JP2001263136A - Air-fuel ratio feedback control device for internal combustion engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve converging responsiveness to a target air-fuel ratio in a device for performing a feedback control for an air-fuel ratio by calculating a linear item and a nonlinear item to be integrated by a sliding mode control. SOLUTION: The nonlinear item UNL is calculated as UNL=(gain 1)×(target air-fuel ratio - actual air-fuel ratio)/(|target air-fuel ratio - actual air-fuel ratio|)+UNL(OLD). The linear item UL is calculated as UL=(gain 2)×(target air-fuel ratio - actual air-fuel ratio)/actual air-fuel ratio. By the addition values, the feedback control is performed for the air-fuel ratio. The value of the nonlinear item UNL when the air-fuel ratio is converted in the vicinity of the target air-fuel ratio is initialized to an initial value for every target air fuel ratio.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an apparatus for feedback-controlling an air-fuel ratio of an internal combustion engine, and more particularly to a technique for performing feedback control using sliding mode control.

[0002]

2. Description of the Related Art In an internal combustion engine for a vehicle, it is general to perform feedback control of an air-fuel ratio of a combustion mixture to a target value for the purpose of purifying exhaust gas and improving fuel efficiency.

For this reason, PID control (proportional / integral / differential) or the like is used so that the air-fuel ratio is sequentially detected by an air-fuel ratio sensor provided in an exhaust passage or the like and the detected air-fuel ratio converges to a target air-fuel ratio. The fuel supply amount is feedback controlled.

On the other hand, a sliding mode control is known as a highly robust control that suppresses the influence of disturbance, and is often used in robot control and the like. Feedback control of the air-fuel ratio is performed using the sliding mode control. It has been proposed to do this (Japanese Unexamined Patent Publication No. Hei 8-23271).
No. 3).

[0005]

By the way, in the sliding mode control, if the system state is quickly put on the switching line (S = 0), the convergence performance is improved, but the nonlinear term is simply set large. Then, an overshoot occurs in the system state, and the system oscillates with a large swing around the switching line, resulting in a large variation in the air-fuel ratio. In the case of the above-mentioned sliding mode control, the deviation between the target air-fuel ratio and the actual air-fuel ratio is used as a switching function, and the feedback gain that switches between positive and negative according to the positive / negative of the switching function is integrated to calculate a nonlinear term. When the change in the air-fuel ratio is large, a delay occurs when the system state reaches the switching line, and the responsiveness deteriorates. If the feedback gain is increased to suppress this, the non-linear term integrated during the detection delay period of the air-fuel ratio sensor increases, and as in the case described above, a large overshoot occurs and the fluctuation of the air-fuel ratio increases.

The present invention has been made in view of such a problem. By appropriately setting a nonlinear term in the sliding mode control, the system state can be quickly put on the switching line while suppressing overshoot. To provide an air-fuel ratio feedback control device with improved convergence to a target air-fuel ratio.

[0007]

SUMMARY OF THE INVENTION Therefore, an invention according to claim 1 is an air-fuel ratio feedback control device for feedback-controlling an air-fuel ratio of a combustion air-fuel mixture of an internal combustion engine to a target air-fuel ratio. The present invention is configured to calculate the air-fuel ratio feedback control amount including the term and the non-linear term to be integrated, and to initialize the non-linear term to a value corresponding to the target air-fuel ratio when the target air-fuel ratio is switched.

With this configuration, when the target air-fuel ratio is switched, the nonlinear term is initialized to a value required to control the target air-fuel ratio. Accordingly, the system state can be quickly put on the switching line (S = 0) while suppressing the overshoot with respect to the change in the target air-fuel ratio, and the convergence to the target air-fuel ratio is improved.

According to the second aspect of the present invention, the non-linear term is set for each target air-fuel ratio and is initialized to a non-linear term corresponding to the target air-fuel ratio after switching. According to this configuration, when the target air-fuel ratio is switched, the value of the nonlinear term is initialized to a value set in advance corresponding to the target air-fuel ratio after switching.

Thus, it is possible to converge to an arbitrary target air-fuel ratio with uniform high response. According to the third aspect of the present invention, a linear term and a non-linear term are calculated by a sliding mode control using a deviation between the air-fuel ratio of the combustion air-fuel mixture and the target air-fuel ratio as a switching function. The non-linear term is calculated by integrating a positive / negative feedback gain.

With this configuration, each time the state of the air-fuel ratio crosses the switching line, the sign of the switching function is inverted, and the non-linear term, which is a value obtained by integrating the feedback gain whose sign is inverted, is set by the nonlinear term. The state is converged on the target air-fuel ratio while being restricted on the switching line.

