Control apparatus for internal combustion engine

Abstract

A control apparatus is applicable to an internal combustion engine having an exhaust passage arranged with an NSR catalyst and an SCR catalyst, wherein when it is necessary to decrease NH.sub.3 adsorbed to the SCR catalyst, then an air-fuel ratio of an air-fuel mixture to be combusted in the internal combustion engine is controlled to a predetermined lean air-fuel ratio which is higher than a theoretical air-fuel ratio if a temperature of the SCR catalyst is not less than a lower limit temperature at which NH.sub.3 can be oxidized, while the air-fuel ratio of the air-fuel mixture to be combusted in the internal combustion engine is controlled to a predetermined weak lean air-fuel ratio which is lower than the predetermined lean air-fuel ratio and which is higher than the theoretical air-fuel ratio if the temperature of the SCR catalyst is less than the lower limit temperature.

Claims

1. A control apparatus for an internal combustion engine which is provided with a first exhaust gas purification apparatus that is arranged in an exhaust passage and includes a NO.sub.x storage reduction catalyst, and a second exhaust gas purification apparatus that is arranged in the exhaust passage downstream from the first exhaust gas purification apparatus and includes a selective catalytic reduction catalyst, wherein the control apparatus comprising a controller including at least one processor configured to: acquire an NH.sub.3 adsorption amount as an amount of NH.sub.3 adsorbed to the selective catalytic reduction catalyst; detect a temperature of the selective catalytic reduction catalyst; and operate when the NH.sub.3 adsorption amount of the selective catalytic reduction catalyst is not less than a predetermined threshold value such that an air-fuel ratio of an air-fuel mixture to be combusted in the internal combustion engine is controlled to a predetermined lean air-fuel ratio which is higher than a theoretical air-fuel ratio if the temperature of the selective catalytic reduction catalyst is not less than a lower limit temperature at which NH.sub.3 can be oxidized, while the air-fuel ratio of the air-fuel mixture to be combusted in the internal combustion engine is controlled to a predetermined weak lean air-fuel ratio which is lower than the predetermined lean air-fuel ratio and which increases an amount of NO.sub.x flowing out from the NO.sub.x storage reduction catalyst per unit time as compared with an amount of NO.sub.x stored in the NO.sub.x storage reduction catalyst per unit time if the temperature of the selective catalytic reduction catalyst is less than the lower limit temperature.

2. The control apparatus for the internal combustion engine according to claim 1, wherein the controller executes a temperature raising process for raising a temperature of the selective catalytic reduction catalyst if the temperature of the selective catalytic reduction catalyst is not less than a predetermined temperature which is higher than the lower limit temperature when the NH.sub.3 adsorption amount of the selective catalytic reduction catalyst is not less than the predetermined threshold value.

3. The control apparatus for the internal combustion engine according to claim 2, wherein the controller performs the temperature raising process by retarding an ignition timing of the internal combustion engine.

4. The control apparatus for the internal combustion engine according to claim 2, wherein the controller performs the temperature raising process by advancing a valve opening timing of an exhaust valve of the internal combustion engine.

5. The control apparatus for the internal combustion engine according to claim 1, wherein the first exhaust gas purification apparatus includes a three-way catalyst in addition to the NO.sub.x storage reduction catalyst.

6. The control apparatus for the internal combustion engine according to claim 2, wherein the first exhaust gas purification apparatus includes a three-way catalyst in addition to the NO.sub.x storage reduction catalyst.

7. The control apparatus for the internal combustion engine according to claim 3, wherein the first exhaust gas purification apparatus includes a three-way catalyst in addition to the NO.sub.x storage reduction catalyst.

8. The control apparatus for the internal combustion engine according to claim 4, wherein the first exhaust gas purification apparatus includes a three-way catalyst in addition to the NO.sub.x storage reduction catalyst.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 shows a schematic arrangement of an internal combustion engine and an exhaust system thereof to which the present invention is applied.

(2) FIG. 2 shows the correlation among the air-fuel ratio of the air-fuel mixture, the NO.sub.X emission amount of the internal combustion engine, the NO.sub.X emission amount of the three-way catalyst, and the NO.sub.X emission amount of the NSR catalyst.

(3) FIG. 3 shows the correlation between the temperature of the SCR catalyst and the decreasing speed of the NH.sub.3 adsorption amount.

(4) FIG. 4 shows a method for determining the target temperature Tstrg in the temperature raising process.

(5) FIG. 5 shows the correlation between the exhaust gas flow rate and the predetermined amount.

(6) FIG. 6 shows a flow chart illustrating a process routine executed by ECU when the NH.sub.3 adsorption amount of the SCR catalyst is decreased.

DESCRIPTION OF EMBODIMENTS

(7) An explanation will be made below on the basis of the drawings about a specified embodiment of the present invention. For example, the dimension or size, the material, the shape, and the relative arrangement of each of constitutive parts or components described in the embodiment of the present invention are not intended to limit the technical scope of the invention only thereto unless specifically noted.

(8) FIG. 1 shows a schematic arrangement of an internal combustion engine and an exhaust system thereof to which the present invention is applied. The internal combustion engine 1 shown in FIG. 1 is an internal combustion engine (gasoline engine) of the spark ignition type which can be operated with the air-fuel mixture having an air-fuel ratio (lean air-fuel ratio) higher than the theoretical air-fuel ratio.

