JPH0666129A - Exhaust emission control device for internal combustion engine - Google Patents

Abstract

PURPOSE:To release sulfur absorbed by an NOx absorber. CONSTITUTION:An NOx absorber 18 is arranged inside an engine exhaust passage for absorbing NOx when an air-fuel ratio of inflow of exhaust gas is lean, and releasing the absorbed NOx when an oxygen density in the inflow of the exhaust gas is decreased. Sulfur absorbing rate of the NOx absorber 18 is estimated. When the estimated sulfer absorbing rate exceeds a specified set value, an electric heater 20 is heated, the air-fuel ratio is switched from the lean air-fuel ratio to a theoretical air-fuel ratio, and the sulfur absorbed by the NOx absorber 18 is released.

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, NOx absorbs NOx when the air-fuel ratio of the inflowing exhaust gas is lean and releases the absorbed NOx when the oxygen concentration in the inflowing exhaust gas decreases. The NOx absorbent absorbs NOx generated when a lean air-fuel mixture is burnt by placing the absorbent in the engine exhaust passage, and the exhaust gas flowing into the NOx absorbent before the NOx absorbent capacity of the NOx absorbent is saturated. The applicant has already proposed an internal combustion engine in which the air-fuel ratio of gas is temporarily made rich to release NOx from the NOx absorbent and reduce the released NOx (Japanese Patent Application No. 3-284095). reference).

[0003]

However, since sulfur is contained in the fuel and the lubricating oil of the engine, sulfur is contained in the exhaust gas, and this sulfur is also absorbed by the NOx absorbent together with NOx. . However, this sulfur is not released from the NOx absorbent even if the air-fuel ratio of the exhaust gas flowing into the NOx absorbent is made rich, so that the amount of sulfur in the NOx absorbent gradually increases. However, N
When the amount of sulfur in the Ox absorbent increases, the amount of NOx that can be absorbed by the NOx absorbent gradually decreases until finally NOx is absorbed.
The problem arises that the absorbent can hardly absorb NOx.

[0004]

In order to solve the above problems, according to the present invention, NOx is absorbed when the air-fuel ratio of the inflowing exhaust gas is lean, and it is absorbed when the oxygen concentration in the inflowing exhaust gas decreases. In an internal combustion engine in which a NOx absorbent that releases NOx is arranged in an engine exhaust passage, a sulfur absorption amount estimating means for estimating a sulfur absorption amount of the NOx absorbent,
And a sulfur release means for releasing sulfur from the NOx absorbent when the amount of absorbed sulfur estimated by the means for estimating amount of absorbed sulfur exceeds a predetermined set amount.

[0005]

When the absorbed amount of sulfur discharged from the engine and absorbed by the NOx absorbent exceeds the set amount, sulfur is released from the NOx absorbent, thereby restoring the NOx absorbent capacity of the NOx absorbent.

[0006]

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 includes an intake duct 12 and an air flow meter 13
Is connected to the air cleaner 14 through the intake duct 12
A throttle valve 15 is arranged inside. On the other hand, the exhaust port 8 is connected to the N via the exhaust manifold 16 and the exhaust pipe 17.
Connected to a casing 19 containing an Ox absorbent 18,
An electric heater 20 for heating the exhaust gas flowing in the exhaust pipe 17 is arranged around the exhaust pipe 17 located upstream of the NOx absorbent 18.

