JPH06221140A - Exhaust gas purifying device for internal combustion engine - Google Patents

Abstract

PURPOSE:To promote reduction operation of NOx discharged during discharge of NOx from an NOx absorbent. CONSTITUTION:When an air-fuel ratio of inflow exhaust gas is lean, NOx is absorbed and when an air-fuel ratio of exhaust gas is brought into a rich state, an NOx absorbent 18 to discharge absorbed NOx is arranged in an engine exhaust gas passage. A CO generating catalyst 21 to partially oxide HC to generate CO is arranged in the engine exhaust gas passage situated upper stream from the NOx absorbent 18. When NOx is discharged from the NOx absorbent 18, an air-fuel ratio of fuel-air mixture fed in a combustion chamber 3 is switched from lean to rich and in this case, an amount of CO in exhaust gas flowing in the NOx absorbent 18 is increased by means of the CO generating catalyst 21.

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, it absorbs NOx when the air-fuel ratio of the inflowing exhaust gas is lean, and absorbs NOx when the inflowing exhaust gas becomes rich or the stoichiometric air-fuel ratio. NO to release
NOx absorbent is placed in the engine exhaust passage to absorb NOx generated when the lean air-fuel mixture is burned, and it is supplied to the combustion chamber before the NOx absorbent capacity of the NOx absorbent is saturated. The air-fuel ratio of the air-fuel mixture is temporarily made rich or the stoichiometric air-fuel ratio to release NOx from the NOx absorbent, and the released NOx is contained in the exhaust gas.
An internal combustion engine that is reduced by C and CO has already been proposed by the present applicant (see Japanese Patent Application No. 3-284095).

[0003]

By the way, when the air-fuel ratio of the air-fuel mixture supplied to the combustion chamber is made rich or the stoichiometric air-fuel ratio, the better the NOx reduction reaction by the unburned HC and CO, the better the effect. The amount of NOx that flows out from the NOx absorbent without being reduced is reduced. However, in the above-mentioned internal combustion engine, there is a problem that the amount of NOx that flows out without being reduced from the NOx absorbent cannot be sufficiently reduced because the reduction reaction of NOx by unburned HC and CO is not sufficiently performed.

[0004]

In order to solve the above problems, according to the present invention, when the air-fuel ratio of the inflowing exhaust gas is lean, NOx is absorbed and the air-fuel ratio of the inflowing exhaust gas is rich or theoretical. A NOx absorbent that releases the absorbed NOx when the air-fuel ratio is reached is arranged in the engine exhaust passage, and a CO generator that partially oxidizes HC to generate CO is provided in the engine exhaust passage upstream of the NOx absorbent. When NOx is to be released from the agent, the air-fuel ratio of the exhaust gas upstream of the CO generating means is switched from lean to rich or stoichiometric air-fuel ratio.

[0005]

[Function] CO has a stronger ability to reduce NOx than HC. Therefore, as described above, CO is generated by partially oxidizing HC by the CO generating means to generate CO in the exhaust gas.
NOx released from NOx absorbent with increasing amount of
Is well reduced in the NOx absorbent and thus N
The amount of NOx that flows out from the Ox absorbent without being reduced is reduced.

[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.
It is connected to a casing 19 containing an Ox 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, a CPU (microprocessor) 34, and an input port 35 which are mutually connected by a bidirectional bus 31. And an output port 36. 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. Also, input port 3
A rotation speed sensor 20 for generating an output pulse representing the engine rotation speed is connected to 5. 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 38, 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 theoretical 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.

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 acceleration 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,
If full load operation is performed, the correction coefficient K is made larger than 1.0. 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, acceleration operation and full load 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 19
The absorbent 18 uses, for example, alumina as a carrier, and potassium K, sodium Na, lithium Li, an alkali metal such as cesium Cs, alkaline earth such as barium Ba, calcium Ca, lanthanum La, yttrium Y is used as a carrier on the carrier. At least one selected from such rare earths and a noble metal such as platinum Pt are supported. The ratio of air and fuel (hydrocarbons) supplied into the engine intake passage and the exhaust passage upstream of the NOx absorbent 18 is called the air-fuel ratio of the exhaust gas flowing into the NOx absorbent 18,
The absorbent 18 absorbs NOx when the air-fuel ratio of the inflowing exhaust gas is lean, and absorbs and releases the NOx that absorbs the NOx when the oxygen concentration in the inflowing exhaust gas decreases. When fuel (hydrocarbon) or air is not supplied into the exhaust passage upstream of the NOx 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, In this case, the NOx absorbent 18 absorbs NOx when the air-fuel ratio of the air-fuel mixture supplied to the combustion chamber 3 is lean, and when the oxygen concentration in the air-fuel mixture supplied to the combustion chamber 3 decreases. It will release NOx.

