Study Of Soot Filtration And Active Regeneration Characteristics For Heavyduty Diesel Engines

Diesel Particulate Filter(DPF) is the most effective aftertreatment device to reduce particulate matter(PM) emissions from diesel engine. When the PM trapped in the DPF reaches a certain amount,it needs to be regenerated to recover its filtration efficiency. Using the heat released by the diesel fuel injected into the exhaust pipe upstream of Diesel Oxidation Catalyst(DOC) to trigger the DPF active regeneration has become one of the key technologies to meet the stringent emission regulations in the future. This thesis studied the DPF soot accumulation process and pressure drop characteristics, the diesel spray characteristics in the exhaust pipe, the diesel oxidation characteristics and the temperature distribution in the DOC, the temperature characteristic of DPF during active regeneration by combining experiments and numericial methods.The pressure drop characteristics and soot trap behavior in the DPF were investigated based on an engine test bench and computational fluid dynamics(CFD) code. A related numerical model was established by FIRE CFD code. The pressure drop of DPF without PM shows a linear growth with the increase of exhaust temperature except for the condition of the exhaust temperature being too high. In deep soot trap stage, the DPF pressure drop rises faster, while the DPF pressure drop increases relatively slower than that in surface soot drap stage. More soot accumulates on both ends of the filter and the centre of the filter crosssection, but less on the middle of the filter and the edge of filter crosssection during the soot trap process. But inhomogeneity of the soot distribution becomes weak with the increasing of the trapped soot.The diesel oxidation characteristic in the DOC was analyzed on an engine test bench. Experiment results show that the DOC outlet exhaust temperature can be improved effectively by means of injecting diesel into the exhaust pipe. Space velocity, the temperature of DOC inlet and diesel injection strategy can affect diesel oxidation characteristic and temperature characteristic obviously. DPF active regeneration requires a higher oxidation performance of DOC.Diesel spray characteristics in the exhaust pipe before Diesel Oxidation Catalyst(DOC) and diesel oxidation process in the DOC were investigated by combining optical experiments with numerical simulation. High-speed photography device was used to capture diesel spray images, and the spray droplet sizes were measured by a Phase Doppler Particle Analyzer(PDPA) to obtain spray droplet distribution. A diesel spray model was established and validated based on the test results. Experiment results indicate that the rate of fuel injection has a great influence on the spray shape and penetration. The spray penetration and spray cone angle increase with the increase of the injection rate. The injection rate has an effect on the shape of the spray edge and the diesel droplet size distribution of the nozzle exit. The SMD along spray axis varies with the number of large droplets.The diesel spray oxidation process, the soot oxidation process in the DPF and the temperature distribution characteristics of the system were analyzed based on a numerical model. Simulation results show that the hydrocarbon distribution in DOC inlet is determined by the diesel spray characteristics along the pipe. The temperature distribution in DOC outlet has a strong correlation with the hydrocarbon distribution in DOC inlet. The impingement and gravity cause more hydrocarbon to accumulate on the bottom of the pipe, resulting in a low temperature area in DOC outlet. The uniformity of gas mixture can be improved by the increasing the pipe length,then the diesel accumulating on the pipe bottom is weakened. During DPF active regeneration, the bottom and backend of the DPF is the area where the temperature is much higher. The space velocity being too high or oxygen concentration being too low will lead to an incomplete DPF regeneration, and the space velocity being too low or oxygen concentration being too high will lead to a much high temperature to overheat the DPF. As a result, the space velocity and oxygen concentration need to be controlled in appropriate range.