JP4496615B2 - Exhaust gas purification device for internal combustion engine - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an exhaust gas purification apparatus for an internal combustion engine, and more particularly to an exhaust gas purification apparatus having a NOx catalyst that removes nitrogen oxides in exhaust gas (hereinafter referred to as “NOx” unless otherwise specified).
[0002]
[Prior art]
As an exhaust gas purification device for purifying exhaust gas of an internal combustion engine, alkaline metals such as potassium K, sodium Na, lithium Li and cesium Cs, alkaline earth materials such as barium Ba and calcium Ca, lanthanum La and yttrium Y An exhaust emission control device using a NOx catalyst comprising at least one selected from rare earths and a noble metal such as platinum Pt is known.
[0003]
The NOx storage reduction catalyst is a catalyst capable of purifying exhaust gas emitted from a lean-ignition internal combustion engine such as a compression ignition internal combustion engine such as a diesel engine or a lean burn gasoline engine.
[0004]
The NOx storage reduction catalyst absorbs NOx when the air-fuel ratio of the exhaust gas flowing into the catalyst (hereinafter referred to as “exhaust air-fuel ratio (exhaust A / F)”) is lean, and the oxygen concentration of the inflowing exhaust gas When it falls, the NOx absorption / release action of releasing the NOx that has been absorbed is performed.
[0005]
In the compression ignition internal combustion engine and the lean combustion internal combustion engine, the exhaust air-fuel ratio in the steady operation state is lean. Therefore, when the NOx storage reduction catalyst is applied to the exhaust gas purification device of these internal combustion engines, NOx in the exhaust gas is reduced. Absorbed by NOx catalyst. However, there is a limit to the absorption capacity, so it will eventually saturate, and it may not be able to absorb any more and NOx may leak.
[0006]
In order to avoid leaks, the NOx absorption capacity must be restored. For this purpose, it is necessary to release the NOx absorbed by the NOx storage reduction catalyst so far and restore the NOx absorption capacity of the NOx storage reduction catalyst.
[0007]
Therefore, before the NOx storage reduction catalyst is saturated, the air-fuel ratio of the inflowing exhaust gas is made rich at a predetermined timing to extremely reduce the oxygen concentration and release NOx from the NOx storage reduction catalyst, thereby absorbing the NOx. Capacity recovery (hereinafter referred to as “regeneration of NOx storage reduction catalyst”) is performed.
[0008]
However, even if the NOx storage reduction catalyst can be regenerated by simply releasing NOx, NOx purification, which is the original function of the NOx catalyst, cannot be achieved, leading to deterioration of emissions. Therefore, for example, a fuel injection valve is provided in the exhaust port, and engine fuel is injected from the fuel injection valve continuously or intermittently according to the operating state of the internal combustion engine, and the engine fuel is added to the NOx storage reduction catalyst as a reducing agent. NOx released by N₂To NOx purification.
[0009]
By the way, since engine fuel contains a large amount of sulfur components, it generates sulfur oxides (hereinafter referred to as “SOx” unless otherwise specified) when burned. SOx is absorbed by the NOx storage reduction catalyst by the same mechanism as NOx.
[0010]
Since SOx forms a stable sulfate, it is generally difficult to be decomposed and released and is easily accumulated in the NOx storage reduction catalyst.
[0011]
As a result, the NOx purification rate of the NOx storage reduction catalyst is reduced by the amount of SOx absorbed, and there is a possibility of causing a phenomenon that hinders NOx purification, which is a function of the NOx storage reduction catalyst, so-called S poisoning. Therefore, it is necessary to recover the NOx storage reduction catalyst in the S poison state, and the process for this is called the S poison recovery processing of the NOx storage reduction catalyst.
[0012]
However, since the sulfate accumulated in the NOx storage reduction catalyst is relatively stable as described above, the NOx storage reduction catalyst was at a temperature (for example, about 500 ° C.) at which it was regenerated. It is difficult to release SOx absorbed by the NOx storage reduction catalyst.
[0013]
In order to perform the S poisoning recovery process, the temperature of the NOx storage reduction catalyst is raised to a high temperature of, for example, about 600 to 700 ° C., which is higher than the temperature at which the NOx storage regeneration catalyst is regenerated, and the exhaust air-fuel ratio is stoichiometrically increased. Or deal with it regularly by making it rich.
[0014]
On the other hand, the diesel engine has a lower exhaust temperature than a conventional gasoline engine, and it is difficult to achieve the high temperature necessary for the S poison recovery process.
[0015]
For example, when the diesel engine is in a high-load operation state, that is, when the diesel engine is operating at a high temperature for the diesel engine, if the exhaust air-fuel ratio is made stoichiometric or rich, soot, unburned fuel components, etc. Particulate matter such as so-called particulate matter (hereinafter referred to as “PM” unless otherwise specified) is easily discharged. Therefore, the exhaust air-fuel ratio cannot be stoichiometric or rich at the high temperature.
[0016]
Therefore, the temperature of the NOx storage reduction catalyst (hereinafter referred to as “catalyst bed temperature”) is raised by some method at the high temperature, and the exhaust air-fuel ratio is made stoichiometric or rich during light load operation where particulates such as soot are difficult to be discharged. To recover S poison.
[0017]
[Problems to be solved by the invention]
However, if the diesel engine is accelerated, for example, to secure the temperature suitable for the S poison recovery process, the fuel injection amount increases to maintain the engine output in the operating state, which is not preferable in terms of fuel consumption. .
