JP2013221442A - Exhaust emission control device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an exhaust emission control device capable of efficiently eliminating the clogging with PM when a recovery control is to be done, which is for recovering the performance of an exhaust gas purifying catalyst such as an NSR catalyst 21, and of precluding an increase of the exhaust pressure loss in an internal combustion engine or a drop of the reliability.SOLUTION: An exhaust emission control device is configured to use a fact that a recovery control (for example, S-poisoning recovery control) of an exhaust gas purifying catalyst causes the catalyst floor temperature to rise owing to the air-fuel ratio turning lean and conduction of a post-injection. When a determination is made that the deposit amount of PM on the front end face of the catalyst has risen beyond a prescribed level, an angle advance is made in the post-injection so that at least a portion of the fuel is combusted in a cylinder. Thus, the temperature of the exhaust gas flowing to the catalyst is increased further, and the PM deposited on the front end face is removed upon oxidation (front end face recovery processing).

Description

  The present invention relates to an exhaust emission control device for an internal combustion engine (hereinafter also referred to as an engine), and particularly relates to an apparatus for performing recovery control for recovering the purification performance of an exhaust purification catalyst.

  2. Description of the Related Art Conventionally, a lean NOx catalyst capable of purifying nitrogen oxides (NOx) even in exhaust gas with a high oxygen concentration, as an exhaust gas purification device for a diesel engine (hereinafter, also simply referred to as an engine) mounted on an automobile or the like, for example A PM filter that collects particulate matter (PM) in exhaust gas is known (see, for example, Patent Document 1 below).

Among lean NOx catalysts, NOx storage reduction (NSR catalyst) stores NOx in a state where the oxygen concentration in the exhaust is high, in other words, in a state where the air-fuel ratio (A / F) of the exhaust is lean. It has NOx storage material. The NOx released from the NOx occlusion material as the oxygen concentration decreases, that is, the air-fuel ratio becomes richer, is reduced to hydrogen (H ₂ ), carbon monoxide (CO), hydrocarbons (HC), etc. in the exhaust gas. It is made harmless by reacting with agent components.

  In such an NSR catalyst, the ability to store NOx in the exhaust gas decreases as the NOx storage amount increases. Therefore, when the storage amount reaches a predetermined threshold, the exhaust air-fuel ratio is intentionally switched to the rich side. Then, NOx reduction control for releasing and reducing the stored NOx is performed. Specifically, a fuel addition valve is provided in the exhaust system to supply fuel, or a small amount of fuel is injected (post-injection) after the main fuel injection into the combustion chamber, thereby enriching the air-fuel ratio of the exhaust. Thus, the NSR catalyst can be brought into a reducing atmosphere.

  In general, in a diesel engine, accompanying combustion, an oxide (SOx) of a sulfur component contained in fuel (light oil) or lubricating oil is also generated. Since this SOx accumulates in the NOx occlusion material as a chemically stable substance such as sulfate, the NOx occlusion capacity gradually decreases as the accumulation amount increases (so-called sulfur poisoning) Hereinafter, it is also referred to as S poison).

  Therefore, if the SOx accumulation amount of the NOx occlusion material reaches a predetermined threshold value, the control is performed to recover the NOx occlusion capability by intentionally enriching the air-fuel ratio of the exhaust gas and releasing the accumulated SOx (hereinafter referred to as “NOx occlusion capacity”). S poisoning recovery control) is performed. This is because the catalyst bed temperature is effectively increased by supplying fuel to the exhaust system or post-injection in a lean state of the air-fuel ratio, and then the air-fuel ratio is enriched to bring the NSR catalyst into a reducing atmosphere. In this way, SOx is released from the NOx occlusion material and reduced and purified.

  On the other hand, as the PM filter, various structures have been proposed for efficiently scavenging (collecting) PM from the flow of exhaust gas passing therethrough, such as DPF (Diesel Particulate Filter) and DPNR (Diesel Particulate-). This is known as NOx Reduction. Thus, in the filter that collects PM, as the amount of collected PM increases, the ventilation resistance of the filter increases and eventually clogging occurs.

  Therefore, if the amount of PM trapped in the filter reaches a predetermined threshold, fuel supply to the exhaust system, post-injection, etc. are performed in the same manner as in the NOx reduction control and S poison recovery control, and thus the exhaust gas is supplied into the exhaust. Filter regeneration control is performed in which the collected PM is oxidized (burned) and removed by raising the filter temperature to a predetermined level or higher by burning the fuel.

JP 2008-45461 A

  By the way, as described above, PM contained in the exhaust gas may be clogged by adhering to and accumulating not only in the PM filter but also in the cell of the NSR catalyst. In particular, when the above-described NOx reduction control or S poison recovery control is performed, hydrocarbons (HC) present in the rich exhaust gas adhere to the front end surface (the upstream end surface of the exhaust gas) of the catalyst carrier, and PM is deposited. It will function as a binder that promotes.

  If the NSR catalyst is clogged by the PM deposited and deposited in this way, the engine output and fuel consumption deteriorate due to an increase in exhaust pressure loss, and the catalyst bed temperature rises excessively when the accumulated PM burns rapidly. There is also a problem of reliability that there is a risk of failure.

  With regard to this point, in Patent Document 1, if clogging due to PM is determined, the temperature of the NSR catalyst is raised to promote HC vaporization, and the electric supercharger is operated to force the flow rate of exhaust gas. A technique for increasing the amount and blowing away PM deposited on the front end face of the catalyst carrier is disclosed.

  However, in order to blow off PM in such a manner, an electric supercharger that can freely increase the flow rate of exhaust gas is required. When PM is blown off from the front end face of the catalyst carrier, this PM is disposed downstream of the cell passage. Since it may be reattached to the wall surface, the method of blowing off PM in the exhaust gas having a high concentration of HC as a binder as described above is not necessarily efficient.

  The present invention has been made in view of such a point, and an object of the present invention is to efficiently prevent clogging by PM in an exhaust purification apparatus that performs control for recovering the performance of an exhaust purification catalyst such as an NSR catalyst. It is to eliminate well and prevent an increase in exhaust pressure loss and a decrease in reliability.

  In order to achieve the above object, the present invention focuses on the fact that the catalyst bed temperature rises due to the leaning of the air-fuel ratio and post-injection during the recovery control of the exhaust purification catalyst, and at least one of the fuel produced by the post-injection. By burning the part in the cylinder, the temperature of the exhaust gas flowing into the catalyst was raised to a predetermined level or more, and PM deposited on the front end face was oxidized and removed.

-Solution-
Specifically, the present invention is directed to an exhaust purification device for an internal combustion engine in which a fuel injection valve is provided facing a cylinder, an exhaust purification catalyst disposed in an exhaust system of the internal combustion engine, and the exhaust purification catalyst In order to recover the purification performance, there is provided control means for performing recovery control for controlling at least the fuel injection valve and switching the air-fuel ratio from the rich side to the lean side. When the air-fuel ratio is switched to the lean side in the recovery control, unburned fuel is supplied to the exhaust purification catalyst by post injection in the latter half of the expansion stroke of the cylinder, and the control means If it is determined that the amount of PM deposited on the catalyst has reached a predetermined amount or more, the post injection is advanced so that at least a part of the fuel burns in the cylinder, and the temperature of the exhaust gas flowing into the exhaust purification catalyst is increased. It was set as the structure raised.

  In the exhaust purification device having such a configuration, when the air-fuel ratio is switched to the lean side in the recovery control of the purification performance of the exhaust purification catalyst, if the PM accumulation amount in the exhaust purification catalyst is equal to or greater than a predetermined amount, the cylinder is usually The post-injection performed in the latter half of the expansion stroke is advanced to burn at least part of the fuel in the cylinder. As a result, the temperature of the exhaust gas flowing into the exhaust purification catalyst becomes higher than a predetermined value (for example, 600 ° C. or higher), and the PM adhering to the front end surface is oxidized and removed.

