JPH0763096A - Air-fuel ratio controller of internal combustion engine - Google Patents

Abstract

PURPOSE:To prevent any increase in an exhaust quantity of NOx in a lean combustion operational range. CONSTITUTION:An exhaust quantity of NOx per specified time is calculated (P9-P14) based on output Van of a NOx sensor provided downstream from a catalyst and an intake air quantity Qa detected by an air-flow meter when an injection quantity Ti is calculated in response to a target air-fuel ratio leaner than a theoretical air-fuel ratio(P6). The target air-fuel ratio is switched from a lean air-fuel ratio to the theoretical air-fuel ratio so as to calculate the infection quantity Ti (P17) when the calculated NOx exhaust quantity is more than the standard exhaust quantity S/L(P15).

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 system for an internal combustion engine, and more particularly, to an increase in NOx emission amount under operating conditions in which a target air-fuel ratio is set to a leaner air-fuel ratio than a stoichiometric air-fuel ratio. In addition, the present invention relates to a technology capable of maximizing fuel efficiency.

[0002]

2. Description of the Related Art Conventionally, as a technique for suppressing the NOx emission amount from an engine, there is one disclosed in, for example, Japanese Utility Model Laid-Open No. 63-108543. In the fuel injection control disclosed in the above publication, the fact that the output of an oxygen sensor that detects the oxygen concentration in the exhaust gas shifts according to the NOx concentration in the exhaust gas is utilized to detect an increase in the NOx concentration during high load immediately after acceleration. At this time, the air-fuel ratio is made rich, and the NOx emission amount immediately after acceleration is reduced.

[0003]

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention
As in the air-fuel ratio control method disclosed in Japanese Patent Application Laid-Open No. 77741, the target air-fuel ratio of the engine is significantly leaner than the theoretical air-fuel ratio and the theoretical air-fuel ratio depending on operating conditions such as engine load and cooling water temperature. A configuration is known in which switching control is performed between an air-fuel ratio (for example, air-fuel ratio = 22).

In such lean air-fuel ratio control, NO
The air-fuel ratio is set as the target air-fuel ratio such that the x emission amount is below the allowable level and the generation of surge torque due to instability of combustion is below the allowable level (see FIG. 11). However, in a configuration including a so-called lean NOx catalyst that performs NOx conversion in the lean air-fuel ratio state (excess oxygen atmosphere), the engine is deteriorated due to such lean NOx catalyst, and due to changes in the ignition timing and the compression ratio of the engine. There is a possibility that a steady increase in the NOx emission amount may occur in the lean air-fuel ratio control state due to the increase change in the NOx emission amount from the fuel cell. In the lean combustion state, the concentration of HC and CO is sufficiently small. The increase of NOx becomes a problem.

In the technique disclosed in Japanese Utility Model Laid-Open No. 63-108543, it is not possible to detect a steady increase in the NOx emission amount in the lean air-fuel ratio combustion state, and therefore the NOx emission amount is not detected. Even if the amount increases, this cannot be dealt with, and a large amount of N is generated under lean air-fuel ratio operating conditions.
There was a fear that Ox would be discharged. Further, in Japanese Utility Model Laid-Open No. 63-108543, the NOx emission amount is reduced by correcting the increase of the fuel injection amount, and when the lean air-fuel ratio is set to the target air-fuel ratio, the leaning direction of the air-fuel ratio is set. Is the direction of decreasing the NOx concentration, but the decreasing change rate is small, and the lean direction also coincides with the increasing direction of the surge torque (see FIG. 11). Therefore, the NOx concentration decreases in the lean combustion state. There is also a problem that it is difficult to reduce NOx even if the fuel injection amount is corrected.

Further, in the lean air-fuel ratio range as described above, it is desired to promote leanness as much as possible to improve the fuel consumption performance when the NOx emission amount is below the allowable level. Since combustion may become unstable and large surge torque may be generated when leaning is advanced, in the past, even if there is a change with time etc., a margin is prepared in advance so that a large surge torque does not occur. Since the lean target air-fuel ratio is set in, there is also a problem that fuel efficiency cannot be maximized.

