Exhaust purification system of internal combustion engine

A hydrocarbon feed valve and an exhaust purification catalyst are arranged in an engine exhaust passage. A first NOX purification method which injects hydrocarbons from the hydrocarbon feed valve by a predetermined period to thereby remove NOX which is contained in the exhaust gas and a second NOX purification method which makes the air-fuel ratio of the exhaust gas which flows into the exhaust purification catalyst rich to make the exhaust purification catalyst release the stored NOX when the NOX which is stored in the exhaust purification catalyst exceeds a first allowable value are selectively used. Hydrocarbons are injected from the hydrocarbon feed valve by the predetermined period, and when the NOX which is stored in the exhaust purification catalyst exceeds a second allowable value which is smaller than the first allowable value, the air-fuel ratio of the exhaust gas which flows into the exhaust purification catalyst is made rich.

TECHNICAL FIELD

The present invention relates to an exhaust purification system of an internal combustion engine.

BACKGROUND ART

Known in the art is an internal combustion engine in which an exhaust purification catalyst is arranged in an engine exhaust passage and a hydrocarbon feed valve is arranged upstream of the exhaust purification catalyst in the engine exhaust passage, the exhaust purification catalyst has the property of reducing the NOxwhich is contained in exhaust gas if making a concentration of hydrocarbons which flow into the exhaust purification catalyst vibrate by within a predetermined range of amplitude and by within a predetermined range of period and has the property of being increased in storage amount of NOXwhich is contained in the exhaust gas if making the vibration period of the hydrocarbon concentration longer than the predetermined range, a first NOxremoval method which injects hydrocarbons from the hydrocarbon feed valve by a predetermined injection period to thereby remove the NOxwhich is contained in the exhaust gas and a second NOxremoval method which makes the air-fuel ratio of the exhaust gas which flows into the exhaust purification catalyst rich to make the exhaust purification catalyst release the stored NOxwhen the NOxwhich is stored in the exhaust purification catalyst exceeds the allowable value are selectively used, and in the second NOxremoval method, the period by which the air-fuel ratio of the exhaust gas which flows into the exhaust purification catalyst is made rich is longer than the above-mentioned predetermined injection period (for example, see Patent Literature 1).

In this internal combustion engine, when the temperature of the exhaust purification catalyst is high, the NOxremoval action by the first NOxremoval method is performed, while when the temperature of the exhaust purification catalyst is low, the NOxremoval action by the second NOxremoval method is performed. In this regard, in this internal combustion engine, when the NOxremoval action by the first NOxremoval method is being performed, the exhaust purification catalyst stores NOx. If the amount of NOxwhich is stored in this exhaust purification catalyst increases, the NOxremoval rate at the time when the NOxremoval action by the first NOxremoval method is being performed ends up falling. Therefore, in this internal combustion engine, when the NOxremoval action by the first NOxremoval method is being performed, when the NOxwhich is stored in the exhaust purification catalyst increases, the amount of injection of hydrocarbons from the hydrocarbon feed valve is increased to make the air-fuel ratio of the exhaust gas which flows into the exhaust purification catalyst rich and thereby make the exhaust purification catalyst release the stored NOx.

CITATIONS LIST

Patent Literature

SUMMARY OF INVENTION

Technical Problem

However, in this internal combustion engine, in the case where the NOxremoval action by the first NOxremoval method is being performed, only restoration of the NOxremoval rate when the NOxremoval rate falls is considered. Further improvement of the NOxpurification rate when the NOxremoval action by the first NOxremoval method is being performed is not considered at all. An object of the present invention is to provide an exhaust purification system of an internal combustion engine which is designed so that a higher NOxpurification rate is obtained compared to when the NOxremoval action by the first NOxremoval method is being performed and when the NOxremoval action by the second NOxremoval method is being performed.

Solution to Problem

According to the present invention, there is provided an exhaust purification system of an internal combustion engine in which an exhaust purification catalyst is arranged in an engine exhaust passage and a hydrocarbon feed valve is arranged in the engine exhaust passage upstream of the exhaust purification catalyst, a precious metal catalyst is carried on an exhaust gas flow surface of the exhaust purification catalyst and a basic exhaust gas flow surface part is formed around the precious metal catalyst, the exhaust purification catalyst has a property of reducing NOxwhich is contained in exhaust gas if making a concentration of hydrocarbons which flow into the exhaust purification catalyst vibrate by within a predetermined range of amplitude and by within a predetermined range of period and has a property of being increased in storage amount of NOXwhich is contained in the exhaust gas if making the vibration period of the hydrocarbon concentration longer than the predetermined range, a first NOxremoval method which injects hydrocarbons from the hydrocarbon feed valve by a predetermined injection period to thereby remove the NOxwhich is contained in the exhaust gas and a second NOxremoval method which makes an air-fuel ratio of the exhaust gas which flows into the exhaust purification catalyst rich to make the exhaust purification catalyst release a stored NOxwhen the NOxwhich is stored in the exhaust purification catalyst exceeds a first allowable value are selectively used, and in the second NOxremoval method, the period by which the air-fuel ratio of the exhaust gas which flows into the exhaust purification catalyst is made rich is longer than the above-mentioned predetermined injection period, wherein temperature regions which the exhaust purification catalyst can take at the time of engine operation are divided into three regions of a low temperature region, an intermediate temperature region, and a high temperature region, in the high temperature region, an NOxremoval action by the first NOxremoval method is performed, in the low temperature region, an NOxremoval action by the second NOxremoval method is performed, and in the intermediate temperature region, hydrocarbons are injected from the hydrocarbon feed valve by the predetermined injection period and, when the NOxwhich is stored in the exhaust purification catalyst exceeds a predetermined second allowable value of a value smaller than the first allowable value, the air-fuel ratio of the exhaust gas which flows into the exhaust purification catalyst is made rich.

Advantageous Effects of Invention

In the intermediate temperature region of the exhaust purification catalyst, it is possible to obtain a higher NOxpurification rate than when the NOxremoval action by the first NOxremoval method is being performed and than when the NOxremoval action by the second NOxremoval method is being performed.

DESCRIPTION OF EMBODIMENTS

FIG. 1is an overall view of a compression ignition type internal combustion engine.

Referring toFIG. 1, 1indicates an engine body,2a combustion chamber of each cylinder,3an electronically controlled fuel injector for injecting fuel into each combustion chamber2,4an intake manifold, and5an exhaust manifold. The intake manifold4is connected through an intake duct6to an outlet of a compressor7aof an exhaust turbocharger7, while an inlet of the compressor7ais connected through an intake air amount detector8to an air cleaner9. Inside the intake duct6, a throttle valve10which is driven by an actuator is arranged. Around the intake duct6, a cooling device11is arranged for cooling the intake air which flows through the inside of the intake duct6. In the embodiment which is shown inFIG. 1, the engine cooling water is guided to the inside of the cooling device11where the engine cooling water is used to cool the intake air.

