Device for Purifying Exhaust Gas of Engine

1. Field of the Invention

The present invention relates to a device for purifying the exhaust gas of an engine.

2. Description of the Related Art

If a ratio of the total amount of air fed to the intake passage, the combustion chamber, and the exhaust passage upstream of a certain position in the exhaust passage to the total amount of fuel fed to the intake passage, the combustion chamber, and the exhaust passage upstream of the above-mentioned position, is referred to as an air-fuel ratio of the exhaust gas flowing through the certain position, it is well known that an engine, in which the lean air-fuel mixture is burned, has a NO.sub.X absorbent arranged in the exhaust passage, the NO.sub.X absorbent absorbing NO.sub.X therein when the air-fuel ratio of the inflowing exhaust gas is lean, and releasing the absorbed NO.sub.X therefrom when the oxygen concentration in the inflowing exhaust gas becomes lower. In the engine, the air-fuel ratio of the exhaust gas flowing the NO.sub.X absorbent is made rich temporarily to thereby release the absorbed NO.sub.X from the NO.sub.X absorbent and reduce the NO.sub.X.

However, fuel and the lubrication oil contain sulphur containing components, and thus the exhaust gas also contains sulphur containing components, such as SO.sub.X. The NO.sub.X absorbent absorbs the SO.sub.X in the form of SO.sub.4.sup.2-, together with NO.sub.X. However, the SO.sub.X is not released from the NO.sub.X absorbent even when the air-fuel ratio of the inflowing exhaust gas is made rich. Thus, the amount of SO.sub.X absorbed in the NO.sub.X absorbent increases gradually. However, if the SO.sub.X amount in the NO.sub.X absorbent increases, the NO.sub.X absorbing capacity of the NO.sub.X absorbent gradually becomes smaller, and at the last, the NO.sub.X absorbent can hardly absorb NO.sub.X therein.

However, the NO.sub.X absorbent releases the absorbed SO.sub.X therefrom in the form of SO.sub.2, for example, when the oxygen concentration in the inflowing exhaust gas becomes lower with the temperature of the NO.sub.X absorbent being higher than the SO.sub.X releasing temperature thereof. Japanese Unexamined Patent Publication No. 6-88518 discloses an exhaust gas purifying device for an engine in which the air-fuel ratio of the exhaust gas flowing to the NO.sub.X absorbent is made rich temporarily when the temperature of the NO.sub.X absorbent is higher than the SO.sub.X releasing temperature.

The device mentioned above does not include a device for heating the NO.sub.X absorbent, such as an electric heater. Thus, the temperature of the NO.sub.X absorbent is only higher than the SO.sub.X releasing temperature when the engine load is high, for example. However, at the high load engine operation, the flow rate of the exhaust gas flowing through the NO.sub.X absorbent is high, i.e., the contact period between the exhaust gas and the NO.sub.X absorbent is short. However, the SO.sub.X releasing rate of the NO.sub.X absorbent is relatively low, and thus SO.sub.X is not released from the NO.sub.X absorbent sufficiently, even when the air-fuel ratio of the inflowing exhaust gas is made rich with the temperature of the NO.sub.X absorbent is higher than the SO.sub.X releasing temperature, as long as the contact period is short. Namely, the air-fuel ratio of the exhaust gas flowing to the NO.sub.X absorbent must be made rich for a long time to release the SO.sub.X from the NO.sub.X absorbent sufficiently, if the contact period is short.

An object of the present invention is to provide a device for purifying an exhaust gas of an engine capable of releasing the absorbed sulphur containing components from the sulphur containing components absorbent quickly and sufficiently.

According to the present invention, there is provided a device for purifying the exhaust gas of an engine having an exhaust passage, comprising: a sulphur containing components absorbent arranged in the exhaust passage, the sulphur containing components absorbent absorbing the sulphur containing components therein when the air-fuel ratio of the inflowing exhaust gas is lean, and releasing the absorbed sulphur containing components therefrom when the oxygen concentration in the inflowing exhaust gas becomes lower with the temperature of the sulphur containing components absorbent being higher than a sulphur containing components releasing temperature of the sulphur containing components absorbent; and releasing means for making the air-fuel ratio of the exhaust gas flowing to the sulphur containing components absorbent stoichiometric or rich temporarily, when the temperature of the sulphur containing components absorbent is higher than the sulphur containing components releasing temperature and when the flow rate of the exhaust gas flowing through the sulphur containing components absorbent is lower than a predetermined flow rate, to release the absorbed sulphur containing components from the sulphur containing components absorbent.

The present invention may be more fully understood from the description of the preferred embodiments of the invention as set forth below, together with the accompanying drawings.

In the drawings:

FIG. 1 is a general view of an engine;

FIG. 2 is a diagram illustrating the basic fuel injection time;

FIG. 3 is a diagram schematically illustrating the concentration of the unburned HC, CO, and oxygen in the exhaust gas from the engine;

FIGS. 4A and 4B illustrate the NO.sub.X absorbing and releasing function of the NO.sub.X absorbent;

FIG. 5 shows a flowchart for controlling the SO.sub.X releasing operation;

FIG. 6 is a diagram illustrating the NO.sub.X absorbent temperature TEXN;

FIGS. 7A and 7B are diagrams illustrating the flow rate SVN;

FIG. 8 shows a flowchart for controlling the NO.sub.X releasing operation;

FIGS. 9A and 9B are diagrams illustrating the inflowing NO.sub.X amount FN;

FIGS. 10A and 10B are diagrams illustrating the released NO.sub.X amount DN;

FIG. 11 shows a flowchart for calculating the fuel injection time TAU;

FIG. 12 is a diagram illustrating relationships between the amount of SO.sub.X released from the NO.sub.X absorbent and the flow rate;

FIG. 13 is a diagram illustrating relationships between the amount of NO.sub.X released from the NO.sub.X absorbent and the flow rate;

FIG. 14 is a general view of an engine according to another embodiment of the present invention;

FIG. 15 is a diagram illustrating the opening VS of the exhaust gas control valve;

FIG. 16 is a diagram illustrating the amount of fuel to be injected secondarily for executing the SO.sub.X releasing operation;

FIG. 17 is a diagram illustrating the amount of fuel to be injected secondarily for executing the NO.sub.X releasing operation;

FIGS. 18 and 19 show a flowchart for controlling the SO.sub.X releasing operation according to the embodiment of FIG. 14;

FIG. 20 shows a flowchart for controlling the NO.sub.X releasing operation according to the embodiment of FIG. 14;

FIG. 21 is a general view of an engine according to another embodiment of the present invention; and

FIGS. 22 and 23 show a flowchart for controlling the SO.sub.X releasing operation according to the embodiment of FIG. 21.

FIG. 1 illustrates a case where the present invention is applied to a spark-ignition engine.

Referring to FIG. 1, a reference numeral 1 designates the engine body, 2 designates a piston, 3 designates a combustion chamber, 4 designates a spark plug, 5 designates an intake valve, 6 designates an intake port, 7 designates an exhaust valve, and 8 designates an exhaust port. The intake ports 6 of each cylinder are connected to a common surge tank 10 via corresponding branches 9. A fuel injector injecting fuel to the corresponding intake ports 6 is arranged in each branch 9. The surge tank 10 is connected to an air cleaner 13 via an intake duct 12. A throttle valve 14 is disposed in the intake duct 12. On the other hand, the exhaust ports 8 of each cylinder are connected to a casing 17 housing a NO.sub.X absorbent 16 therein, via an exhaust manifold 15.

