JP5598370B2 - Control device for internal combustion engine - Google Patents

Description

  The present invention relates to a control device for an internal combustion engine with an EGR device including an EGR catalyst and an EGR cooler, and more particularly to a control device for an internal combustion engine in which rich control for enriching an exhaust air-fuel ratio is performed.

  Some of the internal combustion engines for vehicles used today are equipped with an EGR device that recirculates a part of the exhaust gas to the intake side for the purpose of improving the exhaust gas performance. An EGR device disclosed in Japanese Patent Laid-Open No. 2011-000514 includes an EGR valve for controlling the amount of exhaust gas (EGR gas) recirculated from the exhaust passage to the intake passage, and for cooling the EGR gas. An EGR cooler and an EGR catalyst for purifying EGR gas are provided. The EGR cooler is provided in the EGR passage closer to the exhaust passage than the EGR valve, and the EGR catalyst is provided in the EGR passage further to the exhaust passage than the EGR cooler.

JP 2011-000514 A JP 2006-105048 A

The above-mentioned EGR device can also be used for a lean burn engine. However, in the case of a lean burn engine, there is a problem that the EGR cooler is corroded if the EGR device is continuously used. As a result of investigating the cause of corrosion, it was found that NH ₃ generated in the EGR catalyst is involved in corrosion. More specifically, in a lean burn engine, in order to reduce and remove NOx stored in the catalyst, the exhaust air-fuel ratio is periodically enriched. During such rich control of the exhaust air-fuel ratio, a gas having an air-fuel ratio richer than the stoichiometric gas flows into the EGR catalyst. Then, a lot of NH ₃ is generated by the reaction of N ₂ (or NOx) and H ₂ (or HC) in the exhaust gas. FIG. 7 is a graph showing the results of examining how the amount of NH ₃ generated per unit time (the concentration of NH ₃ in the exhaust gas that has passed through the EGR catalyst) varies depending on the air-fuel ratio of the gas flowing into the EGR catalyst. It is. As shown in this graph, when the air-fuel ratio of the gas flowing into the EGR catalyst becomes richer than stoichiometric, the amount of NH ₃ generated increases rapidly. The NH ₃ generated in this manner reacts with Cl contained in the fuel to produce solid NH ₄ Cl. It remains in the passage to form high-concentration strong acid water, thereby promoting corrosion of the EGR cooler.

The present invention has been made in view of the above-described problems, and relates to an internal combustion engine with an EGR device so that exhaust air-fuel ratio rich control can be performed while suppressing generation of NH _{3 that} causes corrosion of an EGR cooler. The purpose is to.

In order to achieve the above object, the present invention provides a plurality of cylinders divided into at least two groups, an exhaust manifold provided for each of the groups, and an exhaust gas obtained by collecting the exhaust manifolds of each group into one. There is a passage, a catalyst arranged in the exhaust passage, and an EGR passage for extracting exhaust gas from a part of the exhaust manifold and recirculating it to the intake passage, and the EGR passage is equipped with an EGR catalyst and an EGR cooler. A control device for an internal combustion engine comprising:
Request acquisition means for acquiring a request to supply exhaust gas having an air-fuel ratio richer than stoichiometric to the catalyst;
When the request is acquired, a first control is performed to control the air-fuel ratio of each cylinder connected to the exhaust manifold so that the exhaust air-fuel ratio in the exhaust manifold to which the EGR passage is connected is close to the stoichiometric or leaner than the stoichiometric. Air-fuel ratio control means,
When the request is acquired, the exhaust air manifold in the exhaust manifold to which the EGR passage is not connected is made richer than stoichiometric, so that the exhaust air / fuel ratio in the exhaust passage becomes richer than stoichiometric. Second air-fuel ratio control means for controlling the air-fuel ratio of each connected cylinder;
Equipped with a,
A plurality of cylinders are connected to the exhaust manifold to which the EGR passage is connected,
The first air-fuel ratio control means controls the air-fuel ratio of some cylinders to be richer than stoichiometric, and controls the air-fuel ratio of the remaining cylinders to be leaner than stoichiometric. Yes.

