JP2000161045A - Exhaust emission control device for internal combustion engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent discharging of SOx absorbed by occulusion reduction type NOx catalyst when a car is stopped or driven with extremely low speed. SOLUTION: After SOx is absorbed by an NOx catalyst 17 by a specific amount, an ECU 30 determines a discharging timing of SOx. The ECU 30 performs stoichi controlling of an air-fuel ratio of the exhaust gas for discharging SOx from the NOx catalyst 17 while keeping temperature of the exhaust gas at a specified value or higher. That is, it performs high temperature stoichi controlling for discharging SOx. However, when the running speed of a car sensed by a vehicular speed sensor 27 is not higher than a lower limit value, that is, the car is stopped or driven with extremely low speed, the high temperature stoichi controlling is suspended. Low temperature stoichi controlling where the exhaust air-fuel ratio is kept in stoichi state and the exhaust gas temperature is kept lower than SOx discharge reduction temperature is executed, so that SOx discharging from the NOx catalyst is suspended to thereby discontinue a SOx discharging treatment.

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 provided with an SOx absorbent for absorbing sulfur oxides (SOx) contained in exhaust gas of an internal combustion engine.

[0002]

2. Description of the Related Art As an exhaust gas purifying apparatus for purifying NOx from exhaust gas discharged from an internal combustion engine for a vehicle capable of lean combustion, there is a NOx absorbent represented by a NOx storage reduction catalyst. The NOx absorbent absorbs NOx when the air-fuel ratio of the inflowing exhaust gas is lean (that is, under an oxygen-excess atmosphere),
NO absorbed when the oxygen concentration of the inflowing exhaust gas decreases
The NOx storage-reduction type NOx catalyst, which is a type of NOx absorbent, absorbs NOx when the air-fuel ratio of the inflowing exhaust gas is lean (that is, under an oxygen-excess atmosphere), and reduces the inflowing exhaust gas. oxygen concentration is the catalyst for reducing the absorbed NOx release and N ₂ when dropped.

When this storage-reduction type NOx catalyst (hereinafter sometimes simply referred to as a catalyst or NOx catalyst) is disposed in an exhaust passage of an internal combustion engine capable of lean combustion, when exhaust gas having a lean air-fuel ratio flows, the exhaust gas contains NOx is absorbed by the catalyst, and NO is absorbed by the catalyst when exhaust gas having a stoichiometric (stoichiometric air-fuel ratio) or rich air-fuel ratio flows.
x is released as NO ₂ , and HC and C
It is reduced to N ₂ by a reducing component such as O, that is, NOx is purified.

In general, the fuel of an internal combustion engine contains sulfur, and when the fuel is burned in the internal combustion engine, the sulfur in the fuel burns and sulfur oxides such as SO ₂ and SO ₃ (SOx ) Occurs. Since the NOx catalyst absorbs SOx in the exhaust gas by the same mechanism as that for absorbing NOx, if a NOx catalyst is disposed in the exhaust passage of the internal combustion engine, this catalyst will absorb not only NOx but also SOx. Is done. That is, the NOx catalyst as the NOx absorbent
Since it has an x-absorbing function, it can also be called a SOx absorbent.

However, the sulfur absorbed by the NOx catalyst
Ox forms stable sulfate over time,
Under the same conditions as when releasing and reducing NOx from the NOx catalyst, there is a tendency that it is difficult to be decomposed and released and easily accumulated in the NOx catalyst. When the SOx accumulation amount in the NOx storage reduction catalyst increases, the NOx absorption capacity of the catalyst decreases, and it becomes impossible to sufficiently remove NOx from the exhaust gas.
Purification efficiency decreases. This is so-called SOx poisoning. Therefore, in order to maintain the NOx purification ability of the NOx storage reduction catalyst high over a long period of time, the SOx absorbed by the catalyst is required.
Must be released at an appropriate timing.

[0006] The technology for treating SOx release from a storage reduction type NOx catalyst is disclosed in Japanese Patent Publication No. 2605586. In order to release SOx absorbed by the NOx storage reduction catalyst, it is effective to make the air-fuel ratio of the inflowing exhaust gas stoichiometric or rich, and to set the catalyst temperature to a predetermined high temperature higher than that at the time of releasing and reducing NOx. It is considered to be the target.

Therefore, the time when a predetermined amount of SOx is absorbed by the NOx catalyst is determined as the SOx release timing, and at that time, the catalyst temperature is controlled to a temperature at which SOx can be released, and
The air-fuel ratio control is performed so that the air-fuel ratio of the inflowing exhaust gas is kept stoichiometric or rich, and the SOx release process is executed. By this SOx release processing, the sulfate absorbed by the NOx catalyst is decomposed into SO ₃ , and this SO ₃ is reduced by unburned HC and CO in the exhaust gas and released as SO _2. You.

[0008]

Heretofore, there has been a case where the SOx releasing process is executed when the vehicle is stopped. If SOx is discharged from the vehicle when the vehicle is stopped in this way, the vicinity of the rear of the vehicle becomes an atmosphere with a high concentration of SOx, which may cause discomfort to pedestrians near the stopped vehicle. In addition, there is a possibility that air containing high concentration SOx may be introduced into the cabin of the following vehicle that has stopped immediately after the stopped vehicle, which may cause discomfort to the occupants of the following vehicle.

