JP4556364B2 - 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]
Conventionally, when the air-fuel ratio of the inflowing exhaust gas is lean, the NO in the exhaust gas_X NO is absorbed when the oxygen concentration in the exhaust gas that flows in decreases._X NO is released and reduced_X NO is discharged from the internal combustion engine at this time while the absorbent is placed in the exhaust passage of the diesel engine and the engine is operated under a lean air-fuel ratio._X NO_X Absorb to the absorbent, and temporarily run the engine under a rich air-fuel ratio._X NO absorbed by the absorbent_X There is known an internal combustion engine that releases and reduces gas.
[0003]
However, a large amount of smoke is generated when the engine is operated under a rich air-fuel ratio at the time of engine high load high speed operation in a diesel engine._X NO from absorbent_X Cannot be released and reduced.
[0004]
Therefore, NO is present in an oxidizing atmosphere in which secondary supplied urea exists._X A selective reduction catalyst capable of selectively reducing the selective reduction catalyst and a urea supply means capable of secondary supply of urea to the selective reduction catalyst are disposed in the engine exhaust passage, and the engine operation is lean-empty during engine high-load high-speed operation. While performing under the fuel ratio, urea is supplied from the urea supply means to the selective reduction catalyst, and NO is added to the selective reduction catalyst by urea._X An exhaust gas purification device for an internal combustion engine that reduces the amount of gas is known (see Japanese Patent Application Laid-Open No. 2000-265828).
[0005]
In this exhaust emission control device, NO is obtained unless the engine is under high load and high speed operation._X The exhaust gas purification action by the absorbent is performed.
[0006]
[Problems to be solved by the invention]
However, even if the air-fuel ratio of the inflowing exhaust gas is lean, for example, NO_X NO when the absorbent temperature rises considerably_X Absorbent is NO_X No longer absorbs so NO_X A good exhaust gas purification action by the absorbent cannot be secured. Therefore, it is NO when the engine is not at high load and high speed._X If the temperature of the absorbent becomes too high, there is a problem that exhaust gas may no longer be purified well.
[0007]
SUMMARY OF THE INVENTION An object of the present invention is to provide an exhaust gas purification apparatus for an internal combustion engine that can purify exhaust gas satisfactorily regardless of engine operating conditions.
[0009]
[Means for Solving the Problems]
  UpTo solve the problem1According to the second invention, when the air-fuel ratio of the inflowing exhaust gas is lean, the NO in the exhaust gas_XNO is absorbed when the oxygen concentration in the exhaust gas that flows in decreases._XNO is released and reduced_XNO is discharged from the internal combustion engine at this time while the absorbent is disposed in the engine exhaust passage and the engine is operated under a lean air-fuel ratio._XNO_XAbsorb to the absorbent and NO_XTemporarily enrich the air-fuel ratio of the exhaust gas flowing into the absorbent to NO_XNO absorbed by the absorbent_XIn an exhaust gas purification apparatus for an internal combustion engine that performs a first exhaust gas purification action that releases and reduces NO in an oxidizing atmosphere in which a reductant that is secondarily supplied exists_XA selective reduction catalyst capable of selectively reducing the selective reduction catalyst and a reducing agent supply means capable of secondary supply of the reducing agent to the selective reduction catalyst are disposed in the engine exhaust passage, and NO per unit time._XNO flowing into the absorbent_XMeans for determining whether or not the amount is greater than a predetermined upper limit, and NO per unit time_XNO flowing into the absorbent_XWhen the amount of the catalyst is larger than the upper limit value, the reducing agent is supplied from the reducing agent supply means to the selective reduction catalyst while the engine is operated at a lean air-fuel ratio._XThe second exhaust gas purification action is performed to reduce the amount. That is, NO per unit time_XNO flowing into the absorbent_XIf the amount of_XIs NO_XNO absorbed by the absorbent_XThere is a risk of spilling from the absorbent. Therefore, in the second invention, NO per unit time_XNO flowing into the absorbent_XWhen the amount is larger than the upper limit value, the exhaust gas is purified by the selective reduction catalyst, so that the exhaust gas can be reliably purified.
