JP2017025861A - Exhaust emission control device for internal combustion engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an exhaust emission control device for an internal combustion engine that can suppress a decline in exhaust emission control performance of a three-way catalyst by using a method other than reduction treatment.SOLUTION: An exhaust emission control device 2 includes: an upstream catalytic converter (three-way catalyst) 31 provided in an exhaust pipe 11 of an engine 1; an intermediate catalytic converter (NOx catalyst) 32 that is provided in the exhaust pipe 11 and has a carrier comprising zeolite and Pd supported on the carrier; an exhaust flow passage switching mechanism 34 for switching an exhaust passage between a first flow passage passing through the upstream catalytic converter 31 and bypassing the intermediate catalytic converter 32 and a second flow passage passing through both of the upstream catalytic converter 31 and the intermediate catalytic converter 32; and an ECU 6 for switching the exhaust flow passage switching mechanism 34 between the first flow passage side and the second flow passage side in accordance with a predetermined condition.SELECTED DRAWING: Figure 1

Description

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

  A three-way catalyst may be provided in the exhaust passage of the internal combustion engine in order to purify hydrocarbon (HC), carbon monoxide (CO), and nitrogen oxide (NOx) in the exhaust gas. The three-way catalyst reduces NOx using HC and CO in the exhaust as reducing agents. As the three-way catalyst, one having an oxygen storage function for storing oxygen in the exhaust gas is used. For this reason, if a fuel cut that temporarily stops fuel injection in the internal combustion engine during deceleration is performed, the oxygen storage amount of the three-way catalyst becomes the maximum, and the NOx purification performance of the three-way catalyst immediately after the fuel cut is restored is reduced. To do.

  In the invention of Patent Document 1, the NOx of the three-way catalyst immediately after the fuel cut return is obtained by temporarily enriching the air-fuel ratio immediately after the fuel cut return and reducing the oxygen stored in the three-way catalyst in a short time. Reduces purification performance.

Japanese Patent No. 5331931

  However, when the reduction process of the three-way catalyst is performed by enrichment as in the invention of Patent Document 1, if the shift amount in enrichment is excessive, the amount of HC and CO emissions increases or the fuel consumption deteriorates. There is a fear. If the shift amount is too small, NOx emission cannot be sufficiently suppressed.

  An object of the present invention is to provide an exhaust gas purification apparatus for an internal combustion engine that can compensate for a decrease in exhaust gas purification performance of a three-way catalyst by a method other than a reduction process.

  (1) An exhaust purification device (for example, an exhaust purification device 2 to be described later) of an internal combustion engine (for example, an engine 1 to be described later) of the present invention is provided in an exhaust passage (for example, an exhaust pipe 11 to be described later) of the internal combustion engine. A three-way catalyst (for example, an upstream catalytic converter 31 described later), a NOx catalyst (for example, an intermediate catalytic converter 32 described later) provided in the exhaust passage and having a support made of zeolite and Pd supported on the support; Switching to switch the exhaust flow path of the internal combustion engine between a first flow path that passes through the three-way catalyst and bypasses the NOx catalyst, and a second flow path that passes through both the three-way catalyst and the NOx catalyst A mechanism (for example, an exhaust flow path switching mechanism 34 described later) and the switching mechanism according to a predetermined condition are arranged on the first flow path (for example, first flow path F1 described later) side or the second flow path (for example, The second flow path described later Exhaust flow path control means to switch 2) side (for example, characterized in that it comprises, means, etc.) related to execution of the processing of ECU6 and 9 below.

  (2) In this case, the exhaust purification device has catalyst temperature acquisition means (for example, exhaust temperature sensor 36 and ECU 6 described later) for acquiring the temperature of the NOx catalyst, and adsorption for acquiring the NOx adsorption amount in the NOx catalyst. Amount acquisition means (e.g., ECU 6 described later and means for executing the processing of FIG. 11), and the exhaust flow path control means is either or both of the acquired NOx catalyst temperature and NOx adsorption amount. It is preferable to switch the switching mechanism based on the above.

  (3) In this case, the exhaust flow path control means moves the switching mechanism to the second flow path side for a predetermined period (for example, time t1 to t4 in FIG. 15 described later) immediately after the start of the internal combustion engine. It is preferable to set.

  (4) In this case, the exhaust flow path control means activates the switching mechanism during a fuel cut period in which fuel injection in the internal combustion engine is temporarily stopped (for example, at times t0 to t1 in FIG. 16 described later). It is preferable to set on the first flow path side.

  (5) In this case, after the fuel cut period has elapsed (for example, after time t1 in FIG. 16 described later), the exhaust flow path control means moves the switching mechanism from the first flow path side to the second flow path. It is preferable to switch to the channel side.

  (6) In this case, the exhaust purification device executes a NOx catalyst regeneration process for making the temperature of the NOx catalyst higher than a predetermined desorption temperature and making the air-fuel ratio of the exhaust flowing into the NOx catalyst stoichiometric or rich. A regeneration control means (for example, ECU 6 to be described later) for reducing and purifying NOx adsorbed on the NOx catalyst, and when the NOx adsorption amount in the NOx catalyst exceeds a predetermined upper limit threshold, the regeneration control means sends the NOx to the regeneration control means. Regeneration request generating means for generating a request for executing the catalyst regeneration process (for example, an ECU 6 described later and a means for executing the process of FIG. 8), and the exhaust flow path control means is configured to perform the NOx catalyst regeneration process. While the execution is requested, it is preferable to set the switching mechanism on the second flow path side.

  (7) In this case, the exhaust purification device executes a reduction process for shifting the air-fuel ratio of the exhaust gas flowing into the three-way catalyst to a richer side than a predetermined reference, and the oxygen stored in the three-way catalyst is A reduction control means (for example, ECU 6 described later) for reduction is provided, and the exhaust flow path control means is configured to perform the reduction when the temperature of the NOx catalyst is lower than a predetermined desorption temperature during the reduction process. It is preferable to set the switching mechanism on the second flow path side.

  (8) In this case, the exhaust purification device executes a NOx catalyst regeneration process for making the temperature of the NOx catalyst higher than a predetermined desorption temperature and making the air-fuel ratio of the exhaust flowing into the NOx catalyst stoichiometric or rich. A regeneration control means (for example, ECU 6 described later) for reducing and purifying NOx adsorbed on the NOx catalyst, and a reduction process for shifting the air-fuel ratio of the exhaust gas flowing into the three-way catalyst to a richer side than a predetermined reference. Reduction control means (for example, ECU 6 to be described later) that executes and reduces oxygen stored in the three-way catalyst, and the exhaust flow path control means performs the NOx during the reduction process. It is preferable that the switching mechanism is set on the second flow path side when the temperature of the catalyst is lower than a predetermined desorption temperature or when the NOx catalyst regeneration process is performed.

  (9) In this case, when the switching mechanism is set on the second flow path side, the reduction control means is more effective at the time of execution of the reduction process than when set on the first flow path side. It is preferable to reduce the shift amount of the exhaust air-fuel ratio to the rich side.

