JP2021099039A - Exhaust treatment device for engine - Google Patents

Abstract

To appropriately reduce NOX stored in a NOX storage catalyst while appropriately eliminating HC and CO from an exhaust gas.SOLUTION: An exhaust treatment device (1) includes a LNT catalyst (20) provided in an exhaust passage (12) of an engine (10), and a filter (30) provided in the exhaust passage (12) at its portion on a downstream side further than the LNT catalyst (20), the LNT catalyst (20) having a function to store NOX in the exhaust gas when an exhaust air-fuel ratio is lean, and to release the NOX to be reduced and eliminated when the exhaust air-fuel ratio is a theoretical air-fuel ratio or rich, the filter (30) having a function to trap PMs from the exhaust gas. On the filter (30), an OSC material 31 is supported having a function to store and release oxygen. In the LNT catalyst (20), the OSC material is not included.SELECTED DRAWING: Figure 1

Description

The present disclosure relates to an engine exhaust treatment device.

In an engine that can burn in a state that is leaner than the stoichiometric air-fuel ratio (typically a diesel engine), NO _X (nitrogen oxide) is likely to be generated, so purify _{NO X in the exhaust passage. A NO X} storage catalyst for this purpose may be provided. _The NO _X storage catalyst occludes NO _X in the exhaust gas when the exhaust air-fuel ratio is leaner than the stoichiometric air-fuel ratio, and has a function of releasing NO _X when the exhaust gas air-fuel ratio is stoichiometric or rich .. NO _X storage catalyst NO released from _X is reduced is purified HC in the exhaust gas (hydrocarbons) and CO (the carbon monoxide) as the reducing material. The NO _X storage catalyst is also referred to as an LNT catalyst (Lean Nitrogen oxides Trap catalyst). The NO _X storage catalyst also has a function of oxidizing and purifying HC and CO in the exhaust when the exhaust air-fuel ratio is lean. _{Therefore, in an engine equipped with a NO X} storage catalyst in the exhaust passage, NO _X , HC, and CO in the exhaust can be purified by appropriately switching between a state in which the exhaust air-fuel ratio is lean and a state in which the exhaust air-fuel ratio is rich. ..

The JP 2001-276622 (Patent Document 1), an oxygen storage material having a function of storing and releasing oxygen (OSC material, OSC; Oxygen Storage / release Capacity ) NO X storage catalyst that is supported is disclosed ing. NO _{X By} supporting the oxygen storage material on the storage catalyst, even if the exhaust air-fuel ratio becomes slightly leaner or richer than the stoichiometric air-fuel ratio, the oxygen storage material stores or releases oxygen, so that NO _X The exhaust air-fuel ratio around the storage catalyst is maintained near the stoichiometric air-fuel ratio.

Japanese Unexamined Patent Publication No. 2001-276622

Generally, a diesel engine is operated with the air-fuel ratio leaner than the stoichiometric air-fuel ratio. Therefore, in a diesel engine equipped _{with a NO X} storage catalyst in the exhaust passage, when performing reduction purification of NO _{X by the NO X} storage catalyst, the exhaust is exhausted more than _{the NO X} storage catalyst in order to bring the exhaust air-fuel ratio closer to the stoichiometric air-fuel ratio. Control to supply fuel to the upstream side (hereinafter also referred to as "riching control") is performed.

When the exhaust air-fuel ratio was brought close to the stoichiometric air-fuel ratio by this enrichment control, if the NO _X storage catalyst was supported by the oxygen storage material as in the configuration disclosed in Patent Document 1, it was stored in the oxygen storage material. oxygen is delayed switching of a factor to be released during the rich control to the stoichiometric air-fuel ratio of the exhaust air-fuel ratio, that is not possible to adequately reduce and purify NO _X occluded in _the NO _X storage catalyst I am concerned. That is, the exhaust gas when the air-fuel switching of ratio to the stoichiometric air-fuel ratio is delayed, to follow the state exhaust air-fuel ratio is lean, oxygen NO _X reducing agent in the exhaust gas (HC and CO) it can cause been consumed by oxidation , There is a concern that the reduction reaction of _{NO X will be difficult to occur.} The exhaust for air fuel delayed by time period switching of is to be extra feed, the temperature of the NO _X storage catalyst excessively increases, the NO _X storing catalyst NO _X that was stored in the reduction There is a concern that it will be detached as it is without being purified. As a countermeasure, if the _{oxygen storage material is simply removed from the NO X} storage catalyst, the oxygen storage material will not release oxygen when the exhaust air-fuel ratio becomes slightly richer than the stoichiometric air-fuel ratio due to enrichment control. There is concern that HC and CO cannot be oxidized (purified). Patent Document 1 does not mention such a problem and its solution.

The present disclosure has been made to solve the above-mentioned problems, and an object thereof is to appropriately reduce _{NO X} stored in _{a NO X storage catalyst while appropriately purifying HC and CO in the exhaust gas.} It is to provide an exhaust purification device capable of doing so.

