Exhaust Purification Device of Internal Combustion Engine

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

A diesel engine in which an engine exhaust passage is branched to a pair of exhaust branch passages for purifying NO.sub.x in the diesel engine, a switching valve is disposed at the branched portion of these exhaust branch passages, the switching valve is switched each time a predetermined time passes to alternately guide the exhaust gas to one of the exhaust branch passages, and a catalyst which can oxidize and absorb the NO.sub.x is disposed in each of the exhaust branch passages is well known (refer to Japanese Unexamined Patent Publication No. 62-106826). In this diesel engine, the NO.sub.x in the exhaust gas introduced into one exhaust branch passage is oxidized and absorbed by the catalyst disposed in that exhaust branch passage. During this time, the inflow of the exhaust gas to the other exhaust branch passage is stopped and, at the same time, a gaseous reducing agent is fed into this exhaust branch passage. The NO.sub.x accumulated in the catalyst disposed in this exhaust branch passage is reduced by this reducing agent. Subsequently, after the elapse of a predetermined time, the introduction of the exhaust gas to the exhaust branch passage to which the exhaust gas had been introduced heretofore is stopped by the switching function of the switching valve, and the introduction of the exhaust gas to the exhaust branch passage to which the introduction of the exhaust gas had been stopped heretofore is started again. That is, in this diesel engine, seen from the viewpoint of each of the exhaust branch passages, exhaust gas is made to flow for a predetermined time during which the NO.sub.x in the exhaust gas is oxidized and absorbed by the catalyst, then the inflow of exhaust gas is stopped for a predetermined period and a reducing agent is fed, whereby the NO.sub.x accumulated in the catalyst is reduced.

However, in this diesel engine, when the NO.sub.x was to be reduced, there was the problem that the inflow of the exhaust gas to the catalyst was stopped, but that the NO.sub.x was attempted to be reduced while causing exhaust gas to flow into the catalyst. That is, with such a catalyst, the speed of reduction of the NO.sub.x changed depending on the temperature of the catalyst and as the temperature of the catalyst became lower, the speed of reduction of the NO.sub.x became slower. Accordingly, where the temperature of the catalyst is low, if a reducing agent is fed for a predetermined period, when the period of feeding the reducing agent is short, the NO.sub.x absorbed in the catalyst cannot be sufficiently reduced, so a large amount of NO.sub.x remains in the catalyst and therefore the NO.sub.x absorption capacity ends up being reduced. As a result, there was the problem that the NO.sub.x absorption capacity becomes saturated in a short time after the feeding of the reducing agent is stopped and the NO.sub.x absorption action started and therefore the NO.sub. x is released into the atmosphere.

Further, if a large amount of the reducing agent is fed when the temperature of the catalyst is low, only a small amount of the reducing agent is used for the reduction of the NO.sub.x due to the slow speed of reduction of the NO.sub.x, therefore there was the problem that a large amount of the reducing agent is released in the atmosphere. On the other hand, if the amount of the reducing agent is reduced to solve this problem, then even when the temperature of the catalyst is high and therefore the speed of reduction of NO.sub.x is fast, only part of the NO.sub.x is reduced and therefore a large amount of NO.sub.x remains in the catalyst, so the NO.sub.x absorption capacity ends up reduced. As a result, there is the problem that the NO.sub.x absorption capacity ends up saturated in a short time after the feeding of the reducing agent is stopped and the NO.sub.x absorption action is started and therefore the NO.sub.x is released into the atmosphere.

An object of the present invention is to provide an exhaust purification device which suppresses the release of harmful components into the atmosphere at the time of reducing NO.sub.x.

According to the present invention, there is provided an exhaust purification device of an internal combustion engine which has in an engine exhaust passage a NO.sub.x absorbent which absorbs the NO.sub.x when the air-fuel ratio of the inflowing exhaust gas is lean and which releases the absorbed NO.sub.x when the air-fuel ratio of the inflowing exhaust gas becomes rich and which is provided with a temperature detecting means for detecting a temperature representing the temperature of the NO.sub.x absorbent and a NO.sub.x release controlling means for making the air-fuel ratio of the exhaust gas flowing into the NO.sub.x absorbent rich when the NO.sub.x is to be released from the NO.sub.x absorbent and at that time increasing the degree of richness and or for shortening the time for which the air-fuel ratio of the exhaust gas flowing into the NO.sub.x absorbent is made rich as the temperature of the NO.sub.x absorbent increases.

