JPH1193641A - Exhaust emission control device for internal combustion engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the NOx purifying ratio of an exhaust emission control device of an internal combustion engine. SOLUTION: An exhaust pipe 12 is branched into a first exhaust pipe 12a and a second exhaust pipe 12b, a first catalytic converter 21 housing a storing and reducing-type catalyst is provided on the first exhaust pipe 12a, and a second catalytic converter 22 housing a selective reducing-type catalyst is provided on the second exhaust pipe 12b. A selector valve 20 is so switched as to allow to flow exhaust gas to the first exhaust pipe 12a when an engine is in the acceleration state and as to flow exhaust gas to the second exhaust pipe 12b when the engine is in the non-acceleration state. When exhaust gas is flowing in the second exhaust pipe 12b, a reducing agent valve 31 is opened, fuel as reducing agent is added from an injection nozzle 24 to the storing and reducing-type catalyst of the first catalytic converter 21, an EGR valve 27 is opened, and NOx stored in the storing and reducing-type catalyst is discharged and reduced.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an exhaust purification system for an internal combustion engine that burns in a lean air-fuel ratio.

[0002]

2. Description of the Related Art When the air-fuel ratio of inflowing exhaust gas is lean, N
Since the NOx storage reduction catalyst that absorbs Ox and releases the absorbed NOx when the oxygen concentration of the inflowing exhaust gas is reduced has a high NOx purification rate, it is recently discharged from an internal combustion engine that burns in a lean air-fuel ratio state. It is frequently used for NOx purification of exhaust gas.

For example, Japanese Patent Application Laid-Open No. 6-50132 discloses that an exhaust pipe is branched into a plurality of branch exhaust pipes in parallel, and each of the branch exhaust pipes has a different storage temperature range in which the NOx purification rate has a peak. Exhaust gas pursuing an improved NOx purification rate by storing a type catalyst and switching the flow path of the exhaust gas according to the temperature of the exhaust gas to allow the exhaust gas to flow through the occlusion reduction type catalyst in any of the exhaust branch pipes An apparatus is disclosed.

[0004]

However, even if the temperature range in which the NOx purification rate peaks is different, the mechanism of NOx purification of the occlusion reduction type catalyst is the same and has the same characteristics. Even if the catalyst is properly used by switching the flow path according to the exhaust gas temperature, NOx
There are limits to improving the purification rate.

[0005] Further, since the storage reduction catalyst saturates and becomes unable to absorb NOx when it continues to absorb NOx, it is necessary to release and reduce the absorbed NOx at an appropriate timing. In addition, it is necessary to provide a means for adding a reducing agent to the apparatus, and to control the regeneration processing, and there is a problem that the whole system becomes complicated.

[0006] The present invention has been made in view of such problems of the conventional art, and an object of the present invention is to provide an occlusion reduction type catalyst and a selective reduction type catalyst depending on the operating state of the engine. The purpose of the present invention is to improve the NOx purification rate and simplify the exhaust purification system.

[0007]

The present invention has the following features to attain the object mentioned above. The present invention relates to an exhaust gas purifying apparatus for an internal combustion engine, in which exhaust gas discharged from an internal combustion engine burning in an air-fuel ratio lean state passes through an exhaust passage and is purified by a catalyst provided in the exhaust passage. The exhaust gas is branched into a passage and a second exhaust passage. When the exhaust gas temperature changes from high temperature to low temperature, the exhaust gas flows through the second exhaust passage. Otherwise, the exhaust gas flows through the first exhaust passage. Flow path switching means for switching the flow path of the exhaust gas, and NOx absorbed when the air-fuel ratio of the inflowing exhaust gas is lean and NOx absorbed when the oxygen concentration of the inflowing exhaust gas decreases is provided in the first exhaust passage. A selective reduction type catalyst provided in the second exhaust passage for reducing or decomposing NOx in the presence of hydrocarbons in an oxygen-excess atmosphere; and a storage reduction type catalyst. An exhaust gas purifying apparatus for an internal combustion engine, comprising: a reducing agent adding means for adding a reducing agent to the occlusion reduction type catalyst while flowing exhaust gas through the second exhaust passage so as to release and reduce NOx. It is.

When the exhaust gas changes from a high temperature to a low temperature, the exhaust gas is caused to flow through the second exhaust passage by the flow path switching means, and NOx in the exhaust gas is reduced by the selective reduction catalyst.

Except when the exhaust gas changes from a high temperature to a low temperature, the exhaust gas flows into the first exhaust passage by the flow path switching means, and NOx in the exhaust gas is absorbed by the storage reduction catalyst. NOx absorbed by this storage reduction type catalyst
Is discharged and reduced by the reducing agent added from the reducing agent adding means when the exhaust gas is flowing through the second exhaust passage, and the occlusion reduction type catalyst is regenerated to a state capable of absorbing NOx.

