JP2913993B2 - NOx purification device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a so-called lean NOx
The present invention relates to a NOx purification device provided with a catalyst, and more particularly to a NOx purification system for improving a NOx purification rate.

[0002]

_2. Description of the Related Art Internal combustion engines capable of lean burn (including diesel engines) are being developed as automotive internal combustion engines that can contribute to both prevention of global warming and improvement of fuel efficiency by reducing CO ₂ emissions. Some have been put to practical use.

However, since the three-way catalyst has almost no NOx reduction ability in the exhaust gas of a lean burn engine, it is desired to develop a lean NOx catalyst capable of purifying NOx in exhaust gas generated by combustion at a lean air-fuel ratio. It is rare.

As an example of such a lean NOx catalyst, JP-A-1-139145 discloses a lean NOx catalyst in which a transition metal such as Cu is ion-exchanged and supported on zeolite.
A catalyst is proposed. The zeolite-based lean NOx catalyst reduces NOx in the presence of HC in an oxidizing atmosphere (that is, during exhaust from combustion at a lean air-fuel ratio).

In order to use such a lean NOx catalyst as a NOx purification device for an internal combustion engine for a vehicle, it is necessary to develop a system in which the lean NOx catalyst operates at a high NOx purification rate.

In this sense, the applicant has previously arranged a plurality of lean NOx catalysts in an exhaust system of an internal combustion engine in parallel, and provided a space velocity changing means comprising a valve upstream of the lean NOx catalysts. NO that increases or decreases the exhaust gas flow rate flowing through the catalyst at predetermined intervals within a predetermined temperature range and increases or decreases the exhaust space velocity at each lean NOx catalyst at predetermined intervals within a predetermined temperature range.
x purification device was proposed. By doing so, the lean NOx catalyst whose space velocity changes from high speed to low speed can transiently greatly improve the NOx purification rate, and by repeating this, the NOx purification rate of the system improves.

[0007]

However, in a further development test of the present inventor, even if the space velocity is increased or decreased at a constant cycle within a predetermined temperature range as in the prior application, the temperature within the predetermined temperature range is not increased. It has been found that the NOx purification rate of the lean NOx catalyst is lower in the high temperature side portion and the low temperature side portion than in the intermediate portion.

It is an object of the present invention to change the space velocity of the lean NOx catalyst at a constant cycle within a predetermined temperature range to obtain N
In the NOx purifying apparatus for improving the Ox purification rate, the NOx in the high-temperature part and the low-temperature part in the predetermined temperature range is reduced.
x It is to suppress the reduction of the purification rate.

[0009]

According to the present invention, the above object is attained by the following NOx purifying apparatus. That is placed in an exhaust system of an internal combustion engine, the exhaust gas is an oxidizing atmosphere且
A lean NOx catalyst for reducing NOx in the presence of HC
Space velocity changing means for periodically increasing or decreasing the exhaust flow velocity flowing into the lean NOx catalyst ; and increasing or decreasing the space velocity changing cycle by the space velocity changing means when the exhaust temperature is high and increasing the space cycle when the exhaust temperature is low. , NOx purification device characterized by comprising <br/> a space velocity changing means control means for controlling the space velocity changing means.

[0010]

In the development test by the inventor, lean NOx
It was found that when the space velocity of exhaust gas at the catalyst changed, the NOx purification rate of the lean NOx catalyst temporarily changed for several minutes, and eventually returned to the steady NOx purification rate. In particular,
When the space velocity changes from a high speed to a low speed, the NOx purification rate temporarily increases remarkably. When the space velocity changes from a low speed to a high speed, there is almost no improvement in the NOx purification rate, and in some cases, compared with the steady NOx purification rate. And temporarily worsens slightly.

[0011] varying the exhaust flow rate of rie emissions NOx catalysts, NOx purification rate is improved as the system. Sand <br/> KazuSatoshi, spatial velocity increased from fast changes result is much NOx purification ratio at low speed, spatial speed changes from low speed to high speed
Although the NOx purification rate slightly decreases, the effect of improving the NOx purification rate is greater than the effect of decreasing the NOx purification rate, so that the NOx purification rate improves as a whole.

