JP3528315B2 - Engine air-fuel ratio control device - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an engine air-fuel ratio control device for switching the air-fuel ratio of an air-fuel mixture supplied to an engine between a stoichiometric air-fuel ratio and a lean air-fuel ratio according to operating conditions.

[0002]

2. Description of the Related Art Conventionally, for the purpose of improving fuel economy, the air-fuel ratio of an air-fuel mixture supplied to an engine is a theoretical air-fuel ratio (hereinafter also referred to as "stoichi"; A / F = 14.6) under predetermined operating conditions.
Has proposed an air-fuel ratio control device for switching to a lean air-fuel ratio (for example, A / F = 20 to 23).

Specifically, it is disclosed in JP-A-58-48746 and JP-A-59-7741.
In the air-fuel ratio control device disclosed in these publications, when the air-fuel ratio is controlled to switch between the stoichiometric air-fuel ratio and the lean air-fuel ratio according to the operating conditions, the output torque changes abruptly due to the sudden change in the air-fuel ratio. The air-fuel ratio is changed stepwise at the time of switching between the stoichiometric air-fuel ratio and the lean air-fuel ratio in order to eliminate the problem that the vehicle is shocked.

[0004]

However, in the above-mentioned conventional air-fuel ratio control device, 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. Since it is premised that an oxidation catalyst that does not have a reducing function is used, when the stoichiometric air-fuel ratio is changed to a lean air-fuel ratio, purification of NOx by the catalyst cannot be expected at the intermediate air-fuel ratio in the transition process, and the NOx emission amount increases. Therefore, there is a problem that the air-fuel ratio switching speed must be increased to some extent even at the expense of the deterioration of drivability due to the occurrence of shock. Further, the lean air-fuel ratio is also set to 20 to 23, which reduces the NOx emission amount, so that the shock is large.

Therefore, when operating with a lean air-fuel ratio, NOx
Instead of the conventional three-way catalyst, which has a significantly reduced reducing function,
It is conceivable to use a catalyst capable of reducing NOx to some extent (hereinafter referred to as "lean NOx catalyst") during operation at a lean air-fuel ratio. According to this, the NOx emission amount can be suppressed by the reducing function of the catalyst even in the lean air-fuel ratio state. Therefore, even if the lean air-fuel ratio is switched from the theoretical air-fuel ratio to A / F = 18, the NOx is reduced. It is possible to prevent a shock by setting the target lean air-fuel ratio to a richer side at the time of switching the air-fuel ratio without the emission amount significantly increasing.

As the lean NOx catalyst, zeolite is used as a base material of the catalyst, and HC is adsorbed on the zeolite during operation at a stoichiometric air-fuel ratio, and discharged during operation at a lean air-fuel ratio. It is known that NOx is reduced by the generated HC. However,
Even with the lean NOx catalyst, the conversion efficiency changes greatly depending on the history of operating conditions of the engine. Therefore, the lean air-fuel ratio at the time of switching the air-fuel ratio is set to the rich side by adapting to the operating conditions with good conversion efficiency. If so, the NOx emission amount increases under operating conditions with poor conversion efficiency, and conversely, the lean air-fuel ratio during air-fuel ratio switching is made lean so that the NOx emission amount does not increase significantly even under operating conditions with poor conversion efficiency. If it is set to the side, there is a problem that a shock is generated due to a sudden change in torque at the time of switching.

In view of the above-mentioned conventional problems, the present invention provides an air-fuel ratio control for an engine which can achieve both a high level of avoidance of a shock due to a sudden change in torque during air-fuel ratio switching and a suppression of NOx emissions. The purpose is to provide a device.

[0008]

The present invention comprises an air-fuel ratio switching means for switching the air-fuel ratio of an air-fuel mixture supplied to an engine between a stoichiometric air-fuel ratio and a lean air-fuel ratio according to operating conditions, and at least an exhaust system. NO when operating with lean air-fuel ratio
It is premised on an engine equipped with a catalyst capable of reducing x.

