JPH06330741A - Air fuel ratio controller of lean burn engine - Google Patents

Abstract

PURPOSE:To ensure the amount of HC necessary for reduction process for NOx through a lean NOx catalyst in a lean-burn engine provided with the lean NOx catalyst. CONSTITUTION:The discharged amounts of HC and NOx when driven at an aimed lean air fuel ratio and when driven at a predetermined rich air fuel ratio, are estimated. A frequency N for driving periodically at a rich air fuel ratio is set based on the results of estimation and on the ratio of the amount of HC to the amount of NOx required to achieve anticipated invert ratio. A cylinder to be driven periodically at the rich air fuel ratio is generated (S24-S28) during the driving (S21) at the lean air fuel ratio according to the set frequency N.

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 a lean burn engine, and more specifically to NOx in the presence of HC.
The present invention relates to an air-fuel ratio control technique for maintaining the NOx reducing ability of a lean NOx catalyst that reduces NOx.

[0002]

2. Description of the Related Art In recent years, a lean burn engine that burns in an air-fuel ratio range (for example, an air-fuel ratio of about 20 to 22) that is significantly leaner than the stoichiometric air-fuel ratio has been developed. An exhaust purification catalyst device called a lean NOx catalyst is used to purify NOx in the region.

The lean NOx catalyst is mainly composed of zeolite, and is presumed to temporarily adsorb HC in exhaust gas and reduce NOx by this HC. Therefore, N in the lean NOx catalyst is
The presence of HC is indispensable for the purification process of Ox.
As shown in, the ratio of the amount of HC in the exhaust gas to the amount of NOx (HC
/ NOx) becomes equal to or less than the predetermined value R, the conversion rate of NOx decreases.

On the other hand, the HC / NOx concentration in the exhaust gas is shown in FIG.
As shown in, when the target lean air-fuel ratio of lean burn control is set to the rich side, it becomes smaller.Therefore, if the target lean air-fuel ratio is made smaller due to restrictions such as operational stability, the amount of HC becomes insufficient with respect to the amount of NOx. , N in lean NOx catalysts
In some cases, the conversion rate of Ox could not be maintained. As a technique for avoiding a decrease in the NOx conversion rate due to such a shortage of HC, for example, in Japanese Patent Laid-Open No. 3-253744, exhaust gas recirculation (EGR) is performed under operating conditions predicted to be short of HC.
There is disclosed a technique for forcibly increasing the amount of HC in the exhaust gas by forcibly performing the above.

Further, Japanese Laid-Open Patent Publication No. 3-237216 discloses a swirl control valve in which a strong swirl is generated in a cylinder to ensure combustion stability during lean air-fuel ratio operation. There is disclosed a technique in which the swirl adjusted by the valve is forcibly weakened by performing the open control, and the amount of HC in exhaust gas is forcibly increased.

[0006]

However, JP-A-3-253744 and JP-A-3-237216 are known.
In the technique disclosed in Japanese Patent Publication, combustion becomes unstable by performing exhaust gas recirculation or weakening the swirl, which causes unburned residue to be used to forcibly increase the amount of HC in exhaust gas. Lean N
Even if the conversion rate of NOx in the Ox catalyst can be maintained, there is a fear that the operating stability of the engine will be deteriorated due to an increase in surge torque.

Further, the exhaust gas recirculation and swirl control are used to control the HC.
In the control for forcibly increasing the amount, the amount of HC in the exhaust cannot be controlled with high accuracy, and the amount of HC necessary for maintaining the conversion rate of NOx can be reliably obtained to obtain the conversion rate of NOx. It was difficult to keep it stable. The present invention has been made in view of the above problems, and control for forcibly increasing the amount of HC in exhaust gas in order to maintain the NOx conversion rate in a lean NOx catalyst, without causing instability of combustion, However, the purpose is to be able to perform with high accuracy.

[0008]

Therefore, the air-fuel ratio control system for a lean burn engine according to the present invention is operated at a lean air-fuel ratio according to the operating conditions of the engine, while reducing NOx in the presence of HC in the exhaust passage. It is an air-fuel ratio control device for a lean burn engine in which a lean NOx catalyst is installed, which is configured as shown in FIG.

