JP2743764B2 - 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]

2. Description of the Related Art In an internal combustion engine in which a lean air-fuel mixture is burned, when the air-fuel ratio of the inflowing exhaust gas is lean, NOx is absorbed, and when the inflowing exhaust gas becomes stoichiometric or rich, the absorbed NOx is released. The NOx absorbent is disposed in the engine exhaust passage, and the NOx generated when the lean air-fuel mixture is burned is absorbed by the NOx absorbent, and the air-fuel ratio of the air-fuel mixture supplied to the engine cylinder as in the acceleration operation. An internal combustion engine has been proposed by the present applicant that releases NOx from a NOx absorbent and reduces the released NOx when the air-fuel ratio becomes rich or becomes stoichiometric (Japanese Patent Application No. 3-284095). reference).

[0003]

However, if the air-fuel ratio of the air-fuel mixture supplied to the engine cylinder is always set to the stoichiometric air-fuel ratio or rich during the acceleration operation as described above, each time the acceleration operation is performed, Although NOx is released from the NOx absorbent, it is possible to prevent the NOx absorbent from saturating the NOx absorbing ability, but there is a problem that fuel consumption increases. On the other hand, if the air-fuel ratio of the mixture supplied to the engine cylinder is always lean during acceleration operation to reduce fuel consumption, a large amount of NOx is discharged from the engine during acceleration operation. In some cases, the absorption capacity of the NOx absorbent is saturated, thus causing a problem that NOx is released into the atmosphere.

[0004]

According to the present invention, in order to solve the above-described problems, NOx is absorbed when the inflowing exhaust gas is lean, and when the inflowing exhaust gas becomes stoichiometric air-fuel ratio or rich. A NOx absorbent releasing the absorbed NOx is disposed in the engine exhaust passage, and a means for estimating the amount of NOx absorbed by the NOx absorbent and a means for detecting the acceleration operation are provided. When the estimated NOx amount absorbed into the engine cylinder is equal to or less than the set value, the air-fuel ratio of the air-fuel mixture supplied to the engine cylinder is made lean. When the estimated NOx amount is equal to or more than the set value, the air-fuel ratio of the air-fuel mixture supplied into the engine cylinder is reduced. The fuel ratio is set to the stoichiometric air-fuel ratio or rich.

[0005]

When the estimated NOx amount absorbed by the NOx absorbent is equal to or less than the set value, even if the air-fuel ratio of the air-fuel mixture supplied into the engine cylinder during the acceleration operation is lean, the Nx of the NOx absorbent is reduced.
There is no danger of saturation of the Ox absorption capacity. On the other hand, N
When the estimated NOx amount absorbed by the Ox absorbent is equal to or greater than the set value, the acceleration operation is performed. At this time, if the air-fuel ratio of the air-fuel mixture supplied into the engine cylinder is made lean, the NOx absorption capacity of the NOx absorbent is saturated. There is a risk of doing. Therefore, at this time, the air-fuel ratio of the air-fuel mixture supplied into the engine cylinder is made stoichiometric or rich.

[0006]

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 is connected to an air cleaner 13 via an intake duct 12, and a throttle valve 14 is arranged in the intake duct 12. On the other hand, the exhaust port 8 is connected to a casing 18 containing a NOx absorbent 17 via an exhaust manifold 15 and an exhaust pipe 16.

The electronic control unit 30 is composed of a digital computer, and a ROM (read only memory) 32, a RAM (random access memory) 33, a CPU (microprocessor) 34, and an input port 35 interconnected by a bidirectional bus 31. And an output port 36. A pressure sensor 19 that generates an output voltage proportional to the absolute pressure in the surge tank 10 is mounted in the surge tank 10, and the output voltage of the pressure sensor 19 is input to an input port 35 via an AD converter 37. . The throttle valve 14 is provided with a throttle sensor 20 that generates an output pulse each time the throttle valve 14 rotates by a predetermined angle and generates an output signal indicating that the throttle valve 14 is at the idling position. These output pulses and output signals are input to the input port 35, and the CPU 34 calculates the valve opening speed of the throttle valve 14 from the output pulses.

