JP2004360593A - Exhaust emission control device for internal combustion engine - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To desorb and purify adsorbed nitrogen oxide NOx without increasing emission quantity of hydrocarbon HC and carbon monoxide CO. <P>SOLUTION: This device determine whether current air fuel ratio control is lean operation or rich operation(S1), and whether quantity of NOx adsorbed by NOx trap catalyst exceeds threshold vale established beforehand (S3). If the device determines that the current air fuel ratio control is lean operation (S1:YES) and quantity of NOx adsorbed by the NOx trap catalyst exceeds the threshold value established beforehand (S3:YES), the device controls air fuel ratio and period of time to maintain the air fuel ratio (S6, S8, S10, S11) based on temperature of the NOx trap catalyst (S5), concentration of CO flowing in the NOx trap catalyst and quantity of reducing agent (S7) and make NOx adsorbed by the NOx trap catalyst desorb from the same and purifies the same. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an exhaust gas purifying apparatus for an internal combustion engine, and more particularly to an exhaust gas purifying apparatus for an internal combustion engine that can discharge adsorbed nitrogen carbide NOx without increasing the discharge amount of hydrogen carbide HC and carbon monoxide CO.
[0002]
[Prior art]
Conventionally, a three-way catalyst has been used as a means for purifying hydrogen carbide HC, carbon monoxide CO, nitrogen carbide NOx and the like contained in exhaust gas of an internal combustion engine. The three-way catalyst purifies the exhaust gas by simultaneously oxidizing HC and CO and reducing NOx when the air-fuel ratio of the exhaust gas is the stoichiometric air-fuel ratio. However, it is difficult for the three-way catalyst to purify NOx in the exhaust gas to a low level when the air-fuel ratio of the exhaust gas is leaner than the stoichiometric air-fuel ratio. For this reason, a NOx trap catalyst is installed downstream of the three-way catalyst, and NOx that cannot be adsorbed by the three-way catalyst at a lean air-fuel ratio is adsorbed on the NOx trap catalyst so that exhaust gas can be purified in a wide air-fuel ratio region. I have to.
[0003]
For example, in the invention described in Patent Literature 1 in which exhaust gas is purified by a three-way catalyst and a NOx trap catalyst, the air-fuel ratio is controlled in accordance with the temperature of the NOx trap catalyst to reduce NOx adsorbed by the NOx trap catalyst. Efficiency is reduced to prevent deterioration of NOx adsorption efficiency.
[0004]
[Patent Document 1]
JP-A-6-10725 (see paragraph 0013)
[0005]
[Problems to be solved by the invention]
However, the desorption / purification characteristics of NOx in the NOx trap catalyst depend not only on the temperature but also on the amount of a reducing agent such as hydrocarbon HC, hydrogen H, carbon monoxide CO supplied to the catalyst per unit time. In a temperature range of, for example, 200 ° C. or less where the NOx trap catalyst is not sufficiently activated, the CO concentration also has a large effect (the NOx desorption / purification speed is significantly reduced when the CO concentration is high).
[0006]
For this reason, simply controlling the air-fuel ratio based only on the temperature of the NOx trap catalyst as in the related art cannot sufficiently reduce NOx adsorbed on the NOx trap catalyst and purify HC and CO.
[0007]
The present invention has been made in view of such a conventional problem, and it is possible to desorb and purify the adsorbed nitrogen carbide NOx without increasing the emission amount of the hydrocarbons HC and carbon monoxide CO. It is an object of the present invention to provide an exhaust gas purification device for an internal combustion engine that can be used.
