JPH1193745A - Air-fuel ratio control device for lean-burn engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To accurately detect a lean limit during lean-burn operation without causing surge. SOLUTION: An air-fuel ratio correction value setting means M2 sets an air-fuel correction value to correct a fuel injection pulse width set in accordance with a target air-fuel ratio to be rich at two combustion strokes for cylinders and then sets the air-fuel ratio correction value to correct it to be lean at one combustion stroke for cylinders. As a result, in case of an four-cylinder engine, an air-fuel ratio during lean-burn operation is rich-rich-lean against a target air-fuel ratio at the sequential combustion strokes in the same cylinder. A combustion state comparison value setting means M6 sets a combustion state comparison value from the ratio of an average value for combustion pressure during combustion due to the latest and previous rich-correction of all cylinders to an average value for combustion pressure during combustion due to the latest lean-correction of all cylinders and a lean limit judgement means M7 compares a combustion state comparison value with a preset combustion state comparison reference value to judge a lean limit when the combustion state comparison value does not exceed its reference value.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an air-fuel ratio control device for a lean burn engine capable of controlling an air-fuel ratio near a lean limit while suppressing engine speed fluctuations.

[0002]

2. Description of the Related Art Various lean burn engines have been proposed which realize stable combustion with a lean mixture by improving combustion.

In this type of lean-bar engine, swirl vortices (lateral vortices) or tumble vortices (longitudinal vortices) are generated in the intake air to enhance the gas flow in the cylinder, thereby reducing NO.
The air-fuel ratio limit, which is limited by the x-emission amount, is increased to enable stable combustion even in a lean air-fuel ratio region equal to or higher than the NOx emission limit.

[0004] In the lean burn engine, air-fuel ratio control is also performed at the same time in order to perform stable combustion near the lean limit (combustion limit). For example, JP-A-5-2312
Japanese Patent Application Laid-Open No. 10-210, No. 10 discloses a method of detecting a rotational fluctuation rate and a load fluctuation rate of an engine, determining a combustion state by comparing a ratio of the both fluctuation rates and a reference value, and determining whether the combustion state is stable. The target air-fuel ratio gradually shifts to the lean side until the combustion state becomes unstable, and when the combustion state is determined to be unstable, the fuel injection amount is increased to a predetermined amount and the target air-fuel ratio is returned to the rich side. It has been disclosed.

[0005] Also, JP-A-6-159130 discloses that
Technology that calculates a combustion index that is a lean limit from engine rotation fluctuations, compares this combustion index with a preset threshold, and returns the target air-fuel ratio to the rich side when the combustion index exceeds the threshold. It has been disclosed.

[0006]

By the way, when the engine speed fluctuates during the lean burn operation, it is known that low frequency vibration (surge) of about 3 to 7 Hz is generated in the vehicle due to the engine speed fluctuation. ing. Because this surge overlaps the vibration frequency band that humans feel most sensitive,
Even a slight vibration feels uncomfortable.

However, since the engine rotation fluctuation detected in the above two prior arts occurs when the engine speed exceeds the lean limit, it is considered that a surge is generated when the engine rotation fluctuation is detected, and the operation is stopped. Will cause discomfort for a moment. Furthermore, if the air-fuel ratio control based on the target air-fuel ratio is executed simultaneously for all cylinders, for example, when the target air-fuel ratio is set to a value exceeding the lean limit, the combustion state of all cylinders becomes temporarily unstable, Not only does this cause an increase in engine speed fluctuation, but also a strong surge occurs.

In the above two prior arts, the target air-fuel ratio is controlled so as to gradually lean to a region where engine rotation fluctuation occurs. However, as shown in FIG. 17, an air-fuel ratio for detecting engine rotation fluctuation is detected. The region and the target air-fuel ratio near the ideal lean limit do not always match, and it is considered that the combustion state at the time when the engine rotation fluctuation is detected is considered to be significantly unstable. It is difficult to achieve low fuel consumption and low pollution, which are characteristics of lean burn engines, in an ideal form.

SUMMARY OF THE INVENTION In view of the above circumstances, the present invention detects a lean limit properly without giving a driver discomfort, and reflects the detection result in air-fuel ratio control to improve fuel efficiency and reduce exhaust emissions. It is an object of the present invention to provide a lean burn engine air-fuel ratio control device that can be achieved.

[0010]

To achieve the above object, a first air-fuel ratio control apparatus for a lean burn engine according to the present invention comprises an air-fuel ratio correction value for rich correction of a target air-fuel ratio and an air-fuel ratio correction value for lean correction. And a target air-fuel ratio setting means for setting a target air-fuel ratio during lean burn operation based on at least the engine operating state and the air-fuel ratio correction value. Combustion state detection value setting means for detecting a combustion state detection value for each cylinder; and combustion for setting a combustion state comparison value by comparing the combustion state detection value at the time of rich correction with the combustion state detection value at the time of lean correction. It is characterized by comprising a state comparison value setting means, and a lean limit determination means for comparing the combustion state comparison value with a set value to determine a lean limit.

The air-fuel ratio control device for a second lean-burn engine is the air-fuel ratio control device for a first lean-burn engine, wherein the target air-fuel ratio setting means determines the lean limit by the lean limit determination means. It is characterized in that the target air-fuel ratio is corrected to the rich side, and when the lean limit is not determined, the target air-fuel ratio is corrected to the lean side.

A third lean-burn engine air-fuel ratio control device is the first or second lean-burn engine air-fuel ratio control device, wherein the combustion state comparison value setting means sets the combustion state detection value at the time of rich correction. The combustion state comparison value is set by comparing the average value of the set number of cylinders with the average value of the set number of cylinders of the combustion state detection value at the time of lean correction.

A fourth lean-burn engine air-fuel ratio control device is the same as the first to third lean-burn engine air-fuel ratio control devices, wherein the engine is a four-cylinder engine and the air-fuel ratio correction value setting means performs rich correction. The air-fuel ratio correction value is set to two combustion stroke cylinders, and the air-fuel ratio correction value for lean correction is set to one combustion stroke cylinder.

A fifth aspect of the present invention provides an air-fuel ratio control device for an air-fuel ratio correction device for setting an air-fuel ratio correction value for richly correcting a target air-fuel ratio and the air-fuel ratio correction value for lean correction in the order of cylinders in a combustion stroke. Target air-fuel ratio establishing means for setting a target air-fuel ratio during lean burn operation based on at least the engine operating state and the air-fuel ratio correction value, and a combustion state detection value for detecting a combustion state detection value during combustion for each cylinder Setting means for comparing the detected combustion state value at the time of rich correction with the detected combustion state value based on the target air-fuel ratio to set a first combustion state comparison value; And comparing the detected combustion state value based on the target air-fuel ratio during combustion with the detected combustion state value during lean correction to set a second combustion state comparison value.
Comparison value setting means, combustion state final comparison value setting means for comparing the combustion state first comparison value and the combustion state second comparison value to set a combustion state final comparison value, A lean limit determining unit that determines a lean limit by comparing the set value with a set value.

The sixth lean-burn engine air-fuel ratio control device is the fifth lean-burn engine air-fuel ratio control device, wherein the target air-fuel ratio setting means determines the lean limit by the lean limit determining means. It is characterized in that the target air-fuel ratio is corrected to the rich side, and when the lean limit is not determined, the target air-fuel ratio is corrected to the lean side.

The air-fuel ratio control device for a seventh lean-burn engine is the air-fuel ratio control device for the fifth and sixth lean-burn engines, wherein the engine is a four-cylinder engine and the air-fuel ratio correction value setting means performs rich correction. The air-fuel ratio correction value, the air-fuel ratio correction value corresponding to the target air-fuel ratio, and the air-fuel ratio correction value to be subjected to lean correction are set in the order of the combustion stroke cylinder.

