JP2008267177A - Exhaust emission control device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve the cleaning efficiency of exhaust gas at the cold start of an engine. <P>SOLUTION: A three way catalyst 3 and an NOx adsorbing catalyst 4 are arranged in an exhaust passage 2 so that the three way catalyst is arranged in an upstream side and the NOx catalyst is arranged at a downstream side. When an air-fuel ratio of an exhaust gas is rich, active oxygen (ozone) is supplied to the three-way catalyst 3. When the air-fuel ratio of the exhaust gas is lean, the active oxygen is supplied from the downstream side of the three-way catalyst 3 to the NO<SB>x</SB>occlusion catalyst 4. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

    The present invention relates to an exhaust gas purification apparatus for an engine provided with active oxygen supply means for promoting purification of exhaust gas.

    Since the exhaust gas temperature is low when the engine is cold started, it takes time to activate the catalyst, and the exhaust gas purification rate is low. Specifically, it generally takes several tens of seconds from the start of the engine for the temperature of the catalyst to rise to a temperature at which a 50% purification rate called light-off is obtained.

    On the other hand, in Patent Document 1, ozone, which is active oxygen, is used to convert a part of HC (hydrocarbon) in exhaust gas into CO, thereby improving the exhaust gas purification efficiency by the catalyst, and It is described that ozone is generated by silent discharge in the air.

Further, Patent Document 2 describes that plasma is generated in exhaust gas of a diesel engine to generate nitrogen dioxide and ozone, which oxidize particulate matter in the exhaust gas.
JP-A-2005-207316 JP 2004-169643 A

    However, in the case of the active oxygen generator using silent discharge, only a part of oxygen in the air is converted into active oxygen, so the remaining oxygen is supplied to the exhaust passage together with the active oxygen. Therefore, when active oxygen is supplied from the upstream side to the three-way catalyst in the exhaust passage, the three-way catalyst is exposed to oxygen-excess exhaust gas due to the influence of residual oxygen, and NOx (nitrogen oxide) It is disadvantageous for the reduction and purification.

    In particular, when air-fuel ratio modulation control is performed in which the air-fuel ratio of the exhaust gas is changed alternately between lean and rich in order to increase the exhaust gas purification efficiency by the three-way catalyst, the residual oxygen supplied together with the active oxygen during the lean is excessive. It becomes impossible to expect NOx purification by a three-way catalyst.

    On the other hand, it is conceivable that the NOx storage material is included in the three-way catalyst, but such a NOx storage material does not function sufficiently until it reaches a temperature capable of storing NOx.

    Thus, it is not possible to efficiently purify all HC, CO, and NOx simply by supplying active oxygen to the three-way catalyst from the upstream side.

    In order to solve such a problem, the present invention provides a NOx occlusion material in the exhaust passage downstream of the three-way catalyst. When the air-fuel ratio of the exhaust gas is rich, the three-way catalyst has active oxygen from its upstream side. When lean, the active oxygen was supplied to the NOx occlusion material from the downstream side of the three-way catalyst.

That is, the invention according to claim 1 is an exhaust gas purification device in which a three-way catalyst and a NOx occlusion material are sequentially arranged in the exhaust passage of an engine from the upstream side of the exhaust gas flow,
A first active oxygen supply unit disposed in the exhaust passage upstream of the three-way catalyst to supply active oxygen to the three-way catalyst;
A second active oxygen supply unit disposed in the exhaust passage between the NOx storage material and the three-way catalyst to supply active oxygen to the NOx storage material;
The supply of active oxygen by the first active oxygen supply unit is executed when the air-fuel ratio of the exhaust gas discharged from the engine is rich, and the supply of active oxygen by the second active oxygen supply unit is performed by the exhaust gas of the exhaust gas. It is executed when the air-fuel ratio is lean.

    Therefore, when the air-fuel ratio of the exhaust gas is rich, active oxygen is supplied to the three-way catalyst from the upstream side. This active oxygen promotes oxidative combustion of HC and CO in the exhaust gas, and promotes oxidative combustion of HC and CO in the three-way catalyst. Due to the heat generated by this oxidative combustion, the temperature of the exhaust gas flowing through the NOx occlusion material increases, and the temperature of the NOx occlusion material can be raised.