(9) The internal combustion engine 1 is provided with a fuel injection valve 2. The fuel injection valve 2 may be a valve device which injects the fuel into an intake passage (for example, an intake port), or the fuel injection valve 2 may be a valve device which injects the fuel into a cylinder. The internal combustion engine 1 is connected to an exhaust pipe 3. The exhaust pipe 3 is a pipe for allowing the gas (exhaust gas) combusted in the cylinder of the internal combustion engine 1 to flow therethrough. A first catalyst casing 4 is arranged at an intermediate position of the exhaust pipe 3. The first catalyst casing 4 accommodates a three-way catalyst composed of a honeycomb structure which is coated with a coat layer such as alumina or the like and a noble metal (for example, platinum, palladium, or rhodium) which is carried on the coat layer.

(10) A second catalyst casing 5 is arranged in the exhaust pipe 3 downstream from the first catalyst casing 4. The second catalyst casing 5 accommodates a NO.sub.X storage reduction catalyst (NSR catalyst) composed of a honeycomb structure which is coated with a coat layer such as alumina or the like, a noble metal (for example, platinum, palladium, or rhodium) which is carried on the coat layer, and a NO.sub.X occluding or storing agent (for example, alkaline metal or alkaline earth metal) which is carried on the coat layer.

(11) A third catalyst casing 6 is arranged in the exhaust pipe 3 downstream from the second catalyst casing 5. The third catalyst casing 6 accommodates a selective catalytic reduction catalyst (SCR catalyst) composed of a honeycomb structure which comprises cordierite and heat resistant steel based on FeCrAl, a coat layer based on zeolite which coats the honeycomb structure, and a noble metal (for example, platinum or palladium) which is carried on the coat layer.

(12) In this context, the combination of the first catalyst casing 4 and the second catalyst casing 5 corresponds to the first exhaust gas purification apparatus according to the present invention. Further, the third catalyst casing 6 corresponds to the second exhaust gas purification apparatus according to the present invention.

(13) ECU 7 is provided in combination with the internal combustion engine 1 constructed as described above. ECU 7 is an electronic control unit composed of, for example, CPU, ROM, RAM, and backup RAM. ECU 7 is electrically connected to various sensors including, for example, an air-fuel ratio sensor (A/F sensor) 8, an oxygen concentration sensor (O.sub.2 sensor) 9, a first NO.sub.X sensor 10, a second NO.sub.X sensor 11, a temperature sensor 12, an accelerator position sensor 13, a crank position sensor 14, and an air flow meter 15.

(14) The air-fuel ratio sensor 8 is attached to the exhaust pipe 3 upstream from the first catalyst casing 4, and the air-fuel ratio sensor 8 outputs an electric signal which correlates with the air-fuel ratio of the exhaust gas flowing into the first catalyst casing 4. The oxygen concentration sensor 9 is attached to the exhaust pipe 3 between the first catalyst casing 4 and the second catalyst casing 5, and the oxygen concentration sensor 9 outputs an electric signal which correlates with the concentration of oxygen contained in the exhaust gas flowing out from the first catalyst casing 4. The first NO.sub.X sensor 10 is attached to the exhaust pipe 3 between the second catalyst casing 5 and the third catalyst casing 6, and the first NO.sub.X sensor 10 outputs an electric signal which correlates with the concentration of NO.sub.X contained in the exhaust gas flowing out from the second catalyst casing 5 (in other words, the exhaust gas flowing into the third catalyst casing 6). The second NO.sub.X sensor 11 is attached to the exhaust pipe 3 downstream from the third catalyst casing 6, and the second NO.sub.X sensor 11 outputs an electric signal which correlates with the concentration of NO.sub.X contained in the exhaust gas flowing out from the third catalyst casing 6. The accelerator position sensor 13 outputs an electric signal which correlates with the operation amount of an accelerator pedal (accelerator opening degree). The crank position sensor 14 outputs an electric signal which correlates with the rotation position of an output shaft (crank shaft) of the internal combustion engine 1. The air flow meter 15 outputs an electric signal which correlates with the amount of air (intake air amount) sucked into the cylinder of the internal combustion engine 1.

(15) ECU 7 controls the operation state of the internal combustion engine 1 on the basis of the output signals of the various sensors described above. For example, ECU 7 calculates the target air-fuel ratio of the air-fuel mixture on the basis of the engine rotation speed calculated on the basis of the output signal of the crank position sensor 14 and the output signal of the accelerator position sensor 13 (accelerator opening degree). ECU 7 calculates the target fuel injection amount (fuel injection period) of the fuel injection valve 2 on the basis of the target air-fuel ratio and the output signal of the air flow meter 15 (intake air amount), and ECU 7 operates the fuel injection valve 2 in accordance with the target fuel injection amount.

(16) Note that ECU 7 sets the target air-fuel ratio to the lean air-fuel ratio which is higher than the theoretical air-fuel ratio when the operation state of the internal combustion engine 1 belongs to a low rotation-low load area or a moderate rotation-moderate load area (hereinafter referred to as lean operation area). ECU 7 sets the target air-fuel ratio to the theoretical air-fuel ratio or the rich air-fuel ratio which is lower than the theoretical air-fuel ratio when the operation state of the internal combustion engine 1 belongs to a high load area or a high rotation area (hereinafter referred to as rich operation area). In this way, when the operation state of the internal combustion engine 1 belongs to the lean operation area, if the target air-fuel ratio is set to the lean air-fuel ratio, then it is possible to suppress the fuel consumption amount so that the fuel consumption amount is small. Further, ECU 7 performs, for example, the air-fuel ratio feedback control in which the target fuel injection amount is corrected so that the output signal of the air-fuel ratio sensor 8 coincides with the target air-fuel ratio described above, and the learning control for the correction coefficient used for the air-fuel ratio feedback control on the basis of the output signal of the oxygen concentration sensor 9.

