To suppress the NOx desorption from a zeolite catalyst 14 in a period until NOx trap catalyst 12, enabling purification of NOx at high temperatures becomes active, when arranging the zeolite catalyst 14 which is capable of trapping in an exhaust passage of an engine 1 at low temperatures.
A heat balance to zeolite catalyst 14 is obtained, based on the moisture content flowing into the zeolite catalyst 14, exhaust temperature, exhaust flow, etc. The temperature of the zeolite catalyst 14 or increasing ratio of the temperature is gained. This makes suppression of the temperature increasing of the zeolite catalyst 14, when it is determined as being NOx desorption condition. Specifically, the exhaust temperature is lowered by leaning of air/fuel ratio or timing angle of ignition, or the moisture content flowing into the zeolite catalyst 14 is controlled, so as to suppress the content.
TSUCHIDA HIROBUMI
SHIINO SHUNICHI
HOTTA ISAMU
