JP2009185659A - Exhaust emission control device for internal combustion engine - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To burn and remove accumulated PM while inhibiting emission of NOx to the atmospheric air at a low temperature right after start or the like in a diesel engine. <P>SOLUTION: In a structure provided with a filter catalyst having a low temperature NOx trap function, an oxidation catalyst function, and a PM collection function, a quick temperature rise control is made to raise a temperature of the filter catalyst to a second prescribed temperature at which a conversion rate from NO to NO<SB>2</SB>is high and PM is burned and removed by using NO<SB>2</SB>(S1-S5) when the temperature of the filter catalyst reaches a first prescribed temperature near NOx desorption temperature or when an NOx trap quantity reaches an allowable upper limit value. A filter forced regeneration process burning and removing PM by further raising the temperature of the filter catalyst is executed (S6-S11) when the temperature of the filter catalyst reaches a third prescribed temperature higher than the second prescribed temperature. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an exhaust gas purification apparatus for an internal combustion engine, and more particularly to an exhaust gas purification apparatus that removes particulate matter (particulate: PM) and nitrogen oxides (NOx) in exhaust gas in a diesel engine.

A continuous regeneration type DPF is known as an exhaust gas purification device for a diesel engine. For example, in the apparatus described in Patent Document 1, an oxidation catalyst is arranged upstream of a DPF (diesel particulate filter), and NO in the exhaust gas is converted into NO ₂ by this oxidation catalyst, and this NO ₂ is converted into DPF. The filter function is regenerated by burning and removing the accumulated PM.
Japanese Patent No. 3012249

According to the above prior art, it is possible to remove PM deposited on the DPF while removing NOx in the exhaust gas.
However, if the oxidation catalyst needs to be activated and the exhaust temperature is low and the oxidation catalyst is not activated, PM continues to accumulate on the DPF, and NOx in the exhaust is removed. Difficult to do. For this reason, the subject remains about removal of PM deposited on NOx and DPF at a low temperature such as immediately after starting.

  The present invention has been made paying attention to such a problem, and an object of the present invention is to provide an exhaust gas purification apparatus for an internal combustion engine that can effectively remove NOx and PM at a low temperature after starting. .

The present invention traps NOx in exhaust at a low temperature, desorbs trapped NOx when a predetermined desorption temperature is reached, an oxidation catalyst function that converts NO into NO ₂ , and particulate matter ( In a configuration including an exhaust gas purification filter having a PM collection function for collecting particulates (PM), when the temperature of the exhaust gas purification filter reaches a first predetermined temperature in the vicinity of the desorption temperature, or of the exhaust gas purification filter When the NOx trap amount reaches the allowable upper limit, the exhaust purification filter is rapidly heated.

  According to the present invention, accumulated PM can be removed by combustion while suppressing release of NOx into the atmosphere at a low temperature such as immediately after startup.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
FIG. 1 is an overall configuration diagram of an internal combustion engine (diesel engine) according to an embodiment of the present invention. In FIG. 1, an intake compressor 2 of the turbocharger 4 is provided in the intake passage 2 of the engine 1 on the downstream side of the air cleaner 3. The intake compressor 41 is coaxially coupled to an exhaust turbine 42 that is provided in the exhaust passage 5 and is rotationally driven by the energy of the exhaust. The turbocharger 4 includes an intake compressor 41, an exhaust turbine 42, and a variable nozzle 43, and the intake compressor 41 rotates as the exhaust turbine 42 rotates, so that air is compressed and sent. The variable nozzle 43 is driven via an actuator 44 by a control signal from an engine control unit (ECU) 10 to vary the turbine volume.

  An intercooler 6 is provided on the downstream side of the intake compressor 41. The intercooler 6 cools the air (supercharged air) compressed by the intake compressor 41. The supercharged air cooled by the intercooler 6 is further supplied to the combustion chamber of each cylinder of the engine 1 via the intake throttle valve 7, the collector portion 8 and the intake manifold 9 on the downstream side. The intake throttle valve 7 is driven via an actuator 71 by a control signal from the ECU 10.

