JP2004092497A - Exhaust emission control device of diesel engine - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an exhaust gas clean-up device of a diesel engine which can preferably regenerate a particulate filter with an NOx occlusion catalyst, without changing the main combustion of the engine. <P>SOLUTION: In the exhaust gas clean-up device of the invention, a CO generation catalyst 19 which converts HC in the exhaust gas into CO is provided on the inflow side of the particulate filter 18 with an NOx occlusion catalyst, and a burner 22 which raises the temperature of the exhaust gas is provided on the inflow side of the CO generation catalyst 19. Further, an HC addition injector 23 which adds HC in the exhaust gas is provided on the upstream side from the CO generation catalyst 19. When regeneration is demanded, only by controlling the burner 22 and the HC addition injector 23 by an ECU 24 without changing the main combustion of the engine, the occlusion NOx of the NOx occlusion catalyst is designed to be reduced by utilizing the resulting CO. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a diesel engine exhaust purification device that purifies exhaust gas of a diesel engine using a particulate filter with a NOx storage catalyst.
[0002]
[Prior art]
Exhaust gas emitted from diesel engines installed in many trucks and buses contains particulates mainly composed of carbon particles (exhaust particulates: particulate matter; hereinafter, referred to as PM). I have. Further, since a diesel engine burns in a situation with a large amount of air, NOx is likely to be generated due to excess oxygen as in the case of lean burn combustion of a gasoline engine.
[0003]
Therefore, purifying the exhaust gas of a diesel engine requires both removal of PM and purification of NOx.
[0004]
Therefore, in a diesel engine, a particulate filter with a NOx storage catalyst carrying a NOx storage catalyst is provided in the exhaust passage, PM contained in the exhaust gas is removed by the particulate filter, and NOx contained in the exhaust gas is removed. The removal of oxygen by the storage function of a NOx storage catalyst is performed using an atmosphere in which oxygen is excessive.
[0005]
By the way, the particulate filter with a NOx storage catalyst must be regenerated to recover its function when the amount of stored NOx and the amount of deposited PM reach a predetermined amount. In this regeneration, the NOx released from the NOx storage catalyst is reduced by utilizing the characteristic of releasing the stored NOx in an atmosphere in which the oxygen concentration is reduced, which is a characteristic property of the NOx storage catalyst, that is, in a reducing atmosphere. It is purified by reacting (reducing) with HC and CO in the atmosphere. Then, by using the NO ₂ and the heat generated during regeneration of the NOx storage catalyst, oxidizing the PM which is deposited in the particulate filter.
[0006]
Therefore, in order to regenerate the NOx storage catalyst, for example, as shown in Japanese Patent Application Laid-Open No. 2000-186531, the NOx storage catalyst is installed in the exhaust passage of the engine by using the combustion gas of a heating combustion heater mounted on a truck. There has been proposed a structure for regenerating a NOx storage catalyst. This uses a structure in which the exhaust gas of the heating combustion heater is guided to the inlet side of the NOx storage catalyst, and when the NOx storage catalyst is regenerated, the combustion heater controls the air-fuel ratio to the rich side to burn the fuel, This is a technique for supplying this combustion gas to the NOx storage catalyst.
[0007]
[Problems to be solved by the invention]
By the way, in order to regenerate the NOx storage catalyst, it is required that the temperature of the NOx storage catalyst is raised to the activation temperature and that the atmosphere is such that HC and CO reduce the stored NOx.
[0008]
However, since the combustion heater is used for heating, in winter when heating is required, much of the heat of the combustion heater is consumed for heating the vehicle interior through the heat exchanger, so that the amount of heat of the combustion gas is small. Further, even in the summer time when heating is not required, it is difficult to sufficiently warm the NOx storage catalyst by using the combustion heat of the combustion heater because combustion heat is taken from the heat exchanger for heating.
[0009]
In addition, since the main combustion of the diesel engine is always operated with excess oxygen (air), HC and CO may not be used as NOx storage catalysts by changing the air-fuel ratio of the combustion heater to make the combustion rich. It is not possible to secure a reducing atmosphere that can be supplied sufficiently. Particularly, the main combustion hardly becomes rich even if the combustion gas is recirculated by the EGR.