The electronic control unit 30 is composed of a digital computer, and has a ROM (Read Only Memory) 32, a RAM (Random Access Memory) 33 which are mutually connected by a bidirectional bus 31, a backup RAM 33a which is constantly supplied with power, A CPU (microprocessor) 34, an input port 35 and an output port 36 are provided. The air flow meter 13 generates an output voltage proportional to the intake air amount, and this output voltage is input to the input port 35 via the AD converter 37. The engine body 1 has a water temperature sensor 21 for generating an output voltage proportional to the engine cooling water temperature.
Is attached, and the output voltage of the water temperature sensor 21 is input to the input port 35 via the AD converter 38. Also,
The input port 35 is connected to a rotation speed sensor 22 that generates an output pulse indicating the engine rotation speed and a speed sensor 23 that generates an output pulse at a cycle proportional to the vehicle speed.
On the other hand, the output port 36 corresponds to the corresponding drive circuits 39, 40,
The spark plug 4, the fuel injection valve 11 and the electric heater 20 are connected via 41, 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 obtained by an experiment in advance, and the engine load Q / N
As a function of (intake air amount Q / engine speed N) and engine speed N, the ROM 3 is previously stored in the form of a map as shown in FIG.
It is stored in 2. 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, when K <1.0, the air-fuel ratio of the air-fuel mixture supplied into the engine cylinder becomes larger than the stoichiometric air-fuel ratio, that is, becomes lean, and K> 1.0.
If so, the air-fuel ratio of the air-fuel mixture supplied into the engine cylinder becomes smaller than the stoichiometric air-fuel ratio, that is, becomes rich. Figure 1
In the internal combustion engine shown in FIG.
That is, the air-fuel ratio of the air-fuel mixture supplied into the combustion chamber 3 is made lean, so that the lean air-fuel mixture is burned in the combustion chamber 3.

FIG. 3 schematically shows the concentrations of typical components in the exhaust gas discharged from the combustion chamber 3. As can be seen from FIG. 3, the amount 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 amount 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 19
The absorbent 18 uses, for example, alumina as a carrier, on which potassium K, sodium Na, lithium Li, cesium Cs, alkali metals such as barium Ba, calcium Ca, lanthanum La, yttrium Y are contained. At least one selected from the rare earths and a noble metal such as platinum Pt are supported. The ratio of air and fuel supplied into the engine intake passage and the exhaust passage upstream of the NOx absorbent 18 is referred to as the air-fuel ratio of the exhaust gas flowing into the NOx absorbent 18, and the air-fuel ratio of the exhaust gas flowing into the NOx absorbent 18 is called. Is lean, it absorbs NOx, and when the oxygen concentration in the inflowing exhaust gas is low, it absorbs Nx.
It acts to absorb and release NOx which releases Ox. Note that NO
x When no fuel or air is supplied into the exhaust passage upstream of the absorbent 18, 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 therefore NOx absorption in this case. The agent 18 absorbs NOx when the air-fuel ratio of the air-fuel mixture supplied to the combustion chamber 3 is lean, and releases the absorbed NOx when the oxygen concentration in the air-fuel mixture supplied to the combustion chamber 3 decreases. Become.

If the above-mentioned NOx absorbent 18 is arranged in the exhaust passage of the engine, the NOx absorbent 18 actually acts to absorb and release NOx, but the detailed mechanism of this absorbing and releasing effect 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 as an example the 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.

That is, when the inflowing exhaust gas becomes considerably lean, the oxygen concentration in the inflowing exhaust gas greatly increases.
Platinum P in the form of ^- these oxygen O ₂ is O _2, as shown in
Adheres to the surface of t. On the other hand, NO in the inflowing exhaust gas reacts with O ₂ ⁻ on the surface of platinum Pt to become NO ₂ (2N
O + O ₂ → 2NO ₂ ). Then part of the produced NO ₂ while bonding with the barium oxide Ba O is absorbed into the absorbent while being further oxidized on platinum Pt, 4 nitrate as shown in (A) ions NO ₃ ^- in Diffuses into the absorbent in the form. In this way, NOx is absorbed in the NOx absorbent 18.

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 absorbing capacity of the NOx of the absorbent is saturated, NO ₂ is absorbed in the absorbent and nitrate ion NO ₃ ^- is formed. 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 18. As shown in FIG. 3, when the lean degree of the inflowing exhaust gas is low, the oxygen concentration in the inflowing exhaust gas is low, and when the lean degree of the inflowing exhaust gas is low, NOx is released from the NOx absorbent 18. Will be.