On the other hand, when the air-fuel ratio of the exhaust gas is lean, active oxygen is supplied to the NOx storage material from the downstream side of the three-way catalyst. Therefore, NO in the exhaust gas flowing to the NOx storage material without being purified by the three-way catalyst is oxidized to NO ₂ or NO ₃ ⁻ which is easily stored in the NOx storage material by the active oxygen. The NOx occlusion efficiency of the NOx occlusion material is increased by the oxidation of NO and the temperature increase of the NOx occlusion material.

    Moreover, since the active oxygen is not supplied to the three-way catalyst when the air-fuel ratio of the exhaust gas is lean, the atmosphere of the three-way catalyst does not become excessively lean, and the NOx purification efficiency by the three-way catalyst is large. Decreasing is avoided.

The invention according to claim 2 is the invention according to claim 1,
Air-fuel ratio detection means for detecting the air-fuel ratio of the exhaust gas in the exhaust passage upstream from the first active oxygen supply section, and based on the air-fuel ratio of the exhaust gas detected by the air-fuel ratio detection means, The supply of active oxygen by the first and second active oxygen supply units is performed.

    Therefore, according to the change in the air-fuel ratio of the exhaust gas exhausted from the engine, active oxygen is supplied from the upstream side to the three-way catalyst when rich, and active oxygen is supplied to the NOx storage material downstream of the three-way catalyst when lean. The supply of active oxygen can be switched between the first active oxygen supply unit side and the second active oxygen supply unit side so as to be supplied.

The invention according to claim 3 is the invention according to claim 1 or claim 2,
An air-fuel ratio detecting means for detecting an air-fuel ratio of the exhaust gas in the exhaust passage between the three-way catalyst and the second active oxygen supply unit, the air-fuel ratio of the exhaust gas detected by the air-fuel ratio detecting means; The supply amount of active oxygen by the second active oxygen supply unit is corrected according to the degree of lean.

    That is, the active oxygen supply switching can be executed by detecting (estimating) the air-fuel ratio of the exhaust gas based on the engine operating state (fuel supply control), or the engine periodically performs rich / lean. When the control is repeated, the supply can be switched by a timer, or the air-fuel ratio of the exhaust gas upstream of the first active oxygen supply unit can be determined by the air-fuel ratio detection means as in claim 2. The supply switching can be performed upon detection.

    However, the supply of active oxygen from the second active oxygen supply unit is to oxidize NO in the exhaust gas so that it is easily stored in the NOx storage material. Therefore, a three-way catalyst is required to realize efficient NOx storage. The air-fuel ratio of the exhaust gas that has passed through becomes a problem.

    On the other hand, the flow of exhaust gas is not always constant, and the flow rate or flow velocity changes depending on the operating state of the engine. For this reason, the exhaust gas passes through the three-way catalyst, so that the lean state is different from the lean state that is grasped by the operating state of the engine or by the air-fuel ratio detection means etc. upstream from the first active oxygen supply unit. May be.

    Therefore, in the present invention, the supply of active oxygen to be executed during lean is detected by detecting the air-fuel ratio of the exhaust gas that has passed through the three-way catalyst so as to be advantageous for NOx occlusion according to the lean degree. The amount was corrected.

According to a fourth aspect of the present invention, in any one of the first to third aspects,
Exhaust gas temperature detecting means for detecting the temperature of the exhaust gas;
A bypass passage that bypasses the NOx occlusion material from the downstream side of the three-way catalyst and discharges the exhaust gas when the exhaust gas temperature detected by the exhaust gas temperature detection is equal to or higher than a predetermined value. It is characterized by.

    That is, the NOx storage material is for storing NOx when the activity of the three-way catalyst is low, but when the exhaust gas temperature becomes high, the activity of the three-way catalyst becomes high and NOx is reduced and purified. The need for NOx storage materials is reduced. On the contrary, there is a problem that the back pressure of the engine becomes high due to the presence of the NOx occlusion material.

    Therefore, in the present invention, when the exhaust gas temperature exceeds a predetermined value, the exhaust gas is discharged by bypassing the NOx occlusion material by the bypass passage, thereby suppressing the increase in the back pressure and deteriorating the fuel consumption. To avoid.

    As described above, according to the first aspect of the present invention, when the air-fuel ratio of the exhaust gas is rich, active oxygen is supplied from the upstream side to the three-way catalyst, and when the air-fuel ratio is lean, Since the exhaust gas purification device is configured so that the active oxygen is supplied from the downstream side of the original catalyst to the NOx storage material, the active oxygen is used in the case where the air-fuel ratio of the exhaust gas fluctuates during cold start of the engine. While the HC and CO purification efficiency can be increased, the NOx storage efficiency can be increased with the NOx storage material while avoiding a significant decrease in the NOx purification efficiency with the three-way catalyst. This is advantageous in preventing discharge.