[0010]
For this reason, it is difficult to regenerate the NOx storage catalyst with a structure using a combustion-type heater for heating. Therefore, in order to regenerate the NOx storage catalyst, it is necessary to forcibly change the main combustion of the diesel engine to the rich side. However, the change of the main combustion causes a large torque fluctuation and has a disadvantage of generating a shock. Moreover, smoke (black smoke) may be generated.
[0011]
Therefore, an object of the present invention is to provide an exhaust gas purifying apparatus for a diesel engine that can satisfactorily regenerate a particulate filter with a NOx storage catalyst without changing the main combustion of the engine.
[0012]
[Means for Solving the Problems]
In order to achieve the above object, according to the first aspect of the present invention, a CO generation catalyst for converting HC in exhaust gas to CO is provided on the inflow side of the particulate filter with a NOx storage catalyst. A burner for increasing the temperature of the exhaust gas of the engine is provided on the side, and an HC adding means for adding HC to the exhaust gas is provided at a portion of the exhaust passage on the upstream side from the CO generation catalyst, and the burner and the HC adding means are provided. A control unit is provided for controlling the burner and the HC addition means to form a reducing environment in which the NOx stored in the NOx storage catalyst is reduced by CO from the CO generation catalyst when the regeneration process of the NOx storage catalyst is requested. Configuration.
[0013]
With this configuration, when the regeneration process of the particulate filter with the NOx storage catalyst is requested, the catalyst activation temperature required for regeneration is controlled by the operation of the burner, and the amount of HC added required for regeneration is controlled by the operation of the HC addition means. Controlled. That is, the control of the system independent of the main combustion of the diesel engine reduces the NOx stored in the NOx storage catalyst and oxidizes the particulates collected by the particulate filter.
[0014]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, the present invention will be described based on one embodiment shown in FIGS.
[0015]
In FIG. 1, reference numeral 1 denotes an engine body of a traveling diesel engine mounted on a vehicle such as a truck or a bus. The engine body 1 includes, for example, four cylinders 2a to 2d arranged in series. A piston (not shown) is reciprocally housed in each of the cylinders 2a to 2d. In addition, an injector (not shown), intake ports 3a to 3d, exhaust ports 4a to 4d, and intake / exhaust valves (not shown) are provided for each cylinder in a cylinder head (not shown) above each of the cylinders 2a to 2d. It is. A fuel supply device (not shown) is connected to each injector. Thus, the intake / exhaust valves of the cylinders 2a to 2d are opened and closed at a predetermined timing, and the fuel from the fuel supply device is injected from the injectors to the cylinders 2a to 2d at a predetermined timing at a predetermined timing. A predetermined cycle, for example, a cycle of intake, compression, explosion, and exhaust is repeated in each of the cylinders 2a to 2d.
[0016]
An intake manifold 5 is connected to inlets of the intake ports 3a to 3d opened on one side of the engine body 1. The intake manifold 5 is connected to an intake pipe 8 in which an air-cooled intercooler 6 and a compressor section 7a of a turbocharger 7 are installed in the middle, and the intake manifold 8 is used to burn each cylinder 2a to 2d through the intake pipe 8. The air can be introduced. An exhaust manifold 9 is connected to outlets of the exhaust ports 4a to 4d which are open on the other side. The exhaust manifold 9 is connected to an exhaust pipe 10 in which a turbine portion 7b of the turbocharger 7 is installed in the middle, so that exhaust gas from each of the cylinders 2a to 2d that have finished burning can be exhausted to the outside. Like that. The passage from the exhaust ports 4a to 4d to the end of the exhaust pipe 10 is referred to as an exhaust passage in the present embodiment.
[0017]
At the inlet of the intake manifold 5, for example, a solenoid-driven intake throttle valve 11 is installed. Further, for example, an EGR device 13 is provided between the intake manifold 5 and the exhaust manifold 9 so that an EGR passage 13b having an EGR valve 13a bypasses the two.
[0018]
At the rear of the exhaust pipe 10 of such a diesel engine, a main body 15 of the exhaust purification device is provided.
