More Dirt, Less Ash

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It can sometimes seem that advances in heavy-duty engines crawl along like the slow and steady tortoise compared to the sprinting hare of passenger car engines. But heavy-duty lubes are steadily heading in the same direction as light-duty oils, with sulfated ash the latest component of focus, especially for off-highway applications.

While different specifications focus on different aspects of performance—for example, Europe’s ACEA E9 specification puts more emphasis on biodiesel compatibility than other standards—general trends in the latest specs have moved toward improved piston cleanliness, oxidation control and fuel economy, as well as longer drain intervals, according to Ruud ter Rele, Chevron’s industry liaison for heavy-duty OEMs. 

Speaking at the UNITI Mineral Oil Tech­nology Congress last year, Rele noted that modern heavy-duty engines generate less soot than the older models used for engine tests. A more realistic approach to soot-handling requirements in the newest specifications has enabled formulators to improve fuel economy and piston deposit control while extending drain intervals.

Global demand for the latest in engine oil technology is about to take a leap. Outside of more developed markets, sales of newer heavy- and medium-duty vehicles will increase 178 percent from 2019 to 2021 as governments adopt Euro VI type emission standards, according to Rele. Major increases will be seen in China and India, as well as Mexico, as that country adopts standards equivalent to 2010 U.S. laws.

“One obvious change on the horizon is continued fuel economy regulations,” Shawn Whitacre, senior staff engineer with Chevron, told Lubes’n’Greases. Stricter greenhouse gas emissions limits will require changes to engine design and further exploration of lower-viscosity oils than what is currently on the market. 

Lube formulators have been enabling better fuel economy by pushing down viscosity for a few years. Japan launched its JASO DH-2F heavy-duty fuel economy grade in 2015. North America and Europe have both chosen to focus on lower high-temperature high-shear viscosity. API FA-4 debuted in 2016 as SAE XW-30 oil characterized by HTHS viscosity limits of 2.9 to 3.2 mPa. Europe will introduce its own specifications with the same HTHS range, ACEA F8 and F11, sometime this year.

But overall, heavy-duty engine oils have been much more resistant than passenger car lubricants to a shift to lower viscosity grades. According to Chevron, even in North America SAE 5W-30 is only 2 percent of overall heavy-duty engine oil demand. More than 80 percent of the market is SAE 15W-40, and 40-grade oils are used most around the world.

Fuel economy matters less for off-highway applications, and manufacturers and operators in that segment are skeptical that lower-viscosity oils could ever provide sufficient protection for their precious investments.

“Durability and protection of machine assets is critical. We want machines to last forever, and they have to undergo several overhauls and still be productive,” Hind Abi-Akar, fluids technical expert for Caterpillar, told the ICIS Pan American Base Oils & Lubricants Conference in December.

An oil film must be able to stand up to high shear, high temperatures and high pressures, she explained. It also has to be robust enough to withstand chemical contamination and keep dirt particles from causing wear on components—a particular problem for off-highway applications. A dirt particle that passes through bearings just once leaves a mark. “If these machines didn’t work in the dirt, they could use low-viscosity oils, but that’s not the operating environment.”

One opening for thinner oils may be in cold-temperature climates. When large engines start up in temperatures around 5 to 10 degrees Celsius, there is ample opportunity for a thick oil to bypass the oil filter, allowing contaminated lubricant to circulate in the machine, Abi-Akar pointed out.

For Caterpillar, most factory fill is now SAE 10W-30 API CK-4 engine oil. “It can go anywhere in the world from Africa to Antarctica,” she said. However, she noted that customers like 40-weight oils because of the durability they provide. “We will see a shift, but it will be slow. Smaller machines can move much more easily.”

