JP3036080B2 - Exhaust gas purification device for internal combustion engine - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an exhaust gas purifying apparatus for an internal combustion engine according to the present invention.

BACKGROUND ART Japanese Patent Laying-Open No. 4-365920 discloses that a cylinder of a multi-cylinder internal combustion engine is divided into a first cylinder group and a second cylinder group.
An NH ₃ generation catalyst for generating ammonia NH ₃ from nitrogen oxide NO _x in the inflowing exhaust gas when the exhaust air-fuel ratio of the inflowing exhaust gas is rich is arranged in the first exhaust passage connected to the cylinder group of A first exhaust passage downstream of the NH ₃ generation catalyst;
And a second exhaust passage connected to the first cylinder group to form a combined exhaust passage, and perform rich operation in the first cylinder group to supply NH ₃ into the combined exhaust passage. the ₃ discloses an exhaust gas purifying apparatus for an internal combustion engine so as to purify the NO _x in the exhaust gas lean operation is discharged from the second cylinder group to be performed. In this exhaust gas purification apparatus, the number of cylinders constituting the second cylinder group is increased as much as possible so that the fuel consumption rate of the engine is reduced as much as possible.
So that NO _x released into the atmosphere is reduced as much as possible the NO _x generated from the cylinder group to purify the NH _3.

By the way, the above-mentioned NH ₃ generation catalyst includes, for example, palladium P
d, a three-way catalyst supporting a noble metal such as platinum Pt and rhodium Rh is used. However, when the NH ₃ generation catalyst is composed of such a three-way catalyst, if the exhaust air-fuel ratio of the exhaust gas flowing into the NH ₃ generation catalyst is rich for a long time, that is, the exhaust air-fuel ratio is rich in the NH ₃ generation catalyst. If the exhaust gas keeps contacting, the NH ₃ generation catalyst loses its catalytic activity relatively easily, and as a result, it becomes impossible to generate NH ₃ satisfactorily, so that there is a problem that it is not possible to purify NO _x satisfactorily.

DISCLOSURE OF THE INVENTION An object of the present invention is to provide an exhaust gas purification device capable of maintaining good catalytic activity of an NH ₃ generation catalyst.

According to the present invention, there is provided an internal combustion engine having a plurality of cylinders divided into first and second cylinder groups and first and second exhaust passages respectively connected to the first and second cylinder groups. an apparatus for purifying exhaust, equipment generates NH ₃ from at least a portion of the first disposed in the exhaust passage, NO _x in the exhaust the exhaust air-fuel ratio of the exhaust gas flowing flows when the rich and NH ₃ synthesizing catalyst to the first enrichment means to the exhaust gas air-fuel ratio of the exhaust gas flowing into the NH ₃ synthesizing catalyst to produce a NH ₃ rich, a first exhaust passage of the NH ₃ synthesizing catalyst downstream and NH ₃ produced in the NH ₃ synthesizing catalyst and the second exhaust passage are merged with each other, N being discharged from the second exhaust passage
A combined exhaust passage for bringing O _x into contact with each other and purifying the NO _x with the NH ₃ , and a first leaning for temporarily making the exhaust air-fuel ratio of the exhaust gas flowing into the NH ₃ generation catalyst lean. And an apparatus comprising:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an overall view of an internal combustion engine, FIG. 2 is a diagram showing characteristics of a three-way catalyst, FIG. 3 is a diagram showing characteristics of an exhaust purification catalyst, and FIG. 4 is a basic exhaust gas of the present invention. 5A and 5B are diagrams showing the amount of HC discharged from the first cylinder group per unit time, FIG. 6 is a time chart for explaining changes in the target air-fuel ratio, Figure 7 is a flow diagram for execution of the operation control, Figure 8 is a flow chart for calculating the fuel injection time, 9A and 9B are diagrams showing the amount of NO _x exhausted from the second cylinder group per unit time, FIG.
And 10B diagrams showing an amount of NO _x being released from per _the NO _x storage-reduction catalyst unit time, 11 is a line diagram showing the temperature of the exhaust gas flowing into _the NO _x storage reduction catalyst 12 according to another embodiment It is a flowchart for performing operation control.

BEST MODE FOR CARRYING OUT THE INVENTION In general, nitrogen oxides NO _x may include nitric oxide NO, nitrogen dioxide NO ₂ , nitrous oxide tetranitrate N ₂ O ₄ , dinitrogen monoxide N ₂ O and the like. Description will be given of the case where mainly NO, and NO ₂ to NO _x is below, an exhaust purifying apparatus of the present invention can also purify other nitrogen oxides.

Referring to FIG. 1, for example, an engine body 1 for an automobile
Each of the cylinders # 1 to # 1, including one cylinder # 1, # 2, # 2, # 3, # 3, and # 4.
4 are connected to a common surge tank 3 via the corresponding intake branch pipes 2, and the surge tank 3 is connected to an air cleaner (not shown) via the intake duct 4. A fuel injection valve 5 for supplying fuel to a corresponding cylinder is arranged in each intake branch pipe 2. On the other hand, a throttle valve 6 whose opening degree increases as the depression amount of an accelerator pedal increases is arranged in the intake duct 4. In addition,
Each fuel injection valve 5 is controlled based on an output signal from the electronic control unit 20.

On the other hand, the first cylinder # 1 is connected via an exhaust pipe 7 to a catalytic converter 9 having a built-in NH ₃ generating catalyst 8. On the other hand, the second cylinder # 2, the third cylinder # 3 and the fourth cylinder # 4 are connected via a common exhaust manifold 10 to a catalytic converter 12 containing a storage material 11. In the internal combustion engine shown in FIG. 1, the first cylinder # 1 constitutes a first cylinder group 1a, and the second cylinder # 2, the third cylinder # 3 and the fourth cylinder # 4 constitute a second cylinder group 1b. doing. Therefore, the exhaust gas of the first cylinder group 1a is guided to the NH ₃ generation catalyst 8, and the exhaust gas of the second cylinder group 1b is guided to the storage material 11. These catalytic converters 9,
12 is an exhaust purification catalyst 14 through a common merging exhaust pipe 13
Is connected to the catalytic converter 15 having the built-in.

The electronic control unit 20 is composed of a digital computer, and a ROM (read only memory) 22, a RAM (random access memory) 23, a CPU (microprocessor) 24, an input port 2
5, and an output port 26. A pressure sensor 27 that generates an output voltage proportional to the pressure in the surge tank 3 is attached to the surge tank 3, and the output voltage of the pressure sensor 27 is input to an input port 25 via an AD converter 28. The CPU 24 calculates the intake air amount based on the output signal from the AD converter 28. Attached to the exhaust pipe 7 is an air-fuel ratio sensor 29 that generates an output voltage corresponding to the exhaust air-fuel ratio (described later) of the exhaust flowing through the exhaust pipe 7. The output voltage of the air-fuel ratio sensor 29 is an AD converter. Input port 25 through 30
Is input to In addition, in the collecting part of the exhaust manifold 10,
An air-fuel ratio sensor 31 that generates an output voltage according to the exhaust air-fuel ratio of the exhaust gas flowing through the collecting portion of the exhaust manifold 10 is attached, and the output voltage of the air-fuel ratio sensor 31 is supplied to an input port 25 via an AD converter 32. Is entered. In addition, input port 25
Is connected to a crank angle sensor 33 that generates an output pulse every time the crankshaft rotates, for example, by 30 degrees. CPU2
In 4, the engine speed is calculated based on the output pulse. On the other hand, the output ports 26
Is connected to each of the fuel injection valves 5.

In the example shown in FIG. 1, the NH ₃ generation catalyst 8a is composed of a three-way catalyst 8a. The three-way catalyst 8a is formed by supporting a noble metal such as palladium Pd, platinum Pt, and rhodium Rh on a washcoat layer made of, for example, alumina formed on the surface of the carrier.

FIG. 2 shows the purification rate of the three-way catalyst. When the ratio of the total amount of air to the total amount of fuel supplied into the exhaust passage, the combustion chamber, and the intake passage upstream of a certain position in the exhaust passage is referred to as an exhaust air-fuel ratio of exhaust flowing through that position, FIG. When the exhaust air-fuel ratio of the exhaust gas flowing into the three-way catalyst 8a is leaner than the stoichiometric air-fuel ratio (A / F) S (= 14.6, λ = 1.0), the three-way catalyst 8a Pass NO _x ,
When the exhaust gas air-fuel ratio of the exhaust gas flowing into the TW catalyst 8a becomes richer than the stoichiometric air-fuel ratio (A / F) S for converting the NO _x in the exhaust to the ammonia NH _3. Although the NH ₃ generation mechanism in this case is not necessarily clarified, a part of the NO _{x in} the exhaust gas having a rich exhaust air-fuel ratio is expressed by the following formulas (1) to (1).
It is believed that the reaction of (2) is converted to NH ₃ .

5H ₂ + 2NO → 2NH ₃ + 2H ₂ O (1) 7H ₂ + 2NO ₂ → 2NH ₃ + 4H ₂ O (2) On the other hand, the remaining NO _x is expressed by the following formula (3)
It is believed that the reaction (6) reduces to N ₂ .

2CO + 2NO → N ₂ + 2CO ₂ (3) 2H ₂ + 2NO → N ₂ + 2H ₂ O (4) 4CO + 2NO ₂ → N ₂ + 4CO ₂ (5) 4H ₂ + 2NO ₂ → N ₂ + 4H ₂ O (6) Therefore, to the three-way catalyst 8a NO _x exhaust air-fuel ratio of the exhaust gas flowing into the TW catalyst 8a when a rich flowing is converted to either NH ₃ or N _2, i.e. NO _x is the three-way catalyst 8a
Is prevented from being exhausted.

