DE10316830A1 - Device and method for controlling the air / fuel ratio of an internal combustion engine - Google Patents

Abstract

An engine (10) comprises an NOx storage / reduction catalytic converter (22) in an exhaust gas duct (18), which adsorbs a sulfur component. The engine (10) has an operating mode of a lean operating mode, in which the air / fuel ratio of the exhaust gas is a lean air / fuel ratio, and a stoichiometric operating mode, in which the air / fuel ratio of the exhaust gas is the theoretical air / Fuel ratio is. An electronic control unit (40) controls the air / fuel ratio of an air / fuel mixture so that the air / fuel ratio of the exhaust gas becomes a rich air / fuel ratio when the amount of NOx storage / reduction Catalyst adsorbed sulfur component exceeds a predetermined value during the stoichiometric operating mode.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention

The invention relates to a device for controlling the Air / fuel ratio of an internal combustion engine in which a catalytic converter is located in an exhaust gas duct is provided to adsorb the sulfur component in the exhaust gas when the Air / fuel ratio of the exhaust gas is lean, as well as a corresponding Control method.

2. Description of the prior art

Recently, an internal combustion engine running on lean fuel has been proposed, if the air / fuel ratio of the exhaust gas is lean because of the Fuel concentration is lower than the stoichiometric air / fuel ratio (the theoretical air / fuel ratio), which runs in stoichiometric operation, if the air / fuel ratio of the exhaust gas corresponds to the theoretical air / fuel Ratio corresponds. This type of internal combustion engine is a catalytic one NOx storage / reduction device provided in the exhaust duct to the Nitrogen oxide (NOx), which is contained in the exhaust gas during lean operation, is effective Way to eliminate. This catalytic device temporarily adsorbs that Nitrogen oxide (NOx), which is contained in the exhaust gas during lean combustion. If the Combustion under a stoichiometric air / fuel ratio or one rich air / fuel ratio is performed at which the fuel concentration is higher than the stoichiometric air / fuel ratio, reduced and eliminated the catalyst device then the adsorbed NOx by the action of the exhaust gas contained hydrocarbon (HC) and carbon monoxide (CO) and sets the exhaust gas free.

In this catalytic NOx storage / reduction device is by a mechanism that is essentially the same as the mechanism of NOx adsorption is identical, sulfur oxide (SOx) contained in the exhaust gas is adsorbed. This Phenomenon is commonly referred to as SOx poisoning. If the amount of adsorbed SOx (the extent of SOx poisoning) increases, the ability to NOx increases adsorb with increasing amount of adsorbed SOx, resulting in a Deterioration of the NOx adsorbability of the catalytic device leads.

Therefore, as in Japanese Patent Laid-Open No. 2000-80914 and Japanese Patent Laid-Open No. 2000-161107 Sulfur poisoning regeneration control performed, the amount of im Catalyst adsorbed SOx is calculated and the air / fuel ratio of the Exhaust gas is controlled so that it becomes rich if the calculated amount of SOx is equal to or is larger than an allowable amount so that the NOx storage / reduction device can regenerate after sulfur poisoning.

Note that the NOx catalytic storage / reduction device a sulfur component (SOx) contained in the exhaust gas is adsorbed when the air / - The fuel ratio of the exhaust gas is lean. If the said sulfur poisoning Regeneration control is performed, the lean operation of the Internal combustion engine ended, and the fuel concentration of the Air / fuel mixture supplied to the internal combustion engine, i.e. H. the amount of that Fuel supplied to the internal combustion engine is increased. Therefore, they deteriorate Fuel consumption values when the sulfur poisoning regeneration control is carried out.

SUMMARY OF THE INVENTION

It is the subject of the invention, a device for controlling the Air / fuel ratio of an internal combustion engine and a corresponding one Provide control procedures, increasing the likelihood of termination of the Lean operation is reduced and a catalyst is effectively freed from a sulfur component and can cause a deterioration in fuel consumption can be suppressed.

A first aspect of the invention relates to a device for controlling the air / - Fuel ratio of an internal combustion engine, being in the exhaust duct Catalyst is provided which adsorbs the sulfur component in the exhaust gas when that The air / fuel ratio of the exhaust gas is lean. Depending on the operating state of the engine selects this device to control the air / fuel ratio Lean operating mode, in which the air / fuel ratio of the exhaust gas is lean, or the stoichiometric operating mode in which the air / fuel ratio of the Exhaust gas corresponds to the theoretical air / fuel ratio. If the stoichiometric operating mode is selected, controls the device to control the Air / fuel ratio the air / fuel ratio of the Air / fuel mixture that is supplied to the internal combustion engine, so that the Air / fuel ratio of the exhaust gas corresponds to the theoretical air / fuel ratio. About that this device for controlling the air / fuel ratio also includes one Calculating device to determine the amount of adsorbed in the catalyst Calculate sulfur component, as well as a controller to control the air / fuel ratio to control the air / fuel mixture so that the air / fuel ratio of the Exhaust gas becomes rich when the calculated adsorption amount is equal to or greater than one permissible value and the air / fuel ratio of the air / fuel mixture to control so that the air / fuel ratio of the exhaust gas becomes rich when the calculated adsorption amount exceeds a first predetermined value, which is smaller is as the allowable value and the stoichiometric mode of operation is selected.

According to the above configuration, the device for controlling the Air / fuel ratio the air / fuel ratio of the Air / fuel mixture so that the air / fuel ratio of the exhaust gas becomes rich when the calculated amount of the adsorbed sulfur component the first predetermined value exceeds and the stoichiometric operating mode is selected. Accordingly, the Removal of the catalyst from the sulfur component promoted and the frequency, with which the amount of sulfur component adsorbed in the catalyst Lean operating mode exceeds the permissible value, is reduced. Accordingly, the Probability of ending the lean business for the exemption of the Catalyst are reduced from the sulfur component, and the catalyst can be effectively freed from the sulfur component. As a result, one Deterioration in fuel consumption values can be suppressed.

If the calculated amount of the adsorbed sulfur component is the first exceeds the predetermined value and the stoichiometric operating mode is selected is, the control device under the condition that the temperature of the Catalyst is equal to or higher than a second predetermined value, the air / fuel Control the air / fuel ratio so that the air / fuel ratio of the exhaust gas becomes fat. The control device can then also prevent combustion a rich air / fuel ratio if the operating mode of the is stoichiometric operating mode.

