JP4639565B2 - 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 for an internal combustion engine.
[0002]
[Prior art]
As an exhaust purification device that purifies exhaust gas from an internal combustion engine, a catalyst having a plurality of oxidation functions, for example, a NOx catalyst, may be arranged in parallel in an exhaust passage.
[0003]
The NOx catalyst is a catalyst capable of purifying exhaust gas discharged from a lean burnable internal combustion engine such as a diesel engine or a lean burn gasoline engine, and examples thereof include a selective reduction type NOx catalyst and an occlusion reduction type NOx catalyst.
[0004]
For example, the NOx storage reduction catalyst absorbs NOx when the oxygen concentration of the inflowing exhaust gas is high, and releases NOx that has been absorbed when the oxygen concentration of the inflowing exhaust gas decreases. ₂ It absorbs and releases NOx that is reduced to NOx.
[0005]
In the case of this NOx storage reduction catalyst, in the internal combustion engine, the air-fuel ratio of exhaust during normal operation becomes lean, so that NOx in the exhaust is absorbed by the NOx catalyst. However, if the lean air-fuel ratio exhaust gas continues to be supplied to the NOx catalyst, the NOx absorption capacity of the NOx catalyst is saturated, and no more NOx can be absorbed, and NOx is leaked. Therefore, in the NOx catalyst, the oxygen concentration of the inflowing exhaust gas is decreased and the amount of HC component in the exhaust gas is increased at a predetermined timing before the NOx absorption capacity is saturated, and the NOx absorbed in the NOx catalyst is released while Nx is released. ₂ To reduce the NOx absorption capacity of the NOx catalyst.
[0006]
As described above, in the exhaust gas purification apparatus using the lean NOx catalyst, it is necessary to intermittently lower the oxygen concentration of the exhaust gas in order to purify NOx. An example of a method for intermittently lowering the oxygen concentration in the exhaust is addition of fuel in the exhaust.
[0007]
On the other hand, the NOx storage reduction catalyst also absorbs sulfur oxide (SOx) produced by combustion of sulfur contained in the fuel by the same mechanism as NOx. The SOx absorbed in this manner is less likely to be released than NOx and is accumulated in the NOx catalyst. This is called sulfur poisoning (SOx poisoning), and the NOx purification rate decreases. Therefore, it is necessary to perform poisoning recovery processing for recovering from SOx poisoning at an appropriate time. This poisoning recovery process is performed by circulating the exhaust gas having a reduced oxygen concentration while keeping the NOx catalyst at a high temperature (for example, about 600 to 650 ° C.) through the NOx catalyst.
[0008]
However, since the temperature of the exhaust gas during the lean combustion operation is low, it is difficult to raise the temperature of the catalyst to a temperature required for recovery from SOx poisoning.
[0009]
To deal with such a problem, for example, in Japanese Patent No. 2727906, in an exhaust gas purification apparatus for an internal combustion engine in which two particulate filters carrying a NOx absorbent are arranged in parallel in an exhaust pipe, exhaust gas flowing into the NOx absorbent is Each is blocked to release NOx from the NOx absorbent and to eliminate SOx poisoning. In the exhaust gas purification apparatus for an internal combustion engine configured as described above, it is possible to operate the exhaust gas through one NOx absorbent while the other NOx absorbent is being regenerated, so there is no need to reduce the exhaust flow rate as a whole. The output of the internal combustion engine is not reduced. Therefore, it is possible to perform an operation such as regeneration of the NOx absorbent at an arbitrary time regardless of the operating conditions. Then, the particulates burn when the reducing agent is supplied, and the temperature of the filter can be raised to a temperature required for eliminating SOx poisoning.
[0010]
[Problems to be solved by the invention]
However, if the temperature increase control of the NOx absorbent is performed one by one, it takes twice as much time to complete the temperature increase of both NOx absorbents as compared with a system having one NOx catalyst. Therefore, during that period, the purification rate of NOx or the like decreases, and there is a risk that exhaust emissions will deteriorate.
[0011]
Further, the catalyst has a temperature range (temperature window) in which exhaust gas can be effectively purified, and it is important to maintain the temperature of the catalyst within the temperature window range. However, when there is a risk that the temperature of the catalyst may drop and the temperature window may fall off, as described above, if the temperature of the catalyst is increased one by one, the temperature of the other catalyst is increased during the temperature increase of one catalyst. There is a risk that the purification rate will decrease due to the temperature window being removed. The above-described temperature rise control of the catalyst is also performed when oxidizing the accumulated particulates, and it is desired to raise the temperature of the catalyst in a short time.
[0012]
The present invention has been made in order to solve the above problems, and in an exhaust gas purification apparatus for an internal combustion engine provided with a plurality of catalysts having an oxidation function, a technique capable of quickly raising the temperature of the plurality of catalysts. The purpose is to provide.
