JPWO2015181922A1 - Exhaust gas purification system for internal combustion engine - Google Patents

Abstract

An object of the present invention is to provide an exhaust gas purification system for an internal combustion engine that can optimize the PM emission amount and the HC emission amount. The exhaust purification system includes an LNT having a function of oxidizing HC, a fuel injection valve capable of performing main injection and after injection, fuel injection control means for controlling a fuel injection amount and fuel injection timing from the fuel injection valve, Temperature acquisition means for acquiring the carrier temperature of the LNT, and oxidation ability estimation means for estimating the HC oxidation ability of the LNT based on the acquired carrier temperature. After-injection is defined as fuel injection in a region after the main injection and in which the amount of PM combustion in the cylinder increases as the distance from the main injection increases. The fuel injection control means increases the interval between the main injection timing and the after injection timing as the oxidation characteristic parameter Pox indicating the HC oxidation capability increases.

Description

  An exhaust gas purification system for an internal combustion engine purifies HC (hydrocarbon), CO (carbon monoxide), and NOx (nitrogen oxide) contained in the exhaust gas of the engine. Exhaust purification systems are mainly used to purify the ternary components in the exhaust using reactions in various types of catalysts provided in the exhaust passage. As an exhaust purification catalyst, an oxidation catalyst (hereinafter abbreviated as DOC (Diesel Oxidation Catalyst) may be used), a three-way catalyst (hereinafter abbreviated as TWC (Three-Way Catalyst)) may be used. ), Lean NOx catalyst (hereinafter, abbreviated as LNT (Lean NOx Trap)), selective reduction catalyst (hereinafter, abbreviated as SCR catalyst (Selective Catalytic Reduction Catalyst)), etc. Various catalysts with different functions have been proposed.

  The oxidation catalyst has an oxidation function of purifying HC and CO by advancing the oxidation reaction of HC and CO under an exhaust gas containing lean air / fuel ratio (exhaust gas having a lean air / fuel ratio). This oxidation catalyst also has a three-way purification function in which the oxidation reaction of HC and CO and the reduction reaction of NOx proceed simultaneously with high efficiency under exhaust gas with the air-fuel ratio of the air-fuel mixture stoichiometric (exhaust gas with the stoichiometric air-fuel ratio). . The three-way catalyst corresponds to a catalyst obtained by adding an oxygen storage material (hereinafter also referred to as an OSC (Oxygen Storage Capacity) material) to the oxidation catalyst. The window, that is, the air-fuel ratio width that exhibits the three-way purification function is widened. This effect is caused by the fact that the range of the variation in the catalyst air-fuel ratio relative to the variation in the pre-catalyst air-fuel ratio is reduced by the oxygen storage effect of the OSC material.

The selective reduction catalyst reduces NOx in the presence of a reducing agent supplied from the outside such as NH ₃ or HC or present in the exhaust gas. The lean NOx catalyst stores NOx in the exhaust gas under a lean air-fuel ratio exhaust gas, and reduces the NOx stored in the stoichiometric or rich air-fuel ratio exhaust gas with a reducing agent. Exhaust gas purification systems based on lean combustion, such as lean-burn gasoline engines and diesel engines, have these selective reduction catalysts and lean NOx catalysts to ensure NOx purification performance under lean air-fuel ratio exhaust. In many cases, a catalyst called a DeNOx catalyst is used in combination with the above-described oxidation catalyst or three-way catalyst.

  Patent Documents 1 and 2 propose an exhaust purification system that combines a three-way catalyst and a DeNOx catalyst. In the exhaust purification system of Patent Document 1, during a warm-up process in which the DeNOx catalyst provided under the floor in the exhaust passage cannot exhibit sufficient NOx purification performance, a stoichiometric operation is performed in which the air-fuel ratio of the air-fuel mixture is stoichiometric. The reduction of the NOx purification rate during the warm-up process is suppressed by utilizing the three-way purification reaction in the original catalyst. In the exhaust gas purification system of Patent Document 2, stoichiometric operation is performed during transient operation in which the NOx emission amount exceeds a predetermined allowable amount, and NOx is efficiently purified using a three-way purification reaction. Further, in connection with such stoichiometric operation, Patent Document 3 describes that in the SOx release control for desorbing the SOx stored in the DeNOx catalyst, the after-injection is performed after the main injection to reduce the air-fuel ratio of the air-fuel mixture. An exhaust purification system is described.

JP 2010-116781 A JP 2009-293585 A JP 2010-127251 A

  As described above, in the exhaust purification system equipped with the DeNOx catalyst, the stoichiometric operation is performed, so that the NOx emission amount in the entire system is reduced as compared with the case where the lean operation is continued and NOx is purified mainly by the DeNOx catalyst. There are cases where it is possible. However, it is known that when stoichiometric operation is performed, the amount of particulate matter (hereinafter referred to as “PM (Particulate Matter)”) discharged from the internal combustion engine increases in contrast to NOx. For this reason, at the time of stoichiometric operation, it is preferable to perform after injection like the technique of patent document 3. FIG. When the after injection is performed, the PM generated by the main injection can be combusted in the cylinder, so that the amount of PM discharged from the internal combustion engine can be reduced.

  FIG. 32 shows the amount of after injection [mg / str] during the stoichiometric operation or the separation between the main injection and the after injection [deg. ] And PM discharge amount [mg / str] and HC discharge amount [mg / str]. In FIG. 32, the solid line indicates the PM emission amount, and the broken line indicates the HC emission amount. In FIG. 32, the PM emission amount and the HC emission amount are both amounts discharged from the internal combustion engine per unit time.

  As shown by the solid line in FIG. 32, the PM emission amount decreases as the after injection amount increases or the separation of the after injection increases. However, as indicated by the broken line in FIG. 32, the HC emission amount increases as the after injection amount increases or the separation of the after injection increases. That is, during the stoichiometric operation, there is a trade-off relationship between the PM emission amount and the HC emission amount. Conventionally, however, the after-injection amount and after-injection timing are uniquely determined according to the operating state of the internal combustion engine using a predetermined map without paying attention to the relationship between the PM emission amount and the HC emission amount. Since it was determined, it could not be said that the PM emission amount and the HC emission amount were optimal.

  The present invention has been made in view of the above points, and an object of the present invention is to provide an exhaust purification system for an internal combustion engine that can optimize the PM emission amount and the HC emission amount.

  (1) An exhaust purification system (for example, exhaust purification system 2) of an internal combustion engine (for example, engine 1) of the present invention is provided in an exhaust passage (for example, exhaust passage 11) of the internal combustion engine, and oxidizes HC in the exhaust. HC oxidation catalyst (for example, LNT41) having a function to perform fuel injection in a region where the combustion amount of PM in the cylinder increases as the interval between the main injection and the main injection increases. A fuel injection valve (for example, fuel injection valve 13) capable of performing after-injection, fuel injection control means (for example, ECU 3) for controlling the fuel injection amount and fuel injection timing from the fuel injection valve, Based on the temperature of the HC oxidation catalyst (for example, the pre-catalyst temperature sensor 53, the post-catalyst temperature sensor 54, and the ECU 3) and the temperature of the HC oxidation catalyst, the HC An oxidization capacity estimating means for estimating the HC oxidation capacity of the catalyst (for example, ECU 3 and a means for executing the processing of S52 to S54 in FIG. 10 described later), and the fuel injection control means The higher the capacity, the wider the interval between the main injection timing and the after injection timing.

  FIG. 31 is a diagram for explaining the difference between after-injection and post-injection, which are fuel injections performed after the main injection. FIG. 31 shows the relationship between the injection timing and injection amount of fuel injection performed after the main injection, and the PM emission amount (left side) and HC emission amount (right side) from the cylinder. FIG. 31 shows the case where the fuel injection timing after the main injection is changed within a range of about 45 to 75 [degATDC] while the engine operating condition is kept constant, and the fuel injection amount after the main injection is 2 mg. (Thin line), 6 mg (medium thick line), and 8 mg (thick line). In the example shown in FIG. 31, the fuel injection amount after the main injection is changed while keeping the total fuel injection amount constant, and the fuel injection timing after the main injection is changed while keeping the main injection timing constant. .

  As shown on the right side of FIG. 31, the HC emission amount increases as the fuel injection amount after the main injection is increased. Further, the HC emission amount increases as the interval between the main injection timing and the subsequent fuel injection timing is increased. Further, as shown on the left side of FIG. 31, the PM emission amount decreases as the fuel injection amount after the main injection is increased. This decrease in PM emissions is due to a decrease in the amount of PM generated in the cylinder due to a decrease in the main injection amount, and low temperature PM combustion is generated in the cylinder due to fuel additionally supplied after the main injection. By doing.

  Further, as shown on the left side of FIG. 31, when the fuel injection timing after the main injection is retarded, the PM emission amount decreases with the retarding to about 65 degrees (that is, the PM combustion amount in the cylinder) However, after about 65 degrees, PM emission does not decrease even if retarded. This is because if the fuel injection after the main injection is delayed to about 65 degrees, the combustion gas becomes too low in temperature and the low temperature PM combustion is not promoted. That is, even if the fuel injection after the main injection is delayed by more than about 65 degrees, not only a further PM emission reduction effect can be expected, but also the HC emission increases.

  Fuel injection performed after the main injection is classified into after injection and post injection performed after this after injection, but there is no definition that clearly distinguishes both. Therefore, in the present invention, after injection and post-injection are distinguished by the time when the above-described qualitative change appears in the effect produced by retarding the injection time. That is, after the main injection, fuel injection within a region where the PM combustion amount in the cylinder increases as the angle is retarded (45 to 65 [degATDC] in the example of FIG. 31) is defined as after injection. To do. Further, the post-injection is after the main injection and after-injection, and even if retarded, the PM combustion amount in the cylinder does not increase and only the HC emission amount increases (in the example of FIG. 31). , 65 [degATDC] or later) is defined as post-injection.

  (2) In this case, the exhaust passage is further provided with a DeNOx catalyst having a DeNOx function for purifying NOx in the exhaust in the presence of a reducing agent added in the exhaust passage, and the HC oxidation catalyst is stoichiometric. The exhaust purification system has a three-way purification function for purifying HC, CO and NOx in exhaust under an air-fuel ratio exhaust, and the exhaust purification system is responsive to both or both of the state of the engine and the state in the exhaust passage. The fuel injection control means further comprises NOx excess state determination means for determining whether or not the NOx excess state in which the ratio of the NOx amount flowing into the NOx purification catalyst with respect to the NOx amount that can be purified by the DeNOx function is increased. If the NOx excess state is not established, the combustion air-fuel ratio is controlled to be leaner than the stoichiometric state. If the NOx excess state is established, the combustion air-fuel ratio is stoichiometrically controlled. Control it is preferable to.

  (3) In this case, the HC oxidation catalyst (for example, LNT41) includes a three-way purification function for purifying HC, CO, and NOx in the exhaust gas under exhaust at a stoichiometric air-fuel ratio, and a reducing agent added in the exhaust passage. And a DeNOx function for purifying NOx in the exhaust gas in the presence of the exhaust gas, and the exhaust purification system purifies by the DeNOx function according to either or both of the state of the engine and the state in the exhaust passage. NOx excess state determining means for determining whether or not the NOx excess state in which the ratio of the NOx amount flowing into the NOx purification catalyst with respect to the NOx amount that can be generated is in an NOx excess state (for example, execution of ECU3 and S23 to S25 in FIG. 6 described later) The fuel injection control means controls the combustion air-fuel ratio to be leaner than stoichiometric when the NOx excess state is not present. It is preferable to control the combustion air-fuel ratio to the stoichiometric ratio if it is excessive state.

  (4) An exhaust purification system (for example, exhaust purification system 2) of an internal combustion engine (for example, engine 1) of the present invention is provided in an exhaust passage (for example, exhaust passage 11) of the internal combustion engine, and oxidizes HC in the exhaust. An HC oxidation catalyst (for example, LNT41) having an HC oxidation function to perform, a three-way purification function to purify HC, CO, and NOx in the exhaust under exhaust at a stoichiometric air-fuel ratio, main injection, and after the main injection A fuel injection valve (for example, a fuel injection valve 13) capable of performing after injection, which is fuel injection in a region where the amount of PM combustion in the cylinder increases as the interval with the main injection increases. Fuel injection control means (for example, ECU 3) for controlling the fuel injection amount and fuel injection timing from the fuel injection valve, and temperature acquisition means (for example, pre-catalyst temperature control) for acquiring the temperature of the HC oxidation catalyst. 53, the post-catalyst temperature sensor 54, and the ECU 3) and oxidation capacity estimation means (for example, the ECU 3 and S52 in FIG. 10 described later) for estimating the HC oxidation capacity of the HC oxidation catalyst based on the temperature of the HC oxidation catalyst. To the NOx amount that flows into the NOx purification catalyst with respect to the NOx amount that can be purified by the DeNOx function according to either or both of the state of the engine and the state in the exhaust passage. NOx excess state determination means (e.g., means related to execution of S23 to S25 in FIG. 6 to be described later) that determines whether or not the NOx excess state is increased. When it is determined that the NOx excess state is present, the means increases the combustion air-fuel ratio while increasing the after-injection amount as the HC oxidation capacity increases. To control the breath.