On the other hand, the exhaust manifold5is connected to an inlet of an exhaust turbine7bof the exhaust turbocharger7, and an outlet of the exhaust turbine7bis connected through an exhaust pipe12to an inlet of an exhaust purification catalyst13. In an embodiment of the present invention, this exhaust purification catalyst13is comprised of an NOXstorage catalyst13. An outlet of the exhaust purification catalyst13is connected to an inlet of a particulate filter14and, upstream of the exhaust purification catalyst13inside the exhaust pipe12, a hydrocarbon feed valve15is arranged for feeding hydrocarbons comprised of diesel oil or other fuel used as fuel for a compression ignition type internal combustion engine. In the embodiment shown inFIG. 1, diesel oil is used as the hydrocarbons which are fed from the hydrocarbon feed valve15. Note that, the present invention can also be applied to a spark ignition type internal combustion engine in which fuel is burned under a lean air-fuel ratio. In this case, from the hydrocarbon feed valve15, hydrocarbons comprised of gasoline or other fuel used as fuel of a spark ignition type internal combustion engine are fed.

On the other hand, the exhaust manifold5and the intake manifold4are connected with each other through an exhaust gas recirculation (hereinafter referred to as an “EGR”) passage16. Inside the EGR passage16, an electronically controlled EGR control valve17is arranged. Further, around the EGR passage16, a cooling device18is arranged for cooling the EGR gas which flows through the inside of the EGR passage16. In the embodiment which is shown inFIG. 1, the engine cooling water is guided to the inside of the cooling device18where the engine cooling water is used to cool the EGR gas. On the other hand, each fuel injector3is connected through a fuel feed tube19to a common rail20. This common rail20is connected through an electronically controlled variable discharge fuel pump21to a fuel tank22. The fuel which is stored inside of the fuel tank22is fed by the fuel pump21to the inside of the common rail20. The fuel which is fed to the inside of the common rail21is fed through each fuel feed tube19to the fuel injector3.

An electronic control unit30is comprised of a digital computer provided with a ROM (read only memory)32, a RAM (random access memory)33, a CPU (microprocessor)34, an input port35, and an output port36, which are connected with each other by a bidirectional bus31. Downstream of the exhaust purification catalyst13, a temperature sensor23is arranged for detecting the temperature of the exhaust gas flowing out from the exhaust purification catalyst13, and the output signals of this temperature sensor23and intake air amount detector8are input through respectively corresponding AD converters37to the input port35. Further, an accelerator pedal40has a load sensor41connected to it which generates an output voltage proportional to the amount of depression L of the accelerator pedal40. The output voltage of the load sensor41is input through a corresponding AD converter37to the input port35. Furthermore, at the input port35, a crank angle sensor42is connected which generates an output pulse every time a crankshaft rotates by, for example, 15°. On the other hand, the output port36is connected through corresponding drive circuits38to each fuel injector3, the actuator for driving the throttle valve10, hydrocarbon feed valve15, EGR control valve17, and fuel pump21.

FIG. 2schematically shows a surface part of a catalyst carrier which is carried on a substrate of the exhaust purification catalyst13shown inFIG. 1. At this exhaust purification catalyst13, as shown inFIG. 2, for example, there is provided a catalyst carrier50made of alumina on which precious metal catalysts51comprised of platinum Pt are carried. Furthermore, on this catalyst carrier50, a basic layer53is formed which includes at least one element selected from potassium K, sodium Na, cesium Cs, or another such alkali metal, barium Ba, calcium Ca, or another such alkali earth metal, a lanthanide or another such rare earth and silver Ag, copper Cu, iron Fe, iridium Ir, or another metal able to donate electrons to NOX. In this case, on the catalyst carrier50of the exhaust purification catalyst13, in addition to platinum Pt, rhodium Rh or palladium Pd may be further carried. Note that the exhaust gas flows along the top of the catalyst carrier50, so the precious metal catalysts51can be said to be carried on the exhaust gas flow surfaces of the exhaust purification catalyst13. Further, the surface of the basic layer53exhibits basicity, so the surface of the basic layer53is called the “basic exhaust gas flow surface parts54”.

If hydrocarbons are injected from the hydrocarbon feed valve15into the exhaust gas, the hydrocarbons are reformed by the exhaust purification catalyst13. In the present invention, at this time, the reformed hydrocarbons are used to remove the NOXat the exhaust purification catalyst13.FIG. 3schematically shows the reformation action performed at the exhaust purification catalyst13at this time. As shown inFIG. 3, the hydrocarbons HC which are injected from the hydrocarbon feed valve15become radical hydrocarbons HC with a small carbon number due to the precious metal catalyst51.

FIG. 4shows the feed timing of hydrocarbons from the hydrocarbon feed valve15and the change in the air-fuel ratio (A/F) in of the exhaust gas which flows into the exhaust purification catalyst13. Note that, the change in the air-fuel ratio (A/F) in depends on the change in concentration of the hydrocarbons in the exhaust gas which flows into the exhaust purification catalyst13, so it can be said that the change in the air-fuel ratio (A/F) in shown inFIG. 4expresses the change in concentration of the hydrocarbons. However, if the hydrocarbon concentration becomes higher, the air-fuel ratio (A/F) in becomes smaller, so, inFIG. 4, the more to the rich side the air-fuel ratio (A/F) in becomes, the higher the hydrocarbon concentration.

FIG. 5shows the NOXpurification rate R1by the exhaust purification catalyst13with respect to the catalyst temperatures TC of the exhaust purification catalyst13when periodically making the concentration of hydrocarbons which flow into the exhaust purification catalyst13change so as to, as shown inFIG. 4, periodically make the air-fuel ratio (A/F) in of the exhaust gas flowing to the exhaust purification catalyst13rich. In this regard, as a result of a research relating to NOXpurification for a long time, it is learned that if making the concentration of hydrocarbons which flow into the exhaust purification catalyst13vibrate by within a predetermined range of amplitude and within a predetermined range of period, as shown inFIG. 5, an extremely high NOXpurification rate R1is obtained even in a 350° C. or higher high temperature region.