Conversion efficiency ETA when NO _x that has flowed into the three-way catalyst 8a is converted to NH ₃ is larger as the exhaust gas air-fuel ratio of exhaust gas flowing into the three-way catalyst 8a, as shown in FIG. 2 is reduced from the stoichiometric air-fuel ratio It becomes constant when it becomes smaller. In the example shown in FIG. 2, the exhaust air-fuel ratio of the exhaust gas flowing into the three-way catalyst 8a is about 1
When it is smaller than 3.8 (the excess air ratio λ is about 0.95), the conversion efficiency ETA becomes constant.

In the internal combustion engine of FIG. 1, for reasons to be described later, the exhaust gas air-fuel ratio of the exhaust gas flowing into the TW catalyst 8a so is to generate a large amount of NH ₃ as possible when it is rich, as well as possible non燃担rack hydrogen HC simultaneously Preferably, it can be purified. Therefore, as the three-way catalyst 8a, a three-way catalyst supporting palladium Pd having high conversion efficiency of NO _x to NH ₃ and cerium Ce having an oxygen holding function is used. Note that, in the three-way catalyst supporting rhodium Rh, generation of NH ₃ is suppressed. Therefore, the three-way catalyst 8a is preferably a three-way catalyst that does not support rhodium Rh.

On the other hand, storage material 11 is for a large amount of the NO _x in the exhaust purification catalyst 14 is temporarily stored the NO _x in the inflowing exhaust gas is prevented from flowing. The storage material 11 does not necessarily have to have a catalytic function, but in the present embodiment, the storage material 11 is a so-called NO _x storage-reduction catalyst 11a having a function of absorbing and releasing NO _{x and} a function of reducing NO _x . This NO _x storage reduction catalyst 11a
Represents potassium K, sodium N on a washcoat layer made of, for example, alumina formed on the surface of the carrier.
a, lithium Li, alkali metal such as cesium Cs, barium Ba, alkaline earth such as calcium Ca, lanthanum La, rare earth such as yttrium Y, at least one selected from transition metals such as iron Fe, It is formed by supporting a noble metal such as platinum Pt. Absorbing the exhaust air-fuel ratio of the exhaust gas the NO _{x storage-and-reduction} catalyst 11a is flowing into _the NO _x storage reduction catalyst 11a is at the time of lean occludes NO _x, the oxygen concentration in the exhaust gas flowing into the NO _{x storage-and-reduction} catalyst 11a decreases It performs the absorption and release action of NO _x that releases the released NO _x .

If the NO _x storage-reduction catalyst 11a is disposed in the engine exhaust passage, the NO _x storage-reduction catalyst 11a actually performs the NO _x absorption / release operation, but there are some parts where the detailed mechanism of the absorption / release operation is not clear. . However, it is considered that this absorption / release action is performed by the mechanism described below. 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, rare earths, and transition metals.

That is, the exhaust gas air-fuel ratio of the exhaust gas flowing into _the NO _x storage reduction catalyst 11a is becomes lean, i.e. the oxygen concentration in the exhaust gas of these oxygen O ₂ Increasing significantly O ₂ ^- or O platinum Pt in ^2-form Attaches to surface. On the other hand, NO in the exhaust gas reacts with O ₂ ⁻ or O ²⁻ on the surface of platinum Pt to become NO ₂ (2NO + O ₂ → 2
NO ₂ ). Next, a part of the generated NO ₂ is absorbed in the NO _x storage reduction catalyst 11a while being oxidized on the platinum Pt, and barium oxide is absorbed.
Nitrate ions NO ₃ while bonding with the BaO ^- diffuse into _the NO _x storage-reduction catalyst 11a in the form of. In this way, NO _x is stored in the NO _x storage reduction catalyst 11a.

On the other hand, when the oxygen concentration in the exhaust gas flowing into the NO _x storage reduction catalyst 11a decreases and the amount of generated NO ₂ decreases, the reaction proceeds in the reverse direction (NO ₃ ⁻ → NO ₂ ), and thus the NO _x storage occurs. Reduction catalyst 11a
The nitrate ion NO ₃ ⁻ in the inside is released from the NO _x storage reduction catalyst 11a in the form of NO ₂ . That, _the NO _x storage-reduction when the oxygen concentration in the exhaust gas flowing into the catalyst 11a decreases, for example, NO _x when the exhaust gas air-fuel ratio of storage reduction catalyst 11a and flows into the exhaust gas becomes rich from lean NO _{x storage-and-reduction} catalyst 11a from the NO _x Is released.
At this time, if a reducing agent, for example, HC exists around the NO _x storage reduction catalyst 11a, this NO _x is reduced by the HC and purified.

The exhaust gas purifying catalyst 14 used in each of the following embodiments is provided with a wash coat layer such as alumina formed on a carrier, for example, copper Cu, chromium Cr, vanadium V, titanium Ti, iron F
e, nickel Ni, cobalt Co, platinum Pt, palladium Pd,
A catalyst in which one or more substances selected from transition elements included in the fourth period of the periodic table or group VIII such as rhodium Rh and iridium Ir are supported as a catalyst component is used.

When the temperature of the exhaust purification catalyst 14 is within a predetermined temperature range (optimum temperature range described later) determined by the supported catalyst component, the exhaust purification catalyst 14 substantially reduces the NH ₃ component contained in the inflowing oxidizing atmosphere exhaust gas. complete conversion to N _2, and has a function to purify.

The temperature of the exhaust purification catalyst 14 NH ₃ is oxidized NO _x in the inflowing exhaust gas on the exhaust purification catalyst 14 when more than the optimum temperature range is generated, NO _x flows out into the exhaust purification catalyst 14 downstream I will be. That is, in the catalyst temperature region equal to or higher than the optimum temperature range, the next oxidation reaction of NH ₃ (7) and (8) becomes dominant on the exhaust purification catalyst 14, and the NO _x (NO ₂ , NO) component increases.

4NH ₃ + 7O ₂ → 4NO ₂ + 6H ₂ O (7) 4NH ₃ + 5O ₂ → 4NO + 6H ₂ O (8) Furthermore, when the temperature of the exhaust purification catalyst 14 is lower than the optimum temperature range, the ammonia decomposition reaction of the exhaust purification catalyst 14 decreases. ,
Of NH ₃ in the inflowing exhaust gas, the amount of NH ₃ flowing into the downstream side of the catalyst through the exhaust gas purifying catalyst 14 is increased.

FIG. 3 is a diagram schematically showing a change in the exhaust gas purification characteristics depending on the temperature of the exhaust gas purification catalyst 14. As shown in FIG. 3, NH ₃ in the gas at the exhaust purification catalyst 14 an outlet in the case of supplying an oxidizing atmosphere of a gas containing NH ₃ in constant concentration in the exhaust purification catalyst 14
And the concentration of NO _x, shows the relationship between the temperature of the exhaust purification catalyst 14, the horizontal axis represents the catalyst temperature and the vertical axis represents each concentration of each component in the gas, solid line in the figure is a catalyst outlet NH ₃ concentrations, the dotted line shows a catalyst outlet concentration of NO _x is.

As shown in FIG. 3, when the NH ₃ concentration in the gas at the catalyst inlet is kept constant, the region where the catalyst temperature is low (FIG. 3, section I)
Then, the NH ₃ concentration at the catalyst outlet is almost equal to the NH ₃ concentration at the inlet, and conversely, the NO _x concentration is almost zero. That is,
NH ₃ in the inflowing gas passes through the exhaust purification catalyst 14 as it is and flows out downstream.

In a temperature region higher than section I (FIG. 3, section II), the outlet NH ₃ concentration decreases as the temperature increases, but the outlet NO _x
The concentration remains almost zero and does not change. That is, in this region, the proportion of NH ₃ flowing into the exhaust purification catalyst 14 that is converted to N ₂ increases as the temperature of the exhaust purification catalyst 14 increases.

In this state, when the temperature of the exhaust purification catalyst 14 further rises (section III in FIG. 3), the NH ₃ concentration further decreases at the catalyst outlet while the NO _x concentration remains almost zero, and the NH ₃ and NO _x concentrations become In both cases, an almost zero state occurs. That is, in this temperature range,
Almost all of the NH ₃ flowing into the exhaust purification catalyst 14 is converted to N ₂ without generating NO _x and purified.

Further, NO _x concentration at the catalyst outlet when the temperature of the exhaust purification catalyst 14 than the section III is further increased increases with temperature (Fig. 3, section IV), further the temperature of the exhaust purification catalyst 14 rises exhaust purification catalyst The entire amount of NH ₃ flowing into 14 is converted to NO _x (FIG. 3, section V).

In the present specification, the temperature range in which almost all of the NH ₃ component in the gas flowing into the exhaust purification catalyst 14 is converted to N ₂ and NO _x is not generated as shown in a section III of FIG. It is called the optimal temperature range.

This optimum temperature range is determined according to the substance used as the catalyst component, and starts from, for example, a temperature lower than the activation temperature of the three-way catalyst. The optimal temperature range is, for example, about 100 ° C. to 400 ° C. (preferably 100 to 300 ° C., more preferably 100 to 250 ° C.) when platinum Pt, palladium Pd, rhodium Rh or the like is supported as a catalyst component. Yes, Chrome Cr,
Approximately 150 ° C to 650 ° C when copper Cu, iron Fe, etc. are supported
(Preferably 150 to 500 ° C.).

