JP3613660B2 - Exhaust gas purification device for internal combustion engine - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an exhaust emission control device provided with an NOx storage reduction catalyst in an exhaust passage of an internal combustion engine.
[0002]
[Prior art]
An NOx storage reduction catalyst is an exhaust purification device that purifies NOx from exhaust gas discharged from an internal combustion engine for lean combustion. The NOx storage reduction catalyst absorbs NOx when the air-fuel ratio of the inflowing exhaust gas is lean (that is, in an oxygen-excess atmosphere), and releases the absorbed NOx when the oxygen concentration of the inflowing exhaust gas decreases and N₂It is a catalyst that reduces to
[0003]
When this NOx storage reduction catalyst (hereinafter also referred to simply as a catalyst or NOx catalyst) is disposed in the exhaust passage of an internal combustion engine capable of lean combustion, when exhaust gas having a lean air-fuel ratio flows, NOx in the exhaust gas is converted into a catalyst. NOx absorbed by the catalyst when exhaust gas with stoichiometric (theoretical air-fuel ratio) or rich air-fuel ratio flows is NO.₂N is further released by reducing components such as HC and CO in the exhaust gas.₂That is, NOx is purified.
[0004]
By the way, in general, the fuel of the internal combustion engine contains a sulfur content, and when the fuel is combusted in the internal combustion engine, the sulfur content in the fuel is combusted and SO.₂Or SO₃Sulfur oxide (SOx) is generated. The NOx catalyst absorbs SOx in the exhaust gas by the same mechanism that absorbs NOx. If a NOx catalyst is disposed in the exhaust passage of the internal combustion engine, this catalyst absorbs not only NOx but also SOx. Is done.
[0005]
However, since SOx absorbed in the NOx catalyst forms a stable sulfate over time, it is difficult to decompose and release under the same conditions as NOx release / reduction from the NOx catalyst and accumulate in the NOx catalyst. There is a tendency to be easily done. When the amount of SOx accumulated in the NOx storage reduction catalyst increases, the NOx absorption capacity of the catalyst decreases, and NOx removal in the exhaust gas cannot be sufficiently performed, resulting in a decrease in NOx purification efficiency. This is so-called SOx poisoning. Therefore, in order to maintain the NOx purification ability of the NOx storage reduction catalyst high over a long period of time, it is necessary to release SOx absorbed by the catalyst at an appropriate timing.
[0006]
The SOx release treatment technology from the NOx storage reduction catalyst is disclosed in Japanese Patent No. 2745985. In order to release SOx absorbed by the NOx storage reduction catalyst, it is effective to make the air-fuel ratio of the inflowing exhaust gas stoichiometric or rich and to make the catalyst temperature a predetermined high temperature higher than that during NOx release / reduction. Is considered to be
[0007]
Therefore, when the NOx catalyst absorbs a predetermined amount of SOx, it is determined as the SOx release timing. At that time, the catalyst temperature is controlled to a temperature at which SOx can be released, and the air-fuel ratio of the inflowing exhaust gas is stoichiometric or rich. Thus, the air-fuel ratio control of the inflowing exhaust gas is performed so that the SOx is released. By this SOx release treatment, the sulfate absorbed by the NOx catalyst is decomposed and SO2 is released.₃And this SO₃Is reduced by unburned HC and CO in the exhaust gas, and SO₂And released.
[0008]
In the technique disclosed in the above-mentioned patent publication, an electric heater is provided around the exhaust pipe upstream of the catalyst in order to control the temperature of the catalyst, and the exhaust pipe is heated by energizing the electric heater during the SOx release process. Thus, the exhaust gas is heated to heat the catalyst.
[0009]
[Problems to be solved by the invention]
In the above-described conventional exhaust purification device, if HC or CO in the exhaust gas passes through the catalyst while performing the SOx release processing with the air-fuel ratio of the inflowing exhaust gas stoichiometric or rich, there is a risk that it will be released into the atmosphere as it is. This was insufficient in terms of fail-safety.
[0010]
As described above, if the catalyst is heated by the heat transfer route of exhaust pipe → exhaust gas → catalyst, it is difficult to quickly raise the temperature of the catalyst, and the efficiency of SOx release at the initial stage of SOx release processing is poor.
[0011]
In addition, the conventional exhaust purification device has a large catalyst capacity, so that the heat capacity of the catalyst becomes large, and it is difficult to heat the entire catalyst in a short time, which also reduces the SOx release efficiency in the initial stage of the SOx release process. It was a factor.
[0012]
However, if the capacity of the catalyst is reduced in order to reduce the heat capacity of the catalyst, when the SOx release process is executed with the air-fuel ratio of the inflowing exhaust gas stoichiometric or rich, the HC and CO in the exhaust gas will cause the catalyst There is a risk of passing through and polluting the atmosphere, so the catalyst capacity cannot simply be reduced.
[0013]
The present invention has been made in view of such problems of the prior art, and the problem to be solved by the present invention is that SOx can be efficiently released from the NOx storage reduction catalyst, An object of the present invention is to provide an exhaust purification device for an internal combustion engine that is excellent in safety.