  (1) In the present invention, two types of catalysts, a three-way catalyst, a support made of zeolite, and a NOx catalyst having Pd supported on the support, are provided in an exhaust passage through which the exhaust gas of the internal combustion engine can flow. This NOx catalyst has a characteristic of adsorbing NOx at a low temperature and desorbing the adsorbed NOx at a high temperature. Further, in the present invention, by using the switching mechanism, two exhaust flows of the first flow path that passes through the three-way catalyst and bypasses the NOx catalyst, and the second flow path that passes through both the three-way catalyst and the NOx catalyst. A path is set and the first flow path and the second flow path are switched according to a predetermined condition. Thus, for example, when a three-way catalyst can exhibit a sufficient NOx purification performance, the switching mechanism is set to the first flow path side, so that the NOx catalyst is fully utilized while making full use of the NOx purification performance of the three-way catalyst. Deterioration can be suppressed. Further, for example, when the NOx purification performance of the three-way catalyst is insufficient immediately after the fuel cut recovery as described above, the switching mechanism is set to the second flow path side to purify with the three-way catalyst. Since the NOx that could not be made can be adsorbed to the NOx catalyst, it is possible to suppress a decrease in the NOx purification performance of the entire exhaust purification device.

  Note that when two exhaust passages are set as described above, the NOx catalyst is intermittently exposed to the exhaust gas, and therefore the temperature tends to be lower than that of the three-way catalyst. For this reason, immediately after switching the exhaust flow path from the first flow path to the second flow path, the NOx purification performance of the entire exhaust purification device temporarily decreases due to the low temperature of the NOx catalyst. Is concerned. However, the NOx catalyst as described above exhibits a function of adsorbing NOx even in a low-temperature environment before the three-way catalyst is activated, such as immediately after the engine is started. Therefore, immediately after switching the exhaust flow path from the first flow path to the second flow path, NOx that could not be purified by the three-way catalyst can be adsorbed by the NOx catalyst, so this NOx is discharged out of the exhaust purification device. Can be prevented.

  (2) The NOx purification performance of the NOx catalyst varies depending on the temperature and the NOx adsorption amount. In the present invention, the temperature of the NOx catalyst and the NOx adsorption amount are acquired, and the switching mechanism is switched based on either or both of them to switch the exhaust flow path at an appropriate timing so that the performance of the NOx catalyst is utilized. be able to.

  (3) As described above, the NOx catalyst exhibits a function of adsorbing NOx at a low temperature before the three-way catalyst reaches its activity. In the present invention, in a predetermined period immediately after the start of the internal combustion engine, the switching mechanism is set on the second flow path side, and exhaust is passed through both the three-way catalyst and the NOx catalyst, so that the three before the activation is reached. NOx discharged from the original catalyst can be adsorbed by the NOx catalyst.

  (4) In the present invention, the switching mechanism is set to the first flow path side during the fuel cut period so that the high-temperature air flowing through the exhaust flow path does not flow into the NOx catalyst. As a result, the temperature of the NOx catalyst can be maintained in a low temperature range where NOx can be adsorbed, so that NOx can be prevented from being desorbed from the NOx catalyst during the fuel cut period. In addition, since the heat load on the NOx catalyst can be reduced, deterioration thereof can also be suppressed.

  (5) In the present invention, after the fuel cut period has elapsed, the switching mechanism is switched from the first flow path side to the second flow path side, and the exhaust gas is allowed to flow through both the three-way catalyst and the NOx catalyst. Since the air flows through the three-way catalyst during the fuel cut period as described above, the oxygen storage amount becomes maximum immediately after the fuel cut period elapses, and the NOx purification performance is degraded. In the present invention, after the fuel cut period has elapsed, exhaust gas is allowed to flow through both the three-way catalyst and the NOx catalyst, so that NOx that could not be reduced by the three-way catalyst can be adsorbed by the NOx catalyst. Since the high temperature air is not allowed to flow through the NOx catalyst during the fuel cut period as described above, the temperature of the NOx catalyst is maintained at a low temperature. Therefore, the NOx adsorption is performed immediately after the end of the fuel cut period. The function can be demonstrated.

  (6) The NOx adsorption function of the NOx catalyst decreases as the NOx adsorption amount increases. Therefore, in the present invention, when the NOx adsorption amount in the NOx catalyst exceeds a predetermined upper limit threshold, execution of the NOx catalyst regeneration process is requested. In the present invention, while the execution of the NOx catalyst regeneration process is requested, the switching mechanism is set on the second flow path side. The NOx catalyst regeneration process is to remove NOx adsorbed on the NOx catalyst and reduce and purify it by making the temperature of the NOx catalyst higher than the desorption temperature and making the air-fuel ratio stoichiometric or rich. It is processing. Thereby, the NOx purification performance of the NOx catalyst can be maintained high.

  (7) The NOx purification performance of the three-way catalyst decreases as the oxygen storage amount increases. For this reason, oxygen stored in the three-way catalyst is reduced by appropriately performing a reduction process for shifting the air-fuel ratio to a richer side than a predetermined reference. During the reduction process as described above, NOx that could not be purified by the three-way catalyst is discharged, so the switching mechanism is set to the second flow path side, and this unpurified NOx is converted by the NOx catalyst. Adsorption is preferred. However, if exhaust gas is passed through the NOx catalyst in a state where the temperature of the NOx catalyst is higher than the predetermined desorption temperature, a large amount of NOx is desorbed from the NOx catalyst, and NOx purification in the entire exhaust purification device Performance may be degraded. Therefore, in the present invention, during the reduction process, when the temperature of the NOx catalyst is lower than the desorption temperature, the three-way catalyst is reduced by setting the switching mechanism to the second flow path side. It is possible to suppress a decrease in the NOx purification performance of the entire exhaust gas purification apparatus.

  (8) In the present invention, during the reduction process of the three-way catalyst, switching is performed when the temperature of the NOx catalyst is lower than the desorption temperature or when the NOx catalyst regeneration process is performed in parallel with the reduction process. The mechanism is set on the second flow path side. Thereby, it is possible to suppress a decrease in the NOx purification performance of the entire exhaust gas purification apparatus during the reduction process of the three-way catalyst. Further, the reduction process of the three-way catalyst and the NOx catalyst regeneration process of the NOx catalyst can be performed simultaneously.

  (9) In the present invention, when the switching mechanism is set on the second flow path side, the exhaust air-fuel ratio to the rich side at the time of execution of the reduction process is more than when the switching mechanism is set on the first flow path side. Reduce the shift amount. When the switching mechanism is set on the second flow path side, the exhaust flows through both the three-way catalyst and the NOx catalyst, so that the decrease in the NOx purification performance of the three-way catalyst is compensated by the NOx catalyst. For this reason, it is not necessary to increase the amount of shift to the rich side in order to quickly recover the NOx purification performance of the three-way catalyst. In the present invention, when the switching mechanism is set on the second flow path side, the shift amount to the rich side is made smaller than in the case where the switching mechanism is set on the first flow path side. While suppressing a decrease in NOx purification performance, it is possible to prevent HC and CO emissions and suppress unnecessary fuel consumption.

