JP2004346793A - Exhaust emission control device for internal combustion engine - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an exhaust emission control device for an internal combustion engine capable of suppressing worsening of fuel economy while sufficiently attaining the recovery of NOx purifying performance by efficiently purging sulfur in a short time from a low temperature state. <P>SOLUTION: This exhaust emission control device for the internal combustion engine has an NOx storage catalyst 16 interposed in an exhaust pipe 7 to absorb NOx in exhaust gas in a lean gas state with high oxygen concentration while releasing/reducing the NOx in a rich gas state with low oxygen concentration and high reducing agent concentration. The exhaust emission control device is provided with an electronic control unit 30 for performing opening/closing control of a fuel injection valve 21 for injecting fuel to the exhaust pipe 7, to alternately change over to the rich gas state and the lean gas state from the start time to the end time of sulfur purge of desorbing SOx poisoning the NOx storage catalyst 16. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an exhaust purification device for an internal combustion engine in which a NOx storage catalyst is interposed in an exhaust passage.
[0002]
[Prior art]
2. Description of the Related Art In recent years, lean-burn internal combustion engines in which the internal combustion engine is operated at a lean air-fuel ratio to improve fuel efficiency and the like have been put to practical use. This lean-burn internal combustion engine has a problem in that when operated at a lean air-fuel ratio, the three-way catalyst cannot sufficiently purify nitrogen oxides (NOx) in exhaust gas due to its purification characteristics. A NOx storage (type) catalyst that stores NOx in exhaust gas therein and releases and reduces NOx stored during operation at a stoichiometric or rich air-fuel ratio is employed.
[0003]
The NOx storage catalyst, NOx in exhaust gas in an excess oxygen state of the internal combustion engine (lean state) occluded as nitrate (X-NO _3), occluded NOx with a reducing agent carbon monoxide (CO), etc. This is a catalyst having the property of releasing in an excess state (rich gas state) and reducing it to nitrogen (N ₂ ) (carbonate X-CO ₃ is generated at the same time). By the way, the fuel contains a sulfur (S) component, and the S component reacts with oxygen in the lean gas state to form a sulfur oxide (SOx), and this SOx is stored as a sulfate in the form of NOx as well as NOx. Occluded by the catalyst.
[0004]
When the sulfur component is occluded in the NOx storage catalyst in this manner, the NOx is not stored, so that the performance of the NOx storage catalyst is reduced. When the sulfur component ratio is increased, the NOx storage catalyst does not function as a NOx storage catalyst. Therefore, in order to maintain the performance of the NOx storage catalyst, it is necessary to periodically release (S purge) SOx (sulfur component) that has been stored (poisoned).
[0005]
It is said that the S purge is established under a high temperature rich gas condition. Therefore, in Patent Literature 1, at the time of the S purge control, the gas is first switched to the rich gas condition and then switched to the lean gas condition to reduce oxygen in the catalyst casing in which a reducing agent such as hydrocarbon (HC) or CO is suspended and stays. A technique has been disclosed in which the NOx storage catalyst is efficiently heated by supplying and reacting them in the vicinity of the catalyst, and then the NOx storage catalyst is desorbed by the NOx storage catalyst under rich gas conditions. I have.
[0006]
[Patent Document 1]
JP 2000-227022 A
[Problems to be solved by the invention]
However, as a result of various tests performed by the present inventors on the S purge, it was found that high-temperature rich gas conditions were necessary for desorbing SOx. It was found that the NOx storage catalyst could not desorb the poisoned SOx unless oxygen was supplied, for example.
[0008]
That is, in the above-mentioned Patent Document 1, the air-fuel ratio is switched from rich to lean only as a means for raising the temperature of the NOx storage catalyst (exhaust gas). After the temperature is raised, the S purge is performed only under the rich gas condition. Therefore, there is a problem that the S purge cannot be efficiently performed in a short time, the recovery of the NOx purification performance is insufficient, and the fuel efficiency deteriorates.
