Exhaust gas purification device

Abstract

The present invention relates to an exhaust gas purification device, which improves the removal rate of hydrocarbons at a low cost. The exhaust gas purification device (200) comprises: a first catalyst (front stage catalyst 210) arranged in an exhaust pipe (160), a bypass pipe (230) branched from the upstream side of the first catalyst in the exhaust pipe (160) and connected to the upstream side of the first catalyst in the exhaust pipe (160), a second catalyst (auxiliary catalyst 240) arranged in the bypass pipe (230), and a switching control unit (272) for switching the passage path of the exhaust gas to the bypass pipe (230) based on the degree of degradation of the first catalyst.

Description

Technical Field

[0001] The present invention relates to an exhaust gas purification device equipped with a three-way catalyst. Background Art

[0002] In order to remove hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides (NO x ), a three-way catalyst is provided in the exhaust pipe of a vehicle (for example, Patent Document 1). The three-way catalyst oxidizes hydrocarbons to form water and carbon dioxide (CO2), oxidizes carbon monoxide to form carbon dioxide, and reduces nitrogen oxides to form nitrogen (N2).

[0003] Prior art literature

[0004] Patent Literature

[0005] Patent Document 1: Japanese Patent Application Publication No. 2010-253447 Summary of the invention

[0006] Problems to be solved by the invention

[0007] However, when starting the engine, the air-fuel ratio is made rich in order to warm up the engine in advance. Therefore, when starting the engine, a relatively large amount of hydrocarbons is contained in the exhaust gas.

[0008] On the other hand, when the engine is started, the temperature of the exhaust gas is relatively low, so the hydrocarbon removal capability of the three-way catalyst is lower than that during normal operation. In addition, when the three-way catalyst deteriorates, the hydrocarbon removal capability decreases. Therefore, the content of the precious metal in the three-way catalyst is increased so that hydrocarbons can be removed when the engine is started even if the three-way catalyst deteriorates. Therefore, there is a problem that the cost of the three-way catalyst becomes high.

[0009] In view of such problems, an object of the present invention is to provide an exhaust gas purification device capable of improving the removal rate of hydrocarbons at low cost.

[0010] Solutions to Solve Problems

[0011] In order to solve the above-mentioned problems, the present invention provides an exhaust gas purification device, which comprises: a first catalyst, which is arranged in an exhaust pipe; a bypass pipe, which branches from the upstream side of the first catalyst in the exhaust pipe and is then connected to the upstream side of the first catalyst in the exhaust pipe; a second catalyst, which is arranged in the bypass pipe; and a switching control unit, which switches the passage path of the exhaust gas to the bypass pipe based on the degree of degradation of the first catalyst.

[0012] Furthermore, the switching control unit may switch the passage to the bypass pipe when the degree of degradation of the first catalyst is equal to or greater than a degradation threshold.

[0013] Furthermore, the switching control unit may stop the introduction of the exhaust gas into the bypass pipe when the temperature of the first catalyst is equal to or higher than a temperature threshold.

[0014] Alternatively, the temperature threshold may be increased as the degree of degradation of the first catalyst increases.

[0015] In addition, the second catalyst may also contain less OSC material than the first catalyst.

[0016] Effects of the Invention

[0017] According to the present invention, the hydrocarbon removal rate can be improved at low cost. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 It is a diagram for explaining an engine system according to an embodiment.

[0019] Figure 2 It is a diagram for explaining the structure of the exhaust gas purification device according to the embodiment.

[0020] Figure 3 This is a diagram for explaining the switching map.

[0021] Figure 4 This is a flowchart showing the process flow of the exhaust gas purification method.

[0022] Explanation of symbols

[0023] 200 Exhaust gas purification device

[0024] 210 Front stage catalyst (first catalyst)

[0025] 230 Bypass pipe

[0026] 240 Auxiliary catalyst (second catalyst)

[0027] 272 Switching Control Unit DETAILED DESCRIPTION

[0028] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. The dimensions, materials, and other specific numerical values ​​shown in the above-mentioned embodiments are merely examples for easy understanding of the invention, and do not limit the present invention unless otherwise specified. In addition, in this specification and the accompanying drawings, the same symbols are marked for elements having substantially the same functions and structures, and thus, repeated descriptions are omitted, and elements that are not directly related to the present invention are omitted from the drawings.

