JPH06272540A - Exhaust emission control device for internal combustion engine - Google Patents

Abstract

PURPOSE:To release NOx absorbent for a short time by increasing the degree of air fuel ratio rich in exhaust gas flowing into the NOx absorbent in proportion to an amount of NOx estimated to be absorbed in the NOx absorbent. CONSTITUTION:An NOx absorbent 17 for absorbing NOx when inflow exhaust gas is lean an releasing the absorbed NOx when the inflow exhaust gas gets rich is disposed in an exhaust path of an engine. An amount of NOx absorbed in the absorbent 17 is estimated and the degree of richness of air fuel ratio when the air fuel ratio of the exhaust gas flowing into the NOx absorbent 17 to release the NOx from the NOx absorbent 17 gets rich is increased in proportion to an amount of NOx estimated to be absorbed in the NOx absorbent 17. Thus, unburnt HC and CO are blocked from being emitted to the atmosphere while the NOx can be released from the NOx absorbent 17 for a short time.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an exhaust gas purification device for an internal combustion engine.

[0002]

2. Description of the Related Art In an internal combustion engine that burns a lean mixture, a NOx absorbent that absorbs NOx when the air-fuel ratio of the inflowing exhaust gas is lean and releases the absorbed NOx when the inflowing exhaust gas becomes rich. The NOx absorbent, which is arranged in the engine exhaust passage and absorbs NOx generated when the lean air-fuel mixture is burned, absorbs NOx of the NOx absorbent.
An internal combustion engine in which the air-fuel ratio of the exhaust gas flowing into the NOx absorbent is temporarily made rich before the Ox absorbing capacity is saturated to release NOx from the NOx absorbent and reduce the released NOx. It has already been proposed by a person (see Japanese Patent Application No. 3-284095).

[0003]

If the air-fuel ratio of the exhaust gas flowing into the NOx absorbent is made rich, for example, if the air-fuel mixture supplied to the engine cylinder is made rich, a large amount of unburned HC, CO is emitted from the engine. Etc. are discharged and NO
Absorbed NOx is released from the x absorbent. At this time, a part of the unburned HC, CO, etc. discharged from the engine is used to reduce the NOx discharged from the engine, and the remaining unburned HC, CO, etc. are released by the NOx absorbent.
Used to reduce Ox. Therefore, in this case, N
In order to suppress the release of Ox into the atmosphere, it is necessary to discharge from the engine an amount of unburned HC, CO, etc. that can reduce both NOx discharged from the engine and NOx released from the NOx absorbent. is there.

However, as in the above-mentioned internal combustion engine, NO
Even if the air-fuel ratio of the exhaust gas flowing into the NOx absorbent is simply made rich when NOx should be released from the x absorbent, unburned HC, CO, etc. discharged from the engine can reduce all NOx. Less or more than you need. As a result, when the amount of unburned components such as unburned HC and CO is small, all NOx cannot be released from the NOx absorbent, and when the amount of unburned components is large, surplus unburned components are oxidized. The problem arises that the NOx absorbent is exhausted without any trouble.

[0005]

In order to solve the above problems, according to the present invention, NOx is absorbed when the inflowing exhaust gas is lean and the absorbed NOx is absorbed when the inflowing exhaust gas becomes rich. The NOx absorbent to be released is arranged in the engine exhaust passage, and the Nx absorbed by the NOx absorbent is placed.
Equipped with means for estimating the amount of Ox, Nx from NOx absorbent
When the air-fuel ratio of the exhaust gas flowing into the NOx absorbent is made rich in order to release Ox, the rich degree is increased as the NOx amount estimated to be absorbed by the NOx absorbent increases.

[0006]

[Function] As the amount of NOx absorbed in the NOx absorbent increases, the NOx released from the NOx absorbent during NOx release
The amount increases. Therefore, the N absorbed by the NOx absorbent
The greater the amount of Ox, the higher the degree of richness.

