JP2857176B2 - Exhaust gas purification device - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an exhaust gas purification device for an internal combustion engine, and more particularly, to feedback control of an air-fuel ratio to a predetermined value using an air-fuel ratio sensor provided in an engine exhaust system. Further, the present invention relates to an exhaust gas purifying apparatus that maximizes the purifying function of a three-way catalyst provided with an exhaust passage.

[Prior Art] In order to effectively operate a so-called three-way catalyst for removing three harmful components (HC, CO, NO _x ) in exhaust gas of an internal combustion engine, the air-fuel ratio of exhaust gas entering the catalyst must be theoretically determined. It must be kept close to the air-fuel ratio. For this reason, the air-fuel ratio of the intake air-fuel mixture is feedback-controlled using an air-fuel ratio sensor (a so-called O ₂ sensor based on the action of an oxygen concentration cell) in the engine exhaust system regardless of whether it is a carburetor type or a fuel injection type. An exhaust purification system is employed (for example, see Japanese Patent Application Laid-Open No. 52-48738).
No., JP-B-62-12379). A conventional air-fuel ratio control method using such an air-fuel ratio system will be briefly described with reference to FIG. FIGS. 8 (a) and 8 (b) show the relationship between
FIGS. 4B and 4E show output signals of the O ₂ sensor that generates an output voltage corresponding to the O ₂ concentration. FIGS. It is an air-fuel ratio signal whose waveform is compared and shaped. The air-fuel ratio is proportionally integrated (PI controlled) as shown in (c) and (f) with respect to the output signal of the air-fuel ratio sensor whose waveform has been shaped.

The air-fuel ratio sensor detects the concentration of O ₂ in the exhaust gas, and the air-fuel ratio during combustion in the engine combustion chamber is smaller than the stoichiometric air-fuel ratio (usually about 14.7) (hereinafter referred to as a rich state).
This is to determine whether the state is large (hereinafter, referred to as a lean state). The amount of fuel supplied to the engine, that is, the air-fuel ratio λ is controlled in accordance with the determination signal as follows. First, when the engine speed (hereinafter abbreviated as Ne) is low, when the air-fuel ratio sensor output signal is inverted from the lean state to the rich state in FIG. 8B, a feedback control amount signal (FIG. 8C) ) Is delayed by a delay time D, and the lean side is skipped by a skip amount B as a proportional component. Thereafter, the air-fuel ratio signal in FIG. 8B is integrated toward the lean side with an integration constant (slope) C until the air-fuel ratio signal is inverted from rich to lean. Then, when the air-fuel ratio signal in FIG. 8B is inverted from rich to lean, the air-fuel ratio signal is immediately skipped to the rich side by the skip amount B, and integrated to the rich side with a slope C until the air-fuel ratio signal is inverted again from lean to rich. This is repeated below.
8 (e) and 8 (f) show a feedback control amount signal (FIG. 8 (f)) with respect to the air-fuel ratio sensor output signal (FIG. 8 (e)) when the engine speed Ne is high. The control method is the same as in the case of the low rotation described above. The delay time D compensates for the difference in the detection response delay of the output signal of the air-fuel ratio sensor when the atmosphere around the detection unit of the air-fuel ratio sensor changes from lean to rich or from rich to lean. The length is exaggerated for illustration. With this control method, the average air-fuel ratio can be controlled to the stoichiometric air-fuel ratio, and the exhaust gas purifying function of the three-way catalyst can be maximized.

[Problems to be Solved by the Invention] In the case of the above-described conventional air-heat ratio control method, as is clear from FIG. 8, the control cycle T and the control amplitude A are small on the high engine speed and high load side. On the other hand, T,
A also increases. This is due to various transmission delays, such as the intake of the air-fuel mixture into the combustion chamber, combustion, exhaust, and further through the exhaust manifold, from the time the controlled air-fuel ratio is actually inverted to the time the air-fuel ratio sensor output is inverted. This is because there is a factor, and the transmission delay due to the factor becomes large on the low rotation speed and low load side. When the transmission delay further increases, FIG. 8 (c)
As shown in (2), even if the feedback control amount is skipped by B when the air-fuel ratio sensor is inverted, a phenomenon occurs in which the feedback control amount signal is not immediately inverted, but is integrated for a while and finally inverted. Therefore, the control cycle T is increased by the delay time from the inversion of the air-fuel ratio sensor to the inversion of the feedback control amount, and accordingly, the control amplitude A is further increased. As a result, the driver may feel uncomfortable due to engine vibration (hunting) particularly at the time of idling, or the feedback control amount may change from lean to rich or from rich to lean as is apparent in FIGS. 8 (c) and 8 (f). Since the waveform at the time of engine reversal is different between low engine speed and high engine speed, the variation (difference) in the detection response delay of the air-fuel ratio sensor itself at the time of inversion slightly shifts, and the predetermined value shown in FIG. There is a problem that the delay time D for compensating the detection response delay cannot be adapted depending on the engine operating conditions, the control air-fuel ratio deviates from the stoichiometric air-fuel ratio, and the exhaust gas purification characteristics of the three-way catalyst deteriorate.
Furthermore, due to variations in air-fuel ratio sensor characteristics and changes over time, it is difficult to always achieve the maximum purification characteristics that can be achieved for each three-way catalyst. There is also a problem that a large capacity three-way catalyst must be used.

