JP2006104970A - Exhaust emission control device of internal combustion engine - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an exhaust emission control device of an internal combustion engine capable of keeping NOx purifying efficiency high even when a catalyst deteriorates. <P>SOLUTION: This exhaust emission control device is formed by arranging a NOx occlusion reducing catalyst 3 in an exhaust passage 2 for performing reduction processing by occluding NOx exhaust in air-fuel ratio lean combustion of the internal combustion engine 1 and discharging the occluded NOx by temporarily making air-fuel ratio rich; and is constituted by arranging a deterioration detecting means 5 for detecting a deterioration degree of the NOx occlusion reducing catalyst 3 and a plasma device 4 operated in response to the deterioration degree, on the upstream side of the NOx occlusion reducing catalyst 3. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an exhaust gas purification device for an internal combustion engine.

  In a conventional internal combustion engine, particularly a lean burn engine, NOx (nitrogen oxide) is occluded when the air-fuel ratio of exhaust gas is lean (excess oxygen), and occluded when the air-fuel ratio is made the theoretical air amount or rich (oxygen deficient). A NOx occlusion reduction catalyst that releases and purifies the NOx that has been purified is disposed in the exhaust flow path, and the air-fuel ratio of the exhaust gas is temporarily switched from lean to rich over a time interval, and occluded by the NOx occlusion reduction catalyst. Technology that purifies NOx is used. (For example, refer to Patent Document 1).

Further, the NOx storage reduction catalyst gradually deteriorates while being used, and the NOx storage capacity decreases. When the NOx storage capacity decreases, the amount of NOx that can be stored decreases, and the saturated storage state is reached early. After that, NOx is not occluded by the catalyst and discharged downstream.
Therefore, it is considered to detect the degree of deterioration of the catalyst and shorten the cycle of switching the air-fuel ratio of the exhaust gas flowing into the NOx storage reduction catalyst from lean to rich as the degree of deterioration increases. It is also considered that the rich time when switching to rich is shortened as the degree of deterioration increases. (For example, refer to Patent Document 2).

  Further, a method using discharge plasma for purifying exhaust gas from the engine is already known. (For example, refer to Patent Document 3).

Japanese Patent No. 2600492 Japanese Patent No. 2836523 JP 61-31615 A

  In the conventional exhaust purification device, when the NOx occlusion reduction catalyst deteriorates, the NOx occlusion reduction catalyst leans the air-fuel ratio of the exhaust gas to occlude NOx, and when the NOx is reduced by making the air-fuel ratio rich, the catalyst is reduced. Performance deteriorates. The catalyst performance deteriorates not only the amount of NOx that can be stored, but also the speed at which it is stored and reduced. Therefore, when the catalyst is deteriorated, NOx is simply adjusted by temporal adjustment to shorten the cycle for switching the air-fuel ratio from lean to rich as the degree of deterioration increases, or to shorten the rich time when switching from lean to rich. There was a problem that improvement of the purification rate was insufficient.

  Further, when discharge plasma is always used for purifying exhaust gas, there is a problem that energy loss increases and power consumption increases.

  The present invention has been made to solve the above problems, and provides an exhaust purification device for an internal combustion engine that can maintain a high NOx purification rate while suppressing an increase in power consumption even when the catalyst is deteriorated. The purpose is to do.

  An exhaust gas purification apparatus for an internal combustion engine according to the present invention stores NOx discharged when the air-fuel ratio of the internal combustion engine is lean-burned, temporarily enriches the air-fuel ratio, releases the stored NOx, and performs a reduction process. In the exhaust gas purification apparatus, a deterioration detecting means for detecting the deterioration degree of the NOx storage reduction catalyst, and a plasma device that operates in accordance with the deterioration degree are arranged upstream of the NOx storage reduction catalyst. It is arranged.

  Since the exhaust gas purification apparatus for an internal combustion engine according to the present invention is configured as described above, when the catalyst is deteriorated, even in the NOx occlusion period in which the air-fuel ratio is lean, and also in the NOx reduction process period in which the air-fuel ratio is rich. There is an unprecedented remarkable effect that the purification efficiency can be improved.