The present invention has been made in view of the above problems, and under operating conditions in which a lean target air-fuel ratio is set, an increase in NOx emission amount can be avoided, and the maximum NOx emission amount can be suppressed while being suppressed. The purpose is to obtain fuel efficiency.

[0008]

Therefore, an air-fuel ratio control system for an internal combustion engine according to the present invention is constructed as shown in FIG. In FIG. 1, an exhaust purification catalyst is a catalyst that is interposed in an exhaust passage of an engine and converts at least NOx in exhaust gas. NOx in exhaust gas is provided downstream of the exhaust purification catalyst.
An NOx sensor for detecting the concentration is provided.

Further, the lean combustion control means controls the air-fuel ratio of the engine intake air-fuel mixture with the air-fuel ratio leaner than the stoichiometric air-fuel ratio as the target air-fuel ratio in the predetermined lean combustion operation region. Further, an intake air amount detecting means for detecting an intake air amount of the engine is provided, and the NOx emission amount calculating means is provided with the intake air amount detected by the intake air amount detecting means and the NOx.
The NOx emission amount per predetermined time is calculated based on the NOx concentration detected by the sensor.

The air-fuel ratio changing means determines the NOx emission amount per predetermined time period calculated by the NOx emission amount calculating means when the lean air-fuel ratio is controlled by the lean combustion control means. When it is determined that the amount exceeds the target amount, the target air-fuel ratio in the predetermined lean combustion operation region is changed. Here, when the exhaust purification catalyst is a catalyst having a three-way catalytic action, when it is determined that the NOx emission amount per the predetermined time exceeds the predetermined reference emission amount, in the predetermined lean combustion operation region. The target air-fuel ratio may be switched from the lean air-fuel ratio to the stoichiometric air-fuel ratio.

Further, an output fluctuation detecting means for detecting an output fluctuation of the engine is provided, and when the lean air-fuel ratio is controlled to the lean air-fuel ratio by the lean combustion control means, a predetermined time period calculated by the NOx emission amount calculating means is calculated. When it is determined that the NOx emission amount is equal to or less than a predetermined reference emission amount, the target lean air-fuel ratio in the predetermined lean combustion operation region is within a range in which the output fluctuation detected by the output fluctuation detecting means does not exceed an allowable level. It is preferable to provide an air-fuel ratio changing means depending on the output variation that makes the engine lean.

Further, it is preferable to provide a reference emission amount variable setting means for variably setting the predetermined reference emission amount based on engine operating conditions.

[0013]

According to the air-fuel ratio control device having such a configuration, when the NOx emission amount per predetermined time exceeds the reference emission amount when the lean air-fuel ratio is controlled in the predetermined lean combustion operation region, the lean combustion is performed. The target air-fuel ratio is changed in the operating region, and the NOx emission amount can be reduced by changing the air-fuel ratio.

In particular, when the exhaust purification catalyst has a three-way catalytic action, if the target air-fuel ratio is changed from the lean air-fuel ratio to the stoichiometric air-fuel ratio, the theoretical air-fuel ratio combustion and the three-way catalytic action are performed. By combining with, exhaust harmful components including NOx can be purified at a high conversion rate to improve exhaust properties. Also, if the NOx emission amount does not exceed the reference emission amount even if the engine is operated at the initially set lean air-fuel ratio,
Normally, the lean air-fuel ratio is controlled, but at this time, the output fluctuation of the engine is detected, and the lean target air-fuel ratio is made lean in a range in which the output fluctuation does not exceed the allowable level. That is, in the lean air-fuel ratio region, fuel efficiency can be improved by promoting leaning, but on the contrary, combustion becomes unstable, so by advancing leaning while monitoring output fluctuation of the engine, output exceeding the allowable level can be achieved. We have made it possible to maximize leanness while avoiding fluctuations.

Further, the reference emission amount for discriminating the calculated NOx emission amount is variably set based on the engine operating condition, whereby the discriminating accuracy of the NOx emission amount is improved, and only when it is truly necessary, the empty amount is obtained. It is possible to change the fuel ratio.

