JP2658757B2 - 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 purifying apparatus for an internal combustion engine.

[0002]

BACKGROUND OF THE INVENTION Most internal combustion engine so as to allowed to burn a lean mixture in the engine operating region of absorbing the NO _x when the air-fuel ratio of the inflowing exhaust gas is lean,
NO _x absorbed when the oxygen concentration in the incoming exhaust gas decreases
The _the NO _x absorbent to release disposed in the engine exhaust passage and from _the NO _x absorbent and the air-fuel ratio of the exhaust gas flowing into _the NO _x absorbent rich when the combustion period of the lean air-fuel mixture is continued over a certain period of time internal combustion engine so as to reduce the released NO _x with the release of NO _x has been already proposed by the present applicant (see Japanese Patent application No. 3-284095).

[0003] NO from the engine even at the time of NO _x emissions is large engine medium and high load operation of the institutions in this internal combustion engine
Even when _x emissions less engine low load operation of the combustion period of the lean air-fuel mixture air-fuel ratio of exhaust gas flowing into _the NO _x absorbent when continued over a certain period of time is made rich.

[0004]

THE INVENTION Problems to be Solved By the way NO _x large amount of unburned HC is the air-fuel ratio of the inflowing exhaust gas from the engine when the rich to the absorbent, CO and the like are discharged, moreover the oxygen concentration in the inflowing exhaust gas Due to the decrease, the absorbed NO _x is released from the NO _x absorbent. At this time, a part of the unburned HC, CO, etc. discharged from the engine is used to reduce NO _x discharged from the engine, and the remaining unburned HC, C
O and the like are used to reduce NO _x released from the NO _x absorbent. In this case, therefore, NO NO _x is discharged from the engine to be prevented from being released into the atmosphere
it is necessary to discharge _x and NO _x absorbent unburned HC amount that can both reduce NO _x released from the CO and the like from the engine.

[0005] By the way, unburned HC, C discharged from the engine
The amount of O and the like increases as the air-fuel ratio of the air-fuel mixture burned in the engine becomes richer. Thus considerable fuel ratio of the mixture burned in engine when the _the NO _x absorbent when the discharge amount is large engine medium and high load operation of the NO _x so as to release the NO _x as in the above internal combustion engine to a rich If not, the emission of NO _x into the atmosphere cannot be suppressed.

[0006] However, if the air-fuel ratio of the air-fuel mixture burned in the engine is increased, the fuel consumption rate is increased and the torque fluctuation is increased.

[0007]

According to the present invention, in order to solve the above problems, in an internal combustion engine in which a lean air-fuel mixture is burned in most of the engine operating range, the air-fuel ratio of the inflowing exhaust gas is reduced. absorbs NO _x when the lean, the _the NO _x absorbent the oxygen concentration in the inflowing exhaust gas to release NO _x absorbed to decrease placed engine exhaust passage, rich are predetermined by the engine operating condition when there is the operating region is the air-fuel ratio of the exhaust gas flowing into _the NO _x absorbent rich when a low load of less load engine load is predetermined except when its, NO _x from _the NO _x absorbent Is released and the released NO _x is reduced.

[0008]

Air-fuel ratio of the inflowing gas into _the NO _x absorbent when a small amount of NO _x discharged from the [action] engine is rich.
Therefore, at this time, the emission of NO _x to the atmosphere is suppressed even if the degree of the richness is not so increased.

[0009]

Referring to FIG. 1, 1 is an engine body, 2 is a piston, 3 is a combustion chamber, 4 is a spark plug, 5 is an intake valve, 6 is an intake port, 7 is an exhaust valve, and 8 is an exhaust port. Show. The intake port 6 is connected to a surge tank 10 via a corresponding branch pipe 9, and a fuel injection valve 11 for injecting fuel into the intake port 6 is attached to each branch pipe 9. The surge tank 10 includes an intake duct 12 and an air flow meter 13.
Is connected to the air cleaner 14 through the
Inside, a throttle valve 15 is arranged. On the other hand, the exhaust port 8 is connected to the N through the exhaust manifold 16 and the exhaust pipe 17.
It is connected to a casing 19 containing an O _x absorbent 18.

