JP2008255936A - Exhaust emission control device of internal combustion engine - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To prevent a large amount of ammonia generating compound from being discharged from a catalyst even when the easiness of storage of ammonia generating compound by the catalyst is changed. <P>SOLUTION: The catalyst 27 suitable for reducing NOx contained in an exhaust gas using ammonia under excessive oxygen is disposed in the exhaust passage of an engine. A urea solution is supplied from an addition control valve 32 to the catalyst 27 to store a part of the urea supplied to the catalyst 27 in the catalyst 27 and reduce NOx contained in the exhaust gas by the ammonia generated from the urea stored in the catalyst 27. It is determined whether or not the storage capacity of the catalyst 27 is larger than an allowable upper limit predetermined beforehand. When the storage capacity of the catalyst 27 is determined to be larger than the allowable upper limit, the supply of the urea solution is prohibited. The storage capacity of the catalyst 27 is corrected based on the degree of easiness of storage which represents the easiness of storage of the urea solution by the catalyst 27. Then, it is determined whether or not the corrected storage capacity is larger than the allowable upper limit. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

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

  A catalyst suitable for reducing NOx in the exhaust gas with ammonia under an excess of oxygen is disposed in the engine exhaust passage, an aqueous urea solution is supplied to the catalyst, and part of the urea supplied to the catalyst is catalyzed. An internal combustion engine in which NOx in exhaust gas is reduced by ammonia generated from urea stored in a catalyst while being stored in the catalyst is known (see Patent Documents 1 and 2).

US Pat. No. 5,628,186 International Patent Publication No. 99/67511

  In the internal combustion engine described above, a part of the urea supplied to the catalyst is once stored in the catalyst, and a urea-derived substance such as ammonia is generated from the stored urea. A part of this urea-derived substance is used to reduce NOx, and the rest is discharged from the catalyst without being used for NOx reduction. In this case, the amount of urea-derived substances discharged from the catalyst increases as the amount of urea stored in the catalyst increases. Therefore, if a large amount of urea is stored in the catalyst, a large amount of urea-derived substances may be undesirably discharged from the catalyst.

  In this regard, if a urea aqueous solution is supplied to the catalyst in an amount commensurate with the amount of NOx discharged from the engine, it is possible to prevent a large amount of urea from being stored in the catalyst. It is thought that the substance can be prevented from being discharged from the catalyst. However, it is difficult to accurately determine the amount of NOx discharged from the engine or the amount of urea commensurate with it. Alternatively, if the amount of urea stored in the catalyst is obtained and the amount of urea aqueous solution supplied to the catalyst is controlled so that the urea storage amount does not exceed the allowable upper limit, a large amount of urea is stored in the catalyst. It is thought that it can be prevented. However, it is difficult to accurately determine the amount of urea stored in the catalyst. In any case, there is a problem that it is impossible to reliably prevent a large amount of urea-derived material from being discharged from the catalyst. If the amount of the urea aqueous solution supplied to the catalyst is reduced, it is possible to prevent a large amount of urea from being stored in the catalyst, but in this case, NOx cannot be reduced well.

  Further, for example, the ease of storing urea in the catalyst varies depending on the degree of deterioration of the catalyst and the space velocity of the exhaust gas in the catalyst, so the urea supply amount to the catalyst is determined in consideration of the degree of deterioration of the catalyst and the like. There is a need.

  In order to solve the above problems, according to the present invention, a catalyst suitable for reducing NOx in exhaust gas with ammonia under an excess of oxygen is disposed in the engine exhaust passage, and an ammonia generating compound is added to the catalyst. A supply means for supplying and a supply control means for controlling the supply amount of the ammonia generating compound, wherein the catalyst stores at least a part of the ammonia generating compound supplied to the catalyst in the catalyst. And having a function of generating ammonia from the ammonia generating compound stored in the catalyst and reducing NOx in the exhaust gas by the generated ammonia, and the storage capacity of the catalyst is a predetermined allowable upper limit capacity Further comprising a judging means for judging whether or not the storage capacity of the catalyst is larger than the allowable upper limit capacity. An exhaust gas purification apparatus for an internal combustion engine that prohibits the supply of an ammonia generating compound at the time, and further comprises a correcting means for correcting the storage capacity of the catalyst based on the degree of storage indicating the ease of storing the ammonia generating compound of the catalyst. The determining means determines whether the corrected storage capacity is larger than an allowable upper limit capacity.

  Even when the ease of storage of the ammonia-generating compound in the catalyst varies, it is possible to prevent a large amount of the ammonia-generating compound from being discharged from the catalyst.