[0010]
  To solve the above problems2According to the second invention, when the air-fuel ratio of the exhaust gas flowing into the exhaust passage of the internal combustion engine capable of switching between low temperature combustion and normal combustion is lean, the NO in the exhaust gas_XNO is absorbed when the oxygen concentration in the exhaust gas that flows in decreases._XNO is released and reduced_XNO is discharged from the internal combustion engine at this time while arranging the absorbent and performing low-temperature combustion or normal combustion under a lean air-fuel ratio_XNO_XNO when absorbing into the absorbent and performing low temperature combustion_XNO absorbed by the absorbent_XWhen the amount of fuel exceeds a predetermined upper limit, low-temperature combustion is temporarily performed under a rich air-fuel ratio and NO._XNO absorbed in the absorbent_XIn an exhaust gas purification apparatus for an internal combustion engine configured to perform a first exhaust gas purification action that releases and reduces NO in an oxidizing atmosphere in which a reductant supplied secondarily exists_XA selective reduction catalyst capable of selectively reducing the selective reduction catalyst and a reducing agent supply means capable of secondary supply of the reducing agent to the selective reduction catalyst are disposed in the engine exhaust passage and NO is to be used when normal combustion is to be performed._XNO absorbed by the absorbent_XWhen the amount of the catalyst exceeds the upper limit, the reducing agent is supplied from the reducing agent supply means to the selective reduction catalyst while performing normal combustion at a lean air-fuel ratio, and the selective reduction catalyst uses the reducing agent to reduce NO._XThe second exhaust gas purification action is performed to reduce the amount. That is, since it is difficult to perform normal combustion under a rich air-fuel ratio, NO NO_XNO absorbed by the absorbent_XWhen the amount of the fuel exceeds the upper limit value, it is no longer NO_XCannot be reduced satisfactorily. Therefore, in the third aspect of the invention, NO is emitted when normal combustion is performed._XNO absorbed by the absorbent_XWhen the amount of gas exceeds the upper limit, the exhaust gas is purified by the selective reduction catalyst, so that the exhaust gas can be purified reliably.
[0011]
  Also,31st according to the 2nd inventionOr 2Th departureClearlyIn the oxidizing atmosphere in which the selective reduction catalyst is present in the secondary supply of urea._XIs formed from a selective reduction catalyst capable of selectively reducing, and the reducing agent supply means is formed from urea supply means capable of secondary supply of urea to the selective reduction catalyst.
[0012]
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 shows a case where the present invention is applied to a diesel engine. However, the present invention can also be applied to a spark ignition internal combustion engine.
[0013]
Referring to FIG. 1, 1 is an engine body, 2 is a cylinder block, 3 is a cylinder head, 4 is a piston, 5 is a combustion chamber, 6 is an electrically controlled fuel injection valve, 7 is an intake valve, 8 is an intake port, 9 Is an exhaust valve, and 10 is an exhaust port. The intake port 8 is connected to a surge tank 12 via a corresponding intake branch pipe 11, and the surge tank 12 is connected to a compressor 15 of an exhaust turbocharger 14 via an intake duct 13. A throttle valve 17 driven by a step motor 16 is disposed in the intake duct 13, and a cooling device 18 for cooling intake air flowing through the intake duct 13 is disposed around the intake duct 13. In the embodiment shown in FIG. 1, the engine cooling water is guided into the cooling device 18 and the intake air is cooled by the engine cooling water.
[0014]
On the other hand, the exhaust port 10 is connected to an exhaust turbine 21 of an exhaust turbocharger 14 via an exhaust manifold 19 and an exhaust pipe 20, and an outlet of the exhaust turbine 21 is NO._X It is connected to a casing 23 containing the absorbent 22. The casing 23 is connected to a casing 26 containing a selective reduction catalyst 25 through an exhaust pipe 24. A urea supply nozzle 27 for supplying urea to the selective reduction catalyst 25 is disposed in the exhaust pipe 24, and a urea aqueous solution in a urea tank 28 is supplied to the urea supply nozzle 27 by a urea pump 29. Note that the urea pump 29 is normally stopped, so that the supply of urea to the selective reduction catalyst 25 is normally stopped.