By the way, the three-way catalyst is supplied with a slightly rich and slightly lean air-fuel mixture alternately around the stoichiometric air-fuel ratio, compared to a case where the air-fuel mixture with a uniform air-fuel ratio is supplied to the engine. Experimental results have shown that the purification efficiency is increased. Furthermore, changing the amplitude and period of the vibration to various values, according to the result of measuring the purification efficiency characteristics with small capacity of the catalyst, the rich and lean inverted output response is obtained by the O ₂ sensor vibration It has been found that the purification efficiency is improved by supplying rich and lean exhaust gas to the catalyst with a cycle of 1/5 to 1/6 shorter than the cycle, with the stoichiometric air-fuel ratio as the center.

The present invention has been made in view of the above problems and considerations, and provides an air-fuel ratio feedback control that can fully utilize the exhaust purification capability of a three-way catalyst provided in an exhaust system of an internal combustion engine. It is intended to be realized.

[Means for Solving the Problems] An exhaust gas purifying apparatus according to the present invention includes an air-fuel ratio sensor provided in an exhaust system of an engine, reference signal forming means for forming a reference signal, and an output signal of the air-fuel ratio sensor. And a reference signal from the reference signal forming means, comparing means for outputting a deviation signal based on the difference, integrating means for integrating the deviation signal by predetermined integration characteristics, and detecting an operating state of the engine. Operating state detecting means for receiving, an output signal of the integrating means and an output signal of the operating state detecting means,
Integral output correction means for correcting the output of the integrator when a predetermined time has elapsed according to the operating state after the inversion of the integration direction of the integrator is maintained until the integration direction is inverted again. And a vibration signal generating means for generating a vibration signal having a predetermined amplitude and a period shorter than a period in which the deviation signal is inverted with the integrated output signal level corrected by the integrated output correction means as a center; Air-fuel ratio control means for performing feedback control of the air-fuel ratio of the engine based on an addition signal of the signal obtained by proportionally processing the above and the vibration signal.

[Operation] In the present invention, the integrated output obtained when a predetermined time has elapsed after the integration direction of the integration means for integrating the air-fuel ratio deviation signal has been inverted is held until the next integration direction is inverted. A vibration signal having a short cycle centering on the corrected integrated output signal level is generated, and a signal obtained by adding the vibration signal to a signal obtained by proportional processing of the deviation signal is used as an air-fuel ratio feedback control signal to control the air-fuel ratio of the engine.

Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

FIG. 1 is an overall view of the system of the present invention. Organization (1)
Is a known four-cylinder four-cycle spark ignition engine mounted on a normal automobile, and sucks combustion air through an air cleaner (2), an intake pipe (4), and a throttle valve (6).
(3) is an intake air amount sensor for detecting the amount of air to be taken in. Note that an intake pipe pressure sensor (15) may be employed instead of the intake air amount sensor (3). Various sensors such as a potentiometer type, a heat wire type, a Karman vortex type, and an ultrasonic type can be used as the intake air amount sensor (3). (5) is an intake air temperature sensor, and (10) is an engine cooling water temperature sensor, and a thermistor type is generally used.
Fuel is supplied from a fuel system (not shown) through injectors (8) provided for each cylinder. The injector (8) is of a constant injection pressure type, and the injection amount is proportional to the valve opening time. The exhaust gas after combustion is discharged into the atmosphere via an exhaust manifold (11), an exhaust pipe (13), a catalytic converter (14), and the like. The air-fuel ratio sensor (12) provided in the exhaust pipe (13) detects the oxygen concentration in the exhaust gas, that is, the deviation of the actual air-fuel ratio from the stoichiometric air-fuel ratio as a voltage, and is smaller than the stoichiometric air-fuel ratio. (Rich) is about 1V,
When it is larger than the stoichiometric air-fuel ratio (lean), an output of about 0.1 V is obtained. Organic exhaust gas 3 component in the catalytic converter _{(14) (NO x, CO} , HC) is filled with three-way catalyst for simultaneously purifying shows the best purification performance in the vicinity of the stoichiometric air-fuel ratio. The rotation sensor (9) uses the signal of the primary terminal of the ignition coil as a rotation synchronization signal, and controls the injection start timing and calculates the rotation speed using this signal. The control circuit (7) includes the sensors (3), (15), (5),
(9) Based on the signals from (10) and (12) and the battery (16), the optimum fuel injection amount is calculated, and the valve opening time of the injector (8) is controlled.