  (5) In this case, the HC oxidation capacity is determined by the basic factor (Pox_bs) depending on the temperature and exhaust volume of the HC oxidation catalyst and the deterioration factor (Kmod, Kox, Krd, Dtw) depending on the deterioration degree of the HC oxidation catalyst. ) Is preferably quantified.

  (6) In this case, the oxidization capacity estimation means is configured to perform the HC oxidation based on a heat generation coefficient (c) of the HC oxidation catalyst indicating a degree of contribution of the fuel supplied by after injection to the temperature increase of the HC oxidation catalyst. It is preferable to estimate the capacity.

  (7) In this case, the oxidation capacity estimation means is necessary for reducing the amount of reducing agent necessary for desorbing oxygen stored in the HC oxidation catalyst or NOx stored in the HC oxidation catalyst. It is preferable to estimate the HC oxidation ability based on the amount of reducing agent (Crd_hat).

(8) In this case, the exhaust purification system further includes an O ₂ sensor (for example, a post-catalyst O ₂ sensor 52) that detects an oxygen concentration in the exhaust downstream of the HC oxidation catalyst, and the oxidation capacity estimation means Preferably, the HC oxidation capability is estimated based on an air-fuel ratio target value (AFcmd) set so that the output value (Vout) of the O ₂ sensor is maintained at a predetermined target value (Vcmd).

  (1) If the interval between the main injection timing and the after injection timing is widened, the PM emission amount decreases, but the HC emission amount increases. Further, when the interval between the main injection timing and the after injection timing is narrowed, the PM emission amount increases but the HC emission amount decreases (see FIG. 31 described above). In the present invention, the HC oxidation capability of the HC oxidation catalyst is estimated based on the temperature of the HC oxidation catalyst, and the higher the HC oxidation capability, the wider the interval between the main injection timing and the after injection timing. For example, when performing the stoichiometric operation using both main injection and after injection as described above, the interval between the main injection timing and the after injection timing is controlled according to the HC oxidation capability of the HC oxidation catalyst in this way. Thus, it is possible to minimize the amount of PM discharged from the engine while maintaining the amount of unpurified HC discharged downstream of the HC oxidation catalyst within an allowable range.

  (2) In the present invention, the ratio of the amount of NOx flowing into the NOx purification catalyst to the amount of NOx that can be purified by the lean NOx purification function increases in accordance with either or both of the state of the engine and the state in the exhaust passage. It is determined whether or not the NOx is excessive. More specifically, the NOx excess state means, for example, during acceleration operation in which the amount of NOx discharged from the engine increases, or immediately after starting the engine where the temperature of the NOx purification catalyst is low and the lean NO purification function is reduced. . In the present invention, when it is not an excessive NOx state, a lean operation for controlling the combustion air-fuel ratio from lean to lean is performed to purify NOx using a lean NOx purification function. In order to purify NOx using a three-way purification function, a stoichiometric operation is performed in which the combustion air-fuel ratio is controlled stoichiometrically while using both main injection and after-injection. Thus, by switching between the lean operation and the stoichiometric operation according to the state in the engine and the exhaust passage, the NOx purification rate in the entire exhaust purification system can be maintained high. Further, when performing the stoichiometric operation, by controlling the interval between the main injection timing and the after injection timing based on the HC oxidation capability as described above, the unpurified HC discharged downstream of the HC oxidation catalyst is controlled. While maintaining the amount within an acceptable range, the amount of PM discharged from the engine can be kept to a minimum. In summary, according to the present invention, NOx and HC can be efficiently purified while minimizing PM emission.

  (3) The present invention is different from the above invention (2) in that the HC oxidation catalyst has a NOx purification function. Therefore, according to this invention, there exists an effect similar to invention of (2).

  (4) When the after injection amount is increased, the PM emission amount decreases, but the HC emission amount increases. Further, when the after injection amount is reduced, the PM emission amount increases but the HC emission amount decreases (see FIG. 31 described above). In the present invention, when it is determined that the NOx is excessive, a stoichiometric operation for controlling the combustion air-fuel ratio to stoichiometric is performed in order to efficiently purify NOx in the exhaust using the three-way purification function. Further, in the present invention, the after-injection amount is increased as the HC oxidation capability increases while performing the stoichiometric operation. As a result, the amount of PM discharged from the engine can be minimized while maintaining the amount of unpurified HC discharged downstream of the HC oxidation catalyst within an allowable range.

  (5) In the present invention, the basic factor that changes according to the state of the engine at that time, such as the temperature of the oxidation catalyst and the exhaust volume, and the state in the exhaust passage, and the deterioration factor that changes according to the degree of deterioration of the HC oxidation catalyst, Is used to quantify the HC oxidation ability of the HC oxidation catalyst. As a result, the strength of the HC oxidation capability can always be properly grasped, so that the interval between the main injection timing and the after injection timing and the after injection amount can be appropriately controlled.

  (6) When after-injection is performed, PM generated by the main injection is burned, and the PM emission amount is reduced, and at the same time, the HC emission amount is increased. When the HC emission amount increases, the heat generation amount in the HC oxidation catalyst also increases by the HC oxidation function. That is, the heat generation coefficient of the HC oxidation catalyst, which indicates the degree to which the fuel supplied by the after injection contributes to the temperature increase of the HC oxidation catalyst, can be an index indicating the HC oxidation function. In the present invention, by using the heat generation coefficient having such characteristics, it is possible to estimate the HC oxidation capability with high accuracy, and accordingly, it is possible to appropriately control the interval between the main injection timing and the after injection timing and the after injection amount.

  (7) As the HC oxidation catalyst, a catalyst having a so-called storage function for storing oxygen and NOx in exhaust gas may be used. This storage function is estimated to decrease to the same extent as the HC oxidation function as the HC oxidation catalyst deteriorates. Therefore, the amount of reducing agent necessary for desorbing oxygen stored in the HC oxidation catalyst, the amount of reducing agent necessary for reducing NOx stored in the HC oxidation catalyst, etc., depends on the storage function of the HC oxidation catalyst. It can be an index indicating not only the strength of HC but also the strength of the HC oxidation function. In the present invention, by using such a reducing agent amount, it is possible to estimate the HC oxidation ability with high accuracy, and accordingly, the interval between the main injection timing and the after injection timing and the after injection amount can be appropriately controlled.

(8) In the present invention, the HC oxidation capability is estimated based on the air-fuel ratio target value set so that the output value of the O ₂ sensor provided on the downstream side of the HC oxidation catalyst is maintained at a predetermined target value. . Here, the state in which the output value of the O ₂ sensor is maintained at the target value corresponds to a state in which the reducing agent slips slightly to the downstream side of the HC oxidation catalyst. Further, the amount of reducing agent that needs to be supplied from the upstream side in order to maintain such a reducing agent slip state decreases as the reducing ability of the HC oxidation catalyst decreases. As the reduction capability of the HC oxidation catalyst decreases, the air-fuel ratio target value for maintaining the output value of the O ₂ sensor at the target value shifts to the lean side. In addition, it is estimated that the reducing ability of the HC oxidation catalyst decreases to the same extent as the HC oxidation function as the deterioration of the HC oxidation catalyst proceeds. Therefore, the air-fuel ratio target value can be an index indicating not only the strength of the reducing ability of the HC oxidation catalyst but also the strength of the HC oxidation function. In the present invention, by using such an air-fuel ratio target value, it is possible to accurately estimate the HC oxidation capability, and accordingly, the interval between the main injection timing and the after injection timing and the after injection amount can be appropriately controlled.

It is a mimetic diagram showing composition of an engine and its exhaust gas purification system concerning one embodiment of the present invention. It is a figure which shows the relationship between the after-injection amount or after-injection time in stoichi operation, PM emission amount, and HC emission amount. It is a figure which shows the relationship between the HC oxidation performance, and the temperature of the exhaust gas on the inlet side of the direct catalyst, the temperature of the exhaust gas on the outlet side, the carrier temperature of the direct catalyst and the exhaust volume. It is an example of the map which determines the correction coefficient of an after injection amount. It is a main flowchart which shows the specific procedure of fuel-injection control. It is a flowchart which shows the specific procedure of stoichi operating condition judgment processing. It is an example of the map (for the ternary purification mode) for determining the value of the stoichiometric mode flag. It is an example of the map (for combined mode) for determining the value of the stoichiometric mode flag. It is a flowchart which shows the specific procedure of the air-fuel ratio feedback calculation before a catalyst. It is a flowchart which shows the specific procedure which determines the after injection quantity and the after injection timing at the time of stoichi operation. It is an example of the map which determines the reference value of an oxidation characteristic parameter. It is an example of the map which determines the reference value of after-injection amount. It is an example of the map which determines the correction coefficient of an after injection amount. It is an example of the map which determines the reference value of after injection timing. It is an example of the map which determines the corrected amount of after injection timing. It is a flowchart which shows the specific procedure of a post-catalyst air-fuel ratio feedback calculation. It is an example of the map which determines a target air fuel ratio under weak rich mode. It is an example of a map for determining a target value of the output of the post-catalyst O ₂ sensor. It is a time chart showing a specific example of a pre-catalyst LAF sensor output and a change in post-catalyst O ₂ sensor output at the time of air-fuel ratio feedback execution catalyst. It is a flowchart which shows the specific procedure of a direct catalyst reduction characteristic determination process. It is a time chart which shows the specific example of a change of the reduction process completion flag etc. at the time of execution of a direct catalyst reduction characteristic determination process. It is a flowchart which shows the specific procedure of a catalyst ternary characteristic adaptive calculation. It is a figure which shows an example of the shape of a ternary characteristic weight function. It is a flowchart which shows the specific procedure of a catalyst reduction characteristic adaptive calculation. It is an example of the map which determines a reference | standard reducing agent supply amount. It is a figure which shows an example of the shape of a reducing agent supply amount weight function. It is a flowchart which shows the specific procedure of the sequential identification calculation of the thermal model of a direct catalyst. It is a flowchart which shows the specific procedure of a catalyst oxidation characteristic adaptive calculation. It is a figure which shows an example of the shape of an oxidation characteristic weight function. It is a mimetic diagram showing composition of an engine and its exhaust gas purification system concerning one embodiment of the present invention. It is a figure for demonstrating the difference between after injection and post injection. It is a figure which shows the relationship between the after-injection amount in the stoichiometric operation or the separation between the main injection and the after-injection, and the PM emission amount and the HC emission amount.

Hereinafter, an embodiment of the present invention will be described with reference to the drawings.
FIG. 1 is a diagram showing a configuration of an internal combustion engine (hereinafter referred to as “engine”) 1 and its exhaust purification system 2 according to the present embodiment. The engine 1 is based on so-called lean combustion in which the combustion air-fuel ratio is leaner than stoichiometric, more specifically, a diesel engine, a lean burn gasoline engine, or the like.

  The exhaust purification system 2 includes a lean NOx catalyst (hereinafter referred to as “LNT”) 41 and an exhaust purification filter (hereinafter referred to as “DPF”) 43 provided in the exhaust passage 11 of the engine 1, and a reducing agent in the exhaust passage 11. And an electronic control unit (hereinafter referred to as “ECU”) 3 for controlling the engine 1 and the exhaust fuel injection device 45.

  The engine 1 is provided with a fuel injection valve 13 for injecting fuel into each cylinder. These fuel injection valves 13 are connected to the ECU 3 via a driving device (not shown). The ECU 3 determines the fuel injection amount, the fuel injection timing, and the like from the fuel injection valve 13 by fuel injection control which will be described later with reference to FIGS. 2 to 29, and the drive device realizes the determined fuel injection mode. Then, the fuel injection valve 13 is driven.

  The LNT 41 has at least three functions of an oxidation function, a DeNOx function, and a ternary purification function. Here, the oxidation function refers to a function of oxidizing HC and CO contained in exhaust gas during a lean operation in which the combustion air-fuel ratio is leaner than stoichiometric. With the DeNOx function, during lean operation, NOx contained in the exhaust gas is occluded, and when fuel is supplied into the exhaust gas by exhaust fuel injection from the exhaust fuel injection device 45, post injection from the fuel injection valve 13, or the like, This is the function of reducing NOx using this as a reducing agent. The three-way purification function refers to a function for purifying HC, CO, and NOx contained in exhaust gas together during a stoichiometric operation in which the combustion air-fuel ratio is stoichiometric.