The present invention has been made in view of such problems of the conventional technology, and an object of the present invention is to solve the problem by reducing the release of SOx while the vehicle is stopped or running at low speed. The purpose is to consider the environment for pedestrians and occupants of following vehicles.

[0010]

The present invention has the following features to attain the object mentioned above. The exhaust gas purifying apparatus for an internal combustion engine according to the present invention includes: (a) an oxygen concentration of exhaust gas that is disposed in an exhaust passage of an internal combustion engine capable of lean combustion and absorbs SOx when the air-fuel ratio of the exhaust gas is lean and absorbs SOx; A SOx absorbent that releases the absorbed SOx when is low,
(B) SOx releasing means for reducing the oxygen concentration of the exhaust gas flowing into the SOx absorbent in order to release the SOx absorbed by the SOx absorbent; and (c) running speed of a vehicle equipped with the internal combustion engine. A vehicle speed determining means for determining whether or not the vehicle speed is equal to or lower than a predetermined speed; and (d) a vehicle speed determining means for reducing the oxygen concentration of the exhaust gas by the SOx releasing means to release SOx from the SOx absorbent. SOx emission reducing means for reducing the release of SOx from the SOx absorbent when the determining means determines that the traveling speed of the vehicle is equal to or lower than the predetermined speed.

In this exhaust gas purifying apparatus for an internal combustion engine, when the air-fuel ratio of the exhaust gas is lean, the S
Ox is absorbed by the SOx absorbent, and SOx is released from the SOx absorbent when the SOx releasing means reduces the oxygen concentration of the exhaust gas. Then, the SOx release means
If the vehicle speed determining means determines that the traveling speed of the vehicle is equal to or lower than the predetermined speed during the execution of the x release processing, the SOx release reducing means reduces the SOx release from the SOx absorbent. This makes it possible to prevent the vicinity of the rear part of the vehicle from becoming an atmosphere having a high SOx concentration when the vehicle is at a predetermined speed or lower.

Here, in the present invention, the predetermined speed which is a criterion of the vehicle speed determining means is a vehicle stop state (0 km / h).
It is preferable to set appropriately in the range from about to the walking speed of a human. By setting the predetermined speed in this way, it is possible to prevent a pedestrian or the like near the vehicle from feeling uncomfortable due to SOx emission.

The air-fuel ratio of exhaust gas refers to the ratio of air and fuel (hydrocarbon) supplied into the engine intake passage and the exhaust passage upstream of the SOx absorbent.

As the internal combustion engine capable of lean combustion in the present invention, a direct-injection-type lean-burn gasoline engine or a diesel engine can be exemplified. In the case of a lean burn gasoline engine, air-fuel ratio control of exhaust gas can be executed by air-fuel ratio control of air-fuel mixture supplied to a combustion chamber. Regarding the air-fuel ratio control of exhaust gas in the case of a diesel engine, a so-called sub-injection for injecting fuel in an intake stroke, an expansion stroke, or an exhaust stroke is performed, or a reducing agent is injected into an exhaust passage upstream of an SOx absorbent. It can be performed by supplying.

As the SOx absorbent, a storage reduction type NOx catalyst can be exemplified. The storage reduction NOx catalyst is
Air-fuel ratio of the inflowing exhaust gas absorbs NOx when the lean, the oxygen concentration in the inflowing exhaust gas to release NOx absorbed and reduced, is a catalyst for reducing the N _2, the NOx storage reduction The catalyst has the effect of absorbing and releasing SOx in the same manner as absorbing and releasing NOx.

This storage-reduction type NOx 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 or cesium Cs, or an alkaline earth such as barium Ba or calcium Ca. , Lanthanum La, at least one selected from rare earths such as yttrium Y, and a noble metal such as platinum Pt.

The NOx storage reduction catalyst is disposed upstream of the NOx storage reduction catalyst to absorb SOx in the exhaust gas, and
Of course, the absorbent for preventing the catalyst from being poisoned by SOx is also the SOx absorbent of the present invention.

In the present invention, the SOx emission reducing means may be constituted by air-fuel ratio control means for controlling the air-fuel ratio of the exhaust gas of the internal combustion engine lean. When the air-fuel ratio of the exhaust gas is made lean, the SOx
Emission is reduced or stopped, and SO in the exhaust gas is reduced.
x becomes absorbed by the SOx absorbent.

In the present invention, the SOx emission reducing means can be constituted by temperature control means for controlling the temperature of the SOx absorbent to a predetermined temperature or less. This is because a temperature higher than the predetermined temperature is required to release SOx from the SOx absorbent, and when the temperature of the SOx absorbent falls below the predetermined temperature, SOx is not released from the SOx absorbent. The temperature control of the SOx absorbent may be performed by controlling the temperature of the exhaust gas flowing into the SOx absorbent,
Alternatively, a cooling device such as a water-cooled type may be provided in the SOx absorbent or in the exhaust passage upstream of the SOx absorbent, and the operation of the cooling device may be controlled.

[0020]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of an exhaust gas purifying apparatus for an internal combustion engine according to the present invention will be described below with reference to FIGS.

[First Embodiment] FIG. 1 is a diagram showing a schematic configuration in the case where the present invention is applied to a gasoline engine for a vehicle capable of lean combustion. In this figure, reference numeral 1 denotes an engine body, reference numeral 2 denotes a piston, reference numeral 3 denotes a combustion chamber, reference numeral 4 denotes a spark plug, reference numeral 5 denotes an intake valve, reference numeral 6 denotes an intake port, reference numeral 7 denotes an exhaust valve, reference numeral 8 denotes an exhaust port. Are shown respectively. Intake port 6
Are connected to a surge tank 10 via corresponding branch pipes 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 an air flow meter 21, 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 storage-reduction type NOx catalyst (SOx absorbent) 17.
Are connected. Hereinafter, the storage reduction type NOx catalyst 17 is abbreviated as the NOx catalyst 17.

