To meet future stringent emission regulations such as Euro6, the
design and control of diesel exhaust after-treatment systems will
become more complex in order to ensure their optimum operation over
time. Moreover, because of the strong pressure for CO₂ emissions
reduction, the average exhaust temperature is expected to decrease,
posing significant challenges on exhaust after-treatment. Diesel
Oxidation Catalysts (DOCs) are already widely used to reduce CO and
hydrocarbons (HC) from diesel engine emissions. In addition, DOC is
also used to control the NO₂/NOx ratio and to generate the
exothermic reactions necessary for the thermal regeneration of
Diesel Particulate Filter (DPF) and NOx Storage and Reduction
catalysts (NSR). The expected temperature decrease of diesel
exhaust will adversely affect the CO and unburned hydrocarbons
(UHC) conversion efficiency of the catalysts. Therefore, the
development cost for the design and control of new DOCs is
increasing. To select the best quality product at affordable price,
the authors have evaluated exhaust simulation as an additional
development tool.
In this study, a multidimensional exhaust modeling tool based on
physical background was used. The model is able to solve heat and
mass transfer equations under transient conditions.
"Apparent" reaction rates and Langmuir-Hinshelwood rate
expressions are used for the description of the chemical reactions.
The model includes appropriate HC storage models applicable to DOC
with storage capacity.
In principle, exhaust model calibration can be performed using
engine bench experimental data under steady state and transient
conditions with full scale catalysts. However, with this approach
it is indeed difficult to identify chemical mechanisms and to
develop a standard calibration procedure. This paper proposes a new
DOC model calibration methodology based primarily on synthetic gas
tests with small scale catalysts. The main target of this
methodology is to define a process to transfer the information
obtained from synthetic gas tests to full-scale tests with real
exhaust gas, which is not straightforward due to the complex
hydrocarbon speciation. In the present work, the hydrocarbon
speciation and the chemical reaction parameters of the heavy diesel
exhaust hydrocarbons are investigated using well-controlled
experiments with small-scale samples fed by real exhaust gas. The
main target of this approach is to be able to use this calibration
under real exhaust gases under driving cycle application without
any modification. This methodology is expected to be also
applicable to a wide range of DOC technologies.
In this paper, the calibration methodology is presented step by
step. First, the adsorption mechanisms of heavy hydrocarbons (HHC)
were calibrated using storage and release curves at constant flow
and at different temperatures. Then, standard light-off tests were
performed using different gas compositions in order to obtain the
CO and HC oxidation rates. Finally, the small-scale catalyst tests
with real diesel exhaust were performed. The resulting model
calibration was then validated via full scale engine tests under
steady state and under New European Driving Cycle (NEDC)
conditions. In this study, experimental CO, HC and NO conversion
efficiencies were well predicted. Finally this methodology was
successfully applied to a new DOC formulation to monitor the impact
of the exhaust temperature reduction on the CO and HC conversion
efficiency.