JP2009191735A - Exhaust emission control device of internal combustion engine - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To ensure a high NO<SB>x</SB>conversion rate using a NO<SB>x</SB>selective reduction catalyst. <P>SOLUTION: A NO<SB>x</SB>selective reducing catalyst 15 is disposed in an engine exhaust emission passage. A particulate filter 13 is disposed upstream of the NO<SB>x</SB>selective reducing catalyst 15 in the engine exhaust emission passage. A urea aqueous solution is supplied to the NO<SB>x</SB>selective reducing catalyst 15. Ammonia generated from the urea aqueous solution selectively reduces NO<SB>x</SB>contained in exhaust gas. A SO<SB>x</SB>trap catalyst 12 capable of capturing SO<SB>x</SB>contained in the exhaust gas is disposed in the engine exhaust passage upstream of the particulate filter 13. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an exhaust emission control device for an internal combustion engine.

An NO _x selective reduction catalyst is arranged in the engine exhaust passage, an oxidation catalyst is arranged in the engine exhaust passage upstream of the NO _x selective reduction catalyst, and NO contained in the exhaust gas is converted to NO ₂ by this oxidation catalyst, An internal combustion engine in which urea is supplied to an NO _x selective reduction catalyst and NO _x contained in exhaust gas is selectively reduced by ammonia generated from the urea is known (see, for example, Patent Document 1). Thus, when the selectively reducing NO _X by _the NO _X selective reducing catalyst, the ratio of NO ₂ with respect to NO _X in the exhaust gas can be purified best NO _X when the 50%. However, most of the NO _x contained in the exhaust gas is NO. Therefore, in this internal combustion engine, NO is converted into NO ₂ by an oxidation catalyst so that the ratio of NO ₂ is as close to 50 percent as possible.
JP 2006-512529 A

However poisoned oxidation catalyst by SO _X contained in the exhaust gas as this time the internal combustion engine has elapsed, the results NO to function for converting to NO ₂ is weakened gradually _the NO _X selective reducing catalyst by NO _X purification There is a problem that the rate gradually decreases.

In order to solve the above problem, according to the present invention, the NO _x selective reduction catalyst is arranged in the engine exhaust passage, the exhaust purification element having an oxidation function is arranged in the engine exhaust passage upstream of the NO _x selective reduction catalyst, In an exhaust gas purification apparatus for an internal combustion engine in which urea is supplied to an NO _x selective reduction catalyst and NO _x contained in the exhaust gas is selectively reduced by ammonia generated from the urea, the engine exhaust upstream of the exhaust purification element An SO _X trap catalyst capable of capturing SO _X contained in the exhaust gas is disposed in the passage.

Since the exhaust purification element is prevented from receiving SO _x poisoning, a high NO _x purification rate by the NO _x selective reduction catalyst can be ensured over a long period of time.

FIG. 1 shows an overall view of a compression ignition type internal combustion engine.
Referring to FIG. 1, 1 is an engine body, 2 is a combustion chamber of each cylinder, 3 is an electronically controlled fuel injection valve for injecting fuel into each combustion chamber 2, 4 is an intake manifold, and 5 is an exhaust manifold. Respectively. The intake manifold 4 is connected to the outlet of the compressor 7 a of the exhaust turbocharger 7 via the intake duct 6, and the inlet of the compressor 7 a is connected to the air cleaner 9 via the intake air amount detector 8. A throttle valve 10 driven by a step motor is disposed in the intake duct 6, and a cooling device 11 for cooling intake air flowing through the intake duct 6 is disposed around the intake duct 6. In the embodiment shown in FIG. 1, the engine cooling water is guided into the cooling device 11, and the intake air is cooled by the engine cooling water.

