Particulate Matter Emissions from Hybrid Diesel-Electric and Conventional Diesel Transit Buses: Fuel and Aftertreatment Effects

This study compares the particulate emissions from two hybrid diesel-electric (HDE) and two conventional diesel (CD) transit buses of similar performance characteristics measured under three different fuel and aftertreatment configurations over one-year of on-board emissions testing: Phase 1. No. 1 diesel fuel + diesel oxidation catalyst (DOC) aftertreatment; Phase 2. Ultralow sulfur diesel fuel (ULSD) + diesel oxidation catalyst (DOC); and Phase 3. ULSD + diesel particulate filter (DPF), where ULSD used in this study had measured sulfur levels </= 50 ppm. The Allison parallel hybrid buses, in their tested control configuration, did not have any significant particulate emission benefits over the conventional diesel buses, but may have other fuel economy and maintenance benefits that were not addressed by this study. CTTRANSIT should investigate whether a series hybrid design will offer more emissions benefits without sacrificing other advantages of the hybrid bus such as lower noise, smoother rides and performance characteristics comparable to conventional diesel transit buses on freeway commuter routes and routes with high grade (up to 9% in this study). While operation on ULSD did not lower PM emissions relative to No. 1 diesel for the buses tested, operation on ULSD does enable the use of diesel particulate filter aftertreatment, which reduced the particulate emissions by over 95%. The use of DPFs on older engine buses in the CTTRANSIT fleet should be a targeted goal for CTTRANSIT over the next 5-10 years to cover the gap as the current fleet is replaced by newer technologies.