JP6459004B2 - Engine exhaust purification system - Google Patents

Description

  The present invention relates to an exhaust emission control device for an engine, and more particularly, to an exhaust emission control device for an engine provided with an NOx catalyst for purifying NOx in exhaust gas on an exhaust passage.

  Conventionally, NOx in the exhaust gas is occluded in a lean state (λ> 1) where the air-fuel ratio of the exhaust gas is larger than the stoichiometric air-fuel ratio, and this occluded NOx is stored in the vicinity of the stoichiometric air-fuel ratio. NOx occlusion reduction type NOx catalysts that reduce in a certain state (λ≈1) or a rich state (λ <1) smaller than the stoichiometric air-fuel ratio are known. In the normal operating range, from the viewpoint of improving fuel efficiency, the engine is operated with the air-fuel ratio set to a lean state (λ> 1). If this lean operating state continues, the NOx storage of the NOx catalyst is continued. When the amount reaches the limit, the NOx catalyst cannot store NOx in the exhaust gas (in this case, NOx is released). Therefore, the air-fuel ratio is appropriately set to a stoichiometric air-fuel ratio or a richer state (λ ≦ 1) than the stoichiometric air-fuel ratio, so that NOx occluded in the NOx catalyst is reduced. “Λ” is an index indicating the air-fuel ratio expressed with the theoretical air-fuel ratio as a reference, and corresponds to a so-called excess air ratio.

  For example, in Patent Document 1, when the NOx occlusion amount of the NOx catalyst is a predetermined amount or more, fuel injection control is performed so as to enrich the air-fuel ratio of the exhaust gas so as to reduce NOx occluded in the NOx catalyst. Techniques to do are disclosed. On the other hand, in the technique disclosed in Patent Document 2, for example, the air-fuel ratio of the exhaust gas is reduced to λ ≦ 1 during the acceleration operation of the engine in order to improve the deterioration of fuel consumption due to the air-fuel ratio control for the reduction of the NOx catalyst. Control to set to.

JP 2004-360593 A Japanese Patent Laid-Open No. 11-50884

  By the way, as one of the control methods for setting the air-fuel ratio of the exhaust gas to an air-fuel ratio capable of reducing NOx stored in the NOx catalyst (hereinafter referred to as “target air-fuel ratio”), a desired engine torque is output. For this reason, a method of performing post-injection in which fuel is injected at a timing that does not contribute to the output of engine torque (typically an expansion stroke) after main injection in which fuel is injected into the cylinder can be considered. When performing such post-injection, the post-injection amount is increased in reaching the target air-fuel ratio of the exhaust gas. However, if the post-injection amount is increased rapidly, the air-fuel ratio of the exhaust gas is increased. There is a possibility of becoming richer than the target air-fuel ratio. In this case, smoke (soot), HC (corresponding to unburned fuel), etc. are generated, and the emission may be deteriorated.

  The present invention has been made to solve the above-described problems of the prior art, and performs NOx reduction control in which post-injection is performed so as to set the NOx stored in the NOx catalyst to a target air-fuel ratio that can be reduced. An object of the present invention is to suppress the air-fuel ratio of the exhaust gas from becoming richer than the target air-fuel ratio by appropriately changing the post-injection amount in the engine exhaust purification device.

In order to achieve the above object, the present invention is provided on an exhaust passage of an engine, and stores NOx in exhaust gas when the air-fuel ratio of the exhaust gas is leaner than the stoichiometric air-fuel ratio. An engine exhaust purification device that includes a NOx catalyst that reduces the stored NOx when the air-fuel ratio of the exhaust gas is close to or richer than the stoichiometric air-fuel ratio, the NOx occlusion amount of the NOx catalyst NOx reduction control for performing first NOx reduction control for post-injecting from the fuel injection valve so that the air-fuel ratio of the exhaust gas becomes a predetermined air-fuel ratio at which the NOx occluded in the NOx catalyst can be reduced when NO is greater than or equal to a predetermined amount The NOx reduction control means increases the post injection amount so that the air-fuel ratio of the exhaust gas is set to a predetermined air-fuel ratio by the first NOx reduction control, and the post-injection at this time The NOx reduction control means imposes a limit so that the change rate of the amount becomes less than the predetermined change rate, and the NOx stored in the NOx catalyst when the air-fuel ratio of the exhaust gas changes to the rich side due to acceleration of the vehicle. In order to reduce the air-fuel ratio of the exhaust gas, a second NOx reduction control for post-injecting from the fuel injection valve so as to temporarily set the air-fuel ratio of the exhaust gas to a predetermined air-fuel ratio is further executed, and the air-fuel ratio of the exhaust gas is predetermined by the second NOx reduction control When the post injection amount is increased so as to set the air / fuel ratio, the change rate of the post injection amount is higher than when the post injection amount is increased so as to set the air / fuel ratio of the exhaust gas to the predetermined air / fuel ratio by the first NOx reduction control. And the first NOx reduction control is executed for a longer time than the second NOx reduction control.
According to the present invention configured as described above, a limit is imposed on the rate of change of the post injection amount during the first NOx reduction control, that is, control is performed so that the rate of change of the post injection amount does not exceed a predetermined value. By controlling the post injection amount, the air-fuel ratio of the exhaust gas can be accurately set to a predetermined air-fuel ratio. In this case, it is possible to appropriately suppress that the actual air-fuel ratio becomes richer than the predetermined air-fuel ratio during the post injection amount control, that is, the actual air-fuel ratio undershoots the predetermined air-fuel ratio. As a result, it is possible to appropriately suppress the occurrence of smoke and HC due to the actual air-fuel ratio undershooting the predetermined air-fuel ratio during the first NOx reduction control.
Further, according to the present invention, during the second NOx reduction control, the change rate of the post injection amount is made larger than during the first NOx reduction control, so that the actual air-fuel ratio can be made to coincide with the target air-fuel ratio quickly. Thereby, in a relatively short period (acceleration period) in which the second NOx reduction control is performed, the actual air-fuel ratio is immediately set to the target air-fuel ratio, and the reduction of NOx occluded in the NOx catalyst is surely executed. it can.

In the present invention, preferably, the NOx reduction control means imposes a limit on the change rate of the post injection amount so that the air-fuel ratio of the exhaust gas does not become richer than the predetermined air-fuel ratio.
According to the present invention configured as described above, the actual air-fuel ratio is reliably suppressed from undershooting the predetermined air-fuel ratio during the first NOx reduction control, and the occurrence of smoke and HC is effectively suppressed. can do.

In the present invention, preferably, when the first NOx reduction control is executed, the NOx reduction control means performs the post injection at a timing when the post-injected fuel is burned in the cylinder of the engine.
According to the present invention configured as described above, when the first NOx reduction control is executed, from the viewpoint of suppressing the discharge of HC (unburned fuel) and the like, at the timing when the post-injected fuel is burned in the cylinder. Do post injection. In the state where the actual air-fuel ratio undershoots the target air-fuel ratio, smoke tends to be generated when the fuel injected in this way is combusted. In the present invention, however, the change rate of the post-injection amount is increased as described above. On the other hand, a restriction is imposed on the undershoot to suppress the occurrence of such smoke.

In the present invention, preferably, when the second NOx reduction control is performed, the NOx reduction control means performs post-injection at a timing at which post-injected fuel is discharged into the exhaust passage as unburned fuel without being burned in the cylinder of the engine. Perform the injection.
According to the present invention configured as described above, when the second NOx reduction control is executed, the post-injected fuel is burned in the cylinder from the viewpoint of suppressing smoke that may be generated by the combustion of the post-injected fuel. Post injection is performed at the timing when the fuel is discharged as unburned fuel. Therefore, even if the actual air-fuel ratio undershoots the target air-fuel ratio as a result of increasing the change rate of the post injection amount as described above, smoke does not occur.

In the present invention, preferably, when the post-injection amount is increased so that the air-fuel ratio of the exhaust gas is set to a predetermined air-fuel ratio by the second NOx reduction control, the NOx reduction control means corresponds to the change rate of the post-injection amount. Avoid imposing restrictions.
According to the present invention thus configured, the actual air-fuel ratio can be made to reach the predetermined air-fuel ratio more quickly during the second NOx reduction control.

In the present invention, preferably, the NOx reduction control means realizes the predetermined air-fuel ratio based on the air-fuel ratio of the exhaust gas detected by the sensor and the predetermined air-fuel ratio when executing the first and second NOx reduction controls. Thus, when the post-injection amount is F / B controlled and the second NOx reduction control is executed, the gain in the F / B control is made larger than when the first NOx reduction control is executed.
In the present invention configured as described above, the post-injection amount is changed by F / B control based on the air-fuel ratio during the first and second NOx reduction control, and the gain of the F / B control performed during the first NOx reduction control is set to the second NOx. The gain is made smaller than the gain of the F / B control performed during the reduction control. As a result, the actual air-fuel ratio can be appropriately suppressed from undershooting the predetermined air-fuel ratio during the first NOx reduction control, and the actual air-fuel ratio can be quickly reached the predetermined air-fuel ratio during the second NOx reduction control. .

In the present invention, preferably, the NOx reduction control means performs the first NOx reduction only when the engine load is in a middle load range that is equal to or higher than the first predetermined load and less than a second predetermined load that is greater than the first predetermined load. Execute control.
According to the present invention configured as described above, since the first NOx reduction control is executed only in the medium load region, the first NOx reduction control is executed outside the intermediate load region, and the fuel that has been post-injected by this NOx reduction control. It is possible to appropriately suppress the generation of smoke and HC generated by trying to burn the gas. On the other hand, in the present invention, the first NOx reduction control is executed in the middle load range and the second NOx reduction control is executed outside the middle load range, so that the engine operation is suppressed while suppressing the deterioration of fuel consumption due to the NOx reduction control. Emissions in the entire area can be improved appropriately.

In the present invention, it is preferable to further include an SCR catalyst that is provided on the exhaust passage of the engine and purifies NOx in the exhaust gas by reaction with ammonia.
According to the present invention configured as described above, NOx in the exhaust gas can be appropriately purified by the SCR catalyst in the region where the first and second NOx reduction control is not executed. Therefore, emission can be improved.

  According to the present invention, in an exhaust purification device for an engine that performs NOx reduction control for performing post-injection so as to set the target air-fuel ratio at which NOx stored in the NOx catalyst can be reduced, the post-injection amount is appropriately changed. Thus, it is possible to suppress the air-fuel ratio of the exhaust gas from becoming richer than the target air-fuel ratio.

1 is a schematic configuration diagram of an engine system to which an engine exhaust gas purification apparatus according to an embodiment of the present invention is applied. 1 is a block diagram showing an electrical configuration of an engine exhaust gas purification apparatus according to an embodiment of the present invention. It is a flowchart which shows the fuel-injection control by embodiment of this invention. It is a flowchart which shows the post injection amount calculation process for DeNOx by embodiment of this invention. It is explanatory drawing about the driving | operation area | region of the engine which performs each of passive DeNOx control and active DeNOx control in embodiment of this invention. It is explanatory drawing about the temperature range which performs DeNOx control in embodiment of this invention. It is a flowchart which shows the setting process of the passive DeNOx control execution flag by embodiment of this invention. It is a flowchart which shows the passive DeNOx control by embodiment of this invention. It is a time chart which shows the specific example of the 1st post injection F / B control by the embodiment of the present invention. It is a flowchart which shows the setting process of the active DeNOx control execution flag by embodiment of this invention. 4 is a flowchart illustrating active DeNOx control according to an embodiment of the present invention. It is a time chart which shows the specific example of the 2nd post injection F / B control by embodiment of this invention.

