EP2997241A1

EP2997241A1 - Exhaust gas control apparatus for internal combustion engine

Info

Publication number: EP2997241A1
Application number: EP14747700.4A
Authority: EP
Inventors: Hidetaka Shibata; Tomohiko Matsushita; Yasuaki Nakano
Original assignee: Toyota Motor Corp
Current assignee: Toyota Motor Corp
Priority date: 2013-05-14
Filing date: 2014-05-12
Publication date: 2016-03-23
Also published as: JP2014222062A; WO2014184635A1

Abstract

A reduction process is executed when a catalyst bed temperature is lower than a target temperature of the catalyst bed temperature in a burn-off process. When an execution condition for the burn-off process is satisfied, the reduction process is executed, and execution of the burn-off process is prohibited until an accumulation amount of sulfur oxide in the oxidation catalyst becomes equal to or less than an allowable amount through the execution of the reduction process.

Description

EXHAUST GAS CONTROL APPARATUS FOR INTERNAL COMBUSTION ENGINE

BACKGROUND OF THE INVENTION

1. Field of the Invention

[0001] The invention relates to an exhaust gas control apparatus for an internal combustion engine, in which an oxidation catalyst and a filter that traps particulate matter are provided in an exhaust passage.

2. Description of Related Art

[0002] In an exhaust gas control apparatus in which a filter and an oxidation catalyst for purifying exhaust gas are provided in an exhaust passage, the oxidation catalyst oxidizes, and thus purifies, unburned components in exhaust gas, and the filter traps particulate matter. In this kind of exhaust gas control apparatus, when an accumulation amount of particulate matter trapped in the filter is larger than an allowable amount, fuel is added into the exhaust passage to raise the catalyst bed temperature of the oxidation catalyst. The temperature of the filter is raised by the heat of the exhaust gas when the catalyst bed temperature of the oxidation catalyst is raised in this way and the particulate matter is consequently burned off.

[0003] Also, the exhaust gas from an internal combustion engine includes sulfur oxide in addition to unburned components and particulate matter. This sulfur oxide is adsorbed by the oxidation catalyst and when it continues to accumulate, the catalyst function may decline. Therefore, in the exhaust gas control apparatus, in addition to the burn-off process for the particulate matter described above, fuel is added into the exhaust passage to raise the catalyst bed temperature of the oxidation catalyst and place the oxidation catalyst in a low oxygen concentration environment. In this way, the sulfur oxide that has accumulated in the oxidation catalyst is reduced and desorbed by the added fuel.

[0004] For example, with an exhaust gas control apparatus described in Japanese Patent Application Publication No. 2006-291823 (JP 2006-291823 A), when a required temperature increase in the catalyst bed temperature in the burn-off process for particulate matter is higher than a required temperature increase in the reduction process for sulfur oxide, the burn-off process for particulate matter is performed. By performing this kind of burn-off process, particulate matter trapped by the filter is burned off, and in conjunction with this, the sulfur oxide is reduced and desorbed.

SUMMARY OF THE INVENTION

[0005] When a reduction process for sulfur oxide is performed in conjunction with a burn-off process for particulate matter, as described above, in the exhaust gas control apparatus in which an oxidation catalyst is arranged upstream of a filter, white smoke is generated, as described below.

[0006] The catalyst bed temperature of the oxidation catalyst also rises while the burn-off process for particulate matter is being performed. Therefore, sulfur dioxide

(502) that has been reduced and desorbed in the oxidation catalyst may be oxidized again in the oxidation catalyst. If sulfur dioxide is oxidized in this way, sulfate (sulfur trioxide

(503) ) is generated. This sulfate may slip through the filter and become white smoke, which may be discharged out of the exhaust passage.

[0007] Also, the sulfur dioxide that has been desorbed from the oxidation catalyst may flow into the filter and be oxidized again by the heat of the filter. In this case as well, while smoke is generated just as described above. The reoxidation of the sulfur dioxide in the filter is able to be suppressed by increasing to the fuel additive amount and placing the filter in an environment with a low oxygen concentration. However, in this case, there are trade-offs such as the fuel efficiency deteriorating and unburned fuel becoming white smoke and being discharged, so the generation of white smoke is unable to be sufficiently resolved.

[0008] Also, when the sulfur content in the fuel is large such that the amount of sulfur oxide to be reduced is large, the generation of white smoke caused by the generation of sulfate as described above becomes even more significant.

[0009] The invention thus provides an exhaust gas control apparatus for an internal combustion engine, which suppresses white smoke from being generated due to sulfate generated from sulfur oxide that has accumulated in an oxidation catalyst.

[0010] A first aspect of the invention relates to an exhaust gas control apparatus for an internal combustion engine, which includes an oxidation catalyst provided in an exhaust passage of the internal combustion engine, a filter that traps particulate matter, and a controller. The filter is provided in the exhaust passage of the internal combustion engine, and is provided on a downstream side of the oxidation catalyst in an exhaust gas flow direction. The controller is configured to execute a burn-off process and a reduction process. The burn-off process is a process to burn off particulate matter trapped in the filter, by adding fuel upstream of the oxidation catalyst, in the exhaust gas flow direction. The reduction process is a process to reduce and desorb sulfur oxide accumulated in the oxidation catalyst.