In FIG. 1, the first emission amount estimating means is
The amount of NOx and HC generated in the operating state with the lean air-fuel ratio is estimated based on the operating condition and the air-fuel ratio, and the second
The emission amount estimating means estimates the amount of NOx and HC generated in an operating state with a predetermined rich air-fuel ratio based on the operating condition and the air-fuel ratio. The rich operation frequency calculation means causes the operation with the predetermined rich air-fuel ratio during the operation with the lean air-fuel ratio based on the amounts of NOx and HC estimated by the first and second emission amount estimation means. The frequency of periodically generating cylinders is calculated.

Then, the cylinder-by-cylinder air-fuel ratio control means controls the air-fuel ratio to either the lean air-fuel ratio or the predetermined rich air-fuel ratio for each cylinder according to the frequency calculated by the rich operation frequency calculation means. Here, the predetermined rich air-fuel ratio is set so that the generated torque in the cylinder operated at the predetermined rich air-fuel ratio by the individual cylinder air-fuel ratio control means becomes the same as the generated torque in the cylinder operated at the lean air-fuel ratio. It is preferable to provide a rich ignition timing changing means for changing the ignition timing of the cylinder operated at.

Further, it is preferable that the cylinder-by-cylinder air-fuel ratio control means is configured so that the cylinders to be operated at the predetermined rich air-fuel ratio are different for each rich air-fuel ratio operation.

[0012]

According to the air-fuel ratio control device for a lean burn engine having such a configuration, the amount of NOx and HC generated when operating at a lean air-fuel ratio according to the operating conditions, and forcibly operating at a predetermined rich air-fuel ratio. NOx generated when you let
And the amount of HC are respectively estimated based on the operating conditions and the air-fuel ratio. Then, the frequency of occurrence of the cylinders periodically operated at the rich air-fuel ratio becomes the estimation result of the emission amount so that the ratio of the amount of HC to the amount of NOx becomes a value that allows the lean NOx catalyst to exhibit a high conversion rate. It is calculated based on.

That is, when the amount of HC required to reduce NOx cannot be secured when the engine is operated at a desired lean air-fuel ratio, a rich air-fuel ratio operation is periodically performed during lean air-fuel ratio operation. Generate a cylinder to operate at a fuel ratio. And
Due to the periodic rich air-fuel ratio operation, the H
The amount of C was forcibly increased to compensate for the shortage of the amount of HC due to operation with a lean air-fuel ratio. However, since the emission amounts of NOx and HC at each of the lean air-fuel ratio and the rich air-fuel ratio are estimated and the frequency of operation at the rich air-fuel ratio is calculated based on the estimation result, the rich air-fuel ratio is required at the minimum necessary frequency. It is possible to drive.

Further, in the case where the cylinders to be operated at the rich air-fuel ratio are periodically generated during the lean air-fuel ratio operation as described above, there are a cylinder operated at the lean air-fuel ratio and a cylinder operated at the rich air-fuel ratio. There is a step difference in the generated torque between the two. Therefore, the ignition timing of the cylinder operated at the rich air-fuel ratio is changed (retarded) so that the generated torque of the cylinder operated at the rich air-fuel ratio matches the torque generated in the cylinder operated at the lean air-fuel ratio. .

Further, when the rich air-fuel ratio operation is periodically performed during the lean air-fuel ratio operation, if only a specific cylinder is periodically operated at the rich air-fuel ratio, the amount of heat generated between the cylinders is increased. There is a risk of unbalance and distortion due to heat.Therefore, the cylinders operated at the rich air-fuel ratio are made different for each rich air-fuel ratio operation so that the operation opportunities at the rich air-fuel ratio can be made substantially equal in each cylinder. I did it.

[0016]

EXAMPLES Examples of the present invention will be described below. In FIG. 2 showing the system configuration of one embodiment, the engine 1 is
Air is taken in through the air cleaner 2, the intake duct 3, the throttle chamber 4, and the intake manifold 5.

An air flow meter 6 for detecting the intake air flow rate Q of the engine 1 is interposed in the intake duct 3. The throttle chamber 4 is provided with a butterfly-type throttle valve 7 that opens and closes in conjunction with an accelerator pedal (not shown) to adjust the intake air amount of the engine 1. The throttle valve 7 has its opening degree. A potentiometer-type throttle sensor 8 for detecting TVO is attached.

