JP3488982B2 - Air-fuel ratio control 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 air-fuel ratio control device for an internal combustion engine, and more particularly to a device for switching the air-fuel ratio of an engine intake air-fuel mixture between a stoichiometric air-fuel ratio and a lean air-fuel ratio.

[0002]

2. Description of the Related Art Conventionally, air-fuel ratio control devices of this type have been disclosed in, for example, Japanese Patent Application Laid-Open Nos.
There is one disclosed in Japanese Patent Publication No. 9-7741. In the air-fuel ratio control device and method disclosed in the above publications, when the engine intake air-fuel mixture is controlled to be switched between the stoichiometric air-fuel ratio and the lean air-fuel ratio (for example, air-fuel ratio 20 to 23) according to engine operating conditions, In order to solve the problem that the output torque suddenly changes due to the sudden change in the fuel ratio, which causes a shock to the vehicle, the air-fuel ratio is changed stepwise when the stoichiometric air-fuel ratio and the lean air-fuel ratio are switched.

[0003]

However, in the above-mentioned conventional air-fuel ratio control, a general three-way catalyst or NOx in which the NOx conversion efficiency sharply decreases as the exhaust purification catalyst becomes leaner than the stoichiometric air-fuel ratio. As shown in FIG. 26, when the theoretical air-fuel ratio is changed to the lean air-fuel ratio, NOx purification by the catalyst cannot be expected and the NOx emission amount becomes maximum as shown in FIG. It will pass the fuel ratio. Therefore, if the switching speed of the air-fuel ratio is slowed down to alleviate the switching shock, the NOx emission amount increases, so the switching speed of the air-fuel ratio must be increased to some extent even at the expense of the deterioration of drivability due to the shock occurrence. There was a problem.

Here, instead of the conventional three-way catalyst whose NOx reduction function is greatly reduced in the lean air-fuel ratio region, even if a large amount of oxygen is present in the exhaust gas (even during lean combustion).
A catalyst that can reduce NOx to some extent (hereinafter, lean NOx
It is called a catalyst. ) Is used, as shown in FIG.
Even in the air-fuel ratio state in which the NOx amount is maximum at the catalyst inlet, the NOx emission amount can be suppressed by the reducing function of the catalyst, so even if the theoretical air-fuel ratio is switched to the lean air-fuel ratio at a relatively slow speed, Compared to the case of using a general three-way catalyst or oxidation catalyst in which the NOx conversion efficiency decreases sharply in the lean range without significantly increasing the NOx emission amount, the torque is changed slowly at the time of switching the air-fuel ratio, and the shock is generated. It is possible to prevent.

[0005] Incidentally, as the lean NOx catalyst, using zeolite as a matrix for catalysts, which reducing the NO _X is known by the HC adsorbed on the zeolite. However, even with the lean NOx catalyst, the conversion efficiency greatly changes depending on the engine operating conditions. Therefore, if the air-fuel ratio switching speed is set to be relatively slow by adapting to the operating conditions with good conversion efficiency, the conversion will be changed. If the switching speed is set relatively high so that the NOx emission amount increases under inefficient operating conditions and conversely the NOx emission amount does not increase significantly even under operating conditions with poor conversion efficiency, a sudden torque change may occur during switching. There is a problem of shock.

The present invention has been made in view of the above problems, and in an apparatus for controlling the air-fuel ratio of the engine intake air-fuel mixture to switch between the stoichiometric air-fuel ratio and the lean air-fuel ratio, a shock is generated due to a sudden change in torque when the air-fuel ratio is switched. Avoidance of
It is an object of the present invention to provide an air-fuel ratio control device capable of achieving both high NOx control and suppression of NOx emission amount without being affected by changes in catalyst conversion efficiency.

[0007]

Therefore, an air-fuel ratio control system for an internal combustion engine according to the present invention is provided with a catalyst for purifying exhaust gas in an engine exhaust passage, while changing the air-fuel ratio of an engine intake air-fuel mixture according to engine operating conditions. An air-fuel ratio control device for an internal combustion engine, which controls switching between a stoichiometric air-fuel ratio and a lean air-fuel ratio,
Alternatively, it is configured as shown in FIG.

In FIG. 1, the conversion efficiency detecting means detects a parameter corresponding to the NOx conversion efficiency of the catalyst. The switching speed control means variably controls the switching speed between the stoichiometric air-fuel ratio and the lean air-fuel ratio according to the NOx conversion efficiency of the catalyst detected by the conversion efficiency detecting means. Here, it is preferable to provide lean air-fuel ratio varying means for variably setting the air-fuel ratio under the operating condition corresponding to the lean air-fuel ratio according to the NOx conversion efficiency of the catalyst detected by the conversion efficiency detecting means.

Further, as the parameters corresponding to the NOx conversion efficiency, it is preferable to detect parameters corresponding to the exhaust passage velocity and the catalyst bed temperature in the catalyst. As the parameters corresponding to the NOx conversion efficiency, parameters corresponding to the exhaust passage velocity in the catalyst and the engine temperature may be detected.

