초록 ▼

A catalyst suited for reducing NOx in an exhaust gas by ammonia in the presence of excess oxygen is arranged in an engine exhaust passage. An aqueous urea solution is fed from a flow addition valve onto a catalyst, part of the urea fed to the catalyst is stored in the catalyst, and the ammonia gener

A catalyst suited for reducing NOx in an exhaust gas by ammonia in the presence of excess oxygen is arranged in an engine exhaust passage. An aqueous urea solution is fed from a flow addition valve onto a catalyst, part of the urea fed to the catalyst is stored in the catalyst, and the ammonia generated from the urea stored in the catalyst is used to reduce the NOx in the exhaust gas. At the time of engine startup, at the time of cold operation, and at the time of increase of the HC amount, it is determined that the amount of formaldehyde in the exhaust gas flowing into the catalyst exceeds the allowable amount, and, at this time, the feed of urea to the catalyst is prohibited.

대표청구항 ▼

1. 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

1. 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; andcontrolling 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. 2. The method of claim 1, wherein the step of controlling controls the feed controlling device to shut off the feeding the amount of the ammonia generating compound to the catalyst when it is determined that the amount of the formaldehyde in the exhaust gas flowing into the catalyst is greater than the predetermined allowable amount. 3. The method of claim 1, wherein, at a time of the internal combustion engine startup, the step of determining determines whether or not the amount of the formaldehyde in the exhaust gas flowing into the catalyst is greater than the predetermined allowable amount. 4. The method of claim 1, wherein, at a time of cold operation of the internal combustion engine, the step of determining determines whether or not the amount of the formaldehyde in the exhaust gas flowing into the catalyst is greater than the predetermined allowable amount. 5. The method of claim 1, further comprising controlling a hydrocarbon increasing device arranged in the exhaust passage upstream of the catalyst or in a combustion chamber of the internal combustion engine to temporarily increase an amount of hydrocarbon contained in the exhaust gas, wherein the step of determining determines whether or not the amount of the formaldehyde in the exhaust gas flowing into the catalyst is greater than the predetermined allowable amount when the amount of hydrocarbon contained in the exhaust gas is temporarily increased by the hydrocarbon increasing device. 6. The method of claim 1, further comprising arranging another catalyst in the engine exhaust passage upstream of the catalyst. 7. The method of claim 6, wherein the step of feeding feeds the ammonia generating compound into the engine exhaust passage between the another catalyst and the catalyst. 8. The method of claim 6, wherein the another catalyst oxidizes a hydrocarbon, and the step of determining determines whether or not the amount of the formaldehyde in the exhaust gas flowing into the catalyst is greater than the predetermined allowable amount when the another catalyst is at a temperature lower than an activation temperature for the another catalyst. 9. The method of claim 6, wherein the another catalyst is carried on a particulate filter for collecting particulate in the exhaust gas. 10. The method of claim 6, wherein the another catalyst is suitable for reducing NOx in the exhaust gas by a hydrocarbon in the presence of excess oxygen. 11. The method of claim 1, wherein the feed controlling device is controlled to feed the ammonia generating compound to the catalyst in accordance with an amount of NOx flowing into the catalyst or the amount of ammonia stored in the catalyst when the amount of the formaldehyde in the exhaust gas flowing into the catalyst is not determined to be greater than the predetermined allowable amount. 12. The method of claim 1, wherein the ammonia generating compound is urea. 13. The method of claim 12, wherein the step of feeding feeds the urea in a form of an aqueous urea solution. 14. The method of claim 1, wherein the step of feeding feeds the ammonia generating compound in a form of a liquid. 15. The method of claim 1, wherein the catalyst is formed from one of a carrier comprised of titania that carries vanadium oxide, or a carrier comprised of zeolite that carries copper. 16. The method of claim 1, wherein the internal combustion engine is a compression ignition type internal combustion engine.