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Mark Rees has the issue of chemical limits in the formulation of engine oil in sharp focus. The global technology manager for Lubrizol Corp. in Wickliffe, Ohio, told LubesnGreases recently, The emissions strategy of OEMs globally, as they target todays and future emissions limits, is a major driver for lubricant design. For a number of years weve recognized the move toward lower SAPS [Sulfated Ash, Phosphorus, Sulfur] as a key industry trend, and have done a lot of work toward gearing our R&D around understanding what the consequences of low SAPS are, on both formulation development and the development of new additive componentry itself.

Lubrizol is the worlds largest developer and manufacturer of the chemical additives which are blended into base oils to form finished lubricants, and its no surprise that it has jumped into the SAPS issue with both feet.

But it is not alone. The three other major lubricant additive companies – Afton Chemical of Richmond, Va., Chevron Oronite of San Ramon, Calif., and Infineum International in Abingdon, U.K. – also are responding to the SAPS challenge. Theyre all alert to the likelihood that vehicle manufacturers will continue to push down certain chemical limits, both in the United States and Europe.

Phosphorus: Strike One

Phosphorus – the first engine oil component targeted by automakers – has been known to poison the catalysts used in vehicle exhaust systems, shortening their effective life. So automobile manufacturers have insisted on a continuous decrease in the amount of phosphorus in engine oil, starting with a ceiling of 0.12 percent mass in ILSAC GF-1 quality oils, then 0.10 for GF-2 oils, and finally just 0.08 percent in todays ILSAC GF-4 oils.

The GF-1 phosphorus limit in the mid-90s was the first chemical limit, observed Tom Cousineau, director of customer technical service for engine oils at Afton Chemical. It wasnt much of a formulation hurdle for us to meet that initial limit, but with todays GF-4 its a different story. Phosphorus is a major component of zinc dialkyldithiophosphates [ZDDP], the most effective, well-known and cost-effective antioxidant and antiwear agent available. We were concerned about meeting oxidation limits, which we did by adding extra antioxidants.

Many worried that cutting back on ZDDP to meet the phosphorus restriction would hurt the oils antiwear performance. But Cousineaus colleague, consultant Charlie Passut, pointed out, The antioxidant issue was the decisive one. The antiwear protection lined up pretty well because we saw that OEMs were building engines which were much more resistant to wear – the valvetrains, for example. Moreover, the more stringent requirements of the TEOST [high-temperature deposit] test, and not the new IIIG engine test nor ZDDP, turned out to be the limiting factor for antioxidants in passenger car motor oil. More antioxidants were required to meet TEOST limits, and this offset the reduction of ZDDP required by the lower phosphorus limit.

Sulfur: Strike Two

ILSAC GF-4, adopted in January 2004, was the first passenger car motor oil where the phosphorus limit was joined by a sulfur limit. Sulfur is limited to 0.5 percent mass for 0W-XX and 5W-XX oils, and 0.7 percent for 10W multi-grades. Bob Olree of General Motors, who chaired the ILSAC/Oil Committee that developed GF-4, said automakers sought a sulfur limit for the new engine oils because sulfur is a catalyst poison, fuel sulfur levels are being reduced worldwide, and emission regulations are asking for more and more controls.

For GF-4, Olree noted, we recognized the difficulty of meeting the 0.5 percent maximum for all viscosity grades and so we agreed to a 0.7 percent level for the 10W grades, because of the use of Group I base stocks for these grades. However, this is a one-time exception and will not be repeated in any future quality upgrades.

As with earlier phosphorus limits, Aftons Cousineau noted, meeting the new GF-4 sulfur limit was not a significant formulating challenge [for additive companies]. However, the limit was significant for oil companies because the sulfur limits major impact was on the base oil. Roughly two-thirds of the new sulfur limit is contributed by the average additive package, an essential component, which then compelled oil companies to make certain the base stock sulfur contribution didnt take the finished oil above 0.5 percent. That meant reducing the use of higher-sulfur Group I solvent refined base stocks, and increasing sulfur-free hydrotreated (and more expensive) Group II and III base stocks.

Sulfated Ash: At Bat Now

Chemical limits are not only being seen in passenger car engine oils, however. As emissions control devices begin to penetrate the heavy-duty diesel engine market, the same ratcheting down of certain chemicals is being felt.

Modern additive packages contain multiple components based on metals such as calcium, magnesium, zinc, etc. When an oil containing these elements enters the combustion chamber and is burned, the ash that results can contribute to deposits in the crown land above the piston ring as well as to deposits in the ring grooves. These deposits can lead to rubbing wear on the cylinder liner, and piston rings that dont operate freely. Ultimately, as the cylinder liner-to-ring interface is compromised, high oil consumption will result.

However, the more urgent issue is sulfated ashs impact on aftertreatment devices – particularly diesel particulate filters – needed to meet EPA-mandated emission levels for 2007 model year engines as well as the Euro 4 levels which took effect in October. To protect diesel emissions systems, the PC-10 heavy-duty oil now under development will have strict limits of 0.10 percent for phosphorus, 0.4 percent for sulfur and 1 percent for sulfated ash. This will impact the amount of metal-containing detergents that can be used, so alternative detergent and dispersant technology will be needed, Lubrizol research indicates. And at 0.4 percent, the sulfur limit for PC-10 is lower than GF-4s, pointed out Aftons Passut. But we dont anticipate a major problem in meeting it.