An eighth air-fuel ratio control device for a lean burn engine is the air-fuel ratio control device for a fifth or sixth lean burn engine, wherein the first and second combustion state comparison means compares the detected combustion state value during rich correction. A combustion state first comparison value is set by comparing the average value of the set number of cylinders and the average value of the set number of cylinders of the combustion state detection value during combustion based on the target air-fuel ratio, and the combustion state second comparison value is set. The means compares the average value of the set number of cylinders of the combustion state detection value at the time of combustion based on the target air-fuel ratio with the average value of the set number of cylinders of the combustion state detection value at the time of the lean correction to obtain a second combustion comparison value. It is characterized by setting.

A ninth lean-burn engine air-fuel ratio control device is the first, second, fifth, and sixth lean-burn engine air-fuel ratio control devices. A combustion state detection value is set based on an output signal of a pressure sensor for detecting a combustion pressure at the time of combustion.

A tenth lean-burn engine air-fuel ratio control device is the air-fuel ratio control device for the first, second, fifth, and sixth lean-burn engines. The combustion state detection value is set based on the change amount of the crank angular velocity at the time.

An eleventh lean-burn engine air-fuel ratio control system is the tenth lean-burn engine air-fuel ratio control system according to the tenth lean burn engine air-fuel ratio control device, wherein the change in the crank angular speed is the square of the crank angular speed in the latter stage of combustion and the crank angular speed in the early stage of combustion. Is calculated based on the difference from the square of.

In the first air-fuel ratio control device for the lean burn engine, the air-fuel ratio correction value for richly correcting the target air-fuel ratio and the air-fuel ratio correction value for lean correction are set in the order of the combustion stroke cylinder. A target air-fuel ratio during lean burn operation is set based on the state and the air-fuel ratio correction value, and a combustion state detection value during combustion is detected for each cylinder. Then, a combustion state comparison value is set by comparing the combustion state detection value, and the combustion state comparison value is compared with the set value to determine a lean limit.

In the second lean burn engine air-fuel ratio control device, the target air-fuel ratio is set to rich when the lean-burn limit is determined based on the combustion state comparison value in the first lean burn engine air-fuel ratio control device. To the side,
If the lean limit is not determined, the target air-fuel ratio is corrected to the lean side to control the air-fuel ratio during the lean burn operation near the lean limit.

In a third lean-burn engine air-fuel ratio control apparatus, in the first or second lean-burn engine air-fuel ratio control apparatus, the combustion state comparison value is set to a combustion state detection value setting cylinder at the time of rich correction. By comparing and setting the average value of the numbers and the average value of the set number of cylinders of the combustion state detection value at the time of the lean correction, the influence of the temporary combustion fluctuation due to the variation in combustion between the cylinders is eliminated.

In the fourth air-fuel ratio control device for a lean burn engine, in the first to third air-fuel ratio control devices for a lean burn engine, when the engine has four cylinders, the air-fuel ratio correction value for performing the rich correction is set to 2 Set the combustion stroke cylinder,
By setting the air-fuel ratio correction value to be subjected to lean correction to one combustion stroke cylinder, and repeatedly executing this in the combustion stroke cylinder order,
The fuel injection amount to the combustion stroke cylinder before and after the combustion stroke cylinder by the lean correction and the fuel injection amount supplied at the time of the previous and next combustion of the same cylinder are always rich corrected, and the combustion by the lean correction is limited to the lean limit. Vibration that occurs when the frequency exceeds the frequency band can be set to a high frequency band.

In the fifth lean-burn engine air-fuel ratio control apparatus, the air-fuel ratio correction value for rich correction of the target air-fuel ratio and the air-fuel ratio correction value for lean correction are set in the order of the combustion stroke cylinder. A target air-fuel ratio during lean burn operation is set based on the air-fuel ratio correction value, a combustion state detection value during combustion is detected for each cylinder, and then the combustion state detection value during rich correction and the target air-fuel ratio are detected. A combustion state detection value at the time of combustion based on the target air-fuel ratio, and a combustion state detection value at the time of lean correction. And a combustion state second comparison value is set.
Next, the combustion state first comparison value is compared with the combustion state second comparison value to set a combustion state final comparison value, and the combustion state final comparison value is compared with the set value to determine a lean limit.

In the sixth lean-burn engine air-fuel ratio control apparatus, when the lean-burn engine air-fuel ratio control apparatus determines that the lean limit has been reached, the target air-fuel ratio is corrected to the rich side, and the lean lean engine is controlled. If not determined, by correcting the target air-fuel ratio to the lean side,
The air-fuel ratio during lean burn operation is controlled in a stable combustion range.

According to a seventh lean burn engine air-fuel ratio control device, in the fifth and sixth lean burn engine air-fuel ratio control devices, when the engine is a four-cylinder engine, the air-fuel ratio correction value for performing rich correction and the By repeatedly setting the air-fuel ratio correction value corresponding to the target air-fuel ratio and the air-fuel ratio correction value for the lean correction in the order of the combustion stroke cylinder, the fuel injection amount for the next combustion stroke cylinder after the combustion stroke cylinder by the lean correction, In addition, since the fuel injection amount supplied at the next combustion of the same cylinder is always rich-corrected, the vibration generated when the combustion by the lean correction exceeds the lean limit can be set to a high frequency band.

According to an eighth aspect of the present invention, in the fifth and sixth lean-burn engine air-fuel ratio control apparatuses, the average value of the set number of cylinders of the detected combustion state value at the time of rich correction and the target A combustion state detection value at the time of combustion based on the air-fuel ratio is compared with an average of the set number of cylinders to set a combustion state first comparison value, and a combustion state detection value setting cylinder at the time of combustion based on the target air-fuel ratio is set. By comparing the average value of the number and the average value of the set number of cylinders of the combustion state detection value at the time of the lean correction and setting the combustion second comparison value, temporary combustion fluctuation due to variation in combustion between cylinders is reduced. Remove.

In a ninth lean-burn engine air-fuel ratio control system, in the first, second, fifth, and sixth lean-burn engine air-fuel ratio control systems, preferably, the detected combustion state value is determined by each cylinder of the engine. Is set based on the output signal of the pressure sensor for detecting the combustion pressure at the time of combustion provided in the above.

In a tenth lean-burn engine air-fuel ratio control apparatus, in the first, second, fifth, and sixth lean-burn engine air-fuel ratio control apparatuses, preferably, the combustion state detection value is determined by the combustion of each cylinder. It is set based on the change amount of the crank angular velocity at the time.

An eleventh lean-burn engine air-fuel ratio control system is characterized in that in the tenth lean-burn engine air-fuel ratio control system, the change in the crank angular speed is determined by calculating the square of the crank angular speed in the latter stage of combustion and the crank angular speed in the early stage of combustion. By calculating based on the difference from the square of, the torque index value can be derived from the change amount.

[0032]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to FIGS. 1 to 11 show a first embodiment of the present invention.

Reference numeral 1 in FIG. 10 denotes an engine, and in the drawing, a horizontally opposed four-cylinder engine is shown. The engine 1 can select both combustion at a normal stoichiometric air-fuel ratio and lean burn (lean combustion) according to the operating state. When lean combustion is selected, for example, an intake control valve interposed in an intake port A vortex such as a swirl flow or a tumble flow is generated by a not-shown flow to enhance the gas flow in the cylinder, thereby enabling stable combustion with a lean mixture. Incidentally, the above-mentioned intake control valve has been described in detail in Japanese Patent Application Laid-Open No. Hei 7-119472 by the present applicant, and the description thereof will be omitted.

An intake port 2a and an exhaust port 2b communicating with each cylinder are formed in the cylinder head 2 of the engine 1, and each intake port 2 has an intake manifold 3a.
The intake manifold 3 is provided with an air chamber 4 in which intake passages of respective cylinders are gathered. Through the air chamber 4, a throttle chamber 5 and an intake pipe 6 are communicated. An air cleaner 7 is attached to the mouth side.

An exhaust pipe 26 communicates with the exhaust port 2b via an exhaust manifold 25, and a muffler 27 communicates with the exhaust pipe.

An intake air amount sensor 8 of a hot wire type or the like is interposed immediately downstream of the air cleaner 7 in the intake pipe 6. A throttle valve 5 a provided in the throttle chamber 5 has a throttle opening degree. ON when the throttle opening sensor 9a that outputs a voltage corresponding to the throttle valve and the throttle valve is fully closed
And an idle switch 9b having an idle contact.