  As described above, the NOx in the exhaust gas can be purified using the DeNOx function of the LNT 41 during the lean operation, and can be purified using the three-way purification function of the LNT 41 during the stoichiometric operation. Here, comparing the case where NOx is purified using the DeNOx function and the case where NOx is purified using the three-way purification function, the case where the three-way purification function is used is more efficient for NOx. It can be purified. Therefore, for example, when the amount of NOx exhausted from the engine 1 increases during high-load operation, or when the DeNOx function cannot be fully exhibited because the LNT 41 has not reached its activity (NOx excessive state described later). The lean operation is switched to the stoichiometric operation in order to purify the exhaust using the three-way purification function. A specific procedure for switching between the lean operation and the stoichiometric operation will be described in detail later with reference to FIG.

  When the exhaust gas passes through fine holes in the filter wall, the DPF 43 collects PM mainly composed of carbon in the exhaust gas by depositing it on the surface of the filter wall and the holes in the filter wall. As a constituent material of the filter wall, for example, a porous body made of aluminum titanate or cordierite is used.

  By the way, the exhaust passage 11 is divided into a section located in an engine room (not shown) (a section directly under the engine) and a section (under the floor section) located under the floor of a vehicle (not shown). The section immediately below is closer to the engine 1 than the section below the floor. Accordingly, the average temperature in the section immediately below is higher than that in the section under the floor, and the temperature rise after the start of the engine 1 is quicker. Therefore, the LNT 41 is provided in the section immediately below the exhaust passage 11 in order to exhibit the above-described oxidation function, three-way purification function, and DeNOx function as advantageously as possible. As described above, since LNT 41 is provided in the immediately lower section, LNT 41 is hereinafter also referred to as a direct catalyst.

  When PM is collected to the limit of the collection capability of the DPF 43, the pressure loss increases. For this reason, the forced regeneration process which burns and removes the collected PM and regenerates the filter function of the DPF 43 is appropriately executed. In this forced regeneration processing, for example, post injection or fuel injection from the exhaust fuel injection device 45 is executed, and the exhaust gas flowing into the DPF 43 is heated to burn and remove the accumulated PM in a short time.

  The exhaust fuel injection device 45 includes a fuel tank 451 in which fuel is stored, an exhaust fuel injector 452 provided on the upstream side of the LNT 11 in the exhaust passage 11, and pressurization for pressure-feeding the fuel in the fuel tank 451 to the injector 452. A pump 453. The exhaust fuel injector 452 is electromagnetically connected to the ECU 3 via a drive device (not shown). When the exhaust gas is purified by using the DeNOx function of the LNT 41, the ECU 3 determines the exhaust fuel injection amount and the exhaust fuel injection timing from the exhaust fuel injector 452 by exhaust fuel injection control (not shown). The exhaust fuel injector 452 is driven so as to realize the exhaust fuel injection mode.

Incidentally, in recent years, when fuel is injected from the exhaust fuel injector 452 and NOx is reduced using the DeNOx function of the LNT 41, the exhaust fuel injection amount from the exhaust fuel injector 452 is within a predetermined range at a cycle of 5 Hz or more. It is known that when the hydrocarbon concentration of the exhaust gas flowing into the LNT 41 is increased and decreased, an intermediate product derived from hydrocarbons is generated on the LNT 41, and NOx can be purified at a high purification rate by this intermediate product. However, when fuel is injected in the above-described manner in a state where the carrier temperature of LNT41 is about 350 ° C. or less, unnecessary components (for example, N ₂ O) that do not contribute to NOx purification are generated. It may be discharged downstream. Therefore, in the exhaust fuel injection control, the fuel is intermittently injected as described above only when the carrier temperature of the LNT 41 is about 350 ° C. or more and not more than the upper limit temperature of about 630 to 700 ° C.

The ECU 3 includes a pre-catalyst LAF sensor 51, a post-catalyst O ₂ sensor 52, a pre-catalyst temperature sensor 53, a post-catalyst temperature sensor 54, a crank angle as sensors for detecting the state in the exhaust passage 11 and the state of the engine 1. A position sensor 55, an accelerator opening sensor 56, an air flow sensor 57, an atmospheric temperature sensor 58, and the like are connected.

The post-catalyst O ₂ sensor 52 is provided between the LNT 41 and the DPF 43 in the exhaust passage 11. The O ₂ sensor 52 detects the oxygen concentration (air-fuel ratio) of the exhaust gas downstream of the LNT 41, and transmits a signal corresponding to the detected value to the ECU 3. The level of the signal output from the O ₂ sensor 52 is high (eg, 1) when the combustion air-fuel ratio is richer than stoichiometric, and is low (eg, 0) when leaner than stoichiometric. There are substantially binary characteristics as described above (for example, see FIG. 19 described later). Hereinafter, switching of the output of the O ₂ sensor 52 from low to high is referred to as output inversion.

The pre-catalyst LAF sensor 51 is provided upstream of the LNT 41 and the exhaust fuel injector 452 in the exhaust passage 11. The LAF sensor 51 detects the air-fuel ratio of the exhaust before the fuel is injected from the exhaust fuel injector 452 on the upstream side of the LNT 41 and transmits a signal substantially proportional to the detected value to the ECU 3. Unlike the O ₂ sensor 52, the signal output from the LAF sensor 51 has linear characteristics in a wider range of air-fuel ratios from a rich region to a lean region.

  The pre-catalyst temperature sensor 53 is provided upstream of the LNT 41 in the exhaust passage 11, and the post-catalyst temperature sensor 54 is provided downstream of the LNT 41 in the exhaust passage 11. These temperature sensors 53 and 54 detect the temperatures of the exhaust gas flowing into the LNT 41 and the exhaust gas flowing out of the LNT 41, respectively, and transmit a signal substantially proportional to the detected value to the ECU 3. The carrier temperature of the LNT 41 is estimated by the ECU 5 as a weighted average value of the outputs of these temperature sensors 53 and 54.

  The crank angle position sensor 55 detects the rotation angle of the crankshaft of the engine 1, generates a pulse for each predetermined crank angle, and transmits the pulse signal to the ECU 3. The rotational speed of the engine 1 is calculated by the ECU 3 based on this pulse signal. The accelerator opening sensor 56 detects a depression amount (hereinafter referred to as “accelerator opening”) of an accelerator pedal (not shown) of the vehicle, and transmits a detection signal substantially proportional to the detected value to the ECU 3. The ECU 3 calculates the driver request torque according to the accelerator opening and the engine speed. The air flow meter 57 is provided in the intake passage 12. The air flow meter 57 detects the amount of intake air flowing through the intake passage 12 and transmits a signal substantially proportional to the detected value to the ECU 3. The ECU 3 calculates the exhaust volume according to the intake air amount.

Next, the content of the fuel injection control will be described in detail with reference to FIGS.
FIG. 2 is a diagram showing the relationship between the after injection amount [mg / str] or the after injection timing [degATDC] and the PM emission amount [mg / str] and the HC emission amount [mg / str] during the stoichiometric operation. . In FIG. 2, the solid line indicates the PM emission amount, and the broken line indicates the HC emission amount. In FIG. 2, the PM emission amount indicates the amount of PM discharged from the engine per unit time, and the HC emission amount indicates the amount of HC discharged per unit time without being purified downstream of the direct catalyst. Indicates.

  As described with reference to FIG. 31, when the after injection amount is increased or the after injection timing is delayed, the amount of PM discharged from the engine decreases, but the amount of HC discharged from the engine increases. In the exhaust purification system shown in FIG. 1, HC discharged from the engine during the stoichiometric operation is oxidized using a three-way purification function in the direct catalyst. Then, as shown by three broken lines in FIG. 2, the amount of HC discharged to the downstream side of the direct catalyst changes according to the HC oxidation performance of the direct catalyst at that time. That is, as the HC oxidation performance of the direct catalyst increases, the amount of HC discharged to the downstream side of the direct catalyst decreases.

  During the stoichiometric operation, the upper limit of the HC emission amount as shown by the one-dot chain line in FIG. 2 is set for the HC emission amount to the downstream side of the direct catalyst. Further, during the stoichiometric operation, it is preferable that the PM emission amount is always small in order to reduce the burden on the DPF as much as possible. When the above conditions are imposed on the HC emission amount and PM emission amount during stoichiometric operation, an upper limit is set for the after-injection amount and the after-injection timing according to the HC oxidation performance of the direct catalyst at that time. . In other words, in order to keep the HC emission amount below a predetermined upper limit and to reduce the PM emission amount as much as possible, the after injection amount and the after injection timing may be adjusted according to the HC oxidation performance of the direct catalyst. Next, a procedure (STEPs 1 to 3) for determining the after injection amount and the after injection timing in consideration of this point will be briefly described.

  In STEP1, a parameter (corresponding to an oxidation characteristic parameter Pox described later) indicating the HC oxidation performance of the direct catalyst is calculated. Factors that determine the HC oxidation performance of the direct catalyst are mainly divided into the environment in which the direct catalyst is used and the deterioration degree and individual variation of the direct catalyst. In STEP 1, this oxidation characteristic parameter is divided into a basic factor (corresponding to Pox_bs described later) determined by the environment in which the direct catalyst is used, and a deterioration factor (corresponding to Kmod described later) determined by the degree of deterioration of the direct catalyst and individual variations. ) Using two factors.

  FIG. 3 is a diagram showing the relationship between the HC oxidation performance and the temperature of the exhaust on the inlet side of the direct catalyst, the temperature of the exhaust on the outlet side, the carrier temperature of the direct catalyst, the exhaust volume, and the like. As shown in FIG. 3, the higher the inlet side temperature, outlet side temperature, or carrier temperature of the direct catalyst, the HC oxidation performance of the direct catalyst tends to become stronger. On the other hand, as the exhaust volume increases, the HC oxidation performance of the direct catalyst tends to weaken. That is, the temperature on the inlet side, the temperature on the outlet side, the carrier temperature, the exhaust volume, and the like of these direct catalysts can be employed as parameters for determining the above-mentioned basic factors. In STEP 1, these parameters are used as inputs, and a basic factor is determined using a map search or a predetermined arithmetic expression.

  In addition, the degree of deterioration and individual variation of the direct catalyst cannot be directly quantified. Accordingly, the deterioration factor is determined by any one of the following TYPEs 1 to 3, paying attention to the fact that the degree of deterioration has a correlation with various characteristics of the direct catalyst.

  In TYPE1, the deterioration factor is determined using the oxidation characteristics of the direct catalyst. When after-injection is performed, an amount of HC approximately proportional to the after-injection amount flows into the catalyst directly below. In the direct catalyst, the inflowed HC is oxidized and generates heat. For this reason, the heat generation coefficient indicating the degree to which the fuel supplied by after injection contributes to the temperature increase of the direct catalyst can be adopted as a deterioration factor.

  In TYPE2, the deterioration factor is determined using the storage characteristics of the direct catalyst. The direct catalyst has a storage function for storing oxygen and NOx in the exhaust. For this reason, the amount of oxygen and NOx stored in the direct catalyst or the amount of reducing agent necessary for desorbing these oxygen and NOx can be employed as a deterioration factor.

In TYPE3, when the post-catalyst air-fuel ratio feedback control, which will be described later, is executed, the air-fuel ratio target value set so that the output value of the O ₂ sensor provided on the downstream side of the direct catalyst is maintained at a predetermined target value or The deterioration factor is determined using the actual air-fuel ratio.

  In STEP2, based on the oxidation characteristic parameter calculated in STEP1, for example, a map as shown in FIG. 4 is searched to calculate the correction coefficient of the after injection amount and the correction value of the after injection timing. In FIG. 4, the horizontal axis represents the oxidation characteristic parameter calculated in STEP 1. The correction coefficient of the after injection amount on the vertical axis is defined as a positive real number multiplied by a predetermined basic injection amount. The correction value of the after injection timing on the vertical axis is defined as a real number that is subtracted to a predetermined basic time. As shown in FIG. 4, the after injection amount is corrected to the increase side as the HC oxidation performance of the direct catalyst increases. Further, the after injection timing is corrected to the retard side as the HC oxidation performance of the direct catalyst increases. That is, the separation between the main injection and the after injection becomes wider as the HC oxidation performance of the direct catalyst becomes higher.

  In STEP 3, the after injection amount and the after injection timing are determined using the correction coefficient and the correction value calculated in STEP 2. More specifically, the final after injection amount is calculated by multiplying a predetermined basic injection amount by the correction coefficient. The final after injection timing is calculated by subtracting the correction value from a predetermined basic time. Accordingly, the after injection amount and the after injection timing during the stoichiometric operation can be adjusted according to the HC oxidation performance of the direct catalyst so that the HC emission amount is equal to or less than the predetermined upper limit value and the PM emission amount is as small as possible. .

