JPH10184430A - Exhaust gas purifying device for engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To perform proper operation in a rich air-fuel ratio to purify NOX of a three way catalyst for lean combustion and to relieve a torque shock. SOLUTION: During operation in a lean air-fuel ratio, an NOX storage amount of a three way catalyst for lean combustion is estimated by an NOX storage amount estimating part 75. Wen it is decided by an NOX storage capacity saturation deciding part 76 that NOX storage capacity reach saturation, a discharge amount of HC and CO is temporarily increased as a torque shock is relieved in a way that by a rich operation executing part 77 classified by cylinder, operation in a lean air-fuel ratio is switched to operation in a rich-fuel ratio at a given interval classified by a cylinder as a timing is staggered, and NOX in the three way catalyst for lean combustion is reduced and purified. To relive a torque shock in a similar manner described above after completion of purification of NOX, operation in a rich air-fuel ratio is returned to operation in a lean air-fuel ratio at a given interval classified by a cylinder as a timing is staggered.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention
The present invention relates to an exhaust gas purification device for an engine in which a three-way catalyst for storing x and reducing and purifying it with HC and CO is interposed in an exhaust system.

[0002]

2. Description of the Related Art Generally, in the air-fuel ratio control of an engine, in order to purify exhaust gas by a catalyst, a fuel injection amount and the like are controlled so that the air-fuel ratio falls within a region where the exhaust gas purification efficiency of the catalyst is the best. However, the analysis of the combustion process in recent engines has progressed, and engines that use lean combustion (lean burn), which does not misfire even at a lean air-fuel ratio and can burn efficiently with a small amount of fuel, have been developed. ing.

In such a so-called lean burn engine, when the oxygen concentration of the exhaust gas is high, HC and CO are oxidized and reduced and NOx is stored. When the oxygen concentration in the exhaust gas decreases, the stored NOx is released. There is a three-way catalyst that employs a three-way catalyst that reduces and purifies HC and CO that have not been oxidized and reduced, and such a three-way catalyst for lean combustion is discharged during operation at a lean air-fuel ratio. NO
Before the storage amount of x reaches saturation, it is necessary to perform a short-time operation at a rich air-fuel ratio to temporarily increase the emission amounts of HC and CO to reduce and purify NOx.

[0004] When switching from lean operation to rich operation for NOx purification, there is a problem that torque shock occurs due to fluctuations in engine torque. For example, Japanese Patent Application Laid-Open No. 7-26999 discloses a method for purifying NOx. A technique for purifying NOx by performing rich operation only when the engine load is reduced without causing the driver to feel a torque shock is disclosed.

[0005]

SUMMARY OF THE INVENTION However, NOx
If restrictions such as limiting the rich operation for purification to when the engine load is reduced are provided, the frequency of performing the rich operation such as when driving at a constant speed where the operating conditions above the medium load such as an expressway are maintained Under low operating conditions, the NOx storage amount may reach saturation and be discharged without being purified.

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and provides an engine exhaust gas purification apparatus capable of appropriately performing operation at a rich air-fuel ratio for NOx purification and reducing torque shock. It is intended to be.

[0007]

According to the first aspect of the present invention,
In an exhaust gas purifying apparatus for an engine in which a catalyst for reducing and purifying the stored NOx is interposed in the exhaust system when the NOx is stored during the lean operation of the engine and the lean operation is temporarily switched to the rich operation, the engine is temporarily stopped. When switching from the lean operation to the rich operation, a control means for switching at predetermined intervals while shifting the timing for each cylinder is provided.

According to a second aspect of the present invention, in the first aspect of the present invention, when returning from the rich operation to the lean operation, the control means performs the return at predetermined intervals while shifting the timing for each cylinder. And

According to a third aspect of the present invention, in the first aspect of the present invention, the control means estimates the amount of NOx emitted from the exhaust gas during lean operation according to the operating range.
NO integrated into the catalyst by integrating the estimated value of x emissions
means for estimating the amount of x, means for determining that the NOx storage capacity of the catalyst has reached saturation when the estimated value of the NOx storage amount exceeds a set value, and means for determining that the NOx storage capacity of the catalyst has reached saturation. Means for switching at a predetermined interval from the lean operation to the rich operation while shifting the timing for each cylinder.

According to a fourth aspect of the present invention, in the first aspect of the invention, the control means integrates an intake air amount after the start of the rich operation, and when the integrated value of the intake air amount exceeds a set value. Means for determining that the NOx purification of the catalyst has been completed, and means for returning from the rich operation to the lean operation at predetermined intervals while shifting the timing for each cylinder from the rich operation to the lean operation when it is determined that the NOx purification of the catalyst has been completed. It is characterized by having.

According to a fifth aspect of the present invention, when the switching from the lean operation to the rich operation and the return from the rich operation to the lean operation are performed while shifting the timing for each cylinder, the lean operation of the lean operation is performed. The switching interval and the return interval are set to be longer as the operation region is on the high load / high rotation side.

According to a sixth aspect of the present invention, in the invention of the fifth aspect, the switching interval and the return interval are set for every two rotations of the engine according to the ignition order and for one rotation of the engine according to a cylinder order different from the ignition order. It is characterized in that the order is every time or in the order of ignition.

That is, according to the present invention, when temporarily switching from lean operation to rich operation in order to reduce and purify NOx stored in the catalyst during lean operation of the engine, switching is performed at predetermined intervals while shifting the timing for each cylinder.
When returning from rich operation to lean operation,
The cylinders are returned at predetermined intervals while shifting the timing for each cylinder.

The switching from the lean operation to the rich operation is performed when the estimated NOx emission amount in the exhaust gas is integrated during the lean operation to estimate the NOx storage amount, and when the estimated value of the NOx storage amount exceeds the set value. The operation is performed by determining that the NOx storage capacity of the catalyst has reached saturation. The return from the rich operation to the lean operation is performed when the integrated value of the intake air amount after the start of the rich operation exceeds the set value. It is desirable to perform the determination by determining that the purification has been completed, and the switching interval from the lean operation to the rich operation and the return interval from the rich operation to the lean operation are determined every two engine revolutions according to the ignition order, When the engine operating range at a lean air-fuel ratio is higher on the higher load / higher rotation side, for example, every one rotation of the engine according to a cylinder order different from that of It is desirable to increase.

