The Diesel Oxidation Catalyst (DOC) is an important technology
for the removal of CO and hydrocarbons (HC) from the exhaust of
diesel engines, as well as for generating exotherms for active
regeneration, and for producing NO₂ used by downstream
components.
This paper describes the development of a one-dimensional
numerical model for a Pt-Pd DOC for use in designing aftertreatment
systems. The model is based on kinetics developed from laboratory
microreactor data. The model is a significant advance over previous
DOC models we have developed. A much larger experimental matrix was
used enabling the kinetics and inhibition effects to be much better
defined. The experiments included rich conditions enabling the
model to be used in NOX trap systems, where the exhaust becomes
rich during regeneration. Reduction of NO₂ to NO by CO and HC has
been included in the model. As well as converting NO₂ entering the
DOC, this reaction prevents NO₂ formation while significant levels
of CO and HC are present along the catalyst. The conversion of NO
to NO₂ is observed to exhibit hysteresis when the temperature is
ramped up and then down. This has been explained and successfully
modeled in terms of the formation of an oxide layer on the surface
of the catalyst, which is inactive for NO oxidation. Including both
this effect and NO₂ reduction is important for obtaining a good
prediction of the outlet NO₂ concentration, which in turn is
crucial for the performance of any downstream model. The model has
been successfully validated over both NEDC and FTP75 tests.
Finally, it is demonstrated that this model, developed for an
LDD catalyst, is readily adaptable to an HDD catalyst. The HDD
version of the model has been extensively validated against engine
test data for a range of tests (steady state light-off, transient
light-off, HD-FTP, NRTC, WHTC).