JP5169547B2 - Exhaust control device for internal combustion engine - Google Patents

Description

  The present invention relates to an exhaust control device for an internal combustion engine that performs exhaust purification using a catalytic converter, and in particular, a bypass provided with another catalytic converter immediately after a cold start when the main catalytic converter is not activated or during deceleration with a fuel cut. The present invention relates to an improvement of an exhaust control device of a type in which exhaust is guided to a flow path side.

  As conventionally known, in a configuration in which the main catalytic converter is disposed relatively downstream of the exhaust system such as under the floor of a vehicle, the temperature of the catalytic converter rises and is activated after a cold start of the internal combustion engine. In the meantime, a sufficient exhaust purification action cannot be expected. On the other hand, the closer the catalytic converter is to the upstream side of the exhaust system, that is, the internal combustion engine side, the lower the durability due to thermal degradation of the catalyst.

  Therefore, as disclosed in Patent Document 1, a bypass flow path is provided in parallel with the upstream portion of the main flow path including the main catalytic converter, and another bypass catalytic converter is interposed in the bypass flow path. However, an exhaust control device has been conventionally proposed in which the exhaust gas is guided to the bypass flow channel side immediately after the cold start by the flow channel switching valve for switching between the two. In this configuration, the bypass catalytic converter is positioned relatively upstream of the main catalytic converter in the exhaust system and is activated relatively early, so that exhaust gas purification can be started from an earlier stage.

  The operation of the flow path switching valve is, for example, detecting the exhaust temperature upstream (or downstream) of the main catalytic converter corresponding to the catalyst temperature using a temperature sensor, and the active state of the exhaust purification catalyst is determined according to the exhaust temperature. Determined. That is, when the main catalytic converter is in an operating state that can be inactive, the flow path switching valve is closed so as to guide the exhaust to the bypass flow path side, and the main catalytic converter is activated when the exhaust temperature exceeds the catalyst activation determination temperature. Judgment is made and the flow path switching valve is opened.

The flow path switching valve (hereinafter also simply referred to as “switching valve”) is in a closed state immediately after the engine cold start when the main catalytic converter (hereinafter also referred to as “underfloor catalyst”) has not been activated after engine startup. However, the temperature of the underfloor catalyst is low when the engine is idle for a relatively long period of time, such as when there is a traffic jam or when visiting a convenience store, or when the fuel injection is stopped, i.e. when the vehicle is decelerating with fuel cut. May decrease and become inactive again. Therefore, in such a case, it is preferable to close the flow path switching valve again.
Japanese Patent Laid-Open No. 2005-351088

  However, the determination of the activity state of the underfloor catalyst using such a temperature sensor cannot sufficiently consider the influence of the heat of reaction in the underfloor catalyst. For example, in a decelerating operation state that involves a fuel cut, a relatively large amount of oxygen in the underfloor catalyst Is supplied, heat of reaction is generated due to oxidation of HC (hydrocarbon) remaining in the catalyst, and the activity tends to be accelerated by the influence of the heat of reaction. For this reason, as described above, when the activation state of the underfloor catalyst is determined at the exhaust temperature upstream of the underfloor catalyst, it is erroneously determined that the underfloor catalyst is inactive even though it is already activated. The switching valve closes and high-temperature exhaust gas flows to the bypass catalytic converter side, which may accelerate the thermal deterioration of the bypass catalytic converter. In addition, since the engine output is suppressed in a situation where it is determined as inactive, the driver does not have the output intended by the driver, and therefore the accelerator pedal is stepped on excessively, and there is a concern that the fuel consumption may be deteriorated.

  By the way, before and after the underfloor catalyst, that is, upstream and downstream, an air-fuel ratio sensor for outputting a sensor output corresponding to the air-fuel ratio of the exhaust is provided as an air-fuel ratio detecting means for detecting the air-fuel ratio (oxygen gas concentration) of the exhaust. Then, well-known air-fuel ratio feedback control is performed using these sensor outputs.

