Exhaust gas control apparatus for internal combustion engine

An exhaust gas control apparatus includes an exhaust gas control catalyst disposed in an exhaust passage, a filter disposed downstream of the catalyst, a secondary air supply device configured to supply secondary air into exhaust gas flowing into the filter at a location downstream of the catalyst in an exhaust gas flow direction, and an electronic control unit. The electronic control unit is configured to, when a temperature of the catalyst is higher than or equal to an activation temperature and an air-fuel ratio of exhaust gas emitted from an engine body is a rich air-fuel ratio, cause the supply device to supply secondary air into exhaust gas while periodically increasing or reducing the air such that the air-fuel ratio of exhaust gas flowing into the filter alternately varies between rich and lean air-fuel ratios.

INCORPORATION BY REFERENCE

The disclosure of Japanese Patent Application No. 2018-155556 filed on Aug. 22, 2018 including the specification, drawings and abstract is incorporated herein by reference in its entirety.

BACKGROUND

1. Technical Field

The present disclosure relates to an exhaust gas control apparatus for an internal combustion engine.

2. Description of Related Art

An exhaust gas control catalyst for an internal combustion engine including a secondary air supply device is known (for example, Japanese Unexamined Patent Application Publication No. 2010-127147 (JP 2010-127147 A) and Japanese Unexamined Patent Application Publication No. 2007-092713 (JP 2007-092713 A)). The secondary air supply device supplies secondary air into exhaust gas flowing through an exhaust passage of the internal combustion engine. In the exhaust gas control apparatus described in JP 2010-127147 A, a particulate filter (hereinafter, also simply referred to as filter) is provided in the exhaust passage, and the secondary air supply device is configured to supply secondary air into exhaust gas flowing into the filter.

In the thus configured exhaust gas control apparatus described in JP 2010-127147 A, when the amount of particulate matter (hereinafter, also referred to as PM) deposited on the filter increases, secondary air is supplied while the air-fuel ratio of exhaust gas that is emitted from an engine body is kept at an air-fuel ratio richer than a stoichiometric air-fuel ratio (hereinafter, also referred to as rich air-fuel ratio). Thus, the secondary air reacts with unburnt fuel contained in the rich air-fuel ratio exhaust gas on the filter to raise the temperature of the filter, with the result that the PM is burnt and removed.

SUMMARY

Incidentally, it is known that NO2contained in exhaust gas is more reactive with PM than oxygen. For this reason, when exhaust gas containing a large amount of NO2is caused to flow into the filter, the amount of PM removed can be increased. However, in the exhaust gas control apparatus described in JP 2010-127147 A, gas composed of rich air-fuel ratio exhaust gas with a small amount of NO2and added secondary air directly flows into the filter. Therefore, NO2is not so contained in the exhaust gas flowing into the filter, so the burning rate of PM is not increased.

On the other hand, as a method of causing exhaust gas containing a large amount of NO2to flow into the filter, it is conceivable that the air-fuel ratio of exhaust gas that is emitted from the engine body is set to a lean air-fuel ratio and an oxidation catalyst is provided upstream of the filter in an exhaust gas flow direction. Thus, part of NO in exhaust gas emitted from the engine body is converted into NO2by the oxidation catalyst, the NO2is caused to flow into the filter, and PM deposited on the filter is burnt with the NO2.

However, not only NO2but also NO is contained in lean air-fuel ratio exhaust gas flowing into the filter in this case. When the NO flows into the filter, the NO flows out from the filter without reacting with PM. Therefore, when the lean air-fuel ratio exhaust gas is continuously caused to flow into the filter, emissions of exhaust gas flowing out from the filter deteriorate.

The disclosure provides an exhaust gas control apparatus that is able to reduce deterioration of emissions while facilitating removal of PM from a filter.

A first aspect of the disclosure relates to an exhaust gas control apparatus for an internal combustion engine. The exhaust gas control apparatus includes an exhaust gas control catalyst, a particulate filter, an oxygen supply device, and an electronic control unit. The exhaust gas control catalyst is disposed in an exhaust passage of the internal combustion engine. The exhaust gas control catalyst has a catalytic function. The particulate filter is disposed in the exhaust passage at a location downstream of the exhaust gas control catalyst in an exhaust gas flow direction. The oxygen supply device is configured to supply gas containing oxygen into exhaust gas flowing into the particulate filter at a location downstream of the exhaust gas control catalyst in the exhaust gas flow direction. The electronic control unit is configured to regulate an amount of oxygen that is supplied from the oxygen supply device. When a temperature of the exhaust gas control catalyst falls within a predetermined temperature range higher than or equal to an activation temperature and an air-fuel ratio of exhaust gas emitted from a body of the internal combustion engine is a rich air-fuel ratio richer than a stoichiometric air-fuel ratio, the electronic control unit is configured to cause the oxygen supply device to supply oxygen into the exhaust gas while periodically increasing or reducing the oxygen such that an air-fuel ratio of exhaust gas flowing into the particulate filter alternately varies between the rich air-fuel ratio and a lean air-fuel ratio leaner than the stoichiometric air-fuel ratio.

In the exhaust gas control apparatus, the predetermined temperature range may be higher than or equal to 400° C. and lower than or equal to 600° C.