(17) In the meantime, when the target air-fuel ratio is set to the lean air-fuel ratio, the NO.sub.X purification performance of the three-way catalyst accommodated in the first catalyst casing 4 is lowered. On this account, when the target air-fuel ratio is set to the lean air-fuel ratio, it is necessary to purify NO.sub.X contained in the exhaust gas by means of the NSR catalyst of the second catalyst casing 5 and the SCR catalyst of the third catalyst casing 6.

(18) When the oxygen concentration of the exhaust gas flowing into the second catalyst casing 5 is high (when the air-fuel ratio of the exhaust gas is lean), the NSR catalyst stores (occludes) or adsorbs NO.sub.X contained in the exhaust gas. When the oxygen concentration of the exhaust gas flowing into the second catalyst casing 5 is low, and the reducing component such as hydrocarbon (HC), carbon monoxide (CO) or the like is contained in the exhaust gas (when the air-fuel ratio of the exhaust gas is rich), then the NSR catalyst releases NO.sub.X occluded by the NSR catalyst, and released NO.sub.X is reduced into nitrogen (N.sub.2).

(19) Accordingly, ECU 7 executes the rich spike process when the NO.sub.X occlusion amount (storage amount) of the NSR catalyst is not less than a certain amount in the lean operation area. The rich spike process is such a process that the fuel injection amount and/or the intake air amount is/are adjusted so that the oxygen concentration in the exhaust gas is low and the concentrations of HC and CO are raised. More specifically, the rich spike process is such a process that at least one of the process for increasing the fuel injection amount of the fuel injection valve 2 and the process for decreasing the opening degree of the intake throttle valve (throttle valve) is executed. Note that when the fuel injection valve 2 is arranged so that the fuel is directly injected into the cylinder, the rich spike process may be executed by means of a method in which the fuel is injected from the fuel injection valve 2 during the exhaust stroke of the cylinder. Further, the rich spike process may be executed if the operation time after the termination of the previous rich spike process (preferably the operation time in which the target air-fuel ratio is set to the lean air-fuel ratio) is not less than a certain time, or the rich spike process may be executed if the travel distance after the termination of the previous rich spike process (preferably the travel distance in which the target air-fuel ratio is set to the lean air-fuel ratio) is not less than a certain distance.

(20) The SCR catalyst adsorbs ammonia (NH.sub.3) contained in the exhaust gas. The SCR catalyst reduces NO.sub.X into nitrogen (N.sub.2) by reacting NH.sub.3 adsorbed to the SCR catalyst with NO.sub.X contained in the exhaust gas. Note that NH.sub.3, which is supplied to the SCR catalyst, is produced by the three-way catalyst and/or the NSR catalyst. For example, when the rich spike process is executed, a part of NO.sub.X is reduced into NH.sub.3 by the three-way catalyst. A part of NO.sub.X, which flows out from the NSR catalyst, is reduced into NH.sub.3 by the NSR catalyst. In this procedure, the amount of NH.sub.3, which is produced by the NSR catalyst, changes depending on, for example, the interval at which the rich spike process is executed and the air-fuel ratio which is provided when the rich spike process is executed. Therefore, when NH.sub.3 is supplied to the SCR catalyst, then the execution interval of the rich spike process may be set to an interval which is suitable for the production of NH.sub.3, or the air-fuel ratio, which is provided during the execution of the rich spike process, may be set to an air-fuel ratio (for example, about 14.1) which is suitable for the production of NH.sub.3.

(21) In the meantime, when the operation state of the internal combustion engine 1 belongs to the rich operation area, the target air-fuel ratio of the air-fuel mixture is set to the theoretical air-fuel ratio or the rich air-fuel ratio. Therefore, NO.sub.X, which is contained in the exhaust gas, is purified by the three-way catalyst of the first catalyst casing 4. Therefore, the amount of NO.sub.X, which arrives at the SCR catalyst of the third catalyst casing 6, is zero or extremely small, and hence NH.sub.3, which is adsorbed to the SCR catalyst, is scarcely consumed. Further, when the target air-fuel ratio of the air-fuel mixture is set to the theoretical air-fuel ratio or the rich air-fuel ratio, NH.sub.3 is produced by the three-way catalyst of the first catalyst casing 4 and the NSR catalyst of the second catalyst casing 5. Therefore, the amount of NH.sub.3, which is adsorbed to the SCR catalyst of the third catalyst casing 6, is increased. Therefore, if the operation state of the internal combustion engine 1 belongs to the rich operation area in many opportunities, there is a possibility that the ability of the SCR catalyst to adsorb NH.sub.3 may be saturated. If the ability of the SCR catalyst to adsorb NH.sub.3 is saturated, there is a possibility that a relatively large amount of NH.sub.3 may be emitted into the atmospheric air, and the odor may be caused.