Next, a specific procedure of fuel injection control will be described with reference to FIGS.
FIG. 5 is a main flowchart showing a specific procedure of fuel injection control for determining the fuel injection mode by the fuel injection valve of each cylinder. The process shown in FIG. 5 is executed in the ECU in synchronism with the TDC timing of each cylinder for each combustion cycle. In the following description, values that are updated or sampled in TDC synchronization in the ECU will be denoted by parenthesized symbols “k”.

In S1, the basic fuel injection amount Gfuel_bs (k) is determined by searching a predetermined map (not shown) according to the operating state of the engine, and the process proceeds to S2. This basic fuel injection amount corresponds to the fuel injection amount during lean operation (see S13 described later). As will be described in detail later, during the stoichiometric operation, the basic fuel injection amount is multiplied by an air-fuel ratio correction coefficient KAF (k) determined by feedback control based on the outputs of the pre-catalyst LAF sensor and the post-catalyst O ₂ sensor. (See S8 described later). Moreover, it shows the operating state of the engine, and examples of the input parameters used for determining the basic fuel injection amount include a driver request torque and an engine speed.

In S2, it is determined whether or not a device related to fuel injection control is normal. The devices related to the determination in S2 are, for example, an intake throttle and an EGR valve (not shown), a pre-catalyst LAF sensor, a post-catalyst O ₂ sensor, a temperature sensor, and the like that are necessary for performing stoichiometric operation. When the determination of S2 is YES (when the apparatus is normal), the process proceeds to S3, and when NO (when the apparatus is not normal), the process proceeds to S13 and the lean operation is executed.

  In S3, it is determined whether or not the immediately below catalyst is in an active state. More specifically, in S3, an estimated value of the support temperature of the direct catalyst is calculated, and when the estimated value is equal to or higher than a predetermined activation temperature (for example, 200 ° C.), it is determined that it is in an active state. In this case, it is determined that the active state is not established. A specific procedure for calculating the estimated value of the carrier temperature of the direct catalyst will be described in S51 of FIG. If the determination in S3 is YES, the process proceeds to S5, and if NO, the process proceeds to S13 and the lean operation is executed. Even if the stoichiometric operation is performed when the direct catalyst is not in an active state, a sufficient exhaust purification effect may not be obtained.

  In S5, a stoichiometric operation condition determination process for determining whether or not the stoichiometric operation can be executed is executed, and the process proceeds to S6. As will be described later with reference to FIG. 6, in this stoichiometric operation condition determination process, it is determined whether or not the stoichiometric operation condition is suitable for performing the stoichiometric operation in accordance with the operating state of the engine, the state of the catalyst directly under the exhaust passage, and the like. Is judged. As a result, when it is determined that the state is suitable for performing the stoichiometric operation, the stoichiometric mode flag F_Stoic_mode (k) that clearly indicates this is set to “1”, otherwise, the flag F_Stoic_mode ( k) is set to “0”.

  In S6, it is determined whether or not the stoichiometric mode flag F_Stoic_mode (k) is “1”. If the determination in S6 is YES, the process proceeds to S7 and a stoichiometric operation is performed. If the determination is NO, the process proceeds to S13 and a lean operation is performed. In the flowchart of FIG. 5, the processes shown in S7 to S12 correspond to fuel injection control in the stoichiometric operation mode, and the processes shown in S13 to S14 correspond to fuel injection control in the lean operation mode.

  In S7, a pre-catalyst air-fuel ratio feedback calculation described later is executed, and the process proceeds to S8. As will be described in detail later with reference to FIG. 9, in the pre-catalyst air-fuel ratio feedback calculation, the air-fuel ratio for controlling the combustion air-fuel ratio in the vicinity of stoichiometric (stoichiometric or weakly rich) based on the output of the pre-catalyst LAF sensor. A correction coefficient KAF (k) is calculated.

In S8, the basic fuel injection amount Gfuel_bs (k) obtained in S1 is multiplied by an air-fuel ratio correction coefficient KAF (k) to determine the total fuel injection amount Gfuel (k) during stoichiometric operation (the following equation) (Refer to (1)), the process proceeds to S9. Here, the “total fuel injection amount” is the total amount of fuel used for combustion in the cylinder during one combustion cycle, and all the fuels injected by pilot injection, main injection, and after injection are combined. It corresponds to that.

  In S9, the after injection amount Gfuel_aft (k) and the after injection timing Θ_aft (k) are calculated, and the process proceeds to S10. A specific procedure for calculating the after injection amount and timing will be described later in detail with reference to FIG.

  In S10, the fuel amount Gfuel_pi (k) (hereinafter referred to as “pilot injection amount”) to be supplied by pilot injection, the timing Θ_pi (k) (hereinafter referred to as “pilot injection timing”) for performing the pilot injection, and the main injection are determined. The execution time Θ_main (k) (hereinafter referred to as “main injection timing”) is calculated, and the process proceeds to S11. The pilot injection amount Gfuel_pi (k), pilot injection timing Θ_pi (k), and main injection timing Θ_main (k) are the engine speed and load parameters (for example, BMEP. Other, required torque, fuel injection amount, engine A parameter that increases in proportion to the engine load, such as an estimated torque value and an exhaust volume) is used as an input, and is calculated by a known method such as a map search.

  In S12, the fuel amount Gfuel_main (hereinafter referred to as “main injection amount”) supplied by the main injection by subtracting the pilot injection amount Gfuel_pi (k) and the after injection amount Gfuel_aft (k) from the total fuel injection amount Gfuel (k). ”) And the process is terminated.

  In S13, the basic fuel injection amount Gfuel_bs (k) obtained in S1 is set as the total fuel injection amount Gfuel (k) during the lean operation, and the process proceeds to S14. In S14, the fuel injection mode is determined according to the algorithm defined for the lean operation mode, and this process is terminated.

  FIG. 6 is a flowchart showing a specific procedure of stoichiometric operation condition determination processing for updating the stoichiometric mode flag F_Stoic_mode. In other words, FIG. 6 is a flowchart for determining whether to perform stoichiometric operation or lean operation. The process shown in FIG. 6 is executed at the same cycle (TDC synchronization) as a subroutine of the main process shown in FIG.

  In S21, it is determined whether or not a predetermined direct catalyst protection condition set for protecting the direct catalyst from heat is satisfied. When the stoichiometric operation is executed, the exhaust temperature rises and the carrier temperature of the catalyst in the exhaust passage also rises. Since the direct catalyst is close to the engine, the temperature rise during the stoichiometric operation is large. The direct catalyst protection condition is a condition set to prevent the direct catalyst from deteriorating due to a temperature rise. More specifically, in S21, an estimated value of the carrier temperature of the direct catalyst is calculated, and when the estimated value is less than a predetermined catalyst protection temperature set to, for example, about 630 to 700 ° C., the protection condition is set. It is determined that the condition is satisfied, otherwise it is determined that the protection condition is not satisfied. If the determination in S21 is NO, the process moves to S22, the flag F_Stoic_mode is set to “0” to prohibit the stoichiometric operation, and the process returns to S6 in FIG. If the determination in S21 is YES, the process moves to S23.

In S23, the LNT determines whether or not the DeNOx function can be sufficiently exhibited. Here, “the state in which the DeNOx function can be sufficiently exerted” is unnecessary from the LNT in the presence of the reducing agent injected from the exhaust fuel injector in the exhaust purification system of FIG. This refers to a state in which NOx can be purified with an appropriate efficiency without exhausting various components (for example, the above-described N ₂ O). More specifically, in S23, when the estimated value of the carrier temperature of the LNT is equal to or higher than a predetermined purifiable temperature set at, for example, about 350 to 400 ° C., the LNT can sufficiently exhibit the DeNOx function. It is judged that. Note that the case where the determination in S23 is NO includes, for example, a case where the engine is warming up immediately after the start of engine start, a case where the vehicle is running in an urban area, and the temperature of the LNT is lowered.

  If the determination in S23 is NO, that is, if the amount of NOx that can be purified by using the DeNOx function of the direct catalyst is small, the process proceeds to S24, and emphasis is placed on exhaust purification by the three-way purification function rather than the DeNOx function. The stoichiometric mode flag F_Stoic_mode (k) is updated using the map for the three-way purification mode. More specifically, the flag F_Stoic_mode is obtained by acquiring the engine speed and the engine load parameter (for example, BMEP), and searching the map for the three-way purification mode as shown in FIG. 7 using these as input parameters. Determine the value of (k). As shown by broken lines in FIG. 7, when the engine operating state is roughly divided into four regions, a low rotation-high load region and a high rotation-low load region in which the amount of NOx discharged from the engine and flowing into the direct catalyst is large. In addition, the stoichiometric operation is selected in the three regions of the high rotation-high load region (F_Stoic_mode ← 1), and the lean operation is selected in the low rotation-low load region where the amount of NOx flowing into the direct catalyst is small (F_Stoic_mode ← 0). ).

  If the determination in S23 is YES, that is, if the amount of NOx that can be purified using the DeNOx function of the direct catalyst is large, the process proceeds to S25, and the map for the combined mode of the DeNOx function and the three-way purification function is used. Update the stoichiometric mode flag F_Stoic_mode (k). More specifically, the value of the flag F_Stoic_mode (k) is determined by acquiring the engine speed and the load parameter and searching a map for the combined mode as shown in FIG. 8 as these input parameters. When the map for the three-way purification mode in FIG. 7 and the map for the combined mode in FIG. 8 are compared, the region where the stoichiometric operation is selected (F_Stoic_mode ← 1) is the map in the combined mode mode in FIG. Is narrow. This is because when the determination in S23 is YES, the amount of NOx that can be purified using the DeNOx function of the direct catalyst is larger than when NO.

  A state in which the ratio of the amount of NOx flowing into the direct catalyst per unit time to the amount of NOx that can be purified by the DeNOx function of the direct catalyst is greater than a predetermined value is defined as a NOx excess state. In the flowchart of FIG. 6, it is determined whether or not the NOx is excessive by the processes of S23 to S25 and the maps of FIGS. 7 and 8, and if the NOx is excessive, the NOx is purified using the three-way purification function. The stoichiometric operation is selected as much as possible (F_Stoic_mode ← 1), and if the NOx is not excessive, the lean operation is selected so as to purify NOx using the DeNOx function (F_Stoic_mode ← 0).

  FIG. 9 is a flowchart showing a specific procedure of the pre-catalyst air-fuel ratio feedback calculation for determining the air-fuel ratio correction coefficient KAF used during the stoichiometric operation. The process shown in FIG. 9 is executed in the same cycle (TDC synchronization) as a subroutine of the main process shown in FIG.

  In S31, it is determined whether or not the pre-catalyst LAF sensor has reached activity. If the determination in S31 is NO, the correction coefficient KAF (k) = 1 is set without performing the following feedback calculation (S32), and the process returns to S8 in FIG.

If the determination in S31 is YES, the deviation E_af (k) (see the following equation (2-1)) between the output value AFact (k) of the pre-catalyst LAF sensor and the target value AFcmd (k) is 0. Thus, the correction coefficient KAF (k) is determined using a known feedback algorithm (S33), and the process returns to S8 in FIG. Here, the target value AFcmd (k) for the output of the pre-catalyst LAF sensor is a resampled value that is updated at a control cycle of about 10 to 50 ms in a post-catalyst air-fuel ratio feedback calculation (see FIG. 16 described later). Obtained. As an example of the calculation in S33, the following formulas (2-1) to (2-3) show calculation formulas when the correction coefficient KAF (k) is determined using the sliding mode algorithm. In Expression (2-2), “Pole_af” is a switching function setting parameter, and is set to a value larger than −1 and smaller than 0 (for example, −0.65). In the equation (2-3), the two feedback gains “Krch_af” and “Kadp_af” are set to negative values. Note that the compensation speed of the deviation of the pre-catalyst feedback in S31 is preferably set to be higher than the speed of the post-catalyst feedback in S77 of FIG.

  FIG. 10 is a flowchart showing a specific procedure for determining the after injection amount and the after injection timing during the stoichiometric operation. The process shown in FIG. 10 is executed in TDC synchronization as a subroutine of the main process shown in FIG.

In S51, the estimated value Tcc_hat (k) of the carrier temperature of the direct catalyst and the estimated value Gex_hat (k) of the exhaust volume are acquired, and the process proceeds to S52. Note that the estimated value Tcc_hat (k) of the immediately below catalyst temperature is, for example, as shown in the following formula (3), an output Tup (k) of the immediately below catalyst upstream temperature sensor and an output Tds (k) of the immediately below catalyst downstream temperature sensor. It is calculated by weighting under a predetermined weight coefficient Wt (0 ≦ Wt ≦ 1, for example, “0.3”).