  FIG. 1 shows a case where the present invention is applied to a compression ignition type internal combustion engine. The present invention can also be applied to a gasoline 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 via a fuel supply pipe 15, and this common rail 16 is connected to a fuel tank 18 via an electronically controlled variable discharge pump 17. The fuel in the fuel tank 18 is supplied into the common rail 16 by the fuel pump 17, 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 upstream catalytic converter 22 connected to the outlet of the exhaust turbine 7b via an exhaust pipe 21, and a downstream catalytic converter 24 connected to the upstream catalytic converter 22 via an exhaust pipe 23. It has. In the upstream catalytic converter 22, a catalyst 25 and a catalyst 26 are arranged in order from the upstream side, and in the downstream catalytic converter 24, a catalyst 27 and a catalyst 28 are arranged in order from the upstream side. The catalysts 25, 26 and 28 are composed of a catalyst having an oxidation function, for example, an oxidation catalyst or a three-way catalyst. On the other hand, the catalyst 27 is composed of a NOx selective reduction catalyst suitable for reducing NOx in the exhaust gas with ammonia under an excess of oxygen. The catalysts 25, 27, and 28 are carried on a honeycomb carrier, and the catalyst 26 is carried on a particulate filter for collecting fine particles in the exhaust gas. The exhaust pipe 23 is provided with a temperature sensor 29 for detecting the temperature of the exhaust gas flowing into the downstream catalytic converter 24. The temperature of the exhaust gas flowing into the downstream catalytic converter 24 represents the temperature of the catalyst 27.

  On the other hand, a liquid containing an ammonia generating compound that generates ammonia is stored in the tank 30, and the liquid containing the ammonia generating compound stored in the tank 30 is supplied via a supply pump 31 and an electromagnetically controlled addition control valve 32. And is supplied into the exhaust pipe 23.

  The electronic control unit 40 is composed of 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 air flow meter 8 generates an output voltage proportional to the amount of intake air, and this output voltage is input to the input port 45 via the corresponding AD converter 47. On the other hand, the output signal of the temperature sensor 29 is input to the input port 45 via the corresponding AD converter 47. A load sensor 50 that generates an output voltage proportional to the amount of depression of the accelerator pedal 49 is connected to the accelerator pedal 49, and the output voltage of the load sensor 50 is input to the input port 45 via the corresponding AD converter 47. Further, the input port 45 is connected to a crank angle sensor 51 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 3, the throttle valve 10 drive device, the EGR control valve 13, the fuel pump 17, the supply pump 31, and the addition control valve 32 through corresponding drive circuits 48.

  As described above, the liquid containing the ammonia generating compound is supplied into the exhaust pipe 23 upstream of the catalyst 27. There are various types of ammonia generating compounds capable of generating ammonia, and therefore various compounds can be used as the ammonia generating compound. In the embodiment according to the present invention, urea is used as the ammonia generating compound, and an aqueous urea solution is used as the liquid containing the ammonia generating compound. Therefore, hereinafter, the present invention will be described by taking as an example the case of supplying a urea aqueous solution into the exhaust pipe 23 upstream of the catalyst 27.

On the other hand, as described above, the catalyst 27 is composed of a NOx selective reduction catalyst. In the embodiment shown in FIG. 1, the catalyst V ₂ O ₅ / TiO having titania as a carrier as the NOx selective reduction catalyst and vanadium oxide supported on this carrier. ₂ (hereinafter referred to as “vanadium / titania catalyst”) or a catalyst Cu / ZSM5 (hereinafter referred to as “copper zeolite catalyst”) using zeolite as a carrier and copper supported on this carrier is used.

When urea aqueous solution is supplied to exhaust gas containing excess oxygen, NO contained in the exhaust gas is reduced on the catalyst 27 by ammonia NH ₃ generated from urea CO (NH ₂ ) ₂ (for example, 2NH ₃ + 2NO + 1 / 2O ₂ → 2N ₂ + 3H ₂ O).

  That is, urea in the supplied urea aqueous solution first adheres on the catalyst 27. At this time, if the temperature of the catalyst 27 is as high as, for example, approximately 350 ° C. or higher, urea is thermally decomposed at once and ammonia is generated.

  On the other hand, when the temperature of the catalyst 27 is approximately 132 ° C. to approximately 350 ° C., urea is once stored in the catalyst 27, and then ammonia is gradually generated and released from the urea stored in the catalyst 27. The generation of ammonia in this case is considered to be due to the form change of urea on the catalyst 27. That is, urea changes to biuret at approximately 132 ° C., biuret changes to cyanuric acid at approximately 190 ° C., and cyanuric acid changes to cyanic acid or isocyanic acid at approximately 360 ° C. Or as the elapsed time after storing in the catalyst 27 becomes long, urea changes to biuret, biuret changes to cyanuric acid, and cyanuric acid changes to cyanic acid or isocyanic acid. It is considered that ammonia is generated little by little in the process of such morphological change.