In the invention according to claim 1, as shown in FIG. 1 (A), the stoichiometric duration measuring means for measuring the operation duration of the engine at the stoichiometric air-fuel ratio, and the stoichiometric air-fuel ratio to the lean air-fuel ratio At the time of switching, the target lean air-fuel ratio setting means for setting the target lean air-fuel ratio in accordance with the operation continuation time at the stoichiometric air-fuel ratio immediately before that is provided and the air-fuel ratio control device for the engine is configured.

According to the second aspect of the present invention, the target lean air-fuel ratio setting means sets the target lean air-fuel ratio to a richer side as the operation duration time at the stoichiometric air-fuel ratio is longer. Characterize. In the invention according to claim 3, in addition to the configuration of the invention according to claim 1, as shown in FIG. 1 (A), a lean continuation time measuring means for measuring the operation continuation time during operation at a lean air-fuel ratio. And, after switching from the stoichiometric air-fuel ratio to the lean air-fuel ratio, when the operation duration at the current lean air-fuel ratio exceeds a predetermined value, instead of the target lean air-fuel ratio setting means, the target lean air-fuel ratio And a second target lean air-fuel ratio setting means for setting the lean side to a leaner side.

In the invention according to claim 4, as shown in FIG. 1 (B), the stoichiometric duration measuring means for measuring the operating duration at the time of operation at the stoichiometric air-fuel ratio, and the stoichiometric duration time measuring means at the time of operation at the lean air-fuel ratio are provided. Depending on the lean duration measuring means for measuring the operating duration and the operating duration at the stoichiometric air-fuel ratio immediately before and the operating duration at the current lean air-fuel ratio, the target value And a target lean air-fuel ratio setting means for setting a lean air-fuel ratio for the engine.

In the invention according to claim 5, as shown in FIG. 1 (C), the stoichiometric duration measuring means for measuring the operating duration during the operation at the stoichiometric air-fuel ratio, and the stoichiometric duration measuring means during the operation at the lean air-fuel ratio are provided. Lean duration measuring means for measuring the operation duration, and at the time of switching from the stoichiometric air-fuel ratio to the lean air-fuel ratio, depending on the operation duration at the theoretical air-fuel ratio immediately before that,
After switching from the stoichiometric air-fuel ratio to the lean air-fuel ratio after switching from the stoichiometric air-fuel ratio to the lean air-fuel ratio, the target lean air-fuel ratio is set to a higher value when the operation duration time at the current lean air-fuel ratio is within the switching time. A target lean air-fuel ratio setting means for setting the rich target first lean air-fuel ratio and setting the target lean air-fuel ratio to the leaner second target lean air-fuel ratio when the switching time is exceeded. And are provided to constitute an engine air-fuel ratio control device.

[0013]

In the invention according to claim 1, during the operation at the stoichiometric air-fuel ratio (during stoichiometric operation), the operation continuation time (stoichiometric continuation time) is measured and stored. Then, at the time of switching from stoichiometric to lean, the target lean air-fuel ratio is set according to the immediately preceding stoichiometric duration.

The target lean air-fuel ratio is made variable depending on the stoichiometric duration because the NOx conversion efficiency of the catalyst changes depending on the stoichiometric duration. Specifically, the longer the stoichiometric duration, the higher the NOx conversion efficiency of the catalyst. Therefore, by maximizing the NOx conversion efficiency of the catalyst that changes depending on the stoichiometric duration, the target lean air-fuel ratio during lean switching is made as rich as possible to the extent that the NOx emission amount does not increase, thereby increasing the NOx emission amount. It is possible to reduce the switching shock without causing it.

In the invention according to claim 2, since the NOx conversion efficiency of the catalyst is improved as the stoichiometric duration is longer, the target lean air-fuel ratio is set to a richer side as the stoichiometric duration is increased. The switching shock can be reduced without increasing the NOx emission amount. In the invention according to claim 3, the target lean air-fuel ratio is set according to the immediately preceding stoichiometric duration at the time of switching from stoichiometric to lean. However, when the present lean duration exceeds a predetermined value, the target lean air-fuel ratio is set. Set the air-fuel ratio to a leaner side. If lean operation continues for a long time, the NOx conversion efficiency of the catalyst decreases, so the target lean air-fuel ratio is set to the leaner side, and NOx is reduced.
This is because the amount of emission is not increased.

