JP2020139492A - Exhaust gas purification system - Google Patents

Abstract

To suppress exhaust emission which is generated at the detection of an occlusion amount of catalyst oxygen.SOLUTION: An exhaust gas purification system comprises; an exhaust emission purification catalyst; an air-fuel ratio sensor disposed at a downstream side of the exhaust emission purification catalyst; air-fuel ratio active control means which is constructed so as to perform active control; voltage control means which is constructed so as to be variably settable in a sensor application voltage applied to the air-fuel ratio sensor, and also so as to alternately switch the sensor application voltage between a first voltage and a second voltage at the active control; and voltage control means which is constructed so as to correct the first voltage to a high-voltage side when a rich output value of the air-fuel ratio sensor which is detected according to the air-fuel ratio active control means exceeds a preset rich-side threshold to a rich side, and also so as to switch the second voltage to a low-side voltage when a lean output value of the air-fuel ratio sensor which is detected according to the air-fuel ratio active control means exceeds a preset lean-side threshold to a lean side.SELECTED DRAWING: Figure 12

Description

The present invention relates to an exhaust gas purification system.

Conventionally, as described in, for example, Japanese Patent No. 5949957, a technique for detecting the oxygen occlusion amount of a three-way catalyst is known. So-called active control is performed to detect the amount of oxygen stored. The active control is a control in which the occluded oxygen is released from the three-way catalyst or the oxygen is stored in the three-way catalyst by alternately switching the air-fuel ratio between rich and lean.

The operation of switching the air-fuel ratio and releasing and storing oxygen is detected by an air-fuel ratio sensor provided downstream of the three-way catalyst. The air-fuel ratio sensor has a specific applied voltage such that the output current is zero. The above patent publication describes a device constructed so as to alternately apply a voltage having a lean air-fuel ratio and a voltage having a rich air-fuel ratio as the voltage applied to the air-fuel ratio sensor.

Japanese Patent No. 5949957

In the above-mentioned conventional technique, when the state of the catalyst and the variation of the air-fuel ratio sensor are taken into consideration, there is still room for improvement from the viewpoint of suppressing the exhaust emission during the execution of the control for detecting the oxygen storage amount.

The present invention has been made to solve the above-mentioned problems, and an object of the present invention is to provide an exhaust gas purification system capable of suppressing exhaust emissions generated when detecting an occluded amount of catalytic oxygen. To do.

The exhaust gas purification system according to the present invention is
An exhaust purification catalyst installed in the exhaust passage of an internal combustion engine and having an oxygen storage capacity,
An air-fuel ratio sensor provided downstream of the exhaust gas purification catalyst and
An air-fuel ratio active control means constructed so as to carry out active control for alternately switching the air-fuel ratio of the exhaust gas flowing into the exhaust gas purification catalyst between a predetermined lean air-fuel ratio and a predetermined rich air-fuel ratio.
The voltage applied to the sensor applied to the air-fuel ratio sensor is constructed so as to be variable, and is set according to the switching to the predetermined lean air-fuel ratio and the predetermined rich air-fuel ratio. A voltage control means constructed to alternately switch the voltage applied to the sensor between a second voltage lower than the first voltage and a second voltage.
The rich output value of the air-fuel ratio sensor detected in response to the air-fuel ratio control from the predetermined lean air-fuel ratio to the predetermined rich air-fuel ratio by the air-fuel ratio active control means enriches the predetermined rich side threshold value. If it exceeds the side, the first voltage is corrected to the high voltage side, and the air-fuel ratio control from the predetermined rich air-fuel ratio to the predetermined lean air-fuel ratio is performed by the air-fuel ratio active control means. When the lean output value of the air-fuel ratio sensor detected above exceeds a predetermined lean side threshold on the lean side, the voltage control means is constructed so as to correct the second voltage to the low voltage side. ,
To be equipped.

According to the present invention, the voltage applied to the air-fuel ratio sensor can be updated so as to increase the detection sensitivity of the air-fuel ratio sensor based on the output value of the air-fuel ratio sensor according to the previous applied voltage. As a result, the lean air-fuel ratio and the rich air-fuel ratio can be detected with high sensitivity, so that exhaust emissions can be reduced.

It is a figure which shows the structure of the exhaust gas purification system which concerns on embodiment, and the air-fuel ratio sensor used for this. It is a figure for demonstrating the calculation principle of the Cmax method. It is a figure which shows the purification property of a three-way catalyst. It is a figure for demonstrating the effect of OSC in purifying engine exhaust gas. It is a figure for demonstrating the movement of gas in an air-fuel ratio sensor according to three kinds of air-fuel ratio states. It is a graph which shows the relationship between the applied voltage of the downstream air-fuel ratio sensor, and the oxygen concentration on the exhaust side electrode at zero current. It is a graph which shows the correlation of the actual air-fuel ratio with respect to the applied voltage of an air-fuel ratio sensor, and the output current of an air-fuel ratio sensor. It is a figure for demonstrating a technical approach for early detection of a deviation from a catalyst from a stove, and its effect, and is a figure assuming a state in which the downstream of a catalyst switches from stoichiometric to rich. It is a timing chart for demonstrating the operation of the catalyst active OBD which concerns on a comparative example. It is a figure for demonstrating the operation based on the 1st feature control of the exhaust gas purification system which concerns on embodiment. It is a figure for demonstrating the operation based on the 2nd feature control of the exhaust gas purification system which concerns on embodiment. It is a figure for demonstrating the operation based on the 3rd feature control of the exhaust gas purification system which concerns on embodiment. It is a flowchart of the control routine executed by the exhaust gas purification system which concerns on embodiment. It is a flowchart of the control routine executed by the exhaust gas purification system which concerns on embodiment.

