JP2006207549A - Temperature raising method for exhaust emission control device and exhaust emission controlling system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve a purification rate of NOx and PM at a low temperature period of exhaust gas in an exhaust emission control device equipped with a NOx purifying catalyst and a DPF, by making use of a self-heating action during oxygen occlusion of an oxygen storing matter other than during a NOx catalyst regenerating period and a DPF regenerating period, and raising the temperature of the exhaust emission control device. <P>SOLUTION: The exhaust emission control system 1 releases oxygen to the upstream side when an air-fuel ratio of exhaust gas is rich, and oxygen is occluded in a lean state of the air-fuel ratio. The system 1 is equipped with a carrier 20 for carrying the oxygen storing matter which performs self-heating and an exhaust emission control device 10 arranged therein, having at least one of the NOx purifying catalyst and the DPF on the downstream side. The air-fuel ratio of an exhaust gas G flowing into the carrier 20 is controlled so that a rich state and a lean state is alternately repeated in the case where a temperature Tg of the exhaust gas G is a predetermined temperature Tc or less, other than during a regeneration control period of the exhaust emission control device 10. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a NOx purification catalyst for purifying nitrogen oxide (hereinafter referred to as NOx) in exhaust gas of an internal combustion engine and a diesel particulate filter (hereinafter referred to as DPF) for purifying particulate matter (hereinafter referred to as PM). ) And an exhaust gas purification system.

  Various studies and developments have been made on exhaust gas purification devices used in internal combustion engines such as diesel engines and lean burn gasoline engines. In order to purify NOx in exhaust gas, NOx occlusion reduction type catalysts and NOx NOx purification catalysts such as direct reduction catalysts have been proposed, and DPFs have been proposed to purify PM.

  This NOx occlusion reduction type catalyst basically has a noble metal such as platinum (Pt) or palladium (Pd) that promotes oxidation / reduction reaction on a catalyst carrier such as aluminum oxide (alumina), and barium (Ba). And a NOx occlusion material (NOx occlusion material) having a function of occluding and releasing NOx formed from alkaline earth metals such as potassium and alkali metals such as potassium (K).

When the air-fuel ratio of the inflowing exhaust gas is in a lean (excessive oxygen) state where oxygen (O ₂ ) is present in the atmosphere, this NOx occlusion reduction type catalyst converts nitrogen monoxide (NO) in the exhaust gas to a noble metal. It is oxidized by the catalytic action of nitrogen to form nitrogen dioxide (NO ₂ ), and this NO ₂ is accumulated in the NOx storage material as nitrate (Ba ₂ NO ₄ etc.).

Further, when the air-fuel ratio of the inflowing exhaust gas becomes a stoichiometric air-fuel ratio or a rich (low oxygen concentration) state in which oxygen does not exist in the atmosphere, the NOx storage material such as Ba is combined with carbon monoxide (CO), and nitrate NO ₂ is decomposed and released from this, and the released NO ₂ is reduced by unburned hydrocarbons (HC), carbon monoxide (CO), etc. contained in the exhaust gas by the ternary function of precious metals, and nitrogen ( N ₂ ), and various components in the exhaust gas are released into the atmosphere as harmless substances such as carbon dioxide (CO ₂ ), water (H ₂ O), and N ₂ .

  Therefore, in an exhaust gas purification system equipped with a NOx occlusion reduction type catalyst, when the NOx occlusion capacity is close to saturation, the NOx occlusion capacity is recovered by reducing the oxygen concentration of the inflowing exhaust gas by making the air-fuel ratio of the exhaust gas rich. By performing the rich control, the NOx absorbed is released, and the NOx catalyst regeneration operation is performed in which the released NOx is reduced by the noble metal catalyst.

  Further, the NOx direct reduction catalyst (DCR) is a catalyst that directly reduces NOx. For example, a catalyst component such as rhodium (Rh) or palladium (Pd) is supported on a catalyst carrier such as β-type zeolite. It has been made. In addition, cerium (Ce) that contributes to maintaining the NOx reduction ability is reduced by reducing the oxidation action of these metals, or a three-way catalyst is provided in the lower layer to reduce the oxidation reaction, particularly in the rich state of NOx. It is promoted or iron (Fe) is added to the carrier to improve the NOx purification rate.

This NOx direct reduction type catalyst directly reduces NOx to N ₂ in the lean state, but O ₂ is adsorbed on the metal which is an active substance of the catalyst during this reduction, and the reduction performance is reduced. In an exhaust gas purification system equipped with a type catalyst, when the NOx reduction capacity approaches saturation, rich control is performed to restore the NOx purification capacity to reduce the oxygen concentration of the inflowing exhaust gas by making the air-fuel ratio of the exhaust gas rich. Thus, the NOx catalyst regeneration operation is performed in which O ₂ adsorbed from the active material of the catalyst is released to regenerate the active material.

  The DPF is formed of, for example, a monolith honeycomb wall-through type filter in which the inlets and outlets of the porous ceramic honeycomb channels are alternately sealed, and collects PM in the exhaust gas. When using a continuously regenerating DPF in which an oxidation catalyst is provided on the upstream side of the DPF or an oxidation catalyst or a PM oxidation catalyst is supported on the DPF so that the collected PM can be burned and removed even at a low exhaust gas temperature. There are many.