Embodiment 1 FIG.
Embodiment 1 of the present invention will be described below with reference to the drawings. FIG. 1 is a diagram showing a change state of the NOx emission amount between lean and rich in a catalyst that has not deteriorated, in which the horizontal axis represents time and the vertical axis represents NOx emission. In the initial stage when the air-fuel ratio is made lean, the NOx storage capacity of the catalyst is large and the amount of NOx discharged to the catalyst outlet is small, but as NOx is gradually stored, the NOx discharged without being stored increases. .

  When the air-fuel ratio is switched to rich, the NOx stored in the catalyst is released because the oxygen concentration of the exhaust gas decreases, and it is purified by reacting with reducing components such as hydrocarbons contained in the exhaust gas. At this time, since the amount of NOx released from the catalyst is large, NOx that has not fully reacted with the reducing component is discharged as shown in FIG.

  FIG. 2 is a diagram showing a change state of the NOx emission amount between lean and rich of a deteriorated catalyst. The amount of NOx that can be occluded by air-fuel ratio lean (saturated occlusion amount) becomes smaller, so the lean period must be shortened, and the occlusion speed becomes slower, so the catalyst is not deteriorated (FIG. 1). The amount of NOx discharged during the elapsed time also increases. In addition, since the deteriorated catalyst has a smaller amount of NOx occluded during the lean period than the catalyst that has not deteriorated, the air-fuel ratio rich period is also shortened, and the reducing ability in this period is also deteriorated. Therefore, the amount of NOx discharged increases.

FIG. 3 is a schematic diagram showing the configuration of the exhaust gas purification apparatus for an internal combustion engine according to the first embodiment.
A NOx occlusion reduction catalyst 3 is disposed in an exhaust passage 2 through which exhaust gas discharged from the engine 1 flows. A plasma device 4 is provided upstream of the NOx storage reduction catalyst 3. In addition, a deterioration detection device 5 that detects the degree of deterioration of the NOx occlusion reduction catalyst 3 and a plasma control device 6 that controls the plasma device 4 by a signal from the deterioration detection device 5 are provided.

  The plasma control device 6 incorporates a high voltage power source (not shown), and controls the amount of plasma generated by the plasma device 4 by controlling the high voltage power source based on the monitored information on the degree of deterioration of the catalyst. When the deterioration detector 5 detects the deterioration of the catalyst, the plasma controller 4 is operated by the plasma controller 6 during the air-fuel ratio lean period according to the degree of deterioration. The high voltage power source in the plasma apparatus 4 converts the DC voltage into an AC voltage and adjusts the frequency and the AC voltage value to change the input power.

In the case of the air-fuel ratio lean, in addition to NOx, oxygen (O ₂ ) and water vapor (H ₂ O) of about several to 10% are included in the exhaust gas. NOx mainly consists of NO (nitrogen monoxide) and NO ₂ (nitrogen dioxide), and the proportion of NO occupies about 90%. In the NOx occlusion reduction catalyst 3, when NOx is occluded, it is generally occluded as a nitrate with an alkali metal oxide or an alkaline earth metal oxide. In this case, barium and potassium are often used.

In the process of converting to nitrate, NO is oxidized by the noble metal catalyst to change to NO ₂ , and this NO ₂ is occluded in the form of nitrate. When the catalyst deteriorates, the oxidation performance of the noble metal also decreases, and the conversion rate of NO → NO ₂ becomes slow. When the plasma apparatus 4 is operated, the oxygen concentration and the water vapor concentration in the exhaust gas are high, so that the following discharge chemical reaction is affected.

First, when oxygen molecules or water molecules in the exhaust gas are discharged, the following dissociation occurs.
O ₂ → 2O *
H ₂ O → H * + OH *
The O * and OH * are NO ₂ is caused to react with NO.
NO + O * → NO ₂

In this way, at least a part of NO passes through the plasma device 4 and is oxidized to NO ₂ . When the amount of NO ₂ that tends to become nitrate increases, the occlusion effect improves and the NOx that is discharged decreases. The NOx emission preventing effect can also be exhibited by operating the plasma device 4 only in the latter half of the lean period when the exhausted NOx becomes large at the early stage of catalyst deterioration.