An electronic control unit (ECU) 30 for engine control is composed of a digital computer, and is connected to a ROM (Read Only Memory) 32, a RAM (Random Access Memory) 33 and a CPU (Central Processor) by a bidirectional bus 31. Unit) 3
4, an input port 35 and an output port 36 are provided. The air flow meter 21 generates an output voltage proportional to the amount of intake air, and this output voltage is input to an input port 35 via an AD converter 37. An idle switch 20 for detecting that the throttle valve 14 has an idling opening is attached to the throttle valve 14, and an output signal of the idle switch 20 is input to an input port 35.

On the other hand, the exhaust pipe 19 downstream of the casing 18
A temperature sensor 25 that generates an output voltage proportional to the temperature of the exhaust gas is attached to the inside, and the output voltage of the temperature sensor 25 is input to an input port 35 via an AD converter 38. The input port 35 is connected to a rotation speed sensor 26 that generates an output pulse indicating the engine rotation speed, and a vehicle speed sensor 27 that generates an output pulse indicating the vehicle speed. The output ports 36 are connected to the ignition plug 4 and the fuel injection valve 11 via corresponding drive circuits 39, respectively.

In this gasoline engine, the fuel injection time TAU is calculated based on, for example, the following equation. TAU = TP · K Here, TP indicates a basic fuel injection time, and K indicates a correction coefficient. The basic fuel injection time TP indicates a fuel injection time required for setting the air-fuel ratio of the air-fuel mixture supplied into the engine cylinder to the stoichiometric air-fuel ratio. This basic fuel injection time TP is obtained in advance by an experiment, 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.
2 is stored. The correction coefficient K is a coefficient for controlling the air-fuel ratio of the air-fuel mixture supplied to the engine cylinder. 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 into the engine cylinder becomes larger than the stoichiometric air-fuel ratio, that is, lean, and K> 1.
When it becomes 0, the air-fuel ratio of the mixture supplied to the engine cylinder becomes smaller than the stoichiometric air-fuel ratio, that is, becomes rich.

In the gasoline engine of this embodiment,
In the low engine load operation region, the value of the correction coefficient K is set to a value smaller than 1.0.
Is set to 1.0, and the value of the correction coefficient K is set to be larger than 1.0 in the engine full load operation region. In an internal combustion engine, a low-medium load operation is usually most frequently performed, and thus the value of the correction coefficient K is set to be smaller than 1.0 during most of the operation period, so that the lean mixture is fueled.

FIG. 3 schematically shows the concentration of typical components in the exhaust gas discharged from the combustion chamber 3. As can be seen from this figure, 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. The concentration of oxygen O _{2 in} the discharged exhaust gas increases as the air-fuel ratio of the air-fuel mixture supplied into the combustion chamber 3 becomes leaner.

NOx contained in casing 18
The catalyst 17 is made of, for example, alumina as a carrier. On the carrier, for example, potassium K, sodium Na, lithium Li, alkali metal such as cesium Cs, barium Ba, alkaline earth such as calcium Ca, lanthanum La, yttrium Y At least one selected from such rare earths and a noble metal such as platinum Pt are supported. The ratio of air and fuel (hydrocarbon) supplied to the engine intake passage and the exhaust passage upstream of the NOx catalyst 17 is referred to as the air-fuel ratio of the exhaust gas flowing into the NOx catalyst 17 (hereinafter simply referred to as the exhaust air-fuel ratio). The NOx catalyst 17 absorbs NOx when the exhaust air-fuel ratio is lean, and releases the absorbed NOx when the oxygen concentration in the inflowing exhaust gas decreases.
Ox absorbs and releases.

When no fuel (hydrocarbon) or air is supplied into the exhaust passage upstream of the NOx catalyst 17, the exhaust air-fuel ratio matches the air-fuel ratio of the air-fuel mixture supplied into the combustion chamber 3. Therefore, in this case, the NOx catalyst 1
Numeral 7 absorbs NOx when the air-fuel ratio of the mixture supplied to the combustion chamber 3 is lean, and releases the absorbed NOx when the oxygen concentration in the mixture supplied to the combustion chamber 3 decreases. .

If the above-described NOx catalyst 17 is disposed in the engine exhaust passage, the NOx catalyst 17 actually performs the NOx absorbing / releasing action. However, the detailed mechanism of the absorbing / releasing action is not clear in some parts. However, it is considered that this absorption / release action is performed by a mechanism as shown in FIG. Next, this mechanism will be described by taking as an example 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.

That is, when the inflowing exhaust gas becomes considerably lean, the oxygen concentration in the inflowing exhaust gas greatly increases.
As shown in (A), oxygen O ₂ adheres to the surface of platinum Pt in the form of O ₂ ⁻ or O ²⁻ . On the other hand, NO contained in the inflowing exhaust gas reacts with O ₂ ⁻ or O ²⁻ on the surface of the platinum Pt to become NO ₂ (2NO + O ₂ → 2NO ₂ ).