The electronic control unit 30 is composed of a digital computer, and a ROM (Read Only Memory) 32, a RAM (Random Access Memory (RAM)) 33, a CPU (Microprocessor) 34, An input port 35 and an output port 36 are provided. The air flow meter 13 generates an output voltage proportional to the amount of intake air, and the output voltage is input to the input port 35 via the AD converter 37. A throttle sensor 20 that generates an output voltage proportional to the throttle opening is attached to the throttle valve 15, and the output voltage of the throttle sensor 20 is input to an input port 35 via an AD converter 38. A water temperature sensor 21 for generating an output voltage proportional to the engine cooling water temperature is attached to the engine body 1, and the output voltage of the water temperature sensor 21 is converted to an AD converter 39.
Through the input port 35. The input port 35 is connected to a rotation speed sensor 22 that generates an output pulse representing the engine rotation speed. On the other hand, the output port 36 is connected to the ignition plug 4 and the fuel injection valve 11 via corresponding drive circuits 40 and 41, respectively.

In the internal combustion engine shown in FIG. 1, the fuel injection time TAU is calculated based on, for example, the following equation. TAU = TP · K Here, TP indicates a basic fuel injection time, and K indicates a correction coefficient. The basic fuel injection time TP indicates a fuel injection time required for setting the air-fuel ratio of the air-fuel mixture supplied into the engine cylinder to the stoichiometric air-fuel ratio. This basic fuel injection time TP is obtained in advance by an experiment, and the engine load Q / N
As a function of (intake air amount Q / engine speed N) and engine speed N, the ROM 3 is previously stored in the form of a map as shown in FIG.
2 is stored. The correction coefficient K is a coefficient for controlling the air-fuel ratio of the air-fuel mixture supplied to the engine cylinder. If K = 1.0, the air-fuel mixture supplied to the engine cylinder becomes the stoichiometric air-fuel ratio. On the other hand, if K <1.0, the air-fuel ratio of the air-fuel mixture supplied into the engine cylinder becomes larger than the stoichiometric air-fuel ratio, that is, lean, and K> 1.0
, The air-fuel ratio of the air-fuel mixture supplied into the engine cylinder becomes smaller than the stoichiometric air-fuel ratio, that is, becomes rich.

The correction coefficient K is controlled according to the operating state of the engine. FIG. 3 shows an embodiment of the control of the correction coefficient K. In the embodiment shown in FIG. 3, during the warm-up operation, the correction coefficient K is gradually decreased as the engine cooling water temperature increases, and when the warm-up is completed, the correction coefficient K is a constant value smaller than 1.0, that is, the engine cylinder. The air-fuel ratio of the air-fuel mixture supplied to the inside is maintained lean. Next, when the acceleration operation is performed, the correction coefficient K is set to, for example, 1.0, that is, the air-fuel ratio of the air-fuel mixture supplied to the engine cylinder is set to the stoichiometric air-fuel ratio. .0, that is, the air-fuel ratio of the mixture supplied to the engine cylinder is made rich. As can be seen from FIG. 3, in the embodiment shown in FIG. 3, the air-fuel ratio of the air-fuel mixture supplied to the engine cylinder is maintained at a constant lean air-fuel ratio except during warm-up operation, acceleration operation and full load operation. Therefore, the lean mixture is burned in most of the engine operating range.

FIG. 4 schematically shows the concentration of typical components in the exhaust gas discharged from the combustion chamber 3. As can be seen from FIG. 4, the amounts of unburned HC and CO in the exhaust gas discharged from the combustion chamber 3 increase as the air-fuel ratio of the air-fuel mixture supplied into the combustion chamber 3 increases, and the amount of the exhaust gas discharged from the combustion chamber 3 increases. The amount of oxygen O ₂ in the exhaust gas increases as the air-fuel ratio of the air-fuel mixture supplied into the combustion chamber 3 becomes leaner.