In the exhaust passage 5, a filter catalyst (corresponding to the “exhaust purification filter” of the present invention) 11 and a NOx trap catalyst 12 are interposed downstream of the exhaust turbine 42.
The filter catalyst 11 is formed, for example, by coating a ceramic plug-type honeycomb body (PM filter) with zeolite and further coating with alumina carrying a noble metal (platinum Pt, palladium Pd, rhodium Rh, etc.). .

Filter catalyst 11 traps NOx in the exhaust gas at a low temperature, some extent the temperature increases NOx trap ability to desorb trapped NOx (the low temperature NOx trap function), the oxidation catalytic function of converting NO to NO ₂ And a PM trapping function for trapping particulate matter (particulates: PM) in the exhaust gas. Here, the filter catalyst 11 in the present embodiment traps NOx in the exhaust at a low temperature after the start, and starts desorption of the trapped NOx when a predetermined NOx desorption temperature (approximately 170 ° C.) is reached. Further, not only to convert NO to NO _2, oxidation of HC and CO which is other exhaust components are also possible. Furthermore, by raising the temperature of the filter catalyst 11 to 600 ° C. or more, it is possible to regenerate the PM collection function by burning and removing the accumulated PM (forced regeneration).

  The NOx trap catalyst 12 is made of, for example, alumina as a carrier, and selected from alkali metals (K, Na, Li, Cs, etc.), alkaline earths (Ba, Ca, etc.), and rare earths (La, Y, etc.) on the carrier. It is formed by supporting at least one element and a noble metal.

  In the active state, the NOx trap catalyst 12 traps NOx when the air-fuel ratio of the exhaust gas is lean, and releases the NOx trapped so far when the air-fuel ratio is rich or rich (high temperature NOx trap function).

  Accordingly, the NOx in the exhaust is trapped by the filter catalyst 11 at a low temperature after the start, and when the exhaust temperature becomes sufficiently high after a certain time has elapsed from the start, the NOx desorbed from the filter catalyst 11 and the NOx in the exhaust Is trapped by the NOx trap catalyst 12.

  An EGR cooler 14 and an EGR valve 15 are provided in the EGR passage 13 that connects the exhaust passage 5 upstream of the exhaust turbine 42 and the collector portion 8 of the intake passage 2. The EGR cooler 14 cools the exhaust gas recirculated through the EGR passage 13, and the EGR valve 15 is driven to open and close by a control signal from the ECU 10, and the exhaust gas recirculated so as to have a predetermined EGR rate corresponding to the operating state. Adjust.

  The engine 1 also includes a common rail combustion injection device 16. The fuel injection device 16 includes a supply pump 17, a common rail 18, and a fuel injection valve 19. The fuel pumped from the supply pump 17 is stored in the common rail 18. The fuel injection valve 19 is driven to open by the ECU 10 and injects pressurized fuel supplied via the common rail 18 into the cylinder of each cylinder. The combustion injection device 16 (fuel injection valve 19) can perform post injection for injecting a predetermined amount of fuel after main injection in order to raise the temperature of the filter catalyst 11.

  The ECU 10 receives detection signals output from various sensors, and executes various engine controls based on these detection signals. In particular, when the engine is started, the temperature of the filter catalyst 11 and the amount of NOx trap are monitored, and the temperature of the filter catalyst 11 is raised under predetermined conditions.

  The various sensors include an accelerator sensor 21 that detects the accelerator opening (depression amount), a crank angle sensor (which also functions as an engine speed sensor) 22, a water temperature sensor 23 that detects the engine coolant temperature, and an intake air amount. An air flow meter 24 for detecting, an upstream temperature sensor 25 for detecting the inlet exhaust temperature of the filter catalyst 11, a downstream temperature sensor 26 for detecting the outlet exhaust temperature of the filter catalyst 11, and a differential pressure detecting passage 27 for bypassing the filter catalyst 11. There are a differential pressure sensor 28 that detects a pressure difference (pressure loss) between the upstream side and the downstream side of the filter catalyst 11 and an air-fuel ratio sensor 29 that detects an exhaust air-fuel ratio.

Next, control executed by the ECU 10 at the time of starting will be described.
FIG. 2 is a flowchart of the start-up control according to the present embodiment, which starts when starting (for example, turning on the power by operating the start key).