[0029] [Engine system 100]

[0030] Figure 1 : is a diagram for explaining the engine system 100 of the present embodiment. Figure 1 In the figure, the dotted arrows indicate the flow of signals.

[0031] like Figure 1 As shown, an engine system 100 mounted on a vehicle is provided with an ECU (Engine Control Unit) 10 composed of a microcomputer including a central processing unit (CPU), a ROM storing programs, etc., a RAM as a work area, etc. The ECU 10 is used to comprehensively control the entire engine E. However, the following will describe in detail the structure and processing related to the present embodiment, and the description of the structure and processing not related to the present embodiment will be omitted.

[0032] The engine E constituting the engine system 100 includes a cylinder block 102, a crankcase 104, a cylinder head 106, and an oil pan 110. The crankcase 104 is formed integrally with the cylinder block 102. The cylinder head 106 is joined to the side of the cylinder block 102 opposite to the crankcase 104. The oil pan 110 is joined to the side of the crankcase 104 opposite to the cylinder block 102.

[0033] A plurality of cylinder bores 112 are formed in the cylinder block 102. In each of the plurality of cylinder bores 112, a piston 114 is movably supported by a connecting rod 116. In the engine E, a space surrounded by the cylinder bores 112, the cylinder head 106, and the crown surface of the piston 114 is formed as a combustion chamber 118.

[0034] In the engine E, a space surrounded by the crankcase 104 and the oil pan 110 is formed as a crank chamber 120 . A crankshaft 122 is rotatably supported in the crank chamber 120 , and a piston 114 is connected to the crankshaft 122 via a connecting rod 116 .

[0035] In cylinder head 106, intake port 124 and exhaust port 126 are provided in communication with combustion chamber 118. The front end (beam portion) of intake valve 128 is located between intake port 124 and combustion chamber 118, and the front end (beam portion) of exhaust valve 130 is located between exhaust port 126 and combustion chamber 118.

[0036] In addition, an intake cam 134a, a rocker arm 134b, an exhaust cam 136a, and a rocker arm 136b are provided in a space surrounded by the cylinder head 106 and a cylinder head cover (not shown). The intake cam 134a fixed to the intake camshaft abuts against the intake valve 128 via the rocker arm 134b. The intake valve 128 moves axially with the rotation of the intake camshaft to open and close the intake port 124 and the combustion chamber 118. The exhaust cam 136a fixed to the exhaust camshaft abuts against the exhaust valve 130 via the rocker arm 136b. The exhaust valve 130 moves axially with the rotation of the exhaust camshaft to open and close the exhaust port 126 and the combustion chamber 118.

[0037] An intake pipe 140 including an intake manifold is connected to the upstream side of the intake port 124. A throttle valve 142 is provided in the intake pipe 140, and an air filter 144 is provided further upstream of the throttle valve 142. The throttle valve 142 is opened and closed by an actuator according to the opening degree of an accelerator pedal (not shown). The air purified by the air filter 144 is sucked into the combustion chamber 118 through the intake pipe 140 and the intake port 124.

[0038] The injector 150 (fuel injection unit) is provided in the cylinder head 106 in such a manner that the fuel injection port opens toward the combustion chamber 118, and the spark plug 152 is provided in the cylinder head 106 in such a manner that the front end is located in the combustion chamber 118. The fuel injected from the injector 150 to the combustion chamber 118 is mixed with the air supplied to the combustion chamber 118 from the intake port 124 to form a mixed gas. Then, at a predetermined timing, the spark plug 152 is ignited to cause the mixed gas generated in the combustion chamber 118 to burn. Due to this combustion, the piston 114 performs a reciprocating motion, and this reciprocating motion is converted into a rotational motion of the crankshaft 122 through the connecting rod 116.

