Exhaust gas control apparatus for internal combustion engine

A rich control for temporarily declining an air-fuel ratio of exhaust gas discharged from an engine combustion chamber is performed by an additional fuel being injected into a cylinder in an expansion stroke or an exhaust stroke in a state where a throttle opening degree is switched from a base throttle opening degree to a throttle opening degree for the rich control and an EGR rate is switched from a base EGR rate to an EGR rate for the rich control. The rich control is initiated by switching a low pressure side EGR control valve opening degree (VEGRL) to a low pressure side EGR control valve opening degree for the rich control (VEGRLR), then switching a high pressure side EGR control valve opening degree (VEGRH) to a high pressure side EGR control valve opening degree for the rich control (VEGRHR), then controlling the throttle opening degree (VTH), and then initiating the injection of the additional fuel (Qa).

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a national phase application of International Application No. PCT/JP2013/084352, filed Dec. 20, 2013, the content of which is incorporated herein by reference.

TECHNICAL FIELD

The invention relates to an exhaust gas control apparatus for an internal combustion engine.

BACKGROUND ART

An exhaust gas control apparatus for an internal combustion engine is known (refer to PTL 1) in which an engine intake passage and an engine exhaust passage are connected to each other by an exhaust gas recirculation passage, an exhaust gas recirculation control valve controlling an exhaust gas recirculation rate is disposed in the exhaust gas recirculation passage, a rich control for temporarily declining an air-fuel ratio of exhaust gas discharged from an engine combustion chamber is performed in a state where a throttle opening degree is decreased and the exhaust gas recirculation rate is reduced, and an opening degree of the exhaust gas recirculation control valve is decreased along with the decrease in the throttle opening degree, such that the exhaust gas recirculation rate is reduced, when the rich control is initiated.

In addition, another exhaust gas control apparatus for an internal combustion engine is also known in which the air-fuel ratio of the exhaust gas discharged from the engine combustion chamber is temporarily declined by an additional fuel being injected into a cylinder in an expansion stroke or an exhaust stroke in a state where the throttle opening degree is decreased and the exhaust gas recirculation rate is reduced during the rich control. In this exhaust gas control apparatus, the rich control is performed in the state where the throttle opening degree is decreased and the exhaust gas recirculation rate is reduced, and thus an additional fuel amount that is required for the air-fuel ratio of the exhaust gas to be enriched can be reduced.

Furthermore, an internal combustion engine is also known that is provided with an exhaust turbocharger which drives a compressor disposed in an engine intake passage on the upstream side of a throttle valve with an exhaust turbine disposed in an engine exhaust passage, a high pressure exhaust gas recirculation passage which connects the engine exhaust passage on the upstream side of the exhaust turbine and the engine intake passage on the downstream side of the throttle valve to each other, a low pressure exhaust gas recirculation passage which connects the engine exhaust passage on the downstream side of the exhaust turbine and the engine intake passage on the upstream side of the compressor to each other, a high pressure exhaust gas recirculation control valve which is disposed in the high pressure exhaust gas recirculation passage so as to control a high pressure exhaust gas recirculation rate, and a low pressure exhaust gas recirculation control valve which is disposed in the low pressure exhaust gas recirculation passage so as to control a low pressure exhaust gas recirculation rate.

CITATION LIST

Patent Literature

SUMMARY OF THE INVENTION

In the internal combustion engine that is provided with the high pressure exhaust gas recirculation passage and the low pressure side exhaust gas recirculation passage, a simultaneous decrease in the throttle opening degree, a high pressure exhaust gas recirculation control valve opening degree, and a low pressure exhaust gas recirculation control valve opening degree during the initiation of the rich control results in a significant decrease in an in-cylinder gas amount and a significant reduction in a compression end temperature. As a result, the risk of the occurrence of a misfire increases.

An object of the invention is to provide an exhaust gas control apparatus for an internal combustion engine that is capable of suppressing the risk of the occurrence of a misfire when a rich control is initiated.

According to the invention, there is provided an exhaust gas control apparatus for an internal combustion engine having an exhaust turbocharger driving a compressor disposed in an engine intake passage on an upstream side of a throttle valve with an exhaust turbine disposed in an engine exhaust passage, a high pressure exhaust gas recirculation passage connecting the engine exhaust passage on an upstream side of the exhaust turbine and the engine intake passage on a downstream side of the throttle valve to each other, a low pressure exhaust gas recirculation passage connecting the engine exhaust passage on a downstream side of the exhaust turbine and the engine intake passage on an upstream side of the compressor to each other, a high pressure exhaust gas recirculation control valve disposed in the high pressure exhaust gas recirculation passage so as to control a high pressure exhaust gas recirculation rate, and a low pressure exhaust gas recirculation control valve disposed in the low pressure exhaust gas recirculation passage so as to control a low pressure exhaust gas recirculation rate, in which a rich control for temporarily declining an air-fuel ratio of exhaust gas discharged from an engine combustion chamber is performed by an additional fuel being injected into a cylinder in an expansion stroke or an exhaust stroke in a state where a throttle opening degree, which is an opening degree of the throttle valve disposed in the intake passage and controlling a suctioned air amount, is switched from a base throttle opening degree to a throttle opening degree for the rich control lower than the base throttle opening degree, the high pressure exhaust gas recirculation rate is switched from a base high pressure exhaust gas recirculation rate to a high pressure exhaust gas recirculation rate for the rich control different from the base high pressure exhaust gas recirculation rate, and the low pressure exhaust gas recirculation rate is switched from a base low pressure exhaust gas recirculation rate to a low pressure exhaust gas recirculation rate for the rich control different from the base low pressure exhaust gas recirculation rate, a low pressure exhaust gas recirculation control valve opening degree being controlled first such that the low pressure exhaust gas recirculation rate is switched to the low pressure exhaust gas recirculation rate for the rich control, and then a high pressure exhaust gas recirculation control valve opening degree being controlled such that the high pressure exhaust gas recirculation rate is switched to the high pressure exhaust gas recirculation rate for the rich control, and then the throttle opening degree being switched to the throttle opening degree for the rich control, and then the injection of the additional fuel being initiated when the rich control is initiated.

The risk of the occurrence of a misfire can be suppressed when the rich control is initiated.

MODES FOR CARRYING OUT THE INVENTION

An overall view of a compression ignition-type internal combustion engine is illustrated inFIG. 1.

Referring toFIG. 1, 1represents an engine main body,2represents respective combustion chambers of cylinders,3represents electronically controlled fuel injection valves for injecting a fuel into the respective combustion chambers2,4represents an intake manifold, and5represents an exhaust manifold. The intake manifold4is connected to an outlet of a compressor7aof an exhaust turbocharger7via an intake duct6, and an inlet of the compressor7ais connected to an air cleaner9via an intake air introduction pipe8awhere a suctioned air amount detector8is disposed. A throttle valve10that is driven by an actuator is disposed in the intake duct6, and a cooling device11for cooling suctioned air flowing through the intake duct6is disposed around the intake duct6. In the example that is illustrated inFIG. 1, engine cooling water is guided into the cooling device11and the suctioned air is cooled by the engine cooling water. In addition, a pressure sensor4pfor detecting a pressure in the intake manifold4, that is, an intake pressure, is attached to the intake manifold4at a position on the downstream side of the throttle valve10, and a pressure sensor5pfor detecting a pressure in the exhaust manifold5, that is, an exhaust pressure, is attached to the exhaust manifold5. Furthermore, a temperature sensor5tfor detecting the temperature of exhaust gas in the exhaust manifold5is attached to the exhaust manifold5.

The exhaust manifold5is connected to an inlet of an exhaust turbine7bof the exhaust turbocharger7, and an outlet of the exhaust turbine7bis connected to an inlet of an exhaust gas control catalyst13via an exhaust pipe12a. In the example according to the invention, the exhaust gas control catalyst13is a NOx storage catalyst. An outlet of the exhaust gas control catalyst13is connected to a particulate filter14via an exhaust pipe12b. In the exhaust pipe12a, a hydrocarbon supply valve15for supplying hydrocarbons consisting of gas oil and other fuels used as a fuel of the compression ignition-type internal combustion engine is disposed on the upstream side of the exhaust gas control catalyst13. In the example that is illustrated inFIG. 1, the gas oil is used as the hydrocarbon that is supplied from the hydrocarbon supply valve15. An exhaust pipe12cis connected to the particulate filter14. The invention can also be applied to a spark ignition-type internal combustion engine in which combustion is performed at a lean air-fuel ratio. In this case, the hydrocarbon supply valve15supplies hydrocarbons consisting of gasoline and other fuels used as a fuel of the spark ignition-type internal combustion engine.