[0015]

Embodiments of the present invention will be described below with reference to the drawings. 1 to 11 relate to a first embodiment of the present invention, FIG. 1 is a functional block diagram of a control device, FIG. 2 is a flowchart of a rich operation execution permission routine, and FIG. 3 is a flowchart of a # 1 cylinder rich operation execution routine. 4 is a flowchart of a routine for performing # 3 (2, 4) cylinder rich operation, FIG. 5 is a flowchart of a NOx storage amount estimation routine, FIG. 6 is a flowchart of a NOx storage capacity saturation determination routine, and FIG. 7 is a NOx purification end determination routine. 8 is an explanatory diagram of the NOx emission amount estimation map, FIG. 9 is a time chart showing a transition from the lean operation to the rich operation, FIG. 10 is a time chart showing a transition from the rich operation to the lean operation, and FIG. FIG. 2 is a schematic configuration diagram of an engine control system.

In FIG. 11, reference numeral 1 denotes an engine (in this embodiment, a horizontally opposed four-cylinder engine), which is a full operation except for a part of an operation region such as a high load / acceleration operation requiring high output. It is a lean burn engine that performs lean burn in a region. Cylinder heads 2 are provided in both left and right banks of a cylinder block 1a of the engine 1, respectively. In a combustion chamber 3 for each cylinder formed by the cylinder head 2 and the cylinder block 1a,
An intake manifold 5 is communicated via an intake port 4 and an exhaust manifold 7 is communicated via an exhaust port 6.

An injector 8 faces directly upstream of the intake port 4, and a spark plug 9 whose tip is exposed to the combustion chamber 3 is attached to each cylinder of the cylinder head 2. This ignition plug 9 has an ignition coil 1
The ignition coil 10 is connected to an igniter 11.

An air chamber 12 is formed in the upstream gathering portion of the intake manifold 5 and communicates with an intake pipe 13. An air cleaner 14 is attached to an air intake port upstream of the intake pipe 13. A throttle valve 15 is provided. Further, a bypass passage 16 that bypasses the throttle valve 15 is connected to the intake pipe 13, and an idle speed control valve (ISCV) 17 is interposed in the bypass passage 16.

On the other hand, an exhaust pipe 18 is communicated with the exhaust manifold 7, and a catalyst converter 19 containing a three-way catalyst for lean combustion is interposed immediately downstream of the exhaust pipe 18 where the exhaust manifold 7 is gathered. ,further,
A muffler 20 is interposed at the downstream end. The three-way catalyst for lean combustion, for example, an alkali metal, alkaline earth, NOx storage material such as rare earth and a noble metal such as platinum supported on a carrier such as alumina, by the storage function of NOx and O2, When the oxygen concentration of the exhaust gas is high, H
When C and CO are oxidized and reduced, NOx is stored, and when the oxygen concentration in the exhaust gas is reduced, the stored NOx is released and reduced and purified by excess HC and CO that are not oxidized and reduced.

The engine 1 is provided with various sensors, such as a throttle opening sensor connected to the throttle valve 15 for detecting the throttle opening and a throttle fully closed sensor. A throttle sensor 21 having a built-in idle switch to be turned on, a hot wire type or hot film type intake air amount sensor 22 interposed immediately downstream of the air cleaner 14 in the intake pipe 13, both right and left sides of the cylinder block 1a A cooling water temperature sensor 23 facing the cooling water passage 1b communicating with the bank, a crank angle sensor 25 provided on the outer periphery of a crank rotor 24 axially mounted on the crankshaft 1c,
There are a cylinder discriminating sensor 27 provided opposite to a cam rotor 26 connected to the camshaft 1d, a wide-range air-fuel ratio sensor 28 facing the exhaust pipe 18 upstream of the catalytic converter 19, and other various sensors not shown.

The various sensors are provided by an electronic control unit (ECU) 50 for electronically controlling the air-fuel ratio of the engine 1 and the like.
It is connected to the. The ECU 50 includes a CPU 51, R
OM 52, RAM 53, backup RAM 54, I /
A power supply circuit 56 for supplying a stabilized voltage to each unit, a drive circuit 57 for driving actuators by a signal from an output port of the I / O interface 55, and A peripheral circuit such as an A / D converter 58 for converting an analog signal from sensors into a digital signal is incorporated.

The input ports of the I / O interface 55 are connected to the crank angle sensor 25, the cylinder discriminating sensor 27, and other sensors and switches (not shown). The A / D converter 58 Via the throttle sensor 21, the intake air amount sensor 22,
The cooling water temperature sensor 23, the wide area air-fuel ratio sensor 28, and the like are connected. On the other hand, the I / O interface 5
The igniter 11 is connected to the output port 5 and the injector 8, the ISCV 17, and other various actuators (not shown) are connected via the drive circuit 57.

The power supply circuit 56 is connected to a battery 61 via a relay contact of an ECU relay 60, and a relay coil of the ECU relay 60 is connected to the battery 61 via an ignition switch 62. Further, the power supply circuit 56 is directly connected to the battery 6.
1 and the ignition switch 62
Is turned on and the relay contact of the ECU relay 60 is closed, power is supplied from the power supply circuit 56 to each unit, and the ignition switch 62 is turned on and off.
Regardless, the backup RAM 54 is always supplied with power for backup.

In accordance with the control program stored in the ROM 52, the ECU 50 reads output signals from the sensors and switches to detect an engine operating state, and determines an optimal fuel injection amount,
By calculating the ignition timing and the like, the air-fuel ratio is controlled in a lean or stoichiometric manner to improve the fuel efficiency and the exhaust emission and to secure the engine output.

