Exhaust gas control apparatus and method for an internal combustion engine

An exhaust gas control apparatus for an internal combustion engine, including a three-way catalyst, a NOX storage-reduction catalyst, a rich controlling mechanism performing control to make an air-fuel ratio of exhaust gas rich to reduce NOX stored in the NOX storage-reduction catalyst, and a control performing control to ensure that a NOX purifying component is present in the NOX storage-reduction catalyst when reducing the NOX.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to an exhaust gas control apparatus and method for an internal combustion engine, which purifies exhaust gas using a NOXstorage-reduction catalyst.

2. Description of the Related Art

An exhaust gas control apparatus has been proposed which purifies NOXin exhaust gas using a NOXstorage-reduction catalyst provided in an exhaust passage. For example, Japanese Patent Application Publication No. JP-A-2004-84617 describes technology that executes rich spike control (hereinafter also referred to as “RS control”) which makes the air-fuel ratio rich in order to reduce NOXstored in the NOXstorage-reduction catalyst.

In addition, technology has also been proposed to purify NOXin the exhaust gas using ammonia (NH3). Japanese Patent Application Publication No. JP-A-10-47041 describes technology in which a NOXstorage-reduction catalyst is arranged in some of the exhaust ports. NH3is produced by making the air-fuel ratio of the exhaust gas in the cylinders upstream of the NOXstorage-reduction catalyst rich and is used to purify NOXdischarged from the other cylinders in the downstream exhaust gas control catalyst. Also, Japanese Patent Application Publication No. JP-A-2004-218475 describes technology which improves NOXpurifying performance in both low and high temperature regions by providing a NOXselective reduction catalyst upstream of a NOXstorage-reduction catalyst and adding urea into the exhaust passage. In addition, Japanese Patent Application Publication No. JP-A-2005-214098 describes technology that produces NH3by making the air-fuel ratio of the exhaust gas at the NOXcatalyst portion rich and purifies NOXusing that NH3during lean burn operation.

However, with the technology described in the Japanese Patent Application Publication No. JP-A-2004-84617, there were cases in which NOXcomponents leaked out from the NOXstorage-reduction catalyst (hereinafter this may also be referred to as simply “NOXleakage”) in the initial stage of RS control. Such NOXleakage is thought to occur when reducing agent components such as HC, CO, and H2are consumed by oxygen or the like such that exhaust gas with an insufficient amount of the reducing agent is supplied to the NOXstorage-reduction catalyst. Also, with the technologies described in the Japanese Patent Application Publications No. JP-A-10-47041, JP-A-2004-218475, and JP-A-2005-214098, as well, it was difficult to appropriately suppress such NOXleakage from the NOXstorage-reduction catalyst when reducing NOXtherein.

SUMMARY OF THE INVENTION

This invention thus provides an exhaust gas control apparatus and exhaust gas control method for an internal combustion engine, which are capable of suppressing NOXleakage that may occur during NOXreduction, by appropriately executing rich spike control and control to ensure that a NOXpurifying component is present in the NOXstorage-reduction catalyst.

A first aspect of the invention relates to an exhaust gas control apparatus for an internal combustion engine, which has a three-way catalyst and a NOXstorage-reduction catalyst downstream of the three-way catalyst in an exhaust passage. This exhaust gas control apparatus includes rich controlling means for performing control to make an air-fuel ratio of exhaust gas from a combustion chamber of the internal combustion engine rich to reduce NOXstored in the NOXstorage-reduction catalyst, and controlling means for performing control to ensure that a NOXpurifying component capable of purifying the NOXis present in the NOXstorage-reduction catalyst when reducing the NOX.

This exhaust gas control apparatus for an internal combustion engine is preferably used to purify exhaust gas using the three-way catalyst and the NOXstorage-reduction catalyst. More specifically, the rich controlling means performs control to make the air-fuel ratio of the exhaust gas rich to reduce the NOXstored in the NOXstorage-reduction catalyst. Also, the controlling means performs control to ensure that a NOXpurifying component is present in the NOXstorage-reduction catalyst when reducing the NOX. As a result, it is possible to appropriately perform NOXreduction in the NOXstorage-reduction catalyst while efficiently suppressing NOXfrom leaking out of the NOXstorage-reduction catalyst, which may occur during the initial stage of the control to make the air-fuel ratio of the exhaust gas rich.

In the foregoing structure, the NOXpurifying component may be ammonia. In this case, reducing NO and NO2and the like to nitrogen, which is harmless, using ammonia enables the discharge of NO and NO2and the like to be suppressed. Also, the controlling means may include injection controlling means for injecting at least one of urea, ammonia, and fuel into the exhaust passage to ensure that the NOXpurifying component is present in the NOXstorage-reduction catalyst. Incidentally, the injection controlling means may perform the injection into the exhaust passage between the three-way catalyst and the NOXstorage-reduction catalyst.

In the foregoing structure, the injection controlling means may start the injection a first predetermined period of time before a reducing agent produced by making the air-fuel ratio rich reaches the NOXstorage-reduction catalyst. This makes it possible to suppress NOXfrom leaking out of the NOXstorage-reduction catalyst.