The intake air amount of the engine may be detected as a parameter corresponding to the exhaust passage velocity of the catalyst. On the other hand, in FIG. 2, the total NOx emission amount estimating means is configured to output NO at the catalyst outlet when the air-fuel ratio is switched.
x Estimate total emissions. The switching control means based on the NOx amount is the N estimated by the total NOx emission amount estimating means.
The switching speed between the stoichiometric air-fuel ratio and the lean air-fuel ratio is variably controlled according to the total amount of discharged Ox.

Here, the total NOx emission amount estimating means is
Conversion efficiency detecting means for detecting a parameter corresponding to NOx conversion efficiency of the catalyst, intake air amount detecting means for detecting an intake air amount of the engine, and NOx concentration setting means for setting a NOx concentration estimated value according to an air-fuel ratio. It is preferable that the total NOx emission amount is estimated based on the detected intake air amount and conversion efficiency and the NOx concentration estimated value according to the air-fuel ratio.

Further, the switching control means based on the NOx amount sets a longest switching time at which the total NOx emission amount at the time of switching becomes a predetermined amount or less based on the estimated total NOx emission amount, and gradually within this switching time. It is preferable that the air-fuel ratio is changed.

[0013]

According to the air-fuel ratio control device having such a configuration, the air-fuel ratio switching speed between the stoichiometric air-fuel ratio and the lean air-fuel ratio is variably controlled according to the NOx conversion efficiency of the catalyst. The switching speed is increased under the condition that the NOx emission amount increases, and conversely, when the conversion efficiency is high and the NOx emission amount can be suppressed, the switching speed is reduced to avoid the sudden change of the torque. .

Further, if the air-fuel ratio under the operating condition corresponding to the lean air-fuel ratio is variably set according to the conversion efficiency of NOx, it is possible to achieve stable combustion while suppressing the NOx emission amount to a predetermined amount or less. Become. Further, since the NOx conversion efficiency changes depending on the catalyst bed temperature or the engine temperature that correlates with the catalyst bed temperature and the exhaust passage velocity in the catalyst, the conversion efficiency changes by detecting parameters corresponding to these. Can be accurately detected.

As the parameter corresponding to the exhaust gas passage velocity in the catalyst, data of the intake air amount, which can be easily detected, can be used. On the other hand, if the total amount of NOx emissions during the air-fuel ratio switching control is estimated and the air-fuel ratio switching speed is variably controlled accordingly, the switching of the air-fuel ratio is controlled while suppressing the total amount of NOx emissions during the switching below a predetermined amount. become able to.

Here, the total NOx emission amount is the NOx of the catalyst.
It can be estimated based on the conversion efficiency, the intake air amount, and the NOx concentration estimated value corresponding to the air-fuel ratio, and thereby, highly accurate emission amount estimation can be performed corresponding to the NOx conversion efficiency at that time. Further, by setting the longest switching time at which the total NOx emission amount at the time of switching becomes equal to or less than the predetermined amount based on the estimated total NOx emission amount, a long switching time can be achieved while suppressing the NOx emission amount at the time of switching the air-fuel ratio. It can be secured and the drivability can be improved.

[0017]

EXAMPLES Examples of the present invention will be described below. In FIG. 3 showing the system configuration of the embodiment, air is sucked into each cylinder of the V-type internal combustion engine 1 through an air cleaner 2, a throttle valve 3 and an intake manifold 4. An electromagnetic injector 5 is provided at each branch of the intake manifold 4.

Exhaust gas from the engine 1 is exhausted from an exhaust manifold 6
After being collected for each bank by a and 6b, they are guided to the muffler 8 by exhaust pipes 7a and 7b, respectively. Catalysts 9a and 9b are provided in the exhaust pipes 7a and 7b, respectively. The catalysts 9a and 9b use zeolite as a base material of the catalyst and are
O _X is a what reduction, even during lean combustion oxygen is present a lot in the exhaust by such NOx reduction function, as shown in FIG. 26, is a lean NOx catalyst capable of suppressing NOx emissions .

The control unit 10 has a built-in microcomputer, calculates the fuel injection amount Ti by the injector 5 as described later based on the detection signals from various sensors, and opens the injector 5 based on the fuel injection amount Ti. The fuel supply to the engine 1 is electronically controlled by the drive control. As the various sensors, an air flow meter 11 (intake air amount detecting means) for detecting an intake air amount Qa of the engine 1 on the upstream side of the throttle valve 3, a crank angle sensor 12 for extracting a rotation signal from a cam shaft, and a cooling of the engine 1. Water temperature Tw
A cooling water temperature sensor 13 for detecting (a parameter corresponding to the engine temperature), oxygen sensors 14a, 14b provided in a collective portion of the exhaust manifolds 6a, 6b for detecting the oxygen concentration in the exhaust for each bank, and a throttle valve A potentiometer type throttle sensor 15 for detecting the opening degree of 3 and catalyst temperature sensors 16a, 16b for detecting the outlet temperature Te (parameter corresponding to the catalyst bed temperature) of each catalyst 9a, 9b are provided.

Reference numeral 17 is a control valve for adjusting the amount of intake air at the time of idling, and the engine 1 is provided through a bypass passage 18 which bypasses the throttle valve 3.
Adjust the amount of air supplied to the. Here, the configuration of the electronically controlled fuel injection device of the present embodiment is simplified and shown in the block diagram of FIG.