If the temperature of the catalyst is low, the It is difficult to remove the sulfur component from the catalyst, even if that Air / fuel ratio of the air / fuel mixture is controlled so that the Air / fuel ratio of the exhaust gas becomes rich. Therefore, fuel is unnecessarily consumed for the Increase catalyst temperature. Taking this into account when the calculated Amount of sulfur component adsorbed exceeds the first predetermined value and the stoichiometric mode of operation is selected, the air / fuel ratio of the exhaust gas only under the condition that the catalyst temperature is equal to or is higher than the second predetermined value. Accordingly, one becomes unnecessary Fuel consumption and thus a deterioration in fuel consumption values suppressed.

The control device can control the duration of the combustion under a rich air / - Fuel ratio during the stoichiometric operating mode depending on Shorten the degree of fatness of the air / fuel ratio so that it is equal to or becomes shorter than a permissible duration. That is, the control device can control the duration the combustion under a rich air / fuel ratio during the change stoichiometric operating mode depending on how rich the air / fuel ratio of the exhaust gas.

The sulfur component is removed from the catalyst if the air / - Controls the fuel ratio of the exhaust gas so that it becomes rich. If the However, the catalyst temperature is low and the sulfur component is difficult to remove the rich combustion can take a long time, and the fuel consumption values can get worse. With that in mind, the duration of the burn is below a rich air / fuel ratio so limited that it corresponds to the The degree of fatness of the air / fuel ratio is equal to or shorter than the permissible duration. Accordingly, the rich combustion can be prevented from continuing for a long time and deterioration in fuel consumption values can be suppressed.

The control device can control the combustion under a rich air / fuel Perform the ratio during the stoichiometric operating mode so that the degree the enrichment increases when the catalyst temperature drops.

It becomes more difficult to remove the sulfur component from the catalyst when the catalyst temperature drops. As already mentioned, however, if the Fatness level of the air / fuel ratio is variable, so that the fatness level of the Exhaust air / fuel ratio increases as the catalyst temperature drops, the removal of the catalyst from the sulfur component by increasing the Enrichment are encouraged, and the catalyst can become toxic after sulfur regenerate appropriately.

If the internal combustion engine has a plurality of cylinders, the Control device when the calculated adsorption amount is the first set value exceeds and the stoichiometric operating mode is selected during the stoichiometric mode of operation burning under a rich Adjust the air / fuel ratio so that the air / fuel ratio becomes rich in all cylinders.

The control device can adjust the air / fuel ratio of the air / fuel Control the mixture so that the air / fuel ratio of the exhaust gas immediately to the stoichiometric air / fuel ratio will increase after during stoichiometric mode of operation burning under a rich Air / fuel ratio has ended.

A second aspect of the invention relates to a method for controlling the air / - Fuel ratio of the exhaust gas to the relief of the catalyst that is in the Exhaust gas duct of an internal combustion engine is provided by a sulfur component promote. This procedure involves the steps of calculating the amount of im Adsorbed sulfur component, controlling the catalyst Air / fuel ratio of the air / fuel mixture that is supplied to the internal combustion engine, so that the air / fuel ratio of the exhaust gas becomes rich when the calculated amount the adsorbed sulfur component is equal to or larger than an allowable value, and controlling the air / fuel ratio of the air / fuel mixture so that Air / fuel ratio of the exhaust gas becomes rich when the calculated amount of adsorbed sulfur component is equal to or greater than a first fixed value, which is less than the permissible value, and the stoichiometric operating mode as Operating mode of the internal combustion engine is selected.

According to this method, the air / fuel ratio of the air / fuel Mixture controlled so that the air / fuel ratio of the exhaust gas becomes rich when the calculated amount of the adsorbed sulfur component is the first predetermined Value exceeds and the stoichiometric operating mode is selected. Accordingly, the removal of the sulfur component from the catalyst is promoted, and the Frequency with which the amount of the sulfur component adsorbed in the catalyst in Lean operating mode exceeds the permissible value, lowered. Accordingly, the Probability of ending the lean business for the exemption of the Catalyst can be lowered from the sulfur component, and the catalyst can be effectively freed from the sulfur component. As a result, one Deterioration in fuel consumption values can be suppressed.

BRIEF DESCRIPTION OF THE DRAWING

The above and other objects, features and advantages of the invention will be from the following description of the preferred embodiments below Consultation of the accompanying drawing clearly, in which similar numbers are used to designate similar elements, and where:

Fig. 1 is a diagram showing an inventive device for controlling the air / fuel ratio of an internal combustion engine;

Fig. 2 is a flowchart showing the process of poisoning regeneration;

Fig. 3 is a flowchart showing the process of poisoning regeneration;

Fig. 4 explains the relationship between the air-fuel ratio of the exhaust gas and the amount of SOx removed;

Figure 5 explains the relationship between an FB constant and an air / fuel ratio during poisoning regeneration.

Fig. 6A is a diagram showing a delay time set to set a target air / fuel ratio during poisoning regeneration;

Figure 6B is a graph showing an integral constant used to set a target air / fuel ratio during poisoning regeneration; and

Fig. 7 is a diagram showing a relationship between the air / fuel ratio of the exhaust gas and the allowable duration time of the rich combustion.

DETAILED DESCRIPTION OF THE INVENTION

In the following, an embodiment according to the invention is described with reference to the accompanying drawings. Fig. 1 shows the schematic structure of an inventive device for controlling the air / fuel ratio and a four-cylinder gasoline injection engine 10 for a vehicle with cylinder fuel injection (hereinafter simply referred to as "engine"); to which the facility is applied.

As shown in FIG. 1, there are an injector 14 that injects fuel directly into each combustion chamber 12 of each of cylinders 1-4 (only one combustion chamber for one of the cylinders is shown in FIG. 1) and a spark plug 16 that injects the injected one Ignited fuel provided in engine 10 .

Further, a three-way catalytic device 20 (hereinafter simply referred to as "three-way catalyst") and a catalytic NOx storage / reduction device 22 (hereinafter simply referred to as "NOx catalyst") arranged downstream of the three-way catalytic converter 20 are provided in the exhaust duct 18 , which is connected to the combustion chamber 12 . HC (hydrocarbon), CO (carbon monoxide) and NOx (nitrogen oxide) contained in the exhaust gas are removed from the three-way catalyst 20 and the NOx storage / reduction catalyst 22 .

