JP2012036835A - Exhaust emission control device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To accurately determine the existence of failure of an exhaust sensor with a simple configuration concerning an exhaust emission control device.SOLUTION: The exhaust emission control device is provided with: a selective reduction catalyst 5 provided in an exhaust passage 15 of an on-vehicle engine 20, adsorbing ammonia and reducing nitrogen oxide in an exhaust gas by using the ammonia as a reducing agent; and a catalyst temperature detection unit 8 for detecting a catalyst temperature of the selective reduction catalyst 5. In addition, the exhaust emission control device is provided with: an adsorption amount calculation unit 7c for calculating an adsorption amount of ammonia adsorbed on the selective reduction catalyst 5; a condition detection unit 7d for detecting an idle operation condition of the engine 20; and the exhaust gas sensor 9 provided on a downstream side rather than the selective reduction catalyst 5 and detecting nitrogen oxide concentration. Furthermore, the exhaust emission control device is provided with a determining unit 7e for determining the existence of failure of the exhaust sensor 9 on the basis of the catalyst temperature, the adsorption amount and the nitrogen oxide concentration when the engine 20 is in the idle operation condition.

Description

  The present invention relates to an exhaust purification device for purifying nitrogen oxides contained in engine exhaust.

Conventionally, an exhaust purification system using a urea addition type selective reduction catalyst (urea SCR (Selective Catalytic Reduction) catalyst) as a catalyst for removing nitrogen oxide (NOx) contained in exhaust gas is known. That is, in order to selectively react nitrogen oxides with the selective reduction catalyst, an aqueous urea solution as a reducing agent is injected into the exhaust passage upstream of the selective reduction catalyst, and ammonia (NH ₃ ) is produced by hydrolysis of urea. The nitrogen oxide is generated and reduced to nitrogen and water with ammonia. A system using a selective reduction catalyst is effective for purifying nitrogen oxides in an atmosphere having a relatively high oxygen concentration, such as exhaust of a diesel engine, or at a low temperature.

  Some of the exhaust purification systems described above are provided with an exhaust sensor (for example, a NOx sensor) on the downstream side of the catalyst for confirming the component of exhaust actually discharged from the exhaust passage to the outside of the vehicle. For example, it is confirmed that the selective reduction catalyst is functioning normally when the concentration of nitrogen oxide detected by this sensor is less than a predetermined value. However, since the exhaust sensor is provided on the downstream side of the selective reduction catalyst, the function of the selective reduction catalyst cannot be accurately diagnosed unless it is ensured that the exhaust sensor is normal (not faulty).

  Therefore, in the state in which the reduction reaction on the selective reduction catalyst is stopped, the nitrogen oxide concentration is detected or estimated on each of the upstream side and the downstream side of the selective reduction catalyst, and these are compared to detect a failure of the exhaust sensor. A technique for detection has been proposed (see, for example, Patent Document 1).

JP 2009-121413 A (paragraph 0061, etc.)

  However, since the selective reduction catalyst has a characteristic of adsorbing ammonia generated by hydrolysis of urea, even if the addition of urea water is stopped, a reduction reaction of nitrogen oxides by ammonia adsorbed in advance occurs. That is, even if the exhaust sensor has not failed, the nitrogen oxide concentration on the downstream side of the selective reduction catalyst is reduced by the reduction reaction on the selective reduction catalyst, and a value different from the nitrogen oxide concentration on the upstream side is reduced. May be detected.

  Thus, it is difficult for the conventional technology to accurately determine the failure of the exhaust sensor. Therefore, if an exceptional value is detected by the exhaust sensor, it is known that a fault has occurred in the exhaust purification system, but the cause is in the exhaust sensor or in a device other than the exhaust sensor. It is difficult to identify if there is.

The present invention has been devised in view of the above problems, and an object of the present invention is to provide an exhaust emission control device capable of accurately determining the presence or absence of an exhaust sensor failure with a simple configuration.
The present invention is not limited to this purpose, and is a function and effect derived from each configuration shown in the embodiments for carrying out the invention described later, and other effects of the present invention are to obtain a function and effect that cannot be obtained by conventional techniques. Can be positioned.

(1) An exhaust emission control device disclosed herein is provided in an exhaust passage of an engine mounted on a vehicle, adsorbs ammonia, and uses the ammonia as a reducing agent to reduce nitrogen oxide in exhaust, Catalyst temperature detecting means for detecting the catalyst temperature of the selective reduction catalyst.
Further, an adsorption amount calculation means for calculating the adsorption amount of the ammonia adsorbed on the selective reduction catalyst, a state detection means for detecting an idle operation state of the engine, and a downstream of the exhaust passage from the selective reduction catalyst. And an exhaust sensor that detects the concentration of nitrogen oxides contained in the exhaust.
Furthermore, a determination unit that determines whether or not the exhaust sensor has failed is provided based on the catalyst temperature, the adsorption amount, and the nitrogen oxide concentration when the engine is in the idle operation state.
The catalyst temperature detecting means may directly detect the catalyst temperature or may detect an estimated value by calculation.

