Research on particle emissions of light-duty hybrid electric vehicles in real driving

Real driving emission (RDE) assessments were conducted on light-duty hybrid electric vehicles (HEVs) compliant with China-VI standards across various altitudes, including Kaiyuan, Jianshui, and Kunming. Portable Emission Measurement Systems (PEMS) were utilized to analyze instantaneous particulate number (PN) emissions. At the same time, a particle trap was used to collect primary particulate matter (PM). High-resolution Transmission Electron Microscopy (TEM) was employed to capture the micro-morphology and nano-structure of PM. The findings revealed a positive correlation between PN emission factors and altitude, suggesting more stringent regulations are necessary in high-altitude regions. Cold-start conditions exhibited heightened PN emissions, surpassing regulatory thresholds (primarily when the coolant temperature is below 30 °C). Notably, the PN emission rate increases when the vehicle is under light acceleration (0 ≤ a ≤0.5 m/s²) and the coolant temperature is within the range of 20∼30 °C. The PM emitted by HEVs exhibited an unimodal distribution across different driving conditions, with cold-start yielding finer particles. In contrast, urban and rural driving conditions showed enhanced PM aggregation due to more complete fuel combustion. Urban driving conditions, characterized by coolant temperatures exceeding 70 °C and speeds below 60 km/h, produced PM with the largest average particle size (32.68 nm) and the broadest size distribution range (8.99 nm–77.60 nm). Furthermore, increased engine speed and load resulted in elevated average outer shell fringe lengths of primary nanoparticles. The average inner core fringe lengths initially increased, peaked in urban environments (1.04 nm), and then decreased. Fringe separation distances gradually increased, predominantly between 0.2 nm and 0.8 nm. The increase in engine load is the primary factor influencing the growth of microcrystal tortuosity. The study underscores the significance of HEVs in PM emissions, which pose potential risks to both human health and environmental sustainability, emphasizing the need for effective mitigation strategies.