Individual and Synergistic Effects of Lubricant Additive Components on Diesel Particulate Filter Ash Accumulation and Performance

Abstract

The current CJ-4 oil specification places a limit on the oil’s sulfated ash content of 1.0% to reduce the build-up of lubricant-derived ash in the diesel particulate filter (DPF). Lubricant additives, specifically detergents and anti-wear additives, contribute to most of the sulfated ash content in the oil and ash accumulation in the DPF, and hence are studied with increasing interest in the optimization of the combined engine-oil-aftertreatment system. However, characteristics of ash deposits found in the particulate filter, which are affected by a number of parameters, differ markedly from those of the ASTM-method defined sulfated ash. In addition, ash characteristics and effects on DPF performance vary substantially among the different metallic base in the additives, specifically calcium, magnesium, and zinc. Through a series of carefully-controlled tests with specially-formulated lubricant additives, this work quantified the individual and combined effects of the most common detergent and anti-wear additives on the ash properties which directly influence DPF pressure drop. The results show that different lubricant additive formulations (Ca, Zn, Mg) produce profound differences in DPF pressure drop. It was found that DPF ash is a complex mixture of metals (Ca, Zn, Mg) in the form of sulfates, phosphates, and oxides. These ash compounds each have unique physical properties, which affect DPF pressure drop differently. In particular, ash containing calcium sulfate compounds resulted in an increase in filter pressure drop by over a factor of two, relative to the same amount of ash composed only of zinc phosphate or magnesium oxide compounds, at the same ash loading in the DPF. In addition, synergistic effects due to specific additive combinations were also explored and showed significant differences in ash composition and degree of exhaust flow restriction imposed by the ash resulting from specific additive combinations, as opposed to the individual additives themselves. Results are useful not only for lubricant formulators to design oils for improved aftertreatment system compatibility, but also to understand the practical effects of ash in the DPF in relation to the standardized sulfated ash definition in the lubricant specification.

Overview of automotive engine friction and reduction trends–Effects of surface, material, and lubricant-additive technologies

“…Unlike soot, ash cannot be oxidized into gaseous species. In as little as 35,000 miles, there is more ash accumulated in a DPF between regeneration intervals (for active regenerations) than soot [50]. The ratio of ash to soot in the DPF is even higher for continuously regenerated DPFs.…”

“…It has been shown that the engine back pressure doubles in about 180,000 miles of normal operation [51], and that the type of lubricant additive seems to have a difference in how DPFs are affected [52,53].…”

“…A special technique named lean burn [50,51] was developed using more air than is required to burn the fuel in an engine. This is in contrast to most spark-ignition (SI) engines on the road today, which use just enough air to burn the fuel completely.…”

Experimental study of lubricant-derived ash effects on diesel particulate filter performance

“…Among the few studies, several studies focused on the ash effects on DPF pressure drop behaviors. 19–21 Sappok et al 19 and Aravelli et al 20 found that the DPF pressure drop could be reduced when the DPF ash loading was lower than about 10 g/L, while it would increase when DPF ash loading was higher than that. Sappok et al 21 further investigated the effects of ash produced from oils with different formulas on DPF pressure drop and found that the ash generated from the lubricant oil containing calcium-based compounds caused higher DPF pressure drop than that produced from the lubricant oil containing magnesium-based or zinc-based compounds.…”

“…19–21 Sappok et al 19 and Aravelli et al 20 found that the DPF pressure drop could be reduced when the DPF ash loading was lower than about 10 g/L, while it would increase when DPF ash loading was higher than that. Sappok et al 21 further investigated the effects of ash produced from oils with different formulas on DPF pressure drop and found that the ash generated from the lubricant oil containing calcium-based compounds caused higher DPF pressure drop than that produced from the lubricant oil containing magnesium-based or zinc-based compounds. Besides that, Liati and Eggenschwiler 22 characterized the ash and soot deposited in the DPF in macro-, micro-, and nano-scales and found that the ash cake layer on DPF wall acted as the substrate for soot filtration, and the ash plugs at the DPF rear end would reduce the DPF effective volume.…”