The electronic control unit (ECU) 30 is constructed as a digital computer and comprises a read-only memory (ROM) 32, a random-access memory (RAM) 33, a backup RAM 33a to which the electric power is always supplied, the CPU (micro processor) 34, an input port 35, and an output port 36, which are interconnected with each other via a bidirectional bus 31. A pressure sensor 37 generating an output voltage in proportion to the pressure in the surge tank 10 is arranged in the surge tank 10. A water temperature sensor 38 generating an output voltage in proportion to the temperature of the engine cooling water is attached to the engine body 1. The output voltages of the sensors 37 and 38 are input to the input port 35 via corresponding AD converters 39, respectively. The input port 35 is connected to a crank angle sensor 40, which generates a pulse whenever a crankshaft is turned by, for example, 30 degrees. The CPU 34 calculates the intake air amount according to the output voltages from the pressure sensor 37, and calculates the engine speed N according to the pulses from the crank angle sensor 40. The output port 36 is connected to the spark plugs 4 and the fuel injectors 11 via corresponding drive circuits 41, respectively.

In the engine shown in FIG. 1, the fuel injection time TAU is calculated on the basis of the following equation, for example, that is:

where TP and K represent a basic fuel injection time and a correction coefficient, respectively. The basic fuel injection time TP is a fuel injection time required to make the air-fuel ratio of the air-fuel mixture to be fed to the combustion chamber 3 equal to the stoichiometric air-fuel ratio. The basic fuel injection time TP is obtained in advance by experiment, and is stored in the ROM 32 in advance as a function of the engine load Q/N (the intake air amount Q/the engine speed N), in the form of a map as shown in FIG. 2. The correction coefficient K is for controlling the air-fuel ratio of the air-fuel mixture to be fed to the combustion chamber 3. If K=1.0, the air-fuel of the air-fuel mixture to be fed to the combustion chamber 3 is made stoichiometric. If K<1.0, the air-fuel ratio is made larger than the stoichiometric air-fuel ratio, i.e., is made lean. If K>1.0, the air-fuel ratio is made smaller than the stoichiometric air-fuel ratio, i.e., is made rich. In the engine shown in FIG. 1, the correction coefficient K is usually made smaller than 1.0, such as 0.6. Namely, the air-fuel ratio of the air-fuel mixture to be fed to the combustion chamber 3 is usually made lean, and thus the lean air-fuel mixture is usually burned in the combustion chamber 3.

FIG. 3 schematically illustrates the concentration of the representative component in the exhaust gas discharged from the combustion chamber 3. As can be seen from FIG. 3, the amount of the unburned HC and CO in the exhaust gas from the combustion chamber 3 becomes larger as the air-fuel ratio of the air-fuel mixture to be fed to the engine becomes richer, and the amount of oxygen O.sub.2 in the exhaust gas from the combustion chamber 3 becomes larger as the air-fuel ratio of the air-fuel mixture to be fed to the engine becomes leaner.

The NO.sub.X absorbent 16 housed in the casing 17 is comprised of at least one substance selected from alkali metals such as potassium K, sodium Na, lithium Li, and cesium Cs, alkali earth metals such as barium Ba and calcium Ca, rare earth metals such as lanthanum La and yttrium Y, and of precious metals such platinum Pt, which are carried on a carrier such as alumina. The NO.sub.X absorbent 16 performs NO.sub.X absorbing and releasing functions in which the NO.sub.X absorbent 16 absorbs NO.sub.X therein when the air-fuel ratio of the inflowing exhaust gas is lean, and releases the absorbed NO.sub.X therefrom when the oxygen concentration in the inflowing exhaust gas becomes lower. Note that, in a case where no fuel or air is fed to the exhaust passage upstream of the NO.sub.X absorbent 16, the air-fuel ratio of the exhaust gas flowing to the NO.sub.X absorbent 16 conforms to that of the air-fuel mixture to be fed to the combustion chamber 3. Accordingly, the NO.sub.X absorbent 16 absorbs NO.sub.X therein when the air-fuel ratio of the air-fuel mixture to be fed to the combustion chamber 3 is lean, and releases the absorbed NO.sub.X when the oxygen concentration in the air-fuel mixture to be fed to the combustion chamber 3 becomes lower.

When the NO.sub.X absorbent 16 is disposed in the exhaust passage of the engine, the NO.sub.X absorbent 16 actually performs the NO.sub.X absorbing and releasing function, but the function is unclear. However, it is considered that the function is performed according to the mechanism shown in FIGS. 4A and 4B. This mechanism will be explained by using as an example the 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 metal, or rare earth metal is used.

Namely, when the inflowing exhaust gas becomes considerably lean, the oxygen concentration in the inflowing exhaust gas greatly increases, and as shown in FIG. 4A, 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, 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 is absorbed into the absorbent. While bonding with barium oxide BaO, it is diffused in the absorbent in the form of nitric acid ions NO.sub.3.sup.-, as shown in FIG. 4A. In this way, NO.sub.X is absorbed in the NO.sub.X absorbent 16.

As long as the oxygen concentration in the inflowing exhaust gas is high, NO.sub.2 is produced on the surface of the platinum Pt, and as long as the NO.sub.X absorbing capacity of the absorbent is not saturated, NO.sub.2 is absorbed in the absorbent and the nitric acid ions N.sub.3.sup.- are produced. Contrarily, when the oxygen concentration in the inflowing exhaust gas becomes lower 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 is released in the form of NO.sub.2 from the absorbent. Namely, when the oxygen concentration in the inflowing exhaust gas becomes lower, NO.sub.X is released from the NO.sub.X absorbent 16. As shown in FIG. 3, when the degree of leanness of the inflowing exhaust gas becomes low, the oxygen concentration in the inflowing exhaust gas is lowered, and thus NO.sub.X is released from the NO.sub.X absorbent 16 when the degree of leanness of the inflowing exhaust gas is lowered.

On the other hand, if the air-fuel ratio of the inflowing exhaust gas is made rich at this time, a large amount of unburned HC and CO are discharged from the engine, as shown in FIG. 3. The unburned HC and the CO react with oxygen O.sub.2.sup.- or O.sup.2- on the surface of platinum Pt, and are oxidized. Also, when the air-fuel ratio of the inflowing exhaust gas is made rich, the oxygen concentration in the inflowing exhaust gas is extremely lowered. Thus, NO.sub.2 is released from the absorbent and the NO.sub.2 reacts with unburned HC and CO and is reduced as shown in FIG. 4B. In this way, when no NO.sub.2 exists on the surface of platinum Pt, NO.sub.2 is released from the absorbent successively. Therefore, when the air-fuel ratio of the inflowing exhaust gas is rich, NO.sub.X is released from the NO.sub.X absorbent 16 in a short time.

In this way, NO.sub.X is absorbed in the NO.sub.X absorbent 16 when the air-fuel ratio of the inflowing exhaust gas is lean, and NO.sub.X is released from the NO.sub.X absorbent 16 in a short time when the air-fuel ratio of the inflowing exhaust gas is rich. Therefore, in the engine shown in FIG. 1, when an amount of NO.sub.X absorbed in the NO.sub.X absorbent 16 becomes larger than a constant amount, the air-fuel ratio of the air-fuel mixture fed to the combustion chamber 3 is made temporarily rich to release NO.sub.X from the NO.sub.X absorbent 16 and to reduce the NO.sub.X.