Further, in the ECU 50, as shown in FIG. 1, an operation area determination section 71, a lean operation permission section 72,
Cylinder-specific fuel injection control unit 73, cylinder-specific ignition timing control unit 7
4. The functions of the NOx storage amount estimation unit 75, the NOx storage capacity saturation determination unit 76, and the cylinder-specific rich operation execution unit 77 monitor the NOx storage capability of the lean-burn three-way catalyst during operation at the lean air-fuel ratio, When it is determined that the storage capacity has reached saturation, the operation is shifted from lean air-fuel ratio operation to rich air-fuel ratio operation at predetermined intervals for each cylinder with a predetermined interval, which temporarily reduces torque shock by temporarily switching the timing. The amount of HC and CO emissions is increased to reduce and purify NOx in the lean burn three-way catalyst. After the end of the NOx purification, the operation at the rich air-fuel ratio is returned to the operation at the lean air-fuel ratio while shifting the timing at predetermined intervals for each cylinder in order to reduce the torque shock.

That is, when the operation region determined by the operation region determination unit 71 is the lean operation region, the lean operation permission unit 7
2, the lean operation is permitted, and the fuel injection amount for each cylinder in the fuel injection control unit for each cylinder 73 is reduced and corrected.
The cylinder-specific ignition timing in the cylinder-specific ignition timing control unit 74 is corrected according to the lean air-fuel ratio. At the same time, the NOx storage amount estimation unit 75 estimates the NOx storage amount of the lean-burn three-way catalyst after the start of the lean operation. Let it.

The cylinder-by-cylinder fuel injection control unit 73 calculates a basic fuel injection pulse width TP from the engine speed NE and the intake air amount Q (TP ← K × Q / NE; where K is an injector characteristic correction constant). As shown in the following equation (1), the basic fuel injection pulse width TP is set to a correction coefficient COEF based on the coolant temperature TW or the like, and a target air-fuel ratio correction coefficient KTGT # n ( n = 1 to 4), an air-fuel ratio feedback correction coefficient LAMBD for approaching the air-fuel ratio to the target air-fuel ratio based on the output voltage of the wide area air-fuel ratio sensor 28
A, a learning correction coefficient KBLRC for quickly correcting the deviation of the air-fuel ratio due to variations during production of the intake air amount measuring system or the fuel supply system or a change over time, fuel deposition for correcting the effect of fuel deposited on the port wall surface, etc. The effective fuel injection pulse width TE is calculated by correcting with the correction coefficient KX. And
The effective fuel injection pulse width TE, which compensates for the invalid injection time of the injector 8 set based on the battery voltage VB, is added to the effective fuel injection pulse width TE as shown in the following equation (2), and the final fuel The injection pulse width Ti is calculated for each cylinder. TE = TP × COEF × KTGT # n × LAMBDA × KBRRC × KX (1) Ti = TE + TV (2) At this time, the target air-fuel ratio correction coefficient K set for each cylinder
TGT # n adopts a different value between the lean operation and the rich operation. For example, the fuel injection amount of each cylinder is reduced at a lean value of 0.6 to 0.8, and 1.2 to 1.5. The fuel injection amount of each cylinder is increased by the rich value of. It is desirable that the fuel adhesion correction coefficient KX is also set for each cylinder (KX
1 to KX4).

The cylinder-specific ignition timing control section 74 sets the basic ignition timing IGREG for each cylinder by referring to a map using the engine speed NE and the engine load such as the basic fuel injection pulse width Tp as parameters. As shown in the equation (3), the basic ignition timing IGREG is obtained by setting a water temperature correction value ADVTW based on the cooling water temperature TW, an acceleration correction value ADVMAC which is a correction value at the time of acceleration detected based on a signal from the throttle sensor 21, and a knock avoidance. The final ignition timing ADV is calculated for each cylinder by correcting the knock control correction value KB. ADV = IGREG ± ADVTW ± ADVMAC−KB (3) Even when calculating the ignition timing, the basic ignition timing I
GREG has two maps, a basic ignition timing map for lean operation and an ignition timing map for rich operation, corresponding to lean operation and rich operation.
The map is selected according to lean operation / rich operation.

The NOx storage amount estimating section 75 integrates the NOx emission amount in each operation region specified by the engine speed NE and the intake air amount Q at every set time only during the lean operation, and calculates the integrated value. NOx in lean combustion three-way catalyst
Obtained as an estimated storage amount. This estimated NOx storage amount is cleared when the rich operation is performed.

Then, the NOx storage capacity saturation determining section 76
Then, the estimated value of the NOx storage amount estimated by the NOx storage amount estimation unit 75 is compared with a set value, and when the estimated value of the NOx storage amount exceeds the set value, the NOx storage capacity of the lean-burn three-way catalyst is saturated. , And instructs the cylinder-specific rich operation execution unit 77 to start the cylinder-specific rich operation.

In order to purify the NOx stored in the lean combustion three-way catalyst, the cylinder-by-cylinder rich operation execution unit 77 first determines the fuel injection amount and ignition timing of one cylinder from the values during the lean operation during the rich operation. , The rich operation is performed, and then the next cylinder is switched to the rich operation at a predetermined interval, and all the cylinders are sequentially switched to the rich operation at the predetermined interval. At the same time, the intake air amount after the start of the rich operation switching is integrated for each set time, and when the integrated value reaches the set value, the NOx stored in the lean-burn three-way catalyst by switching from the lean operation to the rich operation is switched. Is determined to be completed, and, similarly to the switching from the lean operation to the rich operation, the fuel injection amount and the ignition timing of one cylinder are returned from the value at the time of the rich operation to the value at the time of the lean operation,
At predetermined intervals, the other cylinders are returned to the lean operation one after another, and all the cylinders are returned to the lean operation.

In this embodiment, the switching from the lean operation to the rich operation is performed by switching the # 1 cylinder, the # 3 cylinder, the # 2 cylinder, and the # 4 cylinder at intervals of at least two revolutions (one cycle) of the engine between the cylinders. It is performed according to the ignition order, and from rich operation to lean operation, # 1 cylinder → # 3 cylinder → # 2 cylinder →
In the ignition sequence of the # 4 cylinder, the cylinders are returned at intervals of at least two revolutions of the engine between the cylinders.

