JP2006207512A - Exhaust emission control device and exhaust emission control method for internal combustion engine - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an exhaust emission control device and an exhaust emission control method for an internal combustion engine capable of efficiently purifying NO<SB>X</SB>in exhaust gas. <P>SOLUTION: This exhaust emission control device is provided in an exhaust gas passage of the internal combustion engine and purifies exhaust gas. The exhaust emission control device for the internal combustion engine includes an oxidation catalyst and SCR catalyst arranged on a downstream side of the oxidation catalyst, and is provided with a bypass branching off of an exhaust passage on an upstream side of the oxidation catalyst and merging to the exhaust passage on a downstream side of the oxidation catalyst and between the oxidation catalyst and the SCR catalyst, a temperature sensor for measuring temperature of exhaust gas, a flow rate control valve for changing ratio of flow rate of exhaust gas flowing on an oxidation catalyst side and a bypass side, and a control means for adjusting ratio of flow rate. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

The present invention relates to an exhaust gas purification device and an exhaust gas purification method for exhaust gas discharged from an internal combustion engine, and more particularly to an exhaust gas purification device and an exhaust gas purification method for an internal combustion engine that can efficiently purify NO _x in the exhaust gas. .

Conventionally, in exhaust gas discharged from an internal combustion engine such as a diesel engine black smoke particles (hereinafter, referred to as PM) and nitrogen oxides (hereinafter, referred to as NO _X), etc. are included. Therefore, a diesel particulate filter (hereinafter referred to as DPF), an SCR catalyst (Selective Catalytic Reduction) or the like is provided as an aftertreatment device for an internal combustion engine to take measures to purify exhaust gas.
For example, when a DPF is provided as an aftertreatment device, PM in exhaust gas is adsorbed in the DPF, and periodically burned by a burner, or burned (oxidized) by high-temperature gas that has passed through an oxidation catalyst. By doing so, PM is removed. Further, when the SCR catalyst is provided as the aftertreatment device, NO and NO ₂ in the exhaust gas are adsorbed to the NO _x catalyst, and a NO _x reducing agent such as urea is injected to cause a reduction reaction, so that nitrogen and water It is decomposed and discharged.

As shown in FIG. 5, an exhaust emission control device including an oxidation catalyst and a DPF carrying an NO _x catalyst is disclosed as an exhaust emission control device equipped with such an aftertreatment device. More specifically, the DPF has been carried is NO _X catalyst, an upstream portion of the oxidation catalyst in the exhaust passage downstream of the oxidation catalyst, and the upstream portion of the DPF is connected with a bypass passage The bypass passage is provided with a reducing agent supply means for supplying NO _X reducing agent, and the branch upstream of the oxidation catalyst in the bypass passage has a flow of exhaust to the oxidation catalyst side according to the temperature of the oxidation catalyst. An exhaust purification device provided with a switching valve for switching between circulation to the bypass passage (see Patent Document 1).
JP 2003-13730 A (Claims Fig. 1)

However, the switching valve provided in the exhaust gas purification device disclosed in Patent Document 1 shuts off the bypass passage when the exhaust gas is circulated to the oxidation catalyst side, and flows the exhaust gas to the bypass passage side. Since this is a switching valve that shuts off the oxidation catalyst side, the ratio of NO and NO ₂ in the exhaust gas flowing into the DPF carrying the NO _x catalyst cannot be finely controlled. That is, even though the ratio of NO and NO ₂ in the exhaust gas varies depending on the temperature of the exhaust gas, the operating state of the internal combustion engine, etc., the configuration allows the NO _x catalyst to flow without considering the ratio. It was. Therefore, the ratio of NO and NO ₂ is controlled so that the NO _X purification rate in the NO _X catalyst approaches the maximum value, and NO, NO _2, and the NO _X reducing agent can be efficiently reduced. There was a problem that I could not.
Further, such an exhaust purifying apparatus, as a post-treatment device, and essential and in that it comprises a NO _X catalyst loaded in DPF, be those that are intended to remove the PM and NO _X contained in the exhaust gas simultaneously It was. Therefore, since it is difficult to finely control the ratio of NO and NO ₂ in the exhaust gas, the exhaust gas purification rate may not always be high.

Accordingly, the inventors of the present invention have made diligent efforts to provide a predetermined bypass in the exhaust gas purification device, and control the flow rate ratio of the exhaust gas flowing through the oxidation catalyst side and the bypass side, thereby exhaust gas flowing into the SCR catalyst. It has been found that NO _{x in} exhaust gas can be efficiently purified by finely controlling the ratio of NO and NO ₂ therein, and the present invention has been completed.
That is, an object of the present invention is to provide an exhaust purification device and an exhaust gas that can remarkably improve the NO _x purification rate in the SCR catalyst by controlling the ratio of NO and NO ₂ in the exhaust gas flowing into the SCR catalyst. It is to provide a purification method.

