CONTROL APPARATUS FOR EXHAUST GAS PURIFICATION APPARATUS

Abstract

When the NSR temperature Tnsr is in a warming-up temperature range equal to or higher than the activation start temperature of the NSR catalyst and lower than the activation completion temperature of the NSR catalyst, a control apparatus according to the present invention controls the quantity of fuel supplied to the NSR catalyst by a fuel supply device such that the air-fuel ratio of the exhaust gas flowing into the NSR catalyst while the rich spike process is performed is lower when the NSR temperature Tnsr is lower than a specific temperature Tthr than when the NSR temperature Tnsr is equal to or higher than the specific temperature Tthr.

Claims

1. A control apparatus applied to an exhaust gas purification apparatus which is equipped with an NOx storage reduction catalyst disposed in an exhaust passage of an internal combustion engine, and a fuel supply device that supplies fuel to exhaust gas flowing into the NOx storage reduction catalyst, the control apparatus comprising: a controller comprising at least one processor configured to: obtain an NSR temperature defined as the temperature of the NOx storage reduction catalyst; obtain an NOx storage amount defined as the amount of NOx stored in the NOx storage reduction catalyst; and perform a rich spike process, which is the process of reducing and removing NOx stored in the NOx storage reduction catalyst by supplying fuel through the fuel supply device so as to adjust the air-fuel ratio of exhaust gas flowing into the NOx storage reduction catalyst to a rich air-fuel ratio lower than the theoretical air-fuel ratio when the NOx storage amount is equal to or larger than a predetermined threshold in circumstances in which the NSR temperature is equal to or higher than the activation start temperature of the NOx storage reduction catalyst, wherein when the NSR temperature is in a warming-up temperature range equal to or higher than the activation start temperature of the NOx storage reduction catalyst and lower than the activation completion temperature of the NOx storage reduction catalyst, the controller controls the quantity of fuel supplied through the fuel supply device such that the air-fuel ratio of the exhaust gas flowing into the NOx storage reduction catalyst while the rich spike process is performed is lower when the NSR temperature is lower than a specific temperature than when the NSR temperature is equal to or higher than the specific temperature.

2. A control apparatus for an exhaust gas purification apparatus according to claim 1, wherein the exhaust gas purification apparatus further comprises a selective catalytic reduction catalyst disposed in the exhaust passage downstream of the NOx storage reduction catalyst, the controller is further configured to obtain an SCR temperature defined as the temperature of the selective catalytic reduction catalyst, wherein when the SCR temperature is lower than the activation start temperature of the selective catalytic reduction catalyst, the controller does not perform the rich spike process even if the NSR temperature is in the warming-up temperature range and the NOx storage amount is equal to or larger than the threshold.

3. A control apparatus for an exhaust gas purification apparatus according to claim 2, wherein when the NSR temperature is in the warming-up temperature range, if the SCR temperature at the time when the NOx storage amount becomes equal to or larger than the predetermined threshold is lower than the activation start temperature of the selective catalytic reduction catalyst, the controller performs the rich spike process after performing a heating-up process for raising the temperature of the selective catalytic reduction catalyst until the SCR temperature reaches or exceeds the activation start temperature of the selective catalytic reduction catalyst.

4. A control apparatus for an exhaust gas purification apparatus according to claim 1, wherein the exhaust gas purification apparatus further comprises a selective catalytic reduction catalyst disposed in the exhaust passage downstream of the NOx storage reduction catalyst and an additive supply device that supplied an additive, which is ammonia or a precursor of ammonia, to the selective catalytic reduction catalyst, the controller is further configured to: obtain an SCR temperature defined as the temperature of the selective catalytic reduction catalyst, and obtain an NH.sub.3 adsorption amount defined as the amount of ammonia adsorbed in the selective catalytic reduction catalyst, wherein when the NSR temperature is equal to or higher than the activation start temperature of the NOx storage reduction catalyst and the SCR temperature is equal to or higher than the activation start temperature of the selective catalytic reduction catalyst, if the NH.sub.3 adsorption amount at the time when the NOx storage amount becomes equal to or larger than the predetermined threshold is smaller than a predetermined amount, the controller performs an NH.sub.3 supply process to supply the additive by the additive supply device so as to make the NH.sub.3 adsorption amount in the selective catalytic reduction catalyst equal to or larger than the predetermined amount and performs the rich spike process after completion of the NH.sub.3 supply process.

5. A control apparatus for an exhaust gas purification apparatus according to claim 4, wherein the controller is further configured to calculate a slipping NOx quantity defined as the quantity of NOx which flows into and slips through the NOx storage reduction catalyst per unit time, wherein when the SCR temperature is equal to or higher than an adsorption limit temperature at which the amount of ammonia that the selective catalytic reduction catalyst can adsorb is smaller than the predetermined amount, the controller performs an equivalence ratio control to control the additive supply device such that a quantity of additive of which the equivalence ratio of the quantity of ammonia to the slipping NOx quantity is equal to a predetermined ratio is supplied to the selective catalytic reduction catalyst while the rich spike process is not performed and a quantity of additive of which the equivalence ratio of the quantity of ammonia to the slipping NOx quantity is larger than the predetermined ratio is supplied to the selective catalytic reduction catalyst while the rich spike process is performed.

6. A control apparatus for an exhaust gas purification apparatus according to any one of claim 2, wherein when the NSR temperature is equal to or higher than the activation completion temperature of the NOx storage reduction catalyst, the controller controls the quantity of fuel supplied by the fuel supply device such that the air-fuel ratio of the exhaust gas flowing into the NOx storage reduction catalyst while the rich spike process is performed is higher when the SCR temperature is equal to or higher than the activation start temperature of the selective catalytic reduction catalyst than when the SCR temperature is lower than the activation start temperature of the selective catalytic reduction catalyst.

7. A control apparatus for an exhaust gas purification apparatus according to any one of claim 3, wherein when the NSR temperature is equal to or higher than the activation completion temperature of the NOx storage reduction catalyst, the controller controls the quantity of fuel supplied by the fuel supply device such that the air-fuel ratio of the exhaust gas flowing into the NOx storage reduction catalyst while the rich spike process is performed is higher when the SCR temperature is equal to or higher than the activation start temperature of the selective catalytic reduction catalyst than when the SCR temperature is lower than the activation start temperature of the selective catalytic reduction catalyst.

8. A control apparatus for an exhaust gas purification apparatus according to any one of claim 4, wherein when the NSR temperature is equal to or higher than the activation completion temperature of the NOx storage reduction catalyst, the controller controls the quantity of fuel supplied by the fuel supply device such that the air-fuel ratio of the exhaust gas flowing into the NOx storage reduction catalyst while the rich spike process is performed is higher when the SCR temperature is equal to or higher than the activation start temperature of the selective catalytic reduction catalyst than when the SCR temperature is lower than the activation start temperature of the selective catalytic reduction catalyst.

9. A control apparatus for an exhaust gas purification apparatus according to any one of claim 5, wherein when the NSR temperature is equal to or higher than the activation completion temperature of the NOx storage reduction catalyst, the controller controls the quantity of fuel supplied by the fuel supply device such that the air-fuel ratio of the exhaust gas flowing into the NOx storage reduction catalyst while the rich spike process is performed is higher when the SCR temperature is equal to or higher than the activation start temperature of the selective catalytic reduction catalyst than when the SCR temperature is lower than the activation start temperature of the selective catalytic reduction catalyst.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0027] FIG. 1 is a diagram showing the general configuration of an internal combustion engine to which the present disclosure is applied and its exhaust system in a first embodiment.

