JP3412314B2 - Exhaust gas purification device for internal combustion engine - Google Patents

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 Exhaust gas in an engine exhaust passage is passed through an engine intake passage.
Equipped with an exhaust gas recirculation device for recirculation in the road,
NO when the air-fuel ratio of the inflowing exhaust gas is lean_xSuck
Absorbed and absorbed when the air-fuel ratio of the inflowing exhaust gas becomes rich
NO_xReleases NO _xAbsorbent engine exhaust passage
Place inside, NO_xAbsorbent to NO_xWhen to release
The lean air-fuel mixture supplied to the engine combustion chamber.
NO by switching to_xExhaust gas flowing into the absorbent
NO from the lean air-fuel ratio_xAbsorbent
To NO_xIs emitted and is further supplied to the engine combustion chamber.
Exhaust gas when the air-fuel mixture is switched from lean to rich
The engine output torque is increased by recirculating the exhaust gas into the engine intake passage.
Internal combustion engines are known to prevent the increase
(See JP-A-6-108824).

[0003]

However, if the air-fuel mixture is made rich and the exhaust gas is recirculated, the temperature of the combustion flame is lowered, so that the flame is less likely to propagate and a misfire is likely to occur. That is, when combustion is initiated in the combustion chamber, this heat of combustion is used to heat the recirculated exhaust gas and the fuel that has not yet burned in the combustion chamber, and thus the higher the exhaust gas recirculation rate, the more the combustion flame The lower the temperature and the higher the degree of richness, the lower the temperature of the combustion flame. Therefore, the higher the recirculation rate of the exhaust gas, the more likely it is that misfire will occur, and the greater the degree of richness, the more likely the misfire will occur. Therefore, as the degree of richness increases, misfire will occur unless the recirculation rate of exhaust gas is reduced.

In order to prevent the misfire, it is necessary to determine the rich degree in consideration of the recirculation rate. Therefore, as in the above-described internal combustion engine, if the exhaust gas is simply recirculated when the air-fuel mixture supplied to the combustion chamber is made rich, there is a problem that misfire may occur depending on the recirculation amount and the degree of richness at this time. is there.

[0005]

According to the first aspect of the present invention, in order to solve the above problems, an exhaust gas recirculation device for recirculating exhaust gas in the engine exhaust passage into the engine intake passage is provided. and air-fuel ratio of the inflowing exhaust gas is absorbed NO _x when the lean, the _the NO _x absorbent when the air-fuel ratio of the inflowing exhaust gas to release NO _x which is absorbed and becomes rich in the engine exhaust passage arrangement and, in the internal combustion engine air-fuel ratio is switched from lean to rich exhaust gas is when releasing the NO _x from the NO _x absorbent flowing into _the NO _x absorbent, while recirculating the exhaust gas into the engine intake passage when the _the NO _x absorbent should be released NO _x is to be reduced the degree of the rich exhaust gas flowing into _the NO _x absorbent than when low when a high recirculation rate of the exhaust gas.

In the second invention, in the first invention,
When the exhaust gas recirculation rate is higher than a predetermined allowable recirculation rate when NO _x should be released from the NO _x absorbent, the exhaust gas recirculation rate is lowered below the allowable recirculation rate. There is. In the third invention, in the first invention, when the NO _x absorbent should release NO _x , and the exhaust gas recirculation rate is lower than a predetermined allowable recirculation rate, the NO _x releasing action and the NO _x releasing action are performed. _{x The} exhaust gas recirculation rate is not changed during the emission process.

[0007]

[Action] In the first invention, the exhaust gas flowing into _the NO _x absorbent than when low when a high recirculation rate of the exhaust gas so as not to cause a misfire when releasing the NO _x from the NO _x absorbent The degree of richness is reduced. In the second aspect of the invention, when the NO _x absorbent should release NO _x , when the exhaust gas recirculation rate is higher than a predetermined allowable recirculation rate, the exhaust gas recirculation rate is adjusted so as not to cause misfire. Is reduced below the allowable recirculation rate.

[0008] In the third aspect, the air-fuel ratio in order to release the NO _x from the NO _x absorbent is made rich, the action of releasing this case NO _x is performed when the recirculation rate of the exhaust gas is performed during the action of releasing the recirculation rate is NO _x before the exhaust gas is maintained as it is.

[0009]

1 to 22 show a first embodiment in which the present invention is applied to a cylinder injection type internal combustion engine. First, referring to FIGS. 1 to 10, the cylinder injection type internal combustion engine will be described. The basic operation of the internal combustion engine will be described. 1 to 5, 1 is an engine body, 2 is a cylinder block, 3 is a piston that reciprocates in the cylinder block 2, 4 is a cylinder head fixed on the cylinder block 2, and 5 is a piston 3. A combustion chamber formed between cylinder heads 4, 6a is a first intake valve, 6b is a second intake valve, 7a is a first intake port, 7b is a second intake port, 8 is a pair of exhaust valves, and 9 is a pair of exhaust valves. The exhaust ports are shown respectively. First as shown in FIG.
The intake port 7a is composed of a helical intake port
The intake port 7b is a straight port that extends almost straight. Further, as shown in FIG. 3, a spark plug 10 is arranged at the center of the inner wall surface of the cylinder head 4, and fuel is injected to the peripheral portion of the inner wall surface of the cylinder head 4 between the first intake valve 6a and the second intake valve 6b. A valve 11 is arranged. On the other hand, as shown in FIGS. 4 and 5, a cavity 3a is formed on the top surface of the piston 3. The cavity 3a has a shallow dish portion 12 having a substantially circular contour shape extending from below the fuel injection valve 11 to below the spark plug 10, and a hemispherical deep dish portion 13 formed in the central portion of the shallow dish portion 12. Consists of. Further, a concave portion 14 having a substantially spherical shape is formed at a connection portion between the shallow dish portion 12 and the deep dish portion 13 below the spark plug 10.

