JP4019867B2 - Exhaust gas purification device for internal combustion engine - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an exhaust emission control device for an internal combustion engine.
[0002]
[Prior art]
NO contained in exhaust gas when combustion is performed under lean air-fuel ratio_X As a catalyst for purifying NO, an alkali metal or alkaline earth NO on the surface of a support made of alumina_X A catalyst in which an absorbent layer is formed and a noble metal catalyst such as platinum is supported on the surface of the support is known (see, for example, Patent Document 1). In this catalyst, when the air-fuel ratio of the exhaust gas is lean, NO contained in the exhaust gas_X Is oxidized by platinum and in the form of nitrates NO_X Absorbed in the absorbent. Next, when the air-fuel ratio of the exhaust gas is rich for a short time, NO_X NO absorbed by the absorbent_X NO is released and then reduced, and then the air-fuel ratio of the exhaust gas is returned to lean again._X NO to absorbent_X The absorption action of is started.
[0003]
On the other hand, the exhaust gas contains SO._X Is also included, NO_X NO for absorbent_X In addition to SO_X Is also absorbed. In this case SO_X Is absorbed in the form of sulfate. However, this sulfate is harder to decompose than nitrate, and is not decomposed simply by making the exhaust gas air-fuel ratio rich. Therefore NO_X SO in the absorbent_X As the amount of NO absorbed increases gradually, NO_X Cannot absorb. Therefore, such NO_X Occasional SO when absorbent is used_X Need to be released. By the way, sulfates are easily decomposed when the temperature of the catalyst is 600 ° C. or higher. At this time, if the air-fuel ratio of the exhaust gas is made rich, NO_X SO to SO_X Is released. Therefore, such NO_X NO when using absorbent_X SO to SO_X Is released, the temperature of the catalyst is maintained at 600 ° C. or higher and the air-fuel ratio of the exhaust gas is maintained rich.
[0004]
Now, NO like this_X NO with the absorbent layer_X In addition to SO_X Is absorbed, so SO_X To prevent it from being absorbed_X It is sufficient that no absorbent layer is provided. Therefore, a catalyst in which only platinum is supported on a support made of alumina has been proposed (see Patent Document 2). In Patent Document 2, even when only platinum is supported on a support made of alumina, the catalyst is NO when the air-fuel ratio is lean._X Is captured, and if the air-fuel ratio is switched between lean and rich alternately, NO_X It can be purified.
[0005]
Also, NO generated when combustion is performed under a lean air-fuel ratio_X As a catalyst that can purify NO, lean NO on which transition metal or noble metal is supported on zeolite_X Catalysts are known. This lean NO_X The catalyst is HC and NO in the exhaust gas._X Absorbs NO_X Has the function of reducing oxygen, but NO adsorbs oxygen_X The purification performance of the remarkably deteriorates. Therefore, in order to release this adsorbed oxygen, lean NO_X An internal combustion engine in which the air-fuel ratio of exhaust gas flowing into a catalyst is periodically made rich is known (see Patent Document 3). This lean NO_X The catalyst is NO even when burning under a lean air-fuel ratio_X Can be reduced, but NO in the exhaust gas_X It is necessary to supply HC for reduction, which has the disadvantage that the heat resistance is low and only a purification rate of 50% or less can be obtained.
[0006]
[Patent Document 1]
Japanese Patent No. 2600492
[Patent Document 2]
JP-A-11-285624
[Patent Document 3]
Japanese Patent No. 3154110
[0007]
[Problems to be solved by the invention]
  Now, we have NO on the carrier._XWhile the research on the catalyst which formed the layer of absorbent is advanced, NO on the support_XResearch has also been carried out on catalysts that do not have an absorbent layer. As a result, NO on the carrier_XIn a catalyst having no absorbent layer, for example, a catalyst in which only platinum is supported on a support made of alumina, the air-fuel ratio is temporarily reduced when combustion is performed under a lean air-fuel ratio.TargetWhen rich, the catalyst temperature is 90% or more at low temperatures of 250 ° C or lower._XIt has been found that a purification rate can be obtained.
[0008]
As a result of studying the reason from various angles, the following conclusions were reached. That is, generally speaking, platinum inherently has activity at low temperatures, and NO contained in the exhaust gas._X Is decomposed directly on the surface of platinum or is selectively reduced. In addition, a base point exists on the surface of the support made of alumina, and NO oxidized on the surface of platinum._X Is NO₂ Adsorbed on the support surface in the form of nitrate ion NO_Three ^-It is held on the base point on the surface of the carrier in the form of NO_X When these purifications are performed, these various actions are performed simultaneously, and as a result, a high purification rate of 90% or more can be obtained.
[0009]
By the way, if a catalyst supporting only platinum on a support made of alumina is exposed to a lean air-fuel ratio exhaust gas, NO_X The purification rate gradually decreases. This is because the surface of the platinum is covered with oxygen atoms, i.e. the surface of the platinum is oxygen-poisoned, so on the one hand the NO on the platinum surface._X Direct decomposition or NO_X This is based on the fact that the selective reduction of the compound becomes difficult to occur. In fact, if the air-fuel ratio is temporarily rich at this time, oxygen atoms covering the platinum surface are consumed for oxidation of HC and CO, that is, oxygen poisoning on the platinum surface is eliminated, and then the air-fuel ratio is made lean. NO again when returned_X Direct decomposition and NO_X Is selectively reduced.
[0010]
On the other hand, if the platinum surface is covered with oxygen atoms, NO_X Is susceptible to oxidation on the platinum surface and is therefore adsorbed or retained on the support NO_X The amount of increases. Nevertheless NO_X The reduction in purification rate means NO_X NO for the cleansing action_X Direct decomposition or NO_X The selective reduction is dominant. Therefore, when only platinum is supported on a support made of alumina, it is most important to prevent the entire surface of platinum from being poisoned by oxygen, and therefore, before the entire surface of platinum is subjected to oxygen poisoning. It is necessary to temporarily switch the air-fuel ratio of the exhaust gas from lean to rich.
[0011]
When the air-fuel ratio of the exhaust gas is temporarily switched from lean to rich, NO adsorbed on the carrier_X Or nitrate ion NO retained on the carrier_Three ^-Is reduced by HC and CO. That is, when the air-fuel ratio of the exhaust gas is temporarily switched from lean to rich to eliminate oxygen poisoning on the platinum surface, NO adsorbed or held on the carrier_X NO is removed, so NO again when the air-fuel ratio is returned from rich to lean_X Adsorption or nitrate ion NO_Three ^-The holding action is started.
[0012]
As described above, NO is supported when only platinum is supported on a support made of alumina._X In order to secure a high purification rate, it is necessary to prevent the entire surface of platinum from causing oxygen poisoning. However, neither of Patent Documents 2 and 3 suggests anything about this. That is, Patent Document 2 describes NO._X Is all purified_X Shows the results of an examination based on the premise that it is caused by the adsorption action of platinum._X Not aware that it will dominate the purification rate. Therefore, as a matter of course, Patent Document 2 does not suggest that a high purification rate can be obtained at a low temperature of 250 ° C. or lower.
[0013]
Reference 3 is a lean NO made of zeolite._X Targeting catalyst, this lean NO_X NO adsorption on the catalyst is NO_X Although it is disclosed that the purification rate is affected, oxygen poisoning on the platinum surface is NO._X There is no suggestion about controlling the purification rate. Since this zeolite has no base point, NO is different from when alumina is used._X Not only the way of purification is different but also more than 50% NO_X It is difficult to obtain a purification rate. Therefore, Patent Document 3 cannot be a document that suggests that a high purification rate of 90% or more can be obtained at 250 ° C. or lower.
[0014]
In the present invention, oxygen poisoning of the platinum surface, that is, the noble metal surface is NO._X To control the purification rate of NO, and based on this, high NO_X An object of the present invention is to provide an exhaust gas purification device for an internal combustion engine that ensures a purification rate.