However, the exhaust gas contains sulphur containing components, and thus the NO.sub.X absorbent absorbs not only NO.sub.X, but also sulphur containing components such as SO.sub.X. It is considered that the absorption mechanism of SO.sub.X into the NO.sub.X absorbent 16 is same as that of NO.sub.X.

Namely, when explaining the mechanism by taking an example in which platinum Pt and barium Ba are carried on the carrier, as in the explanation of the NO.sub.X absorption mechanism, oxygen O.sub.2 is deposited on the surface of platinum Pt, in the form of O.sub.2.sup.- or O.sup.2-, when the air-fuel ratio of the inflowing exhaust gas is lean, as mentioned above. SO.sub.X, such as SO.sub.2, in the inflowing exhaust gas reacts with O.sub.2.sup.- or O.sup.2- on the surface of platinum Pt and becomes SO.sub.3. The produced SO.sub.3 is then further oxidized on the platinum Pt and is absorbed into the absorbent. While bonding with barium oxide BaO, it is diffused in the absorbent in the form of sulphuric acid ions SO.sub.4.sup.2-. The sulphuric acid ions SO.sub.4.sup.2- bond with barium ions Ba.sup.2+ to produce sulphate BaSO.sub.4.

However, the sulphate BaSO.sub.4 is difficult to decompose and, if the air-fuel ratio of the inflowing exhaust gas is simply made rich, the sulphate BaSO.sub.4 remains as it is without being decomposed. Accordingly, as the time is elapsed, the amount of the sulphate BaSO.sub.4 in the NO.sub.X absorbent 16 increases, and thus the amount of NO.sub.X that can be absorbed in the NO.sub.X absorbent 16 will be lowered.

However, when the temperature of the NO.sub.X absorbent 16 is higher than the SO.sub.X releasing temperature of the NO.sub.X absorbent 16, the sulphate BaSO.sub.4 produced in the NO.sub.X absorbent 16 can be decomposed by making the air-fuel ratio of the inflowing exhaust gas rich or stoichiometric, and thus the sulphuric acid ions SO.sub.4.sup.2- are released from the absorbent in the form of SO.sub.3. Therefore, in the present embodiment, the air-fuel ratio of the exhaust gas flowing to the NO.sub.X absorbent 16 is made rich or stoichiometric temporarily when the temperature of the NO.sub.X absorbent 16 is higher than the SO.sub.X releasing temperature, to thereby release SO.sub.X from the NO.sub.X absorbent 16. The released SO.sub.3 is reduced to SO.sub.2 immediately by the unburned HC and CO in the inflowing exhaust gas.

In this way, in the present embodiment, the sulphurous component absorbent is formed by the NO.sub.X absorbent 16. Note that it is decided whether the air-fuel ratio of the exhaust gas flowing to the NO.sub.X absorbent 16 is made rich or stoichiometric when SO.sub.X should be released from the NO.sub.X absorbent 16, on the basis of an amount of SO.sub.X to be released from the NO.sub.X absorbent 16 per unit time.

In a case where no heating device, such as an electric heater, is provided for heating the exhaust gas flowing to the NO.sub.X absorbent 16 or the NO.sub.X absorbent 16 directly, as in the engine shown in FIG. 1, the temperature of the NO.sub.X absorbent 16 becomes higher than the SO.sub.X releasing temperature when the engine load is high. However, when the engine load is high, the flow rate of the exhaust gas flowing through the NO.sub.X absorbent 16 is high, and the contact period between the exhaust gas and the NO.sub.X absorbent 16 is short when the engine load is high. However, the SO.sub.X releasing rate of the NO.sub.X absorbent 16 is relatively low, and thus SO.sub.X is not released from the NO.sub.X absorbent sufficiently, even when the air-fuel ratio of the inflowing exhaust gas is made rich, with a condition where the contact period is short. Namely, the air-fuel ratio of the exhaust gas flowing to the NO.sub.X absorbent 16 must be made rich for a long time, or the degree of richness of the exhaust gas flowing to the NO.sub.X absorbent must be larger, to release SO.sub.X from the NO.sub.X absorbent 16 sufficiently, if the contact period is short.

Therefore, in the engine shown in FIG. 1, the air-fuel ratio of the exhaust gas flowing to the NO.sub.X absorbent 16 is made rich temporarily to thereby release S.sub.X from the NO.sub.X absorbent 16, when the flow rate SVN of the exhaust gas flowing through the NO.sub.X absorbent 16 is lower than a predetermined flow rate SVN1, i.e., when the contact period between the exhaust gas and the NO.sub.X absorbent 16 is longer than a period required to release SO.sub.X from the NO.sub.X absorbent 16 sufficiently. In other words, the air-fuel ratio of the exhaust gas flowing to the NO.sub.X absorbent 16 is temporarily made rich when the temperature of the NO.sub.X absorbent 16 is higher than the S.sub.X releasing temperature and the flow rate SVN is lower than the predetermined flow rate SVN1. When the flow rate of the exhaust gas flowing through the NO.sub.X absorbent 16 becomes lower, the contact period between the exhaust passage and the NO.sub.X absorbent 16 becomes longer, and thus the residence time of the exhaust gas in the NO.sub.X absorbent 16 becomes longer. Thus, the exhaust gas is effectively used for releasing SO.sub.X. As a result, a period during which the air-fuel ratio of the exhaust gas flowing to the NO.sub.X absorbent 16 must be made rich can be made shorter, or the degree of richness of the exhaust gas flowing to the NO.sub.X absorbent 16 is kept lower. Note that the temperature of the NO.sub.X absorbent 16 is higher than the SO.sub.X releasing temperature and the flow rate SVN is lower than the predetermined flow rate SVN1, at the engine low load operation just after the engine high load operation, for example.

The inventors of the present invention have found that the sulphate BaSO.sub.4 decomposes relatively easily and is released from the NO.sub.X absorbent 16 if the exhaust gas in the NO.sub.X absorbent 16 contains CO or H.sub.2, and that it is released more easily as the amount of CO or H.sub.2 becomes larger. On the other hand, the exhaust gas which is obtained when the rich air-fuel mixture is burned with the flow rate being low, contains CO and unburned HC of high concentration, as shown in Table 1, and CO and H.sub.2 are produced by oxidation of the unburned HC by oxygen O.sub.2 and NO.sub.X. Namely, the concentration of CO and H.sub.2 in the NO.sub.X absorbent 16 is relatively high when the rich air-fuel mixture is burned with the flow rate of the exhaust gas is low. This is due to the following reasons. When the flow rate of the exhaust gas is high, the exhaust gas discharged from the combustion chamber 3 flows through the exhaust passage upstream of the NO.sub.X absorbent 16 while the temperature thereof is kept high. Thus, the oxidizing reaction of CO and unburned HC occurs in the exhaust passage upstream of the NO.sub.X absorbent 16, and therefore the concentration of CO and unburned HC in the exhaust gas flowing to the NO.sub.X absorbent 16 is lowered. Contrarily, when the flow rate is low, the temperature of the exhaust gas drops quickly when it is discharged from the combustion chamber 3. Thus, the unburned HC and CO reach the NO.sub.X absorbent 16 without being oxidized. Namely, the exhaust gas flows into the NO.sub.X absorbent 16, while the concentration of the unburned HC and CO is kept high. Therefore, in the engine shown in FIG. 1, the air-fuel ratio of the air-fuel mixture to be fed to the combustion chamber 3 is made rich, and the air-fuel mixture is ignited by the spark plug 4 and is burned, when the air-fuel ratio of the exhaust gas flowing to the NO.sub.X absorbent 16 must be made rich. Further, the rich air-fuel mixture is burned with the flow rate of the exhaust gas being low, when SO.sub.X must be released from the NO.sub.X absorbent 16.

