JPH08296471A - Air-fuel ratio controller for engine - Google Patents

Abstract

PURPOSE: To provide reduction in NOx and torque shock by enriching air-fuel ratio at a prescribed enriching speed for returning to a theoretical air-fuel ratio if NOx adsorption quantity exceeds its allowance with a catalyst, and setting the enriching rate based on NOx desorbing rate. CONSTITUTION: The first catalyst for adsorbing NOx in the exhaust gas at the lean side atmosphere, and the second catalyst having an O2 stage function at the atmosphere near theoretical air-fuel ratio are placed in an exhaust gas passage. If a discriminating means 21 discriminates a lean driving region, air-fuel ratio is set at a lean side target value by a setting means 22, and NOx adsorption quantity by the catalyst is computer by a computation means 23. When switching to the driving at the theoretical air-fuel ratio, if a discriminating means 24 discriminates that NOx adsorption quantity exceeds its allowance, an air-fuel ratio control means 25 enriches the air-fuel ratio at a prescribed enriching speed to return it to the theoretical air-fuel ratio, and a setting means 26 sets the enriching rate based on NOx desorbing rate at that time.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an air-fuel ratio control device for operating an engine lean (lean mixture).

[0002]

2. Description of the Prior Art Improving engine fuel economy and at the same time NO
In order to reduce x, the fuel supply amount is controlled so that the air-fuel ratio, which is the ratio of air to fuel, becomes a target value on the lean side of the theoretical air-fuel ratio, and when the operating range where a certain amount of output is required, the theoretical In the engine operating method in which the operation is returned to the air-fuel ratio, NOx generated in the lean combustion region is adsorbed by the storage material of the catalyst, and if it is judged that this NOx adsorption amount has reached the limit, it will be for only a short time. There is one in which the air-fuel ratio is controlled to be richer than the stoichiometric air-fuel ratio (see, for example, Japanese Patent Laid-Open No. 6-66185).

In this case, when it is necessary to once release the NOx adsorbed to the storage substance to the limit, the amounts of HC and CO, which are unburned components in the exhaust gas, are all NOx.
Enrich the air-fuel ratio so that it exceeds the necessary amount for reducing (both NOx released from the occluded substance and NOx in the exhaust gas) without excess or deficiency, and then immediately return to the theoretical air-fuel ratio at a constant recovery speed. ing. Then, after the operation at the stoichiometric air-fuel ratio is continued for a predetermined time, the operation shifts to the lean operation, and NOx is adsorbed again to the empty storage material.

As described above, every time when NOx is adsorbed to the limit, the air-fuel ratio is enriched, and NOx released from the storage material is reduced by the unburned components in the exhaust gas increased by the enrichment of the air-fuel ratio. By doing so, even when the lean operation continues for a long time, it is possible to avoid that the occlusion material of the catalyst cannot adsorb NOx completely and the purification becomes impossible.

Further, O is provided downstream of the catalyst having an occlusion substance.
₂ A catalyst having a storage effect is provided, and O ₂ adsorbed by this catalyst oxidizes and purifies excess unburned components remaining after NOx is reduced when the air-fuel ratio is enriched. It

[0006]

By the way, when the air-fuel ratio enrichment process is graphically shown, it is shown in FIG. 12 that after the enrichment to a target value is made at a predetermined enrichment speed, the recovery speed is immediately followed by a theoretical operation. It returns to the air-fuel ratio (DML = 1.0 line in the figure), and in order to reduce the NOx that escapes in the atmosphere of the theoretical air-fuel ratio, the amount of unburned components (particularly HC) commensurate with this is reduced. Is supplied to the exhaust gas by enriching the exhaust gas. It can be considered that the HC supply amount at this time is almost proportional to the hatched area in the figure. Therefore, the hatched area in the figure indicates the amount of NOx released and the amount of NOx in the exhaust gas.
It is necessary to control the enrichment speed so that the size corresponds to the total amount.

On the other hand, torque shock occurs due to the switching of the air-fuel ratio between the stoichiometric air-fuel ratio and the target value on the lean side. Therefore, when the air-fuel ratio enrichment process is performed as described above, the air-fuel ratio before and after the switching is further increased. The difference in fuel ratio becomes large, and the torque shock is increased.

However, in the conventional device, the torque shock is not taken into consideration at all, and too much emphasis is placed on the reduction of NOx, so that the enrichment speed is kept substantially infinite. Therefore, the air-fuel ratio suddenly becomes rich from the target value on the lean side, and the torque shock is increased by the sudden change in the air-fuel ratio, which is not preferable in terms of drivability.

Therefore, according to the present invention, the enrichment speed is set to NOx.
The purpose is to make the reduction of NOx and the reduction of torque shock compatible by setting based on the amount of separation.

