JP2004239088A - Air fuel ratio control device for engine - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To promote activity of the catalytic purifying action by adsorption-desorption of oxygen of a three way catalyst. <P>SOLUTION: The lean target equivalent ratio ΦL is set on the basis of an engine speed; lean atmosphere control is started for controlling the equivalent ratio Φ detected by a wide area air-fuel ratio sensor in the lean target equivalent ratio ΦL (t1); afterwards, an oxygen adsorption estimate SL adsorbed to the three way catalyst is integrated; and when the oxygen adsorption estimate SL reaches a preset upper limit value SLMAX, the lean atmosphere control is finished (t2); and is transferred to rich atmosphere control. In the rich atmosphere control, the rich target equivalent ratio ΦR is set on the basis of the engine speed; the rich atmosphere control is started; and in that case, a determination estimate SL1 is set by a value of reducing the oxygen adsorption estimate SL by 10 %. An oxygen desorption estimate SR desorbed from the three way catalyst is integrated, and when the oxygen desorption estimate SR reaches the determination estimate SL1, the rich atmosphere control is finished (t3). Since lean/rich of the target equivalent ratio is switched on the basis of a value by estimating an oxygen adsorption-desorption quantity of the three way catalyst, exhaust emission can be reduced by maximally exhibiting purifying capacity of the three way catalyst. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an air-fuel ratio control device for an engine that appropriately controls the amount of oxygen adsorbed and the amount of desorbed oxygen of a three-way catalyst to promote activation of a catalyst purifying action.
[0002]
[Prior art]
As is well known, the three-way catalyst has a function of oxidizing HC (hydrocarbon) and CO (carbon monoxide) and reducing NOx (nitrogen oxide) when the air-fuel ratio is approximately the stoichiometric air-fuel ratio. Therefore, if the air-fuel ratio can be maintained at substantially the stoichiometric air-fuel ratio, HC, CO, and NOx can be simultaneously purified using the three-way catalyst.
[0003]
However, it is difficult to maintain the air-fuel ratio substantially at the stoichiometric air-fuel ratio, and the air-fuel ratio may actually deviate from the stoichiometric air-fuel ratio. However, even if the air-fuel ratio deviates from the stoichiometric air-fuel ratio, HC, CO, and NOx can be purified by the oxygen storage function of the three-way catalyst.
[0004]
That is, when the air-fuel ratio is lean, the three-way catalyst reduces NOx by the excess oxygen in the exhaust gas stored at that time when the air-fuel ratio is lean. On the other hand, when the air-fuel ratio becomes rich, the unburned HC in the exhaust gas is reduced. , CO deprives the three-way catalyst of oxygen, thereby oxidizing unburned HC and CO. Therefore, the purification performance of the catalyst can be improved by maximizing the effect of absorbing and desorbing oxygen in the three-way catalyst.
[0005]
For example, in Japanese Patent Application Laid-Open No. H6-74072, a wide-range air-fuel ratio sensor that linearly detects an air-fuel ratio in exhaust gas is disposed upstream of a three-way catalyst. Calculates the amount of oxygen adsorbed by the three-way catalyst based on the air amount and controls the air-fuel ratio so that the amount of oxygen adsorbed is always zero. A technique has been disclosed in which the ability to adsorb harmful components of the catalyst is ensured and the total amount of exhaust emissions is reduced.
[0006]
[Patent Document 1]
JP-A-6-74072
[0007]
[Patent Document 2]
JP 10-246139 A
[0008]
[Problems to be solved by the invention]
However, in the technology disclosed in the above-mentioned publication, since the amount of change in the amount of oxygen adsorbed on the three-way catalyst is small, activation of the catalyst purification action by oxygen desorption is not promoted, and the purification capacity of the three-way catalyst is not improved. There is an inconvenience that it is not possible to maximize the performance.
[0009]
On the other hand, for example, Japanese Patent Application Laid-Open No. 10-246139 discloses that when performing air-fuel ratio feedback control based on an air-fuel ratio detected by a wide-range air-fuel ratio sensor disposed upstream of a three-way catalyst, the exhaust air-fuel ratio is reduced. There is disclosed a technique for improving the purification performance of a three-way catalyst by effectively performing oxygen absorption / desorption with respect to the three-way catalyst by so-called perturbation control in which the three-way catalyst is inverted in a predetermined cycle.