[0011] This reduction process includes an execution condition for the reduction process, and the execution condition is that a catalyst bed temperature of the oxidation catalyst in the reduction process be lower than a target temperature of the catalyst bed temperature of the oxidation catalyst in the burn-off process. Therefore, the reduction process for sulfur oxide is able to be performed in an environment in which the catalyst bed temperature of the oxidation catalyst and the temperature of the filter are low, compared to when the reduction process for sulfur oxide is performed in conjunction with the burn-off process for particulate matter.

[0012] Therefore, the generation of sulfate caused by sulfur dioxide being reoxidized in the oxidation catalyst is able to be suppressed. Furthermore, the generation of sulfate caused by sulfur dioxide that has flowed into the filter from the oxidation catalyst being oxidized again in the filter is also able to be suppressed.

[0013] Also, the controller is configured to prohibit an execution of the burn-off process in case of an accumulation amount of sulfur oxide in the oxidation catalyst being larger than an allowable amount. Therefore, the burn-off process for particulate matter is performed in an environment in which the accumulation amount of sulfur oxide in the oxidation catalyst is low, so the amount of sulfur dioxide that flows into the filter from the oxidation catalyst while the burn-off process is being executed is able to be reduced. Therefore, even if regeneration of sulfur dioxide has occurred in the filter, the reoxidized amount is decreased, and the generation of sulfate caused by this reoxidation is also able to be suppressed.

[0014] In this way, with the exhaust gas control apparatus described above, it is possible to prevent sulfate from being generated from sulfur oxide that has accumulated in the oxidation catalyst, and thus prevent white smoke from being generated. Also, with the exhaust gas control apparatus described above, the burn-off process for particulate matter is prohibited on a condition that the accumulation amount of sulfur oxide in the oxidation catalyst is larger than the allowable amount. Instead, however, when the execution condition for the burn-off process is satisfied, a timing at which the burn-off process is started may be delayed until the accumulation amount of sulfur oxide in the oxidation catalyst becomes equal to or less than the allowable amount through (or thanks to) the execution of the reduction process. Alternatively, the timing at which the burn-off process for particulate matter is started may be delayed until a period of time for (or during) which the reduction process for sulfur oxide is executed reaches a predetermined period of time.

[0015] In the aspect described above, the burn-off process for particulate matter is performed in an environment in which the accumulation amount of sulfur oxide in the oxidation catalyst is small. Therefore, the amount of sulfur dioxide that flows into the filter from the oxidation catalyst while the burn-off process is being executed is able to be reduced. Therefore, even if regeneration of sulfur dioxide has occurred in the filter, the reoxidized amount is decreased, and the generation of sulfate caused by this reoxidation is also able to be suppressed.

[0016] Also, in the reduction process for sulfur oxide, regeneration of sulfur dioxide in the oxidation catalyst occurs frequently when the catalyst bed temperature of the oxidation catalyst is higher than an upper limit temperature at which the desorbed sulfur dioxide in the oxidation catalyst does not reoxidize. In this case, the generation of sulfate that causes the kind of white smoke described above tends to progress. On the other hand, in view of the generation of sulfate, when the catalyst bed temperature of the oxidation catalyst is equal to or lower than the upper limit temperature, the reoxidized amount of sulfur dioxide is kept within an extremely small range that is negligible. Therefore, the reduction process for sulfur oxide may also be executed when the catalyst bed temperature is equal to or lower than an upper limit temperature at which desorbed sulfur dioxide in the oxidation catalyst does not reoxidize. According to this kind of structure, the generation of sulfate when the reduction process for sulfur oxide is performed is able to be effectively suppressed.

[0017] In the aspect described above, a carried amount of platinum-based metal per unit volume of a carrier of the filter may be less than a carried amount of platinum-based metal per unit volume of a carrier of the oxidation catalyst. By setting the relationship between the carried amounts of platinum-based metal in this way, the oxidative power of the filter is relatively low, while the oxidative power of the oxidation catalyst is relatively high.

[0018] Therefore, in the oxidation catalyst, the amount of sulfur dioxide adsorbed by the carrier of the oxidation catalyst increases, and the amount of sulfur dioxide that flows into the filter without being adsorbed by the oxidation catalyst decreases. As a result, the generation of sulfate caused by sulfur dioxide flowing into the heated filter is able to be suppressed. On the other hand, in the filter, even if sulfur dioxide has flowed in from the oxidation catalyst, the oxidative power of the filter is low, so reoxidation of the sulfur dioxide is able to be suppressed. Therefore, employing this kind of structure enables the amount of sulfate that is discharged from the filter to be further reduced. In the filter, a carrier on which a platinum-based metal is not carried may also be employed.

[0019] Also, in the aspect described above, the oxidation catalyst may be a catalyst in which the carrier of the oxidation catalyst is an alumina-based metal, and the carried amount of platinum-based metal per 1 liter of the carrier of the oxidation catalyst is equal to or greater than 3.0 g. Moreover, the filter may be a filter in which the carried amount of platinum-based metal per 1 liter of the carrier of the filter is equal to or less than 1.0 g.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020] 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 view showing a frame format of the general structure of an internal combustion engine and an exhaust gas control apparatus thereof;

FIG. 2 is a flowchart illustrating a routine for executing a burn-off process and a reduction process;

FIG. 3 is a time chart showing a shift in an execution condition for the burn-off process, an amount of fuel added, a catalyst bed temperature of an oxidation catalyst, an amount of sulfur dioxide that flows into a filter from the oxidation catalyst, an amount of sulfate discharged from the filter, and an accumulation amount of sulfur oxide in the oxidation catalyst;

FIG. 4 is a flowchart illustrating a routine for executing a reduction process;

FIG. 5 is a flowchart illustrating a routine for executing a burn-off process; and FIG. 6 is a time chart showing a shift in a running distance, an accumulation amount of sulfur oxide in the oxidation catalyst, an executing flag for the reduction process, a prohibiting flag for the burn-off process, and an execution mode of the burn-off process. DETAILED DESCRIPTION OF EMBODIMENTS

[0021] Hereinafter, a first example embodiment of the exhaust gas control apparatus will be described with reference to FIGS. 1 to 3.

