JP4853297B2 - Exhaust system for internal combustion engine - Google Patents

Description

  The present invention relates to an exhaust system applied to an internal combustion engine provided with an exhaust gas recirculation device that introduces a part of exhaust gas into an intake passage as EGR gas.

  As an exhaust gas purification device that purifies exhaust gas from internal combustion engines, filters that collect particulates in the exhaust gas and purify the exhaust gas, store nitrogen oxide (NOx) in the exhaust gas, and reduce and purify the stored NOx. Such a NOx storage reduction catalyst is widely known.

  In the case of a filter, when the amount of collected particulates increases, clogging occurs, and the collection function, that is, the exhaust purification function deteriorates. Therefore, in order to recover the function, a filter regeneration process is performed in which the particulates captured are burned by injecting fuel into the exhaust passage to raise the bed temperature of the particulates. Further, in the case of a NOx catalyst, the NOx purification function deteriorates as the NOx occlusion amount increases. Therefore, in order to reduce and purify the stored NOx, fuel is injected into the exhaust passage to temporarily enrich the exhaust air-fuel ratio. Rich spike processing is performed, and the purification function deteriorated thereby is recovered. Further, in the case of a NOx catalyst, a sulfur poisoning regeneration process is performed in which fuel is injected into the exhaust passage in order to recover the purification function deteriorated by sulfur poisoning. These processes for injecting fuel into the exhaust passage are necessary to restore the purification function of the exhaust purification device.

  In an internal combustion engine provided with such an exhaust purification device, an exhaust gas recirculation device that takes out a part of exhaust gas as EGR gas from the exhaust passage and introduces it into the intake passage in order to lower the combustion temperature and reduce the NOx emission amount. Is often installed. For example, an exhaust system provided with an exhaust gas recirculation device that extracts EGR gas from the downstream of a filter and introduces it into an intake passage is known (Patent Document 1). In the filter regeneration process, particulates burn in the filter, so that the oxygen concentration of the gas that has passed through the filter decreases and carbon dioxide increases. That is, since the component ratio of EGR gas differs depending on whether or not the filter is being regenerated, if the EGR gas having the same flow rate is introduced into the intake passage without considering whether or not the filter is being regenerated, the introduction of EGR gas when the filter is regenerated is introduced. As a result, oxygen deficiency and carbon dioxide increase will cause combustion fluctuations such as misfire. In the system of Patent Document 1, the flow rate of EGR gas is controlled in accordance with the oxygen concentration downstream of the filter and the excess air ratio in order to reduce the EGR flow rate during filter regeneration. In addition, there is Patent Document 2 as a prior art document related to the present invention.

JP 2005-069207 A Japanese Patent Laid-Open No. 2001-234772

  By the way, as in Patent Document 1, an exhaust gas recirculation device that extracts EGR gas from the downstream of an exhaust gas purification means such as a filter, and EGR gas is extracted from a position that is not affected by the fuel injection into the exhaust passage and introduced into the intake passage. The exhaust gas recirculation device is used in combination, and the mode in which EGR gas is introduced using both exhaust gas recirculation devices and the mode in which EGR gas is introduced using either one of the exhaust gas recirculation devices are used depending on the situation. Exhaust systems are being considered. In such an exhaust system, the introduction of EGR gas using the former exhaust gas recirculation device at the time of fuel injection into the exhaust passage such as when the filter is regenerated in order to suppress the influence of fuel injection into the exhaust passage during the introduction of EGR gas. It is necessary to control the flow rate of the EGR gas so as to maintain the target EGR rate set according to the operating state of the internal combustion engine using the latter exhaust gas recirculation passage. In order to accurately perform such control, it is assumed that the fuel injected into the exhaust passage is accurately grasped. Since the fuel addition means for injecting fuel into the exhaust passage naturally has a range in which the injection accuracy can be maintained due to structural limitations, an amount of fuel smaller than the lower limit of the range cannot be accurately injected.

  Therefore, an object of the present invention is to provide an exhaust system for an internal combustion engine that can realize accurate control of the EGR rate by maintaining the injection accuracy of the fuel addition means that injects fuel into the exhaust passage.