The outline of the embodiment will be described. The so-called Cmax method is used as a method for determining the deterioration of the three-way catalyst that purifies the exhaust gas of the engine. The Cmax method is a method in which exhaust sensors are provided upstream and downstream of the exhaust purification catalyst, and the oxygen storage amount (OSA) of the exhaust purification catalyst is calculated by changing the air-fuel ratio supplied to the engine.

In this catalyst deterioration determination method, when the deterioration determination is performed, the exhaust gas sensor downstream of the catalyst detects the behavior of changing from rich to lean or the behavior of changing from lean to rich, and switches the air-fuel ratio of the gas flowing into the exhaust purification catalyst. Such an operation has a problem of lowering the emission characteristics. In order to solve the problem of deterioration of emission characteristics in the Cmax method, the embodiments described below are provided.

The system configuration of the internal combustion engine system and the configuration of the air-fuel ratio sensor according to the embodiment may be the same as those described in, for example, Japanese Patent Application Laid-Open No. 2014-145308. Japanese Unexamined Patent Publication No. 2014-145308 describes an internal combustion engine system in which air-fuel ratio sensors are provided upstream and downstream of the three-way catalyst, and a one-cell limit current type air-fuel ratio sensor.

The configuration of the internal combustion engine system may be, for example, an exhaust system in which air-fuel ratio sensors are provided upstream and downstream of the three-way catalyst, as described in International Publication No. 2014/118892.

<Internal combustion engine system configuration>
The internal combustion engine according to the embodiment is a spark-ignition type internal combustion engine having a plurality of cylinders. As an example, gasoline having a stoichiometric air-fuel ratio of 14.6 is used as the fuel used. The exhaust port of each cylinder of the internal combustion engine is connected to the exhaust manifold. The exhaust manifold has a plurality of branches connected to each exhaust port and an aggregate portion in which these branches are aggregated.

The collecting portion of the exhaust manifold is connected to the upstream casing containing the upstream exhaust purification catalyst. The upstream casing is connected to the downstream casing containing the downstream exhaust purification catalyst via an exhaust pipe. The exhaust port, exhaust manifold, upstream casing, exhaust pipe and downstream casing form an exhaust passage.

An "upstream air-fuel ratio sensor" is arranged at the gathering part of the exhaust manifold of the internal combustion engine. The upstream air-fuel ratio sensor detects the air-fuel ratio of the exhaust gas flowing in the exhaust manifold. The exhaust gas flowing in the exhaust manifold is, that is, the exhaust gas flowing into the upstream exhaust purification catalyst.

A "downstream air-fuel ratio sensor" is arranged in the exhaust pipe. The downstream air-fuel ratio sensor detects the air-fuel ratio of the exhaust gas flowing in the exhaust pipe. The exhaust gas flowing in the exhaust pipe is, that is, the exhaust gas that flows out from the upstream exhaust purification catalyst and flows into the downstream exhaust purification catalyst.

The output of the upstream air-fuel ratio sensor and the output of the downstream air-fuel ratio sensor are input to the input port of the electronic control unit (ECU) 31 via the corresponding AD converters.

<Structure of air-fuel ratio sensor and its peripheral circuits>
FIG. 1 is a diagram showing a configuration of an exhaust gas purification system according to an embodiment and a downstream air-fuel ratio sensor 41 used therein. With reference to FIG. 1, the configuration of the downstream air-fuel ratio sensor 41 (hereinafter, also simply referred to as “air-fuel ratio sensor 41”) to which the present embodiment is applied will be described. The upstream air-fuel ratio sensor may have the same structure as the air-fuel ratio sensor 41.

FIG. 1 is a schematic cross-sectional view of the air-fuel ratio sensor 41. As can be seen from FIG. 1, the air-fuel ratio sensor 41 in the present embodiment is a one-cell type air-fuel ratio sensor having one cell composed of a solid electrolyte layer and a pair of electrodes.

As shown in FIG. 1, the air-fuel ratio sensor 41 includes a solid electrolyte layer 51, an exhaust side electrode (first electrode) 52 arranged on one side surface of the solid electrolyte layer 51, and the other of the solid electrolyte layer 51. An atmospheric side electrode (second electrode) 53 arranged on the side surface of the above, a diffusion rate-determining layer 54 for diffusing and controlling the exhaust gas passing through, a protective layer 55 for protecting the diffusion rate-determining layer 54, and an air-fuel ratio sensor 41. A heater unit 56 for heating is provided.

A diffusion-controlled layer 54 is provided on one side surface of the solid electrolyte layer 51, and a protective layer 55 is provided on the side surface of the diffusion-controlled layer 54 opposite to the side surface of the solid electrolyte layer 51. In the present embodiment, the gas chamber 57 to be measured is formed between the solid electrolyte layer 51 and the diffusion-controlled layer 54. The gas to be detected by the air-fuel ratio sensor 41, that is, the exhaust gas, is introduced into the gas chamber 57 to be measured via the diffusion-controlled layer 54.

Further, the exhaust side electrode 52 is arranged in the gas chamber 57 to be measured, and therefore the exhaust side electrode 52 is exposed to the exhaust gas via the diffusion rate-determining layer 54. The gas chamber 57 to be measured does not necessarily have to be provided, and the diffusion-controlled layer 54 may be configured to be in direct contact with the surface of the exhaust side electrode 52.