A temperature sensor 21 for generating an output voltage proportional to the exhaust gas temperature is provided in the exhaust pipe 16 upstream of the casing 18.
The output voltage of the temperature sensor 21 is input to the input port 35 via the AD converter 38. Furthermore,
A transmission 23 connected to the engine crankshaft 22 is provided with a shift change sensor 24 for generating an output signal indicating that a shift operation is being performed. The output signal of the shift change sensor 24 is input to an input port 35. .
The input port 35 is connected to a rotation speed sensor 25 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 a corresponding drive circuit 39, 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,
Are stored in the ROM 32 in advance in the form of a map as shown in FIG. 2 as a function of the absolute pressure PM and the engine speed N. 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, when 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 when K> 1.0, the air-fuel ratio is supplied into the engine cylinder. The air-fuel ratio of the air-fuel mixture is smaller than the stoichiometric air-fuel ratio, that is, rich.

The correction coefficient K is controlled in accordance with the operating state of the engine. FIG. 3 shows an embodiment of the control of the correction coefficient K except during the acceleration operation. 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 becomes a constant value smaller than 1.0, that is, the engine The air-fuel ratio of the air-fuel mixture supplied into the cylinder is maintained lean. Next, if the idling operation is performed, the correction coefficient K becomes, for example, 1.
0, that is, the air-fuel ratio of the air-fuel mixture supplied into the engine cylinder is the stoichiometric air-fuel ratio, and the correction coefficient K is made larger than 1.0 if deceleration operation or shift change of the transmission 23 is performed. That is, the air-fuel ratio of the air-fuel mixture supplied into the engine cylinder is made rich. As can be seen from FIG.
In the embodiment shown in the above, 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, idling operation, deceleration operation and shift change, and therefore, The lean mixture will burn in most engine operating ranges.

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 concentration of unburned HC and CO in the exhaust gas discharged from the combustion chamber 3 increases as the air-fuel ratio of the air-fuel mixture supplied into the combustion chamber 3 increases, and the concentration of the exhaust gas from the combustion chamber 3 increases The concentration 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.

NOx contained in casing 18
The absorbent 17 is made of, for example, alumina as a carrier. On this carrier, for example, alkali metal such as potassium K, sodium Na, lithium Li, cesium Cs, alkaline earth such as barium Ba, calcium Ca, lanthanum La, yttrium Y are used. At least one selected from such rare earths and a noble metal such as platinum Pt are supported. The ratio of air and fuel (hydrocarbon) supplied to the engine intake passage and the exhaust passage upstream of the NOx absorbent 17 is referred to as the air-fuel ratio of the exhaust gas flowing into the NOx absorbent 17.
The x-absorbent 17 absorbs NOx when the air-fuel ratio of the inflowing exhaust gas is lean, and performs an absorption / release operation of the NOx that releases the absorbed NOx when the oxygen concentration in the inflowing exhaust gas decreases. When fuel (hydrocarbon) or air is not supplied into the exhaust passage upstream of the NOx absorbent 17, the air-fuel ratio of the inflowing exhaust gas matches the air-fuel ratio of the air-fuel mixture supplied into the combustion chamber 3. In this case, the NOx absorbent 17 absorbs NOx when the air-fuel ratio of the mixture supplied to the combustion chamber 3 is lean, and absorbs when the oxygen concentration in the mixture supplied to the combustion chamber 3 decreases. NOx will be released.

If the above-mentioned NOx absorbent 17 is arranged in the engine exhaust passage, the NOx absorbent 17 actually performs the NOx absorbing / releasing action, but there are some parts where the mechanism of this absorbing / releasing action is not clear. 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 other noble metals, alkali metals, alkaline earths,
The same mechanism is obtained even when rare earth elements are used.

That is, the exhaust gas flowing in becomes considerably lean.
And the oxygen concentration in the exhaust gas greatly increased, and FIG.
As shown, these oxygen O_TwoIs O_Two ^-Or O^2-In the form of
It adheres to the surface of platinum Pt. On the other hand, N
O is O on the surface of platinum Pt_Two ^-Or O^2-Reacts with NO
_Two(2NO + O_Two→ 2NO_Two). Then generated
NO_TwoSome of the oxygen is oxidized on the platinum Pt and
FIG. 5 shows a state of being absorbed and binding with barium oxide BaO.
As shown in (A), nitrate ion NO_Three ^-Absorbed in the form of
Diffuses into agent. In this way, NOx is converted to NOx absorbent
17 is absorbed.