[0028] FIG. 2 shows the relationship between the NSR temperature Tnsr and the N.sub.2O concentration in the exhaust gas flowing out of the NSR catalyst.

[0029] FIG. 3 is a flow chart of a processing routine executed by an ECU when a rich spike process is performed in the first embodiment.

[0030] FIG. 4 is a diagram showing the general configuration of an internal combustion engine to which the present disclosure is applied and its exhaust system in a second embodiment.

[0031] FIG. 5 is a flow chart of a processing routine executed by the ECU when a rich spike process is performed in the second embodiment.

[0032] FIG. 6 is a flow chart of a processing routine executed by the ECU when a rich spike process is performed in a modification of the second embodiment.

[0033] FIG. 7 is a flow chart of a processing routine executed by the ECU when an equivalence ratio control is performed in the modification of the second embodiment.

DESCRIPTION OF EMBODIMENTS

[0034] In the following, specific embodiments of the present disclosure will be described with reference to the drawings. The dimensions, materials, shapes, relative arrangements, and other features of the components that will be described in connection with the embodiments are not intended to limit the technical scope of the present disclosure only to them, unless particularly stated.

First Embodiment

[0035] A first embodiment of the present disclosure will be described with reference to FIGS. 1 to 3. FIG. 1 is a diagram showing the general configuration of an internal combustion engine to which the present disclosure is applied and its exhaust system. The internal combustion engine 1 shown in FIG. 1 is a compression-ignition internal combustion engine (diesel engine) having a fuel injection valve 2 that injects fuel into a cylinder (not shown). The internal combustion engine 1 may alternatively be a spark-ignition internal combustion engine that operates with air-fuel mixture having a lean air-fuel ratio higher than the theoretical air-fuel ratio.

[0036] The internal combustion engine 1 is connected with an exhaust passage 3. The exhaust passage 3 is a channel through which the gas (exhaust gas) having been burned in the cylinder of the internal combustion engine 1 flows. In the middle of the exhaust passage 3, an exhaust gas purification apparatus is provided. The exhaust gas purification apparatus includes an NSR catalyst 4 provided in the exhaust passage 3 and a fuel addition valve 6 provided in the exhaust passage 3 upstream of the NSR catalyst 4.

[0037] The NSR catalyst 4 is made up of a honeycomb structure coated with a coating layer such as alumina, a noble metal (such as platinum, palladium, or rhodium) supported on the coating layer, and a NOx storage material (such as barium or lithium) supported on the coating layer. The NSR catalyst 4 configured as above takes in NOx in the exhaust gas to store it when the air-fuel ratio of the exhaust gas flowing into the NSR catalyst 4 is a lean air-fuel ratio. The term “store” (along with its derivatives) is used in this specification to express modes in which the NSR catalyst stores NOx chemically and modes in which the NSR catalyst adsorbs NOx physically. When the oxygen concentration in the exhaust gas flowing into the NSR catalyst 4 is low and the concentration of unburned fuel is high, namely when the air-fuel ratio of the exhaust gas flowing into the NSR catalyst 4 is a rich air-fuel ratio, the NSR catalyst 4 desorbs NOx stored therein to allow the desorbed NOx to be reduced by unburned fuel into nitrogen (N.sub.2) and/or ammonia (NH.sub.3). The fuel addition valve 6 is a device used to add fuel to the exhaust gas flowing in the exhaust passage 3 upstream of the NSR catalyst 4. The fuel addition valve constitutes the fuel supply device according to the present disclosure. The fuel supply device may be implemented by fuel injection through the fuel injection valve 2 of the cylinder during the exhaust stroke.

[0038] The exhaust passage 3 upstream of the NSR catalyst 4 is provided with a first A/F sensor 9, a first NOx sensor 10, and a first temperature sensor 11. The first A/F sensor 9 outputs an electrical signal representing the air-fuel ratio of the exhaust gas flowing into the NSR catalyst 4. The first NOx sensor 10 outputs an electrical signal representing the concentration of NOx in the exhaust gas flowing into the NSR catalyst 4. The first temperature sensor 11 outputs an electrical signal representing the temperature of the exhaust gas flowing into the NSR catalyst 4. The exhaust passage 3 downstream of the NSR catalyst 4 is provided with a second temperature sensor 12, a second A/F sensor 13, and a second NOx sensor 14. The second temperature sensor 12 outputs an electrical signal representing the temperature of the exhaust gas flowing out of the NSR catalyst 4. The second A/F sensor 13 outputs an electrical signal representing the air-fuel ratio of the exhaust gas flowing out of the NSR catalyst 4. The second NOx sensor 14 outputs an electrical signal representing the concentration of NOx in the exhaust gas flowing out of the NSR catalyst 4.

[0039] An ECU 8 is provided for the internal combustion engine 1 having the above-described configuration. The ECU 8 is an electronic control unit composed of a CPU, a ROM, a RAM, and a backup RAM etc. The ECU 8 is electrically connected with the first A/F sensor 9, the first NOx sensor 10, the first temperature sensor 11, the second temperature sensor 12, the second A/F sensor 13, and the second NOx sensor 14. The ECU 8 is also electrically connected with various sensors such as an accelerator position sensor 17, a crank position sensor 18, and an air flow meter 19. Signals output from the aforementioned sensors are input to the ECU 8. The accelerator position sensor 17 is a sensor that outputs an electrical signal representing the amount of operation of the accelerator pedal (or the accelerator opening degree). The crank position sensor 18 is a sensor that outputs an electrical signal representing the rotational position of the power output shaft (i.e. crankshaft) of the internal combustion engine 1. The air flow meter 19 is a sensor that outputs an electrical signal representing the intake air quantity of the internal combustion engine 1.

[0040] The ECU 8 is electrically connected with various devices including the fuel injection valve 2, the fuel addition valve 6, and a urea addition valve 7 and adapted to control these devices using signals output from the aforementioned sensors. For example, the ECU 8 controls the fuel injection quantity and fuel injection timing of the fuel injection valve 2 on the basis of the output signal of the accelerator position sensor 17 (accelerator opening degree) and the engine speed that is calculated using the output signal of the crank position sensor 18. The ECU 8 performs a rich spike process for changing the exhaust gas flowing into the NSR catalyst 4 into a gas having a low oxygen concentration and high unburned fuel concentration (namely, a gas having a rich air-fuel ratio) by adding fuel to the exhaust gas through the fuel addition valve 6, when the amount of NOx stored in the NSR catalyst 4 (or the NOx storage amount) is equal to or larger than a predetermined threshold in circumstances in which the temperature of the NSR catalyst 4 (or the NSR temperature) is equal to or higher than the activation start temperature. In the following, how the rich spike process is performed in this embodiment will be described.