  In this continuous regeneration type DPF, when the temperature of the exhaust gas is about 350 ° C. or higher, PM trapped in the DPF is continuously burned and purified, and the DPF self-regenerates, but the temperature of the exhaust gas is low In such a case, PM cannot be removed by oxidation, and clogging of the filter proceeds.

  For this reason, in an exhaust gas purification system equipped with a DPF, when the clogging of the filter proceeds and the exhaust pressure increases, the temperature of the DPF is raised by the rise of the temperature of the exhaust gas or the heating of the heater, etc. DPF regeneration operation is performed to forcibly remove the PM that is burning.

  In the exhaust gas treatment of an internal combustion engine, an exhaust gas purification device of a single NOx purification catalyst, a single DPF, or a combination of a NOx purification catalyst and a DPF is used according to the purpose of purification.

  However, in these exhaust gas purification devices, when NOx and PM are purified, when the temperature of the exhaust gas becomes 300 ° C. or lower, the activity of the catalyst such as the NOx purification catalyst, the oxidation catalyst of the continuous regeneration type DPF, and the PM oxidation catalyst is increased. descend. Therefore, as shown in FIG. 6, when the catalyst temperature decreases, as shown in the relationship between the NOx storage reduction catalyst (No) and the NOx direct reduction catalyst (Nd), and the catalyst temperature, the NOx purification rate decreases significantly. . There is a similar tendency with respect to PM purification.

  Therefore, when the exhaust gas temperature is low, such as during idle operation or low load operation, the purification rate of NOx or PM is significantly reduced. Therefore, the purification performance of the exhaust gas purification device when the exhaust gas temperature is 300 ° C. or less is reduced. Improvement is a big problem.

  On the other hand, there has been proposed an exhaust purification device for an internal combustion engine that carries an oxygen storage component that absorbs oxygen in the exhaust when the air-fuel ratio of the exhaust is lean and releases oxygen that is absorbed when the air-fuel ratio of the exhaust is rich. (For example, refer to Patent Document 1).

This oxygen storage component is cerium (Ce) supported in the form of a ceria-zirconia solid solution. In the lean air-fuel ratio, cerium oxide IV (CeO ₂ ) is formed to store oxygen, and in the rich air-fuel ratio, cerium oxide IV Releases oxygen to cerium oxide III (Ce ₂ O ₃ ). Then, O ₂ released from the oxygen storage component and oxidation reaction of HC and CO components in the exhaust, for example, 2CeO ₂ + CO → Ce ₂ O ₃ + CO ₂ , 2CeO ₂ + H ₂ → Ce ₂ O ₃ + H ₂ Due to the heat generated by O, the catalyst component temperature rises, thereby improving the catalytic activity and improving the NOx purification rate.

Also, an internal combustion engine in which a catalyst having an O ₂ storage function (oxygen storage capacity) is arranged in the exhaust passage to prevent overheating of the catalyst provided in the exhaust passage is proposed (for example, see Patent Document 2). In order to oxidize PM and improve the PM oxidation rate, a filter catalyst for purifying diesel exhaust gas in which a porous oxide containing at least a cerium (Ce) element is supported on an exhaust gas purification layer of a filter partition wall or cerium An exhaust emission control device including an active oxygen generator containing (Ce) element has also been proposed (see, for example, Patent Documents 3 and 4).

  The present inventors have obtained the knowledge that the oxygen storage material that releases oxygen when the air-fuel ratio of the exhaust gas is rich and stores oxygen in the lean state self-heats during oxygen storage, It was considered to be used for raising the temperature.

When this oxygen storage material is formed of cerium oxide IV (CeO ₂ ), in the rich state, CeO ₂ → (1/2) Ce ₂ O ₃ + (1/4) O ₂ reacts with Ce. Becomes trivalent cerium oxide III (Ce ₂ O ₃ ) and is reduced. This reaction is an endothermic reaction, and heat of ΔH = 191 kJ / mol is absorbed. On the other hand, when switching to the lean state, Ce returns to tetravalent cerium oxide IV (CeO ₂ ) and is oxidized by the reaction of (1/2) Ce ₂ O ₃ + (1/4) O ₂ → CeO _2. . This reaction is an exothermic reaction, and heat of ΔH = 191 kJ / mol is generated. These reactions are reversible reactions.

  Gibbs free energy ΔrG ° and equilibrium constant Kp for this reaction are shown in FIGS. FIG. 4 shows the relationship between ΔrG ° and temperature, which is under atmospheric conditions, but the higher the ΔrG ° is on the minus (−) side, the higher the probability that the reaction will occur, and the more the reaction is on the plus (+) side. It does not happen or shows a reverse reaction. FIG. 5 shows the relationship between Kp and temperature. The larger the Kp, the higher the rate of the oxidation reaction. That is, both FIG. 4 and FIG. 5 show that the oxidation reaction occurs more easily and the oxidation reaction is faster at lower temperatures. In this way, when the exhaust gas temperature is low, which requires a temperature increase for the catalytic reaction, the oxidation reaction rate is fast, so that a great effect can be expected in increasing the temperature of the exhaust gas purification device.

  In this rich state, the oxidation reaction (combustion) of oxygen released from the oxygen storage material and the reducing agent generates more heat than is absorbed, and the oxidation reaction heat of the reducing agent can be utilized. In the lean state, the reaction heat of the oxidation reaction when the oxygen storage material occludes oxygen can be used.

