JP5035670B2 - Catalyst deterioration detection device for internal combustion engine - Google Patents

Description

  The present invention relates to an apparatus for detecting 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 adsorbed and held oxygen. Therefore, during normal operation of the internal combustion engine, even if the air-fuel mixture fluctuates to the rich side or the lean side depending on the operating conditions centering on the stoichiometric air-fuel ratio, the catalyst surface is maintained at the stoichiometric air-fuel ratio, and the three-way catalyst has O _2. Due to the storage function, when the air-fuel mixture becomes lean, excess oxygen is adsorbed and held by the catalyst, so NOx is reduced. When the air-fuel mixture becomes rich, oxygen adsorbed and held by the catalyst is released. HC and CO are oxidized, so that NOx, HC and CO can be simultaneously purified.

  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 controlled around the stoichiometric air-fuel ratio, so that even if the fuel is alternately swung to the rich side or the lean side, NOx, HC and CO can be reduced simultaneously. It has become.

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 because they are reactions through noble metals. Therefore, it is possible to detect that the catalyst has deteriorated by detecting that the O ₂ storage function has deteriorated. More specifically, the deterioration of the catalyst can be detected by measuring the oxygen storage capacity as the maximum amount of oxygen that can be stored by the current catalyst.

  As an apparatus for detecting catalyst deterioration based on such a principle, for example, there is one disclosed in Patent Document 1. This apparatus determines an abnormality of the downstream catalyst among the upstream catalyst and the downstream catalyst arranged in series in the exhaust passage of the internal combustion engine. An inter-catalyst sensor that detects an inter-catalyst air-fuel ratio between the upstream catalyst and the downstream catalyst, and a post-catalyst sensor that detects a post-catalyst air-fuel ratio downstream of the downstream catalyst are provided. Active air-fuel ratio control is performed in which the pre-catalyst air-fuel ratio upstream of the upstream catalyst is switched from lean to rich or vice versa in response to switching of the output of the inter-catalyst sensor from rich to lean or vice versa. An abnormality of the downstream catalyst is determined based on the sensor output emitted from the post-catalyst sensor during the execution of the active air-fuel ratio control.

JP 2004-176615 A

By the way, according to the results of the study by the present inventors, when the catalyst is left for a long period (about 1 month or more) and then the deterioration of the catalyst is detected, it is detected that the catalyst is deteriorated even though the catalyst is normal. It was confirmed that Then, if the internal combustion engine is operated for a while and the exhaust gas continues to flow through the catalyst, the deterioration state is eliminated and the original normal state is restored. When this cause was investigated, carbon dioxide CO ₂ in the atmosphere was adsorbed on the oxygen storage component of the catalyst while the catalyst was left unattended, and this adsorbed CO ₂ inhibited the oxygen storage and release action of the catalyst. Was inferred. It was also found that this adsorbed CO ₂ is gradually desorbed under a high catalyst temperature.

The deterioration detected by the CO ₂ adsorption to the catalyst is a temporary deterioration that should not be originally deteriorated, and detecting this as deterioration leads to erroneous detection.

  Therefore, the present invention has been made in view of the above circumstances, and an object of the present invention is to provide a catalyst deterioration detection device for an internal combustion engine that can prevent a temporary deterioration caused by leaving the catalyst for a long time from being erroneously detected as deterioration. It is to provide.

According to the first aspect of the present invention,
An apparatus for detecting deterioration of a catalyst disposed in an exhaust passage of an internal combustion engine,
A suppression means for suppressing contact of the catalyst with the atmosphere while the internal combustion engine is stopped ;
The suppression means comprises rich control means for executing rich control for controlling the exhaust air-fuel ratio to be richer than the stoichiometric air-fuel ratio between the ignition switch off and the internal combustion engine stop,
At least one of the rich control time and the rich amplitude in the rich control is variably set according to the lean operation time before the internal combustion engine is stopped .

