JP2002227688A - Exhaust gas purifying device for diesel engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To effectively execute regeneration of a DPF7 (removal of PM combustion), and desorption and purification of an NOx trapped by an NOx trap catalyst 8 at the same time, when an exhaust gas passage 5 of a diesel engine 1 is provided with a diesel particulate filter(DPF) 7 and a NOx trap catalyst 8. SOLUTION: When an oxidized catalyst 6 is disposed on an upstream side of the exhaust gas passage 5, and the DPF7 is judged to be a time for regeneration, the DPF7 temperature is raised by the reaction heat by an oxidized reaction of HC, CO by the oxidized catalyst 6. In this case, when the NOx trap amount of the NOx trap catalyst 8 is above a specified value, an exhaust to air fuel ratio is repeatedly controlled to a rich and lean value. In case the trap amount is less than the specified value, the exhaust to air fuel ratio is stoically controlled. After a rise in temperature, the exhaust air to the fuel ratio is controlled to the lean value.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a device for purifying exhaust gas of a diesel engine.

[0002]

2. Description of the Related Art In a diesel engine, a diesel particulate filter (DPF) is disposed in an exhaust passage so that particulate matter (PM) contained in exhaust gas is discharged.
It is known that, during the regeneration of the filter, the collected particulates are burnt and removed by oxygen and a predetermined heat load (temperature).

In an internal combustion engine that performs lean combustion,
An alkali metal such as potassium, sodium, lithium and cesium, an alkaline earth such as barium and calcium, or a rare earth such as lanthanum and yttrium is added to the exhaust passage.
A NOx trap catalyst used as an Ox trap agent is disposed, NOx is trapped (adsorbed or absorbed) by making the air-fuel ratio of exhaust gas flowing into this catalyst lean, and trapped NOx is released and purified by making it rich. It is known to

Accordingly, in a diesel engine, by disposing a particulate filter and a NOx trap catalyst in an exhaust passage, particulates in the exhaust gas and N
Ox can be removed. However, the regeneration of the particulate filter is performed at a heat load equal to or higher than a predetermined heat load. When a heat load equal to or higher than the predetermined heat load is applied to the NOx trap catalyst, the NOx trap catalyst naturally releases trapped NOx, and NOx traps. Can not trap. Therefore, when the particulate filter is regenerated, a large amount of NOx spontaneously released from the NOx trap catalyst is discharged into the atmosphere in addition to NOx contained in the exhaust gas.

[0005] Therefore, in the technique described in Japanese Patent Application Laid-Open No. 2000-145506, a method is used in which NOx trapped in a NOx trap catalyst is released and purified in advance, and then the particulate filter is regenerated.

[0006]

However, in this method, after purifying NOx, the air-fuel ratio of the exhaust gas is made lean to supply oxygen to the particulate filter so that the collected particulate matter and oxygen are removed. NOx in the exhaust gas flowing at a lean air-fuel ratio is discharged to the atmosphere without being trapped by the NOx trap catalyst due to the generated thermal load, and the temperature of the particulate filter rises. NOx trapped inside is also discharged.

The present invention has been made in view of the above problems, and has as its object to prevent the emission of NOx downstream of the NOx trap catalyst during the temperature rise of the particulate filter.

[0008]

According to the first aspect of the present invention, there is provided a particulate filter disposed in an exhaust passage of an engine for collecting particulates in exhaust gas,
A NOx trap catalyst disposed in an exhaust passage downstream of the particulate filter for trapping NOx in exhaust when the exhaust air-fuel ratio is lean and releasing and purifying the trapped NOx when the exhaust air-fuel ratio is rich. In the exhaust purification device for a diesel engine, comprising: a regeneration timing determination unit configured to determine a regeneration timing of the particulate filter; and a temperature raising unit configured to increase a temperature of the particulate filter when the regeneration timing is determined. ,
When it is determined that the regeneration time is reached, the exhaust air-fuel ratio is controlled to be rich while the temperature of the particulate filter is raised by the temperature raising means, and the air-fuel ratio is controlled to be lean after the temperature is raised. Means are provided.