According to the present invention , when there is a request to supply exhaust gas having an air-fuel ratio richer than stoichiometric to the catalyst, the exhaust air-fuel ratio in the exhaust passage leading to the catalyst is made richer than stoichiometric while the EGR passage is The exhaust air / fuel ratio in the connected exhaust manifold can be made near the stoichiometric or leaner than the stoichiometric. As a result, the air-fuel ratio of the exhaust gas flowing into the EGR catalyst is prevented from becoming richer than stoichiometric, and the generation of NH3 that causes corrosion of the EGR cooler is suppressed. Further, since a mixture of an exhaust gas having an air-fuel ratio richer than stoichiometric gas and an exhaust gas having a lean air-fuel ratio flows into the EGR catalyst, an exothermic reaction in the EGR catalyst becomes intense. Thereby, the bed temperature of the EGR catalyst rises and the generation of NH3 is further suppressed.

It is a figure which shows the structure of the lean burn engine with an EGR apparatus to which the control apparatus of Embodiment 1 of this invention is applied. It is a flowchart which shows the content of the rich control performed in Embodiment 1 of this invention. It is a figure which shows the air fuel ratio profile at the time of the rich control performed in Embodiment 1 of this invention. It is a flowchart which shows the content of the rich control performed in Embodiment 2 of this invention. It is a figure which shows the air fuel ratio profile at the time of the rich control performed in Embodiment 2 of this invention. It is a graph showing the relationship between the generated amount of bed temperature and NH ₃ in the EGR catalyst. It is a graph showing the relationship between the generation of the exhaust air-fuel ratio and the NH _3.

Embodiment 1 FIG.
Embodiment 1 of the present invention will be described below with reference to the drawings.

  The internal combustion engine to which the control device of Embodiment 1 of the present invention is applied is a spark ignition type gasoline engine capable of lean burn operation. It is a turbocharged engine equipped with a twin entry turbocharger. Furthermore, it is also an engine with an EGR device that can recirculate a part of the exhaust gas to the intake side.

  FIG. 1 is a schematic diagram showing a configuration of an internal combustion engine to which a control device according to Embodiment 1 of the present invention is applied. The internal combustion engine according to the present embodiment includes four cylinders in the engine body 2. The four cylinders included in the engine body 2 are a cylinder group consisting of a first cylinder (# 1) and a fourth cylinder (# 4), and a cylinder group consisting of a second cylinder (# 2) and a third cylinder (# 3). It is divided into and.

  An exhaust manifold is provided for each cylinder group. An exhaust manifold 22 is connected to the group of the first cylinder and the fourth cylinder, and an exhaust manifold 24 is connected to the group of the second cylinder and the third cylinder. The two exhaust manifolds 22 and 24 are connected to the turbine 8 of the turbocharger 4 and are assembled together in the turbine 8. An exhaust passage 20 is connected to the outlet side of the turbine 8. A catalyst 26 is provided in the middle of the exhaust passage 20.

  On the other hand, the compressor 6 of the turbocharger 4 is disposed in the intake passage 10. An intercooler 18 is attached downstream of the compressor 6 in the intake passage 10, and a throttle 16 is disposed further downstream thereof. An intake manifold 14 is provided downstream of the throttle 16 in the intake passage 10, and the intake manifold 14 is connected to each cylinder of the engine body 2.

  The internal combustion engine according to the present embodiment is equipped with an EGR device 30 that recirculates exhaust gas from the exhaust system to the intake system. The EGR device 30 includes an EGR passage 32 that connects the exhaust manifold 24 and the intake manifold 14 of the second and third cylinder groups, an EGR valve 34 that is disposed in the EGR passage 32, an EGR cooler 36, and an EGR catalyst 38. Therefore, it is comprised. The EGR valve 34 is disposed at a position closest to the intake manifold 14, and the EGR catalyst 38 is disposed at a position close to the exhaust manifold 24. The EGR cooler 36 is disposed between the EGR valve 34 and the EGR catalyst 38.

