Viscosity index 2204862 225904910 2008-07-15T23:13:02Z 201.231.239.219 '''Viscosity index''' is a [[petroleum]] [[oil industry|industry]] term. It is a [[lubrication|lubricating]] oil [[quality]] indicator, an arbitrary measure for the change of [[kinematic viscosity]] with temperature. The [[viscosity]] of liquids decreases as [[temperature]] increases. The [[viscosity]] of a [[lubricant]] is closely related to its ability to reduce [[friction]]. Generally, you want the thinnest [[liquid]]/[[oil]] which still forces the two moving surfaces apart. If the [[lubricant]] is too thick, it will require a lot of energy to move the surfaces (such as in [[honey]]); 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. 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 (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 °[[Celsius|C]]) and 210 °F (100 °C). The original scale only stretched between VI=0 (worst oil, naftenic) and VI=100 (best oil, parafinic) but since the conception of the scale better oils have also been produced, leading to VIs greater than 100 (see below). VI improver [[oil additive|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. {{Chem-stub}} [[Category:Petroleum]] [[Category:Lubricants]] [[Category:Oil additives]]