(1) The exhaust gas treatment device of the engine according to the present disclosure includes a first exhaust gas purification device provided in the exhaust passage of the engine and a second exhaust purification device provided in a portion downstream of the first exhaust purification device in the exhaust passage. To be equipped. The first exhaust gas purification device has a function of storing nitrogen oxides in the exhaust when the exhaust air-fuel ratio is lean and releasing nitrogen oxides when the exhaust air-fuel ratio is the stoichiometric air-fuel ratio or rich. Includes NO _X storage catalyst with. The second exhaust gas purification device _{does not contain a NO X} storage catalyst and includes an oxygen storage material having a function of storing and releasing oxygen. The amount of oxygen storage material contained in the second exhaust gas purification device is larger than the amount of oxygen storage material contained in the first exhaust gas purification device.

In the above exhaust gas treatment device, a second exhaust gas purification device containing an oxygen storage material is provided on the downstream side of the first exhaust gas purification device including _{a NO X storage catalyst.} The amount of the oxygen storage material contained in the second exhaust gas purification device is larger than the amount of the oxygen storage material contained in the first exhaust gas purification device. In other words, the oxygen storage material having a function of storing and releasing oxygen is eliminated or reduced from the first exhaust gas purification device containing the _{NO X storage catalyst.} Accordingly, it is possible to perform quick changeover of the air-fuel ratio by the rich control in the first the exhaust gas purifying apparatus, it is possible to appropriately reduce the NO _X occluded in the NO _X storage catalyst. Furthermore, since the second exhaust gas purification device on the downstream side of the first exhaust gas purification device contains an oxygen storage material, even when the exhaust air-fuel ratio becomes slightly richer than the theoretical air-fuel ratio due to the enrichment control. HC and CO that have passed through the first exhaust gas purification device can be appropriately oxidized (purified) by _{O 2 released from the oxygen storage material contained in the second exhaust gas purification device.} Consequently, while appropriately purify HC and CO in the exhaust, the NO _X occluded in the NO _X storage catalyst can be appropriately reduced.

(2) In some embodiments, the first exhaust gas purification device is a NO _X storage catalyst that does not contain an oxygen storage material.

In this embodiment, since the NO _X storage catalyst does not contain the oxygen storage material, the air-fuel ratio can be switched more quickly by the enrichment control as compared with the case where _{the NO X storage catalyst contains some oxygen storage material.} Can be done.

(3) In one embodiment, the second exhaust gas purification device is a filter on which an oxygen storage material is supported to collect particulate matter in the exhaust gas.

In this embodiment, in the second exhaust gas purification device, while collecting particulate matter in the exhaust gas, HC and CO that have passed through the first exhaust gas purification device are appropriately oxidized _{by O 2 released from the oxygen storage material (} Can be purified).

(4) In some embodiments, the exhaust treatment device is based on a fuel supply device that supplies fuel to the exhaust passage and a fuel supply device when predetermined execution conditions are satisfied with the exhaust air-fuel ratio being lean. It is provided with a control device that executes control to bring the exhaust air-fuel ratio closer to the stoichiometric air-fuel ratio as enrichment control by repeating on / off processing for stopping the fuel supply after fuel supply. The control device controls the fuel supply device so that the time of one on / off process is shorter than the time of two rotations of the rotation shaft of the engine during the execution of the enrichment control.

In the above aspect, during the execution of the enrichment control, the time of one on / off process is shorter than the time of two rotations of the rotation shaft of the engine (hereinafter, also referred to as "one cycle time"). As a result, it is possible to secure more opportunities to stop the fuel supply than when the time of one on / off processing is set to one cycle time. Therefore, the air-fuel ratio is made lean and oxygen is added to the oxygen storage material. Can be secured more opportunities to store. Therefore, an amount of oxygen sufficient to oxidize HC and CO passing through the first exhaust gas purification device can be easily stored in the oxygen storage material of the second exhaust gas purification device.

(5) In some embodiments, the control device controls the fuel supply device so that the time for supplying fuel by the fuel supply device and the time for stopping the fuel supply are substantially equal in the on / off process.

In the above aspect, in the on / off process, the time for supplying fuel and the time for stopping the fuel supply are substantially equal. As a result, the amount of fuel supplied at one time can be reduced as compared with the case where the time for supplying fuel is shorter than the time for stopping fuel, and the HC and CO discharged by one fuel supply can be reduced accordingly. The amount of fuel can be reduced. Therefore, an amount of oxygen sufficient to oxidize HC and CO discharged by one fuel supply can be easily stored in the oxygen storage material of the second exhaust gas purification device.

According to the present disclosure, it is possible to provide an exhaust gas purification device capable of appropriately reducing _{NO X} stored in _{a NO X} storage catalyst while appropriately purifying HC and CO in the exhaust.