[0009]
Therefore, an object of the present invention is to purify the exhaust gas of an internal combustion engine in which the S purge can be efficiently performed from a low temperature state in a short time and the NOx purification performance can be sufficiently recovered and the fuel consumption can be prevented from deteriorating. It is to provide a device.
[0010]
[Means for Solving the Problems]
In order to achieve the above object, an exhaust gas purifying apparatus for an internal combustion engine according to claim 1 of the present invention absorbs NOx in exhaust gas in a lean gas state in which oxygen concentration is high in an exhaust passage while reducing oxygen concentration in the exhaust gas. In the exhaust gas purifying apparatus for an internal combustion engine provided with a NOx storage catalyst that releases and reduces NOx in a rich gas state having a high concentration, from the start time to the end time of S purge in which the NOx storage catalyst desorbs poisoned SOx. S purge control means for alternately switching between the rich gas state and the lean gas state is provided.
[0011]
Thereby, the S purge can be efficiently performed in a short time and from a low temperature state.
[0012]
The exhaust gas purifying apparatus for an internal combustion engine according to claim 2 of the present invention is characterized in that the S purge control means alternately switches between the rich gas state and the lean gas state after at least the exhaust gas has been heated to a predetermined temperature or higher. I do.
[0013]
Thus, the S purge can be efficiently performed under favorable conditions.
[0014]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, an exhaust gas purifying apparatus for an internal combustion engine according to the present invention will be described in detail with reference to the drawings by using embodiments.
[0015]
[Example]
Figure 1 is applied schematic diagram to a compression ignition type internal combustion engine the exhaust purification system of an internal combustion engine of the present invention (diesel engine), the flow chart of FIG. 2 also S purge control, FIG. 3 is desorbed in SO ₂ -TPD test FIG. 4 is a table showing the gas condition, FIG. 4 is a graph showing the results of the SO ₂ -TPD test, and FIG. 5 is a graph showing the residual amount of S in the catalyst after the SO ₂ -TPD test.
[0016]
As shown in FIG. 1, each cylinder of the engine body 1 is supplied with intake air from an air cleaner (not shown) via an intake pipe 2 and an intake manifold 3, and is not shown via a common rail 4 and a fuel injection valve 5. Fuel is supplied from a fuel pump. The fuel injection valve 5 is of an electronic control type whose injection amount and injection timing are controlled by an electronic control unit (ECU) 30, which will be described later. Wiring in the drawing is omitted to avoid complication. On the other hand, the exhaust gas discharged from each cylinder of the engine body 1 is discharged to the atmosphere via an exhaust manifold 6 as an exhaust passage and an exhaust pipe 7.
[0017]
A turbocharger 8 is provided between the intake pipe 2 and the exhaust pipe 7, and the intake air is pressurized by a compressor that rotates integrally with a turbine rotated by the exhaust gas and supplied to each cylinder of the engine body 1. It has become. At this time, the pressurized air is cooled by the intercooler 9 interposed in the intake pipe 2, so that the efficiency of charging each cylinder of the engine body 1 is increased.
[0018]
The intake pipe 2 is provided with an air flow sensor 10 located upstream of the turbocharger 8 for detecting the amount of intake air, and an intake throttle valve 11 located downstream of the intercooler 9. The output signal of the airflow sensor 10 is input to the ECU 30, and the opening and closing of the intake throttle valve 11 is controlled by the ECU 30 via an actuator (not shown).
[0019]
The intake manifold 3 and the exhaust manifold 6 are connected by an EGR (exhaust gas recirculation) passage 12, and are controlled at predetermined times by an EGR valve 14 controlled to be opened and closed by an actuator 13 by the ECU 30 according to an engine operating state. A fixed amount of EGR is performed to suppress the generation of NOx as much as possible. The EGR passage 12 is provided with an EGR cooler 15 for cooling the EGR gas.
[0020]
Then, a NOx storage catalyst 16 is interposed in the exhaust pipe 7. The NOx storage catalyst 16 has a characteristic of absorbing NOx in exhaust gas in a lean gas state having a high oxygen concentration, and releasing and reducing this NOx in a rich gas state having a low oxygen concentration and a high concentration of a reducing agent such as HC or CO. .