A second aspect of the disclosure relates to an exhaust gas control apparatus for an internal combustion engine. The exhaust gas control apparatus includes an exhaust gas control catalyst, a particulate filter, an oxygen supply device, and an electronic control unit. The exhaust gas control catalyst is disposed in an exhaust passage of the internal combustion engine. The exhaust gas control catalyst has a catalytic function. The particulate filter is disposed in the exhaust passage at a location downstream of the exhaust gas control catalyst in an exhaust gas flow direction. The oxygen supply device is configured to supply gas containing oxygen into exhaust gas flowing into the particulate filter at a location downstream of the exhaust gas control catalyst in the exhaust gas flow direction. The electronic control unit is configured to regulate an amount of oxygen that is supplied from the oxygen supply device. Under a condition that hydrogen or ammonia is produced in the exhaust gas control catalyst when an air-fuel ratio of exhaust gas emitted from a body of the internal combustion engine is a rich air-fuel ratio richer than a stoichiometric air-fuel ratio, the electronic control unit is configure to cause the oxygen supply device to supply oxygen into the exhaust gas while periodically increasing or reducing the oxygen such that an air-fuel ratio of exhaust gas flowing into the particulate filter alternately varies between the rich air-fuel ratio and a lean air-fuel ratio leaner than the stoichiometric air-fuel ratio.

A third aspect of the disclosure relates to an exhaust gas control apparatus for an internal combustion engine. The exhaust gas control apparatus includes a particulate filter, an oxygen supply device, and an electronic control unit. The particulate filter is disposed in an exhaust passage of the internal combustion engine. The oxygen supply device is configured to supply gas containing oxygen into exhaust gas flowing into the particulate filter. The electronic control unit is configured to regulate an amount of oxygen that is supplied from the oxygen supply device. Under a condition that NO2is produced with oxygen supplied from the oxygen supply device when oxygen that is supplied from the oxygen supply device into exhaust gas is periodically increased or reduced such that an air-fuel ratio of exhaust gas flowing into the particulate filter alternately varies between a rich air-fuel ratio richer than a stoichiometric air-fuel ratio and a lean air-fuel ratio leaner than the stoichiometric air-fuel ratio, the electronic control unit is configured to cause the oxygen supply device to supply oxygen into the exhaust gas while periodically increasing or reducing the oxygen such that an air-fuel ratio of exhaust gas flowing into the particulate filter alternately varies between the rich air-fuel ratio and the lean air-fuel ratio.

In the exhaust gas control apparatus, the electronic control unit may be configured to cause the oxygen supply device to supply oxygen into the exhaust gas while periodically increasing or reducing the oxygen such that an average air-fuel ratio in a plurality of cycles in which an air-fuel ratio of exhaust gas flowing into the particulate filter alternately varies between the rich air-fuel ratio and the lean air-fuel ratio becomes the stoichiometric air-fuel ratio.

In the exhaust gas control apparatus, the electronic control unit may be configured to increase a leanness degree at the time when the air-fuel ratio of exhaust gas flowing into the particulate filter is the leanest as a temperature of the particulate filter decreases.

In the exhaust gas control apparatus, the electronic control unit may be configured to cause the oxygen supply device to supply oxygen into the exhaust gas while periodically increasing or reducing the oxygen such that the air-fuel ratio of exhaust gas flowing into the particulate filter alternately varies between the rich air-fuel ratio and the lean air-fuel ratio with a period shorter than or equal to a period with which exhaust gas having the lean air-fuel ratio flows into the particulate filter before exhaust gas having the rich air-fuel ratio and having flowed into the particulate filter flows out from the particulate filter and exhaust gas having the rich air-fuel ratio flows into the particulate filter before exhaust gas having the lean air-fuel ratio and having flowed into the particulate filter flows out from the particulate filter.

In the exhaust gas control apparatus, the particulate filter may have a catalytic function.

According to the aspects of the disclosure, an exhaust gas control apparatus that reduces deterioration of emissions while facilitating removal of PM in a filter is provided.

DETAILED DESCRIPTION OF EMBODIMENTS

Hereinafter, embodiments of the disclosure will be described in detail with reference to the accompanying drawings. In the following description, like reference numerals denote similar components.

First Embodiment

Overall Description of Internal Combustion Engine

FIG. 1is a view schematically showing an internal combustion engine in which an exhaust gas control apparatus according to a first embodiment is used. As shown inFIG. 1, the reference numeral1denotes an engine body, the reference numeral2denotes a cylinder block, the reference numeral3denotes a piston that reciprocates inside the cylinder block2, the reference numeral4denotes a cylinder head fixed onto the cylinder block2, the reference numeral5denotes a combustion chamber defined between the piston3and the cylinder head4, the reference numeral6denotes an intake valve, the reference numeral7denotes an intake port, the reference numeral8denotes an exhaust valve, and the reference numeral9denotes an exhaust port. The intake valve6opens or closes the intake port7. The exhaust valve8opens or closes the exhaust port9.

As shown inFIG. 1, an ignition plug10is disposed at the center of an inner wall surface of the cylinder head4, and a fuel injection valve11is disposed at an inner wall surface peripheral portion of the cylinder head4. The ignition plug10is configured to generate spark in response to an ignition signal. The fuel injection valve11injects a predetermined amount of fuel into the combustion chamber5in response to an injection signal. The fuel injection valve11may be disposed so as to inject fuel into the intake port7. In the present embodiment, gasoline having a stoichiometric air-fuel ratio of 14.6 is used as a fuel. Alternatively, a fuel other than gasoline or gasoline-mixed fuel may be used in the internal combustion engine in which the exhaust gas control apparatus according to the disclosure is used.