It is a figure which shows the example of the whole structure of the exhaust treatment apparatus schematicly. It is a figure which shows typically the mechanism of _{NO oxidation and NO X} occlusion that occur in an LNT catalyst. The mechanism of the NO _X reduction occurring in LNT catalyst is a view schematically showing. It is a figure which shows typically the mechanism of the reaction which occurs in the LNT catalyst which supported the OSC material. It is a figure which shows the change _{of the O 2} concentration around the LNT catalyst by the enrichment control. It is a figure which shows the change of the exhaust air-fuel ratio around the LNT catalyst, and _{the change of the NO X concentration occluded in the LNT catalyst by the enrichment control.} It is a figure which shows an example of the on-off processing by a comparative example. It is a figure which shows an example of the on-off processing by this embodiment. It is a flowchart which shows an example of the processing procedure performed when the ECU executes enrichment control. It is a figure which shows the modification of the on-off processing time.

Hereinafter, the present embodiment will be described in detail with reference to the drawings. The same or corresponding parts in the drawings are designated by the same reference numerals, and the description thereof will not be repeated.

<Overall configuration>
FIG. 1 is a diagram schematically showing an example of the overall configuration of the exhaust treatment device 1 according to the present embodiment. The exhaust treatment device 1 is a device for purifying the exhaust gas of the engine 10. The exhaust treatment device 1 includes an LNT catalyst (NO _X storage catalyst) 20, a filter (DPF: Diesel Particulate Filter) 30, and an electronic control unit (Electronic Control Unit, hereinafter referred to as “ECU”) 100.

The engine 10 is a general 4-stroke diesel engine mounted on a vehicle or the like. The engine 10 has a plurality of (four in the example shown in FIG. 1) cylinders. The number of cylinders of the engine 10 may be one.

A fuel injection valve 15 is provided in each cylinder of the engine 10. Fuel from a fuel tank is supplied to each fuel injection valve 15 by a fuel pump (not shown). Each fuel injection valve 15 operates (opens) according to a control signal from the ECU 100 to inject fuel into each cylinder.

Each cylinder of the engine 10 is connected to the intake passage 11 via the intake manifold 11a and is connected to the exhaust passage 12 via the exhaust manifold 12a. Inside the exhaust passage 12, an LNT catalyst 20 and a filter 30 are provided.

_{The LNT catalyst occludes NO X} in the exhaust when the exhaust air-fuel ratio is lean (when there is excess oxygen in the surroundings), and when the exhaust air-fuel ratio is the stoichiometric air-fuel ratio or rich (oxygen in the surroundings). It has a function to release _{NO X} when it is not available. NO _X released from _the NO _X storage catalyst when the exhaust air-fuel ratio is stoichiometric or rich is purified by reduction HC in the exhaust gas (hydrocarbons) and CO (the carbon monoxide) as the reducing material .. The LNT catalyst 20 also has a function of oxidizing and purifying HC and CO in the exhaust when the exhaust air-fuel ratio is lean. _{Therefore, the LNT catalyst 20 can purify NO X} , HC, and CO in the exhaust gas by appropriately switching the exhaust air-fuel ratio between a lean state and a rich state.

FIG. 2 is a diagram schematically showing the mechanism of _{NO oxidation and NO X} occlusion that occurs in the LNT catalyst 20 when the exhaust air-fuel ratio is lean. FIG. 3 is a diagram schematically showing the mechanism of _{NO X} reduction that occurs in the LNT catalyst 20 when the exhaust air-fuel ratio is the stoichiometric air-fuel ratio or rich.

LNT catalyst 20 includes a carrier layer 21 formed on the carrier with a porous coating material such as zeolite or alumina porous, and a carrier layer a noble metal 22 supported on the 21 and _the NO _X storage material 23. The noble metal 22 is platinum (Pt), palladium (Pd), or the like having an oxidation catalyst function. The NO _X occlusion material 23 is potassium (K), barium (Ba), lanthanum (La) or the like having _{a NO X occlusion function.}

When the exhaust air-fuel ratio is lean, as shown in FIG. 2, NO (nitric oxide) in the exhaust is oxidized by O ₂ (oxygen) in the _{exhaust by the oxidizing function of the noble metal 22 and NO 2} (nitrogen monoxide). Nitrogen dioxide). The NO ₂ is because it is occluded in the form of nitrates by _the NO _X storage material 23, NO _X in the exhaust gas is purified.

However, if this state continues, the NO _X storage material 23 _{loses the NO X} storage function. Therefore, by changing the operating conditions of the engine 10 (specifically, executing the "riching control" described later), a high-temperature exhaust gas having an exhaust air-fuel ratio of the stoichiometric air-fuel ratio or richness is generated, and this exhaust gas is generated. It is supplied to the LNT catalyst 20.

If the exhaust air-fuel ratio is the stoichiometric air-fuel ratio or rich, eliminates O ₂ in the exhaust, and by the exhaust gas temperature rises, as shown in FIG. 3, NO _X that was stored in the NO _X occluding member 23 Is released as _{NO 2.} The released NO ₂ is purified by being reduced to _{H 2} O (water), CO ₂ (carbon dioxide) and N ₂ (nitrogen) using CO, HC, etc. in the exhaust gas on the precious metal 22 as a reducing agent. Further, by NO _X that was stored in the NO _X occluding member 23 is released, _the NO _X storage capability of the NO _X occluding member 23 is restored.

