JP2012237296A - Controller of internal combustion engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a controller of an internal combustion engine which controls emission of NHand NOx into the air.SOLUTION: When, in a routine shown in Fig.4, a stoichiometric driving is requested, and an attained floor temperature of SCR 20 is estimated to be higher than a set temperature A, three air-fuel ratio controls are made according to the magnitude relation between an SCR occluded NHamount and an NSR occluded NOx amount. When (the SCR occluded NHamount)=(the NSR occluded NOx amount), an operation near the stoichiometry is executed over a predetermined time (Step 140). When (the SCR occluded NHamount)>(the NSR occluded NOx amount), a weak lean operation is executed (Step 160). When (the SCR occluded NHamount)<(the NSR occluded NOx amount), a rich spike is executed until an SCR generated NHamount exceeds the difference between the NSR occluded NOx amount and the SCR occluded NHamount (Step 180, 190).

Description

  The present invention relates to a control device for an internal combustion engine, and more particularly to a control device for an internal combustion engine provided with a conversion device for purifying NOx in exhaust gas.

Conventionally, in order from the upstream side of the exhaust passage, NOx in the exhaust gas is occluded when the exhaust air-fuel ratio is in a predetermined lean air-fuel ratio region, and the stored NOx is released when it is in the predetermined rich air-fuel ratio region. NH ₃ in the reducing to NSR: and (NOx Storage reduction catalysts storage reduction catalyst), the occluding and NH ₃ in the exhaust gas, SCR to reduce NOx in the exhaust gas by NH ₃ occluding (selective catalytic reduction catalysts: selection An internal combustion engine provided with a reduction catalyst) is known.

For example, Patent Document 1 discloses that in such an internal combustion engine, the target air-fuel ratio is controlled based on the amount of stored NH ₃ in the SCR. Specifically, when the amount of stored NH ₃ in the SCR is acquired and it is determined that the stored amount is sufficient, a rich spike that temporarily enriches the target air-fuel ratio is executed. Thereby, NH ₃ is released from the NSR and stored in the SCR. Therefore, it is possible to suppress the release of NH ₃ into the atmosphere while preventing the NOx storage in the NSR and the NH ₃ storage saturation in the SCR.

JP 2010-112345 A JP 2010-116784 A JP 2009-111487 A JP 2009-097471 A

However, the storage capacity of NH _{3 in} the SCR is temperature-dependent, and this storage capacity decreases as the bed temperature of the SCR increases. Therefore, when hot exhaust gas flows into the SCR, there is a problem that the stored NH ₃ is desorbed and released into the atmosphere. This problem is particularly the case storage amount of NH ₃ in the SCR is large, i.e., the amount of storage NH ₃ becomes more remarkable as the case close to the storage capacity.

Under high temperature conditions, NH ₃ and O ₂ react to generate NOx. For this reason, when O ₂ is contained in the high-temperature exhaust gas, there is a problem that NH ₃ and O ₂ desorbed by the high temperature react and NOx is released into the atmosphere. This problem becomes more prominent as the amount of occluded NH ₃ in the SCR is closer to the occlusion allowable amount.

The present invention has been made to solve the above-described problems, and an object of the present invention is to provide a control device for an internal combustion engine that can suppress the release of NH ₃ and NOx into the atmosphere.

In order to achieve the above object, a first invention is a control device for an internal combustion engine,
It is provided in the exhaust passage of the internal combustion engine, and stores NOx in the exhaust gas when the exhaust air-fuel ratio is in a predetermined lean air-fuel ratio range, and releases the stored NOx when the exhaust air-fuel ratio is in a predetermined rich air-fuel ratio range. An occlusion reduction catalyst that reduces to NH ₃ and
Provided in the exhaust passage downstream of said occlusion reduction type catalyst, thereby occluding and NH ₃ in the exhaust gas, a selective reduction catalyst that reduces NOx in exhaust gas by NH ₃ occluding,
An operation switching request determination means for determining whether or not there is an operation switching request for switching from a lean operation in the predetermined lean air-fuel ratio region to a high exhaust temperature operation;
When it is determined that there is an operation switching request, the lean operation is performed from the lean operation to the high exhaust gas temperature according to the magnitude relationship between the storage NOx amount in the storage reduction catalyst and the storage NH ₃ amount in the selective reduction catalyst. Target air-fuel ratio setting means for setting a target air-fuel ratio during switching to operation;
It is characterized by providing.

