JP5090187B2 - Exhaust gas purification device for internal combustion engine - Google Patents

Description

  The present invention relates to an exhaust emission control device for an internal combustion engine. Specifically, the present invention relates to an exhaust gas purification apparatus for an internal combustion engine that includes a NOx purification catalyst that adsorbs or occludes NOx in exhaust gas and reduces the adsorbed or occluded NOx.

  In the present invention, the term “rich” indicates that the air / fuel ratio of the fuel in question (hereinafter referred to as “air-fuel ratio”) is smaller than the stoichiometric air-fuel ratio, and is referred to as “lean”. The term indicates that the air / fuel ratio of the fuel in question is greater than the stoichiometric air / fuel ratio described above. In the following description, the weight ratio of air to fuel in the air-fuel mixture flowing into the engine is referred to as engine air-fuel ratio, and the weight ratio of air in the exhaust pipe to combustible gas is referred to as exhaust air-fuel ratio.

Conventionally, a technique for purifying nitrogen oxide (hereinafter referred to as “NOx”) contained in exhaust gas is known.
For example, in Patent Documents 1 and 2 and Non-Patent Document 1, a NOx occlusion reduction catalyst (hereinafter referred to as “LNT”) is provided in the exhaust passage, and the exhaust gas is passed through an oxidation catalyst during a lean operation in which the exhaust gas is excessive in oxygen. There is shown an exhaust emission control device that stores NOx by reacting with alkali metal or alkaline earth metal and stores the reduced NOx during a rich operation in which the oxygen concentration of the exhaust gas is reduced. In this exhaust purification apparatus, NOx occlusion and NOx reduction can be performed periodically by repeating the lean operation and the rich operation.

  Further, for example, in Non-Patent Document 2, NOx is adsorbed on the catalyst during a lean operation in which the exhaust gas is excessive in oxygen, and then a lean operation is performed to periodically create a state where the oxygen concentration in the exhaust gas is low. A method is shown in which NOx adsorbed during lean operation is periodically reduced by periodically synthesizing and supplying carbon.

More specifically, in the method shown in Non-Patent Document 2, first, during the lean operation in which the exhaust gas is excessive in oxygen, the following formulas (1) to (3) indicate that nitrogen monoxide present in the exhaust gas and Nitrogen dioxide is adsorbed on the catalyst.
NO → NO (adsorption) (1)
2NO + O ₂ → 2NO ₂ (2)
NO ₂ → NO ₂ (Adsorption) (3)
Next, rich operation is performed and carbon monoxide is synthesized. The carbon monoxide synthesized here produces hydrogen by an aqueous gas shift reaction represented by the following formula (4) in an environment where the oxygen partial pressure is low.
CO + H ₂ O → H ₂ + CO ₂ (4)
Further, this hydrogen reacts with nitric oxide in a reducing atmosphere to generate ammonia, and this ammonia is adsorbed on the catalyst by the following formula (5).
5H ₂ + 2NO → 2NH ₃ (adsorption) + 2H ₂ O (5)
As described above, NOx in the exhaust or NOx adsorbed on the catalyst is reduced by the following formulas (6) to (8) using ammonia generated from carbon monoxide as a final reducing agent.
4NH ₃ + 4NO + O ₂ → 4N ₂ + 6H ₂ O (6)
2NH ₃ + NO ₂ + NO → 2N ₂ + 3H ₂ O (7)
8NH ₃ + 6NO ₂ → 7N ₂ + 12H ₂ O (8)

  In addition, for example, in Patent Documents 3 and 4, a fuel reformer in which an LNT is provided in an exhaust passage and further, a hydrocarbon gas is reformed upstream of the LNT to produce a reducing gas containing hydrogen and carbon monoxide. An exhaust purification system is provided with a vacuum vessel. In particular, this exhaust purification system uses a fuel reformer that produces a reducing gas such that hydrogen is larger than carbon monoxide by volume ratio. According to this system, it is possible to selectively reduce NOx in the exhaust gas by adding a reducing gas containing hydrogen from the upstream side of the LNT into the exhaust gas.

Here, for example, as a method for producing a reducing gas from a hydrocarbon fuel, a partial oxidation reaction using oxygen as an oxidant is known, for example, as shown in the following formula (9).
_{C n H m + 1 / 2nO} 2 → nCO + 1 / 2mH 2 (9)
This partial oxidation reaction is an exothermic reaction using fuel and oxygen, and the reaction proceeds spontaneously. For this reason, once reaction starts, it can continue producing hydrogen, without supplying heat from the outside. Further, in such a partial oxidation reaction, when a fuel and oxygen coexist at a high temperature, a combustion reaction as shown in the following formula (10) also proceeds.
_{C n H m + (n +} 1 / 4m) O 2 → nCO 2 + 1 / 2mH 2 O (10)
Further, a steam reforming reaction represented by the following formula (11) using steam as an oxidizing agent is known.
_{C n H m + nH 2 O} → nCO + (n + 1 / 2m) H 2 (11)
This steam reforming reaction is an endothermic reaction using fuel and steam, and is not a reaction that proceeds spontaneously. For this reason, the steam reforming reaction is easily controlled with respect to the partial oxidation reaction described above.
Japanese Patent No. 2,586,738 Japanese Patent No. 2600492 Japanese Patent No. 3642273 JP 2002-89240 A "Development of NOx storage-reduction three-way catalyst system" Proceedings of the Society of Automotive Engineers of Japan Vol.26, No.4, October 1995 “A NOx Reduction System Using Ammonia Storage- Selective Catalytic Reduction in Rich and Lean Poerations”, 15 Aachener Kolloquium Fahrzeug-und Motorentechnik 2006 p. 259-270

However, there are the following problems when the engine is repeatedly operated in a lean operation and a rich operation, for example, as shown in the above-mentioned Patent Documents 1 and 2 and Non-Patent Documents 1 and 2.
That is, when the exhaust air-fuel ratio is made rich in order to reduce the NOx stored in the LNT, if the exhaust gas flowing into the LNT contains a large amount of NOx, the reducing agent mainly contains the NOx in the exhaust gas. It may be consumed for reduction. In such a case, the NOx stored in the LNT is desorbed by making the exhaust air-fuel ratio rich, but it flows out downstream of the LNT without being reduced, and the NOx purification performance may be reduced. .

