Integration And Optimization Of Urea-SCR And DPF System For Heavy-duty Diesel Engines

In Euro VI stage,heavy-duty diesel engines are normally equipped with NOx Selective Catalytic Reduction(SCR) and Diesel Particulate Filter(DPF) simultaneously. The individual aftertreatment technologies including Diesel Oxidation Catalyst(DOC), SCR and DPF, have to be integrated and optimized to meet the requirement of Euro VI. This thesis studied the low temperature performance of the urea SCR system, the characteristics of zeolite SCR catalysts, the filtration and regeneration of the DPF system, and the performance of the SCR-DPF integrated system on a heavy duty diesel engine, in order to reduce the NOx and PM emissions below the Euro VI limits.The urea decomposition behavior in the SCR system is investigated based on an engine test bench, computational fluid dynamics(CFD) code and thermal gravimetric analysis(TGA). The comparison among Vanadium-based SCR, Cu-zeolite SCR, and Fe-zeolite SCR was carried out on a diesel engine test bench to study their steady conversion efficiencies, transient reaction characteristics and secondary products. At low temperatures, the urea solution injected into the exhaust forms the wall film on the exhaust pipe inside, resulting in local low temperature area and urea deposition. The conversion level of the Cu-zeolite SCR at low temperatures is higher than that of the Vanadium-based and Fe-zeolite SCR, but the Cu-zeolite SCR is more likely to produce N2 O in the SCR reaction. The upstream DOC can improve the low temperature performance of the Vanadium-based and Fe-zeolite SCR significantly. The conversion efficiency of the Vanadium-based SCR decreases when the NO2/NOx ratio is above 50%, while the zeolite SCR is almost unaffected. The Cu-Fe compound zeolite SCR catalyst can combine the advantages of the Fe-zeolite and Cu-zeolite SCR catalyst to achieve high conversion efficiency and low N2 O formation.The filtration and regeneration of the particulate filter system consisting of a DOC and a catalyzed diesel particulate filter(CDPF) is studied on an engine test bench. Both the DOC and the CDPF led to high conversion of NO to NO2 for continuous regeneration. The filtration efficiency on solid particle number(SPN) is close to 100%. The particles aftrer the CDPF are mainly in accumulation mode. The downstream SPN was sensitively influenced by the variation of the soot loading. This phenomenon provides a method for determining the balance point temperature by measuring the trend of the SPN concentration. The exhaust temperature for active regeneration is well controlled under various engine conditions by dosing a certain amount of diesel fuel upstream of the DOC when the DOC inlet temperature is above 250℃. The N2 O is generated because of the NOx reduction by HC in the DOC. The N2 O formation is mainly influenced by the DOC outlet temperature.The SCR and DPF systems are integrated on a diesel engine without exhaust gas recirculation(EGR) to meet Euro IV regulations. The interaction of different aftertreatment technologies is studied, and the characteristics of emissions with different aftertreatment combinations are investigated on the engine test bench. The NOx conversion efficiency can be increased with the optimization of the aftertreatment, but not enough for Euro VI due to the high engine-out NOx emission. By recalibrating the engine and redesigning the aftertreatment, the emissions of NOx, PM mass and SPN can satisfy Euro VI requirement.