[0039] An exhaust pipe 160 including an exhaust manifold is connected to the downstream side of the exhaust port 126. An exhaust gas purification device 200 is provided in the exhaust pipe 160. The exhaust gas purification device 200 purifies the exhaust gas discharged from the exhaust port 126. The specific structure of the exhaust gas purification device 200 will be described in detail later. The exhaust gas purified by the exhaust gas purification device 200 is exhausted to the outside through the muffler 164.

[0040] Furthermore, the engine system 100 is provided with an intake air amount sensor 180 , a throttle valve opening sensor 182 , a crank angle sensor 184 , and an accelerator pedal opening sensor 186 .

[0041] The intake air amount sensor 180 detects the amount of intake air flowing into the engine E. The throttle opening sensor 182 detects the opening of the throttle valve 142. The crank angle sensor 184 detects the crank angle of the crankshaft 122. The accelerator pedal opening sensor 186 detects the opening of an accelerator pedal (not shown).

[0042] The intake air amount sensor 180 , the throttle valve opening sensor 182 , the crank angle sensor 184 , and the accelerator pedal opening sensor 186 are connected to the ECU 10 , and output signals indicating detection values ​​to the ECU 10 .

[0043] The ECU 10 acquires signals output from the intake air amount sensor 180, the throttle opening sensor 182, the crank angle sensor 184, the accelerator pedal opening sensor 186, and the air-fuel ratio sensor 260, the downstream air-fuel ratio sensor 262, and the temperature sensor 264 described later, and controls the engine E. When controlling the engine E, the ECU 10 functions as a signal acquisition unit 12 and a drive control unit 14.

[0044] The signal acquisition unit 12 acquires signals indicating values ​​detected by the intake air amount sensor 180, the throttle valve opening sensor 182, the crank angle sensor 184, and the accelerator pedal opening sensor 186. In addition, the signal acquisition unit 12 derives the rotation speed of the engine E (rotation speed of the crankshaft) based on the signal indicating the crank angle obtained from the crank angle sensor 184. In addition, the signal acquisition unit 12 derives the load of the engine E (engine load) based on the signal indicating the intake air amount obtained from the intake air amount sensor 180. The technology for obtaining the engine load from the intake air amount described above can utilize various existing technologies, and therefore, the description thereof is omitted here.

[0045] The drive control unit 14 controls a throttle valve actuator (not shown), the injector 150 , and the spark plug 152 based on the signal acquired by the signal acquisition unit 12 .

[0046] When the ECU 10 functions as the exhaust gas purification device 200, it functions as the signal acquisition unit 12, the degradation degree derivation unit 270, and the switching control unit 272 (see Figure 2 The degradation degree deriving unit 270 and the switching control unit 272 will be described in detail later.

[0047] [Exhaust gas purification device 200]

[0048] Figure 2 2 is a diagram for explaining the structure of the exhaust gas purification device 200 according to the present embodiment. Figure 2 In the figure, the dotted arrows indicate the flow of signals.

[0049] like Figure 2 As shown, the exhaust gas purification device 200 includes: a front-stage catalyst 210, a rear-stage catalyst 220, a bypass pipe 230, an auxiliary catalyst 240, a switching valve 250, an air-fuel ratio sensor 260, a downstream air-fuel ratio sensor 262, a temperature sensor 264, a signal acquisition unit 12, a degradation degree derivation unit 270, and a switching control unit 272.

[0050] The front catalyst 210 (first catalyst) is provided in the exhaust pipe 160. The rear catalyst 220 is provided downstream of the front catalyst 210 in the exhaust pipe 160. In other words, the rear catalyst 220 is provided between the front catalyst 210 and the muffler 164 in the exhaust pipe 160.