The second invention is the first invention, wherein
The target air-fuel ratio setting means sets the target air-fuel ratio to a weak lean air-fuel ratio that is richer than the predetermined lean air-fuel ratio region over a predetermined time when the stored NH ₃ amount is larger than the stored NOx amount. It is characterized by setting.

The third invention is the first invention, wherein
When the occluded NH ₃ amount is smaller than the occluded NOx amount, the target air / fuel ratio setting means temporarily sets the target air / fuel ratio to be rich until the occluded NH ₃ amount becomes larger than the occluded NOx amount. Then, a weak lean air-fuel ratio that is richer than the predetermined lean air-fuel ratio region is set over a predetermined time.

Moreover, 4th invention is 2nd or 3rd invention,
The degree of enrichment of the weak lean air-fuel ratio is weakened as the degree of deterioration of the selective catalytic reduction catalyst increases.

The fifth invention is the first invention, wherein
The target air-fuel ratio setting means sets the target air-fuel ratio in the vicinity of the theoretical air-fuel ratio over a predetermined time when the occluded NH ₃ amount is equal to the occluded NOx amount.

According to the first invention, if it is determined that there is a driving switch request, the amount of NOx stored in the storage reduction catalyst, in accordance with the magnitude relationship between occlusion NH ₃ amount in the selective reduction catalyst, operating The target air-fuel ratio for the switching period can be set. Therefore, for example, when the occluded NH ₃ amount is larger, the target air-fuel ratio for preferentially purifying NH ₃ of the selective catalytic reduction catalyst is set, and when the occluded NH ₃ amount is smaller, the occluded NH ₃ amount is set. It is possible to set the target air-fuel ratio to be released after NH ₃ is released from the reduction catalyst and temporarily stored in the selective reduction catalyst and then purified. Therefore, according to the present invention, it is possible to efficiently reduce the amount of the occluded NH ₃ during the operation switching period, so that NH ₃ and NOx are introduced into the atmosphere as the temperature of the selective catalytic reduction catalyst increases. It can be prevented well from being released.

According to the second invention, when the occluded NH ₃ amount is larger than the occluded NOx amount, the target air-fuel ratio is set to a weak lean air-fuel ratio that is richer than the predetermined lean air-fuel ratio region over a predetermined time. Can be set. If such a weak lean air-fuel ratio is set, the NOx in the exhaust gas can be allowed to pass through the storage-reduction catalyst without being stored. If the weak lean air-fuel ratio is set, part of the NOx stored in the storage reduction catalyst can be released. Therefore, according to the present invention, these NOx can be introduced into the selective catalytic reduction catalyst to efficiently purify the NH ₃ .

According to the third invention, when the occluded NH ₃ amount is smaller than the occluded NOx amount, the target air-fuel ratio is temporarily made rich until the occluded NH ₃ amount becomes larger than the occluded NOx amount. After that, the weak lean air-fuel ratio can be set for a predetermined time. If the target air-fuel ratio is temporarily set to be rich until the occluded NH ₃ amount becomes larger than the occluded NOx amount, calculation of the predetermined time for setting the weak lean air-fuel ratio can be facilitated, and thus the selective catalytic reduction catalyst Of NH ₃ can be efficiently purified.

  According to the fourth invention, the degree of enrichment of the weak lean air-fuel ratio can be reduced as the degree of deterioration of the selective catalytic reduction catalyst increases. When the selective catalytic reduction catalyst deteriorates, the predetermined lean air-fuel ratio region is reduced to the lean side. Further, in the air-fuel ratio range near the weak lean air-fuel ratio, the NOx amount in the exhaust gas exhausted from the engine becomes larger at the leaner air-fuel ratio. Therefore, if the degree of enrichment of the weak lean air-fuel ratio is weakened, more NOx in the exhaust gas can be introduced into the selective reduction catalyst during the setting of the weak lean air-fuel ratio. Therefore, it is possible to shorten the predetermined time and promptly switch to the high exhaust temperature operation.

According to the fifth aspect, when the occluded NH ₃ amount is equal to the occluded NOx amount, the target air-fuel ratio can be set in the vicinity of the theoretical air-fuel ratio over a predetermined time. If it is set in the vicinity of the stoichiometric air-fuel ratio, a part of NOx stored in the storage reduction catalyst can be released. Therefore, according to the present invention, the released NOx can be introduced into the selective reduction catalyst, and the NH ₃ can be efficiently purified.

