Patent Description:
An exhaust gas control catalyst for an internal combustion engine including a secondary air supply device is known (for example, <CIT> (<CIT>) and <CIT> (<CIT>)). 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 <CIT>, 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 <CIT>, 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.

<CIT> discusses a method and a device for the exhaust gas after-treatment of an internal combustion engine.

<CIT> discusses a method for regenerating a arranged in the exhaust system of an internal combustion engine.

<CIT> discusses a method for purifying exhaust gas of a diesel engine.

<CIT> discusses an exhaust emission control device for an engine.

Incidentally, it is known that NO<NUM> contained in exhaust gas is more reactive with PM than oxygen. For this reason, when exhaust gas containing a large amount of NO<NUM> is caused to flow into the filter, the amount of PM removed can be increased. However, in the exhaust gas control apparatus described in <CIT>, gas composed of rich air-fuel ratio exhaust gas with a small amount of NO<NUM> and added secondary air directly flows into the filter. Therefore, NO<NUM> is 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 NO<NUM> to 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 NO<NUM> by the oxidation catalyst, the NO<NUM> is caused to flow into the filter, and PM deposited on the filter is burnt with the NO<NUM>.

However, not only NO<NUM> but 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 invention 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 invention relates to an exhaust gas control apparatus for an internal combustion engine, the exhaust gas control apparatus comprising an exhaust gas control catalyst (<NUM>) disposed in an exhaust passage of the internal combustion engine, the exhaust gas control catalyst having a catalytic function, a particulate filter (<NUM>) disposed in the exhaust passage at a location downstream of the exhaust gas control catalyst (<NUM>) in an exhaust gas flow direction, an oxygen supply device (<NUM>) configured to supply gas containing oxygen into exhaust gas flowing into the particulate filter (<NUM>) at a location downstream of the exhaust gas control catalyst (<NUM>) in the exhaust gas flow direction a catalyst temperature sensor (<NUM>) and an electronic control unit (<NUM>) configured to regulate an amount of oxygen that is supplied from the oxygen supply device (<NUM>), when a temperature of the exhaust gas control catalyst (<NUM>) is detected by the catalyst temperature sensor as fallings within a predetermined temperature range higher than or equal to an activation temperature, an amount of particulate matter deposited on the particular filter is greater than a minimum allowable value, 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 being configured to cause the oxygen supply device (<NUM>) 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 (<NUM>) alternately varies between the rich air-fuel ratio and a lean air-fuel ratio leaner than the stoichiometric air-fuel ratio, wherein the predetermined temperature range is higher than or equal to <NUM> and lower than or equal to <NUM>, wherein the electronic control unit (<NUM>) is configured to increase a leanness degree at the time when the air-fuel ratio of exhaust gas flowing into the particulate filter (<NUM>) is the leanest as a temperature of the particulate filter (<NUM>) decreases, and wherein if the temperature of the exhaust gas control catalyst (<NUM>) is detected by the catalyst temperature sensor as being less than the activation temperature, the electronic control unit is configured to start a temperature raising process to increase the temperature of the exhaust gas control catalyst (<NUM>).

In the exhaust gas control apparatus, the predetermined temperature range is higher than or equal to <NUM> and lower than or equal to <NUM>.

An example of the invention 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.

Another example of the invention 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 NO<NUM> is 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 invention, an exhaust gas control apparatus that reduces deterioration of emissions while facilitating removal of PM in a filter is provided.

In the following description, like reference numerals denote similar components.