Returning to FIG. 1, the filter 30 is provided in a portion of the exhaust passage 12 on the downstream side of the LNT catalyst 20. The filter 30 has a function of collecting particulate matter (PM: Particulate Matter) contained in the exhaust gas. The filter 30 _{does not contain the NO X} storage catalyst (precious metal 22 and NO _X storage material 23) as contained in the LNT catalyst 20.

A fuel addition valve 60 is provided in a portion of the exhaust passage 12 on the upstream side of the LNT catalyst 20. Fuel from a fuel tank is supplied to the fuel addition valve 60 by a fuel pump (not shown). The fuel addition valve 60 operates (opens) by a control signal from the ECU 100, and injects fuel into a portion of the exhaust passage 12 on the upstream side of the LNT catalyst 20.

The engine 10 is provided with an engine rotation speed sensor 50. The engine rotation speed sensor 50 detects the rotation speed NE of the engine 10 and outputs the detection result to the ECU 100.

The ECU 100 includes a CPU (Central Processing Unit), a memory, and input / output ports for inputting / outputting various signals (none of which are shown). The ECU 100 controls each device based on signals from each sensor and device, a program stored in a memory, and the like. Note that various controls are not limited to software processing, but can also be processed by dedicated hardware (electronic circuits).

For example, when the engine 10 is operated, the ECU 100 performs main injection as one cycle of fuel injection to each cylinder. Specifically, the ECU 100 outputs a main injection command according to the required power to the fuel injection valve 15 at a predetermined timing. As a result, "main injection" is performed in which the fuel corresponding to the main injection command is injected from the fuel injection valve 15.

Further, the ECU 100 can also perform post-injection after main injection as one-cycle fuel injection to each cylinder for the purpose of purifying exhaust gas or the like. When performing post-injection, the ECU 100 outputs a post-injection command for injecting a small amount of fuel to the fuel injection valve 15 after outputting the main injection command. As a result, after the main injection, "post injection" is performed in which a small amount of fuel corresponding to the post injection command is injected from the fuel injection valve 15. In addition to the above-mentioned main injection and post injection, another injection (for example, pilot injection before the main injection) may be added as one cycle of fuel injection to each cylinder.

<Enrichment control>
As described above, the engine 10 according to the present embodiment is a general 4-stroke diesel engine. Generally, a diesel engine is operated with a lean air-fuel ratio. Therefore, during normal operation of the engine 10, the exhaust air-fuel ratio becomes lean and the NO _{X in} the exhaust is occluded in the LNT catalyst 20, so that the NO _{X in} the exhaust is purified (see FIG. 2 above). However, if this state continues, the LNT catalyst 20 _{loses the NO X} occlusion function.

Therefore, the ECU 100 performs post-injection by the fuel injection valve 15 or fuel addition by the fuel addition valve 60 in order to switch the exhaust air-fuel ratio to the vicinity of the stoichiometric air-fuel ratio when a predetermined execution condition is satisfied. Controls to generate high-temperature exhaust gas having an exhaust air-fuel ratio of the stoichiometric air-fuel ratio or rich and supply it to the LNT catalyst 20 (hereinafter, also referred to as “riching control”) are executed. Due to the exhaust generated by this enrichment control, O _{2 in} the exhaust disappears and the temperature of the LNT catalyst 20 rises. _{Therefore, NO X} occluded in the LNT catalyst 20 uses CO and HC in the exhaust as reducing agents. It is reduced and purified, and _{the NO X storage function of the NO X} storage material 23 is restored (see FIG. 3 above).

In the following, a case where the enrichment control is executed by adding fuel by the fuel addition valve 60 will be described. However, the fuel supply device used for the enrichment control is not limited to the fuel addition valve 60, and the enrichment control may be executed by post-injection by the fuel injection valve 15.

<Arrangement of OSC material (oxygen storage material)>
When the exhaust air-fuel ratio is switched to the vicinity of the stoichiometric air-fuel ratio by the above-mentioned enrichment control, if it takes time to switch the exhaust air-fuel ratio, _{there is a concern that NO X} cannot be appropriately reduced and purified. That is, if it takes time to switch to near stoichiometric air-fuel ratio of the exhaust air-fuel ratio, since the state exhaust air-fuel ratio is lean (state oxygen is in excess) followed, NO _X reducing agent (HC and CO in the exhaust ) Is _{oxidized by O 2} in the exhaust gas and consumed, and there _{is a concern that the NO X} reduction reaction is unlikely to occur. Further, since it takes time to switch the exhaust air-fuel ratio, excess fuel is injected from the fuel addition valve 60, so that the temperature of the LNT catalyst 20 rises excessively and the NO _X stored in the LNT catalyst 20 is reduced. There is a concern that it will be detached as it is without being purified. Therefore, in order to improve the reduction rate of the NO _X, it is required to perform fast switching of the exhaust air-fuel ratio by the rich control.

