JP5561059B2 - Exhaust gas purification device for internal combustion engine - Google Patents

Description

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

The engine exhaust passage, NO _x storage catalyst air-fuel ratio of the inflowing exhaust gas when the lean of releasing NO _x air-fuel ratio of the exhaust gas which is occluded becomes rich for occluding NO _x contained in the exhaust gas inflow It was placed, an oxidation catalyst is arranged having an adsorbing function in _the NO _x storage catalyst in the engine exhaust passage upstream of the feed hydrocarbon into the engine exhaust passage an oxidation catalyst upstream in when releasing the NO _x from _the NO _x storage catalyst An internal combustion engine in which the air-fuel ratio of the exhaust gas flowing into the NO _x storage catalyst is made rich is known (see, for example, Patent Document 1).

In this internal combustion engine, hydrocarbons supplied when NO _x should be released from the NO _x storage catalyst are turned into gaseous hydrocarbons in the oxidation catalyst, and gaseous hydrocarbons are sent to the NO _x storage catalyst. As a result, the _the NO _x storage NO _x released from the catalyst is caused to favorably reduced.

Patent No. 3969450

However, the NO _x storage catalyst has a problem that the NO _x purification rate decreases at a high temperature.
An object of the present invention is to provide an exhaust purification device for an internal combustion engine that can obtain a high NO _x purification rate even when the temperature of the exhaust purification catalyst becomes high and can reliably detect the base air-fuel ratio. is there.

According to the present invention, the exhaust purification catalyst for reacting NO _x contained in the exhaust gas and the reformed hydrocarbon is disposed in the engine exhaust passage, and on the exhaust gas flow surface of the exhaust purification catalyst. A noble metal catalyst is supported and a basic exhaust gas flow surface portion is formed around the noble metal catalyst. The exhaust purification catalyst has a hydrocarbon concentration flowing into the exhaust purification catalyst within a predetermined range. which has a property for reducing the NO _x to a vibrating with a cycle of the amplitude and predetermined range contained in the exhaust gas, exhaust gas to be longer than the predetermined range vibration period of the hydrocarbon concentration It has a property that occlusion amount increases of the NO _x contained in the air-fuel ratio sensor for detecting an air-fuel ratio of the exhaust gas into the engine exhaust passage for flow of hydrocarbons is disposed, the exhaust purification during engine operation Flow to catalyst The base air-fuel ratio detection enables the hydrocarbon concentration to be oscillated with the amplitude within the above-mentioned predetermined range and the period within the above-mentioned predetermined range, and at this time the base air-fuel ratio can be detected by the air-fuel ratio sensor. An exhaust purification device for an internal combustion engine is provided in which a possible period is obtained and the air-fuel ratio of the exhaust gas detected by the air-fuel ratio sensor within the base air-fuel ratio detectable period is the base air-fuel ratio.

Even when the temperature of the exhaust purification catalyst becomes high, a high NO _x purification rate can be obtained and the base air-fuel ratio can be reliably detected.

1 is an overall view of a compression ignition type internal combustion engine. It is a figure which shows the surface part of a catalyst support | carrier schematically. It is a figure for demonstrating the oxidation reaction in an exhaust purification catalyst. It is a figure which shows the change of the air fuel ratio of the exhaust gas which flows in into an exhaust gas purification catalyst. Is a diagram showing _the NO _x purification rate. It is a figure for demonstrating the oxidation reduction reaction in an exhaust purification catalyst. It is a figure for demonstrating the oxidation reduction reaction in an exhaust purification catalyst. It is a figure which shows the change of the air fuel ratio of the exhaust gas which flows in into an exhaust gas purification catalyst. Is a diagram showing _the NO _x purification rate. It is a time chart which shows the change of the air fuel ratio of the inflow exhaust gas to an exhaust gas purification catalyst. It is a time chart which shows the change of the air fuel ratio of the inflow exhaust gas to an exhaust gas purification catalyst. FIG. 3 is a diagram showing a relationship between an oxidizing power of an exhaust purification catalyst and a required minimum air-fuel ratio X. Obtained of the same of the NO _x purification rate is a diagram showing the relationship between the amplitude ΔH of the oxygen concentration and the hydrocarbon concentration in the exhaust gas. It is a diagram showing a relationship between an amplitude ΔH and _the NO _x purification rate of the hydrocarbon concentration. It is a diagram showing the relationship between the vibration period ΔT and _the NO _x purification rate of the hydrocarbon concentration. It is a figure which shows maps, such as hydrocarbon supply amount W. It is a figure which shows the change of the air fuel ratio etc. of the inflow exhaust gas to an exhaust gas purification catalyst. It is a diagram illustrating a map of exhaust NO _x amount NOXA. It is a figure which shows fuel injection time. It is a figure which shows the map of hydrocarbon supply amount WR. It is a figure which shows maps, such as reference | standard opening degree (theta) of an EGR control valve. It is a figure which shows the change of the output of an air fuel ratio sensor. It is a figure which shows the map etc. of the base air fuel ratio detectable period (DELTA) Dt. It is a flowchart for performing operation control. It is a flowchart for performing learning control. It is a figure which shows the correction coefficient KA etc. It is a flowchart for performing learning control. It is a flowchart for performing learning control.

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 arranged in the intake duct 6, and a cooling device 11 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 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. The outlet of the exhaust turbine 7b is connected to the inlet of the exhaust purification catalyst 13 via the exhaust pipe 12, and the outlet of the exhaust purification catalyst 13 is connected to the particulate filter 14 for collecting particulates contained in the exhaust gas. The In the exhaust pipe 12 upstream of the exhaust purification catalyst 13, a hydrocarbon supply valve 15 for supplying hydrocarbons composed of light oil and other fuels used as fuel for the compression ignition internal combustion engine is disposed. In the embodiment shown in FIG. 1, light oil is used as the hydrocarbon supplied from the hydrocarbon supply valve 15. The present invention can also be applied to a spark ignition type internal combustion engine in which combustion is performed under a lean air-fuel ratio. In this case, hydrocarbons made of gasoline or other fuel used as fuel for the spark ignition type internal combustion engine are supplied from the hydrocarbon supply valve 15.

  On the other hand, 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 16, and an electronically controlled EGR control valve 17 is disposed in the EGR passage 16. A cooling device 18 for cooling the EGR gas flowing in the EGR passage 16 is disposed around the EGR passage 16. In the embodiment shown in FIG. 1, the engine cooling water is guided into the cooling device 18, 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 20 via a fuel supply pipe 19, and this common rail 20 is connected to a fuel tank 22 via an electronically controlled fuel pump 21 with variable discharge amount. The fuel stored in the fuel tank 22 is supplied into the common rail 20 by the fuel pump 21, and the fuel supplied into the common rail 20 is supplied to the fuel injection valve 3 through each fuel supply pipe 19.

  The electronic control unit 30 comprises a digital computer and is connected to each other by a bidirectional bus 31. ROM (read only memory) 32, RAM (random access memory) 33, CPU (microprocessor) 34, input port 35 and output port 36 It comprises. A temperature sensor 23 for detecting the exhaust gas temperature is attached downstream of the exhaust purification catalyst 13. Further, an air-fuel ratio sensor 24 for detecting the air-fuel ratio of the exhaust gas is disposed upstream of the exhaust purification catalyst 13 and downstream of the hydrocarbon feed valve 15, and the exhaust gas is also disposed downstream of the exhaust purification catalyst 13. An air-fuel ratio sensor 25 for detecting the air-fuel ratio is disposed. The output signals of the temperature sensor 23, the air-fuel ratio sensors 24 and 25, and the intake air amount detector 8 are input to the input port 35 via corresponding AD converters 37, respectively.