A bypass passage 10 which bypasses the throttle valve 5a and communicates between the upstream side and the downstream side.
An ISC (idle speed control) valve 11 is interposed. Further, an injector 14 is located immediately upstream of each intake port 2 a of each cylinder of the intake manifold 3.
A spark plug 15a whose tip is exposed to the combustion chamber is attached to each cylinder. An ignition coil 15b is connected to each of the ignition plugs 15a.
The igniter 16 is connected to 5b.

The injector 14 has a fuel supply passage 17.
The fuel tank 18 is in communication with a fuel tank 18 through which an in-tank type fuel pump 19 is provided. The fuel from the fuel pump 19 is fed to the injector 14 and the pressure regulator 21 through a fuel filter 20 interposed in the fuel supply path 17 and is returned from the pressure regulator 21 to the fuel tank 18 to be returned to the fuel tank 18. The fuel pressure to 14 is regulated to a predetermined pressure.

A knock sensor 22 is attached to the cylinder block 1a of the engine 1, and a cooling water passage 23 communicating the left and right banks of the cylinder block 1a.
A cooling water temperature sensor 24 faces the combustion chamber of each cylinder, and a cylinder pressure sensor 28 faces the combustion chamber of each cylinder. Furthermore,
A wide-range air-fuel ratio sensor 29 for continuously detecting the rich to lean air-fuel ratio including the stoichiometric air-fuel ratio is provided at the collecting portion of the exhaust manifold 25, and the catalyst 3
0 is interposed.

A crank rotor 31 is axially mounted on a crankshaft 1b supported by the cylinder block 1a. An electromagnetic pickup or the like is provided on the outer periphery of the crank rotor 31 for detecting a projection corresponding to a predetermined crank angle. A crank angle sensor 32 is provided opposite to a cam rotor 33 connected to a cam shaft 1c that makes a half rotation with respect to the crankshaft 1b. Has been established.

The combustion stroke cylinder order of the engine 1 according to the present embodiment is # 1 → # 3 → # 2 → # 4.
The compression top dead centers (TDC # 1, # 2) of the # 1, # 2 cylinders have the same phase, and the compression top dead center (TDC) of the # 3, # 4 cylinders have the same phase. An angle display section (projection or slit) for displaying a specific set crank angle based on the compression top dead center of each cylinder is formed on the outer periphery of the crank rotor 31. A cylinder discriminating display (projection or slit) is formed on the outer periphery of the cam rotor 33.

An electronic control unit (ECU) 40, which will be described later, calculates a crank angle, an engine speed, and the like from an input interval time of a crank pulse from the crank angle sensor 32 for detecting an angle display section provided on the crank rotor 31. At the same time, cylinder discrimination is performed by interruption of a cam pulse from the cam angle sensor 34 for detecting the cylinder discriminating display section of the cam rotor 33.

As shown in FIG. 11, the electronic control unit 4
0 indicates a CPU 41, a ROM 42, a RAM 43, a backup RAM 44, a counter / timer group 45, and an I / O
An interface 46 is mainly composed of microcomputers connected to each other via a bus line 47, a constant voltage circuit 48 for supplying a stabilized voltage to each section, and a signal from an output port of the I / O interface 46. And a peripheral circuit such as an A / D converter 50 for converting an analog signal input from a sensor into a digital signal.

The counter / timer group 45 includes various timers such as a free-run counter, a counter for counting the input of a cam angle sensor signal, a fuel injection timer, an ignition timer,
Various timers such as a timer for generating a periodic interrupt, a timer for counting an input interval of an output signal of a crank angle sensor, and a watchdog timer for monitoring a system abnormality are collectively referred to for convenience. And various software counters and timers are used.

The constant voltage circuit 48 is connected to a battery 52 via a relay contact of a power relay 51, and a relay coil of the power relay 51 is connected to the battery 52 via an ignition switch 53. or,
The fuel pump 19 is connected to the battery 52 via a relay contact of a fuel pump relay 54. Further, the constant voltage circuit 48 is configured such that the ignition switch 53 is
N, when the contact of the power supply relay 51 is closed,
By stabilizing the voltage of the battery 52, the electronic control unit 40
Supply to each part. Further, the backup RAM 44
Is connected directly to the battery 52 via the constant voltage circuit 48.
Backup power is always supplied regardless of N / OFF.

The input port of the I / O interface 46 includes a vehicle speed sensor 35 and a starter switch 3.
6. The idle switch 9b, the knock sensor 22, the crank angle sensor 32, and the cam angle sensor 34 are connected, the intake air amount sensor 8, the throttle opening sensor 9
a, the cooling water temperature sensor 24, the wide-range air-fuel ratio sensor 29, and the in-cylinder pressure sensor 28 are connected via the A / D converter 50, and the A / D converter 50
2 terminal voltage VB is input and monitored.

On the other hand, the igniter 16 is connected to the output port of the I / O interface 46, and one end of the ISC valve 11, the injector 14, and one end of the relay coil of the fuel pump relay 54 are connected via the drive circuit 49. The other end of the relay coil is connected to the battery 52.

The ROM 42 stores engine control programs, various maps, fixed data such as tables, and the like, and the RAM 43 stores data obtained by processing output signals from the sensors and switches. , And data processed by the CPU 41.
The backup RAM 44 stores control data and the like, and the ignition switch 53 is turned off.
The data is also retained at the time.

The CPU 41 calculates a control amount for the injector 14, the ignition plug 15a, and the ISC valve 11 based on output signals from the sensors and switches according to a program stored in the ROM 42. A lean limit is detected during lean burn, the target air-fuel ratio is corrected to the rich side when the lean limit is reached, and the target air-fuel ratio is corrected when the lean limit is not reached. The air-fuel ratio control which corrects the lean to the lean side is executed.

As shown in FIG. 1, the electronic control unit 40
The functions for executing the air-fuel ratio control of the lean burn engine include an engine operating state determining unit M1, an air-fuel ratio correction value setting unit M2, a target air-fuel ratio setting unit M3, a fuel injection amount setting unit M4, and a combustion state detection value setting unit. M5, combustion state comparison value setting means M6, and lean limit determination means M7 are provided.

The engine operating state determining means M1 sets the engine operating state after the completion of warm-up based on the cooling water temperature, the engine speed, the throttle opening, and the engine load (in this embodiment, the basic fuel injection amount Tp is used instead). It is determined whether the vehicle is in a low speed to medium speed constant speed state.

In the air-fuel ratio correction value setting means M2, a pulse correction value KPULSE as an air-fuel ratio correction value for correcting a target air-fuel ratio A / F to a rich side or a lean side during a lean burn operation, which will be described later, is determined in the combustion stroke cylinder order ( # 1 → # 3 → # 2 → # 4)
Then, the pulse correction value KPULSE for rich correction is continuously set for two combustion stroke cylinders, and then the pulse correction value KPULSE for lean correction is set for one combustion stroke cylinder, and this setting is repeatedly executed. As a result, as shown in FIG. 9, the air-fuel ratio of the combustion stroke cylinder is repeatedly set to the target air-fuel ratio A / F in the order of rich (R) → rich (R) → lean (L).

In the target air-fuel ratio setting means M3, the pulse correction value K is added to the feedforward correction value set based on the parameters detected by the engine operating state detecting means.
PULSE is added, and a lean limit determining means M described later is added.
If the lean limit is not determined at 7, the target air-fuel ratio A /
F is further corrected to the lean side, and when it is determined that it is the lean limit, the target lean reduction coefficient KLEAN for returning to the rich side is subtracted, and the target equivalent ratio KTGT for setting the air-fuel ratio to the target air-fuel ratio A / F. Set.

The fuel injection amount setting means M4 corrects the basic fuel injection amount Tp by feedforward correction and feedback correction, and outputs the fuel injection amount Tp to the injector 14.
Set i.

In the combustion state detection value setting means M5, the combustion pressure P, which is the combustion state detection value near the compression top dead center TDC during combustion, is determined for each cylinder based on the output signal of the in-cylinder pressure sensor 28 provided for each cylinder. To detect.

