The influence of base oil on an engines enduring fuel economy performance is paramount in U.S. and European fuel economy engine tests such as Sequence VID/E and M-111. Friction modifiers make a contribution but primarily as a complement to correctly selected base oils.

A number of factors must be considered in choosing base oil, and one of the most important is Noack volatility. The contribution of base oil viscosity to the finished lube viscosity must remain as constant as possible throughout the drain interval. One way to control this is to ensure that the light end of the base oil molecular weight distribution actually remains in the lubricant and does not evaporate away at engine running temperatures.

If evaporation is not controlled, the base oil effectively thickens from light-end loss with adverse consequences for fuel economy. To minimize this effect, the boiling range of the lightest base stock grade used should ideally be as narrow as technically and economically possible.

A given base stocks kinematic viscosity and viscosity index can be produced with any number of boiling ranges for a given grade, but only the narrow-cut version will have the best achievable Noack. This does have cost implications: After all, what does the base stock manufacturer do with the light and heavy ends it has trimmed off to produce a given narrow-cut viscosity grade? Vacuum columns with the high separation index and good stripping capability needed to achieve narrow-cut base stock grades have special and quite sophisticated column packings that can also be costly.

Very high VI base oils are used because they provide good cold crank performance, assisting cold start fuel economy by lowering instantaneous viscosity. Also, the greater constancy of viscosity throughout the operating temperature range limits how low the high temperature high shear viscosity will drop at a given cold crank viscosity. Since HTHS is the main determinant of fuel economy performance at steady running temperature, the viscosity index benefit can thus be double-edged.

There is, however, a complementary aspect of the high VI and low Noack requirement. Very high VI API Group III base stocks are by their nature highly isoparaffinic and relatively low in naphthenic molecules compared to Group II or Group II+ (ignoring trace aromatics). Group IV (PAOs) are essentially 100 percent isoparaffinic. Isoparaffinic molecules have higher atmospheric boiling points than naphthenics for a given molecular weight or carbon number, and will be relatively less volatile. This means there is a correlation between high VI and low volatility, to the benefit of fuel economy, which has nothing to do with base oil viscometrics.

Fuel economy performance from a finished lube is a composite of several frictional and viscometric energy dissipation effects. Weve already said that HTHS is a dominant factor, but another less well known viscometric feature of base oils is ultrahigh pressure viscosity, measured at many Giga Pascals of pressure. This is the type of viscosity manifest in point or line contacts of nonconformal surfaces such as cam lobe and follower combinations at the top of the engine. In this lubricating regime, base oil exhibits a transient ultrahigh viscosity – almost instantaneous crystallization.

Good data is available about this phenomenon, measured as the pressure/viscosity coefficient or the friction coefficient of the base oil. A high level summary is that the more isoparaffinic and less naphthenic the hydrocarbon base oil, the lower will be the pressure viscosity coefficient and friction coefficient. This translates into high VI, and high isoparaffinic base stocks manifest lower frictional contribution from cam and follower valve gear in the total energy loss in an engine. This property also indicates, however, that high VI base stocks would be a poor choice in any formulation that requires high traction or friction such as in certain continuously variable transmissions that require traction fluids for lubrication.

Returning to the need to keep base oil viscosity constant throughout the drain interval, anything else that contributes to bulk viscosity increase in the finished lube must be managed. Perhaps the first thing to address is base oil oxidation, which we covered in a previous article. Among other things, oxidation leads to polymerization of the base stock and increased lube viscosity, so must be controlled. Also, soot loading in diesel engines, if not properly managed, can lead to aggregated soot that will certainly adversely affect the increase of kinematic viscosity of the lube.

Judicious selection of base stock type based on a formulators expertise will minimize oxidation and allow the ashless dispersant in a diesel lubricant additive package to perform optimally.