[0013]
[Means for Solving the Problems]
In order to achieve the above object, the exhaust gas purification apparatus for an internal combustion engine of the present invention employs the following means. That is,
A plurality of exhaust passages branched in parallel, which are exhaust passages of an internal combustion engine;
A catalyst provided in each of the branched exhaust passages and having an oxidation function;
Reducing agent supply means for supplying a reducing agent to the catalyst;
With
When it is necessary to increase the temperature of the catalyst, the reducing agent supply means supplies the reducing agent to the plurality of catalysts to simultaneously increase the temperature of the plurality of catalysts.
[0014]
The most important feature of the present invention is that, in an exhaust gas purification apparatus for an internal combustion engine provided with a catalyst having a plurality of oxidation functions, a reducing agent is simultaneously supplied to the plurality of catalysts to increase the purification rate of the plurality of catalysts in a short time. It is in.
[0015]
In the exhaust gas purification apparatus for an internal combustion engine configured as described above, exhaust gas flows through a plurality of catalysts during normal operation. Thus, if exhaust gas is simultaneously circulated through a plurality of catalysts, the temperatures of the plurality of catalysts may decrease simultaneously. Thus, when it is necessary to raise the temperature of the catalyst, the reducing agent supply means supplies the reducing agent to a plurality of catalysts. The reducing agent supplied to the catalyst reacts with the catalyst and raises the temperature of the catalyst. In this way, the temperature of the plurality of catalysts can be raised at the same time, and the purification rate of the plurality of catalysts can be increased at an early stage.
[0016]
In the present invention, the exhaust gas purification apparatus for an internal combustion engine includes a catalyst temperature raising means that does not depend on a reducing agent supply means, and the catalyst has a hydrocarbon purification rate that varies depending on the bed temperature, When the bed temperature is lower than the temperature at which hydrocarbons can be purified, the temperature of the plurality of catalysts is raised simultaneously by the catalyst temperature raising means that does not depend on the reducing agent supply means, while the bed temperature of the catalyst is carbonized. When the temperature is higher than the temperature at which hydrogen can be purified, a reducing agent can be supplied to a plurality of catalysts to simultaneously raise the bed temperatures of the plurality of catalysts.
[0017]
Thus, in the exhaust gas purification apparatus for an internal combustion engine thus configured, when the catalyst bed temperature is in a temperature range in which the temperature rise of the catalyst can be expected by supplying the reducing agent, the temperature of the catalyst is controlled by supplying the reducing agent. On the other hand, when the bed temperature of the catalyst is at a temperature lower than the temperature range in which the increase in the temperature of the catalyst can be expected by supplying the reducing agent, for example, an operation for increasing the temperature of the exhaust gas of the internal combustion engine is performed. Increase the temperature. In this way, even when the catalyst is at a low temperature, the temperature can be increased at an early stage.
[0018]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, specific embodiments of an exhaust emission control device for an internal combustion engine according to the present invention will be described with reference to the drawings. Here, the case where the exhaust gas purification apparatus for an internal combustion engine according to the present invention is applied to a diesel engine for driving a vehicle will be described as an example.
[0019]
FIG. 1 is a diagram showing a schematic configuration of an engine 1 to which an exhaust emission control device according to the present invention is applied and an intake / exhaust system thereof.
[0020]
An engine 1 shown in FIG. 1 is a water-cooled four-cycle diesel engine having four cylinders 2.
[0021]
The engine 1 includes a fuel injection valve 3 that injects fuel directly into the combustion chamber of each cylinder 2. Each fuel injection valve 3 is connected to a pressure accumulation chamber (common rail) 4 that accumulates fuel to a predetermined pressure. A common rail pressure sensor 4 a that outputs an electrical signal corresponding to the fuel pressure in the common rail 4 is attached to the common rail 4.
[0022]
The common rail 4 communicates with a fuel pump 6 through a fuel supply pipe 5. This fuel pump 6 is a pump that operates using the rotational torque of the output shaft (crankshaft) of the engine 1 as a drive source, and a pump pulley 6 a attached to the input shaft of the fuel pump 6 is connected to the output shaft (crankshaft) of the engine 1. ) And the belt pulley 7 are connected to each other.
[0023]
In the fuel injection system configured as described above, when the rotational torque of the crankshaft is transmitted to the input shaft of the fuel pump 6, the fuel pump 6 transmits the rotational torque transmitted from the crankshaft to the input shaft of the fuel pump 6. The fuel is discharged at a pressure according to the pressure.
[0024]
The fuel discharged from the fuel pump 6 is supplied to the common rail 4 via the fuel supply pipe 5, accumulated in the common rail 4 up to a predetermined pressure, and distributed to the fuel injection valves 3 of each cylinder 2. When a drive current is applied to the fuel injection valve 3, the fuel injection valve 3 opens, and as a result, fuel is injected from the fuel injection valve 3 into the cylinder 2.
[0025]
Next, an intake branch pipe 8 is connected to the engine 1, and each branch pipe of the intake branch pipe 8 communicates with a combustion chamber of each cylinder 2 via an intake port (not shown).
