Study On The Emission Characteristics Of Light-duty Vehicles In A Plateau City And The Effect Of Idling Emissions From Diesel Engine

Due to the rapid growth of vehicle population in the most developed cities and regions in China,vehicular emissions have become one of the major sources of urban atmospheric pollution.Consolidating the management and control of vehicular pollution is of great significance for ensuring urban air quality.Vehicles with internal combustion engines are key components in the prevention and control of vehicular emissions under the"vehicle-oil-road"system.Thus,it is necessary to study engine emission reduction strategies based on actual emission characteristics,as this could provide a scientific basis for the management of vehicular emissions in China.This paper first analyzes the real driving emission（RDE）characteristics of vehicles in a plateau city（Lhasa）,and further evaluates the effects of adopting single or a combination of measures,such as intake air throttling and post-injection retarding on light-duty diesel engine emissions and exhaust temperature.This study is summarized as follows:（1）To determine the actual road emission characteristics of vehicles in plateau cities,a portable emission measurement system（PEMS）is used measure RDE data of gasoline-,LPG-and diesel-fueled light-duty vehicles in Lhasa.The analysis results show that the respective CO₂,CO,NO_x,and HC emission factors are:159.19±11.81,18.38±9.73,1.53±0.46,and 1.27±0.99 g/km for LPG vehicles;223.51±23.1,1.51±0.68,0.27±0.16,and 0.06±0.04 g/km for gasoline vehicles;and 197.52,3.39,4.48 and 0.11 g/km for diesel vehicles.This indicates that LPG vehicles in high-and low-altitude cities have relatively higher CO,NO_x,and HC emission factors than gasoline vehicles.Additionally,it is noted that NO_x emissions from diesel vehicles are much higher than that of all other types of vehicles,and HC emissions were the lowest in all vehicles.（2）Based on the vehicle specific power（VSP）method,the RDE data for cars with different emission standards is synthesized into NEDC and WLTC driving cycles,and the estimated emission factors of these driving cycles are compared and analyzed.The results indicate that the RDE acceleration range is 3 times that of NEDC and 3.7 times that of WLTC,but the speed range of RDE is slightly lower.This implies that CO₂,CO,NO_xand HC emission factors of the test vehicles as masured in the RDE tests are sequentially higher than the WLTC and NEDC standards.（3）Bench tests are carried out on a light-duty diesel engine at idling speed and 50 MPa rail pressure,to study the influence of intake throttling and post-injection on exhaust temperature,emission,and fuel consumption.The test results show that under the baseline condition(850 r/min,wide open throttle（WOT）and with no post injection,the exhaust temperature is about 100.3℃.Under the WOT condition,post-injection can raise the exhaust temperature by about 20℃,Meanwhile,by heavily throttling intake air,the idle exhaust temperature can be greatly increased by over 60℃.If this is combined with fuel post-injection,the maximum exhaust temperature can exceed 200°C.It should be noted that delaying post injection or increasing intake throttling would increase fuel consumption.（4）The effects of adopting single or combined measures such as intake throttling,fuel post-injection,EGR and intake swirl,etc.on idle exhaust temperature and emissions are studied on a diesel engine operating under the condition of normal/fuel cut-off/cylinder deactivation（CDA）.It is found that when the engine is in normal operation,EGR has little effect on emissions under the WOT condition.However,with a decrease of manifold air pressure,increasing the EGR rate can reduce NO_x emission by about 50%and increase the exhaust temperature by 80℃.In addition,an intake swirl has little impact on emissions.Under the fuel cut-off condition,there is a considerable increase of exhaust temperature during idle operation.Through intake air throttling and post-injection,the exhaust temperature can be reached as high as 300℃.（5）The effects of intake throttling on intake process,combustion characteristics,exhaust temperature and emissions during idle operation,when the engine is operating under the condition of normal/fuel cut-off/CDA,are further studied.The results show that the peak of cylinder pressure during normal operation is lower than that under CDA and fuel cut-off conditions.Compared with normal engine operation,fuel consumption could be reduced by about 10%when operating under fuel cut-off conditions and with an intake manifold pressure that is lower than 75 k Pa;fuel consumption in the CDA state could be reduced by about 33%when operating under the WOT condition,and by 40%when operating under some intake throttling conditions.Therefore,CDA can be an effective fuel-saving measure.However,in comparision to normal engine operation,fuel cut-off and CDA operations can lead to an increase of NO_x emissions.