JP2016176404A - Exhaust emission control system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To effectively prevent deterioration of fuel consumption performance while improving efficiency of NOx purge.SOLUTION: An exhaust emission control system includes: an NOx occlusion and reduction type catalyst 32 for occluding NOx in exhaust gas in an exhaust lean state and reducing and eliminating occluded NOx in an exhaust rich state; first NOx purge control sections 130, 140 for performing NOx purge for making the exhaust gas rich state through at least one of post injection and exhaust pipe injection; a second NOx purge control section 160 for performing engine stop time NOx purge for making the exhaust gas rich state through at least one of the post injection and the exhaust pipe injection when an engine stop request is made; and a prohibition section 162 for prohibiting the performance of the engine stop time NOx purge, when the engine stop request is made and if lapse time from the completion of NOx purge control that has been performed before the engine stop request is made does not reach predetermined threshold time.SELECTED DRAWING: Figure 7

Description

  The present invention relates to an exhaust purification system.

  Conventionally, a NOx occlusion reduction type catalyst is known as a catalyst for reducing and purifying nitrogen compounds (NOx) in exhaust gas discharged from an internal combustion engine. The NOx occlusion reduction catalyst occludes NOx contained in the exhaust when the exhaust is in a lean atmosphere, and harmless NOx occluded by hydrocarbons contained in the exhaust when the exhaust is in a rich atmosphere. And release. For this reason, when the NOx occlusion amount of the catalyst reaches a predetermined amount, so-called NOx purging that makes the exhaust gas rich by exhaust pipe injection or post injection needs to be performed periodically in order to recover the NOx occlusion capacity ( For example, see Patent Documents 1 and 2).

  As a technique for improving the efficiency of such NOx purge, for example, Patent Document 3 discloses that rich injection is performed during engine stop processing in which the rotational speed decreases due to stop of fuel injection of the engine, and the rich exhaust gas is effective in the catalyst. A technique for retaining the target is disclosed.

JP 2008-202425 A JP 2007-16713 A JP 2010-127146 A

  By the way, in the above-described prior art, for example, if the engine stop process is performed in a state where the time has not elapsed since the end of the previous NOx purge, the NOx occlusion amount of the catalyst is reduced by the previous NOx purge. Therefore, there is a problem that the rich injection is performed and the fuel consumption is deteriorated.

  Further, when rich injection is performed by engine stop processing in a state where the catalyst temperature is lower than the activation temperature, there is a problem that NOx reduction efficiency cannot be sufficiently obtained and fuel consumption is deteriorated due to wasteful fuel consumption.

  It is an object of the disclosed system to effectively prevent deterioration of fuel consumption performance while improving the efficiency of NOx purge.

  The disclosed system is provided in an exhaust passage of an engine, and stores NOx in exhaust gas in an exhaust lean state and reduces and purifies NOx stored in an exhaust rich state, a post-injection and A first NOx purge control means for performing NOx purge for reducing and purifying NOx stored in the NOx occlusion reduction catalyst by at least one of exhaust pipe injection to make the exhaust rich, and when the engine stop request is satisfied A second NOx purge control means for performing NOx purge when the engine is stopped to bring the exhaust into a rich state by at least one of post injection and exhaust pipe injection; and when the engine stop request is satisfied, Elapsed time from the end of the NOx purge control by the first NOx purge control means performed before If it does not reach the predetermined threshold time, and a prohibiting means for prohibiting the implementation of the engine stop NOx purge.

  According to the disclosed system, it is possible to effectively prevent the deterioration of fuel consumption performance while improving the efficiency of NOx purge.

1 is an overall configuration diagram showing an exhaust purification system according to an embodiment. It is a functional block diagram which shows the NOx purge control part which concerns on this embodiment. It is a timing chart figure explaining NOx purge control concerning this embodiment. It is a block diagram which shows the starting process of NOx purge control which concerns on this embodiment. It is a block diagram which shows the setting process of the MAF target value at the time of NOx purge lean control which concerns on this embodiment. It is a block diagram which shows the setting process of the target injection quantity at the time of NOx purge rich control which concerns on this embodiment. It is a block diagram explaining execution processing of NOx purge at the time of engine stop concerning this embodiment. It is a timing chart figure explaining NOx purge at the time of engine stop concerning this embodiment. It is a flowchart explaining the execution process of the NOx purge at the time of the engine stop which concerns on this embodiment, and the prohibition process of the said NOx purge. It is a block diagram which shows the process of the injection amount learning correction | amendment of the injector which concerns on this embodiment. It is a flowchart explaining the calculation process of the learning correction coefficient which concerns on this embodiment. It is a block diagram which shows the setting process of the MAF correction coefficient which concerns on this embodiment.

