Viscosity and viscosity index
Viscosity plays a vital role in the functionality of engine oil. But what is viscosity? Viscosity is the internal resistance to flowing of a fluid. To give a concrete example, spilling water or honey on the floor will lead to totally different outcomes. Water has low viscosity, meaning that it has low resistance to flowing. Honey on the other hand has higher viscosity, so it won’t flow as fast as water due to its higher internal resistance.
Engine oils are available with different viscosities to serve all types of engines. The Society of Automotive Engineers, SAE in short, has developed a system to classify fluids into different viscosity categories: the SAE viscosity standard. The higher the number on the scale, the higher the fluid’s resistance to flowing.
Viscosity is a static measurement, meaning that it is a denotation of the fluidity of a fluid at a given moment under given circumstances. The viscosity index adds the difference in viscosity over a change in temperature. Oil with a high viscosity index will retain its viscosity better than oil with a low viscosity index. When temperatures rise, oil will become less viscous.
There are two types of engine oils with completely different viscosity indexes. The first group is monograde oil
. Given their low viscosity index, these oils can only be used for very specific applications where temperature changes are not part of the daily job. Monograde engine oil can be recognised by its name containing SAE followed by a matching viscosity grade, like Wolf’s GuardTech SAE 40
Multigrade engine oils
, the second group, revolutionised the engine oil market with their higher viscosity indexes. These oils combine the viscosities of multiple monograde oils into one product, expanding the applicability over a wider temperature range. Multigrade oil’s viscosity can be recognised by the two SAE numbers separated with a W, an example of this is 0W20
Multigrade oils conquered the market with their improved properties, slowly driving monograde oils away. However, monograde oils still have a specific set of applications. Some solders or sealants of older vehicles require monograde mineral engine oils to stay in optimal condition, otherwise they become brittle.
The viscosity of a lubricant is determined by its base oils and additives, and one of those additives is the viscosity modifier. The secret to stabilising the oil’s viscosity is hidden in the viscosity modifier’s structure, which is capable of raising the engine oil’s viscosity index. This unique property is achieved on molecular level as the molecules contained inside viscosity modifiers expand and contract their protrusions when temperature changes. When temperature rises, the molecules start to expand and they cling together to stabilise the viscosity of the fluid. The molecules contract when the temperature decreases, enabling the other molecules to move more freely.
A new generation of viscosity modifiers entered the market, furthermore, they are already included in our Dexos1Tm Gen 2 Products
! These advanced additives offer a hefty set of advantages compared to the previous generations of viscosity modifiers. They gain these advantages through their revolutionary shape.
Thanks to its star shape the molecules can expand more efficient. The ends of the molecules force each other to expand into different directions, optimising the occupied space. The new generation viscosity modifier’s molecules contract more efficient, enabling the other molecules to flow more freely than ever. The combination of compact contraction and extensive expansion widens the operational temperature for engine oils.
This star structure offers additional shear stability as the increased amount of ends reduce impact of severe pressure. When a previous generation viscosity modifier succumbs to pressure their functionality is heavily impacted. The contraction and expansion capacity of the molecule is reduced and the engine oil will lose its viscosity.
The new generation of viscosity modifiers stays fully operational under high pressure. When one of the ends breaks down, the others will rearrange to compensate the lost protrusion. This improved shear stability enables longer lubrication intervals and longer optimal lubrication under demanding conditions.
These two improvements enable blenders to research new opportunities. First of all, it is now possible to improve the fluidity of engine oils at low temperatures without losing engine protection at operational temperature. Customers living or working in extreme cold conditions won’t have to sacrifice protection at operational temperature to ensure a quick cold start anymore.
Secondly, the possible combinations between the new viscosity modifiers and the base oil groups open a new window of opportunities. A lower-grade synthetic base oils or group II mineral base oil in combination with an innovative viscosity modifier can now be used where previously expensive high-grade synthetic base oil was required.
- Viscosity is the resistance to flowing.
- Viscosity index is the change in resistance to flowing over a change of temperature.
- Viscosity modifiers influence the viscosity index of oils.
- The new generation of viscosity modifiers can serve a wider temperature range and have higher shear resistance.
- The new generation of viscosity modifiers enable lower viscosity engine oils and expands the usability of all groups of base oils.