On the other hand, the exhaust manifold 5 is connected to the inlet of the exhaust turbine 7 b of the exhaust turbocharger 7, and the outlet of the exhaust turbine 7 b is connected to the inlet of the SO _X trap catalyst 12. Further, the outlet of the SO _X trap catalyst 12 is connected to the inlet of an exhaust purification element 13 having an oxidation function, and the outlet of the exhaust purification element 13 is connected via an exhaust pipe 14 when the air-fuel ratio of the exhaust gas is lean. It is coupled to _the NO _X selective reducing catalyst 15 capable of selectively reducing NO _X in the exhaust gas by the ammonia. In the embodiment shown in FIG. 1, the exhaust purification element 13 comprises a particulate filter carrying an oxidation catalyst such as platinum Pt.

A urea water supply valve 16 is disposed in the exhaust pipe 14 upstream of the NO _X selective reduction catalyst 15, and this urea water supply valve 16 is connected to a urea water tank 19 via a supply pipe 17 and a supply pump 18. When the urea water is to be supplied, the urea water stored in the urea water tank 19 is injected from the urea water supply valve 16 into the exhaust gas flowing in the exhaust pipe 14 by the supply pump 18, and the ammonia generated from the urea at this time NO _X contained in the exhaust gas is reduced in the NO _X selective reduction catalyst 15 by ((NH ₂ ) ₂ CO + H ₂ O → 2NH ₃ + CO ₂ ).

  The exhaust manifold 5 and the intake manifold 4 are connected to each other via an exhaust gas recirculation (hereinafter referred to as EGR) passage 20, and an electronically controlled EGR control valve 21 is disposed in the EGR passage 20. A cooling device 22 for cooling the EGR gas flowing in the EGR passage 20 is disposed around the EGR passage 20. In the embodiment shown in FIG. 1, the engine cooling water is guided into the cooling device 22, and the EGR gas is cooled by the engine cooling water. On the other hand, each fuel injection valve 3 is connected to a common rail 24 via a fuel supply pipe 23, and this common rail 24 is connected to a fuel tank 26 via an electronically controlled variable discharge fuel pump 25. The fuel stored in the fuel tank 26 is supplied into the common rail 24 by the fuel pump 25, and the fuel supplied into the common rail 24 is supplied to the fuel injection valve 3 through each fuel supply pipe 23.

As described above, most of the NO _x discharged from the combustion chamber 2 during engine operation is NO. On the other hand, in the NO _x selective reduction catalyst 15, NO _x contained in the exhaust gas is selectively reduced by ammonia NH ₃ generated from urea. At this time, the fastest reaction formula is shown by the following formula.
NO + NO ₂ + 2NH ₃ → 2N ₂ + 3H ₂ O
From the above equation, the ratio of NO and NO ₂ in the exhaust gas 1: When 1, i.e. the ratio of NO ₂ with respect to the exhaust gas (NO + NO _2), in other words the ratio of NO ₂ with respect to NO _X in the exhaust gas It can be seen that the reaction rate is the fastest when NO is 50%, and therefore the NO _x purification rate is the highest.

Incidentally, when NO ₂ in the exhaust gas is excessive, for example, the excess of NO ₂ is reacted according to the following equation with a slow reaction rate,
6NO ₂ + 8NH ₃ → 7N ₂ + 12H ₂ O
When the NO in the exhaust gas is excessive, for example, the excess NO is reacted according to the following equation with a slow reaction rate.
6NO + 4NH ₃ → 5N ₂ + 6H ₂ O
Thus when the reaction rate NO _X purification rate is lowered.

As described above, the particulate filter 13 carries the oxidation catalyst, and the amount of the oxidation catalyst carried is 1: as long as the ratio of NO to NO ₂ in the exhaust gas flowing into the NO _x selective reduction catalyst 15 is as high as possible. The carrying amount is close to 1. Therefore, a good NO _x purification rate can be obtained in the NO _x selective reduction catalyst 15.

However, the exhaust gas contains SO _X, oxidation catalyst this SO _X is carried on the particulate filter 13 when flowing into the particulate filter 13 is poisoned by the SO _X. As a result, the function of converting NO in the exhaust gas into NO ₂ is lowered, and thus the NO _x purification rate by the NO _x selective reduction catalyst 15 is lowered.