As shown in FIGS. 1 to 3, the first intake port 7a and the second intake port 7b of each cylinder are formed in the respective intake branch pipes 15, and the first intake passage 15a and the second intake passage 15b are formed. An intake control valve 17 is arranged in each second intake passage 15b. These intake control valves 17 are connected via a common shaft 18 to an actuator 19 composed of, for example, a step motor. The step motor 19 is controlled based on the output signal of the electronic control unit 30. The surge tank 16 receives the air cleaner 2 through the intake duct 20.
1, a throttle valve 23 is arranged in the intake duct 20 and is driven by, for example, a step motor 22. This step motor 22 is also an electronic control unit 30.
Is controlled based on the output signal of.

On the other hand, the exhaust port 9 of each cylinder is connected to an exhaust manifold 24, and the exhaust manifold 24 has a casing 2 containing an NO _x absorbent 26 therein via an exhaust pipe 25.
Connected to 7. Exhaust manifold 24 and surge tank 1
6 is a recirculation exhaust gas (hereinafter referred to as EGR gas) passage 2
An EGR valve 29 that is connected to each other via 8 and controls the amount of EGR gas is arranged in the EGR gas passage 28. The EGR valve 29 is controlled based on the output signal of the electronic control unit 30. When the EGR valve 29 is closed, only air is supplied into the combustion chamber 5 through the intake ports 7a and 7b, and when the EGR valve 29 is opened, air and EGR gas are supplied to the intake ports 7a and 7b. It is supplied into the combustion chamber 5 via the.

The electronic control unit 30 comprises a digital computer, and a RAM (random access memory) 32 and a ROM connected to each other via a bidirectional bus 31.
A (read only memory) 33, a CPU (microprocessor) 34, an input port 35 and an output port 36 are provided. The accelerator pedal 40 has a load sensor 41 that generates an output voltage proportional to the depression amount of the accelerator pedal 40.
Is connected, and the output voltage of the load sensor 41 is the AD converter 3
It is input to the input port 35 via 7. The top dead center sensor 42 generates an output pulse when, for example, the first cylinder reaches the intake top dead center, and this output pulse is input to the input port 35. The crank angle sensor 43 generates an output pulse each time the crankshaft rotates 30 degrees, for example, and the output pulse is input to the input port 35. In the CPU 34, the output pulse of the top dead center sensor 42 and the crank angle sensor 4
The current crank angle is calculated from the output pulse of No. 3, and the engine speed is calculated from the output pulse of the crank angle sensor 43. On the other hand, the output port 36 is connected to each fuel injection valve 11 and each step motor 19 via the corresponding drive circuit 38.
22 is connected.

[0013] From Figure 1 consists scan word Lumpur valve the fuel injection valve 11 injects while giving swirling force to the fuel in the embodiment shown in FIG. 5, from the fuel injection valve 11 at F in FIGS. 3 and 4 Fuel is injected in a cone as shown. Figure 6
Shows the fuel injection amount from this fuel injection valve 11 and the fuel injection timing. FIG. 7 shows the same fuel injection amount as in FIG. 6, plus the opening of the throttle valve 23 and the opening of the EGR valve 29. The average air-fuel ratio A / F in the combustion chamber 5 and the exhaust gas recirculation rate [EGR gas amount / (EGR gas amount + intake air amount)], that is, the EGR ratio are shown. 6 and 7, L indicates the depression amount of the accelerator pedal 40. As can be seen from FIG. 6, during the engine low load operation in which the depression amount L of the accelerator pedal 40 is smaller than L _1, fuel injection is performed by the injection amount Q _{2 at the} end of the compression stroke. on the other hand,
During engine load operation in which the depression amount L of the accelerator pedal 40 is between L ₁ and L ₂ , fuel is injected by the injection amount Q ₁ during the intake stroke, and fuel is injected by the injection amount Q _{2 at the} end of the compression stroke. . That is, at the time of engine medium load operation, the fuel injection is performed twice in the intake stroke and the end of the compression stroke. Further, during the engine high load operation in which the depression amount L of the accelerator pedal 40 is larger than L _{2, the} fuel is injected by the injection amount Q ₁ during the intake stroke. In FIG. 6, θS1 and θE1 respectively indicate the injection start timing and the injection completion timing of the fuel injection Q ₁ performed during the intake stroke, and θS2 and θE2 indicate the injection start of the fuel injection Q ₂ performed at the end of the compression stroke. The timing and the injection completion timing are shown respectively.

On the other hand, as shown in FIG. 7, the opening degree of the throttle valve 23 is considerably small during engine low and medium load operation in which the depression amount L of the accelerator pedal 40 is smaller than L _2. Is the accelerator pedal 40
The smaller the depression amount L is, the smaller it becomes. On the other hand, when the depression amount L of the accelerator pedal 40 becomes larger than L ₂ , the opening degree of the throttle valve 23 rapidly increases and fully opens. Further, the EGR valve 29 is fully opened during the engine low-medium load operation in which the depression amount L of the accelerator pedal 40 is smaller than L ₂ , and when the depression amount L of the accelerator pedal 40 becomes larger than L ₂ , the opening degree of the EGR valve 29 is increased. Rapidly decreases and closes completely. The average air-fuel ratio in the combustion chamber 5 switches from lean to rich at a certain time point L ₀ in the high load operation region (L> L ₂ ). That is, the average air-fuel ratio A / F becomes lean in the range where the depression amount L of the accelerator pedal 40 is smaller than L ₀ , and the smaller the depression amount L of the accelerator pedal 40 at this time is, the leaner the average air-fuel ratio A / F becomes. Become. On the other hand, when the depression amount L of the accelerator pedal 40 becomes larger than L ₀ , the average air-fuel ratio A / F becomes rich.

On the other hand, the opening degree of the throttle valve 23 becomes smaller as the engine load becomes lower. Therefore, the EGR rate becomes maximum when the engine load becomes the lowest in the region where the EGR valve 29 is fully opened. At this time, in this embodiment, the EGR rate is about 50%. The EGR rate decreases substantially linearly as the depression amount L of the accelerator pedal 40 increases, and becomes zero when the EGR valve 29 is fully closed.