Thereby, it is possible to suppress the adsorption of CO ₂ in the atmosphere to the catalyst while the internal combustion engine is stopped, and prevent erroneous detection caused by the adsorbed CO ₂ even if the catalyst deterioration detection is performed after the operation of the internal combustion engine is resumed. be able to.

When such rich control is executed, the catalyst surface can be coated with rich components (mainly HC, CO) in the exhaust gas. Therefore, contact between the catalyst surface and atmospheric CO ₂ and adsorption of CO ₂ to the catalyst can be suppressed, and erroneous detection due to adsorbed CO ₂ after restarting operation can be prevented.

  The rich amplitude refers to the amplitude of the pre-catalyst exhaust air-fuel ratio with respect to the stoichiometric air-fuel ratio during rich control. For example, if the lean operation time before the internal combustion engine is stopped is long, a large amount of oxygen may be stored in the catalyst, and even if rich control is performed when the internal combustion engine is stopped, the rich components in the exhaust gas react with the stored oxygen. Therefore, there is a possibility that it may not reach the coating. Therefore, in such a case, the rich control time is lengthened or the rich amplitude is increased so that more rich components are supplied to the catalyst. In this way, the catalyst can be more reliably coated with the rich component.

According to a second aspect of the present invention, in the first aspect ,
History determination means for determining the presence or absence of a fuel cut history during resumption of operation when performing catalyst deterioration detection during resumption of operation after stopping the internal combustion engine;
And a prohibiting means for prohibiting the catalyst deterioration detection when the history determining means determines that there is no fuel cut history.

  When the internal combustion engine is restarted after the engine stop with the rich control described above and the operation is restarted, if the deterioration detection is performed while the catalyst is coated with the rich component, an erroneous detection is caused by the influence of the rich component. There is a case. On the other hand, when the fuel cut is executed after the engine operation is resumed, the rich component coated on the catalyst reacts with air and is purged. According to the fourth embodiment, when there is no fuel cut history during resumption of engine operation, that is, when the rich component coated on the catalyst is not purged, the detection of catalyst deterioration is prohibited. It is possible to prevent erroneous detection caused by it.

According to a third aspect of the present invention, in the first aspect ,
History determination means for determining the presence or absence of a fuel cut history during resumption of operation when performing catalyst deterioration detection during resumption of operation after stopping the internal combustion engine;
Lean control means for executing lean control for controlling the exhaust air-fuel ratio to be leaner than the stoichiometric air-fuel ratio when the history determination means determines that there is no fuel cut history;
And a deterioration detection executing means for executing catalyst deterioration detection after completion of the lean control by the lean control means.

  By this lean control, the rich component coated on the catalyst can be purged by reacting with the lean gas. Therefore, by performing catalyst deterioration detection after the end of lean control, erroneous detection due to rich components can be prevented in advance.

  According to the present invention, an excellent effect is exhibited that it is possible to prevent the temporary deterioration caused by leaving the catalyst for a long time from being erroneously detected as deterioration.

  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 the exhaust pipe 6 has a three-way catalyst having an O ₂ storage function (oxygen storage capacity). A catalyst 11 comprising: An exhaust passage is formed by the exhaust port, the branch pipe, and the exhaust pipe 6. Air-fuel ratio sensors for detecting the exhaust air-fuel ratio, that is, a pre-catalyst sensor and post-catalyst sensors 17 and 18 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 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 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 near the 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 air-fuel ratio of the exhaust gas flowing into the catalyst 11, that is, the pre-catalyst air-fuel ratio A / Ffr becomes the stoichiometric air-fuel ratio A / Fs. To do. 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. The oxygen storage component is made of, for example, cerium oxide CeO ₂ or zirconia. 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 / Ffr slightly varies from the stoichiometric 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 / Ffr to oscillate slightly around the stoichiometric air-fuel ratio A / Fs and to repeatedly absorb and release 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 by detecting the oxygen storage capacity of the catalyst 11. Here, the oxygen storage capacity of the catalyst 11 is represented by the size of the oxygen storage capacity (OSC; O ₂ Strage Capacity, the unit is g), which is the maximum amount of oxygen that the current 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 is executed by the ECU 20 when the deterioration of the catalyst 11 is detected. The active air-fuel ratio control is a control for forcibly changing the pre-catalyst air-fuel ratio A / Ffr to a rich side or a lean side with respect to a predetermined center air-fuel ratio A / Fc. The air-fuel ratio when changed to the rich side is referred to as rich air-fuel ratio A / Fr, and the air-fuel ratio when changed to the lean side is referred to as lean air-fuel ratio A / Fl. When the pre-catalyst air-fuel ratio A / Ffr is changed to the rich side or the lean side by this active air-fuel ratio control, the oxygen storage capacity OSC of the catalyst is measured.

