# Patent

## Title
Exhaust purification system of internal combustion engine

## Abstract

An exhaust purification system of an internal combustion engine includes an NOX storage reduction catalyst device which is arranged in an engine exhaust passage. The NOX storage reduction catalyst device stores SOX simultaneously with NOX. When the stored SOX amount exceeds a predetermined allowable amount, the SOX is made to be released by SOX release control which raises the temperature of the NOX catalyst device to the SOX releasable temperature, then makes the air-fuel ratio of the exhaust gas which flows into the NOX catalyst device the stoichiometric air-fuel ratio or rich. The NOX catalyst device has a residual SOX storage amount which finally remains even if performing SOX release control depending on the temperature of the NOX catalyst device when performing SOX release control. The system uses the residual SOX storage amount of the current SOX release control as the basis to calculate the SOX release speed at each timing in the current SOX release control.

## Background
TECHNICAL FIELD
The present invention relates to an exhaust purification system of an internal combustion engine.
BACKGROUND ART
The exhaust gas of diesel engines, gasoline engines, and other internal combustion engines includes, for example, carbon monoxide (CO), unburned fuel (HC), nitrogen oxides (NOX), particulate matter (PM), and other constituents. The internal combustion engines are mounted with exhaust purification systems for removing these constituents.
As one method for removing nitrogen oxides, arrangement of an NOX storage reduction catalyst in an engine exhaust passage has been proposed. The NOX storage reduction catalyst stores NOX when the air-fuel ratio of the exhaust gas is lean. When the storage amount of the NOX reaches an allowable amount, the air-fuel ratio of the exhaust gas may be made rich or the stoichiometric air-fuel ratio so that the stored NOX is released. The released NOX is reduced to N2 by the carbon monoxide or other reducing agent which is contained in the exhaust gas.
Japanese Patent Publication (A) No. 2000-314311 discloses a purification system arranging a purification catalyst of nitrogen oxides in an exhaust gas flow path of the internal combustion engine. The nitrogen oxide purification catalyst has a precious metal and a nitrogen oxide trapping material. It is disclosed that the nitrogen oxide purification catalyst can trap nitrogen oxides as NO2 by a higher air-fuel ratio than the stoichiometric air-fuel ratio. Further, the trapping material of nitrogen oxides traps SOX, but it is disclosed that by rendering the atmosphere a reducing one, the trapped SOX can be removed. Further, it is disclosed that the temperature for removing the trapped SOX is preferably 500° C. or more.
The exhaust gas of an internal combustion engine sometimes contains sulfur oxides (SOX). An NOX storage reduction catalyst stores SOX at the same time as storing NOX. If SOX is stored, the storable amount of NOX falls. In this way, the NOX storage reduction catalyst suffers from so-called “sulfur poisoning”. To eliminate sulfur poisoning, sulfur poisoning recovery treatment is performed for releasing the SOX. In the sulfur poisoning recovery treatment, the NOX storage reduction catalyst is raised in temperature and, in that state, the air-fuel ratio of the exhaust gas is made rich or the stoichiometric air-fuel ratio to release the SOX.
At the time of sulfur poisoning recovery treatment of the NOX storage reduction catalyst, the SOX is released into the atmosphere. If the release speed of the SOX is large, a large amount of SOX ends up being released in a short time, so odor and other problems arise.
On the other hand, an NOX storage reduction catalyst suffers from thermal degradation. If thermal degradation occurs, for example, the NOX storable amount is decreased. Thermal degradation proceeds faster the higher the temperature of the NOX storage reduction catalyst. When performing sulfur poisoning recovery treatment, the temperature elevated state continues for a long time. For this reason, at the time of sulfur poisoning recovery treatment, thermal degradation proceeds relatively fast.
In the prior art, the target temperature and the regeneration time of the NOX storage reduction catalyst are set in advance. During this regeneration time, the sulfur poisoning recovery treatment was performed while maintaining the target temperature. Alternatively, the SOX release speed may be detected by using a map using the fuel injection amount and temperature etc. in the combustion chambers as functions. The SOX release amount can be calculated from the SOX release speed. However, the SOX release speed which is detected by the prior art includes relatively large error. For this reason, at the time of sulfur poisoning recovery treatment, there was a possibility that the NOX storage reduction catalyst would be exposed to a higher temperature atmosphere than required and that thermal degradation would excessively proceed. The SOX release speed when performing sulfur poisoning recovery treatment preferably can be precisely detected.
DISCLOSURE OF INVENTION
The present invention has as its object the provision of an exhaust purification system of an internal combustion engine including an NOX storage reduction catalyst device, which exhaust purification system of an internal combustion engine can precisely calculate an SOX release speed when performing sulfur poisoning recovery treatment.
The exhaust purification system of an internal combustion engine of the present invention arranges in an engine exhaust passage an NOX catalyst device which stores NOX which is contained in exhaust gas when an air-fuel ratio of the inflowing exhaust gas is lean and which releases the stored NOX when the air-fuel ratio of the inflowing exhaust gas becomes a stoichiometric air-fuel ratio or rich and which uses SOX release control which raises a temperature of the NOX catalyst device to an SOX releasable temperature when an SOX amount which is stored in the NOX catalyst device exceeds a predetermined allowable amount and which makes the air-fuel ratio of the exhaust gas which flows into the NOX catalyst device a stoichiometric air-fuel ratio or rich so as to make the stored SOX be released. The NOX catalyst device has a residual SOX storage amount which is dependent on the temperature of the NOX catalyst device when performing SOX release control and finally remains even if performing SOX release control. The system uses the residual SOX storage amount of the current SOX release control as the basis to calculate the SOX release speed at each timing in the current SOX release control. By adopting this configuration, the system precisely calculate the SOX release speed when performing SOX release control.
In the above invention, preferably, in the current SOX release control, the system uses a difference between a SOX storage amount at each timing and the residual SOX storage amount as the basis to calculate the SOX release speed at each timing.
In the above invention, preferably the system uses the SOX release speed which was calculated at each timing of the SOX release control as the basis to calculate a cumulative SOX release amount which is released from the start of SOX release control to the current timing and corrects the calculated SOX release speed at the current timing based on a ratio of a first radius and a second radius where when a releasable SOX amount obtained by subtracting from an SOX storage amount when starting SOX release control the residual SOX storage amount is deemed to correspond to an area of a circle of the first radius, a radius of a circle of an area corresponding to the cumulative SOX release amount is calculated as the second radius.
In the above invention, preferably the NOX catalyst device has a final NOX storable amount at which NOX can be stored when the residual SOX storage amount remains, and the system uses the SOX release speed which was calculated at each timing of the SOX release control as the basis to calculate an NOX recovery amount which is restored from the start of SOX release control to the current timing and corrects the calculated SOX release speed at the current timing based on a ratio of a first radius and a second radius where when a restorable NOX storable amount obtained by subtracting from the final NOX storable amount an NOX storable amount when starting SOX release control is deemed to correspond to an area of a circle of the first radius, a radius of a circle of an area corresponding to the NOX recovery amount is calculated as the second radius.
In the above invention, preferably the system uses the SOX release speed which was calculated at each timing of the SOX release control as the basis to calculate a cumulative SOX release amount which is released from the start of SOX release control to the current timing and corrects the calculated SOX release speed at the current timing based on a ratio of a first radius and a second radius where when a releasable SOX amount obtained by subtracting from an SOX storage amount when starting SOX release control the residual SOX storage amount is deemed to correspond to a volume of a sphere of the first radius, a radius of a sphere of a volume corresponding to the cumulative SOX release amount is calculated as the second radius.
In the above invention, preferably the NOX catalyst device has a final NOX storable amount at which storage of NOX is possible when the residual SOX storage amount remains, and the system uses an SOX release speed which was calculated at the each timing of SOX release control as the basis to calculate a NOX recovery amount which is restored from the start of SOX release control to the current timing and corrects the calculated SOX release speed at the current timing based on a ratio of a first radius and a second radius where when a restorable NOX storable amount obtained by subtracting from the final NOX storable amount an NOX storable amount when starting SOX release control is deemed to correspond to a volume of a sphere of the first radius, a radius of a sphere of a volume corresponding to the NOX recovery amount is calculated as the second radius.



