Exhaust gas purification apparatus for internal combustion engine
A three-way catalyst, an NSR catalyst, and an SCR catalyst are provided in this order for an exhaust gas passage, wherein the air-fuel ratio (AFR) is set to a first AFR which is a rich AFR before the AFR is switched from a theoretical AFR to a lean AFR, and then the AFR is set to a second AFR which is higher than the first AFR and lower than the theoretical AFR if a NOx occlusion amount is less than a threshold value during a period until an NH 3 adsorption amount of the SCR catalyst becomes a predetermined adsorption amount, while the AFR is set to a third AFR which is higher than the first AFR and lower than the second AFR if the NOx occlusion amount is not less than the threshold value.
1. An exhaust gas purification apparatus for an internal combustion engine, comprising:
a three-way catalyst which is provided for an exhaust gas passage of the internal combustion engine, which has an ability to store oxygen, and which produces NH 3 when an air-fuel ratio of an exhaust gas is lower than a theoretical air-fuel ratio;
a storage reduction NOx catalyst which is provided for the exhaust gas passage downstream from the three-way catalyst, which has an ability to store oxygen, which is a catalyst to occlude NOx when the air-fuel ratio of the exhaust gas is a lean air-fuel ratio, and which reduces NOx occluded by the catalyst when the air-fuel ratio of the exhaust gas is not more than the theoretical air-fuel ratio;
a selective catalytic reduction NOx catalyst which is provided for the exhaust gas passage downstream from the storage reduction NOx catalyst and which reduces NOx by using NH 3 as a reducing agent; and
a controller comprising at least one processor configured to adjust the air-fuel ratio in the internal combustion engine, wherein:
the controller farther configured to:
estimate a NOx occlusion amount of the storage reduction NOx catalyst;
estimate an NH 3 adsorption amount of the selective catalytic reduction NOx catalyst; and
operate such that the air-fuel ratio is set to a first air-fuel ratio which is lower than the theoretical air-fuel ratio during a period corresponding to an oxygen storage amount of the three-way catalyst and the storage reduction NOx catalyst before the air-fuel ratio in the internal combustion engine is switched from the theoretical air-fuel ratio to the lean air-fuel ratio; the air-fuel ratio in the internal combustion engine is set to a second air-fuel ratio which is higher than the first air-fuel ratio and which is lower than the theoretical air-fuel ratio until the NH 3 adsorption amount, which is estimated by the controller, becomes a predetermined adsorption amount after the period corresponding to the oxygen storage amount is terminated if the NOx occlusion amount, which is estimated by the controller, is less than an occlusion . amount threshold value at a point in time at which the period corresponding to the oxygen storage amount is terminated; the air-fuel ratio is switched to the lean air-fuel ratio if the NH 3 adsorption amount, which is estimated by the controller, becomes the predetermined adsorption amount; the air-fuel ratio in the internal combustion engine is set to a third air-fuel ratio which is higher than the first air-fuel ratio and which is lower than the second air-fuel ratio until the NH 3 adsorption amount, which is estimated by the controller, becomes the predetermined adsorption amount after the period corresponding to the oxygen storage amount is terminated if the NOx occlusion amount, which is estimated by the controller, is not less than the occlusion amount threshold value at the point in time at which the period corresponding to the oxygen storage amount is terminated; and the air-fuel ratio is switched to the lean air-fuel ratio if the NH 3 adsorption amount, which is estimated by the controller, becomes the predetermined adsorption amount.
2. The exhaust gas purification apparatus for the internal combustion engine according to claim 1 , wherein the second air-fuel ratio is such an air-fuel ratio that an amount of production of NH 3 is increased in the three-way catalyst as compared with the third air-fuel ratio, if the NOx occlusion amount, which is estimated by the controller, is less than the occlusion amount threshold value at the point in time at which the period corresponding to the oxygen storage amount is terminated.
3. The exhaust gas purification apparatus for the internal combustion engine according to claim 1 , further comprising:
an air-fuel ratio sensor which detects the air-fuel ratio of the exhaust gas at the exhaust gas passage downstream from the storage reduction NOx catalyst and upstream from the selective catalytic reduction NOx catalyst, wherein:
the controller farther configured to switch the air-fuel ratio to the second air-fuel ratio or the third air-fuel ratio assuming that the period corresponding to the oxygen storage amount is terminated, if the air-fuel ratio, which is detected by the air-fuel ratio sensor, is changed from the theoretical air-fuel ratio to a rich air-fuel ratio when the air-fuel ratio is set to the first air-fuel ratio.
4. The exhaust gas purification apparatus for the internal combustion engine according to claim 1 , further comprising:
an air-fuel ratio sensor which detects the air-fuel ratio of the exhaust gas at the exhaust gas passage downstream from the storage reduction NOx catalyst and upstream from the selective catalytic reduction NOx catalyst, wherein:
the controller farther configured to:
estimate the oxygen storage amount of the three-way catalyst and the storage reduction NOx catalyst;
switch the air-fuel ratio from the first air-fuel ratio to the second air-fuel ratio or the third air-fuel ratio if the oxygen storage amount, which is estimated by the controller, is not more than a predetermined storage amount when the air-fuel ratio is set to the first air-fuel ratio; and
correct the oxygen storage amount which is estimated by the controller or the predetermined storage amount so that the oxygen storage amount, which is estimated by the controller at a point in time at which the air-fuel ratio detected by the air-fuel ratio sensor is changed from the theoretical air-fuel ratio to a rich air-fuel ratio, is equal to the predetermined storage amount.
5. The exhaust gas purification apparatus for the internal combustion engine according to claim 1 , wherein the controller farther configured to switch the air-fuel ratio to the lean air-fuel ratio without providing the first air-fuel ratio and the second air-fuel ratio or the third air-fuel ratio if the NH 3 adsorption amount, which is estimated by the controller, is not less than an adsorption amount threshold value before the air-fuel ratio in the internal combustion engine is switched from the theoretical air-fuel ratio to the lean air-fuel ratio.