In order to control the amount of auxiliary air supplied to the engine 1 by bypassing the throttle valve 7,
An idle control valve 9 whose opening is adjusted by a control unit 23 which will be described later, a fast idle control valve (FICD) 10 which is opened / closed in response to an air conditioner signal, and an auxiliary air amount is adjusted in response to the temperature of cooling water. A wax pellet type air regulator 11 is provided.

On the other hand, an injector 12 is provided for each cylinder in each branch portion of the intake manifold 5. The injector 12 is an electromagnetic injector that intermittently injects fuel, which is pressure-fed from a fuel tank (not shown) and adjusted to a predetermined pressure by a pressure regulator, to the engine 1 and supplies the fuel to the intake stroke of each cylinder. The air-fuel ratio of the intake air-fuel mixture can be controlled for each cylinder by controlling the opening individually at the same timing.

Further, the spark plug 13 is made to face the combustion chamber of each cylinder.
Is provided, and the high voltage generated in the ignition coil 14 is distributed and supplied to the spark plug 13 according to an ignition timing control signal output from the control unit 23 via a distributor 15. . A crank angle sensor 16 is built in the distributor 15. The crank angle sensor 16 outputs a reference angle signal REF for each reference piston position of each cylinder and a unit angle signal POS for each unit angle. The engine rotation speed Ne can be calculated based on the generation cycle of the reference angle signal REF or the number of generations of the unit angle signal POS within a predetermined time.

Exhaust gas from the engine 1 includes an exhaust manifold 17, an exhaust duct 18, a lean NOx catalyst 19, and a muffler 20.
It is discharged into the atmosphere via. In the exhaust duct 18,
An air-fuel ratio sensor 21 capable of widely detecting the air-fuel ratio based on the oxygen concentration in the exhaust gas, which is closely related to the air-fuel ratio of the intake air-fuel mixture
Is installed. An exhaust gas temperature sensor 22 that detects the exhaust gas temperature is provided downstream of the lean NOx catalyst 19.

The lean NOx catalyst 19 is made of zeolite supporting a transition metal or a noble metal, has a HC adsorbing capacity (HC storage effect), and has a function of reducing NOx in the presence of HC in an oxidizing atmosphere. Is. In addition to the various sensors, the control unit 23 having a built-in microcomputer is supplied with detection signals from a water temperature sensor 24 that detects the cooling water temperature and a knock sensor 25 that detects knocking vibration from the cylinder block. There is. Then, the control unit 23, based on the detection signal from the various sensors,
It has a function of controlling the fuel injection amount by each injector 12, the ignition timing by the spark plug 13, and the amount of auxiliary air adjusted by the idle control valve 9.

In FIG. 2, a canister 26 adsorbs and collects the vaporized fuel in a fuel tank (not shown), and desorbs and supplies the vaporized fuel collected and adsorbed to the collector portion of the intake manifold 5. The engine 1 is a so-called lean burn engine, and the control unit 23 determines a lean burn operating condition as described later, and when the lean burn condition is satisfied, the engine is significantly leaner than the theoretical air-fuel ratio according to the operating condition. While setting the fuel ratio as the target air-fuel ratio, the target air-fuel ratio near the stoichiometric air-fuel ratio is set when the lean burn condition is not satisfied. And
Calculate the fuel injection amount according to the set target air-fuel ratio,
In accordance with the calculated fuel injection amount, each injector 12 is individually drive-controlled in synchronization with the intake stroke of each cylinder (sequential injection control).

By the way, since the lean NOx catalyst 19 reduces NOx due to the presence of HC as described above, if the amount of HC is insufficient with respect to the amount of NOx, the conversion rate of NOx is lowered (see FIG. 7). On the other hand, when the lean burn operation is performed, if the target lean air-fuel ratio is made leaner, it is possible to sufficiently secure the HC necessary for reducing NOx, but in order to ensure the operation stability, the lean burn operation is performed. When the target lean air-fuel ratio at the time is rich-shifted, there is a possibility that the amount of HC required to reduce NOx cannot be secured (see FIG. 8).

Therefore, in this embodiment, as described later, H
Based on the estimation result of the amounts of C and NOx generated, it is possible to determine the shortage state of the HC amount and to eliminate the shortage state of the HC amount.
During the lean burn operation, cylinders that are periodically operated at a rich air-fuel ratio are generated, and when operating at such a rich air-fuel ratio, N
The amount of HC required to purify Ox is ensured.