Note that, in Table 1, the engine speed is 2,800 r.p.m. and the air-fuel ratio of the air-fuel mixture burned in the combustion chamber 3 is 13.0, in each case.

On the other hand, the air-fuel ratio of the exhaust gas flowing to the NO.sub.X absorbent 16 can be made rich by secondarily feeding fuel (gasoline), for example, to the exhaust manifold 15 while the lean air-fuel ratio is burned. However, in this case, the fuel flowing to the NO.sub.X absorbent 16 is a higher hydrocarbon of which the molecular weight is large, and thus CO and H.sub.2 are not produced easily. Contrarily, when the rich air-fuel mixture is burned, the unburned HC flowing to the NO.sub.X absorbent 16 is a lower hydrocarbon of which the molecular weight is small, i.e., a hydrocarbon which is partially oxidized, and thus CO and H.sub.2 are produced easily. Therefore, the combustion of the rich air-fuel mixture is preferable to the secondary feeding of fuel to the exhaust manifold 15, to release the absorbed SO.sub.X from the NO.sub.X absorbent 16 sufficiently. Thus, in the engine shown in FIG. 1, the rich air-fuel mixture is burned to make the air-fuel ratio of the exhaust gas flowing to the NO.sub.X absorbent 16.

FIG. 12 illustrates experimental results showing the relationships between the flow rate SVN and the amount of SO.sub.X released from the NO.sub.X absorbent 16, and FIG. 13 illustrates experimental results showing the relationships between the flow rate SVN and the amount of NO.sub.X released from the NO.sub.X absorbent 16. In FIGS. 12 and 13, the SO.sub.X amount and the NO.sub.X amount when SVN=10,000 (h.sup.-1) are made 1.0, respectively. As can be seen from FIG. 12, the amount of SO.sub.X released from the NO.sub.X absorbent 16 becomes larger, when the flow rate SVN becomes lower. Contrarily, as shown in FIG. 13, the amount of NO.sub.X released from the NO.sub.X absorbent 16 does not vary widely, even though the flow rate SVN varies. This is because the decomposition rate of nitrate is sufficiently high. In other words, the decomposition rate of sulphate considerably low, and thus the released SO.sub.X amount becomes low when the flow rate of the exhaust gas becomes high.

Next, the control of the SO.sub.X releasing operation in the engine shown in FIG. 1 will be explained, in more detail, with reference to FIG. 5. The routine shown in FIG. 5 is executed by interruption every predetermined time.

Referring to FIG. 5, first, in step 50, the temperature TEXN of the exhaust gas flowing to the NO.sub.X absorbent 16 is calculated using the map shown in FIG. 6. The temperature TEXN represents the temperature of the NO.sub.X absorbent 16, and thus TEXN is referred to as a NO.sub.X absorbent temperature, hereinafter. To obtain the NO.sub.X absorbent temperature TEXN, a temperature sensor may be arranged in the inlet of the NO.sub.X absorbent 16, but TEXN can be obtained on the basis of the engine operating condition. Thus, in the engine shown in FIG. 1, the NO.sub.X absorbent temperature TEXN is obtained by experiment in advance, as a function of the engine load Q/N and the engine speed N, and is calculated on the basis of the engine load Q/N and the engine speed N. The NO.sub.X absorbent temperature TEXN is stored in the ROM 32 in advance in the form of the map shown in FIG. 6.

In the following step 51, it is judged whether the NO.sub.X absorbent temperature TEXN is higher than the SO.sub.X releasing temperature TEXN1 of the NO.sub.X absorbent 16, such as 500.degree. C. When TEXN>TEXN1, the routine goes to step 52, where the flow rate SVN of the exhaust gas flowing through the NO.sub.X absorbent 16. To obtain the flow rate SVN, a flow rate sensor may be arranged in the inlet of the NO.sub.X absorbent 16, but SVN can be obtained on the basis of the engine operating condition. Namely, as shown in FIG. 7A in which each curve shows the identical flow rate, the flow rate SVN becomes higher as the engine load Q/N becomes higher, and becomes higher as the engine speed N becomes higher. Thus, in the engine shown in FIG. 1, the flow rate SVN is obtained by experiment in advance, as a function of the engine load Q/N and the engine speed N, and is calculated on the basis of the engine load Q/N and the engine speed N. The flow rate SVN is stored in the ROM 32, in advance, in the form of the map shown in FIG. 7B.

In the following step 53, it is judged whether the flow rate SVN is higher than the predetermined flow rate SVN1, i.e., whether the contact period between the exhaust gas and the NO.sub.X absorbent 16 is longer than a period required to release SO.sub.X from the NO.sub.X absorbent 16 sufficiently. When SVN<SVN1, it is judged that the contact period is enough long for the good SO.sub.X releasing operation, and the routine goes to step 54, where a SO.sub.X release flag is set. The SO.sub.X release flag is set when SO.sub.X is released from the NO.sub.X absorbent 16, and is reset when the SO.sub.X releasing operation is not in process. Namely, when TEXN>TEXN1 and SVN<SVN1, the SO.sub.X release flag is set. When the SO.sub.X flag is set, the air-fuel ratio of the air-fuel mixture to be burned in the combustion chamber 3 is made rich, as explained later. In the following step 55, the counter value CS, which represents a time during which the SO.sub.X releasing operation is in process, is incremented by 1. In the following step 56, it is judged whether the counter value CS is larger than a constant CS1, i.e., whether the SO.sub.X releasing operation is performed for a constant time. When CS.ltoreq.CS1, the processing cycle is ended.

Contrarily, when CS>CS1, i.e., when the SO.sub.X releasing operation is performed for the constant time, the routine goes to step 57, where the SO.sub.X release flag is reset. In the following step 58, the counter value CS is cleared. Then, the processing cycle is ended.

Contrarily, when TEXN.ltoreq.TEXN1 in step 51, or when SVN.gtoreq.SVN1 in step 53, the routine goes to step 57, where the SO.sub.X release flag is reset. Thus, the SO.sub.X releasing operation is stopped.

Next, the control of the NO.sub.X releasing operation in the engine shown in FIG. 1 will be explained in more detail, with reference to FIG. 8. The routine shown in FIG. 8 is executed by interruption every predetermined time.