[0026]
The intake branch pipe 8 is connected to an intake pipe 9, and a compressor housing 15a of a centrifugal supercharger (turbocharger) 15 that operates using the heat energy of exhaust gas as a drive source is provided in the middle of the intake pipe 9. The intake pipe 9 downstream of the compressor housing 15a is provided with an intercooler 16 for cooling the intake air that has been compressed in the compressor housing 15a and has reached a high temperature.
[0027]
In the intake system configured as described above, the intake air flows into the compressor housing 15 a via the intake pipe 9.
[0028]
The intake air flowing into the compressor housing 15a is compressed by the rotation of the compressor wheel built in the compressor housing 15a. The intake air that has been compressed in the compressor housing 15 a and has reached a high temperature is cooled by the intercooler 16 and then flows into the intake branch pipe 8. The intake air that has flowed into the intake branch pipe 8 is distributed to the combustion chambers of the respective cylinders 2 through the respective branch pipes, and is burned using the fuel injected from the fuel injection valves 3 of the respective cylinders 2 as an ignition source.
[0029]
On the other hand, an exhaust branch pipe 18 is connected to the engine 1, and each branch pipe of the exhaust branch pipe 18 communicates with a combustion chamber of each cylinder 2 via an exhaust port (not shown).
[0030]
The exhaust branch pipe 18 is connected to the turbine housing 15 b of the turbocharger 15. The turbine housing 15b is connected to one end of an exhaust pipe 19, and the other end of the exhaust pipe 19 communicates with a muffler (not shown).
[0031]
The exhaust pipe 19 is branched downstream of the turbocharger 15 into a first exhaust pipe 19a and a second exhaust pipe 19b. A first filter 20a is provided in the middle of the first exhaust pipe 19a, while a second filter 20b is provided in the middle of the second exhaust pipe 19b. The first filter 20a and the second filter 20b are particulate filters (hereinafter simply referred to as filters) carrying an NOx storage reduction catalyst. Note that, in the present embodiment, the simple expression “filter 20” indicates both the first filter 20a and the second filter 20b. The first filter 20a and the second filter 20b are provided with filter temperature sensors 24a and 24b that output signals according to the temperature of the filter 20, respectively.
[0032]
The first exhaust pipe 19a downstream of the first filter 20a is provided with a flow path switching valve 22a that is opened and closed by a signal from the ECU 35, while the second exhaust pipe 19b downstream of the second filter 20b is provided with the ECU 35. A flow path switching valve 22b that is opened and closed by a signal from is provided. And after the 1st exhaust pipe 19a and the 2nd exhaust pipe 19b merge downstream of the flow-path switching valves 22a and 22b, it connects with a muffler (illustration omitted). Reducing agent injection valves 28a and 28b are provided upstream of the first filter 20a and the second filter 20b, respectively, which are opened by a signal from the ECU 35 and inject fuel as a reducing agent. Note that, in the present embodiment, the simple expression “flow path switching valve 22” indicates both the flow path switching valve 22a and the flow path switching valve 22b.
[0033]
By the way, although a diesel engine is excellent in economic efficiency, removal of particulate matter (hereinafter referred to as PM) represented by soot, which is a suspended particulate matter contained in exhaust gas, is an important issue. For this reason, in the present embodiment, a particulate filter (hereinafter simply referred to as “filter”) that collects PM is provided in the exhaust system of the diesel engine so that PM is not released into the atmosphere.
[0034]
Next, the filter 20 will be described.
[0035]
FIG. 2 shows the structure of the filter 20. 2A shows a cross-sectional view of the filter 20 in the horizontal direction, and FIG. 2B shows a cross-sectional view of the filter 20 in the vertical direction. As shown in FIGS. 2A and 2B, the filter 20 is a so-called wall flow type having a plurality of exhaust flow passages 50 and 51 extending in parallel with each other. These exhaust flow passages include an exhaust inflow passage 50 whose downstream end is closed by a plug 52 and an exhaust outflow passage 51 whose upstream end is closed by a plug 53. In FIG. 2A, hatched portions indicate plugs 53. Therefore, the exhaust inflow passages 50 and the exhaust outflow passages 51 are alternately arranged via the thin partition walls 54. In other words, the exhaust inflow passage 50 and the exhaust outflow passage 51 are arranged such that each exhaust inflow passage 50 is surrounded by four exhaust outflow passages 51 and each exhaust outflow passage 51 is surrounded by four exhaust inflow passages 50.
[0036]
The filter 20 is made of a porous material such as cordierite, so that the exhaust gas flowing into the exhaust inflow passage 50 is adjacent to the surrounding partition wall 54 as shown by an arrow in FIG. To the exhaust outlet passage 51.
[0037]
In the embodiment according to the present invention, a carrier layer made of alumina, for example, is formed on the peripheral wall surfaces of each exhaust inflow passage 50 and each exhaust outflow passage 51, that is, on both side surfaces of each partition wall 54 and on the pore inner wall surface in each partition wall 54. The NOx storage reduction catalyst is supported on the carrier.
[0038]
Next, the function of the NOx storage reduction catalyst carried by the filter 20 according to the present embodiment will be described.
