Patent Grant
10316776
This application claims the benefit of priority to Japanese Patent Application No. 2015-096560 filed May 11, 2015, which is hereby incorporated by reference in its entirety.
The present disclosure relates to a control apparatus which is applied to an internal combustion engine with an exhaust gas purification device including a NOx storage reduction catalyst (NSR (NOx Storage Reduction) catalyst) arranged in an exhaust passage.
As an internal combustion engine in which the air fuel ratio of an air-fuel mixture can be changed, there has been known one in which an exhaust gas purification device including an NSR catalyst is arranged in an exhaust passage. In such an internal combustion engine, there has been proposed a technology in which at the time when an amount of NOx stored in the NSR catalyst (a storage amount of NOx) becomes equal to or more than a predetermined threshold value when the air fuel ratio of the air-fuel mixture is a lean air fuel ratio which is an air fuel ratio higher than a stoichiometric air fuel ratio, the air fuel ratio of exhaust gas flowing into the NSR catalyst is controlled from the stoichiometric air fuel ratio to a rich air fuel ratio (rich spike processing), so that the NOx stored in the NSR catalyst is reduced and purified (removed). In addition, there has also been proposed a technology in which when the storage amount of NOx in the NSR catalyst is more than a predetermined amount which is smaller than the above-mentioned predetermined threshold value at the time when the air fuel ratio of the air-fuel mixture is changed from a lean air fuel ratio to the stoichiometric air fuel ratio, rich spike processing is carried out (for example, see Patent Literature 1).
Patent Literature 1 Japanese patent laid-open publication No. 2000-064877
However, according to the technology described in the above-mentioned Patent Literature 1, when the air fuel ratio of the air-fuel mixture is changed from the lean air fuel ratio to the stoichiometric air fuel ratio, rich spike processing may be carried out unnecessarily, in spite of the fact that there is room or margin for the NOx storage ability of the NSR catalyst. For that reason, an increase in the amount of fuel consumption resulting from the unnecessary execution of the rich spike processing may be caused.
The present disclosure has been made in view of the above-mentioned actual circumstances, and the object of the present disclosure is to provide a technology in which when the air fuel ratio of an air-fuel mixture is shifted from a lean air fuel ratio to a stoichiometric air fuel ratio, the amount of NOx discharged from an NSR catalyst can be suppressed small, while suppressing an increase in the amount of fuel consumption resulting from the execution of rich spike processing to a small level.
In order to solve the above-mentioned problems, the present disclosure is directed to a control apparatus applied to an internal combustion engine having an exhaust gas purification device which is arranged in an exhaust passage and includes a NOx storage reduction catalyst (an NSR catalyst), wherein at the time of the air fuel ratio of the air-fuel mixture being shifted from a lean air fuel ratio to a stoichiometric air fuel ratio, rich spike processing is carried out when there is no room or margin in the NOx storage ability of the NSR catalyst, and on the other hand, rich spike processing is not carried out when there is room or margin for the NOx storage ability of the NSR catalyst.
In some embodiments, the present disclosure is directed to a control apparatus for an internal combustion engine, the internal combustion engine having an exhaust gas purification device which is arranged in an exhaust passage and includes a NOx storage reduction (NSR) catalyst, the control apparatus comprising; a first detection unit configured to detect a temperature of the NSR catalyst; a second detection unit configured to a NOx storage amount which is an amount of NOx stored in the NSR catalyst; a rich spike unit configured to carry out rich spike processing which is to reduce NOx stored in the NSR catalyst by adjusting an air fuel ratio of exhaust gas flowing into the exhaust gas purification device to a rich air fuel ratio; and a control unit configured, when the air fuel ratio of the air-fuel mixture is shifted from a lean air fuel ratio to the stoichiometric air fuel ratio, to control the rich spike unit in such a manner that the rich spike processing is carried out in a state in which the storage amount of NOx detected by the second detection unit is smaller when the temperature detected by the first detection unit is high in comparison with when the temperature is low, and further control the air fuel ratio of the air-fuel mixture to the stoichiometric air fuel ratio after the end of the rich spike processing.
A maximum value of the amount of NOx which can be stored by the NSR catalyst, in other words, a storage amount of NOx (NOx storage capacity) at the time when the NOx storage ability of the NSR catalyst is saturated, is smaller in the case where the air fuel ratio of exhaust gas flowing into the exhaust gas purification device is the stoichiometric air fuel ratio than in the case where it is the lean air fuel ratio. For that reason, when the air fuel ratio of exhaust gas flowing into the exhaust gas purification device is shifted from the lean air fuel ratio to the stoichiometric air fuel ratio according to the shifting of the air fuel ratio of the air-fuel mixture from the lean air fuel ratio to the stoichiometric air fuel ratio, the NOx storage capacity of the NSR catalyst decreases. Accordingly, when the storage amount of NOx in the NSR catalyst immediately before the air fuel ratio of the air-fuel mixture is shifted from the lean air fuel ratio to the stoichiometric air fuel ratio exceeds the NOx storage capacity of the NSR catalyst after the air fuel ratio of the air-fuel mixture has been shifted from the lean air fuel ratio to the stoichiometric air fuel ratio, NOx will be discharged from the NSR catalyst.
