Exhaust gas purification device

An exhaust gas purification device includes a first catalyst, a second catalyst, a bypass pipe, a hydrocarbon adsorbent, and a switching controller. The first catalyst is provided in an exhaust pipe. The second catalyst is provided downstream of the first catalyst in the exhaust pipe. The bypass pipe branches from a first portion of the exhaust pipe. The first portion is located upstream of the second catalyst. The bypass pipe is recoupled to a second portion of the exhaust pipe. The second portion is located upstream of the second catalyst. The hydrocarbon adsorbent is provided in the bypass pipe. The switching controller is configured to switch a flow path of an exhaust gas to the bypass pipe based on a deterioration degree of the first catalyst.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority from Japanese Patent Application No. 2020-022991 filed on Feb. 14, 2020, the entire contents of which are hereby incorporated by reference.

BACKGROUND

The disclosure relates to an exhaust gas purification device including a three-way catalyst.

A three-way catalyst is provided in an exhaust pipe of a vehicle in order to remove hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides (NOx) contained in an exhaust gas (for example, Japanese Unexamined Patent Application Publication No. 2010-253447). The three-way catalyst oxidizes hydrocarbons into water and carbon dioxide (CO2), oxidizes carbon monoxide into carbon dioxide, and reduces nitrogen oxides into nitrogen (N2).

SUMMARY

An aspect of the disclosure provides an exhaust gas purification device including a first catalyst, a second catalyst, a bypass pipe, a hydrocarbon adsorbent, and a switching controller. The first catalyst is provided in an exhaust pipe. The second catalyst is provided downstream of the first catalyst in the exhaust pipe. The bypass pipe branches from a first portion of the exhaust pipe. The first portion is located upstream of the second catalyst. The bypass pipe is recoupled to a second portion of the exhaust pipe. The second portion is located upstream of the second catalyst. The hydrocarbon adsorbent is provided in the bypass pipe. The switching controller is configured to switch a flow path of an exhaust gas to the bypass pipe based on a deterioration degree of the first catalyst.

An aspect of the disclosure provides an exhaust gas purification device including a first catalyst, a second catalyst, a bypass pipe, a hydrocarbon adsorbent, and circuitry. The first catalyst is provided in an exhaust pipe. The second catalyst is provided downstream of the first catalyst in the exhaust pipe. The bypass pipe branches from a first portion of the exhaust pipe. The first portion is located upstream of the second catalyst. The bypass pipe is recoupled to a second portion of the exhaust pipe. The second portion is located upstream of the second catalyst. The hydrocarbon adsorbent is provided in the bypass pipe. The circuitry is configured to switch a flow path of an exhaust gas to the bypass pipe based on a deterioration degree of the first catalyst.

DETAILED DESCRIPTION

During an engine start, an air-fuel ratio is made rich in order to warm up the engine early. Therefore, during the engine start, an exhaust gas contains a relatively large amount of hydrocarbons.

On the other hand, during the engine start, a temperature of the exhaust gas is relatively low, and thus a hydrocarbon removal capacity of the three-way catalyst is lower than that during normal operation. When the three-way catalyst deteriorates, the hydrocarbon removal capacity decreases. To deal with this issue, a content of precious metal in the three-way catalyst is increased such that hydrocarbons can be removed during the engine start even if the three-way catalyst deteriorates. Therefore, cost of the three-way catalyst may increase.

It is desirable to provide an exhaust gas purification device that can improve a removal rate of hydrocarbons at low cost.

FIG.1illustrates an engine system100according to the present embodiment. InFIG.1, dashed arrows indicate signal flows.

As illustrated inFIG.1, the engine system100mounted on a vehicle is provided with an engine control unit (ECU)10implemented by a microcomputer including a central processing unit (CPU), a ROM storing a program, and a RAM serving as a work area. The overall engine E is controlled by the ECU10in an integrated manner. In the following, configurations and processing related to the present embodiment will be described in detail, and descriptions of configurations and processing unrelated to the present embodiment may be omitted.

The engine E constituting the engine system100includes a cylinder block102, a crankcase104, a cylinder head106, and an oil pan110. The crankcase104is permanently affixed to the cylinder block102. The cylinder head106is joined to the cylinder block102on a side opposite to the crankcase104. The oil pan110is joined to the crankcase104on a side opposite to the cylinder block102.

Multiple cylinder bores112are formed in the cylinder block102. In each cylinder bore112, a piston114is slidably supported by a connecting rod116. In the engine E, a space surrounded by the cylinder bore112, the cylinder head106, and a crown surface of the piston114is a combustion chamber118.