[0019]
The main body 15 will be described. For example, the main body 15 has a cylindrical casing 16 whose both ends are narrowed and whose center is formed to have a larger diameter than the exhaust pipe 10. An inflow port 16a formed at one end of the casing 16 is connected to an exhaust pipe portion extending from the exhaust manifold 9, and an outflow port 16b formed at the other end of the casing 16 opens to the atmosphere. Connected to the exhaust pipe section.
[0020]
In the casing 16, a particulate filter with a NOx storage catalyst (DPF carrying a NOx storage catalyst) 18 is accommodated. The particulate filter 18 has, for example, a filter portion 18a having a polygonal cross section partitioned by a porous partition wall through which exhaust gas can pass and particulates cannot pass through, and formed by the partition wall of the filter portion 18a. A structure is used in which adjacent inlets and outlets of a large number of through holes are alternately sealed with plugs. Thus, when the exhaust gas passes through the partition, particulates (hereinafter, referred to as PM) contained in the exhaust gas are collected. A NOx storage catalyst is carried on the wall surface of the filter portion 18a, and has a structure capable of removing NOx contained in exhaust gas.
[0021]
Further, in the casing 16, a CO generation catalyst 19 is accommodated at an upstream side of the particulate filter 18 with a NOx storage catalyst. The CO generation catalyst 19 is a catalyst having a function of converting HC contained in the exhaust gas into CO. When the particulate filter 18 with the NOx storage catalyst is regenerated, CO that easily reacts with NOx is generated by the CO generation catalyst 19 by utilizing the addition of HC (reducing agent) described later.
[0022]
A temperature sensor 20a for detecting the temperature of the exhaust gas flowing into the catalyst, an A / F sensor 20b for detecting the air-fuel ratio of the exhaust gas flowing into the catalyst, are provided on the inflow side of the particulate filter 18 with a NOx storage catalyst. A pressure sensor 20c for detecting a differential pressure between the entrance and exit of the particulate filter 18 is provided. A NOx sensor 20d for detecting a NOx concentration is provided near the outlet 16b.
[0023]
On the other hand, a combustion type burner 22 (corresponding to the burner of the present application) is mounted near the upstream side of the CO generation catalyst 19, for example, in the exhaust pipe near the inflow port 16a. This combustion type burner 22 is a dedicated device used only to raise the temperature of exhaust gas for catalytic activity. For this purpose, the combustion type burner 22, for example, connects a flame generating combustion head 22a at the tip thereof to the exhaust pipe portion obliquely from the upstream side, and transfers high-temperature exhaust gas generated by burner combustion downstream of the exhaust pipe 10. It has a structure that can be sent to the side.
The combustion head 22 a of the combustion type burner 22 is separated from the CO generation catalyst 19 by interposing a short pipe 10 a between the exhaust pipe 10 and the combustion head 22 a so that the flame does not touch the CO generation catalyst 19.
[0024]
A light oil addition injector 23 is provided as an HC addition means in an exhaust passage upstream of the CO generation catalyst 19, for example, in an exhaust pipe portion upstream of the combustion burner 22. The light oil-added injector 23 has a function of injecting, for example, light oil (HC) as a reducing agent into the exhaust pipe 10. By adding light oil to the exhaust gas, the exhaust gas is forced into a reducing atmosphere. The exhaust gas having the increased HC concentration is supplied to the CO generation catalyst 19, so that a large amount of CO that easily reacts with NOx is stored using the function of converting the HC of the CO generation catalyst 19 into CO. It can be sent to the catalyst.
[0025]
On the other hand, 24 is an ECU (for example, constituted by a microcomputer) as control means. The ECU 24 is connected to the intake throttle valve 11, the EGR valve 13a, various sensors 20a to 20d, and the combustion type burner 22. The ECU 24 is provided with a function for regenerating the particulate filter 18 with the NOx storage catalyst as shown in the block diagram of FIG. That is,
A function of shifting from the normal operation mode in which NOx is stored and PM is collected to the NOx release / reduction mode or the S-poisoning regeneration mode in accordance with the travel time and travel distance of the vehicle, the detection signal of the NOx sensor 21, and the like.