BRIEF DESCRIPTION OF DRAWINGS
FIG. 1 is a schematic view of an internal combustion engine in Embodiment 1.
FIG. 2 is an enlarged schematic cross-sectional view of an NOX storage reduction catalyst device when storing NOX.
FIG. 3 is an enlarged cross-sectional view of an NOX storage reduction catalyst device when storing SOX.
FIG. 4 is a map of an SOX storage amount per unit time as a function of the engine speed and the demanded torque.
FIG. 5 is a time chart for when performing sulfur poisoning recovery treatment.
FIG. 6 is a graph which explains a relationship between an SOX amount which is stored in an NOX storage reduction catalyst device and a SOX release speed in Embodiment 1.
FIG. 7 is a graph of a bed temperature of an NOX storage reduction catalyst device and a finally remaining residual SOX storage amount in Embodiment 1.
FIG. 8 is a view which explains changes in an SOX amount which is stored in an NOX storage reduction catalyst device in SOX release control.
FIG. 9 is a flow chart for when performing SOX release control in Embodiment 1.
FIG. 10 is a graph of a case of using a correction term to calculate an SOX release speed in Embodiment 1 and a comparative example which calculates an SOX release speed without using a correction term.
FIG. 11 is an enlarged schematic view which explains a state where SOX is released at a high temperature from an NOX storage reduction catalyst device.
FIG. 12 is an enlarged schematic view which explains a state where SOX is released at a low temperature from an NOX storage reduction catalyst device.
FIG. 13 is a schematic view which explains an SOX release model.
FIG. 14 is a graph of an SOX release speed when using a calculated correction term for calculation in Embodiment 2.
FIG. 15 is a flow chart for when performing SOX release control in Embodiment 2.
FIG. 16 is a view which explains a change of an NOX storable amount of an NOX storage reduction catalyst device in SOX release control.
FIG. 17 is a graph which explains a relationship between a temperature of an NOX storage reduction catalyst device and a final NOX storable amount for when unreleasable SOX remains in Embodiment 3.
FIG. 18 is a graph which explains a relationship between an SOX storage amount and an NOX storable amount in Embodiment 3.