That is, in the three-way catalyst 20 , HC, CO and NOx contained in the exhaust gas are simultaneously removed by an oxidizing and a reducing action. On the other hand, NOx contained in the exhaust gas NOx in the exhaust gas 22 is once adsorbed during the lean-burn mode (lean-burn mode), and the adsorbed NOx is then reduced using HC and CO serving as a reducing agent during the rich-fatigue mode (the fat-burning mode) (or the stoichiometric operating mode ( of the stoichiometric combustion mode)) are contained in the exhaust gas, reduced and removed.

An oxygen sensor 24 , which detects the oxygen concentration in an exhaust gas component, is provided in the exhaust gas duct 18 , specifically between the three-way catalytic converter 20 and the NOx catalytic converter 22 . Furthermore, a temperature sensor 26 , which detects the catalytic converter temperature, is provided in the NOx catalytic converter 22 .

An engine speed sensor 31 which detects the engine speed, an acceleration sensor 32, the depressing of an accelerator pedal (not shown) is detected and a vehicle speed sensor 33 which detects the traveling speed of a vehicle C (the vehicle speed SPD) are provided in the vehicle C. The detection signals from the oxygen sensor 24 , the temperature sensor 26 and the sensors 31 to 33 are input into an electrical control unit 40 which controls the motor 10 in various ways.

The electronic control unit 40 performs the fuel injection control and the like based on the operating state of the engine 10 and the driving state of the vehicle, which in turn are detected by the oxygen sensor 24 , the temperature sensor 26 , the sensors 31 to 33 and the like. The electronic control unit 40 further contains a memory 41 , in which a program and a map for calculation are stored, which are used to carry out the various types of control, as well as various data which are calculated when the controls are carried out.

In the engine 10 of the present invention, the shape of the fuel injection by the injector 14 , the timing of the ignition by the spark plug 16 or the like is changed so that a stratified operating mode (lean operating mode), a stoichiometric operating mode (normal operating mode), a rich operating mode or a temperature increase is selectively Operating mode (temperature increase combustion mode) is performed.

For example, in the stratified operating mode, the time of the fuel injection is set so that it takes place at a later stage of the compression stroke. Accordingly, only the air / fuel mixture in the vicinity of the spark plug 16 is a combustible air / fuel mixture and is only partially ignited at the time of ignition. In this case, the average air / fuel ratio (A / F) of the air / fuel mixture in each of the cylinders is set 1 to 4 leaner (e.g., A / F = 25 to 50) than the stoichiometric air / fuel ratio ( A / F = 14.5).

In the stoichiometric operating mode, the time of the fuel injection is set so that it takes place during the intake stroke. Accordingly, the air / fuel ratio in the combustion chamber 12 is substantially uniform at the time of ignition, and the air / fuel ratio of the air / fuel mixture in each of the cylinders 1 to 4 is adjusted to be close to the stoichiometric air / Fuel ratio.

The operating mode of the engine is basically switched depending on the operating state of the engine 10 , such as engine load and engine speed. In general, the operating mode of the engine is set to the stratified operating mode in a low load and low speed operating range and to the stoichiometric operating mode in a high load and high speed operating range.

Further, in the rich operation mode, the fuel injection timing is set to take place during the intake stroke as well as in the stoichiometric mode, and the air / fuel ratio of the air / fuel mixture in each of the cylinders 1 to 4 is uniformly set to be is richer than the stoichiometric air / fuel ratio. The rich operation mode is selected at the time of the process (enrichment increase of the process) when the NOx adsorbed in the NOx catalyst 22 by temporarily enriching the exhaust air / fuel ratio by the amounts of HC and CO contained in the exhaust gas is increased, reduced and removed when the amount of NOx adsorbed in the NOx catalyst 22 exceeds a predetermined value.

In contrast, the temperature increase mode of operation is selected when a predetermined condition is met, for example when a temperature increase is necessary for the NOx catalyst 22 to recover after SOx poisoning, ie when the amount of SOx poisoning of the NOx catalyst 22 exceeds a predetermined value.

In the temperature increase operating mode, the air / fuel ratio of one to three of cylinders 1 to 4 is set to be richer than the stoichiometric air / fuel ratio and the air / fuel ratio of the other cylinder (s) becomes set to be leaner than the stoichiometric air / fuel ratio. Furthermore, the average air / fuel ratio of these cylinders 1 to 4 is set to the stoichiometric air / fuel ratio or a rich air / fuel ratio.

Thus, by switching the engine operating mode to the temperature increasing operating mode, the unburned fuel component in the exhaust gas released from one to three of the cylinders 1 to 4 , the air / fuel ratio of which is set to be rich, and the oxygen in the exhaust gas, which is from the or the other cylinders, in which the air / fuel ratio was set lean, is burned by the catalytic action of the three-way catalytic converter 20 or the NOx catalytic converter 22 . As a result, the temperature of the NOx catalyst 22 rises due to the heat of combustion, and the catalyst 22 is freed of SOx that has been adsorbed in the NOx catalyst.

Next, the SOx poisoning regeneration of the NOx catalyst 22 will be described in detail with reference to FIGS. 2 and 3. Figs. 2 and 3 form a flow chart showing the flow of the SOx poisoning recovery. A number of the operations shown in the flowchart are interruption operations performed by the electronic control unit 40 at a predetermined crank angle. Typically, as indicated above, the engine operating mode is switched between the stratified operating mode and the stoichiometric operating mode depending on the operating state of the engine 10 according to a flowchart other than that shown in FIGS . 2 and 3. The flow chart shown in Figs. 2 and 3 is a flow chart for switching the engine operating mode to the temperature increasing operating mode when the stoichiometric operating mode would normally be performed and performing combustion under a rich air / fuel ratio even when the operating mode of the internal combustion engine is the stoichiometric operating mode.