  The accelerator pedal 40 is connected to a load sensor 41 that generates an output voltage proportional to the depression amount L of the accelerator pedal 40. The output voltage of the load sensor 41 is input to the input port 35 via the corresponding AD converter 37. Is done. Further, a crank angle sensor 42 that generates an output pulse every time the crankshaft rotates, for example, 15 ° is connected to the input port 35. On the other hand, the output port 36 is connected to the fuel injection valve 3, the step motor for driving the throttle valve 10, the hydrocarbon supply valve 15, the EGR control valve 17, and the fuel pump 21 through corresponding drive circuits 38.

FIG. 2 schematically shows the surface portion of the catalyst carrier carried on the substrate of the exhaust purification catalyst 13. In this exhaust purification catalyst 13, as shown in FIG. 2, noble metal catalysts 51 and 52 are supported on a catalyst support 50 made of alumina, for example, and further on this catalyst support 50, potassium K, sodium Na, cesium Cs. selected from an alkali metal, barium Ba, alkaline earth metals such as calcium Ca, rare earth and silver Ag, such as lanthanides, copper Cu, iron Fe, the metal which can donate electrons to NO _x, such as iridium Ir, such as A basic layer 53 containing at least one of them is formed. Since the exhaust gas flows along the catalyst carrier 50, it can be said that the noble metal catalysts 51 and 52 are supported on the exhaust gas flow surface of the exhaust purification catalyst 13. Further, since the surface of the basic layer 53 is basic, the surface of the basic layer 53 is referred to as a basic exhaust gas flow surface portion 54.

  On the other hand, in FIG. 2, the noble metal catalyst 51 is made of platinum Pt, and the noble metal catalyst 52 is made of rhodium Rh. That is, the noble metal catalysts 51 and 52 carried on the catalyst carrier 50 are composed of platinum Pt and rhodium Rh. In addition to platinum Pt and rhodium Rh, palladium Pd can be further supported on the catalyst carrier 50 of the exhaust purification catalyst 13, or palladium Pd can be supported instead of rhodium Rh. That is, the noble metal catalysts 51 and 52 supported on the catalyst carrier 50 are composed of platinum Pt and at least one of rhodium Rh and palladium Pd.

When hydrocarbons are injected into the exhaust gas from the hydrocarbon supply valve 15, the hydrocarbons are reformed in the exhaust purification catalyst 13. In the present invention, NO _x is purified in the exhaust purification catalyst 13 by using the reformed hydrocarbon at this time. FIG. 3 schematically shows the reforming action performed in the exhaust purification catalyst 13 at this time. As shown in FIG. 3, the hydrocarbon HC injected from the hydrocarbon feed valve 15 is converted into a radical hydrocarbon HC having a small number of carbons by the catalyst 51.

Even if fuel, that is, hydrocarbons, is injected from the fuel injection valve 3 into the combustion chamber 2 during the latter half of the expansion stroke or during the exhaust stroke, the hydrocarbon is reformed in the combustion chamber 2 or in the exhaust purification catalyst 13 and exhausted. NO _x contained in the gas is purified by the exhaust purification catalyst 13 by the reformed hydrocarbon. Therefore, in the present invention, instead of supplying hydrocarbons from the hydrocarbon supply valve 15 into the engine exhaust passage, it is also possible to supply hydrocarbons into the combustion chamber 2 during the latter half of the expansion stroke or during the exhaust stroke. As described above, in the present invention, hydrocarbons can be supplied into the combustion chamber 2, but the present invention will be described below by taking as an example a case where hydrocarbons are injected from the hydrocarbon supply valve 15 into the engine exhaust passage. .

  FIG. 4 shows the supply timing of hydrocarbons from the hydrocarbon supply valve 15 and the change in the air-fuel ratio (A / F) in of the exhaust gas flowing into the exhaust purification catalyst 13. Since the change in the air-fuel ratio (A / F) in depends on the change in the concentration of hydrocarbons in the exhaust gas flowing into the exhaust purification catalyst 13, the air-fuel ratio (A / F) in shown in FIG. It can be said that the change represents a change in hydrocarbon concentration. However, since the air-fuel ratio (A / F) in decreases as the hydrocarbon concentration increases, the hydrocarbon concentration increases as the air-fuel ratio (A / F) in becomes richer in FIG.

FIG. 5 shows a change in the air-fuel ratio (A / F) in of the exhaust gas flowing into the exhaust purification catalyst 13 as shown in FIG. 4 by periodically changing the concentration of hydrocarbons flowing into the exhaust purification catalyst 13. The NO _x purification rate by the exhaust purification catalyst 13 when it is made to be shown with respect to each catalyst temperature TC of the exhaust purification catalyst 13. The present inventor has conducted research on NO _x purification over a long period of time, and in the course of research, the concentration of hydrocarbons flowing into the exhaust purification catalyst 13 is set to an amplitude within a predetermined range and a predetermined range. As shown in FIG. 5, it was found that an extremely high NO _x purification rate can be obtained even in a high temperature region of 400 ° C. or higher when the vibration is made with the internal period.

Further, at this time, a large amount of reducing intermediates containing nitrogen and hydrocarbons are kept or adsorbed on the surface of the basic layer 53, that is, on the basic exhaust gas flow surface portion 54 of the exhaust purification catalyst 13. It was found that the reducing intermediate plays a central role in obtaining a high NO _x purification rate. Next, this will be described with reference to FIGS. 6 (A) and 6 (B). 6A and 6B schematically show the surface portion of the catalyst carrier 50 of the exhaust purification catalyst 13, and these FIGS. 6A and 6B flow into the exhaust purification catalyst 13. FIG. The reaction is assumed to occur when the concentration of hydrocarbons to be oscillated with an amplitude within a predetermined range and a period within a predetermined range.

  FIG. 6A shows a case where the concentration of hydrocarbons flowing into the exhaust purification catalyst 13 is low, and FIG. 6B shows that hydrocarbons are supplied from the hydrocarbon supply valve 15 and flow into the exhaust purification catalyst 13. It shows when the hydrocarbon concentration is high.

As can be seen from FIG. 4, since the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst 13 is maintained lean except for a moment, the exhaust gas flowing into the exhaust purification catalyst 13 is normally in an oxygen-excess state. Therefore, as shown in FIG. 6A, NO contained in the exhaust gas is oxidized on the platinum 51 to become NO ₂ , and then this NO ₂ is donated with electrons from the platinum 51 to become NO ₂ ⁻ . Therefore, a large amount of NO ₂ ⁻ is generated on the platinum 51. The NO ₂ ^- is strongly active, or the NO ₂ ^- is referred to as the active NO ₂ ^*.