[0039]
The filter 20 uses, for example, alumina as a carrier, and an alkali metal such as potassium (K), sodium (Na), lithium (Li), or cesium (Cs), and barium (Ba) or calcium (Ca). ), An alkaline earth such as lanthanum (La) or yttrium (Y), and a noble metal such as platinum (Pt).
[0040]
The NOx catalyst configured in this way absorbs nitrogen oxide (NOx) in the exhaust when the oxygen concentration of the exhaust flowing into the NOx catalyst is high.
[0041]
On the other hand, the NOx catalyst releases the absorbed nitrogen oxide (NOx) when the oxygen concentration of the exhaust gas flowing into the NOx catalyst decreases. At that time, if a reducing component such as hydrocarbon (HC) or carbon monoxide (CO) is present in the exhaust, the NOx catalyst converts nitrogen oxide (NOx) released from the NOx catalyst to nitrogen (N ₂ ).
[0042]
In the exhaust system configured as described above, the air-fuel mixture (burned gas) combusted in each cylinder 2 of the engine 1 is discharged to the exhaust branch pipe 18 through the exhaust port, and then from the exhaust branch pipe 18 to the turbocharger 15. Into the turbine housing 15b. The exhaust gas flowing into the turbine housing 15b rotates a turbine wheel that is rotatably supported in the turbine housing 15b using the thermal energy of the exhaust gas. At that time, the rotational torque of the turbine wheel is transmitted to the compressor wheel of the compressor housing 15a described above.
[0043]
The exhaust discharged from the turbine housing 15b flows into the filter 20 via the first exhaust pipe 19a and the second exhaust pipe 19b, and PM in the exhaust is collected and harmful gas components are removed or purified. After the PM is collected by the filter 20 and the harmful gas components are removed or purified, the exhaust gas is circulated through the exhaust pipe 19 and then released into the atmosphere via a muffler (not shown). In this case, both the flow path switching valves 22 are opened.
[0044]
The exhaust branch pipe 18 and the intake branch pipe 8 are communicated with each other via an exhaust gas recirculation passage (EGR passage) 25 that recirculates a part of the exhaust gas flowing through the exhaust branch pipe 18 to the intake branch pipe 8. Yes. A flow rate adjusting valve (hereinafter referred to as EGR gas) that is configured by an electromagnetic valve or the like in the middle of the EGR passage 25 and changes the flow rate of exhaust gas (hereinafter referred to as EGR gas) that flows through the EGR passage 25 according to the magnitude of applied power. EGR valve) 26 is provided.
[0045]
An EGR cooler 27 that cools the EGR gas flowing in the EGR passage 25 is provided in the middle of the EGR passage 25 and upstream of the EGR valve 26. The EGR cooler 27 is provided with a cooling water passage (not shown), and a part of the cooling water for cooling the engine 1 circulates.
[0046]
In the exhaust gas recirculation mechanism configured as described above, when the EGR valve 26 is opened, the EGR passage 25 becomes conductive, and a part of the exhaust gas flowing through the exhaust branch pipe 18 flows into the EGR passage 25. Then, it is guided to the intake branch pipe 8 through the EGR cooler 27.
[0047]
At that time, in the EGR cooler 27, heat exchange is performed between the EGR gas flowing in the EGR passage 25 and the cooling water of the engine 1 to cool the EGR gas.
[0048]
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.
[0049]
Here, the EGR gas contains water (H ₂ O) and carbon dioxide (CO ₂ ) And the like, and an inert gas component having endothermic properties is contained in the mixture, so if EGR gas is contained in the mixture, the combustion temperature of the mixture is lowered. Therefore, the amount of nitrogen oxide (NOx) generated is suppressed.
[0050]
The engine 1 configured as described above is provided with an electronic control unit (ECU) 35 for controlling the engine 1. The ECU 35 is a unit that controls the operating state of the engine 1 in accordance with the operating conditions of the engine 1 and the driver's request.
[0051]
Various sensors such as a common rail pressure sensor 4a, filter temperature sensors 24a and 24b, a crank position sensor 33, a water temperature sensor 34, and an accelerator opening sensor 36 are connected to the ECU 35 through electric wiring, and output signals of the various sensors described above. Is input to the ECU 35.
[0052]
On the other hand, the fuel injection valve 3, the flow path switching valve 22, the EGR valve 26, and the like are connected to the ECU 35 via electric wiring, and the ECU 35 can control each of the above parts.
[0053]
Here, as shown in FIG. 3, the ECU 35 includes a CPU 351, a ROM 352, a RAM 353, a backup RAM 354, an input port 356, and an output port 357, which are connected to each other by a bidirectional bus 350. , An A / D converter (A / D) 355 connected to the input port 356 is provided.
[0054]
The input port 356 receives an output signal from a sensor that outputs a digital signal format signal, such as the crank position sensor 33, and transmits the output signal to the CPU 351 and the RAM 353.