The intake port7of each cylinder is coupled to a surge tank14via a corresponding one of intake branch pipes13. The surge tank14is coupled to an air cleaner16via an intake pipe15. The intake ports7, the intake branch pipes13, the surge tank14, and the intake pipe15form an intake passage. A throttle valve18is disposed in the intake pipe15. The throttle valve18is actuated by a throttle valve actuator17. The throttle valve18is able to change the opening area of the intake passage when the throttle valve18is turned by the throttle valve actuator17.

On the other hand, the exhaust port9of each cylinder is coupled to an exhaust manifold19. The exhaust manifold19includes a plurality of branch portions respectively coupled to the exhaust ports9, and a collecting portion that collects these branch portions. The collecting portion of the exhaust manifold19is coupled to an upstream-side casing21. An exhaust gas control catalyst20is incorporated in the upstream-side casing21. The upstream-side casing21is coupled to a downstream-side casing23via an exhaust pipe22. A particulate filter (hereinafter, also simply referred to as filter)24is incorporated in the downstream-side casing23. A secondary air supply device25is provided in the exhaust pipe22between the exhaust gas control catalyst20and the filter24. The secondary air supply device25supplies secondary air into exhaust gas flowing through the exhaust pipe22, that is, exhaust gas flowing into the filter24. The exhaust ports9, the exhaust manifold19, the upstream-side casing21, the exhaust pipe22, and the downstream-side casing23form an exhaust passage.

The exhaust gas control apparatus of the present embodiment includes the secondary air supply device25. Instead, the exhaust gas control apparatus may include another oxygen supply device as long as the oxygen supply device is able to supply gas containing oxygen into exhaust gas flowing into the filter24. Specific examples of the oxygen supply device include a device that supplies only oxygen into exhaust gas.

An electronic control unit (ECU)31is a digital computer, and includes a random access memory (RAM)33, a read only memory (ROM)34, a microprocessor (CPU)35, an input port36, and an output port37connected to one another via a bidirectional bus32.

An air flow meter39is disposed in the intake pipe15. The air flow meter39is used to detect the flow rate of air flowing through the intake pipe15. An output signal of the air flow meter39is input to the input port36via an associated AD converter38. An upstream-side air-fuel ratio sensor40is disposed at the collecting portion of the exhaust manifold19. The upstream-side air-fuel ratio sensor40detects the air-fuel ratio of exhaust gas flowing through the exhaust manifold19(that is, exhaust gas flowing into the exhaust gas control catalyst20). In addition, a downstream-side air-fuel ratio sensor41is disposed in the exhaust pipe22. The downstream-side air-fuel ratio sensor41detects the air-fuel ratio of exhaust gas flowing through the exhaust pipe22(that is, exhaust gas flowing out from the exhaust gas control catalyst20and flowing into the filter24). Output signals of the air-fuel ratio sensors40,41are also input to the input port36via associated AD converters38.

The exhaust gas control catalyst20is provided with a catalyst temperature sensor46. The catalyst temperature sensor46is used to detect the temperature of the exhaust gas control catalyst20. The filter24is provided with a filter temperature sensor47. The filter temperature sensor47is used to detect the temperature of the filter24. A differential pressure sensor48is provided between the exhaust pipe22upstream of the filter24and the exhaust pipe22downstream of the filter24. The differential pressure sensor48is used to detect the differential pressure between the upstream side and downstream side of the filter24. Output signals of the temperature sensors46,47and differential pressure sensor48are also input to the input port36via associated AD converters38.

A load sensor43is connected to an accelerator pedal42. The load sensor43generates an output voltage proportional to a depression amount of the accelerator pedal42. An output voltage of the load sensor43is input to the input port36via an associated AD converter38. A crank angle sensor44generates an output pulse, for example, each time a crankshaft rotates 15 degrees. The output pulse is input to the input port36. The CPU35calculates an engine rotation speed based on the output pulse of the crank angle sensor44.

On the other hand, the output port37is connected to the ignition plugs10, the fuel injection valves11, the throttle valve actuator17, and the secondary air supply device25via associated driving circuits45. Therefore, the ECU31functions as a controller that controls the operations of the ignition plugs10, fuel injection valves11, throttle valve actuator17, and secondary air supply device25.

In the present embodiment, the exhaust gas control catalyst20is a three-way catalyst in which catalytic precious metal (for example, platinum (Pt)) having a catalysis is supported on a carrier made of ceramics. A three-way catalyst has a function of removing unburnt HC, CO, and NOx at the same time when the air-fuel ratio of exhaust gas flowing into the three-way catalyst is kept at the stoichiometric air-fuel ratio. The exhaust gas control catalyst20may be a catalyst other than the three-way catalyst, such as an oxidation catalyst and an NOx storage-reduction catalyst, as long as the catalyst has a structure that a substance having a catalysis is supported.

FIG. 2AandFIG. 2Bare views showing the structure of the filter24.FIG. 2Ais a front view of the filter24.FIG. 2Bis a longitudinal cross-sectional view of the filter24. As shown inFIG. 2AandFIG. 2B, the filter24has a honeycomb structure, and has a plurality of exhaust gas flow passages60,61extending parallel to each other. These exhaust gas flow passages include exhaust gas inflow passages60and exhaust gas outflow passages61. The downstream end of each exhaust gas inflow passage60is closed by a plug62. The upstream end of each exhaust gas outflow passage61is closed by a plug63. InFIG. 2A, the hatched portions represent the plugs63. Therefore, the exhaust gas inflow passage60and the exhaust gas outflow passage61are disposed alternately via a thin partition wall64. In other words, the exhaust gas inflow passages60and the exhaust gas outflow passages61are disposed such that each exhaust gas inflow passage60is surrounded by the four exhaust gas outflow passages61and each exhaust gas outflow passage61is surrounded by the four exhaust gas inflow passages60.