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 there is a part where 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 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, when the inflowing exhaust gas becomes considerably lean, the oxygen concentration in the inflowing exhaust gas greatly increases.
As shown in (A), the oxygen O ₂ is O ₂ ⁻ or O _2.
It attaches to the surface of platinum Pt in the form of ^{2 −} . On the other hand, NO in the inflowing exhaust gas reacts with O ₂ ⁻ or O ^{2 −} on the surface of platinum Pt to become NO ₂ (2NO + O ₂ → 2NO ₂ ). Next, a part of the generated NO ₂ is absorbed on the platinum Pt while being absorbed in the absorbent and combined with barium oxide BaO to be absorbed in the form of nitrate ion NO ₃ ⁻ as shown in FIG. 5 (A). Diffuses in the agent. In this way NOx becomes NOx
It is absorbed in the 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 NOx absorption capacity of the absorbent is saturated, NO ₂ is absorbed by the nitrate ion NO ₃ ^−. 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. 4, if the lean degree of the inflowing exhaust gas is low, the oxygen concentration in the inflowing exhaust gas is low. Therefore, if the leaning degree of the inflow exhaust gas is low, the air-fuel ratio of the inflowing exhaust gas is lean. Even in this case, NOx is released from the NOx absorbent 18.

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 drops extremely, so
O ₂ is released, and this NO ₂ is reduced by reacting with unburned HC and CO as shown in FIG. 5 (B). 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 18 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- is still unburned HC be consumed,
If CO remains, NOx discharged from the absorbent by this unburned HC and CO and NOx discharged from the engine.
Is reduced. Therefore, if the air-fuel ratio of the inflowing exhaust gas is made rich, the NOx absorbed in the NOx absorbent 18 is released within a short time, and the released NOx is released.
x will be reduced. When the air-fuel ratio of the inflowing exhaust gas is made rich in this way, NOx is released from the NOx absorbent 18, and the released NOx is reduced by unburned HC, CO. In this case, the reducing power for NOx is HC, C.
O is quite different. Next, this will be described with reference to FIG.

FIG. 6 shows the relationship between the HC concentration when C ₃ H ₆ is used as the HC and the amount of outflow NOx which flows out from the NOx absorbent 18 without being reduced, and the CO concentration and the outflow N.
As can be seen from FIG. 6, which shows the relationship with the amount of Ox, H
When the concentration of C or the concentration of CO becomes high, the amount of outflowing NOx becomes considerably small, so that most of the NOx is reduced in the NOx absorbent 18. On the other hand, when the concentration of HC or CO becomes low, the outflow NOx amount increases, and particularly when the NOx is reduced by HC, the outflow NOx amount becomes considerably large when the HC concentration becomes low. Thus, the amount of outflowing NOx differs between HC and CO because the reducing power for NOx differs between HC and CO. As can be seen from FIG. 6, CO is much stronger than NO with respect to NOx. It has a reducing power.

By the way, when the degree of richness of the air-fuel mixture supplied into the combustion chamber 3 is determined, the C in the exhaust gas is correspondingly increased.
Since the concentration of O and the concentration of HC are determined, if the degree of richness of the air-fuel mixture is determined, the amount of outflowing NOx without being reduced from the NOx absorbent 18 is determined accordingly. In this case, since CO has a stronger reducing power for NOx than HC, if HC can be converted to CO, that is, if the amount of CO can be increased, the amount of outflowing NOx can be greatly increased even if the degree of richness of the air-fuel mixture is the same. Can be reduced to.

By the way, conventionally used three-way catalysts have a function of partially oxidizing HC to generate CO. Therefore, when the three-way catalyst is arranged upstream of the NOx absorbent 18, CO is generated in the three-way catalyst, so that the amount of CO in the exhaust gas flowing into the NOx absorbent 18 increases, and thus the reduction from the NOx absorbent 18 is performed. Spilled NOx without being done
The amount can be reduced. However, the amount of COx increased by the three-way catalyst is not so large.

However, when at least one selected from silicon Si, titanium Ti, tin Sn and zirconium Zr is mixed with alumina, which is a carrier of a three-way catalyst, to make a catalyst having a stronger oxidant than the three-way catalyst, It was found that the partial oxidation action was strengthened, and as a result, a larger amount of CO was generated than the three-way catalyst. Table 1 shows the difference in the rate of increase in CO between the CO generation catalyst that generates a larger amount of CO than the three-way catalyst and the three-way catalyst. The CO generation catalyst was composed of a monolith type catalyst, and contained 1.5 g of platinum t per 1 liter of monolith volume.
And 0.3 g of rhodium Rh and 0.1 mol of silicon S
i and are included.