An exhaust system for an internal combustion engine according to the present invention is arranged in an exhaust passage of the internal combustion engine to purify exhaust, and injects fuel into the exhaust passage to restore the exhaust purification function of the exhaust purification means. A fuel addition means and an exhaust gas recirculation device that extracts part of the exhaust gas from the exhaust passage as EGR gas and introduces the exhaust gas into the intake passage of the internal combustion engine are provided. A first exhaust gas recirculation device located downstream of each of the purifying means and the fuel addition device, and a second exhaust gas recirculation device whose EGR gas take-out position is located upstream of the fuel addition device are provided. Each of the first exhaust gas recirculation device and the second exhaust gas recirculation device so that the target EGR rate set according to the operating state of the internal combustion engine is maintained. An internal combustion engine exhaust system for controlling the required fuel amount calculation means for calculating the required fuel amount to be injected by the fuel addition means so that the larger the gas flow rate passing through the exhaust purification means, While the exhaust purification function of the exhaust purification unit needs to be restored, the fuel addition unit is controlled so that the fuel of the required fuel amount calculated by the required fuel amount calculation unit is injected into the exhaust passage. When the required fuel amount calculated by the fuel addition control means and the required fuel amount calculation means is less than the lower limit amount of fuel that can be injected while maintaining the injection accuracy, the fuel addition means passes through the exhaust purification means. A distribution of the gas flow rate of the EGR gas introduced by the first exhaust gas recirculation device and the gas flow rate of the EGR gas introduced by the second exhaust gas recirculation device is set so as to increase the gas flow rate. And exhaust gas recirculation control means for changing while maintaining a R rate, to solve the problems described above by providing a (claim 1).

  According to the exhaust system of the present invention, when the required fuel amount calculated as the amount of fuel to be injected by the fuel addition means is smaller than the lower limit amount that the fuel addition means can inject while maintaining the injection accuracy, the exhaust purification means The gas flow rate passing through is controlled to increase. Since the required fuel amount is calculated to increase as the gas flow rate passing through the exhaust purification unit increases, the required fuel amount increases as the gas flow rate through the exhaust purification unit increases. As a result, the required fuel amount is kept above the lower limit amount, so that the injection accuracy by the fuel addition means is maintained. When increasing the flow rate of gas passing through the exhaust gas purification means, the gas flow rate of EGR gas introduced by the first exhaust gas recirculation device and the gas flow rate of EGR gas introduced by the second exhaust gas recirculation device while maintaining the target EGR rate. Therefore, the EGR rate can be accurately controlled.

One embodiment of an exhaust system of the present invention, the exhaust gas recirculation control means, said whether there is a possibility that the combustion variation is caused by the misfire unacceptable in an internal combustion engine with the fuel addition by the fuel addition means When the determination is made based on a physical quantity that affects the combustion fluctuation included in the EGR gas introduced by the first exhaust gas recirculation apparatus, and when it is determined that there is no possibility of causing the combustion fluctuation, the exhaust purification means passes. The target EGR rate is maintained by distributing the gas flow rate of the EGR gas introduced by the first exhaust gas recirculation device and the gas flow rate of the EGR gas introduced by the second exhaust gas recirculation device so that the gas flow rate to be increased increases. However, it may be changed (claim 2). According to this aspect, on the condition that unacceptable combustion fluctuations are not caused, the gas flow rate passing through the exhaust gas purification means is controlled to increase. Can be prevented. Accordingly, it is possible to maintain an appropriate EGR rate while suppressing combustion fluctuations accompanying fuel injection by the fuel addition means. The physical quantity that affects the combustion fluctuation is not particularly limited, and for example, the gas flow rate of carbon dioxide contained in the EGR gas introduced by the first exhaust gas recirculation device can be used as the physical quantity that affects the combustion fluctuation. .

  In this aspect, when the exhaust gas recirculation control means determines that the combustion fluctuation is likely to occur, the first exhaust gas recirculation apparatus is configured so that the gas flow rate passing through the exhaust gas purification means decreases. The distribution of the gas flow rate of the introduced EGR gas and the gas flow rate of the EGR gas introduced by the second exhaust gas recirculation device may be changed while maintaining the target EGR rate (Claim 3). According to this, the distribution of the gas flow rate of the EGR gas introduced by the second exhaust gas recirculation device is relatively increased, while the distribution of the gas flow rate of the EGR gas introduced by the first exhaust gas recirculation device is relatively increased. Therefore, combustion fluctuations can be suppressed while the target EGR rate is maintained.

As one aspect of the exhaust system of the present invention, the fuel addition control means causes the same amount of fuel as the required fuel amount calculated by the required fuel amount calculating means to be injected into the exhaust passage at a predetermined frequency. The fuel adding means is controlled, and when the required fuel amount calculated by the required fuel amount calculating means is smaller than the lower limit amount of fuel that the fuel adding means can inject while maintaining the injection accuracy, The gas flow rate of EGR gas introduced by the first exhaust gas recirculation device with respect to the sum of the gas flow rate of EGR gas introduced by the first exhaust gas recirculation device and the gas flow rate of EGR gas introduced by the second exhaust gas recirculation device ratio further comprising a permissible limit determining means for determining whether the or combustion fluctuation due to a misfire in the internal combustion engine is within acceptable limits without the occurrence of the fuel addition control means, wherein at the allowable limit determination means On condition that if it is determined to be within the acceptable limits, the fuel addition as the fuel of the lower limit amount or more of injection injection amount is injected into the exhaust passage at a lower frequency than the predetermined frequency The means may be controlled (claim 4). According to this aspect, the EGR gas introduced by the first exhaust gas recirculation device with respect to the sum of the gas flow rate of the EGR gas introduced by the first exhaust gas recirculation device and the gas flow rate of the EGR gas introduced by the second exhaust gas recirculation device. On the condition that the ratio of the gas flow rate is within an allowable limit that does not cause combustion fluctuations, the fuel injected by the fuel adding means is increased beyond the lower limit amount that can maintain the injection accuracy. Thereby, the injection accuracy by the fuel addition means can be maintained while preventing combustion fluctuations. In addition, since the injection amount by the fuel addition means is directly increased, there is an advantage that the response is faster than when the required fuel amount is increased by increasing the gas flow rate passing through the exhaust purification means.

