中文摘要：

满足国Ⅳ标准的公交车已被广泛用于北京等大城市，为了评价其相对于国Ⅲ公交车的实际减排效果，使用PEMS（车载排放测试系统）测试了4辆装有SCR（选择性催化还原）系统的国Ⅳ公交车和1辆国Ⅲ公交车在实际运行条件下的污染物排放并进行对比．结果表明：相对于国Ⅲ公交车，装有SCR系统的国Ⅳ公交车在实际运行工况下有效降低了污染物排放，CO、THC和PM分别比国Ⅲ限值降低了60．8％、94．2％和86．2％；但由于实际运行工况中排气温度较低，致使SCR系统效率偏低，只有一辆国Ⅳ公交车NOx排放低于国Ⅲ限值，其他3辆车的NOx排放分别比国Ⅲ限值高187．3％、228．7％和157．3％．国Ⅳ混合动力车的CO2、CO和PM排放比常规动力车分别降低了42．9％、48．8％和89．5％，但NOx排放反而增加了112．1％．实际运行工况下，测试车辆只有12．7％的工况点分布在A转速（1478r／min）以上；而ETC工况中在A转速以上的工况点占整个循环的80．5％，造成了满足ETC等型式认证的车辆在实际工况下NOx排放偏高．因此，需要对公交车的实际排放进行有效监督．

英文摘要：

To reduce urban air pollution, more and more low-emission transit buses have been deployed in cities in China. To assess the effectiveness of the adoption of China IV buses on emissions reductions, real-world pollutant emissions of four China IV buses with SCR （selective catalytic reduction） technology, together with one China Ⅲ bus, were measured and analyzed in this study. PEMS （portable emission measurement system） was used for the measurement. The results demonstrate that China Ⅳ buses with SCR had 60.8% lower CO emissions, 94.2% lower THC emissions, and 86.2% lower PM emissions compared with the China Ill bus. However, because of the lower conversion efficiency of the SCR systems as a result of a low exhaust temperatures in real-world operating conditions, three of the China IV buses emitted higher NO, than the China Ⅲ bus; their NOxemissions were 187.3% , 229.7% and 157.3% greater than the limit value of the current China IV standard. Among the China IV buses, the hybrid bus emitted 42.9% less CO2 , 48.8% less CO, and 89.5% less PM. The hybrid bus encountered higher NO, issues as well. Monitoring of the real-world operating conditions showed that the buses were operating at an engine speed higher than A-speed （1478 r/min） only 12.7% of the time. However, the percentage is as high as 80.5% in the ETC testing procedure. The discrepancy between certification cycles like the ETC and real-world operation leads to high NOx emissions under real-world operating conditions. It was concluded that more effective supervision of heavy-duty vehicle emissions should take more real-world operating conditions into consideration.