1 is a diagram illustrating a configuration of an engine and an exhaust purification device thereof according to an embodiment of the present invention. It is a figure which shows the adsorption | suction and desorption behavior of NOx in a NOx catalyst. It is a figure which compares the adsorption amount and desorption amount of NOx in a NOx catalyst. It is a figure which shows the relationship between the adsorption amount of NOx in a NOx catalyst, and thermal degradation. It is a figure which shows the relationship between the adsorption amount of NOx in a NOx catalyst, and temperature. It is a figure which shows the relationship between the adsorption amount of NOx in a NOx catalyst, and oxygen concentration. It is a figure which shows the relationship between NOx density | concentration in the downstream of a NOx catalyst, and the temperature of a NOx catalyst. It is a flowchart which shows the procedure which determines the timing which generate | occur | produces the regeneration request | requirement of a NOx catalyst. It is a flowchart which shows the procedure which sets an exhaust flow path. It is an example of the map which determines the value of the correction coefficient at the time of weak rich using the estimated value of the maximum storage amount. It is a flowchart which shows the procedure which calculates the estimated value of the NOx adsorption amount in a NOx catalyst. It is an example of the map which calculates NOx adsorption rate. It is an example of the map which calculates the correction coefficient of NOx adsorption rate. It is an example of the map which calculates desorption amount. It is a time chart which shows the operation example immediately after starting of the engine of the exhaust gas purification apparatus which concerns on the said embodiment. It is a time chart which shows the operation example immediately after the fuel cut of the exhaust gas purification apparatus which concerns on the said embodiment.

Hereinafter, an embodiment of the present invention will be described with reference to the drawings.
FIG. 1 is a diagram showing a configuration of an internal combustion engine (hereinafter referred to as “engine”) 1 and an exhaust purification device 2 for purifying exhaust thereof according to the present embodiment.

  The engine 1 is based on so-called lean combustion in which the combustion air-fuel ratio is leaner than stoichiometric, more specifically, a diesel engine, a lean burn gasoline engine, or the like. The engine 1 is provided with a fuel injection valve 17 that injects fuel into each cylinder. The actuator that drives the fuel injection valve 17 is electromagnetically connected to the ECU 6. The ECU 6 determines the fuel injection amount and fuel injection timing from the fuel injection valve 17 under fuel injection control described later, and drives the fuel injection valve 17 so as to realize this.

  The exhaust purification device 3 includes an upstream catalytic converter 31, an intermediate catalytic converter 32, a downstream catalytic converter 33, an exhaust flow path switching mechanism 34, an air-fuel ratio sensor 35, and an exhaust temperature sensor 36, which are provided in the exhaust pipe 11, respectively. The

  The upstream catalytic converter 31 and the downstream catalytic converter 33 are each configured by supporting a three-way catalyst on a base material of a flow-through type honeycomb structure. In a three-way catalyst, under a stoichiometric exhaust ratio, a three-way purification reaction, that is, an oxidation reaction of HC and CO and a reduction reaction of NOx proceed simultaneously. In the three-way catalyst, the oxidation reaction of HC and CO proceeds under a lean air-fuel ratio exhaust.

  The intermediate catalytic converter 32 is provided between the upstream catalytic converter 31 and the downstream catalytic converter 33 in the exhaust pipe 11. The intermediate catalytic converter 32 has a flow-through honeycomb structure as a base material, and a NOx catalyst is supported on the base material. The NOx catalyst includes a support made of zeolite and Pd supported on the support. This NOx catalyst can be completely purified by the three-way catalyst, for example, under relatively low temperature conditions immediately after the engine 1 is started (more specifically, for example, before the three-way catalyst of the upstream catalytic converter 31 reaches the activation temperature). It has the function of adsorbing and reducing and purifying NOx that was not present.

  The zeolite of the NOx catalyst, under stoichiometric or rich air-fuel ratio exhaust, takes and adsorbs HC contained in the exhaust into the pores in the skeleton under low temperature conditions, and adsorbs the adsorbed HC under high temperature conditions. Has the property of desorption. The HC desorption temperature at which HC desorption starts is substantially equal to the NOx desorption temperature at which NOx begins to desorb from Pd described later.

  Zeolite includes ZSM-5, ferrierite, mordenite, Y-type zeolite, beta-type zeolite, and CHA-type zeolite. In the present embodiment, any of these may be used alone, or a plurality of them may be used in combination. By supporting Pd on such a zeolite, excellent NOx adsorption performance is exhibited.

Here, usually, zeolite has a characteristic of adsorbing NOx supplied as NO ₂ in its pores. Therefore, in order to convert NO, which mainly constitutes NOx in the exhaust gas, to NO ₂ , the exhaust gas is made lean, has a high oxygen concentration and a high temperature atmosphere, and further requires active species such as Pt. On the other hand, the NOx catalyst of this embodiment exhibits excellent NOx adsorption performance even when the air-fuel ratio of the exhaust is stoichiometric or rich under low temperature conditions by supporting Pd on the support zeolite. The reason is as follows.

That is, in the NOx catalyst, Pd is arranged in the vicinity of Al which is an acid point among Al, Si and O constituting the zeolite. Therefore, Pd is present as divalent Pd ²⁺ because its electronic state is changed by the interaction with Al. Unlike the conventional NOx adsorption of zeolite, this divalent Pd ²⁺ has a characteristic of adsorbing NO as it is without oxidizing NO to NO ₂ . As a result, the NOx catalyst can obtain excellent NOx adsorption performance even when the air-fuel ratio of the exhaust is stoichiometric or rich under low temperature conditions.

  The content of Pd with respect to the entire NOx catalyst is preferably 0.01 to 10% by mass. If the content of Pd is within this range, excellent NOx adsorption performance can be obtained. A more preferable content is 0.1 to 3% by mass.

Further, the NOx catalyst is not limited to a catalyst in which Pd is supported on a support made of zeolite as described above. In addition to the above Pd, at least one additive element selected from the group consisting of Fe, Ce, Pr, Sr, Ba, La, Ga, In and Mn may be co-supported on the zeolite. That is, at least one additive element selected from the group consisting of Ce, Pr, Sr, Ba, La, Ga, In, and Mn is interposed between Pd, so that divalent Pd ²⁺ is zero-valent. Reduction to Pd ⁰ is suppressed, and movement and aggregation of Pd are suppressed, so that deterioration of dispersibility of Pd is suppressed. Therefore, according to such a NOx catalyst, excellent NOx adsorption performance is maintained, and heat resistance in a low oxygen concentration atmosphere is improved.

  FIG. 2 is a diagram showing NOx adsorption and desorption behavior in the NOx catalyst. This FIG. 2 shows that the model gas having the composition shown below is supplied to the NOx catalyst, and the NOx concentration of the gas flowing into the NOx catalyst and the NOx catalyst are maintained in an oxygen-excess atmosphere (oxygen excess ratio λ = 2). The NOx concentration of the gas flowing out from the NOx catalyst when the temperature of the NOx catalyst is changed is shown. In FIG. 2, the horizontal axis represents time (seconds), the right vertical axis represents the temperature of the NOx catalyst [° C.], and the left vertical axis represents the NOx concentration [ppm].

In the model gas, CO is constant at 1000 ppm, O ₂ is constant at 0.1%, NO is changed in a predetermined manner, and N ₂ is used as a balance gas, so that the total oxygen excess ratio λ = 2. did. Here, the NOx concentration of the model gas flowing into the NOx catalyst is set to a predetermined value larger than 0 until about 200 seconds have elapsed from the start of supply of the model gas, as indicated by a broken line in FIG. 0. Further, the temperature of the NOx catalyst is constant at about 50 ° C. until about 1200 seconds from the start of supply of the model gas, as shown by a one-dot chain line in FIG. 2, and about 500 ° C. after about 1200 seconds. Gradually increased until it reached. In addition, since there is almost no change in the NOx concentration, the temperature of the NOx catalyst, etc. between about 400 and 1000 seconds, the illustration between these is omitted in FIG.