According to a second aspect of the present invention, on the premise of the first aspect, the temperature increasing means is disposed in an exhaust passage upstream of the particulate filter, and is provided when the exhaust air-fuel ratio is rich or stoichiometric. An oxidation catalyst for oxidizing HC and CO therein is provided. According to a third aspect of the present invention, based on the premise of the second aspect, the air-fuel ratio control means may control the NOx trap amount of the NOx trap catalyst to a predetermined value while the temperature raising means raises the temperature of the particulate filter. When the value is equal to or more than the value, the exhaust air-fuel ratio is repeatedly controlled to be rich and lean, and when the NOx trap amount is less than a predetermined value, the exhaust air-fuel ratio is controlled to be stoichiometric.

According to a fourth aspect of the present invention, in the first to third aspects of the present invention, the NOx trap catalyst comprises an alkali metal such as potassium, sodium, lithium and cesium; an alkaline earth such as barium and calcium;
At least one selected from rare earths such as yttrium
And a precious metal such as platinum and palladium.

[0011]

According to the first aspect of the present invention, when it is determined that the particulate filter is to be regenerated, the temperature of the particulate filter is increased. During the temperature increase, the exhaust air-fuel ratio is controlled to the rich side. By doing so, a reducing agent (HC, CO) can be supplied to the NOx trap catalyst to release and purify the NOx trapped therein, and also purify NOx in the exhaust gas. Then, by controlling the exhaust air-fuel ratio to lean after the temperature rise, the particulates trapped in the particulate filter can be reliably and promptly burned and removed by a sufficient supply of oxygen.

As described above, during the temperature rise for regeneration of the particulate filter, NOx in the exhaust gas can be purified in addition to the purification of NOx trapped in the NOx trap catalyst. NOx emissions can be significantly reduced. According to the second aspect of the present invention, an oxidation catalyst is disposed upstream of the particulate filter, and when the exhaust air-fuel ratio is rich or stoichiometric, the reaction heat generated by the oxidation of HC and CO in the exhaust gas by the oxidation catalyst. Thus, by raising the temperature of the particulate filter, the temperature can be effectively increased without providing a heater or the like.

According to the third aspect of the present invention, when the NOx trap amount of the NOx trap catalyst is equal to or greater than a predetermined value while the temperature of the particulate filter is raised, the emission purification of NOx trapped in the NOx trap catalyst is performed. Since a large amount of reducing agent is required, the exhaust air / fuel ratio is controlled to be rich to increase HC and CO in the exhaust gas, and the particulate filter is raised by reaction heat generated by the oxidation reaction of HC and CO in the oxidation catalyst. The NOx release catalyst can be purified by the NOx trap catalyst using HC and CO remaining without being oxidized by the oxidation catalyst as a reducing agent while heating.

However, if the rich state is continued, the reducing agents HC and CO naturally increase, and the purification of the release of NOx trapped in the NOx trap catalyst proceeds. Therefore, the exhaust air-fuel ratio is repeatedly controlled at a proper interval between rich and lean at appropriate intervals. Thereby, HC, C
While securing the oxidation reaction of O at a lean air-fuel ratio, many NO
The supply of the reducing agent to the NOx trap catalyst trapping x can be ensured at a rich air-fuel ratio.

On the other hand, when the NOx trap amount of the NOx trap catalyst is less than a predetermined value, a large amount of reducing agent is not required for NOx release purification, so that the exhaust air-fuel ratio is controlled so as not to excessively supply HC and CO. Is controlled to stoichiometrically, the temperature of the particulate filter is raised by the heat of reaction at the oxidation catalyst, and the required NO at the NOx trap catalyst is increased.
x release purification can be performed.

According to the fourth aspect of the present invention, the NOx trap catalyst can have a lean and reliable NOx trapping function and a rich and reliable NOx purifying function.

[0017]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a system diagram of a diesel engine showing one embodiment of the present invention. In the diesel engine 1, an intake throttle valve 3 is provided on the intake passage 2 side, and air flows into the combustion chamber of each cylinder under the control of the intake throttle valve 3. Fuel is directly injected into the combustion chamber from the fuel injection valve 4 of each cylinder. The air that has flowed into the combustion chamber and the injected fuel burn here by compression ignition, and the exhaust flows out to the exhaust passage 5.

In order to purify exhaust gas, an oxidation catalyst 6, a diesel particulate filter (hereinafter, referred to as DPF) 7, a NOx trap catalyst 8,
Is arranged. The oxidation catalyst 6 can oxidize HC and CO in the exhaust when the exhaust air-fuel ratio is rich or stoichiometric, and generates heat of reaction due to the oxidation reaction.
It is provided upstream of the DPF 7 and is used as a temperature raising means for the DPF 7.