[0010] Examples of the internal combustion engine include a diesel engine and a lean burn gasoline engine. The first exhaust passage and the second exhaust passage are not limited to one each, and a plurality of exhaust passages may be provided in parallel.

In the exhaust gas purifying apparatus for an internal combustion engine according to the present invention, "when the exhaust gas changes from high temperature to low temperature" is set to "non-acceleration state of the internal combustion engine" and "when the exhaust gas changes from high temperature to low temperature. Other times "may be" an acceleration state of the internal combustion engine ". Further, the change in the exhaust gas temperature may be determined by actually detecting the exhaust gas temperature by the temperature detecting means.

In the exhaust gas purifying apparatus for an internal combustion engine according to the present invention, the second reducing method comprises adding a reducing agent to the exhaust gas upstream of the selective reduction catalyst when the exhaust gas is flowing through the second exhaust passage. It is possible to add an agent adding means.
In that case, the second reducing agent adding means can be realized by an expansion stroke injection for injecting a reducing agent (fuel) into the combustion chamber when the internal combustion engine is in the expansion stroke.

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, an embodiment of an exhaust gas purifying apparatus for an internal combustion engine according to the present invention will be described with reference to FIGS. In this embodiment, a diesel engine is used as an internal combustion engine.

FIG. 1 shows an exhaust purification device for a diesel engine for a vehicle. Air is introduced from an intake pipe 2 into a combustion chamber of each cylinder of a diesel engine main body 1 through an intake manifold 3. The fuel in the fuel reservoir tank 4 is sucked up by a fuel supply pump 5 whose discharge pressure can be controlled, and supplied to a fuel accumulator 7 through a fuel supply pipe 6. The fuel pressure accumulating pipe 7 has a pressure accumulating chamber 8 with a constant volume therein, and high-pressure fuel discharged from the fuel supply pump 5 is accumulated in the pressure accumulating chamber 8. The fuel in the accumulator 8 is supplied to the fuel injection valve 9 of each cylinder via a fuel supply pipe 19, and is injected into the combustion chamber of each cylinder at a predetermined timing. Each fuel injection valve 9 is connected to the fuel reservoir tank 4 via a fuel return conduit 10.
It is connected to.

Exhaust gas discharged from each cylinder of the engine body 1 is discharged through an exhaust manifold 11 and an exhaust pipe (exhaust passage) 12 in this order. The exhaust pipe 12 is bifurcated on the way into a first exhaust pipe (first exhaust passage) 12a and a second exhaust pipe (second exhaust passage) 12b, and merges again to form one exhaust pipe 12. It is configured.

The first exhaust pipe 12a is provided with a first catalytic converter 21 having a storage-reduction type catalyst.
2b is a second catalytic converter 2 having a selective reduction catalyst
2 are provided. The storage reduction catalyst and the selective reduction catalyst will be described later in detail.

A portion of the exhaust pipe 12 that branches into the first exhaust pipe 12a and the second exhaust pipe 12b is provided with a damper type switching valve 20 for switching the flow path of the exhaust gas. The switching valve 20 is driven by a valve driving device 23, and is positioned so as to open one of the first exhaust pipe 12a and the second exhaust pipe 12b and close the other. That is, the second exhaust pipe 12b is closed when the exhaust gas flows through the first exhaust pipe 12a, and the first exhaust pipe 12a is closed when the exhaust gas flows through the second exhaust pipe 12b.

An injection nozzle 24 for injecting fuel as a reducing agent into the first catalytic converter 21 is provided inside the first exhaust pipe 12a and upstream of the first catalytic converter 21.

The injection nozzle 24 is connected to the pressure accumulating pipe 7 via a reducing agent pipe 29. A reducing agent valve 31 which is opened and closed by a valve driving device 30 is provided in the middle of the reducing agent pipe 29. I have. The opening of the reducing agent valve 31 is set in advance so that a predetermined flow rate flows.

The downstream side of the first catalytic converter 21 in the first exhaust pipe 12a is connected to the intake pipe 2 via an exhaust gas recirculation pipe 25.
An exhaust gas recirculation control valve (hereinafter abbreviated as EGR valve) 27 driven by the engine 6 is provided.

An electronic control unit (hereinafter abbreviated as ECU) 50 for engine control is composed of a digital computer, and ROMs connected to each other by a bidirectional bus.
(Read-on-memory), RAM (random access memory), CPU (central processor unit), input port, output port, perform basic control such as fuel injection amount control of the engine. The operation of the device is controlled.

A fuel pressure sensor 13 for detecting a fuel pressure in the pressure accumulating chamber 8 and generating an output voltage proportional to the fuel pressure is attached to an end of the fuel pressure accumulating pipe 7.
Is mounted with an accelerator opening sensor 17 which detects the accelerator opening and generates an output voltage proportional to the accelerator opening. A second catalyst is provided downstream of the second catalytic converter 22 in the second exhaust pipe 12b. An output gas temperature sensor 18 that detects an exhaust gas temperature (output gas temperature) at the outlet of the converter 22 and generates an output voltage proportional to the detected temperature is attached. These fuel pressure sensor 13, accelerator opening sensor 17, and outgassing temperature sensor 18 output their respective output signals as E.
Output to CU50. Then, the ECU 50 calculates the engine load L based on the output signal of the accelerator opening sensor 17.