[0015]
[Means for Solving the Problems]
  That is, in the first invention, NO generated when combustion is performed under a lean air-fuel ratio._XIn the exhaust gas purification apparatus for an internal combustion engine that is purified by an exhaust gas purification catalyst disposed in the exhaust passage, a carrier having a base point on the carrier surface is used as the catalyst carrier of the exhaust gas purification catalyst, NO_XCan absorb NO_XDisperse and carry the noble metal catalyst without forming an absorbent layer,Provided with means for calculating an oxygen poisoning amount that is proportional to the oxygen concentration in the exhaust gas and increases as the temperature of the exhaust purification catalyst increases, based on the oxygen concentration in the exhaust gas and the temperature of the exhaust purification catalyst When the amount of oxygen poisoning exceeds a predetermined toleranceThe air-fuel ratio of the exhaust gas is temporarily switched from lean to rich.
[0037]
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 shows a case where the present invention is applied to a compression ignition type internal combustion engine. The present invention can also be applied to a spark ignition type internal combustion engine.
[0038]
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 through the intake duct 6, and the inlet of the compressor 7 a is connected to the air cleaner 8. A throttle valve 9 driven by a step motor is arranged in the intake duct 6, and a cooling device 10 for cooling intake air flowing in the intake duct 6 is arranged around the intake duct 6. In the embodiment shown in FIG. 1, the engine cooling water is guided into the cooling device 10 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 casing 12 containing the exhaust purification catalyst 11. A reducing agent supply valve 13 for supplying a reducing agent made of, for example, hydrocarbons into the exhaust gas flowing through the exhaust manifold 5 is disposed at the outlet of the collecting portion of the exhaust manifold 5.
[0039]
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 14, and an electronically controlled EGR control valve 15 is disposed in the EGR passage 14. A cooling device 16 for cooling the EGR gas flowing in the EGR passage 14 is disposed around the EGR passage 14. In the embodiment shown in FIG. 1, the engine cooling water is guided into the cooling device 16 and the EGR gas is cooled by the engine cooling water. On the other hand, each fuel injection valve 3 is connected to a fuel reservoir, so-called common rail 18 via a fuel supply pipe 17. Fuel is supplied into the common rail 18 from an electronically controlled fuel pump 19 with variable discharge amount, and the fuel supplied into the common rail 18 is supplied to the fuel injection valve 3 via each fuel supply pipe 17.
[0040]
The electronic control unit 30 is composed of a digital computer, and is connected to each other by a bidirectional bus 31. A ROM (read only memory) 32, a RAM (random access memory) 33, a CPU (microprocessor) 34, an input port 35 and an output port 36. It comprises. A temperature sensor 20 for detecting the temperature of the exhaust purification catalyst 11 is attached to the exhaust purification catalyst 11, and an output signal of this temperature sensor 20 is input to the input port 35 via the corresponding AD converter 37. Various sensors 22 are arranged in the exhaust pipe 21 connected to the outlet of the casing 12 as necessary. A load sensor 41 that generates an output voltage proportional to the depression amount L of the accelerator pedal 40 is connected to the accelerator pedal 40, and the output voltage of the load sensor 41 is input to the input port 35 via the corresponding AD converter 37. . Further, the input port 35 is connected to a crank angle sensor 42 that generates an output pulse every time the crankshaft rotates, for example, 15 °. On the other hand, the output port 36 is connected to the fuel injection valve 3, the throttle valve driving step motor 9, the EGR control valve 15, and the fuel pump 19 through corresponding drive circuits 38.
[0041]
The exhaust purification catalyst 11 shown in FIG. 1 is composed of a monolith catalyst, and a catalyst carrier is supported on the base of the exhaust purification catalyst 11. FIG. 2 schematically shows a cross section of the surface portion of the catalyst carrier 50. As shown in FIG. 2, the noble metal catalyst 51 is dispersed and supported on the surface of the catalyst carrier 50. In the present invention, a carrier having a basic point showing basicity on the surface of the carrier 50 is used as the catalyst carrier 50, and alumina is used as the catalyst carrier 50 in the embodiment according to the present invention. In the embodiment according to the present invention, platinum is used as the noble metal catalyst 51.
[0042]
Thus, in the embodiment according to the present invention, only the platinum 51 is supported on the surface of the catalyst carrier 50 made of alumina, and NO made of alkali metal or alkaline earth._X Can absorb NO_X The absorbent layer is not formed. Thus, as a result of examining the exhaust purification catalyst 11 in which only the platinum 51 is supported on the surface of the catalyst carrier 50 made of alumina, the exhaust purification catalyst 11 is emptied when combustion is performed under a lean air-fuel ratio. When the fuel ratio is temporarily rich, the exhaust purification catalyst 11 has a temperature of 90% or more when the temperature of the exhaust purification catalyst 11 is a low temperature of 250 ° C. or lower._X It has been found that a purification rate can be obtained.
[0043]
As a result of examining the reason from various angles, NO_X When purification of NO is performed, NO on the surface of the platinum 51_X Direct decomposition of NO or NO_X Selective reduction action and NO on the catalyst carrier 50_X Adsorption or NO on the catalyst carrier 50_X Are maintained simultaneously in parallel, and these effects occur simultaneously in parallel._X The conclusion was reached that a purification rate could be obtained.
[0044]
That is, platinum 51 inherently has activity at low temperatures, and NO._X The first action that occurs when purifying the exhaust gas is that NO in the exhaust gas when the air-fuel ratio of the exhaust gas is lean_X Is adsorbed on the surface of the platinum 51 in a state of being dissociated into N and O on the surface of the platinum 51, and the dissociated N becomes N₂ And desorbing from the surface of the platinum 51, that is, NO_X It is a direct decomposition action. This direct decomposition action causes some NO_X The purifying action is performed.
[0045]
NO_X The second action that occurs when the purification of NO is performed is that NO adsorbed on the surface of the platinum 51 when the air-fuel ratio of the exhaust gas is lean is adsorbed on the HC in the exhaust gas or the catalyst carrier 50. This is an action that is selectively reduced by HC. This NO_X Some NO by selective reduction action_X The purifying action is performed.
[0046]
On the other hand, NO in exhaust gas_X That is, NO is oxidized on the surface of the platinum 51 and NO.₂ When further oxidized, nitrate ion NO_Three ^-It becomes. NO_X The third effect that occurs when the purification of NO is performed is NO₂ Is the action of adsorbing on the catalyst carrier 50. This adsorption action causes some NO_X The purifying action is performed. Further, a base point exists on the surface of the catalyst carrier 50 made of alumina, and NO._X The fourth action that occurs when the purification of NO is performed is nitrate ion NO_Three ^-Is the action held at the base point on the surface of the catalyst carrier 10. This holding action causes some NO_X The purifying action is performed.
[0047]
NO like this_X When these purifications are performed, these various actions are performed simultaneously, and as a result, a high purification rate of 90% or more can be obtained.
[0048]
By the way, if the exhaust purification catalyst 11 carrying only the platinum 51 on the catalyst carrier 50 made of alumina is exposed to the lean air-fuel ratio exhaust gas, NO._X The purification rate gradually decreases. This is because the surface of the platinum 51 is covered with oxygen atoms, that is, the surface of the platinum 51 is poisoned by oxygen, so that the NO on the surface of the platinum 51 is reduced._X Direct decomposition and NO_X This is based on the fact that the selective reduction of the compound becomes difficult to occur. That is, if the surface of the platinum 51 is covered with oxygen atoms, NO in the exhaust gas cannot be adsorbed on the surface of the platinum 51._X The direct decomposition of NO becomes difficult, and if the surface of platinum 51 is covered with oxygen atoms, NO cannot be adsorbed on the surface of platinum 51._X Is less likely to occur.
[0049]
However, if the air-fuel ratio is temporarily made rich at this time, oxygen atoms covering the surface of the platinum 51 are consumed for the oxidation of HC and CO, that is, oxygen poisoning on the surface of the platinum 51 is eliminated, and thus the air-fuel ratio is reduced. NO again when leaned back_X Direct decomposition and NO_X Is selectively reduced.
[0050]
By the way, if the surface of platinum 51 is covered with oxygen atoms, NO_X Is likely to be oxidized on the surface of the platinum 51, and therefore is adsorbed or retained on the catalyst carrier 50._X The amount of increases. Nevertheless NO_X The reduction in purification rate means NO_X NO for the cleansing action_X Direct decomposition or NO_X The selective reduction is dominant. Therefore, when only the platinum 51 is supported on the catalyst support 50 made of alumina, it is the most important issue to prevent the entire surface of the platinum 51 from causing oxygen poisoning. Before receiving poison, it is necessary to temporarily switch the air-fuel ratio of the exhaust gas from lean to rich.