The filter24is made of a porous material, such as cordierite. Therefore, exhaust gas flowing into the exhaust gas inflow passage60passes through the surrounding partition walls64as represented by the arrows inFIG. 2B, and flows out to the adjacent exhaust gas outflow passages61. In this way, while exhaust gas is flowing through the partition walls64, PM contained in the exhaust gas is trapped by the filter24.

The filter24has supported catalytic precious metal (such as platinum (Pt)) having a catalysis. That is, the filter24has a catalytic function. Therefore, the filter24is able to not only trap PM contained in exhaust gas but also oxidize and remove unburnt HC and CO contained in exhaust gas. The filter24may have another configuration as long as the filter24has a supported substance that traps PM contained in exhaust gas and that has a catalysis. Furthermore, when an exhaust gas control catalyst having a catalysis is disposed between the secondary air supply device25and the filter24, the filter24does not need to have a supported substance having a catalysis.

Filter Regeneration Process

PM trapped in the filter24deposits on the filter24. When the amount of PM deposited on the filter24increases, pores in the partition walls64clog, with the result that a pressure loss of exhaust gas due to the filter24increases. An increase in pressure loss leads to a decrease in output power and deterioration of combustion of the internal combustion engine resulting from difficulty of exhaust gas flow. Therefore, to prevent a decrease in output power and deterioration of combustion of the internal combustion engine, when the amount of PM deposited on the filter24is greater than a limit deposition amount, PM deposited on the filter24needs to be oxidized and removed. A limit deposition amount is an amount as follows. When the amount of PM deposited on the filter24increases to the limit deposition amount or above, a pressure loss due to the filter24increases, leading to, for example, deterioration of the operating state of the internal combustion engine.

In the present embodiment, when the amount of PM deposited on the filter24has increased, a filter regeneration process is performed to oxidize and remove PM. Hereinafter, the filter regeneration process will be described with reference toFIG. 3AandFIG. 3B.FIG. 3AandFIG. 3Bare views schematically showing reactions that take place in the exhaust gas control apparatus when the filter regeneration process is performed.FIG. 3Ashows the case where no secondary air is being supplied from the secondary air supply device25.FIG. 3Bshows the case where secondary air is being supplied from the secondary air supply device25.

In performing the filter regeneration process, initially, the temperature of the exhaust gas control catalyst20is raised to its activation temperature or higher. Specifically, the temperature of the exhaust gas control catalyst20is raised to 300° C. or higher and 700° C. or lower, and is preferably raised to 400° C. or higher and 600° C. or lower.

In addition, in the present embodiment, in performing the filter regeneration process, a fuel injection amount from each fuel injection valve11is regulated such that the air-fuel ratio of exhaust gas that is emitted from the engine body1becomes an air-fuel ratio richer than the stoichiometric air-fuel ratio (hereinafter, also referred to as rich air-fuel ratio). In other words, in performing the filter regeneration process, the fuel injection amount is regulated such that the air-fuel ratio of exhaust gas flowing into the exhaust gas control catalyst20becomes the rich air-fuel ratio. As a result, during the filter regeneration process, exhaust gas having the rich air-fuel ratio flows into the exhaust gas control catalyst20.

Unburnt HC and CO are contained in exhaust gas having the rich air-fuel ratio. In addition, since water is produced as a result of combustion of air-fuel mixture in each combustion chamber5, water is contained in exhaust gas. Therefore, exhaust gas containing unburnt HC and CO and water flows into the exhaust gas control catalyst20.

When the temperature of the exhaust gas control catalyst20falls within the range of 300° C. to 500° C., as exhaust gas containing CO and water flows into the exhaust gas control catalyst20, the water gas shift reaction expressed by the following formula (1) takes place in the exhaust gas control catalyst20under the catalysis of the exhaust gas control catalyst20.
CO+H2O→H2+CO2(1)

When the temperature of the exhaust gas control catalyst20is higher than or equal to 500° C., as exhaust gas containing unburnt HC and water flows into the exhaust gas control catalyst20, the steam-reforming reaction expressed by the following formula (2) or formula (3) takes place in the exhaust gas control catalyst20under the catalysis of the exhaust gas control catalyst20.
CH4+H2O→3H2+CO  (2)
C12H26+12H2O→25H2+12CO  (3)

Therefore, when the temperature of the exhaust gas control catalyst20is higher than or equal to the activation temperature (for example, 300° C.), as exhaust gas having the rich air-fuel ratio flows into the exhaust gas control catalyst20, hydrogen is produced in the exhaust gas control catalyst20.

Even when the air-fuel ratio of exhaust gas emitted from the engine body1is the rich air-fuel ratio, NOx (mainly, NO) is contained in the exhaust gas. When the temperature of the exhaust gas control catalyst20is relatively high, NO contained in exhaust gas in this way reacts with hydrogen into ammonia as expressed by the following formula (4) in the exhaust gas control catalyst20under the catalysis of the exhaust gas control catalyst20. This reaction particularly tends to take place when the temperature of the exhaust gas control catalyst20falls within the range of 400° C. to 600° C.
2NO+5H2→2NH3+2H2O  (4)

Therefore, when the temperature of the exhaust gas control catalyst20is higher than or equal to the activation temperature (particularly, the range of 400° C. to 600° C.), as exhaust gas having the rich air-fuel ratio flows into the exhaust gas control catalyst20, exhaust gas having the rich air-fuel ratio and containing ammonia flows out from the exhaust gas control catalyst20.