FIG. 1 is an overall view of an internal combustion engine; FIG. 2 is a diagram showing a map of a basic fuel injection time; FIG. 3 is a diagram showing a correction coefficient K; FIG. 4 is a graph schematically showing the concentration of unburnt HC, CO, and oxygen in the exhaust gas discharged from the engine; FIG. 5 is a diagram for explaining an absorption and release function of the NO.sub.x ; FIG. 6 is a diagram showing the amount of NO.sub.x discharged from the engine, FIG. 7 is a graph showing the capacity of absorption of NO.sub.x of the NO.sub.x absorbent; FIG. 8 is a graph showing the characteristics of release of NO.sub.x ; FIG. 9 is a graph showing the change in the correction coefficient K; FIG. 10 is a graph showing C.sub.1, C.sub.2, .alpha., and .beta.; FIG. 11 is a diagram showing a map of the exhaust gas temperature T; FIG. 12 and FIG. 13 are flow charts showing a time interruption routine; FIG. 14 is a flow chart for calculating the fuel injection time TAU; and FIG. 15 is an overall view of an internal combustion engine showing another embodiment.

FIG. 1 shows a case where the present invention is applied to a gasoline engine.

Referring 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, respectively. The intake port 6 is connected to a surge tank 10 via a corresponding branch pipe 9, and a fuel injector 11 injecting the fuel toward the interior of the intake port 6 is attached to each branch pipe 9. The surge tank 10 is connected to an air cleaner 13 via an intake duct 12, and a throttle valve 14 is disposed 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 18 including a NO.sub.x absorbent 17.

An electronic control unit 30 comprises a digital computer and is provided with 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, which are interconnected by a bidirectional bus 31. In the surge tank 10 is mounted a pressure sensor 19 for generating an output voltage proportional to the absolute pressure in the surge tank 10. The output voltage of this pressure sensor 19 is input through an AD converter 37 to the input port 35. Further, a temperature sensor 20 which generates an output voltage proportional to the temperature of the exhaust gas is attached in the exhaust pipe 16 upstream of the casing 18. The output voltage of this temperature sensor 20 is input through an AD converter 38 to the input port 35. Further, the input port 35 is connected to a rotational speed sensor 21 for generating an output pulse expressing the engine rotational speed. On the other hand, the output port 36 is connected through a corresponding drive circuit 39 to a spark plug 4 and fuel injector 11.

In the internal combustion engine shown in FIG. 1, the fuel injection time TAU is calculated based on for example the following equation.

where, TP is a basic fuel injection time, and K is a correction coefficient. The basic fuel injection time TP shows the fuel injection time necessary for bringing the air-fuel ratio of an air-fuel mixture fed into the engine cylinder to the stoichiometric air-fuel ratio. This basic fuel injection time TP is found in advance by experiments and is stored in advance in the ROM 32 in the form of a map as shown in FIG. 2 as the function of the absolute pressure PM in the surge tank 10 and the engine rotational speed N. The correction coefficient K is a coefficient for controlling the air-fuel ratio of the air-fuel mixture fed into the engine cylinder, and if K=1.0, the air-fuel mixture fed 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 fed into the engine cylinder becomes larger than the stoichiometric air-fuel ratio, that is, becomes lean, and when K becomes larger than 1.0, the air-fuel ratio of the air-fuel mixture fed into the engine cylinder becomes smaller than the stoichiometric air-fuel ratio, that is, becomes rich.

The value of this correction coefficient K is predetermined in relation to the absolute pressure PM in the surge tank 10 and the engine rotational speed N. FIG. 3 shows an embodiment of the value of the correction coefficient K. In the embodiment shown in FIG. 3, in the region where the absolute pressure PM in the surge tank 10 is relatively low, that is, in the engine low and medium load operation region, the value of the correction coefficient K is made a value smaller than 1.0, therefore at this time the air-fuel ratio of the air-fuel mixture fed into the engine cylinder is made lean. On the other hand, in the region where the absolute pressure PM in the surge tank 10 is relatively high, that is, in the engine high load operation region, the value of the correction coefficient is made 1.0. Accordingly, at this time, the air-fuel ratio of the air-fuel mixture fed into the engine cylinder is made the stoichiometric air-fuel ratio. Further, in the region where the absolute pressure PM in the surge tank 10 becomes the highest, that is, in the engine full load operation region, the value of the correction coefficient is made a value larger than 1.0. Therefore, at this time, the air-fuel ratio of the air-fuel mixture fed into the engine cylinder is made rich. An internal combustion engine is usually operated most frequently with a low and medium load and therefore for the majority of the period of operation a lean air-fuel mixture is burned.

FIG. 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 fed into the combustion chamber 3 becomes richer, and the concentration of the oxygen O.sub.2 in the exhaust gas discharged from the combustion chamber 3 is increased as the air-fuel ratio of the air-fuel mixture fed into the combustion chamber 3 becomes leaner.