In the engine E, a space surrounded by the crankcase104and the oil pan110is a crank chamber120. A crankshaft122is rotatably supported in the crank chamber120. The piston114is coupled to the crankshaft122via the connecting rod116.

The cylinder head106is provided with an intake port124and an exhaust port126such that the intake port124and the exhaust port126communicate with the combustion chamber118. A tip end (that is, a head) of an intake valve128is located between the intake port124and the combustion chamber118. A tip end (that is, a head) of an exhaust valve130is located between the exhaust port126and the combustion chamber118.

An intake cam134a, a rocker arm134b, an exhaust cam136a, and a rocker arm136bare provided in a space surrounded by the cylinder head106and a head cover (not illustrated). The intake cam134afixed to an intake camshaft is in contact with the intake valve128via the rocker arm134b. The intake valve128moves in an axial direction along with rotation of the intake camshaft so as to open and close between the intake port124and the combustion chamber118. The exhaust cam136afixed to an exhaust camshaft is in contact with the exhaust valve130via the rocker arm136b. The exhaust valve130moves in the axial direction along with rotation of the exhaust camshaft so as to open and close between the exhaust port126and the combustion chamber118.

An intake pipe140including an intake manifold communicates with an upstream portion of the intake port124. A throttle valve142and an air cleaner144upstream of the throttle valve142are provided in the intake pipe140. The throttle valve142is opened and closed by an actuator according to an opening degree of an accelerator (not illustrated). Air purified by the air cleaner144is suctioned into the combustion chamber118through the intake pipe140and the intake port124.

The cylinder head106is provided with an injector150(that is, a fuel injector) and an ignition plug152. A fuel injection port of the injector150opens to the combustion chamber118. A tip end of the ignition plug152is located in the combustion chamber118. The fuel injected from the injector150into the combustion chamber118mixes with the air supplied from the intake port124to the combustion chamber118to form an air-fuel mixture. Then, the ignition plug152is ignited at a predetermined timing, and the generated air-fuel mixture is combusted in the combustion chamber118. With such combustion, the piston114reciprocates, and the reciprocation is converted into a rotational movement of the crankshaft122through the connecting rod116.

An exhaust pipe160including an exhaust manifold communicates with a downstream part of the exhaust port126. An exhaust gas purification device200is provided in the exhaust pipe160. The exhaust gas purification device200purifies an exhaust gas discharged from the exhaust port126. A specific configuration of the exhaust gas purification device200will be described in detail later. The exhaust gas purified by the exhaust gas purification device200is exhausted to the outside through a muffler164.

The engine system100is provided with an intake air amount sensor180, a throttle opening degree sensor182, a crank angle sensor184, and an accelerator opening degree sensor186.

The intake air amount sensor180detects an intake air amount flowing into the engine E. The throttle opening degree sensor182detects an opening degree of the throttle valve142. The crank angle sensor184detects a crank angle of the crankshaft122. The accelerator opening degree sensor186detects an opening degree of the accelerator (not illustrated).

The intake air amount sensor180, the throttle opening degree sensor182, the crank angle sensor184, and the accelerator opening degree sensor186are coupled to the ECU10and output signals indicating detected values to the ECU10, respectively.

The ECU10acquires the signals output from the intake air amount sensor180, the throttle opening degree sensor182, the crank angle sensor184, and the accelerator opening degree sensor186, and an air-fuel ratio sensor260, a downstream air-fuel ratio sensor262, and a temperature sensor264, which will be described later, and controls the engine E. The ECU10serves as a signal acquiring unit12and a drive controller14when controlling the engine E.

The signal acquiring unit12acquires the signals indicating the values detected by the intake air amount sensor180, the throttle opening degree sensor182, the crank angle sensor184, and the accelerator opening degree sensor186. The signal acquiring unit12derives a rotation speed of the engine E (that is, a rotation speed of the crankshaft) based on the signal indicating the crank angle acquired from the crank angle sensor184. The signal acquiring unit12also derives a load of the engine E (that is, an engine load) based on the signal indicating the intake air amount acquired from the intake air amount sensor180. Since various existing techniques may be used to calculate the engine load based on the intake air amount, the description thereof will be omitted here.

The drive controller14controls a throttle valve actuator (not illustrated), the injector150, and the ignition plug152based on the signals acquired by the signal acquiring unit12.