FIG. 8 shows the relationship between the opening degree of the intake control valve 17 and the depression amount L of the accelerator pedal 40. As shown in FIG. 8, the intake control valve 17 is held in the fully closed state during engine low load operation in which the depression amount L of the accelerator pedal 40 is smaller than L ₁ , and the depression amount L of the accelerator pedal 40 is less than L ₁ . The intake control valve 17 is opened as the depression amount L of the accelerator pedal 40 increases. When the intake control valve 17 is fully closed, the intake air flows into the combustion chamber 5 while swirling through the first intake port 7a having a helical shape, so that the combustion air in the combustion chamber 5 is indicated by an arrow S in FIG. A powerful swirling flow is generated as shown in. On the other hand, when the intake control valve 17 is opened, intake air also flows into the combustion chamber 5 from the second intake port 7b.

Next, the combustion method will be described with reference to FIGS. 9 and 10. It should be noted that FIG. 9 shows a combustion method during engine low load operation, and FIG. 10 shows a combustion method during engine medium load operation. As shown in FIG. 6, during the engine low load operation in which the depression amount L of the accelerator pedal 40 is smaller than L ₁ , fuel is injected at the end of the compression stroke. At this time, the injected fuel F collides with the peripheral wall surface of the basin portion 13 as shown in FIGS. 9 (A) and 9 (B). The injection amount Q _{2 at} this time is as shown in FIG.
It increases as the depression amount L of 0 increases. The fuel that has collided with the peripheral wall surface of the basin portion 13 is diffused while being vaporized by the swirling flow S, and as a result, as shown in FIG. A combustible mixture G is formed. At this time, the inside of the combustion chamber 5 other than the recess 14 and the basin 13 is filled with air and EGR.
Filled with gas. Next, the air-fuel mixture G is ignited by the spark plug 10.

On the other hand, in FIG. 6, the first fuel injection Q ₁ is performed during the intake stroke during the engine medium load operation in which the depression amount L of the accelerator pedal 40 is between L ₁ and L ₂ , and then the final stage of the compression stroke. The second fuel injection Q ₂ is performed at.
That is, first, as shown in FIG. 10A, fuel injection F is performed toward the inside of the cavity 3a at the beginning of the intake stroke, and the injected fuel forms a lean air-fuel mixture in the entire combustion chamber 5. Next, as shown in FIG. 10B, fuel injection F is performed toward the inside of the cavity 3a at the end of the compression stroke, and as shown in FIG. A flammable air-fuel mixture G is formed in this area. The combustible air-fuel mixture G is ignited by the spark plug 10, and the ignition flame causes combustion chamber 5 to ignite.
The lean air-fuel mixture is burned. In this case, since it is sufficient to generate a spark for the fuel injected at the end of the compression stroke, as shown in FIG. 6, the fuel injection amount at the end of the compression stroke is irrespective of the depression amount L of the accelerator pedal 40 during the engine medium load operation. Q ₂ is kept constant. On the other hand, the fuel injection amount Q _{1 at the} beginning of the intake stroke increases as the depression amount L of the accelerator pedal 40 increases.

In FIG. 6, the fuel is injected only once in the early stage of the intake stroke during the engine high load operation in which the depression amount L of the accelerator pedal 40 is larger than L _2.
A homogeneous mixture is formed therein. At this time, the fuel injection amount at the beginning of the intake stroke is as shown in FIG.
It increases as the amount L of depression increases. The above is the basic combustion method of the direct injection internal combustion engine shown in FIG. Next, an exhaust gas purification method suitable for this cylinder injection internal combustion engine will be described.

Referring first to FIG. 11, this FIG.
Shows schematically the relationship between the concentrations of typical components in the exhaust gas discharged from the combustion chamber 5 and the average air-fuel ratio A / F in the combustion chamber 5. As can be seen from FIG. 11, the combustion chamber 5
The concentration of unburned HC and CO in the exhaust gas discharged from the combustion chamber 5 increases as the average air-fuel ratio A / F in the combustion chamber 5 becomes richer, and oxygen O ₂ in the exhaust gas discharged from the combustion chamber 5 increases.
Is increased as the average air-fuel ratio A / F in the combustion chamber 5 becomes leaner.

On the other hand, the NO _x absorbent 26 contained in the casing 27 shown in FIG. 1 uses, for example, alumina as a carrier, and potassium K, sodium Na,
At least one selected from alkali metals such as lithium Li and cesium Cs, alkaline earths such as barium Ba and calcium Ca, rare earths such as lanthanum La and yttrium Y, and a noble metal such as platinum Pt are supported. ing. Inside the engine intake passage, inside the combustion chamber 5 and NO _x
The NO _x absorbent 26 of Toko to refer to the ratio of the total amount of air supplied to the absorbent 26 upstream of the exhaust passage and the total fuel (hydrocarbon) amount and the air-fuel ratio of the exhaust gas flowing into _the NO _x absorbent 26 flows When the air-fuel ratio of the exhaust gas is lean, it absorbs NO _x ,
NO _x absorbed when the oxygen concentration in the inflowing exhaust gas decreases
Carry out the absorption and release action of NO _x to release. When the fuel (hydrocarbon) or air is not supplied into the exhaust passage upstream of the NO _x absorbent 26, the air-fuel ratio of the inflowing exhaust gas is equal to the average air-fuel ratio A / F in the combustion chamber 5, so that absorbs NO _x when the mean air-fuel ratio a / F is lean in _the NO _x absorbent 26 is the combustion chamber 5 when, the oxygen concentration in the gas in the combustion chamber 5 to release the NO _x absorbed to decrease It will be.

If the above-mentioned NO _x absorbent 26 is placed in the exhaust passage of the engine, the NO _x absorbent 26 actually acts to absorb and release NO _x , but the detailed mechanism of this absorbing and releasing action is not clear. There is also. However, it is considered that this absorbing / releasing action is performed by the mechanism shown in FIG. Next, this mechanism will be described by taking the case where platinum Pt and barium Ba are supported on the carrier as an example, but the same mechanism can be obtained by using other noble metals, alkali metals, alkaline earths and rare earths.

That is, the average air-fuel ratio A / in the combustion chamber 5
F is lean, therefore the oxygen concentration in the inflowing exhaust gas when the inflow exhaust gas is lean is increased, these oxygen O ₂ as shown in FIG. 12 (A) this time is O ₂
^- or O ^2- shape is deposited on the surface of the platinum Pt. On the other hand, NO in the inflowing exhaust gas is O ₂ ⁻ or O ₂ on the surface of platinum Pt.
Reacts with ^2- to become NO ₂ (2NO + O ₂ → 2N
O ₂ ). Then, a part of the generated NO ₂ is oxidized on the platinum Pt and absorbed in the absorbent to be barium oxide BaO.
As shown in FIG. 12 (A), it diffuses into the absorbent in the form of nitrate ion NO ₃ ⁻ while being bound with. In this way, NO _x is absorbed in the NO _x absorbent 26.