The NO.sub.x absorbent 17 contained in the casing 18 uses, for example, alumina as a carrier. On this carrier, at least one substance selected from alkali metals, for example, potassium K, sodium Na, lithium Li, and cesium Cs; alkali earths, for example, barium Ba and calcium Ca; and rare earths, for example, lanthanum La and yttrium Y and a precious metal such as platinum Pt are carried. When referring to the ratio between the air and fuel (hydrocarbons) fed into the intake passage of the engine and the exhaust passage upstream of the NO.sub.x absorbent 17 as the air-fuel ratio of the inflowing exhaust gas flowing into the NO.sub.x absorbent 17, this NO.sub.x absorbent 17 performs the absorption and releasing function of NO.sub.x by absorbing the NO.sub.x when the air-fuel ratio of the inflowing exhaust gas is lean, while releasing the absorbed NO.sub.x when the concentration of oxygen in the inflowing exhaust gas falls. Note that, where the fuel (hydrocarbons) or air is not fed into the exhaust passage upstream of the NO.sub.x absorbent 17, the air-fuel ratio of the inflowing exhaust gas coincides with the air-fuel ratio of the air-fuel mixture fed into the combustion chamber 3, and accordingly in this case, the NO.sub.x absorbent 17 absorbs the NO.sub.x when the air-fuel ratio of the air-fuel mixture fed into the combustion chamber 3 is lean and releases the absorbed NO.sub.x when the concentration of oxygen in the air-fuel mixture fed into the combustion chamber 3 is lowered.

When the above-mentioned NO.sub.x absorbent 17 is disposed in the exhaust passage of the engine, this NO.sub.x absorbent 17 actually performs the absorption and releasing function of NO.sub.x, but there are areas of the exact mechanism of this absorption and releasing function which are not clear. However, it can be considered that this absorption and releasing function is conducted by the mechanism as shown in FIG. 5. This mechanism will be explained by using as an example a case where platinum Pt and barium Ba are carried on the carrier, but a similar mechanism is obtained even if another precious metal, alkali metal, alkali earth, or rare earth is used.

Namely, when the inflowing exhaust gas becomes considerably lean, the concentration of oxygen in the inflowing exhaust gas is greatly increased. As shown in FIG. 5(A), the oxygen O.sub.2 is deposited on the surface of the platinum Pt in the form of O.sub.2.sup.- or O.sup.2-. On the other hand, the NO in the inflowing exhaust gas reacts with the O.sub.2.sup.- or O.sup.2- on the surface of the platinum Pt and becomes NO.sub.2 (2NO+O.sub.2 .fwdarw.2NO.sub.2). Subsequently, a part of the produced NO.sub.2 is oxidized on the platinum Pt and absorbed into the absorbent. While bonding with the barium oxide BaO, it is diffused in the absorbent in the form of nitric acid ions NO.sub.3.sup.- as shown in FIG. 5(A). In this way, NO.sub.x is absorbed into the NO.sub.x absorbent 17.

So long as the oxygen concentration in the inflowing exhaust gas is high, the NO.sub.2 is produced on the surface of the platinum Pt, and so long as the NO.sub.x absorption ability of the absorbent is not saturated, the NO.sub.2 is absorbed into the absorbent and nitric acid ions NO.sub.3.sup.- are produced. Contrary to this, when the oxygen concentration in the inflowing exhaust gas is lowered and the production of NO.sub.2 is lowered, the reaction proceeds in an inverse direction (NO.sub.3.sup.- .fwdarw.NO.sub.2), and thus nitric acid ions NO.sub.3.sup.- in the absorbent are released in the form of NO.sub.2 from the absorbent. Namely, when the oxygen concentration in the inflowing exhaust gas is lowered, the NO.sub.x is released from the NO.sub.x absorbent 17. 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 accordingly when the degree of leanness of the inflowing exhaust gas is lowered, the NO.sub.x is released from the NO.sub.x absorbent 17 even if the air-fuel ratio of the inflowing exhaust gas is lean.

On the other hand, at this time, when the air-fuel ratio of the air-fuel mixture fed 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 is discharged from the engine, and these unburnt HC and CO react with the oxygen O.sub.2.sup.- or O.sup.2- on the platinum Pt and are oxidized. Also, when the air-fuel ratio of the inflowing exhaust gas becomes rich, the oxygen concentration in the inflowing exhaust gas is extremely lowered, and therefore the NO.sub.2 is discharged from the absorbent. This NO.sub.2 reacts with the unburnt HC and CO as shown in FIG. 5(B) and is reduced. In this way, when the NO.sub.2 no longer exists on the surface of the platinum Pt, the NO.sub.2 is successively released from the absorbent. Accordingly, when the air-fuel ratio of the inflowing exhaust gas is made rich, the NO.sub.x is released from the NO.sub.x absorbent 19 in a short time.