The DPF 7 can collect particulates (hereinafter referred to as PM) in the exhaust gas. The NOx trap catalyst 8 comprises at least one selected from alkali metals such as potassium, sodium, lithium and cesium; alkaline earths such as barium and calcium; rare earths such as lanthanum and yttrium; and a noble metal such as platinum and palladium. When the exhaust air-fuel ratio is lean
It has a function of trapping (adsorbing or absorbing) Ox and releasing and purifying trapped NOx when the exhaust air-fuel ratio is rich.

The control unit 10 receives signals from an unillustrated engine speed sensor for detecting the engine speed Ne, an accelerator opening sensor for detecting the accelerator opening APO, and the like for controlling the engine 1. In the present embodiment, in particular, the exhaust pressure sensors 11 and 11 for detecting the exhaust pressures Pin and Pout are provided on the inlet side and the outlet side of the DPF 7, respectively.
12 is provided, and the inlet side (or the inside) of the DPF 7
An exhaust temperature sensor 13 for detecting the exhaust gas temperature Tin is provided at the outlet of the NOx trap catalyst 8.
A NOx sensor 14 for detecting the Ox concentration is provided, and these signals are also input to the control unit 10.

Based on these input signals, the control unit 10 controls the timing of fuel injection to the fuel injection valve 4 and a fuel injection command signal for controlling the injection amount, and the opening for controlling the opening of the intake throttle valve 3. Outputs command signals and the like. Further, in the present embodiment, in particular, it is determined whether or not the regeneration of the DPF 7 is necessary.
In the case of the regeneration time, a predetermined regeneration process is performed, and such DPF regeneration control will be described in detail below.

FIG. 2 is a flowchart of the DPF regeneration control executed by the control unit 10. In step 1 (referred to as S1 in the figure, the same applies hereinafter), the exhaust pressure sensors 11 and 12 determine the exhaust pressure P in on the inlet side and the exhaust pressure P on the outlet side of the DPF 7 in order to determine the regeneration timing of the DPF 7.
out is detected, and the pressure difference ΔP = Pin−Pout is calculated.

In step 2, the pressure difference ΔP
Is compared with a predetermined value P1 to determine whether or not ΔP ≧ P1 (regeneration time). The PPF that is more than the predetermined trapping amount
This is because if M is collected, the pressure difference ΔP increases due to the clogging of the DPF 7. Therefore, Δ
In the case of P <P1, it is determined that it is not the reproduction time yet,
This flow ends. If ΔP ≧ P1, it is determined that it is time to regenerate, and the process proceeds to step 3 and subsequent steps.

In step 3, based on the output of the NOx sensor 14, the NOx in the exhaust at the outlet side of the NOx trap catalyst 8 is determined.
Detect x concentration. In step 4, the detected NOx
It is determined whether the NOx concentration in the exhaust gas at the outlet side of the trap catalyst 8 is equal to or higher than a predetermined concentration Cmax. When the NOx trap amount of the NOx trap catalyst 8 becomes equal to or more than a predetermined trap amount,
Part of the NOx in the exhaust gas passes through the NOx trap catalyst 8 without being trapped by the NOx trap catalyst 8, and the NOx on the outlet side thereof
This is because the concentration increases.

Therefore, N at the NOx trap catalyst 8 outlet side
If the Ox concentration is equal to or higher than the predetermined concentration Cmax, it is determined that the NOx trap amount of the NOx trap catalyst 8 is equal to or higher than the predetermined trap amount (trap limit), and the process proceeds to step 5, and conversely, the NOx trap catalyst 8 exit side When the NOx concentration is less than the predetermined concentration Cmax, the NOx
It is determined that the trap amount is less than the predetermined trap amount, and the process proceeds to step 6.

The NOx emission from the engine 1 estimated from the operating conditions of the engine 1 is integrated to obtain NO
The x trap amount may be calculated, and the determination may be made based on the calculated value. In step 5, since the NOx trap amount of the NOx trap catalyst 8 is equal to or larger than the predetermined trap amount (trap limit), the rich operation and the lean operation are alternately repeated at appropriate short intervals. That is, air-fuel ratio control is performed by, for example, controlling the opening degree of the intake throttle valve 3 so that the exhaust air-fuel ratio alternately becomes rich and lean. However, the total air-fuel ratio is made rich.