  As described above, according to the present invention, when the required fuel amount calculated as the fuel amount to be injected by the fuel adding means is smaller than the lower limit amount that the fuel adding means can inject while maintaining the injection accuracy, Since the gas flow rate passing through the exhaust purification unit is controlled to increase, the required fuel amount increases as the gas flow rate passing through the exhaust purification unit increases. As a result, the required fuel amount is kept above the lower limit amount, so that the injection accuracy by the fuel addition means is maintained. When increasing the flow rate of gas passing through the exhaust gas purification means, the gas flow rate of EGR gas introduced by the first exhaust gas recirculation device and the gas flow rate of EGR gas introduced by the second exhaust gas recirculation device while maintaining the target EGR rate. Therefore, the EGR rate can be accurately controlled.

(First form)
FIG. 1 shows a main part of an internal combustion engine to which an exhaust system according to an embodiment of the present invention is applied. The internal combustion engine 1 is mounted on a vehicle (not shown) as a driving power source, and is configured as an in-line four-cylinder diesel engine in which four cylinders 2 are arranged in a row. An intake passage 3 and an exhaust passage 4 are connected to each cylinder 2. The intake passage 3 is provided with an air flow meter 5 that outputs a signal corresponding to the air flow rate, a throttle valve 6, a compressor 7a of a turbocharger 7, and an intercooler 8 that cools the intake air. On the other hand, the exhaust passage 4 is provided with a turbine 7 b of the turbocharger 7 and an exhaust purification device 9. In FIG. 1, the compressor 7a and the turbine 7b of the turbocharger 7 are shown separately for convenience of illustration, but these are configured to rotate integrally. The turbocharger 7 is a known variable capacity turbocharger having a variable nozzle (not shown) on the turbine 7b side. Although a detailed description is omitted, the turbocharger 7 can appropriately adjust the supercharging pressure by changing the opening of the variable nozzle according to the operating state of the internal combustion engine 1.

  The exhaust purification device 9 includes a casing 10 that forms part of the exhaust passage 4, and an oxidation catalyst 11 and a filter 12 that are accommodated in the casing 10. The filter 12 has a porous base material (not shown) capable of collecting particulates in the exhaust. Thus, the exhaust purification device 9 has an exhaust purification function of collecting particulates by the filter 12. When the amount of collected particulates reaches the limit, the filter 12 is clogged, and the exhaust purification device 9 impairs the exhaust purification function. Therefore, in order to restore the purification function of the exhaust purification device 9 before reaching its limit, a fuel addition valve 13 is provided as a fuel addition means for injecting fuel into the exhaust passage 4. The fuel addition valve 13 is a so-called needle type electromagnetic valve, and the range in which the injection accuracy can be maintained is determined due to structural limitations. The fuel addition valve 13 is disposed downstream of the turbine 7 b of the turbocharger 7 and upstream of the exhaust purification device 9. When fuel is injected from the fuel addition valve 13, the oxidation catalyst 11 generates heat, and the bed temperature of the filter 12 rises due to the heat. Thereby, the particulates collected by the filter 12 are burned, and the purification function of the exhaust purification device 9 is restored. Such an operation is a well-known operation called a filter regeneration process.

  As shown in FIG. 1, a low-pressure exhaust gas recirculation device 14 as an exhaust gas recirculation device and a high-pressure exhaust gas recirculation device are provided in the internal combustion engine 1 in order to extract part of the exhaust gas from the exhaust passage 4 as EGR gas and introduce it into the intake passage 3. 18 are provided. The low-pressure exhaust gas recirculation device 14 corresponds to the first exhaust gas recirculation device according to the present invention, and the high-pressure exhaust gas recirculation device 18 corresponds to the second exhaust gas recirculation device according to the present invention. The low-pressure exhaust gas recirculation device 14 includes a low-pressure EGR passage 15 that connects the exhaust passage 4 and the intake passage 3, an EGR cooler 16 that cools EGR gas that passes through the low-pressure EGR passage 15, and a flow rate of EGR gas that flows through the low-pressure EGR passage 15. Is provided with a low pressure EGR valve 17 that adjusts the opening degree by changing the opening. A connection position between the low pressure EGR passage 15 and the exhaust passage 4, that is, an EGR gas extraction position 15 a is set on the downstream side of the exhaust purification device 9, and a connection position 15 b between the low pressure EGR passage 15 and the intake passage 3 is downstream of the throttle valve 6. And on the upstream side of the compressor 7a of the turbocharger 7. Thereby, the low pressure exhaust gas recirculation device 14 takes out the EGR gas from the downstream sides of the exhaust purification device 9 and the fuel addition valve 13, and introduces the EGR gas upstream of the compressor 7a. The low pressure exhaust gas recirculation device 14 that recirculates exhaust gas through such a route is known as a low pressure loop type exhaust gas recirculation device.

