JP2013238170A - Exhaust cleaning system for internal combustion engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an exhaust cleaning system that can use NOx reduction function at a maximum by an SCR catalyst even while an air-fuel ratio of mixture air is stoichiometry.SOLUTION: An exhaust cleaning system 2 includes: a downstream catalyst converter 33 including an SCR catalyst for reducing NOx under existence of NH; a urea water supply device 4 for supplying urea water to the downstream catalyst converter 33; an upstream catalyst converter 31 including an upstream catalyst having a ternary cleaning function; a secondary air supply device 5 for supplying a secondary air to the downstream catalyst converter 33; an air-fuel controller 61 for changing air-fuel ratio of a mixed air from a lean side to a stoichiometry side when determined as an acceleration operation state; and a secondary air controller 63 that controls the secondary air supply device 5 so that the secondary air is supplied to the downstream catalyst converter 33 while the air-fuel ratio of the mixed air is changed into the stoichiometry side.

Description

The present invention relates to an exhaust gas purification system for an internal combustion engine. More specifically, the present invention relates to an exhaust purification system provided with a selective reduction catalyst that reduces NOx in the presence of NH ₃ .

Conventionally, as one of exhaust purification systems for purifying NOx in exhaust, a system in which a selective reduction catalyst for selectively reducing NOx in exhaust with a reducing agent such as ammonia (NH ₃ ) is provided in the exhaust passage has been proposed. (For example, refer to Patent Document 1). For example, in the exhaust purification system of urea addition type, the NH ₃ by supplying urea water which is a precursor of NH ₃ from the upstream side of the selective reduction catalyst, thermal decomposition or hydrolysis in the exhaust heat from the urea water The NOx in the exhaust gas is selectively reduced by this NH ₃ . In addition to such a urea addition type system, for example, a system in which NH ₃ is generated by heating a NH ₃ compound such as ammonia carbide and this NH ₃ is directly added has also been proposed.

  Selective reduction catalysts have a lean air-fuel ratio that is leaner than stoichiometric and exhibits high NOx purification performance under lean air-fuel ratio exhaust gas that contains a large amount of oxygen. Therefore, lean combustion such as lean-burn gasoline engines and diesel engines Often used in engine exhaust purification systems based on However, during acceleration operation in which the amount of NOx emissions increases, NOx may not be sufficiently purified with only the selective reduction catalyst. Therefore, as in the system disclosed in Patent Document 2, it is conceivable to purify NOx using a three-way purification reaction in a three-way catalyst provided upstream of the selective reduction catalyst during acceleration operation. In the exhaust purification system of Patent Document 2, in a system in which a three-way catalyst is provided on the upstream side of the NOx storage reduction catalyst, the air-fuel ratio of the air-fuel mixture is made lean to use the three-way purification reaction in the three-way catalyst during acceleration operation. Switch from side to stoichiometric.

JP 2012-2065 A JP 2009-293585 A

By the way, in the selective reduction catalyst, the reduction reaction of NOx proceeds using NH ₃ as a reducing agent. Even if a sufficient amount of NH ₃ is present, this reduction reaction is performed by at least one of oxygen and NO _2. If it does not exist, it will not progress. Further, since most of the NOx discharged from the internal combustion engine after warm-up is NO, NO ₂ in the exhaust flowing into the selective reduction catalyst is generated by an oxidation reaction of NO in the upstream three-way catalyst. Most are things. Therefore, when the air-fuel ratio of the air-fuel mixture is stoichiometric in a system in which a three-way catalyst is provided upstream of the selective reduction catalyst, both oxygen and NO ₂ hardly flow into the selective reduction catalyst. The reduction reaction does not proceed.

  The present invention has been made in consideration of the above points, and provides an exhaust purification system that can make maximum use of the NOx reduction function of the selective reduction catalyst even while the air-fuel ratio of the air-fuel mixture is stoichiometric or rich. For the purpose.

(1) An exhaust purification system (for example, exhaust purification systems 2, 2A, 2B, which will be described later) of an internal combustion engine (for example, an engine 1 which will be described later) of the present invention makes the air-fuel ratio of the air-fuel mixture leaner during steady operation. a is provided in an exhaust passage of the engine, selective catalytic reduction (e.g., SCR catalyst downstream catalytic converter 33 described later) for reducing NOx in the presence of NH ₃ and, NH ₃ in the selective reduction catalyst or a A reducing agent supply device (for example, urea water supply device 4 to be described later) for supplying a precursor, and an upstream catalyst (for example, to be described later) provided on the upstream side of the selective reduction catalyst in the exhaust passage. Of the upstream catalytic converter 31) and a secondary air supply device for supplying secondary air containing oxygen from between the upstream catalyst and the selective reduction catalyst in the exhaust passage (for example, a secondary air described later). Supply device 5, 5 </ b> A, 5 </ b> B), acceleration determination means (for example, an accelerator opening sensor 38 and ECUs 6, 6 </ b> B described later) for determining that the vehicle is in an accelerated operation state, Air-fuel ratio control means (for example, an air-fuel ratio controller 61 described later) for changing the air-fuel ratio of the air-fuel mixture to the stoichiometric or rich side, and the selective reduction catalyst while the air-fuel ratio of the air-fuel mixture is changed to the stoichiometric or rich side Secondary air control means (for example, secondary air controllers 63 and 63B described later) for controlling the secondary air supply device so that the secondary air is supplied to the secondary air.

(1) In the present invention, when it is determined by the acceleration determining means that the vehicle is in the acceleration operation state, the air-fuel ratio of the air-fuel mixture is changed from the lean side to the stoichiometric or rich side by the air-fuel ratio control means. As a result, the three-way purification reaction proceeds in the upstream catalyst, and NOx discharged during acceleration is reduced. In addition, the air-fuel ratio of the air-fuel mixture is changed to the stoichiometric or rich side in this way, and while the oxygen from the internal combustion engine hardly contains oxygen, the selective reduction catalyst contains oxygen-containing secondary air. Since it is supplied, the NOx reduction reaction also proceeds in the selective reduction catalyst. Therefore, according to the present invention, NOx can be purified by both the upstream catalyst and the selective reduction catalyst during the acceleration operation state.
When the air-fuel ratio of the air-fuel mixture is set to the rich side, NH ₃ is generated in the upstream catalyst having a three-way purification function. Therefore, during the acceleration operation state, NH ₃ generated in the upstream catalyst flows into the selective reduction catalyst, and accordingly, the supply amount of NH ₃ or the precursor from the reducing agent supply device can be suppressed accordingly.
In general, during the acceleration operation state, the temperature of the exhaust gas rapidly increases, so that the temperature of the selective reduction catalyst also rapidly increases. According to the present invention, by supplying the secondary air from the secondary air supply device in such a state, the rapid increase in the temperature of the selective reduction catalyst can be suppressed by using the secondary air as a cooling gas. A rapid decrease in the NOx purification performance of the catalyst can be suppressed. Further, if the temperature of the selective reduction catalyst increases, the amount of the NH ₃ can occlusion (maximum NH ₃ storage amount) is also decreased, during general acceleration operation state NH ₃ is discharged from the selective reduction catalyst easy. On the other hand, in the present invention, the discharge of NH ₃ can be suppressed by using the secondary air as the cooling gas as described above. Even if NH ₃ is discharged from the selective reduction catalyst, the secondary air supplied from the secondary air supply device functions as a dilution gas for NH ₃ , so that high-concentration NH ₃ is discharged outside the system. Can be prevented.

