Patent ID: 8371106

Claim:
A method for purifying exhaust gas in an internal combustion engine, the method being carried out by an electronic control unit of the internal combustion engine, the method comprising: feeding an amount of an ammonia generating compound to a catalyst arranged in an engine exhaust passage of the internal combustion engine via a feed controlling device, the catalyst being suitable for reduction of NOx in exhaust gas by ammonia in the presence of excess oxygen, the catalyst: i) storing at least a part of the ammonia generating compound in the catalyst, ii) generating ammonia from the ammonia generating compound stored in the catalyst, and iii) using the generated ammonia to reduce the NOx in the exhaust gas; determining, using a formaldehyde-detecting sensor, whether an amount of formaldehyde in the exhaust gas flowing into the catalyst is greater than a predetermined allowable amount, the formaldehyde inhibiting an exhaust purification action in the catalyst by reacting with the ammonia generating compound or a substance derived from the ammonia generating compound to form a urea resin; and controlling the feed controlling device to decrease the amount of the ammonia generating compound fed to the catalyst, when it is determined using the formaldehyde-detecting sensor that the amount of the formaldehyde in the exhaust gas flowing into the catalyst is greater than the predetermined allowable amount.