[0010]

According to the first aspect of the invention, the first catalyst has a function of adsorbing NOx in the exhaust gas in an atmosphere leaner than the stoichiometric air-fuel ratio and releasing NOx in the atmosphere of the stoichiometric air-fuel ratio. And a second catalyst downstream of the catalyst and having an O ₂ storage function in an atmosphere near the stoichiometric air-fuel ratio are arranged in the exhaust passage, while a lean operation that is preset as shown in FIG. Means 21 for determining whether or not it is in the region based on the detection signal of the operating condition, and means 22 for setting the air-fuel ratio to a target value on the lean side of the theoretical air-fuel ratio when it is determined to be in the lean operating region based on the result of this determination,
Means 23 for calculating the NOx adsorption amount of the first catalyst;
A means 24 for determining whether or not the NOx adsorption amount exceeds an allowable value, and a predetermined air-fuel ratio when switching to the stoichiometric air-fuel ratio operation when the NOx adsorption amount exceeds the allowable value based on this determination result. In the air-fuel ratio control device for an engine, which comprises means 25 for returning the stoichiometric air-fuel ratio to the stoichiometric air-fuel ratio immediately after enriching the stoichiometric air-fuel ratio with respect to the theoretical air-fuel ratio. The amount 26 of NOx release) is provided.

According to a second aspect of the present invention, in the first aspect of the present invention, the degree of enrichment when enriching the stoichiometric air-fuel ratio above the stoichiometric air-fuel ratio is increased in accordance with the NOx adsorption amount.

According to a third aspect of the present invention, in the first aspect of the present invention, the degree of enrichment when enriching the stoichiometric air-fuel ratio above the stoichiometric air-fuel ratio is increased in accordance with the enrichment rate.

According to a fourth aspect of the invention, in any one of the first to third aspects of the invention, the NOx withdrawal rate is the N
The value increases as the Ox adsorption amount increases.

In a fifth aspect of the invention, in any one of the first to third aspects of the invention, the NOx release speed is a value that increases as the intake air amount increases.

According to a sixth aspect of the present invention, in any one of the first to third aspects of the present invention, the NOx release speed is a value that increases as the engine load increases.

In a seventh aspect of the invention, in any one of the first to sixth aspects of the invention, the NOx adsorption rate (the NOx adsorption amount per unit time) is added for each unit time during the lean operation range. In addition, the NOx adsorption amount is calculated by subtracting the NOx desorption rate every unit time while in a region other than the lean operation region.

[0017]

When only the reduction of NOx is emphasized, the enrichment speed should be increased, but on the other hand, a sudden change in the air-fuel ratio causes an increase in torque shock, which causes discomfort associated with the torque shock. become. At this time, in the first aspect of the invention, since the enrichment speed is set based on the NOx withdrawal speed, it is possible to provide the enrichment speed with a size commensurate with the NOx withdrawal speed.
Even when the x separation speeds are different, the NOx emission amount can be kept within the allowable value while reducing the torque shock.

If the enrichment degree is made constant regardless of the NOx adsorption amount, for example, when the actual NOx adsorption amount is smaller than the NOx adsorption amount when the enrichment degree is adapted, the HC supply amount becomes excessive than the required value. , On the contrary, the actual N
When the amount of adsorbed Ox is larger than the compatible value, the amount of HC supply becomes insufficient than the required value. However, in the second invention, the degree of enrichment is increased according to the amount of adsorbed NOx, so even when the amount of adsorbed NOx is different. , HC supply amount can be given exactly as required.

If the enrichment degree is made constant regardless of the enrichment rate, for example, when the actual enrichment rate is higher than the enrichment rate when the enrichment degree is adapted, the HC supply amount becomes insufficient than the required value. On the contrary, when the actual enrichment speed is smaller than the compatible value, the HC supply amount becomes excessive than the required value, but in the third invention, the enrichment degree is increased according to the enrichment speed. Even when the values are different, the HC supply amount can be provided as required without excess or deficiency.

On the other hand, the NOx desorption rate is defined as the NOx adsorption amount,
Since the intake air amount and the engine load differ, if the NOx release speed is given regardless of these, NOx
There is an error in the calculation of the departure speed. At this time, in the second invention, the NOx release speed increases as the NOx adsorption amount increases, in the third invention the NOx release speed increases as the intake air amount increases, and in the fourth invention the NOx removal speed increases. Since the value increases as the load increases, the NOx release speed can be accurately obtained even when the NOx adsorption amount, the intake air amount, and the engine load are different.

In the fifth aspect of the invention, the NOx adsorption rate is added for each unit time during the lean operation range, and the NOx desorption rate is subtracted for each unit time during the non-lean operation range to reduce the NOx adsorption amount. Since the calculation is performed, the calculation accuracy of the NOx adsorption amount that responds with a first-order delay is increased, and the determination accuracy of whether the NOx adsorption amount becomes equal to or more than the allowable value is also improved.