On the other hand, when hydrocarbons are supplied from the hydrocarbon supply valve 15, as shown in FIG. 3, the hydrocarbons are reformed in the exhaust purification catalyst 13 and become radicals. As a result, as shown in FIG. 6B, the hydrocarbon concentration around the active NO ₂ ^* increases. However after the active NO ₂ ^* is generated, the active NO ₂ ^* concentration of oxygen around the high state continues for a predetermined time or more active NO ₂ ^* is oxidized, nitrate ions NO ₃ ^- inside the basic layer 53 in the form of Absorbed. However, when the active NO ₂ ^* hydrocarbon concentration around is high before the lapse of the predetermined time FIG 6 (B) the active NO as shown in ₂ ^* reacts with radical hydrocarbons HC on the platinum 51 , Thereby producing a reducing intermediate. This reducing intermediate is attached or adsorbed on the surface of the basic layer 53.

Incidentally, the first produced reducing intermediate this time is considered to be a nitro compound R-NO _2. When this nitro compound R-NO ₂ is produced, it becomes a nitrile compound R-CN, but since this nitrile compound R-CN can only survive for a moment in that state, it immediately becomes an isocyanate compound R-NCO. This isocyanate compound R-NCO becomes an amine compound R-NH _{2 when} hydrolyzed. However, in this case, it is considered that a part of the isocyanate compound R-NCO is hydrolyzed. Therefore, as shown in FIG. 6B, most of the reducing intermediates retained or adsorbed on the surface of the basic layer 53 are considered to be the isocyanate compound R-NCO and the amine compound R-NH _2. .

On the other hand, as shown in FIG. 6B, when the hydrocarbon HC surrounds the generated reducing intermediate, the reducing intermediate is blocked by the hydrocarbon HC and the reaction does not proceed further. In this case, the concentration of hydrocarbons flowing into the exhaust purification catalyst 13 is lowered, and as a result, when the oxygen concentration is increased, hydrocarbons around the reducing intermediate are oxidized. As a result, as shown in FIG. 6 (A), the reducing intermediate and active NO ₂ ^* come to react. At this time, the active NO ₂ ^* reacts with the reducing intermediates R—NCO and R—NH ₂ to become N ₂ , CO ₂ , and H ₂ O, thus purifying NO _x .

Thus, in the exhaust purification catalyst 13, a reducing intermediate is generated by increasing the concentration of hydrocarbons flowing into the exhaust purification catalyst 13, and the concentration of hydrocarbons flowing into the exhaust purification catalyst 13 is decreased to reduce the oxygen concentration. By increasing the NO, active NO ₂ ^* reacts with the reducing intermediate, and NO _x is purified. That is, in order to purify NO _x by the exhaust purification catalyst 13, it is necessary to periodically change the concentration of hydrocarbons flowing into the exhaust purification catalyst 13.

Of course, in this case, it is necessary to increase the hydrocarbon concentration to a concentration high enough to produce a reducing intermediate, and carbonize to a concentration low enough to react the resulting reducing intermediate with active NO ₂ ^*. It is necessary to reduce the concentration of hydrogen. That is, it is necessary to vibrate the hydrocarbon concentration flowing into the exhaust purification catalyst 13 with an amplitude within a predetermined range. In this case, a sufficient amount of the reducing intermediate R-NCO or R-NH ₂ is added on the basic layer 53, that is, basic exhaust gas, until the generated reducing intermediate reacts with active NO ₂ ^*. A basic exhaust gas flow surface portion 24 is provided for this purpose, which must be retained on the gas flow surface portion 24.

On the other hand, if the hydrocarbon supply cycle is lengthened, the period during which the oxygen concentration becomes high after the hydrocarbon is supplied and before the next hydrocarbon is supplied becomes longer, so the active NO ₂ ^* is reduced to the reducing intermediate. Without being generated in the basic layer 53 in the form of nitrate. In order to avoid this, it is necessary to vibrate the concentration of hydrocarbons flowing into the exhaust purification catalyst 13 with a period within a predetermined range.

Therefore, in the embodiment according to the present invention, NO _x contained in the exhaust gas is reacted with the reformed hydrocarbon to generate reducing intermediates R-NCO and R-NH ₂ containing nitrogen and hydrocarbons. Further, noble metal catalysts 51 and 52 are supported on the exhaust gas flow surface of the exhaust purification catalyst 13, and the generated reducing intermediates R-NCO and R-NH ₂ are held in the exhaust purification catalyst 13. Therefore, a basic exhaust gas flow surface portion 54 is formed around the noble metal catalysts 51 and 52, and the reducing intermediates R-NCO and R-NH held on the basic exhaust gas flow surface portion 54 are formed. NO _x is reduced by the reduction action of ₂ , and the vibration period of the hydrocarbon concentration is the vibration period necessary to continue to produce the reducing intermediates R-NCO and R-NH ₂ . Incidentally, in the example shown in FIG. 4, the injection interval is 3 seconds.

If the oscillation period of the hydrocarbon concentration, that is, the supply period of hydrocarbon HC is longer than the period within the above-mentioned predetermined range, the reducing intermediates R-NCO and R-NH ₂ are formed on the surface of the basic layer 53. At this time, the active NO ₂ ^* produced on the platinum Pt 53 is diffused into the basic layer 53 in the form of nitrate ions NO ₃ ⁻ as shown in FIG. That is, at this time, NO _x in the exhaust gas is absorbed in the basic layer 53 in the form of nitrate.

On the other hand, FIG. 7B shows that the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst 13 when the NO _x is absorbed in the basic layer 53 in the form of nitrate is thus the stoichiometric air-fuel ratio or rich. Shows the case. In this case, since the oxygen concentration in the exhaust gas decreases, the reaction proceeds in the reverse direction (NO ₃ ⁻ → NO ₂ ), and thus the nitrates absorbed in the basic layer 53 are successively converted into nitrate ions NO _3. ⁻ And released from the basic layer 53 in the form of NO ₂ as shown in FIG. The released NO ₂ is then reduced by the hydrocarbons HC and CO contained in the exhaust gas.

FIG. 8 shows a case where the air-fuel ratio (A / F) in of the exhaust gas flowing into the exhaust purification catalyst 13 is temporarily made rich slightly before the NO _x absorption capacity of the basic layer 53 is saturated. Yes. In the example shown in FIG. 8, the time interval of this rich control is 1 minute or more. In this case, when the air-fuel ratio (A / F) in of the exhaust gas is lean, the NO _x absorbed in the basic layer 53 temporarily makes the air-fuel ratio (A / F) in of the exhaust gas rich. When released, it is released from the basic layer 53 at once and reduced. Therefore, in this case, the basic layer 53 serves as an absorbent for temporarily absorbing NO _x .

Incidentally, at this time, sometimes the basic layer 53 temporarily adsorbs NO _x, thus using term of storage as a term including both absorption and adsorption In this case the basic layer 53 temporarily the NO _x It plays the role of NO _x storage agent for storage. That is, in this case, the ratio of air and fuel (hydrocarbon) supplied into the exhaust passage upstream of the engine intake passage, the combustion chamber 2 and the exhaust purification catalyst 13 is referred to as the exhaust gas air-fuel ratio. 13, the air-fuel ratio of the exhaust gas is acting as _the NO _x storage catalyst during the lean occludes NO _x, the oxygen concentration in the exhaust gas to release NO _x occluding the drops.

FIG. 9 shows the NO _x purification rate when the exhaust purification catalyst 13 is made to function as a NO _x storage catalyst in this way. The horizontal axis in FIG. 9 indicates the catalyst temperature TC of the exhaust purification catalyst 13. When the exhaust purification catalyst 13 functions as a NO _x storage catalyst, as shown in FIG. 9, an extremely high NO _x purification rate is obtained when the catalyst temperature TC is 300 ° C. to 400 ° C., but the catalyst temperature TC is 400 ° C. _the NO _x purification rate decreases when a high temperature of more.