  Hereinafter, an exhaust purification system according to an embodiment of the present invention will be described with reference to the accompanying drawings.

  As shown in FIG. 1, each cylinder of a diesel engine (hereinafter simply referred to as an engine) 10 is provided with an injector 11 that directly injects high-pressure fuel stored in a common rail (not shown) into each cylinder. The fuel injection amount and fuel injection timing of each injector 11 are controlled in accordance with an instruction signal input from an electronic control unit (hereinafter referred to as ECU) 50.

  An intake passage 12 for introducing fresh air is connected to the intake manifold 10A of the engine 10, and an exhaust passage 13 for leading the exhaust to the outside is connected to the exhaust manifold 10B. In the intake passage 12, an air cleaner 14, an intake air amount sensor (hereinafter referred to as MAF sensor) 40, a compressor 20A of the variable displacement supercharger 20, an intercooler 15, an intake throttle valve 16 and the like are provided in order from the intake upstream side. ing. The exhaust passage 13 is provided with a turbine 20B of the variable displacement supercharger 20, an exhaust aftertreatment device 30 and the like in order from the exhaust upstream side. In FIG. 1, reference numeral 41 denotes an engine speed sensor, reference numeral 42 denotes an accelerator opening sensor, and reference numeral 46 denotes a boost pressure sensor.

  The EGR device 21 includes an EGR passage 22 that connects the exhaust manifold 10B and the intake manifold 10A, an EGR cooler 23 that cools EGR gas, and an EGR valve 24 that adjusts the EGR amount.

  The exhaust aftertreatment device 30 is configured by arranging an oxidation catalyst 31, a NOx occlusion reduction type catalyst 32, and a particulate filter (hereinafter simply referred to as a filter) 33 in order from the exhaust upstream side in a case 30A. The exhaust passage 13 upstream of the oxidation catalyst 31 is provided with an exhaust pipe injection device 34 that injects unburned fuel (mainly HC) into the exhaust passage 13 in accordance with an instruction signal input from the ECU 50. It has been.

  The oxidation catalyst 31 is formed, for example, by carrying an oxidation catalyst component on the surface of a ceramic carrier such as a honeycomb structure. When the unburned fuel is supplied by the post-injection of the exhaust pipe injector 34 or the injector 11, the oxidation catalyst 31 oxidizes this and raises the exhaust temperature.

  The NOx storage reduction catalyst 32 is formed, for example, by supporting an alkali metal or the like on the surface of a ceramic carrier such as a honeycomb structure. The NOx occlusion reduction type catalyst 32 occludes NOx in the exhaust when the exhaust air-fuel ratio is in a lean state, and occludes with a reducing agent (HC or the like) contained in the exhaust when the exhaust air-fuel ratio is in a rich state. NOx is reduced and purified.

  The filter 33 is formed, for example, by arranging a large number of cells partitioned by porous partition walls along the flow direction of the exhaust gas and alternately plugging the upstream side and the downstream side of these cells. . The filter 33 collects PM in the exhaust gas in the pores and surfaces of the partition walls, and when the estimated amount of PM deposition reaches a predetermined amount, so-called filter forced regeneration is performed in which the PM is burned and removed. Filter forced regeneration is performed by supplying unburned fuel to the upstream side oxidation catalyst 31 by exhaust pipe injection or post injection, and raising the exhaust temperature flowing into the filter 33 to the PM combustion temperature.

  The first exhaust temperature sensor 43 is provided on the upstream side of the oxidation catalyst 31 and detects the exhaust temperature flowing into the oxidation catalyst 31. The second exhaust temperature sensor 44 is provided between the NOx storage reduction catalyst 32 and the filter 33 and detects the exhaust temperature flowing into the filter 33. The NOx / lambda sensor 45 is provided on the downstream side of the filter 33, and detects the NOx value and lambda value (hereinafter also referred to as excess air ratio) of the exhaust gas that has passed through the NOx storage reduction catalyst 32.

  The ECU 50 performs various controls of the engine 10 and the like, and includes a known CPU, ROM, RAM, input port, output port, and the like. In order to perform these various controls, sensor values of the sensors 40 to 46 are input to the ECU 50. Further, the ECU 50 includes the NOx purge control unit 100, the MAF follow-up control unit 200, the injection amount learning correction unit 300, and the MAF correction coefficient calculation unit 400 as some functional elements. Each of these functional elements will be described as being included in the ECU 50 which is an integral hardware, but any one of these may be provided in separate hardware.