[0010]
However, according to the technology disclosed in this publication, the lean / rich inversion is controlled only at every predetermined time (SINTIM #) set based on the engine speed and the engine load. The actual amount of oxygen absorbed and desorbed from the catalyst is not known, and therefore, the technique disclosed in the publication can sufficiently promote activation of the catalyst purifying action by oxygen absorption and desorption. Therefore, there is a disadvantage that the purifying ability of the three-way catalyst cannot be maximized.
[0011]
In view of the above circumstances, the present invention promotes the activity of the catalyst purifying action by oxygen desorption of the three-way catalyst, maximizes the purifying ability of the three-way catalyst, and reduces the exhaust air emission of the engine. It is an object to provide a fuel ratio control device.
[0012]
[Means for Solving the Problems]
In order to achieve the above object, an air-fuel ratio control apparatus according to the present invention provides a three-way catalyst and an air-fuel ratio detecting means for linearly detecting an exhaust air-fuel ratio in an exhaust system. Target air-fuel ratio setting means for setting a target air-fuel ratio, subsequently setting a rich target air-fuel ratio, and oscillating the target air-fuel ratio with the lean target air-fuel ratio and the rich target air-fuel ratio as one cycle; Lean atmosphere control means for controlling to a target air-fuel ratio, and rich atmosphere control means for controlling the exhaust air-fuel ratio to a rich target air-fuel ratio after it has been determined that the lean atmosphere control has been completed, wherein the lean atmosphere control means comprises: An oxygen adsorption estimated amount integrating means for integrating an oxygen adsorption estimated amount adsorbed on the three-way catalyst based on the exhaust air-fuel ratio, the lean target air-fuel ratio, and the intake air amount; Lean atmosphere control end timing determining means for determining the end of lean atmosphere control by comparing the estimated arrival amount with a preset set value, wherein the rich atmosphere control means includes the exhaust air-fuel ratio and the rich target air-fuel ratio. Oxygen desorption estimated amount integrating means for integrating the estimated amount of oxygen desorbed from the three-way catalyst based on the intake air amount, and determination estimated amount setting means for setting a determined estimation amount based on the estimated oxygen desorption amount And a rich atmosphere control end timing determining means for comparing the estimated amount of oxygen adsorption with the estimated amount of determination and ending the rich atmosphere control when the estimated amount of oxygen adsorption reaches the estimated amount of determination. Features.
[0013]
In such a configuration, first, a lean target air-fuel ratio is set based on the engine operating state, and lean atmosphere control for controlling the exhaust air-fuel ratio detected by the air-fuel ratio detecting means to the lean target air-fuel ratio is performed. At this time, based on the exhaust air-fuel ratio, the lean target air-fuel ratio, and the intake air amount, the estimated amount of oxygen adsorption adsorbed by the three-way catalyst provided in the exhaust system is integrated, and the estimated oxygen adsorption amount is calculated in advance. When the estimated amount of oxygen adsorption reaches the set value by comparing the set value with the set value, the lean atmosphere control ends, and the process shifts to rich atmosphere control for controlling the exhaust air-fuel ratio to the rich target air-fuel ratio. In the rich atmosphere control, a rich target air-fuel ratio is set based on the engine operating state, and rich atmosphere control for controlling the exhaust air-fuel ratio to the rich target air-fuel ratio is performed. At that time, an estimated amount of oxygen desorbed from the three-way catalyst is integrated based on the exhaust air-fuel ratio, the rich target air-fuel ratio, and the intake air amount, and a determination estimated amount is set based on the integrated estimated oxygen desorbed amount. When the estimated suction amount has reached the estimated determination amount, the rich atmosphere control ends.
[0014]
In this case, preferably, 1) the determination estimation amount setting means sets the determination estimation amount with a value obtained by reducing the oxygen desorption estimation amount by a fixed ratio.
[0015]
2) The lean atmosphere control end timing determination means sets a lean atmosphere control time based on the instantaneous oxygen adsorption amount calculated based on the intake air amount, the stoichiometric air-fuel ratio, and the lean target air-fuel ratio, and controls the lean atmosphere control. When the elapsed time exceeds the lean atmosphere control time, the lean atmosphere control is terminated.
[0016]
3) The rich atmosphere control end timing determining means sets a rich atmosphere control time based on the instantaneous oxygen desorption amount calculated based on the intake air amount, the stoichiometric air-fuel ratio, and the rich target air-fuel ratio, and performs rich atmosphere control. The rich atmosphere control is terminated when the elapsed time exceeds the rich atmosphere control time.
