BACKGROUND OF THE INVENTION

1. Field of the Invention

[0001] The invention relates to an exhaust gas purification apparatus for an internal combustion engine, and a control method thereof.

2. Description of Related Art

[0002] A conventional exhaust gas purification apparatus for an internal combustion engine, as described in Japanese Patent Application Publication No. 2009-138592 (JP 2009-138592 A), for example, includes a selective reduction type NOx catalyst serving as a catalyst that eliminates nitrogen oxide (NOx) contained in exhaust gas, a reducing agent supply mechanism that adds a reducing agent used during NOx elimination by the selective reduction type NOx catalyst into an exhaust passage, and a dispersion plate provided in the exhaust passage to disperse the reducing agent.

[0003] In this exhaust gas purification apparatus, urea water is injected into the exhaust passage from the reducing agent supply mechanism. Atomization of the injected urea water is advanced by the dispersion plate, whereupon the atomized urea water is hydrolyzed by heat from the exhaust gas so as to generate ammonia. The ammonia is then supplied to the catalyst as the reducing agent.

[0004] Note that in the exhaust gas purification apparatus described in JP 2009-138592 A, vaporization of reducing agent adhered to the dispersion plate is promoted by gathering the reducing agent adhered to the dispersion plate near a center of the exhaust passage. The reducing agent gathered near the center of the exhaust passage in this manner is vaporized by exhaust gas near the center, which has a higher temperature than exhaust gas near a wall surface of the exhaust passage.

SUMMARY OF THE INVENTION

[0005] Incidentally, the reducing agent is added to the exhaust passage when a temperature of the catalyst reaches or exceeds an activation temperature. Here, when an outside air temperature or the like is low, for example, a large amount of heat is discharged from the exhaust passage, and therefore the dispersion plate provided in the exhaust passage is likely to decrease in temperature. Hence, the temperature of the dispersion plate may be comparatively low even when the temperature of the catalyst equals or exceeds the activation temperature, and in this case, the reducing agent added to the exhaust passage may remain adhered to the dispersion plate so as to form a deposit thereafter.

[0006] The invention provides an exhaust gas purification apparatus for an internal combustion engine in which deposit formation on a dispersion plate is suppressed, and a control method thereof.

[0007] An exhaust gas purification apparatus for an internal combustion engine according to a first aspect of the invention includes a catalyst, a reducing agent supply mechanism, a dispersion plate, and a control unit. The catalyst is configured to purify exhaust gas by adding a reducing agent. The reducing agent supply mechanism is configured to add the reducing agent into an exhaust passage. The dispersion plate is provided in the exhaust passage and configured to disperse the reducing agent upstream of the catalyst. The control unit is configured to execute addition of the reducing agent by the reducing agent supply mechanism, and to prohibit the addition of the reducing agent when a temperature of the dispersion plate is lower than a predetermined temperature.

[0008] With the exhaust gas purification apparatus according to the first aspect of the invention, when the temperature of the dispersion plate is lower than the predetermined temperature such that the reducing agent is more likely to remain adhered to the dispersion plate, addition of the reducing agent is prohibited. As a result, reducing agent adhesion to the dispersion plate and deposit formation caused by reducing agent adhesion can be suppressed. Note that the predetermined temperature in the configuration described above is preferably set at a temperature at which reducing agent adhesion to the dispersion plate can be suppressed, for example a boiling point of the reducing agent or the like.

[0009] In the exhaust gas purification apparatus according to the first aspect of the invention, the control unit may be configured to lift the prohibition on addition of the reducing agent for a predetermined period such that the reducing agent is added when a reducing agent addition request is issued, even while the temperature of the dispersion plate is lower than the predetermined temperature, the predetermined period being shorter than an addition period employed when the temperature of the dispersion plate equals or exceeds the predetermined temperature.

[0010] According to the configuration described above, when the reducing agent addition request is issued while the temperature of the dispersion plate is lower than the predetermined temperature, the request is prioritized so that the reducing agent is added for the predetermined period. The predetermined period, which serves as an addition period employed when the reducing agent is added while the temperature of the dispersion plate is lower than the predetermined temperature, is set to be shorter than the addition period employed when the temperature of the dispersion plate equals or exceeds the predetermined temperature. By shortening the reducing agent addition period in this manner, the amount of reducing agent adhered to the dispersion plate decreases. Therefore, when the reducing agent addition request is issued while the temperature of the dispersion plate is low, the reducing agent can be added while suppressing reducing agent adhesion to the dispersion plate. As a result, exhaust gas purification through reducing agent addition can be maintained even when the temperature of the dispersion plate is low, for example.

[0011] In the exhaust gas purification apparatus according to the first aspect of the invention, the control unit may be configured to shorten the predetermined period as the temperature of the dispersion plate decreases. According to this configuration, the reducing agent addition period is shortened as the temperature of the dispersion plate decreases such that the likelihood of reducing agent adhesion increases, and therefore the amount of reducing agent adhered to the dispersion plate decreases. As a result, the amount of reducing agent adhered to the dispersion plate can be suppressed appropriately in accordance with the temperature of the dispersion plate.

[0012] In the exhaust gas purification apparatus according to the first aspect of the invention, the exhaust gas purification apparatus may be installed in a vehicle. The control unit may be configured to set the predetermined period on the basis of at least one of an exhaust gas temperature, an outside air temperature, an exhaust gas flow, and a vehicle speed.

[0013] According to the configuration described above, the reducing agent addition period employed when the temperature of the dispersion plate is low is set variably on the basis of at least one of the exhaust gas temperature, the outside air temperature, the exhaust gas flow, and the vehicle speed, all of which contribute to the temperature of the dispersion plate. As a result, the reducing agent addition period can be shortened as the temperature of the dispersion plate decreases without directly detecting the temperature of the dispersion plate.

[0014] In the exhaust gas purification apparatus according to the first aspect of the invention, the control unit may be configured to lift the prohibition on addition of the reducing agent for a predetermined period such that the reducing agent is added when a reducing agent addition request is issued, even while the temperature of the dispersion plate is lower than the predetermined temperature. An amount of the reducing agent added during addition of the reducing agent may be set to be smaller than an addition amount employed when the temperature of the dispersion plate equals or exceeds the predetermined temperature.

[0015] According to the configuration described above, when the reducing agent addition request is issued while the temperature of the dispersion plate is lower than the predetermined temperature, the request is prioritized so that the reducing agent is added for the predetermined period. In this configuration, the addition amount employed when the reducing agent is added while the temperature of the dispersion plate is lower than the predetermined temperature is set to be smaller than the addition amount employed when the temperature of the dispersion plate equals or exceeds the predetermined temperature. By reducing the reducing agent addition amount in this manner, the amount of reducing agent adhered to the dispersion plate decreases. Therefore, when the reducing agent addition request is issued while the temperature of the dispersion plate is low, the reducing agent can be added while suppressing reducing agent adhesion to the dispersion plate. As a result, exhaust gas purification through reducing agent addition can be maintained even when the temperature of the dispersion plate is low, for example.