[0051]
Next, this will be described with reference to experimental results.
[0052]
FIG. 3 shows that the reducing agent is injected from the reducing agent supply valve 13 for the time t1 with a time interval of t2 hours, whereby the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst 11 (intake passage, combustion chamber 2 and exhaust purification). The ratio of the amount of air supplied to the exhaust passage upstream of the catalyst and the amount of fuel and reducing agent) is maintained lean for t2 hours and then rich for t1 hours.
[0053]
FIG. 4 shows the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst 11 before the entire surface of the platinum 51 is subjected to oxygen poisoning in the exhaust purification catalyst 11 carrying only the platinum 51 on the catalyst carrier 50 made of alumina. The temperature TC (° C.) and NO of the exhaust purification catalyst 11 when the engine is temporarily switched from rich to temporarily for t1 time shown in FIG._X The relationship with the purification rate (%) is shown. FIG. 4 shows the case where the coating amount of the catalyst carrier 50 made of alumina is 150 (g) and the loading amount of the platinum 51 is 3 (g).
[0054]
From FIG. 4, the temperature TC of the exhaust purification catalyst 11 is a low temperature of 250 ° C. or lower and NO is close to 90% or more and almost 100%._X It turns out that the purification rate is obtained. When the temperature TC of the exhaust purification catalyst 11 becomes 200 ° C. or lower, NO_X Although the purification rate is slightly reduced, NO is maintained even if the temperature TC of the exhaust purification catalyst 11 is reduced to 150 ° C._X It can be seen that the purification rate is 80% or more and is still high. Further, when the temperature TC of the exhaust purification catalyst 11 becomes higher than 250 ° C., NO_X The purification rate gradually decreases. That is, when the temperature TC of the exhaust purification catalyst 11 becomes high, NO becomes difficult to adsorb on the surface of the platinum 51, and as a result, the NO_X Not only is the direct decomposition action difficult to occur, but also NO_X NO is also less likely to occur, so NO_X The purification rate gradually decreases.
[0055]
Even if the amount of platinum 51 supported exceeds 3 (g), NO_X The purification rate hardly increases, but if the amount of platinum 51 supported is less than 3 (g), NO_X The purification rate decreases.
[0056]
FIG. 4 shows a case where the lean period t2 in which the air-fuel ratio of the exhaust gas is lean is 60 seconds and the rich time t1 in which the air-fuel ratio of the exhaust gas is rich is 3 seconds in FIG. In this case, if the rich time t1 is 3 seconds, the oxygen poisoning on the surface of the platinum 51 can be completely eliminated. From the viewpoint of eliminating the oxygen poisoning, it is meaningful to set the rich time t1 to 3 seconds or more. There is no. On the other hand, if the rich time t1 is shorter than 3 seconds, NO_X The purification rate gradually decreases.
[0057]
Further, as the noble metal catalyst 51, rhodium can be used in addition to platinum. In this case, NO in FIG._X The region of temperature TC (° C) at which the purification rate is 90% or more spreads to the high temperature side, and NO on the high temperature side_X The purification rate increases.
[0058]
Thus, if the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst 11 is temporarily switched from lean to rich before the entire surface of the noble metal catalyst 51 is subjected to oxygen poisoning, 90% or more of NO._X A purification rate can be obtained. If the air-fuel ratio of the exhaust gas is temporarily switched from lean to rich as described above, NO adsorbed on the catalyst carrier 50 is absorbed.₂ Alternatively, nitrate ion NO held on the catalyst carrier 50_Three ^-Is reduced by HC and CO. That is, when the air-fuel ratio of the exhaust gas is temporarily switched from lean to rich in order to eliminate oxygen poisoning on the surface of the noble metal catalyst 51, NO adsorbed or held on the catalyst carrier 50 is retained._X NO is removed, so NO again when the air-fuel ratio is returned from rich to lean₂ Adsorption or nitrate ion NO_Three ^-The holding action is started.
[0059]
As described above, when only the platinum 51 is supported on the catalyst carrier 50 made of alumina, NO._X NO against the purification rate_X Decomposition of NO and NO_X The selective reduction of becomes dominant. However, NO to the catalyst carrier 50₂ Adsorption and nitrate ion NO on the catalyst support 50_Three ^-NO retention is also NO_X It contributes to the purification. By the way, NO in exhaust gas than before₂ More or less NO with any catalyst₂ Is known to adsorb to the catalyst. In the embodiment according to the present invention, as described above, NO in the exhaust gas is oxidized in the platinum 51 to be NO.₂ Is thus generated and thus NO₂ Is adsorbed on the exhaust purification catalyst 11.
[0060]
In contrast, nitrate ion NO_Three ^-Is not retained in any catalyst, nitrate ion NO_Three ^-In order to keep the catalyst on the catalyst, the surface of the catalyst must be basic. In the embodiment according to the present invention, since the catalyst carrier 50 is made of alumina as described above, a basic point having basicity exists on the surface of the catalyst carrier 50, and thus, nitrate ion NO._Three ^-Is held at the base point present on the surface of the catalyst carrier 50.
[0061]
By the way, the basicity of the base point existing on the surface of the catalyst carrier 50 made of alumina is not so strong, and therefore nitrate ion NO._Three ^-The holding power against is not so strong. Therefore, when the temperature TC of the exhaust purification catalyst 11 rises, the NO held in the exhaust purification catalyst 11_X Is desorbed from the exhaust purification catalyst 11. As shown in FIG. 4, NO increases as the temperature TC of the exhaust purification catalyst 11 increases._X It is such NO that the purification rate gradually decreases._X This is also because of the desorption action of.
[0062]
On the other hand, the higher the basicity of the base point on the surface of the catalyst carrier 50, the more nitrate ion NO._Three ^-NO held in the form of_X The amount increases. Therefore, the NO held on the exhaust purification catalyst 11_X In order to increase the amount, the number of base points should be increased or the basicity of the base points should be increased. In this case, an alkali metal such as potassium K, sodium Na, lithium Li, cesium Cs, and rubidium Rb, barium Ba, calcium Ca, and strontium are disposed inside the catalyst support 50 made of alumina as indicated by reference numeral 52 in FIG. Adding at least one selected from alkaline earths such as Sr, rare earths such as lanthanum La, and yttrium Y can increase the number of base points or increase the basicity of the base points. Can do. In this case, the additive 52 such as lanthanum La or barium Ba can be added to the inside of the catalyst carrier 50 so as to constitute a part of the crystal structure of alumina for the purpose of structural stabilization, or the alumina and additive. It can also be added to the inside of the catalyst carrier 50 so as to form a salt with 52. As a matter of course, if the amount of the additive 52 such as lanthanum La or barium Ba is increased, the NO held by the exhaust purification catalyst 11 when the air-fuel ratio of the exhaust gas is lean._X The amount increases.
[0063]
On the other hand, when the basicity of the base point is increased, nitrate ion NO_Three ^-Holding power against becomes stronger. Therefore, nitrate ion NO_Three ^-Is difficult to detach even when the temperature TC of the exhaust purification catalyst 11 rises. Therefore, if the basicity of the base point is increased, NO on the high temperature side in FIG._X The purification rate increases.
[0064]
By the way, the exhaust gas contains SO.₂ Is also included, this SO₂ Is oxidized in platinum 51 and SO_Three It becomes. Then this SO_Three Is further oxidized on platinum 51 and sulfate ions SO_Four ^2- It becomes. If the catalyst is basic, sulfate ion SO_Four ^2- Is retained on the catalyst, and this sulfate ion SO_Four ^2- Is nitrate ion NO_Three ^-It is easier to hold on the catalyst than Therefore, nitrate ion NO_Three ^-Sulfate is retained on the catalyst._Four ^2- Is always retained on the catalyst. In an embodiment according to the present invention, nitrate ion NO_Three ^-Is retained on the catalyst support 50 and, therefore, in the embodiment according to the invention, the sulfate ion SO_Four ^2- Is also held on the catalyst carrier 50.