[0017]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, an embodiment of the present invention will be described with reference to the drawings. FIG. 1 shows a schematic configuration diagram of the engine control system. Reference numeral 1 in the figure denotes an engine, which in this embodiment is a horizontally opposed engine. An intake manifold 2 and an exhaust manifold 3 are connected to the engine 1, respectively.
[0018]
The upstream side of the intake manifold 2 is gathered in an air chamber 4, and an intake pipe 5 is communicated upstream of the air chamber 4, and the intake pipe 5 is communicated with an air cleaner (not shown). Further, a throttle valve 6 is interposed immediately upstream of the air chamber 4 of the intake pipe 5. Although not shown, an intake air amount sensor for detecting an intake air amount is provided immediately downstream of the air cleaner in the intake pipe 5. Further, an injector 7 is disposed immediately upstream of the intake port of each cylinder of the intake manifold 2.
[0019]
Further, an exhaust pipe 8 is communicated with the gathering portion of the exhaust manifold 3, and a muffler 9 is communicated downstream of the exhaust pipe 8. A three-way catalyst 10 is disposed in the exhaust pipe 8. At the inlet of the three-way catalyst 10, an air-fuel ratio (hereinafter referred to as "exhaust air-fuel ratio") from rich to lean including the stoichiometric air-fuel ratio based on the oxygen concentration in the exhaust gas. ) Is provided with a wide-range air-fuel ratio sensor 12 as an air-fuel ratio detecting means for linearly detecting the air-fuel ratio.
[0020]
A crank angle sensor 14 for detecting an engine speed from the rotation of the crank rotor is provided on the outer periphery of a crank rotor (not shown) which is mounted on the crank shaft 1a of the engine 1.
[0021]
On the other hand, reference numeral 21 denotes an electronic control unit (ECU), which includes a CPU 22, a ROM (not shown), a RAM (not shown), and a rewritable nonvolatile memory (for example, an EEP (Electrically Erasable Programmable) ROM, It mainly includes a microcomputer having a flash memory) and a backup RAM. The CPU 22 processes detection signals and the like from each sensor according to a control program stored in the ROM, and stores various data stored in the RAM and various learning value data stored in the nonvolatile memory and the backup RAM. The control of the entire engine, such as air-fuel ratio control for calculating the fuel injection amount corresponding to the target air-fuel ratio, ignition timing control, and the like, is performed.
[0022]
In such an engine control system, in the air-fuel ratio control in the ECU 21, the excess air ratio λ in the exhaust gas at the inlet of the three-way catalyst 10 is detected based on the output value VO [V] of the wide-range air-fuel ratio sensor 12, and this air The estimated amount of oxygen adsorption SL and the estimated amount of oxygen desorption SR in the three-way catalyst 10 are calculated based on the excess ratio λ and the intake air flow rate Qa, and are calculated according to the estimated amount of oxygen adsorption SL and the estimated amount of oxygen desorption SR. The so-called perturbation control, in which the excess air ratio λ in the exhaust gas upstream of the three-way catalyst 10 is slightly vibrated to reverse the exhaust air-fuel ratio between rich and lean, is performed.
[0023]
In the present embodiment, since the air-fuel ratio is expressed on the basis of the stoichiometric air-fuel ratio (λ = 1), in the following description, the air-fuel ratio (exhaust air-fuel ratio) is equivalent to the equivalent ratio (exhaust equivalent ratio) Φ (Φ = 1 / λ), the same result can be obtained by using any of the air-fuel ratio, fuel-air ratio, and excess air ratio.
[0024]
Hereinafter, the routine for setting the air-fuel ratio during the perturbation control executed by the ECU 21 will be described with reference to the flowcharts of FIGS.
[0025]
When the ignition switch is turned on and the power is turned on to the ECU 21, the system is initialized and the program is executed. First, in steps S1 and S2, a target equivalence ratio setting condition is determined.
[0026]
In step S1, it is determined whether the three-way catalyst 10 is active based on whether the air-fuel ratio feedback control is being performed. In step S2, the fuel cut condition or the fuel increase condition is checked to determine whether the fuel cut control is being performed or the fuel increase correction is being performed.