When the electronic control unit 40 proceeds to the routine of the steps shown in FIGS. 2 and 3, the total amount of SOx adsorbed in the NOx catalyst 22 is first calculated in step S100. The electronic control unit 40 adds the amount of SOx that is added during the lean combustion, at which the air / fuel ratio is lean, to the total SOx, and subtracts the amount of SOx, during the rich combustion, at which the Air / fuel ratio is rich, comes off of the total SOx. The amount of SOx to be added is set to a larger value when the amount of intake air increases or the air / fuel ratio decreases (ie, when the fuel concentration increases). This is because the amount of SOx adsorbed per unit time in the NOx catalyst 22 when it comes in contact with the NOx catalyst 22 increases as the amount of air drawn in or the fuel concentration increases. Furthermore, the amount of SOx to be subtracted in the SOx calculation is set to a higher value when the enrichment of the air / fuel ratio increases or when the catalyst bed temperature increases because the release of the NOx catalyst 22 from the SOx by an increase the catalyst bed temperature or by a higher enrichment of the air / fuel ratio. The calculation of the total amount of SOx is influenced by the amount of sulfur contained in the fuel consumed. For example, when high octane gasoline is used, the amount of SOx that is adsorbed per unit time in the NOx catalyst decreases because the amount of sulfur contained in high octane gasoline is smaller than that in low octane gasoline. On the other hand, the amount of SOx that is adsorbed per unit time in the NOx catalyst increases when the low octane gasoline is used because the amount of sulfur contained in the low octane gasoline is larger than that in the high octane gasoline. Note that when using high octane gasoline, the ignition timing of the engine 10 is advanced by an advance timing because knock is difficult to generate. In contrast, when low octane gasoline is used, the ignition timing of the engine 10 tends to be retarded. Thus, this determination of the fuel type can be made based on this tendency, for example. Further, the relationship between the intake air amount and the air / fuel ratio and the amount of SOx to be added, and the relationship between the air / fuel ratio and the catalyst bed temperature and the amount of SOx to be subtracted are calculated in advance through experiments and the like, and stored 41 of the electronic control unit 40 .

After the total amount of SOx has been calculated, it is determined whether Requirement flag that shows that SOx poisoning regeneration is required is set to ON (step S102). If it is determined that the request Indicator for SOx poisoning regeneration is not set to ON (step S102: NO), it goes to step S104. On the other hand, if it is determined that the request indicator for the poisoning regeneration is set to ON (Step S102: YES) proceeds to Step S108.

If it is determined that the poisoning regeneration request flag is not set to ON, it is determined whether the total SOx calculated in the previous step S100 is equal to or larger than an allowable value ASt (step S104). The allowable value ASt is a value that is used to determine whether the extent of the SOx poisoning has risen to a level at which the deterioration in the NOx adsorption capacity can no longer be accepted. If it is determined that the total SOx amount is equal to or larger than the allowable value ASt (step S104: YES), the request flag for the poisoning regeneration is set to ON because SOx poisoning regeneration must be performed (step S106 ). When the engine 10 is running in the lean operating mode, the level of SOx poisoning increases. Accordingly, when it is determined that the total SOx calculated in the previous step S100 is equal to or larger than the allowable value ASt, it is assumed that the engine 10 is running in the lean operating mode. The process proceeds from step S106 to step S108, and the operating mode is switched from the lean operating mode to the temperature increase operating mode. Further, when it is determined in the previous step S104 that the total SOx amount is smaller than the allowable value ASt and the extent of the SOx poisoning is not particularly high (step S104: NO), the process proceeds to step S128.

If it was determined in the previous step S102 that the poisoning regeneration request flag is set to ON, or if the poisoning regeneration request flag is set to ON in step S106, it is determined whether the catalyst bed temperature which is detected by the temperature sensor 26 in step S108, is equal to or higher than the predetermined temperature THc. The predetermined temperature THc is the temperature at which it is determined whether the NOx catalyst 22 can remove SOx depending on its temperature state.

If it is determined that the catalyst bed temperature is lower than the predetermined temperature THc (step S108: NO), the combustion mode is switched from the lean combustion mode to the temperature increase mode in which the air / fuel ratio corresponds to the stoichiometric air / fuel ratio because the catalyst bed temperature must be increased. As previously stated, in the temperature increase mode, the air / fuel ratio of one to three of cylinders 1 to 4 is set to be richer than the stoichiometric air / fuel ratio and the air / fuel ratio of the other The cylinder is adjusted to be leaner than the stoichiometric air / fuel ratio. Further, the average air / fuel ratio of cylinders 1 to 4 is set to correspond to the stoichiometric air / fuel ratio, and the average air / fuel ratio of the exhaust gas reaching the catalysts 20 , 22 is determined by a Air / fuel ratio feedback control using the oxygen sensor 24 is maintained at the stoichiometric air / fuel ratio. By combustion at elevated temperature, the unburned fuel component in the exhaust gas, which is released from one to three of the cylinders 1 to 4 , in which the air / fuel ratio is set to be rich, and the oxygen in the exhaust gas, which is from the or the other cylinders in which the air / fuel ratio is lean is burned by the catalytic action of the three-way catalyst 20 or the NOx catalyst 22 , and the temperature of the NOx catalyst 22 is caused by the heat of combustion elevated.

On the other hand, when the catalyst bed temperature is equal to or higher than the predetermined temperature THc (step S108: YES), since SOx can be removed from the NOx catalyst 22 , rich combustion is performed in step S116 due to the sequence of operations described below, where the average air / fuel ratio in cylinders 1 to 4 is rich. First, before a rich combustion is performed in step S116, a measurement of the duration Te of the rich combustion is started in step S110.

Next, in step S112, the degree of richness of a target air / fuel ratio to be set is determined depending on the catalyst bed temperature. As shown in FIG. 4, the amount of SOx removed from the NOx catalyst 22 tends to increase as the air / fuel ratio fatness increases and also as the catalyst bed temperature increases. When this rich combustion is performed, as shown in FIG. 5, the air / fuel feedback control by means of the oxygen sensor 24 calculates a feedback (FB) coefficient kaf in which the value of the air / fuel ratio is controlled becomes richer than the stoichiometric air / fuel ratio. Namely, as shown in Fig. 5, the coefficient kaf is calculated so that its value is over 1.0. Accordingly, the feedback (F / B) coefficient used in the air / fuel ratio feedback control is set in step S112. Namely, with reference to a map shown in FIG. 6A, a delay time TDR and a delay time TDL are set depending on the catalyst bed temperature. The delay time TDR begins with the reverse timing output from the oxygen sensor 24 (ie, the time when a signal output from the oxygen sensor 24 becomes a lean signal from a rich signal indicating a rich air-fuel ratio of the exhaust gas , which indicates a lean air / fuel ratio of the exhaust gas), and takes until the deflection is added to the rich side, thereby reducing the FB coefficient. The delay time TDL begins with the reversal timing and lasts until the deflection to the lean side, which increases the FB coefficient. With reference to a map shown in FIG. 6B, an integral constant CR that makes the air / fuel ratio rich by increasing the FB coefficient and an integral constant CL that decreases the air / fuel ratio by lowering the F / B Coefficient makes lean, depending on the catalyst bed temperature set. Accordingly, the controlled value of the air / fuel ratio at this time is richer than the controlled value of the stoichiometric air / fuel ratio. In step S112 of the temperature increase mode, the air / fuel ratio of the rich cylinders in which the air / fuel ratio has become rich in the temperature increase mode in the previous step S126 (where the average air / fuel ratio is the stoichiometric air / fuel ratio has become richer, and the air / fuel ratio of the other cylinders is adjusted to approximate the stoichiometric air / fuel ratio. Furthermore, the average air / fuel ratio of cylinders 1 to 4 is set to a rich air / fuel ratio.