It is a figure for demonstrating the structure of Embodiment 1 of this invention. Bed temperature of the SCR and (° C.), a diagram showing the relationship between the NH ₃ storage allowance (g). FIG. 3 is a diagram for explaining acceleration-requested air-fuel ratio control in the first embodiment. 3 is a flowchart showing acceleration required air-fuel ratio control executed by an ECU 30 in the first embodiment. It is the figure which showed the relationship between the NOx storage window of NSR, the amount of engine exhaust gas NOx, and an air fuel ratio. 7 is a timing chart for explaining acceleration-requested air-fuel ratio control in the second embodiment. 6 is a flowchart showing acceleration required air-fuel ratio control executed by an ECU 30 in a second embodiment.

Embodiment 1 FIG.
[Configuration of Embodiment 1]
First, Embodiment 1 of the present invention will be described with reference to FIGS. FIG. 1 is a diagram for explaining the configuration of the present embodiment. As shown in FIG. 1, the system of this embodiment includes an engine 10 as an internal combustion engine. The number of cylinders of the engine and the arrangement form of the cylinders are not limited to the engine 10 of FIG. 1, and may be, for example, a 6-cylinder, 8-cylinder, or 12-cylinder engine. There may be.

An exhaust passage 12 communicates with the engine 10. In the exhaust passage 12, a turbocharger 14, a TWC (Three-Way Catalyst) 16, an NSR 18 and an SCR 20 are arranged in this order. The engine 10 easily discharges HC and CO when the air-fuel ratio is rich, and easily discharges NOx when the air-fuel ratio is lean. TWC16 is the NOx is reduced to _{N 2} while adsorption of _{O 2} in the exhaust gas when the air-fuel ratio is lean, _H 2, HC and CO while releasing _{O 2} when the air-fuel ratio is rich O oxidized to or CO _2. Further, when the air-fuel ratio is rich, N ₂ contained in the exhaust gas by reacting with H _2, the NH ₃ is generated.

The NSR 18 occludes NOx contained in the exhaust gas when the air-fuel ratio is in a predetermined lean air-fuel ratio range. Further, the NSR 18 releases the stored NOx when the air-fuel ratio is in a predetermined rich air-fuel ratio range. NOx released from the NSR 18 is reduced by HC and CO contained in the exhaust gas. In this case, NH ₃ is also generated in the NSR 18 as in the case of the TWC 16.

The SCR 20 has a function of storing NH ₃ generated in the TWC 16 and the NSR 18 and selectively reducing NOx in the exhaust gas using the stored NH ₃ as a reducing agent. According to the SCR 20, it is possible to effectively prevent NH ₃ and NOx that have blown through the subsequent stage of the NSR 18 from being released into the atmosphere.

In addition, the system of the present embodiment includes an ECU (Electronic Control Unit) 30 as a control device. The ECU 30 is electrically connected to a NOx sensor 22 for detecting the NOx concentration upstream of the NSR 18 and an NH ₃ sensor 24 for detecting the NH ₃ concentration upstream of the SCR 20. In addition to the above sensors, various sensors necessary for controlling the engine 10 (for example, a crank angle sensor that detects the engine speed, an air flow meter that detects the intake air amount, and a throttle sensor that detects the opening of the throttle valve). Etc.) are electrically connected to the ECU 30. The ECU 30 controls the operating state of the engine 10 by driving the actuators such as the injector, the spark plug, and the VVT according to the detection result and the request from the driver.

[Basic operation of the first embodiment]
In the present embodiment, the engine 10 is normally operated at a lean air-fuel ratio (lean operation). During lean operation, the engine 10 discharges a relatively large amount of NOx, so that NOx flows into the NSR 18. Therefore, during lean operation, the amount of NOx stored in the NSR 18 (hereinafter also referred to as “NSR stored NOx amount”) increases with the passage of time. However, if the storage limit of the NSR 18 is exceeded, exhaust gas containing NOx blows through to the subsequent stage in large quantities, and exhaust emission deteriorates. Therefore, in the present embodiment, the rich spike is executed when the NSR storage NOx amount approaches this allowable value.

In the rich spike, the engine 10 is temporarily operated at a rich air-fuel ratio. Therefore, NOx stored in the NSR 18 is released, and the amount of NSR stored NOx can be reduced. Further, during execution of the rich spike, the engine 10 exhausts exhaust gas containing a large amount of HC and CO, so that HC and CO are oxidized and NH ₃ is generated in the TWC 16. In addition, NOx released from the NSR 18 reacts with HC and CO to generate NH ₃ . Therefore, a large amount of NH ₃ flows into the SCR 20. As a result, during execution of the rich spike, the amount of occluded NH _{3 in the} SCR 20 (hereinafter also referred to as “SCR occluded NH ₃ amount”) increases with time.