    According to the second aspect of the invention, the air-fuel ratio of the exhaust gas is detected on the upstream side of the first active oxygen supply unit for supplying active oxygen to the three-way catalyst, and based on this, the three-way catalyst is supplied to the three-way catalyst. Since the exhaust gas purification device is configured so that the supply of active oxygen and the supply of active oxygen to the NOx storage material are executed, the three-way operation is performed in the rich state in accordance with the change in the air-fuel ratio of the exhaust gas discharged from the engine. The supply of active oxygen can be switched so that active oxygen is supplied to the catalyst and active oxygen is supplied to the NOx storage material during lean.

According to the invention of claim 3, in claim 1 or claim 2,
The air-fuel ratio of the exhaust gas is detected between the second active oxygen supply unit for supplying active oxygen to the NOx storage material and the three-way catalyst, and the supply of active oxygen to the NOx storage material according to the lean degree Since the exhaust gas purification device is configured so that the amount is corrected, the supply of active oxygen to the NOx occlusion material is appropriately controlled regardless of changes in the flow rate or flow rate of the exhaust gas, and the NOx occlusion material is used. The NOx occlusion efficiency can be increased.

    According to the invention of claim 4, when the exhaust gas temperature is equal to or higher than a predetermined value, the exhaust gas purification device is configured to discharge the exhaust gas by bypassing the NOx storage material by the bypass passage. Increase in engine back pressure can be suppressed and fuel consumption can be avoided.

    Hereinafter, embodiments of the present invention will be described with reference to the drawings. It should be noted that the following description of the preferred embodiment is merely illustrative in nature, and is not intended to limit the present invention, its application, or its use.

    In the engine exhaust gas purification apparatus shown in FIG. 1, reference numeral 1 denotes an engine, and 2 denotes an exhaust passage thereof. In the exhaust passage 2, a three-way catalyst 3 and a NOx storage catalyst 4 are arranged side by side in the exhaust gas flow direction so that the former is on the upstream side of the exhaust gas flow and the latter is on the downstream side. A first active oxygen supply unit 5 is provided in the exhaust passage 2 upstream of the three-way catalyst 3, and a second active oxygen supply unit 6 is provided in the exhaust passage 2 between the three-way catalyst 3 and the NOx storage catalyst 4. Is provided.

    The three-way catalyst 3 is formed by forming a catalyst layer on a honeycomb carrier made of cordierite or the like, and the catalyst layer contains an OSC (oxygen storage material) such as a CeZr-based double oxide and a catalyst metal such as Pt. To do. The NOx storage catalyst 4 is formed by forming a catalyst layer on a honeycomb carrier made of cordierite or the like, and the catalyst layer contains an alkaline earth metal that functions as a NOx adsorbent and a catalyst metal such as Pt.

    The first active oxygen supply unit 5 is for supplying the active oxygen generated by the active oxygen generator 7 to the three-way catalyst 3 from the upstream side, and the second active oxygen supply unit 6 Is supplied to the NOx storage catalyst 4 from the downstream side of the three-way catalyst 3. Both the active oxygen supply sections 5 and 6 are configured by branching an active oxygen supply path 8 extending from the active oxygen generating device 7 in the middle and connecting to the exhaust path 2.

    The active oxygen generator 7 generates ozone as active oxygen by silently discharging air, and is connected to a pump 9 for supplying air to the apparatus 7. The branch portion of the active oxygen supply path 8 is provided with a passage switching valve 11 that switches the supply direction of active oxygen between the first active oxygen supply unit 5 side and the second active oxygen supply unit 6 side. In the active oxygen generator 7, only a part of the oxygen in the air sent by the pump 9 is converted into active oxygen, so the remaining oxygen and nitrogen (hereinafter referred to as “residual air”) are also supplied with the active oxygen. The exhaust gas is supplied to the exhaust passage 2 through the passage 8.

    The exhaust passage 2 is provided with a bypass passage 12 for bypassing the NOx storage catalyst 4 and discharging exhaust gas. The upstream end of the bypass passage 12 is connected to the exhaust passage 2 between the NOx storage catalyst 4 and the second active oxygen supply unit 6, and the downstream end is connected to the exhaust passage 2 downstream of the NOx storage catalyst 4. ing. A flow rate control valve 13 for adjusting the exhaust gas bypass amount is provided at the upstream end of the bypass passage 12.

