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A Three-Parameter Transient 1D Catalyst Model
Abstract:
Interactions between in-cylinder combustion and emission
aftertreatment need to be understood for optimizing the overall
powertrain system. Numerical investigations can aid this process.
For this purpose, simple and numerically fast, but still accurate
models are needed for in-cylinder combustion and exhaust
aftertreatment. The chemical processes must be represented in
sufficient detail to predict engine power, fuel consumption, and
tailpipe emission levels of NOx, soot, CO and unburned
hydrocarbons. This paper reports on a new transient one-dimensional
catalyst model. This model makes use of a detailed kinetic
mechanism to describe the catalytic reactions.
A single-channel or a set of representative channels are used in
the presented approach. Each channel is discretized into a number
of cells. Each cell is treated as a perfectly stirred reactor (PSR)
with a thin film layer for washcoat treatment. Heat and mass
transport coefficients are calculated from Nusselt and Sherwood
laws. Either detailed or global surface chemistry is applied in the
thin film layer. Three global parameters are used to align the
detailed chemistry model with a given catalyst topology and
composition; one parameter for heat transfer, one for mass transfer
and one for overall reaction efficiency. This allows considering
detailed surface chemistry, molecular diffusion and heat
conductivity while maintaining affordable CPU time. Detailed,
usually unknown, specifications of the catalyst material are
insignificant for the presented approach.
The models' applicability is demonstrated for a
single-channel of a NOx-storage catalyst (NSC). The detailed
surface chemistry by Koop and Deutschmann is utilized. Good
agreement between experimental data and model results is achieved.
The investigation of surface site fractions shows, that CO and C₃H₆
from exhaust gases inhibit NO oxidation by the same process; in
both cases surface bound CO blocks the sites for NO oxidation. The
inhibition effect is mainly determined by the total concentration
of carbon atoms contained in CO and HC in the exhaust stream.
Oxidation by surface bound oxygen was further found to be the major
pathway for HC conversion. The lasting inhibition effect of
unburned hydrocarbons on NO oxidation was studied by a transient
calculation. In this test a sudden cutoff of unburned hydrocarbons
in the exhaust stream was assumed. The response time for NO
oxidation was found to be 5.5 seconds. The high response time
proofs the necessity of using a transient model of sufficient
detail to simulate catalytic oxidation during transient engine
processes or under cyclic variations.