As shown in FIG. 4, the control unit 10 is based on the intake air amount Q detected by the air flow meter 11 and the engine speed N calculated based on the output signal from the crank angle sensor 12. And basic fuel injection amount Tp
Of the basic fuel injection amount Tp, and further, various corrections are made to the basic fuel injection amount Tp in accordance with the operating conditions such as the cooling water temperature Tw to obtain the final fuel injection amount T.
It has a function as an injection amount calculation means for calculating i and outputting the calculation result to the injector 5.

Further, in this embodiment, as shown in FIG.
A theoretical air-fuel ratio region that controls the air-fuel ratio of the engine intake air-fuel mixture to the theoretical air-fuel ratio (air-fuel ratio = 14.6) in the operating region that is divided by the engine load represented by the basic fuel injection amount Tp and the engine speed N. And a lean air-fuel ratio region in which the air-fuel ratio is significantly leaner than the stoichiometric air-fuel ratio (for example, air-fuel ratio = 22), and it is determined which region the current operating conditions correspond to, A fuel injection amount corresponding to the set air-fuel ratio in the relevant region is calculated.

Here, when there is a change in operating conditions between the stoichiometric air-fuel ratio region and the lean air-fuel ratio region, it is turned on.
The air-fuel ratio is not switched off, but is gradually switched within a certain period of time so as to suppress a sudden change in the engine output torque due to the switching of the air-fuel ratio. Catalyst outlet temperature T
e, the control unit 10 functions as an air-fuel ratio switching time setting means that is variably set according to the cooling water temperature Tw.
Is equipped with.

That is, the catalyst provided in the engine 1 of the present embodiment exerts the NOx reducing function even with a lean air-fuel ratio, so that the NOx emission amount is reduced even if the air-fuel ratio is switched at a relatively slow speed. Although it does not increase significantly, the conversion efficiency decreases depending on the operating conditions, and the NOx emission amount increases at the switching time suitable for high efficiency. Therefore, as described below, the switching time (switching speed) according to the change in the conversion efficiency is varied so that the suppression of the NOx emission amount and the improvement of the drivability are made compatible at a high level.

The function of the air-fuel ratio switching time setting means is a feature of this embodiment, and the details of the air-fuel ratio switching time setting means will be described below with reference to the flow chart. The program shown in the flow charts of FIGS. 6 and 7 showing the first embodiment is a program that is interrupted every 10 ms.

First, at P1, the air flow meter 11 measures the intake air amount Qa. At the next P2, the engine speed N is measured by the output signal from the crank angle sensor 12. Then, at P3, based on the intake air amount Qa and the engine speed N, the basic fuel injection amount T equivalent to the theoretical air-fuel ratio is obtained.
Calculate p (← K × Qa / N; K is a constant).

At P4, the lean air-fuel ratio region is determined based on the basic fuel injection amount Tp and the engine speed N (see FIG. 5). At P5, it is determined whether or not the latest basic fuel injection amount Tp and the engine speed N are operating conditions included in the lean air-fuel ratio region. If it falls within the lean air-fuel ratio region, the routine proceeds to P6, and it is time to switch from the stoichiometric air-fuel ratio region to the lean air-fuel ratio region, in other words, shift from the stoichiometric air-fuel ratio region to the lean air-fuel ratio region. It is determined whether it is the first time.

If it is the first transition, the routine proceeds to P7, where the switching timer Ta for counting the maximum time (1 sec in this case) of the air-fuel ratio switching control is initialized (T
1 sec is set for a) to prepare for the countdown. At P8, the catalyst outlet temperature Te is detected by the catalyst temperature sensors 16a and 16b. Here, the data of the catalyst outlet temperature Te is output from the respective sensors 16a and 16b, and the average value of these data or the larger or smaller data obtained by comparing the two is used for the following processing. The catalyst outlet temperature Te may be used.

At the next P9, a switching time Tc for gradually switching the air-fuel ratio from the stoichiometric air-fuel ratio to the lean air-fuel ratio based on the catalyst outlet temperature Te and the intake air amount Qa.
(See Figure 13). The switching time Tc is variably set as shown in FIG. 8, corresponding to the intake air flow rate Qa and the catalyst outlet temperature Te. The tendency of the switching time Tc in FIG. 8 is set corresponding to the change in the NOx conversion efficiency of the catalysts 9a and 9b.

That is, since the catalysts 9a and 9b of the present example reduce NO _X by the HC adsorbed on the zeolite even in a lean atmosphere, as shown in FIG.
Even in an air-fuel ratio region leaner than the theoretical air-fuel ratio (a region where the oxygen concentration in the exhaust gas is high), the NOx conversion efficiency can be kept relatively high, but as shown in FIGS. Depending on the space velocity S / V and the catalyst outlet temperature Te, the NOx conversion efficiency η _NO greatly changes even in the same air-fuel ratio state, and as the space velocity S / V becomes slower and the catalyst outlet temperature Te becomes higher, the conversion efficiency becomes higher. Will be shown.

The space velocity S / V means a predetermined surface area,
It is the exhaust flow velocity passing through the volume, which corresponds to the intake air amount Qa at that time, so that the intake air amount Qa data can be represented as the passage flow velocity data.
Therefore, the NO of the catalysts 9a and 9b in the predetermined air-fuel ratio state
As shown in FIG. 12, the x conversion efficiency η _NO can be estimated based on the intake air amount Qa and the catalyst outlet temperature Te at that time, and thus the intake air amount Qa and the catalyst outlet temperature Te The switching time Tc as shown in FIG. 8 is set in correspondence with the conversion efficiency η _NO estimated from.