[0021]
In the exhaust pipe 7 immediately before and immediately after the NOx storage catalyst 16, temperature sensors (catalyst front and rear temperature sensors) 17a and 17b for detecting the temperature of the exhaust gas are provided, and the exhaust pipe 7 immediately after the NOx storage catalyst 16 is provided. Is provided with an oxygen sensor (catalyst outlet oxygen sensor) 18 for detecting the oxygen concentration in the exhaust gas, and the output signals of these sensors 17a, 17b, 18 are input to the ECU 30.
[0022]
The exhaust pipe 7 is provided with an exhaust throttle valve 19 and an oxygen sensor (engine outlet oxygen sensor) 20 for detecting the oxygen concentration in the exhaust gas which are located downstream of the turbocharger 8 in order toward the downstream side. The exhaust throttle valve 19 is controlled to open and close by an ECU 30 via an actuator (not shown), and an output signal of the oxygen sensor 20 is input to the ECU 30.
[0023]
Further, a fuel injection valve 21 whose injection amount and injection timing are controlled by the ECU 30 is provided in the exhaust pipe 7 located upstream of the NOx storage catalyst 16 and downstream of the oxygen sensor 20.
[0024]
The ECU 30 includes a microcomputer (CPU), a memory, and an interface as an input / output signal processing circuit. The input side of the ECU 30 is connected to the air flow sensor 10, the temperature sensors 17a and 17b, and the oxygen sensors 18 and 20, and detects a rotation sensor (not shown) for detecting an engine speed and a vehicle speed. A vehicle speed sensor, an accelerator opening sensor for detecting an accelerator pedal opening, and the like are connected to each other, and engine operation information from these sensors and the like is input. On the other hand, the output side of the ECU 30 is connected to the fuel injection valves 5 and 21, the intake throttle valve 11, the EGR valve 14, the exhaust throttle valve 19, and the like.
[0025]
The ECU 30 controls the injection amount and the injection timing of the fuel injection valve 5 based on the engine operation information from the above-described sensors and the like, and controls the opening and closing of the EGR valve 14 according to the engine operation state to control the PM and NOx. Is suppressed as much as possible.
[0026]
Further, when the exhaust gas is enriched only by in-cylinder combustion, the ECU 30 controls the opening and closing of the fuel injection valve 21 by controlling the opening and closing of the fuel injection valve 21 because there are many problems such as generation of smoke and torque fluctuation particularly in a high load region. By injecting (adding) fuel (light oil) from the injection valve 21 to the exhaust pipe 7, the exhaust gas is enriched, and the NOx storage catalyst 16 purifies NOx.
[0027]
The ECU 30 controls the opening and closing of the fuel injection valve 21 when the poisoning amount (hereinafter referred to as S accumulation amount) reaches a predetermined value in order to desorb the SOx poisoned by the NOx storage catalyst 16. Thus, the S purge control is performed by alternately switching between the rich gas state and the lean gas state (S purge control means).
[0028]
The S purge control will be described in detail with reference to the flowchart of FIG.
First, in step P1, it is determined whether or not the S accumulation amount calculated from the integrated value of the fuel consumption amount of the engine body 1 has reached a predetermined amount for starting the S purge. The S purge target amount is set according to the amount. On the other hand, if NO, the calculation of the S accumulation amount from the integrated value of the fuel consumption amount is continued in Step P3.
[0029]
After step P2, at step P4, the operating state of the engine body 1 (torque, rotation, vehicle speed, accelerator opening, exhaust temperature (detected by the catalyst front-rear temperature sensors 17a, 17b), λ (excess air ratio, engine outlet It is determined whether or not the operating temperature is detected by the oxygen sensor 20) or the like, so that the exhaust gas temperature (the temperature of the NOx storage catalyst 16) can be increased in a high load region or the like where S purging is possible. The target fuel (light oil) addition amount to the exhaust pipe 7 is calculated based on the addition amount map determined by (the temperature of the NOx storage catalyst 16) and λ. On the other hand, if the answer is NO, the process waits until the operation state in which S purge can be performed.