He emphasized, In this new category, sulfated ash is the most critical limit. While theres some zinc in ZDDP, most of the heavy-duty ash is coming from calcium or magnesium.

Infineums Global Heavy Duty Manager Rick Finn agreed, saying, Sulfated ash is where the major action is for the additives industry, and this will drive significant added cost into the new formulations. As with passenger car engine oils, sulfur limits for diesel oils will have an even sharper impact on base stock suppliers, forcing a greater shift to low-sulfur Group II stocks.

Racing to Comply

Technical specifications for PC-10 are being aggressively hammered out in ASTMs Heavy Duty Engine Oil Classification Panel, chaired by Jim McGeehan of ChevronTexaco Global Lubricants. One of the first issues confronting the panel, and resolved in mid-2004, was chemical limits for SAPS.

McGeehan explained, The 2007 on-highway emission standard reduces particulates by 90 percent from the current 2002 limits. Consequently, all on-highway diesel engines will use Diesel Particulate Filters to meet this standard, as the particulate standard cannot be achieved with improvements in in-cylinder combustion alone. In addition, EPA requires that all heavy-duty DPFs must achieve 150,000 miles before cleaning to remove the ash from the engine oil.

He confirmed, Research with DPFs indicated that increasing lubricants ash directly increased the ash in the DPFs.

The Classification Panel agreed there might be a need for chemical limits for PC-10 and formed a task force chaired by Infineums Finn with representatives of vehicle manufacturers, additive suppliers and oil companies. As Finn commented, The task force cast a wide net across the U.S., Europe and Japan in search of data relating oil SAPS to aftertreatment device operation. Only a limited amount of data were provided, so limits were established using best engineering judgment on the data available.

ACEA E6 and API PC-10 are just the first heavy-duty categories with chemical limits, he added. We expect tightening of these limits in the future if NOx traps are introduced, which will pose significant formulation issues as well as an industrywide continuing shift to low-sulfur base oils.

Europe will see an emissions upgrade in 2008 which could force broader lubricant chemical restrictions. GF-5 is scheduled for 2009 and the next heavy-duty upgrade in the U.S. is possible as early as 2010.

The Balancing Act

Over and over, a single concept comes up: balance. Lubrizols Rees put it, As we go down in SAPS, the key area that will be stressed continually will be the oxidation performance of the oil. At the same time were hearing from OEMs that thermal severity will be getting more severe rather than milder so we have to consider that aspect. Thats a balance we need to make.

Another balance to consider when the sulfur level is decreased, is the significant impact on base stock selection, moving from Group I to higher-quality stocks. Those are the primary effects we consider.

A careful balance of elements and measures makes formulating a new engine oil, particularly with several chemical limits, a high-tech, high-wire effort.

Another is the cooperation that must take place among the oil, additive and OEM industries. Wim Van Dam, Chevron Oronites product development engineer for heavy-duty engine oils, illustrates the point: When GF-1 first came out there was a lot of talk about the sensitivity of catalysts to phosphorus, he remarked, but it turned out that we were able to rather easily reformulate, stay within the new limits, and achieve the required performance level.

More recently, in the years leading up to CI-4 heavy-duty engine oils, there was a great deal of concern that lubricants for engines with exhaust gas recirculation would see high levels of acid contamination, leading to unacceptable TBN depletion and significant reductions in drain intervals. That didnt happen. Engine builders found out how to build EGR [exhaust gas recirculation] into their engines so that it was not as severe on acid contamination as initially thought. We were able to use conventional additives boosted with supplemental inhibitors. Weve seen the benefits of the technological and formulation learning curve over and over.

Theres little doubt that vehicle manufacturers will continue to batten down the existing limits on SAPS. But adding more chemicals to the list seems unlikely, according to Infineums Finn. We dont see additional elements being constrained because we havent seen any data that would suggest adding to SAPS. There are some instances of chlorine limits within European OEM specs and in Japan, but without data we would not support any additional chlorine constraints.

According to Finn, SAPS limits are evolving independently in the three main oil-regulating geographic areas – U.S., Europe and Japan. Vehicle manufacturers in each area have adopted different technological approaches to meeting emissions requirements. For example, while PC-10 will have SAPS limits in the U.S., driven by the use of diesel particulate filters, most Euro 4 engines will not require oils with chemical constraints as [these engines] use Selective Catalyst Reduction technology, which is not impacted by lubricant SAPS. Japan is following a path similar to the U.S. and has SAPS limits for JASO DH-2 oils that are designed for EGR/DPF engines.

Aftons Passut added, Some European OEMs have chlorine limits, and weve been dealing successfully with that limit. Weve seen some concern with heavy metals being used in European formulations, but have seen no evidence that this will be an issue in the U.S.

Any Better Way?

From day one of that first ceiling on phosphorus, there has been a universal desire to develop a performance test to serve in lieu of chemical limits.Thats because todays SAPS limits are based to some degree on caution and guesswork, not on complete knowledge of how much or how little of a chemical can be tolerated without harm. Most agree that some limits are in order; few agree on how to set them.

However, the only attempt at developing a performance test in lieu of a chemical limit was the Ford-sponsored Oil Protection of Emission System Test in the mid-1990s, which tried to measure the effects of engine oil phosphorus. Ford made two attempts; both failed. There are no other performance tests in the pipeline to address a SAPS chemical limit, and none are foreseen.

Chemical limits put a restriction on our future formulation innovation capability, Lubrizols Rees states. But chemical limits, its apparent to all, are here to stay.

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