JP2008121562A - Exhaust gas purification device for internal combustion engine - Google Patents

Abstract

An NOx storage reduction catalyst that reliably releases NOx even when the NOx storage reduction catalyst is at a low temperature.
An NOx occlusion reduction catalyst 24 that absorbs NOx in exhaust gas when the air-fuel ratio of the inflowing exhaust gas is lean and releases NOx absorbed when the air-fuel ratio of the inflowing exhaust gas becomes rich is provided in the engine. A fuel addition valve 28 is arranged in the exhaust passage and in the engine exhaust passage upstream of the NOx storage reduction catalyst. When NOx should be released from the NOx storage reduction catalyst, fuel is added from the fuel addition valve so that the air-fuel ratio of the exhaust gas flowing into the NOx storage reduction catalyst becomes temporarily rich. When the temperature of the NOx storage reduction catalyst is lower than a predetermined set temperature, the first temperature increase control for increasing the temperature of the NOx storage reduction catalyst in the first half of the fuel addition period from the fuel addition valve in the presence of a large amount of oxygen is performed. The second temperature increase control is performed to increase the temperature of the NOx storage reduction catalyst in the presence of a small amount of oxygen in the latter stage of the fuel addition period.
[Selection] Figure 1

Description

  The present invention relates to an exhaust emission control device for an internal combustion engine.

  A NOx storage reduction catalyst that absorbs NOx in the exhaust gas when the air-fuel ratio of the inflowing exhaust gas is lean and releases the absorbed NOx when the air-fuel ratio of the inflowing exhaust gas becomes rich is disposed in the engine exhaust passage In addition, a fuel addition device is arranged in the engine exhaust passage upstream of the NOx storage reduction catalyst, and when NOx should be released from the NOx storage reduction catalyst, fuel is added from the fuel addition device and flows into the NOx storage reduction catalyst. An internal combustion engine in which the air-fuel ratio of the engine is temporarily rich is known. In this internal combustion engine, NOx generated when combustion is performed under a lean air-fuel ratio is absorbed by the NOx storage reduction catalyst. On the other hand, when the NOx absorption capacity of the NOx occlusion reduction catalyst approaches saturation, the air-fuel ratio of the exhaust gas is temporarily made rich, and the oxygen concentration in the exhaust gas is lowered, whereby NOx is released from the NOx occlusion reduction catalyst and reduced. Is done.

  However, when the temperature of the NOx occlusion reduction catalyst is low, the added fuel is not sufficiently oxidized or burned by the NOx occlusion reduction catalyst, and the oxygen concentration in the exhaust gas cannot be lowered sufficiently, and therefore the NOx occlusion reduction. There is a possibility that NOx cannot be released satisfactorily from the catalyst. The same applies to the case where an oxidation catalyst is disposed in the exhaust passage upstream of the NOx storage reduction catalyst and fuel is added from the upstream of the oxidation catalyst so that the added fuel is oxidized by the oxidation catalyst.

  Therefore, an internal combustion engine is known in which a component having high low-temperature oxidizability, such as hydrogen, is added to the exhaust passage upstream of the oxidation catalyst at the beginning of the fuel addition period (see Patent Document 1). That is, even when the temperature of the NOx occlusion reduction catalyst is low, hydrogen is favorably oxidized by the oxidation catalyst, so that the temperature of the oxidation catalyst is raised, and the added fuel is favorably oxidized by the heated oxidation catalyst. As a result, the oxygen concentration in the exhaust gas is sufficiently reduced.

Japanese Patent Laid-Open No. 10-30430

  However, since hydrogen reacts at once with the oxidation catalyst, the temperature of the oxidation catalyst is high for only a short time, and therefore the oxygen concentration in the exhaust gas is kept low enough for only a short time. . In order to sufficiently release NOx from the NOx storage reduction catalyst, the oxygen concentration in the exhaust gas must be kept low for a long time.