[0027]
If it is determined in step S1 that the air-fuel ratio feedback control is not being executed or that the fuel cut control or the fuel increase correction is being executed in step S2, the process jumps to step S21 to initialize each parameter. Then, exit the routine. That is, when the fuel cut condition such as during deceleration is satisfied or the fuel increase condition such as the acceleration increase is satisfied, even if the target equivalence ratio is variably set, the feedforward control is eventually performed. In this case, there is no point in setting the target equivalence ratio. In such a case, the perturbation control is stopped. The details of the initialization process will be described later.
[0028]
On the other hand, if it is determined in step S1 that the air-fuel ratio feedback control is being performed, and if it is determined in step S2 that the fuel cut condition or the fuel increase condition is not satisfied, the process proceeds to step S3. The estimation of the oxygen absorption / desorption state is started.
[0029]
First, in step S3, by referring to the value of the lean / rich atmosphere determination flag Fs, it is checked whether the current control state is lean atmosphere control (Fs = 0) or rich atmosphere control (Fs = 1). If the lean atmosphere control is Fs = 0, the process proceeds to step S4. If the rich atmosphere control is Fs = 1, the process jumps to step S12. Note that the initial value of the lean / rich atmosphere determination flag Fs is 0, and is initially set when the ignition switch is ON and in step S21, and is set in step S13 to be described later.
[0030]
When the estimation of the oxygen desorbing state in the three-way catalyst 10 is started after the ignition switch is turned on and in step S3 and thereafter, first, the lean atmosphere control is executed, then the rich atmosphere control is executed, and this cycle is performed for one cycle. The target equivalent ratio is amplitude. Therefore, in the following description, the lean atmosphere control will be described first, and then the rich atmosphere control will be described.
[0031]
In step S3, it is determined that the current control of Fs = 0 is the lean atmosphere control, and when the process proceeds to step S4, the lean target equivalent ratio table is interpolated and calculated based on the engine speed Ne detected by the crank angle sensor 14. With reference to FIG. 7, a lean target equivalent ratio ΦL of the intake air-fuel mixture supplied from the intake system to the cylinder is set. In each area of the lean target equivalent ratio table, a value for gradually setting the lean target equivalent ratio ΦL in the lean direction as the engine speed Ne shifts from the low rotation to the high rotation direction is stored. The lean target equivalence ratio ΦL stored in the area is set to a value such that a change in the engine speed Ne due to a change in the equivalence ratio falls within an allowable range. The symbol ΦS is a theoretical equivalent ratio (ΦS = 1).
[0032]
That is, if the lean target equivalent ratio ΦL is largely shifted to the lean side on the low rotation side, the engine speed Ne greatly fluctuates under the influence of a slight load change or the like. Therefore, at low rotation speed (500 rpm), the lean target equivalent ratio ΦL is set to a value close to the theoretical equivalent ratio ΦS (ΦL = ΦS−0.005 = 0.959), while the high rotation side ( At 2000 rpm), since the engine output is large, even if the lean target equivalent ratio ΦL is relatively largely shifted to the lean side, there is no great effect on the engine rotation fluctuation. Therefore, the lean target equivalent ratio ΦL is changed from the theoretical equivalent ratio ΦS by The value is set by subtracting 0.05 (ΦL = ΦS−0.05 = 0.95).
[0033]
Thereafter, when the process proceeds to step S5, it is determined whether or not the estimated oxygen adsorption amount is initially set to SL = 0 with reference to the value of the estimated oxygen adsorption amount SL. This estimated oxygen adsorption amount SL is initially set when the ignition switch is turned on and in step S21 described later. When SL = 0, this is the first routine after shifting to the lean atmosphere control. Proceeding to S6, the lean atmosphere control time TL [sec] is set. When SL ≠ 0, the lean atmosphere control time TL is continuing, and the process goes to step S7.
[0034]
In step S6, the instantaneous oxygen adsorption amount Qa · (ΦS−ΦL) is calculated by multiplying the intake air flow rate Qa [g / sec] by the difference between the stoichiometric equivalent ratio ΦS and the lean target equivalent ratio ΦL, Using the instantaneous oxygen adsorption amount Qa · (ΦS−ΦL) as a parameter, the lean atmosphere control time TL [sec] is set by referring to the lean atmosphere control time setting table with interpolation calculation.