The NO ₂ is produced on the surface of the platinum Pt so long as the oxygen concentration in the inflowing exhaust gas is high, as long as NO ₂ to NOx absorption ability of the absorbent is not saturated it is absorbed in the absorbent and nitrate ions 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. That is, when the oxygen concentration in the inflowing exhaust gas decreases, NOx is released from the NOx absorbent 17. As shown in FIG. 4, if the degree of leanness of the inflowing exhaust gas decreases, the oxygen concentration in the inflowing exhaust gas decreases. Therefore, if the degree of leanness of the inflowing exhaust gas decreases, the air-fuel ratio of the inflowing exhaust gas decreases. Even NOx absorbent 17
Will release NOx.

On the other hand, at this time, when the air-fuel mixture supplied into the combustion chamber 3 is made rich and the air-fuel ratio of the inflowing exhaust gas becomes rich, as shown in FIG.
C and CO are discharged, and these unburned HC and CO are converted to platinum Pt.
It reacts with the above oxygen O ₂ ^- or O ^2- to be oxidized.
Further, when the air-fuel ratio of the inflowing exhaust gas becomes rich, the oxygen concentration in the inflowing exhaust gas extremely decreases.
O ₂ is released, and this NO ₂ is reduced by reacting with unburned HC and CO as shown in FIG. 5 (B). When NO ₂ is no longer present on the surface of the platinum Pt, NO ₂ is released from the absorbent one after another. Therefore, when the air-fuel ratio of the inflowing exhaust gas is made rich, NOx is released from the NOx absorbent 17 in a short time.

[0017] That is, first of all unburned HC and air-fuel ratio to the rich inflowing exhaust gas, CO is O ₂ on the platinum Pt ^- is allowed or O ^2- and immediately react with oxidized, then O on the platinum Pt ₂ ^- or O ^2- is still unburned HC be consumed,
If CO remains, the unburned HC, NOx released from the absorbent by the CO and NOx discharged from the engine
Is reduced. Therefore, if the air-fuel ratio of the inflowing exhaust gas is made rich, the NOx absorbed in the NOx absorbent 17 is released in a short time, and the released NOx is released.
Since x is reduced, NOx can be prevented from being discharged into the atmosphere. Further, since the NOx absorbent 17 has a function of a reduction catalyst, even if the air-fuel ratio of the inflowing exhaust gas is set to the stoichiometric air-fuel ratio, the NOx released from the NOx absorbent 17 is reduced. However, when the air-fuel ratio of the inflowing exhaust gas is set to the stoichiometric air-fuel ratio, NOx is only gradually released from the NOx absorbent 17, so that N
It takes a slightly longer time to release all the NOx absorbed by the Ox absorbent 17.

If the air-fuel ratio of the inflowing exhaust gas is made rich as described above, NOx is released from the NOx absorbent 17 in a short time, but if the degree of the richness is made too high at this time,
That is, if the amount of unburned HC and CO becomes excessively large, excess unburned HC and CO will be released into the atmosphere. At this time, if the degree of richness is too low, that is, unburned HC, CO
If the amount of CO is too small, the NOx absorbent 1
7 would not be able to release all NOx.
Therefore, in order to reduce all NOx released from the absorbent and all NOx discharged from the engine when the air-fuel ratio of the inflowing exhaust gas is made rich, O ₂ ^- or O ^2- on platinum Pt is required.
Of unburned HC and CO required to consume CO2 and total NOx
It is necessary to control the degree of the richness of the air-fuel ratio of the inflowing exhaust gas so that the unburned HC and CO required for reducing the NOx flow into the NOx absorbent 17.