[0051] The front catalyst 210 and the rear catalyst 220 are three-way catalysts. The front catalyst 210 and the rear catalyst 220 purify (remove) hydrocarbons, carbon monoxide and nitrogen oxides contained in the exhaust gas. The front catalyst 210 and the rear catalyst 220 include precious metal materials, OSC materials and aluminum oxide (Al2O3). The precious metal material includes any one or more of platinum (Pt), palladium (Pd), and rhodium (Rh). The OSC material includes cerium dioxide (cerium (IV) oxide, C e Ceria (セリア) has oxygen storage capacity (OSC: Oxygen Storage Capacity). The amount of precious metal material in the rear-stage catalyst 220 is less than that in the front-stage catalyst 210.

[0052] The bypass pipe 230 branches from the upstream side of the pre-catalyst 210 in the exhaust pipe 160, and is reconnected to the upstream side of the pre-catalyst 210 in the exhaust pipe 160. The branching portion of the bypass pipe 230 is located on the upstream side of the reconnecting portion.

[0053] The auxiliary catalyst 240 (second catalyst) is provided in the bypass pipe 230. The auxiliary catalyst 240 includes palladium and aluminum oxide. In the present embodiment, the auxiliary catalyst 240 does not include an OSC material.

[0054] The switching valve 250 is provided at a branching portion or an intersection portion between the exhaust pipe 160 and the bypass pipe 230. The switching valve 250 switches the passage (flow path) of the exhaust gas to the exhaust pipe 160 and the bypass pipe 230.

[0055] The air-fuel ratio sensor 260 detects the air-fuel ratio of the exhaust gas exhausted from the engine E. In the present embodiment, the air-fuel ratio sensor 260 detects the air-fuel ratio of the exhaust gas between the reconnection portion of the bypass pipe 230 and the pre-catalyst 210 in the exhaust pipe 160 .

[0056] The downstream air-fuel ratio sensor 262 detects the oxygen concentration of the exhaust gas that has passed through the front catalyst 210. In the present embodiment, the downstream air-fuel ratio sensor 262 detects the oxygen concentration of the exhaust gas that has passed between the front catalyst 210 and the rear catalyst 220 in the exhaust pipe 160.

[0057] The temperature sensor 264 detects the temperature of the front catalyst 210. The temperature of the exhaust gas discharged from the front catalyst 210 is actually equal to the temperature of the front catalyst 210. Therefore, in this embodiment, the temperature sensor 264 measures the temperature of the exhaust gas passing between the front catalyst 210 and the rear catalyst 220 in the exhaust pipe 160, and regards it as the temperature of the front catalyst 210.

[0058] The signal acquisition unit 12 acquires signals indicating values ​​detected by the air-fuel ratio sensor 260 , the downstream air-fuel ratio sensor 262 , and the temperature sensor 264 .

[0059] The degradation degree deriving unit 270 derives the degradation degree of the front catalyst 210 based on the detection value of the air-fuel ratio sensor 260 and the detection value of the downstream air-fuel ratio sensor 262. As described above, the front catalyst 210 contains the OSC material. Therefore, when the front catalyst 210 is not degraded, the air-fuel ratio of the exhaust gas becomes the theoretical air-fuel ratio while passing through the front catalyst 210.

[0060] Therefore, for example, the degradation degree deriving unit 270 derives the difference between the air-fuel ratio detected by the air-fuel ratio sensor 260 and the air-fuel ratio derived from the detection value of the downstream air-fuel ratio sensor 262 (hereinafter referred to as "air-fuel ratio difference"). Moreover, the degradation degree deriving unit 270 derives the degradation degree of the front-stage catalyst 210 based on the air-fuel ratio difference. In addition, the greater the degradation degree of the front-stage catalyst 210, the smaller the air-fuel ratio difference. In addition, the degradation degree deriving unit 270 derives the air-fuel ratio difference when the air-fuel ratio detected by the air-fuel ratio sensor 260 is not the theoretical air-fuel ratio. The derived degradation degree of the front-stage catalyst 210 is stored in a memory not shown.

[0061] Then, the switching control unit 272 switches the passage of the exhaust gas to the bypass pipe 230 (auxiliary catalyst 240) based on the degree of degradation of the pre-catalyst 210. In the present embodiment, the switching control unit 272 controls the switching valve 250 based on the degree of degradation.