  In order to solve the above problems, according to the present invention, when the air-fuel ratio of the inflowing exhaust gas is lean, the NOx in the exhaust gas is absorbed, and when the air-fuel ratio of the inflowing exhaust gas becomes rich, the NOx that is absorbed A NOx storage reduction catalyst for releasing NOx is disposed in the engine exhaust passage, and a fuel addition valve is disposed in the engine exhaust passage upstream of the NOx storage reduction catalyst. When NOx should be released from the NOx storage reduction catalyst, the fuel addition valve In an internal combustion engine in which the air-fuel ratio of exhaust gas flowing into the NOx storage reduction catalyst by adding fuel is temporarily rich, when the temperature of the NOx storage reduction catalyst is lower than a predetermined set temperature, In the first half of the fuel addition period from the fuel addition valve, the first temperature rise control is performed to raise the temperature of the NOx storage reduction catalyst in the presence of a large amount of oxygen, and after the fuel addition period. Small has a NOx storage reduction catalyst to perform the second heating control for raising the temperature in the presence of oxygen to.

  Even when the NOx storage reduction catalyst is at a low temperature, NOx can be reliably released from the NOx storage reduction catalyst.

  FIG. 1 shows a case where the present invention is applied to a compression ignition type internal combustion engine. However, the present invention can also be applied to a spark ignition type internal combustion engine.

  Referring to FIG. 1, 1 is an engine body, 2 is a combustion chamber of each cylinder, 3 is an electronically controlled fuel injection valve for injecting fuel into each combustion chamber 2, 4 is an intake manifold, and 5 is an exhaust manifold. Respectively. The intake manifold 4 is connected to the outlet of the compressor 7 a of the exhaust turbocharger 7 via the intake duct 6, and the inlet of the compressor 7 a is connected to the air cleaner 9 via the air flow meter 8. An electrically controlled throttle valve 10 is arranged in the intake duct 6, and a cooling device 11 for cooling intake air flowing in the intake duct 6 is arranged around the intake duct 6. In the embodiment shown in FIG. 1, the engine cooling water is guided into the cooling device 11, and the intake air is cooled by the engine cooling water. On the other hand, the exhaust manifold 5 is connected to the inlet of the exhaust turbine 7 b of the exhaust turbocharger 7, and the outlet of the exhaust turbine 7 b is connected to the exhaust aftertreatment device 20.

  The exhaust manifold 5 and the intake manifold 4 are connected to each other via an exhaust gas recirculation (hereinafter referred to as EGR) passage 12, and an electrically controlled EGR control valve 13 is disposed in the EGR passage 12. A cooling device 14 for cooling the EGR gas flowing in the EGR passage 12 is disposed around the EGR passage 12. In the embodiment shown in FIG. 1, the engine cooling water is guided into the cooling device 14, and the EGR gas is cooled by the engine cooling water. On the other hand, each fuel injection valve 3 is connected to a common rail 16 through a fuel supply pipe 15. Fuel is supplied into the common rail 16 from an electronically controlled fuel pump 17 with variable discharge amount, and the fuel supplied into the common rail 16 is supplied to the fuel injection valve 3 through each fuel supply pipe 15.

  The exhaust aftertreatment device 20 includes an exhaust pipe 21 connected to the outlet of the exhaust turbine 7 b, a catalytic converter 22 connected to the exhaust pipe 21, and an exhaust pipe 23 connected to the catalytic converter 22. A NOx occlusion reduction catalyst 24 is disposed in the catalytic converter 22. A temperature sensor 25 for detecting the temperature Tc of the NOx storage reduction catalyst 24 is attached to the catalytic converter 22, and an air-fuel ratio for detecting the air-fuel ratio AF of the exhaust gas discharged from the catalytic converter 22 is attached to the exhaust pipe 23. A sensor 26 is attached. Further, a fuel addition valve 28 is attached to the exhaust pipe 21. Fuel is supplied to the fuel addition valve 28 from the common rail 16, and fuel is added into the exhaust pipe 21 from the fuel addition valve 28. In an embodiment according to the invention, this fuel consists of light oil. The fuel addition valve 28 can be attached to the exhaust manifold 5.