  Hereinafter, an exhaust emission control device for an engine according to an embodiment of the present invention will be described with reference to the accompanying drawings.

<System configuration>
First, an engine system to which an exhaust emission control device for an engine according to an embodiment of the present invention is applied will be described with reference to FIG. FIG. 1 is a schematic configuration diagram of an engine system to which an engine exhaust gas purification apparatus according to an embodiment of the present invention is applied.

  As shown in FIG. 1, the engine system 200 mainly includes an engine E as a diesel engine, an intake system IN that supplies intake air to the engine E, a fuel supply system FS that supplies fuel to the engine E, An exhaust system EX that exhausts exhaust gas from the engine E, sensors 100 to 119 that detect various states relating to the engine system 200, a PCM (Power-train Control Module) 60 that controls the engine system 200, and an SCR catalyst 47 And a DCU (Dosing Control Unit) 70 for performing control related to the above.

First, the intake system IN has an intake passage 1 through which intake air passes, and an air cleaner 3 that purifies air introduced from the outside in order from the upstream side, and intake air that passes through the intake passage 1. The compressor of the turbocharger 5 that compresses and raises the intake pressure, the intercooler 8 that cools the intake air by outside air or cooling water, and the intake shutter valve 7 that adjusts the intake air flow rate (corresponding to a throttle valve) And a surge tank 12 for temporarily storing the intake air supplied to the engine E.
In the intake system IN, an air flow sensor 101 for detecting the intake air amount and a temperature sensor 102 for detecting the intake air temperature are provided on the intake passage 1 immediately downstream of the air cleaner 3. A pressure sensor 103 for detecting the pressure of the intake air is provided. A temperature sensor 106 for detecting the intake air temperature is provided on the intake passage 1 immediately downstream of the intercooler 8. A position sensor 105 for detecting the opening degree is provided, and the surge tank 12 is provided with a pressure sensor 108 for detecting the pressure of intake air in the intake manifold. Various sensors 101 to 108 provided in the intake system IN output detection signals S101 to S108 corresponding to the detected parameters to the PCM 60, respectively.

  Next, the engine E includes an intake valve 15 for introducing the intake air supplied from the intake passage 1 (specifically, an intake manifold) into the combustion chamber 17, a fuel injection valve 20 for injecting fuel toward the combustion chamber 17, A glow plug 21 provided with a heat generating portion in the combustion chamber 17 that generates heat when energized, a piston 23 that reciprocates by combustion of the air-fuel mixture in the combustion chamber 17, and a crankshaft 25 that is rotated by the reciprocating motion of the piston 23 And an exhaust valve 27 that exhausts exhaust gas generated by combustion of the air-fuel mixture in the combustion chamber 17 to the exhaust passage 41. Further, the engine E is provided with a crank angle sensor 100 that detects a crank angle as a rotation angle with respect to a top dead center in the crankshaft 25, and the crank angle sensor 100 is provided at the detected crank angle. The corresponding detection signal S100 is output to the PCM 60, and the PCM 60 acquires the engine speed based on the detection signal S100.

  The fuel supply system FS includes a fuel tank 30 that stores fuel, and a fuel supply passage 38 that supplies fuel from the fuel tank 30 to the fuel injection valve 20. In the fuel supply passage 38, a low-pressure fuel pump 31, a high-pressure fuel pump 33, and a common rail 35 are provided in order from the upstream side.

  Next, the exhaust system EX has an exhaust passage 41 through which exhaust gas passes. A turbo that is rotated by the exhaust gas passing through the exhaust passage 41 and drives the compressor as described above by this rotation. A turbine of the supercharger 5 is provided. Further, on the exhaust passage 41 on the downstream side of the turbine, NOx catalyst 45 for purifying NOx in the exhaust gas and particulate matter (PM) in the exhaust gas are collected in order from the upstream side. A diesel particulate filter (DPF) 46, a urea injector 51 that injects urea into the exhaust passage 41 downstream of the DPF 46, and urea that is injected from the urea injector 51 are hydrolyzed to generate ammonia. SCR (Selective Catalytic Reduction) catalyst 47 for purifying NOx by reacting (reducing) this NOx with NOx in the exhaust gas, and slip catalyst 48 for oxidizing and purifying unreacted ammonia discharged from SCR catalyst 47 And are provided. The urea injector 51 is controlled to inject urea into the exhaust passage 41 by a control signal S51 supplied from the DCU 70.

  Here, the NOx catalyst 45 will be described more specifically. The NOx catalyst 45 occludes NOx in the exhaust gas in a lean state (λ> 1) in which the air-fuel ratio of the exhaust gas is larger than the stoichiometric air-fuel ratio. This is a NOx storage reduction catalyst (NSC) that reduces in the vicinity (λ≈1) or in a rich state (λ <1) smaller than the stoichiometric air-fuel ratio. The NOx catalyst 45 not only functions as this NSC, but also oxidizes hydrocarbons (HC), carbon monoxide (CO), etc. using oxygen in the exhaust gas to change them into water and carbon dioxide. It is comprised so that it may also have a function as a catalyst (DOC: Diesel Oxidation Catalyst). Specifically, the NOx catalyst 45 is made by coating the surface of a DOC catalyst material layer with an NSC catalyst material.

In the exhaust system EX, a pressure sensor 109 for detecting the pressure of the exhaust gas and a temperature sensor 110 for detecting the temperature of the exhaust gas are provided on the exhaust passage 41 upstream of the turbine of the turbocharger 5. An O ₂ sensor 111 for detecting the oxygen concentration is provided on the exhaust passage 41 immediately downstream of the turbine of the turbocharger 5. Further, the exhaust system EX includes a temperature sensor 112 for detecting the temperature of the exhaust gas immediately upstream of the NOx catalyst 45, a temperature sensor 113 for detecting the temperature of the exhaust gas between the NOx catalyst 45 and the DPF 46, and the DPF 46. Differential pressure sensor 114 for detecting the pressure difference between the exhaust gas immediately upstream and the downstream side, a temperature sensor 115 for detecting the temperature of the exhaust gas immediately downstream of the DPF 46, and the exhaust gas immediately downstream of the DPF 46 NOx sensor 116 for detecting the concentration of NOx in the exhaust gas, temperature sensor 117 for detecting the temperature of the exhaust gas immediately upstream of the SCR catalyst 47, and the concentration of NOx in the exhaust gas immediately downstream of the SCR catalyst 47 And a PM sensor 119 for detecting PM in the exhaust gas immediately upstream of the slip catalyst 48. Various sensor sensors 109 to 119 provided in the exhaust system EX output detection signals S109 to S119 corresponding to the detected parameters to the PCM 60, respectively.

  Further, in the present embodiment, the turbocharger 5 is configured as a two-stage supercharging system that can efficiently obtain high supercharging throughout the entire range from a low rotation range to a high rotation range where the exhaust energy is low. That is, the turbocharger 5 includes a large turbocharger 5a for supercharging a large amount of air in a high rotation range, a small turbocharger 5b capable of efficiently supercharging with low exhaust energy, and a compressor of the small turbocharger 5b. A compressor bypass valve 5c for controlling the flow of intake air to the turbine, a regulator valve 5d for controlling the flow of exhaust gas to the turbine of the small turbocharger 5b, and a waste gate for controlling the flow of exhaust gas to the turbine of the large turbocharger 5a A valve 5e is provided, and the supercharging by the large turbocharger 5a and the small turbocharger 5b is switched by driving each valve according to the operating state (engine speed and load) of the engine E.

  The engine system 200 according to the present embodiment further includes an EGR device 43. The EGR device 43 connects the exhaust passage 41 upstream of the turbine of the turbocharger 5 and the intake passage 1 downstream of the compressor of the turbocharger 5 (specifically, downstream of the intercooler 8). The passage 43a, the EGR cooler 43b that cools the exhaust gas that passes through the EGR passage 43a, the first EGR valve 43c that adjusts the flow rate of the exhaust gas that passes through the EGR passage 43a, and the EGR cooler 43b are bypassed to flow the exhaust gas. The EGR cooler bypass passage 43d and the second EGR valve 43e for adjusting the flow rate of the exhaust gas passing through the EGR cooler bypass passage 43d are provided. The EGR passage 43a corresponds to a “first EGR passage”, and the EGR cooler bypass passage 43d corresponds to a “second EGR passage”.

  Next, with reference to FIG. 2, the electrical configuration of the exhaust emission control device for an engine according to the embodiment of the present invention will be described. FIG. 2 is a block diagram showing an electrical configuration of the engine exhaust gas purification apparatus according to the embodiment of the present invention.

  In addition to the detection signals S100 to S119 of the various sensors 100 to 119 described above, the PCM 60 according to the embodiment of the present invention detects an accelerator opening sensor 150 that detects an accelerator pedal opening (accelerator opening), and a vehicle speed. Based on the detection signals S150 and S151 output from the vehicle speed sensors 151, the control signal S20 is mainly output to control the fuel injection valve 20, and the control signal S7 is output to control the intake shutter valve 7. Then, a control signal S21 is output to control the glow plug 21, and control signals S431 and S432 are output to control each of the first and second EGR valves 43c and 43e.

  In particular, in the present embodiment, the PCM 60 sets the air-fuel ratio of the exhaust gas from the fuel injection valve 20 so as to set the target air-fuel ratio (specifically, near the stoichiometric air-fuel ratio or a predetermined air-fuel ratio smaller than the stoichiometric air-fuel ratio). Control (NOx reduction control) is performed to reduce the NOx occluded in the NOx catalyst 45 by post injection. That is, the PCM 60 adds to the main injection that injects fuel into the cylinders in order to output the engine torque according to the driver's accelerator operation (basically, the air-fuel ratio of the exhaust gas becomes lean in the main injection). After this main injection, post injection is performed to inject fuel at a timing that does not contribute to engine torque output (specifically, an expansion stroke), and the air-fuel ratio of the exhaust gas is reduced. The NOx occluded in the NOx catalyst 45 is reduced so as to be set to a state close to the theoretical air-fuel ratio (λ≈1) or a rich state smaller than the theoretical air-fuel ratio (λ <1). Hereinafter, such control for reducing the NOx stored in the NOx catalyst 45 is referred to as “DeNOx control”. Note that “De” in the word “DeNOx” is a prefix meaning separation or removal.

  Although details will be described later, the PCM 60 functions as “NOx reduction control means”, “execution frequency calculation means”, “EGR control means”, “glow control means”, and the like.

  The PCM 60 stores a CPU, various programs interpreted and executed on the CPU (including a basic control program such as an OS and an application program that is activated on the OS and realizes a specific function), programs, and various data. It is configured by a computer having an internal memory such as a ROM or RAM for storing.

<Fuel injection control>
Next, the fuel injection control according to the embodiment of the present invention will be described with reference to FIG. FIG. 3 is a flowchart (fuel injection control flow) showing the fuel injection control according to the embodiment of the present invention. This fuel injection control flow is started when the ignition of the vehicle is turned on and the PCM 60 is turned on, and is repeatedly executed at a predetermined cycle.

  First, in step S101, the PCM 60 acquires the driving state of the vehicle. Specifically, the PCM 60 is currently set to at least the accelerator opening detected by the accelerator opening sensor 150, the vehicle speed detected by the vehicle speed sensor 151, the crank angle detected by the crank angle sensor 100, and the transmission of the vehicle. Get the gear position.

