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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.
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.
There are several reasons for the reductions. Regenerative braking
contributes significantly to reducing fuel consumption and thereby
improving efficiency. Regenerative braking takes advantage of the
energy storage system to capture the kinetic energy of the vehicle
during braking. This is accomplished by using the drive motors as
generators during braking to recapture the vehicle's kinetic energy
and restore a portion of this energy back to the energy storage
device to be used later -- for example, during acceleration.
Another contributing factor is the fact that, on a series hybrid,
the engine is not directly coupled to the vehicle drivetrain (i.e.,
the electric drive motor alone drives the wheels). This allows the
auxiliary power unit (APU) to operate independently from the vehicle.
This would theoretically allow the engine/generator to operate at
peak efficiency and optimized emission load points. Series hybrid
control strategies typically prevent the engine from operating in
zones where its efficiency may be low and its emissions high.
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