JP3788331B2 - Exhaust gas purification device for internal combustion engine - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an exhaust emission control device for an internal combustion engine.
[0002]
[Prior art]
In recent years, a technique for arranging a NOx catalyst in an exhaust system of a lean combustion internal combustion engine has been proposed for the purpose of improving exhaust emission of the lean combustion internal combustion engine.
[0003]
As such a technique, for example, an “exhaust gas purification device for an internal combustion engine” as described in JP 2000-54824 A is known. The exhaust purification device for an internal combustion engine described in the above publication has a storage reduction type NOx catalyst provided in the exhaust passage of the internal combustion engine and an oxygen storage capacity provided in the exhaust passage upstream of the storage reduction type NOx catalyst. In an exhaust gas purification apparatus for an internal combustion engine equipped with an exhaust purification catalyst, the NOx storage reduction type NOx is integrated by integrating the balance between the NOx amount stored in the NOx storage reduction catalyst and the NOx release amount released from the NOx storage reduction catalyst. The NOx occlusion amount of the catalyst is estimated and calculated, and the NOx occlusion amount estimation calculation is prohibited while the exhaust purification catalyst is performing oxygen absorption and release.
[0004]
Further, the exhaust gas purification apparatus for an internal combustion engine described in the above publication temporarily changes the air-fuel ratio of the exhaust gas discharged from the internal combustion engine when the NOx occlusion amount obtained by the above estimation calculation reaches a predetermined amount. The rich spike control with the rich air-fuel ratio is performed and the NOx occlusion amount estimation calculation is continued. When the NOx occlusion amount obtained by the estimation calculation becomes less than the predetermined amount, the execution of the rich spike control is terminated. It is configured.
[0005]
[Problems to be solved by the invention]
By the way, in the conventional technology described above, the NOx amount stored in the NOx storage reduction catalyst and the NOx amount released from the NOx storage reduction catalyst are estimated using the operating state of the internal combustion engine as a parameter. Since the actual state of the NOx catalyst is not reflected, an error may occur between the actual NOx storage amount and the estimated amount of the NOx storage reduction catalyst.
[0006]
If an error occurs between the actual NOx storage amount and the estimated amount of the NOx storage reduction catalyst, the estimated amount becomes too small or too large relative to the actual NOx storage amount when the rich spike control is executed. There is a possibility that the execution of the spike control is ended before the NOx release of the storage reduction type NOx catalyst is completed, or is ended after an excessive delay from the completion of the NOx release of the storage reduction type NOx catalyst.
[0007]
The present invention has been made in view of the above-described problems, and an object of the present invention is to provide a technique capable of suitably performing rich spike control in an exhaust gas purification apparatus for an internal combustion engine equipped with an NOx storage reduction catalyst. To do.
[0008]
[Means for Solving the Problems]
In order to achieve the above object, the exhaust gas purification apparatus for an internal combustion engine of the present invention employs the following means. That is, the first invention is provided in the exhaust passage of a lean burnable internal combustion engine, and stores NOx when the air-fuel ratio of the inflowing exhaust is lean and stores NOx when the air-fuel ratio of the inflowing exhaust is rich. A NOx storage reduction catalyst that reduces to N _2; an oxygen concentration detection means that detects an oxygen concentration in the exhaust gas downstream of the NOx storage reduction catalyst; a reducing agent supply means that supplies a reducing agent to the NOx storage reduction catalyst; The reducing agent processing execution time determining means for determining whether or not it is time to execute NOx reduction processing for releasing NOx from the NOx storage reduction catalyst and the reducing agent processing execution time determining means are determined to be the execution time. the NOx storage reduction catalyst target reducing the amount of reducing agent supplied from the reducing agent supply means so as to release the NOx stored in the NOx storage reduction catalyst causes the temperature of the when the A reducing agent amount control means for controlling the agent amount, at the time when the reducing agent supplied from the supply means of the target amount of reducing agent reducing agent to the occlusion reduction type the NO x catalyst is completed, detected by the oxygen concentration detection means When the detected value is rich, the amount of the next reducing agent supplied from the reducing agent supply means is reduced, and when the detected value detected by the oxygen concentration detection means is stoichiometric, it is supplied from the reducing agent supply means. And a reducing agent amount correcting means for increasing the amount of the next reducing agent .
[0009]
The most significant feature of the present invention is that, in the exhaust gas purification apparatus for an internal combustion engine, when the NOx stored in the NOx storage reduction catalyst is being reduced, the air-fuel ratio of the exhaust downstream of the NOx storage reduction catalyst becomes stoichiometric. In view of this, the present invention has been made to grasp the excess or deficiency of the reducing agent supply amount based on the oxygen concentration downstream of the NOx storage reduction catalyst when the reducing agent is supplied, and to correct the reducing agent supply amount.
[0010]
In the exhaust gas purification apparatus for an internal combustion engine thus configured, when it is necessary to reduce the NOx stored in the NOx storage reduction catalyst, the reducing agent supply means supplies the reducing agent to the NOx storage reduction catalyst. . Here, when the reducing agent is supplied to the NOx storage reduction catalyst and the NOx stored in the NOx storage reduction catalyst is being reduced, the air-fuel ratio of the exhaust gas downstream of the NOx storage reduction catalyst becomes stoichiometric. On the other hand, an excess relative to the target amount of reducing agent necessary amount, further reducing agent is supplied after the reduction of the NOx occluded in the NOx storage reduction catalyst is completed, occlusion reduction type NOx catalyst downstream of the exhaust The air-fuel ratio changes to a rich air-fuel ratio. Further, if the target reducing agent amount is insufficient with respect to the required amount, the air-fuel ratio of the exhaust gas downstream of the NOx storage reduction catalyst at the end of the reducing agent supply becomes stoichiometric.
[0011]
Therefore, the reducing agent amount correction means determines the excess or deficiency of the reducing agent based on the air-fuel ratio of the exhaust downstream of the NOx storage reduction catalyst when the supply of the reducing agent of the target reducing agent amount is completed , and the reducing agent supply amount Can be corrected. That is, when the air-fuel ratio of the exhaust downstream of the NOx storage reduction catalyst when the supply of the reducing agent of the target reducing agent amount is complete, the next reducing agent amount supplied from the reducing agent supply means is reduced. When the air-fuel ratio is stoichiometric, the next reducing agent amount supplied from the reducing agent supply means is increased.