  That is, by utilizing the temperature rise of the exhaust purification catalyst during the recovery control, the PM on the catalyst front end face can be removed by slightly changing the post injection timing when the air-fuel ratio is switched to the lean side. Therefore, clogging due to PM of the exhaust purification catalyst can be efficiently eliminated, and an increase in exhaust pressure loss and a decrease in reliability can be prevented. In addition, the temperature upstream of the catalyst can be efficiently raised by the oxidation heat of PM, which is advantageous for recovery of the purification performance of this portion.

  Further, since the PM can be efficiently removed when the air-fuel ratio is switched to the lean side, it is possible to allow the generation of PM to be promoted more than before when the air-fuel ratio is switched to the rich side. . Therefore, the recovery control of the exhaust purification catalyst can be executed even in the operation region where the recovery control could not be executed conventionally because of concern that the amount of PM generated will increase.

Furthermore, if the generation of PM can be allowed more than before as described above, the air-fuel ratio can be made richer than before. Therefore, if, for example, S poisoning recovery control (control for releasing the sulfur component accumulated in the exhaust purification catalyst) is performed as recovery control, when the air-fuel ratio is controlled to the rich side, it is richer than the stoichiometric air-fuel ratio. It is preferable to make it. By doing so, reducing agent components such as H ₂ , CO, and HC generated with combustion increase, and the release of SOx can be effectively promoted.

  Further, when the post-injection is advanced during the recovery control so that at least a part of the fuel is burned in the cylinder, the engine torque increases thereby canceling the increase in the torque. Thus, it is preferable to reduce the fuel injection amount of the main injection performed near the compression top dead center of the cylinder.

  Here, it can be determined by the pressure upstream of the exhaust gas from the exhaust purification catalyst that the amount of PM accumulated in the exhaust purification catalyst has reached a predetermined amount or more as described above. That is, as an example, an exhaust pressure sensor is disposed in the exhaust passage on the upstream side of the catalyst, and the clogging of the catalyst can be accurately determined from the output signal. Note that it may be determined from the operation history of the internal combustion engine that the accumulated amount of PM is equal to or greater than a predetermined amount, or may be determined using both the output signal of the exhaust pressure sensor and the operation history of the internal combustion engine. You may make it do.

  By the way, for example, when the air-fuel ratio is enriched in the recovery control such as the S poison recovery control, the generation of PM is promoted. Therefore, a PM filter is disposed downstream of the exhaust purification catalyst. If this is the case, there is a concern that the increase in the amount of PM collected by this PM filter will be accelerated and the ventilation resistance will increase.

  In this regard, in the S poison recovery control, when the air-fuel ratio is switched to the lean side, the unburned fuel supplied to the exhaust system by post-injection burns, so that not only the catalyst bed temperature but also the temperature of the PM filter increases. Therefore, the PM of the PM filter may be oxidized and removed as in the filter regeneration control described above.

  Therefore, if the amount of PM collected in the PM filter is equal to or greater than a predetermined amount, the advance angle of post injection in the recovery control may not be performed. That is, for example, when the amount of PM accumulated in the exhaust purification catalyst becomes equal to or greater than a predetermined determination value, if the amount of PM collected by the PM filter is less than a predetermined amount, the post-injection is advanced. Do not advance the post-injection.

  Further, for example, if the PM accumulation amount in the exhaust purification catalyst reaches a predetermined upper limit value, and the possibility of clogging the front end surface becomes high, the recovery control is performed without considering the PM collection amount of the PM filter. You may make it perform the advance angle of the post injection in. In addition, what is necessary is just to determine about PM collection amount of PM filter based on the differential pressure | voltage of the upstream of a PM filter, and downstream as well-known.

  Further, if the PM collection amount of the PM filter is taken into consideration, the recovery control is not performed to switch the air-fuel ratio to the rich side until the PM collection amount of the PM filter becomes less than a predetermined amount. It may be. By so doing, the air-fuel ratio is made lean in the recovery control, the state in which normal post injection is performed continues, and as a result of effectively raising the temperature of the exhaust system, PM is removed from the PM filter.

  Therefore, regardless of the amount of PM deposited on the exhaust purification catalyst, that is, the case where the amount of PM accumulated is less than a predetermined amount and the advance of the post injection in the recovery control is not performed, If the amount of collected PM is equal to or greater than a predetermined amount, the air-fuel ratio may not be switched to the rich side in the recovery control.

  According to the exhaust emission control device according to the present invention, in the recovery control for recovering the purification performance of the exhaust purification catalyst, when post injection is performed with the air-fuel ratio lean to increase the bed temperature of the catalyst, If the amount of accumulated PM increases, the post-injection is advanced to raise the temperature of the exhaust gas flowing into the catalyst, whereby the PM accumulated on the front end face of the catalyst can be efficiently oxidized and removed. Therefore, clogging of the exhaust purification catalyst can be efficiently eliminated, and an increase in exhaust pressure loss and a decrease in reliability can be prevented.

It is a figure which shows schematic structure of the diesel engine which concerns on embodiment, and its control system. It is a block diagram which shows the structure of control systems, such as ECU. FIG. 3A is a diagram for explaining fuel injection control of an injector. FIG. 3A shows an example of fuel injection in a lean period of S poison recovery control, and FIG. 3B shows heat generation at that time. Each rate is shown. It is a figure explaining the basic operation of S poison recovery control, and is a timing chart showing an example of each change of exhaust air-fuel ratio, catalyst bed temperature, SOx release rate from NSR catalyst, SOx remaining amount of NSR catalyst is there. It is a flowchart figure which shows the procedure of the basic operation | movement of S poison recovery control. It is a flowchart figure which shows the operation | movement procedure of the front end surface recovery process performed in the case of S poison recovery control. It is a figure which shows the PM deposition amount map for calculating | requiring the catalyst front end surface PM deposition amount from a catalyst front pressure. Timing chart showing respective changes in A / F control, exhaust air-fuel ratio, and pre-catalyst pressure when the front end surface recovery process is performed when the S poison recovery control is executed, as compared with the case where the front end surface recovery process is not performed. FIG. FIG. 7 is a view corresponding to FIG. 6 according to a modified example (modified example 1) of the front end face recovery process. It is a figure which shows the change of the throttle opening and the A / F sensor output in the modification (modification 2) of the estimation operation | movement of the catalyst front end surface PM accumulation amount. It is a figure which shows the PM deposition amount map for calculating | requiring the catalyst front end surface PM deposition amount from A / F sensor response time. FIG. 6 is a view corresponding to FIG. 6 according to another embodiment in which the front end face recovery process is periodically performed during the S poison recovery control.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. In the present embodiment, a case where the present invention is applied to a common rail in-cylinder direct injection multi-cylinder (for example, in-line 4-cylinder) diesel engine (internal combustion engine) mounted on an automobile will be described.

-Engine configuration-
First, a schematic configuration of a diesel engine according to the present embodiment will be described with reference to FIG. 1. In this diesel engine 1 (hereinafter, also simply referred to as an engine 1), injection holes are respectively provided in combustion chambers in the cylinders 1a. The injector 2 is arranged facing it. The injector 2 for each cylinder 1a is connected to a common rail 11 common to all the cylinders 1a, and receives supply of fuel sent from the upstream supply pump 10 via the common rail 11.

  The supply pump 10 pressurizes the fuel pumped from the fuel tank and then supplies the fuel to the common rail 11 through the fuel passage 10a. The common rail 11 has a function as a pressure accumulation chamber that relaxes fluctuations in fuel discharged from the supply pump 10 and maintains (accumulates) the fuel pressure at a predetermined value, and distributes the accumulated fuel to each injector 2. The injector 2 is an electromagnetically driven on / off valve that opens when a predetermined voltage is applied and injects fuel into the cylinder 1a. The opening / closing control of the injector 2, that is, the control of the fuel injection amount and the injection timing is performed by an ECU (Electronic Control Unit) 100 as a control means.

  An intake passage 3 and an exhaust passage 4 are connected to the engine 1. In the intake passage 3, an air cleaner 9, an air flow meter 33, a compressor 63 of the turbocharger 6, an intercooler 8, and a throttle valve 5 are arranged in this order from the upstream portion (upstream portion in the intake air flow direction) to the downstream side. Has been. The throttle valve 5 is adjusted in opening degree by a throttle motor 51. This throttle opening is detected by a throttle opening sensor 41. The intake passage 3 is branched corresponding to each cylinder 1a in an intake manifold 3a arranged on the downstream side of the throttle valve 5.

