An exhaust purification device of an engine

BACKGROUND OF THE INVENTION

1. Field of the Invention

0001The present invention relates to an exhaust purification device of an engine.

2. Description of the Related Art

0002Known in the art is an internal combustion engine which provides in the engine exhaust passage an NO_x absorbent which absorbs NO_x when the air-fuel ratio of the inflowing exhaust gas is lean and releases the absorbed NO_x when the air-fuel ratio of the inflowing exhaust gas becomes rich and repeatedly temporarily changes the air-fuel ratio of the exhaust gas flowing into the NO_x absorbent from lean to rich at certain time intervals so as to cause the NO_x absorbed in the NO_x absorbent to be released and reduced (see PCT International Publication WO93/07363).

0003This type of NO_x absorbent, however, gradually deteriorates along with use. The more it deteriorates, the lower the ability of the NO_x absorbent to absorb NO_x. If the ability of the NO_x to absorb NO_x falls, then it will become necessary to release NO_x from the NO_x absorbent while the amount of NO_x absorbed in the NO_x absorbent is still small, so the more deteriorated the NO_x absorbent, the shorter the cycle for changing the air-fuel ratio of the exhaust gas flowing into the NO_x absorbent to rich must be made. In the above-mentioned internal combustion engine, however, no consideration is given at all to the deterioration of the NO_x absorbent. The air-fuel ratio of the exhaust gas flowing into the NO_x absorbent is changed from lean to rich at a predetermined cycle suited to a new NO_x absorbent regardless of the degree of deterioration of the NO_x absorbent. Accordingly, when the degree of deterioration of the NO_x absorbent is high, the ability of the NO_x absorbent to absorb the NO_x will become saturated before the air-flow ratio of the exhaust gas flowing into the NO_x absorbent is changed to rich and accordingly the NO_x will end up being released into the atmosphere without being absorbed by the NO_x absorbent.

0004Further, when the NO_x absorbent deteriorates, the amount of the NO_x absorbed in the NO_x absorbent becomes smaller and accordingly it is necessary to shorten the time the air-fuel ratio of the exhaust gas flowing into the NO_x absorbent is made rich for releasing the total NO_x absorbed in the NO_x absorbent. In the above-mentioned internal combustion engine, however, this rich period is made a constant period suited to a new NO_x absorbent without regard as to the deterioration of the NO_x absorbent. Accordingly, when the degree of deterioration of the NO_x absorbent becomes high, the air-fuel ratio of the exhaust gas flowing into the NO_x absorbent continues to be made rich even after all of the NO_x is released from the NO_x absorbent and therefore not only is the fuel consumption increased, but also a large amount of unburnt hydrocarbons is released into the atmosphere.

SUMMARY OF THE INVENTION

0005An object of the present invention is to provide an exhaust gas purification device capable of preventing NO_x and hydrocarbons from being discharged into the atmosphere and also preventing the fuel consumption from increasing.

0006According to the present invention, there is provided an exhaust purification device of an engine having an exhaust passage, comprising an NO_x absorbent arranged in the exhaust passage, the NO_x absorbent absorbing NO_x therein when an air-fuel ratio of exhaust gas flowing into the NO_x absorbent is lean and releasing absorbed NO_x therefrom when the air-fuel ratio of the exhaust gas flowing into the NO_x absorbent becomes rich; an air-fuel ratio control means for controling the air-fuel ratio of the exhaust gas flowing into the NO_x absorbent to temporarily change the air-fuel ratio of the exhaust gas flowing into the NO_x absorbent from lean to rich at certain time intervals to release NO_x from the NO_x absorbent; a deterioration detecting means for detecting a degree of deterioration of the NO_x absorbent; and a NO_x releasing control means for controlling at least one of a period at which the air-fuel ratio of exhaust gas flowing into the NO_x absorbent is changed from lean to rich and a rich time during which the air-fuel ratio of exhaust gas flowing into the NO_x absorbent ismade rich to make one of the period and the rich time shorter as the degree of deterioration of the NO_x becomes higher.

BRIEF DESCRIPTION OF THE DRAWINGS

0007The present invention may be more fully understood from the description of the preferred embodiments of the invention set forth below with reference to the accompanying drawings, wherein: <ul id="ul0001" list-style="none" compact="compact"><li>Fig. 1 is an overall view of an engine;</li><li>Fig. 2 is a view of a map of a basic fuel injection time;</li><li>Fig. 3 is a view of a correction coefficient K;</li><li>Fig. 4 is a graph schematically showing a concentration of unburnt HC and CO and oxygen in exhaust gas discharged from the engine;</li><li>Figs. 5A and 5B are views for explaining an absorption and releasing action of NO_x;</li><li>Fig. 6 is a view of an amount of absorption of NO_x NOXA;</li><li>Fig. 7 is a time chart of the air-fuel ratio control;</li><li>Figs. 8A and 8B are views of a cycle of making the air-fuel ratio of an air-fuel mixture rich for releasing NO_x and a rich time at this time;</li><li>Fig. 9 is a view of a current flowing between an anode and a cathode of an O₂ sensor;</li><li>Figs. 10 and 11 are time charts showing the change of the value of a current flowing between the anode and cathode of the NO_x sensor;</li><li>Figs. 12 and 13 are flow charts of the control of the air-fuel ratio;</li><li>Fig. 14 is a flow chart of a feedback control I;</li><li>Fig. 15 is a time chart of the change of a feedback correction coefficient FAF;</li><li>Fig. 16 is a flow chart of a feedback control II;</li><li>Fig. 17 is a flow chart of processing for release of NO_x;</li><li>Fig. 18 is a flow chart of a decision of deterioration;</li><li>Figs. 19A and 19B are views of a cycle TL for making the air-fuel ratio of the air-fuel mixture rich for releasing NO_x and the rich time TR;</li><li>Figs. 20 and 21 are flow charts of another embodiment for controlling the air-fuel ratio;</li><li>Fig. 22 is a flow chart for the processing for release of NO_x; and</li><li>Fig. 23 is a flow chart of a decision of deterioration.</li></ul>

DESCRIPTION OF THE PREFERRED EMBODIMENTS

0008Referring to Fig. 1, 1 denotes an engine body, 2, a piston, 3, a combustion chamber, 4, a spark plug, 5, an intake valve, 6, an intake port, 7, an exhaust valve, and 8, an exhaust port. The intake port 6 is connected to a surge tank 10 via a corresponding branch pipe 9, and a fuel injector 11 injecting fuel toward the interior of the input port 6 is attached to each branch pipe 9. The surge tank 10 is connected via an intake duct 12 to an air cleaner 13, and a throttle valve 14 is arranged in the intake duct 12. On the other hand, the exhaust port 8 is connected via an exhaust manifold 15 and an exhaust pipe 16 to a casing 17 containing a NO_x absorbent 18.

0009An electronic control unit 30 comprises a digital computer and is provided with a read only memory (ROM) 32, a random access memory (RAM) 33, a microprocessor (CPU) 34, a back-up RAM 35 continuously connected to a power source, an input port 36, and an exhaust port 37 - all of which are connected to each other by a bi-directional bus 31. In the surge tank 10, a pressure sensor 19 for generating an output voltage proportional to an absolute pressure in the surge tank 10 is arranged. The output voltage of this pressure sensor 19 is input to the input port 36 via a corresponding analog-to-digital (AD) converter 38. An air-fuel ratio sensor (hereinafter referred to as an O₂ sensor) 20 is arranged in the exhaust manifold 15, and the output of this O₂ sensor 20 is input to the input port 36 via the corresponding AD converter 38. Another air-fuel ratio sensor (hereinafter referred to as an O₂ sensor) 22 is arranged in the exhaust pipe 21 downstream of the NO_x absorbent 18. This O₂ sensor 22 is connected to the input port 36 via a corresponding AD converter 38. Further, an engine speed sensor 23 generating an output pulse representing the engine speed and a vehicle speed sensor 24 generating an output pulse representing the vehicle speed are connected to the input port 36. On the other hand, the output port 37 is connected via the corresponding drive circuit 39 to the spark plug 4, fuel injection valve 11, and the alarm lamp 25.