[0007]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 1, 1 is an engine body, 2 is a piston, 3 is a combustion chamber, 4 is a spark plug, 5 is an intake valve, 6 is an intake port, 7 is an exhaust valve, and 8 is an exhaust port. Show. The intake port 6 is connected to the surge tank 10 via a corresponding branch pipe 9, and each branch pipe 9 is provided with a fuel injection valve 11 for injecting fuel into the intake port 6. The surge tank 10 is connected to an air cleaner 13 via an intake duct 12, and a throttle valve 14 is arranged in the intake duct 12. On the other hand, the exhaust port 8 is connected via an exhaust manifold 15 and an exhaust pipe 16 to a casing 18 containing a NOx absorbent 17.

The electronic control unit 30 comprises a digital computer, and a ROM (read only memory) 32, a RAM (random access memory) 33, a CPU (microprocessor) 34, an input port 35 which are mutually connected by a bidirectional bus 31. And an output port 36. A pressure sensor 19 for generating an output voltage proportional to the absolute pressure in the surge tank 10 is attached in the surge tank 10, and the output voltage of the pressure sensor 19 is input to the input port 35 via the AD converter 37. . Attached to the throttle valve 14 is a throttle sensor 20 which generates an output pulse each time the throttle valve 14 rotates by a certain angle and an output signal indicating that the throttle valve 14 is in the idling position. These output pulses and output signals are input to the input port 35, and the valve opening speed of the throttle valve 14 is calculated from the output pulses in the CPU 34.

A temperature sensor 21 for generating an output voltage proportional to the exhaust gas temperature is provided in the exhaust pipe 16 upstream of the casing 18.
Is attached, and the output voltage of the temperature sensor 21 is input to the input port 35 via the AD converter 38. Furthermore,
A transmission 23 connected to the engine crankshaft 22 is provided with a shift change sensor 24 for generating an output signal indicating that a gear change operation is being performed, and the output signal of the shift change sensor 24 is input to an input port 35. .
Further, the input port 35 is connected to a rotation speed sensor 25 that generates an output pulse representing the engine rotation speed. On the other hand, the output port 36 is connected to the spark plug 4 and the fuel injection valve 11 via the corresponding drive circuit 39, respectively.

In the internal combustion engine shown in FIG. 1, the fuel injection time TAU is calculated based on the following equation, for example. TAU = TP · K Here, TP represents the basic fuel injection time, and K represents the correction coefficient. The basic fuel injection time TP indicates the fuel injection time required to make the air-fuel ratio of the air-fuel mixture supplied into the engine cylinder the stoichiometric air-fuel ratio. This basic fuel injection time TP is previously obtained by an experiment, and the surge tank 10
It is previously stored in the ROM 32 in the form of a map as shown in FIG. 2 as a function of the absolute pressure PM and the engine speed N. The correction coefficient K is a coefficient for controlling the air-fuel ratio of the air-fuel mixture supplied to the engine cylinder, and if K = 1.0, the air-fuel mixture supplied to the engine cylinder becomes the stoichiometric air-fuel ratio. On the other hand, if K <1.0, the air-fuel ratio of the air-fuel mixture supplied to the engine cylinder becomes larger than the stoichiometric air-fuel ratio, that is, becomes lean, and if K> 1.0, it is supplied to the engine cylinder. The air-fuel ratio of the air-fuel mixture thus generated becomes smaller than the stoichiometric air-fuel ratio, that is, becomes rich.

The correction coefficient K is controlled according to the operating state of the engine, and FIG. 3 shows an embodiment of the control of the correction coefficient K. In the embodiment shown in FIG. 3, the correction coefficient K is gradually decreased as the engine cooling water temperature increases during warm-up operation, and when warm-up is completed, the correction coefficient K becomes a constant value smaller than 1.0, that is, the engine. The air-fuel ratio of the air-fuel mixture supplied into the cylinder is maintained lean. Next, when the idling operation is performed, the correction coefficient K is set to 1.0, that is, the air-fuel ratio of the air-fuel mixture supplied into the engine cylinder is set to the stoichiometric air-fuel ratio, and when the acceleration operation is performed, the correction coefficient K is 1. The air-fuel ratio is made larger than 0, that is, the air-fuel ratio of the air-fuel mixture supplied into the engine cylinder is made rich. As can be seen from FIG. 3, in the embodiment shown in FIG. 3, the air-fuel ratio of the air-fuel mixture supplied to the engine cylinder is maintained at a constant lean air-fuel ratio except during warm-up operation, idling operation and acceleration operation. Therefore, the lean air-fuel mixture is burned in most engine operating regions.