  On the other hand, the exhaust passage 4 collects exhaust flows from the plurality of cylinders 1a into one by an exhaust manifold 4a connected to each cylinder 1a of the engine 1. In the exhaust passage 4 downstream of the exhaust manifold 4a, a NOx occlusion reduction type catalyst (NOx) capable of purifying NOx even when the exhaust air-fuel ratio (A / F) is lean (the oxygen concentration in the exhaust gas is high). Storage Reduction (hereinafter referred to as NSR catalyst 21) and a PM filter (Diesel Particulate Filter: DPF) 22 that collects particulate matter (Particulate Matter: PM) contained in the exhaust gas are sequentially arranged.

  As an example, the NSR catalyst 21 is a general three-way catalyst carrying alkalis, alkaline earths, and rare earth oxides as NOx storage materials. That is, the carrier made of a porous ceramic structure such as cordierite has a number of elongated cell passages extending in the longitudinal direction, that is, the exhaust flow direction, and the NOx occlusion material as described above is formed on the wall surfaces of these cell passages. And a noble metal such as platinum is supported thereon.

The NSR catalyst 21 stores NOx in the exhaust gas with the NOx storage material while the air-fuel ratio (A / F) of the exhaust gas is lean. On the other hand, when the air-fuel ratio of the exhaust gas becomes rich, the NSR catalyst 21 is released from the NOx storage material. NOx is reacted with reducing agent components such as hydrogen (H ₂ ), carbon monoxide (CO), and hydrocarbon (HC) in the exhaust gas. That is, NOx is reduced to harmless nitrogen N ₂ , and H ₂ , CO, and HC are oxidized to harmless water (H ₂ O) and carbon dioxide (CO ₂ ). In the present embodiment, such adjustment of the air-fuel ratio of the exhaust gas is performed by controlling the injector 2 (post-injection described later) or the like as will be described later.

  On the other hand, the DPF 22 is a porous ceramic structure similar to the NSR catalyst 21 and has a structure in which the front end portion and the rear end portion of adjacent ones of many cells are alternately sealed. Exhaust gas flows into the cell where the end of the upstream side of the DPF 22 is opened, and passes through a porous wall between adjacent cells. At this time, PM in the exhaust gas is collected. The In addition, a noble metal such as platinum is supported on the DPF 22 of the present embodiment, and functions as an oxidation catalyst that promotes an oxidation reaction of the deposited PM during DPF regeneration control described later.

  A / F sensors 36a and 36b are disposed in the exhaust passage 4 on the upstream side (upstream side of the exhaust flow) and the downstream side of the NSR catalyst 21, respectively, and continuously change according to the oxygen concentration in the exhaust gas. Output a signal. Further, exhaust temperature sensors 37a and 37b are also arranged in the exhaust passages 4 on the upstream side and downstream side of the NSR catalyst 21, respectively, and output signals that continuously change according to the exhaust temperature. The A / F sensors 36a and 36b and the exhaust gas temperature sensors 37a and 37b can detect the air-fuel ratio and temperature of the exhaust gas flowing into the NSR catalyst 21 and the DPF 22.

  An exhaust pressure sensor 38 is disposed in the exhaust passage 4 upstream of the NSR catalyst 21, and outputs a signal that continuously changes in accordance with the pressure of the exhaust passage 4 upstream of the NSR catalyst 21. The upstream pressure of the NSR catalyst 21 correlates with the amount of PM deposited on the front end surface (upstream end surface of the exhaust flow) of the NSR catalyst 21, as will be described later. That is, since the pressure on the upstream side of the NSR catalyst 21 increases due to the increase in the ventilation resistance accompanying the increase in the PM deposition amount on the front end face of the NSR catalyst 21, the PM deposition amount is estimated from the pressure on the upstream side of the NSR catalyst 21. be able to.

  In addition, a differential pressure sensor 39 (differential pressure transducer) that detects a differential pressure between the upstream pressure and the downstream pressure of the DPF 22 is also provided, and is continuous according to the pressure difference between the upstream pressure and the downstream pressure of the DPF 22. A signal that changes continuously. Based on this differential pressure signal, the amount of PM trapped in the DPF 22 can be estimated. The arrangement of the A / F sensors 36a, 36b, the exhaust temperature sensors 37a, 37b, etc. is merely an example, and they may be only on the upstream side of the NSR catalyst 21 or only on the downstream side. .

  Furthermore, the engine 1 of this embodiment is equipped with a turbocharger 6. The turbocharger 6 is formed by connecting a turbine 62 in the exhaust passage 4 and a compressor 63 in the intake passage 3 via a rotor shaft 61, and rotates the compressor 63 using an exhaust flow (exhaust pressure) received by the turbine 62. In this way, the intake air is supercharged. The intake air supercharged by the turbocharger 6 is cooled by the intercooler 8 disposed in the intake passage 3.

Further, the engine 1 of the present embodiment is equipped with an EGR device 7. The EGR device 7 recirculates a part of the exhaust from the exhaust passage 4 to the intake passage 3 and recirculates it to the combustion chamber of each cylinder 1a. The EGR device 7 includes an EGR passage 71 that connects the intake manifold 3a and the exhaust manifold 4a, an EGR cooler 73 that cools exhaust gas recirculated to the intake side (hereinafter also referred to as EGR gas), And an EGR valve 72 for adjusting the amount of reflux. The opening degree of the EGR valve 72 is adjusted according to a control command from the ECU 100 described below.
As shown in FIG. 2, the ECU 100 includes a CPU 101, a ROM 102, a RAM 103, a backup RAM 104, and the like. The ROM 102 stores various control programs, maps that are referred to when the control programs are executed, and the like. The CPU 101 executes arithmetic processing based on various control programs and maps stored in the ROM 102. The RAM 103 is a memory that temporarily stores calculation results of the CPU 101, data input from each sensor, and the like. The backup RAM 104 is a non-volatile memory that stores data to be saved when the engine 1 is stopped. is there.

  The CPU 101, ROM 102, RAM 103, and backup RAM 104 are connected to each other via a bus 107, and are connected to an input interface 105 and an output interface 106.

  The input interface 105 includes an engine speed sensor 31 that detects the rotation speed of the crankshaft that is the output shaft of the engine 1, a water temperature sensor 32 that detects the engine water temperature (cooling water temperature), an air flow meter 33 that detects the intake air flow rate, An intake air temperature sensor 34 disposed in the intake manifold 3a, an intake pressure sensor 35 disposed in the intake manifold 3a, the A / F sensors 36a and 36b, exhaust temperature sensors 37a and 37b, an exhaust pressure sensor 38, a differential pressure sensor 39, A rail pressure sensor 40 for detecting the pressure of the high-pressure fuel in the common rail 11, a throttle opening sensor 41, an accelerator opening sensor 42, a vehicle speed sensor 43, and the like are connected, and signals from these sensors are sent to the ECU 100. Entered.

  On the other hand, the output interface 106 is connected to the injector 2, the supply pump 10, the throttle motor 51 of the throttle valve 5, the actuator of the EGR valve 72, and the like. The ECU 100 controls the opening degree of the throttle valve 5 of the engine 1, the fuel injection control (control of the injection amount / injection timing) by the injector 2, and the control of the EGR gas amount based on the output signals of the various sensors described above. Various controls of the engine 1 including the above are executed.

  As an example, the ECU 100 executes pilot injection, main injection (fuel injection contributing mainly to torque generation of the engine 1), and post injection as fuel injection control by the injector 2 as schematically shown in FIG. As is well known, pilot injection is intended to lead to stable diffusion combustion by burning a small amount of fuel injected prior to main injection, thereby suppressing the ignition delay of fuel subsequently injected by main injection. .

  The main injection is generally an injection operation for generating torque of the engine 1, and the injection amount basically depends on the operation state such as the engine speed, the accelerator operation amount, the coolant temperature, the intake air temperature, and the like. The required torque is determined. For example, the higher the engine speed and the greater the accelerator opening, the higher the required torque for the engine 1, and the larger the main injection amount is set accordingly.

