JP2841806B2 - Air-fuel ratio control device for engine - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an air-fuel ratio control device for an engine that controls a fuel injection amount such that an air-fuel ratio of a mixture supplied to an engine becomes a stoichiometric air-fuel ratio. It is.

[Conventional technology]

2. Description of the Related Art Conventionally, there has been disclosed a so-called dither control in which an air-fuel ratio of an air-fuel mixture supplied to an engine is increased toward a rich side and a lean side with a predetermined air-fuel ratio as a median value in order to improve a purification effect of a catalyst (for example, JP-A-62-56335).

[Problems to be solved by the invention]

However, if the dither control for changing the air-fuel ratio between the rich side and the lean side is performed also during acceleration / deceleration, there is a problem that responsiveness and fuel efficiency are deteriorated.

This is because the transient response deteriorates when the air-fuel ratio swings to the lean side by the above-mentioned dither control during acceleration when a large amount of fuel is required. If the air-fuel ratio fluctuates to the rich side by the above dither control during deceleration when a large amount of fuel is not required for the reaction, the fuel is consumed excessively and the fuel efficiency is deteriorated.

The present invention has been made in view of the above circumstances, and has as its object to provide an air-fuel ratio control device for an engine having excellent responsiveness and fuel efficiency.

[Means for solving the problem]

As a means for solving the above-mentioned problems, the present invention provides a catalyst disposed in an exhaust pipe of an engine for purifying exhaust gas, and an air-fuel ratio of an air-fuel mixture disposed upstream of the catalyst and supplied to the engine. An oxygen concentration sensor that outputs a detection signal that is linear with respect to, a dither control unit that dither-controls a target air-fuel ratio of the air-fuel mixture at a predetermined cycle, and the engine according to the detection signal and the target air-fuel ratio. An acceleration / deceleration detecting means for detecting acceleration / deceleration of the engine; a prohibiting means for prohibiting the dither control during acceleration / deceleration. And a target air-fuel ratio setting means for setting the target air-fuel ratio to a predetermined value at the time of deceleration.

[Action]

Thus, when the acceleration / deceleration detecting means detects the acceleration / deceleration, the dither control is prohibited. Then, the target air-fuel ratio is set to a predetermined value, and the fuel injection amount supplied to the engine is set according to the target air-fuel ratio and the detection signal of the oxygen concentration sensor.

〔The invention's effect〕

According to the present invention, by prohibiting dither control during acceleration / deceleration, there is an excellent effect that responsiveness is improved during acceleration and fuel consumption is improved during deceleration.

〔Example〕

In order to further clarify the configuration of the present invention described above,
Hereinafter, an air-fuel ratio control device for an engine as a preferred embodiment of the present invention will be described. FIG. 2 is a schematic configuration diagram showing the engine 10 for which air-fuel ratio control is performed and its peripheral devices. As shown, in the present embodiment, each control of the ignition timing Ig of the engine 10 and the fuel injection amount TAU is performed by an electronic control unit (E
CU) 20.

As shown in FIG. 2, the engine 10 is of a four-cylinder, four-cycle spark ignition type, and its intake air is supplied from an air cleaner 11, an intake pipe 12, a throttle valve 13,
It is sucked into each cylinder via the surge tank 14 and the intake branch pipe 15. On the other hand, fuel is pressure-fed from a fuel tank (not shown), and fuel injection valves 16a, 16b, 16c, 1
It is configured to be injected and supplied from 6d. The engine 10 further includes a distributor 19 that distributes a high-voltage electric signal supplied from an ignition circuit 17 to ignition plugs 18a, 18b, 18c, and 18d of the respective cylinders.
A throttle sensor 30 for detecting the rotation speed Ne of the engine 10, a throttle sensor 31 for detecting the opening TH of the throttle valve 13, an intake pressure sensor 32 for detecting the intake pressure PM downstream of the throttle valve 13, and the engine 10 A warm-up sensor 33 for detecting the cooling water temperature Thw and an intake air temperature sensor 34 for detecting the intake air temperature Tam are provided. The above-described rotation speed sensor 30 is provided for a ring gear that rotates in synchronization with the crankshaft of the engine 10, and is provided in proportion to the rotation speed Ne.
2 rotations, that is, 24 pulse signals are output at 720 ° C. The throttle sensor 31 outputs an on / off signal from an idle switch for detecting that the throttle valve 13 is almost fully closed together with an analog signal corresponding to the throttle opening TH.