A pair of disks 32 and 33 are mounted on the engine crankshaft.
A pair of crank angle sensors 34 and 35 are disposed opposite the toothed outer peripheral surfaces 2 and 33. One of the crank angle sensors 34 outputs an output pulse indicating, for example, that the first cylinder is at the intake top dead center to the ECU 50, and from the output pulse of the crank angle sensor 34, the ECU 50 operates the fuel injection valve 9 of any cylinder. Decide what to do. The other crank angle sensor 35 outputs an output pulse to the ECU 50 every time the crankshaft rotates by a certain angle, and the ECU 50 calculates the engine speed N from the output pulse of the crank angle sensor 35.

The valve driving devices 23, 26 and 30 are controlled by a command signal from the ECU 50 based on the operating state of the engine body 1.

Next, the storage reduction catalyst accommodated in the first catalytic converter 21 and the selective reduction catalyst accommodated in the second catalytic converter 22 will be described. The storage-reduction catalyst uses, for example, alumina as a carrier, and on the carrier, for example, an alkali metal such as potassium K, sodium Na, lithium Li, cesium Cs, an alkaline earth such as barium Ba, calcium Ca, lanthanum La, yttrium. At least one selected from rare earths such as Y and a noble metal such as platinum Pt are supported. When the ratio of air and fuel (hydrocarbon) supplied into the engine intake passage and the exhaust passage upstream of the storage reduction catalyst is referred to as the air-fuel ratio of the exhaust gas flowing into the storage reduction catalyst, the storage reduction catalyst is NOx when the air-fuel ratio of the inflowing exhaust gas is lean
When the oxygen concentration in the inflowing exhaust gas decreases, the absorbed NOx is released.

When no fuel (hydrocarbon) or air is supplied into the exhaust passage upstream of the storage reduction catalyst, the air-fuel ratio of the inflowing exhaust gas matches the air-fuel ratio of the air-fuel mixture supplied into the combustion chamber. In this case, the storage reduction catalyst absorbs NOx when the air-fuel ratio of the mixture supplied to the combustion chamber is lean, and releases the absorbed NOx when the concentration of oxygen in the mixture supplied to the combustion chamber decreases.・ Reduce.

FIG. 2 schematically shows the concentrations of typical components of the exhaust gas discharged from the combustion chamber. As can be seen from FIG. 2, unburned H in the exhaust gas discharged from the combustion chamber
The concentration of C and CO increases as the air-fuel ratio of the air-fuel mixture supplied to the combustion chamber becomes richer, and the concentration of oxygen O ₂ in the exhaust gas discharged from the combustion chamber increases the air-fuel ratio of the air-fuel mixture supplied into the combustion chamber. It increases as the fuel ratio becomes leaner.

NOx absorption / reduction by the storage reduction type catalyst is as follows.
It is believed that this is done by the mechanism shown in FIG. This mechanism is a case where platinum Pt and barium Ba are supported on a carrier, but the same mechanism can be obtained by using other noble metals, alkali metals, alkaline earths, and rare earths.

First, when the exhaust gas becomes considerably lean, the exhaust gas is exhausted.
Since the oxygen concentration in the gas greatly increases, FIG.
Oxygen O as shown in_Two Is O_Two ^-Or O^2-In the form of platinum Pt
Attaches to surface. Next, NO contained in the exhaust gas is white.
O on the surface of gold Pt_Two ^-Or O ^2-Reacts with NO_Two Becomes
(2NO + O_Two → 2NO_Two ).

Thereafter, the generated NO ₂ is absorbed in the catalyst while being oxidized on the platinum Pt, unless the NOx absorption capacity of the occlusion reduction type catalyst is not saturated.
And nitrate ions NO as shown in FIG.
₃ ^- diffuse storage reduction catalyst in 19 in the form of. In this way, NOx is absorbed in the catalyst.

[0031] In contrast, if the oxygen concentration in the exhaust gas decreases, and decreases the amount of NO _2, the reverse of the reaction and the reaction, nitrate ions NO ₃ in the catalyst ^- is, NO ₂
Alternatively, it is released from the storage reduction catalyst in the form of NO.

That is, when the oxygen concentration in the exhaust gas decreases, NOx is released from the storage reduction catalyst. As shown in FIG. 2, when the lean degree of the inflowing exhaust gas decreases, the oxygen concentration in the inflowing exhaust gas decreases,
Therefore, if the lean degree of the inflowing exhaust gas is reduced, NOx will be released from the occlusion reduction type catalyst even if the inflowing exhaust gas has a lean air-fuel ratio.