[0018]
The present invention has been made in view of the above circumstances, and the problem to be solved is to perform the S poison recovery process when the internal combustion engine is decelerating while ensuring the function of the NOx catalyst. It is to control consumption.
[0019]
[Means for Solving the Problems]
In order to solve the above problems, the exhaust gas purification apparatus for an internal combustion engine of the present invention employs the following means.
[0020]
(1) The exhaust purification system for an internal combustion engine of the present invention absorbs NOx in the exhaust when the exhaust air-fuel ratio is lean, and releases the NOx that has been absorbed until then when the oxygen concentration in the exhaust decreases, and the addition of the reducing agent In the case where the NOx catalyst for reducing and purifying NOx and the NO poisoning of this NOx catalyst should be recovered, when the internal combustion engine is in the acceleration operation state or the steady operation state, the temperature of the NOx catalyst is suitable for NOx purification. A temperature raising means for raising the temperature within a range and setting the exhaust air-fuel ratio lean to make the temperature of the NOx catalyst suitable for S poison recovery when the internal combustion engine is in a decelerating operation state. I did it.
[0021]
Here, the ECU that controls the entire internal combustion engine will be briefly described, and the components of this section will be described.
[0022]
The ECU is composed of a digital computer as is well known, and is connected to each other by a CPU as a central processing control device, a ROM as a read-only memory, a RAM as a random access memory, an input port, an output port, etc. Constitute.
[0023]
The input port is electrically connected to various sensors attached to the internal combustion engine and the vehicle. When output signals of these various sensors enter the ECU via the input port, parameters related to these sensors are temporarily stored in the RAM. Remembered.
[0024]
When the CPU executes various application programs stored in the dedicated memory ROM, the parameters stored in the random access memory RAM through the bidirectional bus and the map stored in the ROM are used as necessary. Based on the parameters, the CPU performs a calculation process required by the CPU, and as a result of the calculation process, various components of the internal combustion engine are operated via the output port.
[0025]
Examples of the “NOx catalyst” include a selective reduction type NOx catalyst and a storage reduction type NOx catalyst. The reducing agent for purifying NOx used for these catalysts may be used also as a fuel containing HC components such as light oil and gasoline, that is, an engine fuel used for combustion of an internal combustion engine.
[0026]
As "temperature raising means", when the internal combustion engine is in an acceleration operation state or a steady operation state, the NOx catalyst is raised within a temperature range suitable for NOx purification, and when the internal combustion engine is in a deceleration operation state An application program set to make the temperature of the NOx catalyst suitable for S poison recovery by making the exhaust air-fuel ratio lean. The application program is executed by the CPU, and the attribute of the CPU is in the ECU. Therefore, the ECU can also be referred to as a temperature raising means.
[0027]
“The internal combustion engine is in a steady operation state” means an engine operation state in which the accelerator depression amount is constant and the engine speed is constant.
[0028]
The “temperature range suitable for NOx purification” includes, for example, a temperature of about 200 to 500 ° C.
[0029]
Examples of the “temperature suitable for S poison recovery” include a temperature of about 600 to 700 ° C.
Examples of the “internal combustion engine” include a lean burn gasoline engine that is a lean combustion internal combustion engine and a diesel engine that is a compression ignition internal combustion engine.
[0030]
In the exhaust gas purification apparatus for an internal combustion engine of the present invention having such a configuration, when the S poisoning of the NOx catalyst is to be recovered, when the internal combustion engine is in an acceleration operation state or a steady operation state, the NOx catalyst is subjected to NOx purification. Since the temperature is raised within a suitable temperature range, NOx can be reduced and purified by adding a reducing agent at that time.
[0031]
Further, when the internal combustion engine is in a decelerating operation state, the amount of oxygen in the exhaust gas can be increased by making the exhaust air-fuel ratio lean, so that if the increased oxygen is supplied toward the NOx catalyst, the oxidation reaction is promoted. it can. As a result, the heat of reaction increases and the temperature of the NOx catalyst can be raised. Therefore, if the application program is set so as to increase the amount of oxygen so that the reaction heat can be increased to a temperature suitable for S poison recovery, the catalyst temperature is increased by burning the fuel. There is no need. As a result, even with a small amount of fuel, the NOx catalyst can be raised to a temperature sufficient to recover S poisoning, so that the fuel consumption can be reduced.
[0032]
(2) Further, after the NOx catalyst reaches a temperature suitable for S poison recovery, the air-fuel ratio of the exhaust gas flowing through the NOx catalyst is made rich for a predetermined time, and the temperature raising means includes the NOx catalyst. It is preferable to maintain the temperature at a temperature suitable for the S poison recovery process.
[0033]
Here, the “predetermined time” refers to a time sufficient to recover the NOx storage reduction catalyst in the S poison state.
[0034]
While enriching the air-fuel ratio of the exhaust gas flowing through the NOx catalyst for a predetermined time, the temperature raising means maintains the temperature of the NOx catalyst at a temperature suitable for the S poisoning recovery process, thereby allowing the S poison recovery process. Can be executed reliably.
[0035]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of an exhaust emission control device for an internal combustion engine according to the present invention will be described below with reference to the accompanying drawings.
[0036]
First, an overall configuration of an internal combustion engine that employs an exhaust gas purification apparatus for an internal combustion engine of the present invention will be described with reference to FIG.
[0037]
An internal combustion engine 1 shown in FIG. 1 is a water-cooled four-stroke cycle diesel engine having four cylinders 2.