0010In the engine shown in Fig. 1, a fuel injection time TAU is calculated on the basis of for example the following equation:<maths id="math0001" num=""><math display="block"><mrow><mtext>TAU = TP · K · FAF</mtext></mrow></math><img file="EP0733786A2_D0001.tif" /></maths>

0011Here, TP represents a basic fuel injection time, K, a correction coefficient, and FAF, a feedback correction coefficient, respectively. The basic fuel injection time TP indicates a fuel injection time necessary for making the air-fuel ratio of the air-fuel mixture to be supplied into the engine cylinder the stoichiometric air-fuel ratio. This basic fuel injection time TP is found in advance by experiments and preliminarily stored in the ROM 32 in the form of a map as shown in Fig. 2 as a function of the absolute pressure PM in the surge tank 10 and the engine rotation speed N. The correction coefficient K is a coefficient for controlling the air-fuel ratio of the air-fuel mixture to be supplied into the engine cylinder. If K = 1.0, the air-fuel ratio of the air-fuel mixture to be supplied into the engine cylinder becomes the stoichiometric air-fuel ratio. Contrary to this, when K becomes smaller than 1.0, the air-fuel ratio of the air-fuel mixture to be supplied into the engine cylinder becomes larger than the stoichiometric air-fuel ratio, that is, lean, and when K becomes larger than 1.0, the air-fuel ratio of the air-fuel mixture supplied into the engine cylinder becomes smaller than the stoichiometric air-fuel ratio, that is, rich.

0012The feedback correction coefficient FAF is a coefficient for making the air-fuel ratio accurately coincide with the stoichiometric air-fuel ratio on the basis of the output signal of the O₂ sensor 20 when K = 1.0, that is, when the air-fuel ratio of the air-fuel mixture supplied into the engine cylinder should be made the stoichiometric air-fuel ratio. This feedback correction coefficient FAF moves up or dawn around about 1.0. The FAF is decreased when the air-fuel mixture becomes rich and increased when the air-fuel mixture becomes lean. Note that, when K < 1.0 or K > 1.0, the FAF is fixed to 1.0.

0013The target air-fuel ratio of the air-fuel mixture which should be supplied into the engine cylinder, that is, the value of the correction coefficient K, is changed in accordance with the operating state of the engine. In the embodiment according to the present invention, basically, as shown in Fig. 3, it is determined in advance as a function of the absolute pressure PM in the surge tank 10 and the engine speed N. Namely, as shown in Fig. 3, in a low load operation region on the lower load side from a solid line R, K becomes smaller than 1.0, that is, the air-fuel ratio of the air-fuel mixture is made lean, and in a high load operation region between the solid line R and solid line S, K becomes equal to 1.0, that is, the air-fuel ratio of the air-fuel mixture is made the stoichiometric air-fuel ratio. In the full load operation region on the higher load side from the solid line S, K becomes larger than 1.0, that is, the air-fuel ratio of the air-fuel mixture is made rich.

0014Figure 4 schematically shows the concentration of representative components in the exhaust gas discharged from the combustion chamber 3. As seen from Fig. 4, the concentration of the unburnt HC and CO in the exhaust gas discharged from the combustion chamber 3 is increased as the air-fuel ratio of the air-fuel mixture supplied into the combustion chamber 3 becomes rich, and the concentration of the oxygen O₂ discharged from the combustion chamber 3 is increased as the air-fuel ratio of the air-fuel mixture supplied into the combustion chamber 3 becomes lean.

0015A NO_x absorbent 18 accommodated in the casing 17 uses for example alumina as the carrier. On this carrier, at least one element selected from alkali metals such as for example potassium K, sodium Na, lithium Li, and cesium Cs, alkali earth metals such as barium Ba or calcium Ca, and rare earth metals such as lanthanum La or yttrium Y and a precious metal such as platinum Pt are carried. When the ratio of the air and fuel (hydrocarbon) supplied into the engine intake passage and the exhaust passage upstream of the NO_x absorbent 18 is referred to as the air-fuel ratio of the inflowing exhaust gas into the NO_x absorbent 18, this NO_x absorbent 18 performs the action of absorbing and releasing NO_x so as to absorb the NO_x when the air-fuel ratio of the inflowing exhaust gas is lean and release the absorbed NO_x when the oxygen concentration in the inflowing exhaust gas is lowered. Note that, where the fuel (hydrocarbon) or the air is not supplied into the exhaust passage upstream of the NO_x absorbent 18, the air-fuel ratio of the flowing exhaust gas coincides with the air-fuel ratio of the air-fuel mixture supplied into the combustion chamber 3, and therefore, in this case, the NO_x absorbent 18 absorbs the NO_x when the air-fuel ratio of the air-fuel mixture supplied into the combustion chamber 3 is lean and releases the absorbed NO_x when the oxygen concentration in the air-fuel mixture supplied into the combustion chamber 3 is lowered.

0016When the NO_x absorbent 18 is arranged in the engine exhaust passage, this NO_x absorbent 18 actually performs the absorbing and releasing action of NO_x, but there are areas of uncertainty regarding the detailed mechanism of this absorbing and releasing action. However, it can be considered that this absorbing and releasing action is carried out by the mechanism as shown in Figs. 5A and 5B. Next, an explanation will be made of this mechanism by taking as an example a case where platinum Pt and barium Ba are carried on this carrier, but a similar mechanism is obtained even if an other precious metal or alkali metal, alkali earth metal, and rare earth metal are used.

0017Namely, when the inflowing exhaust gas becomes considerably lean, the oxygen concentration in the inflowing exhaust gas is greatly increased, and as shown in Fig. 5A, the oxygen O₂ is deposited on the surface of the platinum Pt in the form of O₂^- or O^2-. On the other hand, the NO in the inflowing exhaust gas reacts with O₂^-or O^2- on the surface of the platinum Pt and becomes NO₂ (2NO + O₂ → 2NO₂). Subsequently, one part of the generated NO₂ is absorbed into the absorbent while being oxidized on the platinum Pt and bonded to the barium oxide BaO while being diffused in the absorbent in the form of a nitric acid ion NO₃^- as shown in Fig. 5A. In this way, NO_x is absorbed into the NO_x absorbent 18.

0018So far as the oxygen concentration in the inflowing exhaust gas is high, NO₂ is generated on the surface of the platinum Pt, and so far as the NO_x absorbing capability of the absorbent is not saturated, the nitric acid ion NO₃^- formed by absorption of NO₂ into the absorbent is generated. Contrary to this, when the oxygen concentration in the flowing exhaust gas is lowered and the amount of generation of the NO₂ is lowered, the reaction advances in a reverse direction (NO₃^- → NO₂), and thus the nitric acid ion NO₃^- in the absorbent is released from the absorbent in the form of NO₂. Namely, when the oxygen concentration in the flowing exhaust gas is lowered, NO_x will be released from the NO_x absorbent 18. As shown in Fig. 4, when the degree of leanness of the inflowing exhaust gas becomes low, the oxygen concentration in the inflowing exhaust gas is lowered, and therefore when the degree of the leanness of the inflowing exhaust gas is lowered, even if the air-fuel ratio of the inflowing exhaust gas is lean, NO_x will be released from the NO_x absorbent 18.

0019On the other hand, when the air-fuel ratio of the air-fuel mixture supplied into the combustion chamber 3 is made rich and the air-fuel ratio of the inflowing exhaust gas becomes rich, as shown in Fig. 4, a large amount of unburnt HC and CO are discharged from the engine. These unburnt HC and CO react with the oxygen O₂^-or O^2- on the platinum Pt and are oxidized. Further, when the air-fuel ratio of the inflowing exhaust gas becomes rich, the oxygen concentration in the inflowing exhaust gas is extremely lowered, so NO₂ is released from the absorbent. This NO₂ reacts with the unburnt HC and CO and is reduced as shown in Fig. 5B. In this way, when NO₂ no longer exists on the surface of the platinum Pt, the NO₂ is successively released from the absorbent. Accordingly, when the air-fuel ratio of the inflowing exhaust gas is made rich, the NO_x will be released from the NO_x absorbent 18 in a short time.

0020Namely, when the air-fuel ratio of the inflowing exhaust gas is made rich, first of all, the unburnt HC and CO immediately react with O₂^- or O^2- on the platinum Pt and are oxidized, and then even if the O₂^- or O^2- on the platinum Pt is consumed, if the unburnt HC and CO still remain, the NO_x released from the absorbent and the NO_x discharged from the engine are reduced. Accordingly, if the air-fuel ratio of the inflowing exhaust gas is made rich, the NO_x absorbed in the NO_x absorbent 18 is released in a short time, and, in addition, this released NO_x is reduced, so it becomes possible to prevent the NO_x from being discharged into the atmosphere.