However, if the OSC material is supported on the LNT catalyst 20 as in the configuration disclosed in Patent Document 1, the oxygen stored in the OSC material is released during the enrichment control, which is a factor. There is a concern that the switching of the exhaust air-fuel ratio due to the enrichment control will be delayed and _{the reduction rate of NO X will decrease.}

FIG. 4 is a diagram schematically showing the mechanism of the reaction occurring in the LNT catalyst on which the OSC material is supported. Note that FIG. 4 shows a comparative example with respect to the present embodiment. When the OSC material is supported on the LNT catalyst, the switching of the exhaust air-fuel ratio to the theoretical air-fuel ratio is delayed due to the fact _{that the O 2 stored in the OSC material is released into the exhaust during the enrichment control.} .. Furthermore, if CO and HC in the exhaust gas are oxidized and consumed by _{O 2} released from the OSC material _{, CO and HC acting as NO X} reducing agents are insufficient, and the NO _X reduction reaction is unlikely to occur. There is concern that it will become. Therefore, if the OSC material is supported on the LNT catalyst, there is a concern that the reduction rate of _{NO X will decrease.}

As a countermeasure, when simply removing the OSC material from LNT catalyst, since when the exhaust air-fuel ratio by the rich control becomes slightly richer closer than the stoichiometric air-fuel ratio, the O ₂ in the exhaust eliminated, as NO _X reducing agent There is concern that excess HC and CO that did not work cannot be oxidized (purified).

Therefore, in the exhaust treatment device 1 according to the present embodiment, as shown in FIG. 1, the OSC material 31 for the purpose of purifying HC and CO is placed on the exhaust downstream side of the LNT catalyst 20 instead of the LNT catalyst 20. It is supported on the filter 30 to be arranged. By excluding the OSC material for the purpose of purifying HC and CO from the LNT catalyst 20 in this way, the exhaust air-fuel ratio can be quickly switched by the enrichment control. Further, since the OSC material 31 is supported on the filter 30 arranged on the downstream side of the exhaust gas from the LNT catalyst 20, even when the exhaust air-fuel ratio becomes slightly richer than the theoretical air-fuel ratio due to the enrichment control. _{Excess HC and CO that did not act as NO X} reducing agents in the LNT catalyst 20 can be appropriately oxidized (purified) by _{O 2} released from the OSC material 31 supported on the filter 30.

FIG. 5 is a diagram showing changes in _{the O 2} concentration around the LNT catalyst 20 due to enrichment control. In FIG. 5, the horizontal axis represents time and the vertical axis represents O ₂ concentration. _{In FIG. 5, the solid line shows the O 2} concentration when the LNT catalyst 20 (in the present embodiment) in which the OSC material is excluded is used, and the broken line shows the LNT catalyst in which the OSC material is supported (comparative example). The O ₂ concentration in the case is shown.

Time from time t0 that initiated the enrichment control until the time the O ₂ concentration is minimized corresponds to the switching time of the exhaust air-fuel ratio by the rich control. From FIG. 5, when the LNT catalyst 20 from which the OSC material is excluded is used (solid line), the switching time of the exhaust air-fuel ratio by the enrichment control is shorter than when the LNT catalyst on which the OSC material is supported is used (broken line). It can be understood that it will be done.

FIG. 6 is a diagram showing changes in the exhaust air-fuel ratio around the LNT catalyst 20 and the NO _{X concentration occluded in the LNT catalyst 20 by enrichment control.} 6, the horizontal axis represents time and the vertical axis from the top, showing the NO _X concentration which is stored in LNT catalyst 20 near the exhaust air-fuel ratio and LNT catalyst 20,. 6, the solid line indicates the air-fuel ratio and concentration of NO _X in the case of using the LNT catalyst 20 OSC material is eliminated (the present embodiment), LNT catalyst dashed line the OSC material is supported (Comparative Example) The air-fuel ratio and NO _X concentration when used are shown. In FIG. 6, the period from the time t0 to the time t1 is the period during which the enrichment control is executed.

From FIG. 6, when the LNT catalyst 20 from which the OSC material is excluded is used (solid line), the exhaust air-fuel ratio is set earlier by the enrichment control than when the LNT catalyst on which the OSC material is supported is used (broken line). It can be understood that the air-fuel ratio has been switched to the vicinity of the theoretical air-fuel ratio. Further, when the LNT catalyst 20 from which the OSC material is excluded is used (solid line), the NO stored in the LNT catalyst 20 is occluded by the enrichment control as compared with the case where the LNT catalyst on which the OSC material is supported (broken line) is used. _It can be understood that the X concentration is further lowered and _{the reduction rate of NO X is further improved.}

As described above, the exhaust treatment device 1 according to the present embodiment includes the LNT catalyst 20 (first exhaust purification device) provided in the exhaust passage 12 of the engine 10 and a portion downstream of the LNT catalyst 20 in the exhaust passage 12. A filter 30 (second exhaust gas purification device) provided in the above is provided. _{The LNT catalyst 20 has a function of occluding NO X} in the exhaust when the exhaust air-fuel ratio is lean, _{and releasing NO X} when the exhaust air-fuel ratio is the stoichiometric air-fuel ratio or rich for reduction and purification. The filter 30 has a function of collecting PM in the exhaust gas.