[0035]
In this lean atmosphere control time setting table, based on the instantaneous oxygen adsorption amount Qa · (ΦS−ΦL), the equivalence ratio is forcibly set on the lean side from experiments based on the capacity of the three-way catalyst 10 without impairing the operation performance. The lean atmosphere control time that can be controlled is obtained, and the time is stored for each region. The lean atmosphere control time TL [sec] is the amount of oxygen supplied to the three-way catalyst 10 per unit time. As the number increases, the time is gradually set shorter.
[0036]
Thereafter, when the process proceeds from step S5 or step S6 to step S7, an exhaust equivalence ratio Φ (Φ = 1 / λ) is calculated based on the output value VO [V] of the wide area air-fuel ratio sensor 12, and this exhaust equivalence ratio Φ is calculated. The theoretical equivalent ratio ΦS is compared, and when the equivalent ratio of Φ <ΦS is lean, the estimated oxygen adsorption amount SL is integrated from the following equation.
SL = SL + Qa · (ΦS−Φ)
[0037]
When the equivalent ratio of Φ ≧ ΦS is rich, the estimated oxygen desorption amount SR is integrated by the following equation.
SR = SR + Qa · (Φ-ΦS)
[0038]
That is, even during the lean atmosphere control, the equivalence ratio Φ may be instantaneously rich. In such a case, the integration of the estimated oxygen adsorption SL is interrupted, and the estimated oxygen desorption SR is integrated. I do.
[0039]
Next, when the process proceeds to step S8, the estimated oxygen adsorption amount SL and the set upper limit SLMAX are compared to determine the end time of the lean atmosphere. The set upper limit value SLMAX is a fixed value set based on the saturation capacity of the oxygen adsorption amount of the three-way catalyst 10.
[0040]
When SL <SLMAX, the process proceeds to step S9 because there is a margin in the oxygen adsorption amount of the three-way catalyst 10. On the other hand, when SL ≧ SLMAX, since the oxygen adsorption amount of the three-way catalyst 10 is in a saturated state, the process jumps to step S12, ends the lean atmosphere control, and shifts to the rich atmosphere control.
[0041]
In step S9, the elapsed time Tim from the shift to the lean atmosphere control is compared with the lean atmosphere control time TL. If Tim <TL, the routine exits from the routine and the lean atmosphere control is continued. On the other hand, when Tim ≧ TL, since the lean atmosphere control time has reached the allowable time, the flow branches to step S10 to check whether or not the estimated oxygen adsorption amount SL has been integrated. = 0), the process proceeds to step S11, in which the estimated oxygen adsorption amount SL is set to a preset set value SLFIX (SL ← SLFIX).
[0042]
Even during the lean atmosphere control, the equivalent ratio Φ may be in the rich direction and the estimated oxygen adsorption amount SL may not be integrated. In such a case, when the process shifts to the next rich atmosphere control in the state of SL = 0, the accumulated time of the estimated oxygen desorption amount SR corresponding to the estimated oxygen adsorption amount SL integrated in the lean atmosphere control in the rich atmosphere control. Only the rich atmosphere control is executed, so that when SL = 0, it is not possible to substantially shift to the rich atmosphere control.
[0043]
Therefore, the estimated amount of oxygen adsorption SL is set at a preset set value SLFIX, and SL ≠ 0, thereby enabling a transition to rich atmosphere control. Therefore, if SL ≠ 0 in step S10, the process jumps to step S12 and shifts to rich atmosphere control.
[0044]
Then, when the process proceeds from step S3, step S8, step S10 or step S11 to step S12, the estimated amount of oxygen adsorption SL integrated in step S7 is compared with the estimated amount of oxygen desorption SR. Rather than shifting to atmosphere control and accumulating the estimated oxygen desorption amount SR, by accumulating the estimated oxygen adsorption amount SL by executing the lean atmosphere control, it is expected that the catalyst purification action by oxygen desorption is promoted. Therefore, after jumping to step S21 to perform initialization processing, the routine exits.
[0045]
If SR <SL, the process proceeds to step S13, where a lean / rich atmosphere determination flag Fs is set (Fs ← 1), and the process proceeds to step S14, where the rich / rich atmosphere is determined based on the engine speed Ne detected by the crank angle sensor 14. With reference to the target equivalent ratio table with interpolation calculation, a rich target equivalent ratio ΦR of the intake air-fuel mixture supplied from the intake system to the cylinder is set.
[0046]
In each area of the rich target equivalent ratio table, a value for gradually setting the target equivalent ratio ΦR in the rich direction as the engine speed Ne shifts from the low rotation to the high rotation direction is stored. Is set to a value such that the fluctuation of the engine speed Ne due to the change in the equivalence ratio falls within an allowable range.