  In S52, the reference value Pox_bs (k) of the oxidation characteristic parameter Pox (k) of the direct catalyst obtained by quantifying the strength of the HC oxidation ability in the direct catalyst is calculated, and the process proceeds to S53. The oxidation characteristic parameter Pox (k) includes a reference value Pox_bs (k) corresponding to a basic factor depending on the carrier temperature and the exhaust volume of the direct catalyst, and the degree of deterioration of the direct catalyst, as shown in equation (4) described later. And an oxidation characteristic correction coefficient Kmod (k) corresponding to a deterioration factor depending on individual variation. In S52, the reference value Pox_bs (k) (0 ≦ Pox_bs (k) ≦ 1) is searched by searching a predetermined map based on the two estimated values Tcc_hat (k) and Gex_hat (k) acquired in S51. Is calculated.

  FIG. 11 is an example of a map for determining the reference value Pox_bs (k) of the oxidation characteristic parameter. As shown in FIG. 11, the HC oxidation performance of the direct catalyst is exhibited when the carrier temperature exceeds the activation temperature (about 150 to 200 ° C.), and becomes stronger as the temperature increases. In addition, as the exhaust volume increases, that is, as the amount of exhaust passing through the direct catalyst per unit time increases, the HC oxidation performance decreases. Note that the reference value Pox_bs (k) is normalized to be 0 when the direct catalyst does not have the ability to oxidize HC, and is normalized to be a predetermined value (for example, 1) when the direct catalyst has the highest HC oxidation performance. The

  Returning to FIG. 10, in S53, the oxidation characteristic correction coefficient Kmod (k) is calculated, and the flow proceeds to S54. As shown in FIG. 10, there are three types of methods TYPE 1 to 3 for calculating the oxidation characteristic correction coefficient Kmod (k). However, in any method, the correction coefficient Kmod (k) is obtained by re-sampling a parameter updated in TDC synchronization in a process executed in a different cycle from the main process executed in TDC synchronization.

In the TYPE 1 method, the re-sampling of the catalytic oxidation characteristic adaptive coefficient Kox updated at the period tn by the process shown in FIG. 28 described later is used as the correction coefficient Kmod and used for the subsequent processes.
In the TYPE 2 method, a re-sampling of the catalyst reduction characteristic adaptive coefficient Krd updated at the cycle tm by the process shown in FIG. 24 described later is used as the correction coefficient Kmod and used for the subsequent processes.
In the method of TYPE 3, the value obtained by resampling the catalyst ternary characteristic adaptive correction value Dtw updated at the period tm is multiplied by −1 and further added by 1 by the process shown in FIG. The coefficient Kmod (= 1−Dtw) is used for the subsequent processing.

  These coefficients Kox, Krd, and Dtw are values calculated by different methods based on different characteristics of the direct catalyst, but all change depending on the degree of deterioration of the direct catalyst and individual variations. It can be adopted as a factor.

In S54, the oxidation characteristic parameter Pox (k) is calculated by multiplying the basic value Pox_bs (k) by the oxidation characteristic correction coefficient Kmod (k) (see the following equation (4)), and the process proceeds to S55. The after injection amount and the after injection timing are finely adjusted using the oxidation characteristic parameter Pox (k) calculated here.

  In S55, the reference value Gfuel_aft_bs (k) of the after injection amount Gfuel_aft (k) is calculated, and the process proceeds to S56. More specifically, in S55, the reference value Gfuel_aft_bs (k) is calculated by searching a predetermined map based on the engine speed and the load parameter.

  FIG. 12 is an example of a map for determining the reference value Gfuel_aft_bs of the after injection amount. As shown in FIG. 12, the after injection amount is increased as the engine speed increases. Further, the after-injection amount is increased as the engine load increases. This is because the main injection amount increases as the load increases, and the amount of PM to be combusted by after-injection also increases.

  In S56, the correction coefficient Kg_aft (k) of the after injection amount Gfuel_aft (k) is calculated based on the oxidation characteristic parameter Pox (k), and the process proceeds to S57. More specifically, in S56, a correction coefficient Kg_aft (k) is calculated by searching a predetermined map based on the oxidation characteristic parameter Pox (k).

  FIG. 13 is an example of a map for determining the after injection amount correction coefficient Kg_aft (k). The correction coefficient Kg_aft calculated in S56 is used as a multiplicative correction coefficient for the after-injection amount reference value Gfuel_aft_bs (see formula (5) described later). As shown in FIG. 13, the after injection amount is corrected to the increase side as the oxidation characteristic parameter Pox increases, that is, as the HC oxidation performance increases. As a result, the PM emission amount can be suppressed as much as possible in accordance with the HC oxidation performance of the direct catalyst at that time, so that the DPF regeneration interval can be prolonged, and as a result, the total fuel consumption can be improved. Further, as described with reference to FIG. 31, when the after injection amount increases, the amount of HC flowing into the catalyst immediately below increases accordingly. However, by increasing the after injection amount in accordance with the oxidation characteristic parameter Pox in this way, the amount of HC discharged downstream of the direct catalyst can be made equal to or less than a predetermined upper limit.

  In S57, the reference value Θ_aft_bs (k) of the after injection timing Θ_aft (k) is calculated based on the oxidation characteristic parameter Pox (k), and the process proceeds to S58. More specifically, in S57, the reference value Θ_aft_bs (k) is calculated by searching a predetermined map based on the engine speed and the load parameter.

  FIG. 14 is an example of a map for determining the reference value Θ_aft_bs (k) [degATDC] of the after injection timing. As shown in FIG. 14, the reference value for the after injection timing becomes slower as the engine speed increases. Further, the after injection timing becomes later as the engine load increases. This is because the main injection amount increases as the load increases, and the amount of PM to be burned by the after injection also increases.

  In S58, the correction amount D_aft (k) of the after injection timing Θ_aft (k) is calculated based on the oxidation characteristic parameter Pox, and the process proceeds to S59. More specifically, in S58, the correction amount D_aft (k) is calculated by searching a predetermined map based on the oxidation characteristic parameter Pox (k).

  FIG. 15 is an example of a map for determining the correction amount D_aft (k) of the after injection timing. The final after injection timing Θ_aft is calculated by subtracting the correction amount D_aft (k) calculated in S58 from the reference value Θ_aft_bs (see formula (6) described later). As shown in FIG. 15, the after injection timing is corrected to the retard side as the oxidation characteristic parameter Pox increases, that is, as the HC oxidation performance increases. Also, the main injection timing is not corrected by the oxidation characteristic parameter Pox, unlike the after injection timing. Therefore, the correction amount D_aft as shown in FIG. 15 corrects the after injection amount to the retard side as the oxidation characteristic parameter Pox increases. The separation between the main injection timing and the after injection timing increases as the oxidation characteristic parameter Pox increases. Is equivalent to widening. Thereby, since the PM emission amount can be suppressed as much as possible according to the HC oxidation performance of the direct catalyst, the regeneration interval of the DPF can be lengthened, and as a result, the total fuel consumption can be improved. Further, as described with reference to FIG. 31, when the separation of the after injection is expanded, the amount of HC flowing into the catalyst immediately below increases accordingly. However, the amount of HC discharged downstream of the direct catalyst can be reduced to a predetermined upper limit or less by widening the separation of after injection according to the oxidation characteristic parameter Pox in this way.

Returning to FIG. 10, in S59, the after-injection amount Gfuel_aft (k) is calculated by multiplying the reference value Gfuel_aft (k) and the correction coefficient Kg_aft (k) (see the following equation (5)), and in S60 Move.

In S60, the after-injection timing Θ_aft (k) is calculated by subtracting the correction amount D_aft (k) from the reference value Θ_aft_bs (k) (see the following formula (6)), and the process returns to S10 in FIG.

  FIG. 16 is a flowchart showing a specific procedure of the post-catalyst air-fuel ratio feedback calculation for determining the target value AFcmd of the output AFact of the pre-catalyst LAF sensor. The process shown in FIG. 16 is executed at a predetermined control cycle tm (10 to 50 msec) in the ECU. In the following, a value that is updated or sampled at the period tm is denoted by “m” in parentheses.

In S71, it is determined whether or not the post-catalyst O ₂ sensor has reached activity. If the determination in S71 is NO, the target value AFcmd (m) is set to a predetermined reference value AFcmd_bs (fixed value, for example, 14.5) without performing the following feedback calculation (S72). Exit. If the determination in S71 is yes, the process moves to S73.

  In S73, it is determined whether or not the stoichiometric mode flag F_Stoic_mode (m) is 1. If the determination in S73 is YES, the process moves to S74. If the determination is NO, the process moves to S72, and AFcmd (m) = AFcmd_bs is set as described above.

In S74, it is determined whether or not a reduction process completion flag F_CRD_Done (m) described later is 1. As described above, the stoichiometric operation starts when the stoichiometric mode flag F_Stoic_mode (m) is changed from 0 to 1 during the lean operation. However, since the lean operation has been performed so far, a large amount of oxygen is stored in the direct catalyst, and even if the stoichiometric operation is started, the three-way purification function of the direct catalyst cannot be exhibited immediately. For this reason, immediately after the flag F_Stoic_mode (m) is changed from “0” to “1”, the air-fuel ratio is biased slightly richer than stoichiometric (so-called weakly rich) over a predetermined period, so that A reduction process is performed to release the stored oxygen in a short time. This reduction process completion flag F_CRD_Done (m) is a flag indicating that the reduction process immediately after the start of the stoichiometric operation has ended, and is updated by a direct catalyst reduction characteristic determination process shown in FIG. Hereinafter, the operation mode that promotes the reduction of the direct catalyst immediately after the start of the stoichiometric operation is referred to as “weakly rich mode”. An operation mode in which the target air-fuel ratio AFcmd (m) is determined based on the output of the post-catalyst O ₂ sensor during the stoichiometric operation is referred to as “post-catalyst air-fuel ratio feedback mode”.

  When the determination in S74 is NO, the process proceeds to S75, and the target air-fuel ratio AFcmd (m) is determined under the weak rich mode. More specifically, in S75, an estimated value Tcc_hat (m) of the carrier temperature of the direct catalyst and an estimated value Gex_hat (m) of the exhaust volume are acquired, and these two estimated values Tcc_hat (m) and Gex_hat (m) are obtained. Based on this, the predetermined air-fuel ratio AFcmd (m) is determined by searching a predetermined map, and this process is terminated.

  FIG. 17 is an example of a map for determining the target air-fuel ratio AFcmd under the weak rich mode. As shown in FIG. 17, in the weak rich mode, the target air-fuel ratio AFcmd is a value corresponding to the estimated value Tcc_hat of the direct catalyst temperature and the estimated value Gex_hat of the exhaust volume within the weak rich region (about 14.5 to 13.5). Set to More specifically, the target air-fuel ratio AFcmd is set to the rich side in the weakly rich region as the carrier temperature of the direct catalyst increases or the exhaust volume decreases.

Returning to FIG. 16, if the determination in S74 is YES, the process moves to S76, and the target air-fuel ratio AF_cmd (m) is determined under the post-catalyst stoichiometric feedback mode. In S76, the estimated value Tcc_hat (m) of the carrier temperature of the direct catalyst and the estimated value Gex_hat (m) of the exhaust volume are acquired, and are determined in advance based on these two estimated values Tcc_hat (m) and Gex_hat (m). By searching the map, the target value Vcmd (m) for the output Vout (m) of the post-catalyst O ₂ sensor is determined, and the process proceeds to S77.

FIG. 18 is an example of a map for determining the target value Vcmd (m) of the output of the post-catalyst O ₂ sensor. As shown in FIG. 18, the target value Vcmd of the O ₂ sensor, O ₂ inversion judging threshold Vln set to determine the inversion of the output of the sensor (e.g., 0.1) in the region richer than The higher the carrier temperature of the direct catalyst is corrected to the rich side. Further, as the exhaust volume increases (in other words, as the load increases), the amount of NOx discharged from the engine increases and the exhaust passage speed in the LNT increases. As a result, the NOx purification rate in the direct catalyst decreases. . In order to compensate for such a decrease in the NOx purification rate, the target value Vcmd (m) of the O ₂ sensor is corrected to the rich side as the exhaust volume increases, as shown in FIG. 18, and the CO, H on the direct catalyst is corrected. _2. Increase the production of reducing agents such as NH ₃ .

Returning to FIG. 16, in S77, the deviation E_v (m) (see the following formula (7-1)) between the output value Vout (m) of the post-catalyst O ₂ sensor and the target value Vcmd (m) becomes zero. As described above, the target air-fuel ratio AFcmd (m) is determined using a known feedback algorithm, and the process proceeds to S78. As an example of the feedback calculation in S77, the following formulas (7-1) to (7-3) show calculation formulas when the target air-fuel ratio AFcmd (m) is determined using the sliding mode algorithm. In Expression (7-2), “Pole_af” is a switching function setting parameter, and is set to a value larger than −1 and smaller than 0 (for example, −0.85). In the equation (7-3), the two feedback gains “Krch_v” and “Kadp_v” are set to negative values.