(2) The determination means is such that the engine is in the idle operation state, the catalyst temperature is equal to or higher than an activation temperature capable of reducing the nitrogen oxides, and the adsorption amount is equal to or greater than a predetermined amount. And when the said nitrogen oxide density | concentration is less than predetermined concentration, it determines with the said exhaust sensor being normal.
In this case, during the idling operation in which the exhaust flow rate and the exhaust temperature are stable, the nitrogen oxide is reduced by using the ammonia adsorbed on the selective reduction catalyst, thereby improving the purification rate of the nitrogen oxide. Therefore, the nitrogen oxide concentration is less than a predetermined concentration unless the exhaust sensor is out of order.

(3) Further, when the state detection means continues the idle rotation of the engine based on the traveling speed of the vehicle and the engine load (for example, accelerator opening, throttle opening, etc.) for a predetermined time, the idle operation state Is detected.
(4) Moreover, it has an alerting | reporting means which alert | reports the result of determination by the said determination means to a passenger | crew.
(5) Moreover, it comprises an error calculation means for calculating a difference between the nitrogen oxide concentration and the predetermined concentration when the determination means determines that the exhaust sensor has failed, and the determination means includes the error calculation. Based on the difference calculated by the means, the degree of failure of the exhaust sensor is determined.

(1) According to the disclosed exhaust purification device, it is possible to accurately distinguish between a failure of the selective reduction catalyst and the engine provided on the exhaust passage and a failure of the exhaust sensor with a simple configuration. Can be identified. Thereby, the reliability of the value of the nitrogen oxide concentration detected by the exhaust sensor can be improved, and more accurate control is possible.
(2) Further, it is possible to reliably determine whether or not the detection value of the exhaust sensor is appropriate by utilizing the improvement in the purification rate of nitrogen oxides under stable engine operating conditions. A failure of the exhaust sensor can be identified well.

(3) Also, by setting the conditions for the idling operation state, it is possible to accurately grasp the state in which the nitrogen oxide purification rate is improved, and the accuracy of the exhaust sensor failure determination can be further increased.
(4) Further, when it is determined that the exhaust sensor has failed, it is possible to prompt maintenance or replacement of the exhaust sensor by notifying the passenger to that effect.
(5) Further, when a failure of the exhaust sensor is determined by determination by the determination means, the degree of failure and the type of failure can be estimated by calculating the difference between the detected value of the exhaust sensor and a predetermined concentration. It becomes possible.

It is a figure which illustrates typically the whole structure of the exhaust-air-purification device concerning one embodiment. 2 is a graph illustrating the relationship between the catalyst temperature of the selective reduction catalyst of FIG. 1 and the maximum ammonia adsorption amount. 2 is a graph illustrating the relationship between the operating state of the engine of FIG. 1 and the concentration of nitrogen oxides contained in exhaust gas. 2 is a graph illustrating the change over time in the amount of ammonia adsorbed on the selective reduction catalyst of FIG. 1. It is a flowchart which illustrates the control content in the exhaust gas purification apparatus of FIG. 2 is a graph for explaining exhaust performance during a running test of a vehicle equipped with the exhaust emission control device of FIG. 1, (a) is a temporal variation of the engine speed, (b) is a calculated value of engine out NOx and NOx sensor This is a change with time in the concentration of NOx detected in FIG.

  The disclosed exhaust purification device will be described below with reference to the drawings. Note that the embodiment described below is merely an example, and there is no intention to exclude various modifications and technical applications that are not explicitly described in the following embodiment.

[1. Constitution]
[1-1. Intake and exhaust system]
The exhaust emission control device 10 of this embodiment is applied to the vehicle intake / exhaust system illustrated in FIG. An engine 20 in FIG. 1 is a diesel engine using light oil as a fuel, and an exhaust passage 16 and an intake passage 17 are connected to the engine 20. Intake air is introduced into the combustion chamber of each cylinder of the engine 20 via the intake passage 17, and exhaust gas after combustion is discharged to the outside via the exhaust passage 16.

  A turbocharger 18, a DPF (Diesel Particulate Filter) device 1, and an SCR (Selective Catalytic Reduction) device 4 are interposed in the exhaust passage 16 in order from the upstream side of the exhaust flow. The DPF device 1 is a continuous regeneration type filtration device, and the SCR device 4 is a purification device for removing NOx (nitrogen oxide) contained in the exhaust gas.

  The turbocharger 18 is a supercharger interposed so as to straddle the exhaust passage 16 and the intake passage 17. The turbocharger 18 rotates the turbine with the exhaust pressure of the exhaust gas flowing through the exhaust passage 16 and uses the rotational force. By driving the compressor, the intake air from the intake passage 17 is compressed and the engine 20 is supercharged.