According to the present invention, there is provided an exhaust purification device for purifying exhaust gas that is provided in an exhaust passage of an internal combustion engine, and includes an oxidation catalyst and an SCR catalyst disposed on the downstream side of the oxidation catalyst. An exhaust purification device for an internal combustion engine,
A temperature sensor for measuring the temperature of the exhaust gas;
A bypass branching from the exhaust passage upstream of the oxidation catalyst and downstream of the oxidation catalyst and joining the exhaust passage between the oxidation catalyst and the SCR catalyst;
Control means for setting the flow rate ratio of the exhaust gas flowing through the oxidation catalyst side and the bypass side based on the temperature measured by the temperature sensor;
A flow rate ratio control means for changing the flow rate ratio based on a signal from the control means;
An exhaust emission control device for an internal combustion engine is provided, which can solve the above-described problems.

  In configuring the exhaust gas purification apparatus for an internal combustion engine of the present invention, it is preferable that the flow rate ratio control means can change the flow rate ratio continuously or stepwise.

  In constituting the exhaust gas purification apparatus for an internal combustion engine of the present invention, the flow rate ratio control means is preferably a flow path control valve.

  Further, in configuring the exhaust gas purification apparatus for an internal combustion engine according to the present invention, it further includes a detecting means for detecting the operating state of the internal combustion engine, and the control means further includes an operating state of the internal combustion engine detected by the detecting means. Correspondingly, it is preferable to set the flow rate ratio.

Another aspect of the present invention is an exhaust purification device including an oxidation catalyst and an SCR catalyst, a temperature sensor for measuring the temperature of the exhaust gas, and a branch from the exhaust passage upstream of the oxidation catalyst, The flow rate of exhaust gas flowing on the oxidation catalyst side and the bypass side on the downstream side of the oxidation catalyst, based on the bypass that joins the exhaust passage between the oxidation catalyst and the SCR catalyst, and the temperature measured by the temperature sensor In exhaust gas discharged from an internal combustion engine using an exhaust emission control device comprising: a control means for setting a ratio; and a flow rate ratio control means for changing a flow rate ratio based on a signal from the control means. a exhaust gas purifying method for an internal combustion engine for purifying the NO _X,
Measure the exhaust gas temperature with a temperature sensor,
Based on the measured temperature, the control means sets the flow rate ratio of the exhaust gas flowing through the oxidation catalyst side and the bypass side,
Exhaust gas of an internal combustion engine, wherein the ratio of NO to NO ₂ in the exhaust gas flowing into the SCR catalyst is controlled by changing the flow rate ratio by the flow rate ratio control means based on the set value of the flow rate ratio. It is a purification method.

  In carrying out the exhaust gas purification method for an internal combustion engine of the present invention, it is preferable to detect the operating state of the internal combustion engine and set the flow rate ratio by the control means further corresponding to the operating state.

  In carrying out the exhaust gas purification method for an internal combustion engine of the present invention, it is preferable to measure the temperature of the exhaust gas at one of the inlet and the outlet of the oxidation catalyst.

In carrying out the exhaust gas purification method for an internal combustion engine of the present invention, it is preferable that the ratio of NO and NO ₂ in the exhaust gas is made to coincide with the previously obtained ratio at which the reduction rate of NO _x is the fastest.

According to the exhaust gas purification apparatus for an internal combustion engine of the present invention, by providing a predetermined temperature sensor, a bypass, a control means for setting a flow rate ratio, and a flow rate ratio control means, an oxidation is performed corresponding to the temperature of the exhaust gas. It becomes possible to accurately control the flow rate ratio of the exhaust gas flowing to the catalyst side and the bypass side. Therefore, since the ratio of NO and NO ₂ in the exhaust gas flowing into the SCR catalyst can be finely controlled, the NO _x purification rate in the SCR catalyst can be remarkably improved.
It has been found that the PM in the exhaust gas can be efficiently reduced by delaying the fuel injection timing in the internal combustion engine, and the exhaust purification device of the present invention is used. By controlling the injection timing together, PM in exhaust gas discharged from the internal combustion engine can be effectively reduced.

  Further, in the exhaust gas purification apparatus for an internal combustion engine of the present invention, by changing the flow rate ratio continuously or stepwise, it is suitable for the exhaust gas temperature without completely shutting off either the oxidation catalyst side or the bypass side. It becomes easy to control the flow rate.