In the combustion state comparison value setting means M6, the combustion pressure average value PRAVE for all cylinders of the combustion pressure PR when the target air-fuel ratio A / F is rich-corrected and the combustion pressure PL when the lean-correction is performed is calculated. PLAVE is calculated, and the combustion state comparison value P is calculated based on the ratio of the average values PRAVE and PLAVE.
Set x.

The lean limit judging means M7 compares the combustion state comparison reference value Pxs set based on the engine operating state with the combustion state comparison value Px. When Px> Pxs, it is determined that the lean limit has not been reached. And Px ≦ P
If xs, it is determined to be the lean limit.

Hereinafter, the air-fuel ratio control of the lean burn engine executed by the electronic control unit 40 will be described with reference to FIGS.
This will be described according to the flowchart of FIG.

The lean limit correction value setting routine shown in FIG. 2 is started every time a specific crank pulse near the compression top dead center is input.

First, in step S1, the cylinder of the combustion stroke cylinder #i is determined from the interrupt input of the cam pulse. In step S2, the combustion pressure P # i of the combustion stroke cylinder #i is detected. Next, in step S3, it is determined whether the target air-fuel ratio A / F of the combustion stroke cylinder #i is rich-corrected or lean-corrected by referring to the pulse correction value KPULSE, and the pulse correction value KPULSE is determined. Is a positive value (KPULSE>
0), the process proceeds to step S4, and a negative value (KPULSE
If <0), the process proceeds to step S5. The pulse correction value KPULSE is set by a pulse correction value setting routine described later (see FIG. 3).

In step S3, the pulse correction value KPU
When it is determined that LSE is a positive value that is rich correction, the process proceeds to step S4, and when it is determined that LSE is a negative value that is lean correction, the process proceeds to step S5.

In step S4, the latest rich state combustion pressure PR # i of the cylinder stored in the RAM 43 is stored.
Is updated with the previous rich combustion pressure PR # i (-1) of the same cylinder (PR # i (-1) ← PR # i), and the above-described combustion pressure P # i of the cylinder is updated with the combustion pressure P # i detected this time. Rich state combustion pressure PR
#i is updated (PR # i ← P # i), and the process proceeds to step S6.
On the other hand, when the flow proceeds to step S5, the lean combustion pressure PL # i of the cylinder stored in the RAM 43 is updated with the currently detected combustion pressure P # i, and the flow proceeds to step S6.

Then, when the process proceeds to step S6, the above RA
The latest rich state combustion pressures PR # 1, PR # 3, PR # 2, and PR # 4 of all cylinders stored in M43 and the previous rich state combustion pressures PR # 1 (-1) and PR # 3 ( -1), PR # 2 (-1),
PR # 4 (-1) is read, these are averaged, and then the rich state combustion pressure average value PRAVE is calculated (PRAVE ← Σ (P
R # i + PR # i (-1)) / 8), and proceeds to step S7.

At step S7, the latest lean state combustion pressures PL # 1, PL # stored in the RAM 43 are stored.
# 3, PL # 2, and PL # 4 are read, and these are averaged to calculate a lean combustion pressure average PLAVE (PLAVE ← Σ
PL # i / 4), and proceeds to step S8.

In step S8, a combustion state comparison value Px is calculated from the ratio of the lean combustion pressure average PLAVE to the rich combustion pressure average PRAVE (Px ← PLAVE).
/ PRAVE), and proceeds to step S9. The combustion state comparison value Px may be calculated by a well-known weighted averaging process.

In step S9, a combustion state comparison reference value Pxs is set based on the engine operating state. The combustion state comparison reference value Pxs is set by referring to a map with interpolation calculation using, for example, the engine speed NE and the basic fuel injection amount Tp, which is a representative value of the engine load, as shown in FIG. Further, the map stores a combustion state comparison reference value Pxs that gradually increases as the engine speed NE and the basic fuel injection amount Tp shift from a low value to a high value.

In step S10, the combustion state comparison value Px is compared with the combustion state comparison reference value Pxs.
When the lean limit of x ≦ Pxs has been reached, the process proceeds to step S11, where the lean limit correction value KSURGE stored in the RAM 43 is updated with a value obtained by adding the rich correction amount SURGE1 to the lean limit correction value KSURGE, Exit the routine.

In step S10, Px> Pxs
If it is determined that the rich limit has been reached, step S
The flow branches to 12 to update with a value obtained by subtracting the lean correction value SURGE2 from the lean limit correction value KSURGE stored in the RAM 43, and exit the routine.

The lean limit correction value KSURGE is read in a target equivalent ratio setting routine described later.

FIG. 3 shows a pulse correction value setting routine. This routine is started every time a predetermined crank pulse is input to each cylinder.

First, in step S21, it is determined whether or not the cylinder count value n representing the number of rich corrections is 2, and n ≠ 2
That is, when the cylinder count value n is 0 or 1, the process proceeds to step S22, and the pulse correction value KPULSE is changed to a rich correction amount K1 (K1
> O), and in step S23, the cylinder count value n is counted up, and the routine exits.

On the other hand, if it is determined in step S21 that n = 2, the flow branches to step S24, where the pulse correction value KPULSE is set to a lean correction amount K2 (K2 < O) and set in step S
After clearing the cylinder count value n at 25, the routine exits.

As a result, the pulse correction value KPULSE is
Each time the pulse correction value setting routine is executed, the order is set in the order of rich → rich → lean.

The pulse correction value KPULSE is read by a target equivalent ratio setting routine shown in FIG. This routine is started every time a predetermined crank pulse is input to each cylinder.

First, in step S31, a full increase coefficient KFULL is set based on the throttle valve opening, the basic fuel injection amount Tp, or the engine speed NE. The full increase coefficient KFULL is set when the throttle valve is fully opened, when a high load state is detected based on the basic fuel injection amount Tp, or by referring to the table with interpolation calculation using the engine speed NE as a parameter. Is done. As a result, in an operation state where high output is required, such as when the throttle valve is fully opened or during high load operation, the amount of fuel is increased and the output performance is improved. Note that when the throttle valve opening degree is other than the full opening state and other than the high load operation, KFULL ← 0 is set. Therefore, the full increase coefficient KFULL during the lean burn operation is set to zero.

Next, at step S32, the post-start increase coefficient K
AS is set. This post-start increase coefficient KAS is used to secure the stability of the engine speed immediately after the engine starts.
It is set based on the cooling water temperature at the time of starting, etc., and is attenuated every set time until it becomes 0 after the complete explosion.

Then, at step S33, a water temperature increase coefficient KTW is set. The water temperature increase coefficient KTW is an increase coefficient for ensuring operability when the engine is cold.
Based on w, the lower the cooling water temperature Tw, the higher the fuel increase rate is set.

Next, in step S34, the map is referred to by interpolation calculation using the engine speed NE and the basic fuel injection amount Tp as parameters, and the basic lean reduction value KLNMAP is obtained.
Set. This basic lean decrease value KLNMAP is a basic value indicating how much the air-fuel ratio can be made leaner than the stoichiometric air-fuel ratio when setting the target air-fuel ratio A / F during lean burn operation. As shown in FIG. 7, the table stores an optimal basic lean reduction value KLNMAP previously obtained from an experiment or the like for each operation region specified by the engine speed NE and the basic fuel injection amount Tp.

Since the basic lean decrease value KLNMAP is a value set for performing the lean burn operation in a constant speed state from a low speed to a medium speed, as shown in FIG. KLNMAP = 0 is set in the operation region and the high rotation / high load operation region. Note that the basic lean reduction value K set with reference to the map shown in FIG.
FIG. 8 shows an example of the target air-fuel ratio A / F changed by the LNMAP.

In step S35, the basic lean reduction value KLNMAP is corrected by the lean limit correction value KSURGE, and a target lean reduction coefficient KLEAN for setting a basic target air-fuel ratio during lean burn operation is set. (KLEAN
← KLNMAP ・ KSURGE).

