Exhaust gas control apparatus for internal combustion engine

Abstract

A SO.sub.x trap catalyst, an oxidation catalyst, a particulate filter, an aqueous urea supply valve, and a NO.sub.x selective reduction catalyst are arranged in order from upstream to downstream in an engine exhaust passage. It is determined whether a discharge concentration of hydrogen sulfide H.sub.2S will become equal to or greater than a preset maximum concentration when SO.sub.x is released from the SO.sub.x trap catalyst. If it is estimated that the discharge concentration of the hydrogen sulfide H.sub.2S will become equal to or greater than the maximum concentration when SO.sub.x is released, an adsorbed ammonia amount adsorbed on the NO.sub.x selective reduction catalyst is reduced before SO.sub.x is released so that the discharge concentration of the hydrogen sulfide H.sub.2S is less than the maximum concentration when SO.sub.x is released.

Description

INCORPORATION BY REFERENCE

[0001] The disclosure of Japanese Patent Application No. 2007-277724 filed on Oct. 25, 2007, including the specification, drawings and abstract is incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The invention relates to an exhaust gas control apparatus for an internal combustion engine.

[0004] 2. Description of the Related Art

[0005] Japanese Patent Application Publication No. 2006-512529 (JP-A-2006-512529) describes an internal combustion engine which has a NO.sub.x storage catalyst provided in an engine exhaust passage and a NO.sub.x selective reduction catalyst provided downstream of the NO.sub.x storage catalyst in the engine exhaust passage. The NO.sub.x storage catalyst stores NO.sub.x in the exhaust gas when the air-fuel ratio of the inflowing exhaust gas is lean, and releases the stored NO.sub.x when the air-fuel ratio of the inflowing exhaust gas becomes equal to the stoichiometric air-fuel ratio or rich. The NO.sub.x selective reduction catalyst is able to selectively reduce NO.sub.x in the exhaust gas using ammonia when the air-fuel ratio of the exhaust gas is lean. When stored NO.sub.x needs to be released from the NO.sub.x storage catalyst, the internal combustion engine makes the air-fuel ratio of the exhaust gas flowing into the NO.sub.x storage catalyst rich.

[0006] In this internal combustion engine, a large portion of NO.sub.x produced during combustion with a lean air-fuel ratio is stored in the NO.sub.x storage catalyst. The NO.sub.x that is not stored in the NO.sub.x storage catalyst flows into the NO.sub.x selective reduction catalyst located downstream. In this internal combustion engine, however, the NO.sub.x released from the NO.sub.x storage catalyst when the air-fuel ratio of the exhaust gas flowing into the NO.sub.x storage catalyst is rich reacts with large amounts of HC in the exhaust gas, producing ammonia NH.sub.3 which is adsorbed on the NO.sub.x selective reduction catalyst. Accordingly, when combustion is performed with a lean air-fuel ratio, the NO.sub.x that passes through the NO.sub.x storage catalyst is reduced by this adsorbed ammonia in the NO.sub.x selective reduction catalyst such that NO.sub.x is able to be successfully purified.

[0007] Exhaust gas also contains SO.sub.x which also gets stored in the NO.sub.x storage catalyst. As the amount of SO.sub.x stored in the NO.sub.x storage catalyst increases, less NO.sub.x is able to be stored so when a NO.sub.x storage catalyst is used, SO.sub.x needs to occasionally be released from the NO.sub.x storage catalyst. In this case, SO.sub.x can be released from the NO.sub.x storage catalyst by making the air-fuel ratio of the exhaust gas flowing into the NO.sub.x storage catalyst is rich when the temperature of the NO.sub.x storage catalyst is increased to 600.degree. C. or more.

[0008] When SO.sub.x is released from the NO.sub.x storage catalyst, it reacts with the adsorbed ammonia in the NO.sub.x selective reduction catalyst, producing hydrogen sulfide. In this case, however, not much SO.sub.x is released from the NO.sub.x storage catalyst so not much hydrogen sulfide is produced.

[0009] SO.sub.x in the exhaust gas substantially reduces the durability and performance of post-processing apparatuses such as exhaust gas control catalysts so it is necessary to remove it from the exhaust gas. To do this, it is preferable to provide a SO.sub.x trap catalyst capable of trapping the SO.sub.x in the exhaust gas. However, even when such a SO.sub.x trap catalyst is used, the SO.sub.x must be released from the SO.sub.x trap catalyst before the SO.sub.x trap catalyst becomes saturated with SO.sub.x. However, unlike the NO.sub.x storage catalyst, the SO.sub.x trap catalyst is designed to trap SO.sub.x so large amounts of SO.sub.x are trapped in the SO.sub.x trap catalyst.

[0010] Therefore, when SO.sub.x is released from the SO.sub.x trap catalyst, it is released in large amounts. Accordingly, when a SO.sub.x trap catalyst is used, large amounts of hydrogen sulfide are produced in the NO.sub.x selective reduction catalyst In this case, when high concentrations of hydrogen sulfide are discharged into the atmosphere, a very irritating odor is produced. Thus there is a need to keep the concentration of hydrogen sulfide that is discharged down to an allowable concentration at which the irritating odor is almost unnoticeable.

SUMMARY OF THE INVENTION

[0011] Therefore one aspect of this invention relates to an exhaust gas control apparatus for an internal combustion engine, which includes a NO.sub.x selective reduction catalyst which is arranged in an engine exhaust passage and selectively reduces NO.sub.x in exhaust gas using ammonia when an air-fuel ratio of the exhaust gas is lean, a SO.sub.x trap catalyst which is arranged in the engine exhaust passage upstream of the NO.sub.x selective reduction catalyst and traps SO.sub.x in the exhaust gas, and a control apparatus that controls the state of the exhaust gas. The control apparatus i) reduces an adsorbed ammonia amount adsorbed on the NO.sub.x selective reduction catalyst before SO.sub.x is released or ii) reduces the amount of SO.sub.x released from the SO.sub.x trap catalyst when SO.sub.x is released, such that a discharge concentration of hydrogen sulfide will be less than a preset maximum concentration when SO.sub.x is released.

