JP2005240716A - Deterioration diagnostic device for catalyst - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To accurately make deterioration determination of a catalyst even at a lean air fuel ratio. <P>SOLUTION: This deterioration diagnostic device for the catalyst is provided with the catalyst 42 arranged on an exhaust passage 2 and removing harmful components in exhaust gas, an upstream side air fuel ratio detecting means 43 for detecting an air fuel ratio in the catalyst upstream side, and a downstream side air fuel ratio detecting means 44 for detecting an air fuel ratio in the catalyst downstream side. During lean operation, deterioration of the catalyst 42 is determined by comparing a deviation of each excess air ratio calculated from outputs of the upstream side and downstream side air fuel ratio detecting means 43, 44 with a predetermined reference value. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a deterioration diagnosis device for a catalyst used in a diesel engine or a gasoline engine.

In a diesel engine or a gasoline engine, a catalyst is provided in the exhaust passage in order to purify harmful components discharged from the engine. However, if the catalyst deteriorates with use, a regular purification function cannot be obtained. Therefore, there is one that diagnoses whether or not the catalyst has deteriorated (see Patent Document 1).
Japanese Patent Laid-Open No. 7-109918

  By the way, in the apparatus described in the above-mentioned Patent Document 1, it is noticed that the oxidation-reduction reaction decreases as the deterioration of the catalyst proceeds, and even if the oxygen concentration upstream of the catalyst is the same, the downstream oxygen concentration is different. During the control, the deterioration of the catalyst is determined by looking at the deviation from the reference value of the output of the air-fuel ratio sensor on the downstream side of the catalyst.

  However, in this case, it is assumed that the air-fuel ratio is feedback-controlled to the target lean air-fuel ratio, and the determination is based on a deviation from a predetermined reference value. It is not possible to cope with it, and a highly accurate deterioration determination cannot be obtained.

  An object of the present invention is to provide a catalyst deterioration diagnosis apparatus that solves such a problem and that can accurately determine deterioration of a catalyst even at a lean air-fuel ratio.

  The present invention relates to a catalyst deterioration diagnosis device for an internal combustion engine such as a diesel engine or a gasoline engine, which is provided in an exhaust passage and purifies harmful components in exhaust gas, and an upstream side for detecting an air-fuel ratio upstream of the catalyst. Deviations of excess air ratios calculated from outputs of the air-fuel ratio detection means, the downstream air-fuel ratio detection means for detecting the air-fuel ratio downstream of the catalyst, and the upstream and downstream air-fuel ratio detection means during lean operation are predetermined. Deterioration determining means for determining deterioration of the catalyst in comparison with the reference value.

  Based on the phenomenon that HC, CO, etc. in the exhaust gas are oxidized by the catalyst during lean operation, and oxygen ions generated when the resulting moisture adheres to the catalyst decrease with the deterioration of the catalyst, the downstream side and upstream side of the catalyst. Focusing on the fact that the deviation of the excess air ratio on the side becomes smaller as it deteriorates, the deviation of the excess air ratio is compared with a predetermined judgment reference value, and when the deviation is smaller than the reference value, the catalyst is deteriorated. Since the determination is made, it is possible to accurately diagnose the deterioration of the catalyst. In particular, the air-fuel ratio upstream and downstream of the catalyst is detected, and the deviation of the excess air ratio calculated from these is compared with the reference value, so even if there is some variation in the air-fuel ratio at the time of judgment, there are individual differences in the catalyst. However, the deterioration diagnosis of the catalyst can be performed with high accuracy.

  Embodiments of the present invention will be described with reference to the drawings.

  FIG. 1 is a schematic configuration diagram showing an embodiment of the present invention.

  In FIG. 1, reference numeral 1 denotes a diesel engine, and an EGR valve 6 that is driven by a step motor 5 is provided in an EGR passage 4 that connects an exhaust passage 2 and a collector portion 3 a of the intake passage 3. The step motor 5 is driven by a control signal from the engine controller 31 and thereby obtains a predetermined EGR rate corresponding to the operating conditions.