Namely, when the air-fuel ratio of the inflowing exhaust gas is made rich, first of all, the unburnt HC and CO immediately react with the O.sub.2.sup.- or O.sup.2- on the platinum Pt and are oxidized, and subsequently if the unburnt HC and CO still remain even though the O.sub.2.sup.- or O.sup.2- on the platinum Pt is consumed, the NO.sub.x released from the absorbent and the NO.sub.x discharged from the engine are reduced by these unburnt HC and CO. Accordingly, when the air-fuel ratio of the inflowing exhaust gas is made rich, the NO.sub.x absorbed in the NO.sub.x absorbent 17 is released in a short time and in addition this released NO.sub.x is reduced, and therefore the discharge of NO.sub.x into the atmosphere can be blocked. Also, since the NO.sub.x absorbent 17 has the function of a reduction catalyst, even if the air-fuel ratio of the inflowing exhaust gas is made the stoichiometric air-fuel ratio, the NO.sub.x released from the NO.sub.x absorbent 17 can be reduced. However, where the air-fuel ratio of the inflowing exhaust gas is made the stoichiometric air-fuel ratio, the NO.sub.x is released merely gradually from the NO.sub.x absorbent 17, and therefore a slightly long time is required for releasing all NO.sub.x absorbed in the NO.sub.x absorbent 17.

When the degree of leanness of the inflowing exhaust gas is lowered as mentioned before, even if the air-fuel ratio of the inflowing exhaust gas is lean, the NO.sub.x is released from the NO.sub.x absorbent 17. Accordingly, so as to release the NO.sub.x from the NO.sub.x absorbent 17, it is satisfactory if the concentration of oxygen in the inflowing exhaust gas is lowered. Note, even if the NO.sub.x is released from the NO.sub.x absorbent 17, when the air-fuel ratio of the inflowing exhaust gas is lean, the NO.sub.x is not reduced in the NO.sub.x absorbent 17, and accordingly, in this case, it is necessary to provide a catalyst which can reduce the NO.sub.x downstream of the NO.sub.x absorbent 17 or supply a reducing agent downstream of the NO.sub.x absorbent 17. Of course, it is also possible to reduce the NO.sub.x downstream of the NO.sub.x absorbent 17 in this way, but it is rather preferable that the NO.sub.x be reduced in the NO.sub.x absorbent 17. Accordingly, in the embodiment according to the present invention, when the NO.sub.x should be released from the NO.sub.x absorbent 17, the air-fuel ratio of the inflowing exhaust gas is made rich, whereby the NO.sub.x released from the NO.sub.x absorbent 17 is reduced in the NO.sub.x absorbent 17.

However, in the embodiment according to the present invention, as mentioned above, during full load operation, the air-fuel mixture fed into the engine cylinder 3 is made rich and during high load operation, the air-fuel mixture is made the stoichiometric air-fuel ratio, so during the full load operation and the high load operation, the NO.sub.x is released from the NO.sub.x absorbent 17. However, if the frequency of this full load operation or high load operation is small, then even if the NO.sub.x is released from the NO.sub.x absorbent 17 only during full load operation and high load operation, the absorption capacity of the NO.sub.x by the NO.sub.x absorbent 17 will end up becoming saturated during the time when a lean air-fuel mixture is burnt and therefore it will end up becoming impossible for the NO.sub.x absorbent 17 to absorb the NO.sub.x. Accordingly, in the embodiment according to the present invention, when a lean air-fuel mixture continues to be burnt, the air-fuel mixture fed into the combustion chamber 3 is cyclically made rich and during this time the NO.sub.x is released from the NO.sub.x absorbent 17.

In this case, however, if the cycle at which the air-fuel mixture fed into the engine cylinder 3 is made rich is long, then the NO.sub.x absorbing capacity of the NO.sub.x absorbent 17 will end up becoming saturated during the time the lean air-fuel mixture is being burnt and therefore the NO.sub.x can no longer be absorbed in the NO.sub.x absorbent 17, so there will be the problem that NO.sub.x will end up being released into the atmosphere. As opposed to this, even if an engine operating state where a large amount of NO.sub.x is discharged from the engine continues, if the cycle at which the air-fuel mixture is made lean is shortened so that the NO.sub.x is released from the NO.sub.x absorbent 17 before the NO.sub.x absorbing capacity of the NO.sub.x absorbent 17 becomes saturated, then this time the problem will arise of an increase of the amount of fuel consumption.

