Basically two aftertreatment systems are in use today: the NOx adsorber catalyst and DeNOx with SCR.
NOx Adsorbers store the NOx emitted by the Diesel engine during normal lean operation in the catalyst without the need of any additional agent. When the adsorber reaches a certain NOx load, the engine management will switch for a few seconds to a rich understoichiometric air fuel ratio in the exhaust gas for catalyst regeneration. At AVL, this system has been developed for passenger car diesel engines with good results. For commercial Diesel engines the NOx Adsorber is not well suited as it is mostly operated under high load, the rich air fuel ratio needed for regeneration however can only be adjusted during part load.
The selective catalytic reduction (SCR) is preferred by most OEM's today, but requires Urea to be injected upstream of the catalyst for its regeneration. Depending on the temperature level in the test cycle, NOx conversion rates in excess of 80% are achievable even under transient conditions. This allows the base engine to be tuned for good fuel economy and low particulate emission. The operating cost (fuel and Urea) is lower compared to an engine with EGR and DPF, however an area-wide supply of Urea must be ensured as it is the case in Europe already.