  The electronic control unit 30 is composed of a digital computer, and is connected to each other by a bidirectional bus 31. A ROM (Read Only Memory) 32, a RAM (Random Access Memory) 33, a CPU (Microprocessor) 34, an input port 35 and an output port 36. It comprises. Output signals of the air flow meter 8, the temperature sensor 25, and the air-fuel ratio sensor 26 are input to the input port 35 via the corresponding AD converters 37, respectively. The accelerator pedal 39 is connected to a load sensor 40 that generates an output voltage proportional to the amount of depression of the accelerator pedal 39 representing the required torque TQ. The output voltage of the load sensor 40 is input via a corresponding AD converter 37. Input to port 35. Further, the input port 35 is connected to a crank angle sensor 41 that generates an output pulse every time the crankshaft rotates, for example, 15 °. The CPU 34 calculates the engine speed Ne based on the output pulse from the crank angle sensor 41. On the other hand, the output port 36 is connected to the fuel injection valve 3, the throttle valve 10 drive device, the EGR control valve 13, the fuel pump 17, and the fuel addition valve 28 via corresponding drive circuits 38.

  In the embodiment according to the present invention, the NOx storage reduction catalyst 24 has a honeycomb structure and includes a plurality of exhaust gas flow passages separated from each other by thin partition walls. A catalyst carrier made of alumina, for example, is supported on both side surfaces of each partition wall, and FIGS. 2A and 2B schematically show a cross section of the surface portion of the catalyst carrier 65. As shown in FIGS. 2A and 2B, a noble metal catalyst 66 is dispersedly supported on the surface of the catalyst carrier 65, and a layer of NOx absorbent 67 is further formed on the surface of the catalyst carrier 65. Is formed. Note that the NOx occlusion reduction catalyst 24 may be supported on the particulate filter.

  In the embodiment according to the present invention, platinum Pt is used as the noble metal catalyst 66, and the components constituting the NOx absorbent 67 are, for example, alkali metals such as potassium K, sodium Na, cesium Cs, barium Ba, and calcium Ca. At least one selected from rare earths such as alkaline earth, lanthanum La, and yttrium Y is used.

  When the ratio of air and fuel (hydrocarbon) supplied into the engine intake passage, the combustion chamber 2 and the exhaust passage upstream of the NOx storage reduction catalyst 24 is referred to as the air-fuel ratio of the exhaust gas, the NOx absorbent 67 When the fuel ratio is lean, NOx is absorbed, and when the oxygen concentration in the exhaust gas decreases, the NOx is absorbed and released to release the absorbed NOx.

That is, the case where barium Ba is used as a component constituting the NOx absorbent 67 will be described as an example. When the air-fuel ratio of the exhaust gas is lean, that is, when the oxygen concentration in the exhaust gas is high, the NO contained in the exhaust gas 2A is oxidized on platinum Pt66 to become NO ₂ as shown in FIG. 2 (A), and then is absorbed into the NOx absorbent 67 and combined with barium oxide BaO to form a NOx absorbent in the form of nitrate ions NO ₃ ^−. It diffuses in 67. In this way, NOx is absorbed in the NOx absorbent 67. Exhaust oxygen concentration in the _gas, NO ₂ is produced on a high as long as the surface of the platinum PT66, unless NO ₂ to NOx absorbing capability of the NOx absorbent 67 is not saturated is absorbed in the NOx absorbent 67 nitrate ions NO ₃ ^- is Generated.

On the other hand, when the air-fuel ratio of the exhaust gas is made rich or the stoichiometric air-fuel ratio, the oxidation concentration in the exhaust gas decreases, so that the reaction proceeds in the reverse direction (NO ₃ ⁻ → NO ₂ ). As shown in (B), nitrate ions NO ₃ ⁻ in the NOx absorbent 67 are released from the NOx absorbent 67 in the form of NO ₂ . Next, the released NOx is reduced by unburned HC and CO contained in the exhaust gas.