As described above, the parameters corresponding to the NOx conversion efficiency of the catalyst in this embodiment are the intake air amount Qa and the catalyst outlet temperature Te, and the intake air amount Qa
Represents the exhaust passage velocity of the catalyst, and the catalyst outlet temperature Te represents the catalyst bed temperature. The portion P9 includes functions as conversion efficiency detecting means and switching speed control means.

If the stoichiometric air-fuel ratio is switched to the lean air-fuel ratio in the state where the NOx conversion efficiency η _NO is low, the NOx emission amount in the middle air-fuel ratio state increases, but N
If the Ox conversion efficiency η _NO is high, it is possible to avoid a sudden change in the engine output torque by slowly switching from the stoichiometric air-fuel ratio to the lean air-fuel ratio. Therefore, as shown in FIG. 8, when the intake air amount Qa is small, the catalyst outlet temperature Te is high, and the conversion efficiency η _NO is high, the switching time Tc is set to be relatively long to prevent the occurrence of shock during switching. On the contrary, the intake air flow rate Qa is large, and
In the state where the catalyst outlet temperature Te is low and the conversion efficiency η _NO is bad,
The switching time Tc is set to be relatively short so that the NOx emission amount can be kept low even if the drivability is somewhat sacrificed.

Thus, the NOx conversion efficiency η at that time
_If the switching time Tc (switching speed) from the stoichiometric air-fuel ratio to the lean air-fuel ratio is variably set according to _NO , even if the NOx conversion efficiency η _NO changes greatly depending on the operating conditions, a shock due to a sudden torque change at the time of switching the air-fuel ratio. It is possible to achieve both the prevention of the generation and the suppression of the NOx emission amount at a high level.

At P9, as described above, the air-fuel ratio switching time T
When c is variably set according to the NOx conversion efficiency η _NO , in the next P10, the switching timer Tm for measuring the actual switching time is reset to zero to prepare for the count-up. Then, in the first switching to the lean air-fuel ratio region, the routine proceeds to P11, where the injector 5 due to the change in the battery voltage is added to the basic fuel injection amount Tp corresponding to the theoretical air-fuel ratio.
The fuel injection amount Ti is calculated by adding the correction amount Ts for correcting the variation in the injection amount.

Even when it is determined in P5 that it is not the lean air-fuel ratio region but the stoichiometric air-fuel ratio region, P6-P10
Jump to P11. On the other hand, when it corresponds to the lean air-fuel ratio region and is not the first transition to the lean region, the routine proceeds from P6 to P12. At P12, the switching timer Ta is decremented by a predetermined time, and at next P12, it is determined whether or not the switching timer Ta is decremented to zero.

When the switching timer Ta is not zero, the routine proceeds to P14, where the execution cycle time of this program is added to the switching timer Tm to count the elapsed time from the start of switching. Then, at P15, the air-fuel ratio switching time Tc set according to the NOx conversion efficiency η _NO as described above is compared with the actual switching time Tm, and when the actual switching time Tm does not reach the target time Tc. , P16.

At P16, within the air-fuel ratio switching time Tc, the actual elapsed time Tm is set so as to gradually change from the stoichiometric air-fuel ratio (air-fuel ratio 14.6) to the lean air-fuel ratio (air-fuel ratio 22) (see FIG. 13). The basic fuel injection amount Tp is corrected according to the ratio with the target time Tc, and the fuel injection amount Ti is calculated as follows. Ti ← Tp × {1- (1-14.6 / 22) × Tm / Tc} +
On the other hand, when it is determined in P15 that the target switching time Tc has elapsed, and in P13 when it is determined that the maximum switching time by the switching timer Ta has elapsed, P
Proceeding to 17, the basic fuel injection amount Tp corresponding to the theoretical air-fuel ratio calculated in P3 is corrected to an amount corresponding to the set air-fuel ratio (for example, 22) in the lean air-fuel ratio region, and the fuel injection amount Ti is To calculate.

Ti ← Tp × 14.6 / 22 + Ts The above P12 to P17 correspond to the switching speed control means. In addition, except the above-mentioned air-fuel ratio switching state,
Based on the exhaust air-fuel ratio detected by the oxygen sensors 14a, 14b, a correction coefficient for feedback-correcting the actual air-fuel ratio to the target air-fuel ratio (theoretical air-fuel ratio or lean air-fuel ratio) is set, and the basic fuel is thereby set. The injection amount Tp may be corrected.

The switching timer Ta normally counts down to zero after the switching time Tc has elapsed, and when the timer Ta reaches zero, the counting up of the switching timer Tm is omitted. You will be able to proceed to P17. by the way,
In the above embodiment, the intake air amount Qa corresponding to the exhaust passage velocity (space velocity) and the catalyst outlet temperature Te corresponding to the catalyst bed temperature are used as the parameters corresponding to the conversion efficiency of the catalyst. Instead of Te, a cooling water temperature Tw that represents the engine temperature and is correlated with the catalyst outlet temperature Te may be used.