In the foregoing structure, the first predetermined period of time may be set based on a timing at which the injection should be started to ensure that the NOXpurifying component is present in the NOXstorage-reduction catalyst before the reducing agent reaches the NOXstorage-reduction catalyst.

In the foregoing structure, the injection controlling means may end the injection a second predetermined period of time before the control by the rich controlling means ends.

According to the foregoing structure, the injection controlling means ends the injection control before the control by the rich controlling means ends. This is because a sufficient amount of the reducing agent is supplied to the NOXstorage-reduction catalyst once the control by the rich controlling means has been performed for a certain amount of time. Ending the injection in this way makes it possible to prevent the NOXpurifying component from leaking out of the NOXstorage-reduction catalyst.

In the foregoing structure, the second predetermined period of time may be set based on a timing at which enough of the reducing agent, which is produced by making the air-fuel ratio rich, to reduce the NOXis supplied to the NOXstorage-reduction catalyst.

In the foregoing structure, the exhaust gas control apparatus may also include NOXpurifying component concentration obtaining means for obtaining a concentration of the NOXpurifying component in the exhaust gas downstream of the NOXstorage-reduction catalyst, and the injection controlling means may end the injection when the concentration of the NOXpurifying component starts to rise.

According to the foregoing structure, the injection controlling means ends the injection when the concentration of the NOXpurifying component downstream of the NOXstorage-reduction catalyst starts to rise. This makes it possible to more reliably prevent the NOXpurifying component from leaking out of the NOXstorage-reduction catalyst.

In the foregoing structure, the rich controlling means may end the control to make the air-fuel ratio of the exhaust gas rich when the injection controlling means ends the injection. In this case, it is conceivable that NOXreduction in the NOXstorage-reduction catalyst has substantially ended when the concentration of the NOXpurifying component starts to rise so the injection control as well as the control to make the air-fuel ratio of the exhaust gas rich end. As a result, it is possible to suppress a deterioration in fuel efficiency and the like that can occur as a result of the control to make the air-fuel ratio of the exhaust gas rich.

In the foregoing structure, the injection controlling means may execute the injection such that an injection quantity in a later stage of the injection is less than the injection quantity in an initial stage of the injection. That is, the injection is executed in such a manner that the injection quantity during the initial stage of the injection is greater than the injection quantity during the later stage of the injection. The reason for this is as follows. During the initial stage of the control to make the air-fuel ratio of the exhaust rich, NOXthat was stored in the NOXstorage-reduction catalyst is released all at once, while almost no reducing agent is supplied to the NOXstorage-reduction catalyst. Therefore, the injection quantity at that time is made relatively large. On the other hand, after a certain amount of time has passed after the control to make the air-fuel ratio of the exhaust gas rich has started, the reducing agent is supplied to the NOXstorage-reduction catalyst so the injection quantity is reduced so that it is relatively small. Accordingly, it is possible to appropriately suppress both NOXand NOXpurifying component from leaking out of the NOXstorage-reduction catalyst.

A second aspect of the invention relates to an exhaust gas control method for an internal combustion engine having a three-way catalyst and a NOXstorage-reduction catalyst downstream of the three-way catalyst in an exhaust passage. This exhaust gas control method includes performing control to make an air-fuel ratio of exhaust gas from a combustion chamber of the internal combustion engine rich to reduce NOXstored in the NOXstorage-reduction catalyst, and performing control to ensure that a NOXpurifying component capable of purifying the NOXis present in the NOXstorage-reduction catalyst when reducing the NOX.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Hereinafter, first, second, and third example embodiments of the invention will be described with reference to the accompanying drawings.

First Example Embodiment

First, a first example embodiment of the invention will be described.

FIG. 1is a schematic of the structure of a vehicle100to which an exhaust gas apparatus for an internal combustion engine according to the first example embodiment of the invention has been applied. In the drawing, the solid arrows indicate the direction of gas flow, while the broken arrows indicate the direction of signal input and output.

The vehicle100is mainly provided with an intake passage3, a throttle valve6, a surge tank7, an engine (i.e., an internal combustion engine)8, a fuel injection valve9, an exhaust passage18, a three-way catalyst21, a NOXstorage-reduction catalyst22, a urea injection valve24, an O2sensor25, a fuel tank30, a urea water tank31, and an ECU (Engine Control Unit)50.

The intake passage3is a passage through which intake air to be supplied to the engine8flows. The throttle valve6which regulates the amount of intake air supplied to the engine8and the surge tank7which stores intake air to be supplied to the engine8are both provided in the intake passage3. The engine8is supplied with intake air via the intake passage3and fuel which is injected by the fuel injection valve9. More specifically, the intake air and fuel are supplied to a combustion chamber8bof a cylinder8a. In particular, the fuel, which is stored in the fuel tank30, is supplied to the combustion chamber8bby the fuel injection valve9via a fuel supply line30a. The fuel injection valve9is controlled by a control signal supplied from the ECU50.