[0022]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS In FIG. 1, reference numeral 1 is an engine body, a fuel injection valve 7 is provided in an intake passage 8 of the engine downstream of an intake throttle valve 5, and an injection signal from a control unit 2 responds to operating conditions. Fuel is injected and supplied into the intake air so that a predetermined air-fuel ratio is achieved. In the control unit 2, the Ref signal and Pos signal from the crank angle sensor 4, the intake air amount signal from the air flow meter 6, the air-fuel ratio (oxygen concentration) signal from the O ₂ sensor 3 installed in the exhaust passage 9, and the water temperature. Engine cooling water temperature signal from sensor 15, gear position signal from transmission gear position sensor (not shown), vehicle speed sensor 16
A vehicle speed signal from the vehicle is input, and the operating condition is judged based on these signals, and operation is performed at a lean air-fuel ratio in a predetermined operating area where the load is not so large, and the air-fuel ratio is set to the theoretical air-fuel ratio in other operating areas. Control to the fuel ratio or its vicinity.

A three-way catalyst 10 having an O ₂ storage effect is installed in the exhaust passage 9 to reduce NOx in the exhaust gas and HC with maximum conversion efficiency during operation at a stoichiometric air-fuel ratio.
CO is oxidized. However, the three-way catalyst 10 oxidizes HC and CO when the air-fuel ratio is lean, but the NOx reduction efficiency is low. Therefore, the lean NOx catalyst 11 is installed on the upstream side of the three-way catalyst 10, and NOx generated in the lean operation region is adsorbed by the storage substance of the catalyst 11.

However, when the lean operation continues for a long time, the storage material of the catalyst 11 cannot adsorb NOx completely and purification becomes impossible. Therefore, the NOx adsorption amount ABSTC becomes the allowable value RSFTAB # or more in the non-lean operating range. The air-fuel ratio enrichment process is performed. For example, as shown in FIG. 2, the NOx adsorption amount ABST before the non-lean operating range is reached.
If C is greater than or equal to the allowable value RSFTAB #,
At the timing when the operation is switched to the stoichiometric air-fuel ratio, the enrichment process is started, and NO is released from the storage substance of the catalyst 11.
Both x and NOx in the exhaust gas are rapidly reduced by the amount of HC increased (supplied) in the exhaust gas by enriching the air-fuel ratio. Specifically, a value obtained by adding the enrichment degree DMLR to the map fuel-air ratio Mdml (see the chain double-dashed line) at the timing of t1 when switching to the stoichiometric air-fuel ratio is the target enrichment target fuel-air ratio T
The DMLR (see the alternate long and short dash line) is increased stepwise, and a predetermined enrichment speed DDM is applied to this TDMLR.
The fuel-air ratio correction coefficient DML is increased by LR. This DML
From t2 when TDMLR reaches TDMLR, the DML is reduced at the recovery speed DDMLRS, and the enrichment process ends at t3 when DML matches the map fuel-air ratio Mdml (= 1.0).

It should be noted that due to excessive supply of HC, part of the NO
Even if it is left after reducing x, the excess H
C will be oxidized by O ₂ adsorbed on the downstream three-way catalyst 10.

The control shown in FIG. 2 (air-fuel ratio switching processing and air-fuel ratio enrichment processing) is performed by the control unit 2
The contents of these processes will be described with reference to the following flowcharts.

The flowchart of FIG. 3 shows the control operation for calculating and outputting the fuel injection pulse width. First, in step A), the target fuel-air ratio Tfbya is calculated as follows: Tfbya = DML + KTW + KAS + KHOT (1) where DML: fuel-air Ratio correction coefficient KTW: Water temperature increase correction coefficient KAS: Post-start increase correction coefficient KHOT: High water temperature increase correction coefficient

The fuel-air ratio correction coefficient DML of the equation (1) is shown in FIG.
Alternatively, the map shown in FIG. 7 is searched for the fuel-air ratio Mdml, and a predetermined damper operation is performed at the time of switching the air-fuel ratio. In this case, one of the maps is selected depending on whether or not the lean operation condition is satisfied. There is.

Here, the determination of the lean operating condition will be described with reference to the flowcharts of FIGS.

These operations are performed as a background job, and the lean condition is determined in step A) of FIG. 4, and the specific contents therefor are shown in FIG.
The lean condition is determined by checking the contents of steps A) to F) of FIG. 5 one by one. When all of the items are satisfied, the lean operation is permitted, and when any of them is not satisfied, the lean operation is performed. Prohibit

That is, step A): the air-fuel ratio sensor (O ₂ sensor) is activated, step B): engine warm-up is completed, step C): load (Tp) is in a predetermined lean region. Step D): The number of revolutions (N) is in a predetermined lean range, Step E): The gear position is in the second speed or higher, Step F): The vehicle speed is in a predetermined range, and sometimes Step G ) To allow lean operation (FLEA
N = 1), otherwise move to step H) to prohibit lean operation (FLEAN = 0). The above steps A) to F) are conditions for performing stable lean operation without impairing the operation performance.

