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What
are ozone precursors?
Ozone emissions
are regulated by the U.S. Environmental Protection Agency. Ozone
is not attributable to direct emissions, but is instead a function
of ozone precursor emissions. Oxides of nitrogen (NOx) and volatile
organic compounds (VOCs) are regulated as precursors for ozone.
Ozone is a particular problem in urban areas, many of which are
in non-attainment for the ozone.
VOCs are defined
in a regulatory sense as any compound of carbon that participates
in atmospheric photochemical reactions. By definition, VOCs are
the subset of organic compounds that are considered ozone precursors.
This definition exempts several compounds, including many fluorinated
hydrocarbons (primarily refrigerants), methane, ethane, carbon monoxide,
carbon dioxide, acetone and several others. (It should be noted
that this list is updated as compounds are determined to be non-reactive
with respect to ground level ozone.) For transportation purposes,
non-methane organic compounds (NMOC) are measured as a subset of
VOCs excluding methane, which is not considered a VOC.
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Overview
of Results
NOx emissions
from the Orion-LMCS hybrid buses were 30 to 40 percent lower than
a conventional diesel vehicle. This result is interesting given
these hybrid buses used diesel engines certified to the same NOx
standard of 4.0 g/bhp-hr.
Only about a
third of this benefit is attributable to regenerative braking. The
Orion-LMCS hybrid was tested on both the NY Bus cycle and the CBD
cycle with its regenerative braking system turned off and in both
cases NOx emissions were still 20 to 30 percent lower than the conventional
DDC Series 50 diesel engine bus. As these vehicles were not equipped
with aftertreatment for the control of NOx, the remaining NOx benefit
must be attributed to differences in engine operation.
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