Next, a part of the generated NO ₂ is absorbed in the NOx catalyst 17 while being oxidized on the platinum Pt and combined with the barium oxide BaO, and as shown in FIG. NO ₃ ^- diffuses into the NOx catalyst 17 in the form of. In this way, NOx is absorbed in the NOx catalyst 17.

As long as the oxygen concentration in the inflowing exhaust gas is high, NO ₂ is generated on the surface of the platinum Pt,
As long as the absorption capacity is not saturated, NO ₂ is absorbed in the NOx catalyst 17 and nitrate ions NO ₃ ⁻ are generated.

On the other hand, the oxygen concentration in the inflowing exhaust gas
Decreases and NO_TwoThe reaction goes in the reverse direction when the amount of
(NO_Three ^-→ NO_Two), And nitrate ion in the NOx catalyst 17
NO _Three ^-Is NO_TwoFrom the NOx catalyst 17.
That is, when the oxygen concentration in the inflowing exhaust gas decreases, NOx
NOx is released from the catalyst 17. Shown in FIG.
The leanness of the incoming exhaust gas
This reduces the oxygen concentration in the incoming exhaust gas,
The NOx catalyst 1
7, NOx is released.

On the other hand, at this time, when the mixture supplied to the combustion chamber 3 is made stoichiometric or rich and the exhaust air-fuel ratio becomes stoichiometric or rich, as shown in FIG. , CO are discharged, and the unburned HC, CO reacts with oxygen O ₂ ^- or O ^2- on platinum Pt to be oxidized.

When the exhaust air-fuel ratio becomes stoichiometric or rich, the concentration of oxygen in the inflowing exhaust gas is extremely reduced, so that NO ₂ is released from the NOx catalyst 17, and this NO
₂ is reduced by reacting with unburned HC and CO as shown in FIG. 4 (B) to become N ₂ . When NO ₂ is no longer present on the surface of the platinum Pt, NO ₂ is released from the NOx catalyst 17 one after another, and is further reduced to N ₂ . Therefore, when the exhaust air-fuel ratio is set to stoichiometric or rich, the NOx catalyst 17
Ox will be released.

[0036] Thus, NOx when the exhaust air-fuel ratio becomes lean is absorbed in the NOx catalyst 17, NOx when the exhaust air-fuel ratio to the stoichiometric or rich is released in a short time from the NOx catalyst 17, reduced to N ₂ Is done.

In this embodiment, as described above, the mixture supplied to the combustion chamber 3 is made rich at the time of full load operation, and the mixture is made the stoichiometric air-fuel ratio at the time of high load operation. Since the air-fuel mixture is lean during the load operation, NOx in the exhaust gas is absorbed by the NOx catalyst 17 during the low-to-medium-load operation, and N
NOx is released from the Ox catalyst 17 and reduced. However, if the frequency of full-load operation or high-load operation is low, and the frequency of low-medium load operation is high and the operation time is long, the release and reduction of NOx cannot be made in time, and N
The NOx absorption capacity of the Ox catalyst 17 is saturated, and the NOx cannot be absorbed. Therefore, in this embodiment,
When the lean air-fuel mixture is being burned, that is, when the medium-low load operation is being performed, the air-fuel ratio of the air-fuel mixture is controlled so that the stoichiometric or rich air-fuel mixture is burned in a relatively short cycle, NOx is released and reduced in a short cycle. In this way, control is performed such that the exhaust air-fuel ratio (in this embodiment, the air-fuel ratio of the air-fuel mixture) is alternately and repeatedly repeated in a relatively short cycle between "lean" and "stoichiometric or rich" in order to absorb and release NOx. This is referred to as lean-rich spike control in the following description.

On the other hand, the fuel contains sulfur (S), and when the sulfur in the fuel burns, sulfur oxides (SOx) such as SO ₂ and SO ₃ are generated, and the NOx catalyst 17 These SOx also absorb. That is, the NOx catalyst 17
It also functions as an Ox absorbent. It is considered that the SOx absorption mechanism of the NOx catalyst 17 is the same as the NOx absorption mechanism. That is, platinum Pt and barium Ba are deposited on the carrier in the same manner as when the NOx absorption mechanism is described.
As described above, when the exhaust air-fuel ratio is lean, the oxygen O ₂ becomes O ₂ ⁻
Alternatively, SOx (eg, SO ₂ ) in the inflowing exhaust gas is oxidized to Pt _{3 in} the form of O ^{2− on} the surface of platinum Pt of the NOx catalyst 17.

Thereafter, the generated SO ₃ is further oxidized on the surface of the platinum Pt, is absorbed in the NOx catalyst 17 and combines with the barium oxide BaO, and enters the NOx catalyst 17 in the form of sulfate ion SO ₄ ^2−. diffused to form the sulfate BaSO _4. When the amount of BaSO ₄ generated in the NOx catalyst 17 increases, the amount of BaO that can participate in the absorption of the NOx catalyst 17 decreases, and the NOx absorption capacity decreases. This is SOx
Poisoning. Therefore, in order to maintain the NOx absorption capacity of the NOx catalyst 17 high, the NOx
It is necessary to release the SOx absorbed by the catalyst 17.
It is known that SOx can be released from the NOx catalyst 17 by lowering the oxygen concentration of the exhaust gas as in the case of releasing NOx, and that the higher the temperature of the NOx catalyst 17, the easier it is to release. I know.

