Exhaust Emissions of Transit Buses

Some of the exhaust or tailpipe emissions commonly associated with mobile sources are carbon monoxide (CO), hydro-carbons (HC), nitrogen oxides (NOx), and particulate matter (PM). These emissions can cause local air pollution and be a determinant in human health problems (U.S. Environmental Protection Agency 2012a). In many countries, these emissions are regulated through emissions standards that spur motor vehicle technology advancements and improved exhaust after-treatments. Exhaust emissions also produce greenhouse gases (GHG), specifically carbon dioxide, which are not reduced by current exhaust after-treatment technologies. Recent GHG emissions regulations in Europe cover only passenger cars and vans, while in 2011, the United States announced the first-ever GHG regulations and fuel economy standards for heavy-duty engines and vehicles (Lindqvist 2012).

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. Each of the fuels and technologies has its benefits and costs. A statistical meta-analysis technique for combining the results of 24 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.

Overall, the results from the meta-analysis of the compiled studies align with results from studies on individual fuels and technologies. The meta-analysis shows that 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. This analysis aids in understanding these variations in order to more accurately evaluate results from further emissions testing. Technologies are often developed to meet emissions standards, and the results of this study imply that emissions standards are generally effective. However, it is demonstrated that not all buses are meeting their expected emissions standards, specifically for NOx and PM, which also can be associated with wear on the bus.

The analysis also shows that 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. However, because none of the fuels can be classified as the best at reducing all emissions, it is important to consider lifecycle costs and lifecycle emissions for buses in specific locations before making fleet selection decisions. The lifecycle cost and emissions components raise many possible variables, either global or local, which can have an impact on fuel and vehicle recommendations. Understanding how fuels and technologies contribute to exhaust emissions is a first step in understanding the true costs and impacts of urban bus fleets in various urban contexts.