  In the internal combustion engine shown in FIG. 1, combustion is continuously performed under a lean air-fuel ratio. Therefore, the air-fuel ratio of the exhaust gas is lean, so that NOx in the exhaust gas is absorbed in the NOx absorbent 67 at this time. Is done. However, if combustion under a lean air-fuel ratio is continuously performed, the NOx absorbent capacity of the NOx absorbent 67 is saturated during that time, and therefore the NOx absorbent 67 cannot absorb NOx. Therefore, in the embodiment according to the present invention, the air-fuel ratio of the exhaust gas is temporarily made rich by adding fuel from the fuel addition valve 28 before the absorption capacity of the NOx absorbent 67 is saturated, and thereby the NOx absorbent 67 to the NOx. To be released.

  That is, in the embodiment according to the present invention, as shown by X in FIG. 3, every time the NOx integrated value ΣNOx exceeds the allowable value MAX, the fuel is added from the fuel addition valve 28 and the exhaust gas flowing into the NOx absorbent 67. The air-fuel ratio is switched to rich. In this case, the fuel is repeatedly added, for example, in a pulse shape over the fuel addition period. On the other hand, in the embodiment according to the present invention, the NOx amount dNOx absorbed per unit time by the NOx absorbent 67 is stored in advance in the ROM 32 as a function of the required torque TQ and the engine speed N in the form of a map as shown in FIG. The integrated value ΣNOx of the NOx amount absorbed in the NOx absorbent 67 is calculated by integrating the NOx amount dNOx. When fuel addition from the fuel addition valve 28 is performed, the NOx amount integrated value ΣNOx is returned to zero.

  As described at the beginning, when the temperature Tc of the NOx storage reduction catalyst 24 is low, the oxygen concentration in the exhaust gas cannot be lowered sufficiently. Therefore, in the embodiment according to the present invention, when the catalyst temperature Tc is lower than a predetermined set temperature, the NOx storage reduction catalyst 24 is heated.

  Specifically, in the embodiment according to the present invention, the first temperature rise control is performed in the first half of the fuel addition period from the fuel addition valve 28, and the second temperature rise control is performed in the second half of the fuel addition period. That is, when fuel addition from the fuel addition valve 28 is started as indicated by X in FIG. 5A, first temperature increase control is performed first, and then the first temperature increase is indicated as indicated by Y. When the control is stopped, the second temperature raising control is started, and when the fuel addition is completed as indicated by Z, the second temperature raising control is also completed. Next, the first temperature rise control and the second temperature rise control will be described.

  In the first temperature increase control, the NOx storage reduction catalyst 24 is heated in the presence of a relatively large amount of oxygen. That is, when the temperature of the NOx occlusion reduction catalyst 24 is increased, the oxidation reaction of the added fuel is promoted. With the oxidation heat generated at this time, the NOx occlusion reduction catalyst 24 is further heated and the oxidation reaction of the addition fuel is further accelerated. The In this case, if a large amount of oxygen is present in the exhaust gas flowing into the NOx occlusion reduction catalyst 24, the oxidation reaction of the added fuel even if the air-fuel ratio of the exhaust gas flowing into the NOx occlusion reduction catalyst 24 is rich. Is facilitated relatively easily, and thus the oxidation reaction rate of the added fuel is rapidly and significantly increased.

