JP4665858B2 - Catalyst deterioration detection device for internal combustion engine - Google Patents

Description

  The present invention relates to a catalyst deterioration detection device that detects deterioration of a catalyst disposed in an exhaust passage of an internal combustion engine.

Generally, in an internal combustion engine, a catalyst is disposed in an exhaust passage in order to purify exhaust gas. Such a catalyst, for example, a three-way catalyst, adsorbs and holds excess oxygen present in the exhaust gas when the air-fuel ratio of the exhaust gas flowing into the catalyst becomes larger than the stoichiometric air-fuel ratio, that is, when the air-fuel ratio becomes lean. When the air-fuel ratio of the gas becomes smaller than the stoichiometric air-fuel ratio, that is, when the air-fuel ratio becomes rich, it has an O ₂ storage function for releasing the adsorbed oxygen. Therefore, during normal operation of the internal combustion engine, if the air-fuel mixture is alternately swung to the rich side or the lean side centering on the stoichiometric air-fuel ratio, when the air-fuel mixture becomes lean due to the O ₂ storage function of the three-way catalyst, excess NOx is reduced because oxygen is adsorbed and held by the catalyst, and when the air-fuel mixture becomes rich, oxygen adsorbed and held by the catalyst is released, so that HC and CO are oxidized, whereby NOx, HC and CO can be purified at the same time.

  Therefore, an air-fuel ratio sensor for detecting the exhaust air-fuel ratio is disposed in the exhaust passage upstream of the catalyst, and when the exhaust air-fuel ratio becomes lean, the fuel supply amount is increased, and when the exhaust air-fuel ratio becomes rich By reducing the fuel supply amount, the air-fuel ratio is alternately shifted to the rich side or the lean side around the theoretical air-fuel ratio, thereby NOx, HC and CO are simultaneously reduced.

By the way, when the three-way catalyst deteriorates, the exhaust gas purification rate decreases. There is a correlation between the degree of deterioration of the three-way catalyst and the degree of deterioration of the O ₂ storage function. Therefore, it is possible to detect that the catalyst has deteriorated by detecting that the O ₂ storage function has deteriorated.

  As an apparatus for detecting catalyst deterioration based on such a principle, for example, there is one disclosed in Patent Document 1. In this apparatus, a first air-fuel ratio sensor is disposed in the engine exhaust passage upstream of the three-way catalyst, and a second air-fuel ratio sensor is disposed in the engine exhaust passage downstream of the three-way catalyst. In addition, air-fuel ratio switching means for switching the air-fuel ratio upstream of the catalyst from the lean air-fuel ratio to the rich air-fuel ratio with respect to the stoichiometric air-fuel ratio or vice versa is provided. When the air-fuel ratio of the air-fuel mixture supplied into the engine cylinder is changed from the lean air-fuel ratio A / Fl to the rich air-fuel ratio A / Fr, the air-fuel ratio detected by the second air-fuel ratio sensor is equal to the theoretical air for a certain time ΔTr. After maintaining the fuel ratio, the air-fuel ratio changes to the rich air-fuel ratio A / Fr. The absolute amount of oxygen adsorbed and held by the three-way catalyst is determined from the product of the deviation ΔA / Fr of the rich air-fuel ratio A / Fr with respect to the stoichiometric air-fuel ratio, the time ΔTr, and the intake air amount. The degree of deterioration of the catalyst is detected.

JP-A-5-133264

  By the way, according to the results of a test study by the present inventor, when the amount of air sucked into the internal combustion engine is relatively large, a value smaller than the true value is calculated as the oxygen storage capacity of the catalyst. It has been found that there is a possibility that a false detection may occur in the deterioration detection.

  Therefore, the present invention has been made in view of such circumstances, and an object of the present invention is to detect a catalyst deterioration in an internal combustion engine that can prevent erroneous detection when the amount of air taken into the internal combustion engine is relatively large. Is to provide.

In order to achieve the above object, the first invention provides:
An apparatus for detecting deterioration of a catalyst disposed in an exhaust passage of an internal combustion engine,
The exhaust air-fuel ratio on the upstream side of the catalyst is selected from one of a rich air-fuel ratio that is rich from a predetermined center air-fuel ratio by a predetermined rich amplitude and a lean air-fuel ratio that is lean from the central air-fuel ratio by a predetermined lean amplitude. On the other hand, active air-fuel ratio control means for forcibly switching at a predetermined timing,
An intake air amount detection means for detecting the amount of air sucked into the internal combustion engine,
The active air-fuel ratio control means increases the rich amplitude when the intake air amount detected by the intake air amount detection means is large than when the intake air amount is small.

The second invention is the first invention, wherein
The active air-fuel ratio control means increases the lean amplitude when the intake air quantity detected by the intake air quantity detection means is large, compared with when the intake air quantity is small, and has the same intake air quantity The rich amplitude and the lean amplitude with respect to are made equal to each other, or the former is made larger than the latter.

The third invention is the first or second invention, wherein
The active air-fuel ratio control means maximizes the rich amplitude when the exhaust air-fuel ratio upstream of the catalyst is switched to the rich air-fuel ratio, and then gradually decreases the rich amplitude.

Further, a fourth invention is any one of the first to third inventions,
A pre-catalyst air-fuel ratio detecting means for detecting an exhaust air-fuel ratio upstream of the catalyst;
The active air-fuel ratio control means controls the exhaust air-fuel ratio so that the exhaust air-fuel ratio detected by the pre-catalyst air-fuel ratio detection means matches a predetermined target value, and the rich air-fuel ratio is controlled. The target value is calculated by subtracting a predetermined offset value from a predetermined reference value,
The offset value is a value set in advance so as to increase as the intake air amount detected by the intake air amount detection means increases.

The fifth light is any one of the first to third inventions.
A pre-catalyst air-fuel ratio detecting means for detecting an exhaust air-fuel ratio upstream of the catalyst;
The active air-fuel ratio control means controls the exhaust air-fuel ratio so that the exhaust air-fuel ratio detected by the pre-catalyst air-fuel ratio detection means matches a predetermined target value, and the rich air-fuel ratio is controlled. A target value is calculated by multiplying a predetermined reference value by a predetermined coefficient,
The coefficient is a value set in advance so as to become a smaller value than 1 as the intake air amount detected by the intake air amount detecting means is larger.

Also, a sixth invention is any one of the first to third inventions,
The active air-fuel ratio control means sets the rich amplitude to a predetermined reference value when the intake air quantity detected by the intake air quantity detection means does not exceed a predetermined threshold value, and the intake air quantity is set to a predetermined value. When the threshold value is exceeded, the rich amplitude is set to a value larger than the reference value.