FIG. 4 schematically shows the concentrations of typical components in the exhaust gas discharged from the combustion chamber 3. As can be seen from FIG. 4, the concentration of unburned HC and CO in the exhaust gas discharged from the combustion chamber 3 increases as the air-fuel ratio of the air-fuel mixture supplied into the combustion chamber 3 becomes richer, and is discharged from the combustion chamber 3. The concentration of oxygen O _{2 in} the generated exhaust gas increases as the air-fuel ratio of the air-fuel mixture supplied into the combustion chamber 3 becomes leaner.

NOx contained in the casing 18
The absorbent 17 uses, for example, alumina as a carrier, and potassium K, sodium Na, lithium Li, an alkali metal such as cesium Cs, an alkaline earth such as barium Ba, calcium Ca, lanthanum La, and yttrium Y are used on the carrier. At least one selected from such rare earths and a noble metal such as platinum Pt are supported. The ratio of the air and the fuel (hydrocarbon) supplied into the exhaust passage upstream of the engine intake passage and the NOx absorbent 17 is determined by the NOx agent 1
When referred to as the air-fuel ratio of the exhaust gas flowing into 7, the NOx absorbent 17 is NO when the air-fuel ratio of the exhaust gas is lean.
x is absorbed, and when the oxygen concentration in the inflowing exhaust gas is reduced, a NOx absorption / release action is performed to release the absorbed NOx. When fuel (hydrocarbon) or air is not supplied into the exhaust passage upstream of the NOx absorbent 17, the air-fuel ratio of the inflowing exhaust gas matches the air-fuel ratio of the air-fuel mixture supplied into the combustion chamber 3, and In this case, the NOx absorbent 17 is NOx when the air-fuel ratio of the air-fuel mixture supplied into the combustion chamber 3 is lean.
When the oxygen concentration in the air-fuel mixture supplied to the combustion chamber 3 is reduced, the absorbed NOx is released.

If the above-mentioned NOx absorbent 17 is arranged in the engine exhaust passage, the NOx absorbent 17 actually performs the NOx absorption / release operation, but the detailed mechanism of this absorption / release operation is not clear. However, it is considered that this absorbing / releasing action is performed by the mechanism shown in FIG. Next, this mechanism will be described by taking the case where platinum Pt and barium Ba are supported on the carrier as an example, but the same mechanism can be obtained by using other noble metals, alkali metals, alkaline earths and rare earths.

That is, the inflowing exhaust gas becomes considerably lean.
And the concentration of oxygen in the inflowing exhaust gas increased drastically.
As shown in (A), these oxygen O₂Is O₂ ^-Or O
^2-In the form of, it adheres to the surface of platinum Pt. On the other hand, inflow and outflow
NO in the gas is O on the surface of platinum Pt.₂ ^-Or O^2-
Reacts with NO₂Becomes (2NO + O₂→ 2 NO₂).
NO generated next₂Part of is oxidized on platinum Pt
At the same time, it is absorbed in the absorbent and binds to barium oxide BaO.
Meanwhile, as shown in FIG. ₃ ^-
Diffuses into the absorbent in the form of. In this way NOx is N
It is absorbed in the Ox absorbent 17.

As long as the oxygen concentration in the inflowing exhaust gas is high, NO ₂ is produced on the surface of platinum Pt, and unless the NOx absorption capacity of the absorbent is saturated, NO ₂ is absorbed in the absorbent and nitrate ions NO ₃ ⁻ are produced. Is generated. In contrast the reaction with the amount of NO ₂ oxygen concentration is lowered in the inflowing exhaust gas is lowered backward (NO ₃ ^- → NO ₂₎ proceeds to, thus nitrate ions to the absorber NO ₃ ^- Are released from the absorbent in the form of NO ₂ . That is, when the oxygen concentration in the inflowing exhaust gas decreases, NOx is released from the NOx absorbent 17. As shown in FIG. 4, when the lean degree of the inflowing exhaust gas is low, the oxygen concentration in the inflowing exhaust gas is low. Therefore, even if the leaning degree of the inflow exhaust gas is low, the air-fuel ratio of the inflowing exhaust gas is lean. Even NOx absorbent 17
NOx will be released from.