[0062] Switching control unit 272 refers to a switching map stored in a memory to control switching valve 250. The switching map is information in which a degradation threshold value, a degree of degradation, and a temperature threshold value are associated with each other.

[0063] Figure 3 is a diagram illustrating the switching mapping. Figure 3 As shown, when the degree of degradation of the front catalyst 210 is lower than the degradation threshold value Td, the temperature threshold value Tt is set to 0° C. The degradation threshold value Td is set to an upper limit value of the degree of degradation of the front catalyst 210 that can remove hydrocarbons up to a target value even when the engine is started.

[0064] On the other hand, when the degree of degradation of the front-stage catalyst 210 is equal to or higher than the degradation threshold value Td, the temperature threshold value Tt is set higher as the degree of degradation increases. When the temperature threshold value Tt reaches a predetermined switching temperature, the temperature threshold value Tt is maintained at the switching temperature regardless of the degree of degradation of the front-stage catalyst 210. The switching temperature is a lower limit value (for example, a predetermined value of 400° C. or higher and 450° C. or lower) of the temperature of the front-stage catalyst 210 (temperature of the exhaust gas) that can remove hydrocarbons even if the front-stage catalyst 210 is degraded until the target value.

[0065] Moreover, when the degree of degradation of the front-stage catalyst 210 is above the degradation threshold Td and the detection value of the temperature sensor 264 is lower than the temperature threshold Tt, the switching control unit 272 controls the switching valve 250 and switches the passage path of the exhaust gas to the bypass pipe 230 until the detection value of the temperature sensor 264 reaches the temperature threshold Tt.

[0066] On the other hand, when the degree of degradation of the front catalyst 210 is equal to or greater than the degradation threshold value Td and the detection value of the temperature sensor 264 reaches the temperature threshold value Tt (when it is equal to or greater than the temperature threshold value Tt), the switching control unit 272 controls the switching valve 250 and switches the passage of the exhaust gas to the exhaust pipe 160. That is, the switching control unit 272 stops the introduction of the exhaust gas into the bypass pipe 230.

[0067] Furthermore, as described above, when the degree of degradation of the front catalyst 210 is lower than the degradation threshold value Td, the temperature threshold value Tt is set to 0° C. Therefore, when the degree of degradation of the front catalyst 210 is lower than the degradation threshold value Td, the switching control unit 272 sets the passage of the exhaust gas to the exhaust pipe 160 regardless of the detection value of the temperature sensor 264.

[0068] [Exhaust gas purification method]

[0069] Next, an exhaust gas purification method using the exhaust gas purification device 200 will be described. Figure 4 This is a flowchart showing the processing flow of the exhaust gas purification method.

[0070] like Figure 4 As shown, the exhaust gas purification method includes: degradation degree determination processing S110, first temperature determination processing S120, bypass pipe switching processing S130, second temperature determination processing S140, exhaust pipe switching processing S150, condition establishment determination processing S160, derivation determination processing S170, degradation degree derivation processing S180, degradation degree storage processing S190. In addition, the exhaust gas purification method starts when the user receives the engine start input. Below, each process is described.

[0071] [Degradation Degree Determination Process S110]

[0072] The switching control unit 272 determines whether the degree of degradation stored in the memory in the previous operation cycle is greater than the degradation threshold value Td. The operation cycle is the period from when the engine E is started to when it is stopped. As a result, if it is determined that the degree of degradation is greater than the degradation threshold value Td (Yes in S110), the switching control unit 272 transfers the process to the first temperature determination process S120. On the other hand, if it is determined that the degree of degradation is not greater than the degradation threshold value Td, that is, it is less than the degradation threshold value Td (No in S110), the switching control unit 272 transfers the process to the exhaust pipe switching process S150.