Hereinafter, switching from the lean operation to the rich operation after the ECU 50 permits the lean operation, and
The processing related to the return from the rich operation to the lean operation will be described with reference to the flowcharts in FIGS.

The switching from the lean operation to the rich operation after the lean operation is permitted is performed based on the estimated NOx storage amount of the lean-burn three-way catalyst obtained by the NOx storage amount estimation routine in FIG. When the determination routine determines that the NOx storage capacity of the lean-burn three-way catalyst has reached saturation, the rich operation execution flag that permits switching from lean operation to rich operation is set in the rich operation execution permission routine of FIG. It is implemented by being done.
On the other hand, the return from the rich operation to the lean operation is indicated by N in FIG.
When it is determined by the Ox purification completion determination routine that the NOx purification of the lean combustion three-way catalyst has been completed, the rich operation execution flag is cleared by the rich operation execution permission routine of FIG.

The switching from the lean operation to the rich operation and the return from the rich operation to the lean operation of each cylinder are performed by first setting the rich operation execution flag in the rich operation execution routine of FIG. Then, in the rich operation execution routine shown in FIG. 4, the rich operation execution flag and the rich flag indicating the immediately preceding execution of the rich operation of the ignition cylinder (# described later)
1, # 2, # 3, and # 4 rich flags), and sequentially executed to the # 3, 2, and 4 cylinders at intervals of two engine revolutions or more according to the ignition order. Note that the rich operation execution flag and the # 1 to # 4 rich flags of each cylinder have been cleared in advance by initialization at the time of turning on the system power.

First, the rich operation execution permission routine of FIG. 2 will be described. In this routine, step S101
Then, it is determined whether or not the rich operation execution flag is set. If the rich operation execution flag is not set, the process proceeds to step S102 to refer to the determination result of the NOx storage capacity saturation determination routine of FIG. It is checked whether or not the NOx storage capacity of the three-way catalyst is saturated.

If it is determined that the NOx storage capacity is not saturated, the process proceeds from step S102 to step S104 to clear the rich operation execution flag and exit the routine, and the NOx storage of the lean-burn three-way catalyst is performed. If the determination result is that the ability is saturated, the above step S1
From 02, the process proceeds to step S105 to set the rich operation execution flag, and exits the routine.

On the other hand, if the rich operation execution flag is already set in step S101,
The process branches from step S101 to step S103, and NOx in FIG.
With reference to the determination result of the purification end determination routine, the rich operation of the # 1 to # 4 cylinders switched from the lean operation by the rich operation execution routine shown in FIGS. It is checked whether or not the purification of the exhausted NOx has been completed.

As a result, when the NOx purification is not completed, the process proceeds from the step S103 to the above-mentioned step S105 to exit the routine with the rich operation execution flag set, and when the NOx purification is completed, the step S103 is completed.
Then, the process proceeds to step S104 to clear the rich operation execution flag, and exits the routine.

In contrast to the above-described rich operation permission routine, in the rich operation execution routine for the # 1 cylinder shown in FIG. 3, first, in step S201, it is determined whether or not the rich operation execution flag is set. When the lean operation three-way catalyst has not saturated the NOx storage capacity and does not need to switch from the lean operation to the rich operation, the rich operation execution flag is not set, or the lean combustion three-way catalyst has already been set. When the switching from the lean operation to the rich operation is performed for NOx purification of the catalyst, and the rich operation execution flag once set is cleared by the NOx purification end determination, the process proceeds from step S201 to step S201.
Proceeding to S202, the # 1 rich flag indicating the execution of the rich operation of the # 1 cylinder is cleared, and the lean operation of the # 1 cylinder is executed at step S203 to exit the routine.

On the other hand, in step S201, when the NOx storage capacity of the lean-burn three-way catalyst is saturated and the rich operation execution flag for switching from lean operation to rich operation for NOx purification is set, step S201 is performed.
Then, the process proceeds to step S204, where the # 1 rich flag is set. In step S205, rich operation of the # 1 cylinder is performed, and the routine exits.

Next, the routine for performing the rich operation for the # 3, # 2, and # 4 cylinders will be described. The flowchart of FIG. 4 generally describes the algorithm of the lean / rich operation execution processing for the # 3, # 2, and # 4 cylinders, and the description here is based on three cylinders.

The lean operation for each cylinder is performed as follows.
The final fuel injection pulse width Ti is set using the target air-fuel ratio correction coefficient KTGT # n for each cylinder as a lean value, and the basic ignition timing map for lean operation is selected to set the ignition timing ADV to the value during lean operation. This is done by setting Further, the rich operation for each cylinder is performed by setting the fuel injection pulse width Ti with the target air-fuel ratio correction coefficient KTGT # n for each cylinder as a rich value, and selecting a basic ignition timing map for rich operation. This is performed by setting the timing ADV to a value during the rich operation.

In the rich operation execution routine for the # 3 cylinder, first, it is determined whether or not the rich operation execution flag is set in step S301. If NO (the rich operation execution flag is not set), the process proceeds to step S302. Then, it is determined whether or not the # 1 rich flag of the cylinder # 1, which is the immediately preceding ignition cylinder, is cleared.

In this case, when the lean operation is started and the NOx storage capacity of the lean combustion three-way catalyst is not saturated, the rich operation execution flag is not set, and # 1
Since the # 1 rich flag of the cylinder has also been cleared, initially, the steps from step S301 to step S302 are performed.
Proceeding to S303, referring to the timer, it is checked whether or not the elapsed time after the # 1 rich flag has been cleared is equal to or longer than the time corresponding to two revolutions of the engine.

The elapsed time can be known by referring to, for example, a hardware timer or a software timer in the system. When the # 1 rich flag is cleared, clocking is started, and the # 1 rich flag is started. Timing is continued until is reset when is set.

As described above, since the # 1 to # 4 rich flags are cleared by initialization at the time of turning on the system power, when the lean operation is started, the time of two or more engine revolutions after the # 1 rich flag is cleared. Has passed,
In the initial state in which the NOx storage capacity of the lean-burn three-way catalyst is not saturated, the process proceeds from step S303 to step S306 to clear the # 3 rich flag, and executes the lean operation of the # 3 cylinder in step S307. Through.

