A report by the U.S. Dept. of Energy’s (DOE) National Renewable Energy Laboratory has concluded that diesel-electric hybrids produce better fuel economy than compressed natural gas (CNG)-powered buses and conventional diesel buses.
The federal study involved buses run by the New York City Transit (NYCT).
The report, available at www.nrel.gov/vehiclesand fuel/fleettest/pdfs/40125.pdf, describes the evaluation results for new Orion VII low-floor buses at NYCT that are powered by Detroit Diesel Series 50G CNG engines and the same model buses equipped with BAE Systems’ HybriDrive propulsion system.
According to the report, the CNG-powered buses had 25% to 28% lower fuel economy than the diesel buses, while the hybrid system turned in fuel economy figures 37% higher than the diesel buses and 88% higher than the CNG-powered buses.
The stop-and-go duty cycles of transit bus operations are an ideal application for hybrid systems that capture and use energy usually lost during braking, DOE said.
Additionally, the low speeds characteristic of transit bus service also put the CNG engines at a further disadvantage because CNG engines are spark-ignited and typically have lower thermal efficiency at slow speed and load than compression-ignition diesel engines.
According to the study, the hybrid buses, however, had the largest decrease/fluctuation in fuel economy in the summer months, when running the buses’ air conditioning systems increased energy consumption.