NO ₂ is produced on the surface of platinum Pt as long as the oxygen concentration in the inflowing exhaust gas is high, and NO ₂ is absorbed in the absorbent and nitrate ion NO ₃ unless the absorption capacity of NO _x of the absorbent is saturated. ^- 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 . Namely, when the oxygen concentration in the inflowing exhaust gas is released NO _x from the NO _x absorbent 26 when lowered. As can be seen from FIG. 11, when the lean degree of the inflowing exhaust gas becomes low, the oxygen concentration in the inflowing exhaust gas decreases,
Therefore NO _x from _the NO _x absorbent 26 air-fuel ratio to a lean if the inflowing exhaust gas when lowering the lean degree of the inflowing exhaust gas is to be released.

On the other hand, at this time, the average empty space in the combustion chamber 3
The fuel ratio A / F is made rich and the air-fuel ratio of the inflowing exhaust gas is increased.
As shown in Fig. 11, a large amount of
Unburned HC and CO are discharged, and these unburned HC and CO are white
Oxygen O on gold Pt₂ ^-Or O ^2-React with and oxidize
Be done. When the air-fuel ratio of the inflowing exhaust gas becomes rich,
Since the oxygen concentration in the exhaust gas is extremely reduced, the absorbent
To NO₂Is released and this NO₂Is shown in FIG.
Is reacted with unburned HC and CO to be reduced.
It In this way, NO is formed on the surface of platinum Pt.₂Exists
When it disappears, it will be NO one after another from the absorbent₂Is released
It Therefore, if the air-fuel ratio of the inflowing exhaust gas is made rich, it will be
NO in the meantime_xAbsorbent 26 to NO_xIs released
Becomes

That is, the air-fuel ratio of the inflowing exhaust gas is made rich.
Then, first, unburned HC and CO are O on platinum Pt.₂ ^-or
Is O^2-Reacts instantly with and is oxidized, then platinum
O on Pt₂ ^-Or O^2-Is consumed, but unburned HC,
If CO remains, the unburned HC and CO absorb the absorbent.
NO released from_xAnd NO emitted from the engine _x
Is reduced. Therefore, the air-fuel ratio of the inflowing exhaust gas is
No in a short time if you switch_xAbsorbed by the absorbent 26
NO_xIs released, and this released NO
_xNO is reduced to the atmosphere_xIs discharged
You will be able to prevent it. Also, NO_xAbsorbent
Since 26 has a function of a reduction catalyst,
NO even if the air-fuel ratio is the theoretical air-fuel ratio_xRelease from absorbent 26
NO done_xIs reduced. However, inflow and outflow
NO when the air-fuel ratio of air gas is set to the theoretical air-fuel ratio_xabsorption
Agent 26 to NO_xNO is released only gradually
_xAll NO absorbed in the absorbent 26_xTo release
Takes a little longer.

As described above, the average empty space in the combustion chamber 5
NO when the fuel ratio A / F is lean_xIs NO_xabsorption
It is absorbed by the agent 26. But no_xOf absorbent 26
NO _xAbsorption capacity is limited, NO_xAbsorbent 26 N
O_xNO if the absorption capacity is saturated_xAbsorbent 26 is no longer N
O_xCan no longer be absorbed. Therefore NO_xAbsorbent 26 N
O_xNO before the absorption capacity is saturated_xAbsorbent 26 to NO
_xNeed to be released, for which NO_xAbsorbent
26 NO_xEstimate if is absorbed
There is a need. So next this NO_xFor estimating the amount of absorption
explain about.

When the average air-fuel ratio A / F in the combustion chamber 5 is lean, the higher the engine load is, the more the amount of NO _x discharged from the engine per unit time increases. Therefore, the NO _x absorbent 26 is absorbed per unit time. is the amount of NO _x increases, also NO _x to _the amount of NO _x discharged from the higher unit time per engine the engine speed increases is absorbed per unit time _the NO _x absorbent 26 in order to increase is increased. Thus _the amount of NO _x absorbed per unit time per _the NO _x absorbent 26 becomes a function of the engine load and the engine speed. In this case, since the engine load can be represented by the depression amount L of the accelerator pedal 40, the N absorbed by the NO _x absorbent 26 per unit time is N.
O _x amount is the amount of depression function of L and the engine rotational speed N of the accelerator pedal 40. Thus determined by experiment the amount of NO _x A is absorbed per unit time _the NO _x absorbent 26 as a function of the depression amount L and the engine rotational speed N of the accelerator pedal 40 in the embodiment shown in FIG. 1, the amount of NO _x A Is L and N
As a function of the RO in advance in the form of the map shown in FIG.
It is stored in M3 3 .

On the other hand, the average air-fuel ratio A / in the combustion chamber 5
When F becomes the stoichiometric air-fuel ratio or becomes rich, NO _x is released from the NO _x absorbent 26, but the NO _x release amount at this time is mainly affected by the exhaust gas amount and the average air-fuel ratio. Ie, N the amount of NO _x amount exhaust gas is discharged from the unit time per _the NO _x absorbent 26 as to increase is increased, the average air-fuel ratio is discharged from the higher per unit time _the NO _x absorbent 26 becomes rich
The amount of O _x increases. In this case, the exhaust gas amount, that is, the intake air amount is determined by the depression amount L of the accelerator pedal 40 and the engine speed N.
And the average air-fuel ratio A / F is also the accelerator pedal 40
Is a function of the depression amount L and the engine speed N. Therefore, the NO _x amount D released from the NO _x absorbent 26 per unit time is a function of the depression amount L of the accelerator pedal 40 and the engine speed N, and this NO _x amount D is a function of L and N in FIG.
Stored in the 4 (B) in advance ROM3 3 in the form of a map shown.