According to the fourth aspect of the present invention, during lean operation, the target lean air-fuel ratio is set according to the immediately preceding stoichiometric duration and the current lean duration. Specifically, the target lean air-fuel ratio is set to a richer side as the stoichiometric duration is longer, but the target lean air-fuel ratio is set to a leaner side as the lean duration is lengthened. In this way, the switching shock is reduced without increasing the NOx emission amount.

In the invention according to claim 5, at the time of switching from stoichiometric to lean, the switching time is set according to the stoichiometric continuation time immediately before that. Immediately after switching from stoichiometric to lean, the target lean air-fuel ratio is set to the first target lean air-fuel ratio on the richer side, and when the switching time elapses, the target lean air-fuel ratio is set to the second leaner side on the leaner side. Set to the target lean air-fuel ratio of. In this way, the switching shock is reduced without increasing the NOx emission amount.

[0018]

EXAMPLES Examples of the present invention will be described below. First, a first embodiment of the present invention will be described with reference to FIGS. FIG. 2 shows the system configuration. Air is drawn into the combustion chamber of each cylinder of the engine 1 from the air cleaner 2 through the throttle valve 3 and the intake manifold 4. An electromagnetic fuel injection valve 5 is provided in each branch portion of the intake manifold 4, and a fuel-air mixture is generated by the fuel injected from each fuel injection valve 5. Then, the air-fuel mixture is ignited by the spark plug 6 and burned in the combustion chamber.

The fuel injection valve 5 is opened by being energized by a drive pulse signal output at a predetermined timing in synchronization with the engine rotation from a control unit 12 which will be described later.
The fuel adjusted to a predetermined pressure is injected. Therefore, the fuel injection amount is controlled by the pulse width of the drive pulse signal. Exhaust gas from the engine 1 reaches an exhaust pipe 8 through an exhaust manifold 7.

A catalyst 10 is provided in the middle of the exhaust pipe 8. This catalyst 10 uses zeolite as a base material of the catalyst, adsorbs HC on the zeolite during stoichiometric operation, and reduces NOx by the adsorbed HC and HC discharged from the engine during lean operation. It is a lean NOx catalyst capable of suppressing the NOx emission amount even during lean combustion in which a large amount of oxygen exists in the exhaust gas due to its function. Further, as the catalyst 10, such a lean NOx catalyst and an ordinary three-way catalyst may be used in series. Then, the exhaust gas passes through the catalyst 10,
It is discharged through the muffler 11.

A controller for controlling the operation of the fuel injection valve 5.
The unit 12 contains a microcomputer
Thus, signals are input from various sensors. The various
As the sensor of the engine, the engine is provided on the upstream side of the throttle valve 3.
1. An air flow meter 13, which detects the intake air flow rate Q of 1,
Reference crank angle signal and unit from engine 1 camshaft rotation
Outputs a crank angle signal to indirectly detect engine speed N
Crank angle sensor 14 that can be output, engine 1 water
A water temperature sensor 15 for detecting the cooling water temperature Tw in the jacket,
It is attached to the exhaust manifold 7 and sucked into the engine 1.
Corresponding to the oxygen concentration in the exhaust gas related to the air-fuel ratio of the air-fuel mixture
Output voltage signal ₂Sensor 16 etc. are provided
It

Here, the control unit 12
Controls the operation of the fuel injection valve 5 by performing arithmetic processing as described below based on signals from the various sensors. Next, the contents of arithmetic processing of the control unit 12 will be described with reference to the flowcharts of FIGS. It should be noted that this flow is executed every predetermined time Δt (for example, 10 ms).

In step 1 (denoted as S1 in the drawing; the same applies hereinafter), the intake air flow rate Q is detected based on a signal from the air flow meter 13. In step 2, the engine speed N is detected based on the signal from the crank angle sensor 14. In step 3, a basic fuel injection amount Tp = K × Q / N (K is a constant) corresponding to stoichiometry (A / F = 14.6) is calculated from the intake air flow rate Q and the engine speed N.