BEST MODE FOR CARRYING OUT INVENTION
Embodiment 1
Referring to FIG. 1 to FIG. 10, an exhaust purification system of an internal combustion engine in Embodiment 1 will be explained. The internal combustion engine in the present embodiment is arranged in a vehicle. In the present embodiment, the explanation will be given with reference to a compression ignition type diesel engine mounted in a vehicle as an example.
FIG. 1 shows an overall view of the internal combustion engine in the present embodiment. The internal combustion engine is provided with an engine body 1. Further, the internal combustion engine is provided with an exhaust purification system which purifies exhaust gas. The engine body 1 includes cylinders constituted by combustion chambers 2, electronic control type fuel injectors 3 for injecting fuel into the combustion chambers 2, an intake manifold 4, and an exhaust manifold 5.
The intake manifold 4 is connected through an intake duct 6 to an outlet of a compressor 7a of an exhaust turbocharger 7. An inlet of the compressor 7a is connected through an intake air detector 8 to an air cleaner 9. Inside the intake duct 6, a throttle valve 10 which is driven by a step motor is arranged. Furthermore, around the intake duct 6, a cooling device 11 is arranged for cooling the intake air which flows through the inside of the intake duct 6. In the embodiment shown in FIG. 1, the engine cooling water is guided to the cooling device 11. The engine cooling water is used to cool the intake air.
The exhaust manifold 5 is connected to the inlet of an exhaust turbine 7b of the exhaust turbocharger 7. The exhaust purification system in the present embodiment is provided with an NOX catalyst device comprised of an NOX storage reduction catalyst device (NSR) 17 (hereinafter simply referred to as an “NOX storage reduction catalyst”). The NOX storage reduction catalyst 17 is connected to an outlet of the exhaust turbine 7b through an exhaust pipe 12. Downstream of the NOX storage reduction catalyst 17 inside of the engine exhaust passage, a particulate filter 16 is arranged for trapping particulate in the exhaust gas. Further, downstream of the particulate filter 16 inside of the engine exhaust passage, an oxidation catalyst 13 is arranged.
Between the exhaust manifold 5 and the intake manifold 4, an EGR passage 18 is arranged for performing exhaust gas recirculation (EGR). Inside the EGR passage 18, an electronic control type EGR control valve 19 is arranged. Further, around the EGR passage 18, a cooling device 20 is arranged for cooling the EGR gas which flows through the inside of the EGR passage 18. In the embodiment shown in FIG. 1, engine cooling water is guided into the cooling device 20. The engine cooling water is used to cool the EGR gas.
The fuel injectors 3 are connected through fuel feed tubes 21 to a common rail 22. The common rail 22 is connected through an electronic control type variable discharge fuel pump 23 to a fuel tank 24. The fuel which is stored in the fuel tank 24 is supplied by a fuel pump 23 to the inside of the common rail 22. The fuel which is supplied to the inside of the common rail 22 is supplied through the fuel feed tubes 21 to the fuel injectors 3.
The electronic control unit 30 is comprised of a digital computer. The electronic control unit 30 in the present embodiment functions as a control system of the exhaust purification system. The electronic control unit 30 includes constituents which are connected to each other by a bidirectional bus 31 such as a ROM (read only memory) 32, RAM (random access memory) 33, CPU (microprocessor) 34, input port 35, and output port 36.
The ROM 32 is a read only storage device. The ROM 32 stores in advance maps and other information necessary for control. The CPU 34 can perform any computation or judgment. The RAM 33 is a random access storage device. The RAM 33 stores the operating history and other information or temporarily stores results of processing.
Downstream of the NOX storage reduction catalyst 17, a temperature sensor 26 is arranged for detecting the temperature of the NOX storage reduction catalyst 17. Downstream of the oxidation catalyst 13, a temperature sensor 27 is arranged for detecting the temperature of the oxidation catalyst 13 or particulate filter 16. At the particulate filter 16, a differential pressure sensor 28 is attached for detecting the differential pressure before and after the particulate filter 16. The output signals of these temperature sensors 26 and 27, differential pressure sensor 28, and intake air detector 8 are input through the corresponding AD converters 37 to the input port 35.
An accelerator pedal 40 is connected to a load sensor 41 which generates an output voltage proportional to the amount of depression of the accelerator pedal 40. The output voltage of the load sensor 41 is input through a corresponding AD converter 37 to the input port 35. Furthermore, the input port 35 is connected to a crank angle sensor 42 which generates an output pulse every time the crankshaft rotates by for example 15°. The output of the crank angle sensor 42 can be used to detect the speed of the engine body 1.
On the other hand, the output port 36 is connected through corresponding drive circuits 38 to the fuel injectors 3, the step motor for driving the throttle valve 10, the EGR control valve 19, and the fuel pump 23. In this way, the fuel injector 3 and throttle valve 10 etc. are controlled by the electronic control unit 30.
The oxidation catalyst 13 is a catalyst which has an oxidation ability. The oxidation catalyst 13 is, for example, provided with a substrate which has partition walls extending in the flow direction of the exhaust gas. The substrate is, for example, formed in a honeycomb structure. The substrate is for example housed in a tubular case. On the surface of the substrate, for example, a porous oxide powder is used to form a coated layer serving as a catalyst carrier. The coated layer carries a catalyst metal formed by platinum (Pt), rhodium (Rd), palladium (Pd), or other such precious metal. The carbon monoxide or unburned hydrocarbons which are contained in the exhaust gas are oxidized at the oxidation catalyst and converted to water, carbon dioxide, etc.
The particulate filter 16 is a filter for removing carbon particles, sulfates and other ion-based particles, and other particulates contained in the exhaust gas. The particulate filter, for example, has a honeycomb structure and has a plurality of channels extending in the flow direction of the gas. In the plurality of channels, channels with downstream ends which are sealed and channels with upstream ends which are sealed are alternately formed. The partition walls of the channels are formed by cordierite or other such porous material. When the exhaust gas passes through these partition walls, the particulate is trapped.
The particulate matter is trapped and oxidized on the particulate filter 16. The particulate matter which gradually deposits on the particulate filter 16 is removed by oxidation by raising the temperature in an excess air atmosphere to for example 600° C. or so.
FIG. 2 is an enlarged schematic cross-sectional view of an NOX storage reduction catalyst. The NOX storage reduction catalyst 17 is a catalyst which temporarily stores the NOX which is contained in the exhaust gas which is discharged from the engine body 1 and converts the stored NOX to N2 when releasing it.
The NOX storage reduction catalyst 17 is comprised of a substrate on which for example a catalyst carrier 45 comprised of alumina is carried. On the surface of the catalyst carrier 45, a catalyst metal 46 formed by a precious metal is carried dispersed. On the surface of the catalyst carrier 45, a layer of an NOX absorbent 47 is formed. As the catalyst metal 46, for example, platinum Pt is used. As the ingredient forming the NOX absorbent 47, for example, at least one element selected from potassium K, sodium Na, cesium Cs, or other such alkali metal, barium Ba, calcium Ca, or other alkali earth, lanthanum La, yttrium Y, or other such rare earth is used. In the present embodiment, as the ingredient forming the NOX absorbent 47, barium Ba is used.
In the present invention, the ratio of the air and fuel (hydrocarbons) in the exhaust gas which is supplied to the engine intake passage, combustion chambers, or engine exhaust passage is referred to as the “air-fuel ratio of the exhaust gas (A/F)”. When the air-fuel ratio of the exhaust gas is lean (when it is larger than the stoichiometric air-fuel ratio), the NO which is contained in the exhaust gas is oxidized on the catalyst metal 46 and becomes NO2. The NO2 is stored in the form of nitrate ions NO3− in the NOX absorbent 47. As opposed to this, when the air-fuel ratio of the exhaust gas is rich or when it becomes the stoichiometric air-fuel ratio, the nitrate ions NO3− which are stored in the NOX absorbent 47 are released in the form of NO2 from the NOX absorbent 47. The released NOX is reduced to N2 by the unburned hydrocarbons, carbon monoxide, etc. contained in the exhaust gas.
FIG. 3 shows another enlarged schematic cross-sectional view of an NOX storage reduction catalyst. Exhaust gas contains SOX, that is, SO2. If SO2 flows into the NOX storage reduction catalyst 17, it is oxidized at the catalyst metal 46 and becomes SO3. This SO3 is absorbed at the NOX absorbent 47 and for example generates sulfate BaSO4. Sulfate BaSO4 is stable and hard to break down. If just making the air-fuel ratio of the exhaust gas rich, the sulfate BaSO4 remains as it is without being broken down. For this reason, the NOX amount which the NOX storage reduction catalyst can store falls. In this way, the NOX storage reduction catalyst suffers from sulfur poisoning.
To recover from sulfur poisoning, the temperature of the NOX storage reduction catalyst is raised to a temperature where SOX can be released. In this state, SOX release control is performed to make the air-fuel ratio of the exhaust gas which flows into the NOX storage reduction catalyst rich or the stoichiometric air-fuel ratio. By performing this SOX release control, it is possible to make the NOX storage reduction catalyst release SOX.
In the present embodiment, at the time of ordinary operation of the internal combustion engine, the SOX amount which is stored in the NOX storage reduction catalyst is calculated. The SOX storage amount is calculated continuously during operation of the internal combustion engine. The exhaust purification system in the present embodiment is provided with a detection device for the SOX storage amount during ordinary operation. Referring to FIG. 1, the detection device for the SOX storage amount in the present embodiment includes an electronic control unit 30.
FIG. 4 shows a map of the SOX amount which is stored per unit time in the NOX storage reduction catalyst as a function of the engine speed and the demanded torque. By specifying the engine speed N and the demanded torque TQ, it is possible to find the SOX amount SOXZ which is stored in the NOX storage reduction catalyst per unit time. This map is stored in for example the ROM 32 of the electronic control unit 30. The operation is continued and, every predetermined time period, the SOX amount which is stored per unit time is found from the map. The SOX storage amount is for example stored in the RAM 33. It is possible to consider the SOX storage amount which remains at the time of the end of the previous sulfur poisoning recovery treatment and cumulatively add the calculated SOX storage amount so as to detect the SOX storage amount at any timing.
The detection device of the SOX amount which is stored during ordinary operation is not limited to this mode. It is possible to employ any device which can detect the SOX amount which is stored in the NOX storage reduction catalyst.
FIG. 5 shows a time chart for when performing sulfur poisoning recovery treatment. At the timing t0, the SOX storage amount of the NOX storage reduction catalyst reaches the allowable value. From the timing t0, the sulfur poisoning recovery treatment is started. Temperature elevation control is performed to raise the temperature of the NOX storage reduction catalyst from the timing t0. Referring to FIG. 1, the temperature elevation control is, for example, performed by controlling the fuel injectors 3 which inject fuel into the combustion chambers 2. In the combustion chambers 2, it is possible to retard the injection timing of the main injection performed near compression top dead center so as to make the temperature of the exhaust gas rise. Furthermore, by performing after-injection as auxiliary injection at a time at which fuel can be burned after main injection, it is possible to make the temperature of the exhaust gas rise. By the temperature of the exhaust gas rising, the NOX storage reduction catalyst can be raised in temperature.
At the timing ts the bed temperature of the NOX storage reduction catalyst reaches the target temperature at which SOX can be released. SOX release control is performed from the timing ts. In the SOX release control of the present embodiment, the bed temperature of the NOX storage reduction catalyst is maintained at a substantially constant temperature. Furthermore, in the SOX release control, the air-fuel ratio of the exhaust gas which flows into the NOX storage reduction catalyst is made the stoichiometric air-fuel ratio or rich.
In the present embodiment, the injection amount of the after injection is increased to make the air-fuel ratio of the exhaust gas the stoichiometric air-fuel ratio or rich. At this time, the throttle valve 10 which is arranged at the engine intake passage may also be choked. Alternatively, by performing post-injection as auxiliary injection at a time at which fuel cannot be burned after the main injection, the air-fuel ratio of the exhaust gas can be made the stoichiometric air-fuel ratio or rich. The “post-injection” is injection which is performed after the injection timing of the after-injection. By making the air-fuel ratio of the exhaust gas which flows into the NOX storage reduction catalyst the stoichiometric air-fuel ratio or rich, the SOX can be made to be released.
The device which raises the temperature of the NOX storage reduction catalyst and the device which controls the air-fuel ratio of the exhaust gas which flows into the NOX storage reduction catalyst are not limited to this mode. Any device may be employed.
At the timing te, the SOX storage amount reaches the judgment value for ending the SOX release control. At the timing te, the SOX release control is ended and the sulfur poisoning recovery treatment is ended.
When performing SOX release control, the speed by which SOX is released from the NOX storage reduction catalyst is expressed by the following formula. The SOX release speed R becomes a function of the temperature T, the SOX storage amount S of the current timing, and the reducing agent CO which flows into the NOX storage catalyst. The reducing agent includes unburned fuel and carbon monoxide.