In the present embodiment, the engine 10 is operated at the stoichiometric air-fuel ratio (stoichiometric operation) when there is an acceleration request from the driver. That is, when there is an acceleration request, the lean operation is switched to the stoichiometric operation. During the stoichiometric operation, NOx, HC and CO discharged from the engine 10 are purified by the TWC 16. During the stoichiometric operation, part of the NOx occluded in the NSR 18 oozes out and flows into the SCR 20. Therefore, this NOx reacts with NH ₃ stored in the SCR 20. As a result, during the stoichiometric operation, the amount of SCR occluded NH ₃ decreases with the passage of time.

After the rich spike or stoichiometric operation is performed, the engine 10 is leaned again. As described above, since the engine 10 discharges a relatively large amount of NOx during the lean operation, the amount of NSR occluded NOx increases. Further, a part of the NOx flowing into the NSR 18 flows into the SCR 20 without being occluded by the NSR 18. Therefore, NH ₃ occluded in the SCR 20 reacts with this NOx. Therefore, during lean operation, the amount of SCR occluded NH ₃ decreases as during stoichiometric operation.

[Features of Embodiment 1]
By the way, the NH ₃ storage allowance of the SCR has temperature dependency. FIG. 2 is a graph showing the relationship between the SCR bed temperature (° C.) and its NH ₃ storage allowance (g). As shown in FIG. 2, the NH ₃ storage allowance is inversely proportional to the SCR bed temperature, and decreases as the SCR bed temperature increases. Therefore, in the system of the present embodiment, when the bed temperature of the SCR 20 is increased, NH ₃ corresponding to the storage capacity is released to the subsequent stage. This phenomenon becomes more prominent when the SCR storage NH ₃ amount is raised to a high temperature in a state close to the NH ₃ storage allowable amount.

Further, under a high temperature condition, a decomposition reaction of NH ₃ represented by the following formula (1) proceeds.
4NH ₃ + 5O ₂ → 4NO + 6H ₂ O (1)
Therefore, when O ₂ flows into the SCR 20 in a high temperature state, NO is generated from the stored NH ₃ and released to the subsequent stage of the SCR 20. The amount of NO generated by the decomposition reaction increases as the SCR storage NH ₃ amount is closer to the NH ₃ storage allowable amount of the SCR 20. The reason for this is that NH ₃ is more likely to be released as the SCR storage NH ₃ amount is closer to the NH ₃ storage allowable amount.

Here, if the amount of fuel supplied to the engine 10 increases, the exhaust temperature rises and the SCR 20 becomes high temperature. That is, it can be said that the exhaust temperature is likely to rise during execution of rich spike or during stoichiometric operation. On the other hand, since the execution of the rich spike is temporary, the rise in the exhaust temperature is also temporary and has little effect. Therefore, during the stoichiometric operation, there is a high possibility that NH ₃ or NO is released to the subsequent stage of the SCR 20. Therefore, in the present embodiment, the air-fuel ratio during the switching from the lean operation to the stoichiometric operation is controlled in consideration of the SCR storage NH ₃ amount and the NSR storage NOx amount (acceleration required air-fuel ratio control).

The acceleration request air-fuel ratio control in the present embodiment will be described with reference to FIG. In the acceleration request air-fuel ratio control, the three air-fuel ratio controls shown in FIGS. 3A to 3C are executed in accordance with the magnitude relationship between the SCR storage NH ₃ amount and the NSR storage NOx amount.

When NSR occlusion NOx amount <SCR occlusion NH ₃ amount, before switching from lean operation to stoichiometric operation, operation is performed at a weak lean air-fuel ratio (weak lean operation) (FIG. 3A). Here, the weak lean air-fuel ratio is an air-fuel ratio existing in an air-fuel ratio region that deviates from both the NOx purification window in the TWC 16 and the NOx occlusion window in the NSR 18. By adopting such a weak lean air-fuel ratio, as shown in the figure, NOx in the engine exhaust gas can be introduced into the SCR 20 without being purified by the TWC 16 or being purified or occluded by the TWC 16 or the NSR 18. Further, during the weak lean operation, a part of the NOx occluded in the NSR 18 oozes out as in the stoichiometric operation. Therefore, this exudation NOx can also be introduced into the SCR 20. Therefore, the NH ₃ stored in the SCR 20 can be reacted with these NOx, so that the entire amount of NH ₃ stored in the SCR 20 can be consumed.