  As shown in FIG. 2, the NOx concentration of the gas flowing out from the NOx catalyst is about 200 seconds after the NOx catalyst is in a low temperature state of 50 ° C. and the model gas containing NOx starts to be supplied to the NOx catalyst. (Solid line) is lower than the NOx concentration (broken line) of the gas flowing into the NOx catalyst. In particular, immediately after the supply of the model gas is started (about 0 to 100 seconds), the NOx concentration in the exhaust gas flowing out from the NOx catalyst is approximately 0 ppm. This means that almost all NOx (NO) contained in the gas flowing into the NOx catalyst is adsorbed by the NOx catalyst. This result shows that the NOx catalyst of the intermediate catalytic converter can efficiently adsorb NOx (NO) under the low temperature condition of 50 ° C. before the three-way catalyst of the upstream catalytic converter reaches the activity.

  The NOx concentration in the gas flowing out from the NOx catalyst gradually increases until approximately 200 seconds elapse after the supply of the model gas starts, and becomes substantially equal to the NOx concentration of the gas flowing into the NOx catalyst. (See around 200 seconds in FIG. 2). This means that there is a limit to the amount of NOx that can be adsorbed by the NOx catalyst, and as the amount of NOx adsorbed to the NOx catalyst approaches this limit amount, NOx becomes difficult to adsorb (decrease in the NOx adsorption rate). To do. That is, when about 200 seconds have elapsed, the NOx catalyst has adsorbed an amount of NOx that is almost close to the limit amount. Further, the area of the region Tad in FIG. 2 represents the total amount of NOx adsorbed by the NOx catalyst (that is, the NOx adsorption amount).

  When the NOx concentration of the model gas is reduced to 0 ppm when about 200 seconds have elapsed, the NOx concentration of the gas flowing out from the NOx catalyst is immediately reduced to 0 ppm accordingly. Thereafter, as shown in FIG. 2, the NOx concentration of the gas flowing out from the NOx catalyst is approximately 0 ppm. That is, the NOx catalyst has a function of continuously holding NOx adsorbed on the NOx catalyst in 0 to 200 seconds in an oxygen-excess atmosphere.

Further, after the NOx concentration is reduced to 0 ppm, the temperature of the NOx catalyst is increased at a substantially constant rate between about 1200 and 2500 seconds. At this time, the NOx concentration of the gas flowing out from the NOx catalyst starts to increase from 0 ppm in about 1800 seconds as shown in FIG. 2, and returns to 0 ppm again in about 2300 seconds. Note that the temperature of the NOx catalyst is about 250 ° C. for about 1800 seconds and about 450 ° C. for about 2300 seconds. This means that the NOx adsorbed on the NOx catalyst was desorbed between the time when the temperature of the NOx catalyst exceeded about 250 ° C. and the time when it exceeded 450 ° C. Hereinafter, the temperature at which the desorption of NOx adsorbed on the NOx catalyst starts (about 250 ° C. in the example of FIG. 2) is referred to as the NOx desorption temperature. At this time, almost all NOx desorbed from the NOx catalyst was NO, and almost no NO ₂ or N ₂ O was observed. The area of the region Tdes in FIG. 2 represents the total amount of NOx desorbed from the NOx catalyst (that is, the NOx desorption amount).

  FIG. 3 is a diagram comparing the NOx adsorption amount and desorption amount in the NOx catalyst. In FIG. 3, the NOx adsorption amount and the NOx desorption amount were obtained by performing tests for changing the NOx concentration and temperature in accordance with the same procedure as in FIG. 2 using a model gas having a predetermined excess air ratio. The left side of FIG. 3 is a result obtained using a gas having an oxygen excess ratio λ = 2, and the right side of FIG. 3 is a result obtained using a gas having an oxygen excess ratio λ = 0.9.

  As shown on the left side of FIG. 3, under the model gas in an oxygen-excess atmosphere (λ = 2), the NOx desorption amount is almost equal to the NOx adsorption amount. That is, in an oxygen-excess atmosphere, almost all NOx adsorbed on the NOx catalyst at a low temperature is desorbed as it is when the temperature is raised to about 500 ° C.

On the other hand, as shown on the right side of FIG. 3, under the model gas of λ = 0.9, the NOx adsorption amount is almost the same as that of λ = 2, but the NOx desorption amount is the NOx adsorption amount. Significantly less than the amount. That is, in the reducing atmosphere, almost all of the NOx adsorbed on the NOx catalyst at a low temperature is desorbed and reduced to N _{2 while} being heated to about 500 ° C. In other words, the NOx catalyst desorbs HC and CO contained in the exhaust and HC adsorbed on the zeolite as described above as a reducing agent under an appropriate air-fuel ratio exhaust with λ of 1 or less. It means that there is a function to reduce and purify the separated NOx.

  FIG. 4 is a diagram showing the relationship between the NOx adsorption amount [g / L] in the NOx catalyst and the thermal deterioration of the NOx catalyst. In FIG. 4, the left side shows the amount of NOx that can be adsorbed by a new NOx catalyst, and the right side shows the amount of NOx that can be adsorbed by the NOx catalyst after applying a predetermined amount of heat load. As shown in FIG. 4, the amount of NOx that can be adsorbed by the NOx catalyst decreases as the thermal load increases.

  FIG. 5 is a graph showing the relationship between the NOx adsorption amount [g / L] in the NOx catalyst and the temperature of the NOx catalyst. As shown in FIG. 5, the NOx catalyst exhibits a function of adsorbing NOx in the exhaust gas at a desorption temperature of about 250 ° C. or lower, more specifically between about 50 and 200 ° C. Further, the amount of NOx that can be adsorbed by the NOx catalyst decreases as the temperature increases.

  FIG. 6 is a diagram showing the relationship between the NOx adsorption amount [g / L] in the NOx catalyst and the oxygen concentration of the exhaust gas flowing into the NOx catalyst. More specifically, FIG. 6 shows the NOx adsorption amount when NOx is adsorbed under an exhaust gas having an oxygen concentration of 2% (left side) or 0% (right side) while keeping the NOx catalyst at 50 ° C. As described with reference to FIG. 5, the NOx catalyst has a characteristic that the amount of NOx that can be adsorbed decreases as the temperature increases. However, as shown in FIG. 6, the NOx adsorption function of the NOx catalyst does not depend on the oxygen concentration of the exhaust gas flowing into the NOx catalyst.

  FIG. 7 is a graph showing the relationship between the NOx concentration on the downstream side of the NOx catalyst and the temperature of the NOx catalyst. More specifically, it is a diagram showing a change in the NOx concentration on the downstream side of the NOx catalyst when the temperature of the NOx catalyst on which NOx is adsorbed is increased. FIG. 7 shows a case where the oxygen concentration of the exhaust gas flowing into the NOx catalyst is changed in three stages (0.1%, 0.05%, 0.045%). As shown in this figure, when the temperature of the NOx catalyst is raised to a temperature higher than its desorption temperature, the NOx that has been adsorbed until then begins to desorb. At this time, if the air-fuel ratio is lean, NOx desorbed from the NOx catalyst is discharged directly downstream without being reduced. For this reason, when the temperature of the NOx catalyst is raised, the NOx concentration on the downstream side of the NOx catalyst increases. In contrast, when the air-fuel ratio is stoichiometric or rich, NOx desorbed from the NOx catalyst is reduced and purified on the NOx catalyst. For this reason, the NOx concentration on the downstream side of the NOx catalyst hardly changes.