[0012] This aspect makes it possible to make the irritating odor from hydrogen sulfide almost unnoticeable.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] The features, advantages, and technical and industrial significance of this invention will be described in the following detailed description of example embodiments of the invention with reference to the accompanying drawings, in which like numerals denote like elements, and wherein:

[0014] FIG. 1 is an overall view of a compression ignition internal combustion engine;

[0015] FIG. 2 is a sectional view of a surface portion of a substrate of a SO.sub.x trap catalyst;

[0016] FIG. 3A is a graph showing the release rate of SO.sub.x from the SO.sub.x trap catalyst and the like;

[0017] FIG. 3B is a graph showing the release rate of SO.sub.x from the SO.sub.x trap catalyst;

[0018] FIG. 4A is a graph showing an adsorbed ammonia amount and the like;

[0019] FIG. 4B is a chart showing the relationship between the adsorbed ammonia amount and the supply timing of aqueous urea;

[0020] FIG. 4C is a map for calculating the amount of NO.sub.x discharged per unit of time from the engine;

[0021] FIG. 5 is a flowchart of a routine used to control the supply of aqueous urea;

[0022] FIG. 6 is a flowchart of a routine for releasing SO.sub.x;

[0023] FIG. 7 is a graph showing the discharge concentration and the allowable concentration of hydrogen sulfide H.sub.2S;

[0024] FIG. 8 is a flowchart of a routine for executing a second example embodiment of SO.sub.x release control;

[0025] FIG. 9 is a flowchart of a routine for executing a third example embodiment of SO.sub.x release control;

[0026] FIG. 10A is a chart illustrating control to increase the amount of NO.sub.x discharged from the engine;

[0027] FIG. 10B is a map for calculating the amount of NO.sub.x amount discharged per unit of time from the engine;

[0028] FIG. 11 is a flowchart of a routine for executing a fourth example embodiment of SO.sub.x release control;

[0029] FIGS. 12A and 12B are graphs showing the desorption rate of adsorbed ammonia;

[0030] FIG. 13 is a graph showing the allowable concentration of the discharge concentration of hydrogen sulfide H.sub.2S;

[0031] FIG. 14 is a flowchart of a routine for executing a fifth example embodiment of SO.sub.x release control;

[0032] FIG. 15 is a graph showing the allowable concentration of the discharge concentration of hydrogen sulfide H.sub.2S; and

[0033] FIG. 16 is a flowchart of a routine for executing a sixth example embodiment of SO.sub.x release control.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0034] FIG. 1 is an overall view of a compression ignition internal combustion engine. This internal combustion engine includes the engine 1 itself, a combustion chamber 2 in each cylinder, electronically controlled fuel injection valves 3 for injecting fuel into the combustion chambers 2, an intake manifold 4, and an exhaust manifold 5. The intake manifold 4 is connected to an outlet port of a compressor 7a of an exhaust turbocharger 7 via an intake duct 6, and an inlet port of the compressor 7a is connected to an air cleaner 9 via an intake air amount detector 8. A throttle valve 10 which is driven by a step motor is arranged inside the intake duct 6. Further, a cooling apparatus 11 for cooling the intake air that flows through the intake duct 6 is arranged around the intake duct 6. In the first example embodiment shown in FIG. 1, the intake air is cooled by engine coolant that is introduced into the cooling apparatus 11.

[0035] Meanwhile, the exhaust manifold 5 is connected to an inlet port of an exhaust turbine 7b of the exhaust turbocharger 7. An outlet port of the exhaust turbine 7b is connected to an inlet port of a SO.sub.x trap catalyst 12, and an outlet port of the SO.sub.x trap catalyst 12 is connected to an inlet port of an oxidation catalyst 13. Further, an outlet port of the oxidation catalyst 13 is connected to an inlet port of a particulate filter 14, and an outlet port of the particulate filter 14 is connected via an exhaust pipe 15 to a NO.sub.x selective reduction catalyst 16 which is capable of selectively reducing NO.sub.x in the exhaust gas using ammonia when the exhaust gas air-fuel ratio is lean. This NO.sub.x selective reduction catalyst 16 is made from Fe zeolite, for example.

[0036] An aqueous urea supply valve 17 is arranged in the exhaust pipe 15 upstream of the NO.sub.x selective reduction catalyst 16. This aqueous urea supply valve 17 is connected via a supply pipe 18 and a supply pump 19 to an aqueous urea tank 20. When aqueous urea is to be supplied, aqueous urea stored in the aqueous urea tank 20 is injected by the supply pump 19 from the aqueous urea supply valve 17 into the exhaust gas flowing through the exhaust pipe 15. At this time, the NO.sub.x in the exhaust gas is reduced in the NO.sub.x selective reduction catalyst 16 by ammonia ((NH.sub.2).sub.2CO+H.sub.2O.fwdarw.2NH.sub.3+CO.sub.2) produced by the urea.

[0037] The exhaust manifold 5 and the intake manifold 4 are connected together via an exhaust gas recirculation (hereinafter simply referred to as "EGR") passage 21 in which an electronically controlled EGR control valve 22 is arranged. Also, a cooling apparatus 23 for cooling EGR gas flowing through the EGR passage 21 is arranged around the EGR passage 21 In the first example embodiment shown in FIG. 1, the EGR gas is cooled by engine coolant that is introduced into the cooling apparatus 23. Meanwhile, the fuel injection valves 3 are connected to a common rail 25 via fuel supply pipes 24. This common rail 25 is connected to a fuel tank 27 via an electronically controlled variable discharge fuel pump 26 which supplies fuel stored in the fuel tank 27 to the common rail 25. The fuel in the common rail 25 is then supplied to the fuel injection valves 3 via the fuel supply pipes 24.