When NSR occlusion NOx amount = SCR occlusion NH ₃ amount, before switching to stoichiometric operation, operation is performed at an air-fuel ratio in the vicinity of stoichiometric operation (stoichiometric operation) (FIG. 3B). By setting the air-fuel ratio in the vicinity of the stoichiometry, as shown in the figure, the NOx occluded in the NSR 18 can be oozed out and allowed to flow into the SCR 20. Thus, before the exhaust temperature rises, NH ₃ stored in the SCR 20 can be reacted with NOx to consume the entire amount of NH ₃ stored in the SCR 20. Note that NOx discharged from the engine 10 is purified by the TWC 16, and therefore does not flow into the NSR 18 or the SCR 20.

When NSR occlusion NOx amount> SCR occlusion NH ₃ amount, before switching to the stoichiometric operation, a rich spike is executed and then a weak lean operation is performed (FIG. 3C). As shown in the figure, by executing the rich spike, NOx released from the NSR 18 can be reacted with HC and CO to generate NH ₃ and flow into the SCR 20. As a result, the NSR occlusion NOx amount can be decreased and the SCR occlusion NH ₃ amount can be increased. The rich spike is repeatedly executed until the NSR occlusion NOx amount <the SCR occlusion NH ₃ amount, and then the operation is switched to the weak lean operation. This weak lean operation is as described with reference to FIG. Such a series of operation control makes it possible to consume the entire amount of NH ₃ stored in the SCR 20.

Incidentally, the duration of the weak lean operation or near stoichiometric operation (specifically, substance amount of the NH ₃ which is stored in SCR20) SCR occlusion NH ₃ amount depending on, outlet gas NOx amount from the engine 10 and NSR18 Adjust the amount of NOx exuded from the NOx. Here, regarding the amount of outgas NOx, the reaction between NH ₃ and NOx is expressed by, for example, the following formulas (2) and (3).
6NO + 4NH ₃ → 5N ₂ + 6H ₂ O (2)
6NO ₂ + 8NH ₃ → 7N ₂ + 12H ₂ O (3)
The output NOx amount is determined based on the air-fuel ratio set as the weak lean air-fuel ratio and the reactions of the above formulas (2) and (3), and the intake air amount, EGR amount, and VVT valve overlap amount to the engine 10 It is calculated using various parameters such as ignition timing. On the other hand, the amount of NOx exuded is a fixed value.

[Specific Processing in Embodiment 1]
Next, a specific process for realizing the above-described acceleration required air-fuel ratio control will be described with reference to FIG. FIG. 4 is a flowchart showing the acceleration request air-fuel ratio control executed by the ECU 30 in the present embodiment. The routine shown in FIG. 4 is repeatedly executed while the engine 10 is operating.

  In the routine shown in FIG. 4, it is first determined whether or not there is a request for stoichiometric operation (step 100). The presence / absence of a request for stoichiometric operation can be determined, for example, by the presence / absence of a torque request. Specifically, if it is determined that the vehicle is in a high load range based on the throttle opening at that time, it is determined that there is a torque request. On the other hand, if it is determined that the vehicle is in the low / medium load range, it is determined that there is no torque request. If it is determined in this step that there is no request, this routine is terminated.

On the other hand, if it is determined in step 100 that there is a request, it is determined whether or not the estimated bed temperature of the SCR 20 is higher than the set temperature A (step 110). The estimated reached bed temperature of the SCR 20 can be calculated from the operating state such as the engine speed and the throttle opening, for example. The set temperature A is a temperature at which the release of NH ₃ from the SCR 20 starts. For example, the set temperature A can be determined by applying the amount of SCR occluded NH ₃ to the map of the relationship shown in FIG. Incidentally, SCR occlusion amount of NH ₃ is used a value calculated from the separately stored amount models. In this step, when it is determined that the bed temperature of the SCR 20 is lower than the set temperature A, it can be determined that there is no NH ₃ release from the SCR 20 even if the stoichiometric operation is executed. Accordingly, the stoichiometric operation is executed (step 120).