Also, a seventh invention is any one of the first to third inventions,
The active air-fuel ratio control means responds to the intake air amount using a map or function set in advance so that the rich amplitude increases as the intake air amount detected by the intake air amount detection means increases. The rich amplitude is determined.

Further, an eighth invention is any one of the first to seventh inventions,
When the active air-fuel ratio control means switches the exhaust air-fuel ratio to the rich air-fuel ratio, it further comprises torque fluctuation suppression control means for executing predetermined torque fluctuation suppression control when the rich amplitude is larger than a predetermined threshold value. It is characterized by that.

  According to the present invention, it is possible to provide a catalyst deterioration detection device for an internal combustion engine that can prevent erroneous detection when the amount of air taken into the internal combustion engine is relatively large. .

  The best mode for carrying out the present invention will be described below with reference to the accompanying drawings.

  FIG. 1 is a schematic diagram showing the configuration of the present embodiment. As shown in the figure, the internal combustion engine 1 generates power by burning a mixture of fuel and air inside a combustion chamber 3 formed in a cylinder block 2 and reciprocating a piston 4 in the combustion chamber 3. To do. The internal combustion engine 1 is a vehicular multi-cylinder engine (only one cylinder is shown), and is a spark ignition type internal combustion engine, more specifically, a gasoline engine.

  In the cylinder head of the internal combustion engine 1, an intake valve Vi for opening and closing the intake port and an exhaust valve Ve for opening and closing the exhaust port are provided for each cylinder. Each intake valve Vi and each exhaust valve Ve are opened and closed by a camshaft (not shown). A spark plug 7 for igniting the air-fuel mixture in the combustion chamber 3 is attached to the top of the cylinder head for each cylinder. Further, an injector (fuel injection valve) 12 is disposed in the cylinder head for each cylinder so that fuel is directly injected into the combustion chamber 3. The piston 4 is configured as a so-called deep dish top surface type, and a concave portion 4a is formed on the upper surface thereof. In the internal combustion engine 1, fuel is directly injected from the injector 12 toward the concave portion 4 a of the piston 4 in a state where air is sucked into the combustion chamber 3. As a result, a layer of a mixture of fuel and air is formed (stratified) in the vicinity of the spark plug 7 and separated from the surrounding air layer, and stable stratified combustion is executed.

  The intake port of each cylinder is connected to a surge tank 8 serving as an intake air collecting chamber via a branch pipe for each cylinder. An intake pipe 13 that forms an intake manifold passage is connected to the upstream side of the surge tank 8, and an air cleaner 9 is provided at the upstream end of the intake pipe 13. An air flow meter 5 for detecting the intake air amount and an electronically controlled throttle valve 10 are incorporated in the intake pipe 13 in order from the upstream side. An intake passage is formed by the intake port, the surge tank 8 and the intake pipe 13.

On the other hand, the exhaust port of each cylinder is connected to an exhaust pipe 6 forming an exhaust collecting passage through a branch pipe for each cylinder, and a catalyst 11 made of a three-way catalyst having an O ₂ storage function is connected to the exhaust pipe 6. It is attached. An exhaust passage is formed by the exhaust port, the branch pipe, and the exhaust pipe 6. Pre-catalyst sensors and post-catalyst sensors 17 and 18 for detecting the exhaust air-fuel ratio are installed on the upstream side and the downstream side of the catalyst 11, respectively. The pre-catalyst sensor 17 is a so-called wide-area air-fuel ratio sensor, can continuously detect an air-fuel ratio over a relatively wide area, and outputs a current signal proportional to the air-fuel ratio. On the other hand, the post-catalyst sensor 18 is a so-called O ₂ sensor, and has a characteristic that the output voltage changes suddenly at the theoretical air-fuel ratio.

  The spark plug 7, the throttle valve 10, the injector 12, and the like described above are electrically connected to an electronic control unit (hereinafter referred to as ECU) 20 as control means. The ECU 20 includes a CPU, a ROM, a RAM, an input / output port, a storage device, and the like, all not shown. In addition to the air flow meter 5, the pre-catalyst sensor 17, and the post-catalyst sensor 18, the ECU 20 includes a crank angle sensor 14 that detects the crank angle of the internal combustion engine 1 and an accelerator that detects the accelerator opening, as shown in the figure. The opening sensor 15, the intake pressure sensor 16 for detecting the intake pressure, and other various sensors are electrically connected via an A / D converter or the like (not shown). The ECU 20 controls the ignition plug 7, the throttle valve 10, the injector 12, etc. so as to obtain a desired output based on the detection values of various sensors, etc., and the ignition timing, fuel injection amount, fuel injection timing, throttle opening. Control the degree etc. The throttle opening is normally controlled to an opening corresponding to the accelerator opening.

  The catalyst 11 simultaneously purifies NOx, HC and CO when the air-fuel ratio A / F of the exhaust gas flowing into the catalyst 11 is a stoichiometric air-fuel ratio (stoichiometric) A / Fs (for example, 14.6). Correspondingly, during normal operation of the internal combustion engine, the ECU 20 controls the air / fuel ratio so that the exhaust air / fuel ratio upstream of the catalyst, that is, the pre-catalyst air / fuel ratio A / Ffr becomes the stoichiometric air / fuel ratio A / Fs. Specifically, the ECU 20 sets a target air-fuel ratio A / Ft equal to the theoretical air-fuel ratio A / Fs, and the pre-catalyst air-fuel ratio A / Ffr detected by the pre-catalyst sensor 17 matches the target air-fuel ratio A / Ft. Thus, the fuel injection amount injected from the injector 12 is controlled. As a result, the air-fuel ratio of the exhaust gas flowing into the catalyst 11 is kept in the vicinity of the theoretical air-fuel ratio, and the maximum purification performance is exhibited in the catalyst 11.

Here, the catalyst 11 will be described in more detail. As shown in FIG. 2, in the catalyst 11, a coating material 31 is coated on the surface of a carrier base material (not shown), and the coating material 31 is held in a state in which a large number of particulate catalyst components 32 are dispersedly arranged. The catalyst 11 is exposed inside. The catalyst component 32 is mainly composed of a noble metal such as Pt or Pd, and serves as an active point for reacting exhaust gas components such as NOx, HC and CO. On the other hand, the coating material 31 plays the role of a promoter that promotes the reaction at the interface between the exhaust gas and the catalyst component 32 and includes an oxygen storage component capable of absorbing and releasing oxygen according to the air-fuel ratio of the atmospheric gas. Oxygen storage component, for example made of cerium dioxide CeO _2. For example, when the atmosphere gas of the catalyst component 32 and the coating material 31 is richer than the theoretical air-fuel ratio A / Fs, oxygen stored in the oxygen storage component existing around the catalyst component 32 is released, and as a result, release Unburned components such as HC and CO are oxidized and purified by the released oxygen. Conversely, if the atmosphere gas of the catalyst component 32 and the coating material 31 is leaner than the theoretical air-fuel ratio A / Fs, the oxygen storage component present around the catalyst component 32 absorbs oxygen from the atmosphere gas, and as a result, NOx is reduced. Reduced and purified.