When the lean condition is determined in this way,
Returning to steps C) and D) in FIG. 4, when the lean condition is not satisfied, the theoretical fuel air ratio or the map fuel air ratio Mdml which is denser than it is determined in step C), and the characteristic map shown in FIG. It is calculated by searching with the load Tp. On the other hand, in the case of lean conditions, in step D), the map that is leaner than the stoichiometric air-fuel ratio within a predetermined range is used. And search in the same way.

Since the numerical values shown in these maps are relative values with the stoichiometric air-fuel ratio being 1.0, richer values and lean values are indicated.

Next, FIG. 8 is a flow chart showing a damper operation at the time of switching the air-fuel ratio, which basically prevents the sudden change of the torque by gently switching the air-fuel ratio and ensures the stability of the driving performance. It is for.

In step A), the enrichment target fuel-air ratio TD
Set the MLR (see FIG. 2). This will be described with reference to the flowchart of FIG.

In step A) of FIG. 9, the flag FLEAN is set.
When the lean operation is in progress (FLEAN = 1), the flag FRSFTR is set to "0" and the other flag FRSFTS is set to "1" in step B), and the flow of FIG. 9 is ended. The contents of each flag are: FRSFTR: "1" enables enrichment execution, "0" prohibits enrichment execution FRSFTS: "1" completes enrichment execution, "0" does not complete enrichment execution.

On the other hand, when the lean condition is not satisfied, the flag FRSFTR is checked in step C). When the lean condition is not satisfied for the first time this time, since FRSFTR = 0, the process proceeds from step C) to D), and the NOx adsorption amount ABST
C is compared with the allowable value RSFTAB #.

Here, the calculation of the NOx adsorption amount ABSTC is performed at a constant cycle according to the flowchart shown in FIG. Explaining this, when the maximum NOx adsorption amount of the catalyst 11 is ABSFC # and the NOx concentration in the exhaust flowing into the catalyst 11 is constant, the adsorption amount AB
When the response of the STC is modeled with a first-order delay as shown in FIG. 11, the NOx concentration in the exhaust gas is almost proportional to the engine load, and the NOx adsorption rate is the adsorption residual ratio (= (maximum adsorption amount-adsorption amount) / If it is proportional to (maximum adsorption amount),
The NOx adsorption rate DABSR during lean operation is as follows: DABSR = DABSR0 # × (Tp / Tp0 #) × (Q / Q0 #) × (ABSFC # −ABSTC) / ABSFC # (2) where Tp0 #: reference Tp [Msec] Q0 #: Standard Q [L / min] DABSR0 #: Adsorption rate at Tp0 #, Q0 # [% /
Δt] ABSFC #: maximum adsorption amount (= 100) [%] ABSTC: adsorption amount (adsorption ratio) [%] Δt: can be obtained by the formula of the execution cycle of the flow in FIG. 10 (step A in FIG. 10), B)).

In this case, the NOx adsorption rate is the NOx adsorption amount per execution cycle in FIG.
The NOx adsorption amount ABSTC can be obtained by integrating the Ox adsorption rate DABSR for each execution cycle in FIG. 10 (step C in FIG. 10).

Similarly, when the lean condition is not satisfied, NO
x Departure speed DPRGR is calculated by the following formula: DPRGR = DABSR0 # × (Tp / Tp0 #) × (Q / Q0 #) × (ABSTC / ABSFC #) (3)
The value subtracted from BSTC is again designated as ABSTC (steps D and E in FIG. 10)).

In this way, during lean operation, the NOx adsorption amount is obtained by accumulating the NOx adsorption amount per execution cycle for each execution cycle, and during non-lean operation, this time from this NOx adsorption amount to this time. Is NO per execution cycle
By subtracting the x removal amount for each execution cycle, the NOx adsorption amount ABSTC that responds with a primary delay is accurately obtained as shown in FIG.

Returning to step D) in FIG. 9, when the NOx adsorption amount ABSTC is equal to or greater than the allowable value RSFTAB #, the enrichment process is performed. Therefore, the process proceeds to steps E) and F) to set the flag FRSFTR to "1". Another flag FR
Set SFTS to "0".

In steps G) and H), a value obtained by adding the enrichment degree DMLR to the map fuel-air ratio Mdml during non-lean operation is set as the target fuel-air ratio TDML, and the value of TDML at this time is set as the enrichment target fuel-air ratio. The ratio TDMLR is entered, the flow of FIG. 9 is terminated, and the process returns to step B) of FIG.