[0015]
The exhaust manifold 19 and the surge tank 12 are connected to each other via an exhaust gas recirculation (hereinafter referred to as EGR) passage 30, and an electrically controlled EGR control valve 31 is disposed in the EGR passage 30. A cooling device 32 for cooling the EGR gas flowing in the EGR passage 30 is disposed around the EGR passage 30. In the embodiment shown in FIG. 1, the engine cooling water is guided into the cooling device 32, and the EGR gas is cooled by the engine cooling water. On the other hand, each fuel injection valve 6 is connected to a fuel reservoir, so-called common rail 33, through a fuel supply pipe 6a. Fuel is supplied into the common rail 33 from an electrically controlled fuel pump 34 with variable discharge amount, and the fuel supplied into the common rail 33 is supplied to the fuel injection valve 6 via each fuel supply pipe 6a. A fuel pressure sensor 35 for detecting the fuel pressure in the common rail 33 is attached to the common rail 33, and a fuel pump 34 is set so that the fuel pressure in the common rail 33 becomes a target fuel pressure based on an output signal of the fuel pressure sensor 35. The discharge amount is controlled.
[0016]
The electronic control unit 40 comprises a digital computer and is connected to each other by a bidirectional bus 41. A ROM (read only memory) 42, a RAM (random access memory) 43, a CPU (microprocessor) 44, an input port 45 and an output port 46 are connected. It comprises. The output signal of the fuel pressure sensor 35 is input to the input port 45 via the corresponding AD converter 47. Further, a temperature sensor 49 for detecting the temperature of the exhaust gas flowing through the exhaust pipe 24 is attached to the exhaust pipe 24, and an output signal of the temperature sensor 49 is input to the input port 45 via the corresponding AD converter 47. Is input. The temperature of the exhaust gas flowing through the exhaust pipe 24 is NO._X The temperature of the absorbent 22 is shown.
[0017]
On the other hand, a load sensor 51 that generates an output voltage proportional to the depression amount L of the accelerator pedal 50 is connected to the accelerator pedal 50, and the output voltage of the load sensor 51 is input to the input port 45 via the corresponding AD converter 47. Is done. Further, the input port 45 is connected with a crank angle sensor 52 that generates an output pulse every time the crankshaft rotates, for example, 30 °. On the other hand, the output port 46 is connected to the fuel injection valve 6, the throttle valve driving step motor 16, the EGR control valve 31, the urea pump 29, and the fuel pump 34 via corresponding drive circuits 48.
[0018]
NO_X The absorbent 22 has, for example, alumina as a carrier. On this carrier, for example, potassium K, sodium Na, lithium Li, alkali metal such as cesium Cs, alkaline earth such as barium Ba, calcium Ca, lanthanum La, yttrium Y, etc. At least one selected from such rare earths and a noble metal such as platinum Pt, palladium Pd, rhodium Rh, and iridium Ir are supported. It should be noted that NO is applied to the particulate filter for collecting fine particles made of solid carbon contained in the exhaust gas._X The absorbent 22 can also be carried.
[0019]
This NO_X The absorbent 22 is NO when the average air-fuel ratio of the inflowing exhaust gas is lean._X NO and absorbed when the oxygen concentration in the inflowing exhaust gas decreases_X NO release_X Performs absorption and release action. In this specification, the ratio of the air, fuel, and urea supplied to the exhaust passage upstream of a certain position of the exhaust passage, the combustion chamber 5, and the intake passage is referred to as the air-fuel ratio of the exhaust gas.
[0020]
NO_X The detailed absorption and release mechanism of the absorbent 22 is not completely clarified. However, the absorption / release mechanism that is currently considered can be briefly described as an example in which platinum Pt and barium Ba are supported on a carrier.
[0021]
That is, NO_X When the air-fuel ratio of the exhaust gas flowing into the absorbent becomes considerably leaner than the stoichiometric air-fuel ratio, the oxygen concentration in the inflowing exhaust gas greatly increases, and oxygen O₂Is O₂ ⁻Or O^2-It adheres to the surface of platinum Pt. On the other hand, NO in the inflowing exhaust gas is O on the surface of platinum Pt.₂ ⁻Or O^2-Reacts with NO₂(2NO + O₂→ 2NO₂). Then the generated NO₂A part of the catalyst is further oxidized on platinum Pt while being absorbed in the absorbent and combined with the barium oxide BaO.₃ ⁻Diffuses into the absorbent in the form of In this way NO_X Is NO_X Absorbed in the absorbent 22.