Such an effect of improving the NOx purification rate lasts only a few minutes from the time when the space velocity changes, and after that, returns to the steady-state NOx purification rate. Therefore,
When switched periodically, an improvement in the NOx purification rate is repeatedly produced, and an average high NOx purification rate is obtained over a long period of time.

In a development test conducted by the inventor, the improvement of the NOx purification rate of the lean NOx catalyst when the space velocity is switched from high to low is promptly exhibited when the exhaust gas temperature is high, and when the exhaust gas temperature is high. It was found that the return was quick, when the exhaust gas temperature was low, it appeared slowly, and the return to the normal state was also slow.

Therefore, if the fluctuation cycle of the space velocity is made slow at a high exhaust gas temperature, the generation rate of HC and the direct complete oxidation of the generated HC to CO ₂ and H ₂ O become intense, making it difficult to retain the active species. NOx even when space velocity fluctuates
The effect of improving the purification rate is less likely to appear. However, in the present invention, the period of the space velocity fluctuation cycle is shortened at the time of high exhaust gas temperature, so that the retention and utilization of HC becomes appropriate and the NOx purification rate is improved.

If the variation cycle of the space velocity is shortened when the exhaust gas temperature is low, the exhaust gas is switched before the active species is sufficiently generated when the exhaust gas temperature is low even though the generation speed of the active species is low. The amount of active species is not sufficient, and it is difficult to improve the NOx purification rate. However, in the present invention, since the cycle of the space velocity fluctuation cycle is lengthened at a low exhaust temperature, a sufficient amount of HC is generated before switching, and the NOx purification rate is improved.

[0016]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the NOx purifying apparatus according to the present invention will be described below with reference to the drawings. In FIG. 1, an internal combustion engine capable of lean combustion (including a diesel engine)
The second exhaust system 4 is provided with a parallel passage portion 6 having a plurality of parallel passages. In the illustrated example, the parallel passage portion 6
It has a first passage 6a and a second passage 6b which are parallel to each other.

In the parallel passage section 6, a plurality of lean NOx catalysts 8 are provided in parallel. Specifically, the first passage 6a
Is provided with a first lean NOx catalyst 8a,
A second lean NOx catalyst 8b is provided in the passage 6b. Three or more passages may be provided in the parallel passage portion 6, in which case the passages are divided into two groups, and a first lean NOx catalyst is provided in each of the first group of passages, and a second lean NOx catalyst is provided. A second lean NOx catalyst will be provided in each of the group passages.

Space velocity changing means 10 is provided at a branch portion of the parallel passage portion 6 on the upstream side of the plurality of passages 6a and 6b. This space velocity changing means 10 is provided with a first lean NOx
Exhaust Velocity at Catalyst 8a and Second Lean NOx Catalyst 8
b, when one 8a is high speed, the other 8b
Are alternately and periodically increased and decreased so that the one 8a changes to a low speed and the other 8b changes to a high speed when the one 8a changes to a low speed.

For example, in FIG. 1, the space velocity changing means 10 comprises a valve element 10a and an actuator 10b for moving the valve element 10a up and down periodically. The valve element 10a is
The passages 6a and 6b are not completely closed, and the exhaust gas always flows through the passages 6a and 6b during the operation of the engine, so that the flow rate of the exhaust gas flowing through the passages 6a and 6b is increased or decreased.
However, the fluctuation of the space velocity is performed only when the exhaust gas temperature is within a predetermined temperature range (for example, 330 ° C. to 470 ° C.)
If it deviates from that, even if the space velocity is changed, the effect of improving the NOx purification rate, which will be described later, is unlikely to appear, so that it is not changed.