The state of the air-fuel ratio control by the control unit 23 will be described below with reference to the programs shown in the flow charts of FIGS. The program shown in the flowchart of FIG. 3 is for determining lean burn operating conditions. First, in S1,
It is determined whether or not the engine 1 has been started.

When the start is completed, the next S
2 to S4, cooling water temperature Tw, engine speed N
e, it is determined whether or not the engine load is within a predetermined range. Here, it is determined that the lean-burn operating condition is satisfied when the cooling water temperature Tw, the engine rotation speed Ne, and the engine load are all within the predetermined range at the completion of the start. , S5 to set the lean air-fuel ratio operation permission state.

On the other hand, one of the cooling water temperature Tw, the engine rotation speed Ne, and the engine load is being started.
If the lean air-fuel ratio is not included in the predetermined range, it is determined that the lean air-fuel ratio operation condition is not satisfied, and the process proceeds to S6 to set the lean air-fuel ratio operation impossible state. The program shown in the flowchart of FIG. 4 is for calculating the frequency (cycle) of periodically generating the cylinders to be operated at a predetermined rich air-fuel ratio during lean operation.

The function of the control unit 23 shown in the flow chart of FIG. 4 corresponds to the first emission amount estimating means, the second emission amount estimating means and the rich operation frequency calculating means in this embodiment. In the flowchart of FIG. 4, first, in S11, it is determined whether or not the operation by the lean air-fuel ratio is permitted based on the determination result in the flowchart of FIG.

In the lean air-fuel ratio operation impossible state,
If the air-fuel ratio near the stoichiometric air-fuel ratio is to be operated as the target air-fuel ratio, the process proceeds to S12, where the number of cycles N indicating the frequency of periodic rich air-fuel ratio operation during lean air-fuel ratio operation is zero. Reset. On the other hand, if it is determined in S11 that the lean air-fuel ratio operation is permitted, the process proceeds to S13, and first, the target rich air-fuel ratio when periodically operating in the rich air-fuel ratio during lean air-fuel ratio operation is set to It is set according to the target lean air-fuel ratio and the operating conditions (engine load, rotation) at that time.

The rich air-fuel ratio is for the purpose of forcibly increasing the HC in the exhaust gas by operating at the rich air-fuel ratio, and effectively increases the HC from the target lean air-fuel ratio and operating conditions at that time. It is possible to set a rich air-fuel ratio. Next, in S14, the NOx emission amount N when the engine is operated at the target lean air-fuel ratio based on the current target lean air-fuel ratio and the operating conditions (engine load, rotation).
Estimate Ox (L). The estimation of the emission amount NOx (L) is performed by setting the estimated emission amount data in advance in a map using the air-fuel ratio, engine load, and rotation as parameters and referring to the map. good.

Similarly, in the next S15, the HC emission amount HC (L) when the engine is operated at the target lean air-fuel ratio based on the current target lean air-fuel ratio and the operating conditions (engine load, rotation). To estimate. Further, in S16 and S17, based on the rich air-fuel ratio and the operating conditions (engine load, rotation) set in S13, the NOx emission amount NOx (R) and HC emission when operating at the rich air-fuel ratio. Quantity H
Estimate C (R) respectively.

In this embodiment, when the rich air-fuel ratio operation is periodically performed during the lean air-fuel ratio operation, the torque generated in the cylinder operated at the rich air-fuel ratio and the lean air-fuel ratio are used. In order to make the generated torque of the cylinders substantially equal to each other, the ignition timing of the cylinders operated at the rich air-fuel ratio is retarded, as will be described later. Correspondingly, HC and NOx at the rich air-fuel ratio are corrected. The estimation of the emission amount is predicted to occur when the ignition timing of the cylinder operated at the rich air-fuel ratio is retarded, and HC and NOx are predicted to occur.
The amount.

In S18, the estimated lean air-fuel ratio emissions HC (L), NOx (L) and the rich air-fuel ratio emissions HC (R), NOx (R), and the target HC. Based on the ratio R of the amount and the amount of NOx (see FIG. 7), the number N of cycles for continuously operating all the cylinders at the lean air-fuel ratio.
(Integer), in other words, the cycle of the rich air-fuel ratio operation is calculated.