[0014]
In the present invention, when the reducing agent supplied from the supply means of the target amount of reducing agent reducing agent to the occlusion reduction type the NO x catalyst is completed, the detection value detected by the oxygen concentration detection means is rich If the reducing agent amount correcting means, before Symbol longer the rich duration after detected stoichiometric to the oxygen concentration detection means, can be reduced for the next reducing agent supply amount.
[0015]
In the exhaust gas purification apparatus for an internal combustion engine configured as described above, NOx is reduced by stoichiometry, but the air-fuel ratio becomes rich due to the reducing agent supplied after the reduction of NOx is completed. Therefore, the longer the rich duration time, the more the reducing agent is supplied. Therefore, the longer the rich duration time after stoichiometry, the more the reducing agent is reduced during the next reducing agent supply, thereby suppressing the excessive supply. It becomes possible.
[0016]
In order to achieve the above object, the exhaust gas purification apparatus for an internal combustion engine of the present invention employs the following means. That is, the second invention is
An occlusion reduction type that is provided in an exhaust passage of an internal combustion engine capable of lean combustion, stores NOx when the air-fuel ratio of the inflowing exhaust gas is lean, and reduces NOx occluded when the air-fuel ratio of the inflowing exhaust gas is rich to N ₂ NOx catalyst,
Oxygen concentration detection means for detecting the oxygen concentration of the exhaust downstream of the NOx storage reduction catalyst;
Reducing agent supply means for supplying a reducing agent to the NOx storage reduction catalyst;
Reducing agent treatment execution timing determination means for determining whether or not it is time to execute NOx reduction processing for releasing NOx from the NOx storage reduction catalyst;
When the reducing agent treatment execution time determination means determines that it is an execution time, the reduction is performed so that the NOx storage reduction catalyst is heated and the NOx stored in the NOx storage reduction catalyst is released. Reducing agent amount control means for controlling the amount of reducing agent supplied from the agent supply means;
Reducing agent end timing determining means for determining a time when the detection value detected by the oxygen concentration detecting means with respect to the reducing agent amount supplied by the reducing agent amount control means has changed from stoichiometric to rich as a reducing agent supply end timing;
It is provided with.
[0017]
The most significant feature of the present invention is that, in the exhaust gas purification apparatus for an internal combustion engine, when the NOx stored in the NOx storage reduction catalyst is being reduced, the air-fuel ratio of the exhaust downstream of the NOx storage reduction catalyst becomes stoichiometric. In view of this, it is determined that the NOx reduction is completed when the stoichiometric condition is changed to the rich air-fuel ratio, and this time is determined as the reducing agent supply end timing.
[0018]
In the exhaust gas purification apparatus for an internal combustion engine thus configured, when it is necessary to reduce the NOx stored in the NOx storage reduction catalyst, the reducing agent supply means supplies the reducing agent to the NOx storage reduction catalyst. . Here, when the reducing agent is supplied to the NOx storage reduction catalyst and the NOx stored in the NOx storage reduction catalyst is being reduced, the air-fuel ratio of the exhaust gas downstream of the NOx storage reduction catalyst becomes stoichiometric. On the other hand, if the reducing agent is further supplied after the reduction of NOx stored in the NOx storage reduction catalyst is completed, the reducing agent becomes excessive, and the air-fuel ratio of the exhaust downstream of the NOx storage reduction catalyst changes to a rich air-fuel ratio. . If the supply amount of the reducing agent is insufficient with respect to the required amount, the air-fuel ratio of the exhaust gas downstream of the NOx storage reduction catalyst at the end of the supply of the reducing agent becomes stoichiometric.
[0019]
Therefore, the reducing agent end timing determination means determines the end timing of the reducing agent supply, assuming that the reduction of NOx is completed when the air-fuel ratio of the exhaust downstream of the NOx storage reduction catalyst changes from stoichiometric to rich. It becomes possible.
[0020]
Note that the amount of reducing agent corresponding to the time when the air-fuel ratio changes from stoichiometric to rich can be set as a reference for the next reducing agent supply amount.
[0021]
DETAILED DESCRIPTION OF THE INVENTION
<First Embodiment>
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. Here, the case where the exhaust gas purification apparatus for an internal combustion engine according to the present invention is applied to a diesel engine for driving a vehicle will be described as an example.
[0022]
FIG. 1 is a diagram showing a schematic configuration of an engine 1 and an intake / exhaust system to which the exhaust purification apparatus according to the present embodiment is applied.
[0023]
An engine 1 shown in FIG. 1 is a water-cooled four-cycle diesel engine having four cylinders 2.
[0024]
The engine 1 includes a fuel injection valve 3 that injects fuel directly into the combustion chamber of each cylinder 2. Each fuel injection valve 3 is connected to a pressure accumulation chamber (common rail) 4 that accumulates fuel to a predetermined pressure. A common rail pressure sensor 4 a that outputs an electrical signal corresponding to the fuel pressure in the common rail 4 is attached to the common rail 4.
[0025]
The common rail 4 communicates with a fuel pump 6 through a fuel supply pipe 5. This fuel pump 6 is a pump that operates using the rotational torque of the output shaft (crankshaft) of the engine 1 as a drive source, and a pump pulley 6 a attached to the input shaft of the fuel pump 6 is connected to the output shaft (crankshaft) of the engine 1. ) And the belt pulley 7 are connected to each other.
[0026]
In the fuel injection system configured as described above, when the rotational torque of the crankshaft is transmitted to the input shaft of the fuel pump 6, the fuel pump 6 transmits the rotational torque transmitted from the crankshaft to the input shaft of the fuel pump 6. The fuel is discharged at a pressure according to the pressure.
[0027]
The fuel discharged from the fuel pump 6 is supplied to the common rail 4 via the fuel supply pipe 5, accumulated in the common rail 4 up to a predetermined pressure, and distributed to the fuel injection valves 3 of each cylinder 2. When a drive current is applied to the fuel injection valve 3, the fuel injection valve 3 opens, and as a result, fuel is injected from the fuel injection valve 3 into the cylinder 2.
[0028]
Next, an intake branch pipe 8 is connected to the engine 1, and each branch pipe of the intake branch pipe 8 communicates with a combustion chamber of each cylinder 2 via an intake port (not shown).