The NO _x contained in the casing 19
The absorbent 18 is made of, for example, alumina as a carrier. On this carrier, for example, potassium K, sodium Na, lithium Li, alkali metal such as cesium Cs, barium Ba, alkaline earth such as calcium Ca, lanthanum La, yttrium Y At least one selected from such rare earths and a noble metal such as platinum Pt are supported. Engine intake passage and _the NO _x absorbent 18 upstream ratio of the exhaust passages supplied air and fuel into the of Toko called air-fuel ratio of the exhaust gas flowing into _the NO _x absorbent 18 _the NO _x absorbent 18 is the exhaust gas flowing N the air-fuel ratio of the absorbs NO _x when the lean, the oxygen concentration in the inflowing exhaust gas is absorbed and reduced
The O _x performs the absorbing and releasing action of NO _x to be released. Note that NO
_When no fuel or air is supplied into the exhaust passage upstream of the absorbent 18, the air-fuel ratio of the inflowing exhaust gas matches the air-fuel ratio of the air-fuel mixture supplied into the combustion chamber 3, and in this case, NO _x absorbent 18 release the NO _x concentration of oxygen absorbed and reduced in the mixture air-fuel ratio of the mixture supplied into the combustion chamber 3 absorb NO _x when the lean, fed into the combustion chamber 3 Will do.

If the above-described NO _x absorbent 18 is arranged in the engine exhaust passage, the NO _x absorbent 18 actually performs the absorption and release of NO _x , but the detailed mechanism of the absorption and release is not clear. There is also. However, it is considered that this absorption / release action is performed by a mechanism as shown in FIG. Next, this mechanism will be described by taking as an example a case where platinum Pt and barium Ba are supported on a carrier, but the same mechanism can be obtained by using other noble metals, alkali metals, alkaline earths and rare earths.

That is, when the inflowing exhaust gas becomes considerably lean, the oxygen concentration in the inflowing exhaust gas greatly increases.
As shown in (A), these oxygens O ₂ adhere to the surface of platinum Pt in the form of O ₂ ⁻ . On the other hand, N
O is O ₂ on the surface of the platinum Pt ^- reacted with, and _{NO 2 (2NO + O 2 →} 2NO 2). NO ₂ generated
Some of the nitrate ions NO ₃ as shown in FIG. 5 (A) while bonding with the barium oxide BaO is absorbed into the absorbent while being oxidized on the platinum Pt ^- diffused in the absorbent in the form of. In this way, NO _x is absorbed in the NO _x absorbent 18.

As long as the oxygen concentration in the inflowing exhaust gas is high, NO ₂ is generated on the surface of the platinum Pt, and as long as the NO _x absorption capacity of the absorbent is not saturated, NO ₂ is absorbed in the absorbent and the nitrate ion NO ₃ ⁻ Is generated. On the other hand, when the oxygen concentration in the inflowing exhaust gas decreases and the amount of generated NO ₂ decreases, the reaction proceeds in the opposite direction (NO ₃ ⁻ → NO ₂ ), thus the nitrate ion NO ₃ ⁻ in the absorbent. There 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 18 when lowered. Figure 4 is the oxygen concentration of the inflowing exhaust gas when the degree of leanness becomes lower in the inflowing exhaust gas as shown in drops, thus when low lean degree of the inflowing exhaust gas NO _x from the absorbent 18 NO _x is Will be released.