  When the aqueous solution of urea is supplied to the catalyst 27 when the temperature of the catalyst 27 is approximately 132 ° C. or lower, which is the thermal decomposition temperature of urea, urea in the aqueous urea solution is stored in the catalyst 27, and ammonia is stored from the urea stored at this time. It hardly occurs.

  However, after that, for example, when the engine acceleration operation is performed and the temperature of the catalyst 27 increases, the above-described ammonia, biuret, cyanuric acid, cyanic acid, or isocyanic acid is generated from urea stored in the catalyst 27, and In some cases, these products react with hydrocarbon HC in the exhaust gas to produce hydrogen cyanide. A part of the urea-derived substance generated from urea in this way is used to reduce NOx in the exhaust gas, but the rest is discharged from the catalyst 27 without reducing NOx.

  The amount of urea-derived substance discharged from the catalyst 27 increases as the amount of urea stored in the catalyst 27 increases. Therefore, if a large amount of urea is stored in the catalyst 27, a large amount of urea-derived material may be undesirably discharged from the catalyst 27.

  On the other hand, the amount of urea that can be stored in the catalyst 27 is limited, that is, urea can be stored in the catalyst 27 only to the storage capacity. The storage capacity varies depending on the atmosphere of the catalyst 27, for example, the temperature of the catalyst 27. Therefore, when the storage capacity of the catalyst 27 is large, a large amount of urea can be stored in the catalyst 27, and when the storage capacity of the catalyst 27 is small. Only a small amount of urea can be stored in the catalyst 27.

  Then, if the supply of the urea aqueous solution to the catalyst 27 is prohibited when the storage capacity of the catalyst 27 is large, it is possible to prevent a large amount of urea from being stored in the catalyst 27. This is the basic idea of the present invention.

  That is, in the embodiment according to the present invention, it is determined whether or not the storage capacity of the catalyst 27 is larger than a predetermined allowable upper limit capacity, and when it is determined that the storage capacity of the catalyst 27 is larger than the allowable upper limit capacity, the urea aqueous solution Supply is prohibited. In this case, it is difficult to directly determine the storage capacity of the catalyst 27. On the other hand, as shown in FIG. 2, the storage capacity SC of the catalyst 27 is larger when the temperature TC of the catalyst 27 is low than when it is high, and the storage capacity SC of the catalyst 27 is when the temperature TC of the catalyst 27 is TCX. The allowable upper limit amount SCU.

  Therefore, in the embodiment according to the present invention, the temperature TC at which the storage capacity SC of the catalyst 27 reaches the allowable upper limit capacity SCU is set to the set temperature TCX, and the storage of the catalyst 27 is performed when the temperature TC of the catalyst 27 is lower than the set temperature TCX. It is determined that the capacity SC is larger than the allowable upper limit capacity SCU, and at this time, the supply of the urea aqueous solution is prohibited. On the other hand, when the temperature of the catalyst 27 is higher than the set temperature TCX, it is determined that the storage capacity SC of the catalyst 27 is smaller than the allowable upper limit capacity SCU, and at this time, the supply of the urea aqueous solution is permitted. That is, for example, the urea aqueous solution is supplied to the catalyst 27 by an amount corresponding to the amount of NOx discharged from the engine. In this way, the amount of urea stored in the catalyst 27 does not exceed the allowable upper limit capacity SCU. Therefore, it is possible to prevent a large amount of urea from being stored in the catalyst 27 without obtaining the amount of urea actually stored in the catalyst 27.

  The allowable upper limit capacity SCU may be set in any way. For example, the allowable upper limit capacity SCU can be set so that the concentration of at least one of urea-derived substances, for example, ammonia, hydrogen cyanide, and isocyanic acid discharged after the catalyst 27 does not exceed the respective allowable upper limit values. . Alternatively, the allowable upper limit capacity SCU can be set so that the concentrations of ammonia, hydrogen cyanide, and isocyanate all subsequently discharged from the catalyst 27 do not exceed the allowable upper limit values.

  By the way, as shown in FIG. 3, the NOx purification rate EFF of the catalyst 27 becomes lower than the allowable lower limit rate EFFL when the temperature TC of the catalyst 27 is lower than the lower limit temperature TCEL or higher than the upper limit temperature TCEH. When the temperature TC is between the lower limit temperature TCEL and the upper limit temperature TCEH, the allowable lower limit ratio EFFL is higher. The set temperature TCX described above is higher than the lower limit temperature TCEL.