[0029]
The intake branch pipe 8 is connected to an intake pipe 9, and the intake pipe 9 is connected to an air cleaner box 10. An air flow meter 11 that outputs an electric signal corresponding to the mass of the intake air flowing through the intake pipe 9 is attached to the intake pipe 9 downstream of the air cleaner box 10.
[0030]
An intake throttle valve 13 for adjusting the flow rate of the intake air flowing through the intake pipe 9 is provided at a portion of the intake pipe 9 located immediately upstream of the intake branch pipe 8. The intake throttle valve 13 is provided with an intake throttle actuator 14 that is configured by a step motor or the like and that drives the intake throttle valve 13 to open and close.
[0031]
The intake pipe 9 positioned between the air flow meter 11 and the intake throttle valve 13 is provided with a compressor housing 15a of a centrifugal supercharger (turbocharger) 15 that operates using the energy of the exhaust as a drive source. The intake pipe 9 downstream of the housing 15a is provided with an intercooler 16 for cooling the intake air that has been compressed in the compressor housing 15a and has reached a high temperature.
[0032]
In the intake system configured as described above, the intake air that has flowed into the air cleaner box 10 is removed through the intake pipe 9 after dust or dust in the intake air is removed by an air cleaner (not shown) in the air cleaner box 10. It flows into the compressor housing 15a.
[0033]
The intake air flowing into the compressor housing 15a is compressed by the rotation of the compressor wheel built in the compressor housing 15a. The intake air that has been compressed in the compressor housing 15a and has reached a high temperature is cooled by the intercooler 16, and then the flow rate is adjusted by the intake throttle valve 13 as necessary to flow into the intake branch pipe 8. The intake air that has flowed into the intake branch pipe 8 is distributed to the combustion chambers of the respective cylinders 2 through the respective branch pipes, and is burned using the fuel injected from the fuel injection valves 3 of the respective cylinders 2 as an ignition source.
[0034]
On the other hand, an exhaust branch pipe 18 is connected to the engine 1, and each branch pipe of the exhaust branch pipe 18 communicates with a combustion chamber of each cylinder 2 via an exhaust port (not shown).
[0035]
The exhaust branch pipe 18 is connected to the turbine housing 15 b of the centrifugal supercharger 15. The turbine housing 15b is connected to an exhaust pipe 19, and the exhaust pipe 19 is connected to a muffler (not shown) downstream.
[0036]
In the middle of the exhaust pipe 19, a particulate filter (hereinafter simply referred to as a filter) 20 carrying an NOx storage reduction catalyst is provided. An exhaust gas temperature sensor 24 that outputs an electrical signal corresponding to the temperature of the exhaust gas flowing through the exhaust pipe 19 is attached to the exhaust pipe 19 upstream of the filter 20. On the other hand, an air-fuel ratio sensor 37 that outputs an electrical signal corresponding to the air-fuel ratio of the exhaust gas flowing through the exhaust pipe 19 is attached to the exhaust pipe 19 downstream of the filter 20.
[0037]
The exhaust pipe 19 downstream of the air-fuel ratio sensor 37 is provided with an exhaust throttle valve 21 for adjusting the flow rate of the exhaust gas flowing through the exhaust pipe 19. The exhaust throttle valve 21 is provided with an exhaust throttle actuator 22 that is configured by a step motor or the like and that drives the exhaust throttle valve 21 to open and close.
[0038]
In the exhaust system configured as described above, the air-fuel mixture (burned gas) combusted in each cylinder 2 of the engine 1 is discharged to the exhaust branch pipe 18 through the exhaust port, and then centrifugally supercharged from the exhaust branch pipe 18. Flows into the turbine housing 15b of the machine 15. The exhaust gas flowing into the turbine housing 15b rotates the turbine wheel rotatably supported in the turbine housing 15b using the energy of the exhaust gas. At that time, the rotational torque of the turbine wheel is transmitted to the compressor wheel of the compressor housing 15a described above.
[0039]
Exhaust gas discharged from the turbine housing 15b flows into the filter 20 through the exhaust pipe 19, and particulate matter (hereinafter simply referred to as PM) in the exhaust gas is collected and harmful gas components are removed or purified. The Exhaust gas from which PM has been collected by the filter 20 and from which harmful gas components have been removed or purified is adjusted in flow rate by the exhaust throttle valve 21 as necessary, and then released into the atmosphere via the muffler.
[0040]
Further, the exhaust branch pipe 18 and the intake branch pipe 8 have an exhaust recirculation passage (hereinafter referred to as an EGR passage) 25 that recirculates a part of the exhaust gas flowing through the exhaust branch pipe 18 to the intake branch pipe 8. It is communicated through. In the middle of the EGR passage 25, a flow rate adjusting valve is configured with an electromagnetic valve or the like, and changes the flow rate of exhaust gas (hereinafter referred to as EGR gas) flowing through the EGR passage 25 in accordance with the magnitude of applied power. (Hereinafter referred to as an EGR valve) 26 is provided.
[0041]
An EGR cooler 27 that cools the EGR gas flowing in the EGR passage 25 is provided in the middle of the EGR passage 25 and upstream of the EGR valve 26. The EGR cooler 27 is provided with a cooling water passage (not shown), and a part of the cooling water for cooling the engine 1 circulates.
[0042]
In the exhaust gas recirculation mechanism configured as described above, when the EGR valve 26 is opened, the EGR passage 25 becomes conductive, and a part of the exhaust gas flowing through the exhaust branch pipe 18 flows into the EGR passage 25. Then, it is guided to the intake branch pipe 8 through the EGR cooler 27.
[0043]
At that time, in the EGR cooler 27, heat exchange is performed between the EGR gas flowing in the EGR passage 25 and the cooling water of the engine 1 to cool the EGR gas.
[0044]
The EGR gas recirculated from the exhaust branch pipe 18 to the intake branch pipe 8 via the EGR passage 25 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 8.
[0045]
Here, the EGR gas contains an inert gas component that does not burn itself and has a high heat capacity, such as water (H ₂ O) and carbon dioxide (CO ₂ ). When the EGR gas is contained in the air-fuel mixture, the combustion temperature of the air-fuel mixture is lowered, and the amount of nitrogen oxide (NOx) generated is suppressed.
