WO2015049581A1

WO2015049581A1 - Exhaust gas control apparatus for internal combustion engine

Info

Publication number: WO2015049581A1
Application number: PCT/IB2014/002131
Authority: WO
Inventors: Hirohiko Ota
Original assignee: Toyota Jidosha Kabushiki Kaisha
Priority date: 2013-10-04
Filing date: 2014-10-01
Publication date: 2015-04-09
Also published as: EP3052776A1; JP2015071994A; AU2014330887A1

Abstract

An engine is provided with: a urea water supply mechanism that adds ammonia-derived urea water into an exhaust passage; an SCR catalyst that purifies exhaust gas by adding the urea water; an ammonia oxidation catalyst that is provided on a downstream side of the SCR catalyst in the exhaust passage; and a control unit that controls a urea water additive amount by the urea water supply mechanism. Compared to when a temperature of the ammonia oxidation catalyst is lower than a predetermined temperature, when the temperature of the ammonia oxidation catalyst is equal to or higher than the predetermined temperature, the control unit carries out an additive amount reduction process to reduce the urea water additive amount.

Description

EXHAUST GAS CONTROL APPARATUS FOR INTERNAL COMBUSTION ENGINE

BACKGROUND OF THE INVENTION

1. Field of the Invention

[0001] The present invention relates to an exhaust gas control apparatus for an internal combustion engine.

2. Description of Related Art

[0002] For example, as disclosed in Japanese Patent Application Publication No. 2008-115775 (JP 2008-115775 A), an exhaust gas control apparatus has been known that includes: a reduction agent supply mechanism that adds an ammonia-derived reduction agent in an exhaust passage; a reduction catalyst that purifies exhaust gas by addition of the reduction agent; an oxidation catalyst that is provided on a downstream side of the reduction catalyst in a flow direction of the exhaust gas.

[0003] The ammonia-derived reduction agent (such as urea water) that is added in the exhaust passage undergoes hydrolysis caused by exhaust heat and generates ammonia. This ammonia is adsorbed to the reduction catalyst, and thus-adsorbed ammonia removes and purifies NOx in the exhaust gas. In addition, ammonia that slips through the reduction catalyst or ammonia that is desorbed from the reduction catalyst is oxidized by the oxidation catalyst. Thus, release of ammonia into atmosphere is suppressed.

SUMMARY OF THE INVENTION

[0004] By the way, in an internal combustion engine that includes the exhaust gas control apparatus, there is a case where a process to heat the exhaust gas is carried out. When such a heating process is carried out, the oxidation catalyst is heated to a high temperature. When the oxidation catalyst is heated to the high temperature, in some cases, an oxidation reaction of ammonia may occur on the oxidation catalyst to generate NOx.

[0005] The present invention provides an exhaust gas control apparatus for an internal combustion engine that can suppress generation of NOx caused by heating of the oxidation catalyst to a high temperature.

[0006] The exhaust gas control apparatus for the internal combustion engine includes: a reduction agent supply mechanism for adding an ammonia-derived reduction agent into an exhaust passage; a reduction catalyst for purifying exhaust gas by adding the reduction agent; an oxidation catalyst provided on a downstream side of the reduction catalyst in the exhaust passage; and a control section for controlling an additive amount of the reduction agent by the reduction agent supply mechanism. Then, compared to when a temperature of the oxidation catalyst is lower than a predetermined temperature, when the temperature of the oxidation catalyst is equal to or higher than the predetermined temperature, the control section carries out an additive amount reduction process to reduce the additive amount of the reduction agent.

[0007] According to this configuration, when the oxidation catalyst is equal to or higher than the predetermined temperature and NOx is possibly generated in the oxidation catalyst, the additive amount of the ammonia-derived reduction agent that is one of causes to generate NOx is reduced. Due to a reduction of the additive amount, just as described, a desorbed amount of ammonia that is adsorbed to the reduction catalyst and an amount of ammonia that slips through the reduction catalyst are reduced. Consequently, an amount of ammonia that reaches the oxidation catalyst is reduced. Thus, it is possible to suppress generation of NOx that is caused by oxidation of ammonia in the oxidation catalyst that is heated to a high temperature. Here, the above-described predetermined temperature in this configuration is desirably set to a temperature at which it is possible to appropriately determine whether NOx is generated by an oxidation reaction of ammonia in the oxidation catalyst.

[0008] In addition, a filter to collect particulate matter in the exhaust gas is provided on an upstream side of the reduction catalyst in the exhaust passage. When a regeneration process in which the filter is regenerated by heating the exhaust gas is carried out, the temperature of the oxidation catalyst is increased by the exhaust gas at a high temperature. Thus, NOx that is caused by the oxidation of ammonia is likely to be generated in the oxidation catalyst. Thus, the control section may carry out the above-described additive amount reduction process while the regeneration process is carried out.

[0009] According to this configuration, even when the oxidation catalyst is heated to the high temperature that is equal to or higher than the predetermined temperature by the regeneration process of the filter, it is possible to suppress the generation of NOx that is caused by heating of the oxidation catalyst to the high temperature. In addition, in the above-described exhaust gas control apparatus, the control section may reduce a reduction agent additive amount for correction as the additive amount reduction process such that a reduced correction amount of the reduction agent additive amount increases as the exhaust temperature is higher.

[0010] According to this configuration, the reduction agent additive amount is reduced as the exhaust temperature is increased, that is, as NOx is more likely to be generated by promoting the oxidation reaction of ammonia in the oxidation catalyst. Consequently, the amount of ammonia that reaches the oxidation catalyst is reduced. Thus, the generation of NOx can preferably be suppressed even when the exhaust temperature is changed.

[0011] In the above-described exhaust gas control apparatus, the control section may reduce the reduction agent additive amount for correction as the additive amount reduction process such that the reduced correction amount of the reduction agent additive amount increases as the amount of ammonia that is adsorbed to the reduction catalyst is larger.

[0012] According to this configuration, the reduction agent additive amount is reduced as the amount of ammonia that is adsorbed to the reduction catalyst is increased, that is, an amount of ammonia that is desorbed from the reduction catalyst is also increased. Consequently, the amount of ammonia that reaches the oxidation catalyst is reduced. Thus, the generation of NOx can preferably be suppressed even when the amount of ammonia that is adsorbed to the reduction catalyst is changed.