[0014]
[Means for Solving the Problems]
The present invention employs the following means in order to solve the above problems. The exhaust gas purification apparatus for an internal combustion engine according to the present invention is arranged in (i) an exhaust passage of a lean burnable internal combustion engine, and absorbs NOx when the air-fuel ratio of the inflowing exhaust gas is lean. NOx absorbed and reduced when the concentration is low, and a NOx storage reduction catalyst having an integrated heating means; and (b) three exhaust gas passages arranged downstream of the NOx storage reduction catalyst. And original catalyst, The occlusion reduction type NO x The capacity of the catalyst is 20% or less of the displacement of the internal combustion engine, and the NOx storage reduction type x The catalyst support rate of the catalyst is 2 to 8 times the normal catalyst support rate.It is characterized by that.
[0015]
The NOx storage reduction catalyst absorbs NOx when the air-fuel ratio of the inflowing exhaust gas is lean, and releases the absorbed NOx when the oxygen concentration in the inflowing exhaust gas decreases, and N₂The NOx storage reduction catalyst has an SOx absorption / release action in the same manner as NOx absorption / release. When SOx is absorbed and accumulated in the NOx storage reduction catalyst, the NOx absorption capacity decreases and the NOx purification efficiency decreases, so it is necessary to execute the SOx release process at a predetermined time. In the exhaust purification apparatus of the present invention, when the SOx release process is executed, the storage reduction NOx catalyst can be heated to a temperature required for SOx release by operating a heating means provided in the NOx storage reduction catalyst. it can.
[0016]
Since the heating means is integrated with the NOx storage reduction catalyst, the NOx storage reduction catalyst can be directly heated. Therefore, the NOx storage reduction catalyst can be quickly heated to the SOx release temperature, and SOx can be released very efficiently.
[0017]
In addition, the SOx release process is executed with the air-fuel ratio of the exhaust gas being stoichiometric or slightly rich (slight rich). During the SOx release process, HC and CO in the exhaust gas pass through the NOx storage reduction catalyst. Even in the case of stagnation, the HC and CO are purified by a three-way catalyst disposed downstream of the NOx storage reduction catalyst.
[0018]
The air-fuel ratio of exhaust gas refers to the ratio of air and fuel (hydrocarbon) supplied into the exhaust passage upstream of the engine intake passage and the NOx storage reduction catalyst.
As the lean burnable internal combustion engine in the present invention, an in-cylinder direct injection type lean burn gasoline engine or a diesel engine can be exemplified.
[0019]
The NOx storage reduction catalyst is, for example, alumina (Al₂O₃) Coated metal support, and alumina of this metal support, for example, potassium K, sodium Na, lithium Li, alkali metal such as cesium Cs, alkaline earth such as barium Ba, calcium Ca, lanthanum La, yttrium At least one selected from rare earths such as Y and a noble metal such as platinum Pt are supported.
[0020]
The heating means integrated with the NOx storage reduction catalyst may be constituted by an electric heating element that generates heat when energized, or a metal part that is not supported by the catalyst. An electric heating element may be embedded and integrated.
[0021]
In the exhaust gas purification apparatus for an internal combustion engine according to the present invention, the NOx storage reduction catalyst has a small capacity and is highly supported.. this is,This is because the heat capacity can be reduced by reducing the capacity of the NOx storage reduction catalyst and the temperature of the NOx storage reduction catalyst can be raised more quickly. Further, the reason why the catalyst is carried high is to prevent the NOx purification efficiency from being lowered even if the capacity is reduced. The capacity of the NOx storage reduction catalyst can be set to 20% or less of the displacement of the internal combustion engine, and it seems that a reduction in the capacity can be realized to about 5% of the displacement. The “high support” refers to about 2 to 8 times the normal catalyst support rate.
[0022]
In the exhaust gas purification apparatus for an internal combustion engine according to the present invention, (c) an exhaust passage upstream of the NOx storage reduction catalyst and an exhaust passage between the NOx storage reduction catalyst and a three-way catalyst are connected to exhaust gas. And a bypass passage that bypasses the NOx storage reduction catalyst and (d) a bypass flow rate control means that controls the flow rate of the exhaust gas flowing through the bypass passage.
[0023]
In such a configuration, when performing the SOx release process, the bypass flow rate control means is operated so that a predetermined flow rate of exhaust gas flows through the bypass passage, and thereby the exhaust gas flowing through the NOx storage reduction catalyst. Reduce gas flow. Thus, the flow rate of the exhaust gas flowing through the NOx storage reduction catalyst can be reduced without reducing the flow rate of the exhaust gas discharged from the internal combustion engine, and the temperature rise of the NOx storage reduction catalyst can be promoted. The bypass flow rate control means can be constituted by a flow rate control valve.
[0024]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of an exhaust gas purification apparatus for an internal combustion engine according to the present invention will be described below with reference to the drawings of FIGS.
[0025]
[First Embodiment]
FIG. 1 is a diagram showing a schematic configuration of an exhaust purification device of a direct injection type diesel engine for driving a vehicle as an internal combustion engine. In this figure, the engine 1 is an in-line four cylinder, and intake air is supplied to each cylinder via an intake pipe 2 and an intake manifold 3.