Next, the control circuit (7) (hereinafter referred to as ECU) will be described with reference to FIG. (70) is a microcomputer which is a main part of the ECU (7), and has a well-known ROM,
It is composed of RAM, microprocessor, timer controller, etc., and has digital information input / output, calculation and storage functions. The microcomputer (70) receives an intake air temperature sensor (5) and a cooling water temperature sensor (1
0), a detection signal from the battery (16), and a feedback signal based on the air-fuel ratio sensor (12) are input via the input interface (71) and the A / D converter (72). A detection signal from the intake air amount sensor (3) (a pulse signal proportional to the air flow rate in the case of the Karman vortex type), a detection signal from the rotation sensor (9), and an input signal (17) from the key switch are input to another input terminal. The data is input to the input port of the microcomputer (70) via the interface (73). On the other hand, the output signal of the air-fuel ratio sensor (12) is input to one input terminal of the comparator (20), and the output signal _Vref of the reference signal forming circuit (21) is input to another input terminal. . The output signal of the comparator (20) is input to an integration circuit (22), a proportional amplification circuit (23), and an inversion timing detection circuit (24). Inversion timing detection circuit (2
4) Outputs a short-time pulse signal at the timing when the output signal of the comparator (20) switches to “High” or “Low” level, and this output signal is sent to the microcomputer (70). The microcomputer (70) serves as a reference for time measurement when generating a correction signal for time-limiting the operation of an integration correction circuit (25) described later. The vibration signal generation circuit (26) is a so-called voltage controlled oscillator, and outputs the output signal of the integration circuit (22) corrected and changed by the integration correction circuit (25) and the microcomputer (70).
Receives a frequency control signal V _freq sent from the D / A converter (74) through the D / A converter (74), and generates a rectangular wave vibration signal centered on the output signal level of the integration circuit (22). The frequency of the vibration signal is variably set according to the voltage level of the signal _Vfreq . The output signal of the proportional amplification circuit (23) and the output signal of the vibration signal generation circuit (26) are added by an adder (27), and the input interface (71) and the A / D converter ( 72)
Is input to the micron computer (70).

From the microcomputer (70), a control signal for fuel injection calculated by a method described later is output via a driver (75), and sequentially drives the four injectors (8).

FIG. 3 is a functional block diagram showing such fuel injection control (electromagnetic valve drive time control). Ie ECU
(7), first has an injector (8) basic drive time determining means (30) for determining a basic drive time T _B for this basic drive time determining means (30) is the intake air quantity sensor (3 determine the intake air quantity Qa / Ne information per revolution engine from the engine speed Ne information from the intake air quantity Qa information and the rotation sensor (9) from), the basic drive time T _B based on this information
To determine. Subsequently, a cooling water temperature correcting means (31) for setting a correction coefficient K _WT according to the engine cooling water temperature, an intake temperature correcting means (32) for setting a correction coefficient K _AT according to the intake air temperature,
Acceleration increase correction means (33) that sets correction coefficient K _AC for acceleration increase according to the rate of change of Qa / Ne, and sets dead time (invalid time) T _D to correct the drive time according to the battery voltage Each correction coefficient is determined by dead time correction means (34). Further, as a result of the feedback operation based on the detection signal of the O ₂ sensor (12), the correction coefficient K _AF is determined in the same meaning as each of the correction coefficients. Next, the operation will be described with reference to FIG.

