Ash, primarily derived from diesel engine lubricants, accumulates in diesel particulate filters directly affecting the filter's pressure drop sensitivity to soot accumulation, thus impacting regeneration frequency and fuel economy. After approximately 33,000 miles of equivalent on-road aging, ash comprises more than half of the material accumulated in a typical cordierite filter. Ash accumulation reduces the effective filtration area, resulting in higher local soot loads toward the front of the filter. At a typical ash cleaning interval of 150,000 miles, ash more than doubles the filter's pressure drop sensitivity to soot, in addition to raising the pressure drop level itself.
In order to evaluate the effects of lubricant-derived ash on DPF pressure drop performance, a novel accelerated ash loading system was employed to generate the ash and load the DPFs under carefully-controlled exhaust conditions. The ash loading system utilized a conventional CJ-4 oil and was coupled to the exhaust of a Cummins ISB diesel engine, allowing for accelerated ash loading and DPF performance evaluation with realistic exhaust conditions. Following DPF performance evaluation, the filters were subjected to a detailed post-mortem analysis in which key ash properties were measured and quantified.
Measurements of ash properties and distribution provide key information to interpret the experimental results. In parallel with the experiments, theoretical models were developed and utilized to provide additional details regarding the underlying mechanisms responsible for the manner in which ash alters DPF channel geometry and affects the conditions under which soot is accumulated. The results show a significant increase in local soot loads, elevated filter wall velocities, and a resulting increase in the Peclet number with ash accumulation. Based on the DPF post-mortem analysis and theoretical models, explanations for the differences in DPF pressure drop sensitivity to soot for ash-loaded filters and DPFs containing no ash were developed.