[0065]
On the other hand, as described at the beginning, NO consisting of alkali metal or alkaline earth metal on the catalyst support._X When the absorbent layer is formed, SO_X Is NO_X Sulfate forms in the absorbent layer. However, this sulfate is difficult to decompose, and unless the air-fuel ratio of the exhaust gas is made rich with the catalyst temperature raised to 600 ° C. or higher, the SO_X Cannot be released from the catalyst.
[0066]
However, the basicity of the base point existing on the surface of the catalyst support 50 made of alumina in the embodiment of the present invention is NO._X Very low compared to the basicity of the absorbent, so SO_X Is not in the form of sulfate at the base point on the surface of the catalyst carrier 50, but sulfate ion SO._Four ^2- Is held in the form of In this case, sulfate ion SO_Four ^2- The holding power against is quite small.
[0067]
In this way, sulfate ion SO_Four ^2- Sulfate ion SO when holding power against_Four ^2- Decomposes at low temperatures and becomes detached. In fact, in the embodiment according to the present invention, if the temperature TC of the exhaust purification catalyst 11 is raised to about 500 ° C. and the air-fuel ratio of the exhaust gas is made rich, the SO retained in the exhaust purification catalyst 11 is increased._X Can be released from the exhaust purification catalyst 11.
[0068]
As described above, NO on the catalyst carrier_X When the absorbent layer is formed, the catalyst temperature must be raised to 600 ° C or higher._X NO from absorbent_X Is not released. However, in a compression ignition type internal combustion engine having a low exhaust gas temperature, the temperature of the catalyst is raised to 600 ° C. or more and SO_X It is quite difficult to maintain the temperature of the catalyst at 600 ° C. or higher while the release action is being performed. However, even in a compression ignition type internal combustion engine having a low exhaust gas temperature, it is easy to raise the temperature of the catalyst to about 500 ° C., and it is easy to maintain the temperature of the catalyst at about 500 ° C. That is, SO_X It is very significant that the temperature of the catalyst required to release the catalyst can be lowered from 600 ° C. or higher to almost 500 ° C. In the present invention, when only the platinum 51 is supported on the surface of the catalyst carrier 50 made of alumina, if the entire surface of the platinum 51 is not subjected to oxygen poisoning, the NO is extremely high._X It has great significance in finding that a purification rate can be obtained, but SO_X It is also very significant to have found that the temperature of the exhaust purification catalyst 11 necessary for releasing the gas can be lowered to about 500 ° C.
[0069]
By the way, when the basicity of the basic point on the surface of the catalyst carrier 50 is increased by adding an additive 52 such as lanthanum La or barium Ba to the catalyst carrier 50 as described above, the catalyst carrier is used when the air-fuel ratio of the exhaust gas is lean. NO retained on 50_X The amount can be increased, and thus NO particularly on the high temperature side_X The purification rate can be increased. However, when the basicity of the base point on the surface of the catalyst carrier 50 is increased, the SO retained on the catalyst carrier 50 is increased._X The volume increases and SO_X Holding power against increases. As a result, SO_X Of the exhaust purification catalyst 11 required to release_X Release temperature increases.
[0070]
In this case, SO_X If the discharge temperature becomes too high, the temperature of the exhaust purification catalyst 11 is changed to SO._X It is difficult to raise the temperature to the release temperature._X There is a limit from the viewpoint of whether or not the release action can be performed. That is, the limit at which the temperature of the exhaust purification catalyst 11 can be raised relatively easily in a compression ignition type internal combustion engine is approximately 550 ° C. Therefore, when an additive 52 such as lanthanum La or barium Ba is added to the catalyst carrier 50. Is the amount of additive 52 added_X It is preferable that the discharge temperature is not more than an amount at which the discharge temperature is approximately 550 ° C.
[0071]
As the catalyst carrier 50, not only alumina but also various carriers conventionally known can be used as long as the carrier has a base point on the surface of the catalyst carrier.
[0072]
Next NO_X And SO_X This process will be described based on a specific embodiment.
[0073]
First, the oxygen poisoning amount of a noble metal catalyst such as platinum 51 is calculated, and when the calculated oxygen poisoning amount exceeds a predetermined allowable value, the air-fuel ratio of the exhaust gas is switched from lean to rich, A first embodiment in which the oxygen poisoning of the platinum 51 is thereby eliminated will be described.
[0074]
As shown in FIG. 5A, the oxygen poisoning amount W of platinum 51 per unit time is proportional to the oxygen concentration in the exhaust gas. Further, as shown in FIG. 5B, the oxygen poisoning amount W of platinum 51 per unit time increases as the temperature of the exhaust purification catalyst 11 increases. Here, the oxygen concentration in the exhaust gas and the temperature of the exhaust purification catalyst 11 are determined from the operating state of the engine, that is, they are a function of the fuel injection amount Q and the engine speed N, and therefore the oxygen content of the platinum 51 per unit time. The poison amount W is a function of the fuel injection amount Q and the engine speed N. In the first embodiment, the oxygen poisoning amount W of the platinum 51 per unit time according to the fuel injection amount Q and the engine speed N is obtained in advance by experiment, and this oxygen poisoning amount W is determined as the fuel injection amount Q. As a function of the engine speed N, the map is stored in advance in the ROM 32 as shown in FIG.
[0075]
Figure 6 is NO_X And SO_X The time chart of the release control of is shown. As shown in FIG. 6, every time the integrated value ΣW of the oxygen poisoning amount W exceeds the allowable value WX, the reducing agent is supplied from the reducing agent supply valve 13, and the air-fuel ratio A / of the exhaust gas flowing into the exhaust purification catalyst 11. F is temporarily switched from lean to rich. At this time, oxygen poisoning of the platinum 51 is eliminated, and NO adsorbed or held on the catalyst carrier 50_X Is released from the catalyst carrier 50 and reduced.
[0076]
On the other hand, the SO held on the exhaust purification catalyst 11_X The integrated value ΣSOX of the quantity is also calculated, and this SO_X When the integrated value ΣSOX of the amount exceeds the allowable value SX, SO from the exhaust purification catalyst 11_X A release action takes place. That is, first, the temperature TC of the exhaust purification catalyst 11 is SO._X It is raised until the discharge temperature TX is reached. This SO_X The release temperature TX is approximately 500 ° C. when the additive 52 is not added to the catalyst carrier 51, and approximately 500 ° C. to 550 corresponding to the amount of additive 52 added when the additive 52 is added to the catalyst carrier 51. The temperature is between ° C.
[0077]
The temperature TC of the exhaust purification catalyst 11 is SO_X When the discharge temperature TX is reached, the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst 11 is switched from lean to rich, and the SO_X The release of. SO_X While the gas is released, the temperature TC of the exhaust purification catalyst 11 is SO._X The discharge temperature TX is maintained, and the air-fuel ratio of the exhaust gas is maintained rich. Then SO_X When the releasing action is completed, the temperature raising action of the exhaust purification catalyst 11 is stopped, and the air-fuel ratio of the exhaust gas is returned to lean.
[0078]
As described above, from the exhaust purification catalyst 11 to the SO_X The temperature of the exhaust purification catalyst 11 is NO_X It is raised until the discharge temperature TX is reached. Next, a method for increasing the temperature TC of the exhaust purification catalyst 11 will be described with reference to FIG.
[0079]
One effective method for raising the temperature TC of the exhaust purification catalyst 11 is a method of retarding the fuel injection timing until after the compression top dead center. That is, the normal main fuel Q_m Is injected near the compression top dead center as shown in FIG. In this case, as shown in FIG. 7 (II), the main fuel Q_m When the injection timing is delayed, the afterburning period becomes longer, and thus the exhaust gas temperature rises. As the exhaust gas temperature increases, the temperature TC of the exhaust purification catalyst 11 increases accordingly.
[0080]
Further, in order to increase the temperature TC of the exhaust purification catalyst 11, as shown in (III) of FIG._m In addition to the auxiliary fuel Q near the intake top dead center_v Can also be injected. Thus, auxiliary fuel Q_v When additional fuel is injected, auxiliary fuel Q_v Since the amount of fuel combusted by the amount increases, the exhaust gas temperature rises, and thus the temperature TC of the exhaust purification catalyst 11 rises.