[0073] [First Temperature Determination Process S120]

[0074] The switching control unit 272 determines whether the temperature Tcat of the front catalyst 210 (the temperature of the exhaust gas detected by the temperature sensor 264) is less than or equal to the temperature threshold value Tt. As a result, if it is determined that the temperature Tcat is less than or equal to the temperature threshold value Tt (Yes in S120), the switching control unit 272 transfers the process to the bypass pipe switching process S130. On the other hand, if it is determined that the temperature Tcat is not less than or equal to the temperature threshold value Tt, that is, it exceeds the temperature threshold value Tt (No in S120), the switching control unit 272 transfers the process to the exhaust pipe switching process S150.

[0075] The switching control unit 272 executes the first temperature determination process S120 to shorten the time from the previous operation cycle to the current operation cycle, and can avoid introducing the exhaust gas into the auxiliary catalyst 240 when the engine E is already warmed up.

[0076] [Bypass pipe switching process S130]

[0077] The switching control unit 272 controls the switching valve 250 to switch the passage of the exhaust gas to the bypass pipe 230 (the auxiliary catalyst 240 ).

[0078] [Second Temperature Determination Process S140]

[0079] The switching control unit 272 determines whether the temperature Tcat of the front catalyst 210 exceeds the temperature threshold Tt. Then, the switching control unit 272 waits until the temperature Tcat exceeds the temperature threshold Tt (No in S140), and if the temperature Tcat exceeds the temperature threshold Tt (Yes in S140), the switching control unit 272 transfers the process to the exhaust pipe switching process S150.

[0080] [Exhaust pipe switching process S150]

[0081] Switching control unit 272 controls switching valve 250 to switch the passage of the exhaust gas to exhaust pipe 160 .

[0082] [Conditional Establishment Determination Process S160]

[0083] The switching control unit 272 determines whether the derivation condition of the degradation degree of the front catalyst 210 is satisfied. The derivation condition is, for example, when the theoretical air-fuel ratio is not in normal operation. In addition, the switching control unit 272 waits until the derivation condition is satisfied (No in S160), and if the derivation condition is satisfied (Yes in S160), the switching control unit 272 transfers the processing to the derivation determination processing S170.

[0084] [Derivation determination process S170]

[0085] The switching control unit 272 determines whether the derivation of the degree of degradation is completed in this operation cycle. As a result, if it is determined that the derivation is not completed (No in S170), the switching control unit 272 transfers the process to the derivation process of the degree of degradation S180. On the other hand, if it is determined that the derivation is completed (Yes in S170), the switching control unit 272 ends the exhaust gas purification method.

[0086] [Degradation Derivation Process S180]

[0087] The degradation degree deriving unit 270 derives an air-fuel ratio difference from the detection value of the air-fuel ratio sensor 260 and the detection value of the downstream air-fuel ratio sensor 262 , and derives the degradation degree of the front catalyst 210 based on the air-fuel ratio difference.

[0088] [Degradation degree storage process S190]

[0089] The degradation degree deriving unit 270 writes the degradation degree derived in the degradation degree deriving process S180 in the memory, and terminates the exhaust gas purification method.

[0090] As described above, the exhaust gas purification device 200 of the present embodiment purifies the exhaust gas using the front-stage catalyst 210 and the rear-stage catalyst 220 until the front-stage catalyst 210 deteriorates. Furthermore, if the front-stage catalyst 210 deteriorates, the exhaust gas purification device 200 purifies the exhaust gas using the auxiliary catalyst 240 in addition to the front-stage catalyst 210 and the rear-stage catalyst 220 when the engine is started. Thus, the exhaust gas purification device 200 can improve the removal rate of hydrocarbons when the engine is started without increasing the amount of precious metal material of the front-stage catalyst 210. Therefore, the exhaust gas purification device 200 can improve the removal rate of hydrocarbons at a low cost.

[0091] In addition, the auxiliary catalyst 240 is provided to remove hydrocarbons contained in the exhaust gas when the engine is started. Therefore, the auxiliary catalyst 240 does not need to set the catalyst atmosphere to the theoretical air-fuel ratio. Therefore, as described above, the auxiliary catalyst 240 does not contain an OSC material. Thus, the auxiliary catalyst 240 can be manufactured at a low cost.