[0055]
The input port 356 is input via an A / D 355 of a sensor that outputs a signal in an analog signal format, such as the common rail pressure sensor 4a, the filter temperature sensors 24a and 24b, the water temperature sensor 34, and the like, and outputs the output signals. It transmits to CPU351 and RAM353.
[0056]
The output port 357 is connected to the fuel injection valve 3, the flow path switching valve 22, the EGR valve 26, the shutoff valve 31, and the like via electrical wiring, and the control signal output from the CPU 351 is transmitted to the fuel injection valve 3, It transmits to the flow path switching valve 22, the EGR valve 26, the shutoff valve 31 and the like.
[0057]
The ROM 352 stores various application programs.
[0058]
The ROM 352 stores various control maps in addition to the application programs described above.
[0059]
The RAM 353 stores output signals from the sensors, calculation results of the CPU 351, and the like. The calculation result is, for example, the engine speed calculated based on the time interval at which the crank position sensor 33 outputs a pulse signal. These data are rewritten to the latest data every time the crank position sensor 33 outputs a pulse signal.
[0060]
The backup RAM 354 is a non-volatile memory capable of storing data even after the engine 1 is stopped.
[0061]
The CPU 351 operates in accordance with application programs stored in the ROM 352 and executes various controls.
[0062]
By the way, when the engine 1 is operated with lean combustion, the air-fuel ratio of the exhaust discharged from the engine 1 becomes a lean atmosphere, and the oxygen concentration of the exhaust becomes high, so that nitrogen oxide (NOx) contained in the exhaust is NOx. If the lean combustion operation of the engine 1 is continued for a long time, the NOx absorption capacity of the NOx catalyst is saturated and nitrogen oxides (NOx) in the exhaust gas are removed by the NOx catalyst. Without being released into the atmosphere.
[0063]
In particular, in a diesel engine such as the engine 1, the lean air-fuel ratio mixture is combusted in most of the operating region, and the exhaust air-fuel ratio becomes the lean air-fuel ratio in most of the operating region accordingly. The NOx absorption capacity is easily saturated.
[0064]
Therefore, when the engine 1 is in a lean combustion operation, the oxygen concentration in the exhaust gas flowing into the NOx catalyst is reduced and the concentration of the reducing agent is increased and absorbed by the NOx catalyst before the NOx absorption capacity of the NOx catalyst is saturated. The released nitrogen oxide (NOx) needs to be released and reduced.
[0065]
Therefore, the exhaust gas purification apparatus for an internal combustion engine according to the present embodiment includes a reducing agent supply mechanism that adds fuel (light oil) as a reducing agent to the exhaust gas upstream of the filter 20, and the exhaust gas from the reducing agent supply mechanism into the exhaust gas. By adding fuel, the oxygen concentration of the exhaust gas flowing into the filter 20 is lowered and the concentration of the reducing agent is increased.
[0066]
As shown in FIG. 1, the reducing agent supply mechanism is provided in the first exhaust pipe 19a and the second exhaust pipe 19b so that its nozzle hole faces the first exhaust pipe 19a and the second exhaust pipe 19b. A reductant injection valve 28a and 28b for opening the fuel according to a signal from the fuel and injecting fuel; a reductant supply path 29 for guiding the fuel discharged from the fuel pump 6 to the reductant injection valve 28a and 28b; And a shutoff valve 31 provided in the agent supply passage 29 and configured to shut off the fuel flow in the reducing agent supply passage 29.
[0067]
In such a reducing agent supply mechanism, high-pressure fuel discharged from the fuel pump 6 is applied to the reducing agent injection valves 28 a and 28 b via the reducing agent supply path 29. Then, the reducing agent injection valve 28a or 28b is opened by a signal from the ECU 35, and fuel as a reducing agent is injected into the first exhaust pipe 19a or the second exhaust pipe 19b.
[0068]
The reducing agent injected from the reducing agent injection valve 28a or 28b into the first exhaust pipe 19a or the second exhaust pipe 19b forms exhaust having a low oxygen concentration. When the exhaust gas having a low oxygen concentration thus formed flows into the filter 20a or 20b, nitrogen (Nx) is released while releasing nitrogen oxide (NOx) absorbed by the filter 20a or 20b. ₂ ).
[0069]
Thereafter, the reducing agent injection valve 28a or 28b is closed by a signal from the ECU 35, and the addition of the reducing agent into the first exhaust pipe 19a or the second exhaust pipe 19b is stopped.
[0070]
Next, NOx purification control for releasing / reducing NOx from the NOx storage reduction catalyst carried by the filter 20 will be described in detail.
[0071]
In this NOx purification control, the CPU 351 executes so-called rich spike control in which the oxygen concentration in the exhaust gas flowing into the filter 20 is reduced in a spike manner (short time) in a relatively short cycle.