[0016] In the exhaust gas purification apparatus according to the first aspect of the invention, the control unit may be configured to reduce the addition amount as the temperature of the dispersion plate decreases. According to this configuration, the reducing agent addition amount is reduced as the temperature of the dispersion plate decreases such that the likelihood of reducing agent adhesion increases, and therefore the amount of reducing agent adhered to the dispersion plate decreases. As a result, the amount of reducing agent adhered to the dispersion plate can be suppressed appropriately in accordance with the temperature of the dispersion plate.

[0017] In the exhaust gas purification apparatus according to the first aspect of the invention, the exhaust gas purification apparatus may be installed in a vehicle. The control unit may be configured to set the addition amount on the basis of at least one of an exhaust gas temperature, an outside air temperature, an exhaust gas flow, and a vehicle speed.

[0018] According to the configuration described above, the reducing agent addition amount is set variably on the basis of at least one of the exhaust gas temperature, the outside air temperature, the exhaust gas flow, and the vehicle speed, all of which contribute to the temperature of the dispersion plate. As a result, the reducing agent addition amount can be reduced as the temperature of the dispersion plate decreases without directly detecting the temperature of the dispersion plate.

[0019] In the exhaust gas purification apparatus according to the first aspect of the invention, the exhaust gas purification apparatus may be installed in a vehicle. The control unit may be configured to determine whether or not the temperature of the dispersion plate is lower than the predetermined temperature on the basis of at least one of an exhaust gas temperature, an outside air temperature, an exhaust gas flow, and a vehicle speed.

[0020] According to the configuration described above, the determination as to whether or not the temperature of the dispersion plate is lower than the predetermined temperature is made on the basis of parameters that contribute to the temperature of the dispersion plate, namely the exhaust gas temperature, the outside air temperature, the exhaust gas flow, and the vehicle speed. Therefore, the determination as to whether or not the temperature of the dispersion plate is lower than the predetermined temperature can be made without directly detecting the temperature of the dispersion plate.

[0021] In a control method for an exhaust gas purification apparatus for an internal combustion engine according to a second aspect of the invention, the exhaust gas purification apparatus includes a catalyst, a reducing agent supply mechanism, a dispersion plate, and a control unit, the catalyst being configured to purify exhaust gas by adding a reducing agent, the reducing agent supply mechanism being configured to add the reducing agent into an exhaust passage, and the dispersion plate being provided in the exhaust passage and configured to disperse the reducing agent upstream of the catalyst,

the control method including:

excecuting addition of the reducing agent by the control unit; and

prohibiting ,by the control unit, addition of the reducing agent when a temperature of the dispersion plate is lower than a predetermined temperature.

[0022] In the control method according to the second aspect of the invention, when a reducing agent addition request is issued, even while the temperature of the dispersion plate is lower than the predetermined temperature, the prohibition on addition of the reducing agent may be lifted for a predetermined period such that addition of the reducing agent is executed, the predetermined period being shorter than an addition period employed when the temperature of the dispersion plate equals or exceeds the predetermined temperature. In the control method according to the second aspect of the invention, the predetermined period may be shortened as the temperature of the dispersion plate decreases.

[0023] In the control method according to the second aspect of the invention, the exhaust gas purification apparatus may be installed in a vehicle. The predetermined period may be set on the basis of at least one of an exhaust gas temperature, an outside air temperature, an exhaust gas flow, and a vehicle speed. In the control method according to the second aspect of the invention, when a reducing agent addition request is issued, even while the temperature of the dispersion plate is lower than the predetermined temperature, the prohibition on addition of the reducing agent may be lifted for a predetermined period such that addition of the reducing agent is executed, and an amount of the reducing agent added during addition of the reducing agent may be set to be smaller than a reducing agent addition amount employed when the temperature of the dispersion plate equals or exceeds the predetermined temperature. In the control method according to the second aspect of the invention, the addition amount may be reduced as the temperature of the dispersion plate decreases.

[0024] In the control method according to the second aspect of the invention, the exhaust gas purification apparatus may be installed in a vehicle. The addition amount may be set on the basis of at least one of an exhaust gas temperature, an outside air temperature, an exhaust gas flow, and a vehicle speed.

[0025] In the control method according to the second aspect of the invention, the exhaust gas purification apparatus may be installed in a vehicle. A determination as to whether or not the temperature of the dispersion plate is lower than the predetermined temperature may be made on the basis of at least one of an exhaust gas temperature, an outside air temperature, an exhaust gas flow, and a vehicle speed.

BRIEF DESCRIPTION OF THE DRAWINGS

[0026] 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 view showing an internal combustion engine to which a first embodiment of an exhaust gas purification apparatus for an internal combustion engine is applied, and a peripheral configuration thereof;

FIG. 2 is a flowchart showing a series of processing procedures executed when adding urea water, according to the first embodiment;

FIG. 3 is a table showing a relationship between a vehicle speed and a vehicle speed coefficient, according to the first embodiment;

FIG. 4 is a table showing a relationship between an outside air temperature and an outside air temperature coefficient, according to the first embodiment;

FIG. 5 is a table showing a relationship between an exhaust gas flow and an exhaust gas flow coefficient, according to the first embodiment;

FIG. 6 is a flowchart showing a series of processing procedures executed when adding urea water, according to a second embodiment;

FIG. 7 is a flowchart showing procedures of addition processing according to the second embodiment;

FIG. 8 is a table showing a relationship between a dispersion plate temperature and a time coefficient, according to the second embodiment;

FIG. 9 is a flowchart showing procedures of addition processing according to a third embodiment; and

FIG. 10 is a table showing a relationship between the dispersion plate temperature and an addition amount coefficient, according to the third embodiment. DETAILED DESCRIPTION OF EMBODIMENTS

[0027] A first embodiment of an exhaust gas purification apparatus for an internal combustion engine will be described specifically below with reference to FIGS. 1 to 5.

[0028] FIG. 1 shows a configuration of a diesel engine (referred to simply as an "engine" hereafter) serving as the internal combustion engine to which the exhaust gas purification apparatus is applied, and the exhaust gas purification apparatus provided in the engine 1. The engine 1 is provided with a plurality of cylinders #1 to #4. A plurality of fuel injection valves 4a to 4d are attached to a cylinder head 2 in alignment with the respective cylinders #1 to #4. The fuel injection valves 4a to 4d inject fuel into combustion chambers of the respective cylinders #1 to #4. Further, intake ports for introducing fresh air into the cylinders and exhaust ports 6a to 6d for discharging combustion gas to the exterior of the cylinders are provided in the cylinder head 2 in alignment with the respective cylinders #1 to #4.