[0008]
[Means for Solving the Problems]
In order to solve the above-mentioned problems and achieve the object, an exhaust gas purifying apparatus for an internal combustion engine according to the invention adsorbs NOx to an exhaust pipe of the internal combustion engine when the air-fuel ratio is lean and when the air-fuel ratio is rich. An exhaust purification device for an internal combustion engine equipped with a NOx trap catalyst for desorbing and purifying adsorbed NOx, wherein the air-fuel ratio control determining means determines whether the current air-fuel ratio control is a lean operation or a rich operation. Is determined, and the adsorbed NOx amount judging means judges whether or not the amount of NOx adsorbed by the NOx trap catalyst exceeds a preset threshold value. If the control means determines that the current air-fuel ratio control is a lean operation and that the amount of NOx adsorbed by the NOx trap catalyst exceeds a preset threshold value, The air-fuel ratio and the time for maintaining the air-fuel ratio are controlled based on the temperature, the concentration of CO flowing into the NOx trap catalyst, and the amount of the reducing agent, and the NOx adsorbed by the NOx trap catalyst is desorbed and purified from the NOx trap catalyst. .
[0009]
【The invention's effect】
According to the exhaust gas purifying apparatus for an internal combustion engine of the present invention, the air-fuel ratio and the time for maintaining the air-fuel ratio are controlled in consideration of the concentration of CO flowing into the NOx trap catalyst and the amount of the reducing agent. The NOx trapped by the NOx trap catalyst can be efficiently desorbed and purified without increasing the amount of hydrocarbon HC and carbon monoxide CO emitted.
[0010]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, preferred embodiments of an exhaust emission control device according to the present invention will be described in detail with reference to the accompanying drawings. FIG. 1 is a schematic configuration diagram when the exhaust gas purification device according to the present invention is applied to a diesel engine.
[0011]
An intake pipe 12 for taking in air and an exhaust pipe 14 for discharging exhaust gas are connected to the diesel engine 10.
[0012]
An EGR (exhaust gas recirculation device) valve 16 for recirculating a part of the exhaust gas flowing through the exhaust pipe 14 to the intake pipe 12 is attached to the intake pipe 12. The EGR is a device used as one means for reducing NOx in the exhaust gas. By adjusting the opening of the EGR valve 16, an optimum EGR rate (exhaust gas recirculation amount / suction (Air amount + exhaust gas recirculation amount). Further, although not shown, the intake pipe 12 is provided with an intake valve for controlling the amount of air taken into the diesel engine 10 in accordance with the accelerator work of the driver and an air flow meter for detecting the amount of air. ing.
[0013]
The engine 10 is provided with a fuel injection valve 18 for injecting fuel into a cylinder. A fuel pipe 22 connected to a fuel tank 20 is attached to the fuel injection valve 18, and a fuel pump 24 for pumping the fuel sucked from the fuel tank 20 to the fuel injection valve 18 is attached to the fuel pipe 22.
[0014]
The exhaust pipe 14 is provided with a three-way catalyst 26 for purifying harmful three components (HC, CO, NOx) by a catalytic reaction, and a NOx trap catalyst 28 for purifying NOx by a catalytic reaction. On the upstream side of the three-way catalyst 26 has O ₂ sensor 30 is attached for detecting the concentration of oxygen in the exhaust gas. On the upstream side of the NOx trap catalyst 28, a CO sensor 32 for detecting the concentration of carbon monoxide in the exhaust gas and a temperature sensor 34 for detecting the temperature of the exhaust gas flowing into the NOx trap catalyst 28 are provided. I have.
[0015]
EGR valve 16, fuel injection valve 18, fuel pump 24, _{O 2} sensor 30, CO sensor 32, the temperature sensor 34 is connected to a controller 40. The control device 40 is configured by a microcomputer, and includes a CPU, a ROM, and a RAM. Various tables used in the air-fuel ratio enrichment control are stored in the ROM. The control device 40 includes: a detection value of the O ₂ sensor 30, the CO sensor 32, the temperature sensor 34, a rotation speed sensor for detecting the rotation speed of the diesel engine 10, an accelerator opening sensor for detecting the amount of depression of the accelerator, an air flow meter A signal related to the combustion of the diesel engine 10, such as a detected value of the engine, is input. The control device 40 controls the opening degree of the intake valve, the opening degree of the EGR valve 16, the on / off of the fuel pump 24, the amount of fuel injected from the fuel injection valve 18, and the like based on the input signals. The air-fuel ratio control is performed to make the exhaust atmosphere rich, stoichiometric, and lean. The control device 40 has a function of determining whether the current air-fuel ratio control is a lean operation or a rich operation, and the amount of NOx adsorbed by the NOx trap catalyst 28 exceeds a preset threshold. It has a function of judging whether or not it is present, and acts as air-fuel ratio control judging means and adsorbed NOx amount judging means.