  Here, for example, consider the supply start timing of the urea aqueous solution after the engine is started. When the engine is started as indicated by X in FIG. 4, the temperature TC of the catalyst 27 gradually increases. At this time, the supply of the urea aqueous solution is stopped. Next, even if the temperature TC of the catalyst 27 reaches the lower limit temperature TCEL as indicated by Y in FIG. 4, the supply of the urea aqueous solution is not started. Next, as indicated by Z in FIG. 4, when the temperature TC of the catalyst 27 reaches the set temperature TCX, the supply of the urea aqueous solution is started. That is, in the embodiment according to the present invention, even if the catalyst 27 is sufficiently activated for NOx reduction, the supply of the urea aqueous solution is not started, and the temperature TC of the catalyst 27 can prevent the discharge of a large amount of urea-derived substances. At last, the supply of the aqueous urea solution starts.

  Now, the catalyst 27 deteriorates with time. When the catalyst 27 deteriorates, the urea aqueous solution becomes difficult to be stored in the catalyst 27, and the storage capacity SC of the catalyst 27 becomes small. As a result, the set temperature TCX, which is the temperature at which the storage capacity SC of the catalyst 27 becomes the allowable upper limit capacity SCU, becomes lower as the catalyst 27 deteriorates. That is, as shown in FIG. 5, when the set temperature TCX when the catalyst 27 is new, that is, when the deterioration degree DT of the catalyst 27 is zero, is referred to as a basic set temperature TCXB, the deterioration degree DT of the catalyst 27 increases. The set temperature TCX is lower than the basic set temperature TCXB. When the deterioration degree DT of the catalyst 27 is large, the set temperature TCX is lower than when the deterioration degree DT is small.

  Therefore, even when the catalyst 27 is deteriorated, if the supply of the urea aqueous solution is prohibited because the temperature TC of the catalyst 27 is lower than the basic set temperature TCXB, the actual storage capacity SC of the catalyst 27 becomes the allowable upper limit capacity SCU. However, the supply of the urea aqueous solution is prohibited in spite of being less.

  Therefore, in the embodiment according to the present invention, as shown in FIG. 6, when the deterioration degree DT of the catalyst 27 is large, a correction coefficient k that is smaller than that when it is small is introduced, and the basic set temperature TCXB is corrected with this correction coefficient k. Thus, the set temperature TCX is calculated (TCX = TCXB · k). When the temperature TC of the catalyst 27 is lower than the set temperature TCX corrected in this way, the supply of the urea aqueous solution is prohibited. As a result, when the catalyst 27 deteriorates, the supply of the urea aqueous solution is prevented when the storage capacity SC of the catalyst 27 is smaller than the allowable upper limit capacity SCU.

  The deterioration degree DT of the catalyst 27b can be expressed by, for example, an integrated amount of HC flowing into the catalyst 27 or an integrated value of the temperature of the catalyst 27.

  FIG. 7 shows a routine for executing the supply control of the urea aqueous solution according to the embodiment of the present invention, and this routine is executed by interruption every predetermined time.

  Referring to FIG. 7, first, at step 100, the correction coefficient k is calculated from the map of FIG. In the subsequent step 101, the set temperature TCX is corrected (TCX = TCX · k). In the following step 102, it is determined whether or not the temperature TC of the catalyst 27 is lower than the corrected set temperature TCX. When TC ≧ TCX, the routine proceeds to step 103 where the urea aqueous solution is allowed to be supplied. On the other hand, when TC <TCX, the routine proceeds to step 104 where the urea aqueous solution supply is prohibited.

  Next, another embodiment according to the present invention will be described.

  When the space velocity of the exhaust gas in the catalyst 27 increases, the urea aqueous solution becomes difficult to be stored in the catalyst 27, and the storage capacity SC of the catalyst 27 decreases. On the contrary, when the space velocity of the exhaust gas in the catalyst 27 is lowered, the urea aqueous solution is easily stored in the catalyst 27, and the storage capacity SC of the catalyst 27 is increased. As a result, the set temperature TCX, which is the temperature at which the storage capacity SC of the catalyst 27 becomes the allowable upper limit capacity SCU, increases as the space velocity in the catalyst 27 increases, and increases as the space velocity decreases. That is, as shown in FIG. 8, if the set temperature TCX when the exhaust gas space velocity SV is a predetermined basic space velocity SVB is referred to as a basic set temperature TCXB, the space velocity SV increases. The set temperature TCX is lower than the basic set temperature TCXB, and the set temperature TCX increases as the space velocity SV decreases.