[0029] 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 from a fuel tank and supplies high pressure fuel to the common rail 9. The high pressure fuel supplied to the common rail 9 is injected into the cylinders from the respective fuel injection valves 4a to 4d when the fuel injection valves 4a to 4d are opened.

[0030] An intake manifold 7 is connected to the intake ports. The intake manifold 7 is connected to an intake passage 3. An intake air throttle valve 16 that adjusts an intake air amount is provided in the intake passage 3.

[0031] An exhaust manifold 8 is connected to the exhaust ports 6a to 6d. The exhaust manifold 8 is connected to an exhaust passage 26. A turbocharger 11 that turbocharges the intake air introduced into the cylinders using exhaust gas pressure is provided midway in the exhaust passage 26. An intercooler 18 is provided in the intake passage 3 between an intake side compressor of the turbocharger 11 and the intake air throttle valve 16. The intercooler 18 cools the intake air, which is increased in temperature by the turbocharging performed by the turbocharger 11.

[0032] Further, a first purifying member 30 for purifying the exhaust gas is provided midway in the exhaust passage 26 on an exhaust gas downstream side of a discharge side turbine of the turbocharger 11. An oxidation catalyst 31 and a filter 32 are disposed in the interior of the first purifying member 30 in series in an exhaust gas flow direction.

[0033] A catalyst that performs oxidation processing on hydrocarbon (HC) contained in the exhaust gas is carried on the oxidation catalyst 31. The filter 32 is constituted by a porous ceramic member that collects particulate matter (PM) in the exhaust gas. A catalyst that promotes oxidation of the PM is carried on the filter 32, and the PM in the exhaust gas is collected while passing through a porous wall of the filter 32.

[0034] Further, a fuel addition valve 5 that supplies fuel to the oxidation catalyst 31 and the filter 32 as an additive is provided in the vicinity of an aggregation portion of the exhaust manifold 8. The fuel addition valve 5 is connected to the supply pump 10 via a fuel supply pipe 27. Note that a disposal position of the fuel addition valve 5 may be modified appropriately as long as the fuel addition valve 5 remains on an upstream side of the first purifying member 30 within the exhaust system. Furthermore, fuel may be supplied to the oxidation catalyst 31 and the filter 32 as an additive by adjusting a fuel injection timing so that a post injection is performed.

[0035] When an amount of PM collected in the filter 32 exceeds a predetermined value, regeneration processing of the filter 32 is started such that fuel is injected into the exhaust manifold 8 from the fuel addition valve 5. The fuel injected from the fuel addition valve 5 is burned upon reaching the oxidation catalyst 31 , leading to an increase in an exhaust gas temperature. The exhaust gas increased in temperature by the oxidation catalyst 31 flows into the filter 32, causing the filter 32 to increase in temperature, and as a result, the PM collected in the filter 32 is oxidized. Thus, the filter 32 is regenerated.

[0036] Further, a second purifying member 40 for purifying the exhaust gas is provided midway in the exhaust passage 26 on the exhaust gas downstream side of the first purifying member 30. A selective reduction type NOx catalyst (referred to hereafter as an SCR catalyst) 41 that reduces and eliminates NOx contained in the exhaust gas using a reducing agent is disposed in the interior of the second purifying member 40. A carrier of the SCR catalyst 41 is constituted by zeolite, and a catalyst coating layer is formed on the carrier.

[0037] Furthermore, a third purifying member 50 for purifying the exhaust gas is provided midway in the exhaust passage 26 on the exhaust gas downstream side of the second purifying member 40. An ammonia oxidation catalyst 51 that purifies ammonia contained in the exhaust gas is disposed in the interior of the third purifying member 50.

[0038] A urea water supply mechanism 200 is provided in the engine 1 as a reducing agent supply mechanism that adds the reducing agent to the SCR catalyst 41. The urea water supply mechanism 200 is constituted by a tank 210 that stores urea water, a urea addition valve 230 that injects the urea water into the exhaust passage 26, a supply passage 240 that connects the urea addition valve 230 to the tank 210, and a pump 220 provided midway in the supply passage 240.

[0039] The urea addition valve 230 is provided in the exhaust passage 26 between the first purifying member 30 and the second purifying member 40 such that an injection hole thereof opens onto the SCR catalyst 41. When the urea addition valve 230 is opened, urea water is injected into the exhaust passage 26 through the supply passage 240. Note that the urea addition valve 230 is constituted the aforesaid reducing agent injection valve.

[0040] The pump 220 is an electric pump that pumps urea water from the tank 210 toward the urea addition valve 230 while rotating normally. When the pump 220 rotates in reverse, on the other hand, urea water is pumped from the urea addition valve 230 toward the tank 210. In other words, when the pump 220 rotates in reverse, urea water is collected from the urea addition valve 230 and the supply passage 240 and returned to the tank 210.

[0041] Further, a dispersion plate 60 that promotes atomization of the urea water injected from the urea addition valve 230 by dispersing the urea water is provided in the exhaust passage 26 between the urea addition valve 230 and the SCR catalyst 41.

[0042] The urea water injected from the urea addition valve 230 is hydrolyzed by heat from the exhaust gas so as to form ammonia. This ammonia is supplied to the SCR catalyst 41 as a NOx reducing agent. The ammonia supplied to the SCR catalyst 41 is occluded in the SCR catalyst 41 and used during NOx reduction.

[0043] The engine 1 also includes an exhaust gas recirculation apparatus (referred to as an EGR apparatus hereafter). The EGR apparatus reduces an amount of generated NOx by introducing a part of the exhaust gas into the intake air so as to reduce a combustion temperature in the cylinders. The EGR apparatus is constituted by an EGR passage 13 that connects the intake passage 3 to the exhaust manifold 8, an EGR valve 15 and an EGR cooler 14 provided in the EGR passage 13, and so on. By adjusting an opening of the EGR valve 15, an amount of recirculated exhaust gas introduced into the intake passage 3 from the exhaust passage 26, or in other words an external EGR amount, is regulated. Further, the temperature of the exhaust gas flowing through the EGR passage 13 is reduced by the EGR cooler 14.