Inside the combustion chamber8b, a mixture of the intake air (i.e., air) and the fuel is ignited by a spark from a spark plug and combusted. The force generated by this combustion forces a piston8cto move in a reciprocating motion which is transmitted to a crankshaft, not shown, via a connecting rod8dso that the crankshaft rotates. Incidentally, although only one cylinder8ais shown inFIG. 1to facilitate the description, in actuality the engine may be configured to have two or more cylinders. Also, the engine8is not limited to being a direct injection type (in-cylinder type) engine in which fuel is injected directly into the combustion chamber8b. Alternatively, the engine8may be a port injection type engine in which fuel is injected into the intake passage.

Moreover, an intake valve10and an exhaust valve11are provided in the combustion chamber8bof the engine8. The intake valve10is controlled open and closed to selectively allow and prevent communication between the intake passage3and the combustion chamber8b. Similarly, the exhaust valve11is controlled open and closed to selectively allow and prevent communication between the exhaust passage18and the combustion chamber8b.

The exhaust gas produced by combustion in the engine8is discharged into the exhaust passage18in which are provided, in order from upstream to downstream, the three-way catalyst21, the urea injection valve24, the NOXstorage-reduction catalyst22, and the O2sensor25. The three-way sensor21is a catalyst having a precious metal such as platinum or rhodium as the active component and functions to remove oxides of nitrogen (NOX), carbon monoxide (CO), and hydrocarbons (HC) and the like in the exhaust gas. The three-way catalyst21functions as a so-called start catalyst. The urea injection valve24is a valve that injects urea into the exhaust passage18between the three-way catalyst21and the NOXstorage-reduction catalyst22. The urea is stored in the urea water tank31and supplied into the exhaust passage18by the urea injection valve24via a urea supply line31a. The urea injection valve24is controlled by a control signal supplied from the ECU50.

The NOXstorage-reduction catalyst22is a catalyst that serves to both store NOXin the exhaust gas and reduce the stored NOX. More specifically, the NOXstorage-reduction catalyst22basically stores NOXwhen the air-fuel ratio is lean and reduces the stored NOXusing a reducing agent (such as H2, CO, HC or the like) when the air-fuel ratio is rich or a stoichiometric air-fuel ratio. For example, when the NOXstorage-reduction catalyst22is completely saturated with NOX, the stored NOXis reduced by the ECU50executing rich spike control (i.e., RS control) that forcibly makes the air-fuel ratio rich. The NOXstorage-reduction catalyst22functions as a so-called NSR (NOXStorage Reduction) catalyst. The O2sensor25is a sensor that detects the oxygen concentration downstream of the NOXstorage-reduction catalyst22. The oxygen concentration detected by the O2sensor25is supplied to the ECU50and used in air-fuel ratio control and the like, for example.

The ECU50includes a CPU, ROM, RAM, and an A/D converter and the like, none of which are shown. The ECU50performs various controls in the vehicle based on outputs supplied from various sensors in the vehicle. In the first example embodiment, the ECU50executes RS control to reduce NOXin the NOXstorage-reduction catalyst22. In addition, when performing this kind of RS control, the ECU50also performs control to ensure that a NOXpurifying component which can purify NOXis present in the NOXstorage-reduction catalyst22. More specifically, the ECU50performs control to inject urea from the urea injection valve24into the exhaust passage18so that ammonia (NH3), which is a NOXpurifying component, is present in the NOXstorage-reduction catalyst22(hereinafter this control may also be referred to as “urea injection control”). In this way, the ECU50functions as the rich controlling means and the controlling means (injection controlling means) of the invention.

Here, a control method performed by the ECU50will be described in detail with reference toFIGS. 2 to 4. As described above, in the first example embodiment, the ECU50executes the urea injection control together with the RS control. More specifically, the ECU50starts the urea injection control before the RS control starts in order to suppress NOXleakage that may occur during the initial stage of the RS control. That is, NOXis purified (i.e., reduced to nitrogen which is harmless) by the ammonia which is produced from the urea by executing the urea injection control before the RS control.

FIGS. 2A to 2Dare views showing NOXleakage that may occur during the initial stage of the RS control,FIG. 2Ashows the on/off state of the RS control,FIG. 2Bshows the air-fuel ratio,FIG. 2Cshows the amount of reducing agent, andFIG. 2Dshows the amount of NOXdischarged from the NOXstorage-reduction catalyst22(hereinafter, this amount will simply be referred to as the “NOXdischarge amount”). Also, the horizontal axis in each ofFIGS. 2A to 2Drepresents time. Incidentally, inFIG. 2C, the solid line A1basically shows the amount of reducing agent in the exhaust gas discharged from the engine8(i.e., the amount of reducing agent supplied to the three-way catalyst21), the broken line A2shows the amount of reducing agent in the exhaust gas discharged from the three-way catalyst21, and the alternate long and short dash line A3shows the amount of reducing agent supplied to the downstream end portion (i.e., the portion on the downstream side) of the NOXstorage-reduction catalyst22.