  By the way, a method of reducing exhausted NOx by recirculating a part of exhaust to the intake side is known. However, when a large amount of exhaust gas is recirculated, although the amount of NOx discharged can be reduced, the amount of particulate discharged increases. In particular, it is difficult to recirculate a large amount of exhaust gas during high-load operation where the amount of NOx emission tends to increase. Therefore, the amount of NOx flowing out from the LNT may increase as the amount of NOx emission increases. There is.

Further, when the NOx stored in the LNT is reduced, it is necessary to make the exhaust air-fuel ratio rich. As a method for enriching the exhaust air-fuel ratio, a method for enriching the exhaust air-fuel ratio by performing fuel injection that does not contribute to torque (hereinafter referred to as “post-injection”) and causing unburned fuel to flow through the exhaust passage (hereinafter referred to as “fuel injection”) And a method of enriching by adjusting the amount of fuel injection (hereinafter referred to as “main injection”) that contributes to torque (hereinafter referred to as “method of combustion rich”).
However, in the post-rich method, fuel is injected into the cylinder in the latter half of the engine's explosion stroke or in the exhaust stroke, so that part of the fuel adheres to the cylinder wall surface. That is, in the post-rich method, not all of the fuel supplied by post-injection contributes to the enrichment of the exhaust air / fuel ratio. For this reason, there exists a possibility that a fuel consumption may deteriorate compared with the method by combustion rich. Further, the fuel adhering to the cylinder wall may be mixed into the engine oil as it is, and so-called oil dilution may occur.
In addition, the operation condition is limited in the combustion rich method. For example, during high load operation where the combustion is steep, the combustion noise is worsened. Further, when the engine is in a low load operation such as immediately after starting the engine or during idling, the charging efficiency into the cylinder may be reduced and combustion may become unstable.

In addition, the exhaust purification systems of Patent Documents 3 and 4 are different from those shown in Patent Documents 1 and 2 described above, basically, hydrogen in the exhaust gas with excess oxygen regardless of lean operation and rich operation. Carbon monoxide and hydrocarbons are added.
However, in the case where LNT is purified by adding reducing agents such as hydrogen, carbon monoxide, and hydrocarbons to the exhaust gas containing excess oxygen, NOx can be purified only at about 200 ° C. For example, when the temperature of LNT is 200 ° C. or higher, the added hydrogen and carbon monoxide are combusted. For this reason, at such a temperature, the amount of the additive is insufficient, and the reduction reaction of NOx does not proceed sufficiently.

In addition, in the case where a fuel reformer is provided in an exhaust passage in which the exhaust amount constantly varies as in the exhaust gas purification systems of Patent Documents 3 and 4, in order to effectively produce hydrogen with this fuel reformer Therefore, it is necessary to increase the reaction time between the reforming catalyst of the fuel reformer and the exhaust. However, in order to increase the reaction time in this way, it is necessary to enlarge the reforming catalyst, which may be costly.
In order to operate the fuel reformer in a stable state, it is necessary to keep the reaction temperature in the reforming catalyst of the fuel reformer constant. However, when the fuel reformer is provided in the exhaust passage in which the oxygen amount, the water vapor amount, and the temperature constantly change as in the above-described exhaust gas purification systems of Patent Documents 3 and 4, the fuel reformer is operated in a stable state. It becomes difficult to do. When the fuel reformer cannot be stably operated in this way, there is a risk that NOx may not be fully reduced and may flow out downstream of the LNT, such as when the exhaust contains a large amount of NOx as described above.

  The present invention has been made in consideration of the above-described points, and an object of the present invention is to provide an exhaust purification device for an internal combustion engine that can prevent a decrease in NOx purification performance during high-load operation.

  In order to achieve the above object, an invention according to claim 1 is provided in an exhaust passage (4, 5) of an internal combustion engine (1), and an exhaust air / fuel ratio of exhaust gas flowing through the exhaust passage is used as an exhaust air / fuel ratio. An internal combustion engine comprising a NOx purification catalyst (33) that adsorbs or occludes NOx in exhaust when the air-fuel ratio is made lean, and reduces the adsorbed or occluded NOx when the exhaust air-fuel ratio is made rich. In the exhaust emission control device, a catalytic converter (31) provided in the exhaust passage for continuously reducing NOx in the exhaust gas using a reducing gas, and provided separately from the exhaust passage, reforms the fuel. A reducing gas containing hydrogen and carbon monoxide is produced, and this reducing gas is discharged from an inlet (14) provided upstream of the NOx purification catalyst and the catalytic converter in the exhaust passage. A fuel reformer to be supplied into the passage, an operating condition detecting means (40) for detecting an operating condition of the internal combustion engine, and a rich means (40) for enriching the exhaust air-fuel ratio. When the operating condition detected by the operating condition detecting means is a high load operating condition, the converting means supplies the reducing gas into the exhaust passage by the fuel reformer to reduce the exhaust air / fuel ratio. It is characterized by being enriched.

According to a second aspect of the present invention, the exhaust gas purification apparatus for an internal combustion engine according to the first aspect further comprises exhaust gas recirculation rate estimating means (40, 21, 25, 26) for estimating an exhaust gas recirculation rate of the internal combustion engine. When the exhaust gas recirculation rate (REGR) estimated by the exhaust gas recirculation rate estimating device is smaller than a predetermined determination value (REGRH), the enrichment unit reduces the exhaust passage by the fuel reformer. The exhaust air-fuel ratio is enriched by supplying gas.
According to a third aspect of the present invention, in the exhaust gas purification apparatus for an internal combustion engine according to the first or second aspect, the catalytic converter is provided upstream of the NOx purification catalyst in the exhaust passage. To do.
According to a fourth aspect of the present invention, in the exhaust gas purification apparatus for an internal combustion engine according to the third aspect, an exhaust air-fuel ratio for detecting or estimating an exhaust air-fuel ratio between the introduction port and the catalytic converter in the exhaust passage. A detection means (23), wherein the enrichment means estimates or is estimated by the exhaust air / fuel ratio detection means when the exhaust air / fuel ratio is enriched by supplying a reducing gas from the fuel reformer; The supply amount (GRG) of the reducing gas is adjusted so that the detected exhaust air-fuel ratio (AF) matches a predetermined target value (NAFTV, HAFTV).

