Characterization of Emissions from Hybrid-Electric and Conventional Transit Buses

Hybrid-electric transit buses offer benefits over conventional transit buses of comparable capacity. These benefits include reduced fuel consumption, reduced emissions and the utilization of smaller engines. Factors allowing for these benefits are the use of regenerative braking and reductions in engine transient operation through sophisticated power management systems. However, characterization of emissions from these buses represents new territory: the whole vehicle must be tested to estimate real world tailpipe emissions levels and fuel economy. The West Virginia University Transportable Heavy Duty Emissions Testing Laboratories were used to characterize emissions from diesel hybrid-electric powered as well as diesel and natural gas powered transit buses in Boston, MA and New York City. Vehicle emissions (carbon monoxide, oxides of nitrogen, hydrocarbons, particulate matter) and fuel economy were measured and the performance of hybrid-electric transit buses was compared to that of conventional buses over the Central Business District (CBD) and New York Bus driving cycles. Testing was also performed using the new Manhattan cycle that was developed from actual speed-time data from recent conventional and hybrid-electric transit buses in Manhattan, NY. The effect of driving cycles is examined, and the cycle was found to influence the advantage of hybrid buses over conventional buses.