<FIG> is a view schematically showing an internal combustion engine in which an exhaust gas control apparatus according to a first embodiment is used. As shown in <FIG>, the reference numeral <NUM> denotes an engine body, the reference numeral <NUM> denotes a cylinder block, the reference numeral <NUM> denotes a piston that reciprocates inside the cylinder block <NUM>, the reference numeral <NUM> denotes a cylinder head fixed onto the cylinder block <NUM>, the reference numeral <NUM> denotes a combustion chamber defined between the piston <NUM> and the cylinder head <NUM>, the reference numeral <NUM> denotes an intake valve, the reference numeral <NUM> denotes an intake port, the reference numeral <NUM> denotes an exhaust valve, and the reference numeral <NUM> denotes an exhaust port. The intake valve <NUM> opens or closes the intake port <NUM>. The exhaust valve <NUM> opens or closes the exhaust port <NUM>.

As shown in <FIG>, an ignition plug <NUM> is disposed at the center of an inner wall surface of the cylinder head <NUM>, and a fuel injection valve <NUM> is disposed at an inner wall surface peripheral portion of the cylinder head <NUM>. The ignition plug <NUM> is configured to generate spark in response to an ignition signal. The fuel injection valve <NUM> injects a predetermined amount of fuel into the combustion chamber <NUM> in response to an injection signal. The fuel injection valve <NUM> may be disposed so as to inject fuel into the intake port <NUM>. In the present embodiment, gasoline having a stoichiometric air-fuel ratio of <NUM> 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 invention is used.

The intake port <NUM> of each cylinder is coupled to a surge tank <NUM> via a corresponding one of intake branch pipes <NUM>. The surge tank <NUM> is coupled to an air cleaner <NUM> via an intake pipe <NUM>. The intake ports <NUM>, the intake branch pipes <NUM>, the surge tank <NUM>, and the intake pipe <NUM> form an intake passage. A throttle valve <NUM> is disposed in the intake pipe <NUM>. The throttle valve <NUM> is actuated by a throttle valve actuator <NUM>. The throttle valve <NUM> is able to change the opening area of the intake passage when the throttle valve <NUM> is turned by the throttle valve actuator <NUM>.

On the other hand, the exhaust port <NUM> of each cylinder is coupled to an exhaust manifold <NUM>. The exhaust manifold <NUM> includes a plurality of branch portions respectively coupled to the exhaust ports <NUM>, and a collecting portion that collects these branch portions. The collecting portion of the exhaust manifold <NUM> is coupled to an upstream-side casing <NUM>. An exhaust gas control catalyst <NUM> is incorporated in the upstream-side casing <NUM>. The upstream-side casing <NUM> is coupled to a downstream-side casing <NUM> via an exhaust pipe <NUM>. A particulate filter (hereinafter, also simply referred to as filter) <NUM> is incorporated in the downstream-side casing <NUM>. A secondary air supply device <NUM> is provided in the exhaust pipe <NUM> between the exhaust gas control catalyst <NUM> and the filter <NUM>. The secondary air supply device <NUM> supplies secondary air into exhaust gas flowing through the exhaust pipe <NUM>, that is, exhaust gas flowing into the filter <NUM>. The exhaust ports <NUM>, the exhaust manifold <NUM>, the upstream-side casing <NUM>, the exhaust pipe <NUM>, and the downstream-side casing <NUM> form an exhaust passage.

The exhaust gas control apparatus of the present embodiment includes the secondary air supply device <NUM>. 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 filter <NUM>. Specific examples of the oxygen supply device include a device that supplies only oxygen into exhaust gas.

An electronic control unit (ECU) <NUM> is a digital computer, and includes a random access memory (RAM) <NUM>, a read only memory (ROM) <NUM>, a microprocessor (CPU) <NUM>, an input port <NUM>, and an output port <NUM> connected to one another via a bidirectional bus <NUM>.

