Kline data confirms this resistance to migration through their analysis of market development for heavy-duty viscosity grades through 2025 and their data for potential market shifts to API Group III and Group IV base stocks over the same time period. Certainly, the consultancy projects some small percentage growth for these very high viscosity index base stocks, but current estimates place growth in the low to mid-single digits.

Interestingly, current global demand for 15W-XX (primarily 15W-40), which is the major heavy-duty grade in the United States, is not projected to change in the next decade. Whether the U.S. market will mirror this global trend remains to be seen.

Why the Disconnect?

There seems to be some disconnect between the needs expressed by original equipment manufacturers either in public meetings or in industry bodies developing new sequences, compared to likely grade migration and base oil group use. For example, at the 20th ICIS Base Oils and Lubricants Conference in February, Volvos Dr. Anders Pettersson presented a balanced discussion of the potential benefits and risks of formulating lower viscosity grade, fuel efficient heavy-duty engine oils. His main message was that the perceived risks, such as lower oil film thickness leading to possible durability issues, are manageable.

In essence, he was pointing to lower viscosity grades that really are fuel efficient – such as lower high-temperature, high-shear viscosity SAE 5W-30s in the range of 2.9 to 3.2 millipascal-seconds at 150 degrees C. For a number of years, some European OEMs, where lower viscosity grades have had more traction, have adopted their own variants on some ACEA E categories, formulated as 5W-30 oils for use as their initial fill oils.

This shows that they must have confidence in this viscosity grade overall because they were and are offering warrantied engines filled with these fluids. However, even though these 5W-30s were formulated as severe-duty, long-drain oils, they were not full-blown fuel economy oils because they were still ACEA E4-type oils with greater than 3.5 mPa-s HTHS.

Indeed, this is currently the case for all ACEA E engine oils. There is some talk of European OEMs developing a so-called ACEA F type category of genuine fuel-efficient lower HTHS oils for ACEA 2016 or later, but this is currently only at the discussion stage.

Importance of HTHS

HTHS is the accepted determinant of fuel economy performance, especially in the heavy-duty sector – where most of the time these large engines operate fully warmed-up on long haul trips. This means that the W grade – or cold-cranking viscosity specification – during start-up and warm-up can only make a secondary contribution to overall fuel economy.

The bulk of viscous losses in such large engines are hydrodynamic lubrication losses because of their large main bearings and significant piston/liner surface areas. Therefore, it is the HTHS viscosity that will need to be addressed for maximum fuel economy developments. Friction modifiers can be used, but they would likely have limited impact in such large engines because boundary friction, where friction modifiers are active, is a relatively small percentage of such shear energy losses.

Lets look in a little more detail at how to address the specific issues laid on the table. Pettersson voiced a need to use naturally high viscosity index base fluids that should be shear stable with minimal use of advanced polymers. This points to greater use of Group III or at least Group II+ for cost-effective formulations.

Such base stocks can deliver on the viscosity index front relative to Group II, while minimizing viscosity modifier content for a given SAE grade. Further, these base oils enhance long drain capability due to their intrinsically better additive response – especially antioxidant response – compared to Group II or even Group II+.

In the U.S. a larger percentage of the heavy-duty lube market comprises owner-operators than in other developed countries. Quite rightly, these lubricant customers first priority is engine reliability and minimal downtime. This has historically had the consequence of a belief in thicker is better and that the classic 15W-40 multigrade will provide more comprehensive protection.

Returning to Petterssons belief in the naturally high viscosity index of the base fluids, lets analyze what that means. For example, a Group II 15W-40 engine oil has a typical base oil viscosity of around 6.6 centistokes at 100 degrees C. A similar Group III 5W-30 formulation would have a comparable base oil viscosity of about 5.6 cSt.

At the HTHS temperature of 150 degrees C, the comparable base oil viscosities are about 2.8 and 2.6 cSt, respectively, using the typical market viscosity indexes of the relevant quality Group II and Group III stocks. These viscosities are very similar and certainly converge in percentage terms. This shows that the enhanced viscosity index of the Group III formulations narrows the difference in base oil viscosity as conditions become more taxing.