[0041] The quantity of fuel added to the exhaust gas through the fuel addition valve 6 in the rich spike process is generally controlled such that the exhaust gas flowing into the NSR catalyst 4 has a rich air-fuel ratio that is suitable for reduction and removal of NOx stored in the NSR catalyst 4. Specifically, when the NSR temperature is equal to or higher than the activation completion temperature, the fuel addition quantity through the fuel addition valve 6 is controlled so as to adjust the air-fuel ratio of the exhaust gas flowing into the NSR catalyst 4 to a standard rich air-fuel ratio (e.g. 13.5) of a relatively high degree of richness. The aforementioned activation completion temperature is a temperature at which, for example, the NOx removal rate reaches or exceeds 80% when the NSR catalyst 4 is in a rich atmosphere. The activation completion temperature as such is about 350° C. When the NSR temperature is in a warming-up temperature range higher than or equal to the activation start temperature and lower than the aforementioned activation completion temperature, the fuel addition quantity through the fuel addition valve 6 is controlled such that the exhaust gas flowing into the NSR catalyst 4 has a rich air-fuel ratio of a degree of richness smaller than the degree of richness of the standard rich air-fuel ratio, the lower the NSR temperature, the smaller the degree of richness. The aforementioned activation start temperature is a temperature at which, for example, the NOx removal rate reaches or exceeds 20% when the NSR catalyst 4 is in a rich atmosphere. The activation start temperature as such is about 200° C.

[0042] While the NSR catalyst 4 is in the process of warming-up, as is the case when the NSR temperature is in the warming-up temperature range, if the rich spike process is performed, there is a possibility that a portion of NOx stored in the NSR catalyst 4 may not be reduced into nitrogen (N.sub.2), and N.sub.2O may be produced.

[0043] For the purpose of developing an effective method for reducing the amount of N.sub.2O produced in an NSR catalyst during the rich spike process when the NSR temperature is in the warming-up temperature range, the inventor of the present disclosure conducted experiments and verifications strenuously to discover that when the NSR temperature is lower than a specific temperature (e.g. 250° C.), the amount of N.sub.2O produced in the NSR catalyst during the rich spike process is smaller when the air-fuel ratio of the exhaust gas flowing into the NSR catalyst is low (namely, when the degree of richness is large) than when it is high (namely, when the degree of richness is small) and that when the NSR temperature is equal to or higher than the aforementioned specific temperature, the amount of N.sub.2O produced in the NSR catalyst during the rich spike process is smaller when the air-fuel ratio of the exhaust gas flowing into the NSR catalyst is high (namely, when the degree of richness is small) than when it is low (namely, when the degree of richness is large). From this follows that when the NSR temperature is in the warming-up temperature range, changing the air-fuel ratio of the exhaust gas flowing into the NSR catalyst to an air-fuel ratio suitable for removal of NOx described above can lead to an increase in the amount of N.sub.2O produced in the NSR catalyst.

[0044] Given the above discovery, we designed the rich spike process of this embodiment to control the fuel addition quantity through the fuel addition valve 6 such that when the NSR temperature is in the aforementioned warming-up temperature range, the air-fuel ratio of the exhaust gas flowing into the NSR catalyst 4 is made lower when the NSR temperature is lower than the aforementioned specific temperature than when the NSR temperature is equal to or higher than the aforementioned specific temperature. Specifically, in the case where the rich spike process is carried out in circumstances in which the NSR temperature is in the warming-up temperature range and lower than the aforementioned specific temperature, the fuel addition quantity through the fuel addition valve 6 is controlled so as to adjust the air-fuel ratio of the exhaust gas flowing into the NSR catalyst 4 to a first rich air-fuel ratio lower than air-fuel ratios suitable for removal of NOx. In the case where the rich spike process is carried out in circumstances in which the NSR temperature is in the warming-up temperature range and equal to or higher than the aforementioned specific temperature, the fuel addition quantity through the fuel addition valve 6 is controlled so as to adjust the air-fuel ratio of the exhaust gas flowing into the NSR catalyst 4 to a second rich air-fuel ratio higher than air-fuel ratios suitable for removal of NOx and higher than the aforementioned first rich air-fuel ratio.

[0045] FIG. 2 shows the relationship between the NSR temperature and the N.sub.2O concentration in the exhaust gas flowing out of the NSR catalyst 4. The horizontal axis of FIG. 2 represents the NSR temperature Tnsr, and the vertical axis of FIG. 2 represents the N.sub.2O concentration in the exhaust gas flowing out of the NSR catalyst 4. The solid curve in FIG. 2 shows the relationship in a case where the air-fuel ratio of the exhaust gas flowing into the NSR catalyst 4 is equal to the first rich air-fuel ratio A/Fr1. The chain curve in FIG. 2 shows the relationship in a case where the air-fuel ratio of the exhaust gas flowing into the NSR catalyst 4 is equal to the second rich air-fuel ratio A/Fr2. In FIG. 2, Tnsr1 is the activation start temperature with the NSR catalyst 4, Tnsr2 is the activation completion temperature with the NSR catalyst 4, and Tthr is the aforementioned specific temperature.

[0046] As shown in FIG. 2, in the case where the NSR temperature Tnsr is in the warming-up temperature range and lower than the specific temperature Tthr, the N.sub.2O concentration in the exhaust gas flowing out of the NSR catalyst 4 is lower when the air-fuel ratio of the exhaust gas flowing into the NSR catalyst 4 is equal to the first rich air-fuel ratio A/Fr1 than when the air-fuel ratio of the exhaust gas flowing into the NSR catalyst 4 is equal to the second rich air-fuel ratio A/Fr2. In the case where the NSR temperature Tnsr is in the warming-up temperature range and equal to or higher than the specific temperature Tthr, the N.sub.2O concentration in the exhaust gas flowing out of the NSR catalyst 4 is lower when the air-fuel ratio of the exhaust gas flowing into the NSR catalyst 4 is equal to the second rich air-fuel ratio A/Fr2 than when the air-fuel ratio of the exhaust gas flowing into the NSR catalyst 4 is equal to the first rich air-fuel ratio A/Fr1.

[0047] Therefore, in the case where the NSR temperature Tnsr is in the warming-up temperature range, the amount of N.sub.2O produced in the NSR catalyst 4 can be kept small by adjusting the air-fuel ratio of the exhaust gas flowing into the NSR catalyst 4 during the rich spike process to the first rich air-fuel ratio A/Fr1 when the NSR temperature Tnsr is lower than the aforementioned specific temperature Tthr and to the second rich air-fuel ratio A/Fr2 when the NSR temperature Tnsr is equal to or higher than the aforementioned specific temperature Tthr.

[0048] The aforementioned first rich air-fuel ratio A/Fr1 is an air-fuel ratio at which the amount of N.sub.2O produced is considered to be minimized while keeping the NOx removal rate with the NSR catalyst 4 higher than or equal to a desired lower limit value when the NSR temperature Tnsr is equal to or higher than the activation start temperature Tnsr1 and lower than the aforementioned specific temperature Tthr. The second rich air-fuel ratio A/Fr2 is an air-fuel ratio at which the amount of N.sub.2O produced is considered to be minimized while keeping the NOx removal rate with the NSR catalyst 4 higher than or equal to a desired lower limit value when the NSR temperature Tnsr is higher than or equal to the aforementioned specific temperature Tthr and lower than the activation completion temperature Tnsr2. By setting the first rich air-fuel ratio A/Fr1 and the second rich air-fuel ratio A/Fr2 as above, the amount of N.sub.2O produced in the NSR catalyst 4 can be kept small while preventing an excessive increase in the amount of NOx that is not removed in the NSR catalyst 4. The first rich air-fuel ratio A/Fr1 and the second rich air-fuel ratio A/Fr2 that satisfy the above-described conditions are determined experimentally in advance.