Furthermore, it is learned that at this time, a large amount of reducing intermediates which contain nitrogen and hydrocarbons continues to be held or adsorbed on the surface of the basic layer53, that is, on the basic exhaust gas flow surface parts54of the exhaust purification catalyst13, and the reducing intermediates play a central role in obtaining a high NOXpurification rate R1. Next, this will be explained with reference toFIGS. 6A and 6B. Note that, theseFIGS. 6A and 6Bschematically show the surface part of the catalyst carrier50of the exhaust purification catalyst13. TheseFIGS. 6A and 6Bshow the reaction which is presumed to occur when the concentration of hydrocarbons which flow into the exhaust purification catalyst13is made to vibrate by within a predetermined range of amplitude and within a predetermined range of period.

FIG. 6Ashows when the concentration of hydrocarbons which flow into the exhaust purification catalyst13is low, whileFIG. 6Bshows when hydrocarbons are fed from the hydrocarbon feed valve15and the air-fuel ratio (A/F) in of the exhaust gas flowing to the exhaust purification catalyst13is made rich, that is, the concentration of hydrocarbons which flow into the exhaust purification catalyst13becomes higher.

Now, as will be understood fromFIG. 4, the air-fuel ratio of the exhaust gas which flows into the exhaust purification catalyst13is maintained lean except for an instant, so the exhaust gas which flows into the exhaust purification catalyst13normally becomes a state of oxygen excess. At this time, part of the NO which is contained in the exhaust gas deposits on the exhaust purification catalyst13, while part of the NO which is contained in the exhaust gas, as shown inFIG. 6A, is oxidized on the platinum51and becomes NO2. Next, this NO2is further oxidized and becomes NO3. Further, part of the NO2becomes NO2−. Therefore, on the platinum Pt 51, NO2−and NO3are produced. The NO which is deposited on the exhaust purification catalyst13and the NO2−and NO3which are formed on the platinum Pt 51 are strong in activity. Therefore, below, these NO, NO2−, and NO3will be referred to as the “active NOX*”.

On the other hand, if hydrocarbons are fed from the hydrocarbon feed valve15and the air-fuel ratio (A/F) in of the exhaust gas flowing to the exhaust purification catalyst13is made rich, the hydrocarbons successively deposit over the entire exhaust purification catalyst13. The majority of the deposited hydrocarbons successively react with oxygen and are burned. Part of the deposited hydrocarbons are successively reformed and become radicalized inside of the exhaust purification catalyst13as shown inFIG. 3. Therefore, as shown inFIG. 6B, the hydrogen concentration around the active NOX* becomes higher. In this regard, if, after the active NOX* is produced, the state of a high oxygen concentration around the active NOX* continues for a constant time or more, the active NOX* is oxidized and is absorbed in the form of nitrate ions NO3−inside the basic layer53. However, if, before this constant time elapses, the hydrocarbon concentration around the active NOX* becomes higher, as shown inFIG. 6B, the active NOX* reacts on the platinum51with the radical hydrocarbons HC to thereby form the reducing intermediates. The reducing intermediates are adhered or adsorbed on the surface of the basic layer53.

Note that, at this time, the first produced reducing intermediate is considered to be a nitro compound R—NO2. If this nitro compound R—NO2is produced, the result becomes a nitrile compound R—CN, but this nitrile compound R—CN can only survive for an instant in this state, so immediately becomes an isocyanate compound R—NCO. This isocyanate compound R—NCO becomes an amine compound R—NH2if hydrolyzed. However, in this case, what is hydrolyzed is considered to be part of the isocyanate compound R—NCO. Therefore, as shown inFIG. 6B, the majority of the reducing intermediates which are held or adsorbed on the surface of the basic layer53is believed to be the isocyanate compound R—NCO and amine compound R—NH2.

On the other hand, as shown inFIG. 6B, if the produced reducing intermediates are surrounded by the hydrocarbons HC, the reducing intermediates are blocked by the hydrocarbons HC and the reaction will not proceed any further. In this case, if the concentration of hydrocarbons which flow into the exhaust purification catalyst13is lowered and then the hydrocarbons which are deposited around the reducing intermediates will be oxidized and consumed, and thereby the concentration of oxygen around the reducing intermediates becomes higher, the reducing intermediates react with the NOXin the exhaust gas, react with the active NOX*, react with the surrounding oxygen, or break down on their own. Due to this, the reducing intermediates R—NCO and R—NH2are converted to N2, CO2, and H2O as shown inFIG. 6A, therefore the NOXis removed.

In this way, in the exhaust purification catalyst13, when the concentration of hydrocarbons which flow into the exhaust purification catalyst13is made higher, reducing intermediates are produced, and after the concentration of hydrocarbons which flow into the exhaust purification catalyst13is lowered, when the oxygen concentration is raised, the reducing intermediates react with the NOXin the exhaust gas or the active NOX* or oxygen or break down on their own whereby the NOXis removed. That is, in order for the exhaust purification catalyst13to remove the NOX, the concentration of hydrocarbons which flow into the exhaust purification catalyst13has to be periodically changed.

Of course, in this case, it is necessary to raise the hydrocarbon concentration to a concentration sufficiently high for producing the reducing intermediates and it is necessary to lower the hydrocarbon concentration to a concentration sufficiently low for making the produced reducing intermediates react with the NOXin the exhaust gas or the active NOX* or oxygen or break down on their own. That is, it is necessary to make the concentration of hydrocarbons which flow into the exhaust purification catalyst13vibrate by within a predetermined range of amplitude. Note that, in this case, it is necessary to hold these reducing intermediates on the basic layer53, that is, the basic exhaust gas flow surface parts54, until the produced reducing intermediates R—NCO and R—NH2react with the NOXin the exhaust gas or the active NOX* or oxygen or break down themselves. For this reason, the basic exhaust gas flow surface parts54are provided.

On the other hand, if lengthening the feed period of the hydrocarbons, the time until the oxygen concentration becomes higher becomes longer in the period after the hydrocarbons are fed until the hydrocarbons are next fed. Therefore, the active NOX* is absorbed in the basic layer53in the form of nitrates without producing reducing intermediates. To avoid this, it is necessary to make the concentration of hydrocarbons which flow into the exhaust purification catalyst13vibrate by within a predetermined range of period.

Therefore, in the embodiment according to the present invention, to react the NOXcontained in the exhaust gas and the reformed hydrocarbons and produce the reducing intermediates R—NCO and R—NH2containing nitrogen and hydrocarbons, the precious metal catalysts51are carried on the exhaust gas flow surfaces of the exhaust purification catalyst13. To hold the produced reducing intermediates R—NCO and R—NH2inside the exhaust purification catalyst13, the basic exhaust gas flow surface parts54are formed around the precious metal catalysts51. The reducing intermediates R—NCO and R—NH2which are held on the basic exhaust gas flow surface parts54are converted to N2, CO2, and H2O. The vibration period of the hydrocarbon concentration is made the vibration period required for continuation of the production of the reducing intermediates R—NCO and R—NH2. Incidentally, in the example shown inFIG. 4, the injection interval is made 3 seconds.