Further, in the exhaust gas purification apparatus for an internal combustion engine of the present invention, by providing a predetermined flow path control valve, the flow rate ratio of the exhaust gas can be easily adjusted in accordance with the temperature change of the exhaust gas that affects the NO _x reduction rate. can be changed, it is possible to significantly improve the purification rate of the NO _X in the SCR catalyst.

The exhaust gas purification apparatus for an internal combustion engine according to the present invention further includes a predetermined detection means, and sets the fuel ratio while taking into consideration the operation state of the internal combustion engine, so that the SCR catalyst is adapted to the change in the operation state. The NO _x purification rate can be significantly improved.

Further, according to the exhaust gas purification method for an internal combustion engine of the present invention, by setting the flow rate ratio of the exhaust gas flowing to the oxidation catalyst side and the bypass side in accordance with the exhaust gas temperature measured by the temperature sensor, the SCR The ratio of NO and NO ₂ in the exhaust gas flowing into the catalyst can be finely controlled. Therefore, the NO _x purification rate in the SCR catalyst can be remarkably improved.

Further, in the exhaust gas purification method for an internal combustion engine of the present invention, the NO _x purification rate in the SCR catalyst is controlled in accordance with the change in the operating state by controlling the flow rate ratio in consideration of the operating state of the internal combustion engine. It can be significantly improved.

Further, in the exhaust gas purification method for an internal combustion engine according to the present invention, by setting the flow rate ratio based on the temperature of the exhaust gas at a predetermined location, the NO in the exhaust gas is more finely matched to the temperature of the exhaust gas. The ratio with NO ₂ can be controlled.

Further, in the exhaust gas purification method for an internal combustion engine according to the present invention, by controlling the ratio of NO and NO ₂ in the exhaust gas to coincide with a predetermined ratio, it is possible to cope with a change in exhaust gas temperature or a change in operating condition. Thus, the NO _x purification rate in the SCR catalyst can be improved.

  DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments relating to an internal combustion engine exhaust purification apparatus and an internal combustion engine exhaust purification method according to the present invention will be specifically described below with reference to the drawings as appropriate. However, this embodiment shows one aspect of the present invention and does not limit the present invention, and can be arbitrarily changed within the scope of the present invention.

1. Exhaust Purification Device (1) Basic Configuration An exhaust purification device for an internal combustion engine according to the present invention is provided in an exhaust passage 20 of the internal combustion engine 50 as shown in FIGS. An exhaust purification device 10 for purifying, which is an exhaust purification device 10 for an internal combustion engine, including an oxidation catalyst 11 and an SCR catalyst 13 disposed on the downstream side of the oxidation catalyst 11.
Such an exhaust gas purification device 10 for an internal combustion engine includes a temperature sensor 17 for measuring the temperature of exhaust gas, a branch from the exhaust passage 20 upstream of the oxidation catalyst 11, a downstream side of the oxidation catalyst 11, and an oxidation catalyst. Based on the temperature measured by the bypass 21 that joins the exhaust passage 20 between the catalyst 11 and the SCR catalyst 13 and the temperature sensor 17, the flow rate ratio of the exhaust gas that flows through the oxidation catalyst 11 side and the bypass 21 side is set. And a flow rate ratio control means 15 for changing the flow rate ratio based on a signal from the control means 19.
That is, according to such an exhaust purification device, the ratio of NO and NO ₂ in the exhaust gas flowing into the SCR catalyst is controlled by controlling the amount of exhaust gas that passes through the oxidation catalyst based on the temperature of the exhaust gas. Can be controlled. Therefore, the NO _x purification efficiency in the SCR catalyst can be remarkably improved.
Hereinafter, the exhaust emission control device for an internal combustion engine of the present invention will be described separately for each component.

(2) Internal combustion engine As the internal combustion engine 50 that exhausts exhaust gas, a diesel engine or a gasoline engine is typical, but not only the exhaust purification device is indispensable but also the regeneration of the diesel engine is indispensable. It is suitable to do.
Further, the operating state detecting means 53 of the internal combustion engine shown in FIG. 1 is a means for detecting the rotational speed, the combustion temperature, etc. of the internal combustion engine 51, and the flow rate described later in consideration of the detection result. It is preferable that the ratio control means 15 is configured to be controlled.
Further, by implementing the present invention, it is possible to efficiently purify NO _x in the exhaust gas. However, by controlling so as to delay the fuel injection timing in the internal combustion engine, NO contained in the exhaust gas. Content of PM etc. other than _X can be reduced effectively.