[0022]
In contrast, NO_X When the air-fuel ratio of the exhaust gas flowing into the absorbent becomes rich, the oxygen concentration in the exhaust gas decreases and NO₂Production amount decreases and the reaction proceeds in the reverse direction (NO₃ ⁻→ NO₂) And thus nitrate ion NO in the absorbent₃ ⁻Is NO₂Is released from the absorbent in the form of This released NO_X Is reduced by reacting with HC and CO in the exhaust gas. In this way, NO on the surface of platinum Pt.₂NO from the absorbent to the next when no longer exists₂Is released and reduced.
[0023]
On the other hand, in the embodiment shown in FIG. 1, the selective reduction catalyst 25 is a vanadium titania catalyst in which titania is used as a carrier and vanadium oxide is supported on the carrier, or a copper zeolite catalyst in which zeolite is used as a carrier and copper is supported on this carrier. Formed from. The selective reduction catalyst 25 is NO in an oxidizing atmosphere in which secondary supplied urea is present._X Can be selectively reduced. That is, even if the selective reduction catalyst 25 is in an oxidizing atmosphere, if urea is contained in the inflowing exhaust gas, the NO is reduced with this urea._X Can be reduced.
[0024]
The internal combustion engine shown in FIG. 1 can switch between low-temperature combustion and conventional normal combustion. Next, this low temperature combustion method will be briefly described with reference to FIGS.
[0025]
In the internal combustion engine shown in FIG. 1, as the EGR rate (EGR gas amount / (EGR gas amount + intake air amount)) increases, the amount of smoke generated gradually increases and reaches a peak, and the EGR rate is further increased. This time, the amount of smoke generated decreases rapidly. This will be described with reference to FIG. 2 showing the relationship between the EGR rate and smoke when the degree of cooling of the EGR gas is changed. In FIG. 2, curve A shows the case where EGR gas is strongly cooled and the EGR gas temperature is maintained at about 90 ° C., and curve B shows the case where EGR gas is cooled by a small cooling device. Curve C shows the case where the EGR gas is not forcibly cooled.
[0026]
As shown by curve A in FIG. 2, when the EGR gas is strongly cooled, the amount of smoke generated peaks when the EGR rate is slightly lower than 50%. In this case, the EGR rate is increased to about 55% or more. If this is done, smoke will hardly occur. On the other hand, as shown by the curve B in FIG. 2, when the EGR gas is slightly cooled, the amount of smoke generated peaks when the EGR rate is slightly higher than 50%. In this case, the EGR rate is approximately 65% or more. If it is made, smoke will hardly occur. Also, as shown by curve C in FIG. 2, when the EGR gas is not forcibly cooled, the amount of smoke generated reaches a peak when the EGR rate is around 55%. In this case, the EGR rate is approximately 70%. If it is more than a percentage, smoke is hardly generated.
[0027]
As described above, when the EGR gas ratio is 55% or more, smoke is not generated because the endothermic action of the EGR gas does not cause the temperature of the fuel and the surrounding gas to be so high, that is, low-temperature combustion is performed. This is because hydrocarbons do not grow to soot.
[0028]
This low-temperature combustion suppresses the generation of smoke regardless of the air-fuel ratio, while NO._X It has the characteristic that the generation amount of can be reduced. That is, when the air-fuel ratio is made rich, the fuel becomes excessive, but the combustion temperature is suppressed to a low temperature, so that the excessive fuel does not grow to the soot, and thus smoke does not occur. At this time, NO_X However, only a very small amount is generated. On the other hand, even when the average air-fuel ratio is lean, or even when the air-fuel ratio is the stoichiometric air-fuel ratio, a small amount of soot is produced if the combustion temperature is high, but the combustion temperature is suppressed to a low temperature under low-temperature combustion. NO smoke at all, NO_X However, only a very small amount is generated.
[0029]
By the way, when the required torque TQ of the engine is increased, that is, when the fuel injection amount is increased, the temperature of the fuel and the surrounding gas at the time of combustion is increased, so that it is difficult to perform low temperature combustion. That is, low-temperature combustion can be performed only when the engine is in a low load operation where the amount of heat generated by combustion is relatively small. In FIG. 3A, a region I is an operation region in which the first combustion in which the amount of inert gas in the combustion chamber 5 is larger than the amount of inert gas in which the amount of generated soot reaches a peak, that is, low temperature combustion can be performed. Region II indicates the second combustion in which the amount of inert gas in the combustion chamber is smaller than the amount of inert gas in which the amount of soot generation reaches a peak, that is, the operating region in which only normal combustion can be performed. .