Thus, when the target air-fuel ratio is switched, the nonlinear term is initialized to a value corresponding to the switched target air-fuel ratio, and the system state quickly approaches the switching line before integration is performed. The air-fuel ratio can be constrained to the vicinity of the target air-fuel ratio to quickly converge on the target air-fuel ratio.

[0013] In the present invention, the absolute value of the feedback gain is variably set according to the engine operating state. According to this configuration, the engine load and
The nonlinear term of the air-fuel ratio feedback control amount is calculated using a feedback gain whose absolute value is variably set in accordance with the engine operating state such as the rotation speed.

Thus, it is possible to prevent the value of the nonlinear term from being excessively integrated during the detection delay time of the air-fuel ratio, and to appropriately reduce the deviation amount of the actual air-fuel ratio from the target air-fuel ratio while ensuring responsiveness. Thus, stable air-fuel ratio feedback control can be performed.

According to a fifth aspect of the present invention, the linear term is:
The calculation is made as a value proportional to the ratio of the deviation to the air-fuel ratio of the combustion mixture. According to such a configuration,
As the state of the air-fuel ratio moves away from the switching line, a larger linear term is set in proportion to this.

Thus, it is possible to converge quickly on the switching line toward the target air-fuel ratio while suppressing overshoot.

[0017]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described below with reference to the drawings. In FIG. 1 showing a system configuration of an internal combustion engine in the embodiment, an air flow meter 13 for detecting an intake air flow rate Qa and a throttle valve 14 for controlling the intake air flow rate Qa in conjunction with an accelerator pedal are provided in an intake passage 12 of the engine 11. An electromagnetic fuel injection valve 15 is provided for each cylinder in a downstream intake manifold portion.

The fuel injection valve 15 is driven to open by an injection pulse signal from a control unit 16 containing a microcomputer, and is supplied by pressure from a fuel pump (not shown) to inject and supply fuel controlled to a predetermined pressure by a pressure regulator. . Further, a water temperature sensor 17 for detecting a cooling water temperature Tw in a cooling jacket of the engine 11 is provided, and a wide area type air for linearly detecting an air-fuel ratio of a combustion mixture in accordance with an oxygen concentration in exhaust gas in an exhaust passage 18. A fuel ratio sensor 19 is provided.
A three-way catalyst 20 for purifying by oxidizing O and HC and reducing NOx is provided.

Here, the structure of the wide-range air-fuel ratio sensor 19 will be described with reference to FIG. Zirconia (ZrO
On a substrate 31 made of a solid electrolyte member such as 2), a + electrode 32 for measuring oxygen concentration is provided. Further, an air introduction hole 33 into which air is introduced is opened in the substrate 31,
A negative electrode 34 is attached to the air introduction hole 33 so as to face the positive electrode 32.

In this manner, the oxygen concentration detecting section 35 is formed by the substrate 31, the positive electrode 32 and the negative electrode 34.
Further, an oxygen pump section 39 formed by providing a pair of platinum pump electrodes 37 and 38 on both surfaces of a solid electrolyte member 36 made of zirconia or the like is provided.

The oxygen pump section 39 is stacked above the oxygen concentration detection section 35 via a spacer 40 formed in a frame shape of, for example, alumina, and the oxygen pump section 39 is disposed between the oxygen concentration detection section 35 and the oxygen pump section 39. Is provided with a hollow chamber 41, and
Introducing holes 4 for introducing the exhaust of the engine into the hollow chamber 41
2 is formed on the solid electrolyte member 36 of the oxygen pump section 39.

The outer periphery of the spacer 40 is filled with an adhesive 43 made of glass so as to secure the hermeticity of the hollow chamber 41 and to provide the substrate 31 and the spacer 40 with the solid electrolyte 36.
Is fixed by bonding. Here, the spacer 4
Since the substrate 0 and the substrate 31 are simultaneously fired and bonded, the hermeticity of the hollow chamber 41 is ensured by bonding the spacer 40 and the solid electrolyte member 36. Further, the oxygen concentration detection unit 39 has a built-in heater 44 for warming up.

Then, the oxygen concentration of the exhaust gas introduced into the hollow chamber 41 through the introduction hole 42 is detected from the voltage of the + electrode 32. Specifically, an oxygen ion current flows in the substrate 31 according to the concentration difference between the oxygen in the atmosphere in the air introduction hole 33 and the oxygen in the exhaust gas in the hollow chamber 41, and accordingly, the positive electrode 32 Then, a voltage corresponding to the oxygen concentration in the exhaust gas is generated.

Then, according to the detection result, the hollow chamber 41
The value of the current flowing to the oxygen pump section 39 is variably controlled so as to keep the atmosphere in the chamber constant (for example, the stoichiometric air-fuel ratio), and the oxygen concentration in the exhaust is detected from the current value at that time.

