JP3866347B2 - Engine exhaust purification system - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an engine exhaust purification system in which an exhaust system is provided with a three-way catalyst that stores NOx in exhaust gas and reduces and purifies it together with HC and CO.
[0002]
[Prior art]
In general, in the air-fuel ratio control of an engine, the exhaust gas is purified by a catalyst, and thus the fuel injection amount and the like are controlled so that the air-fuel ratio falls within the region where the exhaust gas purification efficiency of the catalyst is the best. Then, the analysis of the combustion process has progressed, and an engine has been developed that employs lean burn that can be burned efficiently with a small amount of fuel without misfiring even at a lean air-fuel ratio.
[0003]
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 occluded. When the oxygen concentration in the exhaust gas decreases, the occluded NOx is released and oxidized. There are those that employ a three-way catalyst that reduces and purifies surplus HC and CO that have not been reduced. In such a three-way catalyst for lean combustion, NOx emitted during operation at a lean air-fuel ratio is reduced. Before the storage amount reaches saturation, it is necessary to perform an operation at a rich air-fuel ratio for a short time to temporarily increase the exhaust amount 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 of torque shock due to fluctuations in engine torque. For example, Japanese Patent Laid-Open No. 7-26999 discloses rich operation for NOx purification. A technique for performing NOx purification without causing the driver to feel a torque shock by performing only when the engine load decreases is disclosed.
[0005]
[Problems to be solved by the invention]
However, if a restriction such as limiting the rich operation for NOx purification when the engine load is reduced is provided, the rich operation is performed as when driving at a constant speed in which the driving condition of medium load or higher such as an expressway is sustained. Under operating conditions that are performed less frequently, the NOx occlusion amount reaches saturation and may be discharged without being purified.
[0006]
The present invention has been made in view of the above circumstances, and an object of the present invention is to provide an engine exhaust purification device that can appropriately perform operation at a rich air-fuel ratio for NOx purification and reduce torque shock. It is said.
[0007]
[Means for Solving the Problems]
According to the first aspect of the present invention, an NOx is occluded during the lean operation of the engine, and when the engine is temporarily switched from the lean operation to the rich operation, a catalyst for reducing and purifying the occluded NOx is disposed in the exhaust system. In an exhaust purification system, when the engine is temporarily switched from lean operation to rich operation, it is switched at predetermined intervals while shifting the timing for each cylinder. And means for integrating 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, it is determined that the NOx purification of the catalyst is completed, and the NOx purification of the catalyst is performed. Means for returning from a rich operation to a lean operation at predetermined intervals while shifting the timing for each cylinder when it is determined that the operation has ended. It is provided with.
[0008]
The invention according to claim 2 In an engine exhaust gas purification apparatus in which NOx is occluded during the lean operation of the engine and the catalyst for reducing and purifying the occluded NOx is temporarily inserted into the exhaust system when the lean operation is temporarily switched to the rich operation, the engine is temporarily When switching from lean operation to rich operation, means for switching at predetermined intervals while shifting the timing for each cylinder, and means for returning at predetermined intervals while shifting the timing for each cylinder when returning from rich operation to lean operation When switching from lean operation to rich operation and returning from rich operation to lean operation while shifting the timing for each cylinder, the switching interval increases as the operating range of lean operation is on the high load / high rotation side. And increase the return interval It is characterized by that.
[0009]
The invention according to claim 3 is the invention according to claim 1, When switching from lean operation to rich operation and returning from rich operation to lean operation while shifting the timing for each cylinder, the switching interval and the return interval are set as the operating range of lean operation is on the high load / high rotation side. Lengthen It is characterized by that.
[0010]
The invention according to claim 4 Claim 2 or claim 3 In the invention, The switching interval and the return interval are set to every two engine revolutions according to the ignition order, every one engine revolution according to a cylinder order different from the ignition order, or the ignition order. It is characterized by that.
[0013]
That is, in the present invention, when temporarily switching from lean operation to rich operation in order to reduce and purify NOx stored in the catalyst during the lean operation of the engine, switching is performed at predetermined intervals while shifting the timing for each cylinder. When returning from lean to lean operation, it is returned at predetermined intervals while shifting the timing for each cylinder.
[0014]
Re It is desirable to carry out the return from the switch operation to the lean operation by determining that the NOx purification of the catalyst has ended when the integrated value of the intake air amount after the start of the rich operation exceeds the set value, The switching interval from the lean operation to the rich operation and the return interval from the rich operation to the lean operation are set every two engine revolutions according to the ignition order, every one engine revolution according to the cylinder order different from the ignition order, or in the ignition order. For example, the longer the engine operating region at the lean air-fuel ratio is on the high load / high rotation side, the longer is desirable.
[0015]
DETAILED DESCRIPTION OF THE INVENTION
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 the # 3 (2, 4) cylinder rich operation execution routine, 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. FIG. 8 is an explanatory diagram of the 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, and FIG. It is a schematic block diagram of an engine control system.
