JP3899945B2 - Exhaust gas purification device for internal combustion engine - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a technique for purifying exhaust gas from an internal combustion engine.
[0002]
[Prior art]
In recent years, internal combustion engines mounted on automobiles and the like have been required to improve exhaust emission. In particular, in a compression ignition type internal combustion engine (diesel engine) using light oil as fuel, purification of particulates (PM: Particulate Matter) such as soot and SOF (Soluble Organic Fraction) and nitrogen oxides (NOx) contained in the exhaust gas The demand for is increasing.
[0003]
In response to such a demand, a NOx catalyst for purifying nitrogen oxide (NOx) in exhaust, a particulate filter for collecting PM in exhaust, or a combination of a NOx catalyst and a particulate filter is used for an internal combustion engine. Various techniques have been proposed that are arranged in an exhaust system to improve the exhaust emission of an internal combustion engine.
[0004]
[Problems to be solved by the invention]
By the way, in order to improve the exhaust emission of the internal combustion engine, it is also important to detect the deterioration of the NOx catalyst.
[0005]
The present invention has been made in view of the above-described circumstances, and an object of the present invention is to provide a technique capable of detecting deterioration of a NOx catalyst in an exhaust gas purification apparatus for an internal combustion engine equipped with a NOx catalyst. .
[0006]
[Means for Solving the Problems]
The present invention employs the following means in order to solve the above-described problems.
That is, the exhaust gas purification apparatus for an internal combustion engine according to the present invention is
A NOx catalyst provided in the exhaust passage of the internal combustion engine and having an oxygen storage capacity;
An oxygen concentration sensor that is provided in an exhaust passage downstream of the NOx catalyst and detects an oxygen concentration in the exhaust;
The time from when the exhaust gas flowing into the NOx catalyst is continuously rich for a predetermined time or more and then switched to lean until the time when the detected value of the oxygen concentration sensor shows a value corresponding to the stoichiometric air-fuel ratio is measured. Measuring means;
A deterioration determining means for determining that the NOx catalyst is deteriorated when the time measured by the measuring means is less than a predetermined time;
I was prepared to.
[0007]
The present invention relates to an exhaust gas purification apparatus for an internal combustion engine comprising a NOx catalyst disposed in an exhaust passage of the internal combustion engine and an oxygen concentration sensor disposed in an exhaust passage downstream of the NOx catalyst. The greatest feature is that the deterioration of the NOx catalyst is detected using the oxygen storage capacity of the catalyst.
[0008]
Here, the oxygen concentration corresponding to the stoichiometric air-fuel ratio corresponds to the oxygen concentration of the exhaust discharged from the internal combustion engine operated at the stoichiometric air-fuel ratio. To make the exhaust gas flowing into the NOx catalyst rich, the internal combustion engine is operated at a rich air-fuel ratio or by adding fuel, a reducing agent, etc. into the exhaust gas in the exhaust passage upstream of the NOx catalyst, It is to make it lower than the oxygen concentration corresponding to the fuel ratio. To make the exhaust gas flowing into the NOx catalyst lean, the internal combustion engine is operated at a lean air-fuel ratio or by adding secondary air or the like into the exhaust gas in the exhaust passage upstream of the NOx catalyst, the oxygen concentration of the exhaust gas is changed to the stoichiometric air-fuel ratio. The higher the corresponding oxygen concentration.
[0009]
In such an exhaust gas purification apparatus for an internal combustion engine, when the exhaust gas flowing into the NOx catalyst is continuously made rich for a predetermined time or more and then switched to lean, the measuring means starts measuring time. Thereafter, when the detection value of the oxygen concentration sensor shows a value corresponding to the stoichiometric air-fuel ratio, the measuring means finishes timing.
[0010]
In other words, the measuring means from the time when the exhaust gas flowing into the NOx catalyst is continuously rich for a certain time or more and then switched to lean until the time when the detected value of the oxygen concentration sensor shows a value corresponding to the stoichiometric air-fuel ratio. Time (hereinafter referred to as stoichiometric arrival time) is measured.
[0011]
Here, if the exhaust gas flowing into the NOx catalyst is continuously rich for a certain time or longer, all of the oxygen stored in the NOx catalyst is released by the oxygen storage capacity of the NOx catalyst.
[0012]
When the exhaust gas flowing into the NOx catalyst is switched from rich to lean after all the oxygen stored in the NOx catalyst is released, the NOx catalyst stores oxygen in the exhaust gas. The oxygen concentration of the exhaust gas is an oxygen concentration corresponding to an air fuel ratio (rich air fuel ratio) lower than the stoichiometric air fuel ratio. This state is continued until the oxygen storage capacity of the NOx catalyst is saturated.
