JP2007255305A - Exhaust emission control device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an exhaust emission control device capable of reducing NOx while suppressing the exhaust of PM even when the temperature of an exhaust purifying catalyst falls lower than active temperature due to the continuation of idle operation over a long period. <P>SOLUTION: An exhaust emission control device is provided with a bypass passage which is branched from an exhaust passage to bypass a catalyst and merged with the exhaust passage on the upper reach of a filter. When the temperature of catalyst is lower than a predetermined active lower limit temperature Ta under an idle operation state of an internal combustion engine (S10), an EGR valve is substantially fully opened and exhaust gas is passed through the bypass passage (S12, S14, S16). <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an exhaust purification device, and more particularly to an exhaust purification technology when an idle operation continues for a long period of time and an exhaust purification catalyst becomes lower than an activation temperature.

An exhaust purification catalyst is generally interposed in the exhaust passage of the internal combustion engine (engine), and harmful substances (CO, HC, NOx, etc.) in the exhaust are oxidized or reduced by the exhaust purification catalyst and removed.
By the way, the exhaust purification catalyst has a characteristic that the catalyst temperature functions well in a predetermined temperature range equal to or higher than the activation temperature (light-off temperature), and the harmful substances cannot be sufficiently purified below the activation temperature, It is necessary to maintain the catalyst temperature above the activation temperature.

In particular, various NOx catalysts such as a storage NOx catalyst capable of purifying NOx and a selective reduction type NOx catalyst have recently been developed and put into practical use. However, when the engine is an engine that easily discharges NOx, such as a diesel engine. In this case, it is required to reliably prevent NOx from diffusing into the atmosphere, and it is desired to maintain the temperature of these NOx catalysts at an activation temperature or higher.
However, when the engine is in an idling state (for example, low idling) for a long period of time, the exhaust purification catalyst is easily cooled because the exhaust temperature is low, so the temperature of the exhaust purification catalyst is lowered below the activation temperature. There is a problem that it is difficult to maintain the exhaust purification catalyst at a temperature higher than the activation temperature.

Therefore, a bypass passage is provided to bypass the exhaust purification catalyst provided in the exhaust passage of the diesel engine, and when the temperature of the exhaust purification catalyst is low or high, the exhaust is released to the bypass passage and the temperature of the exhaust purification catalyst is adjusted. At the same time, a technique has been developed that maintains the exhaust gas recirculation (EGR) amount to ensure a reduced amount of NOx (see Patent Document 1).
JP-A-5-44449

  However, in the technology disclosed in the above-mentioned patent document, it is possible to prevent the exhaust purification catalyst from being lowered in temperature and excessively heated, and to reduce NOx by maintaining the EGR amount while exhaust is released to the bypass passage. However, there is a problem that particulate matter (PM) such as soot that tends to be contained in the exhaust of the diesel engine during this period is directly discharged into the atmosphere through the bypass passage.

This problem is remarkable because PM is more likely to occur as the amount of EGR increases and combustion becomes unstable.
The present invention has been made to solve such problems, and the object of the present invention is when the idling operation continues over a long period of time and the temperature of the exhaust purification catalyst decreases below the activation temperature. However, an object of the present invention is to provide an exhaust purification device capable of reducing NOx while suppressing PM emission.

  In order to achieve the above object, an exhaust emission control device according to claim 1 is provided in an exhaust passage of an internal combustion engine, and is provided in the exhaust passage and located downstream of the catalyst in the exhaust passage. A filter for capturing particulate matter in exhaust, a catalyst temperature detecting means for detecting the temperature of the catalyst, and a bypass that branches from the exhaust passage and bypasses the catalyst, and joins upstream of the filter A switching valve that is provided at a branch portion between the passage and the bypass passage, and switches an exhaust flow between the exhaust passage side and the bypass passage side; and an EGR passage that recirculates part of the exhaust from the exhaust passage to the intake passage And an EGR valve provided in the EGR passage for adjusting the recirculation amount of a part of the exhaust gas, and when the internal combustion engine is in an idling state, is detected by the catalyst temperature detecting means. The switching valve is switched to the exhaust passage side when the catalyst temperature is equal to or higher than a predetermined activation lower limit temperature, and the switching valve is switched to the bypass passage side when the catalyst temperature is lower than the predetermined activation lower limit temperature. And a control means for substantially fully opening the EGR valve.