A heater portion 56 is provided on the other side surface of the solid electrolyte layer 51. A reference gas chamber 58 is formed between the solid electrolyte layer 51 and the heater portion 56, and the reference gas is introduced into the reference gas chamber 58. In the present embodiment, the reference gas chamber 58 is open to the atmosphere, so that the atmosphere is introduced into the reference gas chamber 58 as the reference gas. The atmospheric side electrode 53 is arranged in the reference gas chamber 58, and therefore the atmospheric side electrode 53 is exposed to the reference gas (reference atmosphere). In the present embodiment, since the atmosphere is used as the reference gas, the atmosphere side electrode 53 is exposed to the atmosphere.

A plurality of heaters 59 are provided in the heater unit 56, and the temperature of the air-fuel ratio sensor 41, particularly the temperature of the solid electrolyte layer 51, can be controlled by these heaters 59. The heater unit 56 has a heat generation capacity sufficient to heat the solid electrolyte layer 51 until it is activated.

The solid electrolyte layer 51, ZrO _{2 (zirconia), HfO 2, ThO 2,} Bi 2 O 3 or the like _{CaO, MgO, Y 2 O 3} , Yb 2 O 3 such as oxygen ion-conducting oxides dividends as stabilizer It is formed of a sintered body of. The diffusion-controlled layer 54 is formed of a porous sintered body of a heat-resistant inorganic substance such as alumina, magnesia, silica stone, spinel, and mullite. Further, the exhaust side electrode 52 and the atmosphere side electrode 53 are formed of a noble metal having high catalytic activity such as platinum.

Further, an applied voltage Vr is applied between the exhaust side electrode 52 and the atmosphere side electrode 53 by the voltage applying device 60 mounted on the ECU 31. In addition, the ECU 31 is provided with a current detection device 61 that detects the current (output current) flowing between the electrodes 52 and 53 via the solid electrolyte layer 51 when the applied voltage Vr is applied by the voltage application device 60. .. The current detected by the current detection device 61 is the output current of the air-fuel ratio sensor 41.

As described above, the constant voltage applied current detection circuit is constructed in which the applied voltage Vr is applied between the electrodes of the air-fuel ratio sensor 41 by the voltage applying device 60 and the current is detected by the current detecting device 61. The circuit configuration of the constant voltage applied current detection circuit is constructed so that the applied voltage Vr output by the voltage applying device 60 can be variably set by an analog circuit or microcomputer control. The applied voltage Vr can be controlled to a desired voltage by the ECU 31.

FIG. 2 is a diagram for explaining the calculation principle of the Cmax method. As an example, the Cmax method described in JP-A-5-106493 will be introduced.

The Cmax method makes an accurate determination of catalyst deterioration in consideration of the characteristics of the catalyst's O ₂ storage capacity. The first time measuring means measures the first time from the time when the air-fuel ratio control means changes the air-fuel ratio to the lean side until the output of the downstream O ₂ sensor (that is, the oxygen sensor) reverses from rich to lean. ..

At the same time, the _second time from the time when the air-fuel ratio is changed to the rich side until the output of the downstream O ₂ sensor is reversed from lean to rich is measured by the second time measuring means. The catalyst deterioration determining means determines the deterioration of the catalyst based on whether or not the average value of the first time and the second time becomes shorter than the predetermined time.

The contents of the air-fuel ratio active control, which is a Cmax calculation method, will be described. It is determined that the inversion of the sub O ₂ sensor output = the OSC limit of the catalyst is exceeded, and the target air-fuel ratio is switched at the sub O ₂ output inversion timing. Both the oxygen occlusal amount and the oxygen release amount are obtained, and the average value thereof is regarded as the maximum OSC value under the conditions and is referred to as OSCmax, that is, "Cmax".

OSA is the amount of oxygen stored (O ₂ Storage Amount). The OSA is the sum of the amount of inflow and outflow oxygen to the catalyst from 0 to the maximum, and can be calculated by the following formula (1).
OSA = 0.23 x ΔA / F x fuel injection amount ... (1)

In formula (1), 0.23 is the oxygen mass ratio. ΔA / F is a value obtained by subtracting the stoichiometric air-fuel ratio from the air-fuel ratio sensor output value. Cmax is the average value of osafall and osarise shown in FIG.

As a method of determining the deterioration of the three-way catalyst that purifies the exhaust gas of the engine, exhaust sensors are provided upstream and downstream of the catalyst, and the oxygen storage amount (OSA) of the catalyst is calculated by changing the air-fuel ratio supplied to the engine. Although so-called Cmax is used, it has the following problems.

The problem is that the catalytic exhaust gas emission characteristics when the catalyst active OBD is carried out are deteriorated. A case where this problem occurs is a case where an oxygen (O ₂ ) sensor is used downstream of the catalyst. Oxygen (O ₂ ) sensors do not show stoichiometric points like air-fuel ratio (A / F) sensors. Therefore, when an oxygen (O ₂ ) sensor is used downstream of the catalyst, it is necessary to detect a completely rich or lean state and switch the air-fuel ratio of the gas containing the catalyst. As a result, deterioration of emission characteristics becomes a problem. The "catalyst-containing gas" is a gas that flows into the exhaust gas purification catalyst.