[0013]
After the oxygen storage capacity of the NOx catalyst is saturated, the oxygen concentration in the exhaust downstream of the NOx catalyst becomes an oxygen concentration corresponding to the stoichiometric air-fuel ratio, and then the oxygen concentration corresponding to an air-fuel ratio (lean air-fuel ratio) higher than the stoichiometric air-fuel ratio. It becomes.
[0014]
By the way, since there is a correlation between the purification capacity of the NOx catalyst and the oxygen storage capacity, when the purification capacity of the NOx catalyst deteriorates, the oxygen storage capacity decreases accordingly. When the oxygen storage capacity of the NOx catalyst is reduced, the amount of oxygen that can be stored by the NOx catalyst is reduced, and the above-mentioned stoichiometric time is shortened as compared with the normal time.
[0015]
As a result, when the stoichiometric arrival time is shorter than the predetermined time, it can be said that the purification ability of the NOx catalyst is deteriorated. The predetermined time is preferably determined based on the stoichiometric arrival time when the purification capacity of the NOx catalyst is at the allowable limit.
[0016]
Therefore, in the present invention, the deterioration determining means determines that the NOx catalyst is normal when the stoichiometric arrival time measured by the measuring means is equal to or longer than the predetermined time, while the stoichiometric arrival time is less than the predetermined time. It can be determined that the NOx catalyst has deteriorated.
[0017]
Further, in the exhaust gas purification apparatus for an internal combustion engine according to the present invention, the case where the exhaust gas flowing into the NOx catalyst is continuously rich for a predetermined time or longer is exemplified when the sulfur poisoning recovery process of the NOx catalyst is executed. can do. In this case, the measuring means determines that the detected value of the oxygen concentration sensor becomes the stoichiometric air-fuel ratio from the time when the exhaust gas flowing into the NOx catalyst after the NOx catalyst sulfur poisoning recovery process is switched from rich to lean. What is necessary is just to measure the time to the time of showing the corresponding value.
[0018]
Further, as the NOx catalyst in the exhaust gas purification apparatus for an internal combustion engine according to the present invention, when the oxygen concentration of the inflowing exhaust gas is high, the nitrogen oxide in the exhaust gas is occluded, the oxygen concentration of the inflowing exhaust gas is reduced, and there is a reducing agent. In this case, an NOx storage reduction catalyst that reduces the released nitrogen oxide while releasing it can be exemplified.
[0019]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, specific embodiments of an exhaust emission control device for an internal combustion engine according to the present invention will be described with reference to the drawings.
[0020]
FIG. 1 is a diagram showing a schematic configuration of an internal combustion engine to which an exhaust gas purification apparatus according to the present invention is applied and its intake and exhaust system.
[0021]
An internal combustion engine 1 shown in FIG. 1 is a compression ignition type diesel engine having four cylinders 2. In the internal combustion engine 1, every time a fuel injection valve 3 that directly injects fuel into the combustion chamber of each cylinder 2 and a crankshaft that is an engine output shaft of the internal combustion engine 1 rotate by a predetermined angle (for example, 15 °). A crank position sensor 4 that outputs a pulse signal and a water temperature sensor 5 that outputs an electrical signal corresponding to the temperature of cooling water flowing through a water jacket (not shown) of the internal combustion engine 1 are attached.
[0022]
The fuel injection valve 3 described above is connected to a pressure accumulating chamber (common rail) 7 through a fuel pipe 6. The common rail 7 is connected to a fuel pump 9 attached to the fuel tank 8 via a fuel pipe 10 and to the fuel tank 8 via a return pipe 11.
[0023]
When the fuel pressure in the common rail 7 is lower than a preset maximum pressure, the common rail 7 is closed at the connecting portion of the return pipe 11 to cut off the conduction between the common rail 7 and the return pipe 11. When the internal fuel pressure becomes equal to or higher than the maximum pressure, there is provided a pressure regulating valve 12 that opens and allows the common rail 7 and the return pipe 11 to conduct.
[0024]
A fuel pressure sensor 13 for outputting an electric signal corresponding to the fuel pressure in the common rail 7 is attached to the common rail 7.
[0025]
In the fuel system configured as described above, the fuel pump 9 pumps up the fuel stored in the fuel tank 8 and pumps the pumped up fuel to the common rail 7 through the fuel pipe 10. At that time, the fuel discharge amount of the fuel pump 9 is feedback-controlled based on the output signal value of the fuel pressure sensor 13 described above.
[0026]
The fuel supplied from the fuel pump 9 to the common rail 7 is accumulated until the pressure of the fuel reaches a desired target pressure. The fuel accumulated up to the target pressure in the common rail 7 is distributed to the fuel injection valves 3 of the respective cylinders 2 through the fuel pipe 6. Each fuel injection valve 3 opens when a drive current is applied, and injects fuel at a target pressure supplied from the common rail 7 into the combustion chamber of each cylinder 2.