Thereafter, when the lean operation is continued and the NOx storage capacity of the lean-burn three-way catalyst is saturated and the rich operation execution flag is set by the above-described rich operation execution permission routine of FIG. 2, the rich operation execution flag is set. Is determined in step S301 to determine whether or not is set, and the process proceeds from step S301 to step S304 to determine whether or not the # 1 rich flag is set. If the # 1 rich flag has not been set and the # 1 cylinder has not yet been switched from the lean operation to the rich operation, the process proceeds from step S304 to steps S306 and S30.
After 7, the lean operation of the # 3 cylinder is continued, and the process exits from the routine.

On the other hand, if the # 1 rich flag has been set in step S304 and the # 1 cylinder has already been switched from the lean operation to the rich operation, the process proceeds to step S3.
From step 04, the process proceeds to step S305, and the elapsed time after the # 1 rich flag is set with reference to the timer (the elapsed time after the # 1 cylinder switches from the lean operation to the rich operation) corresponds to two revolutions of the engine. Check if the time has passed.

The elapsed time referenced in step S305 is different from the elapsed time referenced in step S303.
The timer starts when the # 1 rich flag is set, and continues until the timer is reset when the # 1 rich flag is cleared. Here, when the time equal to or more than two revolutions of the engine has not elapsed after the # 1 cylinder switched from the lean operation to the rich operation, the # 3 cylinder is operated in the lean operation through the above-described steps S305 and S306 and S307. Exit the routine while keeping the state, # 1
When the time of two or more revolutions of the engine has elapsed after the cylinder switched from the lean operation to the rich operation, the process proceeds from step S305 to step S308, where the # 3 rich flag is set. The rich operation is performed, the operation is switched from the lean operation to the rich operation, and the routine exits.

Thereafter, when the routine is repeated, the above steps S301 to S304 and S305 are repeated while the rich operation execution flag is set, that is, while the NOx reduction purification of the lean combustion three-way catalyst is not completed. Then, the process proceeds to step S308. In step S309, the rich operation of the # 3 cylinder is continued.

Then, as shown in FIG. 9, after the # 1 cylinder is switched from the lean operation to the rich operation by the NOx storage saturation determination, an interval of two engine revolutions or more is provided.
When the three cylinders are switched from the lean operation to the rich operation, similarly, the # 2 cylinder is switched from the lean operation to the rich operation at an interval of at least two revolutions of the engine.
After the # 2 cylinder is switched to the rich operation, the # 4 cylinder is switched from the lean operation to the rich operation at an interval of at least two engine revolutions.

It should be noted that the routine for performing the rich operation for the # 2 cylinder is described in “# 1 (3, 3)
2) The notation “” indicates the # 3 cylinder that is the immediately preceding ignition cylinder.
3 and the notation "# 3 (2, 4)" may be interpreted as # 2 indicating the # 2 cylinder, which is the self-cylinder, and step S30 is performed.
3. The elapsed time in S305 is the elapsed time after the # 3 rich flag is cleared and the elapsed time after the # 3 rich flag is set, respectively. In the routine for performing the rich operation on the # 4 cylinder, the notation "# 1 (3, 2)" is interpreted as # 2 indicating the immediately preceding ignition cylinder # 2, and "# 3 (2, 4)". The notation “” is the self-cylinder #
It may be interpreted as # 4 indicating four cylinders, and steps S303 and S30
The elapsed time in 5 is the elapsed time after the # 2 rich flag is cleared and the elapsed time after the # 2 rich flag is set, respectively.

Then, all the cylinders are switched from the lean operation to the rich operation, and by performing the rich operation, the NOx stored in the lean combustion three-way catalyst is reduced and purified.
When it is determined that x purification has been completed, the rich operation execution flag is cleared by the above-described rich operation execution permission routine of FIG. 2, and the # 1 cylinder is switched from the rich operation by the above-described rich operation execution routine for the # 1 cylinder of FIG. Return to lean operation.

At this time, in the rich operation execution routine of FIG. 4 for the # 3 cylinder, the process proceeds from step S301 to step S302 by clearing the rich operation execution flag, and the # 1 rich flag of the # 1 cylinder which is the immediately preceding ignition cylinder is cleared. Check if it has been done. When the # 1 rich flag has not been cleared, since the # 1 cylinder has not yet returned from the rich operation to the lean operation immediately after the determination of the end of NOx purification, the # 3 rich flag is set from step S302. Jump to step S308, and through step S309, the rich operation of the # 3 cylinder is continued.

On the other hand, when the # 1 rich flag is cleared in step S302, as described above, the process proceeds from step 302 to step S303, and after the # 1 rich flag has been cleared, the time for two or more engine revolutions has elapsed. It is checked whether or not the time has elapsed. If the time equal to or longer than two engine revolutions has not elapsed, the process proceeds from step S303 to the above-described step
The process jumps to S308, and after step S309, sets the # 3 cylinder to the rich operation execution state and exits the routine.

When the time equal to or more than two revolutions of the engine elapses after the # 1 rich flag is cleared, the process proceeds from step S303 to step S306, where the # 3 rich flag is cleared. In step S307, the # 3 cylinder lean operation is performed. And exit the routine. As a result, as shown in FIG. 10, after the # 1 cylinder returns from the rich operation to the lean operation, the # 3 cylinder returns from the rich operation to the lean operation at an interval of at least two engine revolutions. At intervals of two or more revolutions of the engine, # 2 cylinder and # 4 cylinder sequentially return to lean operation.

As described above, the air-fuel ratio in the cylinder does not switch instantaneously (in a stepwise manner) due to the effect of fuel adhesion to the port wall surface, etc., and the output fluctuation is large, and the output fluctuation is large. In the first cycle immediately after the switching, the operation at the rich air-fuel ratio for NOx purification can be appropriately performed, the torque shock can be reduced, and the drivability can be secured.

Next, NOx in the three-way catalyst for lean combustion
The NOx occlusion amount estimation routine of FIG. 5 for estimating the occlusion amount will be described.