In addition, in the present embodiment, in performing the filter regeneration process, secondary air is intermittently supplied from the secondary air supply device25.FIG. 4is a timing chart of the amount of secondary air supplied from the secondary air supply device25, and the air-fuel ratio of exhaust gas flowing into the filter24. The dashed line X inFIG. 4represents the air-fuel ratio of exhaust gas before secondary air is supplied (in the illustrated example, the air-fuel ratio at this time is 13.6).

As shown inFIG. 4, in the present embodiment, secondary air is intermittently supplied from the secondary air supply device25. In the example shown inFIG. 4, a constant amount of secondary air is supplied from time t0to time t1, from time t2to time t3, from time t4to time t5, and from time t6to time t7. Particularly, in the present embodiment, secondary air is intermittently supplied such that a duration (for example, from time t0to time t1) during which secondary air is being supplied is equal to a duration (for example, from time t1to time t2) during which supply of secondary air is stopped (hereinafter, this duration is also referred to as period).

Particularly, in the present embodiment, the switching period of supply of secondary air is set to at or below a duration during which exhaust gas having a lean air-fuel ratio flows into the filter24before exhaust gas having the rich air-fuel ratio and having flowed into the filter24flows out from the filter24Preferably, the switching period of supply of secondary air is set to at or below a duration during which exhaust gas having the lean air-fuel ratio flows into the filter24before exhaust gas having the rich air-fuel ratio and having flowed into the filter24reaches a center of the filter24in the exhaust gas flow direction.

Similarly, the switching period of supply of secondary air is set to at or below a duration during which exhaust gas having the rich air-fuel ratio flows into the filter24before exhaust gas having the lean air-fuel ratio and having flowed into the filter24flows out from the filter24. Preferably, the switching period of supply of secondary air is set to at or below a duration during which exhaust gas having the rich air-fuel ratio flows into the filter24before exhaust gas having the rich air-fuel ratio and having flowed into the filter24reaches a center of the filter24in the exhaust gas flow direction. Specifically, the switching period of supply of secondary air is set to, for example, about 10 Hz.

As a result of supply of secondary air from the secondary air supply device25in this way, the air-fuel ratio of exhaust gas that flows into the filter24varies alternately between the rich air-fuel ratio and an air-fuel ratio leaner than the stoichiometric air-fuel ratio (hereinafter, also referred to as lean air-fuel ratio) as shown inFIG. 4. Particularly, in the present embodiment, secondary air is intermittently supplied such that a richness degree (for example, a richness degree at time t0, time t2, time t4, or the like) at which the air-fuel ratio of exhaust gas flowing into the filter24is the richest is equal to a leanness degree (for example, a leanness degree at time t1, time t3, time t5, or the like) at which the air-fuel ratio of exhaust gas flowing into the filter24is the leanest.

As a result, in the present embodiment, an average air-fuel ratio of exhaust gas flowing into the filter24over a certain length of time is almost the stoichiometric air-fuel ratio. That is, in the present embodiment, an average air-fuel ratio over multiple cycles in which the air-fuel ratio of exhaust gas flowing into the filter24varies alternately between the rich air-fuel ratio and the lean air-fuel ratio is almost the stoichiometric air-fuel ratio.

In the present embodiment, secondary air is supplied such that a richness degree at the time when the air-fuel ratio of exhaust gas is the richest is equal to a leanness degree at the time when the air-fuel ratio of exhaust gas is the leanest. However, these richness degree and leanness degree do not always need to be equal to each other. Secondary air may be supplied such that any one of the leanness degree and the richness degree is higher than the other. In the present embodiment, secondary air is supplied such that the duration during which secondary air is being supplied is equal to the duration during which supply of secondary air is stopped. However, these durations do not always need to be equal to each other. Secondary air may be supplied such that any one of the durations is longer than the other. In the present embodiment, secondary air is supplied such that the average air-fuel ratio of exhaust gas flowing into the filter24is almost the stoichiometric air-fuel ratio. Instead, secondary air may be supplied such that the average air-fuel ratio of exhaust gas flowing into the filter24is the rich air-fuel ratio or the lean air-fuel ratio.

FIG. 3Ashows the reactions that take place in the exhaust gas control apparatus when no secondary air is being supplied from the secondary air supply device25. As is apparent fromFIG. 3A, unburnt HC and CO contained in exhaust gas emitted from the engine body1flow into the filter24, and ammonia produced in the exhaust gas control catalyst20also flows into the filter24.

Part of NOx contained in exhaust gas emitted from the engine body1also remains without being removed in the exhaust gas control catalyst20. Therefore, NOx also flows into the filter24. NOx having flowed into the filter24is reduced and removed by unburnt HC or CO under the action of the supported catalytic precious metal in the filter24. Therefore, when no secondary air is being supplied, exhaust gas containing unburnt HC, CO, and ammonia flows to the downstream side of the filter24.

On the other hand,FIG. 3Bshows the reactions that take place in the exhaust gas control apparatus when secondary air is being supplied from the secondary air supply device25. In a region upstream of a region where the secondary air supply device25is provided, similar reactions to those in the case where no secondary air is being introduced take place. Therefore, exhaust gas flowing into the filter24contains unburnt HC, CO, ammonia, and NOx (particularly, NO). In addition, when secondary air is being supplied, exhaust gas flowing into the filter24contains air, particularly, oxygen, supplied from the secondary air supply device25.