An air flow meter <NUM> is disposed in the intake pipe <NUM>. The air flow meter <NUM> is used to detect the flow rate of air flowing through the intake pipe <NUM>. An output signal of the air flow meter <NUM> is input to the input port <NUM> via an associated AD converter <NUM>. An upstream-side air-fuel ratio sensor <NUM> is disposed at the collecting portion of the exhaust manifold <NUM>. The upstream-side air-fuel ratio sensor <NUM> detects the air-fuel ratio of exhaust gas flowing through the exhaust manifold <NUM> (that is, exhaust gas flowing into the exhaust gas control catalyst <NUM>). In addition, a downstream-side air-fuel ratio sensor <NUM> is disposed in the exhaust pipe <NUM>. The downstream-side air-fuel ratio sensor <NUM> detects the air-fuel ratio of exhaust gas flowing through the exhaust pipe <NUM> (that is, exhaust gas flowing out from the exhaust gas control catalyst <NUM> and flowing into the filter <NUM>). Output signals of the air-fuel ratio sensors <NUM>, <NUM> are also input to the input port <NUM> via associated AD converters <NUM>.

The exhaust gas control catalyst <NUM> is provided with a catalyst temperature sensor <NUM>. The catalyst temperature sensor <NUM> is used to detect the temperature of the exhaust gas control catalyst <NUM>. The filter <NUM> is provided with a filter temperature sensor <NUM>. The filter temperature sensor <NUM> is used to detect the temperature of the filter <NUM>. A differential pressure sensor <NUM> is provided between the exhaust pipe <NUM> upstream of the filter <NUM> and the exhaust pipe <NUM> downstream of the filter <NUM>. The differential pressure sensor <NUM> is used to detect the differential pressure between the upstream side and downstream side of the filter <NUM>. Output signals of the temperature sensors <NUM>, <NUM> and differential pressure sensor <NUM> are also input to the input port <NUM> via associated AD converters <NUM>.

A load sensor <NUM> is connected to an accelerator pedal <NUM>. The load sensor <NUM> generates an output voltage proportional to a depression amount of the accelerator pedal <NUM>. An output voltage of the load sensor <NUM> is input to the input port <NUM> via an associated AD converter <NUM>. A crank angle sensor <NUM> generates an output pulse, for example, each time a crankshaft rotates <NUM> degrees. The output pulse is input to the input port <NUM>. The CPU <NUM> calculates an engine rotation speed based on the output pulse of the crank angle sensor <NUM>.

On the other hand, the output port <NUM> is connected to the ignition plugs <NUM>, the fuel injection valves <NUM>, the throttle valve actuator <NUM>, and the secondary air supply device <NUM> via associated driving circuits <NUM>. Therefore, the ECU <NUM> functions as a controller that controls the operations of the ignition plugs <NUM>, fuel injection valves <NUM>, throttle valve actuator <NUM>, and secondary air supply device <NUM>.

In the present embodiment, the exhaust gas control catalyst <NUM> is 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 catalyst <NUM> may 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> are views showing the structure of the filter <NUM>. <FIG> is a front view of the filter <NUM>. <FIG> is a longitudinal cross-sectional view of the filter <NUM>. As shown in <FIG>, the filter <NUM> has a honeycomb structure, and has a plurality of exhaust gas flow passages <NUM>, <NUM> extending parallel to each other. These exhaust gas flow passages include exhaust gas inflow passages <NUM> and exhaust gas outflow passages <NUM>. The downstream end of each exhaust gas inflow passage <NUM> is closed by a plug <NUM>. The upstream end of each exhaust gas outflow passage <NUM> is closed by a plug <NUM>. In <FIG>, the hatched portions represent the plugs <NUM>. Therefore, the exhaust gas inflow passage <NUM> and the exhaust gas outflow passage <NUM> are disposed alternately via a thin partition wall <NUM>. In other words, the exhaust gas inflow passages <NUM> and the exhaust gas outflow passages <NUM> are disposed such that each exhaust gas inflow passage <NUM> is surrounded by the four exhaust gas outflow passages <NUM> and each exhaust gas outflow passage <NUM> is surrounded by the four exhaust gas inflow passages <NUM>.

