The value of VI

摘要

How did a cross-industry technical exercise in the 1990s lead to myriad claims about the superiority of some base oils in lubricants? Viscosity index (VI) is described by Wikipedia as "an arbitrary, unit-less measure of a fluid's change in viscosity relative to temperature change." However, arbitrary and unit-less does not equate to a lack of value. As a technical description of the viscosity-temperature relationship of a lubricating fluid, it has great technical value. A high VI fluid can have a lower viscosity at low temperatures or a higher viscosity at high temperatures or both, relative to an isoviscous fluid at an intermediate temperature. This can bring multiple benefits. Lower viscosity at low temperatures allows flow, so cold-start for vehicles or operation of industrial equipment in cold climates is facilitated by a flowing lubricant, rather than metal-on-metal contact causing damage. A higher viscosity at high temperatures results in a thicker oil film, maintaining the hydrodynamic regime to higher temperatures or lower sliding speeds and protecting parts by, again, preventing contact.