When NOx is being released from the NOx absorbent 17, most of the unburned HC and CO are converted to NOx.
Since it is used to reduce the NOx released from the absorbent 17, the degree of richness when the air-fuel ratio of the inflowing exhaust gas is made rich is absorbed by the NOx absorbent 17.
The higher the x amount, the higher the amount of NOx can be satisfactorily released from the NOx absorbent 17 while preventing excess unburned HC and CO from being released into the atmosphere.

Therefore, in the embodiment according to the present invention, NO
the ratio of the amount of NOx absorbed by the x-absorbent 17 to the maximum amount of NOx that the NOx absorbent 17 can absorb, that is, NOx
The saturation S is estimated, and the degree of richness of the inflowing exhaust gas when NOx is to be released from the NOx absorbent 17 is increased as the NOx saturation S increases. That is,
As can be seen from FIG. 6, the air-fuel ratio of the air-fuel mixture normally supplied into the engine cylinder is lean, and at this time, N
Since Ox continues to be absorbed by the NOx absorbent 17, NOx
The saturation S continues to increase.

On the other hand, in the embodiment according to the present invention, the air-fuel ratio of the air-fuel mixture supplied into the engine cylinder is temporarily made rich before the supply of fuel is stopped at the start of deceleration. It increases as the NOx saturation S increases. Therefore, at this time, the NOx saturation S falls to zero. The mixture is also made rich at the time of a shift change, and the richness at this time is also increased as the NOx saturation S increases. Therefore, also at this time, the NOx saturation S falls to zero. During the idling operation, the air-fuel mixture is set to the stoichiometric air-fuel ratio. At this time, NOx is only gradually released from the NOx absorbent 17, so that the NOx saturation S gradually decreases.

Next, the air-fuel ratio control during the acceleration operation will be described. The acceleration operation is roughly classified into a rapid acceleration operation in which a rapid increase in engine output is required and a gentle acceleration operation in which the engine output only needs to increase slowly. At the time of rapid acceleration, a rapid increase in the engine output is required, so that the air-fuel ratio of the air-fuel mixture supplied into the engine cylinder must be set to the stoichiometric air-fuel ratio or rich. Therefore, the embodiment according to the present invention is shown in FIG. As described above, at the time of rapid acceleration, the air-fuel ratio of the air-fuel mixture supplied into the engine cylinder is set to the stoichiometric air-fuel ratio. Therefore, when the rapid acceleration operation is performed, the NOx saturation S gradually decreases. It should be noted that such a rapid acceleration operation is not performed so frequently, and therefore, even if the air-fuel mixture is set to the stoichiometric air-fuel ratio during the rapid acceleration, the fuel consumption is not significantly affected.

On the other hand, the slow acceleration operation is frequently used. Therefore, if the air-fuel ratio of the air-fuel mixture supplied to the engine cylinder is made lean at this time, the fuel consumption can be greatly reduced. Further, since the engine output only needs to rise slowly during the moderate acceleration operation, there is no particular problem in terms of vehicle drivability even if the air-fuel ratio of the air-fuel mixture supplied into the engine cylinder is made lean. Therefore, in the embodiment according to the present invention, basically, the air-fuel ratio of the air-fuel mixture supplied into the engine cylinder during the moderate acceleration operation is made lean.

However, if the air-fuel ratio of the air-fuel mixture supplied to the engine cylinder during the acceleration operation is made lean, a large amount of NOx is discharged from the engine. Therefore, at this time, if the NOx saturation S of the NOx absorbent 17 is high, the NOx absorption capacity of the NOx absorbent 17 is saturated, and thus NOx is released to the atmosphere. Therefore, in the embodiment according to the present invention, when the slow acceleration operation is started, NOx
When the saturation S is higher than the set value S ₀ (FIG. 6) or when the NOx saturation S becomes higher than the set value S ₀ after the slow acceleration operation is performed, the mixture supplied to the engine cylinder is reduced. and the air-fuel ratio at the stoichiometric air-fuel ratio prevents the NOx from being discharged into the atmosphere, when NOx saturation S is lower than the set value S ₀ when slow acceleration operation is performed on this N
Since there is no risk that the NOx absorption capacity of the Ox absorbent 17 is saturated, the air-fuel ratio of the mixture supplied to the engine cylinder is made lean as shown in FIG. 6 in order to reduce the fuel consumption.