By setting the flag FRSFTR to "1" and the flag FRSFTS to "0",
Next, in FIG. 9, the process proceeds from steps C) and I) to step J). At that time, the fuel-air ratio correction coefficient DM
L (set in step D of FIG. 8 described later) or J)) is compared with the target fuel-air ratio TDML. If the process has proceeded to step J) for the first time, DML <TDML, so the flow in FIG. 9 is terminated and the process returns to step B) in FIG. By the processing in FIG. 8, DML becomes large at the enrichment speed DDMLLR. Eventually DML ≧ TDML
If so, it is determined that the DML has reached the peak of enrichment (enriched to a target enrichment degree),
Go to step K) from step J) in FIG.
The RSTFTS is set to "1" to end the flow of FIG. 9 and return to step B) of FIG.

By setting this FRSFTS to "1", the process proceeds from step I) to steps L) and M) in FIG. 9, where the recovery speed DDMLRS is set from the enrichment target fuel-air ratio TDMLR. The reduced value is set again as TDMLR, and the value of TDMLR is transferred to TDML. By the operation of step L), as shown in FIG. 2, TDMLR becomes smaller by DDMLRS.

In step N) of FIG. 9, the target fuel-air ratio TDM
The map fuel air ratio Mdml during non-lean operation is compared with L. When you proceed to step N) for the first time, TDML> M
Since it is dml, the flow of FIG. 9 is terminated and the process returns to step B) of FIG. Eventually, TDML ≦ Mdml
Then, the process proceeds from step N) of FIG. 9 to step O), Mdml is put into TDML, the flow of FIG. 9 is terminated, and the process returns to step B) of FIG.

In this way, the enrichment target fuel air ratio TD
After setting the MLR, return to step B) of FIG.
Looking at the flag FLEAN, step C) during lean operation,
In D), a predetermined value DDLML0 is put into the fuel-air ratio change amount ΔDML calculated once, and this Δ is calculated from the fuel-air ratio correction coefficient DML.
The value obtained by subtracting DML is compared with the map fuel-air ratio Mdml, and the larger one is set as DML.

In step E), DML is compared with 1.0, which is a value corresponding to the stoichiometric air-fuel ratio, and if DML is not 1.0 (that is, the operation is not at the stoichiometric air-fuel ratio), step F) is performed. The air-fuel ratio feedback correction coefficient α is clamped to 1.0, and the flow of FIG. 8 ends. This is because the air-fuel ratio feedback control using the O ₂ sensor cannot be executed when the engine is not operating at the stoichiometric air-fuel ratio, and the air-fuel ratio feedback control is prohibited during the enrichment of the air-fuel ratio.

On the other hand, during non-lean operation, the flags FRFRTR and FRSFTS are checked in steps G) and H) of FIG. 8 only while the air-fuel ratio is being enriched (that is, FRSF).
Only when TR = 1 and FRSFTS = 0), in step I) the enrichment speed DDMLLR is entered, and in step K) otherwise, a predetermined value DDLML0 is entered in ΔDML.

It should be noted that the enrichment speed DDMLRS is also the same as the fuel-air ratio change amount per calculation (that is, the fuel-air ratio change speed), and in the present invention, it is distinguished from the normal air-fuel ratio change speed DDML0 when the air-fuel ratio is switched. It is used to

In step J), the value obtained by adding ΔDML to DML is compared with the target fuel-air ratio TDML, and the smaller one is set as DML.

In FIG. 8, steps B), G),
The operations H), I), and J) are performed in the section from t1 to t2 in FIG. 2, and the operation causes the DML to be TDML.
It grows toward R.

After the fuel-air ratio correction coefficient DML is obtained in this way, returning to FIG. 3, the output of the air flow meter is A / D converted in step B) and linearized to calculate the intake air amount Q. Then, in step C), the basic pulse width Tp given to the fuel injection valve is obtained from the intake air amount Q and the engine speed N as Tp = K × Q / N. Note that K is a constant.

Then, in step D), based on this Tp, the one-time fuel injection pulse width Ti is Ti = Tp × Tfbya × (α + αm−1) × 2 + Ts (4) where α: air-fuel ratio Feedback correction coefficient αm; air-fuel ratio learning correction coefficient Ts; invalid pulse width.

However, under the lean condition, α, αm
Are fixed to predetermined values.

Next, in step F), the fuel cut is judged, and in steps G) and H), the invalid pulse width Ts is stored in the output register if the fuel cut condition is satisfied, and if it is not, the crank angle sensor is stored in the output register. Prepare for injection at a predetermined injection timing according to the output of. The injection here is sequential, and the fuel amount corresponding to Ti in equation (4) is injected at a timing that matches the intake stroke of each cylinder.