0021As mentioned above, when the lean air-fuel mixture is burned, NO_x is absorbed into the NO_x absorbent 18. However, there is a limit to the NO_x absorbing ability of the NO_x absorbent 18. When the NO_x absorbing capability of the NO_x absorbent 18 is saturated, the NO_x absorbent 18 no longer can absorb the NO_x. Accordingly, it is necessary to release the NO_x from the NO_x absorbent 18 before the NO_x absorbing capability of the NO_x absorbent 18 is saturated. For this purpose, it is necessary to estimate to what degree the NO_x has been absorbed in the NO_x absorbent 18. Next, an explanation will be made of the estimation method of this amount of absorption of NO_x.

0022When the lean air-fuel mixture is burned, the higher the engine load, the larger the amount of NO_x discharged from the engine per unit time, so the amount of NO_x absorbed into the NO_x absorbent 18 per unit time is increased. Also, the higher the engine speed, the larger the amount of NO_x discharged from the engine per unit time, so the amount of NO_x absorbed into the NO_x absorbent 18 per unit time is increased. Accordingly, the amount of NO_x absorbed into the NO_x absorbent 18 per unit time becomes a function of the engine load and the engine speed. In this case, the engine load can be represented by the absolute pressure in the surge tank 10, so the amount of NO_x absorbed into the NO_x absorbent 18 per unit time becomes a function of the absolute pressure PM in the surge tank 10 and the engine speed N. Accordingly, in the embodiment according to the present invention, the amount of NO_x absorbed into the NO_x absorbent 18 per unit time is found in advance as a function of the absolute pressure PM and the engine speed N by experiments. These amounts of absorption of NO_x NOXA and PM are stored in advance in the ROM 32 in the form of a map shown in Fig. 6 as a function of PM and N.

0023On the other hand, as mentioned before, during the period where the NO_x is released from the NO_x absorbent 18, the unburnt HC and CO contained in the exhaust gas, that is, the excess fuel, is used for reducing the NO_x released from the NO_x absorbent 18, therefore the amount NOXD of NO_x released from the NO_x absorbent 18 per unit time becomes proportional to the amount of excess fuel supplied per unit time. Note that the amount Q_ex of excess fuel supplied per unit time can be represented by the following equation:<maths id="math0002" num=""><math display="block"><mrow><msub><mrow><mtext>Q</mtext></mrow><mrow><mtext>ex</mtext></mrow></msub><msub><mrow><mtext> = f</mtext></mrow><mrow><mtext>1</mtext></mrow></msub><mtext> · (K - 1.0) · TP · N</mtext></mrow></math><img file="EP0733786A2_D0002.tif" /></maths>

0024Here, f₁ indicates a proportional constant, K, a correction coefficient, TP a basic fuel injection time, and N, an engine speed. On the other hand, when the proportional constant is f₂, the amount NOXD of NO_x released from the NO_x absorbent 18 per unit time can be represented by<maths id="math0003" num=""><math display="block"><mrow><msub><mrow><mtext>NOXD = f</mtext></mrow><mrow><mtext>2</mtext></mrow></msub><msub><mrow><mtext> · Q</mtext></mrow><mrow><mtext>ex</mtext></mrow></msub></mrow></math><img file="EP0733786A2_D0003.tif" /></maths> so if <maths id="math0004" num=""><math display="inline"><mrow><msub><mrow><mtext>f = f</mtext></mrow><mrow><mtext>1</mtext></mrow></msub><msub><mrow><mtext> · f</mtext></mrow><mrow><mtext>2</mtext></mrow></msub></mrow></math><img file="EP0733786A2_D0004.tif" /></maths>, the amount NOXD of NO_x released from the NO_x absorbent 18 per unit time can be represented by the following equation:<maths id="math0005" num=""><math display="block"><mrow><mtext>NOXD = f · (K - 1.0) · TP · N</mtext></mrow></math><img file="EP0733786A2_D0005.tif" /></maths>

0025As mentioned above, when a lean air-fuel mixture is burned, the amount of absorption of NO_x per unit time is represented by NOXD, and when a rich air-fuel mixture is burned, the amount of release of NO_x per unit time is represented by NOXD, therefore the amount ΣNOX of NO_x estimated to be absorbed in the NO_x absorbent 18 will be represented by the following equation:<maths id="math0006" num=""><math display="block"><mrow><mtext>ΣNOX = ΣNOX + NOXA - NOXD</mtext></mrow></math><img file="EP0733786A2_D0006.tif" /></maths>

0026Therefore, in the embodiment according to the present invention, as shown in Fig. 7, when the amount ΣNOX of the NO_x estimated to be absorbed in the NO_x absorbent 18, in practice, the corrected amount of estimation of NO_x ΣNKX mentioned later, reaches the allowable maximum value MAX, the air-fuel ratio of the air-fuel mixture is temporarily made rich, whereby NO_x is released from the NO_x absorbent 18.

0027However, SO_x is contained in the exhaust gas, and not only NO_x, but also SO_x are absorbed into the NO_x absorbent 18. The absorbing mechanism of SO_x to the NO_x absorbent 18 can be considered to be the same as the absorption mechanism of NO_x.

0028Namely, similar to the explanation of the absorbing mechanism of NO_x, when the explanation is made by taking as an example a case where platinum Pt and barium Ba are carried on the carrier, as mentioned before, when the air-fuel ratio of the inflowing exhaust gas is lean, the oxygen O₂ is deposited on the surface of the platinum Pt in the form of O₂^- or O^2-, and the SO₂ in the inflowing exhaust gas reacts with the O₂^- or O^2- on the surface of the platinum Pt and becomes SO₃. Subsequently, one part of the generated SO₃ is absorbed into the absorbent while being further oxidized on the platinum Pt and bonded to the barium oxide BaO while being diffused in the absorbent in the form of a sulfuric acid ton SO₄^2- and stable sulfate BaSO₄ is generated.

0029However, this sulfate BaSO₄ is stable and hard to decompose. Even if the air-fuel ratio of the air-fuel mixture is made rich for just a short time as shown in Fig. 7, most of the sulfate BaSO₄ is not decomposed and remains as it is. Accordingly, the sulfate BaSO₄ is increased in the NO_x absorbent 18 along with the elapse of time, and thus the maximum amount of absorption of NO_x which can be absorbed by the NO_x absorbent 18 will be gradually lowered along with the elapse of time. Namely, in other words, the NO_x absorbent 18 will gradually deteriorate along with the elapse of time. When the maximum amount of absorption of NO_x by the NO_x absorbent 18 is lowered, it is necessary to release the NO_x from the NO_x absorbent 18 in a period when the amount of absorption of the NO_x in the NO_x absorbent 18 is small. For this purpose, first, it becomes necessary to correctly detect the maximum amount of absorption of NO_x possible by the NO_x absorbent 18, that is, the degree of deterioration of the NO_x absorbent 18.

0030In the embodiment according to the present invention, the maximum amount of absorption of NO_x possible by the NO_x absorbent 18, that is, the degree of deterioration of the NO_x absorbent 18, is detected from the air-fuel ratio detected by the O₂ sensor 22. This will be explained later.

0031Namely, when the air-fuel ratio of the air-fuel mixture supplied into the combustion chamber 3 becomes rich, as shown in Fig. 4, the exhaust gas containing the oxygen O₂ and the unburnt HC and CO is discharged from the combustion chamber 3, but this oxygen O₂ and the unburnt HC and CO do not react much at all with each other, and thus this oxygen O₂ passes through the NO_x absorbent 18 and is discharged from the NO_x absorbent 18. On the other hand, when the air-fuel ratio of the air-fuel mixture supplied into the combustion chamber 3 becomes rich, NO_x is released from the NO_x absorbent 18. At this time, the unburnt HC and CO contained in the exhaust gas is used for reducing the released NO_x, so during a period when the NO_x is released from the NO_x absorbent 18, no unburnt HC and CO are discharged from the NO_x absorbent 18. Accordingly, during a period when the NO_x is continuously released from the NO_x absorbent 18, the oxygen O₂ is contained in the exhaust gas discharged from the NO_x absorbent 18, but no unburnt HC and CO are contained, therefore during this term, the air-fuel ratio of the exhaust gas discharged from the NO_x absorbent 18 becomes slightly lean.