[0038]
The internal combustion engine 1 includes an injector 3 that directly injects fuel into the combustion chamber of each cylinder 2, and each injector 3 is connected to a pressure accumulation chamber (common rail) 4 that accumulates fuel to a predetermined pressure. The common rail 4 includes a common rail pressure sensor 4 a that outputs an electric signal corresponding to the fuel pressure in the common rail 4, and communicates with the fuel pump 6 through a fuel supply pipe 5.
[0039]
The fuel pump 6 operates using the rotational torque of the output shaft (crankshaft) of the internal combustion engine 1 as a drive source, and is attached to the pump pulley 6a attached to the input shaft of the fuel pump 6 and the output shaft (crankshaft) of the internal combustion engine 1. The crank pulley 1a is connected by a belt 7.
[0040]
When the crankshaft rotates and the rotational torque is transmitted to the input shaft of the fuel pump 6 via the crank pulley 1a-belt 7-pump pulley 6a, the fuel pump 6 has a rotational torque transmitted from the crankshaft to the input shaft of the fuel pump 6. A pump pressure corresponding to the size is generated, and an amount of fuel corresponding to the pump pressure is sent to the common rail 4 through the fuel supply pipe 5.
[0041]
The fuel sent to the common rail 4 is accumulated up to a predetermined pressure and distributed to the injectors 3 of each cylinder 2. When a drive current is applied to the injector 3, the injector 3 opens and fuel is injected from the injector 3 into the cylinder 2.
[0042]
Next, an intake branch pipe 8 is connected to the internal combustion engine 1, and each branch pipe of the intake branch pipe 8 is connected to a combustion chamber of each cylinder 2 via an intake port (not shown).
[0043]
The intake branch pipe 8 is connected to an intake pipe 9, and the intake pipe 9 has an air cleaner box 10 at its starting end. An air flow meter 11 for outputting an electric signal corresponding to the mass of the intake air flowing in the intake pipe 9 and an electric power corresponding to the temperature of the intake air flowing in the intake pipe 9 are provided in a portion downstream of the air cleaner box 10 in the intake pipe 9. An intake air temperature sensor 12 for outputting a signal is attached.
[0044]
An intake throttle valve 13 for adjusting the flow rate of the intake air flowing through the intake pipe 9 is provided in the intake pipe 9 in the vicinity of the intake branch pipe 8 upstream. The intake throttle valve 13 is connected to an intake throttle actuator 14 composed of a stepper motor or the like, and opens and closes by the operation of the intake throttle actuator 14.
[0045]
A compressor housing 15 a of a centrifugal supercharger (turbocharger) 15 that operates using the heat energy of exhaust gas as a drive source is provided between the air flow meter 11 and the intake throttle valve 13 in the intake pipe 9. The intake air that has flowed into the compressor housing 15a is compressed by the rotation of a compressor wheel built in the compressor housing 15a to become high-temperature intake air. The intake air is cooled by an intercooler 16 provided downstream of the compressor housing 15a in the intake pipe 9. As a result, the air density is prevented from lowering and the internal combustion engine is powered down. The compressor wheel is another component of the centrifugal supercharger 15 and is coaxially connected to the turbine wheel of the turbine housing 15b disposed in the exhaust system.
[0046]
The intake air cooled by way of the intercooler 16 is adjusted in flow rate by the intake throttle valve 13 and flows into the intake branch pipe 8. The intake air that has flowed into the intake branch pipe 8 is distributed to the combustion chamber of each cylinder 2 via each branch pipe, and is used for combustion of fuel injected from the injector 3 of each cylinder 2.
[0047]
On the other hand, the internal combustion engine 1 has an exhaust branch pipe 18 connected to the combustion chamber of each cylinder via an exhaust port (not shown) of each cylinder 2.
[0048]
The exhaust branch pipe 18 is connected to the turbine housing 15 b of the centrifugal supercharger 15, and the turbine housing 15 b is connected to the exhaust pipe 19. A muffler (not shown) is attached downstream of the exhaust pipe 19.
[0049]
In the middle of the exhaust pipe 19, a catalytic converter 20 is disposed as an exhaust purification device that contains an exhaust purification catalyst for purifying harmful gas components in the exhaust. Further, an air-fuel ratio sensor 23 that outputs an electric signal corresponding to the exhaust air-fuel ratio and an exhaust temperature sensor 24 that outputs an electric signal corresponding to the exhaust temperature are attached downstream of the catalytic converter 20 in the exhaust pipe 19. It is.
[0050]
Further, an exhaust throttle valve 21 for adjusting the exhaust flow rate in the exhaust pipe 19 is attached downstream of the air-fuel ratio sensor 23 and the exhaust temperature sensor 24. The exhaust throttle valve 21 is connected to an exhaust throttle actuator 22 constituted by a stepper motor or the like, and opens and closes when the exhaust throttle actuator 14 is operated.
[0051]
Then, the air-fuel mixture burned in each cylinder 2 becomes exhaust gas and is discharged to the exhaust branch pipe 18 via the exhaust port, and then enters the turbine housing 15 b of the centrifugal supercharger 15 from the exhaust branch pipe 18. The exhaust gas flowing into the turbine housing 15b rotates a turbine wheel rotatably supported in the turbine housing 15b, and the intake air is rotated by rotating the compressor wheel of the compressor housing 15a that is coaxially connected to the turbine wheel. To high pressure.
[0052]
Exhaust gas discharged from the turbine housing 15b flows into the catalytic converter 20 through the exhaust pipe 19, where harmful gas components contained in the exhaust gas are removed or purified. Exhaust gas from which harmful gas components have been removed or purified by the catalytic converter 20 is adjusted in flow rate as necessary by the exhaust throttle valve 21, passes through the muffler, and is then released into the atmosphere.