This NOx occlusion amount estimation routine is a periodic processing routine that is interrupted and executed at every set time. In step S401, the intake air amount Q is calculated based on the signal from the intake air amount sensor 22 and the crank angle is calculated. The engine speed NE is calculated based on the signal from the sensor 25, and in step S402, it is checked whether the current operation state is the lean operation.

When the lean operation is not being performed, the process exits the routine from step S402. When the lean operation is being performed, the process proceeds from step S402 to step S403 to check whether the rich operation flag is set. When the rich operation execution flag is not set, the process proceeds to step S404, in which the NOx emission amount according to the operating state is estimated from the NOx emission amount estimation map using the intake air amount Q and the engine speed NE as parameters, and
Proceed to S405.

As shown in FIG. 8, the above-mentioned NOx emission amount estimation map is obtained by analyzing the exhaust gas of the engine for each operating region specified by the engine speed NE and the intake air amount Q. NOx emission from the estimated NO
x is stored in advance as the amount of x emission,
By referring to the x emission estimation map, the NOx emission according to the operating state of the engine can be estimated.

In step S405, the estimated NOx emission amount obtained by referring to the NOx emission amount estimation map is added to the estimated value of the NOx storage amount of the lean-burn three-way catalyst obtained at the time of executing the previous routine. Is updated as the estimated NOx occlusion amount, and the routine exits.

On the other hand, when the rich operation execution flag is set in step S403, the above-described FIGS.
Purification of NOx stored in the lean combustion three-way catalyst has been started by performing the rich operation of the # 1 to # 4 cylinders in the rich operation execution routine of the above. Therefore, the process proceeds from step S403 to step S406, and the NOx storage amount Clear the estimate and exit the routine.

The N calculated by the NOx storage amount estimation routine
The estimated value of the Ox storage amount is referred to in the NOx storage capacity saturation determination routine of FIG. In this NOx storage capacity saturation determination routine, in step S501, the estimated NOx storage amount is compared with a set value, and when the estimated NOx storage amount exceeds the set value,
In step S502, it is determined that the NOx storage capacity is saturated, and the determination result is stored at a predetermined address in the RAM 53. When the estimated value of the NOx storage amount is equal to or smaller than the set value, NOx is determined in step S503.
The storage capacity saturation determination is canceled, and the determination result stored at a predetermined address in the RAM 53 is cleared.

Further, in the NOx purification end determination routine of FIG. 7 for determining the end of NOx purification of the lean combustion three-way catalyst by performing the rich operation of the # 1 to # 4 cylinders, first, step
In step S601, it is checked whether the rich operation execution flag is set. If the rich operation execution flag is not set, the process proceeds to step S602 to clear the NOx purification amount counter, exit from the routine, and set the rich operation execution flag. If so, the process proceeds to step S603, and the current intake air amount Q is added to a NOx purification amount counter that integrates the intake air amount Q after the start of the rich operation.

Then, steps S603 to S6 are performed.
Proceeding to 04, the value of the NOx purification amount counter is compared with the set value of the intake air amount corresponding to the exhaust gas amount necessary for reducing and purifying the stored NOx amount determined to be saturated. If the value exceeds the set value, it is determined in step S605 that the NOx purification has been completed, the determination result is stored at a predetermined address in the RAM 53, and the routine exits.

12 to 16 relate to the second embodiment of the present invention, FIG. 12 is a flowchart of a control selection routine, FIG. 13 is an explanatory view of an area determination table, and FIG. 14 is an explanatory view showing divisions of control areas. FIG. 15 is a time chart showing the transition from the lean operation to the rich operation, and FIG. 16 is a time chart showing the transition from the rich operation to the lean operation.

In this embodiment, the function of the cylinder-specific rich operation execution unit 77 of the above-described first embodiment is slightly changed, and the switching from the lean operation to the rich operation and the return from the rich operation to the lean operation are performed as described above. The torque shock reduction NOx purification control mode in which the lean / rich operation is switched for each cylinder at intervals of one cycle or more as described in the first embodiment, and the normal NOx in which the lean / rich operation is sequentially switched in accordance with the ignition order
The two control modes are selected according to the operation range.

For this reason, in the present embodiment, the switching from the lean operation to the rich operation is performed in the same manner as in the first embodiment by the routine of FIGS. When it is determined from the estimated value that the NOx storage capacity has reached saturation, the operation is performed by setting the rich operation execution flag, and the return from the rich operation to the lean operation is performed by the NOx purification of the lean-burn three-way catalyst. When it is determined that the operation has been completed, the operation is performed by clearing the rich operation execution flag. The actual switching process from the lean operation to the rich operation and the return process from the rich operation to the lean operation for each cylinder are shown in FIG. It is selected by twelve control selection routines.

In the control selection routine shown in FIG.
In S51, the intake air amount Q is calculated based on the signal from the intake air amount sensor 22 and the crank angle sensor 25 is calculated.
, The engine speed NE is calculated based on the signal from
As shown in FIG. 13, the normal N
A reference is made to a region determination table that stores a slice level value of the in-cylinder intake air amount that separates the Ox purification control region and the torque shock reduction NOx purification control region. A comparison is made with the intake air amount Q calculated in step S51 to check whether or not the current operation region is in a preset low rotation / low load region.

That is, in order to release NOx stored in the lean-burn three-way catalyst, it is necessary to make the average air-fuel ratio of all cylinders rich to lower the oxygen concentration in the exhaust gas.
In the torque shock reduction NOx purification control mode in which lean / rich operation is switched for each cylinder at intervals of one cycle or more, when switching from lean operation to rich operation, the rich cylinder and the lean cylinder are temporarily mixed. Although the air-fuel ratio becomes lean and the effect of NOx purification does not increase, there is a possibility that fuel efficiency may deteriorate.

For this reason, as shown in FIG. 14, the engine output is low and the torque
The low rotation region is a region of a normal NOx purification control mode in which the lean / rich operation is sequentially switched according to the ignition order,
In other regions, torque shock reduction NO for switching between lean / rich operation for each cylinder at intervals of one cycle or more
x It is the area of the purification control mode.