However, when lean / rich spike control of the exhaust air-fuel ratio is performed for the NOx absorption / release processing to flow exhaust gas having a low oxygen concentration to the NOx catalyst 17, NOx is released from the NOx catalyst 17. However, SOx is hardly released. This is considered to be due to the fact that the crystal of BaSO ₄ tends to be coarse and relatively stable, so that once generated, it is difficult to be decomposed and released. As described above, in order to release the SOx absorbed in the NOx catalyst 17 in a stable manner, it is necessary to continuously flow exhaust gas having a low oxygen concentration for a long time.

Therefore, in this embodiment, when a predetermined amount of SOx is absorbed by the NOx catalyst 17, the SOx releasing process is executed. In the SOx releasing process, the exhaust gas whose exhaust air-fuel ratio is maintained at the stoichiometric ratio is used. Long-time NOx catalyst 17
To be carried out. However, when the vehicle is stopped or running at extremely low speed, SO
When x is discharged, the vicinity of the rear of the vehicle becomes an atmosphere having a high SOx concentration, and there is a possibility that pedestrians near the vehicle and occupants of following vehicles may be uncomfortable, so in order to prevent this, When the vehicle is stopped or running at a very low speed, the execution of the SOx release process is stopped so that the SOx is not released, and the SOx release process is started or restarted when the vehicle starts running at a predetermined speed or more. Like that.

Next, referring to FIG. 5, a description will be given of a routine for executing the SOx releasing process in this embodiment. A flowchart including the steps constituting this routine is stored in the ROM 32 of the ECU 30, and the processing in each step of the flowchart is entirely performed by the CPU of the ECU 30.
34.

<Step 101> First, the ECU 30
In step 101, the travel distance of the vehicle from the completion of the previous SOx release process to the current time is integrated.

<Step 102> Next, the ECU 30
Proceeds to step 102, the travel distance accumulated value D calculated in step 101 determines whether exceeds the judgment value D _0.
SOx contained in exhaust gas discharged from the engine body 1
Is generated by burning the sulfur (S) component in the fuel, and the SOx amount absorbed by the NOx catalyst 17 is:
There is a correlation with the amount of fuel consumed by combustion in the engine body 1.
Therefore, the amount of SOx absorbed by the NOx catalyst 17 can be calculated based on the integrated value of the fuel consumption,
When the integrated value of the fuel consumption reaches a value corresponding to the predetermined amount of SOx absorption, it is possible to determine the SOx release timing, but there is also a correlation between the fuel consumption and the mileage. Accordingly, in this embodiment, the travel distance is integrated instead of the fuel consumption amount, and when the integrated value of the travel distance exceeds a value (determination value D ₀ ) corresponding to a predetermined amount of SOx absorption. It is determined that it is time to release SOx, and if not, it is determined that it is not time to release SOx. In this embodiment, the determination value D ₀ is 50% of the SOx absorption saturation amount of the NOx catalyst 17.
%.

The part of executing a step 102 in the series of signal processing by the ECU 30 is a step of determining whether it is time to release SOx from the NOx catalyst (NOx absorbent).
This can be referred to as a release timing determination unit. Step 102
When the determination is affirmative, the routine proceeds to step 103, and when the determination is negative, the routine proceeds to step 108.

<Step 103> In step 103, the ECU 30 executes an SOx releasing process on the NOx catalyst 17. The SOx release process controls the exhaust air-fuel ratio to stoichiometric by controlling the air-fuel ratio of the air-fuel mixture supplied to the combustion chamber 3 to stoichiometric. 17 is performed by controlling to a predetermined temperature (for example, 600 to 750 ° C.) at which high temperature deterioration is difficult. In this embodiment, NOx
The temperature of the catalyst 17 is controlled by controlling the temperature of the exhaust gas. Hereinafter, controlling the exhaust air-fuel ratio to stoichiometric and controlling the exhaust gas temperature to the predetermined temperature is referred to as high-temperature stoichiometric control, and the SOx release processing is performed when the high-temperature stoichiometric control is performed. Become.

The part of the series of signal processing by the ECU 30 in which the air-fuel ratio is stoichiometrically controlled in step 103 is the stoichiometric (or air-fuel ratio) of the exhaust gas in order to release the SOx absorbed by the NOx catalyst (SOx absorbent). It can be said that the air-fuel ratio control means performs rich control, and the SOx releasing means reduces the oxygen concentration of the exhaust gas in order to release the SOx absorbed by the NOx catalyst (SOx absorbent).

The ECU 3 controls the exhaust gas temperature.
0 is the NOx catalyst 1 based on the input signal from the temperature sensor 25.
7 is detected, and when the output gas temperature is equal to or lower than the predetermined temperature, the exhaust gas temperature is increased and the hydrocarbon (HC) or carbon monoxide (CO) is increased while the engine load is kept constant. The amount of the reducing agent discharged is increased, and the exhaust gas temperature is controlled so as to be maintained at the predetermined temperature.

Here, as means for increasing the exhaust gas temperature, for example, in this embodiment, it is conceivable to delay the ignition timing. Although not provided in the gasoline engine of this embodiment, if the engine includes an exhaust gas recirculation device (so-called EGR device), the exhaust gas recirculation amount (EGR amount) is increased to increase the exhaust gas recirculation amount. It is also conceivable to raise the temperature. Furthermore, although the internal combustion engine in this embodiment is a gasoline engine, if the internal combustion engine is a diesel engine, the pre-mixing is increased, or a sub-injection for injecting fuel in an intake stroke, an expansion stroke, or an exhaust stroke is performed. It is also conceivable to increase the exhaust gas temperature by performing the following.