  Further, the post injection is normally performed in the latter half of the expansion stroke so that the fuel injected after the main injection does not burn in the cylinder 1a. As described below, post-injection adjusts the air / fuel ratio (exhaust A / F) of exhaust gas without causing torque fluctuations in the engine 1 in NOx reduction control, DPF regeneration control, and S poison recovery control, or NSR. This is used to increase the temperature of the catalyst 21 and the DPF 22.

  Further, the ECU 100 controls the opening degree of the EGR valve 72 according to the operating state of the engine 1 and adjusts the ratio of EGR gas in the intake air that is filled in the cylinder 1a. That is, the ECU 100 controls the opening degree of the EGR valve 72 based on an EGR map that is created in advance by experiments, simulations, or the like and stored in the ROM. The EGR map is a map for determining a suitable EGR amount (EGR rate) using the engine speed and the engine load (which may be a fuel injection amount) as parameters. As an example, the EGR map increases as the engine speed and the engine load increase. The opening of the valve 72 is set to be small.

  As will be described below, when the exhaust air-fuel ratio (exhaust A / F) is switched to rich and lean in the S poison recovery control, the throttle valve 5 is opened and the EGR is reduced when the exhaust gas is rich. In order to reduce the intake air amount while making lean, the opening degree of the throttle valve 5 is increased and EGR is not performed.

-Control operation of exhaust gas purification device-
Hereinafter, the NOx reduction control, the S poison recovery control, and the DPF regeneration control by the exhaust gas purification apparatus of the present embodiment will be described. First, the well-known NOx reduction control and DPF regeneration control will be described, and then the S poison recovery control, which is a feature of this embodiment, will be described.

(NOx reduction control)
In general, in the diesel engine 1, the air-fuel ratio of the exhaust gas is a lean air-fuel ratio in most operating regions, and the ambient atmosphere of the NSR catalyst 21 is in a high oxygen concentration state under normal operating conditions, so that NOx in the exhaust gas is exhausted. Is stored in the NSR catalyst 21. Since the situation where the ambient atmosphere of the NSR catalyst 21 has a low oxygen concentration is very small, the NOx occlusion amount gradually increases, and accordingly, the NOx occlusion capacity of the NSR catalyst 21 decreases.

  Therefore, when the NOx occlusion amount estimated based on the engine operating state or the like reaches a predetermined threshold ((appropriate value before the NOx occlusion capacity of the NSR catalyst 21 is saturated)), post injection from the injector 2 is performed. By supplying fuel into the exhaust gas, the air-fuel ratio (A / F) is temporarily enriched to increase the amount of reducing agent component (HC, etc.). As a result, the NOx occluded in the reducing atmosphere around the NSR catalyst 21 is released and reduced and purified, so that the NOx occlusion ability of the NSR catalyst 21 is restored.

  As a method for estimating the NOx occlusion amount, the NOx occlusion amount corresponding to the engine speed and the fuel injection amount from the injector 2 is obtained in advance through experiments or the like and mapped, and the NOx occlusion amount obtained from this map. The method of integrating | accumulating is mentioned.

(DPF regeneration control)
In general, in the diesel engine 1, PM contained in the exhaust is collected by the DPF 22, but the collected PM accumulates on the wall surface of the cell and obstructs the flow of the exhaust, and gradually increases the ventilation resistance. It will become. Accordingly, the differential pressure between the exhaust pressure upstream of the DPF 22 and the exhaust pressure downstream is increased, so that the amount of PM collected by the DPF 22 can be estimated from the differential pressure.

  Therefore, the ECU 100 refers to the map based on the output signal (differential pressure) of the differential pressure sensor 39 and estimates the amount of PM trapped in the DPF 22. In addition, the map used for the estimation of the amount of PM trapped is a map of values adapted by experiments and calculations in consideration of the correlation between the differential pressure before and after the DPF 22 and the amount of PM trapped. It is stored in the ROM 102 of the ECU 100.

  When the estimated amount of collected PM reaches a predetermined threshold value, ECU 100 supplies DPF regeneration control to burn and remove PM accumulated in DPF 22 by supplying fuel into the exhaust gas by post-injection from injector 2. . That is, the post-injected fuel reaches the NSR catalyst 21 together with the exhaust gas and is oxidized, and the exhaust gas temperature rises due to heat generated by this oxidation reaction, and the temperature of the DPF 22 rises.

  If the temperature of the DPF 22 is raised to a temperature at which PM burns (for example, 600 ° or more), the PM deposited on the DPF 22 starts to burn, and the temperature of the DPF 22 further rises due to heat generated by this combustion. If such a state can be maintained for a predetermined time, the deposited PM is removed, and the PM trapping ability of the DPF 22 is restored.

(S poison recovery control)
Next, the S poison recovery control characteristic of this embodiment will be described. In the NOx reduction control described above, the NOx stored can be released by instantaneously enriching the air-fuel ratio of the exhaust gas flowing into the NSR catalyst 21, but the NSR catalyst 21 stores NOx. Adsorption of sulfur component (SOx) occurs by the same mechanism. Since once adsorbed SOx is harder to desorb than NOx, SOx is not desorbed and gradually accumulates in the NSR catalyst 21 even if the NOx reduction control is performed.

More specifically, the mechanism of S poisoning is as follows. First, when fuel or lubricating oil burns in the cylinder 1a of the engine 1, SOx such as sulfur dioxide (SO ₂ ) and sulfur trioxide (SO ₃ ) is generated. When the oxygen concentration of the exhaust gas flowing into the NSR catalyst 21 is high, SOx such as SO ₂ and SO ₃ in the inflowing exhaust gas is oxidized on the surface of platinum (Pt) and is in the form of sulfate ions, for example. To be adsorbed.

The sulfate ions thus adsorbed on the NSR catalyst 21 combine with barium oxide (BaO) to form sulfate (BaSO ₄ ), which is more stable than barium nitrate (Ba (NO ₃ ) ² ). It is difficult to be disassembled. For this reason, even if the oxygen concentration of the exhaust gas flowing into the NSR catalyst 21 is lowered by the NOx reduction control or the like, the sulfate remains in the NSR catalyst 21 without being decomposed and is accumulated with time.

Thus, as the amount of sulfate (BaSO ₄ ) accumulated in the NSR catalyst 21 increases, the amount of barium oxide (BaO) that can participate in NOx occlusion decreases. The occlusion capacity is reduced, and the NOx purification rate is reduced (S poisoning). The amount of SOx accumulated in the NSR catalyst 21 is measured based on the total fuel injection amount of the injector 2 and the sulfur concentration in the fuel from the end of the previous S poison recovery control.

As a method for eliminating S poisoning as described above, the ambient temperature of the NSR catalyst 21 is raised to a high temperature range of approximately 600 to 700 ° C., and the oxygen concentration of the exhaust gas flowing into the NSR catalyst 21 is lowered (exhaust air). By making the fuel ratio rich), barium sulfate (BaSO ₄ ) adsorbed on the NSR catalyst 21 is thermally decomposed, and hydrogen (H ₂ ), carbon monoxide (CO), and hydrocarbon (HC) in the exhaust gas are exhausted. And a method of reducing to gaseous SO ₂ .

  In this embodiment, the post-injection and the control of the opening degree of the throttle valve 5 and the EGR valve 72 described above are used to switch the air-fuel ratio of the exhaust gas to rich and lean, and by performing post-injection in the lean state, I try to eliminate the poison. Specifically, after the post-injected fuel is oxidized in the NSR catalyst 21 and the bed temperature of the NSR catalyst 21 is increased by the generated heat, the air-fuel ratio is enriched and the reducing agent component is supplied to the NSR catalyst 21. .

  Hereinafter, the basic operation of the S poison recovery control will be described in detail with reference to FIG. 4 in addition to FIG. 3 described above. Here, for easy understanding, a basic operation performed when the amount of PM deposited on the front end face of the NSR catalyst 21 is small and there is no concern about clogging will be described. In addition, when there is a concern about clogging of the front end face of the NSR catalyst 21, an operation (front end face recovery process) performed to remove the accumulated PM will be described later.