[0081]
On the other hand, the auxiliary fuel Q in the vicinity of the intake top dead center in this way._v Is injected by the heat of compression during the compression stroke._v From these, intermediate products such as aldehydes, ketones, peroxides, carbon monoxide and the like are produced, and these intermediate products produce main fuel Q._m The reaction is accelerated. Therefore, in this case, as shown in (III) of FIG._m Even if the injection timing is greatly delayed, good combustion can be obtained without causing misfire. That is, the main fuel Q_m Therefore, the exhaust gas temperature becomes considerably high, so that the temperature TC of the exhaust purification catalyst 11 can be quickly raised.
[0082]
Further, in order to raise the temperature TC of the exhaust purification catalyst 11, as shown in FIG._m In addition to the auxiliary fuel Q during the expansion stroke or the exhaust stroke_p Can also be injected. That is, in this case, most of the auxiliary fuel Q_p Is discharged into the exhaust passage in the form of unburned HC without burning. This unburned HC is oxidized by excess oxygen on the exhaust purification catalyst 11, and the temperature TC of the exhaust purification catalyst 11 is raised by the oxidation reaction heat generated at this time.
[0083]
FIG. 8 shows a poisoning elimination flag indicating that the oxygen poisoning of the platinum 51 should be eliminated, and the SO._X Indicates that should be released_X The control routine of the discharge flag is shown, and this routine is executed by interruption every predetermined time.
[0084]
Referring to FIG. 8, first, at step 100, the oxygen poisoning amount W per unit time is calculated from the map shown in FIG. Next, at step 101, the integrated value ΣW of the oxygen poisoning amount is calculated by adding the oxygen poisoning amount W to ΣW. Next, at step 102, it is determined whether or not the integrated value ΣW of the oxygen poisoning amount exceeds the allowable value WX, that is, whether or not the entire surface of the platinum 51 is just before oxygen poisoning. When ΣW ≦ WX, the routine jumps to step 104. On the other hand, when ΣW> WX, the routine proceeds to step 103, where the poisoning elimination flag is set, and then proceeds to step 104.
[0085]
In step 104, a value k · Q obtained by multiplying the fuel injection amount Q by a constant k is added to ΣSOX. The fuel contains a certain amount of sulfur. Therefore, the SO that is held in the exhaust purification catalyst 11 per unit time._X The quantity can be expressed in k · Q. Therefore, the ΣSOX obtained by adding ΣSOX to k · Q is the SO held on the exhaust purification catalyst 11._X It represents the integrated value of the quantity. Next, at step 105, SO_X It is determined whether or not the integrated value ΣSOX of the quantity exceeds the allowable value SX. When ΣSOX ≦ SX, the processing cycle is completed. When ΣSOX> SX, the routine proceeds to step 106, where SO_X The release flag is set.
[0086]
Next, the reducing agent supply control routine will be described with reference to FIG.
[0087]
Referring to FIG. 9, first, at step 200, it is judged if the poisoning elimination flag is set. When the poisoning elimination flag is not set, the routine jumps to step 208. On the other hand, when the poisoning elimination flag is set, the routine proceeds to step 201, where it is judged if the temperature of the exhaust purification catalyst 11 is lower than the allowable temperature TL. This allowable temperature TL is, for example, NO_X The allowable temperature TL is approximately 400 ° C. when only the platinum 51 is supported on the catalyst carrier 50 made of alumina, which is the temperature TC of the exhaust purification catalyst 11 at which the purification rate becomes 30%. When TC ≧ TL, that is, even if the exhaust gas air-fuel ratio is rich, high NO_X When the purification rate cannot be obtained, the routine jumps to step 208. That is, when the temperature TC of the exhaust purification catalyst 11 exceeds approximately 400 ° C., the switching action of the air-fuel ratio from lean to rich is prohibited. In contrast, when TC <TL, that is, if the air-fuel ratio of the exhaust gas is made rich, the NO_X When the purification rate can be obtained, the routine proceeds to step 202.
[0088]
In step 202, the supply amount of the reducing agent necessary for setting the air-fuel ratio of the exhaust gas to a rich air-fuel ratio of about 13, for example, is calculated. Next, at step 203, the supply time of the reducing agent is calculated. The supply time of this reducing agent is usually 10 seconds or less. Next, at step 204, supply of the reducing agent from the reducing agent supply valve 13 is started. Next, in step 205, it is determined whether or not the reducing agent supply time calculated in step 203 has elapsed. When the supply time of the reducing agent has not elapsed, the routine jumps to step 208, where the supply of reducing agent is continued and the air-fuel ratio of the exhaust gas is maintained at a rich air-fuel ratio of about 13. In contrast, when the supply time of the reducing agent has elapsed, that is, when the oxygen poisoning of the platinum 51 has been eliminated, the process proceeds to step 206, where the supply of the reducing agent is stopped, and then proceeds to step 207, where ΣW and poisoning elimination The flag is cleared. Next, the routine proceeds to step 208.
[0089]
In step 208, SO_X It is determined whether or not the release flag is set. SO_X When the release flag is not set, the processing cycle is completed. On the other hand, SO_X When the release flag is set, the routine proceeds to step 209, where the temperature raising control of the exhaust purification catalyst 11 is performed. That is, the fuel injection pattern from the fuel injection valve 3 is changed to any one of the injection patterns shown in (II) to (IV) of FIG. When the fuel injection pattern is changed to any one of the injection patterns shown in (II) to (IV) of FIG. 7, the exhaust gas temperature rises and thus the temperature of the exhaust purification catalyst 11 rises. Next, the routine proceeds to step 210.
[0090]
In step 210, the temperature TC of the exhaust purification catalyst 11 detected by the temperature sensor 20 is the SO._X It is determined whether or not the discharge temperature TX has been reached. When TC <TX, the processing cycle is completed. On the other hand, when TC ≧ TX, the routine proceeds to step 211, where the supply amount of reducing agent necessary to make the air-fuel ratio of the exhaust gas a rich air-fuel ratio of about 14, for example, is calculated. Next, at step 212, the supply time of the reducing agent is calculated. The supply time of this reducing agent is about several minutes. Next, at step 213, supply of the reducing agent from the reducing agent supply valve 13 is started. Next, at step 214, it is judged if the reducing agent supply time calculated at step 212 has elapsed. When the supply time of the reducing agent has not elapsed, the processing cycle is completed. At this time, the supply of the reducing agent is continued and the air-fuel ratio of the exhaust gas is maintained at a rich air-fuel ratio of about 14. On the other hand, when the supply time of the reducing agent has elapsed, that is, the SO retained in the exhaust purification catalyst 11._X When the release of is completed, the routine proceeds to step 215, where the supply of the reducing agent is stopped. Next, at step 216, the temperature raising action of the exhaust purification catalyst 11 is stopped, and then the routine proceeds to step 217, where ΣSOX, ΣW and SO_X The release flag is cleared.
[0091]
11 and 12 show a second embodiment. In the second embodiment, as the sensor 22 disposed in the exhaust pipe 21, NO in exhaust gas is used._X NO that can detect the concentration_X A density sensor is used. This NO_X The concentration sensor 22 is NO as shown in FIG._X An output voltage V proportional to the concentration is generated.
[0092]
NO when platinum 51 is poisoned_X As a result, the purification rate of NO decreases in the exhaust gas._X The concentration gradually increases. Therefore, the oxygen poisoning amount of precious metal catalysts such as platinum 51 is the NO in the exhaust gas._X It can be estimated from the concentration. In this second embodiment, NO in the exhaust gas._X When the oxygen poisoning amount estimated from the concentration exceeds a predetermined allowable value, that is, as shown in FIG._X When the output voltage V of the concentration sensor 22 exceeds the set value VX, the air-fuel ratio of the exhaust gas is switched from lean to rich.
[0093]
FIG. 12 shows a reducing agent supply control routine in the second embodiment.