As described above, the NO _x purification rate decreases when the catalyst temperature TC exceeds 400 ° C. The nitrate is thermally decomposed and released from the exhaust purification catalyst 13 in the form of NO ₂ when the catalyst temperature TC exceeds 400 ° C. Because. That is, as long as NO _x is occluded in the form of nitrate, it is difficult to obtain a high NO _x purification rate when the catalyst temperature TC is high. However, in the new NO _x purification method shown in FIGS. 4 to 6 (A) and (B), as can be seen from FIGS. 6 (A) and (B), nitrate is not produced or is produced in a very small amount even if produced. Thus, as shown in FIG. 5, a high NO _x purification rate can be obtained even when the catalyst temperature TC is high.

Therefore, in the present invention, the exhaust purification catalyst 13 for reacting NO _x contained in the exhaust gas and the reformed hydrocarbon is disposed in the engine exhaust passage, and the exhaust purification catalyst 13 has an exhaust gas flow surface on the surface. The noble metal catalysts 51 and 52 are supported and a basic exhaust gas flow surface portion 54 is formed around the noble metal catalysts 51 and 52, and the exhaust purification catalyst 13 is a hydrocarbon that flows into the exhaust purification catalyst 13. which has a property for reducing the NO _x to a vibrating with a cycle of the amplitude and a predetermined range within a determined range of concentrations previously contained in the exhaust gas, the pre-vibration period of the hydrocarbon concentration If the length is longer than the predetermined range, the storage amount of NO _x contained in the exhaust gas increases, and the concentration of hydrocarbons flowing into the exhaust purification catalyst 13 during engine operation is within a predetermined range. Amplitude and predetermined And with at periodic caused to oscillate within the range, so that the reduction of NO _x contained in the exhaust gas in the exhaust purification catalyst 13 thereby.

That is, the NO _x purification method shown in FIGS. 4 to 6 (A) and (B) uses an exhaust purification catalyst that supports a noble metal catalyst and forms a basic layer capable of absorbing NO _x . it can be said to be new _the NO _x purification methods so as to purify without NO _x to almost form a nitrate. In fact, when this new NO _x purification method is used, the amount of nitrate detected from the basic layer 53 is very small compared to when the exhaust purification catalyst 13 functions as a NO _x storage catalyst. This new NO _x purification method is hereinafter referred to as a first NO _x purification method.

Next, the first NO _x purification method will be described in a little more detail with reference to FIGS. 10 to 15.
FIG. 10 shows an enlarged view of the change in the air-fuel ratio (A / F) in shown in FIG. As described above, the change in the air-fuel ratio (A / F) in of the exhaust gas flowing into the exhaust purification catalyst 13 simultaneously indicates the change in the concentration of hydrocarbons flowing into the exhaust purification catalyst 13. In FIG. 10, ΔH indicates the amplitude of the change in the concentration of hydrocarbon HC flowing into the exhaust purification catalyst 13, and ΔT indicates the oscillation period of the concentration of hydrocarbon flowing into the exhaust purification catalyst 13.

Further, in FIG. 10, (A / F) b represents the base air-fuel ratio indicating the air-fuel ratio of the combustion gas for generating the engine output. In other words, this base air-fuel ratio (A / F) b represents the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst 13 when the supply of hydrocarbons is stopped. On the other hand, in FIG. 10, X is the air-fuel ratio (A / F) used for the production of the reducing intermediate without the generated active NO ₂ ^* being occluded in the basic layer 53 in the form of nitrate. represents the upper limit of in, and in order to generate a reducing intermediate by reacting active NO ₂ ^* and the reformed hydrocarbon, the air / fuel ratio (A / F) in is set to be higher than the upper limit X of the air / fuel ratio. It needs to be lowered.

In other words, X in FIG. 10 represents the lower limit of the concentration of hydrocarbons required for reacting active NO ₂ ^* with the reformed hydrocarbon to produce a reducing intermediate, which is reducible. In order to produce the intermediate, the hydrocarbon concentration needs to be higher than the lower limit X. In this case, whether or not the reducing intermediate is generated is determined by the ratio of the oxygen concentration around the active NO ₂ ^* and the hydrocarbon concentration, that is, the air-fuel ratio (A / F) in, and the reducing intermediate is generated. The above-described upper limit X of the air-fuel ratio necessary for this is hereinafter referred to as a required minimum air-fuel ratio.

  In the example shown in FIG. 10, the required minimum air-fuel ratio X is rich. Therefore, in this case, the air-fuel ratio (A / F) in is instantaneously required to generate the reducing intermediate. The following is made rich: On the other hand, in the example shown in FIG. 11, the required minimum air-fuel ratio X is lean. In this case, the reducing intermediate is generated by periodically reducing the air-fuel ratio (A / F) in while maintaining the air-fuel ratio (A / F) in lean.

  In this case, whether the required minimum air-fuel ratio X becomes rich or lean depends on the oxidizing power of the exhaust purification catalyst 13. In this case, for example, if the amount of the precious metal 51 supported is increased, the exhaust purification catalyst 13 becomes stronger in oxidizing power, and if it becomes more acidic, the oxidizing power becomes stronger. Therefore, the oxidizing power of the exhaust purification catalyst 13 varies depending on the amount of noble metal 51 supported and the strength of acidity.

  When the exhaust purification catalyst 13 having a strong oxidizing power is used, the air-fuel ratio (A / F) in is periodically decreased while maintaining the air-fuel ratio (A / F) in lean as shown in FIG. When the air-fuel ratio (A / F) in is lowered, the hydrocarbon is completely oxidized, and as a result, a reducing intermediate cannot be generated. On the other hand, when the exhaust purification catalyst 13 having a strong oxidizing power is used, if the air-fuel ratio (A / F) in is periodically made rich as shown in FIG. 10, the air-fuel ratio (A / F) in is rich. The hydrocarbons are partially oxidized without being completely oxidized, i.e., the hydrocarbons are reformed, thus producing a reducing intermediate. Therefore, when the exhaust purification catalyst 13 having a strong oxidizing power is used, the required minimum air-fuel ratio X needs to be made rich.

  On the other hand, when the exhaust purification catalyst 13 having a weak oxidizing power is used, the air-fuel ratio (A / F) in is periodically decreased while maintaining the air-fuel ratio (A / F) in lean as shown in FIG. In this case, the hydrocarbon is not completely oxidized but partially oxidized, that is, the hydrocarbon is reformed, and thus a reducing intermediate is produced. On the other hand, when the exhaust purification catalyst 13 having a weak oxidizing power is used, if the air-fuel ratio (A / F) in is periodically made rich as shown in FIG. 10, a large amount of hydrocarbons are not oxidized. The exhaust gas is simply exhausted from the exhaust purification catalyst 13, and the amount of hydrocarbons that are wasted is increased. Therefore, when the exhaust purification catalyst 13 having a weak oxidizing power is used, the required minimum air-fuel ratio X needs to be made lean.