  The main object of the present invention is to accurately determine the active state of the main catalytic converter using the air-fuel ratio before and after the main catalytic converter and the opening / closing operation of the flow path switching valve.

  A flow switching valve is interposed in the main flow path of the exhaust upstream of the main catalytic converter, and a bypass catalytic converter is interposed in the bypass flow path arranged in parallel with the upstream portion of the main flow path, An exhaust control device for an internal combustion engine configured to allow exhaust to flow to the bypass flow path when the flow path switching valve is closed, comprising air-fuel ratio detection means for detecting the air-fuel ratio before and after the main catalytic converter. When the flow path switching valve is closed, the air-fuel ratio of the exhaust gas is temporarily changed to the rich side or the lean side, and the flow path switching valve is temporarily opened. Based on the change in the air-fuel ratio before and after the catalytic converter, the active state of the main catalytic converter is determined.

  By temporarily changing the air-fuel ratio of the exhaust gas to the rich side or the lean side and then temporarily opening the flow path switching valve, the exhaust gas temporarily enriched or leaned is almost the main flow. It is supplied to the main catalytic converter through the passage and hardly flows to the bypass flow path in which the bypass catalytic converter is interposed. Therefore, since the activation state of the catalyst is determined without being influenced by the bypass catalytic converter and according to the change in the air-fuel ratio before and after the main catalytic converter, the influence of the reaction heat in the catalyst as described above Therefore, the active state of the main catalytic converter can be determined with high accuracy.

  Since the existing air-fuel ratio sensor used for air-fuel ratio feedback control, and the exhaust system flow-path switching valve equipped with a bypass catalytic converter for early catalyst activation immediately after engine startup, etc. are used, It is not necessary to dare to add new parts, and it can be easily applied to existing internal combustion engines.

  Hereinafter, an embodiment in which an exhaust control device according to the present invention is applied to an inline four-cylinder internal combustion engine 1 will be described in detail with reference to the drawings. FIG. 1 is an explanatory diagram schematically showing the piping layout and control system of the internal combustion engine 1. First, the configuration of the internal combustion engine 1 will be described with reference to FIG.

  In the cylinder head 1a of the internal combustion engine 1, exhaust ports 2 of cylinders # 1 to # 4 arranged in series are formed so as to open toward the side surfaces, respectively. In addition, the main passage 3 is connected. The four main passages 3 of the # 1 cylinder to the # 4 cylinder merge into one flow path, and a main catalytic converter (hereinafter also referred to as “underfloor catalyst”) 4 is disposed on the downstream side thereof. Yes. The main catalytic converter 4 has a large capacity arranged under the floor of the vehicle, and includes, for example, a three-way catalyst and an HC trap catalyst as the catalyst. The main passage 3 and the main catalytic converter 4 constitute a main passage through which exhaust flows during normal operation. Further, a flow path switching valve 5 that opens and closes the main passages 3 at the same time is provided at the junction of the four main passages 3 from each cylinder. The flow path switching valve 5 is driven to open and close by an appropriate actuator 5a.

  On the other hand, bypass passages 7 each having a smaller passage sectional area than the main passage 3 are branched from the main passages 3 of the respective cylinders as bypass passages. The branch point 6 that is the upstream end of each bypass passage 7 is set to a position on the upstream side of the main passage 3 as much as possible. The four bypass passages 7 merge into one flow path on the downstream side, and a bypass catalytic converter 8 using a three-way catalyst is interposed immediately after the junction. The bypass catalytic converter 8 has a small capacity as compared with the main catalytic converter 4, and preferably uses a catalyst excellent in low-temperature activity. The downstream end of the bypass passage 7 extending from the outlet side of the bypass catalytic converter 8 is connected to the main passage 3 at the junction 9 upstream of the main catalytic converter 4 in the main passage 3 and downstream of the flow path switching valve 5. Has been.

