DE10011612A1 - Emission control device for an internal combustion engine - Google Patents

Abstract

A NOx absorption-reduction catalyst (3) that absorbs NOx from the exhaust gas when the air-fuel ratio of the exhaust gas is on the lean side, and releases the NOx and causes a reduction of NOx when the oxygen concentration in the exhaust gas decreases , is provided in an exhaust pipe (2) of a lean-burn internal combustion engine (1). A selective urea reduction catalyst (4), which effects a selective reduction due to the addition of urea, is also provided. The two catalysts clean the emissions complementarily to one another essentially over the entire operating range of the internal combustion engine.

Description

The present invention relates to a Emission control device for an internal combustion engine and in particular to a device for essential disposal of NOx and the like from the exhaust gas generated from a Lean-burn engine is ejected.

An emission control device for one Lean-burn engine is used, for example, in the Japanese Patent No. 2605580.

The emission control device described in the Japanese patent is described in an exhaust line Absorbent that absorbs NOx when the air Fuel ratio of the exhaust gas that flows in on the lean side of the theoretical air-fuel ratio and that the absorbed NOx releases when the Oxygen concentration in the incoming exhaust gas decreases. To the Decreasing oxygen concentration in the exhaust gas leads to that Device by a greased pulse control in which Fuel is injected into the internal combustion engine to produce unburned gas (reducing agent). The means that due to the enriched pulse control Reducing agent for reducing NOx above the Internal combustion engine is supplied to the NOx absorbent.

Regarding an emission control device, the one It delivers reducing agents from an internal combustion engine desirable that if the internal combustion engine is a lean engine is the internal combustion engine in a lean burning Condition operated, even at high speeds and high Charges. However, it is in such an operating condition impossible the reducing agent by the enriched pulse to deliver to the NOx absorbent (NOx catalyst).  

To achieve a fuel-rich air-fuel ratio it is necessary to reduce the amount of intake air reduce a throttle valve opening. However, if one such a fuel-rich condition is established when the Internal combustion engine at high speed and high The load is operating in the lean-burn mode Fuel combustion is hampered so that smoke is generated becomes.

To solve this problem, the lean Fuel combustion can be abandoned and instead can the stoichiometric air-fuel ratio (theoretical Air-fuel ratio). This solution away from improving frugal, however Dealing with fuel caused by lean combustion was achieved, d. H. by adjusting a lean burn Internal combustion engine.

It may also be conceivable to use a selective one To provide a reduction catalyst, the HCs or H as Reducing agents used. However, the selective one achieves Reduction catalyst, the HCs or H as a reducing agent used in a high speed range and a high Load state only a low NOx removal rate.

Therefore, it was difficult to get NOx across the board Sufficiently remove the internal combustion engine operating area.

Accordingly, it is an object of the present invention to provide a Emission control device for an internal combustion engine create that is capable of NOx over one if possible wide operating range compared to the state of the art in to eliminate essential.

According to the invention, an emission control device for a Internal combustion engine characterized in that they have a NOx  Absorption-reduction catalyst device, which in an exhaust line of a lean-burn engine is provided where the NOx absorption reduction Catalyst device absorbs NOx when an air Fuel ratio of an exhaust gas from the internal combustion engine on the lean side of a theoretical air Fuel ratio, and the NOx absorption Reduction catalyst device releases NOx and one Reduction of NOx, if any Oxygen concentration in the exhaust gas decreases, and labeled through a selective ammonia compound reduction Catalyst device that undergoes selective reduction the addition of an ammonia compound.

The internal combustion engine to which the invention is applied is a lean-burn diesel or gasoline engine that contains a direct injection into the cylinder.

During the operation of the internal combustion engine in a lean Combustion state, the NOx absorption reduction Catalyst device unable to remove NOx because NOx, that is absorbed in a NOx absorbent, not is released, and therefore the NOx in the Absorbent not reduced. However, the selective one serves Ammonia compound reduction catalyst during one Operating state of the internal combustion engine in the high-load range to substantially eliminate NOx. Therefore, the Emission control device of the invention the operating range, where NOx is eliminated compared to a device which a NOx absorption reduction catalyst and otherwise no other catalyst used.

Although the device of the invention is intended typically to be embodied in a construction in which a reducing agent via the internal combustion engine to the NOx absorption reduction catalyst is supplied Invention further applicable to a device of a type  in which a reducing agent is supplied in an exhaust line that is connected to an internal combustion engine without, that there is some problem with it.

The emission control device can also be a Operating state detection device for detecting a Have operating state of the internal combustion engine, and a Switching device for changing the flow direction of the Exhaust gas either to the NOx absorption reduction Catalyst device or the selective Ammonia compound catalyst device depending on the operating state of the internal combustion engine by the Operating state detection device was detected.