If the vibration period of the hydrocarbon concentration, that is, the injection period of hydrocarbons from the hydrocarbon feed valve15, is made longer than the above predetermined range of period, the reducing intermediates R—NCO and R—NH2disappear from the surface of the basic layer53. At this time, the active NOX* which is produced on the platinum Pt 53, as shown inFIG. 7A, diffuses in the basic layer53in the form of nitrate ions NO3−and becomes nitrates. That is, at this time, the NOXin the exhaust gas is absorbed in the form of nitrates inside of the basic layer53.

On the other hand,FIG. 7Bshows the case where the air-fuel ratio of the exhaust gas which flows into the exhaust purification catalyst13is made the stoichiometric air-fuel ratio or rich when the NOXis absorbed in the form of nitrates inside of the basic layer53. In this case, the oxygen concentration in the exhaust gas falls, so the reaction proceeds in the opposite direction (NO3−→NO2), and consequently the nitrates absorbed in the basic layer53successively become nitrate ions NO3−and, as shown inFIG. 7B, are released from the basic layer53in the form of NO2. Next, the released NO2is reduced by the hydrocarbons HC and CO contained in the exhaust gas.

FIG. 8shows the case of making the air-fuel ratio (A/F) in of the exhaust gas which flows into the exhaust purification catalyst13temporarily rich slightly before the NOXabsorption ability of the basic layer53becomes saturated. Note that, in the example shown inFIG. 8, the time interval of this rich control is 1 minute or more. In this case, the NOXwhich was absorbed in the basic layer53when the air-fuel ratio (A/F) in of the exhaust gas was lean is released all at once from the basic layer53and reduced when the air-fuel ratio (A/F) in of the exhaust gas is made temporarily rich. Therefore, in this case, the basic layer53plays the role of an absorbent for temporarily absorbing NOX.

Note that, at this time, sometimes the basic layer53temporarily adsorbs the NOX. Therefore, if using term of “storage” as a term including both “absorption” and “adsorption”, at this time, the basic layer53performs the role of an NOXstorage agent for temporarily storing the NOX. That is, in this case, if the ratio of the air and fuel (hydrocarbons) which are supplied into the engine intake passage, combustion chambers2, and upstream of the exhaust purification catalyst13in the exhaust passage is referred to as “the air-fuel ratio of the exhaust gas”, the exhaust purification catalyst13functions as an NOXstorage catalyst which stores the NOXwhen the air-fuel ratio of the exhaust gas is lean and releases the stored NOXwhen the oxygen concentration in the exhaust gas falls.

FIG. 9shows the NOXpurification rate R2when making the exhaust purification catalyst13function as an NOXstorage catalyst in this way. Note that, the abscissa of theFIG. 9shows the catalyst temperature TC of the exhaust purification catalyst13. When making the exhaust purification catalyst13function as an NOXstorage catalyst, as shown inFIG. 9, when the catalyst temperature TC is 250° C. to 300° C., an extremely high NOXpurification rate is obtained, but when the catalyst temperature TC becomes a 350° C. or higher high temperature, the NOXpurification rate R2falls.

In this way, when the catalyst temperature TC becomes 350° C. or more, the NOXpurification rate R2falls because if the catalyst temperature TC becomes 350° C. or more, NOXis less easily stored and the nitrates break down by heat and are released in the form of NO2from the exhaust purification catalyst13. That is, so long as storing NOXin the form of nitrates, when the catalyst temperature TC is high, it is difficult to obtain a high NOXpurification rate R2. However, in the new NOXpurification method shown fromFIG. 4toFIGS. 6A and 6B, nitrates are not formed or even if formed are extremely small in amount, and consequently, as shown inFIG. 5, even when the catalyst temperature TC is high, a high NOXpurification rate R1is obtained.

In the embodiment according to the present invention, to be able to purify NOXby using this new NOXpurification method, a hydrocarbon feed valve15for feeding hydrocarbons is arranged in the engine exhaust passage, an exhaust purification catalyst13is arranged in the engine exhaust passage downstream of the hydrocarbon feed valve15, precious metal catalysts51are carried on the exhaust gas flow surfaces of the exhaust purification catalyst13, basic exhaust gas flow surface parts54are formed around the precious metal catalysts51, the exhaust purification catalyst13has the property of reducing the NOXwhich is contained in exhaust gas if the concentration of hydrocarbons which flow into the exhaust purification catalyst13is made to vibrate by within a predetermined range of amplitude and within a predetermined range of period and has the property of being increased in storage amount of NOXwhich is contained in exhaust gas if the vibration period of the hydrocarbon concentration is made longer than this predetermined range, and, at the time of engine operation, the hydrocarbons are injected from the hydrocarbon feed valve15within the predetermined range of period to thereby reduce the NOXwhich is contained in the exhaust gas in the exhaust purification catalyst13.

That is, the NOXpurification method which is shown fromFIG. 4toFIGS. 6A and 6Bcan be said to be a new NOXpurification method designed to remove NOXwithout forming so much nitrates in the case of using an exhaust purification catalyst which carries precious metal catalysts and forms a basic layer which can absorb NOX. In actuality, when using this new NOXpurification method, the nitrates which are detected from the basic layer53become extremely smaller in amount compared with the case where making the exhaust purification catalyst13function as an NOXstorage catalyst. Note that, this new NOXpurification method will be referred to below as the “first NOXpurification method”.

Now, as mentioned before, if the injection period ΔT of the hydrocarbons from the hydrocarbon feed valve15becomes longer, the time period in which the oxygen concentration around the active NOX* becomes higher becomes longer in the time period after the hydrocarbons are injected to when the hydrocarbons are next injected. In this case, in the embodiment shown inFIG. 1, if the injection period ΔT of the hydrocarbons becomes longer than about 5 seconds, the active NOX* starts to be absorbed in the form of nitrates inside the basic layer53. Therefore, as shown inFIG. 10, if the vibration period ΔT of the hydrocarbon concentration becomes longer than about 5 seconds, the NOXpurification rate R1falls. Therefore, the injection period ΔT of the hydrocarbons has to be made 5 seconds or less.

On the other hand, in the embodiment of the present invention, if the injection period ΔT of the hydrocarbons becomes about 0.3 second or less, the injected hydrocarbons start to build up on the exhaust gas flow surfaces of the exhaust purification catalyst13, therefore, as shown inFIG. 10, if the injection period ΔT of the hydrocarbons becomes about 0.3 second or less, the NOXpurification rate R1falls. Therefore, in the embodiment according to the present invention, the injection period of the hydrocarbons is made from 0.3 second to 5 seconds.

