JP4292947B2 - Exhaust gas purification device for internal combustion engine - Google Patents

Description

  The present invention relates to an exhaust gas purification apparatus for an internal combustion engine that purifies exhaust gas using a NOx storage catalyst.

  In an engine (internal combustion engine) mounted on an automobile, as an exhaust gas countermeasure, an exhaust gas purification device using a NOx storage catalyst is equipped to purify NOx contained in the exhaust gas of the engine. In particular, the NOx storage catalyst stores NOx when the air-fuel ratio of the exhaust gas flowing into the NOx storage catalyst is lean (lean than the stoichiometric air-fuel ratio), and the air-fuel ratio of the exhaust gas is rich (excessive concentration including the stoichiometric air-fuel ratio). In general, it is often used in diesel engines, lean burn gasoline engines, and the like that tend to operate with lean air-fuel ratios because they have the characteristic of releasing and reducing the stored NOx.

  By the way, the NOx occlusion catalyst of the exhaust purification device occludes NOx in the exhaust gas in a normal lean operation state, but the occlusion capacity of the NOx occlusion catalyst is limited, and when it reaches a certain occlusion amount, NOx purification is performed. Ability is reduced. Therefore, the NOx storage catalyst performs a rich operation of the engine at a certain time to release and reduce the stored NOx to maintain the NOx purification performance (see, for example, Patent Document 1).

The action of shifting to the rich operation in order to regenerate the NOx storage catalyst is called rich spike (RS), and in many cases, the lean operation is periodically performed, specifically, a certain amount of time obtained by integrating the lean operation time. Then, the engine is switched to rich operation.
JP-A-6-272540

  By the way, the catalyst temperature of the NOx storage catalyst changes according to the operating state of the engine (vehicle). Specifically, the catalyst temperature of the NOx storage catalyst is the temperature range where the catalyst activity rises due to the change in exhaust gas temperature during engine operation (from the temperature at which the catalyst activity starts to the upper limit of activity), and the activity in the higher temperature range The temperature ranges from a low to high temperature range such as a temperature range where the upper limit is maintained, a higher temperature range, specifically, a temperature range where activity is easily lost due to high temperature (a temperature range where activity decreases from the upper limit of activity).

  Since the periodic rich spike is performed for these various catalyst temperatures, the rich spike in the temperature range where the upper limit of the activity is maintained exhibits sufficient NOx reduction performance, but is performed in the rising temperature range. In the case of the rich spike, the catalyst temperature is insufficient, so that the NOx reduction performance is not sufficiently exhibited. In addition, in the case of a rich spike performed in a temperature range where the activity on the high temperature side is likely to be lost (temperature range where the activity decreases from the upper limit temperature of the activity), on the contrary, the catalyst temperature exceeds the temperature at which the catalyst activity is lost due to the reaction heat caused by NOx reduction There is a tendency for an excessive temperature rise phenomenon to occur. For this reason, there is a problem that the region where the purification performance of the NOx storage catalyst can be stably maintained is narrow and the NOx purification performance cannot be satisfactorily exhibited.

  SUMMARY OF THE INVENTION An object of the present invention is to provide an exhaust gas purifying device for an internal combustion engine capable of expanding an area where high NOx purification performance can be stably exhibited.

In order to achieve the above object, according to the present invention, the execution cycle for periodically executing rich spikes during lean operation is changed according to the temperature of the NOx storage catalyst or a parameter corresponding to the temperature.

Further, in the present invention , the execution cycle is shorter than the normal time value at least in the temperature range from the temperature at which the catalyst activity is started to reach the upper limit of the activity, so that stable and stable NOx purification performance is ensured. In the temperature range where the activity is higher than the upper limit temperature and the catalyst activity decreases, the execution cycle is made longer than the normal time value .

According to the present invention, when the rich spike (rich operation) is executed in the temperature range on the low temperature side where the catalyst activity is insufficient, the execution cycle is shortened, and the NOx reduction reaction accompanying the increase of the reducing agent is accelerated. The NOx storage catalyst is heated. Further, when rich spike is executed in the high temperature range where the catalytic activity is likely to be lost , the execution cycle becomes longer, and the NOx reduction reaction accompanying the reduction of the reducing agent is suppressed, so that the temperature rise of the NOx storage catalyst can be suppressed. .