On the other hand, at this time, the air-fuel ratio of the inflowing exhaust gas is reset.
As shown in FIG. 4, a large amount of
Fuel HC and CO are discharged, and these unburned HC and CO are converted to platinum.
Oxygen O on Pt_Two ^-And oxidize. Ma
When the air-fuel ratio of the inflow exhaust gas is made rich, the inflow exhaust gas
NOx from the absorbent because the oxygen concentration in the
_TwoIs released and this NO_TwoIs as shown in FIG.
And reacts with unburned HC and CO to be reduced. This
NO on the surface of platinum Pt_TwoIs no longer present
NO from next to next from absorbent_TwoIs released. Therefore the flow
If the air-fuel ratio of the incoming and exhaust gas is made rich, N
O_xNO from absorbent 18_xWill be released.

That is, the air-fuel ratio of the inflowing exhaust gas is made rich.
First, unburned HC and CO are converted to O on platinum Pt._Two ^-When
Immediately reacted and oxidized, then on platinum Pt
O_Two ^-If unburned HC and CO still remain even if
N released from the absorbent by unburned HC and CO
O_xAnd NO emitted from the engine_xIs reduced
You. Therefore, when the air-fuel ratio of the inflow exhaust gas is made rich,
Total NO released from absorbent_xAnd emissions from institutions
All no_xO on platinum Pt to reduce _Two ^-Turn off
The amount of unburned HC and CO required to spend and total NO_xReturn
Of unburned HC and CO required to recover_xAbsorbent
18 so that the air-fuel ratio of the inflow gas is rich
Must be controlled.

FIG. 6 shows the rich control of the air-fuel ratio of the inflow gas used in the embodiment according to the present invention.
NO _x from the NO _x absorbent 18 in the embodiment shown in FIG. 6
Is to be released, the air-fuel ratio of the mixture supplied to the combustion chamber 3 is made rich by increasing the correction coefficient K used for calculating the fuel injection time TAU to KK (> 1.0). . Next, the correction coefficient K is gradually decreased, and then the correction coefficient K is maintained at 1.0, that is, the air-fuel ratio of the air-fuel mixture supplied into the combustion chamber 3 is maintained at the stoichiometric air-fuel ratio. Next, when the time C ₀ elapses after the rich control is started, the correction coefficient K is made smaller than 1.0 again, and the combustion of the lean mixture is started again.

[0021] NO _x Most of the air-fuel ratio of the mixture supplied into the combustion chamber 3 is absorbed in _the NO _x absorbent 18 becomes rich (K = KK) is suddenly released. The value of the correction coefficient KK is O ₂ on this time the platinum Pt ^- are defined as unburned HC consumed and the amount needed to reduce the total NO _x to, CO is generated. In this case, as the temperature of the NO _x absorbent 18 an exhaust gas temperature becomes high is increased NO
The amount of NO _x released from the _x absorbent 18 increases. Therefore, as shown in FIG. 7A, the value of the correction coefficient KK increases as the exhaust gas temperature T increases. FIG.
The relationship between the correction coefficient KK and the exhaust gas temperature T shown in (A) is stored in the ROM 32 in advance.

In this case, the exhaust gas temperature T can be directly detected, but can also be estimated from the intake air amount Q and the engine speed N. Therefore, in the embodiment according to the present invention, the relationship among the exhaust gas temperature T, the intake air amount Q, and the engine speed N is obtained in advance by experiments, and this relationship is stored in the ROM 32 in advance in the form of a map as shown in FIG. The exhaust gas temperature T is calculated from this map.

On the other hand, if the air-fuel ratio of the air-fuel mixture supplied into the combustion chamber 3 becomes rich (K = KK) as described above, NO
Most of the NO _x absorbed in the _x absorbent 18 is rapidly released, and thereafter, even if the air-fuel ratio is made rich, NO
NO _x is released from the _x absorbent 18 little by little. Therefore, if the air-fuel ratio is kept rich, unburned HC, CO
Will be released to the atmosphere. So after the rich (K = KK) the air-fuel ratio as shown in Figure 6 to reduce the degree of rich little by little, followed by keeping the air-fuel ratio to the stoichiometric air-fuel ratio (K = 1.0) NO _x so that allowed to successively reduce the NO _x released little by little from the absorbent 18.