  Even if the pre-catalyst air-fuel ratio A / F varies somewhat with respect to the theoretical air-fuel ratio A / Fs during the normal air-fuel ratio control, the three exhaust gas components such as NOx, HC and CO can be obtained. Can be purified simultaneously. Therefore, in normal air-fuel ratio control, it is also possible to purify exhaust gas by causing the pre-catalyst air-fuel ratio A / Ff to oscillate slightly around the stoichiometric air-fuel ratio A / Fs and repeat the absorption and release of oxygen.

  By the way, in the catalyst 11 in the new state, as described above, a large number of fine particulate catalyst components 32 are uniformly distributed, and the contact probability between the exhaust gas and the catalyst component 32 is kept high. However, when the catalyst 11 deteriorates, some of the catalyst components 32 are lost, and some of the catalyst components 32 are baked and solidified by exhaust heat (see broken lines in the figure). In this case, the contact probability between the exhaust gas and the catalyst component 32 is lowered, and the purification rate is lowered. In addition to this, the amount of the coating material 31 existing around the catalyst component 32, that is, the amount of the oxygen storage component decreases, and the oxygen storage capacity itself decreases.

Thus, the degree of deterioration of the catalyst 11 and the degree of decrease in the oxygen storage capacity of the catalyst 11 are in a correlation. Therefore, in this embodiment, the degree of deterioration of the catalyst 11 is detected or determined by detecting the oxygen storage capacity of the catalyst 11. Here, the oxygen storage capacity of the catalyst 11 is represented by the amount of oxygen storage capacity (OSC; O ₂ Strage Capacity, the unit is g), which is the amount of oxygen that the catalyst 11 can store.

  Hereinafter, detection of deterioration of the catalyst in the present embodiment will be described.

  In the present embodiment, active air-fuel ratio control by the ECU 20 is executed when the deterioration of the catalyst 11 is detected. Here, the active air-fuel ratio control means that the pre-catalyst air-fuel ratio A / Ffr that is the exhaust air-fuel ratio upstream of the catalyst is changed from one of the predetermined rich air-fuel ratio A / Fr and lean air-fuel ratio A / Fl to a predetermined value. This control is forcibly switched at the timing.

  Here, the deterioration detection of the catalyst 11 is executed when the internal combustion engine 1 is in a steady operation and when the catalyst 11 is in a predetermined activation temperature range. The temperature of the catalyst 11 may be detected directly, but in the present embodiment, it is estimated using a predetermined map or function based on the engine operating state. The detection of the deterioration of the catalyst 11 is performed once for each operation of the engine, and the warning device is activated when it is determined that the catalyst 11 is in a deteriorated state at least twice.

  3A and 3B, the outputs of the pre-catalyst sensor 17 and the post-catalyst sensor 18 when the active air-fuel ratio control is executed are indicated by solid lines, respectively. In FIG. 3A, the target air-fuel ratio A / Ft generated inside the ECU 20 is indicated by a broken line. The outputs of the pre-catalyst sensor 17 and the post-catalyst sensor 18 represent the pre-catalyst air / fuel ratio A / Ffr and the post-catalyst air / fuel ratio A / Frr, respectively.

  As shown in FIG. 3A, the target air-fuel ratio A / Ft is centered on the theoretical air-fuel ratio A / Fs as the center air-fuel ratio, and then has a predetermined amplitude (rich amplitude Ar, Ar> 0) on the rich side. ) Separated by an air-fuel ratio (rich air-fuel ratio A / Fr) and an air-fuel ratio (lean air-fuel ratio A / Fl) separated from the air-fuel ratio by a predetermined amplitude (lean amplitude Al, Al> 0) on the lean side. And alternately. Then, the pre-catalyst air-fuel ratio A / Ffr as an actual value is switched with a slight time delay with respect to the target air-fuel ratio A / Ft so as to follow the switching or vibration of the target air-fuel ratio A / Ft. Accordingly, the pre-catalyst air-fuel ratio A / Ffr is also forcibly and alternately switched between the rich air-fuel ratio A / Fr and the lean air-fuel ratio A / Fl in the same manner as the target air-fuel ratio A / Ft. From this, it will be understood that the target air-fuel ratio A / Ft and the pre-catalyst air-fuel ratio A / Ffr are equivalent except that there is a time delay.

  In the illustrated example, the rich amplitude Ar and the lean amplitude Al are equal. For example, theoretical air fuel ratio A / Fs = 14.6, rich air fuel ratio A / Fr = 14.1, lean air fuel ratio A / Fl = 15.1, rich amplitude Ar = lean amplitude Al = 0.5. Compared with the normal air-fuel ratio control, the active air-fuel ratio control has a larger amplitude of the air-fuel ratio, that is, the values of the rich amplitude Ar and the lean amplitude Al are larger.

  By the way, the timing at which the target air-fuel ratio A / Ft is switched is the timing at which the output of the post-catalyst sensor 18 is switched from rich to lean, or from lean to rich. As shown in the figure, the output voltage of the post-catalyst sensor 18 changes suddenly at the theoretical air-fuel ratio A / Fs, and the post-catalyst air-fuel ratio A / Frr is the rich air-fuel ratio smaller than the theoretical air-fuel ratio A / Fs. When the output voltage becomes equal to or higher than the rich determination value VR, and when the post-catalyst air-fuel ratio A / Frr is the lean air-fuel ratio greater than the theoretical air-fuel ratio A / Fs, the output voltage becomes lower than the lean determination value VL. Here, VR> VL, for example, VR = 0.59 (V) and VL = 0.21 (V).

  As shown in FIGS. 3A and 3B, when the output voltage of the post-catalyst sensor 18 changes from the rich value to the lean value and becomes equal to the lean determination value VL (time t1), the target sky The fuel ratio A / Ft is switched from the lean air-fuel ratio A / Fl to the rich air-fuel ratio A / Fr. Thereafter, when the output voltage of the post-catalyst sensor 18 changes from the lean value to the rich side and becomes equal to the rich determination value VR (time t2), the target air-fuel ratio A / Ft becomes lean from the rich air-fuel ratio A / Fr. The air-fuel ratio is switched to A / Fl.

  While performing the active air-fuel ratio control that performs such an air-fuel ratio change, the oxygen storage capacity OSC of the catalyst 11 is calculated as follows, and the deterioration of the catalyst 11 is detected or diagnosed.