The filter <NUM> is made of a porous material, such as cordierite. Therefore, exhaust gas flowing into the exhaust gas inflow passage <NUM> passes through the surrounding partition walls <NUM> as represented by the arrows in <FIG>, and flows out to the adjacent exhaust gas outflow passages <NUM>. In this way, while exhaust gas is flowing through the partition walls <NUM>, PM contained in the exhaust gas is trapped by the filter <NUM>.

The filter <NUM> has supported catalytic precious metal (such as platinum (Pt)) having a catalysis. That is, the filter <NUM> has a catalytic function. Therefore, the filter <NUM> is able to not only trap PM contained in exhaust gas but also oxidize and remove unburnt HC and CO contained in exhaust gas. The filter <NUM> may have another configuration as long as the filter <NUM> has 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 device <NUM> and the filter <NUM>, the filter <NUM> does not need to have a supported substance having a catalysis.

PM trapped in the filter <NUM> deposits on the filter <NUM>. When the amount of PM deposited on the filter <NUM> increases, pores in the partition walls <NUM> clog, with the result that a pressure loss of exhaust gas due to the filter <NUM> increases. 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 filter <NUM> is greater than a limit deposition amount, PM deposited on the filter <NUM> needs to be oxidized and removed. A limit deposition amount is an amount as follows. When the amount of PM deposited on the filter <NUM> increases to the limit deposition amount or above, a pressure loss due to the filter <NUM> increases, 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 filter <NUM> has increased, a filter regeneration process is performed to oxidize and remove PM. Hereinafter, the filter regeneration process will be described with reference to <FIG> are views schematically showing reactions that take place in the exhaust gas control apparatus when the filter regeneration process is performed. <FIG> shows the case where no secondary air is being supplied from the secondary air supply device <NUM>. <FIG> shows the case where secondary air is being supplied from the secondary air supply device <NUM>.

In performing the filter regeneration process, initially, the temperature of the exhaust gas control catalyst <NUM> is raised to its activation temperature or higher. Specifically, the temperature of the exhaust gas control catalyst <NUM> is raised to <NUM> or higher and <NUM> or lower, and is preferably raised to <NUM> or higher and <NUM> or lower.

In addition, in the present embodiment, in performing the filter regeneration process, a fuel injection amount from each fuel injection valve <NUM> is regulated such that the air-fuel ratio of exhaust gas that is emitted from the engine body <NUM> becomes 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 catalyst <NUM> becomes 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 catalyst <NUM>.

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 chamber <NUM>, water is contained in exhaust gas. Therefore, exhaust gas containing unburnt HC and CO and water flows into the exhaust gas control catalyst <NUM>.

When the temperature of the exhaust gas control catalyst <NUM> falls within the range of <NUM> to <NUM>, as exhaust gas containing CO and water flows into the exhaust gas control catalyst <NUM>, the water gas shift reaction expressed by the following formula (<NUM>) takes place in the exhaust gas control catalyst <NUM> under the catalysis of the exhaust gas control catalyst <NUM>.

CO + H<NUM>O → H<NUM> + CO<NUM>     (<NUM>).

When the temperature of the exhaust gas control catalyst <NUM> is higher than or equal to <NUM>, as exhaust gas containing unburnt HC and water flows into the exhaust gas control catalyst <NUM>, the steam-reforming reaction expressed by the following formula (<NUM>) or formula (<NUM>) takes place in the exhaust gas control catalyst <NUM> under the catalysis of the exhaust gas control catalyst <NUM>.

CH<NUM> + H<NUM>O → <NUM><NUM> + CO     (<NUM>).

C<NUM>H<NUM> + <NUM><NUM>O → <NUM><NUM> + 12CO     (<NUM>).

Therefore, when the temperature of the exhaust gas control catalyst <NUM> is higher than or equal to the activation temperature (for example, <NUM>), as exhaust gas having the rich air-fuel ratio flows into the exhaust gas control catalyst <NUM>, hydrogen is produced in the exhaust gas control catalyst <NUM>.