On the other hand, at this time, the air-fuel ratio of the inflowing exhaust gas is
When the switch is turned on, a large amount of
Burned HC and CO are discharged, and these unburned HC and CO are platinum
Oxygen on Pt O₂ ^-It reacts with and is oxidized. Well
In addition, when the air-fuel ratio of the inflow exhaust gas is made rich, the inflow exhaust gas
Since the oxygen concentration in the gas is extremely low, NO
₂Is released and this NO₂As shown in Figure 4 (B)
It is reduced by reacting with unburned HC and CO. like this
NO on the surface of platinum Pt₂Sucks when no longer exists
NO from collecting agent to another₂Is released. Therefore inflow
If the air-fuel ratio of exhaust gas is made rich, NO
NOx will be released from the x absorbent 18.

Thus, when the air-fuel ratio of the inflowing exhaust gas becomes lean, NOx is absorbed by the NOx absorbent 18, and when the air-fuel ratio of the inflowing exhaust gas is made rich, NOx is released from the NOx absorbent 18 in a short time. It Therefore, in the internal combustion engine shown in FIG. 1, when the NOx absorption amount of the NOx absorbent 18 becomes a certain amount or more, the air-fuel ratio of the air-fuel mixture supplied into the engine cylinder is temporarily made rich so that the NOx absorbent 18 emits NOx. Is to be released.

However, the inflowing exhaust gas contains sulfur, and the NOx absorbent 18 absorbs not only NOx but also sulfur. It is considered that the absorption mechanism of sulfur into the NOx absorbent 18 is the same as the absorption mechanism of NOx. That is, similarly to the case of explaining the NOx absorption mechanism, the case where platinum Pt and barium Ba are carried on the carrier is explained as an example. As described above, when the air-fuel ratio of the inflowing exhaust gas is lean, the oxygen O ₂ is It adheres to the surface of platinum Pt in the form of O ₂ ⁻ , and SO ₂ in the inflowing exhaust gas reacts with O ₂ ^{− on} the surface of platinum Pt to form S 2.
It becomes O ₃ . Then, the generated SO ₃ is further oxidized on platinum Pt and absorbed in the absorbent to be barium oxide Ba.
While bound to O, it diffuses into the absorbent in the form of sulfate ions SO ₄ ²⁻ . Next, the sulfate ion SO ₄ ²⁻ combines with the barium ion Ba ²⁺ to form the sulfate salt Ba SO ₄ .

However, this sulfate Ba SO ₄ is hard to decompose, and even if the air-fuel ratio of the inflowing exhaust gas is made rich, the sulfate Ba SO ₄ remains without being decomposed. Therefore NO
x Sulfate Ba in the absorbent 18 over time
SO ₄ will increase, and thus the amount of NOx that can be absorbed by the NOx absorbent 18 will decrease as time passes.

However, the sulfate Ba SO ₄ produced in the NOx absorbent 18 decomposes when the temperature of the NOx absorbent 18 rises, and sulfate ions SO ₄ ²⁻ are released from the absorbent in the form of SO ₃ . Therefore, in the embodiment according to the present invention, when the NOx absorbent 18 has absorbed a certain amount of sulfur or more, the electric heater 20 arranged around the exhaust pipe 17 is heated to heat the NOx.
The temperature of the exhaust gas flowing into the absorbent 18 is raised, thereby releasing sulfur from the NOx absorbent 18. At this time, if the air-fuel ratio of the inflowing exhaust gas is made rich, the released SO ₃ is immediately oxidized by the unburned HC and CO, but even if the air-fuel ratio of the inflowing exhaust gas is changed to the stoichiometric air-fuel ratio, the released SO ₃ is not released. The fuel is oxidized by HC and CO. Whether the air-fuel ratio of the inflowing exhaust gas is made rich or stoichiometric depends on the amount of sulfur released from the NOx absorbent 18 per unit time, that is, the heating amount of the electric heater 20. Then, the electric heater 20 is heated to about 700 ° C. so that the air-fuel ratio of the inflowing exhaust gas becomes the stoichiometric air-fuel ratio.