FIGS. 2 and 3 show the first lean NOx catalyst 8a or the second lean N
6 shows a pattern of a change in the flow rate of the Ox catalyst 8b. However,
The flow rate of exhaust gas discharged from the internal combustion engine is fixed. As shown in FIGS. 2 and 3, the large flow rate and the small flow rate are alternately and periodically repeated in each of the catalysts 8a and 8b. For example, when a% of exhaust gas flows through one lean NOx catalyst 8a, (100-a)% of exhaust gas flows through the other lean NOx catalyst 8b, and the flow rate of the one lean NOx catalyst 8a decreases. (100-a)%, the other lean N
For example, the flow rate of the Ox catalyst 8b changes to a%.

As shown in FIGS. 2 and 3, at the high temperature side within the predetermined temperature range, the cycle of the space velocity fluctuation cycle is shortened, for example, to about 30 seconds to 1 minute, and at the low temperature side within the predetermined temperature range. Here, the period of the space velocity fluctuation cycle is lengthened, for example, to about 1 minute to 2 minutes.

As shown in FIGS. 2 and 3, in the high-temperature side portion within the predetermined temperature range, the fluctuation difference is increased, for example, the flow rate is largely changed between 95% and 5%. In the low temperature side portion in the region, the fluctuation difference amount is made small, for example, the fluctuation difference is made small between 60% and 40%.

The space velocity changing means 10 is controlled by a command from an electronic control unit (ECU) 12 so as to obtain the above space velocity fluctuation pattern. ECU
A microcomputer 12 includes an input interface, an output interface, a read-only memory (ROM) as a read-only storage element, and a random access memory (RA) as a storage element for temporary storage.
M), and has a central processor unit (CPU) for executing operations.

The input interface of the ECU 12 includes an engine load sensor 14, an engine rotational speed sensor 16,
Further, the output of the exhaust gas temperature sensor 18 is input as needed. However, when the input value is an analog signal, it is converted into a digital signal by an analog / digital converter and input.

The ROM of the ECU 12 stores the control routine of FIG.
Is stored and read out by the CPU to execute the calculation. The control routine of FIG.
Interrupted every time. In FIG. 4, in step 102,
The engine load P which is the output of the engine load sensor 14
M, the engine speed NE which is the output of the engine speed sensor 16 is read. Subsequently, the routine proceeds to step 104, where it is determined whether or not the current engine operating state is in an area where the space velocity fluctuation control is performed using the map of FIG. Proceed to step. The exhaust temperature is high when the PM is high and NE is high, and the exhaust temperature is low when the PM and NE are low. However, when the exhaust temperature is out of the predetermined range, NO is obtained even when the space velocity is changed.
Since it is difficult to improve the x purification rate, the process proceeds to step 106 only when the exhaust gas temperature is within the predetermined range to change the space velocity.

In step 106, it is determined whether or not the previous space velocity fluctuation cycle has completed one cycle. That is, it is determined whether or not the time T counted from the start of the previous space velocity fluctuation has exceeded the cycle S of the previous space velocity fluctuation. If T does not exceed S, step 114
Then, the time is further counted up by ΔT per one interruption. If the time exceeds S, the previous cycle has been completed, so the routine proceeds to step 108, where T is cleared to 0 for the current cycle.

The process proceeds from step 108 to step 110, in which a space speed fluctuation period S corresponding to the current engine load and engine speed is obtained using the map shown in FIG. As shown in the map of FIG. 6, in the space speed fluctuation region, the higher the engine load and the higher the engine rotation speed, that is, the higher the exhaust gas temperature and the space speed fluctuation cycle S are set shorter, the lower the engine load and the lower the engine speed. The space speed fluctuation cycle S is set longer as the engine speed increases, that is, as the exhaust gas temperature decreases.

In step 110, the space velocity fluctuation amount D corresponding to the current engine load and engine speed is obtained using the map shown in FIG. As shown in the map of FIG. 7, in the space velocity fluctuation region, the higher the engine load and the higher the engine rotation speed, that is, the higher the exhaust gas temperature,
The space velocity fluctuation amount D is set large, and the lower the engine load and the lower the engine rotation speed, that is, the lower the exhaust gas temperature, the smaller the space velocity fluctuation amount D is set.

