In a typical diesel engine exhaust aftertreatment system
consisting of a diesel oxidation catalyst (DOC), a diesel
particulate filter (DPF) and a selective catalytic reduction (SCR)
system the main purpose of the DOC, besides the oxidation of CO to
CO₂, is the oxidation of NO to NO₂. The NO to NO₂ conversion is an
essential contribution for the downstream SCR system because the
fast SCR reaction which provides the highest conversion rates of
NOx to H₂O and N₂ works well only under roughly equal
concentrations of NO and NO₂.
The typical amount of NO to NOx ratio produced by the
engine is about 0.95, hence the DOC is necessary to decrease this
coefficient close to 0.50. Due to the temperature dependency of the
DOC reaction mechanism the oxidation of NO to NO₂ takes only place
sufficiently if the temperature of the DOC is higher than 200°C,
which, however, cannot be reached during low engine speed and low
load situations. As a consequence, under these circumstances the
whole reduction system is ineffective and, moreover, the
potentially high raw NOx emissions in this operating
range may cause significant tailpipe emission values.
Against this background this paper presents a strategy to raise
the DOC temperature during lower temperature operating ranges in
its most effective way by acting on the post or the main injection.
The final temperature control approach is designed with respect to
the additionally required fuel amount and the effect of the
different injections on the total NOx emissions.
Finally the control strategy was implemented on a test bench to
evaluate the benefit of the DOC temperature increase and
consequently the gain in SCR conversion efficiency against the fuel
penalty. Of course, the additional consumption must be traded off
against other system modifications, especially in the
aftertreatment case.