[0049] In the following, the procedure of performing the rich spike process according to the first embodiment will be described with reference to FIG. 3. FIG. 3 is a flow chart of a processing routine executed by the ECU 8 at regular intervals while the internal combustion engine 1 is operating. The processing routine is stored in the ROM or other device of the ECU 8.

[0050] In the processing routine in FIG. 3, firstly in the processing of step S101, the ECU 8 obtains the NSR temperature Tnsr. The NSR temperature Tnsr is calculated using as parameters the difference between the measurement value of the first temperature sensor 11 and the measurement value of the second temperature sensor 12 and the exhaust gas flow rate, which is the intake air quantity (i.e. the measurement value of the air flow meter 19) and the fuel injection quantity. Alternatively, the NSR temperature Tnsr may be calculated using as parameters the measurement value of the second temperature senor 12 and the exhaust gas flow rate.

[0051] In the processing of step S102, the ECU 8 determines whether or not the NSR temperature Tnsr calculated in step S101 is equal to or higher than the activation start temperature Tnsr1. If the determination made in step S102 is negative, the NOx removal capability of the NSR catalyst 4 is not active, and therefore the ECU 8 terminates the execution of this processing routine. On the other hand, the determination made in step S102 is affirmative, the ECU 8 executes the processing of step S103 next.

[0052] In the processing of step S103, the ECU 8 obtains or retrieves the NOx storage amount Anox in the NSR catalyst 4. The NOX storage amount Anox in the NSR catalyst 4 is calculated in another process by integrating the quantity of NOx stored into the NSR catalyst 4 per unit time since the end of the previous rich spike process. The quantity of NOx stored into the NSR catalyst 4 per unit time is equal to the difference between the quantity of NOx flowing into the NSR catalyst 4 per unit time and the quantity of NOx flowing out of the NSR catalyst 4 per unit time. The quantity of NOx flowing into the NSR catalyst 4 per unit time can be calculated as the product of the measurement value of the first NOx sensor 10 (namely, the NOx concentration in the exhaust gas flowing into the NSR catalyst 4) and the exhaust gas flow rate. The quantity of NOx flowing out of the NSR catalyst 4 per unit time can be calculated as the product of the measurement value of the second NOx sensor 14 and the exhaust gas flow rate. Alternatively, the quantity of NOx flowing into the NSR catalyst 4 per unit time may be estimated using as parameters operating conditions of the internal combustion engine 1 (such as the engine load and the engine speed etc.).

[0053] In the processing of step S104, the ECU 8 determines whether or not the NOx storage amount Anox obtained in step S103 is equal to or larger than a predetermined threshold Anoxthr. The predetermined threshold Anoxthr is such a value that if the NOx storage amount Anox in the NSR catalyst 4 is equal to or larger than this predetermined threshold Anoxthr at the time when the internal combustion engine 1 is stopped, it is considered that there is a possibility that the NSR catalyst 4 will not be able to exercise a desired NOx storage capability after the internal combustion engine 1 starts to operate next time. If the determination made in step S104 is negative, it is not necessary to perform the rich spike process, and the ECU 8 terminates the execution of this processing routine. On the other hand, if the determination made in step S104 is affirmative, the ECU 8 executes the processing of step S105 next.

[0054] In the processing of step S105, the ECU 8 determines whether or not the NSR temperature Tnsr obtained in the processing of step S101 is equal to or larger than the activation completion temperature Tnsr. If the determination made in step S105 is affirmative, it is considered that the NOx removal capability of the NSR catalyst 4 is sufficiently active. Then, the ECU 8 executes the processing of S106 next, where the ECU 8 performs the rich spike process so as to adjust the air-fuel ratio (A/F) of the exhaust gas flowing into the NSR catalyst 4 to the aforementioned standard rich air-fuel ratio A/Frst. Specifically, the ECU 8 calculates a fuel addition quantity that is needed to adjust the air-fuel ratio (A/F) of the exhaust gas flowing into the NSR catalyst 4 to the aforementioned standard rich air-fuel ratio A/Frst, using as parameters the difference between the air-fuel ratio of the exhaust gas discharged from the internal combustion engine 1 (i.e. the measurement value of the first NOx sensor 10) and the standard rich air-fuel ratio A/Frst and the exhaust gas flow rate. Then, the ECU 8 performs the rich spike process by controlling the fuel addition valve 6 on the basis of the fuel addition quantity thus calculated. Thus, NOx stored in the NSR catalyst 4 can be reduced and removed efficiently. The rich spike process performed by the processing of step S106 may be terminated either at the time when a predetermined length of time has elapsed or at the time when the measurement value of the second A/F sensor 13 becomes equal to or lower than the standard rich air-fuel ratio A/Frst.

[0055] If the determination made in step S105 is negative, the NSR temperature Tnsr is in the warming-up temperature range. Then, if the air-fuel ratio of the exhaust gas flowing into the NSR catalyst 4 is adjusted to an air-fuel ratio suitable for removal of NOx when the rich spike process is performed, there is a possibility that the amount of N.sub.2O produced may increase, as described above. Therefore, in the processing of step S107 onward, the ECU 8 performs the rich spike process while controlling the production of N.sub.2O.

[0056] In the processing of step S107, the ECU 8 determines whether or not the NSR temperature Tnsr obtained in the processing of step S101 is equal to or higher than the specific temperature Tthr. If the determination made in step S107 is affirmative, the NSR temperature Tnsr is in the temperature range equal to or higher than the specific temperature Tthr and lower than the activation completion temperature Tnsr2. In the temperature range equal to or higher than the specific temperature Tthr and lower than the activation completion temperature Tnsr2, the amount of N.sub.2O produced in the NSR catalyst 4 is smaller when the degree of richness of the exhaust gas flowing into the NSR catalyst 4 is small than when it is large, as described above with reference to FIG. 2. Therefore, if the determination made in step S107 is affirmative, the ECU 8 executes the processing of step S108 next, where the ECU 8 performs the rich spike process so as to adjust the air-fuel ratio of the exhaust gas flowing into the NSR catalyst 4 to the second rich air-fuel ratio A/Fr2 of which the degree of richness is larger than the degree of richness of the air-fuel ratio suitable for removal of NOx. As described above, the second rich air-fuel ratio A/Fr2 is an air-fuel ratio at which the amount of N.sub.2O produced is minimized while keeping the NOx removal rate with the NSR catalyst 4 higher than or equal to a desired lower limit value when the NSR temperature Tnsr is higher than or equal to the specific temperature Tthr and lower than the activation completion temperature Tnsr2. Therefore, the amount of N.sub.2O produced in the NSR catalyst 4 can be kept small while preventing an excessive increase in the amount of NOx that is not removed in the NSR catalyst 4. The rich spike process performed by the processing of step S108 may be terminated either at the time when a predetermined length of time has elapsed or at the time when the measurement value of the second A/F sensor 13 becomes equal to or lower than the second rich air-fuel ratio A/Fr2.

[0057] On the other hand, if the determination made in step S107 is negative, the NSR temperature Tnsr is in the temperature range equal to or higher than the activation start temperature Tnsr1 and lower than the specific temperature Tthr. In the temperature range equal to or higher than activation start temperature Tnsr1 and lower than the specific temperature Tthr, the amount of N.sub.2O produced in the NSR catalyst 4 is smaller when the degree of richness of the exhaust gas flowing into the NSR catalyst 4 is large than when it is small, as described above with reference to FIG. 2.