    The operations of the active oxygen generator 7, the pump 9, the passage switching valve 11 and the flow rate control valve 13 are controlled by a controller (engine control unit) 15 using a microcomputer. For this control, between the first air-fuel ratio detection means 16 for detecting the air-fuel ratio of the exhaust gas upstream of the first active oxygen supply unit 5, the three-way catalyst 3 and the second active oxygen supply unit 6. The second air-fuel ratio detection means 17 for detecting the air-fuel ratio of the exhaust gas at the exhaust gas and the temperature of the exhaust gas is detected upstream of the three-way catalyst 3 (between the three-way catalyst 3 and the first active oxygen supply unit 5). Temperature detecting means 18 is provided.

    FIG. 2 is a flowchart showing a mode of control by the controller 15. The exhaust gas temperature T detected by the temperature detection means 18 in step S1 after the start, the upstream air-fuel ratio detected by the first air-fuel ratio detection means 16, and the downstream air-fuel ratio detected by the second air-fuel ratio detection means 17 Is read.

Exhaust gas temperature T in step S2 is equal to or less than a predetermined value To is determined, the process proceeds to step S3 when it is less than the predetermined value T _O, the bypass passage 12 is closed by the flow control valve 13. That is, when the exhaust gas temperature is low, such as when the engine is started, not only the three-way catalyst 3 but also the NOx storage catalyst 4 is used for exhaust gas purification.

    In subsequent step S4, it is determined whether or not the upstream exhaust gas air-fuel ratio detected by the first air-fuel ratio detection means 16 is rich. If it is rich, the process proceeds to step S5, and the passage switching valve 11 is set to the first. Control is performed to supply active oxygen to the active oxygen supply unit 5 side, that is, to the three-way catalyst 3 side. By supplying this active oxygen, oxidative combustion of HC and CO in the exhaust gas proceeds, and oxidative combustion of HC and CO in the three-way catalyst is promoted. Due to the heat generated by the oxidative combustion of HC and CO, the temperature of the exhaust gas flowing through the NOx storage catalyst 4 becomes high, and the temperature of the NOx storage catalyst 4 can be increased.

Exhaust gas temperature T in the subsequent step S6 it is determined whether a predetermined value above T _1, when a predetermined value above T ₁ goes to step S7, is a bypass passage 12 is opened by the flow control valve 13 . Here, T ₁ > T _O. That is, when the exhaust gas temperature T reaches a predetermined value above T ₁ means that the three-way catalyst 3 has become active beyond its light-off temperature, in which case the bypass passage 12 is opened (NOx The passage on the side of the storage catalyst 4 is closed), and the NOx storage catalyst 4 avoids an increase in the engine back pressure, thereby improving the fuel efficiency of the engine.

When it is determined in step S4 that the upstream exhaust gas air-fuel ratio is not rich, the process proceeds to step S8, and it is determined whether or not the air-fuel ratio is lean. When it is lean, the routine proceeds to step S9, and the passage switching valve 11 is controlled to supply active oxygen to the second active oxygen supply unit 6 side, that is, to the NOx storage catalyst 4 side. In step S10, the supply amount of active oxygen (ozone) is corrected according to the detected value (lean degree) of the downstream air-fuel ratio detected by the second air-fuel ratio detection means 17. That is, when the degree of leanness is low, the amount of active oxygen supplied to the NOx storage catalyst 4 is increased or the output voltage is increased by increasing the output voltage of the active oxygen generator 7 to increase the amount of active oxygen generated. At the same time, the amount of air supplied by the pump 9 is increased to increase the amount of active oxygen and the amount of air supplied to the NOx storage catalyst 4. As a result, NO in the exhaust gas flowing into the NOx storage catalyst 4 is easily oxidized to NO ₂ or NO ₃ ⁻ , and is efficiently stored in the NOx storage catalyst 4.

Exhaust gas temperature T in the subsequent step S11 it is determined whether a predetermined value above T _1, when a predetermined value above T ₁ goes to step S7, is a bypass passage 12 is opened by the flow control valve 13 .

In step S8, the process proceeds to step S12 when the upstream air-fuel ratio is determined not to be lean when the exhaust gas temperature T is determined whether a predetermined value above T _1, a predetermined value above T ₁ Advances to step S7, and the bypass passage 12 is opened by the flow control valve 13.