An injector 11 is provided between the DPF device 1 and the SCR device 4 on the exhaust passage 16. The injector 11 is a nozzle that sprays and supplies an aqueous solution of urea [CO (NH ₂ ) ₂ ] into the exhaust gas. Urea added to the exhaust gas is hydrolyzed by exhaust heat to generate ammonia.

A temperature sensor 8 that detects an exhaust gas temperature T ₁ flowing into the SCR catalyst 5 is provided immediately upstream of the SCR device 4. Further, on the downstream side of the SCR device 4, NOx sensor 9 (exhaust gas sensor) is provided for detecting the concentration C ₁ of the NOx contained in the exhaust gas. The exhaust temperature T ₁ and the concentration C ₁ detected by these sensors 8 and 9 are input to the controller 7 described later.

An air flow sensor 14 is provided at an arbitrary position on the intake passage 17 (for example, upstream of the throttle valve). The air flow sensor 14 is a flow rate sensor that detects an intake air amount Q introduced into the cylinder of the engine 20. Further, an accelerator opening sensor 12 and a vehicle speed sensor 19 are provided at arbitrary positions of the intake / exhaust system. The accelerator opening sensor 12 detects the operation amount θ _AC (accelerator opening) of the accelerator pedal by the driver, and the vehicle speed sensor 19 detects the vehicle speed V based on, for example, the rotational speed of the wheel.

Further, in the vicinity of the crankshaft 21 of the engine 20, an engine speed sensor 13 for detecting an engine speed N _e is provided. The accelerator opening θ _AC detected by the accelerator opening sensor 12, the engine speed N _e detected by the engine speed sensor 13, the intake air amount Q detected by the air flow sensor 14, and the vehicle speed V detected by the vehicle speed sensor 19. Is input to the controller 7 described later.

[1-2. DPF device]
The DPF device 1 includes a DOC (Diesel Oxidation Catalyst) catalyst 2 disposed on the upstream side and a filter 3 disposed on the downstream side. This DPF device 1 has both a function of collecting PM (Particulate Matter, particulate matter) contained in exhaust gas and a function of continuously oxidizing and removing the collected PM. In addition, PM is a particulate substance in which fuel, lubricating oil components, sulfur compounds, and the like that remain unburned around black smoke (soot) made of carbon adhere.

The DOC catalyst 2 is an oxidation catalyst having an oxidizing ability with respect to components in exhaust gas, and is a catalyst in which a catalyst material is supported on a honeycomb-shaped carrier made of metal, ceramics or the like. Examples of components in the exhaust gas oxidized by the DOC catalyst 2 include nitrogen monoxide (NO) and hydrocarbons in unburned fuel. For example, when nitrogen monoxide is oxidized by the DOC catalyst 2, nitrogen dioxide (NO ₂ ) is generated. In addition, the chemical reaction formula of the oxidation reaction of nitric oxide in the DOC catalyst 2 is illustrated below.
2NO + O ₂ → 2NO ₂ (Formula 1)

  The filter 3 is a porous filter (for example, a ceramic filter) that collects PM. The interior of the filter 3 is divided into a plurality along the flow direction of the exhaust gas by a porous wall. A large number of pores having a size commensurate with the particulates of PM are formed in the wall. When exhaust passes near or inside the wall, PM is collected on the wall and on the wall surface, and the exhaust is filtered.

In addition, as the filter 3 of the DPF device 1, a filter having a catalytic noble metal supported on the surface thereof may be used. In this case, exhaust particulates are incinerated using nitrogen dioxide or the like in the exhaust as an oxidizing agent. Thereby, PM collected by the filter 3 is removed, and the filter 3 is regenerated and purified. The chemical reaction formula of the PM combustion reaction in the filter 3 is illustrated below.
C + 2NO ₂ → 2NO + CO ₂ (Formula 2)
C + O ₂ → CO ₂ ... (Formula 3)

In this embodiment, the filter 3 is regenerated and purified by a continuous regeneration method in which PM is burned with NO ₂ during normal driving of the vehicle, and the filter 3 is regenerated and purified by a forced regeneration method in which PM is burned with O _{2 as} necessary. Is done. The reaction shown in Formula 2 is a PM combustion reaction at a low temperature, and proceeds mainly during regeneration control by the continuous regeneration system. Moreover, the reaction shown in Formula 3 is a PM combustion reaction at a high temperature, and proceeds mainly during regeneration control by the forced regeneration method.