[0022] As shown in FIG. 1 , an injection valve 12 that injects fuel into a combustion chamber 11 of an internal combustion engine 10 is provided in the internal combustion engine 10. An adding valve 20 that adds fuel into an exhaust passage 13, an oxidation catalyst 21, and a filter 22, are provided in this order from an upstream side in the exhaust passage 13. The oxidation catalyst 21 oxidizes and purifies unburned fuel in the exhaust gas. The filter 22 traps particulate matter that is in the exhaust gas.

[0023] The oxidation catalyst 21 is a catalyst in which a carrier is formed by an alumina-based metal, and 3.5 grams of platinum particles (Pt) per 1 liter are carried on this carrier. Meanwhile, the filter 22 is a filter in which a carrier is formed by cordierite or silicon carbide, and 0.5 grams of platinum particles per 1 liter are carried on this carrier.

[0024] Also, a bed temperature sensor 30 that detects a temperature of the carrier of the oxidation catalyst 21, i.e., a catalyst bed temperature of the oxidation catalyst 21, is mounted to the oxidation catalyst 21. A filter temperature sensor 31 that detects a temperature of the carrier of the filter 22 is mounted to the filter 22. Also, a differential pressure sensor 32 is provided in the exhaust passage 13. This differential pressure sensor 32 detects a difference between an internal pressure of a portion upstream of the filter 22 and an internal pressure of a portion downstream of the filter 22. Detection signals from these sensors are received by a controller 40 of the exhaust gas control apparatus. This controller 40 performs various controls related to exhaust gas control by driving actuators of the injection valve 12 and the adding valve 20 and the like based on the detection signals from these sensors.

[0025] Next, the burn-off process for particulate matter will be described. For example, the controller 40 executes a burn-off process that burns off particulate matter that is trapped in the filter 22. The particulate matter, trapping performance of the filter 22 is recovered by this burn-off process. In this burn-off process, when an execution condition for the burn-off process is satisfied, fuel is added from the adding valve 20 into the exhaust passage 13. By adding fuel in this way, the catalyst bed temperature of the oxidation catalyst 21 is increased to a target temperature, and the filter 22 increases in temperature from the heat of the exhaust gas, thereby causing particulate matter that has accumulated on the filter 22 to be burned off.

[0026] Here, an accumulation amount of particulate matter (ΣΡΜ) on the filter 22 may be obtained by repeating a calculation according to Expression (1) below, for example, at predetermined cycles.

ΣΡΜ (i) <- ΣΡΜ (i - 1) + PMadd - PMsub ... (1)

In Expression (1) above, ΣΡΜ (i) is the accumulation amount of particulate matter on the filter 22 obtained at the current calculation timing, and ΣΡΜ (i - 1) is the accumulation amount of particulate matter on the filter 22 obtained at the most recent (i.e., the last) calculation timing. Also, PMadd is the amount of particulate matter that has newly accumulated on the filter 22 in a period from the most recent calculation timing until the current calculation timing. PMsub is the amount of particulate matter that has been burned off of the filter in the same period. The PMadd and the PMsub are obtained through map calculations using parameter that correlate to the operating state of the engine (i.e., the internal combustion engine). Parameter that correlates to the operating state of the engine (i.e., the internal combustion engine) is a fuel injection quantity, an engine speed, an amount of fuel added by the adding valve 20, the temperature of the filter 22, and the catalyst bed temperature, for example.

[0027] Also, the execution condition for executing the burn-off process is that at least one of i) the accumulation amount of particulate matter obtained by Expression (1) above is larger than an allowable amount, and ii) the differential pressure detected by the differential pressure sensor 32 is equal to or higher than a predetermined value, be satisfied.

[0028] When the execution condition is satisfied and the burn-off process starts, fuel is added from the adding valve 20 such that the catalyst bed temperature of the oxidation catalyst 21 increases. The controller 40 controls the manner in which the fuel is added by the adding valve 20, while monitoring the filter temperature and the catalyst bed temperature of the oxidation catalyst 21, such that the catalyst bed temperature of the oxidation catalyst 21 comes to match the target temperature, and the temperature of the filter 22 does not increase too much. For example, the controller 40 makes the catalyst bed temperature of the oxidation catalyst 21 match the target temperature by intermittently adding fuel and adjusting the lengths of an adding period during which time fuel is added, and a pausing period during which time fuel is not added.

[0029] Next, a reduction process for sulfur oxide will be described. Sulfur dioxide in the exhaust gas is oxidized when the internal combustion engine is operating. The oxidized sulfur dioxide is adsorbed by the oxidation catalyst 21 in the form of a sulfur oxide, for example, aluminum sulfate such as A12 (S04) 3, and accumulates on the oxidation catalyst 21. Therefore, the controller 40 executes a reduction process that reduces and desorbs sulfur oxide that has accumulated in the oxidation catalyst 21 in this way.

