Alternative fuels and technologies offer potential for reducing emissions in public transportation. These potentials were explored by determining emissions levels and fuel consumption from the U.S. transit bus fleet and comparison of hypothetical scenarios in which implementation of specific alternative fuels and technologies is considered. Impacts from current transit bus procurements were also evaluated. Emissions benefits above and beyond the natural course of transit bus procurements were examined for new diesel buses running on ULSD fuel, diesel-electric hybrid buses, gasoline-electric hybrid buses, compressed natural gas and biodiesel. According to the analysis, reductions in emissions of CO, NMHC, NOx, PM and CO2, as well as fuel consumption, may be attained, and diesel hybrid buses yield the largest reductions in CO2 emissions and are the only technology to reduce fuel consumption relative to the present fleet. Introducing diesel-electric hybrid buses in 15% of the U.S. transit bus fleet would reduce annual end-use emissions by nearly 1,800 tons of CO, 400 tons of NMHC, 4,400 tons of NOx, 200 tons of PM, 491,400 tons of CO2, and fuel consumption by 50.66 millions of diesel gallons.

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Finite time thermodynamics is used to solve a new model of an extended Brayton cycle with variable-temperature heat reservoirs and finite size heat exchangers. The model takes into account external and internal entropy generation and handles heat recovery and heat leaks to the environment in a novel way. The extended system considerations are very important for minimizing entropy generation and maximizing second law efficiency, profit and ecological criterion. An optimization analysis was developed on this new model to determine its maximum power and minimum entropy generation, and amid the most important findings were the global maximum net power, global minimum entropy generation, optimum global heat exchangers size distribution, best working fluid specific heat ratio and optimal fluid heat capacities, some of these never having been published previously.

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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 CO 2 emissions, approximately 10-20% lower than conventional diesel. Stoichiometric CNG buses demonstrated extremely low emissions of NO x , while conventional, lean-burn CNG had the highest NO x 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 NO x emissions levels and significantly lower PM compared to conventional diesel.