Thereafter, the pulse correction value KPULSE is read in step S36, and in step S37, the target equivalent ratio is determined based on the full increase coefficient KFULL, the increase coefficient after starting KAS, the water temperature increase KTW, the target lean decrease coefficient KLEAN, and the pulse correction value KPULSE. Set KTGT from the following equation and exit the routine. KTGT ← 1 + KFULL + KAS + KTW-KLEAN + KPULSE

The target equivalent ratio KTGT is a correction coefficient for performing feed-forward control of the air-fuel ratio (in the present embodiment, the air-fuel ratio is controlled by increasing or decreasing the fuel injection amount) according to the engine operating state. Target lean weight loss coefficient KLE
AN is a parameter for setting the air-fuel ratio to the target air-fuel ratio A / F during lean burn operation.

The pulse correction value KPULSE is a variable intentionally added to detect a lean limit during lean burn operation. Therefore, during operation based on the stoichiometric air-fuel ratio, the target lean reduction coefficient KLEAN and the pulse correction value KPULSE
Are both fixed to 0. Note that the fuel injection control based on the stoichiometric air-fuel ratio is performed by normal control, and thus the description thereof will be omitted.

FIG. 5 shows a fuel injection pulse width setting routine. This routine is executed every set crank angle cycle.

First, in step S41, a basic fuel injection amount Tp is calculated based on the intake air amount Q and the engine speed NE (Tp ← K · Q / NE; K: injector characteristic correction constant).

Next, at step S42, the target equivalent ratio KTG
T is read, and an air-fuel ratio feedback correction coefficient LAMBDA for converging the actual air-fuel ratio to the target air-fuel ratio A / F, which is detected based on the output value from the wide area air-fuel ratio sensor 29 in step S43, is calculated.

In step S44, based on the engine speed NE and the basic fuel injection amount Tp representing the engine load, the variation in the production of the intake air measurement system such as the intake air amount sensor and the fuel supply system such as the injector is determined. The learning value K is referred to by referring to the air-fuel ratio learning map in the backup RAM 44 in which the result of learning the deviation of the air-fuel ratio due to aging is stored.
LR is searched and the air-fuel ratio learning correction coefficient KBLRC is calculated by interpolation.
Set.

Then, at step S45, the intake air amount Q,
Using the coolant temperature Tw and the engine speed NE as parameters, a fuel adhesion correction coefficient Kx for correcting the amount of fuel adhering to the intake port 2a into the cylinder is set by a map search or the like, and based on the battery voltage VB in step S46. The voltage correction coefficient TS for compensating the invalid injection time of the injector 14 is set by referring to the table.

In step S47, the target air-fuel ratio A / F is calculated by multiplying the basic fuel injection amount Tp by the target equivalent ratio KTGT.
, The air-fuel ratio feedback correction coefficient LAMBDA, the air-fuel ratio learning correction coefficient KBLR
C: The air-fuel ratio is corrected by multiplying the fuel adhesion correction coefficient Kx, and the voltage correction coefficient Ts is added to set the final fuel injection pulse width Ti to be supplied to each cylinder. Ti ← Tp ・ KTGT ・ LAMBDA ・ KBLRC ・ Kx + T
s

Then, in step S48, the fuel injection pulse width Ti is set in the injection timer of the combustion stroke cylinder, and the routine exits.

As a result, the injection timer is started at a predetermined timing, a drive pulse signal having the fuel injection pulse width Ti is output to the injector 14 of the combustion stroke cylinder, and a predetermined amount of fuel is injected from the injector 14. You.

The fuel injection pulse width Ti during the lean burn operation is controlled by the pulse correction value KPULSE so that the air-fuel ratio becomes rich (R) in the order of the cylinders in the combustion stroke (# 1 → # 3 → # 2 → # 4).
→ Rich (R) → Lean (L)
As shown in FIG. 9, the air-fuel ratio of the same cylinder is also rich (R) →
It is set in the order of rich (R) → lean (L).

When the combustion in the cylinder in which fuel is injected with the fuel injection pulse width Ti lean-corrected to the target air-fuel ratio A / F exceeds the lean limit, the combustion state becomes extremely unstable and the combustion pressure P decreases. On the other hand, in the cylinder in which fuel is injected with the fuel injection pulse width Ti richly corrected with respect to the target air-fuel ratio A / F, the combustion state is good, and thus the combustion pressure P does not decrease. As a result, the target air-fuel ratio The combustion pressure PL # i of the combustion stroke cylinder and the richly corrected fuel pulse width Ti based on the fuel injection pulse width Ti corrected leaner than the A / F
The difference from the combustion pressure PR # i of the combustion stroke cylinder due to
The lean limit can be detected with high accuracy.

Also, the fuel injection pulse width Ti that has been richly corrected
Is injected twice while the fuel injection pulse width Ti with lean correction is injected once, so that even if the combustion with the fuel injection pulse width Ti with lean correction exceeds the lean limit, Since the fuel injection pulse width Ti that has been richly corrected is continuously supplied twice, even when the combustion of the cylinder is deteriorated due to the lean correction, engine rotation fluctuation occurs in a high frequency band (for example, 22.2 Hz at 2000 rpm). Does not cause discomfort to the person.

Further, since the combustion state comparison value Px is calculated based on the ratio of the rich state combustion pressure average value PRAVE and the lean state combustion pressure average value PLAVE of all cylinders, the temporary combustion due to the variation in combustion between the cylinders. It is hardly affected by the fluctuation, and the influence of the temporary combustion fluctuation due to the fluctuation of the combustion between the cylinders is eliminated, so that the lean limit can be detected with high accuracy.

Also, the latest rich state combustion pressure average value PR
Since the combustion state comparison value Px is calculated by comparing AVE with the lean state combustion pressure average value PLAVE, the lean limit can be detected even in the transient state in which the operating state changes, and the lean burn operation can be performed. The air-fuel ratio at the time can be appropriately controlled.

In this embodiment, the combustion state comparison value Px
The target air-fuel ratio A / F during the lean burn operation is set based on the comparison result between the target air-fuel ratio A / F and the combustion state comparison reference value Pxs. Can be variably set.

Next, FIGS. 12 to 16 show a second embodiment of the present invention. In the first embodiment described above, the fuel injection pulse width Ti during the lean burn operation is richly corrected for two combustion stroke cylinders in succession, and the fuel injection pulse width Ti for one subsequent combustion stroke cylinder is lean corrected. Mean rich combustion pressure PRAVE and mean combustion pressure PLA lean
Although the lean limit is determined from the ratio with VE,
In this embodiment, the fuel injection pulse width Ti at the time of lean burn operation is set to rich (R) → target air-fuel ratio (A / F) → lean ( L) is repeatedly set in this order, and the lean limit is determined from the change in the angular velocity of the engine rotation at this time. Therefore, in the present embodiment, the in-cylinder pressure sensor becomes unnecessary.

As shown in FIG. 12, the electronic control unit 40 (see FIG. 11) has an air-fuel ratio correction value setting shown in FIG. 1 of the first embodiment as a function of executing the air-fuel ratio control of the lean burn engine. Means M2 and combustion state detection value setting means M5
Instead of the above, an air-fuel ratio correction value setting means M2 'and a combustion state detection value setting means M5' are provided, and the combustion state comparison value setting means M6 is further provided with a combustion state first comparison value setting means M6a and a combustion state second It comprises a comparison value setting means M6b and a combustion state final comparison value setting means M6c.

In the air-fuel ratio correction value setting means M2 ', a pulse correction value for performing rich correction → non-correction (target air-fuel ratio) → lean correction of a target air-fuel ratio A / F during lean burn operation, which will be described later, in the order of cylinders in the combustion stroke. Set KPULSE. as a result,
As shown in FIG. 16, the air-fuel ratio (fuel injection pulse width Ti) of the combustion stroke cylinder is rich (R) with respect to the target air-fuel ratio A / F.
→ The target air-fuel ratio (A / F) → Lean (L) are repeatedly set in this order.