For example, if the operating status of the Internal combustion engine, which is affected by the operating Detection device is detected, a condition that below a predetermined high speed and one predetermined high load, the NOx absorption Reduction catalyst selected and if the detected Operating state of the internal combustion engine a state with the predetermined high speed and the predetermined high Load exceeds the selective Ammonia compound reduction catalyst device selected.

The operating state to be recorded is one Operating range in which the NOx absorption reduction Catalyst is unable to reduce perform. In the invention, when the Internal combustion engine in a predetermined high load and high-speed operating condition is considered, the Reduction is impracticable and the emissions regulation will carried out by the selective ammonia compound Reduction catalyst device is used. To the It is to detect the aforementioned operating state therefore possible to use parameters directly or  indirectly the impracticable reduction range of the NOx Absorption-reduction catalyst show how for example the engine speed and / or the engine load, or the air intake amount, or the degree of throttle opening or similar.

The NOx absorption reduction catalyst and the selective one Ammonia compound reduction catalyst can be used in the Exhaust line can be arranged in series.

With this arrangement it is possible to use the selective Ammonia compound reduction catalyst downstream of the NOx absorption-reduction catalyst in the exhaust system to arrange. It is also possible to use the selective Ammonia compound reduction catalyst upstream of the NOx absorption-reduction catalytic converter in the exhaust system to arrange.

Together with the serial arrangement of the catalysts a bypass passage can be provided that a bypasses upstream catalyst and the exhaust gas to downstream catalyst conducts. The flow direction of the Exhaust gas is changed by a switching device that the Bypass passage opens and closes.

The NOx absorption reduction catalyst and the selective one Ammonia compound reduction catalyst can also be used be arranged in parallel in the exhaust line.

If the catalysts are arranged in parallel, can also the emission control device have a construction in which the exhaust line into a first exhaust line and one second exhaust line forks, the NOx absorption Reduction catalyst is arranged in the first exhaust line, and the selective ammonia compound reduction catalyst is arranged in the second exhaust line, wherein a Changeover valve as changeover device at the branch point  between the first exhaust line and the second exhaust line is arranged. Depending on the operating status at least one of the first and second exhaust lines through the Switch valve selected.

The construction described above enables the NOx Absorption-reduction catalyst and the selective Complementary ammonia compound reduction catalyst towards each other depending on the operating state function. Therefore, the emission control device is in able to regulate emissions across the widest possible Operating area compared to a Device that is only one of the emission control catalysts used.

The emission control device may further include an added one Ammonia compound amount determination device included to to estimate an amount of the ammonia compound corresponding to the selective ammonia compound reduction catalyst must be added based on an amount of NOx that in the exhaust gas that is present in the selective ammonia compounds Reduction catalyst flows, and an intake air amount that is sucked into the internal combustion engine. That's why it will possible an amount of ammonia compound added should be easy to determine.

The emission control device can also be a Ammonia compound detection device included to a Ammonia compound coming from the selective Ammonia compound reduction catalyst is released, too detect, and a control device for correcting a quantity the ammonia compound added in an appropriate amount based on an amount of the ammonia compound, detected by the ammonia compound detection device has been. Therefore it becomes possible to regulate the Amount of ammonia compound to be added is more accurate  regulate, and therefore the emissions regulation more effectively perform.

Examples of the ammonia compound, i.e. i.e. one Reducing agent that together with the selective Ammonia compound reduction catalyst is used contain urea, ammonium carbamate and the like.

The constructions according to the invention described above can essentially in some way can be combined with each other.

The above task and features and advantages of present invention will become apparent from the following Description of preferred embodiments under Reference to the accompanying drawings, the same reference numerals are used to denote the same To represent elements.

Fig. 1 is a schematic representation of a first embodiment of the emission control device for an internal combustion engine of the invention.

Fig. 2A is a diagram showing a NOx absorption and - shows release, which are performed by the NOx absorption-reduction catalyst, wherein the air-fuel ratio of the stoichiometric air-fuel ratio state of the exhaust gas flowing to the lean side.

Fig. 2B is a graph showing the NOx-absorbing and - shows release caused by the will NOx absorption-reduction catalyst performed wherein the air-fuel ratio of the inflowing exhaust gas at the stoichiometric air-fuel ratio or to the rich side thereof lies.

FIG. 3 is a diagram schematically showing the concentrations of unburned HCs, CO and oxygen in the exhaust gas discharged from the internal combustion engine.

Fig. 4 is a diagram showing the emission control ranges of the catalysts in relation representing the engine speed and engine load.

Fig. 5 is a diagram showing a relationship between the exhaust gas temperature and the emission control rate.

Fig. 6 shows a second embodiment of the invention.

Fig. 7 shows a third embodiment.

Fig. 8 illustrates a fourth embodiment.

Preferred embodiments of the Invention in detail with reference to the accompanying Described drawings. Use the embodiments Urea as an ammonia compound.

A first embodiment of the invention will be described with reference to FIGS. 1 to 5.