In step 4, the cooling water temperature Tw is detected based on the signal from the water temperature sensor 15. In step 5, it is determined whether the cooling water temperature Tw is, for example, 75 ° C. or higher,
When the temperature is lower than 75 ° C., the operation is stoichiometric, so the process proceeds to step 8. When the cooling water temperature Tw is 75 ° C. or higher, the process proceeds to step 6 in order to realize the air-fuel ratio switching control according to the operating region.

In step 6, the regions (stoichiometric region / lean region) on the map of FIG. 5 are detected based on the engine speed N and the basic fuel injection amount (load) Tp, and the process proceeds to step 7. In step 7, it is determined whether or not it is in the lean region. If NO (stoichiometric region), the process proceeds to step 8 for stoichiometric operation, and if YES (lean region), the process proceeds to step 14 for lean operation. [In the case of stoichiometric operation] In step 8, the value of the lean flag FL (FL = 0 during stoichiometric operation, FL = 1 during lean operation) is determined.

When FL = 1, it means that the lean-to-stoichi switching command is issued during the lean operation at present, and at this time, the routine proceeds to step 9 and the lean flag FL = 0 is set. Further, in the next step 10, the timer TMS for measuring the stoichiometric duration is cleared (TMS = 0). FL =
When it is 0, the stoichiometric operation is continuing, so step
The process proceeds to step 11 and the timer TMS for measuring the stoichiometric duration is counted up by the execution time interval Δt of this routine (TM
S = TMS + Δt).

After that, the routine proceeds to step 12, where the air-fuel ratio feedback correction coefficient α is calculated based on the signal from the O ₂ sensor 16. In the next step 13, from the basic fuel injection amount Tp equivalent to stoichiometry, the air-fuel ratio feedback correction coefficient α, and the voltage correction amount (ineffective injection time) Ts set based on the battery voltage, the fuel injection is performed according to the following equation. The amount Ti is calculated, and this routine ends.

Ti = Tp × α + Ts When the fuel injection amount Ti is calculated, this is set in a predetermined register, and a drive pulse signal having a pulse width of Ti is set at a predetermined timing in synchronization with the engine rotation. 5, and fuel injection is performed. At this time, the air-fuel ratio is controlled stoichiometrically. [For lean operation] In step 14, lean flag F
Value of L (FL = 0 during stoichiometric operation, F during lean operation
L = 1) is determined.

When FL = 0, it means that a stoichiometric → lean switching command is being issued during the current stoichiometric operation. At this time, the routine proceeds to step 15, where the lean flag FL = 1 is set. Further, in the next step 16, the lean continuous time measuring timer TML is cleared (TML = 0). Further, in the next step 17, as shown in FIG. 6, the target lean air-fuel ratio AF at the time of lean switching according to the stoichiometric duration TMS.
The target lean air-fuel ratio AF1 is searched from the actual stoichiometric duration TMS with reference to the map defining 1. Here, the longer the stoichiometric duration TMS, the more rich the target lean air-fuel ratio AF1 is set.

When FL = 1, the lean operation is continuing, so the routine proceeds to step 18, and the timer TML for measuring the lean continuation time is counted up by the execution time interval Δt of this routine. After this, the routine proceeds to step 19, where the current lean continuation time TML is compared with a predetermined value C _0, and TML ≦ C
_{If 0} , go to step 20.

In step 20, the basic fuel injection amount Tp is corrected to the target lean air-fuel ratio AF1 as shown in the following equation, the fuel injection amount Ti is calculated, and this routine is ended. If Ti = Tp × (14.6 / AF1) + Ts TML> C ₀ , proceed to step 21. Step 21
Then, as shown in the following expression, the leaner target lean air-fuel ratio AF2 (for example, A / F
= 22) After making a corresponding correction, calculate the fuel injection amount Ti,
This routine ends.

Ti = Tp × (14.6 / AF2) + Ts When the fuel injection amount Ti is calculated, this is set in a predetermined register, and a driving pulse having a pulse width of Ti is set at a predetermined timing in synchronization with the engine rotation. A signal is output to the fuel injection valve 5 and fuel injection is performed. At this time, the air-fuel ratio is controlled to be lean (target lean air-fuel ratio AF1 or AF2).