Even when the air-fuel ratio of exhaust gas emitted from the engine body <NUM> is the rich air-fuel ratio, NOx (mainly, NO) is contained in the exhaust gas. When the temperature of the exhaust gas control catalyst <NUM> is relatively high, NO contained in exhaust gas in this way reacts with hydrogen into ammonia as expressed by the following formula (<NUM>) in the exhaust gas control catalyst <NUM> under the catalysis of the exhaust gas control catalyst <NUM>. This reaction particularly tends to take place when the temperature of the exhaust gas control catalyst <NUM> falls within the range of <NUM> to <NUM>.

2NO + <NUM><NUM> → 2NH<NUM> + <NUM><NUM>O     (<NUM>).

Therefore, when the temperature of the exhaust gas control catalyst <NUM> is higher than or equal to the activation temperature (particularly, the range of <NUM> to <NUM>), as exhaust gas having the rich air-fuel ratio flows into the exhaust gas control catalyst <NUM>, exhaust gas having the rich air-fuel ratio and containing ammonia flows out from the exhaust gas control catalyst <NUM>.

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

As shown in <FIG>, in the present embodiment, secondary air is intermittently supplied from the secondary air supply device <NUM>. In the example shown in <FIG>, a constant amount of secondary air is supplied from time t<NUM> to time t<NUM>, from time t<NUM> to time t<NUM>, from time t<NUM> to time t<NUM>, and from time t<NUM> to time t<NUM>. Particularly, in the present embodiment, secondary air is intermittently supplied such that a duration (for example, from time t<NUM> to time t<NUM>) during which secondary air is being supplied is equal to a duration (for example, from time t<NUM> to time t<NUM>) 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 filter <NUM> before exhaust gas having the rich air-fuel ratio and having flowed into the filter <NUM> flows out from the filter <NUM> Preferably, 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 filter <NUM> before exhaust gas having the rich air-fuel ratio and having flowed into the filter <NUM> reaches a center of the filter <NUM> in 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 filter <NUM> before exhaust gas having the lean air-fuel ratio and having flowed into the filter <NUM> flows out from the filter <NUM>. 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 filter <NUM> before exhaust gas having the rich air-fuel ratio and having flowed into the filter <NUM> reaches a center of the filter <NUM> in the exhaust gas flow direction. Specifically, the switching period of supply of secondary air is set to, for example, about <NUM>.

As a result of supply of secondary air from the secondary air supply device <NUM> in this way, the air-fuel ratio of exhaust gas that flows into the filter <NUM> varies 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 in <FIG>. Particularly, in the present embodiment, secondary air is intermittently supplied such that a richness degree (for example, a richness degree at time t<NUM>, time t<NUM>, time t<NUM>, or the like) at which the air-fuel ratio of exhaust gas flowing into the filter <NUM> is the richest is equal to a leanness degree (for example, a leanness degree at time t<NUM>, time t<NUM>, time t<NUM>, or the like) at which the air-fuel ratio of exhaust gas flowing into the filter <NUM> is the leanest.

As a result, in the present embodiment, an average air-fuel ratio of exhaust gas flowing into the filter <NUM> over 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 filter <NUM> varies 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 filter <NUM> is 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 filter <NUM> is the rich air-fuel ratio or the lean air-fuel ratio.

<FIG> shows the reactions that take place in the exhaust gas control apparatus when no secondary air is being supplied from the secondary air supply device <NUM>. As is apparent from <FIG>, unburnt HC and CO contained in exhaust gas emitted from the engine body <NUM> flow into the filter <NUM>, and ammonia produced in the exhaust gas control catalyst <NUM> also flows into the filter <NUM>.

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

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

When the temperature of the filter <NUM> is higher than or equal to <NUM> at this time, NO<NUM> is produced from ammonia and oxygen through the reaction expressed by the following formula (<NUM>) on the filter <NUM> having supported catalytic precious metal.