[0047]
That is, if the target equivalence ratio ΦR is largely shifted to the rich side on the low rotation side, the engine speed Ne fluctuates greatly due to a change in the engine output, so that the target equivalence ratio ΦR cannot be largely shifted to the rich side. Therefore, at low rotation speed (500 rpm), the target equivalence ratio ΦR is set to a value close to the theoretical equivalence ratio ΦS (ΦL = ΦS + 0.005 = 1.005). On the other hand, at the high rotation speed (2000 rpm), the engine output is large. Even if the target equivalence ratio ΦR is shifted to a rich side relatively large, there is no great effect on the engine rotation fluctuation. Therefore, 0.05 is added to the theoretical equivalence ratio ΦS by adding the target equivalence ratio ΦR (ΦR = ΦS + 0.05 = 1.05).
[0048]
Then, when the process proceeds to step S15, it is determined whether or not the estimated oxygen desorption amount is initially set to SR = 0 by referring to the value of the estimated oxygen desorption amount SR. This estimated oxygen desorption amount SR is initially set when the ignition switch is turned on and in step S21 described later. When SR = 0, this is the first routine after the transition to the rich atmosphere control, and therefore, step S16 Then, the rich atmosphere control time TR [sec] is set. When SR ≠ 0, since the rich atmosphere control time TR is continuing, the process jumps to step S17.
[0049]
In step S16, the instantaneous oxygen desorption amount Qa · (ΦR−ΦS) is calculated by multiplying the intake air flow rate Qa [g / sec] by the difference between the rich target equivalent ratio ΦR and the theoretical equivalent ratio ΦS. Using the instantaneous oxygen desorption amount Qa · (ΦR−ΦS) as a parameter, the rich atmosphere control time TR [sec] is set by referring to the rich atmosphere control time setting table with interpolation calculation.
[0050]
In this rich atmosphere control time setting table, based on the instantaneous oxygen desorption amount Qa · (ΦR−ΦS), the equivalence ratio is forcibly set on the rich side without impairing the operation performance from experiments or the like based on the capacity of the three-way catalyst 10. A rich atmosphere control time that can be controlled is obtained, and the time is stored for each region. This rich atmosphere control time TR [sec] is defined as the oxygen per unit time released from the three-way catalyst 10. As the volume increases, the time is gradually set shorter.
[0051]
Thereafter, when the process proceeds from step S15 or step S16 to step S17, an exhaust equivalence ratio Φ (Φ = 1 / λ) in the exhaust gas is calculated based on the output value VO [V] of the wide area air-fuel ratio sensor 12, and this exhaust is calculated. The equivalent ratio Φ is compared with the theoretical equivalent ratio ΦS. When the equivalent ratio of Φ <ΦS is lean, the estimated oxygen adsorption amount SL is integrated from the following equation.
SL = SL + Qa · (ΦS−Φ)
[0052]
When the equivalent ratio of Φ ≧ ΦS is rich, the estimated oxygen desorption amount SR is integrated by the following equation.
SR = SR + Qa · (Φ-ΦS)
[0053]
That is, even during the rich atmosphere control, the equivalent ratio Φ may instantaneously become lean. In such a case, the estimated oxygen adsorption amount SL is integrated.
[0054]
Next, when the process proceeds to step S18, a value obtained by reducing the estimated oxygen adsorption amount SL by several tens of percent (10% in the present embodiment) is set as the estimated estimation amount SL1 (SL1 = 0.9 · SL), and is continued. In step S19, the estimated amount of oxygen desorption SR is compared with the estimated amount of determination SL1. If SR ≧ SL1, the process branches to step S21, performs initialization, ends rich atmosphere control, and exits the routine. . In step S19, the estimated amount of oxygen desorption SR and the estimated amount of oxygen adsorption SL are not compared with the estimated amount of determination SL1 in which the estimated amount of oxygen adsorption SL is reduced by several tens% (10% in the present embodiment). Thus, the integrated amount of the estimated oxygen desorption amount SR can always be set to a value slightly smaller than the estimated oxygen adsorption amount SL.