In this embodiment, steps 5, 6, 7
Corresponds to the air-fuel ratio switching means, and steps 8, 9, 1
The 0, 11 portion corresponds to the stoichiometric duration measuring means, the steps 14, 15, 16, 18 correspond to the lean duration measuring means, and the steps 17, 20 correspond to the target lean air-fuel ratio setting means. The steps 19 and 21 correspond to second target lean air-fuel ratio setting means.

As described above, in this embodiment, when the stoichiometric mode is switched to the lean mode, the stoichiometric duration time TM immediately before that is changed.
A target lean air-fuel ratio AF1 is set according to S. That is, the longer the stoichiometric duration TMS, the more the target lean air-fuel ratio AF1 is set to the rich side. The target lean air-fuel ratio AF1 is set according to the stoichiometric duration TMS because the NOx conversion efficiency of the lean NOx catalyst changes depending on the stoichiometric duration TMS.

The reason will be described with reference to FIG. The solid line and the dotted line in FIG. 7 indicate the case where the stoichiometric operation is started at the same time and the lean operation is performed at different timings, and the stoichiometric duration is different. The longer the stoichiometry duration (dotted line), the more HC is adsorbed by the zeolite, and the NOx conversion efficiency during lean operation temporarily rises. Therefore, according to the stoichiometric duration TMS, the stoichiometric duration TM
The longer S is, the richer the target lean air-fuel ratio AF1 at the time of lean switching can be reduced the switching shock without increasing the NOx emission amount.

[0036] The target air-fuel ratio AF1 is a predetermined time C ₀ operating at the air-fuel ratio, adsorbed to stoichiometric operation HC
The air-fuel ratio is determined so that it will be used up. In the present embodiment, the target lean air-fuel ratio is set to a leaner side AF2 (for example, A / F = 22) when the lean continuation time TML exceeds the predetermined value C ₀ after switching from stoichiometric to lean. .

This is because if the lean operation continues for a long time, the HC adsorbed during the stoichiometric operation will be used up and the NOx conversion efficiency of the lean NOx catalyst will decrease, so that the air-fuel ratio will be made leaner and the NOx emissions will be reduced. This is because the amount is not increased. That is, as the conversion efficiency of the lean NOx catalyst is reduced, the air-fuel ratio is made lean to reduce the NOx generation amount itself and prevent the final NOx emission amount from increasing. The relationship between the air-fuel ratio and the NOx generation amount is shown in FIG.

FIG. 9 shows the effect of this embodiment. this is,
This is an example of stoichiometry duration TMS = 5 minutes (see FIG. 6). Compared to the conventional example indicated by the solid line, in the present embodiment indicated by the dotted line, the change width of the air-fuel ratio is small, so the torque change is also small,
It can be seen that the NOx emissions are the same. Next, a second embodiment of the present invention will be described with reference to FIGS.

FIG. 10 is a flow chart of the second embodiment. However, the flowchart of FIG. 10 is executed instead of that of FIG. 4 following the flowchart of FIG. 3 of the first embodiment. In the second embodiment, during lean operation, the target lean air-fuel ratio is set according to the immediately preceding stoichiometric duration and the current lean duration.

Therefore, the difference from the flowchart of FIG. 4 is only in the lean operation, and step 14 and the subsequent steps in the lean operation will be described. At step 14, the value of the lean flag FL (FL =
0, FL = 1) is determined during lean operation. When FL = 0, it means that the stoichiometric-to-lean switching command is being issued during the current stoichiometric operation. At this time, the routine proceeds to step 15, where the lean flag FL = 1 is set. Also, the next step
At 16, the timer TML for measuring the lean continuous time is cleared (TML = 0).