On the other hand, at this time, when the air-fuel mixture supplied to the combustion chamber 3 is made rich and the air-fuel ratio of the inflowing exhaust gas becomes rich, a large amount of unburned H is emitted from the engine as shown in FIG.
C and CO are discharged, and these unburned HC and CO are platinum Pt.
It is oxidized by reacting with the oxygen O ₂ ⁻ or O ^{2 −} above. Further, when the air-fuel ratio of the inflowing exhaust gas becomes rich, the oxygen concentration in the inflowing exhaust gas is extremely lowered, so NO ₂ is released from the absorbent, and this NO ₂ is unrecovered as shown in FIG. 5 (B). It is reduced by reacting with fuel HC and CO.
When NO ₂ is no longer present on the surface of platinum Pt in this way, NO ₂ is released one after another from the absorbent. Therefore, when the air-fuel ratio of the inflowing exhaust gas is made rich, NOx is released from the NOx absorbent 17 within a short time.

That is, when the air-fuel ratio of the inflowing exhaust gas is made rich, first, the unburned HC and CO immediately react with O ₂ ⁻ or O ^{2 −} on the platinum Pt to be oxidized, and then the O on the platinum Pt is oxidized. ₂ ^- or O ^{2 -} yet be consumed unburned H
If C and CO remain, NOx emitted from the absorbent by the unburned HC and CO and N emitted from the engine.
Ox is reduced. Therefore, if the air-fuel ratio of the inflowing exhaust gas is made rich, the NOx absorbed in the NOx absorbent 17 is released within a short time, and the released NOx is reduced, so that the NOx is exhausted to the atmosphere. Can be prevented. Further, since the NOx absorbent 17 has the function of a reduction catalyst, the NOx released from the NOx absorbent 17 can be reduced even if the air-fuel ratio of the inflowing exhaust gas is changed to the stoichiometric air-fuel ratio. However, when the air-fuel ratio of the inflowing exhaust gas is set to the stoichiometric air-fuel ratio, NOx
Since NOx is gradually released from the absorbent 17, it takes a slightly longer time to release all the NOx absorbed in the NOx absorbent 17.

If the air-fuel ratio of the inflowing exhaust gas is made rich in this way, NOx is released from the NOx absorbent 17 within a short time, but if the rich degree is too high at this time,
That is, if the amount of unburned HC and CO becomes excessively large, surplus unburned HC and CO are released into the atmosphere. At this time, if the degree of richness is too low, that is, unburned HC and CO,
If the amount of CO is too small, NOx absorbent 1
Therefore, all NOx cannot be released from 7.
Therefore, in order to reduce all NOx emitted from the absorbent and all NOx emitted from the engine when the air-fuel ratio of the inflowing exhaust gas is made rich, O ₂ ⁻ or O on platinum Pt can be reduced.
² -The amount of unburned HC and CO required to consume
The amount of unburned HC and CO required to reduce Ox is NO
It is necessary to control the rich degree of the air-fuel ratio of the inflowing gas so that the inflowing gas flows into the x absorbent 17.

By the way, when NOx is released from the NOx absorbent 17, most of unburned HC and CO are NOx.
Since it is used to reduce the NOx released from the absorbent 17, the degree of richness when the air-fuel ratio of the inflowing exhaust gas is made rich is NO absorbed by the NOx absorbent 17
The higher the x amount is, the higher the x amount becomes, so that it is possible to satisfactorily release all NOx from the NOx absorbent 17 while preventing excess unburned HC and CO from being released into the atmosphere.

Therefore, in the embodiment according to the present invention, NO is actually set.
The ratio of the amount of NOx absorbed in the x-absorbent 17 to the maximum amount of NOx absorbed by the NOx-absorbent 17, that is, NOx.
The saturation S is estimated, and the rich degree of the inflowing exhaust gas when NOx is to be released from the NOx absorbent 17 is set to increase as the NOx saturation S increases. That is,
As can be seen from FIG. 6, the air-fuel ratio of the air-fuel mixture normally supplied into the engine cylinder is lean, and at this time, N
Since NOx is continuously absorbed by the NOx absorbent 17, NOx
The saturation S continues to rise.