Referring to FIG. 1, a NOx absorption-reduction catalyst 3 and a selective Urea-reduction catalyst 4, that is a selective ammonia compound reduction catalyst, arranged in series in an exhaust pipe 2 of a direct-injection lean burn gasoline engine 1.

With regard to the internal combustion engine 1 , a fuel injection period TAU is calculated according to the following equation:

TAU = TP. K

In the above equation, TP represents one Base fuel injection duration and K represents one Correction coefficients. The basic fuel injection duration TP is a fuel injection period that is needed to the theoretical air-fuel ratio in the mixture that delivered in the cylinder to achieve. The Base fuel injection duration TP is previously determined by experiments is determined and stored in a ROM as a function of engine load Q / N (amount of intake air Q / engine speed N) and the Engine speed N previously stored in the form of a table.

The correction coefficient K is a coefficient for control the air-fuel ratio of the mixture contained in the Cylinder of the internal combustion engine is delivered. If K = 1.0, then the air-fuel ratio of the mixture that is in the cylinder of the internal combustion engine is delivered, equal to that theoretical air-fuel ratio. If K <1.0, it will Air-fuel ratio of the mixture in the cylinder of the internal combustion engine is delivered larger than that theoretical air-fuel ratio, d. that is, the mixture skinny. When K becomes <1.0, the air-fuel ratio becomes of the mixture in the cylinder of the internal combustion engine is delivered less than the theoretical air Fuel ratio, d. that is, the mixture becomes fat.

In the engine 1 , the correction coefficient K is set to a value less than 1.0 to perform lean air-fuel ratio control in a low to medium engine load operating range. In a high-load operating range of the engine, during the warm-up phase of the engine after starting, during acceleration or during a constant speed of the vehicle, for example, 120 km / h or more, the correction coefficient K is set to 1.0 to perform stoichiometric operation. In a full-load operating range of the internal combustion engine, the correction coefficient K is set to a value larger than 1.0 to perform rich air-fuel ratio control.

Usually the internal combustion engine is most common at operated low to medium loads so that mostly during the operation of the internal combustion engine Correction coefficient K is set to less than 1.0 and therefore a lean fuel mixture is burned.

The NOx absorption-reduction catalyst 3 has a carrier, for example an aluminum carrier, which is coated with a noble metal such as platinum Pt or the like and with at least one element from the group of, for example, alkali metals, such as potassium (K), sodium (Na) , Lithium (Li), cesium (Cs) and the like, is selected from alkaline earths such as barium (Ba), calcium (Ca) and the like and from rare earths such as lanthanum (La), yttrium (Yi) and the like. Hereinafter, the ratio between the air and the fuel (hydrocarbons) supplied into the intake passage of the internal combustion engine 1 and into a portion of the exhaust line upstream of the NOx absorption-reduction catalyst 3 is referred to as the air-fuel ratio of the exhaust gas, that flows into the NOx absorption-reduction catalyst 3 . When the air-fuel ratio of the inflowing exhaust gas is on the lean side of the theoretical air-fuel ratio, the NOx absorption-reduction catalyst 3 absorbs the NOx. When the oxygen concentration in the inflowing exhaust gas decreases, the NOx absorption-reduction catalyst 3 releases the absorbed NOx.

In a case where no fuel (hydrocarbons) or no air is supplied to the portion of the exhaust line upstream of the NOx absorption-reduction catalyst 3 , the air-fuel ratio of the inflowing exhaust gas is equal to the air-fuel ratio of the mixture that is delivered to the combustion chamber. In this case, therefore, the NOx absorption reduction catalyst 3 absorbs the NOx when the air-fuel ratio of the mixture supplied into the combustion chamber is on the lean side of the theoretical air-fuel ratio and the NOx absorption reduction -Catalyst 3 releases the absorbed NOx when the oxygen concentration in the mixture supplied to the combustion chamber decreases.

It is considered that the absorption and reduction of the NOx caused by the NOx absorption-reduction catalyst 3 occurs by a mechanism as shown in Figs. 2A and 2B. Although the mechanism is illustrated in the case of a catalyst device in which a support is loaded with platinum (Pt) or barium (Ba), the same mechanism applies essentially to a catalyst which uses a noble metal other than platinum or an alkaline metal, a contains alkaline earth, or a rare earth.

When the exhaust gas becomes noticeably lean, the oxygen concentration in the exhaust gas increases considerably, so that oxygen (O ₂ ) is deposited on the surfaces of platinum (Pt) in the form of O ₂ ^- or O ² , as in Fig. 2A is shown. Nitrogen oxides No contained in the exhaust gas react with O ₂ ^- or O ^2- on the surfaces of platinum (Pt) to generate NO ₂ (2NO + O ₂ → 2NO ₂ ).

