Reducing ultra-high particulate number (PN) emissions from high-mileage vehicles: The effective role of compressed natural gas (CNG).

Vehicle particle number (PN) emissions pose a serious threat to human health and the environment, making it crucial to investigate their emission characteristics and control strategies in depth. In this study, high- and low-mileage bi-fuel (gasoline/CNG) taxis that use conventional gasoline, ethanol-containing gasoline, and CNG were tested via a chassis dynamometer. The results indicate that the PN emission factor is significantly affected by the fuel type and driving conditions. Compared with conventional gasoline, both ethanol-containing gasoline and CNG effectively reduce PN emissions, with CNG showing a particularly prominent effect. For high-mileage Vehicle 2, using CNG instead of gasoline reduced PN emissions by three orders of magnitude. Furthermore, driving conditions significantly impact PN emissions, which have a clear relationship with vehicle specific power (VSP). Moreover, PN emission rates and the modified combustion efficiency (MCE) are combined, revealing a negative correlation and explaining the low PN emissions of CNG. Specifically, high-mileage Vehicle 2 had much lower MCE levels with liquid fuels (0.4-1) than with CNG. In addition, the results of this study show that CNG reduced CO, THC, and NMHC emissions from high-mileage Vehicle 2 by up to 95.3 %, 70.6 %, and 89.9 %, respectively. However, it increased CH4 and NOx emissions, with NOₓ levels rising over 180 times in high-mileage Vehicle 2. Therefore, promoting CNG requires balancing its emission reduction benefits with the challenge of controlling NOₓ.