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In the immediate future the introduction of a wider variety of fuel types will play a significant role in reducing emissions and in solving the energy needs of the transportation industry. Both compressed natural gas, CNG, and hydrogen are expected to play very large roles, and the present paper shows that these fuels, when used together, can offer large benefits in NOx emissions. Significant reductions in NOx emissions will be required for CNG transit buses and heavy duty trucks, if they are to meet the future stringent emissions standards that come into effect in the year 2007. In the present paper a detailed engine model was used to understand and predict the results from engine dynamometer tests from a production automotive engine over a range of hydrogen/CNG gas fuel mixtures.

California Air Resources Board's transit bus fleet regulation required public transit fleets in California to reduce emissions starting in 2002 [1]. In response to this rule, San Francisco Municipal Railway (Muni) launched the “Alternative Fuel Pilot Project”. The objective of the project is to compare the on-road performance, emissions, maintainability, safety, and costs of advanced diesel and alternative fuel buses over a two-year period. This paper discusses the preliminary emissions results from chassis dynamometer testing conducted during the first six months of the emissions study. The California Truck Testing Service's dynamometer facility tested four types of buses: conventional diesel, conventional diesel with particulate traps, compressed natural gas, and hybrid-electric transit buses.

At the present time there are rapid changes occurring in the fleets of transit buses that are used in cities. These changes involve improvements in conventional diesel buses, Compressed Natural Gas, CNG, and more recently hybrid electric vehicles. In order to evaluate the performance of the transit buses, driving cycles have been developed, and two of the most popular are the New York City, NYC, and the Central Business District, CBD. These cycles have proven to be very valuable for predicting both performance and emissions of the transit buses, however they do not well characterize some of the unique characteristics of certain cities, such as San Francisco with its hills and high grade. In this paper we present the results of Chassis dynamometer measurements and modeling of the performance of four different types of transit buses on the typical grades that exist in San Francisco.

In the immediate future the introduction of a wider variety of fuel types will play a significant role in reducing emissions and in solving the energy needs of the transportation industry. Both compressed natural gas, CNG, and hydrogen are expected to play significant roles, and the present paper shows that these fuels, when used together, can offer large benefits in NOx emissions. Significant reductions in NOx emissions will be required for CNG transit buses and heavy duty trucks, if they are to meet the future stringent emissions standards that come into effect in the year 2007. In the present paper we have applied a newly developed engine model with detailed chemical reactions to predict the “in cylinder” production to NOx under realistic engine conditions.

This paper presents model-based predictions of the performance of diesel, compressed natural gas (CNG), and hybrid buses on bus routes in the City of San Francisco. The bus route details were obtained by recording time-series measurements of speed and grade during actual runs of buses on the city streets under different traffic conditions. The transit buses' physical and mechanical characteristics were obtained from manufacturers' data and chassis dynamometer testing of the buses on different city cycles. Both the bus routes and the bus performance characteristics were put into the simulation package ADVISOR from the National Renewal Energy Laboratory (NREL). The most extreme results were for the San Francisco routes that have high grades. The high grades cause performance and emissions problems for both the diesel and CNG buses relative to the hybrid bus.