On-road measurement of regulated pollutants from diesel and CNG buses with urea selective catalytic reduction systems

In this study, emissions from 13 buses operated in Beijing, including two Euro-III diesel buses, four Euro-IV diesel buses, three Euro-V diesel buses and four Euro-V CNG buses, were characterized in real world conditions. All of the buses tested were fitted with selective catalytic reduction (SCR) systems except for the Euro-III diesel buses. A SEMTECH-DS was used for testing the gaseous pollutants, and an electric low pressure impactor (ELPI) was used for measuring of particle numbers and size distributions. A comparison was made based on emission performance of these buses by employing the VSP approach and fuel- based emissions factors. Diesel buses emitted less CO and THC but more NO_x and PM pollutants than CNG buses. The NO_x reduction efficiencies of the SCR systems for CNG buses were higher because of the high exhaust temperature and high NO₂/NO_x ratio, whereas the efficiencies for diesel buses were lower. This resulted in extremely low NO_x emissions from CNG buses, but the high NO₂/NO_x ratio needs further study. Failures of urea injection in the SCR systems were detected in this research, which resulted in very high NO_x emissions. The CNG buses also emitted smaller numbers of particles and less particle mass with the presence of oxidation catalysts. Diesel buses satisfying the Euro-V standard performed better than Euro-IV and Euro-III diesel buses in terms of emission performance, except for more nuclei mode particles. Most of time, the Euro-IV diesel buses show no advantages in CO and NO_x emissions compared with the Euro-III diesel buses.