Referring to FIG. 8, first, in step 60, it is judged whether the SO.sub.X release flag, which is set or reset in the routine shown in FIG. 5, is set. When the SO.sub.X release flag is reset, the routine goes to step 61, where it is judged whether a NO.sub.X release flag is set. The NO.sub.X release flag is set when NO.sub.X is released from the NO.sub.X absorbent 16 and is reduced, and is reset when the NO.sub.X releasing operation is not in process. When the NO.sub.X release flag is reset, i.e., when both of the SO.sub.X release flag and the NO.sub.X release flag are reset, the routine goes to step 62. When both of the SO.sub.X release flag and the NO.sub.X release flag are reset, the air-fuel ratio of the exhaust gas flowing to the NO.sub.X absorbent 16 is made lean, as explained later, and thus the NO.sub.X absorbing operation is in process in the NO.sub.X absorbent 16.

The steps 62 and 63 are for obtaining the amount SN of NO.sub.X absorbed in the NO.sub.X absorbent 16. It is difficult to obtain the absorbed NO.sub.X amount SN directly, and thus the absorbed NO.sub.X amount SN is estimated on the basis of the amount of NO.sub.X discharged from the engine 1, i.e., the engine operating condition, in the engine shown in FIG. 1. Namely, in step 62, an amount FN of NO.sub.X flowing to the NO.sub.X absorbent 16 per unit time is calculated. As shown in FIG. 9A in which each curve shows the identical inflowing NO.sub.X amount, the inflowing NO.sub.X amount FN becomes larger as the engine load Q/N becomes higher, and becomes larger as the engine speed N becomes higher. Thus, in the engine shown in FIG. 1, the inflowing NO.sub.X amount FN is obtained, by experiment in advance, as a function of the engine load Q/N and the engine speed N, and is calculated on the basis of the engine load Q/N and the engine speed N. The inf lowing NO.sub.X amount FN is stored in the ROM 32 in advance in the form of the map shown in FIG. 9B. In the following step 63, the absorbed NO.sub.X amount SN is calculated on the basis of the following equation.

where DLT represents a period from the last processing cycle to the present processing cycle, and thus the FN.multidot.DLT represents the amount of NO.sub.X absorbed in the NO.sub.X amount from the last processing cycle to the present processing cycle. In the following step 64, it is judged whether the absorbed NO.sub.X amount SN is larger than a predetermined amount SN1. The predetermined amount SN1 corresponds to about 30% of the maximum NO.sub.X amount which the NO.sub.X absorbent 16 can absorb therein, for example. When SN.ltoreq.SN1, the processing cycle is ended. When SN>SN1, the routine goes to step 65, where the NO.sub.X release flag is set. In the following step 66, the absorbed NO.sub.X amount SN, when the NO.sub.X release flag is set, is memorized as an initial absorbed amount SNI.

When the NO.sub.X release flag is set, the routine goes from step 61 to step 67. When the NO.sub.X release flag is set, the air-fuel ratio of the exhaust gas flowing to the NO.sub.X absorbent 16 is made rich, as explained later, and thus the NO.sub.X releasing operation is in process in the NO.sub.X absorbent 16. In step 67, the amount DN of NO.sub.X released from the NO.sub.X absorbent 16 per unit initial absorbed NO.sub.X amount and per unit time is calculated.

FIGS. 10A and 10B illustrate experimental results showing the amount of NO.sub.X released from the NO.sub.X absorbent 16 per unit time and per unit initial absorbed NO.sub.X amount, when the air-fuel ratio of the exhaust gas flowing to the NO.sub.X absorbent 16 is made rich. In FIG. 10A, the solid line represents the case where the NO.sub.X absorbent temperature TEXN is high, and the dotted line represents the case where the NO.sub.X absorbent temperature TEXN is low. Further, in FIG. 10A, t represents a time from when the air-fuel ratio of the exhaust gas flowing to the NO.sub.X absorbent 16 is made rich. When the NO.sub.X absorbent temperature TEXN becomes high, the decomposition rate of nitrate in the NO.sub.X absorbent 16 becomes high. Thus, as shown in FIG. 10A, the released NO.sub.X amount DN becomes larger as the NO.sub.X absorbent temperature TEXN becomes higher. The released NO.sub.X amount DN is stored in the ROM 32 in advance as a function of the NO.sub.X absorbent temperature TEXN and the time t, in the form of a map shown in FIG. 10B. In the following step 68, the absorbed NO.sub.X amount SN is calculated on the basis of the following equation.

where DN.multidot.SNI represents an amount of NO.sub.X released from the NO.sub.X absorbent 16 per unit time, and DN.multidot.SNI.multidot.DLT represents an amount of NO.sub.X released from the NO.sub.X absorbent 16 from the last processing cycle to the present processing cycle. In the following step 69, it is judged whether the absorbed NO.sub.X amount is smaller or equal to zero. When SN>0, the processing cycle is ended. When SN.ltoreq.0, the routine goes to step 70, the NO.sub.X release flag is reset.

Contrarily, when the SO.sub.X release flag is set, the routine goes from step 60 to step 67. When the SO.sub.X release flag is set, the air-fuel ratio of the exhaust gas flowing to the NO.sub.X absorbent 16 is made rich, as explained later, and thus the SO.sub.X releasing operation is in process together with the NO.sub.X releasing process, in the NO.sub.X absorbent 16.

FIG. 11 shows a routine for calculating the fuel injection time TAU. The routine is executed by interruption every predetermined crank angle.

Referring to FIG. 11, first, in step 80, the basic fuel injection time TP is calculated using the map shown in FIG. 2. In the following step 81, it is judged whether the SO.sub.X releasing flag is reset. When the SO.sub.X releasing flag is reset, the routine goes to step 83, where it is judged whether the NO.sub.X release flag is reset. When the NO.sub.X release flag is reset, the routine goes step 83, where the correction coefficient K is made 0.6, for example. In the following step 84, the fuel injection time TAU is calculated by multiplying K by TP. Accordingly, the air-fuel mixture fed to the combustion chamber 3 at this time is made lean and the lean air-fuel mixture is burned, and thereby the air-fuel ratio of the exhaust gas flowing to the NO.sub.X absorbent 16 is made lean.

Contrarily, when the SO.sub.X release flag or the NO.sub.X release flag is set in step 81 or 82, the routine goes to step 85, where the correction coefficient K is made 1.3, for example, and then the routine goes to step 84. Accordingly, the air-fuel mixture fed to the combustion chamber 3 at this time is made rich and the rich air-fuel mixture is burned, and thereby the air-fuel ratio of the exhaust gas flowing to the NO.sub.X absorbent 16 is made rich.

FIG. 14 illustrates a case where the present invention is applied to the diesel engine. In FIG. 14, components similar to those in FIG. 1 are depicted by the same reference numerals.

Referring to FIG. 14, the fuel injector 11 is arranged in the combustion chamber 3, and injects fuel into the combustion chamber 3 directly. On the other hand, the exhaust manifold 15 is connected to a casing 22, housing a SO.sub.X absorbent 21 therein, via an exhaust pipe 20 and the casing 22 is connected to the casing 17, housing the NO.sub.X absorbent 16 therein, via an exhaust pipe 23. A bypass pipe 24 bypassing the SO.sub.X absorbent 21 is provided between the exhaust pipes 20 and 23. Further, an exhaust gas control valve 26 is arranged in the exhaust pipe 20 downstream of the inlet of the bypass pipe 24, and is driven by an actuator 25.

