Fuel dilution of engine oil has always been an issue with diesels, but never more so than when late post-injection and biodiesel are used. Chevron Oronite Company LLC has developed an engine oil additive to address biodiesel fuel dilution.

Late post-injection is employed in some newer-model diesels as part of an emissions control strategy to periodically help "regenerate," or burn off, soot accumulated in diesel particulate filters (DPF). Injected late in the combustion cycle, the fuel does not combust but vaporizes and is carried downstream through the exhaust to create an exothermic reaction burning off the collected soot. Original equipment manufacturers can choose to utilize either post-injection or fuel injection downstream, directly in the exhaust system. In general, however, post-injection is the more cost-effective approach since no additional hardware is needed to perform DPF regeneration.

The physical properties of biodiesel facilitate fuel dilution in the crankcase and increase the potential for engine wear. Mineral diesel dilutes engine oil too but it gets volatilized and is eventually released through the breather system. For biodiesel, however, the story is significantly different. Biodiesel has a higher and narrower boiling range than regular diesel fuel, and because of its molecular structure droplet sizes coming out of the fuel injectors are larger.

So, while the mineral diesel portion of the fuel blend vaporizes and follows its destination to the exhaust stream, the piston is at the bottom of the cylinder and the methyl ester fraction-with its higher, narrower boiling range and larger droplet size-remains in liquid form, collecting along the exposed surface area of the cylinder wall. As the piston rises, much of the biodiesel can bypass the oil rings and enter the crankcase. Once there, biodiesel doesn't volatilize as does mineral diesel. Combined with heat inside the crankcase, there is concern about oxidation of the oil/biodiesel mix and subsequent engine wear from the organic acids.

Chevron Oronite developed a unique engine oil additive to help combat serious engine wear associated with methyl ester dilution in the crankcase. "In the case of biodiesel, once it gets in the crankcase and as it starts to degrade, it forms organic acids and starts to polymerize," said Gary Parsons, global OEM and industry liaison manager for Chevron Oronite. "The organic acids can aggressively attack certain metals, particularly lead in the bearings. As it polymerizes and oxidizes, it can lead to increased deposits in the engine-in particular on the pistons." This engine oil additive is designed to counter the effect of the acids so they don't aggressively attack the metal, prevent oxidation and the subsequent formation of deposits.

Chevron Oronite research engineer Michelle Morcos said, "The formulation we've developed is a combination of different detergents and antioxidants that would help counter some of the corrosion and control some of the deposits formed when biodiesel is used."

Historically in the context of acids, the lubricant additive business has largely been focused on formulations that help combat sulfuric acid damage. Before many of the recent sulfur limitations on diesel fuel went into effect, sulfur content in diesel fuel ranged from unlimited to 5,000 parts per million (ppm) to 500 ppm. Now, however, on-road diesel fuel
contains a maximum of only 15 ppm sulfur.

"Much of the historical effort had been in neutralizing sulfuric acids, and now much of the sulfur issues have gone away," Parsons said. "But now we're talking about putting this organic material in the oil, which forms organic acids, so part of what we've done is tailored our formulation to address those organic acids in order to prevent oxidation of the fatty acid methyl ester in the oil. That's why it's called for special research and development in that area-because it's different than what's been done historically."

The product, which is commercially available and marketed under the Oronite Lubricating Oil Additive, or OLOA, trademark, has been receiving global attention since its commercial debut last year. "People are starting to see that there's going to be more biodiesel in the market, along with more exposure and potential risk, so we're seeing more and more interest," Parsons said.