R=f(T,S,CO)  (1)

The SOX release speed R can, for example, be specifically expressed by the following formula. The next formula applies the Arrhenius equation.

R=A×exp(−Ea/RT)×[SOX][CO]  (2)

Here, the coefficient A is a frequency factor and is a physical value. A can be found experimentally. The constant Ea is the activation energy and is a known physical property. The variable T is the absolute temperature. The coefficient R is the gas constant. The variable [SOX] shows the concentration of sulfates. The variable [CO] shows the concentration of the reducing agent which flows into the NOX storage reduction catalyst.
Formula (2) shows that for example the higher the temperature, the greater the SOX release speed becomes and that the greater the SOX storage amount, the greater the SOX release speed becomes. Furthermore, this shows that the greater the amount of the reducing agent, the greater the SOX release speed.
The inventors discovered that even if performing sulfur poisoning recovery treatment, sometimes it is not possible to make all of the SOX which is stored in the NOX storage reduction catalyst be released. In the present invention, the SOX amount which finally remains even if performing sulfur poisoning recovery treatment is called the “residual SOX storage amount”.
FIG. 6 is a graph which explains the relationship between the SOX storage amount and SOX release speed of the NOX storage reduction catalyst. The abscissa shows the SOX storage amount of the NOX storage reduction catalyst, while the ordinate shows the SOX release speed. FIG. 6 shows an example of performing SOX release control at a bed temperature of the NOX storage reduction catalyst of 650° C., 620° C., or 580° C. It is learned that the greater the SOX storage amount, the larger the SOX release speed.
It is learned that when the bed temperature of the NOX storage reduction catalyst is 650° C., the SOX release speed is larger than zero until the SOX storage amount becomes substantially zero. That is, when the bed temperature of the NOX storage reduction catalyst is 650° C., it is possible to release substantially all of the stored SOX. As opposed to this, as the bed temperature of the NOX storage reduction catalyst becomes lower, cases appear where the SOX release speed becomes zero despite SOX remaining at the NOX storage reduction catalyst. In this way, at a predetermined temperature or less, even if performing SOX release control, SOX remains at the NOX storage reduction catalyst
FIG. 7 shows the relationship between the bed temperature of the NOX storage reduction catalyst and the residual SOX storage amount. The abscissa shows the bed temperature of the NOX storage reduction catalyst when performing SOX release control. The ordinate shows the residual SOX storage amount which finally remains even if performing SOX release control. When the temperature of the NOX storage reduction catalyst is low, the residual SOX storage amount becomes larger. As the temperature of the NOX storage reduction catalyst becomes higher, the residual SOX storage amount becomes smaller. In this way, the inventors clarified that sometimes SOX is not completely released and remains at the NOX storage reduction catalyst. Further, the inventors clarified that the residual SOX storage amount depends on the temperature of the NOX storage reduction catalyst when performing SOX release control.
FIG. 8 schematically shows the SOX amount which remains at the NOX storage reduction catalyst when performing SOX release control. The timing ts is the timing when starting SOX release control. The timing te is the timing of ending the SOX release control. In the present embodiment, the time when the SOX storage amount becomes the residual SOX storage amount is made the end timing te. The timing t1 is any timing when performing SOX release control.
The total NOX storable amount Qtotal is the maximum amount of NOX which the NOX storage reduction catalyst can store. The NOX storage reduction catalyst stores NOX and stores SOX. At the timing ts, the NOX storage reduction catalyst stores the initial SOX storage amount S0 of SOX. By performing SOX release control, SOX is released. The SOX storage amount St1 at the timing t1 becomes smaller than the initial SOX storage amount S0. In the present embodiment, the system detects when the SOX storage amount reaches the residual SOX storage amount Se and ends SOX release control.
In the present embodiment, the system precisely detects the amount of SOX which is released from the NOX storage reduction catalyst, that is, the SOX release amount. It precisely detects the timing te when the SOX storage amount St1 of the NOX storage reduction catalyst becomes the residual SOX storage amount Se.
In the present embodiment, when performing SOX release control, the system calculates the SOX release speed at every predetermined interval. It is possible to multiply the calculated SOX release speed with predetermined intervals to calculate the SOX amount which is released at predetermined intervals. By cumulatively adding the calculated SOX release amount, it is possible to calculate the cumulative SOX release amount Mt1 from the start of the SOX release control to any timing. It is possible to subtract from the initial SOX storage amount S0 the cumulative SOX release amount Mt1 to thereby calculate the SOX storage amount St1 at any timing.
In the present embodiment, the system considers the finally remaining residual SOX storage amount Se to calculate the SOX release speed. In the present embodiment, when calculating the SOX release speed R, the SOX storage amount St1 of the NOX storage reduction catalyst is used to calculate the SOX storage amount St1* when corrected by the following formula (3):

St1*=St1×(1−Se/St1=St1−Se  (3)

For example, in the formula (1) or formula (2), the SOX storage amount St1* after correction is entered instead of the SOX storage amount St1 so as to calculate the SOX release speed at the current timing. That is, the SOX release speed Rt1 at the timing t1 can be expressed by the following formula by modifying the formula (1).

Rt1=f(Tt1,St1*,COt1)  (4)

In this way, the difference between the SOX storage amount at each timing and the residual SOX storage amount can be used as the basis to calculate the SOX release speed at each timing.
FIG. 9 is a flow chart of the time when performing SOX release control in the present embodiment. When the SOX amount which is stored in the NOX storage reduction catalyst exceeds the allowable value, the sulfur poisoning recovery treatment is started. Temperature elevation control is performed, then, at step 101, SOX release control is started.
Next, at step 102, the residual SOX storage amount Se is detected. First, the temperature of the NOX storage reduction catalyst is detected. Referring to FIG. 1, the temperature of the NOX storage reduction catalyst 17 can be detected, for example, by a temperature sensor 26 which is arranged downstream of the NOX storage reduction catalyst 17. As explained above, the residual SOX storage amount depends on the temperature. The exhaust purification system of an internal combustion engine in the present embodiment is provided with a map of the residual SOX storage amount as a function of the temperature of the NOX storage reduction catalyst. The map of the residual SOX storage amount is, for example, stored in the ROM 32 of the electronic control unit 30. The temperature of the NOX storage reduction catalyst 17 and map are used to detect the residual SOX storage amount Se.
Next, at step 103, the SOX storage amount St1 at the current timing t1 is read. Right after the SOX release control is started, the initial SOX storage amount S0 which is stored in the NOX storage reduction catalyst becomes the SOX storage amount St1 of the current timing.
Next, at step 104, to calculate the SOX release speed, the corrected SOX storage amount S1t is calculated. The SOX storage amount St1 at the timing t1 and the residual SOX storage amount Se can be used to calculate the SOX storage amount St1* after correction by the formula (3).
Next, at step 105, the SOX storage amount St1* after correction is used to calculate the SOX release speed Rt1, at the timing t1 by, for example, formula (4).
Alternatively, when using the formula (2) to calculate the SOX release speed, it is possible to find the concentration of sulfates [SOX] from the SOX storage amount St1* after correction so as to calculate the SOX release speed Rt1. The concentration [CO] of the reducing agent can for example be calculated from the amount of fuel which is injected into the combustion chambers, the intake air flow amount, the temperature of the exhaust gas, etc.
Next, at step 106, the SOX release amount ΔMt during a micro time Δt is calculated.