  That is, it can be seen that the required minimum air-fuel ratio X needs to be lowered as the oxidizing power of the exhaust purification catalyst 13 becomes stronger, as shown in FIG. In this way, the required minimum air-fuel ratio X becomes lean or rich due to the oxidizing power of the exhaust purification catalyst 13, but hereinafter the case where the required minimum air-fuel ratio X is rich is taken as an example. The amplitude of the change in the concentration of the inflowing hydrocarbon and the oscillation period of the concentration of the hydrocarbon flowing into the exhaust purification catalyst 13 will be described.

When the base air-fuel ratio (A / F) b increases, that is, when the oxygen concentration in the exhaust gas before the hydrocarbons are supplied increases, the air-fuel ratio (A / F) in is set to be equal to or less than the required minimum air-fuel ratio X. As a result, the amount of hydrocarbons necessary for the increase increases, and the amount of excess hydrocarbons that did not contribute to the production of the reducing intermediate also increases. In this case, in order to remove the NO _x well, it is necessary to oxidize the excess hydrocarbons as described above, therefore in order to remove the NO _x well large amounts of higher hydrocarbons the amount of the surplus is large Oxygen is needed.

In this case, the amount of oxygen can be increased by increasing the oxygen concentration in the exhaust gas. For to remove the NO _x well, therefore, it is necessary to increase the oxygen concentration in the exhaust gas after the hydrocarbon feed when the oxygen concentration in the exhaust gas before the hydrocarbons are fed is high. That is, it is necessary to increase the amplitude of the hydrocarbon concentration as the oxygen concentration in the exhaust gas before the hydrocarbon is supplied is higher.

FIG. 13 shows the relationship between the oxygen concentration in the exhaust gas before the hydrocarbon is supplied and the amplitude ΔH of the hydrocarbon concentration when the same NO _x purification rate is obtained. FIG. 13 shows that in order to obtain the same NO _x purification rate, it is necessary to increase the amplitude ΔH of the hydrocarbon concentration as the oxygen concentration in the exhaust gas before the hydrocarbon is supplied is higher. That is, in order to obtain the same NO _x purification rate, it is necessary to increase the amplitude ΔT of the hydrocarbon concentration as the base air-fuel ratio (A / F) b increases. In other words, in order to remove the NO _x well it can reduce the amplitude ΔT of the hydrocarbon concentration as the base air-fuel ratio (A / F) b becomes lower.

By the way, the base air-fuel ratio (A / F) b becomes the lowest during the acceleration operation. At this time, if the amplitude ΔH of the hydrocarbon concentration is about 200 ppm, NO _x can be purified well. The base air-fuel ratio (A / F) b is usually larger than that during acceleration operation. Therefore, as shown in FIG. 14, when the hydrocarbon concentration amplitude ΔH is 200 ppm or more, a good NO _x purification rate can be obtained. become.

On the other hand, it is known that when the base air-fuel ratio (A / F) b is the highest, a good NO _x purification rate can be obtained by setting the amplitude ΔH of the hydrocarbon concentration to about 10000 ppm. Therefore, in the present invention, the predetermined range of the amplitude of the hydrocarbon concentration is set to 200 ppm to 10000 ppm.

Further, when the vibration period ΔT of the hydrocarbon concentration becomes longer, the oxygen concentration around the active NO ₂ ^* becomes higher while the hydrocarbon is supplied after the hydrocarbon is supplied. In this case, when the vibration period ΔT of the hydrocarbon concentration becomes longer than about 5 seconds, the active NO ₂ ^* begins to be absorbed in the basic layer 53 in the form of nitrate, and therefore the vibration of the hydrocarbon concentration as shown in FIG. When the period ΔT is longer than about 5 seconds, the NO _x purification rate decreases. Therefore, the vibration period ΔT of the hydrocarbon concentration needs to be 5 seconds or less.

On the other hand, when the vibration period ΔT of the hydrocarbon concentration becomes approximately 0.3 seconds or less, the supplied hydrocarbon begins to accumulate on the exhaust gas flow surface of the exhaust purification catalyst 13, and accordingly, the vibration of the hydrocarbon concentration as shown in FIG. When the period ΔT is approximately 0.3 seconds or less, the NO _x purification rate decreases. Therefore, in the present invention, the vibration period of the hydrocarbon concentration is set to be between 0.3 seconds and 5 seconds.

  In the present invention, the hydrocarbon supply amount and the injection timing from the hydrocarbon supply valve 15 are changed so that the amplitude ΔH and the vibration period ΔT of the hydrocarbon concentration become optimal values according to the operating state of the engine. The In this case, in the embodiment according to the present invention, the hydrocarbon supply amount W capable of obtaining this optimum hydrocarbon concentration amplitude ΔH is shown as a function of the injection amount Q from the fuel injection valve 3 and the engine speed N as shown in FIG. ) In advance in the form of a map as shown in FIG. Further, the vibration amplitude ΔT of the optimum hydrocarbon concentration, that is, the hydrocarbon supply period ΔT is also stored in the ROM 32 in advance in the form of a map as shown in FIG. 16B as a function of the injection amount Q and the engine speed N. It is remembered.

Next, the NO _x purification method when the exhaust purification catalyst 13 functions as a NO _x storage catalyst will be specifically described with reference to FIGS. Hereinafter, the NO _x purification method when the exhaust purification catalyst 13 functions as the NO _x storage catalyst will be referred to as a second NO _x purification method.

In this second NO _x purification method, as shown in FIG. 17, the exhaust gas flowing into the exhaust purification catalyst 13 when the occluded NO _x amount ΣNOX occluded in the basic layer 53 exceeds a predetermined allowable amount MAX. The air-fuel ratio (A / F) in of the gas is temporarily made rich. When the air-fuel ratio (A / F) in of the exhaust gas is made rich, NO _x occluded in the basic layer 53 from the basic layer 53 when the air-fuel ratio (A / F) in of the exhaust gas is lean It is released at once and reduced. As a result, NO _x is purified.

Occluded amount of NO _x ΣNOX is calculated from the amount of NO _x exhausted from the engine, for example. In the embodiment according to the present invention is stored in advance in the ROM32 in the form of a map as shown in FIG. 18 as a function of the discharge amount of NO _x NOXA the injection quantity Q and the engine speed N which is discharged from the engine per unit time, The occluded NO _x amount ΣNOX is calculated from this exhausted NO _x amount NOXA. In this case, as described above, the period during which the air-fuel ratio (A / F) in of the exhaust gas is made rich is usually 1 minute or more.

In this second NO _x purification method, as shown in FIG. 19, the exhaust gas flowing into the exhaust purification catalyst 13 by injecting the additional fuel WR from the fuel injection valve 3 into the combustion chamber 2 in addition to the combustion fuel Q. The air / fuel ratio (A / F) in of the gas is made rich. Note that the horizontal axis of FIG. 19 indicates the crank angle. This additional fuel WR is injected when it burns but does not appear as engine output, that is, slightly before ATDC 90 ° after compression top dead center. This fuel amount WR is stored in advance in the ROM 32 as a function of the injection amount Q and the engine speed N in the form of a map as shown in FIG. Of course, in this case, the air-fuel ratio (A / F) in of the exhaust gas can be made rich by increasing the amount of hydrocarbons supplied from the hydrocarbon supply valve 15.