The exhaust manifold5on the upstream side of the exhaust turbine7band the intake manifold4on the downstream side of the throttle valve10are connected to each other via a high pressure exhaust gas recirculation (hereinafter, referred to as EGR) passage16H, and an electrically controlled high pressure EGR control valve17H is disposed in the high pressure EGR passage16H. In addition, a cooling device18H for cooling EGR gas flowing through the high pressure EGR passage16H is disposed around the high pressure EGR passage16H. In addition, an exhaust throttle valve29is disposed in the exhaust pipe12c. The exhaust pipe12con the upstream side of the exhaust throttle valve29and the intake air introduction pipe8aon the downstream side of the suctioned air amount detector8are connected to each other via a low pressure EGR passage16L, and an electrically controlled low pressure EGR control valve17L is disposed in the low pressure EGR passage16L. In addition, a cooling device18L for cooling the EGR gas flowing through the low pressure EGR passage16L is disposed around the low pressure EGR passage16L. In another example, the exhaust throttle valve is omitted. A pressure sensor8pfor detecting a pressure in the intake air introduction pipe8ais attached to the intake air introduction pipe8aon the downstream side of the suctioned air amount detector8, and a pressure sensor12pfor detecting a pressure in the exhaust pipe12cis attached to the exhaust pipe12con the upstream side of the exhaust throttle valve29.

Each of the fuel injection valves3is connected to a common rail20via a fuel supply pipe19, and this common rail20is connected to a fuel tank22via a fuel pump21that is electronically controlled and has a variable discharge amount. The fuel that is stored in the fuel tank22is supplied into the common rail20by the fuel pump21, and the fuel supplied into the common rail20is supplied to the fuel injection valves3via the respective fuel supply pipes19.

An electronic control unit30consists of a digital computer and is provided with a read-only memory (ROM)32, a random access memory (RAM)33, a CPU (microprocessor)34, an input port35, and an output port36, which are connected to one another by a bidirectional bus31. A temperature sensor24for detecting the temperature of the exhaust gas flowing out from the exhaust gas control catalyst13is attached to the exhaust pipe12bon the downstream side of the exhaust gas control catalyst13. The temperature of the exhaust gas flowing out from the exhaust gas control catalyst13represents the temperature of the exhaust gas control catalyst13. In addition, a differential pressure sensor26for detecting a differential pressure across the particulate filter14is attached to the particulate filter14. Output signals of the temperature sensor24, the differential pressure sensor26, the pressure sensors4p,5p,8p,12p, the temperature sensor5t, and the suctioned air amount detector8are input to the input port35via respectively corresponding AD converters37. In addition, a load sensor41that generates an output voltage which is proportional to a depression amount L of an accelerator pedal40is connected to the accelerator pedal40, and the output voltage of the load sensor41is input to the input port35via the corresponding AD converter37. Furthermore, a crank angle sensor42is connected to the input port35, and the crank angle sensor42generates an output pulse each time a crankshaft rotates by, for example, 15°. The output port36is connected to the fuel injection valves3, the actuator that drives the throttle valve10, the hydrocarbon supply valve15, the high pressure EGR control valve17H, the low pressure EGR control valve17L, the exhaust throttle valve29, and the fuel pump21via corresponding drive circuits38.

FIG. 2schematically shows a surface part of a catalyst carrier that is supported on a substrate of the exhaust gas control catalyst13which is illustrated inFIG. 1. In this exhaust gas control catalyst13, noble metal catalysts51consisting of platinum Pt are supported on a catalyst carrier50consisting of, for example, alumina and a basic layer53is formed on the catalyst carrier50as illustrated inFIG. 2. The basic layer53contains at least one selected from an alkali metal such as potassium K, sodium Na, and cesium Cs, an alkaline earth metal such as barium Ba and calcium Ca, a rare earth such as lanthanoid, and a metal capable of donating an electron to NOx such as silver Ag, copper Cu, iron Fe, and iridium Ir. Ceria CeO2is contained in this basic layer53, and thus the exhaust gas control catalyst13has an oxygen storage capacity. In addition, rhodium Rh or palladium Pd can be supported, in addition to the platinum Pt, on the catalyst carrier50of the exhaust gas control catalyst13. Because the exhaust gas flows along the top of the catalyst carrier50, it can be said that the noble metal catalysts51are supported on an exhaust gas flow surface of the exhaust gas control catalyst13. A surface of the basic layer53exhibits basicity, and thus the surface of the basic layer53will be referred to as a basic exhaust gas flow surface part54.

When the hydrocarbon is injected into the exhaust gas from the hydrocarbon supply valve15, the hydrocarbon is reformed in the exhaust gas control catalyst13. In the invention, NOx is removed in the exhaust gas control catalyst13by the use of the hydrocarbon reformed at this time.FIG. 3schematically shows a reforming action that is performed in the exhaust gas control catalyst13at this time. As illustrated inFIG. 3, the hydrocarbon HC injected from the hydrocarbon supply valve15is turned into radical hydrocarbons HC with a small carbon number by the noble metal catalysts51.

FIG. 4shows a timing of the hydrocarbon supply from the hydrocarbon supply valve15and a change in an air-fuel ratio (A/F)in of the exhaust gas flowing into the exhaust gas control catalyst13. The change in this air-fuel ratio (A/F)in depends on a change in the concentration of the hydrocarbon in the exhaust gas flowing into the exhaust gas control catalyst13, and thus it can be said that the change in the air-fuel ratio (A/F)in that is illustrated inFIG. 4represents the change in the concentration of the hydrocarbon. Nevertheless, because the air-fuel ratio (A/F)in decreases as the hydrocarbon concentration increases, the hydrocarbon concentration is higher when the air-fuel ratio (A/F)in is on a rich side inFIG. 4.

FIG. 5shows, with respect to respective catalyst temperatures TC of the exhaust gas control catalyst13, a NOx removal rate in the exhaust gas control catalyst13at a time when the air-fuel ratio (A/F)in of the exhaust gas flowing into the exhaust gas control catalyst13is periodically enriched as illustrated inFIG. 4by the concentration of the hydrocarbons flowing into the exhaust gas control catalyst13being periodically changed. As a result of long-term researches on NOx removal, it has been found that an extremely high NOx removal rate can be obtained, even in a high-temperature region of at least 400° C. as illustrated inFIG. 5, when the concentration of the hydrocarbons flowing into the exhaust gas control catalyst13is vibrated at an amplitude within a range determined in advance and a cycle within a range determined in advance.

It has also been found that a large amount of reducing intermediates containing nitrogen and hydrocarbons continue to be held or adsorbed on the surface of the basic layer53, that is, on the basic exhaust gas flow surface part54of the exhaust gas control catalyst13at this time and the reducing intermediate plays a central role for the achievement of the high NOx removal rate. Hereinafter, this will be described with reference toFIGS. 6A and 6B.FIGS. 6A and 6Bschematically show the surface part of the catalyst carrier50of the exhaust gas control catalyst13, and a reaction that is estimated to occur when the concentration of the hydrocarbons flowing into the exhaust gas control catalyst13is vibrated at the amplitude within the range determined in advance and the cycle within the range determined in advance is illustrated inFIGS. 6A and 6B.

FIG. 6Ashows a time when the concentration of the hydrocarbons flowing into the exhaust gas control catalyst13is low, andFIG. 6Bshows a time when the air-fuel ratio (A/F)in of the exhaust gas flowing into the exhaust gas control catalyst13has become rich with the hydrocarbons supplied from the hydrocarbon supply valve15, that is, a time when the concentration of the hydrocarbons flowing into the exhaust gas control catalyst13is high.