  Next, in step S102, the PCM 60 sets a target acceleration based on the driving state of the vehicle including the operation of the accelerator pedal acquired in step S101. Specifically, the PCM 60 determines the acceleration corresponding to the current vehicle speed and gear stage from acceleration characteristic maps (created in advance and stored in a memory or the like) defined for various vehicle speeds and various gear stages. A characteristic map is selected, and a target acceleration corresponding to the current accelerator opening is determined with reference to the selected acceleration characteristic map.

  Next, in step S103, the PCM 60 determines a target torque of the engine E for realizing the target acceleration determined in step S102. In this case, the PCM 60 determines a target torque within the range of torque that can be output by the engine E based on the current vehicle speed, gear stage, road surface gradient, road surface μ, and the like.

  Next, in step S104, the PCM 60 calculates the fuel injection amount to be injected from the fuel injection valve 20 based on the target torque and the engine speed so as to output the target torque determined in step S103 from the engine E. . This fuel injection amount is a fuel injection amount (main injection amount) applied in main injection.

  On the other hand, in parallel with the processing of steps S102 to S104 described above, in step S105, the PCM 60 sets a fuel injection pattern according to the operating state of the engine E. Specifically, when performing the above-described DeNOx control, the PCM 60 sets a fuel injection pattern for performing at least post injection in addition to main injection. In this case, the PCM 60 also determines the fuel injection amount (post injection amount) applied in the post injection and the timing (post injection timing etc.) for performing the post injection. Details of these will be described later.

  After steps S104 and S105, the process proceeds to step S106, where the PCM 60 is based on the main injection amount calculated in step S104 and the fuel injection pattern set in step S105 (when post injection is performed, the post injection amount or post injection). The fuel injection valve 20 is controlled. That is, the PCM 60 controls the fuel injection valve 20 so that a desired amount of fuel is injected in a desired fuel injection pattern.

  Next, a method for calculating the post injection amount (hereinafter referred to as “DeNOx post injection amount”) applied during DeNOx control in the embodiment of the present invention will be described with reference to FIG. 4. FIG. 4 is a flowchart (DeNOx post injection amount calculation flow) showing the DeNOx post injection amount calculation processing according to the embodiment of the present invention. This DeNOx post-injection amount calculation flow is repeatedly executed by the PCM 60 at a predetermined cycle, and is executed in parallel with the fuel injection control flow shown in FIG. That is, while the fuel injection control is being performed, the post injection amount for DeNOx is calculated as needed.

First, in step S201, the PCM 60 acquires the operating state of the engine E. Specifically, the PCM 60 determines at least the intake air amount (fresh air amount) detected by the air flow sensor 101, the oxygen concentration of the exhaust gas detected by the O ₂ sensor 111, and the main calculated in step S104 of FIG. Get the injection amount. In addition, the PCM 60 also obtains an exhaust gas amount (EGR gas amount) recirculated to the intake system IN by the EGR device 43, which is obtained by a predetermined model or the like.

  Next, in step S202, the PCM 60 calculates the amount of air (that is, the filling amount) introduced into the engine E based on the fresh air amount and the EGR gas amount acquired in step S201. In step S203, the PCM 60 calculates the oxygen concentration of the air introduced into the engine E from the filling amount calculated in step S202.

Next, in step S204, the PCM 60 performs post-injection in addition to main injection, so that the NOx occluded in the NOx catalyst 45 is reduced, so that the air-fuel ratio of the exhaust gas is near or below the stoichiometric air-fuel ratio. A post injection amount (DeNOx post injection amount) necessary to set the air-fuel ratio is calculated. That is, the PCM 60 determines how much post injection amount should be applied in addition to the main injection amount in order to set the air-fuel ratio of the exhaust gas to the target air-fuel ratio. In this case, the PCM 60 considers the difference between the oxygen concentration acquired in step S201 (the oxygen concentration detected by the O ₂ sensor 111) and the oxygen concentration calculated in step S203, and determines the post-injection amount for DeNOx. calculate. Specifically, the PCM 60 appropriately performs feedback processing according to the difference between the detected oxygen concentration and the calculated oxygen concentration from the air-fuel ratio of the exhaust gas generated when the main injected fuel is burned. Then, the post-injection amount for DeNOx for setting the air-fuel ratio of the exhaust gas to the target air-fuel ratio is calculated. By calculating the post-injection amount for DeNOx in this way, the air-fuel ratio of the exhaust gas is accurately set to the target air-fuel ratio by post-injection in DeNOx control, and NOx occluded in the NOx catalyst 45 is reliably reduced. I am doing so.

<DeNOx control>
Hereinafter, DeNOx control according to the embodiment of the present invention will be specifically described.

(Basic concept)
First, the basic concept of DeNOx control according to an embodiment of the present invention will be described.

  In this embodiment, the PCM 60 reduces the NOx occluded in the NOx catalyst 45 to approximately 0 when the NOx occlusion amount of the NOx catalyst 45 is equal to or greater than a predetermined amount, typically when the NOx occlusion amount is near the limit. DeNOx control in which post-injection is performed from the fuel injection valve 20 so as to continuously set the air-fuel ratio of the exhaust gas to a target air-fuel ratio in the vicinity of the stoichiometric air-fuel ratio or lower than the stoichiometric air-fuel ratio (“first NOx reduction in the present invention”). In the following, it is referred to as “active DeNOx control” as appropriate). By so doing, NOx stored in the NOx catalyst 45 in a large amount is forcibly reduced, and the NOx purification performance of the NOx catalyst 45 is reliably ensured.

  Further, in this embodiment, even if the NOx occlusion amount of the NOx catalyst 45 is less than a predetermined amount, the PCM 60 is occluded in the NOx catalyst 45 when the air-fuel ratio of the exhaust gas changes to the rich side due to vehicle acceleration. In order to reduce the NOx, the DeNOx control (which corresponds to the “second NOx reduction control” in the present invention, which is post-injected from the fuel injection valve 20 so as to temporarily set the air-fuel ratio of the exhaust gas to the target air-fuel ratio, is described below. Appropriately called “passive DeNOx control”). In this passive DeNOx control, the air-fuel ratio is adjusted to a target air-fuel ratio in the vicinity of the stoichiometric air-fuel ratio or below the stoichiometric air-fuel ratio by taking advantage of the situation where the main injection amount increases during acceleration and the air-fuel ratio of the exhaust gas decreases. Since the post-injection is performed so as to be set, the post-injection amount for setting the air-fuel ratio to the target air-fuel ratio is smaller than in the case where the DeNOx control is performed in a situation where the air-fuel ratio of the exhaust gas does not decrease (that is, during non-acceleration) Become. Further, since the passive DeNOx control is performed by taking advantage of acceleration of the vehicle, it is performed at a relatively high frequency.

  In the present embodiment, by applying such passive DeNOx control, DeNOx is performed at a high frequency while suppressing deterioration in fuel consumption due to DeNOx. Although the passive DeNOx control is performed only for a relatively short period, it is performed at a high frequency, so that the NOx occlusion amount of the NOx catalyst 45 can be efficiently reduced. As a result, the NOx occlusion amount of the NOx catalyst 45 is less likely to exceed a predetermined amount, so that the frequency of execution of active DeNOx control that requires a larger amount of post-injection than passive DeNOx control can be reduced, and fuel consumption deterioration due to DeNOx can be reduced. It becomes possible to improve effectively.

  Further, in the present embodiment, when the above-described active DeNOx control is executed, the post-injected fuel is burned in the cylinder of the engine E, so that the air-fuel ratio of the exhaust gas is set to the target air-fuel ratio. In this case, the PCM 60 performs the post-injection at the timing when the post-injected fuel is combusted in the cylinder. Specifically, the PCM 60 sets a predetermined timing in the first half of the expansion stroke of the engine E as a post injection timing in the active DeNOx control. By applying such post injection timing in the active DeNOx control, the post-injected fuel is discharged as it is as unburned fuel (that is, HC), and the oil dilution by the post-injected fuel is suppressed. ing.

  On the other hand, in the present embodiment, when the above-described passive DeNOx control is performed, the PCM 60 causes the post-injected fuel to be discharged into the exhaust passage 41 as unburned fuel without burning in the cylinder of the engine E. Thus, the air-fuel ratio of the exhaust gas is set to the target air-fuel ratio. In this case, the PCM 60 performs the post-injection at the timing when the post-injected fuel is discharged into the exhaust passage 41 as unburned fuel without being burned in the cylinder. Specifically, the PCM 60 sets a predetermined timing in the latter half of the expansion stroke of the engine E as a post injection timing in the passive DeNOx control. In principle, the post-injection timing in the passive DeNOx control is set to be retarded from the post-injection timing in the active DeNOx control. By applying such post-injection timing in passive DeNOx control, it is possible to prevent the post-injected fuel from burning in the cylinder and generating smoke.

  Here, with reference to FIG. 5, the operation area | region of the engine E which performs each of passive DeNOx control and active DeNOx control in embodiment of this invention is demonstrated. FIG. 5 shows the engine speed on the horizontal axis and the engine load on the vertical axis. In FIG. 5, a curve L1 indicates the maximum torque line of the engine E.

  As shown in FIG. 5, in the present embodiment, the PCM 60 is in an intermediate load range where the engine load is equal to or higher than the first predetermined load Lo1 and lower than the second predetermined load Lo2 (> first predetermined load Lo1), and When the engine speed is in the middle engine speed range that is greater than or equal to the first predetermined engine speed N1 and less than the second predetermined engine speed N2 (> the first predetermined engine speed N1), that is, the engine load and the engine engine speed are indicated by reference numeral R12. When included in the region (hereinafter referred to as “active DeNOx execution region R12”), the active DeNOx control is executed. The reason for adopting such an active DeNOx execution region R12 is as follows.

  As described above, when executing the active DeNOx control, the post-injected fuel is reduced from the viewpoint of suppressing the generation of HC due to the post-injected fuel being discharged as it is or the oil dilution by the post-injected fuel. Post injection is performed at the timing of combustion in the cylinder. In this case, in this embodiment, when the post-injected fuel is burned, the generation of smoke is suppressed and the generation of HC (that is, the discharge of unburned fuel due to incomplete combustion) is suppressed. Specifically, the generation of smoke and HC is suppressed by maximizing the time until the post-injected fuel burns as much as possible, that is, ignition occurs in a state where air and fuel are properly mixed. Yes. For this reason, the ignition of post-injected fuel is effectively delayed by introducing an appropriate amount of EGR gas during active DeNOx control.

The reason for suppressing the generation of HC during the active DeNOx control is that when EGR gas is introduced as described above, HC is also recirculated to the intake system IN as EGR gas, and this HC becomes a binder and is combined with soot. This is to prevent the gas passage from being blocked. In addition, when active DeNOx control is performed in an area where the temperature of the NOx catalyst 45 is low and HC purification performance (HC purification performance by DOC in the NOx catalyst 45) is not ensured, HC is not purified. This is to prevent discharge. Incidentally, the active DeNOx execution region R12 includes a region where the temperature of the NOx catalyst 45 where the HC purification performance is not ensured is relatively low.
The reason for suppressing the generation of smoke during active DeNOx control is that PM corresponding to the smoke is collected by the DPF 46, but DPF regeneration for burning and removing the PM collected by the DPF 46 (similar to DeNOx control). This is to prevent the fuel efficiency and the like from being deteriorated by frequently performing post-injection control).