On the other hand, at this time, when the air-fuel mixture supplied to the combustion chamber is made rich and the air-fuel ratio of the exhaust gas becomes rich, a large amount of unburned HC and CO is discharged from the engine as shown in FIG. You. These unburned HC and CO are platinum P
It reacts immediately with oxygen O ₂ ^- or O ^2- on t and is oxidized.

Further, when the air-fuel ratio of the inflowing exhaust gas becomes rich, the oxygen concentration in the exhaust gas extremely decreases, so that the storage reduction catalyst releases NO ₂ or NO. This N
O ₂ or NO, as shown in FIG.
It is reduced by reacting with C and CO. Thus, platinum P
When NO ₂ or NO on t disappears, NO ₂ or NO is released from the storage reduction catalyst one after another. Therefore, when the air-fuel ratio of the inflowing exhaust gas is made rich, NOx is released from the storage reduction catalyst within a short time. Or O ^2- unburned HC also consume, ^- O ₂ on the platinum Pt
If CO remains, N released from the storage reduction catalyst
Ox and NOx exhausted from the engine
Reduced by C and CO.

Therefore, if the air-fuel ratio of the inflowing exhaust gas is made rich, the NOx absorbed by the occlusion reduction type catalyst is released within a short time, and the released NOx is reduced. NOx can be prevented from being discharged.

By the way, in the case of a diesel engine,
Since combustion is performed in a much leaner region than the stoichiometric ratio (the stoichiometric air-fuel ratio, A / F = 13 to 14), the air-fuel ratio of the exhaust gas flowing into the first catalytic converter 21 is very lean in a normal engine operating state. NOx in the exhaust gas is absorbed by the storage reduction catalyst, and the amount of NOx released from the catalyst is extremely small.

In the case of a gasoline engine, as described above, the air-fuel ratio of the exhaust gas is made rich by enriching the air-fuel mixture supplied to the combustion chamber, and the oxygen concentration in the exhaust gas is lowered to store the gas. NOx absorbed in the reduction catalyst can be released and regenerated, but in the case of a diesel engine, if the air-fuel mixture supplied to the combustion chamber is made rich, soot is generated during combustion. Can not be adopted. Therefore, in a diesel engine, it is necessary to directly supply the fuel as a reducing agent separately from the air-fuel mixture for combustion to the occlusion reduction type catalyst for regeneration.

On the other hand, the selective reduction catalyst accommodated in the second catalytic converter 22 is provided with NOx in the presence of hydrocarbons (HC) in a lean air-fuel ratio exhaust (oxygen excess atmosphere).
Is defined as a catalyst that reduces or decomposes. Such a selective reduction catalyst includes a catalyst in which a transition metal such as Cu is ion-exchanged and supported on zeolite, a catalyst in which a noble metal is supported on zeolite or alumina, and the like.

The selective reduction type catalyst has a high NOx purification rate in a temperature distribution state in which the exhaust gas has a low space velocity and the catalyst temperature is higher on the catalyst outlet side than on the catalyst inlet side, and the NOx purification rate is high during acceleration when the NOx emission amount is large. It is known that the rate decreases. Here, the temperature distribution where the catalyst temperature is higher on the catalyst outlet side than on the catalyst inlet side is when the exhaust gas temperature changes from high temperature to low temperature, which corresponds to almost non-acceleration.

On the other hand, the occlusion reduction type catalyst housed in the first catalytic converter 21 has a high NOx purification rate even when the NOx emission amount is large, such as during acceleration, which the selective reduction type catalyst is not good at. The relatively high catalyst temperature (approximately 4
(Up to about 00 ° C).

Therefore, in this exhaust gas purifying apparatus, two types of catalysts are selectively used depending on the operation state of the diesel engine by utilizing the characteristics of these catalysts. FIG. 4 is a schematic flowchart of an operation method of the exhaust gas purification apparatus. First, it is determined whether the engine body 1 is in an accelerated state or a non-accelerated state (step 100).
By closing the second exhaust pipe 12b and opening the first exhaust pipe 12a, the exhaust gas is passed through the first catalytic converter 21 (step 101), and NOx in the exhaust gas is absorbed by the storage reduction catalyst.

On the other hand, in the non-acceleration state, the first exhaust pipe 12a is closed by the switching valve 20 and the second exhaust pipe 12a is closed.
Opening b, the exhaust gas is passed through the second catalytic converter 22 (step 102). Here, the second catalytic converter 22
In order to reduce NOx by the selective reduction type catalyst, HC as a reducing agent is required. Therefore, the outgassing temperature T of the second catalytic converter 22 falls within a predetermined temperature range (T1 <T
<T2) is determined (step 10).
3) If it is within the temperature range, a sub-injection is performed in the expansion stroke to inject fuel as a reducing agent into the exhaust gas (step 104), and NOx in the exhaust gas is converted into a selective reduction catalyst of the second catalytic converter 22. To reduce.

