The catalyzed particulate filter (CPF), used in conjunction with
a diesel oxidation catalyst (DOC) is an important aftertreatment
device used to meet Environmental Protection Agency (EPA)
heavy-duty diesel emission standards for particulate matter (PM).
Numerical modeling of these exhaust after-treatment devices
decreases the time and cost of development involved. Modeling CPF
active regeneration gives insight into the PM oxidation kinetics,
which helps in reducing the regeneration fuel penalty. As seen from
experimental data, active regeneration of the CPF results in a
significant temperature increase into the CPF (up to 8°C/sec) which
affects the oxidation rate of particulate matter (PM). PM oxidation
during active regeneration was determined to be a function of
filter PM loading, inlet temperature and inlet hydrocarbon
concentration.
This paper focuses on the development and calibration of a
1-dimensional, MATLAB/Simulink DOC model and its use with a CPF
model appropriate for studying active regeneration strategies. The
DOC model accurately captures DOC behavior when the exhaust gas HC
concentration is at the elevated levels observed during CPF active
regeneration. Experimental data were used to calibrate the DOC
model during CPF active regeneration using a numerical optimization
approach. The DOC model is used to predict the time-varying outlet
HC concentrations which are then used as input to a 1-D CPF
model.
The CPF model is an improved version of the model described in
Premchand et al., with several important differences including: HC
and CO oxidation, reversible NO oxidation/NO2 dissociation
reaction, PM cake layer permeability, PM cake layer density,
improvements in the filtration model, wall and gas temperature
prediction and time-varying HC concentration into the CPF.
The CPF modeling effort focuses on using the DOC outlet HC
concentration and calibrating the sub-models that have a
significant effect on the active regeneration pressure drop across
the CPF, including: HC oxidation, PM cake permeability, substrate
wall PM and PM cake oxidation and filtration. Comparisons are made
between simulated and experimental values of the pressure drop
across the CPF and the PM oxidized during loading, regeneration and
post-loading, CPF outlet temperatures and CPF outlet gaseous
emission concentrations for the selected load cases. An
optimization has been done using the reaction rate from the CPF
model to find thermal PM activation energy and pre-exponential
factors.
Active regeneration experimental data acquired by Chilumukuru et
al., with a 2007 production Cummins regenerative particulate filter
has been used for this study. The experimental matrix consists of
three CPF inlet temperatures (525, 550 and 600°C), three filter
loadings (1.1, 2.2 and 4.1 g/l) and PM oxidation levels of 40 and
70% of the PM retained at the end of loading. Accurate estimates of
the DOC inlet HC levels are critical for accurate DOC and CPF model
temperature predictions. A combined direct/indirect HC measurement
approach was used. Emission FID HC measurements were adjusted using
a DOC/CPF energy balance based on DOC and CPF inlet and outlet
temperature measurements.