[0016]

EXAMPLES Examples of the present invention will be described below. In FIG. 2 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 fuel injection valve 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. Catalytic converters (exhaust gas purification catalysts) 9a and 9b are provided in the exhaust pipes 7a and 7b, respectively. The catalytic converters 9a, 9b are configured to reduce NOx, HC, and
It has both a three-way catalytic action for simultaneously purifying CO at a high conversion rate and a lean NOx catalytic action for reducing (converting) NOx in an oxygen excess state (lean air-fuel ratio combustion state).

The control unit 10 has a built-in microcomputer, calculates a fuel injection amount Ti (injection pulse width) by the fuel injection valve 5 based on detection signals from various sensors, and calculates the fuel injection amount Ti. The fuel supply to the engine 1 is electronically controlled by controlling the open drive of the fuel injection valve 5 based on The engine of this embodiment is a so-called lean burn engine that burns at an air-fuel ratio (for example, 22) that is significantly leaner than the stoichiometric air-fuel ratio in a predetermined operating range.
Except for the lean combustion operation region, combustion is performed with the stoichiometric air-fuel ratio as the target air-fuel ratio to ensure acceleration performance and the like.

The various sensors include a throttle valve 3
Upstream, the air flow meter 11 as an intake air amount detecting means for detecting the intake air amount Qa of the engine 1, a crank angle sensor 12 for extracting an engine rotation signal from a cam shaft, and a water temperature sensor for detecting a cooling water temperature Tw of the engine 1. 13. Oxygen sensors 14a and 14b, which are respectively provided in the collecting portion of the exhaust manifolds 6a and 6b and detect the oxygen concentration in the exhaust gas for each bank,
A potentiometer-type throttle sensor 15 for detecting the opening of the throttle valve 3 is provided.

Further, each of the catalytic converters 9a and 9b is provided with NOx sensors 16a and 16b on the downstream side thereof. The NOx sensors 16a and 16b are semiconductor type A
It is a thin film type sensor of g _0.04 V ₂ O ₅ . This NOx sensor is a known sensor whose resistance value changes as shown in FIG. 3 due to the adsorption of NOx on the sensor surface.

Reference numeral 17 is a control valve for adjusting the intake air amount 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 auxiliary air supplied to the. Here, the structure of the fuel injection control by the control unit 10 is simplified and shown in the block diagram of FIG.

As shown in FIG. 4, the control unit
Reference numeral 10 is a function as a basic fuel injection amount calculation means A for calculating the basic fuel injection amount Tp based on the detection signals of the crank angle sensor 12 and the air flow meter 11, the NOx sensor 16a,
NO based on the detection signals of 16b and the air flow meter 11
The function as the NOx emission amount calculation means B for calculating the x emission amount, the final fuel injection based on the calculation results by the basic fuel injection amount calculation means A and the NOx emission amount calculation means B, and the detection signal of the water temperature sensor 13. A function as an injection amount calculation unit C (including a function as a lean combustion control unit and an air-fuel ratio changing unit) that calculates the amount Ti, and further, a fuel based on the fuel injection amount Ti calculated by the injection amount calculation unit C Injection valve 5
It has a function as an injection valve drive control means D for driving and controlling.

Next, the state of fuel injection control (air-fuel ratio control) performed according to the configuration shown in FIG. 4 will be described in detail with reference to the flowchart of FIG. In the flowchart of FIG. 5, at P1, the intake air amount Qa of the engine 1 is detected based on the detection signal of the air flow meter 11. Next, at P2, the engine speed Ne is detected based on the detection signal of the crank angle sensor 12.

At P3, the basic injection amount (basic injection pulse width) Tp = K × Qa / Ne in the fuel injection valve 5 is calculated based on the engine speed Ne and the intake air amount Qa.
(K is a proportional constant corresponding to the flow rate characteristic of the injection valve 5.) is calculated. The basic injection amount Tp calculated in P3 is calculated as a stoichiometric air-fuel ratio equivalent amount.

At the next P4, the cooling water temperature Tw is detected based on the detection signal of the water temperature sensor 13. Then, in P5, it is determined whether or not the cooling water temperature Tw detected in P4 exceeds a predetermined temperature (for example, 70 ° C.). Here, the cooling water temperature Tw is a parameter for determining an operation region in which lean combustion is performed in this embodiment.
In addition to the cooling water temperature Tw, the engine load and the engine speed Ne
The lean combustion region may be determined based on the above.

