Emission factors and fuel consumption of CNG buses in real driving conditions

This paper describes the results of an experimental campaign carried out on three urban buses fuelled by natural gas, aimed at measuring pollutant emissions and fuel consumption in real driving conditions. The buses, used for public transport in three Italian cities, were equipped with a Portable Emission Measurement System. Emission tests were performed over the routes on which the buses operate in their service, and cold start emissions were also investigated. The results showed that the differences in CO and NOx emissions can be attributed to the calibration of the fuel supply system. PM emissions were low and similar to those reported in the literature for other CNG buses, while CO2 and fuel consumption were affected by the driving cycle. Cold start strongly impacted CO and NOx emissions because of the effect of temperature on catalytic converter efficiency, with cold over-emissions strongly dependent on ambient and coolant temperatures at engine start.