(22) In view of the above, in this embodiment, if the NH.sub.3 adsorption amount of the SCR catalyst is not less than a predetermined threshold value, a process (hereinafter referred to as NH.sub.3 decreasing process), in which the NH.sub.3 adsorption amount of the SCR catalyst is decreased, is executed. Note that the term predetermined threshold value referred to herein is the amount obtained by subtracting a predetermined margin from the NH.sub.3 adsorption amount at which it is assumed that the amount of NH.sub.3 desorbed or released from the SCR catalyst and emitted or discharged into the atmospheric air exceeds a permissible amount (for example, a maximum value of the amount at which any stimulative odor is not caused) if the internal combustion engine 1 is operated under an operation condition in which NH.sub.3 adsorbed to the SCR catalyst is easily desorbed when the NH.sub.3 adsorption amount of the SCR catalyst is not less than the predetermined threshold value. The predetermined threshold value is previously determined by means of an adaptation operation by utilizing, for example, an experiment. Note that the operation condition in which NH.sub.3 adsorbed to the SCR catalyst is easily desorbed referred to herein is such an operation condition that the flow rate of the exhaust gas is apt to be suddenly increased and the temperature of the exhaust gas is apt to be suddenly raised, for example, as in the acceleration operation.

(23) A method for executing the NH.sub.3 decreasing process according to this embodiment will be described below. At first, when the NH.sub.3 decreasing process is executed, ECU 7 acquires the NH.sub.3 adsorption amount of the SCR catalyst. The NH.sub.3 adsorption amount of the SCR catalyst is determined by adding up values each of which is obtained by subtracting the NH.sub.3 consumption amount (amount of NH.sub.3 which contributes to the reduction of NO.sub.X) and the NH.sub.3 slip amount (amount of NH.sub.3 which passes through the SCR catalyst) from the amount of NH.sub.3 which is supplied to the SCR catalyst of the third catalyst casing 6.

(24) In this procedure, the amount of NH.sub.3, which is supplied to the SCR catalyst, is the total amount of the amount of NH.sub.3 which is produced by the three-way catalyst of the first catalyst casing 4 and the amount of NH.sub.3 which is produced by the NSR catalyst of the second catalyst casing 5. The amount of NH.sub.3, which is produced by the three-way catalyst, correlates with the air-fuel ratio of the exhaust gas, the flow rate of the exhaust gas, and the temperature of the three-way catalyst. Therefore, if the correlation thereamong is previously determined, it is possible to determine the amount of NH.sub.3 which is produced by the three-way catalyst by using the arguments of the air-fuel ratio of the exhaust gas, the flow rate of the exhaust gas, and the temperature of the three-way catalyst. On the other hand, the amount of NH.sub.3, which is produced by the NSR catalyst, correlates with the air-fuel ratio of the exhaust gas, the flow rate of the exhaust gas, and the temperature of the NSR catalyst. Therefore, if the correlation thereamong is previously determined, it is possible to determine the amount of NH.sub.3 which is produced by the NSR catalyst by using the arguments of the air-fuel ratio of the exhaust gas, the flow rate of the exhaust gas, and the temperature of the NSR catalyst.

(25) The NH.sub.3 consumption amount is calculated by using the parameters of the amount of NO.sub.X flowing into the SCR catalyst (NO.sub.X inflow amount) and the NO.sub.X purification rate of the SCR catalyst. In this procedure, the NO.sub.X inflow amount is calculated by multiplying the measured value of the first NO.sub.X sensor 10 (NO.sub.X concentration of the exhaust gas flowing into the third catalyst casing 6) and the flow rate of the exhaust gas (total amount of the measured value of the air flow meter 15 and the fuel injection amount). On the other hand, the NO.sub.X purification rate of the SCR catalyst is calculated by using the parameters of the flow rate of the exhaust gas and the temperature of the SCR catalyst. Note that the correlation among the NO.sub.X purification rate of the SCR catalyst, the flow rate of the exhaust gas, and the temperature of the SCR catalyst is previously determined experimentally.

(26) The NH.sub.3 slip amount is determined by using the parameters of the previous calculated value of the NH.sub.3 adsorption amount, the temperature of the SCR catalyst, and the flow rate of the exhaust gas. In this procedure, if the flow rate of the exhaust gas is constant, the following relationship holds. That is, the larger the NH.sub.3 adsorption amount is and/or the higher the temperature of the SCR catalyst is, the higher the concentration of NH.sub.3 of the exhaust gas flowing out from the SCR catalyst is. Further, if the NH.sub.3 concentration of the exhaust gas flowing out from the SCR catalyst is constant, the following relationship holds. That is, the larger the flow rate of the exhaust gas is, the larger the NH.sub.3 slip amount per unit time is. Based on the correlations as described above, the NH.sub.3 slip amount can be determined by determining the NH.sub.3 concentration of the exhaust gas flowing out from the SCR catalyst by using the parameters of the previous calculated value of the NH.sub.3 adsorption amount and the temperature of the SCR catalyst, and then multiplying the NH.sub.3 concentration by the flow rate of the exhaust gas.

