JP5052208B2 - Exhaust gas purification device for internal combustion engine - Google Patents

Description

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

A NO _x selective reduction catalyst is disposed in the engine exhaust passage, and NO _x contained in the exhaust gas is occluded in the form of nitrate in the engine exhaust passage upstream of the NO _x selective reduction catalyst and supplied as a reducing agent. the the NO _X storing catalyst to release the _X located, an internal combustion engine which is adapted to selectively reduce NO _X contained in exhaust gas by ammonia by supplying urea to the NO _X selective reducing catalyst generated from the urea known (For example, see Patent Document 1). Urea supply amount in consideration of the amount of NO _X released from the amount of NO _X and _the NO _X storage catalyst is occluded in the NO _X storage catalyst in this internal combustion engine is determined. For example, when NO _X is released from the NO _X storage catalyst, the urea supply amount is increased by the reduction amount of the released NO _X amount.
JP 2005-2925 A

Incidentally NO to the NO _X storing catalyst in an internal combustion engine described above _the amount of NO _X amount of NO _X that can be adsorbed to thermal degradation is occluded in the NO _X storage catalyst to reduce changes are further released from the NO _X storing catalyst _X amount also changes. Therefore, in determining the urea supply amount, it is necessary to consider the thermal deterioration of the NO _x storage catalyst. However no consideration of the thermal deterioration of the NO _X storage catalyst in the above-mentioned internal combustion engine, therefore when _the NO _X storage catalyst is thermally degraded, there is a problem that not E maintained at optimum amount of urea supply amount.

In the above-described internal combustion engine, when a reducing agent, that is, a fuel is supplied to release NO _X from the NO _X storage catalyst, a part of the stored NO _X is released from the NO _X storage catalyst in the form of NO or NO _2. However, some occluded NO _x is further reduced than NO and released in the form of ammonia NH ₃ . In this case, it is not clear how much the stored NO _x is released as NO _x and how much it is released as ammonia NH ₃ . In this case, if the amount released as ammonia NH ₃ is large, the released NO _x is reduced by this ammonia NH ₃ , so that it is not necessary to increase the urea supply amount.

However, the above-mentioned internal combustion engine is based on the premise that all of the stored NO _x is released as NO _x, and therefore the urea supply amount is increased by the reduced amount of the released NO _x amount, so the urea supply amount is increased. There is a problem that it becomes excessive. Such a problem occurs as long as the NO _x storage catalyst is used.
An object of the present invention is to provide an exhaust purification device for an internal combustion engine in which various problems as described above do not occur by using a NO _x adsorption catalyst capable of releasing NO _x without supplying a reducing agent.

That is, according to the present invention, the NO _x selective reduction catalyst is arranged in the engine exhaust passage, urea is supplied to the NO _x selective reduction catalyst, and the NO _x contained in the exhaust gas is selectively selected by ammonia generated from the urea. in the exhaust purification system of an internal combustion engine which is adapted to reduce, NO and NO _X adsorbing catalyst disposed in the _X selective reducing catalyst in the engine exhaust passage upstream of, NO _X adsorber NO _X as NO _X temperature decreases adsorption catalyst properties _the amount of NO _X temperature of the NO _X adsorbing catalyst is adsorbed to the NO _X trap catalyst rises with the maximum adsorption amount has the property of increasing emits NO _X which is adsorbed to exceed the maximum amount of adsorption has a further NO _X adsorbing catalyst has the property of gradually gradually decreases the maximum adsorption amount with the progress of heat deterioration of the NO _X adsorbing catalyst, the amount of NO _X that is adsorbed is calculated in the NO _X adsorbing catalyst progress of the Rutotomoni heat deterioration Maximum adsorption amount is calculated that with decreased adsorption amount of NO _X in the time less than the maximum adsorption amount adsorbed amount of NO _X is calculated, which is calculated based on the amount of NO _X discharged from the engine to the NO _X adsorbing catalyst together but are calculated, when more than the maximum adsorption amount of adsorbed amount of NO _X is calculated, which is calculated is calculated released amount of NO _X from the NO _X adsorbing catalyst, is calculated from the amount of NO _X discharged from the engine adsorption NO NO _X amount in the exhaust gas flowing out from the NO _X adsorbing catalyst _{by X} amount by subtracting or adding the release NO _X amount calculated NO _X amount exhausted from the engine is calculated, this NO _The supplied urea amount is calculated from the _X amount.