According to a second aspect of the present invention, in the first aspect, the catalyst is an occlusion type NOx catalyst.
According to a third aspect of the present invention, in the first aspect, the catalyst is a selective reduction type NOx catalyst.

  According to the exhaust emission control device of the first aspect, the bypass passage is provided that branches from the exhaust passage and bypasses the catalyst and then merges upstream of the filter. When the internal combustion engine is in an idle operation state, the temperature of the catalyst has a predetermined activity. When the temperature is lower than the lower limit temperature, the switching valve is switched to the exhaust passage side, and when the catalyst temperature is lower than the predetermined activation lower limit temperature, the switching valve is switched to the bypass passage side and the EGR valve is substantially fully opened. In the case where the temperature of the exhaust purification catalyst decreases below the activation temperature for a long period of time (idle), a large amount of EGR is introduced into the intake side to suppress the generation of NOx and flow exhaust through the bypass passage In addition, the temperature of the catalyst can be prevented from being lowered by being cooled by low-temperature exhaust gas. It can be surely suppressed.

  According to the exhaust gas purification apparatus of claim 2, NOx to the NOx storage type catalyst that has become less than the activation temperature by flowing exhaust gas through the bypass passage while introducing a large amount of EGR to the intake side and suppressing NOx generation. Occlusion (including adsorption) of the NOx catalyst can be suppressed to prevent breakthrough of the NOx catalyst (NOx breakthrough), the temperature of the NOx catalyst can be prevented from lowering due to cooling with low temperature exhaust gas, The PM in the exhaust gas generated during this time can be captured by the filter, and PM emission can be reliably suppressed.

  According to the exhaust emission control device of claim 3, a large amount of EGR is introduced to the intake side to suppress the generation of NOx, and the exhaust gas is allowed to flow to the bypass passage, so that the selective reduction type NOx catalyst that has become less than the activation temperature can be obtained. Inflow of NOx can be suppressed and temperature reduction of the storage NOx catalyst due to cooling with low-temperature exhaust gas can be prevented. Further, PM in the exhaust gas generated during this period is captured by a filter to reliably suppress PM emission. be able to.

Hereinafter, an embodiment of an exhaust emission control device according to the present invention will be described with reference to the drawings.
FIG. 1 is a system configuration diagram of an entire internal combustion engine including an exhaust purification apparatus according to the present invention. In FIG. 1, reference numeral 1 indicates a common rail diesel engine, for example, and reference numeral 10 forms a main part of the engine control apparatus. An electronic control unit (hereinafter referred to as ECU) is shown.
Although not shown in detail, a common rail diesel engine (hereinafter simply referred to as an engine) 1 includes a needle valve and a fuel injector having a fuel chamber and a control chamber provided on the distal end side and the proximal end side of the needle valve. Provided for each cylinder, the fuel chamber and the control chamber are connected to a pressure accumulating chamber via a fuel passage, and the control chamber is connected to a fuel tank via a fuel return passage. Under the control of the ECU 10, when the solenoid valve provided in the fuel injector is opened, the high-pressure fuel supplied from the pressure accumulating chamber is injected into the combustion chamber of the engine 1 through the fuel injector, and when the solenoid valve is closed, the fuel injection is finished. Thus, the fuel injection start / end timing (fuel injection amount) is adjusted by controlling the opening / closing timing of the solenoid valve.

