Patent Publication Number: US-2021180488-A1

Title: Exhaust purification system for internal combustion engine

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application claims priority under 35 U.S.C. § 119 to Japanese Patent Application No. 2019-224033, filed Dec. 11, 2019. The contents of this application are incorporated herein by reference in their entirety. 
     TECHNICAL FIELD 
     The present disclosure relates to a system for purifying exhausts from an internal combustion engine (hereinafter also referred to simply as an “engine”). 
     BACKGROUND 
     JP2009-209788A discloses an exhaust purifying device including a filter which is configured to trap particulate matters contained in emissions from the engine (hereinafter also referred to as a “PM”). This conventional device estimates an amount of the PM burned in the filter during an engine stop. The burning amount of the PM is estimated based on temperatures of the filter immediately before the engine stop and those when an engine operation is restarted. 
     SUMMARY 
     However, the conventional device lacks a perspective of actively removing the PM deposited on the filter during the engine stop. Therefore, the filter may become clogged when a situation where the PM could not be removed during the engine operation has been repeated for a long time. Accordingly, it is desirable to make an improvement from a viewpoint of not missing opportunities to remove the PM. 
     With respect to this improvement, intentional oxygenation to the filter during the engine stop allows for an active elimination of the PM. However, when the PM reacts with oxygen, heat is generated. This heat of the reaction is also generated when the oxygen is supplied to the filter during the engine operation. However, an amount of the heat carried away by gases passing through the filter during engine stop is usually less than that during the engine operation. Therefore, when oxygen is intentionally supplied to the filter during the engine stop, temperature of the filter is easily reach the one at which an exhaust purifying function of the filter is impaired in a short time. Therefore, it is also desirable to make an improvement from another viewpoint of suppressing an excessive rise in the temperature of the filter. 
     It is an object of the present disclosure to provide a novel technique to remove the PM on the filter actively during the engine stop. Another object of the present disclosure is to reduce the excessive rise in the temperature of the filter associated with the removal of the PM performed during the engine stop. 
     The present disclosure is an exhaust purification system for internal combustion engine and has the following features. 
     The exhaust purification system comprises a filter, an oxygen supply device, and a controller. 
     The filter is configured to trap particulate matters contained in exhaust gas of the internal combustion engine. 
     The oxygen supply device is configured to supply oxygen contained in intake air of the internal combustion engine to the filter. 
     The controller is configured to execute filter regeneration processing to oxidize and remove the particulate matters deposited on the filter. 
     The filter regeneration processing includes regeneration processing during an engine stop that is executed during a shut-down of the internal combustion engine. 
     In the regeneration processing during the engine stop, the controller is configured to: 
     calculate a future temperature of the filter based on an accumulated amount of the particulate matters deposited on the filter, a present temperature of the filter, and an estimated pass amount of oxygen passing through the filter; and 
     variably set an operation amount of the oxygen supply device based on a result of a comparison between the future temperature and an upper limit temperature of the filter. 
     In the regeneration processing during the engine stop, the controller may be configured to: 
     if the future temperature is higher than the upper limit temperature, set the operation amount such that oxygen is not supplied to the filter. 
     In the regeneration processing during the engine stop, the controller may be configured to: 
     if the future temperature is lower than the upper limit temperature, set the operation amount such that oxygen is supplied to the filter. 
     In the regeneration processing during the engine stop, the controller may be configured to: 
     set the operation amount to an upper limit operation amount of the oxygen supply device. 
     According to present disclosure, the regeneration processing during the engine stop is executed. According to the regeneration processing during the engine stop, the operation amount of the oxygen supply device is variably set based on the comparison result between the future temperature and the upper limit temperature. If the operation amount is variably set, oxygen may be or may not be supplied to the filter. When oxygen is supplied to the filter, the PM is oxidized and removed. Therefore, it is possible to remove the PM actively during the engine stop. On the other hand, if no oxygen is supplied to the filter, an oxidation reaction of the PM does not proceed. Therefore, it is also possible to suppress the excessive rise of the temperature of the filter associated with the removal of the PM during the engine stop. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a diagram showing a configuration example of an exhaust purification system for internal combustion engine according to an embodiment. 
         FIG. 2  is a flow chart for explaining a processing flow of filter regeneration processing. 
         FIG. 3  is a flow chart describing a processing flow of the regeneration processing executed during an engine operation. 
         FIG. 4  is a diagram showing an example of a threshold map. 
