Comparison on real-world driving emission characteristics of CNG, LNG and Hybrid-CNG buses

Abstract

This study used a portable emission measurement system (PEMS) to investigate the emission characteristics of three Euro V buses (a CNG bus, an LNG bus and a hybrid-CNG bus) under real-traffic conditions. The results showed that the emission factor of gaseous pollutants decreased gradually with increasing driving speed. The LNG bus showed the highest emission characteristics, especially the emissions of CO, NOx, PM and BC. Although the NOx emission factor for the LNG bus was the highest (23.56 g/km), its NO2 emission factor (0.82 g/km) was lower than that of the CNG bus (1.58 g/km). Moreover, the particle size of the particulate matter emitted by the LNG buses was large, and the ratio of BC/PM was about 40%, which was much higher than those of the other two buses (7.59% and 33.11% for the CNG bus and hybrid-CNG buses, respectively). Compared with the CNG bus, the hybrid-CNG bus could effectively reduce particle number emissions by 59.1%, mainly by reducing the emission of nucleation-mode particles with particle size less than 30 nm. However, the emissions factors of other pollutants of the hybrid-CNG bus were higher than those of the CNG bus, especially the emission of black carbon, which was 66% higher.