As is apparent fromFIG. 4, the air-fuel ratio of the exhaust gas that flows into the exhaust gas control catalyst13is maintained lean with the exception of one moment, and thus the exhaust gas that flows into the exhaust gas control catalyst13is usually in a hyperoxic state. At this time, some of the NO that is contained in the exhaust gas adheres onto the exhaust gas control catalyst13, and some of the NO that is contained in the exhaust gas becomes NO2after being oxidized on the platinum51as illustrated inFIG. 6A. Then, this NO2is further oxidized and becomes NO3. In addition, some of the NO2becomes NO2—. Accordingly, NO2— and NO3are generated on the platinum Pt 51. The NO adhering on the exhaust gas control catalyst13and the NO2— and the NO3generated on the platinum Pt 51 have high levels of activity, and thus these NO, NO2—, and NO3will be referred to as active NOx* hereinbelow.

When the air-fuel ratio (A/F)in of the exhaust gas flowing into the exhaust gas control catalyst13is enriched with the hydrocarbon supplied from the hydrocarbon supply valve15, this hydrocarbon adheres to the entire exhaust gas control catalyst13in order. Most of these adhering hydrocarbons react with oxygen and are burned in order, and some of the adhering hydrocarbons are reformed in the exhaust gas control catalyst13and become radical in order as illustrated inFIG. 3. Accordingly, the hydrocarbon concentration around the active NOx* increases as illustrated inFIG. 6B. When a state where the concentration of the oxygen around the active NOx* is high continues for a certain period of time or longer after the active NOx* generation, the active NOx* is oxidized and is absorbed into the basic layer53in the form of a nitrate ion NO3—. When the hydrocarbon concentration around the active NOx* is increased before the elapse of the certain period of time, however, the active NOx* reacts with the radical hydrocarbon HC on the platinum51and then the reducing intermediate is generated as illustrated inFIG. 6B. This reducing intermediate adheres or is adsorbed onto the surface of the basic layer53.

It is conceivable that the reducing intermediate that is first generated at this time is a nitro compound R—NO2. Once generated, this nitro compound R—NO2becomes a nitrile compound R—CN. However, this nitrile compound R—CN can survive only for an instant in that state, and immediately becomes an isocyanate compound R—NCO. When hydrolyzed, this isocyanate compound R—NCO becomes an amine compound R—NH2. In this case, however, it is conceivable that it is a part of the isocyanate compound R—NCO that is hydrolyzed. Accordingly, it is conceivable that most of the reducing intermediates held or adsorbed on the surface of the basic layer53are the isocyanate compound R—NCO and the amine compound R—NH2as illustrated inFIG. 6B.

When the hydrocarbons HC adhere around the generated reducing intermediates as illustrated inFIG. 6B, the reducing intermediates are hampered by the hydrocarbons HC and no further reaction proceeds. In this case, the concentration of the hydrocarbons flowing into the exhaust gas control catalyst13declines, and then the hydrocarbons adhering around the reducing intermediates are oxidized and disappear. Once the concentration of the oxygen around the reducing intermediates increases as a result, the reducing intermediates react with the NOx and the active NOx* in the exhaust gas, react with the ambient oxygen, or autolyze. Then, the reducing intermediates R—NCO and R—NH2are converted to N2, CO2, and H2O as illustrated inFIG. 6A, which causes the NOx to be removed.

As described above, in the exhaust gas control catalyst13, the reducing intermediates are generated by the concentration of the hydrocarbons flowing into the exhaust gas control catalyst13being increased, the reducing intermediates react with the NOx, active NOx*, and oxygen in the exhaust gas or autolyze when the oxygen concentration is increased after the decline in the concentration of the hydrocarbons flowing into the exhaust gas control catalyst13, and then the NOx is removed. In other words, when the NOx is removed by the exhaust gas control catalyst13, the concentration of the hydrocarbons flowing into the exhaust gas control catalyst13needs to be periodically changed.

As a matter of course, in this case, the hydrocarbon concentration needs to be raised to a concentration that is sufficiently high for the reducing intermediate generation and the hydrocarbon concentration needs to be lowered to a concentration that is sufficiently low for the generated reducing intermediates to react with the NOx, active NOx*, and oxygen in the exhaust gas or autolyze. In other words, the concentration of the hydrocarbons flowing into the exhaust gas control catalyst13is required to be vibrated at the amplitude within the range determined in advance. In this case, these reducing intermediates should be held on the basic layer53, that is, on the basic exhaust gas flow surface part54, until the generated reducing intermediates R—NCO and R—NH2react with the NOx, active NOx*, and oxygen in the exhaust gas or autolyze. This is a reason why the basic exhaust gas flow surface part54is disposed.

When the hydrocarbon supply cycle is extended, a period in which the oxygen concentration increases between the hydrocarbon supply and the next hydrocarbon supply is lengthened, and thus the active NOx* is absorbed into the basic layer53in the form of nitrate without generating the reducing intermediate. For this to be avoided, the concentration of the hydrocarbons flowing into the exhaust gas control catalyst13needs to be vibrated at the cycle within the range determined in advance.

In the example according to the invention, the noble metal catalysts51are supported on the exhaust gas flow surface of the exhaust gas control catalyst13so that the reducing intermediates R—NCO and R—NH2containing the nitrogen and the hydrocarbons are generated by the NOx contained in the exhaust gas and the reformed hydrocarbons reacting with each other, the noble metal catalysts51includes the basic exhaust gas flow surface part54so that the generated reducing intermediates R—NCO and R—NH2are held in the exhaust gas control catalyst13, the reducing intermediates R—NCO and R—NH2held on the basic exhaust gas flow surface part54are converted to N2, CO2, and H2O, and the hydrocarbon concentration vibration cycle is a vibration cycle that is required for the generation of the reducing intermediates R—NCO and R—NH2to continue. In this regard, the example that is illustrated inFIG. 4has an injection interval of three seconds.

When the cycle of the vibration of the hydrocarbon concentration, that is, the cycle of the injection of the hydrocarbons HC from the hydrocarbon supply valve15, exceeds the cycle within the range determined in advance described above, the reducing intermediates R—NCO and R—NH2disappear from the top of the surface of the basic layer53. At this time, the active NOx* generated on the platinum Pt 51 diffuses in the basic layer53in the form of the nitrate ion NO3— as illustrated inFIG. 7Aand becomes nitrate. In other words, at that time, the NOx in the exhaust gas is absorbed into the basic layer53in the form of nitrate.

FIG. 7Bshows a case where the air-fuel ratio of the exhaust gas flowing into the exhaust gas control catalyst13becomes a stoichiometric air-fuel ratio or is enriched when the NOx is absorbed into the basic layer53in the form of the nitrate as described above. In this case, the concentration of the oxygen in the exhaust gas declines, and thus the reaction proceeds in the reverse direction (NO3—→NO2). Accordingly, the nitrates absorbed in the basic layer53become the nitrate ions NO3— in order and are released from the basic layer53in the form of NO2as illustrated inFIG. 7B. Then, the released NO2is reduced by the hydrocarbons HC and CO contained in the exhaust gas.

FIG. 8shows a case where the air-fuel ratio (A/F)in of the exhaust gas flowing into the exhaust gas control catalyst13is temporarily enriched shortly before the saturation of the NOx absorption capacity of the basic layer53. In the example that is illustrated inFIG. 8, this rich control has a time interval of at least one minute. In this case, the NOx absorbed into the basic layer53when the air-fuel ratio (A/F)in of the exhaust gas is lean is released at once from the basic layer53and is reduced when the air-fuel ratio (A/F)in of the exhaust gas is temporarily enriched. Accordingly, in this case, the basic layer53plays the role of an absorbent for temporary NOx absorption.

At this time, the basic layer53temporarily adsorbs the NOx in some cases. Accordingly, using the term of storage as a term including both absorption and adsorption, the basic layer53at this time plays the role of a NOx storing agent for temporary NOx storage. In other words, referring to the ratio of the air and the fuel (hydrocarbon) supplied into the engine intake passage, the combustion chambers2, and the exhaust passage on the upstream side of the exhaust gas control catalyst13as the air-fuel ratio of the exhaust gas, the exhaust gas control catalyst13in this case functions as a NOx storage catalyst that stores the NOx when the air-fuel ratio of the exhaust gas is lean and releases the stored NOx once the oxygen concentration in the exhaust gas declines.