In particular, by actively utilizing the self-heating of the oxygen storage material that occurs when switching from the rich state to the lean state, the temperature of the carrier carrying the oxygen storage material of the exhaust gas purification device is raised, Since the temperature of the exhaust gas passing through the body can be raised to raise the temperature of the NOx purification catalyst and the DPF, the exhaust gas purification rate can be improved.
Japanese Patent No. 3370957 JP 2000-54887 A JP 2003-190793 A JP 2003-193822 A

  The present invention has been made in order to obtain the above knowledge and solve the above problems. The object of the present invention is to perform NOx catalyst regeneration and DPF regeneration in an exhaust gas purification apparatus equipped with a NOx purification catalyst and a DPF. At other times, the temperature of the exhaust gas flowing into the exhaust gas purification device is raised by utilizing the self-heating action during the oxygen storage of the oxygen storage material, and the purification rate of NOx and PM when the exhaust gas is low An object of the present invention is to provide an exhaust gas purification device temperature raising method and an exhaust gas purification system that can be improved.

  In order to achieve the above object, the exhaust gas purifying apparatus is provided with a method for raising the temperature of an oxygen storage device that releases oxygen when the air-fuel ratio of the exhaust gas is rich, stores oxygen in a lean state, and self-heats. In an exhaust gas purification system comprising a carrier carrying a substance and having an exhaust gas purification device provided with at least one of a NOx purification catalyst or DPF on the downstream side, an air-fuel ratio state of exhaust gas flowing into the carrier Is controlled to repeat the rich state and the lean state alternately when the temperature of the exhaust gas is equal to or lower than a predetermined temperature except during the regeneration control of the exhaust gas purification device.

  In the temperature raising method of the exhaust gas purifying apparatus, a substance containing a cerium element is used as the oxygen storage substance.

  Further, in the temperature raising method of the exhaust gas purifying apparatus, the ratio of the rich state time to the lean state time is set to rich state: lean state = 3: 57 to 3: 3.

  An exhaust gas purification system for achieving the above object is an exhaust gas purification system in which an exhaust gas purification device provided with at least one of a NOx purification catalyst and a DPF is disposed. A carrier carrying an oxygen storage substance that releases oxygen in a state and absorbs oxygen in a lean state and self-heats, and has an air-fuel ratio state of the exhaust gas flowing into the carrier, Except at the time of regeneration control of the gas purification device, when the temperature of the exhaust gas is equal to or lower than a predetermined temperature, exhaust gas air-fuel ratio control means for controlling to alternately repeat the rich state and the lean state is configured.

  In the exhaust gas purifying apparatus described above, a substance containing a cerium element is used as the oxygen storage substance.

  In the exhaust gas purifying apparatus, the exhaust gas air-fuel ratio control means sets the ratio of the rich state time and the lean state time to rich state: lean state = 3: 57 to 3: 3. Composed.

  Furthermore, in the said support body, it is comprised so that the load of the substance containing the said cerium element may be 50 g / L-200 g / L.

  That is, in an exhaust gas purification device using a NOx purification catalyst or DPF, the exhaust gas air-fuel ratio state is made rich or alternately by regeneration control of the exhaust gas purification device such as NOx catalyst regeneration control or DPF regeneration control. When the exhaust gas temperature other than during regeneration control of the exhaust gas purification device is lower than a predetermined temperature, the post-injection in the cylinder (cylinder) fuel injection may be performed. By adjusting the amount of oxygen in the exhaust gas by direct fuel injection in the exhaust pipe and repeatedly switching between the rich state of the reducing atmosphere and the lean state of the oxidizing atmosphere, a reversible oxidation reaction of the oxygen storage material occurs repeatedly. The heat of the reaction is used to raise the temperature of the oxygen storage material carrier. The temperature of the exhaust gas that passes through the carrier and then flows into the downstream NOx purification catalyst and DPF is raised.

  Here, the air-fuel ratio state of the exhaust gas does not necessarily mean the state of the air-fuel ratio in the cylinder, but the amount of air and the amount of fuel (cylinder (cylinder) supplied to the exhaust gas flowing into the exhaust gas purification device) This is the ratio to the amount of combustion in the cylinder).

When cerium oxide IV (CeO ₂ ) is employed as the oxygen storage material, in a rich state (reducing atmosphere), CeO ₂ → (1/2) Ce ₂ O ₃ + (1/4) O ₂ reacts with Ce. Becomes trivalent cerium oxide III (Ce ₂ O ₃ ) and is reduced. This reaction is an endothermic reaction, and heat of ΔH = 191 kJ / mol is absorbed. On the other hand, when switching to the lean state (oxidizing atmosphere), Ce returns to tetravalent cerium oxide IV (CeO ₂ ) by the reaction of (1/2) Ce ₂ O ₃ + (1/4) O ₂ → CeO _2. Is oxidized. This reaction is an exothermic reaction, and heat of ΔH = 191 kJ / mol is generated. These reactions are reversible reactions.