Therefore, in the present invention, the amount of NO.sub.x which is absorbed in the NO.sub.x absorbent 17 is found and the air-fuel mixture is made rich when the amount of the NO.sub.x absorbed in the NO.sub.x absorbent 17 exceeds a predetermined allowable value. If the air-fuel mixture is made rich when the amount of the NO.sub.x absorbed in the NO.sub.x absorbent 17 exceeds a predetermined allowable value, then the NO.sub.x absorbing capacity of the NO.sub.x absorbent 17 will never become saturated, so the NO.sub.x will no longer be released into the atmosphere and, further, the frequency at which the air-fuel mixture is made rich can be reduced as well, so it is possible to suppress an increase in the amount of the fuel consumption.

However, when finding the amount of NO.sub.x being absorbed in the NO.sub.x absorbent 17, it is difficult to directly find the amount of NO.sub.x being absorbed in the NO.sub.x absorbent 17. Therefore, in the embodiment according to the present invention, the amount of the NO.sub.x absorbed in the NO.sub.x absorbent 17 is estimated from the amount of NO.sub.x in the exhaust gas discharged from the engine. That is, the higher the rotational speed N of the engine, the larger the amount of exhaust gas discharged per unit time from the engine, so as the engine rotational speed N becomes higher, the amount of NO.sub.x discharged from the engine per unit time increases. Further, the higher the engine load, that is, the higher the absolute pressure PM in the surge tank 10, the greater the amount of the exhaust gas discharged from the combustion chambers 3 and further the higher the combustion temperature, so the higher the engine load, that is, the higher the absolute pressure PM in the surge tank 10, the greater the amount of NO.sub.x discharged from the engine per unit time.

FIG. 6(A) shows the relationship between the amount of the NO.sub.x discharged from the engine per unit time, the absolute pressure PM in the surge tank 10, and the engine rotational speed N as found by experiments. In FIG. 6(A), the curves show the identical amounts of NO.sub.x. As shown in FIG. 6(A), the amount of NO.sub.x discharged from the engine per unit time becomes larger the higher the absolute pressure PM in the surge tank 10 and becomes larger the higher the engine rotational speed N. Note that the amount of NO.sub.x shown in FIG. 6(A) is stored in the ROM 32 in advance in the form of a map as shown in FIG. 6(B).

On the other hand, FIG. 7 shows the relationship between the absorption capacity NO.sub.x CAP which can be absorbed by the NO.sub.x absorbent 17 and the temperature T of the exhaust gas, which represents the temperature of the NO.sub.x absorbent 17. If the temperature of the NO.sub.x absorbent 17 becomes lower, that is, the temperature T of the exhaust gas becomes lower, the oxidation action of the NO.sub.x (2NO+O.sub.2 -2NO.sub.2) is weakened, and therefore the NO.sub.x absorption capacity NO.sub.x CAP is lowered. Further, if the temperature of the NO.sub.x absorbent 17 becomes higher, that is, the temperature T of the exhaust gas becomes higher, the NO.sub.x absorbed in the NO.sub.x absorbent 17 is decomposed and naturally released, so the NO.sub.x absorption capacity NO.sub.x CAP is lowered. Accordingly, the NO.sub.x absorption capacity NO.sub.x CAP becomes larger between about 300.degree. C. to about 500.degree. C.

On the other hand, FIG. 8 shows the results of experiments on the amount of NO.sub.x released from the NO.sub.x absorbent 17 when switching the air-fuel ratio of the exhaust gas flowing into the NO.sub.x absorbent 17 from lean to rich. Note that the solid line in FIG. 8 shows the state when the temperature of the NO.sub.x absorbent 17, that is, the temperature T of the exhaust gas, is high, while the broken line shows when the temperature of the NO.sub.x absorbent 17, that is, the temperature T of the exhaust gas, is low. The rate of decomposition of the NO.sub.x in the NO.sub.x absorbent 17 becomes faster the higher the temperature of the NO.sub.x absorbent 17. Therefore, when the temperature of the NO.sub.x absorbent 17 is high, as shown by the solid line in FIG. 8, that is, when the temperature T of the exhaust gas is high, a large amount of NO.sub.x is released from the NO.sub.x absorbent 17 in a short time, while when the temperature of the NO.sub.x absorbent 17, that is, the temperature T of the exhaust gas, is low, as shown by the broken line in FIG. 8, a small amount of NO.sub.x is continually released from the NO.sub.x absorbent 17 over a long period. That is, the higher the temperature T of the exhaust gas, the greater the amount of NO.sub.x released from the NO.sub.x absorbent 17 per unit time and the shorter the release time of the NO.sub.x.