  When the deterioration progresses, the NOx emission preventing effect is maintained by extending the time for which the plasma apparatus 4 is operated. In this way, energy saving can be realized by limiting the operation period of the plasma device 4 to a period having an NOx emission preventing effect.

When the air-fuel ratio is rich, the oxygen concentration in the exhaust gas decreases to about 0.1%, and reducing components such as hydrocarbon (HC) increase. However, depending on the operating mode of the engine, the hydrocarbons in the exhaust gas may be mostly saturated HC, and saturated HC is compared to unsaturated HC, carbon monoxide (CO), aldehyde, and hydrogen (H ₂ ). Since the reactivity with NOx is low, the NOx reduction efficiency may decrease. When the plasma apparatus 4 is operated with rich air-fuel ratio according to the degree of deterioration of the catalyst, dissociation of water vapor proceeds.

H ₂ O → H * + OH *
This OH * reacts with saturated HC, and finally, unsaturated HC, CO, formaldehyde, acetaldehyde, H _{2 and the} like are generated.
Saturated HC + OH * → unsaturated HC, CO, aldehyde, H ₂

NOx released from the catalyst due to a decrease in oxygen concentration reacts with these unsaturated HC, CO, aldehyde, and H ₂ to reduce the amount of NOx that is purified and discharged. The period during which the plasma apparatus 4 is operated may be a part of the air-fuel ratio rich period, for example, only when the amount of exhausted NOx has a peak when the degree of deterioration of the catalyst is small. If the deterioration has progressed, the plasma apparatus 4 may be operated over the entire rich period.

  FIG. 4 is a partially broken perspective view showing the plasma device 4 more specifically. The plasma apparatus 31 includes an exhaust pipe 21 and has a structure in which exhaust gas passes through the exhaust pipe 21. For example, the exhaust gas can be passed from left to right in FIG. At both ends of the exhaust pipe 21, flanges 28 are provided so that the exhaust flow path 2 can be connected. The material of the exhaust pipe 21 may be an insulating material and is not limited to one. For example, ceramic such as aluminum oxide can be used.

  Inside the exhaust pipe 21, a high voltage electrode 22 fixed by a mesh 23 is disposed so as to be coaxial with the exhaust pipe 21. In order to supply a voltage to the high voltage electrode 22, the high voltage electrode terminal 22 a and the power supply terminal 22 b are connected by a high voltage cable 26. The power supply terminal 22b is connected to the plasma control device 6 by a cable (not shown). A ground electrode 24 is attached to the periphery (outer peripheral surface) of the exhaust pipe 21 so as to contact the exhaust pipe 21.

  The ground electrode 24 is fastened with a fixing screw 27. Further, the above-described cable (not shown) for connecting to the plasma control device 6 is also attached by the fixing screw 27. In addition, the material of the mesh 23 should just be an insulator, and although it is not limited to one, For example, ceramics, such as aluminum oxide, can be used. Further, the material of the high voltage electrode 22 and the ground electrode 24 may be a conductor and is not limited to one. For example, stainless steel can be used.

  In the plasma apparatus 31 configured as described above, an AC high voltage or a pulsed high voltage is applied between the high voltage electrode 22 and the ground electrode 24, whereby a space between the high voltage electrode 22 and the ceramic exhaust pipe 21 is formed. Generate silent discharge. The length in the gas flow direction of the space where silent discharge occurs is the same as the length of the ground electrode 24 in the gas flow direction. That is, silent discharge occurs in the space surrounded by the ground electrode 24. The exhaust gas introduced into the plasma apparatus 31 passes through the mesh 23 and passes through the silent discharge space. In this passage process, the exhaust gas undergoes a chemical reaction by the plasma.