[NOx purge control]
The NOx purge control unit 100 restores the NOx occlusion ability of the NOx occlusion reduction catalyst 32 by making the exhaust atmosphere rich and detoxifying and releasing NOx occluded in the NOx occlusion reduction catalyst 32 by reduction purification. The NOx purge control is executed.

  In the present embodiment, the NOx purge control unit 100 includes a NOx purge start processing unit 110, a NOx purge lean control unit 130, a NOx purge rich control unit 140, and an NOx purge control unit 160 when the engine is stopped, as shown in FIG. As part of the functional elements. In the present embodiment, the NOx purge lean control unit 130 and the NOx purge rich control unit 140 constitute the first NOx purge control means of the present invention, and the NOx purge control unit 160 at the time of engine stop constitutes the second NOx purge control means of the present invention. To do. Details of each of these functional elements will be described below.

[NOx purge control start processing]
FIG. 4 is a block diagram showing a start process by the NOx purge start processing unit 110. The NOx purge start determination unit 111 includes (1) a first start condition in which the NOx occlusion amount estimated value _{m_NOx of the} NOx occlusion reduction type catalyst 32 reaches a predetermined NOx occlusion amount threshold value _{STR_thr_NOx} , and (2) the NOx occlusion reduction type catalyst. A second start condition in which the NOx purification rate _{Pur_NOx% of} 32 is reduced to a predetermined purification rate threshold value _{Pur_thr_NOx%} , (3) an elapsed time _{Int_Time} measured by a timer built in the ECU from the end of the previous NOx purge control is a predetermined value When any of the third start conditions reaching the interval target value _{Int_tgr} is satisfied, the NOx purge flag F _NP is turned on (F _NP = 1) to start the NOx purge control (see time t _{1 in} FIG. 3).

The NOx occlusion amount estimated value _{m_NOx} used for the determination of the first start condition is estimated by the NOx occlusion amount estimation unit 113. The NOx occlusion amount estimated value _{m_NOx} may be calculated based on, for example, a map or a model formula that includes the operating state of the engine 10, the sensor value of the NOx / lambda sensor 45, and the like as input signals. NOx storage amount threshold _{STR _Thr_NOx} is set in storage amount threshold map 114 referenced based on the estimated catalyst temperature _{Temp _LNT} the NOx occlusion-reduction catalyst 32. The estimated catalyst temperature _{Temp_LNT} is estimated by the catalyst temperature estimation unit 115. The estimated catalyst temperature _{Temp_LNT} is estimated _based on, for example, the inlet temperature of the oxidation catalyst 31 detected by the first exhaust temperature sensor 43, the HC / CO heat generation amount inside the oxidation catalyst 31 and the NOx storage reduction catalyst 32, and the like. do it. The NOx occlusion amount threshold value _{STR_thr_NOx} set based on the occlusion amount threshold value map 114 is corrected by an occlusion amount threshold value correction unit 116, which will be described in detail later.

The NOx purification rate _{NOx_pur%} used for the determination of the second start condition is calculated by the purification rate calculation unit 117. The NOx purification rate _{NOx_pur%} is obtained, for example, by _dividing the NOx amount on the downstream side of the catalyst detected by the NOx / lambda sensor 45 by the NOx emission amount on the upstream side of the catalyst estimated from the operating state of the engine 10 or the like. That's fine.

The interval target value _{Int_tgr} used for the determination of the third start condition is set in the interval target value map 118 that is referred to based on the engine speed Ne and the accelerator opening Q. This interval target value _{Int_tgr} is corrected by an interval target value correction unit 119, which will be described in detail later.

[Occlusion amount threshold correction]
The NOx occlusion capacity of the NOx occlusion reduction type catalyst 32 decreases with the progress of aging degradation, thermal degradation, and the like. For this reason, if the NOx occlusion amount threshold value _{NOx_thr_val} is set to a fixed value, the execution frequency of NOx purge according to aging deterioration, thermal deterioration, etc. cannot be secured, and exhaust emission may be deteriorated.