By the way, when the engine load increases, the air introduced into the engine E in order to achieve the target air-fuel ratio is reduced, so that there is not enough oxygen necessary to properly burn the post-injected fuel, so that smoke and HC Tend to occur. In particular, when the engine load increases, the in-cylinder temperature rises and the time until the post-injected fuel is ignited cannot be properly secured, and ignition occurs midway, that is, the air and fuel are mixed properly. Combustion occurs in a state where it is not performed, and smoke and HC are generated. On the other hand, in a region where the engine load is considerably low, the temperature of the NOx catalyst 45 is low, and the NOx reduction function of the NOx catalyst 45 is not sufficiently exhibited. In addition, in this region, the post-injected fuel does not burn properly, that is, misfire occurs.
In addition, although the phenomenon regarding an engine load was described above, the same phenomenon arises also about an engine speed.

  From the above, in this embodiment, the operation region of the engine E corresponding to the medium load region and the medium rotation region is employed as the active DeNOx execution region R12 for executing the active DeNOx control. In other words, in this embodiment, the active DeNOx control is executed only in the active DeNOx execution region R12, and the execution of the active DeNOx control is prohibited in the operation region other than the active DeNOx execution region R12. Thus, in the operation region of the engine E for which execution of the active DeNOx control is prohibited, particularly in the region on the higher load side or the higher rotation side than the active DeNOx execution region R12 (region denoted by reference numeral R13), the SCR Since the NOx purification performance of the catalyst 47 is sufficiently secured, the SCR catalyst 47 purifies NOx, and NOx emission from the vehicle can be prevented without executing DeNOx control.

Further, in this embodiment, in the region on the higher load side than the region R13 in which the SCR catalyst 47 purifies NOx (the region denoted by reference numeral R11, hereinafter referred to as “passive DeNOx execution region R11”). Since the amount of exhaust gas increases and the SCR catalyst 47 cannot completely purify NOx, passive DeNOx control is executed. In this passive DeNOx control, as described above, post-injection is performed at a timing at which the post-injected fuel is discharged into the exhaust passage 41 as unburned fuel without being combusted in the cylinder. In the passive DeNOx execution region R11, the temperature of the NOx catalyst 45 is sufficiently high, and the HC purification performance (HC purification performance by DOC in the NOx catalyst 45) is ensured, so the unburned fuel discharged in this way Can be appropriately purified by the NOx catalyst 45.
In passive DeNOx control, smoke is generated when post-injected fuel is combusted in the cylinder as in active DeNOx control. As described above, the reason is the same as the reason why the execution of the active DeNOx control is prohibited when the engine load increases. Therefore, in passive DeNOx control, the post-injected fuel is discharged into the exhaust passage 41 as unburned fuel without burning in the cylinder.

  Here, a specific example of active DeNOx control when the operating state of the engine changes as indicated by an arrow A11 in FIG. 5 will be described. First, when the operating state of the engine enters the active DeNOx execution region R12 (see symbol A12), the PCM 60 executes active DeNOx control. When the engine operating state deviates from the active DeNOx execution region R12 (see A13), the PCM 60 temporarily stops the active DeNOx control. At this time, the SCR catalyst 47 purifies NOx. When the engine operating state reenters the active DeNOx execution region R12 (see reference A14), the PCM 60 resumes active DeNOx control. By doing so, the active DeNOx control is not terminated until the NOx occluded in the NOx catalyst 45 drops to almost zero.

  Next, a temperature range in which passive DeNOx control and active DeNOx control are performed in the embodiment of the present invention will be described with reference to FIG. FIG. 6 shows the relationship between the catalyst temperature (horizontal axis) and the exhaust gas purification rate (vertical axis).

  Specifically, the graph G11 shows the relationship between the temperature of the NOx catalyst 45 (hereinafter referred to as “NOx catalyst temperature”) and the NOx purification rate by the NOx catalyst 45, and the graph G12 shows the temperature of the SCR catalyst 47. (Hereinafter referred to as “SCR temperature”) and the NOx purification rate by the SCR catalyst 47 are shown. Further, the range indicated by the symbol R21 indicates a temperature range in which the NOx catalyst 45 can obtain a NOx purification rate equal to or higher than a predetermined value according to the NOx purification characteristics of the NOx catalyst 45 as shown in the graph G11. The range indicated by the symbol R22 The temperature range in which the SCR catalyst 47 obtains a NOx purification rate equal to or higher than a predetermined value according to the NOx purification characteristics of the SCR catalyst 47 as shown in the graph G12 is shown. The latter temperature range R22 is defined by the temperature T11 of the SCR catalyst 47. This temperature T11 is a temperature that is at least included in the activation temperature range of the SCR catalyst 47, and corresponds to, for example, a lower boundary value of the temperature range R12 at which the SCR catalyst 47 obtains a predetermined or higher NOx purification rate. Hereinafter, the temperature T11 is appropriately referred to as “SCR determination temperature”.

  As shown in FIG. 6, the NOx catalyst 45 exhibits NOx purification performance in a relatively low temperature range, and the SCR catalyst 47 exhibits NOx purification performance in a relatively high temperature range. Therefore, NOx in the exhaust gas is purified by the NOx catalyst 45 when the exhaust gas is in a relatively low temperature region, and NOx in the exhaust gas is purified by the SCR catalyst 47 when the exhaust gas is in a relatively high temperature region. It is better to purify.

  In the present embodiment, the PCM 60 executes passive DeNOx control and active DeNOx control only when the SCR temperature is lower than the SCR determination temperature T11. When the SCR temperature is equal to or higher than the SCR determination temperature T11, the PCM 60 performs passive DeNOx control. In addition, execution of active DeNOx control is prohibited. This is because when the SCR temperature is equal to or higher than the SCR determination temperature T11, NOx in the exhaust gas can be appropriately purified by the SCR catalyst 47, so that the NOx purification performance by the NOx catalyst 45 should be ensured. This is because there is no need to perform DeNOx control. Therefore, in the present embodiment, when the SCR temperature is equal to or higher than the SCR determination temperature T11, execution of the DeNOx control is prohibited, and deterioration in fuel consumption caused by the execution of the DeNOx control is suppressed.

  Hereinafter, the passive DeNOx control and the active DeNOx control according to the above-described embodiment of the present invention will be specifically described.

(Passive DeNOx control)
First, the passive DeNOx control according to the embodiment of the present invention will be specifically described.

  First, with reference to FIG. 7, the setting process of the passive DeNOx control execution flag used in order to determine the necessity of execution of the passive DeNOx control according to the embodiment of the present invention will be described. FIG. 7 is a flowchart (passive DeNOx control execution flag setting flow) showing the setting process of the passive DeNOx control execution flag according to the embodiment of the present invention. This passive DeNOx control execution flag setting flow is repeatedly executed by the PCM 60 at a predetermined cycle, and is executed in parallel with the fuel injection control flow shown in FIG. 3 and the DeNOx post-injection amount calculation flow shown in FIG. The

First, in step S301, the PCM 60 acquires various information on the vehicle. Specifically, the PCM 60 calculates at least the NOx catalyst temperature, the SCR temperature, the target torque determined by the fuel injection control flow shown in FIG. 3, and the DeNOx post injection amount calculation flow shown in FIG. The obtained DeNOx post injection amount, the NOx occlusion amount of the NOx catalyst 45, and the value of the active DeNOx control execution flag used for determining whether or not the active DeNOx control needs to be executed are acquired. In this case, the NOx catalyst temperature is estimated based on, for example, the temperature detected by the temperature sensor 112 provided immediately upstream of the NOx catalyst 45 (a temperature sensor provided between the NOx catalyst 45 and the DPF 46). The temperature detected by 113 may also be used). The SCR temperature is estimated based on the temperature detected by the temperature sensor 117 provided immediately upstream of the SCR catalyst 47, for example. Further, the NOx occlusion amount is obtained by, for example, estimating the NOx amount in the exhaust gas based on the operating state of the engine E, the flow rate of the exhaust gas, the temperature of the exhaust gas, and the like, and integrating this NOx amount. It is done. The active DeNOx control execution flag is set in an active DeNOx control execution flag setting flow (see FIG. 10) described later.
In addition, in step S301, the PCM 60 also acquires the execution frequency of passive DeNOx control within a predetermined period. Specifically, the PCM 60 acquires, as the execution frequency of the passive DeNOx control, the number of times that the passive DeNOx control has been executed during a predetermined period (for example, several seconds or several minutes).

Next, in step S302, the PCM 60 determines whether or not the SCR temperature acquired in step S301 is lower than the SCR determination temperature T11. As a result of this determination, when the SCR temperature is lower than the SCR determination temperature T11 (step S302: Yes), the process proceeds to step S303. On the other hand, when the SCR temperature is equal to or higher than the SCR determination temperature T11 (step S302: No), the process proceeds to step S309. In this case, since NOx in the exhaust gas can be appropriately purified by the SCR catalyst 47, the PCM 60 sets the passive DeNOx control execution flag to “0” in order to prohibit the execution of the passive DeNOx control ( Step S309). Then, the process ends.
In step S302, it may be determined whether or not the SCR temperature is lower than the SCR determination temperature T11 and whether or not the exhaust gas flow rate is a predetermined amount or more. In this case, even when the SCR temperature is equal to or higher than the SCR determination temperature T11, if it is determined that the exhaust gas flow rate is equal to or higher than the predetermined amount, the passive DeNOx control execution flag is not set to “0”. Good. This is because the NOx purification performance of the NOx catalyst 45 is ensured by performing passive DeNOx control in consideration of the decrease in the NOx purification rate of the SCR catalyst 47 as the exhaust gas amount increases.

  Next, in step S303, the PCM 60 determines whether or not the target torque acquired in step S301 is equal to or greater than a predetermined torque. This determination corresponds to determining whether or not the air-fuel ratio of the exhaust gas when the fuel corresponding to the target torque is injected falls below a predetermined value on the rich side. In other words, this is equivalent to determining whether or not the driving state (predetermined acceleration state) is such that passive DeNOx control can be executed while suppressing deterioration in fuel consumption. If the result of determination in step S303 is that the target torque is greater than or equal to the predetermined torque (step S303: Yes), processing proceeds to step S304. On the other hand, when the target torque is less than the predetermined torque (step S303: No), the process proceeds to step S309. In this case, the PCM 60 prohibits the execution of the passive DeNOx control, and sets the passive DeNOx control execution flag to “0” in order to suppress the deterioration in fuel consumption caused by the execution of the passive DeNOx control (step S309). . Then, the process ends.

  Next, in step S304, the PCM 60 determines whether the execution frequency of the passive DeNOx control acquired in step S301 is less than a predetermined frequency determination value. As a result of the determination in step S304, when the execution frequency of the passive DeNOx control is less than the frequency determination value (step S304: Yes), the process proceeds to step S305. On the other hand, when the execution frequency of the passive DeNOx control is equal to or higher than the frequency determination value (step S304: No), the process proceeds to step S309. In this case, the PCM 60 sets the passive DeNOx control execution flag to “0” in order to prohibit the execution of the passive DeNOx control (step S309).

  When the passive DeNOx control has been performed relatively frequently so far, when the passive DeNOx control is executed from now on, there is a high possibility that oil dilution due to post injection occurs. Therefore, in this embodiment, when the execution frequency of the passive DeNOx control is equal to or higher than the frequency determination value (step S304: No), the execution of the passive DeNOx control is prohibited and the oil resulting from the post injection in the passive DeNOx control. Dilution is controlled. On the other hand, when passive DeNOx control has hardly been performed so far (that is, when the frequency of passive DeNOx control is relatively low), even if passive DeNOx control is executed from now on, oil dilution caused by post-injection will occur. Is unlikely to occur. Therefore, in this embodiment, when the execution frequency of the passive DeNOx control is less than the frequency determination value (step S304: Yes), the execution of the passive DeNOx control is not prohibited.