Incidentally To purify NO _x by using the first NO _x purification method, it is necessary to supply a certain amount or more of hydrocarbons even at low concentration of NO _x in the exhaust gas in shorter periods. Thus _the NO _x purification efficiency is deteriorated when a low concentration of NO _x in the exhaust gas. On the other hand, in the second NO _x purification method, when the NO _x concentration in the exhaust gas is low, the time until the occluded NO _x amount ΣNOX reaches the allowable value MAX becomes long, so the air-fuel ratio (A / F of the exhaust gas) ) Only the period for making in rich becomes longer, and the NO _x purification efficiency is not particularly deteriorated. Thus when a low concentration of NO _x in the exhaust gas can be said preferable to use a second of the NO _x purification method than the first NO _x purification method. That is, which of the first NO _x purification method and the second NO _x purification method should be used varies depending on the operating state of the engine.

Next, an engine operation control method will be briefly described.
That is, in the embodiment according to the present invention, the injection amount Q of the combustion fuel to be injected into the combustion chamber 2 is determined based on the depression amount L of the accelerator pedal 40 and the engine speed N as in a general compression ignition internal combustion engine. The fuel is injected from the fuel injection valve 3 into the combustion chamber 2 in accordance with the calculated injection amount Q. In this case, the base air-fuel ratio (A / F) b indicating the ratio (GA / Q) of the intake air amount GA into the combustion chamber 2 with respect to the injection amount Q has an optimum value that changes according to the operating state of the engine. The opening degree of the throttle valve 10 and the opening degree of the EGR control valve 17 are controlled so that the base air-fuel ratio (A / F) b becomes the optimum value.

  FIG. 21A shows the reference opening θ of the EGR control valve 17 necessary for setting the base air-fuel ratio (A / F) b to an optimum value. In the embodiment according to the present invention, this reference opening θ is As shown in FIG. 21A, it is stored in advance in the ROM 32 in the form of a map as a function of the injection amount Q and the engine speed N. Further, the optimum base air-fuel ratio (A / F) b is also stored in advance in the ROM 32 in the form of a map as a function of the injection amount Q and the engine speed N as shown in FIG.

Now, the base air-fuel ratio is not only caused a problem that the combustion is deteriorated at the optimal base air-fuel ratio (A / F) deviates from the combustion chamber 2 from b, _the NO _x purification process by the first NO _x purification method The problem is that the optimum amount of reducing intermediate cannot be generated because the minimum air-fuel ratio of the inflow air-fuel ratio (A / F) in to the exhaust purification catalyst 13 that is periodically oscillated is changed. Is produced. In order to prevent such a problem from occurring, it is necessary to return the base air-fuel ratio to the optimal base air-fuel ratio (A / F) b when the base air-fuel ratio deviates from the optimal base air-fuel ratio (A / F) b. For this purpose, it is necessary to detect the actual base air-fuel ratio.

In this case, if the supply of hydrocarbons is stopped, the air-fuel ratio of the exhaust gas becomes the base air-fuel ratio, and therefore the actual base air-fuel ratio can be easily detected by the air-fuel ratio sensor 24 or 25. However, the exhaust gas air-fuel ratio fluctuates drastically when hydrocarbons are periodically supplied, such as when NO _x purification processing is performed by the first NO _x purification method. The fuel ratio does not easily match the base air-fuel ratio. This will be described with reference to FIG.

  FIG. 22 shows the relationship between the hydrocarbon supply timing from the hydrocarbon supply valve 15 and the output of the air-fuel ratio sensor 24. FIG. 22 shows a case where an air-fuel ratio sensor is used as the air-fuel ratio sensor 24, in which the output voltage (V) increases as the air-fuel ratio of the exhaust gas becomes leaner. As shown in FIG. 22, hydrocarbons are instantaneously injected from the hydrocarbon feed valve 15. However, this hydrocarbon diffuses into the exhaust gas, and since there is a detection delay in the air-fuel ratio sensor 24, the output voltage (V) of the air-fuel ratio sensor 24 does not change instantaneously.

  Further, in this case, when hydrocarbons adhere to the air-fuel ratio sensor 24 in the form of droplets, the output voltage (V) of the air-fuel ratio sensor 24 shows a richer air-fuel ratio until the adhering hydrocarbons evaporate. Therefore, even if the hydrocarbon is supplied instantaneously, the output voltage (V) of the air-fuel ratio sensor 24 changes slowly as shown in FIG.

  In FIG. 22, ΔDt represents a period during which the output voltage (V) of the air-fuel ratio sensor 24 indicates the base air-fuel ratio, and ΔAt represents that the output voltage (V) of the air-fuel ratio sensor 24 is richer than the base air-fuel ratio. This represents a period indicating the air-fuel ratio. That is, during the period ΔAt, the base air-fuel ratio cannot be detected by the air-fuel ratio sensor 24, and the base air-fuel ratio can be detected by the air-fuel ratio sensor 24 only during the period ΔDt. Therefore, hereinafter, this period ΔDt is referred to as a base air-fuel ratio detectable period in which the base air-fuel ratio can be detected by the air-fuel ratio sensor 24. Therefore, in the present invention, the base air-fuel ratio is detected by the air-fuel ratio sensor 24 within the base air-fuel ratio detectable period ΔDt.

  That is, in the present invention, the air-fuel ratio sensors 24 and 25 for detecting the air-fuel ratio of the exhaust gas are disposed in the engine exhaust passage through which hydrocarbons flow, and the hydrocarbons flowing into the exhaust purification catalyst 13 during engine operation The base air-fuel ratio can be detected by the air-fuel ratio sensors 24 and 25 at this time, and the base air-fuel ratio can be detected by the air-fuel ratio sensors 24 and 25 at the same time. A period ΔDt is obtained, and the air-fuel ratio of the exhaust gas detected by the air-fuel ratio sensors 24 and 25 within this base air-fuel ratio detectable period ΔDt is set as the base air-fuel ratio.

  This base air-fuel ratio detectable period ΔDt varies depending on the operating state of the engine. In one embodiment according to the present invention, this base air-fuel ratio detectable period ΔDt is obtained in advance by experiments, and this base air-fuel ratio detectable period ΔDt is stored in advance for the operating state of each engine. For example, as shown in FIG. 23A, the base air-fuel ratio detectable period ΔDt is stored in advance in the ROM 32 in the form of a map as a function of the injection amount Q and the engine speed N. In FIG. 23A, a cross indicates that the base air-fuel ratio detectable period ΔDt does not exist.

In the present invention, the air-fuel ratio sensor 24 is obtained even when the hydrocarbon supply interval is short, such as when the first NO _x purification method is used by obtaining such a base air-fuel ratio detectable period ΔDt. , 25 can detect the base air-fuel ratio. In this case, the base air-fuel ratio is detected using one or both of the air-fuel ratio sensors 24 and 25.

  In the embodiment according to the present invention, the learning control of the base air-fuel ratio is performed based on the air-fuel ratio detected by the air-fuel ratio sensor so that the base air-fuel ratio becomes the optimum base air-fuel ratio (A / F) b. In this case, in the embodiment according to the present invention, when the base air-fuel ratio deviates from the optimum base air-fuel ratio (A / F) b, the intake air amount is set so that the base air-fuel ratio becomes the optimum base air-fuel ratio (A / F) b. The intake air amount is adjusted by adjusting the opening degree of the EGR control valve 17.

  That is, in the present invention, as shown by the broken lines in FIGS. 23B and 23C, the operation region determined from the injection amount Q and the engine speed N is divided into a plurality of operation regions. Each operation region is a learning region for the base air-fuel ratio. As shown in FIG. 23B, in each learning region, the reference opening degree θ of the EGR control valve 17 (FIG. 21) required to set the base air-fuel ratio to the optimum base air-fuel ratio (A / F) b. A correction value for (A)), that is, a learning value Δθij is stored.