As described above, when the average air-fuel ratio A / F in the combustion chamber 5 is lean, the NO _x absorption amount per unit time is represented by A, and the average air-fuel ratio A in the combustion chamber 5 is A.
When / F is the stoichiometric air-fuel ratio or rich, the NO _x release amount per unit time is represented by D, so the NO _x amount ΣNOx estimated to be absorbed by the NO _x absorbent 26 can be calculated using the following equation. become.

ΣNOX = ΣNOX + A-D Figure 13 shows this NO_xAmount ΣNOX and flatness in combustion chamber 5
The relationship with the air-fuel ratio A / F is shown. You can see from Figure 7
Engine load L is L₀Inside the combustion chamber 5 when lower than
The average air-fuel ratio A / F in
Sometimes NO_xIs NO_xAs it is absorbed by the absorbent 26,
NO as shown in 13_xThe amount ΣNOX increases. one
However, as shown in FIG. 7, the engine load L is L₀Higher than
Then, the average air-fuel ratio A / F in the combustion chamber 5 becomes rich.
NO to become_xAbsorbent 26 to NO_xIs released.
Therefore, as shown by X in FIG. 13, the engine load L is
L ₀And the average air-fuel ratio A / F becomes richer than
And NO_xThe amount ΣNOX decreases.

On the other hand, when the average air-fuel ratio A / F is continuously made lean and the NO _x amount ΣNOX exceeds the allowable value MAX, the average air-fuel ratio A / F in the combustion chamber 5 is shown by Y in FIG. Is forcibly made rich. When the average air-fuel ratio A / F is made rich, NO _x is rapidly released from the NO _x absorbent 26, and as shown in FIG.
The amount of O _x ΣNOX rapidly decreases. Then NO _x amount ΣN
When OX decreases to the lower limit value MIN, the average air-fuel ratio A / F is returned from rich to lean.

In the embodiment according to the present invention, as shown in FIG. 7, the EGR gas is recirculated during engine low load operation or engine medium load operation.
O _x amount ΣNOX average air-fuel ratio A / F in the combustion chamber 5 in a state in which the recirculation of EGR gas exceeds the allowable value MAX is being performed is made rich. However, in this case, when the combustion is started in the combustion chamber 5 as described above, this combustion heat is used to heat the EGR gas and the injected fuel that has not yet burned, so the higher the EGR rate, the lower the combustion temperature. The combustion temperature becomes lower as the rich degree of the average air-fuel ratio A / F becomes larger. As a result, the higher the EGR rate, the more likely misfire will occur, and the average air-fuel ratio A /
The higher the degree of F richness, the more likely misfire will occur. Therefore, as the degree of richness of the average air-fuel ratio A / F increases, unless the EGR rate is lowered, misfire will occur.

The broken line in FIG. 15 shows the misfire limit value indicating the limit at which misfire occurs. If the combination of the air-fuel ratio and the EGR rate exists in the region above this misfire limit value, no misfire will occur, whereas the combination of the air-fuel ratio and the EGR rate will exist in the region below this misfire limit value. If present, it will cause a misfire. From FIG. 15, in order to prevent misfire, the average air-fuel ratio A increases as the EGR rate increases.
It can be seen that the degree of richness in / F must be reduced.

By the way, as shown in FIG. 15, NO_xEmission
For a certain lean air-fuel ratio (A / F) _LFrom a certain rich sky
Fuel ratio (A / F)_RWhen switching to, NO_xTo the release action of
From the theoretical air-fuel ratio to the rich air-fuel ratio (A / F)_R
Is the fuel amount S required for_xEmission
In order to use the fuel effectively, the fuel amount S can be obtained.
The air-fuel ratio (A / F)_Rof
It is preferable to make the rich degree as large as possible.
That is, the rich air-fuel ratio (A / F)_RThe degree of richness is a solid line
S₀As shown in, it can be done within the range where the misfire limit is not exceeded.
It is preferable to approach the fire extinguishing limit value. Therefore, the present invention
In the embodiment according to, the degree of richness with respect to the EGR rate is indicated by a solid line.
S₀The control is performed so that the value becomes.

The solid line in FIG. 16 indicates the EGR during normal operation.
Rate, average air-fuel ratio A / F in the combustion chamber 5, throttle valve 23
16 and the opening degree of the EGR valve 29 are shown in FIG.
The broken line is NO_xAbsorbent 26 to NO_xBurn to release
EG when the average air-fuel ratio A / F in the chamber 5 is made rich
R ratio, average air-fuel ratio A / F in combustion chamber 5, throttle valve 2
3 and the EGR valve 29 are shown. Well
17 to 19 is NO_xAbsorbent 26 to NO_xTo
NO to release_xThailand when the emission flag is set
It shows a chart. Next, these FIG. 16 to FIG.
NO while referring to _xNO from absorbent 26_xFor controlled release
explain about.

As described above, the fuel is injected at the end of the compression stroke during engine low load operation in which the depression amount L of the accelerator pedal 40 is smaller than L ₁ . At this time, the air-fuel mixture is formed only in the cavity 3a, and the regions other than the cavity 3a are substantially filled with air and EGR gas, so that the stratification degree is extremely high and a strong stratification state is obtained. On the other hand, the amount of depression L of the accelerator pedal 40 is lean is formed on the whole in the combustion chamber 5 at which during the time of load operation the engine by the intake stroke injection between L ₁ and L _2, the lean mixture by the compression stroke injection A mixture richer than air is formed in the cavity 3a. Therefore, at this time, the degree of stratification is lower than that of the strong stratification during low engine load operation. Further, during the engine high load operation in which the depression amount L of the accelerator pedal 40 is larger than L _2, only the intake stroke injection is performed, and therefore, the homogeneous air-fuel mixture is formed in the combustion chamber 5 at this time.

FIG. 17 shows NO _x during engine low load operation.
The time chart of release control is shown. In the embodiment according to the present invention, the NO _x absorbents 26 to N
When O _x has to be released, the strongly stratified state is switched to the homogeneous mixture state, and the average air-fuel ratio A / F is made rich by increasing the injection amount in the homogeneous mixture state from the NO _x absorbent 26. It is designed to release NO _x . Further, the output torque of the engine increases when the average air-fuel ratio A / F is made rich in order to release the NO _x from the NO _x absorbent 26. However, in the embodiment according to the present invention, this increase in output torque is due to the increase in the combustion chamber 5. When the output torque is suppressed by reducing the amount of intake air supplied to the engine, the output torque is prevented from increasing by, for example, retarding the ignition timing.

