Evaluation of Emissions from New and In-Use Transit Buses in Mexico City, Mexico

The West Virginia University Transportable Heavy-Duty Emissions Testing Laboratory was used to evaluate exhaust emissions from nine transit buses from six separate manufacturers, as commissioned by the Mexico City, Mexico, Secretariat of the Environment. The vehicles included a hybrid-drive diesel bus, two buses with lean-burn spark-ignited compressed natural gas (CNG) engines, and six buses powered by conventional diesel engines. Vehicle testing weights (curb weight plus passenger weight) ranged from 26,996 lb (12,256 kg) to 57,025 lb (25,889 kg), and passenger capacities ranged from 85 to 161. Two driving cycles, the European Transient Cycle (ETC, also known as the FIGE transient cycle) and a new, three-mode Mexico City Schedule, were used to simulate in-use driving conditions during emissions measurements. Diesel fuels with three different sulfur concentrations (15, 150, and 350 ppm) were also examined, and the lowest-sulfur fuel (15 ppm) was not found to have an appreciable direct effect on emissions. When the ETC was used, oxides of nitrogen (NOx) emissions varied from 5.5 to 11.6 g/mi, whereas particulate matter emissions varied from 0.02 to 0.86 g/mi. Significant differences in emissions and fuel economy for the two CNG buses raised the issue that fuel delivery systems must be optimized for operation at high altitudes. The differences in fuel economy and NOx emissions from the hybrid bus for congested and noncongested driving conditions were significantly less than those from the conventional diesel and CNG buses.