  Returning to FIG. 1, the exhaust pipe 11 includes a main exhaust pipe 111 provided with the upstream catalytic converter 31 and the downstream catalytic converter 33, and a bypass pipe 112 branched from the main exhaust pipe 111 between the upstream catalytic converter 31 and the downstream catalytic converter 33. And comprising. The intermediate catalytic converter 33 is provided in the bypass pipe 112.

  The exhaust flow path switching mechanism 34 includes a flow path switching valve 341 that can be opened and closed in the exhaust pipe 11 and an actuator 342 that opens and closes the flow path switching valve 341 in response to a command from the ECU 6. The flow path switching valve 341 constitutes a part of the flow path of the exhaust discharged from the engine 1, and forms different exhaust flow paths F1 and F2 in a closed state and an open state, respectively.

  When the flow path switching valve 341 is closed, the bypass pipe 112 is shut off, so that it passes through the upstream catalytic converter 31 and the downstream catalytic converter 33 provided in the main exhaust pipe 111 and is intermediate in the bypass pipe 112. An exhaust passage (hereinafter referred to as a “first passage”) F1 that bypasses the catalytic converter 32 is formed. When the flow path switching valve 341 is opened, the main exhaust pipe 111 and the bypass pipe 112 are connected, so that the exhaust flow path (through the upstream catalytic converter 31, the intermediate catalytic converter 32, and the downstream catalytic converter 33 ( Hereinafter, this is referred to as a “second flow path”.

  The ECU 6 opens and closes the flow path switching valve 341 with reference to the value of a flow path flag F_PZ, which will be described later, updated by the process of FIG. 9, which will be described later. More specifically, when the flag F_PZ is “0”, the ECU 6 closes the flow path switching valve 341, sets the first flow path F1 as the exhaust flow path, and the flag F_PZ is “1”. In this case, the flow path switching valve 341 is opened, and the second flow path F2 is used as the exhaust flow path.

  The air-fuel ratio sensor 35 is provided on the upstream side of the upstream catalytic converter 31 in the main exhaust pipe 11. The air-fuel ratio sensor 35 detects the air-fuel ratio of the exhaust gas flowing into the upstream catalytic converter 35 (ratio of fuel components (HC, CO, etc.) to oxygen in the exhaust gas), and transmits a signal substantially proportional to the detected value to the ECU 6. . As the air-fuel ratio sensor 35, for example, a sensor having a linear output characteristic from a rich region to a lean region is used.

  The exhaust temperature sensor 36 is provided on the downstream side of the intermediate catalytic converter 32 in the bypass pipe 112. The exhaust temperature sensor 36 detects the temperature of the exhaust gas flowing out from the intermediate catalytic converter 32 and transmits a signal substantially proportional to the detected value to the ECU 6. The temperature of the NOx catalyst of the intermediate catalytic converter 32 is estimated by calculation in the ECU 6 based on the output of the exhaust temperature sensor 36, for example.

  The ECU 6 includes an I / O interface that performs A / D conversion on the detection signal of the sensor, a CPU that executes fuel injection control described below, and processes according to flowcharts shown in FIGS. The microcomputer includes a drive circuit that drives various devices in the mode determined in (1), and a RAM, a ROM, and the like that store various data.

  In the fuel injection control in the ECU 6, four types of operation modes are defined: a lean operation mode, a stoichiometric operation mode, a weak rich operation mode, and a regeneration operation mode. Under the selected operation mode, A fuel injection amount from the fuel injection valve 17 is determined.

The lean operation mode is an operation mode in which the air-fuel ratio is made leaner than stoichiometry, and is the most basic operation mode. When the lean operation mode is selected, the ECU 6 determines the basic injection amount Gfuel_bs calculated by searching a map (not shown) as the fuel injection amount Gfuel (see the following formula (1)).
Gfuel = Gfuel_bs (1)

The stoichiometric operation mode is an operation mode in which the air-fuel ratio is stoichiometric by feedback control using the output of the air-fuel ratio sensor 35, and is selected when a predetermined condition is satisfied. When the stoichiometric operation mode is selected, the ECU 6 determines the fuel injection amount Gfuel by multiplying the basic injection amount Gfuel_bs by a predetermined feedback correction coefficient KAF as shown in the following equation (2). In the following equation (2), the correction coefficient KAF is calculated according to a known feedback control law so that the output of the air-fuel ratio sensor 35 becomes stoichiometric.
Gfuel = Gfuel_bs × KAF (2)

  The weak rich operation mode is an operation mode in which the air-fuel ratio is slightly richer than stoichiometric (so-called weak rich), and is selected when a reduction request flag F_CRD described later is “1”. The three-way catalyst provided in the upstream catalytic converter 31 has a function of storing oxygen in the exhaust. Therefore, the oxygen storage amount of the three-way catalyst is maximum immediately after the fuel cut is temporarily stopped due to deceleration, or immediately after the transition from the lean operation mode to the stoichiometric operation mode. The purification performance is temporarily reduced.

In the weak rich operation mode, the oxygen stored in the three-way catalyst is reduced by performing a reduction process that weakens the air-fuel ratio of the exhaust gas flowing into the three-way catalyst. When the weak rich operation mode is selected, the ECU 6 performs known feedback control so that the output of the air-fuel ratio sensor 35 becomes weakly rich with respect to the basic injection amount Gfuel_bs as shown in the following equation (3). A fuel injection amount Gfuel is determined by multiplying the feedback correction coefficient KAF calculated according to the law and a predetermined weak rich correction coefficient KCRD. This correction coefficient KCRD is a positive real number of 1 or less introduced to finely adjust the rich shift amount, and is calculated according to the procedure shown in FIG.
Gfuel = Gfuel_bs × KAF × KCRD (3)

  The regeneration operation mode is an operation mode in which the temperature rise control is executed while making the air-fuel ratio stoichiometric or richer than the stoichiometry, and is selected when a regeneration request flag F_rgn described later is “1”. The NOx catalyst provided in the intermediate catalytic converter 32 has a characteristic of adsorbing NOx at a low temperature and desorbing the adsorbed NOx at a high temperature as described with reference to FIGS. However, if the NOx adsorption amount in the NOx catalyst increases, the adsorption performance also decreases. Therefore, if the NOx adsorption amount increases to some extent, it is preferable to raise the temperature of the NOx catalyst and forcibly desorb NOx. .

In the regeneration operation mode, NOx adsorbed on NOx is reduced and purified by performing a regeneration process in which the temperature of the NOx catalyst is made higher than the desorption temperature and the air-fuel ratio of the exhaust gas flowing into the NOx catalyst is stoichiometric or rich. . When this regeneration operation mode is selected, the ECU 6 is known so that the output of the air-fuel ratio sensor 35 becomes richer than the stoichiometric value or the basic injection amount Gfuel_bs as shown in the following equation (4). A fuel injection amount Gfuel is determined by multiplying the feedback correction coefficient KAF calculated according to the feedback control law. Further, the temperature raising control of the exhaust gas is realized, for example, by retarding the injection timing of post injection, after injection, main injection, and the like with respect to the combustion parameters of the engine in a normal operation state.
Gfuel = Gfuel_bs × KAF (4)

  As described above, when the reduction request flag F_CRD becomes “1”, the weak rich operation mode is selected, and when the regeneration request flag F_rgn becomes “1”, the regeneration operation mode is selected, but both flags are simultaneously set to “1”. Sometimes it becomes. That is, there is a case where the reduction request for the three-way catalyst and the regeneration request for the NOx catalyst occur at the same time. In such a case, the reduction process for the three-way catalyst and the regeneration process for the NOx catalyst are performed simultaneously. The fuel injection amount is preferably determined according to the above equation (3) and simultaneously the temperature raising control is executed.