On the other hand, when it is determined in step 110 that the bed temperature of the SCR 20 is higher than the set temperature A, it can be determined that there is a high possibility that NH ₃ is released from the SCR 20. Therefore, it is determined whether or not the SCR storage NH ₃ amount = the NSR storage NOx amount is satisfied (step 130). In this step, each of the SCR occlusion NH ₃ amount and the NSR occlusion NOx amount is a value obtained by separately applying the detection values of the NOx sensor 22 and the NH ₃ sensor 24 to the occlusion amount model. When it is determined that satisfies the SCR occlusion amount of NH ₃ = NSR absorbed NOx amount is near stoichiometric operation is performed for a predetermined time (step 140), then, the stoichiometric operation is performed (step 120) .

On the other hand, when it is determined in step 130 that the SCR storage NH ₃ amount = the NSR storage NOx amount is not satisfied, it is determined whether or not the SCR storage NH ₃ amount> the NSR storage NOx amount is satisfied (step 150). When it is determined that the SCR storage NH ₃ amount> the NSR storage NOx amount is satisfied, the target air-fuel ratio is set to the weak lean air-fuel ratio, and execution of the weak lean operation is started (step 160). As described above, when the weak lean operation is executed, NOx in the engine exhaust gas or NOx exuding from the NSR 18 is introduced into the SCR 20.

Subsequently, it is determined whether or not the SCR occlusion NH ₃ amount ≦ the SCR introduction NOx amount is satisfied (step 170). Here, since the SCR introduction NOx amount is proportional to the duration of the weak lean operation, whether or not the SCR storage NH ₃ amount ≦ the SCR introduction NOx amount has passed the above predetermined time. It can be judged by whether or not. Therefore, when it is determined that the predetermined time has elapsed, the weak lean operation is switched to the stoichiometric operation (step 120).

On the other hand, when it is determined in step 150 that the SCR storage NH ₃ amount> the NSR storage NOx amount is not satisfied, a rich spike is executed (step 180), and whether or not the SCR generated NH ₃ amount> the excess NOx amount is satisfied. Is determined (step 190). Here, the excessive NOx amount corresponds to the difference between the NSR storage NOx amount and the SCR storage NH ₃ amount, and the SCR generated NH ₃ amount is the NH ₃ material amount generated in the NSR 18 during execution of the rich spike. It corresponds to. If it is determined that the amount of SCR generated NH ₃ > the amount of excess NOx is not satisfied, the process returns to step 180 and a rich spike is executed. That is, the rich spike is repeatedly executed until the amount of SCR generated NH ₃ becomes larger than the amount of excess NOx. If it is determined that SCR generated NH ₃ amount> excess NOx amount is satisfied, the routine proceeds to step 160 where the weak lean operation is executed for a predetermined time. The processing after step 160 is as described above.

As described above, according to the routine shown in FIG. 4, the air-fuel ratio during switching to the stoichiometric operation can be controlled according to the magnitude relationship between the SCR storage NH ₃ amount and the NSR storage NOx amount. Therefore, without depending on the SCR occlusion amount of NH _3, prior to the high temperature of SCR20, the NH ₃ that is occluded it can be consumed by reacting with NOx. Therefore, it is possible to satisfactorily prevent NH ₃ and NO from being released to the subsequent stage as the temperature of the SCR 20 increases.

  In the first embodiment described above, when there is a request for stoichiometric operation, the air-fuel ratio is controlled while switching to the stoichiometric operation. However, such air-fuel ratio control is a request for operations other than stoichiometric operation. It can also be applied when there is a problem. That is, if there is a request for an operation in which the exhaust temperature is increased, the SCR 20 is increased in temperature by executing the operation. Therefore, when there is a request for such a high exhaust temperature operation, the same effect as in the present embodiment can be obtained by controlling the air-fuel ratio in the same way as the acceleration-requested air-fuel ratio control. Note that this modification can also be applied to Embodiment 2 described later.

  In the first embodiment described above, the ECU 30 executes the process of step 100 in FIG. 4 so that the “operation switching request determination unit” in the first aspect of the invention executes the processes of steps 130 to 190 in FIG. Thus, the “target air-fuel ratio setting means” in the first aspect of the present invention is realized.

Embodiment 2. FIG.
Next, a second embodiment of the present invention will be described with reference to FIGS. This embodiment is realized by causing the ECU 30 to execute a routine shown in FIG. 7 described later in the configuration of FIG. Therefore, in this embodiment, the description will be focused on the differences from the first embodiment, and the description of the same matters will be simplified or omitted.

