配置SCR的柴油货车道路运行NO_x排放分析

Analysis of NO_x emission of diesel truck with SCR in actual driving and transportation

为了研究重型柴油货车在不同道路运行工况下的NO_x排放特性,以一辆配置选择催化还原(Selective CatalyticReduction,SCR)净化系统的国V排放标准的重型柴油货车为研究对象,开展实际道路运行车载排放测试,通过车载排放测试系统实时采集车辆行驶速度、NO_x排放体积分数与排气温度等数据,分析车速、排气温度、路况等对NO_x排放的影响。结果表明,车速低于40 km/h,NO_x排放随车速增大稍有增加; 40～70 km/h,随车速增大NO_x排放降低;高于70 km/h,随车速增大NO_x排放显著降低。车速与排气温度呈线性正相关,排气温度高于150℃,SCR才能显示出对NO_x的净化效果。市区工况车速低,排温低于150℃,SCR不能有效工作;市郊、高速工况下排温高于150℃,SCR催化效率提高,车速增大,排温升高,NO_x排放降低,因此NO_x排放市郊工况低于市区工况,高速工况低于市郊工况。配置SCR的重型柴油货车NO_x高排放区主要集中在中低车速、加速区间内。

Intended to trace the specific features of authentic road driving NOx emission of the heavy-duty diesel truck,we have done a series of road-in-service emission tests on the typical truck routes by taking a specific heavy-duty diesel truck equipped with the selective catalytic reduction(SCR)device as the research sample.In so doing,we have managed to collect the related experimental data,such as the instantaneous NOx emission volume fraction,the exhaust temperature and the vehicle speed by means of a portable emission measurement system(PEMS).The results of tests are that the exhaust temperature tends to increase linearly with that of the vehicle speed,and the catalytic purification effect of SCR on NOx become apparent when the exhaust temperature is higher than about 150℃.More specifically speaking,when the vehicle speed is lower than 40 km/h,the NOx emission would increase with the rise of the speed;during 40-70 km/h,NOx emission decreases with the increase of speed.However,when the travelling speed is higher than 70 km/h,the NOx emission may turn to decrease significantly.This may indicate that NOs emission of the heavyduty trucks tends to get greatest in the urban road operating condition,which is followed by the suburban road condition and gets lowest under the highway condition.The above analysis shows that the vehicle speed affects the exhaust temperature,which in turn affects the catalytic purification effect of SCR on NOx However,the SCR system cannot work effectively on the urban road operating condition when its vehicle speed is relatively low and the ex-haust temperature is lower than about 150℃.When the exhaust temperature is higher than about 150℃,the increase of the exhaust temperature with the vehicle speed can improve the efficiency of NOx purification by SCR system,thus,the NOx emission on the suburban operating conditions tends to be lower than that on the urban road conditions.What's more,highway operation conditions may experience higher exhaust temperature,with its SCR purification efficiency being further improved.Thus,its NOx emission tends to be lower than on the suburban roads.Thus,all in all,the high NOx emission region of heavyduty trucks with SCR in the reality driving should be mainly concentrated on the condition of low or medium speed range in its acceleration.