  The engine 1 has an intake pipe (intake passage) 12 connected to an intake manifold 11 and an exhaust pipe (exhaust passage) 14 connected to an exhaust manifold 13. In the middle of the intake pipe 12, a compressor 21, an intercooler 31, and an intake throttle valve 32 of the supercharger 20 are arranged. The opening degree of the intake throttle valve 32 is variably adjusted by the ECU 10. On the other hand, a turbine 22 of the supercharger 20, an exhaust brake 15, a light oil addition injector 50, an aftertreatment device 40 and a muffler (not shown) are provided in the middle of the exhaust pipe 14.

  In FIG. 1, reference numeral 36 denotes an EGR passage extending from the exhaust manifold 13 to the intake pipe 12, and a part of the exhaust gas is supplied to the engine 1 as EGR gas through the EGR passage 36. In the middle of the EGR passage 36, an EGR cooler 37 that cools the EGR gas to increase the gas filling density of the engine 1 and an EGR valve 38 for supplying and shutting off the supply of the EGR gas to the engine 1 are provided. . The EGR valve 38 is controlled to be opened or closed or adjusted by the ECU 10.

  The post-processing device 40 is a diesel particulate filter (DPF) 41 that collects particulate matter (PM) such as soot and burns and removes it, and an exhaust gas in an oxidizing atmosphere (lean air-fuel ratio) that is positioned in front of the DPF 41. NOx occlusion catalyst (occlusion type NOx catalyst) 42 that occludes NOx contained therein and releases NOx occluded in a reducing atmosphere (rich air-fuel ratio) to reduce and remove (NOx purge); And a post-stage catalyst 43 that oxidizes and removes HC and CO. A light oil addition injector 50 is provided in the front stage of the aftertreatment device 40.

  The light oil addition injector 50 generates a reducing atmosphere for the NOx occlusion catalyst 42 during NOx purging of the NOx occlusion catalyst 42 and serves as a reducing agent for NOx, or during the forced regeneration of the DPF 41 that burns and removes PM trapped in the DPF 41. In order to raise the temperature of the DPF 41 by raising the temperature of the NOx occlusion catalyst 42, light oil (HC) is injected into the exhaust gas and is driven and controlled by the ECU 10.

A bypass pipe (bypass passage) 18 branches from the exhaust pipe 14 so as to bypass the NOx storage catalyst 42, and the end of the bypass pipe 18 joins the exhaust pipe 14 upstream of the DPF 41. Is connected to the upstream portion of the DPF 41.
A switching valve 16 that switches the flow of exhaust gas between the exhaust pipe 14 side, that is, the aftertreatment device 40 side and the bypass pipe 18 side, is provided at a branch portion between the exhaust pipe 14 and the bypass pipe 18. The switching valve 16 is also controlled to be switched by the ECU 10.

In FIG. 1, reference numeral 60 denotes a catalyst outlet exhaust temperature sensor (catalyst temperature detection means), and the catalyst outlet exhaust temperature sensor 60 has a temperature detection end inserted between the NOx storage catalyst 42 and the DPF 41, The exhaust temperature (catalyst temperature, particularly the temperature of the NOx storage catalyst 42) on the outlet side of the NOx storage catalyst 42 can be detected.
Further, various sensors such as a load sensor 61 and a crank angle sensor 62 are connected to the ECU 10. The load sensor 61 detects the amount of depression of an accelerator pedal (not shown), that is, the accelerator opening, as an engine load, and the crank angle sensor 62 detects the crank angle to detect the rotation of the crankshaft (not shown) of the engine 1 in the engine. This is detected as the rotational speed Ne.

Thus, the ECU 10 determines the operating region of the engine 1 based on the engine load detected by the load sensor 61 and the engine rotational speed Ne detected by the crank angle sensor 62, and the engine 1 of the engine 1 is determined according to the engine operating region. The fuel injection timing and the fuel injection amount can be controlled by turning on and off the solenoid valve of each fuel injector.
Hereinafter, the operation and effect of the exhaust emission control device according to the present invention configured as described above will be described.