  The deterioration detection of the catalyst 11 is performed when the internal combustion engine 1 is in a steady operation and when the catalyst 11 is in the active temperature range. The temperature of the catalyst 11 may be detected directly using a temperature sensor, but in the present embodiment, it is estimated from the operating state of the internal combustion engine. For example, the ECU 20 uses a map or function set in advance through experiments or the like based on the intake air amount GA detected by the air flow meter 5 and the engine speed NE calculated based on the output of the crank angle sensor 14. Then, the temperature of the catalyst 11 is estimated.

  Detection of catalyst deterioration is performed at least once per trip of the internal combustion engine. When catalyst deterioration is detected for at least two consecutive trips, a final determination is made that catalyst deterioration has occurred, and a warning device such as a check lamp is activated. It is done. One trip means a period from the start to the stop of the engine once.

  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 output values of the pre-catalyst sensor 17 and the post-catalyst sensor 18 represent the values of 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. Following the switching of the target air-fuel ratio A / Ft, the pre-catalyst air-fuel ratio A / Ffr as an actual value is also switched with a slight time delay with respect to 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 measured as follows, and the deterioration of the catalyst 11 is determined.

  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 at one time is used and compared with a predetermined deterioration determination value. If the oxygen storage capacity OSC1 exceeds the deterioration determination value, the oxygen storage capacity OSC1 is normal. The deterioration of the catalyst can be determined such that the deterioration is below the deterioration determination 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 determination is performed by comparing the value with the deterioration determination 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 an average value OSCav thereof is compared with a predetermined deterioration determination value OSCs. If the average value OSCav exceeds the deterioration determination value OSCs, the catalyst 11 is determined to be normal, and if the average value OSCav is equal to or less than the deterioration determination 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 the catalyst deterioration, such as a vehicle travel distance. 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, a method is shown in which the value of the oxygen storage capacity OSC is measured as an index value relating to the degree of catalyst deterioration, and this measured value is compared with a predetermined deterioration determination value to determine catalyst deterioration. However, other deterioration detection methods can be employed, and the deterioration index value is not limited to the oxygen storage capacity OSC. For example, as disclosed in Patent Document 1, the manner in which the post-catalyst air-fuel ratio A / Frr vibrates when the pre-catalyst air-fuel ratio A / Ffr is forcibly vibrated in a short cycle varies depending on the degree of catalyst deterioration. Therefore, a method may be adopted in which the ratio of the output trajectory lengths of the sensors before and after the catalyst is measured as a deterioration index value, and this measured value is compared with a predetermined deterioration determination value to determine catalyst deterioration.

As described above, when the deterioration detection of the catalyst is performed after leaving the catalyst for a long period (about 1 month or longer), carbon dioxide CO ₂ in the atmosphere adsorbed to the oxygen storage component of the catalyst during the leaving is left in the catalyst. The oxygen storage / release action is hindered, and the measured value of the oxygen storage capacity OSC becomes smaller than the true value. In some cases, the measured value is lower than the deterioration determination value, and it may be erroneously detected as deterioration despite the normal catalyst. On the other hand, the adsorbed CO ₂ is slowly desorbed over a relatively long time (about 10 minutes) under a high catalyst temperature condition of about 600 ° C. or higher. That is, the deterioration of the catalyst due to the adsorbed CO ₂ is temporary deterioration that should not be originally deteriorated, and setting this as deterioration leads to erroneous detection. Therefore, in order to prevent this erroneous detection, in the present embodiment, there is provided suppression means for suppressing the catalyst from coming into contact with the atmosphere while the internal combustion engine is stopped. Thereby, it is possible to suppress the adsorption of CO ₂ in the atmosphere to the catalyst while the internal combustion engine is stopped, and prevent erroneous detection caused by the adsorbed CO ₂ even if the catalyst deterioration detection is performed after the operation of the internal combustion engine is resumed. be able to.