[0058] Therefore, the ECU 8 executes the processing of step S109 next, where the ECU 8 performs the rich spike process so as to adjust the air-fuel ratio of the exhaust gas flowing into the NSR catalyst 4 to the first rich air-fuel ratio A/Fr1 lower than the air-fuel ratio suitable for removal of NOx. As described above, the first rich air-fuel ratio A/Fr1 is an air-fuel ratio at which the amount of N.sub.2O produced is minimized while keeping the NOx removal rate with the NSR catalyst 4 higher than or equal to a desired lower limit value when the NSR temperature Tnsr is higher than or equal to the activation start temperature Tnsr1 and lower than the specific temperature Tthr. Therefore, the amount of N.sub.2O produced in the NSR catalyst 4 can be kept small while preventing an excessive increase in the amount of NOx that is not removed in the NSR catalyst 4. The rich spike process performed by the processing of step S109 may be terminated either at the time when a predetermined length of time has elapsed or at the time when the measurement value of the second A/F sensor 13 becomes equal to or lower than the first rich air-fuel ratio A/Fr1.

[0059] With the above-described embodiment, when the rich spike process is performed, the amount of N.sub.2O produced in the NSR catalyst 4 can be kept as small as possible while preventing an excessive increase in the amount of NOx that is not removed in the NSR catalyst 4.

Second Embodiment

[0060] Next, a second embodiment of the present disclosure will be described with reference to FIGS. 4 and 5. In the following, only features that are different from those in the above-described first embodiment will be described, and like features will not be described.

[0061] The second embodiment differs from the first embodiment in that the exhaust gas purification apparatus has, in addition to an NSR catalyst 4, an SCR catalyst 5 arranged in the exhaust passage 3 downstream of the NSR catalyst 4, and even when the temperature of the NSR catalyst 4 is in the warming-up temperature range, the rich spike process is not performed if the SCR catalyst 5 is not active.

[0062] FIG. 4 is a diagram showing the general configuration of an internal combustion engine and its exhaust system according to the second embodiment. As shown in FIG. 4, the exhaust gas purification apparatus of this embodiment has, in addition to an NSR catalyst 4 and a fuel injection valve 6, an SCR catalyst 5 arranged in the exhaust passage 3 downstream of the NSR catalyst 4 and an addition valve 7 arranged in the exhaust passage 3 between the NSR catalyst 4 and the SCR catalyst 5.

[0063] The SCR catalyst 5 is made up of a honeycomb structure made of cordierite or an Fe-Cr-Al heat-resisting steel, an alumina-based or zeolite-based coating layer on the honeycomb structure, and a noble metal (such as platinum or palladium) supported on the coating layer. The SCR catalyst 5 configured as above adsorbs NH.sub.3 contained in the exhaust gas and reduces NOx in the exhaust gas using the adsorbed NH.sub.3 as reducing agent to remove NOx.

[0064] The addition valve 7 is a valve device used to add a reducing agent such as NH.sub.3 or a precursor of NH.sub.3 to the exhaust gas. The reducing agent may be urea solution (aqueous solution of urea) or NH.sub.3 gas. In this embodiment, urea solution is used as the reducing agent. Accordingly, the addition valve 7 will be referred to as the urea addition valve 7. Urea solution added by the urea addition valve 7 is thermally decomposed in the exhaust gas or in the SCR catalyst 5 and hydrolyzed in the SCR catalyst 5 to produce NH.sub.3. NH.sub.3 thus produced is adsorbed by the SCR catalyst 5. The urea addition valve 7 constitutes the additive supply device according to the present disclosure.

[0065] The exhaust passage 3 downstream of the SCR catalyst 5 is provided with a third NOx sensor 15 and a third temperature sensor 16. The third NOx sensor 15 outputs an electrical signal representing the NOx concentration in the exhaust gas flowing out of the SCR catalyst 5. The third temperature sensor 16 outputs an electrical signal representing the temperature of the exhaust gas flowing out of the SCR catalyst 5. Signals output from these sensors are input to the ECU 8.

[0066] In the system shown in FIG. 4, in the case where the NSR temperature Tnsr is in the warming-up temperature range, if the rich spike process is performed in the same procedure as in the first embodiment when the NOx storage amount Anox in the NSR catalyst 4 is equal to or larger than the aforementioned predetermined threshold Anoxthr, NOx that has not been removed by the NSR catalyst 4 is removed by the SCR catalyst 5. Therefore, an increase in the amount of NOx that is not removed by the NSR catalyst 4 or the SCR catalyst 5 can be prevented. However, when the temperature Tscr of the SCR catalyst 5 (which will be referred to as the “SCR temperature”) at the time when the rich spike process is performed is lower than the activation start temperature Tscr1 of the SCR catalyst 5, NOx that has not been removed by the NSR catalyst 4 is not removed by the SCR catalyst 5 either.

[0067] Therefore, in this embodiment, even when the NSR temperature Tnsr is in the warming-up temperature range and the NOx storage amount Anox in the NSR catalyst 4 is equal to or larger than the aforementioned specific threshold Anoxthr, the rich spike process is not performed if the SCR temperature Tscr is lower than the activation start temperature Tscr1 of the SCR catalyst 5. With this feature, when the NSR temperature Tnsr is in the warming-up temperature range, the rich spike process is performed on condition that the SCR temperature Tscr is equal to or higher than the activation start temperature Tscr1 of the SCR catalyst 5. Therefore, even if the air-fuel ratio of the exhaust gas flowing into the NSR catalyst 4 is adjusted to an air-fuel ratio at which the NOx removal rate with the NSR catalyst 4 is lower than the aforementioned lower limit during the rich spike process, an increase in the amount of NOx that is not removed by the exhaust gas purification apparatus can be prevented. Thus, in this embodiment, the first rich air-fuel ratio A/Fr1 is set to an air-fuel ratio at which the amount of N.sub.2O produced in the NSR catalyst 4 is minimized when the NSR temperature Tnsr is in the temperature range equal to or higher than the activation start temperature Tnsr1 of the NSR catalyst and lower than the specific temperature Tthr. The first rich air-fuel ratio A/Fr1 as such is set to, for example, about 13.5. Similarly, the second rich air-fuel ratio A/Fr2 in this embodiment is set to an air-fuel ratio at which the amount of N.sub.2O produced in the NSR catalyst 4 is minimized when the NSR temperature Tnsr is in the temperature range equal to or higher than the specific temperature Tthr and lower than the activation completion temperature Tnsr2 of the NSR catalyst 4. The second rich air-fuel ratio A/Fr2 as such is set to, for example, approximately 14.0. By setting the first rich air-fuel ratio A/Fr1 and the second rich air-fuel ratio A/Fr2 as above, the amount of N.sub.2O produced in the NSR catalyst 4 can be kept small with higher reliability while preventing an increase in the amount of NOx that is not removed by the exhaust gas purification apparatus during the rich spike process.

