Study Of Optimized Engine-bench Evaluation Methods Of Urea-SCR Catalyst For Diesel Vehicle Applications

Shortly after the implementation of China stage IV diesel emission regulation,almost all medium and heavy-duty diesel vehicles sold in the market adopted selective catalytic reduction(SCR)as after-treatment technology,and lots of attention has been paid to real-world NOx emission reduction ability of SCR catalysts as well as its evaluation methods.Engine bench testings are popular and widely used in SCR catalysts evaluations,although there are still some disadvantages as: 1)Impacted by urea solution mixing and decomposition conditions,SCR bench evaluation results are normally with deviation and uncertainty;2)The existing SCR bench evaluation methods are usually used to reflect the impact of one single exhaust condition variable on catalyst performance,it is difficult to predict catalyst performance under regulated emission testing cycles or real-word vehicle operations which combines the actions of multiple exhaust variables.In view of these,optimized bench evaluation method for SCR catalystic researched in this paper.To eliminate the impact to SCR performance evaluations by urea solution mixing conditions,a dedicated engine bench is designed and set up,velocity mixer and ammonia concentration mixer are fixed up on the engine exhaust pipe.It is considered that at the inlet of SCR catalyst the two-phase flow of urea solution and diesel engine exhaust gas is approximately uniform and with same flow velocity,and the test results based on this engine bench are only influenced by urea solution decomposition rate.As urea solution injected into the exhaust pipe is not always able to decompose to ammonia completely and timely,parameters as theoretical ammonia supply concentration represented by CUrea(t),urea decomposition ratio by ?Urea(t),deposited quantity by QUrea_BP(t)and decomposition rate by ?Urea_BP(Q)are introduced and the SCR inlet gaseous ammonia supply equation is built by the author.On the basis of ammonia balance relationship within SCR reaction system,the calculation methods of real ammonia nitrogen ratio ANR(t),excess ammonia concentration factor ?,catalyst conversion efficiency and calculated ammonia storage capacity are put forward,urea solution decomposition rate analysis and catalyst steady-state efficiency bench test results screening methods based on above parameters are also discussed in this paper.Series of tests are conducted on the dedicated engine bench to investigate how SCR evaluation results and urea decomposition rates are affected by engine exhaust conditions,i.e.,SCR inlet exhaust temperature,urea solution injection rate,exhaust flow rate and catalyst space velocity,etc.Test results show that lower urea decomposition rate ?Urea_BP(Q)leads to unstable gaseous ammonia supply to SCR reaction system,and causes sustained oscillations of catalyst conversion efficiency ? during the test process.Exhaust flow rate has a significant effect on urea decomposition rate,?Urea_BP(Q)is lower when engine exhaust flow rate stands at lower level,and this influence cannot be completely eliminated by rising the exhaust temperature only.As a result,for the actual condition of vehicle operations,more attention is required to the so-called "Double-Low" operation condition(i.e.,low exhaust temperature and low exhaust flow rate),and SCR bench test evaluation should be conducted with reasonable larger exhaust flow rate.In order to eliminate the influence of urea decomposition conditions,and improve the accurate and repeatable level of engine bench evaluation results,a correction method combined with catalyst bench test results screening and catalyst performance simulations has been established.Starting from the BOOST default values of the parameters of SCR catalytic reaction rate equation,catalytic reaction parameter value estimations and function assignment are performed regarding the sample SCR catalyst.Applying the real ammonia nitrogen ratio(ANR),SCR steady-state conversion efficiency simulation results are highly accorded with engine bench testings,therefore the simulation can be used for corrections of the test results of catalyst steady-state conversion efficiency.As catalyst's inlet gaseous ammonia concentration is assumed to be constant,SCR conversion efficiency simulations are not affected by urea decomposition rate.A quantitative analysis method for urea decomposition rate under different exhaust conditions is proposed by comparing the simulation results with the test results.Since it is difficult for using existing bench evaluation method to effectively predict SCR performance under vehicle operations,a representation method of SCR dynamic conversion efficiency temperature window is proposed based on reaction balance duration,and the nominal ESC conversion efficiency temperature window is established according to engine exhaust conditions under ESC cycles.This E-T window could be used in ESC emission prediction accurately.Additionally,in order to make SCR bench evaluation results more helpful in vehicle real-word emission prediction and control,an advanced off-cycle emission evaluation method is also put forward which uses diesel engine work-based emission intensity and work density as the feed forward,with existing vehicle running data as input,this method provides a new solution for real-time vehicle emission calculation,supervision and OBD application as well.