The solid line inFIG. 9represents the NOx removal rate at a time when the exhaust gas control catalyst13is allowed to function as the NOx storage catalyst as described above. The horizontal axis inFIG. 9represents the catalyst temperature TC of the exhaust gas control catalyst13. In a case where the exhaust gas control catalyst13is allowed to function as the NOx storage catalyst as described above, an extremely high NOx removal rate can be obtained when the catalyst temperature TC is 300° C. to 400° C. but the NOx removal rate declines once the catalyst temperature TC reaches a high temperature of at least 400° C. as illustrated with the solid line inFIG. 9. InFIG. 9, the NOx removal rate that is illustrated inFIG. 5is illustrated with a dashed line.

The above-described decline in the NOx removal rate at the catalyst temperature TC of 400° C. or higher is because the nitrate is thermally decomposed and is released from the exhaust gas control catalyst13in the form of NO2once the catalyst temperature TC becomes equal to or higher than 400° C. In other words, it is difficult to obtain a high NOx removal rate when the catalyst temperature TC is high insofar as the NOx is stored in the form of the nitrate. By the novel NOx removal control that is illustrated inFIGS. 4 to 6B, however, no nitrate is generated or an extremely small amount of the nitrate is generated even if the nitrate is generated as is apparent fromFIGS. 6A and 6B, and thus a high NOx removal rate can be obtained as illustrated inFIG. 5even when the catalyst temperature TC is high.

In the example according to the invention, the hydrocarbon supply valve15for supplying the hydrocarbons is disposed in the engine exhaust passage, the exhaust gas control catalyst13is disposed in the engine exhaust passage on the downstream side of the hydrocarbon supply valve15, the noble metal catalysts51are supported on the exhaust gas flow surface of the exhaust gas control catalyst13, and the noble metal catalysts51includes the basic exhaust gas flow surface part54so that NOx is removed by the use of this novel NOx removal control. The exhaust gas control catalyst13has the property of reducing the NOx that is contained in the exhaust gas when the concentration of the hydrocarbons flowing into the exhaust gas control catalyst13is vibrated at the amplitude within the range determined in advance and the cycle within the range determined in advance and the property of having an increasing amount of storage of the NOx contained in the exhaust gas when the hydrocarbon concentration vibration cycle exceeds this range determined in advance. While the engine is in operation, the hydrocarbons are injected from the hydrocarbon supply valve15at a cycle determined in advance, and then the NOx contained in the exhaust gas is reduced in the exhaust gas control catalyst13.

In other words, it can be said that the NOx removal control that is illustrated inFIGS. 4 to 6Bis a novel NOx removal control by which the NOx is removed with little nitrate formation in a case where the exhaust gas control catalyst where the basic layer is formed to be capable of supporting the noble metal catalyst and absorbing the NOx is used. In actuality, compared to a case where the exhaust gas control catalyst13is allowed to function as the NOx storage catalyst, the nitrate that is detected from the basic layer53has an extremely small amount in a case where this novel NOx removal control is used. Hereinafter, this novel NOx removal control will be referred to as a first NOx removal control.

As described above, when the cycle ΔT of the hydrocarbon injection from the hydrocarbon supply valve15is extended, the period in which the oxygen concentration around the active NOx* increases is lengthened between the hydrocarbon injection and the next hydrocarbon injection. In this case, in the example that is illustrated inFIG. 1, the active NOx* begins to be absorbed into the basic layer53in the form of the nitrate once the hydrocarbon injection cycle ΔT exceeds approximately five seconds. Accordingly, as illustrated inFIG. 10, the NOx removal rate declines once the hydrocarbon concentration vibration cycle ΔT exceeds approximately five seconds. Hence, in the example that is illustrated inFIG. 1, the hydrocarbon injection cycle ΔT is required to be five seconds or less.

In the example according to the invention, the injected hydrocarbons begin to be deposited on the exhaust gas flow surface of the exhaust gas control catalyst13once the hydrocarbon injection cycle ΔT becomes approximately 0.3 seconds or less. Accordingly, as illustrated inFIG. 10, the NOx removal rate declines once the hydrocarbon injection cycle ΔT becomes approximately 0.3 seconds or less. In this regard, in the example according to the invention, the hydrocarbon injection cycle is between 0.3 seconds and five seconds.

In the example according to the invention, the air-fuel ratio (A/F)in of the exhaust gas flowing into the exhaust gas control catalyst13and the injection cycle ΔT are controlled to have optimum values in accordance with engine operation states by the quantity and timing of the hydrocarbon injection from the hydrocarbon supply valve15being changed. In this case, in the example according to the invention, an optimal hydrocarbon injection quantity W at a time when a NOx removal action according to the first NOx removal control is performed is stored in advance in the ROM32in the form of the map which is illustrated inFIG. 11and as a function of the depression amount L of the accelerator pedal40and an engine rotation speed N and an optimal hydrocarbon injection cycle ΔT at that time is also stored in advance in the ROM32in the form of a map and as the function of the depression amount L of the accelerator pedal40and the engine rotation speed N.

Hereinafter, a NOx removal control in a case where the exhaust gas control catalyst13is allowed to function as the NOx storage catalyst will be described in detail with reference toFIGS. 12 to 15. The NOx removal control in the case where the exhaust gas control catalyst13is allowed to function as the NOx storage catalyst as described above will be referred to as a second NOx removal control hereinbelow.

In this second NOx removal control, the air-fuel ratio (A/F)in of the exhaust gas flowing into the exhaust gas control catalyst13is temporarily enriched, as illustrated inFIG. 12, when the amount ΣNOX of the NOx stored in the basic layer53exceeds an allowable amount MAX determined in advance. Once the air-fuel ratio (A/F)in of the exhaust gas is enriched, the NOx stored in the basic layer53when the air-fuel ratio (A/F)in of the exhaust gas is lean is released at once from the basic layer53and is reduced. This causes the NOx to be removed.

The stored NOx amount ΣNOX is calculated from, for example, the amount of the NOx that is discharged from the engine. In the example according to the invention, the amount NOXA of the NOx discharged from the engine per unit time is stored in advance in the ROM32in the form of the map which is illustrated inFIG. 13and as the function of the depression amount L of the accelerator pedal40and the engine rotation speed N, and the stored NOx amount ΣNOX is calculated from this discharged NOx amount NOXA. In this case, the cycle in which the air-fuel ratio (A/F)in of the exhaust gas is enriched as described above is usually at least one minute.

According to the second NOx removal control, the air-fuel ratio (A/F)in of the exhaust gas flowing into the exhaust gas control catalyst13is enriched by an additional fuel Qa as well as a fuel for combustion, that is, a main fuel Qm, being injected from the fuel injection valves3into the combustion chambers2as illustrated inFIG. 14. The horizontal axis inFIG. 14represents a crank angle. As an example, this additional fuel Qa is injected after a compression top dead center and slightly before the ATDC 90°. This additional fuel amount Qa is stored in advance in the ROM32in the form of the map which is illustrated inFIG. 15and as the function of the depression amount L of the accelerator pedal40and the engine rotation speed N.

In the example according to the invention, the NOx removal action according to the first NOx removal control and a NOx removal action according to the second NOx removal control are selectively performed. Whether to perform the NOx removal action according to the first NOx removal control or the NOx removal action according to the second NOx removal control is determined, for example, as follows. In other words, the NOx removal rate at a time when the NOx removal action according to the first NOx removal control is performed begins to decline rapidly, as illustrated inFIG. 5, once the temperature TC of the exhaust gas control catalyst13becomes equal to or lower than a limit temperature TX. In contrast, the NOx removal rate at a time when the NOx removal action according to the second NOx removal control is performed declines relatively slowly, as illustrated inFIG. 9, when the temperature TC of the exhaust gas control catalyst13declines. Accordingly, in the example according to the invention, the NOx removal action according to the first NOx removal control is performed when the temperature TC of the exhaust gas control catalyst13is higher than the limit temperature TX and the NOx removal action according to the second NOx removal control is performed when the temperature TC of the exhaust gas control catalyst13is lower than the limit temperature TX.