[0055]
As a result, oxygen always remains in the three-way catalyst 10, and the amount of CO emission can be suppressed accordingly. Further, since the estimated amount of oxygen desorption SR and the estimated amount of oxygen adsorption SL are not actually measured values of the amount of oxygen desorbed in the three-way catalyst 10, the estimated amount of oxygen desorption SR is set to be slightly smaller. In addition, it is possible to prevent untreated gas from being discharged from the three-way catalyst 10 beyond the catalyst purification ability. Note that the estimated estimation amount SL1 does not need to be 90% of the oxygen adsorption estimated amount SL, and an optimal value is set by experiments or the like.
[0056]
When SR <SL1, since the amount of oxygen released from the three-way catalyst 10 has a margin, the process proceeds to step S20, and the elapsed time Tim from the transition to the rich atmosphere control is compared with the rich atmosphere control time TR. , Tim <TR, the routine exits the routine and the rich atmosphere control is continued. On the other hand, when Tim ≧ TR, since the rich atmosphere control time has reached the allowable time, the process branches to step S21, executes initialization processing, and exits the routine.
[0057]
In step S21, an initialization process for clearing the lean / rich atmosphere determination flag Fs, the estimated oxygen adsorption amount SL, the estimated oxygen desorption amount SR, the estimated determination amount SL1, and the elapsed time Tim is performed (Fs ← 0, SL ← 0, SR ← 0, SL1 ← 0, Tim ← 0).
[0058]
As described above, in the present embodiment, when performing the perturbation control, the estimated amount SL of oxygen adsorbed on the three-way catalyst 10 during the lean atmosphere control and the estimated amount of oxygen desorption from the three-way catalyst 10 during the rich atmosphere control. Since the lean / rich of the exhaust air-fuel ratio is inverted in response to SR, the lean / rich of the exhaust air-fuel ratio can be inverted in a cycle close to the amount of oxygen that the three-way catalyst 10 can actually absorb and desorb. And the activity of the catalyst purifying action can be promoted. As a result, by maximizing the purifying ability of the three-way catalyst, it is possible to efficiently reduce the exhaust emission without increasing the capacity of the three-way catalyst 10.
[0059]
When the rich atmosphere control is ended, the estimated oxygen desorption amount SR and the estimated oxygen adsorption SL are not compared with the estimated estimation amount SL1 set to several tens% of the estimated oxygen adsorption SL. Therefore, even when the rich atmosphere control is finished, since oxygen always remains in the three-way catalyst 10, the amount of CO emission can be suppressed, and the estimated oxygen desorption amount SR and the estimated oxygen adsorption can be reduced. Even when the amount SL slightly deviates from the actual oxygen desorption amount and the oxygen adsorption amount, it is possible to prevent the untreated gas from being discharged from the three-way catalyst 10 beforehand.
[0060]
Next, an example of the air-fuel ratio control according to the present embodiment will be described with reference to a time chart shown in FIG.
[0061]
When the target equivalent ratio setting condition is satisfied, first, the target equivalent ratio is set to the lean target equivalent ratio ΦL (t1). As a result, in the fuel injection control, the fuel injection amount is set based on the lean target equivalent ratio ΦL, so that the exhaust equivalent ratio is made lean. As a result, the exhaust equivalence ratio Φ detected by the wide area air-fuel ratio sensor 12 satisfies Φ <ΦS, so the inside of the three-way catalyst 10 calculated based on the intake air flow rate Qa, the exhaust equivalence ratio Φ, and the stoichiometric equivalence ratio ΦS. Is estimated, and the estimated amount of oxygen adsorption SL is gradually increased.
[0062]
Then, when the estimated oxygen adsorption amount SL reaches the set upper limit SLMAX (t2), the target equivalent ratio is switched to the rich target equivalent ratio ΦR. At this time, since the integration of the estimated oxygen adsorption amount SL is stopped, the value at that time is maintained. On the other hand, in the fuel injection control, since the fuel injection amount is set based on the rich target equivalent ratio ΦR, the exhaust equivalent ratio is made rich. As a result, the exhaust equivalence ratio Φ detected by the wide area air-fuel ratio sensor 12 satisfies Φ ≧ ΦS, so the inside of the three-way catalyst 10 calculated based on the intake air flow rate Qa, the exhaust equivalence ratio Φ, and the stoichiometric equivalence ratio ΦS. Is estimated, and the estimated oxygen departure amount SR is gradually increased.