When the target air-fuel ratio on the rich side is set, an allowable duration Tp of the rich combustion is set in step S114. If the air / fuel ratio of the exhaust gas is set on the rich side and a rich combustion is currently being carried out, the fuel consumption values and the exhaust gas values deteriorate. If the duration Te of the rich combustion continues to increase, the fuel consumption values and the exhaust gas values continue to deteriorate. Accordingly, the duration Tp of the allowable rich combustion in S 114 is set based on the air-fuel ratio at the time of the rich combustion with reference to a map shown in FIG. 7.

After the allowable duration Tp of the rich combustion is set, as shown in FIG. 5, the air / fuel ratio of the exhaust gas is controlled to the target air / fuel ratio on the rich side, and rich combustion is performed (Step S116).

When rich combustion is performed, it is determined whether the duration Te the rich combustion is equal to or longer than the permissible duration Tp, which in previous step S114 has been set (step S118). If it is determined that the Duration Te is shorter than the allowable duration Tp (step S118: NO) becomes step S120 passed. On the other hand, if it is determined that the duration Te is equal to or is greater than the allowable duration Tp (step S118: YES) becomes step S122 passed.

If it is determined that the duration Te is shorter than the allowable duration Tp, it is determined whether the total SOx amount calculated in the previous step S100 is equal to or larger than a predetermined value ASb (step S120). The predetermined value ASb is a value with which it is determined whether the extent of the SOx poisoning has decreased by performing poisoning regeneration and whether the NOx adsorbing ability of the NOx catalyst 22 has recovered. The predetermined value ASb is set to ASt>ASb> 0.

If it is determined that the total SOx amount is equal to or larger than the predetermined value ASb (step S120: YES), the SOx poisoning regeneration must be continued and the sequence of operations is ended this time. On the other hand, when it is determined in the previous step S120 that the total SOx amount is smaller than the predetermined value ASb and the NOx adsorbability of the NOx catalyst 22 is restored (step S120: NO), the process proceeds to step S122.

If it was determined in the previous step S118 that the duration Te is equal or longer than the allowable duration Tp, or when it is determined in step S120 that the total SOx amount is smaller than the predetermined value ASb, it becomes Poisoning regeneration request flag set to OFF to End SOx poisoning regeneration (step S122).

Then in the next step S 124 the air / fuel ratio of the exhaust gas controlled to the lean air / fuel ratio, and stratified combustion is carried out. Further, if it was determined in the previous step S104, that the amount of SOx is less than the predetermined value ASt, and the extent of the SOx poisoning is not particularly large, determines whether the operating mode to this Time is the stoichiometric operating mode (step S128). If it is determined that the operating mode is not the stoichiometric operating mode (step S128: NO), the lean combustion becomes below the lean air / fuel ratio continued since it is not yet necessary to regenerate SOx poisoning and the sequence of operations is ended for this time. If it is determined that the operating mode is the stoichiometric operating mode (step S128: YES) proceeded to step S130.

In step S130, it is determined whether the total SOx amount calculated in the previous step S100 is larger than a second predetermined value AS0 (step S130). The second predetermined value AS0 is a value with which it is determined whether the NOx adsorbability of the NOx catalyst 22 is restored. The second predetermined value AS0 is set to 0 in this embodiment.

If it is determined that the total SOx amount is equal to or less than the predetermined value AS0 and the NOx adsorbability of the NOx catalyst 22 is restored (step S130: NO), the sequence of operations is ended this time.

If it is determined in the previous step S130 that the total SOx amount exceeds the predetermined value AS0 (step S130: YES), the process proceeds to step S132. In step S132, it is determined whether the catalyst bed temperature determined by the temperature sensor 26 is equal to or higher than the predetermined temperature THc at which SOx can be removed from the catalyst. If it is determined that the catalyst bed temperature is lower than the predetermined temperature THc (step S132: NO), the sequence of operations for this time is ended.

On the other hand, if the catalyst bed temperature is equal to or higher than the predetermined temperature THc (step S132: YES) because SOx can be removed from the NOx catalyst 22 , the measurement of the rich combustion duration Te is started (step S134).

Next, in step S136, the target air / fuel ratio is set depending on the catalyst bed temperature on the rich side. If the air / fuel ratio of the exhaust gas is set on the rich side and a rich combustion is being carried out, the fuel consumption values and the exhaust gas values deteriorate. The fuel consumption values and the exhaust gas values deteriorate further as the duration Te of the rich combustion increases. Accordingly, the allowable duration Tp1 of the rich combustion is set based on the air-fuel ratio at the time of the rich combustion with reference to the map shown in FIG. 7.

After the allowable duration Tp1 of the rich combustion is set, the recirculation (FB) coefficient kaf is calculated as shown in Fig. 5, the air / fuel ratio of the exhaust gas is set to the target air / fuel ratio on the rich side is set, and rich combustion is performed in all cylinders (step S140).

If rich combustion is performed in this way, it is determined whether the duration Te of the rich combustion is equal to or longer than the permissible duration Tpl set in the previous step S138 (step S142). If it is determined that the duration Te is shorter than the allowable duration Tp1 (step S142: NO), the sequence of procedures is ended for this time. If determined against becomes that the duration Te is equal to or longer than the allowable duration Tp1 (step S142: YES), it goes to step S144.