[0044] Various sensors for detecting engine operating conditions are attached to the engine 1. For example, an air flow meter 19 detects an intake air amount GA. A throttle valve opening sensor 20 detects an opening of the intake air throttle valve 16. An engine rotation speed sensor 21 detects a rotation speed of a crankshaft, or in other words an engine rotation speed NE. An accelerator sensor 22 detects a depression amount of an accelerator pedal, or in other words an accelerator depression amount ACCR An outside air temperature sensor 23 detects an outside air temperature THout. A vehicle speed sensor 24 detects a vehicle speed SPD of a vehicle in which the engine 1 is installed. The engine 1 is also provided with an ignition switch (referred to as an IG switch hereafter) 25 that is operated by a driver of the vehicle to start and stop the engine 1, and the engine 1 is started and stopped in accordance with an operating position of the IG switch 25.

[0045] Further, a first exhaust gas temperature sensor 100 provided upstream of the oxidation catalyst 31 detects a first exhaust gas temperature TH1, which 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 of an exhaust gas pressure upstream and downstream of the filter 32.

[0046] A second exhaust gas temperature sensor 120 and a first NOx sensor 130 are provided in the exhaust passage 26 between the first purifying member 30 and the second purifying member 40 upstream of the urea addition valve 230. The second exhaust gas temperature sensor 120 detects a second exhaust gas temperature TH2, which is a temperature of the exhaust gas before the exhaust gas flows into the SCR catalyst 41. The first NOx sensor 130 detects a first NOx concentration Nl, which is a concentration of the NOx contained in the exhaust gas before the exhaust gas flows into the SCR catalyst 41.

[0047] A second NOx sensor 140 that detects a second NOx concentration N2, which is a concentration of the NOx contained in the exhaust gas purified by the SCR catalyst 41, is provided in the exhaust passage 26 downstream of the third purifying member 50.

[0048] Outputs from these various sensors and so on are input into a control apparatus 80 constituting a control unit. The control apparatus 80 is configured about a microcomputer including 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 stores calculation results generated by the CPU and the like temporarily, a timer counter, an input interface, an output interface, and so on.

[0049] The control apparatus 80 performs various types of control on the engine 1, such as fuel injection control of the fuel injection valves 4a to 4d and the fuel addition valve 5, discharge pressure control of the supply pump 10, drive amount control of an actuator 17 that opens and closes the intake air throttle valve 16, and opening control of the EGR valve 15, for example. The control apparatus 80 also performs various types of exhaust gas purification control such as the aforesaid regeneration processing for burning the PM collected in the filter 32.

[0050] As another type of exhaust gas purification control, the control apparatus 80 controls urea water addition by the urea addition valve 230. In this addition control, an amount of urea required to reduce the NOx discharged from the engine 1 is calculated on the basis of the engine operating conditions and so on. Further, an amount of urea required to maintain an amount of ammonia adsorbed to the SCR catalyst 41 at a predetermined amount is calculated. Note that the ammonia adsorption amount is estimated using an appropriate method. For example, the ammonia adsorption amount is estimated on the basis of parameters that correlate with the ammonia adsorption amount, such as the urea addition amount, the exhaust gas temperature, and an exhaust gas flow.

[0051] A sum of the amount of urea required to reduce the NOx and the amount of urea required to maintain the ammonia adsorption amount is then calculated as a urea addition amount QE, whereupon a driving condition of the urea addition valve 230 is controlled such that the urea addition amount QE is injected from the urea addition valve 230. Note that in this embodiment, when urea water is injected into the exhaust passage 26, the urea addition valve 230 is opened and closed repeatedly on a periodical basis. In so doing, the urea water is injected intermittently such that atomization thereof in the exhaust passage 26 is promoted. Incidentally, by maintaining the urea addition valve 230 in an open condition, the urea water may be injected continuously rather than intermittently.

[0052] The urea water is injected when the temperature of the SCR catalyst 41 reaches or exceeds an activation temperature. Here, when an outside air temperature or the like is low, for example, a large amount of heat is discharged from the exhaust passage 26, and therefore the dispersion plate 60 provided in the exhaust passage 26 is likely to decrease in temperature. Hence, the temperature of the dispersion plate 60 may be comparatively low even when the temperature of the SCR catalyst 41 is at or above the activation temperature, and in this case, the urea water added to the exhaust passage 26 may remain adhered to the dispersion plate 60 so as to form a deposit on a surface of the dispersion plate 60 thereafter.

[0053] Furthermore, when the temperature of the dispersion plate 60 is comparatively low, a part of the urea water added to the exhaust passage 26 may reach the SCR catalyst 41 in liquid form, i.e. without being dispersed by the dispersion plate 60. When moisture adheres to the zeolite-based SCR catalyst 41, the zeolite forming the carrier becomes brittle, leading to an increase in the likelihood of the coating layer peeling away.

[0054] In this embodiment, therefore, deposit formation on the dispersion plate 60 and peeling away of the coating layer on the SCR catalyst 41 are suppressed by performing processing shown in FIG. 2 using the control apparatus 80.

[0055] As shown in FIG. 2, when the processing is started, the control apparatus 80 determines whether or not a urea water addition request has been issued (SI 00). The urea water addition request is issued when the amount of ammonia adsorbed to the SCR catalyst 41 is insufficient, the NOx discharge amount increases rapidly, and so on.

[0056] When a urea water addition request has not been issued (SI 00: NO), the control apparatus 80 terminates the processing. When a urea water addition request has been issued (SI 00: YES), on the other hand, the control apparatus 80 calculates a dispersion plate temperature TB serving as an estimated temperature of the dispersion plate 60 on the basis of the second exhaust gas temperature TH2, which indicates the exhaust gas temperature in the vicinity of the dispersion plate 60, the vehicle speed SPD, the outside air temperature THout, and an exhaust gas flow GEX (SI 10). More specifically, a value obtained by correcting the second exhaust gas temperature TH2 using the vehicle speed SPD, the outside air temperature THout, and the exhaust gas flow GEX is set as the estimated temperature of the dispersion plate 60, and the dispersion plate temperature TB is calculated on the basis of Equation (1), shown below.

[0057] TB = TH2 x l x K2 x K3 (1)

TB: dispersion plate temperature

TH2: second exhaust gas temperature

Kl : vehicle speed coefficient (0 < Kl < 1)

K2: outside air temperature coefficient (0 < K2 < 1)

K3: exhaust gas flow coefficient (0 < K3 < 1)

FIG. 3 shows a setting of the vehicle speed coefficient Kl . The vehicle speed coefficient Kl is set variably within a range larger than "0" and no larger than "1". A value of the vehicle speed coefficient Kl is set to increase as the vehicle speed SPD decreases. The reason for this is that as the vehicle speed SPD decreases, a cooling effect of a traveling wind on the exhaust passage 26 decreases, leading to an increase in the temperature of the dispersion plate 60.