When an air-fuel ratio sensor is used downstream of the catalyst as in a system using two air-fuel ratio sensors, the stoichiometric point can be detected with good responsiveness. Therefore, the gas containing the catalyst can be switched near the extreme castoiki point. However, even in the case of the air-fuel ratio sensor, since the gas containing the catalyst is switched after detecting the state of being rich or lean from the stoichiometric point, the deterioration of the emission characteristics becomes a problem. Further, there is a problem that the emission characteristics are further deteriorated due to the responsiveness variation of the A / F sensor.

FIG. 3 is a diagram showing the purification characteristics of the three-way catalyst. The purification characteristics and oxygen storage capacity of the three-way catalyst will be described. The three-way catalyst (TWC) is generally endowed with oxygen storage capacity (hereinafter referred to as OSC: Oxygen Storage Capacity), and absorbs A / F fluctuations of the catalyst-containing gas to improve the catalyst purification capacity. There is. In the example of FIG. 4, the stoichiometric air-fuel ratio exists near 14.6, and the purification rate (Conversion) shows around 100%.

FIG. 4 is a diagram for explaining the effect of OSC when purifying engine exhaust gas. The effect of OSC is shown in FIG. When the catalyst-filled gas A / F flowing into the upstream of the catalyst becomes rich, the stored oxygen is released (Oxygen discharged). On the contrary, when it becomes lean, oxygen occluded (Oxygen occluded) is performed. As a result, the inside of the catalyst can be maintained at the stoichiometric air-fuel ratio. However, for example, when the oxygen in the catalyst is saturated, the NOx purification characteristics as shown in FIG. 3 are exhibited, so that suppression of oxygen saturation in the catalyst is important for reducing NOx.

FIG. 5 is a diagram for explaining the movement of gas in the air-fuel ratio sensor according to the three types of air-fuel ratio states. FIG. 5 and the following description describe the basic drive circuit of the air-fuel ratio sensor described in the non-patent document (Ueno, Sasayama: T.IEE Japan, Vol.114-C, No 7/8, '94). It is based on the operating principle. The exhaust side electrode is coated with a porous gas diffusion rate-determining film, and oxygen around the exhaust electrode of the concentration cell is ionized by the catalytic ability of platinum and pumped to the atmosphere side.

To summarize the relationship between the air-fuel ratio and the movement of gas, first, in the lean range (Lean range) of FIG. 5, residual oxygen diffuses in the membrane, and this flows to the atmosphere side as ions in the concentration cell. At the stoichiometric point of FIG. 5, there are no unburned components or residual oxygen in the exhaust. Therefore, neither diffusion in the film nor movement of oxygen ions in the concentration cell occurs. In the rich range of FIG. 5, unburned components diffuse in the membrane and oxidize them, so that oxygen flows from the atmosphere side as ions.

FIG. 6 is a graph showing the relationship between the applied voltage of the downstream air-fuel ratio sensor 41 (also referred to as “rear A / F sensor 41”) and the oxygen concentration on the exhaust side electrode at zero current. According to the principle described in FIG. 3, the oxygen partial pressure (concentration) on the exhaust electrode side where the sensor output current becomes zero when the applied voltage Vr is applied to the air-fuel ratio sensor has the correlation as shown in FIG.

FIG. 7 is a graph showing the correlation between the actual air-fuel ratio and the output current of the air-fuel ratio sensor with respect to the applied voltage of the air-fuel ratio sensor. From the correlation of FIG. 6, the output current of the air-fuel ratio sensor when the applied voltage is increased (for example, 0.6V) and when the applied voltage is decreased (for example, 0.2V) with reference to 0.45V has a base characteristic (for example, 0.2V). It has the characteristics shown in FIG. 7 with respect to 0.45 V applied).

Since there is a tendency in FIG. 7, by switching the voltage Vr applied to the air-fuel ratio sensor, the sensitivity of the air-fuel ratio sensor can be adjusted especially with respect to the low oxygen concentration gas near the stoichiometric value detected by the air-fuel ratio sensor downstream of the three-way catalyst. It will be possible.

That is, the general air-fuel ratio sensor characteristic to which 0.45 V is applied shows stoichiometric (air-fuel ratio = 14.6) at zero sensor current. On the other hand, for example, an air-fuel ratio sensor to which 0.6 V is applied exhibits a minute richness as compared with a general air-fuel ratio sensor to which 0.45 V is applied when the sensor current is zero. Conversely, for example, an air-fuel ratio sensor to which 0.2 V is applied exhibits a minute lean state as compared with a general air-fuel ratio sensor to which 0.45 V is applied when the sensor current is zero.

In the embodiment, a novel catalytic active OBD method in an exhaust gas purification system is provided by utilizing the characteristics depending on the applied voltage Vr as described above.

FIG. 8 is a diagram for explaining a technical approach for early detection of deviation of the catalyst from stoichiometric and its effect, and is a diagram assuming a state in which the downstream of the catalyst switches from stoichiometric to rich. When the catalyst active OBD is executed and the catalyst-containing gas control target is rich (oxygen desorption state in the catalyst), the applied voltage Vr of the downstream air-fuel ratio sensor 41 is set to a low voltage value. When the control target of the gas containing the catalyst is lean (oxygen storage state in the catalyst), the applied voltage Vr of the downstream air-fuel ratio sensor 41 is set to a high voltage value.

As a result, by detecting the state of switching from stoichiometric to rich and lean with high response, the decrease in emissions during catalytic active OBD execution can be suppressed to the utmost as the first effect. Further, as a second effect, the influence of variation on the OSA due to the air-fuel ratio sensor response delay is reduced, and the OBD detection accuracy is also improved. The outline of the catalyst active OBD is also illustrated in FIGS. 9 and 10 below.