Further, the exhaust purification catalyst 14 is used as a tandem catalyst that supports a noble metal-based catalyst component such as platinum Pt on the downstream side in the exhaust flow direction and a base metal-based catalyst component such as chromium Cr on the upstream side. The temperature range can be expanded.

When the exhaust purification catalyst 14 is in the above limited temperature range, the reason is as follows.
At present, it is completely impossible to convert NH ₃ in the incoming gas almost completely to N ₂ without generating NO _x, and to convert NH ₃ to NO _x in the temperature range higher than that. Is not clear. However, this is because the exhaust purification catalyst 14
It is considered that the following reaction occurs in the optimum temperature range of.

That is, in the region where the catalyst temperature is in the optimum temperature range, the following denitration reactions (9) and (10) occur in addition to the above-described NH ₃ oxidation reactions (7) and (8).

8NH ₃ + 6NO ₂ → 12H ₂ O + 7N ₂ (9) 4NH ₃ + 4NO + O ₂ → 6H ₂ O + 4N ₂ (10) Therefore, it was produced in the oxidation reactions (7) and (8).
Since NO _x sequential reaction are decomposed generated by the reaction immediately denitration reaction with NH ₃ in the exhaust (9) and (10), the total amount of NH ₃ is believed to be converted to N ₂ as a result.

On the other hand, when the temperature of the exhaust purification catalyst 14 is higher than the optimum temperature range, the oxidizing reactions (7) and (8) is large proportion of those is converted to NO _x of NH ₃ in the inflowing exhaust gas becomes active (9) and (10) are less likely to occur. Therefore, the generated NO _x is to flow out as it is from the exhaust purification catalyst 14 without being reduced by the denitrating reactions (9) and (10) is at a temperature higher than the optimum temperature range.

At a temperature lower than the optimum temperature range, the oxidation reaction (7)
And (8) denitration reaction product amount of the NO _x to become inactive is reduced (9) and (10) is less likely to occur. For this reason, at a temperature lower than the optimum temperature, the exhaust purification catalyst 14 does not consume NH ₃ due to the denitration reactions (9) and (10).
It is thought that it will come out from.

As described above, the optimal temperature range of the exhaust purification catalyst 14 is NH 3
₃ of the oxidation reaction (7) and (8) is balanced with the denitrating reactions (9) and (10), the generated NO _x components with the NH ₃ by immediately successive reaction the reaction, the temperature region as reduced Conceivable. For this reason, the optimal temperature range is determined by the oxidizing power of the catalyst and the temperature change characteristics of the oxidizing power. As described above, when a catalyst having a strong oxidizing power such as platinum Pt is used, a relatively high temperature such as chromium Cr is used. It is considered that the optimum temperature range tends to be on the high temperature side as compared with the case where the catalyst having weak oxidizing power is used.

Although at present the reason as described above has not been fully elucidated, actually an exhaust purification catalyst 14 NH ₃ is almost completely in the gas oxidizing atmosphere flows used in the optimum temperature range converted to N ₂ Has been confirmed to be.

In connection with this, the following three points have been confirmed when the exhaust purification catalyst 14 is used in the above-mentioned optimum temperature range.

That is, the first point is that if the exhaust gas flowing into the exhaust purification catalyst 14 is an oxidizing atmosphere, that is, if the exhaust air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst 14 is leaner than the stoichiometric air-fuel ratio, the NH ₃ of is completely converted into N _2, the lean degree of the exhaust gas air-fuel ratio of the inflowing exhaust gas is that unaffected.

The second point is that in the exhaust gas flowing into the exhaust purification catalyst 14,
When NO _x is contained together with NH ₃ , NH ₃ is
₃ NO _x is also cleaned with, NO _x concentration at the catalyst outlet is that becomes substantially zero. In this case, the ratio between the amount of NH ₃ in the exhaust gas flowing into the exhaust purification catalyst 14 and the amount of NO _x, that is, NO ₂ or NO, is determined by the equivalent ratio (4: 3) in the above-described denitration reactions (9) and (10). Or 1: 1), the amount of NH ₃ contained in the inflowing exhaust gas is reduced to the amount of NO ₂ in the inflowing exhaust gas.
If the amount is larger than necessary to reduce NO, NO _x (NO ₂ , NO) in the inflowing exhaust gas is completely purified. Further, as described above, if the exhaust air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst 14 is lean, the excess NH ₃ is completely purified by the exhaust purification catalyst 14, so that the excess NH ₃ flows out downstream of the catalyst. Never.

However, in the exhaust gas flowing into the exhaust purification catalyst 14, NH ₃ and NO _x
Contains NO _x in the exhaust gas when both contain a section (FIG. 3, section IV) the concentration of NO _x in catalyst outlet shown in FIG. 3 increases, which flows into the exhaust purification catalyst 14 with N without
It will be started more from the low temperature side as compared with the case that contains the H _3, thus the optimum temperature range becomes narrow.

This is because the NO _x produced by the oxidation of NH ₃ in a high temperature region if already contains NO _x in the exhaust gas flowing into the exhaust purification catalyst 14, both the NO _x in the inflowing exhaust gas This is because it is necessary to purify and NH ₃ shortage is likely to occur. Conventionally, vanadium oxide / titania (V ₂ O ₅ / TiO ₂ ) has been used as a catalyst for causing a denitration reaction between NH ₃ and NO _x in exhaust gas.
Although such systems the catalyst is known, NH ₃ in the exhaust purification catalyst 14
And NO _x in order to prevent excess NH ₃ and NO _x from flowing out downstream of the catalyst, so that NH ₃ and N
It was necessary to strictly adjust the ratio of the amount to O _x to the equivalent ratio in the denitration reaction. That is, when both NO ₂ and NO are contained in the exhaust gas, the amount of NH ₃ has to be strictly adjusted to the sum of 4/3 times NO ₂ and 1 time NO in the exhaust gas. On the other hand, in the exhaust purification catalyst 14 of the present embodiment, the amount of NH ₃ is equal to or more than the above-mentioned equivalent ratio with respect to the amount of NO _x (NO ₂ , NO) in the exhaust gas, and the exhaust air of the inflowing exhaust gas is discharged. If the fuel ratio is lean, both NO _x and NH ₃ in the inflowing exhaust gas can be completely purified, and there is a great difference in that they do not flow out downstream of the catalyst.

The third point is that the exhaust gas flowing into the exhaust purification catalyst 14
Even when HC and CO components are contained, if the exhaust air-fuel ratio is lean, the HC and CO components in the exhaust gas are oxidized by the exhaust purification catalyst 14 and do not flow out to the downstream side of the catalyst.

Incidentally, as described above, the exhaust purification catalyst 14 completely decomposes NH ₃ in the inflowing exhaust gas in the optimum temperature range.
In the temperature range lower than the optimal temperature range as described in
NH ₃ in the inflowing exhaust gas flows out downstream of the catalyst without being purified.

On the other hand, acidic inorganic components (Zeolite, silica SiO ₂ , silica alumina SiO ₂ · Al ₂ O ₃ , Brønsted acids such as titania, and oxides of transition metals such as copper Cu, cobalt Co, nickel Ni, iron Fe, etc. (Including Lewis acids) are known to adsorb NH _{3 in the} low temperature range. Therefore, when the above-mentioned acidic inorganic component is carried on the exhaust purification catalyst 14, or when the porous material is formed from a material containing the above-mentioned acidic inorganic component and used as the carrier itself, the gas flows in a temperature range lower than the optimum temperature range. Excess NH _{3 in} the exhaust gas is adsorbed on the catalyst carrier. As a result, the amount of unpurified NH ₃ flowing downstream of the catalyst in a temperature range lower than the optimum temperature range can be reduced. Further, NH ₃ adsorbed by these acidic inorganic components is released when the temperature of the exhaust purification catalyst 14 rises or the NH ₃ concentration in the inflowing exhaust gas decreases. NH ₃ released from the exhaust purification catalyst 14 is
In the case where the temperature of the exhaust purification catalyst 14 fluctuates because it is decomposed by 4, the use of the acidic carrier as described above can improve the NH ₃ purification efficiency as a whole.

In the internal combustion engine of FIG. 1, the fuel injection time TAU is calculated based on the following equation.
Is calculated.

TAU = TB ・ ((A / F) S / (A / F) T) ・ FAF If the air-fuel ratio of the air-fuel mixture in the combustion chamber of each cylinder is called the engine air-fuel ratio, TB is the stoichiometric air-fuel ratio. (A / F) The basic fuel injection time that is optimal for S, and is determined by the following equation.

TB = (Q / N) · K Here, Q is the intake air amount, N is the engine speed, and K is a constant. Therefore, the basic fuel injection time TB is a constant for the intake air amount per engine rotation. It is obtained as a result of multiplication.

(A / F) T represents a control target value of the engine air-fuel ratio.
If the control target value (A / F) T is increased to make the engine air-fuel ratio leaner than the stoichiometric air-fuel ratio (A / F) S, the fuel injection time TAU
Becomes smaller, the fuel injection amount is reduced, and if the control target value (A / F) T is reduced to make the engine air-fuel ratio richer than the stoichiometric air-fuel ratio (A / F) S, the fuel injection time TAU increases. The fuel injection amount is increased. In this embodiment, the control target value (A / A) of the engine air-fuel ratio of each cylinder of the second cylinder group 1b is set.
F) T is the same for each cylinder.