Cooling water temperature Tw instead of catalyst outlet temperature Te
An example of using is shown in the flowchart of FIG. In the flowchart of FIG. 14, P8 ′, P
Other than 9'is the same as the flowchart of FIG. 6,
If the determination is NO at P6 in the flowchart of FIG. 14, the process may proceed to P12 in the flowchart of FIG.

In P8 'of the flow chart of FIG. 14,
The cooling water temperature sensor 13 detects the cooling water temperature Tw of the engine 1. Then, at the next P9 ′, as shown in FIG. 15, referring to a map in which the air-fuel ratio switching time Tc is set corresponding to the cooling water temperature Tw and the intake air amount Qa, the current NO
The optimum switching time Tc is set by the x conversion efficiency η _NO .

Map of air-fuel ratio switching time Tc according to the cooling water temperature Tw and the intake air amount Qa (see FIG. 15)
Is the cooling water temperature Tw and the intake air amount Qa as shown in FIG.
The NOx conversion efficiency η _NO is set according to the characteristics of Since the cooling water temperature sensor 13 is a sensor that is generally provided in an electronically controlled fuel injection device in order to use it for correction control of the fuel injection amount during cooling, the catalyst temperature sensor
This is advantageous in terms of cost as compared with the case where 16a and 16b are provided for setting the air-fuel ratio switching time Tc.

In the above embodiment, the control when the operating condition shifts from the stoichiometric air-fuel ratio region to the lean air-fuel ratio region and the stoichiometric air-fuel ratio is switched to the lean air-fuel ratio has been explained.
When switching from the lean air-fuel ratio to the stoichiometric air-fuel ratio, it is advisable to gradually switch the air-fuel ratio within the switching time Tc as in the above-described embodiment. The programs shown in the flow charts of FIGS. 17 and 18 set the air-fuel ratio switching time Tc when switching from the lean air-fuel ratio region to the stoichiometric air-fuel ratio region using the catalyst outlet temperature Te. Is a program corresponding to.

Here, the programs shown in the flow charts of FIGS. 17 and 18 are P5 ', P6', P11 ', P16', and P5 in comparison with the programs shown in the flow charts of FIGS.
Only the 17 'part is different, so only the different steps are described below. When the lean air-fuel ratio region is determined in P4, it is determined in P5 'whether or not the current operating condition is included in the stoichiometric air-fuel ratio region. When it is not the stoichiometric air-fuel ratio region but the lean air-fuel ratio region, the routine proceeds to P11 ′, where the basic fuel injection amount Tp equivalent to the stoichiometric air-fuel ratio is corrected to the amount corresponding to the lean air-fuel ratio (air-fuel ratio = 22) and the fuel is corrected. Injection amount Ti (←
Tp × 14.6 / 22 + Ts) is calculated.

On the other hand, when it is judged at P5 'that it is in the stoichiometric air-fuel ratio region, the routine proceeds to P6', where it is judged whether or not it is the first transition to the stoichiometric air-fuel ratio region.
Various processes (setting of the air-fuel ratio switching time Tc, etc.) for controlling the air-fuel ratio switching time are executed at P7 to P10, and the process proceeds from P6 'to P12 from the next. Then, from the start of the air-fuel ratio switching control until the switching time Tc corresponding to the NOx conversion efficiency η _NO elapses, the fuel injection amount T is calculated according to the following equation.
By calculating i, the lean air-fuel ratio is gradually changed to the stoichiometric air-fuel ratio within the switching time Tc (P1
6 ').

Ti ← Tp × {14.6 / 22- (1-14.6 / 2
2) × Tm / Tc} + Ts Further, when the air-fuel ratio switching time Tc has passed or when the maximum switching time has elapsed (Ta = φ), the routine proceeds to P17 ′, where the theoretical air-fuel ratio is reached. The fuel injection amount Ti (← Tp + Ts) is calculated using the basic fuel injection amount Tp calculated as a corresponding value as it is.

By the way, as described above, the air-fuel ratio switching time Tc depends on the NOx conversion efficiency η _NO when the air-fuel ratio is switched.
The variable setting is to suppress the NOx emission amount at the time of switching the air-fuel ratio, to secure the switching time as long as possible,
The purpose is to prevent sudden changes in output torque. Therefore, the integrated value of NOx emissions within the switching time (total amount)
May be estimated, and the air-fuel ratio switching time Tc may be variably set according to the estimation of the NOx emission amount. Such an embodiment will be described below.

The program shown in the flow chart of FIG. 19 differs from the programs shown in the flow charts of FIGS. 6 and 7 only in the step portion of P9 for setting the air-fuel ratio switching time Tc. If NO is determined, the process shown in FIG.
Go to P12 in the flowchart. The setting of the air-fuel ratio switching time Tc in the flowchart of FIG.
In P9-1, the NOx conversion efficiency η _NO of the catalysts 9a and 9b is obtained based on the catalyst outlet temperature Te and the intake air amount Qa (Fig.
See 12). This part corresponds to the conversion efficiency detecting means.

Next, at P9-2, the data of the intake air amount Qa and the NOx conversion efficiency η _{NO and} the data of the NOx concentration in each air-fuel ratio state stored in advance (the storage of this concentration data is stored in the NOx concentration setting means). (Corresponding)), the integrated value (total amount) of NOx discharged when the air-fuel ratio is changed from the stoichiometric air-fuel ratio to the lean air-fuel ratio is calculated.