[0016]
In FIG. 11, reference numeral 1 denotes an engine (in this embodiment, a horizontally opposed type four-cylinder engine), which is dilute in all operation areas except for a part of operation areas such as high load / acceleration operation where high output is required. This is a lean burn engine that performs combustion (lean burn). Cylinder heads 2 are provided in both the left and right banks of the cylinder block 1a of the engine 1, and a combustion chamber 3 for each cylinder formed by the cylinder head 2 and the cylinder block 1a is connected via an intake port 4. An intake manifold 5 is communicated, and an exhaust manifold 7 is communicated via an exhaust port 6.
[0017]
An injector 8 faces directly upstream of the intake port 4, and a spark plug 9 is attached to the cylinder head 2 for each cylinder, the tip of which is exposed to the combustion chamber 3. An ignition coil 10 is connected to the ignition plug 9, and an igniter 11 is connected to the ignition coil 10.
[0018]
An air chamber 12 is formed in the upstream manifold portion of the intake manifold 5 and communicates with the intake pipe 13. An air cleaner 14 is attached to the upstream side of the intake pipe 13, and a throttle valve 15 is provided midway. Is intervening. 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.
[0019]
On the other hand, an exhaust pipe 18 is communicated with the exhaust manifold 7, and a catalytic converter 19 containing a lean combustion three-way catalyst is disposed immediately downstream of the exhaust manifold 7 in the exhaust pipe 7. A muffler 20 is interposed at the downstream end. The above three-way catalyst for lean combustion comprises, for example, a NOx occlusion material such as alkali metal, alkaline earth, rare earth and a noble metal such as platinum supported on a support such as alumina, and by the storage function of NOx and O2, When the oxygen concentration of the exhaust gas is high, HC and CO are oxidized and reduced and NOx is occluded. When the oxygen concentration in the exhaust gas is lowered, the stored NOx is released, and the HC, Reduce and purify with CO.
[0020]
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 an idle that is turned on when the throttle is fully closed. A throttle sensor 21 having a built-in switch, a hot wire type or hot film type intake air amount sensor 22 interposed immediately downstream of the air cleaner 14 of the intake pipe 13, and both the left and right banks of the cylinder block 1a communicate with each other. A cooling water temperature sensor 23 facing the cooling water passage 1b, a crank angle sensor 25 facing the outer periphery of the crank rotor 24 mounted on the crankshaft 1c, and a cam rotor 26 connected to the camshaft 1d. Cylinder discrimination sensor 27 and the exhaust pipe 18 facing the upstream side of the catalytic converter 19 Air-fuel ratio sensor 28, and other, there is a variety of sensors (not shown).
[0021]
The various sensors are connected to an electronic control unit (ECU) 50 that electronically performs air-fuel ratio control and the like of the engine 1. The ECU 50 is mainly configured by a microcomputer including a CPU 51, a ROM 52, a RAM 53, a backup RAM 54, an I / O interface 55, etc., and a power supply circuit 56 for supplying a stabilized voltage to each part and an output port of the I / O interface 55 Peripheral circuits such as a drive circuit 57 for driving the actuators by signals from the A / D converter 58 and an A / D converter 58 for converting analog signals from the sensors to digital signals are incorporated.
[0022]
The input port of the I / O interface 55 is connected to the crank angle sensor 25, the cylinder discrimination sensor 27, and other sensors and switches (not shown), and further via the A / D converter 58. 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 igniter 11 is connected to the output port of the I / O interface 55, and the injector 8, the ISCV 17, and other various actuators (not shown) are connected via the drive circuit 57. .
[0023]
The power 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. The power supply circuit 56 is directly connected to the battery 61. When 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 part. On the other hand, regardless of whether the ignition switch 62 is ON or OFF, the backup RAM 54 is always supplied with backup power.
[0024]
In the ECU 50, in accordance with a control program stored in the ROM 52, the output signals from the sensors and switches are read to detect the engine operating state, and the optimum fuel injection amount, ignition timing, etc. are calculated from the detected engine operating state. The air / fuel ratio control is performed lean or stoichiometric to improve fuel efficiency and exhaust emissions, and secure engine output.
[0025]
Further, in the ECU 50, as shown in FIG. 1, the operation region determination unit 71, the lean operation permission unit 72, the cylinder specific fuel injection control unit 73, the cylinder specific ignition timing control unit 74, the NOx occlusion amount estimation unit 75, and the NOx occlusion. When the functions of the capacity saturation determination unit 76 and the cylinder-specific rich operation execution unit 77 monitor the NOx storage capacity of the lean combustion three-way catalyst during operation at a lean air-fuel ratio, and determine that the NOx storage capacity has reached saturation, Switching from lean air-fuel ratio operation to rich air-fuel ratio operation by shifting the timing with a predetermined interval for each cylinder, temporarily reducing HC and CO emissions while reducing torque shock and diluting NOx in the combustion three-way catalyst is reduced and purified. Then, after the NOx purification is completed, the operation at the rich air-fuel ratio is returned to the operation at the lean air-fuel ratio while shifting the timing at a predetermined interval for each cylinder in order to reduce the torque shock.
[0026]
That is, when the operation region determined by the operation region determination unit 71 is the lean operation region, the lean operation permission unit 72 permits the lean operation, and the cylinder-specific fuel injection control unit 73 corrects the fuel injection amount for each cylinder to be reduced. The cylinder specific ignition timing control unit 74 corrects the cylinder specific ignition timing according to the lean air-fuel ratio, and at the same time, the NOx storage amount estimation unit 75 determines the NOx storage amount of the lean three-way catalyst after the lean operation is started. Let me estimate.