FAF represents a feedback correction coefficient for making the actual engine air-fuel ratio coincide with the control target value (A / F) T. The feedback correction coefficient FAF is determined by the fuel injection time TAU of the cylinders constituting the first cylinder group 1a, that is, the first cylinder # 1.
When calculating the fuel injection time TAU of each of the cylinders constituting the second cylinder group 1b, namely, the second cylinder # 2, the third cylinder # 3, and the fourth cylinder # 4,
AFB. The feedback correction coefficient FAFA is determined based on the output signal of the air-fuel ratio sensor 29, and the feedback correction coefficient FAFB is determined based on the output signal of the air-fuel ratio sensor 31. In the internal combustion engine shown in FIG.
Is one-to-one with the exhaust air-fuel ratio over a wide range of exhaust air-fuel ratios.
Each is constituted by a so-called full-range air-fuel ratio sensor that generates a corresponding continuous signal. The exhaust air-fuel ratio of the exhaust gas in the exhaust pipe 7 detected by the whole-area air-fuel ratio sensor 29 is the first cylinder group 1a.
When the engine air-fuel ratio detected by the full-range air-fuel ratio sensor 29 is leaner than the control target value (A / F) T, the feedback correction coefficient FAFA is increased to increase the fuel injection. The engine air-fuel ratio detected by the full-range air-fuel ratio sensor 29 is increased to the control target value (A /
F) When it is richer than T, the fuel injection amount is reduced by reducing the feedback correction coefficient FAFA. Thus, the engine air-fuel ratio of the first cylinder group 1a is reduced to the control target value (A / F) T. Is matched.

The exhaust air-fuel ratio of the exhaust in the exhaust manifold 10 is the second
And the engine air-fuel ratio detected by the full-range air-fuel ratio sensor 31 is equal to the control target value (A /
F) When leaner than T, the fuel injection amount is increased by increasing the feedback correction coefficient FAFB, and the engine air-fuel ratio detected by the full-range air-fuel ratio sensor 31 is smaller than the control target value (A / F) T. When the engine is rich, the fuel injection amount is reduced by reducing the feedback correction coefficient FAFB. Thus, the engine air-fuel ratio of each cylinder of the second cylinder group 1b is made to match the control target value (A / F) T. Can be
Note that these feedback correction coefficients FAFA and FAFB fluctuate around 1.0, respectively.

In order to better match the engine air-fuel ratio with the control target value (A / F) T, the second cylinder group in the combined exhaust pipe 13
An additional air-fuel ratio sensor is provided in a position where exhaust gas from 1b does not contact, for example, in the merged exhaust pipe 13 immediately downstream of the three-way catalyst 8a, and the air-fuel ratio sensor 29 is deteriorated in accordance with the output signal of this air-fuel ratio sensor. Compensates for the deviation of the engine air-fuel ratio of the first cylinder group 1a from the control target value (A / F) T based on the position of the first cylinder group 1a, or the position within the merged exhaust pipe 13 where the exhaust from the first cylinder group 1b does not contact. , ie for example additional in _the NO _x storage-reduction confluence exhaust pipe 13 immediately downstream of the catalyst 11a air-fuel ratio provided sensor, the engine air-fuel ratio based on the deterioration of the air-fuel ratio sensor 31 in accordance with the output signal of the air-fuel ratio sensor A deviation from the control target value (A / F) T may be compensated. As these air-fuel ratio sensors, besides the full-range air-fuel ratio sensor, for example, a so-called Z-characteristic oxygen sensor whose output voltage changes stepwise when the exhaust air-fuel ratio increases or decreases beyond the stoichiometric air-fuel ratio can be used. Further, the deterioration of the catalyst located between the air-fuel ratio sensors may be detected based on the output signals of the plurality of air-fuel ratio sensors arranged in series in the exhaust passage.

The internal combustion engine shown in FIG. 1 is not provided with a device for secondary supply of fuel or air into the exhaust passage. Thus, the exhaust gas air-fuel ratio of the exhaust gas flowing into the TW catalyst 8a coincides with the engine air-fuel ratio, i.e. the engine air-fuel ratio of the first cylinder # 1 of the first cylinder group 1a, flows into _the NO _x storage reduction catalyst 11a exhaust Is equal to the engine air-fuel ratio of the second cylinder group 1b, that is, the engine air-fuel ratio of the second cylinder # 2, the third cylinder # 3, and the fourth cylinder # 4.

Next, a basic exhaust gas purification method of the internal combustion engine of FIG. 1 will be described first with reference to FIG.

Exhaust gas air-fuel ratio of the exhaust gas flowing into the TW catalyst 8a is an internal combustion engine of FIG. 1 is rich, the exhaust gas air-fuel ratio of the exhaust gas flowing into _the NO _x storage reduction catalyst 11a is made lean. Exhaust gas air-fuel ratio of the exhaust gas flowing into the TW catalyst 8a is NH ₃ is generated from the NO _x in the exhaust gas flowing in when it is made rich three-way catalyst 8a. This NH ₃
Reaches the exhaust purification catalyst 14 via the merged exhaust pipe 13. on the other hand,
In NO _x when the exhaust gas air-fuel ratio of the exhaust gas occluding flow into the reduction catalyst 11a is made lean NO _{x storage-and-reduction} catalysts 11a, NO _x in the exhaust gas flowing into the NO _{x storage-and-reduction} catalyst 11a is in _the NO _x storage reduction catalyst 11a Occluded. In this case, not necessarily _the NO _x storage total amount of the NO _x flowing into the reduction catalyst 11a is not necessarily occluded in _the NO _x storage reduction catalyst 11a, NO _x Some of the NO _x out of the NO _x flowing into the storage-reduction catalyst 11a Is stored in the NO _x storage reduction catalyst 11a
It leaks out of the O _x storage reduction catalyst 11a. This NO _x then reaches the exhaust purification catalyst 14 via the combined exhaust pipe 13.

The exhaust gas flowing through the three-way catalyst 8a and the NO
_Although the exhaust gas flowing through the _x storage reduction catalyst 11a flows in, in this embodiment, the exhaust air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst 14 is always kept lean. Therefore, the exhaust purification catalyst 14 purifies NO _x and NH ₃ in the exhaust gas flowing into the exhaust purification catalyst 14 by the reactions of the above equations (7) to (10). Therefore, the NO _x and NH ₃ is discharged into the atmosphere is prevented. The exhaust purification catalyst
Hydrocarbons HC in the exhaust gas flowing into the 14, carbon monoxide CO or hydrogen H _2, is also included. These HC and CO act as reducing agents in the same manner as NH _3, and NO
It is believed that some of _x is reduced. However, N
The reducing power of H ₃ is stronger than those of HC and CO, and therefore, NO _x can be reliably reduced by using NH ₃ as a reducing agent.

In order to make the exhaust air-fuel ratio of the exhaust gas flowing into the three-way catalyst 8a rich, rich operation in which the engine air-fuel ratio is richer than the stoichiometric air-fuel ratio (A / F) S is performed on the cylinders of the first cylinder group 1a. I am trying to do it. That is, if the control target value of the engine air-fuel ratio of each cylinder is referred to as a target air-fuel ratio (A / F) T, the target air-fuel ratio (A / F) T of the first cylinder # 1 is calculated as the stoichiometric air-fuel ratio (A / F). ) S
By setting the rich air-fuel ratio (A / F) R to be richer, the exhaust air-fuel ratio of the exhaust gas flowing into the three-way catalyst 8a becomes rich.

Further, in order to make the exhaust air-fuel ratio of the exhaust gas flowing into the NO _x storage reduction catalyst 11a lean, each cylinder of the second cylinder group 1b has:
Lean operation is performed in which the engine air-fuel ratio is leaner than the stoichiometric air-fuel ratio (A / F) S. That is, the target air-fuel ratio (A / F) T of the second cylinder # 2, the third cylinder # 3, and the fourth cylinder # 4 is set to a lean air-fuel ratio that is leaner than the stoichiometric air-fuel ratio (A / F) S. exhaust gas air-fuel ratio of the exhaust gas flowing into _the NO _x storage reduction catalyst 11a by the (a / F) L is set to be lean.

In order to make the exhaust air-fuel ratio of the exhaust gas flowing into the three-way catalyst 8a rich, for example, a secondary fuel supply device for secondary supply of fuel into the exhaust pipe 7 upstream of the three-way catalyst 8a is provided. The secondary air supply of fuel from the secondary fuel supply device while performing the lean operation of the first cylinder group 1a so that the exhaust air-fuel ratio of the exhaust gas flowing into the three-way catalyst 8a becomes rich. Can also.

Rich air-fuel ratio (A / F) R and lean air-fuel ratio (A / F) L
May be varied according to the engine operating state, but in this embodiment, it is substantially constant regardless of the engine operating state. That is, the rich air-fuel ratio (A / F) R is about 14.0, and the lean air-fuel ratio (A / F) L is about 25.0. Accordingly, the target air-fuel ratio (A / F) T of the first cylinder is maintained at about 14.0, and the target air-fuel ratios (A / F) of the second cylinder # 2, the third cylinder # 3, and the fourth cylinder # 4 are maintained. T is maintained at about 25.0.

By the way, the fuel consumption rate for performing the lean operation can be reduced. Therefore, as in the case of the internal combustion engine shown in FIG. 1, the second cylinder group 1b basically performs the lean operation, whereby the fuel consumption rate of the engine 1 can be reduced while the exhaust gas is favorably purified. In particular, in the internal combustion engine of FIG. 1, the number of cylinders in the second cylinder group 1b is a majority of all cylinders,
Therefore, the fuel consumption rate of the engine 1 can be favorably reduced.