The regeneration process for releasing and reducing the NOx stored in the storage reduction catalyst of the first catalytic converter 21 is performed during non-acceleration when exhaust gas is not passed through the first exhaust pipe 12a. At 105, it is determined whether or not the occlusion reduction type catalyst of the first catalytic converter 21 has been regenerated, and if not, a regeneration process for the first catalytic converter 21 is executed (step 106).

The exhaust gas is passed through the first exhaust pipe 12a and the NOx is stored in the first catalytic converter 21 by the occlusion reduction catalyst.
When x is being absorbed, the air-fuel ratio of the exhaust gas needs to be lean, so no sub-injection is performed.

Next, the operation control of the exhaust gas purifying apparatus will be described in more detail with reference to FIGS. FIG.
6 is an operation control routine of the exhaust gas purification apparatus, and this operation control routine is executed by the ECU 50 at every constant crank angle.

First, at step 200, the reproduction execution flag F
It is determined whether 1 is 1 or not. When the regeneration execution flag F1 is 1, it means that the occlusion reduction type catalyst of the first catalytic converter 21 is being regenerated, and when the regeneration execution flag F1 is 0, it means that the occlusion reduction type catalyst has been regenerated. I do. The reproduction execution flag F1 is 0 in the initial routine.

If NO is determined in the step 200, it is determined whether the current running state of the vehicle is an acceleration state or a non-acceleration state. That is, the accelerator opening α1 is detected by the accelerator opening sensor 17 (step 201), and then the accelerator opening α2 is detected once more (step 20).
2) The difference (α2−α1) in accelerator opening is calculated, and it is determined whether or not the difference is positive (step 203).

If the difference in accelerator opening is positive (α2−α1> 0), it is determined whether the determination timer is running (step 204), and if it is, the determination timer value is updated (step 204). Step 205), it is determined whether or not the updated determination timer value exceeds a preset determination value t (step 206).

If YES is determined in step 206,
Since the increasing state of the accelerator opening has continued for a predetermined time, it is determined that the vehicle is accelerating, and the routine proceeds to step 207, where the acceleration flag F2 is set to 1, the switching valve 20 is switched, the first exhaust pipe 12a is opened and the second exhaust gas is opened. The pipe 12b is closed and the exhaust gas flows through the first exhaust pipe 12a (step 208), and NOx in the exhaust gas is absorbed by the storage reduction catalyst of the first catalytic converter 21. Then, the amount of NOx absorbed by the storage reduction catalyst of the first catalytic converter 21 is integrated.

Incidentally, it is difficult to directly detect the total NOx amount absorbed by the storage reduction type catalyst. Therefore, here, the NOx emission amount in the exhaust gas discharged from the engine is estimated, and the NOx absorption amount absorbed by the storage reduction catalyst is estimated from the exhausted NOx amount.

That is, as the engine speed N increases, the amount of exhaust gas discharged from the engine per unit time increases. Therefore, the NOx amount discharged from the engine increases as the engine speed N increases. Also, the engine load L
The higher the engine load L, the higher the combustion temperature. Therefore, the higher the engine load L, the higher the NO discharged per unit time.
x amount increases. Therefore, the engine load L
The relationship between these parameters and the amount of NOx emitted from the engine per unit time is determined and mapped using the parameters and the engine speed N as a parameter, and this NOx emission map is stored in the ROM of the ECU 50.

The NOx emission amount is determined from the engine speed N obtained based on the output pulse of the crank angle sensor 35 and the engine load L obtained based on the accelerator opening α detected by the accelerator opening sensor 17. The engine emission NOx amount Nij per unit time is read out with reference to the map (step 209), and the NOx amount absorbed by the occlusion reduction type catalyst until the next execution of this operation control routine is obtained. Q is updated (step 210), and the execution of this routine ends.

From the next time on, the present operation control routine is executed, and as long as it is determined in steps 200 to 206 that the vehicle is in the accelerated state, the exhaust gas is flown to the first exhaust pipe 12a and stored in steps 209 and 210. The accumulation of the NOx amount absorbed by the reduction catalyst is continued.

If the determination in step 203 is NO, it means that the vehicle has been switched to the non-acceleration state, and the flow advances to step 211 to reset the determination timer.
By switching the switching valve 20, the second exhaust pipe 12b is opened, the first exhaust pipe 12a is closed, and the exhaust gas flows through the second exhaust pipe 12b (step 212), and NOx in the exhaust gas is selectively reduced by the second catalytic converter 22. Reduction with a type catalyst.