FIG. 8 shows the reasons why the first effect and the second effect can be obtained. The effect of operating the applied voltage Vr of the sensor will be described. Even if an attempt is made to instantly detect a deviation from the stoichiometric value, the sensor tolerance and the sensor response delay (Ts) remain. Ts is the sum of the gas arrival time and the sensor reaction time.

On the other hand, the gas sensitivity of the downstream air-fuel ratio sensor 41 can be controlled by operating the applied voltage Vr. As a result, the response delay of the deviation from the stoichiometric can be improved to be substantially zero. The sensor output minus (rich) in FIG. 8 corresponds to the deviation determination point from the stoichiometric. The deviation determination point is generally determined from the sensor structure, circuit tolerance, and the like.

Here, as a first supplementary explanation, the reason for detecting and controlling the zero current in the above control will be described below. The first reason is that the error of the circuit or the like is small because the current does not flow at the zero point of the sensor current.

The second reason is that the current flowing through the air-fuel ratio sensor is set to zero when the air-fuel ratio sensor is cold when the engine is started, and a technique for performing learning control of circuit error in this state has been proposed. This is because the error at zero current can be made extremely small by combining with such a technique.

The third reason is that it is possible to control the slightly rich and slightly lean states that sandwich the stoichiometric. When the sensor current other than zero (that is, a predetermined A / F) in the air-fuel ratio sensor is targeted, the oxygen concentration changes greatly including an error, not the minimum change in oxygen concentration near the stoichiometric voltage that can be controlled by the applied voltage. A state in which the downstream of the catalyst is completely rich or lean is detected, and the emission of exhaust gas downstream of the catalyst cannot be suppressed.

Further, as a second supplementary explanation, a control target of the output of the downstream air-fuel ratio sensor 41 will be described. As described above, it is best to control the zero current of the downstream air-fuel ratio sensor 41 as a target, but in reality, there are minute errors in the performance of the air-fuel ratio sensor and peripheral circuits, so they are used. The output current when the output of the downstream air-fuel ratio sensor 41 is actively switched may be a positive current or a negative current in consideration of the error of.

FIG. 9 is a timing chart for explaining the operation of the catalyst active OBD according to the comparative example. In the comparative example, the applied voltage Vr for determining the stoichiometric point is constant, for example, 0.45V. The OSA is determined by integrating the amount of inflow and outflow of oxygen to the catalyst between zero and maximum.

In the comparative example, the state deviated from the stoichiometric state is detected at the time of the rich lean switching determination, and then the switching is performed. As a result, the downstream air-fuel ratio sensor 41 shows a large richness and leanness as shown by reference numerals Q1 and Q2 in FIG. As a result, the emission characteristics may be significantly reduced.

<Operation of the device of the embodiment>
In the comparative example of FIG. 9 above, the exhaust emission characteristics are deteriorated. The stoichiometric output value (for example, zero current) of the air-fuel ratio sensor is a reference point for calculating the catalyst OSA (oxygen storage amount) of the air-fuel ratio sensor downstream of the catalyst when the catalyst active OBD is executed.

Therefore, in the present embodiment, the gas sensitivity at this stoichiometric output value (for example, zero current) is manipulated by manipulating the voltage Vr applied to the air-fuel ratio sensor downstream of the catalyst. This provides an exhaust gas purification system that can quickly detect when the air-fuel ratio of the catalyst-containing gas changes from stoichiometric to rich or lean, and effectively reduce emissions when performing catalytic active OBD. To.

(First feature control)
FIG. 10 is a diagram for explaining an operation based on the first feature control of the exhaust gas purification system according to the embodiment. According to the first feature control of the embodiment, when the catalyst-containing gas becomes lean, the voltage Vr applied to the downstream air-fuel ratio sensor 41 is switched to a high voltage (see, for example, reference numeral Q5 in FIG. 10). This high voltage is a predetermined value, and may be 0.6 V or the like as an example.

Further, according to the first feature control of the embodiment, when the catalyst-containing gas becomes rich, the voltage Vr applied to the downstream air-fuel ratio sensor 41 is switched to a low voltage (see, for example, reference numeral Q6 in FIG. 10). .. This high voltage is a predetermined value, and may be 0.2 V or the like as an example. As described above, according to the first feature control of the embodiment, the ECU 31 controls the voltage application device 60 so that the applied voltage Vr alternately switches between the predetermined high voltage and the predetermined low voltage. ..

By controlling the gas sensitivity of the air-fuel ratio sensor by this voltage control, it is possible to quickly detect a state deviated from the stoichiometric state and switch the control content when switching between rich and lean. As a result, as shown by reference numerals Q3 and Q4 in FIG. 10, the amount of rich deviation and the amount of lean deviation from the stoichiometric can be significantly suppressed. Therefore, it is possible to suppress a decrease in emissions.

(Second feature control)
FIG. 11 is a diagram for explaining an operation based on the second feature control of the exhaust gas purification system according to the embodiment. When the catalyst active OBD is executed by the above-mentioned first feature control, the air-fuel ratio sensor output may fluctuate at the moment when the applied voltage Vr is switched. Explaining the factors that cause the output of the air-fuel ratio sensor to fluctuate, an electric circuit such as an air-fuel ratio sensor contains an R component (resistance component) and a C component (capacitive component). Due to this resistance component and capacitance component, when the applied voltage Vr is switched, the sensor output fluctuates due to the change in the air-fuel ratio sensor output current.

