- Viscosity index
Viscosity index is a
petroleum industry term. It is a lubricating oilquality indicator, an arbitrary measure for the change ofkinematic viscosity with temperature. Theviscosity of liquids decreases astemperature increases. Theviscosity of alubricant is closely related to its ability to reducefriction . Generally, you want the thinnestliquid /oil which still forces the two moving surfaces apart. If thelubricant is too thick, it will require a lot of energy to move the surfaces (such as inhoney ); if it is too thin, the surfaces will rub and friction will increase.As stated above, the Viscosity Index highlights how a lubricant's
viscosity changes with variations in temperature. Manylubricant applications require thelubricant to perform across a wide range of conditions: for example, in anengine . Automotive lubricants must reducefriction betweenengine components when it is started from cold (relative to engine operating temperatures) as well as when it is running (up to 200 °C). The best oils (with the highest VI) will not vary much in viscosity over such a temperature range and therefore will perform well throughout.The VI scale was set up by the
Society of Automotive Engineers (SAE). The temperatures chosen arbitrarily for reference are 100 °Fahrenheit (40 °C) and 210 °F (100 °C). The original scale only stretched between VI=0 (worst oil, naphthenic) and VI=100 (best oil, paraffinic) but since the conception of the scale better oils have also been produced, leading to VIs greater than 100 (see below).VI improver additives and higher quality base oils are widely used nowadays which increase the VIs attainable beyond the value of 100. The Viscosity Index of
synthetic oil s ranges from 80 to over 400.
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