Thus to perform the rich operation to the first cylinder group 1a, since the if ask performed lean operation NO _x and NH ₃ in the air is released is blocked by the second cylinder group 1b, first If the first cylinder group 1a is always operated rich, and the second cylinder group 1b is always operated lean, emission of NO _x and NH ₃ into the atmosphere is always prevented. However, according to the inventors of the present application, if the three-way catalyst 8a is kept in contact with exhaust gas having a rich exhaust air-fuel ratio for a long time, the three-way catalyst 8a
It has been confirmed that the catalytic activity gradually decreases. In particular, it has been confirmed that in the case of a three-way catalyst carrying palladium Pd, the catalytic activity is significantly reduced. For this reason, if the first cylinder group 1a is constantly operated to perform the rich operation so that the exhaust air-fuel ratio of the exhaust gas flowing into the three-way catalyst 8a is always rich, the catalytic activity of the three-way catalyst 8a gradually decreases. As a result, NO _x and NH ₃ will be released into the atmosphere.

On the other hand, when the exhaust gas with a lean exhaust air-fuel ratio is brought into contact with the three-way catalyst 8a, which has been in contact with the exhaust gas with a rich exhaust air-fuel ratio for a long time and has reduced catalytic activity, the catalytic activity of the three-way catalyst 8a is restored. Has also been confirmed. Therefore, in the present embodiment, the exhaust air-fuel ratio of the exhaust gas flowing into the three-way catalyst 8a is temporarily made lean, thereby recovering the catalytic activity of the three-way catalyst 8a. As a result, good catalytic activity of the three-way catalyst 8a is ensured, thus good purification of the NO _x and NH ₃ is ensured at all times.

In order to make the exhaust air-fuel ratio of the exhaust gas flowing into the three-way catalyst 8a lean, a secondary air supply device for supplying air to the three-way catalyst 8a in a secondary manner is provided to perform rich operation on the first cylinder group 1a. You may make it supply secondary air temporarily by a secondary air supply apparatus while performing it. On the other hand, as described above, the engine air-fuel ratio of the first cylinder group 1a matches the exhaust air-fuel ratio of exhaust flowing into the three-way catalyst 8a. Thus, in this embodiment, the first cylinder group 1a is caused to temporarily perform the lean operation so that the exhaust air-fuel ratio of the exhaust gas flowing into the three-way catalyst 8a temporarily becomes lean. That is, in the present embodiment, the first cylinder group 1a is basically made to perform the rich operation, and the three-way catalyst
Lean operation is performed temporarily when the catalyst activity of 8a decreases.

More specifically, when it is determined that the catalytic activity of the three-way catalyst 8a has dropped below a predetermined allowable value, for example, the target air-fuel ratio (A) of the first cylinder # 1 for a predetermined set time t. / F) T is a lean air-fuel ratio (A / F) LL that is leaner than the stoichiometric air-fuel ratio (A / F) S, thereby causing the # 1 cylinder # 1 to perform lean operation. When the first cylinder # 1 performs a lean operation and the exhaust air-fuel ratio of the exhaust gas flowing into the three-way catalyst 8a becomes lean, the catalytic activity of the three-way catalyst 8a is restored. In this embodiment, the lean air-fuel ratio (A / F) LL is constant irrespective of the engine operating state and is, for example, 2
5.0. However, lean air / fuel ratio (A / F) LL,
For example, it may be appropriately changed according to the catalytic activity of the three-way catalyst 8a, the operating state of the engine, and the like.

By the way, it is difficult to directly determine the catalytic activity of the three-way catalyst 8a. Therefore, in the present embodiment, the catalytic activity of the three-way catalyst 8a is estimated from the amount of HC flowing into the three-way catalyst 8a. That is, although the exhaust gas with a rich exhaust air-fuel ratio is circulated to the three-way catalyst 8a, the mechanism when the catalytic activity of the three-way catalyst 8a decreases is not clear, but the three-way catalyst is not clear.
It is considered that when the amount of HC flowing into 8a increases, the HC covers the surface of the catalyst component, for example, palladium Pd or platinum Pt, and thus the catalytic activity of the three-way catalyst 8a decreases. Therefore, if the amount of HC necessary for the catalytic activity of the three-way catalyst 8a to decrease beyond the allowable value is determined in advance, the three-way catalyst
The catalytic activity of the three-way catalyst 8a can be estimated by calculating the amount of HC flowing into the 8a. In addition, the engine operation state,
The catalytic activity of the three-way catalyst 8a can be estimated from the engine operating time, the amount of exhaust gas passing through the three-way catalyst 8a, and the like.

FIG. 5A shows the HC amount Q (HC) discharged from the first cylinder group 1a per unit time, the engine load Q / N, and the engine speed at a constant rich air-fuel ratio (A / F) R, which are experimentally obtained. The relationship with N is shown. In FIG. 5A, each curve shows the same HC amount. As the engine speed N increases, the amount of exhaust discharged from the engine per unit time increases, and as the engine load Q / N increases, the amount of exhaust discharged from the engine increases. Therefore, as shown in FIG. 5A, the HC amount Q (HC) discharged from the first cylinder group 1a per unit time, that is, the HC amount flowing into the three-way catalyst 8a per unit time is equal to the engine load Q /
It increases as N increases and increases as the engine speed N increases. Note that the HC amount Q shown in FIG.
(HC) is stored in the ROM 22 in advance in the form of a map as shown in FIG. 5B.

Therefore, the HC amount S (HC) flowing into the three-way catalyst 8a while the first cylinder # 1 is performing the rich operation is determined by the following equation.

S (HC) = S (HC) + Q (HC) .DELTAa Here, DELTAa represents a detection time interval of Q (HC).
Therefore, Q (HC) .DELTAa has flowed into the three-way catalyst 8a between the previous processing routine and the present processing routine.
Shows the amount of HC.

The time chart of FIG. 6 shows a change in the engine air-fuel ratio of the first cylinder group 1a. In FIG. 6, the time zero indicates the time when the rich operation was started in the first cylinder group 1a, that is, the first cylinder # 1. In this case, the target air-fuel ratio (A / F) T of the first cylinder # 1 is maintained at the rich air-fuel ratio (A / F) R as shown in FIG. When the rich operation of the first cylinder # 1 starts, the HC amount S (HC) gradually increases, and the time a
Then, the allowable maximum value TM is exceeded. The permissible maximum value TM is the amount of HC necessary to reduce the catalytic activity of the three-way catalyst 8a beyond the permissible value described above, and is obtained in advance by an experiment. That is, when S (HC)> TM, it can be determined that the catalytic activity of the three-way catalyst 8a has decreased, and in this case, the target air-fuel ratio (A / F) T of the first cylinder # 1 becomes equal to the lean air-fuel ratio (A / F). F) LL. The target air-fuel ratio (A / F) T is maintained at the lean air-fuel ratio (A / F) LL for a set time t. The set time t is a lean operation time required for restoring the catalytic activity of the three-way catalyst 8a, and is determined in advance by an experiment. When the target air-fuel ratio (A / F) T is set to the lean air-fuel ratio (A / F) LL and the set time t has elapsed, the target air-fuel ratio (A / F) T of the first cylinder # 1 becomes the rich air-fuel ratio ( A / F) R. Note that S (HC) is reset at time a, and when rich operation of the first cylinder # 1 is restarted at time a + t, calculation of S (HC) is restarted.

By the way, when the first cylinder group 1a performs the lean operation as described above, the NO _x discharged from the first cylinder group 1a at this time is
Reaches the exhaust purification catalyst 14 without being converted into NH ₃ or N ₂ in the three-way catalyst 8a. However, the NO _x from the first cylinder group 1a to the exhaust purification catalyst 14 at this time the second cylinder group 1b is performing the lean operation, the second cylinder group 1b
And NO _x from will be to flow, resulting NO _x in the exhaust purification catalyst 14 is to be released to the atmosphere without being purified. On the other hand, come leak without part of HC discharged from the second cylinder group 1b at this time when to perform rich operation in the second cylinder group 1b is oxidized in _the NO _x storage reduction catalyst 11a. Therefore, in this embodiment, when the first cylinder group 1a is to perform the lean operation, the second cylinder group 1b is caused to perform the rich operation, thereby supplying HC to the exhaust purification catalyst 14 and using the HC to perform the first operation. From cylinder group 1a
And so as to purify the NO _x. As a result, even if the first cylinder group 1a performs the rich operation or the lean operation, the NO _x
Can be satisfactorily purified.

That is, as shown in FIG. 6, when the first cylinder group 1a temporarily performs the lean operation, the target air-fuel ratio (A / F) T of each cylinder of the second cylinder group 1b is temporarily Theoretical air-fuel ratio (A
/ F) A rich air-fuel ratio (A / F) RR, which is richer than S, is used to temporarily cause the second cylinder group 1b to perform rich operation. This rich air-fuel ratio (A /
F) In this embodiment, RR is constant irrespective of the engine operating state and is, for example, 14.0.

Incidentally, _the NO _x storage exhaust gas air-fuel ratio of the exhaust gas flowing into the reduction catalyst 11a is occluded in _the NO _x storage reduction catalyst 11a when the rich NO _x is released. Thus, in this way the second cylinder group 1b is at least a portion of the NO _x occluded in the Doing rich operation _the NO _x storage reduction catalyst 11a is released, _the NO _x storage resulting NO _{x storage-and-reduction} catalyst 11a Your ability is restored. Therefore, NO _x storage-reduction catalyst 11a NO _x flowing into even be prevented from flowing a large amount of the NO _x in the exhaust purification catalyst 14 as rapidly increases, NO _x in the exhaust purification catalyst 14 and thus
Is well purified. Incidentally, NO _x exhaust air-fuel ratio of the exhaust gas flowing into _the NO _x storage reduction catalyst 11a is released from _the NO _x storage reduction catalyst 11a when the rich is reduced due exhaust HC, CO flowing.