Next, a method of calculating the amount of NOx absorbed by the NOx absorbent 17 will be described with reference to FIGS.
FIG. 7 shows a routine for calculating the NOx absorption amount, and this routine is executed by interruption every predetermined time. Referring to FIG. 7, first, at step 100, it is determined whether or not the correction coefficient K for the basic fuel injection time TP is zero. When K = 0, that is, when the fuel injection is stopped, the processing cycle is completed. On the other hand, when K = 0, that is, when fuel injection is being performed, the routine proceeds to step 101, where it is determined whether the correction coefficient K is smaller than 1.0.

When K <1.0, that is, when the lean air-fuel mixture is being burned, the routine proceeds to step 102, where the NOx absorption amount A estimated to be absorbed by the NOx absorbent 17 during the time interruption interval is calculated. Is calculated. As shown in FIG. 8A, the amount of NOx discharged from the engine increases as the absolute pressure PM in the surge tank 10 increases.
As shown in (B), the number increases as the engine speed N increases. This NOx absorption amount A is stored in the ROM 32 in advance in the form of a map as shown in FIG. 8C as a function of the absolute pressure PM in the surge tank 10 and the engine speed N. Therefore, in step 102, the NOx absorption amount A is calculated from the map shown in FIG. 8C based on the output signals of the pressure sensor 19 and the rotation speed sensor 25. Next, at step 103, the NOx absorption amount A is added to the total NOx absorption amount C, and then the routine proceeds to step 106. Therefore, when the lean air-fuel mixture is being burned, the NOx absorbent 17
Is gradually increased.

On the other hand, in step 101, K ≧ 1.0
When it is determined that the air-fuel ratio of the air-fuel mixture supplied into the engine cylinder is the stoichiometric air-fuel ratio or rich, the process proceeds to step 104, where it is estimated that the air-fuel mixture is released from the NOx absorbent 17 during the time interruption interval. The calculated NOx release amount B is calculated. As shown in FIG. 9A, the amount of NOx released from the NOx absorbent 17 increases as the temperature of the NOx absorbent 17, that is, the exhaust gas temperature T increases.
As shown in (B), as the correction coefficient K increases,
That is, the higher the degree of richness of the air-fuel mixture supplied into the engine cylinder, the greater the amount. This NOx release amount B is stored in advance in the ROM 32 in the form of a map as shown in FIG. 9C as a function of the exhaust gas temperature T and the correction coefficient K. Therefore, in step 104, based on the output signal of the temperature sensor 21 and the correction coefficient K, N is calculated from the map shown in FIG.
The Ox release amount B is calculated. Next, at step 104, the NOx release amount B is subtracted from the total NOx absorption amount C, and then the routine proceeds to step 106. Therefore, when the air-fuel ratio of the air-fuel mixture supplied into the engine cylinder becomes the stoichiometric air-fuel ratio or rich, the total NOx absorption amount C absorbed by the NOx absorbent 17 is reduced.

In step 106, it is determined whether or not the total NOx absorption amount C has become negative. When C ≧ 0, the process jumps to step 108, and when C <0, the process proceeds to step 10.
After setting C = 0 in step 7, the process proceeds to step 108. In step 108, the total NOx absorption amount C absorbed by the NOx absorbent 17 and the maximum N that can be absorbed by the NOx absorbent 17
NOx saturation S, which is the ratio of the Ox absorption C ₀ is calculated. The value of the correction coefficient K for enriching the air-fuel ratio of the air-fuel mixture supplied into the engine cylinder when NOx is to be released from the NOx absorbent 17 is a function of the NOx saturation S, and the correction coefficient K Is N as shown in FIG.
It increases as the Ox saturation S increases.

FIGS. 11 to 13 show a routine for calculating the fuel injection time TAU. 11 to 1
Referring to FIG. 3, first in step 201, FIG.
The basic fuel injection time TP is calculated from the map shown in FIG.
Next, at step 202, the target correction coefficient K ₀ is calculated. The target correction coefficient K ₀ is basically has set to K ₀ <1.0, for example K ₀ = 0.6 to perform the lean mixture combustion, K ₀ when the specific operating conditions such as idling = 1.0. Next, the routine proceeds to step 203.