[Features of Embodiment 2]
In the first embodiment, the target air-fuel ratio during the weak lean operation, that is, the weak lean air-fuel ratio, is set to the air-fuel ratio in the air-fuel ratio region outside the NOx storage window of the NSR 18. However, the NSR 18 deteriorates with long-term use. If the NSR 18 deteriorates, the NOx storage window is reduced. Therefore, in the present embodiment, the target air-fuel ratio at the time of the weak lean operation is changed according to the degree of reduction of the NOx storage window.

This change of the target air-fuel ratio will be described with reference to FIG. FIG. 5 is a graph showing the relationship between the NSR NOx occlusion window and the engine exhaust gas NOx amount and the air-fuel ratio. The solid line shown in FIG. 5 is the NOx occlusion window before deterioration. Further, what is indicated by a dotted line in FIG. 5 is a NOx occlusion window after deterioration. As can be seen from these NOx storage windows, when the NSR deteriorates, the NOx storage window is further reduced to the lean side. In other words, the air-fuel ratio region outside the NSR NOx storage window is expanded. Therefore, the target air-fuel ratio after deterioration can be set to a value larger than the weak lean air-fuel ratio (air-fuel ratio r _A ) before deterioration.

Further, as shown in FIG. 5, the amount of engine output NOx increases as it goes from the stoichiometric side to the lean side, and decreases with a certain maximum value as a boundary. Therefore, in the present embodiment, the target air-fuel ratio after deterioration is set to the air-fuel ratio (air-fuel ratio r _B ) that maximizes the amount of engine output NOx in the region outside the NOx storage window of the NSR 18. If such a target air-fuel ratio is set, more NOx can be introduced into the SCR 20 than before deterioration, so that the duration of the weak lean operation can be shortened. Therefore, since it becomes possible to switch to the stoichiometric operation more quickly, it is possible to secure the torque for the acceleration request. In addition, deterioration of the TWC 16 due to execution of weak lean operation can be suppressed.

The above will be further described with reference to FIG. FIG. 6 is a timing chart for explaining the acceleration request time air-fuel ratio control in the present embodiment. FIG. 6A shows acceleration required air-fuel ratio control before deterioration, and FIG. 6B shows acceleration required air-fuel ratio control after deterioration. FIGS. 6A and 6B show the acceleration required air-fuel ratio when the magnitude relationship between the SCR occlusion NH ₃ amount and the NSR occlusion NOx amount at the time of acceleration request is NSR occlusion NOx amount <SCR occlusion NH ₃ amount. Represents control.

As shown in FIG. 6A, when the target air-fuel ratio (A / F) is set to the air-fuel ratio r _A at the time t ₀ and the weak lean operation is started, the amount of SCR occluded NH ₃ begins to decrease. . This is because NOx exuding from the NSR 18 and NOx in the engine exhaust gas that has passed through the TWC 16 and the NSR 18 are introduced into the SCR 20. Therefore, the NSR occlusion NOx amount decreases in proportion to the exuded NOx amount. At time t ₁ when the NSR storage NOx amount becomes equal to the SCR storage NH ₃ amount, A / F is set to stoichiometric and the stoichiometric operation is started. That is, the duration of the weak lean operation before deterioration is represented by T _A (= t ₁ −t ₀ ).

On the other hand, as shown in FIG. 6B, when the target air-fuel ratio (A / F) is set to the air-fuel ratio r _B and the weak lean operation is started, the SCR is more effective than the case where the air-fuel ratio r _A is set. The decrease rate of the amount of occluded NH ₃ increases. This is because the amount of NOx in the gas leaving the engine by setting the air-fuel ratio r _B is increased. Therefore, at an early time _{t 2} than the time _{t 1,} NSR occluded NOx amount becomes equal to the SCR occlusion NH ₃ amount, stoichiometric operation A / F is set to the stoichiometric is started. In other words, the duration _T B of the weak lean operation after degradation ₍₌ t 2 -t ₀₎ is the duration of the weak lean operation before deterioration is shorter than _{T A.} Therefore, it is possible to obtain effects such as securing the torque and suppressing deterioration of the TWC 16 described above.

[Specific Processing in Second Embodiment]
Next, a specific process for realizing the acceleration required air-fuel ratio control of the present embodiment will be described with reference to FIG. FIG. 7 is a flowchart showing the acceleration request air-fuel ratio control executed by the ECU 30 in the present embodiment. Note that the routine shown in FIG. 7 is repeatedly executed while the engine 10 is in operation.