On the other hand, in the embodiment according to the present invention, at the start of deceleration, the air-fuel ratio of the air-fuel mixture supplied into the engine cylinder is temporarily made rich before the fuel supply is stopped. It increases as the NOx saturation S increases. Therefore, at this time, the NOx saturation S falls to zero. Further, the air-fuel mixture is made rich during acceleration operation or shift change, and the rich degree at this time is NOx.
The higher the saturation S, the higher the saturation. Therefore, also at this time, the NOx saturation S falls to zero. Further, during the idling operation, the air-fuel mixture is set to the stoichiometric air-fuel ratio, and at this time NO
Since NOx is gradually released from the x absorbent 17, the NOx saturation S gradually decreases. Further, when the lean air-fuel mixture continues to burn and the NOx saturation S exceeds the maximum allowable saturation So, the air-fuel mixture is forcibly made rich,
The rich degree at this time is a degree according to the maximum allowable saturation So. Therefore, also at this time, the NOx saturation S falls to zero.

Next, a method of calculating the amount of NOx absorbed in the NOx absorbent 17 will be described with reference to FIGS. 7 to 10.
FIG. 7 shows a routine for calculating the NOx absorption amount, and this routine is executed by interruption at regular time intervals. Referring to FIG. 7, first, at step 100, it is judged if the correction coefficient K for the basic fuel injection time TP is zero. When K = 0, that is, when the fuel injection is stopped, the processing cycle is completed. On the other hand, if K = 0, that is, if fuel injection is being performed, the routine proceeds to step 101, where it is judged if the correction coefficient K is smaller than 1.0.

When K <1.0, that is, when the lean air-fuel mixture is being combusted, the routine proceeds to step 102, where the NOx absorption amount A estimated to be absorbed by the NOx absorbent 17 during the time interruption interval is It is calculated. As shown in FIG. 8 (A), the amount of NOx discharged from the engine increases as the absolute pressure PM in the surge tank 10 increases.
As shown in (B), it increases as the engine speed N increases. The NOx absorption amount A is stored in advance in the ROM 32 in the form of a map as shown in FIG. 8C as a function of the absolute pressure PM in the surge tank 10 and the engine speed N. Therefore, in step 102, the NOx absorption amount A is calculated from the map shown in FIG. 8C based on the output signals of the pressure sensor 19 and the rotation speed sensor 25. Next, at step 103, the NOx absorption amount A is added to the total NOx absorption amount C, and then the routine proceeds to step 106. Therefore, when the lean air-fuel mixture is being burned, the total NOx absorption amount C absorbed by the NOx absorbent 17 is gradually increased.

On the other hand, in step 101, K ≧ 1.0
When it is determined that the air-fuel ratio of the air-fuel mixture supplied to the engine cylinder is the stoichiometric air-fuel ratio or rich, the routine proceeds to step 104, where it is estimated that the NOx absorbent 17 is released during the time interruption interval. The NOx emission amount B to be calculated is calculated. As shown in FIG. 9A, the amount of NOx released from the NOx absorbent 17 increases as the temperature of the NOx absorbent 17, that is, the exhaust gas temperature T, increases.
As the correction coefficient K increases as shown in (B),
That is, the higher the degree of richness of the air-fuel mixture supplied into the engine cylinder, the greater the amount. This NOx emission amount B is stored in advance in the ROM 32 in the form of a map as shown in FIG. 9C as a function of the exhaust gas temperature T and the correction coefficient K. Therefore, in step 104, based on the output signal of the temperature sensor 21 and the correction coefficient K, NO is determined from the map shown in FIG.
The x emission amount B is calculated. Next, at step 104, the NOx release amount B is subtracted from the total NOx absorption amount C, and then the routine proceeds to step 106. Therefore, when the air-fuel ratio of the air-fuel mixture supplied into the engine cylinder becomes the stoichiometric air-fuel ratio or rich, N
The total amount C of NOx absorbed by the Ox absorbent 17 is reduced.

