![]() ![]() This provides consistent engine performance within the normal working conditions. The best oils with the highest VI will remain stable and not vary much in viscosity over the temperature range. Many lubricant applications require the lubricant to perform across a wide range of conditions, for example, automotive lubricants are required to reduce friction between engine components when the engine is started from cold (relative to the engine's operating temperatures) up to 200 ☌ (392 ☏) when it is running. ![]() If the lubricant is too viscous, it will require a large amount of energy to move (as in honey) if it is too thin, the surfaces will come in contact and friction will increase. Generally, the least viscous lubricant which still forces the two moving surfaces apart to achieve " fluid bearing" conditions is desired. ![]() The viscosity of a lubricant is closely related to its ability to reduce friction in solid body contacts. The VI was originally measured on a scale from 0 to 100 however, advancements in lubrication science have led to the development of oils with much higher VIs. The higher the VI, the more stable the viscosity remains over some temperature range. The lower the VI, the more the viscosity is affected by changes in temperature. It is mostly used to characterize the viscosity-temperature behavior of lubricating oils. The viscosity index ( VI) is an arbitrary, unit-less measure of a fluid's change in viscosity relative to temperature change. JSTOR ( May 2018) ( Learn how and when to remove this template message).Unsourced material may be challenged and removed. Please help improve this article by adding citations to reliable sources. The viscosity of water is taken as 1 centipoise at 25 oC.This article needs additional citations for verification. Water is usually chosen as a reference liquid. And viscosity is calculated by using the formula: For this purpose time for the flow of reference and test liquid between two marks, a & b made on the viscometer is noted. In this method relative viscosity is measured. Viscosity is usually measured by an Oswald viscometer (See Figure given below). The coefficient of viscosity is defined as the force required to maintain a difference of velocity of 1m/s between two parallel layers of liquid one metre apart. Viscosity is measured in terms of the coefficient of viscosity (η). Sulphur molecules are ring-shaped at 140 oC and offer less resistance to each other’s flow while the molecules at 190 oC are tangled and offer more resistance to flow. viscosity of liquid Sulphur is higher at 190 oC than that at 140 oC. Irregular-shaped molecules offer more resistance to each other’s flow as compared to regular-shaped molecules e.g. viscosity of sulphuric acid is higher than water due to the higher density of sulphuric acid. Higher is the density high will be the viscosity and vice versa e.g. mobile oil, grease etc have high viscosity, and they consist of larger hydrocarbon molecules. Molecules having bigger sizes or more molecular mass offer more resistance to each other’s flow, so more will be the viscosity e.g. In the refrigerator due to the low-temperature viscosity of honey was high. it is easy to pour honey from a bottle placed outside as compared to a bottle placed in the refrigerator. Higher is the temperature low will be the viscosity and vice versa e.g. the viscosity of glycerin is higher than in water because hydrogen bonding in glycerin is stronger than in water. Stronger intermolecular forces among molecules of liquid higher will be the viscosity and vice versa e.g. ![]() This internal resistance that one layer offers to the flow of the other layer is called viscosity. Liquid molecule’s layers slide one past the other and they offer resistance to each other. It means liquid flows in the form of layers. Water flowing in the river runs fast in the middle but slowly along the bank. ![]()
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