Sappok, A., Govani, I., Kamp, C., Wang, Y. et al., "In-Situ Optical Analysis of Ash Formation and Transport in Diesel Particulate Filters During Active and Passive DPF Regeneration Processes," SAE Int. J. Fuels Lubr. 6(2):336-349, 2013, doi:10.4271/2013-01-0519.
The formation and transport processes governing the build-up of incombustible ash deposits in diesel particulate filters (DPF) are influenced to a large extent by the filter's operating history. More specifically, the regeneration process, whether active, passive, or some variation of the two, has long been assumed to exert significant influence on the resulting ash characteristics. Until recently, only limited circumstantial evidence was available to describe differences in ash properties and distribution impacting DPF performance for filters subjected to different regeneration strategies. This work presents, for the first time, results from a comprehensive series of evaluations with optically-accessible DPF core samples showing the processes controlling the formation, transport, and interaction of the soot and ash deposits over a range of DPF regeneration conditions.DPF sample fixtures with optical access were employed with core samples from new and aged (ash-loaded) catalyzed DPFs to visualize the soot oxidation and ash formation processes occurring during filter regeneration. The filter core samples were loaded with soot utilizing a small diesel engine. Regeneration studies were carried out on a bench reactor allowing for careful control of exhaust conditions (temperature, flow, and gas composition). A stereo microscope provided high resolution videos of the processes occurring during the regeneration event in a small portion of a single DPF channel. Measurements of filter pressure drop and gaseous emissions via FTIR provide additional data, correlated to the microscope videos, to quantify the soot oxidation processes.Results of this study clearly show strong interactions between the ash and soot during the regeneration event. In particular, soot oxidation in the catalyzed DPF is observed to proceed locally with a certain degree of mobility of the soot cake as it is oxidized, depending on the specific regeneration pathway. Regeneration with relatively thick soot cake layers further shows the manner in which ash precursors are concentrated in the remaining soot and ultimately begin to agglomerate and form larger ash particles in the DPF. Particle mobility and transport of ash and soot from the channel walls to the back of the DPF were also explored during the regeneration event and following partial regenerations. The results shed considerable light on the processes occurring in the DPF and are extremely useful to describe the physical mechanisms governing ash formation and transport in the DPF, and ultimately explain the impact of ash on DPF performance.