  FIG. 4 shows the exhaust air-fuel ratio, the catalyst bed temperature, the SOx release rate from the NSR catalyst 21 (SOx release amount per unit rotation angle of the crankshaft), and the SOx remaining amount of the NSR catalyst 21 when the S poison recovery control is executed. It is a timing chart figure showing an example of change of.

In the S poison recovery control, a lean period for setting the exhaust air-fuel ratio to lean (a period for controlling the exhaust air-fuel ratio to lean) and a rich period for setting the exhaust air-fuel ratio to rich (a period for controlling the exhaust air-fuel ratio to rich) ) Appear alternately, and during the lean period, the bed temperature of the NSR catalyst 21 is raised to a temperature at which SOx can be released (a temperature at which barium sulfate (BaSO ₄ ) can be thermally decomposed). On the other hand, in the rich period, by making the exhaust air-fuel ratio rich, SOx is released from the NSR catalyst 21 and reacted with hydrogen (H ₂ ), carbon monoxide (CO), and hydrocarbon (HC) in the exhaust. Reduce and purify.

  As an example, the timing for switching from the lean period to the rich period is a time point when the bed temperature of the NSR catalyst 21 reaches a temperature sufficient to release SOx (for example, 680 ° C.). On the other hand, the switching timing from the rich period to the lean period is a time point when the bed temperature of the NSR catalyst 21 is lowered to a predetermined temperature (for example, 630 ° C.). In this way, the lean period and the rich period are switched and alternately repeated, whereby SOx is released from the NSR catalyst 21.

  More specifically, referring also to FIG. 3, when the S poisoning amount (SOx accumulation amount) reaches a predetermined value and the S poisoning recovery control is started, first, in the lean period of the air-fuel ratio, FIG. As indicated by the solid line in a), after the main injection is performed, the post-injection is performed in the second half of the expansion stroke of the cylinder 1a (for example, ATDC 100 ° CA), and the catalyst bed temperature rises. That is, in normal post-injection, fuel does not burn in the cylinder 1a (no heat is generated as shown by the solid line waveform in FIG. 3B), flows into the exhaust passage 4, and is oxidized while passing through the NSR catalyst 21. This effectively increases the catalyst bed temperature.

  Thus, when the bed temperature of the NSR catalyst 21 rises and reaches a predetermined temperature (for example, 680 ° C.), the lean period is switched to the rich period (timing T1 in FIG. 4). In this rich period, the injection amount of the main injection is increased or the sub-injection is performed at a timing at which the fuel burns after the main injection, while the opening of the throttle valve 5 is reduced to a predetermined opening and EGR is also executed. Thus, the amount of air (fresh air) sucked into the cylinder 1a is reduced. As a result, the air-fuel ratio in the cylinder 1a becomes richer than the stoichiometric air-fuel ratio (for example, A / F is 14 or less), and the air-fuel ratio of the exhaust also becomes rich.

  As a result, in the rich period, the exhaust air-fuel ratio becomes rich while the bed temperature of the NSR catalyst 21 is sufficiently increased (rising to a temperature at which SOx can be released), so that SOx is released from the NSR catalyst 21. Will be. That is, as shown in the graph of the SOx release rate in FIG. 4, SOx is released from the NSR catalyst 21, and the SOx remaining amount in the NSR catalyst 21 gradually decreases. At the same time, the bed temperature of the NSR catalyst 21 gradually decreases during the rich period, and when the bed temperature reaches a predetermined temperature (for example, 630 ° C.), the rich period is switched to the lean period (timing T2).

  In this way, the air-fuel ratio of the exhaust gas is switched to rich and lean according to the change in the bed temperature while the bed temperature of the NSR catalyst 21 is maintained in the high temperature range of 600 to 700 ° C., so that the NSR catalyst 21 SOx is gradually released from the catalyst, and the amount of SOx accumulated in this NSR catalyst 21 (SOx remaining amount) decreases. Then, when the SOx accumulation amount becomes less than the predetermined amount, the S poison recovery control ends.

  Note that the SOx release amount from the NSR catalyst 21 is measured based on the bed temperature of the NSR catalyst 21 and the rich period. That is, the higher the bed temperature of the NSR catalyst 21 and the longer the rich period, the greater the SOx release amount. Therefore, by measuring the bed temperature and rich period of the NSR catalyst 21, the SOx is released. The amount released will be required. When the SOx release amount coincides with the SOx adsorption amount at the start of the S poison recovery control (the SOx adsorption amount measured based on the total fuel injection amount of the injector 23 and the sulfur concentration in the fuel), the NSR The S poison recovery control is terminated assuming that substantially the entire amount of SOx in the catalyst 21 has been released.

  The above is the basic operation of the S poison recovery control.

  By the way, since the air-fuel ratio in the cylinder 1a becomes richer than the stoichiometric air-fuel ratio in the rich period of the S poison recovery control as described above, the amount of PM generated with combustion becomes considerably large. Moreover, since HC contained in the rich exhaust adheres to the front end face of the NSR catalyst 21 and functions as a binder, PM in the exhaust tends to adhere to and accumulate on the front end face of the NSR catalyst 21.

  Thus, as the amount of PM deposited and deposited increases, the ventilation resistance of the NSR catalyst 21 increases, and the output and fuel consumption of the engine 1 deteriorate due to an increase in exhaust pressure loss. Further, when the accumulated PM burns rapidly, the bed temperature of the NSR catalyst 21 may be excessively increased.

  Therefore, in the present embodiment, it is determined that the amount of PM deposited on the front end face of the NSR catalyst 21 (hereinafter referred to as catalyst front end face PM deposition amount) is equal to or greater than a predetermined amount, and the above S The post injection timing in the lean period of the poisoning recovery control is advanced to the first half of the expansion stroke of the cylinder 1a (for example, about ATDC 30 to 60 °). As a result, at least a part of the post-injected fuel is combusted in the cylinder 1a, and the temperature of the exhaust gas flowing into the NSR catalyst 21 rises to a predetermined level or more, thereby oxidizing and removing the accumulated PM. (Hereinafter referred to as front end face recovery processing).

-Specific control procedure-
Hereinafter, a specific operation procedure of the S poison recovery control of the present embodiment will be described with reference to the flowcharts of FIGS. 5 and 6. FIG. 5 shows a basic procedure of the above-described S poisoning recovery control, and FIG. 6 shows a procedure of front end face recovery processing. These flowcharts are executed every predetermined period (for example, every several msec or every predetermined rotation angle of the crankshaft) after the engine 1 is started.

(Basic operation of S poison recovery control)
First, in step ST1 of FIG. 5, it is determined whether or not the S poison recovery control execution flag stored in advance in the ECU 100 is ON. This S-poisoning recovery control execution flag is turned on with the start of the S-poisoning recovery control, and when this S-poisoning recovery control is completed (substantially the entire amount of SOx in the NSR catalyst 21 has been released, the S-poisoning recovery control execution flag is set). It is turned off (after recovery control is finished). When the vehicle starts running or immediately after the previous S poisoning recovery control is completed, the sulfur component accumulation amount (S poisoning amount) in the NSR catalyst 21 is small, so the S poisoning recovery control is started. The S poison recovery control execution flag is OFF.

  If the S poison recovery control execution flag is OFF and NO is determined in step ST1, the process proceeds to step ST2 to determine whether or not the S poison amount is equal to or greater than the recovery control start amount. This recovery control start amount is set based on experiments and simulations in advance as a value that requires S poison recovery control when the S poison amount reaches the recovery control start amount. When the vehicle starts running or immediately after the previous S poison recovery control is completed, this S poison amount is not equal to or greater than the recovery control start amount. Return.

  When the operation of the engine 1 is continued, the S poisoning amount of the NSR catalyst 21 increases, and when it becomes equal to or greater than the recovery control start amount, YES is determined in step ST2, and the process proceeds to step ST3. In step ST3, the S poison recovery control described above is started, and the S poison recovery control execution flag is switched from OFF to ON. Then, the process proceeds to step ST4, and as described above, the exhaust air / fuel ratio switching operation for alternately switching the air / fuel ratio to rich and lean according to the bed temperature of the NSR catalyst 21 is executed.