(3) Exhaust passage The exhaust passage 20 is connected to an exhaust port of the internal combustion engine, and the oxidation catalyst 11 and the SCR catalyst 13 are disposed in the middle thereof. The cross-sectional shape of the exhaust passage 20 is not particularly limited as long as it is a circular, elliptical, or prismatic exhaust passage 20.

(4) Temperature sensor A temperature sensor 17 for measuring the temperature of the exhaust gas is provided in the middle of the exhaust passage 20. Based on the temperature of the exhaust gas measured by the temperature sensor 17, the oxidation reaction rate from NO to NO ₂ in the oxidation catalyst 11 can be estimated, and the NO and NO ₂ in the exhaust gas flowing into the SCR catalyst 13 can be estimated. This is because the ratio can be predicted.
The temperature sensor 17 is most preferably provided in the oxidation catalyst 11, but in reality, since it is difficult to provide the temperature sensor in the oxidation catalyst 11, an inlet and an outlet of the oxidation catalyst 11 are provided. It is preferable to be provided in either one of them.

(5) Oxidation catalyst The oxidation catalyst 11 disposed in the exhaust passage 20 of the exhaust gas discharged from the internal combustion engine 50 is not particularly limited, and is a known one, for example, one in which platinum is supported on alumina. In addition, a material to which a predetermined amount of a rare earth element such as cerium is added can be used.
In particular, in the exhaust emission control device according to the present invention, it is preferable to use an oxidation catalyst having a relatively strong oxidizing ability in a low temperature region, for example, by increasing the amount of platinum to be supported. The reason for this is that when the temperature of the exhaust gas is in the low temperature region, NO can be effectively oxidized to NO ₂ , while the sulfur content in the fuel is likely to generate sulfate, For example, even if it is in the temperature range of 300 ° C. or higher, the generation of such sulfate can be suppressed by bypassing the oxidation catalyst by bypass.

(6) SCR catalyst The SCR catalyst 13 is a catalyst that is disposed on the downstream side of the oxidation catalyst 11 and purifies NO _x contained in the exhaust gas by using urea (NH ₃ ). That is, NO or NO ₂ contained in the exhaust gas is adsorbed by the catalyst, and urea (NH ₃ ) is injected to the NO or NO ₂ , so that nitrogen (N ₂ ), water (H ₂ O), Disassemble and discharge.
The NO _x storage catalyst used for the SCR catalyst 13 is not particularly limited, and is known, for example, an alkaline earth metal such as strontium or barium as an active component, and magnesium on a porous carrier. Alternatively, those containing rare earth metals such as cerium and lanthanum, noble metals such as platinum and rhodium, and the like can be used.
Further, the aspect of urea as a reducing agent injected to NO or NO ₂ in the SCR catalyst 13 is not particularly limited, and urea water, urea powder, ammonia gas, ammonium salt, etc. are preferably used. can do.
Further, these urea, the injection means when injection against above of the NO _X storage catalyst can also be used known ones such as an injector.

(7) Bypass Further, the exhaust passage 20 is branched from the exhaust passage 20 on the upstream side of the oxidation catalyst 11 and on the downstream side of the oxidation catalyst 11 between the oxidation catalyst 11 and the SCR catalyst 13. A bypass 21 that joins the exhaust passage 20 is provided. By providing such a bypass 21, part or all of the exhaust gas discharged from the internal combustion engine 50 and passing through the exhaust passage 20 can flow into the SCR catalyst 13 by bypassing the oxidation catalyst 11. It becomes possible.
More specifically, when NO _x is decomposed in the SCR catalyst 13, the reduction reaction rate of NO, NO ₂ and NH ₃ greatly depends on the ratio of NO and NO ₂ contained in the exhaust gas. Further, the ratio of NO to NO ₂ in the exhaust gas depends on the flow rate of the exhaust gas to the oxidation catalyst 11 because NO in the exhaust gas is oxidized by the oxidation catalyst 11 to generate NO ₂ . Furthermore, the conversion rate at which NO in the exhaust gas passing through the oxidation catalyst 11 is oxidized and NO ₂ is generated depends on the temperature of the exhaust gas. Therefore, by providing such a bypass 21, a part or all of the exhaust gas can bypass the oxidation catalyst 11 and pass through the bypass 21 side based on the temperature of the exhaust gas, and flow into the SCR catalyst 13. The ratio of NO and NO ₂ in the exhaust gas to be made can be finely controlled.