However, the NOx storage capacity of the NSR catalyst changes not only with the air fuel ratio of exhaust gas flowing into the exhaust gas purification device but with the temperature of the NSR catalyst. That is, when the temperature of the NSR catalyst is high, the NOx storage capacity of the NSR catalyst becomes smaller, in comparison with when it is low. In view of such a characteristic of the NSR catalyst, when the temperature of the NSR catalyst is relatively high at the time of the shifting of the air fuel ratio of the air-fuel mixture from the lean air fuel ratio to the stoichiometric air fuel ratio, an amount of margin of the NOx storage ability after the air fuel ratio of the air-fuel mixture has been shifted from the lean air fuel ratio to the stoichiometric air fuel ratio becomes small. For that reason, when the temperature of the NSR catalyst is relatively high at the time of the shifting of the air fuel ratio of the air-fuel mixture from the lean air fuel ratio to the stoichiometric air fuel ratio, NOx tends to be easily discharged from the NSR catalyst after the air fuel ratio of the air-fuel mixture has been shifted from the lean air fuel ratio to the stoichiometric air fuel ratio, even if the storage amount of NOx in the NSR catalyst is in a relatively small state. On the other hand, when the temperature of the NSR catalyst is relatively low at the time of the shifting of the air fuel ratio of the air-fuel mixture from the lean air fuel ratio to the stoichiometric air fuel ratio, the amount of margin of the NOx storage ability after the air fuel ratio of the air-fuel mixture has been shifted from the lean air fuel ratio to the stoichiometric air fuel ratio tends to become large. For that reason, when the temperature of the NSR catalyst is relatively low at the time of the shifting of the air fuel ratio of the air-fuel mixture from the lean air fuel ratio to the stoichiometric air fuel ratio, NOx tends to be hardly discharged from the NSR catalyst after the air fuel ratio of the air-fuel mixture has been shifted from the lean air fuel ratio to the stoichiometric air fuel ratio, even if the storage amount of NOx in the NSR catalyst is in a relatively large state.
In contrast to this, according to the control apparatus for an internal combustion engine according to the present disclosure, when the temperature of the NSR catalyst is high at the time of the shifting of the air fuel ratio of the air-fuel mixture from the lean air fuel ratio to the stoichiometric air fuel ratio, the rich spike processing will be carried out in a state in which the storage amount of NOx detected by the second detection unit is smaller when the temperature detected by the first detection unit is high in comparison with when the temperature is low, and the air fuel ratio of the air-fuel mixture will be shifted to the stoichiometric air fuel ratio after the end of the rich spike processing, without being returned to the lean air fuel ratio. As a result, when the temperature of the NSR catalyst is relatively high at the time of the shifting of the air fuel ratio of the air-fuel mixture from the lean air fuel ratio to the stoichiometric air fuel ratio (i.e., when the amount of margin of the NOx storage ability is small), the rich spike processing will be carried out even in a state in which the storage amount of NOx in the NSR catalyst is relatively small, and the air fuel ratio of the air-fuel mixture will be shifted to the stoichiometric air fuel ratio after the execution of the rich spike processing, without being returned to the lean air fuel ratio. On the other hand, when the temperature of the NSR catalyst is relatively low at the time of the shifting of the air fuel ratio of the air-fuel mixture from the lean air fuel ratio to the stoichiometric air fuel ratio (i.e., when the amount of margin of the NOx storage ability is large), even if the storage amount of NOx in the NSR catalyst is in a relatively large state, the air fuel ratio of the air-fuel mixture will be shifted to the stoichiometric air fuel ratio, without the rich spike processing being carried out. Accordingly, when the air fuel ratio of the air-fuel mixture is shifted from the lean air fuel ratio to the stoichiometric air fuel ratio, the amount of NOx discharged from the NSR catalyst after the air fuel ratio of the air-fuel mixture has been shifted from the lean air fuel ratio to the stoichiometric air fuel ratio can be suppressed to a small level, while suppressing unnecessary execution of the rich spike processing. In addition, according to the control apparatus for an internal combustion engine of the present disclosure, the opportunity for the rich spike processing to be carried out in the state where the temperature of the NSR catalyst is relatively low can be decreased. Here, when the temperature of the NSR catalyst is relatively low, the NOx removing or reducing ability of the NSR catalyst may become low. For that reason, when the rich spike processing is carried out in the state where the temperature of the NSR catalyst is relatively low, the amount of NOx, which is not reduced in the NSR catalyst, may be increased. On the other hand, when the opportunity for the rich spike processing to be carried out in the state where the temperature of the NSR catalyst is relatively low becomes smaller, the opportunity for the amount of NOx not reduced in the NSR catalyst to increase can also be decreased.
The control unit of the present disclosure may control the rich spike unit, when the air fuel ratio of the air-fuel mixture is shifted from the lean air fuel ratio to the stoichiometric air fuel ratio, in such a manner that the rich spike processing is carried out when the storage amount of NOx detected by the second detection unit is larger than a predetermined NOx amount, and to change the predetermined NOx amount so as to be smaller when the temperature detected by the first detection unit is high in comparison with when the detected temperature is low.
According to such a construction, when the temperature of the NSR catalyst is high at the time of the shifting of the air fuel ratio of the air-fuel mixture from the lean air fuel ratio to the stoichiometric air fuel ratio, the predetermined NOx amount is made to be a smaller value, in comparison with when the temperature is low. For that reason, when the temperature of the NSR catalyst is relatively high at the time of the air fuel ratio of the air-fuel mixture being shifted from the lean air fuel ratio to the stoichiometric air fuel ratio, the storage amount of NOx becomes more than the predetermined NOx amount, even if the storage amount of NOx in the NSR catalyst is in a relatively small state. As a result, the air fuel ratio of the air-fuel mixture will be shifted to the stoichiometric air fuel ratio, after the rich spike processing has been carried out. On the other hand, when the temperature of the NSR catalyst is relatively low at the time of the air fuel ratio of the air-fuel mixture being shifted from the lean air fuel ratio to the stoichiometric air fuel ratio, the storage amount of NOx becomes equal to or less than the predetermined NOx amount, even if the storage amount of NOx in the NSR catalyst is in a relatively large state. As a result, the air fuel ratio of the air-fuel mixture will be shifted from the lean air fuel ratio to the stoichiometric air fuel ratio, without the rich spike processing being not carried out.