More specifically, after the control circuit 45 amplifies the voltage of the positive electrode 32, the voltage detection resistor 46
Is applied between the electrodes 37 and 38 to keep the oxygen concentration in the hollow chamber 41 constant.

For example, when detecting the air-fuel ratio in a lean region where the oxygen concentration in the exhaust gas is high, the outer pump electrode 3
A voltage is applied using 7 as an anode and the pump electrode 38 on the hollow chamber 41 side as a cathode. Then, oxygen (oxygen ion O ²⁻ ) proportional to the current is pumped out of the hollow chamber 41 to the outside. When the applied voltage exceeds a predetermined value, the flowing current reaches a limit value. By measuring the limit current value by the control circuit 45, the oxygen concentration in the exhaust gas, in other words, the air-fuel ratio can be detected.

Conversely, the pump electrode 7 is used as a cathode,
If oxygen is pumped into the hollow chamber 41 using the anode 8 as an anode, detection can be performed in an air-fuel ratio rich region where the oxygen concentration in the exhaust gas is low.

The limit current is determined by the voltage detection resistor 46.
Is detected from the output voltage of the differential amplifier 47 for detecting the voltage between the terminals. Returning to FIG. 1, a distributor (not shown) has a built-in crank angle sensor 21 that counts a crank unit angle signal output from the crank angle sensor 21 in synchronization with engine rotation for a certain period of time, or The engine rotation speed Ne is detected by measuring the cycle of the crank reference angle signal.

Then, the control unit 16
Calculates and controls the amount of fuel injected from the fuel injection valve 15 and the ignition timing. Here, in the air-fuel ratio feedback control region, the air-fuel ratio (actual air-fuel ratio) detected by the air-fuel ratio sensor 19 is made to coincide with the target air-fuel ratio according to the operating condition by the sliding mode control according to the present invention. Perform fuel ratio feedback control.

FIG. 3 is a block diagram showing the air-fuel ratio feedback control by the sliding mode control. In the non-linear term calculation unit 101, a nonlinear term U _NL feedback control amount is calculated by the following equation.

U _NL = (gain 1) × (target air-fuel ratio−actual air-fuel ratio) / (| target air-fuel ratio−actual air-fuel ratio |) + _{UNL (OLD) In} the above equation, U _{NL (OLD)} is This is the previous value of the nonlinear term _UNL .

That is, in the sliding mode control, the switching function S is changed to the switching function S by the direct switching function method.
= Target air-fuel ratio-actual air-fuel ratio, the switching line (S
= 0) is a desired state, ie, the actual air-fuel ratio = the target air-fuel ratio, and the direction of increase / decrease (positive / negative) of the feedback gain is switched every time the switching line is crossed. Then, the nonlinear term _UNL is calculated as a value obtained by integrating the feedback gain for switching the increase / decrease direction (positive / negative) by the switching line (S = 0).

Further, the linear term calculation unit 102, the linear term U _L of the feedback control amount is calculated by the following equation. _UL = (gain 2) × (target air-fuel ratio−actual air-fuel ratio) / actual air-fuel ratio Note that the gain 1 and the gain 2 are negative values.

[0034] Then, the linear term control amount U _L, the added value and a non-linear term control amount U _NL, the air-fuel ratio feedback correction median coefficient alpha (value corresponding to no feedback correction: 1.0) to After adding, the limiter 103 performs limiter processing so as to be equal to or smaller than the upper limit and equal to or larger than the lower limit, and then output.

Hereinafter, in the same manner as the normal air-fuel ratio control, the basic fuel injection amount Tp calculated based on the intake air amount and the engine speed is corrected to a value corrected by various correction coefficients COEF as described above. The calculated feedback correction coefficient α
, And further adds a battery voltage correction Ts to calculate a fuel injection amount (fuel injection pulse width) Ti (Ti = T
p × COEF × α + Ts), the drive signal to the fuel injector 15
(Pulse) to output fuel.

Here, the basic fuel injection amount Tp is set as a value corresponding to the stoichiometric air-fuel ratio, and the feedback correction coefficient α is set as follows. α = 1.0 (the median) + linear term _UL + nonlinear term U _NL Also, the gain 1 of the non-linear term U _NL is corrected in response to the air-fuel ratio detection delay time varying with the intake air amount. A gain correction value calculating unit 104 is provided for correcting the absolute value of the gain 1 so that the smaller the intake air amount with the longer delay time, the smaller the absolute value.

Instead of correcting the gain 1 according to the intake air amount, the absolute value of the gain 1 may be corrected according to the engine speed, or a combination of the engine load and the engine speed may be used. In any case, the absolute value of the gain 1 is corrected to be smaller as the delay time becomes longer.