Referring to the ratio of the amount GeH of the EGR gas that is supplied from the high pressure EGR passage16H into the combustion chambers2to a total amount G of the gas that is supplied into the combustion chambers2as a high pressure EGR rate REGRH (=GeWG), the actual high pressure EGR rate REGRH is calculated and an opening degree VEGRH of the high pressure EGR control valve17H is controlled in the example according to the invention such that the actual high pressure EGR rate REGRH corresponds to a target high pressure EGR rate REGRHT. In addition, referring to the ratio of the amount GeL of the EGR gas that is supplied from the low pressure EGR passage16L into the combustion chambers2to the total amount G of the gas that is supplied into the combustion chambers2as a low pressure EGR rate REGRL (=GeUG), the actual low pressure EGR rate REGRL is calculated and an opening degree VEGRL of the low pressure EGR control valve17L and an opening degree of the exhaust throttle valve29are controlled in the example according to the invention such that the actual low pressure EGR rate REGRL corresponds to a target low pressure EGR rate REGRLT.

The total amount G of the gas that is supplied into the combustion chambers2is calculated based on the intake pressure detected by the pressure sensor4p. The amount GeH of the EGR gas from the high pressure EGR passage16H is calculated based on the intake pressure detected by the pressure sensor4p, the exhaust pressure detected by the pressure sensor5p, and the opening degree of the high pressure EGR control valve17H. The amount GeL of the EGR gas from the low pressure EGR passage16L is calculated based on the pressure detected by the pressure sensor8p, a pressure detected by a pressure sensor17p, and the opening degree of the low pressure EGR control valve17L. Accordingly, the high pressure EGR rate REGRH and the low pressure EGR rate REGRL are calculated. A total amount Ge of the EGR gas that is supplied into the combustion chambers2is represented by GeH+GeL, and thus an EGR rate REGR, which is the ratio of the total amount of the EGR gas that is supplied into the combustion chambers2to the total amount of the gas that is supplied into the combustion chambers2, is represented by Ge/G.

In the example according to the invention, the rich control for temporarily declining the air-fuel ratio of the exhaust gas that is discharged from the combustion chambers2is performed so that the air-fuel ratio (A/F)in of the exhaust gas flowing into the exhaust gas control catalyst13is enriched with regard to the second NOx removal control as described above. In this case, the rich control is performed by the additional fuel Qa being injected into the combustion chambers2. In a case where the hydrocarbon is not supplied from the hydrocarbon supply valve15, the air-fuel ratio (A/F)in of the exhaust gas flowing into the exhaust gas control catalyst13corresponds to the air-fuel ratio of the exhaust gas that is discharged from the combustion chambers2.

FIG. 16is a time chart showing a time when the rich control is initiated. Time ta1inFIG. 16represents a timing at which a signal is issued for the initiation of the rich control.

Before time ta1, that is, during a normal control in which the rich control is not performed, the target low pressure EGR rate REGRLT and the target high pressure EGR rate REGRHT are set to a base low pressure EGR rate REGRLB and a base high pressure EGR rate REGRHB, respectively. In other words, the low pressure EGR control valve opening degree VEGRL becomes a base low pressure EGR control valve opening degree VEGRLB that is required for the actual low pressure EGR rate REGRL to become the base low pressure EGR rate REGRLB, and the high pressure EGR control valve opening degree VEGRH becomes a base high pressure EGR control valve opening degree VEGRHB that is required for the actual high pressure EGR rate REGRH to become the base high pressure EGR rate REGRHB. As a result, the low pressure EGR rate REGRL and the high pressure EGR rate REGRH become the base low pressure EGR rate REGRLB and the base high pressure EGR rate REGRHB, respectively. The base low pressure EGR rate REGRLB and the base high pressure EGR rate REGRHB are stored in advance in the ROM32as the function of the depression amount L of the accelerator pedal40and the engine rotation speed N and in the form of the maps which are illustrated inFIGS. 17 and 18, respectively.

In addition, a throttle opening degree VTH is set to a base throttle opening degree VTHB. As a result, a suctioned air amount Ga becomes a base suctioned air amount GaB that is determined in accordance with the base throttle opening degree VTHB. The base throttle opening degree VTHB is stored in advance in the ROM32in the form of the map which is illustrated inFIG. 19and as the function of the depression amount L of the accelerator pedal40and the engine rotation speed N.

Furthermore, the additional fuel amount Qa is set to zero. In other words, the additional fuel Qa is not injected. As a result, the air-fuel ratio (A/F)in of the exhaust gas becomes a base air-fuel ratio AFB that is leaner than a stoichiometric air-fuel ratio AFS.

Moreover, the main fuel Qm is set to a base main fuel amount QmB. The base main fuel amount QmB is the amount of the fuel that is required for the generation of a required output. The base main fuel amount QmB is stored in advance in the ROM32in the form of the map which is illustrated inFIG. 20and as the function of the depression amount L of the accelerator pedal40and the engine rotation speed N.

Moreover, a main fuel injection timing θm is set to a base injection timing θmB. The base injection timing θmB is stored in advance in the ROM32in the form of the map which is illustrated inFIG. 21and as the function of the depression amount L of the accelerator pedal40and the engine rotation speed N.

In this case, an intake pressure Pin becomes a base intake pressure PinB and an exhaust pressure Pex becomes a base exhaust pressure PexB. Accordingly, a pump loss PL (=Pex-Pin) that is represented by the difference between the exhaust pressure Pex and the intake pressure Pin becomes a base pump loss PLB (=PexB-PinB). Considering that the intake pressure Pin and the exhaust pressure Pex are determined in accordance with the throttle opening degree VTH, the low pressure EGR rate REGRL, and the high pressure EGR rate REGRH, the base pump loss PLB is determined in accordance with the base throttle opening degree VTHB, the base low pressure EGR rate REGRLB, and the base high pressure EGR rate REGRHB.

A compression end temperature TCE becomes a base compression end temperature TCEB. Considering that the compression end temperature TCE is determined in accordance with an in-cylinder gas amount and the in-cylinder gas amount is determined in accordance with the throttle opening degree VTH, the low pressure EGR rate REGRL, and the high pressure EGR rate REGRH, the base compression end temperature TCEB is determined in accordance with the base throttle opening degree VTHB, the base low pressure EGR rate REGRLB, and the base high pressure EGR rate REGRHB.

Once the signal for the initiation of the rich control is issued at time ta1, the target low pressure EGR rate REGRLT is first switched from the base low pressure EGR rate REGRLB to a low pressure EGR rate REGRLR for the rich control that is different from the base low pressure EGR rate REGRLB. Then, the low pressure EGR control valve opening degree VEGRL is switched from the base low pressure EGR control valve opening degree VEGRLB to a low pressure EGR control valve opening degree VEGRLR for the rich control. In the example that is illustrated inFIG. 16, the target low pressure EGR rate REGRLT is reduced to zero, and thus the low pressure EGR control valve opening degree VEGRL is reduced to zero. The low pressure EGR rate REGRLR for the rich control is stored in advance in the ROM32in the form of the map which is illustrated inFIG. 22and as the function of the depression amount L of the accelerator pedal40and the engine rotation speed N.

At this time, the target high pressure EGR rate REGRHT is maintained at the base high pressure EGR rate REGRHB, and thus the high pressure EGR control valve opening degree VEGRH is maintained at the base high pressure EGR control valve opening degree VEGRHB. Accordingly, the high pressure EGR rate REGRH is maintained at the base high pressure EGR rate REGRHB.

In addition, the throttle opening degree VTH is maintained at the base throttle opening degree VTHB as well. Accordingly, the suctioned air amount Ga is maintained at the base suctioned air amount GaB.

Furthermore, the additional fuel amount Qa is maintained at zero. In other words, the injection of the additional fuel Qa has yet to be initiated.