The diameter and length of the bypass and the number of bypasses are not particularly limited and can be selected as appropriate. For example, when two bypasses are provided, the flow rate ratio of the exhaust gas flowing through the oxidation catalyst side and the bypass side can be more precisely controlled by a flow rate ratio control means described later. In addition, when two bypasses are provided, even if it is necessary to repair and inspect one of them, the flow rate can be continuously controlled using the other one.
In addition, it is also preferable to provide means for supplying urea for the reduction reaction of NO _x during the bypass. The reason for this is that by arranging the injection means for supplying urea and the like in this way, at the place where the bypass joins the exhaust passage, the exhaust gas that has passed through the bypass side and has been mixed with urea in advance and the oxidation catalyst side are connected. This is because urea can be uniformly dispersed in the exhaust gas when the exhaust gas that has passed through is mixed.

(8) Control Means In the exhaust purification apparatus 10 of the present invention, the flow rate ratio of the exhaust gas flowing through the oxidation catalyst 11 side and the bypass 21 side is set based on the temperature measured by the temperature sensor 17 described above. A control means 19 is provided. That is, in order to adjust the flow rate ratio control means 15 in order to make the ratio of NO and NO ₂ in the exhaust gas flowing into the SCR catalyst 13 coincide with the ratio at which the NO _X reduction rate in the SCR catalyst 13 becomes the fastest. It is the control means 19 of. For example, based on the temperature measured by the oxidation catalyst 11, the temperature sensor 17 provided at the inlet or outlet of the oxidation catalyst 11, and the exhaust gas temperature estimated from the operating state of the internal combustion engine 51, The flow rate ratio control means 15 can be controlled by sending a signal to the flow rate ratio control means 15 after setting the flow rate ratio to a predetermined ratio.

(9) Flow Rate Ratio Control Unit A flow rate ratio control unit 15 is provided at a branch point where the bypass 21 branches from the exhaust passage 20 on the upstream side of the oxidation catalyst 11. The flow rate ratio control means 15 can change the flow ratio of the exhaust gas flowing through the oxidation catalyst 11 side and the bypass 21 side based on the signal from the control means 19 for setting the flow rate ratio described above. It is configured. That is, in correspondence with the temperature of the exhaust gas, the ratio of NO and NO ₂ contained in the exhaust gas by controlling the rate at which reduction rate is fastest of the NO _X in the SCR catalyst 13, the purification rate of the NO _X It is for improving. Further, with this configuration, even when the oxidation catalyst 11 having a high oxidation capability is used, an abnormal oxidation reaction occurs in a high temperature region, the temperature of the exhaust gas rises, and the SCR catalyst 13 is thermally damaged. This is because it can be prevented.

The flow rate ratio control means 15 can be suitably used as long as the flow rate ratio can be changed based on the signal sent from the control means 19 described above. For example, the flow rate control valve 15 It can be. As shown in FIGS. 1A to 1C, the flow control valve 15 changes the flow rate ratio of the exhaust gas flowing through the oxidation catalyst 11 side and the bypass 21 side by adjusting the opening degree. Can do. That is, when all of the exhaust gas flows into the oxidation catalyst 11, the valve body may be disposed so as to close the inlet on the bypass 21 side, as shown in FIG. Further, when all of the exhaust gas flows into the bypass 21 side, the valve body may be disposed so as to close the inlet on the oxidation catalyst 11 side, as shown in FIG. Further, when the exhaust gas is divided into the oxidation catalyst 11 side and the bypass 21 side and is allowed to flow in at a desired flow rate ratio, as shown in FIG. What is necessary is just to change an arrangement position, controlling.
Such a flow rate ratio control means can be composed of one valve element 15a as shown in FIG. 2 (a), or can be composed of two valve elements 15b as shown in FIG. 2 (b). Furthermore, as shown in FIG. 2C, the shutter member 15c may be used.

2. Exhaust Purification Method Next, an exhaust purification method for purifying NO _x in the exhaust gas using the exhaust purification device 10 having the above-described configuration will be described in detail.
In such an exhaust gas purification method for an internal combustion engine, the temperature of the exhaust gas is measured by a temperature sensor, and the flow rate ratio of the exhaust gas flowing through the oxidation catalyst side and the bypass side is set by the control means based on the measured temperature. The ratio of NO and NO ₂ in the exhaust gas flowing into the SCR catalyst is controlled by changing the flow rate ratio by the flow rate control means based on the set value of the ratio.
In the following description, an example in which a flow control valve is used as the flow rate control means will be described.