In this embodiment, after operating one cylinder at the rich air-fuel ratio set in S13, the number of cylinders Nc × N
The lean air-fuel ratio is continuously operated for only one cycle, and the air-fuel ratio switching control for again operating only one cylinder at the rich air-fuel ratio is repeated. The cycle number N is set to the NOx and HC emission amounts. By variably setting according to the estimation result, the frequency of generating the cylinders operated at the rich air-fuel ratio is changed, and thus the lean NOx catalyst is
Ensure that the HC / NOx ratio that can secure the NOx conversion rate at 19 is obtained.

The rich air-fuel ratio in S13 is set so that the target HC / NOx ratio can be obtained by appropriately setting the cycle number N in the air-fuel ratio switching control as described above. . The number of cycles N is
Specifically, it is calculated by the following formula. The following number 1
In INTG (), INTG () represents an arithmetic characteristic that the number of cycles N is calculated by rounding off the data after the decimal point in parentheses.

[0037]

[Equation 1]

That is, the frequency of the cylinder operated at the lean air-fuel ratio is N _L , and the frequency of the cylinder operated at the rich air-fuel ratio is N _R.
(= 1), the total amount of HC is N _L · HC (L) +
N _R · HC (R), and the total amount of NOx is N _L
・ NOx (L) + N _R・ NOx (R). And
In the present embodiment, since the target is HC / NOx ≧ R (see FIG. 7), it is necessary to set the frequencies N _L and N _R that satisfy the following equation.

[0039]

[Equation 2]

Here, the frequency N _{R is set} to 1 and N _L
= Cylinder number Nc × cycle number N, and when expanded to an equation for calculating the cycle number N, the arithmetic expression shown in the equation 1 for obtaining the cycle number N for obtaining the HC / NOx ratio = R is obtained.
Therefore, if the cylinders to be operated at the rich air-fuel ratio are periodically generated according to the cycle number N calculated according to the above equation 1, the amount of HC insufficient for the NOx reduction process in the operation at the target lean air-fuel ratio will be changed to the cycle. The HC / NOx ratio can be maintained at a value capable of exhibiting a NOx conversion rate equal to or higher than a predetermined value R by being supplemented by HC relatively generated in a cylinder operated at a rich air-fuel ratio.

The program shown in the flow chart of FIG. 5 is for actually controlling the air-fuel ratio for each cylinder in accordance with the number of cycles N (frequency of rich air-fuel ratio operation) calculated according to the flow chart of FIG. The control unit 23 shown in the flowchart of FIG.
This function corresponds to the cylinder-by-cylinder air-fuel ratio control means in this embodiment.

The program shown in the flow chart of FIG. 5 has a reference angle signal REF corresponding to the injection interval for each cylinder.
It is executed every time. First, in S21, it is determined whether or not the condition is that the air-fuel ratio is operated at the lean air-fuel ratio.
Then, if the operating condition is such that lean air-fuel ratio operation is not possible, the control proceeds to S22, where after the cylinder counter n is reset to zero, all cylinders are operated in accordance with the target air-fuel ratio near the stoichiometric air-fuel ratio in S23. Control).

That is, when the target air-fuel ratio is set near the stoichiometric air-fuel ratio, the injectors 12 provided for each cylinder are driven and controlled based on the target air-fuel ratio equivalent amount.
Make all cylinders operate with the same air-fuel ratio. on the other hand,
If it is determined in S21 that the lean air-fuel ratio operation is permitted, the process proceeds to S24 and the cylinder counter n
Increase by 1.

Then, in the next S25, the cylinder counter n counted up in S24 is a value obtained by adding 1 to the value obtained by multiplying the cycle number N calculated in the flowchart of FIG. 4 by the cylinder number Nc ( It is determined whether or not (N · Nc + 1) or more. The determination in S25 is for determining whether or not it is time to generate a cylinder that is periodically operated at a rich air-fuel ratio during lean air-fuel ratio operation.

In the flow chart of FIG. 4, N
After the operation with the lean air-fuel ratio is continued by the number of cylinders indicated by Nc, one cylinder is operated to operate with the rich air-fuel ratio. However, if N · Nc is the number of cylinders with which the lean operation is continued, Only one specific cylinder is operated at the rich air-fuel ratio, which causes an imbalance in the amount of heat generated between the cylinders. Therefore, the cycle of the lean air-fuel ratio operation is set to N · Nc + 1, and the cylinders that are cyclically operated at the rich air-fuel ratio are sequentially changed for each rich air-fuel ratio operation.

