Study On The Key Scientific Problems Of The Diesel Exhaust Aftertreatment Systems

With the more stringent emission regulations on the diesel engines, we have toinduce the aftertreatment systems of diesel engines to satisfy the regulations. However,the emissions of the diesel vehicles on the road and on the verification bench aredifferent because their diving conditions are not same. Thus, in the application of theaftertreatment systems there may be some issues like the “blow-off” issue of POC, thedeposits and the worse low-temperature performance of urea-SCR system. This thesistook a heavy-duty diesel engine as the research object, investigated the key scientificproblems above comprehensively on a diesel engine bench.A particulate oxidation catalyst (POC) was employed to perform experiments onthe engine test bench to evaluate the effects on the nitrogen dioxide (NO2) andparticulate matter (PM) emissions from diesel engine. The results showed that POCincreased the ratios of NO2/nitrogen oxides (NOx) significantly in the middle and highload, the ratio of NO2/NOx increased4.5times on average under all experiment modeswith POC. In the continuous run under the steady mode, abnormal particle emissionsafter POC happened several times, and the average PN concentration of theseabnormal emission increased two orders of magnitude. The PN concentrations tendedto increase over time, which indicated that the performance of POC decreased overtime.The composition and cause of formation of the deposits from urea-SCR systemwere investigated by Fourier transform infrared (FTIR) spectroscopy and thermalgravimetric (TG) analysis. The test results indicate that the main components of thedeposits near the nozzle are urea, biuret and cyanuric acid. The main component of thedeposits in the exhaust pipe is cyanuric acid. The causes of deposits formation near thenozzle are low temperature and lack of air flow disturbances, the causes of depositsformation in the exhaust pipe wall are low temperature, low exhaust flow, urea sprayedon the wall and high temperature polymerization of isocyanic acid. The experimental research results on the low-temperature performance ofurea-SCR systems show that, the NOx conversion efficiency was lower at temperaturelower than200°C, the NOx conversion efficiency was lower than25%when the NSR（Normalized Stoichiometric Ratio）was1.0. The NSR was no obvious effect on theNOx conversion efficiency at the temperatures lower than222°C, the NOx conversionefficiency increased with the NSR at the temperatures234°C~270°C. In the ESCexperiments, the NOx conversion efficiencies of the vanadium-based SCR were higherunder all the conditions except the A25, B25and C25conditions. The NOx efficienciesof the Cu-Fe SCR under these three engine conditions were higher than those of thevanadium-based SCR, improved7.9%,17.4%and24.1%respectively.The study on the factors affecting the ammonia slip and ammonia storagecharacteristics of urea SCR system was studied. The results show that under the fixedspace velocity, the time from the injection start to the maximum of NOx conversionefficiency and the time from zero to the saturated amount of storage ammonia tendedto decrease with the temperature increase, and the saturated amount of storageammonia tended to decrease with the temperature increase. The time from the injectionstart to the maximum of NOx conversion efficiency and the time from zero to thesaturated amount of storage ammonia decreased with the space velocity increase, andthe saturated amount of storage ammonia decreased with the space velocity increase.The amount of ammonia slip with various NSR tend to increase before decreasing withthe temperature increase. Under the fixed inlet temperature, the amount of ammoniaslip increased with the space velocity increase.Established the SCR reaction model based on the AVL Boost software platform,and the urea injection amount control model based on the MATLAB/Simulink softwareplatform, introduced the method of the chemical kinetic optimization. The off-linesimulation of the transient engine mode was conducted through the reaction model andthe calculated injection amount. The simulation and experiment results of ETC andWHTC modes indicate that, the average temperature of the WHTC was lower than thatof ETC mode. The average NOx conversion efficiency of the ETC mode was83.88%, the average NOx conversion efficiency of the hot-start and the cold-start WHTC was76.57%and51.82%respectively.The effects of the EHC on the SCR low-temperature performance on dieselengines were studied. The results show that under the space velocity of20000h-1, withEHC, the inlet temperature of SCR increased about20°C on average. The inlettemperature increase and the increasing rate of the NOx conversion efficiency bothdecreased with the increasing space velocity, the average increases of the NOxconversion efficiency tended to increase with the increasing inlet temperature. TheEHC showed better performance on improving the low-temperature NOx conversionefficiency under lower space velocity modes and the engine modes at temperatureshigher than180°C.