[0046]
Further, when the EGR gas is cooled in the EGR cooler 27, the temperature of the EGR gas itself is reduced and the volume of the EGR gas is reduced. Therefore, when the EGR gas is supplied into the combustion chamber, the atmospheric temperature in the combustion chamber is reduced. Is not increased unnecessarily, and the amount of fresh air (volume of fresh air) supplied into the combustion chamber is not unnecessarily reduced.
[0047]
Next, the filter 20 according to the present embodiment will be described.
[0048]
FIG. 2 is a cross-sectional view of the filter 20. FIG. 2A is a diagram illustrating a cross-section in the horizontal direction of the filter 20. FIG. 2B is a view showing a longitudinal section of the filter 20.
[0049]
As shown in FIGS. 2A and 2B, the filter 20 is a so-called wall flow type having a plurality of exhaust flow passages 50 and 51 extending in parallel with each other. These exhaust flow passages include an exhaust inflow passage 50 whose downstream end is closed by a plug 52 and an exhaust outflow passage 51 whose upstream end is closed by a plug 53. In FIG. 2A, hatched portions indicate plugs 53. Therefore, the exhaust inflow passages 50 and the exhaust outflow passages 51 are alternately arranged via the thin partition walls 54. In other words, the exhaust inflow passage 50 and the exhaust outflow passage 51 are arranged such that each exhaust inflow passage 50 is surrounded by four exhaust outflow passages 51 and each exhaust outflow passage 51 is surrounded by four exhaust inflow passages 50.
[0050]
The filter 20 is made of a porous material such as cordierite, so that the exhaust gas flowing into the exhaust inflow passage 50 is adjacent to the surrounding partition wall 54 as shown by an arrow in FIG. To the exhaust outlet passage 51.
[0051]
In the embodiment according to the present invention, a carrier layer made of alumina, for example, is formed on the peripheral wall surfaces of each exhaust inflow passage 50 and each exhaust outflow passage 51, that is, on both side surfaces of each partition wall 54 and on the pore inner wall surface in each partition wall 54. The NOx storage reduction catalyst is supported on the carrier.
[0052]
Next, the function of the NOx storage reduction catalyst carried by the filter 20 according to the present embodiment will be described.
[0053]
The filter 20 uses, for example, alumina as a carrier, and an alkali metal such as potassium (K), sodium (Na), lithium (Li), or cesium (Cs), and barium (Ba) or calcium (Ca). ), An alkaline earth such as lanthanum (La) or yttrium (Y), and a noble metal such as platinum (Pt). In the present embodiment, occlusion configured by supporting barium (Ba) and platinum (Pt) on a support made of alumina and further adding ceria (Ce ₂ O ₃ ) having O ₂ storage capability. A reduced NOx catalyst was employed.
[0054]
The NOx catalyst configured in this way occludes (absorbs and adsorbs) nitrogen oxides (NOx) in the exhaust when the oxygen concentration of the exhaust flowing into the NOx catalyst is high.
[0055]
On the other hand, the NOx catalyst releases the stored nitrogen oxide (NOx) when the oxygen concentration of the exhaust gas flowing into the NOx catalyst decreases. At that time, if a reducing component such as hydrocarbon (HC) or carbon monoxide (CO) is present in the exhaust, the NOx catalyst converts nitrogen oxide (NOx) released from the NOx catalyst to nitrogen (N ₂ ) can be reduced.
[0056]
By the way, when the engine 1 is operated with lean combustion, the air-fuel ratio of the exhaust discharged from the engine 1 becomes a lean atmosphere, and the oxygen concentration of the exhaust becomes high, so that nitrogen oxide (NOx) contained in the exhaust is NOx. When the lean combustion operation of the engine 1 is continued for a long period of time, the NOx storage capacity of the NOx catalyst is saturated, and nitrogen oxides (NOx) in the exhaust gas are removed by the NOx catalyst. Without being released into the atmosphere.
[0057]
In particular, in a diesel engine such as the engine 1, the lean air-fuel ratio mixture is combusted in most of the operating region, and the exhaust air-fuel ratio becomes the lean air-fuel ratio in most of the operating region accordingly. The NOx occlusion capacity is easily saturated. Here, the lean air-fuel ratio means, for example, 20 to 50 in a diesel engine, and a region in which NOx cannot be purified by a three-way catalyst.
[0058]
Therefore, when the engine 1 is in lean burn operation, before the NOx occlusion capacity of the NOx catalyst is saturated, the oxygen concentration in the exhaust gas flowing into the NOx catalyst is lowered and the concentration of the reducing agent is increased and occluded in the NOx catalyst. It is necessary to reduce the formed nitrogen oxides (NOx).
[0059]
As a method for reducing the oxygen concentration in this way, after adding fuel in the exhaust gas or increasing the amount of recirculated EGR gas to increase the amount of soot generation to a maximum, the amount of EGR gas is further increased. Methods such as low-temperature combustion (Japanese Patent No. 3116876) and fuel injection during the expansion stroke into the cylinder 2 are conceivable. In the present embodiment, a reducing agent supply mechanism for adding fuel (light oil) as a reducing agent to the exhaust gas flowing through the exhaust pipe 19 upstream from the filter 20 is provided, and the fuel is added from the reducing agent supply mechanism into the exhaust gas. As a result, the oxygen concentration of the exhaust gas flowing into the filter 20 is decreased and the concentration of the reducing agent is increased.
[0060]
As shown in FIG. 1, the reducing agent supply mechanism is attached so that its injection hole faces the exhaust branch pipe 18, and is opened by a signal from the ECU 35 to inject fuel and a reducing agent injection valve 28. A reducing agent supply path 29 that guides the fuel discharged from the fuel pump 6 to the reducing agent injection valve 28, and a shutoff that is provided in the reducing agent supply path 29 and blocks the flow of fuel in the reducing agent supply path 29. And a valve 31.
[0061]
In such a reducing agent supply mechanism, high-pressure fuel discharged from the fuel pump 6 is applied to the reducing agent injection valve 28 via the reducing agent supply path 29. Then, the reducing agent injection valve 28 is opened by a signal from the ECU 35 and fuel as a reducing agent is injected into the exhaust branch pipe 18.
[0062]
The reducing agent injected into the exhaust branch pipe 18 from the reducing agent injection valve 28 reduces the oxygen concentration of the exhaust gas flowing from the upstream side of the exhaust branch pipe 18.
