How does a Selective Catalytic Reduction (SCR) system work?

SCR reduces NOx emissions in diesel exhaust using a urea solution (AdBlue/DEF) injected upstream of a catalyst. The urea decomposes to ammonia, which reacts with NOx on the catalyst to form nitrogen and water. Key components include DEF tank, dosing module, mixer, SCR catalyst, and ammonia slip catalyst. Control requires precise dosing based on NOx sensors, exhaust temperature, and flow. SCR can achieve >95% NOx reduction and enables more efficient engine calibration with higher NOx-out.