Research On Optimization For SCR Flow Characteristics Of Diesel Engine And The Influence On NO_x Conversion

Since the Industrial Revolution,the invention of the internal combustion engine had greatly promoted the development of productivity,however,its negative effects on the environment are becoming increasingly apparent at the same time.The number of nitrogen oxides produced by internal combustion engines is several hundred times that of the previous,selective catalytic re-duction?SCR?technology was subsequently applied to the internal combustion engine.In this paper,bench test of air assisted urea injection and non-air assisted urea injection is carry out to study the influence of different exhaust flow temperature,different Urea/NOx ratio on NOx conversion.The results show that air assisted urea injection has achieved a good performance in mixing exhaust flow and urea thus obtaining higher NOx convertion;Uniformity of exhaust flow at the entrance of SCR declined with engine speed increasing,while air assisted urea injection is able to compensate flow ununiformity caused by the increased exhaust flow;NOx conversion increases with Urea/NOx ratio increasing when Urea/NOx ratio is in the range of 1.0 to 1.6.When Urea/NOx ratio is above 1.6,NOx conversion deceases with Urea/NOx ratio increasing.NOx con-version is lower at temperature below 200?,meanwhile,when stopping inject urea,the recovery of NOx convertion becomes slow apperantly.NOx emission is greatly reduced and achieved 86%of NOx convertion finally when European Stationary Cycle?ESC?test cycle is introduced.Based on the bench test above,the uniformity of exhaust flow is studied by different SCR mixers and rectifiers?which are Original Model equipped a 4-blade mixer with 8 circular holes distributed on the rectifier plate of the rectifier;Model A equipped a same mixer as Original model with 5 through holes distributed on the rectifier plate of the rectifier;Model B does not equipped a mixer,5 through holes are distributed on the rectifier plate of the rectifier;As a a comparison plan,Model C does not equipped a mixer and a rectifier.?with a three-dimensional computational fluid dynamics?CFD?simulation model constructed by AVL FIRE software.The results show that the uniformity index of single/dual catalyst SCR with blade mixer is higher than single/dual cata-lyst SCR without blade mixer;the descending order of uniformity index of 4 different SCR de-signing at 4 different operating points is Model A,Model B,Original Model,Model C,the uni-formity index of mixer equipped Model A is similar to mixer equipped Model B;Pressure drop of single catalyst SCR is higher than dual catalyst SCR at operating point A,B,C.However,pressure drop of dual catalyst SCR is high than single catalyst SCR at operating point D.Researches are carried out in terms of the influence of different SCR mixers and rectifiers on NH₃ and HNCO distribution and NOx conversion.The results show that the average concentration of NH₃ of single/dual catalyst for Original Model and Model A at operating point F and H is lower than Model B and Model C,NH₃ of Model B and Model C mostly concentrate on the front of catalyst;4 catalyst Models of single/dual catalyst obtain lowest NOx conversion and lowest reac-tion duration under operating point E;At operating point E,peak NOx conversion of single catalyst of Original Model reaches 73.2%and of Model A reaches 74.4%while at operating point F,Orig-inal Model reaches 87.6%and Model A reaches 91.5%;Improved flow turbulence can be seen at the entry of the catalyst when applying newly designed mixer named Model D.Uniformity index of Model D is the highest among the provious 4 Models and it keeps above 0.8 at both 4 operating points.NOx conversion keeps above 80%at operating point F,G and H,however,NOx conversion at operating point E is much better than other Models.A three-dimensional CFD simulation model for researching the influence of different nega-tive valve overlaps?NVOs?on NOx emission is constructed,in addition,coupled with SCR Model D,simulation results are used as boundary conditions to study NOx conversion.The results show that in-cylinder pressure and temperature decrease with NVO increases,its peak values are lagged as NVO increases and ignition delay is prolonged.NO emission increases,Soot emission decreases and its time of peak value advanced as NVO decreases,time of peak value of CO emission is advanced and peak value is desreased;NOx conversion increases as NVO decreases when NVO is combined with SCR Model D,which demonstrates that engine with negative valve overlap is able to obtain relatively low NO emission when combined with SCR aftertreatment equipment.