When the current operation range is in the preset low rotation speed / low load range, the process proceeds from step S52 to step S53, where the normal NOx purification control mode is selected and the routine is exited. If the region is on the high rotation / high load side outside the preset low rotation / low load region, the process proceeds from step S52 to step S54 to select the torque shock reduction NOx purification control mode and exit the routine.

In the normal NOx purification control mode, the processing by the rich operation execution routine of FIG. 3 is applied to the # 1 cylinder which is the first operation switching cylinder, and # 3, #
For the # 2 and # 4 cylinders, the processing from Step S203 and S205 for leaving an interval of two or more revolutions of the engine between the cylinders from the rich operation execution routine of FIG. 4 is omitted.

That is, as shown in FIG. 15, when the # 1 cylinder is switched from the lean operation to the rich operation by the NOx storage capacity saturation determination, the # 3 cylinder → the # 2 cylinder → the # 4 cylinder sequentially performs the lean operation in accordance with the ignition order. To rich operation. When it is determined that the NOx purification has ended after all the cylinders have been switched to the rich operation, FIG.
As shown in FIG. 6, the # 1 cylinder returns from the rich operation to the lean operation, and then returns from the rich operation to the lean operation in the order of # 3 cylinder → # 2 cylinder → # 4 cylinder.

The torque shock reduction NOx purification control mode is the same as that described in the first embodiment. In this embodiment, the drivability is maintained by selecting the normal NOx purification control mode in the low rotation speed and low load range. Fuel economy deterioration can be minimized as it is.

FIGS. 17 to 21 relate to a third embodiment of the present invention. FIG. 17 is a flowchart of a control selection routine, FIG. 18 is a flowchart of a # 3 (2, 4) cylinder rich operation execution routine, and FIG. FIG. 20 is an explanatory diagram showing the division of the control region, FIG. 20 is a time chart showing the transition from the lean operation to the rich operation, and FIG. 21 is a time chart showing the transition from the rich operation to the lean operation.

This embodiment is different from the above-described second embodiment in that a second torque shock reduction NOx purification control mode in which lean / rich operation is switched for each cylinder at intervals shorter than one cycle in a cylinder order different from the ignition order. In addition, the second torque shock reduction NOx purification control mode and the above-described torque shock reduction NOx switching between lean / rich operation for each cylinder with an interval of one or more cycles in accordance with the ignition order.
An x purification control mode (first torque shock reduction NOx purification control mode) and the above-mentioned normal NOx purification control mode in which lean / rich operation is sequentially switched in accordance with the ignition order are selected according to the operation range. .

Control in the above three modes is performed by a control selection routine of FIG. 17 instead of the control selection routine of FIG. 12 of the second embodiment. In this routine, step S6
In step 1, the intake air amount Q is calculated based on the signal from the intake air amount sensor 22, and the engine speed NE is calculated based on the signal from the crank angle sensor 25. In step S62, the engine speed NE is calculated. It is checked whether or not the current operation region specified from the intake air amount Q is a high rotation / high load operation region.

When the current operation region is in the high rotation / high load operation region, the above-described steps S62 to S62 are executed.
Proceeding to 66, the first torque shock reduction NOx purification control mode is selected and the routine is exited. If the current operation region is not in the high rotation / high load operation region, the process proceeds from step S62 to step S63, and furthermore, It is determined whether or not the current operation range is in the low rotation speed / low load operation range.
When the purging control mode is selected and the routine is exited, and when the vehicle is not in the low rotation speed / low load operation region, the second torque shock reduction NOx purification control mode is selected in step S65 and the routine is exited.

That is, as shown in FIG. 19, when the operation region specified by the engine speed NE and the intake air amount Q is on the low load / low rotation side, the switching time from the lean operation to the rich operation and the rich The normal NOx purification control region mode in which the return time from the operation to the lean operation is short is selected, and the normal NOx
The second torque shock reduction NOx purification control mode in which the switching time and the recovery time are longer than the purification control mode, and the first torque shock in which the switching time and the recovery time are longer than the second torque shock reduction NOx purification control mode The reduced NOx purification control mode is selected.

In the second torque shock reduction NOx control mode, the switching from the lean operation to the rich operation is carried out at intervals of about one revolution of the engine between cylinders # 1 cylinder → #
The ignition sequence of the four cylinders # 2 cylinders # 3 cylinders is performed in a different order from the ignition sequence. Also, from the rich operation to the lean operation, the ignition sequence of the # 1 cylinders # 4 cylinders # 2 cylinders # 3 cylinders The cylinders are returned in a different order with an interval of about one engine revolution between the cylinders.

In this case, the process according to the rich operation execution routine of FIG. 3 is applied to the # 1 cylinder which is the first cylinder to be switched for operation, and the # 3, # 2, and # 4 cylinders are applied to the # 1 cylinder. The processing by the rich operation execution routine shown in FIG. 18 is applied.

The flowchart of the rich operation routine shown in FIG. 18 is similar to the flowchart of the rich operation routine of FIG. 4 described in the first embodiment.
It generally describes the algorithm of the lean / rich operation execution processing for the # 2 and # 4 cylinders, and the contents of the processing conform to the rich operation execution routine of FIG.

That is, at the time of processing for the # 3 cylinder, the notation of "# 2 (4, 1)" in the figure is interpreted as # 2 indicating the immediately preceding operation switching cylinder # 2, and "# 3" is also indicated. The notation “(2, 4)” is interpreted as # 3 indicating the # 3 cylinder, which is a self-cylinder, and the elapsed time of one revolution of the engine in steps S352 and S355 is elapsed after the # 2 rich flag is cleared, respectively. Time, the elapsed time after the # 2 rich flag is set.