[0058] FIG. 4 shows a setting of the outside air temperature coefficient K2. The outside air temperature coefficient K2 is likewise set variably within a range larger than "0" and no larger than "1". A value of the outside air temperature coefficient K2 is set to decrease as the outside air temperature THout decreases. The reason for this is that as the outside air temperature THout decreases, the temperature of the exhaust passage 26 decreases, leading to a reduction in the temperature of the dispersion plate 60.

[0059] FIG. 5 shows a setting of the exhaust gas flow coefficient K3. The exhaust gas flow coefficient K3 is likewise set variably within a range larger than "0" and no larger than "1". A value of the exhaust gas flow coefficient K3 is set to decrease as the exhaust gas flow GEX decreases. The reason for this is that as the exhaust gas flow GEX decreases, an amount of heat moving from the exhaust gas to the dispersion plate 60 decreases, leading to a reduction in the temperature of the dispersion plate 60. Note that the exhaust gas flow GEX may be estimated on the basis of the intake air amount GA and so on. Accordingly, the exhaust gas flow coefficient K3 may be set on the basis of the intake air amount GA and so on. Further, the exhaust gas flow GEX may be estimated on the basis of the engine rotation speed NE and a fuel injection amount Q from the respective fuel injection valves 4a to 4d. Accordingly, the exhaust gas flow coefficient K3 may be set on the basis of the engine rotation speed NE and the fuel injection amount Q.

[0060] When the dispersion plate temperature TB has been calculated in this manner, the control apparatus 80 determines whether or not the dispersion plate temperature TB equals or exceeds a determination value A serving as a predetermined temperature (S120). The determination value A is set at a temperature at which urea water adhesion to the dispersion plate 60 can be suppressed. For example, a boiling point of urea water or a temperature in the vicinity of the boiling point may be set as the determination value A.

[0061] When the dispersion plate temperature TB equals or exceeds the determination value A (S120: YES), the control apparatus 80 permits urea addition (S130), whereby urea water is injected. The control apparatus 80 then terminates the processing.

[0062] When the dispersion plate temperature TB is lower than the determination value A (SI 20: NO), on the other hand, the control apparatus 80 prohibits urea addition even after the urea water addition request is issued (S140). The control apparatus 80 then terminates the processing.

[0063] Next, actions of this embodiment will be described. When the estimated dispersion plate temperature TB is lower than the determination value A (SI 20: NO) such that it can be determined that the actual temperature of the dispersion plate 60 is lower than the determination value A, urea water is likely to remain adhered to the dispersion plate 60, and therefore urea addition is prohibited (S140). Hence, urea water adhesion to the dispersion plate 60 is suppressed, with the result that deposit formation caused by urea water adhesion is suppressed.

[0064] Further, when the dispersion plate temperature TB is lower than the determination value A (SI 20: NO), a part of the urea water added to the exhaust passage 26 may reach the SCR catalyst 41 in liquid form, and therefore urea addition is prohibited (S140). As a result, peeling of the coating layer due to moisture adhesion to the zeolite-based SCR catalyst 41 is also suppressed.

[0065] Furthermore, the dispersion plate temperature TB serving as the estimated temperature of the dispersion plate 60 is calculated on the basis of the second exhaust gas temperature TH2, the outside air temperature THout, the exhaust gas flow GEX, and the vehicle speed SPD, all of which contribute to the temperature of the dispersion plate 60 (SI 10). A determination is then made as to whether or not the dispersion plate temperature TB is lower than the determination value A (S 120). Hence, the determination as to whether or not the temperature of the dispersion plate 60 is lower than the determination value A is made using a value estimated on the basis of the second exhaust gas temperature TH2, the outside air temperature THout, the exhaust gas flow GEX, and the vehicle speed SPD. As a result, the determination as to whether or not the temperature of the dispersion plate 60 is lower than the predetermined value can be made without directly detecting the temperature of the dispersion plate 60. [0066] According to this embodiment, as described above, following effects can be obtained. (1) When the temperature of the dispersion plate 60 is lower than the determination value A, urea water addition to the exhaust passage 26 is prohibited. As a result, deposit formation due to urea water adhesion to the dispersion plate 60 and peeling of the coating layer due to moisture adhesion to the zeolite-based SCR catalyst 41 are suppressed.

[0067] (2) The determination as to whether or not the temperature of the dispersion plate 60 is lower than the determination value A is made using a value estimated on the basis of the second exhaust gas temperature TH2, the outside air temperature THout, the exhaust gas flow GEX, and the vehicle speed SPD. Therefore, the determination as to whether or not the temperature of the dispersion plate 60 is lower than a predetermined value can be made without directly detecting the temperature of the dispersion plate 60.

(Second Embodiment)

Next, a second embodiment of the exhaust gas purification apparatus for an internal combustion engine will be described with reference to FIGS. 6 to 8.

[0068] In the first embodiment, when the temperature of the dispersion plate 60 is lower than the determination value A, urea water addition is prohibited even after the urea water addition request is issued. However, when urea water addition is prohibited completely regardless of whether or not the urea water addition request has been issued, a NOx elimination rate decreases.

[0069] In this embodiment, therefore, when the urea water addition request is issued, the prohibition on urea water addition is lifted for a predetermined period so that urea water is added, even while the temperature of the dispersion plate 60 is lower than the determination value A. The predetermined period is set to be shorter than an addition period employed when the temperature of the dispersion plate 60 equals or exceeds the determination value A.

[0070] This embodiment is implemented by modifying a part of the series of processes shown in FIG. 2. More specifically, as shown in FIG. 6, this embodiment differs from the first embodiment in that when a negative determination is made in step SI 20 of FIG. 2, processing of a new step S200 is executed, and after executing the processing of step S200, the processing of step SI 40 is performed to prohibit urea addition. The following description of this embodiment centers on these differences with the first embodiment.

[0071] FIG. 6 shows a series of processing procedures performed by the control apparatus 80. Likewise in this embodiment, when the urea water addition request is issued (SI 00: YES), the control apparatus 80 calculates the dispersion plate temperature TB on the basis of the second exhaust gas temperature TH2, the outside air temperature THout, the exhaust gas flow GEX, and the vehicle speed SPD, all of which contribute to the temperature of the dispersion plate 60 (SI 10).

[0072] When the dispersion plate temperature TB is lower than the determination value A (SI 20: NO), the control apparatus 80 executes different processing to the first embodiment, more specifically addition processing for adding urea water for the predetermined period (S200).