[0053]
Further, the exhaust branch pipe 18 and the intake branch pipe 8 are exhaust recirculation passages (hereinafter referred to as “EGR passages”) 25 for recirculating a part of the exhaust gas flowing in the exhaust branch pipe 18 to the intake branch pipe 8. linked. The EGR passage 25 has a flow rate adjusting valve (hereinafter referred to as “EGR valve”) 26 for changing the flow rate of exhaust gas recirculation gas (hereinafter referred to as “EGR gas”) flowing therethrough. The EGR valve 26 is configured by an electromagnetic valve or the like, and its opening degree is determined according to the magnitude of applied power, and the flow rate of EGR gas changes according to the opening degree.
[0054]
Further, an EGR cooler 27 that cools the EGR gas flowing in the EGR passage 25 is provided upstream of the EGR valve 26 in the EGR passage 25.
[0055]
When the EGR valve 26 is opened, the EGR passage 25 becomes conductive, and a part of the exhaust gas flowing in the exhaust branch pipe 18 flows into the EGR passage 25 and flows into the intake branch pipe 8 via the EGR cooler 27.
[0056]
At that time, in the EGR cooler 27, heat exchange is performed between the EGR gas flowing in the EGR passage 25 and a predetermined refrigerant to cool the EGR gas.
[0057]
The EGR gas recirculated from the exhaust branch pipe 18 to the intake branch pipe 8 via the EGR passage 25 is guided to the combustion chamber of each cylinder 2 while being mixed with fresh air flowing from the upstream side of the intake branch pipe 8. Is used for combustion of the injected fuel. However, EGR gas contains water (H₂O) and carbon dioxide (CO₂) And the like, and since an inert gas component having endothermic properties is contained without being combusted by itself, if the EGR gas is contained in the mixture, the combustion temperature of the mixture decreases, Reduces the amount of nitrogen oxide (NOx) generated.
[0058]
Further, when the EGR gas is cooled by the EGR cooler 27, the temperature is lowered and the volume of the EGR gas is reduced. For this reason, when the EGR gas is supplied into the combustion chamber, the increase in the atmospheric temperature in the combustion chamber is suppressed, and the amount of fresh air supplied into the combustion chamber is not extremely reduced. The operation of the engine 1 is not hindered.
[0059]
The internal combustion engine 1 also includes a reducing agent supply mechanism that adds fuel (light oil) as a reducing agent to the exhaust gas flowing through the exhaust passage upstream of the catalytic converter 20.
[0060]
As shown in FIG. 1, this reducing agent supply mechanism is attached to the cylinder head of the internal combustion engine 1 so that its injection hole faces the exhaust branch pipe 18 and opens when fuel of a predetermined pressure or higher is applied. An injection valve 28, a reducing agent supply path 29 for guiding the fuel discharged from the fuel pump 6 described above to the reducing agent injection valve 28, and an inside of the reducing agent supply path 29 provided in the reducing agent supply path 29. A flow rate adjusting valve 30 that adjusts the flow rate of the flowing fuel, and a shutoff valve 31 that is provided in the reducing agent supply path 29 upstream from the flow rate adjusting valve 30 and blocks the fuel flow in the reducing agent supply path 29 are provided. ing.
[0061]
Note that the reducing agent injection valve 28 has the injection hole of the reducing agent injection valve 28 downstream of the connection portion of the EGR passage 25 in the exhaust branch pipe 18, and is most at the gathering portion of the four branch pipes in the exhaust branch pipe 18. It is preferable to attach to the cylinder head so that it protrudes to the exhaust port of the near cylinder 2 and faces the collecting portion of the exhaust branch pipe 18.
[0062]
This prevents the reducing agent (unburned fuel component) injected from the reducing agent injection valve 28 from flowing into the EGR passage 25, and the centrifugal supercharger without the reducing agent remaining in the exhaust branch pipe 18. This is to reach the 15 turbine housings 15b.
[0063]
In the example shown in FIG. 1, the first (# 1) cylinder 2 of the four cylinders 2 of the internal combustion engine 1 is in the position closest to the collecting portion of the exhaust branch pipe 18 so that the first (# 1) The reducing agent injection valve 28 is attached to the exhaust port of the cylinder 2, but when the cylinders 2 other than the first (# 1) cylinder 2 are located closest to the aggregate portion of the exhaust branch pipe 18, the cylinder 2 A reducing agent injection valve 28 is attached to the exhaust port.
[0064]
In such a reducing agent supply mechanism, when the flow rate adjustment valve 30 is opened, the high-pressure fuel sent from the fuel pump 6 is applied to the reducing agent injection valve 28 via the reducing agent supply path 29. When the pressure of the fuel applied to the reducing agent injection valve 28 reaches a predetermined pressure or higher, the reducing agent injection valve 28 is opened to inject fuel as a reducing agent into the exhaust branch pipe 18.
[0065]
The reducing agent injected into the exhaust branch pipe 18 from the reducing agent injection valve 28 flows into the turbine housing 15 b together with the exhaust gas flowing from the upstream side of the exhaust branch pipe 18. The exhaust gas and the reducing agent that have flowed into the turbine housing 15b are agitated by the rotation of the turbine wheel and mixed homogeneously to form a rich air-fuel ratio exhaust gas.
[0066]
The exhaust gas flows into the catalytic converter 20 from the turbine housing 15b through the exhaust pipe 19, and releases nitrogen oxides (NOx) stored in the exhaust purification catalyst of the catalytic converter 20 while nitrogen (N₂).