[0038] An electronic control unit (ECU) 30 is formed of a digital computer and includes ROM (read only memory) 32, RAM (random access memory) 33, a CPU (a microprocessor) 34, an input port 35, and an output port 36, all of which are connected together via a bidirectional bus 31. A temperature sensor 28 for detecting the bed temperature of the SO.sub.x trap catalyst 12 is mounted to the SO.sub.x trap catalyst 12, and a temperature sensor 29 for detecting the bed temperature of the NO.sub.x selective reduction catalyst 16 is mounted to the NO.sub.x selective reduction catalyst 16. Output signals from these temperature sensors 28 and 29 and the intake air amount detector 8 are input to the input port 35 via corresponding AD converters 37. Also, a load sensor 41 that generates an output voltage proportional to a depression amount L of an accelerator pedal 40 is connected to the accelerator pedal 40, and the output voltage of this load sensor 41 is input to the input port 35 via a corresponding AD converter 37. Further, a crank angle sensor 42 that generates an output pulse every time a crankshaft rotates 15.degree., for example, is connected to the input port 35. Meanwhile, the output port 36 is connected to the fuel injection valves 3, the step motor for driving the throttle valve 10, the aqueous urea supply valve 17, the supply pump 19, the EGR control valve 22, and the fuel pump 26 via corresponding drive circuits 38. The ECU 30 controls, for example, the exhaust gas temperature or the exhaust gas air-fuel ratio via the fuel injection valves 3, the aqueous urea supply valve 17 and the like.

[0039] First the SO.sub.x trap catalyst 12 will be described. This SO.sub.x trap catalyst 12 is a monolith catalyst having, for example, a honeycomb structure with multiple exhaust gas holes extending in straight lines in the axial direction of the SO.sub.x trap catalyst 12. FIG. 2 illustrates a cross-section of a surface portion of a substrate 50 of the SOX trap catalyst 12. As shown in the drawing, a coat layer 51 is formed on the surface of the substrate 50, and a precious metal catalyst 52 is carried dispersed on the surface of this coat layer 51.

[0040] In the first example embodiment shown in FIG. 2, platinum Pt is used as the precious metal catalyst 52. The component forming the coat layer 51 may be at least one selected from the group consisting of an alkali metal, an alkali earth, and a rare earth. The alkali metal is, for example, kalium K, natrium Na, or cesium Cs. The alkali earth is, for example, barium Ba or calcium Ca. The rare earth is, for example, lanthanum La or yttrium Y That is, the coat layer 51 of the SO.sub.x trap catalyst 12 is strongly basic.

[0041] The SO.sub.x in the exhaust gas, i.e., SO.sub.2, oxidizes on the platinum Pt, as shown in FIG. 2, and then becomes trapped in the coat layer 51. That is, the SO.sub.2 diffuses in the coat layer 51 in the form of sulfate ions SO.sub.4.sup.2-, thus forming hydrosulfate. Incidentally, as described above, the coat layer 51 is strongly basic so some of the SO.sub.2 in the exhaust gas becomes trapped directly in the coat layer 51, as shown in FIG. 2.

[0042] The shading in the coat layer 51 in FIG. 2 indicates the concentration of trapped SO.sub.x. As shown in FIG. 2, the SO.sub.x concentration in the coat layer 51 is highest * in the area near the surface of the coat layer 51 and becomes gradually lower deeper down. When the SO.sub.x concentration near the surface of the coat layer 51 becomes high, the surface of the coat layer 51 becomes less basic and its ability to trap SO.sub.x diminishes. However, even if its ability to trap SO.sub.x diminishes in this way, the SO.sub.x trapping ability is restored when the temperature of the SO.sub.x trap catalyst 12 rises when combustion is performed with a lean air-fuel ratio.

[0043] That is, when the temperature of the SO.sub.x trap catalyst rises, the SO.sub.x that is concentrated near the surface of the coat layer 51 diffuses inward in the coat layer 51 so that the SO.sub.x concentration in the coat layer 51 evens out. That is, the hydrosulfate produced in the coat layer 51 changes from an unstable state in which it is concentrated near the surface of the coat layer 51 to a stable state in which it is evenly dispersed throughout the entire coat layer 51. When the SO.sub.x near the surface of the coat layer 51 diffuses inward in the coat layer 51, the SO.sub.x concentration near the surface of the coat layer 51 drops so the SO.sub.x trapping ability is restored

[0044] In this way, the SO.sub.x trap catalyst 12 continues to trap SO.sub.x while repeating this process to restore the SO.sub.x trapping ability. However, as the SO.sub.x trap catalyst 12 becomes saturated with SO.sub.x (i.e., as the SO.sub.x trap catalyst 12 nears the point where it is no longer able to trap any more SO.sub.x), the ability of the SO.sub.x trap catalyst 12 to trap SO.sub.x no longer able to be restored. At this time, the temperature of the SO.sub.x trap catalyst 12 is raised to 600.degree. C. or more and the air-fuel ratio of the exhaust gas flowing into the SO.sub.x trap catalyst 12 is made rich. As a result, trapped SO.sub.x is released from the SO.sub.x trap catalyst 12, thereby restoring the ability of the SO.sub.x trap catalyst 12 to trap SO.sub.x.

[0045] FIG. 3A is a graph showing the relationship between i) the amount of SO.sub.x released per unit of time from the SO.sub.x trap catalyst 12, i.e., the SO.sub.x release rate W (g/sec), when the air-fuel ratio of the exhaust gas is a reference rich air-fuel ratio such as 135 and ii) the bed temperature TC of the SO.sub.x trap catalyst 12. FIG. 3B is a graph showing the relationship between the SO.sub.x release rate K from the SO.sub.x trap catalyst 12 and the air-fuel ratio of the exhaust gas. The amount (g/sec) of SO.sub.x that is released per unit of time from the SO.sub.x trap catalyst 12 is expressed by the product of the SO.sub.x release rate W and the SO.sub.x release rate K.

[0046] Therefore, the amount (g/sec) of SO.sub.x that is released per unit of time from the SO.sub.x trap catalyst 12 rapidly increases when the catalyst bed temperature TC reaches 600.degree. C. or higher, as shown in FIG. 3A, and also increases when the air-fuel ratio of exhaust gas is reduced, i.e., when the degree of richness of the exhaust gas air-fuel ratio increases, as shown in FIG. 3B. Incidentally, the temperature of the SO.sub.x trap catalyst 12 is increased by, for example, retarding the fuel injection timing or injecting supplemental fuel during the exhaust stroke. Also, the air-fuel ratio of the exhaust gas that flows into the SO.sub.x trap catalyst 12 is made rich by supplying additional fuel during the exhaust stroke, for example.