Therefore, in the second feature control according to the embodiment, the output of the downstream air-fuel ratio sensor 41 is held at the value before switching (that is, the output) until a predetermined time (for example, 500 ms) elapses after the applied voltage Vr is switched. Update is prohibited) (reference numeral Q7 in FIG. 11). At the same time, once the applied voltage Vr is switched, the re-switching of the applied voltage Vr is prohibited until a predetermined time elapses (reference numeral Q8 in FIG. 11).

After a predetermined time has elapsed after the applied voltage is switched, the output fluctuation of the downstream air-fuel ratio sensor 41 converges. The second feature control of the embodiment starts reading the output of the downstream air-fuel ratio sensor 41 after the convergence, and permits switching control of the applied voltage.

(Third feature control)
FIG. 12 is a diagram for explaining an operation based on the third feature control of the exhaust gas purification system according to the embodiment. There is room for deterioration of emission characteristics due to factors such as catalyst conditions and variations in air-fuel ratio sensor characteristics.

Therefore, did the output of the downstream air-fuel ratio sensor 41 deviate from the stoichiometric output value range including the tolerance after switching the catalyst-containing gas-controlled air-fuel ratio based on the output of the downstream air-fuel ratio sensor 41 during the execution of the catalyst active OBD? Detect whether or not. The stoichiometric output value range is the stoichiometric determination range shown by reference numeral Q11 in FIG. 12, and is a range determined by the stoichiometric output tolerance of the air-fuel ratio sensor.

When the deviation from the stoichiometric determination area Q11 is detected, the applied voltage Vr is corrected so as to reduce the emission on the side deviating from the stoichiometric output value at the next catalyst active execution. The correction of the applied voltage Vr is to increase or decrease the voltage value of the applied voltage Vr to the side where the stoichiometric deviation can be detected and determined more quickly.

Specifically, the third feature control can first realize the control operations shown by the reference numerals Q9 and Q12 in FIG. It is assumed that when the catalyst-containing gas is switched from lean to rich, the gas is so rich that it exceeds the stoichiometric determination region Q11 as shown by reference numeral Q9 in FIG. In this case, in order to suppress the emission emission, the high voltage value of the applied voltage Vr is increased and corrected to a higher voltage value at the time of the next gas lean control with a catalyst. As a result, as shown by reference numeral Q12 in FIG. 12, the corrected applied voltage Vr is set to a value higher than the previous applied voltage Vr.

Further, the third feature control can also realize the control operation shown by the reference numerals Q10 and Q13 in FIG. It is assumed that when the catalyst-containing gas is switched from rich to lean, the gas is so lean that it exceeds the stoichiometric determination region Q11 as shown by reference numeral Q10 in FIG. In this case, in order to suppress the emission emission, the low voltage value of the applied voltage Vr is reduced and corrected to a lower voltage value. As a result, as shown by reference numeral Q13 in FIG. 12, the corrected applied voltage Vr is set to a value lower than the previous applied voltage Vr.

According to the third feature control, the catalyst active OBD can be executed while correcting the high voltage value and the low voltage value as described above. FIG. 12 shows the state of the catalyst active OBD when the third feature control is executed.

The changed applied voltage Vr value may be stored in the ECU 31 as a learning value for each predetermined operating region or the like. As a result, it is possible to immediately suppress the deterioration of emissions by reflecting the learning value at the start of the next catalyst active OBD execution.

<Specific control of the embodiment>
13 and 14 are flowcharts of the control routine executed in the exhaust gas purification system according to the embodiment. The routines of FIGS. 13 and 14 are executed by the ECU 31 to perform a catalytic active OBD.

In the routine of FIG. 13, it is first determined whether or not the sensor has failed after the engine has started (step S100). If the engine has been started or a sensor failure has been found, the determination result in step S100 is negative (NO), and the current routine ends.

If the determination result in step S100 is affirmative (YES), then the control of the downstream air-fuel ratio sensor 41 is executed (step S101). That is, heater control or the like is performed to drive the air-fuel ratio sensor 41, and an applied voltage Vr of a normal A / F control value (for example, 0.45 V or the like) is applied to the air-fuel ratio sensor 41.

Next, it is determined whether or not the execution condition of the catalyst active OBD is satisfied (step S102). Specific examples of this execution condition include a first condition of whether or not the engine and the exhaust gas purification catalyst have been warmed up, a second condition of whether or not the catalyst temperature is within a predetermined temperature range, and an air-fuel ratio feedback. It includes a third condition of whether or not control is in progress, a fourth condition of whether or not feedback is being cut, and a fifth condition of whether or not the air-fuel ratio sensor 41 is in an active state.

For example, when all of the above first to fifth conditions are satisfied, the determination result in step S102 may be affirmative (YES). If at least one of the above plurality of conditions is unsuccessful, the determination result in step S102 is negated (NO), and the current routine ends.

If the determination result in step S102 is affirmative (YES), then the applied voltage learning value is read (step S103). In this step, the lean learning value VRAFSLEANG and the rich learning value VRAFSRICHG, which are the applied voltage learning values of the downstream air-fuel ratio sensor 41, are read respectively. The applied voltage learning value is read separately for each operating condition.

Here, the applied voltage Vr on the lean side is also referred to as an applied voltage VRAFSLEAN. The applied voltage Vr on the rich side is also referred to as an applied voltage VRAFSRICH. That is, the applied voltage Vr is a value possessed by a plurality of parameters having a lean side parameter and a rich side parameter.