  Therefore, when the catalyst temperature is low, the NOx reduction performance is maximized, and when the catalyst temperature is high, the NOx reduction performance can be prevented from being lowered. Therefore, the region where high NOx purification performance can be stably exhibited is expanded. Can be achieved.

Furthermore, according to the present invention, there is an effect that the NOx purification performance can be secured at a high level by changing the execution cycle of the rich spike (rich operation) with respect to the change of the catalyst temperature .

[First Embodiment]
Hereinafter, the present invention will be described based on a first embodiment shown in FIGS.

  FIG. 1 shows a main part of an internal combustion engine (internal combustion engine), for example, a diesel engine, which is mounted on an automobile (vehicle). In FIG. 1, reference numeral 1 denotes an engine body composed of a cylinder block 2 and a cylinder head 3. , 4 is a cylinder formed in the cylinder block 2, 5 is a piston provided in a reciprocating manner in the cylinder 4, 6 and 7 are intake / exhaust ports provided in the cylinder head 3, and 8 and 9 are intake / exhaust ports The intake / exhaust valves 10 for opening and closing the exhaust ports 6 and 7 are injectors provided in the cylinder head 3. Among these, the intake port 6 is connected to the discharge part of the compressor 14 of the turbocharger 13 via the first intake passage 12 extending from the intake port 6. In addition, the suction part of the compressor 14 is connected to the 2nd intake passage 15 which goes to an air cleaner (not shown). However, 17 is an intercooler interposed in the first intake passage 12. The exhaust port 9 is connected to an inlet portion of the turbine 19 of the turbocharger 13 via a first exhaust passage 18 extending from the exhaust port 9. A second exhaust passage 20 that is open to the atmosphere is connected to the outlet of the turbine 19. Moreover, the injector 10 is connected to ECU21 which comprises a control part. The injection operation of the injector 10 is controlled according to the injection timing and the fuel injection amount that are set in advance in the ECU 21 according to the operating state of the engine, and the engine performs a predetermined cycle (for example, suction, compression, expansion, (Exhaust cycle 4) (Normal operation: lean operation (because the excess air ratio is usually large)).

  Note that, between the downstream side of the first intake passage 12 and the upstream side of the first exhaust passage 18, each device constituting the EGR device 22, for example, an EGR passage 24 in which an EGR cooler 23 is interposed, and the EGR passage are provided. An EGR valve 25 that opens and closes 24 is provided, and an electric throttle valve 26 is provided in an upstream intake passage portion that merges with an outlet of the EGR passage 24.

  An exhaust gas purification device 30 is assembled in the exhaust system of such a diesel engine. The exhaust gas purification apparatus 30 uses a configuration in which a NOx storage catalyst 33 with a built-in casing 32, a reducing agent addition unit 34 that supplies a reducing agent, and a control system 35 that performs regeneration control are combined.

  That is, the NOx storage catalyst 33 is interposed in the middle of the second exhaust passage 20. As the structure, for example, a structure in which a carrier is supported with a noble metal such as platinum (Pt) and barium (Ba) as an occlusion agent is used. With this structure, when the air-fuel ratio of the exhaust gas flowing into the NOx storage catalyst 33 is lean (lean than the stoichiometric air-fuel ratio), NOx in the exhaust reacts with oxygen on platinum (Pt), and in the form of nitrate ions When the air-fuel ratio of the exhaust gas absorbed by barium (Ba) and flowing into the NOx storage catalyst 33 is rich (overconcentration including theoretical air fuel), the nitrate ions in barium (Ba) are released in the form of NOx. In addition, the released NOx reacts with unburned HC, CO, etc. in the exhaust gas on platinum (Pt) to reduce it to nitrogen.

  In the reducing agent adding section 34, for example, a reducing agent injector 36 for injecting a reducing agent, for example, fuel (for example, light oil in this case) into the exhaust passage 20 from the upstream side of the NOx storage catalyst 33, and the previous EGR device 22. A structure using a combination of and is used. That is, the rich operation is performed by a technique in which fuel (reducing agent) is added to the exhaust gas flowing through the second exhaust passage 20 and the EGR gas is recirculated to the engine intake side. Thus, during normal operation of the diesel engine (lean operation), when necessary, rich exhaust gas containing a large amount of unburned HC and CO serving as a reducing agent flows into the NOx storage catalyst 33. It is configured.