  Next, in step S305, it is determined whether or not the NOx occlusion amount acquired in step S301 is greater than or equal to the first occlusion amount determination value. For example, the first storage amount determination value is set to a value that is approximately half the limit value of the NOx storage amount. As a result of this determination, when the NOx storage amount is equal to or greater than the first storage amount determination value (step S305: Yes), the process proceeds to step S306. On the other hand, when the NOx storage amount is less than the first storage amount determination value (step S305: No), the process proceeds to step S309. In this case, the PCM 60 prohibits the execution of useless passive DeNOx control and sets the passive DeNOx control execution flag to “0” in order to suppress deterioration in fuel consumption due to the execution of passive DeNOx control (step S309). ). Then, the process ends.

  Next, in step S306, the PCM 60 determines whether or not the active DeNOx control execution flag acquired in step S301 is “0”. That is, the PCM 60 determines whether or not the situation is that the active DeNOx control should be executed. As a result of this determination, when the active DeNOx control execution flag is “0” (step S306: Yes), the process proceeds to step S307. On the other hand, when the active DeNOx control execution flag is not “0”, that is, when it is “1” (step S306: No), the process proceeds to step S309. In this case, the PCM 60 prohibits the execution of the passive DeNOx control and sets the passive DeNOx control execution flag to “0” in order to preferentially execute the active DeNOx control (step S309). That is, even if the execution condition of the passive DeNOx control is satisfied, the active DeNOx control is preferentially executed when the execution condition of the active DeNOx control is satisfied. Then, the process ends.

  Next, in step S307, the PCM 60 determines whether or not the DeNOx post injection amount acquired in step S301 is less than the first post injection amount determination value. As a result of the determination, when the DeNOx post injection amount is less than the first post injection amount determination value (step S307: Yes), the process proceeds to step S308. The condition that the DeNOx post injection amount in step S307 is less than the first post injection amount determination value is basically satisfied when the vehicle is accelerating (specifically, the vehicle is accelerating). (This is the premise that the condition is satisfied). In this case, since all of the above-described conditions of steps S302 to S307 are satisfied, the PCM 60 sets a passive DeNOx control execution flag to “1” in order to permit execution of the passive DeNOx control (step S308). Then, the process ends. On the other hand, when the DeNOx post injection amount is equal to or greater than the first post injection amount determination value (step S307: No), the process proceeds to step S309. In this case, since a relatively large amount of fuel is post-injected and the possibility of oil dilution increases, the PCM 60 controls the passive DeNOx control in order to inhibit the oil dilution by prohibiting the execution of the passive DeNOx control. The execution flag is set to “0” (step S309). Then, the process ends.

Note that the first post-injection amount determination value used in the determination in step S307 is preferably set based on the post-injection amount that is highly likely to cause oil dilution in one example. In another example, the first post-injection amount determination value may be set from the viewpoint of suppressing deterioration in fuel consumption caused by the execution of passive DeNOx control in addition to the viewpoint of suppressing oil dilution. In such a case, it is determined in step S307 whether or not the DeNOx post injection amount is less than the first post injection amount determination value because the air-fuel ratio of the exhaust gas when the fuel corresponding to the target torque is injected is predetermined. This is equivalent to determining whether or not the value drops to a value or less. In other words, it is almost synonymous with determining whether or not the target torque in step S303 is equal to or greater than a predetermined torque. For this reason, the determination in step S303 and the determination in step S307 overlap. For example, the determination in step S307 may be performed without performing the determination in step S303, and the passive DeNOx control execution flag may be set.
The first post injection amount determination value is not limited to a fixed value, and the first post injection amount determination value may be changed according to the in-cylinder temperature. Specifically, the first post injection amount determination value may be changed to a larger value as the in-cylinder temperature becomes higher. As the in-cylinder temperature rises, the post-injected fuel is vaporized and oil dilution is less likely to occur. Therefore, the restriction on the DeNOx post-injection amount in performing the passive DeNOx control may be relaxed, that is, the DeNOx post What is necessary is just to enlarge the upper limit of the injection amount.

  Next, the passive DeNOx control according to the embodiment of the present invention, which is executed based on the passive DeNOx control execution flag set as described above, will be described with reference to FIG. FIG. 8 is a flowchart (passive DeNOx control flow) showing passive DeNOx control according to the embodiment of the present invention. This passive DeNOx control flow is repeatedly executed by the PCM 60 at a predetermined cycle, and the fuel injection control flow shown in FIG. 3, the DeNOx post injection amount calculation flow shown in FIG. 4, and the passive DeNOx control shown in FIG. It is executed in parallel with the execution flag setting flow.

  First, in step S401, the PCM 60 acquires various information on the vehicle. Specifically, the PCM 60 at least sets the DeNOx post injection amount calculated in the DeNOx post injection amount calculation flow shown in FIG. 4 and the passive DeNOx control execution flag setting flow shown in FIG. And the value of the DeNOx control execution flag.

  Next, in step S402, the PCM 60 determines whether or not the passive DeNOx control execution flag acquired in step S401 is “1”. That is, the PCM 60 determines whether or not it is a situation where passive DeNOx control should be executed. As a result of this determination, when the passive DeNOx control execution flag is “1” (step S402: Yes), the process proceeds to step S403. On the other hand, when the passive DeNOx control execution flag is “0” (step S402: No), the process ends without executing the passive DeNOx control.

  Next, in step S403, the PCM 60 sets the post injection timing to be applied in the passive DeNOx control. A method for setting the post injection timing will be specifically described.

  As described above, in the present embodiment, when the passive DeNOx control is executed, the post-injected fuel is not burned in the cylinder without being burned from the viewpoint of suppressing smoke caused by the combustion of the post-injected fuel. By discharging the fuel into the exhaust passage 41 as fuel, the air-fuel ratio of the exhaust gas is set to the target air-fuel ratio. In order to discharge such post-injected fuel as unburned fuel without burning in the cylinder, post-injection may be performed at a relatively retarded timing in the expansion stroke. However, if the post injection timing is delayed too much, oil dilution by post injection occurs. Therefore, in this embodiment, from the viewpoint of suppressing smoke and oil dilution, an appropriate timing in the latter half of the expansion stroke is basically adopted as the post injection timing in the passive DeNOx control. In this embodiment, the post-injection timing at least in the latter half of the expansion stroke is set to be more retarded as the in-cylinder temperature becomes higher. This is because when the in-cylinder temperature becomes high, the post-injected fuel is further vaporized and oil dilution is less likely to occur even if the post injection timing is delayed.

  As described above, since it is difficult to detect the in-cylinder temperature with a sensor or accurately estimate the in-cylinder temperature, the PCM 60 uses various indexes that reflect the in-cylinder temperature, What is necessary is just to set a post injection timing. For example, the PCM 60 sets the post injection timing based on the engine water temperature and / or the intake air temperature. In this example, the PCM 60 retards the post injection timing as the engine water temperature increases, or retards the post injection timing as the intake air temperature increases.

  Further, in the above, the post injection timing is changed according to the in-cylinder temperature, but in another example, the post injection timing is not changed according to the in-cylinder temperature or the like, but a fixed value (that is, expansion) (Fixed timing in the second half of the stroke) may be applied.

Next, in step S404, the PCM 60 controls the fuel injection valve 20 to inject the DeNOx post-injection amount acquired in step S401 at the post-injection timing set in step S403, so that the air-fuel ratio of the exhaust gas is reached. Is set to the target air-fuel ratio, and NOx occluded in the NOx catalyst 45 is reduced. Specifically, the PCM 60 corresponds to the detection value of the O ₂ sensor 111 provided on the exhaust passage 41 in order to cope with detection variations of various sensors and variations in the fuel injection amount of the fuel injection valve 20. Based on the air-fuel ratio (actual air-fuel ratio) and the target air-fuel ratio, the post-injection amount injected from the fuel injection valve 20 is F / B controlled so that the actual air-fuel ratio matches the target air-fuel ratio. Hereinafter, the F / B control of the post injection amount performed during the passive DeNOx control is appropriately referred to as “first post injection F / B control”. In this first post-injection F / B control, not only F / B control but also F / F control is performed. However, because F / B control is mainly performed, the term “F / B control” is used for convenience of explanation. Is used.
Specifically, the PCM 60 first sets a relatively small air-fuel ratio (an air-fuel ratio with a relatively large rich degree) as a target value, and performs F / F control on the post-injection amount to be injected from the fuel injection valve 20, and thereafter Based on the actual air-fuel ratio and the target air-fuel ratio, the post-injection amount injected from the fuel injection valve 20 is F / B controlled using a relatively large F / B gain. In this way, the actual air-fuel ratio is made to quickly coincide with the target air-fuel ratio during passive DeNOx control that is performed for a relatively short time.
Actually, the PCM 60 executes the process of step S404 in step S106 of the fuel injection control flow shown in FIG.

  Next, in step S405, the PCM 60 determines whether or not the passive DeNOx control execution flag has become “0”. That is, the PCM 60 determines whether or not to end the passive DeNOx control. As a result of this determination, when the passive DeNOx control execution flag becomes “0” (step S405: Yes), the process ends. In this case, the PCM 60 ends the passive DeNOx control. On the other hand, when the passive DeNOx control execution flag is not “0” (step S405: No), that is, when the passive DeNOx control execution flag is maintained at “1”, the process returns to step S403, The processes after step S403 are performed again. In this case, the PCM 60 continues the passive DeNOx control. That is, the PCM 60 continues the passive DeNOx control until the passive DeNOx control execution flag is switched from “1” to “0”.

  Next, a specific example of the first post injection F / B control performed during the passive DeNOx control according to the embodiment of the present invention will be described with reference to FIG. FIG. 9 is a time chart showing a specific example of the first post-injection F / B control according to the embodiment of the present invention. FIG. 9 shows, in order from the top, the passive DeNOx control execution flag, the post injection amount, and the actual air-fuel ratio (λ).

  As shown in FIG. 9, when the passive DeNOx control execution flag is switched from “0” to “1”, the PCM 60 sets the actual air-fuel ratio to the target air-fuel ratio in order to reduce NOx occluded in the NOx catalyst 45. Thus, passive DeNOx control for post-injection from the fuel injection valve 20 is started. Specifically, the PCM 60 performs the first post-injection F / B control based on the actual air-fuel ratio and the target air-fuel ratio so that the actual air-fuel ratio quickly matches the target air-fuel ratio. Specifically, the PCM 60 first sets a relatively small air-fuel ratio (an air-fuel ratio with a relatively high degree of richness) as a target value, and performs F / F control on the post-injection amount injected from the fuel injection valve 20 (arrow A21). reference). As a result, the post injection amount is greatly increased, and the actual air-fuel ratio is rapidly changed to the rich side. Thereafter, the PCM 60 performs F / B control of the post-injection amount to be injected from the fuel injection valve 20 using a relatively large F / B gain based on the actual air-fuel ratio and the target air-fuel ratio (see arrow A22). As a result, the post-injection amount greatly changes, so that the actual air-fuel ratio once becomes richer than the target air-fuel ratio, that is, after the actual air-fuel ratio temporarily undershoots the target air-fuel ratio (see arrow A23). ), The actual air-fuel ratio becomes equal to the target air-fuel ratio.

  As described above, in the present embodiment, the first post-injection F / B control is executed during the passive DeNOx control, so that the post-injection amount is greatly changed so that the actual air-fuel ratio can be quickly matched with the target air-fuel ratio. ing. By doing so, the actual air-fuel ratio is immediately set to the target air-fuel ratio in a short period in which passive DeNOx control is performed, and the reduction of NOx occluded in the NOx catalyst 45 is surely executed.