The applied voltage VRAFSLEAN is set based on the lean learning value VRAFSLEANG. The lean side applied voltage VRAFSLEAN is a voltage applied to the downstream side air-fuel ratio sensor 41 when the catalyst-containing gas A / F is lean, that is, when measuring the oxygen storage amount.

The applied voltage VRAFSRICH is set based on the rich learning value VRAFSRICHG. The applied voltage VRAFSRICH is a voltage applied to the downstream air-fuel ratio sensor 41 when the catalyst-containing gas A / F is rich, that is, when measuring the amount of oxygen release (elimination).

Next, a process of switching the control A / F of the catalyst-containing gas to lean is executed (step S104). In this step, the control A / F of the catalyst-containing gas is switched to a predetermined lean A / F at the time of executing the catalyst active. This predetermined lean A / F may be, for example, 15.1.

Next, switching of the applied voltage Vr is performed (step S105). In this step, the output value AFSO of the downstream air-fuel ratio sensor 41 immediately before the applied voltage switching is performed is acquired. Based on the learned value VRAFSLEANG, the applied voltage VRAFSLEAN is set to, for example, 0.6V.

Next, it is determined whether or not a predetermined time has elapsed since the applied voltage Vr was switched (step S106). The predetermined time is a predetermined value, and may be, for example, 500 ms.

If it is determined in step S106 that the predetermined time has not elapsed, the process proceeds to step S108. In step S108, the output value of the downstream air-fuel ratio sensor 41 is fixed, and switching of the applied voltage Vr is prohibited. For example, a fixed output flag and a voltage switching prohibition flag are prepared, and in step S108, a process may be constructed such that these flags are turned on. As a result, the above-mentioned second feature control is realized. The process loops in steps S106 and S108 until the predetermined time elapses.

When it is determined in step S106 that the predetermined time has elapsed, next, the output update of the downstream air-fuel ratio sensor 41 is permitted, and the switching of the applied voltage Vr is also permitted (step S107). For example, the above-mentioned output fixing flag and voltage switching prohibition flag are turned off, and the output fixing and voltage switching prohibition are canceled.

Next, a process of updating the minimum value of the output of the downstream air-fuel ratio sensor 41 at any time is executed (step S109). In this step, after the applied voltage Vr of the downstream air-fuel ratio sensor 41 is switched, the minimum value of the output of the downstream air-fuel ratio sensor 41 from the time when the output update is permitted to the next switching of the applied voltage Vr. AFSOMN is acquired and will be updated from time to time.

Next, it is determined whether or not the output value of the downstream air-fuel ratio sensor 41 indicates a lean output (step S110). In this step, it is determined whether or not the air-fuel ratio deviates to the lean side from the stoichiometric determination region Q11 (see FIG. 12), which is the stoichiometric output value range including the tolerance. When the condition of step S110 is not satisfied, the process proceeds to step S111. As long as the condition of step S110 is not satisfied, the OSA integration in step S111 is continued.

If the determination result in step S110 is affirmative (YES), the integration of OSA is completed, and the integrated value is stored in the storage area of the ECU 31 as OSARISE (step S112).

Next, it is determined whether or not the minimum value AFSOMN of the output of the downstream air-fuel ratio sensor 41 acquired by updating at any time in step S109 shows a rich output (step S113). Specifically, it is determined whether or not the minimum value AFSOMN exceeds the rich side boundary value of the stoichiometric determination area Q11 on the rich side.

If the condition of step S113 is not satisfied, the process proceeds to step S115 of FIG.

When the condition of step S113 is satisfied, the current applied voltage Vr is corrected and stored as a learning value (step S114). That is, a process is executed in which the current applied voltage VRAFSRICH is corrected to the low voltage side according to the following equation (2) and then stored as the corrected learning value VRAFSRICHG. The predetermined value VRR0 in the formula (2) is a predetermined value, and may be, for example, 0.02V. After that, the process proceeds to step S115 of FIG.
VRAFSRICHG ← VRAFSRICH-VRR0 ... (2)

In step S115 of FIG. 14, the control A / F of the catalyst-containing gas is switched. In this step, the control A / F of the catalyst-containing gas is switched to the predetermined rich A / F at the time of executing the catalyst active. The predetermined rich A / F may be, for example, 14.1.

Next, the output of the downstream air-fuel ratio sensor 41 is acquired and the applied voltage Vr is switched (step S116). In this step, the output value AFSO of the downstream air-fuel ratio sensor 41 immediately before the switching of the applied voltage is performed is acquired. The applied voltage Vr is switched based on the learned value VRAFSTRICHG, and the initial value of the applied voltage VRAFSTRICH may be, for example, 0.2V.

Next, it is determined whether or not a predetermined time has elapsed since the applied voltage Vr was switched (step S117). The predetermined time is a predetermined value, and may be, for example, 500 ms.

If it is determined in step S117 that the predetermined time has not elapsed, the process proceeds to step S118. In step S118, the output value of the downstream air-fuel ratio sensor 41 is fixed (masked) as the value AFSO acquired in step S116, and its update is prohibited.

With the update prohibition, the switching of the applied voltage Vr is prohibited. For example, a fixed output flag and a voltage switching prohibition flag are prepared, and in step S118, these flags are turned on. As a result, the above-mentioned second feature control is realized. The process loops in steps S117 and S118 until the predetermined time elapses.

