18 February 2000

The Northeast Advanced Vehicle Consortium (NAVC) has released a report that summarizes results from a year-long testing project which compares the energy efficiency and emission performance of hybrid-electric, compressed natural gas, and low-sulfur diesel buses with those from conventional heavy-duty diesel vehicles. The study has found that all of the new technologies offer significant benefits in lower emissions and improved fuel economy over current diesel buses. The project was funded by the US Defense Advanced Research Projects Agency (DARPA).

The Hybrid-Electric Drive Heavy-Duty Vehicle Testing Project was the first independent demonstration of near-term commercial diesel hybrid-electric technology in real world urban driving cycles. The project was managed by the NAVC and conducted by an independent team of engineers and scientists from M.J. Bradley & Associates and West Virginia University, with participation from hybrid bus manufacturers and transit operators in Boston and New York City. Two hybrid bus models, three CNG bus models, and one diesel bus model were selected for testing, with each model representing the most current, commercially-available version of each technology. The buses were evaluated in over six different emission test cycles with average speeds ranging from 3 to 17 mph and with duty cycles ranging from 4 to 18 stops per mile. In addition, various fuel types were used in order to evaluate the effects of fuel sulfur levels with respect to particulate emissions. The tests measured nitrogen oxides (NOx), carbon monoxide (CO), carbon dioxide (CO2), volatile organic compounds (VOC) and particulate matter (PM). Fuel economy for each vehicle was calculated on a mile per gallon basis.

The program results demonstrate that diesel hybrid-electric and CNG vehicles offer significant emission reductions from that of conventional diesel buses. The hybrids were equipped with diesel particulate filters that showed the greatest benefits when they were operated on low-sulfur fuel. PM emissions from the low-sulfur diesel hybrids were 50 to 70% lower than conventional diesel while NOx emissions were 30 to 40% lower than conventional diesel. The hybrid electric buses also exhibited the lowest CO emission of any of the buses tested, with a 70% reduction from a conventional diesel bus. The testing also verified the significant benefits that CNG bus technologies offer in lowering emissions, making CNG a good choice in transit fleets. For example, PM emissions from the CNG buses showed a 80 to 90% reduction from conventional diesel, and CNG NOx emssions were 50 to 60% lower than conventional diesel.

The project demonstrated significant fuel economy benefits for hybrids with 30 to 65% fuel economy improvements over conventional diesel and as much as 100% over a comparable CNG bus when operated on severe duty cycles. Total greenhouse gas emissions, including carbon dioxide and methane, were found to be similar in conventional diesel and CNG buses, but significantly reduced in diesel hybrid vehicles.

The hybrid results were found to be especially encouraging since heavy-duty hybrid technology is relatively new and will improve as the technology matures.

The NAVC is a public-private partnership of companies, public agencies, and university and federal laboratories working together to promote advanced vehicle technologies in the Northeast United States. The NAVC receives funding from the DARPA Electric and Hybrid-Electric Vehicle Program and the Department of Transportation's Advanced Vehicle Technologies Program, as well as other sources.

For more information, contact Tom Webb, 617.482.1770, tomwebb@navc.org

Download the report (pdf, 3.6 MB)

Source: Northeast Advanced Vehicle Consortium