Study on the influence of urea injection and ambient humidity on PN emissions in the new energy vehicle

This study systematically investigates the significant impact of urea injection and ambient humidity on Particulate Number (PN) emissions from diesel engines in new energy vehicles, utilizing WHTC (World Harmonized Transient Cycle) and WHSC (World Harmonized Steady Cycle) test protocols. The analysis focuses on the effects of urea injection quantity, urea quality, and intake air humidity. Key findings reveal that urea injection is a primary source of PN emissions, particularly under transient conditions like the WHTC cycle. Injecting urea solution can increase PN emissions by more than tenfold, contributing approximately 80% of the total PN particle count in the exhaust of China VI and China VII compliant diesel engines. Furthermore, urea quality substantially influences PN results, with variations in test outcomes reaching up to 7.6 times on bench equipment and 10.3 times using PEMS (Portable Emissions Measurement Systems), attributed to impurities in inferior urea promoting particle formation under high temperatures. Ambient humidity is also identified as a critical factor; a 10% increase in intake humidity leads to an average rise exceeding 50% in PN emissions. This effect is likely due to water vapor facilitating particulate nucleation and growth while potentially interfering with urea solution evaporation and decomposition processes. This research provides a crucial theoretical foundation for developing strategies to control PN emissions in modern diesel engines.