ΔMt=Rt1×Δt  (5)

The micro time Δt used may be any time. The micro time Δt is the length of the interval for calculating the SOX release speed. The micro time Δt is the time from when calculating the SOX release speed to when calculating the next SOX release speed.
Next, at step 107, the current SOX storage amount is reduced by the SOX release amount ΔMt of the micro time Δt so as to calculate the new SOX storage amount.
Next, at step 108, it is judged if the calculated SOX storage amount St1 is the residual SOX storage amount Se or less. When the SOX storage amount St1 becomes larger than the residual SOX storage amount Se, the routine returns to step 103 where this calculation is repeated. In this way, it is possible to calculate the SOX storage amount St1 at any timing t1.
At step 108, when the SOX storage amount St1 is the residual SOX storage amount Se or less, the routine proceeds to step 109 where the SOX release control is ended. In this way, the fact of the SOX storage amount reaching the residual SOX storage amount is detected.
FIG. 10 shows a graph of the SOX release speed which is calculated by the method of calculation in the present embodiment and a graph of a comparative example where the calculation is performed without considering the residual SOX storage amount. Further, FIG. 10 shows the points of examples measuring the SOX release speed by experiments.
In the comparative example, the calculation is performed without correction of the SOX storage amount St1 shown in formula (3). In the graph of the comparative example, there is an SOX release speed until the SOX storage amount of the NOX storage reduction catalyst becomes zero. As opposed to this, in the example of calculation in the present embodiment, if the SOX storage amount of the NOX storage reduction catalyst becomes the residual SOX storage amount, the SOX release speed becomes zero. It is learned that the examples of calculation of the present embodiment match with the actually measured values well.
In the present embodiment, the residual SOX storage amount of the current SOX release control is used as the basis to calculate the SOX release speed at each timing in the current SOX release control. By adopting this configuration, when performing SOX release control, the remaining SOX is considered and the SOX release speed can be calculated precisely. In particular, in the present embodiment, the difference between the SOX storage amount at each timing in the current SOX release control and the residual SOX storage amount is used as the basis to calculate the SOX release speed at each timing. Due to this configuration it is possible to calculate the SOX release speed precisely by simple control.
Further, in the present embodiment, to calculate the SOX release speed at each timing, it is possible to precisely calculate the SOX release amount from the NOX storage reduction catalyst. Alternatively, it is possible to precisely calculate the SOX storage amount which remains at the NOX storage reduction catalyst. It is possible to precisely judge the end timing of the SOX release control. As result, it is possible to avoid the time for SOX release control becoming longer than necessary. It is possible to suppress thermal degradation of the NOX storage reduction catalyst. Alternatively, it is possible to avoid fuel being consumed more than necessary when performing auxiliary injection at the combustion chambers.
In the present embodiment, the SOX release control is ended when the SOX storage amount becomes the residual SOX storage amount, but the invention is not limited to this mode. It is possible to make the SOX release control end at any SOX storage amount.
Further, the formula for calculating the SOX release speed is not limited to the formula (2). It is possible to apply the correction term of the formula (3) in the present embodiment to any formula (1) for calculating the SOX release speed. Further, the correction of the SOX release speed is not limited to the mode. It is possible to employ any correction considering the residual SOX storage amount.
The sulfur poisoning recovery treatment is performed each time the SOX amount which is stored in the NOX storage catalyst increases and reaches the allowable value. When performing the sulfur poisoning recovery treatment a plurality of times, the temperature of the NOX storage reduction catalyst at the time when performing the SOX release control may be changed each time.
Embodiment 2
Referring to FIG. 1, FIG. 6, FIG. 8, and FIG. 11 to FIG. 15, an exhaust purification system of an internal combustion engine in Embodiment 2 will be explained. In the present embodiment, the formula for calculating the SOX release speed is used corrected.
Referring to FIG. 6, the SOX release speed is decreased in accordance with a decrease of the SOX storage amount of the NOX storage catalyst. It is learned that the trend of decrease of the SOX release speed at this time differs according to the bed temperature of the NOX storage reduction catalyst. For example, when the bed temperature of the NOX storage reduction catalyst is 650° C., the graph of the SOX release speed becomes substantially linear. In this regard, if the bed temperature of the NOX storage reduction catalyst becomes lower, the graph of the SOX release speed becomes curved. When the bed temperature of the NOX storage reduction catalyst is low, there is the trend that after the release of SOX is started, the SOX release speed rapidly decreases, then the SOX release speed gradually decreases. In the present embodiment, a correction term for calculating this trend is incorporated into the formula for calculating the SOX release speed.
FIG. 11 is an enlarged schematic view of an NOX storage reduction catalyst in the present embodiment. FIG. 11 is an enlarged schematic view of when performing SOX release control until the SOX storage amount becomes the residual SOX storage amount. The NOX storage reduction catalyst contains the catalyst metal 46. SOX 50 is contained in the NOX absorbent in the form of sulfate. If performing SOX release control, near the catalyst metal 46, a large amount of SOX 50 is released. In this regard, at a position a predetermined distance from the catalyst metal 46, a large amount of SOX 50 remains. It is learned that along with the distance from the catalyst metal 46, the remaining SOX gradually increases.
FIG. 12 shows another enlarged schematic view of an NOX storage reduction catalyst in the present embodiment. FIG. 12 is an enlarged schematic view of the time when performing SOX release control at a lower temperature than the temperature of the NOX storage reduction catalyst in FIG. 11. By rendering the bed temperature of the NOX storage reduction catalyst a low temperature to perform the SOX release control, the SOX 50 which is released is decreased. Even near the catalyst metal 46, SOX 50 remains. In the case of this example as well, it is learned that the along with the distance from the catalyst metal 46, the remaining SOX gradually increases.
Referring to FIG. 11 and FIG. 12, it is learned that if performing SOX release control, SOX is released centered about the catalyst metal 46. Further, it is learned that the distance from the catalyst metal 46 at which SOX is completely released becomes longer the higher the temperature of the NOX storage reduction catalyst. In this way, it is learned that the higher the temperature of the NOX storage reduction catalyst, the more possible it is to release SOX at a position distant from the catalyst metal 46. In the present embodiment, the distance from the catalyst metal 46 is used to create a model of release of SOX.
FIG. 13 shows a schematic view of a model of the release of SOX. In the first release model in the present embodiment, circles are defined centered about the catalyst metal 46. The areas of the circles are deemed to correspond to the SOX release amount.
A circle of a first radius of a radius r1 is defined centered about the catalyst metal 46. Further, a circle of a second radius of a radius r2 is defined centered about the catalyst metal 46. In this release model, the release of the SOX proceeds from the catalyst metal 46 toward the outside. The inside of the circle of the radius r1 centered about the catalyst metal 46 corresponds to the region where the SOX can be released. The outside of the circle of the radius r1 centered about the catalyst metal 46 corresponds to the region where SOX cannot be released and SOX remains. The radius r1 depends on the bed temperature of the NOX storage reduction catalyst when performing SOX release control. The inside of the circle of the radius r2 is a region releasing SOX up to any timing. The radius r2 gradually becomes larger as the SOX release control proceeds. The radius r2 can become larger up to the radius r1.
When considering the release model of FIG. 13, the concentration of the sulfate BaSO4 which can be involved in the reduction reaction is calculated by the following formula:

[BaSO4]*=[BaSO4](1−r2/r1)  (6)

The concentration of sulfates is multiplied with the correction term (1−r2/r1) to calculate the concentration of sulfates after correction. Similarly, the SOX release speed Rt1* after correction is expressed by the following formula using the SOX release speed Rt1 before correction.