Therefore, in this embodiment of the present invention to capture SO _X contained in the exhaust gas by this _the SO _X trap catalyst 12 by arranging _the SO _X trap catalyst 12 upstream of the particulate filter 13, SO thereby the particulate filter 13 _X is prevented from flowing in. Next, the SO _X trap catalyst 12 will be described.

The SO _X trap catalyst 12 is made of, for example, a monolith catalyst having a honeycomb structure, and has a large number of exhaust gas flow holes extending straight in the axial direction of the SO _X trap catalyst 12. FIG. 2 schematically shows a cross section of the surface portion of the base 30 of the SO _X trap catalyst 12. As shown in FIG. 2, a coat layer 31 is formed on the surface of the substrate 30, and a noble metal catalyst 32 is dispersed and supported on the surface of the coat layer 31.

In the embodiment shown in FIG. 2, platinum is used as the noble metal catalyst 32, and the components constituting the coating layer 31 are alkali metals such as potassium K, sodium Na and cesium Cs, barium Ba and calcium Ca. At least one selected from rare earths such as alkaline earth, lanthanum La, and yttrium Y is used. That is, the coat layer 31 of the SO _X trap catalyst 12 exhibits strong basicity.

Now, SO _x contained in the exhaust gas, that is, SO ₂ is oxidized in platinum Pt 32 as shown in FIG. 2 and then trapped in the coating layer 31. That is, SO ₂ diffuses into the coat layer 31 in the form of sulfate ions SO ₄ ^2- to form sulfate. As described above, the coat layer 31 has a strong basicity. Therefore, a part of SO ₂ contained in the exhaust gas is directly captured in the coat layer 31 as shown in FIG.

Shading shows the concentration of trapped SO _X in the coat layer 31 in FIG. 2. As can be seen from FIG. 2, the SO _X concentration in the coat layer 31 is highest near the surface of the coat layer 31, and gradually decreases toward the back. When the SO _X concentration in the vicinity of the surface of the coat layer 31 is increased, the basicity of the surface of the coat layer 31 is weakened and the SO _X capturing ability is weakened. However, in this case, the SO _X trap rate recovers when the temperature of the SO _X trap catalyst 12 rises while the air-fuel ratio of the exhaust gas is lean.

That, SO _X air temperature of lean under _the SO _X trap catalyst 12 is present in a concentrated manner in the vicinity of the surface of the coating layer 31 when increase in the exhaust gas from the uniform SO _X concentration in the coat layer 31 It spreads toward the back of the coat layer 31 so as to be. That is, the nitrate produced in the coat layer 31 changes from an unstable state concentrated near the surface of the coat layer 31 to a stable state uniformly dispersed throughout the coat layer 31. SO _X present near the surface of the coating layer 31 is reduced SO _X concentration near the surface of the coating layer 31 when dispersed toward the inner portion of the coating layer 31, _the SO _X trap of _the SO _X trap catalyst 12 and thus The rate will recover.

Accordingly, the SO _X trap rate is recovered every time the temperature of the SO _X trap catalyst 12 becomes higher during engine operation, and therefore SO _X always flows into the particulate filter 13 by the SO _X trap catalyst 12. Be blocked. As a result, a high NO _x purification rate by the NO _x selective reduction catalyst 15 can be ensured.

FIG. 3 shows another embodiment. It is composed of an oxidation catalyst the exhaust purification catalyst 13 having an oxidation function is a noble metal catalyst such as platinum Pt in this embodiment, the oxidation catalyst 13 by _the SO _X trap catalyst 12 is subjected to a SO _X poisoning in this embodiment Is blocked. Also in this embodiment, the amount of the precious metal catalyst supported on the oxidation catalyst is such that the ratio of NO to NO ₂ in the exhaust gas flowing into the NO _x selective reduction catalyst 15 is as close to 1: 1 as possible. It is said that. Therefore, a good NO _x purification rate can be obtained in the NO _x selective reduction catalyst 15.