In order to reduce NOx with a selective reduction catalyst, it is necessary to add HC as a reducing agent to the exhaust gas. However, the molecular size of HC added here is relatively small (the number of C Is 8 or less), the NOx purification rate is higher. Diesel fuel contains a large amount of large-molecule-size HC with a larger amount of C, so that it is injected into the cylinder of the cylinder in the expansion and exhaust strokes rather than being supplied as it is immediately upstream of the selective reduction catalyst ( Hereinafter, this is referred to as an auxiliary injection),
Purification efficiency is higher when the catalyst is thermally decomposed by high-temperature exhaust gas and supplied as small molecular HC to the selective reduction catalyst.

Therefore, in this embodiment, step 2
In step 13, it is determined whether or not the catalyst outlet gas temperature T detected by the outlet gas temperature sensor 18 is within a predetermined temperature range (T1 <T <T2) (for example, 200 to 400 ° C.), and if YES is determined. In step 21, the sub-injection condition is determined (step 21).
4) The sub-injection is executed under these conditions (step 21).
5), HC was added to the exhaust gas.

Part of the HC supplied to the selective reduction catalyst by the sub-injection is partially oxidized to generate active species, and the active species reacts with NOx to reduce NOx, and N ₂ , H ₂
Produces ₂ O, O, CO ₂ . The setting method of the sub injection condition will be described later in detail.

After executing the sub-injection in step 215, the process proceeds to step 216. If the determination in step 213 is NO, the process proceeds to step 216 without executing the sub-injection because the condition for executing the sub-injection is not satisfied.

At step 216, it is determined whether or not the acceleration flag F2 is 1. Since the acceleration flag F2 has already been set to 1 at step 207, at step 216, F2 = 1.
It proceeds to step 217.

At step 217, it is determined whether or not the regeneration execution flag F1 is 1. Since the regeneration execution flag F1 is 0 in the initial routine, NO is determined at step 217.
It proceeds to step 218.

[0061] At step 218, the NOx integrated value Q calculated in step 210, determines whether the over reproduction permission NOx value Q ₀ which is set in advance. Note that the regeneration permission NOx
The value Q ₀ is stored in advance in the ROM of the ECU 50 as a minimum NOx absorption amount sufficient for regeneration.

If it is determined in step 218 that the NOx integrated value Q does not exceed the regeneration-permitted NOx value Q ₀ , the regeneration is not required because the NOx integrated value required for regeneration has not been reached, and this routine is executed. finish.

When it is determined that the NOx integrated value Q exceeds the regeneration determination NOx value Q ₀ , the process proceeds to step 219 and thereafter, and the regeneration process is performed on the occlusion reduction type catalyst of the first catalytic converter 21. .

First, at step 219, the reproduction time corresponding to the NOx integrated value Q is read out with reference to the reproduction time map stored in the ROM of the ECU 50. The regeneration time map indicates the amount of NOx absorbed in the storage reduction catalyst of the first catalytic converter 21 and the reducing agent valve 31 necessary to inject the amount of fuel for releasing and reducing all the NOx by experiments in advance. Is obtained by mapping the relationship with the valve opening time, and is stored in the ROM of the ECU 50 in advance.

Next, the valve driving device 30 is driven to open the reducing agent valve 31, and the first catalytic converter 21
The fuel is injected into the inside, and the EGR valve 27 is opened by driving the valve driving device 26 (step 220), and the inside of the first catalytic converter 21 is set to a reducing atmosphere to release NOx absorbed by the occlusion reduction type catalyst.・ Reduce.

Here, the valve opening of the EGR valve 27 is
Is determined based on the normal EGR control according to the operating state of the EGR valve 27. However, only during the regeneration process of the first catalytic converter 21, the opening of the EGR valve 27 in the EGR control is equal to or less than a predetermined set opening (fully closed). ) Is issued,
The control is performed such that priority is given to maintaining the valve opening state of the set opening degree.

During the regeneration process of the first catalytic converter 21,
By opening the EGR valve 27, the first catalytic converter 21
Of the first catalytic converter 21
NOx absorbed in the NOx storage reduction catalyst is easily released. In addition, since the leakage occurs in the switching valve 20 that closes the first exhaust pipe 12a and the flow of the exhaust gas is appropriately generated in the first exhaust pipe 12a, the fuel injected from the injection nozzle 24 is stored in the storage reduction catalyst. In a short period of time, it becomes easy to diffuse and spreads to every corner, thereby promoting regeneration of the storage reduction catalyst.

Thereafter, the reproduction timer is started (step 221), the reproduction execution flag F1 is set to 1 (step 222), and the execution of this routine is terminated. In the next execution of the operation control routine, the regeneration execution flag F1 is determined to be 1 in step 200, so the process proceeds from step 200 to step 212, and further proceeds to steps 213 to 21.
After step 6, the process proceeds to step 217.

In step 217, the reproduction execution flag F1 =
Therefore, the process proceeds to step 223, and
It is determined whether the reproduction set time read in step 9 has elapsed. If the reproduction time has not elapsed, the execution of this routine is terminated, and the reproduction is continued. Step 22
If it is determined in step 3 that the reproduction time has elapsed, step 22
4 and the fuel injection from the injection nozzle 24 is stopped by closing the reducing agent valve 31 and the EGR valve 27 is closed,
Subsequent opening control of the EGR valve 27 is entrusted to normal EGR control. Thus, the regeneration processing of the storage reduction catalyst of the first catalytic converter 21 is completed.