  On the other hand, the high-pressure exhaust gas recirculation device 18 includes a high-pressure EGR passage 19 that connects the exhaust passage 4 and the intake passage, and a high-pressure EGR valve 20 that adjusts the flow rate of EGR gas flowing through the high-pressure EGR passage 19 by changing the opening. Yes. Note that an EGR cooler that cools the EGR gas may be provided in the high-pressure EGR passage 19 as in the low-pressure exhaust gas recirculation device 14. The connection position between the high-pressure EGR passage 19 and the exhaust passage 4, that is, the EGR gas take-out position 19a is set on the upstream side of the turbine 7b and the fuel addition valve 13 of the turbocharger 7, and the high-pressure EGR passage 19 and the intake passage 3 The connection position 19b is set on the downstream side of the compressor 7a of the turbocharger 7. As a result, the high-pressure exhaust gas recirculation device 18 takes out the EGR gas from the upstream side of the fuel addition valve 13 and introduces the EGR gas downstream of the compressor 7a. Therefore, there is no possibility that the fuel injected by the fuel addition valve 18 will enter the intake passage 3. The high-pressure exhaust gas recirculation device 18 that recirculates exhaust gas through such a path is known as a high pressure loop type exhaust gas recirculation device.

  The operations of the fuel addition valve 13, the low pressure exhaust gas recirculation device 14, and the high pressure exhaust gas recirculation device 18 are controlled by an engine control unit (ECU) 21 that is a computer provided to properly control the internal combustion engine 1. The ECU 21 includes a microprocessor and a storage device such as a ROM and a RAM necessary for its operation. The ECU 21 includes a signal NE related to the rotational speed (rotational speed) of the internal combustion engine 1 in addition to a signal from the air flow meter 5, exhaust gas purification. Various signals such as a signal DP relating to the differential pressure across the apparatus 9, a signal TO relating to the opening of the throttle valve 6, a signal EL relating to the opening of the low pressure EGR valve 17, and a signal EH relating to the opening of the high pressure EGR valve 20 are input. The ECU 21 controls various parameters such as the fuel injection amount and fuel injection timing supplied to each cylinder 2 of the internal combustion engine 1 according to various programs held in the storage device while referring to these various signals. Although the control performed by the ECU 21 is diverse, here, the control related to the present invention will be mainly described.

  Execution and prohibition of the introduction of EGR gas into the intake passage 3 are determined according to the operating state of the internal combustion engine 1. FIG. 2 is a conceptual diagram showing an example of a mode selection map in which a plurality of types of modes for executing the introduction of EGR gas are associated with operating states. As shown in this figure, when the introduction of EGR gas is executed, the ECU 21 determines the low-pressure exhaust gas recirculation device 14 according to the operating state of the internal combustion engine 1, here according to the rotational speed and load of the internal combustion engine 1. LPL mode in which EGR gas is introduced using only the gas, HPL mode in which EGR gas is introduced using only the high-pressure exhaust gas recirculation device 18, and EGR gas using both of these exhaust gas recirculation devices 14, 18. An appropriate mode is selected from the MPL modes to be introduced.

  Regardless of which mode is selected, the low pressure exhaust gas recirculation device is controlled while the ECU 21 controls the opening degree of the throttle valve 6 so as to be maintained at the target EGR rate set according to the operating state of the internal combustion engine 1. 14 and the high-pressure exhaust gas recirculation device 18 are controlled. As is well known, the EGR rate is defined as the ratio of EGR gas to the intake gas sucked into each cylinder 2. Since the LPL mode and the HPL mode are the same as the known EGR rate control except that either the low pressure EGR 17 valve or the high pressure EGR valve 20 is fully closed, the description is omitted here, and the control in the MPL mode is mainly described below. Explained.