[0027]
The cylinder specific fuel injection control unit 73 calculates the 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 equation (1), the basic fuel injection pulse width TP is set to a correction coefficient COEF based on the coolant temperature TW and the like, and a target air-fuel ratio correction coefficient KTGT # n (n = 1) set for each cylinder of # 1 to # 4. -4), air-fuel ratio feedback correction coefficient LAMBDA for bringing the air-fuel ratio closer to the target air-fuel ratio based on the output voltage of the wide-range air-fuel ratio sensor 28, variation during production of the intake air amount measurement system and fuel supply system, or changes over time The effective fuel injection pulse width TE is calculated by correcting with a learning correction coefficient KBLRC for quickly correcting the deviation of the air-fuel ratio and the fuel adhesion correction coefficient KX for correcting the influence of the fuel adhering to the port wall surface. . Then, as shown in the following equation (2), the effective fuel injection pulse width TE is added with an invalid injection pulse width TV that compensates for the invalid injection time of the injector 8 set based on the battery voltage VB. A fuel injection pulse width Ti is calculated for each cylinder.
TE = TP × COEF × KTGT # n × LAMBDA × KBLRC × KX (1)
Ti = TE + TV (2)
At that time, the target air-fuel ratio correction coefficient KTGT # n set for each cylinder has a different value between the lean operation and the rich operation. For example, each of the cylinders has a lean value of 0.6 to 0.8. The fuel injection amount is decreased, and the fuel injection amount of each cylinder is increased by a rich value of 1.2 to 1.5. The fuel adhesion correction coefficient KX is preferably set for each cylinder (KX1 to KX4).
[0028]
The cylinder specific ignition timing control unit 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, and the following equation (3) As shown in FIG. 4, the basic ignition timing IGREG is set to a water temperature correction value ADVTW based on the cooling water temperature TW, an acceleration correction value ADVMAC that is a correction value detected based on a signal from the throttle sensor 21, and to avoid knocking. The final ignition timing ADV is calculated for each cylinder by correcting with the knock control correction value KB.
ADV = IGREG ± ADVTW ± ADVMAC−KB (3)
Also in the calculation of the ignition timing, the basic ignition timing IGREG corresponds to the two maps of the lean operation basic ignition timing map and the rich operation ignition timing map corresponding to the lean operation and the rich operation. Is prepared, and a map is selected according to lean operation / rich operation.
[0029]
In addition, the NOx occlusion amount estimation unit 75 integrates the NOx emission amount for each operation region specified by the engine speed NE and the intake air amount Q for each set time only during lean operation, and this accumulated value is lean-burned. This is obtained as the estimated NOx storage amount of the three-way catalyst. This estimated NOx storage amount is cleared when the rich operation is performed.
[0030]
Then, the NOx occlusion capacity saturation determination unit 76 compares the estimated NOx occlusion amount estimated by the NOx occlusion amount estimation unit 75 with a set value, and when the estimated NOx occlusion amount exceeds the set value, the lean combustion three It is determined that the NOx storage capacity of the original catalyst has reached the saturation state, and the cylinder-specific rich operation execution unit 77 is instructed to start the rich operation for each cylinder.
[0031]
In order to purify the NOx occluded in the lean combustion three-way catalyst, the rich operation execution unit 77 for each cylinder first changes the fuel injection amount and ignition timing of one cylinder from the values during lean operation to the values during rich operation. The rich operation is performed by switching, 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 a predetermined interval. At the same time, the intake air amount after the start of the rich operation switching is integrated every set time, and when this integrated value reaches the set value, the NOx stored in the lean combustion three-way catalyst by switching from the lean operation to the rich operation In the same way as switching from lean operation to rich operation, the fuel injection amount and ignition timing of one cylinder are returned from the values during rich operation to the values during lean operation, with a predetermined interval. Return the other cylinders to lean operation one after another, and return all cylinders to lean operation.
[0032]
In this embodiment, switching from lean operation to rich operation is performed according to the ignition sequence of # 1 cylinder → # 3 cylinder → # 2 cylinder → # 4 cylinder with an interval of at least two engine revolutions (one cycle) between each cylinder. In addition, the engine is returned from the rich operation to the lean operation with an interval of at least two engine rotations between the cylinders in the order of ignition from # 1 cylinder → # 3 cylinder → # 2 cylinder → # 4 cylinder.
[0033]
Hereinafter, processing related to switching from lean operation to rich operation after the lean operation permission by the ECU 50 and returning from rich operation to lean operation will be described with reference to the flowcharts of FIGS.
[0034]
Switching from lean operation to rich operation after permitting lean operation is performed by the NOx storage capacity saturation determination routine of FIG. 6 from the estimated NOx storage amount of the lean combustion three-way catalyst obtained by the NOx storage amount estimation routine of FIG. When it is determined that the NOx storage capacity of the three-way catalyst for lean combustion has reached saturation, a rich operation execution flag for permitting switching from lean operation to rich operation is set in the rich operation execution permission routine of FIG. Will be implemented. On the other hand, the return from the rich operation to the lean operation is performed by the rich operation execution permission routine of FIG. 2 when the NOx purification end determination routine of FIG. 7 determines that the NOx purification of the lean combustion three-way catalyst has ended. This is performed by clearing the execution flag.