    When the NOx occlusion amount of the NOx occlusion catalyst 4 increases, the NOx is released when the air-fuel ratio of the exhaust gas becomes stoichiometric or rich, but the NOx is a catalytic metal of the NOx occlusion catalyst 4 with increased activity. Is reduced and purified.

    FIG. 3 shows the change with time of the air-fuel ratio of the exhaust gas on the downstream side of the second active oxygen supply unit 6 by the above control by a solid line. A two-dot chain line is an air-fuel ratio change of the exhaust gas discharged from the engine 1 by the air-fuel ratio modulation control.

    That is, as indicated by the two-dot chain line, when the air-fuel ratio of the engine exhaust gas alternately changes to rich and lean at a constant period (for example, 1 Hz), the NOx storage catalyst 4 is activated downstream of the three-way catalyst 3 during lean. By supplying oxygen and residual air, the degree of leanness of the exhaust gas air-fuel ratio increases. At this time, NOx in the exhaust gas is oxidized by the active oxygen on the downstream side of the three-way catalyst 3, so that the NOx storage efficiency by the NOx storage catalyst 4 is increased.

    On the other hand, when rich, active oxygen and residual air are supplied to the three-way catalyst 3, whereby the exhaust gas air-fuel ratio rises to the stoichiometric level. At this time, HC and CO in the exhaust gas supplied to the three-way catalyst 3 are oxidized by active oxygen, thereby increasing the exhaust gas temperature. Therefore, even in the three-way catalyst, oxidation combustion of HC and CO and reduction of NOx proceed. . Furthermore, since the temperature of the exhaust gas flowing through the NOx storage catalyst 4 becomes high, the temperature of the NOx storage catalyst 4 can be increased, which is advantageous for storing NOx.

Incidentally, when the exhaust gas temperature T reaches a predetermined value above T _1, as in the exhaust gas passing through the NOx storage catalyst 4 is the flow rate is reduced, it may flow a portion of the exhaust gas into the bypass passage 12 .

It is a figure which shows the structure of the exhaust-gas purification apparatus which concerns on this invention. It is a control flow figure of the same device. It is a figure which shows the time-dependent change of the exhaust gas air fuel ratio by the same control.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Engine 2 Exhaust passage 3 Three-way catalyst 4 NOx storage catalyst 5 1st active oxygen supply part 6 2nd active oxygen supply part 7 Active oxygen production | generation apparatus 11 Passage switching valve 12 Bypass passage 13 Flow control valve 15 Controller 16 1st air fuel ratio Detection means 17 Second air-fuel ratio detection means 18 Exhaust gas temperature detection means

Claims (4)

An exhaust gas purification device in which a three-way catalyst and a NOx occlusion material are arranged in order from the upstream side of the exhaust gas flow in the exhaust passage of the engine,
A first active oxygen supply unit disposed in the exhaust passage upstream of the three-way catalyst to supply active oxygen to the three-way catalyst;
A second active oxygen supply unit disposed in the exhaust passage between the NOx storage material and the three-way catalyst to supply active oxygen to the NOx storage material;
The supply of active oxygen by the first active oxygen supply unit is executed when the air-fuel ratio of the exhaust gas discharged from the engine is rich, and the supply of active oxygen by the second active oxygen supply unit is performed by the exhaust gas of the exhaust gas. An exhaust gas purification device that is executed when the air-fuel ratio is lean. In claim 1,
Air-fuel ratio detection means for detecting the air-fuel ratio of the exhaust gas in the exhaust passage upstream from the first active oxygen supply section, and based on the air-fuel ratio of the exhaust gas detected by the air-fuel ratio detection means, An exhaust gas purifying apparatus characterized in that the supply of active oxygen by the first and second active oxygen supply units is executed. In claim 1 or claim 2,
An air-fuel ratio detecting means for detecting an air-fuel ratio of the exhaust gas in the exhaust passage between the three-way catalyst and the second active oxygen supply unit, the air-fuel ratio of the exhaust gas detected by the air-fuel ratio detecting means; An exhaust gas purifying apparatus, wherein the amount of active oxygen supplied by the second active oxygen supply unit is corrected in accordance with the degree of lean. In any one of Claim 1 thru | or 3,
Exhaust gas temperature detecting means for detecting the temperature of the exhaust gas;
A bypass passage that bypasses the NOx occlusion material from the downstream side of the three-way catalyst and discharges the exhaust gas when the exhaust gas temperature detected by the exhaust gas temperature detection is equal to or higher than a predetermined value. An exhaust gas purification device characterized by the above.

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