  In the rich state, the oxidation reaction (combustion) between oxygen released from the oxygen storage material and the reducing agent generates more heat than is absorbed, and uses the oxidation reaction heat of the reducing agent. Since oxygen (O) released by the endothermic reaction during the reduction includes radical oxygen, reducing agents such as HC, CO, and H2 in the rich exhaust gas can be burned very efficiently. Therefore, it is possible to increase the temperature by offsetting the endotherm generated by the oxygen releasing reaction. In the lean state, the oxygen storage material uses the reaction heat of the oxidation reaction when storing oxygen. In this case, the temperature is raised by self-heating. Therefore, as a whole, the calorific value exceeds and the temperature rises.

  By actively using the heat generated when switching from the rich state to the lean state, the temperature of the NOx purification catalyst and the oxygen storage material carrier disposed upstream of the DPF is raised, and the NOx purification catalyst and DPF The exhaust gas purification rate can be improved by raising the temperature of the exhaust gas flowing into the exhaust gas.

The exothermic reaction that occurs when switching from the rich state to the lean state progresses slowly because the amount of oxygen in the exhaust gas is small in a gasoline engine, but in lean burn gasoline engines and diesel engines, the amount of oxygen in the exhaust gas increases. It progresses rapidly because there are many. Therefore, the surface temperature of the catalyst and the support can be easily raised, for example, by about 60 ° C. by self-heating of the oxygen storage material such as CeO ₂ .

  When the temperature of the exhaust gas is lower than the predetermined temperature, that is, when the temperature of the exhaust gas is low, the temperature of the catalyst such as the NOx purification catalyst, the oxidation catalyst of the continuous regeneration type DPF or the PM oxidation catalyst is low, and the activity of the catalyst When this is low, these catalysts are activated to improve the NOx purification rate and the PM self-combustion rate. The predetermined temperature is set to a temperature at which these catalysts are not activated when the exhaust gas temperature is equal to or lower than the predetermined temperature, that is, a temperature requiring a temperature raising action (for example, 300 ° C.). The

  The exhaust gas temperature sensor may determine whether or not the exhaust gas temperature is lower than a predetermined temperature. However, the exhaust gas temperature may be determined based on whether or not the catalyst temperature is lower than the predetermined temperature. It may be determined whether the temperature is lower than a predetermined temperature. Further, when the temperature of the exhaust gas is lower than a predetermined temperature, it is an idle operation, a low load operation, or the like. In the present invention, since the purpose is to activate the catalyst by raising the temperature of the catalyst, it is preferable to judge whether or not the temperature of the catalyst is lower than a predetermined temperature (activation temperature). Since it is difficult to measure the catalyst temperature, the exhaust gas temperature is used instead.

  In addition, a NOx catalyst regeneration operation for recovering the NOx storage capacity of the NOx storage catalyst, a NOx catalyst regeneration operation for regenerating the active material of the NOx direct reduction catalyst, or a DPF that forcibly burns and removes PM collected in the DPF. Even during the regeneration operation, by repeatedly switching from the rich state to the lean state, the exothermic reaction that occurs when switching from the rich state to the lean state can be used. It can also contribute greatly to high temperatures.

When a material containing cerium element is used as the oxygen storage material, the heat generated by the oxidation reaction of 2Ce ₂ O ₃ + O ₂ → 4CeO ₂ can be used. Since this cerium has a large oxygen storage capacity, it can produce a great effect.

  The rich state is 1.1 to 0.8 in terms of excess air ratio (λ), and the lean state is 1.8 to 1.0 in terms of excess air ratio (λ). However, the lean time is made smaller than the rich time. Furthermore, it is preferable that the rich state is 1.0 to 0.8 in terms of excess air ratio (λ), and the lean state is in the range of 1.1 to 1.0 lean in terms of excess air ratio (λ). However, even in this case, the lean state time is made smaller than the rich state time.

  When the ratio of rich state (R): lean state (L) is changed, the amount of heat generation is different. This is also due to the amount of oxygen in the oxygen storage material affected by the excess air ratio λ, although there is an influence of the remaining amount of the reducing agent such as HC. That is, as the time of the rich state increases, the enrichment in the oxygen storage material becomes deeper, the amount of released oxygen increases, and the oxidation reaction occurs more strongly.

  As a preferable ratio of the rich state (R) / lean state (L), 3:57 to 3: 3 has been experimentally obtained. Therefore, the ratio between the rich state time and the lean state time is set to the rich state ( R): Lean state (L) = 3: 57 to 3: 3. Thereby, the temperature rise of the carrier can be efficiently achieved in relation to the fuel consumption for the rich state and the degree of the temperature rise.

  Furthermore, in the carrier that supports the oxygen storage material, the amount of the substance containing the cerium element is set to 50 g / L to 200 g / L. When the amount is less than 50 g / L, the heat generation amount is reduced and the effect is reduced. Moreover, when it exceeds 200 g / L, it will saturate. In addition, this carrying amount is obtained by dividing the carrying amount (mass) of the substance containing the cerium element by the volume (L: liter) of the entire outer shape of the carrying body.

  If the exhaust gas purification device includes at least one of a NOx purification catalyst and a DPF, the exhaust gas temperature rises due to the temperature rise of the carrier, particularly during NOx catalyst regeneration or DPF regeneration. When the exhaust temperature is low, it is possible to improve the NOx purification rate by activating the NOx purification catalyst and to activate the self-combustion of PM by activating the oxidation catalyst or the PM oxidation catalyst of the continuous regeneration type DPF.