  (2) In this case, a filter (for example, an exhaust purification filter 32 described later) for collecting particulate matter in the exhaust is provided between the upstream catalyst and the selective reduction catalyst, and the secondary air supply device Preferably, secondary air is supplied from between the upstream catalyst and the filter in the exhaust passage.

  (2) During the acceleration operation state, the temperature of the exhaust gas rises, and the temperature of the particulate matter deposited on the filter can also rise to the combustible temperature. In such a state, by supplying oxygen-containing secondary air from the secondary air supply device to the upstream side of the filter, while supplying oxygen to the selective reduction catalyst on the downstream side of the filter, at the same time, particulates accumulated on the filter The substance can be burned. In general, particulate matter deposited on the filter needs to be burned and removed using unburned fuel supplied to the filter by post injection. In contrast, in the present invention, by causing the particulate matter to burn during the acceleration operation state, it is possible to suppress the deterioration of fuel consumption and the occurrence of oil dilution accompanying the execution of post injection.

  (3) In this case, it is preferable that an oxidation catalyst is supported on the filter.

(3) When the oxidation catalyst is supported on the filter, the particulate matter is easily burned from a lower temperature. For this reason, particulate matter can be burned more efficiently during the acceleration operation state. In addition, by carrying an oxidation catalyst on the filter and supplying oxygen-containing secondary air to the upstream side of the filter during the acceleration operation state, a reaction of oxidizing a part of NO in the exhaust to NO ₂ proceeds. To do. Therefore, since the NO ₂ generated by the filter flows into the selective reduction catalyst in the accelerated operation state in addition to oxygen in the secondary air, the selective reduction catalyst uses the oxygen to reduce NO ( In addition to the so-called Standard-SCR reaction, a reaction (so-called Fast-SCR reaction) that simultaneously reduces NO and NO ₂ , which have a higher reaction rate than this reaction, also proceeds. Therefore, the NOx purification rate in the selective reduction catalyst can be further improved.

  (4) In this case, the exhaust purification system further includes temperature acquisition means (for example, an exhaust temperature sensor 36 and ECUs 6 and 6B described later) for acquiring the temperature of the filter, and the secondary air supply device includes the upstream An upstream supply unit that supplies secondary air between the catalyst and the filter (for example, an upstream secondary air supply valve 531B described later) and a downstream that supplies secondary air between the filter and the selective reduction catalyst A supply unit (for example, a downstream secondary air supply valve 532B, which will be described later), and the secondary air control means performs secondary operation by the downstream supply unit until the filter temperature reaches the combustion temperature of the particulate matter. It is preferable to supply air and supply secondary air by the upstream supply unit after the filter temperature reaches the combustion temperature.

  (4) In the present invention, the air-fuel ratio of the air-fuel mixture is changed to the stoichiometric or rich side in response to the acceleration operation state, and when the secondary air is supplied to the selective reduction catalyst, the filter temperature is the particulate matter. The secondary air is supplied by the downstream supply unit until the combustion temperature is reached, and the secondary air is supplied by the upstream supply unit after the filter temperature reaches the combustion temperature of the particulate matter. As described above, the secondary air also functions as a cooling gas. Therefore, according to the present invention, it is possible to prevent the temperature rise of the filter from being delayed due to the supply of secondary air while supplying the oxygen necessary for reducing NOx to the selective reduction catalyst. That is, the temperature of the filter can be quickly raised during the acceleration operation state.

  (5) In this case, the exhaust purification system includes an air-fuel ratio sensor (for example, an air-fuel ratio sensor 35 to be described later) for detecting the air-fuel ratio of the exhaust, and the air-fuel ratio of the air-fuel mixture is changed from lean to And a stoichiometric timing determination means (for example, ECUs 6 and 6B described later) for determining a timing at which the selective reduction catalyst switches to a stoichiometric or rich atmosphere based on the output of the air-fuel ratio sensor after being changed to the rich side. The secondary air control means preferably starts the supply of secondary air based on the determination by the stoichiometric timing determination means.

  (5) Since there is a distance from the exhaust port of the internal combustion engine to the selective reduction catalyst and an upstream catalyst, a filter and the like are provided, after changing the air-fuel ratio of the air-fuel mixture from the lean side to the stoichiometric or rich side, There is a delay before the selective reduction catalyst actually switches to a stoichiometric or rich atmosphere. In the present invention, the timing for switching to the stoichiometric or rich atmosphere as described above is determined based on the output of the air-fuel ratio sensor that detects the actual air-fuel ratio of the exhaust gas, and the supply of secondary air is started based on this determination. Thus, the secondary air can be supplied at an appropriate time so that the NOx purification rate in the selective reduction catalyst does not decrease. Further, the delay from the switching of the air-fuel ratio to the change in the atmosphere of the selective reduction catalyst varies each time an oxygen storage / release material is provided in the upstream catalyst, filter, or the like. In such a case, by providing the air-fuel ratio sensor on the downstream side of the oxygen storage / release material, the secondary air can be supplied at an appropriate time according to the actual air-fuel ratio fluctuation.

  (6) In this case, the secondary air control means continues supplying the secondary air for a predetermined time after the air-fuel ratio of the air-fuel mixture is changed from the stoichiometric or rich side to the lean side by the air-fuel ratio control means. It is preferable.

  (6) Since an upstream catalyst having a distance and a three-way purification function is provided from the exhaust port of the internal combustion engine to the selective reduction catalyst, the air-fuel ratio of the air-fuel mixture is changed from the stoichiometric or rich side to the lean side. There is a delay between the change and the selective reduction catalyst actually switching to the lean atmosphere. In the present invention, even after the air-fuel ratio of the air-fuel mixture is changed to the lean side, the supply of secondary air is continued for a predetermined time, so that the oxygen is continuously reduced so that the NOx purification rate in the selective reduction catalyst does not decrease. Can continue to supply.