  As a control system 35, the ECU 21 has a function of periodically shifting from lean operation to rich operation to perform rich spikes during lean operation (usually), and a transition period for shifting to rich operation, and the temperature of the NOx storage catalyst 33. The function to change according to is set.

  Among these functions, for the function of periodically shifting to rich operation, for example, a timer is used to accumulate the lean operation time, and the accumulated time is set to a predetermined interval, that is, a value (time) of a predetermined RS (rich spike) interval trs. When reaching, the operation is switched to the rich operation, and the operation is performed for a period of time corresponding to the NOx occlusion amount estimated by the accumulated time, so that the rich spike is periodically executed.

For the control to change the execution cycle of the rich spike , for example, a map for determining the execution cycle in which the catalyst temperature is associated with the rich spike interval (hereinafter referred to as the RS interval trs ) is used. Control is used to change the RS interval ( trs ).

That is, in changing the RS interval (trs), considering the NOx reduction characteristics of the NOx storage catalyst 33 shown in FIG. 4, the low temperature side from the temperature at which the catalytic activity of the NOx storage catalyst 33 starts to the upper limit of activity is reached. In the temperature range α, the RS interval (trs) is shorter than that in normal time (the temperature range in which the activity upper limit continues), higher than the activity upper limit temperature, and in the temperature range β on the high temperature side where the catalytic activity decreases, RS The interval (trs) is set longer than that in the normal state. Therefore, a quadratic curve diagram in which the RS interval (trs) is changed in accordance with the catalyst temperature as shown in the map of step S5 in FIG. 2 is used in the execution cycle determination map. . Specifically, when the temperature at which the NOx storage catalyst 33 is sufficiently activated , the time value of the RS interval (trs) that has been normally used until now is set, and as the temperature becomes lower than the temperature, A characteristic diagram is used in which the time value of the normal RS interval (trs) becomes gradually shorter and becomes higher than the temperature, and the time value of the normal RS interval (trs) becomes gradually longer. The RS interval (trs) is determined by, for example, exhaust gas temperature sensors 39 and 40 (temperature detecting means for detecting the catalyst temperature) provided on both the entrance and exit sides of the NOx storage catalyst 33 after the end of the previous rich spike, for example. The catalyst temperature (Tc) representing the NOx storage catalyst 33 is detected, and the time value of the corresponding RS interval (trs) is selected from the map (step S5) based on the detected catalyst temperature (Tc). The control for executing the rich spike according to the time value of the selected RS interval (trs) is used. Furthermore, a function for correcting the selected time value is added to the control. This is formed by the function of re-determining the time value of the determined RS interval (trs) when the catalyst temperature (Tc) changes after the determination of the RS interval (trs) and before the rich spike is performed. ing. Specifically, for example, after the RS interval (trs) is determined and before the rich spike is executed, the catalyst temperature is detected again, and the detected catalyst temperature and the previous RS interval (trs) are detected again. The RS interval ( trs ) is re-determined only when the difference (absolute value) from the catalyst temperature detected in this determination becomes larger than a predetermined temperature difference value ΔT that greatly affects NOx reduction performance. . Thereby, the temperature change of the catalyst temperature (Tc) is reflected in the setting of the RS interval ( trs ) as much as possible.

  Such periodic rich spike control is illustrated in the flowchart of FIG. If the operation of the exhaust gas purifying device 30 is described with reference to the flowchart of FIG. 2, it is assumed that the previous rich spike has ended during the normal operation (lean operation) of the diesel engine.

  Then, first, the ECU 21 resets the timer that defined the execution time (t = 0) as shown in step S1 with the end of the execution of the rich spike. Then, it progresses to step S2, and the catalyst temperature (Tc) of the NOx storage catalyst 33 is detected by the outputs from the exhaust gas temperature sensors 39 and 40. The detected catalyst temperature (Tc) is extremely high or extremely low, that is, a high temperature or NOx occlusion catalyst 33 that causes thermal degradation of the NOx occlusion catalyst 33 by the determination processing shown in steps S3 and S4. If it is determined that the temperature is not low and the temperature is not active, it is determined that the rich spike can be transferred, and the processing enters the processing for determining the rich spike cycle (RS interval) in the next step S5.

