Viscosity index (VI)
is an arbitrary measure for the change of viscosity with variations in temperature.
It is used to characterize viscosity changes with relation to temperature in lubricating oil.
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 (as in honey
if it is too thin, the surfaces will come in contact and friction will increase.);
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 °C or 392 °F when it is running.
The best oils with the highest VI will remain stable and not vary much in viscosity over the temperature range. This allows for consistent engine performance within the normal working conditions.
The VI scale was set up by the Society of Automotive Engineers (SAE).
The temperatures chosen arbitrarily for reference are 100 and 210 °F (38 and 99 °C).
The original scale only stretched between VI=0 (lowest VI oil, naphthenic) and VI=100 (best
oil,paraffinnic) but since the conception of the scale better oils have also been produced, leading to VIs
greater than 100
VISCOSITY INDEX BASE OILS
|| VERY HIGH
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