• A) Wenn eine Schwefelvergiftungs-Regenerierungssteuerung des NOx-Katalysators 22 durchgeführt wird, wird die zulässige Dauer Tp der fetten Verbrennung abhängig vom Luft/Kraftstoff-Verhältnis des Abgases während der Schwefelvergiftungs-Regenerierungssteuerung eingestellt. Wenn die Dauer Te der fetten Verbrennung während der Schwefelvergiftungs-Regenerierungssteuerung die zulässige Dauer Tp erreicht, wird die Schwefelvergiftungs-Regenerierungssteuerung zwangsbeendet. Demgemäß wird verhindert, daß die Schwefelvergiftungs-Regenerierungssteuerung über längere Zeit fortgesetzt wird, und eine Verschlechterung der Kraftstoff-Verbrauchswerte und der Emissionswerte kann unterdrückt werden.
• B) Darüber hinaus kann, da die zulässige Dauer Tp der fetten Verbrennung abhängig vom Luft/Kraftstoff-Verhältnis des Abgases eingestellt wird, das von der Schwefelvergiftungs-Regenerierungssteuerung gesteuert wird, eine Verschlechterung der Kraftstoff-Verbrauchswerte und der Emissionswerte bei der Schwefelvergiftungs-Regenerierungssteuerung unabhängig vom Grad der Anfettung des Luft/Kraftstoff-Verhältnisses angemessen unterdrückt werden.
• C) Die Kraftstoff-Verbrauchswerte und die Emissionswerte neigen während der Schwefelvergiftungs-Regenerierungssteuerung zur Verschlechterung. Das Luft/- Kraftstoff-Verhältnis des Abgases, das von der Schwefelvergiftungs-Regenerierungssteuerung gesteuert wird, ist jedoch variabel, so daß der Grad der Anfettung steigt, wenn die Katalysatortemperatur sinkt. Demgemäß kann eine Verschlechterung der Kraftstoff-Verbrauchswerte und der Emissionswerte unterdrückt werden.
• D) Weiter wird, wenn die SOx-Gesamtmenge im NOx-Katalysator 22 den vorbestimmten Wert AS0 (< zulässiger Wert ASt) überschreitet, und wenn der stöchiometrische Betriebsmodus des Motors 10 ausgewählt ist, das Luft/Kraftstoff-Verhältnis des Abgases auf das fette Luft/Kraftstoff-Verhältnis gesteuert. Zu diesem Zeitpunkt wird die Befreiung des Katalysators von der Schwefelkomponente gefördert und die Häufigkeit, mit der die Menge der Schwefelkomponente, die im Katalysator adsorbiert ist, den zulässigen Wert überschreitet, wird gesenkt. Demgemäß kann die Wahrscheinlichkeit einer Beendigung des Magerbetriebs für die Befreiung des Katalysators von der Schwefelkomponente gesenkt werden, und die Schwefelkomponente kann wirksam vom Katalysator entfernt werden. Infolgedessen kann eine Verschlechterung der Kraftstoff-Verbrauchswerte unterdrückt werden.
• E) Darüber hinaus wird, wenn der stöchiometrische Betriebsmodus ausgewählt ist, unter der Bedingung, daß die Katalysatorbett-Temperatur gleich oder höher ist die vorbestimmte Temperatur THc, die Steuerung so durchgeführt, daß das Luft/- Kraftstoff-Verhältnis des Abgases fett wird, wenn normalerweise der stöchiometrische Betriebsmodus ausgewählt würde. Demgemäß wird ein unnötiger Kraftstoffverbrauch, um die Katalysatorbett-Temperatur zu erhöhen, unterdrückt, und die Verschlechterung der Kraftstoff-Verbrauchswerte kann unterdrückt werden.
• F) Mit sinkender Katalysatortemperatur läßt sich die Schwefelkomponente schwerer vom NOx-Katalysator 22 entfernen. Das fette Luft/Kraftstoff-Verhältnis ist jedoch variabel, wenn normalerweise der stöchiometrische Betriebsmodus ausgewählt würde, so daß der Grad der Anfettung erhöht wird, wenn die Temperatur des Katalysators sinkt. Demgemäß kann die Befreiung des NOx-Katalysators von der Schwefelkomponente gefördert werden, und die Schwefelvergiftungsregenerierung des NOx-Katalysators 22 kann auf angemessene Weise durchgeführt werden.

Then, in the next step S144, the air / fuel ratio of the exhaust gas is controlled to the stoichiometric air / fuel ratio, stoichiometric combustion is performed, and the SOx poisoning regeneration is ended. According to the above-mentioned embodiment, the following effects can be achieved.

• A) When performing a sulfur poisoning regeneration control of the NOx catalyst 22 , the allowable duration Tp of the rich combustion is set depending on the air / fuel ratio of the exhaust gas during the sulfur poisoning regeneration control. When the rich combustion period Te during the sulfur poisoning regeneration control reaches the allowable duration Tp, the sulfur poisoning regeneration control is forcibly ended. Accordingly, the sulfur poisoning regeneration control is prevented from continuing for a long time, and deterioration in fuel consumption values and emission values can be suppressed.
• B) In addition, since the allowable duration Tp of the rich combustion is set depending on the air-fuel ratio of the exhaust gas controlled by the sulfur poisoning regeneration control, deterioration in the fuel consumption values and the emission values in the sulfur poisoning regeneration control can be independent adequately suppressed by the degree of enrichment of the air / fuel ratio.
• C) The fuel consumption values and the emission values tend to deteriorate during the sulfur poisoning regeneration control. However, the air / fuel ratio of the exhaust gas controlled by the sulfur poisoning regeneration control is variable so that the degree of enrichment increases as the catalyst temperature decreases. Accordingly, deterioration in fuel consumption values and emission values can be suppressed.
• D) Further, when the total amount of SOx in the NOx catalyst 22 exceeds the predetermined value AS0 (<allowable value ASt), and when the stoichiometric operating mode of the engine 10 is selected, the air / fuel ratio of the exhaust gas to the rich air becomes / Fuel ratio controlled. At this time, the catalyst component is released from the sulfur component, and the frequency with which the amount of the sulfur component adsorbed in the catalyst exceeds the allowable value is decreased. Accordingly, the likelihood of termination of the lean operation for the removal of the catalyst from the sulfur component can be reduced, and the sulfur component can be effectively removed from the catalyst. As a result, deterioration in fuel consumption values can be suppressed.
• E) Furthermore, when the stoichiometric mode of operation is selected, under the condition that the catalyst bed temperature is equal to or higher than the predetermined temperature THc, the control is performed so that the air / fuel ratio of the exhaust gas becomes rich when normally the stoichiometric operating mode would be selected. Accordingly, unnecessary fuel consumption to raise the catalyst bed temperature is suppressed, and deterioration in fuel consumption values can be suppressed.
• F) As the catalyst temperature decreases, the sulfur component is more difficult to remove from the NOx catalyst 22 . However, the rich air / fuel ratio is variable when the stoichiometric mode of operation would normally be selected, so that the degree of enrichment increases as the temperature of the catalyst drops. Accordingly, the liberation of the NOx catalyst from the sulfur component can be promoted, and the sulfur poisoning regeneration of the NOx catalyst 22 can be performed appropriately.