[0030] In this reduction process, fuel is added into the exhaust passage 13 to increase the catalyst bed temperature of the oxidation catalyst 21 and place the oxidation catalyst 21 in an environment with a low oxygen concentration. As a result, sulfur oxide that has accumulated on the oxidation catalyst 21 is reduced and desorbed by the added fuel.

[0031] Also, the reduction process performed by this exhaust gas control apparatus is executed when the catalyst bed temperature of the oxidation catalyst 21 shifts into a temperature range that is lower than the target temperature of the catalyst bed temperature of the oxidation catalyst 21 in the burn-off process. More specifically, the reduction process is executed under the condition that, with a temperature lower than the target temperature as an upper limit temperature of the temperature range, the catalyst bed temperature of the oxidation catalyst 21 be equal to or lower than this upper limit temperature.

[0032] Here, this upper limit temperature is a temperature at which sulfur dioxide desorbed from the oxidation catalyst 21 does not reoxidize in the oxidation catalyst 21. That is, when the catalyst bed temperature of the oxidation catalyst 21 is higher than this upper limit temperature, reoxidation of sulfur dioxide does occur frequently in the oxidation catalyst 21. In such a case, the generation of sulfate that causes white smoke such as that described above tends to progress. In contrast, from the viewpoint of the generation of sulfate, when the catalyst bed temperature of the oxidation catalyst 21 is equal to or lower than the upper limit temperature, the reoxidation amount of sulfur dioxide is able to be kept to an extremely small range that is negligible.

[0033] Also, when the execution condition is satisfied and the reduction process starts, the catalyst bed temperature of the oxidation catalyst 21 rises. The controller 40 controls the manner in which the fuel is added by the adding valve 20 such that the catalyst bed temperature of the oxidation catalyst 21 shifts into a temperature range that is equal to or lower than the upper limit temperature, while monitoring the catalyst bed temperature of the oxidation catalyst 21. For example, the controller 40 causes the catalyst bed temperature of the oxidation catalyst 21 to shift into the temperature range that is equal to or lower than the upper limit temperature by adjusting the lengths of the adding period during which time fuel is added and the pausing period during which time fuel is not added. Then the accumulation amount of sulfur oxide in the oxidation catalyst 21 is reduced by this reduction process being performed.

[0034] Here, an accumulation amount of sulfur oxide (∑SO) in the oxidation catalyst 21 may be obtained by repeating a calculation according to Expression (2) below, for example, at predetermined cycles.

∑SO (i) <-∑SO (i - 1) + SOadd - SOsub ... (2)

In Expression (2) above,∑SO (i) is the accumulation amount obtained at the current calculation timing, and∑SO (i - 1) is the accumulation amount obtained at the most recent calculation timing. Also, SOadd is the amount of sulfur oxide that has newly accumulated on the oxidation catalyst 21 in a period from the most recent calculation timing until the current calculation timing. SOsub is the amount of sulfur oxide that has been reduced and desorbed by the oxidation catalyst 21 in the same period. The SOadd and the SOsub are obtained through map calculations using a parameter that correlates to the concentration of sulfur in the fuel currently being used, in addition to parameter that correlates to the operating state of the engine (i.e., the internal combustion engine). Parameter that correlates to the operating state of the engine (i.e., the internal combustion engine) is a fuel injection quantity, an engine speed, the amount of fuel added by the adding valve 20, the temperature of the filter 22, and the catalyst bed temperature of the oxidation catalyst 21, for example.

[0035] Next, a particular routine of the burn-off process and the reduction process described above that is executed will be described with reference to FIG. 2.

[0036] As shown in FIG. 2, when the execution condition for the burn-off process is satisfied (i.e., YES in step S201), it is then determined whether the catalyst bed temperature of the oxidation catalyst 21 is equal to or lower than the upper limit temperature described above (step S202).

[0037] If the catalyst bed temperature of the oxidation catalyst 21 is equal to or lower than the upper limit temperature (i.e., YES in step S202), the reduction process is executed (step S203). Next, the accumulation amount of sulfur oxide in the oxidation catalyst 21 is calculated (step S204). If this accumulation amount is equal to or less than an allowable amount (i.e., YES in step S205), the bum-off process starts (step S206). The bum-off process is continually executed until the accumulation amount of particulate matter on the filter 22 is zero ("0").

[0038] Here, the allowable amount is a threshold value for limiting the accumulation amount of sulfur oxide while the bum-off process is being executed, so that even if the bum-off process is started and the catalyst bed temperature rises such that sulfur dioxide desorbed from the oxidation catalyst 21 flows into the filter 22 and is reoxidized, it will not affect the generation of white smoke.

[0039] On the other hand, if the execution condition for the bum-off process is not satisfied, or if the catalyst bed temperature of the oxidation catalyst 21 is higher than the upper limit temperature, or if an accumulation amount of the sulfur oxide in the oxidation catalyst 21 is larger than the allowable amount (i.e., NO in step S201 , NO in step S202, or NO in step S205), this cycle of the routine ends.