         FIG. 5  is a flow chart explaining a processing flow of the filter regeneration processing executed during an engine stop. 
     
    
    
     EMBODIMENT 
     Hereinafter, an embodiment of the present disclosure will be described with reference to drawings. 
     1. System Configuration 
     An exhaust purification system for internal combustion engine according to the embodiment of the present disclosure (hereinafter simply referred to as a “system”) is mounted on a conventional vehicle powered by the engine (hereinafter referred to as an “engine vehicle”) or on a hybrid vehicle powered by the engine and a motor.  FIG. 1  is a diagram illustrating an configuration example of the system according to the embodiment of the present disclosure. A system  100  shown in  FIG. 1  includes an engine  10  as a power source. An example of the engine  10  includes a gasoline engine. There is no particular limitation on number and arrangement of a cylinder of the engine  10 . 
     The engine  10  includes an injection device  11 , an ignition apparatus  12 , a VVT (Variable Valve Timing)  13 , and a crank angle sensor  14 . The injection device  11  is configured to inject fuels into the cylinder of the engine  10 . The ignition apparatus  12  is configured to ignite a mixed gas containing fuel and air. The VVT  13  is a variable valve timing mechanism in which an electric motor is used as an actuator, To the VVT  13 , a known structure is applied. The VVT  13  is configured to change a valve timing of at least one of an intake air valve and an exhaust valve of the engine  10  by energizing the electric motor. As a result, a valve overlapping period OL in which the intake air valve and the exhaust valve are in an open state at the same time is changed. The crank angle sensor  14  is configured to detect rotation angle of a crank shaft. 
     The engine  10  also includes an intake pipe  20 . An inlet portion of the intake pipe  20 , an airflow sensor  21  is provided. The air flow sensor  21  is configured to measure a flow amount of an intake air (fresh air) flowing into the intake pipe  20  from an outside of the engine  10 . In a middle of the intake pipe  20 , an electronically controlled throttle valve  22  is provided. The throttle valve  22  is configured to regulate an amount of air (the intake air) flowing into the engine  10 . This regulation is performed by changing an opening degree of the throttle valve  22  (hereinafter also referred to as a “throttle opening degree”). On a downstream of throttle valve  22 , a pressure sensor  23  is provided. The pressure sensor  23  is configured to detect a pressure (hereinafter also referred to as an “intake pressure”) Pi of the gas flowing through the intake pipe  20 . 
     The engine  10  also includes an exhaust pipe  30 . An exhaust air from the engine  10  flows through the exhaust pipe  30 . In a middle of the exhaust pipe  30 , a three-way catalyst  31  is provided. The three-way catalyst  31  has a honeycomb-shaped and has a plurality of internal passages formed in a flow direction of the exhaust gas. Each of partition walls that divides these internal passages has a metal or a metal compound that purifies harmful components contained in the exhaust gas hydrogen carbon, carbon monoxide and nitrogen oxide, hereinafter referred to as an “exhaust element”). 
     On a downstream of the three-way catalyst  31 , a filter  32  is provided. The filter  32  has a honeycomb-shaped and has a plurality of internal passages. Each of partition wall that divides these internal passages has a metal or a metal compound for purifying the exhaust element. The configuration up to this point is the same as that of the three-way catalyst  31 . Unlike the three-way catalyst  31 , the filter  32  has sealing members on an upstream end or a downstream end of the internal passage. The internal passage having the sealing member on the upstream end and that having the sealing member on the downstream end are arranged alternately and adjacently. According to such the configuration, the filter  32  traps the PM contained in the exhaust gas. 
     To the filter  32 , a temperature sensor  33  which is configured to detect an actual temperature TFa of the filter  32  is attached. 
     The system  100  also includes an ECU (Electric Control Unit)  40  as a controller. The ECU  40  is a microcomputer that includes at least a processor  41  and a memory  42 . The processor  41  executes various processing by executing computer programs. The various processing include filter regeneration processing. The detail of the filter regeneration processing will be described later. The memory  42  stores the computer programs, various databases and the like. The memory  42  also stores various kinds of data. The various kinds of data include rotation angle data from the crankshaft angle sensor  14 , air flow amount data from the air flow sensor  21 , and actual temperature data from the temperature sensor  33 . The various kinds of data also include intake pressure data from the pressure sensor  23  and information on valve overlapping period OL (hereinafter also referred to as “overlapping information.”) 