  After the exhaust air / fuel ratio switching operation for recovering the S poison starts in this way, it is determined in step ST5 whether or not the S emission amount (departure amount) from the NSR catalyst 21 has reached a predetermined amount. The predetermined amount is an S discharge amount for ending the S poison recovery control, and is, for example, a value corresponding to the SOx adsorption amount at the start of the S poison recovery control, or the S poison recovery control. It is set to a value slightly smaller than the accumulated amount of SOx at the start. The S emission amount from the NSR catalyst 21 is obtained from the floor temperature and the exhaust air / fuel ratio as described above.

  If the S discharge amount from the NSR catalyst 21 has not reached the predetermined amount, NO is determined in step ST5 and the process returns. In this case, in the next routine, YES is determined in step ST1, and the operations of steps ST1, ST4, and ST5 are repeated, and the S poison recovery control is continued until the S discharge amount from the NSR catalyst 21 reaches a predetermined amount. It will be.

  As a result, when the amount of S discharged from the NSR catalyst 21 reaches a predetermined amount and a YES determination is made in step ST5, the process proceeds to step ST6, where an end process of the S poison recovery control is performed. That is, the air-fuel ratio rich / lean switching and post-injection are stopped, and the S poison recovery control execution flag is turned OFF. As a result, the S poison recovery control is completed, and the normal engine control is shifted to the state where the NOx storage capability of the NSR catalyst 21 is recovered.

(Front end face recovery process)
Next, a specific operation procedure of the front end face recovery process will be described with reference to the flowchart of FIG. First, in step ST11, it is determined by the S poison recovery control execution flag whether or not the S poison recovery control is being executed. If the S poisoning recovery control execution flag is OFF and it is determined NO, the process directly returns. On the other hand, if the S poisoning recovery control execution flag is ON and it is determined YES, the process proceeds to step ST12 to estimate the catalyst front end face PM accumulation amount.

  As an example, this estimation is performed based on the exhaust pressure upstream of the NSR catalyst 21 (hereinafter referred to as pre-catalyst pressure) detected by an exhaust pressure sensor 38 disposed upstream of the NSR catalyst 21. Specifically, a PM accumulation amount map that preliminarily defines the relationship between the operating state of the engine 1, the pre-catalyst pressure, and the catalyst front end face PM accumulation amount is stored in the ROM of the ECU 100, and is detected in this PM accumulation amount map. By applying the pre-catalyst pressure, the catalyst front end face PM deposition amount is estimated. Needless to say, the front end face of the NSR catalyst 21 on which PM is deposited does not mean a geometric end face of the catalyst carrier but includes a predetermined range in the vicinity of the end face.

  FIG. 7 shows an example of the PM accumulation amount map. In this PM accumulation amount map, when the pre-catalyst pressure is PA in the figure, the catalyst front end face PM accumulation quantity (mass) is obtained as A in the figure, and the catalyst pre-pressure is PB in the figure. The catalyst front end face PM accumulation amount is obtained as B in the figure, and when the catalyst front pressure is PC in the figure, the catalyst front end face PM accumulation amount is obtained as C in the figure. This C is an upper limit value that is likely to clog the front end face of the NSR catalyst 21 and that PM needs to be removed quickly. B is for determining whether or not to start the front end face recovery process. This is a preset start determination value (determination value).

  As a method for estimating the catalyst front end face PM accumulation amount, the catalyst front end face PM accumulation amount may be estimated based on the differential pressure before and after the NSR catalyst 21 (differential pressure between the upstream side and the downstream side). That is, a differential pressure sensor (differential pressure transducer) (not shown) that detects a pressure difference between the upstream side and the downstream side of the NSR catalyst 21 is provided, and the catalyst front end face PM accumulation amount is calculated based on an output signal from the differential pressure sensor. Can be sought. It is determined that the catalyst front end face PM deposition amount increases as the differential pressure increases.

  After estimating the catalyst front end face PM deposition amount in step ST12, the process proceeds to step ST13, where it is determined whether or not the current catalyst front end face PM deposition amount is equal to or greater than a predetermined amount. As an example, in this embodiment, if the pre-catalyst pressure is equal to or greater than a predetermined pressure (pressure PB corresponding to the start determination value B of the catalyst front end face PM accumulation amount in FIG. 7), the determination is YES and the process proceeds to step ST14. Execute surface recovery processing. That is, the post injection timing in the lean period is advanced in the exhaust air / fuel ratio switching operation (step ST4) of the S poison recovery control.

  Specifically, as shown by the broken line in FIG. 3A, after the main injection is performed, the post-injection is performed in the first half of the expansion stroke where the temperature state of the cylinder 1a is high, and a part of the injected fuel is cylinder 1a. Will start to burn. As a result, combustion heat is generated following the main combustion (advanced post-combustion) as shown by the broken line waveform in FIG. 3B, and the exhaust flowing into the NSR catalyst 21 together with the combustion in the exhaust passage 4 thereafter. Is raised to a predetermined temperature (for example, 600 ° C. or higher), and PM deposited on the front end face of the catalyst is oxidized and removed.

  That is, in the S poison recovery control in which the NSR catalyst 21 is maintained in a high temperature range and SOx is released, the post-injection timing in the lean period is slightly changed to oxidize and remove PM deposited on the catalyst front end face. can do.

  Since part of the post-injected fuel burns in the cylinder 1a and generates heat as described above, the main injection amount is corrected to decrease so as to offset the torque increase of the engine 1 due to this (see FIG. 3 (a) and (b) are indicated by broken lines). This reduction correction amount is obtained by calculating the torque increase amount from the post injection timing and the injection amount in advance for each operating state of the engine 1 through experiments or the like and calculating a reduction amount of the main injection that cancels this. Create it and use this map.

  On the other hand, if it is determined in step ST13 that the pre-catalyst pressure is less than the predetermined pressure PB (NO) and the current catalyst front end face PM accumulation amount is less than the start determination value B, the advance angle of the post injection is not executed. (Step ST15: The front end face recovery process is not performed). In this case, post-injection is performed at normal timing, and the temperature of the NSR catalyst 21 is raised by combustion of fuel supplied to the exhaust passage 4 in an unburned state.

  Then, it progresses to step ST16 and it is determined whether PM collection amount in DPF22 is more than predetermined amount. In this determination as well as the relationship between the catalyst front pressure and the catalyst front end face PM accumulation amount, a PM collection amount map in which the relationship between the differential pressure before and after the DPF 22 and the PM collection amount is defined in advance is stored in the ROM of the ECU 100. Then, the differential pressure before and after the DPF 22 detected by the differential pressure sensor 39 is applied to the PM trapping amount map to determine whether the PM trapping amount of the DPF 22 is equal to or larger than a predetermined amount.

  When the differential pressure before and after the DPF 22 is less than a predetermined value and NO, that is, when it is determined that the amount of collected PM is less than the predetermined amount, the process returns (normal S poison recovery control is performed), while the differential pressure before and after If YES in a predetermined value or more, that is, if it is determined that the PM trapping amount is a predetermined amount or more, the process proceeds to step ST17, and switching to the rich period in the S poison recovery control is prohibited. Thereby, in the S poison recovery control described above, the lean period continues, and as a result of the NSR catalyst 21 and the DPF 22 being heated by the post injection, the PM deposited on the DPF 22 gradually decreases. Become.

  FIG. 8 shows A / F control (air / fuel ratio rich / lean switching), exhaust air / fuel ratio change, catalyst pre-pressure, and the like when front end face recovery processing is performed as necessary during S poison recovery control. That is, it is a timing chart showing the change in the catalyst front end face PM deposition amount in comparison with the case where the front end face recovery process is not performed.

  First, during the lean period of the S poison recovery control, the throttle valve 5 is opened relatively large so that the air-fuel ratio becomes lean, and the EGR valve 72 is closed, so the air (fresh air) sucked into the cylinder 1a Even if the amount increases and the fuel injection amount is determined according to the required torque, the air-fuel ratio of the exhaust gas becomes lean (in the example of the figure, it changes between A / F = 20-25).