  (7) In this case, it is preferable that the upstream catalyst is provided with an oxygen storage / release material.

  (7) Since the upstream catalyst's three-way purification window can be widened by providing an oxygen storage / release material in the upstream catalyst, the upstream catalyst stably exhibits the three-way purification function while the air-fuel ratio is controlled stoichiometrically. be able to. However, when the air-fuel ratio is greatly changed from the lean side to the stoichiometric side or from the stoichiometric to the lean side as in the present invention, the air-fuel ratio fluctuation is absorbed by the oxygen storage / release material, and the atmosphere of the selective reduction catalyst changes. Will be delayed. In contrast, in the present invention, as described above, the secondary air supply start timing is determined based on the output of the air-fuel ratio sensor, and the secondary air supply stop timing is delayed by a predetermined time even after the air-fuel ratio change. In order to prevent the NOx purification rate in the selective reduction catalyst from being lowered when the air-fuel ratio is switched, the secondary air can be supplied or stopped at an appropriate time according to the actual atmospheric fluctuation of the selective reduction catalyst. That is, according to the present invention, it is possible to improve both the purification performance of the upstream catalyst during stoichiometric control and the purification performance of the selective reduction catalyst during air-fuel ratio switching.

(8) In this case, the exhaust gas purification system detects an NH ₃ concentration in the exhaust gas downstream of the selective reduction catalyst using an electromotive force generated between the electrodes (for example, NH ₃ described later). ₃ sensor 37) and reducing agent control means (for example, urea water controller 62 described later) for controlling the supply amount of NH ₃ or precursor from the reducing agent supply device based on the output of the NH ₃ sensor. It is preferable to further provide.

(8) By controlling the supply of the reducing agent using the output of the NH ₃ sensor provided on the downstream side of the selective reduction catalyst, an appropriate amount is set so that an excessive amount of NH ₃ is not discharged from the selective reduction catalyst. Can be supplied. However, the NH ₃ sensor that detects NH ₃ using the electromotive force generated between the electrodes cannot change the NH ₃ concentration properly because its output characteristics change greatly in a low oxygen concentration atmosphere. In contrast, by the present invention is to provide a secondary air oxygenated, since NH ₃ sensors can be prevented from falling into undetectable state as described above, in the accelerating operation state is also the NH ₃ sensor The supply of the reducing agent can be appropriately controlled based on the output.

  (9) In this case, the secondary air control means is configured to supply oxygen to the selective reduction catalyst while the air-fuel ratio of the air-fuel mixture is changed to the rich side, and to oxidize HC with the oxidation catalyst of the filter It is preferable to supply secondary air from between the upstream catalyst and the filter so that the air travels.

  (9) When the air-fuel ratio of the air-fuel mixture is changed to the rich side, HC that could not be oxidized in the upstream catalyst flows into the filter. In the present invention, the air-fuel ratio of the air-fuel mixture is changed to the stoichiometric or rich side according to the acceleration operation state, and the secondary air is supplied from between the upstream catalyst and the filter when supplying the secondary air to the selective reduction catalyst. HC can be oxidized by the oxidation catalyst of the filter while supplying oxygen necessary for reducing NOx to the selective reduction catalyst.

(10) In this case, the secondary air control means supplies oxygen and NH ₃ generated by the upstream catalyst to the selective reduction catalyst while the air-fuel ratio of the air-fuel mixture is changed to the rich side. As described above, it is preferable to supply secondary air from between the filter and the selective reduction catalyst.

(10) When the air-fuel ratio of the air-fuel mixture is changed to the rich side, NH _{3 serving as a} reducing agent is generated in the selective reduction catalyst in the upstream catalyst. However, if the filter carrying the oxidation catalyst is in an oxidizing atmosphere, NH ₃ produced in the upstream catalyst may be oxidized when passing through the filter, and may not reach the selective reduction catalyst as a reducing agent. In the present invention, the air-fuel ratio of the air-fuel mixture is changed to the stoichiometric or rich side in response to the acceleration operation state, and the secondary air is supplied to the selective reduction catalyst so that the filter is not in an oxidizing atmosphere. By supplying secondary air to and from the selective catalytic reduction catalyst, the oxygen necessary for reducing NOx is supplied to the selective catalytic reduction catalyst, and the NH ₃ produced in the upstream catalyst is selected without being oxidized by the filter. The reduction catalyst can be supplied.

It is a mimetic diagram showing composition of an engine and its exhaust purification system concerning a 1st embodiment of the present invention. Is a diagram showing the _{O 2} concentration dependence of the NOx purification rate by Standard-SCR reaction. It is a main flowchart which shows the specific procedure of the secondary air control which concerns on the said embodiment. It is a flowchart which shows the procedure which updates the secondary air supply flag which concerns on the said embodiment. It is a schematic diagram which shows the structure of the engine and exhaust gas purification system which concern on 2nd Embodiment of this invention. It is a schematic diagram which shows the structure of the engine and exhaust gas purification system which concern on 3rd Embodiment of this invention. It is a main flowchart which shows the specific procedure of the secondary air control which concerns on the said embodiment. It is a flowchart which shows the procedure which updates the filter temperature increase completion flag which concerns on the said embodiment.

<First Embodiment>
Hereinafter, a first embodiment of the present invention will be described with reference to the drawings.
FIG. 1 is a schematic diagram showing a configuration of an internal combustion engine (hereinafter referred to as “engine”) 1 and an exhaust purification system 2 thereof according to the present embodiment. The engine 1 is based on so-called lean combustion in which the air-fuel ratio of the air-fuel mixture is leaner than the stoichiometric state during steady operation, more specifically, a diesel engine, a lean burn gasoline engine, or the like.

  The exhaust purification system 2 includes a catalyst purification device 3 provided in an exhaust passage 11 extending from an exhaust port of the engine 1, an electronic control unit (hereinafter referred to as "ECU") 6 that controls the engine 1 and the catalyst purification device 3, Is provided.

  The engine 1 is provided with a fuel injection valve that injects fuel into each cylinder (not shown). The actuator that drives the fuel injection valve is electromagnetically connected to the ECU 6. The air-fuel ratio of the air-fuel mixture of the engine 1 includes the amount of fresh air introduced into the cylinder, the amount of EGR gas introduced into the cylinder via an exhaust gas recirculation device (not shown), the amount of fuel injected from the fuel injection valve, etc. Is controlled by adjusting.

