Study Of Fuel Injection Strategy And EGR Collaborative Optimization For Light-duty Diesel Vehicle With Low NO_X Emission

The continuous expansion of the market scale of light-duty diesel vehicles has led to increasingly serious emission pollution,and the corresponding emission regulations are becoming increasingly stringent.In view of the control difficulties of NO_X and PM emissions of diesel engines,limited to the chassis space and cost constraints of light-duty diesel vehicles,the main emission control technology route was:turbocharged inter-cooling+EGR+DOC+DPF.The emission control strategy of the technology route was reducing NO_X emissions through internal measures and controlling PM emissions by aftertreatment technology.In addition,the emission of light-duty diesel vehicle was measured through the NEDC test cycle,but the early calibration of each control strategy of the engine was carried out on the engine test bench.To getting the relationship between the vehicle test and the engine test has an important engineering meaning,which could improve the efficiency of the vehicle calibration and reduce the calibration cost.In this paper,the 4D25 diesel engine used for light-duty vehicle was the research object.The engine working conditions which can characterize the NEDC cycle test results were studied and determined by using cluster analysis;Through experimental research and theoretical analysis,the control strategy of NO_X emission reduction was studied based on the internal purification measures which was fuel injection strategy coupled EGR.The main contents of this paper were as follows.The whole NEDC test cycle was discrete to 1180 vehicle speed points as the time interval of1s.The scatter points of the NEDC test cycle were transformed into engine working conditions based on the transmission relationship of the engine and the vehicle including the transmission ratio of the gearbox,the transmission efficiency the wheel size and other parameters.The engine working conditions were divided into 7 regions by cluster analysis.The only center mode of each region was determined based on the Euclidean method and the change trend of the emissions and fuel consumption vs vehicle speed under NEDC test cycle.The weight coefficients of each region were calculated according to the vehicle running time of the cluster region under NEDC test cycle.The emissions and fuel consumption were measured by the engine test bench and NEDC cycle test.The test results showed that the NO_X,PM emission values and fuel consumption calculated by the 7 engine working conditions have good correspondence with the test values of the vehicle NEDC cycle,and the deviation was less than 5%.Therefore,the identified 7 engine operating conditions can predict the economic and emission performances of vehicle NEDC cycle.By this method,the calibration efficiency of the whole vehicle can be improved and the calibration cost can be reduced.Through experimental research and theoretical analysis,the effects of fuel injection strategy on NO_X emission and other performances were studied under light and heavy load conditions respectively.The results showed that the earlier pilot injection timing and larger fuel quantity can significantly reduce NO_X and smoke under light load condition.The effect of pilot injection on reducing NO_X emission was not obvious under heavy load conditions,but the earlier pilot injection timing and appropriate fuel quantity had obvious effect on reducing smoke.In order to avoid the sharp deterioration of the smoke and BSFC,the main injection timing should be advanced with the increasing load.The reasonable post injection timing and fuel quantity can effectively reduce the smoke and improve the NO_X emission slightly under two load conditions.To reducing NO_X emission and taking account of smoke,the strategy of fuel injection coupling EGR was that the earlier pilot injection timing and appropriate fuel quantity coupling2%-4%EGR rate should be adopted under low speed and light load conditions with low NO_X and smoke emissions,the pilot-main-post fuel injection coupling 6%-9%EGR rate should be adopted under middle speed and load conditions,the post fuel injection coupling 4%-6%EGR rate should be adopted under high speed and load conditions with much NO_X and smoke emissions,the main fuel injection coupling the 2.5%-4.5%EGR rate should be adopted under around rated speed and heavy load conditions.The emission verification with NEDC cycle was carried out.The test results showed that the NO_X emission was 175 mg·km^-1,which could meet the national V emission limit of light-duty diesel vehicle.Therefore,the strategy of coupling EGR with fuel injection strategy can effectively reduce the NO_X,and reached the expected research goal.