When FL = 1, the lean operation is continuing, so the routine proceeds to step 18, and the timer TML for measuring the lean continuation time is incremented by the execution time interval Δt of this routine. After this, proceed to step 22. Step 22
Then, as shown in FIG. 11, the actual stoichiometric duration TMS and the lean duration are referred to by referring to a map in which the target lean air-fuel ratio AF during lean operation is determined according to the stoichiometric duration TMS and the lean duration TML. The target lean air-fuel ratio AF is searched from TML. Here, as the stoichiometric duration time TMS is longer, the target lean air-fuel ratio AF is set to a richer side, and as the lean duration time TML becomes longer,
The target lean air-fuel ratio AF is set to the leaner side.

In the next step 23, the basic fuel injection amount Tp is corrected to a value corresponding to the target lean air-fuel ratio AF, and the fuel injection amount Ti is calculated, as shown in the following equation, and this routine ends. Ti = Tp × (14.6 / AF) + Ts When the fuel injection amount Ti is calculated, this is set in a predetermined register, and the drive pulse signal having the pulse width of Ti is fueled at a predetermined timing in synchronization with the engine rotation. The fuel is injected by being output to the injection valve 5. At this time, the air-fuel ratio is controlled to be lean (target lean air-fuel ratio AF).

In the present embodiment, steps 5, 6, 7
Corresponds to the air-fuel ratio switching means, and steps 8, 9, 1
The 0, 11 portion corresponds to the stoichiometric duration measuring means, the steps 14, 15, 16, 18 correspond to the lean duration measuring means, and the steps 22 and 23 correspond to the target lean air-fuel ratio setting means. To do. As described above, in the present embodiment, during lean operation, the longer the stoichiometry duration TMS, the richer the lean duration and the lean duration depending on the immediately preceding stoichiometry duration TMS and the current lean duration TML. As the TML becomes longer, the target lean air-fuel ratio AF is set on the leaner side.

Specifically, for example, the stoichiometric duration is 2
If it is minutes, the target lean air-fuel ratio after switching from stoichiometric to lean is as shown in the dotted line in Fig. 11, immediately after switching = 19, about 1.5.
After 20 minutes, after about 3 minutes = 21, and after about 5.5 minutes = 22, it changes gradually. As a result, the switching shock can be reduced without increasing the NOx emission amount.

Next, a third embodiment of the present invention will be described with reference to FIGS.
Will be described. FIG. 12 is a flowchart of the third embodiment. However, the flowchart of FIG. 12 follows the flowchart of FIG. 3 of the first embodiment and replaces FIG.
To be executed. In the third embodiment, at the time of switching from stoichiometric to lean, depending on the stoichiometric duration immediately before that,
After setting the switching time and after switching from stoichiometric to lean, the target lean air-fuel ratio is first set to the first target lean air-fuel ratio on the richer side, and when the switching time elapses, the target lean air-fuel ratio becomes leaner. The second target lean air-fuel ratio on the side is set.

Therefore, the difference from the flowchart of FIG. 4 is only in the lean operation, and step 14 and the subsequent steps in the lean operation will be described. At step 14, the value of the lean flag FL (FL =
0, FL = 1) is determined during lean operation. When FL = 0, it means that the stoichiometric-to-lean switching command is being issued during the current stoichiometric operation. At this time, the routine proceeds to step 15, where the lean flag FL = 1 is set. Also, the next step
At 16, the timer TML for measuring the lean continuous time is cleared (TML = 0).

Further, in the next step 24, as shown in FIG. 13, the actual stoichiometric continuation time TMS is switched with reference to a map in which the switching time C ₀ for lean switching is determined according to the stoichiometric continuation time TMS. Search for time C ₀ . Here, the longer the stoichiometric duration TMS, the longer the switching time C ₀ is set. When FL = 1, the lean operation is continuing, so the routine proceeds to step 18, where the lean continuation time measuring timer TML is incremented by the execution time interval Δt of this routine.

After this, the process proceeds to step 25, where the current
Change the lean continuation time TML to the switching time C ₀Compared with TML
≤C₀If No, go to step 26. In step 26,
As shown in the following equation, the basic fuel injection amount Tp is
1st target lean air-fuel ratio AF1 (eg A / F
= 18 to 20) Corrected and calculated the fuel injection amount Ti
Then, this routine is finished.

