Viscosity temperature behaviour

    Characteristics: Viscosity index (VI)

    Oil viscosity changes with temperature:

    Viscosity decreases with rising temperature,
    and it increases with falling temperature.

    An oil can be used for practical applications if oil viscosity changes as little as possible with changes in temperature (oil B). This ensures a flawless cold start and sufficient carrying capacity of the lubricating film even at high oil temperature.

    Note:
    An oil can be used for practical applications if oil viscosity changes as little as possible with the changes in temperature.

     

    Viscosity index (VI)

    The viscosity index (VI) characterises the viscosity temperature behaviour of the oil:

    • The index value increases with the flattening of the gradient (oil B - see Figure).
    • The index value decreases with the steepening of the gradient (oil A - see Figure).

    VI is not measured but calculated based on two viscosity measurements at 40°C and at 100°C.

    In the refining process, the viscosity index increases together with the improvement of the aging properties and dissolving of aromatic compounds. In general, well refined base oils have a VI value of approx. 90 to 100. Hydrocrack oils and synthetic hydrocarbons have even higher values - PAO (poly-alpha-olefin) with ca. 110 to 150.

    Viscosity temperature behaviour

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