[0040] Next, the operation of this exhaust gas control apparatus will be described with reference to FIG. 3. As shown in FIG. 3, when the execution condition for the bum-off process is satisfied at timing tl (execution condition for the bum-off process in FIG. 3), the catalyst bed temperature of the oxidation catalyst is below the upper limit temperature, so the reduction process starts and fuel starts to be intermittently added by the adding valve 20 (amount of fuel added in FIG. 3). When fuel is added in this way, the catalyst bed temperature of the oxidation catalyst gradually rises (catalyst bed temperature of the oxidation catalyst in FIG. 3).

[0041] Then sulfur oxide that is adsorbed by the oxidation catalyst 21 is reduced by the fuel and desorbed, thus generating sulfur dioxide, and this sulfur dioxide flows into the filter 22 from the oxidation catalyst 21 (amount of sulfur dioxide that flows into a filter from the oxidation catalyst, in FIG. 3). Here, the catalyst bed temperature of the oxidation catalyst 21 is below the upper limit temperature, so reoxidation of the sulfur dioxide in the oxidation catalyst 21 is suppressed. Also, the temperature of the filter 22 is also low, which also suppresses the reoxidation of the sulfur dioxide that has flowed into the filter 22.

[0042] When the sulfur dioxide is desorbed from the oxidation catalyst 21 in this way, the accumulation amount of sulfur oxide in the oxidation catalyst 21 gradually decreases (accumulation amount in oxidation catalyst in FIG. 3). Then when the accumulation amount of the sulfur oxide becomes equal to or less than the allowable amount at timing t2, the reduction process ends and the burn-off process begins. As a result, the catalyst bed temperature of the oxidation catalyst 21 rises even higher and gradually converges on a target temperature of the burn-off process (catalyst bed temperature of oxidation catalyst in FIG. 3). In this way, the accumulation amount of sulfur oxide in the oxidation catalyst 21 decreases due to the reduction process that is performed before the burn-off process. Therefore, even if the burn-off process is performed and the catalyst bed temperature rises, sulfur dioxide that flows from the oxidation catalyst 21 into the heated filter 22 also becomes an extremely limited amount. Therefore, the discharge of white smoke caused by sulfate being generated in the filter 22 is able to be suppressed (amount of sulfate discharged from the filter in FIG. 3).

[0043] In this way, with this exhaust gas control apparatus, when the execution condition for the burn-off process is satisfied, the timing at which the burn-off process is started is delayed until the accumulation amount of sulfur oxide becomes equal to or less than the allowable amount through or due to the execution of the reduction process.

[0044] The exhaust gas control apparatus described above is able to take the effects described below. The reduction process is executed when the catalyst bed temperature of the oxidation catalyst 21 is lower than the target temperature of the catalyst bed temperature of the oxidation catalyst 21 in the burn-off process. Therefore, the reduction process is able to be performed in an environment in which the catalyst bed temperature of the oxidation catalyst 21 and the filter temperature are low, compared to when the reduction process is performed in conjunction with the burn-off process. Therefore, the generation of sulfate caused by sulfur dioxide being reoxidized in the oxidation catalyst 21 is able to be suppressed. Furthermore, the generation of sulfate caused by sulfur dioxide that has flowed into the filter 22 from the oxidation catalyst 21 being oxidized again in the filter 22 is also able to be suppressed.

[0045] Also, when the execution condition for the burn-off process is satisfied, the timing at which the bum-off process is started is delayed until the accumulation amount of sulfur oxide in the oxidation catalyst 21 becomes equal to or less than the allowable amount through the execution of the reduction process. Therefore, the burn-off process is performed in an environment in which the accumulation amount of sulfur oxide in the oxidation catalyst 21 is low, so the amount of sulfur dioxide that flows into the filter 22 from the oxidation catalyst 21 while the burn-off process is being executed is able to be reduced. Therefore, the generation of sulfate caused by sulfur dioxide being oxidized again in the filter 22 is also able to be suppressed.

[0046] According to this exhaust gas control apparatus, it is possible to suppress sulfate from being generated from sulfur oxide that has accumulated in the oxidation catalyst, and thereby suppress white smoke from being generated. Also, the reduction process is executed when the catalyst bed temperature of the oxidation catalyst is equal to or less than the upper limit temperature at which sulfur dioxide desorbed from the oxidation catalyst 21 does not reoxidize in the oxidation catalyst 21. Therefore, the reoxidation amount of sulfur dioxide in the oxidation catalyst 21 is able to be suppressed to an extremely small range that is negligible, and as a result, the generation of sulfate when the reduction process is performed is able to be effectively suppressed.

[0047] Moreover, the carried amount of the platinum particles per unit volume of the carrier of the filter 22 is made less than the carried amount of platinum particles per unit volume of the carrier of the oxidation catalyst 21. Therefore, the oxidative power of the filter 22 is relatively low, while the oxidative power of the oxidation catalyst 21 is relatively high.

[0048] And therefore, with the oxidation catalyst 21, the amount of sulfur dioxide adsorbed by the catalyst of the oxidation catalyst 21 increases, and the amount of sulfur dioxide that flows into the filter 22 without being adsorbed by the oxidation catalyst 21 decreases. As a result, the generation of sulfate caused by sulfur dioxide flowing into the heated filter 22 is able to be suppressed. On the other hand, with the filter 22, even if sulfur dioxide has flowed into the filter 22 from the oxidation catalyst 21, the oxidative power of the filter 22 is low, so reoxidation of the sulfur dioxide is able to be suppressed. Therefore, the amount of sulfate discharged from the filter 22 is able to be further reduced.