[0016]
The three-way catalyst 26 has at least one or more noble metals such as Pt, Pd, and Rh supported on a carrier such as activated alumina, and then mixed and pulverized, and then supported on a cordierite-based monolithic carrier. On top of this, at least one of rare earth elements such as lanthanum (La), cerium (Ce), and yttrium (Y) is supported and formed. The three-way catalyst 26 can effectively remove the three harmful components (HC, CO, NOx) at a high purification rate by performing a predetermined air-fuel ratio control.
[0017]
The NOx trap catalyst 28 has at least one or more noble metals such as Pt, Pd, and Rh supported on a carrier such as activated alumina, then mixed and pulverized, and supported on a cordierite-based monolithic carrier. From above, for example, alkali metals such as potassium (K), sodium (Na), lithium (Li) and cesium (Cs), and alkaline earth metals such as barium (Ba), calcium (Ca) and strontium (Sr) , Lanthanum (La), cerium (Ce), and yttrium (Y). In the NOx trap catalyst 28, in a lean exhaust atmosphere, NOx in the exhaust gas becomes nitrate ions ( _NO3- ) on the noble metal of the NOx trap catalyst 28, and the nitrate ions are carried as, for example, a NOx trap agent. NOx is adsorbed in order to combine with the Ba oxide or the Ba carbonate. On the other hand, in the enriched exhaust atmosphere, nitrate ions on the noble metal of the NOx trap catalyst 28 decrease, and nitrate ions combined with the trapping agent are released as nitrogen dioxide (NO ₂ ). NOx is desorbed, and the NOx trap catalyst 28 is regenerated.
[0018]
By the way, the characteristics of NOx adsorption, desorption and purification in the NOx trap catalyst 28 differ significantly depending on the temperature. For example, at a temperature of 250 ° C. or higher where the NOx trap catalyst 28 is sufficiently activated, the adsorbed NOx can be quickly and sufficiently desorbed and purified. Therefore, when it is attempted to treat NOx by increasing the richness of the air-fuel ratio, even if the rich holding time (time for maintaining the elementary air-fuel ratio) is set short, the adsorbed NOx can be sufficiently desorbed and removed. Can be purified. On the other hand, at a temperature of 200 ° C. or lower where the NOx trap catalyst 28 is not sufficiently activated, the adsorbed NOx cannot be sufficiently desorbed and purified. Therefore, when it is attempted to treat NOx by increasing the richness of the air-fuel ratio, even if the rich holding time is set long, the adsorbed NOx cannot be sufficiently desorbed and purified. This is because when the air-fuel ratio is enriched, the CO concentration is high (see FIG. 4), and the adsorbed NOx is less likely to be released due to the influence of CO poisoning. For this reason, NOx is slowly desorbed and purified in a temperature range where the NOx trap catalyst 28 is not sufficiently activated. However, in the present invention, the NOx trap catalyst 28 By reducing the CO concentration in the inflowing exhaust gas and setting the rich retention time sufficiently long, the adsorbed NOx can be sufficiently desorbed and purified, and the unpurified hydrocarbon HC is discharged. Is also controlled.
[0019]
The CO sensor 32 detects the concentration of carbon monoxide in the exhaust gas. When the CO concentration is high, the main injection timing of the fuel injected from the fuel injection valve 18 is delayed to perform the main injection and the pilot injection. The CO concentration discharged from the diesel engine 10 is reduced by lengthening the interval between them and reducing the EGR rate. Since the CO concentration can be predicted to some extent from the air-fuel ratio detected by the O ₂ sensor 30, in the present invention, the CO concentration is 4% or less in a temperature region where the NOx trap catalyst 28 is not sufficiently activated. The rich degree of the air-fuel ratio is set so that Then, the rich holding time in this temperature range is set longer than the rich holding time in a state where the NOx trap catalyst 28 is sufficiently activated. Further, the amount of the reducing agent such as hydrocarbons HC, hydrogen H, and carbon monoxide CO in this temperature range is set to be smaller than the amount of the reducing agent when the NOx trap catalyst 28 is sufficiently activated. Since the three-way catalyst 26 located on the upstream side of the NOx trap catalyst 28 also consumes a reducing agent such as hydrocarbons HC, hydrogen H, and carbon monoxide CO, the richness of the air-fuel ratio becomes 14 or less. To do.
