Particulate emission risks of HEVs during cold-starts and warm-restarts: Characterisation of particulate number under fresh and aged aftertreatment systems

Particle number (PN), which was regulatorily capped to alleviate ultrafine particle exposure, is a key challenge per the current and forthcoming regulations in both the EU and China. Several previous studies have necessitated the research on real-driving PN emissions from hybrid electric vehicles (HEVs), particularly in China, where HEVs are increasingly dominating the market. In this study, to better understand the risk of PN deterioration in real application, a China 6-compliant HEV was tested over real driving emission (RDE) and RTS95 cycle, with fresh and bench-aged after-treatment systems separately. The results indicate that the test vehicle exhibited a trimodal particle number size distribution (PNSD), predominantly characterized by accumulation-mode particles with a peak diameter of approximately 90 nm. The majority of emissions originated from urban and highway segments. The high PN emissions from the test HEV were primarily attributed to engine start events during both cold and warm operation. Catalyst aging reduced the oxidation efficiency but increased the filtration efficiency of gasoline particulate filter (GPF) due to the establishment of ash layer, resulting in improved overall PN removal efficiency of the aged aftertreatment system. Besides, optimizing the fuel enrichment strategy during engine warm-starts could further reduce the PN emissions with the aged aftertreatment system.