In steps 203 to 214, during deceleration operation, first, the air-fuel ratio of the air-fuel mixture supplied into the engine cylinder is made rich for a certain period of time, and then the processing of stopping the supply of fuel is performed. That is, in step 203, it is determined whether or not a cut flag indicating that fuel injection should be stopped is set. When the cut flag is reset, the routine proceeds to step 204, where it is determined whether or not the throttle valve 14 is at the idling opening based on the output signal of the throttle sensor 20. Step 2 when the throttle valve 14 is at the idling opening
Proceeding to 05, it is determined whether or not the engine speed N is higher than the fuel cut speed, for example, 1100 rpm. Throttle valve 14 is not idling, or N ≦ 1
If it is 100 rpm, the process jumps to step 215. On the other hand, when the throttle valve 14 has the idling opening degree and N> 1100 rpm, it is determined that the fuel injection is to be stopped during the deceleration operation, and the routine proceeds to step 206.

At step 206, a cut flag indicating that fuel injection should be stopped is set, then at step 207, a rich flag 1 indicating that the air-fuel mixture should be made rich is set, and then at step 208 NOx
The correction coefficient K is calculated from the saturation S (FIG. 10). As will be described later, the mixture is made rich while both the cut flag and the rich flag 1 are set, and then the fuel injection is stopped when the rich flag 1 is reset.

When the cut flag is set, step 2
From 03, the routine proceeds to step 209, where it is determined whether or not the throttle valve 14 is at the idling opening. If the throttle valve 14 is at the idling opening, step 21
0, the engine speed N becomes the return speed, for example, 800 r.
It is determined whether it is lower than pm. N ≧ 800
At rpm, the routine proceeds to step 211, where it is determined whether or not a predetermined time has elapsed since the rich flag 1 was set. If the fixed time has not elapsed, step 215
When the predetermined time has elapsed, the routine proceeds to step 212, where the rich flag 1 is reset. Therefore, the rich flag 1 is kept set for a certain period of time, during which the mixture is made rich.

On the other hand, if the throttle valve 14 is opened during the deceleration operation, the routine proceeds from step 209 to step 2
When the engine speed N becomes equal to or less than 800 rpm, the process proceeds from step 210 to step 213. In step 213, the cut flag is reset,
In step 214, the rich flag 1 is reset.

In steps 215 to 217, a process for enriching the air-fuel ratio of the air-fuel mixture at the time of the shift change of the transmission 23 is performed. That is, in step 215, the transmission 23 is controlled based on the output signal of the shift change sensor 24.
It is determined whether or not the shift change operation is performed. When the shift change operation is being performed, the routine proceeds to step 216, where a rich flag 2 indicating that the air-fuel mixture should be made rich is set. Then, at step 217, the correction coefficient K is calculated from the NOx saturation S (FIG. 10). You. Next, the routine proceeds to step 219. On the other hand, when the shift change is not performed, the process proceeds to step 218, and the rich flag 2 is reset. Therefore, it is understood that the rich flag 2 is set when the shift change is performed.

In step 219, it is determined whether any one of the rich flag 1 and the rich flag 2 is set. If any rich flag is set, the process jumps to step 231. On the other hand, if no rich flag is set, the process proceeds to step 220. Step 220
In step 230, the process of the air-fuel ratio of the air-fuel mixture during acceleration is performed. That is, in step 220, the valve opening speed θ of the throttle valve 14, that is, the degree of acceleration is calculated from the output pulse of the throttle sensor 20. Next, at step 221, it is determined whether or not the valve opening speed θ of the throttle valve 14 is faster than the set value L, that is, whether or not the vehicle is undergoing rapid acceleration. When θ ≦ L, that is, when the vehicle is not in the rapid acceleration operation, the routine proceeds to step 222, where it is determined whether or not the valve opening speed θ of the throttle valve 14 is higher than the set value M (M <L), ie, whether the vehicle is in the moderate acceleration operation. It is determined whether or not it is.