0032Subsequently, when all of the NO_x absorbed in the NO_x absorbent 18 is released, the unburnt HC and CO contained in the exhaust gas are not used for the reduction of the O₂ in the NO_x absorbent 18 but are discharged as they are from the NO_x absorbent 18. Accordingly, the air-fuel ratio of the exhaust gas discharged from the NO_x absorbent 18 becomes rich at this time. Namely, when all of the NO_x absorbed in the NO_x absorbent 18 is released, the air-fuel ratio of the exhaust gas discharged from the NO_x absorbent 18 changes from lean to rich. Accordingly, all of the NO_x absorbed in the NO_x absorbent 18 is released from the NO_x absorbent 18 during the time elapsing from when the air-fuel ratio of the exhaust gas flowing into the NO_x absorbent 18 is changed from lean to rich to when the air-fuel ratio of the exhaust gas discharged from the NO_x absorbent 18 becomes rich. Therefore, from this, the amount of NO_x absorbed in the NO_x absorbent 18 is seen. This will be explained in slightly detail more next.

0033The O₂ sensor 22 shown in Fig. 1 comprises a cup-like cylindrical body made of zirconia arranged in the exhaust passage. An anode made of a thin platinum film is formed on an inside surface of this cylindrical body and a cathode made of a thin platinum film is formed on an outside surface of this cylindrical body, respectively. The cathode is covered by a porous layer. Constant voltage is applied between the cathode and anode. In this O₂ sensor 22, as shown in Fig. 9, a current I (mA) proportional to the air-fuel ratio A/F flows between the cathode and anode. Note that, in Fig. 9, I₀ indicates the current when the air-fuel ratio A/F is the stoichiometric air-fuel ratio (= 14.6). As seen from Fig. 9, when the air-fuel ratio A/F is lean, the current I is increased as the air-fuel ratio A/F becomes larger within a range where I > I₀, and the current I becomes zero when the air-fuel ratio A/F becomes rich of almost 13.0 or less.

0034Figure 10 shows the change of the air-fuel ratio (A/F)_in of the exhaust gas flowing into the NO_x absorbent 18, the change of the current I flowing between the cathode and anode of the O₂ sensor 22, and the change of the air-fuel ratio (A/F) of the exhaust gas flowing out from the NO_x absorbent 18. As shown in Fig. 10, when the air-fuel ratio (A/F) of the exhaust gas flowing into the NO_x absorbent 18 is changed from lean to rich and the NO_x releasing action from the NO_x absorbent 18 is started, the air-fuel ratio (A/F)_out of the exhaust gas flowing out from the NO_x absorbent 18 abruptly becomes small to near the stoichiometric air-fuel ratio, and therefore the current I is abruptly decreased to near I₀. Subsequently, in a term when the NO_x releasing action from the NO_x absorbent 18 is carried out, the air-fuel ratio (A/F)_out of the exhaust gas flowing out from the NO_x absorbent 18 is held in a slightly lean state, and therefore the current I is held at a value slightly larger than the I₀. Subsequently, when all of the NO_x absorbed in the NO_x absorbent 18 is released, the air-fuel ratio (A/F) of the exhaust gas flowing out from the NO_x absorbent 18 abruptly becomes small and becomes rich, and therefore the current I abruptly falls to zero.

0035Figure 11 shows the change of the current I where the amount of NO_x contained in the NO_x absorbent 18 differs. Note that, in Fig. 11, the numerical values indicate the amount of NO_x absorbed in the NO_x absorbent 18. As shown in Fig. 11, when the amount of NO_x absorbed in the NO_x absorbent 18 is different, along with this, an elapsed time t from when the air-fuel ratio (A/F)_in of the exhaust gas flowing into the NO_x absorbent 18 is changed from lean to rich to when the current I becomes almost zero changes. The smaller the amount of NO_x absorbed in the NO_x absorbent 18, the shorter this elapsed time. NO_x is continuously released from the NO_x absorbent 18 for almost this elapsed time t. If the entire amount of NO_x released during this elapsed time t is found, the entire amount of NO_x absorbed in the NO_x absorbent 18 will be seen.

0036Note that, as mentioned before, the amount of release of NOXD of NO_x released from the NO_x absorbent 18 is represented by the following equation:<maths id="math0007" num=""><math display="block"><mrow><msub><mrow><mtext>NOXD = f</mtext></mrow><mrow><mtext>1</mtext></mrow></msub><mtext> · (K - 1.0) · TP · N</mtext></mrow></math><img file="EP0733786A2_D0007.tif" /></maths>

0037Accordingly, if the total sum of the amount of release of NO_x NOXD during the elapsed time t is found, the entire amount of NO_x actually absorbed in the NO_x absorbent 18 can be detected.

0038By the say, to detect the maximum amount of absorption of NO_x possible by the NO_x absorbent 18, that is, the degree of deterioration of the NO_x absorbent 18, at detection, the amount of absorption ΣNOX of NO_x of the NO_x absorbent 18 must become the maximum amount of absorption of NO_x. Namely, when assuming that the VNO_x indicated by the broken line in Fig. 7 is the maximum amount of absorption of NO_x which is actually possible, when the amount of absorption of NO_x ΣNOX of the NO_x absorbent 18 is smaller than this VNO_x, even if all of the NO_x is released from the NO_x absorbent 18, the maximum amount of absorption of NO_x VNO_x cannot be found. This is because the entire amount of NO_x released at this time is smaller than the maximum amount of absorption of NO_x.

0039Contrary to this, when the NO_x is released from the NO_x absorbent 18 when the absorbing capability of the NO_x absorbent 18 is saturated, the entire amount of NO_x released at this time represents the maximum amount of absorption of NO_x VNO_x. Therefore, in the embodiment according to the present invention, a decision level SAT which is slightly larger than the value near the maximum amount of absorption of NO_x VNO_x at present is set, and as shown in Fig. 7. When the amount of absorption of NO_x ΣNOX of the NO_x absorbent 18 reaches this decision level SAT, the entire NO_x is released from the NO_x absorbent 18, whereby the actual amount of absorption of NO_x VNO_x, that is, the degree of deterioration of the NO_x absorbent 18 at this time, is found.

0040Note that, as shown in Fig. 7, the allowable maximum value MAX with respect to the amount of NO_x ΣNOX is set to a value smaller than the maximum amount of absorption of NO_x VNO_x, and when the ΣNOX reaches the allowable maximum value MAX, the decision of deterioration of the NO_x absorbent 18 is not carried out, and only the action of releasing NO_x from the NO_x absorbent 18 is carried out. The frequency of only the action of releasting the NO_x from the NO_x absorbent 18 being carried out is higher than the frequency of the decision of deterioration of the NO_x absorbent 18 being carried out, and therefore for a period after the decision of deterioration of the NO_x absorbent 18 is carried out and until the next decision of deterioration of the NO_x absorbent 18 is carried out, a number of actions of releasing NO_x are carried out.

0041The amount of absorption of NO_x ΣNOX of the NO_x absorbent 18 is, however, an estimated amount as mentioned before, and therefore this amount of absorption of NO_x ΣNOX does not always represent the actual amount of absorption of NO_x. In this case, if for example the amount of absorption of NO_x ΣNOX indicates a considerably higher value than the actual amount of absorption of NO_x, even if the amount of absorption of NO_x ΣNOX reaches the decision level SAT, the actual amount of absorption of NO_x does not reach the actual maximum amount of absorption of NO_x VNO_x, and thus there arises a problem in that the actual maximum amount of absorption of NO_x VNO_x cannot be correctly detected.