The heating action of such an electric heater 20 is N
This is done when the amount of sulfur absorbed in the Ox absorbent 18 exceeds a certain amount, but it is difficult to detect the amount of sulfur actually absorbed in the NOx absorbent 18, and therefore it is absorbed in the NOx absorbent 18. We have to estimate the amount of sulfur that is stored. However, the amount of sulfur absorbed by the NOx absorbent 18 depends on the traveling distance of the vehicle, and therefore the amount of sulfur absorbed by the NOx absorbent 18 can be estimated from the traveling distance of the vehicle.

FIG. 5 shows the relationship between the amount of absorbed sulfur and the traveling distance of the vehicle, which is obtained by experiments. In FIG. 5, S ₀ represents the cumulative traveling distance of the vehicle, and S is NOx.
The travel distance of the vehicle after the release of sulfur from the absorbent 18 is shown. The curve f (S ₀ ) shows that sulfur occupies 80% of the absorption capacity of the NOx absorbent 18. When a new vehicle is driven, both driving distances S
₀ and S are equal to each other, and in the embodiment according to the present invention, when the both traveling distances S ₀ and S increase to reach the curve f (S ₀ ) as shown by the chain line, the first NOx absorbent 18 is obtained. The action of releasing sulfur from is performed. When the release action of sulfur is performed, the traveling distance S becomes zero as shown by the chain line, and when the point determined by the cumulative traveling distance S ₀ and the traveling distance S reaches the curve f (S ₀ ), the second sulfur The release action takes place. Similarly, the release action of sulfur is sequentially performed.
The relationship between the curve f (S ₀ ) shown in FIG. 5 and the traveling distances S ₀ , S is stored in the ROM 32 in advance.

Next, a first embodiment of sulfur emission control according to the present invention will be described with reference to FIGS. 6 and 7. Figure 6
Shows a time interruption routine. First, at step 100, the traveling distance ΔS from the time of the previous interruption to the time of this interruption is calculated from the output pulse of the speed sensor 23, and this traveling distance ΔS is the accumulated traveling distance S _0. Is added to. Next, at step 101, ΔS is added to the traveling distance S. The travel distances S ₀ and S calculated in steps 100 and 101 are stored in the backup RAM 33a. Next, at step 102, each traveling distance S ₀ , S
It is determined whether or not the point defined by exceeds the curve f (S ₀ ) in FIG. 5, and if not, the process proceeds to step 103.

In step 103, the NOx absorbent 18 is absorbed.
The stored NOx amount W is calculated. That is, the combustion chamber 3
The amount of NOx discharged from the vehicle increases as the intake air amount Q increases.
N increases as the engine load Q / N increases.
The NOx amount W absorbed in the Ox absorbent 18 is W and k.
To be represented by the sum of Q and Q / N (k is a constant)
Become. Next, at step 104, the NOx absorbent 18 is absorbed.
The NOx amount W that is stored is the preset amount W₀Yo
It is determined whether or not it is larger than that. This set amount W ₀Is like
For example, the maximum NOx amount that can be absorbed by the NOx absorbent 18 is 30 parts.
It is about a cent. W ≦ W₀Then the processing cycle
Completed, W> W₀If so, proceed to step 105 and NO.
The x emission flag is set.

Next, at step 106, it is judged whether or not a fixed time, for example, 3 seconds has elapsed since the NOx release flag was set, and when 3 seconds have passed, the routine proceeds to step 107, where the NOx release flag is reset. N
While the Ox release flag is set, the air-fuel mixture supplied into the combustion chamber 3 is made rich as described later, and therefore the air-fuel mixture is made rich for 3 seconds. During this time N
NOx absorbed in the Ox absorbent 18 is released.
Next, at step 108, the NOx amount W absorbed by the NOx absorbent 18 is made zero. This NOx releasing action is performed at relatively short time intervals.