[0068] If an engine operation state in which the exhaust gas temperature is relatively low while the NSR temperature Tnsr is in the aforementioned warming-up temperature range and the NOx storage amount Anox in the NSR catalyst 4 is equal to or larger than the aforementioned predetermined threshold Anoxthr continues, the rate of rise in the SCR temperature Tscr may be so low that the rich spike process may not be performed for a long period of time. Then, the NOx storage capability of the NSR catalyst 4 may be saturated. To address this problem, in this embodiment, if the SCR temperature Tscr is lower than the activation start temperature Tscr1 of the SCR catalyst 5 at the time when the NOx storage amount Anox in the NSR catalyst 4 reaches or exceeds the aforementioned specific threshold Anoxthr while the NSR temperature Tnsr is in the warming-up temperature range, a process for heating up the SCR catalyst 5 (which will be hereinafter referred to as the “heating-up process”) is performed until the SCR temperature Tscr reaches or exceeds the activation start temperature Tscr1 of the SCR catalyst 5, and the rich spike process is performed after the completion of the heating-up process. With this feature, the period over which the rich spike process is not performed while the NOx storage amount Anox in the NSR catalyst 4 is equal to or larger than the specific threshold Anoxthr can be prevented from continuing for a long time. Therefore, saturation of the NOx storage capability of the NSR catalyst 4 can be prevented.

[0069] The heating-up process mentioned above can be carried out by supplying fuel to the NSR catalyst 4 through the fuel addition valve 6 to cause oxidation of fuel in the NSR catalyst 4, thereby heating up the exhaust gas flowing into the SCR catalyst 5 by the reaction heat generated thereby. However, if the air-fuel ratio of the exhaust gas flowing into the NSR catalyst 4 during the heating-up process becomes equal to or lower than the theoretical air-fuel ratio, there is a possibility that NOx stored in the NSR catalyst 4 may be desorbed undesirably. Therefore, the quantity of fuel supplied from the fuel addition valve 6 to the NSR catalyst 4 in the heating-up process is to be controlled such that the air-fuel ratio of the exhaust gas flowing into the NSR catalyst 4 will be higher than the theoretical air-fuel ratio. By performing the heating-up process in this way, the temperature of the SCR catalyst 5 can be raised without causing undesirable desorption of NOx stored in the NSR catalyst 4. In cases where the exhaust gas purification apparatus is equipped with an electric heater capable of heating the SCR catalyst 5, the heating-up process may be performed by heating the SCR catalyst 5 by the heater.

[0070] Even in the case where the SCR temperature Tscr is equal to or higher than the activation start temperature Tscr1 of the SCR catalyst 5 at the time when the NOx storage amount Anox in the NSR catalyst 4 reaches or exceeds the aforementioned specific threshold Anoxthr while the NSR temperature Tnsr is in the warming-up temperature range, there is a possibility that a portion of NOx that has not been removed in the NSR catalyst 4 during the rich spike process may not be removed by the SCR catalyst 5 either, if the amount Anh3 of NH.sub.3 adsorbed in the SCR catalyst 5 is small. To address this problem, in this embodiment, if the amount Anh3 of NH.sub.3 adsorbed in the SCR catalyst 5 is smaller than a specific amount Anh3thr at the time when a condition for performing the rich spike process is met, a process of supplying urea solution through the urea addition valve 7 (which will be referred to as the “NH.sub.3 supply process”) so as to make the amount Anh3 of NH.sub.3 adsorbed in the SCR catalyst 5 larger than or equal to the aforementioned specific amount Anh3thr is performed, and the rich spike process is performed thereafter. The aforementioned condition for performing the rich spike process is that the NSR temperature Tnsr is equal to or higher than the activation start temperature Tnsr1 of the NSR catalyst 4, the SCR temperature Tscr is equal to or higher than the activation start temperature Tscr1 of the SCR catalyst 5, and the NOx storage amount Anox in the NSR catalyst 4 is equal to or larger than the specific threshold Anoxthr. The specific amount Anh3thr mentioned above is an amount of NH.sub.3 that is needed to remove NOx flowing out of the NSR catalyst 4 in its entirety by the SCR catalyst 5 in circumstances in which the quantity of NOx flowing out of the NSR catalyst 4 during the rich spike process is considered to be largest. The specific amount Anh3thr as such is determined in advance by an adaptation process based on an experiment. With this feature, NOx that has not been removed by the NSR catalyst 4 in the rich spike process can be removed by the SCR catalyst 5 with improved reliability.

[0071] In the following, the procedure of performing the rich spike process according to the second embodiment will be described with reference to FIG. 5. FIG. 5 is a flow chart of a processing routine executed by the ECU 8 at regular intervals while the internal combustion engine 1 is operating. The processing routine is stored in the ROM or other device of the ECU 8. The processings that are the same as those in the processing routine shown in FIG. 3 according to the above-described first embodiment are denoted by the same reference signs.

[0072] In the processing routine in FIG. 5, firstly in the processing of step S201, the ECU 8 obtains the NSR temperature Tnsr and the SCR temperature Tscr. The NSR temperature Tnsr is obtained in the same manner as in the first embodiment. The SCR temperature Tscr is calculated using as parameters the difference between the measurement value of the second temperature sensor 12 and the third temperature sensor 16 and the exhaust gas flow rate. Alternatively, the SCR temperature Tscr may be calculated using as parameters the measurement value of the third temperature sensor 16 and the exhaust gas flow rate.

[0073] After executing the processing of step S201, the ECU 8 executes the processing of steps S102 trough S104. If the determination made in step S104 is affirmative, the ECU 8 executes the processing of step S202 next, where the ECU 8 determines whether or not the SCR temperature Tscr obtained in step S201 is equal to or higher than the activation start temperature Tscr1 of the SCR catalyst 5. If the determination made in step S202 is affirmative, the ECU 8 executes the processing of step S203 next.

[0074] In the processing of step S203, the ECU 8 retrieves the NH.sub.3 adsorption amount Anh3 in the SCR catalyst 5 (or the amount of NH.sub.3 adsorbed in the SCR catalyst 5). The NH.sub.3 adsorption amount Anh3 in the SCR catalyst 5 is determined in another process by the method described below and written in a specific memory area of the RAM or backup RAM. The NH.sub.3 adsorption amount in the SCR catalyst 5 is calculated by integrating the value obtained by subtracting the NH.sub.3 consumption (i.e. the quantity of NH.sub.3 that contributes to reduction of NOx in the SCR catalyst 5) per unit time and the slipping NH.sub.3 quantity (i.e. the quantity of NH.sub.3 slipping through the SCR catalyst 5) per unit time from the quantity of NH.sub.3 supplied to the SCR catalyst 5 per unit time. The quantity of NH.sub.3 supplied to the SCR catalyst 5 per unit time is calculated using as parameters the quantity of urea solution added through the fuel addition valve 7 per unit time. The NH.sub.3 consumption per unit time is calculated using as parameters the quantity of NOx flowing into the SCR catalyst 5 (also referred to as the inflowing NOx quantity) per unit time and the NOx removal rate with the SCR catalyst 5. The aforementioned inflowing NOx quantity per unit time is calculated as the product of the measurement value of the second NOx sensor 14 and the exhaust gas flow rate. The NOx removal rate of the SCR catalyst 5 is calculated using as parameters the exhaust gas flow rate and the SCR temperature Tscr. The relationship between the NOx removal rate with the SCR catalyst, the exhaust gas flow rate, and the SCR temperature is determined experimentally in advance. The slipping NH.sub.3 quantity per unit time is calculated using as parameters the value of the NH.sub.3 adsorption amount calculated last time, the SCR temperature, and the exhaust gas flow rate.