[0072]
Here, when both the flow path switching valves 22 a and 22 b are opened, substantially the same amount of exhaust gas is flowing through the filter 20. In such a case, if a reducing agent is supplied into the exhaust gas, a large amount of reducing agent is required to reach the required oxygen concentration, and thus fuel consumption deteriorates. Therefore, in the present embodiment, one flow path switching valve is closed, and the reducing agent is supplied to the filter while reducing the amount of exhaust gas flowing through the one filter. In this way, the required oxygen concentration can be obtained with a small amount of reducing agent by reducing the flow rate of the exhaust gas.
[0073]
In the present embodiment, the release / reduction of NOx absorbed by the filter 20a will be described, but the same processing can be performed in the filter 20b.
[0074]
In the rich spike control described above, the CPU 351 determines whether or not the rich spike control execution condition is satisfied every predetermined cycle. As the rich spike control execution condition, for example, the filter 20a is in an active state, the output signal value (exhaust temperature) of the exhaust temperature sensor is equal to or less than a predetermined upper limit, the poisoning elimination control is not executed, etc. These conditions can be exemplified.
[0075]
When it is determined that the rich spike control execution condition as described above is satisfied, the CPU 351 closes the flow path switching valve 22a. Then, by controlling the reducing agent injection valve 28a so as to inject the fuel as the reducing agent in a spike manner from the reducing agent injection valve 28a, the air-fuel ratio of the exhaust gas flowing into the filter 20a is temporarily set to a predetermined target rich air-fuel ratio. And
[0076]
Specifically, the CPU 351 stores the engine speed stored in the RAM 353, the output signal of the accelerator opening sensor 36 (accelerator opening), the output signal value (intake air amount) of an air flow meter (not shown), the air-fuel ratio. Read the sensor output signal, fuel injection amount, etc.
[0077]
The CPU 351 accesses the reducing agent addition amount control map in the ROM 352 using the engine speed, the accelerator opening, the intake air amount, and the fuel injection amount as parameters, and sets the air-fuel ratio of the exhaust to a preset target air-fuel ratio. The amount of addition of the reducing agent (target addition amount) required above is calculated.
[0078]
Subsequently, the CPU 351 accesses the reducing agent injection valve control map in the ROM 352 using the target addition amount as a parameter, and the reducing agent injection valve 28a necessary for injecting the reducing agent with the target addition amount from the reducing agent injection valve 28a. The valve opening time (target valve opening time) is calculated.
[0079]
When the target valve opening time of the reducing agent injection valve 28a is calculated, the CPU 351 opens the reducing agent injection valve 28a.
[0080]
The CPU 351 closes the reducing agent injection valve 28a when the target valve opening time elapses from the time when the reducing agent injection valve 28a is opened.
[0081]
When the reducing agent injection valve 28a is thus opened for the target valve opening time, a target addition amount of fuel is injected from the reducing agent injection valve 28a into the first exhaust pipe 19a. The reducing agent injected from the reducing agent injection valve 28a is mixed with the exhaust gas flowing from the upstream side of the first exhaust pipe 19a to form an air-fuel mixture having a target air-fuel ratio. Thereafter, the exhaust gas flows into the filter 20.
[0082]
As a result, the oxygen concentration of the air-fuel ratio of the exhaust gas flowing into the filter 20a changes in a relatively short cycle, so that the filter 20 alternately absorbs and releases / reduces nitrogen oxide (NOx). Will be repeated in a short period.
[0083]
On the other hand, the PM collected by the filter is burned and removed by the high-temperature exhaust discharged when the engine is operated in a high rotation and high load region. However, since a certain amount of time is required for the combustion of the PM, the PM remains unburned if the engine operating region deviates from the high rotation / high load region before the PM is completely burned and removed. Further, if the light load state continues for a long time, PM is accumulated without burning. Since it is difficult to maintain an engine operating state suitable for PM combustion for a long period of time, unburned PM gradually accumulates on the filter, causing clogging of the filter.
[0084]
In this way, addition of fuel to the exhaust gas is also effective as one method for effectively removing unburned PM.
[0085]
When fuel is added to the exhaust, the temperature of the filter 20 rises due to a catalytic reaction (oxidation reaction). At that time, the active oxygen is released by the fuel that has flowed into the filter 20, so that the PM is easily oxidized and the oxidizable removal amount per unit time is improved. Further, the addition of fuel removes oxygen poisoning of the catalyst and increases the activity of the catalyst, so that active oxygen is easily released. Then, PM is oxidized and burned by active oxygen and removed.
[0086]
By the way, in the present embodiment, during normal operation, both the flow path switching valves 22a and 22b are opened, and exhaust gas is circulated through the first exhaust pipe 19a and the second exhaust pipe 19b, and exhausted by two filters at the same time. To purify. In this way, the flow rate of the exhaust gas flowing through each filter can be halved, so that the filter capacity can be reduced. However, when the temperature of the exhaust gas is low, the temperatures of both filters are lowered at the same time, and the purification efficiency is lowered. The NOx storage reduction catalyst supported on the filter 20 has different purification rates of NOx, hydrocarbons (HC), etc. depending on its temperature. Therefore, it is important to maintain the temperature of the NOx storage reduction catalyst at a high purification rate of NOx, hydrocarbon (HC), and the like.