The combustion state detection value setting means M5 'calculates the combustion initial crank angular velocity ω1 from the crank pulse input interval at the beginning of combustion (near the compression top dead center) of each combustion stroke cylinder #I, and calculates the late combustion (ATDC40 to ATDC40). 60 ° CA
(The vicinity) of the crank pulse input interval, the latter-stage crank angular speed ω2 is calculated.
Based on the following equation, the torque index value TIN # i of each cylinder is calculated from the following equation. TIN # i ← ω2 ² −ω1 ²

Incidentally, assuming that the mass of the torque T is m and the radius of rotation of the crankshaft is r, T = m · r · ω ² . In this case, since the radius r and the mass m of the crankshaft are constant, the square of the crank angular velocity ω is a function with respect to the torque T, and the difference between the square torque and the initial torque and the final torque during combustion of the cylinder is determined from the difference. A certain torque index value is derived.

In the combustion state first comparison value setting means M6a,
The rich state torque index value TRINAVE is calculated based on the latest rich state torque index value TRIN # i for all cylinders at the time of combustion by the rich correction, and is also calculated for all cylinders from the latest combustion state at the target air-fuel ratio A / F. Target air-fuel ratio torque index value TNIN
The target air-fuel ratio torque index value average value TNINAVE is calculated based on #i, and then the two torque index value average values TRINAVE,
A first combustion state comparison value IRN is calculated from the difference of TNINAVE.

In the combustion state second comparison value setting means M6b,
The lean state torque index value average value TLINAVE is calculated based on the latest lean state torque index value TLIN # i for all cylinders of the combustion by lean correction, and then the target air-fuel ratio torque index value average value TNINAVE and the lean state torque index are calculated. The second combustion state comparison value INL is calculated from the difference from the average value TLINAVE.

The final combustion state comparison value setting means M6c calculates the final combustion state comparison value IRN / NL from the ratio of the first combustion state comparison value IRN to the second combustion state comparison value INL.

Then, the lean limit determining means M7 compares the lean determination reference value TO set based on the engine operating state with the combustion state final comparison value IRN / NL,
When IRN / NL> TO, it is determined that the lean limit has not been reached, and when IRN / NL ≦ TO, the lean limit is determined.

The air-fuel ratio control of the lean burn engine executed by the electronic control unit 40 will be described below with reference to the flowcharts of FIGS.

As shown in FIG. 16, the crank angular velocity calculation routine shown in FIG. 13 inputs the crank pulse of the initial combustion θTDC (near the compression top dead center) and the late combustion θATDC (ATD).
It is activated every time a crank pulse of C40 to 60 ° CA is input.

First, in step S51, it is determined whether the input crank pulse is the initial combustion θTDC or the late combustion θATDC from the interruption of the cam pulse.
In the case of a crank pulse indicating, the process proceeds to step S52 to calculate an initial crank angular velocity ω1 from the current crank pulse (a crank pulse indicating the initial combustion θTDC), and then exits the routine.

On the other hand, in the above step S51, the latter stage of combustion θAT
If it is determined that the crank pulse indicates DC, step S
Branching to 53, this crank pulse (late combustion θATDC
From the crank pulse indicating the late crank angle velocity ω
2 is calculated and the routine exits.

The lean limit correction value setting routine shown in FIG. 14 is started every time a crank pulse subsequent to the crank pulse indicating the latter period θATDC is input.

First, in step S61, the cylinder of the combustion stroke cylinder #i is discriminated from the interrupt input of the cam pulse, and in step S62, the late crank angular velocity ω2 and the initial crank angular velocity ω1 calculated in the crank angular velocity calculation routine are calculated. Read the two crank angular speeds ω2, ω
From the square of the difference between 1 and calculates the torque index value TIN # i of the combustion stroke cylinder ^{#i (TIN # i ← ω2 2} -ω1 2).

Next, in step S63, a pulse correction value setting routine, which will be described later, determines whether the combustion state of the combustion stroke cylinder #i is combustion with the target air-fuel ratio A / F, combustion with rich correction, or combustion with lean correction. The determination is made with reference to the value of the pulse correction value KPULSE set in (see FIG. 15).

When the combustion is performed at the target air-fuel ratio A / F of KPULSE = 0, the process proceeds to step S64, where the target air-fuel ratio of the combustion stroke cylinder #i stored in the RAM 43 with the torque index value TIN # i. Update the torque index value TNIN # i,
Proceed to step S67. When the combustion is performed by lean correction of KPULSE <0, the process proceeds to step S65, and the lean state torque index value TLIN # i of the combustion stroke cylinder #i stored in the RAM 43 is updated with the torque index value TIN # i. Then, the process proceeds to step S67. When the combustion is performed by the rich correction of KPULSE> 0, the process proceeds to step S66, and the rich state torque index value TRIN # i of the combustion stroke cylinder #i stored in the RAM 43 is updated with the torque index value TIN # i. Then, the process proceeds to step S67.

In step S67, the latest rich state torque index value T of all cylinders stored in the RAM 43 is set.
RIN # i average (rich state torque index average) TR
INAVE is calculated (TRINAVE ← ΣTRIN # i / 4), and the process proceeds to step S68.

In step S68, the latest target air-fuel ratio torque index value T for all cylinders stored in the RAM 43 is set.
Average value of NIN # i (average target air-fuel ratio torque index value) TN
INAVE is calculated (TNINAVE ← ΣTNIN # i / 4), and the process proceeds to step S69.

At step S69, the latest lean state torque index value T of all cylinders stored in the RAM 43 is set.
LIN # i average (lean state torque index average) TL
INAVE is calculated (TLINAVE ← ΣTLIN # i / 4), and the process proceeds to step S70.

In step S70, a first combustion state comparison value IRN is calculated from the difference between the rich state torque index average value TRINAVE and the target air-fuel ratio torque index average value TNINAVE (IRN ← TRINAVE-TNINAVE). ), And proceed to step S71.

In step S71, a second combustion state index value INL is calculated from the difference between the target air-fuel ratio torque index value average value TNINAVE and the lean state torque index value average value TLINAVE (INL ← TNINAVE-TINAVE).

Thereafter, the flow advances to step S72 to calculate a final combustion state comparison value IRN / NL from the ratio of the first combustion state comparison value IRN to the second combustion state index value INL (IRN / NL ←
IRN / INL), and proceeds to step S73. The combustion state final comparison value IRN / NL may be calculated by a well-known weighted average process.

In step S73, the combustion state final comparison value IRN / NL is compared with the combustion state comparison reference value Is. If the lean limit of IRN / NL ≦ Is has been reached, the flow proceeds to step S74. The lean limit correction value KSURGE stored in the RAM 43 is replaced with the lean limit correction value KSUR.
Update with the value obtained by adding the rich correction amount SURGE1 to GE,
Exit the routine.

In step S73, IRN / NL> I
If it is determined that the rich limit of s has been reached, the process branches to step S75, where the lean limit correction value KSURGE stored in the RAM 43 is replaced with the lean limit correction value KSURGE.
Is updated with the value obtained by subtracting the lean correction value SURGE2 from
Exit the routine.

FIG. 15 shows a pulse correction value setting routine. This routine is started every time a predetermined crank pulse is input.

First, the cylinder count value n is identified in steps S81 and S82. When n = 0, the process proceeds to step S83, and when n = 1, the process proceeds to step S84.
When n = 2, the process proceeds to step S85.

For example, when the cylinder count value n is 0, the process proceeds to step S83, in which the pulse correction value KPULSE is set by a rich correction amount K1 (K1> O) set in advance for each operation region. In S86, the cylinder count value n (n = 0 in this case) is counted up, and the routine exits.

Thereafter, when this routine is started by the input of a specific crank pulse for the next cylinder, the cylinder count value n at this time is 1, so the flow advances to step S84 to clear the pulse correction value KPULSE (KPULSE ← 0). ), And returns to step S86. In step S86, the cylinder count value n (in this case, n = 1) is counted up, and the routine exits.

Then, when the present routine is started by input of a specific crank pulse of the next cylinder, the cylinder count value n at this time is 2, the process proceeds to step S85, and the pulse correction value KPULSE is set for each operation region. Set with a preset lean correction amount K2 (where K2 <O),
In step S87, the cylinder count value n is cleared.
Exit the routine.