  Referring to FIG. 3, the target air-fuel ratio A / Ft is set to the lean air-fuel ratio A / Fl before time t1, and the lean gas flows into the catalyst 11. At this time, the catalyst 11 continues to absorb oxygen, but when it fully absorbs oxygen, it can no longer absorb oxygen, and the lean gas flows through the catalyst 11 and flows downstream of the catalyst 11. When this happens, the post-catalyst air-fuel ratio A / Frr changes to the lean side, and when the output voltage of the post-catalyst sensor 18 reaches the lean determination value VL (t1), the target air-fuel ratio A / Ft becomes the rich air-fuel ratio A / Fr. Or reversed. In this way, the target air-fuel ratio A / Ft is reversed using the output of the post-catalyst sensor 18 as a trigger.

  This time, rich gas flows into the catalyst 11. At this time, the oxygen stored in the catalyst 11 continues to be released from the catalyst 11. Therefore, the exhaust gas of the theoretical air-fuel ratio A / Fs flows out to the downstream side of the catalyst 11 and the post-catalyst air-fuel ratio A / Frr does not become rich, so the output of the post-catalyst sensor 18 is not reversed. When oxygen is continuously released from the catalyst 11, all of the stored oxygen is eventually released from the catalyst 11, and at that time, no more oxygen can be released, and the rich gas flows through the catalyst 11 and flows downstream of the catalyst 11. When this happens, the post-catalyst air-fuel ratio A / Frr changes to the rich side, and when the output voltage of the post-catalyst sensor 18 reaches the rich determination value VR (t2), the target air-fuel ratio A / Ft becomes the lean air-fuel ratio A / Fl. Can be switched to.

  The larger the oxygen storage capacity OSC, the longer the time during which oxygen can be absorbed or released. That is, when the catalyst is not deteriorated, the inversion cycle of the target air-fuel ratio A / Ft (for example, the time from t1 to t2) becomes longer, and the inversion cycle of the target air-fuel ratio A / Ft becomes shorter as the deterioration of the catalyst proceeds. .

  Therefore, using this fact, the oxygen storage capacity OSC is calculated as follows. As shown in FIG. 4, immediately after the target air-fuel ratio A / Ft is switched to the rich air-fuel ratio A / Fr at time t1, the pre-catalyst air-fuel ratio A / Ffr as the actual value is slightly delayed with the rich air-fuel ratio A / Fr. Switch to Fr. Then, from the time t11 when the pre-catalyst air-fuel ratio A / Ffr reaches the stoichiometric air-fuel ratio A / Fs to the time t2 when the target air-fuel ratio A / Ft next reverses, the oxygen storage capacity for every minute time is given by the following equation (1). dC is calculated, and the oxygen storage capacity dC for each minute time is integrated from time t11 to time t2. In this way, the oxygen storage capacity OSC1, that is, the amount of released oxygen in this oxygen release cycle is calculated.

  Here, Q is the fuel injection amount, and the excess air amount can be calculated by multiplying the air-fuel ratio difference ΔA / F by the fuel injection amount Q. K is the proportion of oxygen contained in the air (about 0.23).

  Basically, the oxygen storage capacity OSC1 calculated once is used and compared with a predetermined threshold value (catalyst deterioration determination threshold value). If the oxygen storage capacity OSC1 exceeds the threshold value, For example, the deterioration of the catalyst can be determined such that the deterioration is normal and the oxygen storage capacity OSC1 is lower than the threshold value. However, in this embodiment, in order to improve the accuracy, the oxygen storage capacity (oxygen absorption amount in this case) is calculated on the lean side as well, and the calculation is repeated a plurality of times on the rich side and the lean side as necessary. The final deterioration judgment is performed by comparing the value with a threshold value.

  Specifically, as shown in FIG. 4, after the target air-fuel ratio A / Ft is switched to the lean air-fuel ratio A / Fl at time t2, the oxygen storage capacity dC for every minute time is calculated by the previous equation (1). And the oxygen storage capacity dC for each minute time from the time t21 when the pre-catalyst air-fuel ratio A / Ffr reaches the stoichiometric air-fuel ratio A / Fs, and then when the target air-fuel ratio A / Ft reverses to the rich side Integration is performed until t3. Thus, the oxygen storage capacity OSC2, that is, the amount of absorbed oxygen in this oxygen absorption cycle is calculated. The oxygen storage capacity OSC1 of the previous cycle and the oxygen storage capacity OSC2 of the current cycle should be approximately equal. Thus, a plurality of oxygen storage capacities OSC1, OSC2,... OSCn (for example, n is 5 or more) are repeatedly calculated, and the average value OCCSav is compared with a predetermined threshold value OSCs. If the average value OCCSav exceeds the threshold value OSCs, the catalyst 11 is determined to be normal, and if the average value OCCSav is equal to or less than the threshold value OSCs, the catalyst 11 is determined to be deteriorated.

  Note that the number of times n of calculation of the oxygen storage capacity OSC may be changed in accordance with a parameter that correlates with the progress of catalyst deterioration, such as the travel distance of the vehicle. For example, if it is assumed that the travel distance is relatively small and the deterioration is not significantly advanced, the value of n is set to a small value. If the travel distance is relatively large and the deterioration may be advanced to a considerable degree, the value of n is increased. And

  Here, the relationship between the oxygen storage capacity OSC and the catalyst temperature is shown in FIG. As can be seen, the oxygen storage capacity OSC tends to increase as the catalyst temperature increases. The oxygen storage capacity OSC is the maximum for a new catalyst, and decreases as the catalyst deteriorates. When the oxygen storage capacity OSC becomes equal to or less than the threshold value OSCs, the catalyst 11 is determined to be deteriorated.

  According to the results of the test research by the present inventor, when the amount of air sucked into the internal combustion engine is relatively large, a value smaller than the true value is calculated as the oxygen storage capacity OSC of the catalyst. It has been found that there is a possibility that a false detection may occur in the deterioration detection. The reason is considered that when the amount of intake air is large, the flow rate of the exhaust gas passing through the catalyst is high, and the exhaust gas passes through before the oxygen absorption / release reaction at the catalyst is sufficiently performed. In addition, a certain amount of time is required for each component in the exhaust gas to undergo an oxidation or reduction reaction. However, when the exhaust gas flows into the catalyst at a high speed, the amount of unreacted gas that passes through the catalyst increases, and apparently. It is thought that the above reaction rate decreases. FIG. 6 shows the test results of examining the relationship between the oxygen storage capacity OSC and the intake air amount GA. As can be seen from the solid line, the oxygen storage capacity OSC tends to be calculated as a smaller value as the intake air amount GA increases.

  Therefore, in order to prevent such erroneous detection of catalyst deterioration detection, in the present embodiment, the following amplitude increase control is executed in the above-described active air-fuel ratio control.