[0026]
The fuel (light oil) in the fuel tank 4 is sucked up by a fuel supply pump 5 capable of controlling the discharge pressure and supplied to the common rail 6. The engine control electronic control unit (ECU) 50 controls the operation of the fuel supply pump 5 so that the fuel pressure in the common rail 6 becomes a predetermined pressure value according to the operating state of the engine 1.
[0027]
Further, the engine 1 is provided with a fuel injection valve 7 for injecting fuel supplied from the common rail 6 into each cylinder, and the opening timing and the opening period of the fuel injection valve 7 are the operating state of the engine 1. Is controlled by the ECU 50 accordingly.
[0028]
In the combustion chamber of each cylinder, fuel is mainly injected from the corresponding fuel injection valve 7 in the vicinity of the compression top dead center in each cylinder, and exhaust gas generated by the explosion of this fuel is discharged to the exhaust pipe 9 through the exhaust manifold 8. Is done.
[0029]
Further, in the engine 1, fuel is sub-injected into the cylinder from the fuel injection valve 7 of the corresponding cylinder in the expansion stroke or exhaust stroke of the predetermined cylinder at a predetermined time. The HC component of the sub-injected fuel is reformed into light HC by the heat of the explosion stroke, and is discharged together with the exhaust to the exhaust pipe 9 through the exhaust manifold 8, and is supplied to the NOx storage reduction catalyst 21 described later. The
[0030]
In the middle of the exhaust pipe 9, a first catalytic converter 20 containing an NOx storage reduction catalyst (hereinafter abbreviated as NOx catalyst) 21 and a second catalytic converter 30 containing a three-way catalyst 31 are installed. Has been.
[0031]
The NOx catalyst 21 of the first catalytic converter 20 is alumina (Al₂O₃) Coated metal support, and alumina of this metal support, for example, potassium K, sodium Na, lithium Li, alkali metal such as cesium Cs, alkaline earth such as barium Ba, calcium Ca, lanthanum La, yttrium At least one selected from rare earths such as Y and a noble metal such as platinum Pt are supported.
[0032]
The first catalytic converter 20 is smaller than a normal one, and the catalyst is carried higher than usual. The small size and high carrying capacity will be described below with numerical values. Conventionally, for example, in the case of an engine with a total displacement of 2000 cc, the capacity of the NOx catalyst (occlusion reduction type NOx catalyst) requires about 2000 cc, which is almost the same as the total displacement of the engine, and the catalyst carrying rate is, for example, Pt / Ba In the case of a catalyst, Pt is about 1 to 2 g / l, and Ba is about 0.1 to 0.2 mol / l. In contrast, in the first catalytic converter 20 of the present embodiment, the capacity of the NOx catalyst 21 is about 20% of the normal capacity, that is, 400 cc, and the catalyst supporting rate is about 5% of the normal supporting rate. Double, i.e. Pt is 5 to 10 g / l and Ba is 0.5 to 1.0 mol / l. The first catalytic converter 20 has a smaller heat capacity because the capacity of the NOx catalyst 21 is reduced.
[0033]
The metal carrier of the NOx catalyst 21 has a heater function. When a current is passed, the metal carrier generates heat and heats the NOx catalyst 21. That is, the NOx catalyst 21 is a catalyst in which heating means are integrated. The metal carrier of the NOx catalyst 21 is electrically connected to the heater drive circuit 10, and this heater drive circuit 10 is ON / OFF controlled by a command signal from the ECU 50, whereby the heater made of the metal carrier is ON / OFF controlled. It has come to be.
[0034]
Further, the first catalytic converter 20 is provided with a catalyst temperature sensor 11 that outputs an output signal corresponding to the temperature of the NOx catalyst 21 to the ECU 50.
The action of the NOx catalyst 21 (NOx absorption / release action and SOx absorption / release action) will be described in detail later.
[0035]
The ECU 50 is composed of a digital computer, and includes a ROM (Read On Memory), a RAM (Random Access Memory), a CPU (Central Processor Unit), an input port, and an output port that are connected to each other via a bidirectional bus. In addition to performing basic control such as injection amount control, in this embodiment, SOx release processing control of the first catalytic converter 20 is performed.
[0036]
For these controls, an input signal from the accelerator opening sensor 14 and an input signal from the crank angle sensor 15 are input to the input port of the ECU 50. The accelerator opening sensor 14 outputs an output voltage proportional to the accelerator opening to the ECU 50, and the ECU 50 calculates the engine load based on the output signal of the accelerator opening sensor 14. The crank angle sensor 15 outputs an output pulse to the ECU 50 every time the crankshaft rotates by a certain angle, and the ECU 50 calculates the engine rotation speed based on the output pulse. The engine operating state is determined based on the engine load and the engine speed.
[0037]
The NOx catalyst 21 housed in the first catalytic converter 20, that is, the NOx storage reduction catalyst, absorbs NOx when the air-fuel ratio of the inflowing exhaust (hereinafter referred to as the exhaust air-fuel ratio) is lean, When the fuel ratio is stoichiometric or rich and the oxygen concentration in the exhaust gas decreases, the NOx absorbing and releasing action is performed to release the absorbed NOx. Here, the exhaust air-fuel ratio means the ratio of the total amount of air supplied to the exhaust passage, engine combustion chamber, intake passage, etc. upstream of the NOx catalyst 21 and the total fuel (hydrocarbon). And Therefore, when fuel, a reducing agent, or air is not supplied into the exhaust passage upstream of the NOx catalyst 21, the exhaust air-fuel ratio matches the air-fuel ratio of the air-fuel mixture supplied into the engine combustion chamber.