0042Therefore, in the embodiment according to the present invention, a correction value KX with respect to the amount of absorption of NO_x ΣNOX is introduced. Whenever the amount of absorption of NO_x ΣNOX reaches the allowable maximum value MAX and the release of NO_x from the NO_x absorbent 18 is carried out, the actual amount of absorption of NO_x XNO_x is calculated on the basis of the output signal of the NO_x sensor 22, and the correction value KX is updated on the basis of the following equation:<maths id="math0008" num=""><math display="block"><mrow><msub><mrow><mtext>KX = KX · (XNO</mtext></mrow><mrow><mtext>x</mtext></mrow></msub><mtext>/ΣNOX)</mtext></mrow></math><img file="EP0733786A2_D0008.tif" /></maths>

0043In this case, the corrected estimated amount of NO_x is represented by <maths id="math0009" num=""><math display="inline"><mrow><mtext>ΣNKX (= KX · ΣNOX)</mtext></mrow></math><img file="EP0733786A2_D0009.tif" /></maths>. Namely, where for example the estimated amount of absorption of NO_x ΣNOX becomes smaller than the actual amount of absorption of NO_x XNO_x, the value of the correction value KX is increased with respect to the value of the correction value KX which has been used heretofore so that <maths id="math0010" num=""><math display="inline"><mrow><mtext>ΣNKX (= KX · ΣNOX)</mtext></mrow></math><img file="EP0733786A2_D0010.tif" /></maths> coincides with XNO_x. Accordingly, in the embodiment according to the present invention, in actuality, not when the estimated amount of NO_x ΣNOX reaches MAX, but when the corrected estimated amount of NO_x ΣNOX reaches the allowable maximum value MAX, the action of releasing NO_x is carried out.

0044When the maximum amount of absorption of NO_x VNO_x becomes small, that is, when the degree of deterioration of the NO_x absorbent 18 becomes high, the allowable maximum value MAX becomes small, and thus as seen from Fig. 7, a cycle at which the air-fuel ratio is made rich for releasing NO_x becomes short. Further, when the degree of deterioration of the NO_x absorbent 18 becomes high and the allowable maximum value MAX becomes small, the time required for the release of NO_x becomes short, so the time during which the air-fuel ratio is maintained rich becomes short. Accordingly, when the degree of deterioration of the NO_x absorbent 18 is low, as shown in Fig. 8A, a cycle t₁ when the air-fuel ratio is made rich and the time t₂ during which the air-fuel ratio is maintained rich are relatively long, and when the degree of deterioration of the NO_x absorbent 18 becomes high, as shown in Fig. 8B, a cycle when the air-fuel ratio is made rich and the time during which the air-fuel ratio is maintained rich become short.

0045As mentioned above, in the embodiment according to the present invention, the actual amount of NO_x VNO_x and XNO_x are calculated on the basis of the current I flowing between the cathode and anode of the O₂ sensor 22 and the air-fuel ratio is controlled for releasing NO_x on the basis of these values VNO_x and XNO_x. In this case, the current I flowing between the cathode and anode of the O₂ sensor 22 is converted to a voltage and input into the input port 36. In the electronic control unit 30, this voltage is converted to the corresponding current I again and the air-fuel ratio is controlled on the basis of the current value I.

0046Figure 12 and Fig. 13 show a routine for control of the air-fuel ratio. This routine is executed by interruption at every predetermined time interval.

0047Referring to Fig. 12 and Fig. 13, first of all, at step 100, a basic fuel injection time TP is calculated from the relationship shown in Fig. 2. Subsequently, at step 101, it is determined whether or not a decision of deterioration flag indicating that the degree of deterioration of the NO_x absorbent 18 should be decided has been set. When the decision of deterioration flag has not been set, the processing routine proceeds to step 102, where it is determined whether or not the NO_x releasing flag indicating that the NO_x should be released from the NO_x absorbent 18 has been set. When the NO_x releasing flag has not been set, the processing routine proceeds to step 103.

0048At step 103, the correction coefficient K is calculated on the basis of Fig. 3. Subsequently, at step 104, it is determined whether or not the correction coefficient K is 1.0. When K = 1.0, that is, when the air-fuel ratio of the air-fuel mixture should be made the stoichiometric air-fuel ratio, the processing routine proceeds to step 126, at which the feedback control I of the air-fuel ratio is carried out. This feedback control I is shown in Fig. 14. On the other hand, when K does not equal 1.0, the processing routine proceeds to step 105, at which it is determined whether or not the correction coefficient K is smaller than 1.0. When K &lt; 1.0, that is, when the air-fuel ratio of the lean air-fuel mixture should be made lean, the processing routine proceeds to step 127, at which the feedback control II of the air-fuel ratio is carried out. This feedback control II is shown in Fig. 16. On the other hand, when K is not smaller than 1.0, the processing routine proceeds to step 106, at which FAF is fixed to 1.0, and then the processing routine proceeds to step 107. At step 107, the fuel injection time TAU is calculated on the basis of the following equation:<maths id="math0011" num=""><math display="block"><mrow><mtext>TAU = TP · K · FAF</mtext></mrow></math><img file="EP0733786A2_D0011.tif" /></maths>

0049Subsequently, at step 108, it is determined whether or not the correction coefficient K is smaller than 1.0. When K &lt; 1.0, that is, when a lean air-fuel mixture should be burned, the processing routine proceeds to step 109, at which the amount of absorption of NO_x NOXA is calculated from Fig. 6. Subsequently, at step 110, the amount of absorption of NO_x NOXD is made zero, and then the processing routine proceeds to step 113. Contrary to this, at step 108, when K ≧ 1.0 is determined, that is, when an air-fuel mixture of the stoichiometric air-fuel ratio or the rich air-fuel mixture should be burned, the processing routine proceeds to step 111, at which the amount of absorption of NO_x NOXD is calculated on the basis of the following equation:<maths id="math0012" num=""><math display="block"><mrow><mtext>NOXD = f · (K - 1) · TP · N</mtext></mrow></math><img file="EP0733786A2_D0012.tif" /></maths>

0050Subsequently, at step 112, the amount of absorption of NO_x NOXA is made zero, and then the processing routine proceeds to step 113. At step 113, the amount ΣNOX estimated to be absorbed in the NO_x absorbent 18 is calculated on the basis of the following equation.<maths id="math0013" num=""><math display="block"><mrow><mtext>ΣNOX = ΣNOX + NOXA - NOXD</mtext></mrow></math><img file="EP0733786A2_D0013.tif" /></maths>

0051Subsequently, at step 114, by multiplying the estimated amount of NO_x ΣNOX by KX, the corrected estimated amount of NO_x, that is, the actual amount of NO_x ΣNKX is calculated. Subsequently, at step 115, it is determined whether or not the ΣNOX becomes negative. When ΣNOX becomes smaller than 0, the processing routine proceeds to step 116, at which the ΣNOX is made zero. Subsequently, at step 117, a current vehicle speed SP is added to ΣSP. This ΣSP indicates the cumulative traveling distance of the vehicle. Subsequently, at step 118, it is determined whether or not the cumulative travelling distance ΣSP is larger than the set value SP₀. When ΣSP ≦ SP₀, the processing routine proceeds to step 119, at which it is determined whether or not the ΣNKX exceeds the allowable maximum value MAX (Fig. 7). When ΣNKX becomes larger than MAX, the processing routine proceeds to step 120, at which the NO_x releasing flag is set.

0052On the other hand, when it is determined at at step 118 that ΣSP > SP₀, the processing routine proceeds to step 121, at which it is determined whether or not the amount of NO_x ΣNKX becomes larger than SAT (Fig. 7). When ΣNKX becomes larger than SAT, the processing routine proceeds to step 122, at which the decision of deterioration flag is set, and then at step 123, ΣSP is made zero.

0053When the decision of deterioration flag is set, the processing routine goes from step 101 to step 124, at which the decision of deterioration is carried out. This decision of deterioration is shown in Fig. 18. On the other hand, when the NO_x releasing flag is set, the processing routine proceeds from step 102 to step 125, at which the processing for release of NO_x is performed. This processing for release of NO_x is shown in Fig. 17.

0054Next, an explanation will be made of the feedback control I to be carried out at step 126 of Fig. 12, that is, the feedback control for maintaining the air-fuel ratio at the stoichiometric air-fuel ratio on the basis of the output signal of the O₂ sensor 22 referring to Fig. 14 and Fig. 15.

0055As shown in Fig. 15, the O₂ sensor 20 generates an output voltage V of about 0.9V when the air-fuel ratio of the air-fuel mixture is rich and generates an output voltage V of about 0.1V when the air-fuel ratio of the air-fuel mixture is lean. The feedback control I shown in Fig. 14 is carried out on the basis of the output signal of this O₂ sensor 20.