As shown inFIG. 2A, at time t1a RS control request is output and the RS control starts. As a result, the air-fuel ratio changes from lean to rich, as shown inFIG. 2B. Also, as shown by the solid line A1inFIG. 2C, the amount of reducing agent in the exhaust gas discharged from the engine8increases. Incidentally, the RS control is executed during the period from time t1until time t2. When the RS control is executed in this manner, it is evident fromFIG. 2Dthat NOXleaks out during the initial stage of the RS control.

Here, cause of this NOXleakage will be explained simply. When the air-fuel ratio is made rich by starting the RS control, NOXthat was stored in the NOXstorage-reduction catalyst22tends to be released from the catalyst surface. At this time, the gas exhausted from the engine8contains large amounts of reducing agents (see the solid line A1inFIG. 2C). However, during the initial stage of the RS control, these reducing agents are consumed in the three-way catalyst21, and as a result, almost none are supplied to the NOXstorage-reduction catalyst22(see the broken line A2and the alternate long and short dash line A3inFIG. 2C). Therefore, as described above, the NOXreleased from the NOXstorage-reduction catalyst22ends up being discharged as it is (seeFIG. 2D). That is, NOXleakage occurs. Then when the oxygen and the like of the three-way catalyst21is gone, the reducing agents (in this specification, these reducing agents may also be referred to in the singular to simplify the description) are supplied to the NOXstorage-reduction catalyst22and used to reduce oxygen and NOXin the NOXstorage-reduction catalyst22.

In the first example embodiment, both the RS control and the urea injection control are executed to appropriately suppress NOXleakage which may occur during the initial stage of the RS control. In this case, the NOXthat leaks out during the initial stage of the RS control is purified by the ammonia produced by injecting urea. More specifically, the urea injection control starts to be executed a predetermined period of time before the reducing agent produced by the RS control reaches the NOXstorage-reduction catalyst22. More specifically, in the first example embodiment, the urea injection control starts before the RS control starts. Incidentally, The first predetermined period of time is set based on the timing at which the urea injection control should be started to ensure that there is ammonia in the NOXstorage-reduction catalyst22before the reducing agent reaches the NOXstorage-reduction catalyst22.

Also in the first example embodiment, the urea injection control ends before the RS control ends to prevent ammonia from leaking out of the NOXstorage-reduction catalyst22. More specifically, the urea injection control ends a second predetermined period of time before the RS control ends. This second predetermined period of time is set based on the timing at which enough of the reducing agent to reduce NOXis supplied to the NOXstorage-reduction catalyst22. That is, the urea injection control ends when enough of the reducing agent to reduce the NOXin the NOXstorage-reduction catalyst22is supplied to the NOXstorage-reduction catalyst22.

Furthermore, in the first example embodiment, the urea injection control is executed in such a manner that the amount of urea injected is less in the later stage of injection than it is in the initial stage of injection taking into account the release characteristics of NOXduring the RS control described above. That is, during the initial stage of the RS control, NOXthat was stored in the NOXstorage-reduction catalyst22is released all at once, while almost no reducing agent is supplied to the NOXstorage-reduction catalyst22. Therefore, the amount of urea that is injected is relatively large at first. Then after a certain amount of time has passed after the RS control starts, the reducing agent is supplied to the NOXstorage-reduction catalyst22so the amount of urea injected is reduced to a relatively small amount.

Here, the method for purifying NOXwith ammonia will be described. First, ammonia (NH3) is produced from urea ((NH2)2CO) according to Reaction formula (1) below, for example. Incidentally, the reaction in Reaction formula (1) progresses appropriately at the temperature of the exhaust gas of a typical gasoline engine. That is, ammonia can be sufficiently produced in a typical gasoline engine.
(NH2)2CO+H2O→2NH3+CO2Reaction formula (1)

Then the NO and NO2in the exhaust gas are purified by the ammonia produced from Reaction formula (1). More specifically, NO and NO2are reduced to nitrogen (N2) according to Reaction formulas (2) to (4) below.
4NH3+4NO+O2→4N2+6H2O  Reaction formula (2)
2NH3+NO+NO2→2N2+3H2Reaction formula (3)
4NH3+2NO2+O2→3N2+6H2O  Reaction formula (4)

Incidentally, the temperature range for these reactions substantially matches the NOXpurifying temperature range in the NOXstorage-reduction catalyst22. Therefore, it can be said that reducing NOXusing ammonia is suitable for the NOXstorage-reduction catalyst22. Also, of Reaction formulas (2) to (4), the reaction rate of Reaction formula (3) is the fastest. That is, a ratio of NO:NO2=1:1 is desirable to efficiently reduce NOX. Although approximately 95% of the NOXin exhaust gas from a typical gasoline engine is NO, with the system structure in this example embodiment, the production of NO2progresses in the three-way catalyst21upstream of the NOXstorage-reduction catalyst22so it is quite likely that the reaction in Reaction formula (3) will take place. That is, according to the system structure in this example embodiment, NOXcan be reduced efficiently using ammonia.

