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NAVC
Hybrid Transit Bus Emissions Project
In February,
2000, the NAVC released the results of the year-long Hybrid-Electric
Drive Heavy-Duty Vehicle Testing Project which put hybrid-electric,
compressed natural gas, and low-sulfur fuel diesel buses through
extensive tests to demonstrate energy efficiency and emission performance
in comparison with conventional diesel heavy-duty vehicles. 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.
Overview
of Results
The program
results demonstrate that hybrid-electric buses offer significant
emission reductions from that of conventional diesel buses. Hybrids
showed the greatest benefits when they were operated on low-sulfur
fuel. PM emissions from the low-sulfur diesel hybrids were 50 to
70 percent lower than conventional diesel while NOx emissions were
30 to 40 percent lower than conventional diesel. The hybrid electric
buses also exhibited the lowest CO emission of any of the buses
tested, with a 70 percent reduction from a conventional diesel bus.
The hybrid results are especially encouraging since heavy-duty hybrid
technology is relatively new and will improve as the technology
matures.
The project
demonstrated significant fuel economy benefits for hybrids with
30 to 65 percent fuel economy improvements over conventional diesel
and as much as 100 percent over a comparable CNG bus when operated
on severe duty cycles. The hybrid vehicle also demonstrated significantly
lower total greenhouse gas emissions than that of a conventional
diesel or CNG bus.
Why
are hybrid bus emissions different?
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