  A bypass air-fuel ratio sensor 10 is disposed at a position upstream of the bypass catalytic converter 8 in the bypass passage 7 or the main passage 3, that is, at a position adjacent to the inlet portion 8 a of the bypass catalytic converter 8. Further, in the main passage 3, at a position downstream of the portion where the branched bypass passage 7 joins and upstream of the main catalytic converter 4, that is, a position adjacent to the inlet 4 a of the main catalytic converter 4. An upstream air-fuel ratio sensor 11 is arranged. Further, the downstream air-fuel ratio sensor 12 is disposed at a position downstream of the main catalytic converter 4 in the main passage 3, that is, at a position adjacent to the outlet portion 4 b of the main catalytic converter 4.

  The upstream air-fuel ratio sensor 11 and the downstream air-fuel ratio sensor 12 are for performing known air-fuel ratio feedback control after the activation of the main catalytic converter 4. The engine air-fuel ratio (fuel injection amount) is controlled by the fuel-fuel ratio sensor 11, and the output signal of the downstream-side air-fuel ratio sensor 12 downstream of the main catalytic converter 4 is used in an auxiliary manner for correcting variations in the control characteristics. . That is, in this case, the output signal of the downstream air-fuel ratio sensor 11 is not necessarily essential.

  Similarly, the bypass air-fuel ratio sensor 10 is for performing known feedback control when the bypass catalytic converter 8 is used together with the upstream air-fuel ratio sensor 11. The engine air-fuel ratio (fuel injection amount) is controlled by the bypass-side air-fuel ratio sensor 10, and the output signal of the upstream-side air-fuel ratio sensor 11 downstream of the bypass catalytic converter 8 is supplemented to correct variations in the control characteristics. Is used. That is, in this case, the output signal of the upstream air-fuel ratio sensor 11 is not necessarily essential.

  Here, the known air-fuel ratio feedback control is a feedback control of the air-fuel ratio of the internal combustion engine to the stoichiometric air-fuel ratio. More specifically, the air-fuel ratio of the internal combustion engine is rich or lean, and the stoichiometric air-fuel ratio is sandwiched. It is a step change to rich or lean on the opposite side. Then, when correcting the air-fuel ratio feedback control based on the sensor output signal on the downstream side of the catalytic converter, if the sensor that is the source of the sensor output signal on the upstream side of the catalytic converter is switched, the downstream side of the catalytic converter is adjusted accordingly. The sensor that is the source of the sensor output signal is also switched. The known stop conditions for air-fuel ratio feedback control are when starting, when the water temperature is low, when the engine is heavily loaded, when decelerating, when the air-fuel ratio sensor is abnormal.

  As these air-fuel ratio sensors 10 to 12, either a so-called wide-range air-fuel ratio sensor having a substantially linear output characteristic corresponding to the exhaust air-fuel ratio or a so-called oxygen sensor having a binary output characteristic of rich or lean is used. However, from the viewpoint of control in the above-described air-fuel ratio control, the bypass air-fuel ratio sensor 10 and the upstream air-fuel ratio sensor 11 are preferably wide-area air-fuel ratio sensors, and the downstream air-fuel ratio sensor is also preferable. As the sensor 12, an oxygen sensor can be used from the viewpoint of component cost and the like.

  The internal combustion engine 1 includes a spark plug 21, and a fuel injection valve 23 is disposed in the intake passage 22. Furthermore, a so-called electronically controlled throttle valve 24 that is opened and closed by an actuator such as a motor is disposed upstream of the intake passage 22, and an air flow meter 25 that detects the intake air amount is provided downstream of the air cleaner 26. Yes.

  Various control parameters of the internal combustion engine 1, for example, the fuel injection amount by the fuel injection valve 23, the ignition timing by the spark plug 21, the opening degree of the throttle valve 24, the open / close state of the flow path switching valve 5, etc. It is controlled by an engine control unit (ECU) 27. In addition to the sensors described above, the engine control unit 27 includes a coolant temperature sensor 28, an accelerator opening sensor 29 that detects the opening (depression amount) of the accelerator pedal operated by the driver, and the main catalytic converter 4. Detection signals of various sensors such as an exhaust temperature sensor 30 that is disposed on the upstream side and detects the exhaust temperature upstream of the catalyst are input.