When the temperature of the filter24is higher than or equal to 300° C. at this time, NO2is produced from ammonia and oxygen through the reaction expressed by the following formula (5) on the filter24having supported catalytic precious metal.
4NH3+5O2→4NO2+6H2O  (5)

NO2produced in this way is more reactive with PM deposited on the filter24than oxygen. Therefore, when the temperature of the filter24is about 300° C., NO2oxidizes and removes PM (containing carbon C as a main ingredient) through the reaction expressed by the following formulae (6) to (9).
2NO2+2C→2CO2+N2(6)
NO2+C→CO+NO  (7)
O2+C→CO2(8)
O2+2C→2CO  (9)

When secondary air is being supplied, unburnt HC and CO contained in exhaust gas emitted from the exhaust gas control catalyst20react with supplied oxygen on the filter24, so unburnt HC and CO are removed. Similarly, CO produced in the above formula (7) and formula (9) also reacts with supplied oxygen and is removed. On the other hand, NO contained in exhaust gas emitted from the exhaust gas control catalyst20is not removed and remains because of a lean atmosphere caused by supply of secondary air. Therefore, when secondary air is being supplied, exhaust gas containing NO flows into the region downstream of the filter24.

As described with reference toFIG. 2AandFIG. 2B, exhaust gas flows through the thin partition walls64in the filter24. In the present embodiment, the air-fuel ratio of exhaust gas flowing into the filter24alternately varies between the rich air-fuel ratio and the lean air-fuel ratio with a relatively short period. As a result, even when exhaust gas having the rich air-fuel ratio and exhaust gas having the lean air-fuel ratio alternately flow in at the inlet of the filter24, these exhaust gases are mixed with each other at the outlet of the filter24.

As described above, when no secondary air is being supplied, exhaust gas containing unburnt HC, CO, and ammonia flows into the region downstream of the filter24. On the other hand, when secondary air is being supplied, exhaust gas containing NO or oxygen flows into the region downstream of the filter24. These exhaust gases mix with each other in the region downstream of the filter24, so unburnt HC, CO, and ammonia react with NO and oxygen. As a result, unburnt HC, CO, ammonia, and NO are removed.

As described above, in the present embodiment, when secondary air is being supplied, exhaust gas containing a large amount of NO2flows into the filter24, so removal of PM deposited on the filter24is facilitated. In addition, in the present embodiment, exhaust gas having the rich air-fuel ratio and exhaust gas having the lean air-fuel ratio are caused to alternately flow into the filter24with a short period, so unburnt HC, CO, NO, and other substances, in exhaust gas are removed. Thus, deterioration of exhaust emissions is reduced.

In summary, in the present embodiment, when the temperature of the exhaust gas control catalyst20falls within a predetermined temperature range higher than or equal to the activation temperature and the air-fuel ratio of exhaust gas emitted from the engine body1is the rich air-fuel ratio, secondary air (oxygen) is supplied from the secondary air supply device25into the exhaust gas while being periodically increased or reduced such that the air-fuel ratio of exhaust gas flowing into the filter24alternately varies between the rich air-fuel ratio and the lean air-fuel ratio.

From another viewpoint, in the present embodiment, under the condition that hydrogen or ammonia is produced in the exhaust gas control catalyst20when the air-fuel ratio of exhaust gas emitted from the engine body1is the rich air-fuel ratio, secondary air (oxygen) is supplied from the secondary air supply device25into the exhaust gas while being periodically increased or reduced such that the air-fuel ratio of exhaust gas flowing into the filter24alternately varies between the rich air-fuel ratio and the lean air-fuel ratio.

From further another viewpoint, in the present embodiment, under the condition that NO2is produced by secondary air (oxygen) supplied from the secondary air supply device25when oxygen that is supplied from the secondary air supply device25into exhaust gas is periodically increased or reduced such that the air-fuel ratio of exhaust gas flowing into the filter24alternately varies between the rich air-fuel ratio and the lean air-fuel ratio, secondary air (oxygen) is supplied from the secondary air supply device25into the exhaust gas while being periodically increased or reduced such that the air-fuel ratio of exhaust gas flowing into the filter24alternately varies between the rich air-fuel ratio and the lean air-fuel ratio.

According to the present embodiment, when secondary air supplied from the secondary air supply device25is regulated in this way, deterioration of exhaust emissions is reduced while facilitating removal of PM deposited on the filter24.

Specific Control

Next, specific control in the filter regeneration process according to the present embodiment will be described with reference toFIG. 5.FIG. 5is a flowchart showing the control routine of the filter regeneration process according to the present embodiment. The illustrated control routine is executed at set time intervals.

First, in step S11, it is determined whether a regeneration flag is in an on state. The regeneration flag is set to the on state when filter regeneration is being performed; otherwise, the regeneration flag is set to an off state. When it is determined in step S11that the regeneration flag is in the off state, the process proceeds to step S12.

In step S12, it is determined whether a condition for performing the process of regenerating the filter24is satisfied. The condition for performing the regeneration process is satisfied, for example, when the amount of PM deposited on the filter24is greater than the limit deposition amount. Specifically, when the differential pressure between the upstream side and downstream side of the filter24, detected by the differential pressure sensor48, is greater than a limit differential pressure, it is determined that the amount of PM deposited on the filter24is greater than the limit deposition amount. On the other hand, when the differential pressure between the upstream side and downstream side of the filter24, detected by the differential pressure sensor48, is less than or equal to the limit differential pressure, it is determined that the amount of PM deposited on the filter24is less than or equal to the limit deposition amount. The amount of PM deposited on the filter24may be detected or estimated by other methods without using the differential pressure sensor48.