In the exhaust treatment device 1 according to the present embodiment, the OSC material for purifying HC and CO is supported not on the LNT catalyst 20 but on the filter 30 arranged on the exhaust downstream side of the LNT catalyst 20. By excluding the OSC material for the purpose of purifying HC and CO from the LNT catalyst 20 in this way, the exhaust air-fuel ratio can be quickly switched by the enrichment control. Further, since the OSC material is supported on the filter 30 arranged on the downstream side of the exhaust gas from the LNT catalyst 20, even when the exhaust air-fuel ratio becomes slightly richer than the theoretical air-fuel ratio due to the enrichment control, the LNT _{Excess HC and CO that did not act as NO X} reducing agents in the catalyst 20 can be appropriately oxidized (purified) by _{O 2} released from the OSC material 31 supported on the filter 30. As a result, the exhaust treatment device 1 according to the present embodiment can appropriately reduce the _{NO X} occluded in the LNT catalyst 20 while appropriately purifying the HC and CO in the exhaust.

In the present embodiment, the OSC material is not supported on the LNT catalyst 20, but some OSC material may be supported on the LNT catalyst 20 as well. For example, if the pressure loss of the filter 30 is excessively increased when all the amount of OSC material required for purification of HC and CO is supported on the filter 30, the switching of the exhaust air-fuel ratio by the enrichment control is extremely high. A small amount of OSC material may be supported on the LNT catalyst 20 as long as it does not slow down. However, even in this case, it is desirable that the amount of the OSC material contained in the filter 30 is larger than the amount of the OSC material contained in the LNT catalyst 20.

<On / off processing during enrichment control>
During the execution of the enrichment control, the ECU 100 does not always add fuel by the fuel addition valve 60, but stops the fuel addition by the fuel addition valve 60 after performing the fuel addition by the fuel addition valve 60 for a predetermined on-time ton. By repeating the process of turning off for a predetermined off time (hereinafter, also referred to as “on / off process”), the average value of the exhaust air-fuel ratio during the execution period of the enrichment control is brought close to the theoretical air-fuel ratio.

In a 4-stroke diesel engine such as the engine 10, four steps of a suction stroke, a compression stroke, an expansion (combustion) stroke, and an exhaust stroke are performed in this order in one cycle of each cylinder. During one cycle, the piston of each cylinder reciprocates twice in the cylinder, so that the crankshaft of the engine 10 makes two rotations (the crank angle advances by 720 °). In the case of a 4-cylinder engine, the cycle of each cylinder is configured every 180 ° (= 720 ° / 4 cylinders) in terms of crank angle. Hereinafter, a case where the expansion steps of the 4-cylinder engine are performed in the order of # 1, # 2, # 3, # 4 will be described.

If the on / off process is performed once per cycle (while the rotation shaft of the engine 10 rotates twice) during the enrichment control, the O ₂ is stored in the OSC material 31 supported on the filter 30. Is not sufficiently secured, and it is assumed that HC and CO are not properly purified.

FIG. 7 shows the time Tonoff (the sum of one on-time Ton and off-time Tof) of one on-off process during the enrichment control, and one cycle time TS (the period during which the crankshaft of the engine 10 rotates twice). ) Is set to be a diagram schematically showing the state of _{O 2} release and O ₂ storage by the OSC material 31 of the filter 30. In FIG. 7, the horizontal axis represents the crank angle CA (corresponding to time). In FIG. 7, in one cycle time TS (crank angle 0 ° to 720 °), the crank 0 ° to 180 ° at which the expansion stroke of the cylinder # 1 is performed is set to the on time Ton, and the other period is set to the off time Tof. .. Note that FIG. 7 is a diagram showing only an example of on / off processing according to a comparative example with respect to the present embodiment.

In the comparative example shown in FIG. 7, the on / off process is performed once in one cycle time TS. In this case, the off-time Toff in which the air-fuel ratio becomes lean and O ₂ is stored in the OSC material 31 is secured only once in one cycle time TS. As a result, there is a concern that a sufficient amount of oxygen cannot be released from the OSC material 31 in the on-time ton where the air-fuel ratio becomes rich and the emissions of HC and CO increase.

Further, the on-time Ton is also secured only once in one cycle time TS. Therefore, the amount of fuel required to bring the average value of the air-fuel ratio in one cycle time TS close to the theoretical air-fuel ratio (hereinafter, also referred to as "required fuel amount FS") is all injected in one on-time ton. Therefore, the amount of fuel injected in one on-time ton increases, and a large amount of HC and CO are discharged in a short time accordingly. In particular, in the comparative example shown in FIG. 7, since the on-time ton is shorter than the off-time ton, the required fuel amount FS is injected in a shorter time, and more HC and CO are discharged in a shorter time. It will be. _{On the other hand, the capacity of O 2 that} can be stored in the OSC material 31 of the filter 30 is fixed, and even if one off-time Toff is lengthened, one off-time Toff _{can store more O 2 in the OSC material 31 than the capacity.} It cannot be stored. Therefore, in the comparative example shown in FIG. 7, an amount of oxygen sufficient to oxidize HC and CO supplied to the filter 30 in one on-time ton is stored in the OSC material 31 in one off-time ton. It's difficult to keep.