Next, a control method according to the first example embodiment will be described more specifically with reference toFIGS. 3A to 3E.FIG. 3Ashows the on/off state of the RS control,FIG. 3Bshows the air-fuel ratio,FIG. 3Cshows the amount of reducing agent,FIG. 3Dshows the on/off state of the urea injection control, andFIG. 3Eshows the amount of NOXdischarged (i.e., the NOXdischarge amount). Also, the horizontal axis in each ofFIGS. 3A to 3Erepresents time. Incidentally, inFIG. 3C, the solid line B1basically shows the amount of reducing agent in the exhaust gas discharged from the engine8, the broken line B2shows the amount of reducing agent in the exhaust gas discharged from the three-way catalyst21, and the alternate long and short dash line B3shows the amount of reducing agent supplied to the downstream end portion (i.e., the portion on the downstream side) of the NOXstorage-reduction catalyst22.

In this case, the urea injection control starts at time t11(seeFIG. 3D) before time t12when the RS control starts. That is, the urea injection control starts earlier than the RS control does. Then the RS control starts (seeFIGS. 3A and 3B) at time t12after a certain amount of time has passed after the urea injection control started. The NOXdischarge amount when the urea injection control is executed in this way is shown by the solid line B5inFIG. 3E. In contrast, the broken line B6inFIG. 3Eshows the NOXdischarge amount obtained when only the RS control is executed without the urea injection control being executed (this is the same as the graph inFIG. 2D). It is evident from the broken line B6that there is NOXleakage in this case. When comparing the solid line B5with the broken line B6, the NOXleakage is greatly suppressed by the execution of the urea injection control according to the first example embodiment. This substantial suppression of NOXleakage is thought to be because the NOXwas purified by the ammonia produced from the urea.

Then at time t13the urea injection control ends (seeFIG. 3D). That is, the urea injection control ends before the RS control does. This is because at time t13a sufficient amount of reducing agent is supplied to the downstream end portion of the NOXstorage-reduction catalyst22, as shown by the alternate long and short line B33inFIG. 3C. Ending the urea injection control in this way makes it possible to prevent ammonia from leaking out. Then at time t14after time t13, the RS control ends (seeFIGS. 3A and 3B).

Next, an example of the amount of urea injected (i.e., the urea injection quantity) in the urea injection control will be described with reference toFIG. 4. InFIG. 4, the horizontal axis represents time and the vertical axis represents the urea injection quantity. In this case, the urea injection control starts at time T1and ends at time T2. In the first example embodiment, the urea injection control is executed so that the urea injection quantity is less in the later stage of injection than it is in the initial stage of injection. In other words, the urea injection control is executed so that more urea is injected in the initial stage of injection than in the later stage of injection. The reason for this is as follows. In the initial stage of the RS control, almost no reducing agent is supplied to the NOXstorage-reduction catalyst22so the amount of urea injected is made relatively large. On the other hand, after a certain amount of time has passed after the RS control starts, the reducing agent is supplied to the NOXstorage-reduction catalyst22so the amount of urea injected is reduced to a relatively small amount. As a result, the urea injection control and the RS control enable NOXin the NOXstorage-reduction catalyst22to be efficiently reduced while appropriately suppressing NOXand ammonia from leaking out. Incidentally, substantially the same effects as those described above can also be obtained when the urea injection control is executed according to the injection characteristics denoted by the broken line C2inFIG. 4.

(Routine According to the First Example Embodiment)

Next, a routine according to the first example embodiment will be described with reference to the flowchart shown inFIG. 5. This routine is executed repeatedly at predetermined cycles by the ECU50described above.

First in step S101, the ECU50obtains the operating state of the engine8, after which the process proceeds on to step S102. In step S102, the ECU50determines whether the engine8is operating in a lean burn mode. If the engine8is operating in the lean burn mode (i.e., Yes in step S102), the process proceeds on to step S103. If on the other hand, the engine8is not operating in the lean burn mode (i.e., No in step S104), the routine immediately ends.

In step S103, the ECU50obtains from the operating state the period of time for which the RS control is to be executed (hereinafter referred to as the “RS period”) and the interval at which the RS control is to be executed (hereinafter referred to as the “RS interval”). Then the process proceeds on to step S104. In step S104, the ECU50determines whether there is a demand for the RS control. The ECU50determines whether the RS control should be executed based on, for example, the period of time that has passed after the RS control was executed last or the amount of NOXin the exhaust gas supplied to the NOXstorage-reduction catalyst22or the like. If there is a demand for the RS control (i.e., Yes in step S104), the process proceeds on to step S105. If, on the other hand, there is not a demand for the RS control (i.e., No in step S104), the routine immediately ends.

