The United States Environmental Protection Agency (EPA) has
finalized a reporting rule for the Greenhouse Gases (GHGs)
emissions including Nitrous Oxide (N₂O) from all sectors of the
economy. In addition, EPA and the National Highway Traffic Safety
Administration (NHTSA) have been working together on developing a
National Program of harmonized regulations to reduce GHGs emissions
and improve fuel economy of light-duty vehicles (LDV). As a
consequence, the limiting value for N₂O emission from LDV is set to
0.01 g/mile. Considering this regulatory limit of N₂O emission from
LDV, if the exhaust gas is diluted and stored in a sample storage
bag, the concentration of N₂O becomes very low which requires a
highly sensitive analyzer for accurate measurement.
In the previous study, an instrument based on Quantum Cascade
Mid-IR Laser (QCL-Mid IR) Spectroscopy has been developed for
measuring ultra-low level of N₂O in automobile exhaust gas sampled
in a sample storage bag. In this study, the instrument has been
used for measuring total and modal mass emissions of N₂O from four
different LDVs, all meeting regional emission standards applicable
for respective model years. The vehicles included a direct
injection (DI) gasoline vehicle, a direct injection diesel vehicle,
and two conventional port injection (PI) gasoline vehicles. The
main objective is to examine the measurement capability of the
instrument and to develop a better understanding of N₂O emissions
from diverse catalyst-equipped vehicles. Measurements are conducted
on a chassis dynamometer under FTP75 driving cycle both at cold and
hot start conditions. Samples are stored in sample storage bags for
measurement after being diluted in a Constant Volume Sampling (CVS)
system.
It is found from the investigation that the DI gasoline vehicle
emits higher N₂O (approximately double) than conventional PI
gasoline vehicles. N₂O emission is found to be increased during
nitrogen oxides (NOx) conversion in the catalytic
converter. The concentration of N₂O in the sample storage bag
becomes very low upon dilution. The laser-based motor exhaust gas
analyzer showed strong potential for measuring this ultra-low
concentration of N₂O in sample storage bag.