  FIG. 3 is a flowchart showing an example of a control routine of the filter regeneration process, and FIG. 4 is a flowchart showing an example of an operation control routine of the low pressure exhaust gas recirculation device 14 and the high pressure exhaust gas recirculation device 18 in the MPL mode. These control routine programs are repeatedly executed at a predetermined calculation cycle. As shown in FIG. 3, the ECU 21 determines the necessity of the filter regeneration process in step S1. This determination is performed based on a physical quantity that correlates with the amount of patty cue rate collected by the filter 12. For example, the necessity of the filter regeneration process can be determined based on the differential pressure across the exhaust purification device 9 including the filter 12 or the travel distance of the vehicle on which the internal combustion engine 1 is mounted. When the filter regeneration process is necessary, the process proceeds to step S2, and a regeneration request flag F1 for managing the necessity of the filter regeneration process is set, that is, F1 = 1. The regeneration request flag F1 is assigned to a predetermined area of a storage device built in the ECU 21. On the other hand, if the filter regeneration process is not necessary, the process proceeds to step S3, where the regeneration request flag F1 is cleared, that is, F1 = 0, and this routine is finished.

  In step S4, the gas flow rate Gcat passing through the exhaust purification device 9 is acquired. Gcat may be obtained directly from detection means such as a flow meter. Also, the intake air flow rate is Ga, the flow rate of EGR gas passing through the low pressure EGR passage 15 is Glpl, the gas flow rate of gas discharged from the fuel injected into the cylinder 2 into the exhaust passage 4 is Gfuel, and fuel addition When the gas flow rate of the gas derived from the fuel injected by the valve 13 is Gpstfuel, Gcat may be obtained by calculation based on the following equation (1). Of these physical quantities, the intake air flow rate Ga can be detected by the air flow meter 5. The gas flow rate Glpl, the gas flow rate Gfuel, and the gas flow rate Gpstfuel are the rotational speed of the internal combustion engine 1, the opening degree of the low pressure EGR valve 17, the opening degree of the high pressure EGR valve 20, the opening degree of the throttle valve 5, the bed temperature of the filter 12, etc. It can be derived by a predetermined arithmetic expression based on various physical quantities.

      Gcat = Ga + Glpl + Gfuel + Gpstfuel (1)

  Next, in step S5, the ECU 21 calculates the required fuel amount Q to be injected by the fuel addition valve 13 based on the difference between the gas flow rate Gcat and the current bed temperature of the filter 12 and the target bed temperature. When it is assumed that the difference between the bed temperature of the filter 12 and the target bed temperature is the same, the ECU 21 calculates the required fuel amount Q so as to increase as the gas flow rate Gcat increases. Next, in step S <b> 6, the ECU 21 determines whether it is the fuel injection timing by the fuel addition valve 13. This injection timing is managed by the crank angle of the internal combustion engine 1, and the injection timing is set to arrive at every 720 ° C. If it is not the injection timing, the subsequent processing is skipped and the current routine is terminated. If it is the injection timing, the process proceeds to step S7, and it is determined whether or not the regeneration execution flag F2 is set. The regeneration execution flag F2 is set in the routine of FIG. 4 to be described later, and manages whether fuel injection by the fuel addition valve 13 can be executed. The regeneration execution flag F2 is assigned to a predetermined area of a storage device built in the ECU 21. When the regeneration execution flag F2 is set, the routine proceeds to step S8, where the fuel addition valve 13 is controlled so that the fuel of the same amount as the required fuel amount Q is injected from the fuel addition valve 13. On the other hand, if the regeneration execution flag F2 is not set, fuel injection is not performed, and the subsequent routine is terminated by skipping the subsequent processing.

  Next, at step S9, the regeneration execution flag F2 is cleared (F2 = 0), the required fuel amount Q is cleared, and the current routine is terminated. Note that, unless the required fuel amount Q is cleared, the integrated value is calculated as the required fuel amount Q in step S5.

  By repeatedly executing the routine of FIG. 3, filter regeneration processing is performed as necessary, and the purification function of the exhaust purification device 9 is restored.

  Next, the control routine of FIG. 4 will be described. First, the ECU 21 sets a target EGR rate in step S21. The target EGR rate is appropriately set according to the operating state of the internal combustion engine 1. Next, in step S22, it is determined whether or not the above-described reproduction request flag F1 is set. If the regeneration request flag F1 is not set, the filter regeneration process is not executed, and no fuel is injected from the fuel addition valve 13, so the routine proceeds to step S23 and normal control is executed. This normal control is performed as follows. First, the gas flow rate of the EGR gas that provides the target EGR rate set in step S21 is determined based on the intake air flow rate Ga. Next, in order to obtain the gas flow rate of the EGR gas, the distribution of the gas flow rate of the EGR gas introduced by the low-pressure exhaust gas recirculation device 14 and the EGR gas flow rate introduced by the high-pressure exhaust gas recirculation device 18 is determined as the operating state of the internal combustion engine 1. Set according to. Then, the respective opening degrees of the low pressure EGR valve 17 and the high pressure EGR valve 20 are controlled so that EGR gas is introduced into the intake passage 3 from each of the low pressure EGR passage 15 and the high pressure EGR passage 19 according to the set distribution. After such normal control is performed, the current routine is terminated.