[0073] FIG 7 shows detailed processing procedures of the addition processing performed in step S200. When the addition processing is started, the control apparatus 80 sets a low temperature addition period TL on the basis of the dispersion plate temperature TB (S300). The low temperature addition period TL takes a value corresponding to the aforesaid predetermined period in which the prohibition on urea water addition is lifted so that urea water is added, and is calculated on the basis of Equation (2), shown below.

[0074] low temperature addition period TL = addition period T x time coefficient K4 (2)

The addition period T is a urea addition period employed when the temperature of the dispersion plate 60 equals or exceeds the determination value A, and constitutes a basic addition period used during normal urea addition.

[0075] The time coefficient K4 is a coefficient for correcting the addition period T in accordance with the dispersion plate temperature TB. FIG. 8 shows a setting of the time coefficient 4. The time coefficient K4 is set variably within a range larger than "0" and no larger than "1". A value of the time coefficient 4 is set to decrease as the dispersion plate temperature TB decreases. The reason for this is that as the dispersion plate temperature TB decreases, the amount of urea water adhered to the dispersion plate 60 can be reduced by shortening the urea water addition period.

[0076] Note that the low temperature addition period TL may be determined directly from a one-dimensional map of the dispersion plate temperature TB or the like instead of calculating Equation (2). For example, the low temperature addition period TL may be set directly on the basis of the dispersion plate temperature TB such that as the dispersion plate temperature TB decreases, the low temperature addition period TL becomes shorter. Further, a maximum value of the variably set low temperature addition period TL is set to be no larger than the addition period T. In so doing, a similar period to the low temperature addition period TL obtained from Equation (2) can be set.

[0077] When the low temperature addition period TL has been set in this manner, the control apparatus 80 executes urea addition (S310). Incidentally, the amount of urea water added during this urea addition operation is the aforesaid urea addition amount QE.

[0078] Next, the control apparatus 80 determines whether or not the low temperature addition period TL has elapsed following the start of urea addition in step S310 (S320). When the low temperature addition period TL has not elapsed (S320: NO), urea addition is continued by repeating the processing of steps S310 and S320 until the low temperature addition period TL elapses.

[0079] When the low temperature addition period TL has elapsed following the start of urea addition in step S310 (S320: YES), on the other hand, the control apparatus 80 terminates this processing and then performs the processing of step SI 40 in FIG. 6. As a result of the processing of step S 140, urea addition is prohibited. Hence, the urea addition started in step S310 of FIG. 7 is executed until the low temperature addition period TL elapses, and terminated when urea addition is prohibited in step SI 40 of FIG. 6.

[0080] Next, actions obtained specifically from this embodiment will be described. When the urea water addition request has been issued (SI 00 in FIG. 6: YES), the addition request is prioritized even if the temperature of the dispersion plate 60 is lower than the determination value A (S I 20 in FIG. 6: NO), and accordingly, the prohibition on urea water addition is lifted for the predetermined period so that urea addition is performed. More specifically, urea water is added until the low temperature addition period TL elapses even when the temperature of the dispersion plate 60 is lower than the determination value A (S200 in FIG. 6, S300 to S320 in FIG. 7).

[0081] Here, the low temperature addition period TL employed when urea water is added while the temperature of the dispersion plate 60 is lower than the determination value A is calculated on the basis of Equation (2), and therefore the low temperature addition period TL is shorter than the addition period T employed when the temperature of the dispersion plate 60 equals or exceeds the determination value A. By shortening the urea water addition period in this manner, the amount of urea water adhered to the dispersion plate 60 is reduced. Hence, when the urea water addition request is issued while the temperature of the dispersion plate 60 is low, urea water can be added while suppressing urea water adhesion to the dispersion plate 60. Therefore, the amount of ammonia adsorbed to the SCR catalyst 41 can be increased even when the temperature of the dispersion plate 60 is low, for example, and as a result, NOx elimination through urea water addition can be maintained.

[0082] Further, by setting the time coefficient K4 to be smaller as the dispersion plate temperature TB decreases, as shown in FIG. 8, the low temperature addition time TL becomes shorter as the temperature of the dispersion plate 60 decreases. Hence, the low temperature addition period TL becomes steadily shorter as the temperature of the dispersion plate 60 decreases such that the likelihood of urea water adhesion increases, and therefore the amount of urea water adhered to the dispersion plate 60 decreases. As a result, the amount of urea water adhered to the dispersion plate 60 can be suppressed appropriately in accordance with the temperature of the dispersion plate 60. [0083] Furthermore, the time coefficient K4 is set variably on the basis of the dispersion plate temperature TB. Here, the dispersion plate temperature TB is estimated on the basis of the second exhaust gas temperature TH2, the outside air temperature THout, the exhaust gas flow GEX, and the vehicle speed SPD. Therefore, when at least one of the second exhaust gas temperature TH2, the outside air temperature THout, the exhaust gas flow GEX, and the vehicle speed SPD varies, the dispersion plate temperature TB also varies, leading to variation in the time coefficient K4. Hence, in this embodiment, the low temperature addition period TL is set variably on the basis of at least one of the second exhaust gas temperature TH2, the outside air temperature THout, the exhaust gas flow GEX, and the vehicle speed SPD, all of which contribute to the temperature of the dispersion plate 60. As a result, the low temperature addition period TL can be shortened steadily as the temperature of the dispersion plate 60 decreases without directly detecting the temperature of the dispersion plate 60.

[0084] According to this embodiment, as described above, following effects can be obtained in addition to effects (1) and (2) described above. (3) When the urea water addition request is issued, even while the temperature of the dispersion plate 60 is lower than the determination value A, the prohibition on urea water addition is lifted such that urea is added for the low temperature addition period TL. Moreover, the low temperature addition period TL is set to be shorter than the addition period T employed when the temperature of the dispersion plate 60 equals or exceeds the determination value A. Hence, when the urea water addition request is issued while the temperature of the dispersion plate 60 is low, urea water can be added while suppressing urea water adhesion to the dispersion plate 60. As a result, exhaust gas purification through urea water addition can be maintained even when the temperature of the dispersion plate 60 is low, for example.

[0085] (4) The low temperature addition period TL is set to be steadily shorter as the temperature of the dispersion plate 60 decreases. As a result, the amount of urea water adhered to the dispersion plate 60 can be suppressed appropriately in accordance with the temperature of the dispersion plate 60.

[0086] (5) The low temperature addition period TL is set variably on the basis of at least one of the second exhaust gas temperature TH2, the outside air temperature THout, the exhaust gas flow GEX, and the vehicle speed SPD. As a result, the low temperature addition period TL can be shortened steadily as the temperature of the dispersion plate 60 decreases without directly detecting the temperature of the dispersion plate 60.

