JP2009203919A - Air-fuel ratio control method and air-fuel ratio control device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To restrain discharge of NOx and HC, after performing a fuel cut. <P>SOLUTION: When the fuel cut of an engine 2 is caused, output of a rear O<SB>2</SB>sensor 12 arranged downstream of a catalyst 5 is controlled in a back-stepping system with a lower shelf value higher than a minimum value measurable when continuing the fuel cut and lower than a value corresponding to the stoichiometric air-fuel ratio as a target. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to control of an air-fuel ratio after execution of fuel cut in an engine such as an automobile.

Generally, in the exhaust pipe of an automobile or the like, HC in exhaust gas discharged from an engine, a catalyst to harmless by oxidation / reduction of NO _x it is mounted. In order to maintain the exhaust gas purification efficiency by the catalyst, it is necessary to converge the air-fuel ratio downstream of the catalyst to a certain range (called a window) near the theoretical air-fuel ratio. For this purpose, an O ₂ sensor or an air-fuel ratio sensor is arranged on each of the upstream and downstream sides of the catalyst, and a double feedback loop using the output signals of both the front and rear sensors is configured to control the air-fuel ratio. (See, for example, the following patent document).
JP 2006-329005 A JP 2006-329006 A

  In an automobile or the like, fuel supply to the engine may be temporarily stopped under predetermined conditions. This fuel cut occurs, for example, when the driver's foot leaves the accelerator pedal and enters deceleration while the engine speed exceeds a predetermined fuel cut set speed. With the fuel cut, the catalyst is filled with air that does not contain fuel.

Thereafter, when the engine speed becomes equal to or lower than the resupply speed, fuel supply is resumed. At this time, the air-fuel ratio downstream of the catalyst remains lean and does not readily recover, and NO _x may be spiked out at the same time as the fuel cut ends, or a small amount of NO _x may continue to be discharged during idle rotation.

  On the other hand, if rich correction is performed in order to quickly control the air-fuel ratio within the window, overcorrection is caused instead, and another problem of HC discharge is caused.

Although the present invention has been made in view of, the to suppress the emission of the NO _x and HC after fuel cut and intended purpose.

In order to solve the above problems, when controlling the air-fuel ratio on the downstream side of the catalyst mounted on the exhaust pipe of the engine, when a fuel cut of the engine occurs, the rear O ₂ sensor provided downstream of the catalyst The output is controlled to be a value higher than the minimum value that can be measured when the fuel cut is continued and lower than a value corresponding to the theoretical air-fuel ratio.

The O ₂ sensor outputs a voltage signal corresponding to the oxygen concentration in the exhaust gas and has a non-linear output characteristic with respect to the air-fuel ratio. As shown in FIG. 2, the output of the O ₂ sensor has a large and steep slope with respect to the air-fuel ratio in the window range, and gradually approaches a low saturation value in the lean region where the air-fuel ratio is larger than that. In a rich region where the air-fuel ratio is small, it gradually approaches a high saturation value.

However, in detail, when returning from the lean region to the window region, it passes through a shelf-like value range (referred to as a lower shelf) with an output value slightly higher than the lower saturation value and a small change rate. In the present invention, after the fuel cut is executed, control is performed so that the air-fuel ratio downstream of the catalyst, that is, the output of the rear O ₂ sensor quickly follows the value of the lower shelf. This can reduce the emission of re acceleration or idling of NO _x. In addition, since unnecessary rich correction is not performed, the problem of HC discharge due to overcorrection can be avoided.

In a universal LQI control system or the like, an integrator integrates a deviation between a control output and its target value. Then, the overshoot and undershoot of the control output may be repeated due to the influence of the integration. In order to make the output of the rear O ₂ sensor follow the target as quickly as possible, it is effective to control this by using a backstepping controller without an integrator.

According to the present invention, NO _x and HC emissions after fuel cut can be suppressed.

Hereinafter, embodiments of the present invention will be described with reference to the drawings. The air-fuel ratio control apparatus 1 of the present embodiment controls the air-fuel ratio in the catalyst 3 for detoxifying harmful substances NO _x and HC generated by burning fuel in the engine 2, as shown in FIG. Further, a front O ₂ sensor 11 that outputs an output signal corresponding to the air-fuel ratio or oxygen concentration on the upstream side of the catalyst 3, and a rear O ₂ that outputs an output signal corresponding to the air-fuel ratio or oxygen concentration on the downstream side of the catalyst 3. The sensor 12 includes a detection unit 13 that detects output signals of both the sensors 11 and 12, and a control unit 14 that controls the output signals of both the sensors 11 and 12 to target values thereof.