  According to the temperature raising method and the exhaust gas purification system of the exhaust gas purification apparatus according to the present invention, the exhaust gas provided with at least one of a NOx purification catalyst for purifying NOx in the exhaust gas and a DPF for purifying PM. In the purifying apparatus, the oxygen storage substance can be obtained by repeatedly changing the air-fuel ratio of the exhaust gas to the lean state and the rich state, that is, by repeating the lean / rich cycle, except during NOx catalyst regeneration, DPF regeneration, etc. The temperature of the carrier can be increased by utilizing the self-heating action during oxygen storage. Thereby, the temperature of the exhaust gas passing through the carrier can be increased.

  The exhaust gas temperature can be raised even when the exhaust gas temperature is low, such as during idling or low-load operation, so that the NOx and PM purification rates can be improved. In the case where both the purification catalyst and the DPF are provided, both the NOx reduction and PM oxidation reactions can be improved simultaneously.

  Since the oxygen storage substance carrier is formed separately from the exhaust gas purification device, it is only necessary to add the oxygen storage material to the exhaust passage. Purification performance can be improved.

  Hereinafter, a temperature raising and exhaust gas purification system of an exhaust gas purification apparatus according to an embodiment of the present invention will be described with reference to the drawings. Note that the air-fuel ratio state of the exhaust gas here does not necessarily mean the state of the air-fuel ratio in the cylinder (cylinder), but the amount of air supplied to the exhaust gas flowing into the exhaust gas purification device and the fuel This is the ratio to the amount (including the amount burned in the cylinder).

  FIG. 1 shows a configuration of an exhaust gas purification system 1 according to a first embodiment of the present invention. The exhaust gas purification system 1 is configured by disposing an exhaust gas purification device 10 in an exhaust passage 3 of an engine (internal combustion engine) 2. The exhaust gas purification device 10 includes one or both of a NOx purification catalyst and a DPF (diesel particulate filter).

  The NOx purification catalyst is formed of, for example, a NOx occlusion reduction type catalyst or a NOx direct reduction type catalyst. This NOx occlusion reduction type catalyst is formed of a monolith catalyst, and a catalyst coat layer is provided on a carrier such as aluminum oxide or titanium oxide, and a catalyst metal such as platinum (Pt) palladium (Pd) and barium are provided on the catalyst coat layer. A NOx occlusion material (NOx occlusion material) such as (Ba) is supported.

  In this NOx occlusion reduction type catalyst, the NOx occlusion material occludes NOx in the exhaust gas G when the exhaust gas is in a high oxygen concentration state (lean air-fuel ratio state), thereby purifying the NOx in the exhaust gas G. In the exhaust gas state where the oxygen concentration is low or zero, the stored NOx is released and the released NOx is reduced by the catalytic action of the catalytic metal to obtain a purified exhaust gas Gc, which is released into the atmosphere. Prevent NOx outflow.

  Further, the NOx direct reduction catalyst (DCR) is a catalyst that directly reduces NOx. For example, a catalyst component such as rhodium (Rh) or palladium (Pd) is supported on a catalyst carrier such as β-type zeolite. Let it form. Furthermore, in order to reduce the oxidation action of these metals and to add cerium (Ce) that contributes to maintaining the NOx reduction ability, or to promote the oxidation-reduction reaction, particularly the reduction reaction of NOx in a rich state, a ternary layer is provided. In order to provide a catalyst or to improve the NOx purification rate, iron (Fe) is added to the support.

This NOx direct reduction type catalyst directly reduces NOx to N ₂ in a lean state, but O ₂ is adsorbed on the metal that is the active substance of the catalyst during this reduction, and the reduction performance is reduced. Is close to saturation, the rich control is performed to restore the NOx purification ability to reduce the oxygen concentration of the exhaust gas G flowing in the rich state by making the air-fuel ratio of the exhaust gas G rich, and the O ₂ adsorbed from the active substance of the catalyst is removed. Release. This regenerates the active material.

  In the case of a continuous regeneration type DPF, it is composed of an upstream oxidation catalyst and a downstream DPF, or a DPF with a catalyst.

The oxidation catalyst is formed of a monolith catalyst having a number of polygonal cells formed of a structural material such as cordierite, silicon carbide (SiC), or stainless steel. The inner wall of the cell has a catalyst coat layer having a large surface area, and a large surface is loaded with a catalyst metal such as platinum (Pt) or palladium (Pd). This oxidation catalyst oxidizes reducing agents such as HC and CO in the exhaust gas G and raises the exhaust gas temperature by heat generated by this oxidation reaction, or oxidizes NO in the exhaust gas G. It generates NO ₂ and promotes the oxidation of PM trapped in the DPF with this NO ₂ .

  The DPF is formed of a monolith honeycomb type wall-through type filter or the like in which the inlet and outlet of a porous ceramic honeycomb channel are alternately sealed, and collects and removes PM in the exhaust gas G. This DPF may be coated with an oxidation catalyst or a catalyst layer carrying a PM oxidation catalyst in order to promote the combustion removal of PM.

In the present invention, oxygen (O ₂ ) is released to the upstream side of the exhaust gas purifying device 10 when the exhaust gas G is rich, and oxygen is occluded and generates heat when the exhaust gas G is lean. A carrier (catalytic converter) 20 carrying a substance is provided. Examples of the oxygen storage material include a material containing a cerium (Ce) element as a base metal.