  FIG. 6A shows the experimental results of the air-fuel ratio AF of the exhaust gas exhausted from the NOx storage reduction catalyst 24 when fuel addition is performed when the catalyst temperature Tc is lower than the set temperature TX. When the temperature of the NOx occlusion reduction catalyst 24 is raised in the presence of a large amount of oxygen, as shown by the solid line in FIG. 6 (A), the air-fuel ratio AF of the exhaust gas rapidly and significantly decreases, and the air-fuel ratio AF of the exhaust gas The downward peak value AFP becomes rich beyond the set air-fuel ratio AFX set richer than the theoretical air-fuel ratio. Further, the air-fuel ratio AF of the exhaust gas is rich beyond the set air-fuel ratio AFX for the rich time tR. Therefore, it can be seen that the oxygen concentration in the exhaust gas is sufficiently reduced. On the other hand, if the temperature increase control of the NOx storage reduction catalyst 24 is not performed, the air-fuel ratio peak value AFP of the exhaust gas remains leaner than the set air-fuel ratio AFX, as shown by the broken line in FIG. Yes, the oxygen concentration in the exhaust gas is not sufficiently reduced.

  On the other hand, in the second temperature increase control, the NOx storage reduction catalyst 24 is heated in the presence of a relatively small amount of oxygen. In this way, although the oxidation reaction rate of the added fuel is increased as compared with the case where the temperature is not increased, the oxidation reaction rate is suppressed as compared with the case where the first temperature increase control is performed. As a result, the oxygen concentration in the exhaust gas is slightly higher than when the first temperature increase control is performed, but is kept relatively low for a long time.

  FIGS. 6A and 6B show the experimental results of the air-fuel ratio AF of the exhaust gas exhausted from the NOx occlusion reduction catalyst 24 when fuel is added when the catalyst temperature Tc is lower than the set temperature TX. . 6A and 6B, the dotted line indicates the case where the temperature increase control of the NOx storage reduction catalyst 24 is not performed, the broken line indicates the case where only the first temperature increase control is performed steadily, and the solid line indicates the second case. Each of the cases where only the temperature increase control is performed regularly is shown. When the first temperature raising control is performed, as indicated by broken lines in FIGS. 6A and 6B, the air-fuel ratio AF of the exhaust gas decreases rapidly and significantly, and the air-fuel ratio AF of the exhaust gas decreases downward. The peak value AFP becomes rich beyond the set air-fuel ratio AFX. Further, the air-fuel ratio AF of the exhaust gas becomes rich beyond the set air-fuel ratio AFX for the rich time tR. Therefore, it can be seen that the oxygen concentration in the exhaust gas is sufficiently reduced. On the other hand, if the temperature raising control is not performed, the exhaust gas air-fuel ratio peak value AFP remains leaner than the set air-fuel ratio AFX, as indicated by the dotted line in FIG. The oxygen concentration is not lowered sufficiently.

  Further, when the second temperature raising control is performed, as shown by the solid line in FIG. 6B, the peak value AFP of the exhaust gas air-fuel ratio becomes larger than when the first temperature raising control is performed. However, the rich time tR becomes longer.

  Therefore, in the embodiment according to the present invention, the first temperature rise control is performed in the first half of the fuel addition period from the fuel addition valve 28 so that the oxygen concentration in the exhaust gas is quickly and sufficiently lowered. Next, the control is switched to the second temperature rise control so that the oxygen concentration in the exhaust gas is kept low for a long time. As a result, even when the catalyst temperature Tc is low, the oxygen concentration in the exhaust gas can be kept sufficiently low for a long time, and therefore NOx can be reliably and sufficiently released from the NOx storage reduction catalyst 24. become able to. Further, as shown in FIG. 5, by raising the temperature of the NOx occlusion reduction catalyst 24 in synchronism with the addition of fuel, it is necessary for raising the catalyst temperature as compared with the case where the NOx occlusion reduction catalyst 24 is continuously raised. The amount of energy required can be greatly reduced.