  Note that when the actual air-fuel ratio undershoots the target air-fuel ratio as described above, the amount of unburned fuel discharged from the engine E to the exhaust passage 41 increases, but the HC purification performance of the NOx catalyst 45 during passive DeNOx control. Since the HC purification performance by the DOC in the NOx catalyst 45 is sufficiently secured, and since the execution period of the passive DeNOx control is relatively short, the unburned fuel discharged in this way is transferred to the NOx catalyst 45. It can be properly purified and does not cause any problems.

(Active DeNOx control)
Next, the active DeNOx control according to the embodiment of the present invention will be specifically described.

  First, with reference to FIG. 10, an active DeNOx control execution flag setting process used to determine whether or not active DeNOx control is necessary according to an embodiment of the present invention will be described. FIG. 10 is a flowchart showing an active DeNOx control execution flag setting process (active DeNOx control execution flag setting flow). The active DeNOx control execution flag setting flow is repeatedly executed by the PCM 60 at a predetermined cycle, and the fuel injection control flow shown in FIG. 3, the DeNOx post injection amount calculation flow shown in FIG. 4, and the flow shown in FIG. It is executed in parallel with the passive DeNOx control execution flag setting flow.

  First, in step S501, the PCM 60 acquires various information on the vehicle. Specifically, the PCM 60 acquires at least the NOx catalyst temperature, the SCR temperature, and the NOx occlusion amount of the NOx catalyst 45. Note that the method for obtaining the NOx catalyst temperature, the SCR temperature, and the NOx occlusion amount is as described in the description of step S301 in the “passive DeNOx control” section.

  Next, in step S502, the PCM 60 determines whether or not the SCR temperature acquired in step S501 is lower than the SCR determination temperature T11. As a result of this determination, when the SCR temperature is lower than the SCR determination temperature T11 (step S502: Yes), the process proceeds to step S503. On the other hand, when the SCR temperature is equal to or higher than the SCR determination temperature T11 (step S502: No), the process proceeds to step S509. In this case, since NOx in the exhaust gas can be appropriately purified by the SCR catalyst 47, the PCM 60 sets the active DeNOx control execution flag to “0” in order to prohibit the execution of the active DeNOx control ( Step S509). Then, the process ends.

  Next, in step S503, the PCM 60 determines whether or not the NOx catalyst temperature acquired in step S501 is equal to or higher than a predetermined temperature. When the NOx catalyst temperature is low, the NOx catalyst 45 hardly reduces the stored NOx even if the air-fuel ratio of the exhaust gas is set to the target air-fuel ratio. Therefore, in step S503, it is determined whether or not the NOx stored in the NOx catalyst 45 can be reduced. Therefore, the predetermined temperature used in the determination in step S503 is set based on the NOx catalyst temperature that can reduce the NOx stored in the NOx catalyst 45. As a result of the determination in step S503, when the NOx catalyst temperature is equal to or higher than the predetermined temperature (step S503: Yes), the process proceeds to step S504. On the other hand, when the NOx catalyst temperature is lower than the predetermined temperature (step S503: No), the process proceeds to step S509. In this case, the PCM 60 sets an active DeNOx control execution flag to “0” in order to prohibit execution of active DeNOx control (step S509).

  Next, in step S504, the PCM 60 determines whether or not active DeNOx control has never been executed after the engine is started. In step S504, if the active DeNOx control has never been executed after the engine is started, the execution conditions of the active DeNOx control are relaxed compared to the case where the active DeNOx control has been executed after the engine is started. The purpose is to preferentially execute the active DeNOx control. Specifically, when the active DeNOx control has been executed after the engine is started, the execution conditions of step S507 and the execution conditions of step S508, which are relatively severe, are used, whereas the active DeNOx control is executed after the engine is started. Is not executed at all, only the execution condition of step S505, which is relatively mild, is used (details thereof will be described later). As a result of the determination in step S504, when the active DeNOx control is not executed after the engine is started (step S504: Yes), the process proceeds to step S505.

  Next, in step S505, the PCM 60 determines whether or not the NOx storage amount acquired in step S501 is greater than or equal to the second storage amount determination value. For example, the second storage amount determination value is set to a value that is somewhat lower than the limit value of the NOx storage amount. If the result of this determination is that the NOx storage amount is greater than or equal to the second storage amount determination value (step S505: Yes), the process proceeds to step S506. In this case, the PCM 60 sets the active DeNOx control execution flag to “1” in order to permit execution of the active DeNOx control (step S506). In this way, the active DeNOx control is executed after the engine is started to forcibly reduce NOx occluded in the NOx catalyst 45 to some extent, thereby ensuring the NOx purification performance of the NOx catalyst 45 reliably. On the other hand, if the NOx occlusion amount is less than the second occlusion amount determination value (step S505: No), the process proceeds to step S509. In this case, the PCM 60 sets the active DeNOx control execution flag to “0” in order to prohibit the execution of useless active DeNOx control (step S509). Then, the process ends.

  On the other hand, if the result of determination in step S504 is that active DeNOx control has been executed after engine startup (step S504: No), the process proceeds to step S507. In step S507, the PCM 60 determines whether or not the NOx storage amount acquired in step S501 is equal to or greater than a third storage amount determination value (a value greater than the second storage amount determination value). For example, the third storage amount determination value is set to a value near the limit value of the NOx storage amount. As a result of this determination, if the NOx storage amount is equal to or greater than the third storage amount determination value (step S507: Yes), the process proceeds to step S508. On the other hand, when the NOx occlusion amount is less than the third occlusion amount determination value (step S507: No), the process proceeds to step S509. In this case, the PCM 60 sets the active DeNOx control execution flag to “0” in order to prohibit the execution of useless active DeNOx control (step S509). Then, the process ends.

  Next, in step S508, the PCM 60 determines whether the travel distance from the previous execution time of the active DeNOx control is equal to or greater than a predetermined determination distance. As a result of the determination in step S508, when the travel distance from the previous execution time of the active DeNOx control is equal to or greater than the determination distance (step S508: Yes), the process proceeds to step S506. In this case, the PCM 60 sets the active DeNOx control execution flag to “1” in order to permit execution of the active DeNOx control (step S506). In this way, the active DeNOx control is executed to forcibly reduce the NOx occluded in a large amount in the NOx catalyst 45, thereby ensuring the NOx purification performance of the NOx catalyst 45 reliably. On the other hand, when the travel distance from the previous execution time of the active DeNOx control is less than the determination distance (step S508: No), the process proceeds to step S509. In this case, the PCM 60 sets an active DeNOx control execution flag to “0” in order to prohibit execution of active DeNOx control (step S509). Then, the process ends.

  When the active DeNOx control is executed in a situation where the travel distance from the previous execution time of the active DeNOx control is short (that is, when the execution interval of the active DeNOx control is short), there is a high possibility that oil dilution due to post injection occurs. Therefore, in this embodiment, when the travel distance from the previous execution time of the active DeNOx control is less than the determination distance (step S508: No), the execution of the active DeNOx control is prohibited and the post injection in the active DeNOx control is performed. The oil dilution caused by this is suppressed. On the other hand, when the travel distance from the previous execution time of the active DeNOx control is long (that is, when the execution interval of the active DeNOx control is long), even if the active DeNOx control is executed from now on, the oil dilution caused by the post injection is not performed. It is unlikely to occur. Therefore, in this embodiment, when the travel distance from the previous execution time of the active DeNOx control is equal to or more than the determination distance (step S508: Yes), the execution of the active DeNOx control is not prohibited.

  Next, the active DeNOx control according to the embodiment of the present invention executed based on the active DeNOx control execution flag set as described above will be described with reference to FIG. FIG. 11 is a flowchart (active DeNOx control flow) showing active DeNOx control according to the embodiment of the present invention. This active DeNOx control flow is repeatedly executed by the PCM 60 at a predetermined cycle, and the fuel injection control flow shown in FIG. 3, the DeNOx post injection amount calculation flow shown in FIG. 4, and the active DeNOx control shown in FIG. It is executed in parallel with the execution flag setting flow.

  Here, in the active DeNOx control flow according to the present embodiment, in parallel with fuel injection control in active DeNOx control (fuel injection control in which post-injection is performed in order to set the air-fuel ratio of the exhaust gas to the target air-fuel ratio), Control for plug 21 (glow control) and EGR gas control (EGR control) are performed. The glow control is performed in order to improve the ignitability of the post-injected fuel in the active DeNOx control by controlling the energization of the glow plug 21 during the active DeNOx control. In addition, the EGR control controls the recirculation of an appropriate amount of EGR gas during the active DeNOx control, and delays the ignition of the post-injected fuel, thereby ensuring the combustion stability of the post-injected fuel and reducing the smoke. This is done to suppress the occurrence. In this embodiment, the fuel injection control in the active DeNOx control is started after the state of the glow plug 21 and the state of the EGR gas by the glow control and the EGR control are stabilized.

  The active DeNOx control flow in FIG. 11 will be specifically described. First, in step S601, the PCM 60 acquires various types of information on the vehicle. Specifically, the PCM 60 includes at least the engine load, the engine speed, the NOx catalyst temperature, the DeNOx post injection amount calculated in the DeNOx post injection amount calculation flow shown in FIG. And the value of the active DeNOx control execution flag set in the active DeNOx control execution flag setting flow shown. Further, the PCM 60 acquires the in-cylinder oxygen concentration obtained by the estimation.

  Next, in step S602, the PCM 60 determines whether or not the active DeNOx control execution flag acquired in step S601 is “1”. That is, the PCM 60 determines whether or not it is a situation in which active DeNOx control should be executed. As a result of this determination, when the active DeNOx control execution flag is “1” (step S602: Yes), the process proceeds to step S603. On the other hand, when the active DeNOx control execution flag is “0” (step S602: No), the process ends without executing the active DeNOx control.

  Next, in step S603, the PCM 60 determines whether or not the engine operating state (engine load and engine speed) is included in the active DeNOx execution region R12 (see FIG. 5). In this case, the PCM 60 expands the active DeNOx execution region R12 to the high load side and the high rotation side when the NOx catalyst temperature is equal to or higher than the predetermined temperature and / or when the NOx occlusion amount is equal to or higher than the predetermined amount. Good. As a result of the determination in step S603, if the engine operating state is included in the active DeNOx execution region R12 (step S603: Yes), the process proceeds to step S605. On the other hand, when the operating state of the engine is not included in the active DeNOx execution region R12 (step S603: No), the process proceeds to step S604.

  In step S604, the PCM 60 does not include the post injection without performing the active DeNOx control, that is, without performing the fuel injection control including the post injection for setting the air-fuel ratio of the exhaust gas to the target air-fuel ratio. The fuel injection control is performed (step S604). Basically, the PCM 60 performs only control for main injection of the fuel injection amount corresponding to the target torque. Actually, the PCM 60 executes the process of step S604 in step S106 of the fuel injection control flow shown in FIG. And a process returns to step S603 and performs determination of above-described step S603 again. That is, when the active DeNOx control execution flag is “1”, the PCM 60 performs normal fuel injection control while the engine operating state is not included in the active DeNOx execution region R12. When the operating state is included in the active DeNOx execution region R12, the normal fuel injection control is switched to the fuel injection control in the active DeNOx control. For example, when the operating state of the engine deviates from the active DeNOx execution region R12 during the fuel injection control in the active DeNOx control, the PCM 60 interrupts the fuel injection control and performs the normal fuel injection control. When the state enters the active DeNOx execution region R12, the fuel injection control in the active DeNOx control is resumed.