[0027]
In the fuel system described above, when the fuel pressure in the common rail 7 becomes higher than the maximum pressure, the pressure adjustment valve 12 is opened. In this case, a part of the fuel stored in the common rail 7 is returned to the fuel tank 8 through the return pipe 11, and the fuel pressure in the common rail 7 is reduced.
[0028]
Next, an intake branch pipe 14 formed so that a plurality of branch pipes merge into one collecting pipe is connected to the internal combustion engine 1. Each branch pipe of the intake branch pipe 14 communicates with the combustion chamber of each cylinder 2 via an intake port (not shown). A collecting pipe of the intake branch pipe 14 is connected to an intake pipe 15, and the intake pipe 15 is connected to an air cleaner box 16.
[0029]
An air flow meter 17 that outputs an electric signal corresponding to the mass of the intake air flowing in the intake pipe 15 and a temperature of the intake air flowing in the intake pipe 15 are disposed in the intake pipe 15 immediately downstream of the air cleaner box 16. And an intake air temperature sensor 18 for outputting an electrical signal corresponding to the above.
[0030]
A compressor housing 19a of a centrifugal supercharger (turbocharger) 19 that operates using the thermal energy of exhaust gas discharged from the internal combustion engine 1 as a drive source is provided in a portion of the intake pipe 15 downstream of the air flow meter 17. Yes.
[0031]
An intercooler 20 for cooling fresh air that has been compressed in the compressor housing 19a and has reached a high temperature is provided in a portion of the intake pipe 15 downstream of the compressor housing 19a.
[0032]
An intake throttle valve 21 for adjusting the flow rate of the intake air flowing through the intake pipe 15 is provided in a portion of the intake pipe 15 downstream of the intercooler 20. An intake throttle actuator 21a that opens and closes the intake throttle valve 21 and an intake throttle valve opening sensor 21b that outputs an electrical signal corresponding to the opening of the intake throttle valve 21 are attached to the intake throttle valve 21. It has been.
[0033]
In the intake system configured as described above, fresh air that has flowed into the air cleaner box 16 is removed through the intake pipe 15 after dust or dirt in the fresh air is removed by a filter (not shown) in the air cleaner box 16. It flows into the compressor housing 19a of the centrifugal supercharger 19.
[0034]
The fresh air flowing into the compressor housing 19a is compressed by the rotation of the compressor wheel built in the compressor housing 19a. The fresh air that has been compressed in the compressor housing 19 a and has reached a high temperature is cooled by the intercooler 20.
[0035]
The fresh air cooled by the intercooler 20 is guided to the intake branch pipe 14 with the flow rate adjusted by the intake throttle valve 21 as necessary. The fresh air guided to the intake branch pipe 14 is distributed from the collecting pipe of the intake branch pipe 14 to each branch pipe and is guided to the combustion chamber of each cylinder 2.
[0036]
The fresh air distributed to the combustion chamber of each cylinder 2 is compressed by a piston (not shown), and burns using the fuel injected from the fuel injection valve 3 as an ignition source.
[0037]
Next, an exhaust branch pipe 24 formed so that a plurality of branch pipes merge into one collecting pipe is connected to the internal combustion engine 1. Each branch pipe of the exhaust branch pipe 24 communicates with the combustion chamber of each cylinder 2 via an exhaust port (not shown). The collecting pipe of the exhaust branch pipe 24 is connected to the exhaust pipe 25 a via the turbine housing 19 b of the centrifugal supercharger 19.
[0038]
A portion of the exhaust branch pipe 24 located immediately upstream of the turbine housing 19b and a portion of the exhaust pipe 25a located immediately downstream of the turbine housing 19b are connected by a turbine bypass passage 26 that bypasses the turbine housing 19b. Has been.
[0039]
A waste gate valve 27 including a valve body 27a for opening and closing the turbine bypass path 26 and an actuator 27b for opening and closing the valve body 27a is attached to the turbine bypass path 26.
[0040]
The actuator 27b is connected to the intake pipe 15 located immediately downstream of the compressor housing 19a via the working pressure passage 28, and the pressure of fresh air flowing in the intake pipe 15 immediately downstream of the compressor housing 19a, in other words, The valve body 27a is driven to open and close using the pressure (supercharging pressure) of fresh air compressed in the compressor housing 19a.
[0041]
The exhaust pipe 25a is connected to an exhaust purification mechanism 29 that purifies harmful gas components in the exhaust, particularly nitrogen oxides (NOx). The exhaust purification mechanism 29 is connected to an exhaust pipe 25b, and the exhaust pipe 25b is connected downstream to a muffler (not shown). Hereinafter, the exhaust pipe 25a upstream from the exhaust purification mechanism 29 is referred to as an upstream exhaust pipe 25a, and the exhaust pipe 25b downstream from the exhaust purification mechanism 29 is referred to as a downstream exhaust pipe 25b.
