Ukunciphisa okukhethiweyo kwe-catalytic (SCR) isetyenziselwa ukulawula i-NOx kwi-exhaust ye-injini ye-diesel.I-NH3 okanye i-urea (idla ngokuba yi-urea ene-aqueous solution enobunzima be-32.5%) isetyenziswa njengento yokunciphisa.Ngaphantsi kwemeko yokuba i-O2 i-concentration ingaphezulu kwemiyalelo emibini yobukhulu obuphezulu kunogxininiso lwe-NOx, phantsi kwesenzo sobushushu obuthile kunye ne-catalyst, i-NH3 isetyenziselwa ukunciphisa i-NOx kwi-N2 kunye ne-H2O.Ngenxa yokuba i-NH3 inciphisa ngokukhetha i-NOx ngaphandle kokusabela nge-O2 kuqala, Ngoko ke, ibizwa ngokuba "ukunciphisa i-catalytic ekhethiweyo".