  There are various methods for the first temperature rise control. For example, a method of heating the NOx storage reduction catalyst 24 by attaching an electric heater to the NOx storage reduction catalyst 24, a method of raising the temperature of the NOx storage reduction catalyst 24 by burning auxiliary fuel in the NOx storage reduction catalyst 24, NOx There is a method of raising the temperature of the exhaust gas flowing into the storage reduction catalyst 24. As a method for combusting the auxiliary fuel by the NOx storage reduction catalyst 24, for example, as shown in FIG. 7A, in addition to the main injection Qm performed around the compression top dead center (TDC), the end stage of the engine expansion stroke or the exhaust stroke The auxiliary fuel Qa is injected from the fuel injection valve 3. This auxiliary fuel Qa is partially oxidized in the exhaust passage upstream of the combustion chamber 2 or the NOx storage reduction catalyst 24. As a result, the activated auxiliary fuel is supplied to the NOx storage reduction catalyst 24. Therefore, the NOx storage reduction catalyst 24 burns relatively easily. On the other hand, as a method for raising the temperature of the exhaust gas flowing into the NOx storage reduction catalyst 24, for example, as shown in FIG. 7B, the auxiliary fuel Qa is injected from the fuel injection valve 3 from the initial stage to the middle stage of the expansion stroke. The auxiliary fuel Qa is burned in the exhaust passage upstream of the combustion chamber 2 or the NOx storage reduction catalyst 24, and thus the temperature of the exhaust gas is raised.

  On the other hand, there are various methods for the second temperature increase control, for example, a method of increasing the temperature of the NOx storage reduction catalyst 24 in the presence of oxygen reduced by correcting an increase in EGR gas, or a decrease correction of the amount of new air. There is a method of raising the temperature of the NOx storage reduction catalyst 24 in the presence of reduced oxygen. In this case, the opening degree of the EGR control valve 13 can be corrected to increase, or the opening degree of the throttle valve 10 can be controlled to decrease. Considering this point, it can be said that in the first temperature increase control, the NOx occlusion reduction catalyst 24 is heated without performing the increase correction of the EGR gas or the decrease correction of the fresh air amount. Alternatively, in the first temperature increase control, the NOx occlusion reduction catalyst 24 is heated without suppressing the participation reaction rate of the added fuel, and in the second temperature increase control, the NOx occlusion is performed while the oxidation reaction rate of the added fuel is suppressed. The view that the temperature of the reduction catalyst 24 is raised can also be considered.

  By the way, even if the temperature increase control is not performed, if the air-fuel ratio AF of the exhaust gas when the added fuel is performed is sufficiently rich, it is not necessary to perform the first temperature increase control. Further, if the rich time tR at this time is sufficiently long, it is not necessary to perform the second temperature increase control, and if the rich time tR is short, it is sufficient to perform only the second temperature increase control. Similarly, if the rich time tR when only the first temperature increase control is performed is sufficiently long, it is not necessary to perform the second temperature increase control.

  Therefore, in the embodiment according to the present invention, when the catalyst temperature Tc is lower than the set temperature TX, the fuel addition is performed without first performing the temperature rise control, and the peak value AFP of the exhaust gas air-fuel ratio at this time is the set air-fuel ratio. When the fuel is leaner than AFX, only the first temperature raising control is performed as shown in FIG. In addition, when the rich time tR when only the first temperature raising control is performed at the time of fuel addition is shorter than a predetermined set time tX, the first temperature raising is performed as shown in FIG. Control and second temperature rise control are performed.

  On the other hand, if the peak value AFP of the air-fuel ratio of the exhaust gas when the fuel addition is performed without performing the temperature increase control is richer than the set air-fuel ratio AFX, the first temperature increase is performed at the next fuel addition. There is no control. However, when the rich time tR at this time is shorter than the set time tX, only the second temperature raising control is performed as shown in FIG. 8B at the next fuel addition.

  FIG. 9A shows another example of the first temperature rise control, and FIG. 9B shows another example of the second temperature rise control. In the example shown in FIG. 9A, the first temperature rise control is started prior to the start of fuel addition. On the other hand, in the example shown in FIG. 9B, even if the fuel addition is completed, the second temperature rise control is continued and then completed.

  FIG. 10 shows a NOx release control routine. This routine is executed by interruption every predetermined time.