  FIG. 8 is a flowchart showing a procedure for determining the timing for generating a regeneration request for the NOx catalyst. The process of FIG. 8 is repeatedly executed in the ECU under a predetermined control cycle in response to an ignition switch (not shown) for starting the engine being turned on.

  As described below, in the process of FIG. 8, the reproduction request flag F_rgn is set to either “1” or “0” according to a predetermined condition. This flag F_rgn is a flag that clearly indicates that the execution of the regeneration process of the NOx catalyst is requested, and is reset to “0” when the engine is cranked. That is, when the flag F_rgn is set to “1” in the process of FIG. 8, the NOx catalyst regeneration process is executed as described above.

  In S1, the ECU determines whether or not the start flag F_STMOD is “1”. This flag F_STMOD is a flag that clearly indicates that the engine has just started, is set to “1” when the engine is cranked, and is reset to “0” in S3 described later. The flag F_STMOD is maintained at “0” after being reset at S3 to be described later until it is set to “1” again during engine cranking.

  If the determination in S1 is YES, that is, immediately after starting the engine, the ECU sets the regeneration request flag F_rgn to “1” (see S2), and resets the start flag F_STMOD to “0”. This process ends (see S3). That is, in the process of FIG. 8, immediately after the start of the engine, the flag F_rgn is forcibly set to “1” to start the regeneration process of the NOx catalyst. When the determination in S1 is NO, the ECU determines whether or not the regeneration request flag F_rgn is “1” (see S4).

  If the determination in S4 is YES, that is, if a regeneration request for the NOx catalyst is generated, the ECU obtains an estimated value ΣNOx of the current NOx adsorption amount of the NOx catalyst, and regenerates using this estimated value ΣNOx. It is determined whether or not it is time to end the process. More specifically, it is determined whether or not the estimated value ΣNOx is smaller than a predetermined lower limit threshold Lim_L (see S5). Note that the estimated value ΣNOx of the current NOx adsorption amount of the NOx catalyst is updated every predetermined control cycle in the process of FIG. 11 described later. When the regeneration process of the NOx catalyst is executed, the temperature of the NOx catalyst rises, and the NOx that has been adsorbed so far is desorbed and reduced and purified. Therefore, while the flag F_rgn is set to “1”, the NOx adsorption amount of the NOx catalyst gradually decreases. If the determination in S5 is NO, the ECU determines that the NOx adsorbed on the NOx catalyst has not been sufficiently reduced and purified, and ends this process while maintaining the flag F_rgn at "1". . If the determination in S5 is YES, the ECU determines that NOx has been sufficiently reduced and purified, resets the flag F_rgn to “0” to end the regeneration process (see S6), and ends this process. .

  When the determination in S4 is NO, that is, when the regeneration request for the NOx catalyst has not occurred, the ECU acquires the estimated value ΣNOx of the NOx adsorption amount and starts the regeneration process using this estimated value ΣNOx. It is determined whether or not it has been reached. More specifically, it is determined whether or not the estimated value ΣNOx is smaller than the upper limit threshold Lim_H set to a value larger than the lower limit threshold Lim_L (see S7). If the determination in S7 is YES, the ECU determines that it is not necessary to start the regeneration process yet, and ends this process while maintaining the flag F_rgn at “0”. If the determination in S7 is NO, the ECU determines that the NOx adsorption function of the NOx catalyst needs to be restored, sets the flag F_rgn to “1” to start the regeneration process (see S8), This process ends. Further, here, in response to the flag F_rgn being set to “1”, the exhaust passage is set to the second passage (see S15 or S18 in FIG. 9 described later).

  FIG. 9 is a flowchart showing a procedure for setting the exhaust passage. Similar to the process of FIG. 8, the process of FIG. 9 is repeatedly executed in the ECU under a predetermined control cycle in response to an ignition switch (not shown) for starting the engine being turned on.

  As described below, in the process of FIG. 9, the flow path flag F_PZ is set to either “1” or “0” according to a predetermined condition. This flag F_PZ is a flag that clearly indicates that the NOx catalyst is required to flow exhaust gas. That is, when the second flow path via the NOx catalyst is selected as the exhaust flow path, the flag F_PZ is set to “1”. When the first flow path that bypasses the NOx catalyst is selected as the exhaust flow path, the flag F_PZ is set to “0”.

  In S11, the ECU determines whether or not a fuel cut flag F_FC is “1”. This fuel cut flag F_FC is a flag indicating that fuel cut is being executed to temporarily stop fuel injection during deceleration, and is updated by a process (not shown) according to the running state of the vehicle. If the determination in S11 is YES, that is, if the fuel cut period is currently in progress, the flag F_PZ is set to “0” (see S12), and this process ends. Therefore, during the fuel cut period, the exhaust flow path is set to the first flow path that bypasses the NOx catalyst regardless of the state of the NOx catalyst. As a result, the thermal load on the NOx catalyst during the fuel cut period can be reduced, and the temperature rise of the NOx catalyst can be suppressed in preparation for returning from the fuel cut.

  When the determination in S11 is NO, the ECU determines whether or not a reduction request flag F_CRD is “1” (see S13). The reduction request flag F_CRD is a flag that clearly indicates that the upstream three-way catalyst reduction process is requested, and is reset to “0” when the engine is cranked. The flag F_CRD is changed from “0” to “1” in response to the end of the fuel cut (that is, in response to the flag F_FC being reset from “1” to “0” by a process not shown). Set to Further, after the flag F_CRD is set to “1”, the flag F_CRD is reset to “0” when a predetermined time has elapsed.

  When the determination in S13 is NO, the ECU determines whether or not the regeneration request flag F_rgn is “1” (see S14). If the determination in S14 is YES, that is, if the regeneration process of the NOx catalyst is requested, the ECU sets the flag F_PZ to “1” (see S15) and ends this process. Thereby, while the regeneration process is being performed, the exhaust passage is set to the second passage through the NOx catalyst. If the determination in S14 is NO, the ECU sets the flag F_PZ to “0” (see S16) and ends this process.

  When the determination in S13 is YES, the ECU determines whether or not the regeneration request flag F_rgn is “1” (see S17). When the determination in S17 is YES, that is, when both the reduction process of the three-way catalyst and the regeneration process of the NOx catalyst are requested, the ECU sets the flag F_PZ to “1” (see S18). The process proceeds to S30 described later. When the determination in S17 is NO, that is, when only the execution of the reduction process is requested, the ECU acquires the temperature of the NOx catalyst and determines whether or not the catalyst temperature is lower than the NOx desorption temperature. (See S19). If the determination in S19 is YES, the ECU sets the flag F_PZ to “0” (see S20), and proceeds to S30 described later. If the determination in S19 is NO, the ECU sets the flag F_PZ to “1”. The setting is performed (see S21), and the process proceeds to S31 described later. Therefore, when the above-described processes of S17 to S21 are summarized, the NOx catalyst temperature is lower than the desorption temperature and the NOx catalyst can adsorb NOx while the reduction process of the three-way catalyst is performed, or the NOx catalyst Only when the regeneration process is performed, the exhaust passage is set to the second passage.