Now, in the embodiment according to the present invention, when the NOXpurification action by the first NOXpurification method is performed, by controlling the injection amount and injection timing of hydrocarbons from the hydrocarbon feed valve15, the air-fuel ratio (A/F) in of the exhaust gas flowing into the exhaust purification catalyst13and the injection period ΔT of the hydrocarbons are controlled so as to become the optimal values for the engine operating state. In this case, in the embodiment according to the present invention, the optimum hydrocarbon injection amount WT when the NOXpurification action by the first NOXpurification method is performed is stored as a function of the injection amount Q from fuel injectors3and the engine speed N in the form of a map such as shown inFIG. 11Ain advance in the ROM32. Further, the optimum injection period ΔT of the hydrocarbons at this time is also stored as a function of the injection amount Q from the fuel injectors3and the engine speed N in the form of a map such as shown inFIG. 11Bin advance in the ROM32.

Next, referring toFIG. 12toFIG. 15, an NOXpurification method when making the exhaust purification catalyst13function as an NOXstorage catalyst will be explained specifically. The NOXpurification method in the case of making the exhaust purification catalyst13function as an NOXstorage catalyst in this way will be referred to below as the “second NOXpurification method”.

In this second NOXpurification method, as shown inFIG. 12, when the stored NOXamount ΣNOXof NOXwhich is stored in the basic layer53exceeds a first predetermined allowable amount MAX1, the air-fuel ratio (A/F) in of the exhaust gas flowing into the exhaust purification catalyst13is temporarily made rich. If the air-fuel ratio (A/F) in of the exhaust gas is made rich, the NOXwhich was stored in the basic layer53when the air-fuel ratio (A/F) in of the exhaust gas was lean is released from the basic layer53all at once and reduced. Due to this, the NOXis removed.

The stored NOXamount ΣNOXis, for example, calculated from the amount of NOXwhich is exhausted from the engine. In this embodiment according to the present invention, the exhausted NOXamount NOXA of NOXwhich is exhausted from the engine per unit time is stored as a function of the injection amount Q and engine speed N in the form of a map such as shown inFIG. 13in advance in the ROM32. The stored NOXamount ΣNOXis calculated from this exhausted NOXamount NOXA. In this case, as explained before, the period at which the air-fuel ratio (A/F) in of the exhaust gas is made rich is usually 1 minute or more.

In this second NOXpurification method, as shown inFIG. 14, by injecting an additional fuel WR into each combustion chamber2from the fuel injector3in addition to the combustion-use fuel Q, the air-fuel ratio (A/F) in of the exhaust gas which flows into the exhaust purification catalyst13is made rich. Note that, inFIG. 14, the abscissa indicates the crank angle. This additional fuel WR is injected at a timing at which it will burn, but will not appear as engine output, that is, slightly before ATDC90° after compression top dead center. This fuel amount WR is stored as a function of the injection amount Q and engine speed N in the form of a map such as shown inFIG. 15in advance in the ROM32. In this way, in case where the second NOXpurification method is performed, when the air-fuel ratio (A/F) in of the exhaust gas flowing into the exhaust purification catalyst13should be made rich, the air-fuel ratio (A/F) in of the exhaust gas discharged from the combustion chamber2is made rich by feeding the additional fuel WR to the combustion chamber2.

FIG. 16shows together the NOxremoval rate R1when the NOxremoval action by the first NOxremoval method is being performed and the NOXremoval rate R2when the NOxremoval action by the second NOxremoval method is being performed.

As shown inFIG. 16, the NOxremoval rate R1when the NOxremoval action by the first NOxremoval method is being performed becomes extremely high when the catalyst temperature TC is 350° C. or more and falls along with the fall of the catalyst temperature TC when the catalyst temperature becomes 350° C. or less. On the other hand, the NOxremoval rate R2when the NOxremoval action by the second NOxremoval method is being performed becomes extremely high when the catalyst temperature TC is 250° C. to 300° C., starts to fall a bit at a time along with the catalyst temperature becoming higher when the catalyst temperature TC becomes 300° C. or more, and rapidly falls along with the rise of the catalyst temperature TC when the catalyst temperature TC becomes 350° C. or more.

InFIG. 16, T1shows the catalyst temperature when the NOxremoval rate R2starts to fall when the catalyst temperature TC rises in the case where the NOxremoval action by the second NOxremoval method is being performed, while T2shows the catalyst temperature when the NOxremoval rate R2becomes zero when the catalyst temperature TC further rises in the case where the NOxremoval action by the second NOxremoval method is being performed. In this embodiment according to the present invention, the temperature region where the catalyst temperature TC is the temperature T1or less is called the “low temperature region”, the temperature region where the catalyst temperature TC is between the temperature T1and the temperature T2is called the “intermediate temperature region”, and the temperature region where the catalyst temperature TC is the temperature T2or more is called the “high temperature region”. Therefore, in this embodiment according to the present invention, the intermediate temperature region shows the temperature range where the NOxremoval rate R2continues to fall if the temperature TC of the exhaust purification catalyst13rises in the case where the NOxremoval action by the second NOxremoval method is being performed.

As shown inFIG. 16, in the high temperature region where the catalyst temperature TC is higher than the temperature T2, the NOxremoval rate R2becomes zero. With the second NOxremoval method, NOxcannot be removed. Therefore, in this embodiment according to the present invention, at this time, that is, in the high temperature region, the NOxremoval action by the first NOxremoval method is performed. On the other hand, in the low temperature region where the catalyst temperature TC is lower than the temperature T1, the NOxremoval rate R2becomes high. Therefore, in this embodiment according to the present invention, at this time, that is, in the low temperature region, the NOxremoval action by the second NOxremoval method is performed. As opposed to this, when the catalyst temperature TC is between the temperature T1and the temperature T2, that is, in the intermediate temperature region, the NOxremoval rate R1falls in part of the temperature region and the NOxremoval rate R2falls in a considerably broad temperature region. Therefore, in this case, even if using either of the first and the second NOxremoval methods, the NOxremoval rate will fall in some temperature region.