Now, when the air-fuel ratio enrichment process is taken out and shown as a model in FIG. 12, when the NOx adsorption amount exceeds the allowable value, after enriching to a target value at a predetermined enrichment speed DDMLLR, Immediate recovery speed DDML
It is returned to the stoichiometric air-fuel ratio (DML = 1.0 line in the figure) by RS, and in order to reduce NOx that desorbs in the atmosphere of the stoichiometric air-fuel ratio, the amount of HC commensurate with this is made rich in the air-fuel ratio. Therefore, it is necessary to supply it into the exhaust gas. In this case, it can be considered that the HC supply amount is almost proportional to the hatched area in the figure. Therefore, it is necessary to control the fuel-air ratio correction coefficient DML so that the shaded area in FIG. 11 becomes a size commensurate with the NOx release amount.

In this case, focusing on the enrichment speed,
When prioritizing the reduction of NOx, it is necessary to increase the enrichment speed as the NOx withdrawal speed is faster. On the other hand, as the enrichment speed is increased, the torque shock accompanying the enrichment becomes more strongly felt by the driver. Specifically, when only the reduction of NOx is emphasized, it is necessary to increase the enrichment speed as indicated by the broken line at the top of FIG. 13, but in the present invention, the top of FIG. 13 is also provided in order to reduce torque shock. As shown by the solid line in, the enrichment speed is reduced (slowed). Due to the slowed enrichment rate, a large amount of NOx is exhausted due to the delay in the supply of HC, but it is possible to determine the enrichment rate so that the NOx emission concentration is still below the allowable value. In the second stage from the bottom of FIG. 13, the broken line is the amount that is not reduced due to the supply delay of HC, and the dashed-dotted line is the amount that is reduced due to the enrichment of the air-fuel ratio. The NOx emission concentration when the temperature is delayed is certainly within the allowable emission concentration. That is, in order to achieve both the reduction of NOx and the reduction of torque shock, it is sufficient to make the enrichment speed as slow as possible within the range where the NOx emission concentration falls within the allowable value. Note that the stoichiometric air-fuel ratio is abbreviated in FIG. 13 (also in FIG. 11).

Therefore, in the present invention, the enrichment speed is set to NOx.
Set based on departure speed.

FIG. 14 is a flow chart for setting the enrichment speed DDMLLR, which is executed at regular intervals.

In step A), the flag FLEAN is checked, and this flow is ended as it is during lean operation.

At the time of non-lean operation, in step B) the NOx adsorbing amount ABSTC, the basic injection pulse width Tp and the intake air amount Q are used to determine the NOx releasing speed (NOx releasing amount per execution cycle in FIG. 14) DPRGR as described above. (3)
It is obtained in the same manner as the equation, and in step C) this NO
A table of characteristics shown in the frame of step C) is searched from the x departure speed DPRGR to obtain the enrichment speed DDMLLR, and this is stored in the memory.

The characteristics shown in the frame of step C) are adapted as follows. First, when the drivability is emphasized, a value (comfort limit) that is an index of how much the DDMLLR is comfortable even if it is increased is obtained. That is, if the value of DDMLLR is within the comfort limit, no discomfort due to torque shock is felt, and conversely, if the value exceeds the comfort limit, discomfort due to torque shock is felt. On the other hand, the minimum required value of DDMLLR for the NOx emission concentration to fall below the allowable value (that is, NOx
The required value) is represented by a straight line proportional to DPRGR (see the broken line in the figure). In the case of DDMLLR below the broken line in the figure, the enrichment speed is too slow and the NOx emission concentration exceeds the allowable value.

From the above two requirements, the region that must be adopted as the DDMLLR is the region below the comfort limit and above the NOx request value, so NOx
Until the required value exceeds the comfort limit, the NOx is reduced as much as possible and the drivability is kept as good as possible, and the value is set to an intermediate value between them (see the solid line from DPRGR to A in the figure).
In the area where DPRGR is A to B, the comfort limit is matched. On the other hand, the DPRG whose NOx requirement exceeds the comfort limit
In the R range, the NOx request value is traced because the reduction of NOx is prioritized.

In this way, the enrichment speed DDMLLR
Then, in step D) of FIG. 13, in proportion to the enrichment rate DDMLLR and the NOx adsorption amount ABSTC, that is, DMLR = K1 × DDMLLR × ABSTC / ABSFC # (5) where K1: proportional constant formula The enrichment degree DMLR is obtained by the following, and this is stored in the memory.

The enrichment degree DMLR is set to the NOx adsorption amount AB
The reason for obtaining in proportion to the STC and the enrichment speed DDMLLR is as follows. First, the required value of the HC supply amount is proportional to the NOx adsorption amount, and the enrichment degree represents the height of the shaded area of the triangle shown in FIG. 12 (the shaded area is proportional to the HC supply amount). NOx is the required degree of chemical conversion
This is because it increases in proportion to the amount of adsorption. On the other hand, when the degree of enrichment is constant, the time to reach the peak of enrichment (T2 in FIG. 12) decreases in inverse proportion to the increase in the enrichment speed DDMLLR, and the HC supply amount accordingly. Will decrease. Therefore, in order to match the HC supply amount with the required value, it is necessary to set the enrichment degree DMLR to a value proportional to the enrichment speed.