On the other hand, in step 102, the traveling distance S
_When it is determined that the point defined by ₀ and S exceeds the curve f (S ₀ ) in FIG. 5, that is, the absorption capacity of the NOx absorbent 18 is 8
When it is determined that 0% is occupied by sulfur, the routine proceeds to step 109, where the sulfur release flag is set, and then at step 110, energization of the electric heater 20 is started. Next, at step 111, for a certain period of time after the sulfur emission flag is set and the energization is started,
For example, it is determined whether or not 10 minutes have elapsed. When 10 minutes have elapsed, the routine proceeds to step 112, where the sulfur release flag is reset, and then, at step 113, the power supply to the electric heater 20 is stopped.

While the sulfur release flag is set, the air-fuel mixture supplied into the combustion chamber 3 has a stoichiometric air-fuel ratio as described later, and therefore the air-fuel mixture has a stoichiometric air-fuel ratio for 10 minutes and the electric heater 20 It will be energized for 10 minutes.
During this period, the sulfur absorbed in the NOx absorbent 18 is released, and at the same time, the NO absorbed in the NOx absorbent 18 is released.
x is also emitted. Next, at step 114, the traveling distance S
NOx absorbed by the NOx absorbent 18 is set to zero.
The quantity W is set to zero.

FIG. 7 shows a routine for calculating the fuel injection time TAU, and this routine is repeatedly executed. Referring to FIG. 7, first of all in FIG.
The basic fuel injection time TP is calculated from the map shown in FIG.
Next, at step 151, it is judged if the sulfur release flag is set or not. When the sulfur release flag is not set, the routine proceeds to step 152, where NOx
It is determined whether or not the release flag is set. N
When the Ox release flag is not set, the routine proceeds to step 153, where the correction coefficient K is set to, for example, 0.6, and then at step 154, the basic fuel injection time TP is multiplied by the correction coefficient K to calculate the fuel injection time TAU. It Therefore, at this time, the air-fuel mixture supplied into the combustion chamber 3 becomes lean, and therefore the lean air-fuel mixture is burned.

On the other hand, when the NOx releasing flag is set, the routine proceeds from step 152 to step 155, where the correction coefficient K
Is set to, for example, 1.3, and then the process proceeds to step 154.
Therefore, at this time, the air-fuel mixture supplied into the combustion chamber 3 becomes rich. On the other hand, when the sulfur release flag is set, the routine proceeds from step 151 to step 156, the correction coefficient K is set to 1.0, and then the routine proceeds to step 154.
Therefore, at this time, the air-fuel mixture supplied into the combustion chamber 3 has the stoichiometric air-fuel ratio.

FIG. 8 and FIG. 9 show another embodiment. Figure 8
In this example, the inlet of the NOx absorbent 18 is referred to
The first NOx concentration sensor 23 is attached to the
The second NOx concentration sensor 24 is attached to the outlet of the collecting agent 18.
Be kicked. Each of these NOx concentration sensors 23, 24 is N
Generate an output voltage proportional to the Ox concentration, and
Are input ports via the corresponding AD converters 42 and 43, respectively.
Input to the printer 35. Large absorption capacity of NOx absorbent 18
When sulfur occupies the part, NOx by NOx absorbent 18
The absorption capacity is reduced, and thus the NOx absorbent 18 flows into the
Some of the NOx is absorbed by the NOx absorbent 18.
It is discharged from the NOx absorbent 18 without any notice. Therefore this fruit
In the embodiment, N detected by the first NOx concentration sensor 23
NOx concentration C at the inlet of the Ox absorbent 18₁And the second
NOx absorbent 1 detected by NOx concentration sensor 24
NOx concentration C at the outlet of 8₂Difference ΔC (= C₁−
C ₂) Is calculated and the difference ΔC is averaged for one minute.
Average value ΔC_m(=¹/_nΣΔC) is calculated, and this average value ΔC_m
Is the set value C₀, NOx concentration C at the inlet, for example₁Of 50
If it is lower than the cent, the absorption capacity of the NOx absorbent 18
It is judged that sulfur occupies most of the amount, and the mixture is treated.
The air-fuel ratio should be adjusted and the electric heater 20 should be energized.
is doing.

