Characterization of pollutant emissions and fuel use for CNG transit buses

Abstract

Transit agencies have historically relied on diesel buses to move their passengers; however, many agencies have adopted alternative fuels such as compressed natural gas (CNG). New knowledge is needed to accurately assess the environmental impacts of CNG as a transportation fuel. The goal of this research is to characterize the air pollutant emissions and fuel use of CNG buses. This dissertation focuses on real-world data collection of emissions and fuel use rates and vehicle activity data for CNG buses. The case study fleet included five big capacity (32-seat) CNG buses and five small capacity (19-seat) CNG buses, and three (27-seat) diesel buses. The diesel bus data was used as a baseline to compare the environmental impacts of CNG buses. The central component of the field data collection process was a portable emissions measurement system (PEMS). Monitored pollutants included carbon dioxide (CO2), hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides (NOx). Vehicle activity data included, engine speed in revolutions per minute, intake air temperature, manifold absolute pressure, engine load and vehicle speed. The major outcome of the research is new knowledge that may lead to improved environmental and fleet management practices as well as improved decision making capacity for stakeholders.