[0038]
Although the detailed mechanism of the absorption / release action of the NOx catalyst 21 is not clear, it is considered that the mechanism is as shown in FIG. Next, this mechanism will be described by taking as an example the case where platinum Pt and barium Ba are supported on the support, but the same mechanism can be obtained by using other noble metals, alkali metals, alkaline earths, and rare earths.
[0039]
That is, when the air-fuel ratio of the inflowing exhaust gas becomes considerably lean, the oxygen concentration in the inflowing exhaust gas greatly increases, and as shown in FIG.₂Is O₂ ⁻Or O^2-It adheres to the surface of platinum Pt. On the other hand, NO contained in the inflowing exhaust gas is O on the surface of platinum Pt.₂ ⁻Or O^2-Reacts with NO₂(2NO + O₂→ 2NO₂).
[0040]
Then the generated NO₂Is oxidized on platinum Pt and absorbed in the NOx catalyst 21 and combined with barium oxide BaO, as shown in FIG.₃ ⁻In the form of NOx catalyst 21. In this way, NOx is absorbed into the NOx catalyst 21.
[0041]
As long as the oxygen concentration in the inflowing exhaust gas is high, NO on the surface of platinum Pt₂NOx is not generated and the NOx absorption capacity of the NOx catalyst 21 is not saturated.₂Is absorbed in the NOx catalyst 21 and nitrate ions NO.₃ ⁻Is generated.
[0042]
On the other hand, when the exhaust air-fuel ratio becomes rich or stoichiometric, the oxygen concentration in the inflowing exhaust gas decreases.₂Production amount decreases and the reaction proceeds in the reverse direction (NO₃ ⁻→ NO₂), And nitrate ion NO in the NOx catalyst 21₃ ⁻Is NO₂Alternatively, it is released from the NOx catalyst 21 in the form of NO. That is, when the oxygen concentration in the inflowing exhaust gas decreases, NOx is released from the NOx catalyst 21.
[0043]
On the other hand, at this time, HC and CO in the exhaust gas are oxygen O on platinum Pt.₂ ⁻Or O^2-It reacts with and is oxidized. Further, the NOx released from the NOx catalyst 21 due to the decrease in the oxygen concentration in the inflowing exhaust gas.₂Alternatively, NO is reduced by reacting with unburned HC and CO as shown in FIG.₂It becomes.
[0044]
That is, HC and CO in the inflowing exhaust gas are first oxygen O on platinum Pt.₂ ⁻Or O^2-Immediately reacts with and oxidizes, then oxygen O on platinum Pt.₂ ⁻Or O^2-If HC and CO still remain after the consumption of NOx, NOx released from the NOx catalyst and NOx discharged from the engine by the HC and CO are N₂Reduced to
[0045]
In this way, NO on the surface of platinum Pt.₂Or, when NO is no longer present, NOx from the NOx catalyst 21 to NO₂Or NO is released and N₂To be reduced. Therefore, when the exhaust air-fuel ratio is made stoichiometric or rich, NOx is released from the NOx catalyst 21 in a short time, and N₂Reduced to
[0046]
Thus, when the exhaust air-fuel ratio becomes lean, NOx is absorbed by the NOx catalyst 21, and when the exhaust air-fuel ratio is made stoichiometric or rich, NOx is released from the NOx catalyst 21 in a short time, and N₂Reduced to Accordingly, NOx emission into the atmosphere can be prevented.
[0047]
By the way, in the case of a diesel engine, combustion is performed in a much leaner range than stoichiometric (theoretical air-fuel ratio, A / F = 13 to 14), so that the exhaust air flowing into the NOx catalyst 21 in the normal engine operating state is exhausted. The fuel ratio is very lean, and NOx in the exhaust is absorbed by the NOx catalyst 21 and the amount of NOx released from the NOx catalyst 21 is extremely small.
[0048]
In the case of a gasoline engine, the air-fuel ratio of the exhaust gas is stoichiometric or rich by making the air-fuel mixture supplied to the combustion chamber stoichiometric or rich, and the oxygen concentration in the exhaust gas is reduced, so that the NOx catalyst 21 The absorbed NOx can be released, but in the case of a diesel engine, if the air-fuel mixture supplied to the combustion chamber is stoichiometric or rich, there is a problem that soot is generated at the time of combustion. Can not.
[0049]
Therefore, in the diesel engine, fuel as a reducing agent is supplied directly to the NOx catalyst 21 separately from the combustion air-fuel mixture at a predetermined timing before the NOx absorption capacity of the NOx catalyst 21 saturates to release NOx. There is a need to do. Therefore, in this embodiment, when NOx is released from the NOx catalyst 21, the air-fuel ratio of the exhaust gas flowing into the NOx catalyst 21 is reduced by sub-injecting fuel into the cylinder during the expansion stroke or exhaust stroke of the engine 1. Make it stoic or rich.