(Third Embodiment)

Next, a third embodiment of the exhaust gas purification apparatus for an internal combustion engine will be described with reference to FIGS. 9 and 10. [0087] In the addition processing of the second embodiment, shown in FIG. 7, the amount of urea water adhered to the low-temperature dispersion plate 60 is reduced by shortening the urea water addition period. In this embodiment, on the other hand, the amount of urea water adhered to the low-temperature dispersion plate 60 is reduced by reducing the urea water addition amount itself. In other words, in this embodiment, the content of the addition processing performed in step S200 of FIG. 6 differs from the processing content of the second embodiment, shown in FIG. 7. The addition processing according to this embodiment will be described below.

[0088] FIG. 9 shows procedures of the addition processing according to this embodiment. The processing of step S200 in FIG. 6 is likewise executed as the addition processing of this embodiment. When the addition processing is started, the control apparatus 80 sets a low temperature urea water addition amount QEL on the basis of the dispersion plate temperature TB (S400). The low temperature addition amount QEL is calculated on the basis of Equation (3), shown below.

[0089] low temperature addition amount QEL = urea addition amount QE x addition amount coefficient K5 (3)

As described above, the urea addition amount QE is the sum of the amount of urea required for the NOx reduction processing and the amount of urea required to maintain the ammonia adsorption amount, and is calculated on the basis of the engine operating conditions and so on.

[0090] The addition amount coefficient 5 is a coefficient for correcting the urea addition amount QE in accordance with the dispersion plate temperature TB. FIG. 10 shows a setting of the addition amount coefficient K5. The addition amount coefficient K5 is set variably within a range larger than "0" and no larger than "1". A value of the addition amount coefficient K5 is set to decrease as the dispersion plate temperature TB decreases. The reason for this is that as the dispersion plate temperature TB decreases, the amount of urea water adhered to the dispersion plate 60 can be reduced by reducing the amount of added urea water.

[0091] Note that the low temperature addition amount QEL may be determined directly from a one-dimensional map of the dispersion plate temperature TB or the like instead of calculating Equation (3). For example, the low temperature addition amount QEL may be set directly on the basis of the dispersion plate temperature TB such that as the dispersion plate temperature TB decreases, the low temperature addition amount QEL decreases. Further, a maximum value of the variably set low temperature addition amount QEL is set to be no larger than the urea addition amount QE set on the basis of the engine operating conditions and so on. In so doing, a similar addition amount to the low temperature addition amount QEL obtained from Equation (3) can be set. [0092] When the low temperature addition amount QEL has been set in this manner, the control apparatus 80 executes urea addition by drive-controlling the urea addition valve 230 in accordance with the low temperature addition amount QEL (S410).

[0093] Next, the control apparatus 80 determines whether or not the currently calculated dispersion plate temperature TB is lower than a determination value B (S420). Note that the determination value B is set at a slightly lower temperature than the determination value A for determining whether or not the temperature of the dispersion plate 60 is low (for example, a temperature approximately 5°C lower than the determination value A, or the like).

[0094] When the dispersion plate temperature TB equals or exceeds the determination value B (S420: NO), the control apparatus 80 continues urea addition by repeating the processing of steps S400 to S420 until the dispersion plate temperature TB falls below the determination value B.

[0095] When the dispersion plate temperature TB is lower than the determination value B (S420: YES), on the other hand, this indicates that the temperature of the dispersion plate 60 is excessively low, and therefore, if urea addition is continued further, the amount of urea water adhered to the dispersion plate 60 may exceed an allowable amount. Hence, when an affirmative determination is made in step S420, the control apparatus 80 terminates this processing and then performs the processing of step S140 in FIG. 6. As a result of the processing of step SI 40, urea addition is prohibited. The urea addition started in step S410 of FIG. 9 is thereby executed for a predetermined period extending until the dispersion plate temperature TB falls below the determination value B, and terminated when urea addition is prohibited in step SI 40 of FIG. 6.

[0096] Note that instead of performing urea addition until the dispersion plate temperature TB falls below the determination value B, urea addition may be executed by drive-controlling the urea addition valve 230 in accordance with the low temperature addition amount QEL for a predetermined period.

[0097] Next, actions obtained specifically from this embodiment will be described. When the urea water addition request has been issued (SI 00 in FIG. 6: YES), the addition request is prioritized even if the temperature of the dispersion plate 60 is lower than the determination value A (SI 20 in FIG. 6: NO), and accordingly, the prohibition on urea water addition is lifted for the predetermined period so that urea water is added. More specifically, the prohibition on urea water addition is lifted so that urea water is added for a period in which the dispersion plate temperature TB is within a temperature range below the determination value A and equal to or above the determination value B (S200 in FIG. 6, S400 to S420 in FIG. 9).

[0098] Here, the low temperature addition amount QEL employed when urea water is added while the temperature of the dispersion plate 60 is lower than the determination value A is calculated on the basis of Equation (3) such that the low temperature addition amount QEL is smaller than the urea addition amount QE employed normally when the temperature of the dispersion plate 60 equals or exceeds the determination value A. By reducing the urea water addition amount in this manner, the amount of urea water adhered to the dispersion plate 60 decreases. Hence, when the urea water addition request is issued while the temperature of the dispersion plate 60 is low, urea water can be added while suppressing urea water adhesion to the dispersion plate 60. Therefore, the amount of ammonia adsorbed to the SCR catalyst 41 can be increased even when the temperature of the dispersion plate 60 is low, for example, and as a result, NOx elimination through urea water addition can be maintained.

[0099] Further, by reducing the addition amount coefficient K5 as the dispersion plate temperature TB decreases, as shown in FIG. 10, the low temperature addition amount QEL decreases steadily as the temperature of the dispersion plate 60 decreases. Accordingly, the low temperature addition amount QEL decreases as the temperature of the dispersion plate 60 decreases such that the likelihood of urea water adhesion increases, and therefore the amount of urea water adhered to the dispersion plate 60 decreases. As a result, the amount of urea water adhered to the dispersion plate 60 can be suppressed appropriately in accordance with the temperature of the dispersion plate 60.

[0100] Furthermore, the addition amount coefficient K5 is set variably on the basis of the dispersion plate temperature TB. Here, the dispersion plate temperature TB is estimated on the basis of the second exhaust gas temperature TH2, the outside air temperature THout, the exhaust gas flow GEX, and the vehicle speed SPD. Therefore, when at least one of the second exhaust gas temperature TH2, the outside air temperature THout, the exhaust gas flow GEX, and the vehicle speed SPD varies, the dispersion plate temperature TB also varies, leading to variation in the addition amount coefficient K5. Hence, in this embodiment, the low temperature addition amount QEL is set variably on the basis of at least one of the second exhaust gas temperature TH2, the outside air temperature THout, the exhaust gas flow GEX, and the vehicle speed SPD, all of which contribute to the temperature of the dispersion plate 60. As a result, the low temperature addition amount QEL can be reduced steadily as the temperature of the dispersion plate 60 decreases without directly detecting the temperature of the dispersion plate 60.