[0092] In addition, the auxiliary catalyst 240 contains palladium having high hydrocarbon-removing performance. Therefore, the auxiliary catalyst 240 can efficiently remove hydrocarbons contained in the exhaust gas.

[0093] In addition, as described above, when the temperature threshold value Tt is exceeded, the temperature of the front catalyst 210 reaches the activation temperature, so the front catalyst 210 can purify the exhaust gas even if it is degraded. Therefore, the switching control unit 272 stops the introduction of the exhaust gas to the auxiliary catalyst 240 when the temperature threshold value Tt is exceeded, thereby preventing the auxiliary catalyst 240 from being degraded and preventing the leakage of hydrocarbons.

[0094] In addition, as described above, the bypass pipe 230 is provided on the upstream side of the front catalyst 210. That is, the auxiliary catalyst 240 is provided on the upstream side of the front catalyst 210. Thus, the exhaust gas purification device 200 can warm up the auxiliary catalyst 240 in advance when the engine is started. Therefore, the auxiliary catalyst 240 can remove hydrocarbons immediately when the engine is started.

[0095] The preferred embodiments of the present invention are described above with reference to the accompanying drawings, but the present invention is not limited to the above-mentioned embodiments. Those skilled in the art will know that various variations or modifications can be thought of within the scope of the claims, and they will also fall within the technical scope of the present invention.

[0096] In addition, in the above embodiment, the following case is exemplified: when the temperature of the front-stage catalyst 210 is above the temperature threshold value Tt, the switching control unit 272 stops the introduction of the exhaust gas into the bypass pipe 230 (auxiliary catalyst 240). However, the switching control unit 272 may allow the exhaust gas to pass through the front-stage catalyst 210 and the auxiliary catalyst 240 when the temperature of the front-stage catalyst 210 is above the temperature threshold value Tt.

[0097] In the above embodiment, the case where the temperature threshold Tt is set higher as the degree of degradation of the front catalyst 210 increases is exemplified. However, the temperature threshold Tt may be a fixed value. For example, the temperature threshold Tt may be set to a switching temperature.

[0098] In addition, in the above-mentioned embodiment, the case where the OSC material includes the ceria-zirconia composite is exemplified, but the OSC material only needs to include at least ceria.

[0099] In the above embodiment, the auxiliary catalyst 240 does not contain an OSC material. However, the auxiliary catalyst 240 only needs to contain less OSC material than the pre-catalyst 210. In this case, the auxiliary catalyst 240 can be manufactured at low cost. In addition, the auxiliary catalyst 240 may also contain an OSC material.

[0100] In the above embodiment, an example is given in which the auxiliary catalyst 240 includes palladium. However, the auxiliary catalyst 240 may include either or both of rhodium and platinum instead of or in addition to palladium.

[0101] In addition, in the above embodiment, an example is given of a case where the degradation degree deriving unit 270 derives the degradation degree of the front-stage catalyst 210 based on the air-fuel ratio difference. However, the method of deriving the degradation degree of the front-stage catalyst 210 by the degradation degree deriving unit 270 is not limited. For example, the degradation degree deriving unit 270 may also derive the degradation degree of the front-stage catalyst 210 based on the following time: the time for maintaining the air-fuel ratio derived based on the oxygen concentration detected by the downstream air-fuel ratio sensor 262 at the theoretical air-fuel ratio. In this case, the greater the degradation degree of the front-stage catalyst 210, the shorter the time for maintaining the theoretical air-fuel ratio.