That is, as shown in FIG. 17, in the engine low load operating state before the NO _x release flag is set, that is, in the strong stratified operating state, the intake stroke injection is not performed, and only the compression stroke injection is performed. The average air-fuel ratio A / F in the combustion chamber 5
Is lean. At this time, the throttle valve 23
The EGR valve 29 is opened. Then N
When the NO _x release flag is set to release NO _x from the O _x absorbent 26, the EGR valve 29 is closed and the throttle valve 23 is also closed. Throttle valve 23
Is closed, the compression stroke injection amount is slightly increased so that the output torque of the engine does not decrease.

Next, when t time has elapsed after the NO _x release flag was set, the compression stroke injection is stopped and the intake stroke injection is started. That is, the strongly stratified state is switched to the homogeneous mixture state, and at this time, the average air-fuel ratio A / in the combustion chamber 5 is changed.
The injection amount is increased to make F rich. By the way, as shown by the solid line in FIG. 16, the EGR rate is considerably high when the engine is in a strong stratified state at low load. At this time, there is sufficient air, so E
Even if the GR rate is increased, good combustion can be obtained without causing misfire. However, when the average air-fuel ratio A / F is made rich under a uniform mixture state, misfire occurs if the EGR rate is made too high, and in this case, the allowable EGR does not occur.
The rate is about 20%.

Therefore, when the strongly stratified state is changed to the homogeneous air-fuel mixture state, the EGR rate is reduced to about 20% as shown by the broken line in FIG. That is,
When the EGR rate is higher than the allowable EGR rate, the EGR rate is reduced to the allowable EGR rate or less. This effect of decreasing the EGR rate is EG as shown by the broken line in FIG.
This is performed by reducing the opening degree of the R valve 29. The degree of richness at this time is determined from the EGR rate in FIG.
The value on the solid line S _{0 of} 5 is set. Therefore, at this time, good combustion is performed without causing misfire.

When the average air-fuel ratio A / F is made rich, NO
_The action of releasing NO _x from the _x absorbent 26 is started. Then, when the action of releasing NO _x from the NO _x absorbent 26 is completed, the strong stratified state is restored again in the reverse order. That is,
When the NO _x releasing action is completed, the intake stroke injection is stopped and the compression stroke injection is started. When the time t has elapsed after the NO _x releasing action was completed, the throttle valve 23 and the EGR are completed.
The valve 29 is opened and the NO _x release flag is reset.

FIG. 18 shows NO _x during engine medium load operation.
The time chart of release control is shown. When the NO _x releasing flag is set during the engine medium load operation, the compression stroke injection is stopped and the intake stroke injection amount is increased. That is, N
When the _Ox release flag is set, the weak stratification state is switched to the homogeneous mixture state, and the average air-fuel ratio A / F in the combustion chamber 5 is changed.
There is a rich, releasing action of the NO _x from _the NO _x absorbent 26 is started. When the NO _x releasing action is completed, the homogeneous mixture state is returned to the weakly stratified state.

As shown in FIG. 16, the EGR rate is higher than the allowable EGR rate as shown by the solid line when the engine load is low even during the engine medium load operation. Therefore, at this time, FIGS. As indicated by the broken line in FIG. 11, the EGR valve 29 is slightly closed when NO _x is released, and the EGR rate is made equal to or lower than the allowable EGR rate. Other EGR rate in _the NO _x releasing action since the EGR rate is below the allowable EGR rate at the time of the _the NO _x releasing action before EG
It is maintained at the same value as the R rate. At this time, the EGR rate and the degree of rich are on the solid line S _{0 in} FIG.
As can be seen from 6, the richer the degree, the higher the EGR rate.

FIG. 19 shows NO _x during engine high load operation.
The time chart of release control is shown. As shown in FIG. 19, during engine high load operation before the NO _x release flag is set, only intake stroke injection is performed and homogeneous mixture combustion is performed, and when the NO _x release flag is set. The intake stroke injection amount is increased and the average air-fuel ratio A / F is made rich.

Next, a basic routine for controlling fuel injection will be described first with reference to FIGS. 20 and 21. It should be noted that this routine is executed, for example, by interruption at regular intervals. Referring to FIGS. 20 and 21, first, at step 100, it is judged if the NO _x releasing flag is set. When the NO _x release flag is not set, step 101
6 stored in advance in the ROM 33 based on the depression amount L of the accelerator pedal 40 and the engine speed N.
The intake stroke injection amount Q ₁ and the compression stroke injection amount Q ₂ are calculated from the relationship shown in FIG. Next, at step 102, the injection timing is calculated from the relationship shown in FIG. 6 stored in advance in the ROM 33 based on the depression amount L of the accelerator pedal 40 and the engine speed N, and then at step 103, the depression amount L of the accelerator pedal 40 is calculated. Based on the engine speed N and the engine speed N, the opening degree of the throttle valve 23 is calculated from the relationship shown in FIG.
In 04, the opening degree of the EGR valve 29 is calculated from the relationship stored in advance in the ROM 33 based on the depression amount L of the accelerator pedal 40 and the engine speed N, and then step 1
In 05, the opening degree of the intake control valve 17 is calculated based on the depression amount L of the accelerator pedal 40 from the relationship shown in FIG.

Next, at step 106, it is judged if the depression amount L of the accelerator pedal 40 is lower than L ₀ (FIG. 7). When L <L _0, the routine proceeds to step 107, where the NO _x emission amount A is calculated from the map shown in FIG. Next, at step 108, the NO _x release amount D is made zero, and then the routine proceeds to step 111. On the other hand, step 1
If it is determined in L06 that L ≧ L ₀ , the routine proceeds to step 109, where NO is determined from the map shown in FIG.
_{The x} release amount D is calculated. Next, at step 110, N
The _Ox absorption amount A is made zero, and then the routine proceeds to step 111. _The amount of NO _x .SIGMA.NOX which is estimated to be absorbed in _the NO _x absorbent 26 in step 111 (= ΣNOX + A-
D) is calculated. Next, at step 112, ΣNOX
Is determined to be negative, and when ΣNOX <0, the routine proceeds to step 113, where ΣNOX is made zero. Next, at step 114, the NO _x amount ΣNO _x is shown in FIG.
It is determined whether or not the allowable value MAX shown in 3 is exceeded,
When ΣNOX> MAX, the routine proceeds to step 115, where the NO _x release flag is set.