If lean combustion is executed when the cooling water temperature Tw is lower than a predetermined temperature, the stability of engine operation is greatly deteriorated. Therefore, the routine proceeds to P17, where the basic fuel injection amount Tp calculated in P3 is used. Then, the fuel injection amount Ti with the stoichiometric air-fuel ratio (= 14.6) as the target air-fuel ratio is calculated. The calculation of the fuel injection amount Ti in P17 is performed by the cooling water temperature Tw.
Various correction coefficients COEF that are set by including a correction coefficient and an acceleration increase correction coefficient, etc., and a correction set to correct the invalid injection time of the injection valve 5 based on the battery voltage that is the power source of the injection valve The basic fuel injection amount Tp calculated in P3 is corrected by the amount Ts or the like (Ti
= Tp x COEF + Ts).

On the other hand, when it is determined at P5 that the cooling water temperature Tw exceeds the predetermined temperature, it is determined that the operating range is in the lean combustion mode. In this case, the process proceeds to P6 and the operation is performed at P3. Based on the calculated basic fuel injection amount Tp, calculation of the fuel injection amount Ti with the initially set lean target air-fuel ratio (22 in this embodiment) as the target air-fuel ratio (Ti = (14.6 /
22) × Tp × COEF + Ts).

At the next P7, the time measured by the timer Tm
Is determined for a predetermined time (for example, 4 seconds) or more. When the measured time Tm is equal to or longer than the predetermined time, the measured time Tm is reset to zero at P8, and then the process proceeds to P9. In P9, the NOx sensors 16a, 16b
Of the outputs V _Rn and V _Ln are measured, and at P10, an average value V _An (= (V _Rn + V _Ln ) / 2) of the outputs V _Rn and V _Ln is calculated.

The subscript n of the output indicates a value corresponding to the measurement timing. At the next P11, the moving average value KV _An of the average value V _An is calculated according to the following equation. KV _An = (79/80) × KV _An-1 + (1/80) × V _{An In the} above equation, KV _An-1 is the previous value of the moving average value KV _An . Further, in the present embodiment, the program shown in the flowchart of FIG. 5 is set to be interrupted every 50 ms, and since the density average value for 4 seconds is calculated, the number of data of density detection values is 4 sec / 50 ms. = 80, the above weighting coefficient is used.

Further, in P12, the moving average value KQa of the intake air amount Qa is calculated in accordance with the following equation, as in P11. KQa = (79/80) × KQa ₋₁ + (1/80) × Qa In P13, in the measurement time Tm, the execution cycle of this program
Add 50 ms to count up the measurement time Tm.

At the next P14, as a result of the count-up at P13, it is judged whether or not the measured time Tm exceeds 4 seconds which is the calculation cycle of the NOx emission amount. When the measurement time Tm is less than 4 seconds, the moving average value KV of the NOx concentration
_The present program is terminated without calculating the NOx emission amount based on the calculation result of _An and the moving average value KQa of the intake air amount Qa.

On the other hand, when the measured time Tm exceeds 4 seconds, the routine proceeds to P15, where the reference emission amount S / L for discriminating the NOx emission amount is previously set to the engine speed Ne and the basic fuel injection amount Tp. It is determined by referring to a map (see FIG. 6) in which the reference emission amount S / L is stored for each of the operation regions divided into a plurality of regions based on the above. The reference emission amount S / L is made to correspond to the allowable level of NOx emission amount for each operating condition, and thus the reference emission amount S / L is variably set according to the engine operating condition. The target air-fuel ratio, which will be described later, can be changed only when it is truly necessary. The function of P15 corresponds to the reference emission amount variable setting means in this embodiment.

At the next P16, the NOx emission amount (= KV _An × KQa) obtained by multiplying the moving average value KV _An of the NOx concentration by the moving average value KQa of the intake air amount Qa, that is, in 4 seconds. NOx emission amount and the reference emission amount S
Compare with / L. Here, if it is determined that the NOx emission amount predicted and calculated from the detection result of the NOx concentration and the intake air amount Qa is equal to or less than the reference emission amount S / L, NO
Since it indicates that the x emission amount is within the allowable level, this program is ended as it is in order to continue the lean air-fuel ratio combustion.