  In the routine shown in FIG. 7, first, it is determined whether or not there is a request for stoichiometric operation (step 200). The processing in this step is the same as that in step 100 in FIG. If it is determined in step 200 that there is a request for stoichiometric operation, it is determined whether or not the estimated reached bed temperature of the SCR 20 is higher than the set temperature A (step 210). In this step, when it is determined that the bed temperature of the SCR 20 is lower than the set temperature A, a stoichiometric operation is executed (step 220). The reason why such processing is performed is as described in the description of step 120 in FIG.

If it is determined in step 210 that the bed temperature of the SCR 20 is higher than the set temperature A, the NSR 18 is subjected to a deterioration test (step 230). Specifically, the deterioration test is performed according to the following procedure. That is, first, a rich spike for verification is executed, and the detection value of the NH ₃ sensor 24 at that time is acquired. Next, the sensor detection value with respect to the initial value (setting value) is calculated to obtain the NSR deterioration degree.

  Subsequently, the target air-fuel ratio at the time of weak lean operation is calculated (step 240). Specifically, the target air-fuel ratio is obtained by applying the NSR deterioration degree obtained in step 230 to the map data showing the relationship between the NSR deterioration degree and the target air-fuel ratio. It is assumed that this map data is stored in the ECU 30 in advance. The target air-fuel ratio obtained in this step is rewritten as a new weak lean air-fuel ratio. The processing in steps 250 to 310 after rewriting the target air-fuel ratio is the same processing as steps 130 to 190 in FIG.

  As described above, according to the routine shown in FIG. 7, since the air-fuel ratio is changed to the leaner target air-fuel ratio in accordance with the NSR deterioration degree, the duration of the weak lean operation can be shortened. Therefore, in addition to the effects of the first embodiment, it is possible to obtain effects such as securing torque and suppressing deterioration of the TWC 16.

10 Engine 12 Exhaust passage 14 Turbocharger 16 TWC
18 NSR
20 SCR
22 NOx sensor 24 NH ₃ sensor 30 ECU

Claims (5)

It is provided in the exhaust passage of the internal combustion engine, and stores NOx in the exhaust gas when the exhaust air-fuel ratio is in a predetermined lean air-fuel ratio range, and releases the stored NOx when the exhaust air-fuel ratio is in a predetermined rich air-fuel ratio range. An occlusion reduction catalyst that reduces to NH ₃ and
Provided in the exhaust passage downstream of said occlusion reduction type catalyst, thereby occluding and NH ₃ in the exhaust gas, a selective reduction catalyst that reduces NOx in exhaust gas by NH ₃ occluding,
An operation switching request determination means for determining whether or not there is an operation switching request for switching from a lean operation in the predetermined lean air-fuel ratio region to a high exhaust temperature operation;
When it is determined that there is an operation switching request, the lean operation is performed from the lean operation to the high exhaust gas temperature according to the magnitude relationship between the storage NOx amount in the storage reduction catalyst and the storage NH ₃ amount in the selective reduction catalyst. Target air-fuel ratio setting means for setting a target air-fuel ratio during switching to operation;
A control device for an internal combustion engine, comprising: The target air-fuel ratio setting means includes
When the occluded NH ₃ amount is larger than the occluded NOx amount, the target air-fuel ratio is set to a weak lean air-fuel ratio that is richer than the predetermined lean air-fuel ratio region over a predetermined time. Item 2. A control device for an internal combustion engine according to Item 1. The target air-fuel ratio setting means includes
When the occluded NH ₃ amount is smaller than the occluded NOx amount, the target air / fuel ratio setting means temporarily sets the target air / fuel ratio to be rich until the occluded NH ₃ amount becomes larger than the occluded NOx amount. 2. The control apparatus for an internal combustion engine according to claim 1, wherein after that, a weak lean air-fuel ratio that is richer than the predetermined lean air-fuel ratio region is set over a predetermined time.   4. The control apparatus for an internal combustion engine according to claim 2, wherein the degree of enrichment of the weak lean air-fuel ratio is weakened as the degree of deterioration of the selective catalytic reduction catalyst is larger. The target air-fuel ratio setting means includes
2. The control device for an internal combustion engine according to claim 1, wherein when the occluded NH ₃ amount is equal to the occluded NOx amount, the target air-fuel ratio is set in the vicinity of the theoretical air-fuel ratio over a predetermined time.

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