Patent Application
20110214417
The present invention relates generally to exhaust gas purification systems for internal combustion engines, and more particularly, to an internal combustion engine exhaust gas purification system including an NOx storage reduction catalyst and an NOx selective reduction.
A system including an NOx storage reduction catalyst (hereinafter, referred to as the NSR catalyst) in the exhaust passageway of an internal combustion engine, as disclosed in Japanese Laid-Open Patent Application Publication JP-2001-271679A, is traditionally known. The NSR catalyst has a catalytic function that stores into the catalyst the nitrogen oxides (NOx) contained in combustion gas emissions from the internal combustion engine, and a catalytic function that purifies NOx and hydrocarbons (HC). The exhaust gases containing a large amount of NOx are emitted during the operation of the internal combustion engine under a lean air-fuel mixture atmosphere. The NSR catalyst stores the NOx into the catalyst and thus suppresses the situation of the NOx being released to the downstream side relative to the catalyst.
The NOx that has been stored into the NSR catalyst is next purified in predetermined timing. More specifically, the above conventional system executes rich-spike control to temporarily release unburnt constituents from the internal combustion engine. The NOx and unburnt constituents in the NSR catalyst are thus made to react in the catalyst.
Upon a large quantity of unburnt constituents being released from the internal combustion engine by the start of rich-spike control, the exhaust gases emitted to the downstream side relative to the NSR catalyst will be a stoichiometric atmosphere while NOx to be reduced by the unburnt constituents remain in the catalyst. After that, upon the reduction of the NOx stored within the catalyst, the unburnt constituents will be released to the downstream side of the catalyst, thus changing the exhaust gases into a rich atmosphere. In the above conventional system, such a change of the exhaust gases on the downstream side of the catalyst into a rich atmosphere is detected from an oxygen concentration and the NOx concentration, and in this detection timing, rich-spike control is terminated. This prevents the execution of excessive rich-spike control and hence suppresses the degradation of fuel economy.
The above-discussed conventional system is claimed Lo execute rich-spike control to treat the NOx stored within the NSR catalyst. During the execution of rich-spike control, part of the stored NOx may become released to the downstream side of the catalyst. That is to say, when rich-spike control is executed and the unburnt constituents that are a reductant are introduced into the NSR catalyst, the stored NOx will become desorbed to react on the catalyst. The NOx desorbed, however, will include that blown through the NSR catalyst to the downstream side thereof without being purified on the catalyst. The blow-through of the NOx is considered to be suppressible by expanding the capacity of the catalyst for an increased reaction field size. Expanding the catalyst capacity, however, causes a significant increase in costs due to an increase in noble metal content. A system capable of suppressing the degradation of emissions due to the blow-through of NOx without involving the expansion of a catalyst capacity has been desired for such a reason.
The present invention has been made for solving the foregoing problem, and an object of the invention is to provide an exhaust gas purification system for an internal combustion engine, adapted to suppress degradation of emissions due to blow-through of NOx.
In order to attain the above object, a first aspect of the present invention is an exhaust gas purification system for an internal combustion engine which operates in lean-burn mode, the system comprising: an NOx storage reduction catalyst (hereinafter, referred to as an NSR catalyst) disposed in an exhaust passageway of the internal combustion engine; an NOx selective catalytic reduction catalyst (hereinafter, referred to as an SCR) disposed on a downstream side relative to the NSR catalyst; and rich- spike means for executing rich-spike control at a predetermined time interval during lean-burn operation. In the system, the rich-spike means includes: first-rule acquisition means for acquiring as a first rule a relationship between the time interval and an amount of NH3 generated in the NSR catalyst during rich-spike control; second-rule acquisition means for acquiring as a second rule a relationship between the time interval and an amount of NOx blown through the NSR catalyst to the downstream side thereof during rich-spike control; and time interval setting means for setting the time interval on the basis of the first rule and the second rule so that a total of the amount of NOx blown through will be reduced and purified in the SCR.
A second aspect of the present invention is that the time interval setting means in the first aspect sets the time interval so that a ratio of the amount of NH3 generation to the amount of NOx blow-through is greater than a predetermined ratio.
A third aspect of the present invention is that the time interval setting means in the first or second aspect sets the time interval in a range that an increase rate of the amount of NH3 generation in the first rule becomes larger than a predetermined rate.
A fourth aspect of the present invention is that the time interval setting means in the first or second aspect sets the time interval in a range that the amount of NH3 generation in the first rule becomes larger than a predetermined amount.
