Patent ID: 8448428

Claim:
An exhaust gas purification system for an internal combustion engine, comprising: a NO x selective reduction catalyst that is disposed in an exhaust passage of the engine, the NO x selective reduction catalyst being supplied with ammonia that is adsorbed on the NO x selective reduction catalyst, and selectively reducing NO x in exhaust gases with the adsorbed ammonia, there existing a first adsorption state and a second adsorption state of the ammonia: in the first adsorption state, when a temperature of the NO x selective reduction catalyst being in a low-temperature region, the ammonia being adsorbed onto the NO x selective reduction catalyst and, upon a temperature increase of the NO x selective reduction catalyst, a desorption amount of the ammonia reaching maximum in the low-temperature region; and in the second adsorption state, when the temperature of the NO x selective reduction catalyst being in a high-temperature region and being higher than the highest temperature in the low-temperature region, the ammonia being adsorbed onto or having already been adsorbed on the NO x selective reduction catalyst, and upon the temperature increase of the NO x selective reduction catalyst, the desorption amount of the ammonia reaching maximum in the high-temperature region, the ammonia, which has been in the first adsorption state and which is desorbed from the NO x selective reduction catalyst, being more likely to pass through the NO x selective reduction catalyst without purifying NO x than the ammonia, which has been in the second adsorption state, and which is desorbed from the NO x selective reduction catalyst; and a control section that estimates the adsorption amount of the ammonia in the first adsorption state and controls the adsorption amount of the ammonia in the first adsorption state, the control section estimates the adsorption amount of the ammonia in the second adsorption state and controls a supply amount of the ammonia on the basis of the estimated adsorption amount of the ammonia in the first adsorption state and the estimated adsorption amount of the ammonia in the second adsorption state, a main target ammonia adsorption amount is set for a total adsorption amount of the ammonia in the first adsorption state and the second adsorption state, a sub target ammonia adsorption amount is set for the adsorption amount of the ammonia in the first adsorption state, and the sub target ammonia adsorption amount is set smaller than the main target ammonia adsorption amount at a same temperature of the NO x selective reduction catalyst, and the control section controls the supply amount of the ammonia such that the adsorption amount of the ammonia in the first adsorption state is smaller than the sub target ammonia adsorption amount and that the total ammonia adsorption amount is approximately the same as the main target ammonia adsorption amount.