When the target air-fuel ratio is switched, the non-linear term initialization section 105 performs a process of initializing the non-linear term _UNL to an initial value set corresponding to the target air-fuel ratio after the switching.

The initialization process by the nonlinear term initialization unit 105 will be described with reference to the flowchart of FIG. In the flowchart of FIG. 4, in step S1, it is determined whether or not the target air-fuel ratio (target A / F) has changed. When the target air-fuel ratio changes, the process proceeds to step S2.

In step S2, from the initial values set in the initial value table for each target air-fuel ratio, the initial values stored corresponding to the switched target air-fuel ratio are read. Specifically, the initial value of the nonlinear term is calculated by the following equation, and a table as shown in FIG. 5 is obtained for each target air-fuel ratio.

Initial value = (14.7 / target air-fuel ratio) -1
However, the stoichiometric air-fuel ratio = 14.7 In this way, the nonlinear term U _{NL is} changed by the change of the target air-fuel ratio.
Is initialized, the feedback gain is integrated at a predetermined time period, and the nonlinear term _UNL is updated while switching between positive and negative every time the switching line is crossed.

FIG. 6 shows the control of the air-fuel ratio according to the present embodiment, in which the evaporated fuel temporarily stored in the canister is purged into the intake system according to the operating conditions, and the purge is performed by changing the operating conditions. The state when stopping (cutting) is shown.

When the purge is started, the nonlinear term is gradually reduced so as to suppress the enrichment of the air-fuel ratio due to the purge of the evaporated fuel, and the operating conditions change to change the target air-fuel ratio (for example, the lean air-fuel ratio is increased). When a command to stop purging is output together with the target air-fuel ratio, the nonlinear term is switched to an initial value corresponding to the target air-fuel ratio after the change (see the thick line in FIG. 6).

In the conventional control, even if the target air-fuel ratio changes, the feedback correction coefficient α is not initialized, but gradually converges to the changed target air-fuel ratio by integration. (See thin line in Fig. 6)
In the above embodiment, the nonlinear term is quickly switched to the initial value corresponding to the target air-fuel ratio after the change,
The deviation of the air-fuel ratio can be prevented.

[Brief description of the drawings]

FIG. 1 is a diagram showing a system configuration of an internal combustion engine according to an embodiment.

FIG. 2 is a diagram illustrating an air-fuel ratio sensor and peripheral circuits according to the embodiment.

FIG. 3 is a control block diagram showing air-fuel ratio feedback control in the embodiment.

FIG. 4 is a flowchart illustrating initialization control of a nonlinear term according to the embodiment;

FIG. 5 is a table showing an initial value of a nonlinear term for each target air-fuel ratio in the embodiment.

FIG. 6 is a time chart showing how an air-fuel ratio changes during purge control of evaporated fuel according to the embodiment.

[Explanation of symbols]

 11 internal combustion engine 15 fuel injection valve 16 control unit 19 air-fuel ratio sensor

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 3G301 JA07 JA11 KA11 MA01 NB11 NC08 ND02 ND05 ND45 PA01Z PE01Z 5H004 GA02 GA03 GB12 GB20 HA04 HA13 HB04 HB13 JA03 KA62 KA74 KB04 9A001 HH34 JJ77

Claims (5)

[Claims] An air-fuel ratio feedback control device for feedback-controlling an air-fuel ratio of a combustion mixture of an internal combustion engine to a target air-fuel ratio, wherein an air-fuel ratio feedback control amount including a linear term and a non-linear term is calculated by sliding mode control. An air-fuel ratio feedback control device for an internal combustion engine, wherein the non-linear term is initialized to a value corresponding to the target air-fuel ratio when the target air-fuel ratio is switched. 2. The air-fuel ratio feedback control device for an internal combustion engine according to claim 1, wherein the non-linear term is set for each target air-fuel ratio, and is initialized to a non-linear term corresponding to the target air-fuel ratio after switching. . 3. A linear mode and a non-linear term are calculated by a sliding mode control using a deviation between an air-fuel ratio of a combustion air-fuel mixture and a target air-fuel ratio as a switching function, and the sign is switched according to the sign of the switching function. 3. The air-fuel ratio feedback control device for an internal combustion engine according to claim 1, wherein the nonlinear term is calculated by integrating a feedback gain obtained. 4. The air-fuel ratio feedback control device for an internal combustion engine according to claim 3, wherein the absolute value of the feedback gain is variably set according to an engine operating state. 5. An air-fuel ratio feedback control system for an internal combustion engine according to claim 3, wherein said linear term is calculated as a value proportional to the ratio of said deviation to the air-fuel ratio of the combustion air-fuel mixture.