  In such a configuration, basically, at the stage where the engine temperature or the exhaust temperature after the cold start is low, the flow path switching valve 5 is closed via the actuator 5a (the flow path switching valve 5 is in the closed position), The main passage 3 is blocked. Therefore, the entire amount of exhaust discharged from each cylinder flows from the branch point 6 to the bypass catalytic converter 8 through the bypass passage 7. The bypass catalytic converter 8 is located upstream of the exhaust system, that is, at a position close to the exhaust port 2 and is small in size, so that it is activated quickly and exhaust purification is started at an early stage.

  On the other hand, if the engine warm-up progresses and the engine temperature or the exhaust temperature becomes sufficiently high, it is considered that the catalyst of the main catalytic converter 4 is activated as one of the trigger conditions, and the flow path switching valve 5 is opened. (The flow path switching valve 5 is in the open position). Thus, the exhaust discharged from each cylinder mainly passes through the main catalytic converter 4 from the main passage 3. At this time, the bypass passage 7 side is not particularly cut off, but the bypass passage 7 side has a smaller passage cross-sectional area than the main passage 3 side and the bypass catalytic converter 8 is interposed. Due to the difference, most of the exhaust flow passes through the main passage 3 side and hardly flows into the bypass passage 7 side. As a result, as a means for switching the flow of exhaust gas, a complicated switching valve is not required, and the flow path switching valve 5 can be configured to simply close or open the main passage 3. Further, the thermal deterioration of the bypass catalytic converter 8 can be sufficiently suppressed.

  FIG. 5 is a timing chart showing the opening / closing operation of the switching valve 5 in the vicinity of the vehicle deceleration state with fuel cut according to the reference example. In this reference example, the upstream side of the temperature underfloor catalyst 4 detected by the temperature sensor 30. The exhaust gas temperature determines the active state of the catalyst, that is, the switching timing of the switching valve 5 from closing to opening.

  When the main catalytic converter 4 is in a predetermined operating state that can be inactive, such as during vehicle deceleration with fuel cut, the temperature condition flag is set to “1” and the switching valve 5 is closed (CLOSE). Accordingly, the exhaust gas flows to the bypass passage 7 side, and the exhaust gas is purified by the bypass catalytic converter 8 provided in the bypass passage 7. Therefore, even if the main catalytic converter 4 is in an inactive state, the exhaust gas is exhausted. Reduction of emissions can be prevented. For example, the flag is set when the fuel injection amount TP, the throttle opening TVO, the engine water temperature, or the like becomes a predetermined value or less.

  Here, during vehicle deceleration accompanied by fuel cut, oxygen in the lean gas supplied to the main catalytic converter 4 undergoes an oxidation reaction with HC remaining inside the catalyst of the main catalytic converter 4, and the temperature sensor 30 is generated by this reaction heat. Compared to the exhaust gas temperature upstream of the catalyst detected by the above, the actual actual catalyst temperature of the main catalytic converter 4 does not decrease immediately but tends to decrease with a delay.

  When a predetermined fuel cut recovery condition such as detection of accelerator pedal depression or a decrease in engine speed is satisfied in such a vehicle deceleration state involving fuel cut, fuel injection is resumed, and the exhaust temperature rises accordingly. To go. When the exhaust temperature upstream of the catalyst by the temperature sensor 30 reaches a predetermined activation temperature equivalent value α1 corresponding to the catalyst activation temperature, it is determined that the main catalytic converter 4 is in an activated state, and the switching valve is “closed”. To “open”.