     2. Filter Regeneration Processing 
     The filter regeneration processing is processing to oxidize and remove the PM trapped by the filter  32 . When the filter regeneration processing is executed, a function of the filter  32  to trap the PM is regenerated. The filter regeneration processing includes regeneration processing during an engine operation and regeneration processing during an engine stop. The regeneration processing during the engine operation is carried out during the engine is operated. The regeneration processing during the engine stop is carried out during the engine  10  is shut down. A distinction between the operation and the shut-down is determined by whether rotational speed Ne of the engine  10  is higher than a threshold THNe. An example of the threshold THNe includes rotational speed when the rotation of the crankshaft is substantially in a shut-down state. 
     The regeneration processing during the engine operation is executed regardless of the type of the vehicle (i.e., the gasoline vehicle and the hybrid vehicle) on which the system  100  is mounted. If the system  100  is mounted on the hybrid vehicle, the rotational speed Ne decreases less than or equal to the threshold THNe during the hybrid vehicle is powered only by the motor, Therefore, when the system  100  is mounted on the hybrid vehicle, the regeneration processing during the engine stop is also executed while traveling only with the power from the motor. The regeneration processing during the engine stop may be executed when the vehicle on which the system  100  is mounted is being towed by another vehicle. 
       FIG. 2  is a flow chart for explaining a processing flow of the filter regeneration processing. The routine shown in  FIG. 2  is repeatedly executed at a predetermined control cycle. 
     In the routine shown in  FIG. 2 , first, an accumulated amount APM is calculated (step S 10 ). The accumulated amount APM is an amount of the PM deposited on the filter  32 . 
     The accumulated amount APM is calculated, for example, based on an operation history of the engine  10 . According to the operation history, a total amount EPM of the PM discharged from the engine  10  and a total amount RPM of the PM removed from the filter  32  in the filter regeneration processing are estimated. The accumulated amount APM is calculated, for example, from the following formula (1). 
         APM=EPM*RF−RPM   (1)
 
     In the formula (1), “RF2 denotes a trap rate of the PM in the filter  32 . 
     In another example, the accumulated amount APM is calculated from a difference between pressure of the gas on the upstream of the filter  32  and that on the downstream of the filter  32 . This pressure difference is calculated by detecting the pressure of the gas on the upstream of the filter  32  and that on the downstream thereof. 
     Subsequent to the step S 10 , present temperature TFp is obtained (step S 11 ). The present temperature TFp is calculated based on actual temperature data. 
     Subsequent to the step S 11 , it is determined whether or not the rotational speed Ne is equal to or less than the threshold THNe (step S 12 ). The rotational speed Ne is calculated based on the rotation angle data. 
     If the determination result of the step S 12  is negative, the regeneration processing during the engine operation is executed (step S 13 ). If the determination result of the judgement result of the step S 12  is positive, the regeneration processing during the engine stop is executed (step S 14 ). Hereinafter, the regeneration processing during the engine operation and the regeneration processing during the engine stop will be described. 
     2-1. Regeneration Processing During Engine Operation 
       FIG. 3  is a flow chart for explaining processing flow of the regeneration processing during the engine operation. In the routine shown in  FIG. 3 , first, it is determined whether or not a condition C 1  is satisfied (step S 20 ). The condition C 1  is a condition to determine whether or not to allow an oxidation of the PM deposited on the filter  32 . The condition C 1  includes the following conditions C 11  to C 13 . 
     C 11 : The vehicle on which the system  100  is mounted is in a decelerating travel. 
     C 12 : The present temperature TFp of the filter  32  is higher than a lower limit temperature TFL. 
     C 13 : A future temperature TFf of the filter  32  is lower than an upper limit temperature TFH. 
     Regarding the condition C 11 , whether or not the vehicle on which the system  100  is mounted is in the decelerating travel is determined based on data detected by a vehicle speed sensor (or a wheel speed sensor). 
     Regarding the condition C 12 , an example of the lower limit temperature TFL includes temperature (e.g., 500 degree C.) at which a progress of the oxidation reaction of the PM on the filter  32  is ensured. For the present temperature TFp, the temperature calculated in the step S 11  is used. 
     For the condition C 13 , the upper limit temperature TFH is set to a higher temperature than the lower limit temperature TFL. An example of the upper limit temperature TFH includes temperature at which a purification function of the filter  32  toward the exhaust element is ensured (e.g., 800 degree C.). 
