| Diesel engine has the advantages of strong power,low fuel consumption and high thermal efficiency.It is widely used in medium and heavy road vehicles,construction machinery,ships and other fields.However,the environmental pollution caused by nitrogen oxides(NOx)emitted by diesel engine is becoming more and more serious.In order to reduce the NOx emitted by diesel engines,countries all over the world have introduced more and more stringent emission regulations.In China,selective catalytic reduction(SCR)has been widely used to remove NOx from diesel engines since the China IV emissions.At this stage,national six regulations had made strict restrictions on tail exhaust NOx.Urea catalytic reduction(urea SCR)technology based on copper based molecular sieve was considered to be one of the main technologies that can meet the NOx emission standards of China Ⅵ heavy for duty diesel engines.China has conducted some research on copper based molecular sieve SCR,which has laid a solid foundation for the implementation of China Ⅵ.However,there were still a lack of systematic research on the core technologies of China Ⅵ,such as the research on high-precision computational ammonia storage,engine exhaust heat management,Diesel Particulate Filter(DPF)active regeneration and the influence of hydrothermally aging catalyst on NOx,The current China Ⅵ issues and the subsequent stricter China Ⅶ emissions need to be further studied.Based on this,this paper studied and analyzed the technical priorities and difficulties faced by SCR through engine bench test and vehicle test.Before the dew point detection of the on-board NOx sensor,the NOx concentration used to calculate the urea injection cannot be measured,which posed a challenge to the tail exhaust NOx of the cold start WHTC or low-temperature Portable Emission Measurement System(PEMS).An original NOx model with high accuracy,low computation and integration into Electronic Control Unit(ECU)is the key to solve the problem.This paper described a semi empirical model for physical prediction of NOx emissions,which can predict the steady-state and transient NOx emissions of diesel engines only with the help of vehicle sensors.The NOx original emission model mainly considered the factors closely related to emissions,such as EGR(Exhaust Gas Recirculation)rate,post inter-cooler temperature,intake pressure,fuel rail pressure,altitude,etc.,and modeled the relationship between these factors and NOx generation.The NOx emission model was tested on the bench and the vehicle.The results showed that under the bench World Harmonized Steady Cycle(WHSC),the cumulative deviation between the NOx concentration predicted by the model and the actual NOx concentration was-1.07%;Under the World Harmonized Transient Cycle(WHTC),the cumulative deviation between the two was 4.43%;In the vehicle test,the cumulative deviation between the two was 4.94%.The test results showed that the NOx original emission prediction model can provide the system with high precision model prediction value when the on-board NOx sensor was not working,so it can provide accurate input for calculating the urea injection amount.Engine exhaust heat management is very important for China Ⅵ emission.The relationship between after-treatment temperature and exhaust NOx is nonlinear,and quantifying the relationship between them is the focus of current research.In this paper,the engine thermal management was studied under transient and steady-state conditions.The effects of fuel rail pressure,injection advance angle and intake throttle valve on engine thermal management under steady-state conditions were analyzed under medium and low loads.Under transient conditions,the exhaust temperature rise effect is used to evaluate the engine exhaust heat management under WHTC,and the influence of intake system on after-treatment temperature and exhaust NOx emission under no-load condition was revealed;The effects of no-load exhaust heat management on engine exhaust temperature,SCR ammonia storage and exhaust NOx were quantitatively analyzed.The results showed that no-load heat management had a great influence on diesel oxidation catalyst(DOC)and SCR temperature and NOx in tail gas;It had little impact on ammonia storage and NO2.DPF active regeneration not only affected the engine intake and fuel system,but also affected the efficiency of SCR conversion of NOx,which made the control of tailpipe NOx emission and ammonia leakage difficult.In this paper,the air system,fuel system and SCR of the engine during DPF active regeneration were tested on the bench.The effects of DPF active regeneration mode on SCR temperature,ammonia storage,urea injection,tail NOx and ammonia leakage were quantitatively analyzed under WHTC cycle.The results showed that the average SCR temperature increased from 245℃ to 502℃ during DPF active regeneration;High temperature affected the SCR conversion efficiency,especially when the temperature exceeded 550℃,the SCR conversion efficiency decreased significantly,and the exhaust NOx changed from 0.12(g/kW·h)to 0.65(g/kW·h).SCR conversion efficiency was less sensitive to temperature and more sensitive to space velocity;During DPF active regeneration,a method of pre controlling urea injection was proposed to control ammonia leakage.SCR reaction model is the basis for calculating urea injection and ammonia storage.This paper described a copper based SCR reaction model,which mainly considered the factors closely related to NOx conversion efficiency,such as temperature,ammonia storage,space velocity,etc.In this paper,the ammonia loss factor was proposed,and the specific algorithm was given.The ammonia loss factor was used to correct the ammonia storage and NOx conversion efficiency calculated by the model.The model was tested on the bench and the vehicle.The results showed that under the bench WHTC,the cumulative deviation between the NOx conversion efficiency predicted by the model and that calculated by the sensor was-0.2%;Under the WHSC,the cumulative deviation between the two was-0.41%;Under the vehicle condition,the cumulative deviation between the two was 0.36%,indicating that the SCR reaction model modified by ammonia storage consumption factor predicted the downstream NOx of SCR with high accuracy.The hydrothermally aging copper based SCR catalyst showed an irreversible phenomenon in the conversion of NOx,resulting in a large number of NOx and ammonia gas in the downstream of SCR,resulting in many difficulties in the controls of NOx in the tail gas and On-Board Diagnostics(OBD)monitoring of hydrothermal aging SCR.In this paper,the performance and diagnosis of hydrothermally aging SCR fault parts required by regulations were studied.The effects of hydrothermally aging SCR on ammonia storage,NOx in tail gas,urea injection rate and OBD diagnosis were quantitatively analyzed.In terms of SCR conversion efficiency and OBD detection,compared with performance fresh parts,hydrothermally aging parts were more sensitive to space velocity than to temperature.When different OBD monitoring release conditions were selected,the NOx conversion efficiency in SCR monitoring area was quite different.Therefore,the accuracy of SCR efficiency diagnosis can be improved through appropriate release conditions. |
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