  As can be seen from FIG. 23B, in the embodiment according to the present invention, these learning values Δθij are stored only in the learning region where the base air-fuel ratio detectable period ΔDt exists, and the existence of the base air-fuel ratio detectable period ΔDt exists. The learning value Δθij in the learning region not to be calculated is calculated using an interpolation method. During engine operation, the opening degree of the EGR control valve 17 is an opening degree obtained by adding the learning value Δθij to the reference opening degree θ, and at this time, the base air-fuel ratio becomes the optimum base air-fuel ratio (A / F) b.

FIG. 24 shows an engine operation routine.
Referring to FIG. 24, first, the injection amount Q is calculated in step 60, and then in step 61, the fuel for combustion of the injection amount Q is injected from the fuel injection valve 3. Next, at step 62, the opening degree of the throttle valve 10 is controlled. Then calculated reference opening θ of the EGR control valve 17 from the map shown in step 63 FIG. 21 (A), the then learned value Δθ from the map shown in step 6 4 Figure 23 (B) is calculated. Then the final opening degree θt (= θ + Δθ) is calculated by adding the learning value [Delta] [theta] to the reference opening theta Step 6 5, the opening degree of Step 6 6 the EGR control valve 17 is set to the opening degree [theta] t The Next, at step 6 7 supply control of hydrocarbons from the hydrocarbon feed valve 15 is performed.

FIG. 25 shows a learning control routine for the base air-fuel ratio, and this routine is executed by interruption every predetermined time.
Referring to FIG. 25, first, at step 70, it is judged if a predetermined learning time has been reached. When the predetermined learning time is reached, the routine proceeds to step 71, where it is judged if the engine is in a steady operation state. When the engine is in a steady operation state, the routine proceeds to step 72, where whether or not the base air-fuel ratio detectable period ΔDt exists from the map shown in FIG. 23A, that is, whether or not the base air-fuel ratio can be learned. Is determined.

  When learning is possible, the routine proceeds to step 73, where it is determined whether or not it is within the period currently allowed to learn, that is, whether or not it is within the base air-fuel ratio detectable period ΔDt. Currently, when it is within the base air-fuel ratio detectable period ΔDt, the routine proceeds to step 74 where the air-fuel ratio (A / F) t of the exhaust gas is detected by the air-fuel ratio sensor 24, for example. Next, at step 75, the optimum base air-fuel ratio (A / F) b is calculated from the map shown in FIG.

Next, at step 76, the proportionality constant Kij is calculated. At step 77, the learning value Δθij of the opening degree of the EGR control valve 17 is calculated from the following equation using this proportionality constant Kij.
Δθij ← Kij · ((A / F) t− (A / F) b)
Here, the proportionality constant Kij is the correction amount of the opening degree of the EGR control valve 17 necessary for setting the base air-fuel ratio to the optimum base air-fuel ratio (A / F) b, that is, the learning value Δθij and the optimum base air-fuel ratio ( A / F) b shows the relationship with the amount of deviation of air-fuel ratio ((A / F) t- (A / F) b), and this proportionality constant Kij represents each learning as shown in FIG. It is preset for the area.

  When the learning value Δθij is calculated in step 77, the learning value Δθij in the corresponding learning region is updated in FIG. Next, at step 78, it is determined whether or not learning has been completed in all the areas that can be learned. When learning has been completed in all the areas that can be learned, the routine proceeds to step 79 where the next learning time is set.

Next, another embodiment shown in FIGS. 26 to 28 will be described.
As described above, ΔAt in FIG. 22 indicates a period during which the output voltage (V) of the air-fuel ratio sensors 24, 25 is richer than the base air-fuel ratio. The base air-fuel ratio cannot be detected by the fuel ratio sensors 24 and 25. Therefore, in the present invention, this period ΔAt is referred to as a base air-fuel ratio detection inhibition period in which detection of the base air-fuel ratio is prohibited because the base air-fuel ratio cannot be detected.

  In the embodiment according to the present invention, this base air-fuel ratio detection inhibition period ΔAt is stored in advance in the ROM 32 in the form of a map as a function of the injection amount Q and the engine speed N as shown in FIG. On the other hand, in the present invention, the hydrocarbon feed cycle ΔT is also stored in advance as shown in FIG. Therefore, in this embodiment, the base air-fuel ratio detectable period ΔDt (= ΔT−ΔAt) is calculated by subtracting the base air-fuel ratio detection inhibition period ΔAt from the hydrocarbon supply cycle ΔT, and basically the calculated base air-fuel ratio is calculated. The base air-fuel ratio learning control is performed using the fuel ratio detectable period ΔDt.

  Further, in this embodiment, the injection pressure of hydrocarbons from the hydrocarbon feed valve 15 is taken into consideration. That is, when the injection pressure is increased with the same amount of hydrocarbons supplied, the hydrocarbon injection time is shortened, and thus the hydrocarbons are injected at a high density. When the density of the injected hydrocarbons increases, the injected hydrocarbons also flow into the downstream portion of the exhaust purification catalyst 13, and as a result, the production region of the reducing intermediate increases. That is, when the injection pressure is increased, the production amount and the retention amount of the reducing intermediate are increased. Accordingly, at this time, even if the supply cycle ΔT is lengthened, a sufficient amount of the reducing intermediate can be generated and retained.

  In this embodiment, therefore, the hydrocarbon supply cycle ΔT is lengthened as the hydrocarbon injection pressure is increased. More specifically, a correction coefficient KA to be multiplied by the supply cycle ΔT shown in FIG. 16B is stored in advance as a function of the injection pressure P as shown in FIG. The final supply cycle ΔT is calculated by multiplying the coefficient KA. In FIG. 26 (A), Po indicates the injection pressure when the supply cycle ΔT shown in FIG. 16 (B) is obtained.

  On the other hand, when the injection pressure of hydrocarbons is increased and the injection time is shortened, the hydrocarbons adhere to the air-fuel ratio sensors 24 and 25 instantaneously. As a result, the higher the injection pressure, the shorter the time from the start of the attachment of hydrocarbons to the air-fuel ratio sensors 24, 25 to the completion of evaporation of the attached hydrocarbons, that is, the time during which the base air-fuel ratio cannot be detected.

  Therefore, in this embodiment, the base air-fuel ratio detection inhibition period ΔAt is shortened as the hydrocarbon injection pressure is increased. Specifically, the correction coefficient KB to be multiplied by the base air-fuel ratio detection inhibition period ΔAt shown in FIG. 26 (B) is stored in advance as a function of the injection pressure P as shown in FIG. The final base air-fuel ratio detection prohibition period ΔAt is calculated by multiplying ΔAt by this correction coefficient KB. In FIG. 26C, Po indicates the injection pressure when the base air-fuel ratio detection inhibition period ΔAt shown in FIG. 26B is obtained.

  As can be seen from FIG. 26 (A), the hydrocarbon supply cycle ΔT becomes longer as the injection pressure P becomes higher, and as can be seen from FIG. 26 (C), the base air-fuel ratio detection inhibition period ΔAt decreases as the injection pressure P becomes higher. . Therefore, the base air-fuel ratio detectable period ΔDt (= ΔT−ΔAt) is increased as the injection pressure P increases.