FIG. 4 (a) shows an input signal to the comparator (20), wherein a solid line is a detection output of the air-fuel ratio sensor (12), and a dashed line is a reference signal for determining a rich / lean determination level. (V _ref ). Rich (“Low”) in the comparator (20)
The air-fuel ratio signal (FIG. 4 (b)) determined to be (output) or lean (“High” output) is output by the integrator (22) in FIG. 4 (c).
Is integrated as shown by On the other hand, a pulse indicating the inversion timing of the air-fuel ratio determination signal is output from the inversion timing detection circuit (24) as shown in FIG. 4 (d) and input to the microcomputer (70). In the microcomputer (70), the operation state, for example, the intake air flow rate Q
a, the time _Td set and stored according to the engine speed Ne or the like is read from the memory, and the time _Td is measured using a built-in timer with reference to the input time of the pulse, and the time _Td elapses. Then, an integration correction signal as shown in FIG. 4 (e) is output to the integration correction circuit (25). The integration correction circuit (25)
In response to the correction signal, the integrated signal (c) is changed to that shown in FIG.
The correction processing is performed as shown by the solid line. Vibration signal generation circuit (2
In 6), a square wave vibration signal is generated around the output signal level of the integration correction circuit (25) and output as shown by the broken line in FIG. 4 (f). FIG. 4 (g) shows the sum of the vibration signal and the signal obtained by proportionally amplifying the output signal (FIG. 4 (b)) of the comparator (20) with a predetermined proportional gain. This is the signal output from (27).

As described above, the air-fuel ratio feedback control signal (solid waveform in FIG. 4 (g)) is generated. This signal is a voltage signal corresponding to the air-fuel ratio correction coefficient _KAF , and is input to the microcomputer (70) as a digital signal via the interface (71) and the A / D converter (72).

FIG. 5 is an operation waveform diagram for explaining that the generation frequency of the vibration signal described above changes according to the operating state. FIG. 5 (a) shows the detection output of the air-fuel ratio sensor (12) as in FIG. 4 (a). FIG. 5 (b) shows a frequency control voltage signal V _freq input from the microcomputer (70) to the vibration signal generation circuit (26) via the D / A converter (74). A voltage proportional to the number Ne is transmitted. Note that a voltage proportional to the air flow rate Qa sucked by the engine may be used instead of the engine speed Ne. Further according to the coolant temperature T _w in addition to the above,
It is possible to make the frequency of the vibration signal change. Then, it is possible to adapt to the change in the temperature characteristic of the catalytic action, and the effect of improving the purifying action is further increased. The vibration signal generation circuit (26) performs the frequency control signal V _freq
In response to this, the frequency is changed and controlled according to the engine speed Ne. FIG. 5 (c) shows how the vibration frequency increases as the engine speed Ne increases.

With the above operation, the injector drive time T _inj can be calculated according to the following equation. Then, during this drive time T _inj , the injector (8) _{opens the valve} at a predetermined timing, in this case, twice per rotation in synchronization with the rotation of the engine, and supplies fuel to the engine.

T _inj = T _B × K _WT × K _AT × K _AC × K _AFS + T _{D The} above-described fuel injection control (air-fuel ratio control) uses the voltage signal generation corresponding to the air-fuel ratio correction coefficient K _AF described with reference to FIG. Except for the feedback control circuit, the circuit is realized by a known technique, for example, the method described in Japanese Patent Publication No. 62-12379, and the description of the operation of the program by a flowchart or the like is omitted here.

In this embodiment, the operation for realizing the feedback operation shown in FIG. 4 in an analog manner has been described. For example, the input signal of the comparator (20) is output in a time sufficiently shorter than the response time of the air-fuel ratio sensor. If the A / D conversion is performed and the subsequent operation, that is, the operation of the integrating circuit (22), is performed digitally in the microcomputer (70) in synchronization with the A / D conversion, the operation is somewhat discrete. However, it goes without saying that the operation exactly the same as the operation shown in FIG. 4 can be realized, and the same exchange can be obtained.

FIG. 6 is an example of an experimental result showing the effect of the above embodiment. The three-way catalyst used is the same as that currently in practical use, and its capacity is reduced. This three-way catalyst shows a purification efficiency of 90% or more in a normal capacity. As shown in this figure, a harmful components HC, CO, NO _x
Although the aspect of the purification efficiency characteristics differs depending on the type, in any case, the purification efficiency when the average air-fuel ratio is maintained near the stoichiometric air-fuel ratio is improved as compared with the conventional one.