[0021]

[Table 1]

As can be seen from Table 1, when the degree of richness is particularly low, the CO generation catalyst generates a larger amount of CO than the three-way catalyst. As shown in FIG. 1, in the embodiment according to the present invention, a catalytic converter 22 containing this CO generating catalyst 21 is arranged between the exhaust manifold 16 and the exhaust pipe 17. Therefore, in the embodiment shown in FIG. 1, when the air-fuel mixture supplied into the combustion chamber 3 becomes rich, the CO generation catalyst 21
Since a large amount of CO is generated in the NOx absorbent 18, the NOx released from the NOx absorbent 18 is well reduced in the NOx absorbent 18, and thus the NOx absorbent 1
It is possible to suppress the release of NOx released from No. 8 into the atmosphere.

Since the NOx absorbent 18 has the function of a reduction catalyst, the NOx released from the NOx absorbent 18 can be reduced even if the air-fuel ratio of the air-fuel mixture is changed to the stoichiometric air-fuel ratio. However, when the air-fuel ratio of the air-fuel mixture is set to the stoichiometric air-fuel ratio, NOx is gradually released from the NOx absorbent 18, and therefore the total N absorbed in the NOx absorbent 18 is increased.
It takes a little longer time to release Ox.

In the embodiment according to the present invention, as described above, the air-fuel mixture supplied into the combustion chamber 3 is rich during full-load operation, and the air-fuel mixture has a stoichiometric air-fuel ratio during acceleration operation. NOx during acceleration and acceleration
NOx will be released from the absorbent 18. However, if the frequency of such full load operation or acceleration operation is low, even if NOx is released from the NOx absorbent 18 only during full load operation and acceleration operation, while the lean air-fuel mixture is being burned. NOx absorbent 18
The NOx absorption capacity due to is saturated, and thus N
NOx cannot be absorbed by the Ox absorbent 18. Therefore, in the embodiment according to the present invention, when the lean air-fuel mixture is continuously burned, the air-fuel ratio of the air-fuel mixture supplied to the combustion chamber 3 is periodically made rich as shown in FIG. 7 (A). Alternatively, as shown in FIG. 7B, the air-fuel ratio of the air-fuel mixture is periodically set to the stoichiometric air-fuel ratio.

By the way, the action of releasing NOx from the NOx absorbent 18 is such that when a certain amount of NOx is absorbed by the NOx absorbent 18, for example, 50% of the absorption capacity of the NOx absorbent 18 is reached.
It is performed when NOx is absorbed. The amount of NOx absorbed by the NOx absorbent 18 is proportional to the amount of exhaust gas discharged from the engine and the NOx concentration in the exhaust gas. In this case, the exhaust gas amount is proportional to the intake air amount, and NOx
Since the concentration is proportional to the engine load, the NOx amount absorbed by the NOx absorbent 18 is exactly proportional to the intake air amount and the engine load. Therefore, the amount of NOx absorbed in the NOx absorbent 18 can be estimated from the cumulative value of the product of the intake air amount and the engine load. However, in the embodiment of the present invention, the NOx absorption is simplified from the cumulative value of the engine speed. The amount of NOx absorbed in the agent 18 is estimated.

Next, with reference to FIGS. 8 and 9, one embodiment of the control of the absorption and release of the NOx absorbent 18 according to the present invention will be described. FIG. 8 shows an interrupt routine executed at regular time intervals. Referring to FIG. 8, first of all, step 1
At 00, the correction coefficient K for the basic fuel injection time TP
Is less than 1.0, that is, whether the lean air-fuel mixture is burned. When K <1.0, that is, when the lean air-fuel mixture is being combusted, the routine proceeds to step 101, where ΣNE is added to the current engine speed NE.
The result of adding is set to ΣNE. Therefore, this ΣNE indicates the cumulative value of the engine speed NE. Next, at step 102, it is judged if the cumulative rotation speed ΣNE is larger than a fixed value SNE. This constant value SNE indicates the cumulative rotational speed at which it is estimated that the NOx absorbent 18 has absorbed, for example, 50% of NOx amount of its NOx absorbing capacity. When ΣNE ≦ SNE, the processing cycle is completed,
When ΣNE> SNE, that is, when the NOx absorbent 18 has the N
When it is estimated that the NOx amount of 50% of the Ox absorbing capacity is absorbed, the routine proceeds to step 103, where the NOx releasing flag is set. When the NOx releasing flag is set, the air-fuel mixture supplied into the engine cylinder is made rich as will be described later.