According to a fifth aspect of the present invention, in the exhaust gas purification apparatus for an internal combustion engine according to any one of the first to fourth aspects, the predetermined target value is obtained by supplying a reducing gas from the fuel reformer. The case where the exhaust air-fuel ratio is enriched is different from the case where the exhaust air-fuel ratio is enriched without supplying the reducing gas by the fuel reformer.
According to a sixth aspect of the present invention, in the exhaust gas purification apparatus for an internal combustion engine according to any one of the first to fifth aspects, the catalytic converter includes ceria, platinum, and rhodium.
According to a seventh aspect of the present invention, in the exhaust gas purification apparatus for an internal combustion engine according to any one of the first to sixth aspects, the fuel reformer is operated from an atmospheric pressure by a partial oxidation reaction between hydrocarbon fuel and air. It is characterized by producing a reducing gas having a high pressure and containing carbon monoxide as a main component.

According to the first aspect of the present invention, when the exhaust gas air-fuel ratio is enriched, if the operating state of the internal combustion engine is a high load operating state, the reducing gas produced by the fuel reformer is exhausted. Supply into the passage. As a result, even when the engine is operating at a high load as described above and it is difficult to make the engine air-fuel ratio rich, the engine air-fuel ratio is made lean and the combustion in the internal combustion engine is kept in an optimal state while flowing into the NOx purification catalyst. The exhaust air-fuel ratio of the exhaust gas to be exhausted can be made rich, and NOx reduction in the NOx purification catalyst can be promoted. Here, since the engine air-fuel ratio can be kept lean, it is possible to prevent the oil dilution as described above from occurring and the combustion of the internal combustion engine from becoming unstable.
In addition, by providing a fuel reformer that produces reducing gas separately from the exhaust passage, the reducing gas is always supplied with optimum efficiency regardless of the operating state of the internal combustion engine, the oxygen concentration and the water vapor concentration of the exhaust gas, etc. While being able to be manufactured, this reducing gas can be supplied into the exhaust passage. Further, by providing the fuel reformer separately from the exhaust passage, it is possible to supply reducing gas as required, such as the operating state of the internal combustion engine.
On the other hand, when the fuel reformer is provided in the exhaust passage, it is necessary to enlarge the fuel reformer so that the fuel reformer can be operated without affecting the exhaust gas component, temperature, and flow velocity. According to this invention, by providing the fuel reformer separately from the exhaust passage, stable operation can be performed without increasing the size of the apparatus. Further, by providing the fuel reformer separately from the exhaust passage, it is possible to activate the catalyst included in the fuel reformer at an early stage by performing control of a system different from the control of the internal combustion engine.
Moreover, the reduction reaction rate in the NOx purification catalyst can be improved by supplying a reducing gas containing hydrogen. Thereby, when the exhaust air-fuel ratio is made rich, NOx adsorbed or stored in the NOx purification catalyst can be prevented from being desorbed without being reduced.

According to the second aspect of the present invention, when the exhaust gas recirculation rate is smaller than a predetermined judgment value when enriching the exhaust air-fuel ratio, the reducing gas produced by the fuel reformer is used. Is supplied to the exhaust passage to enrich the exhaust air-fuel ratio.
By the way, if the engine air-fuel ratio is made rich while the exhaust gas is recirculated to the intake side, hydrocarbons in the exhaust gas may accumulate in the intake passage and block the intake passage. According to the invention of claim 2, by supplying the reducing gas, the exhaust air-fuel ratio is enriched without blocking the intake passage as described above, and the NOx purification performance in the NOx purification device can be improved.
According to the invention of claim 3, by providing the catalytic converter upstream of the NOx purification catalyst, the NOx concentration of the exhaust gas flowing into the NOx purification catalyst during the rich can be reduced. That is, the amount of NOx flowing into the NOx purification catalyst during richness can be reduced. Therefore, when the exhaust air-fuel ratio is made rich, NOx adsorbed or stored in the NOx purification catalyst can be prevented from being desorbed without being fully reduced.

According to the fourth aspect of the present invention, when the exhaust air-fuel ratio is enriched by supplying the reducing gas, the supply amount of the reducing gas is set so that the exhaust air-fuel ratio matches a predetermined target value. Is adjusted. As a result, the exhaust gas having the optimum exhaust air / fuel ratio can always be caused to flow into the NOx purification catalyst.
According to the fifth aspect of the present invention, there are different targets for the case where the exhaust air-fuel ratio is enriched by supplying the reducing gas and the case where the exhaust air-fuel ratio is enriched without supplying the reducing gas. The exhaust air / fuel ratio is enriched based on the value. As a result, the exhaust gas having the optimum exhaust air / fuel ratio can always be caused to flow into the NOx purification catalyst.

According to the sixth aspect of the present invention, by including ceria in the catalytic converter, it is possible to further improve the NOx reduction ability of the catalytic converter and to suppress sintering of the precious metal active species. In addition, by including rhodium having oxygen storage capacity in the catalytic converter, the reaction in the catalytic converter can be stabilized even if the oxygen concentration in the exhaust gas changes abruptly. Further, by including platinum in the catalytic converter, it is possible to oxidize and adsorb or occlude NO and reduce NOx.
According to the seventh aspect of the present invention, the fuel reformer can be made small by producing a reducing gas by a partial oxidation reaction. That is, as described above, the partial oxidation reaction is an exothermic reaction, and once the reaction starts, the reaction proceeds spontaneously, so that it is not necessary to provide a device for supplying extra energy from the outside. Further, there is no need to provide a converter or system for concentrating hydrogen such as shift reaction. Moreover, the light-off time of the fuel reformer can be shortened by reducing the size of the fuel reformer. Therefore, reducing gas can be quickly supplied into the exhaust passage as necessary.
Further, by producing the reducing gas having a pressure higher than the atmospheric pressure, the produced reducing gas can be supplied into the exhaust passage without adding an extra device.
In addition, light hydrocarbons produced as a secondary in the partial oxidation reaction can also be introduced into the catalytic converter together with carbon monoxide and hydrogen and used for NOx reduction.