[0013] In the above-described exhaust gas control apparatus, when the reduction agent supply mechanism adds the reduction agent intermittently by repeating an addition of the reduction agent and a stop of the addition of the reduction agent with a predetermined cycle, the control section may carry out the additive amount reduction process such that the predetermined cycle is longer as the exhaust temperature is higher.

[0014] According to this configuration, the addition cycle during intermittent addition is extended as the exhaust temperature is increased, that is, NOx is more likely to be generated by promoting the oxidation reaction of ammonia in the oxidation catalyst. Consequently, the number of addition of the reduction agent in a predetermined period is reduced. When the number of addition of the reduction agent is reduced just as described, the total additive amount of the reduction agent in the predetermined period is reduced. Consequently, the amount of ammonia that reaches the oxidation catalyst is reduced. Thus, the generation of NOx can preferably be suppressed even when the exhaust temperature is changed.

[0015] In the above-described exhaust gas control apparatus, when the reduction agent supply mechanism adds the reduction agent intermittently by repeating an addition of the reduction agent and a stop of the addition of the reduction agent with a predetermined cycle, the control section may carry out the additive amount reduction process such that the predetermined cycle is longer as an amount of ammonia that is adsorbed to the reduction catalyst is larger.

[0016] According to this configuration, the addition cycle during the intermittent addition is extended as the amount of ammonia that is adsorbed to the reduction catalyst is increased, that is, the amount of ammonia that is desorbed from the reduction catalyst is increased. Consequently, the number of addition of the reduction agent in the predetermined period is reduced. When the number of addition of the reduction agent is reduced just as described, the total additive amount of the reduction agent in the predetermined period is reduced. Consequently, the amount of ammonia that reaches the oxidation catalyst is reduced. Thus, the generation of NOx can preferably be suppressed even when the amount of ammonia that is adsorbed to the reduction catalyst is changed.

[0017] In the above-described exhaust gas control apparatus, as the additive amount reduction process, the control section may prohibit an addition of the reduction agent while the temperature of the oxidation catalyst is equal to or higher than the above-described predetermined temperature. According to this configuration, the addition of the reduction agent is prohibited when the oxidation catalyst is equal to or higher than the predetermined temperature and NOx is possibly generated in the oxidation catalyst. Since the addition of reduction agent is prohibited, just as described, the amount of ammonia that reaches the oxidation catalyst is sufficiently reduced. Thus, it is possible to further appropriately suppress the generation of NOx that is caused by the oxidation of ammonia in the oxidation catalyst that is heated to the high temperature.

[0018] In the above-described exhaust gas control apparatus, as the additive amount reduction process, the control section may set a prohibition period in which an addition of the reduction agent is prohibited during an addition period of the reduction agent. According to this configuration, the prohibition period in which the addition of the reduction agent is prohibited is set in the addition period of the reduction agent. Thus, compared to when such a prohibition period is not set, the total additive amount of the reduction agent in the addition period is reduced. Consequently, the amount of ammonia that reaches the oxidation catalyst is reduced. Thus, it is possible to suppress the generation of NOx that is caused by the oxidation of ammonia in the oxidation catalyst that is heated to the high temperature.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Features, advantages, and technical and industrial significance of exemplary embodiments of the invention will be described below with reference to the accompanying drawings, in which like numerals denote like elements, and wherein:

FIG 1 is a schematic drawing of a first embodiment of an exhaust gas control apparatus for an internal combustion engine and shows an internal combustion engine to which the first embodiment of the exhaust gas control apparatus is applied and a peripheral configuration of the internal combustion engine;

FIG. 2 is a timing chart that illustrates a mode of addition of urea water in the same embodiment;

FIG. 3 is a flowchart that illustrates procedures of an additive amount reduction process in the same embodiment;

FIG. 4 is a table that represents a relationship between a second exhaust temperature and a first urea correction amount in the same embodiment;

FIG. 5 is a table that represents a relationship between an ammonia adsorption amount and a second urea correction amount in the same embodiment;

FIG. 6A and FIG. 6B are timing charts that illustrate modes of addition of the urea water in a second embodiment, in which FIG. 6A is a timing chart that illustrates the mode of addition before an addition interval is corrected, and FIG. 6B is a timing chart that illustrates the mode of addition after the addition interval is corrected;

FIG. 7 is a flowchart that illustrates procedures of an additive amount reduction process in the same embodiment;

FIG. 8 is a table that represents a relationship between a second exhaust temperature and a first interval correction amount in the same embodiment;

FIG. 9 is a table that represents a relationship between an ammonia adsorption amount and a second interval correction amount in the same embodiment;

FIG. 10 is a flowchart that illustrates procedures of an additive amount reduction process in a modified example of the first embodiment; and

FIG. 11 is a timing chart that illustrates a mode of addition of the urea water in the modified example of the first embodiment.

DETAILED DESCRIPTION OF EMBODIMENTS [0020] (First Embodiment) A description will hereinafter be made on a first embodiment, in which an exhaust gas control apparatus for an internal combustion engine is embodied, with reference to FIG. 1 to FIG. 5.

[0021] FIG. 1 shows configurations of a diesel engine (hereinafter, simply referred to as an "engine") as the internal combustion engine, to which the exhaust gas control apparatus is applied, and the exhaust gas control apparatus that is provided in this engine 1. The engine 1 is provided with plural cylinders #1 to #4. Plural fuel injection valves 4a to 4d are attached to a cylinder head 2, so as to respectively correspond to the cylinders #1 to #4. These fuel injection valves 4a to 4d each injects fuel into a combustion chamber in each of the cylinders #1 to #4. In addition, the cylinder head 2 is provided with intake ports for introducing fresh air into the cylinders and exhaust ports 6a to 6d for discharging combustion gas to the outside of the cylinders, the intake ports and the exhaust ports 6a to 6d respectively corresponding to the cylinders #1 to #4.

[0022] The fuel injection valves 4a to 4d are connected to a common rail 9 that accumulates high-pressure fuel. The common rail 9 is connected to a supply pump 10. The supply pump 10 suctions fuel in a fuel tank and supplies the high-pressure fuel to the common rail 9. The high-pressure fuel, which is supplied to the common rail 9, is injected from each of the fuel injection valves 4a to 4d into the cylinder when each of the fuel injection valves 4a to 4d is opened.