At S25, the cylinder counter n is set to n <N.N.
If it is determined to be c + 1, it is regarded as a condition for continuing the lean air-fuel ratio operation, the process proceeds to S26, and the lean air-fuel ratio set according to the operating condition is set as the target air-fuel ratio, and each cylinder is set. To form the air-fuel mixture. On the other hand, S25
When it is determined that n ≧ N · Nc + 1, it is determined that it is time to operate one cylinder at the rich air-fuel ratio, and the process proceeds to S27 to measure the next lean air-fuel ratio operation period. After resetting the cylinder counter n to zero, the process proceeds to S28, the injection amount corresponding to the rich air-fuel ratio is calculated to form the air-fuel mixture of the set rich air-fuel ratio, and at that time, the injection amount is calculated according to the injection amount. The rich control for controlling the injector 12 corresponding to the cylinder, which is the injection timing, is executed.

According to this structure, in the case of operating the lean air-fuel ratio as the target air-fuel ratio, and in the continuous operation with the target lean air-fuel ratio, the HC required for the NOx reduction process by the lean NOx catalyst 19 is required. When the amount cannot be secured, it is possible to generate a cylinder operated at a rich air-fuel ratio at a cycle necessary and sufficient to supplement the amount of HC. Therefore, even if the balance between the HC amount and the NOx amount at the target air-fuel ratio in the lean air-fuel ratio operation cannot maintain a high NOx conversion rate, the periodic rich air-fuel ratio operation causes exhaust gas It is possible to increase the amount of HC in the medium and reduce NOx at a high conversion rate.

Naturally, the cycle of operation at the rich air-fuel ratio is the estimation result of HC and NOx emissions at the target lean air-fuel ratio, and the estimation of HC and NOx emissions at the rich air-fuel ratio set for increasing HC. Since it is set based on the result, it is possible to perform the operation with the rich air-fuel ratio without excess or deficiency. Further, since the control for forcibly increasing the amount of HC in the exhaust gas is performed by enriching the intake air-fuel mixture, combustion does not become unstable due to the increase control of the amount of HC.

The control for periodically operating the rich air-fuel ratio during lean operation as described above is preferably executed only during steady operation. By the way, as described above, when the operation with the rich air-fuel ratio is periodically performed during the lean air-fuel ratio operation, the torque generated between the cylinder operated with the lean air-fuel ratio and the cylinder operated with the rich air-fuel ratio In order to prevent a large step from occurring in the control unit 23, the control unit 23 variably controls the ignition timing according to the air-fuel ratio as shown in the flowchart of FIG.

The function of the control unit 23 shown in the flow chart of FIG. 6 corresponds to the rich ignition timing changing means in this embodiment. In the flowchart of FIG. 6, in S31, it is determined whether or not the conditions for performing the lean air-fuel ratio operation are satisfied. Then, if the operating conditions for performing the lean air-fuel ratio operation are not satisfied, the process proceeds to S32, and ignition is preset which is adapted to the case where the target air-fuel ratio near the stoichiometric air-fuel ratio (stoichiometric air-fuel ratio) is set. The ignition control is performed by referring to the timing map.

On the other hand, if it is determined in S31 that the lean operation condition is satisfied, the routine proceeds to S33, in the air-fuel ratio control for periodically performing the rich air-fuel ratio operation during the lean air-fuel ratio operation, the lean air-fuel ratio control is performed. It is determined whether or not the air-fuel ratio operation is being performed. Then, when the engine is actually operated with the lean air-fuel ratio, the routine proceeds to S34, where ignition control is executed with reference to an ignition timing map which is previously adapted to the operation with the lean air-fuel ratio.

If it is determined in S33 that the air-fuel ratio operation is not being performed but the air-fuel ratio operation is being performed periodically at the rich air-fuel ratio, the routine proceeds to S35, where the operation is performed at the rich air-fuel ratio. Ignition control is performed with reference to the ignition timing map adapted to. Here, in the ignition timing map for the rich air-fuel ratio, the maximum torque is obtained so that the generated torque in the cylinder operated at the lean air-fuel ratio and the generated torque in the cylinder operated at the rich air-fuel ratio are substantially equal to each other. The ignition timing retarded with respect to the ignition timing is set (see FIGS. 9 and 10).