Therefore, in the ECU 100 according to the present embodiment, one on / off processing time Tonoff (total of one on time Ton and off time Toff) is one cycle time TS (rotation of the engine 10) during execution of enrichment control. The fuel addition valve 60 is controlled so as to be shorter than the time required for the shaft to make two rotations.

_{FIG. 8 shows the O 2} concentration around the LNT catalyst 20 and the vicinity of the OSC material 31 of the filter 30 when the one-time on / off processing time Tonoff is set to half of the one cycle time TS during execution of the enrichment control. the O ₂ concentration is a diagram schematically illustrating. In FIG. 8, the horizontal axis represents the crank angle CA (corresponding to time). Note that FIG. 8 is a diagram showing an example of on / off processing performed by the ECU 100 during the enrichment control according to the present embodiment.

In the example shown in FIG. 8, one on / off processing time Tonoff is set to half of one cycle time TS (corresponding to a crank angle of 360 °). Specifically, in the one cycle time TS, the period during which the expansion stroke of cylinder # 1 is performed (the period from the crank angle 0 ° to 180 °) is set as the on-time Ton, and the expansion stroke of the next cylinder # 2 is performed. The period (the period from the crank angle of 180 ° to 360 °) is set as the off time Toff, and the period during which the expansion stroke of the next cylinder # 3 is performed (the period from the crank angle of 360 ° to 540 °) is the on time. The off time is set to Ton, and the period during which the next expansion stroke of cylinder # 4 is performed (the period from the crank angle of 540 ° to 720 °) is set to Off. That is, two on / off processes are performed in one cycle time TS.

In this case, the air-fuel ratio becomes lean and the _{off-time Toff in which O 2} is stored in the OSC material 31 is secured twice in one cycle, and the OSC material 31 is compared with the comparative example shown in FIG. The opportunity for O _{2 to} be stored is secured once more. Further, in one cycle time TS, the required fuel amount FS can be dispersed and injected in two on-time tons. Therefore, as compared with the comparative example shown in FIG. 7, the amount of fuel injected in one on-time ton can be reduced, and the amount of HC and CO discharged in one on-time ton is correspondingly reduced. Can be reduced. In particular, in the example shown in FIG. 8, the on-time Ton and the off-time Toff are substantially equal. As a result, the amount of fuel injected in one on-time ton can be reduced as compared with the case where the on-time ton is shorter than the off-time ton, and the fuel is discharged in one on-time ton by that amount. The amount of HC and CO produced can be reduced. Therefore, in the example shown in FIG. 8, an amount of oxygen sufficient to oxidize HC and CO supplied to the filter 30 in one on-time ton can be easily stored in the OSC material 31 in one off-time ton. can do.

As a result of the above, as shown in FIG. 8, in the filter 30, the O _{2 storage by the OSC material 31 at one on-time Ton and the O 2} release by the OSC material 31 at one off-time Ton are balanced. It can be done well and can properly purify HC and CO. _{The O 2} concentration in the filter 30 is stabilized at a value corresponding to the stoichiometric air-fuel ratio by the action of _{O 2} storage and O ₂ release by the OSC material 31.

On the other hand, in the LNT catalyst 20, as described above, since there is no OSC material _{and the action of O 2} storage and O ₂ release by the OSC material does not occur, the rich state at the on-time Ton and the lean state at the off-time Tof Can be switched quickly.

FIG. 9 is a flowchart showing an example of a processing procedure performed when the ECU 100 executes the enrichment control. This flowchart is repeatedly executed every time a predetermined condition is satisfied (for example, every predetermined cycle).

First, the ECU 100 determines whether or not the enrichment control is being executed (step S10).

When the enrichment control is not being executed (NO in step S10), that is, when the engine 10 is in the normal operation state and the exhaust air-fuel ratio is lean, the ECU 100 satisfies the predetermined enrichment control execution conditions. It is determined whether or not the engine is used (step S12). For example, the ECU 100 determines that the execution condition of the enrichment control is satisfied when the elapsed time since the end of the previous enrichment control exceeds a predetermined time. When the execution condition of the enrichment control is not satisfied (NO in step S12), the ECU 100 shifts the process to the return without executing the enrichment control.

When the execution condition of the enrichment control is satisfied (YES in step S12), the ECU 100 executes the enrichment control described above (step S14). That is, as described above, the ECU 100 repeats the "on-off process" in which the fuel addition by the fuel addition valve 60 is performed on-time ton and then the fuel addition by the fuel addition valve 60 is stopped for off-time ton, thereby performing enrichment control. Bring the average value of the exhaust air-fuel ratio during execution close to the theoretical air-fuel ratio. At this time, as shown in FIG. 8, the ECU 100 sets one on-off processing time Tonoff to half of one cycle time TS, and further sets the on-time Ton and the off-time Toff to substantially the same time (on-off processing time Tonoff). Set to half).