In S78, a catalyst three-way characteristic adaptive calculation described later with reference to FIG. 22 is executed, and this process is terminated. As described above, in the post-catalyst air-fuel ratio feedback mode, the target air-fuel ratio AFcmd for the output of the pre-catalyst LAF sensor is determined based on the output of the post-catalyst O ₂ sensor. Therefore, the state of the catalyst immediately below is reflected in the target air-fuel ratio AFcmd. In the catalyst three-way characteristic adaptive calculation, statistical processing is performed on the target air-fuel ratio AFcmd determined in the post-catalyst air-fuel ratio feedback mode using such characteristics, which corresponds to a deterioration factor of the HC oxidation performance of the direct catalyst. The catalyst three-way characteristic adaptive correction value Dtw is calculated. Note that this catalyst ternary characteristic adaptive calculation in S78 can be omitted when the oxidation characteristic correction coefficient Kmod is determined by a method different from TYPE3 in S53 of FIG.

FIG. 19 is a time chart showing a specific example of changes in the output AFact of the pre-catalyst LAF sensor and the output Vout of the post-catalyst O ₂ sensor when the target air-fuel ratio AFcmd is determined by the post-catalyst air-fuel ratio feedback calculation as described above. is there. FIG. 19 shows a case where the stoichiometric mode flag F_Stoic_mode changes from 0 to 1 at time t1.

As described with reference to FIG. 16, immediately after the start of the stoichiometric operation (time t1 in FIG. 19), the target air-fuel ratio AFcmd is set to be slightly rich (see S75 in FIG. 16), and is thereby stored in the catalyst immediately below. The released oxygen is released and used to oxidize the reducing agent supplied by weakening. At time t2, the reduction process completion flag F_CRD_Done switches from 0 to 1 (see FIG. 20 described later) in response to the determination that the output value Vout of the post-catalyst O ₂ sensor is inverted, and a weak rich mode (rich bias). ) Is canceled. Further, after time t2, the target air-fuel ratio AFcmd is determined so that the output Vout of the post-catalyst O ₂ sensor becomes the target value Vcmd determined according to the operating state.

  FIG. 20 is a flowchart illustrating a specific procedure of the direct catalyst reduction characteristic determination process for updating the reduction process completion flag F_CRD_Done. The process shown in FIG. 20 is executed in the ECU at the same control cycle tm (10 to 50 msec) as the post-catalyst air-fuel ratio feedback calculation of FIG. In the direct catalyst reduction characteristic determination process of FIG. 20, the reduction process completion flag F_CRD_Done is updated while estimating the amount of reducing agent supplied to the direct catalyst. Hereinafter, a specific procedure of the direct catalyst reduction characteristic determination process will be described with reference to the time chart of FIG.

  In S81, it is determined whether or not the stoichiometric mode flag F_Stoic_mode (m) is 1. If the determination in S81 is NO, that is, if the lean operation mode is being performed, the process proceeds to S82, and if the determination in S81 is YES, that is, if the stoichiometric operation is being performed, the process proceeds to S86.

  In S82, an estimated value Rd_hat (m) (hereinafter referred to as “instantaneous reducing agent amount estimated value”) of the amount of reducing agent supplied to the direct catalyst during the control period tm and a provisional value of the instantaneous reducing agent amount estimated value. Both Rd_hat_tmp (m) and the instantaneous reducing agent amount estimated value integrated value Crd_hat (m) (hereinafter referred to as “reducing agent supply amount estimated value”) are reset to 0, and the process proceeds to S83. During the lean operation mode, almost no reducing agent is supplied to the direct catalyst. In S83, the catalyst reduction characteristic update completion flag F_CrdAdp_done (m) is set to 0, and the process proceeds to S84. The flag F_crdAdp_done (m) is used to clearly indicate that the update of the parameter related to the reduction characteristic of the direct catalyst has been completed by executing the catalyst reduction characteristic adaptation calculation in S92 described later. In S84, the reduction process completion flag F_CRD_Done (m) is reset to 0, and this process ends.

In S86, an instantaneous reducing agent amount estimated value Rd_hat (m) is calculated based on the output value AFact (m) of the pre-catalyst LAF sensor, and the process proceeds to S87. More specifically, when the output value AFact (m) of the pre-catalyst LAF sensor is lower than the reference value AFcmd_bs, a value obtained by multiplying this surplus by the estimated value Gex_hat (m) of the exhaust volume is used as the instantaneous reducing agent amount. The estimated value is Rd_hat (m). Specifically, the provisional value Rd_hat_tmp (m) is introduced and expressed by the following formulas (8-1) and (8-2).

In S87, it is determined whether or not the output value Vout (m) of the post-catalyst O ₂ sensor is smaller than the inversion determination threshold value Vln. As described with reference to FIGS. 16 to 19, immediately after the start of the stoichiometric operation mode, the air-fuel ratio is set to be slightly rich, and the oxygen stored in the immediately below catalyst is released downstream. Therefore, whether or not the reduction process is completed can be determined by whether or not the output value Vout (m) of the post-catalyst O ₂ sensor exceeds the inversion determination threshold value Vln. When the determination in S87 is YES, the process proceeds to S88, and the instantaneous reducing agent amount estimated value Rd_hat (m) is integrated to calculate the reducing agent supply amount estimated value Crd_hat (m) (the following equation (9) ) And FIG. 21), this processing is terminated.

  If the determination in S87 is NO, a reduction process completion flag F_CRD_Done (m) is set to 1 to clearly indicate that the reduction process immediately after the start of the stoichiometric operation has been completed (S90), and the process proceeds to S91. Thereby, in the post-catalyst air-fuel ratio feedback calculation, the mode is switched from the weak rich mode to the catalytic air-fuel ratio feedback mode (see S74 in FIG. 16).

In S91, it is determined whether or not the catalyst reduction characteristic update completion flag F_CrdAdp_done (m) is 1. If the determination in S91 is NO, a catalyst reduction characteristic adaptive calculation described later with reference to FIG. 24 is executed (S92), and this process is terminated. If the determination in S91 is YES, and if the catalyst reduction characteristic adaptive calculation has been executed, this process is immediately terminated. As described above, immediately after the start of the stoichiometric operation, the air-fuel ratio is set to be slightly rich until the output of the post-catalyst O ₂ sensor is reversed. Accordingly, the estimated value Crd_hat of the amount of reducing agent supplied to the direct catalyst during this time reflects the state of the direct catalyst. In the catalytic reduction characteristic adaptive calculation, by applying statistical processing to the reducing agent supply amount estimated value Crd_hat calculated during the weak rich mode using such characteristics, it corresponds to a deterioration factor of the HC oxidation performance of the direct catalyst. Calculate the catalytic reduction characteristic adaptation coefficient Krd. Note that this catalyst reduction characteristic adaptation calculation in S92 can be omitted when the oxidation characteristic correction coefficient Kmod is determined by a method different from TYPE2 in S53 of FIG.

  FIG. 22 is a flowchart showing a specific procedure of the catalyst three-way characteristic adaptive calculation for calculating the catalyst three-way characteristic adaptive correction value Dtw. The processing shown in FIG. 22 is executed at a cycle tm in the ECU during execution of the post-catalyst air-fuel ratio feedback mode as a subroutine of post-catalyst feedback calculation shown in FIG.

In the catalyst three-way characteristic adaptive calculation, the target air-fuel ratio AFcmd calculated by the post-catalyst feedback calculation (see FIG. 16) during the stoichiometric feedback mode is subjected to statistical processing as described below, which corresponds to a deterioration factor. The catalyst three-way characteristic adaptive correction value Dtw is calculated. The correction value Dtw corresponds to a deviation from the reference value AFcmd_bs of the target air-fuel ratio AFcmd for maintaining the output value Vout of the post-catalyst O ₂ sensor at the target value Vcmd during the stoichiometric feedback mode. The state in which the output value Vout of the post-catalyst O ₂ sensor is maintained at the target value Vcmd larger than the inversion determination threshold value Vln means that a slight amount of reducing agent according to the target value Vcmd slips downstream of the catalyst immediately below. It corresponds to the state. At this time, if the oxidation ability or reduction ability of the direct catalyst decreases, the amount of reducing agent to be supplied to the direct catalyst in order to maintain the output Vout at the target value Vcmd decreases, and consequently the target calculated by the post-catalyst feedback calculation. The air-fuel ratio AFcmd shifts to the lean side.

  However, the target air-fuel ratio shift amount (AFcmd-AFcmd_bs) may differ depending on, for example, the carrier temperature of the direct catalyst. In other words, even if the degree of deterioration of the direct catalyst is the same, the shift amount may be larger when the support temperature of the direct catalyst is high than when it is low. In addition, this shift amount does not necessarily decrease uniformly in all temperature ranges according to the degree of deterioration of the direct catalyst. For this reason, it is preferable to remove the temperature dependence by performing statistical processing as described below, rather than adopting this shift amount as a deterioration factor proportional to the degree of deterioration as it is. In the following calculation, in order to remove the temperature dependence from the shift amount, a plurality of weight functions Wtw_i defined on a one-dimensional straight line based on the carrier temperature and a local adaptive coefficient Dtw_i associated with each weight function are introduced. Then, the catalyst ternary characteristic adaptive correction value Dtw is calculated by statistical processing weighted using these.

  In S151, an estimated value Tcc_hat (m) of the support temperature of the direct catalyst is acquired, and a predetermined map is searched based on the estimated value Tcc_hat (m), thereby obtaining each ternary characteristic weight function value Wtw_i ( m) (i is a positive integer) is calculated, and the process proceeds to S152.

  FIG. 23 is a diagram showing an example of the map for calculating the ternary characteristic weight function value, that is, the shape of the ternary characteristic weight function Wtw_i. In the following description, a case where i = 1, 2, and 3, that is, a case where the number of weight functions is 3, will be described, but the present invention is not limited to this. The number of weight functions can be easily generalized even when the number is 4 or more.

  As shown in FIG. 23, the first to third weight functions are defined with respect to the estimated value Tcc_hat of the carrier temperature of the direct catalyst. Hereinafter, regions in which the respective weight functions are defined (that is, regions where the weight function value is not 0) are defined as first to third regions, respectively. In the example shown in FIG. 23, the first region is about 300 ° C. or lower, the second region is about 150 to 450 ° C., and the third region is about 300 ° C. or higher. Each area is set to overlap each other. In the example shown in FIG. 23, the first region and the second region overlap at about 150 to 300 ° C., and the second region and the third region overlap at about 300 to 450 ° C.

  Further, the height of each weight function Wtw_i is normalized so that the sum of all weight function values becomes 1 at any temperature. This is realized by setting a function that is 1 in a region that does not overlap with another region and monotonously increases or decreases in a region that overlaps with another region. In the example shown in FIG. 23, the first weighting function Wtw_1 is defined as a function that is 1 below about 150 ° C. and monotonously decreases from 1 to 0 between about 150 ° C. and about 300 ° C. The second weighting function Wtw_2 is defined as a function that monotonically increases from 0 to 1 between about 150 ° C. and about 300 ° C. and monotonically decreases from 1 to 0 between about 300 ° C. and about 450 ° C. The third weighting function Wtw_3 is defined as a function that monotonically increases from 0 to 1 between about 300 ° C. and about 450 ° C. and is 1 at about 300 ° C. or higher.

Returning to FIG. 22, in S152, the sum of products of the weight function value Wtw_i (m) calculated in S151 and the local adaptive coefficient Dtw_i (m) is calculated as shown in the following equation (10), and this is used as a catalyst. The three-way characteristic adaptive correction value Dtw (m) is used. These local adaptive coefficients Dtw_i are calculated, for example, by integration so that an initial value is 0 and an adaptive error signal E_adp ′ (m) described later becomes 0.

In S153, the adaptive target air-fuel ratio AFcmd_adp (m) is calculated by adding the reference value AFcmd_bs and the catalyst three-way characteristic adaptive correction value Dtw (m) (see the following equation (11)).

In S154, an adaptive error signal Eadp '(m) is calculated by subtracting the adaptive target air-fuel ratio AFcmd_adp (m) from the target air-fuel ratio AF_cmd (m) (see the following equation (12-1)), and this adaptation is further performed. The local adaptive error signal E_adp′_i (m) is calculated by distributing the error signal Eadp ′ (m) to each region. More specifically, the adaptive error signal Eadp ′ (m) multiplied by each weight function value Wtw_i (m) is used as the local adaptive error signal E_adp′_i (m) (see the following equation (12-2)). .

In S155, negative adaptation is applied to the local adaptive error signal E_adp'_i (m), for example, as shown in the following equation (13) so that the local adaptive error signal E_adp'_i (m) calculated for each region becomes zero. The local catalyst ternary characteristic adaptive correction value Dtw_i (m) is calculated by integrating the gain Kadp_t.

  In the above-described catalyst three-way characteristic adaptive calculation, the first to third regions overlapping each other are defined on a one-dimensional straight line based on the carrier temperature of the direct catalyst, and the weight function Wtw_i is defined for each region. The invention is not limited to this. For example, a plurality of overlapping regions may be defined on a two-dimensional plane based on the carrier temperature and exhaust volume of the direct catalyst, and a weight function Wtw_ij may be defined for each region on the two-dimensional plane.