[0023] Each of the intake ports is connected to an intake manifold 7. The intake manifold 7 is connected to an intake passage 3. An intake throttle valve 16 for adjusting an intake air amount is provided in this intake passage 3.

[0024] Each of the exhaust ports 6a to 6d is connected to an exhaust manifold 8. The exhaust manifold 8 is connected to an exhaust passage 26. In a middle of the exhaust passage 26, a turbocharger 11 is provided to supercharge^ the intake air that is introduced into the cylinders by using exhaust pressure. An intercooler 18 is provided at a position in the intake passage 3 that is between an intake-side compressor of the turbocharger 11 and the intake throttle valve 16. This inter cooler 18 cools the intake air that is heated by supercharging of the turbocharger 11. [0025] A first purification member 30 that purifies exhaust gas is provided in a middle of the exhaust passage 26 and on a downstream of an exhaust-side turbine of the turbocharger 11. In this first purification member 30, an oxidation catalyst 31 and a filter 32 are disposed in series with respect to a flow direction of the exhaust gas.

[0026] The oxidation catalyst 31 carries out an oxidation process of HC in the exhaust gas. The filter 32 is a filter to collect particulate matter (PM) in the exhaust gas, is formed of porous ceramic, and further carries a catalyst to promote oxidation of PM. PM in the exhaust gas is collected when passing through a porous wall of the filter 32. This filter 32 constitutes the above-described exhaust purification member.

[0027] In the vicinity of a combined section of the exhaust manifold 8, a fuel addition valve 5 is provided to supply the fuel as an additive to the oxidation catalyst 31 and the filter 32. This fuel addition valve 5 is connected to the supply pump 10 via a fuel supply pipe 27. It should be noted that a position to dispose the fuel addition valve 5 can appropriately be changed as long as the fuel addition valve 5 is in an exhaust system and is located on an upstream side of the first purification member 30. In addition, fuel injection timing may be adjusted to carry out post injection, so as to supply the fuel as the additive to the oxidation catalyst 31 and the filter 32.

[0028] When an amount of PM that is collected by the filter 32 exceeds a predetermined value, a regeneration process of the filter 32 is initiated, and the fuel is injected from the fuel addition valve 5 into the exhaust manifold 8. This fuel, which is injected from the fuel addition valve 5, is combusted once reaching the oxidation catalyst 31, and the exhaust gas is thereby heated. Then, since the exhaust gas, which is heated by the oxidation catalyst 31 , flows into the filter 32, the filter 32 is heated. Accordingly, PM that is accumulated in the filter 32 undergoes the oxidation process, and thus the filter 32 is regenerated.

[0029] A second purification member 40 that purifies the exhaust gas is provided in the middle of the exhaust passage 26 and on a downstream side of the first purification member 30. In the second purification member 40, an NOx catalyst of selective reduction type (hereinafter referred to as an SCR catalyst) 41 as a reduction catalyst is disposed to reduce and purify NOx in the exhaust gas by using a reduction agent.

[0030] Furthermore, a third purification member 50 that purifies the exhaust gas is provided in the middle of the exhaust passage 26 and on a downstream side of the second purification member 40. In the third purification member 50, an ammonia oxidation catalyst 51 is disposed to purify ammonia in the exhaust gas.

[0031] The engine 1 is provided with a urea water supply mechanism 200 as a reduction agent supply mechanism that adds urea water as an ammonia-derived reduction agent into the exhaust passage 26. The urea water supply mechanism 200 is configured by including a tank 210 that stores the urea water, a urea addition valve 230 that injects and supplies the urea water into the exhaust passage 26, a supply passage 240 that connects the urea addition valve 230 and the tank 210, and a pump 220 that is provided in a middle of the supply passage 240.

[0032] The urea addition valve 230 is provided at a position in the exhaust passage 26 that is between the first purification member 30 and the second purification member 40, and an injection hole thereof is opened toward the SCR catalyst 41. When this urea addition valve 230 is opened, the urea water is injected and supplied into the exhaust passage 26 via the supply passage 240.

[0033] The pump 220 is an electric pump and feeds the urea water from the tank 210 toward the urea addition valve 230 during forward rotation thereof. On the contrary, during reverse rotation, the pump 220 feeds the urea water from the urea addition valve 230 toward the tank 210. In other words, during the reverse rotation of the pump 220, the urea water is collected from the urea addition valve 230 and the supply passage 240 and returns to the tank 210.

[0034] A dispersion plate 60 is provided at a position in the exhaust passage 26 between the urea addition valve 230 and the SCR catalyst 41. The dispersion plate 60 disperses the urea water, which is injected from the urea addition valve 230, so as to promote atomization of the urea water. [0035] The urea water, which is injected from the urea addition valve 230, undergoes hydrolysis caused by heat of the exhaust gas and generates ammonia. This ammonia is supplied as the reduction agent of NOx to the SCR catalyst 41. Ammonia, which is supplied to the SCR catalyst 41, is adsorbed to the SCR catalyst 41 and used for reduction of NOx.

[0036] In addition to the above, the engine 1 includes an exhaust gas recirculation device (hereinafter referred to as an "EGR device"). This EGR device lowers a combustion temperature in the cylinder by partially introducing the exhaust gas to the intake air, so as to reduce an generation amount of NOx. This EGR device is configured by including an EGR passage 13 that communicates between the intake passage 3 and the exhaust manifold 8, an EGR valve 15 that is provided in the EGR passage 13, an EGR cooler 14, and the like. When an opening amount of the EGR valve 15 is adjusted, a reflux amount of the exhaust gas that is introduced from the exhaust passage 26 to the intake passage 3, that is, an outer EGR amount is adjusted. In addition, a temperature of the exhaust gas that flows through the EGR passage 13 is lowered by the EGR cooler 14.

[0037] Various sensors are attached to the engine 1 to detect an engine operation state. For example, an airflow meter 19 detects an intake air amount GA. A throttle valve opening amount sensor 20 detects an opening amount of the intake throttle valve 16. An engine speed sensor 21 detects a rotational speed of a crankshaft, that is, an engine speed NE. An accelerator sensor 22 detects a depression amount of an accelerator pedal, that is, an accelerator operation amount ACCP. An outside temperature sensor 23 detects an outside temperature THout. A vehicle speed sensor 24 detects a vehicle speed SPD of a vehicle in which the engine 1 is installed. In addition, the engine 1 is also provided with an ignition switch (hereinafter referred to as an IG switch) 25 that is operated by a driver of the vehicle to start or stop the engine 1. The engine start or the engine stop is carried out in accordance with an operation position of this IG switch 25.