[0063]
Then, when the estimated oxygen desorption amount SR reaches the estimated estimation amount SL1 obtained by reducing the estimated oxygen adsorption amount SL by several tens of percent (10% in the present embodiment) (t3), the estimated oxygen adsorption amount SL and the oxygen The departure estimation amount SR is cleared together (SL ← 0, SR ← 0), and one cycle of the perturbation control ends. Then, when the target equivalence ratio setting condition is satisfied again, the next cycle of the perturbation control is started.
[0064]
【The invention's effect】
As described above, according to the present invention, the activity of the catalyst purifying action by the oxygen desorption of the three-way catalyst is promoted, the purifying ability of the three-way catalyst is maximized, and the exhaust emission is efficiently reduced. And other effects can be obtained.
[Brief description of the drawings]
FIG. 1 is a schematic configuration diagram of an engine control system.
FIG. 2 is a flowchart showing an air-fuel ratio setting routine during the perturbation control (part 1);
FIG. 3 is a flowchart illustrating an air-fuel ratio setting routine during the perturbation control (part 2);
FIG. 4 is a time chart showing a setting of a target equivalence ratio and a change in an estimated oxygen adsorption amount and an estimated oxygen desorption amount during the perturbation control.
[Explanation of symbols]
1 engine
10 Three-way catalyst
12 Wide area air-fuel ratio sensor (air-fuel ratio detecting means)
Qa Intake air flow rate
Qa ・ (ΦS-ΦL) Instantaneous oxygen adsorption
Qa ・ (ΦR-ΦS) Instantaneous oxygen desorption
SL1 Judgment amount
SR oxygen desorption estimated amount
TL lean atmosphere control time
TR Rich atmosphere control time
Φ Exhaust equivalent ratio
ΦL Lean target equivalent ratio
ΦR Rich target equivalent ratio

Claims (4)

In an engine in which an exhaust system is provided with a three-way catalyst and air-fuel ratio detection means for linearly detecting the exhaust air-fuel ratio,
Target air-fuel ratio setting means for first setting a lean target air-fuel ratio based on the engine operating state, subsequently setting a rich target air-fuel ratio, and oscillating the target air-fuel ratio with the lean target air-fuel ratio and the rich target air-fuel ratio as one cycle When,
Lean atmosphere control means for controlling the exhaust air-fuel ratio to a lean target air-fuel ratio,
Rich atmosphere control means for controlling the exhaust air-fuel ratio to a rich target air-fuel ratio after it is determined that the lean atmosphere control has been completed,
With
The lean atmosphere control means includes:
Oxygen adsorption estimated amount integrating means for integrating the oxygen adsorption estimated amount adsorbed on the three-way catalyst based on the exhaust air-fuel ratio, the lean target air-fuel ratio, and the intake air amount;
Lean atmosphere control end timing determining means for comparing the estimated amount of oxygen adsorption with a preset set value to determine the end of lean atmosphere control,
With
The rich atmosphere control means includes:
Oxygen desorption estimated amount integrating means for integrating an estimated amount of oxygen desorbed from the three-way catalyst based on the exhaust air-fuel ratio, the rich target air-fuel ratio, and the intake air amount,
Determination estimation amount setting means for setting a determination estimation amount based on the oxygen desorption estimation amount,
Comparing the oxygen adsorption estimation amount and the determination estimation amount, and completing the rich atmosphere control when the oxygen adsorption estimation amount reaches the determination estimation amount, rich atmosphere control end timing determination means,
An air-fuel ratio control device for an engine, comprising: 2. The air-fuel ratio control device for an engine according to claim 1, wherein the determination estimation amount setting means sets the determination estimation amount at a value obtained by reducing the oxygen desorption estimation amount by a fixed ratio. The lean atmosphere control end timing determination means sets a lean atmosphere control time based on the instantaneous oxygen adsorption amount calculated based on the intake air amount, the stoichiometric air-fuel ratio, and the lean target air-fuel ratio, and sets a lean atmosphere control time. 3. The air-fuel ratio control device for an engine according to claim 1, wherein the lean atmosphere control is terminated when the elapsed time exceeds the lean atmosphere control time. The rich atmosphere control end timing determining means sets a rich atmosphere control time based on the instantaneous oxygen desorption amount calculated based on the intake air amount, the stoichiometric air-fuel ratio, and the rich target air-fuel ratio, and performs rich atmosphere control during rich atmosphere control. The air-fuel ratio control device for an engine according to any one of claims 1 to 3, wherein the rich atmosphere control is terminated when the elapsed time exceeds the rich atmosphere control time.

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