Transient On-Road Emission Reduction of an LNT + SCR Aftertreatment System 2008-01-2641
An LNT + SCR diesel aftertreatment system was developed in order to meet the 2010 US HD EPA on-road, and tier 4 US HD EPA off-road emission standards. This system consists of a fuel reformer (REF), lean NOx trap (LNT), catalyzed diesel particulate filter (DPF), and selective catalytic reduction (SCR) catalyst arranged in series to reduce tailpipe nitrogen oxides (NOx) and particulate matter (PM). This system utilizes a REF to produce hydrogen (H2), carbon monoxide (CO) and heat to regenerate the LNT, desulfate the LNT, and actively regenerate the DPF. The NOx stored on the LNT is reduced by the H2 and CO generated in the REF converting it to nitrogen (N2) and ammonia (NH3). NH3, which is normally an undesired byproduct of LNT regeneration, is stored in the downstream SCR which is utilized to further reduce NOx that passes through the LNT. Engine exhaust PM is filtered and trapped by the DPF reducing the tailpipe PM emissions. The REF is used to periodically generate heat to raise the DPF temperature and oxidize the accumulated soot. This paper addresses the issues resulting from transient operation and control of the above mentioned aftertreatment system. These results were obtained from urban and highway drive cycles using a 2007 compliant medium duty test vehicle equipped with an LNT + SCR aftertreatment system. The system performed LNT regenerations, DPF regenerations, and LNT desulfations as needed under a variety of transient conditions. An average system NOx conversion of 86% was demonstrated for a variety of transient driving conditions.