The oxidation catalyst (NSC) close to the engine can temporarily store nitrogen oxides up until the regeneration stage. This catalyst is effective even at low engine operating temperatures, for instance following a cold start. Regeneration occurs by means of a short-term mixture enrichment triggered by the engine control unit. Thus, besides storing and subsequently neutralizing the nitrogen oxides, the catalyst oxidizes unburned hydrocarbons and carbon monoxide into carbon dioxide and water vapor, using the oxygen molecules of the temporarily stored NOx.
Another stage to reduce nitrogen oxides is triggered by the injection of the AdBlue additive. Because this aqueous urea solution is injected into the exhaust system at two points where temperatures differ, using one dosing module at each point, the total system is referred to as twin dosing. Subsequently, the chemical process of urea thermolysis occurs in the exhaust system, which converts the AdBlue additive into ammonia. The ammonia reacts with the SCR-coated diesel particulate filter (SDPF) close to the engine, and on the second SCR catalyst, located further downstream in the exhaust system, with nitrogen oxides that have not been converted yet. This results in the formation of water and elemental nitrogen, which accounts for about four fifths of our Earth’s atmosphere.