[0035]
For switching from lean operation to rich operation and return from rich operation to lean operation for each cylinder, first refer to the rich operation execution flag in the rich operation execution routine of FIG. 3 for the # 1 cylinder. Next, in the rich operation execution routine shown in FIG. 4, a rich operation execution flag and a rich flag indicating execution of rich operation of the immediately preceding ignition cylinder (# 1, # 2, # 3, # 4 rich described later) With reference to the flag), the engine is sequentially performed in the order of # 3, 2, 4 cylinders with an interval of at least two engine revolutions according to the firing order. Note that the rich operation execution flag and the # 1 to # 4 rich flags of each cylinder are cleared in advance upon initialization when the system power is turned on.
[0036]
First, the rich operation execution permission routine of FIG. 2 will be described. In this routine, it is checked in step S101 whether or not the rich operation execution flag is set. If the rich operation execution flag is not set, the routine proceeds to step S102, and the determination result by the NOx storage capacity saturation determination routine of FIG. To check whether the NOx storage capacity of the three-way catalyst for lean combustion is saturated.
[0037]
When it is determined that the NOx storage capacity is not saturated, the routine proceeds from step S102 to step S104, the rich operation execution flag is cleared, the routine is exited, and the NOx storage capacity of the lean combustion three-way catalyst is saturated. If it is determined that the operation is in progress, the routine proceeds from step S102 to step S105, the rich operation execution flag is set, and the routine is exited.
[0038]
On the other hand, when the rich operation execution flag is already set in step S101, the process branches from step S101 to step S103, and the determination result of the NOx purification end determination routine of FIG. It is checked whether or not the purification of NOx stored in the three-way catalyst for lean combustion has been completed by performing the rich operation of the # 1 to # 4 cylinders switched from the lean operation by the rich operation execution routine shown.
[0039]
As a result, when NOx purification has not ended, the routine proceeds from step S103 to step S105 described above, exits the routine while the rich operation execution flag is set, and when NOx purification ends, from step S103 to step S105 described above. Proceed to S104 to clear the rich operation execution flag and exit the routine.
[0040]
In contrast to the above-described rich operation execution permission routine, in the rich operation execution routine for the # 1 cylinder in FIG. 3, it is first checked whether or not the rich operation execution flag is set in step S201. And, when the NOx occlusion capacity of the lean combustion three-way catalyst is not saturated and there is no need to switch from lean operation to rich operation, or when the rich operation execution flag is not set, or already the lean combustion three way When the switching from the lean operation to the rich operation is performed for the NOx purification of the catalyst, and the rich operation execution flag once set is cleared by the NOx purification completion determination, the process proceeds from step S201 to step S202 and # 1 The # 1 rich flag indicating execution of the rich operation of the cylinder is cleared, the lean operation of # 1 cylinder is executed in step S203, and the routine is exited.
[0041]
On the other hand, when the NOx occlusion capability of the lean combustion three-way catalyst is saturated and the rich operation execution flag for switching from lean operation to rich operation is set for NOx purification in step S201, step S201 to step S204 are performed. The # 1 rich flag is set, and the rich operation of the # 1 cylinder is performed in step S205, and the routine is exited.
[0042]
Next, the rich operation execution routine 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 represented by three cylinders.
[0043]
The lean operation for each cylinder is performed by setting the final fuel injection pulse width Ti with the target air-fuel ratio correction coefficient KTGT # n for each cylinder as the lean value and selecting the basic ignition timing map for the lean operation. The ignition timing ADV is set to a value during lean operation. 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 the basic ignition timing map for the rich operation and performing ignition. This is done by setting the time ADV to the value during rich operation.
[0044]
In the rich operation execution routine for # 3 cylinder, first, it is checked in step S301 whether or not the rich operation execution flag is set. If NO (the rich operation execution flag is not set), the process proceeds to step S302. It is checked whether or not the # 1 rich flag of the # 1 cylinder which is the immediately preceding ignition cylinder is cleared.
[0045]
In this case, when the lean operation is started and the NOx occlusion capacity of the lean combustion three-way catalyst is not saturated, the rich operation execution flag is not set, and the # 1 rich flag of the # 1 cylinder is also cleared. Therefore, at first, the process proceeds from step S301 to step S303 through step S302, and whether or not the elapsed time after the # 1 rich flag is cleared with reference to the timer has passed more than the time corresponding to the engine two revolutions. Check out.
[0046]
The elapsed time can be known by referring to, for example, a hardware timer or software timer in the system. Time counting starts when the # 1 rich flag is cleared, and the # 1 rich flag is set. The time is kept until it is reset.
[0047]
As described above, the # 1 to # 4 rich flags are cleared by initialization when the system power is turned on. Therefore, at the start of lean operation, a time of two or more revolutions has elapsed after the # 1 rich flag is cleared. In the initial state where the NOx occlusion capacity of the three-way catalyst for lean combustion is not saturated, the process proceeds from step S303 to step S306, the # 3 rich flag is cleared, and the lean operation of the # 3 cylinder is performed in step S307. Exit the routine.
