Impact Of Road Grade And Traffic Condition On Real Driving Emission Of Light-duty Gasoline Vehicles

In order to reflect the real road driving emission of vehicles,the Real Driving Emission?RDE?test is adopted by the China 6 Emission Regulations as a supplementary program of the laboratory certification cycle test.RDE tests are affected by many factors,resulting in poor repeatability of RDE test results.Therefore,the regulations set many boundary conditions for RDE tests to ensure repeatability of test results.At present,less attention has been paid to the influence of road grade and traffic conditions on the RDE test results of light gasoline vehicles.Based on Chongqing's special landform and urban traffic conditions,two light-duty gasoline vehicles meeting the China 6 Emission standards were selected,and the Portable Emission Measurement System?PEMS?was used to carry out the test on the routes conforming to the RDE test regulations,focusing on the analysis of the impact of road grade and traffic conditions on the real driving emissions of the vehicles in the test route.The specific work content is as follows:Four test routes with different grade characteristics were developed,and the grade characteristics of each routes were evaluated by cumulated altitude gain,so as to compare the real driving emission differences of vehicles on the test routes with different cumulated altitude gain.The result showed that the CO,CO₂ and PN emission factors measured in the test route with large cumulated altitude gain were larger,but the emission differences of the three pollutant emission factors in the test route with different cumulated altitude gain are all within 50%,while the NOx emission factor was less affected by the cumulated altitude gain.It show that the grade characteristics of different test routes developed in this paper have little influence on the repeatability of RDE test results.The relationship between cumulated altitude gain and pollutant emission factors in urban,rural and motorway sections of the four test routes was further analyzed.The results show that the cumulated altitude gain has little influence on CO and PN emission in urban section due to traffic condition and other factors,but the relationship between CO,CO₂ and PN emission factors and cumulated altitude gain is obvious in rural and motorway sections.Therefore,it is suggested that the regulation should consider limiting the grade of rural and motorway sections of the test route to avoid excessive grade of rural and motorway sections.The"segement method"was used to calculate the specific grade of the test route and analyze the relationship between the grade and pollutant emission factors.The results show that CO,CO₂ and PN emission factors of vehicles in urban,rural and motorway sections are linearly correlated with road grade,but the correlation between CO emission factors of urban sections and road grade is poor.The second order polynomial function can be used to describe the relationship between the NOx emission factor and the road grade of rural and motorway sections,while the urban sections show a downward trend,but the correlation between them is poor in three sections.In addition,CO,CO₂ and PN emissions of vehicles when climbing are about twice as much as those when descending.The INRIX index was used to evaluate the traffic conditions in the RDE test,and the influence of different traffic conditions on the real driving emissions of vehicles was analyzed.The results show that the emission factors of CO,CO₂ and PN are larger than the corresponding pollutant emission factors of unblocked road sections.In the same section,with the deterioration of traffic conditions,CO,CO₂ and PN emission factors increase,and all show a strong linear correlation,while NOx emission factors show an irregular rule with the change of traffic conditions.It indicates that the vehicle RDE test on the same test section is greatly affected by traffic conditions,resulting in a large difference in vehicle emissions.In the RDE test,the influence of traffic conditions on the repeatability of RDE test results should be avoided as far as possible.The vehicle idle emission under different traffic conditions was analyzed,and the CO₂ moving average window method is used to compare pollutant emission factors including idle emission and eliminating idle emission.The results show that:the heavier the traffic,the higher the proportion of pollutant emission in idling stage in the total emission of urban road sections,but the smaller the proportion of idling emission.By comparing the emission factors of various pollutants when the CO₂ moving average window method includes and excludes idle speed data,it is found that after the idle speed emissions are included in the calculation,the emission factors will increase when they are not included in the calculation,and the growth rate of CO,CO₂ and PN emissions factors are significantly greater when the traffic condition is relatively poor than when the traffic condition is relatively good.In order to avoid the influence of idling emission differences caused by traffic conditions on the repeatability of RDE test results,it is more reasonable to exclude idling emission when evaluating the emission results.