As a result, the pulse correction value KPULSE is
Each time the pulse correction value setting routine is executed, the setting is repeatedly performed in the order of rich → no correction (target air-fuel ratio) → lean.

The lean limit correction value KSURGE and the pulse correction value KPULSE are read by the target equivalent ratio setting routine (see FIG. 4) described in the first embodiment. This target equivalence ratio setting routine is the same as that of the first embodiment, and the description is omitted.

When the combustion using the fuel injection pulse width Ti corrected with the air-fuel ratio by the pulse correction value KPULSE for lean-correcting the air-fuel ratio exceeds the lean limit, the combustion becomes remarkably unstable, and the amount of increase in the crank angular velocity in the initial stage of the combustion is reduced. descend. Accordingly, the difference between the torque index value of the cylinder based on the target air-fuel ratio A / F and the torque index value of the cylinder based on the lean correction (torque index value difference) is calculated by comparing the torque index value of the cylinder based on the rich correction with the target air-fuel ratio A / F. Is larger than the difference from the torque index value of the cylinder (torque index value difference). In the present embodiment, by controlling the target air-fuel ratio A / F so that the ratio of the difference between the two torque index values converges to a certain value, torque fluctuation during lean burn operation is suppressed and combustion is stabilized. To reduce NOx emissions and improve fuel efficiency.

As described above, according to the present embodiment, the combustion state of each cylinder is calculated based on the crank angular velocity ω1 in the early stage of combustion and the crank angular velocity ω2 in the late stage of combustion. The need for the in-cylinder pressure sensor adopted in the above is unnecessary, and the present invention can be applied to an existing engine, thereby improving convenience. Further, since the fluctuation of the combustion is determined from the torque index value, the combustion state of each cylinder during the lean burn operation can be grasped more accurately.

Since the target air-fuel ratio A / F is repeatedly set in the order of rich (R) → no correction (A / F) → lean (L), the combustion state of the cylinder by the lean correction is set. , The engine rotation fluctuation occurs in a high frequency band (for example, 22.2 Hz at 2000 rpm), so that the driver does not feel uncomfortable.

Furthermore, since the combustion state comparison value, which is the difference between the torque index values, is calculated from the average value of the torque index values of all cylinders, it is less susceptible to temporary combustion fluctuations due to variations in combustion between the cylinders. The combustion state comparison value can be accurately calculated.

Further, since the latest rich state torque index value, target air-fuel ratio torque index value, and lean state torque index value are always compared, the lean limit is accurately set even in a transient state in which the operating state of the engine changes. Can be detected.

[0135]

According to the first aspect of the present invention, the air-fuel ratio correction value for richly correcting the target air-fuel ratio and the air-fuel ratio correction value for lean correction during the lean burn operation are set in the order of the combustion stroke cylinder. A combustion state comparison value is set by comparing the combustion state detection value at the time of rich correction of the combustion with the fuel injection amount corrected by the fuel ratio correction value and the combustion state detection value at the time of lean correction. Since the lean limit is determined by comparing with the set value, for example, if the combustion at the time of lean exceeds the lean limit, the combustion state of the combustion stroke cylinder becomes significantly unstable. The value indicates a large deviation value. Therefore, the lean limit during the lean burn operation can be detected with high accuracy based on the combustion state comparison value.

According to the second aspect of the present invention, in the first aspect of the present invention, when it is determined that the lean limit is set based on the combustion state comparison value, the target air-fuel ratio is corrected to the rich side, and the lean limit is set. If it is not determined that the target air-fuel ratio is corrected to the lean side, the air-fuel ratio during the lean burn operation can be stably burned near the lean limit, thereby reducing NOx emissions and improving fuel efficiency. be able to.

According to a third aspect of the present invention, in the first or second aspect of the present invention, the combustion state comparison value is calculated based on an average value of the set number of cylinders of the combustion state detection value at the time of rich correction and a combustion state detection value at the time of lean correction. By setting the combustion state detection value by comparing it with the average value of the set number of cylinders, a temporary combustion fluctuation due to a variation in combustion between cylinders is eliminated, and the lean limit can be detected more accurately.

According to the invention described in claim 4, claims 1 to 1 are provided.
In the invention described in Item 3, when the engine to be employed is a four-cylinder engine, the rich-corrected air-fuel ratio correction value is set to two combustion stroke cylinders, and the lean-corrected air-fuel ratio correction value is set to one combustion stroke cylinder. By repeatedly executing the cylinder order, the fuel injection amount to the combustion stroke cylinder before and after the combustion stroke cylinder by the lean correction and the fuel injection amount supplied at the time of the previous and next combustion of the same cylinder are always rich corrected. Therefore, the frequency band of the combustion fluctuation is higher than the frequency band in which the surge occurs, and the driver does not feel uncomfortable.

According to the fifth aspect of the present invention, the fuel injection amount set based on the target air-fuel ratio during the lean burn operation is changed for the actual combustion stroke cylinder to the air-fuel ratio corresponding to the target air-fuel ratio. The air-fuel ratio correction value for rich correction of the fuel ratio correction value and the fuel injection amount based on the target air-fuel ratio and the air-fuel ratio correction value for lean correction are set in the order of combustion stroke cylinder,
The combustion state detection value at the time of rich correction is compared with the combustion state detection value at the time of combustion based on the target air-fuel ratio to set a combustion state first comparison value, and the combustion state detection value at the time of combustion based on the target air-fuel ratio is set. A combustion state detection value is compared with the combustion state detection value at the time of lean correction to set a combustion state second comparison value, and the combustion state is compared by comparing the combustion state first comparison value with the combustion state second comparison value. Since the state final comparison value is set and the combustion state final comparison value is compared with the set value to determine the lean limit, for example, when the combustion of the combustion stroke cylinder by the lean correction exceeds the lean limit. Since the combustion state of the combustion stroke cylinder becomes extremely unstable, the final comparison value of the combustion state shows a large deviation value, and therefore, based on the final comparison value of the combustion state, the lean state during the lean burn operation is determined. High limit It is possible to detect in.

According to the invention of claim 6, in the invention of claim 5, the target air-fuel ratio is corrected to a rich side when it is determined that the engine is lean, and when the engine is not determined to be lean, the target air-fuel ratio is corrected. By correcting the target air-fuel ratio to the lean side, it is possible to control the air-fuel ratio during the lean burn operation in a stable combustion region, thereby reducing NOx emissions and improving fuel efficiency.

According to a seventh aspect of the present invention, in the fifth or sixth aspect, the air-fuel ratio correction value for rich-correcting the air-fuel ratio correction value corresponding to the target air-fuel ratio and the target air-fuel ratio are adjusted. By repeatedly setting the corresponding air-fuel ratio correction value and the air-fuel ratio correction value corresponding to the target air-fuel ratio to lean-correct the air-fuel ratio correction value in the order of the combustion stroke cylinder, the next combustion stroke cylinder after the lean correction is performed. Since the fuel injection amount for the combustion stroke cylinder and the fuel injection amount supplied at the next combustion of the same cylinder are always rich corrected,
The frequency band of the combustion fluctuation is higher than the frequency band in which the surge occurs, and the driver does not feel uncomfortable.

According to an eighth aspect of the present invention, in the fifth or sixth aspect of the present invention, the combustion state during combustion based on the average value of the set number of cylinders of the combustion state detection value during rich correction and the target air-fuel ratio. The detected value is compared with the average value of the set number of cylinders to set a combustion state first comparison value, and the average value of the set number of combustion state detected values during combustion based on the target air-fuel ratio and the lean correction time are set. By setting the combustion second comparison value by comparing the combustion state detection value with the average value of the set number of cylinders, a temporary variation in combustion between cylinders is eliminated, and the lean limit is more accurately detected. Can be.

According to the ninth aspect of the present invention, the first aspect,
In the invention described in 2, 5, or 6, the combustion state is directly grasped by setting the combustion state detection value based on an output signal of a pressure sensor for detecting a combustion pressure at the time of combustion provided in each cylinder of the engine. Can be.