On the other hand, if it is determined that the predicted NOx emission amount exceeds the reference emission amount S / L,
If the lean air-fuel ratio combustion is continued as it is, NOx in an amount exceeding the allowable level will be emitted, so the routine proceeds to P17, in which the fuel injection amount Ti with the theoretical air-fuel ratio as the target air-fuel ratio is calculated, The injection control is switched to the stoichiometric air-fuel ratio as the target air-fuel ratio in the original lean combustion operation region.

That is, in the lean combustion operation region, the amount of NOx exceeding the allowable level is reduced due to a decrease in lean NOx catalytic action in the catalytic converters 9a and 9b and an increase in NOx emission amount from the engine in the lean combustion state.
When the exhaust gas becomes exhausted, the target air-fuel ratio in the lean-burn operation region is switched from an air-fuel ratio leaner than the stoichiometric air-fuel ratio to the stoichiometric air-fuel ratio, and combustion by the stoichiometric air-fuel ratio and three-way catalytic action are combined to generate NOx. It is intended to obtain good exhaust gas properties by converting exhaust gas harmful components including the above at a high conversion rate.

Therefore, under the condition that the NOx emission amount does not exceed the allowable level even if the lean combustion is performed, good fuel economy performance can be obtained by the lean combustion, and the NOx emission can be achieved by the lean combustion. When the amount exceeds the permissible level, the stoichiometric air-fuel ratio is switched to maintain good exhaust properties. By the way, in the above embodiment, when the NOx emission amount predicted and calculated from the NOx concentration and the intake air amount of the engine is lower than the reference emission amount, the lean combustion is performed with the initially set lean air-fuel ratio as the target air-fuel ratio. However, even in the same lean combustion state, the fuel efficiency can be further improved by promoting the lean lean target air-fuel ratio.
However, as the lean system progresses, there is a fear that combustion becomes more unstable and a large surge torque is generated (see Fig. 11).

Therefore, a second embodiment will be described below in which the target lean air-fuel ratio can be made leaner while suppressing the generation of surge torque. The block diagram of FIG. 7 shows the control unit in the second embodiment.
FIG. 3 is a diagram showing a simplified configuration of fuel injection control by 10. In FIG. 7, an output fluctuation detecting means E for detecting an output fluctuation (stability) of the engine based on the output of the crank angle sensor 12 is added to the configuration block diagram of FIG. 4 corresponding to the first embodiment described above. And an injection calculation means C
Is configured to calculate the injection amount based on the outputs of the NOx emission amount calculation means B, the basic fuel injection amount calculation means A, the output fluctuation detection means E, and the water temperature sensor 13. Incidentally, in the second embodiment, the function as the air-fuel ratio changing means due to the output fluctuation is included in the injection amount calculating means C shown in FIG.

Next, details of the injection amount control (air-fuel ratio control) in the second embodiment will be described with reference to the flowchart of FIG. In the flowchart of FIG. 8, at P21, the intake air amount Qa of the engine 1 is detected based on the detection signal of the air flow meter 11. Next, at P22, the engine speed Ne is detected based on the detection signal of the crank angle sensor 12.

At P23, the basic injection amount (basic injection pulse width) Tp = K × Qa / Ne in the fuel injection valve 5 is calculated based on the engine speed Ne and the intake air amount Qa.
(K is a proportional constant corresponding to the flow rate characteristic of the injection valve 5.) is calculated. The basic injection amount Tp calculated in P23 is calculated as the theoretical air-fuel ratio equivalent amount.

At the next P24, it is determined whether or not the absolute value of the variation ΔTp of the basic fuel injection amount Tp per unit time is less than or equal to a predetermined value. When the absolute value of the change amount ΔTp exceeds a predetermined value, it is determined that the engine is in acceleration / deceleration operation. In this case, the routine proceeds to P38, where the basic fuel injection amount Tp calculated in P23 is set. Based on the theoretical air-fuel ratio (= 14.6)
Fuel injection amount Ti (= Tp × COEF
+ Ts) is calculated.