The exhaust gas control valve 26 is usually fully opened and thus almost all of the exhaust gas discharged from the engine flows to the SO.sub.X absorbent 21. Contrarily, when the valve 26 is closed, a part of the exhaust gas discharged from the engine flows to the bypass pipe 24, i.e., bypasses the SO.sub.X absorbent 21, and then flows to the NO.sub.X absorbent 16. The remaining exhaust gas flows to the SO.sub.X absorbent 21 and then to the NO.sub.X absorbent 16. Namely, when the valve 26 is closed, the amount of the exhaust gas flowing through the SO.sub.X absorbent 21 is reduced.

Referring further to FIG. 1, an electric heater 27 is attached to the SO.sub.X absorbent 21, and is electrically connected to a battery 29 via a relay 28. The relay 28 is usually turned off. When the relay 28 is turned on, the electric power is supplied to the heater 27 and thereby the SO.sub.X absorbent 21 is heated. Note that the actuator 25 and the relay 28 are controlled on the basis of the output signals from the ECU 30.

A depression sensor 42 generates an output voltage in proportion to the depression of the acceleration pedal (not shown), and the output voltages of the sensor 42 is input to the input port 35 of the ECU 30 via the corresponding AD converter 39. Further, the input port 35 is connected to a speed sensor 43, which generates a pulse representing the speed of the vehicle. The output port 36 is connected to the actuator 25 and the relay 28 via the corresponding drive circuits 41, respectively.

In the diesel engine as shown in FIG. 14, the mean air-fuel ratio of the air-fuel mixture to be burned in the combustion chamber 3 is usually kept lean, to reduce the undesirable smoke and particulate discharged from the engine. Thus, NO.sub.X discharged from the engine is usually absorbed in the NO.sub.X absorbent 16.

As mentioned above, it is not preferable that SO.sub.X is absorbed in the NO.sub.X absorbent 16. Thus, in the present embodiment, the SO.sub.X absorbent 21 is arranged in the exhaust passage upstream of the NO.sub.X absorbent 16 to prevent SO.sub.X from flowing to the NO.sub.X absorbent 16. The SO.sub.X absorbent 21 absorbs SO.sub.X therein when the air-fuel ratio of the inflowing exhaust gas is lean, and releases the absorbed SO.sub.X therefrom when the oxygen concentration in the inflowing exhaust gas becomes lower with the temperature of the SO.sub.X absorbent 21 being higher than a SO.sub.X releasing temperature of the SO.sub.X absorbent 21.

As mentioned above, if SO.sub.X is absorbed in the NO.sub.X absorbent 16, a stable sulphate BaSO.sub.4 is formed, and as a result, the SO.sub.X is hardly released from the NO.sub.X absorbent 16, even when the air-fuel ratio of the exhaust gas flowing to the NO.sub.X absorbent 16 is simply made rich. Thus, to allow the SO.sub.X to be released from the SO.sub.X absorbent 21 easily when the air-fuel ratio of the exhaust gas flowing to the SO.sub.X absorbent 21 is made rich, it is necessary that the absorbed SO.sub.X exists in the absorbent in the form of the sulphuric acid ion SO.sub.4.sup.2-, or, even if the sulphate BaSO.sub.4 is produced, the sulphate BaSO.sub.4 exists in the absorbent in an unstable state. As the SO.sub.X absorbent 21 allowing this, an absorbent carrying at least one selected from lithium Li and a transition metal such as iron Fe, manganese Mn, nickel Ni, and tin Sn, on a carrier made of alumina, for example, can be used.

In the SO.sub.X absorbent 21, when the air-fuel ratio of the exhaust gas flowing to the SO.sub.X absorbent 21 is lean, SO.sub.X in the exhaust gas is oxidized on the surface of the absorbent and absorbed in the absorbent in the form of the sulphuric acid ion SO.sub.4.sup.2-, and then diffused in the absorbent. In this case, when platinum Pt is carried on the carrier of the SO.sub.X absorbent 21, SO.sub.X easily adheres to platinum Pt in the form of SO.sub.3.sup.2-, and thus SO.sub.2 is easily absorbed in the absorbent in the form of the sulphuric acid ion SO.sub.4.sup.2-. Therefore, it is preferable to use the SO.sub.X absorbent 21 carrying platinum Pt to promote the absorption of the SO.sub.2.

As mentioned above, the air-fuel ratio of the exhaust gas flowing into the SO.sub.X absorbent 21 is usually lean and thus SO.sub.X discharged from the engine is absorbed in the SO.sub.X absorbent 21 and only NO.sub.X is absorbed in the NO.sub.X absorbent 16.

However, the SO.sub.X absorbent 21 has a SO.sub.X absorbing capacity. Thus, it is necessary to release SO.sub.X from the SO.sub.X absorbent 21 before it is saturated with SO.sub.X. In the present embodiment, the temperature of the SO.sub.X absorbent 21 is made higher than the SO.sub.X releasing temperature of the SO.sub.X absorbent 21 temporarily and the air-fuel ratio of the exhaust gas flowing to the SO.sub.X absorbent 21 is made rich temporarily, to thereby release the SO.sub.X from the SO.sub.X absorbent 21, when an amount of SO.sub.X absorbed in the SO.sub.X absorbent 21 becomes larger than a constant amount. In this way, the SO.sub.X absorbent 21 constitutes the sulphur containing components absorbent in the present embodiment.

If the flow rate of the exhaust gas flowing through the SO.sub.X absorbent 21 is made lower when SO.sub.X is to be released from the SO.sub.X absorbent 21, the absorbed SO.sub.X is quickly released from the SO.sub.X absorbent 21, as in the above-mentioned embodiment. Thus, in the present embodiment, when SO.sub.X is to be released from the SO.sub.X absorbent 21, the flow rate SVS of the exhaust gas flowing through the SO.sub.X absorbent 21 is made lower than a predetermined flow rate SVS1, i.e., the contact period between the exhaust gas and the SO.sub.X absorbent 21 is made longer than a period required to release SO.sub.X from the SO.sub.X absorbent 21 sufficiently. Accordingly, in the present embodiment, when SO.sub.X is to be released from the SO.sub.X absorbent 21, the temperature of the SO.sub.X absorbent 21 is made higher than the SO.sub.X releasing temperature, and the air-fuel ratio of the inflowing exhaust gas is made rich, and the flow rate SVS is made lower than the predetermined flow rate SVS1. Next, the SO.sub.X releasing operation and the NO.sub.X releasing operation according to the present embodiment will be explained in more detail.

In the present embodiment, the SO.sub.X releasing operation of the SO.sub.X absorbent 21 is performed when the amount of SO.sub.X absorbed in the SO.sub.X absorbent 21 becomes larger than a constant amount, as mentioned above. It is difficult to obtain the absorbed SO.sub.X amount directly, and thus the absorbed SO.sub.X amount is estimated on the basis of the amount of SO.sub.X discharged from the engine 1, i.e., the vehicle driving distance. Namely, the absorbed SO.sub.X amount becomes larger, as the cumulative value SDD of the vehicle driving distance becomes larger. Thus, the SO.sub.X releasing operation is performed when the cumulative value SDD becomes larger than a predetermined value SDD1. The predetermined value SDD1 corresponds to about 30% of the maximum SO.sub.X amount which the SO.sub.X absorbent 21 can absorb therein, for example.