According to the tenth aspect of the invention, in the first, second, fifth or sixth aspect of the invention, the combustion state is set based on the amount of change in the crank angular velocity of each cylinder, so that the pressure sensor or the like can be used. Even if the engine is not equipped with parts, it is possible to easily detect the combustion state,
Not only can the present invention be widely applied,
Manufacturing costs can be reduced.

According to the eleventh aspect, in the first aspect,
0, the amount of change in the crank angular speed is calculated based on the difference between the square of the crank angular speed in the latter stage of combustion and the square of the crank angular speed in the early stage of combustion, whereby the torque index at the time of combustion of the combustion stroke cylinder is calculated. Values can be derived.

[Brief description of the drawings]

FIG. 1 is a functional block diagram of an air-fuel ratio control device for a lean burn engine according to a first embodiment;

FIG. 2 is a flowchart showing a lean limit correction value setting routine.

FIG. 3 is a flowchart showing a pulse correction value setting routine.

FIG. 4 is a flowchart showing a target equivalent ratio setting routine;

FIG. 5 is a flowchart showing a fuel injection pulse width setting routine;

FIG. 6 is a conceptual diagram of a map that stores a combustion comparison reference value.

FIG. 7 is a conceptual diagram of a map for storing a basic lean weight loss value.

FIG. 8 is a characteristic diagram showing an example of a target air-fuel ratio that changes according to a basic lean reduction value.

FIG. 9 is a timing chart showing the relationship between the in-cylinder pressure and the fuel injection pulse width.

FIG. 10 is an overall configuration diagram of the engine.

FIG. 11 is a circuit diagram of the electronic control device.

FIG. 12 is a functional block diagram of an air-fuel ratio control device for a lean burn engine according to a second embodiment.

FIG. 13 is a flowchart showing a crank angular velocity calculation routine.

FIG. 14 is a flowchart showing a lean limit correction value setting routine.

FIG. 15 is a flowchart showing a pulse correction value setting routine.

FIG. 16 is a timing chart showing the relationship between the in-cylinder pressure and the fuel injection pulse width.

FIG. 17 shows engine speed, fuel efficiency, and NOx with respect to the air-fuel ratio.
Is a characteristic diagram showing the relationship with the amount of export

[Explanation of symbols]

28 ... pressure sensor A / F ... target air-fuel ratio INL ... combustion state second comparison value IRN ... combustion state first comparison value IRN / NL ... combustion state final comparison value KPULSE ... air-fuel ratio correction value (pulse correction value) M2, M2 '... Air-fuel ratio correction value setting means M3 ... Target air-fuel ratio setting means M5, M5' ... Combustion state detection value setting means M6 ... Combustion state comparison value setting means M6a ... Combustion state first comparison value setting means M6b ... Combustion state second Comparison value setting means M6c: Combustion state final comparison value setting means M7: Lean limit determination means P: Combustion state detection value (in-cylinder pressure) PLAVE: Average value of combustion state detection values during lean correction (lean state combustion pressure average value) PRAVE: average value of combustion state detection values during rich correction (rich state combustion pressure average value) Px: combustion state comparison value Pxs, Is: set value (combustion state comparison reference value) Ti: fuel injection amount (fuel injection amount) Scan width) TIN # i ... (crank angular speed) variation (torque index value) .omega.1, .omega.2 ... crank angular speed

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁶ Identification code FI F02D 45/00 362 F02D 45/00 362J 368 368S

Claims (11)

[Claims] 1. An air-fuel ratio correction value setting means for setting an air-fuel ratio correction value for rich correction of a target air-fuel ratio and an air-fuel ratio correction value for lean correction in the order of a combustion stroke cylinder, at least an engine operating state and the air-fuel ratio correction value. Target air-fuel ratio setting means for setting a target air-fuel ratio at the time of lean burn operation, combustion state detection value setting means for detecting a combustion state detection value at the time of combustion for each cylinder, and combustion state detection at the time of rich correction Combustion state comparison value setting means for setting a combustion state comparison value by comparing the combustion state detection value at the time of lean correction with the combustion state detection value; and lean for judging a lean limit by comparing the combustion state comparison value with the set value. An air-fuel ratio control device for a lean burn engine, comprising: a limit determination unit. 2. The target air-fuel ratio setting means corrects the target air-fuel ratio to the rich side when the lean limit is determined by the lean limit determination means, and corrects the target air-fuel ratio when the lean limit is not determined. 2. The air-fuel ratio control device for a lean burn engine according to claim 1, wherein the air-fuel ratio is corrected to the lean side. 3. The combustion state comparison value setting means compares the average value of the set number of cylinders of the combustion state detection value during rich correction with the average value of the set number of cylinders of the combustion state detection value during lean correction. The air-fuel ratio control device for a lean burn engine according to claim 1 or 2, wherein the combustion state comparison value is set by using the combustion condition comparison value. 4. The engine has four cylinders, and the air-fuel ratio correction value setting means sets the air-fuel ratio correction value for rich correction to two combustion stroke cylinders and sets the air-fuel ratio correction value for lean correction to one combustion stroke cylinder. The air-fuel ratio control device for a lean burn engine according to any one of claims 1 to 3, wherein: 5. An air-fuel ratio correction value setting means for setting an air-fuel ratio correction value for richly correcting a target air-fuel ratio and the air-fuel ratio correction value for lean correction in the order of a combustion stroke cylinder, and at least an engine operating state and the air-fuel ratio correction. Target air-fuel ratio setting means for setting a target air-fuel ratio during lean burn operation based on the value, combustion state detection value setting means for detecting a combustion state detection value for each cylinder during combustion, and the combustion state during rich correction A first combustion state which sets a first combustion state comparison value by comparing a detected value with the combustion state detection value at the time of combustion based on the target air-fuel ratio.
A comparison value setting means for comparing the combustion state detection value during combustion based on the target air-fuel ratio with the combustion state detection value during lean correction to set a combustion state second comparison value. Setting means; combustion state final comparison value setting means for comparing the combustion state first comparison value with the combustion state second comparison value to set a combustion state final comparison value; And a lean limit judging means for judging a lean limit by comparing the air-fuel ratio with a lean-burn engine. 6. The target air-fuel ratio setting means corrects the target air-fuel ratio to the rich side when the lean limit is determined by the lean limit determining means, and sets the target air-fuel ratio when the lean limit is not determined. 6. The air-fuel ratio control device for a lean burn engine according to claim 5, wherein is corrected to the lean side. 7. An engine having four cylinders, wherein said air-fuel ratio correction value setting means sets said air-fuel ratio correction value for rich correction, said air-fuel ratio correction value corresponding to said target air-fuel ratio, and said air-fuel ratio correction value for lean correction. 7. The air-fuel ratio control device for a lean burn engine according to claim 5, wherein the values are set in the order of the combustion stroke cylinders. 8. An average value of the set number of cylinders of the detected combustion state based on the target air-fuel ratio and an average value of the set number of cylinders of the detected combustion state based on the target air-fuel ratio. The combustion state second comparison value setting means sets the average value of the set number of cylinders of the combustion state detection value at the time of combustion based on the target air-fuel ratio and the lean correction time. 7. The air-fuel ratio control device for a lean burn engine according to claim 5, wherein a second combustion comparison value is set by comparing the combustion state detection value with the average value of the set number of cylinders. 9. The combustion state detection value setting means sets a combustion state detection value based on an output signal of a pressure sensor provided in each cylinder of the engine and detecting a combustion pressure during combustion. 7. The air-fuel ratio control device for a lean burn engine according to any one of 1, 2, 5, and 6. 10. The combustion state detection value setting means sets a combustion state detection value based on a change amount of a crank angular velocity at the time of combustion of each cylinder.
7. The air-fuel ratio control device for a lean burn engine according to any one of 6. 11. The lean burn according to claim 10, wherein the amount of change in the crank angular speed is calculated based on the difference between the square of the crank angular speed in the late stage of combustion and the square of the crank angular speed in the early stage of combustion. Engine air-fuel ratio control device.