  FIG. 1 is a diagram showing a configuration of an internal combustion engine and an exhaust purification device thereof according to an embodiment of the present invention. An internal combustion engine (hereinafter referred to as “engine”) 1 is a diesel engine that directly injects fuel into the combustion chamber of each cylinder 7, and each cylinder 7 is provided with a fuel injection valve (not shown). These fuel injection valves are electrically connected by an electronic control unit (hereinafter referred to as “ECU”) 40, and the valve opening time and valve closing time of the fuel injection valve are controlled by the ECU 40.

  The engine 1 includes an intake pipe 2 through which intake air circulates, an exhaust pipe 4 through which exhaust gas circulates, an exhaust gas recirculation passage 6 that recirculates part of the exhaust gas in the exhaust pipe 4 to the intake pipe 2, and intake air into the intake pipe 2. And a supercharger 8 for pressure-feeding.

  The intake pipe 2 is connected to the intake port of each cylinder 7 of the engine 1 through a plurality of branch portions of the intake manifold 3. The exhaust pipe 4 is connected to the exhaust port of each cylinder 7 of the engine 1 through a plurality of branch portions of the exhaust manifold 5. The exhaust gas recirculation passage 6 branches from the exhaust manifold 5 and reaches the intake manifold 3.

  The supercharger 8 includes a turbine (not shown) provided in the exhaust pipe 4 and a compressor (not shown) provided in the intake pipe 2. The turbine is driven by the kinetic energy of the exhaust flowing through the exhaust pipe 4. The compressor is rotationally driven by the turbine, pressurizes the intake air, and pumps it into the intake pipe 2. The turbine includes a plurality of variable vanes (not shown), and is configured to change the turbine rotation speed (rotational speed) by changing the opening of the variable vanes. The vane opening degree of the turbine is electromagnetically controlled by the ECU 40.

  A throttle valve 9 for controlling the intake air amount GA of the engine 1 is provided upstream of the supercharger 8 in the intake pipe 2. The throttle valve 9 is connected to the ECU 40 via an actuator, and the opening degree is electromagnetically controlled by the ECU 40. In addition, an intercooler 11 for cooling the intake air pressurized by the supercharger 8 is provided on the downstream side of the supercharger 8 in the intake pipe 2.

  The exhaust gas recirculation passage 6 connects the exhaust manifold 5 and the intake manifold 3 and recirculates part of the exhaust discharged from the engine 1. The exhaust gas recirculation passage 6 is provided with an EGR cooler 12 that cools the exhaust gas that is recirculated, and an EGR valve 13 that controls the flow rate of the recirculated exhaust gas. The EGR valve 13 is connected to the ECU 40 via an actuator (not shown), and the valve opening degree is electromagnetically controlled by the ECU 40.

  The exhaust pipe 4 is provided with a catalytic converter 31, a particulate matter collection device (hereinafter referred to as “DPF” (Diesel Particulate Filter)) 32, and a NOx purification catalyst 33 in this order from the upstream side.

The catalytic converter 31 includes a three-way catalyst that continuously reduces NOx in the exhaust gas using a reducing gas supplied from a fuel reformer 50 described later. The catalytic converter 31 includes platinum (Pt) and rhodium (Rh) having an ability to reduce NOx in exhaust gas, and ceria (CeO ₂ ) having an oxygen storage ability.
In this embodiment, as the catalytic converter 31, platinum (Pt) is 2.4 (g / L), rhodium is 1.2 (g / L), and palladium (Pd) is 6.0 (g / L). By stirring and mixing 50 (g / L) of ceria, 150 (g / L) of alumina (Al ₂ O ₃ ), and 10 (g / L) of binder together with an aqueous medium with a ball mill. A slurry is prepared, and this slurry is coated on an Fe—Cr—Al alloy support, and then dried and fired at 600 ° C. for 2 hours.

  When the exhaust gas passes through fine holes in the filter wall, the DPF 32 deposits soot as particulates mainly composed of carbon in the exhaust gas on the surface of the filter wall and the holes in the filter wall. Collect. As a constituent material of the filter wall, for example, ceramics such as silicon carbide (SiC) or a porous metal body is used.

The NOx purification catalyst 33 acts as a catalyst supported on a support of alumina (Al ₂ O ₃ ), ceria (CeO ₂ ), and a composite oxide of cerium and rare earth (hereinafter referred to as “ceria-based composite oxide”). Platinum (Pt), ceria or ceria-based composite oxide having NOx adsorption capability, and zeolite having a function of holding ammonia (NH ₃ ) generated in the catalyst as ammonium ions (NH ₄ ⁺ ) .

In the present embodiment, the NOx purification catalyst 33 is formed by supporting a NOx reduction catalyst having two layers on a catalyst carrier.
The lower layer of the NOx reduction catalyst is platinum (4.5 g / L), ceria 60 (g / L), alumina 30 (g / L), and Ce—Pr—La—Ox 60 (g). / L) and 20 (g / L) of Zr-Ox together with an aqueous medium are put into a ball mill, stirred and mixed to produce a slurry, and this slurry is coated on a catalyst carrier. Formed.
Moreover, the upper layer of the NOx reduction catalyst is 75 (g / L) obtained by ion-exchange of β-type zeolite with iron (Fe) and cerium (Ce), 7 (g / L) alumina, and 8 binders. A material composed of (g / L) is put into a ball mill together with an aqueous medium, stirred and mixed to produce a slurry, and this slurry is coated on the lower layer.