[0020]
FIG. 2 is an operation flowchart of the air-fuel ratio enrichment control of the exhaust gas purification apparatus according to the present invention. This operation flowchart is executed in order to allow NOx adsorbed on the NOx trap catalyst 28 to be efficiently discharged even in a relatively low temperature range.
[0021]
First, the control unit 40, O ₂ air-fuel ratio control from the air-fuel ratio detected by the sensor 30 is currently performing to determine whether the rich operation whether a lean operation (S1). If the current air-fuel ratio control is a rich operation (S1: NO), the process ends because there is no need to perform the air-fuel ratio enrichment control. On the other hand, if the current air-fuel ratio control is a lean operation, the amount of NOx flowing into the NOx trap catalyst 28 is determined. The amount of NOx flowing into the NOx trap catalyst 28 is stored as a table in the ROM of the control device 40 in relation to the air-fuel ratio control of the diesel engine 10. The NOx amount flowing into the NOx trap catalyst 28 is integrated, and the NOx amount ΣNE currently adsorbed on the NOx trap catalyst 28 can be grasped (S2).
[0022]
Next, the control device 40 determines whether or not the NOx amount ΣNE adsorbed on the NOx trap catalyst 28 exceeds a preset threshold value SNE. The threshold value SNE set in advance is a limit NOx amount at which the NOx trap catalyst 28 can adsorb. When the control device 40 determines that the NOx amount ΣNE adsorbed on the NOx trap catalyst 28 does not exceed the threshold value SNE (S3: NO), the NOx trap catalyst 28 can still adsorb NOx. To continue lean operation. On the other hand, when it is determined that the NOx amount ΣNE adsorbed on the NOx trap catalyst 28 exceeds the threshold value SNE (S3: YES), the NOx trap catalyst 28 cannot further adsorb NOx, so that the NOx trap catalyst A NOx flag is set to start control for desorbing and purifying NOx adsorbed on the NOx 28 (S4).
[0023]
The control device 40 detects the temperature T of the exhaust gas (substantially equivalent to the temperature of the NOx trap catalyst 28) by the temperature sensor 34, and determines whether or not this temperature T exceeds a preset threshold temperature Ta. A determination is made (S5). The preset threshold temperature Ta is a temperature at which the NOx trap catalyst 28 is in a sufficiently activated state, and is set to 250 ° C. in the present embodiment. If the temperature T of the exhaust gas exceeds the threshold temperature Ta (S5: NO), the control device 40 calls the reference rich degree and the reference rich holding time stored in the ROM so that the reference rich degree is obtained. The air-fuel ratio control is performed during the reference rich holding time. That is, when the NOx trap catalyst 28 is in a sufficiently activated state, the rich operation of the predetermined reference is performed (S6). On the other hand, if the temperature T of the exhaust gas does not exceed the threshold temperature Ta (S5: YES), the NOx trap catalyst 28 is not in a sufficiently activated state. 32, the concentration C of carbon monoxide in the exhaust gas is detected, and it is determined whether this concentration C exceeds a preset threshold concentration Ca (S7).
[0024]
The reason why the concentration C of carbon monoxide is detected when the temperature of the NOx trap catalyst 28 is relatively low is as follows. As shown in FIG. 4, the concentration of carbon monoxide C contained in the exhaust gas differs depending on the value of the air-fuel ratio (A / F). If the concentration of carbon monoxide C differs, the efficiency of NOx absorption by the NOx trap catalyst 28 changes. Generally, as the concentration of carbon monoxide C increases, the NOx absorption efficiency decreases. Therefore, when the temperature of the NOx trap catalyst 28 is relatively low, the concentration of carbon monoxide C must be considered in order to prevent the NOx absorption efficiency from decreasing too much.