On the other hand, when the enrichment control is being executed (YES in step S10), the ECU 100 determines whether or not a predetermined enrichment control end condition is satisfied (step S16). For example, the ECU 100 determines that the end condition of the enrichment control is satisfied when the elapsed time from the start of the enrichment control this time exceeds a predetermined time. When the end condition of the enrichment control is not satisfied (NO in step S16), the ECU 100 shifts the process to step S14 and continues the execution of the enrichment control. On the other hand, when the end condition of the enrichment control is satisfied (YES in step S16), the ECU 100 ends the enrichment control (step S18).

As described above, the ECU 100 repeats the "on-off process" in which the fuel addition by the fuel addition valve 60 is toned for the on-time and then the fuel addition by the fuel addition valve 60 is stopped for the off-time ton during the execution of the enrichment control. By doing so, the average value of the exhaust air-fuel ratio during the execution of the enrichment control is brought close to the theoretical air-fuel ratio. At this time, the ECU 100 sets the one on / off processing time Tonoff to half of the one cycle time TS, and further sets the on time Ton and the off time Toff to substantially equal times. As a result, the air-fuel ratio becomes lean and the _{off-time Toff in which O 2} is stored in the OSC material 31 is secured twice in one cycle time TS, and the HC and CO discharged in one on-time ton. The amount is reduced. Therefore, the amount of oxygen sufficient to oxidize the HC and CO supplied to the filter 30 in one on-time ton can be easily stored in the OSC material 31 in one off-time ton.

<Modification example of on / off processing time Tonoff>
In FIG. 8 above, an example in which the on / off processing time Tonoff is set to half of the one cycle time TS (corresponding to a crank angle of 360 °) is shown, but the on / off processing time Tonoff is at least one cycle time TS (rotating shaft of the engine 10). The value may be shorter than the time required for two rotations, that is, equivalent to a crank angle of 720 °), and is not necessarily limited to half of the one cycle time TS.

FIG. 10 is a diagram showing a modified example of the on / off processing time Tonoff. As shown in FIG. 10, the on / off processing time Tonoff may be shorter than the crank angle 360 ° (for example, equivalent to the crank angle 240 ° or 180 °), or the on / off processing time Tonoff may be longer than the crank angle 360 °. It may be time (for example, equivalent to a crank angle of 480 ° or 540 °).

In the modified example shown in FIG. 10, the on-time Ton and the off-period To are set to substantially equal times, but the on-time Ton and the off-period To are not necessarily substantially equal to each other. For example, the on-time Ton may be shorter than the off-time To.

It should be considered that the embodiments disclosed this time are exemplary in all respects and not restrictive. The scope of the present disclosure is indicated by the scope of claims rather than the above description, and is intended to include all modifications within the meaning and scope equivalent to the scope of claims.

1 Exhaust treatment device, 10 Engine, 11 Intake passage, 11a Intake manifold, 12 Exhaust passage, 12a Exhaust manifold, 15 Fuel injection valve, 20 LNT catalyst, 21 Support layer, 22 Noble metal, 23 NO _X storage material, 30 Filter, 31 OSC material, 50 engine rotation speed sensor, 60 fuel addition valve, 100 ECU.

Claims (5)

It is an engine exhaust treatment device
The first exhaust purification device provided in the exhaust passage of the engine and
A second exhaust gas purification device provided in a portion downstream of the first exhaust purification device in the exhaust passage is provided.
The first exhaust gas purification device absorbs nitrogen oxides in the exhaust when the exhaust air-fuel ratio is lean, and releases and reduces the nitrogen oxides when the exhaust air-fuel ratio is the stoichiometric air-fuel ratio or rich. _{Contains a NO X} storage catalyst that has a purifying function,
The second exhaust gas purification device _{does not contain the NO X} storage catalyst and contains an oxygen storage material having a function of storing and releasing oxygen.
An engine exhaust treatment device in which the amount of the oxygen storage material contained in the second exhaust gas purification device is larger than the amount of the oxygen storage material contained in the first exhaust gas purification device. Said first exhaust gas purification unit, the a the the NO _X storing catalyst containing no oxygen storage material, the exhaust treatment device for an engine according to claim 1. The exhaust gas treatment device for an engine according to claim 1 or 2, wherein the second exhaust gas purification device is a filter that collects particulate matter in the exhaust on which the oxygen storage material is supported. The exhaust treatment device is
A fuel supply device that supplies fuel to the exhaust passage and
When the predetermined execution conditions are satisfied while the exhaust air-fuel ratio is lean, the exhaust air-fuel ratio is adjusted by repeating the on / off process of stopping the fuel supply after the fuel is supplied by the fuel supply device. It is equipped with a control device that executes control that approaches the stoichiometric air-fuel ratio as enrichment control.
The control device controls the fuel supply device so that the time of one on / off process is shorter than the time of two rotations of the rotation shaft of the engine during the execution of the enrichment control. The engine exhaust treatment device according to any one of. The engine according to claim 4, wherein the control device controls the fuel supply device so that the time for supplying fuel by the fuel supply device and the time for stopping the fuel supply are substantially equal in the on / off process. Exhaust treatment equipment.

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