Further, the exhaust pipe 35 of the engine 10 is provided with a three-way catalyst 38 for reducing harmful components (CO, HC, NOx, etc.) in the exhaust gas discharged from the engine 10. further,
An air-fuel ratio sensor 36, which is a first oxygen concentration sensor that outputs a linear detection signal according to the air-fuel ratio λ of the air-fuel mixture supplied to the engine 10, is provided upstream of the three-way catalyst 38. air-fuel ratio lambda Do rich relative to the stoichiometric air-fuel ratio lambda ₀ of the mixture supplied to the engine 10 on the downstream side of the original catalyst 38 is the second oxygen concentration sensor that outputs a detection signal according to whether lean O ₂ sensor 37 is provided.

The electronic control unit 20 is configured as an arithmetic and logic operation circuit centered on the well-known CPU 21, ROM 22, RAM 23, backup RAM 24, and the like, and an input port for performing input from each of the above-described sensors.
Output port that outputs control signals to 25 and each actuator
26 and the like are interconnected via a bus 27. The electronic control unit 20 controls the intake pressure PM and the intake temperature via the input port 25.
Tam, throttle opening TH, cooling water temperature Thw, air-fuel ratio λ, rotation speed Ne, etc. are input, and based on these, the fuel injection amount TA
U, calculates the ignition timing Ig, and outputs a control signal to each of the fuel injection valves 16a to 16d and the ignition circuit 17 via the output port 26. Among these controls, the air-fuel ratio control will be described below.

The electronic control unit 20 is designed in advance by using a setting method disclosed in Japanese Patent Application Laid-Open No. 64-110853 in order to perform air-fuel ratio control.

Modeling of Controlled Object In the present embodiment, an autoregressive moving average model of order 1 having a dead time P = 3 is used as a model of a system for controlling the air-fuel ratio λ of the engine 10 and further approximated by taking a disturbance d into consideration. I have.

First, a model of a system for controlling an air-fuel ratio λ using an autoregressive moving average model can be approximated by λ (k) = a · λ (k−1) + b · FAF (k−3) (1) Here, λ is an air-fuel ratio, FAF is an air-fuel ratio correction coefficient, a and b are constants, and k is a variable indicating the number of controls from the start of the first sampling. Further, considering the disturbance d, the model of the control system can be approximated as follows: λ (k) = a · λ (k−1) + b · FAF (k−3) + d (k−1) (2)

For the model approximated as described above, the constants a and b are determined by discretizing the rotation-synchronized (360 ° A) sampling using the step response, that is, the transfer function G of the system for controlling the air-fuel ratio λ is determined. It is easy to ask.

How to display the state variable quantity The above equation (2) is converted to the state variable quantity (k) = [X ₁ (k), X
_{2 (k), X 3 (} k), is rewritten with X ₄ (k)] ^T, Get.

Becomes

Designing the regulator When the regulator is designed for the equations (5) and (6), the optimal feedback gain Becomes Further, an integral term Z ₁ (k) for absorbing an error
Was _{added, FAF (k) = K 1} · λ (k) + K 2 · FAF (k-3) + K 3 · FAF (k-2) + K 4 · FAF (k-1) + Z 1 (k) ... (6 ), The air-fuel ratio λ and the correction coefficient FAF can be obtained.

The integral term Z _I (k) is a value determined from the deviation between the target air-fuel ratio λ _TG and the actual air-fuel ratio λ (k) and the integral constant Ka, and is obtained by the following equation.

Z ₁ (k) = Z ₁ (k−1) + Ka · (λ _TG −λ (k)) (7) Optimal feedback gain And the integral constant Ka Optimal feedback gain And the integration constant Ka are, for example, an evaluation function J expressed by the following equation:
Can be set by minimizing.

Here, the evaluation function J is intended to minimize the deviation between the air-fuel ratio λ (k) and the target air-fuel ratio λ _TG while restricting the movement of the air-fuel ratio correction coefficient FAF (k). The weighting of the constraint on the fuel ratio correction coefficient FAF (k) can be changed by the values of the weighting parameters Q and R. Therefore, the simulation is repeated until the optimum control characteristics are obtained by changing the values of the weight parameters Q and R variously, and the optimum feedback gain is obtained. And the integral constant Ka may be determined.