[0047] Next, the NO.sub.x selective reduction catalyst 16 will be described. The NO.sub.x selective reduction catalyst 16 adsorbs ammonia NH.sub.3. In FIG. 4A, reference character Qmax indicates the maximum amount of ammonia able to be adsorbed on the NO.sub.x selective reduction catalyst 16 (hereinafter simply referred to as the "maximum adsorbable ammonia amount Qmax"). As is evident from the drawing, the maximum adsorbable ammonia amount Qmax decreases as the bed temperature TS of the NO.sub.x selective reduction catalyst 16 increases. The NO.sub.x in the exhaust gas is reduced by the ammonia NH.sub.3 that is adsorbed on the NO.sub.x selective reduction catalyst 16 so it is necessary to make sure that a sufficient amount of ammonia NH.sub.3 is always adsorbed on the NO.sub.x selective reduction catalyst 16.

[0048] Therefore in the first example embodiment of the invention, an adsorbed ammonia amount Qt that is only slightly less than the maximum adsorbable ammonia amount Qmax is set beforehand as a reference adsorbed ammonia amount, as shown in FIG. 4A. The amount of aqueous urea that is supplied is controlled so that the adsorbed ammonia amount Q comes to match this reference adsorbed ammonia amount Qt. For example, when the adsorbed ammonia amount Q is less than the reference adsorbed ammonia amount Qt, aqueous urea is supplied intermittently, and when the adsorbed ammonia amount Q exceeds the reference adsorbed ammonia amount Qt, the supply of aqueous urea is stopped, as shown in FIG. 4B.

[0049] In the first example embodiment of the invention, the amount Q adsorbed on the NO.sub.x selective reduction catalyst 16 (i.e., the adsorbed ammonia amount Q) is calculated from the amount of aqueous urea supplied from the aqueous urea supply valve 17 and the amount of NO.sub.x discharged from the engine. That is, generally speaking, the amount of ammonia newly adsorbed on the NO.sub.x selective reduction catalyst 16 is proportional to the amount of aqueous urea supplied, and the amount of adsorbed ammonia that is consumed is proportional to the amount of NO.sub.x that is discharges Therefore, the adsorbed ammonia amount Q is calculated from the amount of aqueous urea supplied and the amount of NO.sub.x discharged. Incidentally, the amount NOXA of NO.sub.x discharged per unit of time from the engine (hereinafter also simply referred to as the "discharged NO.sub.x amount NOXA") is stored in the ROM 32 in advance in the form of a map shown in FIG. 4C as a function of the required torque TQ and the engine speed N.

[0050] FIG. 5 is a flowchart of a routine to control the supply of aqueous urea, i.e., an aqueous urea supply control routine, which is an interrupt processing routine executed at fixed intervals of time. Referring to FIG. 5, first in step 60, the amount NOXA of NO.sub.x discharged per unit of time from the engine is calculated from the map shown in FIG. 4C. Next in step 61, of the ammonia NH.sub.3 that is adsorbed on the NO.sub.x selective reduction catalyst 16, the amount .DELTA.Q of ammonia NH.sub.3 consumed per unit of time by the NO.sub.x (hereinafter also simply referred to as the "consumed ammonia amount .DELTA.Q") is calculated based on the discharged NO.sub.x amount NOXA. Then in step 62, the consumed ammonia amount .DELTA.Q is subtracted from the adsorbed ammonia amount Q.

[0051] Next in step 63, it is determined whether a command to stop the supply of aqueous urea is being output. Normally this command is not being output so the process proceeds on to step 64 where it is determined whether the adsorbed ammonia amount Q is less than the reference adsorbed ammonia amount Qt. If the adsorbed ammonia amount Q is less than the reference adsorbed ammonia amount Qt, i.e., Q<Qt, the process proceeds on to step 65 where aqueous urea continues to be intermittently supplied. Then in step 66, an ammonia amount Qd that is newly adsorbed is added to the adsorbed ammonia amount Q. If, on the other hand, it is determined in step 64 that the adsorbed ammonia amount Q is equal to or greater than the reference adsorbed ammonia amount Qt, i.e., Q.gtoreq.Qt, the process proceeds on to step 67 where the supply of aqueous urea is stopped.

[0052] In this way, if a command to stop the supply of aqueous urea is not being output, the adsorbed ammonia amount Q is maintained at the reference adsorbed ammonia amount Qt. If, on the other hand, a command to stop the supply of aqueous urea is being output, the process proceeds on to step 67 where the supply of aqueous urea is stopped.

[0053] FIG. 6 is a flowchart of a routine to release SO.sub.x from the SO.sub.x trap catalyst 12. This routine is also an interrupt processing routine executed at fixed intervals of time. Referring to FIG. 6, first in step 70, the amount .DELTA.SOX of SO.sub.x trapped per unit of time in the SO.sub.x trap catalyst 16 (hereinafter also simply referred to as the "trapped SO.sub.x amount .DELTA.SOX") is calculated. The fuel contains a fixed percentage of sulfur so in step 70 the trapped SO.sub.x amount .DELTA.SOX per unit of time is calculated by multiplying the fuel injection quantity Qf per unit of time by a constant C. Then in step 71, an integrated value .SIGMA.SOX of the trapped SO.sub.x amount is calculated by adding .SIGMA.SOX to .DELTA.SOX.

[0054] Next in step 72, it is determined whether the integrated value .SIGMA.SOX of the trapped SO.sub.x amount is more than an allowable value MAX at which the SO.sub.x trapping ability starts to decrease. If .SIGMA.SOX is equal to or less than MAX, i.e., .SIGMA.SOX.gtoreq.MAX, then the process jumps ahead to step 74. If, on the other hand, .SIGMA.SOX is greater than MAX, i.e., .SIGMA.SOX>MAX, then the process proceeds on to step 73 where a SO.sub.x release flag indicating that SO.sub.x should be released from the SO.sub.x trap catalyst 12 is set, after which the process proceeds on to step 74.

[0055] In step 74, it is determined whether a command to allow SO.sub.x to be released from the SO.sub.x trap catalyst 12 is being output. If this command is not being output, this cycle of the routine ends. If, on the other hand, this command is being output, the process proceeds on to step 75 where a SO.sub.x release process is executed to release trapped SO.sub.x from the SO.sub.x trap catalyst 12 by raising the temperature of the SO.sub.x trap catalyst 12 to 600.degree. C. or more and making the air-fuel ratio of the exhaust gas that flows into the SO.sub.x trap catalyst 12 rich. Then in step 76, the SO.sub.x release flag is reset and in step 77, .SIGMA.SOX is cleared.