Next, at step 104, the count value C is incremented by 1. Next, at step 105, it is judged if the count value C has become larger than a constant value C ₀ , that is, if 5 seconds have elapsed, for example. C
When ≦ C ₀ , the processing routine is completed, and when C> C ₀ , the routine proceeds to step 106, where the NOx releasing flag is reset. When the NOx release flag is reset, the air-fuel mixture supplied to the engine cylinder is switched from rich to lean as described later, and thus the air-fuel mixture supplied to the engine cylinder is made rich for 5 seconds. Become. Next, at step 107, the cumulative rotation speed ΣNE and the count value C are made zero.

On the other hand, in step 100, K ≧ 1.0
If 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 108, where K ≧ 1.0 is maintained for a certain time, for example, 10 seconds. It is determined whether or not. K ≧ 1.0
When the state of No. does not continue for a certain period of time, the processing cycle is completed, and when the state of K ≧ 1.0 continues for a certain period of time, the routine proceeds to step 109, where the cumulative rotational speed ΣNE is made zero. That is, it is considered that most of the NOx absorbed in the NOx absorbent 18 is released when the time period in which the air-fuel mixture supplied to the engine cylinder is at the stoichiometric air-fuel ratio or rich continues for about 10 seconds, Therefore, in this case, the cumulative rotational speed ΣNE is made zero in step 109.

FIG. 9 shows a routine for calculating the fuel injection time TAU, and this routine is repeatedly executed. Figure 9
Referring to FIG. 2, first, at step 200, the basic fuel injection time TP is calculated from the map shown in FIG. Next, at step 201, it is judged if the operating state is one in which the lean mixture should be burned. Step 20 when the lean air-fuel mixture is not in an operating state in which combustion should be performed, that is, during warm-up operation, acceleration operation or full load operation
Proceeding to 2, the correction coefficient K is calculated. During the engine warm-up operation, this correction coefficient K is a function of the engine cooling water temperature, and K ≧
In the range of 1.0, it becomes smaller as the engine cooling water temperature becomes higher. The correction coefficient K is set to 1.0 during acceleration operation,
The correction coefficient K is set to a value larger than 1.0 during full load operation. Next, at step 203, the correction coefficient K is set to Kt, then at step 204, the fuel injection time TA
U (= TP · Kt) is calculated. At this time, the air-fuel mixture supplied into the engine cylinder is at the stoichiometric air-fuel ratio or rich.

On the other hand, when it is judged at step 201 that the engine is in the operating state in which the lean mixture should be burned, the routine proceeds to step 205, where it is judged if the NOx releasing flag is set or not. When the NOx release flag is not set, the routine proceeds to step 206, where the correction coefficient K is set to, for example, 0.6, then at step 207 the correction coefficient K is set to Kt, and then the routine proceeds to step 204. Therefore, at this time, the lean air-fuel mixture is supplied into the engine cylinder. On the other hand, if it is determined at step 205 that the NOx releasing flag is set, then the routine proceeds to step 208, where a predetermined value KK is set to Kt, and then the routine proceeds to step 204. This value KK is the air-fuel ratio of the air-fuel mixture supplied into the engine cylinder from 12.0 to 13.5.
The value is about 1.1 to 1.2. Therefore, at this time, the rich air-fuel mixture is supplied to the engine cylinder,
NOx absorbed by the NOx absorbent 18 thereby
Will be released. The value of KK is set to 1.0 when the air-fuel mixture has a stoichiometric air-fuel ratio when NOx is released.

[0031]

EFFECT OF THE INVENTION To reduce the NOx amount flowing out from the NOx absorbent without being reduced when the NOx absorbed in the NOx absorbent is released from the NOx absorbent while the lean air-fuel mixture is being burned. You can

[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 diagram showing an outflow NOx amount outflowing from a NOx absorbent.

FIG. 7 is a diagram showing NOx release timing.

FIG. 8 is a flowchart of a time interrupt routine.

FIG. 9 is a flowchart for calculating a fuel injection time TAU.

[Explanation of symbols]

 19 ... Exhaust manifold 18 ... NOx absorbent 21 ... CO generation catalyst 22 ... Catalytic converter

Claims (1)

[Claims] 1. 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 air-fuel ratio of the inflowing exhaust gas becomes rich or at the stoichiometric air-fuel ratio in the engine exhaust passage. And the NO
x Partially oxidizes HC in the engine exhaust passage upstream of the absorbent to C
A CO generating means for generating O is provided to remove N from the NOx absorbent.
An exhaust gas purification device for an internal combustion engine, wherein the air-fuel ratio of exhaust gas upstream of the CO generation means is switched from lean to rich or stoichiometric air-fuel ratio when Ox is to be released.