Therefore, even during lean air-fuel ratio operation,
When the engine is periodically operated with the rich air-fuel ratio, the ignition timing corresponding to the cylinder that has been operated with the lean air-fuel ratio until then is changed to the retard side and the ignition is controlled. Next, another embodiment of the air-fuel ratio control that causes the cylinders to be periodically operated at the rich air-fuel ratio as described above will be described according to the program shown in the flowchart of FIG.

In the program shown in the flow chart of FIG. 11, every time the lean air-fuel ratio operation is performed,
It is configured to determine whether or not the HC amount is insufficient, and it is possible to control with higher accuracy. The flowchart of FIG. 11 is executed for each reference angle signal REF corresponding to the injection interval for each cylinder.
Then, the cylinder number n _C is increased by 1, and in the next S42, the cylinder number n _C and the number of cylinders Nc are increased by 1 in the above.
Comparing the door, when the cylinder number n _C is larger than the number of cylinders Nc resets the cylinder number n _C to 1 in S43.

That is, by the processing in S41 to S43,
The cylinder number n _C is incremented by 1 as an initial value for each injection control for each cylinder, and is reset to the initial value in one cycle in which the injection of all cylinders ends. In S44, it is determined whether or not the conditions for operating the lean air-fuel ratio are satisfied. Then, if it is not the condition to operate with the lean air-fuel ratio, the routine proceeds to S45, S46, and the integrated value ΣHC (L) of the HC emission amount HC (L) and the NOx emission amount NOx (L) estimated during the lean air-fuel ratio control. ) And ΣNOx (L) are each reset to zero.

In the next step S47, injection control (stoichiometric control) is executed in which the target air-fuel ratio is an air-fuel ratio near the stoichiometric air-fuel ratio. On the other hand, if it is determined in S44 that the condition for operating with the lean air-fuel ratio is satisfied, the process proceeds to S48,
It is determined whether or not the ratio of the amount of HC to the amount of NOx is a value R capable of maintaining a high NOx conversion rate in the lean NOx catalyst 19.

Specifically, the HC emission amount HC that is predicted to be generated when one cylinder is operated at the lean air-fuel ratio
(L) and NOx emission amount NOx (L) respectively integrated for each lean air-fuel ratio operation ΣHC (L), ΣNOx
(L) and the HC emission amount HC (R) and the NOx emission amount N predicted when the operation was performed at the rich air-fuel ratio last time.
Based on Ox (R) and R, which is the HC / NOx ratio required to maintain a high NOx conversion rate in the lean NOx catalyst 19, whether or not the relationship shown in the following Expression 3 is established. Determine whether.

[0058]

[Equation 3]

That is, it is considered that the amount of increase in the HC amount by the previous operation with the rich air-fuel ratio is applied to the shortage of the HC amount in the subsequent continuous operation with the lean air-fuel ratio, It is determined whether or not the shortage of the amount of HC cannot be compensated by the amount of HC increased by the rich air-fuel ratio of No. 3 by whether or not the relation of the above-mentioned equation 3 is established.

When it is determined in S48 that the relation of the above-mentioned equation 3 is established, the HC conversion rate of NOx can be maintained.
It is estimated that the actual amount of HC is insufficient with respect to the amount of fuel, and conversely, when it is determined that the relationship of the above equation 3 is not established, the amount of HC that can maintain the conversion rate of NOx is It has been secured. Therefore, when it is determined that the HC amount required for the NOx reduction process is secured, the routine proceeds to S49, S50, where the HC emission amount HC (L) predicted in the lean air-fuel ratio operation this time and NOx emissions NOx
Based on (L), the cumulative emissions ΣHC (L) and ΣNOx (L) during the continuous lean air-fuel ratio operation are updated and set.

Then, in S51, the operation with the lean air-fuel ratio is continued. On the other hand, if it is determined in S48 that the amount of HC is insufficient, the process proceeds to S52, in which the cylinder CYL for which operation at the rich air-fuel ratio is designated and the current cylinder number n _C
It is determined whether and match. Where CYL =
If it is not n _C , the process proceeds to S49, and the execution of the rich air-fuel ratio operation is postponed after the next time.