  In the active air-fuel ratio control of the present embodiment, at least the rich amplitude Ar is changed according to the intake air amount GA detected by the air flow meter 5. In particular, when the detected intake air amount is large, at least the rich amplitude Ar is made larger than when the intake air amount is small. That is, control is performed to increase the so-called rich depth.

  7 to 10 show first to fourth modes of such rich amplitude increase control. In the figure, a broken line indicates a change in the target air-fuel ratio A / Ft when the intake air amount is small (small), and a solid line indicates a change in the target air-fuel ratio A / Ft when the intake air amount is large (large). When the intake air amount is small, the target air-fuel ratio A / Ft is switched to the rich side or lean side by the same amplitudes Ar and Al with the theoretical air-fuel ratio A / Fs as the center, as in the case shown in FIG. It is assumed that the target air-fuel ratio A / Ft is switched between the rich air-fuel ratio A / Fr and the lean air-fuel ratio A / Fl, which is the reference state. Note that the theoretical air-fuel ratio A / Fs = 14.6, the rich air-fuel ratio A / Fr = 14.1, the lean air-fuel ratio A / Fl = 15.1, and the rich amplitude Ar = lean amplitude Al = 0.5. Although illustration is omitted, it will be easily understood that the pre-catalyst air-fuel ratio A / Ffr as the actual value or the pre-catalyst sensor detection value is switched in the same manner following the switching of the target air-fuel ratio A / Ft.

  The first mode shown in FIG. 7 is a mode in which both the rich amplitude and the lean amplitude are increased equally with respect to the reference values Ar and Al. Here, ΔAr = ΔAl when the increase amounts with respect to the reference values Ar and Al are ΔAr and ΔAl, and Arx = Alx when the rich and lean amplitudes after increase are Arx and Alx. As a result, the target air-fuel ratio A / Ft is greatly swung to the rich side and the lean side by ΔAr and ΔAl from the reference state. For example, ΔAr = ΔAl = 0.3 can be set, and in this case, the rich air-fuel ratio A / Fr = 13.8 and the lean air-fuel ratio A / Fl = 15.4 after the amplitude increase.

  The second mode shown in FIG. 8 is a mode in which both the rich amplitude and the lean amplitude are increased from the reference values Ar and Al, but the increased rich amplitude Arx is larger than the increased lean amplitude Alx (Arx> Alx). The increase amounts ΔAr and ΔAl satisfy ΔAr> ΔAl. As a result, the target air-fuel ratio A / Ft is swung larger than the reference state, but the rich side is swung larger than the lean side.

  The third mode shown in FIG. 9 is a mode in which the lean amplitude is made equal to the reference value Al while the rich amplitude is increased from the reference value Ar. For increasing amounts ΔAr and ΔAl, ΔAr> 0 and ΔAl = 0. As a result, the target air-fuel ratio A / Ft is swung larger than the reference state only on the rich side.

  The fourth mode shown in FIG. 10 is a mode in which at least the rich amplitude Ar is changed over time in one cycle. That is, in the reference state, the rich amplitude Ar is constant with respect to time, whereas in the amplitude increasing state, the increased rich amplitude Arx is maximized at the time of switching (t1) and then gradually decreased. In the illustrated example, the rich amplitude Arx is decreased at a constant rate with respect to time. As a result, the waveform of the target air-fuel ratio A / Ft is rectangular in the reference state, but is substantially sawtooth in the increased amplitude state. According to the results of test studies by the present inventors, it has been found that increasing the initial amplitude at the time of switching is effective in preventing false detection. Therefore, such an aspect of amplitude change is also effective.

  In the illustrated example, as in the first aspect of FIG. 7, the lean amplitude Al is also increased and changed over time in the same manner as the rich amplitude Ar. The lean amplitude Alx at the time of switching (t2) is equal to the rich amplitude Arx, and the lean amplitude Al when a predetermined time has elapsed from the time of switching is also equal to the rich amplitude Ar at that time. Although not shown in the drawings, it will be understood that the amplitude increase amounts ΔAr and ΔAl between the rich side and the lean side may be made different as in the second mode of FIG. 8 or the third mode of FIG.

  According to the results of a test study by the present inventors, it has been found that such an increase in rich amplitude can suppress a decrease in the oxygen storage amount calculated value based on an increase in the intake air amount. The reason is considered to be that when a richer rich gas flows into the catalyst, the reaction is promoted and oxygen release is facilitated. On the other hand, it has also been found that increasing the rich amplitude is more effective than increasing the lean amplitude. Therefore, by increasing at least the rich amplitude in this way, it becomes possible to calculate the oxygen storage capacity of the catalyst closer to the true value even when the amount of intake air is large. Can be prevented.

  Next, an aspect of increasing the rich amplitude with respect to the intake air amount GA will be described. The first aspect is an aspect in which the rich amplitude is increased stepwise with a predetermined threshold GAs of the intake air amount GA as a boundary.

  FIG. 11 shows a flowchart of control for implementing the first aspect. First, in step S101, it is determined whether or not active air-fuel ratio control is being executed. If it is not being executed, this flow is terminated. If it is being executed, it is determined in step S102 whether or not the intake air amount GA detected by the air flow meter 5 exceeds a predetermined threshold GAs. If the intake air amount GA does not exceed the threshold GAs, the rich amplitude and the lean amplitude are set to the reference values Ar and Al in step S104. On the other hand, if the intake air amount GA exceeds the threshold GAs, only the rich amplitude is increased (for example, in the case of the third mode shown in FIG. 9) or the rich amplitude and the lean amplitude are increased in step S103. Both are increased (for example, in the case of the first and second modes shown in FIGS. 7 and 8).

  According to the first aspect, as shown by the alternate long and short dash line I or II in FIG. 6, the tendency for the oxygen storage capacity calculated value OSC to decrease when the intake air amount GA is larger than the threshold GAs is suppressed or improved. And erroneous detection of catalyst deterioration can be prevented in advance. The number of threshold values GAs may be two or more, and the rich amplitude increase amount (and the lean amplitude increase amount) may be increased every time the intake air amount GA exceeds the threshold value. As a threshold setting method, the relationship between the oxygen storage capacity calculation value and the intake air amount as shown in FIG. 6 is examined in advance by an actual machine test, and inhalation at the boundary where the decrease tendency of the oxygen storage capacity calculation value becomes large There is a method of grasping the air amount and setting the intake air amount as a threshold value.