Then, the process proceeds to a step 112, wherein the space velocity changing means 10 (the valve 10a and the actuator 10b)
Is turned on, and the variation of the space velocity for one cycle is executed in accordance with S and D obtained in step 110. Step 1
From 12 the process proceeds to the end step. In the interruption to the subsequent routine, the process continues from step 106 through step 114 until the change in the space velocity for the previous cycle is completed. The process proceeds to 108, 110, and 112, and the fluctuation of the space velocity is started again. Hereinafter, this is repeated.

Next, the operation of the above embodiment will be described. As shown in FIG. 8, the inventor has found that when the space velocity of the exhaust gas in the lean NOx catalyst 8 changes, the NOx purification rate changes for several minutes immediately after the change.

More specifically, when the space velocity in the lean NOx catalyst 8 changes from high to low, the NOx
The purification rate is remarkably transiently improved as compared with the steady-state NOx purification rate, and thus the NOx concentration decreases. Conversely, when the space velocity in the lean NOx catalyst 8 changes from a low speed to a high speed, the NOx purification rate hardly increases as compared with the steady-state NOx purification rate, and in some cases, deteriorates by a small amount.

The reason why such a phenomenon occurs is presumed as follows. The reason why the NOx purification rate is improved when changing from high speed to low speed is that at the high speed, the exhaust gas flows through without contacting the catalyst pore surface sufficiently, so that the active point for NOx purification on the lean NOx catalyst increases. It remains without being consumed too much,
The number of active points increases. However, when the speed is switched to a low speed, the exhaust gas sufficiently contacts the surface of the catalyst pores, and the increased active points are consumed at a high speed. Therefore, the NOx purification rate is temporarily improved, but when the stored active points are exhausted,
It returns to the normal NOx purification rate. However, even if the speed changes from a low speed to a high speed, the active point does not increase at the low speed, so that the NOx purification rate does not increase immediately after the high speed switching but slightly decreases. Therefore, immediately after switching to the high speed, the exhaust gas temperature and the catalyst bed temperature increase, but the NOx purification rate does not increase until the improvement of the NOx purification rate by the temperature raising process exceeds the transient NOx purification rate decrease at which the space velocity increases. Slightly lower.

In the above embodiment of the present invention, since the space velocity of the exhaust gas in the first lean NOx catalyst 8a and the second lean NOx catalyst 8b is periodically changed, the space velocity changes from high to low. Large NOx with one lean NOx catalyst
The purification rate is improved, and the NOx purification rate of the entire system is also improved.

For example, the state where 0.8 flows through the first lean NOx catalyst 8a and 0.2 flows through the second lean NOx catalyst 8b is reduced to 0.2% through the first lean NOx catalyst 8a.
2. It is assumed that the state changes to a state of flowing to the second lean NOx catalyst 8b at a ratio of 0.8. In this case, the first lean NOx
Since the catalyst 8a changes from a high speed to a low speed, the NOx purification rate is greatly improved. For example, if the NOx purification rate temporarily changes to 85% compared to 45% in the steady state, the effect on the entire system is (85). % −45%) × 0.2 = 8
% NOx purification rate. On the other hand, the second lean NO
Since the x catalyst 8b changes from a low speed to a high speed, a slight NO
Assuming that the x purification rate is lowered, for example, the purification rate is temporarily changed to 42% compared to the steady-state NOx purification rate of 45%, the effect on the entire system is (42% −45%) × 0.8 = −.
2.4%. Therefore, as a whole system,
8% -2.4% = 5.6% NOx purification rate improvement can be seen.

However, when the space velocity is kept constant regardless of a change in the temperature, the improvement in the NOx purification rate as described above can be achieved in the central part of the predetermined temperature range, for example, in the area where the space velocity fluctuation control is performed in FIG. Although it is obtained in the central part, in the high temperature side part and the low temperature side part in the above-mentioned region,
It has been found that the NOx purification rate improving effect decreases.