During the heating of the DPF 7, NOx
When the NOx trap amount of the trap catalyst 8 is equal to or more than the predetermined trap amount, a large amount of reducing agent is required for purifying the release of NOx trapped in the NOx trap catalyst 8,
By controlling the total exhaust air-fuel ratio to be rich, HC and CO in the exhaust are increased, and HC and C in the oxidation catalyst 6 are increased.
While raising the temperature of the DPF 7 by the heat of reaction of the O oxidation reaction, the NOx trap catalyst 8 performs NOx release purification using HC and CO remaining unoxidized by the oxidation catalyst 6 as reducing agents.

The reason why the exhaust air-fuel ratio is repeatedly controlled to be rich and lean at appropriate intervals is that if the total exhaust air-fuel ratio is to be made rich only by rich operation, it is natural that HC, which is a reducing agent, is used. This is because the amount of CO increases and the release and purification of NOx trapped in the NOx trap catalyst 8 proceeds, but the oxidation reaction decreases due to lack of oxygen in the oxidation catalyst 6 and a sufficient temperature rise cannot be obtained.

Thus, the reducing agent for the NOx trap catalyst 8 trapping a large amount of NOx can be secured at a rich air-fuel ratio while ensuring the oxidation reaction of HC and CO at a lean air-fuel ratio. In step 6, the Nx of the NOx trap catalyst 8
Since the Ox trap amount is less than the predetermined trap amount, the stoichiometric operation is performed. That is, for example, the air-fuel ratio control is performed by controlling the opening degree of the intake throttle valve 3 so that the exhaust air-fuel ratio becomes stoichiometric.

When the NOx trapping amount of the NOx trapping catalyst 8 is less than the predetermined trapping amount, since a large amount of reducing agent is not required for NOx release purification, the exhaust air-fuel ratio is adjusted so as not to supply HC and CO excessively. By performing the stoichiometric control, the NOx trap catalyst 8 performs necessary NOx release purification while raising the temperature of the DPF 7 by the reaction heat of the oxidation catalyst 6.

As described above, during the temperature rise for the regeneration of the DPF 7, the NOx trapped in the NOx trap catalyst 8
x, as well as NOx in exhaust gas
7, NOx emission during regeneration can be significantly reduced. After step 5 or 6, the process proceeds to step 7. In step 7, the exhaust gas temperature Tin on the inlet side (or inside) of the DPF 7 is detected by the exhaust gas temperature sensor 13.

In step 8, the exhaust gas temperature T
in is higher than a predetermined temperature Tmax, that is, D
It is determined whether the temperature of the PF 7 has risen to a temperature required for PM combustion. If the result of this determination is that Tin <Tmax, the process returns to step 5 or 6 because the temperature rise is insufficient. That is, if the NOx trap amount is equal to or more than the predetermined trap amount and the rich operation and the lean operation are repeated, the process returns to step 5 to continue the repetition of the rich operation and the lean operation, and the NOx trap amount is less than the predetermined trap amount. If stoichiometric operation is performed in step 6,
Then, the stoichiometric operation is continued and the temperature of the DPF 7 is increased.

On the other hand, Tin ≧ Tmax, that is, D
If it is determined that the temperature of the PF 7 has risen to a temperature required for PM combustion, the process proceeds to step 9. In step 9, the operation is switched to the lean operation. That is, for example, the air-fuel ratio control is performed by controlling the opening degree of the intake throttle valve 3 so that the exhaust air-fuel ratio becomes lean. As described above, after the temperature is increased, the exhaust air-fuel ratio is controlled to be lean, so that the DP
The PM trapped in F7 is reliably and promptly burned and removed.

In step 10, the exhaust pressure sensors 11 and 12 determine whether or not the regeneration of the DPF 7 has been completed and the DPF 7 has recovered to a state in which PM can be collected again.
Exhaust pressure Pin on the inlet side and exhaust pressure Pout on the outlet side of PF7
And the pressure difference ΔP = Pin−Pout is calculated.
Then, in step 11, the pressure difference ΔP is compared with a predetermined value P2 to determine whether ΔP ≦ P2 (end of regeneration). This is because if the PM trapped in the DPF 7 is burned off, the clogging of the DPF 7 is eliminated and the pressure difference ΔP decreases. Incidentally, it is natural that P2 <P1.

