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 catalyst were tested both in fresh and hydrothermally aged conditions in order to have a better understanding relative to robustness and durability of these newer catalyst.
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 release 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.