When it is determined in step S12that the condition for performing the regeneration process is not satisfied, the control routine is ended. On the other hand, when it is determined in step S12that the condition for performing the regeneration process is satisfied, the process proceeds to step S13. In step S13, the regeneration flag is set to the on state, and the control routine is ended.

When the regeneration flag is set to the on state, the process proceeds from step S11to step S14in the next control routine. In step S14, it is determined whether a temperature Tc of the exhaust gas control catalyst20is higher than or equal to an activation temperature Tact (for example, 300° C.). The temperature of the exhaust gas control catalyst20is detected by the catalyst temperature sensor46provided in the exhaust gas control catalyst20.

As described above, when the temperature Tc of the exhaust gas control catalyst20is lower than the activation temperature Tact, hydrogen or ammonia cannot be produced in the exhaust gas control catalyst20. Therefore, when it is determined in step S14that the temperature of the exhaust gas control catalyst20is lower than the activation temperature Tact, the process proceeds to step S15, and the process of raising the temperature of the exhaust gas control catalyst20is performed. Examples of the process of raising the temperature of the exhaust gas control catalyst20include dither control in which the air-fuel ratio of air-fuel mixture that is supplied to the combustion chamber5is set to the rich air-fuel ratio in part of the plurality of cylinders and the air-fuel ratio of air-fuel mixture that is supplied to the combustion chambers5of the remaining cylinders is set to the lean air-fuel ratio. Through the dither control, exhaust gas containing unburnt HC and CO emitted from the cylinders of the rich air-fuel ratio and exhaust gas containing a large amount of oxygen emitted from the cylinders of the lean air-fuel ratio mix with each other and react with each other on the exhaust gas control catalyst20. For this reason, the temperature of the exhaust gas control catalyst20is raised by the use of heat of the reaction at this time. In the process of raising the temperature of the exhaust gas control catalyst20, existing temperature raising control other than dither control may be used instead of dither control.

When the temperature Tc of the exhaust gas control catalyst20rises to the activation temperature Tact or higher as a result of the process of raising the temperature of the exhaust gas control catalyst20, the process proceeds from step S14to step S16in the next control routine. In step S16, it is determined whether the current amount Dpm of PM deposited on the filter24is greater than a minimum allowable value Dmin. The minimum allowable value Dmin is a predetermined constant value close to zero. As described above, the amount Dpm of PM deposited on the filter24is estimated based on the differential pressure between the upstream side and downstream side of the filter24, detected by the differential pressure sensor48as described above.

When it is determined in step S16that the amount of PM deposited on the filter24is greater than the minimum allowable value Dmin, the process proceeds to step S17. In step S17, rich air-fuel ratio control for regulating the fuel injection amount from each fuel injection valve11is executed such that the air-fuel ratio of exhaust gas emitted from the engine body1becomes the rich air-fuel ratio. A target air-fuel ratio at this time is set to, for example, 13.6.

Subsequently, in step S18, secondary air is intermittently supplied by the secondary air supply device25. The period and amount of supply of secondary air are set such that the air-fuel ratio of exhaust gas flowing into the filter24varies as shown inFIG. 4. Thus, PM deposited on the filter24gradually reduces.

After that, when the amount Dpm of PM deposited on the filter24reduces and becomes less than or equal to the minimum allowable value Dmin, the process proceeds from step S16to step S19in the next control routine. In step S19, rich air-fuel ratio control is stopped, and the air-fuel ratio of exhaust gas that is emitted from the engine body1is set to an air-fuel ratio during normal operation (for example, the air-fuel ratio is kept around the stoichiometric air-fuel ratio). Subsequently, in step S20, supply of secondary air from the secondary air supply device25is stopped. Subsequently, in step S21, the regeneration flag is set to the off state, and the control routine is ended.

Alternative Embodiment

Next, an alternative embodiment to the first embodiment will be described with reference toFIG. 6. In the first embodiment, the filter regeneration process is performed while a vehicle equipped with an internal combustion engine is being driven. However, the filter regeneration process may be performed at a maintenance shop or another place.

In this case, the vehicle is provided with an alarm lamp (not shown) that alarms a driver that the filter regeneration process is required, and the alarm lamp is connected to the output port of the ECU31via the driving circuit45. The alarm lamp lights up when the differential pressure between the upstream side and downstream side of the filter24, detected by the differential pressure sensor48, becomes greater than the limit differential pressure. When the alarm lamp lights up, the driver drives the vehicle to a maintenance shop.

In this case, the exhaust gas control apparatus for an internal combustion engine does not need to include the secondary air supply device25when the exhaust pipe22has an opening (not shown) for attaching a secondary air supply device. At the maintenance shop, a cover attached to the opening of the exhaust pipe22is detached, the secondary air supply device is attached to this opening, and the secondary air supply device is electrically connected to the output port of the ECU. After that, the filter regeneration process is performed.

FIG. 6is a flowchart showing the control routine of the filter regeneration process according to the present alternative embodiment. The illustrated control routine is executed at set time intervals after the secondary air supply device25is attached.

First, it is determined in step S31whether the temperature Tc of the exhaust gas control catalyst20is higher than or equal to the activation temperature Tact. When the temperature Tc is lower than the activation temperature Tact, the process proceeds to step S32, and the process of raising the temperature of the exhaust gas control catalyst20is performed. It is conceivable that the process of raising the temperature of the exhaust gas control catalyst20is, for example, attaching an electric heater around the exhaust gas control catalyst20and then supplying electric power to the electric heater.

After that, when the temperature Tc of the exhaust gas control catalyst20rises to the activation temperature Tact or higher, the process proceeds from step S31to step S33in the next control routine. Step S33to step S37are basically similar to step S16to step S20ofFIG. 5, so the description thereof is omitted.