[0048]
After that, when the lean operation is continued, the NOx occlusion capacity of the lean combustion three-way catalyst is saturated and the rich operation execution flag is set by the rich operation execution permission routine of FIG. 2, the rich operation execution flag is set. In step S301, the process proceeds from step S301 to step S304, and it is checked whether the # 1 rich flag is set. When the # 1 rich flag is not set and the # 1 cylinder has not yet switched from the lean operation to the rich operation, the lean operation of the # 3 cylinder is performed from the above step S304 through the above steps S306 and S307. To exit the routine.
[0049]
On the other hand, if the # 1 rich flag is set in step S304 and the # 1 cylinder has already switched from lean operation to rich operation, the process proceeds from step S304 to step S305, and the timer is referred to # 1. It is checked whether or not the elapsed time after the rich flag is set (the elapsed time after the # 1 cylinder is switched from lean operation to rich operation) has exceeded a time corresponding to two engine revolutions.
[0050]
The elapsed time referred to in step S305 is started when the # 1 rich flag is set with respect to the elapsed time referred to in step S303, and is reset when the # 1 rich flag is cleared. It keeps timing until it is done. Here, when the time for more than two engine revolutions has not elapsed since the switching of the # 1 cylinder from the lean operation to the rich operation, the # 3 cylinder is subjected to the lean operation from the above step S305 through the aforementioned steps S306 and S307. If the time is longer than 2 revolutions after the # 1 cylinder switches from lean operation to rich operation, the routine proceeds from step S305 to step S308 and the # 3 rich flag is set. In step S309, the # 3 cylinder rich operation is performed, and the lean operation is switched to the rich operation to exit the routine.
[0051]
Thereafter, when the routine is repeated, while the rich operation execution flag is set, that is, while the reduction and purification of NOx of the lean combustion three-way catalyst is not completed, the process goes from step S301 to steps S304 and S305 to step S308. The rich operation of the # 3 cylinder is continued in step S309.
[0052]
Then, as shown in FIG. 9, after the # 1 cylinder switches from lean operation to rich operation by NOx occlusion saturation determination, the # 3 cylinder switches from lean operation to rich operation with an interval of two or more revolutions of the engine. Similarly, the # 2 cylinder is switched from lean operation to rich operation with an interval of at least two revolutions of the engine, and after the # 2 cylinder is switched to rich operation, the intervals of at least two revolutions of the engine are left. The # 4 cylinder is switched from lean operation to rich operation.
[0053]
In the rich operation execution routine for the # 2 cylinder, the expression “# 1 (3, 2)” in the flowchart of FIG. 4 is interpreted as # 3 indicating the immediately preceding ignition cylinder # 3, and “# 3 ( 2, 4) ”may be interpreted as # 2 indicating the # 2 cylinder which is the self-cylinder. The elapsed time in steps S303 and S305 is the elapsed time after the # 3 rich flag is cleared, 3 is the elapsed time after the rich flag is set. In the rich operation execution routine for the # 4 cylinder, the notation “# 1 (3, 2)” is interpreted as # 2 indicating the immediately preceding ignition cylinder # 2, and “# 3 (2, 4) "4" indicating the self-cylinder # 4 cylinder, and the elapsed time in steps S303 and S305 are respectively the elapsed time after the # 2 rich flag is cleared and the # 2 rich flag. Elapsed time after is set.
[0054]
Then, when all the cylinders are switched from the lean operation to the rich operation, and NOx stored in the lean combustion three-way catalyst is reduced and purified by the execution of the rich operation, and it is determined that the NOx purification is completed, the above-described FIG. The rich operation execution flag is cleared by the rich operation execution permission routine, and the # 1 cylinder returns from the rich operation to the lean operation by the rich operation execution routine for the # 1 cylinder in FIG.
[0055]
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 is the # 1 rich flag of the # 1 cylinder being the immediately preceding ignition cylinder cleared? Check for no. Then, when the # 1 rich flag is not cleared, it is the time when the # 1 cylinder has not yet returned from the rich operation to the lean operation immediately after the NOx purification end determination, so the # 3 rich flag is set from step S302. Step S308 is jumped to, and after step S309, the rich operation of the # 3 cylinder is continued.
[0056]
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 is cleared, a time of two or more engine revolutions has elapsed. When the time equal to or longer than two engine revolutions has not elapsed, the routine jumps from step S303 to step S308 described above, and after step S309, the # 3 cylinder is set in the rich operation execution state and the routine is exited.
[0057]
Then, if the time of more than 2 engine revolutions has elapsed after the # 1 rich flag is cleared, the process proceeds from step S303 to step S306, the # 3 rich flag is cleared, and the lean operation of the # 3 cylinder is performed in step S307. 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 with an interval of more than two engine rotations. The # 2 and # 4 cylinders sequentially return to lean operation after an interval of at least two engine revolutions.
[0058]
As described above, the air-fuel ratio in the cylinder does not switch instantaneously (in a stepwise manner) due to the effect of fuel adhering to the wall of the port, etc. In this cycle, it is possible to appropriately perform the operation at the rich air-fuel ratio for NOx purification, and it is possible to reduce the torque shock and ensure drivability.
