or viscous flow of honey
Viscosity of honey shown

The measure of a fluid's resistance to flow is called viscosity. You already know this, but by a different name - stickiness. The thicker a liquid is, the more viscous you can call it - like honey versus water. Honey resists its flow more than many other common fluids like water or juice, and is 'stickier,' so you would call it a more viscous fluid.

But wait, you might say - what is the viscosity index, and how does it relate to viscosity? Temperature plays an important role in a fluid's viscosity, so the world needed a way to measure it with changes in the temperature. The viscosity index is a measure of change in viscosity with fluctuations in temperature. In simpler terms, the viscosity index tells you how well a fluid can protect its viscosity during changes in temperature.

Where Does Viscosity Come From?

Before understanding why temperature affects viscosity, it would help to understand where the viscosity of a fluid comes from. Take any river you may have seen. Now try and picture it as a stack of hundreds of super-thin sheets of water, flowing together - each with their own speed. Each sheet has millions of molecules, and all the molecules are flowing into sheets above and below. The difference in speeds between the sheets due to friction, combined by the millions of molecular actions taking place, brings about viscosity. The movement of sheets at different speeds also causes a force or stress between them known as shear stress.

Theory of viscosity & laminar flow
Diagram of Viscous flow and shear stress

Temperature and Viscosity

So what about temperature? When you measure temperature, you are basically measuring the average kinetic energy of all the molecules of that piece of matter. The hotter something is, the more energy and movement the molecules have. Take boiling water for example - as the water molecules are heated, their energy and movement rise to the point of bubbling.

You can now visualize clearly that a change in the energy of the molecules (temperature) will bring about a change in the rate of movement of molecules between the sheets of a liquid (viscosity). Generally, for increases in temperature, the 'stickiness' or viscosity of a liquid decreases. Since every fluid is different, the reaction to the same temperature is different which gives the viscosity index its significance. The same fluid can have a different viscosity at a different temperature, so it makes sense to specify the temperature while mentioning a fluid's viscosity. For example, the viscosity of a lubricating oil at 100 degrees Fahrenheit would be very different than at 200 degrees Fahrenheit.

The Equation

Viscosity indexes are used most commonly in the case of hydraulic and lubricating oils. For any oil, the viscosity index or VI is given by the equation :

Formula for Viscosity Index
Viscosity Index Equation

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