[0067]
Thereafter, when the flow rate adjusting valve 30 is closed and the supply of the reducing agent from the fuel pump 6 to the reducing agent injection valve 28 is cut off, the pressure of the fuel applied to the reducing agent injection valve 28 becomes less than the predetermined pressure. As a result, the reducing agent injection valve 28 is closed and the addition of the reducing agent into the exhaust branch pipe 18 is stopped.
[0068]
The internal combustion engine 1 has an electronic control unit (ECU) 35 that controls the internal combustion engine 1. The ECU 35 is a unit that controls the operation state of the internal combustion engine 1 in accordance with the operation conditions of the internal combustion engine 1 and the driver's request, and is a central processing control unit CPU that is mutually connected by a bidirectional bus (not shown). , Read-only memory ROM, random access memory RAM, input interface circuit, output interface circuit, and the like.
[0069]
The input interface circuit includes a common rail pressure sensor 4a, an air flow meter 11, an intake air temperature sensor 12, an intake pipe pressure sensor 17, an air-fuel ratio sensor 23, an exhaust gas temperature sensor 24, a reducing agent pressure sensor 32, a crank position sensor 33, and a water temperature sensor 34. These are electrically connected to various sensors such as the accelerator opening sensor 36. When the output signals of these various sensors enter the ECU 35 from the input interface circuit, these parameters are temporarily stored in the random access memory RAM. The random access memory RAM stores the arithmetic processing required by the CPU based on these parameters in accordance with various application programs stored in the read-only memory ROM. The map stored in the parameter or ROM is called up as necessary.
[0070]
The output interface circuit is electrically connected to various devices such as an injector 3, an intake throttle actuator 14, an exhaust throttle actuator 22, an EGR valve 26, a flow rate adjusting valve 30, and a shutoff valve 31 that operate based on output signals of the various sensors. These various devices are appropriately operated based on the calculation result of the CPU.
[0071]
Next, the catalytic converter 20 that is an exhaust purification device will be described in detail.
[0072]
The catalytic converter 20 contains a NOx catalyst for purifying nitrogen oxide (NOx) in the exhaust gas in the presence of a reducing agent in the case. Examples of such a NOx catalyst include a selective reduction type NOx catalyst and an occlusion. Although a reduction type NOx catalyst etc. can be illustrated, an occlusion reduction type NOx catalyst will be described as an example here.
[0073]
The NOx storage reduction catalyst is, for example, alumina (Al₂O_Three) As a carrier, and an alkali metal such as potassium (K), sodium (Na), lithium (Li), and cesium (Cs) and an alkali such as barium (Ba) and calcium (Ca) on the carrier. It is configured to carry at least one selected from earth and a rare earth such as lanthanum (La) yttrium (Y) and a noble metal such as platinum (Pt).
[0074]
The NOx storage reduction catalyst configured as described above stores nitrogen oxides (NOx) in the exhaust when the exhaust air-fuel ratio is a lean air-fuel ratio, and the oxygen concentration of the exhaust gas flowing into the NOx storage reduction catalyst has an oxygen concentration. When it is reduced and a reducing agent is present, it is reduced and purified while releasing nitrogen oxides (NOx) that have been occluded.
[0075]
The exhaust air-fuel ratio here means the ratio of the total amount of air supplied to the exhaust pipe 19 upstream of the catalytic converter 20, the combustion chamber, the intake pipe 9 and the like and the total amount of fuel (hydrocarbon). It shall be. Therefore, unless the fuel, reducing agent, or air is supplied to the exhaust system upstream from the catalytic converter 20, the exhaust air-fuel ratio matches the air-fuel ratio of the air-fuel mixture supplied to the combustion chamber.
[0076]
Here, the NOx absorption / release mechanism of the NOx storage reduction catalyst will be described by taking as an example an NOx storage reduction catalyst in which platinum (Pt) and barium (Ba) are supported on a support made of alumina.
[0077]
It is considered that the NOx absorption / release action of the NOx storage reduction catalyst is performed by the mechanism shown in FIG.
[0078]
First, the NOx storage reduction catalyst, when the oxygen concentration in the exhaust gas increases as the air-fuel ratio of the exhaust gas flowing into the NOx storage catalyst becomes a lean air-fuel ratio, as shown in FIG. Oxygen (O₂) Is O₂ ^-Or O^2-The nitrogen monoxide (NO) in the exhaust gas is deposited on the surface of platinum (Pt) in the form of₂ ^-Or O^2-Reacts with nitrogen dioxide (NO₂) (2NO + O₂→ 2NO₂).
[0079]
Nitrogen dioxide (NO₂) Is oxidized on the surface of platinum (Pt) and combined with barium oxide (BaO) to form nitrate ions (NO_Three ^-). Thus, nitrogen oxides (NOx) in the exhaust gas are nitrate ions (NO_3-) Is stored in the NOx storage reduction catalyst.
[0080]
Such an action is called a NOx occlusion action, and the NOx occlusion action is continued as long as the exhaust air-fuel ratio flowing into the occlusion reduction type NOx catalyst is lean and the NOx occlusion capacity of the occlusion reduction type NOx catalyst is not saturated.
[0081]
In contrast, when the oxygen concentration in the inflowing exhaust gas decreases, nitrogen dioxide (NO₂Nitrate ions (NO) bound to barium oxide (BaO)._3-) On the contrary, nitrogen dioxide (NO₂) Or nitrogen monoxide (NO) and is released from the NOx storage reduction catalyst.