FIG. 9 shows the time interrupt routine. In addition,
Also in this embodiment, the fuel injection time TAU is calculated by the routine shown in FIG. Referring to FIG. 9, first, at step 200, it is judged if the sulfur release flag is set or not. When the sulfur release flag is not set, the routine proceeds to step 201, where it is judged if the condition for detecting the amount of absorbed sulfur is satisfied.
For example, when the air-fuel mixture becomes lean for the first time after the engine is started and then the lean state continues for 1 minute or more, it is determined that the detection condition of the sulfur absorption amount is satisfied. When the detection condition of the sulfur absorption amount is not satisfied, the routine proceeds to step 205, and when the detection condition of the sulfur absorption amount is satisfied, the routine proceeds to step 202.

In step 202, the first NOx concentration sensor
NOx concentration C detected by 23 ₁And the second NOx concentration
NOx concentration C detected by the sensor 24₂Difference with ΔC
(= C₁-C₂) Is calculated. Then step 203
Then, the average value ΔC of the differences ΔC between n interrupts._m(=
¹/_nΣΔC) is calculated. Then in step 204
Average value ΔC_mIs the set value C₀Is less than
And ΔC_m≧ C₀If, then the process proceeds to step 205.

In step 205, the NOx amount W absorbed in the NOx absorbent 18 is calculated. Next, at step 206, it is judged if the NOx amount W absorbed by the NOx absorbent 18 is larger than a preset set amount W ₀ . The set amount W ₀ is, for example, about 30% of the maximum NOx amount that the NOx absorbent 18 can absorb.
If W ≦ W ₀ , the processing cycle is completed, and if W> W ₀ , the routine proceeds to step 207, where the NOx releasing flag is set.

Next, at step 208, it is judged if a fixed time, for example, 3 seconds has elapsed since the NOx releasing flag was set, and if 3 seconds have passed, the routine proceeds to step 209, where the NOx releasing flag is reset. N
While the Ox release flag is set, the air-fuel mixture supplied into the combustion chamber 3 is made rich as described above, and therefore the air-fuel mixture is made rich for 3 seconds. During this time N
NOx absorbed in the Ox absorbent 18 is released.
Next, at step 210, the NOx amount W absorbed by the NOx absorbent 18 is made zero. This NOx releasing action is performed at relatively short time intervals.

On the other hand, in step 204, ΔC _m <C
_When it is judged to be _0, that is, when it is judged that most of the absorption capacity of the NOx absorbent 18 is occupied by sulfur, the routine proceeds to step 211, where the sulfur release flag is set, and then at step 212, the electric heater 20 is set. Energization is started. Next, at step 213, it is judged whether or not a fixed time, for example, 10 minutes have elapsed since the sulfur emission flag was set and the energization was started. When 10 minutes have not elapsed, the processing cycle is completed. When the sulfur release flag is set, the routine jumps from step 200 to step 213 in the next processing cycle.

When it is determined in step 213 that the sulfur release flag has been set and 10 minutes have elapsed from the start of energization of the electric heater 20, step 2
In step 14, the sulfur release flag is reset, and then in step 215, the electric heater 20 is deenergized. While the sulfur release flag is set, the air-fuel mixture supplied into the combustion chamber 3 is at the stoichiometric air-fuel ratio as described above, so the air-fuel mixture is at the stoichiometric air-fuel ratio for 10 minutes, and the electric heater 20 is energized for 10 minutes. Will be done. During this time, the sulfur absorbed in the NOx absorbent 18 is released, and at the same time, the NOx absorbed in the NOx absorbent 18 is also released. Next, at step 216, NOx absorbent 1
The NOx amount W absorbed in 8 is made zero.

In this embodiment, the NOx concentration C ₁ at the inlet of the NOx absorbent 18 is detected by the first NOx concentration sensor 23.
The NOx concentration C _{1 at} the inlet of is a function of the engine load Q / N and the engine speed N. Therefore, the NOx concentration C ₁ at the inlet of the NOx absorbent 18 is experimentally obtained in advance as a function of the engine load Q / N and the engine speed N, and the relationship between them is stored in advance in the ROM 3 in the form of a map as shown in FIG.
It is also possible to store it in _{No. 2} and obtain the NOx concentration C ₁ from this map. In this case, the first NO
It is not necessary to provide the x concentration sensor 23.

