Ka whakamahia te whakahekenga catalytic whiriwhiri (SCR) ki te whakahaere i te NOx i roto i te paura miihini diesel.Ko te NH3, ko te Urea ranei (ko te nuinga o nga wa ka whakamahia he wairewa urea me te ōwehenga papatipu 32.5%) hei matū whakaheke.I raro i te tikanga ko te kukū O2 neke atu i te rua ota teitei ake i te kukū NOx, i raro i te mahi a etahi pāmahana me te whakakorikori, ka whakamahia te NH3 hei whakaiti i te NOx ki te N2 me te H2O.Na te mea ko te NH3 te whakaiti i te NOx me te kore e aro ki te O2 i te tuatahi, Na reira, ka kiia ko te "whakaiti catalytic whiriwhiri".