Rt1*=Rt1×(1−r2/r1)  (7)

Formula (7) shows that as the radius r2 approaches the radius r1, the SOX release speed approaches zero. That is, this shows that as the SOX storage amount St1 approaches the residual SOX storage amount Se, the SOX release speed approaches zero. Further, the formula (7) shows that even with the same value of the radius r2, if the radius r1 is large, the SOX release speed Rt1* after correction becomes larger. That is, this shows that even if the SOX storage amount St1 is the same, if the NOX storage reduction catalyst is a high temperature, the SOX release speed Rt1* after correction becomes larger. Further, this shows that the SOX release speed Rt1* after correction decreases linearly along with a decrease of the SOX storage amount when the radius r1 is large.
Next, the ratio of the radius r1 and the radius r2 included in the formula (7) is calculated. In the first release model, the SOX release amount is made to correspond to the area of the circle shown in FIG. 13. That is, the SOX release amount is given by the following formula:

πr2∝SOX release amount  (8)

Referring to FIG. 8 and FIG. 13, the area of the circle of the radius r1 corresponds to the releasable SOX amount (final SOX release amount) Me. The releasable SOX amount Me is the value of the SOX storage amount S0 when starting the SOX release control minus the residual SOX storage amount Se. Further, the area of the circle of the radius r2 corresponds to the cumulative SOX release amount Mt1 which is released from the timing ts to the timing t1. It is possible to use formula (8) to calculate the radius r1.

πr12∝Me  (9)

πr12=kMe(k:constant)

r1=(k/π×Me)1/2  (10)

Next, in the same way as deriving the radius r1, the formula (8) may be used to calculate the radius r2.

πr22∝Mt1  (11)

πr22=kMt1(k:constant)

r2=(k/π×Mt1)1/2  (12)

From formula (10) and formula (12), the ratio of the radius r1 and the radius r2 can be calculated by the following formula:

r2/r1=(Mt1/Me)1/2  (13)

In this way, the ratio of the radius r1 and the radius r2 can be calculated from the releasable SOX amount Me and the cumulative SOX release amount Mt1 which is released from the timing ts to the timing t1. Furthermore, it is possible to enter the value calculated by the formula (13) into the formula (7) so as to calculate the SOX release speed Rt1* after correction.

Rt1*=Rt1×(1−(Mt1/Me)1/2  (14)

FIG. 14 shows a graph of the results of calculations performed by the first release model of the present embodiment. The abscissa shows the SOX storage amount of the NOX storage reduction catalyst, while the ordinate shows the SOX release speed. When the SOX storage amount is large, a trend is shown where the SOX release speed greatly decreases along with the decrease of the SOX storage amount. If the SOX storage amount becomes smaller, a trend is shown where the SOX release speed decreases slightly along with the decrease of the SOX storage amount. Further, the higher the bed temperature of the NOX storage reduction catalyst, the greater this trend and the more curved the graph shown.
In this way, in the first release model, the calculated SOX release speed may be corrected based on the radius r1 and radius r2 so as to precisely calculate the SOX release speed.
FIG. 15 shows a flow chart for when performing the SOX release control in the present embodiment. At step 101, the SOX release control is started. At step 102, the residual SOX storage amount Se is detected. Step 101 and step 102 are similar to Embodiment 1.
Next, at step 111, the initial SOX storage amount S0 is reduced by the residual SOX storage amount Se to calculate the releasable SOX amount Me (see FIG. 8). Next, at step 103, the SOX storage amount St1 at the current timing t1 is detected.
Next, at step 112, the detected SOX storage amount St1 is used to calculate the SOX release speed Rt1 before correction by the formula (1). Further, at step 113, the initial SOX storage amount S0 is reduced by the SOX storage amount St1 at the timing t1 to calculate the cumulative SOX release amount Mt1.
Next, at step 114, the SOX release speed Rt1* after correction is calculated. The releasable SOX amount Me and the cumulative SOX release amount Mu can be used to calculate the SOX release speed Rt1* after correction by the above formula (14).
Next, at step 115, the SOX release speed Rt1* after correction is used to calculate the SOX release amount (ΔMt) of the micro time Δt. Next, at step 107, the current SOX storage amount may be reduced by the released SOX amount to calculate a new SOX storage amount. Step 107 to step 109 are similar to Embodiment 1.
In this way, in the present embodiment, it is possible to use the SOX release speed after correction to calculate the SOX release amount to thereby calculate a more accurate SOX release amount. Alternatively, it is possible to precisely calculate the SOX storage amount which is stored in the NOX storage catalyst.
Next, the second release model in the present embodiment will be explained. In the second release model in the present embodiment, a sphere is defined centered about the catalyst metal 46. That is, the range of release of SOX defined in the first release model is made not a circle, but a sphere. In the second release model, the SOX release amount is deemed to correspond to the volume of the sphere. That is, the SOX release amount is given by the following formula:

(4/3)πr3∝SOX release amount  (15)

In the second release model, the volume of the sphere of the first radius comprised of the radius r1 corresponds to the releasable SOX amount Me. The volume of the sphere of the second radius comprised of the radius r2 corresponds to the cumulative SOX release amount Mt1 which was released from the timing ts to the timing t1. The formula (15) is used to derive the following formulas:

(4/3)πr13=kMe(k:constant)  (16)

(4/3)πr23=kMt1(k:constant)  (17)

From formula (16) and formula (17), the ratio of the radius r1 and the radius r2 can be calculated by the following formula:

r2/r1=(Mt1/Me)1/3  (18)

The ratio of the radius r1 and the radius r2 can be calculated by the releasable SOX amount Me and the cumulative SOX release amount Mt1 which was released from the timing ts to the timing t1. Furthermore, formula (18) may be entered into the formula (7) so as to calculate the SOX release speed Rt1* after correction.