[0030]
After step P5, fuel injection (light oil addition) is started from the fuel injection valve 21 in step P6 to shift to a rich gas state in which the oxygen concentration is low and the reducing agent concentration is high, and then in step P7, the catalyst outlet oxygen sensor 18 It is determined whether or not the exhaust gas is in a rich state (rich gas state) based on the signal from.
[0031]
If it is possible in step P7, it is determined in step P8 whether the S purge operation can be continued based on the operating state (torque, rotation, vehicle speed, accelerator opening) of the engine body 1. If NO in step P7, the amount of fuel injection from the fuel injection valve 21 is corrected (increased) in step P9.
[0032]
If the answer is yes in step P8, it is determined whether or not the end time (for example, 30 seconds) of the rich gas state set by the timer in step P10 has come. If NO in step P8, the fuel injection from the fuel injection valve 21 is stopped in step P11, and the process returns to step P4 and waits until the operating state in which the S purge can be performed.
[0033]
If the answer is yes in step P10, the fuel injection from the fuel injection valve 21 is stopped in step P12 to shift to a lean gas state in which the oxygen concentration is high. If NO in step P10, the process returns to step P8, but the S purge amount during that time is stored. This S purge amount is calculated based on a SOx desorption amount map determined by the temperature of the NOx storage catalyst 16 estimated (calculated) from the exhaust gas temperature detected by the before and after catalyst temperature sensors 17a and 17b. The stored S purge amount is deducted when the S purge control is executed again after the S purge control is interrupted in the step P8.
[0034]
After the step P12, it is determined whether or not the end time (for example, 30 seconds) of the lean gas state set by the timer has come in a step P13. If it is possible, the set S purge target amount is set in a step P14. Determine if it has been achieved. If the answer is YES in Step P14, the present S purge control ends. If the answer is NO, the process returns to Step P4 to repeatedly execute the S purge control.
[0035]
As described above, in the present embodiment, the oxygen is supplied periodically by alternately switching the rich gas state and the lean gas state from the start time to the end time of the S purge control. The S purge can be performed efficiently from the state, the NOx purification performance can be sufficiently recovered, and the deterioration of fuel efficiency can be suppressed. Further, for example, S purge can be performed from about 500 ° C., and the temperature of the NOx storage catalyst 16 does not need to be raised more than necessary, so that the thermal deterioration of the NOx storage catalyst 16 can be prevented.
[0036]
The effect of such S purge control has been clarified in a study of the desorption characteristics of SO ₂ performed using a SO ₂ -TPD (Temperature Programmed Desorption) apparatus described later.
[0037]
That is, the test was performed in such a manner that after poisoning the catalyst in the TPD device, the temperature was increased to 800 ° C. under the five kinds of gas conditions shown in FIG. 3 and the SO ₂ desorption characteristics were measured. FIG. 4 shows the results. The temperature rise pattern is shown in the figure, and the temperature was held for a certain period of time in order to focus on the desorption characteristics at 600 ° C. in consideration of the thermal degradation of the catalyst. As is clear from these results, desorption of SO ₂ is scarcely observed only by raising the temperature with only the CO-rich condition (1), and H ₂ , which is said to be effective for desorption of SO ₂ , is obtained. Desorption of SO ₂ is not recognized only under the coexisting rich condition ( ₂ ). On the other hand, if the temperature is raised while switching between rich (rich gas state) and lean (lean gas state) under conditions (3), (4), and (5) (referred to as O ₂ spike), SO ₂ is desorbed. I understood.
In addition, there is no difference in SO ₂ desorption between only CO of the condition (3) and H ₂ of the condition (4), and the desorption of SO ₂ from low temperature due to the coexistence of CO and H _{2 under} the condition (5). It was also clear that it was going on.