  Referring to FIG. 10, first, at step 100, the NOx amount ΣNOx absorbed in the NOx absorbent 67 is calculated. In the embodiment according to the present invention, the NOx amount dNOx absorbed per unit time is calculated from the map shown in FIG. 4, and this dNOx is added to the NOx amount ΣNOx absorbed in the NOx absorbent 67. Next, at step 101, it is determined whether or not the absorbed NOx amount ΣNOX exceeds the allowable value MAX. When ΣNOX> MAX, the routine proceeds to step 102, where it is determined whether or not the catalyst temperature Tc is lower than the set temperature TX. . When Tc ≧ TX, the routine proceeds to step 103 where fuel addition is performed without performing temperature rise control. Next, the routine proceeds to step 110. On the other hand, when Tc <TX, the routine proceeds to step 104 where it is determined whether or not the second temperature raising flag X2 is set. The second temperature raising flag X2 is set when the second temperature raising control is to be performed (X2 = 1), and otherwise it is reset (X2 = 0). When the second temperature increase flag X2 is reset, the routine proceeds to step 104, where it is determined whether or not the first temperature increase flag X1 is set. The first temperature raising flag X1 is set when the first temperature raising control is to be performed (X1 = 1), and is otherwise reset (X1 = 0). When the first temperature increase flag X1 is reset, the routine proceeds to step 103 where fuel addition is performed without performing temperature increase control. Next, the routine proceeds to step 110. On the other hand, when the first temperature raising flag X1 is set, the routine proceeds from step 105 to step 106, where fuel is added and only the first temperature raising control is performed. Next, the routine proceeds to step 110.

  On the other hand, when the second temperature increase flag X2 is set, the routine proceeds from step 104 to step 107, where it is determined whether or not the first temperature increase flag X1 is set. When the first temperature raising flag X1 is reset, the routine proceeds to step 108 where fuel addition is performed and only the second temperature raising control is performed. Next, the routine proceeds to step 110. On the other hand, when the first temperature increase flag X1 is set, the routine proceeds from step 107 to step 109, where fuel is added and first temperature increase control and second temperature increase control are performed. Next, the routine proceeds to step 110.

  In step 110, the NOx amount integrated value ΣNOx is cleared (ΣNOx = 0).

  FIG. 11 shows a flag control routine. This routine is executed by interruption every predetermined time.

  Referring to FIG. 11, first, at step 120, it is judged if fuel addition has been performed from the fuel addition valve 28. When fuel addition is performed, the routine proceeds to step 121, where it is determined whether or not the first temperature raising flag X1 is set. When the first temperature raising flag X1 is reset, the routine proceeds to step 122 where the peak value AFP of the exhaust gas air-fuel ratio is detected. In the following step 123, it is determined whether or not the detected peak value AFP is larger than the set air-fuel ratio AFX. When AFP> AFX, that is, when the peak value AFP is leaner than the set air-fuel ratio AFX, the routine proceeds to step 124 where the first temperature increase flag X1 is set.

  On the other hand, when the first temperature increase flag X1 is set at step 121, or when AFP ≦ AFX at step 123, that is, when the peak value AFP is richer than the set air-fuel ratio AFX, the routine proceeds to step 125. The rich time tR is detected. In the following step 126, it is determined whether or not the rich time tR is shorter than the set time tRX. When tR <tRX, the routine proceeds to step 127, where the second temperature raising flag X2 is set. That is, when the process proceeds from step 121 to step 125, the first temperature increase flag X1 and the second temperature increase flag X2 are set, and when the process proceeds from step 123 to step 125, only the second temperature increase flag is set. . On the other hand, when tR ≧ tRX, the routine proceeds from step 126 to step 128, where both the first temperature increase flag X1 and the second temperature increase flag X2 are reset.

  By the way, as the engine operation time becomes longer, the NOx occlusion reduction catalyst 24 gradually deteriorates. As a result, even if, for example, the first temperature increase control is simply performed, the peak value AFP of the exhaust gas air-fuel ratio is set higher than the set air-fuel ratio AFX. May not be rich.