0056Referring to Fig. 14, first of all, it is determined at step 130 whether or not the output voltage V of the O₂ sensor 20 is smaller than a reference voltage Vr of about 0.45V. When V ≦ Vr, that is, when the air-fuel ratio is lean, the processing routine proceeds to step 131, at which the delay count CDL is decremented exactly by one. Subsequently, at step 132, it is determined whether or not the delay count CDL becomes smaller than the minimum value TDR. When CDL becomes smaller than TDR, the processing routine proceeds to step 133, at which the CLD is made TDR and then the processing routine proceeds to step 137. Accordingly, as shown in Fig. 15, when V becomes equal to or smaller than Vr, the delay count value CDL is gradually decreased, and subsequently, the CDL is maintained at the minimum value TDR.

0057On the other hand, when it is determined at step 130 that V > Vr, that is, when the air-fuel ratio is rich, the processing routine proceeds to step 134, at which the delay count CDL is incremented exactly by one. Subsequently, at step 135, it is determined whether or not the delay count CDL becomes larger than the maximum value TDL. When CDL becomes larger than TDL, the processing routine proceeds to step 136, at which the CDL is made TDL and then the processing routine proceeds to step 137. Accordingly, as shown in Fig. 15, when V becomes larger than Vr, the delay count CDL is gradually increased, and then CDL is maintained at the maximum value TDL.

0058At step 137, it is determined whether or not the sign of the delay count CDL is inverted from positive to negative or from negative to positive in a period from the previous processing cycle to this processing cycle. When the sign of the delay count CDL is inverted, the processing routine proceeds to step 138, at which it is determined whether or not it is an inversion from positive to negative, that is, whether or not it is an inversion from rich to lean. When it is an inversion from rich to lean, the processing routine proceeds to step 139, at which the rich skip value RSR is added to the feedback correction coefficient FAF and thus, as shown in Fig. 15, the FAF is abruptly increased exactly by the rich skip value RSR. Contrary to this, at the time of an inversion from lean to rich, the processing routine proceeds to step 140, at which the lean skip value RSL is subtracted from the FAF, and thus as shown in Fig. 15, the FAF is abruptly decreased exactly by the lean skip value RSL.

0059On the other hand, when it is determined at step 137 that the sign of the delay count CDL is not inverted, the processing routine proceeds to step 141, at which it is determined whether or not the delay count CDL is negative. When CDL ≦ 0, the processing routine proceeds to step 142, at which the rich integration value KIR (KIR < RSR) is added to the feedback correction coefficient FAF, and thus as shown in Fig. 15, the FAF is gradually increased. On the other hand, when CDL > 0, the processing routine proceeds to step 143, at which rich integration value KIL (KIL < RSL) is subtracted from FAF, and thus the FAF is gradually decreased as shown in Fig. 15. In this way, the air-fuel ratio is controlled to the stoichiometric air-fuel ratio.

0060Next, an explanation will be made of the feedback control for maintaining the air-fuel ratio to the target lean air-fuel ratio corresponding to the correction coefficient K on the basis of the feedback control II carried out at step 127 of Fig. 12, that is, the current I of the O₂ sensor 22, referring to Fig. 16.

0061Referring to Fig. 16, first of all, at step 150, the target current value I₀ corresponding to the target lean air-fuel ratio is calculated from the relationship shown in Fig. 9. Subsequently, at step 151, it is determined whether or not the current I of the O₂ sensor 22 is larger than the target current I₀. When I > I₀, the processing routine proceeds to step 152, at which a constant value ΔF is added to the feedback correction coefficient FAF, and when I ≦ I₀, the processing routine proceeds to step 153, at which the constant value ΔF is subtracted from the feedback correction coefficient FAF. In this way, the air-fuel ratio is maintained at the target lean air-fuel ratio.

0062Next, an explanation will be made of the control for release of NO_x carried out at step 125 of Fig. 12 referring to Fig. 17.

0063Referring to Fig. 17, first of all, at step 160, the correction coefficient K is made a constant value KK of for example about 1.3. Subsequently, at step 161, the fuel injection time TAU is calculated on the basis of the following equation:<maths id="math0014" num=""><math display="block"><mrow><mtext>TAU = TP · K</mtext></mrow></math><img file="EP0733786A2_D0014.tif" /></maths>

0064Accordingly, when the processing for release of NO_x is started, the feedback control of the air-fuel ratio is stopped, and the air-fuel ratio of the air-fuel mixture is made rich. Subsequently, at step 162, the amount of release NOXD of the NO_x released from the NO_x absorbent 18 per unit time is calculated as follows:<maths id="math0015" num=""><math display="block"><mrow><mtext>NOXD = f · (K - 1.0) · TP · N</mtext></mrow></math><img file="EP0733786A2_D0015.tif" /></maths>

0065Subsequently, at step 163, the amount of release XNO_x of NO_x actually released from the NO_x absorbent 18 is calculated on the basis of the following equation. Note that, in the following equation, Δt represents the interval of the time interruption.<maths id="math0016" num=""><math display="block"><mrow><msub><mrow><mtext>XNO</mtext></mrow><mrow><mtext>x</mtext></mrow></msub><msub><mrow><mtext> = XNO</mtext></mrow><mrow><mtext>x</mtext></mrow></msub><mtext> + NOXD · Δt</mtext></mrow></math><img file="EP0733786A2_D0016.tif" /></maths>

0066Subsequently, at step 164, it is determined whether or not the current I of the O₂ sensor 22 becomes lower than the predetermined constant value α (Fig. 11). When I becomes smaller than α, the processing routine proceeds to step 165, at which it is determined whether or not the absolute value |XNO_x - ΣNKX| of the difference between the actual amount of release of NO_x XNO_x and the corrected estimated amount of absorption of NO_x ΣNKX is larger than the constant value β. When <maths id="math0017" num=""><math display="inline"><mrow><msub><mrow><mtext>|XNO</mtext></mrow><mrow><mtext>x</mtext></mrow></msub><mtext> - ΣNKX| ≦ β</mtext></mrow></math><img file="EP0733786A2_D0017.tif" /></maths>, the processing routine jumps to step 167. Contrary to this, when <maths id="math0018" num=""><math display="inline"><mrow><msub><mrow><mtext>|XNO</mtext></mrow><mrow><mtext>x</mtext></mrow></msub><mtext> - ΣNKX| > β</mtext></mrow></math><img file="EP0733786A2_D0018.tif" /></maths>, the processing routine proceeds to step 166, at which the correction value KX is corrected on the basis of the following equation:<maths id="math0019" num=""><math display="block"><mrow><msub><mrow><mtext>KX = KX · XNO</mtext></mrow><mrow><mtext>x</mtext></mrow></msub><mtext> / ΣNKX</mtext></mrow></math><img file="EP0733786A2_D0019.tif" /></maths>

0067Subsequently, at step 167, the NO_x releasing flag is reset, and thus the air-fuel ratio of the air-fuel mixture is changed to the air-fuel ratio determined according to the operating state at that time, usually lean. Subsequently, at step 168, XNO_x and ΣNOX are made zero.

0068Next, an explanation will be made of the decision of deterioration carried out at step 124 of Fig. 12 referring to Fig. 18.

0069Referring to Fig. 18, first of all, at step 170, the correction coefficient K is made the constant value KK of for example about 1.3. Subsequently, at step 171, the fuel injection time TAU is calculated on the basis of the following equation:<maths id="math0020" num=""><math display="block"><mrow><mtext>TAU = TP · K</mtext></mrow></math><img file="EP0733786A2_D0020.tif" /></maths>

0070Accordingly, when the decision of deterioration is started, the feedback control of the air-fuel ratio is stopped, and the air-fuel ratio of the air-fuel mixture is made rich. Subsequently, at step 172, the amount of release NOXD of NO_x released from the NO_x absorbent 18 is calculated based on the following equation:<maths id="math0021" num=""><math display="block"><mrow><mtext>NOXD = f · (K - 1.0) · TP · N</mtext></mrow></math><img file="EP0733786A2_D0021.tif" /></maths>

0071Subsequently, at step 173, the amount of release VNO_x of NO_x actually released from the NO_x absorbent 18 is calculated on the basis of the following equation. Note that, in the following equation, Δt represents the interval of the time interruption.<maths id="math0022" num=""><math display="block"><mrow><msub><mrow><mtext>VNO</mtext></mrow><mrow><mtext>x</mtext></mrow></msub><msub><mrow><mtext> = VNO</mtext></mrow><mrow><mtext>x</mtext></mrow></msub><mtext> + NOXD · Δt</mtext></mrow></math><img file="EP0733786A2_D0022.tif" /></maths>

0072Subsequently, at step 174, it is determined whether or not the current I of the O₂ sensor 22 becomes lower than the predetermined constant value α (Fig. 11). When I becomes smaller than α, the processing routine proceeds to step 175, at which by multiplying the VNO_x by a constant value larger than 1.0, for example 1.1, the decision level SAT (= 1.1 · VNO_x) is calculated. In this way, the decision level SAT is set to a value larger than VNO_x, so this VNO_x represents the maximum amount of absorption of NO_x possible by the NO_x absorbent 18. Namely, if VNO_x represents an amount of absorption of NO_x smaller than the maximum amount of absorption of NO_x, the decision level SAT becomes large whenever the decision of deterioration is carried out, and thus finally the VNO_x represents the maximum amount of absorption of NO_x, that is, the degree of deterioration of the NO_x absorbent 18.