The ECU10serves as the signal acquiring unit12, a deterioration degree derivation unit270, and a switching controller272when serving as the exhaust gas purification device200(seeFIG.2). The deterioration degree derivation unit270and the switching controller272will be described later in detail.

Exhaust Gas Purification Device200

FIG.2illustrates the configuration of the exhaust gas purification device200according to the present embodiment. InFIG.2, dashed arrows indicate signal flows.

As illustrated inFIG.2, the exhaust gas purification device200includes a front stage catalyst210, a rear stage catalyst220, a bypass pipe230, a hydrocarbon adsorbent240, a switching valve250, the air-fuel ratio sensor260, the downstream air-fuel ratio sensor262, the temperature sensor264, the signal acquiring unit12, the deterioration degree derivation unit270, and the switching controller272.

The front stage catalyst210is provided in the exhaust pipe160. In one embodiment, the front stage catalyst210may serve as a “first catalyst”. The rear stage catalyst220is provided downstream of the front stage catalyst210in the exhaust pipe160. In one embodiment, the rear stage catalyst220may serve as a “second catalyst”. In other words, the rear stage catalyst220is provided between the front stage catalyst210and the muffler164in the exhaust pipe160.

The front stage catalyst210and the rear stage catalyst220are three-way catalysts. The front stage catalyst210and the rear stage catalyst220purify (remove) hydrocarbons, carbon monoxide, and nitrogen oxides contained in the exhaust gas. The front stage catalyst210and the rear stage catalyst220contain a precious metal material, an oxygen storage capacity (OSC) material, and alumina (Al2O3). The precious metal material contains any one or more of platinum (Pt), palladium (Pd), or rhodium (Rh). The OSC material contains a ceria (that is, cerium oxide (IV) (CeO2))-zirconia (that is, zirconium dioxide (ZrO2)) composite. Ceria has an oxygen storage capacity (OSC). The rear stage catalyst220has a smaller amount of precious metal material than that of the front stage catalyst210.

The bypass pipe230branches from between the front stage catalyst210and the rear stage catalyst220in the exhaust pipe160, and is recoupled to between the front stage catalyst210and the rear stage catalyst220in the exhaust pipe160. A branch position of the bypass pipe230is located upstream of a recoupling position of the bypass pipe230.

The hydrocarbon adsorbent240is provided in the bypass pipe230. The hydrocarbon adsorbent240adsorbs hydrocarbons at a predetermined adsorption temperature. The hydrocarbon adsorbent240desorbs hydrocarbons at a predetermined desorption temperature. The adsorption temperature is, for example, normal temperature (25° C.) or more and 200° C. or less. The desorption temperature is, for example, 300° C. or more and 400° C. or less. In the present embodiment, the hydrocarbon adsorbent240is zeolite (for example, molecular sieve).

The switching valve250is provided at the branch position between the exhaust pipe160and the bypass pipe230. The switching valve250switches a flow path of the exhaust gas between the exhaust pipe160and the bypass pipe230.

The air-fuel ratio sensor260detects an air-fuel ratio of the exhaust gas exhausted from the engine E. In the present embodiment, the air-fuel ratio sensor260detects the air-fuel ratio of the exhaust gas passing through an upstream side of the front stage catalyst210in the exhaust pipe160.

The downstream air-fuel ratio sensor262detects an oxygen concentration of the exhaust gas that has passed through the front stage catalyst210. In the present embodiment, the downstream air-fuel ratio sensor262detects the oxygen concentration of the exhaust gas passing between the front stage catalyst210and the branch position of the bypass pipe230in the exhaust pipe160.

The temperature sensor264detects a temperature of the exhaust gas introduced into the hydrocarbon adsorbent240. In the present embodiment, the temperature sensor264measures the temperature of the exhaust gas passing between the front stage catalyst210and the branch position of the bypass pipe230in the exhaust pipe160.

The signal acquiring unit12acquires signals indicating values detected by the air-fuel ratio sensor260, the downstream air-fuel ratio sensor262, and the temperature sensor264.

The deterioration degree derivation unit270derives a deterioration degree of the front stage catalyst210based on the detected value of the air-fuel ratio sensor260and the detected value of the downstream air-fuel ratio sensor262. As described above, the front stage catalyst210contains the OSC material. Therefore, when the front stage catalyst210is not deteriorated, the air-fuel ratio of the exhaust gas becomes a theoretical air-fuel ratio in a process in which the exhaust gas passes through the front stage catalyst210.