As long as the NOx absorption capacity of the NOx absorption-reduction catalyst 3 is not saturated, NO ₂ generated as described above is absorbed in the catalyst while oxidation takes place on the platinum (Pt) and binds Barium oxide (BaO) and then diffuses into the NOx absorption-reduction catalyst 3 in the form of nitrate ions (NO ₃ ^- ). In this way, NOx is absorbed in the NOx absorption-reduction catalyst 3 .

However, when the oxygen concentration in the exhaust gas decreases, the production of NO ₂ also decreases, so that through a reaction in the opposite direction to that described above, nitrate ions (NO ₃ ^- ) from the NOx absorption-reduction catalyst 3 in the form of NO ₂ or NOx are released.

That is, NOx is released from the NOx absorption-reduction catalyst 3 when the oxygen concentration in the exhaust gas decreases. As shown in FIG. 3, the oxygen concentration in the incoming exhaust gas decreases as the degree of leanness of the exhaust gas flowing into the NOx absorption-reduction catalyst 3 decreases. That is, by reducing the leanness of the incoming exhaust gas, NOx is released from the NOx absorption-reduction catalyst 3 even if the air-fuel ratio of the incoming exhaust gas is on the lean side of the theoretical air-fuel ratio.

When the air-fuel ratio of the mixture supplied into the combustion chamber changes to the stoichiometric air-fuel ratio or the rich side thereof, and accordingly the air-fuel ratio of the exhaust gas changes to the stoichiometric air-fuel ratio or the rich side thereof large amounts of unburned HCs and CO are emitted from the internal combustion engine, as shown in FIG. 3. In this case, unburned HCs and CO oxidize directly through reactions with O ₂ ^- or O ² on the platinum (Pt).

Furthermore, the oxygen concentration in the exhaust gas becomes very low when the air-fuel ratio of the exhaust gas flowing into the NOx absorption-reduction catalyst 3 shifts to the stoichiometric air-fuel ratio or the rich side thereof, so that the NOx absorption - Reduction catalyst 3 releases NO ₂ or NO. The NO ₂ or NO released in this way is reduced by the reactions with unburned HCs and CO, as shown in Fig. 2B. If NO ₂ or NO on platinum (Pt) is removed in this way, the catalyst releases NO ₂ or NO successively. Therefore, the NOx absorption-reduction catalyst 3 releases NOx in a short time by changing the air-fuel ratio of the exhaust gas flowing into the NOx absorption-reduction catalyst 3 to the rich side of the theoretical air-fuel ratio . If unburned HCs and CO remain after the O ₂ ^- or O ^2- consumed on the platinum (Pt), NOx that has been released from the NOx absorption-reduction catalyst 3 and NOx from the internal combustion engine 1 is emitted, reduced by the remaining unburned HCs and CO.

Therefore, the NOx absorbed in the NOx absorption-reduction catalyst 3 is released in a short time by shifting the air-fuel ratio of the exhaust gas that flows into the NOx absorption-reduction catalyst 3 toward the rich side. Furthermore, the NOx released in this way is reduced, so that the emission of NOx into the atmosphere is prevented.

Further, NOx released from the NOx absorption-reduction catalyst 3 can also be reduced to the theoretical air-fuel ratio by shifting the air-fuel ratio of the exhaust gas flowing into the catalyst 3 because the NOx absorption ratio Reduction catalyst 3 also has the function of a reduction catalyst. However, when the air-fuel ratio of the inflowing exhaust gas is shifted to the theoretical air-fuel ratio, the NOx absorption-reduction catalyst 3 releases the NOx only gradually, so that it takes a long time to remove the total amount of the NOx Absorption-reduction catalyst 3 to release absorbed NOx.

By reducing the leanness of the exhaust gas flowing into the NOx absorption reduction catalyst 3 , NOx is released from the NOx absorption reduction catalyst 3 even if the air-fuel ratio of the exhaust gas flowing therein is on the lean side of the theoretical air-fuel ratio lies. Therefore, NOx can be released from the NOx absorption-reduction catalyst 3 by only reducing the oxygen concentration in the exhaust gas flowing into the catalyst 3 .

The selective urea reduction catalyst 4 is formed by adding urea to a NOx catalyst for selective reduction. An example of the NOx catalyst mentioned here is a zeolite catalyst which contains an oxide of a transition element of the fourth, fifth or sixth period and / or an oxide of a rare earth. A more preferred example is a catalyst in which Al ₂ O _{3 is} charged with Ti and V.

When an aqueous solution of the urea is added to the catalyst, oxides of nitrogen in the exhaust gas are reduced at a predetermined exhaust gas temperature according to the following reaction equations:

(NH ₂ ) 2CO + H ₂ O → 2NH ₃ ^x + CO ₂

4NH ₃ ^x + 4NO + O ₂ → 4N ₂ 6H ₂ O

In order to cause the NOx absorption-reduction catalyst 3 and the selective urea reduction catalyst 4 to function complementarily to one another, this exemplary embodiment uses a NOx sensor 5 and a urea addition control valve 6 , which are located in a section of the exhaust line 2 are provided, which is located downstream of the NOx absorption-reduction catalyst 3 and upstream of the selective urea reduction catalyst 4 . The NOx sensor 5 is arranged immediately downstream from the NOx absorption reduction catalyst 3 and upstream from the urea addition control valve 6 . Furthermore, a catalyst inflow gas temperature sensor 7 is arranged immediately upstream of the selective urea reduction catalyst 4 , and an ammonia sensor 8 is arranged downstream of the selective urea reduction catalyst 4 .

