I-Selective catalytic reduction (SCR) isetshenziselwa ukulawula i-NOx ekukhishweni kwenjini yedizili.I-NH3 noma i-urea (imvamisa isixazululo se-urea esinamanzi esinesilinganiso esikhulu esingu-32.5%) sisetshenziswa njengento enciphisayo.Ngaphansi kwesimo sokuthi ukuhlushwa kwe-O2 kungaphezu kwama-oda amabili obukhulu obuphezulu kunokugxila kwe-NOx, ngaphansi kwesenzo sokushisa okuthile kanye ne-catalyst, i-NH3 isetshenziselwa ukunciphisa i-NOx ku-N2 ne-H2O.Ngoba i-NH3 inciphisa ngokukhetha i-NOx ngaphandle kokusabela nge-O2 kuqala, Ngakho-ke, ibizwa ngokuthi "ukunciphisa okukhethiwe kwe-catalytic".