A Constant Volume Sampler (CVS) over dilutes the exhaust gas
sample when testing Plug-In Hybrid Electric Vehicles (PHEV). This
is because the CVS continues to fill the sample bag when the engine
is shutdown. With a PHEV, it is possible to complete an FTP test
with the engine running less than 20% of the time, resulting in a
CVS bag dilution ratio in the range of 100 to 300. The CVS dilution
ratio should be in the range of 5-25 for accurate results. At
higher dilution ratios, the gas concentrations of CO, NOx and THC
approach the ambient background level in the test cell. At a
dilution of 100, the CO₂ concentration in the sample bag is about
0.13%, which is only 3 times the air background concentration. The
measurement errors caused by over dilution create errors of 10% to
30% in the calculated mass of CO₂, CO, and NOx. Estimated errors
for THC are in the range of 200%.
Many of the limitations of a CVS can be eliminated by using a
partial flow sampling system, such as a bag minidilutor (BMD),
which maintains a constant dilution ratio of 5:1 at all exhaust
flow rates. A BMD dilutes the exhaust gas with pure air, so no
background correction for ambient air contamination is necessary.
Testing a PHEV still poses measurement issues with a BMD, because
the internal combustion engine may not run long enough during a
test for the BMD to acquire a volume of exhaust gas sample
sufficient for the gas analyzers to read accurately.
Modifications have been made to the CVS and BMD hardware and
software to improve the measurement accuracy for PHEV testing.
Vehicle emission data obtained with the modified test equipment is
compared to data obtained previously with standard CVS and BMD
sampling systems.