The NOx sensor 5 , the urea addition control valve 6 , the catalyst inflow gas temperature sensor 7 and the ammonia sensor 8 are electrically connected to a control unit (ECU) 9 , which is formed by a computer. An engine speed sensor for detecting the speed of the internal combustion engine 1 is also provided and connected to the control unit (ECU) 9 .

On the basis of the information from these sensors and the like, the states of the catalysts and therefore the operation of the internal combustion engine 1 are detected. An operating state detection device 10 for detecting the operating state of the internal combustion engine 1 on the basis of the data input from the sensors and the like is implemented in the computer of the control unit (ECU) 9 . Furthermore, a urea addition quantity control device 11 for issuing an urea addition instruction to the urea addition control valve 6 is implemented as a function of the detected operating state and under control of the amount of the added area in the computer of the control unit (ECU) 9 . A reducing agent indicator 12 which, when urea is added by the urea adding amount control device 11 , indicates to a driver that urea is added is provided in a display panel or the like 13 .

The NOx sensor 5 detects the concentration of NOx in the exhaust gas that is output from the NOx absorption-reduction catalyst 3 , that is, the NOx concentration in the exhaust gas that flows into the selective urea reduction catalyst 4 . The urea addition quantity control device 11 includes a urea addition quantity determination device for determining an amount of NOx emitted from the internal combustion engine 1 based on the NOx concentration detected by the NOx sensor 5 and the amount of air detected by an air flow meter ( not shown) is detected and is intended to estimate an amount of urea on the basis of the detected amount of NOx, which must be added to the selective urea reduction catalyst 4 . The urea adding amount control device 11 issues an instruction to add an amount of urea depending on the value estimated by the urea adding amount determining device.

In order to estimate the amount of urea, it is advantageous to do so save as a table in a ROM in advance, the one predetermined ratio between the detected amount of NOx and the amount of urea to be added.

Instead of using the NOx sensor 5 , it is also possible to use an accelerator depression degree (which can therefore be replaced by the fuel injection amount), the engine speed, and the amount of EGR (exhaust gas recirculation) provided by an EGR controller to control the Estimate the amount of NOx. The amount of air drawn into the engine 1 may also be determined based on the amount of throttle opening, instead of being sensed by the air flow meter.

The catalyst inflow gas temperature sensor 7 serves as a catalyst temperature detection device for detecting the temperature of the exhaust gas flowing into the selective urea reduction catalyst 4 . An activation state of the selective urea reduction catalyst 4 can be determined on the basis of the detected exhaust gas temperature.

When the temperature of the exhaust gas flowing into the catalyst 4 is relatively low, the exhaust gas control ability of the catalyst 4 is low, so that the amount of urea added by the urea adding amount control device 11 is reduced. A relationship between the temperature of the exhaust gas flowing into the catalyst 4 and the amount of urea added is previously stored as a table in the ROM.

The ammonia sensor 8 is used to correct the amount of urea to be added. That is, when ammonia is detected by the ammonia sensor 8 located downstream of the selective urea reduction catalyst 4 , the detection means that the amount of urea added is too large compared to the amount of NOx. Therefore, a feedback control device is provided to return the amount of ammonia detected by the ammonia sensor 8 back to the urea addition amount control device 11 , thereby correcting the amount of urea to be added to an appropriate target value. The feedback control device is implemented as a portion of the urea addition amount control device 11 in the computer of the control unit (ECU) 9 .

An emission control according to the Described embodiment.

As a result of the combustion of fuel in the cylinder during the operation of the internal combustion engine 1 , exhaust gas is discharged from the internal combustion engine 1 and flows through the exhaust pipe 2 sequentially over the NOx absorption-reduction catalyst 3 and the selective urea reduction catalyst 4 and then flows through the silencer not shown. Thus, exhaust gas is released into the atmosphere.

This exemplary embodiment cleans the emissions over the widest possible operating range of the internal combustion engine by means of the selective urea reduction catalyst 4 , which functions in a high-load and high-speed range which lies behind the emission control range of the NOx absorption-reduction catalyst 3 .

While the operating state of the internal combustion engine is not within a predetermined high-load and high-speed range, the emission control is performed by repeatedly causing the absorption and reduction of NOx in the NOx absorption-reduction catalyst 3 according to the principle described above. That is, when NOx is released from the NOx absorption-reduction catalyst 3 , the air-fuel ratio of the exhaust gas is shifted to the rich side so that NOx that is released is released through the NOx absorption-reduction catalyst 3 is reduced.