  Further, as shown in FIG. 1, an HC supply valve (fuel injector) 30 for supplying a reducing agent such as hydrocarbon (HC) is provided in the exhaust passage 3 upstream of the carrier 20. The HC supply valve 30 directly injects hydrocarbons (HC) such as light oil as engine fuel from a fuel tank (not shown) into the exhaust passage 3 to adjust the air-fuel ratio of the exhaust gas G to a rich state or a stoichiometric state (theoretical). Air-fuel ratio state). Note that this HC supply valve 30 can be omitted when air-fuel ratio control is performed by post-injection (post-injection) in fuel injection in the cylinder of the engine 2.

As shown in FIG. 1, upstream of the carrier 20, the upstream side exhaust temperature sensor 21, the upstream side NOx concentration sensor 22, the upstream side λ sensor 23, and the downstream side of the exhaust gas purification device 10. When the exhaust gas temperature sensor 24, the downstream NOx concentration sensor 25, and the downstream O ₂ sensor 26 are provided and the exhaust gas purification device 10 includes a DPF, the differential pressure for monitoring the clogged state of the DPF A sensor 27 is provided.

  Further, as shown in FIG. 1, a control device (ECU: engine control unit) 40 that performs overall control of the operation of the engine 2 and also performs recovery control of NOx purification capability of the NOx purification catalyst and regeneration control of the DPF. Provided. Detection values from the sensors 21 to 27 and the like are input to the control device 40. The fuel injection valve and the intake throttle valve (intake air valve) of the EGR valve of the engine 2 and the common rail electronic control fuel injection device for fuel injection are input from the control device 40. A signal for controlling a throttle valve is output.

  The control device 40 controls the exhaust gas purification system 1 in parallel with the operation control of the engine 2. This control device 40 is an exhaust gas for PM regeneration control for removing PM trapped in the DPF, NOx regeneration control for recovering NOx storage ability of the NOx purification catalyst, and desulfurization control for recovering sulfur poisoning of the NOx purification catalyst. Various controls of the purification system 1 are performed.

In this PM regeneration control, when the accumulated amount of PM in the DPF increases and the clogged state worsens, the EGR valve is controlled to increase the EGR amount, or the intake throttle valve is controlled to decrease the new intake amount. The fuel is added to the exhaust gas G by raising the temperature of the exhaust gas G, or by post-injection in the exhaust pipe by the HC supply valve 30 or in-cylinder injection. Then, the fuel is oxidized with an oxidation catalyst, the temperature of the exhaust gas G is raised using the heat of the oxidation reaction, and NO ₂ generated by the oxidation reaction of NO in the exhaust gas G is used. Then, the PM collected in the DPF is oxidized and removed.

  In addition, in the NOx regeneration control of the NOx purification catalyst, the NOx purification rate is calculated from the NOx concentration detected by the NOx concentration sensors 22 and 25 disposed on the upstream side and the downstream side of the NOx purification catalyst, and this NOx purification rate is a predetermined determination. When the value becomes lower than the value, regeneration of the NOx catalyst is started. Alternatively, the NOx emission amount ΔNOx per unit time is calculated from the operating state of the engine, and the regeneration is started when the NOx accumulated value ΣNOx obtained by accumulating the NOx exceeds a predetermined determination value Cn.

  Then, the air-fuel ratio of the exhaust gas G is controlled to a stoichiometric air-fuel ratio (theoretical air-fuel ratio) or a rich state. In this control, the EGR valve is controlled to increase the EGR amount, or the intake throttle valve is controlled to newly The amount of intake air is reduced, the temperature of the exhaust gas G is increased, the fuel is added to the exhaust gas G by post-injection in the exhaust pipe by the HC supply valve 30 or in the cylinder, and the like. Reduce the fuel ratio.

  With these controls, the state of the exhaust gas G is set to a predetermined target air-fuel ratio state and is set to a predetermined temperature range (approximately 200 ° C. to 600 ° C. depending on the catalyst) to recover the NOx purification capacity, Regenerate the catalyst.

  In addition, the sulfur desulfurization control of the NOx purification catalyst is performed by accumulating the sulfur (sulfur) accumulation amount, and if the sulfur accumulation amount exceeds a predetermined determination value, it is assumed that sulfur has accumulated until the NOx occlusion capacity decreases, and desulfurization (sulfur purge). Start control. In this desulfurization control, although sulfates differ depending on the catalyst, they are not decomposed and released unless the rich conditions of about 600 ° C. to 700 ° C. are used. Therefore, from the viewpoint of efficient operation of energy, prior to this desulfurization control, DPF PM regeneration control is performed to raise the temperature of the exhaust gas G due to PM combustion and raise the temperature of the NOx purification catalyst.

  In the present invention, the control device 40 has a temperature (exhaust temperature) Tg of the exhaust gas G other than during the regeneration control of the exhaust gas purification device 10 such as the above DPF regeneration control, NOx catalyst regeneration control, desulfurization control and the like. When the catalyst is below a predetermined temperature (for example, 300 ° C.) Tc at which the catalyst is not activated at a temperature lower than that, the exhaust gas G is repeatedly increased between the rich state (R) and the lean state (L). A temperature increase control means for performing temperature control is provided.