Since the urea supply amount is calculated in consideration of the thermal deterioration of the NO _X adsorption catalyst, even if the NO _X adsorption catalyst is thermally deteriorated, the urea supply amount can be maintained at an optimum amount. Further, since NO or NO ₂ is released from the NO _x adsorption catalyst, the urea supply amount necessary for the reduction of NO _x can be accurately calculated.

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

On the other hand, the exhaust manifold 5 is connected to the inlet of the exhaust turbine 7 b of the exhaust turbocharger 7, and the outlet of the exhaust turbine 7 b is connected to the inlet of the NO _x adsorption catalyst 12. This is downstream of the NO _X adsorption catalyst 12 is arranged a particulate filter 13 for trapping particulate matter contained in the exhaust gas adjacent to the NO _X adsorption catalyst 12, the outlet of the particulate filter 13 The exhaust gas is connected to the inlet of the NO _x selective reduction catalyst 15 through the exhaust pipe 14. An oxidation catalyst 16 is connected to the outlet of the NO _x selective reduction catalyst 15.

A urea water supply valve 17 is disposed in the exhaust pipe 14 upstream of the NO _X selective reduction catalyst 15, and this urea water supply valve 17 is connected to a urea water tank 20 via a supply pipe 18 and a supply pump 19. The urea water stored in the urea water tank 20 is injected into the exhaust gas flowing in the exhaust pipe 14 from the urea water supply valve 17 by the supply pump 19, and ammonia ((NH ₂ ) ₂ CO + H ₂ O generated from urea. → 2NH ₃ + CO ₂ ) NO _X contained in the exhaust gas is reduced in the NO _X selective reduction catalyst 15.

  The exhaust manifold 5 and the intake manifold 4 are connected to each other via an exhaust gas recirculation (hereinafter referred to as EGR) passage 21, and an electronically controlled EGR control valve 22 is disposed in the EGR passage 21. A cooling device 23 for cooling the EGR gas flowing in the EGR passage 21 is disposed around the EGR passage 21. In the embodiment shown in FIG. 1, the engine cooling water is guided into the cooling device 23, and the EGR gas is cooled by the engine cooling water. On the other hand, each fuel injection valve 3 is connected to a common rail 25 via a fuel supply pipe 24, and this common rail 25 is connected to a fuel tank 27 via an electronically controlled fuel pump 26 with variable discharge amount. The fuel stored in the fuel tank 27 is supplied into the common rail 25 by the fuel pump 26, and the fuel supplied into the common rail 25 is supplied to the fuel injection valve 3 through each fuel supply pipe 24.

The electronic control unit 30 is composed of a digital computer, and is connected to each other by a bidirectional bus 31. A ROM (read only memory) 32, a RAM (random access memory) 33, a CPU (microprocessor) 34, an input port 35 and an output port 36. It comprises. The the NO _X adsorption catalyst 12 is attached a temperature sensor 28 for detecting the bed temperature of the NO _X adsorption catalyst 12, the output signal of the temperature sensor 28 and intake air amount detector 8 AD converters 37 respectively corresponding to To the input port 35. A load sensor 41 that generates an output voltage proportional to the depression amount L of the accelerator pedal 40 is connected to the accelerator pedal 40, and the output voltage of the load sensor 41 is input to the input port 35 via the corresponding AD converter 37. Is done. Further, the input port 35 is connected to a crank angle sensor 42 that generates an output pulse every time the crankshaft rotates, for example, 15 °. On the other hand, the output port 36 is connected to the fuel injection valve 3, the step motor for driving the throttle valve 10, the urea water supply valve 17, the supply pump 19, the EGR control valve 22, and the fuel pump 26 through corresponding drive circuits 38. .

The base of the NO _x adsorption catalyst 12 is made of cordierite or zeolite having a large number of pores, and a layer of a catalyst support made of alumina, for example, is formed on the base, and a noble metal such as platinum is formed on the catalyst support. A catalyst is supported. On the other hand, the particulate filter 13 may be a particulate filter that does not carry a catalyst, or a particulate filter that carries a precious metal catalyst such as platinum. Further, the NO _x selective reduction catalyst 15 can be composed of an ammonia adsorption type Fe zeolite having a high NO _x purification rate at a low temperature, or can be composed of a titania / vanadium catalyst having no ammonia adsorption function. . The oxidation catalyst 16 carries a noble metal catalyst made of, for example, platinum, and this oxidation catalyst 16 functions to oxidize ammonia leaked from the NO _x selective reduction catalyst 15.