[0056] When SO.sub.x is released from the SO.sub.x trap catalyst 12, this SO.sub.x reacts with the ammonia NH.sub.3 adsorbed on the NO.sub.x selective reduction catalyst 16, producing hydrogen sulfide H.sub.2S as a result. Generally speaking, the concentration of the hydrogen sulfide H.sub.2S produced at this time is proportional to the adsorbed ammonia amount Q, and proportional to the concentration of SO.sub.x in the exhaust gas flowing into the NO.sub.x selective reduction catalyst 16, i.e., the concentration DS of the SO.sub.x released from the SO.sub.x trap catalyst 12. FIG. 17 shows a graph with equal concentration curves a, b, c, d, and e of hydrogen sulfide H.sub.2S in the exhaust gas that flows out from the NO.sub.x selective reduction catalyst 16 and is discharged into the atmosphere. The concentration DN of the hydrogen sulfide H.sub.2S gradually increases from curve a toward curve e in FIG. 7.

[0057] When the concentration DN of the hydrogen sulfide H.sub.2S discharged into the atmosphere becomes high, a very irritating odor is produced. Therefore it is necessary to keep the discharged concentration DN of hydrogen sulfide H.sub.2S at an allowable concentration or lower where the irritating odor is almost unnoticeable. The allowable concentration where the irritating odor is almost unnoticeable is indicated by the broken line DNO in the drawing. Thus in this example embodiment of the invention, the discharge concentration DN of hydrogen sulfide H.sub.2S is kept to the allowable concentration DNO or lower.

[0058] In this case, the discharge concentration DN of the hydrogen sulfide H.sub.2S will decrease even if the adsorbed ammonia amount Q adsorbed on the NO.sub.x selective reduction catalyst 16 is simply reduced or the SO.sub.x release concentration DS from the SO.sub.x trap catalyst 12, i.e., the amount of SO.sub.x released from the SO.sub.x trap catalyst 12, is simply reduced. Accordingly, in this example embodiment of the invention, when releasing SO.sub.x from the SO.sub.x trap catalyst 12, the adsorbed ammonia amount Q adsorbed on the NO.sub.x selective reduction catalyst 16 is either reduced before SO.sub.x is released or, when SO.sub.x is released, the amount of SO.sub.x that is released from the SO.sub.x trap catalyst 12 is reduced so that the discharge concentration DN of hydrogen sulfide H.sub.2S becomes less than the preset allowable concentration DNO when SO.sub.x is released from the SO.sub.x trap catalyst 12.

[0059] Next, various example embodiments will be described with reference to FIGS. 8 to 15. In a second example embodiment of the invention, an electronic control unit (ECU) 30 that functions as an estimating apparatus is provided which estimates whether the discharge concentration DN of hydrogen sulfide H.sub.2S will become equal to or greater than the allowable concentration DNO when SO.sub.x is released from the SO.sub.x trap catalyst 12. If it is estimated that the discharge concentration DN of hydrogen sulfide H.sub.2S will become equal to or greater than the allowable concentration DNO when SO.sub.x is released, the ECU 30 reduces the adsorbed ammonia amount Q adsorbed on the NO.sub.x selective reduction catalyst 16 before SO.sub.x is released so that the discharge concentration DN of hydrogen sulfide H.sub.2S will be less than the allowable concentration DNO when SO.sub.x is released.

[0060] Incidentally, in this case, when aqueous urea stops being supplied, the ammonia NH.sub.3 that is adsorbed is gradually consumed by the NO.sub.x in the exhaust gas so the adsorbed ammonia amount Q gradually decreases. Therefore, in this second example embodiment, the adsorbed ammonia amount Q is reduced by stopping the supply of aqueous urea. Incidentally, in this case, the adsorbed ammonia amount Q can still be reduced even if the amount of aqueous urea supplied is simply reduced. Therefore, the amount of aqueous urea supplied can also just be reduced instead of being stopped entirely.

[0061] When the discharge concentration DN of the hydrogen sulfide H.sub.2S is less than the allowable concentration DNO, the irritating odor becomes almost unnoticeable. Therefore in the second example embodiment, SO.sub.x is released from the SO.sub.x trap catalyst when the discharge concentration DN of the hydrogen sulfide H.sub.2S is less than the allowable concentration DNO.

[0062] FIG. 8 is a flowchart of a SO.sub.x release control routine that is executed in addition to the routines in FIGS. 5 and 6 to carry out this second example embodiment. This routine is also an interrupt processing routine executed at fixed intervals of time. Referring to FIG. 8, first it is determined in step 80 whether the SO.sub.x release flag is set. If the SO.sub.x release flag is not set, this cycle of the routine ends. If, on the other hand, the SO.sub.x release flag is set, then the process proceeds on to step 81 where the adsorbed ammonia amount Q calculated in the routine shown in FIG. 5 is read.

[0063] Next in step 82, the concentration DS of released SO.sub.x (hereinafter also simply referred to as the "released SO.sub.x concentration DS") when SO.sub.x is released from the SO.sub.x trap catalyst 12 is estimated. That is, the amount of SO.sub.x released (g/sec) per unit of time when SO.sub.x is released from the SO.sub.x trap catalyst 12 is expressed by the product W.times.K of the SO.sub.x release rate W (g/sec) shown in FIG. 3A multiplied by the SO.sub.x release rate K shown in FIG. 3B. Therefore, the concentration DS of SO.sub.x released from the SO.sub.x trap catalyst 12 can be estimated by dividing the amount of released SO.sub.x (i.e., W.times.K) by the volumetric flow rate G (l/sec) of exhaust gas per unit of time (i.e., DS=(W.times.K)/G). Incidentally, the volumetric flow rate G of the exhaust gas is stored in the ROM 32 in advance as a function of the required torque TQ and the engine speed N.

[0064] Next in step 83, the discharge concentration DN of hydrogen sulfide H.sub.2S is estimated from the relationship shown in FIG. 7 based on the adsorbed ammonia amount Q read in step 81 and the released SO.sub.x concentration DS estimated in step 82. Then in step 84, it is estimated whether the discharge concentration DN of hydrogen sulfide H.sub.2S is less than the allowable concentration DNO shown in FIG. 7. If the discharge concentration DN of hydrogen sulfide H.sub.2S is equal to or greater than the allowable concentration DNO, i.e., DN.gtoreq.DNO, then the process proceeds on to step 87.