[0042]
The exhaust purification mechanism 29 is an embodiment of the NOx catalyst according to the present invention. When the oxygen concentration of the exhaust flowing into the exhaust purification mechanism 29 is high, the exhaust purification mechanism 29 occludes nitrogen oxide (NOx) in the exhaust, When the oxygen concentration of the exhaust gas flowing into the exhaust gas purification mechanism 29 decreases and a reducing agent (for example, hydrocarbon (HC)) exists, nitrogen (Nx) is released while releasing the stored nitrogen oxides (NOx). ₂ ) And the like. The exhaust purification mechanism 29 occludes oxygen in the exhaust when the oxygen concentration of the exhaust flowing into the exhaust purification mechanism 29 is high, and occludes when the oxygen concentration of the exhaust flowing into the exhaust purification mechanism 29 decreases. It has the ability to store oxygen to release the oxygen it had stored. Hereinafter, the exhaust purification mechanism 29 is referred to as a storage reduction type NOx catalyst 29.
[0043]
The downstream exhaust pipe 25b is provided with an oxygen concentration sensor 38 that outputs an electrical signal corresponding to the oxygen concentration in the exhaust gas flowing through the downstream exhaust pipe 25b. Further, an exhaust throttle valve 33 for adjusting the flow rate of the exhaust gas flowing through the downstream exhaust pipe 25b is attached to the downstream exhaust pipe 25b. An exhaust throttle actuator 34 that opens and closes the exhaust throttle valve 33 is attached to the exhaust throttle valve 33.
[0044]
In the exhaust system configured as described above, the burned gas burned in the combustion chamber of each cylinder 2 of the internal combustion engine 1 is discharged to the exhaust branch pipe 24 through the exhaust port of each cylinder 2 and then the exhaust branch pipe. It flows into the turbine housing 19b of the centrifugal supercharger 19 from each branch pipe of 24 through the collecting pipe.
[0045]
When the exhaust gas flows into the turbine housing 19b of the centrifugal supercharger 19, the heat energy of the exhaust gas is converted into the rotational energy of the turbine wheel that is rotatably supported in the turbine housing 19b. The rotational energy of the turbine wheel is transmitted to the compressor wheel of the aforementioned compressor housing 19a, and the compressor wheel compresses fresh air by the rotational energy transmitted from the turbine wheel.
[0046]
At that time, when the pressure (supercharging pressure) of fresh air compressed in the compressor housing 19 a rises to a predetermined pressure or higher, the supercharging pressure is applied to the actuator 27 b of the wastegate valve 27 through the operating pressure passage 28. The actuator 27b drives the valve body 27a to open.
[0047]
When the valve element 27a of the wastegate valve 27 is opened, a part of the exhaust gas flowing through the exhaust branch pipe 24 flows to the upstream exhaust pipe 25a via the turbine bypass passage 26, so that the exhaust gas flowing into the turbine housing 19b is discharged. The flow rate decreases, and the thermal energy of the exhaust gas flowing into the turbine housing 19b, in other words, the thermal energy converted into the rotational energy of the turbine wheel in the turbine housing 19b decreases. As a result, rotational energy transmitted from the turbine wheel to the compressor wheel is reduced, and an excessive increase in supercharging pressure is suppressed.
[0048]
Exhaust gas discharged from the turbine housing 19b to the upstream side exhaust pipe 25a and exhaust gas guided from the turbine bypass passage 26 to the upstream side exhaust pipe 25a flow into the NOx storage reduction catalyst 29 from the upstream side exhaust pipe 25a. . At this time, if the oxygen concentration in the exhaust gas is high, nitrogen oxide (NOx) in the exhaust gas is stored in the NOx storage reduction catalyst 29, and if the oxygen concentration in the exhaust gas is low and a reducing agent is present, the NOx storage reduction catalyst 29 The stored nitrogen oxides (NOx) are released and reduced. The exhaust gas from which nitrogen oxide (NOx) has been removed or purified in the NOx storage reduction catalyst 29 is discharged from the NOx storage reduction catalyst 29 to the downstream exhaust pipe 25b, and the flow rate is reduced by the exhaust throttle valve 33 as necessary. After being squeezed, it is released into the atmosphere.
[0049]
Further, an exhaust gas recirculation passage (EGR passage) 100 is connected to the exhaust branch pipe 24, and the EGR passage 100 is connected to the intake branch pipe 14. An EGR valve 101 that opens and closes an opening end of the EGR passage 100 in the intake branch pipe 14 is provided at a connection portion between the EGR passage 100 and the intake branch pipe 14. The EGR valve 101 is composed of an electromagnetic valve or the like, and the opening degree can be changed according to the magnitude of applied power.