FIG. 7 is an overall system diagram showing another embodiment of the present invention. 1 is the same as FIG. 1 except that the location of the air-fuel ratio sensor (12) is different.
In this embodiment, the air-fuel ratio sensor (12) is installed downstream of the exhaust system so as to be located in the exhaust gas that has been partially subjected to the purifying action of the catalyst. By changing the set value of the time _Td shown in FIG. 4 (e), the rich / lean reversal cycle shown in FIG. 4 (b) becomes longer, but the air-fuel ratio supplied to the engine becomes longer. It is possible to make the fluctuation width hardly increase, and the same effect as in the case of FIG. 1 can be obtained. In the embodiment shown in FIG. 7, since the air-fuel ratio sensor (12) is provided in the exhaust gas which has been partially subjected to the purifying action of the catalyst, the deterioration of the air-fuel ratio sensor itself with the passage of time in FIG. Therefore, it can be expected that the fluctuation and variation of the air-fuel ratio control characteristics become smaller.

[Effects of the Invention] As described above, according to the present invention, it is possible to feedback-control the air-fuel ratio so as to make full use of the exhaust gas purifying ability achievable by the three-way catalyst provided in the exhaust system of the internal combustion engine. This makes it possible to maintain high exhaust gas purification efficiency over a wide control range. Therefore, the catalyst capacity can be reduced.

[Brief description of the drawings]

FIG. 1 is an overall configuration diagram of an exhaust gas purifying apparatus according to an embodiment of the present invention, FIG. 2 is a configuration diagram of a control circuit in the embodiment, and FIG. 3 is a function for explaining the operation of the control circuit in the embodiment. FIG. 4 is a block diagram, FIG. 4 and FIG. 5 are operation waveform diagrams for explaining an air-fuel ratio feedback control operation based on an air-fuel ratio sensor detection signal in the embodiment, and FIG. FIG. 7 is an overall configuration diagram of another embodiment of an exhaust gas purification apparatus according to the present invention, and FIG. 8 is an operation waveform of feedback control using an air-fuel ratio sensor in a conventional apparatus. FIG. In the figure, (1) is an engine, (3) is an intake air amount sensor,
(7) is a control circuit, (8) is an injector, (9) is a rotation sensor, (12) is an air-fuel ratio sensor, (14) is a catalytic converter, (15) is an intake pipe pressure sensor, and (20) is a comparator. ,
(21) is a reference signal forming circuit, (22) is an integrating circuit, (23)
Is a comparison amplifier circuit, (25) is an integration correction circuit, (26) is a vibration signal generation circuit, and (27) is an adder.

──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Toru Hashimoto 5-33-8 Shiba, Minato-ku, Tokyo Inside Mitsubishi Motors Corporation (72) Mitsuhiro Miyake 5-33-8 Shiba, Minato-ku, Tokyo Inside Mitsubishi Motors Corporation (72) Inventor Minoru Nishida 8-1-1 Tsukaguchi Honmachi, Amagasaki City, Hyogo Mitsubishi Electric Corporation Industrial Systems Research Institute (56) References JP-A-64-66441 (JP, A) JP-A-56-23531 (JP, A) JP-A-53-48915 (JP, U) Patent 2728744 (JP, B2) JP-B-7-33788 (JP, B2) (58) Fields investigated (Int. . ⁶ , DB name) F02D 41/14

Claims (1)

(57) [Claims] An air-fuel ratio sensor disposed in an exhaust system of an engine, reference signal forming means for forming a reference signal, and an output signal of the air-fuel ratio sensor and a reference signal from the reference signal forming means are compared. Comparison means for outputting a deviation signal based on the deviation, integration means for integrating the deviation signal by a predetermined integration characteristic, operation state detection means for detecting an operation state of the engine, and an output signal of the integration means Receiving the output signal of the operating state detecting means, and inverting the output of the integrating means when a predetermined time elapses according to the operating state after inverting the integrating direction of the integrating means, and then inverting the integrating direction. An integral output correction means for correcting so as to hold the signal until a predetermined period is reached, and a cycle shorter than a cycle in which the deviation signal is inverted with respect to the integrated output signal level corrected by the integral output corrector. Vibration signal generation means for generating a vibration signal having an amplitude; and air-fuel ratio control means for feedback-controlling the air-fuel ratio of the engine based on an addition signal of a signal obtained by proportionally processing the deviation signal and the vibration signal. An exhaust purification device characterized by the above-mentioned.