If Ti = Tp × (14.6 / AF1) + Ts TML> C ₀ , the process proceeds to step 27. Step 27
Then, as shown in the following equation, the fuel injection amount Ti is calculated after correcting the basic fuel injection amount Tp to a predetermined second lean lean target air-fuel ratio AF2 (for example, A / F = 22). This routine ends.

Ti = Tp × (14.6 / AF2) + Ts When the fuel injection amount Ti is calculated, this is set in a predetermined register, and a drive pulse having a pulse width of Ti is set at a predetermined timing in synchronization with the engine rotation. A signal is output to the fuel injection valve 5 and fuel injection is performed. At this time, the air-fuel ratio is lean (first or second target lean air-fuel ratio AF1,
AF2) is controlled.

In the present embodiment, steps 5, 6, 7
Corresponds to the air-fuel ratio switching means, and steps 8, 9, 1
The portion 0, 11 corresponds to the stoichiometric duration measuring means, the portion of steps 14, 15, 16, 18 corresponds to the lean duration measuring means, the portion of step 24 corresponds to the switching time setting means, and the step 25 , 26 and 27 correspond to the target lean air-fuel ratio setting means.

As described above, in this embodiment, the longer the stoichiometric duration time TMS, the longer the switching time C _0, and the longer the time during which the rich target side lean target air-fuel ratio AF1 is stopped. Then, the switching shock is reduced without increasing the NOx emission amount. Switching time C here
₀ may be determined as the time for which the adsorbed HC is exhausted during stoichiometric operation when the engine is operated at the first target lean air-fuel ratio AF1.

[0053]

As described above, according to the first aspect of the present invention, when the stoichiometric mode is switched to the lean mode, the target lean air-fuel ratio is set in accordance with the immediately preceding stoichiometric duration, so that the NOx emission is reduced. Without increasing the amount
The effect that the switching shock can be reduced can be obtained.

According to the second aspect of the invention, the NOx conversion efficiency of the catalyst improves as the stoichiometric duration increases, so the target lean air-fuel ratio is set to a richer side as the stoichiometric duration increases. Thus, it is possible to reliably reduce the switching shock without increasing the NOx emission amount. According to the third aspect of the present invention, when the lean duration time exceeds the predetermined value, the target lean air-fuel ratio is set to a leaner side, whereby the catalyst NO
Even if the x conversion efficiency decreases, the effect of not increasing the NOx emission amount can be obtained.

According to the fourth aspect of the present invention, during lean operation, the NOx emission amount is increased by setting the target lean air-fuel ratio according to the immediately preceding stoichiometric duration and the current lean duration. It is possible to obtain the effect that it is possible to reduce the switching shock.
According to the invention of claim 5, at the time of switching from stoichiometric to lean, the switching time is set in accordance with the immediately preceding stoichiometric duration, and during the switching time, the target lean air-fuel ratio on the richer side is set. By doing so, it is possible to obtain the effect that the switching shock can be reduced without increasing the NOx emission amount.

[Brief description of drawings]

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

FIG. 2 is a system configuration diagram of the first embodiment.

FIG. 3 is a flowchart of the first embodiment (part 1).

FIG. 4 is a flowchart of the first embodiment (part 2).

FIG. 5 is a diagram showing a map for determining a stoichio-lean area.

FIG. 6 is a diagram showing a map for setting a target lean air-fuel ratio.

FIG. 7 is an explanatory diagram of NOx conversion efficiency when switching from stoichio to lean.

FIG. 8 is a diagram showing a relationship between an air-fuel ratio and a NOx emission amount.

FIG. 9 is a diagram showing an effect.

FIG. 10 is a flowchart of the second embodiment.

FIG. 11 is a diagram showing a map for setting a target lean air-fuel ratio.

FIG. 12 is a flowchart of the third embodiment.

FIG. 13 is a diagram showing a switching time setting map.

