Viscosity Index Improver: What it is and how it works

Understanding Viscosity Index Improvers and Their Role in Engine Performance

Viscosity Index Improvers (VIIs) are special polymer additives used in multi-grade engine oils to enhance the oil’s ability to maintain a consistent viscosity across a wide range of temperatures. These polymers are long-chain molecules that expand when heated and contract when cooled. This expansion and contraction help the oil maintain its viscosity, providing optimal lubrication under varying operating conditions.

VIIs are typically added to base oils to create multi-grade oils, which are designed to perform well in both cold and hot temperatures. For example, a common multi-grade oil, like 10W-40, uses VIIs to ensure it remains thin enough to flow during cold starts (represented by the “10W”) while still being thick enough to provide proper lubrication at high engine temperatures (the “40” part).‬

Understanding Viscosity Index

The Viscosity Index is a measure that indicates how much an oil’s viscosity changes with temperature. Oils with a high VI exhibit less change in viscosity when subjected to temperature fluctuations, while oils with a low VI show a more significant change. Maintaining a stable viscosity is essential for engine oils because it ensures consistent lubrication, minimizing wear and tear on engine components. So oil blenders add a viscosity index improver to expand the viscosity index of their oil.

How does VII work?

VII additive polymer molecules are coiled and spherical in shape at cool/cold temperatures. The molecule deforms into a long chain as the polymer heats up and is pressurized. The elongated long-chain molecules impede oil flow, increasing the viscosity index. The increased viscosity allows the oil to support a higher load than otherwise possible. The molecule returns to its coiled/spherical shape when the oil cools down.

What is Oil shear?

Oil shearing occurs when mechanical stresses and extreme temperatures are created in an engine. That causes the oil to experience intense pressure between moving parts such as pistons, bearings, and gears. These combined factors cause the oil’s molecular structure to break down, leading to a loss of its original viscosity. Oil shear also damages the VII polymer, cutting it into shorter pieces

Temporary viscosity loss due to shear forces

During operation, the crankshaft and camshaft shafts spin the oil, creating internal friction, heat, and some thinning. However, when the engine is under load, the oil in the clearances is stressed/sheared even further, causing it to thin beyond what its temperature would normally indicate. When the shear stress is removed, the distorted polymer molecule resumes its original spherical shape, and the oil returns to its original viscosity. That’s what this type of shear thinning is referred to as “temporary viscosity loss.”

Permanent oil shear

Permanent oil shear is defined as the permanent thinning of the oil caused by the destruction of the large viscosity index improver (VII) molecules.

VII destruction happens during extreme shear conditions when the deformed polymer molecules are literally pulled apart into two smaller chains, which can then be cut again into four chains (and so on and so on).

viscosity index improver

Smaller polymer chains have less impact on the oil’s viscosity than a single large chain, so the oil’s overall viscosity index falls. Unlike temporary viscosity loss, where the oil returns to its original viscosity once the stress is removed, broken polymer chains don’t reform into a single large chain, and oil doesn’t return to its original viscosity. At this point, the oil is permanently damaged and can no longer maintain its high-temperature viscosity or provide the same level of protection to metal components.

©, 2023 Rick Muscoplat