Specifically, for example, the air-fuel ratios 16, 17, 18, 1
Data of NOx concentration in each state of 9, 20, 21 NOx (1
6) to NOx (21) can be multiplied by the intake air amount Qa to obtain the NOx emission amount in each air-fuel ratio state, and if these integrated values are multiplied by (1-NOx conversion efficiency η _NO ). , NOx exhausted without being treated by the catalyst
It is possible to roughly determine the total amount ΣNOx of.

ΣNOx ← {Qa × NOx (16) + Qa × NOx (17) + ... + Qa × NOx (21)} × (1-η _NO ) The above P9-1 to P9-2 are It corresponds to a total NOx emission amount estimation means. The total NOx emission amount ΣNOx obtained as described above is the air-fuel ratio from the stoichiometric air-fuel ratio to the lean air-fuel ratio (or in the opposite direction) within the reference switching time.
NOx expected to be emitted when switching
As the NOx conversion efficiency η _NO deteriorates, the total amount ΣNOx increases. Therefore, this total NOx emission amount Σ
When the amount of NOx is large, the NOx emission amount cannot be suppressed within the allowable level unless the air-fuel ratio switching time Tc is set to be short.

Therefore, in the next P9-3, as shown in FIG. 20, the air-fuel ratio switching time Tc is set shorter as the total NOx emission amount ΣNOx increases. Specifically, the N
NOx at the time of switching based on the total Ox emission amount ΣNOx
The longest switching time Tc that can suppress the emission amount within the allowable level is set, in other words, the air-fuel ratio switching time Tc when the total NOx emission amount becomes a predetermined value is set, and the NOx emission amount is suppressed and the drivability is secured. Is compatible. P9- above
The portion 3 corresponds to the switching control means according to the NOx amount.

The air-fuel ratio switching control based on the air-fuel ratio switching time Tc is performed in the same manner as in the above-mentioned embodiment, and therefore its explanation is omitted. NOx when switching as described above
Estimate the total emission amount, and based on this, the switching control time T
If the configuration is such that c is set, NOx when switching the air-fuel ratio
Since the emission amount is quantitatively grasped, it is possible to flexibly meet the regulatory requirement for the NOx emission amount.

By the way, NOx in the catalysts 9a and 9b
As shown in FIG. 9, the conversion efficiency η _{NO of} is maintained to some extent in a state where the oxygen is leaner than the stoichiometric air-fuel ratio and has a relatively large amount of oxygen, but a higher value is obtained in the state where the oxygen amount is stoichiometric. Show. Therefore, when the catalyst bed temperature is low or the intake air amount Qa (exhaust flow velocity) is large and the conversion efficiency of the catalyst is greatly reduced, it is better to control the air-fuel ratio of the engine intake air-fuel mixture to the theoretical air-fuel ratio. A high NOx conversion efficiency can be secured.

Therefore, burning under the initially set lean air-fuel ratio (air-fuel ratio 22) under the operating condition where the conversion efficiency of the catalyst is the lowest may cause an increase in NOx emission amount. It is preferable to burn at the stoichiometric air-fuel ratio rather than the fuel ratio. On the contrary, when the conversion efficiency of the catalyst is high, even if the lean degree of the air-fuel ratio is suppressed, the NOx emission amount can be suppressed within the allowable level, so that it is slightly richer than the initially set lean air-fuel ratio. Combustion can be stabilized by using the lean air-fuel ratio corrected to the side as the set air-fuel ratio.

As described above, it is preferable to change the air-fuel ratio in the lean air-fuel ratio region according to the conversion efficiency of the catalyst at that time. Such an embodiment will be described below. 21 and 22
The program shown in the flowchart of FIG.
Only the part of P17 is modified, and the modified part will be mainly described below.

In the flowcharts of FIGS. 21 and 22,
At P9 ′, the air-fuel ratio switching time Tc is set based on the intake air amount Qa and the catalyst outlet temperature Te as in the above embodiment, and the intake air amount Qa and the catalyst outlet temperature Te are set.
Based on and, the air-fuel ratio T _AF in the lean air-fuel ratio region corresponding to the conversion efficiency at that time is variably set. The variable setting function of the air-fuel ratio T _AF at P9 ′ corresponds to lean air-fuel ratio changing means.

The air-fuel ratio T _AF is set with the characteristics shown in FIG. 23, for example. That is, under the condition that the intake air amount Qa is large and the catalyst outlet temperature Te is low and the conversion efficiency is the worst, the theoretical air-fuel ratio (air-fuel ratio = 14.6) is obtained as the air-fuel ratio T _AF even in the lean air-fuel ratio region. Is set. In order to maintain the conversion efficiency as high as possible even under the condition that the conversion efficiency is significantly lowered, it is preferable to burn at an air-fuel ratio with a small amount of oxygen, which is convenient for exerting a reducing action.

Further, when the conversion efficiency is improved as compared with the case of the lowest conversion efficiency in which the air-fuel ratio T _AF is the stoichiometric air-fuel ratio, in this case, stable combustion is barely obtained in order to suppress the NOx concentration at the catalyst inlet as much as possible. The lean air-fuel ratio (for example, air-fuel ratio = 22) is set as the air-fuel ratio T _AF . this is,
Since the conversion efficiency of the catalyst is not very high, NO from the engine
This is because the x emission amount is suppressed so that the final NOx emission amount from the catalyst does not become unacceptably high.

