Study On SCR Thermal Management For Heavy-Duty Diesel Engines

Selective catalytic reduction(SCR)is one of the most effective techniques to reducing the NO_x emission of diesel engines.However,temperature has an important effect on the conversion efficiency of the SCR system.When the engine is running under low load conditions,the exhaust temperature is very low,as a result,the NO_x conversion efficiency is very low too,in this case,most of the NO_x is discharged directly into the atmosphere.In addition,during the cold-start process,the SCR system can not work in the first state of the start process.So,it is of great significance to study the thermal management technology for engine aftertreatment system.Focused on this core issue,the article carries out a detailed study.Firstly,the operating condition of the engine is determined by examining the properties of the test cycle,and according to the SCR temperature variation trend in the WHTC test cycle,it is considered that the SCR temperature can reach 190? within 600 s.On the basis of this,the effects of injection timing,variable geometry turbocharger(VGT),retarded intake valve close timing(RIVCT),post injection,exhaust throttling and exhaust gas recirculation(EGR)are studied detailed with regard to emissions and engine operation economy.Experiment research shows that under low load conditions,retard injection timing can barely improve exhaust temperature.Post injection strategy is very helpful to improve SCR temperature,while it will cause engine specific fuel consumption to ascend.By taking advantage of VGT and RIVCT to reduce air input can make great contributions to improve combustion temperature and exhaust temperature.With introducing the EGR,engine exhaust temperature decreases first and then increases,high EGR ratio must be adopt so as to improve exhaust temperature,in that case,reduced fresh air input and EGR latency would cause engine torque characteristics deteriorate under transient operating conditions.Steady test results show that the post injection strategy and the VGT-RIVCT control strategy can both significantly improve the SCR temperature.In order to compare the two strategies,cold-start WHTC testes were carried out.It turns out that both strategies can make SCR temperature reach the given temperature within 600 seconds.However,the application of the post injection would result in a sharp deterioration of the fuel consumption.Then,through the contrast test of different VGT opening,a suitable control strategy is obtained,with this strategy,NO_x emission reduced to 0.929g/kWh,and the engine fuel consumption just increased by 1.4%.