When the amount of the unburnt HC and CO discharged from the engine, however, is smaller than the amount which can reduce the total NO.sub.x released from the NO.sub.x absorbent 17, part of the NO.sub.x is released into the atmosphere without being reduced, while when the amount of unburnt HC and CO discharged from the engine is greater than the amount able to reduce the total NO.sub.x released from the NO.sub.x absorbent 17, the excess unburnt HC and CO are released into the atmosphere. Accordingly, to prevent the NO.sub.x and the unburnt HC and CO from being released into the atmosphere, it is necessary to discharge exactly the amount of the unburnt HC and CO from the engine needed to reduce the NO.sub.x released from the NO.sub.x absorbent 17. Toward this end, it becomes necessary to increase the amount of the unburnt HC and CO in accordance with the curve shown in FIG. 8.

As mentioned earlier, however, the amount of the unburnt HC and CO discharged from the engine is proportional to the degree of richness of the air-fuel mixture fed into the combustion chamber 3. Therefore, in the present invention, as shown in FIG. 9, the value of the correction coefficient k with respect to the basic fuel injection time TP, that is, the degree of richness of the air-fuel mixture, is made to change in accordance with a pattern as close as possible to the pattern of change of the concentration of NO.sub.x shown in FIG. 8. Note that here, the correction coefficient k has the relationship K=1+k with the above-mentioned correction coefficient K and therefore when k=0, the air-fuel mixture becomes the stoichiometric air-fuel ratio while when k>0, the air-fuel mixture becomes rich. Therefore, it is understood from FIG. 9 that the higher the temperature of the NO.sub.x absorbent 17, the higher the degree of richness and the shorter the time the ratio is made rich.

As shown by the solid line in FIG. 9, when the NO.sub.x is to be released from the NO.sub.x absorbent 17, the correction coefficient k is made to rise by .alpha. increments with each passing of the unit time until the time C reaches C.sub.1. Next, when the time C is between C.sub.1 and C.sub.2, the correction coefficient k is held constant, then when the time C exceeds C.sub.2, the correction coefficient k is made to descend in .beta. decrements with each unit time. The values of these .alpha., .beta., C.sub.1, and C.sub.2 are set so that the pattern of change of the correction coefficient k becomes as close as possible to the pattern of change of the concentration of NO.sub.x shown by the solid line in FIG. 8.

On the other hand, the pattern of change of the correction coefficient k when the temperature of the NO.sub.x absorbent 17, that is, the temperature T of the exhaust gas, is low, is also set so that it becomes as close as possible to the pattern of change of the concentration of NO.sub.x when the temperature T of the exhaust gas is low, as shown by the broken line in FIG. 8. In this case, to make the pattern of change of the correction coefficient k in FIG. 9 like the broken line, it is understood that it is sufficient to make both .alpha. and .beta. smaller and make C.sub.1 and C.sub.2 larger. That is, to make the pattern of change of the correction coefficient k close to the pattern of change of the concentration of NO.sub.x shown in FIG. 8, it is sufficient to make .alpha. and .beta. larger and make C.sub.1 and C.sub.2 smaller as the temperature T of the exhaust gas becomes higher, as shown in FIG. 10. Note that the relationship between C.sub.1, C.sub.2 , .alpha., and .beta. and the temperature T of the exhaust gas shown in FIG. 10 is stored in advance in the ROM 32.

Note that in the embodiment according to the present invention, provision is made of a temperature sensor 25 for detecting the temperature T of the exhaust gas and accordingly the NO.sub.x absorption capacity NO.sub.x CAP shown in FIG. 7 and the .alpha., .beta., C.sub.1, and C.sub.2 shown in FIG. 10 are determined based on the temperature T of the exhaust gas detected by this temperature sensor 25. The temperature T of the exhaust gas, however, can be estimated from the absolute pressure PM in the surge tank 10 and the engine rotational speed N. Therefore, instead of providing the temperature sensor 25, it is possible to store the temperature T of the exhaust gas in the ROM 32 in advance in the form of a map as shown in FIG. 11 and determine the NO.sub.x absorption capacity NO.sub.x CAP and .alpha., .beta., C.sub.1, and C.sub.2 based on the temperature T of the exhaust gas obtained from this map.

Next, an explanation will be made of the first embodiment of control of the release of NO.sub.x with reference to FIG. 12 to FIG. 14.

FIG. 12 and FIG. 13 show a time interruption routine executed by interruption every predetermined time.

Referring to FIG. 12 and FIG. 13, first, at step 100, it is judged if a NO.sub.x release flag showing that the NO.sub.x should be released from the NO.sub.x absorbent 17 is set or not. When the NO.sub.x release flag is not set, the routine proceeds to step 101, where it is judged if the correction coefficient K is smaller than 1.0, that is, if the operating state is one in which the air-fuel mixture should be made lean. When K<1.0, that is, when the operating state is one in which the air-fuel mixture should be made lean, the routine proceeds to step 102, where the count D is made zero, then the routine proceeds to step 103.