The occlusion amount threshold value correction unit 116 _{decreases the} NOx occlusion amount threshold value _{NOx_thr_val} set by the occlusion amount threshold map 114 as the deterioration degree of the NOx occlusion reduction type catalyst 32 increases in order to prevent such deterioration of exhaust emission. Perform a decrease correction. More specifically, this reduction correction is performed by multiplying the NOx occlusion amount threshold _{NOx_thr_val} by a deterioration correction coefficient (deterioration degree) obtained by the deterioration degree estimation unit 120. The deterioration correction coefficient includes, for example, a decrease in the HC / CO heat generation amount inside the NOx storage reduction catalyst 32, a heat history of the NOx storage reduction catalyst 32, a decrease in the NOx purification rate of the NOx storage reduction catalyst 32, a vehicle travel distance, and the like. Find it based on this.

[Interval target value correction]
If the interval target value _{Int_tgr} is set to a fixed value, the NOx purge execution frequency cannot be ensured when the NOx storage reduction catalyst 32 progresses over time, heat deterioration, etc., and exhaust emission may be deteriorated. .

The interval target value correction unit 119 _{shortens the} interval target value _{Int_tgr} set by the interval target value map 118 as the deterioration degree of the NOx storage reduction catalyst 32 increases in order to prevent such deterioration of exhaust emission. Perform correction. More specifically, this shortening correction is performed by multiplying the NOx occlusion amount threshold value _{NOx_thr_val} by a deterioration correction coefficient (deterioration degree) obtained by the deterioration degree estimation unit 120. The deterioration correction coefficient includes, for example, a decrease in the HC / CO heat generation amount inside the NOx storage reduction catalyst 32, a heat history of the NOx storage reduction catalyst 32, a decrease in the NOx purification rate of the NOx storage reduction catalyst 32, a vehicle travel distance, and the like. Find it based on this.

[NOx purge lean control]
When the NOx purge flag F _NP is turned on, the NOx purge lean control unit 130 sets the excess air ratio on the lean side from the stoichiometric air fuel ratio equivalent value (about 1.0) from the steady operation (for example, about 1.5). The NOx purge lean control is performed to reduce the first target excess air ratio (for example, about 1.3). Details of the NOx purge lean control will be described below.

FIG. 5 is a block diagram showing a setting process of the MAF target value MAF _{NPL_Trgt} at the time of NOx purge lean control. The first target excess air ratio setting map 131 is a map that is referred to based on the engine speed Ne and the accelerator opening Q, and during NOx purge lean control corresponding to the engine speed Ne and the accelerator opening Q. An excess air ratio target value λ _{NPL_Trgt} (first excess air ratio) is set in advance based on experiments or the like.

First, from the first target excess air ratio setting map 131, the excess air ratio target value λ _{NPL_Trgt} at the time of NOx purge lean control is read using the engine speed Ne and the accelerator opening Q as input signals, and the MAF target value calculation unit 132 reads it. Entered. Further, the MAF target value calculation unit 132 calculates the MAF target value MAF _{NPL_Trgt} at the time of NOx purge lean control based on the following formula (1).

MAF _{NPL_Trgt} = λ _{NPL_Trgt} × Q _{fnl_corrd} × Ro _Fuel × AFR _sto / _{Maf_corr} (1)
In Equation (1), Q _{fnl_cord} represents a learning-corrected fuel injection amount (excluding post-injection) described later, Ro _Fuel represents fuel specific gravity, AFR _sto represents a theoretical air-fuel ratio, and _{Maf_corr} represents a MAF correction coefficient described later. Yes.

The MAF target value MAF _{NPL_Trgt} calculated by the MAF target value calculation unit 132 is input to the ramp processing unit 133 when the NOx purge flag F _NP is turned on (see time t _{1 in} FIG. 3). The ramp processing unit 133 reads the ramp coefficient from the ramp coefficient maps 133A, _133B using the engine speed Ne and the accelerator opening Q as input signals, and _{sets the} MAF target ramp value MAF _{NPL_Trgt_Ramp} to which the ramp coefficient is added as a valve control unit 134. To enter.

The valve control unit 134 throttles the intake throttle valve 16 to the _close side and opens the EGR valve 24 to the open side so that the actual MAF value MAF _Act input from the MAF sensor 40 becomes the MAF target ramp value MAF _{NPL_Trgt_Ramp.} Execute control.

Thus, in the present embodiment, the MAF target value MAF _{NPL_Trgt} is set based on the excess air ratio target value λ _{NPL_Trgt} read from the first target excess air ratio setting map 131 and the fuel injection amount of each injector 11, The air system operation is feedback-controlled based on the MAF target value MAF _{NPL_Trgt} . Thus, without providing a lambda sensor upstream of the NOx storage reduction catalyst 32, or even when a lambda sensor is provided upstream of the NOx storage reduction catalyst 32, the sensor value of the lambda sensor is not used. It is possible to effectively reduce the exhaust gas to a desired excess air ratio required for NOx purge lean control.