[1-3. SCR device]
The SCR device 4 incorporates an SCR catalyst 5 (selective reduction catalyst) disposed on the upstream side and a CUC (Clean Up Catalyst) catalyst 6 disposed on the downstream side thereof.
The SCR catalyst 5 is a urea-added nitrogen oxide selective reduction catalyst that adsorbs ammonia supplied from the upstream side and reduces NOx in the exhaust gas to nitrogen using the ammonia as a reducing agent. As shown in FIG. 2, the maximum value X _{MAX of the} amount of ammonia adsorbed on the SCR catalyst 5 increases as the catalyst temperature of the SCR catalyst 5 decreases, and decreases as the temperature increases. Further, the catalyst temperature is above a predetermined desorption temperature T _A (e.g., above about 400 ° C.) becomes, the adsorption amount of ammonia becomes substantially zero.

Further, the reduction reaction of NOx in the SCR catalyst 5, catalyst temperature of SCR catalyst 5 is higher than a predetermined activation temperature T _B (e.g., above about 200 ° C.) occurs when a reaction rate higher catalyst temperature is a high temperature Rises. A chemical reaction formula of urea hydrolysis and NOx reduction reaction in the SCR catalyst 5 is illustrated below.
CO (NH ₂ ) ₂ + H ₂ O → 2NH ₃ + CO ₂ (Formula 4)
NO + NO ₂ + 2NH ₃ → 2N ₂ + 3H ₂ O (Formula 5)

The CUC catalyst 6 is an oxidation catalyst for removing excess ammonia (slip NH ₃ ) in the reduction reaction of the SCR catalyst 5. In addition, the chemical reaction formula of the oxidation reaction of ammonia in the CUC catalyst 6 is illustrated below.
4NH ₃ + 3O ₂ → 2N ₂ + 6H ₂ O (Formula 6)

[2. controller]
The air-fuel ratio of the air-fuel mixture of the engine 20 and the intake air amount, fuel injection amount, fuel injection timing, ignition timing, exhaust temperature, etc. related to the combustion reaction in the cylinder are controlled by a controller 7 [ECU, Engine (electronic) Control Unit]. Be controlled. The controller 7 is an electronic control unit that comprehensively manages the intake / exhaust system including the engine 20, and is an LSI device in which a microprocessor, ROM, RAM, and the like are integrated.

  The temperature sensor 8, NOx sensor 9, accelerator opening sensor 12, engine speed sensor 13, air flow sensor 14, and vehicle speed sensor 19 are connected to the input side of the controller 7. Further, for example, a notification device 15 (notification means) attached to an instrument panel in the passenger compartment is connected to the output side of the controller 7. The notification device 15 includes a display device such as a display and a lamp and an acoustic device such as a speaker and a buzzer.

  In the present embodiment, a diagnosis control function of the NOx sensor 9 mainly relating to catalyst control will be described among the functions implemented in the controller 7. The diagnostic control of the NOx sensor 9 is a control for diagnosing whether or not the NOx sensor 9 has failed from the detected value of the NOx sensor 9 and notifying the passenger when there is a possibility of failure. .

  As shown in FIG. 1, since the NOx sensor 9 is provided on the most downstream side of the exhaust passage 16, in the failure diagnosis based on the detection value of the NOx sensor 9, the DPF device 1 and the SCR device arranged on the upstream side thereof. It is required to make a judgment by separating it from the four failures. The controller 7 of this embodiment has a function for identifying the cause of such a failure and distinguishing the failure of the NOx sensor 9 from other failures.

[3. Controller functions]
Functions programmed as software or hardware circuits inside the controller 7 are schematically shown in FIG. In the case of software, the function described below is realized by recording the software in a memory or storage device (not shown) and reading it to a CPU (Central Processing Unit) (not shown) as needed.

The controller 7 is provided with a catalyst temperature detection unit 7a, an engine exhaust NOx amount estimation unit 7b, an ammonia adsorption amount calculation unit 7c, an idle operation state detection unit 7d, and a failure determination unit 7e.
Catalyst temperature detecting section 7a (catalyst temperature detection means), based on the exhaust temperatures T ₁ detected by the temperature sensor 8 is for detecting and estimating the catalyst temperature T _C of the SCR catalyst 5. The catalyst temperature detector 7a calculates the catalyst temperature T _C using the exhaust temperature T ₁ and its time variation gradient, the heat capacity, temperature characteristics, etc. of the SCR catalyst 5 stored in advance.

The method for calculating the catalyst temperature T _C is not limited to this. For example, the temperature on the downstream side of the SCR catalyst 5 may be used, or calculation may be performed using both of them. Alternatively, the surface temperature of the SCR catalyst 5 may be detected directly. The catalyst temperature T _C is used for determining whether or not the SCR catalyst 5 has reached the activation temperature T _B , and is used for estimating the amount of ammonia adsorbed on the SCR catalyst 5.