At step 103, the NO.sub.x amount Nij discharged from the engine per unit time is calculated from the map shown in FIG. 6(B) based on the absolute pressure PM in the surge tank 10, detected by the pressure sensor 19, and the engine rotational speed N. Next, at step 104, the NO.sub.x amount Nij is multiplied by the interruption time interval At and the product Nij.multidot..DELTA.T is added to .SIGMA.NO.sub.x. The product Nij.multidot..DELTA.t shows the amount of the NO.sub.x discharged from the engine during the interruption time interval .DELTA.t. At this time, the NO.sub.x discharged from the engine is absorbed by the NO.sub.x absorbent 17, so .SIGMA.NO.sub.x shows the estimated value of the amount of NO.sub.x absorbed in the NO.sub.x absorbent 17.

Next, at step 105, the NO.sub.x absorption capacity NO.sub.x CAP is calculated from the relationship shown in FIG. 7 based on the temperature T of the exhaust gas detected by the temperature sensor 25. Next, at step 106, it is judged if the estimated value .SIGMA.NO.sub.x of the amount of NO.sub.x absorbed in the NO.sub.x absorbent 17 has exceeded the NO.sub.x absorption capacity NO.sub.x CAP. When .SIGMA.NO.sub.x .ltoreq.NO.sub.x CAP, the processing cycle is completed. At this time, a lean air-fuel mixture is burned and the NO.sub.x discharged from the engine is absorbed in the NO.sub.x absorbent 17.

On the other hand, if it is judged at step 106 that .SIGMA.NO.sub.x >NO.sub.x CAP, that is, the NO.sub.x absorption capacity of the NO.sub.x absorbent 17 is saturated, the routine proceeds to step 107, where the NO.sub.x release flag is set. Next, at step 108, C.sub.1, C.sub.2, .alpha., and .beta. are calculated from the relation shown in FIG. 10 based on the temperature T of the exhaust gas and the processing cycle is ended. If the NO.sub.x release flag is set, at the next processing cycle, the routine proceeds from step 100 to step 109, where the count C is incremented by one. Next, at step 110, it is judged if the count C is smaller than C.sub.1. When C<C.sub.1, the routine proceeds to step 111, where .alpha. is added to the correction coefficient k. Next, the processing cycle is ended. The action of addition of .alpha. to the correction coefficient k is performed continuously until C.gtoreq.C.sub.1. Accordingly, the value of the correction coefficient k during this time continues to increase as shown in FIG. 9.

On the other hand, if it is judged at step 110 that C.gtoreq.C.sub.1, the routine proceeds to step 112, where it is judged if the count C has become smaller than C.sub.2. When C<C.sub.2, the processing cycle is ended. Therefore, as shown in FIG. 9, the correction coefficient k is held constant until C.gtoreq.C.sub.2.

Next, at step 112, when it is judged that C.gtoreq.C.sub.2, the routine proceeds to step 113, where .beta. is subtracted from the correction coefficient k. Next, at step 113, it is judged if the correction coefficient k has become zero or a negative number. When k>0, the processing cycle is ended. Accordingly, as shown in FIG. 9, the correction coefficient k is reduced until k.ltoreq.0. Note that, as mentioned later, if k>0, the air-fuel mixture fed to the combustion chamber 3 is made rich and during this time the degree of richness is changed by the pattern shown in FIG. 9.

On the other hand, if it is judged at step 114 that K.ltoreq.0, the routine proceeds to step 115, where the NO.sub.x release flag is reset. Next, at step 116, .SIGMA.NO.sub.x is made zero. That is, at this time, it is considered that all of the NO.sub.x which had been absorbed in the NO.sub.x absorbent 17 is released, so the estimated value .SIGMA.NO.sub.x of the NO.sub.x absorbed in the NO.sub.x absorbent 17 is made zero. Next, at step 117, the count C and the correction coefficient k are made zero and the processing cycle is ended.

On the other hand, if it is judged at step 101 that k.gtoreq.1.0, that is, when the engine operating state is one in which the air-fuel mixture should be made rich or the stoichiometric air-fuel ratio, the routine proceeds to step 118, where the count D is incremented by one. Next, at step 119, it is judged if the count D has become larger than the constant value D.sub.0. When D>D.sub.0, the routine proceeds to step 120, where .SIGMA.NO.sub.x is made zero. That is, when the combustion of the rich air-fuel ratio or the stoichiometric air-fuel ratio continues for a certain time, it may be considered that all of the NO.sub.x has been released from the NO.sub.x absorbent 17, so at this time the estimated value .SIGMA.NO.sub.x of the amount of NO.sub.x absorbed in the NO.sub.x absorbent 17 is made zero.