Therefore, in the present invention, when the catalyst temperature TC is in the intermediate temperature region, the first NOxremoval method and the second NOxremoval method are jointly used so as to obtain an NOxremoval rate which is higher than the NOxremoval rate R1when the NOxremoval action by the first NOxremoval method is performed and the NOxremoval rate R2when the NOxremoval action by the second NOxremoval method is performed. Next, this will be explained with reference toFIGS. 17A and 17B. Note that,FIGS. 17A and 17Bshow the exhaust purification catalyst13. InFIGS. 17A and 17B, X shows when the exhaust purification catalyst13is not storing NOx. On the other hand, inFIGS. 17A and 17B, Y shows when the exhaust purification catalyst13is storing NOx, and the hatchings inFIGS. 17A and 17Bshow the ratio of the NOxwhich is actually stored with respect to the total amount of NOxwhich the exhaust purification catalyst13can store, that is, the NOxstorage ratio.

FIG. 17Ashows when the NOxremoval action by the second NOxremoval method is being performed. At this time, the state which is shown by X and the state which is shown by Y are repeated. That is, at this time, as shown inFIG. 17Aby Y, if the NOxamount which is stored in the exhaust purification catalyst13approaches saturation, that is, exceeds the first allowable value MAX which is shown inFIG. 12, the air-fuel ratio of the exhaust gas which flows into the exhaust purification catalyst13is made rich. Due to this, as shown inFIG. 17Aby X, the NOxstorage ratio in the exhaust purification catalyst13is made zero. Next, again, the amount of NOxstored in the exhaust purification catalyst13increases.

As opposed to this,FIG. 17Bshows the case where the first NOxremoval method and the second NOxremoval method are jointly used in the intermediate temperature region. In this case, inFIG. 17B, the state which is shown by X and the state which is shown by Y are repeated. That is, at this time, as shown inFIG. 17Bby Y, if the NOxamount which is stored in the exhaust purification catalyst13becomes the second allowable value SX which is smaller than the first allowable value MAX, that is, in the example which is shown inFIG. 17B, if the NOxstorage ratio becomes 50 percent, the air-fuel ratio of the exhaust gas which flows into the exhaust purification catalyst13is made rich. Due to this, as shown inFIG. 17Bby X, the NOxstorage ratio in the exhaust purification catalyst13is made zero. Next, again, the amount of NOxwhich is stored in the exhaust purification catalyst13increases. In this way, in the example which is shown inFIG. 17B, this second allowable value SX is made the stored NOxamount when the NOxstorage ratio is 50 percent.

That is, if the exhaust purification catalyst13stores NOxwhen the NOxremoval action by the first NOxremoval method is being performed, NOxbecomes harder to stick to or be adsorbed in the form of a reducing intermediate at the surface part of the basicity layer53where the NOxis stored. Therefore, if the amount of NOxwhich is stored at the exhaust purification catalyst13increases, the amount of NOxwhich can be removed by the NOxremoval action by the first NOxremoval method decreases. Therefore, to remove NOxwell by using the NOxremoval action by the first NOxremoval method even if the exhaust purification catalyst13stores NOx, it is necessary to prevent the exhaust purification catalyst13from storing a large amount of NOx. In this case, compared with the maximum stored NOxamount when the NOxremoval action by the second NOxremoval method is being performed, which is shown inFIG. 17Aby Y, if limiting the maximum stored NOxamount when the NOxremoval action by the first NOxremoval method is being performed to a small amount such as shown inFIG. 17Bby Y, it is possible to sufficiently secure an area of the surface part of the basicity layer53at which the reducing intermediate can easily stick or be adsorbed and, therefore, when the NOxremoval action by the first NOxremoval method is performed, a good NOxremoval action is performed.

Therefore, in the example which is shown inFIG. 17B, in the intermediate temperature region, to limit the maximum stored NOxamount to a small amount such as shown inFIG. 17Bby Y, when the stored NOxamount in the exhaust purification catalyst13becomes the second allowable value SX which is smaller than the first allowable value MAX, the air-fuel ratio of the exhaust gas which flows into the exhaust purification catalyst13is made rich. That is, in the intermediate temperature region, the NOxremoval action by the first NOxremoval method is performed, and when the stored NOxamount in the exhaust purification catalyst13becomes the second allowable value SX, the air-fuel ratio of the exhaust gas which flows into the exhaust purification catalyst13is made rich. By doing this, if jointly using the first NOxremoval method and the second NOxremoval method, the NOxremoval action by the second NOxremoval method is superposed in form over the NOxremoval action by the first NOxremoval method, so a high NOxremoval rate such as shown inFIG. 16by R is obtained.

Therefore, in the present invention, there is provided an exhaust purification system of an internal combustion engine in which an exhaust purification catalyst13is arranged in an engine exhaust passage and a hydrocarbon feed valve15is arranged in the engine exhaust passage upstream of the exhaust purification catalyst13, a precious metal catalyst51is carried on an exhaust gas flow surface of the exhaust purification catalyst13and a basic exhaust gas flow surface part54is formed around the precious metal catalyst51, the exhaust purification catalyst13has the property of reducing the NOxwhich is contained in exhaust gas if making a concentration of hydrocarbons which flow into the exhaust purification catalyst13vibrate by within a predetermined range of amplitude and by within a predetermined range of period and has the property of being increased in storage amount of NOXwhich is contained in the exhaust gas if making the vibration period of the hydrocarbon concentration longer than the predetermined range, a first NOxremoval method which injects hydrocarbons from the hydrocarbon feed valve15by a predetermined injection period to thereby remove the NOxwhich is contained in the exhaust gas and a second NOxremoval method which makes the air-fuel ratio of the exhaust gas which flows into the exhaust purification catalyst13rich to make the exhaust purification catalyst13release the stored NOxwhen the NOxwhich is stored in the exhaust purification catalyst13exceeds a predetermined first allowable value MAX are selectively used, and in the second NOxremoval method, the period by which the air-fuel ratio of the exhaust gas which flows into the exhaust purification catalyst13is made rich is longer than the above-mentioned predetermined injection period, in which exhaust purification system of an internal combustion engine, temperature regions which the exhaust purification catalyst can take at the time of engine operation are divided into the three regions of a low temperature region, an intermediate temperature region, and a high temperature region, in the high temperature region, an NOxremoval action by the first NOxremoval method is performed, in the low temperature region, an NOxremoval action by the second NOxremoval method is performed, and in the intermediate temperature region, hydrocarbons are injected from the hydrocarbon feed valve15by the predetermined injection period and, when the NOxwhich is stored in the exhaust purification catalyst13exceeds a predetermined second allowable value SX of a value smaller than the first allowable value MAX, the air-fuel ratio of the exhaust gas which flows into the exhaust purification catalyst13is made rich.