As a result, the EGR gas amount decreases, and thus the intake pressure Pin declines from the base intake pressure PinB. In addition, since the EGR gas amount decreases, the exhaust gas temperature rises, and thus the exhaust pressure Pex rises from the base exhaust pressure PexB. Accordingly, the pump loss PL increases from the base pump loss PLB. As a result, an engine output temporarily declines when the rich control is initiated, and an engine output fluctuation might increase. In this regard, in the example that is illustrated inFIG. 16, the main fuel amount Qm is increased by an increment dQm with respect to the base main fuel amount QmB. As a result, an increase in the engine output fluctuation at a time of the initiation of the rich control is blocked.

The increment dQm is set based on a deviation dPL of the pump loss PL with respect to the base pump loss PLB (=PL−PLB). Specifically, the increment dQm is set to decrease as the deviation dPL decreases. The increment dQm is stored in advance in the ROM32in the form of the map which is illustrated inFIG. 27.

As a result, the air-fuel ratio (A/F)in of the exhaust gas declines by the increment dQm of the main fuel Qm as illustrated inFIG. 16.

In addition, the in-cylinder gas amount decreases, and thus the compression end temperature TCE declines from the base compression end temperature TCEB. As a result, a timing of the combustion of the main fuel Qm is delayed. Accordingly, the engine output temporarily declines when the rich control is initiated, and the engine output fluctuation might increase. In this regard, in the example that is illustrated inFIG. 16, the main fuel injection timing θm is switched from the base main fuel injection timing θmB to an injection timing θmR for the rich control. In the example that is illustrated inFIG. 16, the main fuel injection timing θm is advanced. As a result, the timing of the combustion of the main fuel Qm is advanced, and the increase in the engine output fluctuation immediately after a termination of the rich control is blocked. The injection timing θmR for the rich control is stored in advance in the ROM32in the form of the map which is illustrated inFIG. 26and as the function of the depression amount L of the accelerator pedal40and the engine rotation speed N.

In the example that is illustrated inFIG. 16, the low pressure EGR rate REGRLR for the rich control is set to zero. In other words, the EGR gas supply from the low pressure EGR passage16L is stopped during the rich control. In another example, the low pressure EGR rate REGRLR for the rich control is set to exceed zero and the EGR gas is supplied from the low pressure EGR passage16L during the rich control.

Then, once the low pressure EGR rate REGRL is switched to the low pressure EGR rate REGRLR for the rich control at time ta2, the target high pressure EGR rate REGRHT is switched from the base high pressure EGR rate REGRHB to a high pressure EGR rate REGRHR for the rich control that is different from the base high pressure EGR rate REGRHB, and this causes the high pressure EGR control valve opening degree VEGRH to be switched from the base high pressure EGR control valve opening degree VEGRHB to a high pressure EGR control valve opening degree VEGRHR for the rich control. The high pressure EGR control valve opening degree VEGRHR for the rich control is a high pressure EGR control valve opening degree that is required for the high pressure EGR rate REGRH to become the high pressure EGR rate REGRHR for the rich control. In the example that is illustrated inFIG. 16, the target high pressure EGR rate REGRHT is reduced, and thus the high pressure EGR control valve opening degree VEGRH is reduced. As a result, the high pressure EGR rate REGRH declines. The high pressure EGR rate REGRHR for the rich control is stored in advance in the ROM32in the form of the map which is illustrated inFIG. 23and as the function of the depression amount L of the accelerator pedal40and the engine rotation speed N.

As a result, the pump loss PL further increases, and thus the increment dQm of the main fuel Qm further increases. In addition, the air-fuel ratio (A/F)in of the exhaust gas further declines. The throttle opening degree VTH is still maintained at the base throttle opening degree VTHB, and the injection of the additional fuel Qa remains stopped.

When the amount of the EGR gas from the low pressure EGR passage16L and the amount of the EGR gas from the high pressure EGR passage16H are decreased at the same time, the in-cylinder gas is subjected to a significant decrease and the compression end temperature TCE declines significantly. As a result, the risk of the occurrence of a misfire increases. The length of time that is required for the amount of the EGR gas supplied into the combustion chambers2to decrease after a decline in the low pressure EGR control valve opening degree VEGRL exceeds the length of time that is required for the amount of the EGR gas supplied into the combustion chambers2to decrease after a decline in the high pressure EGR control valve opening degree VEGRH. In this regard, in the example that is illustrated inFIG. 16, the high pressure EGR control valve opening degree VEGRH is declined after the low pressure EGR control valve opening degree VEGRL is declined.

In the example that is illustrated inFIG. 16, the high pressure EGR rate REGRHR for the rich control is set to exceed zero. In other words, the EGR gas is supplied from the low pressure EGR passage16L during the rich control. In another example, the high pressure EGR rate REGRHR for the rich control is set to zero and the EGR gas supply from the high pressure EGR passage16H is stopped during the rich control.

Then, once the high pressure EGR rate REGRH is switched to the high pressure EGR rate REGRHR for the rich control at time ta3, that is, once both the low pressure EGR rate REGRL and the high pressure EGR rate REGRH are respectively switched to the low pressure EGR rate REGRLR for the rich control and the high pressure EGR rate REGRHR for the rich control at time ta3, the throttle opening degree VTH is switched from the base throttle opening degree VTHB to a throttle opening degree VTHR for the rich control that is lower than the base throttle opening degree VTHB. As a result, the suctioned air amount Ga decreases to a suctioned air amount GaR for the rich control. The throttle opening degree VTHR for the rich control is stored in advance in the ROM32in the form of the map which is illustrated inFIG. 24and as the function of the depression amount L of the accelerator pedal40and the engine rotation speed N.

A simultaneous decrease in the EGR gas amount and the suctioned air amount Ga might lead to a significant decrease in the concentration of the oxygen in the in-cylinder gas amount. As a result, the risk of the occurrence of the misfire increases. In this regard, in the example that is illustrated inFIG. 16, the throttle opening degree VTH is declined after the low pressure EGR control valve opening degree VEGRL and the high pressure EGR control valve opening degree VEGRH are declined.

Then, once the suctioned air amount Ga is switched to the suctioned air amount GaR for the rich control determined in accordance with the throttle opening degree VTHR for the rich control at time ta4, the injection of the additional fuel Qa is initiated. In this case, the additional fuel Qa is injected in a state where the suctioned air amount Ga is decreased and the EGR rate is declined, and thus the additional fuel Qa that is required for the enrichment of the air-fuel ratio (A/F)in of the exhaust gas can be reduced.

In addition, the main fuel amount Qm is switched to a main fuel amount QmR for the rich control. In the example that is illustrated inFIG. 16, a slight engine output is generated by some of the additional fuel Qa is burned in the combustion chambers2. In this regard, the main fuel amount QmR for the rich control is slightly reduced compared to the base main fuel amount QmB such that the actual engine output corresponds to the required output. The main fuel amount QmR for the rich control is stored in advance in the ROM32in the form of the map which is illustrated inFIG. 25and as the function of the depression amount L of the accelerator pedal40and the engine rotation speed N. Once the main fuel amount Qm is switched to the main fuel amount QmR for the rich control, an increase in the main fuel Qm based on the increment dQm is stopped.

As a result, the air-fuel ratio (A/F)in of the exhaust gas declines significantly. In the example that is illustrated inFIG. 16, the air-fuel ratio (A/F)in of the exhaust gas becomes richer than the stoichiometric air-fuel ratio AFS.

FIG. 28is a time chart showing a time when the rich control is terminated. Once a signal for the termination of the rich control is issued at time tb1, the target low pressure EGR rate REGRLT and the target high pressure EGR rate REGRHT are returned from the low pressure EGR rate REGRLR for the rich control and the high pressure EGR rate REGRHR for the rich control to the base low pressure EGR rate REGRLB and the base high pressure EGR rate REGRHB, respectively. This causes the low pressure EGR control valve opening degree VEGRL and the high pressure EGR control valve opening degree VEGRH to be returned from the low pressure EGR control valve opening degree VEGRLR for the rich control and the high pressure EGR control valve opening degree VEGRHR for the rich control to the base low pressure EGR control valve opening degree VEGRLB and the base high pressure EGR control valve opening degree VEGRHB, respectively. As a result, the low pressure EGR rate REGRL and the high pressure EGR rate REGRH rise and are returned to the base low pressure EGR rate REGRLB and the base high pressure EGR rate REGRHB, respectively.