  First, the temperature of the exhaust gas is measured by temperature sensors arranged at the inlet and outlet of the oxidation catalyst, and the measured value is transmitted to the control means for setting the flow rate ratio. The exhaust gas temperature to be measured at this time is measured either in the oxidation catalyst or at either the inlet or outlet of the oxidation catalyst, or the exhaust gas temperature is further measured in consideration of the operating state of the internal combustion engine. You can also However, since it is often difficult to incorporate a temperature sensor in the oxidation catalyst, it is often difficult to measure the temperature of the exhaust gas passing through the oxidation catalyst by measuring the temperature of the inlet and outlet of the oxidation catalyst. Preferably, in order to measure the exhaust gas temperature more accurately, it is more preferable to measure the exhaust gas temperature in consideration of the operating state of the internal combustion engine detected by the detection means.

Next, in the control means for setting the flow rate ratio, the flow rate ratio of the exhaust gas flowing into the oxidation catalyst side and the bypass side is set based on the measured temperature of the exhaust gas. Then, the flow rate ratio of the exhaust gas is changed by controlling the arrangement of the flow rate control valve based on the setting of the flow rate ratio. This makes it possible to control the ratio of NO and NO ₂ in the exhaust gas flowing into the SCR catalyst so as to coincide with the ratio at which the reduction rate of NO _x becomes the fastest, and the NO _x purification rate in the SCR catalyst. Can be improved.

Here, the relationship between the temperature of the exhaust gas, the ratio of NO and NO ₂ contained in the exhaust gas, and the NO _x purification rate will be described with reference to FIG. In FIG. 3, the broken line A shows the relationship between the temperature of the exhaust gas and the ratio of NO to NO ₂ (NO ₂ / NO) contained in the exhaust gas when an oxidation catalyst having a high oxidation capacity is used. The broken line C shows the relationship between the exhaust gas temperature and the NO _x purification rate in the SCR catalyst in that case. A solid line B shows the relationship between the temperature of the exhaust gas and the ratio of NO to NO ₂ (NO ₂ / NO) contained in the exhaust gas when an oxidation catalyst having a normal oxidation capacity is used. D shows the relationship between the temperature of the exhaust gas in that case and the NO _x purification rate of the SCR catalyst. Further, the dotted line E shows the relationship between the temperature of the exhaust gas and the NO _x purification rate at the SCR catalyst when the exhaust gas is allowed to flow without using the oxidation catalyst.
However, since these relationships may change depending on the measurement conditions, FIG. 3 is merely shown as an example, and does not limit the temperature region for changing the flow rate ratio in the present invention.

As shown in FIG. 3, in the temperature range where the exhaust gas temperature is less than about 220 ° C., an oxidation catalyst having a higher oxidation ability was used than when an oxidation catalyst having a normal oxidation ability was used (solid line D). In the case (broken line C), the NO _x purification rate in the SCR catalyst is higher.
More specifically, even if an oxidation catalyst having a normal oxidation ability is used in such a relatively low temperature range, the oxidation catalyst is not sufficiently activated, so that NO in the exhaust gas is oxidized and NO _2. Is rarely generated. Therefore, the ratio (NO ₂ / NO) of NO and NO ₂ contained in the exhaust gas becomes small (solid line B), and the temperature of the exhaust gas flowing into the SCR catalyst cannot be raised. NO _X purification cannot be performed efficiently (solid line D). Therefore, by using a high oxidation ability oxidizing catalyst, NO in the exhaust gas efficiently oxidized to produce the NO _2, increase the ratio of NO and _{NO 2 (NO 2 / NO)} ( broken line A) At the same time, the exhaust gas temperature is raised by the reaction heat, and the NO _x purification efficiency in the SCR catalyst can be improved (dashed line C). At this time, if unburned fuel (raw gas: HC) is present in the exhaust gas, the exhaust gas is passed through the oxidation catalyst, so that HC is oxidized by the oxidation catalyst. The temperature of the exhaust gas can be further raised by the generated reaction heat.

However, in general, it has been found that when the ratio of NO to NO ₂ (NO ₂ / NO) contained in the exhaust gas is close to 1, the reduction reaction rate in the SCR catalyst is the fastest. Therefore, even when an oxidation catalyst having a high oxidation capacity is used in a low temperature region, the SCR is increased from about 200 ° C. where the ratio (NO ₂ / NO) exceeds 1 and approaches 2 (broken line A). The NO _x purification efficiency in the catalyst is starting to decrease (dashed line C).
Therefore, even in a temperature range of less than about 220 ° C., it is effective to intentionally reduce the rate at which NO is oxidized and NO ₂ is produced in a range of more than about 200 ° C. This indicates that in such a region, it is effective to bypass the oxidation catalyst for a part of the exhaust gas while using an oxidation catalyst having a high oxidation ability.