  Next, the deterioration detection device 5 will be described. As an example of the deterioration detection device 5, there is a method of detecting the degree of deterioration of the oxygen storage function substance carried on the catalyst. The oxygen storage amount is calculated based on a signal from an air-fuel ratio sensor (not shown) installed on the upstream side or downstream side of the catalyst to detect the degree of deterioration. The air-fuel ratio sensor uses a zirconia limit current type. Stabilized zirconia has oxygen ion conductivity. When a voltage is applied to both ends of an electrode having platinum or the like attached thereto, oxygen becomes oxygen ions and passes through the stabilized zirconia when the measurement gas is in an oxygen atmosphere. The current value at this time is proportional to the oxygen concentration.

By calculating the oxygen concentration from the current value detected by the air-fuel ratio sensor and calculating the oxygen concentration difference between the upstream side and the downstream side of the catalyst, the oxygen stored in the catalyst can be calculated. It is possible to detect the deterioration of the catalyst by comparing this oxygen storage amount with that in the initial stage.
Further, the degree of deterioration may be detected based on a signal from a NOx sensor (not shown) installed on the downstream side of the NOx storage reduction catalyst 3.

FIG. 5 shows a change state of the NOx emission amount when the first embodiment is operated in the air-fuel ratio lean and rich. Due to the effect of the plasma device 4, the occlusion effect is improved because the ratio of NO ₂ increases in the lean period, and the occluded NOx is improved in the rich period because saturated HC is reformed to a highly reactive substance such as unsaturated HC. The purification effect of NOx is improved, and the exhausted NOx is almost the same level as in the case of the non-degraded catalyst shown in FIG.

It is a figure which shows the change condition of the discharge | emission amount of NOx in the lean and rich of the catalyst which has not deteriorated. It is a figure which shows the change condition of the discharge | emission amount of NOx in the lean and rich of the deteriorated catalyst. 1 is a schematic diagram illustrating a configuration of an exhaust gas purification apparatus for an internal combustion engine according to Embodiment 1. FIG. FIG. 4 is a perspective view showing a specific configuration of the plasma device shown in FIG. It is a figure which shows the change condition of the discharge | emission amount of NOx at the time of operating Embodiment 1 in an air fuel ratio lean and rich.

Explanation of symbols

1 engine, 2 exhaust flow path, 3 NOx storage reduction catalyst, 4 plasma device,
5 Deterioration detection device, 6 Plasma control device, 21 Exhaust pipe, 22 High voltage electrode,
22a high voltage electrode terminal, 22b power supply terminal, 23 mesh,
24 ground electrode, 26 high voltage cable, 27 fixing screw, 28 flange,
31 Plasma device.

Claims (5)

  Exhaust gas purifying apparatus for storing NOx occluded and reduced in an exhaust passage for storing NOx exhausted during air-fuel ratio lean combustion of an internal combustion engine and releasing the stored NOx by temporarily enriching the air-fuel ratio The exhaust of the internal combustion engine, wherein a deterioration detecting means for detecting a deterioration degree of the NOx storage reduction catalyst and a plasma device that operates in accordance with the deterioration degree are arranged upstream of the NOx storage reduction catalyst. Purification equipment.   2. The exhaust emission control device for an internal combustion engine according to claim 1, wherein the plasma device is operated at least during a time when the air-fuel ratio is lean according to the degree of deterioration. The plasma apparatus, at least a portion of the exhaust gas purifying apparatus for an internal combustion engine according to claim 2, wherein the converting NO to NO ₂ among NOx contained in the exhaust gas air-fuel ratio lean.   2. The exhaust emission control device for an internal combustion engine according to claim 1, wherein the plasma device is operated at least during a time when the air-fuel ratio is rich according to the degree of deterioration.   5. The internal combustion engine according to claim 4, wherein the plasma device reforms at least a part of saturated hydrocarbons contained in the air-fuel ratio rich exhaust gas into unsaturated hydrocarbons, carbon monoxide, aldehydes, or hydrogen. Exhaust purification equipment.

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