Real-world emission characteristics of natural gas-gasoline bi-fuel vehicles

The CO2,CO,NOx and total hydrocarbon(THC) emissions of 25 compressed natural gas (CNG)-gasoline bi-fuel taxies were tested by portable emissions measurement system (PEMS) to investigate the impact of CNG on the emissions of light-duty vehicles.Results showed that CO2 and CO emissions from original equipment manufacturer (OEM) vehicles on average were reduced by 22% and 24%,respectively,when the fuel was switched from gasoline to CNG.In comparison,NOx and THC emissions increased by 27% and 22%,respectively,as a result of fuel switch.Using CNG for converted vehicles led to decrease of CO2 and CO emissions by 25% and 32% respectively,but increase of NOx and THC emissions by about 200% compared to gasoline.Changes of CO,NOx and THC emissions with gasoline-to-CNG switch may be related to the conversion of engines,exhaust composition and the performance of three way catalysts.