Here, note that the predetermined NOx amount may be changed according to the NOx storage capacity of the NSR catalyst after the air fuel ratio of the air-fuel mixture has been shifted from the lean air fuel ratio to the stoichiometric air fuel ratio. In that case, the control unit for an internal combustion engine of the present disclosure may be further provided with an estimation unit configured to estimate a NOx storage capacity which is an amount of NOx able to be stored by the NOx storage reduction catalyst after a shifting of the air fuel ratio of the air-fuel mixture from the lean air fuel ratio to the stoichiometric air fuel ratio, before the shifting, wherein the estimation unit estimates the NOx storage capacity to be small when the temperature detected by the first detection unit is high in comparison with when the temperature is low; wherein the control unit is configured, when the air fuel ratio of the air-fuel mixture is shifted from the lean air fuel ratio to the stoichiometric air fuel ratio, to control the rich spike unit in such a manner that the rich spike processing is carried out when the storage amount of NOx detected by the second detection unit is larger than a predetermined NOx amount, and to change the predetermined NOx amount so as to be smaller when the NOx storage capacity estimated by the estimation unit is small in comparison with when the NOx storage capacity is large.
According to such a construction, in cases where the storage amount of NOx before the air fuel ratio of the air-fuel mixture is shifted from the lean air fuel ratio to the stoichiometric air fuel ratio is larger than the NOx storage capacity after the shifting, the rich spike processing will be carried out in a more reliable manner. On the other hand, in cases where the storage amount of NOx before the air fuel ratio of the air-fuel mixture is shifted from the lean air fuel ratio to the stoichiometric air fuel ratio is equal to or less than the NOx storage capacity after the shifting, the rich spike processing will not be carried out in a more reliable manner. Accordingly, at the time when the air fuel ratio of the air-fuel mixture is shifted from the lean air fuel ratio to the stoichiometric air fuel ratio, unnecessary execution of the rich spike processing can be suppressed in a more reliable manner, and at the same time, the amount of NOx discharged from the NSR catalyst after the air fuel ratio of the air-fuel mixture has been shifted from the lean air fuel ratio to the stoichiometric air fuel ratio can be suppressed to be small in a more reliable manner.
Here, the NOx storage capacity of the NSR catalyst may also change with the concentration of NOx contained in the exhaust gas, in addition to the air fuel ratio of exhaust gas flowing into the exhaust gas purification device or the temperature of the NSR catalyst. For example, when the concentration of NOx in the exhaust gas flowing into the exhaust gas purification device is low, the NOx storage capacity of the NSR catalyst may become smaller, in comparison with when the concentration of NOx is high. Accordingly, the estimation unit may be configured to predict a concentration of NOx in the exhaust gas flowing into the exhaust gas purification device after the shifting, estimate the NOx storage capacity to be smaller when the NOx concentration is low in comparison with when the NOx concentration is high while estimating the NOx storage capacity to be smaller when the temperature detected by the first detection unit is high in comparison with when the detected temperature is low.
Next, the exhaust gas purification device may be equipped with an NSR catalyst and a selective catalytic reduction catalyst (SCR (Selective Catalytic Reduction) catalyst) that is arranged at the downstream side of the NSR catalyst. In the arrangement in which the SCR catalyst is arranged at the downstream side of the NSR catalyst, at least a part of NOx discharged from the NSR catalyst after the air fuel ratio of the air-fuel mixture has been shifted from the lean air fuel ratio to the stoichiometric air fuel ratio reacts with NH3 adsorbed to the SCR catalyst, so that it is thereby reduced and removed. For that reason, incases where the amount of NOx discharged from the NSR catalyst after the air fuel ratio of the air-fuel mixture has been shifted from the lean air fuel ratio to the stoichiometric air fuel ratio is equal to or less than an amount of NOx (hereinafter, referred to as an “NOx reducible amount”) which can be reduced or removed by NH3 adsorbed to the SCR catalyst, even when the air fuel ratio of the air-fuel mixture is shifted from the lean air fuel ratio to the stoichiometric air fuel ratio in a state where the storage amount of NOx in the NSR catalyst is more than the predetermined NOx amount, the NOx discharged from the NSR catalyst after the shifting will be reduced and removed by the SCR catalyst. On the other hand, in the case where the amount of NOx discharged from the NSR catalyst after the air fuel ratio of the air-fuel mixture has been shifted from the lean air fuel ratio to the stoichiometric air fuel ratio is more than the NOx reducible amount, when the air fuel ratio of the air-fuel mixture is shifted from the lean air fuel ratio to the stoichiometric air fuel ratio in a state where the storage amount of NOx in the NSR catalyst is more than the predetermined NOx amount, a part of the NOx discharged from the NSR catalyst after the shifting will not be reduced and removed by the SCR catalyst, so that it will be discharged into the atmosphere.
Accordingly, in cases where the exhaust gas purification device is equipped with the NSR catalyst and the SCR catalyst, the control apparatus may be further provided with a third detection unit configured to detect an amount of NH3 adsorption which is an amount of NH3 adsorbed to the selective catalytic reduction catalyst. Then, the control unit may control the rich spike unit so that the rich spike processing is carried out when the storage amount of NOx detected by the second detection unit is more than the predetermined NOx amount and a difference between the storage amount of NOx detected by the second detection unit and the predetermined NOx amount is more than an amount of NOx which can be reduced by the amount of NH3 adsorption detected by the third detection unit.
According to such a construction, even in the case where the storage amount of NOx in the NSR catalyst is more than the predetermined NOx amount, when the difference between the storage amount of NOx and the predetermined NOx amount is equal to or less than the NOx reducible amount in the SCR catalyst, the rich spike processing will not be carried out. For that reason, the opportunity for the rich spike processing to be carried out unnecessarily can be decreased in a more reliable manner. As a result, an increase in the amount of fuel consumption resulting from the unnecessary execution of the rich spike processing can be reduced in a more reliable manner.