[0049] In addition, the carrier of the oxidation catalyst 21 is formed by an alumina-based metal, so sulfur dioxide in the exhaust gas is able to be effectively adsorbed by this carrier. Therefore, it is possible to inhibit sulfur dioxide from flowing into the heated filter 22 from the oxidation catalyst 21 while the burn-off process is being executed, so it is possible to inhibit sulfate from being generated in the filter 22.

[0050] Next, a second example embodiment of the exhaust gas control apparatus will be described with reference to FIGS. 4 to 6. The structure of the exhaust gas control apparatus, such as the adding valve 20, the oxidation catalyst 21, and the filter 22, as well as the method used when calculating the accumulation amount of particulate matter, the method used when calculating the accumulation amount of sulfur oxide, and the manner in which fuel is added in the burn-off process and the reduction process, are the same as they are in the first example embodiment. Parts of the second example embodiment that are the same as those of the first example embodiment will be omitted. Also, this example presumes a case in which an internal combustion engine to which the exhaust gas control apparatus is applied is mounted in a vehicle.

[0051] In the exhaust gas control apparatus according to the first example embodiment, the timing at which the burn-off process is started is delayed until the accumulation amount of sulfur oxide in the oxidation catalyst 21 becomes equal to or less than the allowable amount. However, in the exhaust gas control apparatus of this example embodiment, the burn-off process is prohibited when the accumulation amount of sulfur oxide in the oxidation catalyst 21 is larger than the allowable amount.

[0052] Also, in the exhaust gas control apparatus according to the first example embodiment, the reduction process is executed in a predetermined environment when the execution condition for the burn-off process is satisfied. However, in the exhaust gas control apparatus of this example embodiment, the reduction process is executed each time the vehicle runs a predetermined distance.

[0053] Hereinafter, a reduction process and burn-off process that are performed by the exhaust gas control apparatus according to this example embodiment will be described with reference to FIGS. 4 and 5. First, a particular routine of the reduction process will be described.

[0054] As shown in FIG. 4, when the running distance of the vehicle reaches a predetermined distance (i.e., YES in step S401), the running distance is reset to "0" (step S402). Then an executing flag for the reduction process is set to "ON" (step S403), and the process proceeds on to step S404. On the other hand, if the running distance of the vehicle has not reached the predetermined distance (i.e., NO in step S401), the process similarly proceeds on to step S404.

[0055] Here, the predetermined distance is set to a value that is sufficiently shorter than an average distance that the vehicle runs during a period from when the amount of particulate matter that accumulates on the filter 22 is "0" until it reaches an allowable amount.

[0056] In step S404, an accumulation amount of sulfur oxide in the oxidation catalyst 21 is calculated. Then, if this accumulation amount is larger than an allowable amount (i.e., YES in step S405), a prohibiting flag for the burn-off process is set to "ON" (step S406). Next, when the executing flag for the reduction process is "ON" (i.e., YES in step S407), it is then determined whether the catalyst bed temperature of the oxidation catalyst 21 is equal to or less than an upper limit temperature (step S408). When the catalyst bed temperature of the oxidation catalyst 21 is equal to or less than the upper limit temperature (i.e., YES in step S408), the reduction process is executed (step S409).

[0057] Also, when the executing flag for the reduction process is "OFF" (i.e., NO in step S407), or when it is determined that the catalyst bed temperature of the oxidation catalyst 21 is higher than the upper limit temperature (i.e., NO in step S408), this cycle of the routine ends. On the other hand, when the accumulation amount is equal to or less than the allowable amount (i.e., NO in step S405), the prohibiting flag is set to "OFF" (step S416), and then the executing flag for the reduction process is set to "OFF" (step S417), and this cycle of the routine ends.

[0058] In this way, in this exhaust gas control apparatus, the prohibiting flag for the burn-off process is set to "ON" when the accumulation amount is larger than the allowable amount, and is set to "OFF" when the accumulation amount is equal to or less than the allowable amount. Also, the executing flag for the reduction process is set to "ON" when the running distance of the vehicle reaches a predetermined distance, and is set to "OFF" when the accumulation amount is equal to or less than the allowable amount.

[0059] Next, a particular routine of the burn-off process will be described. As shown in FIG. 5, when the execution condition for the burn-off process is satisfied (i.e., YES in step S501), then it is determined whether the prohibiting flag for the burn-off process set in the reduction process before is "OFF" (step S502). When the prohibiting flag is "OFF", the burn-off process starts. Similar to in the first example embodiment, the burn-off process is continually executed until the accumulation amount of the particulate matter on the filter 22 becomes "0".

[0060] On the other hand, if the execution condition for the burn-off process is not satisfied (i.e., NO in step S501), or if the prohibiting flag is "ON" (i.e., NO in step S502), this cycle of the routine ends. That is, even if the foregoing execution condition for the bum-off process is satisfied (i.e., YES in step S501), the bum-off process is not executed if the prohibiting flag is "ON" (i.e., NO in step S502). That is, in this exhaust gas control apparatus, the bum-off process is prohibited when the accumulation amount of sulfur oxide in the oxidation catalyst 21 is larger than the allowable amount.