• A) In der Ausführungsform kann ein Luft/Kraftstoff-Verhältnissensor, der das Luft/- Kraftstoff-Verhältnis des Abgases linear bestimmt, im NOx-Katalysator anstelle des Sauerstoffsensors 24, der zwischen dem Dreiwegekatalysator 20 und dem NOx-Katalysator 22 bereitgestellt ist, vorgesehen sein. In diesem Fall kann das Ziel-Luft/Kraftstoff-Verhältnis der fetten Verbrennung während der Vergiftungsregenerierung direkt eingestellt werden.
• B) In der Ausführungsform wird, wenn die SOx-Gesamtmenge gleich oder größer ist als der zulässige Wert ASt und die Katalysatorbett-Temperatur gleich oder höher ist als THc, dadurch eine fette Verbrennung durchgeführt, daß man das Luft/- Kraftstoff-Verhältnis im fetten Zylinder während der Temperaturerhöhungs-Verbrennung fetter macht und das Luft/Kraftstoff-Verhältnis im mageren Zylinder zur fetten Seite hin verschiebt. Eine fette Verbrennung, bei der das Luft/Kraftstoff-Verhältnis aller Zylinder fett ist, kann jedoch auch durchgeführt werden.
• C) In der Ausführungsform sind der Dreiwegekatalysator 20 und der NOx- Speicherungs/Reduktions-Katalysator 22 im Abgaskanal 18 bereitgestellt. In diesem Fall kann während des stöchiometrischen Verbrennungsbetriebs eine Verschlechterung der HC- und CO-Emissionswerte dadurch vermieden werden, daß das Abgas vom NOx-Speicherungs/Reduktions-Katalysator gereinigt wird.
• D) In der Ausführungsform liegt eine Abgas-Steuerungseinrichtung vor, bei der der Kraftstoff direkt von der Einspritzvorrichtung 14 in die Brennkammer 12 gespritzt wird. Die Abgas-Steuerungseinrichtung kann jedoch auch auf einen Motor mit einer Ansaugkanal-Kraftstoffeinspritzung angewandt werden, bei der der Kraftstoff in den Ansaugkanal gespritzt wird.
• E) In der Ausführungsform wird ein Motor beschrieben, der als geschichtete Verbrennung nur die Verbrennung durchführt, bei der der Zeitpunkt der Kraftstoffeinspritzung auf ein späteres Stadium des Kompressionshubs gesetzt ist (stark geschichtete Verbrennung). Eine erfindungsgemäße Abgas-Steuerungseinrichtung kann jedoch auch auf einen Motor angewandt werden, der zusätzlich zur stark geschichteten Verbrennung eine Verbrennung durchgeführt, bei der die Schichtungsstärke abgeschwächt ist, beispielsweise durch Aufteilen des Kraftstoffs in einen Kraftstoff für den Ansaughub und einen Kraftstoff für den Kompressionshub und durch respektives Einspritzen des aufgeteilten Kraftstoffs (schwach geschichtete Verbrennung). Eine erfindungsgemäße Abgas-Steuerungseinrichtung kann auch auf einen Motor angewandt werden, der als homogene Verbrennung zusätzlich zu einer Verbrennung, bei der das Luft/Kraftstoff-Verhältnis auf das stöchiometrische Luft/Kraftstoff-Verhältnis eingestellt ist (homogene stöchiometrische Verbrennung), eine Verbrennung durchführt, bei der das Luft/Kraftstoff-Verhältnis so eingestellt ist, daß es magerer ist als das stöchiometrische Luft/Kraftstoff-Verhältnis (magere homogene Verbrennung).
• F) In der Ausführungsform wird die Temperatur des NOx-Katalysators 22 direkt vom Temperatursensor 26 erfaßt. Die Temperatur des Katalysators kann jedoch auch aufgrund des Betriebszustands des Verbrennungsmotors bestimmt (geschätzt) werden, vorzugsweise aufgrund der Betriebshistorie des Verbrennungsmotors.

Note that the design of the embodiment can be changed as follows:

• A) In the embodiment, an air-fuel ratio sensor that linearly determines the air-fuel ratio of the exhaust gas may be provided in the NOx catalyst instead of the oxygen sensor 24 provided between the three-way catalyst 20 and the NOx catalyst 22 his. In this case, the target air / fuel ratio of the rich combustion can be set directly during the poisoning regeneration.
• B) In the embodiment, when the total amount of SOx is equal to or larger than the allowable value ASt and the catalyst bed temperature is equal to or higher than THc, rich combustion is carried out by making the air / fuel ratio rich Makes the cylinder richer during the temperature increase combustion and shifts the air / fuel ratio in the lean cylinder to the rich side. However, rich combustion, in which the air / fuel ratio of all cylinders is rich, can also be carried out.
• C) In the embodiment, the three-way catalyst 20 and the NOx storage / reduction catalyst 22 are provided in the exhaust passage 18 . In this case, during the stoichiometric combustion operation, deterioration of the HC and CO emission values can be avoided by cleaning the exhaust gas from the NOx storage / reduction catalyst.
• D) In the embodiment, there is an exhaust gas control device in which the fuel is injected directly from the injection device 14 into the combustion chamber 12 . However, the exhaust control device can also be applied to an engine with an intake port fuel injection in which the fuel is injected into the intake port.
• E) In the embodiment, an engine is described which, as stratified combustion, only carries out the combustion in which the time of fuel injection is set to a later stage of the compression stroke (strongly stratified combustion). However, an exhaust gas control device according to the invention can also be applied to an engine which, in addition to the stratified combustion, carries out a combustion in which the stratification is weakened, for example by dividing the fuel into a fuel for the intake stroke and a fuel for the compression stroke and through respectful injection of the divided fuel (weak stratified combustion). An exhaust gas control device according to the invention can also be applied to an engine which performs combustion as a homogeneous combustion in addition to a combustion in which the air / fuel ratio is set to the stoichiometric air / fuel ratio (homogeneous stoichiometric combustion), where the air / fuel ratio is set to be leaner than the stoichiometric air / fuel ratio (lean homogeneous combustion).
• F) In the embodiment, the temperature of the NOx catalyst 22 is detected directly by the temperature sensor 26 . However, the temperature of the catalytic converter can also be determined (estimated) on the basis of the operating state of the internal combustion engine, preferably on the basis of the operating history of the internal combustion engine.