  Next, in step S605, the PCM 60 determines whether or not a predetermined time has passed since the glow plug 21 was energized by the glow control described above, that is, whether or not the energization time of the glow plug 21 has reached a predetermined time. Determine. In step S605, it is determined whether or not the energized glow plug 21 is in a stable state. The predetermined time used in step S605 is set based on the energization time required for the glow plug 21 to reach a desired temperature, for example. As a result of the determination in step S605, when the energization time of the glow plug 21 has reached the predetermined time (step S605: Yes), the process proceeds to step S606. On the other hand, when the energization time of the glow plug 21 has not reached the predetermined time (step S605: No), the process returns to step S603. In this case, it waits until the energization time of the glow plug 21 reaches a predetermined time.

  Next, in step S606, the PCM 60 causes the in-cylinder oxygen concentration obtained by the above-described EGR control to be the target in-cylinder oxygen concentration to be set in the fuel injection control (including post injection) in the active DeNOx control. It is determined whether or not it has almost reached. For example, the PCM 60 determines whether or not the difference (absolute value) between the in-cylinder oxygen concentration and the target in-cylinder oxygen concentration has become a predetermined value or less. In step S606, it is determined whether or not the amount of EGR gas introduced by the EGR control has reached a stable state, in other words, whether or not EGR gas having a desired flow rate has been introduced. As a result of the determination in step S606, if the in-cylinder oxygen concentration has substantially reached the target in-cylinder oxygen concentration by EGR control (step S606: Yes), the process proceeds to step S607. On the other hand, when the in-cylinder oxygen concentration has not substantially reached the target in-cylinder oxygen concentration (step S606: No), the process returns to step S603. In this case, the system waits until the in-cylinder oxygen concentration substantially reaches the target in-cylinder oxygen concentration by EGR control.

  Next, in step S607, the PCM 60 sets a post injection timing to be applied in the active DeNOx control. A method for setting the post injection timing will be specifically described.

  As described above, in the present embodiment, when executing the active DeNOx control, the air-fuel ratio of the exhaust gas is set to the target air-fuel ratio by burning the post-injected fuel in the cylinder. In order to burn the post-injected fuel in the cylinder, the post-injection may be performed at a relatively advanced timing in the expansion stroke. However, if the post injection timing is advanced too much, ignition occurs in a state where air and fuel are not properly mixed, and smoke is generated. Therefore, in the present embodiment, the post injection timing is appropriately set to the advance side, specifically, an appropriate timing in the first half of the expansion stroke is adopted as the post injection timing in the active DeNOx control, and an appropriate amount is set during the active DeNOx control. By introducing the EGR gas, the ignition of the post-injected fuel is delayed to suppress the occurrence of smoke and the like. In this embodiment, the post-injection timing at least in the first half of the expansion stroke is set to be more retarded as the engine load increases. This is because the amount of fuel injection increases as the engine load increases and smoke is likely to be generated, so that the post injection timing is retarded as much as possible. In this case, if the post injection timing is retarded too much, the post-injected fuel will not burn (misfire) and HC will be generated. Therefore, in this embodiment, the post injection timing is retarded appropriately. ing.

  In the present embodiment, the post injection timing is set to the advance side as the engine speed increases, that is, the degree of delay of the post injection timing is reduced. If fuel is injected at the same crank angle as when the engine speed is low when the engine speed is high, misfire may occur due to a short time until the fuel ignites. In order to ensure combustion stability, the post injection timing is set to the advance side as the engine speed increases.

  Next, in step S608, the PCM 60 determines whether or not the DeNOx post injection amount acquired in step S601 is less than a second post injection amount determination value. The second post injection amount determination value is set to a value larger than the first post injection amount determination value (see step S307 in FIG. 7) used in the above passive DeNOx control. In this way, it is possible to inject a larger amount of post-injection in the active DeNOx control than in the passive DeNOx control, regardless of the operating state of the engine E (for example, in a situation where the air-fuel ratio decreases during acceleration). If not), the air-fuel ratio of the exhaust gas can be reliably set to the target air-fuel ratio.

As a result of the determination in step S608, when the DeNOx post injection amount is less than the second post injection amount determination value (step S608: Yes), the process proceeds to step S609. In step S609, the PCM 60 controls the fuel injection valve 20 to inject the DeNOx post-injection amount acquired in step S601 at the post-injection timing set in step S607, and targets the air-fuel ratio of the exhaust gas. The air-fuel ratio is set so that the NOx stored in the NOx catalyst 45 is reduced. Specifically, the PCM 60 corresponds to the detection value of the O ₂ sensor 111 provided on the exhaust passage 41 in order to cope with detection variations of various sensors and variations in the fuel injection amount of the fuel injection valve 20. Based on the air-fuel ratio (actual air-fuel ratio) and the target air-fuel ratio, the post-injection amount injected from the fuel injection valve 20 is F / B controlled so that the actual air-fuel ratio matches the target air-fuel ratio. Hereinafter, the F / B control of the post injection amount performed during the active DeNOx control is appropriately referred to as “second post injection F / B control”. In this second post-injection F / B control, not only the F / B control but also the F / F control is performed. However, since the F / B control is mainly performed, the term “F / B control” is used for convenience of explanation. Is used.
Specifically, the PCM 60 first sets a relatively large air-fuel ratio (an air-fuel ratio with a relatively small rich degree) as a target value, performs F / F control on the post-injection amount to be injected from the fuel injection valve 20, and thereafter Based on the actual air-fuel ratio and the target air-fuel ratio, the post-injection amount injected from the fuel injection valve 20 is F / B controlled using a relatively small F / B gain. By so doing, the actual air-fuel ratio is gradually changed toward the target air-fuel ratio during active DeNOx control, and the actual air-fuel ratio is prevented from becoming richer than the target air-fuel ratio. Undershooting the target air-fuel ratio is suppressed.
Actually, the PCM 60 executes the process of step S609 in step S106 of the fuel injection control flow shown in FIG.

On the other hand, when the DeNOx post injection amount is equal to or greater than the second post injection amount determination value (step S608: No), the process proceeds to step S610. In step S610, the PCM 60 sets the air-fuel ratio of the exhaust gas by the post injection amount that does not exceed the second post injection amount determination value (specifically, the second post injection amount determination value itself is applied as the DeNOx post injection amount). In order to set the target air-fuel ratio, control is performed to reduce the oxygen concentration of the air introduced into the engine E. In this case, the PCM 60 executes at least one of the control for driving the intake shutter valve 7 in the valve closing direction, the control for increasing the EGR gas amount, and the control for decreasing the supercharging pressure by the turbocharger 5. Thus, the oxygen concentration of the air introduced into the engine E is reduced, that is, the filling amount is reduced. For example, the PCM 60 obtains the supercharging pressure required to bring the air-fuel ratio of the exhaust gas to the target air-fuel ratio by the DeNOx post-injection amount to which the second post-injection amount determination value is applied, and realizes this supercharging pressure. In addition, the intake shutter valve 7 is controlled to a desired opening on the closing side based on the actual supercharging pressure (pressure detected by the pressure sensor 108) and the EGR gas amount. Then, the process proceeds to step S611.
Note that the intake shutter valve 7 is set to be fully open in the normal operation state of the engine E. On the other hand, at the time of DeNOx, DPF regeneration, idle operation, etc., the intake shutter valve 7 is basically set to a predetermined base opening. In the operation state where EGR gas is not introduced, the intake shutter valve 7 is feedback-controlled based on the supercharging pressure.

  Next, in step S611, the PCM 60 applies the second post-injection amount determination value to the DeNOx post-injection amount, that is, sets the DeNOx post-injection amount to the second post-injection amount determination value. The fuel injection valve 20 is controlled so that the injection amount is injected at the post injection timing set in step S607, the air-fuel ratio of the exhaust gas is set to the target air-fuel ratio, and the NOx stored in the NOx catalyst 45 is stored. Try to reduce. Specifically, the PCM 60 performs the second post-injection F / B control as in step S609. That is, the PCM 60 first sets a relatively large air-fuel ratio (an air-fuel ratio with a relatively small rich degree) as a target value, and performs F / F control on the post-injection amount to be injected from the fuel injection valve 20, and thereafter Based on the actual air-fuel ratio and the target air-fuel ratio, the post-injection amount injected from the fuel injection valve 20 is F / B controlled using a relatively small F / B gain. Actually, the PCM 60 executes the process of step S611 in step S106 of the fuel injection control flow shown in FIG.

  After step S609 or step S611 described above, the process proceeds to step S612. In step S612, the PCM 60 determines whether or not the NOx occlusion amount of the NOx catalyst 45 has become substantially zero. Specifically, in the PCM 60, the NOx occlusion amount estimated based on the operating state of the engine E, the flow rate of exhaust gas, the temperature of exhaust gas, and the like is almost zero, and the NOx provided immediately downstream of the DPF 46 When the detection value of the sensor 116 changes, it is determined that the NOx occlusion amount of the NOx catalyst 45 has become almost zero. When the NOx occlusion amount of the NOx catalyst 45 becomes almost zero (step S612: Yes), the process ends. In this case, the PCM 60 ends the active DeNOx control. Further, the PCM 60 resets the NOx occlusion amount used in the active DeNOx control flow and the active DeNOx control execution flag setting flow of FIG.

  On the other hand, when the NOx occlusion amount of the NOx catalyst 45 is not substantially zero (step S612: No), the process returns to step S603. In this case, the PCM 60 continues the active DeNOx control. That is, the PCM 60 continues the active DeNOx control until the NOx occlusion amount of the NOx catalyst 45 becomes substantially zero. In particular, the PCM 60 does not satisfy the execution condition of the active DeNOx control (for example, the condition in step S603) during the active DeNOx control, and when the execution condition of the active DeNOx control is satisfied after that even if the active DeNOx control is interrupted. Then, the active DeNOx control is restarted promptly so that the NOx occlusion amount of the NOx catalyst 45 becomes substantially zero.

  Here, the reason why it is possible to determine that the NOx occlusion amount of the NOx catalyst 45 has become substantially zero based on the detection value of the NOx sensor 116 is as follows. Since the NOx sensor 116 also has a function as an oxygen concentration sensor, the detected value of the NOx sensor 116 corresponds to the air-fuel ratio of the exhaust gas supplied to the NOx sensor 116. While the NOx catalyst 45 is being reduced, that is, when the NOx occlusion amount of the NOx catalyst 45 is not substantially zero, oxygen produced by the reduction of NOx is supplied to the NOx sensor 116. On the other hand, when the NOx occlusion amount of the NOx catalyst 45 becomes substantially zero, oxygen generated by such reduction is not supplied to the NOx sensor 116. Therefore, at the timing when the NOx occlusion amount of the NOx catalyst 45 becomes substantially zero, the detected value of the NOx sensor 116 changes as the air-fuel ratio of the exhaust gas supplied to the NOx sensor 116 decreases.

  The second post injection amount determination value used in the above-described active DeNOx control flow is not limited to a fixed value, and the second post injection amount determination value may be changed according to the in-cylinder temperature. Specifically, in the same way as the first post-injection amount determination value described above, the in-cylinder temperature is reduced in consideration that the fuel in the post-injection is vaporized and oil dilution is less likely to occur as the in-cylinder temperature increases. What is necessary is just to change a 2nd post injection amount determination value to a large value, so that it becomes high.