If the target air-fuel ratio (A / F) T is extremely lean, such as 25.0, and a mixture is formed that fills the combustion chamber almost uniformly, this mixture is extremely lean and is ignited by a spark plug (not shown). However, it does not ignite, resulting in a misfire. Therefore, in the internal combustion engine shown in FIG. 1, when a lean operation is to be performed, an ignitable air-fuel mixture is formed in a limited area in the combustion chamber, and the other area is filled with only air or only air and EGR gas to fill the air-fuel mixture. It is ignited by a spark plug. As a result, even when the engine air-fuel ratio is extremely lean, misfire of the engine is prevented. Alternatively, it is also possible to prevent a misfire by forming a uniform mixture in the combustion chamber and forming a swirling flow.

Next, a routine for executing the above-described embodiment will be described with reference to FIGS.

FIG. 7 shows an operation control routine. This routine is executed by interruption every predetermined set time.

Referring to FIG. 7, first, in step 40, the first cylinder group is set.
It is determined whether or not a flag that is set when the lean operation should be performed in 1a is set. If this flag has not been set, the process proceeds to step 41, where Q (HC) is calculated from the map shown in FIG. 5B based on the engine load Q / N and the engine speed N. In the following step 42, the HC amount S (HC) is calculated using the following equation.

S (HC) = S (HC) + Q (HC) · DELTAa Next, the routine proceeds to step 43, where the HC amount S (HC)
Is larger than the allowable maximum value TM. S
When (HC)> TM, the process proceeds to step 44, where a flag is set. That is, S (CH)>
At the time of TM, it is determined that the catalytic activity of the three-way catalyst 8a has decreased, and the first cylinder group 1a is operated in a lean operation. In the following step 45, the HC amount S (HC) is reset. Next, the processing cycle ends. On the other hand, in step 43, S
When (HC) ≦ TM, the processing cycle ends. That is, when S (HC) ≦ TM, it is determined that the catalytic activity of the three-way catalyst 8a has not decreased, and the rich operation of the first cylinder group 1a is continued.

When the flag is set, the process proceeds from step 40 to step 46. In step 46, the flag is set and the timer count value C representing the time during which the first cylinder group 1a is performing the lean operation is incremented by one. In the following step 47, it is determined whether or not the timer count value C is larger than a predetermined set value CMAX. This CMAX corresponds to the set time t described above. When C ≦ CMAX, the three-way catalyst
It is determined that regeneration of the catalyst activity of 8a is insufficient, and the processing cycle is ended, that is, the lean operation of the first cylinder group 1a is continued. On the other hand, when C> CMAX, it is determined that the set time t has elapsed from the start of the lean operation of the first cylinder group 1a, that is, it is determined that the catalytic activity of the three-way catalyst 8a has been recovered, and step 48 is performed. Go to to reset the flag. Therefore, the lean operation of the first cylinder group 1a is stopped, and the rich operation is restarted. Next, the routine proceeds to step 49, where the timer count value C is cleared. Next, the processing cycle ends.

FIG. 8 shows a routine for calculating the fuel injection time TAU. This routine is executed by interruption every fixed crank angle.

Referring to FIG. 8, first, at step 60, the intake air amount Q
The basic fuel injection time TB is calculated based on the following equation from the engine speed N.

TB = (Q / N) · K In the following step 61, the fuel injection time TAU determined in the current processing cycle is the fuel injection time for the first cylinder group 1a or the fuel injection time TAU for the second cylinder group 1b. It is determined whether it is the fuel injection time. If it is determined that the fuel injection time TAU determined in the current processing cycle is the fuel injection time for the first cylinder group 1a, that is, the first cylinder # 1, then the routine proceeds to step 62, and in step 62, the first cylinder The feedback correction coefficient FAFA for the group 1a is calculated. In the following step 63, FAFA is made FAF. In the following step 64, FIG.
It is determined whether the flag to be set or reset in the routine is set. When the flag is set, that is, when the first cylinder group 1a is to perform the lean operation, the process proceeds to step 65, where the target air-fuel ratio (A / F) T is set to the lean air-fuel ratio (A / F).
LL. In the present embodiment, the lean air-fuel ratio (A / F) LL is fixed at 25.0 regardless of the engine operating state, and therefore, in step 65, (A / F) T = 25.0. Next, the routine proceeds to step 72. On the other hand, when the flag is reset in step 64, that is, when the first cylinder group 1a is to perform the rich operation, the process proceeds to step 66, where the target air-fuel ratio (A / F) T is set to the rich air-fuel ratio (A / F). F) R. In the present embodiment, the rich air-fuel ratio (A / F) R is set to a constant 14.0 regardless of the engine operating state. Therefore, in step 66, (A / F) T is set to 14.0. Next, the routine proceeds to step 72.

In step 61, when it is determined that the fuel injection time TAU determined in the current processing cycle is the fuel injection time for the second cylinder group 1b, that is, the second cylinder # 2, the third cylinder # 3, the fourth cylinder When it is determined that it is the fuel injection time for any one of the cylinders # 4, the process proceeds to step 67. In step 67, a feedback correction coefficient FAFB for the second cylinder group 1b is calculated. In the following step 68, FAFB is set to FAF. In the following step 69, it is determined whether or not the flag is set. When the flag is set, that is, when the second cylinder group 1b is to perform the rich operation, the process then proceeds to step 70,
The target air-fuel ratio (A / F) T is set to the rich air-fuel ratio (A / F) RR. In this embodiment, the rich air-fuel ratio (A / F) RR is set to a constant 14.0 irrespective of the operating state of the engine. Therefore, in step 70, (A / F) TR = 14.0. Then step 7
Proceed to 2. On the other hand, when the flag is reset, that is, when the second cylinder group 1b is to perform the lean operation, the routine proceeds to step 71, where the target air-fuel ratio (A / A
F) T is the lean air-fuel ratio (A / F) L. In this embodiment, the lean air-fuel ratio (A / F) L is fixed at 25.0 regardless of the engine operating state.
F) T = 25.0. Next, the routine proceeds to step 72.

In step 72, the fuel injection time TAU is calculated based on the following equation.

TAU = TB ・ ((A / F) S / (A / F) T ・ FAF Each fuel injection valve 5 injects fuel for the fuel injection time TAU.

 Next, another embodiment of the operation control method will be described.

In the above-described embodiment, when the exhaust air-fuel ratio of the exhaust gas flowing into the three-way catalyst 8a is rich, the catalyst of the three-way catalyst 8a is larger when the integrated HC amount S (HC) flowing into the three-way catalyst 8a is larger than when it is smaller. Focusing on the decrease in activity, when the integrated HC amount S (HC) exceeds the allowable maximum value TM, the exhaust air-fuel ratio of the exhaust gas flowing into the three-way catalyst 8a is temporarily made lean to activate the catalytic activity of the three-way catalyst 8a. Is trying to recover. In other words, as long as the integrated HC amount S (HC) does not exceed the allowable maximum value TM, that is, unless the catalytic activity of the three-way catalyst 8a decreases to the allowable value, the three-way catalyst
The catalytic activity recovery effect of 8a is not performed. However, the catalytic activity of the three-way catalyst 8a may be restored at a relatively short time interval before the catalytic activity of the three-way catalyst 8a does not decrease to the allowable value. If the catalytic activity of the three-way catalyst 8a is restored at such a short time interval, the catalytic activity of the three-way catalyst 8a can be maintained at a high level.

On the other hand, the size of the exhaust purification device is preferably as small as possible, therefore it is not preferable to increase the volume of the NO _x storage-reduction catalyst 11a. However, NO _x storage reduction catalyst 1
_The NO _x storage reduction catalyst 11a is easily saturated with NO _x since A small volume of the 1a is NO _{x the} NO _x storage capacity of the storage reduction catalyst 11a becomes smaller. Thus, with a relatively short time interval to release the NO _x occluded in the NO _{x storage-and-reduction} catalysts 11a, need to temporarily rich exhaust air-fuel ratio of the exhaust gas flowing into _the NO _x storage reduction catalyst 11a There is. in this case,
Seeking amount of NO _x occluded in the NO _{x storage-and-reduction} catalyst 11a of the exhaust air-fuel ratio of the exhaust gas flowing into _the NO _x storage reduction catalyst 11a when the increase exceeds the maximum amount the NO _x is a predetermined temporarily NO _x that are occluded from _the NO _x storage reduction catalyst 11a if rich is released, NO _x storage-reduction catalyst 11a and thus N
O _x storage capacity is secured.

Incidentally, the three-way catalyst when the rich exhaust air-fuel ratio of the exhaust gas flowing into _the NO _x storage reduction catalyst 11a as described above
It is preferable for the exhaust gas purification that the exhaust air-fuel ratio of the exhaust gas flowing into 8a be lean. Therefore, the NO _x storage reduction catalyst 1
_The NO _x storage-reduction catalyst 11a in accordance with the amount of NO _x occluded in the 1a
Temporarily rich exhaust air-fuel ratio of the exhaust gas flowing into the NO _{x storage-and-reduction} catalyst occluding capacity of the NO _x storage-reduction catalyst 11a when the exhaust gas air-fuel ratio of the exhaust gas flowing into the three-way catalyst 8a with it and lean Therefore, the exhaust gas can be satisfactorily purified, and the catalytic activity of the three-way catalyst 8a can be recovered at a relatively short time interval. And in this embodiment,
Temporarily rich exhaust air-fuel ratio of the exhaust gas seeking amount of NO _x occluded in the NO _{x storage-and-reduction} catalyst 11a amount this NO _x flows into _the NO _x storage reduction catalyst 11a After increases beyond the maximum value of the above At the same time, the exhaust air-fuel ratio of the exhaust gas flowing into the three-way catalyst 8a is temporarily made lean.