According to the present disclosure, when the air fuel ratio of an air-fuel mixture is shifted from a lean air fuel ratio to a stoichiometric air fuel ratio, the amount of NOx discharged from an NSR catalyst can be suppressed small, while suppressing an increase in the amount of fuel consumption resulting from the execution of rich spike processing to a small level.
Further features of the present disclosure will become apparent from the following description of exemplary embodiments with reference to the attached drawings.
Hereinafter, predetermined embodiments of the present disclosure will be described based on the attached drawings. However, the dimensions, materials, shapes, relative arrangements and so on of component parts described in the embodiments are not intended to limit the technical scope of the present disclosure to these alone in particular as long as there are no predetermined statements.
First, reference will be made to a first embodiment of the present disclosure based on
The internal combustion engine 1 is provided with fuel injection valves 2 for supplying fuel to individual cylinders, respectively. Each of the fuel injection valves 2 may be a valve mechanism which serves to inject fuel into an intake port of each corresponding cylinder, or may be a valve mechanism which serves to inject fuel into each corresponding cylinder.
An exhaust pipe 3 is connected to the internal combustion engine 1. The exhaust pipe 3 is a pipe having a passage through which a gas (exhaust gas) combusted or burned in the interior of each cylinder of the internal combustion engine 1 flows. A first catalyst casing 4 is arranged in the middle of the exhaust pipe 3. The first catalyst casing 4 receives a three-way catalyst. Specifically, the first catalyst casing 4 receives a honeycomb structured body covered with a coat layer such as alumina, a precious metal (platinum (Pt), palladium (Pd), etc.) supported by the coat layer, and a promoter or co-catalyst such as ceria (CeO2) supported by the coat layer.
A second catalyst casing 5 is arranged in the exhaust pipe 3 at the downstream side of the first catalyst casing 4. The second catalyst casing 5 receives an NSR catalyst that is equipped with a NOx occlusion or storage material. Specifically, the second catalyst casing 5 receives a honeycomb structured body covered with a coat layer such as alumina, a precious metal (platinum (Pt), palladium (Pd), etc.) supported by the coat layer, a promoter or co-catalyst such as ceria (CeO2) supported by the coat layer, and a NOx occlusion or storage material (alkalines, alkaline earths, etc.) supported by the coat layer. The second catalyst casing 5 corresponds to an “exhaust gas purification device” according to the present disclosure.
In the internal combustion engine 1 constructed in this manner, there is arranged in combination therewith an ECU (Electronic Control Unit) 6 for controlling the internal combustion engine 1. The ECU 6 is an electronic control unit which is composed of a CPU, a ROM, a RAM, a backup RAM, and so on. The ECU 6 corresponds to a control apparatus according to the present disclosure. The ECU 6 is electrically connected to various kinds of sensors such as an air fuel ratio sensor (A/F sensor) 7, an oxygen concentration sensor (oxygen sensor) 8, a NOx sensor 9, an exhaust gas temperature sensor 10, an accelerator position sensor 11, a crank position sensor 12, an air flow meter 13, and so on.
The air fuel ratio sensor 7 is mounted on the exhaust pipe 3 at a location upstream of the first catalyst casing 4, and outputs an electric signal correlated with an air fuel ratio of the exhaust gas which flows into the first catalyst casing 4. The oxygen concentration sensor 8 is mounted on the exhaust pipe 3 at a location between the first catalyst casing 4 and the second catalyst casing 5, and outputs an electric signal correlated with a concentration of oxygen contained in the exhaust gas which flows out from the first catalyst casing 4. The NOx sensor 9 is mounted on the exhaust pipe 3 at a location between the first catalyst casing 4 and the second catalyst casing 5, and outputs an electric signal correlated with a concentration of NOx in the exhaust gas which flows into the second catalyst casing 5. The exhaust gas temperature sensor 10 is mounted on the exhaust pipe 3 at a location downstream of the second catalyst casing 5, and outputs an electric signal correlated with a temperature of the exhaust gas flowing in the interior of the exhaust pipe 3. The accelerator position sensor 11 is mounted on an accelerator pedal, and outputs an electric signal correlated with an amount of operation of the accelerator pedal (i.e., a degree of accelerator opening). The crank position sensor 12 is mounted on the internal combustion engine 1, and outputs an electric signal correlated with a rotational position of an engine output shaft (crankshaft). The air flow meter 13 is mounted on an intake pipe (not shown) of the internal combustion engine 1, and outputs an electric signal correlated with an amount (mass)) of fresh air (i.e., air) flowing in the intake pipe.
The ECU 6 controls the operating state of the internal combustion engine 1 based on the output signals of the above-mentioned variety of kinds of sensors. For example, the ECU 6 calculates a target air fuel ratio of the air-fuel mixture based on an engine load calculated from the output signal of the accelerator position sensor 11 (the accelerator opening degree) and an engine rotational speed calculated from the output signal of the crank position sensor 12. The ECU 6 calculates a target amount of fuel injection (a fuel injection period) based on the target air fuel ratio and the output signal of the air flow meter 13 (the amount of intake air), and controls the fuel injection valves 2 according to the target amount of fuel injection thus calculated.
Specifically, the ECU 6 sets the target air fuel ratio to a lean air fuel ratio which is higher than the stoichiometric air fuel ratio, in cases where the operating condition of the internal combustion engine 1, which is decided from the engine load and the engine rotational speed, belongs to a low rotation and low load region or in a middle rotation and middle load region (hereinafter, these operating regions are referred to as a lean operating region). In addition, the ECU 6 sets the target air fuel ratio to the stoichiometric air fuel ratio (or a rich air fuel ratio which is lower than the stoichiometric air fuel ratio), in cases where the operating condition of the internal combustion engine 1 belongs to a high load region or a high rotation region (hereinafter, these operating regions are referred to as a stoichiometric operating region). Thus, when the operating condition of the internal combustion engine 1 belongs to the lean operating region, the target air fuel ratio is set to a lean air fuel ratio, so that the internal combustion engine 1 is operated in a lean burn state, thereby making it possible to suppress the amount of fuel consumption to a low level.