Therefore, for example, the deterioration degree derivation unit270derives a difference (hereinafter, referred to as “air-fuel ratio difference”) between the air-fuel ratio detected by the air-fuel ratio sensor260and an air-fuel ratio derived from the detected value of the downstream air-fuel ratio sensor262. Then, the deterioration degree derivation unit270derives the deterioration degree of the front stage catalyst210based on the air-fuel ratio difference. The air-fuel ratio difference decreases as the deterioration degree of the front stage catalyst210increase. The deterioration degree derivation unit270derives the air-fuel ratio difference when the air-fuel ratio detected by the air-fuel ratio sensor260is not the theoretical air-fuel ratio. The derived deterioration degree of the front stage catalyst210is stored in a memory (not illustrated).

Then, the switching controller272switches the flow path of the exhaust gas to the bypass pipe230based on the deterioration degree of the front stage catalyst210. In the present embodiment, the switching controller272controls the switching valve250based on the deterioration degree.

The switching controller272controls the switching valve250with reference to switching information stored in the memory. The switching information is information indicating a deterioration threshold Td and a temperature threshold Tt. The deterioration threshold Td is set to an upper limit value of the deterioration degree of the front stage catalyst210at which the front stage catalyst210can remove hydrocarbons to a target value even during the engine E start. The temperature threshold Tt is set to the desorption temperature of the hydrocarbon adsorbent240(for example, a predetermined value in a range of 300° C. (inclusive) to 400° C. (inclusive).

Then, when the deterioration degree of the front stage catalyst210is equal to or higher than the deterioration threshold Td and the temperature of the exhaust gas detected by the temperature sensor264is less than the temperature threshold Tt, the switching controller272controls the switching valve250to switch the flow path of the exhaust gas to the bypass pipe230until the temperature of the exhaust gas detected by the temperature sensor264reaches the temperature threshold Tt.

On the other hand, when the deterioration degree of the front stage catalyst210is equal to or higher than the deterioration threshold Td and the detected value of the temperature sensor264reaches the temperature threshold Tt (that is, the detected value is equal to or higher than the temperature threshold Tt), the switching controller272controls the switching valve250to switch the flow path of the exhaust gas to the exhaust pipe160. That is, the switching controller272stops introduction of the exhaust gas into the bypass pipe230.

When the deterioration degree of the front stage catalyst210is less than the deterioration threshold Td, the switching controller272sets the flow path of the exhaust gas to the exhaust pipe160regardless of the temperature of the exhaust gas detected by the temperature sensor264.

Method for Purifying Exhaust Gas

Next, a method for purifying an exhaust gas with the exhaust gas purification device200will be described. FIG. is a flowchart illustrating the method for purifying an exhaust gas. As illustrated inFIG.3, the method for purifying an exhaust gas includes a process of making a determination based on the deterioration degree (S110), a first temperature determination process (S120), a process of switching to the bypass pipe (S130), a second temperature determination process (S140), a process of switching to the exhaust pipe (S150), a process of determining if a condition is satisfied (S160), a process of determining whether to derive the deterioration degree (S170), a process of deriving the deterioration degree (S180), and a process of storing the deterioration degree (S190). The method for purifying an exhaust gas is started when receiving engine start input from the user. Hereinafter, each process will be described.

Process of Making Determination based on Deterioration Degree (S110)

The switching controller272determines whether the deterioration degree stored in the memory in a previous operation cycle is equal to or higher than the deterioration threshold Td. The term “operation cycle” refers to a period from a time of starting the engine E to a time of stopping the engine E. As a result, when determining that the deterioration degree is equal to or higher than the deterioration threshold Td (YES in S110), the switching controller272proceeds to the first temperature determination process (S120). On the other hand, when determining that the deterioration degree is not equal to or higher than the deterioration threshold Td, that is, is less than the deterioration threshold Td (NO in S110), the switching controller272proceeds to the process of switching to the exhaust pipe (S150).

First Temperature Determination Process (S120)

The switching controller272determines whether a temperature Tex of the exhaust gas introduced into the hydrocarbon adsorbent240(a temperature of the exhaust gas detected by the temperature sensor264) is equal to or less than the temperature threshold Tt. As a result, when determining that the temperature Tex is equal to or less than the temperature threshold Tt (YES in S120), the switching controller272proceeds to the process of switching to the bypass pipe (S130). On the other hand, when determining that the temperature Tex is not equal to or less than the temperature threshold Tt, that is, exceeds the temperature threshold Tt (NO in S120), the switching controller272proceeds to the process of switching to the exhaust pipe (S150).