Also in the present embodiment, the first cylinder group 1a performs a rich operation in order to make the exhaust air-fuel ratio of the exhaust gas flowing into the three-way catalyst 8a rich, and the exhaust air-fuel ratio of the exhaust gas flowing into the three-way catalyst 8a is made lean. Therefore, the first cylinder group 1a is caused to perform a lean operation. Further, to perform the rich operation to the second cylinder group 1b to the rich exhaust air-fuel ratio of the exhaust gas flowing into _the NO _x storage reduction catalyst 11a, the exhaust air-fuel ratio of the exhaust gas flowing into _the NO _x storage reduction catalyst 11a In order to achieve a lean operation, the second cylinder group 1b is operated to perform a lean operation. That is, in the present embodiment, the first cylinder group 1a performs the rich operation, and the second cylinder group 1b performs the lean operation.
Amount of NO _x occluded in the NO _{x storage-and-reduction} catalyst 11a is to perform temporarily lean operation to the first cylinder group 1a After increases beyond the maximum value when the cylinder group 1b is performing lean operation At the same time, the second cylinder group 1b is caused to temporarily perform the rich operation. When the second cylinder group 1b temporarily performs the rich operation and the first cylinder group 1a temporarily performs the lean operation, the target air-fuel ratio of each cylinder of the second cylinder group 1b ( A / F) T can be the above-mentioned rich air-fuel ratio (A / F) RR, and the target air-fuel ratio (A / F) T of the cylinders of the first cylinder group 1a is the above-mentioned lean air-fuel ratio (A / F). F) Can be LL.

As described above, the operating period when the second cylinder group 1b temporarily performs the rich operation and the first cylinder group 1a temporarily performs the lean operation may be determined in any manner. the second cylinder group 1b Once lowered beyond the minimum amount NO _x which the second cylinder group 1b is occluded in _the NO _x storage reduction catalyst 11a when performing a rich operation is determined in advance in a manner
Restarts the lean operation, and accordingly, the first cylinder group 1a
To restart the rich operation.

However, NO _x storage-reduction catalyst 11a NO occluded in the _x
It is difficult to determine the quantity directly. Therefore _the amount of NO _x flowing into _the NO _x storage reduction catalyst 11a in the present embodiment, that is, the second
_The NO _x storage-reduction catalyst 11 from the NO _x amount exhausted from the cylinder group 1b
The amount of NO _x stored in a is estimated. Namely, the inflow amount of NO _x flowing per unit time in _the NO _x storage reduction catalyst 11a as the exhaust amount discharged per unit time from the engine as the engine speed N becomes higher the engine speed N becomes higher because the increased Increase. Also, as the engine load Q / N increases, the amount of exhaust discharged from the engine increases,
In addition, the combustion temperature increases, so that the inflow NO _x amount increases.

FIG. 9A shows the NO _x amount Q (NO _x ) exhausted from the second cylinder group 1b per unit time, the engine load Q / N, and the engine load at a constant lean air-fuel ratio (A / F) L obtained by an experiment. Revolution N
Shows the relationship between, each curve in FIG. 9A same NO _x
Indicates the amount. _The amount of NO _x exhausted from the second cylinder group 1b per unit time as shown in FIG. 9A Q (NO _x) becomes increases as the higher the engine load Q / N, increases as the engine speed N becomes higher Become. The NO _x amount Q (NO _x ) shown in FIG. 9A is stored in the ROM 22 in advance in the form of a map as shown in FIG. 9B.

That, NO _x amount S that is occluded in _the NO _x storage reduction catalyst 11a when the second cylinder group 1b is performing lean operation
(NO _x ) increases by Q (NO _x ) per unit time. Therefore, when the second cylinder group 1b is performing the lean operation
_The NO _x storage occluded in the reduction catalyst 11a NO _x amount S (NO _x) is expressed by the following equation.

_{S (NO x) = S (} NO x) + Q (NO x) · DELTAna where DELTAna represents the detection time interval of Q (NO _x), thus Q (NO _x) · DELTAn from the previous processing routine Until this processing routine, the NO _x storage reduction catalyst 1
Represents the occluded amount of NO _x in 1a. And the second cylinder group 1b performs the lean operation the first cylinder group 1a exceeds the maximum value MAX (NO _x) to the amount of NO _x S (NO _x) is predetermined when performing rich operation When it is increased, the second cylinder group 1b is made to perform the rich operation, and the first cylinder group 1a is made to perform the lean operation.

On the other hand, FIG. 10A is released from the unit time per _the NO _x storage-reduction catalyst 11a determined by experiment _the amount of NO _x D (NO _x)
Is shown. The solid line in FIG. 10A is _the NO _x storage catalyst 11
The case where the temperature of a is high is shown, and the broken line shows the case where the temperature of the NO _x storage reduction catalyst 11a is low. Further, after the rich operation period TR is started from TIME, that is, after the target air-fuel ratio (A / F) T is switched from the lean air-fuel ratio (A / F) L to the rich air-fuel ratio (A / F) R. Indicates time. N
The decomposition rate of NO _x in the O _x storage reduction catalyst 11a increases as the temperature of the NO _x storage reduction catalyst 11a increases. Therefore, NO _x storage-reduction catalyst 11a as indicated by the solid line in FIG. 10A
When the temperature is high, that _the NO _x storage-reduction when a high exhaust temperature TNC of the exhaust gas flowing into the catalyst 11a _the NO _x storage exhaust air-fuel ratio at the surface of the reduction catalyst 11a is a large amount while sufficiently not rich NO _x is NO released from _x storage-reduction catalyst 11a, the temperature of the NO _x storage-reduction catalyst 11a, i.e. the exhaust gas temperature TNC
Is low, a small amount of NO _x as shown by the dashed line in FIG.
Is released from the NO _x storage reduction catalyst 11a. In other words,
Exhaust gas temperature TNC NO per more unit time becomes higher _x storing discharged from the reduction catalyst 11a is _the amount of NO _x D (NO _x) is increased. This NO _x amount D (NO _x ) is stored in the ROM 22 in advance in the form of a map shown in FIG. 10B as a function of the exhaust gas temperature TNC and the time TIME.

_The NO _x storage reduction catalyst 11a temperature TNC of the exhaust gas flowing into the may be determined by a sensor, but in this embodiment is estimated from the engine operating state, that the engine load Q / N and the engine speed N. That is, the TNC is obtained in advance by an experiment, and is stored in the ROM 22 in advance in the form of a map shown in FIG.

It is discharged from the second cylinder group 1b per unit time.
The NO _x amount Q (NO _x ) varies according to the engine air-fuel ratio. Therefore, when the lean air-fuel ratio (A / F) L is varied according to, for example, the engine operating state, Q (NO _x ) obtained from the map of FIG. 9B is changed according to the lean air-fuel ratio (A / F) L Need to be corrected. Alternatively, the lean air-fuel ratio (A / F) L and Q (N
It is necessary to find Q (NO _x) by using a map representing the relationship between the O _x).

That, NO _x amount S that is occluded in _the NO _x storage reduction catalyst 11a when the second cylinder group 1b is performing the rich operation
(NO _x ) decreases by D (NO _x ) per unit time. Therefore, NO _x amount occluded in the NO _{x storage-and-reduction} catalyst 11a when the second cylinder group 1b is performing the rich operation S (NO _x)
Is represented by the following equation.

_{S (NO x) = S (} NO x) -D (NO x) · DELTAnd where DELTAnd represents the detection time interval of D (NO _x), thus D (NO _x) · DELTAnd the previous processing routine NO during the period from to the present processing routine _x storage-reduction catalyst 1
It represents the amount of NO _x released from 1a. Second cylinder group 1b
Performs the rich operation and the first cylinder group 1a performs the lean operation, and if the NO _x amount S (NO _x ) falls below a predetermined minimum value MIN (NO _x ), the second The cylinder group 1b is caused to perform the lean operation, and the first cylinder group 1a is caused to perform the rich operation.

FIG. 12 shows an operation control routine according to the above embodiment. This routine is executed by interruption every predetermined set time.

Referring to FIG. 12, first, at step 80, the three-way catalyst 8a
It is determined whether or not a flag that is set when the first cylinder group 1a is to perform the lean operation and the second cylinder group 1b is to perform the rich operation in order to recover the catalyst activity of the first cylinder group 1a. . If the flag has not been set, the process proceeds to step 81. In step 81, Q (NO _x ) is calculated from the map of FIG. 9B. In the following step 82, the stored NO _x amount S (NO _x ) is calculated based on the following equation.

_{S (NO x) = S (} NO x) + Q (NO x) · DELTAna where DELTAad is the time interval from the previous processing cycle to the present processing cycle. Next, the routine proceeds to step 83, where the NO _x amount S (NO _x ) is changed to the NO _x storage reduction catalyst 1
It is determined whether or not it is larger than a maximum value MAX (NO _x ) determined according to the storage capacity of 1a. When S (NO _x ) ≦ MAX (NO _x ), the processing cycle ends. That is, S (NO _x )
≦ MAX (NO _x) to continue the lean operation of the second cylinder group 1b is determined that the still greater _the NO _x storage capacity of the NO _x storage-reduction catalyst 11a when the continuation of the rich operation of the first cylinder group 1a I do.

In contrast, in step 83, S (NO _x )> MAX (N
In the case of _Ox ), the process then proceeds to step 84, where the flag is set and the processing cycle ends. That is, S (NO _x )
If> MAX (NO _x ), it is determined that the NO _x storage capacity of the NO _x storage reduction catalyst 11a has become small, and the lean operation of the second cylinder group 1b is terminated, and the rich operation is started. At this time, the rich operation of the first cylinder group 1a is ended, and the lean operation is started.

