Viscosity index

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The viscosity index (VI) describes the temperature dependence of the kinematic viscosity (m 2 · s −1 ) of a lubricating oil , but not its actual viscosity. Oils with a low viscosity index show a greater temperature-dependent change in viscosity than those with a high viscosity index.

definition

The viscosity index is based on a conventional scale, in which two different types of oil with significantly different 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 = lower change in the Viscosity). The viscosity index is calculated from the comparison of the viscosities of an oil to be described with these two reference oils.

In accordance with the measurement method for kinematic viscosity customary in the USA at the time of definition, the determination was made in the unit of measurement Saybolt Universal seconds at 100 and 210 degrees Fahrenheit , as defined by the Society of Automotive Engineers (SAE). In DIN 51563, the values ​​of the two reference oil series are listed in the standardized unit of kinematic viscosity.

Practical meaning

For the operation of engines, the lowest possible temperature-related change in viscosity of the oils is desired, i.e. those with high viscosity indices. Motor oils with a high viscosity index can therefore be used both in summer and in winter. In summer, the lubricating effect of such oils is still sufficient and in winter the oils are not too thick. Base oils with a significantly higher viscosity index than the previous maximum of 100 have been available for several years . The viscosity index of motor oils can also be increased by additives , so-called viscosity index improvers. So today there are lubricating oils with a viscosity index over 100. The viscosity index of synthetic lubricating oils is between 80 and over 400.

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