  When the amount of adsorbed ammonia in the NOx purification catalyst 33 decreases, the NOx purification capability decreases, so that a reducing agent is supplied to the NOx purification catalyst 33 (hereinafter referred to as “reduction”) in order to reduce NOx as appropriate. . In this reduction, the reducing agent is supplied to the NOx purification catalyst 33 by making the air-fuel ratio (engine air-fuel ratio) of the air-fuel mixture in the combustion chamber of the engine 1 richer than the stoichiometric ratio. That is, by enriching the air-fuel ratio (exhaust air-fuel ratio) of the exhaust discharged from the engine 1, the concentration of the reducing agent in the exhaust flowing into the NOx purification catalyst 33 becomes higher than the oxygen concentration, and reduction is performed. The

The NOx purification in the NOx purification catalyst 33 will be described.
First, when the engine air-fuel ratio is set leaner than the stoichiometric ratio and so-called lean burn operation is performed, the concentration of the reducing agent in the exhaust gas flowing into the NOx purification catalyst 33 becomes lower than the oxygen concentration. As a result, nitrogen monoxide (NO) and oxygen (O ₂ ) in the exhaust gas react with each other by the action of the catalyst, and are adsorbed as NO ₂ on the ceria or ceria-based composite oxide. In addition, carbon monoxide (CO) that has not reacted with oxygen is also adsorbed to ceria or ceria-based composite oxide.

Next, so-called rich operation is performed in which the engine air-fuel ratio is set to be richer than the stoichiometric ratio, and the exhaust air-fuel ratio is enriched. That is, when reduction is performed to make the reducing agent concentration in the exhaust gas higher than the oxygen concentration, carbon monoxide (CO) in the exhaust gas reacts with water (H ₂ O), and carbon dioxide (CO ₂ ) and hydrogen ( H ₂ ) is generated, and hydrocarbons (HC) in the exhaust gas react with water to generate hydrogen together with carbon monoxide (CO) and carbon dioxide (CO ₂ ). Furthermore, NOx contained in the exhaust gas, NOx (NO, NO ₂ ) adsorbed on ceria or ceria-based complex oxide (and platinum), and the produced hydrogen react with each other by the action of the catalyst, and ammonia (NH ₃ ) and water are produced. Further, the ammonia generated here is adsorbed on the zeolite in the form of ammonium ions (NH ₄ ⁺ ).

Next, when lean burn operation is performed in which the engine air-fuel ratio is set leaner than the stoichiometric ratio, and the reducing agent concentration in the exhaust gas flowing into the NOx purification catalyst 33 is set lower than the oxygen concentration, ceria or ceria NOx is adsorbed on the system complex oxide. Further, when ammonium ions are adsorbed on the zeolite, NOx and oxygen in the exhaust gas react with ammonia to generate nitrogen (N ₂ ) and water.

  Thus, according to the NOx purification catalyst 33, ammonia generated during the supply of the reducing agent is adsorbed by the zeolite, and the ammonia adsorbed during the lean burn operation reacts with NOx, so that the NOx purification can be performed efficiently. Can do.

A fuel reformer 50 that reforms the fuel gas to produce a reformed gas containing hydrogen (H ₂ ) and carbon monoxide (CO) is disposed upstream of the catalytic converter 31 in the exhaust pipe 4. It is connected. The fuel reformer 50 supplies the produced reformed gas as a reducing gas into the exhaust pipe 4 from the inlet 14 formed on the upstream side of the catalytic converter 31 in the exhaust pipe 4.

  The fuel reformer 50 is provided in the gas passage 51, a gas passage 51 whose one end is connected to the exhaust pipe 4, a fuel gas supply device 52 that supplies fuel gas from the other end of the gas passage 51, and the gas passage 51. And a reforming catalyst 53 as a reforming catalyst.

  The fuel gas supply device 52 mixes fuel stored in the fuel tank and air supplied by the compressor at a predetermined ratio to produce fuel gas, and supplies the fuel gas to the gas passage 51. The fuel gas supply device 52 is connected to the ECU 40, and the fuel gas supply amount and the mixing ratio thereof are controlled by the ECU 40. Further, by controlling the supply amount of the fuel gas, the supply amount GRG of the reducing gas supplied to the exhaust pipe 4 (the amount of reducing gas supplied into the exhaust pipe 4 per unit time) can be controlled. It is possible.

  The reforming catalyst 53 contains rhodium and ceria. The reforming catalyst 53 is a catalyst that reforms the fuel gas supplied from the fuel gas supply device 52 to produce a reformed gas containing hydrogen, carbon monoxide, and hydrocarbons. More specifically, the reforming catalyst 53 has a pressure higher than atmospheric pressure due to a partial oxidation reaction between the hydrocarbon fuel constituting the fuel gas and air, and has a volume ratio of carbon monoxide higher than hydrogen. A reformed gas containing a large amount of is produced. Further, as described above, the partial oxidation reaction is an exothermic reaction. Thus, the fuel reformer 50 can supply reducing gas having a temperature higher than that of the exhaust gas in the vicinity of the inlet 14 in the exhaust pipe 4 into the exhaust pipe 4.

  The reforming catalyst 53 is connected to a heater (not shown) including a glow plug, a spark plug, and the like, so that the reforming catalyst 53 can be heated when the fuel reformer 50 is started. It is possible. The fuel reformer 50 is provided separately from the exhaust pipe 4. That is, the fuel gas supply device 52 and the reforming catalyst 53 of the fuel reformer 50 are not provided in the exhaust pipe 4.

The ECU 40 includes a crank angle position sensor (not shown) for detecting the rotation angle of the crankshaft of the engine 1, an accelerator sensor (not shown) for detecting an accelerator pedal depression amount AP of a vehicle driven by the engine 1, An air flow meter 21 that detects an intake air amount GA of the engine 1 (amount of air newly sucked into the engine 1 per unit time), and an oxygen concentration of exhaust gas between the inlet 14 and the catalytic converter 31 in the exhaust pipe 4 That is, a UEGO sensor 23 for detecting the exhaust air-fuel ratio AF, a pressure in the intake manifold 3, that is, a boost pressure sensor 25 for detecting the boost pressure PB, and a temperature sensor 26 for detecting the temperature TI of the intake manifold 3 are connected. The detection signals of these sensors are supplied to the ECU 40.
Here, the rotational speed NE of the engine 1 is calculated by the ECU 40 based on the output of the crank angle position sensor.

