Investigation and Simulation of CNG Bus Emissions Based on Real-World Emission Measurement

The regulated gaseous emissions from 2 China-V compressed natural gas (CNG) buses and 2 China-V diesel buses were investigated using a portable emissions measurement system (PEMS) under real road driving conditions. Compared to diesel buses, CNG buses emit less NOx pollutants, but more HC and CO pollutants based on the test results obtained in this paper. In order to evaluate the pollutant emission status of CNG buses in Beijing, an instantaneous emission model as a function of vehicle speed and vehicle specific power (VSP) was developed and validated based on emission data taken from one CNG bus. The input of the instantaneous emission model consists of driving cycle, vehicle parameters, road conditions, ambient conditions and accessory use, all of which were used to calculate the instantaneous vehicle specific power (VSP). For the core model, a group of pollutant emission maps represented as functions of vehicle speed and VSP were used to calculate the second by second emission rates. Finally, the instantaneous emission rates, emission factors and fuel consumption over the selected driving cycle could be obtained as the model outputs. The predicted results for the emissions and fuel consumption of the CNG bus were very close to the tested emission data. The prediction errors for emission factors and fuel consumption varied in the range of -1.62% to -5.8%.