In step S5, an RS interval ( trs ) corresponding to the detected catalyst temperature (Tc) is selected from the map. At this time, if the detected catalyst temperature (Tc) is, for example, the temperature in the upper limit temperature range of the NOx storage catalyst 33 where the NOx reduction performance is sufficiently exhibited, a request for a rich spike has been made regularly until now. In addition, a time value having the same RS interval ( trs ) as the normal period is selected. If the temperature is in the temperature range α from the start of activity to the upper limit of activity, the time value ( trs 1) corresponding to the temperature value in the temperature range α, which is shorter than the normal RS interval ( trs ) time value, is selected. It is. If the temperature is in the temperature range β lowering from the upper limit of activity, a time value ( trs 2) corresponding to the temperature value in the temperature range β that is longer than the time value of the normal RS interval ( trs ) is selected. This selection changes the RS interval (trs) is performed (changing the execution cycle of the rich spike).

When the RS interval ( trs ) is determined, the ECU 21 proceeds to step S7 through subsequent step S6, starts a timer, and starts to accumulate lean operation time after execution of rich spike.

  While this timing is being performed, a process for detecting a change in the catalyst temperature is performed. Specifically, the process of detecting the catalyst temperature (Tc ′) again performed in step S10, the difference between the previous catalyst temperature (Tc) performed in the next step S11 and the re-detected catalyst temperature (Tc ′) ( (Absolute value) is calculated to determine whether or not the temperature has changed from a predetermined temperature difference value (ΔT), and the degree of change in the catalyst temperature is detected.

At this time, if the calculated temperature value is equal to or less than the temperature difference value (ΔT), the ECU 21 determines that there is no change in the catalyst temperature that causes a change in the RS interval ( trs ). When the calculated temperature value exceeds the temperature difference value (ΔT), it is determined that a change in the catalyst temperature that causes a change in the RS interval (trs) has occurred, and the re-detected catalyst temperature as in step S11. After replacing Tc ′ with the initial catalyst temperature Tc, the process returns to steps S3 to S6 again, and the RS interval ( trs ) is determined again from the map. Such correction of the RS interval ( trs ) is repeatedly performed until the rich spike is performed.

Thereafter, when the timed timer time (t) exceeds the RS interval (trs), the process proceeds to step S9, and the ECU 21 sets the RS interval ( trs ) that does not require correction or the corrected RS interval ( trs ). Instructs execution of rich spike.

Thereby, when the catalyst temperature is not sufficient for the activation temperature, specifically, for example, the temperature range α in FIG. 4 (temperature range from the temperature at which the catalyst activity starts to the activation upper limit: for example, 150 ° C. to 200 ° C. ) Is shorter than that in normal time (temperature range in which the upper limit of activity continues), for example, with a short execution cycle such as the RS interval ( trs 1: 30 seconds) as shown by the broken line in FIG. Are executed periodically.

Further, when the catalyst temperature is high and the activity tends to lose its activity, specifically, in the temperature range β in FIG. 4 (temperature range falling from the upper limit of activity: for example, 450 ° C. to 550 ° C.), the normal time (the upper limit of activity continues). Rich spikes are periodically executed with a long execution period longer than that in the temperature range (eg, RS interval ( trs 2: 90 seconds, for example)) as indicated by a solid line in FIG.

  Here, looking at the reduction characteristics of the NOx occlusion catalyst 33, as shown in FIG. 5, the NOx occlusion catalyst increases the reaction heat (temperature increase range) as the reduction reaction proceeds as the amount of reducing agent input increases. If it occurs and the input amount of the reducing agent is reduced, the reduction reaction is suppressed and the reaction heat (temperature rise width) is reduced.

  At this time, the rich spike with a short execution cycle has more opportunities to execute the rich spike than the spike at the normal cycle, and the amount of reducing agent to be introduced is larger than the normal spike. As a result, the temperature of the NOx storage catalyst 33 is greatly increased. That is, the NOx storage catalyst 33 is heated to the upper limit temperature at which the catalytic activity can be sufficiently exerted. As a result, the NOx reduction performance of the NOx storage catalyst 33 is exhibited to the maximum even in a temperature range where the low-temperature side catalyst activity starts to rise where the activity is insufficient.