When the flag is set, the process proceeds from step 80 to step 85. In step 85, the exhaust temperature is obtained from the map in FIG.
TNC is calculated. In the following step 86, D (NO _x ) is calculated from the map of FIG. NO which is stored in _the NO _x storage-reduction catalyst 11a based on the subsequent equation in step 87
_{The x} amount S (NO _x ) is calculated.

_{S (NO x) = S (} NO x) -D (NO x) · DELTAnd where DELTAnd is the time interval from the previous processing cycle to the present processing cycle. Next, the routine proceeds to step 88, where the NO _x amount S (NO _x ) is the minimum value MIN (NO _x ).
It is determined whether it is smaller than. S (NO _x ) ≧ MIN (NO
_{In the case of x} ), the processing cycle ends. That is, S
When (NO _x ) ≧ MIN (NO _x ), the N of the NO _x storage reduction catalyst 11a
Judge that the O _x storage capacity is not sufficiently large
The rich operation of the first cylinder group 1a is continued, and the lean operation of the first cylinder group 1a is continued.

On the other hand, in step 88, S (NO _x ) <MIN (N
In the case of _Ox ), the process then proceeds to step 89, where the flag is reset and the processing cycle ends. That is, S (N
_{O x) <MIN (NO x} ) NO x of the NO _x storage-reduction catalyst 11a when the
Judging that the storage capacity has become sufficiently large, the second cylinder group 1b
Finishes the rich operation and starts the lean operation. Also,
At this time, the lean operation of the first cylinder group 1a ends, and the rich operation starts. The fuel injection time TAU is calculated in the routine of FIG. 8 based on whether the flag controlled by the routine of FIG. 12 is set or reset.

In the embodiment described with reference to FIGS. 1 to 7, the integrated HC amount S (HC) flowing into the three-way catalyst 8a is determined, and this S (H) is calculated.
The operating state of the first and second cylinder groups 1a and 1b is changed according to C). That is, the first cylinder group
The operation state of 1a is changed from rich operation to lean operation,
The operation state of the second cylinder group 1b is changed from the lean operation to the rich operation. Further, in the embodiment described with reference to FIGS. 8 to 12, the NO stored in the NO _x storage reduction catalyst 11a is
_{An x} amount S (NO _x ) is obtained, and the operating state of the first and second cylinder groups 1a and 1b is changed according to the S (NO _x ). However, ternary cumulative HC amount flowing into the catalyst 8a and S (HC), NO _x storage-reduction catalyst 11a NO occluded in the _x
Quantity (S (NO _x )) to determine these S (HC) and S
(NO _x ) according to one of the first and second cylinder groups 1
The operation state of a and 1b may be changed. In this case, when S (HC) exceeds the allowable maximum value TM or S (NO _x ) exceeds the maximum value MAX, the operating state of the first and second cylinder groups 1a and 1b can be changed.
Or, the properties, materials, capacity of each catalyst or occlusion material,
First depending on the exhaust air-fuel ratio or flow rate of the inflowing exhaust
It is also possible to select whether to change the operating state of the second cylinder group 1a and 1b.

Also, the allowable maximum value TM for the integrated HC amount S (HC),
The maximum values MAX and MIN for the NO _x amount S (NO _x ) should be changed according to the properties, materials, capacity, exhaust air-fuel ratio or flow rate of the inflowing exhaust gas, engine operating conditions, etc. You may.

Further, in the embodiments described so far, the first cylinder group 1a is constituted by one cylinder, and the second cylinder group 1b is constituted by three cylinders. However, the first cylinder group
The first cylinder 1a may be composed of a plurality of cylinders, and the second cylinder group 1b may be composed of one cylinder. However, it is preferable that the fuel consumption rate be as small as possible. Therefore, it is preferable that the number of cylinders of the second cylinder group 1b in which the lean operation is basically performed is as large as possible. When the first cylinder group 1a is composed of a plurality of cylinders, the control target value (A / F) T of the engine air-fuel ratio of each cylinder of the first cylinder group 1a is the same for each cylinder. You.

Continued on the front page (51) Int.Cl. ⁷ Identification code FI F01N 3/28 301 B01D 53/36 102B 102H (72) Inventor Naoto Suzuki 1 Toyota Town, Toyota City, Aichi Prefecture Inside Toyota Motor Corporation (72) Inventor Takehisa Yaegashi 1 Toyota Town, Toyota City, Aichi Prefecture Inside Toyota Motor Corporation (72) Inventor Toshiaki Tanaka 1 Toyota Town, Toyota City, Aichi Prefecture Inside Toyota Motor Corporation (72) Inventor Naoto Miyoshi Toyota City, Aichi Prefecture No. 1, Toyota Town Inside Toyota Motor Corporation (56) References JP-A-4-365920 (JP, A) JP-A-7-185344 (JP, A) JP-A-5-131118 (JP, A) (58) ) Surveyed field (Int.Cl. ⁷ , DB name) F02D 41/14 F02D 41/02 F01N 3/08 F01N 3/20 F01N 3/24 B01J 23/40-23/58

Claims (12)

(57) [Claims] An exhaust of an internal combustion engine having a plurality of cylinders divided into first and second cylinder groups and first and second exhaust passages respectively connected to the first and second cylinder groups. an apparatus for purifying apparatus is disposed in the first exhaust passage, the exhaust gas air-fuel ratio of the exhaust gas flowing to generate NH ₃ from at least a portion of the NO _x in the exhaust gas flowing when the rich NH ₃ and generating catalyst, a first enrichment means to the exhaust gas air-fuel ratio of the exhaust gas flowing into the NH ₃ synthesizing catalyst to produce a NH ₃ rich, a first exhaust passage of the NH ₃ synthesizing catalyst downstream the a second exhaust passage are merged with each other NH ₃ NH ₃ produced in the generation catalyst
When, and NO _x exhausted from the second exhaust passage into contact with each other, and junction exhaust passage of the NO _x to be purified by the NH _3, the exhaust air-fuel ratio of the exhaust gas flowing into the NH ₃ synthesizing catalyst A first leaning means for temporarily leaning, wherein the hydrocarbon (HC) flowing into the NH ₃ generation catalyst when the exhaust air-fuel ratio of the inflowing exhaust gas is rich
The first leaning means performs a leaning operation when the integrated HC amount exceeds a predetermined upper limit threshold value. 2. A second control unit for controlling an engine air-fuel ratio of a second cylinder group.
2. The apparatus according to claim 1, further comprising: engine air-fuel ratio control means for controlling the engine air-fuel ratio of the second cylinder group to lean. 3. disposed in the second exhaust passage, the exhaust gas air-fuel ratio of the exhaust gas flowing together with the exhaust air-fuel ratio of the exhaust gas flowing to occlude NO _x when the lean is occluded when the rich and _the NO _x storage-reduction catalyst that reduces by releasing NO _x, the NO _x
And a second enrichment means to temporarily rich exhaust air-fuel ratio of the exhaust gas flowing into _the NO _x storage reduction catalyst to release the NO _x that is occluded from the occlusion-reduction catalyst, the first
3. The apparatus of claim 2, wherein the leaning means performs a leaning action while the second enriching means performs a richening action. 4. The fuel cell system according to claim 1, further comprising an estimating means for estimating the stored NO _x amount of the NO _x storing and reducing catalyst, wherein the second enrichment means sets the estimated NO _x amount based on a predetermined threshold value. 4. The apparatus according to claim 3, wherein the enrichment operation is performed when the value of the parameter is also large. 5. The method according to claim 1, wherein the second enrichment means includes the estimated NO _x
4. The device according to claim 3, wherein the enrichment operation is stopped when the amount falls below a predetermined lower threshold. Wherein said second enrichment means, according to claim 3, the exhaust air-fuel ratio rich of the exhaust gas flowing into the NO _{x storage-and-reduction} catalyst by controlling the second engine air-fuel ratio control means Equipment. 7. The NO _x storage reduction catalyst comprises at least one selected from the group consisting of alkali metals composed of potassium, sodium, lithium and cesium, alkaline earth metals composed of barium and calcium, and rare earth metals composed of lanthanum and yttrium. The apparatus according to claim 3, comprising a noble metal composed of palladium, platinum, and rhodium. 8. A first control unit for controlling an engine air-fuel ratio of a first cylinder group.
Engine air-fuel ratio control means, wherein at least one of the first enrichment means and the first lean means controls the first engine air-fuel ratio control means, so that the NH ₃ generation catalyst The device according to claim 1, wherein the exhaust air-fuel ratio of the inflowing exhaust gas is made rich or lean. 9. The method according to claim 9, wherein the NH ₃ generating catalyst is palladium, platinum,
The apparatus according to claim 1, wherein the three-way catalyst includes at least one of noble metals made of rhodium. 10. The apparatus according to claim 1, wherein said leaning means leans the exhaust air-fuel ratio of the exhaust gas flowing into the NH ₃ generation catalyst for a predetermined time. 11. The apparatus according to claim 1, wherein an exhaust purification catalyst for purifying NH ₃ and NO _x in the inflowing exhaust gas is disposed in the combined exhaust passage. 12. The exhaust purification catalyst according to claim 1, wherein the exhaust purification catalyst is at least one selected from a transition metal consisting of copper, chromium, vanadium, titanium, iron, nickel and cobalt, or a noble metal consisting of platinum, palladium, rhodium and iridium.
The device according to claim 11, comprising two substances.