[0094]
Referring to FIG. 12, first, at step 300, NO is determined._X It is determined whether or not the output voltage V of the density sensor 22 exceeds the set value VX. When V ≦ VX, the routine jumps to step 208 in FIG. On the other hand, when V> VX, the routine proceeds to step 301, where the supply amount of reducing agent required to make the air-fuel ratio of the exhaust gas a rich air-fuel ratio of about 13, for example, is calculated. Next, at step 302, the supply time of the reducing agent is calculated. The supply time of this reducing agent is usually 10 seconds or less. Next, at step 303, supply of the reducing agent from the reducing agent supply valve 13 is started. Next, in step 304, it is determined whether or not the reducing agent supply time calculated in step 302 has elapsed. When the supply time of the reducing agent has not elapsed, the routine jumps to step 208 in FIG. 10, and at this time, the supply of the reducing agent is continued and the air-fuel ratio of the exhaust gas is maintained at a rich air-fuel ratio of about 13. On the other hand, when the supply time of the reducing agent has elapsed, that is, when the oxygen poisoning of the platinum 51 has been eliminated, the process proceeds to step 305 to stop the supply of the reducing agent, and then proceeds to step 208 in FIG.
[0095]
In the second embodiment, the flag control routine shown in FIG. 8 is used. However, in the second embodiment, since it is not necessary to calculate the oxygen poisoning amount W, the flag control routine shown in FIG. Only steps 104 to 106 are executed. In the second embodiment, as described above, the routine shown in FIG. 10 is executed following the routine shown in FIG. 12, but in step 217 in the routine shown in FIG. 10, ΣSOX and SO_X Only the release flag is cleared.
[0096]
13 and 14 show a third embodiment. In this third embodiment, it is determined whether the oxygen poisoning of the platinum 51 is eliminated when the air-fuel ratio of the exhaust gas is made rich in order to eliminate the oxygen poisoning of the noble metal catalyst, for example, the platinum 51. When it is determined that the oxygen poisoning 51 has been eliminated, the air-fuel ratio of the exhaust gas is switched from rich to lean.
[0097]
Specifically, in the third embodiment, an air-fuel ratio sensor for detecting the air-fuel ratio of the exhaust gas flowing out from the exhaust purification catalyst 11 is used as the sensor 22 disposed in the exhaust pipe 21. As shown in FIG. 13, when the air-fuel ratio (A / F) in of the exhaust gas flowing into the exhaust purification catalyst 11 is switched from lean to rich, that is, when the reducing agent is supplied from the reducing agent supply valve 13, the reducing agent is supplied. That is, hydrocarbons are oxidized by oxygen on the platinum 51, and while oxygen is present on the platinum 51, the air-fuel ratio (A / F) out of the exhaust gas flowing out from the exhaust purification catalyst 11 is maintained substantially at the stoichiometric air-fuel ratio. The Next, when the oxygen on the platinum 51 runs out, the hydrocarbons pass through the exhaust purification catalyst 11, so that the air-fuel ratio (A / F) out of the exhaust gas flowing out from the exhaust purification catalyst 11 becomes rich. Therefore, after the air-fuel ratio (A / F) in of the exhaust gas flowing into the exhaust purification catalyst 11 is switched from lean to rich, the air-fuel ratio (A / F) out of the exhaust gas flowing out from the exhaust purification catalyst 11 becomes rich. It can be determined that the oxygen poisoning of the platinum 51 has been eliminated.
[0098]
FIG. 14 shows a reducing agent supply control routine in the third embodiment.
[0099]
Referring to FIG. 14, first, at step 400, it is judged if the poisoning elimination flag is set. When the poisoning elimination flag is not set, the process jumps to step 208 in FIG. On the other hand, when the poisoning elimination flag is set, the routine proceeds to step 401, where the supply amount of the reducing agent necessary for setting the air-fuel ratio of the exhaust gas to a rich air-fuel ratio of about 13, for example, is calculated. Next, the routine proceeds to step 402 where the supply of the reducing agent from the reducing agent supply valve 13 is started. Next, at step 403, it is judged if the air-fuel ratio (A / F) out of the exhaust gas detected by the air-fuel ratio sensor 22 has become rich. When the air-fuel ratio (A / F) out is not rich, the routine jumps to step 208 in FIG. On the other hand, when the air-fuel ratio (A / F) out becomes rich, that is, when the oxygen poisoning of the platinum 51 is eliminated, the routine proceeds to step 404 where the supply of the reducing agent is stopped, and then proceeds to step 405 and ΣW and The poisoning elimination flag is cleared. Next, the routine proceeds to step 208 in FIG.
[0100]
Next, a fourth embodiment using a particulate filter instead of the exhaust purification catalyst 11 will be described.
[0101]
FIGS. 15A and 15B show the structure of the particulate filter 11. 15A shows a front view of the particulate filter 11, and FIG. 15B shows a side sectional view of the particulate filter 11. As shown in FIG. As shown in FIGS. 15A and 15B, the particulate filter 11 has a honeycomb structure, and includes a plurality of exhaust flow passages 60 and 61 extending in parallel with each other. These exhaust flow passages include an exhaust gas inflow passage 60 whose downstream end is closed by a plug 62 and an exhaust gas outflow passage 61 whose upstream end is closed by a plug 63. In addition, the hatched part in FIG. Therefore, the exhaust gas inflow passages 60 and the exhaust gas outflow passages 61 are alternately arranged via the thin partition walls 64. In other words, each of the exhaust gas inflow passages 60 and the exhaust gas outflow passages 61 is surrounded by four exhaust gas outflow passages 61, and each exhaust gas outflow passage 61 is surrounded by four exhaust gas inflow passages 60. Arranged so that.
[0102]
The particulate filter 11 is made of, for example, a porous material such as cordierite. Therefore, the exhaust gas flowing into the exhaust gas inflow passage 60 is contained in the surrounding partition wall 64 as indicated by an arrow in FIG. Through the exhaust gas outflow passage 61 adjacent thereto.
[0103]
In this embodiment, a layer of a catalyst carrier made of alumina is formed on the peripheral wall surfaces of the exhaust gas inflow passages 60 and the exhaust gas outflow passages 61, that is, on both side surfaces of the partition walls 64 and on the pore inner wall surfaces of the partition walls 64. The noble metal catalyst is supported on the catalyst carrier. In this embodiment, platinum Pt is used as a noble metal catalyst.
[0104]
Also in this embodiment, platinum is supported on a catalyst support made of alumina, and the same supply control of the reducing agent as in the first embodiment is performed. Therefore, in this embodiment as well, the NO shown in FIG._X A purification rate is obtained.
[0105]
In this embodiment, the particulates contained in the exhaust gas are captured in the particulate filter 11, and the captured particulates are sequentially burned by the exhaust gas heat. If a large amount of particulates accumulates on the particulate filter 11, the injection pattern is switched from one of the injection patterns (II) to (IV) in FIG. 7, and the exhaust gas temperature is raised. Thereby, the accumulated particulates are ignited and burned.
[0106]
16 and 17 show other embodiments of the compression ignition type internal combustion engine.
[0107]
In the embodiment shown in FIG. 16, the same exhaust purification catalyst, particulate filter, or NO as the exhaust purification catalyst 11 in the exhaust passage upstream of the exhaust purification catalyst 11._X Has the function of selectively reducing NO_X NO has no function to absorb_X A selective reduction catalyst 23 is disposed, and in the embodiment shown in FIG. 17, a particulate filter or NO in the exhaust passage downstream of the exhaust purification catalyst 11._X Has the function of selectively reducing NO_X NO has no function to absorb_X A selective reduction catalyst 23 is arranged.
[0108]
If the same exhaust purification catalyst 23 as the exhaust purification catalyst 11 is arranged in the exhaust passage upstream of the exhaust purification catalyst 11, the temperature of the downstream exhaust purification catalyst 11 is lower than that of the upstream exhaust purification catalyst 23. The temperature of the exhaust purification catalyst 23 becomes higher and NO_X High NO in the exhaust purification catalyst 11 on the downstream side when the purification rate decreases_X A purification rate can be obtained. The particulate filter 23 may have no noble metal catalyst and a catalyst carrier, or may have a noble metal catalyst and a catalyst carrier. NO_X As the selective reduction catalyst 23, a Cu-zeolite catalyst can be used. However, since the Cu-zeolite catalyst 23 has low heat resistance, when the Cu-zeolite catalyst 23 is used, the Cu-zeolite catalyst 23 is preferably arranged downstream of the exhaust purification catalyst 11 as shown in FIG. In the embodiments shown in FIGS. 16 and 17, the reducing agent supply control is performed in the same manner as in the first embodiment.