FIG. 18shows an embodiment in which the second allowable value SX is changed in accordance with the temperature TC of the exhaust purification catalyst13. Note that,FIG. 18also shows the changes in the NOxremoval rates R1, R2, and R. Now then, in the intermediate temperature region, if the catalyst temperature TC becomes higher, the amount of NOxwhich can be stored in the exhaust purification catalyst13becomes smaller. When the amount of NOxwhich can be stored in the exhaust purification catalyst13becomes smaller, unless the exhaust purification catalyst13is made to release NOxwhile the amount of NOxwhich is stored in the exhaust purification catalyst13is small, NOxcan no longer be stored. Therefore, when the amount of NOxwhich can be stored in the exhaust purification catalyst13becomes small, it is necessary to make the air-fuel ratio of the exhaust gas which flows into the exhaust purification catalyst13rich while the amount of storage of NOxis small. Therefore, in the embodiment which is shown inFIG. 19, the second allowable value SX is made smaller as the temperature TC of the exhaust purification catalyst13becomes higher. If the amount of NOxwhich can be stored in the exhaust purification catalyst13becomes smaller, the amount of NOxwhich can be removed by the NOxremoval action by the second NOxremoval method is decreased and the amount of NOxwhich is removed by the NOxremoval action by the first NOxremoval method is increased. That is, in the embodiment which is shown inFIG. 19, in the intermediate temperature region, as the temperature TC of the exhaust purification catalyst13becomes higher, the amount of NOxwhich is removed by the NOxremoval action by the second NOxremoval method is decreased and the amount of NOxwhich is removed by the NOxremoval action by the first NOxremoval method is increased.

FIG. 19shows a time chart of NOxpurification control in the intermediate temperature region. Note that,FIG. 19shows the hydrocarbon feed signal from the hydrocarbon feed valve15, the feed signal of the additional fuel WR from the fuel injector3, the change in the NOxamount ΣNOX which is stored in the exhaust purification catalyst13, and the change in the air-fuel ratio (A/F) in of the exhaust gas which flows into the exhaust purification catalyst13. Further,FIG. 18shows the first allowable value MAX and the second allowable value SX. FromFIG. 18, it will be understood that the second allowable value SX is considerably smaller compared with the first allowable value MAX.

As will be understood fromFIG. 19, when the stored NOxamount ΣNOX is smaller than the second allowable value SX, hydrocarbons are injected in accordance with the hydrocarbon feed signal from the hydrocarbon feed valve15by a predetermined injection period and the NOxremoval action by the first NOxremoval method is performed. As opposed to this, when the stored NOxamount ΣNOX exceeds the second allowable value SX, additional fuel WR is injected from the fuel injector3over a certain time period in accordance with the additional fuel feed signal and thereby the air-fuel ratio (A/F) in of the exhaust gas which flows into the exhaust purification catalyst13is made rich. When the additional fuel WR finishes being injected, the stored NOxfinishes being released, so the stored NOxamount ΣNOX becomes zero. Note that, when additional fuel WR is injected from the fuel injector3and the air-fuel ratio (A/F) in of the exhaust gas which flows into the exhaust purification catalyst13becomes rich, if hydrocarbons are injected from the hydrocarbon feed valve15, the air-fuel ratio (A/F) in of the exhaust gas which flows into the exhaust purification catalyst13will become too rich, the hydrocarbons will slip through the exhaust purification catalyst13, and there is the danger of white smoke being generated. Therefore, as shown inFIG. 18by PH, in the intermediate temperature region, while the feed of additional fuel WR from the fuel injector3is being used to make the air-fuel ratio of the exhaust gas which flows into the exhaust purification catalyst13rich, injection of hydrocarbons from the hydrocarbon feed valve15is suspended.

On the other hand, in this embodiment according to the present invention, when the NOxremoval action by the first NOxremoval method is being performed, the NOxamount NOX which is stored per unit time in the exhaust purification catalyst13is calculated based on the following formula:
NOX=(NOXA−RR)·KR
Here, NOXA shows the amount of NOxwhich is exhausted per unit time from the engine which is shown inFIG. 13, RR shows the amount of reduction of NOxper unit time by hydrocarbons which are injected from the hydrocarbon feed valve15, and KR shows the storage rate of NOxin the exhaust purification catalyst13. As shown inFIGS. 11A and 11B, the injection amount WT and injection period ΔT of hydrocarbons from the hydrocarbon feed valve15are determined in advance in accordance with the operating state of the engine. Therefore, the NOxreduction amount RR which is reduced per unit time of injection by the hydrocarbons which are injected from the hydrocarbon feed valve15is also determined in advance in accordance with the operating state of the engine. Therefore, in this embodiment according to the present invention, this NOxreduction amount per unit time is stored as a function of the amount of injection Q from the fuel injector3and the engine speed N in the form of a map such as shown inFIG. 20Ain advance in the ROM32. On the other hand, the NOxstorage rate KR shows the ratio of the amount of NOxwhich is stored in the exhaust purification catalyst13in the amount of NOx(NOXA−RR) which could not be reduced by the hydrocarbons injected from the hydrocarbon feed valve15. This NOxstorage rate KP, as shown inFIG. 20B, falls as the temperature TC of the exhaust purification catalyst13becomes higher.

If the NOxamount NOX which is stored in the exhaust purification catalyst13per unit time is calculated, the NOxamount ΣNOX which is stored in the exhaust purification catalyst13is calculated by cumulatively adding this NOxamount NOX. In this way, in this embodiment according to the present invention, in the intermediate temperature region, when the NOxremoval action by the first NOxremoval method is being performed, the NOxamount ΣNOX which is stored in the exhaust purification catalyst13is calculated from the NOxamount NOXA which is exhausted from the engine, the NOxreduction amount RR which is determined from the operating state of the engine, and the NOxstorage rate KP which is determined from the temperature TC of the exhaust purification catalyst13and, when the calculated NOxamount ΣNOX exceeds the second allowable value SX, the air-fuel ratio of the exhaust gas which flows into the exhaust purification catalyst13is made rich.

FIG. 21andFIG. 22show the NOxpurification control routine for performing the NOxremoval method which is shown inFIG. 19. This routine is executed by interruption every certain time period.

Referring toFIG. 21, first, to start, at step60, the detected value of the temperature sensor23is used as the basis to calculate the temperature TC of the exhaust purification catalyst13. Next, at step61, it is judged if the catalyst temperature TC is lower than the temperature T1. When the catalyst temperature TC is lower than the temperature T1, that is, when in the low temperature region, it is judged that the NOxremoval action by the second NOxremoval method should be performed. The routine proceeds to step62where the NOxremoval action by the second NOxremoval method is performed.

