|The fuel-economy of the hybrids surpassed that of the diesel and CNG buses. Click to enlarge.|
The US Department of Energy’s National Renewable Energy Laboratory (NREL) has published the final version of a study comparing 10 new compressed natural gas (CNG) and next-generation diesel series-hybrid electric bus propulsion systems operated at New York City Transit (NYCT) against a baseline of conventional diesel buses.
The report is part of a series of evaluations from NREL that track and evaluate new propulsion systems in transit buses and trucks. DOE/NREL evaluated the original 10 prototype diesel-hybrid buses from Orion and BAE Systems (model Orion VI buses) operated at New York City Transit (NYCT) .
That evaluation was reported in July 2002 and provided results from the prototype buses from 1998 through 2001. This report focuses on 10 new compressed natural gas (CNG) buses and hybrid propulsion systems.
This report describes the evaluation results for new Orion VII low floor buses at NYCT with CNG propulsion (equipped with Detroit Diesel Corporation Series 50G CNG engine) and new hybrid propulsion (equipped with BAE Systems’ HybriDrive propulsion system). These final results represent a 12-month evaluation of these two groups of buses (October 2004 through September 2005).
The buses evaluated are the same age, the same bus platform, have been operated on similar duty-cycles, and experienced similar maintenance practices.
Among the study’s findings:
With a fuel economy of 1.70 miles per diesel-equivalent gallon, the CNG study group has a 25%-28% lower fuel economy than the diesel buses. The low average speed of the NYCT operation is the key to this significantly lower fuel economy—spark ignited (SI) natural gas engines typically have a lower thermal efficiency at low speed and load than compression ignition (CI) diesel engines. Consequently, lower natural gas fuel economy is expected in this type of operation.
The hybrid study fleet—with average fuel economy of 3.19 mpg) achieved 37% higher average fuel economy that the diesel buses, and exhibited 88% higher fuel economy than the CNG study fleet.
The hybrid bus average fuel economy had a much larger decrease/fluctuation in fuel economy in the summer months (June through September) than any of the other three study bus groups. According to BAE Systems, much of this decrease is due to an increase in energy consumption for air conditioning.
New York City Transit (NYCT) Hybrid (125 Order) and CNG Transit Buses (NREL/NREL/TP-540-40125)