Furthermore, under conditions of good conversion efficiency, even if the NOx discharged from the engine increases, the increased amount can be treated by the reducing function of the catalyst, so it is necessary to set the air-fuel ratio lean to near the limit of stable combustion. The air-fuel ratio T _AF is slightly richer than the lean limit (for example, the air-fuel ratio 20.5) as the air-fuel ratio T _AF . Further, under the condition of the highest conversion efficiency, the rich shift from the lean limit is further advanced, and the air-fuel ratio 19 is set as the air-fuel ratio T _AF .

When the set air-fuel ratio in the lean air-fuel ratio region is changed by the conversion efficiency of the catalyst as described above, the injection amount Ti during the switching from the stoichiometric air-fuel ratio to the lean air-fuel ratio is the air-fuel ratio after switching. The target is the air-fuel ratio T _AF
Therefore, P16 'is calculated as follows. Ti ← Tp × {1- (1-14.6 / T _AF ) × Tm / Tc}
+ Ts On the other hand, the injection amount T in the lean air-fuel ratio region after the switching is completed
i is calculated in P17 'as follows.

Ti ← Tp × 14.6 / T _AF + Ts That is, in the arithmetic expression of the injection amount Ti when the set air-fuel ratio in the lean air-fuel ratio region is 22, the air-fuel ratio T _AF is set instead of the air-fuel ratio = 22. Just do it. Although the catalyst outlet temperature Te corresponding to the catalyst bed temperature is used in the flowchart of FIG. 21, a cooling water temperature Tw representative of the engine temperature may be used instead of the outlet temperature Te. Is shown in the flowchart of FIG.

In the flowchart of FIG. 24, the catalyst outlet temperature Te in the flowchart of FIG. 21 is all replaced with the cooling water temperature Tw, and the conversion efficiency of the catalyst is estimated based on the intake air amount Qa and the cooling water temperature Tw. The air-fuel ratio in the lean air-fuel ratio region is variably set accordingly. still,
The setting characteristic of the air-fuel ratio T _AF when the cooling water temperature Tw is used as a parameter is as shown in FIG. 25, for example.

As described above, if the air-fuel ratio in a stable state in the lean air-fuel ratio region is variably set according to the conversion efficiency of the catalyst, not only when switching, but also in the lean air-fuel ratio region, continuous operation is performed. The NOx emission amount at this time can be suppressed, and the total NOx emission amount can be controlled to the limit value according to the change in the conversion efficiency. The air-fuel ratio switching control as in the present embodiment may be applied to a system using a three-way catalyst in which the NOx conversion efficiency sharply decreases with a lean air-fuel ratio, but with such a three-way catalyst, NOx conversion in the lean region is possible. The efficiency is originally small, and even if the conditions of the catalyst bed temperature and the intake air amount change, the conversion efficiency does not change significantly, so it is difficult to obtain a large effect, and the NOx reduction function is exhibited in the lean region. Preference is given to application to a system with the resulting catalyst.

[0066]

As described above, according to the present invention,
It becomes possible to balance the suppression of the NOx emission amount at the time of switching and the deterioration of the drivability due to the sudden change in the output torque due to the switching of the air-fuel ratio, without being influenced by the change in the conversion efficiency of the catalyst. Further, if the set air-fuel ratio is changed according to the conversion efficiency under the operating condition of controlling to the lean air-fuel ratio, the NOx emission amount in the lean air-fuel ratio region can be suppressed to the maximum not only at the time of switching. Like

[Brief description of drawings]

FIG. 1 is a block diagram showing the basic configuration of the present invention.

FIG. 2 is a block diagram showing the basic configuration of the present invention.

FIG. 3 is a schematic diagram showing a system configuration of an embodiment.

FIG. 4 is a block diagram showing a basic configuration of an electronically controlled fuel injection device according to an embodiment.

FIG. 5 is a diagram showing a stoichiometric air-fuel ratio region and a lean air-fuel ratio region of the embodiment.

FIG. 6 is a flowchart showing air-fuel ratio switching control of the first embodiment.

FIG. 7 is a flowchart showing the air-fuel ratio switching control of the first embodiment.

FIG. 8 is a diagram showing a setting characteristic of a switching time Tc in the first embodiment.

FIG. 9 is a diagram showing the relationship between the air-fuel ratio A / F and the NOx conversion efficiency.

FIG. 10 is a graph showing the relationship between space velocity (passage velocity) S / V and NOx conversion efficiency.

FIG. 11 is a graph showing the relationship between catalyst outlet temperature Te and NOx conversion efficiency.

FIG. 12 is a graph showing NOx conversion efficiency corresponding to intake air amount Qa and catalyst outlet temperature Te.

FIG. 13 is a time chart showing the air-fuel ratio switching characteristic of the first embodiment.

FIG. 14 is a flowchart showing an embodiment in which a cooling water temperature Tw is used instead of the catalyst outlet temperature Te.

FIG. 15 is a diagram showing a characteristic of a switching time Tc using the cooling water temperature Tw as a parameter.