  The catalyst purification device 3 includes an upstream catalytic converter 31, an exhaust purification filter 32, a downstream catalytic converter 33, a urea water supply device 4, and a secondary air supply device 5. The upstream catalytic converter 31 is provided immediately below the engine 1 in the exhaust passage 11. The downstream catalytic converter 33 is provided downstream of the upstream catalytic converter 31 in the exhaust passage 11. The exhaust purification filter 32 is provided between the upstream catalytic converter 31 and the downstream catalytic converter 33 in the exhaust passage 11. These upstream catalytic converter 31 and downstream catalytic converter 33 are provided with a catalyst for promoting a reaction for purifying components such as CO, HC and NOx contained in the exhaust of the engine 1.

  As the upstream catalyst included in the upstream catalytic converter 31, a catalyst having at least a three-way purification function is used. The ternary purification function refers to a function in which a ternary purification reaction, that is, a reaction in which oxidation of HC and CO and reduction of NOx are performed simultaneously proceeds under a stoichiometric atmosphere. Examples of the catalyst having such a three-way purification function include an oxidation catalyst and a three-way catalyst. As the upstream catalyst, an oxidation catalyst or a three-way catalyst is preferably used.

The oxidation catalyst (DOC) purifies HC, CO, and NOx with high efficiency by the above three-way purification reaction under a stoichiometric atmosphere. The oxidation catalyst purifies HC, CO, and NOx under a rich atmosphere and generates NH ₃ . Further, the oxidation catalyst purifies by oxidizing HC and CO in a lean atmosphere, and also oxidizes a part of NO in the exhaust to NO ₂ to improve the NOx purification rate in the downstream catalytic converter 33 described later. A three-way catalyst (TWC) corresponds to an oxidation catalyst added with an oxygen storage / release material. The three-way catalyst and the oxidation catalyst have the same basic purification function. However, the three-way catalyst is superior to the oxidation catalyst in that it has an oxygen storage / release material and has a wide three-way purification window.

  The exhaust purification filter 32 collects particulate matter in the exhaust. The exhaust purification filter 32 preferably carries the oxidation catalyst described above so that the accumulated particulate matter can be burned and removed from a lower temperature.

The downstream catalytic converter 33 includes a selective reduction catalyst (hereinafter referred to as “SCR catalyst”) that reduces NOx in the presence of NH ₃ .
In this downstream catalytic converter 33, in the presence of NH ₃ , a Fast-SCR reaction (see the following formula (1)), a Standard-SCR reaction (see the following formula (2)), and a Slow-SCR reaction (the following formula (3) )) Can proceed.
NO + NO ₂ + 2NH ₃ → 2N ₂ + 3H ₂ O (1)
4NO + 4NH ₃ + O ₂ → 4N ₂ + 6H ₂ O (2)
6NO ₂ + 8NH ₃ → 7N ₂ + 12H ₂ O (3)

The Fast-SCR reaction is a reaction in which NO and NO ₂ in exhaust gas are simultaneously reduced by NH ₃ , and has the fastest reaction rate among the three types of reactions. Therefore, when the ratio of NO and NO _{2 in} the exhaust gas flowing into the downstream catalytic converter 33 becomes equal, the Fast-SCR reaction proceeds mainly, so that the NOx purification rate by the downstream catalytic converter 33 becomes the highest. However, since most of NOx discharged from the engine 1 is composed of NO, the upstream catalytic converter 31, the exhaust purification filter 32, or the like is used in order to make the Fast-SCR reaction proceed in the downstream catalytic converter 33. , it is necessary to oxidize a portion of the NO in the exhaust to NO _2. Therefore, the catalyst amount and composition of the upstream catalytic converter 31 and the exhaust purification filter 32 are adjusted so that the ratio of NO and NO ₂ of exhaust flowing into the downstream catalytic converter 33 during the lean operation described later is approximately 1: 1. ing.

The Slow-SCR reaction is a reaction in which only NO ₂ in the exhaust gas is reduced by NH ₃ , and the reaction rate is slower than the Fast-SCR reaction. When the ratio of NO ₂ to NO in the exhaust gas flowing into the downstream catalytic converter 33 increases, NO ₂ surplus in the Fast-SCR reaction is reduced by the Slow-SCR reaction.

The Standard-SCR reaction is a reaction in which only NO in exhaust gas is reduced with NH ₃ . As shown in the above formula (2), O ₂ is required only for the Standard-SCR reaction.
FIG. 2 is a diagram showing the O ₂ concentration dependency of the NOx purification rate by the Standard-SCR reaction. More specifically, an exhaust gas having a NO to NO ₂ ratio of 1: 0 is allowed to flow into the SCR catalyst, and the O ₂ concentration in the exhaust gas is changed under conditions that allow only the Standard-SCR reaction to proceed. It is a figure which shows the change of the NOx (NO) purification rate in the case of. As shown in this figure, the NOx purification rate by the Standard-SCR reaction greatly decreases in a stoichiometric or rich atmosphere that hardly contains oxygen, but increases in a lean atmosphere that contains a lot of oxygen. That is, even in a situation where the air-fuel ratio of the air-fuel mixture of the engine 1 is stoichiometric or rich and the exhaust gas exhausted from the engine 1 contains almost no oxygen, the secondary air containing oxygen is separated from the exhaust gas. Is supplied to the downstream catalytic converter 33 so that the downstream catalytic converter 33 can reduce NOx.

The urea water supply device 4 includes a urea water tank 41 and a urea water injector 42. The urea water tank 41 stores urea water that is a precursor of the reducing agent (NH ₃ ) in the SCR catalyst. The urea water tank 41 is connected to a urea water injector 42 via a urea water supply path 43 and a urea water pump (not shown). The urea water injector 42 opens and closes when driven by an actuator (not shown), and injects urea water supplied from the urea water tank 41 to the upstream side of the downstream catalytic converter 33 in the exhaust passage 11. The urea water injected from the injector 42 is hydrolyzed into NH ₃ in the exhaust gas or in the downstream catalytic converter 33 and consumed for the reduction of NOx. The actuator of the urea water injector 42 is electromagnetically connected to the ECU 6. The ECU 6 determines the urea water injection amount by urea water injection control described later, and drives the urea water injector 42 so that this amount of urea water is injected.

  The secondary air supply device 5 includes a secondary air pump 51, a secondary air tank 52, and a secondary air supply valve 53. The secondary air pump 51 compresses outside air as secondary air. The compressed air supplied from the secondary air pump 51 is stored in the secondary air tank 52. The secondary air supply valve 53 is provided between the upstream catalytic converter 31 and the exhaust purification filter 32 in the exhaust passage 11. When driven by an actuator (not shown), the secondary air supply valve 53 opens and closes, and the secondary air supplied from the secondary air tank 52 is supplied from between the upstream catalytic converter 31 and the exhaust purification filter 32 in the exhaust passage 11. Supply. The secondary air supplied from the secondary air supply valve 53 is supplied to the exhaust purification filter 32 and the downstream catalytic converter 33 downstream thereof. The ECU 6 determines a secondary air supply amount by secondary air supply control described later, and drives the secondary air supply valve 53 so that this amount of secondary air is supplied.

