CNG and Diesel Transit Bus Emissions in Review

Over the past three years, the California Air Resources Board (CARB), in collaboration with the University of California and other entities, has investigated the tailpipe emissions from three different latemodel, in-use heavy-duty transit buses in five different configurations. The study has focused on the measurement of regulated emissions (NOX, HC, CO, total PM), other gaseous emissions (CO2, NO2, CH4, NMHC), a number of pollutants of toxic risk significance (aromatics, carbonyls, PAHs, elements), composition (elemental and organic carbon), and the physical characterization (size-segregated number count and mass) of the particles in the exhaust aerosol. Emission samples are also tested in a modified Ames assay. The impact of oxidation catalyst control for both diesel and compressed natural gas (CNG) buses and a passive diesel particulate filter (DPF) were evaluated over multiple driving cycles (idle, 55 mph cruise, CBD, UDDS, NYBC) using a chassis dynamometer. For brevity, only CBD results are discussed in this paper and particle sizing results are omitted. The database of results is large and some findings have been reported already at various forums including last year’s DEER conference. The goal of this paper is to offer an overview of the lessons learned and attempt to draw overall conclusions and interpretations based on key findings to date.