[0063]
The exhaust gas having a low oxygen concentration thus formed flows into the filter 20 and is reduced to nitrogen (N ₂ ) while releasing nitrogen oxides (NOx) stored in the filter 20.
[0064]
Thereafter, the reducing agent injection valve 28 is closed by a signal from the ECU 35, and the addition of the reducing agent into the exhaust branch pipe 18 is stopped.
[0065]
The engine 1 configured as described above is provided with an electronic control unit (ECU) 35 for controlling the engine 1. The ECU 35 is a unit that controls the operating state of the engine 1 in accordance with the operating conditions of the engine 1 and the driver's request.
[0066]
Various sensors such as a common rail pressure sensor 4a, an air flow meter 11, an exhaust temperature sensor 24, a crank position sensor 33, an accelerator opening sensor 36, an air-fuel ratio sensor 37, and the like are connected to the ECU 35 through electric wiring. The output signal is input to the ECU 35.
[0067]
On the other hand, the ECU 35 is connected to the fuel injection valve 3, the intake throttle actuator 14, the exhaust throttle actuator 22, the reducing agent injection valve 28, the EGR valve 26, the shutoff valve 31 and the like via electrical wiring, The ECU 35 can be controlled.
[0068]
Here, as shown in FIG. 3, the ECU 35 includes a CPU 351, a ROM 352, a RAM 353, a backup RAM 354, an input port 356, and an output port 357, which are connected to each other by a bidirectional bus 350. , An A / D converter (A / D) 355 connected to the input port 356 is provided.
[0069]
The input port 356 receives an output signal of a sensor that outputs a digital signal format signal like the crank position sensor 33 and transmits the output signal to the CPU 351 and the RAM 353.
[0070]
The input port 356 is connected via an A / D 355 of a sensor that outputs a signal in the form of an analog signal, such as the common rail pressure sensor 4a, the air flow meter 11, the exhaust gas temperature sensor 24, the accelerator opening sensor 36, and the air / fuel ratio sensor 37. And output those output signals to the CPU 351 and RAM 353.
[0071]
The output port 357 is connected to the fuel injection valve 3, the intake throttle actuator 14, the exhaust throttle actuator 22, the EGR valve 26, the reducing agent injection valve 28, the shutoff valve 31, etc. via electrical wiring, and is output from the CPU 351. The control signal is transmitted to the fuel injection valve 3, the intake throttle actuator 14, the exhaust throttle actuator 22, the EGR valve 26, the reducing agent injection valve 28, or the shutoff valve 31.
[0072]
The ROM 352 controls a fuel injection control routine for controlling the fuel injection valve 3, an intake throttle control routine for controlling the intake throttle valve 13, an exhaust throttle control routine for controlling the exhaust throttle valve 21, and an EGR valve 26. Application programs such as an EGR control routine for performing the above and a NOx purification control routine for adding the reducing agent to the filter 20 to reduce the stored NOx are stored.
[0073]
The ROM 352 stores various control maps in addition to the application programs described above. The control map is, for example, a fuel injection amount control map indicating the relationship between the operating state of the engine 1 and the basic fuel injection amount (basic fuel injection time), and the fuel indicating the relationship between the operating state of the engine 1 and the basic fuel injection timing. The injection timing control map, the intake throttle valve opening control map showing the relationship between the operating state of the engine 1 and the target opening of the intake throttle valve 13, the relationship between the operating state of the engine 1 and the target opening of the exhaust throttle valve 21 Exhaust throttle valve opening control map, EGR valve opening control map showing the relationship between the operating state of the engine 1 and the target opening of the EGR valve 26, the operating state of the engine 1 and the target addition amount of reducing agent (or the exhaust gas A reducing agent addition amount control map showing the relationship with the target air-fuel ratio), a reducing agent injection valve control map showing the relationship between the target addition amount of the reducing agent and the opening time of the reducing agent injection valve 28, and the like.
[0074]
The RAM 353 stores output signals from the sensors, calculation results of the CPU 351, and the like. The calculation result is, for example, an engine speed calculated based on a time interval at which the crank position sensor 33 outputs a pulse signal. These data are rewritten to the latest data every time the crank position sensor 33 outputs a pulse signal.
[0075]
The backup RAM 354 is a non-volatile memory capable of storing data even after the engine 1 is stopped.
[0076]
The CPU 351 operates in accordance with an application program stored in the ROM 352 and executes fuel injection valve control, intake throttle control, exhaust throttle control, EGR control, NOx purification control, and the like.
[0077]
For example, in the NOx purification control, the CPU 351 executes so-called rich spike control in which the oxygen concentration in the exhaust gas flowing into the filter 20 is reduced in a spike manner (short time) in a relatively short cycle.
[0078]
In the rich spike control, the CPU 351 determines whether or not the rich spike control execution condition is satisfied every predetermined cycle. As this rich spike control execution condition, for example, the filter 20 is in an active state, the output signal value (exhaust temperature) of the exhaust temperature sensor 24 is equal to or lower than a predetermined upper limit value, poisoning recovery control is not executed, Etc. can be exemplified.
[0079]
When it is determined that the rich spike control execution condition as described above is satisfied, the CPU 351 controls the reducing agent injection valve 28 to inject fuel as a reducing agent from the reducing agent injection valve 28 in a spike manner. As a result, the air-fuel ratio of the exhaust gas flowing into the filter 20 is temporarily set to a predetermined target rich air-fuel ratio.
[0080]
Specifically, the CPU 351 stores the engine speed stored in the RAM 353, the output signal of the accelerator opening sensor 36 (accelerator opening), the output signal value of the air flow meter 11 (intake air amount), and the output of the air-fuel ratio sensor. Read signal, fuel injection amount, etc.
[0081]
The CPU 351 accesses the reducing agent addition amount control map in the ROM 352 using the engine speed, the accelerator opening, the intake air amount, and the fuel injection amount as parameters, and sets the air-fuel ratio of the exhaust to a preset target air-fuel ratio. The amount of addition of the reducing agent (target addition amount) required above is calculated.
[0082]
Subsequently, the CPU 351 accesses the reducing agent injection valve control map of the ROM 352 using the target addition amount as a parameter, and the reducing agent injection valve 28 necessary for injecting the reducing agent with the target addition amount from the reducing agent injection valve 28. The valve opening time (target valve opening time) is calculated.
[0083]
When the target valve opening time of the reducing agent injection valve 28 is calculated, the CPU 351 opens the reducing agent injection valve 28.
