Research on emission and fuel consumption characteristics of plug-in hybrid electric vehicles in charge-sustaining mode under plateau environment: A comprehensive analysis based on EPA five-cycle test

The number of hybrid vehicles is increasing in high-altitude regions; however, systematic research on emissions and fuel consumption under EPA five-cycle testing in charge-sustaining mode remains limited. In this study, the EPA five-cycle test, including UDDS, US06, SC03, HFEDS, and FTP (−7°C), was conducted on a plug-in hybrid electric vehicle at altitudes of 0 m and 1914 m using a light-duty vehicle emission testing system. The effects of altitude and driving cycles on pollutant emissions and fuel consumption were analyzed. The results show that emissions and fuel consumption are strongly affected by both the driving cycle and altitude. The FTP (−7°C) cycle at 1914 m was the most unfavorable condition because of the combined effects of low temperature and high-altitude hypoxia, whereas the lowest emissions and fuel consumption occurred during the steady-state HFEDS cycle at 0 m, mainly due to favorable thermal management. Under UDDS, frequent start–stop events caused intermittent pulsed CO and THC emissions, with higher peaks and longer durations at high altitude. Under the high-load transient US06 cycle, CO emissions increased further due to mixture enrichment in the plateau environment. The highest NO X emissions were observed under SC03, where air-conditioning demand forced sustained high-load engine operation. Compared with low-altitude conditions, low temperature and high-altitude hypoxia had a significant synergistic adverse effect on fuel economy. This study provides a basis for optimizing hybrid vehicle calibration in plateau regions and highlights the need to improve combustion stability during low-temperature cold starts and the low-temperature adaptability of aftertreatment systems.