[0038]
Next, the S content after the SO ₂ -TPD test was investigated. The result is shown in FIG. And S content of each catalyst, a good correlation is observed in the results of the TPD in FIG. 4, the catalyst is SO ₂ are most desorbed (▲ 3 ▼, ▲ 4 ▼ , ▲ 5 ▼) as the residual S content Is running low.
[0039]
In the above-described embodiment, the rich gas state and the lean gas state are alternately switched from the start time to the end time of the S purge control in which the S accumulation amount has reached the predetermined amount. 16) may be alternately switched between the rich gas state and the lean gas state after the temperature is raised to a predetermined temperature (for example, 500 ° C.) or higher. According to this, it is possible to efficiently perform the S purge under favorable conditions, and it is possible to further suppress the deterioration in fuel efficiency.
[0040]
Further, it is needless to say that the present invention is not limited to the above embodiments, and various changes can be made without departing from the spirit of the present invention. For example, in the above embodiment, the exhaust gas is enriched by fuel injection into the exhaust passage, but may be enriched by combustion control of the engine body. Note that the exhaust gas purification apparatus for an internal combustion engine of the present invention can also be applied to a spark ignition type internal combustion engine (gasoline engine). In this case, the exhaust gas can be enriched by in-cylinder combustion.
[0041]
【The invention's effect】
As described above, according to the first aspect of the present invention, in the exhaust gas purification apparatus for an internal combustion engine in which the NOx storage catalyst is interposed in the exhaust passage, the start timing of the S purge for releasing the SOx poisoned by the NOx storage catalyst S purge control means for alternately switching between the rich gas state and the lean gas state from the end to the end time is provided, so that the S purge can be efficiently performed from a low temperature state in a short time and the NOx purification performance can be sufficiently recovered. And deterioration of fuel economy can be suppressed. Further, thermal deterioration of the NOx storage catalyst can also be prevented.
[0042]
According to the second aspect of the present invention, since the S purge control unit alternately switches between the rich gas state and the lean gas state after at least the exhaust gas has been heated to a predetermined temperature or more, the S purge can be efficiently performed under favorable conditions. In addition, deterioration of fuel economy can be further suppressed.
[Brief description of the drawings]
FIG. 1 is a schematic configuration diagram in which an exhaust gas purification device for an internal combustion engine of the present invention is applied to a compression ignition type internal combustion engine (diesel engine).
FIG. 2 is a flowchart of S purge control.
FIG. 3 is a table showing desorbed gas conditions in an SO ₂ -TPD test.
FIG. 4 is a graph showing the results of an SO ₂ -TPD test.
FIG. 5 is a graph showing the residual amount of S in the catalyst after the SO ₂ -TPD test.
[Explanation of symbols]
1 engine body, 2 intake pipes, 3 intake manifolds, 4 common rails, 5 fuel injection valves, 6 exhaust manifolds, 7 exhaust pipes, 8 turbochargers, 9 intercoolers, 10 air flow sensors, 11 intake throttle valves, 12 EGR passages, Reference Signs List 13 actuator, 14 EGR valve, 15 EGR cooler, 16 NOx storage catalyst, 17a, 17b temperature sensor, 18 oxygen sensor (catalyst outlet oxygen sensor), 19 exhaust throttle valve, 20 oxygen sensor (engine outlet oxygen sensor), 21 fuel injection Valve, 30 Electronic control unit (ECU)

Claims (2)

The exhaust gas of the internal combustion engine is provided with a NOx storage catalyst in the exhaust passage, which absorbs NOx in the exhaust gas in a lean gas state having a high oxygen concentration while releasing and reducing this NOx in a rich gas state having a low oxygen concentration and a high reducing agent concentration. The purifying apparatus further comprises an S purge control unit that alternately switches the rich gas state and the lean gas state from a start time to an end time of the S purge for desorbing the SOx poisoned by the NOx storage catalyst. Exhaust purification device for an internal combustion engine. 2. The exhaust gas purifying apparatus for an internal combustion engine according to claim 1, wherein the S purge control unit alternately switches between the rich gas state and the lean gas state after at least the temperature of the exhaust gas is raised to a predetermined temperature or higher.

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