[0065] In step 87 a command to allow the release of SO.sub.x is cancelled. That is, the command to allow the release of SO.sub.x is not output. Accordingly, as is shown in the routine for releasing SO.sub.x shown in FIG. 6, the SO.sub.x is not released at this time. Instead, the SO.sub.x release process is placed on standby. Next in step 88, a command to stop the supply of aqueous urea is output such that the supply of aqueous urea is stopped at this time, as is shown in the routine to control the supply of aqueous urea shown in FIG. 5. When the SO.sub.x release process is placed on standby and the supply of aqueous urea is stopped in this way, the adsorbed ammonia amount Q gradually decreases. As a result, the estimated value of the discharge concentration DN of hydrogen sulfide H.sub.2S also gradually decreases.

[0066] If, on the other hand, it is determined in step 84 that the discharge concentration DN of hydrogen sulfide H.sub.2S is less than the allowable concentration DNO, i.e., DN<DNO, then the process proceeds on to step 85 where the command to allow the release of SO.sub.x is output. As a result, the process to release SO.sub.x is executed, as is shown in the routine in FIG. 6. At this time, the discharge concentration of hydrogen sulfide H.sub.2S drops below the allowable concentration DNO. Then in step 86, the command to stop supply aqueous urea is cancelled, and the supply of aqueous urea is now restarted. The supply of aqueous urea is preferably restarted after the amount of released SO.sub.x has dropped somewhat after the command to allow the release of SO.sub.x is output.

[0067] FIGS. 9 and 10 show a third example embodiment of the invention. Step 89 is the only step of the SO.sub.x release control routine shown in FIG. 9 for carrying out this third example embodiment that differs from the routine shown in FIG. 8. All of the other steps, i.e., steps 80 to 88, are the same as they are in the routine shown in FIG. 8. Therefore, only step 89 in the SO.sub.x release control routine shown in FIG. 9 will be described. Descriptions of the other steps, i.e., steps 80 to 88, will be omitted.

[0068] In this third example embodiment, the amount of NO.sub.x discharged from the engine is increased in order to rapidly reduce the adsorbed ammonia amount Q when the process to release SO.sub.x from the SO.sub.x trap catalyst 12 is on standby. That is, in this third example embodiment, control to increase the amount of NO.sub.x that is discharged is performed in step 89 in FIG. 9.

[0069] The control to increase the amount of NO.sub.x that is discharged is performed by, for example, advancing the fuel injection timing of fuel from the fuel injection valves 3 or reducing the EGR efficiency. Also in this third example embodiment, when the amount of discharged NO.sub.x is increased, the NO.sub.x amount NOXA that is discharged per unit of time from the engine is stored in the ROM 32 in advance in the form of a map shown in FIG. 10B as a function of the required torque TQ and the engine speed N. When the control to increase the amount of NO.sub.x that is discharged is being performed, the NO.sub.x amount NOXA is calculated from the map shown in FIG. 10B in step 60 shown in FIG. 5.

[0070] When the SO.sub.x release process has been on standby for an extended period of time after the SO.sub.x release flag has been set, the SO.sub.x trap catalyst 12 may become saturated with SO.sub.x, such that SO.sub.x may flow out of the SO.sub.x trap catalyst 12 when the air-fuel ratio is lean. In this case, if the amount of NO.sub.x discharged is increased as in the third example embodiment, the adsorbed ammonia amount will rapidly decrease so the amount of time that the SO.sub.x release process is on standby can be reduced. As a result, it is possible to prevent SO.sub.x from flowing out of the SO.sub.x trap catalyst 12 when the air-fuel ratio is lean.

[0071] FIGS. 11 and 12 show a fourth example embodiment of the invention Steps 99 to 101 are the only steps of the SO.sub.x release control routine shown in FIG. 11 for carrying out this fourth example embodiment that differ from the routine shown in FIG. 8. All of the other steps, i.e., steps 80 to 88, are the same as they are in the routine shown in FIG. 8. Therefore, only steps 99 to 101 in the SO.sub.x release control routine shown in FIG. 11 will be described. Descriptions of the other steps, i.e., steps 80 to 88, will be omitted.

[0072] In this fourth example embodiment, the temperature of the NO.sub.x selective reduction catalyst 16 is increased in order to rapidly reduce the adsorbed ammonia amount Q when the process to release SO.sub.x from the SO.sub.x trap catalyst 12 is on standby. That is, in this fourth example embodiment, control to raise the temperature of the NO.sub.x selective reduction catalyst 16 is performed in step 99 of FIG. 11. This control to raise the temperature of the NO.sub.x selective reduction catalyst 16 is performed by, for example, retarding the fuel injection timing which raises the temperature of the exhaust gas with a lean air-fuel ratio.

[0073] FIGS. 12A and 12B both show desorption rates K1 and K2 of adsorbed ammonia NH.sub.3 from the NO.sub.x selective reduction catalyst 16. As shown in FIG. 12A, the desorption rate K1 of the adsorbed ammonia NH.sub.3 raises rapidly when the bed temperature TS of the NO.sub.x selective reduction catalyst 16 becomes high. Therefore, the adsorbed ammonia amount Q can be rapidly reduced by raising the temperature of the NO.sub.x selective reduction catalyst 16. Also, as shown in FIG. 12B, the desorption rate K2 increases as the volumetric flow rate G of the exhaust gas increases.

[0074] The desorption amount of the adsorbed ammonia can be obtained by multiplying the desorption rates K1 and K2 by the adsorbed ammonia amount Q. Therefore, in the fourth example embodiment, the desorption rates K1 and K2 are calculated from FIGS. 12A and 12B in step 100 when the control to raise the temperature of the NO.sub.x selective reduction catalyst 16 is performed in step 99 as shown in FIG. 11. Then the desorption amount (K1.times.K2.times.Q) is subtracted from the adsorbed ammonia amount Q in step 101. Accordingly, the adsorbed ammonia amount Q gradually decreases.

