Experimental And Simulation Researches Of EGR-LNT Synergetic Control For NO_x Emission Of Lean-burn Gasoline Engine

The ICE brings convinence and comfort to people's life,and it also caused a series of energy and environment issues simutaneously.In recent years,petroleum shortage and environmental pollution promote the gradual implementation of increasingly strigent emission regulations,which challengs the engine related researchers and also provides an opportunity to develop advanced technology of in-cylinder and aftertreatment system of engines.Gasoline engines are the main power resource in our country because of its preferable performance of PPL,NVH and acceleration performance,etc.Most of passenger vehicles operates at medium-low load area where gasoline engines usually have a relative worse BSFC because of its pumping loss.The application of lean-burn technology on gasoline engine could compensate its drawback in fuel economy.But conventional TWC of gasoline engines cannot control NO_x emission efficienctly under an ultra lean AFR because of its internal oxygen inhibition phenomenon,which makes NO_x control technology in lean-burn gasoline engines become one of the highlights in the current research field.An EGR-LNT synergetic control system was proposed to solve NO_x emission issue of lean-burn gasoline engines under the premise of lean-burn modification on conventional productive gasoline engines,which could be used for compensating the drawback of in-cylinder tolerance of EGR,higher cost of LNT and complicated control system,etc.Research results showed that there is a best matching interval between upstream EGR rate and physicochemical characteristic parameters of downstream LNT when EGR-LNT synergetic control system was applied on lean-burn gasoline engines for controlling NO_x emission.Excessive or deficient upstream EGR will deteriorate NO_x control effect of EGR-LNT synergetic control system,which is NO_x conversion efficiency.The approprite upstream EGR rate could make NO_x control effect of EGRLNT synergetic control system better than that of sole EGR and sole LNT.Although downstream LNT could ensure a better NO_x control effect under less upstream EGR rate,inefficient NO_x control of upstream EGR finally becomes the major factor in deteriorating NO_x control effect of EGR-LNT synergetic control system.In addition,although more upstream EGR rate could ensure an efficient NO_x control of in-cylinder,downstream LNT cannot maintain the higher NO_x conversion efficiency any more,which finally becomes the major factor in deteriorating NO_x control effect of EGRLNT synergetic control system.The deterioration of NO_x control effect caused by deficient upstream EGR rate is an intrinsic factor drawback of in-cylinder combustion,but the ineffcient NO_x control of downstream LNT is caused by CO2,H2O(g),O2,temperature and pressure in exhaust emission which vary with upstream EGR.The existence of CO2 in LNT could compete with NO_x for the adsorption sites and excessive CO2 will cause NO_x slip.H2O(g)itself also could compete with NO_x for adsorption sites.Although its competitive intensity is not as higher as that of CO2,the adsorption sites inactivation phenomenon caused by it is harder to be regenerated and it also could strengthen NO_x control deterioration when it coexists with CO2.In addition,H2O(g)could achieve a serious NO_x control deterioration than CO2 at lower temperature.The existence of O2 in LNT is benerficial to NO_x adsorption process,but it also could deteriorate NO_x desorption and NO_x reduction process,which indicates lean-AFR and rich-AFR should be selected with NO_x adsorption,NO_x desorption,NO_x reduction and fuel economy balanced simutaneously.Inlet temperature of LNT beyond 390? will cause NO_x breakthrough.Small amount of NO_x breakthrough is beneficial to NO_x reduction,but a higher NO_x breakthrough will result in NO_x slip because of its insufficient contact with reductants.