FIG. 14 shows the routine for calculation of the fuel injection time TAU. This routine is repeatedly executed.

Referring to FIG. 14, first, at step 150, a basic fuel injection time TP is calculated from a map indicated in FIG. 2. Subsequently, at step 151, the correction coefficient K shown in FIG. 3, which is determined in accordance with the operating state of the engine, is calculated. Next, at step 152, it is judged if the NO.sub.x release flag is set or not. When the NO.sub.x release flag is not set, the routine proceeds to step 153, where the correction coefficient K is made K.sub.t. Next, at step 155, K.sub.t is multiplied with the basic fuel injection time TP, whereby the fuel injection time TAU is calculated. Accordingly, at this time, the air-fuel mixture which is fed into the combustion chamber 3 is made lean, the stoichiometric air-fuel ratio, or rich in accordance with the operating state of the engine as shown in FIG. 3.

On the other hand, when it is judged at step 152 that the NO.sub.x release flag is set, the routine proceeds to step 154, where K.sub.t is made the sum (k+1) of correction coefficient k calculated by the routine shown in FIG. 12 and FIG. 13 and 1, then the routine proceeds to step 155. Next, at this time, the air-fuel mixture fed to the combustion chamber 3 is made rich, then the degree of richness is changed by the pattern shown in FIG. 9.

FIG. 15 shows another embodiment of the internal combustion engine. In this embodiment, an outlet side of a casing 18 is connected through an exhaust pipe 22 to a catalytic converter 24 housing a three-way catalyst 23. This three-way catalyst 23, as is well known, exhibits a high purification efficiency with respect to CO, HC, and NO.sub.x when the air-fuel ratio is maintained near the stoichiometric air-fuel ratio, but the three-way catalyst 23 also has a high purification efficiency with respect to NO.sub.x even when the air-fuel ratio becomes rich to a certain extent. In the embodiment shown in FIG. 15, a three-way catalyst 23 is provided downstream of the NO.sub.x absorbent 17 so as to remove the NO.sub.x using this characteristic.

That is, as is mentioned above, if the air-fuel mixture fed into the engine cylinder is made rich to release the NO.sub.x from the NO.sub.x absorbent 17, the NO.sub.x absorbed in the NO.sub.x absorbent 17 is rapidly released from the NO.sub.x absorbent 17. At this time, the NO.sub.x is reduced during its release, but there is a possibility that all of the NO.sub.x will not be reduced. If the three-way catalyst 23 is disposed downstream of the NO.sub.x absorbent 17, however, the NO.sub.x which could not be reduced at the time of the release of the NO.sub.x is reduced by the three-way catalyst 23. Accordingly, by disposing the three-way catalyst 23 downstream of the NO.sub.x absorbent 17, it becomes possible to improve considerably the purification performance of the NO.sub.x.

In the embodiments discussed up to here, use was made, as the NO.sub.x absorbent, of a NO.sub.x absorbent 17 comprised of at least one of an alkali metal, alkali earth, and rare earth and a precious metal carried on alumina. Instead of using such a NO.sub.x absorbent 17, however, it is also possible to use a complex oxide of an alkali earth and copper, that is, a NO.sub.x absorbent of the Ba--Cu--O system. As such a complex oxide of an alkali earth and copper, use may be made for example of MnO.sub.2.BaCuO.sub.2. In this case, it is also possible to add platinum Pt or cerium Ce. In a NO.sub.x absorbent of the MnO.sub.2.BaCuO.sub.2 system, the copper Cu performs the same catalytic function as the platinum Pt in the NO.sub.x absorbent 17 spoken of up to now. When the air-fuel ratio is lean, the NO.sub.x is oxidized by the copper (2NO+O.sub.2 .fwdarw.2NO.sub.2) and dispersed in the absorbent in the form of nitric acid ions NO.sub.3.sup.- .

On the other hand, if the air-fuel ratio is rich, similarly, NO.sub.x is released from the absorbent. This NO.sub.x is reduced by the catalytic action of the copper Cu. The NO.sub.x reducing ability of copper Cu, however, is weaker than the NO.sub.x reducing ability of platinum Pt and therefore when using an absorbent of the Ba--Cu--O system, the amount of NO.sub.x which is not reduced at the time of release of the NO.sub.x becomes somewhat greater than with the NO.sub.x absorbent 17 discussed up to now. Therefore, when using an absorbent of the Ba--Cu--O system, as shown in FIG. 21, it is preferable to dispose a three-way catalyst 23 downstream of the absorbent.