(27) If the NH.sub.3 adsorption amount determined by the method described above arrives at the value of not less than the predetermined threshold value, ECU 7 executes the NH.sub.3 decreasing process. At first, if the temperature of the SCR catalyst is less than the lower limit temperature at which NH.sub.3 can be oxidized, ECU 7 sets the target air-fuel ratio of the air-fuel mixture to the predetermined weak lean air-fuel ratio which is higher than the theoretical air-fuel ratio. The weak lean air-fuel ratio referred to herein is such an air-fuel ratio that the amount of NO.sub.X purified by the three-way catalyst per unit time is smaller than the amount of NO.sub.X flowing into the three-way catalyst per unit time (the amount of NO.sub.X flowing out from the three-way catalyst is larger than zero), and the amount of NO.sub.X flowing out from the NSR catalyst per unit time is larger than the amount of NO.sub.X stored or occluded by the NSR catalyst per unit time at the air-fuel ratio. Preferably, as shown in FIG. 2, the predetermined weak lean air-fuel ratio is such an air-fuel ratio that the amount of NO.sub.X purified by the three-way catalyst per unit time is smaller than the amount of NO.sub.X flowing into the three-way catalyst per unit time, and the amount of NO.sub.X flowing out from the NSR catalyst is maximized at the air-fuel ratio (A/Fmax shown in FIG. 2) within a range of the air-fuel ratio (range of A/F1 to A/F2 shown in FIG. 2) in which the amount of NO.sub.X flowing out from the NSR catalyst per unit time is larger than the amount of NO.sub.X stored or occluded by the NSR catalyst per unit time. When the air-fuel ratio A/Fmax as described above is decided as the predetermined weak lean air-fuel ratio, even if the NH.sub.3 decreasing process is executed in a state in which the NO.sub.X storage amount (occlusion amount) of the NSR catalyst is relatively small, then a relatively large amount of NO.sub.X can be supplied to the SCR catalyst of the third catalyst casing 6. When the temperature of the SCR catalyst is less than the lower limit temperature, if the relatively large amount of NO.sub.X is supplied to the SCR catalyst, then the amount of NH.sub.3, which is consumed for the reduction of NO.sub.X in the SCR catalyst, is increased. As a result, it is possible to effectively decrease the NH.sub.3 adsorption amount of the SCR catalyst. In the meantime, when only the second catalyst casing 5 is arranged upstream from the third catalyst casing 6, the predetermined weak lean air-fuel ratio may be set to such an air-fuel ratio that the amount of NO.sub.X flowing out from the NSR catalyst is maximized within a range of the air-fuel ratio in which the amount of NO.sub.X flowing out from the NSR catalyst per unit time is larger than the amount of NO.sub.X stored or occluded by the NSR catalyst per unit time. Further, when only the first catalyst casing 4 is arranged upstream from the third catalyst casing 6, the predetermined weak lean air-fuel ratio may be set to such an air-fuel ratio that the amount of NO.sub.X flowing out from the three-way catalyst is maximized within a range of the air fuel ratio in which the amount of NO.sub.X purified by the three-way catalyst per unit time is smaller than the amount of NO.sub.X flowing into the three-way catalyst per unit time.

(28) In the next place, when the NH.sub.3 adsorption amount is not less than the predetermined threshold value, if the temperature of the SCR catalyst is not less than the lower limit temperature, then ECU 7 sets the target air-fuel ratio of the air-fuel mixture to the predetermined lean air-fuel ratio which is higher than the predetermined weak lean air fuel ratio. The predetermined lean air-fuel ratio referred to herein may be the highest air-fuel ratio within a range of the air-fuel ratio in which it is possible to secure the combustion stability of the internal combustion engine 1 and the drivability, or the predetermined lean air-fuel ratio may be the highest air fuel ratio within a range in which the exhaust emission other than NH.sub.3 does not exceed the regulation value. Note that if the operation state of the internal combustion engine 1, which is provided when the NH.sub.3 decreasing process is executed, belongs to the lean operation area, the predetermined lean air-fuel ratio may be set equivalently to the air-fuel ratio which is determined depending on the operation state of the internal combustion engine 1. When the target air-fuel ratio of the air-fuel mixture is set to the predetermined lean air-fuel ratio, the exhaust gas, which has a high oxygen concentration, flows into the SCR catalyst. When the temperature of the SCR catalyst is not less than the lower limit temperature, if the exhaust gas, which has the high oxygen concentration, is supplied to the SCR catalyst, then the oxidation of NH.sub.3 adsorbed to the SCR catalyst is facilitated. In this procedure, when NH.sub.3, which is adsorbed to the SCR catalyst, is oxidized, NO.sub.X is produced. NO.sub.X, which is produced as described above, is reacted with NH.sub.3 adsorbed to the SCR catalyst, and NO.sub.X is reduced. Therefore, if the oxidation of NH.sub.3 adsorbed to the SCR catalyst is facilitated by setting the air-fuel ratio of the air-fuel mixture to the predetermined lean air-fuel ratio when the temperature of the SCR catalyst is not less than the lower limit temperature, then it is possible to effectively decrease the NH.sub.3 adsorption amount of the SCR catalyst.

(29) In the meantime, there is such a tendency that the amount of NH.sub.3 capable of being adsorbed by the SCR catalyst (adsorption capacity) is more decreased as the temperature of the SCR catalyst is more raised. On this account, as shown in FIG. 3, if the temperature of the SCR catalyst is not less than a predetermined temperature (Tsm shown in FIG. 3) which is higher than the lower limit temperature (Tsl shown in FIG. 3), the amount of NH.sub.3 desorbed from the SCR catalyst per unit time (desorption speed) is larger than the amount of NH.sub.3 oxidized per unit time by the SCR catalyst (oxidation speed). Accordingly, ECU 7 executes the process for raising the temperature of the SCR catalyst (temperature raising process) if the temperature of the SCR catalyst is not less than the predetermined temperature Tsm which is higher than the lower limit temperature Tsl.