  The engine 1 includes a common rail fuel injection device 10. The fuel injection device 10 mainly includes a fuel tank (not shown), a supply pump 14, a common rail (pressure accumulation chamber) 16, and a nozzle 17 provided for each cylinder. The fuel pressurized by the supply pump 14 is accumulated in the pressure accumulation chamber 16. And the high pressure fuel in the pressure accumulating chamber 16 is distributed to the nozzles 17 provided in each cylinder.

  The nozzle 17 (fuel injection valve) includes an electromagnetically driven needle valve. When the solenoid is OFF, the needle valve is in a seating state, but when the solenoid is in the ON state, the needle valve rises and the nozzle hole at the tip of the nozzle More fuel is injected. The fuel injection start timing is adjusted by switching the solenoid from OFF to ON, and the fuel injection amount is adjusted by the ON time. If the pressure in the pressure accumulating chamber 16 is the same, the fuel injection amount increases as the ON time increases.

  The exhaust passage 2 downstream of the opening of the EGR passage 4 is provided with a variable capacity turbocharger 21 in which a turbine 22 that converts heat energy of exhaust gas into rotational energy and a compressor 23 that compresses intake air are connected coaxially. A variable nozzle that is driven by an actuator (not shown) is provided at the scroll inlet of the turbine 22, and the engine controller 31 allows the variable nozzle to obtain a predetermined supercharging pressure from the low rotational speed range. The nozzle opening degree for increasing the flow rate of the exhaust gas introduced into the turbine 22 is controlled to the nozzle opening degree (fully opened state) by introducing the exhaust gas into the turbine 22 without resistance on the high rotational speed side.

  An intake throttle valve 8 driven by an actuator (not shown) is provided at the collector 3a inlet.

  An engine to which signals from an accelerator sensor 32 that detects an accelerator opening, a sensor 33 that detects an engine speed and a crank angle, a water temperature sensor 34 that detects an engine coolant temperature, and an air flow meter 35 that detects an intake air amount are input. Based on these signals, the controller 31 performs EGR control and supercharging pressure control in a coordinated manner so that an optimal target EGR rate and target supercharging pressure can be obtained according to the operating state.

  A filter 41 that collects particulates in the exhaust is installed in the exhaust passage 2. When the accumulated amount of particulates in the filter 41 reaches a predetermined value, the exhaust gas temperature is raised and the particulates accumulated in the filter 41 are burned and removed to be regenerated. Switch to rich air-fuel ratio.

  An HC trap catalyst 42 is provided on the upstream side of the filter 41. The HC trap catalyst 42 traps HC when the exhaust gas is at a low temperature, and oxidizes and purifies by using oxygen in the exhaust gas while desorbing and releasing the trapped HC when the exhaust gas exceeds a predetermined temperature. It is what has. In addition, it functions as a normal oxidation catalyst after catalytic activity.

  Wide-range air-fuel ratio sensors 43 and 44 are provided before and after the HC trap catalyst 42, and a wide-area air-fuel ratio sensor (hereinafter simply referred to as “upstream sensor”) 43 upstream of the catalyst 42 and a wide-area air-fuel ratio sensor (hereinafter simply referred to as “upstream sensor”). Based on the output of the “downstream sensor” 44, the engine now roller 31 performs catalyst deterioration diagnosis. In this case, since the diesel engine is mainly operated at a lean air-fuel ratio, the HC trap catalyst is based on the excess air ratio before and after the catalyst detected by the upstream sensor 43 and the downstream sensor 44 when operating at the lean air-fuel ratio. It is determined whether or not 42 has deteriorated. A temperature sensor 36 detects the temperature of the catalyst 42.

  The deterioration diagnosis principle of the catalyst 42 is as follows.

  The reason why the deterioration diagnosis of the HC trap catalyst 42 can be performed at the time of operation at a lean air-fuel ratio is based on new knowledge obtained by the present inventors through experiments on the catalyst 42. To explain this, FIG. 2 shows how the excess air ratio at the catalyst inlet and the excess air ratio at the catalyst outlet actually change when the engine is operated according to a predetermined lean operation mode, including the passage of time. It shows that.

  When the exhaust air-fuel ratio is controlled to a substantially constant value on the lean side, the excess air ratio on the downstream side becomes larger than the excess air ratio on the upstream side of the catalyst. Moreover, the difference in the excess air ratio tends to be smaller when the catalyst is deteriorated than when the catalyst is new.