When the SO.sub.X releasing operation is to be started, first, the exhaust gas control valve 26 is closed to make the flow rate SVS lower than the predetermined flow rate SVS1. In this case, the opening VOP of the valve 26 is made VS, which is an opening required to make the flow rate SVS lower than the predetermined flow rate SVS1, and is obtained by experiments as a function of the depression DEP of the acceleration pedal and the engine speed N. This VS is stored in the ROM 32 in advance in the form of the map shown in FIG. 15.

Then, the air-fuel ratio of the exhaust gas flowing to the SO.sub.X, absorbent 21 is made rich. To this end, the fuel injector 11 injects fuel secondarily at the power stroke or the exhaust stroke of the engine. The secondary fuel injection is different from the usual fuel injection performed around the top dead center of the compression stroke, and does not contribute to the engine output. In this case, the amount of the secondary fuel injection QSF is made QSR, which is a fuel injection amount required to make the air-fuel ratio of the exhaust gas flowing to the SO.sub.X absorbent 21 equal to the rich air-fuel ratio suitable for the SO.sub.X releasing operation, and is obtained, by experiment, as a function of the depression DEP and the engine speed N. This QSR is stored in the ROM 32 in advance in the form of the map shown in FIG. 16.

The secondary fuel injection provides a partial oxidation of fuel in the combustion chamber 3, and thus the fuel flow to the SO.sub.X absorbent 21 in the form of the lower hydrocarbon. As a result, CO and H.sub.2 are easily produced as mentioned above, and thus the sulphate BaSO.sub.4 in the SO.sub.X absorbent 21 is easily decomposed.

After the flow rate of the exhaust gas flowing through the SO.sub.X absorbent 21 is made lower and the air-fuel ratio of the exhaust gas flowing to the SO.sub.X absorbent 21 is made rich, the SO.sub.X absorbent 21 is heated. However, just after the secondary fuel injection is started, oxygen remains on the surface of the SO.sub.X absorbent 21. At this time, even though the temperature of the SO.sub.X absorbent 21 is made higher than the SO.sub.X releasing temperature, SO.sub.X is not released sufficiently. Thus, in the present embodiment, after a constant time has passed since the secondary fuel injection is started, the heating of the SO.sub.X absorbent 21 is started, i.e., the relay 28 is turned on and thus the electric heater 27 is turned on.

After this, when the temperature of the SO.sub.X absorbent 21 becomes higher than the SO.sub.X releasing temperature, the absorbed SO.sub.X is released from the SO.sub.X absorbent 21. At this time, the air-fuel ratio of the exhaust gas flowing to the NO.sub.X absorbent 16 is also rich, and thus the SO.sub.X released from the SO.sub.X absorbent 21 passes through the NO.sub.X absorbent 16 without being absorbed. Further, the NO.sub.X releasing and reducing operation of the NO.sub.X absorbent 16 is also in process at this time.

After a constant time has passed since the relay 28 is turned on, it is judged that almost all of the SO.sub.X is released from the SO.sub.X absorbent 21, and thus the SO.sub.X releasing operation is stopped. Namely, the relay 28 is turned off, and the secondary fuel injection is stopped, and the exhaust gas control valve 26 is fully opened.

The heating of the SO.sub.X absorbent 21 is started after the amount of the exhaust gas flowing through the SO.sub.X absorbent 21 is made low. Thus, the energy required to make the temperature of the SO.sub.X absorbent 21 higher than the SO.sub.X releasing temperature can be reduced.

On the other hand, when the absorbed NO.sub.X amount SN of the NO.sub.X absorbent 16 becomes higher than the predetermined amount SN1, the secondary fuel injection is performed to thereby perform the NO.sub.X releasing and reducing operation of the NO.sub.X absorbent 16. In this case, the amount of the secondary fuel injection QSF is made QNR, which is a fuel injection amount required to make the air-fuel ratio of the exhaust gas flowing to the NO.sub.X absorbent 16 equal to the rich air-fuel ratio suitable for the NO.sub.X releasing and reducing operation, and is obtained, by experiment, as a function of the depression DEP and the engine speed N. This QNR is stored in the ROM 32 in advance in the form of the map shown in FIG. 17.

Note that the SO.sub.X absorbent 21 absorbs not only SO.sub.X, but also NO.sub.X, therein when the air-fuel ratio of the inflowing exhaust gas is lean. The absorbed NO.sub.X is released therefrom and is reduced when the air-fuel ratio of the inflowing exhaust gas is made rich, i.e., when the SO.sub.X releasing operation of the SO.sub.X absorbent 21 or the NO.sub.X releasing operation of the NO.sub.X absorbent 16 is in process.

FIGS. 18 and 19 show a routine for controlling the SO.sub.X releasing operation according to the present embodiment. The routine is executed by interruption every predetermined time.

Referring to FIGS. 18 and 19, first, in step 100, it is judged whether a SO.sub.X release flag is set. The SO.sub.X release flag is set when the SO.sub.X is released from the SO.sub.X absorbent 21, and is reset when the SO.sub.X releasing operation is not in process. When the SO.sub.X release flag is reset, the routine goes to step 101, where the vehicle driving distance DD from the last processing cycle to the present processing cycle is calculated on the basis of the output pulses from the speed sensor 43. In the following step 102, the cumulative value SDD of the vehicle driving distance is calculated (SDD=SDD+DD). In the following step 103, it is judged whether the cumulative value SDD is larger than the predetermined value SDD1. When SDD.ltoreq.SDD1, the processing cycle is ended. When SDD>SDD1, the routine goes to step 104, where the SO.sub.X releasing operation of the SO.sub.X absorbent 21 is started.

Namely, first, in step 104, the opening VS for making the exhaust gas control valve 26 closed is calculated using the map shown in FIG. 15. In the following step 105, the opening VOP of the valve 26 is made equal to VS. In the following step 106, the fuel injection amount QSR, for making the air-fuel ratio of the exhaust gas flowing to the SO.sub.X absorbent 21 rich, is calculated using the map shown in FIG. 16. In the following step 107, the secondary fuel injection amount QSF is made equal to QSR. In the following step 108, the counter value CSR, which represents a time from the air-fuel ratio of the exhaust gas flowing to the SO.sub.X absorbent 21 is made rich, is incremented by 1. In the following step 109, it is judged whether the counter value CSR is larger than the constant CSR1. When CSR.ltoreq.CSR1, the processing cycle is ended. When CSR>CSR1, i.e., when the constant time has passed since the air-fuel ratio of the exhaust gas flowing to the SO.sub.X absorbent 21 is made rich, the routine goes to step 110, where the SO.sub.X release flag is reset. In the following step 111, the relay 28 is turned on. Thus, the heating of the SO.sub.X absorbent 21 is started.

When the SO.sub.X release flag is set, the routine goes from step 100 to step 112, where VS is calculated using the map shown in FIG. 15 and, in the following step 113, the opening VOP of the exhaust gas control valve 26 is made VS. In the following step 114, the counter value CSS, which represents a time during which the SO.sub.X release flag is set, is incremented by 1. In the following step 115, it is judged whether the counter value CSS is larger than a constant CSS1. When CSS.ltoreq.CSS1, the processing cycle is ended. Thus, the SO.sub.X releasing operation is continued. Contrarily, when CS>CS1, it is judged that almost all of the SO.sub.X is released from the SO.sub.X absorbent 21, and thus the routine goes to step 116, where the SO.sub.X release flag is reset. In the following step 117, the relay 28 is turned off. In the following step 118, the secondary fuel injection amount QSF is made zero, i.e., the secondary fuel injection is stopped. In the following step 119, the opening VOP of the exhaust gas control valve 26 is made FL, which represents the full open. In the following step 120, the cumulative value SDD is cleared. In the following step 121, the counter value CSR is cleared. In the following step 122, the counter value CSS is cleared.

