Studies On The Mechanisms Of Ash Effects On DPF Performance And Its Related Application Issues

Diesel particulate filter?DPF?can effectively reduce particulate matters?PM?emissions from diesel engines,and it is indispensable for diesel engines to meet the increasingly stringent emission regulations.A large amount of ash would accumulate in the DPF over time,which would significantly affect the DPF performance.In this thesis,the mechanisms of ash effects on DPF pressure drop,filtration efficiency,and regeneration process were investigated,and some of the application issues related to DPF ash were also studied.The apparent effects of ash accumulation on DPF pressure drop and filtration efficiency were systematacially studied on the diesel engine test bench,and its underlying mechanisms were subsequently revealed with the combination of X-ray computed tomography?X-ray CT?,optical microscope,and numerical simulations.In the initial stage of ash accumulation in the DPF,the accumulated ash deposits along the DPF channel wall and forms ash cake layer,which prevents the depth filtration of the soot in the DPF wall,thus reducing the DPF pressure drop.The compact ash cake layer could also significantly improve the efficiency of soot intercept filtration and diffusion filtration in the DPF,thus greatly improving the overall filtration efficiency of the DPF.However,when the DPF ash loading exceeds about 10 g/L,the thickness of the ash cake layer almost no longer increases,and most of the ash tends to deposit at the rear end of the DPF channel and form long ash plug.The long ash plugs would significantly reduce the DPF effective length and increase the wall velocity of the gas flow as well as the thickness of the soot cake layer,as a consequence the DPF pressure drop would increase.The increased wall velocity of the gas flow would also decrease the efficiency of the soot diffusion filtration in the DPF,thusing slightly decreasing the overall filtration efficiency of the DPF.The effects of ash presence on diesel soot oxidation process under O₂ and NO₂atmosphere were quantitatively studied with the thermal gravimetric analyzer?TGA?,and the chemical kinetics parameters of diesel soot oxidation in the presence of ash were obtained.The mechanisms of ash effects on DPF actual regeneration process were analyzed based on the TGA results.Ash has little direct effect on the soot oxidation rates under O₂ and NO₂ atmosphere,but it could indirectly affect DPF passive and active regeneration in other ways.The ash effects on DPF temperature field during active regeneration were aslo investigated with the combination of engine bench tests and three-dimensional numerical simulations.The results show that the DPF peak temperature and maximum temperature gradient during active regeneration were primarily controlled by the actual local soot loading density in the DPF prior to regeneration.Under the condition of uniform distribution of DPF rear end ash plugs,the higher the actual local soot loading density,the higher the DPF peak temperature and maximum temperature gradient during active regeneration.On the basis of knowing the underlying mechanisms of ash effects on DPF performance,some ash related application issues founded in the actual use of the DPF were also studied in this thesis.The effect of ash accumulation in DPF on engine fuel economy was quantitatively studied,and the results show that the engine fuel penalty caused by ash generally ranges from 0.12%to 0.28%.The potential of reducing DPF size using low ash lubricat oil was analyzed,and the DPF size can be reduced by about 6%,14%,22%,and 7%accordingly when the lubricant oil ash content drops from 1.00%to0.75%,0.50%,0.25%,and 0.00%respectively.Moreover,a self-adaptive control strategy for DPF active regeneration control considering ash effects was proposed,and its effectiveness was validated with simulation.