Referring to FIG. 2, a control routine for low idle control executed by the ECU 10 in the exhaust purification apparatus according to the present invention is shown in a flowchart (control means), and will be described below based on the flowchart.
In step S10, based on the information from the crank angle sensor 62, it is determined whether or not the engine rotational speed Ne is at a normal idle rotational speed Ni and the engine 1 is in a low idle operation state. Here, the low idle operation state is an idle operation state with a low engine speed Ne that is not a high idle operation state for driving an auxiliary machine or the like. If the determination result is true (Yes) and it is determined that the engine is in the low idle operation state, the process proceeds to step S12.

  In step S12, based on the temperature information from the catalyst outlet exhaust temperature sensor 60, it is determined whether or not the catalyst temperature is lower than the activation lower limit temperature Ta. That is, in the low idle operation, unlike the high idle operation, the exhaust temperature is low, and therefore the catalyst temperature is likely to decrease. Therefore, it is determined whether or not the catalyst temperature is lower than the activation lower limit temperature Ta. If the determination result is true (Yes) and the catalyst temperature is lower than the activation lower limit temperature Ta, the process proceeds to step S14.

In step S <b> 14, the opening degree of the EGR valve 38 is controlled to the maximum or substantially maximum so that a large amount of EGR gas flows through the EGR passage 36.
If a large amount of EGR gas is recirculated to the intake side in this way, the amount of fresh air decreases, the combustion in the cylinder slows down, the premixed combustion amount increases, and the combustion temperature still decreases. , NOx generation can be greatly suppressed.

In step S <b> 16, the switching valve 16 that is normally switched to the exhaust pipe 14 side is switched to the bypass pipe 18 side so that the exhaust gas flows through the bypass pipe 18.
When exhaust gas is caused to flow through the bypass pipe 18 in this way, the exhaust gas does not flow through the NOx storage catalyst 42 that is lower than the activation lower limit temperature Ta, so that NOx is not stored in the NOx storage catalyst 42 (including adsorption). The breakthrough of the NOx storage catalyst 42 (NOx breakthrough) can be prevented. Further, since the exhaust temperature is low in the low idle operation state as described above, the catalyst temperature is likely to be further lowered by cooling with the low temperature exhaust gas. However, the temperature reduction of the NOx storage catalyst 42 due to such low temperature exhaust gas is prevented. be able to.

  Furthermore, since the bypass pipe 18 is configured to join the exhaust pipe 14 upstream of the DPF 41, if a large amount of EGR gas is recirculated, combustion becomes unstable and PM is likely to be generated. During the period when the flow is switched to the bypass pipe 18 side, the PM can be well captured by the DPF 41, and the emission of PM into the atmosphere can be reliably prevented.

  If the determination result in step S10 is false (No), and it is determined that the engine speed Ne has increased due to acceleration running or the like and the engine 1 is no longer in the low idle operation state, the catalyst temperature is the lower activation limit temperature. It is expected to increase to Ta or higher, and the process proceeds to step S18, where the opening degree of the EGR valve 38 is set to a normal opening degree, and in step S20, the switching valve 16 is switched to the exhaust pipe 14 side so that the flow of exhaust gas is reduced. A normal flow through the NOx storage catalyst 42 is assumed.

  Further, if the determination result of step S10 is true (Yes) and the engine temperature is equal to or higher than the activation lower limit temperature Ta even if the engine 1 is in the low idle operation state, NOx is well purified by the NOx occlusion catalyst 42. In this case as well, the process proceeds to step S18, and the opening degree of the EGR valve 38 is set to the normal opening degree. In step S20, the switching valve 16 is switched to the exhaust pipe 14 side to flow the exhaust gas. Is the normal flow through the NOx storage catalyst 42.