[0087]
Here, if the temperature rise control is performed for each filter in the same manner as the NOx release / reduction described above, the other filter is left with a reduced purification rate until the temperature rise of one filter is completed. Become. Therefore, it takes time to raise the temperature of the two filters until the temperature of both filters rises and the purification rate of NOx and hydrocarbons (HC) increases, and during that time, the NOx purification rate decreases and the exhaust gas is exhausted. There was a risk of worsening emissions.
[0088]
Therefore, in the present embodiment, when the temperature of the filter is lowered, the temperature of both filters is raised at the same time, and the time required for raising the temperature of the filter can be halved.
[0089]
By the way, in the NOx storage reduction catalyst according to the present embodiment, NOx and hydrocarbon (HC) have different temperature windows, and hydrocarbon (HC) can be purified from a lower temperature. Therefore, there is a temperature range in which NOx cannot be sufficiently purified even at a temperature at which hydrocarbon (HC) can be purified. In such a temperature range, even if fuel is supplied to the filter 20, a temperature increase due to the oxidation reaction heat of the fuel cannot be expected.
[0090]
Therefore, in the present embodiment, when the temperature of the NOx storage reduction catalyst is in a temperature range where hydrocarbons (HC) cannot be sufficiently purified, the exhaust gas temperature is raised by the method described below to filter the exhaust gas. Increase the temperature by 20. On the other hand, when the temperature of the NOx storage reduction catalyst is in a temperature range in which hydrocarbon (HC) can be purified, the fuel is supplied to the filter, and the temperature is raised to the NOx purification temperature at an early stage. The reason why the temperature of the exhaust gas is not raised to a temperature range where NOx can be purified is that the amount of fuel required to supply fuel directly to the filter in the temperature range where hydrocarbons (HC) can be oxidized. This is because a small amount is sufficient and deterioration of fuel consumption can be suppressed.
[0091]
Next, a method for increasing the temperature of the exhaust will be described.
[0092]
In the present embodiment, in order to raise the temperature of the exhaust, sub-injection is used in which fuel is injected again at a time that does not become the engine output after the main injection that injects fuel for engine output to the engine 1. it can.
[0093]
The reason why the fuel is secondarily injected in the expansion stroke in this way is that when the fuel injection amount by the main injection is increased, the engine output is increased and the operating state may be deteriorated. The fuel injected by the sub-injection burns in the cylinder 2 and raises the gas temperature in the cylinder 2. The gas whose temperature has increased becomes exhaust gas and reaches the turbocharger 15, and the temperature of the turbocharger 15 can be increased to remove PM such as SOF.
[0094]
If the relationship between the accelerator opening, the engine speed, the sub-injection amount or the sub-injection timing is mapped in advance and stored in the ROM 352, the map, the accelerator opening, and the engine rotation can be obtained. It can be calculated from the number.
[0095]
Further, the temperature of the exhaust gas can also be raised by delaying the timing of main injection of fuel from the fuel injection valve 3 (hereinafter referred to as delayed injection). This is because if the fuel injection timing is delayed than usual, the amount of energy consumed for the movement of the piston is less than the fuel injected at the normal timing, and therefore the temperature of the exhaust gas becomes higher. . However, when the operation is performed by the delayed fuel injection, the combustion state becomes unstable, so that the period that can be delayed is limited. Therefore, in the present embodiment, fuel is injected when the piston at the end of the exhaust stroke is near top dead center. If it does in this way, in the subsequent intake stroke and compression stroke, fuel will evaporate and it will become easy to ignite, and it can stabilize combustion. Therefore, the fuel injection timing can be further delayed, so that the exhaust temperature can be further increased.
[0096]
Further, the EGR amount may be increased. Since the temperature of EGR gas is high, the temperature of the intake air rises, and the temperature of the exhaust gas rises accordingly. In order to increase the amount of EGR gas, for example, the pressure inside the intake branch pipe 8 is decreased by closing the intake throttle valve 13 to thereby increase the differential pressure with the exhaust branch pipe 18; By closing the flow path switching valves 22a and 22b and increasing the pressure inside the exhaust branch pipe 18, a method for increasing the pressure difference between the intake branch pipe 8 and the like can be raised.
[0097]
As described above, when the fuel injection timing is delayed during the light load operation, the secondary injection after the main injection (sub-injection), and the EGR amount are increased, the temperature of the exhaust gas can be raised. Therefore, the temperature of the filter 20 can be raised, and the temperature can be raised to a temperature at which hydrocarbons (HC) can be purified.
[0098]
Next, the filter temperature increase control according to the present embodiment when the temperature of the filter is low when the engine 1 is started or when the light load operation is continued will be described.
[0099]
FIG. 4 is a flowchart showing a flow of filter temperature raising control.
[0100]
In step S101, the temperature of the exhaust discharged from the engine 1 is increased.
As a method for increasing the temperature of the exhaust gas, for example, the above-described sub-injection, delayed injection, increase of the EGR amount, or the like can be used.