[0082]
That is, when the oxygen concentration in the exhaust gas flowing into the NOx storage reduction catalyst decreases, nitrate ions (NO_3-) In the form of nitrogen oxides (NOx) stored in the NOx storage reduction catalyst.₂) And nitrogen monoxide (NO) and released from the NOx storage reduction catalyst.
[0083]
As shown in FIG. 2B, the nitrogen oxide (NOx) released from the NOx storage reduction catalyst is reduced by a reducing component (for example, oxygen O on platinum (Pt) of the NOx storage reduction catalyst)._2-Or O^2-Reacts with partially oxidized hydrocarbons (HC), carbon monoxide (CO) and other active species) to react with nitrogen (N₂) Etc.
[0084]
That is, hydrocarbons (HC) and carbon monoxide (CO) in the exhaust gas are oxidized on platinum (Pt).₂ ^-Or O^2-Reacts with and oxidizes, thereby O on platinum (Pt)₂ ^-Or O^2-If hydrocarbons (HC) and carbon monoxide (CO) still remain after the consumption of NO, these hydrocarbons (HC) and carbon monoxide (CO) are released from the NOx storage reduction catalyst. Oxides (NOx) and nitrogen oxides (NOx) discharged from the internal combustion engine 1 are nitrogen (N₂).
[0085]
Therefore, by reducing the air-fuel ratio of the exhaust gas flowing into the NOx storage reduction catalyst to the stoichiometric or rich air-fuel ratio, the NOx stored in the NOx storage reduction catalyst is reduced while being released. Is possible.
[0086]
By the way, since the NOx storage capacity of the NOx storage reduction catalyst is limited, when the lean air-fuel ratio exhaust gas flows into the NOx storage reduction catalyst over a long period of time, the NOx storage capacity of the NOx storage reduction catalyst is saturated and the exhaust The nitrogen oxide (NOx) is released into the atmosphere without being removed or purified by the NOx storage reduction catalyst.
[0087]
However, for example, in a diesel engine, the lean air-fuel ratio mixture is burned in the most operating region, and the exhaust air-fuel ratio becomes the lean air-fuel ratio in the most operating region accordingly. There is a possibility that the NOx occlusion capacity is saturated and no more can be absorbed and NOx leaks.
[0088]
In order to avoid leakage, the NOx absorption capacity must be restored. To that end, the NOx absorption capacity of the NOx storage reduction catalyst must be restored by releasing the NOx that had previously been absorbed by the NOx storage reduction catalyst. There is.
[0089]
Therefore, before the NOx storage reduction catalyst is saturated, the air-fuel ratio of the inflowing exhaust gas is made rich at a predetermined timing to extremely reduce the oxygen concentration and release NOx from the NOx storage reduction catalyst, thereby absorbing the NOx. It is necessary to regenerate the NOx storage reduction catalyst that recovers its capacity.
[0090]
However, if the NOx storage reduction catalyst can be regenerated by simply releasing NOx, NOx purification, which is the original function of the NOx catalyst, cannot be achieved, leading to deterioration of emissions. Therefore, for example, a fuel injection valve is provided in the exhaust port, and engine fuel is injected from the fuel injection valve continuously or intermittently according to the operating state of the internal combustion engine, and the engine fuel is added to the NOx storage reduction catalyst as a reducing agent. As a result, the released NOx is reduced to N₂NOx purification is carried out by reducing the amount.
[0091]
By the way, since engine fuel contains a large amount of sulfur components, it generates sulfur oxides (hereinafter referred to as “SOx” unless otherwise specified) when burned. SOx is absorbed by the NOx storage reduction catalyst by the same mechanism as NOx.
[0092]
Since SOx forms a stable sulfate, it is difficult to be decomposed and released, and is easily accumulated in the NOx storage reduction catalyst. As a result, the NOx purification rate of the NOx storage reduction catalyst is reduced by the amount of SOx absorbed by the NOx storage reduction catalyst, causing a phenomenon that hinders NOx purification, which is a function of the NOx storage reduction catalyst, so-called S poisoning. There is a fear. Therefore, it is necessary to recover the NOx storage reduction catalyst in the S poison state, and the process for this is called the S poison recovery processing of the NOx storage reduction catalyst.
[0093]
However, since the sulfate accumulated in the NOx storage reduction catalyst is relatively stable as described above, it was at a temperature at which the NOx storage reduction catalyst regenerates it (for example, about 250 to 500 ° C.). Therefore, it is difficult to release SOx absorbed by the NOx storage reduction catalyst.
[0094]
In order to perform the S poisoning recovery process, the temperature of the NOx storage reduction catalyst is raised to a high temperature of, for example, about 600 to 700 ° C., which is higher than the temperature at which the NOx storage regeneration catalyst is regenerated, and the exhaust air-fuel ratio is stoichiometrically increased. Or deal with it regularly by making it rich.
[0095]
On the other hand, the diesel engine has a lower exhaust temperature than a conventional gasoline engine, and it is difficult to achieve the high temperature necessary for the S poison recovery process. For example, when the diesel engine is in a high load operation state, that is, when the diesel engine is operating at a high temperature for the diesel engine, if the exhaust air-fuel ratio is made stoichiometric or rich, soot, unburned fuel components, etc. Particulate matter such as PM is easily discharged. Therefore, the exhaust air-fuel ratio cannot be stoichiometric or rich at the high temperature.
[0096]
Therefore, the sulfur poisoning is recovered by raising the catalyst bed temperature, which is the temperature of the NOx storage reduction catalyst at the high temperature, and making the exhaust air / fuel ratio stoichiometric or rich at the time of light load operation in which particulates such as soot are difficult to be discharged. Can be considered.