_Further, by using the fuel injection amount Q _{fnl_corrd} after learning correction as the fuel injection amount of each injector 11, the MAF target value MAF _{NPL_Trgt} can be set by feedforward control, and the aging deterioration and characteristic change of each injector 11 can be _achieved. Etc. can be effectively eliminated.

Further, by adding a ramp coefficient that is set according to the operating state of the engine 10 to the MAF target value MAF _{NPL_Trgt} , it is possible to prevent misfire of the engine 10 due to a sudden change in the intake air amount, deterioration of drivability due to torque fluctuation, and the like. It can be effectively prevented.

[NOx purge rich control fuel injection amount setting]
When the NOx purge flag F _NP is turned on, the NOx purge rich control unit 140 reduces the excess air ratio from the first target excess air ratio to the rich second target excess air ratio (for example, about 0.9). The NOx purge rich control is executed. Details of the NOx purge rich control will be described below.

FIG. 6 is a block diagram showing processing for setting the target injection amount Q _{NPR_Trgt} (injection amount per unit time) of exhaust pipe injection or post injection in NOx purge rich control. The second target excess air ratio setting map 145 is a map that is referred to based on the engine speed Ne and the accelerator opening Q, and during NOx purge rich control corresponding to the engine speed Ne and the accelerator opening Q. The air excess rate target value λ _{NPR_Trgt} (second target air excess rate) is preset based on experiments or the like.

First, the excess air ratio target value λ _{NPR_Trgt} at the time of NOx purge rich control is read from the second target excess air ratio setting map 145 using the engine speed Ne and the accelerator opening Q as input signals, and the injection amount target value calculation unit 146 is read. Is input. Further, the injection amount target value calculation unit 146 calculates a target injection amount Q _{NPR_Trgt} at the time of NOx purge rich control based on the following formula (2).

Q _{NPR_Trgt} = MAF _{NPL_Trgt} × _{Maf_corr} / (λ _{NPR_Trgt} × Ro _Fuel × AFR _sto ) −Q _{fnl_corrd} (2)
In Formula (2), MAF _{NPL_Trgt} is a NOx purge lean MAF target value, and is input from the above-described MAF target value calculation unit 72. Q _{fnl_cord} is a fuel injection amount (excluding post-injection) before application of learning corrected MAF tracking control described later, Ro _Fuel is fuel specific gravity, AFR _sto is a theoretical air-fuel ratio, and _{Maf_corr} is a MAF correction coefficient described later. Show.

The target injection amount Q _{NPR_Trgt} calculated by the injection amount target value calculation unit 146 is transmitted as an injection instruction signal to the exhaust pipe injection device 33 or each injector 11 when the NOx purge flag F _NP is turned on (time t in FIG. 3). ₁ ). The transmission of the injection instruction signal is continued until the NOx purge flag F _NP is turned off (time t _{2 in} FIG. 3) due to the end determination of NOx purge control described later.

Thus, in the present embodiment, the target injection amount Q _{NPR_Trgt} is set based on the air excess rate target value λ _{NPR_Trgt} read from the second target air excess rate setting map 145 and the fuel injection amount of each injector 11. It has become. Thus, without providing a lambda sensor upstream of the NOx storage reduction catalyst 32, or even when a lambda sensor is provided upstream of the NOx storage reduction catalyst 32, the sensor value of the lambda sensor is not used. It is possible to effectively reduce the exhaust gas to a desired excess air ratio required for NOx purge rich control.

_Further, by using the fuel injection amount Q _{fnl_corrd} after learning correction as the fuel injection amount of each injector 11, the target injection amount Q _{NPR_Trgt} can be set by feedforward control, and the aging deterioration and characteristic change of each injector 11 can be _achieved. Etc. can be effectively eliminated.

[NOx purge control when the engine is stopped]
FIG. 7 is a block diagram for explaining exhaust rich injection execution processing by the NOx purge control unit 160 when the engine is stopped.

  When the engine is stopped, the NOx purge control unit 160 establishes an engine stop request for turning off the ignition switch 161 by the driver, and none of the rich injection prohibition conditions (1) to (3), which will be described in detail later. In this case, NOx purge is executed when the engine is stopped to bring the exhaust to a rich state by post injection or exhaust pipe injection.

