Emission characteristics of CNG/gasoline dual-fuel taxi

To investigate the real emission level of compressed natural gas(CNG)/gasoline dual-fuel taxis, Portable emission measurement system(PEMS) testing techniques was applied to measure instantaneous CO2, CO, NOx and HC emissions. Comparison between CNG and gasoline was set up to analyze the influence of speed and driving modes on the gas pollutants emissions under the real-world operating conditions. Results show that the average speed of the tested taxis is 27.75 km/h and the speed has a relationship with emission level. Taxi fueled with CNG emit less CO2 and CO but more NOx and HC than taxi fueled with gasoline at all speeds below 45 km/h. CO2, CO and HC emission factor from CNG taxi were lower whilst NOx were higher, NOx emission rate of CNG taxi was higher contrast to the taxi with gasoline. The emission level of CNG taxi is better than gasoline taxi when the driving speed is low.