  Therefore, in the embodiment according to the present invention, as shown in FIG. 9, a correction coefficient k is introduced which becomes smaller when the space velocity SV of the exhaust gas in the catalyst 27 is high than when it is low. The set temperature TCX is calculated by correcting TCXB (TCX = TCXB · k). When the temperature TC of the catalyst 27 is lower than the set temperature TCX corrected in this way, the supply of the urea aqueous solution is prohibited. That is, in another embodiment according to the present invention, the correction coefficient k is calculated from the map of FIG. 9 in step 100 of FIG.

  Here, if the ease of storing the ammonia generating compound of the catalyst 27 is expressed by the ease of storage, the degree of storage of the catalyst 27 becomes smaller when the degree of deterioration of the catalyst 27 is large than when the degree of deterioration is small. When the gas space velocity is high, the degree of storage of the catalyst 27 becomes smaller than when the gas space velocity is low. Then, in general terms, the storage capacity of the catalyst 27 is corrected based on the storage ease of the catalyst 27, and it is determined whether or not the corrected storage capacity is larger than the allowable upper limit capacity. In this case, the storage capacity is corrected so as to be smaller when the degree of storage is small than when the degree of storage is large.

1 is an overall view of an internal combustion engine. It is a diagram which shows the storage capacity of a catalyst. It is a diagram which shows a NOx purification rate. It is a time chart for demonstrating the Example by this invention. It is a diagram which shows the storage capacity of the catalyst which fluctuates according to the deterioration degree of a catalyst. It is a diagram which shows the map of a correction coefficient. It is a flowchart which shows the supply control routine of urea aqueous solution. It is a diagram which shows the storage capacity of the catalyst which fluctuates according to the space velocity of the exhaust gas in a catalyst. It is a diagram which shows the map of the correction coefficient in another Example by this invention.

Explanation of symbols

1 Engine Body 5 Exhaust Manifold 27 Catalyst 29 Temperature Sensor 32 Addition Control Valve

Claims (7)

  A catalyst suitable for reducing NOx in the exhaust gas with ammonia under an excess of oxygen is disposed in the engine exhaust passage, supply means for supplying the ammonia generating compound to the catalyst, Supply control means for controlling a supply amount, and the catalyst stores at least a part of the ammonia generating compound supplied to the catalyst in the catalyst and is stored in the catalyst. A determination means for determining whether or not the storage capacity of the catalyst is larger than a predetermined allowable upper limit capacity, having a function of generating ammonia from the catalyst and reducing NOx in the exhaust gas by the generated ammonia; And the supply control means prohibits the supply of the ammonia generating compound when it is determined that the storage capacity of the catalyst is larger than the allowable upper limit capacity. An exhaust gas purification apparatus for an internal combustion engine, further comprising a correcting means for correcting the storage capacity of the catalyst based on the storage ease indicating the ease of storing the ammonia generating compound of the catalyst, wherein the determining means comprises the correction An exhaust purification device for an internal combustion engine that determines whether or not the stored storage capacity is greater than an allowable upper limit capacity.   2. The exhaust gas purification apparatus for an internal combustion engine according to claim 1, wherein the correction unit corrects the storage capacity so that the storage capacity is smaller when the storage ease is smaller than when the storage ease is large.   2. The exhaust gas purification apparatus for an internal combustion engine according to claim 1, wherein the ease of storage is smaller when the degree of deterioration of the catalyst is large than when the degree of deterioration of the catalyst is small.   2. The exhaust gas purification apparatus for an internal combustion engine according to claim 1, wherein the ease of storage is smaller when the space velocity of the exhaust gas in the catalyst is high than when the space velocity is low.   The storage capacity of the catalyst is larger when the temperature of the catalyst is lower than when the temperature of the catalyst is high, and the judging means determines that the temperature of the catalyst is higher than the allowable upper limit capacity of the catalyst. 2. The exhaust gas purification apparatus for an internal combustion engine according to claim 1, wherein the catalyst storage capacity is determined to be larger than the allowable upper limit capacity when the temperature is lower than a set temperature set to be a temperature.   The correction means corrects the set temperature based on the storage ease, and the determination means has a storage capacity of the catalyst that is higher than the allowable upper limit capacity when the temperature of the catalyst is lower than the corrected set temperature. The exhaust emission control device for an internal combustion engine according to claim 5, wherein the exhaust gas purification device is determined to be larger.   2. The exhaust gas purification apparatus for an internal combustion engine according to claim 1, wherein the supply control unit permits the supply of the ammonia generating compound when it is determined that the corrected storage capacity of the catalyst is smaller than the allowable upper limit capacity.

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