An air-fuel ratio sensor 35, an exhaust temperature sensor 36, an NH ₃ sensor 37, an accelerator opening sensor 38, and the like are connected to the ECU 6 as sensors for detecting the states of the exhaust purification system 2 and the engine 1.

  The air-fuel ratio sensor 35 detects the air-fuel ratio (oxygen concentration) of the exhaust gas flowing between the upstream catalytic converter 31 and the exhaust purification filter 32 in the exhaust passage 11, and transmits a signal substantially proportional to the detected value to the ECU 6. . The air-fuel ratio sensor 35 is a proportional air-fuel ratio sensor (LAF sensor) that outputs a signal proportional to the rich air-fuel ratio to the lean air-fuel ratio.

  The exhaust gas temperature sensor 36 detects the temperature of the exhaust gas flowing between the exhaust gas purification filter 32 and the downstream catalytic converter 33 in the exhaust gas passage 11, and transmits a signal substantially proportional to the detected value to the ECU 6. Based on the output of the exhaust temperature sensor 36, the ECU 6 calculates (acquires) the temperature of the exhaust purification filter 32 and the temperature of the downstream catalytic converter 33 by processing not shown.

The NH ₃ sensor 37 detects the NH ₃ concentration in the exhaust on the downstream side of the downstream catalytic converter 33 in the exhaust passage 11 and transmits a signal substantially proportional to the detected value to the ECU 6. The NH ₃ sensor 37 is configured to generate an electromotive force (EMF) corresponding to the NH ₃ concentration between the electrodes.

  The accelerator opening sensor 38 detects the depression amount of the accelerator pedal, and transmits a signal substantially proportional to the detected value to the ECU 6. Based on the output of the accelerator opening sensor 38, the ECU 6 determines that the vehicle is in an acceleration operation state by a process (not shown).

  The ECU 6 shapes input signal waveforms from various sensors, corrects the voltage level to a predetermined level, converts an analog signal value into a digital signal value, and a central processing unit (hereinafter “ CPU ”), a storage circuit that stores various calculation programs executed by the CPU, calculation results, and the like, a fuel injection valve of the engine 1, a urea water injector 42 of the urea water supply device 4, and a secondary air supply device 5 Output circuit for outputting a control signal to the secondary air supply valve 53 and the like.

  The ECU 6 includes an air-fuel ratio controller 61 related to execution of the air-fuel ratio control of the engine 1, a urea water controller 62 related to execution of urea water injection control by the urea water supply device 4, and secondary air supply from the secondary air supply device 5. A control block such as a secondary air controller 63 related to execution of control is configured.

  The air-fuel ratio controller 61 determines an appropriate operation mode of the engine 1 and adjusts a fresh air amount, an EGR amount, a fuel injection amount, and the like according to an algorithm defined for each operation mode. To control the air-fuel ratio. Two operation modes, a lean operation mode and a stoichiometric operation mode, are set.

In the lean operation mode, the air-fuel ratio controller 61 advances the CO and HC oxidation reaction in the upstream catalytic converter 31 and the exhaust purification filter 32 and the downstream catalytic converter 33 advances the NOx reduction reaction. Is controlled leaner than stoichiometric.
In the stoichiometric operation mode, the air-fuel ratio controller 61 controls the air-fuel ratio of the air-fuel mixture to stoichiometric based on the output of the air-fuel ratio sensor 35 in order to advance the three-way purification reaction in the upstream catalytic converter 31.

  The air-fuel ratio controller 61 determines whether the engine 1 is in a steady operation state or an acceleration operation state based on the output of the accelerator opening sensor 38. If the engine 1 is in a steady operation state, the operation mode of the engine 1 is determined. Is set to the lean operation mode, and in the acceleration operation state, the operation mode is set to the stoichiometric operation mode.

The urea water controller 62 controls the NH ₃ sensor 37 so that an amount of NH ₃ necessary for reducing NOx is supplied to the downstream catalytic converter 33 and an excessive amount of NH ₃ is not discharged from the downstream catalytic converter 33. Based on the output, the injection amount of urea water from the urea water injector 42 is controlled. More specifically, as the urea water controller 62, while estimating the NH ₃ storage amount and the maximum NH ₃ storage capacity of the SCR catalyst, the NH ₃ storage amount is maintained in the vicinity of the maximum storage capacity, NH ₃ Based on the output of the sensor 37, the injection amount of urea water is determined. The detailed algorithm of the urea water injection control as described above is described in detail in, for example, International Publication No. 2008/57628 by the applicant of the present application, and therefore, detailed description thereof is omitted here.

  The secondary air controller 63 is connected from the secondary air supply valve 53 so that the secondary air is supplied to the downstream catalytic converter 33 while the operation mode of the engine 1 is switched from the lean operation mode to the stoichiometric operation mode. The secondary air supply amount is controlled. Hereinafter, the secondary air supply control procedure executed in the secondary air controller 63 will be described with reference to a flowchart.

FIG. 3 is a main flowchart showing a specific procedure of secondary air control. The process shown in FIG. 2 is executed in the secondary air controller 63 at a predetermined control cycle.
In S1, the secondary air controller 63 determines whether or not the secondary air supply flag is 1. This secondary air supply flag is a flag indicating that it is a time suitable for supplying secondary air from the secondary air supply valve, and is updated by a process shown in FIG. 4 described later.

If the determination in S1 is NO, the secondary air controller 63 stops the supply of secondary air (S2) and ends this process.
When the determination in S1 is YES, the secondary air controller 63 determines that it is a time suitable for supplying the secondary air, and supplies a predetermined amount of secondary air (S3). In subsequent S4, the secondary air controller 63 determines whether or not excessive NH ₃ slip has occurred based on the output of the NH ₃ sensor. If the determination in S4 is YES and it is determined that excessive NH ₃ slip has occurred, the secondary air controller 63 uses the secondary air to dilute NH ₃ discharged from the downstream catalytic converter. Is increased from the specified amount determined in S3, and this process is terminated (S5).

FIG. 4 is a flowchart showing a procedure for updating the secondary air supply flag. The process shown in FIG. 4 is executed in the secondary air controller 63 at a predetermined control cycle.
In S11, it is determined whether or not the current engine operation mode is the stoichiometric operation mode. If the determination in S11 is YES, the process proceeds to S12, and if NO, the process proceeds to S14.

