To prevent the drop of exhaust emission control action of a catalyst when urea is supplied to the catalyst.
The catalyst 22 suitable for reducing NOx in exhaust gas by ammonia under excess oxygen condition is arranged in an engine exhaust passage. Urea water solution is supplied to the catalyst 22 from a flow rate addition valve 33, part of urea supplied to the catalyst is stored in the catalyst, and NOx in exhaust gas is reduced by ammonia generated from urea stored in the catalyst. It is determined that formaldehyde quantity in exhaust gas flowing in the catalyst 22 exceed the allowable limit at a time of engine start, cold operation and HC increase, and supply of urea to the catalyst 22 is prohibited.
COPYRIGHT: (C)2007,JPO&INPIT
Jun Tsuruta
Tetsuro Shimada
Shinji Mitsuhashi