The engine exhaust NOx amount estimation unit 7b estimates and calculates the NOx amount (that is, the engine out NOx amount) contained in the exhaust immediately after being exhausted from the engine 20. As shown in FIG. 3, the engine exhaust NOx amount estimation unit 7b stores a map that describes the correspondence between the operating state of the engine 20 and the NOx amount (NOx concentration) exhausted from the engine 20 at that time. Yes. Engine exhaust NOx amount estimating section 7b is detected by the map and the air flow sensor intake air quantity Q, the engine rotational speed detected by the accelerator opening theta _AC and the engine speed sensor 13 detected by the accelerator opening sensor 12 N Based on _e , the engine-out NOx amount is calculated.

The engine out NOx amount tends to increase as the load acting on the engine 20 increases (the torque generated by the engine 20 increases). Further, as shown by the dashed line in FIG. 3, when conveniently divided into three regions according to the operating state to the magnitude of the engine speed N _e, in the operating state of low rotation region and the high rotation speed region, the middle There is a tendency for the NOx concentration in the exhaust to increase compared to the operating state in the middle rotation region. Such a tendency becomes more prominent as the load acting on the engine 20 is larger.

  The ammonia adsorption amount calculation unit 7c (adsorption amount calculation means) calculates the adsorption amount X of ammonia adsorbed on the SCR catalyst 5. Here, the ammonia adsorption amount X is calculated on the assumption that ammonia is sufficiently adsorbed on the SCR catalyst 5 when urea water is injected from the injector 11.

The maximum value X _MAX of the ammonia adsorption amount X depends on the catalyst temperature T _C as shown in FIG. Therefore, the adsorption amount X at that time is calculated from the catalyst temperature T _C when urea water is injected from the injector 11. Note that the target adsorption amount of ammonia that should be adsorbed on the SCR catalyst 5 when urea water is injected from the injector 11 is the maximum value X _MAX obtained for each catalyst temperature T _C at that time.

  Assuming that the amount of ammonia required to reduce all of the engine-out NOx amount calculated by the ammonia adsorption amount calculation unit 7c and the engine exhaust NOx amount estimation unit 7b has been consumed on the SCR catalyst 5, The ammonia adsorption amount X is updated by subtracting the adsorption amount X at the time. Thereby, when urea water is not injected from the injector 11, the ammonia adsorption amount X on the SCR catalyst 5 gradually decreases.

When the maximum value X _MAX becomes smaller than the adsorption amount X at that time due to the decrease in the catalyst temperature T _C , the adsorption amount X is updated to the maximum value X _MAX at that time. In this case, a part of the adsorbed ammonia slips downstream. Note that the slipped ammonia is purified by the CUC catalyst 6.

When the adsorption amount X becomes less than the preset minimum value X _MIN , the ammonia adsorption amount calculation unit 7c outputs a control signal to a urea injection control unit (not shown) in the controller 7 and the like to the injector 11. Urea water is injected. The minimum value X _MIN may be a value set as a fixed value, or may be set each time according to the operating state of the engine 20 and the catalyst temperature T _C.

Therefore, as shown in FIG. 4, the adsorption amount X varies in a generally decreasing direction between the minimum value X _MIN and the maximum value X _MAX , and when ammonia is almost insufficient, it is replenished to the maximum value X _MAX by supplying urea water. Is done. In other words, the ammonia adsorption amount X periodically varies according to the consumption rate of ammonia within the range of the fluctuation range in the vertical axis direction defined by the catalyst temperature T _C.

The idle operation state detection unit 7d (state detection means) detects the idle operation state of the engine 20. The idle operation state is a state in which the idle rotation of the engine 20 continues for a predetermined time (for example, about 0.1 to several seconds) or more. Further, the idle rotation of the engine 20 is determined based on the vehicle speed V and the accelerator opening θ _AC . For example, when the vehicle speed V is approximately 0 [Km / h] and the accelerator pedal is not depressed, it is determined that the engine 20 is idling.

The failure determination unit 7e (determination means) determines a failure of the NOx sensor 9 based on a predetermined failure determination condition. Here, when all of the following conditions are satisfied, it is determined that the NOx sensor 9 has failed, and the notification device 15 is controlled to notify the passenger of the failure.
[Condition A] The engine 20 is in an idling state. [Condition B] The catalyst temperature T _C of the SCR catalyst 5 is equal to or higher than the activation temperature T _B and lower than the desorption temperature T _A [Condition C] detected by the NOx sensor 9 NOx concentration C ₁ is less than predetermined concentration C ₀ [Condition D] Adsorption amount X is greater than or equal to predetermined adsorption amount X ₀

The predetermined density C _{0 according} to the above condition C is 0 or a minute value close to 0. The specific value varies depending on the performance of the NOx sensor 9, the type of the engine 20, the structure of the exhaust passage 16, etc., but may be set within a range of, for example, about 0 to 100 [ppm].
Further, the predetermined adsorption amount X _{0 according} to the above condition D is an arbitrary value not less than ₀ and less than the minimum value X _MIN . In this embodiment, when the adsorption amount X decreases, urea water is automatically supplied from the injector 11 and ammonia is replenished, so that ammonia is always adsorbed on the SCR catalyst 5. Therefore, the adsorption amount X does not become less than the minimum value _XMIN , and the above condition D is always satisfied.