Furthermore, the optimal feedback gain And the integration constant Ka depend on the model constants a and b. Therefore, in order to guarantee the stability (robustness) of the system with respect to the variation (parameter variation) of the system that controls the actual air-fuel ratio λ, the optimal feedback gain is set in consideration of the variation of the model constants a and b. And the integral constant Ka must be designed. Therefore, the simulation is performed in consideration of the actual fluctuations of the model constants a and b, and the optimal feedback gain satisfying the stability is obtained. And the integration constant Ka are determined.

The modeling of the controlled object, the method of displaying the state variable amount, the design of the regulator, and the determination of the optimal feedback gain and the integration constant have been described above. These are set in advance. Control is performed using only equations (6) and (7).

Hereinafter, the air-fuel ratio control will be described based on the flowcharts shown in FIGS.

FIG. 3 shows a process for setting the fuel injection amount TAU, which is executed in synchronization with the rotation (every 360 ° A).

First, in step 101, the basic fuel injection amount Tp is calculated according to the intake pressure PM, the rotation speed Ne, and the like. In the following step 102, it is detected whether or not the feedback condition of the air-fuel ratio λ is satisfied. Here, as is well known, the feedback condition is that the cooling water temperature Thw is equal to or higher than a predetermined value, and is not a high load and a high rotation. If the feedback condition of the air-fuel ratio λ is not satisfied in step 102, the air-fuel ratio correction coefficient FAF is set to 1 in step 103, and the routine proceeds to step 106.

When the feedback condition for the air-fuel ratio λ is satisfied in step 102, the target air-fuel ratio λ _TG is set in step 104 (details will be described later). Then, in step 105, the air-fuel ratio correction coefficient FA is set so that the air-fuel ratio λ becomes the target air-fuel ratio λ _TG.
F is set. More specifically, the air-fuel ratio correction coefficient FAF is calculated according to the equations (6) and (7) according to the target air-fuel ratio λ _TG and the air-fuel ratio λ (k) detected by the air-fuel ratio sensor 36.

Then, in step 106, the basic fuel injection amount Tp is corrected by the following equation according to the air-fuel ratio correction coefficient FAF and another correction coefficient FALL, and the fuel injection amount TAU is set.

TAU = FAF × Tp × FALL An operation signal corresponding to the fuel injection amount TAU set as described above is output to the fuel injection valves 16a to 16d.

Next, the setting of the target air-fuel ratio λ _TG will be described with reference to the flowchart shown in FIG.

In step 301, it is determined whether or not the engine 10 is operating in a steady state, for example, from the intake pressure change rate ΔPM. Intake pressure change rate ΔPM
When the absolute value | ΔPM | is larger than the predetermined value P1, that is, when the vehicle is not in the steady state, the routine proceeds to step 302, where it is determined whether the intake pressure change is positive or negative by determining whether the vehicle is accelerating or decelerating. Depending on whether there is. When the intake pressure change ΔPM is positive, it is determined that the vehicle is accelerating, and when it is negative, it is determined that the vehicle is decelerating. If it is determined that the vehicle is accelerating, the routine proceeds to step 303, where the target air-fuel ratio λ _TG
Is set to the target air-fuel ratio during acceleration λ _AC . λ _AC is set to about 12.5-14.0 in consideration of output torque and emission. If it is determined in step 302 that the vehicle is decelerating, the routine proceeds to step 304, where the target air-fuel ratio λ _TG is reduced to the target air-fuel ratio λ _DC during deceleration.
Set to. λ _DC is set to about 16.0 in consideration of drivability during deceleration and the presence or absence of misfire.

As described above, the target air-fuel ratio λ _TG is set to the target air-fuel ratios λ _AC and λ _DC during acceleration and deceleration during acceleration and deceleration, and the dither control described later is not performed.

If it is determined in step 301 that the state is the steady state, step 305
To execute the setting process of the steady-state target air-fuel ratio λ _ST .

In the above process, first, the median value λ _TGC of the target air-fuel ratio is set so as to correct the difference between the actual air-fuel ratio and the detection signal of the air-fuel ratio sensor 36 based on the detection signal of the O ₂ sensor 37. More specifically, when the detection signal of the O ₂ sensor 37 is rich, the central value λ _TGC is set lean by a predetermined value λ _M.
Conversely, when the detection signal of the O ₂ sensor 37 is lean, the median λ
The _TGC by a predetermined value λ _M is set to the rich. Here, the characteristics of the purification rate η of the three-way catalyst 38 with respect to the air-fuel ratio λ are shown in FIG. As will be described later, the control is performed within a range of a catalyst window W (hatched portion in the figure) shown in FIG. Since the catalyst window W is approximately 1%, as the predetermined value lambda _M described above, is set to be smaller than this value.