  The following reasons can be considered for this.

The harmful components discharged from the diesel engine 1 are mainly HC and CO at low temperatures, and are converted to harmless H ₂ O and CO ₂ by oxidizing them with a catalyst. In this case, H ₂ O, that is, moisture adheres to a noble metal (for example, platinum) as a catalyst and is ionized into hydrogen H ₂ and oxygen ions O ^2−, and this oxygen ion content is detected by the downstream wide area air-fuel ratio sensor 44. Since it is felt as oxygen, it is considered that the downstream sensor 44 outputs a higher oxygen concentration than the upstream sensor 43. In other words, in the region where the excess air ratio exceeds 1.0, a difference in excess air ratio occurs before and after the catalyst due to oxygen ion-derived components such as H ₂ O.

Then, as the catalyst is new, more hydrogen H ₂ and oxygen ions O ²⁻ are ionized from H ₂ O adhering to the catalyst metal. It can be assumed that it will grow.

  In order to confirm whether the characteristics of the catalyst shown in FIG. 2 are unique to the HC trap catalyst 42, the same experiment was conducted for other catalysts (NOx trap catalyst, three-way catalyst, oxidation catalyst, etc.). As a result, it has been found that the same characteristics can be obtained with any catalyst regardless of the type of the catalyst. Therefore, in this embodiment, an example of application to a diesel engine that operates mainly at a lean air-fuel ratio will be described, but the present invention can also be applied to a gasoline engine that may be operated at a lean air-fuel ratio and that has a catalyst in the exhaust passage. .

  The upstream sensor 43 and the downstream sensor 44 (wide air-fuel ratio sensor) function as upstream and downstream air-fuel ratio detection means and oxygen ion-derived component detection means, respectively, and the output is the stoichiometric air-fuel ratio. Even an oxygen sensor that changes relatively greatly at the boundary has some output change depending on the oxygen concentration in the lean region, so it can also be used as an air-fuel ratio detection means and oxygen ion-derived component detection means. is there.

  Here, the diagnostic contents of the catalyst executed by the engine controller 31 will be described in detail with reference to the flowcharts of FIGS. First, FIG. 3 is for starting the catalyst deterioration diagnosis, and is repeatedly executed at regular intervals.

  In step S1, the operation state is read based on the signals of the various sensors described above, and in step S2, it is determined whether the exhaust flow rate is larger than a predetermined flow rate, that is, the exhaust flow rate as shown in FIG. When the flow rate is smaller than the predetermined flow rate, the process proceeds to step S4 to prohibit the catalyst deterioration determination. When the flow rate is smaller than the flow rate, the HC conversion rate before and after catalyst deterioration does not change, and the oxygen ion generation rate downstream of the catalyst changes depending on the HC conversion rate. Therefore, if the conversion rate is the same, even if the catalyst is deteriorated, there is no difference in the determination results, that is, accurate determination cannot be performed.

  When the flow rate is higher than the predetermined flow rate, the process proceeds to step S3 to determine whether or not the catalyst temperature is higher than the predetermined temperature. When the temperature is higher than the predetermined temperature, the process proceeds to step S5, and the deterioration determination of the catalyst 42 is prohibited. As shown in FIG. 6, the predetermined temperature is a region where the conversion rate of HC before and after the deterioration of the catalyst 42 does not change when the temperature is higher than that temperature. This is because there is no difference.

  When the temperature is lower than the predetermined temperature, the process proceeds to step S6 and the catalyst deterioration diagnosis is started.

  FIG. 4 is for performing catalyst deterioration diagnosis, and is repeatedly executed at regular intervals.

  In step S11, it is determined whether or not the lean operation condition is satisfied. Since there is little change in the excess air ratio before and after the catalyst 42 except for lean, an accurate deterioration determination cannot be performed, and thus the routine is terminated without performing diagnosis. .

  If the lean operation condition is in step S11, the process proceeds to step S12, and in order to determine the catalyst deterioration, first, based on the outputs of the upstream sensor 43 and the downstream sensor 44 of the catalyst 42, the excess air in the upstream and downstream respectively. The ratios λF and λR are calculated, and Δλ, which is the deviation between the downstream excess air ratio λR and the upstream excess air ratio λF, is calculated as Δλ = λR−λF.