An outline of the hardware configuration is shown in FIG. The engine 2 is, for example, a multi-cylinder fuel injection engine for automobiles. Combustion gas generated by the engine 2 is released from the exhaust port into the atmosphere through the exhaust manifold 41, the exhaust pipe 42, and the catalyst 3. The front O ₂ sensor 11 and the rear O ₂ sensor 12 output a voltage signal corresponding to the oxygen concentration in the exhaust gas by reacting in contact with the exhaust gas.

The front O ₂ sensor 11 and the rear O ₂ sensor 12 are connected to the electronic control unit 5 together with various sensors (not shown) such as an intake pressure sensor, a rotation speed sensor, a vehicle speed sensor, a water temperature sensor, a cam position sensor, and a throttle sensor. Connected. The electronic control unit 5 includes a processor 51, a RAM 52, a ROM (or flash memory) 53, an I / O interface 54, and the like. The I / O interface 54 is responsible for receiving output signals of various sensors and transmitting control signals, and includes an A / D conversion circuit and / or a D / A conversion circuit. A program to be executed by the processor 51 is stored in the ROM 53, and is read from the ROM 53 into the RAM 52 and decoded by the processor 51 at the time of execution. Thus, the electronic control device 5 exhibits functions as the detection unit 13 and the control unit 14 shown in FIG. 1 according to the program.

The detection unit 13 detects signals output from the front O2 sensor 11, the rear O2 sensor 12, and other sensors via the I / O interface 54. Then, the control unit 14 calculates the fuel injection amount so as to achieve the target air-fuel ratio, inputs a control signal corresponding to the fuel injection amount to the fuel injection valve via the I / O interface 54, and supplies the fuel of the engine 2. Control the injection. The fuel injection amount is obtained by referring to the intake pipe pressure, engine speed, etc., and the basic injection amount is corrected according to the engine temperature (cooling water temperature), the required amount, the exhaust gas air-fuel ratio, etc. And finally decide. In this embodiment, as shown in FIG. 4, a double feedback loop that refers to the output signals of the O ₂ sensors 11 and 12 provided before and after the catalyst 3 is constructed, and the air-fuel ratio upstream of the catalyst 3 is set to the front. The controller 141 controls the downstream air-fuel ratio with the rear controller 142. In FIG. 4, TTAUS represents the basic injection amount. Front A / F and Rear A / F are the air-fuel ratios of the exhaust gas on the upstream side and downstream side of the catalyst 3, respectively. The front O ₂ sensor 11 outputs a voltage OXAD, and the rear O ₂ sensor 12 outputs a voltage OX2AD. The output FAF of the front controller 141 represents how much the fuel injection correction amount, that is, the basic injection amount is corrected. The output FACF of the rear controller 142 corrects the control error in the front controller 141. OXAD ^* is a target value for OXAD, and OX2AD ^* is a target value for OX2AD. In normal times, the target value OX2AD ^* of the post-catalyst air-fuel ratio OX2AD is set to a value corresponding to the air-fuel ratio in the window.

The air-fuel ratio control of this embodiment is characterized by the rear controller 142. The rear controller 142 performs backstepping control, which is one of controls without an integrator. The rear controller 142 outputs a correction amount FACF to be added to OXAD ^* so as to control the sensor output OX2AD with OX2AD ^* as a target. That is, the front controller 141 controls the sensor output OXAD with (OXAD ^* + FACF) as a target. Regarding the calculation of FACF by the rear controller 142, two state quantities x ₁ and x ₂ are considered.
_{x 1 (i) = OX2AD (} i) -OX2AD *
_{x 2 (i) = OX2AD (} i + 1) -OX2AD *
The rear controller 142 calculates the correction amount FACF by controlling the state quantities x ₁ and x ₂ so as to be zero with state feedback.

  The controlled object can be modeled by, for example, a quadratic linear state equation (Equation 1).

The state quantities x ₁ (i + 1) and x ₂ (i + 1) are linear sums of x ₁ (i), x ₂ (i) and FACF, respectively. The equation of state (Equation 1) can be divided into two subsystems. That is,
x ₁ (i + 1) = x ₂ (i)
x ₂ (i + 1) = a ₁ x ₁ (i) + a ₂ x ₂ (i) + bFACF (i)
In the first step of the backstepping control, x ₂ is considered as a virtual input α so that x ₁ becomes zero. Using a constant K _c whose absolute value is less than 1,
α (i) = K _c x ₁ (i)
If x is satisfied, x ₁ can be suitably converged to the target value 0.

