Flow distribution effects in the loading and catalytic regeneration of wall-flow diesel particulate filters

The study of catalytic regeneration characteristics of porous ceramic diesel particulate filters (DPFs) is of growing interest to industry as diesel soot emissions are limited by legislation to levels below 0.01 g/km (for passenger cars). More specifically, pressure drop computations and correlations are important factors employed in the design and control of diesel filter systems. However, in numerous cases, computations and models fail to match the experimentally observed evolution of soot combustion in the filter. In this paper, the role of flow maldistribution in this issue is investigated, by means of full-scale tests of the loading and regeneration behaviour of a particulate filter installed on a modern diesel engine run on catalyst-doped fuel. Loading tests were performed at three characteristic engine operation points with markedly different levels of engine exhaust gas mass flowrate. In these tests, it becomes apparent that complex flow maldistribution phenomena exist during the loading phase, which are not directly reflected by the behaviour of the pressure drop versus time curve. However, these phenomena are shown to affect the distribution of collected soot mass in the different channels of the filter and, consequently, the regeneration behaviour. The evolution of flow maldistribution was also studied in a number of regeneration experiments. It was confirmed that the variation of the volatile organic fraction in the filter and the associated partial catalytic regenerations at low temperatures interact with flow and soot maldistribution in a complex way. The conclusions from this study set the scene for future, more detailed investigations that are expected to improve understanding and modelling of diesel filter pressure drop and regeneration characteristics.