  By this temperature increase control, when the exhaust gas G is rich in the carrier 20, the reducing agent in the exhaust gas G is oxidized by the oxygen released from the oxygen storage material, and the oxygen storage material absorbs the oxygen during the oxygen release. Oxidation heat exceeding the amount of heat is generated to raise the temperature of the carrier 20 carrying the oxygen storage material. When the exhaust gas G is in a lean state, the temperature of the carrier 20 carrying the oxygen storage material is raised by the heat generated when the oxygen storage material occludes oxygen in the exhaust gas G.

  The exhaust gas G passing through the carrier 20 is heated by the temperature rise of the carrier 20. The heated exhaust gas G flows into the exhaust gas purification device 10 to raise the temperature of each catalyst in the exhaust gas purification device 10 to the activation temperature or higher. When the temperature of the exhaust gas G is low except at the time of regeneration control, PM oxidation removal and NOx purification performance are improved.

  The exhaust gas temperature raising control repeats the rich state and the lean state. In the rich state, the air-fuel ratio on the inlet side of the carrier 20 is converted to the rich side where λR = 1.1 to 0.8 in terms of excess air ratio. In order to achieve this, control is performed for a predetermined time by supplying a reducing agent into the exhaust gas G to create a rich atmosphere by in-pipe injection or post-injection.

  Then, after the rich state is performed for a predetermined time, the lean state is set. In this lean state, the injection in the exhaust pipe or the post-injection is stopped, and λ L = 1.8 to 1. Control is performed so that the lean side is 0 (where λR <λL). These controls are repeated alternately. Note that the effect is large when λR = 1.0 to 0.8 and λL = 1.8 to 1.0 (where λR <λL).

  This air-fuel ratio control of the exhaust gas G is supplied into the exhaust gas G so that the output value of the upstream excess air ratio sensor 23 on the inlet side of the carrier 20 becomes the respective target excess air ratios λR and λL. This is done by adjusting and controlling the amount of the reducing agent. As for the air-fuel ratio on the inlet side, an air-fuel ratio calculated from the intake intake air amount and the load (fuel injection amount) can be used instead of the output value of the upstream excess air ratio sensor 23.

  In this temperature increase control, the time ratio between the control to make the rich air-fuel ratio and the control to make the lean air-fuel ratio is set to rich side control (R): lean side control (L) = R: L = 3: 57-3: 3. Note that the time for one cycle of the control to make the rich air-fuel ratio and the control to make the lean air-fuel ratio is, for example, the time for the control to make the rich side is about 3 s to 5 s, and the time for the control to make the lean side is It is about 3 to 60 s, 1 to 20 times. This rich / lean cycle is performed while the temperature of the catalyst needs to be increased.

  FIG. 2 shows an example of a control flow of the exhaust gas purification system 1 including this temperature rise control. In this control flow, in step S11, it is determined whether or not it is the regeneration time of the exhaust gas purification device 10. This regeneration time includes the regeneration time of the DPF, the regeneration time of the NOx purification catalyst, and the regeneration (desulfurization) time from sulfur poisoning.

  If it is determined in step S11 that it is the reproduction time, the process goes to step S12 to perform the respective reproduction control, and the process returns to step S11. Conventional techniques can be used for the determination of the regeneration timing and the regeneration control. If it is determined that it is not the reproduction time, the process goes to step S13 as it is.

  In step S13, it is determined whether or not the exhaust temperature (exhaust gas temperature) Tg is lower than a predetermined temperature Tc. If it is higher, normal control for normal engine operation is performed in step S14 for a predetermined time (time related to an interval for judging regeneration timing and checking exhaust temperature), and control step S11 is executed. Return. If it is low, the process goes to step S15, where the rich state is set to the rich state (R) for a predetermined rich duration (for example, 3s to 5s) and the lean state for a predetermined lean duration (for example, 3s to 60s). The temperature rise control is performed to perform one to several cycles of the rich / lean cycle including the lean control, and the process returns to step S11.

  Steps S11 to S15 are repeated until the engine is stopped. When the engine is stopped, an interrupt is generated and the control end operation in step S16 is performed to stop (stop) and end (end) the control.

  With this control flow, when the temperature Tg of the exhaust gas G is equal to or lower than the predetermined temperature Tc, even when the exhaust gas purification device 10 is not under regeneration control, the rich state (R) and lean state (L) of the exhaust gas G are changed. Repeated temperature rise control can be performed.

  According to the method for raising the temperature of the exhaust gas purification device and the exhaust gas purification system 1, exhaust gas purification provided with at least one of a NOx purification catalyst for purifying NOx in exhaust gas and a DPF for purifying PM. In the apparatus 10, oxygen storage is performed by repeatedly changing the air-fuel ratio state of the exhaust gas G to a lean state and a rich state, that is, by repeating a lean / rich cycle, other than during NOx catalyst regeneration, DPF regeneration, or the like. The temperature of the carrier 20 can be increased by utilizing the self-heating action during oxygen storage of the substance. Thereby, the temperature of the exhaust gas passing through the carrier 20 can be increased.

  The exhaust gas temperature can be raised even when the exhaust gas temperature is low, such as during idling or low-load operation, so that the NOx and PM purification rates can be improved. In the case where both the purification catalyst and the DPF are provided, both the NOx reduction and PM oxidation reactions can be improved simultaneously.

