Optimization And Control Strategy Of Ultra-low NOx Aftertreatment System For Compression Ignition Engines

Compression ignition engines such as diesel engines have advantages in thermal efficiency and fuel consumption with lean burn mode,while nitrogen oxide（NOx）and particulate matter（PM）emission need to be reduced.Selective catalytic reduction（SCR）and diesel particulate filter（DPF）are effective to reduce NOx and PM emissions from heavy-duty engine,and it is indispensable to satisfy ChinaⅥemission legislation with diesel oxidation catalyst（DOC）and DPF located upstream SCR.Under such condition,NOx conversion efficiency during cold start is limited.Close coupled SCR（cc-SCR）is a smaller SCR catalyst located close to the exhaust,which has the characteristics of smaller heat capacity and faster temperature increasing rate especially during cold start.Therefore,it is feasible to improve NOx conversion efficiency and meet increasingly stringent emission legislations with cc-SCR and integrated DOC+DPF+SCR aftertreatment system.Compared with diesel engines,lower fuel injection pressure and PM emission are observed with gasoline compression ignition（GCI）engines.In this thesis,the effect of dual-SCR aftertreatment system to achieve ultra-low NOx emission is investigated,and several researches such as the characteristics of cc-SCR,the optimal ammonia storage of cc-SCR,the control strategy of dual-SCR aftertreatment system and the adaptability of dual-SCR aftertreatment system to GCI engines are carried out.The effect of temperature,space velocity,ammonia nitrogen ratio and urea spray characteristics on cc-SCR are investigated by numerical simulation and engine bench test.It can be concluded the air-assisted urea nozzle is more suitable to be arranged upstream cc-SCR compared to airless urea nozzle considering its higher urea decomposition rate,NH₃ concentration and NOx conversion efficiency.Based on the experimental results of sample test and engine bench test,SCR chemical reaction kinetics model is established and calibrated.Furthermore,a calculation model on ammonia slip in the rapid heating process is proposed and calibrated.The optimal ammonia storage of cc-SCR catalyst is analyzed combined with multi-objective genetic algorithm.It can be concluded that optimal ammonia storage of cc-SCR catalyst is affected by both temperature and temperature increasing rate,and the effect of temperature increasing rate on the optimal ammonia storage of cc-SCR catalyst is more significant at lower temperature.Based on the calculation results,the temperature-temperature increasing rate-optimal ammonia storage diagram is constructed,and the criterion for determining the applicability of ammonia storage model in transient state is proposed.Based on the optimal ammonia storage of cc-SCR and SCR,urea injection strategy for dual-SCR aftertreatment system is proposed and validated.To reduce the impact of severe temperature decrease on NOx conversion efficiency in federal test procedure（FTP）transient test cycle,the supplemental urea injection strategy and the feedback control strategy based on ammonia slip are proposed and validated.Combined with engine thermal management optimization calibration,tailpipe NOx and soot emission under FTP cycle are less than 0.027 g/k W·h and 0.0068 g/k W·h respectively,which indicates that tailpipe emissions for heavy-duty engine satisfy the California ultra-low NOx emission limit.Based on the the dual-SCR aftertreatment system,the emission characteristics and the adaptability of the aftertreatment system are studied on a GCI engine in FTP transient test cycle.It can be concluded that tailpipe NOx emission of gasoline compression ignition engine is close to that of diesel engine,tailpipe PM and PN emissions are lower than that of diesel engine.However,HC emission is higher than that of diesel engine,which can be appropriately reduced by optimized oxidation catalyst,such as increasing noble metal content or adjusting noble metal components.