Moreover, as shown in FIG. 1, the failure determination part 7e has the error calculating part 7f. The error calculation unit 7f (error calculation means) determines the degree of failure when it is determined that the NOx sensor 9 has failed based on the above failure determination condition. That is, when the failure determination unit 7e determines that the NOx sensor 9 has failed, the error calculation unit 7f determines the difference ΔC (= C ₁ − between the NOx concentration C ₁ detected by the NOx sensor 9 and the predetermined concentration C _0. C ₀ ) is periodically calculated, and the degree of failure is determined based on the difference ΔC.

  For example, if the difference ΔC is always greater than or equal to a predetermined value, it is determined that the NOx sensor 9 has definitely failed. If the difference ΔC is always less than the predetermined value, it is determined that the NOx sensor 9 has a sign of failure. Further, when the state where the difference ΔC is 0 and the state where the difference ΔC is equal to or greater than a predetermined value are alternately repeated, there is a possibility that the NOx sensor 9 has some kind of temporary failure (such as poor contact). Is determined.

[4. flowchart]
FIG. 5 is a flowchart for explaining an example of the control in the exhaust emission control device 10. This flow is repeatedly performed inside the controller 7.
In step A10, the exhaust temperatures T ₁ detected by the temperature sensor 8 is inputted to the controller 7, the catalyst temperature T _C of the SCR catalyst 5 in the catalyst temperature detector 7a is calculated. In Step A20, the failure determination unit 7e determines whether the catalyst temperature T _C calculated in the previous step is equal to or higher than the activation temperature T _{B and} lower than the ammonia desorption temperature T _A. That is, here, the condition B is determined.

Here, if T _B ≦ T _C <T _A , the process proceeds to step A30, and the failure determination is continued. On the other hand, if T _C <T _B , or T _A ≦ T _C , this flow is terminated assuming that an accurate failure determination cannot be expected. Note for this flow is repeated even in this case, the re-catalyst temperature T _C in the next cycle is determined, the process proceeds to step A30 if the condition of step A20 is established.

  In step A30, the engine exhaust NOx amount estimation unit 7b estimates the engine out NOx amount based on the map shown in FIG. In addition, since the engine 20 corresponds to the lower left side (below the low rotation region) in FIG. 3, the engine out NOx amount is small as compared with other operation states when the engine 20 is in the idle operation state.

  In Step A40, the ammonia adsorption amount calculation unit 7c calculates the adsorption amount X of ammonia adsorbed on the SCR catalyst 5. Here, the ammonia amount corresponding to the engine-out NOx amount is subtracted from the adsorption amount X up to this point and updated. The ammonia amount corresponding to the NOx amount can be obtained from, for example, the above equation 5. Further, the adsorption amount X obtained in this step varies with time according to the operating state of the engine 20, as shown in FIG.

In step A50, the ammonia adsorption amount calculating section 7c, the target adsorption amount X _MAX ammonia according to the catalyst temperature T _C is set. In subsequent step A60, it is determined whether or not the updated adsorption amount X is less than the minimum value _XMIN . Here, if X <X _MIN, it is considered that the adsorption amount X is becoming insufficient, and the process proceeds to step A70.

In step A70, for example in the urea injection control unit, the injector 11 is controlled so adsorbed amount X of ammonia equal to the target adsorption amount X _MAX, urea water is sprayed supplied to the exhaust passage 16 upstream of the SCR catalyst 5 The Thereby, the ammonia adsorption amount X in the SCR catalyst 5 is supplemented, and the process proceeds to Step A90.
If the determination result in step A60 is X ≧ _XMIN , it is considered that a sufficient adsorption amount X is secured, and the process proceeds to step A90. Therefore, regardless of the determination result of step A60, the above condition D is satisfied at the time of step A90.

In step A90, the operating state of the engine 20 is detected by the idle operating state detection unit 7d. Here, the operating state is determined based on the accelerator opening degree θ _AC and the vehicle speed V. In subsequent step A100, the failure determination unit 7e determines whether or not the engine 20 operation state is an idle operation state. That is, here, the above condition A is determined, and if this is satisfied, the process proceeds to step A110. On the other hand, if not established, this flow is terminated assuming that an accurate failure determination cannot be expected.

In step A <b> 110, the concentration C ₁ detected by the NOx sensor 9 is input to the controller 7. In step A 120, the failure determination section 7e, the concentration C ₁ of the NOx is equal to or less than the predetermined concentration C ₀ is determined. Here, if C ₁ <C ₀ , the process proceeds to step A130, where it is determined that the NOx sensor 9 is not broken (normal).