The NOx purge when the engine is stopped is an instruction to cause the injector 11 or the exhaust pipe injector 33 to perform a predetermined amount of fuel injection if the rich injection prohibition flag F _{Pro_NP} is OFF when the ignition switch 161 is switched from ON to OFF. initiated by sending a signal (see time t ₁ in FIG. 8). Engine injection quantity and injection period of stop NOx purge, for example, the ignition switch 161 is set based off has been occluded amount of NOx estimated value _{m _NOx} or estimated catalyst temperature _{Temp _LNT} like when. The NOx occlusion amount estimation value _{m_NOx} and the estimated catalyst temperature _{Temp_LNT} are input from the NOx occlusion amount estimation unit 113 and the catalyst temperature estimation unit 115, respectively.

The engine stop NOx purge prohibiting unit 162 is a prohibiting means of the present invention. When the ignition switch 161 is turned off, (1) the elapsed time from the end of the previous NOx purge control is a predetermined lower limit interval value _{Int_min.} (2) The second prohibition condition in which the NOx occlusion amount estimated value _{m_NOx} has not increased to the predetermined upper limit threshold _{NOx_thr_max} , and (3) the estimated catalyst temperature _{Temp_LNT} is the predetermined catalyst activation temperature. If any of the third prohibition conditions lower than _{T_activ} is satisfied, the rich injection prohibition flag _{FPro_NP} is turned on to prohibit the execution of NOx purge when the engine is stopped.

As the lower limit interval value _{Int_min} used for the determination of the first prohibition condition, for example, the interval target value _{Int_tgr} set by the interval target value map 118 (shown in FIG. 4) described above may be used. You may set as time shorter than target value _{Int_tgr} . For example, the upper limit threshold _{NOx_thr_max} used for the determination of the second prohibition condition is set based on the upper limit value of the NOx occlusion capacity of the NOx occlusion reduction type catalyst 32 when the catalyst temperature is lowered, for example, in order to prevent NOx discharge during engine restart do it.

  Hereinafter, based on the flow of FIG. 9, the engine stop NOx purge execution process and the NOx purge prohibition process will be described.

When the ignition switch 161 of the engine 10 is turned off in step S100, it is determined in step S110 whether the elapsed time from the end of the previous NOx purge control exceeds the lower limit interval value _{Int_min} . When the elapsed time exceeds the lower limit interval value _{Int_min} (Yes), the process proceeds to step S120. When the elapsed time is shorter than the lower interval value _{Int _min} (No), the process proceeds to step S170, the rich injection prohibition flag _{F Pro_NP} is turned on. In step S120, it is determined whether or not the NOx occlusion amount estimated value _{m_NOx} has increased to the upper limit threshold _{NOx_thr_max} . When the NOx occlusion amount estimated value _{m_NOx} exceeds the upper limit threshold _{NOx_thr_max} (Yes), the process proceeds to step S130. When the NOx occlusion amount estimated value _{m_NOx} is smaller than the upper limit threshold _{NOx_thr_max} (No), the process proceeds to step S170, and the rich injection prohibition flag _{FPro_NP} is turned on.

In step S130, it is determined whether or not the estimated catalyst temperature _{Temp_LNT} is at the catalyst activation temperature _{T_active} . When the estimated catalyst temperature _{Temp_LNT} is at the catalyst activation temperature _{T_active} (Yes), the process proceeds to step S140. When the estimated catalyst temperature _{Temp_LNT} is not at the catalyst activation temperature _{T_activ} (No), the process proceeds to step S170, and the rich injection prohibition flag _{FPro_NP} is turned on.

In step S140, the rich injection prohibition flag F _{Pro_NP} is turned off in response to all the determinations in steps S110 to 130 being affirmed. Furthermore, in step S150, NOx purge is executed when the engine is stopped, in which a predetermined amount of fuel is injected by post injection or exhaust pipe injection, and then this control is terminated.

  As described above in detail, according to the present embodiment, the fuel is effectively retained in the exhaust passage 13 by performing the engine stop NOx purge that makes the exhaust gas rich when the engine stops when the exhaust speed becomes slow. , NOx reduction efficiency can be improved. Also, when the engine is stopped, if the elapsed time from the end of the previous NOx purge control is short, if the NOx occlusion amount is small, or if the catalyst temperature is low, NOx purge is prohibited when the engine is stopped. It becomes possible to reliably prevent fuel consumption deterioration due to fuel consumption.