  In S12, the secondary air controller 63 determines whether or not the output value of the air-fuel ratio sensor has fallen below a predetermined threshold from the previous control cycle to the current control cycle in order to determine when the downstream catalytic converter switches to the stoichiometric atmosphere. Determine. Here, the fact that the output value of the air-fuel ratio sensor has fallen below the threshold means that the operation mode has been switched from the lean operation mode to the stoichiometric operation mode, and the air-fuel ratio of the engine air-fuel mixture has been switched from lean to stoichiometric. This means that the air-fuel ratio of the exhaust gas has actually switched from the lean side to the stoichiometric. That is, it is considered that the inside of the downstream catalytic converter will soon be switched from a lean atmosphere containing a lot of oxygen to a stoichiometric atmosphere containing little oxygen after the determination in S12 is YES. Therefore, the secondary air controller 63 sets the secondary air supply flag from 0 to 1 so that the downstream catalytic converter does not fall into a low oxygen concentration state (S13), and ends this process. Here, by setting the secondary air supply flag to 1, supply of secondary air is started by the processing shown in FIG. In the downstream catalytic converter, the Standard-SCR reaction shown in the above equation (2) proceeds using oxygen contained in the secondary air to purify NOx in the exhaust.

  If the determination in S12 is NO, it means that there is still time before the downstream catalytic converter actually switches to the stoichiometric atmosphere, or that the downstream catalytic converter is in a state after switching to the stoichiometric atmosphere. This means that the processing is immediately terminated to maintain the secondary air supply flag at the value at the previous control.

  In S14, the secondary air controller 63 determines whether or not a value of a later-described delay counter is zero. This delay counter is a timer for determining that the time required from when the operation mode is switched from the stoichiometric operation mode to the lean operation mode until when the downstream catalytic converter actually switches from the stoichiometric atmosphere to the lean atmosphere has elapsed. Equivalent to. The delay counter is reset to a predetermined initial value in S18 described later, and subtraction from the initial value is started in S16 described later.

  If the determination in S14 is NO, the secondary air controller 63 determines whether or not the engine operation mode has been switched from the stoichiometric operation mode to the lean operation mode from the previous control cycle to the current control cycle (S15). If the determination in S15 is YES, the secondary air controller 63 starts subtraction of the value of the delay counter (S16). Thereafter, by a process not shown, the value of the delay counter is subtracted every control cycle until it becomes zero. If the determination in S15 is NO, it means that there is still time before the downstream catalytic converter actually switches to the lean atmosphere, or that the downstream catalytic converter is in the state after switching to the lean atmosphere. Therefore, this process is immediately terminated to maintain the secondary air supply flag at the value at the previous control.

  When the determination in S14 is YES, the secondary air controller 63 determines whether the secondary air controller 63 has changed to the secondary value according to the passage of the time corresponding to the initial value of the delay counter after the operation mode is switched from the stoichiometric operation mode to the lean operation mode. In order to stop the air supply, the secondary air supply flag is reset from 1 to 0 (S17), and the value of the delay counter is reset to a predetermined positive initial value (S18). Thus, the supply of secondary air is continued for a time corresponding to the initial value of the delay counter even after the engine operation mode is switched from the stoichiometric operation mode to the lean operation mode.

According to the exhaust purification system 2 of the present embodiment, the following effects are obtained.
(A) In this embodiment, it is determined that the vehicle is in the acceleration operation state, and the air-fuel ratio of the air-fuel mixture is changed from the lean side to the stoichiometric state by the air-fuel ratio controller. Thereby, the three-way purification reaction proceeds in the upstream catalytic converter, and NOx discharged during acceleration is reduced. During this time, since oxygen-containing secondary air is supplied to the SCR catalyst of the downstream catalytic converter, the NOx reduction reaction also proceeds in the downstream catalytic converter. Therefore, according to this embodiment, NOx can be purified by both the upstream catalytic converter and the downstream catalytic converter during the acceleration operation state.
(B) According to the present embodiment, by supplying the secondary air from the secondary air supply valve, it is possible to suppress a rapid temperature increase of the downstream catalytic converter using the secondary air as a cooling gas. In the NOx purification performance can be suppressed. Further, by using the secondary air as the cooling gas, a decrease in the maximum NH ₃ storage amount of the downstream catalytic converter is also suppressed, so that the discharge of NH ₃ from the downstream catalytic converter can be suppressed. Even if NH ₃ is discharged from the downstream catalytic converter, it is possible to prevent high concentration NH ₃ from being discharged out of the system by increasing the amount of secondary air supplied from the secondary air supply valve.
(C) In this embodiment, oxygen is supplied to the downstream catalytic converter by supplying oxygen-containing secondary air from the secondary air supply valve to the upstream side of the exhaust purification filter even in a stoichiometric or rich atmosphere during acceleration operation. While supplying, the particulate matter deposited on the exhaust purification filter can be burned simultaneously. Accordingly, it is possible to extend the period until the forced regeneration of the particulate matter becomes necessary, and it is possible to suppress the deterioration of fuel consumption and the occurrence of oil dilution caused by the execution of the post injection performed at the forced regeneration.
(D) In this embodiment, oxygen-containing secondary air is supplied to the upstream side of the exhaust purification filter coated with the oxidation catalyst during the acceleration operation state, so that part of the NO in the exhaust gas is removed in the exhaust purification filter. oxidized to NO _2, it can be supplied to the downstream catalytic converter the NO ₂ with oxygen. Thereby, in the downstream catalytic converter, the Standard-SCR reaction and the Fast-SCR reaction can be advanced, so that the NOx purification rate in the downstream catalytic converter can be further improved.
(E) In the present embodiment, the timing at which the inside of the downstream catalytic converter is switched to the stoichiometric atmosphere is determined based on the output of the air-fuel ratio sensor provided on the downstream side of the upstream catalytic converter, and the supply of secondary air is based on this determination. As a result, the secondary air can be supplied at an appropriate time so that the NOx purification rate in the downstream catalytic converter does not decrease. When using an upstream catalyst to which an oxygen storage / release material is added, the timing for switching to the stoichiometric atmosphere varies each time depending on the state of the oxygen storage / release material, but according to this embodiment, it does not depend on such a change. Secondary air can be supplied at an appropriate time.
(F) In this embodiment, after the operation mode is changed from the stoichiometric operation mode to the lean operation mode, the supply of secondary air is continued for a predetermined time so that the NOx purification rate in the downstream catalytic converter does not decrease. It is possible to continue supplying oxygen.
(G) In the present embodiment, by supplying the oxygen-containing secondary air, it is possible to prevent the NH ₃ sensor from entering a state where it cannot be detected, and therefore based on the output of the NH ₃ sensor even during the acceleration operation state. Therefore, the urea water injection control can be performed appropriately.