[0026]
In the NOx release / reduction mode, the temperature of the exhaust gas is raised by the operation of the combustion type burner 22 to activate the NOx storage catalyst and the CO generation catalyst 19 at an activation temperature suitable for the mode (for example, a temperature near 300 ° C.). And a control to change the A / F (air-fuel ratio) of the exhaust gas to the rich side by the operation of the light oil addition injector 23 to release the stored NOx with CO from the CO generation catalyst 19. The ability to create an environment for
[0027]
In the S-poisoning regeneration mode, the temperature of the exhaust gas is increased by the operation of the combustion type burner 22 to activate the NOx storage catalyst and the CO generation catalyst 19 at an activation temperature suitable for the mode (for example, about 600 ° C. to 650 ° C.). And the control to change the A / F of the exhaust gas to the rich side by the operation of the light oil-added injector 23. Similarly, by the reduction reaction using CO from the CO generation catalyst 19, the S poisoning is performed. A function of forming an environment for releasing S (sulfur) from the NOx storage catalyst of the present invention.
[0028]
Is set. Note that both modes, from the viewpoint of forming an effective reducing environment, by operating the intake throttle valve 11 and EGR valve 13a, in a range that does not cause torque fluctuation and smoke emissions, reduced as much as possible O ₂ in the exhaust gas It is desirable to make it.
[0029]
In addition, the ECU 24 also includes
When the detection signal of the pressure sensor 20 detects that excessive PM has accumulated, a warning light 25 (warning means) connected to the ECU 24 informs the driver of the fact, and forcibly regenerates the PM. A function for prompting the operation of the forced regeneration switch 26 for starting the mode.
[0030]
A function of executing the PM forced regeneration mode when the vehicle is idling when the forced regeneration switch 26 is operated.
[0031]
A function of raising the temperature of the exhaust gas to a temperature suitable for oxidizing the deposited PM (for example, about 600 ° C. to 650 ° C.) by operating the combustion burner 22 in the PM forced regeneration mode.
[0032]
Is set. In this case, in order to effectively oxidize PM, idling is set to operate lean.
[0033]
The detailed control contents of each mode are shown in the flowcharts of FIGS. The purifying action of the exhaust gas purifying apparatus will be described with reference to the flowcharts of FIGS. 3 to 5. Now, under the control of the ECU 24, the intake throttle valve 11 is opened, the EGR valve 13 a is closed, and the diesel engine responds to the operating state. It is assumed that the engine is operated at a predetermined injection timing and a fuel injection amount so that a required torque is generated.
[0034]
Then, the exhaust gas discharged from the diesel engine is introduced into the particulate filter 18 with the NOx storage catalyst through the exhaust manifold 9, the turbine section 7b of the turbocharger 7, the inlet 16a of the casing 16, and the CO generation catalyst 19. You.
[0035]
Here, the main combustion of the diesel engine is always operated with excess oxygen (air), so that the A / F of the exhaust gas is lean (oxidizing atmosphere).
[0036]
Therefore, the exhaust emission control device operates in the normal operation mode. That is, while the exhaust gas passes from the inlet to the outlet of the particulate filter 18 with the NOx storage catalyst, PM contained in the exhaust gas is collected (filtered) by the partition walls forming the through holes.
At the same time, NOx contained in the exhaust gas is stored in the NOx storage catalyst. This storage is oxygen attached to the surface of the NOx storage catalyst, is conducted by the oxygen is diffused react to become NO ₂ and NO in the exhaust gas, the final NOx absorbent in the form of nitrate ions .
[0037]
When the removal of NOx in the exhaust gas proceeds and the ECU 24 determines that the NOx has reached a certain amount of occlusion and accumulation based on the traveling time and traveling distance of the vehicle, the detection signal of the NOx sensor 21, and the like, the ECU 24 in FIG. As shown in step S1, the flag indicating that the NOx release / reduction is performed is turned on, and the NOx release / reduction mode shown in FIG. 3 is executed.
[0038]
This NOx release / reduction mode is performed by setting the exhaust gas to a reducing atmosphere in which the activation temperature of the catalyst required for the reaction and the amount of the reducing agent required for the reduction reaction are secured.