[0050]
In the middle of the EGR passage 100, an EGR cooler 103 for cooling the exhaust gas flowing in the EGR passage 100 (hereinafter referred to as EGR gas) is provided.
[0051]
Two pipes 104 and 105 are connected to the EGR cooler 103, and these two pipes 104 and 105 are connected to a radiator 106 for radiating the heat of the cooling water of the internal combustion engine 1 into the atmosphere. ing.
[0052]
One of the two pipes 104 and 105 described above is a pipe for guiding a part of the cooling water cooled in the radiator 106 to the EGR cooler 103, and the other pipe 105 is This is a pipe for guiding the cooling water after circulating through the EGR cooler 103 to the radiator 106. Hereinafter, the pipe 104 will be referred to as a cooling water introduction pipe 104 and the pipe 105 will be referred to as a cooling water outlet pipe 105.
[0053]
In the middle of the cooling water outlet pipe 105, an on-off valve 107 for opening and closing a flow path in the cooling water outlet pipe 105 is provided. The on-off valve 107 is composed of an electromagnetically driven valve that opens when drive power is applied.
[0054]
In the exhaust gas recirculation mechanism (EGR mechanism) configured as described above, when the EGR valve 101 is opened, the EGR passage 100 becomes conductive, and a part of the exhaust gas flowing in the exhaust branch pipe 24 passes through the EGR passage 100. It is led to the intake branch pipe 14.
[0055]
At this time, if the on-off valve 107 is in the open state, the circulation path connecting the radiator 106, the cooling water introduction pipe 104, the EGR cooler 103, and the cooling water outlet pipe 105 becomes conductive, and the cooling water cooled by the radiator 106 Circulates through the EGR cooler 103. As a result, in the EGR cooler 103, heat exchange is performed between the EGR gas flowing in the EGR passage 100 and the cooling water circulating in the EGR cooler 103, thereby cooling the EGR gas.
[0056]
The EGR gas recirculated from the exhaust branch pipe 24 to the intake branch pipe 14 via the EGR passage 100 is guided to the combustion chamber of each cylinder 2 while being mixed with fresh air flowing from the upstream side of the intake branch pipe 14. The fuel injected from the fuel injection valve 3 is burned using an ignition source.
[0057]
Here, the EGR gas contains water (H ₂ O) and carbon dioxide (CO ₂ ) And the like, if EGR gas is contained in the air-fuel mixture, the combustion temperature of the air-fuel mixture is lowered, and thus nitrogen oxides (NO) _x ) Is suppressed.
[0058]
Further, when the EGR gas is cooled in the EGR cooler 103, the temperature of the EGR gas itself is lowered and the volume of the EGR gas is reduced. Therefore, when the EGR gas is supplied into the combustion chamber, the atmosphere in the combustion chamber is reduced. The temperature will not increase unnecessarily, and the amount of fresh air (volume of fresh air) supplied into the combustion chamber will not unnecessarily decrease.
[0059]
The internal combustion engine 1 configured as described above is provided with an electronic control unit (ECU) 35 for controlling the internal combustion engine 1. The ECU 35 is a unit that controls the operation state of the internal combustion engine 1 in accordance with the operation conditions of the internal combustion engine 1 and the request of the driver.
[0060]
In addition to the crank position sensor 4, the water temperature sensor 5, the fuel pressure sensor 13, the air flow meter 17, the intake air temperature sensor 18, the intake throttle valve opening sensor 21b, and the oxygen concentration sensor 38, the ECU 35 is provided in the vehicle interior. An accelerator position sensor 37 that outputs an electrical signal corresponding to the operation amount (accelerator opening) of the accelerator pedal 36 is electrically connected, and the output signals of the sensors described above are input to the ECU 35.
[0061]
On the other hand, the ECU 35 is electrically connected to the fuel injection valve 3, the fuel pump 9, the intake throttle actuator 21a, the exhaust throttle actuator 34, the EGR valve 101, the on-off valve 107, and the like, and the ECU 35 controls the above-described parts. It is possible.
[0062]
Here, as shown in FIG. 2, the ECU 35 includes a CPU 41, a ROM 42, a RAM 43, a backup RAM 44, an input port 45, and an output port 46 connected to each other by a bidirectional bus 40. And an A / D converter (A / D) 47 connected to the input port 45.
[0063]
The input port 45 receives an output signal of a sensor that outputs a signal in the form of a digital signal like the crank position sensor 4 and transmits the output signal to the CPU 41 and the RAM 43 via the bidirectional bus 40.
[0064]
The input port 45 has an analog signal format such as a water temperature sensor 5, a fuel pressure sensor 13, an air flow meter 17, an intake air temperature sensor 18, an intake throttle valve opening sensor 21b, an accelerator position sensor 37, an oxygen concentration sensor 38, and the like. Output signals of sensors that output signals are input via the A / D 47, and those output signals are transmitted to the CPU 41 and the RAM 43 via the bidirectional bus 40.