  FIG. 24 is a flowchart showing a specific procedure of the catalyst reduction characteristic adaptation calculation for calculating the catalyst reduction characteristic adaptation coefficient Krd. The process shown in FIG. 24 is executed at the control cycle tm in the ECU every time the mode is switched from the weak rich mode to the post-catalyst air-fuel ratio feedback mode as a subroutine of the direct catalyst reduction characteristic determination process shown in FIG.

In the catalytic reduction characteristic adaptive coefficient calculation, the deterioration factor is calculated based on the estimated value Crd_hat (m) of the reducing agent supplied to the catalyst immediately after the start of the stoichiometric operation until the output of the post-catalyst O ₂ sensor is reversed. The catalyst reduction characteristic adaptive correction coefficient Krd (m) corresponding to is calculated. For example, it is considered that when the oxidation ability or reduction ability of the direct catalyst decreases, the storage function of the direct catalyst also decreases. Therefore, it is considered that the amount of the reducing agent that needs to be supplied to the catalyst immediately before the output of the O ₂ sensor after the catalyst is reversed decreases as the oxidation capacity and the reduction capacity decrease.

  However, as will be described later with reference to FIG. 25, this supply amount of the reducing agent varies depending on the carrier temperature of the direct catalyst, for example. Further, the reducing agent supply amount does not necessarily decrease uniformly in all temperature ranges according to the degree of deterioration of the direct catalyst. For this reason, it is preferable to remove the temperature dependency by performing statistical processing as described below, rather than adopting the above-described reducing agent supply amount estimated value Crd_hat as a deterioration factor proportional to the degree of deterioration. Therefore, in the following calculation, similarly to the catalyst ternary characteristic adaptive calculation described with reference to FIG. 22, the temperature is defined on a one-dimensional straight line based on the carrier temperature in order to remove the temperature dependence from the estimated value Crd_hat. A plurality of weighting functions Wrd_i and a local adaptive coefficient Krd_i associated with each weighting function are introduced, and the catalyst reduction characteristic adaptive correction value Krd is calculated by statistical processing weighted using these.

  In S101, the estimated value Tcc_hat (m) of the carrier temperature of the direct catalyst is calculated, and a predetermined map is searched based on the estimated value Tcc_hat (m), thereby reducing the reducing agent supply amount estimated value Crd_hat (m ) Is calculated as a reference reducing agent supply amount Crd_bs (m).

FIG. 25 is an example of a map for determining the reference reducing agent supply amount Crd_bs (m). As shown in FIG. 25, the reference reducing agent supply amount Crd_bs (m) is set to increase as the carrier temperature of the direct catalyst increases. This is because the higher the carrier temperature, the higher the oxidizing ability and reducing ability of the direct catalyst, and as a result, the amount of reducing agent required until the output of the O ₂ sensor after the catalyst is reversed increases.

  Referring back to FIG. 24, in S102, each reducing agent supply amount weight function value Wrd_i (m) is calculated by searching a predetermined map based on the estimated value Tcc_hat (m) of the carrier temperature of the direct catalyst. Then, the process proceeds to S103.

  FIG. 26 is a diagram illustrating an example of a map for calculating a reducing agent supply amount weighting function value, that is, a shape of the reducing agent supply amount weighting function Wrd_i. Note that the shape of these reducing agent supply amount weight functions Wrd_i is the same as the weight function Wtw_i described with reference to FIG.

Returning to FIG. 24, in S103, as shown in the following equation (14), the sum of products of the weight function value Wrd_i (m) calculated in S102 and the local adaptive coefficient Krd_i (m) is calculated. The catalytic reduction characteristic adaptation coefficient is Krd (m). The initial value of the local adaptive coefficient Krd_i is 1.

In S104, the reducing agent supply amount adaptation value Crd_adp (m) is calculated by multiplying the reference reducing agent supply amount Crd_bs (m) calculated in S101 by the catalyst reduction characteristic adaptation coefficient Krd (m) (the following formula ( 15)).

In S105, the adaptive error signal Eadp (m) is calculated by subtracting the adaptive value Crd_adp (m) from the estimated value Crd_hat (m) calculated in S88 of FIG. 20 (see the following equation (16-1)). Further, the local adaptive error signal E_adp_i (m) is calculated by distributing the adaptive error signal Eadp (m) to each region (see the following equation (16-2)).

In S106, the local adaptive error signal E_adp_i (m) is multiplied by a negative adaptive gain Kadp_c as shown in the following equation (17) so that the local adaptive error signal E_adp_i (m) calculated for each region becomes zero. The local adaptive coefficient Krd_i (m) is calculated by integrating the obtained values.

  In S107, it is determined whether or not the reduction characteristics have been updated. More specifically, when the local adaptive error signal E_adp_i becomes smaller than a predetermined threshold value, or when a predetermined time or more has elapsed after starting the processing of FIG. It is determined that the Krd_i update process has been completed. If the determination in S107 is YES, the flag F_CrdAdp_done (m) is set to 1 (S108), and this process ends. If the determination in S107 is NO, this process ends while the flag F_CrdAdp_done (m) is maintained at 0 to continue the process of FIG.

  Note that in the above-described catalytic reduction characteristic adaptive calculation, the first to third regions that overlap each other are defined on a one-dimensional straight line based on the carrier temperature of the catalyst directly below, and the weight function Wrd_i is defined for each region. Is not limited to this. For example, a plurality of overlapping areas may be defined on a two-dimensional plane based on the carrier temperature and exhaust volume of the direct catalyst, and a weight function Wrd_ij may be defined for each area on the two-dimensional plane.

  FIG. 27 is a flowchart showing a specific procedure of the sequential identification calculation of the thermal model of the direct catalyst. In this sequential identification calculation, a plurality of model parameters included in the thermal model of the direct catalyst (see formula (18) described later) are identified. The processing shown in FIG. 27 is executed at a predetermined control cycle tn (200 to 500 msec) in the ECU. In the following description, the value updated or sampled at the period tn is denoted by “n” in parentheses. The process shown in FIG. 27 can be omitted when the oxidation characteristic correction coefficient Kmod is determined by a method different from TYPE 1 in S53 of FIG.

The following equation (18) is a thermal model equation of the direct catalyst. In other words, the following expression (18) is an arithmetic expression for calculating the estimated value Tds_hat (n) of the temperature of the exhaust gas downstream of the direct catalyst based on the known value. The carrier temperature of the direct catalyst provided in the exhaust pipe (and the temperature of the exhaust on the downstream side thereof) changes not only by heat exchange with the exhaust flowing in the exhaust pipe but also by heat exchange with the outside air outside the exhaust pipe. . Further, if HC is contained in the exhaust gas flowing into the direct catalyst, the direct catalyst generates heat due to oxidation of HC.

  In the following equation (18), the first term on the right side is the estimated temperature value Tds_hat (n-1) before the control cycle tn, and the second to fourth terms on the right side are the temperatures from before the control cycle tn to the present. It corresponds to the increase. More specifically, the second term on the right side is a heat release term, that is, a term indicating the contribution due to heat transfer between the direct catalyst and the outside air. The estimated value Tds_hat (n-1) of the carrier temperature of the direct catalyst and the outside air It is proportional to the difference from the temperature Ta (n). The proportional coefficient a in the second term may be a fixed value determined by system identification performed in advance or a value determined by scheduling according to the output Tds of the downstream temperature sensor.

  The third term on the right-hand side is the heat transfer term, that is, the term indicating the contribution due to the heat transfer between the direct catalyst and the exhaust. The estimated value Gex_hat (n) of the exhaust volume and the output Tup (n) of the upstream temperature sensor Proportional to product. The value of the proportional coefficient b (n−1) of the heat transfer term is updated every period tn by the process in S123 of FIG.

  The fourth term on the right side is a heat generation term, that is, a term indicating the contribution due to the combustion of HC contained in the exhaust gas flowing into the direct catalyst at the direct catalyst. As described above, when the after injection amount is increased, the amount of HC contained in the exhaust gas is also increased. Therefore, this heat generation term is proportional to the fuel amount Gfuel_aft_tn (n) supplied by after-injection during the period tn. Further, the value of the proportional coefficient c (n−1) of the heat generation term is updated every cycle tn by the process in S124 of FIG. The exothermic term increases as the after injection amount increases, and increases as the HC oxidation performance of the direct catalyst increases. Therefore, this coefficient c is a parameter indicating the degree to which the fuel supplied by after injection contributes to the temperature increase of the direct catalyst, and is a parameter indicating the HC oxidation performance of the direct catalyst. Hereinafter, this coefficient c is referred to as a heat generation coefficient. Hereinafter, a specific procedure for updating the values of the coefficient b and the heat generation coefficient c will be described.

  First, in S121, it is determined whether or not the upstream temperature sensor and the downstream temperature sensor necessary for identifying the coefficients b and c are normal. If the determination in S121 is YES, the process proceeds to S122, and if it is NO, the process of FIG. 27 is immediately terminated.

  In S122, it is determined whether or not the after injection has been executed during the period tn, that is, whether or not the after injection amount Gfuel_aft_tn (n) is zero. If the determination in S122 is YES, the process moves to S123, the value of the coefficient b (n) is updated according to the following formulas (19) and (20), and this process ends. When the determination in S122 is NO, the value of the heat generation coefficient c (n) is updated according to the following formulas (21) and (22) (see S124), and the catalytic oxidation characteristic adaptive correction value Kox (n) is set. The calculated catalytic oxidation characteristic adaptive calculation is executed (see S125 and FIG. 28 described later), and this process is terminated. That is, the value of the coefficient b (n) is updated when the after injection is not performed, and the value of the heat generation coefficient c (n) is updated only when the after injection is performed.

In S123, the value of the coefficient b (n) is updated by applying a predetermined parameter identification algorithm to Equation (18) while maintaining the value of the heat generation coefficient c (n) at the previous value. Hereinafter, an example of the specific procedure will be described. First, the downstream temperature sensor output Tds (n), the atmospheric temperature sensor output Ta (n), the coefficient b (n-1), the exhaust volume estimated value Gex_hat (n), and the upstream temperature sensor output Tup (n ), The virtual output W (n) defined by the following equations (19-1) to (19-3) and the estimated value W_hat (n) are calculated.

Here, when the determination in S122 is YES, since the after injection amount Gfuel_aft_tn (n) = 0, the fourth term on the right side of Expression (18) becomes 0 when S123 is executed. Therefore, the estimated value W_hat (n) defined by the above equation (19-2) corresponds to the estimated value of the virtual output W (n) as in the following equation (20). Therefore, the value of the coefficient b is set so that the difference between the virtual output W (n) defined by the equation (19-1) and the estimated value W_hat (n) defined by the equation (19-2) is minimized. Updating is equivalent to updating the value of the coefficient b (n) so that the difference between the output Tds (n) of the downstream temperature sensor and the estimated value Tds_hat (n) is minimized.

Also, the coefficient b (n) for realizing this is obtained by adding W (n) and W_hat to the variable gain KP1 (n) sequentially updated by the equation (21-2) as shown in the following equation (21-1), for example. It is calculated by integrating the value multiplied by the difference from (n-1). In equation (21-2), the coefficient P1 is a predetermined identification gain. The following formulas (21-1) and (21-2) are algorithms called a fixed gain algorithm among parameter identification algorithms generalized as a so-called sequential least squares algorithm.

In S123, the value of the coefficient c is updated by applying a predetermined parameter identification algorithm to Equation (18) while maintaining the value of the coefficient b at the previous value. Hereinafter, an example of the specific procedure will be described. First, based on the output Tds of the downstream temperature sensor, the output Ta of the atmospheric temperature sensor, the coefficients b and c, the estimated value Gex_hat of the exhaust volume, and the output Tup of the upstream temperature sensor, the following equations (22-1) to (22) -2) Calculate the virtual output W (n) defined in (2) and its estimated value W_hat (n).

Here, when the formula (22-2) is transformed by the formula (18), the following formula (23) is derived. Therefore, the estimated value R_hat (n) defined by the formula (22-2) is a virtual output. It corresponds to the estimated value of R (n). Therefore, the value of the coefficient c is set so that the difference between the virtual output R (n) defined by the equation (22-1) and the estimated value R_hat (n) defined by the equation (22-2) is minimized. Updating is equivalent to updating the value of the coefficient c so that the difference between the output Tds (n) of the downstream temperature sensor and the estimated value Tds_hat (n) is minimized.

The coefficient c (n) for realizing this is obtained by changing the variable gain KP2 (n) sequentially updated by the equation (24-2), for example, as shown in the following equation (24-1): R (n) and R_hat It is calculated by integrating the value obtained by multiplying the difference from (n-1). In Expression (24-2), the coefficient P2 is a predetermined identification gain.