[0039]
That is, as shown in steps S2 to S4 in FIG. 3, first, for example, a ratio of less than 60% to the rated engine speed and a ratio of 90% to the full load of the engine torque are set so that unnecessary load is not applied to the catalyst. %, The combustion type burner 22 is ignited, and the burner 22 is operated at λ = 1, A / F = 14.6 (ideal mixture ratio) and at full load. Then, the high-temperature exhaust gas of the burner 22 is sent into the exhaust pipe 10 and mixed with the exhaust gas of the diesel engine (mixed exhaust) until the temperature of the exhaust gas reaches the activation temperature of the CO generation catalyst 19 and the NOx storage catalyst. To raise. When the activation temperature is secured, control is performed to change the exhaust gas to a rich side, for example, less than the ideal air-fuel ratio (A / F <14.6), as shown in steps S5 to S8. This determines the assist amount (HC amount) of light oil required to enrich the exhaust gas of the engine operating according to the current required torque (for example, the engine exhaust flow rate, burner flow rate, efficiency of the CO generating catalyst, etc.). In accordance with this calculated value, the light oil addition injector 23 is operated to spray light oil into the exhaust passage, here the exhaust pipe portion on the upstream side of the combustion type burner 22. By this light oil spray, the exhaust gas becomes λ <1, and the HC concentration increases. This rich exhaust gas is introduced into the CO generation catalyst 19. Then, while passing through the CO catalyst 19, HC in the exhaust gas is converted into CO that easily undergoes a reduction reaction with NOx, and this CO is introduced into the particulate filter 18 with a NOx storage catalyst.
[0040]
Since the oxygen concentration decreases, NO ₂ (NOx) stored in the NOx storage catalyst is released. This NO ₂ reacts with CO in the exhaust gas and is reduced to N ₂ and CO ₂ to purify NOx in the exhaust gas (regeneration of the NOx storage catalyst). The filter portion 18a is preheated by the reaction heat. Further, the NO ₂ released from the NOx storage catalyst oxidizes with the PM mainly composed of carbon trapped by the filter portion 18a to oxidize and raise the preheated PM to the ignition temperature, thereby reducing the PM. Combustion (oxidation reaction).
[0041]
Thereby, regeneration of the NOx storage catalyst and the filter portion 18a is performed. When the regeneration time exceeds a predetermined time, for example, 4 s, the operation of the combustion type burner 22 is stopped, the operation of the light oil addition injector 23 is stopped, the NOx release reduction flag is turned off, the regeneration is completed, and the normal operation mode is set. To return.
[0042]
On the other hand, if it is determined that the NOx storage catalyst needs to be regenerated for S poisoning based on the traveling time or the traveling distance of the vehicle, the regeneration of S poisoning is performed as shown in step S13 in FIG. The flag is turned on, and the S poisoning regeneration mode shown in FIG. 4 is executed.
[0043]
This S poisoning regeneration mode secures the temperature of the catalyst required for regeneration and the amount of reducing agent required for the reduction reaction, and releases S poisoning from the NOx storage catalyst, as in the previous NOx release and reduction mode. This is performed by reducing S with CO in exhaust gas. However, the flowchart of the S-poisoning regeneration mode shown in FIG. 4 is different from the NOx emission reduction mode in that the temperature of the exhaust gas is raised to about 600 ° C. (step S22) and the regeneration time is set to 10 s (step S28). The only difference is that the flag for ending the S poisoning regeneration is turned off (step S30). For this reason, in the flowchart of FIG. 4, the same parts as those of FIG.
[0044]
Note that the ECU 24 determines from the differential pressure across the filter portion 18a detected by the pressure sensor 20c that continuous regeneration does not proceed, that is, the PM deposition on the particulate filter portion is faster than the regeneration speed, and excessive PM If it is determined that the PM has accumulated, the PM forced regeneration mode shown in FIG. 5 is executed. That is, the ECU 24 turns on, for example, the warning light 25 in step S39 in accordance with the detection value of the pressure sensor 20, and prompts the operation of the forced regeneration switch 26. When the forced regeneration switch 26 is turned on, when the engine is in the idling state (lean) as shown in steps S40 to S45, the combustion type burner 22 is operated, and the temperature suitable for the oxidation (combustion) of PM (for example, The temperature of the exhaust gas of the engine is raised to about 600 ° C.) to forcibly burn the PM deposited on the filter portion 18a. As a result, regeneration of excessively deposited PM is performed. Then, when a predetermined regeneration time (for example, 10S) elapses as shown in steps S46 to S50, the operation of the combustion type burner 22 is stopped, the PM forced regeneration flag is turned off, and the operation returns to the normal operation mode.
