1. Field of the Invention
The present invention relates to an exhaust purification system for an internal combustion engine. More specifically, it relates to an exhaust purification system for an internal combustion engine equipped with a selective reduction catalyst that selectively reduces nitrogen oxides (NOx) in the exhaust under the presence of a reducing agent.
2. Related Art
As one exhaust purification system that purifies NOx in exhaust, a system has thus far been proposed in which a selective reduction catalyst that selectively reduces NOx in the exhaust by way of a reducing agent such as ammonia (NH3) is provided in an exhaust channel. For example, with an exhaust purification system of urea addition type, urea water, which is a NH3 precursor, is supplied from an upstream side of the selective reduction catalyst, NH3 is generated by thermal decomposition or hydrolysis of this urea water by the heat of the exhaust, and the NOx in the exhaust is selectively reduced by this NH3. In addition to such a system of urea addition type, for example, a system has also been proposed that generates NH3 by heating a compound of NH3 such as ammonia carbide, and directly adds this NH3. A system of urea addition type will be explained hereinafter.
The NOx purification rate of the selective reduction catalyst is known to change depending on the proportions of nitric monoxide (NO) and nitrogen dioxide (NO2) constituting the NOx of the exhaust flowing thereinto. More specifically, when the NO2—NOx ratio of the exhaust flowing into the selective reduction catalyst (mole ratio of NO2 to NOx, which combines NO and NO2) is 0.5, i.e. when the ratio of NO to NO2 is 1:1, the NOx purification rate is a maximum.
An exhaust purification device is proposed in Japanese Unexamined Patent Application Publication No. 2008-231950 (hereinafter referred to as “Patent Document 1”) that is configured such that the NO2—NOx ratio of the exhaust flowing into the selective reduction catalyst is 0.5 in order to maximize the performance of such a selective reduction catalyst. With this exhaust purification device, it is configured such that the NO2—NOx ratio of the exhaust flowing into the selective reduction catalyst is always maintained at 0.5 by controlling an EGR amount, fuel injection timing, etc. in a feed-forward manner by searching a map established in advance, based on the operating state of the internal combustion engine.