Examples of such suppression means include an exhaust shutter that opens and closes an exhaust passage on the downstream side of the catalyst. By closing the exhaust shutter while the internal combustion engine is stopped, it is possible to suppress the atmosphere that has entered from the open downstream end of the exhaust passage, and thus the atmospheric CO ₂ from coming into contact with the catalyst, thereby adsorbing the CO ₂ to the catalyst. Can be prevented, and erroneous detection after restarting engine operation can be prevented.

On the other hand, this embodiment does not employ such an exhaust shutter, but instead employs rich control means for executing rich control for controlling the exhaust air-fuel ratio to be richer than the stoichiometric air-fuel ratio when the internal combustion engine is stopped. In this way, it is not necessary to add another part such as an exhaust shutter, and an increase in cost can be avoided. When such rich control is executed, the catalyst surface can be coated and protected with rich components (mainly HC and CO), or the catalyst surface can be richly poisoned. As a result, contact between the catalyst surface and atmospheric CO ₂ and adsorption of CO ₂ to the catalyst can be suppressed, and erroneous detection after restarting operation can be prevented.

FIG. 5 shows the contents of the CO ₂ adsorption suppression process according to this embodiment. The illustrated process is repeatedly executed by the ECU 20 every predetermined calculation cycle.

First, in step S101, it is determined whether or not an ignition switch (IG) (not shown) is turned off. If it is not turned off, this process is terminated. If it is turned off, the process proceeds to step S102 and rich control is immediately started. In this rich control, the air-fuel ratio of the air-fuel mixture is forcibly controlled to be richer than the stoichiometric air-fuel ratio for a very short predetermined time trs (this is called the rich control time). For example, the air-fuel ratio is controlled to a predetermined value of 14 or less for about 1 second. As a result, exhaust gas containing a large amount of rich components is supplied to the catalyst, and the catalyst surface is coated with the rich components, so that CO ₂ in the atmosphere can be prevented from contacting and adsorbing to the catalyst surface while the engine is stopped.

  In the next step S103, it is determined whether or not a predetermined time trs has elapsed since the start of the rich control. If the predetermined time trs has not elapsed, the process ends. If the predetermined time trs has elapsed, the engine is stopped in step S104, and the process ends.

  Incidentally, it is preferable that at least one of the rich control time trs and the rich amplitude Arr in the rich control is variably set according to the lean operation time before the engine is stopped. Here, the rich amplitude Arr is the amplitude or depth of the pre-catalyst exhaust air-fuel ratio A / Ffr with respect to the theoretical air-fuel ratio A / Fs during rich control (Arr = A / Fs−A / Ffr). For example, if the lean operation time before the engine stops is long, the catalyst may be in a state where a large amount of oxygen is occluded, and even if rich control is performed, the rich component in the exhaust gas reacts with the occluded oxygen, resulting in coating There is a risk of not reaching. Therefore, in such a case, the rich control time is lengthened or the rich amplitude is increased so that more rich components are supplied to the catalyst. In this way, the catalyst can be more reliably coated with the rich component. As can be seen from this description, as the lean operation time is longer, the rich control time is set to a longer value, and the rich amplitude is set to a larger value.

  Since the rich control is performed from the ignition switch off to the engine stop, the rich control time is naturally limited, and the rich control time is preferably as short as possible to prevent the driver from feeling uncomfortable. Therefore, when there is a limit on the rich control time, it is preferable to cope with an increase in rich amplitude.