[0097]
However, if the diesel engine is accelerated, for example, to secure the temperature suitable for the S poison recovery process, the fuel injection amount increases to maintain the engine output in the operating state, which is not preferable in terms of fuel consumption. . The above problems are as already described in the description of the prior art.
[0098]
In contrast, the internal combustion engine 1 according to the present embodiment improves the fuel consumption by performing the S poison recovery process when the internal combustion engine is decelerating while ensuring the function of the NOx storage reduction catalyst. Technology.
[0099]
That is, when the internal combustion engine 1 is in an acceleration operation state or a steady operation state, the NOx catalyst is raised within a temperature range suitable for NOx purification, and when the internal combustion engine 1 is in a deceleration operation state, the exhaust air-fuel ratio is increased. The internal combustion engine 1 has a temperature raising means for making the temperature of the NOx catalyst suitable for S poison recovery by making lean.
[0100]
Note that “in the steady operation state” of “the internal combustion engine 1 is in the acceleration operation state or the steady operation state” means that the engine is operated in a state where the amount of depression of the accelerator by the driver is constant and the engine speed is constant. Means state.
[0101]
Next, a program for realizing the S poison recovery process execution routine of the engine 1 by the ECU 35 will be described with reference to the flowchart of FIG.
[0102]
This program includes steps 101 to 108 described below. Moreover, the program which consists of these steps is memorize | stored in ROM of ECU35, and is called as needed. All processes in the steps are performed by the CPU of the ECU 35. For example, step 101 is abbreviated as S101 using the symbol S.
[0103]
This routine estimates the S poisoning amount of the NOx storage reduction catalyst from the fuel consumption, and if it reaches a predetermined value or more, it is determined that the NOx storage reduction catalyst should recover the S poisoning. Migrate to
[0104]
That is, in S101, it is determined whether or not there is a need for the S poison recovery process. In order to recover S poison, it is necessary that the catalyst bed temperature is 600 ° C. or higher and the exhaust air / fuel ratio is 14.5 or lower. If the conditions are not satisfied, S poison cannot be recovered.
[0105]
In S102, first, the catalyst bed temperature is detected, and when the temperature is 500 ° C. or lower, temperature increase control is performed. That is, it is determined whether the temperature is higher than 500 ° C. If a negative determination is made, the process proceeds to S103. If an affirmative determination is made, this process is repeated until a negative determination is made.
[0106]
When the catalyst bed temperature is higher than 500 ° C., the process after S103 is prevented from proceeding to the combustion of fuel by making the exhaust air-fuel ratio lean when the internal combustion engine 1 is in the decelerating operation state. This is because the technical idea of the present invention is that the temperature of the NOx catalyst is set to the temperature of 600 ° C. or more, which is suitable for the recovery of S poison.
[0107]
In addition, useless deterioration of fuel consumption is prevented by setting the determination value to 500 ° C., which is not far from 600 ° C., which is a condition for recovery from S poisoning. Further, 500 ° C. is a limit value for the deterioration of the reduction performance of the NOx catalyst 20, and although the reduction performance deteriorates at a temperature higher than that, it is a temperature suitable for NOx purification. A temperature range suitable for NOx purification is 250 to 500 ° C.
[0108]
In S103, when the catalyst bed temperature is 500 ° C. or lower, the temperature increase control is executed, and the temperature increase control by the fuel injected from the reducing agent injection valve 28 according to the operating state of the internal combustion engine, Either the so-called post-injected fuel temperature rise control, which is a secondary injection other than the main injection injected by the injector 3, or the so-called engine rich temperature rise control that makes the air-fuel ratio rich, or a combination thereof, brings the engine into operation. Perform the temperature rise control accordingly. By this temperature increase control, the NOx storage reduction catalyst is raised to a temperature range suitable for NOx purification.
[0109]
In S104, it is determined whether the internal combustion engine 1 is in an acceleration operation state or a steady operation state. If an affirmative determination is made, the process returns to S102. If a negative determination is made, that is, if the internal combustion engine 1 is in a deceleration operation state, the process returns to S105. move on.
[0110]
In S105, which proceeds when the internal combustion engine 1 is in a decelerating operation state, the exhaust air-fuel ratio is instantaneously leaned. By doing so, the NOx storage reduction catalyst is instantaneously placed in an oxygen-excess atmosphere. That is, since the amount of oxygen in the exhaust gas increases, the increased oxygen is supplied toward the NOx storage reduction catalyst. As a result, the oxidation reaction is promoted to increase the heat of reaction, and the temperature of the NOx storage reduction catalyst can be raised.
[0111]
In S106, it is determined whether or not the catalyst bed temperature is higher than the above-mentioned 600 ° C. necessary for S poison recovery. If an affirmative determination is made, the process proceeds to S107. The process is repeated. In this way, the temperature of the catalyst bed is controlled to rise to 600 ° C. S106 is a process performed in a state where the exhaust air-fuel ratio is instantaneously lean when the internal combustion engine 1 is in a decelerating operation state (see S104 and S105). Therefore, S104 to S106 can be said to be temperature raising means for making the temperature of the NOx storage reduction catalyst suitable for S poison recovery by making the exhaust air-fuel ratio lean when the internal combustion engine 1 is in the decelerating operation state. Further, this program including S104 to S106 is stored in the ROM, and the attribute of the ROM is in the ECU 9. Therefore, when the internal combustion engine 1 is in the decelerating operation state, the ECU 35 makes the exhaust air-fuel ratio lean so that the NOx storage reduction type NOx. It can be said that the temperature of the catalyst is raised to a temperature suitable for S poison recovery. The temperature raising means is also a temperature raising means for maintaining the NOx catalyst at a temperature suitable for NOx purification when the internal combustion engine 1 is in an acceleration operation state or a steady operation state (see S102, S103, and S104).