In addition, the ECU 6 carries out rich spike processing in an appropriate manner, when the operating condition of the internal combustion engine 1 is in the above-mentioned lean operating region. The rich spike processing referred to herein is processing in which the exhaust gas flowing into the second catalyst casing 5 is made into a state where the concentration of oxygen is low and the concentration of hydrocarbon or carbon monoxide is high. That is, the rich spike processing is processing in which the air fuel ratio of the exhaust gas flowing into the second catalyst casing 5 is made to be a rich air fuel ratio lower than the stoichiometric air fuel ratio. The NSR catalyst received in the second catalyst casing 5 stores or adsorbs NOx in the exhaust gas, when the oxygen concentration of the exhaust gas flowing into the second catalyst casing 5 is high (i.e., when the air fuel ratio of the exhaust gas is a lean air fuel ratio). Moreover, the NSR catalyst releases the NOx stored in the NSR catalyst so as to reduce the NOx thus released to nitrogen (N2) or ammonia (NH3), when the oxygen concentration of the exhaust gas flowing into the second catalyst casing 5 is low, and when reducing components such as hydrocarbon (HC), carbon monoxide (CO), etc., are contained in the exhaust gas (i.e., when the air fuel ratio of the exhaust gas is a rich air fuel ratio).
Accordingly, the ECU 6 carries out rich spike processing, when the operating condition of the internal combustion engine 1 belongs to the lean operating region and when the storage amount of NOx in the NSR catalyst becomes more than a predetermined threshold value. The “predetermined threshold value” referred to herein is an amount which is obtained by subtracting a margin from a maximum value of the amount of NOx which is able to be occluded or stored by the NSR catalyst, in other words, a storage amount of NOx (NOx storage capacity) at the time when the NOx storage ability of the NSR catalyst is saturated. The storage amount of NOx in the NSR catalyst is obtained by a method of integrating an amount of NOx flowing into the first catalyst casing 4 per unit time from a point in time at which the last rich spike processing has ended. At that time, the amount of NOx flowing into the second catalyst casing 5 per unit time is assumed to be obtained by multiplying a measured value of the NOx sensor 9 (NOx concentration) and a flow rate of the exhaust gas (a total amount of a measured value of the air flow meter 13 (an amount of intake air) and an amount of fuel injection). Here, note that the amount of NOx flowing into the second catalyst casing 5 per unit time may be estimated by using the operating condition of the internal combustion engine 1 (the engine load, the engine rotation speed, etc.) as a parameter.
Here, note that as a predetermined method of carrying out the rich spike processing, there can be used a method of decreasing the air fuel ratio of the air-fuel mixture to a rich air fuel ratio lower than the stoichiometric air fuel ratio thereby to make the air fuel ratio of the exhaust gas flowing into the second catalyst casing 5 to be a rich air fuel ratio, by carrying out at least one of processing to increase the target amount of fuel injection for the fuel injection valves 2, and processing to decrease the opening degree of an intake air throttle valve (throttle valve). Here, note that in an arrangement in which each of the fuel injection valves 2 injects fuel directly into a corresponding cylinder, the rich spike processing may be carried out by a method of injecting fuel from each fuel injection valve 2 in the exhaust stroke of the corresponding cylinder.
As described above, when the rich spike processing is carried out in an appropriate manner at the time when the operating condition of the internal combustion engine 1 belongs to the lean operating region, the amount of NOx discharged into the atmosphere can be decreased, while suppressing the NOx storage ability of the NSR catalyst from being saturated. Here, note that the rich spike processing may be carried out, when the operating period of time of the internal combustion engine 1 from the last end time of the rich spike processing (in some embodiments, the operating period of time in which the target air fuel ratio has been set to a lean air fuel ratio) becomes equal to or more than a fixed period of time, or when the travel distance of a vehicle, on which the internal combustion engine 1 is mounted, from the last end time of the rich spike processing (in some embodiments, the travel distance within which the target air fuel ratio has been set to the lean air fuel ratio) becomes equal to or more than a fixed distance.
However, when the lean burn operation of the internal combustion engine 1 is carried out in a state where the NOx storage ability of the NSR catalyst has not been activated, NOx discharged from the internal combustion engine 1 may not be stored in the NSR catalyst. For that reason, the lean burn operation of the internal combustion engine 1 is assumed to be carried out on the condition that the NOx storage ability of the NSR catalyst has been activated.