  Here, as described above, in the vehicle deceleration state accompanied by the fuel cut, the actual decrease in the catalyst temperature tends to be delayed due to the reaction heat inside the catalyst, etc., with respect to the decrease in the exhaust temperature upstream of the catalyst detected by the temperature sensor 30. In particular, when the fuel cut recovery is performed relatively early after the fuel cut, the determination by the temperature sensor 30 does not actually reduce the catalyst temperature to the activation temperature, and the hatching region β in FIG. As shown in FIG. 4, the start of switching from the closing to the opening of the switching valve 5 is delayed despite the fact that the underfloor catalyst 4 is in an active state, and the exhaust gas flows unnecessarily to the bypass passage 7 side, causing thermal deterioration of the bypass catalytic converter 8. There is a risk of promoting. Therefore, in this embodiment, as will be described later, the air-fuel ratio sensors 11 and 12 before and behind the underfloor catalyst 4 and the flow path switching valve 5 are used without depending on only the catalyst activity determination by the temperature sensor 30. It detects the active state of the underfloor catalyst with high accuracy.

  FIG. 2 is a flowchart showing the control flow of this embodiment, and this routine is stored and repeatedly executed by the ECU 27. In step S11, it is determined whether an engine operating condition for closing the flow path switching valve 5 such as immediately after a cold start or during a deceleration operation with fuel cut is satisfied, that is, whether the temperature condition flag is “1”. judge. When the condition is satisfied, that is, when the flow path switching valve 5 is closed, the routine proceeds to step S12 and after, otherwise, when this flow path switching valve 5 is open, this routine is ended.

  In step S12, the in-catalyst oxygen storage amount of the main catalytic converter 4 is read. This amount of oxygen storage in the catalyst is constantly calculated and monitored during engine operation based on the outputs of the air-fuel ratio sensors 11 and 12 before and after the catalyst by other routines (not shown). In step S13, it is determined whether the oxygen storage amount in the catalyst is leaner. For example, it is determined whether the lean side is higher than a predetermined threshold value on the lean side. If it is determined that the vehicle is leaner, the process proceeds to step S14 where, for example, a so-called rich spike is performed in which the fuel injection amount is temporarily increased to temporarily change the air-fuel ratio to the rich side temporarily.

  In step S15, it is determined whether the oxygen storage amount in the catalyst is close to rich. For example, it is determined whether the rich side is greater than a predetermined threshold value on the rich side. If it is not determined to be rich, the process proceeds to step S20 described later. If it is determined that the engine is close to rich and it is not determined that the engine has just started in step S16, the process proceeds to step S17, where the air-fuel ratio is temporarily greatly changed to the lean side by temporarily reducing the fuel injection amount, for example. Do the so-called lean spike. On the other hand, if it is determined in step S16 that the engine has just started, the process proceeds to step S18 without performing the lean spike in step S17.

  In step S18, the switching valve 5 is temporarily opened. That is, the switching valve 5 is opened for a predetermined extremely short time and then closed. Here, when the rich / lean spike in step S14 or S17 is performed, it corresponds to the temporary fluctuation of the air-fuel ratio of the exhaust gas due to this spike, that is, the distance that the changed exhaust gas reaches the switching valve 5 Considering the time, the opening timing and opening time of the switching valve 5 are set temporarily.

In step S19, the output difference ΔA / F between the air-fuel ratio sensors 11 and 12 before and after the underfloor catalyst 4 (| the output of the upstream air-fuel ratio sensor 11−the output of the downstream air-fuel ratio sensor 12) is a predetermined determination value α2. It is determined whether it is less than. If the output difference ΔA / F is greater than or equal to the determination value α2, it is determined that the underfloor catalyst 4 is activated, the process proceeds to step S21, and the switching valve 5 is opened. If the output difference ΔA / F is less than the determination value α2, the process proceeds to step S20.

  In step S20, it is determined whether the exhaust temperature upstream of the catalyst detected by the exhaust temperature sensor 30 exceeds a predetermined activation temperature equivalent value α1 corresponding to the activation temperature of the underfloor catalyst 4. If the activation temperature equivalent value α1 is exceeded, it is determined that the underfloor catalyst 4 is active, and the routine proceeds to step S21 where the switching valve 5 is opened.