On the other hand, when it is judged at step 100 that the NO _x releasing flag is set, step 1
16, the rich process shown in FIGS. 17 to 19 is performed. This rich process is executed by the routine shown in FIG. That is, referring to FIG. 22, first, at step 150, it is judged if the release completion flag indicating that the release of NO _x is completed is set. Immediately after the rich process is started, the release completion flag is not set, so the routine proceeds to step 151, where the pre-process until the average air-fuel ratio A / F is made rich is performed. In the low load state shown in FIG. 17, this pre-processing is processing for closing the throttle valve 23 and the EGR valve 29 when the NO _x release flag is set, that is, when the rich processing is started. This pretreatment is not performed during the medium load shown in 18 and during the high load shown in FIG.

Next, at step 152, it is judged if this pre-processing is completed. When the pre-processing is completed, the routine proceeds to step 153, where rich processing for making the average air-fuel ratio A / F rich is performed. Next, at step 154, the NO _x release amount D ′ during the NO _x release action is calculated. This NO _x release amount D'is stored in advance in the ROM 33 in the form of a map as shown in FIG. 23 as a function of the depression amount L of the accelerator pedal 40 and the engine speed N. Then the amount of NO _x ΣNOX step 155 _the NO _x releasing amount D 'is subtracted, then in step 156 _the amount of NO _x Σ
It is determined whether or not NOX has become smaller than the lower limit value MIN. When ΣNOX <MIN, it is determined that the NO _x releasing action is completed, and the routine proceeds to step 157, where the releasing completion flag is set. Then, it proceeds to step 158. Note that once the release completion flag is set, the routine jumps from step 150 to step 158.

In step 158, post-treatment is performed from the completion of the NO _x releasing action to the return to the original state. In the case shown in FIG. 17, this post-processing includes processing for switching from intake stroke injection to compression stroke injection when the NO _x releasing action is completed, and processing for opening the throttle valve 23 and the EGR valve 29. In the case shown in FIG. 18 and FIG. 19, this post-processing consists of processing to restore the injection form. Next, at step 159, it is judged if this post-processing is completed or not, and when the post-processing is completed, the routine proceeds to step 160, where the NO _x releasing flag and the releasing completion flag are reset.

In FIG. 24, the fuel injection valve 11 is attached to each intake branch pipe 15.
Shows the case where the present invention is applied to an internal combustion engine in which a lean air-fuel mixture is burned in the combustion chamber 5 in a stratified state or in a homogeneous air-fuel mixture state. In this embodiment, the opening of the throttle valve 23 is directly controlled by the accelerator pedal, and the intake duct 20 and the air cleaner 2 are also controlled.
An air flow meter 44 for detecting the intake air amount is arranged between the two.

In this embodiment, the fuel injection time TAU is basically calculated based on the following equation. TAU = K · TP where TP is the basic fuel injection time, and K is the correction coefficient. The basic fuel injection time TP indicates the fuel injection time required to make the air-fuel ratio of the air-fuel mixture supplied into the engine cylinder the stoichiometric air-fuel ratio. This basic fuel injection time TP is previously obtained by an experiment, and the engine load Q / N (intake air amount Q /
FIG. 25 as a function of engine speed N) and engine speed N
It is previously stored in the ROM in the form of a map as shown in FIG. The correction coefficient K is a coefficient for controlling the air-fuel ratio of the air-fuel mixture supplied to the engine cylinder, and if K = 1.0, the air-fuel mixture supplied to the engine cylinder becomes the stoichiometric air-fuel ratio. On the other hand, if K <1.0, the air-fuel ratio of the air-fuel mixture supplied to the engine cylinder becomes larger than the stoichiometric air-fuel ratio, that is, becomes lean, and if K> 1.0, it is supplied to the engine cylinder. The air-fuel ratio of the air-fuel mixture thus generated becomes smaller than the stoichiometric air-fuel ratio, that is, becomes rich.

The target air-fuel ratio of the air-fuel mixture to be supplied into the engine cylinder, that is, the value of the correction coefficient K is changed according to the operating state of the engine. In this embodiment, basically, as shown in FIG. It is predetermined as a function of the engine load Q / N and the engine speed N. That is, as shown in FIG. 27, K <in the low load operation region on the low load side of the solid line R.
1.0, that is, the air-fuel mixture is lean, and K = 1.0, that is, the air-fuel ratio of the air-fuel mixture is the theoretical air-fuel ratio in the high-load operating region between the solid line R and the solid line S, and the load is higher than that of the solid line S. In the full load operation region on the side, K> 1.0, that is, the air-fuel mixture is rich.

FIG. 26 shows the EGR rate, and the numbers in FIG. 26 show the EGR rate. As shown in FIG. 26, the EGR rate is a function of the engine load Q / N and the engine speed N, and this EGR rate is stored in the ROM in advance. The opening degree of the EGR valve 29 is controlled so that the EGR rate shown in FIG. 26 is obtained. As shown in FIG. 26, in this example, the maximum value of the EGR rate is 20%, and therefore, when the NO _x absorbent 26 should release NO _x ,
The R rate is maintained as it is. FIG. 28 shows the NO _x absorbent 26
4 shows the relationship between the correction coefficient K and the EGR rate when the air-fuel mixture is made rich in order to release NO _x from. The solid line shown in FIG. 28 corresponds to the misfire limit value S ₀ shown in FIG. 15. Therefore, also in this embodiment, the rich degree of the air-fuel mixture becomes smaller as the EGR rate becomes higher.

FIG. 29 shows a fuel injection control routine, which is executed, for example, by interruption at regular time intervals. Referring to FIG. 29, first, at step 200, the basic fuel injection time TP is calculated from the map shown in FIG. Next, at step 201, FIG.
The EGR rate is obtained from the above, and the opening degree of the EGR valve 29 is controlled so as to obtain this EGR rate. Then step 2
At 02, it is judged if the NO _x releasing flag is set. When the NO _x releasing flag is not set, the routine proceeds to step 203, where the correction coefficient K is calculated based on FIG. Next, at step 207, from the basic fuel injection time TP and the correction coefficient K, the fuel injection time TAU (= K
-TP) is calculated.