[0038] A first exhaust temperature sensor 100 that is provided on an upstream side of the oxidation catalyst 31 detects a first exhaust temperature THl that is a temperature of the exhaust gas before the exhaust gas flows into the oxidation catalyst 31. A differential pressure sensor 110 detects a pressure difference ΔΡ between the exhaust pressure on an upstream side and that on a downstream side of the filter 32.

[0039] A second exhaust temperature sensor 120 and a first NOx sensor 130 are provided at positions in the exhaust passage 26 that are between the first purification member 30 and the second purification member 40 and on an upstream side of the urea addition valve 230. The second exhaust temperature sensor 120 detects a second exhaust temperature TH2 that is a temperature of the exhaust gas before the exhaust gas flows into the SCR catalyst 41. The first NOx sensor 130 detects first NOx concentration Nl that is NOx concentration in the exhaust gas before the exhaust gas flows into the SCR catalyst 41.

[0040] A second NOx sensor 140 is provided on a downstream side of the third purification member 50 in the exhaust passage 26 to detect second NOx concentration N2 that is the NOx concentration of the exhaust gas that has been purified in the SCR catalyst 41.

[0041] A control unit 80 as a control section receives output from each of such various sensors and the like. This control unit 80 is mainly formed of a microcomputer that includes a central processing unit (CPU), a read only memory (ROM) that stores various programs, maps, and the like in advance, a random access memory (RAM) that temporarily stores arithmetic results and the like of the CPU, a timer counter, an input interface, an output interface, and the like.

[0042] The control unit 80 executes various types of control of the engine 1 that include, for example, fuel injection amount control and fuel injection timing control of each of the fuel injection valves 4a to 4d and the fuel addition valve 5, discharge pressure control of the supply pump 10, driving amount control of an actuator 17 that opens and closes the intake throttle valve 16, opening amount control of the EGR valve 15, and the like.

[0043] In addition, various types of exhaust purification control such as the regeneration process described above, in which PM collected by the filter 32 is combusted, are also executed by the control unit 80. As one type of the exhaust purification control, the control unit 80 executes urea water addition control by the urea addition valve 230.

[0044] In this addition control, an amount of urea that is required for a reduction process of NOx discharged from the engine 1 is calculated on the basis of the engine operation state and the like. In addition, an amount of urea that is required to maintain an amount of ammonia adsorbed to the SCR catalyst 41 to a predetermined amount is calculated. Here, an ammonia adsorption amount NHR of the SCR catalyst 41 is estimated by an appropriate method. For example, the ammonia adsorption amount NHR is estimated on the basis of parameters that correlate with the ammonia adsorption amount, such as a urea additive amount, the exhaust temperature, and an exhaust flow rate.

[0045] Then, a sum of the amount of urea that is required for the reduction process of NOx and the amount of urea that is required to maintain the ammonia adsorption amount is calculated as a urea additive amount QE. A driving state of the urea addition valve 230 is controlled such that the urea water in the urea additive amount QE is injected from the urea addition valve 230.

[0046] As shown in FIG. 2, in this embodiment, the urea water is added intermittently. More specifically, opening and closing of the urea addition valve 230 are repeatedly carried out in a predetermined addition cycle (hereinafter referred to as an addition interval) INT. Accordingly, addition and stop of the addition of the urea water are repeated at the predetermined addition interval INT. The atomization of the urea water into the exhaust passage 26 is promoted by such intermittent addition of the urea water.

[0047] By the way, ammonia that is desorbed from the SCR catalyst 41 or ammonia that slips through the SCR catalyst 41 is oxidized by the ammonia oxidation catalyst 51 and converted to nitrogen and water. Accordingly, release of ammonia into the atmosphere is suppressed. However, when the exhaust gas is heated and the ammonia oxidation catalyst 51 is thus heated to the high temperature, in some cases, NOx and water may be generated, instead of nitrogen and water, by the oxidation of ammonia in the ammonia oxidation catalyst 51.

[0048] For example, when the above-described regeneration process of the filter 32 is carried out, a heating process of the exhaust gas is also carried out. Thus, compared to when the regeneration process is not carried out, a temperature of the ammonia oxidation catalyst 51 is increased to an extremely high temperature. Consequently, during the regeneration process of the filter 32, ammonia-derived NOx may be generated in the ammonia oxidation catalyst 51.

[0049] Here, there is a tendency that a desorbed amount of ammonia that is adsorbed to the SCR catalyst 41 and an amount of ammonia that slips through the SCR catalyst 41 are reduced by reducing a urea water additive amount. Considering this, in this embodiment, an additive amount reduction process to reduce the urea water additive amount is carried out, so as to reduce an amount of ammonia that reaches the ammonia oxidation catalyst 51. Accordingly, generation of NOx in the ammonia oxidation catalyst 51 at the high temperature is suppressed.

[0050] FIG. 3 shows procedures of an additive amount reduction process described above. This process is carried out by the control unit 80. Once this process is initiated, the control unit 80 first determines whether the filter 32 currently undergoes the regeneration process (SI 00).

[0051] If the filter 32 does not undergo the regeneration process (S 100: NO), the control unit 80 determines whether the second exhaust temperature TH2 is equal to or higher than a determination temperature a (SI 10). The second exhaust temperature TH2 is used as a substitute value of the temperature of the ammonia oxidation catalyst 51. However, a temperature of another portion may be set as the substitute value. For example, when a temperature of the filter 32 is detected or estimated, the temperature of the filter 32 may be set as the substitute value of the temperature of the ammonia oxidation catalyst 51. Alternatively, the temperature of the ammonia oxidation catalyst 51 may directly be detected by a sensor or the like. Here, the determination temperature a is a value that is used to determine whether NOx is possibly generated in the ammonia oxidation catalyst 51. If the second exhaust temperature TH2 is equal to or higher than the determination temperature a, the temperature of the ammonia oxidation catalyst 51 is equal to or higher than a predetermined temperature, and the ammonia oxidation catalyst 51 is at the high temperature. Thus, it is determined that NOx is possibly generated in the ammonia oxidation catalyst 51.