By executing the high-temperature stoichiometric control, the exhaust gas having a high temperature and a low oxygen concentration flows continuously to the NOx catalyst 17, whereby the sulfate absorbed by the NOx catalyst 17 is decomposed into SO ₃ , This SO ₃ is reduced by unburned HC and CO in the exhaust gas, and is released as SO ₂ .

<Step 104> The ECU 30
Proceeds to step 104, and the vehicle speed detected by the vehicle speed sensor 27 is set at a predetermined speed (for example, 5 km / h).
It is determined whether or not: Hereinafter, this predetermined speed is referred to as a lower limit speed. The part that executes step 104 in the series of signal processing by the ECU 30 can be referred to as vehicle speed determining means for determining whether the running speed of the vehicle is equal to or lower than a predetermined speed.

<Step 105> When the affirmative determination is made in step 104, the ECU 30 proceeds to step 105, stops the high-temperature stoichiometric control, and executes the lean / rich spike control. Stopping the high-temperature stoichiometric control and executing the lean-rich spike control means stopping the execution of the SOx release process and causing the NOx catalyst 17 to perform the NOx absorption / release action. That is,
By stopping the high-temperature stoichiometric control, the NOx catalyst 17
SOx is not released from the lean time by executing the lean-rich spike control is absorbed NOx in the exhaust gas in the NOx catalyst 17, NOx absorbed in the NOx catalyst 17 during the stoichiometric or rich is released, reduced to N ₂ Be purified. Unless the SOx absorption capacity of the NOx catalyst 17 is saturated, SOx in the exhaust gas is absorbed by the NOx catalyst 17 during lean operation. The part that executes step 105 in the series of signal processing by the ECU 30 is NO
It can be said to be SOx emission reduction means for reducing the emission of SOx from the x catalyst (SOx absorbent).

<Step 106> Next, the ECU 30
Proceeding from step 105 to step 106, it is determined whether the vehicle speed detected by the vehicle speed sensor 27 is equal to or lower than the lower limit speed. If an affirmative determination is made in step 106, the routine proceeds to step 105, where the lean / rich spike control is continued. On the other hand, if a negative determination is made in step 106, the routine proceeds to step 103, in which the lean / rich spike control is stopped, the high-temperature stoichiometric control is restarted, and the execution of the SOx release process is restarted.

That is, from step 103 to step 10
By executing step 6, when the vehicle is stopped or running at extremely low speed below the lower limit speed when starting the execution of the high-temperature stoichiometric control, the execution of the high-temperature stoichiometric control is not performed, and the lean-rich spike control is continued, When the vehicle speed becomes higher than the lower limit speed, the execution of the high-temperature stoichiometric control is started. The execution of the high-temperature stoichiometric control is temporarily interrupted until the speed becomes higher than the lower limit speed, and the execution is switched to the execution of the lean / rich spike control. When the vehicle speed becomes higher than the lower limit speed, the execution is switched again to the high-temperature stoichiometric control.

As a result, while the vehicle is stopped or running at a very low speed, the SOx releasing process is not executed, and therefore, the SOx is not released from the vehicle. Can be prevented. As a result, the pedestrian near the vehicle or the occupant of the following vehicle does not feel uncomfortable due to the release of SOx from the vehicle.

<Step 107> The ECU 30
If a negative determination is made in step 104, that is, if it is determined that the vehicle is traveling faster than the lower limit speed, the process proceeds to step 107, where the NOx catalyst 17
It is determined whether or not the Ox release has been completed. The determination as to whether or not the SOx release has been completed is made based on the history of the engine operation under the high-temperature stoichiometric control, and more specifically, whether the operating time of the engine under the high-temperature stoichiometric control has been continuously performed for a predetermined time. Is determined by Here, the operating time of the engine by the high-temperature stoichiometric control is, when the engine is stopped halfway, when the vehicle is stopped such as during idling, or when the vehicle is traveling at an extremely low speed below the lower limit speed,
It is assumed that the time during which the high-temperature stoichiometric control is executed excluding these times is integrated.

The "predetermined time", which is a determination value for determining whether or not it is the time to release SOx,
2 according to the magnitude of the determination value D ₀ of the SOx release timing,
Alternatively, since it differs depending on the magnitude of the target temperature in the execution of the high-temperature stoichiometric control, it cannot be said unconditionally, but it is on the order of several minutes to several hours or several days. Stoichiometric or rich retention time of 100
More than double.

Until the operating time of the engine under the high-temperature stoichiometric control reaches a predetermined time, step 107 is executed.
Is negative, and the execution of the high-temperature stoichiometric control in step 103 is continued.

When the operating time of the engine under the high-temperature stoichiometric control exceeds a predetermined time, it is regarded that SOx absorbed by the NOx catalyst 17 has been almost completely released, and the ECU 30 makes an affirmative determination (SOx
Then, the process proceeds to step 108.

<Step 108> In step 108, the ECU 30 executes lean / rich spike control. During execution of the lean / rich spike control, NOx and SOx in the exhaust gas are reduced by the NOx catalyst 17 during lean operation.
NOx catalyst 17 during stoichiometric or rich
It absorbed NOx is released and reduced and purified to N ₂ in.
During stoichiometric or rich operation during the execution of the lean-rich spike control, SOx absorbed by the NOx catalyst 17 is hardly released because the stoichiometric or rich holding time is short.