The switching controller272executes the first temperature determination process S120, so that it is possible to avoid a situation in which the exhaust gas is introduced into the hydrocarbon adsorbent240when the engine E is already warmed up since a time from the previous operation cycle to a current operation cycle is short.

Process of Switching to Bypass Pipe (S130)

The switching controller272controls the switching valve250to switch the flow path of the exhaust gas to the bypass pipe230(that is, the hydrocarbon adsorbent240). Second Temperature Determination Process (S140)

The switching controller272determines whether the temperature Tex of the exhaust gas introduced into the hydrocarbon adsorbent240exceeds the temperature threshold Tt. Then, the switching controller272waits until the temperature Tex exceeds the temperature threshold Tt (NO in S140), and once the temperature Tex exceeds the temperature threshold Tt (YES in S140), the switching controller272proceeds to the process of switching to the exhaust pipe (S150).

Process of Switching to Exhaust Pipe (S150)

The switching controller272controls the switching valve250to switch the flow path of the exhaust gas to the exhaust pipe160.

Process of Determining if Condition is Satisfied (S160)

The switching controller272determines whether a condition to be satisfied when the deterioration degree the front stage catalyst210is derived is satisfied. Hereinafter, the condition to be satisfied when the deterioration degree the front stage catalyst210is derived will be referred to as a “derivation condition”. The derivation condition is, for example, that the air-fuel ratio is not the theoretical air-fuel ratio for a normal operation. Then, the switching controller272waits until the derivation condition is satisfied (NO in S160), and once the derivation condition is satisfied (YES in S160), the switching controller272proceeds to the process of determining whether to derive the deterioration degree (S170).

Process of Determining Whether to Derive Deterioration Degree (S170)

The switching controller272determines whether the deterioration degree is already derived in the current operation cycle. When determining that the deterioration degree is not derived yet (NO in S170), the switching controller272proceeds to the process of deriving the deterioration degree (S180). On the other hand, when determining that the deterioration degree is already derived (YES in S170), the switching controller272ends the method for purifying an exhaust gas.

Process of Deriving Deterioration Degree (S180)

The deterioration degree derivation unit270derives the air-fuel ratio difference based on the detected value of the air-fuel ratio sensor260and the detected value of the downstream air-fuel ratio sensor262, and derives the deterioration degree of the front stage catalyst210based on the air-fuel ratio difference.

Process of Storing Deterioration Degree (S190)

The deterioration degree derivation unit270overwrites the deterioration degree derived in the process of deriving the deterioration degree (S180) in the memory, and ends the method for purifying an exhaust gas.

As described above, the exhaust gas purification device200of the present embodiment purifies the exhaust gas with the front stage catalyst210and the rear stage catalyst220until the front stage catalyst210deteriorates. Then, if the front stage catalyst210deteriorates, the exhaust gas purification device200causes the hydrocarbon adsorbent240to adsorb hydrocarbons that cannot be purified by the front stage catalyst210during the engine start. With this configuration, the exhaust gas purification device200can improve a removal rate of hydrocarbons during the engine start without increasing an amount of the precious metal material of the front stage catalyst210. Therefore, the exhaust gas purification device200can improve the removal rate of hydrocarbons at low cost.

As described above, when the temperature threshold Tt is exceeded, since the temperature of the front stage catalyst210reaches an activation temperature, the front stage catalyst210can purify the exhaust gas even if the front stage catalyst210deteriorates. Therefore, when the temperature threshold Tt is exceeded, the switching controller272stops introduction of the exhaust gas into the hydrocarbon adsorbent240, so that it is possible to prevent a leakage of hydrocarbons while preventing deterioration of the hydrocarbon adsorbent240.

When the exhaust gas introduced into the hydrocarbon adsorbent240reaches about the temperature threshold Tt, hydrocarbons are desorbed from the hydrocarbon adsorbent240. At about the temperature threshold Tt, the rear stage catalyst220can purify the exhaust gas, so that the hydrocarbons desorbed from the hydrocarbon adsorbent240is purified by the rear stage catalyst220.

As described above, the bypass pipe230is provided between the front stage catalyst210and the rear stage catalyst220. That is, the hydrocarbon adsorbent240is provided between the front stage catalyst210and the rear stage catalyst220. With this configuration, the hydrocarbon adsorbent240can simply adsorb hydrocarbons that cannot be purified by the front stage catalyst210. Therefore, the exhaust gas purification device200can reduce a size of the hydrocarbon adsorbent240.