[Explanation of symbols]

1 Engine 5 Fuel Injection Valve 10 Catalyst (Lean NOx Catalyst) 12 Control Unit 13 Air Flow Meter 14 Crank Angle Sensor 15 Water Temperature Sensor 16 O ₂ Sensor

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Claims (5)

(57) [Claims] 1. An air-fuel ratio switching means for switching an air-fuel ratio of an air-fuel mixture supplied to an engine between a stoichiometric air-fuel ratio and a lean air-fuel ratio according to operating conditions, and NOx at the time of operation at least in the lean air-fuel ratio in an exhaust system. In an engine equipped with a catalyst that can reduce the stoichiometric air-fuel ratio, a stoichiometric duration measuring means for measuring the operating duration when operating at the stoichiometric air-fuel ratio, and the stoichiometric air-fuel ratio immediately before the stoichiometric air-fuel ratio switching to the lean air-fuel ratio The air-fuel ratio can be switched according to the operating duration of
The target value that is the target value of the lean air-fuel ratio that is switched by the means.
A target lean air-fuel ratio setting means for setting the over down air and provided, during operation at a lean air-fuel ratio, the target air-fuel ratio
Target lean air-fuel ratio set by lean air-fuel ratio setting means
An air-fuel ratio control device for an engine, which is characterized in that 2. The target lean air-fuel ratio setting means sets the target lean air-fuel ratio to a richer side as the operation duration time at the stoichiometric air-fuel ratio is longer. Engine air-fuel ratio control device. 3. A lean duration measuring means for measuring the operation duration of the lean air-fuel ratio during operation, and a predetermined operation duration at the lean air-fuel ratio at present after switching from the stoichiometric air-fuel ratio to the lean air-fuel ratio. when it exceeds the value, in place of the target lean air-fuel ratio setting means, target lean air
The second target lean air-fuel ratio setting means for setting the fuel ratio to the leaner side, and the air-fuel ratio control device for the engine according to claim 1 or 2. 4. An air-fuel ratio switching means for switching the air-fuel ratio of the air-fuel mixture supplied to the engine between a stoichiometric air-fuel ratio and a lean air-fuel ratio according to operating conditions, and NOx at the time of operation at least in the lean air-fuel ratio in the exhaust system. In an engine equipped with a catalyst capable of reducing hydrogen, a stoichiometric duration measuring means for measuring the operating duration when operating at the stoichiometric air-fuel ratio and a lean duration measuring means for measuring the operating duration when operating at the lean air-fuel ratio And when operating at a lean air-fuel ratio, depending on the operating duration at the stoichiometric air-fuel ratio immediately before that and the operating duration at the current lean air-fuel ratio, the lean air-fuel ratio switching means switches the lean air-fuel ratio.
A target lean air-fuel ratio setting means for setting a target lean air-fuel ratio which is a target value of the air-fuel ratio is provided , and when operating at a lean air-fuel ratio,
Target lean air-fuel ratio set by lean air-fuel ratio setting means
An air-fuel ratio control device for an engine, which is characterized in that 5. An air-fuel ratio switching means for switching the air-fuel ratio of the air-fuel mixture supplied to the engine between a stoichiometric air-fuel ratio and a lean air-fuel ratio according to operating conditions, and NOx at the time of operation at least in the lean air-fuel ratio in the exhaust system. In an engine equipped with a catalyst capable of reducing hydrogen, a stoichiometric duration measuring means for measuring the operating duration when operating at the stoichiometric air-fuel ratio and a lean duration measuring means for measuring the operating duration when operating at the lean air-fuel ratio When switching from the stoichiometric air-fuel ratio to the lean air-fuel ratio, the switching time setting means for setting the switching time according to the operation duration at the stoichiometric air-fuel ratio immediately before that, and the switching from the stoichiometric air-fuel ratio to the lean air-fuel ratio After that, when the operation duration time at the current lean air-fuel ratio is within the switching time,
Target of lean air-fuel ratio switched by the air-fuel ratio switching means
The target lean air-fuel ratio which is a value is set to the first target lean air-fuel ratio on the richer side, and when the switching time is exceeded,
Target lean air-fuel ratio setting means for setting the target lean air-fuel ratio to the second target lean air-fuel ratio on the leaner side is provided , and when operating at a lean air-fuel ratio, the target air-fuel ratio is set to the above-mentioned target.
Target lean air-fuel ratio set by lean air-fuel ratio setting means
An air-fuel ratio control device for an engine, which is characterized in that