     Further, regarding the condition C 13 , the future temperature TFf is the temperature of the filter  32  that is expected to rise during the filter regeneration processing. The future temperature TFf is calculated based on the accumulated amount APM, the present temperature TFp, and an estimated pass amount AO 2 . For the accumulated amount APM, the one calculated in the step S 10  of  FIG. 2  is used. For the present temperature TFp, the one calculated in the step S 11  is used. 
     The estimated pass amount AO 2  is an amount of oxygen that is estimated to pass through the filter  32  during the filter regeneration processing. The estimated pass amount AO 2  is calculated based on the air flow amount data. The estimated pass amount AO 2  may be calculated based on the intake pressure data and the overlapping information. The estimated pass amount AO 2  may be calculated based on a difference between the intake pressure Pi and an exhaust pressure Pe, and the overlapping information. Note that the exhaust pressure Pc is obtained by detecting the pressure of the gas on the upstream of the three-way catalyst  31 . 
       FIG. 4  is a diagram for explaining the future temperature TFf. The x-axis of  FIG. 4  represents the accumulated amount APM, the y-axis represents the present temperature TFp of the filter  32 , and the z-axis represents the estimated pass amount AO 2 . The oxidation reaction of the PM is an exothermic reaction. Therefore, as the present temperature TFp increases, the oxidative reaction of the PM tends to proceed, and the future temperature TFf tends to increase. Also, the more the PM or oxygen (i.e., the accumulated amount APM or the estimated pass amount AO 2 ) that is a reactant, the more likely the future temperature TFf tends to increase. Therefore, it can be seen that when the accumulated amount APM and the present temperature TFp are fixed, the more the estimated pass amount AO 2 , the higher the future temperature TFf becomes. Thus, a future temperature TFf 3  is higher than a future temperature TFf 2  and the future temperature TFf 2  is higher than a future temperature TFf 1 . 
     In the present embodiment, a three-dimensional data map defining a relationship among the accumulated amount APM, the present temperature TFp, the estimated pass amount AO 2 , and the future temperature TFf is stored in the memory  42 , In the step S 20 , the future temperature TFf is calculated by referring to the three-dimensional data map using the accumulated amount APM, the present temperature TFp and the estimated pass amount AO 2  as inputs thereto. The figure temperature TFf may be calculated by referring to a two-dimensional data map defining a relationship among the accumulated amount APM, the present temperature TFp, and the future temperature TFf. 
     If the determination result of the step S 20  is positive, fuel-cut operation is started (step S 21 ). In the fuel-cut operation, fuel injection from the injection device  11  is prohibited. In the fuel-cut operation, an energization of the ignition apparatus  12  is also prohibited. When the fuel-cut operation is executed, oxygen that has passed through the engine  10  flows into the filter  32 , thereby the oxidative reaction of the PM proceeds. Note that a stoichiometric operation is executed prior to the execution of the fuel-cut operation. In the stoichiometric operation, all the oxygen is consumed in the cylinder of the engine  10 . Therefore, when the stoichiometric operation is executed, oxygen does not flow into the filter  32  and the oxidation reaction of the PM does not proceed. 
     Subsequent to the step S 21 , it is determined whether or not the condition C 1  is satisfied (step S 22 ). The content of the processing of the step S 22  is the same as that in the step S 20 . For example, when a driver of the vehicle depresses an accelerator pedal, the condition C 11  is not satisfied. When the future temperature TFf is equal to or larger than the upper limit temperature TFH, the condition C 13  is not satisfied. The reason why the condition C 13  is not satisfied is as follows. That is, during the processing of the routine shown in  FIG. 3 , the calculation of the accumulated amount APM and the estimated pass amount AO 2  is repeatedly performed. In addition, the calculation of the future temperature TFf based on these calculated values and the present temperature TFp is also repeatedly performed. Therefore, the condition C 13  cannot be satisfied when the future temperature TFf becomes equal to or larger than the upper limit temperature TFH. 
     The processing of the step S 22  is repeatedly executed until a negative determination result is obtained. If the determination result of the step S 22  is negative, the execution of the fuel-cut operation is ended (step S 23 ). After the fuel-cut operation is ended, the stoichiometric operation is executed. 