[0050]
On the other hand, the fuel contains sulfur (S), and when the sulfur in the fuel burns, SO₂Or SO₃Sulfur oxide (SOx) is generated, and the NOx catalyst 21 also absorbs these SOx in the exhaust gas. The SOx absorption mechanism of the NOx catalyst 21 is considered to be the same as the NOx absorption mechanism. That is, the case where platinum Pt and barium Ba are supported on the carrier as in the case of explaining the NOx absorption mechanism will be described as an example. As described above, when the exhaust air-fuel ratio is lean, oxygen O 2₂Is O₂ ⁻Or O^2-Is attached to the surface of the platinum Pt of the NOx catalyst 21 in the form of SOx (for example, SO₂) Is oxidized on the surface of platinum Pt to form SO.₃It becomes.
[0051]
Then the generated SO₃Is absorbed in the NOx catalyst 21 while being further oxidized on the surface of platinum Pt, and is combined with barium oxide BaO to form sulfate ions SO.₄ ^2-In the form of NOx catalyst 21 and sulfate BaSO₄Form. BaSO₄Since crystals tend to coarsen and are relatively stable, once they are produced, they are difficult to decompose and release. For this reason, the BaSO in the NOx catalyst 21 over time.₄When the production amount of NO increases, the amount of BaO that can participate in NOx absorption decreases, and the NOx absorption capacity decreases. This is SOx poisoning. Therefore, in order to maintain the NOx absorption capacity of the NOx catalyst 21 high, it is necessary to release the SOx absorbed by the NOx catalyst 21 at an appropriate timing.
[0052]
In order to release SOx from the NOx catalyst 21, it has been found that the oxygen concentration of the exhaust gas should be lowered as in the case of releasing NOx, and the higher the temperature of the NOx catalyst 21, the easier it is to release. I know that.
[0053]
Therefore, in this embodiment, the SOx release process is executed when a predetermined amount of SOx is absorbed by the NOx catalyst 21, and the SOx release process is performed similarly to the NOx release process. Alternatively, by sub-injecting fuel into the cylinder in the exhaust stroke, the air-fuel ratio of the exhaust gas flowing into the NOx catalyst 21 is controlled to be stoichiometric or rich, and at the same time, current is passed through the metal carrier of the NOx catalyst 21 to generate heat. Thus, the temperature of the NOx catalyst 21 is increased to a predetermined temperature at which SOx can be released.
[0054]
Next, the operation of the exhaust emission control device in the present embodiment will be described.
<NOx absorption / release treatment>
First, the NOx absorption / release process by the NOx catalyst 21 of the first catalytic converter 20 will be described.
As described above, since the engine 1 is a diesel engine, the air-fuel ratio of the exhaust gas is lean and the oxygen concentration is very high in a normal operation state. Therefore, when this exhaust gas is passed through the NOx catalyst 21 of the first catalytic converter 20, NOx in the exhaust gas is absorbed by the NOx catalyst 21.
[0055]
When the air-fuel ratio of the exhaust gas is lean, the three-way catalyst 31 of the second catalytic converter 30 hardly functions as a catalyst, and the exhaust gas from which NOx has been removed by the first catalytic converter 20 almost passes through the second catalytic converter 30. Just pass through.
[0056]
Here, in this embodiment, when NOx in the exhaust gas is absorbed by the NOx catalyst 21, the ECU 50 controls the fuel injection valve 7 so as to execute only the main injection and not perform the sub-injection.
[0057]
If the air-fuel ratio lean exhaust gas is allowed to flow to the first catalytic converter 20, the amount of NOx absorbed by the NOx catalyst 21 increases, and if it continues as it is, the NOx absorption capacity will be saturated. Therefore, the NOx release process is executed on the first catalytic converter 20 at a predetermined time before the NOx absorption capacity of the NOx catalyst 21 is saturated. Here, the predetermined time when the NOx releasing process is executed may be, for example, the time when the operation time of the engine 1 is accumulated by the ECU 50 and the accumulated time reaches the predetermined time, or the operation state of the engine 1 by the ECU 50. The NOx absorption amount may be estimated from the history, and the estimated absorption amount may reach a predetermined amount.
[0058]
When executing the NOx releasing process, the ECU 50 controls the fuel injection valve 7 so as to execute both the main injection and the sub-injection. The number of times is controlled.
By sub-injecting fuel in the expansion stroke or the exhaust stroke, the air-fuel ratio of the exhaust gas is controlled to stoichiometric or slightly rich (slight rich), and the exhaust gas having the stoichiometric or slightly rich air-fuel ratio is supplied to the first catalytic converter 20. By flowing, NOx absorbed in the NOx catalyst 21 is released and reduced, and N₂To the atmosphere.
[0059]
Since the capacity of the NOx catalyst 21 of the first catalytic converter 20 is small, a reducing agent such as HC and CO may remain in the exhaust gas that has passed through the first catalytic converter 20 during the execution of the NOx release process. The reducing agent in the exhaust gas is oxidized by oxygen adsorbed on the three-way catalyst 31 when passing through the second catalytic converter 30, and H₂0 or CO₂To be purified.