In addition, the throttle opening degree VTH is returned from the throttle opening degree VTHR for the rich control to the base throttle opening degree VTHB. As a result, the suctioned air amount Ga increases and is returned to the base suctioned air amount GaB.

Furthermore, the additional fuel amount Qa becomes zero. In other words, the injection of the additional fuel Qa is stopped.

Moreover, the main fuel amount Qm is returned from the main fuel amount QmR for the rich control to the base main fuel amount QmB. Moreover, the main fuel injection timing θm is returned from the main fuel injection timing θmR for the rich control to the base main fuel injection timing θmB. As a result, the air-fuel ratio (A/F)in of the exhaust gas is returned to the base air-fuel ratio AFB.

As a result, the intake pressure Pin rises and is returned to the base intake pressure PinB. In addition, the exhaust pressure Pex declines and is returned to the base exhaust pressure PexB. Accordingly, the pump loss PL decreases and is returned to the base pump loss PLB. Furthermore, the compression end temperature TCE rises and is returned to the base compression end temperature TCEB.

The rich control is terminated in this manner, and the normal control is initiated.

FIG. 29shows a routine for executing a NOx removal control according to the example of the invention. This routine is executed by interruption at regular time intervals.

Referring toFIG. 29, whether to perform the NOx removal action according to the first NOx removal control or the NOx removal action according to the second NOx removal control is determined first in Step100. Then, in Step101, it is determined whether or not the NOx removal action according to the first NOx removal control should be performed. When the NOx removal action according to the first NOx removal control should be performed, the processing proceeds to Step102and the NOx removal action according to the first NOx removal control is performed. In other words, the hydrocarbons are injected from the hydrocarbon supply valve15, by the injection quantity W illustrated inFIG. 11, at the injection cycle ΔT determined in advance in accordance with the operation states of the engine.

When it is determined in Step101that the NOx removal action according to the second NOx removal control should be executed, the processing proceeds to Step103and a routine for executing the NOx removal action according to the second NOx removal control is executed. This routine is illustrated inFIG. 30.

FIG. 30shows the routine for executing the NOx removal action according to the second NOx removal control. This routine is executed in Step103inFIG. 29.

Referring toFIG. 30, in Step200, the discharged NOx amount NOXA per unit time is calculated first from the map which is illustrated inFIG. 13. Then, in Step201, the stored NOx amount ΣNOX is calculated by the discharged NOx amount NOXA being integrated (ΣNOX=ΣNOX+NOXA). Then, in Step202, it is determined whether or not the stored NOx amount ΣNOX exceeds the allowable value MAX. The processing cycle is terminated when the ΣNOX is equal to or smaller than the MAX.

Once the ΣNOX exceeds the MAX, the processing proceeds to Step203from Step202and a routine for executing the rich control is executed. This routine is illustrated inFIGS. 31 and 32. Then, in Step204, the stored NOx amount ΣNOX is cleared.

FIGS. 31 and 32show the routine for executing the rich control. This routine is executed in Step203inFIG. 30.

Referring toFIGS. 31 and 32, in Step300, it is first determined whether or not a flag X is set. This flag X is set (X=1) when the injection of the additional fuel Qa should be performed and is reset (X=0) otherwise. When the flag X1is reset, the processing proceeds to Step301from Step300. In Step301, the low pressure EGR rate REGRLR for the rich control is calculated from the map inFIG. 22and the target low pressure EGR rate REGRLT is set to the low pressure EGR rate REGRLR for the rich control. Then, in Step302, it is determined whether or not the low pressure EGR rate REGRL has been switched to the low pressure EGR rate REGRLR for the rich control. When the low pressure EGR rate REGRL has not been switched to the low pressure EGR rate REGRLR for the rich control, the processing proceeds to Step303from Step302. In Step303, the base high pressure EGR rate REGRHB is calculated from the map inFIG. 18and the target high pressure EGR rate REGRHT is set to the base high pressure EGR rate REGRHB. Then, in Step304, the base throttle opening degree VTHB is calculated from the map inFIG. 19and the throttle opening degree VTH is set to the base throttle opening degree VTHB. Then, in Step305, the base main fuel amount QmB is calculated from the map inFIG. 20, the increment dQm is calculated from the map inFIG. 27, and the main fuel amount Qm is calculated (Qm=QmB+dQm). Then, the processing jumps to Step313.

Once the low pressure EGR rate REGRL is switched to the low pressure EGR rate REGRLR for the rich control, the processing proceeds to Step306from Step302. In Step306, the high pressure EGR rate REGRHR for the rich control is calculated from the map inFIG. 23and the target high pressure EGR rate REGRHT is set to the high pressure EGR rate REGRHR for the rich control. Then, in Step307, it is determined whether or not the high pressure EGR rate REGRH has been switched to the high pressure EGR rate REGRHR for the rich control. The processing proceeds to Step304from Step307when the high pressure EGR rate REGRH has not been switched to the high pressure EGR rate REGRHR for the rich control. The processing proceeds to Step308from Step307when the high pressure EGR rate REGRH has been switched to the high pressure EGR rate REGRHR for the rich control. In Step308, the throttle opening degree VTHR for the rich control is calculated from the map inFIG. 24and the throttle opening degree VTH is set to the throttle opening degree VTHR for the rich control.

Then, in Step309, it is determined whether or not the suctioned air amount Ga has been switched to the suctioned air amount GaR for the rich control. The processing proceeds to Step305from Step309when the suctioned air amount Ga has not been switched to the suctioned air amount GaR for the rich control. The processing proceeds to Step310from Step309when the suctioned air amount Ga has been switched to the suctioned air amount GaR for the rich control. In Step310, the injection of the additional fuel Qa is performed. Then, in Step311, the main fuel amount QmR for the rich control is calculated from the map inFIG. 25and the main fuel amount Qm is set to the main fuel amount QmR for the rich control. Then, in Step312, the flag X is set (X=1). Then, the processing proceeds to Step313.

In Step313, the main fuel injection timing θmR for the rich control is calculated from the map inFIG. 26and the main fuel injection timing θm is set to the main fuel injection timing θmR for the rich control.

Then, in Step314, it is determined whether or not the rich control should be terminated. The processing proceeds to Step315when it is not determined that the rich control should be terminated. In Step315, the low pressure EGR rate REGRLR for the rich control is calculated from the map inFIG. 22and the target low pressure EGR rate REGRLT is set to the low pressure EGR rate REGRLR for the rich control. Then, in Step316, the high pressure EGR rate REGRHR for the rich control is calculated from the map inFIG. 23and the target high pressure EGR rate REGRHT is set to the high pressure EGR rate REGRHR for the rich control. Then, in Step317, the throttle opening degree VTHR for the rich control is calculated from the map inFIG. 24and the throttle opening degree VTH is set to the throttle opening degree VTHR for the rich control. Then, in Step318, the main fuel amount QmR for the rich control is calculated from the map inFIG. 25and the main fuel amount Qm is set to the main fuel amount QmR for the rich control. Then, in Step319, the main fuel injection timing θmR for the rich control is calculated from the map inFIG. 26and the main fuel injection timing θm is set to the main fuel injection timing θmR for the rich control. Then, in Step320, the injection of the additional fuel Qa is performed.

After the rich control is performed for a certain period of time, for example, it is determined that the rich control should be terminated. When it is determined that the rich control should be terminated, the processing proceeds to Step321from Step314. In Step321, the base low pressure EGR rate REGRLB is calculated from the map inFIG. 17and the target low pressure EGR rate REGRLT is set to the base low pressure EGR rate REGRLB. Then, in Step322, the base high pressure EGR rate REGRHB is calculated from the map inFIG. 18and the target high pressure EGR rate REGRHT is set to the base high pressure EGR rate REGRHB. Then, in Step323, the base throttle opening degree VTHB is calculated from the map inFIG. 19and the throttle opening degree VTH is set to the base throttle opening degree VTHB. Then, in Step324, the base main fuel amount QmB is calculated from the map inFIG. 20and the main fuel amount Qm is set to the base main fuel amount QmB. Then, in Step325, the base main fuel injection timing θmB is calculated from the map inFIG. 21and the main fuel injection timing θm is set to the base main fuel injection timing θmB. Then, in Step326, the injection of the additional fuel Qa is stopped. Then, in Step327, the flag X is reset (X=0).