[0065]
The output port 46 is electrically connected to the fuel injection valve 3, the fuel pump 9, the intake throttle actuator 21a, the exhaust throttle actuator 34, the EGR valve 101, the on-off valve 107, etc. via a drive circuit (not shown), and the CPU 41 The control signal output from is transmitted to each unit described above.
[0066]
The ROM 42 stores various application programs such as a fuel injection control routine, an intake throttle control routine, an exhaust throttle control routine, an EGR control routine, a rich spike control routine, an SOx poisoning recovery control routine, and various control maps. is doing.
[0067]
The RAM 43 stores output signals from the sensors, calculation results of the CPU 41, and the like. The calculation result is, for example, an engine speed calculated based on a time interval at which the crank position sensor 4 outputs a pulse signal. These data are rewritten to the latest data every time the crank position sensor 4 outputs a pulse signal.
[0068]
The backup RAM 44 is a non-volatile memory capable of storing data even after the operation of the internal combustion engine 1 is stopped.
[0069]
The CPU 41 operates in accordance with an application program stored in the ROM 42, and in addition to well-known controls such as fuel injection control, intake throttle control, exhaust throttle control, EGR control, rich spike control, SOx poisoning recovery control, The catalyst deterioration detection control for detecting the deterioration of the NOx storage reduction catalyst 29 which is the gist of the invention is executed.
[0070]
In general, the NOx storage reduction catalyst 29 having oxygen storage capacity is operated at an oxygen concentration corresponding to the rich air-fuel ratio (that is, the rich air-fuel ratio) in the exhaust gas flowing into the NOx storage reduction catalyst 29. The oxygen stored in the NOx storage reduction catalyst 29 is released when the oxygen concentration is the same as the oxygen concentration of the exhaust gas exhausted from the internal combustion engine 1 at the time (hereinafter referred to as the rich corresponding concentration). If such a state continues for a certain period of time or longer, all of the oxygen stored in the NOx storage reduction catalyst 29 is released.
[0071]
For example, the NOx storage capacity of the NOx storage reduction catalyst 29 is regenerated, or the NOx storage capacity of the NOx storage catalyst 29 is recovered by the sulfur oxide (SOx) poisoning (SOx poisoning). When the oxygen concentration (upstream oxygen concentration) of the exhaust gas to be controlled is controlled to a desired target rich correspondence concentration, as shown in FIG. 3, the oxygen stored in the NOx storage reduction catalyst 29 is converted to the NOx storage reduction NOx. Since the reducing component such as hydrocarbon (HC) contained in the exhaust gas is discharged from the catalyst 29 and consumed for the reduction of nitrogen oxide (NOx) and / or sulfur oxide (SOx), the NOx storage reduction catalyst The oxygen concentration (downstream oxygen concentration) of the exhaust gas flowing out from the engine 29 does not decrease to the target rich-corresponding oxygen concentration, but corresponds to the stoichiometric air-fuel ratio (that is, the internal combustion engine when operating at the stoichiometric air-fuel ratio) Oxygen concentration and the same concentration of oxygen discharged from the substantially equal density and referred to as a stoichiometric corresponding concentration) in the following (time in Figure 3: time to t1 to t2).
[0072]
All the oxygen stored in the NOx storage reduction catalyst 29 is released, and all the nitrogen oxides (NOx) and / or sulfur oxides (SOx) stored in the NOx storage reduction catalyst 29 are released. When reduced, oxygen is not added from the NOx storage reduction catalyst 29 into the exhaust gas, and the amount of hydrocarbons (HC) consumed for the reduction of nitrogen oxides (NOx) and sulfur oxides (SOx) decreases. The downstream oxygen concentration decreases to the target rich correspondence concentration (time in FIG. 3: period from t2 to t3).
[0073]
In this way, the oxygen concentration on the upstream side is switched from the target rich corresponding concentration to the lean corresponding concentration in a situation where oxygen, nitrogen oxide (NOx), and sulfur oxide (SOx) are not stored in the NOx storage reduction catalyst 29. (Time in FIG. 3: t3), the oxygen in the exhaust is stored in the NOx storage reduction catalyst 29, so that the downstream oxygen concentration is lower than the stoichiometric oxygen concentration (in FIG. 3). Time: period from t3 to t4).
[0074]
Thereafter, when the oxygen storage capacity of the NOx storage reduction catalyst 29 is saturated (time: t4 in FIG. 3), the oxygen in the exhaust gas is not stored in the NOx storage reduction catalyst 29, so the downstream oxygen concentration corresponds to the stoichiometry. After reaching the concentration, it rises to a lean-corresponding concentration (time in FIG. 3: period after t4).