That is, in the embodiment according to the present invention, the hydrocarbon injection pressure P is detected when the hydrocarbon supply control is performed by the first NO _x purification method, and the detected hydrocarbon injection pressure P increases. The base air-fuel ratio detectable period ΔDt is increased. Specifically, in this embodiment, the vibration period ΔT of the hydrocarbon concentration and the base air-fuel ratio detection prohibition period ΔAt are stored in advance according to the operating state of the engine, and the vibration period ΔT and the base air-fuel ratio detection are prohibited. By correcting the period ΔAt with the hydrocarbon injection pressure P, the base air-fuel ratio detectable period ΔDt is calculated.

FIG. 27 and FIG. 28 show a base air-fuel ratio learning control routine for executing this embodiment, and this routine is executed by interruption at regular intervals.
Referring to FIG. 27, first, at step 80, it is judged if a predetermined learning time has been reached. When the predetermined learning time is reached, the routine proceeds to step 81 where it is determined whether or not the engine is in a steady operation state. When the engine is in a steady operation state, the routine proceeds to step 82 where the hydrocarbon injection pressure P is detected.

Next, at step 83, the value of the correction coefficient KA is calculated from the relationship shown in FIG. Next, at step 84, the base air-fuel ratio detection prohibition period ΔAtij is calculated from the map shown in FIG. Next, at step 85, the value of the correction coefficient KB is calculated from the relationship shown in FIG. Next, at step 86, a base air-fuel ratio detectable period ΔDtij is calculated based on the following equation.
ΔDtij ← KA ・ ΔT−KB ・ ΔAtij
Next, at step 87, it is judged if the base air-fuel ratio detectable period ΔDt is positive, that is, whether the base air-fuel ratio can be learned.

  When learning is possible, the routine proceeds to step 88, where it is determined whether or not it is within the period currently allowed to learn, that is, whether or not it is within the base air-fuel ratio detectable period ΔDtij. If it is currently within the base air-fuel ratio detectable period ΔDtij, the routine proceeds to step 89 where the air-fuel ratio sensor 24 detects the air-fuel ratio (A / F) t of the exhaust gas, for example. Next, at step 90, the optimum base air-fuel ratio (A / F) b is calculated from the map shown in FIG.

Next, at step 91, the proportionality constant Kij is calculated from the map shown in FIG. 23C. At step 92, the learning value Δθij of the opening degree of the EGR control valve 17 is calculated from the following equation using this proportionality constant Kij.
Δθij ← Kij · ((A / F) t− (A / F) b)

  When the learning value Δθij is calculated, the learning value Δθij in the corresponding learning region is updated in FIG. Next, at step 93, it is determined whether or not learning has been completed in all the areas that can be learned. When learning has been completed in all the areas that can be learned, the routine proceeds to step 94 where the next learning time is set.

  As another embodiment, an oxidation catalyst for reforming hydrocarbons may be disposed in the engine exhaust passage upstream of the exhaust purification catalyst 13.

4 Intake manifold 5 Exhaust manifold 7 Exhaust turbocharger
12 Exhaust pipe
13 Exhaust gas purification catalyst
14 Particulate filter
15 Hydrocarbon supply valve
24, 25 Air-fuel ratio sensor

Claims (8)

A hydrocarbon supply valve for supplying hydrocarbons is arranged in the engine exhaust passage, and the hydrocarbons injected from the hydrocarbon supply valve and partially oxidized hydrocarbons react with NO _x contained in the exhaust gas to react with nitrogen and An exhaust purification catalyst for producing a reducing intermediate containing hydrocarbon is disposed downstream of the hydrocarbon feed valve , and a noble metal catalyst is supported on the exhaust purification catalyst and the reducing property around the noble metal catalyst . A basic layer for holding an intermediate is formed, and the exhaust purification catalyst is a hydrocarbon that injects hydrocarbons from a hydrocarbon supply valve at a predetermined supply interval and flows into the exhaust purification catalyst. which has a property for reducing the NO _x to a vibrating with a cycle of the amplitude and a predetermined range within a determined range of concentrations previously contained in the exhaust gas, 該予Me a supply interval of the hydrocarbon Determined It has a property that storage amount of the NO _x contained as longer than the sheet interval in the exhaust gas is increased, the vibration period of the predetermined range of the hydrocarbon concentration of the reducing on the basic layer In the exhaust gas purification apparatus for an internal combustion engine, which is a period necessary for continuing to generate the intermediate, the concentration of hydrocarbons flowing into the exhaust gas purification catalyst by injecting hydrocarbons from the hydrocarbon supply valve at the predetermined supply interval. a first NO _x purification method for reducing the NO _x contained in the exhaust gas by oscillating with a cycle of the amplitude and the predetermined range within which said predetermined, the exhaust purification catalyst Using the second NO _x purification method for purifying NO _x by switching the air-fuel ratio of the inflowing exhaust gas from lean to rich at an interval longer than the predetermined supply interval , Engine exhaust An air-fuel ratio sensor for detecting the air-fuel ratio of the exhaust gas is arranged in the road, and the concentration of hydrocarbons flowing into the exhaust purification catalyst during engine operation is determined within the predetermined range and the predetermined range. When the first NO _x purification method is performed by oscillating with an internal period, a base air-fuel ratio detectable period in which the base air-fuel ratio can be detected by the air-fuel ratio sensor is obtained. An exhaust gas purification apparatus for an internal combustion engine, wherein an air-fuel ratio of exhaust gas detected by the air-fuel ratio sensor within a detectable period is a base air-fuel ratio.   The hydrocarbon injection pressure is detected when the concentration of hydrocarbons flowing into the exhaust purification catalyst during engine operation is vibrated with an amplitude within the predetermined range and a period within the predetermined range. The exhaust gas purification apparatus for an internal combustion engine according to claim 1, wherein the base air-fuel ratio detectable period is increased as the detected injection pressure of the hydrocarbon is increased.   A vibration cycle of the hydrocarbon concentration and a base air-fuel ratio detection prohibition period in which detection of the base air-fuel ratio is prohibited because the base air-fuel ratio cannot be detected are stored in advance according to the operating state of the engine. 3. The exhaust gas purifying apparatus for an internal combustion engine according to claim 2, wherein the base air-fuel ratio detectable period is calculated by correcting the base air-fuel ratio detection prohibited period by the hydrocarbon injection pressure.   2. The exhaust gas purification apparatus for an internal combustion engine according to claim 1, wherein the base air-fuel ratio detectable period is stored in advance for the operating state of each engine.   The exhaust purification device of an internal combustion engine according to claim 1, wherein learning control of the base air-fuel ratio is performed based on the air-fuel ratio detected by the air-fuel ratio sensor. The exhaust gas purification apparatus for an internal combustion engine according to claim 1 , wherein the vibration period of the hydrocarbon concentration is between 0.3 seconds and 5 seconds . The exhaust purification device for an internal combustion engine according to claim 1 , wherein the noble metal catalyst is composed of platinum Pt and at least one of rhodium Rh and palladium Pd . The basic layer comprising a metal which can donate electrons to the alkali metal or alkaline earth metal or rare earth or NO _x in the exhaust gas flow on the surface of the exhaust purification catalyst is formed, the surface of the base layer is the base The exhaust gas purification apparatus for an internal combustion engine according to claim 1 , wherein the exhaust gas circulation surface portion is formed .

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