The above embodiment describes, as an example, the configuration in which the bypass pipe230is provided between the front stage catalyst210and the rear stage catalyst220in the exhaust pipe160. However, an installation position of the bypass pipe230is not limited to this configuration as long as the bypass pipe230branches from a first portion of the exhaust pipe160that is located upstream of the rear stage catalyst220and is recoupled to a second portion of the exhaust pipe160that is located upstream of the rear stage catalyst220.

Modification

FIG.4illustrates an exhaust gas purification device300of a modification. InFIG.4, signal flows are indicated by dashed arrows. As illustrated inFIG.4, the exhaust gas purification device300includes the front stage catalyst210, the rear stage catalyst220, a bypass pipe330, the hydrocarbon adsorbent240, the switching valve250, the air-fuel ratio sensor260, the downstream air-fuel ratio sensor262, a temperature sensor364, the signal acquiring unit12, the deterioration degree derivation unit270, and the switching controller272. Elements that are substantially the same as those of the exhaust gas purification device200are designated by the same reference numerals, and description thereof will be omitted.

In the modification, the bypass pipe330branches from a first portion of the exhaust pipe160that is located upstream of the front stage catalyst210and is recoupled to a second portion of the exhaust pipe160that is located upstream of the front stage catalyst210.

The temperature sensor364measures a temperature of an exhaust gas passing through an upstream of a branch position of the bypass pipe330in the exhaust pipe160.

As described above, the exhaust gas purification device300of the modification includes the bypass pipe330branching from the first portion of the exhaust pipe160which is located upstream of the front stage catalyst210and recoupled to the second portion of the exhaust pipe which is located upstream of the front stage catalyst210. Therefore, the hydrocarbon adsorbent240can adsorb most of hydrocarbons contained in the exhaust gas during the engine start. Accordingly, the exhaust gas purification device300can reduce an amount of a precious metal material of the rear stage catalyst220and improve a removal rate of hydrocarbons.

The embodiment of the disclosure has been described above with reference to the accompanying drawings. It is needless to say that the disclosure is not limited to such an embodiment. It is apparent that those skilled in the art would conceive various changes and modifications within the scope of the appended claims, and it is to be understood that such changes and modifications also fall within the technical scope of the disclosure.

The above embodiment describes, as an example, that the switching controller272stops the introduction of the exhaust gas into the bypass pipe230(that is, the hydrocarbon adsorbent240) when the temperature Tex of the exhaust gas introduced into the hydrocarbon adsorbent240is equal to or higher than the temperature threshold Tt. However, when the temperature of the front stage catalyst210is equal to or higher than the temperature threshold Tt, the switching controller272may allow the exhaust gas to pass through the front stage catalyst210and the hydrocarbon adsorbent240.

The above embodiment describes, as an example, that the OSC material contains the ceria-zirconia composite. However, the OSC material may simply contain ceria.

The above embodiment describes, as an example, that the deterioration degree derivation unit270derives the deterioration degree of the front stage catalyst210based on the air-fuel ratio difference. However, a method of deriving the deterioration degree of the front stage catalyst210by the deterioration degree derivation unit270is not limited to this method. For example, the deterioration degree derivation unit270may derive the deterioration degree of the front stage catalyst210based on a time during which the air-fuel ratio derived based on the oxygen concentration detected by the downstream air-fuel ratio sensor262is maintained at the theoretical air-fuel ratio. In this case, the time during which the air-fuel ratio is maintained at the theoretical air-fuel ratio decreases as the deterioration degree of the front stage catalyst210increases.

The above embodiment describes, as an example, that the exhaust gas purification device200includes the air-fuel ratio sensor260and the downstream air-fuel ratio sensor262. However, the exhaust gas purification device200is not limited in configuration as long as an oxygen concentration (air-fuel ratio) upstream of the front stage catalyst210and an oxygen concentration (air-fuel ratio) downstream of the front stage catalyst210can be measured. For example, the exhaust gas purification device200may include an oxygen sensor instead of the air-fuel ratio sensor260. The exhaust gas purification device200may include an oxygen sensor instead of the downstream air-fuel ratio sensor262. The exhaust gas purification device200may include a NOxsensor instead of the air-fuel ratio sensor260and the downstream air-fuel ratio sensor262.

The above embodiment and modification describe, as examples, that the exhaust gas purification devices200and300include the temperature sensors264and364, respectively. However, none of the temperature sensors264and364may be provided. For example, the exhaust gas purification devices200and300may estimate the temperature of the exhaust gas based on a combustion state of the engine E, so as to estimate the temperature of the exhaust gas introduced into the bypass pipes230and330.