装配SCR系统的城市公交车NO_x排放特性

Characters of NO_x emission from transit bus with SCR

利用车载测试系统,对4种类型车辆进行了整车排放测试.以SCR的起燃特性为理论基础,分析了装配SCR的国Ⅳ常规车及国Ⅳ混合动力车NOx排放偏高的原因.并结合试验所采集到的数据,采用怠速工况及工况片段2种分析方法,分析了排气温度对NOx排放的影响.研究结果表明:SCR技术的应用能一定程度上降低NOx排放量,装配SCR的城市公交车当排气温度介于130～230℃之间时,每提高10℃排气温度就可以提高约6%的NOx转化率;配备SCR的公交车,尤其是混合动力公交车,230℃及以上排气温度的所占比例非常低,导致NOx转化率偏低,NOx排放偏高;应提高城市公交排气温度(如排气加热系统等)或提高SCR的低温转化效率.

The emission test on four different vehicles was conducted by using on-board test system.Based on the theory of SCR,the reason of higher NOx emission from State Ⅳ conventional bus and hybrid bus equiped with SCR was analysed.By combining the data from the test and the methods of ′idle′ and ′segment′,the relationship between exhaust temperature and NOx emission was studied.The results show that the use of SCR technology can reduce NOx emissions to a certain extent.When the exhaust temperature is between 130 ℃ and 230 ℃,the NOx conversion efficiency increases about 6% with temperature increase of about 10 ℃.For transit bus with SCR,especially HEV,the proportion of the exhaust temperature above 230 ℃ is very small,which causes the lower conversion efficiency and higher NOx emission.Some techno-logy（e.g.exhaust heating system） should be used or the transformation efficiency under low temperature should be increased.