     Incidentally, in the routine shown in  FIG. 3 , the fuel-cut operation is executed when the condition C 1  is satisfied. However, a lean-burn operation may be executed when the condition C 1  is satisfied. When the lean-burn operation is performed, oxygen that has not been consumed in the cylinder of the engine  10  flows into the filter  32 , thereby the oxidation reaction of the PM proceeds. Note that the estimated pass amount  402  when the lean-burn operation is executed differs from that when the fuel-cut operation is executed. Therefore, when the lean-burn operation is executed, the future temperature. TFf is calculated by referring to a data map that is different from the data map described above. 
     2-2. Regeneration Processing During Engine Stop 
       FIG. 5  is a flow chart for explaining processing flow of the regeneration processing during the engine stop. In the routine shown in  FIG. 5 , first, it is determined whether or not the condition C 2  is satisfied (step S 30 ), The condition C 2  is a condition to determine whether or not to allow the oxidation of the PM deposited on the filter  32 . The condition C 2  includes the following conditions C 21  and C 22 . 
     C 21 : The present temperature TFp is higher than the lower limit temperature TFL 
     C 22 : The future temperature TFf is lower than the upper limit temperature TFH 
     The condition C 21  is the same as the condition C 12 . The condition C 22  is basically the same as the condition C 13 . However, in the regeneration processing during the engine stop, control of the VVT  13  is executed when the condition C 2  is satisfied. Therefore, the estimated pass amount AO 2  used for the calculation of the future temperature TFf of the condition C 22  is calculated based on the intake pressure data and the overlapping information. The estimated pass amount AO 2  may be calculated based on the difference between the intake pressure Pi and the exhaust pressure Pe, and the overlapping information. 
     If the determination result of the step S 30  is positive, the control of the VVT  13  is started (step S 31 ). Specifically, an operation amount of the VVT  13  is set such that the valve overlapping period OL is longer than a reference value. An example of the reference value includes the valve overlapping period OL in which relative phase to the crankshaft with respect to the intake and exhaust cam shafts are zero. When the valve overlapping period OL becomes longer than the reference value, oxygen that has passed through engine  10  flows into the filter  32  thereby the oxidation reaction proceeds. 
     The operation amount of the VVT  13  may be set to a period corresponding to an upper limit operation amount of the VVT  13 . An example of the upper limit operation amount includes an operation amount corresponding to a maximum advance value of the intake cam phase and a operation amount corresponding to a largest retard value of the exhaust earn phase. If the operation amount of the VVT  13  is set to the upper limit operation amount, it is possible to remove the PM in a short time. 
     If a throttle opening degree is zero (i.e., the gas flow from upstream to downstream of the throttle valve  22  is blocked by the throttle valve  22 ), an operation amount of the throttle valve  22  is set such that the throttle opening degree is greater than zero. Note that the throttle opening degree is calculated based on detected data from a throttle sensor. 
     Subsequent to the step S 31 , it is determined whether or not the condition C 2  is satisfied (step S 32 ). The content of the processing of the step S 32  is the same as that of the step S 30 . For example, when the hybrid vehicle travels only by the operation of the motor and the present temperature TFp drops below the lower limit temperature TFL, the condition C 21  is not satisfied. When the future temperature TTf is equal to or greater than the upper limit temperature TFH, the condition C 22  is not satisfied. The reason why the condition C 22  is not satisfied is the same as that of the condition C 13 . 
     The processing of the step S 32  is repeatedly executed until the negative determination result is obtained. If the determination result of the step S 32  is negative, the control of the VVT  13  is ended (step S 33 ). If the control of the throttle valve  22  is executed in parallel with that of the VVT  13 , both are ended. 
     3. Effect 
     According to the embodiment described above, the filter regeneration processing is executed not only during the operation of the engine  10  but also during the shut-down of the engine  10 , Therefore, it is possible to remove the PM actively. In particular, according to regeneration processing during the engine stop, even if the regeneration processing during the engine operation cannot be executed for a long period, it is possible to remove the PM during the shut-down of the engine  10  and suppress a clogging of the filter  32 . 
     Further, according to the filter regeneration processing, when it is determined during the processing that the future temperature TFf is equal to or greater than the upper limit temperature TFH, the execution of the processing is immediately ended. Therefore, it is possible to suppress an excessive rise in the temperature of the filter  32  caused by the execution of the filter regeneration processing. Therefore, it is possible to prevent the purification function of the filter  32  toward the exhaust element from being impaired, 
     4. Correspondence Between Embodiment and Present Disclosure 
     In the embodiment described above, the VVT  13  or a combination of the VVT  13  and the throttle valve  22  corresponds to the “oxygen supply device” of the present disclosure,