In addition, the difference between the actual air-fuel ratio and the detection signal of the air-fuel ratio sensor differs depending on the rotation speed Ne and the intake pressure PM. That is, the air-fuel ratio at which the purification rate η becomes maximum differs depending on the rotation speed Ne and the intake pressure PM. Therefore, as an initial value of the median value λ _TGC, the air-fuel ratio at which the purification rate η becomes the maximum is obtained in advance from the rotation speed Ne and the intake pressure PM, and stored in the ROM 22. Then, at the start of feedback, the information may be read from the ROM 22. The initial value of this median λ _TGC is the rotation speed Ne, the intake pressure PM
Has a characteristic that becomes richer as the value becomes larger.

Next, the target air-fuel ratio λ is periodically (dither period T _DZA ) at a predetermined amplitude (dither amplitude) λ _DTZ in the catalyst window W range with respect to the median value λ _TGC set as described above.
Change _TG (dither control). Where the dither amplitude λ
_{Also for DTZ} and dither period T _DZA , the optimum value at which the purification rate η becomes maximum differs depending on the rotation speed Ne and the intake pressure PM. Therefore, the optimum values of the dither amplitude λ _DTZ and the dither period T _DZA are previously obtained from the rotation speed Ne and the intake pressure PM and stored in the ROM ₂₂ . Then, it may be read from the ROM 22 one after another.

The setting of the target air-fuel ratio lambda _ST at more constant will be described with reference to a flowchart shown in Figure 6.

Steps 201 to 203 are processing for setting the above-described median value λ _TGC of the target air-fuel ratio. First, in step 201
Detection signals from the O ₂ sensor 37 detects rich or lean. Here, when the detection signal from the O ₂ sensor 37 is rich, large median lambda _TGC predetermined value lambda _M in step 202, i.e., set to a lean _{(λ TGC ← λ TGC + λ} M). Further, in step 201, if the detection signal from the O ₂ sensor 37 is lean, it reduces the median lambda _TGC predetermined value lambda _M in step 203, i.e., set to a lean (λ _TGC ← λ _TGC -
λ _M ).

Steps 204 to 213 are the aforementioned dither control. In step 204, it is detected whether or not the counter CDZA is _{equal to} or _longer than the dither period _TDZA . Here, the counter CDZA counts the dither period _TDZA . Where the counter CD
If ZA is less than the dither period T _DZA , the counter CDZA is counted up in step 205 (CDZA ← CDZA + 1), and the process proceeds to step 213.

In step 204, the counter CDZA sets the dither period T _DZA
For more, performs processing for causing stepwise changes the steady target air-fuel ratio lambda _ST at step 206 to step 212. First, in step 206, the counter CDZA is reset (C
DZA = 0). In step 207, the dither amplitude λ _DZA is set. Specifically, as described above, the dither amplitude λ _DZA is determined in advance to an optimum value according to the rotation speed Ne and the intake pressure PM,
A two-dimensional map of the rotation speed Ne and the intake pressure PM is stored in the ROM 22.

Then, the dither amplitude λ _DZA is read from the _successive ROM ₂₂ .
Subsequently, in step 208, a dither cycle T _DZA is set. The dither cycle T _DZA is also stored in the ROM ₂₂ as a two-dimensional map of the rotation speed Ne and the intake pressure PM, similarly to the dither amplitude λ _DZA . Then, the dither cycle T _DZA is read from the _successive ROM ₂₂ .

Next, at step 209, it is determined whether or not the flag XDZR is set. Here, when the flag XDZR is set (XDZR = 1), it indicates that the steady-state target air-fuel ratio λ _ST is set to be rich with respect to the median value λ _TGC . In step 209, the flag XDZR is set (XDZR =
If it is determined that 1), that is, if the steady-state target air-fuel ratio λ _ST has been set to be rich relative to the median value λ _TGC until the previous control timing, the steady-state target air-fuel ratio λ _ST The flag XDZR is reset so that the value λ _TGC is set lean by the dither amplitude λ _DZA (XDZR ← 0). If it is determined in step 209 that the flag XDZR has been reset (XDZR = 1), that is, the steady-state target air-fuel ratio λ _ST has been set lean relative to the median value λ _TGC until the previous control timing. In step 211, the flag XDZR is set so that the target air-fuel ratio λ _TG is set to be rich with respect to the median value λ _TGC by the dither amplitude λ _DZA in step 211.
Is set (XDZR ← 1). In the following step 212, the dither amplitude λ _DZA is _set to a negative number, and the process proceeds in step 213.