  On the other hand, if the regeneration request flag F1 is set, the process proceeds to step S24, and the above-described regeneration execution flag F2 is set (F2 = 1). Next, in step S25, a gas flow rate Gco2 of carbon dioxide (CO2) passing through the low pressure EGR passage 15 is calculated. The gas flow rate Gco2 is derived from the gas flow rate of the EGR gas passing through the low pressure EGR passage 15 as Glpl, the gas flow rate of the gas derived from the fuel injected into the cylinder 2 as derived from Gfuel and the fuel injected by the fuel addition valve 13. The gas flow rate of the gas is calculated based on Gpstfuel. The gas flow rate Gco2 is a physical quantity that affects the combustion fluctuation of the internal combustion engine 1 when the gas that has passed through the exhaust purification device 9 after the fuel is injected from the fuel addition valve 13 enters the intake passage 3. If the gas flow rate Gco2 exceeds the limit, unacceptable combustion fluctuations such as misfires are caused.

  Therefore, in the following step S26, it is determined whether or not the gas flow rate Gco2 is smaller than a threshold value Gco2th which is a lower limit amount that causes such combustion fluctuation. If the gas flow rate Gco2 is greater than or equal to the threshold value Gco2th, combustion fluctuations may occur. Therefore, the process proceeds to step S27, the flow rate of gas passing through the exhaust purification device 9 is decreased, and then the process proceeds to step S28. This decrease in the gas flow rate is achieved by relatively reducing the distribution of the gas flow rate of the EGR gas introduced by the low pressure exhaust gas recirculation device 14 while maintaining the target EGR rate, and the gas of the EGR gas introduced by the high pressure exhaust gas recirculation device 18. This is realized by the ECU 21 changing the opening degree of each of the low pressure EGR valve 17 and the high pressure EGR valve 20 so that the flow rate distribution is relatively increased. As a result, the absolute amount of EGR gas passing through the low pressure EGR passage 15 in a state where the target EGR rate is maintained also decreases, so that the gas flow rate Gco2 of carbon dioxide described above decreases with the decrease, and the internal combustion engine 1 The combustion fluctuation of can be suppressed.

  On the other hand, when the gas flow rate Gco2 is smaller than the threshold value Gco2th, the process proceeds to step S28, and it is determined whether or not the required fuel amount Q is not less than the lower limit amount QL of the range in which the fuel addition valve 13 can inject while maintaining the injection accuracy. judge. When the required injection amount Q is equal to or greater than the lower limit amount QL, the injection accuracy of the fuel addition valve 13 can be maintained, and the current routine is terminated. On the other hand, when the required injection amount Q is smaller than the lower limit amount QL, since the injection accuracy of the fuel addition valve 13 cannot be maintained, the process proceeds to step S29 and the flow rate of gas passing through the exhaust purification device 9 is increased. Proceed to step S26. In contrast to the step S27, this increase causes a relative increase in the distribution of the gas flow rate of the EGR gas introduced by the low pressure exhaust gas recirculation device 14 while the target EGR rate is maintained, and the high pressure exhaust gas recirculation device 18 introduces the increase. This is realized by the ECU 21 changing the respective opening degrees of the low pressure EGR valve 17 and the high pressure EGR valve 20 so that the distribution of the gas flow rate of the EGR gas is relatively reduced. As a result, the required fuel amount Q calculated in step S5 of FIG. 3 increases, and the injection accuracy of the fuel addition valve 13 is maintained.

  According to the above embodiment, the gas flow rate of the EGR gas introduced by the low pressure exhaust gas recirculation device 14 and the high pressure exhaust gas recirculation device so that the injection accuracy of the fuel addition valve 13 is maintained when the filter regeneration process is performed in the MPL mode. Since the distribution of the gas flow rate of the EGR gas introduced by 18 is changed in a state where the target EGR rate is maintained, the EGR rate can be accurately controlled. Further, since the increase in the flow rate of the gas passing through the exhaust purification device 9 is performed when there is no possibility that the combustion fluctuation occurs (step S26 in FIG. 4), it is possible to prevent the combustion fluctuation from being caused by the increase. Thereby, it is possible to maintain an appropriate EGR rate while suppressing combustion fluctuations associated with the filter regeneration process.

(Second form)
Next, a second embodiment of the present invention will be described with reference to FIG. This form is the same as the first form except for the control contents in the MPL mode. Therefore, FIGS. 1 and 2 are referred to for the entire configuration of the exhaust system of this embodiment, and FIG. 3 is referred to for the filter regeneration process. FIG. 5 is a flowchart showing an example of an operation control routine of the low pressure exhaust gas recirculation device 14 and the high pressure exhaust gas recirculation device 18 in the MPL mode according to the second embodiment. The routine of FIG. 5 is obtained by adding a process between step S28 and step S29 of FIG. 4. The same process as that of FIG.

