Exhaust Emissions of Transit Buses (Working Paper)

By Erin Cooper, Magdala Arioli, Aileen Carrigan & Umang Jain

As part of EMBARQ’s Sustainable Urban Transportation Fuels and Vehicles (SUTFV) program funded by FedEx, this report compiles a large data set of in-use transit bus emissions tests for use in a meta-analysis to define ranges of exhaust emissions for urban bus fuel and technology combinations. The analysis also looks at both local and global emissions to understand their impact on human health and the environment. Some of the exhaust or tailpipe emissions commonly associated with mobile sources are carbon monoxide (CO), hydro-carbons (HC), nitrogen oxides (NOx), particulate matter (PM) and carbon dioxide (CO2). The fuels considered in this analysis are diesel with various concentrations of sulfur, biodiesel (100 percent and 20 percent blend with diesel), compressed natural gas (CNG), liquefied natural gas (LNG), and ethanol. The technologies considered are standard internal combustion engines (ICEs) and hybrid ICE-electric, in combination with a variety of exhaust after-treatment technologies. A statistical meta-analysis technique for combining the results of more than 25 independent studies was used to find a range of emissions values for different fuel and technology combinations.

The analysis looked at many factors for which data were available, including specific fuel type and relevant technologies, emissions standards, field tests vs. lab tests, drive cycles, CO2 equivalent emissions, mileage, and altitude. Key findings of the study include: * There is a wide range of emissions values even for the same fuel and technology. * Many of the factors explored, such as altitude and drive cycle, do have an impact on emissions. * No single fuel is best in all categories of emissions if the appropriate exhaust after-treatment technologies are used, which means that these technologies are key to reducing emissions. * The technologies that show the lowest emissions for key pollutants, such as NOx, PM, and CO2 equivalence, are compressed natural gas with a three-way catalyst, 100 percent biodiesel, and ultra-low sulfur diesel with selective catalyst reduction.

If you have any questions or comments, or would like to know more about this research, contact Umang Jain at umang.jain@wri.org.