[0101] According to this embodiment, as described above, following effects can be obtained in addition to effects (1) and (2) described above. (6) When the urea water addition request is issued, even while the temperature of the dispersion plate 60 is lower than the determination value A, the prohibition on urea water addition is lifted for the period extending to the point at which the temperature of the dispersion plate 60 falls below the determination value B so that urea addition is performed. The low temperature addition amount QEL is set to be smaller than the urea addition amount QE employed when the temperature of the dispersion plate 60 equals or exceeds the determination value A. Hence, when the urea water addition request is issued while the temperature of the dispersion plate 60 is low, urea water can be added while suppressing urea water adhesion to the dispersion plate 60. As a result, exhaust gas purification through urea water addition can be maintained even when the temperature of the dispersion plate 60 is low, for example.

[0102] (7) The low temperature addition amount QEL is reduced as the temperature of the dispersion plate 60 decreases. As a result, the amount of urea water adhered to the dispersion plate 60 can be suppressed appropriately in accordance with the temperature of the dispersion plate 60.

[0103] (8) The low temperature addition amount QEL is set variably on the basis of at least one of the second exhaust gas temperature TH2, the outside air temperature THout, the exhaust gas flow GEX, and the vehicle speed SPD. As a result, the low temperature addition amount QEL can be reduced steadily as the temperature of the dispersion plate 60 decreases without directly detecting the temperature of the dispersion plate 60.

[0104] Note that the respective embodiments described above may be implemented with following modifications.

The temperature of the dispersion plate 60 is estimated, but instead of using an estimated value, the temperature of the dispersion plate 60 may be detected directly using a temperature sensor or the like.

[0105] The temperature of the dispersion plate 60 is estimated on the basis of the second exhaust gas temperature TH2, the outside air temperature THout, the exhaust gas flow GEX, and the vehicle speed SPD. Instead, however, the temperature of the dispersion plate 60 may be estimated on the basis of at least one of the second exhaust gas temperature TH2, the outside air temperature THout, the exhaust gas flow GEX, and the vehicle speed SPD. Note that at least the second exhaust gas temperature TH2 and the outside air temperature THout are preferably included in the parameters for estimating the temperature of the dispersion plate 60.

[0106] The temperature of the dispersion plate is estimated in order to determine that the temperature of the dispersion plate 60 is low enough for urea water to adhere to the dispersion plate 60. However, since the respective parameters including the second exhaust gas temperature TH2, the outside air temperature THout, the exhaust gas flow GEX, and the vehicle speed SPD are values that contribute to the temperature of the dispersion plate 60, the determination as to whether or not the temperature of the dispersion plate 60 is low may be made directly on the basis of at least one of these parameters. For example, it may be determined that the temperature of the dispersion plate 60 is low enough for urea water to adhere to the dispersion plate 60 when the outside air temperature THout is lower than a predetermined determination temperature.

[0107] In the second embodiment, the low temperature addition period TL is set variably on the basis of the dispersion plate temperature TB, but the low temperature addition period TL may be set at a fixed value. Note, however, that likewise in this case, the low temperature addition period TL is shorter than the addition period T used during normal urea addition. Effect (3) can be obtained similarly with this modified example.

[0108] In the second embodiment, the dispersion plate temperature TB is the parameter used to set the low temperature addition period TL variably. However, since the respective parameters used to calculate the dispersion plate temperature TB, namely the second exhaust gas temperature TH2, the outside air temperature THout, the exhaust gas flow GEX, and the vehicle speed SPD, are values that contribute to the temperature of the dispersion plate 60, as described above, the time coefficient 4 may be set variably directly on the basis of at least one of these parameters. For example, the time coefficient K4 may be set variably on the basis of the outside air temperature THout such that the time coefficient K4 decreases as the outside air temperature THout decreases. Likewise with this modified example, similar actions and effects to those occurring when the time coefficient K4 is set variably using the dispersion plate temperature TB can be obtained.

[0109] Further, the low temperature addition period TL may be set variably directly on the basis of at least one of the respective parameters including the second exhaust gas temperature TH2, the outside air temperature THout, the exhaust gas flow GEX, and the vehicle speed SPD. For example, the low temperature addition period TL may be set variably on the basis of the outside air temperature THout such that the low temperature addition period TL becomes shorter as the outside air temperature THout decreases. Likewise with this modified example, similar actions and effects to those occurring when the time coefficient 4 is set variably using the dispersion plate temperature TB can be obtained.

[0110] In the third embodiment, the low temperature addition amount QEL is set variably on the basis of the dispersion plate temperature TB, but the low temperature addition amount QEL may be set at a fixed value. Note, however, that likewise in this case, the low temperature addition amount QEL is smaller than the urea addition amount QE used during normal urea addition. Effect (6) can be obtained similarly with this modified example.

[0111] In the third embodiment, the dispersion plate temperature TB is the parameter used to set the low temperature addition period TL variably. However, since the respective parameters including the second exhaust gas temperature TH2, the outside air temperature THout, the exhaust gas flow GEX, and the vehicle speed SPD are values that contribute to the temperature of the dispersion plate 60, as described above, the addition amount coefficient K5 may be set variably directly on the basis of at least one of these parameters. For example, the addition amount coefficient 5 may be set variably on the basis of the outside air temperature THout such that the addition amount coefficient K5 decreases as the outside air temperature THout decreases. Likewise with this modified example, similar actions and effects to those occurring when the addition amount coefficient K5 is set variably using the dispersion plate temperature TB can be obtained.

[0112] Further, the low temperature addition amount QEL may be set variably directly on the basis of at least one of the respective parameters including the second exhaust gas temperature TH2, the outside air temperature THout, the exhaust gas flow GEX, and the vehicle speed SPD. For example, the low temperature addition amount QEL may be set variably on the basis of the outside air temperature THout such that the low temperature addition amount QEL decreases as the outside air temperature THout decreases. Likewise with this modified example, similar actions and effects to those occurring when the addition amount coefficient K5 is set variably using the dispersion plate temperature TB can be obtained.

[0113] The SCR catalyst 41 is carried on a zeolite-based carrier, but may be carried on another carrier. Likewise in this case, according to the respective embodiments described above, at least deposit formation caused by urea water adhesion to the dispersion plate 60 is suppressed.

[0114] Urea water is used as the reducing agent, but another liquid reducing agent may be used instead.