When the air-fuel ratio is made rich, NO _x
The amount of the NO _x released as from then _the NO _x absorbent 18 an amount of the NO _x often released from the absorbent 18 is decreased, therefore _the NO _x absorbent 18 is shorter the time until you release the NO _x It becomes scary. The amount of the NO _x released from _the NO _x absorbent 18 when the air-fuel ratio as the exhaust gas temperature T becomes higher as described above was rich is increased, thus 7
As shown in (B), the time C ₀ from when the air-fuel ratio is made rich until it is returned to lean again becomes shorter as the exhaust gas temperature T becomes higher. Note that the relationship between the time C ₀ and the exhaust gas temperature T shown in FIG. 7B is stored in the ROM 32 in advance.

As shown in FIG. 3, in the embodiment according to the present invention, the mixture supplied to the combustion chamber 3 is made rich at the beginning of the warm-up operation and at the time of full load operation, and the mixture is made stoichiometric during the acceleration operation. Fuel ratio. Therefore, N
NO _x is released from the O _x absorbent 18. However the NO _x from the NO _x absorbent 18 only in such a rich operation region when the frequency is less engine is operated in the rich operation region is predetermined by the operating state of the engine such as during full-load operation If the release is performed, the NO _x absorption capacity of the NO _x absorbent 18 becomes saturated during the lean operation, so that the NO _x absorbent 18 cannot absorb NO _x . Therefore, in this embodiment of the present invention by the air-fuel ratio of the inflowing exhaust gas even when these non-rich operation region rich _the NO _x absorbent 1
8 to release NO _x .

However, in an internal combustion engine, the engine load increases.
And NO emitted from the engine_xThe amount of
NO when load is high_xNO from absorbent 18_xReleased
When the air-fuel ratio of the inflow exhaust gas is made rich to
All NO_xThe amount of unburned HC and CO to reduce
Must be increased, ie in the embodiment according to the invention
Increase the degree of richness of the air-fuel mixture supplied into the combustion chamber 3
You have to do it. However, combustion chamber 3
Increasing the richness of the air-fuel mixture supplied to the
The fuel consumption rate worsens and the engine output fluctuates widely
Must avoid increasing the degree of richness in
Absent. Therefore, in the present invention, the NO from the engine is _xLow emissions
Rich engine air-fuel ratio during low engine load operation
I try to. Thus, when the engine is operating at low load,
If the air-fuel ratio of the inflow exhaust gas is made rich, all NO_xReduce
You don't have to increase the richness
Thus, the fuel consumption rate can be improved, and
Thus, fluctuations in the engine output torque can be suppressed. This is
This is the basis of the rich control according to the present invention.

In FIG. 6, the longer the time C ₀ , the worse the fuel consumption rate becomes. Therefore, to improve the fuel consumption rate, it is preferable to make the time C ₀ as short as possible. It is sufficient to increase the _the NO _x releasing amount from _the NO _x absorbent 18 when the inflow exhaust gas rich in order to short the time C _0, the exhaust when the exhaust gas flowing into the rich for its gas temperature T and _the NO _x absorbent 1
It is only necessary to raise the temperature of 8. Therefore, in the embodiment according to the present invention, the inflow exhaust gas is made rich during the low-load operation immediately after the throttle valve 15 is closed. That is, before the throttle valve 15 is closed, the engine is operated with a high engine load, so that the exhaust gas temperature T is high, and therefore the temperature of the NO _x absorbent 18 is also high. Therefore, immediately after the throttle valve 15 is closed, the temperature of the NO _x absorbent 18 is high, and the temperatures of the combustion chamber 3 and the exhaust system are high, so that the exhaust gas temperature T is also considerably high. Thus the throttle valve 15 to the air-fuel ratio of the inflowing gas immediately after closing rich large amount of the NO _x is released from _the NO _x absorbent 18, will be able thus to shorter time C ₀ in.