  Further, during the lean period, post injection is performed in the latter half of the expansion stroke of the cylinder 1a, and the NSR catalyst 21 is effectively heated as a whole by combustion of unburned fuel in the exhaust system. The temperature should not be too high. For this reason, PM deposited on the front end face of the NSR catalyst 21 is not removed, and the pre-catalyst pressure corresponding to the catalyst front end face PM deposition amount hardly changes as shown in the figure.

  On the other hand, in the rich period, a target air-fuel ratio richer than the stoichiometric air-fuel ratio is set (indicated by a broken line in the figure), and the throttle opening is relatively reduced and EGR is also performed. The amount of generated PM is considerably increased, and the amount of HC in the rich exhaust functions as a binder, so that the amount of PM deposited on the catalyst front end surface increases.

  Therefore, if the front end face recovery process is not performed during the S poison recovery control, the catalyst front end face PM accumulation amount increased in the rich period of the exhaust air-fuel ratio switching operation does not decrease in the lean period. The amount of PM deposited increases and eventually exceeds the upper limit C (the pre-catalyst pressure exceeds PC as indicated by the phantom line in the figure).

  On the other hand, in this embodiment, when the catalyst front end face PM accumulation amount that increases in the rich period reaches the start determination value B (timing T1 in the figure) and the catalyst front pressure becomes equal to or higher than the predetermined pressure PB, the front end face recovery process is performed. The execution flag is turned ON. For this reason, the front end face recovery process is started in the subsequent lean period, the temperature of the exhaust gas flowing into the NSR catalyst 21 becomes higher than a predetermined value by the advance angle of the post injection, and the PM deposited on the front end face of the NSR 21 is oxidized. To be removed.

  As a result, the catalyst front end face PM accumulation amount rapidly decreases, and when this becomes less than the end determination value (timing T2), the front end face recovery process ends (the execution flag of the front end face recovery process is OFF), and then the rich period again. Further, the catalyst front end face PM deposition amount gradually increases as the lean period is repeated. Note that the end determination value of the catalyst front end face PM accumulation amount may be, for example, A in FIG. 7. In this case, the front end face recovery process ends when the catalyst front pressure decreases to PA in the figure. .

  Therefore, according to the exhaust gas purification apparatus of the present embodiment, if the amount of PM deposited on the front end surface of the catalyst increases during the S poison recovery control in which the NSR catalyst 21 is maintained in a high temperature range and SOx is released, the catalyst bed temperature By advancing the post-injection during the air-fuel ratio lean period that raises the temperature, the temperature of the exhaust gas flowing into the NSR catalyst 21 can be raised, and the PM deposited on the front end face can be oxidized and removed. Therefore, the clogging of the NSR catalyst 21 can be efficiently eliminated, and an increase in exhaust pressure loss and a decrease in reliability of the engine 1 can be prevented.

  Moreover, when the post injection in the lean period is advanced as described above to oxidize PM accumulated on the front end surface of the NSR catalyst 21, the temperature of the upstream portion of the NSR catalyst 21 is increased by the reaction heat. As a result, the SOx can be effectively released from this portion. In other words, by performing the recovery process of the front end face of the NSR catalyst 21 during the S poison recovery control, the recovery from the S poison in the upstream portion of the catalyst, which has been difficult until now, can be performed synergistically. Effects can be obtained.

  That is, until now, in the S poison recovery control, although the bed temperature of the NSR catalyst 21 can be effectively increased as a whole by the post injection in the lean period, exhaust gas containing unburned fuel is considered in detail. As a result of being oxidized while flowing through the cell passage of the catalyst carrier, the temperature tends to rise as intended in the downstream portion, whereas the temperature tends to hardly rise in the upstream portion. As a result, the actual situation is that the accumulated SOx cannot be sufficiently released from the upstream portion of the NSR catalyst 21.

  In this regard, when the front end face recovery process is performed in a lean period as in the present embodiment and the PM deposited on the front end face of the NSR catalyst 21 is oxidized, the upstream portion of the NSR catalyst 21 is effectively removed by the reaction heat. The temperature can be raised, and the release of SOx from this portion (recovery from S poisoning) can be sufficiently performed.

Furthermore, since PM deposited by the front end face recovery process in the lean period can be removed in this way, in this embodiment, even if there is a risk that PM deposition on the NSR catalyst 21 may be promoted in the rich period, the S coverage is increased. The poison recovery control can be executed more frequently, in other words, in a wider operating range of the engine 1. Further, during the rich period, the air-fuel ratio is made richer than the stoichiometric air-fuel ratio, so that the amount of reducing agent components such as H ₂ , CO, and HC in the exhaust gas can be sufficiently increased, which is also advantageous for promoting the release of SOx. work.

(Modification 1)
Next, Modification 1 will be described. This modification 1 is a modification of the front end face recovery process, and other configurations and operations are the same as those of the above-described embodiment, so only the front end face recovery process will be described here.

  In the first modification, whether or not to start the front end face recovery process is determined in consideration of not only the amount of PM deposited on the front end face of the NSR catalyst 21, but also the amount of PM trapped by the DPF 22. As an example, as shown in the flowchart of FIG. 9, in steps ST21 and ST22, it is determined that the S poison recovery control is being executed as in steps ST11 and ST12 (see FIG. 6, the same applies hereinafter), and the NSR catalyst 21 After estimating the catalyst front end face PM deposition amount, in step ST23, it is determined whether or not the current catalyst front end face PM deposition amount is equal to or greater than the upper limit C.

  That is, if it is determined that the pre-catalyst pressure is equal to or higher than the predetermined pressure (PC in FIG. 7) based on the signal from the atmospheric pressure sensor 38 as in step ST13, the process proceeds to step ST24, and the front end face recovery is performed as in step ST14. Execute the process. On the other hand, if the pre-catalyst pressure is less than the predetermined pressure, the catalyst front end face PM accumulation amount is less than the upper limit value C. Therefore, the process proceeds to step ST25 and this time the catalyst front end face PM accumulation amount is equal to or greater than the judgment value B It is determined whether or not the pressure has become equal to or higher than a predetermined pressure (PB in FIG. 7).

  If this judgment is YES, it will progress to step ST26 and will judge whether the amount of PM collection of DPF22 is more than predetermined amount like step ST16. That is, if it is determined that the differential pressure before and after the DPF 22 is less than a predetermined value and NO, that is, the amount of collected PM is less than a predetermined amount, the process proceeds to step ST24 to execute the front end face recovery process. On the other hand, if the differential pressure across the front and rear is greater than or equal to a predetermined value and YES, that is, it is determined that the amount of PM trapped is greater than or equal to the predetermined amount, the front end face recovery process is not performed (step ST27). Similarly to the above, switching to the rich period in the S poison recovery control is prohibited.

  That is, even if the catalyst front end face PM accumulation amount is equal to or greater than the determination value B, the front end face recovery process is not executed when the DPF 22 has a large PM collection amount, and the DPF 22 PM collection amount further increases. In order to prevent this, enrichment of the air-fuel ratio is prohibited.

  Even when it is determined in step ST25 that the catalyst front end face PM accumulation amount is less than the determination value B, the front end face recovery process is not executed (step ST29), and the process proceeds to step ST30. It is determined whether the amount of PM trapped is a predetermined amount or more. If this determination is NO, the process returns. On the other hand, if the determination is YES and the amount of PM trapped is greater than a predetermined amount, the process proceeds to step ST28 to switch to the rich period in the S poison recovery control. Ban.

  Other operations are the same as those in the above-described embodiment. However, according to the first modification, the start of the front end surface recovery process is determined in consideration of not only the catalyst front end surface PM accumulation amount but also the PM collection amount of the DPF 22. Since the switch to the rich period in the S poison recovery control is not performed as necessary, not only the front end face of the NSR catalyst 21 but also the DPF 22 suppresses an increase in ventilation resistance due to an increase in PM. Suitable S poisoning recovery control can be performed.

(Modification 2)
Next, Modification 2 will be described. This modified example 2 is a modified example of the catalyst front end face PM accumulation amount estimation operation, and the other configurations and operations are the same as those of the above-described embodiment. explain. In this second modification, the catalyst front end face PM accumulation amount is estimated based on the response time of the A / F sensor 36a disposed on the upstream side of the NSR catalyst 21.