[0025]
If the concentration C of carbon monoxide in the exhaust gas exceeds the threshold concentration Ca (S7: NO), the controller 40 corrects the fuel injection amount by the fuel injection valve 18 so that the degree of richness decreases. The air-fuel ratio control is performed (S8). If the concentration C of carbon monoxide in the exhaust gas does not exceed the threshold concentration Ca (S7: YES), the control device 40 controls the carbonization to be supplied to the NOx trap catalyst 28 based on the currently performed air-fuel ratio control. The amounts of reducing agents such as hydrogen HC, hydrogen H, and carbon monoxide CO are determined. The amount of the reducing agent is determined in consideration of the rotation speed of the diesel engine 10, the fuel injection amount of the fuel injection valve 18, the fuel injection timing, the EGR rate, and the like. The control device 40 sets the obtained amount of the reducing agent (S9), and sets the rich holding time from the amount of the reducing agent. This rich retention time is set to a time at which the amount of reducing agents such as hydrocarbons HC, hydrogen H, and carbon monoxide CO does not exceed the amounts of these reducing agents when the NOx trap catalyst 28 is sufficiently activated. (S10).
[0026]
As described above, when the temperature of the NOx trap catalyst 28 is relatively low, the amount of the reducing agent is considered, as shown in FIG. 3, the NOx absorption efficiency of the NOx trap catalyst 28 changes depending on the amount of the reducing agent. Because. As shown in the figure, as the amount of the reducing agent increases, the NOx absorption efficiency improves, but during this time the current amount is limited by the rich holding time.
[0027]
When the current air-fuel ratio control is continued for the rich holding time (S11), the control device 40 resets the NOx flag and sets the NOx amount ΣNE adsorbed on the NOx trap catalyst 28 to 0 (S12). . The control device 40 determines whether or not to continue the above processing (S13). If the control device 40 determines that all the processes have been completed (S13: YES), the control device 40 ends the process, and if it determines that all the processes have not been completed, the process returns to the step S1. The process is performed again (S13: NO).
[0028]
According to the exhaust gas purification apparatus for an internal combustion engine according to the present invention, the air-fuel ratio and the time for maintaining the air-fuel ratio are determined by taking into account the temperature of the NOx trap catalyst, the concentration of CO flowing into the NOx trap catalyst, and the amount of the reducing agent. Since the control is performed, the NOx adsorbed from the NOx trap catalyst can be efficiently desorbed and purified from the NOx trap catalyst in a wide temperature range without increasing the emission amount of hydrocarbon HC and carbon monoxide CO. .
[0029]
According to the second aspect of the present invention, when the temperature of the NOx trap catalyst is in a temperature range where the NOx trap catalyst is not sufficiently activated, the concentration of CO in the exhaust gas flowing into the NOx trap catalyst is reduced. Since the air-fuel ratio is controlled so as to be 4% or less, the NOx trapped by the NOx trap catalyst can be efficiently desorbed and purified without increasing the emissions of hydrocarbons HC and carbon monoxide CO. Can be.
[0030]
According to the third aspect of the invention, when the temperature of the NOx trap catalyst is in a temperature range where the NOx trap catalyst is not sufficiently activated, the amount of the reducing agent flowing into the NOx trap catalyst is determined by the NOx trap catalyst. Since it is smaller than the amount of the reducing agent that flows into the temperature region where the temperature is sufficiently activated, the NOx trap catalyst can remove the hydrocarbon HC and the carbon monoxide CO without increasing the emission amount. The adsorbed NOx can be efficiently desorbed and purified.
[0031]
According to the fourth aspect of the invention, when the temperature of the NOx trap catalyst is in a temperature range where the NOx trap catalyst is not sufficiently activated, the air-fuel ratio is increased and the time for maintaining the air-fuel ratio is increased. Therefore, the NOx adsorbed by the NOx trap catalyst can be efficiently desorbed and purified from the NOx trap catalyst without increasing the emissions of hydrocarbon HC and carbon monoxide CO.