[0052] If the second exhaust temperature TH2 is lower than the determination temperature a (SI 10: NO), the control unit 80 terminates this process. On the other hand, if the filter 32 undergoes the regeneration process (SI 00: YES), or the second exhaust temperature TH2 is equal to or higher than the determination temperature a (SI 10: YES), the temperature of the ammonia oxidation catalyst 51 is equal to or higher than a predetermined temperature, and the ammonia oxidation catalyst 51 is at the high temperature. Thus, it is determined that NOx is possibly generated in the ammonia oxidation catalyst 51, and the process of step SI 20 onward is carried out.

[0053] In step SI 20, based on the second exhaust temperature TH2, the control unit 80 sets a first urea correction amount QEH1 (S120). The first urea correction amount QEH1 is a reduced correction amount that is used to reduce a basic urea additive amount QEB for correction, and a value that is equal to or higher than "0" is variably set on the basis of the second exhaust temperature TH2. The basic urea additive amount QEB is a base value of the above-described urea additive amount QE, and is set on the basis of the engine operation state, the ammonia adsorption amount NHR, and the like.

[0054] As shown in FIG. 4, the first urea correction amount QEH1 is set to be a larger value as the second exhaust temperature TH2 is increased. Next, based on the ammonia adsorption amount NHR, the control unit 80 sets a second urea correction amount QEH2 (SI 30). The second urea correction amount QEH2 is also a reduced correction amount that is use to reduce the basic urea additive amount QEB for correction, and a value that is equal to or higher than "0" is variably set on the basis of the ammonia adsorption amount NHR.

[0055] As shown in FIG. 5, the second urea correction amount QEH2 is set to be a larger value as the ammonia adsorption amount NHR is increased. Next, based on the first urea correction amount QEH1 and the second urea correction amount QEH2, the control unit 80 corrects the basic urea additive amount QEB and thereby calculates the urea additive amount QE (SI 40). In this step SI 40, as expressed by the following equation (1), a value that is obtained by subtracting the first urea correction amount QEH1 and the second urea correction amount QEH2 from the basic urea additive amount QEB is set as the urea additive amount QE.

[0056] QE = QEB - QEH1 - QEH2 ...(1) As apparent from this equation (1), the urea additive amount QE is reduced as a value of the first urea correction amount QEH1 is increased. In addition, the urea additive amount QE is reduced as a value of the second urea correction amount QEH2 is increased.

[0057] After the urea additive amount QE is calculated just as described, the control unit 80 terminates this process. Next, operations of the above-described additive amount reduction process will be described. Unlike when the second exhaust temperature TH2 is lower than the determination temperature oc (SI 10: NO), when the second exhaust temperature TH2 is equal to or higher than the determination temperature a (SI 10: YES), the processes in step SI 20 to SI 40 are carried out. Accordingly, the basic urea additive amount QEB is reduced for correction, and the urea additive amount QE is thereby reduced. Just as described, compared when it is determined that the temperature of the ammonia oxidation catalyst 51 is lower than the predetermined temperature, when it is determined that the temperature of the ammonia oxidation catalyst 51 is equal to or higher than the predetermined temperature and thus the ammonia oxidation catalyst 51 is at the high temperature, the urea additive amount is reduced. Since the ammonia-derived urea water additive amount, which is one of causes that generate NOx, is reduced, the desorbed amount of ammonia that is adsorbed to the SCR catalyst 41 and the amount of ammonia that slips through the SCR catalyst 41 are also reduced. Consequently, the amount of ammonia that reaches the ammonia oxidation catalyst 51 is reduced. Thus, it is possible to suppress the generation of NOx that is caused by the oxidation of ammonia in the ammonia oxidation catalyst 51 that is heated to the high temperature. [0058] In addition, when the filter 32 undergoes the regeneration process (SI 00: YES), the processes in step SI 20 to step SI 40 are also carried out. Accordingly, the basic urea additive amount QEB is reduced for correction, and the urea additive amount QE is thereby reduced. Thus, when it can be determined that, due to the regeneration process of the filter 32, the temperature of the ammonia oxidation catalyst 51 is equal to or higher than the predetermined temperature and thus the ammonia oxidation catalyst 51 is at the high temperature, the urea water additive amount is also reduced. Consequently, the amount of ammonia that reaches the ammonia oxidation catalyst 51 is reduced. Thus, it is possible to suppress the generation of NOx that is caused by heating of the ammonia oxidation catalyst 51 to the high temperature by the regeneration process of the filter 32.

[0059] In step S140, the basic urea additive amount QEB is reduced for correction. Then, the first urea correction amount QEH1 that is used for the reduced correction is set to be larger as the second exhaust temperature TH2 is increased. Accordingly, the urea additive amount QE is reduced as the exhaust temperature is increased, that is, as NOx is more likely to be generated by promoting the oxidation reaction of ammonia in the ammonia oxidation catalyst 51. Consequently, the amount of ammonia that reaches the ammonia oxidation catalyst 51 is reduced. Thus, the generation of NOx is preferably suppressed even when the exhaust temperature is changed.

[0060] Meanwhile, the second urea correction amount QEH2, which is used to reduce the basic urea additive amount QEB for correction, is set to be larger as an ammonia adsorption amount to the SCR catalyst 41 is increased. Accordingly, the urea additive amount QE is reduced as the ammonia adsorption amount to the SCR catalyst 41 is increased, that is, an ammonia desorption amount from the SCR catalyst 41 is increased. Consequently, the amount of ammonia that reaches the ammonia oxidation catalyst 51 is reduced. Thus, the generation of NOx is preferably suppressed even when the ammonia adsorption amount to the SCR catalyst 41 is changed. [0061] As it has been described so far, the following effects can be obtained according to this embodiment. (1) Compared to when it can be determined that the ammonia oxidation catalyst 51 is lower than the predetermined temperature, when it can be determined that the ammonia oxidation catalyst 51 is equal to or higher than the predetermined temperature, the additive amount reduction process is carried out to reduce the urea additive amount. Thus, it is possible to suppress the generation of NOx that is caused by the oxidation of ammonia in the ammonia oxidation catalyst 51 that is heated to the high temperature.

[0062] (2) The above-described additive amount reduction process is also carried out when the filter 32 undergoes the regeneration process. Thus, even when the ammonia oxidation catalyst 51 is heated to the high temperature that is equal to or higher than the predetermined temperature by the regeneration process of the filter 32, it is possible to suppress the generation of NOx that is caused by heating of the ammonia oxidation catalyst 51 to the high temperature.