  The ECU 40 shapes an input signal waveform from various sensors, corrects a voltage level to a predetermined level, converts an analog signal value into a digital signal value, and a central processing unit (hereinafter, “ CPU ”). In addition, the ECU 40 is a storage circuit that stores various calculation programs executed by the CPU, calculation results, and the like, a fuel reformer 50, a throttle valve 9, an EGR valve 13, a supercharger 8, and a fuel injection of the engine 1. An output circuit for outputting a control signal to a valve or the like.

  The engine 1 is normally operated with the engine air-fuel ratio set to a lean side with respect to the stoichiometric ratio, and rich control for setting the engine air-fuel ratio to the rich side with respect to the stoichiometric ratio is periodically performed.

FIG. 2 is a flowchart showing a procedure of enrichment control by the ECU. As shown in FIG. 2, the normal enrichment control and the H ₂ enrichment control can be selectively executed according to a predetermined condition.

  In step S1, it is determined whether or not to perform enrichment. If this determination is YES, the process proceeds to step S2, and if NO, the enrichment control is terminated. More specifically, the NOx adsorption amount TRN adsorbed on the NOx purification catalyst is estimated based on the intake air amount GA and the fuel injection amount of the engine, and the NOx adsorption amount TRN is equal to or greater than a predetermined determination amount TRNTH. In some cases, enrichment control is executed.

In step S2, it is determined whether or not the engine is in a high load operation state based on the fuel injection amount and the engine speed NE. If this determination is YES, the process proceeds to step S5, and if NO, the process proceeds to step S3.
FIG. 3 is a diagram showing an operating state of the engine. In FIG. 3, the horizontal axis represents the engine speed, and the vertical axis represents the fuel injection amount of the engine. As shown in FIG. 3, the engine operating state is divided into a high-load operating state and a low-load operating state by a curve 91 using the rotation speed and the fuel injection amount as parameters. In step S2 described above, the operating state of the engine is determined based on such a control map using the fuel injection amount and the rotational speed as parameters.

  In step S3, an exhaust gas recirculation rate REGR (hereinafter referred to as “EGR rate REGR”) is estimated, and it is determined whether or not the EGR rate REGR is smaller than a predetermined determination value REGRTH. If this determination is YES, the process proceeds to step S5, and if NO, the process proceeds to step S4. Here, the EGR rate includes the intake air amount GA detected by the air flow meter, the boost pressure PB detected by the boost pressure sensor, the intake manifold temperature TI detected by the temperature sensor 26, and the engine speed NE. Is estimated based on

In step S4, normal enrichment control is executed, and after completion, the enrichment control is terminated. More specifically, in the normal enrichment control, the exhaust air / fuel ratio AF is controlled to be equal to the predetermined first target value NAFTV by setting the engine air / fuel ratio to be richer than the stoichiometric ratio. .
In step S5, the H ₂ enrichment control is executed, and after the completion, the enrichment control is terminated. More specifically, in this H ₂ enrichment control, the reducing gas produced by the fuel reformer is supplied to the exhaust pipe, and the exhaust air-fuel ratio AF is adjusted by adjusting the reducing gas supply amount GRG. Control is performed so as to match a predetermined target value HAFTV.

As described in detail above, according to the present embodiment, when the exhaust air-fuel ratio is enriched, if the operating state of the engine 1 is a high-load operating state, the reduction produced by the fuel reformer 50 is performed. A sex gas is supplied into the exhaust pipe 4. As a result, even when the engine is operating at a high load as described above and it is difficult to make the engine air-fuel ratio rich, the engine air-fuel ratio is made lean and the combustion in the internal combustion engine is kept in an optimal state, while the NOx purification catalyst 33 is maintained. The exhaust air-fuel ratio of the inflowing exhaust gas can be made rich, and NOx reduction in the NOx purification catalyst 33 can be promoted. Here, since the engine air-fuel ratio can be kept lean, it is possible to prevent the oil dilution as described above from occurring and the combustion of the engine 1 from becoming unstable.
Further, by providing the fuel reformer 50 for producing reducing gas separately from the exhaust pipe 4, the reducing performance can be always reduced with optimum efficiency regardless of the operating state of the engine 1 and the oxygen concentration and water vapor concentration of the exhaust gas. Gas can be produced and this reducing gas can be supplied into the exhaust pipe 4. Further, by providing the fuel reformer 50 separately from the exhaust pipe 4, it is possible to supply reducing gas as required, such as the operating state of the engine 1.
On the other hand, when the fuel reformer 50 is provided in the exhaust pipe 4, it is necessary to make the fuel reformer 50 large so that the fuel reformer 50 can be operated without affecting the exhaust components, temperature, and flow velocity. According to the present embodiment, by providing the fuel reformer 50 separately from the exhaust pipe 4, a stable operation can be performed without increasing the size of the apparatus. Further, by providing the fuel reformer 50 separately from the exhaust pipe 4, the reforming catalyst 53 provided in the fuel reformer 50 is activated at an early stage by controlling the system different from the control of the engine 1. It is also possible.
In addition, the reduction reaction rate in the NOx purification catalyst 33 can be improved by supplying a reducing gas containing hydrogen. Thereby, when the exhaust air-fuel ratio is made rich, NOx adsorbed or stored in the NOx purification catalyst 33 can be prevented from being desorbed without being reduced.