[0084]
When the target valve opening time has elapsed from the time when the reducing agent injection valve 28 is opened, the CPU 351 closes the reducing agent injection valve 28.
[0085]
Thus, when the reducing agent injection valve 28 is opened for the target valve opening time, a target addition amount of fuel is injected into the exhaust branch pipe 18 from the reducing agent injection valve 28. The reducing agent injected from the reducing agent injection valve 28 mixes with the exhaust gas flowing from the upstream side of the exhaust branch pipe 18 to form an air-fuel mixture having a target air-fuel ratio and flows into the filter 20.
[0086]
As a result, the air-fuel ratio of the exhaust gas flowing into the filter 20 changes its oxygen concentration in a relatively short cycle, and thus the filter 20 alternately shortens the storage and reduction of nitrogen oxides (NOx). It will repeat periodically.
[0087]
As described above, the air-fuel ratio of the exhaust gas flowing into the filter 20 is spiked to the target rich air-fuel ratio, and the nitrogen oxide (NOx) absorbed by the NOx storage reduction catalyst can be reduced.
[0088]
By the way, during the reduction of the NOx stored in the NOx storage reduction catalyst by the rich spike control, the exhaust of the exhaust gas flowing out from the filter 20 due to the NOx and oxygen O ₂ released from the NOx storage reduction catalyst is reduced. The fuel ratio becomes stoichiometric. That is, the air-fuel ratio of the exhaust gas downstream of the filter 20 becomes stoichiometric while the reducing agent is being added and until the reduction of NOx is completed. Therefore, NOx is reduced only during the period during which the stoichiometry continues. On the other hand, when the reducing agent is further added even though the reduction of NOx is completed, the reducing agent passes through the filter 20 without reacting with the NOx storage reduction catalyst, so the air-fuel ratio of the exhaust gas downstream of the filter 20 is rich. It becomes an air fuel ratio.
[0089]
Here, in the exhaust gas purification apparatus of the conventional internal combustion engine, the target rich air-fuel ratio related to the rich spike control is a fixed value determined on the assumption that the reducing agent supply mechanism and the filter 20 are in a typical state. Therefore, if the performance of the storage reduction type NOx catalyst, the characteristics of the reducing agent supply mechanism, the characteristics of the fuel injection valve 3 and the like change due to the passage of time of use or environmental changes, the absorption reduction type NOx catalyst absorbs them. There is a risk that the amount of the reducing agent added will be excessive or insufficient with respect to the amount of nitrogen oxide (NOx).
[0090]
Further, in a conventional internal combustion engine exhaust gas purification apparatus, the amount of reducing agent added during rich spike control is calculated based on the engine operating state up to that point. Such control was performed in an open loop, and a sufficient amount of reducing agent was added. Therefore, there is a risk of excess or deficiency of the reducing agent, and when it is excessively added, the reducing agent that is not oxidized is released into the atmosphere. On the other hand, when the addition amount is too low, the NOx storage reduction catalyst is saturated. There is a risk of NOx being released into the atmosphere. Furthermore, there is a possibility that fuel consumption may deteriorate due to excessive addition of a reducing agent.
[0091]
Accordingly, in the present embodiment, focusing on the fact that the air-fuel ratio of the exhaust downstream of the NOx storage reduction catalyst becomes stoichiometric when NOx stored in the NOx storage reduction catalyst is reduced, the exhaust gas downstream of the filter 20 is noticed. The excess / deficiency of the reducing agent addition amount is estimated by measuring the air-fuel ratio. In accordance with this, the next reducing agent addition amount is corrected to suppress excessive or insufficient reducing agent addition amount.
[0092]
Here, FIG. 4 is a time chart showing the transition of the air-fuel ratio of the exhaust downstream of the filter 20 before and after the addition of the reducing agent.
[0093]
Region {circle around (1)} is a state in which no reducing agent is added and operation is performed at a normal lean air-fuel ratio. In such a state, NOx in the exhaust is stored in the NOx storage reduction catalyst.
[0094]
Addition of the reducing agent is started at the boundary between the region (1) and the region (2).
[0095]
In region (2), the addition of a reducing agent releases nitrogen oxides (NOx) stored in the NOx storage reduction catalyst, and the released nitrogen oxides (NOx) are reduced to nitrogen (N ₂ ). It is in a state. While the balance between the reducing agent addition amount and the NOx reduction amount is maintained, the air-fuel ratio sensor 37 outputs stoichiometry.
[0096]
Reduction of NOx stored in the NOx storage reduction catalyst is completed at the boundary between region (2) and region (3). At this time, it is desirable to end the addition of the reducing agent.
[0097]
Region (3) is a state in which a reducing agent is added even though NOx is not stored in the NOx storage reduction catalyst. The excessively added reducing agent does not react with NOx in the NOx storage reduction catalyst, passes through the filter 20, and the output of the air-fuel ratio sensor 37 becomes a rich air-fuel ratio.
[0098]
The addition of the reducing agent is completed at the boundary between the region (3) and the region (4). Therefore, FIG. 4 shows the output signal of the air-fuel ratio sensor 37 when the amount of the reducing agent corresponding to the region (3) is excessively added.
[0099]
After the region (4), NOx is occluded in the NOx storage reduction catalyst.
[0100]
As described above, the air-fuel ratio of the exhaust downstream of the filter 20 varies when the reducing agent is added. Here, since the reduction of the NOx stored in the NOx storage reduction catalyst is completed at the boundary between the region (2) and the region (3), if the addition of the reducing agent is terminated at this point, excessive or insufficient It is possible to add a reducing agent that does not exist. If the addition of the reducing agent is completed before this time, NOx remains in the NOx storage reduction catalyst, and the NOx storage capacity decreases. On the other hand, when the addition of the reducing agent is finished thereafter, there is a risk that the reducing agent is released into the atmosphere.
[0101]
In the present embodiment, based on the output signal of the air-fuel ratio sensor 37, when the air-fuel ratio of the exhaust gas at the end of addition of the reducing agent is stoichiometric , increase correction is performed assuming that the reducing agent is insufficient with respect to the required amount. On the other hand, when the air-fuel ratio of the exhaust gas at the end of addition of the reducing agent is rich, the amount of reducing agent is excessive with respect to the required amount, and the amount of reduction is corrected. As described above, the amount of reducing agent (which may be the reducing agent addition time) is feedback controlled while determining whether or not the reducing agent necessary for reducing NOx stored in the NOx storage reduction catalyst is added. .