[0059]
Next, the NOx occlusion amount estimation routine of FIG. 5 for estimating the NOx occlusion amount in the three-way catalyst for lean combustion will be described.
[0060]
This NOx occlusion amount estimation routine is a periodic processing routine that is interrupted 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 sensor 25 Based on this signal, the engine speed NE is calculated, and it is checked in step S402 whether or not the current operating state is in the lean operation.
[0061]
When the lean operation is not being executed, the routine is exited from step S402. When the lean operation is being executed, the routine proceeds from step S402 to step S403 to check whether the rich operation execution flag is set, and the rich operation is executed. When the flag is not set, the process proceeds to step S404, the NOx emission amount corresponding 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 the process proceeds to step S405.
[0062]
As shown in FIG. 8, the NOx emission amount estimation map shows NOx emission per unit time obtained by analyzing engine exhaust gas for each operation region specified by the engine speed NE and intake air amount Q. The amount is stored in advance as an estimated NOx emission amount, and the NOx emission amount according to the operating state of the engine can be estimated by referring to the NOx emission amount estimation map.
[0063]
In step S405, the estimated NOx emission amount obtained by referring to the NOx emission amount estimation map is added to the NOx occlusion amount estimation value of the lean combustion three-way catalyst obtained at the time of the previous routine execution to thereby determine the current NOx emission amount. Update the estimated amount of occlusion and exit the routine.
[0064]
On the other hand, when the rich operation execution flag is set in step S403, the rich operation of the # 1 to # 4 cylinders by the rich operation execution routine of FIGS. Since the purification of the NOx that has been started has started, the routine proceeds from step S403 to step S406 to clear the NOx occlusion amount estimated value, and the routine is exited.
[0065]
The NOx occlusion amount estimated value obtained in the NOx occlusion amount estimation routine is referred to in the NOx occlusion capacity saturation determination routine of FIG. In this NOx occlusion capacity saturation determination routine, the NOx occlusion amount estimated value is compared with the set value in step S501, and when the NOx occlusion amount estimated value exceeds the set value, the NOx occlusion capacity is determined to be saturated in step S502. Is stored at a predetermined address in the RAM 53, and when the estimated NOx storage amount is equal to or smaller than the set value, the NOx storage capacity saturation determination is canceled in step S503, and the determination result stored at the predetermined address in the RAM 53 is cleared.
[0066]
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, is the rich operation execution flag set first in step S601? If the rich operation execution flag is not set, the process proceeds to step S602 to clear the NOx purification amount counter and exit the routine.When the rich operation execution flag is set, the process proceeds to step S603, The current intake air amount Q is added to the NOx purification amount counter that integrates the intake air amount Q after the start of the rich operation.
[0067]
Then, the process proceeds from step S603 to step S604, and 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, When the set value is not exceeded, the routine is exited. When the set value is exceeded, it is determined in step S605 that NOx purification has been completed, the determination result is stored at a predetermined address in the RAM 53, and the routine is exited.
[0068]
FIGS. 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 diagram of an area determination table, FIG. 14 is an explanatory diagram showing divisions of control areas, and FIG. Is a time chart showing the transition from lean operation to rich operation, and FIG. 16 is a time chart showing the transition from rich operation to lean operation.
[0069]
In this embodiment, the function of the cylinder-by-cylinder rich operation execution unit 77 of the first embodiment described above is slightly changed to switch from lean operation to rich operation and return from rich operation to lean operation. This is divided into a torque shock reducing NOx purification control mode for switching lean / rich operation for each cylinder with an interval of one cycle or more, and a normal NOx purification control mode for sequentially switching lean / rich operation according to the ignition sequence. Two control modes are selected according to the operation region.
[0070]
For this reason, in the present embodiment, switching from lean operation to rich operation is performed from the estimated NOx storage amount of the lean combustion three-way catalyst by the routines of FIGS. When it is determined that the NOx storage capacity has reached saturation, the rich operation execution flag is set, and the return from the rich operation to the lean operation is completed when the NOx purification of the lean three-way catalyst is completed. When the determination is made, the rich operation execution flag is cleared, and the actual switching process from lean operation to rich operation and return processing from rich operation to lean operation are performed in the control shown in FIG. Selected by selection routine.
[0071]
In the control selection routine of FIG. 12, in step S51, 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 S52, using the engine speed NE as a parameter, as shown in FIG. 13, for each engine speed NE, a slice of the intake air amount in the cylinder that divides the normal NOx purification control region and the torque shock reduction NOx purification control region. The area determination table in which the level value is stored is referred to, and the reference value (slice level value) in this area determination table is compared with the intake air amount Q calculated in step S51, and the current operation area is set in advance. Check if it is in the low rotation / low load range.
[0072]
That is, in order to release NOx stored in the three-way catalyst for lean combustion, it is necessary to reduce the oxygen concentration in the exhaust gas by making the average air-fuel ratio of all the cylinders rich, but the cylinders are spaced at intervals of one cycle or more. Separately, in the torque shock reduction NOx purification control mode for switching between lean / rich operation, the rich air cylinder and the lean cylinder are temporarily mixed when switching from lean operation to rich operation, so the average air-fuel ratio becomes lean, and NOx While the purification effect does not increase, there is a risk that fuel efficiency will deteriorate.