The obtained enrichment degree DMLR does not become too large within a predetermined range (0 ≦ DMLR ≦ DM
LRMX #, but DMLRMX # is the maximum value), and the flow of FIG. 14 is ended.

Enrichment speed DDM obtained in this way
The LLR is read from the memory in step I) of FIG. 8 described above, and the enrichment degree DMLR is read from the memory in step G) of FIG. 9 described above and used.

In the present invention, as described above in step C) of FIG. 14, the enrichment speed DDMLLR is set to NOx.
Since it is set based on the disengagement speed DPRGR, it is possible to provide a rich speed that is commensurate with the NOx disengagement speed, which allows the NOx emission amount while reducing the torque shock even when the NOx disengagement speed is different. It can be contained within the value.

Further, if the enrichment degree is set to be constant regardless of the NOx adsorption amount and the enrichment rate, for example, the actual NOx adsorption amount is obtained from the NOx adsorption amount when the enrichment degree is adapted.
When the x adsorption amount is small, the HC supply amount becomes excessive, and when the actual enrichment speed is higher than the enrichment speed when the enrichment degree is adapted, the HC supply amount becomes insufficient. Since the enrichment degree is proportional to the product of the NOx adsorption amount and the enrichment rate, the required value of the HC supply amount can be given in just proportion even when the NOx adsorption amount and the enrichment rate are different.

Further, since the NOx adsorption amount, the intake air amount corresponding to the exhaust amount flowing into the catalyst 11 and the engine load are different from each other, if the NOx release speed is calculated regardless of these, there is an error in the NOx release speed. Although it occurs, in the present invention, the NOx desorption rate is given in proportion to the product of the NOx adsorption amount, the intake air amount and the engine load. Therefore, even when the NOx adsorption amount, the intake air amount or the engine load is different, The detachment speed can be accurately obtained.

[0072]

According to the first aspect of the present invention, a first catalyst having a function of adsorbing NOx in exhaust gas in an atmosphere leaner than the stoichiometric air-fuel ratio and desorbing NOx in an atmosphere of stoichiometric air-fuel ratio, and this catalyst A second catalyst having an O ₂ storage function in an atmosphere downstream of the stoichiometric air-fuel ratio is arranged in the exhaust passage, and whether or not the lean operation region is set in advance is determined based on the operation condition detection signal. And means for setting the air-fuel ratio to a target value leaner than the stoichiometric air-fuel ratio when it is determined to be in the lean operation region based on the determination result, and means for calculating the NOx adsorption amount of the first catalyst. The means for determining whether or not the NOx adsorption amount exceeds the allowable value, and the air-fuel ratio when switching to operation at the stoichiometric air-fuel ratio when the NOx adsorption amount exceeds the allowable value based on this determination result. In the air-fuel ratio control device for an engine, which comprises means for returning the stoichiometric air-fuel ratio to the stoichiometric air-fuel ratio at a predetermined recovery speed immediately after enriching the stoichiometric air-fuel ratio at the enrichment speed, the enrichment speed is set based on the NOx release speed. Since the means is provided, the NOx emission amount can be kept within the allowable value while reducing the torque shock even when the NOx release speeds are different.

In the second aspect of the invention, in the first aspect of the invention, the enrichment degree in the case of enriching the stoichiometric air-fuel ratio at the enrichment rate is increased in accordance with the NOx adsorption amount, so the NOx adsorption amount is different. Even at this time, it is possible to supply the HC supply amount as required without excess or deficiency.

In a third aspect of the invention, in the first aspect of the present invention, the enrichment rate in the case of enrichment beyond the stoichiometric air-fuel ratio at the enrichment rate is increased in accordance with the enrichment rate, so the enrichment rates differ. Even at this time, it is possible to supply the HC supply amount as required without excess or deficiency.

According to a fourth aspect of the invention, in any one of the first to third aspects of the invention, the NOx withdrawal rate is the N
Since the value increases as the Ox adsorption amount increases, N
Even when the Ox adsorption amount is different, the NOx desorption rate can be accurately obtained.

According to a fifth aspect of the invention, in any one of the first to third aspects of the invention, the NOx releasing speed is a value that increases as the intake air amount increases, so that even when the intake air amount is different, the NOx is different. The detachment speed can be accurately obtained.

In a sixth aspect of the invention, in any one of the first to third aspects of the invention, the NOx withdrawal speed is a value that increases as the engine load increases, so that the NOx withdrawal speed is different even when the engine load is different. Can be obtained accurately.