FIG. 20 shows a routine for controlling the NO.sub.X releasing operation according to the present embodiment. The routine is executed by interruption every predetermined time.

Referring to FIG. 20, first, in step 140, it is judged whether the SO.sub.X release flag, which is set or reset in the routine shown in FIGS. 18 and 19, is set. When the SO.sub.X release flag is reset, i.e., when the SO.sub.X releasing operation is not in process, the routine goes to step 141, where it is judged whether a NO.sub.X release flag is set. The NO.sub.X release flag is set when the NO.sub.X is released from the NO.sub.X absorbent 16 and reduced, and is reset when the NO.sub.X releasing operation is not in process. When the NO.sub.X release flag is reset, i.e., when both of the SO.sub.X release flag and the NO.sub.X release flag are reset, the routine goes to step 142, where the inflowing NO.sub.X amount FN is calculated using the map shown in FIG. 9B. In the following step 143, the absorbed NO.sub.X amount SN is calculated (SN=SN+FN.multidot.DLT). In the following step 144, it is judged whether the absorbed NO.sub.X amount FN is larger than the predetermined amount SN1, mentioned above. When SN.ltoreq.SN1, the processing cycle is ended. Contrarily, when SN>SN1, the routine goes to step 145, where the NO.sub.X release flag is set. In the following step 146, the fuel injection amount QNR, for making the air-fuel ratio of the exhaust gas flowing to the NO.sub.X absorbent 16 rich, is calculated using the map shown in FIG. 17. In the following step 147, the secondary fuel injection amount QSF is made equal to QNR.

When the NO.sub.X release flag is set, the routine goes from step 141 to step 148, where the counter value CN, which represents a time during which the NO.sub.X release flag is set, is incremented by 1. In the following step 149, it is judged whether the counter value CN is larger than a constant CN1. When CN.ltoreq.CN1, the processing cycle is ended. Contrarily, when CN.gtoreq.CN1, it is judged that almost all of the NO.sub.X is released from the NO.sub.X absorbent 16, and thus the routine goes to step 150, where the secondary fuel injection amount QSR is made zero. In the following step 151, the NO.sub.X release flag is reset. In the following step 152, the absorbed NO.sub.X amount SN is cleared. In the following step 153, the counter value CN is cleared.

Contrarily, when the SO.sub.X release flag is set, the routine goes from step 140 to steps 151 to 153. As mentioned above, when the SO.sub.X releasing operation of the SO.sub.X absorbent 21 is in process, the NO.sub.X releasing operation of the NO.sub.X absorbent 16 is also in process. Further, when the SO.sub.X releasing operation is finished, the NO.sub.X releasing operation is also finished. Thus, when the SO.sub.X release flag is set, the NO.sub.X release flag is reset or kept reset, and the absorbed NO.sub.X amount SN and the counter value CN are cleared.

FIG. 21 illustrates another embodiment.

The present embodiment is different from the embodiment shown in FIG. 14 in the point that the electric heater 27, the relay 28, and the battery 29 are not provided.

When the secondary fuel injection is performed, a part of the secondary fuel is burned in the combustion chamber 3 or the exhaust passage. Thus, the temperature of the exhaust gas flowing to the SO.sub.X absorbent 21 is made higher by increasing the amount of the secondary fuel to be burned in the combustion chamber 3 or the exhaust passage. Therefore, in the present embodiment, the timing of the secondary fuel injection when the SO.sub.X releasing operation of the SO.sub.X absorbent 21 is in process is made earlier or more advanced than that when the NO.sub.X releasing operation of the NO.sub.X absorbent 16 is in process.

Namely, the secondary fuel injection timing RTD for the NO.sub.X releasing operation is set between 180 to 210.degree. crank angle after the top dead center of the compression stroke. Contrarily, the secondary fuel injection timing ADV for the SO.sub.X releasing operation is set between 90 to 180.degree. crank angle after the top dead center of the compression stroke. As a result, the temperature of the SO.sub.X absorbent 21 is made higher than the SO.sub.X releasing temperature without the electric heater.

FIGS. 22 and 23 show a routine for controlling the SO.sub.X releasing operation according to the present embodiment. The routine is executed by interruption every predetermined time. Note that the routine for controlling the NO.sub.X releasing operation shown in FIG. 20 is executed in the present embodiment.

Referring to FIGS. 22 and 23, first, in step 170, it is judged whether a SO.sub.X release flag is set. The SO.sub.X release flag is set when the SO.sub.X is released from the SO.sub.X absorbent 21, and is reset when the SO.sub.X releasing operation is not in process. When the SO.sub.X release flag is reset, the routine goes to step 171, where the vehicle driving distance DD from the last processing cycle to the present processing cycle is calculated on the basis of the output pulses from the speed sensor 43. In the following step 172, the cumulative value SDD of the vehicle driving distance is calculated (SDD=SDD+DD). In the following step 173, it is judged whether the cumulative value SDD is larger than the predetermined value SDD1. When SDD.ltoreq.SDD1, the processing cycle is ended. When SDD>SDD1, the routine goes to step 174. where the SO.sub.X release flag is set.

When the SO.sub.X release flag is set, the routine goes from step 170 to step 175, where the opening VS for making the exhaust gas control valve 26 closed is calculated using the map shown in FIG. 15. In the following step 176, the opening VOP of the valve 26 is made equal to VS. In the following step 177, the secondary fuel injection timing ITS is made equal to ADV which is set in the advanced side. In the following step 178, the fuel injection amount QSR for making the air-fuel ratio of the exhaust gas flowing to the SO.sub.X absorbent 21 rich, is calculated using the map shown in FIG. 16. In the following step 179, the secondary fuel injection amount QSF is made equal to QSR. In the following step 180, the counter value CSS, which represents a time during which the SO.sub.X release flag is set, is incremented by 1. In the following step 181, it is judged whether the counter value CSS is larger than a constant CSS2. When CSS.ltoreq.CSS2, the processing cycle is ended. Contrarily, when CS>CS2, it is judged that almost all of the SO.sub.X is released from the SO.sub.X absorbent 21, and thus the routine goes to step 182, where the SO.sub.X release flag is reset. In the following step 183, the secondary fuel injection timing ITS is made equal to RTD which is set to the retarded side. Thus, when the NO.sub.X releasing operation of the NO.sub.X absorbent 16 is started, the secondary fuel injection is performed with the timing RTD. In the following step 184, the secondary fuel injection amount QSF is made zero, i.e., the secondary fuel injection is stopped. In the following step 185, the opening VOP of the exhaust gas control valve 26 is made FL, which represents full open. In the following step 186, the cumulative value SDD is cleared. In the following step 187, the counter value CSS is cleared.

According to the present invention, it is possible to provide a device for purifying an exhaust gas of an engine capable of releasing the absorbed sulphur containing components from the sulphur containing components absorbent rapidly and sufficiently.

While 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.