[0030]
Therefore, in the internal combustion engine shown in FIG. 1, low temperature combustion is performed in the operation region I, and normal combustion is performed in the operation region II.
[0031]
3B shows the target air-fuel ratio A / F when low temperature combustion is performed in the operation region I, and FIG. 4 shows the throttle valve 17 according to the required torque TQ when low temperature combustion is performed in the operation region I. The opening, the opening of the EGR control valve 31, the EGR rate, the air-fuel ratio, the injection start timing θS, the injection completion timing θE, and the injection amount are shown. FIG. 4 also shows the opening degree of the throttle valve 17 and the like during normal combustion performed in the operation region II.
[0032]
That is, in the operation region I, low-temperature combustion is performed under a lean air-fuel ratio, and at this time NO emitted from the internal combustion engine_X Is NO_X Absorbed by the absorbent 22. Also, when performing low temperature combustion, NO_X NO absorbed by the absorbent_X Amount of absorbed NO_X When the amount Q exceeds a predetermined upper limit QM, the air-fuel ratio is temporarily switched to rich while continuing low temperature combustion, and NO._X NO absorbed in the absorbent 22_X Is released and reduced. This way NO_X Absorbent 22 is NO_X No longer saturates. As described above, smoke does not occur in low-temperature combustion even if the air-fuel ratio is rich.
[0033]
On the other hand, in the operation region II, normal combustion is performed under a lean air-fuel ratio, and at this time NO emitted from the internal combustion engine_X NO_X Absorbed by the absorbent 22. However, in normal combustion, smoke is generated when the air-fuel ratio is made rich._X NO from absorbent 22_X Can not be released.
[0034]
Therefore, in the embodiment according to the present invention, NO is emitted when normal combustion is performed._X Absorption NO of absorbent 22_X It is determined whether or not the amount Q exceeds the upper limit value QM. When Q> QM when normal combustion is performed, the urea supply nozzle 27 continues while normal combustion is continued under a lean air-fuel ratio. Urea is supplied to the selective reduction catalyst 25 from the NO, and the selective reduction catalyst 25 makes NO with the urea._X To reduce.
[0035]
In this case, absorption NO_X NO even if quantity Q exceeds upper limit QM_X The absorbent 22 is actually NO_X Until it is saturated with NO_X Is NO_X Absorbed by absorbent 22, NO_X The absorbent 22 is actually NO_X NO when saturated with_X NO flowing out from the absorbent 22_X Is reduced in the selective reduction catalyst 25. Therefore, NO in operation region II_X Absorbent 22 is NO_X NO even if saturated_X Can be prevented from being discharged into the atmosphere.
[0036]
Next, when returning from the operation region II to the operation region I, low temperature combustion is performed under a rich air-fuel ratio, and thus NO._X NO absorbed from absorbent 22_X Is released and reduced.
[0037]
Where absorption NO_X There are various methods for determining whether or not the amount Q is larger than the upper limit value QM. For example, NO_X NO in exhaust gas flowing out from absorbent 22_X NO to detect concentration_X Concentration sensor is provided and detected NO_X It can be determined that Q> QM when the concentration exceeds a predetermined upper limit value, or absorption NO_X This amount of absorption NO_X The amount can also be compared with the upper limit value QM. This absorption NO_X In order to determine the quantity Q, for example NO_X NO in the exhaust gas flowing into the absorbent 22_X NO to detect concentration_X A density sensor can also be provided.
[0038]
In the internal combustion engine shown in FIG. 1, NO exhausted from the internal combustion engine per unit time._X Is stored in advance in the form of a map shown in FIG. 5 as a function of the engine operating state, for example, the required torque TQ and the engine speed N, and this ΔQ is obtained when the engine is operating at a lean air-fuel ratio. Absorbing NO by integrating_X The quantity Q is calculated.
