Currently, heavy-duty applications on both on-road and off-road vehicles may continue to use the same engine oil specifications. However, at some point, it might become necessary to consider a specialized off-road performance specification or develop unique products that are better suited to this market. This, in itself, also presents certain challenges.

Sometimes, the development of heavy-duty engines seems like that of a slow and steady tortoise, while the development of passenger-car engines resembles that of a swiftly darting hare. However, heavy-duty engine oils are steadily moving in the same direction as their lighter-duty counterparts, with sulfuric ash content emerging as a newer focus—particularly in off-road applications.

Fuel economy boost

According to Lü De Ter Reel, an industry liaison for Chevron’s heavy-duty original equipment manufacturers: Although different oil specifications emphasize different performance characteristics—for example, the European ACEA E9 specification places greater emphasis on biodiesel compatibility than other standards—in fact, the general trend in newer lubricant specifications has been toward improving piston cleanliness, oxidation control, fuel economy, and extending service intervals between emissions-related maintenance.

Rael pointed out that modern heavy-duty engines produce less particulate matter than older models used for engine testing. A more realistic approach to particulate matter control under the newer technical standards enables formulation designers to extend emission intervals while improving fuel economy and controlling piston deposits.

Global demand for newer engine oil technologies is poised for a dramatic surge. According to forecasts, outside more developed markets, as governments around the world adopt Euro VI emission standards, sales of new heavy- and medium-duty vehicles are expected to grow by 178% from 2019 to 2021. Moreover, with China, India, and Mexico adopting standards equivalent to those stipulated by U.S. law in 2010, these two markets will experience substantial growth.

Sean Whittaker, a senior staff engineer at Chevron, stated: “A notable change is the continued implementation of fuel economy regulations.” Stricter greenhouse gas emission limits will necessitate changes in engine design and further development of low-viscosity motor oils based on current market conditions.

In recent years, lubricant formulation designers have been enhancing fuel economy by reducing viscosity. In 2015, Japan introduced the JASODH-2F heavy-duty fuel-economy rating. Both North America and Europe have chosen to focus on motor oils with lower high-temperature high-shear (HTHS) viscosity. The API FA-4 specification was first released in 2016 as SAEXW-30 oil, with an HTHS viscosity range of 2.9 to 3.2 megapascals. This year, Europe will introduce specifications—ACEAF8 and F11—that are identical to the HTHS series.

However, overall, demand for heavy-duty engine oils in this regard is rather subdued compared to the preference for low-viscosity oils in passenger-car engine oils. According to Chevron, even in North America, SAE 5W-30 accounts for only 2% of total demand for heavy-duty engine oils. More than 80% of the market is dominated by SAE 15W-40, making it the most widely used engine oil worldwide.

Off-road application development

Fuel economy has a relatively minor impact on off-road applications, and manufacturers and operators in this sector are skeptical about whether low-viscosity engine oils can provide sufficient protection for their valuable investments.

“The durability and protection of machinery assets are crucial. Hedd Abia Carr, a lubrication technology expert at Caterpillar, said: ‘We want our machines to keep running—and of course, they’ll need several major overhauls to continue producing.’”

She explained that the oil film must be able to withstand high shear forces, high temperatures, and high pressures. It must also be sufficiently robust to resist chemical contamination and prevent dust particles from causing component wear—a particular challenge in off-road applications. Each particle of dirt that passes through the bearings leaves a mark behind. If these machines were operating in clean environments, they could use low-viscosity oils; however, the actual operating conditions are far from ideal.

Under cold climatic conditions, the oil film may develop isolated, thinner oil pockets. Abia Carl pointed out that when large engines are started at temperatures around 5 to 10 degrees Celsius, there is ample opportunity for the viscous engine oil to bypass the oil filter, causing contaminated lubricant to circulate throughout the machinery.

As for Caterpillar, most of its plants now use SAE 10W-30 API-4 engine oil. “It’s highly versatile and can be used anywhere in the world—from Africa to Antarctica,” she said. However, she pointed out that customers prefer heavier-grade oils in the 40 viscosity class precisely because of their durability. “We’ll see a shift, but it’ll be a slow one. Smaller machines may be more likely to undergo changes sooner.”

Viscosity changes may not occur rapidly in the non-road sector, but particle emission regulations—initially introduced for on-road applications—are now targeting non-road operations in the North American market. The emission standards set by the U.S. Environmental Protection Agency (commonly referred to as Tier 4) have been gradually implemented for non-road vehicles since 2011, reaching stricter limits by 2014.

Tier4 has had a profound impact on consumers, particularly large enterprises—such as those in the construction and mining industries—which tend to upgrade their equipment.

Emission Control Research

To meet Tier 4 standards, new equipment is beginning to employ advanced emission control systems, such as exhaust gas recirculation, diesel particulate filters, and selective catalytic reduction. These systems require more stringent monitoring of lubricating oils. For fuels, this means lower sulfur content and reduced particulate contamination. As for lubricating oils, they need to have lower levels of sulfate ash to ensure the proper functioning of the emission control systems.

Diesel particulate filters are effective. This device can capture 98% of particulates, and when the engine temperature is appropriate, these particulates will periodically or continuously burn off. However, diesel particulate filters (DPFs) are so efficient that they can also trap components from consumed lubricating oil—components that might otherwise slip past the piston rings into the combustion chamber. Some of these materials are non-combustible and remain behind in the filter, much like ash in a fireplace, eventually causing blockages that prevent the DPF from functioning properly.

James Booth, Manager of Chevron’s North American Commercial Division, stated: Different types of metal additives produce ash at varying rates. Sulfate ash is the sum total of non-combustible materials, and it has a direct correlation with the degree of filter clogging.

Sulfate ash originates from the functional metallic components in lubricant additive packages, most notably detergents—typically calcium- and magnesium-based—that neutralize acids, control piston deposits, and enhance overall engine cleanliness.

Anti-wear chemical additives represent another influential factor—particularly zinc dialkyldithiophosphates, which are high-performance, cost-effective, and multipurpose additives used to control wear and enhance oxidative stability. They are now widely incorporated into most engine oils available on today’s market. Since the late 1970s, however, their use in passenger-car engine oils has been restricted due to the phosphorus they contain, which can poison catalysts in lightweight emission-control systems.

To ensure the filter functions properly, the ash in the diesel particulate filter (DPF) must be removed periodically. For trucks, this process is relatively straightforward. However, for off-road equipment, it can disrupt the normal maintenance routine. Such equipment typically operates in remote areas, 24 hours a day, seven days a week. These machines need to be adjusted to meet facility maintenance requirements, and before maintenance can begin, they may require an entire day to cool down—a task that is extremely difficult to accomplish with a diesel particulate filter (DPF). In short, cleaning or replacing the DPF could result in a three-day downtime.

Chevron stated that this assumes the diesel particulate filter (DPF) is washable; typically, each filter costs around $700. The cost of putting a new filter into operation ranges from $3,000 to $7,000.