That is, in the second embodiment, the stoichiometric air-fuel ratio is controlled as the target air-fuel ratio during the acceleration / deceleration operation of the engine, and the lean combustion operation is limited to the steady operation so that the lean air-fuel ratio is lean. The deterioration of the combustion stability due to the combustion is discriminated. When it is determined at P24 that the engine is in the steady operation state from the change amount ΔTp, the routine proceeds to P25, where the cooling water temperature Tw is detected based on the detection signal of the water temperature sensor 13.
To detect.

Then, in P26, it is determined whether or not the cooling water temperature Tw detected in P25 exceeds a predetermined temperature (for example, 70 ° C.). If lean combustion is executed when the cooling water temperature Tw is lower than or equal to a predetermined temperature, the stability of engine operation is greatly deteriorated. Therefore, the routine proceeds to P38, and the theoretical empty amount is calculated based on the basic fuel injection amount Tp calculated in P23. Fuel ratio (= 14.6)
Is calculated as the target air-fuel ratio.

On the other hand, when it is determined at P26 that the cooling water temperature Tw exceeds the predetermined temperature, it is determined that the operating region is in the lean combustion mode. In this case, the process proceeds to P27 and the operation is performed at P23. Based on the calculated basic fuel injection amount Tp (theoretical air-fuel ratio equivalent value), calculation of the fuel injection amount Ti with the initially set lean air-fuel ratio (22 in this embodiment) as the target air-fuel ratio (Ti = (14.6 / 22) x Tp x COEF x βn + Ts)
To perform.

It should be noted that βn (initial value = 1.
0) is a coefficient for correcting the initially set lean target air-fuel ratio (= 22), and is variably set based on the output variation detection result in the lean combustion state, as will be described later. . N from next P28 to P37
The estimation calculation of the Ox emission amount is the same as that of P7 to P16 in the flowchart of FIG. 5 described above, and thus the description thereof is omitted here.

In P37, the estimated NOx emission amount (=
KV _An × KQa) and the standard emission amount S / L are compared,
When the NOx emission amount exceeds the reference emission amount, the routine proceeds to P38, where the target air-fuel ratio in the lean combustion region is forcibly switched to the stoichiometric air-fuel ratio to reduce the NOx emission amount. On the other hand, in P37, the estimated NOx emission amount (= KV _An ×
When it is determined that KQa) is equal to or less than the reference emission amount S / L, it is in a state where the lean combustion can be continued as it is, but in the present embodiment, the process proceeds to P39.

At P39, the rotation fluctuation (ωn) is calculated from the output of the crank angle sensor 12. As shown in FIG. 9, the difference Δω between the maximum rotation speed (maximum rotation speed) and the minimum rotation speed (minimum rotation speed) during the explosion cycle of each cylinder is correlated with the indicated torque value (output fluctuation), and the difference Standard deviation of Δω σΔω
Based on the above, the stability of the engine can be determined as shown in FIG.

Therefore, at P40, the standard deviation σΔω is compared with a predetermined value, and when it exceeds the predetermined value, that is, when the output fluctuation of the engine is large and unstable, the lean air-fuel ratio is set as the target air-fuel ratio. The coefficient βn used in the calculation of the injection amount (P27) is increased by a predetermined value Δβ to correct the air-fuel ratio to the rich side. The rich correction stabilizes the combustion and ensures the stability of engine operation.

On the other hand, when it is judged at P40 that the standard deviation σΔω is less than or equal to the predetermined value, that is, when the output fluctuation of the engine is in a sufficiently small stable state, the coefficient βn is reduced by the predetermined value Δβ. , By modifying the air-fuel ratio to the lean side, we will promote leanness and improve fuel efficiency. By controlling the coefficient βn, it is possible to make the target lean air-fuel ratio lean to the maximum while avoiding the occurrence of output fluctuation exceeding the allowable level.