In step S105, the ECU50determines whether the RS control is being executed. If the RS control is being executed (i.e., Yes in step S105), the process proceeds on to step S111. If, on the other hand, the RS control is not being executed (i.e., No in step S105), the process proceeds on to step S106. In step S106, the ECU50determines whether the urea injection control is being executed. If the urea injection control is being executed (i.e., Yes in step S106), the process proceeds on to step S108. If, on the other hand, the urea injection control is not being executed (i.e., No in step S106), the process proceeds on to step S107.

In step S107, the ECU50starts the urea injection control. In this case, there is a demand for the RS control and the urea injection control is not yet being executed so the ECU50starts the urea injection control. Then the process proceeds on to step S108. In step S108, the ECU50measures the period of time for which the urea injection control is executed (hereinafter referred to as the “urea injection period”). Then the process proceeds on to step S109.

In step S109, the ECU50determines whether the RS control can be executed without adverse effects resulting. More specifically, the ECU50determines whether the urea injection period obtained in step S108has exceeded a predetermined period of time α. This predetermined period of time α corresponds to a period of time that should be allowed to pass after the urea injection control has started before starting the RS control to ensure that a NOXpurifying component is present in the NOXstorage-reduction catalyst22before the reducing agent from the RS control reaches the NOXstorage-reduction catalyst22. The predetermined period of time α is obtained from a map specified based on the operating conditions or from an operational expression based on the operating conditions or the like. Incidentally, the first predetermined period of time described above corresponds to a period of time which is equal to the sum of the period of time that it takes for the reducing agent to reach the NOXstorage-reduction catalyst22after the RS control has started plus the predetermined period of time α.

If the urea injection period exceeds the predetermined period of time α (i.e., Yes in step S109), the process proceeds on to step S110. In this case, it may be said that the conditions are such that the RS control may be executed without adverse effects resulting. Accordingly, in step S110, the ECU50starts the RS control. Then the process proceeds to step S111. On the other hand, if the urea injection period has not exceeded the predetermined period of time α (i.e., No in step S109), the routine immediately ends. In this case, only the urea injection control is executed without starting the RS control.

In step S111, the ECU50reads the urea injection period. Then the process proceeds on to step S1112where the ECU50determines whether there is a demand to end the urea injection control. In this case, the ECU50determines whether the urea injection period obtained in step S111has exceeded a predetermined period of time β. The predetermined period of time β is set based on the timing at which enough of the reducing agent to reduce NOXis supplied to the NOXstorage-reduction catalyst22. More specifically, the predetermined period of time β is obtained from a map specified based on the operating conditions or from an operational expression based on the operating conditions or the like. Incidentally, the second predetermined period of time described above corresponds to a period of time which is equal to the difference of the period of time between the start of the urea injection control and end of the RS control minus the predetermined period of time β.

If the urea injection period exceeds the predetermined period of time β (i.e., Yes in step S112), the process proceeds on to step S113. In this case, it may be said that enough of the reducing agent to reduce NOXis being supplied to the NOXstorage-reduction catalyst22. Accordingly, in step S113, the ECU50ends the urea injection control. Then the process proceeds to step S114. On the other hand, if the urea injection period has not exceeded the predetermined period of time β (i.e., No in step S112), the routine immediately ends. In this case, the urea injection control continues to be executed.

In step S114, the ECU50reads the period of time for which the RS control is being executed (hereinafter referred to as the “RS control period”), after which the process proceeds on to step S115. In step S115, the ECU50determines whether it is time to end the RS control based on the RS control period obtained in step S114. More specifically, the ECU50determines whether the RS control period is longer than the RS period obtained in step S103.

If it is time to end the RS control (i.e., Yes in step S115), the process proceeds on to step S116. In step S116, the ECU50ends the RS control, after which the routine immediately ends. In contrast, if it is not time to end the RS control (i.e., No in step S115), the routine immediately ends. In this case, the RS control is continued.

According to the routine described above, NOXin the NOXstorage-reduction catalyst22can be appropriately reduced while effectively suppressing NOXleakage by executing the RS control and the urea injection control. Furthermore, ammonia can also be appropriately prevented from leaking out which may otherwise occur due to the injection of urea.

Second Example Embodiment

Next, a second example embodiment of the invention will be described. In the foregoing first example embodiment, the urea injection control ends a second predetermined period of time before the RS control ends. That is, the urea injection control ends when enough of the reducing agent to reduce NOXis supplied to the NOXstorage-reduction catalyst22. However, in the second example embodiment, the urea injection control ends when the ammonia concentration (which corresponds to the NOXpurifying component concentration) downstream of the NOXstorage-reduction catalyst22starts to rise. In other words, in the second example embodiment, the urea injection control is ended taking into account not only the urea injection period but also the ammonia concentration. This is done to more reliably prevent ammonia from leaking out of the NOXstorage-reduction catalyst22.

Here, ammonia leakage from the NOXstorage-reduction catalyst22will be described with reference toFIGS. 6A to 6D.FIG. 6Ashows the on/off state of the RS control,FIG. 6Bshows the air-fuel ratio,FIG. 6Cshows the on/off state of the urea injection control, andFIG. 6Dshows the amount of ammonia discharged from the NOXstorage-reduction catalyst22(hereinafter, also simply referred to as the “ammonia discharge amount”).