[0039]
By the way, NO_X Absorbent 22 contains NO_X In addition to sulfur, for example, SOX contained in the fuel is absorbed. However, this SOX is NO_X In order to release from the absorbent 22, not only the air-fuel ratio is switched to rich, but also NO._X It is necessary to raise the temperature of the absorbent 22 to about 600 ° C., for example. Therefore, NO_X NO is released when SOX is released from the absorbent 22_X The temperature of the absorbent 22 is increased. Or NO_X When the absorbent 22 is supported on the particulate filter, NO is removed if the particulates deposited on the particulate filter are removed by oxidation._X The temperature of the absorbent 22 is increased.
[0040]
However, as shown in FIG._X NO when the temperature T of the absorbent 22 becomes considerably high_X NO of absorbent 22_X Absorption capacity AC gradually decreases. Therefore, NO_X When the temperature T of the absorbent 22 becomes considerably high, NO discharged from the internal combustion engine_X Not only can not absorb_X NO absorbed from absorbent 22_X Is also released, at this time NO_X NO in absorbent 22_X Is not reduced. Such a phenomenon can occur during low temperature combustion and normal combustion.
[0041]
Therefore, in an embodiment according to the present invention, NO_X It is determined whether or not the temperature T of the absorbent 22 is higher than a predetermined upper limit value TM. Urea is supplied to 25, and NO in the selective reduction catalyst 25 by urea._X To reduce. As a result, NO_X Even if the temperature T of the absorbent 22 becomes considerably high, NO_X Can be prevented from being discharged into the atmosphere.
[0042]
On the other hand, for example, during engine high-load operation, NO discharged from the internal combustion engine per unit time_X The amount of NO is significantly increased, so NO per unit time_X NO flowing into the absorbent 22_X The amount of However, at this time NO_X Is NO_X NO absorbed by the absorbent 22_X There is a risk of spilling from the absorbent 22. Such a phenomenon can occur during low temperature combustion and normal combustion.
[0043]
Therefore, in the embodiment according to the present invention, NO per unit time is obtained._X NO flowing into the absorbent 22_X It is determined whether or not the amount is greater than a predetermined upper limit value, and NO per unit time_X NO flowing into the absorbent 22_X Is greater than the upper limit value, urea is supplied from the urea supply nozzle 27 to the selective reduction catalyst 25 while continuing the engine operation at a lean air-fuel ratio._X To reduce. As a result, NO_X NO from absorbent 22_X NO even if it leaks_X Can be prevented from being discharged into the atmosphere.
[0044]
NO per unit time_X NO flowing into the absorbent 22_X There are various methods for determining whether or not the amount is greater than the upper limit value. For example, the absorption NO described above_X As with quantity Q, NO_X NO upstream or downstream of the absorbent 22_X A concentration sensor can be attached.
[0045]
Alternatively, when low-temperature combustion is performed, NO per unit time when the number of times the air-fuel ratio is switched to rich per unit time is greater than a predetermined upper limit value._X NO flowing into the absorbent 22_X It can also be determined that the amount of is greater than the upper limit. That is, NO per unit time_X NO flowing into the absorbent 22_X When the amount of increases, NO_X Absorption NO of absorbent 22_X The quantity Q becomes greater than the upper limit QM in a short time, so NO_X NO from absorbent 22_X The air-fuel ratio switching action for releasing the fuel is frequently performed. For this reason, the number of air-fuel ratio switching operations per unit time is NO per unit time._X NO flowing into the absorbent 22_X Represents the amount of. From this point of view, NO per unit time_X NO flowing into the absorbent 22_X In the selective reduction catalyst 25 when the amount of NO is large._X Therefore, it is not necessary to frequently perform the air-fuel ratio switching action. Therefore, drivability is improved.
[0046]
In the internal combustion engine shown in FIG. 1, NO exhausted from the internal combustion engine per unit time._X NO per unit time using the amount ΔQ (FIG. 5) of_X NO flowing into the absorbent 22_X It is determined whether or not the amount is greater than the upper limit value ΔQM.
[0047]
FIG. 7 is a flowchart for executing the above-described engine operation control and urea supply control method. This flowchart is executed by interruption every predetermined time.