Note that the flow after step A110 is executed in an idle operation state in which the exhaust flow rate and the exhaust temperature are stable. In addition, ammonia that is sufficiently adsorbed by the SCR catalyst 5 is used for the NOx reduction reaction. As a result, the NOx purification rate in the SCR catalyst 5 is improved, and the detected NOx concentration C ₁ is less than the predetermined concentration C ₀ unless the NOx sensor 9 has failed.
On the other hand, if C ₁ ≧ C ₀ , the process proceeds to step A140, where it is determined that the NOx sensor 9 may have failed. At this time, the error calculator 7f calculates the difference ΔC between the NOx concentration C ₁ and the predetermined concentration C ₀ .

In the subsequent step A150, the notification device 15 is controlled by the controller 7, and the fact that the failure of the NOx sensor 9 has been detected is displayed on a display device such as a display or a lamp. In addition, an acoustic device such as a speaker or a buzzer issues an announcement or warning sound that prompts the passenger to replace or maintain the NOx sensor 9.
Note that the error calculation unit 7f determines the degree of failure based on the difference ΔC calculated while the failure of the NOx sensor 9 is detected. The degree of failure determined here is also displayed on the display device and notified from the acoustic device.

[5. effect]
FIG. 6A shows the change over time in the engine speed during a running test of a vehicle equipped with this intake / exhaust system, and FIG. 6B shows the calculated value of the engine out NOx and the NOx detected by the NOx sensor 9. is a graph showing the time variation of the concentration C ₁ of.

The minimum value N _e1 of the engine speed N _e shown in FIG. _6A is the engine speed during idling of the engine 20. In other words, between time t ₁ ~t _2, between time t ₃ ~t _4, period no fluctuation of the engine speed N _e such between time t ₅ ~t ₆ is a period during which the vehicle is temporarily stopped or signal wait, Corresponds to almost idle state.

Referring to FIG. 6 (b), the engine-out NOx is continuously detected even after the engine speed _Ne is idle. On the other hand, the NOx concentration C ₁ detected by the NOx sensor 9 rapidly decreases immediately after the engine rotation speed _Ne becomes idle rotation, and it can be read that almost all NOx is purified.
In other words, the exhaust flow rate, the exhaust temperature, and the catalyst temperature are all low and stable in the idle operation state (the state where the engine speed _Ne is maintained at the idle rotation for a predetermined time), so that ammonia is adsorbed in the SCR catalyst 5. If so, the ammonia is efficiently consumed in the reduction reaction of NOx, and a NOx purification rate close to 100% can be obtained.

  Therefore, by referring to the detected value of the NOx sensor 9 under such conditions, the failure of the SCR device 4 and the engine 20 provided on the exhaust passage 16 and the failure of the NOx sensor 9 can be accurately distinguished. Thus, the failure of the NOx sensor 9 can be specified. Thereby, the reliability of the value of the NOx concentration detected by the NOx sensor 9 can be improved, and more accurate control is possible.

  Furthermore, the determination of the conditions for performing the failure determination can be realized with a simple configuration, and there is an advantage that it can be easily applied to a conventional intake / exhaust system. In addition, since the failure of the NOx sensor 9 can be determined, it is possible to prevent erroneous recognition of the quality of the urea water. Compared with a conventional SCR system that uses a urea identification sensor as a quality control sensor for urea water, the above system uses a highly versatile NOx sensor 9 instead of the urea identification sensor. It is possible. Therefore, the cost of the entire system can be reduced.

  Further, the exhaust purification device 10 described above determines whether or not the detected value of the NOx sensor 9 is appropriate using the characteristic that the NOx purification rate is improved under the stable operating state of the engine 20. Yes. That is, as a condition (failure determination condition) for reliably determining whether or not the detection value of the NOx sensor 9 is appropriate, disturbance elements caused by elements other than the NOx sensor 9 can be eliminated as much as possible. The failure of the NOx sensor 9 can be identified well.

In the exhaust emission control device 10 described above, the idle operation state detection unit 7d determines the determination condition for the idle operation state. As a result, it is possible to accurately grasp the state in which the NOx purification rate is improved, and the accuracy of the failure determination of the NOx sensor 9 can be further increased. In particular, the idle operation state detection unit 7d does not determine that the engine 20 is in the idle operation state unless the idle rotation of the engine 20 continues for a predetermined time or more, so the times t ₁ , t ₃ , t ₅ shown in FIG. of, the failure determination in a state where not completely still lower the concentration C ₁ of the NOx is not as performed immediately after. Therefore, the accuracy of failure determination can be improved.