FIG. 2 shows another embodiment of the compression ignition type internal combustion engine. In this embodiment, the particulate filter 13 is disposed downstream of the oxidation catalyst 16. Therefore, in this embodiment, the outlet of the NO _X adsorption catalyst 12 is connected to the inlet of the NO _X selective reduction catalyst 15 through the exhaust pipe 14.

Meanwhile _the NO _X selective reducing catalyst 15 does not become substantially 200 ° C. or higher when not activated, thus after the engine start, purification action of the NO _X by _the NO _X selective reducing catalyst 15 to a temperature of the NO _X selective reducing catalyst 15 is raised Cannot be expected. However, the NO _x adsorption catalyst 12 increases the amount of adsorbable NO _x as the temperature of the NO _x adsorption catalyst 12 decreases. Therefore NO _X in the exhaust gas when the NO _X adsorption catalyst 12 upstream of _the NO _X selective reducing catalyst 15 is disposed _the NO _X selective reducing catalyst 15 is not activated, as shown in FIGS. 1 and 2 It is adsorbed by the NO _x adsorption catalyst 12 and thus NO _x is suppressed from being released into the atmosphere.

Then NO _X NO _X from NO _X adsorption catalyst 12 to _the amount of NO _X that can be adsorbed and the temperature comes to rise to a decrease in the adsorption catalyst 12 is released. On the other hand, the temperature of the NO _X selective reducing catalyst 15 when the temperature of the NO _X adsorption catalyst 12 rises even _the NO _X selective reducing catalyst 15 is activated in order to increase, the NO _X released from the NO _X adsorption catalyst 12 and thus The NO _x selective reduction catalyst 15 is purified.

Next, the maximum adsorption amount of NO _{x that} can be adsorbed by the NO _x adsorption catalyst 12 will be described with reference to FIG.
The vertical axis in FIG. 3 (A) shows the NO _X adsorption amount ΣNOX of the NO _X adsorption catalyst 12, the horizontal axis represents the bed temperature TC of the NO _X adsorption catalyst 12. Further, NMAX ₀ in FIG. 3 (A) shows the maximum adsorption amount of the NO _X when NO _X adsorption catalyst 12 when the new, i.e. the NO _X adsorption catalyst 12 is not thermally deteriorated, NMAX (r ) Shows the maximum adsorption amount of NO _x when the NO _x adsorption catalyst 12 is thermally deteriorated. It can be seen from FIG. 3A that the maximum adsorption amount decreases when the NO _x adsorption catalyst 12 deteriorates.

Further, as can be seen from FIG. 3A, the maximum adsorption amounts NMAX ₀ and NMAX (r) are low in the bed temperature TC of the NO _x adsorption catalyst 12 regardless of whether the NO _x adsorption catalyst 12 is thermally deteriorated. Indeed it increases, thus NO _X adsorption catalyst 12 when the bed temperature TC of the NO _X adsorption catalyst 12 is low, it can be seen that has a function of adsorbing a large amount of the NO _X for example when the engine is started. On the other hand, as the NO _x adsorption catalyst 12 deteriorates, the maximum adsorption amount NMAX (r) at the time of deterioration decreases with respect to NMAX _0. Therefore, as the NO _x adsorption catalyst 12 deteriorates, the amount of NO _{x that} can be adsorbed at engine startup becomes smaller. Less.

Thermal deterioration of the NO _X adsorption catalyst 12 is dependent on the thermal history applied to the NO _X adsorption catalyst 12, the higher heat is applied to the NO _X adsorption catalyst 12, or the longer the The times involved NO _X adsorbing catalyst No. 12 is thermally deteriorated. Therefore, in the embodiment according to the present invention, the deterioration rate R (%) of the NO _x adsorption catalyst 12 is calculated based on the following equation.

R (%) = K ₁ · t ₁ + K ₂ · t ₂ + K ₃ · t ₃
Here, K ₁ , K ₂ , and K ₃ are constants, and t ₁ , t ₂ , and t ₃ represent the total time so far shown below.
Bed temperature TC of the NO _X adsorption catalyst 12 to date total time 600 ° C. of to 650 ° C. t ₁
650 ° C to 700 ° C t ₂
700 ℃ or higher t ₃
Note that K ₁ , K ₂ , and K ₃ have a relationship of K ₁ <K ₂ <K ₃ .