[0048]
Referring to FIG. 7, first, at step 100, the map of FIG._X Absorption NO of absorbent 22_X A quantity Q is calculated. In the subsequent step 101, it is determined from the map of FIG. 3A whether or not the current operation region is the region II. When the current operation region is the region I, the routine proceeds to step 102 where low temperature combustion is performed under a lean air-fuel ratio. In the following step 103, NO_X Absorption NO of absorbent 22_X It is determined whether or not the amount Q is larger than the upper limit value QM. When Q ≦ QM, the processing cycle is terminated. When Q> QM, the routine proceeds to step 104, where NO_X It is determined whether or not the temperature T of the absorbent 22 is equal to or lower than the upper limit value TM. When T> TM, the routine jumps to step 112. When T ≦ TM, the routine proceeds to step 105, where NO per unit time._X NO flowing into the absorbent 22_X It is determined whether or not the amount ΔQ is less than or equal to the upper limit value ΔQM. When ΔQ> ΔQM, the routine jumps to step 111. When ΔQ ≦ ΔQ, the routine proceeds to step 106 where low temperature combustion is performed under a rich air-fuel ratio.
[0049]
In contrast, when the current operation region is region II in step 101, the routine proceeds to step 107, where normal combustion is performed under a lean air-fuel ratio. In the following step 108, NO_X Absorption NO of absorbent 22_X It is determined whether or not the amount Q is larger than the upper limit value QM. When Q> QM, the routine proceeds to step 111. When Q ≦ QM, the routine proceeds to step 109, where it is judged if T ≦ TM. When T ≦ TM, the processing cycle ends. When T> TM, the process proceeds to step 110, where it is determined whether ΔQ ≦ ΔQM. When ΔQ ≦ ΔQM, the processing cycle is terminated, and when ΔQ> ΔQM, the routine proceeds to step 111. In step 111, the urea aqueous solution is supplied from the urea supply nozzle 27 to the selective reduction catalyst 25.
[0050]
FIG. 8 shows another embodiment according to the present invention.
[0051]
In the example shown in FIG._X A bypass pipe 60 that bypasses the absorbent 22 and a bypass control valve 61 that is opened when urea is to be supplied to the selective reduction catalyst 25 and closed otherwise are provided. When the bypass control valve 61 is opened, exhaust gas flows into the bypass pipe 60, and NO_X The absorbent 22 is bypassed.
[0052]
In the example shown in FIG._X The absorbent 22 and the selective reduction catalyst 25 are arranged in parallel. That is, a branch pipe 62 is connected to the outlet of the exhaust turbine 21 (FIG. 1), and NO is connected to each branch pipe of the branch pipe 62._X The absorbent 22 and the selective reduction catalyst 25 are connected. In the branch pipe 62, the exhaust gas is guided to the selective reduction catalyst 25 when urea is to be supplied to the selective reduction catalyst 25, and the exhaust gas is NO._X A switching valve 63 for guiding to the absorbent 22 is arranged.
[0053]
In the example shown in FIG._X An oxidation catalyst 64 is disposed in the exhaust passage upstream of the absorbent 22. The oxidation catalyst 64 carries a noble metal such as platinum, and oxidizes HC and CO in the inflowing exhaust gas, and also oxidizes NO to NO2 to oxidize NO._X NO to absorbent 22_X Promotes absorption. Further, when the particulate filter is provided, if the particulate filter is provided integrally with the oxidation catalyst 64, the particulates can be quickly removed by oxidation.
[0054]
In the embodiments described so far, NO_X A selective reduction catalyst 25 and a urea supply nozzle 27 are arranged downstream of the absorbent 22. However, the selective reduction catalyst 25 and the urea supply nozzle 27 are set to NO._X It can also be arranged upstream of the absorbent 22.
[0055]
Also, in the embodiments described so far, when low temperature combustion is performed, NO_X In order to make the air-fuel ratio of the exhaust gas flowing into the absorbent 22 rich, the average air-fuel ratio in the combustion chamber 5 is made rich. However, NO_X In order to make the air-fuel ratio of the exhaust gas flowing into the absorbent 22 rich, for example, additional fuel can be injected into the combustion chamber 5 during the latter half of the expansion stroke or during the exhaust stroke._X Additional fuel can also be injected into the exhaust passage upstream of the absorbent 22. These methods can also be applied to an internal combustion engine that cannot perform low-temperature combustion.
[0056]
Furthermore, in the embodiments described so far, urea is used as the reducing agent supplied to the selective reduction catalyst 25. However, other ammonia compounds, hydrocarbons, or hydrogen can also be used.