[0109]
FIG. 18 shows still another embodiment of the compression ignition type internal combustion engine.
[0110]
In this embodiment, NO in the exhaust passage downstream of the exhaust purification catalyst 11._X Has the function of selectively reducing NO_X NO has no function to absorb_X A selective reduction catalyst 24 is arranged. This NO_X As the selective reduction catalyst 24, a catalyst V in which titania is used as a carrier and vanadium oxide is supported on this carrier.₂ O_Five / TiO₂ (Hereinafter referred to as a vanadium / titania catalyst) or a catalyst Cu / ZSM 5 (hereinafter referred to as a copper zeolite catalyst) in which a zeolite is used as a carrier and copper is supported on the carrier is used.
[0111]
NO_X A urea supply valve 25 for supplying a urea aqueous solution is disposed in the exhaust passage between the selective reduction catalyst 24 and the exhaust purification catalyst 11, and the urea aqueous solution is supplied to the urea supply valve 25 by a supply pump 26. Further, an intake air amount detector 27 is disposed in the intake passage, and the sensor 22 disposed in the exhaust pipe 21 is NO._X A density sensor is used.
[0112]
When the urea aqueous solution is supplied into the exhaust gas from the urea supply valve 25 when the air-fuel ratio of the exhaust gas is lean, NO contained in the exhaust gas is NO_X On the selective reduction catalyst 24, urea CO (NH₂ )₂ Ammonia generated from NH_Three (Eg 2NH_Three + 2NO + 1 / 2O² → 2N₂ + 3H₂ O). In this case, NO contained in the exhaust gas_X NO in exhaust gas_X A certain amount of urea is required to completely remove NO._X The amount of urea required to reduce and completely remove urea / NO_X This is referred to as the amount of urea having an equivalent ratio of 1. Urea / NO_X The equivalent ratio of 1 is hereinafter simply referred to as equivalent ratio = 1.
[0113]
The solid line in FIG. 19 is the same as the value shown in FIG._X The relationship between the purification rate and the temperature TC of the exhaust purification catalyst 11 is shown, and the broken line in FIG. 19 indicates NO in the exhaust gas._X NO when the urea aqueous solution is supplied so that the amount of urea is equivalent to 1 with respect to the amount_X Purification rate and NO_X The relationship with the temperature TC of the selective reduction catalyst 24 is shown. From FIG. 19, NO in the exhaust gas_X When the urea aqueous solution is supplied so that the urea amount is equivalent ratio = 1 to the amount, NO_X When the temperature TC of the selective reduction catalyst 24 reaches approximately 300 ° C. or higher, NO_X The purification rate is almost 100%, NO_X NO decreases as the temperature TC of the selective reduction catalyst 24 decreases._X It turns out that a purification rate falls.
[0114]
In this embodiment, in the region I where the temperature TC of the exhaust purification catalyst 11 in FIG. 19 is lower than a set temperature TL, for example, 300 ° C., the flag control routine shown in FIG. 8 and the reducing agent supply control routine shown in FIGS. Supply control of the reducing agent from the reducing agent supply valve 13 is performed. Therefore, in the region I, the exhaust purification catalyst 11 has a higher NO._X A purification rate is obtained. In this case, as can be seen from FIG. 19, the TL in step 201 in FIG. 9 is 300.degree.
[0115]
On the other hand, NO in FIG._X In the region where the temperature TC of the selective reduction catalyst 24 is higher than the set temperature TN (<TL), the urea aqueous solution is supplied by the urea aqueous solution supply control routine shown in FIG._X NO by selective reduction catalyst 24_X Purification is performed.
[0116]
That is, referring to FIG. 20, first, in step 500, NO is determined._X It is determined whether or not the temperature TC of the selective reduction catalyst 24 is higher than a set temperature TN, for example, 250 ° C. When TC ≦ TN, the processing cycle is completed. On the other hand, when TC> TN, the routine proceeds to step 501 and NO._X NO detected by the concentration sensor 22_X NO discharged from the combustion chamber 2 per unit time from the concentration and the intake air amount detected by the intake air amount detector 27_X The amount is sought and this NO_X NO based on quantity_X The amount of urea per unit time with an equivalent ratio = 1 to the amount is calculated. Next, at step 502, the supply amount of the urea aqueous solution is calculated from the calculated urea amount. Next, at step 503, the urea aqueous solution of the amount calculated at step 502 is supplied from the urea supply valve 13. Therefore, NO in Region II_X High NO with selective reduction catalyst 24_X A purification rate is obtained.
[0117]
As can be seen from FIG. 19, in the region where region I and region II overlap, NO by the exhaust purification catalyst 11_X Purification and NO_X NO by selective reduction catalyst 24_X Purifying action and therefore NO in this region_X The purification rate is almost 100%. Therefore, high NO over a wide temperature range_X A purification rate can be obtained.
[0118]
Next, a description will be given of a low-temperature combustion method suitable for raising the temperature of the exhaust purification catalyst 11 and the like to make the exhaust gas air-fuel ratio rich.
[0119]
In the compression ignition type internal combustion engine shown in FIG. 1, as the EGR rate (EGR gas amount / (EGR gas amount + intake air amount)) increases, the amount of smoke generated gradually increases and reaches a peak. As you increase the amount of smoke, the amount of smoke generated decreases rapidly. This will be described with reference to FIG. 21 showing the relationship between the EGR rate and smoke when the degree of cooling of the EGR gas is changed. In FIG. 21, curve A shows the case where the EGR gas is cooled strongly and the EGR gas temperature is maintained at about 90 ° C., and curve B shows the case where the EGR gas is cooled by a small cooling device. Curve C shows the case where the EGR gas is not forcibly cooled.
[0120]
As shown by curve A in FIG. 21, when the EGR gas is cooled strongly, the amount of smoke generated peaks when the EGR rate is slightly lower than 50%. In this case, the EGR rate is increased to about 55% or more. If this is done, smoke will hardly occur. On the other hand, when the EGR gas is slightly cooled as shown by curve B in FIG. 21, the amount of smoke generated reaches a peak when the EGR rate is slightly higher than 50%. In this case, the EGR rate is approximately 65% or more. If it is made, smoke will hardly occur. Further, as shown by the curve C in FIG. 21, when the EGR gas is not forcibly cooled, the amount of smoke generated reaches a peak when the EGR rate is around 55%. In this case, the EGR rate is approximately 70%. If it is more than a percentage, smoke is hardly generated.
[0121]
As described above, when the EGR gas ratio is 55% or more, smoke is not generated because the endothermic action of the EGR gas does not cause the temperature of the fuel and the surrounding gas to be so high, that is, low-temperature combustion is performed. This is because hydrocarbons do not grow to soot.
[0122]
This low-temperature combustion suppresses the generation of smoke regardless of the air-fuel ratio, while NO._X It has the characteristic that the generation amount of can be reduced. That is, when the air-fuel ratio is made rich, the fuel becomes excessive, but the combustion temperature is suppressed to a low temperature, so that the excessive fuel does not grow to the soot, and thus smoke does not occur. At this time, NO_X However, only a very small amount is generated. On the other hand, even when the average air-fuel ratio is lean, or even when the air-fuel ratio is the stoichiometric air-fuel ratio, a small amount of soot is produced if the combustion temperature is high, but the combustion temperature is suppressed to a low temperature under low-temperature combustion. NO smoke at all, NO_X However, only a very small amount is generated.
[0123]
On the other hand, when this low-temperature combustion is performed, the temperature of the fuel and the surrounding gas decreases, but the exhaust gas temperature increases. This will be described with reference to FIGS. 22 (A) and 22 (B).
[0124]
The solid line in FIG. 22A shows the relationship between the average gas temperature Tg in the combustion chamber 5 and the crank angle when low-temperature combustion is performed, and the broken line in FIG. 22A shows normal combustion. The relationship between the average gas temperature Tg in the combustion chamber 5 and the crank angle is shown. Also, the solid line in FIG. 22B shows the relationship between the fuel and the surrounding gas temperature Tf and the crank angle when low-temperature combustion is performed, and the broken line in FIG. 22B shows normal combustion. The relationship between the fuel and the surrounding gas temperature Tf when broken and the crank angle is shown.