  The operation when the control of this embodiment is applied will be described with reference to the timing charts of FIGS.

  FIG. 3 shows a transition period from a vehicle deceleration state accompanied by a fuel cut to a fuel cut recovered state. In the vehicle deceleration state accompanied by the fuel cut, the temperature drop condition is satisfied and the switching valve 5 is closed, and the oxygen storage amount in the catalyst is largely biased to the lean side due to the fuel cut, so a rich spike is performed (S14, F1), the switching valve 5 is temporarily opened according to the fluctuation of the air-fuel ratio (S18, F2). As a result, most of the exhaust gas whose air-fuel ratio has temporarily changed due to the rich spike does not flow to the bypass passage 7 side, but passes through the main passage 3a downstream of the hatched switching valve in FIG. Therefore, the fluctuation of the air-fuel ratio is not absorbed or blunted by the bypass catalytic converter 8 interposed in the bypass passage 7. As a result, the fluctuation of the air-fuel ratio appears favorably in the output of the upstream air-fuel ratio sensor 11 (F3).

On the other hand, the output of the downstream side air-fuel ratio sensor 12 differs depending on the active state of the underfloor catalyst 4, and if the catalyst is active, it reacts with oxygen stored in the catalyst, and the fluctuation of the air-fuel ratio due to rich spikes is caused. On the other hand, if the catalyst is inactive, the fluctuation due to the rich spike appears as it is, as shown by the broken line F4. Therefore, if the output difference ΔA / F between the air-fuel ratio sensors 11 and 12 before and after the catalyst is equal to or larger than the determination value α2, it is determined that the catalyst is activated, and the switching valve 5 is opened (S19, S21). Thus, even if the exhaust temperature upstream of the catalyst does not reach the activation temperature equivalent value α1, the switching valve 5 opens quickly, and the above-described thermal deterioration of the bypass catalytic converter 8 can be suppressed.

  Further, it is possible to determine the catalyst activity using existing components such as the air-fuel ratio sensors 11 and 12 before and after the catalyst used for the air-fuel ratio feedback control, and the flow path switching valve 5 for exhaust improvement by early activation of the catalyst. It is easy to apply, without the need to add new parts.

  Furthermore, the amount of oxygen storage in the catalyst can be kept in a neutral state by switching the rich / lean spike according to the bias of the amount of oxygen storage in the catalyst toward the lean side or rich side. That is, it is possible to easily determine the activity of the catalyst by using the rich / lean spike for transitioning the oxygen storage amount in the catalyst to the neutral state.

  In addition, in such a vehicle deceleration state with fuel cut, a rich spike is normally performed during fuel cut recovery. Therefore, the catalytic activity is more easily determined by using such a rich spike. It is also possible.

  On the other hand, as shown in FIG. 4, when the in-catalyst oxygen storage amount is biased toward the rich side, a lean spike is performed (S17, F5). In this case, compared with the case where rich spike is performed, it is possible to further suppress a decrease in exhaust emission and fuel consumption performance.

  As shown in FIG. 5, when the switching valve is closed immediately after the engine is started, the main passage 3a on the downstream side of the switching valve 5 is filled with air, that is, lean gas. By temporarily opening the valve 5, lean gas is supplied to the main catalytic converter 4, and the air-fuel ratio of the exhaust gas supplied to the main catalytic converter 4 is temporarily set as in the case where the lean spike is performed. It can be changed to the lean side. Therefore, immediately after the engine is started and the oxygen storage amount in the catalyst is biased toward the rich side, the air-fuel ratio can be temporarily changed by opening the switching valve 5 temporarily. Since the lean spike as shown by the broken line F6 can be omitted, the catalyst activity diagnosis can be performed more simply.

