Evaluating real-world CO₂ and NO_X emissions for public transit buses using a remote wireless on-board diagnostic...

The challenge to mitigate real-world emissions from vehicles calls for powerful in-use compliance supervision. The remote on-board diagnostic (OBD) approach, with wireless data communications, is one of the promising next-generation monitoring methods. We collected second-by-second profiles of carbon dioxide (CO₂) and nitrogen oxides (NO_X) emissions, driving conditions and engine performance for three conventional diesel and three hybrid diesel buses participating in a remote OBD pilot program in Nanjing, China. Our results showed that the average CO₂ emissions for conventional diesel and hybrid diesel buses were 816 ± 83 g km^-1 and 627 ± 54 g km^-1, respectively, under a typical driving pattern. An operating mode binning analysis indicated that CO₂ emissions reduction by series-parallel hybrid technology was largely because of the significant benefits of the technology under the modes of low speed and low power demand. However, significantly higher CO₂ emissions were observed for conventional diesel buses during rush hours, higher than 1200 g km^-1. The OBD data suggested no improvement in NO_X emission reduction for hybrid buses compared with conventional buses; both were approximately 12 g km^-1 because of poor performance of the selective catalyst reduction (SCR) systems in the real world. Speed-dependent functions for real-world CO₂ and NO_X emissions were also constructed. The CO₂ emissions of hybrid buses were much less sensitive to the average speed than conventional buses. If the average speed decreased from 20 km h^-1 to 10 km h^-1, the estimated CO₂ emission factor for conventional buses would be increased by 34%. Such a change in speed would increase NO_X emissions for conventional and hybrid buses by 38% and 56%, respectively. This paper demonstrates the useful features of the remote OBD system and can inform policy makers how to take advantage of these features in monitoring in-use vehicles.