In the second stage of backstepping control, x ₂ is actually made equal to α. If the deviation between x2 and α is σ,
x ₁ (i + 1) = x ₂ (i) = α (i) + σ (i)
σ (i + 1) = a ₁ x ₁ (x) + a ₂ σ (i) + bFACF (i) −α (i + 1) + a ₂ α (i)
Is established. The above two equations are both functions of x ₁ (i) and σ (i). The rear controller 142 calculates the FACF as a linear sum as shown in the following equation (Formula 2) so that the three values of the integrated values of x1, σ, and σ converge simultaneously to 0.

K ₁ , K ₂ , and K ₃ are feedback gains. The feedback gain can be determined with reference to a map stored in advance in the RAM 51 or the ROM 52 based on, for example, the operating state of the engine 2 and / or the state of the catalyst 3. However, the feedback gain may be calculated using an optimum regulator.

In addition, in the present embodiment, the air-fuel ratio downstream of the catalyst 3 after the fuel cut is performed, that is, the target value OX2AD ^* of the output OX2AD of the rear O ₂ sensor 12 is a minimum that can be measured when the fuel cut is continued. The value is set higher than the value and lower than the value corresponding to the theoretical air-fuel ratio.

The output characteristics of the O ₂ sensor are illustrated in FIG. The output of the O ₂ sensor has a large and steep slope with respect to the air-fuel ratio in the window range (around 0.6 V), and a low saturation value (about 0 V in the lean region where the air-fuel ratio is larger than that. ) And draws a so-called Z characteristic curve that gradually approaches a high saturation value (about 1 V) in a rich region where the air-fuel ratio is small. In addition, when returning from the lean area to the window area, the output value is slightly higher than the low saturation value (about 0.1 to 0.2 V) and passes through a shelf-like value area and lower shelf that have a small change rate. .

When the fuel cut occurs, the catalyst 3 is filled with air containing no fuel. Thereafter, even if the fuel supply is resumed, the air-fuel ratio downstream of the catalyst 3 remains lean and does not recover immediately. Then, NO _x is exhausted. On the other hand, conventionally, by performing rich correction in which the target value OX2AD ^* is set to a value slightly higher than the value corresponding to the theoretical air-fuel ratio (about 0.6 to 0.7 V), the air-fuel ratio downstream of the catalyst 3 is set. Was trying to quickly control within the window. However, it was overcorrected and sometimes caused HC emissions.

In the present embodiment, the control unit 14 (the rear controller 142), after executing the fuel cut, sets the output OX2AD of the rear O ₂ sensor 12 between the lower saturation value and the theoretical air-fuel ratio corresponding value. Is controlled as a target. With such a configuration, it is possible to sufficiently reduce NO _x emission during re-acceleration after resumption of fuel supply or idling. In addition, since unnecessary rich correction is not performed, the problem of HC discharge due to overcorrection can be avoided. Moreover, since the control unit 14 controls the output OX2AD of the rear O ₂ sensor 12 using the backstepping controller 142 that does not have an integrator, the output OX2AD of the rear O ₂ sensor 12 can quickly follow the lower shelf. This is possible and contributes to the suppression of NO _x and HC emissions.

  The present invention is not limited to the embodiment described in detail above. In particular, the design method of the backstepping controller is only an example. Of course, it is also possible to model the controlled object by a higher-order equation of the third order or higher in consideration of dead time and the like.

  In addition, the specific configuration of each unit, the processing procedure, and the like can be variously modified without departing from the spirit of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. Diagram illustrating the output characteristics of the rear O ₂ sensor. The hardware resource block diagram of the same air fuel ratio control apparatus. The block diagram of a back stepping control system.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Air-fuel ratio control apparatus 13 ... Detection part 14 ... Control part 142 ... Back stepping controller (rear controller)

Claims (3)

It controls the air-fuel ratio in the catalyst mounted on the exhaust pipe of the engine,
When the engine fuel cut occurs, the output of the rear O ₂ sensor provided downstream of the catalyst is higher than the minimum value that can be measured when the fuel cut is continued and lower than the value corresponding to the stoichiometric air-fuel ratio. An air-fuel ratio control method characterized by controlling the value as a target. The air-fuel ratio control method according to claim 1, wherein the output of the rear O ₂ sensor is controlled using a back stepping controller having no integrator. A detection unit for detecting an output signal of a rear O ₂ sensor provided downstream of the catalyst mounted on the exhaust pipe of the engine;
Control that controls the output of the rear O ₂ sensor to a value that is higher than the minimum value that can be measured when the fuel cut is continued and lower than the value corresponding to the theoretical air-fuel ratio when the engine fuel cut occurs An air-fuel ratio control device.

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