At step 106, it is judged if the total NOx absorption amount C has become negative. When C ≧ 0, jump to step 108, and when C <0, step 10
After setting C = 0 in step 7, the process proceeds to step. In step 108, the total NOx absorption amount C absorbed by the NOx absorbent 17 and the maximum N that the NOx absorbent 17 can absorb.
The NOx saturation S, which is the ratio with the Ox absorption amount Co, is calculated. Next, at step 109, it is judged if the NOx saturation S is larger than the maximum allowable saturation So (FIG. 6). When S ≦ So, the processing cycle is completed. On the other hand, when S> So, the routine proceeds to step 110, where the rich flag 1 is set, and then at step 111, the correction coefficient K, that is, the rich degree is calculated. The correction coefficient K increases as the NOx saturation S increases, as shown in FIG. When the rich flag 1 is set, the air-fuel ratio of the air-fuel mixture supplied into the engine cylinder is made rich with a rich degree corresponding to the correction coefficient K as described later.

11 to 13 show a routine for calculating the fuel injection time TAU. 11 to 1
Referring first to FIG.
The basic fuel injection time TP is calculated from the map shown in FIG.
Next, at step 201, the target correction coefficient Ko is calculated. This target correction coefficient Ko is Ko <1.0, for example Ko = 0.6, in an operating state in which a lean air-fuel mixture should be burned.
And K = 1.0 during idling operation.
Then, it proceeds to step 202.

From step 202 to step 204, N
A process for continuing to set the rich flag 1 set when the Ox saturation S exceeds the maximum allowable saturation So for a certain period of time is performed. That is, at step 202, it is judged if the rich flag 1 is set, and if the rich flag 1 is reset, the routine jumps to step 205. On the other hand, when the rich flag 1 is set, the routine proceeds to step 203, where it is judged if a fixed time has elapsed since the rich flag 1 was set. When the fixed time has elapsed, the routine proceeds to step 204, where the rich flag 1 is reset, and thus the rich flag 1 is continuously set for the fixed time.

In steps 205 to 216, during deceleration operation, the air-fuel ratio of the air-fuel mixture supplied into the engine cylinder is first made rich for a certain period of time, and then the process of stopping the fuel supply is performed. That is, at step 205, it is judged if the cut flag indicating that the fuel injection should be stopped is set. When the cut flag is reset, the routine proceeds to step 206, where it is judged based on the output signal of the throttle sensor 20 whether or not the throttle valve 14 is at the idling opening degree. When the throttle valve 14 is at the idling opening, step 20
7, the engine speed N is the fuel cut speed, for example 1
It is determined whether it is higher than 100 rpm. If the throttle valve 14 is not at the idling opening, or if N ≦ 11
When it is 00r.pm, the process jumps to step 217.
On the other hand, when the throttle valve 14 is at the idling opening and N> 1100 rpm, it is determined that the deceleration operation in which fuel injection should be stopped is in progress, and the routine proceeds to step 208.

In step 208, a cut flag indicating that fuel injection should be stopped is set, then in step 209 a rich flag 2 indicating that the air-fuel mixture should be rich is set, and then in step 210 NOx.
The correction coefficient K is calculated from the saturation S (FIG. 10). As will be described later, while the cut flag and the rich flag 2 are both set, the air-fuel mixture is made rich, and then when the rich flag 2 is reset, the fuel injection is stopped.

When the cut flag is set, step 2
From 05, the routine proceeds to step 211, where it is judged if the throttle valve 14 is at the idling opening degree. If the throttle valve 14 is at the idling opening, step 21
2, the engine speed N is returned to the return speed, for example 800r.
It is determined whether it has become lower than pm. N ≧ 800
When the speed is rpm, the routine proceeds to step 213, where it is judged if a fixed time has elapsed since the rich flag 2 was set. Step 21 if the fixed time has not elapsed
7 and when a certain time has elapsed, the routine proceeds to step 214, where the rich flag 2 is reset. Therefore, the rich flag 2 is continuously set for a certain period of time, during which the air-fuel mixture is made rich.

On the other hand, when the throttle valve 14 is opened during the deceleration operation, step 211 to step 2
When the engine speed N becomes 800 rpm or less, the routine proceeds from step 212 to step 215. In step 215, the cut flag is reset,
In step 216, the rich flag 2 is reset.