Moreover, the NOx storage capacity of the NSR catalyst changes according to the air fuel ratio of the exhaust gas flowing into the second catalyst casing 5. That is, the NOx storage capacity of the NSR catalyst becomes smaller in the case where the air fuel ratio of the exhaust gas flowing into the second catalyst casing 5 is low than in the case where it is high. For that reason, in cases where the operating condition of the internal combustion engine 1 is shifted from the lean operating region to the stoichiometric operating region, when the air fuel ratio of the air-fuel mixture is shifted from a lean air fuel ratio to the stoichiometric air fuel ratio, the air fuel ratio of the exhaust gas accordingly changes from a lean air fuel ratio to the stoichiometric air fuel ratio, so that the NOx storage capacity of the NSR catalyst may become smaller. Then, even in cases where the NOx storage capacity of the NSR catalyst before the shifting is larger than the storage amount of NOx therein, the NOx storage capacity after the shifting may become smaller than the storage amount of NOx. When such a situation occurs, a part of the NOx stored in the NSR catalyst is discharged from the NSR catalyst, immediately after the air fuel ratio of the air-fuel mixture has been shifted from the lean air fuel ratio to the stoichiometric air fuel ratio. As a result, immediately after the air fuel ratio (A/F) of the air-fuel mixture has been shifted from the lean air fuel ratio to the stoichiometric air fuel ratio, the NOx concentration of the exhaust gas discharged from the first catalyst casing 4 increases, as shown in
With respect to the problem as mentioned above, there can be considered a method in which when the storage amount of NOx in the NSR catalyst is more than a predetermined NOx amount, at the time of the air fuel ratio of the air-fuel mixture being shifted from the lean air fuel ratio to the stoichiometric air fuel ratio, rich spike processing is carried out before the air fuel ratio of the air-fuel mixture is changed from the lean air fuel ratio to the stoichiometric air fuel ratio, and the air fuel ratio of the air-fuel mixture is controlled to the stoichiometric air fuel ratio, without being returned to the lean air fuel ratio after the end of the rich spike processing, whereby the amount of NOx discharged from the NSR catalyst is suppressed to a small level. When rich spike processing is carried out before the air fuel ratio of the air-fuel mixture is shifted from the lean air fuel ratio to the stoichiometric air fuel ratio, as shown in
However, the NOx storage capacity of the NSR catalyst changes not only with the air fuel ratio of exhaust gas flowing into the second catalyst casing 5 but with the temperature of the NSR catalyst. For example, as shown in
Accordingly, in this embodiment, based on the characteristic shown in the above-mentioned
The predetermined NOx amount set by the above-mentioned method becomes a larger value in the case where the temperature of the NSR catalyst is low than in the case where it is high, as shown in
In the following, reference will be made to an execution procedure for the rich spike processing at the time when the air fuel ratio of the air-fuel mixture is shifted from the lean air fuel ratio to the stoichiometric air fuel ratio, in line with
In the processing routine of
In the processing of step S102, the ECU 6 reads in the temperature Tnsr of the NSR catalyst. The temperature Tnsr of the NSR catalyst may be calculated based on the measured value of the exhaust gas temperature sensor 10 (i.e., the temperature of the exhaust gas) and the flow rate of the exhaust gas (i.e., the total amount of the measured value of the air flow meter 13 (the amount of intake air) and the amount of fuel injection). Here, note that the measured value of the exhaust gas temperature sensor 10 may be substituted as the temperature Tnsr of the NSR catalyst. In this manner, by carrying out the processing of step S102 by the ECU 6, a “first detection unit” according to the present disclosure is achieved.
In the processing of step S103, the ECU 6 calculates the above-mentioned predetermined NOx amount ANOXthr. Specifically, the ECU 6 calculates the NOx storage capacity of the NSR catalyst after the air fuel ratio of the air-fuel mixture has been shifted from the lean air fuel ratio to the stoichiometric air fuel ratio, by accessing the map or the functional expression in which the above-mentioned correlation shown in
In the processing of step S104, the ECU 6 reads in the storage amount of NOx Anox in the NSR catalyst. Here, it is assumed that the storage amount of NOx Anox in the NSR catalyst has been calculated by the method of integrating the amount of NOx flowing into the second catalyst casing 5 per unit time from the point in time at which the last rich spike processing has ended, and has then been stored in the backup RAM of the ECU 6, etc. In this manner, by carrying out the processing of step S104 by the ECU 6, a “second detection unit” according to the present disclosure is achieved. The routine of the ECU 6 goes to the processing of step S105, after the processing of step S104 has been carried out.
In the processing of step S105, the ECU 6 determines whether the storage amount of NOx Anox read in the above-mentioned processing of step S104 is more than the predetermined NOx amount Anoxthr which has been calculated in the above-mentioned processing of step S103. In cases where an affirmative determination is made in the processing of step S105 (Anox>Anoxthr), the NOx storage capacity after the air fuel ratio of the air-fuel mixture has been shifted from the lean air fuel ratio to the stoichiometric air fuel ratio may become smaller than the storage amount of NOx Anox, and accordingly, it can be considered that NOx may be discharged from the NSR catalyst. Accordingly, in cases where an affirmative determination is made in the processing of step S105, the routine of the ECU 6 goes to the processing of step S106, and carries out rich spike processing. The execution period of time of the rich spike processing in that case may be a period of time required for reducing an amount of NOx (e.g., a difference between the storage amount of NOx Anox and the predetermined NOx amount Anoxthr) which is expected to be discharged from the NSR catalyst, or may be a period of time required for reducing all the NOx stored in the NSR catalyst. In this manner, by carrying out the processing of step S106 by the ECU 6, a “rich spike unit” according to the present disclosure is achieved. After completing the execution of the rich spike processing, the routine of the ECU 6 goes to the processing of step S107, where the air fuel ratio (A/F) of the air-fuel mixture is controlled to the stoichiometric air fuel ratio, without being returned to the lean air fuel ratio. When the air fuel ratio (A/F) of the air-fuel mixture is shifted from the lean air fuel ratio to the stoichiometric air fuel ratio according to such a procedure, the amount of NOx discharged from the NSR catalyst after the shifting of the air fuel ratio of the air-fuel mixture can be suppressed to be small, as described in the above-mentioned explanation of
On the other hand, in cases where a negative determination is made in the above-mentioned processing of step S105 (Anox≤Anoxthr), it can be assumed that the NOx storage capacity after the air fuel ratio (A/F) of the air-fuel mixture has been shifted from the lean air fuel ratio to the stoichiometric air fuel ratio is equal to or more than the storage amount of NOx Anox. For that reason, even if the rich spike processing is not carried out in the process in which the air fuel ratio (A/F) of the air-fuel mixture is shifted from the lean air fuel ratio to the stoichiometric air fuel ratio, the amount of NOx discharged from the NSR catalyst after the shifting of the air fuel ratio of the air-fuel mixture becomes small. Accordingly, in cases where an affirmative determination is made in the processing of step S105, the ECU 6 carries out the processing of step S107, skipping the processing of step S106. When the air fuel ratio (A/F) of the air-fuel mixture is shifted from the lean air fuel ratio to the stoichiometric air fuel ratio according to such a procedure, it is possible to suppress unnecessary execution of the rich spike processing, without increasing the amount of NOx discharged from the NSR catalyst after the shifting of the air fuel ratio of the air-fuel mixture.