[0061] Next, the operation of this exhaust gas control apparatus is described with reference to FIG. 6. As shown in FIG. 6, when the running distance reaches a predetermined distance at timing tl (running distance in FIG. 6), the executing flag for the reduction process is switched from "OFF" to "ON" (executing flag for reduction process in FIG. 6). Therefore, the reduction process is executed if another execution condition for the reduction process is satisfied.

[0062] Here, the prohibiting flag for the burn-off process is "ON" (prohibiting flag for burn-off process in FIG. 6), so the burn-off process is not executed (execution of burn-off process in FIG 6) even if this execution condition is satisfied. That is, in this example embodiment, as a precondition for executing the burn-off process, at least one of two conditions must be satisfied. These two conditions are i) that the accumulation amount of particulate matter described above be larger than the allowable amount, and ii) that differential pressure detected by the differential pressure sensor 32 be equal to or greater than a predetermined value. In addition to this precondition being satisfied, an essential execution condition for executing the burn-off process is that the prohibiting flag for the burn-off process be "OFF",

[0063] Then, as a result of the reduction process being performed, the accumulation amount of sulfur oxide in the oxidation catalyst 21 gradually decreases (accumulation amount in oxidation catalyst in FIG. 6), and when the accumulation amount becomes equal to or less than the allowable amount at timing t2, the executing flag for the reduction process is set to "OFF" and the reduction process ends. Meanwhile, the prohibiting flag for the burn-off process is set to "OFF" at timing t2. Therefore, the burn-off process is started when the precondition for the bum-off process described above is satisfied.

[0064] As described above, this exhaust gas control apparatus is able to take the effects described below, in addition to the effects of the first example embodiment.

[0065] Execution of the bum-off process is prohibited when the accumulation amount of sulfur oxide in the oxidation catalyst 21 is larger than the allowable amount. Therefore, the bum-off process is performed in an environment in which the accumulation amount of sulfur oxide in the oxidation catalyst 21 is small, so the amount of sulfur dioxide that flows into the filter 22 from the oxidation catalyst 21 while the bum-off process is being executed is able to be reduced. Therefore, the generation of sulfate caused by sulfur dioxide being reoxidized in the filter 22 is also able to be suppressed.

[0066] The reduction process is executed when the catalyst bed temperature of the oxidation catalyst is equal to or less than the upper limit temperature. Therefore, it may not be possible to ensure sufficient opportunity to execute the reduction process in the first example embodiment in which the reduction process is able to be executed when the execution condition for the burn-off process is satisfied.

[0067] However, with this example embodiment, the reduction process is executed each time the vehicle runs a predetermined distance. Furthermore, this predetermined distance is set to a value that is sufficiently shorter than the average distance that the vehicle runs during a period from when the amount of particulate matter that accumulates on the filter 22 is "0" until it reaches the allowable amount. Therefore, there are a plurality of opportunities for the burn-off process to be executed during the period after the execution condition for the reduction process is no longer satisfied until this execution condition is satisfied. As a result, the number of opportunities to execute the reduction process is able to be increased, so the number of opportunities to execute the burn-off process is also able to be increased.

[0068] The example embodiments described above may also be carried out in any of the manners described below. Also, the example embodiments and modified structures described below may also be carried out in combination as appropriate.

[0069] In the first example embodiment, the accumulation amount of sulfur oxide is monitored, and the timing at which the burn-off process is started is delayed until the accumulation amount becomes equal to or less than the allowable amount, when the execution condition for the burn-off process is satisfied. In contrast to the first example embodiment, the timing at which the burn-off process is started may also be delayed until the period of time for which or during which the reduction process is executed reaches a predetermined period of time, for example. With this structure as well, the burn-off process is able to be performed in an environment in which the accumulation amount of sulfur oxide in the oxidation catalyst 21 is small, by setting the predetermined period of time to a period of time sufficient for the accumulation amount of sulfur oxide to be reduced. Therefore, the same effect as that obtained by the first example embodiment is able to be obtained.

[0070] In this modified example, when the reduction process is executed intermittently, the timing at which the burn-off process is started may be delayed until the cumulative of this execution period reaches a predetermined period. Alternatively, the timing at which the burn-off process is started may be delayed until a period of time during which the reduction process is continually executed reaches a predetermined period of time.

[0071] Also, in the modified example described above, the predetermined period may be changed according to the accumulation amount of sulfur oxide at the time the reduction process started. If the predetermined period of time becomes longer as the accumulation amount becomes larger, the accumulation amount can be appropriately reduced through the reduction process regardless of the accumulation amount of sulfur oxide. Therefore, the burn-off process is able to be performed in an environment in which the accumulation amount of sulfur oxide is small.

[0072] In each of the example embodiments above, the amount of sulfur oxide that accumulates in the oxidation catalyst 21 is monitored, but the total amount of sulfur oxide that accumulates on both the oxidation catalyst 21 and the filter 22 may also be monitored, for example. There is a correlation between the total amount of sulfur oxide that accumulates on both the oxidation catalyst 21 and the filter 22 and the amount of sulfur oxide that accumulates on the oxidation catalyst 21. Therefore, the effect of the example embodiments is able to be taken even when the control of the example embodiments is executed based on this total amount.

[0073] Fuel is added in the reduction process and the burn-off process using the adding valve 20. In contrast, in addition to adding fuel using the adding valve 20, fuel may be also be added into the exhaust passage 13 by an injection by the injection valve 12 that is performed during the exhaust stroke (this injection is also referred to as an "after injection"). Fuel can be added by an after injection by the injection valve 12 if the exhaust gas control apparatus is not provided with the adding valve 20.