When the air-fuel ratio is set on the rich side, for example, by reducing the throttle valve opening to reduce the amount of intake air when the operation of the engine 1 is in the predetermined high-load and high-speed range, the reduced amount of intake air thus discharges (Amount of oxygen) a little unburned fuel, so that a smoke, commonly referred to as smoke, is generated. Therefore, the reduction of NOx in the predetermined high-load and high-speed operating range cannot be performed in the NOx absorption-reduction catalyst 3 .

In response to the operating state detection device 10 , which determines that the engine 1 is in the above-described operating state, the urea addition quantity control device 11 issues a urea addition instruction to the urea addition control valve 6 so that the urea aqueous solution is injected from the urea addition control valve 6 becomes. Therefore, the emission control is carried out according to the principle described above.

In the meantime, the concentration of NOx flowing out of the NOx absorption reduction catalyst 3 and not treated by it is detected by the NOx sensor 5 . As a result of the input of the value detected by the NOx sensor 5, the urea Zufügemengensteuerungsvorrichtung 11 determines the control unit (ECU) 9, the amount of NOx, the air aspirated by the engine 1 on the basis of the NOx concentration and the amount of the internal combustion engine 1 Air was expelled. Based on the thus determined amount of NOx, the urea addition amount control device 11 estimates an amount of urea to be added to the selective urea reduction catalyst 4 by using the urea addition amount determination device. The urea addition amount control device 11 then instructs the urea addition control valve 6 to add an amount of urea depending on the estimated value.

Furthermore, the temperature of the gas flowing into the NOx sensor 5 is measured by the catalyst inflow gas temperature sensor 7 . Depending on the magnitude of the detected inflow gas temperature, the urea addition amount control device 11 of the control unit (ECU) 9 changes the amount of urea to be added.

Furthermore, the ammonia concentration in the gas flowing out of the selective urea reduction catalyst 4 is detected by the ammonia sensor 8 . Based on the value detected by the ammonia sensor 8 , the feedback control device of the urea adding amount control device 11 corrects the amount of fuel to be added in such a direction that the ammonia concentration in the exhaust gas resulting from the selective urea reduction Catalyst 4 flows out, will decrease.

In the manner described above, the emission control is performed by the NOx absorption-reduction catalyst 3 and the selective urea reduction catalyst 4 . The complementary ratio between the NOx absorption reduction catalyst 3 and the selective urea reduction catalyst 4 is shown in FIG. 4. In Fig. 4, "A" indicates an area in which the NOx absorption-reduction catalyst 3 is capable of performing emission control, and "B" indicates an area in which the selective urea reduction catalyst 4 is able to perform emission control.

Fig. 5 shows relationships between the exhaust temperature and the control rate in the NOx absorption-reduction catalyst 3 and the selective reduction catalyst Urea. 4 In Fig. 5, "A" denotes an area in which the NOx absorption-reduction catalyst 3 is capable of performing emission control, and "B" indicates an area in which the selective urea-reduction catalyst 4 is capable is to perform emission control. As can be understood from FIG. 5, the NOx absorption-reduction catalytic converter 3 functions in a relatively low exhaust gas temperature range and the selective urea reduction catalytic converter 4 functions in a relatively high exhaust gas temperature range.

In this embodiment, the positional relationship between the NOx absorption reduction catalyst 3 and the selective urea reduction catalyst 4 can be reversed.

A second exemplary embodiment according to the invention is described with reference to FIG. 6.

The embodiment shown in FIG. 6 has a starting catalyst device 21 in addition to a construction as described in connection with the first embodiment. The starting catalyst device 21 is a NOx catalyst that is provided in a portion of an exhaust pipe 2 that is as close as possible to the internal combustion engine. The starting catalyst device 21 is capable of substantially removing NOx from the exhaust gas of the internal combustion engine before the NOx absorption-reduction catalyst 3 is warmed up after the internal combustion engine is started. Since the starting catalyst device 21 is arranged in a portion of the exhaust pipe 2 adjacent to the internal combustion engine, which extends upstream of a NOx absorption-reducing catalyst 3 , the starting catalyst device 21 is quickly heated to a control temperature range by the exhaust gas after the engine is started.

A third exemplary embodiment of the invention is described below with reference to FIG. 7.

An emission control device of the embodiment shown in FIG. 7 has a bypass line 31 that bypasses an upstream, side-mounted NOx absorption-reduction catalyst 3 and directs exhaust gas to a downstream catalyst, in addition to a construction as shown in FIG Fig. 1 is shown. A branch point between the bypass line 31 and an exhaust pipe 2 is provided with a switching device, that is, a switching valve 32 , which changes the exhaust gas flow path by closing and opening the bypass line 31 .