Therefore, it can be seen that the deterioration rate R increases as the bed temperature TC of the NO _x adsorption catalyst 12 increases, and increases as the total time increases. When the deterioration rate R is obtained, the maximum adsorption amount NMAX (r) at the time of deterioration at each bed temperature TC is calculated based on the following equation.
NMAX (r) = NMAX _0. (1-R (%) / 100 (%))

In the above-described equation representing the deterioration rate R, it is the third term K ₃ · t ₃ on the right side that has the most influence on the deterioration rate R, so K ₁ · t ₁ and K ₂ · t ₂ are omitted in the above equation. You can also.

FIG. 3B shows the current maximum adsorption amount NMAX (r) calculated using the deterioration rate R. Now, _let us assume that the NO _X adsorption catalyst 12 is in the state indicated by point A in FIG. 3B, that is, the bed temperature TC is relatively low and the NO _X adsorption amount ΣNOX is relatively large. When the bed temperature TC rises from this state to the temperature indicated by point B, the excess NO _x adsorption amount ΔNX with respect to the maximum adsorption amount NMAX (r) is released from the NO _x adsorption catalyst 12 at this time. Thus unlike _the NO _X storage catalyst that occludes NO _X in the form of nitrates, NO _X adsorbing be upstream of the catalyst 12 without supplying a reducing agent of the NO _X adsorption catalyst 12 bed temperature TC is NO if elevated NO _X is released from the _X adsorption catalyst 12.

Well, NO _X adsorbing catalyst 12 adsorbs NO _X at a low temperature say schematically, the nature of releasing NO _X at a high temperature, ie NO _X NO _X contained in the exhaust gas in accordance with the temperature of the adsorption catalyst 12 has the property that decreases gradually possible amount adsorbed nO _X is not accompanied with the progress of heat deterioration of the nO _X adsorption catalyst 12 which has the property of releasing the nO _X that is or adsorbed to the adsorption. In this case, _the amount of NO _X can be adsorbed amount of NO _X is adsorbed in the NO _X adsorption catalyst 12 Of course if reduction is lowered, the amount of NO _X adsorbed in the NO _X adsorption catalyst 12 is reduced NO _X The amount of NO _x released from the adsorption catalyst 12 also decreases.

Thus the adsorption amount of NO _X and NO _X adsorption in the present invention to NO _X adsorption catalyst 12 based on the adsorbable amount of NO _X capable of adsorbing amount of NO _X decreases In conjunction that is to the estimated estimation progression of thermal degradation calculating the discharge amount of NO _X from the catalyst 12 to calculate the urea supply amount using these adsorption amount of NO _X and release amount of NO _X.

Next, the urea supply amount will be described in a little more detail. That, NO _X amount in the exhaust gas flowing out from the NO _X adsorption catalyst 12 at the time when a certain amount of the NO _X from the engine is discharged to NO _X is adsorbed in the NO _X adsorption catalyst 12 is reduced, NO _X There amount of NO _X in the exhaust gas flowing out from the NO _X adsorption catalyst 12 is increased when you are released from the NO _X adsorption catalyst 12. In this case, amount of urea required for reducing the NO _X is decreased with decreasing _the amount of NO _X in the exhaust gas flowing out from the NO _X adsorption catalyst 12, NO _X NO in the exhaust gas flowing out from the adsorption catalyst 12 _It increases as the amount of _X increases. On the other hand, Sadamari is NO _X emissions from the engine when the determined operating state of the engine, thus the urea supply amount required to reduce emissions NO _X from the engine when the operating condition is determined for the engine is determined.

Therefore, in the present invention to reduce the reduction amount corresponding urea supply amount of adsorbed amount of NO _X is calculated to urea supply amount determined from the engine operating state, release NO _X calculated to urea supply amount determined from the engine operating state The urea supply amount is increased by the amount of reduction.

Next, an embodiment of the urea supply method according to the present invention will be described with reference to FIGS.
NO _X exhausted from the engine as described above is determined in accordance with the engine operating state. In the embodiment according to the present invention, the NO _X amount NOXA discharged from the engine per unit time is stored in advance in the ROM 32 as a function of the required torque TQ and the engine speed N in the form of a map shown in FIG.

On the other hand, NO _X exhausted in the adsorption catalyst adsorption rate of the NO _X adsorbing to 12 NO _X adsorbing catalyst NO _X adsorption amount adsorbed by the 12 .SIGMA.NOX and NO _X adsorption catalyst 12 the gas out of the discharged NO _X from the engine It is a function of the space velocity of the flow. That is, as indicated by K1 in FIG. 4B, the adsorption rate decreases as the NO _X adsorption amount ΣNOX adsorbed on the NO _X adsorption catalyst 12 increases, and as indicated by K2 in FIG. 4C. Further, the adsorption rate decreases as the space velocity of the exhaust gas flow in the NO _x adsorption catalyst 12, that is, the intake air amount Ga increases. These adsorption rates K1 and K2 are stored in the ROM 32 in advance. In the embodiment according to the present invention _the amount of NO _X NOXA · K1 · K2 adsorbed in unit time per NO _X adsorption catalyst 12 by multiplying the discharge amount of NO _X NOXA adsorption rate K1 and K2 from the engine is calculated.