Viscosity shifts may not be in the off-highway cards anytime soon, but particulate emissions regulations that began with on-highway applications have now caught up to off-highway operations in the North American market. Emissions requirements set by the United States Environmental Protection Agency, known as Tier 4, began phasing in for off-highway vehicles in 2011, with the strictest limits reached in 2014. Tier 4 has had “a profound recent impact on consumers, especially large companies such as construction and mining operations weighted towards newer pieces of equipment,” said Whitacre.

To meet Tier 4 standards, new equipment began using advanced emissions control systems such as exhaust gas recirculation, diesel particulate filters and selective catalytic reduction. These systems require greater scrutiny of fluids, he explained. For fuels, this means lower sulfur content and particulate contamination. For lubricants, lower levels of sulfated ash are required to keep emissions systems running properly.

Diesel particulate filters are very effective, according to Whitacre. The devices trap 98 percent of particulates, which burn off periodically or continuously when the engine temperature is right. However, DPFs are so efficient they also trap components of consumed lubricating oil that can slip past rings in the combustion chamber. Some of that material is incombustible and stays behind on the filter, much like ash in a fireplace, plugging it up and keeping the DPF from working properly. 

Different types of metallic additives create ash at different rates, according to James Booth, Chevron’s North American commercial sector manager. Sulfated ash is the total amount of noncombustible material, and it has a direct correlation to the level of filter plugging. 

Sulfated ash comes from functional metallic components in the oil’s additive package, most notably detergent, which is typically calcium and magnesium based and is used to neutralize acids, control piston deposits and improve overall engine cleanliness, Whitacre said. Antiwear chemistry is another contributor, especially zinc dialkyldithiophosphate, which is a performance- and cost-effective, multifunctional additive for wear control and oxidation stability used in most engine oils on the market today. Its levels have been constrained in passenger car engine oils since the late 1970s because of the phosphorus it contains, which can poison catalysts in light-duty emissions systems.

Periodically, ash must be cleaned from DPFs in order for the filters to function properly. For trucks, this process is straightforward. But for off-highway equipment, it can be disruptive to the normal maintenance process. Equipment often runs in remote locations nearly 24 hours a day, seven days a week. The machines must be driven to a maintenance facility and can take as much as a day to cool down before they can be serviced, at which point the DPFs are often difficult to reach. In all, cleaning or changing a DPF can add up to three days out of service.

That’s assuming the DPF can be cleaned, which typically costs about $700 per filter, according to Chevron. A new filter runs between $3,000 and $7,000.

The API CJ-4 heavy-duty engine oil service category reduced ash levels to 1 percent—enough for the functional components to do their job but low enough to allow for a reasonable maintenance interval. This requirement remains the same in the most recent API CK-4 and FA-4 twin performance categories. 

In December, Chevron debuted a heavy-duty motor oil aimed at the off-highway market with just 0.4 percent sulfated ash. Marketed as Delo 600 ADF, the oil reduced metallic additives by 60 percent from the company’s Delo 400 products, according to Booth.

Formulators used organic chemistry to replicate the way metallic additives protect against wear and meet other performance goals. While cam wear performance was strong from the beginning, Booth said, other targets were more elusive. The company ran into “some challenges along the way, requiring some different thinking and looking at different chemistries and combinations of chemistries to deliver in all of the performance areas required for API and OEM specifications.”

The hope is that with less ash, the oil will reduce the need for DPF cleaning, increasing uptime and cutting maintenance costs. Less filter plugging could also translate into fuel economy advantages by keeping emissions control systems clear and allowing engines to operate more efficiently, he continued.

The new oil can also be used for on-highway vehicles and could be particularly beneficial for garbage and delivery trucks, for example, which have duty cycles including more idle time and stop-start driving. Such conditions contribute to DPF clogging because engines spend less time at the ideal operating temperature for passive regeneration cycles (burning off).

For now, on-highway and off-highway heavy-duty applications will likely continue to use the same oil specifications. But “there may be a point in time when we need to think about a dedicated off-highway performance specification or something more uniquely tailored to that market,” Whitacre commented. “But that presents some challenges of its own.”