[0032]
According to the fifth and sixth aspects of the present invention, the reducing agent contains any of hydrocarbon HC, hydrogen H, and carbon monoxide CO, and the air-fuel ratio is set to 14 or less. Even if a three-way catalyst is arranged upstream of the NOx trap, the reducing agent can be sufficiently contained, and the NOx adsorbed by the NOx trap catalyst can be efficiently desorbed and purified.
[Brief description of the drawings]
FIG. 1 is a schematic configuration diagram when an exhaust gas purification device according to the present invention is applied to a diesel engine.
FIG. 2 is an operation flowchart of air-fuel ratio enrichment control of the exhaust gas purification apparatus according to the present invention.
FIG. 3 is a graph showing a relationship between carbon monoxide CO concentration and NOx desorption / purification efficiency for each air-fuel ratio.
FIG. 4 is a graph showing a relationship between an air-fuel ratio and a carbon monoxide CO concentration.
[Explanation of symbols]
10. Diesel engine,
12 ... intake pipe,
14 ... exhaust pipe,
16 EGR valve,
18. Fuel injection valve,
20 ... fuel tank,
22 ... fuel pipe,
24 ... Fuel pump,
26 ... Three-way catalyst,
28 ... NOx trap catalyst,
30 ... _{O 2} sensor,
32 ... CO sensor,
34 ... temperature sensor,
40 ... Control device.

Claims (6)

An exhaust purification device for an internal combustion engine including a NOx trap catalyst for adsorbing NOx in an exhaust pipe of an internal combustion engine when the air-fuel ratio is in a lean state and desorbing and purifying the adsorbed NOx when the air-fuel ratio is in a rich state, ,
Air-fuel ratio control determining means for determining whether the current air-fuel ratio control is a lean operation or a rich operation,
Adsorbed NOx amount determining means for determining whether the amount of NOx adsorbed by the NOx trap catalyst exceeds a preset threshold value,
A temperature sensor for detecting a temperature of the NOx trap catalyst;
A CO sensor for detecting the concentration of CO flowing into the NOx trap catalyst;
If it is determined that the current air-fuel ratio control is a lean operation and that the amount of NOx adsorbed by the NOx trap catalyst exceeds a preset threshold, the temperature of the NOx trap catalyst And controlling the air-fuel ratio and the time for maintaining the air-fuel ratio based on the concentration of CO flowing into the NOx trap catalyst and the amount of the reducing agent, and desorbing and purifying the NOx adsorbed by the NOx trap catalyst from the NOx trap catalyst. An exhaust purification device for an internal combustion engine, comprising: The control means,
When the temperature of the NOx trap catalyst is in a temperature range where the NOx trap catalyst is not sufficiently activated, the air-fuel ratio is controlled so that the concentration of CO in the exhaust gas flowing into the NOx trap catalyst becomes 4% or less. The exhaust gas purifying apparatus for an internal combustion engine according to claim 1, wherein: The control means,
When the temperature of the NOx trap catalyst is in a temperature range where the NOx trap catalyst is not sufficiently activated, the amount of the reducing agent flowing into the NOx trap catalyst is set in a temperature range where the NOx trap catalyst is sufficiently activated. 2. The exhaust gas purifying apparatus for an internal combustion engine according to claim 1, wherein the amount of the reducing agent flowing into the internal combustion engine at a certain time is smaller than the amount of the reducing agent. The control means,
When the temperature of the NOx trap catalyst is in a temperature region where the NOx trap catalyst is not sufficiently activated, the air-fuel ratio is larger than when the temperature of the NOx trap catalyst is in a temperature region where the NOx trap catalyst is sufficiently activated. The exhaust gas purifying apparatus for an internal combustion engine according to claim 1, wherein the holding time is extended. The exhaust gas purifying apparatus for an internal combustion engine according to claim 3, wherein the reducing agent includes one of hydrocarbon HC, hydrogen H, and carbon monoxide CO. The exhaust gas purifying apparatus for an internal combustion engine according to claim 4, wherein the air-fuel ratio is 14 or less.

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