FIG. 10 and FIG. 11 show still another embodiment. Referring to FIG. 10, in this embodiment, NOx absorbent 1
The first temperature sensor 25 is attached to the inlet of 8
x A second temperature sensor 26 is attached to the outlet of the absorbent 18. These temperature sensors 25 and 26 generate output voltages proportional to the exhaust gas temperature, and these output voltages are input to the input port 35 via the corresponding AD converters 44 and 45, respectively.

As described above, NOx is the NOx absorbent 18
Since NOx is oxidized when absorbed inside, the exhaust gas temperature at the outlet of the NOx absorbent 18 becomes considerably higher than the exhaust gas temperature at the inlet of the NOx absorbent 18, for example, about 100 ° C. due to this heat of oxidation. . However NOx
If sulfur occupies most of the absorption capacity of the absorbent 18, NO
The NOx absorption capacity of the x absorbent 18 decreases, and thus the amount of NOx oxidation heat generated decreases, so that the exhaust gas temperature at the outlet of the NOx absorbent 18 decreases. Therefore, in this embodiment, the exhaust gas temperature t ₁ at the inlet of the NOx absorbent 18 detected by the first temperature sensor 25 and the exhaust gas temperature t _{2 at} the outlet of the NOx absorbent 18 detected by the second temperature sensor 26. Of the difference Δt (= t ₂ −t ₁ ) is calculated, and the average value Δt _m (= t _m (=
¹ / _n ΣΔt) is calculated, and when this average value Δt _m is lower than a set value t ₀ , for example, 50 ° C., the NOx absorbent 1
It is determined that sulfur occupies most of the absorption capacity of No. 8 and the air-fuel ratio is set to the stoichiometric air-fuel ratio, and the electric heater 20 is energized.

FIG. 11 shows the time interrupt routine. Also in this embodiment, the fuel injection time TAU is calculated by the routine shown in FIG. Referring to FIG. 11, first, at step 300, it is judged if the sulfur release flag is set or not. When the sulfur release flag is not set, the routine proceeds to step 301, where it is judged if the condition for detecting the amount of absorbed sulfur is satisfied. For example, the mixture becomes lean only after starting the engine,
After that, when the lean state continues for one minute or more, it is determined that the detection condition of the sulfur absorption amount is satisfied. If the conditions for detecting the amount of absorbed sulfur are not satisfied, step 3
05, the routine proceeds to step 302 if the conditions for detecting the amount of absorbed sulfur are satisfied.

In step 302, the first temperature sensor 25
Exhaust gas temperature t detected by₁And the second temperature sensor 26
Exhaust gas temperature t detected by₂Difference Δt (= t₂-T
₁) Is calculated. Next, at step 303, interrupt n times
Average value Δt of the differences Δt during_m(=¹/_nΣΔt)
Is calculated. Next, at step 304, the average value Δt _m
Is the set value t₀Is smaller than Δt, Δt_m≧
t₀If, then the process proceeds to step 305.

In step 305, the NOx amount W absorbed in the NOx absorbent 18 is calculated. Next, at step 306, it is judged if the NOx amount W absorbed in the NOx absorbent 18 is larger than a preset set amount W ₀ . The set amount W ₀ is, for example, about 30% of the maximum NOx amount that the NOx absorbent 18 can absorb.
If W ≦ W ₀ , the processing cycle is completed, and if W> W ₀ , the routine proceeds to step 307, where the NOx releasing flag is set.

