Viscosity index

Definition

The viscosity index is based on a conventional scale, in two different types of oils with significantly deviating viscosity-temperature behavior, a viscosity index of 0 (LVI = Low Viscosity Index = strong change in viscosity ) and a viscosity index of 100 ( HVI = High Viscosity Index = less change in viscosity) has been assigned. From the comparison of the viscosities of the oil to be described with reference the two oils at its viscosity index is calculated.

In accordance with the practice at the time of the definition in the U.S. measuring method of kinematic viscosity determination was carried out in the unit Saybolt universal seconds at 100 and 210 degrees Fahrenheit, set by the Society of Automotive Engineers (SAE ). In DIN 51563, the values ​​of the reference oil are listed in rows of the standard unit of kinematic viscosity.

Practical significance

For the operation of engines lowest possible temperature-induced change in viscosity of the oils is desired, ie, those with high viscosity indices. Engine oils with a high viscosity index can thus be used both in summer and in winter. In summer, the lubricating effect of such oils is still adequate and in winter the oils are not too thick. For several years, base oils are available with significantly higher viscosity index than the previous maximum of 100. Further, the viscosity index of the engine oil with additives, so-called viscosity index improvers, can be increased. Today, there are therefore lubricating oils having a viscosity index higher than 100, the viscosity index of synthetic lubricating oils is from 80 to about 400