[0073]
For this reason, as shown in FIG. 14, in the low load / low rotation region where the engine output is low and the torque shock is small, the normal NOx purification control mode where the lean / rich operation is sequentially switched according to the ignition order This region is a torque shock reduction NOx purification control mode in which the lean / rich operation is switched for each cylinder with an interval of one cycle or more.
[0074]
Then, when the current operation region is in the preset low rotation / low load region, the process proceeds from step S52 to step S53, and the normal NOx purification control mode is selected and the routine is exited. When it is on the high rotation / high load side outside the set low rotation / low load region, the routine proceeds from step S52 to step S54, selects the torque shock reduction NOx purification control mode, and exits the routine.
[0075]
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 the # 3, # 2, # 4 cylinders are From step 4 of the rich operation execution routine, processing in which steps S203 and S205 for leaving an interval of at least two engine revolutions between the cylinders is omitted is applied.
[0076]
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 lean operation and the rich operation are sequentially performed from # 3 cylinder → # 2 cylinder → # 4 cylinder according to the ignition sequence. Can be switched to. When it is determined that the NOx purification has been completed after all the cylinders have been switched to the rich operation, as shown in FIG. 16, the # 1 cylinder returns from the rich operation to the lean operation, and then the # 3 cylinder → # Return from rich operation to lean operation in the order of 2 cylinders to # 4 cylinders.
[0077]
The torque shock reduction NOx purification control mode is as described in the first embodiment. In this embodiment, the fuel consumption deteriorates while maintaining the drivability by selecting the normal NOx purification control mode in the low rotation / low load region. Can be minimized.
[0078]
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 a time chart showing the transition from lean operation to rich operation, and FIG. 21 is a time chart showing the transition from rich operation to lean operation.
[0079]
The present embodiment adds a second torque shock reduction NOx purification control mode that switches lean / rich operation for each cylinder with an interval shorter than one cycle in a cylinder order different from the ignition order, compared to the above-described second form. This second torque shock reduction NOx purification control mode and the aforementioned torque shock reduction NOx purification control mode (first torque shock reduction NOx purification) that switches between lean / rich operation for each cylinder at intervals of one cycle or more according to the ignition sequence. Control mode) and the above-described normal NOx purification control mode in which the lean / rich operation is sequentially switched in accordance with the ignition order, depending on the operation region.
[0080]
The 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, in step S61, 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, It is checked whether or not the current operation region specified from the engine speed NE and the intake air amount Q is a high rotation / high load operation region.
[0081]
When the current operation region is in the high rotation / high load operation region, the process proceeds from step S62 to step S66 to select the first torque shock reduction NOx purification control mode and exit the routine. Is not in the high rotation / high load operation region, the process proceeds from step S62 to step S63, and further checks whether the current operation region is in the low rotation / low load operation region. If the normal NOx purification control mode is selected in step S64, the routine is exited. If not in the low rotation / low load operation range, the second torque shock reduction NOx purification control mode is selected in step S65. Then exit the routine.
[0082]
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 lean operation to rich operation and from rich operation to lean The normal NOx purification control region mode in which the return time to operation is short is selected, and the second torque shock reduction NOx purification control in which the switching time and the return time are longer than the normal NOx purification control mode as the load increases and the rotation speed increases. Mode, and further, the first torque shock reduction NOx purification control mode having a longer switching time and return time than the second torque shock reduction NOx purification control mode is selected.
[0083]
In the second torque shock reduction NOx control mode, switching from lean operation to rich operation is performed with an interval of about one rotation of the engine between each cylinder: # 1 cylinder → # 4 cylinder → # 2 cylinder → # 3 cylinder The order of ignition is different from the order of ignition. Also, from rich operation to lean operation, the engine turns about 1 revolution between each cylinder in an order different from the order of ignition of cylinder # 1, cylinder # 4, cylinder # 2, and cylinder # 3. Return with an interval of.
[0084]
In this case, the processing by the rich operation execution routine of FIG. 3 is applied to the # 1 cylinder which is the first cylinder to switch the operation, and FIG. 18 is applied to the # 3, # 2 and # 4 cylinders. The processing by the rich operation execution routine is applied.
[0085]
The flowchart of the rich operation execution routine shown in FIG. 18 summarizes the algorithm of the lean / rich operation execution processing for the # 3, # 2, and # 4 cylinders, similarly to the flowchart of the rich operation execution routine of FIG. 4 described in the first embodiment. The contents of the processing are in accordance with the rich operation execution routine of FIG.
[0086]
That is, in the process for the # 3 cylinder, the notation “# 2 (4, 1)” in the drawing is interpreted as # 2 indicating the # 2 cylinder which is the immediately preceding operation switching cylinder, and “# 3 (2, 4) “” is interpreted as # 3 indicating the # 3 cylinder that is the self-cylinder, and the elapsed time of one engine revolution in steps S352 and S355 is the elapsed time after the # 2 rich flag is cleared, # 2 The elapsed time after the rich flag is set.
