Laboratory characterization of VOC evaporative emissions from light-duty plug-in hybrid electric vehicles: Environmental impact comparison with conventional vehicles

Plug-in hybrid electric vehicles (PHEVs) are effective in reducing fuel consumption and tailpipe emissions. However, a comprehensive understanding of their contribution to evaporative emissions, including the nature of volatile organic compounds (VOCs) released and their subsequent environmental benefits, is lacking. To fill this gap, evaporative emission tests were performed on light-duty PHEVs using the variable temperature sealed housing evaporative determination (VT-SHED) method. This investigation characterized VOC evaporative emission factors (EFs), source profiles, and atmospheric reactivity across various operating conditions. These findings were then compared against conventional internal combustion engine vehicles (ICEVs) adhering to the same China VI emission standards, allowing for an assessment of VOC evaporative emission reductions and the overall environmental advantages provided by PHEVs. Key findings indicate that PHEVs generated total VOC (TVOC) EFs of 0.07 ± 0.04 g/h for hot soak loss (HSL) and 0.03 ± 0.01 g/d for diurnal breathing loss (DBL). In both HSL and DBL procedures, light hydrocarbons (C₂, C₄, and C₅ species) with high saturation pressures were the dominant VOC contributors. Furthermore, the OH loss rate (L_OH) and secondary pollution potential of vehicle VOC emissions exhibited an exponential increase with elevated evaporation temperature and prolonged duration. These results demonstrate that a single PHEV annually reduced TVOC emissions by 54.44% when compared to ICEVs. The evaporative control system in PHEVs successfully reduced aromatic hydrocarbon emissions, resulting in a 66.08% decrease in ozone formation potential (OFP) and a 93.85% decrease in secondary organic aerosol potential (SOAP). Importantly, a key difference is observed when PHEVs operate in electric mode for extended periods, during which their VOC emissions may considerably exceed those of ICEVs. This highlights an important yet often overlooked aspect of their emission behavior. These results advance our understanding of PHEVs’ emission profiles and inform future vehicle design strategies and environmental policy initiatives.