[0075]
By the way, since there is a correlation between the purification capacity of the NOx storage reduction catalyst 29 and the oxygen storage capacity, when the purification capacity deteriorates, the oxygen storage capacity decreases accordingly. When the oxygen storage capacity is reduced with the deterioration of the purification capacity of the NOx storage reduction catalyst 29, the amount of oxygen that can be stored in the NOx storage reduction catalyst 29 is reduced.
[0076]
Here, the upstream oxygen concentration of the NOx storage reduction catalyst 29 is continuously rich for a certain period of time (in this case, the time required for releasing all the oxygen stored in the NOx storage reduction catalyst 29). The time from when the upstream oxygen concentration is changed from the rich corresponding oxygen concentration to the lean corresponding oxygen concentration after reaching the corresponding oxygen concentration until the downstream oxygen concentration reaches the stoichiometric concentration (hereinafter referred to as stoichiometric arrival time) Depends on the oxygen storage capacity of the NOx storage reduction catalyst 29, so that the stoichiometric arrival time becomes shorter as the oxygen storage capacity of the NOx storage reduction catalyst 29 decreases.
[0077]
Therefore, as shown in FIG. 4, when the purification capacity of the NOx storage reduction catalyst 29 deteriorates, the stoichiometric arrival time is shortened according to the degree of deterioration.
[0078]
Therefore, in the catalyst deterioration detection control according to the present embodiment, the CPU 41 is switched to the lean corresponding concentration after the upstream oxygen concentration of the NOx storage reduction catalyst 29 is continuously made the rich corresponding oxygen concentration for a certain time or more. In this case, the time from when the upstream oxygen concentration is switched from the rich-corresponding concentration to the lean-corresponding concentration until the output signal value (downstream oxygen concentration) of the oxygen concentration sensor 38 reaches the stoichiometric concentration is measured. If the measurement time is less than the predetermined time, it is determined that the purification capacity of the NOx storage reduction catalyst 29 has deteriorated. The predetermined time is a time determined based on the stoichiometric arrival time when the purification capacity of the NOx storage reduction catalyst 29 is at the allowable limit.
[0079]
However, even when the NOx storage reduction catalyst 29 is SOx poisoned, the oxygen storage capacity is reduced. Therefore, when the catalyst deterioration detection control is executed in a state where the NOx storage reduction catalyst 29 is SOx poisoned, It becomes difficult to determine whether the decrease in oxygen storage capacity is due to SOx poisoning or deterioration. In contrast, in the present embodiment, the catalyst deterioration detection control is executed immediately after the SOx poisoning recovery control is executed.
[0080]
Hereinafter, the catalyst deterioration detection control according to the present embodiment will be specifically described with reference to FIG.
FIG. 5 is a flowchart showing a catalyst deterioration detection control routine. The catalyst deterioration detection control routine is a routine that is stored in the ROM 42 in advance, and is a routine that is repeatedly executed by the CPU 41 at regular intervals (for example, every time the crank position sensor 4 outputs a pulse signal).
[0081]
In the catalyst deterioration detection control routine, first, in S501, the CPU 41 determines whether or not a deterioration detection condition is satisfied. An example of the deterioration detection condition is that the SOx poisoning recovery control is being executed.
[0082]
If it is determined in S501 that the deterioration detection condition is not satisfied, the CPU 41 once ends the execution of this routine.
[0083]
On the other hand, if it is determined in S501 that the deterioration detection condition is satisfied, the CPU 41 proceeds to S502 and determines whether or not the upstream oxygen concentration is switched from the rich corresponding concentration to the lean corresponding concentration.
[0084]
If it is determined in S502 that the upstream oxygen concentration has not been switched from the rich-corresponding concentration to the lean-corresponding concentration, the CPU 41 regards that the execution of the SOx poisoning recovery control has not ended, and the above-described S501 and subsequent steps Run the process again.
[0085]
If it is determined in S502 that the upstream oxygen concentration has been switched from the rich corresponding concentration to the lean corresponding concentration, the CPU 41 regards that the execution of the SOx poisoning recovery control has ended, and proceeds to S503.
[0086]
In S503, the CPU 41 activates the stoichiometric arrival time counter: C. This stoichiometric arrival time counter: C indicates that the output signal value of the oxygen concentration sensor 38 has reached the stoichiometric response concentration immediately after execution of the SOx poisoning recovery control and the upstream oxygen concentration is switched from the rich corresponding concentration to the lean corresponding concentration. For example, the counter measures the time up to the point in time, and is composed of, for example, a register built in the CPU 41.
[0087]
In S504, the CPU 41 inputs an output signal value: OX of the oxygen concentration sensor 38.
[0088]
In S505, the CPU 41 determines whether or not the output signal value: OX of the oxygen concentration sensor 38 input in S504 is not less than the stoichiometric concentration.