Rt1*=Rt1×(1−(Mt1/Me)1/3)  (19)

In the second release model as well, the calculated SOX release speed may be corrected based on the radius r1 and the radius r2 to precisely calculate the SOX release speed. Further, the corrected formula of the SOX release speed may be used to calculate the SOX release amount to enable more accurate calculation of the SOX release amount. Alternatively, it is possible to precisely calculate the SOX storage amount which is stored in the NOX storage catalyst.
The rest of the configuration, action, and effects are similar to those of Embodiment 1, so the explanations will not be repeated here.
Embodiment 3
Referring to FIG. 1, FIG. 7, FIG. 8, and FIG. 16 to FIG. 18, an exhaust purification system of an internal combustion engine in Embodiment 3 will be explained. In the present embodiment, the correction term of the SOX release speed which was explained in Embodiment 2 is calculated using the NOX storable amount of the NOX storage reduction catalyst. That is, the ratio of the radius r1 and the radius r2 is calculated from the NOX storable amount which shows the amount of NOX which can be stored.
FIG. 16 schematically shows the NOX storable amount when performing SOX release control in the sulfur poisoning recovery treatment. The timing ts is the timing when starting the SOX release control, while the timing te is the timing when ending the SOX release control. In the present embodiment, the time when the SOX storage amount becomes the residual SOX storage amount is made the end timing te. The timing t1 is any timing when performing the SOX release control.
The NOX storage reduction catalyst has an initial NOX storable amount Q0 at the timing ts. By performing SOX release control, the SOX is released. The NOX storable amount Qt1 at the timing t1 becomes larger than the initial NOX storable amount Q0. That is, the NOX storable amount is restored. When performing the SOX release control until the SOX storage amount becomes the residual SOX storage amount Se, the NOX storable amount becomes the final NOX storable amount Qe.
In the first release model in the present embodiment, in the same way as the first release model in Embodiment 2, a circle is defined centered about the catalyst metal 46. The area of the circle is deemed to correspond to the SOX release amount (see FIG. 13). Furthermore, in the present embodiment, the SOX release amount is replaced with the NOX recovery amount to calculate the ratio of the radius r1 and the radius r2. The ratio of the radius r1 and the radius r2 becomes the following formula.

r2/r1=(Nt1/Ne)1/2  (20)

Here, the variable Ne is the recoverable NOX storable amount (final NOX recovery amount) which shows the recovery amount when performing SOX release control from the timing ts to when the SOX storage amount becomes the residual SOX storage amount Se. The variable Nt1 is the NOX storable amount which is recovered from the timing ts to the timing t1 and is called the “NOX recovery amount”.
FIG. 17 shows a graph of the relationship between the final NOX storable amount and the bed temperature of the NOX storage reduction catalyst when performing SOX release control. It is learned that as the temperature of the NOX storage reduction catalyst becomes higher, the final NOX storable amount Qe becomes larger. As shown in FIG. 7, by the temperature of the NOX storage reduction catalyst becoming higher, the residual SOX storage amount Se becomes smaller, so this trend appears.
In the present embodiment, the relationship shown in FIG. 17 is used as the basis to prepare in advance a map of the final NOX storable amount Qe as a function of the bed temperature of the NOX storage reduction catalyst. This is stored in the electronic control unit 30. It is possible to detect the temperature of the NOX storage reduction catalyst and use the map of the NOX storable amount so as to detect the final NOX storable amount Qe.
Alternatively, the final NOX storable amount Qe can be calculated by subtracting from the total NOX storable amount Qtotal an amount corresponding to the residual SOX storage amount Se. The total NOX storable amount Qtotal is stored in advance in the electronic control unit 30. The residual SOX storage amount Se can for example be detected from a map of the residual SOX storage amount as a function of temperature. The total NOX storable amount Qtotal and the residual SOX storage amount Se can be used to calculate the final NOX storable amount Qe.
By subtracting from the final NOX storable amount Qe the initial NOX storable amount Q0, it is possible to calculate the restorable NOX storable amount Ne. The initial NOX storable amount Q0 can be calculated by subtracting from the final NOX storable amount Qe the initial SOX storage amount S0.
FIG. 18 shows a graph of the NOX storable amount of the NOX storage reduction catalyst with respect to the SOX storage amount. It is learned that the greater the SOX storage amount, the smaller the NOX storable amount becomes. The relationship shown in FIG. 18 is used as the basis to prepare in advance a map of an NOX storable amount as a function of the SOX storage amount and store it in the electronic control unit 30. By calculating the SOX storage amount St1 at any timing t1, it is possible to detect the NOX storable amount Qt1 at the timing t1. By subtracting from the NOX storable amount Qt1 at the timing t1 the initial NOX storable amount Q0 when starting the SOX release control, it is possible to calculate the NOX recovery amount Nt1 at the timing t1.
Alternatively, referring to FIG. 16 and FIG. 8, the NOX recovery amount Nt1 corresponds to the cumulative SOX release amount Mt1. From the cumulative SOX release amount Mt1 up to the timing t1, it is possible to calculate the NOX recovery amount Nt1 up to the timing t1. Alternatively, it is possible at step 115 of the flow chart shown in FIG. 15 to calculate the NOX recovery amount which was restored during Δt from the SOX release amount during Δt and cumulatively add this NOX recovery amount to calculate the NOX recovery amount Nt1 at the timing t1.
By entering the calculated restorable NOX storable amount Ne, and NOX recovery amount Nt1 into formula (20), the ratio of the radius r1 and the radius r2 can be calculated. By entering the ratio of the radius r1 and the radius r2 into the formula (7), it is possible to calculate the SOX release speed Rt1* after correction.
Next, the second release model in the present embodiment will be explained. In the second release model in the present embodiment, in the same way as the second release model in Embodiment 2, a sphere is defined centered about the catalyst metal 46. The volume of the sphere is deemed to correspond to the SOX release amount. Furthermore, the SOX release amount is replaced with the NOX recovery amount to calculate the ratio of the radius r1 and the radius r2.
In the case of the second release model in the present embodiment, the following formula may be used to find the ratio of the radius r1 and the radius r2.

r2/r1=(Nt1/Ne)1/2  (21)

By entering the value calculated at formula (21) into the formula (7), it is possible to calculate the SOX release speed Rt1* after correction.
In the present embodiment, it is possible to precisely calculate the SOX release speed. By using the formula of the SOX release speed after correction to calculate the SOX release amount, it is possible to calculate a more accurate SOX release amount. Alternatively, it is possible to precisely calculate the SOX storage amount which is stored in the NOX storage catalyst.
Further, the exhaust purification system of an internal combustion engine in the present embodiment can replace the SOX amount which is stored in the NOX storage reduction catalyst with the NOX amount for management and control.
The rest of the configuration, action, and effects are similar to those of Embodiment 1 or 2, so the explanations will not be repeated here.
The above embodiments may be suitably combined. In the above figures, the same or corresponding parts are assigned the same reference notations. Note that the above embodiments are illustrations and do not limit the invention. Further, the embodiments include changes shown in the claims.
LIST OF REFERENCE NUMERALS


    1 . . . engine body
    2 . . . combustion chamber
    3 . . . fuel injector
    17 . . . NOX storage reduction catalyst
    30 . . . electronic control unit
    45 . . . catalyst carrier
    46 . . . catalyst metal
    47 . . . NOX absorbent

## Claims

1. An exhaust purification system of an internal combustion engine which arranges in an engine exhaust passage an NOX catalyst device which stores NOX which is contained in exhaust gas when an air-fuel ratio of the inflowing exhaust gas is lean and which releases the stored NOX when the air-fuel ratio of the inflowing exhaust gas becomes a stoichiometric air-fuel ratio or rich and which uses SOX release control which raises a temperature of the NOX catalyst device to an SOX releasable temperature when an SOX amount which is stored in the NOX catalyst device exceeds a predetermined allowable amount and which makes the air-fuel ratio of the exhaust gas which flows into the NOX catalyst device a stoichiometric air-fuel ratio or rich so as to make the stored SOX be released,
an exhaust purification system of an internal combustion engine characterized in that
the NOX catalyst device has a residual SOX storage amount which is dependent on the temperature of the NOX catalyst device when performing SOX release control and finally remains even if performing SOX release control,
the system uses the residual SOX storage amount of the current SOX release control as the basis to calculate the SOX release speed at each timing in the current SOX release control,
the system uses the SOX release speed which was calculated at each timing of the SOX release control as the basis to calculate a cumulative SOX release amount which is released from the start of SOX release control to the current timing and
corrects the calculated SOX release speed at the current timing based on a ratio of a first radius and a second radius where when a releasable SOX amount obtained by subtracting from an SOX storage amount when starting SOX release control said residual SOX storage amount is deemed to correspond to an area of a circle of the first radius, a radius of a circle of an area corresponding to said cumulative SOX release amount is calculated as the second radius.