  If it is determined in step S28 that the required injection amount Q is smaller than the lower limit amount QL, the process proceeds to step S30. In step S30, it is determined whether or not the ratio of the gas flow rate Glpl of the EGR gas passing through the low pressure EGR passage 15 to the gas flow rate Gcyl of the intake gas sucked into the cylinder 2 is within an allowable limit. This permissible limit means the limit of the LPL rate that does not cause combustion fluctuation when the fuel injection amount from the fuel addition valve 13 increases with the processing of step S31 described later. Accordingly, when the LPL rate is within this allowable limit, there is no possibility of causing combustion fluctuations, so the process proceeds to step S31, and the regeneration execution flag F2 is cleared so that the fuel injection amount by the fuel addition valve 13 increases. Since the regeneration execution flag F2 is cleared, a negative determination is made in step S7 of FIG. 3 and step S8 is skipped, so that fuel injection at the injection timing is omitted.

  FIG. 6 is an explanatory view for explaining the state of fuel injection by the fuel addition valve 13 when step S31 of FIG. 5 is executed. As shown in FIG. 6, when the required fuel amount Qn at a certain injection timing θn is below the lower limit amount QL and the LPL rate is within the allowable limit, the regeneration execution flag F2 is cleared in step S31 of FIG. Fuel injection with the required fuel amount Qn is omitted. At the next injection timing θn + 1, as described above, the required fuel amount Qn + 1 is calculated in a state where the previous required fuel amount Qn is integrated. If the accumulated required fuel amount Qn + 1 is equal to or greater than the lower limit amount QL, an affirmative determination is made in step S28 of FIG. 5, and thus the regeneration execution flag F2 remains set. Therefore, in step S8 of FIG. 3, the same amount of fuel as the required fuel amount Qn + 1 is injected from the fuel addition valve 13. Thereby, the fuel addition valve 13 is controlled so that the fuel of the lower limit amount QL or more is injected at a lower frequency (in the case of FIG. 6, every 1440 ° C. A) than the frequency of injection at every 720 ° C. become. If the required fuel amount Q does not exceed the lower limit amount QL due to the omission of one fuel injection, the fuel injection is further omitted, and when the integrated required fuel amount Q becomes equal to or higher than the lower limit amount QL. Injection will be executed. Thus, the injection accuracy of the fuel addition valve 13 can be maintained while the combustion fluctuation is prevented by increasing the fuel injection amount by the fuel addition valve 13.

  Returning to FIG. 5, if the LPL rate exceeds the allowable limit in step S30, the flow rate of gas passing through the exhaust purification device 9 by proceeding to step S29 may cause combustion fluctuation due to the increase in the injection amount as described above. And the process returns to step S26.

  According to the second embodiment, in order to directly increase the injection amount by the fuel addition valve 13, the required fuel amount Q is increased by increasing the gas flow rate passing through the exhaust purification device 9 as in the first embodiment. There is an advantage that the response is faster than in the case of making it.

  In each of the above embodiments, the ECU 21 executes the present invention by executing steps S6 to S10 in FIG. 3 as the required fuel amount calculating means according to the present invention by executing steps S4 and S5 in FIG. By executing the control routine of FIG. 4 or FIG. 5 as the fuel addition control means according to FIG. 5, and by performing step S30 of FIG. 5 as the exhaust gas recirculation control means according to the present invention, the allowable limit determination according to the present invention is performed. Each functions as a means.

  However, the present invention is not limited to the above embodiments, and can be implemented in various forms within the scope of the gist of the present invention. In each of the above embodiments, the present invention is applied to an internal combustion engine with a turbocharger. However, the internal combustion engine to be applied may not have a turbocharger.

  Further, by coating the base material of the filter 12 shown in FIG. 1 with an occlusion reduction type NOx catalyst substance, the exhaust purification device 9 stores NOx in addition to the particulate collection function and reduces the exhaust purification function. May be given. When the exhaust purification device 9 has such a function, before the NOx occlusion amount reaches the limit, fuel is injected from the fuel addition valve 13 to temporarily set the air-fuel ratio of the exhaust to the rich side. A rich spike process is performed to reduce the NOx occluded by. In addition, when the exhaust purification device 9 is poisoned with sulfur, a known sulfur poisoning regeneration process in which fuel is injected from the fuel addition valve 13 is also performed in order to eliminate the sulfur poisoning and restore the exhaust purification function. Since the rich spike process and the sulfur poisoning regeneration process are the same as the filter regeneration process described above in that fuel is injected into the exhaust passage 4, the same control as that shown in FIGS. 4 and 5 is performed during the rich spike process. Alternatively, the exhaust system of the present invention may be implemented in a form that is executed during the sulfur poisoning regeneration process.