0073So as to find the decision level SAT, of course it is also possible to multiply another numerical value other than 1.1 with VNO_x, and the decision level SAT can be found by multiplying any number of 1.0 or more with VNO_x. Note, if the numerical value to be multiplied with VNO_x is made too large, the time from when the amount of absorption of NO_x of the NO_x absorbent 18 becomes the maximum amount of absorption of NO_x to when the action of release of NO_x is carried out becomes too long, so the amount of NO_x discharged to the atmosphere is increased. Accordingly, it is not preferred that the numerical value to be multiplied with VNO_x be set too large. This numerical value is preferably about 1.3 or less.

0074When the decision level SAT is calculated at step 175, the processing routine proceeds to step 176, at which by multiplying a positive numerical value of 1.0 or less, for example 0.8, with the VNO_x, the allowable maximum value MAX (= 0.8 · VNO_x) is calculated. Namely, the allowable maximum value MAX is also updated in accordance with the degree of deterioration of NO_x absorbent 18. Subsequently, at step 177, it is determined whether or not the maximum amount of absorption of NO_x VNO_x reaches the predetermined minimum value MIN. When VNO_x becomes smaller than MIN, the processing routine proceeds to step 178, at which the alarm lamp 25 is turned on. Subsequently, at step 179, the decision of deterioration flag is reset. When the decision of deterioration flag is reset, the air-fuel ratio of the air-fuel mixture is changed to the air-fuel ratio in accordance with the operating state at that time, usually lean. Subsequently, at step 180, VNO_x and ΣNOX are made zero.

0075Figure 19 to Fig. 23 show another embodiment. Also in this embodiment, the decision of deterioration of the NO_x absorbent 18 is carried out when the corrected amount of absorption of NO_x ΣNKX exceeds the decision level SAT, but control for release of NO_x from the decision of deterioration to when the next decision of deterioration is carried out can be carried out by a simpler method compared with the first embodiment. Namely, in this embodiment, as shown in Figs. 19A and 19B, a cycle TL at which the air-fuel ratio of the air-fuel mixture is made rich so as to release the NO_x from the NO_x absorbent 18 and the rich time TR of the air-fuel mixture at this time are determined in accordance with the maximum amount of absorption of NO_x VNO_x, that is, the degree of deterioration of the NO_x absorbent 18. Namely, as shown in Fig. 19A, the lower the maximum amount of absorption of NO_x VNO_x, in other words, the larger the degree of deterioration of the NO_x absorbent 18, the shorter the cycle TL at which the air-fuel ratio of the air-fuel mixture is made rich, and as shown in Fig. Fig. 19B, the lower the maximum amount of absorption of NO_x VNO_x, in other words, the larger the degree of deterioration of the NO_x absorbent 18, the shorter the rich time TR of the air-fuel mixture. Note that, the relationships shown in Figs. 19A and 19B are preliminarily stored in the ROM 32.

0076Figure 20 and Fig. 21 show the routine for control of the air-fuel ratio for this second embodiment. This routine is executed by interruption at every predetermined time interval.

0077Referring to Fig. 20 and Fig. 21, first of all, at step 200, the basic fuel injection time TP is calculated from the relationship shown in Fig. 2. Subsequently, at step 201, it is determined whether or not the decision of deterioration flag indicating that the degree of deterioration of the NO_x absorbent 18 should be decided has been set. When the decision of deterioration flag has not been set, the processing routine proceeds to step 202, at which it is determined whether or not the NO_x releasing flag indicating that the NO_x should be released from the NO_x absorbent 18 has been set. When the NO_x releasing flag has not been set, the processing routine proceeds to step 203.

0078At step 203, the correction coefficient K is calculated on the basis of Fig. 3. Subsequently, at step 204, it is determined whether or not the correction coefficient K is 1.0. When K = 1.0, that is, when the air-fuel ratio of the air-fuel mixture is made the stoichiometric air-fuel ratio, the processing routine proceeds to step 228, at which the feedback control I of the air-fuel ratio is carried out. This feedback control I is shown in Fig. 14. On the other hand, when K is not equal to 1.0, the processing routine proceeds to step 205, at which it is determined whether or not the correction coefficient K is smaller than 1.0. When K &lt; 1.0, that is, when the air-fuel ratio of the lean air-fuel mixture should be made lean, the processing routine proceeds to step 229, at which the feedback control II of the air-fuel ratio is carried out. This feedback control II is shown in Fig. 16. On the other hand, when K is not smaller than 1.0, the processing routine proceeds to step 206, at which the FAF is fixed to 1.0, and then the processing routine proceeds to step 207. At step 207, the fuel injection time TAU is calculated on the basis of the following equation:<maths id="math0023" num=""><math display="block"><mrow><mtext>TAU = TP · K · FAF</mtext></mrow></math><img file="EP0733786A2_D0023.tif" /></maths>

0079Subsequently, at step 208, it is determined whether or not the correction coefficient K is smaller than 1.0. When K &lt; 1.0, that is, when the lean air-fuel mixture should be burned, the processing routine proceeds to step 209, at which the amount of absorption of NO_x NOXA is calculated from Fig. 6. Subsequently, at step 210, the amount of release of NO_x NOXD is made zero. Subsequently, at step 211, the interval Δt of the time interruption is added to the count value TC. Accordingly, this count TC represents the elapsed time.

0080At step 211, when the estimated time TC is calculated, the processing routine proceeds to step 215, at which the amount ΣNOX of NO_x estimated to be absorbed in the NO_x absorbent 18 is calculated on the basis of the following equation:<maths id="math0024" num=""><math display="block"><mrow><mtext>ΣNOX = ΣNOX + NOXA - NOXD</mtext></mrow></math><img file="EP0733786A2_D0024.tif" /></maths>

0081On the other hand, when it is determined at step 208 that K ≧ 1.0, that is, when the air-fuel mixture of stoichiometric air-fuel ratio or the rich air-fuel mixture should be burned, the processing routine proceeds to step 212, at which the amount of release of NO_x NOXD is calculated on the basis of the following equation:<maths id="math0025" num=""><math display="block"><mrow><mtext>NOXD = f · (K - 1.0) · TP · N</mtext></mrow></math><img file="EP0733786A2_D0025.tif" /></maths>

0082Subsequently, at step 213, the amount of absorption of NO_x NOXA is made zero, and then at step 214, the elapsed time TC is made zero. Subsequently, the processing routine proceeds to step 215, at which the estimated amount of NO_x ΣNOX is calculated.

0083Subsequently, at step 216, by multiplying the estimated amount of NO_x ΣNOX by the correction value KX, the corrected estimated amount of NO_x, that is, the actual amount of NO_x ΣNKX is calculated. Subsequently, at step 217, it is determined whether or not the ΣNOX becomes negative. When ΣNOX becomes smaller than 0, the processing routine proceeds to step 218, at which ΣNOX is made zero. Subsequently, at step 219, the current vehicle speed SP is added to ΣSP. This ΣSP indicates the cumulative travelling distance of the vehicle. Then, at step 220, it is determined whether or not the cumulative travelling distance ΣSP is larger than the set value SP₀. When ΣSP ≦ SP₀, the processing routine proceeds to step 221, at which it is determined whether or not the elapsed time TC exceeds the cycle TL shown in Fig. 19A in accordance with the maximum amount of absorption of NO_x VNO_x. When TC becomes larger than TL, the processing routine proceeds to step 222, at which the NO_x releasing flag is set.