[0089]
If it is determined in S505 that the output signal value of the oxygen concentration sensor 38: OX is less than the stoichiometric concentration, the CPU 41 executes the above-described processing subsequent to S504 again.
[0090]
If it is determined in S505 that the output signal value OX of the oxygen concentration sensor 38 is greater than or equal to the stoichiometric concentration, the CPU 41 proceeds to S506 and stops the stoichiometric arrival time counter: C.
[0091]
In S507, the CPU 41 determines whether or not the stoichiometric arrival time counter: C counter value (stoichi arrival time): C is less than a predetermined time: T.
[0092]
When it is determined in S507 that the stoichiometric arrival time counting counter: C counter value (stoichi arrival time): C is less than the predetermined time: T, the CPU 41 proceeds to S508, where the NOx storage reduction catalyst 29 It is determined that the oxygen storage capacity is below the allowable limit, and accordingly the degree of deterioration of the purification capacity of the NOx storage reduction catalyst 29 exceeds the allowable limit. In this case, a warning light or the like may be provided in the vehicle interior in advance, and the CPU 41 may light the warning light to notify the vehicle driver of the deterioration of the NOx storage reduction catalyst 29.
[0093]
On the other hand, if it is determined in S507 that the stoichiometric arrival time counting counter: C counter value (stoichi arrival time): C is equal to or greater than the predetermined time: T, the CPU 41 proceeds to S509 and stores the NOx storage reduction catalyst. It is determined that the oxygen storage capacity 29 is higher than the allowable limit, and accordingly the degree of deterioration of the purification capacity of the NOx storage reduction catalyst 29 is within the allowable range.
[0094]
After completing the processing of S508 or S509, the CPU 41 proceeds to S510, resets the stoichiometric arrival time counter: C, and ends the execution of this routine.
[0095]
Thus, when the CPU 41 executes the catalyst deterioration detection control routine, the measuring means and the deterioration determining means according to the present invention are realized.
[0096]
Therefore, according to the exhaust gas purification apparatus for an internal combustion engine according to the present embodiment, it is possible to detect deterioration of the NOx storage reduction catalyst when the NOx storage reduction catalyst is provided in the exhaust system of the internal combustion engine. It becomes.
[0097]
【The invention's effect】
According to the exhaust gas purification apparatus for an internal combustion engine according to the present invention, it is possible to detect deterioration of the NOx catalyst in the exhaust gas purification apparatus for an internal combustion engine provided with the NOx catalyst.
[Brief description of the drawings]
FIG. 1 is a diagram showing a schematic configuration of an internal combustion engine to which the present invention is applied.
FIG. 2 is a block diagram showing the internal configuration of the ECU
FIG. 3 is a diagram showing the behavior of oxygen concentration on the upstream side and downstream side of the NOx storage reduction catalyst.
FIG. 4 is a graph showing the relationship between the deterioration degree of the oxygen storage capacity of the NOx storage reduction catalyst and the stoichiometric time.
FIG. 5 is a flowchart showing a catalyst deterioration detection control routine.
[Explanation of symbols]
1 ... Internal combustion engine
29 ... NOx storage reduction catalyst (NOx catalyst)
35 ... ECU
38 ... Oxygen concentration sensor
41 ... CPU

Claims (3)

A NOx catalyst provided in the exhaust passage of the internal combustion engine and having an oxygen storage capacity;
An oxygen concentration sensor that is provided in an exhaust passage downstream of the NOx catalyst and detects an oxygen concentration in the exhaust;
Rie order to occlude the oxygen in the the NO x catalyst after the exhaust gas flowing into the NOx catalyst is rich or more consecutive time it takes to release all the oxygen stored in the the NO x catalyst Measuring means for measuring the time from when the oxygen concentration sensor is detected to a value at which the detected value of the oxygen concentration sensor indicates a value corresponding to the theoretical air-fuel ratio;
A deterioration determining means for determining that the NOx catalyst is deteriorated when the time measured by the measuring means is less than a predetermined time;
An exhaust emission control device for an internal combustion engine, comprising:   The measurement means is such that the detected value of the oxygen concentration sensor corresponds to the stoichiometric air-fuel ratio from the time when the exhaust gas flowing into the NOx catalyst after the NOx catalyst sulfur poisoning recovery processing is switched from rich to lean. 2. The exhaust gas purification apparatus for an internal combustion engine according to claim 1, wherein a time until a point of time when the measured value is indicated is measured.   The NOx catalyst occludes nitrogen oxides in the exhaust when the oxygen concentration of the inflowing exhaust gas is high, and releases the nitrogen oxides occluded when the oxygen concentration of the inflowing exhaust gas decreases and a reducing agent is present. 3. The exhaust gas purification apparatus for an internal combustion engine according to claim 1, wherein the NOx storage reduction type NOx catalyst is reduced while reducing the internal combustion engine.

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