Then, in step 213, the steady-state target air-fuel ratio λ _ST is set by the following equation.

λ _ST = λ _TGC + λ _DZA Accordingly, when the steady-state target air-fuel ratio λ _ST is established lean by the dither amplitude λ _{DZA with} respect to the median value λ _TGC ,
In step 213, the steady-state target air-fuel ratio λ _ST is set by the following equation.

λ _ST = λ _TGC + λ _{DZA When} the steady-state target air-fuel ratio λ _ST is set to be rich from the median value λ _TGC by the dither amplitude λ _DZA , the dither amplitude λ _DZA is set to a negative value in step 212. Therefore, in step 213, the steady-state target air-fuel ratio λ _ST is set by the following equation.

λ _ST = λ _TGC -λ _{DZA When the} steady-state target air-fuel ratio λ _ST is determined, the process returns to FIG. 5, and in step 306, the above-mentioned λ _ST is set to the target air-fuel ratio λ _TG .

FIG. 7 shows a correction coefficient FAF,
6 is a time chart showing the operation of the target air-fuel ratio λ _TG and the actual air-fuel ratio λ, and the generation status of NO _X and CO. At the initial stage of acceleration, the air-fuel ratio λ shifts to the lean side, but at the time of acceleration, the target air-fuel ratio λ _TG is set to the rich side (λ _AC ), and the dither control stops. Converges to λ _AC . Therefore, over-leaning during acceleration is suppressed, which has conventionally occurred during acceleration.
It is possible to reduce the NO _X, also improves acceleration performance.

At the initial stage of acceleration, the air-fuel ratio λ shifts to the rich side, but at the time of deceleration, the target air-fuel ratio λ _TG is set to the lean side (λ _DC ), and the dither control stops. Converge to the side (λ _DC ). Therefore, over-rich during deceleration is suppressed, CO that has been generated during deceleration in the past can be reduced, and fuel efficiency also improves.

As described above, in the embodiment, during acceleration / deceleration, the target air-fuel ratio is set to the target air-fuel ratio during acceleration λ _AC and the target air-fuel ratio during deceleration λ _DC which have been determined in advance. However, as shown in the flowchart of FIG. During acceleration, the target air-fuel ratio λ is subtracted by a predetermined value α from the target air-fuel ratio λ _{TG in} the steady state.
_TG may be set to the rich side (step 401), and the target air-fuel ratio λ _TG may be set to the lean side by subtracting a predetermined value β during deceleration (step 402).

As still another embodiment, the above-described dither control may be stopped during acceleration / deceleration to fix the target air-fuel ratio to the logical air-fuel ratio.

In this embodiment, the target air-fuel ratio λ _TG is changed in a square wave at the time of acceleration / deceleration, but the target air-fuel ratio λ _TG is increased during acceleration and during deceleration as shown by B and C in FIG. The target air-fuel ratio before deceleration may be gradually or rapidly returned.

[Brief description of the drawings]

FIG. 1 is a diagram corresponding to claims, FIG. 2 is a block diagram of an embodiment of the present invention, FIGS. 3, 5, and 6 are flowcharts for explaining the operation of the embodiment, and FIG. FIG. 7 is a characteristic chart of the purification rate of the catalyst, FIG.
FIG. 9 is a time chart of another embodiment, and FIG. 9 is a flowchart for explaining the operation of the other embodiment. 16a to 16d: fuel injection valve, 20: ECU, 36: air-fuel ratio sensor, 38: three-way catalyst.

Claims (1)

(57) [Claims] 1. A catalyst disposed in an exhaust pipe of an engine for purifying exhaust gas, and a catalyst disposed upstream of the catalyst and linearly detecting an air-fuel ratio of an air-fuel mixture supplied to the engine. An oxygen concentration sensor that outputs a signal, dither control means that dither-controls a target air-fuel ratio of the air-fuel mixture at a predetermined cycle, and a fuel injection amount supplied to the engine according to the detection signal and the target air-fuel ratio. An acceleration / deceleration detection means for detecting acceleration / deceleration of the engine; a prohibition means for inhibiting the dither control during acceleration / deceleration; and a target air / fuel ratio during acceleration / deceleration. And a target air-fuel ratio setting means for setting the air-fuel ratio to a predetermined value.