Similarly, at the time of processing for the # 2 cylinder, the notation of "# 2 (4, 1)" is interpreted as # 4 indicating the immediately preceding operation switching cylinder # 4, and "# 2
The notation “3 (2,4)” is interpreted as # 2 indicating the self-cylinder # 2 cylinder, and the elapsed time in steps S352 and S355 is the elapsed time after the # 4 rich flag is cleared, #
It is the elapsed time after the 4 rich flag is set. Further, at the time of processing for the # 4 cylinder, “# 2 (4,
The notation “1)” is interpreted as # 1 indicating the # 1 cylinder which is the immediately preceding operation switching cylinder, and the notation “# 3 (2, 4)” is interpreted as # 4 indicating the # 4 cylinder which is the self-cylinder. Then, the elapsed time of one revolution of the engine in steps S352 and S355 is represented by the elapsed time after the # 1 rich flag is cleared,
It is the elapsed time after the 1 rich flag is set.

As a result, as shown in FIG.
When the # 1 cylinder is switched from the lean operation to the rich operation according to the storage saturation determination, the next ignition cylinder # 3 in the same cycle and the next ignition cylinder # 2 in the same cycle remain in the lean operation during the cycle. Is the last ignition cylinder of the #
The four cylinders are switched from the lean operation to the rich operation, and in the next cycle, the # 2 cylinder is switched from the lean operation to the rich operation.

Finally, when the # 3 cylinder is switched from the lean operation to the rich operation and the NOx purification end is determined by performing the rich operation of all the cylinders, the # 1 cylinder is operated in the rich operation as shown in FIG. , The # 4 cylinder returns from the rich operation to the lean operation in the same cycle, the # 2 cylinder returns from the rich operation to the lean operation in the next cycle, and # 3 in the next cycle.
The cylinder returns from the rich operation to the lean operation.

In this embodiment, the switching time from the lean operation to the rich operation and the return time from the rich operation to the lean operation can be changed according to the engine operating state, as compared with the first and second embodiments. Thus, the degree of freedom of control is increased, and the effect of minimizing fuel consumption deterioration while maintaining drivability can be maximized.

[0091]

As described above, according to the present invention, N
Excellent effects such as operation at a rich air-fuel ratio for Ox purification can be appropriately performed, and deterioration of fuel efficiency can be minimized while torque drivability is reduced and drivability is maintained. can get.

[Brief description of the drawings]

FIG. 1 is a functional block diagram of a control device according to a first embodiment of the present invention.

FIG. 2 is a flowchart of a rich operation permission routine;

FIG. 3 is a flowchart of a # 1 cylinder rich operation execution routine of the first embodiment;

FIG. 4 is a flowchart of a routine for performing a # 3 (2, 4) cylinder rich operation.

FIG. 5 is a flowchart of a NOx occlusion amount estimation routine.

FIG. 6 is a flowchart of a NOx storage capacity saturation determination routine;

FIG. 7 is a flowchart of a NOx purification end determination routine;

FIG. 8 is an explanatory diagram of a NOx emission amount estimation map;

FIG. 9 is a time chart showing a transition from lean operation to rich operation.

FIG. 10 is a time chart showing a transition from rich operation to lean operation.

FIG. 11 is a schematic configuration diagram of an engine control system according to the third embodiment;

FIG. 12 is a flowchart of a control selection routine according to the second embodiment of the present invention.

FIG. 13 is an explanatory diagram of an area determination table.

FIG. 14 is an explanatory diagram showing control area divisions according to the embodiment.

FIG. 15 is a time chart showing a transition from lean operation to rich operation.

FIG. 16 is a time chart showing a transition from rich operation to lean operation.

FIG. 17 is a flowchart of a control selection routine according to the third embodiment of the present invention.

FIG. 18 is a flowchart of a routine for performing a # 3 (2, 4) cylinder rich operation.

FIG. 19 is an explanatory diagram showing control area divisions according to the embodiment.

FIG. 20 is a time chart showing a transition from lean operation to rich operation.

FIG. 21 is a time chart showing a transition from rich operation to lean operation;

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Engine 19 ... Catalytic converter 50 ... ECU 75 ... NOx storage amount estimation part 76 ... NOx storage capacity saturation determination part 77 ... Cylinder rich operation execution part

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁶ Identification code FI F02D 41/04 305 F02D 41/04 305B 330 330 J

Claims (6)

[Claims] An exhaust gas purifying apparatus for an engine in which a catalyst for reducing and purifying the stored NOx is interposed in an exhaust system when storing NOx during lean operation of the engine and temporarily switching from lean operation to rich operation. The engine exhaust purification system according to any one of claims 1 to 3, further comprising control means for switching at a predetermined interval while shifting the timing for each cylinder when the engine is temporarily switched from the lean operation to the rich operation. 2. The control device according to claim 1, wherein when returning from the rich operation to the lean operation, the control means returns at a predetermined interval while shifting the timing for each cylinder.
An exhaust gas purifying device for an engine according to the above. 3. The control means estimates the NOx emission amount in the exhaust gas during the lean operation according to the operating range.
The estimated value of the Ox emission is integrated and the N stored in the catalyst is calculated.
Means for estimating the amount of Ox, means for determining that the NOx storage capacity of the catalyst has reached saturation when the estimated value of the NOx storage amount exceeds a set value, and means for determining that the NOx storage capacity of the catalyst has reached saturation. And means for switching at predetermined intervals while shifting the timing from lean operation to rich operation for each cylinder when it is determined that the operation has been performed.
An exhaust gas purifying device for an engine according to the above. 4. The control means accumulates the intake air amount after the start of the rich operation, and, when the integrated value of the intake air amount exceeds a set value, determines that the NOx purification of the catalyst has been completed. 2. The engine according to claim 1, further comprising means for returning from rich operation to lean operation at predetermined intervals while shifting the timing for each cylinder when it is determined that the NOx purification of the catalyst is completed. Exhaust gas purification device. 5. When the switching from the lean operation to the rich operation and the return from the rich operation to the lean operation are performed while shifting the timing for each cylinder, the switching is performed as the operating region of the lean operation is on the high load / high rotation side. 3. The exhaust gas purifying apparatus for an engine according to claim 2, wherein the interval and the return interval are lengthened. 6. The switching interval and the return interval are set for every two revolutions of the engine according to the ignition order, every one revolution of the engine according to a cylinder order different from the ignition order, or for the ignition order. Item 6. An exhaust gas purifying apparatus for an engine according to Item 5.