Next, in a temperature range where the temperature of the exhaust gas exceeds about 220 ° C., when an oxidation catalyst having a normal oxidation ability (solid line D) is used rather than using an oxidation catalyst having a high oxidation ability (dashed line C) This shows a higher NO _x purification rate in the SCR catalyst.
More specifically, in this temperature range, both the oxidation reaction from NO to NO ₂ in the oxidation catalyst and the NO _x reduction reaction in the SCR catalyst are performed relatively efficiently. Therefore, when an oxidation catalyst having a high oxidation capacity is used, NO in the exhaust gas is efficiently oxidized and NO ₂ is generated more than necessary, and the ratio of NO to NO ₂ (NO ₂ / NO) is excessively high. (Broken line A). In general, the reduction reaction rate in the SCR catalyst is the fastest when NO ₂ : NO is 1: 1, so the reduction efficiency of NO _{x in} the SCR catalyst decreases. (Dashed line C). Therefore, by using an oxidation catalyst having a normal oxidation capacity, the ratio of NO to NO ₂ (NO ₂ / NO) contained in the exhaust gas is maintained at about 1 (solid line B), and NO _X in the SCR catalyst is maintained. The purification efficiency can be improved (solid line D). This indicates that the same result can be obtained by bypassing the oxidation catalyst for a part of the exhaust gas while using an oxidation catalyst having a high oxidation ability.

However, in a temperature range where the temperature of the exhaust gas exceeds about 300 ° C., the NO _x purification rate is 90% even when the exhaust gas is directly allowed to flow into the SCR catalyst without passing through the oxidation catalyst. A higher value is shown (dotted line E). In addition, when high-temperature exhaust gas is passed through an oxidation catalyst, sulfate (SO ₄ ² ) is generated from sulfur dioxide (SO ₂ ) contained in the exhaust gas, or exhaust gas that has passed through the oxidation catalyst There is also a problem that the temperature becomes abnormally high and the SCR catalyst is thermally damaged.
Therefore, it is effective not to allow the exhaust gas to pass through the oxidation catalyst in a region exceeding about 300 ° C.

From the above, based on FIG. 3, in order to most efficiently perform NO _x in the exhaust gas using the SCR catalyst, the flow rate ratio as shown in FIG. Thus, it is effective to allow the exhaust gas to pass through the oxidation catalyst side.
That is, for example, when the temperature of the exhaust gas is less than about 200 ° C., all exhaust gas is allowed to pass through the oxidation catalyst side (100% to the oxidation catalyst side), thereby efficiently oxidizing NO and NO. ₂ and the temperature of the exhaust gas can be raised, and the NO _x purification rate in the SCR catalyst can be improved. Therefore, in such a temperature range, as shown in FIG. 1A, the flow rate control valve 15 is disposed so as to close the inlet on the bypass 21 side and allow all exhaust gas to flow into the oxidation catalyst 11 side. Is preferred.
Even when the temperature of the exhaust gas is excessively low and is lower than the temperature (about 160 ° C.) necessary for producing ammonia necessary for NO _x reduction, by arranging in this way, The temperature of the exhaust gas is raised, and the SCR catalyst can be activated to be heated.

Further, when the temperature of the exhaust gas is in the range of about 200 to 300 ° C., the oxidation catalyst such that the NO ₂ : NO ratio in the exhaust gas flowing into the SCR catalyst approaches 1: 1, for example. By controlling the flow rate of the exhaust gas passing through the side (0 to 100% on the oxidation catalyst side), the NO _x purification rate in the SCR catalyst can be improved. Therefore, in such a temperature range, the control means sets the flow rate ratio of the exhaust gas flowing through the oxidation catalyst side and the bypass side to a desired ratio in accordance with the temperature of the exhaust gas, and based on the set value, As shown in FIG. 1B, it is preferable to control the arrangement or opening of the flow control valve 15.
For example, the arrangement of the flow control valves can be controlled as follows so that the ratio of NO ₂ : NO contained in the exhaust gas flowing into the SCR catalyst becomes a desired ratio. That is, when the current rate of oxidation reaction from NO to NO ₂ is x and the target conversion rate is X, the ratio a of the flow rate of exhaust gas passing through the oxidation catalyst side in the total flow rate of exhaust gas is expressed as a = Min (1, max (0, X / x)) or a = max (0, min (1, X / x)) by controlling the arrangement of the flow control valve so that The NO _x purification rate can be improved.
The flow rate ratio varies depending on the oxidation ability of the oxidation catalyst used.