The figure which showed the principal part of the internal combustion engine to which the exhaust system which concerns on one form of this invention was applied. The conceptual diagram which showed an example of the mode selection map with which the multiple types of mode for performing introduction of EGR gas was matched with the driving | running state. The flowchart which showed an example of the control routine of filter reproduction | regeneration processing. The flowchart which showed an example of the operation control routine of the low pressure exhaust gas recirculation apparatus and high pressure exhaust gas recirculation apparatus in MPL mode. The flowchart which showed an example of the operation control routine of the low pressure exhaust gas recirculation apparatus and high pressure exhaust gas recirculation apparatus which concern on a 2nd form. Explanatory drawing explaining the mode of the fuel injection by the fuel addition valve which concerns on a 2nd form.

Explanation of symbols

1 Internal combustion engine 3 Intake passage 4 Exhaust passage 9 Exhaust purification device (exhaust purification means)
13 Fuel addition valve (fuel addition means)
14 Low pressure exhaust gas recirculation device (first exhaust gas recirculation device)
18 High pressure exhaust gas recirculation device (second exhaust gas recirculation device)
21 ECU (required fuel amount calculation means, fuel addition control means, exhaust gas recirculation control means, allowable limit determination means)

Claims (4)

An exhaust gas purification unit disposed in an exhaust passage of the internal combustion engine for purifying exhaust; a fuel addition unit for injecting fuel into the exhaust passage to recover an exhaust gas purification function of the exhaust gas purification unit; and an exhaust gas from the exhaust passage And an exhaust gas recirculation device for taking out a part of the exhaust gas as EGR gas and introducing it into the intake passage of the internal combustion engine. A first exhaust gas recirculation device located on the downstream side, and a second exhaust gas recirculation device in which the EGR gas take-out position is located upstream of the fuel addition means, and the internal combustion engine is in an operating state. An exhaust system for an internal combustion engine that controls each of the first exhaust gas recirculation device and the second exhaust gas recirculation device so as to maintain a target EGR rate set accordingly. There,
It is necessary to restore the required fuel amount calculation means for calculating the required fuel amount to be injected by the fuel addition means so that the larger the gas flow rate passing through the exhaust purification means, and the exhaust purification function of the exhaust purification means. Meanwhile, a fuel addition control means for controlling the fuel addition means so that fuel of the required fuel quantity calculated by the required fuel quantity calculation means is injected into the exhaust passage, and the required fuel quantity calculation means. When the required fuel amount calculated in step ( b) is less than the lower limit amount of fuel that can be injected while maintaining the injection accuracy of the fuel addition means, the flow rate of the gas passing through the exhaust purification means is increased. Exhaust gas recirculation control means for changing the distribution of the gas flow rate of EGR gas introduced by the exhaust gas recirculation device and the gas flow rate of EGR gas introduced by the second exhaust gas recirculation device while maintaining the target EGR rate; Internal combustion engine exhaust system, characterized in that it comprises a. The exhaust gas recirculation control means is introduced by the first exhaust gas recirculation apparatus as to whether there is a risk of causing a combustion fluctuation due to misfire that cannot be allowed in the internal combustion engine with the fuel addition by the fuel addition means. When the determination is made based on a physical quantity that affects the combustion fluctuation included in the EGR gas and it is determined that the combustion fluctuation is not likely to be caused, The distribution of the gas flow rate of EGR gas introduced by the first exhaust gas recirculation device and the gas flow rate of EGR gas introduced by the second exhaust gas recirculation device is changed while maintaining the target EGR rate. Item 6. An exhaust system for an internal combustion engine according to Item 1.   The exhaust gas recirculation control means determines the amount of EGR gas introduced by the first exhaust gas recirculation device so that the flow rate of gas passing through the exhaust gas purification means decreases when it is determined that the combustion fluctuation may be caused. The exhaust system for an internal combustion engine according to claim 2, wherein the distribution of the gas flow rate and the gas flow rate of the EGR gas introduced by the second exhaust gas recirculation device is changed while maintaining the target EGR rate. The fuel addition control means controls the fuel addition means so that fuel of the same amount as the required fuel amount calculated by the required fuel amount calculating means is injected into the exhaust passage at a predetermined frequency. Yes,
The EGR gas introduced by the first exhaust gas recirculation device when the required fuel amount calculated by the required fuel amount calculating means is smaller than the lower limit amount of fuel that the fuel adding means can inject while maintaining the injection accuracy. combustion variation rate of the gas flow rate of the EGR gas first exhaust gas recirculation system introduces to the total gas flow rate and gas flow rate of the EGR gas second exhaust gas recirculation device is introduced by a misfire in the internal combustion engine of It further comprises a tolerance limit judging means for judging whether or not it is within a tolerance limit that does not occur,
The fuel addition control means, on condition that the ratio by the permissible limit determining means is determined to be within the acceptable limits, the fuel of the lower limit amount or more of injection injection amount is lower than the predetermined frequency 2. An exhaust system for an internal combustion engine according to claim 1, wherein the fuel adding means is controlled so as to be injected into the exhaust passage at a frequency.

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