[0087]
Similarly, in the process for the # 2 cylinder, the notation “# 2 (4,1)” is interpreted as # 4 indicating the immediately preceding operation switching cylinder # 4, and “# 3 (2,4) "2" 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, after the # 4 rich flag is set The elapsed time. Further, in the process for the # 4 cylinder, the expression “# 2 (4, 1)” is interpreted as # 1 indicating the immediately preceding operation switching cylinder # 1, and “# 3 (2, 4)”. Is interpreted as # 4 indicating the # 4 cylinder that is the self-cylinder, and the elapsed time of one engine revolution in steps S352 and S355 is set to the elapsed time after the # 1 rich flag is cleared, and the # 1 rich flag is set The elapsed time after
[0088]
Accordingly, as shown in FIG. 20, when the # 1 cylinder is switched from lean operation to rich operation by the NOx occlusion saturation determination, the # 3 cylinder, which is the next ignition cylinder in the same cycle, and the next ignition cylinder. While the # 2 cylinder remains in the lean operation, the # 4 cylinder, which is the last ignition cylinder in the cycle, is 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.
[0089]
Finally, when the # 3 cylinder is switched from the lean operation to the rich operation and the NOx purification end determination is made by performing the rich operation of all the cylinders, as shown in FIG. 21, the # 1 cylinder is changed from the rich operation to the lean operation. After returning to, # 4 cylinder returns from rich operation to lean operation in the same cycle, # 2 cylinder returns from rich operation to lean operation in the next cycle, and # 3 cylinder performs rich operation in the next cycle. Return to lean operation.
[0090]
In this embodiment, compared with the first and second embodiments described above, the switching time from lean operation to rich operation and the return time from rich operation to lean operation can be changed according to the engine operating state. The degree of freedom becomes greater, and it is possible to maximize the effect of minimizing fuel consumption deterioration while maintaining drivability.
[0091]
【The invention's effect】
As described above, according to the present invention, it is possible to appropriately perform operation at a rich air-fuel ratio for NOx purification, and to minimize deterioration in fuel consumption while reducing torque shock and maintaining drivability. Excellent effects such as being able to be suppressed are obtained.
[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 execution permission routine
FIG. 3 is a flowchart of a routine for performing a # 1 cylinder rich operation.
FIG. 4 is a flow chart of a # 3 (2,4) cylinder rich operation execution routine, same as above.
FIG. 5 is a flowchart of the 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 the NOx emission estimation map
FIG. 9 is a time chart showing the transition from lean operation to rich operation.
FIG. 10 is a time chart showing the transition from rich operation to lean operation.
FIG. 11 is a schematic diagram of the engine control system
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 as above.
FIG. 14 is an explanatory diagram showing control area divisions
FIG. 15 is a time chart showing the transition from lean operation to rich operation.
FIG. 16 is a time chart showing the 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 the # 3 (2,4) cylinder rich operation execution routine.
FIG. 19 is an explanatory diagram showing control area divisions
FIG. 20 is a time chart showing the transition from lean operation to rich operation.
FIG. 21 is a time chart showing the transition from rich operation to lean operation.
[Explanation of symbols]
1 ... Engine
19 ... Catalytic converter
50 ... ECU
75 ... NOx occlusion amount estimation unit
76 ... NOx storage capacity saturation determination unit
77 ... Cylinder rich operation execution part

Claims (4)

In an engine exhaust purification system in which NOx is occluded during the lean operation of the engine and a catalyst for reducing and purifying the stored NOx when temporarily switching from lean operation to rich operation is interposed in the exhaust system,
Means for switching at predetermined intervals while shifting the timing for each cylinder when temporarily switching the engine from lean operation to rich operation ;
Means for integrating the intake air amount after the start of the rich operation, and determining that the NOx purification of the catalyst is completed when the integrated value of the intake air amount exceeds a set value;
An engine exhaust gas purification apparatus comprising: means for returning from a rich operation to a lean operation at predetermined intervals while shifting the timing for each cylinder when it is determined that the NOx purification of the catalyst has been completed . In an engine exhaust purification system in which NOx is occluded during the lean operation of the engine and a catalyst for reducing and purifying the stored NOx when temporarily switching from lean operation to rich operation is interposed in the exhaust system,
Means for switching at predetermined intervals while shifting the timing for each cylinder when temporarily switching the engine from lean operation to rich operation;
Means for returning at a predetermined interval while shifting the timing for each cylinder when returning from rich operation to lean operation;
When switching from lean operation to rich operation and returning from rich operation to lean operation while shifting the timing for each cylinder, the switching interval and the return interval are set as the operating range of lean operation is on the high load / high rotation side. An exhaust emission control device for an engine characterized by being lengthened . When switching from lean operation to rich operation and returning from rich operation to lean operation while shifting the timing for each cylinder, the switching interval and the return interval are set as the operating range of lean operation is on the high load / high rotation side. 2. The engine exhaust gas purification device according to claim 1, wherein the exhaust gas purification device is long. 3. The switching interval and the return interval are set to every two engine revolutions according to the ignition order, every one engine revolution according to a cylinder order different from the ignition order, or the ignition order. 3. An exhaust emission control device for an engine according to 3 .

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