In this case, the urea injection control is executed from time t3until time t6(seeFIG. 6C), and the RS control is executed from time t4until time t6(seeFIGS. 6A and 6B). When the urea injection control is executed as shown inFIG. 6C, the ammonia discharge amount increases from time t5, as shown inFIG. 6D. This indicates that ammonia is leaking out of the NOXstorage-reduction catalyst22. This kind of phenomenon is thought to occur because the urea injection control continues to be executed even though NOXreduction in the NOXstorage-reduction catalyst22has substantially ended. That is, it is thought that ammonia is discharged without being used to reduce NOXbecause the urea continues to be injected even though there is almost no NO or NO2present to be reduced using the ammonia. Accordingly, in the second example embodiment, the urea injection control ends when the ammonia concentration downstream of the NOXstorage-reduction catalyst22starts to increase.

Hereinafter, the second example embodiment of the invention will be described in more detail with reference toFIGS. 7 to 9.

FIG. 7is a schematic of the structure of a vehicle101to which an exhaust gas control apparatus for an internal combustion engine according to the second example embodiment of the invention has been applied. Here, constituent elements that are the same as those of the vehicle100described above (seeFIG. 1) will be denoted by like reference numerals and descriptions of those elements will be omitted. Incidentally, inFIG. 7, the solid arrows indicate the direction of gas flow, while the broken arrows indicate the direction of signal input and output.

The vehicle101has a NH3sensor28downstream of the NOXstorage-reduction catalyst22in the exhaust passage18. This NH3sensor28is a sensor that detects the ammonia concentration and supplies a detection signal indicative of the detected ammonia concentration to an ECU51. Incidentally, a NOXsensor may be used instead of the NH3sensor28because NOXsensors can also detect the ammonia concentration.

The ECU51includes a CPU, ROM, RAM, and an A/D converter and the like, none of which are shown. The ECU51functions as rich controlling means and controlling means (i.e., injection controlling means) and executes RS control and urea injection control, similar to the ECU50described above. In particular, the ECU51ends the urea injection control based on the ammonia concentration obtained from the NH3sensor28. More specifically, the ECU51ends the urea injection control when the ammonia concentration starts to rise.

Next, a control method according to the second example embodiment will be described in detail with reference toFIGS. 8A to 8D.FIG. 8Ashows the on/off state of the RS control,FIG. 8Bshows the air-fuel ratio,FIG. 8Cshows the on/off state of the urea injection control, andFIG. 8Dshows the ammonia discharge amount.

In this example embodiment, the urea injection control starts from time t21(seeFIG. 8C) and the RS control starts from time t22(seeFIGS. 8A and 8B). When the urea injection control is executed as shown inFIG. 8C, the ammonia discharge amount starts to increase at time t23, as is shown inFIG. 8D. In the second example embodiment, the urea injection control ends when the ammonia discharge amount starts to increase in this manner, as shown inFIG. 8C. As a result, the ammonia discharge amount becomes substantially 0 after time t23, as shown inFIG. 8D. This indicates that leakage of ammonia from the NOXstorage-reduction catalyst22is being suppressed.

Next, a routine according to the second example embodiment will be described with reference to the flowchart shown inFIG. 9. This routine is repeatedly executed at predetermined cycles by the ECU51described above. Incidentally steps S201to S211shown inFIG. 9are the same as steps S101to S111shown inFIG. 6so a description of these steps will be omitted. Here, the steps from step S212on in this routine will be described.

In step S212, the ECU51reads the output (referred to as “VNH3”) from the NH3sensor28. VNH3corresponds to the ammonia concentration in the exhaust passage18downstream of the NOXstorage-reduction catalyst22. When this step ends, the process proceeds on to step S213.

In step S213, the ECU51determines whether there is a demand to end the urea injection control. In this case, the ECU51determines whether VNH3is greater than a predetermined value γ or whether the urea injection period has exceeded the predetermined period of time β. That is, the ECU51determines whether the urea injection control should be ended by determining whether the ammonia concentration is starting to rise and determining whether enough of the reducing agent to reduce NOXis being supplied to the NOXstorage-reduction catalyst22. Incidentally, the predetermined period of time β is set according to the method described above.

If VNH3is greater than the predetermined value γ or the urea injection period exceeds the predetermined period of time β (i.e., Yes in step S213), the process proceeds on to step S214. In this case, it may be said that ammonia is starting to leak from the NOXstorage-reduction catalyst22or there is enough of the reducing agent to reduce NOXbeing supplied to the NOXstorage-reduction catalyst22. Therefore, in step S214the ECU51ends the urea injection control and the process proceeds on to step S215. On the other hand, if VNH3is equal to or less than the predetermined value γ and the urea injection period has not exceeded the predetermined period of time β (i.e., No in step S213), the routine immediately ends. In this case, the urea injection control is continued.