[0102]
Here, FIG. 5 is a time chart showing the relationship between the reducing agent addition time and the air-fuel ratio. The upper part shows the case where the addition time of the reducing agent is insufficient, the middle part shows the case where the addition time of the reducing agent is appropriate, and the lower part shows the case where the addition time of the reducing agent is excessively long.
[0103]
When the upper reducing agent addition time is short, the reducing agent shortage up to the required addition time is obtained, and this reducing agent shortage is used as a correction value. On the other hand, when the lower-stage reducing agent addition time is too long, the excess reducing agent is obtained and this value is used as the correction value.
[0104]
Next, the calculation method of the reducing agent addition amount (it is good also as reducing agent addition time) by this Embodiment is demonstrated.
[0105]
FIG. 6 is a flowchart showing a flow for calculating the reducing agent addition amount.
[0106]
In step S101, the reducing agent addition amount τ is calculated from the map, and rich spike control based on the calculated value is performed.
[0107]
The CPU 351 accesses the reducing agent addition amount control map in the ROM 352 using the engine speed, the accelerator opening, the intake air amount, and the fuel injection amount as parameters, and sets the air-fuel ratio of the exhaust to a preset target air-fuel ratio. The target reducing agent addition amount τmap required above is calculated and added. The calculated target reducing agent addition amount τmap is stored in the backup RAM 354 as the reducing agent addition amount τ.
[0108]
The actual NOx occlusion amount may be obtained by a NOx counter that counts the NOx amount occluded in the NOx storage reduction catalyst, and a reducing agent corresponding to the NOx occlusion amount may be supplied.
[0109]
In step S102, it is determined whether the air-fuel ratio of the exhaust after addition of the reducing agent is rich. In the present embodiment, when the air-fuel ratio after addition of the reducing agent is stoichiometric, the reducing agent is increased, while when the air-fuel ratio after addition of the reducing agent is a rich air-fuel ratio, the reducing agent is reduced.
[0110]
If an affirmative determination is made in step S102, the process proceeds to step S104. On the other hand, if a negative determination is made, the process proceeds to step S103.
[0111]
In step S103, the reducing agent increase correction is performed. The CPU 351 reads the current command reducing agent addition amount τ stored in the backup RAM 354 and adds a predetermined value 1 to this value. Here, the predetermined value 1 is smaller than the reducing agent addition amount τ. In this way, rich spike control is performed with a reducing agent in an amount obtained by adding the predetermined value 1 to the reducing agent addition amount τ. Also, a value obtained by adding the predetermined value 1 to the current command reducing agent addition amount τ is newly stored as the current command reducing agent addition amount τ.
[0112]
In step S104, reduction of the reducing agent is corrected. The CPU 351 reads the current command reducing agent addition amount τ stored in the backup RAM 354 and subtracts the predetermined value 1 from this value. Here, the predetermined value 1 is smaller than the reducing agent addition amount τ. In this way, rich spike control is performed with an amount of reducing agent obtained by subtracting the predetermined value 1 from the reducing agent addition amount τ. Further, a value obtained by subtracting the predetermined value 1 from the current command reducing agent addition amount τ is newly stored as the current command reducing agent addition amount τ. Then, it returns to step S102.
[0113]
In step S105, it is determined whether or not the air-fuel ratio of the exhaust after addition of the reducing agent is stoichiometric.
[0114]
If an affirmative determination is made in step S105, the process proceeds to step S106, whereas if a negative determination is made, the process proceeds to step S107.
[0115]
In step S106, reduction of the reducing agent is corrected. The CPU 351 reads the current command reducing agent addition amount τ stored in the backup RAM 354 and subtracts the predetermined value 2 from this value. Here, the predetermined value 2 is smaller than the predetermined value 1. In this way, rich spike control is performed with an amount of reducing agent obtained by subtracting the predetermined value 2 from the reducing agent addition amount τ. Further, a value obtained by subtracting the predetermined value 2 from the current command reducing agent addition amount τ is newly stored as the current command reducing agent addition amount τ. Thereafter, the process returns to step S105.
[0116]
In step S107, it is determined whether or not the counter value indicating the number of corrections is equal to or greater than a predetermined value N. When the correction of the reducing agent addition amount τ is performed a predetermined number of times, it is treated that the reducing agent addition amount τ has converged.
[0117]
If an affirmative determination is made in step S107, this routine is terminated. On the other hand, if a negative determination is made, the process proceeds to step S108.
[0118]
In step S108, the reducing agent increase correction is performed. The CPU 351 reads the current command reducing agent addition amount τ stored in the backup RAM 354 and adds a predetermined value 2 to this value. Here, the predetermined value 2 is smaller than the predetermined value 1. In this way, rich spike control is performed with a reducing agent in an amount obtained by adding the predetermined value 2 to the reducing agent addition amount τ. Further, a value obtained by adding the predetermined value 2 to the current command reducing agent addition amount τ is newly stored as the current command reducing agent addition amount τ. Thereafter, the process returns to step S105.
[0119]
In this way, the reducing agent addition amount τ can be converged, and the reducing agent addition amount can be corrected based on the output signal of the air-fuel ratio sensor 37.
[0120]
Here, in the conventional exhaust gas purification apparatus for an internal combustion engine, open loop control is performed in which the amount of reducing agent obtained from the NOx occlusion amount is calculated using a map and added. In such control, there is a possibility that the amount of addition of the reducing agent may be excessive or insufficient due to aging of the reducing agent injection valve, the NOx storage reduction catalyst, the air flow meter, and the like.
[0121]
In that regard, in the exhaust gas purification apparatus for an internal combustion engine according to the present embodiment, when the NOx stored in the NOx storage reduction catalyst is reduced, the air-fuel ratio of the exhaust gas downstream of the NOx storage reduction catalyst becomes stoichiometric. Paying attention, the excess / deficiency of the reducing agent addition amount is estimated by measuring the air-fuel ratio of the exhaust downstream of the filter 20. That is, the excess / deficiency amount of the reducing agent can be determined depending on whether the air-fuel ratio of the exhaust downstream of the NOx storage reduction catalyst at the end of the supply of the reducing agent is stoichiometric or rich, and the reducing agent addition amount is feedback controlled. It is possible.