Second Embodiment
Next, a second embodiment of the present invention will be described with reference to the drawings. In the following description of the present embodiment, the same components as those in the first embodiment are denoted by the same reference numerals, and detailed description thereof is omitted.

  FIG. 5 is a schematic diagram showing the configuration of the engine 1 and its exhaust purification system 2A according to the present embodiment. The exhaust purification system 2A of the present embodiment differs from the first embodiment in the configuration of the secondary air supply device 5A. The secondary air supply valve 53 </ b> A of the secondary air supply device 5 </ b> A is provided between the exhaust purification filter 32 and the downstream catalytic converter 33 in the exhaust passage 11. That is, the exhaust purification system 2A of the present embodiment differs from the first embodiment in the supply location of the secondary air.

According to the exhaust purification system 2A of the present embodiment, in addition to the effects (A), (B), (E)-(G), the following effects can be obtained.
(H) According to the present embodiment, by supplying the secondary air only to the downstream catalytic converter 33, the downstream catalytic converter 33 is supplied with an amount of oxygen necessary to advance the Standard-SCR reaction in the stoichiometric operation mode. It can be supplied with high accuracy. Further, according to the present embodiment, as compared with the first embodiment, the secondary air having a temperature lower by the amount not passing through the exhaust purification filter 32 can be supplied, and therefore, downstream in the stoichiometric operation mode. The cooling effect of the catalytic converter 33 is high. Therefore, as compared with the first embodiment, the amount of NH ₃ slipping from the downstream catalytic converter 33 can be suppressed, and the NOx purification rate in the downstream catalytic converter 33 can be increased.

<Third Embodiment>
Next, a third embodiment of the present invention will be described with reference to the drawings. In the following description of the present embodiment, the same components as those in the first embodiment are denoted by the same reference numerals, and detailed description thereof is omitted.
FIG. 6 is a schematic diagram showing the configuration of the engine 1 and its exhaust purification system 2B according to the present embodiment. The exhaust purification system 2B of the present embodiment is different from the first embodiment in the configuration of the secondary air supply device 5B and the configuration of the secondary air controller 63B of the ECU 6B.

  The secondary air supply device 5B is different from the first embodiment in that it includes two secondary air supply valves, an upstream secondary air supply valve 531B and a downstream secondary air supply valve 532B. The upstream secondary air supply valve 531 </ b> A is provided between the upstream catalytic converter 31 and the exhaust purification filter 32 in the exhaust passage 11. The downstream secondary air supply valve 532 </ b> A is provided between the exhaust purification filter 32 and the downstream catalytic converter 33 in the exhaust passage 11.

  The secondary air controller 63B includes an upstream secondary air supply valve 531B so that the secondary air is supplied to the downstream catalytic converter 33 while the operation mode of the engine 1 is switched from the lean operation mode to the stoichiometric operation mode. Alternatively, the secondary air supply amount from the downstream secondary air supply valve 532B is controlled.

FIG. 7 is a main flowchart showing a specific procedure of secondary air control. The process shown in FIG. 7 is executed at a predetermined control cycle in the secondary air controller 63B.
In S21, the secondary air controller 63B determines whether or not the secondary air supply flag is 1. This secondary air supply flag is a flag indicating that it is a time suitable for supplying secondary air from the upstream secondary air supply valve 531B or the downstream secondary air supply valve 532B. According to the procedure shown in FIG. Updated.

  When the determination in S21 is NO, the secondary air controller 63B stops the supply of secondary air from the upstream secondary air supply valve 531B and the downstream secondary air supply valve 532B (S22), and ends this process. .

  When the determination in S21 is YES, the secondary air controller 63B determines whether or not the filter temperature increase completion flag is 1 (S23). This filter temperature increase completion flag indicates that the temperature of the exhaust purification filter has increased to the combustion temperature of particulate matter after the engine operation mode has been switched from the lean operation mode to the stoichiometric operation mode. This flag is updated by the process shown in FIG.

  If the determination in S23 is NO, that is, if the temperature of the exhaust gas purification filter has not yet reached the combustion temperature of the particulate matter, the secondary air controller 63B downstream without disturbing the temperature rise of the exhaust gas purification filter. In order to supply secondary air to the catalytic converter, a predetermined amount of secondary air is supplied from the downstream secondary air supply valve (S24), and the process proceeds to S26.

  When the determination in S23 is YES, that is, when the temperature of the exhaust purification filter reaches the combustion temperature of the particulate matter, the secondary air controller 63B downstream while burning the particulate matter deposited on the exhaust purification filter. In order to supply secondary air to the catalytic converter, a predetermined amount of secondary air is supplied from the upstream secondary air supply valve (S25), and the process proceeds to S26.

In S26, the secondary air controller 63B determines whether or not an excessive NH ₃ slip has occurred based on the output of the NH ₃ sensor. If the determination in S26 is YES and it is determined that excessive NH ₃ slip has occurred, the secondary air controller 63B causes the secondary air to dilute NH ₃ discharged from the downstream catalytic converter. Is increased from the specified amount determined in S24 or S25, and this process is terminated.

FIG. 8 is a flowchart showing a procedure for updating the filter temperature increase completion flag. The process shown in FIG. 8 is executed at a predetermined control cycle in the secondary air controller 63B.
In S31, the secondary air controller 63B determines whether or not the secondary air supply flag is 1. If the determination in S31 is NO, the secondary air controller 63B resets the filter temperature increase completion flag to 0 (S32), and ends this process. When the determination in S31 is YES, the secondary air controller 63B determines whether or not the temperature of the exhaust purification filter is higher than the combustion temperature of the particulate matter (S33). If the determination in S33 is NO, the secondary air controller 63B moves to S32 described above. If the determination in S33 is YES, the secondary air controller 63B sets the filter temperature increase completion flag to 1 (S34), and ends this process. Here, by setting the filter temperature increase completion flag to 1, the supply source of the secondary air is switched from the downstream secondary air supply valve to the upstream secondary air supply valve.

According to the exhaust purification system 2B of the present embodiment, in addition to the effects (A) to (H), the following effects can be obtained.
(I) In the present embodiment, the secondary air is supplied by the downstream secondary air supply valve until the temperature of the exhaust purification filter reaches the combustion temperature of the particulate matter, and the temperature of the exhaust purification filter is changed to the particulate matter. After reaching the combustion temperature, secondary air is supplied by the upstream secondary air supply valve. Thereby, it is possible to prevent the temperature rise of the exhaust purification filter from being delayed by the supply of secondary air while supplying oxygen necessary for reducing NOx to the downstream catalytic converter. That is, the temperature of the exhaust gas purification filter can be quickly raised during the acceleration operation state.