Further, when it is determined that the NOx sensor 9 has failed, the notification device 15 informs the occupant accordingly, so that maintenance such as replacement or maintenance of the NOx sensor 9 can be urged, and the vehicle intake / exhaust system can be It can make it easy to be in a healthy state.
Further, the exhaust gas purification device 10 not only determines whether or not the NOx sensor 9 has failed, but when the NOx sensor 9 has been determined to be defective, the error calculation unit 7f uses the detected value of the NOx sensor 9 and a predetermined value. The degree of failure is determined by calculating a difference ΔC from the value. As a result, it is possible to estimate whether the failure of the NOx sensor 9 is reliable or indicates a sign of failure.

[6. Modified example]
Regardless of the embodiment described above, various modifications can be made without departing from the spirit of the invention. Each structure of this embodiment can be selected as needed, or may be combined appropriately.

  In the above-described embodiment, the control is such that the condition D is always satisfied among the four types of failure determination conditions. However, the specific control mode is appropriately changed according to the type of vehicle and the type of intake / exhaust system. Can do. For example, in an intake / exhaust system in which control is not performed so that ammonia is not automatically replenished even when the adsorption amount X decreases, it is conceivable to determine the condition D before performing the failure determination of the NOx sensor 9. . In this case, a step of determining the condition D in a process prior to step A120 in FIG.

In the above-described embodiment, the air flow sensor 14 provided in the intake passage 17 is used to detect the intake air amount Q, and this is used to estimate the engine out NOx amount. An exhaust flow sensor may be provided, or a value corresponding to this may be calculated using other parameters. Similarly, the load acting on the engine 20 may be detected using a torque sensor or the like.
Further, in the above-described embodiment, an example in which the idle operation state of the engine 20 is detected based on the vehicle speed V and the accelerator opening θ _AC is illustrated, but the specific operation state determination method of the engine 20 is also limited to this. Not.

The exhaust purification device 10 of the above-described embodiment is exemplified by the DPF device 1 and the SCR device 4 arranged in series on the exhaust passage 16, but is an intake / exhaust system including at least the SCR catalyst 4 and the NOx sensor 9. If there is, it is possible to realize a device that exhibits the above technical effects.
Moreover, although what applied this invention to the exhaust system of the diesel engine illustrated in the above-mentioned embodiment, the application to the exhaust system of a gasoline engine is also possible.

DESCRIPTION OF SYMBOLS 1 DPF apparatus 2 DOC catalyst 3 Filter 4 SCR apparatus 5 SCR catalyst 6 CUC catalyst 7 Controller 7a Catalyst temperature detection part (catalyst temperature detection means)
7b Engine exhaust NOx amount estimation unit 7c Ammonia adsorption amount calculation unit (adsorption amount calculation means)
7d Idle operation state detection unit (state detection means)
7e Failure determination unit (determination means)
7f Error calculation unit (error calculation means)
8 Temperature sensor 9 NOx sensor (exhaust sensor)
DESCRIPTION OF SYMBOLS 10 Exhaust gas purification apparatus 11 Injector 12 Accelerator opening degree sensor 13 Engine speed sensor 14 Airflow sensor 15 Notification apparatus (notification means)
16 Exhaust passage 17 Intake passage 18 Turbocharger 19 Vehicle speed sensor 20 Engine 21 Crankshaft

Claims (5)

A selective reduction catalyst that is provided in an exhaust passage of an engine mounted on a vehicle and that adsorbs ammonia and reduces nitrogen oxide in exhaust using the ammonia as a reducing agent;
Catalyst temperature detecting means for detecting a catalyst temperature of the selective reduction catalyst;
An adsorption amount calculating means for calculating an adsorption amount of the ammonia adsorbed on the selective reduction catalyst;
State detecting means for detecting an idle operation state of the engine;
An exhaust sensor that is provided downstream of the selective reduction catalyst in the exhaust passage and detects the concentration of nitrogen oxides contained in the exhaust;
Exhaust gas comprising: determination means for determining whether or not the exhaust sensor has failed based on the catalyst temperature, the adsorption amount, and the nitrogen oxide concentration when the engine is in the idle operation state. Purification equipment. The determination means includes
The engine is in the idle operation state, and
The catalyst temperature is equal to or higher than an activation temperature capable of reducing the nitrogen oxide, and
The amount of adsorption is greater than or equal to a predetermined amount, and
The exhaust emission control device according to claim 1, wherein when the nitrogen oxide concentration is less than a predetermined concentration, it is determined that the exhaust sensor is normal. The exhaust according to claim 1 or 2, wherein the state detection means detects the idle operation state when idle rotation of the engine based on a traveling speed of the vehicle and an engine load continues for a predetermined time. Purification equipment. The exhaust control device according to any one of claims 1 to 3, further comprising notification means for notifying an occupant of a result of determination by the determination means. An error calculating means for calculating a difference between the nitrogen oxide concentration and the predetermined concentration when the determining means determines that the exhaust sensor has failed;
The determination means is
The exhaust control device according to any one of claims 1 to 4, wherein a degree of failure of the exhaust sensor is determined based on the difference calculated by the error calculation means.

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