Hereinafter, another example of the invention will be described.

In the example that is illustrated inFIG. 16, the low pressure EGR control valve opening degree VEGRL is first switched to the low pressure EGR control valve opening degree VEGRLR for the rich control when the rich control is initiated, and then the high pressure EGR control valve opening degree VEGRH is switched to the high pressure EGR control valve opening degree VEGRHR for the rich control once the low pressure EGR rate REGRL is switched from the base low pressure EGR rate REGRLB to the low pressure EGR rate REGRLR for the rich control.

In this case, the base low pressure EGR rate REGRLB and the low pressure EGR rate REGRLR for the rich control at the point in time when the rich control is initiated are determined in accordance with the engine operation states at that point in time. Accordingly, the amount of change dREGRL (=REGRLB−REGRLR) from the base low pressure EGR rate REGRLB to the low pressure EGR rate REGRLR for the rich control is small in some cases and large in the other cases.

When this amount of change dREGRL is large, a long period of time is required for the switching from the low pressure EGR rate REGRL to the low pressure EGR rate REGRLR for the rich control, and thus a long period of time is required for the switching from the high pressure EGR control valve opening degree VEGRH to the high pressure EGR control valve opening degree VEGRHR for the rich control. As a result, a long period of time is required for the additional fuel Qa to be injected after the signal for the initiation of the rich control is issued, and thus a long period of time is required for the air-fuel ratio (A/F)in of the exhaust gas to be switched to a rich one.

In this regard, in another example according to the invention, the target low pressure EGR rate REGRLT and the target high pressure EGR rate REGRHT are respectively set to the low pressure EGR rate REGRLR for the rich control and the high pressure EGR rate REGRHR for the rich control at the same time when the amount of change dREGRL in the low pressure EGR rate exceeds a limit amount dREGRLX determined in advance, and this causes the low pressure EGR control valve opening degree VEGRL and the high pressure EGR control valve opening degree VEGRH to be respectively set to the low pressure EGR control valve opening degree VEGRLR for the rich control and the high pressure EGR control valve opening degree VEGRHR for the rich control at the same time. As a result, the length of time that is required for the air-fuel ratio (A/F)in of the exhaust gas to be switched to a rich one can be shortened.

FIG. 33shows a case where the amount of change dREGRL is larger than the limit amount dREGRLX. Once the signal for the initiation of the rich control is issued at time tc1, the target low pressure EGR rate REGRLT is switched from the base low pressure EGR rate REGRLB to the low pressure EGR rate REGRLR for the rich control, and this causes the low pressure EGR control valve opening degree VEGRL to be switched from the base low pressure EGR control valve opening degree VEGRLB to the low pressure EGR control valve opening degree VEGRLR for the rich control. At the same time, the target high pressure EGR rate REGRHT is switched from the base high pressure EGR rate REGRHB to the high pressure EGR rate REGRHR for the rich control, and this causes the high pressure EGR control valve opening degree VEGRH to be switched from the base high pressure EGR control valve opening degree VEGRHB to the high pressure EGR control valve opening degree VEGRHR for the rich control. In addition, an increase in the amount of the main fuel Qm is initiated at this time, and the main fuel injection timing θm is switched to the injection timing θmR for the rich control from the base main fuel injection timing θmB.

In the example that is illustrated inFIG. 17, the high pressure EGR rate REGRH is switched to the high pressure EGR rate REGRHR for the rich control at the subsequent time tc2. Then, at time tc3, the low pressure EGR rate REGRL is switched to the low pressure EGR rate REGRLR for the rich control. In another example, the low pressure EGR rate REGRL is switched to the low pressure EGR rate REGRLR for the rich control, and then the high pressure EGR rate REGRH is switched to the high pressure EGR rate REGRHR for the rich control. In yet another example, the low pressure EGR rate REGRL and the high pressure EGR rate REGRH are respectively switched to the low pressure EGR rate REGRLR for the rich control and the high pressure EGR rate REGRHR for the rich control almost at the same time.

In any case, the throttle opening degree VTH is switched from the base throttle opening degree VTHB to the throttle opening degree VTHR for the rich control once the low pressure EGR rate REGRL and the high pressure EGR rate REGRH are switched to the low pressure EGR rate REGRLR for the rich control and the high pressure EGR rate REGRHR for the rich control, respectively.

Then, the injection of the additional fuel Qa is initiated once the suctioned air amount Ga is switched to the suctioned air amount GaR for the rich control determined in accordance with the throttle opening degree VTHR for the rich control at time tc4. In addition, the increase in the amount of the main fuel Qm is stopped at this time, and the main fuel Qm is switched to the main fuel amount QmR for the rich control.

When the amount of change dREGRL is smaller than the limit amount dREGRLX, the high pressure EGR control valve opening degree VEGRH is switched to the high pressure EGR control valve opening degree VEGRHR for the rich control after the low pressure EGR control valve opening degree VEGRL is switched to the low pressure EGR control valve opening degree VEGRLR for the rich control as in the example that is illustrated inFIG. 16.

FIGS. 34 to 36show a routine for executing the rich control according to another example of the invention. This routine is executed in Step203inFIG. 30. The routine that is illustrated inFIGS. 34 to 36differs from the routine that is illustrated inFIGS. 31 and 32in the following aspects. In other words, when the flag is reset (X=0) in Step300, the processing proceeds to the subsequent Step300a. In Step300a, the base low pressure EGR rate REGRLB is calculated from the map inFIG. 17, the low pressure EGR rate REGRLR for the rich control is calculated from the map inFIG. 22, and the amount of change dREGRL is calculated (dREGRL=REGRLB−REGRLR). Then, in Step300b, it is determined whether or not the amount of change dREGRL is larger than the limit amount dREGRLX. When the dREGRLX is equal to or larger than the dREGRL, the processing proceeds to the subsequent Step301.

In contrast, when the dREGRL is larger than the dREGRLX, the processing proceeds to the subsequent Step300c, in which the target low pressure EGR rate REGRLT is set to the low pressure EGR rate REGRLR for the rich control calculated in Step300a. In addition, the high pressure EGR rate REGRHR for the rich control is calculated from the map inFIG. 23and the target high pressure EGR rate REGRHT is set to the high pressure EGR rate REGRHR for the rich control. Then, in Step300d, it is determined whether or not the low pressure EGR rate REGRL has been switched to the low pressure EGR rate REGRLR for the rich control and the high pressure EGR rate REGRH has been switched to the high pressure EGR rate REGRHR for the rich control. The processing proceeds to the subsequent Step304when the low pressure EGR rate REGRL has not been switched to the low pressure EGR rate REGRLR for the rich control or the high pressure EGR rate REGRH has not been switched to the high pressure EGR rate REGRHR for the rich control. In contrast, the processing proceeds to Step308when it is determined that the low pressure EGR rate REGRL has been switched to the low pressure EGR rate REGRLR for the rich control and the high pressure EGR rate REGRH has been switched to the high pressure EGR rate REGRHR for the rich control.

In each of the examples of the invention described above, the rich control is performed so that the NOx is released from the exhaust gas control catalyst13. In another example, the rich control is performed so that SOx is released from the exhaust gas control catalyst13. During the rich control in this case, the air-fuel ratio (A/F)in of the exhaust gas flowing into the exhaust gas control catalyst13is allowed to remain richer than the stoichiometric air-fuel ratio while the temperature of the exhaust gas control catalyst13is maintained at or above a SOx release temperature (such as 600° C.). In yet another example, the rich control is performed so that the temperature of the exhaust gas control catalyst13is raised. During the rich control in this case, the air-fuel ratio (A/F)in of the exhaust gas flowing into the exhaust gas control catalyst13is allowed to remain leaner than the stoichiometric air-fuel ratio.

As another example, an oxidation catalyst for reforming the hydrocarbon can also be disposed in the engine exhaust passage on the upstream side of the exhaust gas control catalyst13.

REFERENCE SIGNS LIST