Next, at step 308, it is judged whether or not a fixed time, for example, 3 seconds has elapsed since the NOx release flag was set, and when 3 seconds have passed, the routine proceeds to step 309, where the NOx release flag is reset. N
While the Ox release flag is set, the air-fuel mixture supplied into the combustion chamber 3 is made rich as described above, and therefore the air-fuel mixture is made rich for 3 seconds. During this time N
NOx absorbed in the Ox absorbent 18 is released.
Next, at step 310, the NOx amount W absorbed by the NOx absorbent 18 is made zero. This NOx releasing action is performed at relatively short time intervals.

On the other hand, at step 304, Δt _m <t
_When it is judged to be _0, that is, when it is judged that most of the absorption capacity of the NOx absorbent 18 is occupied by sulfur, the routine proceeds to step 311, where the sulfur release flag is set, and then at step 312, the electric heater 20 is set. Energization is started. Next, at step 313, it is judged whether or not a fixed time, for example, 10 minutes has elapsed since the sulfur emission flag was set and the energization was started. When 10 minutes have not elapsed, the processing cycle is completed. When the sulfur release flag is set, the routine jumps from step 300 to step 313 in the next processing cycle.

When it is determined in step 313 that 10 minutes have elapsed since the sulfur release flag was set and the power supply to the electric heater 20 was started, step 3
In step 14, the sulfur release flag is reset, and then in step 315, the power supply to the electric heater 20 is stopped. While the sulfur release flag is set, the air-fuel mixture supplied into the combustion chamber 3 is at the stoichiometric air-fuel ratio as described above, so the air-fuel mixture is at the stoichiometric air-fuel ratio for 10 minutes, and the electric heater 20 is energized for 10 minutes. Will be done. During this time, the sulfur absorbed in the NOx absorbent 18 is released, and at the same time, the NOx absorbed in the NOx absorbent 18 is also released. Next, at step 316, NOx absorbent 1
The NOx amount W absorbed in 8 is made zero.

In this embodiment, the exhaust gas temperature t ₁ at the inlet of the NOx absorbent 18 is detected by the first temperature sensor 25, but the exhaust gas temperature t _{1 at} the inlet of the NOx absorbent 18 is It is a function of the engine load Q / N and the engine speed N. Therefore, the exhaust gas temperature t ₁ at the inlet of the NOx absorbent 18 is set to the engine load Q / N and the engine speed N.
It is also possible to obtain the exhaust gas temperature t ₁ from this map by preliminarily empirically obtaining it as a function of the above, and storing these relationships in advance in the ROM 32 in the form of a map as shown in FIG. In this case, it is not necessary to provide the first temperature sensor 25.

[0045]

The sulfur absorbed by the NOx absorbent can be released, so that the NOx absorbent capacity of the NOx absorbent can be maintained for a long period of time.

[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] 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. 4 is a diagram for explaining the action of absorbing and releasing NOx.

FIG. 5 is a diagram showing the amount of sulfur absorbed.

FIG. 6 is a flowchart of an interrupt routine.

FIG. 7 is a flowchart for calculating a fuel injection time TAU.

FIG. 8 is an overall view of another embodiment of the internal combustion engine.

FIG. 9 is a flowchart of an interrupt routine.

FIG. 10 is an overall view of still another embodiment of the internal combustion engine.

FIG. 11 is a flowchart of an interrupt routine.

FIG. 12 is a diagram showing a map of NOx concentration C ₁ .

FIG. 13 is a diagram showing a map of exhaust gas temperature t ₁ .

[Explanation of symbols]

 17 ... Exhaust pipe 18 ... NOx absorbent 20 ... Electric heater

Claims (1)

[Claims] 1. An internal combustion engine in which an NOx absorbent that absorbs NOx when the air-fuel ratio of the inflowing exhaust gas is lean and releases the absorbed NOx when the oxygen concentration in the inflowing exhaust gas decreases is arranged in the engine exhaust passage. , A sulfur absorption amount estimating means for estimating the sulfur absorption amount of the NOx absorbent, and when the sulfur absorption amount estimated by the sulfur absorption amount estimating means exceeds a preset set amount, sulfur is removed from the NOx absorbent. An exhaust emission control device for an internal combustion engine, comprising: sulfur releasing means for releasing the sulfur.