  As described above, the present invention has been described based on the specific embodiments. However, the present invention is not limited to the above-described embodiments, and includes various modifications and changes without departing from the spirit of the present invention. . For example, in steps S14 and S17, rich / lean spikes may be performed using increase / decrease in the intake air amount by a throttle valve or a variable valve mechanism. For simplification, the exhaust gas temperature sensor 30 may be omitted, and the process of step S20 may be omitted.

1 is a configuration diagram showing an internal combustion engine to which an exhaust control device according to an embodiment of the present invention is applied. The flowchart which shows the flow of control of a present Example. The timing chart at the time of vehicle deceleration accompanying the fuel cut which concerns on a present Example. The timing chart at the time of the vehicle deceleration accompanying the fuel cut similarly concerning a present Example. The timing chart at the time of engine starting which concerns on a present Example. The timing chart at the time of vehicle deceleration accompanying the fuel cut which concerns on a reference example.

Explanation of symbols

DESCRIPTION OF SYMBOLS 3 ... Main passage 4 ... Main catalytic converter 5 ... Flow path switching valve 7 ... Bypass passage 8 ... Bypass catalytic converter 10 ... Upstream air-fuel ratio sensor (air-fuel ratio detection means)
11: Downstream air-fuel ratio sensor (air-fuel ratio detection means)
27 ... Engine control unit (control unit)
30 ... Exhaust temperature sensor

Claims (5)

A flow switching valve is interposed in the main flow path of the exhaust upstream of the main catalytic converter, and a bypass catalytic converter is interposed in the bypass flow path arranged in parallel with the upstream portion of the main flow path, In an exhaust control device for an internal combustion engine configured to allow exhaust to flow to a bypass flow path when the flow path switching valve is closed,
An upstream air-fuel ratio sensor for detecting the air-fuel ratio before and after the main catalytic converter and a downstream air-fuel ratio sensor;
The upstream air-fuel ratio sensor is disposed downstream of a portion of the main flow path where the branched bypass passages merge, and upstream of the main catalytic converter,
Furthermore, when the flow path switching valve is closed, the air-fuel ratio of the exhaust gas is temporarily changed to the rich side or the lean side, and the flow path switching valve is temporarily opened, and this temporary opening is accompanied. A control unit for determining an active state of the main catalytic converter based on a change in the air-fuel ratio before and after the main catalytic converter ;
An exhaust control device for an internal combustion engine, comprising:   The control unit changes the air-fuel ratio of the exhaust gas to a rich side when the oxygen storage amount in the main catalytic converter is close to lean, and changes the air-fuel ratio of the exhaust gas to a lean side when the oxygen storage amount is close to rich. The exhaust control device for an internal combustion engine according to claim 1, wherein   The control unit temporarily changes the air-fuel ratio of the exhaust gas to the rich side by a rich spike that temporarily increases the fuel injection amount when recovering the fuel cut from the vehicle deceleration state accompanied by the fuel cut. The exhaust control device for an internal combustion engine according to claim 1 or 2.   4. The control unit according to claim 1, wherein the control unit temporarily changes the air-fuel ratio of the exhaust gas to the lean side by temporarily opening the flow path switching valve immediately after the engine is started. An exhaust control device for an internal combustion engine according to any one of the above. The main flow path is formed by joining the upstream main passage for each cylinder connected to each cylinder and the plurality of upstream main passages into one flow path, and the main catalytic converter is interposed. A downstream main passage,
The bypass passage branches from an upstream portion of the upstream main passage, and has an upstream bypass passage for each cylinder having a smaller passage cross-sectional area than the upstream main passage, and the plurality of upstream bypass passages are 1 A downstream bypass passage having a downstream end connected to the downstream main passage at a position upstream of the main catalytic converter and having the bypass catalytic converter interposed therebetween. ,
The flow path switching valve is provided at a merging portion of a plurality of upstream main passages, and when the flow path switching valve is closed, the upstream main passages are not in communication with each other. An exhaust control device for an internal combustion engine according to any one of claims 1 to 4.

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