  Therefore, the degree of deterioration of the NOx storage reduction catalyst 24 is detected, and the amount of energy applied to the NOx storage reduction catalyst 24 in the first temperature increase control is compared with when the degree of deterioration of the NOx storage reduction catalyst 24 is large compared to when it is small. You can make more.

  That is, in the example shown in FIG. 12A, when the deterioration degree DD of the NOx storage reduction catalyst 24 increases, the applied energy amount E1 in the first temperature increase control is linearly increased. In the example shown in FIG. 12B, the applied energy amount E1 is kept constant until the deterioration degree DD of the NOx storage reduction catalyst 24 exceeds the threshold value DD1, and when the deterioration degree DD exceeds the threshold value DD1, the applied energy E1 is When the degree of deterioration DD is further increased in a step-like manner, the applied energy amount E1 is linearly increased. In the example shown in FIG. 12C, the applied energy E1 is kept constant until the deterioration degree DD exceeds the threshold, and the applied energy E1 is increased stepwise every time the deterioration degree DD exceeds the threshold.

  Here, the increase correction of the applied energy in the first temperature raising control is performed, for example, by increasing and correcting the energization amount in the electric heater or the amount of the auxiliary fuel Qa (see FIGS. 7A and 7B). . Further, the deterioration degree DD of the NOx occlusion reduction catalyst 24 may be detected in any way. For example, the increase range of the catalyst temperature Tc when the auxiliary fuel Qa is supplied or the added fuel is added, or the air-fuel ratio of the exhaust gas. Detection is possible based on the behavior of AF.

1 is an overall view of a compression ignition type internal combustion engine. It is sectional drawing of the surface part of a catalyst support | carrier. It is a time chart for demonstrating NOx discharge | release control. It is a figure which shows the map of NOx absorption amount dNOx per unit time. It is a time chart for demonstrating the execution pattern of 1st temperature rising control and 2nd temperature rising control. It is a time chart for demonstrating 1st temperature rising control and 2nd temperature rising control. It is a time chart for demonstrating auxiliary fuel Qa. It is a time chart for demonstrating another execution pattern of 1st temperature rising control and 2nd temperature rising control. It is a time chart for demonstrating another Example of 1st temperature rising control and 2nd temperature rising control. It is a flowchart for performing NOx release control. It is a flowchart for performing flag control. It is a map which shows the applied energy amount of 1st temperature rising control in another Example by this invention.

Explanation of symbols

5 Exhaust manifold 20 Exhaust aftertreatment device 21, 23 Exhaust pipe 24 NOx occlusion reduction catalyst 28 Fuel addition valve

Claims (2)

  A NOx storage reduction catalyst that absorbs NOx in the exhaust gas when the air-fuel ratio of the inflowing exhaust gas is lean and releases the absorbed NOx when the air-fuel ratio of the inflowing exhaust gas becomes rich is disposed in the engine exhaust passage In addition, a fuel addition valve is disposed in the engine exhaust passage upstream of the NOx storage reduction catalyst, and when NOx should be released from the NOx storage reduction catalyst, fuel is added from the fuel addition valve and flows into the NOx storage reduction catalyst. In an internal combustion engine in which the air-fuel ratio of the gas is temporarily rich, when the temperature of the NOx storage reduction catalyst is lower than a predetermined set temperature, a large amount of oxygen is added during the first period of the fuel addition period from the fuel addition valve. The first temperature increase control is performed to raise the temperature of the NOx storage reduction catalyst in the presence of the NOx storage reduction catalyst in the presence of a small amount of oxygen later in the fuel addition period. Exhaust purification device to perform the second heating control for temperature.   A means for detecting the degree of deterioration of the NOx storage reduction catalyst is provided, and the amount of applied energy in the first temperature increase control is increased when the degree of deterioration of the NOx storage reduction catalyst is large compared to when it is small. An exhaust gas purification apparatus for an internal combustion engine as described.

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