(30) If the temperature raising process is executed when the temperature of the SCR catalyst is not less than the predetermined temperature Tsm, then the amount of NH.sub.3 desorbed from the SCR catalyst per unit time is increased, and hence it is possible to decrease the NH.sub.3 adsorption amount of the SCR catalyst. In the meantime, if the temperature of the SCR catalyst is raised by the temperature raising process, the adsorption capacity of the SCR catalyst is decreased in accordance therewith. On this account, there is a possibility that the adsorption capacity, which is provided after raising the temperature of the SCR catalyst by the temperature raising process, may be smaller than the NH.sub.3 adsorption amount (present NH.sub.3 adsorption amount) provided before the execution of the temperature raising process. In this procedure, if the difference is excessively increased between the present NH.sub.3 adsorption amount and the adsorption capacity provided after raising the temperature of the SCR catalyst by the temperature raising process, there is a possibility that the amount of NH.sub.3 desorbed from the SCR catalyst during the execution of the temperature raising process may be excessively increased. Then, if the amount of NH.sub.3, which is desorbed from the SCR catalyst during the execution of the temperature raising process, is excessively increased, there is a possibility that that NH.sub.3 emitted into the atmospheric air may cause the stimulative odor. On this account, the temperature raising process is performed so that the amount of NH.sub.3 desorbed from the SCR catalyst during the execution of the temperature raising process is included in the amount (equal to or not more than the permissible amount described above) which does not cause the stimulative order. Specifically, as shown in FIG. 4, ECU 7 subtracts a predetermined amount from the present NH.sub.3 adsorption amount of the SCR catalyst, and ECU 7 calculates the target value of the adsorption amount of the SCR catalyst after the execution of the temperature raising process (target capacity as shown in FIG. 4). The predetermined amount referred to herein is the amount obtained by subtracting a predetermined margin from the permissible amount. Note that the predetermined amount may be a fixed value. Alternatively, the predetermined amount may be a variable value which is changed depending on the exhaust gas flow rate. For example, the order of NH.sub.3, which is caused when the exhaust gas is emitted into the atmospheric air, is more intensified as the concentration of NH.sub.3 contained in the exhaust gas is more raised. On this account, as shown in FIG. 5, the predetermined amount may be set to a smaller value as the exhaust gas flow rate is more decreased. In the next place, ECU 7 determines the temperature of the SCR catalyst (Tstrg shown in FIG. 4) at which the adsorption capacity of the SCR catalyst is equal to the target capacity described above, and ECU 7 sets the temperature Tstrg as the target temperature of the SCR catalyst in the temperature raising process. In this procedure, the correlation between the adsorption capacity of the SCR catalyst and the temperature of the SCR catalyst as shown in FIG. 4 is previously determined experimentally. In the next place, ECU 7 may calculate the amount of retard of the ignition timing and the amount of advance of the valve opening timing of the exhaust valve on the basis of the difference between the present temperature of the SCR catalyst (Tspt shown in FIG. 4) and the target temperature Tstrg. Note that the amount of retard of the ignition timing and the amount of advance of the valve opening timing the exhaust valve may be previously determined fixed values. In this case, ECU 7 may terminate the temperature raising process at the point in time at which the temperature of the SCR catalyst arrives at the target temperature Tstrg. When the temperature raising process is executed in accordance with the method as described above, it is possible to decrease the NH.sub.3 adsorption amount of the SCR catalyst without causing the stimulative odor.

(31) Note that the target air-fuel ratio of the air-fuel mixture, which is provided when the temperature raising process is executed, may be set to the air-fuel ratio which is determined depending on the operation state of the internal combustion engine 1. Alternatively, the target air-fuel ratio may be set to the predetermined lean air-fuel ratio. When the temperature raising process is executed, if the target air-fuel ratio of the air-fuel mixture is set to the predetermined lean air-fuel ratio, then the desorption of NH.sub.3 adsorbed to the SCR catalyst is facilitated, and the oxidation of NH.sub.3 adsorbed to the SCR catalyst or NH.sub.3 desorbed from the SCR catalyst is facilitated. As a result, it is possible to quickly decrease the NH.sub.3 adsorption amount of the SCR catalyst, while suppressing the amount of NH.sub.3 emitted into the atmospheric air to be not more than the permissible amount. Further, if the air-fuel ratio of the air-fuel mixture is set to the predetermined lean air-fuel ratio during the execution of the temperature raising process, it is also possible to suppress the deterioration of the fuel consumption ratio so that the deterioration is small.

(32) In the meantime, if a method for increasing the fuel injection amount or a method for adding the fuel into the exhaust gas is used as the method for raising the temperature of the SCR catalyst, there is a possibility that the fuel consumption ratio of the internal combustion engine 1 may be deteriorated. On this account, it is desirable that the temperature raising process is performed by means of a method for retarding the ignition timing of the internal combustion engine 1 or a method for advancing the valve opening timing of the exhaust valve of the internal combustion engine 1. If the temperature raising process is performed in accordance with the method as described above, it is possible to suppress the deterioration of the fuel consumption ratio which would be otherwise caused by the execution of the temperature raising process, so that the deterioration is small.

(33) An explanation will now be made with reference to FIG. 6 about a procedure for executing the NH.sub.3 decreasing process in this embodiment. FIG. 6 shows a flow chart illustrating a process routine executed by ECU 7 when the NH.sub.3 adsorption amount of the SCR catalyst is decreased. This process routine is previously stored in ROM of ECU 7, and the process routine is periodically executed by ECU 7.

(34) In the process routine shown in FIG. 6, ECU 7 firstly reads the NH.sub.3 adsorption amount Adnh3 of the SCR catalyst in a process of S101. As described above, the NH.sub.3 adsorption amount Adnh3 of the SCR catalyst can be determined by adding up values each of which is obtained by subtracting the NH.sub.3 consumption amount and the NH.sub.3 slip amount from the total amount of NH.sub.3 produced by the three-way catalyst and the NSR catalyst. The acquirer according to the present invention is realized by determining the NH.sub.3 adsorption amount of the SCR catalyst by means of the method as described above.

