Automotive

What got me going on this topic was a note from a reader, John Green from Chevron, responding to Octobers column (Lubeorama) in which I highlighted some of the ongoing issues with ASTM D5800, the Noack volatility test method. I replied to John that the whole question of volatility is crucial to successful engine operation. And as we move to lighter weight engine oil viscosities, such as SAE 0W-XX grades for passenger cars and 10W- or even 5W-XX for heavy-duty vehicles, the impact of volatility on successful engine protection becomes even more critical.

This exchange got me thinking about the subject of oil volatility and where we are going with it. Lower finished-oil viscosity typically means lower base stock viscosity, and that results in higher volatility. Unless volatility is controlled, lighter viscosity products will evaporate more quickly, escaping past the engines piston rings and valves and leaving less engine oil behind. The remaining oil thickens and has a shorter useful life. Volatility is a key reason we check oil levels and top-off the system, to restore whats lost.

I can tell you that 40 years ago, volatility was just becoming an issue in the lubricants industry. The ASTM Sequence IIIC engine wear test had an operational requirement that oil consumption not exceed 64 ounces during the test. The consumption losses were measured every eight hours. Failing this part of the test was not considered a failure of the oil but was noted as a mechanical failure.

One oil source, I recall, made extensive use of a 90 neutral base stock (no longer available) to formulate the then-popular SAE 10W-40 grade of engine oil. Unfortunately, this 90 neutral oil was very volatile. In fact, you could look into a engine test bay and know what oil was being tested by the presence of a blue haze in the air.

Since then, volatility has played an increasingly important role in engine oil development. Limits on volatility first appeared with the ILSAC GF-1 and API SH oil categories in the early 1990s. The limits have been tightened since then, as the table below shows, yet no real change has been made since the introduction of GF-3 nearly 15 years ago. Were left to wonder why volatility was included in the categories, as well as why no change has been made in the limits despite having two more upgrades.

Some drivers for change were there for a long time but many are new. In no particular order Ive identified the following needs from the voices in the industry as well as my understanding of whats happening.

First, oil volatility obviously has an impact on oil make-up and how often oil needs to be changed. Going back to my blue-smoke story, the volumes of oil consumed in that test would probably represent oil make-up rates in the 1-quart-per-1,000-mile range, which is unacceptable to modern engine manufacturers (and most consumers).

Every time an engine runs with less than normal oil levels, the remaining oil is stressed even more with the result that it needs to be changed more frequently. Again, not what original equipment manufacturers want to see and certainly not in line with the oil change habits of most motorists.

Second, oil volatility can wreak havoc in an engine in other ways. Currently, the Auto-Oil Advisory Panel, the inter-industry group creating the GF-6 oil upgrade, is designing a new engine test to address the phenomenon known as low-speed pre-ignition. LSPI involves the small-displacement, direct injected and turbocharged engines which are beginning to take a larger place in the market. These engines offer good performance coupled with excellent fuel economy, which is the ongoing goal of OEMs everywhere.

Unfortunately, these engines also seem to be sensitive to oil getting into the combustion chamber and triggering pre-ignition events (actual detonations). It doesnt take very many of these events, especially if they are significant, to destroy an engine. What role does oil volatility play in this problem? Its unclear yet, but I suspect the LSPI test may shed some light.

Certainly, volatility is an important part of the engine oil oxidation and wear test, currently the ASTM Sequence IIIG. Volatility strongly affects engine oil viscosity, and it may be even more of a problem in the IIIGs associated low-temperature pumpability test measurement.

I do have some concerns about how the oil industry can handle the continued dual quest for lower viscosity coupled with lower volatility. Lets face it, base oil refiners are reluctant to reduce volatility because their yields suffer. So they wont lower it without getting real value in return (i.e., a better price). Simply put, if you reduce volatility you are removing some of the lighter ends from the base oil. However, that means you have to take some of the heavier ends out as well if you want to meet the same targeted viscosity. Bottom line is that you create a narrower cut – and make a lot of other stuff (like gas oil) that may be less valuable or cant find a home.

Lubricant blenders have some tricks up their sleeves to find a partial solution to the viscosity/volatility dilemma. They can reduce the amount of light neutral, and blend in something heavier to get the viscosity right. Thats whats called a dumbbell blend. Sometimes they use high-vis polyalphaolefin added at low treat rates to API Group II and III to improve the viscosity index, while also reducing the volatility of the finished fluid. The positives of a workaround for blenders are that they can use available components, and probably do it at less expense than with a narrow(er) cut.

I expect well continue to see efforts by OEMs to lower viscosity and volatility in engine oils. To start, General Motors proprietary Dexos1 specification has a 13 percent Noack limit (two points below the ILSAC spec). So do Europes ACEA specifications.

Most recently, the Italian truck engine builder Iveco worked with Petronas Lubricants to develop an SAE 0W-20 heavy-duty engine oil. Called Urania Next, this product launched commercially in December. It demonstrates up to 2.5 percent better fuel economy, which is quite appealing if you own a truck fleet.

Iveco and Petronas worked on and road-tested this oil for more than five years, which makes me think theyre pretty secure with it. The key ingredient is Group III+ base oil from Petronas refinery in Melaka, Malaysia – which not incidentally has a viscosity index of 130 and Noack volatility of 5.8 percent. (Thats for the 6 cSt grade; Noack for the 4 cSt grade is 11.8 percent.)

Another example is Mercedes-Benzs NanoSlide technology, described in our September issue. NanoSlide is a microns-thin, low-friction coating that is spray-welded inside the cylinders of MBs light-duty diesels. It delivers a boost in fuel economy and also strong wear protection, all while using Mercedes normal top-tier, PAO-based SAE 5W-20 engine oils. The oil specification, MB 229.51, requires Noack volatility of less than 10 percent, and allows drain intervals of 30,000 kilometers (18,000 miles). If the automaker switches to a lighter grade for factory fill or wants to strive for longer drain intervals, I can see it taking another look at the volatility.

As if that werent enough to challenge us all, there is an effort under way to displace the traditional volatility tests (most notably ASTM D5800, the Noack test) with a thermo-gravimetric method which many say will give much better results. Another method, HPLC or high performance liquid chromatography, also gives good results and is part of the API category requirements.

Certainly, the direction of engine oil viscosity is towards lower values. All you need to determine that is the introduction of the SAE 16 grade into SAE J300, the document that governs engine oil viscosity classification system. Next, a request is reportedly in front of SAE to define SAE 8 and SAE 12 grades as well.

As viscosities keep going down, how much longer can volatility remain static in the ILSAC and API categories?

The viscosity of finished engine oils will keep going down until there is insufficient lubricity to protect surfaces in contact. The additive guys can make it better with the right chemistry but they are locked into a detergent/dispersant/antiwear/

antioxidant/friction modifier system. There might be a better way although I dont know what it is. (But Ive had inklings from some of my sources.)

As for base oils, they will be lighter and narrower cut to get the viscosity right. When the cut itself is deemed too volatile by the OEMs, we will have reached the end.

The bottom line is that both base oil and additive chemistry is in play here and the winner will be the one who can couple both. Look for a black swan to bring it to the industry.

Industry consultant Steve Swedberg has over 40 years experience in lubricants, most notably with Pennzoil and Chevron Oronite. He is a longtime member of the American Chemical Society and SAE International, where he was chairman of Technical Committee 1 on automotive engine oils. He can be reached at steveswedberg@cox.net.