During lean-burn operation of an internal combustion engine, the NOx that has been emitted from the engine is stored into an NSR catalyst (NOx storage reduction catalyst). When rich-spike control is next executed, the NSR catalyst purifies the stored NOx and generates NH3. The generated NH3 flows through an exhaust passageway and is stored into an NOx selective catalytic reduction (SCR) disposed on a downstream side. The SCR then uses the stored NH3 to selectively reduce the NOx blown through the NSR catalyst to the downstream side thereof.
The execution of rich-spike control at longer intervals causes a greater amount of NOx to be stored into the NSR catalyst. As the time interval is extended, therefore, a greater amount of NH3 will be generated during rich-spike control. Meanwhile, as a greater amount of NOx is stored into the NSR catalyst, that is, as the time interval is extended, a greater amount of NOx will be blown through the NSR catalyst during rich-spike control. According to the first aspect of the present invention, on the basis of the first rule, that is, the relationship between the time interval and the amount of NH3 generated, and the second rule, that is, the relationship between the time interval and the amount of NOx blown through, the time interval is set so that the SCR will reduce and purify the total amount of NOx blown through. According to the invention, therefore, since the SCR purifies the total amount of NOx blown through, the degradation of emissions is effectively suppressed.
According to the second aspect of the present invention, the time interval of the rich-spike control is set so that the ratio of the amount of NH3 generation to the amount of NOx blow-through is greater than the predetermined value. According to the invention, therefore, a shortage of NH3 in the SCR is avoided and thus a situation under which the system could fail to reduce and purify the total amount of NOx blown through is effectively avoided.
According to the third aspect of the present invention, the time interval of the rich-spike control is set in the range that the increase rate of the amount of NH3 generated in the first rule will be greater than the predetermined value. According to the invention, therefore, since a large amount of NH3 can be stored into the SCR, a situation under which the system could fail to reduce and purify the total amount of NOx blown through is effectively avoided.
According to the fourth aspect of the present invention, the time interval of the rich-spike control is set in the range that the increase rate of the amount of NH3 generated in the first rule will be greater than the predetermined value. According to the invention, therefore, since a desired amount of NH3 can be stored into the SCR, a situation under which the system could fail to reduce and purify the total amount of NOx blown through is effectively avoided.
Hereunder, an embodiment of the present invention will be described in accordance with the accompanying drawings. Elements common to each drawing are each assigned the same reference number or symbol, and overlapped description of the common elements is omitted. In addition, the following embodiment does not limit the invention.
Starting catalysts (hereinafter, termed “the SCs”) 141 and 142 are arranged as three-way catalysts in the exhaust passageways 121 and 122, respectively. In addition, an NOx storage reduction catalyst (hereinafter, termed “the NSR catalyst”) 16 is disposed in the exhaust passageway 123 positioned on a downstream side relative to the SCs 141 and 142. Furthermore, an NOx selective catalytic reduction catalyst (hereinafter, termed the “SCR”) 18 is disposed on a downstream side relative to the NSR catalyst 16 in the exhaust passageway 123. Hereinafter, unless otherwise distinguished, the SCs 141 and 142 will be collectively termed “the SC 14”.
The internal combustion engine 10 tends to emit HC and CO under a rich air-fuel mixture atmosphere. The engine 10 tends to emit NOx under a lean air-fuel mixture atmosphere. Under the lean atmosphere, the SC 14 reduces NOx (purifies to N2) while adsorbing oxygen (O2). Under the rich atmosphere, the SC 14 oxidizes HC and CO (purifies to H2O and CO2) while releasing oxygen. Additionally, under the rich atmosphere, nitrogen contained in exhaust gases reacts with hydrogen to generate ammonia (NH3).
Under the lean atmosphere, the NSR catalyst 16 stores the NOx contained in the exhaust gases. The NSR catalyst 16 also releases the NOx stored in the catalyst under the rich atmosphere. The NOx that has been released under the rich atmosphere is reduced via HC and CO. At this time, as with the SC 14, the NSR catalyst 16 generates NH3.
The SCR 18, constructed as an iron(Fe)-based zeolite catalyst, stores the NH3 generated under the rich atmosphere by the SC 14 and the NSR catalyst 16. Under the lean atmosphere, the SCR 18 also functions to use NH3 as a reductant to selectively reduce the NOx contained in the exhaust gases. A situation in which the NH3 and NOx blown through the NSR catalyst 16 to the downstream side thereof will be released to the air can be effectively avoided by using the SCR 18.