2. An exhaust purification system of an internal combustion engine which arranges in an engine exhaust passage an NOX catalyst device which stores NOX which is contained in exhaust gas when an air-fuel ratio of the inflowing exhaust gas is lean and which releases the stored NOX when the air-fuel ratio of the inflowing exhaust gas becomes a stoichiometric air-fuel ratio or rich and which uses SOX release control which raises a temperature of the NOX catalyst device to an SOX releasable temperature when an SOX amount which is stored in the NOX catalyst device exceeds a predetermined allowable amount and which makes the air-fuel ratio of the exhaust gas which flows into the NOX catalyst device a stoichiometric air-fuel ratio or rich so as to make the stored SOX be released,
an exhaust purification system of an internal combustion engine characterized in that
the NOX catalyst device has a residual SOX storage amount which is dependent on the temperature of the NOX catalyst device when performing SOX release control and finally remains even if performing SOX release control,
the system uses the residual SOX storage amount of the current SOX release control as the basis to calculate the SOX release speed at each timing in the current SOX release control, characterized in that
the NOX catalyst device has a final NOX storable amount at which NOX can be stored when said residual SOX storage amount remains, and
the system uses the SOX release speed which was calculated at each timing of the SOX release control as the basis to calculate an NOX recovery amount which is restored from the start of SOX release control to the current timing and
corrects the calculated SOX release speed at the current timing based on a ratio of a first radius and a second radius where when a restorable NOX storable amount obtained by subtracting from said final NOX storable amount an NOX storable amount when starting SOX release control is deemed to correspond to an area of a circle of the first radius, a radius of a circle of an area corresponding to said NOX recovery amount is calculated as the second radius.

3. An exhaust purification system of an internal combustion engine as set forth in claim 1, characterized in that the system
uses the SOX release speed which was calculated at each timing of the SOX release control as the basis to calculate a cumulative SOX release amount which is released from the start of SOX release control to the current timing and
corrects the calculated SOX release speed at the current timing based on a ratio of a first radius and a second radius where when a releasable SOX amount obtained by subtracting from an SOX storage amount when starting SOX release control said residual SOX storage amount is deemed to correspond to a volume of a sphere of the first radius, a radius of a sphere of a volume corresponding to said cumulative SOX release amount is calculated as the second radius.

4. An exhaust purification system of an internal combustion engine as set forth in claim 2, characterized in that
the NOX catalyst device has a final NOX storable amount at which storage of NOX is possible when said residual SOX storage amount remains, and
the system uses the SOX release speed which was calculated at the each timing of SOX release control as the basis to calculate an NOX recovery amount which is restored from the start of SOX release control to the current timing and
corrects the calculated SOX release speed at the current timing based on a ratio of a first radius and a second radius where when a restorable NOX storable amount obtained by subtracting from said final NOX storable amount an NOX storable amount when starting SOX release control is deemed to correspond to a volume of a sphere of the first radius, a radius of a sphere of a volume corresponding to said NOX recovery amount is calculated as the second radius.

5. An exhaust purification system for an internal combustion engine, comprising:
an engine exhaust passage,
an NOx catalyst device present in the engine exhaust passage, and
an electronic control unit;
wherein the electronic control unit controls the NOx catalyst device and includes operable instructions:
(i) to store NOx present in an inflowing exhaust gas in the engine exhaust passage when an air-fuel ratio of the inflowing exhaust gas is lean,
(ii) to release NOx stored in the NOx catalyst device when the air-fuel ratio of the inflowing exhaust gas is a stoichiometric air-fuel ratio or a rich air-fuel ratio,
(iii) to release stored SOx in the NOx catalyst device, at a release speed, by raising the temperature of the NOx catalyst device to an SOx releasable temperature when an amount of the stored SOx present in the NOx catalyst device exceeds a predetermined allowable amount,
(iv) to make the air-fuel ratio of the inflowing exhaust gas a stoichiometric air-fuel ratio or rich air-fuel ratio so as to release at least a portion of the amount of the stored SOx from the NOx catalyst device, and
(v) to leave a residual amount of the stored SOx in the NOx catalyst device dependent on the temperature of the NOx catalyst device when releasing the stored, SOx and
wherein the operable instruction (iii) to release the stored SOx calculates the release speed of the stored SOx based on a difference between a total amount of stored SOx in the NOx catalyst device and the residual amount of the stored SOx in the NOx catalyst device after releasing the stored SOx.

6. The exhaust purification system of claim 5, wherein the electronic control unit further includes operable instructions (vi) to control the release of the stored by calculating a cumulative amount of released SOx based on the release speed of the stored SOx and the time from a start of the release of the stored SOx, and (vii) to correct speed of release of the stored SOx based on a ratio of a first radius and a second radius wherein a releasable SOx amount is calculated by subtracting an amount of the stored SOx at a start of the release of the stored SOx and the residual amount of the stored SOx, and the residual amount of the stored SOx corresponds to an area of a circle of the first radius, and the a circle of an area of the circle of the second radius corresponds to said cumulative amount of the released SOx.

7. The exhaust purification system of claim 5, wherein the NOx catalyst device has a final NOx storable amount at which NOx can be stored and the residual amount of the stored SOx storage amount remains, and the electronic control unit further contains operable instructions (vii) to calculate a NOx recovery amount based on the speed of the release of the stored SOx calculated at release of the stored SOx and (viii) to calculate an NOx recovery amount which is restored from the start of the release of the stored SOx and (ix) to correct the calculated speed of the release of the stored SOx based on a ratio of a first radius and a second radius wherein an area of a first circle of the first radius corresponds to a restorable NOX storable amount obtained by subtracting from said final NOX storable amount an NOX storable amount when starting SOX release control, and an area of a second circle corresponds to second radius corresponding to said NOX recovery amount.

8. The exhaust purification system of claim 5, wherein the electronic control unit further includes operable instructions (x) to calculate a cumulative SOx release amount based on the speed of the release of the stored SOx wherein the cumulative SOx release amount is an amount of SOx is released from the start of the release of the stored SOx to the current timing and (xi) to correct the speed of the release of the stored SOx based on a ratio of a first radius and a second radius wherein a volume of a first sphere of the first radius corresponds to-a releasable SOx amount calculated by subtracting from an SOx storage amount when starting to release the stored SOx said residual SOx storage amount, and a volume of a second sphere of a second radius corresponds to said cumulative SOx release amount.

9. The exhaust purification system of claim 5, wherein the NOx catalyst device has a final NOx storable amount at which storage of NOx is possible when said residual amount of the stored SOx remains, and the electronic control unit further includes operable instructions
(xii) to correct the speed of the release of the stored SOx calculated at the SOx release control as a basis to calculate an NOx recovery amount which is restored from the start of SOx release control and corrects the calculated SOx release speed at the current timing based on a ratio of a first radius and a second radius wherein a first volume of a first sphere of the first radius corresponds to a restorable NOx storable amount obtained by subtracting from said final NOx storable amount an NOx storable amount when starting SOx release control, and a second volume of a second radius of a second sphere corresponds to said calculated NOx recovery amount.