As described above, in the second embodiment described above, even if the stoichiometric air-fuel ratio is not switched in the lean combustion region,
When the amount of Ox emission is sufficiently low, it is judged from the output fluctuation at that time whether or not the lean conversion can be further advanced, and the lean conversion is advanced until the output fluctuation exceeds the allowable level to maximize the fuel efficiency. So that you can pull out.

It should be noted that the detection of the engine output fluctuation is not limited to the method based on the rotational speed described above, and the cylinder internal pressure or the output shaft torque detected by the torque sensor may be detected as a parameter. It is possible to use the various output fluctuation detection methods described above.

[0051]

As described above, according to the present invention,
It is possible to prevent NOx from being emitted in excess of the allowable amount in the lean combustion operation region, and to maintain good exhaust properties,
By correcting the lean target air-fuel ratio from the output fluctuation, NO
x Promoting leanness while suppressing emissions and output fluctuations,
This has the effect of improving fuel efficiency.

[Brief description of drawings]

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

FIG. 2 is a schematic diagram showing a system configuration of an embodiment.

FIG. 3 is a diagram showing characteristics of a NOx sensor.

FIG. 4 is a block diagram showing the configuration of the first embodiment.

FIG. 5 is a flowchart showing injection control of the first embodiment.

FIG. 6 is a diagram showing a map that stores NOx standard emission amounts.

FIG. 7 is a block diagram showing the configuration of a second embodiment.

FIG. 8 is a flowchart showing injection control of the second embodiment.

FIG. 9 is a diagram showing how the rotation changes.

FIG. 10 is a diagram showing a relationship between rotational fluctuation and engine stability.

FIG. 11 is a diagram showing the relationship between NOx concentration, surge torque, and air-fuel ratio.

[Explanation of symbols]

 1 Internal Combustion Engine 5 Fuel Injection Valve 9a, 9b Catalytic Converter (Exhaust Gas Purification Catalyst) 10 Control Unit 11 Air Flow Meter 12 Crank Angle Sensor 13 Water Temperature Sensor 16a, 16b NOx Sensor

Claims (4)

[Claims] 1. An exhaust purification catalyst which is provided in an exhaust passage of an engine and which converts at least NOx in exhaust gas, and an engine intake mixing which uses an air-fuel ratio leaner than a theoretical air-fuel ratio as a target air-fuel ratio in a predetermined lean combustion operation region. Lean combustion control means for controlling the air-fuel ratio of air; NOx sensor for detecting the NOx concentration in the exhaust gas downstream of the exhaust purification catalyst; intake air amount detection means for detecting the intake air amount of the engine; The intake air amount detected by the air amount detecting means and the N
NOx emission amount calculation means for calculating the NOx emission amount per predetermined time based on the NOx concentration detected by the Ox sensor; and the NOx emission amount calculation when the lean air-fuel ratio is controlled by the lean combustion control means. An air-fuel ratio changing means for changing the target air-fuel ratio in the predetermined lean combustion operation range when it is determined that the NOx emission amount per predetermined time calculated by the means exceeds a predetermined reference emission amount. An air-fuel ratio control device for an internal combustion engine, which is configured. 2. The exhaust purification catalyst is a catalyst having a three-way catalytic action, and when the air-fuel ratio changing means determines that the NOx emission amount per the predetermined time exceeds a predetermined reference emission amount, The air-fuel ratio control device for an internal combustion engine according to claim 1, wherein the target air-fuel ratio in the predetermined lean combustion operation region is switched from the lean air-fuel ratio to the stoichiometric air-fuel ratio. 3. An output fluctuation detecting means for detecting an output fluctuation of the engine, and NOx emission per predetermined time period calculated by the NOx emission amount calculating means when the lean air-fuel ratio is controlled by the lean combustion control means. When it is determined that the amount is less than or equal to a predetermined reference emission amount, the target lean air-fuel ratio in the predetermined lean combustion operation region is lean within a range in which the output fluctuation detected by the output fluctuation detecting means does not exceed an allowable level. 3. An air-fuel ratio control device for an internal combustion engine according to claim 1, further comprising: 4. The internal combustion engine according to claim 1, further comprising a reference emission amount variable setting means for variably setting the predetermined reference emission amount based on engine operating conditions. Air-fuel ratio control system for engines.