4NH<NUM> + 5O<NUM> → 4NO<NUM> + <NUM><NUM>O     (<NUM>).

NO<NUM> produced in this way is more reactive with PM deposited on the filter <NUM> than oxygen. Therefore, when the temperature of the filter <NUM> is about <NUM>, NO<NUM> oxidizes and removes PM (containing carbon C as a main ingredient) through the reaction expressed by the following formulae (<NUM>) to (<NUM>).

2NO<NUM> + 2C → 2CO<NUM> + N<NUM>     (<NUM>).

When secondary air is being supplied, unburnt HC and CO contained in exhaust gas emitted from the exhaust gas control catalyst <NUM> react with supplied oxygen on the filter <NUM>, so unburnt HC and CO are removed. Similarly, CO produced in the above formula (<NUM>) and formula (<NUM>) also reacts with supplied oxygen and is removed. On the other hand, NO contained in exhaust gas emitted from the exhaust gas control catalyst <NUM> is 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 filter <NUM>.

As described with reference to <FIG>, exhaust gas flows through the thin partition walls <NUM> in the filter <NUM>. In the present embodiment, the air-fuel ratio of exhaust gas flowing into the filter <NUM> alternately 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 filter <NUM>, these exhaust gases are mixed with each other at the outlet of the filter <NUM>.

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 filter <NUM>. On the other hand, when secondary air is being supplied, exhaust gas containing NO or oxygen flows into the region downstream of the filter <NUM>. These exhaust gases mix with each other in the region downstream of the filter <NUM>, 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 NO<NUM> flows into the filter <NUM>, so removal of PM deposited on the filter <NUM> is 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 filter <NUM> with 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 catalyst <NUM> falls 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 body <NUM> is the rich air-fuel ratio, secondary air (oxygen) is supplied from the secondary air supply device <NUM> into the exhaust gas while being periodically increased or reduced such that the air-fuel ratio of exhaust gas flowing into the filter <NUM> alternately 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 catalyst <NUM> when the air-fuel ratio of exhaust gas emitted from the engine body <NUM> is the rich air-fuel ratio, secondary air (oxygen) is supplied from the secondary air supply device <NUM> into the exhaust gas while being periodically increased or reduced such that the air-fuel ratio of exhaust gas flowing into the filter <NUM> alternately 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 NO<NUM> is produced by secondary air (oxygen) supplied from the secondary air supply device <NUM> when oxygen that is supplied from the secondary air supply device <NUM> into exhaust gas is periodically increased or reduced such that the air-fuel ratio of exhaust gas flowing into the filter <NUM> alternately varies between the rich air-fuel ratio and the lean air-fuel ratio, secondary air (oxygen) is supplied from the secondary air supply device <NUM> into the exhaust gas while being periodically increased or reduced such that the air-fuel ratio of exhaust gas flowing into the filter <NUM> alternately 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 device <NUM> is regulated in this way, deterioration of exhaust emissions is reduced while facilitating removal of PM deposited on the filter <NUM>.

Next, specific control in the filter regeneration process according to the present embodiment will be described with reference to <FIG> is 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 S11 that 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 filter <NUM> is satisfied. The condition for performing the regeneration process is satisfied, for example, when the amount of PM deposited on the filter <NUM> is greater than the limit deposition amount. Specifically, when the differential pressure between the upstream side and downstream side of the filter <NUM>, detected by the differential pressure sensor <NUM>, is greater than a limit differential pressure, it is determined that the amount of PM deposited on the filter <NUM> is greater than the limit deposition amount. On the other hand, when the differential pressure between the upstream side and downstream side of the filter <NUM>, detected by the differential pressure sensor <NUM>, is less than or equal to the limit differential pressure, it is determined that the amount of PM deposited on the filter <NUM> is less than or equal to the limit deposition amount. The amount of PM deposited on the filter <NUM> may be detected or estimated by other methods without using the differential pressure sensor <NUM>.