  The execution of the NOx purge when the engine is stopped is not limited to the time when the ignition switch 161 is turned off. If it is in a state where the engine speed becomes slow, it can be widely applied even when other engine stop requests are established.

[MAF tracking control]
The MAF follow-up control unit 80 includes (1) a switching period from the lean state in the normal operation to the rich state by the NOx purge control, and (2) a switching period from the rich state to the lean state in the normal operation by the NOx purge control. Control (MAF follow-up control) for correcting the fuel injection timing and the fuel injection amount of the injector 11 according to the MAF change is executed.

[Injection amount learning correction]
As shown in FIG. 10, the injection amount learning correction unit 300 includes a learning correction coefficient calculation unit 310 and an injection amount correction unit 320.

The learning correction coefficient calculation unit 310 is a fuel injection amount learning correction coefficient F based on an error Δλ between the actual lambda value λ _Act detected by the NOx / lambda sensor 45 and the estimated lambda value λ _Est during the lean operation of the engine 10. Calculate _Corr . When the exhaust is in a lean state, the HC concentration in the exhaust is very low, so that the change in the exhaust lambda value due to the oxidation reaction of HC at the oxidation catalyst 31 is negligibly small. Therefore, the actual lambda value λ _Act in the exhaust gas that passes through the oxidation catalyst 31 and is detected by the downstream NOx / lambda sensor 45 matches the estimated lambda value λ _Est in the exhaust gas discharged from the engine 10. Conceivable. That is, when an error Δλ occurs between the actual lambda value λ _Act and the estimated lambda value λ _Est , it can be assumed that the difference is between the commanded injection amount for each injector 11 and the actual injection amount. Hereinafter, the learning correction coefficient calculation processing by the learning correction coefficient calculation unit 310 using the error Δλ will be described with reference to the flow of FIG.

  In step S300, based on the engine speed Ne and the accelerator opening Q, it is determined whether or not the engine 10 is in a lean operation state. If it is in the lean operation state, the process proceeds to step S310 to start the calculation of the learning correction coefficient.

In step S310, an error Δλ obtained by subtracting the actual lambda value λ _Act detected by the NOx / lambda sensor 45 from the estimated lambda value λ _Est is multiplied by the learning value gain K ₁ and the correction sensitivity coefficient K ₂ to thereby obtain the learning value F _CorrAdpt is calculated (F _CorrAdpt = (λ _Est −λ _Act ) × K ₁ × K ₂ ). The estimated lambda value λ _Est is estimated and calculated from the operating state of the engine 10 according to the engine speed Ne and the accelerator opening Q. Further, the correction sensitivity coefficient _{K 2} is read the actual lambda value lambda _Act detected by the NOx / lambda sensor 45 from the correction sensitivity coefficient map 310A shown in FIG. 10 as an input signal.

In step S320, it is determined whether or not the absolute value | F _CorrAdpt | of the learning value F _CorrAdpt is within the range of the predetermined correction limit value A. If the absolute value | F _CorrAdpt | exceeds the correction limit value A, the present control is returned to stop the current learning.

In step S330, it is determined whether or not the learning prohibition flag F _{Pro_Corr} is off. The learning prohibition flag F _{Pro_Corr} corresponds to, for example, the transient operation of the engine 10 or the NOx purge control (F _NP = 1). This is because when these conditions are satisfied, the error Δλ increases due to a change in the actual lambda value λ _Act , and accurate learning cannot be performed. Whether or not the engine 10 is in a transient operation state is determined based on, for example, the time change amount of the actual lambda value λ _Act detected by the NOx / lambda sensor 45 when the time change amount is larger than a predetermined threshold value. What is necessary is just to determine with a transient operation state.

In step S340, the learning value map 310B (see FIG. 10) referred to based on the engine speed Ne and the accelerator opening Q is updated to the learning value F _CorrAdpt calculated in step S310. More specifically, a plurality of learning regions divided according to the engine speed Ne and the accelerator opening Q are set on the learning value map 310B. These learning regions are preferably set to have a narrower range as the region is used more frequently and to be wider as a region is used less frequently. As a result, learning accuracy is improved in regions where the usage frequency is high, and unlearning can be effectively prevented in regions where the usage frequency is low.

In step S350, the learning correction coefficient F _Corr is calculated by adding “1” to the learned value read from the learned value map 310B using the engine speed Ne and the accelerator opening Q as input signals (F _Corr = 1 + F). _CorrAdpt ). This learning correction coefficient F _Corr is input to the injection amount correction unit 320 shown in FIG.