When it is determined in step S117 that the predetermined time has elapsed, next, the output update of the downstream air-fuel ratio sensor 41 is permitted, and the switching of the applied voltage Vr is also permitted (step S119). For example, the fixed output flag and the voltage switching prohibition flag illustrated in step S118 are turned off, and the fixed output and the voltage switching prohibition are canceled.

Next, a process of updating the maximum value of the output of the downstream air-fuel ratio sensor 41 at any time is executed (step S120). In this step, after the applied voltage Vr of the downstream air-fuel ratio sensor 41 is switched, the maximum value of the output of the downstream air-fuel ratio sensor 41 from the time when the output update is permitted to the next switching of the applied voltage Vr. AFSOMX is acquired and will be updated from time to time.

Next, it is determined whether or not the output value of the downstream air-fuel ratio sensor 41 shows a rich output (step S121). In this step, it is determined whether or not the air-fuel ratio detected by the downstream air-fuel ratio sensor 41 deviates from the stoichiometric determination region Q11 (see FIG. 12), which is the stoichiometric output value range including the tolerance, to the rich side. When the condition of step S121 is not satisfied, the process proceeds to step S122. As long as the condition of step S121 is not satisfied, the OSA integration in step S122 is continued.

If the determination result in step S121 is affirmative (YES), the OSA integration is completed, and the integrated value is stored as OSAFALL in the storage area of the ECU 31 (step S123).

Next, it is determined whether or not the maximum value AFSOMX of the output of the downstream air-fuel ratio sensor 41 acquired by updating at any time in step S120 indicates a lean output (step S124). Specifically, it is determined whether or not the maximum value AFSOMX exceeds the lean side boundary value of the stoichiometric determination area Q11 to the lean side.

If the condition of step S124 is not satisfied, the process proceeds to step S126.

When the maximum value AFSOMX deviates from the stoichiometric determination area Q11 to the lean side in step S124, the condition of step S124 is satisfied, so that the current applied voltage Vr is corrected, and this is used as the learning value. It is stored (step S125).

That is, a process is executed in which the current applied voltage VRAFSLEAN is corrected to the high voltage side according to the following equation (3) and then stored as the corrected learning value VRAFSLEANG. The predetermined value VRL0 of the formula (3) is a predetermined value, and may be, for example, 0.02V. After that, the process proceeds to step S126.
VRAFSLEANG ← VRAFSLEAN + VRL0 ・ ・ ・ (3)

In step S126, it is determined whether or not OSARISE and OSAFALL are calculated a predetermined number of times. The predetermined number of times is a predetermined value, and may be a condition that OSARISE and OSAFALL are calculated three times each.

If the determination result in step S126 is negative (NO), the process jumps to step S103 in FIG. If the determination result in step S126 is affirmative (YES), the process proceeds to step S127.

In step S127, the applied voltage is switched and the feedback control is started. That is, the applied voltage Vr of the downstream air-fuel ratio sensor 41 is switched to the applied voltage for normal A / F control (for example, 0.45 V). Next, the sensor output is masked for a predetermined time, and the sub F / B control is stopped. Feedback control is started after a lapse of a predetermined time.

Next, the Cmax value is calculated (step S128). Specifically, the Cmax value is calculated from the average value of OSARISE and OSAFALL calculated by the remaining two active controls excluding the first value.

Next, the failure determination of the catalyst is carried out from the Cmax value (step S129). After that, this routine ends.

31 Electronic control unit (ECU)
41 Air-fuel ratio sensor (downstream air-fuel ratio sensor)
51 Solid electrolyte layer 52 Exhaust side electrode 53 Atmosphere side electrode 54 Diffusion rate control layer 55 Protective layer 56 Heater part 57 Measured gas chamber 58 Reference gas chamber 59 Heater 60 Voltage application device 61 Current detector Q11 Stoiki judgment area Vr Applied voltage VRAFSLEAN Voltage (lean)
VRAFSRICH applied voltage (rich)
VRAFSLEANG lean learning value VRAFSRICHG rich learning value

Claims (1)

An exhaust purification catalyst installed in the exhaust passage of an internal combustion engine and having an oxygen storage capacity,
An air-fuel ratio sensor provided downstream of the exhaust gas purification catalyst and
An air-fuel ratio active control means constructed so as to carry out active control in which the air-fuel ratio of the exhaust gas flowing into the exhaust gas purification catalyst is alternately switched between a predetermined lean air-fuel ratio and a predetermined rich air-fuel ratio.
The voltage applied to the sensor applied to the air-fuel ratio sensor is variably constructed so as to be set according to the switching to the predetermined lean air-fuel ratio and the first voltage set according to the switching to the predetermined rich air-fuel ratio. A voltage control means constructed so as to alternately switch the voltage applied to the sensor between a second voltage lower than the first voltage and a second voltage.
The rich output value of the air-fuel ratio sensor detected in response to the air-fuel ratio control from the predetermined lean air-fuel ratio to the predetermined rich air-fuel ratio by the air-fuel ratio active control means enriches the predetermined rich side threshold value. If it exceeds the side, the first voltage is corrected to the high voltage side, and the air-fuel ratio control from the predetermined rich air-fuel ratio to the predetermined lean air-fuel ratio is performed by the air-fuel ratio active control means. When the lean output value of the air-fuel ratio sensor detected above exceeds a predetermined lean side threshold on the lean side, the voltage control means is constructed so as to correct the second voltage to the low voltage side. ,
Exhaust gas purification system equipped with.

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