Valuable Aid in Meeting EPA Fuel Economy Standards
Viscosity modifiers are polymers that play a critical role in modern multi-grade engine oils. Part of EPA fuel economy testing is a cold start and drive test, and carmakers such as Honda have been able to increase their overall new car mileage ratings by blending super-thin, multi-grade oils such as 0W-20 and 0W-16. Thinner oil creates less drag inside the engine, freeing up power in cold start situations as well as in overall driving and ultimately increasing fuel mileage. High quality base stocks and engine oil viscosity modifiers combine to make this possible.
Base oils comprise the majority of a finished engine oil formulation and the required attributes such as good oxidative stability are easier to explain, but what role do viscosity modifiers play? In simplistic terms, viscosity modifiers are polymers that are semi-fluid to a solid rubber like substance at room temperature. These polymers are dissolved into base oil and typically constitute less than 1% of the overall oil formulation. Although the concentration is low, the affect imparted to a finished oil formulation is substantial. In short, they thicken the oil and raise the viscosity index of the finished oil formulation. Oil with a higher viscosity index maintains a more stable viscosity (thickness) over a wider range of temperatures. That is to say it does not turn into a brick when cold and it does not get thin at higher temperatures such as those encountered in an engine (see graph). This results in a more ideal viscosity regime and better lubrication of engine components over a broader temperature range. When heated, viscosity modifier molecules uncoil like a spring, taking up more space and thickening the oil. When the oil is cold, the molecules contract and do not allow base oil molecules to stack on one another, this has an overall effect of keeping the oil thinner so it can be quickly pumped to engine parts upon cold starting.
Viscosity modifiers are not without fault however. When heated, the uncoiled molecules become very large and are susceptible to shearing in close tolerance areas of an engine. When the molecules shear the viscosity of the oil degrades,
so essentially oil that is supposed to be a 10W-30 could “shear out” to a lighter weight such as 10W-20. Engine oils that are properly formulated with high quality viscosity modifiers are designed to “stay in grade”. There are oil industry standards to address this. In fact, General Motors has set standards for its Dexos engine oils above that of the general overall oil standards that call for a specific level of shear stability necessitating the use of high-quality viscosity modifiers.
While viscosity modifiers are playing an important role in modern OEM engine oils by helping them meet stricter fuel mileage standards, there are still applications where viscosity modifiers are not desirable. A viscosity modified molecule is very large and parts with close tolerances such as gears and some types of bearings will easily shear them, thinning the oil. Base oil molecules don’t shear and there are some applications where manufacturers desire this over ultimate efficiency and choose to utilize mono grade engine oils that do not contain viscosity modifiers.
As viscosity modifiers start to play a bigger role in fuel efficiency numbers, attention given to them and their continued development will increase. They are just one of the tools oil formulators are using to engineer fluids to gain more efficiency and meet EPA fuel standards while maintaining maximum engine protection.