Therefore, if ΔP> P2, it is determined that regeneration has not yet been completed, and the routine returns to step 9 to continue lean combustion. However, if ΔP ≦ P2, regeneration is determined to be complete, and this flow is executed. finish. By performing the above control, the regeneration of the DPF 7 (burning out of the PM) and the purification of the release of NOx trapped in the NOx trap catalyst 8 are performed almost simultaneously, so that the DPF 7 can collect PM again. The NOx trap catalyst 8 also recovers to a state where NOx can be trapped.

Steps 1 and 2 correspond to the reproduction timing judging means.
Parts 6 and 9) correspond to the air-fuel ratio control means.

[Brief description of the drawings]

FIG. 1 is a system diagram of a diesel engine showing an embodiment of the present invention.

FIG. 2 is a flowchart of DPF regeneration control.

[Explanation of symbols]

 Reference Signs List 1 diesel engine 2 intake passage 3 intake throttle valve 4 fuel injection valve 5 exhaust passage 6 oxidation catalyst 7 DPF 8 NOx trap catalyst 10 control unit 11, 12 exhaust pressure sensor 13 exhaust temperature sensor 14 NOx sensor

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) F01N 3/02 321 F01N 3/02 321K 4D048 3/08 ZABA 3/08 ZAB 3/18 B 3/18 3 / 24 E 3/24 R 3/28 301C 3/28 301 301E F02D 41/02 355 F02D 41/02 355 45/00 314Z 45/00 314 B01D 53/36 103B F term (reference) 3G084 AA01 AA03 BA05 BA09 BA24 DA10 DA27 EA11 EB01 EB22 FA00 FA10 FA27 FA28 FA33 3G090 AA01 CA01 CA02 CA03 DA04 DA10 DA12 DA18 DA20 EA02 EA04 3G091 AA02 AA18 AA28 AB02 AB06 AB09 AB13 BA00 BA14 BA15 BA19 BA38 CA18 CB02 CB03 EA07 DB02 EA07 FB11 FB12 GB02Y GB03Y GB04Y GB05W HA10 HA15 HA16 HA18 HA36 HA37 HA42 3G301 HA02 HA04 HA06 JA24 JA25 JA26 JB09 LA03 LB11 MA01 NA 06 NA07 NA08 NE13 NE14 NE15 PD01B PD01Z PD11B PD11Z PD14B PD14Z PE01B PE01Z PF03B PF03Z 4D019 AA01 BC07 4D048 AA06 AA13 AA14 AA18 AB01 AB02 AB07 BA01 DABA02X BA14X BA15X BA18X BA19 DA03 BA03 DA01 BA30Y BA30 BAY DA30 DA20 EA04

Claims (4)

[Claims] A particulate filter disposed in an exhaust passage of an engine for collecting particulates in exhaust gas; and a particulate filter disposed in an exhaust passage downstream of the particulate filter, the exhaust filter being provided when the exhaust air-fuel ratio is lean. NOx in
A NOx trap catalyst for trapping and releasing NOx trapped when the exhaust air-fuel ratio is rich;
A regeneration timing determining means for determining a regeneration timing of the particulate filter; a temperature raising means for increasing a temperature of the particulate filter when the regeneration timing is determined; The air-fuel ratio control means controls the exhaust air-fuel ratio to be rich while the temperature raising means raises the temperature of the particulate filter, and controls the exhaust air-fuel ratio to lean after the temperature rise. And an exhaust purification device for a diesel engine, comprising: 2. The fuel cell system according to claim 2, further comprising an oxidation catalyst disposed in an exhaust passage upstream of the particulate filter and oxidizing HC and CO in the exhaust when the exhaust air-fuel ratio is rich or stoichiometric. The exhaust gas purifying apparatus for a diesel engine according to claim 1, wherein: 3. The air-fuel ratio control means, when the temperature of the particulate filter is raised by the temperature raising means, when the NOx trap amount of the NOx trap catalyst is a predetermined value or more, the exhaust air-fuel ratio is increased. 3. The exhaust gas purifying apparatus for a diesel engine according to claim 2, wherein the control is repeatedly performed to control the exhaust air-fuel ratio to stoichiometric when the NOx trap amount is less than a predetermined value. 4. The NOx trap catalyst according to claim 1, wherein the NOx trap catalyst contains at least one selected from an alkali metal, an alkaline earth, and a rare earth, and a noble metal.
The exhaust gas purification device for a diesel engine according to any one of claims 1 to 3.