[0060]
<SOx release treatment>
Next, the SOx release process for the NOx catalyst 21 of the first catalytic converter 20 will be described with reference to the flowchart shown in FIG. This flowchart is stored in the ROM of the ECU 50, and all processes in the steps of the flowchart are executed by the CPU of the ECU 50.
[0061]
<Step 101>
First, in step 101, the ECU 50 integrates the travel distance of the vehicle from the completion of the previous SOx release process to the current time during the execution of the NOx absorption / release process described above.
[0062]
<Step 102>
Next, the ECU 50 proceeds to step 102 where the travel distance integrated value D obtained in step 101 is a determination value (determination criterion) D.₀It is determined whether or not it exceeds
The SOx contained in the exhaust gas discharged from the engine body 1 is produced by burning sulfur (S) in the fuel, and the amount of SOx absorbed by the NOx catalyst 21 is determined by the engine body. There is a correlation with the amount of fuel consumed by combustion at 1. Therefore, the amount of SOx absorbed in the NOx catalyst 21 can be calculated based on the integrated value of the fuel consumption, and the time when the integrated value of the fuel consumption reaches a value corresponding to the predetermined amount of SOx absorption is determined as SOx. Although it is possible to set the release time, since there is a correlation between the fuel consumption and the travel distance, in this embodiment, the travel distance is integrated instead of the fuel consumption integration. The integrated value of the travel distance corresponds to a predetermined amount of SOx absorption amount (determination value D₀), It is determined that it is time to release SOx, and when it does not exceed, it is determined that it is not time to release SOx.
[0063]
The portion of the series of signal processing performed by the ECU 50 that executes step 102 can be referred to as SOx release timing determination means that determines whether it is time to release SOx from the NOx catalyst (NOx absorbent).
If an affirmative determination is made in step 102, the process proceeds to step 103, and if a negative determination is made, the process proceeds to step 105.
[0064]
<Step 103>
In step 103, the ECU 50 performs SOx release processing on the NOx catalyst 21. The SOx release process is executed by air-fuel ratio control of exhaust gas by sub-injection of fuel and temperature control of the NOx catalyst 21. During the execution of the SOx release process, the ECU 50 controls the operation of the fuel injection valve 7 so as to execute both the main injection and the sub injection.
[0065]
Regarding the sub-injection control, the ECU 50 opens the fuel injection valve 7, the valve opening period, the number of sub-injections, etc. so that the air-fuel ratio of the exhaust gas flowing into the first catalytic converter 20 becomes stoichiometric or slightly rich. To control.
[0066]
For temperature control of the NOx catalyst 21, the ECU 50 detects the temperature of the NOx catalyst 21 by the catalyst temperature sensor 11, determines whether the temperature of the NOx catalyst 21 is equal to or higher than the set temperature, and is determined to be less than the set temperature. When the heater drive circuit 10 is turned on, the metal carrier of the NOx catalyst 21 is energized to heat the metal carrier, the temperature of the NOx catalyst 21 is raised, and when it is determined that the temperature is equal to or higher than the set temperature, the heater drive circuit 10 is turned off. To do. Thereby, during the SOx release process, the temperature of the NOx catalyst 21 is controlled so as to be maintained at the set temperature or higher. The set temperature is set to be equal to or higher than the decomposition temperature of sulfate, and is preferably set to a predetermined temperature at which the NOx catalyst 21 hardly deteriorates at high temperature (for example, 600 to 750 ° C.).
[0067]
Since the first catalytic converter 20 is small and has a small heat capacity, the temperature of the NOx catalyst 21 is rapidly increased, and the temperature of the NOx catalyst 21 can be brought to the set temperature as soon as the SOx release process is started.
[0068]
In addition, the ECU 50 executes intake air amount reduction control in order to quickly raise the temperature of the NOx catalyst 21. This is because if the flow rate of exhaust gas flowing through the NOx catalyst 21 is large, it takes a long time to raise the temperature of the NOx catalyst 21.
[0069]
By executing the exhaust gas air-fuel ratio control and the temperature control of the NOx catalyst 21 as described above, the stoichiometric or slightly rich exhaust gas having a low oxygen concentration flows to the NOx catalyst 21 controlled to be equal to or higher than the set temperature. As a result, the sulfate absorbed by the NOx catalyst 21 is efficiently decomposed and SO₃And this SO₃Is reduced by unburned HC and CO in the exhaust gas, and SO₂And released.
[0070]
Since the capacity of the NOx catalyst 21 of the first catalytic converter 20 is small, a reducing agent such as HC or CO may remain in the exhaust gas that has passed through the first catalytic converter 20 during execution of the SOx release process. The reducing agent in the exhaust gas is oxidized by oxygen adsorbed on the three-way catalyst 31 when passing through the second catalytic converter 30, and H₂0 or CO₂To be purified.
[0071]
<Step 104>
The ECU 50 proceeds from step 103 to step 104, and determines whether or not the SOx release from the NOx catalyst 21 is completed. Whether or not the SOx release has been completed is determined by whether or not the execution of the SOx release process has been performed for a predetermined time.
