Diesel engines require a different approach due to their high levels of particulate matter and NOx. Diesel oxidation catalysts (DOCs) are used to oxidize CO and HC into CO₂ and H₂O. Additionally, Selective Catalytic Reduction (SCR) systems are employed to reduce NOx emissions by injecting a reductant, typically urea, into the exhaust stream, which reacts with NOx in the presence of a catalyst to form N₂ and H₂O.
