Exhaust purification system of internal combustion engine

Abstract

The exhaust purification system of an internal combustion engine is provided with an NO X selective reduction catalyst which is arranged in an engine exhaust passage and which has the function of absorbing NO X which is contained in exhaust gas when an air-fuel ratio of inflowing exhaust gas is lean, releasing the absorbed NO X when the air-fuel ratio of the inflowing exhaust gas becomes a stoichiometric air-fuel ratio or rich, and selectively reducing the NO X and with a fuel addition valve which feeds fuel to the NO X selective reduction catalyst. In the case of the region near the stoichiometric air-fuel ratio in the region where the air-fuel ratio of the exhaust gas flowing into the NO X selective reduction catalyst is lean, the fuel addition valve is used to feed fuel to the NO X selective reduction catalyst to selectively reduce the NO X .

1 . An exhaust purification system of an internal combustion engine which is provided with
 an NO X  reduction catalyst which is arranged in an engine exhaust passage and which has the function of absorbing NO X  which is contained in exhaust gas when an air-fuel ratio of inflowing exhaust gas is lean, releasing the absorbed NO X  when the air-fuel ratio of the inflowing exhaust gas becomes a stoichiometric air-fuel ratio or rich, and selectively reducing the NO X  and   a reducing agent feed device which feeds a reducing agent to the NO X  reduction catalyst, wherein,   in the case of the region near the stoichiometric air-fuel ratio in the region where the air-fuel ratio of the exhaust gas flowing into the NO X  reduction catalyst is lean, the reducing agent feed device is used to feed reducing agent to the NO X  reduction catalyst to selectively reduce the NO X .   
     
     
         2 . An exhaust purification system of an internal combustion engine as set forth in  claim 1 , further provided with a three-way catalyst which is arranged downstream of the NO X  reduction catalyst in the engine exhaust passage, wherein
 the reducing agent feed device includes a fuel addition valve which feeds fuel to the engine exhaust passage at the upstream side of the NO X  reduction catalyst, and,   if increasing the amount of fuel which is injected at the combustion chambers of the engine body so as to lower the air-fuel ratio of the exhaust gas flowing into the three-way catalyst to the stoichiometric air-fuel ratio or less, the system feeds fuel from the fuel addition valve to selectively reduce the NO X  at the NO X  reduction catalyst when the air-fuel ratio of the exhaust gas flowing into the NO X  reduction catalyst is inside the region near the stoichiometric air-fuel ratio.   
     
     
         3 . An exhaust purification system of an internal combustion engine which is provided with
 an NO X  reduction catalyst which is arranged in an engine exhaust passage and which has the function of absorbing NO X  which is contained in exhaust gas when an air-fuel ratio of inflowing exhaust gas is lean, releasing the absorbed NO X  when the air-fuel ratio of the inflowing exhaust gas becomes a stoichiometric air-fuel ratio or rich, and selectively reducing the NO X , a three-way catalyst which is arranged downstream of the NO X  reduction catalyst, a reducing agent feed device which feeds a reducing agent to the NO X  reduction catalyst, and an air-fuel ratio reducing device which reduces the air-fuel ratio of the exhaust gas flowing into the three-way catalyst, wherein,   in the operating region of the internal combustion engine, there is a specific operating region in which the air-fuel ratio of the exhaust gas flowing into the NO X  reduction catalyst is lean, at the NO X  reduction catalyst, the NO X  purification rate due to the selective reduction gradually decreases along with a temperature rise and, at the three-way catalyst, the NO X  purification rate gradually increases along with a temperature rise, and, in this specific operating region, when the NO X  purification rate of the NO X  reduction catalyst becomes smaller than a predetermined judgment value, the reducing agent feed device feeds the reducing agent to the NO X  reduction catalyst for selective reduction of NO X  and the air-fuel ratio reducing device makes the air-fuel ratio of the exhaust gas flowing into the three-way catalyst the stoichiometric air-fuel ratio or rich for reduction of NO X .   
     
     
         4 . An exhaust purification system of an internal combustion engine as set forth in  claim 3 , further provided with
 a temperature raising device which raises a temperature of an NO X  reduction catalyst and   an absorption amount detection device which detects an absorption amount of NO X  of an NO X  reduction catalyst, wherein,   if the air-fuel ratio of the exhaust gas flowing into the NO X  reduction catalyst is lean, when a temperature of an NO X  reduction catalyst is lower than a judgment value of a low temperature side for selective reduction and an absorption amount of NO X  of an NO X  reduction catalyst is an allowable value or more, the temperature raising device is used to raise the NO X  reduction catalyst, then the reducing agent feed device feeds the reducing agent to the NO X  reduction catalyst for selective reduction of the NO X .   
     
     
         5 . An exhaust purification system of an internal combustion engine as set forth in  claim 3 , wherein,
 if the air-fuel ratio of the exhaust gas flowing into the NO X  reduction catalyst is lean, when the temperature of an NO X  reduction catalyst is higher than a judgment value of a high temperature side for selective reduction, the system makes the air-fuel ratio of the exhaust gas flowing into the three-way catalyst the stoichiometric air-fuel ratio or rich to reduce the NO X  at the three-way catalyst.