When it is determined in step S12 that 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 S12 that 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 S11 to step S14 in the next control routine. In step S14, it is determined whether a temperature Tc of the exhaust gas control catalyst <NUM> is higher than or equal to an activation temperature Tact (for example, <NUM>). The temperature of the exhaust gas control catalyst <NUM> is detected by the catalyst temperature sensor <NUM> provided in the exhaust gas control catalyst <NUM>.

As described above, when the temperature Tc of the exhaust gas control catalyst <NUM> is lower than the activation temperature Tact, hydrogen or ammonia cannot be produced in the exhaust gas control catalyst <NUM>. Therefore, when it is determined in step S14 that the temperature of the exhaust gas control catalyst <NUM> is lower than the activation temperature Tact, the process proceeds to step S15, and the process of raising the temperature of the exhaust gas control catalyst <NUM> is performed. Examples of the process of raising the temperature of the exhaust gas control catalyst <NUM> include dither control in which the air-fuel ratio of air-fuel mixture that is supplied to the combustion chamber <NUM> is 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 chambers <NUM> of 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 catalyst <NUM>. For this reason, the temperature of the exhaust gas control catalyst <NUM> is raised by the use of heat of the reaction at this time. In the process of raising the temperature of the exhaust gas control catalyst <NUM>, existing temperature raising control other than dither control may be used instead of dither control.

When the temperature Tc of the exhaust gas control catalyst <NUM> rises to the activation temperature Tact or higher as a result of the process of raising the temperature of the exhaust gas control catalyst <NUM>, the process proceeds from step S14 to step S16 in the next control routine. In step S16, it is determined whether the current amount Dpm of PM deposited on the filter <NUM> is 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 filter <NUM> is estimated based on the differential pressure between the upstream side and downstream side of the filter <NUM>, detected by the differential pressure sensor <NUM> as described above.

When it is determined in step S16 that the amount of PM deposited on the filter <NUM> is 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 valve <NUM> is executed such that the air-fuel ratio of exhaust gas emitted from the engine body <NUM> becomes the rich air-fuel ratio. A target air-fuel ratio at this time is set to, for example, <NUM>.

Subsequently, in step S18, secondary air is intermittently supplied by the secondary air supply device <NUM>. The period and amount of supply of secondary air are set such that the air-fuel ratio of exhaust gas flowing into the filter <NUM> varies as shown in <FIG>. Thus, PM deposited on the filter <NUM> gradually reduces.

After that, when the amount Dpm of PM deposited on the filter <NUM> reduces and becomes less than or equal to the minimum allowable value Dmin, the process proceeds from step S16 to step S19 in 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 body <NUM> is 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 device <NUM> is stopped. Subsequently, in step S21, the regeneration flag is set to the off state, and the control routine is ended.

Next, an alternative embodiment to the first embodiment will be described with reference to <FIG>. 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 ECU <NUM> via the driving circuit <NUM>. The alarm lamp lights up when the differential pressure between the upstream side and downstream side of the filter <NUM>, detected by the differential pressure sensor <NUM>, 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 device <NUM> when the exhaust pipe <NUM> has an opening (not shown) for attaching a secondary air supply device. At the maintenance shop, a cover attached to the opening of the exhaust pipe <NUM> is 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> is 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 device <NUM> is attached.

First, it is determined in step S31 whether the temperature Tc of the exhaust gas control catalyst <NUM> is 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 catalyst <NUM> is performed. It is conceivable that the process of raising the temperature of the exhaust gas control catalyst <NUM> is, for example, attaching an electric heater around the exhaust gas control catalyst <NUM> and then supplying electric power to the electric heater.

After that, when the temperature Tc of the exhaust gas control catalyst <NUM> rises to the activation temperature Tact or higher, the process proceeds from step S31 to step S33 in the next control routine. Step S33 to step S37 are basically similar to step S16 to step S20 of <FIG>, so the description thereof is omitted.