  The second aspect of the increase in the rich amplitude with respect to the intake air amount is the detected intake air using a map (which may be a function) that predetermines the relationship between the intake air amount GA and the rich amplitude Ar as shown in FIG. In this aspect, the rich amplitude Ar corresponding to the amount GA is obtained. In this map, the relationship between the two is set so that the rich amplitude Ar increases as the intake air amount GA increases. Accordingly, by obtaining the rich amplitude Ar corresponding to the detected intake air amount GA from this map, a larger rich amplitude Ar is obtained as the intake air amount GA increases, and is always appropriate regardless of the magnitude of the intake air amount GA. Thus, the rich amplitude Ar that can prevent erroneous detection can be obtained. Then, as shown by the alternate long and short dash line III or IV in FIG. 6, the decrease tendency of the oxygen storage capacity calculation value OSC can be suppressed or improved in the entire intake air amount GA, and erroneous detection of catalyst deterioration can be prevented. be able to.

  Here, in the case where the lean amplitude is increased in addition to the rich amplitude (for example, in the case of the first and second modes shown in FIGS. 7 and 8), similarly, the intake air as shown in FIG. A lean amplitude Al corresponding to the detected intake air amount GA may be obtained using a map (which may be a function) that predetermines the relationship between the amount GA and the lean amplitude Al.

  Next, a method for calculating the target air-fuel ratio will be described. The first mode for calculating the target air-fuel ratio is a mode for calculating the target air-fuel ratio by adding or subtracting a predetermined offset value to or from a predetermined reference value of the target air-fuel ratio. The rich target air-fuel ratio is calculated by subtracting the offset value from the reference value, and the lean target air-fuel ratio is calculated by adding the offset value to the reference value.

  The offset value is a value set in advance so as to increase as the detected intake air amount increases. FIG. 13 shows a map (which may be a function) in which the relationship between the two is set in advance. Therefore, by obtaining the offset value OF corresponding to the detected intake air amount GA from this map, the offset value OF can be increased as the intake air amount GA increases, and the rich amplitude or the lean amplitude can be increased.

  For example, as shown in FIGS. 7 and 13, it is assumed that the rich target air-fuel ratio is A / Fr when the offset value OF1 when the intake air amount GA1 is used. When the intake air amount is increased from GA1 to GA2 and the offset value OF2 corresponding to GA2 is used, the larger offset value OF2 is subtracted, so the rich side target air-fuel ratio becomes a smaller value A / Frx. As a result, the rich amplitude is increased. On the other hand, since a larger offset value OF2 is added on the lean side, the lean side target air-fuel ratio becomes a larger A / Flx, and as a result, the lean amplitude is increased.

  Next, the second mode of calculating the target air-fuel ratio is a mode in which the target air-fuel ratio is calculated by multiplying a predetermined reference value of the target air-fuel ratio by a predetermined coefficient. FIG. 14 shows a map (which may be a function) in which the relationship between the intake air amount GA and the coefficient B is determined in advance. In this map, the relationship between the two coefficients B, such as the rich coefficient Br and the lean coefficient Bl, and the intake air amount GA is preset, and the rich coefficient Br becomes a smaller value than 1 as the intake air amount GA increases. Thus, the lean coefficient Bl is a larger value than 1. Therefore, by obtaining the coefficients Br and Bl corresponding to the detected intake air amount GA from this map, a smaller target air-fuel ratio can be obtained as the intake air amount GA increases on the rich side, thereby increasing the rich amplitude. On the other hand, on the lean side, a larger target air-fuel ratio can be obtained as the intake air amount GA increases, and thereby the lean amplitude can be increased.

  For example, as shown in FIGS. 7 and 14, it is assumed that the rich target air-fuel ratio is A / Fr when using the rich coefficient Br1 when the intake air amount GA1 is used. When the intake air amount is increased from GA1 to GA2 and the rich coefficient Br2 corresponding to GA2 is used, a smaller rich coefficient Br2 is multiplied, so the rich target air-fuel ratio becomes a smaller A / Frx, As a result, the rich amplitude is increased. On the other hand, since the lean side is multiplied by a larger lean coefficient Br2, the lean side target air-fuel ratio becomes a larger A / Flx, and as a result, the lean amplitude is increased.

  In the first mode, maps such as those shown in FIG. 13 are prepared separately for the rich side and the lean side, and the offset value OF is made different for the same intake air amount GA, as shown in FIG. As described above, the amplitude may be changed between the rich side and the lean side. Further, in the map shown in FIG. 14, the rich coefficient Br and the lean coefficient Bl are set to be symmetric with respect to 1. However, the present invention is not limited to this. For example, as shown in FIG. When increasing, the absolute value of the difference of the rich coefficient Br with respect to 1 can be made larger than the absolute value of the difference of the lean coefficient Bl with respect to 1 for the same intake air amount GA.

  The first and second modes of calculating the target air-fuel ratio can also be applied when the target air-fuel ratio is set so as to gradually decrease the amplitude as shown in FIG. In this case, as shown in FIG. 10, when the target air-fuel ratio A / Ft is switched from the lean air-fuel ratio A / Fl to the rich air-fuel ratio A / Fr (for example, t1) or vice versa (for example, t2). The first and second modes can be applied to the calculation of the first target air-fuel ratio A / Ft immediately after switching.

  By the way, since the active air-fuel ratio control is a control for largely switching the target air-fuel ratio A / Ft as described above, the fuel injection amount also changes greatly with this, and a large torque fluctuation may occur and a torque shock may occur. Concerned. Such a torque shock is not preferable in terms of drivability.

  Therefore, in the present embodiment, in order to suppress such torque fluctuations and thus torque shocks, the following torque fluctuation suppression control is executed.

  As shown in FIG. 15, the first mode of torque fluctuation suppression control retards the ignition timing simultaneously with switching of the target air-fuel ratio A / Ft. That is, for example, when the target air-fuel ratio A / Ft is switched from the lean side to the rich side (t1), if the rich amplitude determined based on the intake air amount is larger than the predetermined threshold value Ars (Arx> Ars) ), The ignition timing is retarded by a delay amount ΔIG from the basic ignition timing determined based on the operating state (for example, the rotational speed NE and the load factor KL). If the rich amplitude is equal to or less than the threshold value Ars at the switching time t1, the retardation is not executed. The retardation amount ΔIG is maximized at the switching timing t1, and then rapidly decreased to finally become zero. That is, since torque fluctuation occurs instantaneously when the target air-fuel ratio A / Ft is switched, the ignition delay is also instantaneously performed only at the time of switching.

  In the illustrated example, when the target air-fuel ratio A / Ft is switched from the rich side to the lean side (t2), similarly, if the lean amplitude is larger than the predetermined threshold value Als (when Alx> Als), the ignition timing is delayed. Try to run the corner. This ignition timing retardation is possible in any of the modes shown in FIGS.

  The values of threshold values Ars, Als and retardation amount ΔIG are set to optimum values based on actual machine tests and the like. The threshold values Ars and Als may be constant values, or may be values that change according to values such as the intake air amount. The same applies to the retardation amount ΔIG.