That is, at step62, the NOxamount NOXA which is exhausted per unit time is calculated from the map which is shown inFIG. 13. Next at step63, the exhausted NOxamount NOXA is added to ΣNOX to calculate the stored NOxamount ΣNOX. Next, at step64, it is judged if the stored NOxamount ΣNOX exceeds the first allowable value MAX. When ΣNOX>MAX, the routine proceeds to step65where the amount of additional fuel WR is calculated from the map which is shown inFIG. 15. Next, at step66, the action of injection of additional fuel is performed. At this time, the air-fuel ratio (A/F) in of the exhaust gas which flows into the exhaust purification catalyst13is made rich. Next, at step67, it is judged if the exhaust purification catalyst13has finished being regenerated. When it is judged that the exhaust purification catalyst13has finished being regenerated, the routine proceeds to step68where ΣNOX is cleared.

On the other hand, at step61, when it is judged that the exhaust temperature TC is higher than the temperature T1, the routine proceeds to step69where it is judged if the catalyst temperature TC is higher than the temperature T2. When the catalyst temperature T is higher than the temperature T2, that is, when in the high temperature region, it is judged that the NOxremoval action by the first NOxremoval method should be performed and the routine proceeds to step70where the NOxremoval action by the first NOxremoval method is performed. That is, at step70, the injection period ΔT of hydrocarbons is read fromFIG. 11B. Next, at step71, it is judged if the injection timing has been reached. When the injection timing has been reached, the routine proceeds to step72where the amount of injection WT of hydrocarbons is calculated fromFIG. 11A. Next, at step73, the injection amount WT which is calculated at step72is used to inject hydrocarbons from the hydrocarbon feed valve15.

On the other hand, when it is judged at step69that the catalyst temperature TC is lower than the temperature T2, that is, when it is the intermediate temperature region, the routine proceeds to step74ofFIG. 22where it is judged if the injection prohibit flag which shows that injection of hydrocarbons from the hydrocarbon feed valve15should be prohibited has been set. When the injection prohibit flag is not set, the routine proceeds to step75where the NOxamount NOXA which is exhausted per unit time is calculated from the map which is shown inFIG. 13, the NOxreduction amount RR is calculated from the map which is shown inFIG. 20A, and the NOxstorage rate KP is calculated fromFIG. 20B. Next, at step76, the NOxamount NOX which is stored per unit time is calculated based on the following formula:
NOX=(NOXA−RR)·KR
Next at step77, the NOxamount ΣNOX which is stored in the exhaust purification catalyst13is calculated based on the following formula:
ΣNOX=ΣNOX+NOX

Next, at step78, the second allowable value SX which is shown inFIG. 18is calculated. Next, at step79, it is judged if the stored NOxamount ΣNOX exceeds the second allowable value SX. When the stored NOxamount ΣNOX does not exceed the second allowable value SX, the routine proceeds to step80where the NOxremoval action by the first NOxremoval method is performed. That is, at step80, the injection period ΔT of the hydrocarbons is read fromFIG. 11B. Next, at step81, it is judged if the injection timing has been reached. When the injection timing has been reached, the routine proceeds to step82where the injection amount WT of hydrocarbons is calculated fromFIG. 11A. Next, at step83, the injection amount WT which is calculated at step82is used to injection hydrocarbons from the hydrocarbon feed valve15. Next, at step84, the NOxamount CNO which is released from the exhaust purification catalyst13at the time of injection of hydrocarbons from the hydrocarbon feed valve15, is subtracted from the stored NOxamount ΣNOX.

On the other hand, when it is judged at step79that the stored NOxamount ΣNOX exceeds the second allowable value SX, the routine proceeds to step85where the injection prohibit flag is set, next the routine proceeds to step86. If the injection prohibit flag is set, at the next processing cycle, the routine jumps from step74to step86. At step86, the amount of additional fuel WRL which is required for making the stored NOxbe released is calculated. Next, at step87, the action of injection of additional fuel to the inside of the combustion chamber2is performed. At this time, the air-fuel ratio (A/F) in of the exhaust gas which flows into the exhaust purification catalyst13is made rich. Next, at step88, it is judged if the exhaust purification catalyst13has finished being regenerated. When it is judged that the exhaust purification catalyst13has finished being regenerated, the routine proceeds to step89where the injection prohibit flag is reset. Next at step90, ΣNOX is cleared.

In this regard, when the catalyst temperature TC is maintained at the intermediate temperature region, a large temperature difference does not arise between the upstream side and the downstream side of the exhaust purification catalyst13. On the other hand, for example, if the amount of injection of hydrocarbons from the hydrocarbon feed valve15for regeneration of the particulate filter is increased, the temperature TC of the exhaust purification catalyst13becomes higher and the catalyst temperature TC shifts from the intermediate temperature region to the high temperature region. Next, if the particulate filter finishes being regenerated, the catalyst temperature TC falls and the catalyst temperature TC again becomes the intermediate temperature region. In this regard, when the particulate filter finishes being regenerated and the catalyst temperature TC falls, the exhaust purification catalyst13is cooled from the upstream side. Therefore, at this time, as shown inFIG. 23, the downstream side becomes higher than the upstream side. That is, when the particulate filter finishes being regenerated, a large temperature difference arises between the upstream side and the downstream side of the exhaust purification catalyst13. At this time, the catalyst temperature TC which is calculated from the detected value of the temperature sensor23becomes the mean temperature such as shown by Tm inFIG. 23.

On the other hand, as explained above, in the intermediate temperature region, the NOxamount NOX which is stored per unit time is calculated based on the following formula:
NOX=(NOXA−RR)·KR
The NOxreduction amount RR in this case is made the amount at the mean temperature in the intermediate temperature region. In this regard, this NOxreduction amount RR increases the higher the catalyst temperature TC becomes. Therefore, as shown inFIG. 23, when there is a part in the exhaust purification catalyst13where the temperature TC is high, the NOxreduction amount RR increases. Therefore, in this embodiment, the NOxamount NOX which is stored per unit time is calculated based on the following formula, and when in an operating state where there is a part in the exhaust purification catalyst13where the temperature is high as shown inFIG. 23, the value of the increase coefficient ZK, which is usually made 1.0, is increased.
NOX=(NOXA−RR·ZK)·KR
When, in this way, in this embodiment, a temperature difference arises in the exhaust purification catalyst13and there is a temperature region which is higher than the temperature TC of the exhaust purification catalyst13which is detected in the exhaust purification catalyst13, the NOxreduction amount RR increases.

Note that, as another embodiment, it is also possible to arrange an oxidation catalyst for reforming the hydrocarbons in the engine exhaust passage upstream in the exhaust purification catalyst13.

REFERENCE SIGNS LIST