Next, at step 208, it is judged if the correction coefficient K is smaller than 1.0. When K <1.0, the routine proceeds to step 209, where the NO _x release amount A is calculated from the map as shown in FIG. Next, at step 210, the NO _x release amount D is made zero, and then the routine proceeds to step 213. On the other hand, in step 208, K
When it is determined that ≧ 1.0, the routine proceeds to step 211, where the NO _x release amount D is calculated from the map as shown in FIG. 14 (B). Next, at step 212, the NO _x absorption amount A is made zero, and then the routine proceeds to step 213.

In step 213, the NO _x amount ΣNOX (= ΣNO) estimated to be absorbed by the NO _x absorbent 26 is calculated.
X + A-D) is calculated. Next, at step 214, it is judged if ΣNOX becomes negative, and ΣNOX <0.
Is reached, the routine proceeds to step 215, where ΣNOX is made zero. Next, at step 216, ΣNOX> MAX
If ΣNOX> MAX, the routine proceeds to step 217, where the NO _x release flag is set.

On the other hand, when it is determined in step 202 that the NO _x releasing flag is set, step 2
04, the correction coefficient K (> 1.
0) is calculated. Air-fuel ratio of the mixture at this time is made rich, releasing action of the NO _x from _the NO _x absorbent 26 is started and thus. Next, at step 205, it is judged if ΣNOX has become smaller than the lower limit MIN, and Σ
When NOX <MIN, the routine proceeds to step 206, where NO _x
The release flag is reset.

[0059]

[Effect of the Invention] The mixture so as to release the NO _x in the state where the recirculated EGR gases from _the NO _x absorbent can be prevented misfire that occurs when the rich.

[Brief description of drawings]

FIG. 1 is an overall view of an internal combustion engine showing a cross section of an engine body.

FIG. 2 is an overall view of an internal combustion engine.

FIG. 3 is a plan sectional view of a cylinder head.

FIG. 4 is a plan view of a piston top surface.

5 is a side sectional view of FIG.

FIG. 6 is a diagram showing a fuel injection amount and a fuel injection timing.

FIG. 7 is a fuel injection amount, throttle opening, EGR valve opening,
It is a figure which shows the average air-fuel ratio and EGR rate in a combustion chamber.

FIG. 8 is a diagram showing an opening of an intake control valve.

FIG. 9 is a diagram for explaining a combustion method during low load operation.

FIG. 10 is a diagram for explaining a combustion method during medium load operation.

FIG. 11 is a diagram showing the concentrations of CO, HC, and O ₂ in exhaust gas.

FIG. 12 is a diagram for explaining the NO _x absorption and release action of a NO _x absorbent.

FIG. 13 is a time chart of NO _x release control.

FIG. 14 is a diagram showing a map of a NO _x absorption amount and a NO _x release amount.

FIG. 15 is a diagram showing a misfire limit.

FIG. 16 is a diagram showing an EGR rate, an average air-fuel ratio A / F, a throttle opening and an EGR valve opening.

FIG. 17 is a time chart of NO _x release control during engine low load operation.

FIG. 18 is a time chart of NO _x release control during engine medium load operation.

FIG. 19 is a time chart of NO _x release control during engine high load operation.

FIG. 20 is a flowchart for performing injection control.

FIG. 21 is a flowchart for performing injection control.

FIG. 22 is a flowchart for performing rich control.

FIG. 23 is a diagram showing a map of NO _x release amount.

FIG. 24 is an overall view showing another embodiment of the internal combustion engine.

FIG. 25 is a diagram showing a map of a basic fuel injection time TP.

FIG. 26 is a diagram showing an EGR rate.

FIG. 27 is a diagram showing a correction coefficient K.

FIG. 28 is a diagram showing a correction coefficient K.

FIG. 29 is a flowchart for performing injection control.

FIG. 30 is a flowchart for performing injection control.

[Explanation of symbols]

16 ... Surge tank 24 ... Exhaust manifold 26 ... NO _x absorbent 28 ... EGR gas passage 29 ... EGR valve

Continuation of front page (51) Int.Cl. ⁷ Identification symbol FI F02D 41/14 310 F02D 41/14 310C 43/00 301 43/00 301E 301N F02M 25/07 550 F02M 25/07 550R (58) Fields investigated (Int.Cl. ⁷ , DB name) F02D 41/00-45/00 F01N 3/08 F01N 3/24 F02M 25/07

Claims (3)

(57) [Claims] 1. The exhaust gas in the engine exhaust passage is passed through the engine intake passage.
Equipped with an exhaust gas recirculation device for recirculation in the road,
NO when the air-fuel ratio of the inflowing exhaust gas is lean_xSuck
Absorbed and absorbed when the air-fuel ratio of the inflowing exhaust gas becomes rich
NO _xReleases NO_xAbsorbent engine exhaust passage
Place inside, NO_xAbsorbent to NO_xWhen to release
Is NO_xThe air-fuel ratio of the exhaust gas flowing into the absorbent is lean.
Exhaust gas in an internal combustion engine that is switched from rich to rich
NO while recirculating into the engine intake passage_xAbsorbent to NO
_xWhen the exhaust gas recirculation rate is high
NO when compared to low_xExhaust gas flowing into the absorbent
Exhaust from an internal combustion engine designed to reduce the degree of richness
Purification device. 2. When the NO _x absorbent should release NO _x, and when the exhaust gas recirculation rate is higher than a predetermined allowable recirculation rate, the exhaust gas recirculation rate is made equal to or lower than the allowable recirculation rate. The exhaust gas purification device for an internal combustion engine according to claim 1, wherein the exhaust gas purification device is lowered. Wherein the exhaust gas in the NO _x releasing action before and NO _x releasing action when NO recirculation rate of the exhaust gas when the _x absorbent should be released NO _x is less than the recirculation rate to a predetermined The exhaust gas purification device for an internal combustion engine according to claim 1, wherein the recirculation rate of the exhaust gas is not changed.