  In the process of S30 or S31, the ECU determines the value of the weak rich correction coefficient KCRD between 0 and 1. The correction coefficient KCRD is used by multiplying the basic injection amount as described above in order to finely adjust the rich shift amount from the stoichiometry when the air-fuel ratio is made slightly rich during the reduction process. That is, the smaller the value of the correction coefficient KCRD, the smaller the rich shift amount.

  When the determination of S17 is YES or when the determination of S19 is NO, that is, the ECU is performing NOx catalyst regeneration processing or the temperature of the NOx catalyst is higher than the desorption temperature, the NOx catalyst is adsorbed by NOx. When the function cannot be exhibited, the correction coefficient KCRD is set to 1 which is the maximum value in order to quickly recover the NOx purification performance of the three-way catalyst (see S30), and this process is terminated.

  Further, when the determination of S17 is NO and the determination of S19 is YES, that is, when the NOx catalyst can compensate for the decrease in the NOx purification performance of the three-way catalyst, the ECU sets the rich shift amount. The process proceeds to S31 for fine adjustment. In S31, the ECU obtains an estimated value of the maximum oxygen storage amount in the three-way catalyst or an estimated value of the degradation parameter correlated therewith by a known method, and uses this estimated value to set the value of the correction coefficient KCRD to 1. A smaller value is set (see S31), and this process ends.

  FIG. 10 is an example of a map for determining the value of the correction coefficient KCRD using the estimated maximum storage amount. As shown in FIG. 10, the value of the correction coefficient KCRD is set to be smaller as the maximum storage amount is smaller within a range smaller than 1.

  According to the processing of S30 to S31 described above, the rich shift amount at the time of execution of the reduction processing is as follows when the exhaust passage is set on the second passage side and the regeneration processing of the NOx catalyst is not performed. The exhaust flow path is set smaller than when the first flow path is set.

  FIG. 11 is a flowchart showing a procedure for calculating the estimated value ΣNOx of the NOx adsorption amount in the NOx catalyst. The process of FIG. 11 is repeatedly executed in the ECU under a predetermined control cycle in response to the ignition switch (not shown) for starting the engine being turned on, similarly to the processes of FIGS. 8 and 9. The

  In S51, the ECU determines whether or not the flow path flag F_PZ is “1”. If the determination in S51 is NO, that is, if the exhaust flow path is set to the first flow path, the ECU determines that the NOx adsorption amount does not change and maintains the estimated value ΣNOx at the previous value. This process ends.

  If the determination in S51 is YES, the ECU acquires the current temperature of the NOx catalyst and determines whether this is lower than the desorption temperature (see S52). When the determination in S52 is YES, that is, in a state where NOx can be adsorbed to the NOx catalyst, the ECU updates the estimated value ΣNOx to the increasing side by performing the following processes of S53 to S54. Further, when the determination of S52 is NO, that is, when the NOx can be desorbed from the NOx catalyst, the ECU updates the estimated value ΣNOx to the decreasing side by performing the following processes of S55 to S56.

In S53, the ECU calculates a new adsorption amount adnox in the NOx catalyst, and proceeds to S54. The new adsorption amount adnox corresponds to the amount of NOx newly adsorbed on the NOx catalyst during the control cycle from the previous time to the current time. The new adsorption amount adnox is calculated by multiplying the NOx inflow amount innox by the NOx adsorption rate ηnox and the adsorption rate correction coefficient kηnox, for example, as shown in the following equation. In S54, the ECU updates the estimated value ΣNOx by adding the new adsorption amount adnox to the previous value of the estimated value ΣNOx, and ends this process.
adnox = innox × ηnox × kηnox

  Here, the NOx inflow amount innox specifically corresponds to the amount of NOx that flows into the NOx catalyst during the control cycle from the previous time to the current time. This inflow amount innox is calculated by searching a predetermined map (not shown) based on, for example, the engine speed and load. Further, the adsorption rate ηnox is calculated by searching a predetermined map as shown in FIG. 12 using the temperature of the NOx catalyst. Further, the NOx adsorption rate in the NOx catalyst decreases as the NOx adsorption amount increases. The correction coefficient kηnox is a coefficient introduced in order to consider a change in the NOx adsorption rate according to the NOx adsorption amount. A specific value of the correction coefficient kηnox is calculated by searching a map as shown in FIG. 13 using the estimated NOx adsorption amount ΣNOx. For example, the correction coefficient kηnox is set to 1 which is the maximum value when the estimated value ΣNOx is 0 (see FIG. 13).

  In S55, the ECU calculates a new desorption amount denox in the NOx catalyst, and proceeds to S56. This new desorption amount denox corresponds to the amount of NOx newly desorbed from the NOx catalyst during the control cycle from the previous time to this time. This desorption amount denox is calculated by searching a predetermined map as shown in FIG. 14 using the temperature of the NOx catalyst. As shown in FIG. 14, the amount of NOx desorbed from the NOx catalyst increases as the temperature of the NOx catalyst increases. In S56, the ECU updates the estimated value ΣNOx by subtracting the new desorption amount denox from the previous value of the estimated value ΣNOx, and ends this process.

  Next, a specific operation example of the exhaust purification apparatus as described above will be described with reference to a time chart.

  FIG. 15 is a time chart showing an operation example of the exhaust emission control device immediately after the engine is started. FIG. 15 shows, in order from the top, various flags (F_STMOD, F_rgn, F_PZ), the estimated NOx adsorption amount ΣNOx, the temperature of the NOx catalyst, and the NOx amount exhausted from the tailpipe in the exhaust purification system of this embodiment. Show. Further, in FIG. 15, the NOx amount in the conventional exhaust purification apparatus using only the three-way catalyst is shown by a broken line for comparison.

  First, when the engine is started by operating the ignition switch at time t0, the start flag F_STMOD is set to “1”. Thus, at time t1, the regeneration request flag F_rgn is set to “1” (see S1 → S2 in FIG. 8), and the flow path flag F_PZ is also set to “1” (S11 → S13 → S14 → S15 in FIG. 9). reference).

  In the conventional exhaust gas purification apparatus, NOx that could not be purified by the three-way catalyst until time t3 when the three-way catalyst reaches the activation temperature may be discharged from the tail pipe (see the broken line in FIG. 15). On the other hand, in the exhaust gas purification apparatus of the present embodiment, the exhaust flow path displays the NOx catalyst from time t1 to time t4 when the flag F_rgn is reset to “0” in response to the flag being set as described above. A regeneration process is performed in which the second flow path is set and the air-fuel ratio is stoichiometric and temperature rise control is performed. For this reason, NOx that could not be purified by the three-way catalyst is adsorbed by the NOx catalyst, so that the discharge from the tail pipe can be suppressed.

  As shown in FIG. 15, the temperature of the NOx catalyst exceeds the desorption temperature at time t2 by executing the temperature rise control. Therefore, at time t2, the NOx adsorption amount in the NOx catalyst starts to decrease. At this time, by making the air-fuel ratio of the exhaust gas flowing into the NOx catalyst stoichiometric, NOx desorbed from the NOx catalyst is not reduced and is not discharged from the tail pipe.

  Thereafter, at time t4, the regeneration request flag F_rgn is reset from “1” to “0” in response to the estimated adsorption amount ΣNOx being lower than the lower limit threshold Lim_L, thereby completing the regeneration process (FIG. 8). (See S1-> S4-> S5-> S6). Further, in response to the reset of the flag F_rgn to “0”, the flow path flag F_PZ is also set to “0” (see S11 → S13 → S14 → S16 in FIG. 9). Thereafter, NOx exhausted from the engine is reduced by the three-way catalyst that has reached activity. Further, immediately after the engine is started, after the regeneration process of the NOx catalyst is completed, the heat load on the NOx catalyst can be reduced by switching the exhaust passage to the first passage.