Further, according to the present embodiment, when the exhaust air-fuel ratio is enriched, if the estimated EGR rate REGR is smaller than the predetermined determination value REGRTH, the reducing gas produced by the fuel reformer 50 is reduced. By supplying the exhaust pipe 4, the exhaust air-fuel ratio is enriched.
By the way, if the engine air-fuel ratio is made rich while the exhaust gas is recirculated to the intake side, hydrocarbons in the exhaust gas may accumulate on the intake pipe 2 and block the intake pipe 2. According to this embodiment, by supplying the reducing gas, the exhaust air-fuel ratio can be enriched without blocking the intake pipe 2 as described above, and the NOx purification performance of the NOx purification catalyst 33 can be improved.
Further, according to the present embodiment, by providing the catalytic converter 31 on the upstream side of the NOx purification catalyst 33, the NOx concentration of the exhaust gas flowing into the NOx purification catalyst 33 while being rich can be reduced. That is, the amount of NOx flowing into the NOx purification catalyst 33 during the rich can be reduced. Therefore, when the exhaust air-fuel ratio is made rich, the NOx adsorbed or stored in the NOx purification catalyst 33 can be prevented from being desorbed without being fully reduced.

Further, according to the present embodiment, when the exhaust air-fuel ratio is enriched by supplying the reducing gas, the reducing gas is supplied so that the exhaust air-fuel ratio AF matches the predetermined target values NAFTV and HAFTV. The amount GRG is adjusted. As a result, the exhaust gas having the optimum exhaust air / fuel ratio can always flow into the NOx purification catalyst 33.
According to the present embodiment, the exhaust air-fuel ratio is enriched by supplying reducing gas, and the exhaust air-fuel ratio is enriched without supplying reducing gas, based on different target values. As a result, the exhaust air-fuel ratio is enriched. As a result, the exhaust gas having the optimum exhaust air / fuel ratio can always flow into the NOx purification catalyst 33.

Further, according to the present embodiment, the inclusion of ceria in the catalytic converter 31 can further improve the NOx reduction ability of the catalytic converter 31 and further suppress the sintering of the precious metal active species. Further, by including rhodium having an oxygen storage capacity in the catalytic converter 31, the reaction in the catalytic converter 31 can be stabilized even if the oxygen concentration in the exhaust gas changes abruptly. Further, by including platinum in the catalytic converter, it is possible to oxidize and adsorb or occlude NO and reduce NOx.
Further, according to this embodiment, the fuel reformer 50 can be made small by producing a reducing gas by a partial oxidation reaction. That is, as described above, the partial oxidation reaction is an exothermic reaction, and once the reaction starts, the reaction proceeds spontaneously, so that it is not necessary to provide a device for supplying extra energy from the outside. Further, there is no need to provide a converter or system for concentrating hydrogen such as shift reaction. Moreover, the light-off time of the fuel reformer 50 can be shortened by reducing the size of the fuel reformer 50 in this way. Therefore, the reducing gas can be quickly supplied into the exhaust pipe 4 as necessary.
Further, by producing the reducing gas having a pressure higher than the atmospheric pressure, the produced reducing gas can be supplied into the exhaust pipe 4 without adding an extra device.
In addition, light hydrocarbons produced as a secondary in this partial oxidation reaction can also be introduced into the catalytic converter 31 together with carbon monoxide and hydrogen and used for NOx reduction.

  In the present embodiment, the ECU 40 constitutes the enrichment means, part of the operating state detection means, and part of the exhaust gas recirculation rate estimation means. Specifically, the means according to steps S1 to S5 in FIG. 2 corresponds to the enrichment means, the means according to step S2 corresponds to the operating state detection means, the means according to step S3, the air flow meter 21, and the supercharging pressure. The sensor 25 and the temperature sensor 26 correspond to exhaust gas recirculation rate estimation means.

  Next, a NOx purification evaluation test for verifying the effect of supplying the reducing gas to the exhaust pipe as in the above embodiment will be described with reference to FIGS.

[NOx purification performance evaluation test method]
FIG. 4 is a schematic diagram showing the configuration of the test apparatus 80 for the NOx purification performance evaluation test.
The test apparatus 80 includes a supply device 81 for supplying a model gas with a predetermined composition, a heater 83 in which an adsorbent 84 is provided, a gas analyzer 85 for analyzing the model gas, and a data acquisition computer 86. , Including.

The supply device 81 supplies a model gas composed of N ₂ , CO ₂ , O ₂ , H ₂ O, CO, HC, NOx, and H ₂ to the heater 83. The supply device 81 can adjust the flow rate of each component of the model gas.
The heater 83 includes a reactor 82 and an adsorbent 84 therein, and heats the reactor 82 and the adsorbent 84. The reactor 82 mixes the model gas supplied from the supply device 81 and supplies it to the adsorbent 84.

  The adsorbent 84 is composed of a three-way catalyst and a NOx purification catalyst. Here, the same three-way catalyst and NOx purification catalyst as those of the catalytic converter 31 and the NOx purification catalyst 33 (see FIG. 1 described above) described in the above embodiment are used, and thus description thereof is omitted. To do.

In the heater 83, the gas analyzer 85 is supplied from the reactor 82 to the adsorbent 84 and measures the NOx concentration of the model gas that has passed through the adsorbent 84. The data acquisition computer 86 processes the data related to the NOx concentration and calculates the NOx purification rate for each temperature. The NOx purification rate is calculated based on the following equation.
NOx purification rate [%] = (Cin−Cout) / Cin × 100
Here, Cin is the NOx concentration of the model gas at the inlet of the adsorbent 84, and Cout is the NOx concentration of the model gas at the outlet of the adsorbent 84. The NOx concentration of the model gas was measured by the chemical luminescence method.

In this evaluation test, in the test apparatus 80 configured as described above, while supplying the model gas, the model gas passed through the adsorbent 84 while being heated from 50 ° C. to 450 ° C. at 20 ° C./min. The NOx purification rate of gas was measured.
As the model gas, a model gas having a lean atmosphere composition and a model gas having a rich atmosphere composition were alternately supplied over 55 seconds and 5 seconds, respectively.

