Analysis of the Performance and Emissions of Different Bus Technologies on the City of San Francisco Routes

Abstract

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. A large portion of the performance and emissions problems can be directly related to the hydraulic torque converter that is currently used on most diesel and CNG city buses. At conditions that are typical of high grades, the hydraulic torque converters have quite poor efficiencies. Since the hybrid bus has a direct electric drive system, it does not suffer the efficiency problems to the same degree as the diesel and CNG buses. This paper also illustrates that bus performance modeling has reached a stage where it can help city bus systems evaluate bus performance before the buses are put into service. For example, it is expected that global positioning systems (GPS) will provide detailed time, speed, location, and grade information for all the bus routes in typical cities during different times of the day with different traffic conditions. This information can then be used with both old and new specifications for bus technologies to predict performance on the actual routes for which they could be used. In this way, a bus transit system could optimize bus routing assignments to minimize cost and emissions on a wide variety of routes.