In a seventh aspect of the present invention, in any one of the first to sixth aspects of the invention, the NOx adsorption rate is added for each unit time during the lean operation range, and while the NOx adsorption rate is outside the lean operation range. Since the NOx adsorption amount is calculated by subtracting the NOx desorption rate for each unit time, the calculation accuracy of the NOx adsorption amount that responds with a first-order delay is increased, and the determination accuracy of whether the NOx adsorption amount becomes the allowable value or more. Also improves.

[Brief description of drawings]

FIG. 1 is a control system diagram of an embodiment.

FIG. 2 is a waveform diagram for explaining processing when switching the air-fuel ratio.

FIG. 3 is a flowchart of a 10 msec job.

FIG. 4 is a flowchart of a background job.

FIG. 5 is a flowchart for explaining determination of lean conditions.

FIG. 6 is a characteristic diagram showing the contents of a lean map.

FIG. 7 is a characteristic diagram showing the contents of a non-lean map.

FIG. 8 is a flowchart of a 180-degree job.

FIG. 9 is a flow chart for explaining the setting of the enrichment target fuel air ratio TDMLR.

FIG. 10 is a flowchart for explaining calculation of a NOx adsorption amount ABSTC.

FIG. 11 is a waveform diagram for explaining the characteristics of the first-order lag of the NOx adsorption amount ABSFC.

FIG. 12 is a waveform diagram for explaining the enrichment process when switching to the stoichiometric air-fuel ratio.

FIG. 13 is a waveform diagram showing changes over time of the fuel-air ratio correction coefficient DML, the actual air-fuel ratio, the NOx emission concentration (tail), and the HC emission concentration (catalyst inlet).

FIG. 14 is a flowchart for explaining the setting of the enrichment speed DDMLLR.

FIG. 15 is a diagram corresponding to claims of the first invention.

[Explanation of symbols]

1 Engine Body 2 Control Unit 3 O ₂ Sensor 4 Crank Angle Sensor 6 Air Flow Meter 7 Fuel Injection Valve 10 Three-Way Catalyst (Second Catalyst) 11 Lean NOx Catalyst (First Catalyst) 21 Lean Operating Region Judgment Means 22 Air-Fuel Ratio Target value setting means 23 NOx adsorption amount calculating means 24 Determination means 25 Air-fuel ratio control means 26 Enrichment speed setting means

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification number Internal reference number FI Technical display location F01N 3/24 F01N 3/24 R ZAB ZABC F02D 41/14 ZAB F02D 41/14 ZAB 310 310D

Claims (7)

[Claims] 1. A first catalyst having a function of adsorbing NOx in exhaust gas in an atmosphere leaner than the stoichiometric air-fuel ratio and desorbing NOx in an atmosphere of the stoichiometric air-fuel ratio, and a theoretical catalyst located downstream of this catalyst. A second catalyst having an O ₂ storage function in an atmosphere near the air-fuel ratio is disposed in the exhaust passage, and means for determining whether or not a preset lean operation region is based on a detection signal of operating conditions; Based on this determination result, when it is determined to be in the lean operation region, a unit that sets the air-fuel ratio to a target value that is leaner than the theoretical air-fuel ratio, a unit that calculates the NOx adsorption amount of the first catalyst, and this NOx adsorption amount is Means for determining whether or not the value exceeds the allowable value, and based on this determination result, when the NOx adsorption amount exceeds the allowable value, the air-fuel ratio is set to a predetermined enrichment speed when the operation is switched to the stoichiometric air-fuel ratio. In the air-fuel ratio control device for an engine, which includes means for returning to the stoichiometric air-fuel ratio at a predetermined recovery speed immediately after enriching the stoichiometric air-fuel ratio, means for setting the enrichment speed based on the NOx release speed is provided. An air-fuel ratio control device for an engine, characterized in that 2. The air-fuel ratio control device for an engine according to claim 1, wherein the degree of enrichment when enriching the stoichiometric air-fuel ratio above the stoichiometric air-fuel ratio is increased in accordance with the NOx adsorption amount. . 3. The air-fuel ratio control device for an engine according to claim 1, wherein the degree of enrichment in the case of enriching the stoichiometric air-fuel ratio beyond the theoretical air-fuel ratio is increased in accordance with the enrichment speed. . 4. The air-fuel ratio control device for an engine according to claim 1, wherein the NOx desorption speed has a value that increases as the NOx adsorption amount increases. 5. The air-fuel ratio control device for an engine according to claim 1, wherein the NOx release speed has a value that increases as the intake air amount increases. 6. The engine air-fuel ratio control device according to claim 1, wherein the NOx release speed has a value that increases as the engine load increases. 7. The amount of NOx adsorbed by adding the NOx adsorption speed for each unit time during the lean operation region and subtracting the NOx desorption speed for each unit time during the region other than the lean operation region. The air-fuel ratio control device for the engine according to any one of claims 1 to 6, wherein the air-fuel ratio control device calculates.