[Example]
In the model gas of the example, the following gas is modeled on the one obtained by adding the reducing gas (including carbon monoxide, hydrogen, and hydrocarbon) produced by the fuel reformer according to the above embodiment to the exhaust gas. A model gas having a composition was used.
Lean atmosphere NO: 90ppm
CO: 6000 ppm
HC (propylene): 500 ppmC (C ₃ H ₆ )
O ₂ : 6%
CO ₂ : 6%
H ₂ O: 7%
N ₂ : Balance gas H ₂ : 5000 ppm
SV = 50000h-1
Rich atmosphere NO: 90ppm
CO: 2.1%
HC (propylene): 500 ppmC (C ₃ H ₆ )
O ₂ : 0%
CO ₂ : 6%
H ₂ O: 7%
N ₂ : Balance gas H ₂ : 6000 ppm
SV = 50000h-1

[Comparative example]
As a model gas of the comparative example, a model gas having the following composition was used as a model of exhaust gas to which the above-described reducing gas was not added.
Lean atmosphere NO: 90ppm
CO: 1000 ppm
HC (propylene): 500 ppmC (C ₃ H ₆ )
O ₂ : 6%
CO ₂ : 6%
H ₂ O: 7%
N ₂ : Balance gas H ₂ : 0 ppm
SV = 50000h-1
Rich atmosphere NO: 90ppm
CO 2%
HC (propylene): 500 ppmC (C ₃ H ₆ )
O ₂ : 0%
CO ₂ : 6%
H ₂ O: 7%
N ₂ : Balance gas H ₂ : 0 ppm
SV = 50000h-1

[Test results]
FIG. 5 is a diagram showing test results of Examples and Comparative Examples. In FIG. 5, the horizontal axis represents the temperature of the model gas, and the vertical axis represents the NOx purification rate. Moreover, a black circle shows the relationship between the gas temperature and the NOx purification rate in the example, and a white circle shows the relationship between the gas temperature and the NOx purification rate in the comparative example.
Comparing the NOx purification rate of the example and the NOx purification rate of the comparative example, the NOx purification rate of the example is substantially constant over the entire temperature range, whereas the comparative example has a particularly small NOx purification rate in the low temperature range. It has become. Therefore, it was verified that the NOx purification performance in the low temperature range can be improved by adding the reducing gas containing carbon monoxide and hydrogen as in the embodiment.

The present invention is not limited to the embodiment described above, and various modifications can be made.
The present invention can also be applied to an exhaust emission control device such as a marine vessel propulsion engine such as an outboard motor having a vertical crankshaft.

1 is a diagram illustrating a configuration of an internal combustion engine and an exhaust purification device thereof according to an embodiment of the present invention. It is a flowchart which shows the procedure of the enrichment control by ECU which concerns on the said embodiment. It is a figure which shows the driving | running state of the engine which concerns on the said embodiment. It is the schematic which shows the structure of the test apparatus of a NOx purification performance evaluation test. It is a figure which shows the test result of an Example and a comparative example.

Explanation of symbols

1 engine (internal combustion engine)
4 Exhaust pipe (exhaust passage)
5 Exhaust manifold (exhaust passage)
14 Introducing port 23 UEGO sensor (exhaust air / fuel ratio detecting means)
31 catalytic converter 33 NOx purification catalyst 40 electronic control unit (riching means, operating state detecting means, exhaust gas recirculation rate estimating means)
50 Fuel reformer

Claims (7)

The exhaust air-fuel ratio is provided in the exhaust passage of the internal combustion engine and adsorbs or occludes NOx in the exhaust when the exhaust air-fuel ratio is made lean, with the air-fuel ratio of the exhaust gas flowing through the exhaust passage being made lean. In an exhaust gas purification apparatus for an internal combustion engine comprising a NOx purification catalyst that reduces the adsorbed or occluded NOx when the gas is made rich.
A catalytic converter that is provided in the exhaust passage and continuously reduces NOx in the exhaust using a reducing gas;
Provided separately from the exhaust passage, the fuel is reformed to produce a reducing gas containing hydrogen and carbon monoxide, and this reducing gas is upstream of the NOx purification catalyst and the catalytic converter in the exhaust passage. A fuel reformer that is supplied into the exhaust passage from an inlet provided on the side;
An operating state detecting means for detecting an operating state of the internal combustion engine;
Enriching means for enriching the exhaust air-fuel ratio,
When the operation state detected by the operation state detection unit is a high load operation state, the enrichment unit supplies the reducing gas to the exhaust passage by the fuel reformer. An exhaust gas purification apparatus for an internal combustion engine characterized by enriching the fuel ratio. An exhaust gas recirculation rate estimating means for estimating an exhaust gas recirculation rate of the internal combustion engine;
When the exhaust gas recirculation rate estimated by the exhaust gas recirculation rate estimating unit is smaller than a predetermined determination value, the enrichment unit supplies the reducing gas to the exhaust passage by the fuel reformer. The exhaust gas purification apparatus for an internal combustion engine according to claim 1, wherein the exhaust air-fuel ratio is enriched.   The exhaust purification device for an internal combustion engine according to claim 1 or 2, wherein the catalytic converter is provided upstream of the NOx purification catalyst in the exhaust passage. An exhaust air / fuel ratio detecting means for detecting or estimating an exhaust air / fuel ratio between the inlet and the catalytic converter in the exhaust passage;
When the exhaust air-fuel ratio is enriched by supplying a reducing gas from the fuel reformer, the enrichment means has a predetermined exhaust air-fuel ratio detected or estimated by the exhaust air-fuel ratio detection means. The exhaust gas purification apparatus for an internal combustion engine according to claim 3, wherein the supply amount of the reducing gas is adjusted so as to coincide with the target value.   The predetermined target value is obtained when the exhaust air-fuel ratio is enriched by supplying a reducing gas from the fuel reformer, and when the exhaust air-fuel ratio is not supplied by the fuel reformer. The exhaust gas purification apparatus for an internal combustion engine according to any one of claims 1 to 4, wherein the exhaust gas purification apparatus is different from a case where the engine is enriched.   The exhaust purification device of an internal combustion engine according to any one of claims 1 to 5, wherein the catalytic converter contains ceria, platinum, and rhodium.   The fuel reformer produces a reducing gas having a pressure higher than atmospheric pressure and mainly composed of carbon monoxide by a partial oxidation reaction between a hydrocarbon fuel and air. The exhaust gas purification apparatus for an internal combustion engine according to any one of 1 to 6.

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