[0057]
【The invention's effect】
Exhaust gas can be purified well regardless of engine operating conditions.
[Brief description of the drawings]
FIG. 1 is an overall view of an internal combustion engine.
FIG. 2 is a diagram showing the amount of smoke generated.
FIG. 3 is a diagram showing an engine operating region and the like.
FIG. 4 is a diagram showing changes in throttle valve opening and the like.
FIG. 5 shows NO discharged from the internal combustion engine per unit time._X FIG.
FIG. 6 NO_X Absorbent NO_X It is a diagram which shows absorption capacity.
FIG. 7 is a flowchart for executing an engine operation control method and a urea supply control method.
FIG. 8 shows another embodiment according to the present invention.
[Explanation of symbols]
1 ... Internal combustion engine
22 ... NO_X Absorbent
25 ... Selective reduction catalyst
27 ... Urea supply nozzle

Claims (3)

Air-fuel ratio of the inflowing exhaust gas to absorb NO _X in the exhaust gas when the lean, the _the NO _X absorbent to the oxygen concentration in the exhaust gas flowing to the reducing by releasing NO _X that is absorbed decreases disposed in the engine exhaust passage, while the engine operates at a lean air-fuel ratio as well as absorb the NO _X discharged from this time the internal combustion engine in the NO _X absorbent, the exhaust gas flowing into the NO _X absorbent in the exhaust purification apparatus temporarily internal combustion engine to perform the first exhaust gas purifying function of the allowed and reduction and release the NO _X that is absorbed in the NO _X absorbent to a rich air-fuel ratio, secondarily supplied and selectively reducible selective reduction catalyst the NO _X in an oxidizing atmosphere in which a reducing agent is present, which is, in the reducing agent to the selective reduction catalyst secondarily can be supplied reducing agent supply means and the engine exhaust passage Place and unit time Comprising means an amount of the NO _X determines whether more than a predetermined upper limit value flowing into _the NO _X absorbent to Ri, the amount of the NO _X flowing into _the NO _X absorbent per unit time When the number is larger than the upper limit value, the reducing agent is supplied from the reducing agent supply means to the selective reduction catalyst while the engine is operated at a lean air-fuel ratio, and NO _X is reduced by the reducing agent in the selective reduction catalyst. An exhaust gas purification apparatus for an internal combustion engine that performs the exhaust gas purification action of the engine. The low temperature combustion and the normal in the exhaust passage of an internal combustion engine capable of switching between the combustion air-fuel ratio of the exhaust gas flowing absorbs NO _X in the exhaust gas when the lean, the oxygen concentration in the exhaust gas flowing lowered then place the _the NO _X absorbent to reduce by releasing NO _X being absorbed, NO _X absorbed NO _X discharged from this time the internal combustion engine while performing low temperature combustion or the normal combustion under a lean air-fuel ratio while absorbing the agent, the temporarily rich air-fuel ratio of the low temperature combustion when it is more than the upper limit value amounts predetermined of the NO _X that is absorbed in the NO _X absorbent when it should perform the low temperature combustion is also first in an exhaust gas purification apparatus for an internal combustion engine to perform the exhaust gas purification effect, secondarily it supplied reducing agent is allowed and reduction and release the NO _X that is absorbed in the NO _X absorbent carried out in the Oxidation present And selectively reducible selective reduction catalyst the NO _X, and the reducing agent to the selective reduction catalyst secondarily can be supplied reducing agent supply means is disposed in the engine exhaust passage in囲気 should perform normal combustion supplying a reducing agent to the selective reduction catalyst from a reducing agent supply means while normal combustion under a lean air-fuel ratio when the amount of the NO _X that is absorbed in the NO _X absorbent becomes more than the upper limit value when and an exhaust purification device of an internal combustion engine to perform the second exhaust gas purifying effect of reducing the NO _X by the reducing agent in the selective reduction catalyst. The NO _X to form a selectively reducible selective reduction catalyst in an oxidizing atmosphere urea said supplied to the selective reduction catalyst secondarily exists, secondary urea to the selective reduction catalyst to the reducing agent supply means The exhaust gas purification device for an internal combustion engine according to claim 1 or 2, wherein the exhaust gas purification device is formed from urea supply means capable of supplying to the internal combustion engine.

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