[0063] (3) As the additive amount reduction process, the urea additive amount is reduced for correction. Then, the first urea correction amount QEH1 that is the reduced correction amount is increased as the second exhaust temperature TH2 is higher. Thus, the generation of NOx can preferably be suppressed even when the exhaust temperature is changed.

[0064] (4) The second urea correction amount QEH2 that is the reduced correction amount is increased as the ammonia adsorption amount NHR is increased. Thus, the generation of NOx can preferably be suppressed even when the ammonia adsorption amount to the SCR catalyst 41 is changed.

[0065] (Second Embodiment) Next, a description will be made on a second embodiment that embodies an exhaust gas control apparatus for an internal combustion engine with reference to FIG. 6 A to FIG. 9.

[0066] In the above-described first embodiment, when the ammonia oxidation catalyst 51 is at the high temperature, the basic urea additive amount QEB is reduced for correction, so as to reduce the urea water additive amount. Meanwhile, in this embodiment, a total additive amount of the reduction agent in a predetermined period is reduced by extending the above-described addition interval INT for correction.

[0067] FIG. 6A and FIG. 6B show modes of addition of the urea water. While FIG. 6A shows the mode of addition before the addition interval is corrected, FIG. 6B shows the mode of addition after the addition interval is corrected. As shown in FIG. 6B, when the addition interval INT is extended for correction, the number of addition of the urea water in the predetermined period is reduced. For example, in a state shown in FIG. 6A, the urea water is added for six times in a certain predetermined period. Meanwhile, in a state shown in FIG. 6B, the urea water is added for three times in a similar predetermined period. When the number of addition of the urea water is reduced, just as described, the total additive amount of the urea water in the predetermined period is also reduced. Accordingly, also in this embodiment, the desorbed amount of ammonia that is adsorbed to the SCR catalyst 41 and the amount of ammonia that slips through the SCR catalyst 41 are reduced. Consequently, the amount of ammonia that reaches the ammonia oxidation catalyst 51 is reduced. Thus, also in this embodiment, it is possible to suppress the generation of NOx that is caused by the oxidation of ammonia in the ammonia oxidation catalyst 51 that is heated to the high temperature.

[0068] FIG. 7 shows the procedures of the additive amount reduction process in this embodiment. In this process, steps in which the same processes as those in the first embodiment are carried out are denoted by the same step numbers. Once this process is initiated, the control unit 80 first determines whether the filter 32 currently undergoes the regeneration process (SI 00).

[0069] If the filter 32 does not undergo the regeneration process (SI 00: NO), the control unit 80 determines whether the second exhaust temperature TH2 is equal to or higher than the determination temperature a (SI 10). Also in this embodiment, the second exhaust temperature TH2 is used as the substitute value of the temperature of the ammonia oxidation catalyst 51. However, the temperature of the other portion may be set as the substitute value. For example, in the case where the temperature of the filter 32 is detected or estimated, the temperature of the filter 32 may be set as the substitute value of the temperature of the ammonia oxidation catalyst 51. Alternatively, the temperature of the ammonia oxidation catalyst 51 may directly be detected by the sensor or the like. Here, the determination temperature a is the value that is used to determine whether NOx is possibly generated in the ammonia oxidation catalyst 51. If the second exhaust temperature TH2 is equal to or higher than the determination temperature a, the temperature of the ammonia oxidation catalyst 51 is equal to or higher than the predetermined temperature, and the ammonia oxidation catalyst 51 is at the high temperature. Thus, it is determined that NOx is possibly generated in the ammonia oxidation catalyst 51.

[0070] If the second exhaust temperature TH2 is lower than the determination temperature a (SI 10: NO), the control unit 80 terminates this process. On the other hand, if the filter 32 undergoes the regeneration process (SI 00: YES), or the second exhaust temperature TH2 is equal to or higher than the determination temperature a (SI 10: YES), the temperature of the ammonia oxidation catalyst 51 is equal to or higher than a predetermined temperature, and the ammonia oxidation catalyst 51 is at the high temperature. Thus, it is determined that NOx is possibly generated in the ammonia oxidation catalyst 51, and the process of step S220 onward is carried out.

[0071] In step S200, based on the second exhaust temperature TH2, the control unit 80 sets a first interval correction amount INTHl (S200). The first interval correction amount ΓΝΤΗ1 is a correction amount that is used to correct a basic addition interval INTB, and a value that is equal to or higher than "0" is variably set on the basis of the second exhaust temperature TH2. The basic addition interval INTB is a base value of the above-described addition interval INT, and an appropriate value is set therefor on the basis of the engine operation state and the like.

[0072] As shown in FIG. 8, for the first interval correction amount INTHl, a longer time is set as the second exhaust temperature TH2 is increased. Next, based on the ammonia adsorption amount NHR, the control unit 80 sets a second interval correction amount INTH2 (S210). The second interval correction amount INTH2 is also a correction amount that is used to correct the basic addition interval INTB, and a value that is equal to or higher than "0" is variably set on the basis of the ammonia adsorption amount NHR.

[0073] As shown in FIG. 9, for the second interval correction amount INTH2, a longer time is set as the ammonia adsorption amount NHR is increased. Next, based on the first interval correction amount INTH1 and the second interval correction amount INTH2, the control unit 80 corrects the basic addition interval INTB and thereby calculates the addition interval INT (S220). In this step S220, as expressed by the following equation (2), a value that is obtained by adding the first interval correction amount INTH1 and the second interval correction amount INTH2 to the basic addition interval INTB is set as the addition interval INT.

[0074] INT = INTB + ΓΝΤΗ1 + INTH2 ... (2) As apparent from this equation (2), the addition interval INT is extended as a value of the first interval correction amount INTH1 is increased. In addition, the addition interval INT is extended as a value of the second interval correction amount INTH2 is increased.

[0075] After the addition interval INT is calculated just as described, the control unit 80 terminates this process. Next, operations of the above-described additive amount reduction process will be described.

[0076] Unlike when the second exhaust temperature TH2 is lower than the determination temperature a (SI 10: NO), when the second exhaust temperature TH2 is equal to or higher than the determination temperature a (SI 10: YES), the processes in step S200 to S220 are carried out. Accordingly, the basic addition interval INTB is corrected, and the addition interval INT is thereby extended. Just as described, when the addition interval INT is extended, the total additive amount of the urea water in the predetermined period is reduced.

