Real-world emission characteristics of China IV buses

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 III 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_x than the China III bus; their NO_x emissions 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_x 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 NO_x 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.