On the other hand, as described with reference to FIG. 3B, when the NO _x adsorption catalyst 12 rises from the state indicated by point A to the temperature indicated by point B, the excess NO _x adsorption amount with respect to the maximum adsorption amount NMAX (r). ΔNX is released from the NO _x adsorption catalyst 12. In this case, the excess NO _X adsorption amount ΔNX is not being released all at once, the space velocity of the exhaust gas in the NO _X adsorption amount ΣNOX and NO _X adsorption catalyst 12 to the NO _X adsorption catalyst 12, i.e. the intake air amount It is gradually released at a rate according to Ga. That is, as shown in FIG. 5A, the NO _x desorption speed W at a certain intake air amount Ga, that is, the NO _x amount W released from the NO _x adsorption catalyst 12 per unit time is the NO _x adsorption amount ΣNOX. The higher it is, the higher it becomes. That is, as the NO _X adsorption amount ΣNOX increases, a larger amount of NO _X is released.

On the other hand, the desorption rate of NO _x desorbed from the NO _x adsorption catalyst 12 increases as the intake air amount Ga increases as shown in FIG. In this case, the actual NO _x desorption rate, that is, the amount of NO _x actually desorbed per unit time from the NO _x adsorption catalyst 12 is shown in FIG. 5 (B) as the desorption rate W shown in FIG. 5 (A). The value WD obtained by multiplying the desorption rate D to be obtained is obtained. The desorption speed W and desorption rate D are stored in the ROM 32 in advance.

FIG. 6 shows a routine for controlling the supply of urea. This routine is executed by interruption every predetermined time.
Discharge amount of NO _X NOXA per unit time from the engine from the map shown in FIG. 4 (A), first, at step 50 a reference is calculated to FIG. Next, at step 51, the deterioration rate R (= K _t · t ₁ + K ₂ · t ₂ + K ₃ · t ₃ ) is calculated. Specifically, the total time t ₁ , t ₂ , when the bed temperature TC of the NO _x adsorption catalyst 12 is in the corresponding temperature range from when the use of the vehicle is started until now, t ₃ is calculated, and the deterioration rate R is calculated using these total times t ₁ , t ₂ , and t ₃ .

Next, at step 52, the maximum adsorption amount NMAX (r) at the time of deterioration at each bed temperature TC is calculated based on the following equation.
NMAX (r) = NMAX _0. (1-R (%) / 100 (%))
Then adsorption amount of NO _X ΣNOX to NO _X adsorption catalyst 12 at step 53 whether less than the maximum adsorption amount NMAX (r) is determined. When ΣNOX <NMAX (r), that is, when there is still room for adsorbing NO _X , the routine proceeds to step 54.

At step 54, the adsorption rate K1 is calculated from the relationship shown in FIG. 4B, and then at step 55, the adsorption rate K2 is calculated from the relationship shown in FIG. 4C. Next, at step 56, the NO _X adsorption amount ΣNOX is calculated by adding the NO _X amount NOXA · K1 · K2 actually adsorbed to the NO _X adsorption catalyst 12 per unit time to ΣNOX. Next, at step 57, the NO _X amount NOXZ in the exhaust gas flowing out from the NO _X storage catalyst 12 per unit time by subtracting the NO _X amount NOXA · K1 · K2 actually adsorbed per unit time from the exhausted NO _X amount NOXA. Is calculated.

Then NO _X in the exhaust gas flowing out from the NO _X adsorption catalyst 12 at step 62, i.e. amount of urea required for reducing NO _X in the exhaust gas flowing into the NO _X selective reducing catalyst 15 is calculated. In the embodiment according to the present invention, the urea amount is set so that the equivalent ratio = 1 with respect to the NO _x amount to be reduced. Next, at step 63, the urea water supply action from the urea water supply valve 17 is performed.