  On the other hand, a rich spike with a long execution cycle has fewer opportunities to carry out a rich spike than a spike at a normal cycle, and the amount of reducing agent to be introduced is smaller than that in a normal cycle. Is suppressed, and the temperature rise of the NOx storage catalyst 33 is suppressed. That is, the NOx storage catalyst 33 suppresses the behavior that the temperature rises to a temperature higher than the temperature at which the activity is lost (overheating phenomenon) even in a high temperature range close to the overactive temperature, and the behavior that the NOx reduction performance is lost (decreased) is suppressed. Is done. Thereby, the NOx storage catalyst 33 continues to ensure the NOx reduction performance even in the high temperature side catalyst temperature range where the activity tends to be lost. In addition, thermal deterioration of the NOx storage catalyst 33 due to excessive temperature rise can be prevented.

Therefore, the NOx occlusion catalyst 33 can expand the region where it can continue to exhibit stable and high NOx purification performance by changing the execution cycle of the rich spike according to the catalyst temperature. In particular the execution period in the temperature range α of at least the temperature at which the catalytic activity of the NOx storage catalyst 33 starts to the active upper, shorter than normal, the execution period in the temperature range β descending from the active upper, longer than normal If it sets so, expansion of the stable NOx purification performance can be ensured easily.

[Second Embodiment]
FIG. 6 shows a second embodiment of the present invention. The present embodiment does not change the execution cycle of the rich spike (rich operation) according to the catalyst temperature of the NOX storage catalyst as in the first embodiment, but uses a parameter corresponding to the catalyst temperature to perform the rich spike. The execution cycle of is changed .

Specifically, in the present embodiment, instead of the catalyst temperature, parameters such as the engine load (for example, accelerator opening) and the engine speed as shown in FIG. By adopting control to change the execution cycle, the rich spike (rich operation) execution cycle is made shorter than normal when the engine operating state corresponds to when the catalyst temperature rises, and the catalyst temperature falls from the upper limit of activity. When the engine operating state is equivalent, the execution cycle of the rich spike (rich operation) is to be set longer than usual.

  Even if it does in this way, there exists an effect similar to 1st Embodiment.

  However, since this embodiment is the same as the first embodiment except that the engine load and the engine speed are used, other parts are diverted from the first embodiment and the drawings are omitted. .

  In addition, this invention is not limited to each embodiment mentioned above, You may implement in various changes within the range which does not deviate from the main point of this invention. For example, in any of the above-described embodiments, the present invention is applied to a diesel engine. However, the present invention is not limited thereto, and the present invention may be applied to other engines such as a gasoline engine.

The figure which shows together schematic structure of the exhaust gas purification apparatus which concerns on the 1st Embodiment of this invention. The flowchart for demonstrating the control which changes the execution period of a rich spike according to a catalyst temperature. The diagram for demonstrating the change condition of the rich spike period (RS interval) for every catalyst temperature. The diagram for demonstrating the change of NOx reduction performance with respect to the catalyst temperature of a NOx storage catalyst. The diagram for demonstrating the temperature rising condition with respect to the reducing agent injection quantity of a NOx storage catalyst. The diagram for demonstrating the principal part of the 2nd Embodiment of this invention.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Engine main body, 20 ... 2nd exhaust passage (exhaust passage), 21 ... ECU (rich spike means, period variable means), 33 ... NOx occlusion catalyst.

Claims (1)

NOx storage catalyst provided in the exhaust passage of the internal combustion engine, which stores NOx in the exhaust gas when the internal combustion engine is in a lean operation state, and releases and reduces the stored NOx when the internal combustion engine is in a rich operation state When,
Rich spike means for periodically executing a rich spike during the lean operation of the internal combustion engine;
A cycle changing means for changing the execution cycle for executing the rich spike according to the temperature of the NOx storage catalyst or a parameter corresponding to the temperature ;
In addition, when the temperature of the NOx storage catalyst is in the temperature range α from the temperature at which the catalytic activity of the NOx storage catalyst is started to the upper limit of activity, the cycle changing means sets the temperature value in the temperature range α. A time value shorter than the corresponding normal time value is selected as the execution period, and when the temperature of the NOx storage catalyst is higher than the upper limit temperature of activation and in the temperature range β where the activity decreases, the temperature range β An exhaust gas purification apparatus for an internal combustion engine, wherein a time value longer than a normal time value according to the temperature value is selected as the execution cycle .

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