Further, when the throttle valve 15 is closed, the engine output torque decreases. Therefore, at this time, even if the air-fuel ratio of the inflowing exhaust gas is made rich, the driver does not feel the fluctuation of the engine output torque. Further, in the internal combustion engine in which the fuel injection is stopped when the throttle valve 15 is closed and the deceleration operation is started, the rich control of the inflowing exhaust gas is performed immediately after the throttle valve 15 is closed. Even if the injection of fuel is stopped, the driver does not feel the fluctuation of the engine output torque. Therefore, it is preferable to perform the rich control of the inflowing exhaust gas especially immediately after the shift to the engine low load operation even during the engine low load operation.

Further, if the rich control of the inflowing exhaust gas is performed every time the engine is shifted to the low load operation, the fuel consumption rate inevitably deteriorates. Therefore, in this embodiment of the present invention _the NO _x absorbent 1
Estimating the amount of NO _x absorbed in the 8, NO _x absorbent 1
After _the amount of NO _x absorbed in the 8 is estimated to exceed a certain amount, and to perform rich control of the inflowing exhaust gas when the transition to engine low load operation.

Next, with reference to FIGS. 9 to 12, one embodiment of the control of the absorption and release of the NO _x absorbent 18 according to the present invention will be described. FIG. 9 shows a processing routine for a fuel cut flag indicating that fuel injection should be stopped, and this routine is executed by interruption every predetermined time. Referring to FIG. 9, first, in step 50, it is determined whether or not a cut condition satisfaction flag indicating that a condition for stopping fuel injection has been satisfied has been set. If the cut condition satisfaction flag is not set, step 51
Then, it is determined whether or not the throttle valve 15 is at the idling opening based on the output signal of the throttle sensor 20. If it is the idling opening, the routine proceeds to step 52, where the engine speed N is set to a predetermined set value, for example, 12
00r. p. It is determined whether or not it is higher than m, and N> 120
0r. p. If it is m, the process proceeds to step 53. That is, when the throttle valve 15 has the idling opening degree and N> 120
0r. p. If m, it is determined that the vehicle is in deceleration operation, and the routine proceeds to step 53, where a cut condition satisfaction flag is set.

Next, at step 54, the NO _x absorbent 18
Run flag indicating that the rich control is performed for releasing the NO _x is whether it is set is determined from the. This execution flag is controlled in the routine shown in FIGS. If the execution flag has not been set, the routine proceeds to step 55, where the fuel cut flag is set. When the fuel cut flag is set, fuel injection is stopped. On the other hand, when it is determined in step 54 that the execution flag is not set, the processing cycle is completed, and therefore, at this time, the fuel injection is not stopped.

When the cut condition satisfaction flag is set, the routine proceeds to step 56, where it is determined whether or not the throttle valve 15 has been opened based on the output signal of the throttle sensor 20.
If the throttle valve 15 has not been opened, step 5
7, the engine speed N becomes a predetermined constant value, for example, 900 r. p. It is determined whether it has become lower than m. N ≧ 900r. p. If m, the processing cycle is completed. On the other hand, when it is determined in step 56 that the throttle valve 15 has been opened, or in step 5
7, N <900r. p. When it is determined that m is satisfied, the routine proceeds to step 58, where the cutting condition satisfaction flag is reset. Next, at step 59, the fuel cut flag is reset, and fuel injection is started.

FIGS. 10 and 11 show a routine for calculating the correction coefficient KK during the rich control. This routine is executed by interruption every predetermined time. Referring to FIGS. 10 and 11, first, at step 60, it is determined whether or not the correction coefficient K is smaller than 1.0, that is, whether or not the lean air-fuel mixture is being burned. K ≧
When 1.0, that is, when the air-fuel mixture supplied into the combustion chamber 3 is at the stoichiometric air-fuel ratio or rich, the processing cycle is completed. When K <1.0 In contrast, i.e. _the amount of NO _x W that is absorbed in the NO _x absorbent 18 proceeds to step 61 is calculated when the lean air-fuel mixture is burned. That is, the NO _x amount discharged from the combustion chamber 3 increases as the intake air amount Q increases, and increases as the engine load Q / N increases, so that the NO _x amount W absorbed by the NO _x absorbent 18 is W And k ₁ · Q · Q / N (k ₁ is a constant).