[0102] In addition, in the above-mentioned embodiment, an example is given of a case where the exhaust gas purification device 200 is provided with an air-fuel ratio sensor 260 and a downstream air-fuel ratio sensor 262. However, as long as the exhaust gas purification device 200 can measure the oxygen concentration (air-fuel ratio) on the upstream side of the front-stage catalyst 210 and the oxygen concentration (air-fuel ratio) on the downstream side of the front-stage catalyst 210, the structure is not limited. For example, the exhaust gas purification device 200 may also be provided with an oxygen sensor instead of the air-fuel ratio sensor 260. In addition, the exhaust gas purification device 200 may also be provided with an oxygen sensor instead of the downstream air-fuel ratio sensor 262. In addition, the exhaust gas purification device 200 may also be provided with a NOx sensor instead of the air-fuel ratio sensor 260 and the downstream air-fuel ratio sensor 262.

[0103] In addition, in the above embodiment, an example is given of a case where the temperature sensor 264 measures the temperature of the exhaust gas passing between the front catalyst 210 and the rear catalyst 220 in the exhaust pipe 160. However, the temperature sensor 264 only needs to be able to obtain the temperature of the front catalyst 210. For example, the temperature sensor 264 may also measure the temperature of the front catalyst 210. In addition, the temperature sensor 264 may also measure the temperature of the exhaust gas on the upstream side of the front catalyst 210.

[0104] In addition, in the above embodiment, an example is given of a case where the exhaust gas purification device 200 is provided with the temperature sensor 264. However, the temperature sensor 264 is not a necessary structure. For example, the exhaust gas purification device 200 may also estimate the temperature of the exhaust gas based on the combustion state of the engine E, thereby estimating the temperature of the pre-stage catalyst 210.

[0105] Industrial Applicability

[0106] The present invention can be applied to an exhaust gas purification device equipped with a three-way catalyst.

Claims

1. An exhaust gas purification device, comprising: a first catalyst disposed in the exhaust pipe; a bypass pipe branched from the upstream side of the first catalyst in the exhaust pipe and connected to the upstream side of the first catalyst in the exhaust pipe; a second catalyst disposed in the bypass pipe; a switching valve that switches a passage path of exhaust gas on the upstream side of the first catalyst in the exhaust pipe between a first path that does not pass through the bypass pipe and a second path that passes through the bypass pipe; a downstream air-fuel ratio sensor that is disposed on the downstream side of the first catalyst in the exhaust pipe; a degradation degree deriving unit configured to derive a degradation degree of the first catalyst; a switching control unit that controls the switching valve based on the degree of degradation; a memory storing the degradation degree; as well as a temperature sensor, which detects the temperature of the exhaust gas, wherein, when the degree of degradation stored in the memory is lower than a degradation threshold, the switching control unit controls the switching valve to set the passage of the exhaust gas to the first passage regardless of the detection value of the temperature sensor; The switching control unit controls the switching valve to set the passage of the exhaust gas to the second passage when the degree of degradation stored in the memory is equal to or greater than the degradation threshold and the detection value of the temperature sensor is lower than the temperature threshold. The switching control unit controls the switching valve to set the passage of the exhaust gas to the first passage when the degree of degradation stored in the memory is equal to or greater than the degradation threshold and the detection value of the temperature sensor is equal to or greater than the temperature threshold. The degradation degree deriving unit derives the degradation degree of the first catalyst based on: based on an upstream and downstream air-fuel ratio difference derived from detection values ​​of both an upstream air-fuel ratio sensor disposed on the upstream side of the first catalyst in the exhaust pipe and a downstream air-fuel ratio sensor disposed on the downstream side of the first catalyst in the exhaust pipe, or The time during which the air-fuel ratio derived based on the oxygen concentration detected by the downstream air-fuel ratio sensor disposed on the downstream side of the first catalyst in the exhaust pipe is maintained at the stoichiometric air-fuel ratio.

2. The exhaust gas purification device according to claim 1, wherein: The temperature threshold is set to become higher as the degree of degradation stored in the memory increases, and has a predetermined switching temperature as an upper limit.

3. The exhaust gas purification device according to claim 2, wherein: The switching temperature is a lower limit value of the temperature of the first catalyst at which hydrocarbons can be removed up to a target value even if the first catalyst is degraded.

4. The exhaust gas purification device according to any one of claims 1 to 3, wherein: The second catalyst does not comprise an OSC material.

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