On the other hand, when it is determined at step 53 that ΣNOX ≧ NMAX (r), the routine proceeds to step 58 where the desorption speed W is calculated from the relationship shown in FIG. Next, at step 59, the desorption rate D is calculated from the relationship shown in FIG. Then NO _X adsorption amount .SIGMA.NOX is calculated by subtracting actually _the amount of NO _X W · D desorbed per unit in step 60 the time from .SIGMA.NOX. Then calculated _the amount of NO _X NOXZ in the exhaust gas flowing out per unit time from the NO _X adsorption catalyst 12 by adding the amount of NO _X W · D actually desorbed per unit time in step 61 the discharge amount of NO _X NOXA Is done. Next, at step 62 the amount of urea required for reducing the NO _X is calculated.

Thus the release _the amount of NO _X W · D to the NO _X amount NOXA is discharged from subtracting or institutions adsorption amount of NO _X NOXA · K1 · K2 from _the amount of NO _X NOXA exhausted from the engine in the embodiment according to the present invention By adding, the NO _X amount NOXZ in the exhaust gas flowing out from the NO _X adsorption catalyst 12 is calculated, and the supplied urea amount is calculated from this NO _X amount NOXZ.

Urea required the use of NO _X adsorption catalyst 12 from the NO _X adsorption catalyst 12 for reducing the NO _X because it is released in the form of the NO _X without NO _X being adsorbed is converted into ammonia (NH ₃₎ The amount can be calculated accurately. Therefore, NO _x can be well purified by the ammonia generated from the supplied urea, and excess ammonia can be prevented from flowing out from the NO _x selective reduction catalyst 15.

1 is an overall view of a compression ignition type internal combustion engine. It is a general view which shows another Example of a compression ignition type internal combustion engine. It is a diagram showing the maximum adsorption amount of the NO _X of the NO _X adsorbing catalyst. It is a diagram showing a map or the like of the NO _X amount NOXA exhausted from the engine. Is a diagram illustrating the desorption speed of NO _X. It is a flowchart for performing supply control of urea.

Explanation of symbols

4 Intake manifold 5 Exhaust manifold 7 Exhaust turbocharger 12 NO _x adsorption catalyst 13 Particulate filter 15 NO _x selective reduction catalyst 17 Urea water supply valve

Claims (2)

A NO _x selective reduction catalyst is disposed in the engine exhaust passage, and urea is supplied to the NO _x selective reduction catalyst so that NO _x contained in the exhaust gas is selectively reduced by ammonia generated from the urea. In the exhaust gas purification apparatus for an internal combustion engine, a NO _x adsorption catalyst is disposed in the engine exhaust passage upstream of the NO _x selective reduction catalyst, and the NO _x adsorption catalyst has a maximum adsorption amount of NO _x as the temperature of the NO _x adsorption catalyst decreases. has but a property of releasing NO _X which _the amount of NO _X temperature of the NO _X adsorbing catalyst is adsorbed in the NO _X adsorbing catalyst increases with having a property of increasing is adsorbed to exceed said maximum adsorption amount , further the NO _X adsorbing catalyst has a property of said maximum adsorption amount gradually decreases with the progress of heat deterioration of the NO _X adsorbing catalyst, NO _X amount which is adsorbed in the NO _X adsorbing catalyst is calculated low with the progress of thermal degradation along with the It said maximum adsorption amount is calculated for the adsorption NO _X to NO _X adsorbing catalyst based on _the amount of NO _X discharged from the engine when less than the maximum adsorption amount of adsorbed amount of NO _X was issued the calculated that issued the calculated as the amount is calculated, the calculated discharge amount of NO _X from the NO _X adsorbing catalyst when greater than the maximum adsorption amount of adsorbed amount of NO _X was issued the calculated that issued the calculated, _the amount of NO _X discharged from the engine _the amount of NO _X in the exhaust gas flowing out from the NO _X adsorbing catalyst by adding the release amount of NO _X issued the calculated in subtracting the adsorbed amount of NO _X issued the calculated or amount of NO _X discharged from the engine from the An exhaust emission control device for an internal combustion engine, which is calculated and the supplied urea amount is calculated from the NO _x amount. Intake air with NO _X adsorption amount per unit time when fewer than the maximum adsorption amount of adsorbed amount of NO _X was issued the calculated that issued the calculated, the adsorption amount of NO _X that is calculated is made to decrease as increasing the amount is made to decrease as increasing the release amount of NO _X per unit time when more than the maximum adsorption amount of adsorbed amount of NO _X was issued the calculated that issued the calculated, the adsorption amount of NO _X is calculated 2. The exhaust emission control device for an internal combustion engine according to claim 1, wherein the exhaust gas purification device is increased as the intake air amount is increased and is increased as the intake air amount is increased .

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