  Next, with reference to FIG. 12, a specific example of the second post-injection F / B control performed during the active DeNOx control according to the embodiment of the present invention will be described. FIG. 12 is a time chart showing a specific example of the second post injection F / B control according to the embodiment of the present invention. FIG. 12 shows the active DeNOx control execution flag, the post injection amount, and the actual air-fuel ratio (λ) in order from the top.

  As shown in FIG. 12, when the active DeNOx control execution flag is switched from “0” to “1”, the PCM 60 sets the actual air-fuel ratio to the target air-fuel ratio in order to reduce NOx stored in the NOx catalyst 45. Thus, active DeNOx control for post-injection from the fuel injection valve 20 is started. Specifically, the PCM 60 performs the second post-injection F / B control so as to make the actual air-fuel ratio coincide with the target air-fuel ratio by gradually changing the actual air-fuel ratio based on the actual air-fuel ratio and the target air-fuel ratio. To do. Specifically, the PCM 60 first sets an air-fuel ratio (air-fuel ratio with a large rich degree) smaller than the above-described first post-injection F / B control (see FIG. 9) as a target value and injects from the fuel injection valve 20. The post injection amount to be performed is F / F controlled (see arrow A31). As a result, the post injection amount gradually increases, and the actual air-fuel ratio gradually changes to the rich side (see arrow A33). Thereafter, the PCM 60 causes the fuel injection valve 20 to inject based on the actual air-fuel ratio and the target air-fuel ratio using an F / B gain smaller than the first post-injection F / B control (see FIG. 9). The post-injection amount is F / B controlled (see arrow A32). As a result, the post-injection amount increases more gradually, and the actual air-fuel ratio gradually changes toward the target air-fuel ratio, so that the actual air-fuel ratio does not undershoot the target air-fuel ratio (see arrow A34). The actual air fuel ratio becomes equal to the target air fuel ratio.

  As described above, in the present embodiment, the second post-injection F / B control is executed during the active DeNOx control, so that the post-injection amount change speed is limited, that is, the post-injection amount change speed is a predetermined speed. Do not exceed this. In particular, in the second post-injection F / B control, a limit is imposed on the change rate of the post-injection amount so that the actual air-fuel ratio does not undershoot the target air-fuel ratio. In principle, in the second post-injection F / B control performed during active DeNOx control, the change rate of the post-injection amount is made smaller than in the second post-injection F / B control performed during passive DeNOx control (in other words, In the second post-injection F / B control performed during the passive DeNOx control, the change rate of the post-injection amount is made larger than that in the second post-injection F / B control performed during the active DeNOx control. In the present embodiment, by executing such second post-injection F / B control, the actual air-fuel ratio undershoots the target air-fuel ratio during active DeNOx control, and a large amount of smoke and HC are generated. I try to suppress it reliably.

<Effect>
Next, the operation and effect of the engine exhaust gas purification apparatus according to the embodiment of the present invention will be described.

  According to the present embodiment, the second post-injection F / B control is executed during the active DeNOx control to limit the rate of change of the post-injection amount, so that the actual air-fuel ratio is the target during the post-injection amount control. It is possible to appropriately suppress the rich side from the air-fuel ratio, that is, the actual air-fuel ratio undershooting the target air-fuel ratio. As a result, it is possible to appropriately suppress the occurrence of smoke and HC due to the actual air-fuel ratio undershooting the target air-fuel ratio during active DeNOx control.

  Further, according to the present embodiment, when the active DeNOx control is executed, the post injection is performed at the timing when the post-injected fuel is burned in the cylinder. When the post-injected fuel is burned with the actual air-fuel ratio undershooting the target air-fuel ratio, smoke tends to be generated, but in the present embodiment, as described above, the post-injection amount changes with respect to the change rate. Since a restriction is imposed to suppress undershoot, occurrence of such smoke can be appropriately suppressed.

  Further, according to the present embodiment, the first post-injection F / B control is executed during the passive DeNOx control, and the change rate of the post-injection amount is made larger than during the active DeNOx control. It is possible to quickly match the fuel ratio. Thus, in a relatively short period (acceleration engine) in which passive DeNOx control is performed, the actual air-fuel ratio is immediately set to the target air-fuel ratio, and the NOx occluded in the NOx catalyst 45 is reliably reduced. it can.

  Further, according to the present embodiment, when the passive DeNOx control is performed, the post-injected fuel is discharged without being burned in the cylinder and discharged as unburned fuel. As a result of increasing the change rate of the injection amount, smoke does not occur even if the actual air-fuel ratio undershoots the target air-fuel ratio. Further, when the actual air-fuel ratio undershoots the target air-fuel ratio, the amount of unburned fuel that is discharged increases. However, since the purification performance of the NOx catalyst 45 is sufficiently ensured during the passive DeNOx control, Since the execution period is relatively short, the unburned fuel discharged in this way can be appropriately purified by the NOx catalyst 45, and there is no particular problem.

  In the present embodiment, the post-injection amount is changed by the second post-injection F / B control based on the air-fuel ratio during active DeNOx control, and the post-injection is performed by the first post-injection F / B control based on the air-fuel ratio during passive DeNOx control. Change the amount. In this embodiment, the F / B gain of the second post injection F / B control performed during the active DeNOx control is made smaller than the F / B gain of the first post injection F / B control performed during the passive DeNOx control. Thus, it is possible to appropriately suppress the actual air-fuel ratio from undershooting the target air-fuel ratio during active DeNOx control, and it is possible to quickly reach the target air-fuel ratio during passive DeNOx control.

  Further, in the present embodiment, since the active DeNOx control is executed only in the active DeNOx execution region R12 as the medium load / medium rotation region, the active DeNOx control is executed outside the active DeNOx execution region R12, and the post is performed by this DeNOx control. Smoke and HC generated by trying to burn the injected fuel can be appropriately suppressed. On the other hand, in this embodiment, active DeNOx control is executed in the active DeNOx execution region R12, and passive DeNOx control is executed outside the active DeNOx execution region R12. Emission in the entire operation region of E can be appropriately improved. In particular, in the present embodiment, since the SCR catalyst 47 is provided on the exhaust passage 41 in addition to the NOx catalyst 45, NOx in the exhaust gas is converted by the SCR catalyst 47 in a region where the passive DeNOx control and the active DeNOx control are not executed. It can be properly purified.

<Modification>
In the above-described embodiment, the execution of the passive DeNOx control is completely prohibited when the execution frequency of the passive DeNOx control is equal to or higher than the frequency determination value. However, the present invention is not limited to this. In short, when the execution frequency of the passive DeNOx control is equal to or higher than the frequency determination value, the execution of the passive DeNOx control may be appropriately limited as compared with the case where the execution frequency of the passive DeNOx control is less than the frequency determination value. For example, when the execution frequency of the passive DeNOx control is equal to or higher than the frequency determination value, the number of post injections may be reduced as compared with the case where the execution frequency of the passive DeNOx control is less than the frequency determination value.
Similarly, when the travel distance since the previous execution of the active DeNOx control is less than the determination distance, or when the elapsed time since the previous execution of the active DeNOx control is less than the determination time, the execution of the active DeNOx control is performed. Is not limited to completely prohibiting, and execution of active DeNOx control may be appropriately limited.

  Further, in the above-described embodiment, the EGR gas recirculation to the intake system IN is completely prohibited during the passive DeNOx control. However, within the range in which the passage due to the deposit caused by the recirculation of the EGR gas does not occur. A small amount of EGR gas may be refluxed during passive DeNOx control. In this case, at least during passive DeNOx control, the EGR gas amount should be smaller than during active DeNOx control.

DESCRIPTION OF SYMBOLS 1 Intake passage 5 Turbocharger 7 Intake shutter valve 17 Combustion chamber 20 Fuel injection valve 41 Exhaust passage 43 EGR device 43a EGR passage 43b EGR cooler 43c First EGR valve 43d EGR cooler bypass passage 43e 2nd EGR valve 45 NOx catalyst 46DP
47 SCR catalyst 60 PCM
111 O ₂ sensor 116 NOx sensor 200 Engine system E Engine EX Exhaust system IN Intake system

Claims (8)

NOx in the exhaust gas is occluded when the air-fuel ratio of the exhaust gas is leaner than the stoichiometric air-fuel ratio, provided on the exhaust passage of the engine. An engine exhaust purification device comprising a NOx catalyst that reduces when the fuel is in the vicinity of the fuel ratio or richer than the stoichiometric air-fuel ratio,
When the NOx occlusion amount of the NOx catalyst is greater than or equal to a predetermined amount, the first NOx is post-injected from the fuel injection valve so that the air-fuel ratio of the exhaust gas becomes the predetermined air-fuel ratio that can reduce the NOx occluded in the NOx catalyst. NOx reduction control means for executing reduction control,
The NOx reduction control means increases the post injection amount so that the air-fuel ratio of the exhaust gas is set to the predetermined air-fuel ratio by the first NOx reduction control, and the change rate of the post injection amount at this time is less than the predetermined change rate Impose restrictions to be
The NOx reduction control means includes
When the air-fuel ratio of the exhaust gas changes to the rich side due to acceleration of the vehicle, the fuel is set so that the air-fuel ratio of the exhaust gas is temporarily set to the predetermined air-fuel ratio so as to reduce the NOx stored in the NOx catalyst. Further executing second NOx reduction control for post injection from the injection valve;
When increasing the post injection amount so that the air-fuel ratio of the exhaust gas is set to the predetermined air-fuel ratio by the second NOx reduction control, the air-fuel ratio of the exhaust gas is set to the predetermined air-fuel ratio by the first NOx reduction control. Increase the rate of change of the post injection amount than when increasing the post injection amount,
Executing the first NOx reduction control for a longer time than the second NOx reduction control;
An exhaust emission control device for an engine.   2. The engine exhaust purification device according to claim 1, wherein the NOx reduction control means imposes a limit on a change speed of the post injection amount so that an air-fuel ratio of the exhaust gas does not become richer than the predetermined air-fuel ratio. .   3. The engine exhaust purification according to claim 1, wherein when performing the first NOx reduction control, the NOx reduction control means performs post injection at a timing at which post-injected fuel is combusted in a cylinder of the engine. apparatus.   The NOx reduction control means, when executing the second NOx reduction control, performs post injection at a timing when the post-injected fuel is discharged into the exhaust passage as unburned fuel without being burned in the cylinder of the engine. Item 4. The engine exhaust gas purification device according to any one of Items 1 to 3.   The NOx reduction control means does not impose a limit on the change rate of the post injection amount when increasing the post injection amount so that the air-fuel ratio of the exhaust gas is set to the predetermined air-fuel ratio by the second NOx reduction control. The exhaust emission control device for an engine according to any one of claims 1 to 4.   When performing the first and second NOx reduction controls, the NOx reduction control means performs post-processing so as to realize the predetermined air-fuel ratio based on the air-fuel ratio of the exhaust gas detected by the sensor and the predetermined air-fuel ratio. 6. The gain in the F / B control is made larger when the injection amount is F / B controlled and the second NOx reduction control is executed than when the first NOx reduction control is executed. The exhaust emission control device for an engine according to one item.   The NOx reduction control means executes the first NOx reduction control only when the engine load is in a middle load range that is equal to or higher than the first predetermined load and less than a second predetermined load that is greater than the first predetermined load. The exhaust emission control device for an engine according to any one of claims 1 to 6.   The engine exhaust gas purification apparatus according to any one of claims 1 to 7, further comprising an SCR catalyst that is provided on an engine exhaust passage and purifies NOx in the exhaust gas by reaction with ammonia.

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