As described above, according to this embodiment, the SOx releasing process for the NOx catalyst 17 is executed at the optimal timing.
Can be released almost completely, so that the NOx purification rate of the NOx catalyst 17 can always be kept high.

Further, according to this embodiment, when the vehicle is stopped or running at an extremely low speed, the SOx releasing process is not executed, and SOx is not released from the vehicle.
The vicinity of the rear portion of the vehicle can be prevented from becoming an atmosphere having a high SOx concentration. Therefore, the pedestrian near the vehicle or the occupant of the following vehicle does not feel discomfort due to SOx.

During the execution of the high-temperature stoichiometric control in step 103 or the execution of the lean / rich spike control in step 108, if a high load operation is required for the engine, the stoichiometric control of the air-fuel mixture is interrupted with priority. Thus, when full load operation is requested, the rich control of the air-fuel mixture is interrupted with priority.

FIG. 6 shows an example of the air-fuel ratio control in the first embodiment. In this embodiment, in the lean-rich spike control, for example, 6
Lean operation holding time of 40 at constant speed running at 0 km / h
This is alternately repeated with the stoichiometric operation holding time of about 2 seconds. On the other hand, the continuous operation time in the high-temperature stoichiometric control is about 1 hour, during which the high-temperature stoichiometric control is interrupted once.

[Second Embodiment] FIG. 7 is a view showing an SOx release processing execution routine in an exhaust gas purification apparatus for an internal combustion engine according to a second embodiment of the present invention.

The difference between the second embodiment and the first embodiment in the flowchart of FIG.
5 is the same as the first embodiment in other respects.

In the second embodiment, when it is determined in step 104 that the vehicle speed is equal to or lower than the lower limit speed and the routine proceeds to step 105, the ECU 30 controls the NOx catalyst while controlling the exhaust air-fuel ratio to stoichiometric in step 105. By controlling the temperature of the NOx catalyst 17 to a predetermined temperature at which SOx is not easily released from the NOx catalyst 17 (this temperature is referred to as a SOx release reduction temperature in the following description), the NOx catalyst 1
7, the release of SOx is reduced, and the execution of the SOx release process is temporarily suspended. Also in the second embodiment, the temperature control of the NOx catalyst 17 is performed by controlling the temperature of the exhaust gas. Hereinafter, controlling the exhaust gas temperature to a low temperature equal to or lower than the SOx emission reduction temperature while controlling the exhaust air-fuel ratio to stoichiometric is referred to as low-temperature stoichiometric control. The SOx emission reduction temperature is, for example, about 400 ° C.

To lower the exhaust gas temperature, for example,
A water-cooled cooler unit may be provided around the casing 18 containing the NOx catalyst 17 or around the exhaust pipe 16 upstream of the casing 18, and operating this cooler unit may lower the exhaust gas temperature. Conceivable. In the case of an engine equipped with an EGR device, it is conceivable that the exhaust gas temperature is reduced by reducing the EGR amount.

The part where the low-temperature stoichiometric control is executed in step 105 can be regarded as SOx emission reduction means for reducing the release of SOx from the NOx catalyst 17 (SOx absorbent).

When the temperature of the NOx catalyst 17 is lower than the SOx emission reduction temperature, SOx is hardly released from the NOx catalyst 17 even if the exhaust air-fuel ratio is maintained at stoichiometric.
However, when the stoichiometric exhaust gas is passed through the NOx catalyst 17 in such a low temperature state, the progress of SOx poisoning can be stopped, and the deposited SOx poisoning component (that is, sulfate) gradually decreases. It is thought to change to a form that is easily decomposed. Therefore, if the low-temperature stoichiometric control of the exhaust air-fuel ratio is executed in step 105, NO
When the temperature of the x catalyst 17 rises to a predetermined temperature or higher (that is, when the high-temperature stoichiometric control in step 103 is restarted), SOx is released and reduced very efficiently.

The ECU 30 proceeds from step 105 to step 106 and determines whether or not the vehicle speed is equal to or lower than the lower limit speed.
If the determination is affirmative, the low-temperature stoichiometric control in step 105 is continued, and if the determination is negative, step 103 is performed.
And restart the high-temperature stoichiometric control.

[Other Embodiments] According to the present invention, in step 105 in the first embodiment, a lean
Instead of the rich spike control, a continuous lean control that performs control so as to continuously maintain a lean air-fuel ratio without performing an intermittent rich air-fuel ratio is also established. If the exhaust air-fuel ratio is controlled to be lean by increasing the intake air amount, the exhaust gas temperature can also be reduced, and the Sx from the NOx catalyst 17 can be reduced.
Ox emissions are reduced.

In the above embodiment, the present invention is applied to a gasoline engine. However, it goes without saying that the present invention can be applied to a diesel engine. In the case of a diesel engine, the combustion in the combustion chamber is performed in a much leaner region than the stoichiometric region, so that the air-fuel ratio of the exhaust gas flowing into the NOx catalyst 17 is very lean under a normal engine operating condition, and NOx and Although SOx is absorbed, NOx and SOx are hardly released.

In the case of a gasoline engine, as described above, the air-fuel ratio of the exhaust gas is made stoichiometric or rich by making the air-fuel mixture supplied to the combustion chamber 3 stoichiometric or rich, thereby reducing the oxygen concentration in the exhaust gas. And N
Although NOx and SOx absorbed in the Ox catalyst 17 can be released, in the case of a diesel engine, if the air-fuel mixture supplied to the combustion chamber is made stoichiometric or rich, soot is generated during combustion. And cannot be adopted.