  As shown in FIG. 16, the second mode of torque fluctuation suppression control is to change the throttle opening TH by a predetermined amount simultaneously with the switching of the target air-fuel ratio A / Ft, that is, increase (open) or decrease (close). It is. The value of the throttle opening TH shown in (A) is the value of the throttle opening instruction value given from the ECU 20 to the throttle valve 10.

  For example, when the target air-fuel ratio A / Ft is switched from the lean side to the rich side (t1) and the rich amplitude determined based on the intake air amount GA is larger than the predetermined threshold value Ars (when Arx> Ars), The throttle opening TH is reduced by a predetermined amount ΔTH from the basic throttle opening determined based on the detected accelerator opening AC. That is, in this case, switching to the rich air-fuel ratio is performed, and the fuel injection amount is increased, so that the throttle opening is decreased in order to cancel the acceleration side torque fluctuation. If the rich amplitude is less than or equal to the threshold value Ars at the switching time t1, the throttle opening reduction is not executed. As in the case of the ignition timing, the throttle opening change amount ΔTH is maximized at the switching timing t1, and then rapidly decreased to finally become zero. That is, the throttle opening is changed instantaneously only when the target air-fuel ratio is switched.

  In the illustrated example, similarly, when the target air-fuel ratio A / Ft is switched from the rich side to the lean side (t2), if the lean amplitude is larger than the predetermined threshold value Als (when Alx> Als), the throttle The opening degree is changed. In this case, contrary to the previous case, switching to the lean air-fuel ratio is performed, and the fuel injection amount is reduced, so that the throttle opening is increased to cancel the deceleration side torque fluctuation.

  As described above, the thresholds Ars and Als and the throttle opening change amount ΔTH are set to optimum values based on actual machine tests and the like. The threshold values Ars and Als may be constant values, or may be values that change according to values such as the intake air amount. The same applies to the throttle opening change amount ΔTH. In particular, the throttle opening change amount ΔTH is a value depending on one or more parameters (rotational speed NE, load factor KL, intake air amount GA, vehicle speed, catalyst temperature, etc.) representing the engine operating state and the vehicle traveling state. The relationship between the parameter and the throttle opening change amount ΔTH may be determined from a predetermined map or function. It is assumed that the deceleration side torque fluctuation at the time of switching to the lean air-fuel ratio is smaller than the acceleration side torque fluctuation at the time of switching to the rich air-fuel ratio. Therefore, if necessary, the throttle opening increase amount in the former case can be made smaller than the throttle opening decrease amount in the latter case, or the throttle opening increase itself in the former case can be omitted. This change in the throttle opening is possible in any of the modes shown in FIGS.

  The throttle opening change control can take another form as follows. As shown in FIG. 17, here, in response to switching of the post-catalyst air-fuel ratio or the post-catalyst sensor output A / Frr, first the throttle opening TH is changed, and then the target air-fuel ratio A / F is passed through a predetermined delay time Δt. Ft is switched.

  If the ECU 20 gives a drive signal to the injector 12, the air-fuel ratio of the air-fuel mixture in the combustion chamber 3 can be changed immediately. However, the throttle valve 10 is actually set to the command value after the opening command signal is given. There may be a delay in operation before it reaches. Therefore, in order to cancel this operation delay, the operation of the throttle valve 10 is started first, and the target air-fuel ratio A / Ft is switched after that. By doing so, the throttle opening can be changed in accordance with the timing at which the air-fuel ratio of the air-fuel mixture in the combustion chamber 3 is switched, and torque fluctuation and torque shock can be effectively prevented and smooth drivability is achieved. Can be achieved.

  The value of the delay time Δt can be optimally set based on an actual machine test or the like. Further, at the time of executing the active air-fuel ratio control, learning may be performed for each operation state.

  The example shown in FIG. 17 will be described. For example, when the output voltage of the post-catalyst sensor 18 becomes equal to the lean determination value VL (time t1), the rich amplitude after switching based on the detected intake air amount GA, that is, A rich air-fuel ratio is calculated. If the rich amplitude exceeds the threshold value Ars (when Arx> Ars), there is a possibility that a large torque fluctuation may occur. Therefore, the throttle opening TH is reduced by ΔTH in the throttle valve 10 at that time t1. An opening degree command signal for sending is sent. After this, the throttle valve 10 will decrease its opening according to the command value with the aforementioned operation delay. The amount of decrease ΔTH is initially maximum and then decreases rapidly. When the delay time Δt has elapsed from time t1, the target air-fuel ratio A / Ft is switched to the increased rich air-fuel ratio A / Frx. At this time, the actual opening of the throttle valve 10 is decreased by exactly ΔTH, and the torque shock is suppressed. When the rich amplitude calculated at time t1 is equal to or smaller than the threshold value Ars, such throttle opening reduction is not executed.

  In the illustrated example, similar control is executed when the target air-fuel ratio A / Ft is switched to the lean side. At the time t2 when the output voltage of the post-catalyst sensor 18 reaches the rich determination value VL, an opening command signal for increasing the throttle opening TH by ΔTH is sent, and then after the delay time Δt has elapsed, the target air-fuel ratio A / Ft is switched to the increased lean air-fuel ratio A / Flx.

  The throttle opening change control may take another aspect as follows. As shown in FIG. 18, here, control is performed to slow down the rising speed of the throttle opening. Since a torque shock may occur due to a sudden change in the throttle opening, this mode is effective in this case.

  The mode shown in FIG. 18 is basically the same as the mode shown in FIG. The difference is that when the throttle opening TH is changed by a predetermined amount ΔTH, a predetermined smoothing process is executed and the throttle opening waveform is gradually raised. In the illustrated example, the same smoothing process is executed when the target air-fuel ratio A / Ft is switched to the rich side and when it is switched to the lean side. It is also possible to combine the control for slowing the rising speed with the mode shown in FIG.

  In the above-described embodiment, the ECU 20 and the injector 12 constitute active air-fuel ratio control means according to the present invention, and the air flow meter 5 constitutes intake air amount detection means according to the present invention. Alternatively, the ECU 20 and the throttle valve 10 constitute the torque fluctuation suppression control means referred to in the present invention. The pre-catalyst sensor 17 constitutes a pre-catalyst air-fuel ratio detecting means for detecting the exhaust air-fuel ratio upstream of the catalyst, and the post-catalyst sensor 18 detects a post-catalyst air-fuel ratio detecting means for detecting the exhaust air-fuel ratio downstream of the catalyst. Configure. As the post-catalyst sensor 18, an air-fuel ratio sensor similar to the pre-catalyst sensor 17 can be used.