  Since the oxygen storage substance carrier 20 is formed separately from the exhaust gas purification device 10, the oxygen storage material carrier 20 is simply provided in addition to the exhaust passage 3, so that the exhaust gas from the exhaust gas purification device 10 already installed is provided. The purification performance at low gas temperature can be improved.

  Also, by providing the carrier 20, the NOx catalyst regeneration operation for recovering the NOx storage capacity of the NOx storage catalyst in the exhaust gas purification device 10, the NOx catalyst regeneration operation for regenerating the active material of the NOx direct reduction catalyst, the DPF Even during a DPF regeneration operation that forcibly removes PM that has been trapped, the exothermic reaction that occurs when switching from the rich state to the lean state is repeated by repeatedly switching from the rich state to the lean state. Therefore, the exhaust gas temperature can greatly contribute to a high temperature of 500 ° C. or higher.

  In order to confirm the effect of switching between the rich state (R) and the lean state (L) of the oxygen storage material and the exhaust gas G of the present invention, experiments were conducted in the following examples.

  First, as an example, in the exhaust passage 3 of the diesel engine 2, the carrier 20 of the present invention is disposed on the upstream side of the exhaust gas purification device 10 provided with a NOx purification catalyst. Compared to this embodiment, the NOx purification catalyst when the time interval of the rich state (R) / lean state (L) is repeatedly changed at R / L = 3 s / 57 s, 3 s / 45 s, 3 s / 30 s. The temperature Tn of the inflowing exhaust gas was measured by the exhaust temperature sensor 28, and the NOx purification rate was measured.

  Moreover, the same experiment and measurement were performed in the case of only the exhaust gas purification device 10 as the comparative example 1 and in the case where the oxidation catalyst was disposed on the upstream side of the exhaust gas purification device 10 as the comparative example 2. The result is shown in FIG.

  According to FIG. 3, compared with Comparative Example 1, it can be seen that Comparative Example 2 shows an increase in the temperature of exhaust gas and an improvement in the NOx purification rate, and the oxidation catalyst is also effective. It can be seen that an effect larger than 2 can be achieved.

  In FIG. 3, 3/57 to 3/30 are shown, but in the case of 3/10, 5/5, and 3/3, the same result as 3/30 was obtained. In the normal case, 3/30 is sufficient, but in the case where it is necessary to raise the temperature rapidly or in extremely cold regions, 3/3 control may be used as necessary.

It is a figure which shows the structure of the exhaust gas purification system of embodiment which concerns on this invention. It is a figure which shows an example of the control flow of the exhaust-gas purification system which concerns on this invention. It is a figure which shows the change of the NOx purification catalyst inlet_port | entrance temperature and NOx purification rate of an Example and Comparative Examples 1 and 2. FIG. It is a figure which shows the relationship between Gibbs free energy (DELTA) rGdegree and temperature regarding the oxygen storage reaction of cerium oxide. It is a figure which shows the relationship between the equilibrium constant Kp regarding oxygen storage reaction of cerium oxide, and temperature. It is a figure which shows the relationship between the purification rate of NOx occlusion reduction type catalyst and NOx direct reduction type catalyst, and catalyst temperature.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Exhaust gas purification system 2 Engine 3 Exhaust passage 10 Exhaust gas purification apparatus 20 Carrier 30 HC supply valve 40 Control apparatus

Claims (7)

  The upstream side is provided with a carrier that releases oxygen when the air-fuel ratio of the exhaust gas is rich and stores oxygen in a lean state and carries an oxygen storage material that self-heats, and a NOx purification catalyst on the downstream side, or In an exhaust gas purification system in which an exhaust gas purification device including at least one of the DPFs is arranged, the air-fuel ratio state of the exhaust gas flowing into the carrier is set to the temperature of the exhaust gas except during the regeneration control of the exhaust gas purification device. When the temperature is equal to or lower than a predetermined temperature, the exhaust gas purification device temperature increasing method is controlled so as to alternately repeat the rich state and the lean state.   The method for raising the temperature of an exhaust gas purification apparatus according to claim 1, wherein a substance containing a cerium element is used as the oxygen storage substance.   The method of raising a temperature of an exhaust gas purifying apparatus according to claim 1 or 2, wherein a ratio of the rich state time and the lean state time is rich state: lean state = 3:57 to 3: 3. .   In an exhaust gas purification system provided with an exhaust gas purification device provided with at least one of a NOx purification catalyst or a DPF, oxygen is released when the air-fuel ratio of the exhaust gas is rich, and oxygen is occluded and stored in a lean state. A carrier supporting an exothermic oxygen storage material is provided on the upstream side, and the air-fuel ratio of the exhaust gas flowing into the carrier is set at a predetermined exhaust gas temperature except during regeneration control of the exhaust gas purification device. An exhaust gas purification system comprising exhaust gas air-fuel ratio control means for performing control so that the rich state and the lean state are alternately repeated when the temperature is equal to or lower than the above temperature.   The exhaust gas purification system according to claim 4, wherein a substance containing a cerium element is used as the oxygen storage substance.   6. The exhaust gas air-fuel ratio control means sets the ratio of the rich state time and the lean state time to rich state: lean state = 3: 57 to 3: 3. Exhaust gas purification system.   The exhaust gas purification system according to any one of claims 4 to 6, wherein a loading amount of the substance containing the cerium element in the carrier is 50 g / L to 200 g / L.

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