  The operation of the internal combustion engine according to the present embodiment is controlled by an ECU (Electronic Control Unit) (not shown). The ECU periodically performs rich control of the air-fuel ratio during the lean burn operation of the internal combustion engine. The rich control is a process for reducing and removing NOx stored in the catalyst 26 during the lean burn operation (so-called rich spike), or a process for removing sulfur components adhering to the catalyst 26 (so-called S purge). The air-fuel ratio control of the cylinders controls the air-fuel ratio of each cylinder so that the air-fuel ratio of the exhaust gas supplied to the catalyst 26 becomes richer than the stoichiometry. In the conventional rich control, the air-fuel ratios of all the cylinders are uniformly enriched. However, in the rich control according to the present embodiment, the following air-fuel ratio control is performed for each cylinder group.

  FIG. 2 is a flowchart showing the contents of the rich control executed in the present embodiment. In the first step S11, it is determined whether or not there is a request to supply the catalyst 26 with rich gas (exhaust gas whose air-fuel ratio is richer than stoichiometric). This determination is performed even when the conventional rich control is performed. Specifically, it is determined whether or not the conditions for performing the rich spike and the S purge are satisfied.

  If the determination result of step S11 is affirmative, the process of step S12 is performed. The content performed in this step corresponds to the content of rich control according to the present embodiment. In this step, the group of the second cylinder and the third cylinder controls its air-fuel ratio to stoichiometric, and the group of the first cylinder and the fourth cylinder controls its air-fuel ratio stronger and richer than stoichiometric. The group of the second cylinder and the third cylinder is a cylinder group in which the exhaust manifold 24 is connected to the EGR passage 32. By stopping the exhaust air-fuel ratio of this group from being stoichiometric without being enriched, it is possible to prevent the rich gas from flowing into the EGR catalyst 38. Further, since the exhaust manifolds 22 of the groups of the first cylinder and the fourth cylinder are not connected to the EGR passage 32, even if the air-fuel ratio is greatly enriched, the rich gas does not flow through the EGR catalyst 38.

  The contents of the process performed in step S12 are shown in more detail in FIG. FIG. 3 is a diagram showing an air-fuel ratio profile during rich control executed in the present embodiment. 3 shows the air-fuel ratio profile of the exhaust gas exhausted from the second cylinder and the third cylinder. The middle stage shows the air-fuel ratio profile of the exhaust gas discharged from the first cylinder and the fourth cylinder. In the lowermost stage, an air-fuel ratio profile of the exhaust gas flowing into the catalyst 26 is shown. Further, in the figure, the target air-fuel ratio at the time of rich control is shown in contrast with stoichiometry. As shown in this figure, the air-fuel ratio control is performed on the second cylinder and the third cylinder so that the air-fuel ratio of the exhaust gas discharged from them becomes stoichiometric. On the other hand, with respect to the first cylinder and the fourth cylinder, the air-fuel ratio control is performed so that the air-fuel ratio of the exhaust gas discharged from them becomes further richer than the target air-fuel ratio at the time of rich control. More specifically, the air-fuel ratio after the exhaust gases in the two exhaust manifolds 22 and 24 are mixed, that is, the air-fuel ratio of the exhaust gas flowing into the catalyst 26 is matched with the target air-fuel ratio at the time of rich control. Air-fuel ratio control of the first cylinder and the fourth cylinder is performed.

According to such rich control, the air-fuel ratio of the exhaust gas flowing into the EGR catalyst 38 is prevented from becoming richer than the stoichiometric gas while supplying the exhaust gas having an air-fuel ratio richer than the stoichiometric gas to the catalyst 26. can do. As can be seen from the relationship between the exhaust air-fuel ratio and the amount of generated NH ₃ shown in the graph of FIG. 7, if the air-fuel ratio of the exhaust gas flowing into the EGR catalyst 38 is near the stoichiometric or leaner than the stoichiometric, NH ₃ is generated. The amount can be kept low. Therefore, according to the present embodiment, NH ₃ that causes corrosion of the EGR cooler 36 is suppressed from being generated in the EGR catalyst 38 while achieving the original purpose of rich control such as rich spike and S purge. Can do.

Embodiment 2. FIG.
Next, a second embodiment of the present invention will be described with reference to the drawings.