In step S215, the ECU51reads the RS control period, after which the process proceeds on to step S216. In step S216the ECU51determines whether it is time to end the RS control based on the RS control period obtained in step S215. More specifically, the ECU51determines whether the RS control period has exceeded the RS period obtained in step S203.

If it is time to end the RS control (i.e., Yes in step S216), the process proceeds on to step S217. In step S217, the ECU51ends the RS control, after which the routine immediately ends. If, on the other hand, it is not time to end the RS control (i.e., No in step S216), the routine immediately ends. In this case, the RS control is continued.

With the routine according to the second example embodiment described above, the urea injection control ends when the ammonia concentration starts to rise so it is possible to more reliably prevent ammonia from leaking out of the NOXstorage-reduction catalyst22.

Third Example Embodiment

Next, a third example embodiment of the invention will be described. In the foregoing second example embodiment, only the urea injection control ends when the ammonia concentration downstream of the NOXstorage-reduction catalyst22starts to rise. However, in the third example embodiment, the RS control as well as the urea injection control ends when the ammonia concentration starts to rise. This is because it is conceivable that NOXreduction in the NOXstorage-reduction catalyst22has substantially ended when the ammonia concentration starts to rise so it is not necessary to execute the RS control or the urea injection control. That is, it is not necessary to continue to supply the reducing agent according to the RS control.

Here, a control method according to the third example embodiment will be described in detail with reference toFIGS. 10A to 10E.FIG. 10Ashows the on/off state of the RS control,FIG. 10Bshows the air-fuel ratio,FIG. 10Cshows the fuel injection quantity,FIG. 10Dshows the on/off state of the urea injection control, andFIG. 10Eshows the ammonia discharge amount.

In this example embodiment, the urea injection control starts from time t31(seeFIG. 10D) and the RS control starts from time t32(seeFIGS. 10A to 10C). When the urea injection control is executed as shown inFIG. 10D, the ammonia discharge amount starts to rise at time t33, as is shown inFIG. 10E. In the third example embodiment, the urea injection control ends when the ammonia discharge amount starts to increase in this manner, as shown inFIG. 10D. As a result, the ammonia discharge amount becomes substantially 0 after time t33, as shown inFIG. 10E. Furthermore, in the third example embodiment, the RS control ends, as shown inFIGS. 10A to 10C, when the ammonia discharge amount starts to rise (time t33). In this case, the NOXreduction in the NOXstorage-reduction catalyst22has ended so it is not necessary to supply the reducing agent according to the RS control, thus the RS control ends.

Next, a routine according to the third example embodiment will be described with reference to the flowchart inFIG. 11. This routine is repeatedly executed at predetermined cycles by the ECU51described above (seeFIG. 7). Incidentally, steps S301to S313shown inFIG. 11are the same as steps S201to S213shown inFIG. 9so descriptions thereof will be omitted. Here, the steps from step S314on in this routine will be described.

Step S314is executed when VNH3is greater than the predetermined value γ or the urea injection period has exceeded the predetermined period of time β (i.e., Yes in step S313). In this case, it may be said that the conditions are such that the urea injection control should be ended. Accordingly, in step S314, the ECU51ends the urea injection control. Then the process proceeds to step S315.

In step S315, the ECU51ends the RS control. In this case, NOXreduction in the NOXstorage-reduction catalyst22has substantially ended so it is no longer necessary to execute the RS control. Accordingly, the RS control ends immediately after the urea injection control ends. After this step has ended, the routine ends.

With the foregoing routine according to the third example embodiment, the RS control is ended when the ammonia concentration starts to rise, which makes it possible to suppress a deterioration in fuel efficiency and the like due to the RS control.

Modified Example

Heretofore, example embodiments have been described in which control for injecting urea (i.e., urea injection control) is performed to ensure that a NOXpurifying component (i.e., ammonia) capable of purifying NOXis present in the NOXstorage-reduction catalyst22. However, the invention is not limited to this. In another example, it is possible to perform control for injecting one or more of urea, ammonia, and fuel (HC) instead of injecting only urea. In this case as well, it is possible to ensure that an appropriate amount of NOXpurifying component is present in the NOXstorage-reduction catalyst22.

Also, in the example embodiments described above, both urea injection control and RS control are executed to reduce NOXin the NOXstorage-reduction catalyst22. However, the invention is not limited to this. In another example, only the urea injection control can be executed without executing the RS control in order to reduce NOXin the NOXstorage-reduction catalyst22. In this case, it is possible to suppress torque shock (in particular, shock that may occur due to a torque step which is caused by a difference in output when the air-fuel ratio changes from lean to rich or from rich to lean) and the like caused by the RS control.

Furthermore, in the example embodiments described above, the urea injection control starts before the RS control starts. However, the invention is not limited to this. That is, as long as ammonia is present in the NOXstorage-reduction catalyst22before the reducing agent reaches the NOXstorage-reduction catalyst22, the urea injection control does not have to be started before the RS control is started. For example, the urea injection control can be executed at the same time as or after the RS control is started when ammonia is already present in the NOXstorage-reduction catalyst22.