Fuel economy and emissions analysis of conventional diesel, diesel-electric hybrid, biodiesel and natural gas powered transit buses

Abstract

Alternative fuels, emissions control technologies and advanced propulsion technologies offer great potential for reducing emissions from, and increasing fuel economy of, buses employed in public transportation. The use of fuels such as Compressed Natural Gas (CNG) and biodiesel, emissions controls such as diesel particulate filters (DPF) and diesel oxidation catalysts (DOC), and the use of advanced propulsion systems such as hybrid-electric diesel have great potential for decreasing emissions from public transit vehicles and potentially increasing fuel economy. The focus of this paper is to assess the environmental benefits of alternative fuels and advanced hybrid drive technologies in transit vehicles through experimental testing and analysis. Results show that hybrid-electric diesel and CNG buses yield significant reductions in CO2 emissions, approximately 10-20% lower than conventional diesel. Stoichiometric CNG buses demonstrated extremely low emissions of NOx, while conventional, lean-burn CNG had the highest NOx emissions, approximately twice that of hybrid technologies and conventional diesel engines. The hybrid-electric technology demonstrated the highest fuel economy, while CNG has the lowest fuel economy. The use of a 20% biodiesel blend (B20) demonstrated no discernable differences in fuel economy, while showing slightly higher NOx emissions levels and significantly lower PM compared to conventional diesel.