Next, the regeneration execution flag F1 and the acceleration flag F2
Are set to 0 (step 225), the NOx integrated value Q is reset (step 226), the regeneration timer is reset (step 227), and the execution of the present operation control routine is terminated.

If it is determined in step 204 that the determination timer has not started, the process proceeds to step 212 after activating the determination timer in step 228, and the determination timer value does not exceed the predetermined time t in step 206. When the determination is made, the process proceeds to step 212, and in any case, the exhaust gas is controlled to flow through the second exhaust pipe 12b.

As can be seen from the above description, in this exhaust gas purification apparatus, the regeneration timing of the occlusion reduction type catalyst of the first catalytic converter 21 was absorbed during acceleration regardless of whether the catalyst was saturated with NOx. NOx is immediately released and reduced at the time of non-acceleration after the end of acceleration, so that the occlusion-reduction catalyst is always in a regenerated state to prepare for NOx absorption at the next acceleration.

In addition, while it is advantageous to regenerate the occlusion reduction type catalyst at a higher temperature, in this embodiment, the regeneration of the occlusion reduction type catalyst is maintained at a relatively high temperature. Since it is performed immediately after acceleration, the release and reduction of NOx are performed quickly.

In this embodiment, the flow path switching means is realized by a part of the series of signal processing performed by the switching valve 20, the valve driving device 23, and the ECU 50, which executes steps 201 to 208 and 212. Injection nozzle 24, valve driving device 26, and reducing agent valve 3
Step 1 of a series of signal processing by ECU 1 and ECU 50
The portion from Step 17 to Step 224 implements a reducing agent adding means.

Next, a method of setting the above-described sub-injection condition will be briefly described. In the sub-injection, when fuel is injected into a cylinder in the expansion and exhaust strokes, if the injection amount is too small, the amount of HC in the exhaust gas becomes insufficient, and the NOx reduction by the selective reduction catalyst becomes insufficient. On the other hand, if the injection amount is too large, HC becomes larger than that consumed for NOx reduction, and excess HC is discharged, resulting in deterioration of HC emission and reduction of fuel consumption. Further, the engine operating state (the engine speed N,
The generated NOx amount changes according to the engine load L),
Since the required HC amount for reducing x also changes, an optimum amount of sub-injection must be executed according to the engine operating state. This sub-injection condition is determined by the condition setting routine of FIG. The condition setting routine of FIG. 7 is executed by the ECU 50 at every constant crank angle.

First, at step 2151, the engine speed N calculated from the output signal of the crank angle sensor 35 and the engine load L calculated from the output signal of the accelerator opening sensor 17 are read.

Next, the routine proceeds to step 2152, where the cylinder number for performing the sub-injection is determined. That is, the crank angle sensor 34
By knowing, for example, when the first cylinder comes to the top dead center from the output pulse, and by knowing from the output signal of the crank angle sensor 35, for example, the number of rotations of the first cylinder from the top dead center position. The current crank angle can be calculated, and if the stroke of the first cylinder is known, the strokes of the other cylinders can be known, and which cylinder is in the expansion and exhaust strokes, and therefore the cylinder number (the expansion , The number of the cylinder in any of the exhaust strokes).

Next, the routine proceeds to step 2153, where the necessary sub-injection amount q is determined. That is, the engine operating state is substantially determined from the current engine speed N and the engine load L, and N
HC for determining the amount of generated Ox and purifying it
The amount or HC concentration is also determined. This is the target HC concentration HC1
When the target HC concentration HC1 is determined, the necessary sub-injection amount q for obtaining the target HC concentration HC1 is also determined. Therefore, the relationship among the engine rotation speed N, the engine load L, and the required sub-injection amount q is obtained by an experiment, and this is stored as a map in the ROM of the ECU 50 in advance. Step 2153
Then, the necessary sub-injection amount q is determined from the current engine speed N and the engine load L with reference to the map. still,
As the engine rotational speed N increases and the engine load L increases, the required sub-injection amount q also increases.

Next, the routine proceeds to step 2154, where the fuel pressure P of the sub-injection and the injection period Tm are determined. That is, since the sub-injection amount is determined by the injection period Tm of the fuel injection valve 9 and the fuel pressure P, first, the fuel pressure P according to the engine operating state is determined from the period rotation speed N and the period load L. The injection period Tm is determined from the fuel pressure P and the necessary sub-injection amount q. The relationship between the period rotation speed N, the period load L, and the fuel pressure P is stored in the ROM of the ECU 50 in advance as a map, and the relationship between the fuel pressure P, the required sub-injection amount q, and the injection period Tm is mapped in advance. These maps are referred to when the fuel pressure P and the injection period Tm are determined.