[0125]
When low-temperature combustion is performed, the amount of EGR gas is larger than when normal combustion is performed. Therefore, as shown in FIG. 22A, before compression top dead center, that is, during the compression stroke, a solid line The average gas temperature Tg at the time of low-temperature combustion indicated by is higher than the average gas temperature Tg at the time of normal combustion indicated by a broken line. At this time, as shown in FIG. 22B, the fuel and the surrounding gas temperature Tf are substantially the same as the average gas temperature Tg.
[0126]
Next, combustion is started in the vicinity of the compression top dead center. In this case, when low-temperature combustion is performed, as shown by the solid line in FIG. It wo n’t be that expensive. On the other hand, when normal combustion is performed, since a large amount of oxygen exists around the fuel, the fuel and the surrounding gas temperature Tf become extremely high as shown by the broken line in FIG. . In this way, when normal combustion is performed, the temperature of the fuel and the surrounding gas temperature Tf is considerably higher than when low temperature combustion is performed, but the temperature of the other gases which occupy most is low temperature combustion. Therefore, the average gas in the combustion chamber 2 in the vicinity of the compression top dead center as shown in FIG. 22A is lower than that in the case where normal combustion is performed. The temperature Tg is higher when low-temperature combustion is performed than when normal combustion is performed. As a result, as shown in FIG. 22 (A), the burned gas temperature in the combustion chamber 2 after the completion of the combustion is higher when the low-temperature combustion is performed than when the normal combustion is performed. Therefore, when the low temperature combustion is performed, the exhaust gas temperature becomes high.
[0127]
By the way, when the required torque TQ of the engine is increased, that is, when the fuel injection amount is increased, the temperature of the fuel and the surrounding gas at the time of combustion is increased, so that it is difficult to perform low temperature combustion. That is, low-temperature combustion can be performed only when the engine is in a low load operation where the amount of heat generated by combustion is relatively small. In FIG. 23, a region I indicates an operation region in which the first combustion in which the amount of inert gas in the combustion chamber 5 is larger than the amount of inert gas in which the amount of generated soot reaches a peak, that is, low-temperature combustion can be performed. Region II shows an operation region in which only the normal combustion can be performed, ie, the second combustion in which the amount of inert gas in the combustion chamber is smaller than the amount of inert gas in which the amount of generated soot reaches a peak.
[0128]
FIG. 24 shows the target air-fuel ratio A / F when low temperature combustion is performed in the operation region I, and FIG. 25 shows the opening of the throttle valve 9 according to the required torque TQ when performing low temperature combustion in the operation region I. The opening of the EGR control valve 15, the EGR rate, the air-fuel ratio, the injection start timing θS, the injection completion timing θE, and the injection amount are shown. FIG. 25 also shows the opening of the throttle valve 9 and the like during normal combustion performed in the operation region II.
[0129]
24 and 25, it is understood that when the low temperature combustion is performed in the operation region I, the EGR rate is 55% or more and the air-fuel ratio A / F is a lean air-fuel ratio of about 15.5 to 18. As described above, when low temperature combustion is performed in the operation region I, smoke is hardly generated even if the air-fuel ratio is rich.
[0130]
Thus, when low-temperature combustion is performed, the air-fuel ratio can be made rich with almost no smoke. Therefore, elimination of oxygen poisoning or SO_X When the air-fuel ratio of the exhaust gas should be made rich in order to release the exhaust gas, low-temperature combustion can be performed, and the air-fuel ratio can be made rich under the low-temperature combustion.
[0131]
Further, as described above, when low-temperature combustion is performed, the exhaust gas temperature rises. So SO_X Low temperature combustion can be performed when the exhaust gas temperature is to be increased in order to release or to ignite and burn the accumulated particulates.
[0132]
【The invention's effect】
High NO_X A purification rate can be obtained.
[Brief description of the drawings]
FIG. 1 is an overall view of a compression ignition type internal combustion engine.
FIG. 2 is a view schematically showing a cross section of a carrier surface portion of an exhaust purification catalyst.
FIG. 3 is a graph showing changes in the air-fuel ratio of exhaust gas due to supply of a reducing agent.
FIG. 4 NO_X It is a figure which shows a purification rate.
FIG. 5 is a diagram showing an oxygen poisoning amount per unit time.
FIG. 6 NO_X And SO_X It is a figure which shows the time chart of discharge | release control.
FIG. 7 shows various fuel injection patterns.
FIG. 8 is a flowchart for controlling each flag.
FIG. 9 is a flowchart for controlling the supply of a reducing agent.
FIG. 10 is a flowchart for controlling supply of a reducing agent.
FIG. 11 is a view for explaining air-fuel ratio control of exhaust gas.
FIG. 12 is a flowchart for performing supply control of a reducing agent.
FIG. 13 is a diagram showing changes in the air-fuel ratio of exhaust gas.
FIG. 14 is a flowchart for controlling the supply of the reducing agent.
FIG. 15 is a diagram illustrating a particulate filter.
FIG. 16 is an overall view showing another embodiment of the compression ignition type internal combustion engine.
FIG. 17 is an overall view showing still another embodiment of a compression ignition type internal combustion engine.
FIG. 18 is an overall view showing still another embodiment of the compression ignition type internal combustion engine.
FIG. 19 NO_X It is a figure which shows a purification rate.
FIG. 20 is a flowchart for controlling the supply of an aqueous urea solution.
FIG. 21 is a diagram showing the amount of smoke generated.
FIG. 22 is a diagram showing gas temperature and the like in the combustion chamber.
FIG. 23 is a diagram showing operation regions I and II.
FIG. 24 is a diagram showing an air-fuel ratio A / F.
FIG. 25 is a diagram showing changes in the throttle valve opening and the like.
[Explanation of symbols]
3 ... Fuel injection valve
4 ... Intake manifold
5 ... Exhaust manifold
7 ... Exhaust turbocharger
11 ... Exhaust gas purification catalyst
13 ... Reducing agent supply valve

Claims (7)

In the exhaust purification system of an internal combustion engine so as to purify the exhaust gas purifying catalyst of NO _X which is disposed in the exhaust passage generated when burning fuel under a lean air-fuel ratio is being performed, the catalyst of the exhaust purification catalyst using a carrier in the presence of a base point on the support surface as a carrier, on the carrier surface, a noble metal catalyst is dispersed and supported without forming a layer of the NO _X absorbent capable of absorbing NO _X, in the exhaust gas Is provided with means for calculating the oxygen poisoning amount that is proportional to the oxygen concentration of the exhaust gas and increases as the temperature of the exhaust purification catalyst increases, based on the oxygen concentration in the exhaust gas and the temperature of the exhaust purification catalyst. An exhaust gas purification apparatus for an internal combustion engine, wherein the air-fuel ratio of exhaust gas is temporarily switched from lean to rich when the poisoning amount exceeds a predetermined allowable value . The exhaust gas purification apparatus for an internal combustion engine according to claim 1 , wherein the oxygen poisoning amount is stored in advance as a function of the fuel injection amount and the engine speed . The exhaust purification device for an internal combustion engine according to claim 1 , wherein the catalyst carrier is made of alumina . The alkali metal, alkaline earth metal or rare earth is contained inside the catalyst carrier, thereby increasing the number of basic points on the surface of the catalyst carrier or strengthening the basicity at the basic point. Exhaust gas purification device for internal combustion engine. The exhaust purification device for an internal combustion engine according to claim 1 , wherein the noble metal catalyst is made of platinum . A means for judging whether or not oxygen poisoning of the noble metal catalyst has been eliminated is provided, and when it is judged that oxygen poisoning of the noble metal catalyst has been eliminated, the air-fuel ratio of the exhaust gas is switched from rich to lean. 2. An exhaust gas purification apparatus for an internal combustion engine according to 1. An air-fuel ratio sensor for detecting the air-fuel ratio of the exhaust gas flowing out from the exhaust purification catalyst is provided, and after the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst is switched from lean to rich, it flows out from the exhaust purification catalyst The exhaust emission control device for an internal combustion engine according to claim 6 , wherein it is determined that oxygen poisoning of the noble metal catalyst has been eliminated when the air-fuel ratio of the exhaust gas becomes rich .

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