On the other hand, in this embodiment, the outlet of the NO _x selective reduction catalyst 15 is connected to the inlet of the particulate filter 28 via the exhaust pipe 27. That is, in this embodiment, the particulate filter 28 is disposed in the engine exhaust passage downstream of the NO _x selective reduction catalyst 15.

  In this embodiment, the particulate filter 28 is provided with a temperature raising means for raising the temperature of the particulate filter 28 during regeneration of the particulate filter 28. As the temperature raising means, a method of arranging a heater at the inlet of the particulate filter 28 is conceivable, and a method of supporting the particulate filter 28 on a metal thin plate assembly constituting an electric heater is also conceivable. Further, as shown in FIG. 3, a reducing agent supply valve 29 is provided in the exhaust pipe 27, and the temperature of the particulate filter 28 is raised by the reducing agent supplied from the reducing agent supply valve 29, for example, the heat of oxidation reaction of the fuel. You can also.

In the embodiment shown in FIG. 3, when the air-fuel ratio of the exhaust gas flowing into the particulate filter 28 is lean, the air-fuel ratio of the exhaust gas that stores NO _x contained in the exhaust gas and flows in is the stoichiometric air-fuel ratio. Alternatively, a NO _x storage catalyst that releases the stored NO _x when it becomes rich can be supported. In this case, the exhaust gas _the NO _X storage catalyst flows into the NO _X storage catalyst prior to saturated with NO _X, that is, the air-fuel ratio of the exhaust gas flowing into the particulate filter 28 is made rich, whereby _the NO _X storage NO _x is released from the catalyst. As is shown in FIG. 3, for example in this case, the reducing agent supply means for supplying a reducing agent to the NO _X storage catalyst to release the NO _X from the NO _X storing catalyst, for example a reducing agent feed valve 29 is provided .

1 is an overall view of a compression ignition type internal combustion engine. FIG. 3 is a cross-sectional view of the surface portion of the base of the SO _X trap catalyst. It is a general view which shows another Example of a compression ignition type internal combustion engine.

Explanation of symbols

4 Intake manifold 5 Exhaust manifold 7 Exhaust turbocharger 12 SO _X trap catalyst 13 Exhaust purification element 15 NO _X selective reduction catalyst 16 Urea water supply valve

Claims (6)

The _the NO _X selective reducing catalyst disposed in an engine exhaust passage, arranged an exhaust purification element having an oxidation function in _the NO _X selective reducing catalyst in the engine exhaust passage upstream of, the urea supplying urea to _the NO _X selective reducing catalyst In the exhaust gas purification apparatus for an internal combustion engine in which NO _x contained in the exhaust gas is selectively reduced by ammonia generated from the exhaust gas, SO _x contained in the exhaust gas is contained in the engine exhaust passage upstream of the exhaust purification element. An exhaust purification system for an internal combustion engine in which a trapped SO _X trap catalyst is arranged.   2. The exhaust gas purification apparatus for an internal combustion engine according to claim 1, wherein the exhaust gas purification element comprises a particulate filter carrying an oxidation catalyst.   The exhaust gas purification device for an internal combustion engine according to claim 1, wherein the exhaust gas purification element comprises an oxidation catalyst. The exhaust purification device for an internal combustion engine according to claim 3, wherein a particulate filter is disposed in the engine exhaust passage downstream of the NO _x selective reduction catalyst.   The exhaust gas purification apparatus for an internal combustion engine according to claim 4, wherein a temperature raising means is provided in the particulate filter for raising the temperature of the particulate filter. The particulate filter occludes NO _x contained in the exhaust gas when the air-fuel ratio of the inflowing exhaust gas is lean, and releases the occluded NO _x when the air-fuel ratio of the inflowing exhaust gas becomes the stoichiometric air-fuel ratio or rich. the NO _X storing catalyst carries a, internal combustion according to claim 4, which comprises a reducing agent supply means for supplying a reducing agent to the NO _X storage catalyst to release the NO _X from the NO _X storing catalyst Engine exhaust purification system.

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