FIG. 16 is a graph showing NOx conversion efficiency corresponding to intake air amount Qa and cooling water temperature Tw.

FIG. 17 is a flowchart showing control at the time of switching from the lean region to the stoichiometric air-fuel ratio region.

FIG. 18 is a flowchart showing control for switching from a lean air-fuel ratio to a stoichiometric air-fuel ratio.

FIG. 19 is a flowchart showing an example in which the switching time Tc is set based on the total NOx emission amount.

FIG. 20 is a graph showing the relationship between the total NOx emission amount ΣNOx and the air-fuel ratio switching time Tc.

FIG. 21 is a flowchart showing an example in which the air-fuel ratio in the lean region is set according to the conversion efficiency.

FIG. 22 is a flowchart showing an example in which the air-fuel ratio in the lean region is set according to the conversion efficiency.

FIG. 23 is a diagram showing a characteristic of a lean region air-fuel ratio corresponding to a catalyst outlet temperature Te and an intake air amount Qa.

FIG. 24 is a flowchart showing an example in which the lean region air-fuel ratio is set according to the conversion efficiency obtained by using the cooling water temperature as a parameter.

FIG. 25 is a diagram showing characteristics of a lean region air-fuel ratio corresponding to a cooling water temperature Tw and an intake air amount Qa.

FIG. 26 is a diagram showing the NOx conversion efficiency of a catalyst having a NOx reduction function and a general three-way catalyst in the lean range of the example.

[Explanation of symbols]

1 Internal combustion engine 5 injectors 9a, 9b catalyst 10 Control unit 11 Air flow meter 12 crank angle sensor 13 Cooling water temperature sensor 16a, 16b Catalyst temperature sensor

─────────────────────────────────────────────────── ─── Continuation of front page (58) Fields surveyed (Int.Cl. ⁷ , DB name) F02D 41/14 310 F01N 3/24 F02D 45/00 301 F02D 45/00 314 F01N 3/20

Claims (8)

(57) [Claims] 1. An air-fuel ratio of an internal combustion engine, wherein an engine exhaust passage is provided with a catalyst for purifying exhaust gas, and an air-fuel ratio of an engine intake air-fuel mixture is controlled to be switched between a theoretical air-fuel ratio and a lean air-fuel ratio according to engine operating conditions. The control device is a conversion efficiency detecting means for detecting a parameter corresponding to the NOx conversion efficiency of the catalyst, and a theoretical air-fuel ratio and a lean air-fuel ratio according to the NOx conversion efficiency of the catalyst detected by the conversion efficiency detecting means. And a switching speed control means for variably controlling the switching speed between the internal combustion engine and the air-fuel ratio control apparatus for the internal combustion engine. 2. A lean air-fuel ratio varying means for variably setting an air-fuel ratio under an operating condition corresponding to the lean air-fuel ratio according to the NOx conversion efficiency of the catalyst detected by the conversion efficiency detecting means. The air-fuel ratio control device for an internal combustion engine according to claim 1. 3. The conversion efficiency detecting means detects, as the parameters corresponding to the NOx conversion efficiency, parameters corresponding to an exhaust passage velocity of the catalyst and a catalyst bed temperature, respectively. An air-fuel ratio control device for an internal combustion engine according to any one of 1. 4. The conversion efficiency detecting means detects, as parameters corresponding to NOx conversion efficiency, parameters respectively corresponding to an exhaust passage velocity in a catalyst and an engine temperature. An air-fuel ratio control device for an internal combustion engine according to any one of claims. 5. The air-fuel ratio control device for an internal combustion engine according to claim 3, wherein the intake air amount of the engine is detected as a parameter corresponding to the passage velocity of exhaust gas in the catalyst. 6. An air-fuel ratio of an internal combustion engine, wherein an engine exhaust passage is provided with a catalyst for purifying exhaust gas, and the air-fuel ratio of the engine intake air-fuel mixture is controlled to be switched between a theoretical air-fuel ratio and a lean air-fuel ratio according to engine operating conditions. The control device is a total NOx emission amount estimating means for estimating the total NOx emission amount at the catalyst outlet when the air-fuel ratio is switched, and a theoretical empty amount according to the total NOx emission amount estimated by the total NOx emission amount estimating means. An air-fuel ratio control device for an internal combustion engine, comprising: a switching control means for variably controlling a switching speed between a fuel ratio and a lean air-fuel ratio, the switching control means being based on an NOx amount. 7. The total NOx emission amount estimating means, a conversion efficiency detecting means for detecting a parameter corresponding to the NOx conversion efficiency of the catalyst, an intake air amount detecting means for detecting an intake air amount of the engine, and an air-fuel ratio NOx concentration setting means for setting a NOx concentration estimated value according to the above, and N based on the detected intake air amount and conversion efficiency and the NOx concentration estimated value according to the air-fuel ratio.
The air-fuel ratio control device for an internal combustion engine according to claim 6, wherein the total amount of Ox emissions is estimated. 8. A switching control means based on the NOx amount comprises:
7. The longest switching time for which the total NOx emission amount at the time of switching becomes a predetermined amount or less is set based on the estimated total NOx emission amount, and the air-fuel ratio is gradually changed within this switching time. 8. An air-fuel ratio control device for an internal combustion engine according to any one of 7.