  That is, when PM is deposited on the end face of the NSR catalyst 21, it is assumed that PM is also deposited on the surface of the A / F sensor 36a disposed on the upstream side. The response time of the A / F sensor 36a (after the exhaust air / fuel ratio of the exhaust passage 4 has changed, the output signal of the A / F sensor 36a is output as a signal corresponding to the exhaust air / fuel ratio after the change. The longer the time until the PM deposition amount is, the longer it takes. That is, it is considered that there is a correlation between the response time and the catalyst front end face PM accumulation amount.

  Specifically, the A / F sensor 36a is covered with a cover formed of a heat-resistant metal such as stainless steel, and the cover has a vent hole for allowing exhaust gas to flow into the sensing unit. Yes. In a situation where PM accumulates around the vent hole, the response time of the A / F sensor 36a becomes longer as the PM accumulation amount increases. By utilizing this fact, in Modification 2, it is estimated that the catalyst front end face PM accumulation amount increases as the response time of the A / F sensor 36a increases.

  FIG. 10 is a diagram showing changes in the accelerator opening and the output of the A / F sensor 36a in this modification. From the state where the accelerator opening is relatively large and the exhaust air-fuel ratio is rich, the A / F is actually started from the time when the accelerator opening becomes small and the exhaust air-fuel ratio becomes lean (accelerator OFF timing in the figure). When the output signal of the sensor 36a is relatively short as a period until it is output as a value corresponding to the case where the exhaust air-fuel ratio is lean (threshold in the figure) (in the case of an output change indicated by a solid line in the figure); When the response time is TA in the figure), it is estimated that the PM accumulation amount on the A / F sensor 36a is small and the catalyst front end face PM accumulation amount is also small.

  Similarly, depending on the length of the period from when the exhaust air-fuel ratio becomes lean to when the output signal of the A / F sensor 36a is actually output, for example, the output change indicated by a one-dot chain line in the figure In this case, it is estimated that the PM deposition amount on the A / F sensor 36a is increased and the catalyst front end surface PM deposition amount is also increased corresponding to the response time of TB in the figure. Further, for example, in the case of an output change indicated by a two-dot chain line in the figure, the PM accumulation amount on the A / F sensor 36a further increases corresponding to the response time of TC in the figure, and the catalyst front end face PM accumulation. The amount is estimated to be even higher.

  FIG. 11 shows an example of a PM accumulation amount map for obtaining the catalyst front end face PM accumulation amount from the air-fuel ratio sensor response times TA, TB, and TC. In this PM accumulation amount map, when the air-fuel ratio sensor response time is TA in the figure, the catalyst front end face PM accumulation amount (mass) is obtained as A in the figure, and the air-fuel ratio sensor response time in the figure In the case of TB, the catalyst front end face PM accumulation amount is obtained as B in the figure, and in the case where the air-fuel ratio sensor response time is TC in the figure, the catalyst front end face PM accumulation amount is indicated as C in the figure. Desired.

  Other operations are the same as those in the above-described embodiment. However, according to the second modification, it is possible to estimate the catalyst front end face PM accumulation amount by using the existing A / F sensor 36a. The sensor 38 becomes unnecessary, and the system configuration can be simplified and the cost can be reduced.

-Other embodiments-
In the above-described embodiments and modifications, the case where the present invention is applied to the common rail in-cylinder direct injection multi-cylinder diesel engine 1 mounted on an automobile has been described. However, the present invention is not limited to an automobile. It can also be applied to engines used for other purposes. Further, the number of cylinders and the engine type (separate type engine, V-type engine, horizontally opposed engine, etc.) are not particularly limited, and the in-cylinder injection type is likely to generate PM with combustion. It can also be applied to a gasoline engine.

  In the embodiment and the modification, the catalyst front end face PM accumulation amount of the NSR catalyst 21 is estimated based on the catalyst front pressure and the response time of the A / F sensor 36a, and the catalyst front end face PM accumulation amount becomes a predetermined amount or more. However, the present invention is not limited to this.

  For example, as shown in FIG. 12, when the S poison recovery control is executed (YES in step ST31), the amount of PM trapped is a predetermined amount or more based on the differential pressure before and after the DPF 22 as in steps ST16, ST26, and ST30. (Step ST32) If this determination is YES, switching to the rich period is prohibited (step ST33) as in steps ST17 and ST28. On the other hand, if the determination in step ST32 is NO and the amount of collected PM is small, periodic front end face recovery processing may be performed (step ST34).

  Here, the periodic front end face recovery process means that this process is executed once every predetermined number of executions of the S poison recovery control, or a predetermined operation time has elapsed since the end of the previous front end face recovery process. Or the front end face recovery process is executed when the vehicle has traveled a predetermined distance. This eliminates the need for estimating the PM accumulation amount based on the pre-catalyst pressure and the like, and the front end face recovery process is executed regardless of the estimation accuracy, so that the NSR catalyst 21 can be more reliably prevented from being clogged.

  The present invention can efficiently remove PM accumulated on an exhaust purification catalyst, eliminate clogging thereof, and prevent an increase in exhaust pressure loss and a decrease in reliability of an engine. This is extremely useful when applied to diesel engines.

1 Diesel engine (internal combustion engine)
1a Cylinder 2 Injector (fuel injection valve)
4 Exhaust passage (exhaust system)
21 NSR (Exhaust gas purification catalyst)
22 DPF (PM filter)
38 Exhaust pressure sensor 39 Differential pressure sensor 100 ECU (control means)

Claims (7)

An exhaust gas purification apparatus for an internal combustion engine provided with a fuel injection valve facing a cylinder,
An exhaust purification catalyst disposed in the exhaust system of the internal combustion engine;
In order to recover the purification performance of the exhaust purification catalyst, it comprises control means for performing recovery control to control at least the fuel injection valve to switch the air-fuel ratio to the rich side and the lean side,
When the air-fuel ratio is switched to the lean side in the recovery control, unburned fuel is supplied to the exhaust purification catalyst by post injection in the latter half of the expansion stroke of the cylinder.
If the control means determines that the amount of PM deposited on the exhaust purification catalyst has reached a predetermined amount or more, the control means advances the post injection so that at least a part of the fuel burns in the cylinder, and the exhaust purification catalyst. An exhaust emission control device characterized in that the temperature of the exhaust gas flowing into the catalyst is increased. The exhaust emission control device according to claim 1,
The recovery control is for releasing the sulfur component accumulated in the exhaust purification catalyst, and is for making the air-fuel ratio richer than the stoichiometric air-fuel ratio when controlling the air-fuel ratio to the rich side. . The exhaust emission control device according to claim 1 or 2,
When the post injection is advanced in the recovery control, the control means is configured to reduce the fuel injection amount of the main injection performed in the vicinity of the compression top dead center of the cylinder so as to offset the increase in engine torque caused thereby. , Exhaust purification device. The exhaust emission control device according to any one of claims 1 to 3,
The control means is configured to determine that when the pressure upstream of the exhaust gas from the exhaust purification catalyst becomes equal to or higher than a predetermined pressure, the amount of PM deposited on the exhaust purification catalyst becomes equal to or higher than a predetermined amount. Purification equipment. The exhaust emission control device according to any one of claims 1 to 4,
The exhaust system downstream of the exhaust purification catalyst is provided with a PM filter for collecting PM in the exhaust,
The exhaust gas purification apparatus is configured such that the control means does not advance the post-injection in the recovery control if the amount of PM collected in the PM filter is equal to or greater than a predetermined amount. The exhaust emission control device according to claim 5,
The exhaust gas purification apparatus is configured such that the control means does not perform switching to the rich side of the air-fuel ratio in the recovery control until the collected amount of PM in the PM filter becomes less than a predetermined amount. The exhaust emission control device according to any one of claims 1 to 6,
The exhaust system downstream of the exhaust purification catalyst is provided with a PM filter for collecting PM in the exhaust,
If the amount of PM trapped in the PM filter is greater than or equal to a predetermined amount, the control means does not switch the air-fuel ratio to the rich side in the recovery control.

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