[0077] As described above, compared to when it is determined that the temperature of the ammonia oxidation catalyst 51 is lower than the predetermined temperature, when it is determined that the temperature of the ammonia oxidation catalyst 51 is equal to or higher than the predetermined temperature and thus the ammonia oxidation catalyst 51 is at the high temperature, the total additive amount of the urea water in the predetermined period is reduced. Since the ammonia-derived urea water additive amount, which is one of the causes that generate NOx, just as described, is reduced, the desorbed amount of ammonia that is adsorbed to the SCR catalyst 41 and the amount of ammonia that slips through the SCR catalyst 41 are also reduced. Consequently, the amount of ammonia that reaches the ammonia oxidation catalyst 51 is reduced. Thus, it is possible to suppress the generation of NOx that is caused by the oxidation of ammonia in the ammonia oxidation catalyst 51 that is heated to the high temperature.

[0078] In addition, in the case where the filter 32 undergoes the regeneration process (SI 00: YES), the processes in step S200 to step S220 are also carried out. Accordingly, the basic addition interval INTB is corrected, and the addition interval INT is thereby extended. Thus, in the case where it can be determined that, due to the regeneration process of the filter 32, the temperature of the ammonia oxidation catalyst 51 is equal to or higher than the predetermined temperature and thus the ammonia oxidation catalyst 51 is at the high temperature, the urea water additive amount is also reduced. Consequently, the amount of ammonia that reaches the ammonia oxidation catalyst 51 is reduced. Thus, it is possible to suppress the generation of NOx that is caused by heating of the ammonia oxidation catalyst 51 to the high temperature by the regeneration process of the filter 32.

[0079] In step S220, the basic addition interval INTB is corrected. Then, the first interval correction amount INTH1 that is used for the correction is set to be larger as the second exhaust temperature TH2 is increased. Accordingly, the addition interval INT during the intermittent addition is extended as the exhaust temperature is increased, that is, as NOx is more likely to be generated by promoting the oxidation reaction of ammonia in the ammonia oxidation catalyst 51. Consequently, the number of addition of the urea water in the predetermined period is reduced. Just as described, when the number of addition of the urea water is reduced, the total additive amount of the urea water in the predetermined period is also reduced. Consequently, the amount of ammonia that reaches the ammonia oxidation catalyst 51 is reduced. Thus, the generation of NOx is preferably suppressed even when the exhaust temperature is changed.

[0080] Meanwhile, the second interval correction amount INTH2, which is used to correct the basic addition interval INTB, is set to be larger as an ammonia adsorption amount to the SCR catalyst 41 is increased. Accordingly, the addition interval INT during the intermittent addition is extended as the ammonia adsorption amount to the SCR catalyst 41 is increased, that is, the ammonia desorption amount from the SCR catalyst 41 is increased. Consequently, the number of addition of the urea water in the predetermined period is reduced. Just as described, when the number of addition of the urea water is reduced, the total additive amount of the urea water in the predetermined period is also reduced. Consequently, the amount of ammonia that reaches the ammonia oxidation catalyst 51 is reduced. Thus, the generation of NOx is preferably suppressed even when the ammonia adsorption amount to the SCR catalyst 41 is changed.

[0081] As it has been described so far, the following effects can be obtained in addition to the above-described effects of (1) and (2) according to this embodiment. (5) As the additive amount reduction process, the addition interval INT is corrected. Then, the first interval correction amount INTHl is increased as the second exhaust temperature TH2 is increased. Consequently, the addition interval INT is extended. Thus, the generation of NOx can preferably be suppressed even when the exhaust temperature is changed.

[0082] (6) The second interval correction amount INTH2 is extended as the ammonia adsorption amount NHR is increased. Consequently, the addition interval INT is extended. Thus, the generation of NOx can preferably be suppressed even when the ammonia adsorption amount to the SCR catalyst 41 is changed.

[0083] Here, each of the above embodiments can be modified and carried out as described below. In the first embodiment, the first urea correction amount QEH1 may not be set, or the second urea correction amount QEH2 may not be set.

[0084] In the first embodiment, in the case where the filter 32 undergoes the regeneration process (SI 00: YES), or in the case where the second exhaust temperature TH2 is equal to or higher than the determination temperature a (SI 10: YES), the process of step SI 20 onward is carried out. However, the determination process in step SI 00 may not be carried out. In the case where the second exhaust temperature TH2 is equal to or higher than the determination temperature a (SI 10: YES), the process of step SI 20 onward may be carried out. Alternatively, the determination process in step SI 10 may not be carried out. In the case where the filter 32 does not undergo the regeneration process (SI 00: NO), the additive amount reduction process may be terminated.

[0085] In the second embodiment, the first interval correction amount INTH1 may not be set, or the second interval correction amount INTH2 may not be set. In the second embodiment, in the case where the filter 32 undergoes the regeneration process (SI 00: YES), or in the case where the second exhaust temperature TH2 is equal to or higher than the determination temperature a (SI 10: YES), the process of step S200 onward is carried put. However, the determination process in step SI 00 may not be carried out. In the case where the second exhaust temperature TH2 is equal to or higher than the determination temperature a (SI 10: YES), the process of step S200 onward may be carried out. Alternatively, the determination process in step SI 10 may not be carried out. In the case where the filter 32 does not undergo the regeneration process (SI 00: NO), the additive amount reduction process may be terminated.

[0086] In the first embodiment, the urea water is added intermittently. However, the urea water may not be added intermittently but may be added continuously by maintaining the opening state of the urea addition valve 230.

[0087] In the first embodiment, as the additive amount reduction process, the additive amount is reduced for correction. However, while the temperature Of the ammonia oxidation catalyst 51 is equal to or higher than the above-described predetermined temperature and thus the ammonia oxidation catalyst 51 is at the high temperature, the addition of the urea water may be prohibited.

[0088] FIG. 10 shows an example of the procedures of the additive amount reduction process in this modified example. In this process shown in FIG. 10, steps in which the same processes as those in the first embodiment are carried out are denoted by the same step numbers.