0084On the other hand, when it is determined at step 220 that ΣSP > SP₀, the processing routine proceeds to step 223, at which it is determined whether or not the ΣNKX becomes larger than the decision level SAT (Fig. 7). When ΣNKX becomes larger than SAT, the processing routine proceeds to step 224, at which the decision of deterioration flag is set, and then, at step 225, ΣSP is made zero.

0085When the decision of deterioration flag is set, the processing routine goes from step 201 to step 226, at which the decision of deterioration is carried out. This decision of deterioration is shown in Fig. 23. On the other hand, when the NO_x releasing flag is set, the processing routine goes from step 202 to step 227, at which the processing for release of NO_x is carried out. This processing for release of NO_x is shown in Fig. 22.

0086Next, an explanation will be made of the control for releasing NO_x carried out at step 227 of Fig. 20 referring to Fig. 22.

0087Referring to Fig. 22, first of all, at step 230, the correction coefficient K is made the constant value KK of for example about 1.3. Subsequently, at step 231, the fuel injection time TAU is calculated on the basis of the following equation:<maths id="math0026" num=""><math display="block"><mrow><mtext>TAU = TP · K</mtext></mrow></math><img file="EP0733786A2_D0026.tif" /></maths>

0088Accordingly, when the processing for release of NO_x is started, the feedback control of the air-fuel ratio is stopped, and the air-fuel ratio of the air-fuel mixture is made rich. Subsequently, at step 232, the amount of release NOXD of NO_x released from the NO_x absorbent 18 per unit time is calculated on the basis of the following equation:<maths id="math0027" num=""><math display="block"><mrow><msub><mrow><mtext>NOXD = f</mtext></mrow><mrow><mtext>1</mtext></mrow></msub><mtext> · (K - 1.0) · TP · N</mtext></mrow></math><img file="EP0733786A2_D0027.tif" /></maths>

0089Subsequently, at step 233, the amount of release XNO_x of NO_x actually released from the NO_x absorbent 18 is calculated on the basis of the following equation. Note that, in the following equation, Δt represents the interval of the time interruption.<maths id="math0028" num=""><math display="block"><mrow><msub><mrow><mtext>XNO</mtext></mrow><mrow><mtext>x</mtext></mrow></msub><msub><mrow><mtext> = XNO</mtext></mrow><mrow><mtext>x</mtext></mrow></msub><mtext> + NOXD · Δt</mtext></mrow></math><img file="EP0733786A2_D0028.tif" /></maths>

0090Subsequently, at step 234, it is determined whether or not a rich time TR shown in Fig. 19B in accordance with the maximum amount of absorption of NO_x VNO_x elapses from when the processing for release of NO_x is started. When the rich time TR is elapsed, the processing routine proceeds to step 235, at which it is determined whether or not absolute value |XNO_x - ΣNKX| of the difference between the actual amount of release of NO_x XNO_x and the corrected estimated amount of absorption of NO_x ΣNKX is larger than the constant value β. When <maths id="math0029" num=""><math display="inline"><mrow><msub><mrow><mtext>|XNO</mtext></mrow><mrow><mtext>x</mtext></mrow></msub><mtext> - ΣNKX| ≦ β</mtext></mrow></math><img file="EP0733786A2_D0029.tif" /></maths>, the processing routine jumps to step 237. Contrary to this, when <maths id="math0030" num=""><math display="inline"><mrow><msub><mrow><mtext>|XNO</mtext></mrow><mrow><mtext>x</mtext></mrow></msub><mtext> - ΣNKX| > β</mtext></mrow></math><img file="EP0733786A2_D0030.tif" /></maths>, the processing routine proceeds to step 236, at which the correction value KX is corrected based on the following equation:<maths id="math0031" num=""><math display="block"><mrow><msub><mrow><mtext>KX = KX · XNO</mtext></mrow><mrow><mtext>x</mtext></mrow></msub><mtext> / ΣNKX</mtext></mrow></math><img file="EP0733786A2_D0031.tif" /></maths>

0091Subsequently, at step 237, the NO_x releasing flag is reset, and thus the air-fuel ratio of the air-fuel mixture is changed from rich to the air-fuel ratio determined according to the operating state at that time, usually lean. Subsequently, at step 238, TC, XNO_x, and ΣNOX are made zero.

0092Next, an explanation will be made of the decision of deterioration carried out at step 226 of Fig. 20 referring to Fig. 23.

0093Referring to Fig. 23, first of all, at step 240, the correction coefficient K is made the constant value KK of for example about 1.3. Subsequently, at step 241, the fuel injection time TAU is calculated on the basis of the following equation:<maths id="math0032" num=""><math display="block"><mrow><mtext>TAU = TP · K</mtext></mrow></math><img file="EP0733786A2_D0032.tif" /></maths>

0094Accordingly, when the decision of deterioration is started, the feedback control of the air-fuel ratio is stopped, and the air-fuel ratio of the air-fuel mixture is made rich. Subsequently, at step 242, the amount of release NOXD of NO_x released from the NO_x absorbent 18 per unit time is calculated on the basis of the following equation:<maths id="math0033" num=""><math display="block"><mrow><mtext>NOXD = f · (K - 1.0) · TP · N</mtext></mrow></math><img file="EP0733786A2_D0033.tif" /></maths>

0095Subsequently, at step 243, the amount of release VNO_x of NO_x actually released from the NO_x absorbent 18 is calculated on the basis of the following equation. Note that, in the following equation, Δt represents the interval of the time interruption.<maths id="math0034" num=""><math display="block"><mrow><msub><mrow><mtext>VNO</mtext></mrow><mrow><mtext>x</mtext></mrow></msub><msub><mrow><mtext> = VNO</mtext></mrow><mrow><mtext>x</mtext></mrow></msub><mtext> + NOXD · Δt</mtext></mrow></math><img file="EP0733786A2_D0034.tif" /></maths>

0096Subsequently, at step 244, it is determined whether or not the current value I of the O₂ sensor 22 becomes lower than the predetermined constant value α (Fig. 11). When I becomes smaller than α, the processing routine proceeds to step 245, at which by multiplying a constant value larger than 1.0, for example, 1.1, with VNO_x, the decision level SAT (= 1.1 · VNO_x) is calculated. In this case, as mentioned before, VNO_x represents the maximum amount of absorption of NO_x, that is, the degree of deterioration of the NO_x absorbent 18. Subsequently, at step 246, on the basis of the maximum amount of absorption of NO_x VNO_x, a cycle TL making the air-fuel ratio of the air-fuel mixture rich is calculated from the relationship shown in Fig. 19A, and then at step 247, the rich time TR of the air-fuel mixture is calculated from the relationship shown in Fig. 19B on the basis of the maximum amount of absorption of NO_x VNO_x .

0097Subsequently, at step 248, it is determined whether or not the maximum amount of absorption of NO_x VNO_x becomes lower than the predetermined minimum value MIN. When VNO_x becomes smaller than MIN, the processing routine proceeds to step 249, at which the alarm lamp 25 is turned on. Subsequently, at step 250, the decision of deterioration flag is reset. When the decision of deterioration flag is reset, the air-fuel ratio of the air-fuel mixture is changed to the air-fuel ratio in accordance with the operating state at that time, usually lean. Subsequently, at step 251, the VNO_x and ΣNOX are made zero.

0098As mentioned above, according to the present invention, by making the cycle for making the air-fuel ratio rich for releasing the NO_x from the NO_x absorbent or the rich time at that time shorter along with the deterioration of the NO_x absorbent, it becomes possible to not only reduce the ratio of fuel consumption, but also to prevent the release of a large amount of unburnt hydrocarbons, CO, and NO_x into the atmosphere at the time of the release of the NO_x.

0099While the invention has been described by reference to specific embodiments chosen for purposes of illustration, it should be apparent that numerous modifications could be made thereto by those skilled in the art without departing from the basic concept and scope of the invention.

0100An exhaust gas purification device of an engine providing a NO_x absorbent arranged in the exhaust passage. An O₂ sensor is arranged in the engine exhaust passage downstream of the NO_x absorbent. The degree of deterioration of the NO_x absorbent is detected from the output signal of the O₂ sensor. The cycle for making the air-fuel ratio of the air-fuel mixture rich for causing the release of the NO_x from the NO_x absorbent and the rich time at that time are shortened the greater the deterioration of the NO_x absorbent.