  Next, in step S13, a reference value Δλt for determining deterioration of the catalyst 42 is calculated. This Δλt is a value between the deviation of the upstream excess air ratio and the downstream excess air ratio. The determination reference value Δλt is set in consideration of the specification of the catalyst 12 and the like.

  In step S14, the above-mentioned Δλ is compared with a predetermined value Δλt. If Δλ is larger than the predetermined value Δλt, the process proceeds to step S15 and the catalyst 12 is not deteriorated, and the catalyst deterioration flag is not set. If the function of the catalyst 42 is normal, as described above, the oxygen ion generation rate in the catalyst 42 is increased, and the oxygen concentration in the downstream is higher than that in the upstream of the catalyst. Therefore, Δλ, which is the difference between the excess air ratio λR on the downstream side and the excess air ratio λF on the upstream side, increases as the catalyst 42 becomes new, and decreases as the catalyst 42 deteriorates. Therefore, when Δλ is larger than the predetermined value Δλt, it is possible to determine that the catalyst 42 is in a normal state with no deterioration.

  On the other hand, when Δλ is not larger than Δλt in step S15, it proceeds to step S16, determines that the catalyst 42 has deteriorated, sets a catalyst deterioration flag in step S17, and based on this, sets the catalyst deterioration flag. Inform and warn of deterioration.

  Thus, the catalyst deterioration diagnosis is performed during operation at the lean air-fuel ratio, the excess air ratio is calculated based on the upstream and downstream exhaust air-fuel ratios of the catalyst 42, and the downstream and upstream excess air ratios are calculated. Is larger than Δλt, which is a deterioration criterion determination value, it is determined that the rate of oxygen ion generation by the catalyst is high, and the catalyst 42 is functioning normally, whereas Δλ is When it is smaller than Δλt, the rate of generation of oxygen ions by the catalyst 42 is small, and it is determined that the catalyst 42 has deteriorated.

In the catalyst deterioration diagnosis of FIG. 4 described above, the deterioration diagnosis is started during the operation at the lean air-fuel ratio. Thus, the lean air-fuel ratio may be set to an air-fuel ratio that contains more oxygen ion-derived components in the exhaust gas, that is, contains more HC as a reducing agent. That is, it does not increase until the stoichiometric air-fuel ratio, but by setting a slightly leaner air-fuel ratio, if the amount of HC and CO contained in the exhaust gas flowing into the catalyst 42 increases, the amount of H ₂ generated during the catalytic reaction increases accordingly. O also increases, and the amount of oxygen ions that ionize by touching the catalyst metal increases. For this reason, even when the air-fuel ratio is the same, the deviation of the excess air ratio between the upstream and downstream of the catalyst 42 becomes large, and the deterioration determination of the catalyst 42 can be performed with higher accuracy.

  Note that the reducing agent, which is unburned from the diesel engine, can be increased by increasing the air-fuel ratio in the lean region. For example, CO (carbon monoxide) can be increased by combustion in the engine combustion chamber. The produced HC (hydrocarbon) can also be produced by injecting fuel into the exhaust passage.

  As described above, according to the present embodiment, HC, CO, etc. in the exhaust gas are oxidized by the catalyst 42 during the lean operation, and oxygen ions generated when the moisture generated thereby adheres to the catalyst 42 together with the deterioration of the catalyst. Focusing on the fact that the deviation of the excess air ratio between the downstream side and the upstream side of the catalyst becomes smaller as it deteriorates, based on the phenomenon of decreasing, the deviation of the excess air ratio is compared with a predetermined criterion value, However, when the deviation is small, it is determined that the catalyst 42 has deteriorated, so that the deterioration of the catalyst 42 can be accurately diagnosed. In particular, the air-fuel ratio upstream and downstream of the catalyst is detected, and these deviations are compared with a reference value. Therefore, even if there is a slight variation in the air-fuel ratio at the time of determination or there is an individual difference in the catalyst 42, the catalyst 42 Can be accurately diagnosed.

