Currently, two consolidated aftertreatment technologies are
available for the reduction of NOx emissions from diesel engines:
Urea SCR (Selective Catalytic Reduction) systems and LNT (Lean NOx
Trap) systems. Urea SCR technology, which has been widely used for
many years at stationary sources, is becoming nowadays an
attractive alternative also for light-duty diesel applications.
However, SCR systems are much more effective in NOx reduction
efficiency at high load operating conditions than light load
condition, characterized by lower exhaust gas temperatures. One
possible solution to improve the low temperature behavior, is the
use of newly developed Advanced Diesel Oxidation Catalysts (A-DOC)
which are capable to store NOx at low exhaust temperatures (typical
of urban driving conditions) when SCR efficiency is low, and to
release the stored NOx at higher temperatures (i.e., during
extra-urban driving conditions) where the urea injected is
effectively forming ammonia for the subsequent NOx conversion.
Experimental tests were therefore carried out in order to assess
the performance of an A-DOC when exposed at the emissions coming
from a modern Euro 5, 2.0 L displacement turbocharged Common Rail
DI diesel engine for a typical European passenger car: the engine
features a DOC and a DPF in close-coupled position, hosted into an
on-purpose-designed dismountable canning, thus allowing an easy
switch between different components.
The characterization of these newer DOC formulations was
performed over NEDC cycles. Moreover, the catalysts were tested
both in fresh and hydrothermally aged conditions in order to have a
better understanding relative to robustness and durability of these
newer catalysts.
NOx storage capability, which was found to be impressively high
for a fresh A-DOC, significantly decreased after aging, thus
leading to a final NOx cumulated emissions figure which equals the
engine-out value for the aged A-DOC.
Nevertheless, since most of the NOx released from the A-DOC
occurs during the EUDC segment, when a downstream SCR would likely
have reached appreciable NOx reduction efficiencies, even an aged
A-DOC could provide significant benefits in terms of NOx emissions
reduction.
However, the analysis of the NO/NO₂ share downstream of the DPF,
which is of crucial importance for SCR efficiency at low
temperature, revealed that the overall conversion efficiency for NO
over NEDC was negative, while on the contrary the conversion
efficiency for NO₂ was remarkably high. As a result, the NO₂/NOx
ratio downstream of the DPF (i.e., at the inlet of a downstream
SCR) remained significantly low during the whole EUDC segment, thus
hindering the achievement of high NOx conversion efficiencies and
the full exploitation of a synergetic combination of the A-DOC with
a downstream SCR.