Next, an exhaust gas control apparatus for an internal combustion engine according to a second embodiment will be described with reference to <FIG>. 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 filter <NUM> is 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 to <FIG>.

<FIG> is a graph showing the relationship between an amplitude and a temperature of the filter <NUM>. In the graph, Tact denotes the activation temperature (for example, <NUM>) of the filter <NUM>, Tpm denotes a lowest temperature (for example, <NUM>) at or above which PM can be removed by the use of oxygen, and Tot denotes a limit temperature (for example, <NUM>) at or above which the filter <NUM> breaks 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 from <FIG>, when the temperature of the filter <NUM> is 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 filter <NUM> rises. As the temperature of the filter <NUM> rises, 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 filter <NUM> finally 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 filter <NUM> is 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 filter <NUM> through 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 filter <NUM> rises 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 filter <NUM> does 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 filter <NUM>, so the temperature of the filter <NUM> does 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 filter <NUM> is the leanest is varied with the temperature of the filter <NUM>. <FIG> is a graph showing the relationship between a temperature of the filter <NUM> and an amplitude. As shown in <FIG>, in the present embodiment, basically, as the temperature of the filter <NUM> decreases, 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 filter <NUM> is 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 filter <NUM> is the richest needs to be increased. However, if the air-fuel ratio of air-fuel mixture that is supplied to each combustion chamber <NUM> is 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 filter <NUM> is 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 filter <NUM> is 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 filter <NUM>.

For the above reasons, in the present embodiment, as shown in <FIG>, the amplitude is increased with a decrease in the temperature of the filter <NUM>, 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 filter <NUM> is lower than the temperature at the predetermined amplitude ΔLref.

<FIG> is 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 S41 to step S47 in <FIG> are similar to step S11 to step S17 in <FIG>, and step S49 to step S52 in <FIG> are similar to step S18 to step S21 in <FIG>, so the description thereof is omitted.

Claim 1:
An exhaust gas control apparatus for an internal combustion engine, the exhaust gas control apparatus comprising:
an exhaust gas control catalyst (<NUM>) disposed in an exhaust passage of the internal combustion engine, the exhaust gas control catalyst having a catalytic function;
a particulate filter (<NUM>) disposed in the exhaust passage at a location downstream of the exhaust gas control catalyst (<NUM>) in an exhaust gas flow direction;
an oxygen supply device (<NUM>) configured to supply gas containing oxygen into exhaust gas flowing into the particulate filter (<NUM>) at a location downstream of the exhaust gas control catalyst (<NUM>) in the exhaust gas flow direction;
a catalyst temperature sensor (<NUM>); characterized in that the exhaust gas control apparatus comprises
an electronic control unit (<NUM>) configured to regulate an amount of oxygen that is supplied from the oxygen supply device (<NUM>),
when a temperature of the exhaust gas control catalyst (<NUM>) is detected by the catalyst temperature sensor as falling within a predetermined temperature range higher than or equal to an activation temperature, an amount of particulate matter deposited on the particular filter is greater than a minimum allowable value, 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 being configured to cause the oxygen supply device (<NUM>) 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 (<NUM>) alternately varies between the rich air-fuel ratio and a lean air-fuel ratio leaner than the stoichiometric air-fuel ratio, wherein the predetermined temperature range is higher than or equal to <NUM> and lower than or equal to <NUM>,
wherein the electronic control unit (<NUM>) is configured to increase a leanness degree at the time when the air-fuel ratio of exhaust gas flowing into the particulate filter (<NUM>) is the leanest as a temperature of the particulate filter (<NUM>) decreases, and
wherein if the temperature of the exhaust gas control catalyst (<NUM>) is detected by the catalyst temperature sensor as being less than the activation temperature, the electronic control unit is configured to start a temperature raising process to increase the temperature of the exhaust gas control catalyst (<NUM>).