  Further, in the deterioration diagnosis of the catalyst 42, in the region where the exhaust gas flow rate is not more than a predetermined value and the HC conversion rate does not change regardless of the deterioration of the catalyst 42, the amount of HC that affects the generation of oxygen ions in the catalyst 42 is determined. Since there is no change, an accurate deterioration diagnosis cannot be performed. Therefore, by prohibiting the diagnosis, the erroneous diagnosis can be prevented and the reliability can be improved.

  Similarly, in the region where the exhaust gas temperature at which the conversion rate of HC in the catalyst 42 becomes the same is higher than a predetermined value, the diagnosis is prohibited to avoid erroneous diagnosis and improve reliability.

  In addition, for the deterioration determination of the catalyst 42, the more HC and CO adhering to the catalyst 42, that is, the more the reducing agent, the higher the generation rate of oxygen ions, and the larger the deviation of the excess air ratio between upstream and downstream, The accuracy of determination is increased. Therefore, for example, by controlling the combustion air-fuel ratio in the engine or injecting fuel into the exhaust passage so that the amount of reducing agent in the exhaust gas is as much as possible within the range of the lean air-fuel ratio at the time of deterioration determination, It becomes possible to perform deterioration determination with high accuracy.

  In the above embodiment, an example in which the present invention is applied to a diesel engine has been described. However, the present invention is not limited to this, and can naturally be applied to a gasoline engine that performs lean burn. Further, as the catalyst, not only the HC trap catalyst but also a NOx trap catalyst, a three-way catalyst, an oxidation catalyst, and the like can be similarly subjected to degradation diagnosis.

  The catalyst deterioration diagnosis device of the present invention can be applied to a diesel engine, a gasoline engine, or the like.

It is a schematic structure figure showing one embodiment of the present invention. It is a wave form diagram which shows the change of the excess air ratio of a catalyst inlet, and the excess air ratio of a catalyst exit when driving a diesel engine according to a predetermined operation mode. It is a flowchart which shows the control content of deterioration determination of a catalyst. It is also a flowchart. FIG. 6 is a characteristic diagram showing an exhaust flow rate characteristic at the start of catalyst deterioration determination. It is a characteristic view which shows the catalyst temperature characteristic at the time of a catalyst deterioration determination start.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Engine 2 Exhaust passage 31 Engine controller 33 Crank angle sensor 42 HC trap catalyst 43 Upstream side sensor (Upstream side air-fuel ratio detection means, oxygen ion origin component detection means)
44 Downstream sensor (downstream air-fuel ratio detection means, oxygen ion-derived component detection means)

Claims (8)

In internal combustion engines,
A catalyst provided in the exhaust passage for purifying harmful components in the exhaust;
Upstream air-fuel ratio detecting means for detecting the air-fuel ratio upstream of the catalyst;
Downstream air-fuel ratio detecting means for detecting the air-fuel ratio downstream of the catalyst;
And a deterioration determining means for determining the deterioration of the catalyst by comparing the deviation of each excess air ratio calculated from the outputs of the upstream and downstream air-fuel ratio detecting means during lean operation with a predetermined reference value. Catalyst deterioration diagnosis device.   2. The catalyst deterioration diagnosis apparatus according to claim 1, wherein the downstream air-fuel ratio detection means is a wide-range air-fuel ratio sensor or an oxygen sensor that detects an air-fuel ratio containing an oxygen ion-derived component.   The catalyst deterioration diagnosis device according to claim 1 or 2, wherein when the exhaust flow rate is smaller than a predetermined flow rate, the catalyst deterioration determination is prohibited.   The catalyst deterioration diagnosis device according to any one of claims 1 to 3, wherein determination of deterioration of the catalyst is prohibited when the temperature of the catalyst is higher than a predetermined temperature.   The catalyst deterioration diagnosis apparatus according to any one of claims 1 to 4, wherein the catalyst deterioration determination is performed in a state where the amount of reducing agent in the exhaust gas is larger than a normal lean air-fuel ratio.   The catalyst deterioration diagnosis apparatus according to claim 5, wherein the reducing agent is at least one of carbon monoxide and hydrocarbon.   The catalyst deterioration diagnosis apparatus according to claim 6, wherein the carbon monoxide is generated by combustion of an engine.   The catalyst deterioration diagnosis apparatus according to claim 6, wherein the hydrocarbon is generated by fuel injection into an exhaust passage.

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