[0075] FIGS. 13 and 14 show a fifth example embodiment of the invention. In this fifth example embodiment, the discharge concentration DN of hydrogen sulfide H.sub.2S is less than the allowable concentration DNO in order to release SO.sub.x from the SO.sub.x trap catalyst 12 using a simple method. The allowable adsorption amount QX with respect to only the adsorbed ammonia amount Q is set irrespective of the released SO.sub.x concentration DS, as shown in FIG. 13.

[0076] That is, in the fifth example embodiment, when SO.sub.x is to be released from the SO.sub.x trap catalyst 12, the supply of aqueous urea is stopped before SO.sub.x is released when it is determined by the ECU 30 that the adsorbed ammonia amount Q that is adsorbed on the NO.sub.x selective reduction catalyst 16 is equal to or greater than the allowable adsorption amount QX which is set beforehand. In this case as well, the amount of aqueous urea supplied may also be reduced instead of completely stopping the supply of aqueous urea.

[0077] FIG. 14 is a flowchart illustrating a SO.sub.x release control routine that is executed in addition to the routines shown in FIGS. 5 and 6 in order to carry out the fifth example embodiment. This routine is also an interrupt processing routine executed at fixed intervals of time. Referring to FIG. 14, first in step 200, it is determined whether the SO.sub.x release flag is set. If the SO.sub.x release flag is not set, this cycle of the routine ends. If, however, the SO.sub.x release flag is set, the process proceeds on to step 201 where the adsorbed ammonia amount Q calculated in the routine shown in FIG. 5 is read.

[0078] Next in step 202, it is determined whether the adsorbed ammonia amount Q is less than the allowable adsorption amount QX. If the adsorbed ammonia amount Q is equal to or greater than the allowable adsorption amount QX, i.e., Q.gtoreq.QX, then the process proceeds on to step 205 where a command to allow the release of SO.sub.x is canceled. That is, a command to allow SO.sub.x to be released is not output. Accordingly, the process to release SO.sub.x is not executed at this time, as is shown in the routine for releasing SO.sub.x in FIG. 6. Then in step 206, the command to stop the supply of aqueous urea is output so that the supply of aqueous urea is stopped at this time, as is shown in the routine to control the supply of aqueous urea shown in FIG. 5.

[0079] If, on the other hand, it is determined in step 202 that the adsorbed ammonia amount Q is less than the allowable adsorption amount QX, i.e., Q<QX, then the process proceeds on to step 203 where a command to allow the release of SO.sub.x is output. As a result, the process to release SO.sub.x is executed, as is shown in the routine in FIG. 6. Next in step 204, a command to atop supply of aqueous urea is cancelled so aqueous urea starts to be supplied again.

[0080] On the other hand, when the released SO.sub.x concentration DS when SO.sub.x is released from the SO.sub.x trap catalyst 12 is low, the discharge concentration DN of hydrogen sulfide H.sub.2S is less than the allowable concentration DNO or lower irrespective of the adsorbed ammonia amount Q, as shown in FIG. 7. Therefore, in a sixth example embodiment of the invention, when SO.sub.x is released from the SO.sub.x trap catalyst 12, the amount of SO.sub.x that is released from the SO.sub.x trap catalyst 12 is reduced so that the discharge concentration DN of the hydrogen sulfide H.sub.2S becomes less than the allowable concentration DNO.

[0081] That is, in the sixth example embodiment, as shown in FIG. 15, the allowable adsorption amount QX of the adsorbed ammonia amount Q at which the discharge concentration DN of hydrogen sulfide H.sub.2S is less than the allowable concentration DNO is set beforehand irrespective of the released SO.sub.x concentration DS when SO.sub.x is released, and the allowable concentration DX of the released NO.sub.x concentration SO.sub.x at which the discharge concentration DN of hydrogen sulfide H.sub.2S is less than allowable concentration DNO is set beforehand irrespective of the adsorbed ammonia amount Q when SO.sub.x is released. Then SO.sub.x release control is performed using the allowable absorption amount QX and the allowable concentration DX.

[0082] That is, in the region where Q is less than QX, i.e., Q<QX, in FIG. 15, DN is less than DNO, i.e., DN<DNO, regardless of the released SO.sub.x concentration DS. Therefore, in this sixth example embodiment, when Q is less than QX, the air-fuel ratio is made a target air-fuel ratio with a high degree of richness in order to release a large amount of SO.sub.x from the SO.sub.x trap catalyst 12. On the other hand, in the region where Q is equal to or greater than QX, i.e., Q.gtoreq.QX, the degree of richness of the air-fuel ratio is reduced so that the released SO.sub.x concentration DS becomes equal to the allowable concentration DX. The air-fuel ratio at this time is calculated as follows.

[0083] That is, as described above, the amount of SO.sub.x released (g/sec) per unit of time when SO.sub.x is released from the SO.sub.x trap catalyst 12 is expressed by the product K.times.W of the SO.sub.x release rate W (g/sec) shown in FIG. 3A multiplied by the SO.sub.x release rate K shown in FIG. 3B. Accordingly, the released SO.sub.x concentration DS (=W.times.K/G) from the SO.sub.x trap catalyst 12 can be calculated by dividing that SO.sub.x release amount W.times.K by the volumetric flow rate G (l/sec) of the exhaust gas per unit of time. Therefore, to bring the released SO.sub.x concentration DS down to the allowable concentration DX, all that need be done is to make the SO.sub.x release rate K equal (DX.times.G/W), and the air-fuel ratio can be calculated using the relationship shown in FIG. 3B from this SO.sub.x release rate K

[0084] FIG. 16 is a flowchart illustrating a SO.sub.x release control routine that is executed in addition to the routines shown in FIGS. 5 and 6 for carrying out the sixth example embodiment. This routine is also an interrupt processing routine executed at fixed intervals of time. Referring to FIG. 16, first in step 210 it is determined whether the SO.sub.x release flag is set. If the SO.sub.x release flag is not set, this cycle of the routine ends. If, on the other hand, the SO.sub.x release flag is set, the process proceeds on to step 211 where the adsorbed ammonia amount Q calculated in the routine in FIG. 5 is read.