  Thus, according to the exhaust gas purification apparatus of the present invention, when the engine 1 is in the low idle operation state and the catalyst temperature is lower than the activation lower limit temperature Ta, a large amount of EGR gas is recirculated to the intake side. Exhaust gas is allowed to flow through the bypass pipe 18, thereby preventing inflow of NOx into the NOx storage catalyst 42 to prevent breakthrough of the NOx storage catalyst 42, and NOx storage without depending on the NOx storage catalyst 42. While further reducing the temperature, the temperature of the NOx storage catalyst 42 can be prevented from further decreasing, and the PM generated during this time can be reliably suppressed from being discharged into the atmosphere.

  Here, referring to FIG. 3, the load (accelerator opening degree), exhaust temperature, exhaust pipe outlet NOx (solid line) and PM (dashed line), EGR amount, and time variation of exhaust pipe presence / absence when low idle control is performed. As shown in the figure, when the engine 1 is in the low idle operation state and the catalyst temperature is lower than the activation lower limit temperature Ta, the exhaust gas temperature raising control or the like is forcibly performed. In addition, while reducing NOx (solid line), it is possible to reliably suppress the emission of PM into the atmosphere (broken line).

Although the description of one embodiment of the exhaust emission control device according to the present invention is finished above, the embodiment of the present invention is not limited to the above.
For example, in the above embodiment, the NOx catalyst is constituted by the NOx storage catalyst 42, but instead of the NOx storage catalyst 42, a selective reduction type NOx catalyst (Selective Catalytic Reduction NOx catalyst, SCR catalyst for short) is used. It may be. Even in this case, when the engine 1 is in the low idle operation state and the catalyst temperature is lower than the activation lower limit temperature Ta, a large amount of EGR gas is recirculated to the intake side and the exhaust gas is supplied to the bypass pipe 18. By flowing, it is possible to prevent NOx from flowing into the SCR catalyst and prevent NOx from being reduced without relying on the SCR catalyst, while preventing further temperature drop of the SCR catalyst. It is possible to suppress the discharge reliably.

  In the above embodiment, the rear catalyst 43 is provided at the rear stage of the DPF 41. However, the rear catalyst 43 may be omitted.

1 is a system configuration diagram of an entire internal combustion engine including an exhaust purification device according to the present invention. It is a flowchart which shows the control routine of the low idle control in the exhaust gas purification apparatus which concerns on this invention. It is a time chart which shows the time change at the time of carrying out the low idle control concerning the present invention (accelerator opening), exhaust temperature, exhaust pipe outlet NOx (solid line) and PM (broken line), EGR amount, exhaust pipe presence / absence. .

Explanation of symbols

1 engine (diesel engine)
10 Electronic control unit (ECU)
16 Switching valve 18 Bypass pipe (bypass passage)
40 Aftertreatment device 42 NOx storage catalyst (catalyst)
60 Catalyst outlet exhaust temperature sensor (catalyst temperature detection means)

Claims (3)

A catalyst provided in the exhaust passage of the internal combustion engine for purifying harmful substances in the exhaust;
A filter provided in the exhaust passage at a position downstream of the catalyst and capturing particulate matter in the exhaust;
Catalyst temperature detecting means for detecting the temperature of the catalyst;
A bypass passage that branches off from the exhaust passage and bypasses the catalyst, and joins upstream of the filter;
A switching valve provided at a branch portion with the bypass passage, for switching the flow of exhaust gas to the exhaust passage side and the bypass passage side;
An EGR passage that recirculates part of the exhaust from the exhaust passage to the intake passage;
An EGR valve that is provided in the EGR passage and adjusts a recirculation amount of a part of the exhaust;
When the internal combustion engine is in an idling state, the switching valve is switched to the exhaust passage side when the temperature of the catalyst detected by the catalyst temperature detecting means is equal to or higher than a predetermined activation lower limit temperature, and the temperature of the catalyst is Control means for switching the switching valve to the bypass passage side when the temperature is lower than the activation lower limit temperature and substantially fully opening the EGR valve;
An exhaust emission control device comprising:   The exhaust emission control device according to claim 1, wherein the catalyst is an occlusion type NOx catalyst.   The exhaust emission control device according to claim 1, wherein the catalyst is a selective reduction type NOx catalyst.

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