Furthermore, when the temperature of the exhaust gas exceeds about 300 ° C., all the exhaust gas is passed through the bypass side (0% on the oxidation catalyst side) to prevent the generation of sulfate and thermal damage of the SCR catalyst. Meanwhile, the NO _x purification rate in the SCR catalyst can be improved. Therefore, in such a temperature range, as shown in FIG. 1C, the flow rate control valve 15 is disposed so as to close the inlet on the oxidation catalyst 11 side and allow all exhaust gas to flow into the bypass 21 side. Is preferred.

As described above, according to the present invention, the exhaust gas purification apparatus having the predetermined temperature sensor, the bypass, and the flow rate ratio control unit bypasses the oxidation catalyst for all or part of the exhaust gas, thereby By controlling the abundance ratio between NO and NO ₂ flowing into the exhaust gas, NO _x in the exhaust gas can be effectively purified in a wider temperature range than before. Accordingly, NO _x in the exhaust gas discharged from the internal combustion engine can be efficiently purified.

(A)-(c) is a figure which shows the exhaust gas purification apparatus provided with the flow control valve which consists of one valve body. (A)-(c) is a figure which shows the example of a flow rate ratio control means, respectively. With respect to the temperature of the exhaust gas is a diagram illustrating the purification rate of the present ratio and NO _X in the NO and NO _2. It is a figure which shows the relationship between the temperature of exhaust gas, and the flow volume ratio of the exhaust gas which passes the oxidation catalyst side. It is a figure provided in order to demonstrate the structure of the conventional exhaust gas purification apparatus.

Explanation of symbols

10: exhaust purification device 11: oxidation catalyst 13: SCR catalyst 15: flow rate control means 17: temperature sensor 19: control means 20: exhaust passage 21: bypass 50: internal combustion engine

Claims (8)

An exhaust purification device for purifying exhaust gas, provided in an exhaust passage of an internal combustion engine, comprising an oxidation catalyst and an SCR catalyst disposed downstream of the oxidation catalyst. In
A temperature sensor for measuring the temperature of the exhaust gas;
A bypass branched from the exhaust passage on the upstream side of the oxidation catalyst and downstream of the oxidation catalyst and joined to the exhaust passage between the oxidation catalyst and the SCR catalyst;
Based on the temperature measured by the temperature sensor, control means for setting a flow rate ratio of the exhaust gas flowing through the oxidation catalyst side and the bypass side;
A flow rate ratio control means for changing the flow rate ratio based on a signal from the control means;
An exhaust emission control device for an internal combustion engine, comprising:   2. The exhaust gas purification apparatus for an internal combustion engine according to claim 1, wherein the flow rate ratio control means can change the flow rate ratio continuously or stepwise.   The exhaust purification device for an internal combustion engine according to claim 1 or 2, wherein the flow rate ratio control means is a flow path control valve.   The apparatus further comprises detection means for detecting the operating state of the internal combustion engine, and the control means sets the flow rate ratio further corresponding to the operating state of the internal combustion engine detected by the detection means. An exhaust emission control device for an internal combustion engine according to any one of claims 1 to 3. An exhaust emission control device including an oxidation catalyst and an SCR catalyst, a temperature sensor for measuring the temperature of the exhaust gas, a branch from the exhaust passage upstream of the oxidation catalyst, and a downstream side of the oxidation catalyst The flow rate ratio of the exhaust gas flowing through the oxidation catalyst side and the bypass side based on the bypass measured by the temperature sensor and the bypass that joins the exhaust passage between the oxidation catalyst and the SCR catalyst. In exhaust gas exhausted from an internal combustion engine using an exhaust emission control device comprising control means for setting and flow rate ratio control means for changing the flow rate ratio based on a signal from the control means in the exhaust purification method for an internal combustion engine for purifying the NO _X,
Measuring the temperature of the exhaust gas by the temperature sensor;
Based on the measured temperature, the control means sets the flow rate ratio of the exhaust gas flowing through the oxidation catalyst side and the bypass side,
The ratio of NO and NO ₂ in the exhaust gas flowing into the SCR catalyst is controlled by changing the flow rate ratio by the flow rate control means based on the set value of the flow rate ratio. An exhaust purification method for an internal combustion engine.   6. The exhaust gas purification method for an internal combustion engine according to claim 5, wherein the operating state of the internal combustion engine is detected, and the flow rate ratio is set by the control means in further correspondence with the operating state.   The exhaust gas purification method for an internal combustion engine according to claim 5 or 6, wherein the temperature of the exhaust gas is measured at one or both of an inlet and an outlet of the oxidation catalyst. 8. The internal combustion engine according to claim 5, wherein the ratio of NO to NO ₂ in the exhaust gas is made to coincide with a ratio obtained in advance so that the reduction rate of NO _x becomes the fastest. Engine exhaust purification method.

Images