  FIG. 28 is a flowchart showing a specific procedure of the catalyst oxidation characteristic adaptation calculation for calculating the catalyst oxidation characteristic adaptation coefficient Kox. The process shown in FIG. 28 is executed in the ECU at the control cycle tn only when after-injection is being performed as a subroutine for the sequential identification calculation of the thermal model in FIG.

  As described above, the exothermic coefficient c (n) updated according to the equations (18) to (24-2) has a characteristic that it increases as the HC oxidation performance of the direct catalyst increases. Therefore, the deviation of the heat generation coefficient c (n) from the predetermined reference value C_bs can be adopted as a deterioration factor of the direct catalyst. However, the exothermic coefficient c (n) varies depending on the support temperature of the direct catalyst. In addition, the heat generation coefficient c (n) does not necessarily decrease uniformly in all temperature ranges according to the degree of deterioration of the direct catalyst. For this reason, in this catalytic oxidation characteristic adaptive calculation, as in the catalyst ternary characteristic adaptive calculation described with reference to FIG. 22, the carrier temperature of the immediately below catalyst is used as a basis in order to remove the temperature dependence from the heat generation coefficient c. By introducing multiple weighting functions Wox_i defined on a one-dimensional straight line and local adaptive coefficient Kox_i associated with each weighting function, and applying weighted statistical processing using these, it corresponds to the degradation factor of the direct catalyst A catalytic oxidation characteristic adaptive correction value Kox is calculated.

  In S141, an estimated value Tcc_hat (m) of the carrier temperature of the direct catalyst is calculated, and a predetermined map is searched based on the estimated value Tcc_hat (m), whereby each oxidation characteristic weight function value Wox_i (n ) (I is a positive integer) is calculated, and the process proceeds to S142.

  FIG. 29 is a diagram illustrating an example of the shape of the oxidation characteristic weight function Wox_i, that is, the map for calculating the oxidation characteristic weight function value. Note that the shape of these weighting functions Wox_i is the same as the weighting function Wtw_i described with reference to FIG.

Returning to FIG. 28, in S142, as shown in the following equation (25), the sum of the products of the weight function value Wox_i (n) calculated in S141 and the local adaptive coefficient Kox_i is calculated, and this is used as the catalytic oxidation characteristic. The adaptive correction value Kox (n) is used. The initial value of the local adaptive coefficient Kox_i is 1.

In S143, the heat generation coefficient adaptive value C_adp (n) is calculated by multiplying the predetermined reference heat generation coefficient C_bs by the catalytic oxidation characteristic adaptive correction value Kox (n) (see the following equation (26)). The reference heat generation coefficient C_bs corresponds to the heat generation coefficient of the catalyst directly under the reference product without any predetermined deterioration, and is determined by a system identification performed in advance. In the following description, the reference heat generation coefficient C_bs is described as a fixed value that does not depend on temperature, but is not limited thereto. The reference heat generation coefficient C_bs may be determined by searching a predetermined map based on the carrier temperature of the direct catalyst.

In S144, an adaptive error signal E_adp '' (n) is calculated by subtracting the adaptive value C_adp (n) from the heat generation coefficient c (n) (see the following formula (27-1)), and this adaptive error is further calculated. The local adaptive error signal E_adp ″ _i (n) is calculated by distributing the signal E_adp ′ (m) to each region (see the following equation (27-2)). In S145, the local adaptive error signal E_adp '' _ i (n) as shown in the following equation (27-3) is set so that the local adaptive error signal E_adp '' _ i (n) calculated for each region becomes 0, for example. The local adaptive correction value Kox_i (n) is calculated by integrating the product of n) multiplied by the negative adaptive gain Kadp_o.

  Note that in the above-described catalytic oxidation characteristic adaptive calculation, the first to third regions that overlap each other are defined on a one-dimensional straight line based on the carrier temperature of the direct catalyst, and the weight function Wox_i is defined for each region. Is not limited to this. For example, a plurality of regions that overlap each other on a two-dimensional plane based on the carrier temperature of the direct catalyst and the exhaust volume may be defined, and the weight function Wox_ij may be defined in each region on the two-dimensional plane.

  In the above embodiment, the exhaust purification system (FIG. 1) using the DeNOx function of the LNT has been described as an example, but the present invention is not limited to this. For example, the present invention is also effective for an exhaust purification system 2A as shown in FIG. 30 that uses the DeNOx function of an NH3 selective reduction catalyst. 30, different reference numerals are given only to configurations different from those of the exhaust purification system 2 of FIG. 1.

  In the exhaust passage 11 of the exhaust purification system 2A, an oxidation catalyst (hereinafter referred to as “DOC”) 61, a DPF 62, a reducing agent injection device 63, and a selective reduction catalyst (hereinafter referred to as “SCR catalyst”) are sequentially arranged from the upstream side of the exhaust. And 64).

  The oxidation catalyst 61 has at least two functions of an oxidation function and a three-way purification function. Further, the oxidation catalyst 61 may be replaced with an LNT having three functions of an oxidation function, a DeNOx function, and a three-way purification function.

The SCR catalyst 64 has a DeNOx function that selectively reduces NOx in the exhaust gas in an atmosphere in which a reducing agent such as NH ₃ exists. Specifically, when urea water is injected by the reducing agent injection device 63, the urea water is thermally decomposed or hydrolyzed by the heat of the exhaust to generate NH ₃ . The generated NH ₃ is supplied to the SCR catalyst 64, and NO _{3 in} the exhaust gas is selectively reduced by the NH ₃ . As shown in FIG. 30, the oxidation catalyst 61 and the DPF 62 are provided in the immediately lower section, and the SCR 64 is provided in the underfloor section.

  The urea water injection device 63 includes a urea water tank 631 and a urea water injector 633. The urea water tank 631 stores urea water, and is connected to the urea water injector 633 via the urea water pump 635. The urea water injector 633 is connected to the ECU 3A via a drive device (not shown). The ECU 3A determines the urea water injection amount and the urea water injection timing from the urea water injector 633 by urea water injection control (not shown), and the drive device causes the urea water injector 633 to realize the determined urea water injection mode. Drive.

  In the exhaust purification system 2A as described above, the NOx in the exhaust can be purified using the DeNOx function of the SCR catalyst 64 during the lean operation, and can be purified using the three-way purification function of the DOC 61 during the stoichiometric operation. That is, in the system of FIG. 1, the LNO 41 is responsible for the DeNOx function during the lean operation, and in the system of FIG. 30, the SCR catalyst 64 is responsible for the DeNOx function during the lean operation. Therefore, almost all of the processing described with reference to FIGS. 2 to 29 can be executed by the exhaust purification system 2A of FIG. However, in this case, since the device responsible for the DeNOx function is different as described above, the process of S23 of the stoichiometric operation condition determination process of FIG. 6 is replaced as follows. In S23 of FIG. 6, it is determined whether or not the DeNOx function can be sufficiently exhibited. In the system of FIG. 30, the determination of S23 of FIG. 6 is more specifically performed when the estimated value of the carrier temperature of the SCR catalyst is equal to or higher than a predetermined purifiable temperature set to about 180 to 200 ° C., for example. Determines that the SCR catalyst 64 is in a state where the DeNOx function can be sufficiently exerted. When the SCR catalyst 64 is lower than the purifiable temperature, hydrolysis of urea water injected from the urea water injector 633 is difficult to proceed.

1. Engine (internal combustion engine)
DESCRIPTION OF SYMBOLS 11 ... Exhaust passage 13 ... Fuel injection valve 2, 2A ... Exhaust gas purification system 41 ... LNT (HC oxidation catalyst)
3, 3A ... ECU (fuel injection control means, temperature acquisition means, oxidation capacity estimation means, NOx excess state determination means)
53 ... Pre-catalyst temperature sensor (temperature acquisition means)
54 ... Post-catalyst temperature sensor (temperature acquisition means)
61 ... DOC (HC oxidation catalyst)
64 ... SCR catalyst (DeNOx catalyst)

Claims (8)

An HC oxidation catalyst provided in the exhaust passage of the internal combustion engine and having a function of oxidizing HC in the exhaust;
A fuel injection valve capable of performing after-injection, which is fuel injection within a region in which the amount of PM combustion in the cylinder increases as the interval between the main injection and the main injection increases after the main injection;
An exhaust purification system for an internal combustion engine, comprising: a fuel injection control means for controlling a fuel injection amount and fuel injection timing from the fuel injection valve,
Temperature acquisition means for acquiring the temperature of the HC oxidation catalyst;
An oxidation capacity estimation means for estimating the HC oxidation capacity of the HC oxidation catalyst based on the temperature of the HC oxidation catalyst,
The exhaust gas purification system for an internal combustion engine, wherein the fuel injection control means widens the interval between the main injection timing and the after injection timing as the HC oxidation capability increases. The exhaust passage is further provided with a DeNOx catalyst having a DeNOx function for purifying NOx in the exhaust in the presence of a reducing agent added in the exhaust passage,
The HC oxidation catalyst has a three-way purification function that purifies HC, CO, and NOx in exhaust under exhaust at a stoichiometric air-fuel ratio,
Whether the ratio of the NOx amount flowing into the NOx purification catalyst with respect to the NOx amount that can be purified by the DeNOx function is increased according to the state of the engine and / or the state in the exhaust passage. NOx excess state determination means for determining whether or not
The fuel injection control means controls the combustion air-fuel ratio to be leaner than stoichiometric when the NOx is not excessive, and controls the combustion air-fuel ratio to stoichiometric when the NOx is excessive. 2. An exhaust gas purification system for an internal combustion engine according to 1. The HC oxidation catalyst purifies NOx in the exhaust gas in the presence of a three-way purification function for purifying HC, CO and NOx in the exhaust gas under the stoichiometric air-fuel ratio exhaust and the reducing agent added in the exhaust passage. A DeNOx function,
Whether the ratio of the NOx amount flowing into the NOx purification catalyst with respect to the NOx amount that can be purified by the DeNOx function is increased according to the state of the engine and / or the state in the exhaust passage. NOx excess state determination means for determining whether or not
The fuel injection control means controls the combustion air-fuel ratio to be leaner than stoichiometric when the NOx is not excessive, and controls the combustion air-fuel ratio to stoichiometric when the NOx is excessive. 2. An exhaust gas purification system for an internal combustion engine according to 1. HC oxidation catalyst provided in an exhaust passage of an internal combustion engine and having an HC oxidation function for oxidizing HC in exhaust and a three-way purification function for purifying HC, CO and NOx in exhaust under exhaust at a stoichiometric air-fuel ratio When,
A fuel injection valve capable of performing after-injection, which is fuel injection within a region in which the amount of PM combustion in the cylinder increases as the interval between the main injection and the main injection increases after the main injection;
An exhaust purification system for an internal combustion engine, comprising: a fuel injection control means for controlling a fuel injection amount and fuel injection timing from the fuel injection valve,
Temperature acquisition means for acquiring the temperature of the HC oxidation catalyst;
Oxidation capacity estimation means for estimating the HC oxidation capacity of the HC oxidation catalyst based on the temperature of the HC oxidation catalyst;
Whether the ratio of the NOx amount flowing into the NOx purification catalyst with respect to the NOx amount that can be purified by the DeNOx function is increased according to the state of the engine and / or the state in the exhaust passage. NOx excess state determination means for determining whether or not
The internal combustion engine characterized in that the fuel injection control means controls the combustion air-fuel ratio to stoichiometric while increasing the after-injection amount as the HC oxidation capacity increases when it is determined that the NOx excess state is present. Exhaust purification system.   5. The HC oxidation capability is quantified by a basic factor depending on a temperature and an exhaust volume of the HC oxidation catalyst and a deterioration factor depending on a deterioration degree of the HC oxidation catalyst. An exhaust purification system for an internal combustion engine according to any one of the above.   The oxidation capacity estimating means estimates the HC oxidation capacity based on a heat generation coefficient of the HC oxidation catalyst indicating a degree of contribution of fuel supplied by after-injection to a temperature increase of the HC oxidation catalyst. An exhaust purification system for an internal combustion engine according to any one of claims 1 to 5.   The oxidation capacity estimation means is based on the amount of reducing agent necessary for desorbing oxygen stored in the HC oxidation catalyst or the amount of reducing agent necessary for reducing NOx stored in the HC oxidation catalyst. 6. The exhaust gas purification system for an internal combustion engine according to claim 1, wherein the HC oxidation capacity is estimated. An O ₂ sensor that detects an oxygen concentration in the exhaust gas downstream of the HC oxidation catalyst;
2. The oxidation capacity estimating means estimates the HC oxidation capacity based on an air-fuel ratio target value set so that an output value of the O ₂ sensor is maintained at a predetermined target value. 6. An exhaust gas purification system for an internal combustion engine according to any one of items 1 to 5.

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