The changeover valve 32 is an electromagnetic valve that is electrically connected to a control unit (ECU) 9 and is thereby controlled. If it is determined by an operating state detection device 10 that the present operating range of an internal combustion engine is such a range in which the NOx absorption reduction catalyst 3 is able to function, the changeover valve 32 closes the bypass line 31 so that the exhaust gas flows to the NOx absorption-reduction catalyst 3 . When it determines that the operation of the engine has been shifted to a high-load and high-speed range, the switching valve 32 opens the bypass line 31 and closes the exhaust line that extends to the NOx absorption-reduction catalyst 3 so that the exhaust gas passes through the Bypass line 31 flows to the selective urea reduction catalyst 4 .

Therefore, the control unit (ECU) 9 instructs the changeover valve 32 to close the bypass line 31 when the operating state of the engine detected by the operating state detection device 10 is not the high-load and high-speed range, so that the exhaust gas is in the NOx absorption -Reduction catalyst 3 flows. When the operating state of the internal combustion engine, which is detected by the operating state detection device 10 , is in the high-load and high-speed range, the control unit (ECU) 9 instructs the changeover valve 32 to open the bypass line 31 and to close the exhaust line, which leads to the NOx Absorption-reduction catalyst 3 extends so that the exhaust gas flows via the bypass line 31 directly to the selective urea reduction catalyst 4 . In response to the determination by the operating state detection device 10 that the operating state of the internal combustion engine is in the high-load and high-speed range, the urea addition quantity control device 11 issues a urea addition instruction to the urea addition control valve 6 , so that an aqueous solution of urea is injected from the urea addition control valve 6 . In this way, emission control is carried out depending on the principle described above.

Next, a fourth embodiment of the invention will be described with reference to FIG. 8.

In an emission control device of the embodiment shown in FIG. 8, an exhaust pipe branches into a first exhaust pipe 41 and a second exhaust pipe 42 which are arranged in parallel. The first exhaust pipe 41 is provided with a NOx absorption-reduction catalyst 3 . The second exhaust line 42 is provided with a selective urea reduction catalyst 4 .

A changeover valve 32 , ie a changeover device, is arranged at a branch point between the first exhaust line 41 and the second exhaust line 42 .

As in the construction shown in FIG. 1, a NOx sensor 5 is provided upstream of the selective urea reduction catalyst 4 , and a catalyst inflow gas temperature sensor 7 is provided immediately upstream of the selective urea reduction catalyst 4 . Furthermore, an ammonia sensor 8 is provided downstream of the selective urea reduction catalyst 4 , and an urea addition control valve 6 is provided immediately upstream of the selective urea reduction catalyst 4 .

The changeover valve 32 is an electromagnetic valve that is electrically connected to a control unit (ECU) 9 and is thereby controlled. When it is determined by the operating state detection device 10 that the present operating range of an internal combustion engine is such a range that the NOx absorption-reduction catalyst 3 is able to function, the switching valve 32 selects the first exhaust pipe 41 so that the exhaust gas flows to the NOx absorption-reduction catalyst 3 . The NOx absorption-reduction catalyst 3 performs emission control as described above.

When it is determined that the operation of the internal combustion engine has shifted to the high-load and high-speed range, the changeover valve 32 selects the second exhaust pipe 42 so that the exhaust gas flows into the selective urea reduction catalyst 4 . When the selective urea reduction catalyst 4 is selected in this way, the urea addition amount control device 11 issues an urea addition instruction to the urea addition control valve 6 so that an aqueous solution of urea is injected from the urea addition control valve 6 . In this way, the emission control is carried out according to the principle described above.

Although the foregoing embodiments of the invention have been described in connection with a gasoline engine 1 , it should be understood that the invention is also applicable to a diesel engine. In the case of a diesel engine, fuel combustion in the combustion chambers is performed in a lean (or high) air-fuel ratio range that is far from the stoichiometric air-fuel ratio. Therefore, the air-fuel ratio of the exhaust gas flowing into the NOx absorption-reduction catalyst 3 becomes very lean (or high) during a normal operating state of the diesel engine, so that NOx is absorbed by the catalyst 3 but hardly released.

Therefore, when used in a diesel engine, a Exhaust gas recirculation device (generally as an EGR device used) as an example, so that the NOx, that is absorbed in the catalyst by shifting the Air-fuel ratio of the exhaust gas to the stoichiometric Air-fuel ratio or to the rich side thereof Introducing a large amount of recirculated exhaust gas into the Combustion chambers are released.

While the present invention with reference to the has been described, which is currently preferred Embodiments thereof is considered, it should be clear that the present invention is not based on the disclosed  Embodiments or constructions is limited. in the On the contrary, the present invention intends various modifications and equivalent arrangements to cover. While the various elements of the disclosed Invention in various combinations and configurations shown that are exemplary are others Combinations and configurations that are more, less or only contain a single embodiment, also in Scope of the present invention included.