[0112]
In S107, the exhaust air-fuel ratio is made rich for a predetermined time by performing the same temperature rise control as the temperature rise control performed in S103 by executing fuel injection from the reducing agent injection valve 28 or executing engine rich.
[0113]
In S108, the predetermined time in S107 is counted by an appropriate timer (not shown). This predetermined time is obtained by experiment and using an appropriate arithmetic expression. Then, after the predetermined time has elapsed, the S poison recovery control ends.
[0114]
The internal combustion engine 1 having such a configuration has the following operational effects.
[0115]
When S poisoning of the NOx catalyst is to be recovered, when the internal combustion engine 1 is in the acceleration operation state or the steady operation state, the NOx storage reduction catalyst is raised to, for example, 500 ° C. which belongs to a temperature range suitable for NOx purification. Therefore, NOx can be reduced and purified by adding a reducing agent at that time.
[0116]
In addition, when the internal combustion engine 1 is in a decelerating operation state, the amount of oxygen in the exhaust can be increased by making the exhaust air-fuel ratio lean, so that if the increased oxygen is supplied toward the NOx storage reduction catalyst, oxidation will occur. The reaction can be promoted. Therefore, since the reaction heat increases and the temperature of the NOx catalyst can be increased, the oxygen amount is increased so that the reaction heat can be increased to a temperature suitable for S poison recovery (see S105). There is no need to intend to increase the catalyst temperature by burning the fuel as in this technique. As a result, even with a small amount of fuel, the NOx catalyst can be raised to a temperature sufficient to perform the S poisoning recovery process, so that the fuel consumption can be reduced.
[0117]
In addition, the timer counts the predetermined time, during which the air-fuel ratio of the exhaust gas flowing through the NOx storage reduction catalyst is made rich, and the ECU 35 as the temperature raising means sets the temperature of the NOx storage reduction catalyst. By maintaining the temperature suitable for the S poison recovery process, the S poison recovery process can be performed reliably.
[0118]
【The invention's effect】
According to the exhaust gas purification apparatus for an internal combustion engine of the present application, fuel consumption can be suppressed by performing the S poison recovery process when the internal combustion engine is running at a reduced speed while ensuring the function of the NOx catalyst.
[Brief description of the drawings]
FIG. 1 is a diagram showing an overall configuration of an internal combustion engine to which an exhaust gas purification apparatus for an internal combustion engine of the present invention is applied.
FIG. 2 NOx absorption / release action diagram of NOx storage reduction catalyst
FIG. 3 is a flowchart for explaining a program for realizing an S poison recovery process execution routine of the internal combustion engine;
[Explanation of symbols]
1 Internal combustion engine
2-cylinder
3 Injector
4 Common rail
4a Common rail pressure sensor
5 Fuel supply pipe
6 Fuel pump
6a Pump pulley
1a Crank pulley
7 Belt
8 Intake branch pipe
9 Intake pipe
10 Air cleaner box
11 Air flow meter
12 Intake air temperature sensor
13 Inlet throttle valve
14 Inlet throttle actuator
15 Centrifugal supercharger
15a Compressor housing
16 Intercooler
15b Turbine housing
18 Exhaust branch pipe
19 Exhaust pipe
20 Catalytic converter
23 Air-fuel ratio sensor
24 Exhaust temperature sensor
22 Exhaust throttle actuator
21 Exhaust throttle valve
25 EGR passage
26 EGR valve
27 EGR cooler
28 Reducing agent injection valve
29 Reducing agent supply path
30 Flow control valve
31 Shut-off valve
35 ECU
17 Intake pipe pressure sensor
32 Reducing agent pressure sensor
33 Crank position sensor
34 Water temperature sensor
35 ECU (temperature raising means)
36 Accelerator position sensor

Claims (3)

A NOx catalyst that absorbs NOx in the exhaust when the exhaust air-fuel ratio is lean and releases the NOx that has been absorbed when the oxygen concentration in the exhaust decreases, and performs NOx reduction purification by adding a reducing agent;
In the case where the S poisoning of the NOx catalyst is to be recovered, when the internal combustion engine is in an acceleration operation state or a steady operation state, the NOx catalyst is within a range suitable for NOx purification by combustion of fuel in the NOx catalyst . And the exhaust air-fuel ratio is made lean when the internal combustion engine is in a decelerating operation state so as not to exceed a temperature lower than the temperature suitable for S poison recovery of the NOx catalyst. a temperature increase device that the temperature of the NOx catalyst to a temperature suitable for the S poisoning elimination,
An exhaust gas purification apparatus for an internal combustion engine having The temperature within the range suitable for the NOx purification and lower than the temperature suitable for the S poison recovery of the NOx catalyst is the upper limit temperature for the deterioration of the reduction performance of the NOx catalyst.
  The exhaust emission control device for an internal combustion engine according to claim 1. After the NOx catalyst reaches a temperature suitable for S poison recovery, the air-fuel ratio of the exhaust gas flowing through the NOx catalyst is made rich for a predetermined time, and the temperature raising means sets the temperature of the NOx catalyst to S. The exhaust gas purification apparatus for an internal combustion engine according to claim 1 or 2, wherein the exhaust gas purification apparatus is maintained at a temperature suitable for poisoning recovery processing.

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