[0085] Next in step 212, it is determined whether the adsorbed ammonia amount Q is lower than the allowable adsorption amount QX. If the adsorbed ammonia amount Q is lower than the allowable adsorption amount QX, i.e., Q<QX, the process proceeds on to step 213 where the air-fuel ratio when the SO.sub.x is released is made the target air-fuel ratio with a high degree of richness, after which the process proceeds on to step 216. If, on the other hand, the adsorbed ammonia amount Q is equal to or greater than the allowable adsorption amount QX, i.e., Q.gtoreq.QX, the process proceeds on to step 214 where the SO.sub.x release rate K (=KD.times.G/W) is calculated. Then in step 215 the air-fuel ratio when the SO.sub.x is released is calculated based on the relationship shown in FIG. 3B from this SO.sub.x release rate K, after which the process proceeds on to step 216.

[0086] In step 216, a command allowing the release of SO.sub.x is output. As a result, the process to release SO.sub.x is executed, as is shown in the routine in FIG. 6. Incidentally, in this sixth example embodiment, the command to stop the supply of aqueous urea is not output.

Claims

1. An exhaust gas control apparatus for an internal combustion engine, comprising: a NO.sub.x selective reduction catalyst which is arranged in an engine exhaust passage and selectively reduces NO.sub.x in exhaust gas using ammonia when an air-fuel ratio of the exhaust gas is lean; a SO.sub.x trap catalyst which is arranged in the engine exhaust passage upstream of the NO.sub.x selective reduction catalyst and traps SO.sub.x in the exhaust gas; and a control apparatus that controls the state of the exhaust gas, wherein the control apparatus i) reduces an adsorbed ammonia amount adsorbed on the NO.sub.x selective reduction catalyst before SO.sub.x is released or ii) reduces the amount of SO.sub.x released from the SO.sub.x trap catalyst when SO.sub.x is released, such that a discharge concentration of hydrogen sulfide is less than a preset maximum concentration when SO.sub.x is released.

2. The exhaust gas control apparatus according to claim 1, further comprising: an estimating apparatus that estimates whether the discharge concentration of hydrogen sulfide will be equal to or greater than the maximum concentration when SO.sub.x is released from the SO.sub.x trap catalyst, wherein when it is estimated that the discharge concentration of hydrogen sulfide will be equal to or greater than the maximum concentration when SO.sub.x is released, the control apparatus i) reduces the adsorbed ammonia amount adsorbed on the NO.sub.x selective reduction catalyst before SO.sub.x is released or ii) reduces the amount of SO.sub.x released from the SO.sub.x trap catalyst when SO.sub.x is released, such that the discharge concentration of hydrogen sulfide is less than the maximum concentration when SO.sub.x is released.

3. The exhaust gas control apparatus according to claim 1, wherein when releasing SO.sub.x from the SO.sub.x trap catalyst, the control apparatus reduces the adsorbed ammonia amount adsorbed on the NO.sub.x selective reduction catalyst before SO.sub.x is released until the adsorbed ammonia amount is less than a preset maximum adsorption amount.

4. The exhaust gas control apparatus according to claim 3, further comprising: a determining apparatus that determines whether the adsorbed ammonia amount adsorbed on the NO.sub.x selective reduction catalyst is equal to or greater than the maximum adsorption amount when SO.sub.x is released from the SO.sub.x trap catalyst, wherein when it is determined that the adsorbed ammonia amount adsorbed on the NO.sub.x selective reduction catalyst is equal to or greater than the maximum adsorption amount, the control apparatus reduces the adsorbed ammonia amount until the adsorbed ammonia amount adsorbed on the NO.sub.x selective reduction catalyst is less than the maximum adsorption amount before releasing SO.sub.x.

5. The exhaust gas control apparatus according to claim 1, wherein when releasing SO.sub.x from the SO.sub.x trap catalyst, the control apparatus sets an air-fuel ratio to a first target air-fuel ratio when the adsorbed ammonia amount adsorbed on the NO.sub.x selective reduction catalyst is less than a preset maximum adsorption amount; and the control apparatus sets the air-fuel ratio to a second target air-fuel ratio which is greater than the first target air-fuel ratio when the adsorbed ammonia amount adsorbed on the NO.sub.x selective reduction catalyst is equal to or greater than a maximum adsorption amount and a concentration of released SO.sub.x is larger than a threshold value.

6. The exhaust gas control apparatus according to claim 5, wherein if the adsorbed ammonia amount adsorbed on the NO.sub.x selective reduction catalyst is less than the maximum adsorption amount, the discharge concentration of hydrogen sulfide is less than the maximum concentration regardless of the concentration of released SO.sub.x; and if the concentration of released SO.sub.x is equal to or less than the threshold value, the discharge concentration of hydrogen sulfide is less than the maximum concentration regardless of the adsorbed ammonia amount.

7. The exhaust gas control apparatus according to claim 1, wherein when SO.sub.x is to be released from the SO.sub.x trap catalyst, the control apparatus makes the air-fuel ratio of the exhaust gas flowing into the SO.sub.x trap catalyst rich.

8. The exhaust gas control apparatus according to claim 1, further comprising: an aqueous urea supply valve arranged in the engine exhaust passage upstream of the NO.sub.x selective reduction catalyst, wherein when it is estimated that the discharge concentration of hydrogen sulfide will be equal to or greater than the maximum concentration when SO.sub.x is released from the SO.sub.x trap catalyst, the control apparatus i) reduces the amount of aqueous urea supplied before releasing SO.sub.x or ii) stops the supply of aqueous urea before releasing SO.sub.x before releasing SO.sub.x, such that the discharge concentration of hydrogen sulfide is less than the maximum concentration when SO.sub.x is released.

9. The exhaust gas control apparatus according to claim 8, wherein when reducing the amount of aqueous urea supplied or stopping the supply of aqueous urea, the control apparatus increases the amount of NO.sub.x discharged from the engine.

10. The exhaust gas control apparatus according to claim 8, wherein when reducing the amount of aqueous urea supplied or stopping the supply of aqueous urea, the control apparatus raises the temperature of the NO.sub.x selective reduction catalyst.

11. The exhaust gas control apparatus according to claim 8, wherein when it is determined that the adsorbed ammonia amount adsorbed on the NO.sub.x selective reduction catalyst is equal to or greater than the maximum adsorption amount when SO.sub.x is released from the SO.sub.x trap catalyst, the control apparatus reduces the amount of supplied aqueous urea before releasing SO.sub.x or stops the supply of aqueous urea before releasing SO.sub.x.