[0075] After the completing the processing of step S203, the ECU 8 executes the processing of step S204 next, where the ECU 8 determines whether or not the NH.sub.3 adsorption quantity Anh3 obtained in step S203 is equal to or larger than the aforementioned specific amount Anh3rthr. If the determination made in step S204 is negative, the ECU 8 executes the processing of step S205 next, where the ECU 8 performs the aforementioned NH.sub.3 supply process. In this process, the quantity of urea solution supplied through the urea addition valve 7 is set to a value obtained by converting the difference between the NH.sub.3 adsorption quantity Anh3 retrieved in the processing of step S203 and the aforementioned specific amount Anh3thr (=Anh3thr−Anh3) into an equivalent quantity of urea solution.

[0076] After executing the processing of step S205, the ECU 8 executes the processing of step S206 next. If the determination made in step S204 is affirmative, the ECU 8 skips the process of step S205 and executes the processing of step S206 next. In the processing of step S206, the ECU 8 determines whether or not the NSR temperature Tnsr obtained in the processing of step S201 is equal to or higher than the activation completion temperature Tnsr2 of the NSR catalyst 4.

[0077] If the determination made in step S206 is affirmative, the ECU 8 executes the processing of step S207 next, where the ECU 8 performs the rich spike process so as to adjust the air-fuel ratio of the exhaust gas flowing into the NSR catalyst 4 to a third rich air-fuel ratio A/Fr3. The third rich air-fuel ratio A/Fr3 mentioned above is an air-fuel ratio higher than the aforementioned standard rich air-fuel ratio A/Frst and suitable for reduction of production of N.sub.2O. In cases where the NSR temperature Tnsr is equal to or higher than the activation completion temperature Tnsr2 of the NSR catalyst 4, NOx stored in the NSR catalyst 4 can be removed effectively by adjusting the air-fuel ratio of the exhaust gas flowing into the NSR catalyst 4 during the rich spike process to the standard rich air-fuel ratio A/Frst suitable for removal of NOx. However, even in cases where the NSR temperature Tnsr is equal to or higher than the activation completion temperature Tnsr2 of the NSR catalyst 4, if the air-fuel ratio of the exhaust gas flowing into the NSR catalyst 4 during the rich spike process is adjusted to the standard rich air-fuel ratio A/Frst, a small quantity of N.sub.2O can be produced in the NSR catalyst 4. By adjusting the air-fuel ratio of the exhaust gas flowing into the NSR catalyst 4 to the third rich air-fuel ratio A/Fr3 suitable for reduction of production of N.sub.2O, the quantity of N.sub.2O produced in the NSR catalyst 4 can be kept small with improved reliability. NOx, which has been stored in the NSR catalyst 4 and flows out of the NSR catalyst 4 without being removed by the NSR catalyst 4, is removed by the SCR catalyst 5. Therefore, the quantity of N.sub.2O produced in the NSR catalyst 4 can be kept small with improved reliability, while preventing an increase in the amount of NOx that is not removed by the NSR catalyst 4 or the SCR catalyst 5. The rich spike process performed by the processing of step S207 may be terminated either at the time when a predetermined length of time has elapsed or at the time when the measurement value of the second A/F sensor 13 becomes equal to or lower than the third rich air-fuel ratio A/Fr3.

[0078] If the determination made in step S206 is negative, the ECU 8 executes the processing of steps S107 to S109. Then, in the processing of step S108 or step S109, the rich spike process is performed so as to adjust the air-fuel ratio of the exhaust gas flowing into the NSR catalyst 4 to the first rich air-fuel ratio A/Fr1 or the second rich air-fuel ratio A/Fr2. In this case, each of the first rich air-fuel ratio A/Fr1 or the second rich air-fuel ratio A/Fr2 is set to an air-fuel ratio at which the amount of N.sub.2O produced in the NSR catalyst 4 is minimized, as described above. Therefore, the amount of N.sub.2O produced in the NSR catalyst 4 during the processing of step S108 or S109 can be kept small with improved reliability. When the processing of step S108 or step S109 is executed, NOx that is not removed by the NSR catalyst 4 is removed by the SCR catalyst 5. Therefore, an increase in the amount of NOx that is not removed by the NSR catalyst 4 or the SCR catalyst 5 can be prevented.

[0079] If the determination made in step S202 is negative, the ECU 8 executes the processing of step S105 next. Then, if the determination made in step S105 is affirmative, the ECU 8 executes the processing of step S106 next, as in the above-described first embodiment. On the other hand, if the determination made in step S105 is negative, the system is in the state in which although the NOx storage amount Anox in the NSR catalyst 4 is equal to or larger than the predetermined threshold Anoxthr while the NSR temperature Tnsr is in the aforementioned warming-up temperature range, the NOx remove capability of the SCR catalyst 5 is not active. If the rich spike process is performed in the same manner as the first embodiment in this state, while the amount of N.sub.2O produced in the NSR catalyst 4 can be kept small, the amount of NOx that is not removed by the exhaust gas purification apparatus including the NSR catalyst 4 and the SCR catalyst 5 may increase. To address this problem, in this second embodiment, if the determination made in step S105 is negative, the aforementioned heating-up process is performed without performing the rich spike process. Specifically, firstly in the processing of step S208, the ECU 8 starts the heating-up process. Then, the ECU 8 executes the processing of step S209 next, where the ECU 8 obtains the SCR temperature Tscr again. Then, the ECU 8 executes the processing of step S210 next, where the ECU 8 determines whether or not the SCR temperature Tscr obtained in the processing of step S209 has reached or exceeded the activation start temperature Tscr1 of the SCR catalyst 5. If the determination made in step S210 is negative, the ECU 8 returns to the processing of step S209. If the determination made in step S210 is affirmative, the ECU 8 terminates the heating-up process by the processing of step S211, and then the ECU 8 executes the processing of step S203 next. The heating-up process performed in the above-described manner can shorten the length of time through which the SCR catalyst 5 is not active in the state in which the NSR temperature Tnsr is in the warming-up temperature range and the NOx storage amount Anox in the NSR catalyst 4 is equal to or larger than the aforementioned predetermined threshold Anoxthr. Therefore, saturation of the NOx storage capability of the NSR catalyst 4 can be prevented.

[0080] According to the above-described embodiment, when the rich spike process is performed, the amount of N.sub.2O produced in the NSR catalyst 4 can be kept small with improved reliability while keeping the amount of NOx that is not removed by the exhaust gas purification apparatus including the NSR catalyst 4 and the SCR catalyst 5 small.

Modification of Second Embodiment

[0081] When the SCR temperature Tscr reaches or exceeds an adsorption limit temperature Tscrmax higher than the activation start temperature Tscr1 of the SCR catalyst 5, the NH.sub.3 adsorption capacity of the SCR catalyst 5 becomes smaller than the aforementioned specific amount Anh3thr. Therefore, when the SCR temperature Tscr is equal to or higher than the aforementioned adsorption limit temperature Tscrmax, it is necessary to control the quantity of urea solution added to the exhaust gas through the urea addition valve 7 per unit time according to the quantity of NOx flowing into the SCR catalyst 5 per unit time. It is possible to calculate the quantity of NOx flowing into the SCR catalyst 5 per unit time from the measurement value of the second NOx sensor 14. However, in cases where the second NOx sensor 14 and the urea addition valve 7 are located close to each other, it is difficult to add a quantity of urea solution adapted to the calculated quantity of NOx by the urea addition valve 7 by the time when the calculated quantity of NOx passes the neighborhood of the urea addition valve 7. Therefore, when the SCR temperature Tscr is equal to or higher than the aforementioned adsorption limit temperature Tscrmax, it is necessary to estimate the quantity of NOx which flows into and slips through the NSR catalyst 4 per unit time (or the slipping NOx quantity Anoxslp), and to control the urea addition valve 7 so as so supply the SCR catalyst 5 with a quantity of urea solution of which the equivalence ratio Er of the quantity of NH.sub.3 to the slipping NOx quantity Anoxslp is equal to a specific ratio Erst (e.g. equal to 1).