[0072]
The “predetermined time” that is the determination value is the determination value D of the SOx release timing in step 102.₀The optimum time is obtained in advance by experiment and stored in the ROM of the ECU 50, depending on the magnitude of the temperature or the set temperature in the temperature control of the NOx catalyst 21.
[0073]
Then, a negative determination is made in step 104 until the execution time of the SOx release process reaches a predetermined time, and execution of the SOx release process in step 103 is continued. When the execution time of the SOx release process exceeds the predetermined time, the ECU 50 makes an affirmative determination (determination of SOx release completion) in step 104 and proceeds to step 105.
[0074]
<Step 105>
In step 105, the ECU 50 stops executing the SOx release process. That is, the fuel sub-injection is stopped to stop the air-fuel ratio control of the exhaust gas, the heater drive circuit 10 is turned OFF, the intake air amount decrease control is stopped, the temperature control of the NOx catalyst 21 is stopped, and the NOx absorption and release is stopped. Air-fuel ratio control of exhaust gas during processing is executed.
[0075]
As described above, according to this embodiment, the SOx releasing process for the NOx catalyst 21 is executed at the optimum timing, and the SOx releasing process is started by reducing the size of the first catalytic converter 20 and reducing the heat capacity. As a result, the NOx catalyst 21 can be immediately heated to a temperature necessary for releasing SOx, and SOx can be released very efficiently, so that the NOx purification rate of the NOx catalyst 21 can always be kept high. it can.
[0076]
Further, since the first catalytic converter 20 is downsized, HC and CO in the exhaust gas may pass through the first catalytic converter 20 without being consumed by the NOx catalyst 21 during the NOx releasing process or the SOx releasing process. Since the second catalytic converter 30 containing the three-way catalyst 31 is disposed downstream of the first catalytic converter 20, the HC and CO that have passed through the first catalytic converter 20 are also three-way catalyst 31 of the second catalytic converter 30. It can be oxidized and purified by oxygen adsorbed on the surface, and air pollution by HC and CO can be prevented.
[0077]
[Second Embodiment]
FIG. 4 is a configuration diagram of an internal combustion engine exhaust gas purification apparatus according to a second embodiment of the present invention.
The second embodiment is different from the first embodiment in the following points. A bypass pipe (bypass passage) that bypasses the first catalytic converter 20 by connecting the upstream of the first catalytic converter 20 and between the downstream of the first catalytic converter 20 and the upstream of the second catalytic converter 30 to the exhaust pipe 9. ) 12 is connected, and a bypass flow control valve (bypass flow control means) 13 is provided in the middle of the bypass pipe 12. The bypass flow rate control valve 13 can drive a valve body (not shown) by the drive unit 16 to adjust the valve opening, and controls the flow rate of exhaust gas flowing through the bypass pipe 12. The operation of the drive unit 16 of the bypass flow control valve 13 is controlled by the ECU 50. Since other configurations are the same as those of the first embodiment, the same reference numerals are given to the same mode portions in the drawing, and the description thereof is omitted.
[0078]
Next, the operation of the second embodiment will be described. In the first embodiment, when the SOx release process is performed, the intake air amount decrease control is executed to speed up the temperature increase in the temperature control of the NOx catalyst 21, but in the second embodiment, Instead of performing the reduction control of the intake air amount, the flow rate of the exhaust gas flowing through the first catalytic converter 20 is decreased by flowing a part of the exhaust gas through the bypass pipe 12 so as to promote the temperature rise of the NOx catalyst 21. I have to.
[0079]
In step 103 of the flowchart in FIG. 3, the ECU 50 executes the SOx release processing by the exhaust gas air-fuel ratio control by the sub-injection of fuel and the temperature control of the NOx catalyst 21. The temperature control of the NOx catalyst 21 is performed by the NOx catalyst 21. The metal carrier is energized to heat the metal carrier, and the drive unit 16 of the bypass flow rate control valve 13 is driven to open the valve body at a predetermined opening, so that the exhaust gas having a predetermined flow rate flows through the bypass pipe 12. This is performed by reducing the flow rate of the exhaust gas flowing through the first catalytic converter 20. The degree of opening of the bypass flow rate control valve 13 is set in advance according to the exhaust gas flow rate flowing through the first catalytic converter 20.
The HC and CO contained in the exhaust gas flowing through the bypass pipe 12 are oxidized by oxygen adsorbed on the three-way catalyst 31 of the second catalytic converter 30, and H₂0 or CO₂To be purified.
[0080]
Then, the ECU 50 determines that the SOx release has been completed in step 104, and if the execution of the SOx release process is stopped in step 105, the ECU 50 stops the fuel sub-injection and stops the exhaust gas air-fuel ratio control, The heater drive circuit 10 is turned off, the bypass flow rate control valve 13 is fully closed, the temperature control of the NOx catalyst 21 is stopped, and the air-fuel ratio control of the exhaust gas during the NOx intake / release process is executed. Therefore, the exhaust gas does not flow through the bypass pipe 12 during the NOx absorption / release process.
