Importance

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Abstract

The temperature of a downstream catalyst is effectively increased while avoiding temperature increase of an upstream catalyst, so as to remove SOx from the downstream catalyst. For this purpose, a first front three-way catalyst 21 of a first exhaust gas passage 5a connected to a first cylinder group, a second front three-way catalyst 22 of a second exhaust gas passage 5b connected to a second cylinder group, and a rear three-way catalyst 23 installed in an exhaust gas passage 5c which combines the exhaust of both of these exhaust gas passages, are provided. When the conditions hold for performing temperature increase of the rear three-way catalyst 23, the control unit 11 sets the air-fuel ratio of the exhaust supplied to one front catalyst to richer than the stoichiometric air-fuel ratio, and sets the air-fuel ratio of the exhaust gas supplied to the other front catalyst to leaner than the stoichiometric air-fuel ratio. Due to this, unburnt fuel and oxygen which passed through the front catalysts flow into the rear catalyst, promote the reactions in the rear catalyst so as to raise its temperature, and thereby promote discharge and reduction of SOx.

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First Claim