[0082] As described in the description of the second embodiment, if the rich spike process is performed such that the air-fuel ratio of the exhaust gas flowing into the NSR catalyst 4 is adjusted to an air-fuel ratio suitable for reduction of production of N.sub.2O, there is a possibility that a portion of NOx stored in the NSR catalyst 4 may not be removed by the NSR catalyst 4 and flow into the SCR catalyst 5. Therefore, if the rich spike process is performed such that the air-fuel ratio of the exhaust gas flowing into the NSR catalyst 4 is adjusted to an air-fuel ratio suitable for reduction of production of N.sub.2O when the SCR temperature Tscr is equal to or higher than the aforementioned adsorption limit temperature Tscrmax, there is a possibility that a quantity of NOx larger than the aforementioned slipping NOx quantity Anoxslp may flow into the SCR catalyst 5. To address this problem, in this modification, when the SCR temperature Tscr is equal to or higher than the aforementioned adsorption limit temperature Tscrmax, the equivalence ratio control for the urea addition valve 7 is performed such that a quantity of urea solution having an equivalence ratio equal to the aforementioned specific ratio is supplied to the SCR catalyst 5 while the rich spike process is not performed and that a quantity of urea solution larger than the quantity of the aforementioned specific ratio is supplied to the SCR catalyst 5 while the rich spike process is performed.

[0083] In the following, the procedure of performing the rich spike process and the equivalence ratio control according to this modification will be described with reference to FIGS. 6 and 7. FIG. 6 is a flow chart of a processing routine executed by the ECU 8 when performing the rich spike process. In FIG. 6, the processings that are the same as those in the processing routine shown in FIG. 5 according to the above-described second embodiment are denoted by the same reference signs.

[0084] In the processing routine shown in FIG. 6, after executing the processing of step S202, the ECU 8 executes the processing of step S301. In the processing of step S301, the ECU 8 determines whether or not the SCR temperature Tscr obtained in the processing of step S201 is lower than the aforementioned adsorption limit temperature Tscrmax. If the determination made in step S301 is affirmative, it may be considered that the NH.sub.3 adsorption capacity of the SCR catalyst 5 is equal to or larger than the aforementioned specific amount Anh3thr. Then, the ECU 8 executes the processing of step S203 and the subsequent steps as in the above-described second embodiment. On the other hand, if the determination made in step S301 is negative, it may be considered that the NH.sub.3 adsorption capacity of the SCR catalyst 5 is smaller than the aforementioned specific amount Anh3thr. Then, the ECU 8 skips the processing of steps S203 to S205 and executes the processing of step S206 next. In this case, supply of urea solution to the SCR catalyst 5 is carried out by the processing routine shown in FIG. 7.

[0085] The processing routine shown in FIG. 7 is stored in the ROM or other device of the ECU 8 and executed by the ECU 8 at regular intervals while the internal combustion engine 1 is operating.

[0086] In the processing routine shown in FIG. 7, firstly in the processing of step S401, the ECU 8 obtains the SCR temperature Tscr. The method of determining the SCR temperature Tscr is the same as that in the processing of step S201 in the above-described processing routines shown in FIGS. 5 and 6. After executing the processing of step S401, the ECU 8 executes the processing of step S402 next.

[0087] In the processing of step S402, the ECU 8 determines whether or not the SCR temperature Tscr obtained in the processing of step S401 is equal to or higher than the aforementioned adsorption limit temperature Tscrmax. If the determination made in step S402 is negative, it is not necessary to perform the equivalence ratio control, and the ECU 8 terminates the execution of this processing routine. If the determination made in step S402 is affirmative, it is necessary to perform the equivalence ratio control, and the ECU 8 executes the processing of step S403 and the subsequent steps.

[0088] In the processing of step S403, the ECU 8 calculates the slipping NOx quantity Anoxslp. The slipping NOx quantity Anoxslp depends on the NOx storage amount Anox in the NSR catalyst 4, the NSR temperature Tnsr, the air-fuel ratio of the exhaust gas flowing into the NSR catalyst 4, and the exhaust gas flow rate. Therefore, the relationship between these values is stored in the ROM as a map or a function expression in advance. The ECU 8 is configured to calculate the slipping NOx quantity Anoxslp using as arguments the NOx storage amount Anox, the NSR temperature Tnsr, the air-fuel ratio of the exhaust gas flowing into the NSR catalyst 4, and the exhaust gas flow rate.

[0089] In the processing of step S404, the ECU 8 determines whether or not the rich spike process is being performed. This determination can be made with reference to a flag that is turned on at the time when the rich spike process starts and turned off at the time when the rich spike process ends.

[0090] If the determination made in step S404 is affirmative, the rich spike process is being performed in circumstances in which the SCR temperature Tscr is equal to or higher than the activation start temperature Tscr1 of the SCR catalyst 5. Then, it may be considered that the air-fuel ratio of the exhaust gas flowing into the NSR catalyst 4 is suitable for reduction of production of N.sub.2O. In this case, as described above, the exhaust gas flowing into the SCR catalyst 5 contains a portion of NOx having been stored in the NSR catalyst 4 in addition to the slipping NOx quantity Anoxslp. Therefore, if the determination made in step S404 is affirmative, the ECU 8 executes the processing of step S405 next, where the ECU 8 controls the urea addition valve 7 so as to supply the SCR catalyst 5 with a quantity of urea solution of which the equivalence ratio Er of the quantity of NH.sub.3 to the aforementioned slipping NOx quantity Anoxslp is equal to a ratio Er1. This ratio Er1 is larger than the aforementioned specific ratio Erst and is determined by an adaptation process based on, for example, an experiment.

[0091] If the determination made in step S404 is negative, the ECU 8 executes the processing of step S406 next, where the ECU 8 controls the urea addition valve 7 such that the SCR catalyst 5 is supplied with a quantity of urea solution of which the equivalence ratio Er of the quantity of NH.sub.3 to the aforementioned slipping NOx quantity Anoxslp is equal to the aforementioned specific ratio Erst.

[0092] With the above-described modification, even in cases where the rich spike process is performed in circumstances in which the SCR temperature Tscr is equal to or higher than the adsorption limit temperature Tscrmax, the amount of N.sub.2O produced can be kept small while preventing an increase in the amount of NOx that is not removed by the exhaust gas purification apparatus including the NSR catalyst 4 and the SCR catalyst 5.

[0093] While in the cases described in the first and second embodiments, the fuel addition valve 6 is used as the fuel supply device according to the present disclosure, the fuel supply device according to the present disclosure may be implemented alternatively by injection of fuel through the fuel injection valve 2 during the exhaust stroke.

[0094] While the present disclosure has been described with reference to exemplary embodiments, it is to be understood that the disclosure is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions.

[0095] This application claims the benefit of Japanese Patent Application No. 2016-085173, filed on Apr. 21, 2016, which is hereby incorporated by reference herein in its entirety.