  FIG. 16 is a time chart showing an operation example of the exhaust purification apparatus immediately after the fuel cut. FIG. 16 shows, in order from the top, various flags (F_FC, F_CRD, F_PZ), the amount of NOx discharged from the tail pipe, and the fuel injection amount in the exhaust purification system of this embodiment. Further, in FIG. 15, the NOx amount and the fuel injection amount in a conventional exhaust purification device using only a three-way catalyst are shown by broken lines for comparison.

  First, when the fuel cut flag F_FC is set to “1” at time t0 and the fuel cut is started, the exhaust gas purification apparatus of the present embodiment sets the flow path flag F_PZ to “0” and forcibly sets the exhaust flow path. Set to the first flow path. During this time, high-temperature exhaust does not flow into the NOx catalyst, so that the thermal load on the NOx catalyst can be reduced.

  Next, at time t1, the fuel cut flag F_FC is reset from “1” to “0”, and when the fuel cut period elapses, the reduction request flag F_CRD is set to “0” to start the reduction process of the three-way catalyst as described above. "1" is set to "1". When a reduction request is generated, in the exhaust purification apparatus of this embodiment, whether or not the NOx catalyst is being regenerated (see S17 in FIG. 9), and whether or not the NOx catalyst can adsorb NOx (see FIG. 9). If all of these determination results are affirmative, the flow path flag F_PZ is set to “1” (see S21 in FIG. 9), and the exhaust flow path is set to the second flow. Switch to the road.

  As indicated by a broken line in FIG. 16, NOx that cannot be sufficiently reduced and purified by the three-way catalyst during the reduction process in the conventional exhaust purification device may be discharged from the tail pipe as it is. On the other hand, in the exhaust purification apparatus of the present embodiment, after the fuel cut period has elapsed, the NOx catalyst that could not be purified by the three-way catalyst is switched by switching the exhaust passage to the second passage that passes through the NOx catalyst. Therefore, the amount of NOx discharged from the tailpipe can be suppressed more than before.

  Further, in the exhaust purification device of the present embodiment, when the exhaust flow path is set to the second flow path while the air-fuel ratio is weakly rich in order to reduce the oxygen stored in the three-way catalyst, when the exhaust gas is weakly rich, The value of the correction coefficient KCRD is set to a value smaller than 1, which is the maximum value, and the rich shift amount is reduced (see S31 in FIG. 9). Thereby, as shown in FIG. 16, the fuel injection amount concerning the reduction process of a three-way catalyst can be suppressed compared with the past.

  As mentioned above, although embodiment of this invention was described, this invention is not restricted to this. Within the scope of the gist of the present invention, the detailed configuration may be changed as appropriate.

1. Engine (internal combustion engine)
11 ... Exhaust pipe (exhaust passage)
2 ... Exhaust gas purification device 31 ... Upstream catalytic converter (three-way catalyst)
32 ... Intermediate catalytic converter (NOx catalyst)
34 ... Exhaust flow path switching mechanism (switching mechanism)
36 ... Exhaust temperature sensor (catalyst temperature acquisition means)
6 ... ECU (exhaust flow path control means, catalyst temperature acquisition means, adsorption amount acquisition means, regeneration control means, regeneration request generation means, reduction control means)

Claims (9)

A three-way catalyst provided in the exhaust passage of the internal combustion engine;
An exhaust gas purification apparatus for an internal combustion engine, comprising: a support made of zeolite provided in the exhaust passage and a NOx catalyst having Pd supported on the support,
Switching to switch the exhaust flow path of the internal combustion engine between a first flow path that passes through the three-way catalyst and bypasses the NOx catalyst, and a second flow path that passes through both the three-way catalyst and the NOx catalyst Mechanism,
An exhaust emission control device for an internal combustion engine, comprising: an exhaust passage control means for switching the switching mechanism on the first passage side or the second passage side according to a predetermined condition. Catalyst temperature acquisition means for acquiring the temperature of the NOx catalyst;
An adsorption amount obtaining means for obtaining an NOx adsorption amount in the NOx catalyst,
2. The exhaust gas purification apparatus for an internal combustion engine according to claim 1, wherein the exhaust flow path control unit switches the switching mechanism based on or both of the acquired temperature of the NOx catalyst and NOx adsorption amount.   3. The exhaust gas purification of an internal combustion engine according to claim 1, wherein the exhaust flow path control unit sets the switching mechanism to the second flow path side in a predetermined period immediately after the start of the internal combustion engine. apparatus.   The exhaust gas flow path control means sets the switching mechanism to the first flow path side during a fuel cut period in which fuel injection in the internal combustion engine is temporarily stopped. An exhaust purification device for an internal combustion engine according to claim 1.   The exhaust purification of an internal combustion engine according to claim 4, wherein the exhaust flow path control means switches the switching mechanism from the first flow path side to the second flow path side after the fuel cut period has elapsed. apparatus. A NOx catalyst regeneration process is performed to increase the temperature of the NOx catalyst above a predetermined desorption temperature and to make the air-fuel ratio of the exhaust gas flowing into the NOx catalyst stoichiometric or rich, and the NOx adsorbed on the NOx catalyst is reduced and purified Playback control means to
A regeneration request generating means for generating an execution request for the NOx catalyst regeneration process in the regeneration control means when the NOx adsorption amount in the NOx catalyst exceeds a predetermined upper limit threshold;
6. The exhaust gas flow path control means sets the switching mechanism to the second flow path side while the execution of the NOx catalyst regeneration process is required. 6. An exhaust purification device for an internal combustion engine. A reduction process for reducing the oxygen stored in the three-way catalyst by performing a reduction process for shifting the air-fuel ratio of the exhaust gas flowing into the three-way catalyst to a richer side than a predetermined reference;
The exhaust flow path control means sets the switching mechanism to the second flow path side when the temperature of the NOx catalyst is lower than a predetermined desorption temperature during the reduction process. An exhaust emission control device for an internal combustion engine according to any one of claims 1 to 5. A NOx catalyst regeneration process is performed to increase the temperature of the NOx catalyst above a predetermined desorption temperature and to make the air-fuel ratio of the exhaust gas flowing into the NOx catalyst stoichiometric or rich, and the NOx adsorbed on the NOx catalyst is reduced and purified Playback control means to
Reduction control means for performing a reduction process for shifting the air-fuel ratio of the exhaust gas flowing into the three-way catalyst to a richer side than a predetermined reference, and for reducing oxygen stored in the three-way catalyst,
The exhaust flow path control means sets the switching mechanism when the temperature of the NOx catalyst is lower than a predetermined desorption temperature or when the NOx catalyst regeneration process is being performed during the reduction process. The exhaust gas purification apparatus for an internal combustion engine according to any one of claims 1 to 5, wherein the exhaust gas purification apparatus is set on the second flow path side.   When the switching mechanism is set on the second flow path side, the reduction control means is more rich in the air-fuel ratio of the exhaust when the reduction process is performed than when the switching mechanism is set on the first flow path side. The exhaust purification device for an internal combustion engine according to claim 7 or 8, wherein the shift amount to the side is reduced.

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