  FIG. 6 shows a diagram relating to a method for setting the rich control time trs and the rich amplitude Arr. When the pre-catalyst air-fuel ratio detected by the pre-catalyst sensor 17 is leaner than the stoichiometry during engine operation before the ignition switch is turned off (IG OFF), the lean counter provided in the ECU 20 is integrated, and conversely, the pre-catalyst air-fuel ratio is integrated. When is richer than stoichiometric, the lean counter is subtracted. The lean counter is held when the pre-catalyst air-fuel ratio is stoichiometric. Then, based on the value of the lean counter at the time when the ignition switch is turned off, the ECU 20 sets the rich control time tr and the rich amplitude Arr using a predetermined map or the like. As the lean counter value is larger, the rich control time trs is set to a longer value, and the rich amplitude Arr is set to a larger value. When the rich control time trs reaches the predetermined upper limit value, the rich control time trs cannot be increased any further, and the rich amplitude Arr is increased accordingly.

  In this example, both the rich control time and the rich amplitude are variably set according to the lean operation time before the engine is stopped, but only one may be fixed and only the other may be variably set.

  By the way, when the engine is restarted after such rich control is performed and the engine is restarted and the operation is resumed, if the deterioration detection is performed while the catalyst is coated with the rich component, it is also true. An oxygen storage capacity smaller than the oxygen storage capacity is measured, which may lead to erroneous detection. Therefore, in order to prevent this, in this embodiment, when detecting the deterioration of the catalyst during the restart of the engine operation, the presence or absence of the fuel cut history is determined, and when there is no fuel cut history, the detection of the deterioration of the catalyst is prohibited. I have to. When the fuel cut is executed after the engine operation is resumed, the rich component coated on the catalyst reacts with air and is purged or removed. Therefore, in other words, if the fuel cut is not executed after the engine operation is resumed, the rich component remains in the catalyst, and the oxygen absorption / release action of the catalyst may be hindered. Therefore, by prohibiting the detection of deterioration in such a case, erroneous detection can be prevented in advance.

FIG. 7 shows a first mode of the deterioration detection process after restarting the engine operation. The illustrated process is repeatedly executed by the ECU 20 every predetermined calculation cycle, and is executed during the next restart of engine operation when the above-described CO ₂ adsorption suppression process is performed when the engine is stopped.

  First, in step S201, it is determined whether or not a predetermined precondition for detecting catalyst deterioration is satisfied. For example, if the engine is in a steady operation state and the catalyst temperature is within a predetermined activation temperature range, such as the intake air amount GA and the engine rotational speed NE are substantially constant, the precondition is satisfied. If the precondition is not satisfied, detection of catalyst deterioration is prohibited in step S204. On the other hand, if the condition is satisfied, the process proceeds to step S202.

  In step S202, it is determined whether or not there is a history of fuel cut (FC), that is, a fuel cut history after the engine is started for restarting engine operation. Here, since the purge of the rich component requires a certain time, it is determined that there is a fuel cut history when there is a history of fuel cut executed for a predetermined time (for example, 1 second). If there is a fuel cut history, it is considered that the rich component coated on the catalyst has been purged, and catalyst deterioration detection is executed in step S203. On the other hand, if there is no fuel cut history, it is considered that the rich component coated on the catalyst may still remain, and detection of catalyst deterioration is prohibited in step S204.

  Note that step S204 for prohibiting detection of catalyst deterioration may be omitted. Even if it does in this way, since it does not pass through step S203 and the catalyst deterioration detection is not executed, the detection of the catalyst deterioration is naturally prohibited.

  Next, another method for preventing erroneous detection due to rich component adhesion to the catalyst will be described. Here, when detecting the deterioration of the catalyst during resumption of engine operation, it is determined whether or not there is a fuel cut history, and when there is no fuel cut history, the exhaust air / fuel ratio is forcibly controlled from the stoichiometric air / fuel ratio to the lean side. The lean control is executed, and the deterioration detection of the catalyst is executed after the end of the lean control. By this lean control, the rich component coated on the catalyst can be purged or removed by reacting with the lean gas. Therefore, by detecting the deterioration of the catalyst after the end of lean control, erroneous detection can be prevented in advance. Even when there is no fuel cut history, the lean control is executed to detect the deterioration of the catalyst, so that more deterioration detection frequencies can be secured than in the first aspect.