A NOx absorption-reduction catalyst 3 that absorbs NOx from the exhaust gas when the air-fuel ratio of the exhaust gas is on the lean side, and releases NOx and causes a reduction of NOx when the oxygen concentration in the exhaust gas decreases provided in an exhaust pipe 2 of a lean-burn internal combustion engine 1 . A selective urea reduction catalyst 4 , which effects a selective reduction due to the addition of urea, is also provided. The two catalysts clean the emissions complementarily to one another essentially over the entire operating range of the internal combustion engine.

Claims (12)

1. Emission control device for an internal combustion engine, characterized by the following components:
a NOx absorption-reduction catalyst ( 3 ) provided in an exhaust pipe ( 2 ) of a lean-burn internal combustion engine ( 1 ), the NOx absorption-reduction catalyst ( 3 ) absorbing NOx when an air-fuel ratio of an exhaust gas from the internal combustion engine is on a lean side of a theoretical air-fuel ratio, and wherein the NOx absorption-reduction catalyst ( 3 ) releases NOx and causes a reduction in NOx when an oxygen concentration in the exhaust gas decreases; and a selective ammonia compound reduction catalyst ( 4 ) which causes a selective reduction due to the addition of an ammonia compound. 2. Emission control device according to claim 1, characterized by the further components:
an operating state detection device ( 10 ) for detecting an operating state of the internal combustion engine ( 1 ) and
a switching device ( 32 ) for switching a flow direction of the exhaust gas of either the NOx absorption-reduction catalyst ( 3 ) or the selective ammonia compound reduction catalyst ( 4 ), depending on the operating state of the internal combustion engine ( 1 ) caused by the Operating state detection device ( 10 ) was detected. 3. Emission control device according to claim 1, characterized in that the NOx absorption reduction catalyst ( 3 ) and the selective ammonia compound reduction catalyst ( 4 ) are arranged in series in the exhaust line ( 2 ). 4. Emission control device according to claim 3, characterized in that the selective ammonia compound reduction catalyst ( 4 ) is arranged downstream of the NOx absorption reduction catalyst ( 3 ) in the exhaust duct ( 2 ). 5. Emission control device according to claim 3, characterized in that the selective ammonia compound reduction catalyst ( 4 ) is arranged upstream of the NOx absorption reduction catalyst ( 3 ) in the exhaust line ( 2 ). 6. Emission control device according to claim 1 or 2, characterized in that the NOx absorption reduction catalyst ( 3 ) and the selective ammonia compound reduction catalyst ( 4 ) are arranged in parallel in the exhaust pipe ( 2 ). 7. Emission control device according to claim 3, further characterized by a bypass line ( 31 ) which bypasses an upstream catalyst and conducts the exhaust gas to a downstream catalyst, the switching device ( 32 ) changing the flow direction of the exhaust gas by opening and closing the bypass line ( 31 ) changed. 8. Emission control device according to claim 6, characterized in that
the exhaust pipe ( 2 ) forks into a first exhaust pipe ( 41 ) and a second exhaust pipe ( 42 ) which are arranged in parallel;
that the NOx absorption-reduction catalyst ( 3 ) is arranged in the first exhaust line ( 41 ) and that the selective ammonia compound reduction catalyst ( 4 ) is arranged in the second exhaust line ( 42 ); and
that a changeover valve is arranged as a changeover device ( 32 ) at a branching point between the first exhaust line ( 41 ) and the second exhaust line ( 42 ). The emission control device according to one of claims 1 to 8, further characterized by an additional ammonia compound amount determination device ( 11 ) for estimating an amount of the ammonia compound to be added to the selective ammonia compound reduction catalyst ( 4 ) based on an amount of NOx that is in the exhaust gas flow that flows into the selective ammonia compound reduction catalyst ( 4 ) and based on the intake air that is drawn into the internal combustion engine ( 1 ). 10. Emission control device according to claim 9, further characterized by a
Ammonia compound detection device ( 8 ) for detecting an ammonia compound released from the selective ammonia compound reduction catalyst ( 4 ); and
a control device for correcting an amount of the ammonia compound to be added in an appropriate amount based on an amount of ammonia compound detected by the ammonia compound detection device ( 8 ). 11. Emission control device according to one of claims 1 to 10, further characterized by
a catalyst temperature detection device ( 7 ) for detecting a temperature state of the selective ammonia compound reduction catalyst ( 4 ); and
an ammonia compound addition amount control device ( 11 ) for changing an ammonia compound addition amount provided for the selective ammonia compound reduction catalyst ( 4 ) depending on the detected catalyst temperature. 12. Emission control device according to one of claims 1 to 11, further characterized by a third catalyst, which is provided in a portion of the exhaust pipe ( 2 ), which is upstream of the NOx absorption-reduction catalyst ( 3 ) and the selective ammonia compound -Reduction catalyst ( 4 ) extends.