Next, at step 62, the execution flag is set.
It is determined whether or not it has been turned on. Execution flag set
If not, go to step 63 and throttle
The opening degree θ of the throttle valve 15 based on the output signal of the sensor 20
Is the preset opening θ ₀Is smaller than
Separated. Note that this set opening θ₀Is the idling opening
And slightly larger than θ, so that θ <θ₀Is low load
It means that driving is being performed. θ ≦ θ₀Noto
When the processing cycle is completed, θ <θ₀When
Proceed to step 64.

In step 64, NO_xAbsorbed by absorbent 18
NO_xThe amount W is a predetermined set amount W₀Than
Is also determined. This set amount W₀Is for example
NO _xMaximum NO that can be absorbed by the absorbent 18_x30 par of quantity
It is about a cent. W ≦ W₀If so, complete the processing cycle
W> W₀If so, proceed to step 65 to execute
Is set. So the execution flag is set
Is W> W when shifting to low load operation₀Is
It is. Note that the execution flag is set as shown in FIG.
If this is done, the conditions under which fuel injection should be stopped at this time
Fuel injection cannot be stopped even if
No.

When the execution flag is set, step 66 is executed.
In, the correction coefficient KK is calculated from the relationship shown in FIG. 7A and the map shown in FIG. Next, at step 67, the final correction coefficient KK is calculated by multiplying KK by k ₂ · W (k ₂ is a constant). That is, as the amount of NO _x W that is absorbed in the NO _x absorbent 18 is small degree of rich (KK) is reduced. Then step 68
Then, the time C ₀ is calculated from the relationship shown in FIG. 7B and the map shown in FIG. Next, at step 69, the final time C ₀ is calculated by multiplying C ₀ by k ₃ · W (k ₃ is a constant). That, NO _x absorbent 18 NO _x amount W smaller the time C ₀ has been absorbed is short hidden. Then the processing cycle is completed.

When the execution flag is set, in the next processing cycle, from step 62 in FIG. 10 to step 7 in FIG.
_The NO _x releasing flag is set the routine proceeds to 0. Next, at step 71, the count value C is incremented by one. Next, at step 72, the count value C is changed to the time C _0.
Then, it is determined whether or not the time _C0 has elapsed since the start of the rich control. C ≦ C ₀
In step 73, the routine proceeds to step 73, where a constant value α is subtracted from the correction coefficient KK. Next, at step 74, the correction coefficient K
It is determined whether K has become smaller than 1.0. K
When K ≦ 1.0, the routine proceeds to step 75, where KK becomes 1.0
It is said. Accordingly, as shown in FIG. 6, the correction coefficient KK gradually decreases, and is then maintained at 1.0.

Next, when C> C ₀ , the process proceeds from step 72 to step 76 where the execution flag is reset, and then at 77 the NO _x release flag is reset. Then the amount of NO _x W that is absorbed in the NO _x absorbent 18 at step 78 is zero, then the count value C is made zero at step 79. Next, at step 80, it is determined whether or not the cut condition satisfaction flag is set. If the cut condition satisfaction flag is set, the routine proceeds to step 81, where the fuel cut flag is set. That is, if the condition for stopping the fuel injection is still satisfied, the routine proceeds to step 81, where the fuel cut flag is set, and the fuel injection is stopped.