In steps 217 to 219, a process for making the air-fuel ratio of the air-fuel mixture rich during acceleration is performed.
That is, at step 217, it is judged from the valve opening speed of the throttle valve 14 calculated based on the output pulse of the throttle sensor 20 whether or not it is during acceleration. During acceleration, the routine proceeds to step 218, where the rich flag 3 indicating that the air-fuel mixture should be made rich is set, and then at step 219, the NOx saturation S (FIG. 10) is used to determine the correction coefficient K.
Is calculated. Then, it proceeds to step 221. On the other hand, it is understood that when the acceleration is not being performed, the routine proceeds to step 220, where the rich flag 3 is reset, and accordingly, when the acceleration operation is being performed, the rich flag 3 is set.

From step 221 to step 222, processing is performed to make the air-fuel ratio of the air-fuel mixture rich when the transmission 23 is shift-changed. That is, in step 221, it is determined based on the output signal of the shift change sensor 24 whether or not the shift change action of the transmission 23 is being performed. When the shift change operation is being performed, the routine proceeds to step 222, where the rich flag 4 indicating that the air-fuel mixture should be rich is set, and then at step 223, the correction coefficient K is calculated from the NOx saturation S (FIG. 10). It Then, it proceeds to step 225. On the other hand, when the shift change is not being performed, the routine proceeds to step 224, where the rich flag 4 is reset, so that it is understood that the rich flag 4 is set when the shift change is being performed.

At step 225, it is judged if any one of rich flag 1, rich flag 2, rich flag 3 and rich flag 4 is set. If any rich flag is set, the routine jumps to step 227. On the other hand, when neither rich flag is set, the routine proceeds to step 226, where the target coefficient Ko is made the correction coefficient K,
Then, it proceeds to step 227.

At step 227, it is judged if the cut flag is set. When the cut flag is not set, the routine jumps to step 230 and the fuel injection time TAU (= K · TP) is calculated. On the other hand, when the cut flag is set, the routine proceeds to step 228, where it is judged if the rich flag 2 during deceleration operation is set. When the rich flag 2 is set, the routine proceeds to step 230, where the air-fuel ratio of the air-fuel mixture is made rich. Next, when the rich flag 2 is reset, the routine proceeds to step 229, where the correction coefficient K is made zero, and thus fuel injection is stopped.

[0037]

INDUSTRIAL APPLICABILITY NOx can be released from the NOx absorbent in a short time while preventing the release of unburned HC and CO into the atmosphere.

[Brief description of drawings]

FIG. 1 is an overall view of an internal combustion engine.

FIG. 2 is a diagram showing a map of a basic fuel injection time.

FIG. 3 is a diagram showing changes in a correction coefficient K.

FIG. 4 Unburned HC and C in exhaust gas discharged from the engine
It is a diagram which shows the concentration of O and oxygen roughly.

FIG. 5 is a diagram for explaining the action of absorbing and releasing NOx.

FIG. 6 is a time chart showing changes in the air-fuel ratio and the NOx saturation S.

FIG. 7 is a flowchart for calculating a NOx absorption amount.

FIG. 8 is a diagram showing a NOx absorption amount.

FIG. 9 is a diagram showing the amount of NOx released.

FIG. 10 is a diagram showing the relationship between the NOx saturation S and the correction coefficient K.

FIG. 11 is a flowchart for calculating a fuel injection time TAU.

FIG. 12 is a flowchart for calculating a fuel injection time TAU.

FIG. 13 is a flowchart for calculating a fuel injection time TAU.

[Explanation of symbols]

 15 ... Exhaust manifold 17 ... NOx absorbent

Claims (1)

[Claims] 1. A NOx absorbent that absorbs NOx when the inflowing exhaust gas is lean and releases the absorbed NOx when the inflowing exhaust gas becomes rich is arranged in the engine exhaust passage, and the NOx absorbent is also disposed. A means for estimating the amount of NOx absorbed in the NOx absorbent is provided, and the rich degree when the air-fuel ratio of the exhaust gas flowing into the NOx absorbent is made rich in order to release NOx from the NOx absorbent is absorbed in the NOx absorbent. An exhaust emission control device for an internal combustion engine, which is designed to be increased as the NOx amount estimated to be increased.