The injection amount correction unit 320 multiplies each basic injection amount of pilot injection Q _Pilot , pre-injection Q _Pre , main injection Q _Main , after-injection Q _After , and post-injection Q _{Post by} a learning correction coefficient F _Corr. The injection amount is corrected.

In this way, by correcting the fuel injection amount to each injector 11 with the learning value corresponding to the error Δλ between the estimated lambda value λ _Est and the actual lambda value λ _Act , the aging deterioration, characteristic change, individual difference of each injector 11 is corrected. It is possible to effectively eliminate such variations.

[MAF correction coefficient]
MAF correction coefficient calculation unit 400 calculates the MAF correction coefficient _{Maf _Corr} used to set the MAF target value _{MAF NPL_Trgt} and the target injection amount _{Q NPR_Trgt} during NOx purge control.

In the present embodiment, the fuel injection amount of each injector 11 is corrected based on the error Δλ between the actual lambda value λ _Act and the estimated lambda value λ _Est detected by the NOx / lambda sensor 45. However, since lambda is the ratio of air to fuel, the factor of error Δλ is not necessarily only the effect of the difference between the commanded injection amount and the actual injection amount for each injector 11. That is, there is a possibility that the error of not only each injector 11 but also the MAF sensor 40 affects the lambda error Δλ.

FIG. 12 is a block diagram showing the setting process of the MAF correction coefficient _{Maf_corr} by the MAF correction coefficient calculation unit 400. The correction coefficient setting map 410 is a map that is referred to based on the engine speed Ne and the accelerator opening Q, and indicates a MAF that indicates the sensor characteristics of the MAF sensor 40 corresponding to the engine speed Ne and the accelerator opening Q. The correction coefficient _{Maf_corr} is set in advance based on experiments or the like.

The MAF correction coefficient calculation unit 400 reads the MAF correction coefficient _{Maf_corr} from the correction coefficient setting map 410 using the engine speed Ne and the accelerator opening Q as input signals, and _{uses the} MAF correction coefficient _{Maf_corr} as the MAF target value calculation unit 132 and It transmits to the injection quantity target value calculating part 146. As a result, the sensor characteristics of the MAF sensor 40 can be effectively reflected in the settings of the MAF target value MAF _{NPL_Trgt} and the target injection amount Q _{NPR_Trgt} during the NOx purge control.

[Others]
In addition, this invention is not limited to the above-mentioned embodiment, In the range which does not deviate from the meaning of this invention, it can change suitably and can implement.

DESCRIPTION OF SYMBOLS 10 Engine 11 Injector 12 Intake passage 13 Exhaust passage 16 Intake throttle valve 24 EGR valve 31 Oxidation catalyst 32 NOx occlusion reduction type catalyst 33 Filter 34 Exhaust pipe injection device 40 MAF sensor 45 NOx / lambda sensor 50 ECU

Claims (4)

An NOx occlusion reduction catalyst that is provided in the exhaust passage of the engine and occludes NOx in the exhaust in an exhaust lean state and reduces and purifies NOx occluded in the exhaust rich state;
First NOx purge control means for performing NOx purge for reducing and purifying NOx stored in the NOx storage reduction catalyst by making exhaust rich by at least one of post injection and exhaust pipe injection;
A second NOx purge control means for performing NOx purge when the engine is stopped to bring the exhaust into a rich state by at least one of post injection and exhaust pipe injection when the engine stop request is established;
When the engine stop request is satisfied, if the elapsed time from the end of the NOx purge control by the first NOx purge control means performed before the stop request is satisfied does not reach a predetermined threshold time, An exhaust purification system comprising: prohibiting means for prohibiting execution of NOx purge when the engine is stopped. A NOx occlusion amount estimating means for estimating the NOx occlusion amount of the NOx occlusion reduction type catalyst;
The prohibiting means prohibits execution of the NOx purge when the engine is stopped even when the NOx storage amount estimated by the NOx storage amount estimating means is smaller than a predetermined threshold amount when the engine stop request is established. The exhaust purification system according to claim 1. A catalyst temperature estimating means for estimating a catalyst temperature of the NOx storage reduction catalyst;
The prohibiting means prohibits the execution of the NOx purge when the engine is stopped even when the catalyst temperature estimated by the catalyst temperature estimating means is lower than a predetermined catalyst activation temperature when the engine stop request is established. The exhaust gas purification system according to claim 1 or 2. The exhaust gas purification system according to any one of claims 1 to 3, wherein the engine stop request is established by turning off an ignition switch.

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