[0101]
In step S102, it is determined whether or not the bed temperature of the filter 20 is a temperature that can sufficiently purify hydrocarbon (HC). The CPU 351 reads the output signals from the filter temperature sensors 24 a and 24 b and determines the temperature of the filter 20. Here, when the temperature of the filter 20 is lower than the temperature at which the hydrocarbon (HC) can be sufficiently purified, if fuel is supplied from the reducing agent injection valves 28a and 28b, the fuel that is not purified by the NOx storage reduction catalyst is obtained. It passes through the filter 20 and is released into the atmosphere. Therefore, at such a temperature, the temperature of the filter 20 is raised by raising the temperature of the exhaust, thereby suppressing the deterioration of the exhaust emission. On the other hand, if the bed temperature of the filter 20 reaches a temperature window that can sufficiently purify hydrocarbons (HC), the temperature of the filter 20 is increased by supplying the reducing agent from the reducing agent supply valves 28a and 28b. It is possible to suppress the deterioration of fuel consumption.
[0102]
If an affirmative determination is made in step S102, the process proceeds to step S103, whereas if a negative determination is made, the process proceeds to step S101.
[0103]
In step S103, fuel is injected from the reducing agent injection valves 28a and 28b, and the temperature rise control of the filter 20 is performed.
[0104]
Thus, in this embodiment, it is possible to raise the temperature of both filters at the same time, and it is possible to shorten the time until the temperature rises to a temperature that can sufficiently purify NOx.
[0105]
Note that the present invention can also be applied to temperature increase control for regenerating accumulated PM.
[0106]
【The invention's effect】
In the exhaust gas purification apparatus for an internal combustion engine according to the present invention, it is possible to simultaneously raise the temperature of a plurality of catalysts having an oxidation function, and therefore it is possible to shorten the time required for temperature rise.
[0107]
From the above, it becomes possible to shorten the time when exhaust purification becomes insufficient, suppress the deterioration of exhaust emission, and suppress the deterioration of fuel consumption.
[Brief description of the drawings]
FIG. 1 is a schematic configuration diagram illustrating an engine to which an exhaust gas purification apparatus for an internal combustion engine according to an embodiment of the present invention is applied and an intake / exhaust system thereof.
FIG. 2A is a diagram showing a transverse cross section of a particulate filter. (B) is a figure which shows the longitudinal direction cross section of a particulate filter.
FIG. 3 is a block diagram showing an internal configuration of an ECU.
FIG. 4 is a flowchart showing a flow of filter temperature raising control according to the present invention.
[Explanation of symbols]
1 ... Engine
1a ... Crank pulley
2. Cylinder
3. Fuel injection valve
4 ... Common rail
4a ... Common rail pressure sensor
5. Fuel supply pipe
6. Fuel pump
6a ... Pump pulley
8 ... Intake branch pipe
9. Intake pipe
15 ... Turbocharger
15a ・ Compressor housing
15b. Turbine housing
16 ... Intercooler
18 ... Exhaust branch pipe
19 ... Exhaust pipe
20 ... Particulate filter
24 ... Filter temperature sensor
25 ... EGR passage
26 ... EGR valve
27 ... EGR cooler
28 ... Reducing agent injection valve
29 ... Reducing agent supply path
31 ... Shut-off valve
33 ... Crank position sensor
34 ... Water temperature sensor
35 ... ECU
36 Accelerator opening sensor

Claims (2)

A plurality of exhaust passages branched in parallel, which are exhaust passages of an internal combustion engine;
A catalyst provided in each of the branched exhaust passages and having an oxidation function;
And respectively supplying the reducing agent supply means of the reducing agent in each of the catalyst,
A flow path switching valve provided in each of the plurality of exhaust passages to open and close;
With
If it becomes necessary to raise the temperature of the catalyst occurs, the reducing agent supply means at the same time raises the temperature of the plurality of catalyst by supplying respective reducing agent to a plurality of catalysts,
At this time, the flow path switching valve provided in one exhaust passage is closed, the flow path switching valve provided in the other exhaust passage is opened, and the catalyst is provided to the catalyst provided in the one exhaust passage. exhaust purification system of an internal combustion engine, wherein least to Rukoto than the amount of reducing agent supplied to the catalyst provided the reducing agent amount to other exhaust passage.   The exhaust gas purification apparatus of the internal combustion engine includes a catalyst temperature raising means that does not depend on a reducing agent supply means, and the catalyst has a hydrocarbon purification rate that varies depending on the bed temperature, and the bed temperature of the catalyst is a hydrocarbon. When the temperature is lower than the temperature capable of purifying the catalyst, the temperature of the plurality of catalysts is simultaneously raised by the catalyst temperature raising means not based on the reducing agent supply means, while the catalyst bed temperature is capable of purifying hydrocarbons. 2. The exhaust gas purification apparatus for an internal combustion engine according to claim 1, wherein when the temperature is higher than the temperature, a reducing agent is supplied to the plurality of catalysts to simultaneously raise the bed temperatures of the plurality of catalysts.

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