Second Embodiment

Next, an exhaust gas control apparatus for an internal combustion engine according to a second embodiment will be described with reference toFIG. 7. The configuration and control of the exhaust gas control apparatus according to the second embodiment are basically similar to the configuration and control of the exhaust gas control apparatus according to the first embodiment, so portions different from the exhaust gas control apparatus according to the first embodiment will be mainly described below.

Incidentally, where a leanness degree at the time when the air-fuel ratio of exhaust gas flowing into the filter24is the leanest is referred to as amplitude when secondary air is being intermittently supplied, the amount of PM that is removed per unit time in the filter regeneration process (PM removal amount) varies with the amplitude. This will be described with reference toFIG. 7.

FIG. 7is a graph showing the relationship between an amplitude and a temperature of the filter24. In the graph, Tact denotes the activation temperature (for example, 300° C.) of the filter24, Tpm denotes a lowest temperature (for example, 500° C.) at or above which PM can be removed by the use of oxygen, and Tot denotes a limit temperature (for example, 950° C.) at or above which the filter24breaks because of melting, or the like.

The plurality of solid lines in the graph represent equal removal amount lines on each of which the PM removal amount is equal. Among the plurality of solid lines, the solid line located on the upper side represents the equal removal amount line for a greater PM removal amount. Therefore, as is apparent fromFIG. 7, when the temperature of the filter24is constant, the PM removal amount increases as the amplitude increases. When the amplitude is constant, the PM removal amount increases as the temperature of the filter24rises. As the temperature of the filter24rises, the same amount of PM can be removed by a smaller amplitude.

The region A in the graph represents a region as follows. When the filter regeneration process is continued in a state within the region, the temperature of the filter24finally becomes higher than or equal to the limit temperature Tot. Therefore, when the filter regeneration process is performed, the amplitude needs to be regulated so as not to become the state within the region A.

Even when the temperature of the filter24is lower than the lowest temperature Tpm, exhaust gas having the rich air-fuel ratio and exhaust gas having the lean air-fuel ratio alternately flow into the filter24through the filter regeneration process. With this, unburnt HC and CO contained in exhaust gas react with oxygen. Because of the heat of reaction, the temperature of the filter24rises to the lowest temperature Tpm or higher. The region B in the graph represents a region as follows. Even when the filter regeneration process is continued in a state within the region, the temperature of the filter24does not rise to the lowest temperature Tpm or higher. In addition, the region C in the graph represents a region as follows. Even when the filter regeneration process is continued in a state within the region, no reaction of unburnt HC and CO with oxygen occurs on the filter24, so the temperature of the filter24does not rise to the activation temperature Tact or higher. Therefore, when the filter regeneration process is performed, the amplitude needs to be regulated such that the state does not fall within the region B or the region C.

In the present embodiment, the leanness degree (amplitude) at the time when the air-fuel ratio of exhaust gas flowing into the filter24is the leanest is varied with the temperature of the filter24.FIG. 8is a graph showing the relationship between a temperature of the filter24and an amplitude. As shown inFIG. 8, in the present embodiment, basically, as the temperature of the filter24decreases, the amplitude is increased. Thus, while the PM removal amount per unit time is kept at a large amount, an excessive increase in the temperature of the filter24is inhibited.

If the amplitude is excessively increased, when the final average air-fuel ratio is intended to be set to almost the stoichiometric air-fuel ratio, the richness degree at the time when the air-fuel ratio of exhaust gas flowing into the filter24is the richest needs to be increased. However, if the air-fuel ratio of air-fuel mixture that is supplied to each combustion chamber5is excessively decreased, it leads to deterioration of combustion, so the richness degree cannot be increased so much.

In the case where the richness degree at the time when the air-fuel ratio of exhaust gas flowing into the filter24is the richest is not varied, if the amplitude is excessively increased, the duration during which the air-fuel ratio of exhaust gas flowing into the filter24is the rich air-fuel ratio needs to be excessively extended when the final average air-fuel ratio is intended to be set to almost the stoichiometric air-fuel ratio. However, if the duration is excessively extended so much, exhaust gas having the rich air-fuel ratio and exhaust gas having the lean air-fuel ratio do not mix with each other in the filter24.

For the above reasons, in the present embodiment, as shown inFIG. 8, the amplitude is increased with a decrease in the temperature of the filter24, and, when the amplitude reaches a predetermined amplitude ΔLref, the amplitude is kept at the predetermined amplitude ΔLref in the range in which the temperature of the filter24is lower than the temperature at the predetermined amplitude ΔLref.

FIG. 9is a flowchart showing the control routine of the filter regeneration process according to the present embodiment. The illustrated control routine is executed at set time intervals. Step S41to step S47inFIG. 9are similar to step S11to step S17inFIG. 5, and step S49to step S52inFIG. 9are similar to step S18to step S21inFIG. 5, so the description thereof is omitted.

When rich air-fuel ratio control is executed in step S47, a target value of the amplitude is subsequently calculated in step S48. A target value of the amplitude is, for example, calculated based on the temperature of the filter24, detected by the filter temperature sensor47, by consulting a map as shown inFIG. 8. Subsequently, in step S49, secondary air is intermittently supplied by the secondary air supply device25such that the amplitude becomes the target value calculated in step S48. When the richness degree of the air-fuel ratio of exhaust gas that is emitted from the engine body1is varied with the amplitude, the fuel injection amount that is injected from each fuel injection valve11is also regulated for the amplitude calculated in step S48.