Therefore, when the present invention is applied to a diesel engine, in order to make the exhaust air-fuel ratio stoichiometric or rich, a reducing agent (for example, fuel) is used separately from burning fuel to obtain engine output. (Light oil) must be supplied to the exhaust gas. The supply of the reducing agent to the exhaust gas can be performed by sub-injecting the fuel into the cylinder during the intake stroke, the expansion stroke, or the exhaust stroke, or the reducing agent can be supplied into the exhaust passage upstream of the NOx catalyst 17. It is also possible by supplying.

If a diesel engine is provided with an exhaust gas recirculation device (so-called EGR device),
By introducing a large amount of exhaust gas recirculation gas into the combustion chamber, the exhaust air-fuel ratio can be made stoichiometric or rich.

[0077]

According to the exhaust gas purifying apparatus for an internal combustion engine according to the present invention, the exhaust gas which is arranged in the exhaust passage of the internal combustion engine capable of lean combustion absorbs SOx and flows in when the air-fuel ratio of the flowing exhaust gas is lean. SOx absorbent for releasing SOx absorbed when the oxygen concentration of the gas is low, and SOx releasing means for reducing the oxygen concentration of exhaust gas flowing into the SOx absorbent to release SOx absorbed by the SOx absorbent A vehicle speed determining means for determining whether or not a running speed of a vehicle equipped with the internal combustion engine is equal to or lower than a predetermined speed; and reducing the oxygen concentration of exhaust gas by the SOx releasing means to release SOx from the SOx absorbent. During the execution of the SOx release process, when the vehicle speed determination unit determines that the traveling speed of the vehicle is equal to or lower than the predetermined speed, the SOx release that reduces the release of SOx from the SOx absorbent is performed. By providing the reduction means, it is possible to reduce the release of SOx from the SOx absorbent at the time of stopping the vehicle or running at a low speed, and to provide an excellent effect that the environment around the vehicle can be maintained in a good environment. .

[Brief description of the drawings]

FIG. 1 shows a first embodiment of an exhaust gas purification apparatus for an internal combustion engine according to the present invention.
It is a schematic structure figure of an embodiment.

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

FIG. 3 Unburned HC in exhaust gas discharged from the engine,
FIG. 3 is a diagram schematically showing the concentrations of CO and oxygen.

FIG. 4 is a diagram for explaining the NOx absorbing / releasing action of a NOx storage reduction catalyst.

FIG. 5 is an SOx release processing execution routine of the first embodiment.

FIG. 6 is a diagram showing an example of air-fuel ratio control in the first embodiment.

FIG. 7 shows a second embodiment of the exhaust gas purifying apparatus for an internal combustion engine according to the present invention.
9 is a SOx release processing execution routine according to the embodiment.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Engine main body (internal combustion engine) 3 Combustion chamber 4 Spark plug 11 Fuel injection valve 16 Exhaust pipe (exhaust passage) 17 Storage reduction type NOx catalyst (SOx absorbent) 18 Casing 19 Exhaust pipe (exhaust passage) 27 Vehicle speed sensor (vehicle speed judgment) Means) 30 ECU

Continued on the front page F-term (reference) 3G091 AA02 AA11 AA12 AA17 AA18 AA23 AA24 AB06 BA11 BA14 BA20 BA33 BA36 CA08 CA18 CB02 CB03 DA01 DA02 DA04 DB06 DB10 DB13 EA01 EA05 EA07 EA17 EA30 EA39 FA06 GB10 FB10 GB04W GB05W GB06W GB10X GB16X HA37 HB05 3G301 HA01 HA02 HA04 HA06 HA13 HA15 JA15 JA25 JA33 JB09 KA21 KA28 LB02 LB11 MA01 MA11 MA18 NA08 NB12 NE13 NE14 NE15 PA01A PA01Z PA11A PA11Z PD11A PD11Z PE01A08Z

Claims (3)

[Claims] 1. An exhaust gas system which is disposed in an exhaust passage of an internal combustion engine capable of lean combustion and has a lean air-fuel ratio.
an SOx absorbent that absorbs x and releases SOx absorbed when the oxygen concentration of the exhaust gas flowing in is low; and an oxygen of exhaust gas that flows into the SOx absorbent to release the SOx absorbed by the SOx absorbent. SOx releasing means for reducing the concentration, vehicle speed determining means for determining whether or not the running speed of a vehicle equipped with the internal combustion engine is equal to or lower than a predetermined speed; and reducing the oxygen concentration of the exhaust gas by the SOx releasing means. During the execution of the SOx releasing process for releasing SOx from the SOx absorbent, when the vehicle speed determining means determines that the traveling speed of the vehicle is equal to or lower than the predetermined speed, the SOx reducing the SOx release from the SOx absorbent. An exhaust emission control device for an internal combustion engine, comprising: emission reduction means. 2. The exhaust gas purifying apparatus for an internal combustion engine according to claim 1, wherein said SOx emission reducing means comprises air-fuel ratio control means for controlling an air-fuel ratio of exhaust gas of the internal combustion engine lean. 3. The exhaust gas purifying apparatus for an internal combustion engine according to claim 1, wherein said SOx emission reducing means comprises temperature control means for controlling the temperature of the SOx absorbent to a predetermined temperature or lower.