FIG. 12 shows a routine for calculating the fuel injection time TAU, and this routine is repeatedly executed. Referring to FIG. 12, first in step 90, FIG.
The basic fuel injection time TP is calculated from the map shown in FIG.
Next, at step 91, it is determined from the output signal of the water temperature sensor 21 whether or not the warm-up operation is being performed. During the warm-up operation, the routine proceeds to step 93, where the value A, which is a function of the engine cooling water temperature, is set as the correction coefficient K, and then the routine proceeds to step 96. on the other hand,
When the warm-up operation is completed, the routine proceeds to step 92, where NO
It is determined whether or not the _x release flag is set.
Is a predetermined value B by the operation state of the engine proceeds to step 94 is the correction coefficient K when the _the NO _x releasing flag has not been set. For example, K =
1.0, K = 1.3 during full load operation, and K = 0.6 in other operation states. Next, the routine proceeds to step 96.

On the other hand, is the correction coefficient KK is K which has been calculated by the routine of FIG. 10 and FIG. 11 proceeds to step 95 when the _the NO _x releasing flag is judged as being set in step 92, then step 9
Proceed to 6. At step 96, it is determined whether or not the fuel cut flag is set. If the fuel cut flag is not set, the routine jumps to step 98.
On the other hand, when the fuel cut flag is set, the routine proceeds to step 97, where the correction coefficient K is made zero, and then proceeds to step 98. In step 98, the fuel injection time TAU is calculated by multiplying the basic fuel injection time TP by the correction coefficient K.

[0041]

Without deteriorating the fuel consumption rate, according to the present invention, moreover it is possible to release the NO _x from the NO _x absorbent without causing large torque fluctuation.

[Brief description of the drawings]

FIG. 1 is an overall view of an internal combustion engine.

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

FIG. 3 is a diagram showing a change in a correction coefficient K;

FIG. 4 shows unburned HC and C in exhaust gas discharged from the engine.
FIG. 3 is a diagram schematically showing the concentrations of O and oxygen.

FIG. 5 is a diagram for explaining a NO _x absorption / release effect.

FIG. 6 is a diagram showing a change in a correction coefficient K during rich control.

FIG. 7 is a diagram showing a relationship between a correction coefficient KK, a time C ₀ and an exhaust gas temperature T.

FIG. 8 is a view showing a map of an exhaust gas temperature T;

FIG. 9 is a flowchart for processing a fuel cut flag.

FIG. 10 is a flowchart for calculating a correction coefficient KK.

FIG. 11 is a flowchart for calculating a correction coefficient KK.

FIG. 12 is a flowchart for calculating a fuel injection time TAU.

[Explanation of symbols]

11: fuel injection valve 15: throttle valve 16: exhaust manifold 18: NO _x absorbent

──────────────────────────────────────────────────の Continuation of the front page (51) Int.Cl. ⁶ Identification number Agency reference number FI Technical display F01N 3/20 F02D 45/00 301G 3/28 301 B01D 53/34 129A F02D 41/04 305 53 / 36 101Z 45/00 301 (72) Inventor Kenji Kato 1 Toyota Town, Toyota City, Aichi Prefecture Toyota Motor Corporation (72) Inventor Tetsu Iguchi 1 Toyota Town Toyota City, Aichi Prefecture Toyota Motor Corporation (56 References JP-A-3-135417 (JP, A) JP-A-1-247710 (JP, A) JP-A-62-117620 (JP, A) JP-A-62-106826 (JP, A) 62-97630 (JP, A)

Claims (1)

(57) [Claims] 1. A internal combustion engine so as to allowed to burn a lean mixture in most engine operating region of the air-fuel ratio of the inflowing exhaust gas is absorbed NO _x when the lean,
NO _x absorbed when the oxygen concentration in the incoming exhaust gas decreases
The _the NO _x absorbent to release disposed in the engine exhaust passage and a low-load engine load when there is a rich operation region is predetermined except when its load less than or equal to a predetermined by the engine operating condition the air-fuel ratio of the exhaust gas flowing into _the NO _x absorbent rich when they become, the exhaust purification system of an internal combustion engine so as to reduce the released NO _x with the release of NO _x from the NO _x absorbent.