  FIG. 8 shows a second mode of the deterioration detection process after restarting the engine operation. The second mode is almost the same as the first mode shown in FIG. 7, and the description of the same steps is omitted by changing the reference number from the 200th to the 300th. The following points are different.

  If it is determined in step S302 that there is no fuel cut history, the process proceeds to newly added step S305, where lean control is executed. In this lean control, the air-fuel ratio of the air-fuel mixture is forcibly controlled to be leaner than the stoichiometric air-fuel ratio for a very short predetermined time tls (this is called a lean control time). For example, the air-fuel ratio is controlled to a predetermined value of 15 or more for about 1 second or more. As a result, exhaust gas containing a large amount of lean components (mainly NOx) can be supplied to the catalyst, and the rich components coated on the catalyst surface can be purged by the lean components.

  In the next step S306, it is determined whether or not a predetermined time tls has elapsed since the start of lean control. If the predetermined time tls has not elapsed, the process proceeds to step S304, where catalyst deterioration detection is prohibited. If the predetermined time tls has elapsed, catalyst deterioration detection is executed in step S303.

Although the embodiment of the present invention has been described in detail above, 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. In the above embodiment, a so-called O ₂ sensor is used as the post-catalyst sensor 18, but an air-fuel ratio sensor similar to the pre-catalyst sensor 17 can also be used. Similarly, a so-called O ₂ sensor can be used as the pre-catalyst sensor 17. The type of the internal combustion engine is not particularly limited, and may be, for example, a compression ignition type internal combustion engine (diesel engine). The type of catalyst is not particularly limited, and may be, for example, a NOx catalyst.

  The present invention includes all modifications, applications, and equivalents included in the spirit 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 one Embodiment of this invention. It is a schematic sectional drawing which shows the structure of a catalyst. It is a time chart for demonstrating 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. Is a flowchart showing the contents of CO ₂ adsorption suppression treatment. It is a time chart regarding the setting method of rich control time and rich amplitude. It is a flowchart which shows the 1st aspect of the deterioration detection process after engine driving | operation restarting. It is a flowchart which shows the 2nd aspect of the deterioration detection process after engine driving | operation restarting.

Explanation of symbols

1 Internal combustion engine 6 Exhaust pipe 11 Catalyst 12 Injector 17 Pre-catalyst sensor 18 Post-catalyst sensor 20 Electronic control unit (ECU)

Claims (3)

An apparatus for detecting deterioration of a catalyst disposed in an exhaust passage of an internal combustion engine,
A suppression means for suppressing contact of the catalyst with the atmosphere while the internal combustion engine is stopped;
The suppressing means, Ri Do the rich control means for performing rich control for controlling the rich side of the exhaust air-fuel ratio than the stoichiometric air-fuel ratio during the period from the ignition switch off until the engine is stopped,
At least one of the rich control time and the rich amplitude in the rich control is variably set according to the lean operation time before the internal combustion engine is stopped .   History determination means for determining the presence or absence of a fuel cut history during resumption of operation when performing catalyst deterioration detection during resumption of operation after stopping the internal combustion engine;
  Prohibiting means for prohibiting detection of catalyst deterioration when the history determining means determines that there is no fuel cut history;
  The catalyst deterioration detecting device for an internal combustion engine according to claim 1, wherein   History determination means for determining the presence or absence of a fuel cut history during resumption of operation when performing catalyst deterioration detection during resumption of operation after stopping the internal combustion engine;
  Lean control means for executing lean control for controlling the exhaust air-fuel ratio to be leaner than the stoichiometric air-fuel ratio when the history determination means determines that there is no fuel cut history;
  Deterioration detection execution means for executing catalyst deterioration detection after completion of lean control by the lean control means;
  The catalyst deterioration detecting device for an internal combustion engine according to claim 1, wherein

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