  The preferred embodiment of the present invention has been described in detail above, but various other embodiments of the present invention are conceivable. For example, although the above-described internal combustion engine is a direct injection type, the present invention is also applicable to an intake port (intake passage) injection type or a dual injection type internal combustion engine having both injection types. Further, instead of the intake air amount, another value (such as an engine load factor) correlated with the intake air amount may be used.

  The embodiment of the present invention is not limited to the above-described embodiment, and includes all modifications, applications, and equivalents included in the concept of the present invention defined by the claims. Therefore, the present invention should not be construed as being limited, and can be applied to any other technique belonging to the scope of the idea of the present invention.

It is the schematic which shows the structure of this embodiment. It is a schematic sectional drawing which shows the structure of a catalyst. 3 is a time chart for explaining the basics of active air-fuel ratio control. FIG. 4 is a time chart similar to FIG. 3 for illustrating a method for calculating the oxygen storage capacity. It is a graph which shows the relationship between oxygen storage capacity and catalyst temperature, and is a figure for demonstrating the catalyst degradation determination method. It is a graph which shows the relationship between oxygen storage capacity and the amount of intake air. It is a time chart which shows the 1st aspect of rich amplitude increase. It is a time chart which shows the 2nd aspect of rich amplitude increase. It is a time chart which shows the 3rd aspect of rich amplitude increase. It is a time chart which shows the 4th aspect of rich amplitude increase. It is a flowchart of control which performs rich amplitude increase. It is the map which defined the relationship between intake air amount and rich amplitude. It is the map which defined the relationship between the amount of intake air and an offset value. It is the map which defined the relationship between the amount of intake air and a coefficient. It is a time chart which shows the 1st aspect of torque shock suppression control. It is a time chart which shows the one aspect | mode of the 2nd aspect of torque shock suppression control. It is a time chart which shows another aspect of the 2nd aspect of torque shock suppression control. It is a time chart which shows another aspect of the 2nd aspect of torque shock suppression control.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Internal combustion engine 3 Combustion chamber 5 Air flow meter 6 Exhaust pipe 7 Spark plug 10 Throttle valve 11 Catalyst 12 Injector 15 Accelerator opening sensor 17 Pre-catalyst sensor 18 Post-catalyst sensor 20 Electronic control unit (ECU)
A / F Air-fuel ratio A / Ffr Pre-catalyst air-fuel ratio A / Frr Post-catalyst air-fuel ratio A / Ft Target air-fuel ratio A / Fs Theoretical air-fuel ratio A / Fr Rich air-fuel ratio A / Fl Lean air-fuel ratio A / Frx After amplitude increase Rich air-fuel ratio A / Flx Lean air-fuel ratio Ar after amplitude increase Rich amplitude Al Lean amplitude Ars Rich amplitude threshold Als Lean amplitude threshold Arx Rich amplitude Alx after amplitude increase Lean amplitude ΔAr Rich amplitude after amplitude increase Increase amount ΔAl Lean amplitude increase amount OSC Catalyst oxygen storage capacity OSCs Deterioration determination threshold GA Intake air amount GAs Intake air amount threshold TH Throttle opening ΔTH Throttle opening change amount ΔIG Ignition timing retardation amount OF Offset value B coefficient Br rich coefficient Bl lean coefficient VR rich judgment value VL lean judgment value

Claims (8)

An apparatus for detecting deterioration of a catalyst disposed in an exhaust passage of an internal combustion engine,
The exhaust air-fuel ratio on the upstream side of the catalyst is selected from one of a rich air-fuel ratio that is rich from a predetermined center air-fuel ratio by a predetermined rich amplitude and a lean air-fuel ratio that is lean from the central air-fuel ratio by a predetermined lean amplitude. On the other hand, active air-fuel ratio control means for forcibly switching at a predetermined timing,
An intake air amount detection means for detecting the amount of air sucked into the internal combustion engine,
The active air-fuel ratio control means increases the rich amplitude when the intake air amount detected by the intake air amount detection means is large than when the intake air amount is small. Catalyst deterioration detector.   The active air-fuel ratio control means increases the lean amplitude when the intake air quantity detected by the intake air quantity detection means is large, compared with when the intake air quantity is small, and has the same intake air quantity 2. The catalyst deterioration detection device for an internal combustion engine according to claim 1, wherein the rich amplitude and the lean amplitude with respect to are equal to each other, or the former is made larger than the latter.   2. The active air-fuel ratio control means maximizes the rich amplitude when the exhaust air-fuel ratio on the upstream side of the catalyst is switched to the rich air-fuel ratio, and then gradually decreases the rich amplitude. Or the catalyst deterioration detection apparatus of the internal combustion engine of 2. A pre-catalyst air-fuel ratio detecting means for detecting an exhaust air-fuel ratio upstream of the catalyst;
The active air-fuel ratio control means controls the exhaust air-fuel ratio so that the exhaust air-fuel ratio detected by the pre-catalyst air-fuel ratio detection means matches a predetermined target value, and the rich air-fuel ratio is controlled. The target value is calculated by subtracting a predetermined offset value from a predetermined reference value,
The offset value is a value set in advance so as to increase as the intake air amount detected by the intake air amount detection means increases. The catalyst deterioration detection apparatus of the internal combustion engine. A pre-catalyst air-fuel ratio detecting means for detecting an exhaust air-fuel ratio upstream of the catalyst;
The active air-fuel ratio control means controls the exhaust air-fuel ratio so that the exhaust air-fuel ratio detected by the pre-catalyst air-fuel ratio detection means matches a predetermined target value, and the rich air-fuel ratio is controlled. A target value is calculated by multiplying a predetermined reference value by a predetermined coefficient,
The coefficient is a value set in advance so that the larger the intake air amount detected by the intake air amount detection means, the smaller the value becomes than 1. The catalyst deterioration detection apparatus of the internal combustion engine described in 1.   The active air-fuel ratio control means sets the rich amplitude to a predetermined reference value when the intake air quantity detected by the intake air quantity detection means does not exceed a predetermined threshold value, and the intake air quantity is set to a predetermined value. 4. The catalyst deterioration detection apparatus for an internal combustion engine according to claim 1, wherein when the threshold value is exceeded, the rich amplitude is set to a value larger than the reference value.   The active air-fuel ratio control means responds to the intake air amount using a map or function set in advance so that the rich amplitude increases as the intake air amount detected by the intake air amount detection means increases. 4. The catalyst deterioration detection device for an internal combustion engine according to claim 1, wherein the rich amplitude is determined. When the active air-fuel ratio control means switches the exhaust air-fuel ratio to the rich air-fuel ratio, it further comprises torque fluctuation suppression control means for executing predetermined torque fluctuation suppression control when the rich amplitude is larger than a predetermined threshold value. The catalyst deterioration detecting device for an internal combustion engine according to any one of claims 1 to 7.

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