[0045]
As described above, the regeneration of the NOx storage catalyst and the regeneration of the filter portion 18a are both controlled by a system independent of the main combustion of the diesel engine, that is, by controlling the catalyst activation temperature by the combustion type burner 22, and by adding the HC by the light oil addition injector 23. Control of the amount can be performed sufficiently.
[0046]
Therefore, the particulate filter 18 with the NOx storage catalyst can be regenerated without changing the main combustion of the diesel engine, and good regeneration can be realized without generating torque shock or smoke. Particularly, in the NOx reduction of the NOx storage catalyst, since a structure in which CO is easily reacted with NOx instead of HC is used for the reduction, the regeneration efficiency is high.
[0047]
The present invention is not limited to the above-described embodiment, and may be implemented with various modifications without departing from the gist of the present invention. For example, in one embodiment, the example in which light oil is added to the exhaust pipe portion near the combustion type burner by the light oil addition injector is described. However, the present invention is not limited to this, and the exhaust port 4a is indicated by a two-dot chain line in FIG. 4d, for example, the light oil addition injector 23 may be installed to inject light oil into the exhaust port 4d, or the light oil addition injector 23 may be installed to inject light oil into the exhaust manifold 9. Needless to say, the HC adding means is not light oil but may be another means for injecting HC.
[0048]
【The invention's effect】
As described above, according to the present invention, a system independent of the main combustion of the diesel engine, that is, the control of the temperature rise of the exhaust gas by the burner and the control of the amount of HC added by the HC adding means alone can be used. The regeneration of the curated filter can be performed.
[0049]
Therefore, the regeneration does not need to change the main combustion of the engine, there is no possibility of occurrence of torque shock or generation of smoke, and the filter with the NOx storage catalyst can be favorably regenerated. In particular, in the reduction of NOx, since the reduction is performed using CO, which is liable to react with NOx, instead of HC, the regeneration efficiency is high.
[Brief description of the drawings]
FIG. 1 is a configuration diagram showing an exhaust gas purification device according to an embodiment of the present invention, together with an exhaust system of a diesel engine equipped with the exhaust gas purification device.
FIG. 2 is a block diagram showing a relationship between respective regeneration modes in the exhaust gas purification device.
FIG. 3 is a flowchart showing control contents of a NOx release / reduction mode.
FIG. 4 is a flowchart showing control contents of an S poisoning regeneration mode.
FIG. 5 is a flowchart showing control contents in a PM forced regeneration mode.
[Explanation of symbols]
1. Engine body (diesel engine)
10. Exhaust pipe (exhaust passage)
18 ... Particulate filter with NOx storage catalyst 19 ... CO generation catalyst 22 ... Combustion burner (burner)
23… light oil addition injector (HC addition means)
24 ... ECU (control means)

Claims (1)

A particulate filter provided with a NOx storage catalyst provided in an exhaust passage of a diesel engine, carrying a NOx storage catalyst, and removing NOx in the inflowing exhaust gas and collecting particulates;
A CO generation catalyst provided on the inflow side of the particulate filter with a NOx storage catalyst for converting HC in the inflowing exhaust gas into CO;
A burner that is provided on the inflow side of the CO generation catalyst and increases the temperature of the inflowing exhaust gas;
HC addition means provided in an exhaust passage upstream of the CO generation catalyst and adding HC to exhaust gas;
When the regeneration process of the particulate filter with the NOx storage catalyst is requested, the burner and the HC adding means are configured to form a reducing environment in which the NOx stored in the NOx storage catalyst is reduced by CO from the CO generation catalyst. And a control means for controlling exhaust gas.

Images