As described above, a “control unit” according to the present disclosure is achieved by the ECU 6 carrying out the processing routine of
Here, note that in this embodiment, there has been described an example in which at the time of obtaining the NOx storage capacity of the NSR catalyst after the air fuel ratio of the air-fuel mixture has been shifted from the lean air fuel ratio to the stoichiometric air fuel ratio, the temperature of the NSR catalyst is used as a parameter, but in addition to the temperature of the NSR catalyst, there can also be used, as a parameter, the concentration of NOx in the exhaust gas flowing into the second catalyst casing 5 after the air fuel ratio of the air-fuel mixture has been shifted from the lean air fuel ratio to the stoichiometric air fuel ratio. At that time, in the case where the concentration of NOx in the exhaust gas flowing into the second catalyst casing 5 is low after the air fuel ratio of the air-fuel mixture has been shifted from the lean air fuel ratio to the stoichiometric air fuel ratio, it is only necessary to make the NOx storage capacity of the NSR catalyst smaller, in comparison with the case where the concentration of NOx is high. Also, note that after the air fuel ratio of the air-fuel mixture has been shifted from the lean air fuel ratio to the stoichiometric air fuel ratio, most of the NOx discharged from the internal combustion engine 1 is reduced by the three-way catalyst of the first catalyst casing 4. For that reason, the concentration of NOx in the exhaust gas flowing into the second catalyst casing 5 after the air fuel ratio of the air-fuel mixture has been shifted from the lean air fuel ratio to the stoichiometric air fuel ratio may also be assumed to be zero or a value approximate to zero. In addition, in an arrangement in which the first catalyst casing 4 is not disposed in the exhaust pipe 3 at a location upstream of the second catalyst casing 5, it is only necessary to calculate (estimate) the concentration of NOx in the exhaust gas flowing into the second catalyst casing 5 after the air fuel ratio of the air-fuel mixture has been shifted from the lean air fuel ratio to the stoichiometric air fuel ratio by using, as a parameter, the operating condition (the engine load, the engine rotation speed, etc.) of the internal combustion engine 1. When the NOx storage capacity is obtained by taking into consideration the concentration of NOx in the exhaust gas flowing into the second catalyst casing 5 after the air fuel ratio of the air-fuel mixture has been shifted from the lean air fuel ratio to the stoichiometric air fuel ratio, in addition to the temperature of the NSR catalyst, it is possible to obtain the NOx storage capacity of the NSR catalyst after the air fuel ratio of the air-fuel mixture has been shifted from the lean air fuel ratio to the stoichiometric air fuel ratio in a more precise manner.
In addition, in this embodiment, there has been described an example in which when the storage amount of NOx in the NSR catalyst is more than the predetermined NOx amount, at the time of the air fuel ratio of the air-fuel mixture being shifted from the lean air fuel ratio to the stoichiometric air fuel ratio, rich spike processing is carried out, but when the temperature of the NSR catalyst is higher than the predetermined temperature, rich spike processing may be carried out. The “predetermined temperature” referred to herein corresponds to Tnsr0 (i.e., a temperature at which the predetermined NOx amount becomes equal to the storage amount of NOx) shown in the above-mentioned
Next, reference will be made to a second embodiment of the present disclosure based on
The third catalyst casing 14 receives an SCR catalyst. Specifically, the third catalyst casing 14 receives a honeycomb structured body made of cordierite or Fe—Cr—Al based heat resisting steel, a zeolite based coat layer covering the honeycomb structured body, and a transition metal (copper (Cu), iron (Fe), etc.) supported by the coat layer. The combination of this third catalyst casing 14 and the second catalyst casing 5 corresponds to an “exhaust gas purification device” according to the present disclosure.
In addition, a NOx sensor 15, in addition to the above-mentioned exhaust gas temperature sensor 10, is arranged in the exhaust pipe 3 at a location between the second catalyst casing 5 and the third catalyst casing 14. Further, a NOx sensor 16 is arranged in the exhaust pipe 3 at the downstream side of the third catalyst casing 14. Hereinafter, the NOx sensor 9 arranged in the exhaust pipe 3 at a location between the first catalyst casing 4 and the second catalyst casing 5 is referred to as a “first NOx sensor 9”. Moreover, the NOx sensor 15 arranged in the exhaust pipe 3 at a location between the second catalyst casing 5 and the third catalyst casing 14 is referred to as a “second NOx sensor 15”. Further, the NOx sensor 16 arranged in the exhaust pipe 3 at the downstream side of the third catalyst casing 14 is referred to as a “third NOx sensor 16”.
In the arrangement as mentioned above, the NOx discharged from the NSR catalyst after the air fuel ratio of the air-fuel mixture has been shifted from the lean air fuel ratio to the stoichiometric air fuel ratio may be reduced by the SCR catalyst in the third catalyst casing 14. Specifically, in cases where the storage amount of NOx in the NSR catalyst at the time of the air fuel ratio of the air-fuel mixture being shifted from the lean air fuel ratio to the stoichiometric air fuel ratio is more than the above-mentioned predetermined NOx amount, the NOx discharged from the NSR catalyst is reduced and removed by the SCR catalyst, when an amount of NOx (NOx reducible amount) which can be reduced by an amount of NH3 adsorbed to the SCR catalyst is larger, in comparison with the difference between the storage amount of NOx and the predetermined NOx amount (i.e., this difference being an amount of NOx which is considered to be discharged from the NSR catalyst after the air fuel ratio of the air-fuel mixture has been shifted from the lean air fuel ratio to the stoichiometric air fuel ratio, and being referred to as an “estimated amount of discharge”), or when the difference and the NOx reducible amount are equal to each other. Accordingly, in this second embodiment, even in cases where the storage amount of NOx in the NSR catalyst at the time of the air fuel ratio of the air-fuel mixture being shifted from the lean air fuel ratio to the stoichiometric air fuel ratio is more than the predetermined NOx amount, rich spike processing is not carried out, when the NOx reducible amount is equal to or more than the estimated amount of discharge.