[0122]
As described above, according to the exhaust gas purification apparatus for an internal combustion engine according to the present embodiment, the reducing agent addition amount for reducing NOx from the filter 20 is corrected based on the output signal of the air-fuel ratio sensor 37 downstream of the filter 20. This makes it possible to supply exhaust gas having a target air-fuel ratio to the filter 20.
<Second Embodiment>
This embodiment is different from the first embodiment in the following points.
[0123]
That is, in the first embodiment, the reducing agent addition amount control map in the ROM 352 is accessed using the engine speed, accelerator opening, intake air amount, and fuel injection amount as parameters, and the air-fuel ratio of the exhaust gas is set in advance. A target reducing agent addition amount τmap necessary for setting the target air-fuel ratio is calculated and added. The correction amount of the reducing agent is calculated by subsequent feedback control.
[0124]
On the other hand, in the present embodiment, the addition of the reducing agent is started without obtaining the initial reducing agent addition amount. Then, feedback control is performed based on the reducing agent addition amount (reducing agent addition time) when the air-fuel ratio becomes rich from stoichiometric.
[0125]
In the present embodiment, the method of calculating the initial reducing agent addition amount is different from that in the first embodiment, but the basic configuration of the engine 1 and other hardware to be applied is described in the first embodiment. Since it is common to the first embodiment, the description is omitted.
[0126]
In the present embodiment, fuel addition is started when it is necessary to add a reducing agent to the filter 20. The CPU 351 monitors the output signal of the air-fuel ratio sensor 37 at that time, and if the air-fuel ratio changes from stoichiometric to rich, the fuel addition is terminated assuming that the reduction of NOx occluded in the filter 20 is completed. At this time, the addition amount (addition time) of the added fuel is set as the reducing agent addition amount τ in step S101 of FIG. 6 according to the first embodiment, and feedback control is performed in the same manner as in step S102 and subsequent steps of FIG. .
[0127]
Thus, according to the present embodiment, when the air-fuel ratio of the exhaust gas changes richly during the supply of the reducing agent, the time when the addition of the reducing agent ends can be obtained based on the air-fuel ratio of the exhaust gas downstream of the filter 20. it can.
[0128]
Moreover, according to this Embodiment, it is not necessary to memorize | store the reducing agent addition amount previously, and simplification of an apparatus can be achieved.
[0129]
【The invention's effect】
In the exhaust gas purification apparatus for an internal combustion engine according to the present invention, it is possible to obtain the reduction completion timing of the stored NOx depending on whether the air-fuel ratio of the exhaust downstream of the NOx storage reduction catalyst is stoichiometric. The reducing agent addition amount can be corrected based on the NOx reduction completion timing thus obtained. Therefore, it is possible to supply the amount of reducing agent required when performing NOx reduction, and rich spike control can be performed with high accuracy.
[Brief description of the drawings]
FIG. 1 is a schematic configuration diagram showing an engine to which an exhaust gas purification apparatus for an internal combustion engine according to an embodiment of the present invention is applied and an intake / exhaust system thereof.
FIG. 2A is a diagram showing a transverse cross section of a particulate filter. (B) is a figure which shows the longitudinal direction cross section of a particulate filter.
FIG. 3 is a block diagram showing an internal configuration of an ECU.
FIG. 4 is a time chart showing the transition of the air-fuel ratio of the exhaust downstream of the filter 20 before and after the supply of reducing agent.
FIG. 5 is a time chart showing the relationship between the reducing agent addition time and the air-fuel ratio.
FIG. 6 is a flowchart showing a flow for calculating a reducing agent addition amount.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... Engine 1a ... Crank pulley 2 ... Cylinder 3 ... Fuel injection valve 4 ... Common rail 4a ... Common rail pressure sensor 5 ... Fuel supply pipe 6 ... .... Fuel pump 6a ... pump pulley 8 ... intake branch pipe 9 ... intake pipe 18 ... exhaust branch pipe 19 ... exhaust pipe 20 ... particulate filter 21 ... exhaust throttle Valve 24 ... Exhaust temperature sensor 24 ... Exhaust temperature sensor 26 ... EGR valve 27 ... EGR cooler 28 ... Reducing agent injection valve 29 ... Reducing agent supply path 31 ... Shut-off valve 33 ... Crank position sensor 35 ... ECU
36 Accelerator opening sensor 37 Air-fuel ratio sensor

Claims (2)

An occlusion reduction type that is provided in an exhaust passage of a lean burnable internal combustion engine and stores NOx when the air-fuel ratio of the inflowing exhaust is lean and reduces NOx stored when the air-fuel ratio of the inflowing exhaust is rich to N ₂ NOx catalyst,
Oxygen concentration detection means for detecting the oxygen concentration of the exhaust downstream of the NOx storage reduction catalyst;
Reducing agent supply means for supplying a reducing agent to the NOx storage reduction catalyst;
Reducing agent treatment execution timing determination means for determining whether or not it is time to execute NOx reduction processing for releasing NOx from the NOx storage reduction catalyst;
When the reducing agent treatment execution time determination means determines that it is the execution time, the reducing agent is configured to raise the temperature of the NOx storage reduction catalyst and to release NOx stored in the NOx storage reduction catalyst. Reducing agent amount control means for controlling the amount of reducing agent supplied from the supply means to a target reducing agent amount ;
The reducing agent at the time the feed was finished the target reducing agent amount of the reducing agent from the supply means to the storage reduction the NO x catalyst, the when the detected value detected by the oxygen concentration detection means is rich the reducing agent A reducing agent that reduces the amount of the next reducing agent supplied from the supplying unit and increases the amount of the next reducing agent supplied from the reducing agent supplying unit when the detected value detected by the oxygen concentration detecting unit is stoichiometric. An exhaust emission control device for an internal combustion engine, comprising: an amount correction means. Wherein when the supply is finished for the target amount of reducing agent reducing agent from the reducing agent supply means to the storage reduction the NO x catalyst, when the detected value detected by the oxygen concentration detection means is rich, the reducing agent the amount correction means, the longer the rich duration after detected stoichiometric to the previous SL oxygen concentration-detecting means, the exhaust gas of the internal combustion engine according to claim 1, characterized in that the reduction of the next reducing agent supply amount Purification equipment.

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