Although the first to third embodiments have been described above, the present invention is not limited to these embodiments. For example, in the above embodiment, the air-fuel ratio controller controls the air-fuel ratio of the air-fuel mixture to stoichiometric in the stoichiometric operation mode, but may control the air-fuel ratio of the air-fuel mixture to be richer than stoichiometric in the stoichiometric operation mode. In this case, compared with the case where the stoichiometric control is performed, the purification rate of CO and HC in the upstream catalytic converter is reduced, but NH _{3 serving} as a NOx reducing agent in the downstream catalytic converter 33 is generated.

Therefore, when the air-fuel ratio of the air-fuel mixture is controlled to the rich side in the stoichiometric operation mode in this way, the NH ₃ generated in the upstream catalytic converter 31 is actively sent to the downstream catalytic converter 33 without being oxidized by the exhaust purification filter 32. When supplying to the secondary air, the secondary air is preferably supplied from the downstream secondary air supply valve 532B. Further, in the stoichiometric operation mode, by controlling the air-fuel ratio of the air-fuel mixture to the rich side, when the HC that has not been oxidized by the upstream catalytic converter 31 is actively oxidized by the exhaust purification filter 32, the secondary air is It is preferable to supply from the upstream secondary air supply valve 531B.

1. Engine (internal combustion engine)
2, 2A, 2B ... Exhaust purification system 3 ... Catalyst purification device 31 ... Upstream catalytic converter (upstream catalyst)
32. Exhaust gas purification filter (filter)
33 ... Downstream catalytic converter (selective reduction catalyst)
35 ... Air-fuel ratio sensor 36 ... Exhaust temperature sensor (temperature acquisition means)
37 ... NH ₃ sensor 38 ... Accelerator opening sensor (acceleration determining means)
4 ... Urea water supply device (reducing agent supply device)
5, 5A, 5B ... secondary air supply device 53, 53A ... secondary air supply valve 531B ... upstream secondary air supply valve 532B ... downstream secondary air supply valve 6, 6B ... ECU (temperature acquisition means, acceleration determination means)
61: Air-fuel ratio controller (air-fuel ratio control means)
62 ... Urea water controller (reducing agent control means)
63, 63B ... Secondary air controller (secondary air control means)

Claims (10)

An exhaust purification system for an internal combustion engine that makes the air-fuel ratio of the air-fuel mixture lean during steady operation,
A selective reduction catalyst that is provided in an exhaust passage of the engine and reduces NOx in the presence of NH ₃ ;
A reducing agent supply device for supplying NH ₃ or a precursor thereof to the selective reduction catalyst;
An upstream catalyst provided on the upstream side of the selective reduction catalyst in the exhaust passage and having a three-way purification function;
A secondary air supply device for supplying oxygen-containing secondary air from between the upstream catalyst and the selective reduction catalyst in the exhaust passage;
Acceleration determining means for determining that the vehicle is in an accelerated operation state;
An air-fuel ratio control means for changing the air-fuel ratio of the air-fuel mixture to stoichiometric or rich when it is determined that it is in an accelerated operation state;
Secondary air control means for controlling the secondary air supply device so that secondary air is supplied to the selective reduction catalyst while the air-fuel ratio of the air-fuel mixture is changed to the stoichiometric or rich side. An exhaust gas purification system for an internal combustion engine. Between the upstream catalyst and the selective reduction catalyst, a filter for collecting particulate matter in the exhaust is provided,
The exhaust gas purification system for an internal combustion engine according to claim 1, wherein the secondary air supply device supplies secondary air from between the upstream catalyst and the filter in the exhaust passage.   The exhaust purification system for an internal combustion engine according to claim 2, wherein an oxidation catalyst is supported on the filter. A temperature acquisition means for acquiring the temperature of the filter;
The secondary air supply device includes an upstream supply unit that supplies secondary air between the upstream catalyst and the filter, and a downstream supply unit that supplies secondary air between the filter and the selective reduction catalyst. With
The secondary air control means supplies the secondary air by the downstream supply unit until the filter temperature reaches the combustion temperature of the particulate matter, and after the filter temperature reaches the combustion temperature, the upstream air is supplied to the upstream side. The exhaust gas purification system for an internal combustion engine according to claim 2 or 3, wherein secondary air is supplied by the supply unit. An air-fuel ratio sensor for detecting the air-fuel ratio of the exhaust;
A stoichiometric timing for determining when the selective reduction catalyst is switched to a stoichiometric or rich atmosphere based on an output of the air / fuel ratio sensor after the air / fuel ratio of the air-fuel mixture is changed from lean to stoichiometric or rich by the air / fuel ratio control means. And a determination means,
5. The exhaust gas purification system for an internal combustion engine according to claim 1, wherein the secondary air control unit starts the supply of secondary air based on the determination by the stoichiometric timing determination unit.   The secondary air control means continues supplying the secondary air for a predetermined time after the air-fuel ratio of the air-fuel mixture is changed from the stoichiometric or rich side to the lean side by the air-fuel ratio control means. Item 6. An exhaust purification system for an internal combustion engine according to any one of Items 1 to 5.   The exhaust purification system for an internal combustion engine according to claim 5 or 6, wherein the upstream catalyst is provided with an oxygen storage / release material. An NH ₃ sensor for detecting the NH ₃ concentration in the exhaust gas downstream of the selective reduction catalyst using an electromotive force generated between the electrodes;
The reducing agent control means for controlling the amount of NH ₃ or precursor supplied from the reducing agent supply device based on the output of the NH ₃ sensor, further comprising: a reducing agent control means. An exhaust purification system for an internal combustion engine as described.   While the air-fuel ratio of the air-fuel mixture is changed to the rich side, the secondary air control means is configured so that oxygen is supplied to the selective reduction catalyst and the HC oxidation reaction proceeds in the oxidation catalyst of the filter. The exhaust gas purification system for an internal combustion engine according to claim 3, wherein secondary air is supplied from between the upstream catalyst and the filter. The secondary air control means includes the filter and the filter so that oxygen and NH ₃ generated by the upstream catalyst are supplied to the selective reduction catalyst while the air-fuel ratio of the air-fuel mixture is changed to the rich side. The exhaust gas purification system for an internal combustion engine according to claim 3, wherein secondary air is supplied from between the selective reduction catalysts.

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