In the following, reference will be made to an execution procedure for the rich spike processing at the time when the air fuel ratio of the air-fuel mixture is shifted from the lean air fuel ratio to the stoichiometric air fuel ratio, in line with
The difference between the processing routine of
Here, note that the amount of NH3 to be supplied to the SCR catalyst is a total amount of an amount of NH3 to be produced in the three-way catalyst of the first catalyst casing 4 and an amount of NH3 to be produced in the NSR catalyst of the second catalyst casing 5. The amount of NH3 to be produced in the three-way catalyst is correlated with the air fuel ratio of the exhaust gas, the flow rate of the exhaust gas, and the temperature of the three-way catalyst. For that reason, when the correlation has been obtained in advance, the amount of NH3 to be produced in the three-way catalyst can be obtained by using as arguments the air fuel ratio of the exhaust gas, the flow rate of the exhaust gas, and the temperature of the three-way catalyst. On the other hand, the amount of NH3 to be produced in the NSR catalyst is correlated with the air fuel ratio of the exhaust gas, the flow rate of the exhaust gas, and the temperature of the NSR catalyst. For that reason, when this correlation has been obtained in advance, the amount of NH3 to be produced in the NSR catalyst can be obtained by using as arguments the air fuel ratio of the exhaust gas, the flow rate of the exhaust gas, and the temperature of the NSR catalyst.
The amount of NH3 consumption is calculated by using as parameters the amount of NOx flowing into the SCR catalyst (the amount of inflowing NOx) and the NOx reduction rate of the SCR catalyst. The amount of inflowing NOx in that case is calculated by multiplying the measured value of the second NOx sensor 15 (the concentration of NOx in the exhaust gas flowing into the third catalyst casing 14) and the flow rate of the exhaust gas. On the other hand, the rate of NOx reduction used for the calculation of the amount of NH3 consumption is calculated by using as parameters the flow rate of the exhaust gas and the temperature of the SCR catalyst. At that time, the correlation among the flow rate of the exhaust gas, the temperature of the SCR catalyst, and the NOx reduction rate of the SCR catalyst has been obtained experimentally in advance.
The amount of NH3 slip is obtained by using as parameters the last calculated value of the amount of NH3 adsorption, the temperature of the SCR catalyst, and the flow rate of the exhaust gas. Here, when the flow rate of the exhaust gas is constant, the concentration of NH3 in the exhaust gas flowing out from the SCR catalyst becomes higher in accordance with the increasing amount of NH3 adsorption and/or the higher (rising) temperature of the SCR catalyst. In addition, when the concentration of NH3 in the exhaust gas flowing out from the SCR catalyst is constant, the amount of NH3 slip per unit time increases in accordance with the increasing flow rate of the exhaust gas. Based on these correlations, the amount of NH3 slip can be obtained by calculating the concentration of NH3 in the exhaust gas flowing out from the SCR catalyst, using as parameters the amount of NH3 adsorption in the SCR catalyst and the temperature of the SCR catalyst, and subsequently by multiplying the flow rate of the exhaust gas to the concentration of NH3.
Here, returning to the processing routine of
In the processing of step S203, the ECU 6 calculates the above-mentioned estimated amount of discharge (=(Anox−Anoxthr)) by subtracting the predetermined NOx amount Anoxthr from the storage amount of NOx ANOX. Then, the ECU 6 determines whether the NOx reducible amount Aprnox calculated in the above-mentioned processing of step S202 is smaller than the estimated amount of discharge. In cases where an affirmative determination is made in the processing of step S203, it can be assumed that the entire amount of NOx discharged from the NSR catalyst after the air fuel ratio (A/F) of the air-fuel mixture has been shifted from the lean air fuel ratio to the stoichiometric air fuel ratio is not reduced by the SCR catalyst. For that reason, in cases where an affirmative determination is made in the processing of step S203, the routine of the ECU 6 goes to the processing of step S106, where rich spike processing is carried out. On the other hand, in cases where a negative determination is made in the processing of step S203, it can be assumed that the entire amount of NOx discharged from the NSR catalyst after the air fuel ratio (A/F) of the air-fuel mixture has been shifted from the lean air fuel ratio to the stoichiometric air fuel ratio is reduced by the SCR catalyst. For that reason, in cases where a negative determination is made in the processing of step S203, the routine of the ECU 6 goes to the processing of step S107, while skipping the processing of step S106.
As described above, when the ECU 6 carries out the processing routine of
Here, note that in this second embodiment, the above-mentioned predetermined NOx amount is set based on the NOx storage capacity of the NSR catalyst after the air fuel ratio of the air-fuel mixture has been shifted from the lean air fuel ratio to the stoichiometric air fuel ratio, but the predetermined NOx amount may be set based on the NOx storage capacity of the NSR catalyst and the NOx reducible amount of the SCR catalyst after the air fuel ratio of the air-fuel mixture has been shifted from the lean air fuel ratio to the stoichiometric air fuel ratio. That is, a total amount of the NOx storage capacity and the NOx reducible amount (or an amount which is obtained by subtracting a margin from the total amount) may be set as the predetermined NOx amount. The predetermined NOx amount in that case becomes smaller in the case where the temperature of the NSR catalyst at the time of the shifting of the air fuel ratio of the air-fuel mixture from the lean air fuel ratio to the stoichiometric air fuel ratio is high, than in the case where it is low, and also becomes smaller in the case where the amount of NH3 adsorption in the SCR catalyst is small than in the case where it is large. Thus, in the case of using the predetermined NOx amount set in this manner, it is only necessary to carry out the rich spike processing according to the same procedure as shown in the above-mentioned processing routine of
While the present disclosure has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions.
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