  The control device of the second embodiment of the present invention is applied to the internal combustion engine having the configuration shown in FIG. 1 as in the case of the first embodiment. The difference between the present embodiment and the first embodiment is that there is a difference in the function of the ECU as a control device, more specifically, the content of rich control performed during lean burn operation.

  The content of the rich control executed in the present embodiment is shown by the flowchart in FIG. In the first step S21, it is determined whether or not there is a request to supply the rich gas to the catalyst 26. If the result of this determination is affirmative, the process of step S22 is performed. In this step, the air-fuel ratio of the second cylinder is controlled to be slightly richer than stoichiometric, the air-fuel ratio of the third cylinder is controlled to be slightly leaner than stoichiometric, and the first and fourth cylinder groups are The fuel ratio is controlled to be stronger and richer than stoichiometric.

  The details of the process performed in step S22 are shown in more detail in FIG. FIG. 5 is a view showing an air-fuel ratio profile during rich control executed in the present embodiment. 5 shows the air-fuel ratio profile of the exhaust gas discharged from the second cylinder, and the next stage shows the air-fuel ratio profile of the exhaust gas discharged from the third cylinder. The third stage shows the air-fuel ratio profile of the exhaust gas discharged from the first cylinder and the fourth cylinder. In the lowermost stage, an air-fuel ratio profile of the exhaust gas flowing into the catalyst 26 is shown. Further, in the figure, the target air-fuel ratio at the time of rich control is shown in contrast with stoichiometry. As shown in this figure, regarding the second and third cylinders, the air-fuel ratio of the exhaust gas discharged from one of them is richer than the stoichiometric, and the air-fuel ratio of the exhaust gas discharged from the other is higher than the stoichiometric. The air-fuel ratio control is performed so that the average of both becomes lean while leaning. On the other hand, with respect to the first cylinder and the fourth cylinder, the air-fuel ratio control is performed so that the air-fuel ratio of the exhaust gas discharged from them becomes further richer than the target air-fuel ratio at the time of rich control. More specifically, the air-fuel ratio after the exhaust gases in the two exhaust manifolds 22 and 24 are mixed, that is, the air-fuel ratio of the exhaust gas flowing into the catalyst 26 is matched with the target air-fuel ratio at the time of rich control. Air-fuel ratio control of the first cylinder and the fourth cylinder is performed.

According to such rich control, exhaust gas slightly richer than stoichiometric and exhaust gas slightly leaner than stoichiometric are exhausted alternately into the exhaust manifold 24 to which the second and third cylinders are connected. Then, a mixture of them flows into the EGR catalyst 38 through the EGR passage 32. The air-fuel ratio of the exhaust gas flowing into the EGR catalyst 38 is an average of the air-fuel ratio of the exhaust gas discharged from the second cylinder and the air-fuel ratio of the exhaust gas discharged from the third cylinder, which is almost stoichiometric. Therefore, the amount of NH ₃ generated in the EGR catalyst 38 can be kept low.

Further, in the case of the present embodiment, the exhaust gas flowing into the EGR catalyst 38 is a mixed gas of rich gas and lean gas. Therefore, compared with the case of the first embodiment, the exhaust gas contains more unburned HC. And oxygen. For this reason, the exothermic reaction occurring in the EGR catalyst 38 becomes intense, and the bed temperature of the EGR catalyst 38 rises. FIG. 6 is a graph showing the results of examining how the amount of NH ₃ generated per unit time (the concentration of NH ₃ in the exhaust gas that has passed through the EGR catalyst) varies depending on the bed temperature of the EGR catalyst 38. As shown in this graph, a large amount of NH ₃ is generated in the temperature range from 300 ° C. to 700 ° C. If the temperature is higher than that, the amount of NH ₃ generated can be kept low. Therefore, according to the present embodiment, it is possible to further suppress the generation of NH ₃ by promoting an exothermic reaction on the EGR catalyst 38 and increasing the bed temperature of the EGR catalyst 38. That is, according to the present embodiment, NH ₃ that causes corrosion of the EGR cooler 36 is further suppressed from being generated in the EGR catalyst 38 while achieving the original purpose of rich control such as rich spike and S purge. be able to.