[0074] In the reduction process and the burn-off process, the catalyst bed temperature of the oxidation catalyst and the filter temperature are controlled by adding fuel intermittently, and adjusting the lengths of the adding period during which time fuel is added, and the pausing period during which time fuel is not added, but the manner in which fuel is added is not limited to this. For example, the catalyst bed temperature of the oxidation catalyst and the filter temperature may also be controlled by adjusting the amount of fuel added by the adding valve 20 and adjusting the injection quantity by the injection valve 12 if there is an after injection.

[0075] In the second example embodiment, an opportunity to execute the reduction process arises each time the running distance of the vehicle reaches the predetermined distance, but an opportunity to execute the reduction process may also be provided each time the cumulative operating time of the internal combustion engine 10 reaches a predetermined time, for example. Alternatively, the reduction process may be executed when the accumulation amount of sulfur oxide in the oxidation catalyst 21 increases to equal to or greater than a predetermined amount (> the allowable amount), or when the catalyst bed temperature of the oxidation catalyst becomes equal to or less than the upper limit temperature.

[0076] One execution condition of the reduction process is that the catalyst bed temperature of the oxidation catalyst be equal to or lower than the upper limit temperature at which desorbed sulfur dioxide in the oxidation catalyst is not reoxidized. In contrast to this, the reduction process may also be executed when the catalyst bed temperature of the oxidation catalyst is higher than the upper limit temperature, if the catalyst bed temperature of the oxidation catalyst is lower than a target temperature of the catalyst bed temperature of the oxidation catalyst in the burn-off process. With this structure as well, the same effects displayed by the first example embodiment, except for having the reoxidation amount fall within a small range, are able to be displayed.

[0077] In the oxidation catalyst 21, a carrier on which 3.5 grams of platinum particles (Pt) per 1 liter is carried is employed. This carried amount may be changed appropriately as long as it is equal to or greater than 3.0 grams. Also, in the filter 22, a carrier on which 0.5 grams of platinum particles (Pt) per 1 liter is carried is employed, but this carried amount, may be changed appropriately as long as it is equal to or greater than 1.0 grams. Also, for the filter 22, a carrier on which no platinum particles are carried may also be employed.

[0078] Moreover, the carried amount of platinum particles on the carrier of the filter 22 may be equal to or greater than the carried amount in the oxidation catalyst 21. With this structure as well, all of the effects, except for the effect of making the oxidative power of the filter 22 relatively low and making the oxidative power of the oxidation catalyst 21 relatively high described in the first example embodiment, are able to be displayed. The platinum particles carried on the carrier of the filter 22 may be particles of a platinum-based metal, such as palladium (Pd) or rhodium (Rh).

[0079] In the oxidation catalyst 21, a carrier of an alumina-based metal is employed, but a carrier made from another metal, such as titanium oxide, for example, may also be employed.

Claims

CLAIMS:

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

an oxidation catalyst provided in an exhaust passage of the internal combustion engine;

a filter that traps particulate matter, the filter being provided in the exhaust passage of the internal combustion engine, and being provided on a downstream side of the oxidation catalyst in an exhaust gas flow direction; and

a controller configured to:

(a) execute a burn-off process, the burn-off process being a process to burn off particulate matter trapped in the filter, by adding fuel upstream of the oxidation catalyst in the exhaust gas flow direction;

(b) execute a reduction process, the reduction process being a process to reduce and desorb sulfur oxide accumulated in the oxidation catalyst, the reduction process including an execution condition for the reduction process, the execution condition being that a catalyst bed temperature of the oxidation catalyst in the reduction process be lower than a target temperature of the catalyst bed temperature of the oxidation catalyst in the burn-off process; and

(c) prohibit execution of the burn-off process on a condition that an accumulation amount of sulfur oxide in the oxidation catalyst being larger than an allowable amount.

2. The exhaust gas control apparatus according to claim 1, wherein

when the execution condition for the burn-off process is satisfied, a timing at which the burn-off process is started is delayed until the accumulation amount of sulfur oxide in the oxidation catalyst becomes equal to or less than the allowable amount- through the execution of the reduction process.

3. The exhaust gas control apparatus according to claim 1, wherein

when the execution condition for the burn-off process is satisfied, a timing at which the burn-off process is started is delayed until a period of time for which the reduction process is executed reaches a predetermined period of time.

4. The exhaust gas control apparatus according to any one of claims 1 to 3, wherein the reduction process includes, as the execution condition for the reduction process, a condition that the catalyst bed temperature of the oxidation catalyst be lower than the target temperature, and equal to or lower than an upper limit temperature at which desorbed sulfur dioxide in the oxidation catalyst does not reoxidize.

5. The exhaust gas control apparatus according to any one of claims 1 to 4, wherein a carried amount of platinum-based metal per unit volume of a carrier of the filter is less than a carried amount of platinum-based metal per unit volume of a carrier of the oxidation catalyst.

6. The exhaust gas control apparatus according to claim 5, wherein

the carrier of the oxidation catalyst is an alumina-based metal, and the carried amount of platinum-based metal per 1 liter of the carrier of the oxidation catalyst is equal to or greater than 3.0 g; and

the carried amount of platinum-based metal per 1 liter of the carrier of the filter is equal to or less than 1.0 g.