Definitions for viscosity index
Random House Webster's College Dictionary
an arbitrary scale for lubricating oils that indicates the extent of variation in viscosity with variation of temperature.
Category: Machinery, Automotive
Viscosity index is an arbitrary measure for the change of viscosity with temperature. It is used to characterize lubricating oil in the automotive industry. 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; if it is too thin, the surfaces will come in contact and friction will increase. As stated above, the Viscosity Index highlights how a lubricant's viscosity changes with variations in temperature. Many lubricant applications require the lubricant to perform across a wide range of conditions: for example, in an engine. Automotive lubricants must reduce friction between engine components when it is started from cold as well as when it is running. The best oils 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. The temperatures chosen arbitrarily for reference are 100 and 210 °F. The original scale only stretched between VI=0 and VI=100 but since the conception of the scale better oils have also been produced, leading to VIs greater than 100.
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"viscosity index." Definitions.net. STANDS4 LLC, 2013. Web. 4 Dec. 2013. <http://www.definitions.net/definition/viscosity index>.