Evaluating the Performance of a Conventional and Hybrid Bus Operating on Diesel and B20 Fuel for Emissions and Fuel Economy 2020-01-1351

With ongoing concerns about the elevated levels of ambient air pollution in urban areas and the contribution from heavy-duty diesel vehicles, hybrid electric vehicles are considered as a potential solution as they are perceived to be more fuel efficient and less polluting than their conventional engine counterparts. However, recent studies have shown that real-world emissions may be substantially higher than those measured in the laboratory, mainly due to operating conditions that are not fully accounted for in dynamometer test cycles. At the U.S. EPA National Fuel and Vehicle Emissions Laboratory (NVFEL) the in-use criteria emissions and energy efficiency of heavy-duty class 8 vehicles (up to 36280 kg) can be evaluated under controlled conditions in the heavy-duty chassis dynamometer test. The present study evaluated the performance of a conventional bus and a hybrid bus for emissions and fuel economy under representative test cycles (including cold start and hot start conditions) with Diesel (#2) and Biodiesel (B20) fuel. The conventional bus was equipped with a Cummins ISL 8.3L engine and a Diesel Particulate Filter (DPF) and Diesel Oxidation Catalyst (DOC) aftertreatment, while the hybrid bus had a Cummins ISB 6.7L engine with a Selective Catalytic Reduction (SCR), DPF and DOC aftertreatment. Generally, the hybrid bus showed higher fuel economy (lower CO₂ emissions) as compared to the conventional bus for both cold start and hot start test cycles. However, NOx emissions were observed to be somewhat higher for the hybrid bus over the cold start test cycle. In general, lower emissions were observed for both bus configurations when operating on B20 over diesel fuel, while fuel economy was comparable. To confirm these trends on in-use vehicles and to understand if chassis dynamometer testing accounts for real world operation appropriately, some initial results from on-road testing of the buses are reported for comparison purposes.