The system shown in
The system according to the present embodiment further includes an electronic control unit (ECU) 30 as shown in
(Rich-spike control)
First, rich-spike control that the system of the present embodiment executes is described below referring to
The ECU 30 usually operates the internal combustion engine 10 under a lean air-fuel mixture atmosphere (this operation mode is referred to as lean operation). During lean operation, NOx and other oxidants are emitted in larger quantities than HC, CO, and other reductants. For this reason, even in an attempt to use a three-way catalyst to purify the exhaust gases, not all NOx can be purified if a shortage of reductants occurs. Accordingly, the system of the present first embodiment includes the NSR catalyst 16 in the exhaust passageway 123. As shown in section (A) of
The NOx storage performance of the NSR catalyst 16, however, decreases with increases in the amount of NOx stored. This blows non-stored NOx through the catalyst to the downstream side thereof if lean operation is continued over a long time. Accordingly, the system of the present first embodiment executes the rich-spike control in which the NOx stored within the NSR catalyst 16 is treated by periodic desorbing. More specifically, a rich exhaust air-fuel (A-F) mixture is temporarily created in the internal combustion engine 10 by, for example, obtaining a rich A-F ratio of 12:1 in predetermined timing of the NSR catalyst 16 decreasing in NOx storage performance. As shown in section (B) of
(NOx purification by SCR)
Next, functions of the SCR 18 are described below. As described above, the NOx storage performance of the NSR catalyst 16 is effectively restored to the original level by the execution of the rich-spike control. When the rich-spike control is executed, however, part of the NOx desorbed from the NSR catalyst 16 will be directly blown through to the downstream side without being purified. Additionally, as described above, there will exist the NOx blown through to the downstream side without being stored into the NSR catalyst 16 during lean operation. Emissions will degrade if these blown-through NOx substances are released intact to the air.
For this reason, the system of the present first embodiment includes the SCR 18 for treating the NOx blown through the NSR catalyst 16 to the downstream side thereof. As described above, the SCR 18 internally holds the NH3 generated under the rich atmosphere by the SC 14 and the NSR catalyst 16. This enables the SCR 18 to selectively reduce with the NH3, and purify, the NOx blown through the NSR catalyst 16 to the downstream side thereof. The situation where emissions are degraded by the release of the NOx to the air is thus avoided effectively.
In the present inventor's view, active reducing reactions in the SCR 18 can be generated by controlling a bed temperature of the SCR 18 to be equal to or less than 500° C., preferably, around 300° C. In the system of the present first embodiment, the SCR 18 has its position adjusted for a bed temperature around 300° C., whereby the situation of NOx being released to the downstream side of the SCR 18 is effectively suppressed.
Next, characteristic operation of the present embodiment is described below with reference to
In order for the SCR 18 to purify a total amount of NOx blown through, it is demanded that the system should generate large enough an amount of NH3 to purify the total amount of NOx. Under an environment heavily laden with NOx and reduced in catalyst activity, NH3 is generated in large quantities, such that extending the time interval of the rich-spike control is considered to be a useful method for effectively increasing the amount of NH3 generated. This method prolongs a learn operation period, thus increasing the amount of NOx stored into the NSR catalyst 16 and hence the amount of NH3 generated.
Extending the time interval, however, results in an increased amount of NOx blow-through. This is because, as described above, the increase in the amount of NOx storage into the NSR catalyst 16 also increases the amount of NOx blow-through. For effective purification of the blown-through NOx, therefore, a need arises to assign a time interval that allows for a balance between the amount of NH3 generation and that of NOx blow-through.
After repeating energetic studies with focus placed on the relationship between the time interval of the rich-spike control and the amount of NH3 generated, the present inventor has found that there exists an interval period during which the amount of NH3 generation suddenly increases.
Next,
In this way, the relationship between the amount of NOx blow-through and that of NH3 generation changes according to the particular setting of the rich-spike control time interval. Accordingly, the system of the present first embodiment sets the time interval of the rich-spike control so that the above-described relationship in
Further extending the time interval causes too much storage of NOx into the NSR catalyst 16, resulting in an extreme increase in the amount of NOx blow-through. In order to avoid this, the time interval is desirably set to stay within a range that the amount of NH3 generation does not run short for that of NOx blow-through.
In the system of the present first embodiment, while a time interval in the region of the NH3 generation quantity abruptly increasing is set to avoid its shortage, this does not limit the way of setting the time interval. That is to say, the region in which the amount of NH3 generation does not run short for that of NOx blow-through may be identified by computing a ratio of the two quantities from the relationships shown in
In addition, the system of the present first embodiment provides feed-forward control of the time interval, based on the relationships shown in
In the above-described first embodiment, the NSR catalyst 16 is equivalent to the NSR catalyst used in the first aspect of the present invention, and the SCR 18 to the SCR used in the first aspect of the invention. A solid line in
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP2009/069281 | 11/12/2009 | WO | 00 | 2/28/2011 |
