Matching Design Of Aftertreatment System For Heavy Duty Diesel Engine Based On World Harmonized Steady Cycle

The increasingly stringent emission regulations put forward higher requirements for the matching design of aftertreatment system for diesel engines,and it's essential to use the composite post processing technology to reduce the emission of diesel engines.The matching design of compound aftertreatment system has important influence on the power,economic performance and emission performance of diesel engine.As the volume of the post-processing system increases,the space velocity of the exhaust flow decreases through the catalyst,and the reaction time is increased,thus aftertreatment system has higher conversion efficiency.However,the larger the volume of the post-processing system is,the harder the layout is.Besides,the exhaust backpressure of the diesel engine is greater,so the performance of the diesel engine will be the worse.Therefore,it is necessary to study the matching technology of compound aftertreatmentsystem and diesel engine,so as to provide guidance for the design of diesel engine aftertreatment system.This paper focuses on the matching design of a heavy-duty diesel engine aftertreatment system.First,according to the interaction between the diesel engine and the post-processing system,the research order of diesel engine back pressure-DOC-DPF-SCR is determined.Then,based on the experimental data,three dimensional model and other configuration parameters of the diesel engine,a reliable diesel engine simulation model is established using GT-Power.The effect of exhaust back pressure on the performance of the diesel engine under rated operating conditions is studied.When the exhaust back pressure increased from 22kPa to 45kPa,the engine power and economy deteriorats by about 5%,which provides a pressure drop requirement for the matching design of the post processing system.The DOC simulation model is established to study the influence of DOC carrier specification on the conversion efficiency of HC,CO and NO,pressure drop,active regeneration process temperature rise rate,HC secondary pollution and so on.With carrier volume and cell density increasing,the conversion efficiency of HC,CO and NO increases significantly.Different specifications of DOC carrier can meet the six emission requirements of HC and CO,but when the carrier specification is 267mm x 102mm x 400 mesh,DOC has an ideal conversion efficiency for NO of 43.0%,which can improve the passive regeneration rate of DPF.As for active regeneration,with the increase of carrier volume and cell density,the conversion efficiency of HC increases,so the temperature of DOC outlet increases.However,due to the thermal inertia of the carrier,the rate of heating up decreases with the increase of carrier volume.In comprehensive consideration,when the DOC carrier is 267mm x 102mm x 400mesh,it not only has good HC/CO reduction and active regeneration performance,but also can greatly increase the proportion of NO2 at the entrance of DPF.The DPF simulation model is established,and the particle capture efficiency of DPF carriers with different specifications under WHSC cycle is studied.With the increasing of DPF filter volume,cell density and diameter,DPF trapping efficiency increases gradually.The DPF carrier of 267mm x 305mm x 200 mesh and 286mm x 305mm x 200 mesh can meet the emission regulation.The PM emission after deterioration are 7.7 and 6.3 mg·(kW·h)-1 respectively.As for pressure drop,the increasement of the carrier cell density and length will cause greater pressure drop,but when the length of the carrier is fixed,the pressure drop reduces as the diameter increases,because of the decreasing of the exhaust velocity.The pressure drop of DPF under different carbon load is studied.The pressure drop of the two schemes at 4g/L are 31 kPa and 25kPa respectively,indicating that no frequent regeneration will occur.In order to improve the efficiency of SCR reaction,the influence of the amount of coated PGM in DPF on the NO2 ratio of SCR inlet and secondary pollution is studied.The SCR simulation model is established and coupled with DOC and DPF models,which provides more accurate boundary conditions for SCR simulation.The effects of SCR carrier specifications on NOx conversion efficiency,NH3 leakage and pressure drop are studied.The results show that the conversion efficiency of NOx and the pressure drop increase with the increasing of the volume and cell density of SCR carriers,while NH3 leakage decreases as volume and cell density of SCR carriers increase.When the volume of the carrier is fixed,the carrier diameter has little effect on the NOx conversion efficiency and NH3 leakage,but the pressure drop increases significantly with the decreasing of the carrier diameter.Choose the SCR carrier with specification of 267mm x 305mm x 400 mesh,after optimizing the urea injection strategy,the NOx specific emission and NH3 leakage are 0.3 7 mg·(kW·h)-1 and 9.2ppm under the WHSC.The influences of aftertreatment system layout on engine and aftertreatment system performance are studied.When SCR is placed behind DPF,the heating rate of DPF entry is higher when active regeneration accurs,and fuel economy is saved by 3.7%.When SCR is prepositioned,the NOx emission reduces by 2.7%.The SCR postposition scheme is selected and the test bench is built to verify the performance of the aftertreatment system.The results show that the matching scheme of theaftertreatment system can meet emission standards of stage VI under the WHSC cycle.