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Characteristics and Effects of Lubricant Additive Chemistry on Ash Properties Impacting Diesel Particulate Filter Service Life
Abstract:
Ash accumulation in diesel particulate filters, mostly from essential lubricant additives, decreases the filter's soot storage capacity, adversely affects fuel economy, and negatively impacts the filter's service life. While the adverse effects of ash accumulation on DPF performance are well known, the underlying mechanisms controlling these effects are not. To address these issues, results of detailed measurements with specially formulated lubricants, correlating ash properties to individual lubricant additives and their effects on DPF pressure drop, are presented.
Investigations using the specially-formulated lubricants showed ash consisting primarily of calcium sulfates to exhibit significantly increased flow resistance as opposed to ash primarily composed of zinc phosphates. Furthermore, ash accumulated along the filer walls was found to be packed approximately 25% denser than ash accumulated in the channel end-plugs. Both lubricant chemistry and the filter's thermal history were correlated to ash packing density and distribution following DPF post-mortem analysis.
In this study, a heavy-duty diesel engine was utilized in conjunction with a rapid lubricant degradation and aftertreatment ash loading system, which has been validated in detail previously against engine-out exhaust and ash accumulated in the field. Results with specially-formulated lubricants containing only calcium-based detergents, only ZDDP additives, and conventional CJ-4 oils were compared utilizing an aging cycle simulating 180,000 miles of on-road use. On-engine DPF performance evaluation, coupled with subsequent filter post-mortem analysis, clearly indicate the manner in which lubricant chemistry and exhaust conditions influence ash properties and DPF performance.
The results of this work demonstrate the effects of individual lubricant additives on the resulting ash properties controlling DPF pressure drop, including ash packing density and permeability. Results will be useful in optimizing the design of the combined engine-aftertreatment-lubricant system for future diesel engines, balancing the requirements of additives for adequate engine protection with the requirements for robust aftertreatment systems.