Research On Test Boundary And Evaluation Method Of Real Driving Emissions From Light-Duty Vehicles

Along with the increase of vehicle ownership,the road traffic pollution becomes one of the main reasons for air pollution.In order to control vehicle emissions and improve air quality,with reference to EU emission standards,China introduced RDE（real driving emission）test procedure in 2016,and added RDE test as type II test to the type inspection items of new vehicles.From the practical situation,the boundary conditions in the process of RDE test have some randomness,such as the driving behavior,atmospheric environment,road conditions,test altitude,traffic conditions.These uncertain test boundaries have a great impact on the test results.Although these test boundaries are specified in detail by RDE regulations,these measures are still difficult to eliminate the influence of the uncertainty of test boundary conditions on real driving emission test of vehicles.Taking light-duty gasoline vehicles as the research object,combined with the current RDE test evaluation standards in China,this paper deeply researches the influence of test boundary conditions such as cold start condition,geographical condition of driving route and trip dynamics on real driving emission test.Scientific and effective technical support is provided for China’s motor vehicle pollution prevention and control policy.The main research contents and results are as follows:Ⅰ.At the same time,PEMS equipment,full flow exhaust dilution system and CVS are used to measurement the emissions of three different light-duty gasoline vehicles on the chassis dynamometer,and the emission test results are analyzed.The results show that although the test results obtained by PEMS raw sampling have a certain deviation from the laboratory reference value,the deviations are within the error range specified in the standard.The test results obtained by CVS and PEMS are in good agreement.It can be seen from the instantaneous emission rate curve of various pollutants that the time response of CVS and PEMS equipment to various pollutants is basically synchronous in the whole measurement process.Ⅱ.The data processing program is compiled according to the RDE test data processing method specified in GB 18352.6-2016 and European Commission Directive2017/115 and its amendments.Ten RDE test data are selected,and several RDE test data processing and evaluation methods are compared and analyzed.The results show that the emission results of CO,NO_x and PN calculated by MAW method of China standard are less than those calculated by the cumulative average method of European standard for most vehicles.The proportion of cold start emissions to total trip emissions and the high emission data with high calculation times are important factors affecting the difference of results.MAW method and cumulative average method are used to calculate the full trip CO,NO_x and PN emissions and emission difference rate of 10 tests excluding cold start data and including cold start data respectively.The emission difference rate calculated by cumulative average method is greater than that calculated by MAW method.Cumulative average method is more sensitive to cold start data.Ⅲ.In order to evaluate the cold start emission of vehicles more reasonably,four light-duty vehicles were selected to complete the real driving emission test.The analysis and evaluation of RDE test results show that the cold start emission accounts for a large proportion of urban trip emission,and MAW method can not correct the RDE test data containing cold start emission data.In addition,three RDE tests were conducted,including one cold start test and two hot start tests under different vehicle hot states.The results show that the thermal state of TWC catalytic converter has a more direct and important impact on cold start emission in RDE test.It is inferred that in RDE test,the impact of driving behavior on emission under cold start condition is more important than that under non cold start condition.Ambient temperature is a secondary factor affecting cold start emissions relative to driving behavior.The future RDE standard should set the trip dynamics conditions more reasonably and effectively,especially during cold start.Ⅳ.The cumulative positive elevation gain is an important test boundary for the real driving emission test of vehicles.Because it is difficult to separate its impact from other test boundary effects of real driving emission test in the test process,the impact of cumulative positive elevation gain on real driving emission test is difficult to be accurately and comprehensively evaluated.Therefore,two light-duty gasoline vehicles were selected to carry out real driving emission tests on four different test routes respectively.Innovatively,it is proposed to expand the correlation analysis of emission data from the total trip and section level to the window level step by step.With the help of the large expansion of the sample space of emission data at the window level,the complex coupling influence mechanism of trip dynamic characteristics and cumulative positive elevation gain on real driving emissions is clearly displayed.Ⅴ.The test boundary conditions and data processing methods specified in the existing emission standards are difficult to effectively eliminate the significant impact of different driving styles on the RDE test results.Therefore,RDE comparative tests of different driving styles were carried out on three light-duty gasoline vehicles meeting China’s phase VI emission standard.In order to analyze the influence of trip dynamics on RDE test,the concept of window v·a_pos[95]was introduced to represent the trip dynamics characteristics of data windows at the data window level.Based on the statistically significant self correlation between window v·a_pos[95]and window pollutant emission factors of a large number of data window samples,an adaptive normalization evaluation method is proposed.The window pollutant emission factors of the test vehicle under the trip dynamic of WLTC baseline are calculated,and further the pollutant emission level of the road section and the total trip is evaluated.It is verified that this adaptive normalization evaluation method can effectively reduce the impact of driving style on RDE test results.