If it is determined that CYL = n _C , the process proceeds to S53, and when operating at the rich air-fuel ratio next time, the cylinder different from this time is operated so as to operate at the rich air-fuel ratio. Increase the rich operation designated cylinder CYL by 1.
Then, in the next S54, the designated cylinder C which has been increased by 1 is added.
It is determined whether or not YL exceeds the number of cylinders Nc, and if it exceeds, the process proceeds to S55 to set the designated cylinder CYL to the initial value 1
Reset to.

In this way, the cylinders to be operated at the rich air-fuel ratio can be switched in sequence for each rich air-fuel ratio control. In S58, in response to the determination result in S48, in order to increase the amount of HC in the exhaust gas, the target air-fuel ratio in the injection control cylinder at this time is set to the rich air-fuel ratio, and drive control of the corresponding injector 12 is performed.

Also in the embodiment shown in the flow chart of FIG. 11, the ignition timing control shown in the flow chart of FIG. 6 is executed so that the generated torque does not vary.

[0065]

As described above, according to the present invention,
In a lean burn engine configured with a lean NOx catalyst that reduces NOx in the presence of HC in the exhaust passage, a rich air-fuel ratio operation is performed during lean air-fuel ratio operation at a cycle based on the estimation result of HC and NOx emissions. Therefore, the amount of HC necessary for purifying NOx in the lean NOx catalyst can be reliably ensured, and H
Combustion is not made unstable to secure the C content,
There is an effect that the purification performance of NOx can be maintained at a high level without deteriorating the drivability.

[Brief description of drawings]

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

FIG. 2 is a system configuration diagram of an embodiment of the present invention.

FIG. 3 is a flowchart showing air-fuel ratio control in the embodiment.

FIG. 4 is a flowchart showing air-fuel ratio control in the embodiment.

FIG. 5 is a flowchart showing air-fuel ratio control in the embodiment.

FIG. 6 is a flowchart showing air-fuel ratio control in the embodiment.

FIG. 7 is a diagram showing the relationship between the HC / NOx ratio and the NOx conversion rate.

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

FIG. 9 is a diagram showing a relationship between an air-fuel ratio and generated torque.

FIG. 10 is a diagram showing a relationship between ignition timing and generated torque.

FIG. 11 is a flowchart showing air-fuel ratio control in another embodiment.

[Explanation of symbols]

 1 engine 6 air flow meter 12 injector 16 crank angle sensor 19 lean NOx catalyst 23 control unit

Continuation of the front page (51) Int.Cl. ⁵ Identification code Office reference number FI technical display location F02D 41/14 310 D 8011-3G 41/34 L 8011-3G 41/36 B 8011-3G 43/00 301 B E

Claims (3)

[Claims] 1. An air-fuel ratio control device for a lean burn engine, which is operated at a lean air-fuel ratio according to operating conditions of an engine and has a lean NOx catalyst for reducing NOx in the presence of HC in an exhaust passage. First, the amount of NOx and HC generated in the operating state with the lean air-fuel ratio is estimated based on the operating condition and the air-fuel ratio.
And a second emission amount estimating means for estimating the amounts of NOx and HC generated in an operating state with a predetermined rich air-fuel ratio based on the operating condition and the air-fuel ratio, and the first and second NOx estimated by the emission estimation means of
And rich operation frequency calculation means for calculating the frequency of periodically generating cylinders to be operated at the predetermined rich air-fuel ratio during operation at the lean air-fuel ratio, based on the amount of HC, and rich operation frequency calculation A lean air-fuel ratio control means for controlling the air-fuel ratio for each cylinder to either the lean air-fuel ratio or the predetermined rich air-fuel ratio according to the frequency calculated by the means, and a lean air-fuel ratio control means. Air-fuel ratio controller for burn engine. 2. The predetermined torque is controlled by the cylinder-by-cylinder air-fuel ratio control means so that the torque generated in the cylinder operated at the predetermined rich air-fuel ratio is the same as the torque generated in the cylinder operated at the lean air-fuel ratio. 2. The air-fuel ratio control apparatus for a lean burn engine according to claim 1, further comprising rich ignition timing changing means for changing the ignition timing of the cylinder operated at the rich air-fuel ratio. 3. The cylinder-by-cylinder air-fuel ratio control means makes the cylinder operated at the predetermined rich air-fuel ratio different for each rich air-fuel ratio operation. Air-fuel ratio controller for lean burn engine.