Viscosity temperature behaviour

    In case of multipurpose engine oils with VI improvement additives, besides temperature and pressure, viscosity also varies with the shear rate.

    If the lubrication gap and the lubricating film is magnified several thousand times, the oil particles located at the non-moving part (e.g. bearing cup, cylinder lining) have zero speed. The oil particles at the moving parts will have the speed of the respective component (e.g. the peripheral speed of the bearing journal of the crankshaft or the piston speed). This process can be theoretically presented with the velocity triangle.

    Definition: Shear rate
    The term shear rate refers to the speed at the moving parts [m/s] divided by the thickness of the lubricating film [m].

    The term shear rate refers to the speed at the moving parts [m/s] divided by the thickness of the lubricating film [m]. The resulting shear rate [s-1] of combustion engines, in the area of the main bearing, piston and cylinder lining, is approx. 10-5 s-1 at idle speed and up to 10-6 s-1 at maximum rpm.

    Shear rate

    Enlarge image

    Lubricating oils, the viscosity of which only change under the influence of temperature and pressure, but not due to changing shear rate, are called Newtonian liquids (oil A). These include oils without VI improvers.

    If viscosity varies in relation to shear rate too, the oil is deemed a non-Newtonian or pseudoplastic liquid (oil B). This applies to all multipurpose oils with VI improvers.

    The viscosity of these oils decreases more or less steeply at constant temperature and pressure but with increasing shear rate (high shear due to increasing rpm) depending on the type and quantity of VI improvers. This is also called reversible or temporary shear loss. 

    Viscosity temperature behaviour

    Enlarge image