[0081]
In the second embodiment described above, the flow rate of the exhaust gas flowing through the first catalytic converter 20 is reduced by performing the bypass flow rate control by the bypass flow rate control valve 13 instead of performing the reduction control of the intake air amount. However, it is possible to reduce the flow rate of the exhaust gas flowing through the first catalytic converter 20 and promote the temperature rise of the NOx catalyst 21 by using the reduction control of the intake air amount and the bypass flow rate control by the bypass flow rate control valve 13 together. Is also possible.
[0082]
In the above-described embodiment, the present invention has been described with reference to an example in which the present invention is applied to a diesel engine. However, the present invention can also be applied to a gasoline engine capable of lean combustion. In the case of a gasoline engine, the air-fuel ratio supplied to the combustion chamber is stoichiometric or rich, so that the exhaust air-fuel ratio is stoichiometric or rich, and the oxygen concentration in the exhaust gas is lowered and absorbed by the NOx catalyst 21. NOx and SOx can be released.
[0083]
Further, when the present invention is applied to a gasoline engine, in order to promote the temperature rise of the NOx catalyst 21, fuel is sub-injected in the expansion stroke, or in the case of a multi-cylinder engine, some cylinders are The amount of the reducing agent in the exhaust gas is increased by performing so-called bank control to make the air-fuel mixture rich, for example. The reducing agent is oxidized in the NOx catalyst 21, and the temperature of the NOx catalyst 21 is increased by the reaction heat. It can also be promoted.
[0084]
【The invention's effect】
According to the exhaust gas purification apparatus for an internal combustion engine according to the present invention, (i) the exhaust gas that is disposed in the exhaust passage of the lean burnable internal combustion engine and that absorbs NOx and flows in when the air-fuel ratio of the inflowing exhaust gas is lean. NOx absorbed and reduced when the oxygen concentration of the catalyst is low, and is stored in the exhaust passage downstream of the NOx storage reduction catalyst and (b) the NOx storage reduction catalyst. The three-way catalyst can efficiently release SOx absorbed by the NOx storage reduction catalyst immediately after the start of the SOx release process, and has an excellent effect of improving the SOx release efficiency. Is done. Further, even when HC and CO in the exhaust gas pass through the NOx storage reduction catalyst as they are, the HC and CO are purified by the downstream three-way catalyst, so that release to the atmosphere can be prevented. An excellent effect of being able to be produced.
[0085]
When the NOx storage reduction catalyst has a small capacity and a high capacity, the NOx storage reduction catalyst can be heated more quickly and the SOx release efficiency is further improved.
And (c) connecting an exhaust passage upstream of the NOx storage reduction catalyst and an exhaust passage between the NOx storage reduction catalyst and the three-way catalyst to flow exhaust gas bypassing the NOx storage reduction catalyst. In the case of comprising a bypass passage and (d) a bypass flow rate control means for controlling the flow rate of the exhaust gas flowing through the bypass passage, the flow rate of the exhaust gas discharged from the internal combustion engine during the execution of the SOx release process is reduced. It is possible to reduce the flow rate of the exhaust gas flowing through the NOx storage reduction catalyst without reducing the temperature, thereby increasing the temperature of the NOx storage reduction catalyst more quickly and further improving the SOx release efficiency.
[Brief description of the drawings]
FIG. 1 is a schematic configuration diagram of a first embodiment of an exhaust gas purification apparatus for an internal combustion engine according to the present invention.
FIG. 2 is a view for explaining the NOx absorption / release action of the NOx storage reduction catalyst.
FIG. 3 is a SOx release process execution routine of the first embodiment;
FIG. 4 is a schematic configuration diagram of a second embodiment of an exhaust gas purification apparatus for an internal combustion engine according to the present invention.
[Explanation of symbols]
1 Diesel engine (internal combustion engine)
7 Fuel injection valve
9 Exhaust pipe (exhaust passage)
12 Bypass passage (bypass pipe)
13 Bypass flow control valve (Bypass flow control means)
21 NOx storage reduction catalyst
31 Three-way catalyst
50 ECU

Claims (3)

(B) Located in the exhaust passage of an internal combustion engine capable of lean combustion, absorbs NOx when the air-fuel ratio of the inflowing exhaust gas is lean, and releases and reduces the absorbed NOx when the oxygen concentration of the inflowing exhaust gas is low And (b) a three-way catalyst disposed in the exhaust passage downstream of the NOx storage reduction catalyst, and having an integrated heating means ,
Capacity of the storage reduction the NO x catalyst, along with the more than 20% of the emissions of the internal combustion engine, the catalyst loading of the storage reduction the NO x catalyst is twice the conventional catalyst-supporting ratio 8x An exhaust gas purification apparatus for an internal combustion engine characterized by having a loading rate .   Occlusion reduction type NO x The catalyst has a metal carrier,
  2. The exhaust gas purification apparatus for an internal combustion engine according to claim 1, wherein the integrated heating means is the metal carrier having a heater function that generates heat when electricity is supplied. (C) a bypass passage that connects an exhaust passage upstream of the NOx storage reduction catalyst and an exhaust passage between the NOx storage reduction catalyst and a three-way catalyst to flow exhaust gas bypassing the NOx storage reduction catalyst; And (d) bypass flow rate control means for controlling the flow rate of the exhaust gas flowing through the bypass passage.

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