Others.
The present invention is not limited to the above-described embodiment, and various modifications can be made without departing from the spirit of the present invention. For example, the following modifications can be made.

  In the first embodiment, the air-fuel ratio of the second cylinder and the third cylinder during the rich control may be leaner than the stoichiometry. For example, regarding the second cylinder and the third cylinder, the air-fuel ratio at the time of lean burn operation may be maintained as it is. However, in order to set the exhaust air-fuel ratio of the catalyst inflow gas to the target air-fuel ratio at the time of rich control, the air-fuel ratio of the first cylinder and the fourth cylinder becomes more lean as the air-fuel ratio of the second cylinder and the third cylinder becomes leaner. Must be richer. In that case, if the air-fuel ratio is excessively enriched, the combustion limit is exceeded and misfire occurs. Therefore, the air-fuel ratio that can be taken by the second cylinder and the third cylinder has a lean limit determined by the target air-fuel ratio at the time of rich control and the combustion limit air-fuel ratio of the first cylinder and the fourth cylinder. As long as it is within the range from the lean limit to the stoichiometric range, the air-fuel ratio of the second cylinder and the third cylinder during the rich control can be arbitrarily set.

  In the second embodiment, the average of the air-fuel ratio of the second cylinder and the air-fuel ratio of the third cylinder may be leaner than the stoichiometry. However, if it is desired to cause an exothermic reaction in the EGR catalyst 38 and thereby increase the bed temperature of the EGR catalyst 38, it is desirable that the air-fuel ratio of either the second cylinder or the third cylinder is richer than the stoichiometry. . As for the cylinders that make the air-fuel ratio rich among the second cylinder and the third cylinder, as long as the average of the air-fuel ratios of the second and third cylinders is leaner than the stoichiometry, it reaches the limit of the combustion limit. Can be rich.

  The internal combustion engine shown in FIG. 1 is a supercharged engine equipped with a twin-entry turbocharger, but the present invention can also be applied to a naturally aspirated engine. Further, the present invention can be applied not only to a spark ignition type gasoline engine but also to a diesel engine.

2 Engine body 4 Turbocharger 10 Intake passage 14 Intake manifold 16 Throttle 18 Intercooler 20 Exhaust passage 22 Exhaust manifold (# 1, # 4 side)
24 Exhaust manifold (# 2, # 3 side)
26 Catalyst 30 EGR device 32 EGR passage 34 EGR valve 36 EGR cooler 38 EGR catalyst

Claims (1)

A plurality of cylinders divided into at least two groups;
An exhaust manifold provided for each of the groups;
An exhaust passage formed by collecting the exhaust manifolds of each group into one;
A catalyst disposed in the exhaust passage;
An EGR device having an EGR passage for extracting exhaust gas from a part of the exhaust manifold and recirculating it to the intake passage, and equipped with an EGR catalyst and an EGR cooler in the EGR passage;
In a control device for an internal combustion engine comprising:
Request acquisition means for acquiring a request to supply exhaust gas having an air-fuel ratio richer than stoichiometric to the catalyst;
When the request is acquired, a first control is performed to control the air-fuel ratio of each cylinder connected to the exhaust manifold so that the exhaust air-fuel ratio in the exhaust manifold to which the EGR passage is connected is close to the stoichiometric or leaner than the stoichiometric. Air-fuel ratio control means,
When the request is acquired, the exhaust air manifold in the exhaust manifold to which the EGR passage is not connected is made richer than stoichiometric, so that the exhaust air / fuel ratio in the exhaust passage becomes richer than stoichiometric. Second air-fuel ratio control means for controlling the air-fuel ratio of each connected cylinder;
Equipped with a,
A plurality of cylinders are connected to the exhaust manifold to which the EGR passage is connected,
The first air-fuel ratio control means controls the air-fuel ratio of some cylinders to be richer than stoichiometric, and controls the air-fuel ratio of the remaining cylinders to be leaner than stoichiometric. A control device for an internal combustion engine.

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