Ultra-low Ash Lubes Benefit Engines

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Ultra-low Ash Lubes Benefit Engines
A line of semi tractor trailer trucks on a lot in Indianapolis, Indiana. © Jonathan Weiss

Leveraging ultra-low ash lubricants’ benefits in diesel engines – extended diesel particulate filter maintenance intervals and enhanced fuel economy retention – could become more pivotal as emission standards and engines’ useful life requirements become more stringent, a Chevron official said in a presentation at an SAE International online event last week.

The industry has learned more about the effects of ash accumulation over the years, Shawn Whitacre, a senior staff engineer at Chevron, said during his presentation, “Reduced Maintenance and Fuel Consumption from Ultra-low Ash Lubricants,” at SAE International’s Powertrains, Fuels and Lubricants Digital Summit held Sept. 28-30.

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“One of the things I’ve done is try to quantify the impact on fuel economy performance,” Whitacre said. “We know the ash clogs the [diesel particulate filter]. That can do a couple of things – it has an impact on back pressure, exerted by that system, and we know that increases in back pressure reduce fuel economy performance directly.”

He added that this can also interfere with regeneration cycles and frequency, “so the more often that you’re regenerating, the more fuel you’re burning,” he said. “Both of those impacts can contribute to fuel economy penalty over time unless other things are done to temper that effect.”

Ultra-low ash oil can help alleviate those fuel economy impacts. Testing found that the back pressure impacts alone contributed to a lifecycle fuel economy retention advantage on or about 0.7%, he said, which came in addition to advantages seen through keeping regeneration frequency minimized, that was an additional 2.3%. “So the net effect of those two improvements reduced fuel consumption over the lifetime of the system of 3%,” Whitacre said. “That’s in addition to the maintenance-related advantage an ultra-low ash oil can bring.”

Whitacre said the industry collectively has done a remarkable job over the years in developing drastic improvements to reduce emissions from diesel engines. “One of the significant challenges we’ve faced as industry is how to make things efficient and elegant and make sure it doesn’t contribute to significant increases in costs,” Whitacre said.

These costs come in various forms both up front and ongoing because of various components that are used that can be costly, he said. These include upfront, maintenance and operational costs. The upfront costs are associated with precious metal loaded emission control devices, along with such components as dosing units and sensors. The maintenance costs include diesel particulate filter ash cleaning, along with repair and replacement. The operational costs are associated with diesel exhaust fluid for selective catalytic reduction system operation and fuel economy penalty.

He noted that fuel burned and consumed in the engine is not intended for making power. “Instead, a lot of the time it is focused on increasing the thermal profile of the exhaust system to make these systems more active, more efficient,” Whitacre explained. “There’s literature that suggests 4%-5% of fuel use is due to emission control system management. That’s something that, depending on the situation, can get worse over time. And a lot of these considerations have motivated us, both as industry and more specifically at Chevron, to look at ways to further optimize these systems.”

Whitacre explained that lubricant formulations typically are about 75% or so base oil and around 25% chemical additives. “Those additives we’ve seen constraints placed on it over the years, about some of the things that can be incompatible with after treatment systems,” he added.

Engine oil formulators have had to learn how to do more with less, he said. “There’s been constraints placed on levels of sulfur, phosphorous and something called sulfated ash,” Whitacre noted. “That, since about 2007, has been front and center in prevailing standards both industry and [original equipment manufacturer] specifications that put limits on how much of those things we can utilize in lube oils,” he said. “And to a large extent most oil formulators take full advantage of that budget, so a lot of products you see on market do have up to the top end of the allowable limit for this property called sulfated ash, because we need all that chemistry to make high performing oils against these very demanding standards.”

He pointed out that the limits are in place because of the impacts on the exhaust system, most noticeably on the diesel particulate filter. He explained that the fuel economy penalty increases over time as the filter fills with ash. The engine burns more fuel to push exhaust gas and raise temperature for regeneration. “These are highly efficient systems that trap the particulate matter in the exhaust, and when the exhaust system conditions are favorable, it burns that off either continuously or discretely through a regeneration of that,” Whitacre explained. “What doesn’t burn is the incombustible fraction of the lube oil, a little bit of which ends up in the exhaust over time through normal oi consumption. Therefore, something needs to be done.”

Most commonly, he said, the system is removed, and the ash is blown out. “That’s primarily what has motivated the limitations that we have on sulfated ash, and certainly what motivated us to look at maybe pushing the boundaries on that, looking at what we’re calling ultra-low ash engine oils,” added.

He added that ultra-low ash lubricants also have equal benefits for the off-highway market, even if that market has been lagging in terms of emission standards. “Even though the core principles are the same, some of the value drivers are different in that [off-highway] market, both in terms of the way those systems are designed, and in the ways those operators value downtime,” Whitacre said.