Next, the routine proceeds to step 2155, where the sub-injection start timing τ _s is determined based on the injection period T _m calculated in step 2154. That is, the entire amount of the fuel injected by the sub-injection must be injected into the cylinder during the expansion and exhaust strokes of the engine, and furthermore, both the intake valve and the exhaust valve near the end of the exhaust stroke are opened. The sub-injection must end before the opening time. If the end timing of the sub-injection is determined so as not to overlap the intake / exhaust valve opening overlap timing, the timing earlier by _Tm than that is the sub-injection start timing τ _s .

The sub-injection is performed in accordance with the sub-injection amount q, the injection period T _m , and the injection start timing τ _s thus obtained.

[0082]

According to the present invention, there is provided an exhaust gas purifying apparatus for an internal combustion engine in which exhaust gas discharged from an internal combustion engine burning in an air-fuel ratio lean state passes through an exhaust passage and is purified by a catalyst provided in the exhaust passage. The passage branches midway into a first exhaust passage and a second exhaust passage, and when the exhaust gas temperature changes from high temperature to low temperature, the exhaust gas flows through the second exhaust passage. Otherwise, the exhaust gas flows through the first exhaust passage. Flow path switching means for switching the flow path of the exhaust gas so as to allow the exhaust gas to flow therethrough; provided in the first exhaust passage, absorbing NOx when the air-fuel ratio of the inflowing exhaust gas is lean and reducing the oxygen concentration of the inflowing exhaust gas A NOx storage-reduction catalyst that releases NOx absorbed when it is released, a selective reduction-type catalyst that is provided in the second exhaust passage, and reduces or decomposes NOx in the presence of hydrocarbons in an oxygen-excess atmosphere. A reducing agent adding means for adding a reducing agent to the occlusion reduction type catalyst while discharging exhaust gas through the second exhaust passage so as to release and reduce NOx absorbed by the catalyst. The type catalyst and the selective reduction type catalyst can be selectively used according to their respective characteristics, and the NOx purification rate can be improved.
Further, the entire exhaust gas purification system can be simplified.

[Brief description of the drawings]

FIG. 1 is an overall configuration diagram of an exhaust gas purifying apparatus for an internal combustion engine according to an embodiment of the present invention.

FIG. 2 is a diagram showing a relationship between an air-fuel ratio of exhaust gas and concentrations of main components in the exhaust gas.

FIG. 3 is a diagram illustrating the principle of NOx absorption and NOx release / reduction of a storage reduction catalyst.

FIG. 4 is a flowchart illustrating an outline of an operation control procedure of the exhaust gas purification device for an internal combustion engine of the present invention.

FIG. 5 is a detailed flowchart (part 1) illustrating an operation control procedure in the embodiment of the exhaust gas purification device for an internal combustion engine of the present invention.

FIG. 6 is a detailed flowchart (part 2) illustrating an operation control procedure in the embodiment of the exhaust gas purification device for an internal combustion engine of the present invention.

FIG. 7 is a flowchart showing a sub-injection condition setting procedure in one embodiment of the exhaust gas purifying apparatus for an internal combustion engine of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Diesel engine main body (internal combustion engine) 12 Exhaust pipe (exhaust passage) 12a First exhaust pipe (first exhaust passage) 12b Second exhaust pipe (second exhaust passage) 20 Switching valve (flow path switching means) 21 First catalyst Converter (storage reduction type catalyst) 22 Second catalytic converter (selective reduction type catalyst) 23 Valve driving device (flow path switching unit) 24 Injection nozzle (reducing agent adding unit) 30 Valve driving device (reducing agent adding unit) 31 Reducing agent Valve (reducing agent addition means)

Claims (1)

[Claims] An exhaust gas purifying apparatus for an internal combustion engine, wherein exhaust gas discharged from an internal combustion engine burning in an air-fuel ratio lean state passes through an exhaust passage and is purified by a catalyst provided in the exhaust passage. The exhaust gas is branched into an exhaust passage and a second exhaust passage.
Exhaust gas is allowed to flow through the exhaust passage.
Flow path switching means for switching the flow path of the exhaust gas so as to allow the exhaust gas to flow through the exhaust path; and provided in the first exhaust path to absorb NOx when the air-fuel ratio of the inflowing exhaust gas is lean and to reduce the oxygen in the inflowing exhaust gas. A storage-reduction catalyst that releases NOx absorbed when the concentration is reduced; a selective reduction catalyst that is provided in the second exhaust passage and that reduces or decomposes NOx in the presence of hydrocarbons in an oxygen-excess atmosphere; Reducing means for adding a reducing agent to the occlusion reduction type catalyst while flowing exhaust gas through the second exhaust passage in order to release and reduce NOx absorbed by the type catalyst. Exhaust purification device for an internal combustion engine.