[0089] Once this process is initiated, the control unit 80 first determines whether the filter 32 currently undergoes the regeneration process (SI 00). If the filter 32 does not undergo the regeneration process (SI 00: NO), the control unit 80 determines whether the second exhaust temperature ΊΉ2 is equal to or higher than the determination temperature a (SI 10). Also in this modified example, the second exhaust temperature TH2 is used as the substitute value of the temperature of the ammonia oxidation catalyst 51. However, the temperature of the other portion may be set as the substitute value. For example, in the case where the temperature of the filter 32 is detected or estimated, the temperature of the filter 32 may be set as the substitute value of the temperature, of the ammonia oxidation catalyst 51. Alternatively, the temperature of the ammonia oxidation catalyst 51 may directly be detected by the sensor or the like. Here, the determination temperature a is the value that is used to determine whether NOx is possibly generated in the ammonia oxidation catalyst 51. If the second exhaust temperature TH2 is equal to or higher than the determination temperature a, the temperature of the ammonia oxidation catalyst 51 is equal to or higher than the predetermined temperature, and the ammonia oxidation catalyst 51 is at the high temperature. Thus, it is determined that NOx is possibly generated in the ammonia oxidation catalyst 51.

[0090] If the second exhaust temperature TH2 is lower than the determination temperature a (SI 10: NO), the control unit 80 permits the addition of urea (S310) and terminates this process. On the other hand, if the filter 32 undergoes the regeneration process (SI 00: YES), or the second exhaust temperature TH2 is equal to or higher than the determination temperature a (SI 10: YES), the temperature of the ammonia oxidation catalyst 51 is equal to or higher than a predetermined temperature, and the ammonia oxidation catalyst 51 is at the high temperature. Thus, it is determined that NOx is possibly generated in the ammonia oxidation catalyst 51. Then, the control unit 80 prohibits the addition of urea (S300) and terminates this process. [0091] In this modified example, the addition of the urea water is prohibited while the temperature of the ammonia oxidation catalyst 51 is equal to or higher than the predetermined temperature and thus the ammonia oxidation catalyst 51 is at the high temperature, and also while NOx is possibly generated in the ammonia oxidation catalyst 51. Since the addition of urea is prohibited, just as described, the amount of ammonia that reaches the ammonia oxidation catalyst 51 is sufficiently reduced. Thus, it is possible to further appropriately suppress the generation of NOx that is caused by the oxidation of ammonia in the ammonia oxidation catalyst 51 that is heated to the high temperature.

[0092] In the first embodiment, as the additive amount reduction process, the additive amount is reduced for correction. However, as the additive amount reduction process, a prohibition period in which the addition of the urea water is prohibited during an addition period of the urea water may be provided.

[0093] FIG. 11 shows a mode of addition of the urea water in this modified example. As shown in FIG. 11, when the filter 32 undergoes the regeneration process during a period in which a request to add the urea water is made and the urea water is added on the basis of this request, or when the second exhaust temperature TH2 is equal to or higher than the determination temperature a, a prohibited period PP in which the addition of the urea water is prohibited is set. Thus, even during an addition period of the urea water on the basis of the request to add the urea water, the addition of the urea water is prohibited during this prohibited period PP. Such a prohibited period PP may be set for a plurality of times. In addition, the prohibited period PP may be set longer as the exhaust temperature is increased. Alternatively, the prohibited period PP may be set longer as the ammonia adsorption amount NHR is increased.

[0094] According to such a modified example, compared to when the prohibited period PP is not provided, the total additive amount of the urea water in the addition period is reduced. Consequently, the amount of ammonia that reaches the ammonia oxidation catalyst 51 is reduced. Thus, also in this modified example, it is possible to suppress the generation of NOx that is caused by the oxidation of ammonia in the ammonia oxidation catalyst 51 that is heated to the high temperature.

[0095] Although the urea water is used as the reduction agent, another type of the ammonia-derived reduction agent may be used.

Claims

1. An exhaust gas control apparatus for an internal combustion engine, the exhaust gas control apparatus comprising:

a reduction agent supply mechanism for adding an ammonia-derived reduction agent into an exhaust passage;

a reduction catalyst for purifying exhaust gas by adding the reduction agent;

an oxidation catalyst provided on a downstream side of the reduction catalyst in the exhaust passage; and

a control section configured to

(a) control an additive amount of the reduction agent by the reduction agent supply mechanism, and

(b) carry out an additive amount reduction process to reduce the additive amount of the reduction agent when a temperature of the oxidation catalyst is equal to or higher than a predetermined temperature, compared to when the temperature of the oxidation catalyst is lower than the predetermined temperature.

2. The exhaust gas control apparatus according to claim 1 further comprising:

a filter provided on an upstream side of the reduction catalyst in the exhaust passage, the filter collecting particulate matter in the exhaust gas, wherein

the control section carries out the additive amount reduction process while a regeneration process in which the exhaust gas is heated to regenerate the filter is carried out.

3. The exhaust gas control apparatus according to claim 1 or 2, wherein, the control section reduces the additive amount for correction as the additive amount reduction process such that a reduced correction amount of the additive amount increases as an exhaust temperature is higher.

4. The exhaust gas control apparatus according to any one of claims 1 to 3, wherein, the control section reduces the additive amount for correction as the additive amount reduction process such that a reduced correction amount of the additive amount increases as an amount of ammonia that is adsorbed to the reduction catalyst is larger.

5. The exhaust gas control apparatus according to claim 1 or 2, wherein the reduction agent supply .mechanism adds the reduction agent intermittently by repeating an addition of the reduction agent and a stop of the addition of the reduction agent with a predetermined cycle, and the control section carries out the additive amount reduction process such that the predetermined cycle is longer as an exhaust temperature is higher.

6. The exhaust gas control apparatus according to any one of claims 1, 2, 5, wherein the reduction agent supply mechanism adds the reduction agent intermittently by repeating an addition of the reduction agent and a stop of the addition of the reduction agent with a predetermined cycle, and the control section carries out the additive amount reduction process such that the predetermined cycle is longer as an amount of ammonia that is adsorbed to the reduction catalyst is larger.

7. The exhaust gas control apparatus according to claim 1 or 2, wherein, the control section prohibits an addition of the reduction agent while the temperature of the oxidation catalyst is equal to or higher than the predetermined temperature as the additive amount reduction process.

8. The exhaust gas control apparatus according to claim 1 or 2, wherein, the control section sets a prohibition period in which an addition of the reduction agent is prohibited during an addition period of the reduction agent as the additive amount reduction process.