Claims (14)

1. A device for controlling the air / fuel ratio of an internal combustion engine, which is applied to an internal combustion engine ( 10 ), in which a catalyst ( 22 ) is provided in the exhaust passage, which adsorbs a sulfur component in the exhaust gas when the air / fuel ratio of the exhaust gas is a lean air / fuel ratio and, as the operating mode of the internal combustion engine ( 10 ), has a lean operating mode in which the air / fuel ratio of the exhaust gas is a lean air / fuel ratio, as well as a stoichiometric operating mode, in which the air / fuel ratio of the exhaust gas corresponds to the theoretical air / fuel ratio and which selects the operating mode on the basis of the operating state of the engine, and which the air / fuel ratio of the air / fuel mixture which the internal combustion engine ( 10 ) is supplied so that the air / fuel ratio of the exhaust gas controls the theoretical air / fuel ratio when the stoichiometric mode of operation is selected, characterized in that it comprises:
a computing device ( 40 ) for calculating the amount of the sulfur component adsorbed in the catalyst ( 22 ); and
a control device that controls the air / fuel ratio of the air / fuel mixture so that the air / fuel ratio of the exhaust gas becomes a rich air / fuel ratio when the calculated adsorption amount is equal to or larger than an allowable value, and which controls the air / fuel ratio of the air / fuel mixture so that the air / fuel ratio of the exhaust gas becomes the rich air / fuel ratio when the calculated adsorption amount exceeds a first predetermined value which is smaller than that permissible value, and the stoichiometric operating mode is selected. 2. The air / fuel ratio control device according to claim 1, wherein the control device ( 40 ) controls the air / fuel ratio of the air / fuel mixture provided that the catalyst temperature is equal to or higher than a second predetermined value controls so that the air / fuel ratio of the exhaust gas becomes a rich air / fuel ratio when the calculated adsorption amount exceeds the first predetermined value and the stoichiometric operating mode is selected. 3. The air / fuel ratio control device according to claim 1 or 2, wherein the control device ( 40 ) performs combustion under a rich air / fuel ratio even when the operating mode is the stoichiometric operating mode. 4. The air / fuel ratio control device according to claim 3, wherein the control device ( 40 ) limits the duration of the combustion under the rich air / fuel ratio during the stoichiometric operating mode in accordance with the enrichment of the air / fuel ratio it becomes equal to or shorter than a permissible duration. 5. The air / fuel ratio control device according to claim 3 or 4, wherein the control device ( 40 ) performs the combustion under the rich air / fuel ratio during the stoichiometric operating mode so that the degree of enrichment of the air / Exhaust gas fuel ratio is increased when the catalyst temperature is lowered. 6. The air-fuel ratio control device according to any one of claims 3 to 5, wherein in the case that the internal combustion engine has a plurality of cylinders, the control device ( 40 ) performs combustion under a rich air / fuel ratio that the air / fuel ratio becomes rich in all cylinders when the calculated adsorption amount exceeds the first predetermined value and the stoichiometric mode of operation is selected. 7. The air / fuel ratio control device according to one of claims 3 to 6, wherein the control device ( 40 ) controls the air / fuel ratio of the air / fuel mixture so that the air / fuel ratio of the exhaust gas immediately after stopping combustion under the rich air / fuel ratio during the stoichiometric mode of operation becomes the stoichiometric air / fuel ratio. 8. A method of controlling the air / fuel ratio of the exhaust gas to promote the release of a sulfur component from a catalyst ( 22 ) provided in the exhaust passage of an internal combustion engine, characterized in that it comprises the following steps:
Calculating an adsorption amount of the sulfur component adsorbed in the catalyst (S100);
Controlling the air / fuel ratio of an air / fuel mixture supplied to the internal combustion engine so that the air / fuel ratio of the exhaust gas becomes a rich air / fuel ratio when the adsorption amount is equal to or larger than an allowable value (S110 to S116), and
Controlling the air / fuel ratio of the air / fuel mixture so that the air / fuel ratio of the exhaust gas becomes a rich air / fuel ratio when the calculated adsorption amount exceeds a first predetermined value which is less than the allowable value , and the stoichiometric operating mode is selected as the operating mode of the internal combustion engine ( 5128 to 5140 ). 9. The method of claim 8, further comprising the steps of: determining whether a catalyst temperature is equal to or higher than a second predetermined one Value (S 132) and characterized in that when the calculated Adsorption amount exceeds the first predetermined value and the Catalyst temperature is equal to or higher than a second predetermined value, the air / - The fuel ratio of the air / fuel mixture is controlled so that the Air / fuel ratio of the exhaust gas becomes a rich air / fuel ratio (S134 to 5140). 10. The method according to any one of claims 8 or 9, wherein the combustion under the rich air / fuel ratio is performed even when the operating mode is the stoichiometric mode of operation. 11. The method of claim 10, wherein the duration of the combustion under the rich Air / fuel ratio during stoichiometric operating mode depending on the rich air / fuel ratio is limited to the same or shorter than a permissible duration (S 138 to 5142). 12. The method according to any one of claims 10 or 11, wherein the combustion under the rich air / fuel ratio during the stoichiometric Operating mode is carried out so that the degree of enrichment of the The air-fuel ratio of the exhaust gas increases when the temperature of the catalyst drops (S136). 13. The method according to any one of claims 10 to 12, wherein in the case where the Internal combustion engine has a plurality of cylinders that are under combustion the rich air / fuel ratio during the stoichiometric Operating mode is performed so that the air / fuel ratio in all cylinders becomes bold (S140). 14. The method according to any one of claims 10 to 13, wherein the air / fuel Air / fuel mixture ratio is controlled so that Air / fuel ratio of the exhaust gas becomes the stoichiometric air / fuel ratio (S144) as soon as the combustion is under the rich air / fuel ratio stopped during stoichiometric operation.