When θ> M, that is, when the slow acceleration operation is being performed, the routine proceeds to step 223. N In step 223 when going to slow acceleration operation after NOx saturation S is a transition to or slow acceleration operation has been exceeded the set value S ₀
Identification flag indicating that the Ox saturation S exceeds the set value S ₀ is whether it is set or not. When the process proceeds to step 223 for the first time after shifting to the slow acceleration operation, the process proceeds to step 224 because the identification flag has been reset.
It is determined whether the NOx saturation S is greater than the set value S _O. S> identification flag proceeds to step 225 when the S _O is set, then the correction coefficient K proceeds to step 226 is 1.0. That is, the air-fuel ratio of the air-fuel mixture supplied into the engine cylinder is set to the stoichiometric air-fuel ratio. Next, the routine proceeds to step 231. S ≦ S _O correction coefficient K is the target air-fuel ratio K proceeds to step 230 when the hand
_O. Therefore, at this time, the air-fuel ratio of the air-fuel mixture supplied to the engine cylinder is made lean unless idling operation is performed, and the stoichiometric air-fuel ratio of the air-fuel mixture supplied to the engine cylinder is produced if idling. .

Once the identification flag is set, as long as the slow acceleration operation is being performed, steps 223 to 22 are performed.
6, K = 1.0 is maintained during the slow acceleration operation. On the other hand, when it is determined in step 221 that θ> L, that is, during rapid acceleration operation, step 227 is performed.
And the correction coefficient K is set to 1.0.
Step 2 after the identification flag is reset at 28
Go to 31. Therefore, during the rapid acceleration operation, the air-fuel ratio of the air-fuel mixture supplied into the engine cylinder is set to the stoichiometric air-fuel ratio. Further, when it is determined that the theta ≦ M in step 222, i.e., when the acceleration operation is not performed identification flag proceeds to step 229 is reset, then the K = K _O proceeds to step 230. Therefore, at this time, the air-fuel ratio of the air-fuel mixture supplied into the engine cylinder is maintained at the target air-fuel ratio corresponding to the target correction coefficient K _O.

At step 231, it is determined whether or not the cut flag has been set. If the cut flag has not been set, the routine jumps to step 234, where the fuel injection time TAU (= K · TP) is calculated. On the other hand, when the cut flag is set, the routine proceeds to step 232, where it is determined whether or not the rich flag 1 during deceleration operation is set. When the rich flag 1 is set, the routine proceeds to step 234, where the air-fuel ratio of the air-fuel mixture is made rich. Next, when the rich flag 1 is reset, the routine proceeds to step 233, where the correction coefficient K is made zero, and the fuel injection is stopped.

[0039]

The fuel consumption can be reduced while preventing NOx from being released to the atmosphere.

[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 the absorption / release action of NOx.

FIG. 6 is a time chart showing changes in air-fuel ratio and NOx saturation S;

FIG. 7 is a flowchart for calculating a NOx absorption amount.

FIG. 8 is a diagram showing the amount of NOx absorption.

FIG. 9 is a diagram showing the amount of released NOx.

FIG. 10 is a diagram showing a relationship between a NOx saturation S and a correction coefficient K.

FIG. 11 is a flowchart for calculating a fuel injection time TAU.

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

FIG. 13 is a flowchart for calculating a fuel injection time TAU.

[Explanation of symbols]

 15 Exhaust manifold 17 NOx absorbent

Continued on the front page (51) Int.Cl. ⁶ Identification code FI F02D 45/00 314 F02D 45/00 314E

Claims (1)

(57) [Claims] An NOx absorbent that absorbs NOx when the inflowing exhaust gas is lean, and releases the absorbed NOx when the inflowing exhaust gas has a stoichiometric air-fuel ratio or rich, is disposed in the engine exhaust passage. A means for estimating the amount of NOx absorbed by the NOx absorbent and a means for detecting the acceleration operation. When the estimated amount of NOx absorbed by the NOx absorbent during the acceleration operation is equal to or less than a set value, the amount is supplied to the engine cylinder. An exhaust gas purification device for an internal combustion engine in which the air-fuel ratio of the mixture is made lean and the air-fuel ratio of the mixture supplied to the engine cylinder is made stoichiometric or rich when the estimated NOx amount is equal to or more than a set value. .