(35) In a process of S102, ECU 7 judges whether or not the NH.sub.3 adsorption amount Adnh3, which is read in the process of S101 described above, is not less than the predetermined threshold value Adthre1. As described above, the predetermined threshold value Adthre1 is the amount which obtained by subtracting the predetermined margin from the NH.sub.3 adsorption amount at which it is assumed that the amount of NH.sub.3 desorbed from the SCR catalyst and emitted into the atmospheric air exceeds the permissible amount if the internal combustion engine 1 is operated under the operation condition in which NH.sub.3 adsorbed to the SCR catalyst is easily desorbed when the NH.sub.3 adsorption amount of the SCR catalyst is not less than the predetermined threshold value Adthre1. If the affirmative judgment is made in the process of S102 (Adnh3Adthre1) in the process of S102, then it can be regarded that NH.sub.3 in an amount of not less than the permissible amount may be possibly desorbed from the SCR catalyst, if the internal combustion engine 1 is operated under the operation condition in which NH.sub.3 adsorbed to the SCR catalyst is easily desorbed. Therefore, ECU 7 executes the NH.sub.3 decreasing process in a process of S103 and the followings.

(36) At first, ECU 7 reads the temperature Ts of the SCR catalyst in the process of S103. Specifically, ECU 7 reads the measured value of the temperature sensor 12 as the temperature Ts of the SCR catalyst. Note that when a temperature sensor is also attached to the exhaust pipe 3 between the second catalyst casing 5 and the third catalyst casing 6, the temperature Ts of the SCR catalyst may be calculated from the difference between the measured value of the temperature sensor and the measured value of the temperature sensor 12. The detector according to the present invention is realized by determining the temperature Ts of the SCR catalyst in accordance with the methods as described above.

(37) In a process of S104, ECU 7 judges whether or not the temperature Ts, which is read in the process of S103, is less than the lower limit temperature Tsl. As described above, the lower limit temperature Ts1 is the lower limit value of the temperature at which NH.sub.3 can be oxidized. If the affirmative judgment is made in the process of S104 (Ts<Tsl), ECU 7 proceeds to a process of S105.

(38) In the process of S105, ECU 7 sets the target air-fuel ratio A/Ftrg of the air-fuel mixture to the predetermined weak lean air-fuel ratio A/Fsl. As described in the foregoing explanation about FIG. 2, the predetermined weak lean air-fuel ratio A/Fsl is such an air-fuel ratio that the amount of NO.sub.X flowing out from the three-way catalyst is larger than zero, and the amount of NO.sub.X flowing out from the NSR catalyst is maximized at the air-fuel ratio (A/Fmax shown in FIG. 2) within a range of the air-fuel ratio in which the amount of NO.sub.X flowing out from the NSR catalyst per unit time is larger than the amount of NO.sub.X stored or occluded by the NSR catalyst per unit time. If the target air-fuel ratio A/Ftrg of the air-fuel mixture is set to the predetermined weak lean air-fuel ratio A/Fsl as described above, the exhaust gas flowing out from the NSR catalyst contains a relatively large amount of NO.sub.X. If the exhaust gas containing the relatively large amount of NO.sub.X flows into the SCR catalyst, the amount of NH.sub.3, which contributes to the reduction of NO.sub.X in the SCR catalyst, is increased. As a result, it is possible to effectively decrease the NH.sub.3 adsorption amount Adnh3 of the SCR catalyst. If the process of S105 is completely executed, ECU 7 once terminates the execution of this process routine.

(39) Further, if the negative judgment is made in the process of S104 described above (TsTsl), then ECU 7 proceeds to a process of S106 to judge whether or not the temperature Ts read in the process of S103 is less than the predetermined temperature Tsm. That is, ECU 7 judges whether or not the temperature Ts of the SCR catalyst belong to the temperature range which is not less than the lower limit temperature Tsl and which is less than the predetermined temperature Tsm. As described above, the predetermined temperature Tsm is the minimum value of the temperature at which the amount of NH.sub.3 desorbed from the SCR catalyst per unit time is larger than the amount of NH.sub.3 oxidized per unit time by the SCR catalyst. If the affirmative judgment is made in the process of S106 (Ts<Tsm), ECU 7 proceeds to a process of S107.

(40) In the process of S107, ECU 7 sets the target air-fuel ratio A/Ftrg of the air-fuel mixture to the predetermined lean air-fuel ratio A/Fl. As described above, the predetermined lean air-fuel ratio A/Fl is the air-fuel ratio which is higher than the predetermined weak lean air-fuel ratio A/Fsl, and the predetermined lean air-fuel ratio A/Fl is the highest air-fuel ratio within a range of the air-fuel ratio in which it is possible to secure the combustion stability of the internal combustion engine 1 and the drivability (or the highest air-fuel ratio within a range in which the exhaust emission other than NH.sub.3 does not exceed the regulation value). Note that if the operation state of the internal combustion engine 1 at the present point in time belongs to the lean operation area described above, the predetermined lean air-fuel ratio A/Fl may be an air-fuel ratio which is determined depending on the operation state of the internal combustion engine. If the target air-fuel ratio A/Ftrg of the air-fuel mixture is set to the predetermined lean air-fuel ratio A/Fl as described above, the exhaust gas, which contains a relatively large amount of oxygen, is supplied to the SCR catalyst. In this case, NH.sub.3, which is adsorbed to the SCR catalyst, is reacted with oxygen to produce NO.sub.X, and NO.sub.X is reacted with NH.sub.3 which is adsorbed to the SCR catalyst. As a result, it is possible to effectively decrease the NH.sub.3 adsorption amount Adnh3 of the SCR catalyst. If the process of S107 is completely executed, ECU 7 once terminates the execution of this process routine.