The reason is presumed as follows. That is, the relationship between the amount of the active species after changing the space velocity from the high speed to the low speed and the elapsed time t is as shown in FIG. On the high temperature side, active species are generated and consumed promptly as in characteristic A, but on the low temperature side, active species are generated and consumed slowly as in characteristic B. When the exhaust gas temperature is intermediate temperature, the characteristics are as shown in C. When the space velocity fluctuation period t ₀ is set in accordance with this, when the exhaust gas temperature is high, it is difficult to retain the active species, and it is consumed and the amount of retained Space velocity fluctuation is performed when the NOx decreases, so that high NOx
It is difficult to obtain a purification rate. Also, when the exhaust gas temperature is low, the space velocity is switched and fluctuated at a stage where active species are not sufficiently generated during the process of generating active species,
Since the amount of active species is small, a sufficient improvement in the NOx purification rate cannot be obtained.

However, in the present invention, the period of the space velocity fluctuation is not constant (t ₀ ), and the period is shifted to the left in FIG. When the temperature is low, the cycle is shifted to the right in FIG. 5 so as to be controlled so as to approach the peak point of the amount of active species at the time of low temperature of the exhaust gas. In addition, the effect of improving the NOx purification rate due to the space velocity fluctuation is kept high.

[0038]

According to the present invention, there is provided an exhaust system for an internal combustion engine.
NOx is reduced in an oxidizing atmosphere and in the presence of HC
Lean NOx catalyst and flowing into the lean NOx catalyst
A space velocity changing means for periodically increasing and decreasing the exhaust flow velocity, and
Exhaust gas temperature
Shorter when the exhaust gas temperature is high and longer when the exhaust temperature is low.
Space velocity changing means to control space velocity changing means
Since the control means is provided, the effect of improving the transient NOx purification rate due to the space velocity fluctuation can be obtained at high and low temperatures, and a high transient NOx purification rate can be obtained over a relatively wide temperature range. .

[Brief description of the drawings]

FIG. 1 is a system diagram of a NOx purification device according to one embodiment of the present invention.

FIG. 2 is a space velocity fluctuation pattern diagram at the time of high exhaust temperature in each lean NOx catalyst of the NOx purification device of FIG.

FIG. 3 is a space velocity fluctuation pattern diagram at a low exhaust gas temperature in each lean NOx catalyst of the NOx purification device of FIG. 1;

FIG. 4 is a flowchart of a control routine of a space velocity changing means control means of the NOx purification device of FIG. 1;

FIG. 5 is a map diagram for obtaining a region in which space velocity fluctuation control is performed based on an engine load and an engine rotation speed, which is used in the control routine of FIG. 4;

FIG. 6 is a map diagram for calculating a period of space velocity fluctuation from an engine load and an engine rotation speed, which is used in the control routine of FIG. 4;

FIG. 7 is a map used in the control routine of FIG. 4 to obtain a space velocity fluctuation amount from an engine load and an engine rotation speed.

FIG. 8 is a graph showing NOx concentration and temperature versus time when the space velocity changes in the NOx purification device of FIG. 1;

FIG. 9 is a graph showing the amount of active species versus time when the space velocity of the lean NOx catalyst changes.

[Explanation of symbols]

 Reference Signs List 2 internal combustion engine 4 exhaust system 6 parallel passage 6a first passage 6b second passage 8 lean NOx catalyst 8a first lean NOx catalyst 8b second lean NOx catalyst 10 space velocity changing means 10a valve body 10b actuator 12 ECU 14 engine load sensor 16 engine speed sensor 18 exhaust temperature sensor

Claims (1)

(57) [Claims] 1. A is placed in an exhaust system of an internal combustion engine, the exhaust acid
NO to reduce NOx in oxidizing atmosphere and in the presence of HC
an x catalyst, a space velocity changing means for periodically increasing and decreasing the exhaust flow velocity flowing into the lean NOx catalyst , and a space velocity changing cycle by the space velocity changing means is shortened when the exhaust temperature is high, and is decreased when the exhaust temperature is low. as long, NO, characterized in that a space velocity changing means control means for controlling the space velocity changing means
x purification device.