This convergence is partially the basis of the argument for using naturally higher viscosity index base stocks. Simply put, base oil viscosity will always be present, but the contribution from the viscosity modifier could be more transient, depending on the polymers shear stability index and the extent of reversible shear thinning during use. Therefore, thicker is not necessarily very much better, if at all – unless it comes from the base stock.

Retaining Fuel Economy

If we are serious about fuel-economy heavy-duty oils, we also require fuel economy retention. Current API CJ-4, 10W-30 grades have a Noack specification of 15 percent maximum weight loss, whereas other grades have a 13 percent maximum weight loss. Thus, the specification could be viewed as being written to match what could be achieved with the stocks available, rather than for what should be achieved.

This is not the case with the new FA-4 specification, which has a Noack requirement of 13 percent for SAE 0W, 5W and 10W-XX. Therefore, the very highest viscosity index base stocks with commensurate high boiling ranges will be needed. For FA-4, this requirement will be most easily fulfilled by high-quality, low Noack Group III or Group III+ base oils that minimize any light-ends evaporation leading to premature finished lube thickening and fuel economy losses.

Looking at the combined CK-4 and FA-4 specifications with a 13 percent Noack for 0W-XX, 5W-XX and 10W-XX grades, the higher the base stock viscosity index, the better the chance of attaining and retaining the finished lube viscosity. In addition, such higher viscosity index components are less prone to oxidative thickening because they respond better to antioxidant additives. Hence, even 10W-40 or 15W-40 could benefit from some Group III content.

With an emphasis on fuel economy and fuel economy retention in FA-4, we must not overlook every opportunity to minimize evaporative losses in heavy-duty formulations. This provides the incentive to use low Noack Group III as a diluent oil for viscosity modifier concentrates where possible, rather than more conventional Group I, which also affects sulfur limits.

Even Group II diluent oils will not provide the same Noack benefit. Group IIs have marginally more solvency for viscosity modifiers than Group III due to their chemical composition, but this is not usually a constraining issue for using Group III as diluent oil. All-in-all, this suggests it will be more effective to formulate with Group III/III+ rather than Group II.

The FA-4 specification has an HTHS range of 2.9 to 3.2 mPa-s for 5W-30 and 10W-30 oils, a definite first for an API heavy-duty category. The industry has understandable reservations about moving to lower viscosity grade oils even with their fuel economy benefit. Perhaps the difference here is that such grade migration is an elective process, in contrast to the mandated migration to chemically constrained oils starting with API CJ-4 (reduced sulfur, ash and phosphorus). These limits remain the same for CK-4/FA-4.

Chemical constraints to protect emission control devices potentially have equally significant impacts on lubricant performance, oil drain interval and engine life. However, chemically constrained API CJ-4 oils in more conventional 15W-40 viscosity grades have proven themselves in the field relative to their predecessors.

In fact, they proved themselves well before introduction to customers in the form of statistically designed and controlled field trials run by additive companies and lubricant marketers. This is in addition to the sequence engine tests that they passed during new formulation developments.

The same preparation will precede the introduction of low-viscosity grade/low-HTHS FA-4 oils as well as the more conventional viscosity grade CK-4 oils. Returning to Petterssons comments, such fuel-efficient versions will need to be supported with state-of-the-art antiwear and extreme pressure additives. We can be confident they will.

Look to the Future

There was a time, around a decade ago, when the 0W-20 grade was seen as revolutionary for light-duty diesel engines. It is now a routine factory-fill grade for these engines.

Currently, OEMs are pushing for SAE 16, 12 and even 8 grades for ultra-fuel efficient light-duty engines. With greatly improved manufacturing tolerances on highly automated engine production lines, there is no reason why the industry cant fully embrace the opportunities offered by advanced base stocks to enable greater fuel economy advantage in the heavy-duty diesel sector as well.

The hope is that API FA-4 and CK-4 will demonstrate that we can enjoy both excellent fuel economy lubricants and reliable engines. The world will learn a lot from the API and the U.S. in terms of heavy-duty fuel-efficient oils. Even if the uptake is small in the first instance, the huge customer base will constitute a large trial of the acceptability of fuel economy engine oils in heavy-duty engines.