(41) Further, if the negative judgment is made in the process of S106 described above (TsTsm), ECU 7 proceeds to a process of S108. In the process of S108, ECU 7 executes the temperature raising process. Specifically, ECU 7 facilitates the temperature rising of the SCR catalyst by executing the process for retarding the ignition timing of the internal combustion engine 1 or the process for advancing the valve opening timing of the exhaust valve of the internal combustion engine 1. In this procedure, as described in the foregoing explanation about FIG. 4, ECU 7 calculates the target temperature Tstrg of the SCR catalyst so that the amount of NH.sub.3 desorbed from the SCR catalyst during the execution of the temperature raising process is included in the range of not more than the predetermined amount. Then, ECU 7 executes the temperature raising process so that the temperature Ts of the SCR catalyst is raised to the target temperature Tstrg. When the temperature raising process is executed as described above, it is possible to decrease the NH.sub.3 adsorption amount of the SCR catalyst without causing the stimulative odor. Note that ECU 7 may set the target air-fuel ratio of the air-fuel mixture to the predetermined lean air-fuel ratio when the temperature raising process is executed. When the target air-fuel ratio of the air-fuel mixture during the execution of the temperature raising process is set to the predetermined lean air-fuel ratio, then it is possible to facilitate the desorption of NH.sub.3 adsorbed to the SCR catalyst, and it is also possible to facilitate the oxidation of NH.sub.3 adsorbed to the SCR catalyst and NH.sub.3 desorbed from the SCR catalyst. Therefore, even when the internal combustion engine 1 is operated under the operation condition in which NH.sub.3 adsorbed to the SCR catalyst is easily desorbed after the execution of the temperature raising process, the amount of NH.sub.3 emitted into the atmospheric air can be more reliably suppressed to be not more than the permissible amount. Further, it is also possible to suppress the deterioration of the fuel consumption ratio so that the deterioration is small. If the process of S108 is completely executed, ECU 7 once terminates the execution of this process routine.

(42) Further, if the negative judgment is made in the process of S102 described above (Adnh3<Adthre1), it is possible to regard that NH.sub.3 in an amount of not less than the permissible amount is not desorbed from the SCR catalyst, even when the internal combustion engine 1 is operated under the operation condition in which NH.sub.3 adsorbed to the SCR catalyst is easily desorbed. On this account, ECU 7 proceeds to a process of S109 to judge whether or not the NH.sub.3 adsorption amount Adnh3 of the SCR catalyst read in the process of S101 is not more than a predetermined termination judgment value Adthre2 which is smaller than the predetermined threshold value Adthre1. If the negative judgment is made in the process of S109 (Adnh3>Adthre2), ECU 7 once terminates the execution of this process routine. In this procedure, if the NH.sub.3 decreasing process is being executed, the NH.sub.3 decreasing process is continuously executed. On the other hand, if the affirmative judgment is made in the process of S109 (Adnh3Adthre2), ECU 7 proceeds to a process of S110.

(43) When the process of S110 is executed, if the NH.sub.3 decreasing process is being executed, then ECU 7 terminates the NH.sub.3 decreasing process by restoring the target air-fuel ratio A/Ftrg of the air-fuel mixture to the air-fuel ratio which corresponds to the operation state of the internal combustion engine 1. Further, when the process of S110 is executed, if the NH.sub.3 decreasing process is not executed, then ECU 7 continues the state in which the NH.sub.3 decreasing process is not executed.

(44) ECU 7 executes the process routine shown in FIG. 6 as described above, and thus the controller according to the present invention is realized. Therefore, it is possible to decrease the NH.sub.3 adsorption amount of the SCR catalyst when the NH.sub.3 adsorption amount of the SCR catalyst is not less than the predetermined threshold value. As a result, the NH.sub.3 adsorption amount of the SCR catalyst is suppressed from exceeding the predetermined threshold value. Therefore, the amount of NH.sub.3 emitted from the SCR catalyst can be suppressed to be not more than the permissible amount. Further, when the NH.sub.3 adsorption amount of the SCR catalyst is not less than the predetermined threshold value, if the temperature of the SCR catalyst is not less than the lower limit temperature, then the NH.sub.3 adsorption amount of the SCR catalyst is decreased by allowing the air-fuel ratio of the air-fuel mixture to be the predetermined lean air-fuel ratio which is higher than the predetermined weak lean air-fuel ratio. Therefore, it is also possible to suppress the opportunities to decrease the air-fuel ratio of the air-fuel mixture to the predetermined weak lean air-fuel ratio so that there are few opportunities. As a result, the amount of NH.sub.3 emitted from the SCR catalyst can be suppressed to be not more than the permissible amount, while suppressing the deterioration of the fuel consumption ratio so that the deterioration is small.

REFERENCE SIGNS LIST

(45) 1 internal combustion engine 2 fuel injection valve 3 exhaust pipe 4 first catalyst casing 5 second catalyst casing 6 third catalyst casing 7 ECU 8 air-fuel ratio sensor 10 first NO.sub.X sensor 11 second NO.sub.X sensor 12 temperature sensor