  In FIG. 2, in step S1, the temperature of the filter catalyst 11 is detected. Specifically, the temperature of the filter catalyst 11 is calculated based on the detected temperatures of the upstream temperature sensor 25 and the downstream temperature sensor 26. However, the filter catalyst 11 may be calculated (estimated) by searching a map or the like based on various operation parameters (intake air amount, accelerator opening, engine speed, etc.). Here, the temperature detected by the upstream temperature sensor 25 may be detected as the temperature of the filter catalyst 11.

  In step S2, the NOx trap amount of the filter catalyst 11 is calculated. Various methods for calculating the amount of NOx trap have been conventionally known, and any of them may be used. For example, the amount of NOx discharged from the engine 1 (for example, the integrated value of the amount of NOx discharged per unit revolution), the NOx trap performance of the filter catalyst 11 (for example, trap efficiency according to the temperature of the filter catalyst 11), and The NOx trap amount can be calculated based on the NOx desorption amount from the filter catalyst 11 (for example, the integrated value of the NOx amount desorbed per unit time when the filter catalyst 11 becomes equal to or higher than the desorption temperature). .

  In step S3, it is determined whether or not the temperature of the filter catalyst 11 has reached the first predetermined temperature. The first predetermined temperature is set to a NOx desorption temperature at which NOx trapped in the filter catalyst 11 starts desorption or a temperature in the vicinity thereof (preferably slightly lower). In the present embodiment, the first predetermined temperature is set to 170 ° C. If the temperature of the filter catalyst 11 is lower than the first predetermined temperature, the process proceeds to step S4. If the temperature of the filter catalyst 11 is the first predetermined temperature, the process proceeds to step S5.

  In step S4, it is determined whether or not the NOx trap amount of the filter catalyst 11 has reached a predetermined upper limit allowable value. This upper limit allowable value is set in advance as the allowable NOx trap amount of the filter catalyst 11 or as the NOx trap amount in which the NOx trap performance (efficiency) approaches and decreases rapidly. If the NOx trap amount of the filter catalyst 11 is the upper limit allowable value, the process proceeds to step S5, and if it is less than the upper limit allowable value, the process returns to step S1.

In step S5, since the temperature of the filter catalyst 11 is the first predetermined temperature or the NOx trap amount of the filter catalyst 11 is the upper limit allowable value, NOx in the exhaust gas is not trapped by the filter catalyst 11 (trapped). NOx is desorbed), and rapid temperature increase control of the filter catalyst 11 is executed. More specifically, this rapid temperature rise control is performed by post-injection in which a predetermined amount of fuel is injected after main injection, and the exhaust gas temperature is raised to change the temperature of the filter catalyst 11 from NO in the filter catalyst 11 to NO _2. The temperature is raised to a second predetermined temperature (> first predetermined temperature) with a high conversion rate. Then, after reaching the second predetermined temperature, the post injection is adjusted so as to maintain the second predetermined temperature. Note that when the temperature of the filter catalyst 11 is increased, the intake air amount may be reduced by controlling the intake throttle valve 7 in the closing direction together with the post injection.

As shown in FIG. 3, the conversion rate from NO to NO ₂ in the filter catalyst 11 changes according to the temperature of the filter catalyst 11. And in the filter catalyst 11 in this embodiment, it has been confirmed that the conversion rate from NO to NO ₂ becomes maximum at about 300 to 350 ° C. Therefore, in the present embodiment, the second predetermined temperature is a temperature region in which the conversion rate from NO to NO ₂ is a predetermined value (for example, 50%) or more, specifically 250 to 400 ° C. (more preferably 300 ° C.). ~ 350 ° C).

  Here, the post-injection amount is basically adjusted according to the exhaust gas temperature, but since the exhaust gas temperature can be estimated from the operating state of the engine, in this embodiment, the main injection amount (accelerator opening degree and And is calculated by searching a map as shown in FIG. 4 based on the engine speed.

By such rapid temperature increase control, the filter catalyst 11 can generate NO ₂ positively and effectively, and the temperature of the filter catalyst 11 at this time is a temperature at which PM and NO ₂ can react. The PM accumulated on the filter catalyst 11 can be burned by this NO ₂ . Thereby, PM accumulated on the filter catalyst 11 can be removed while suppressing the release of NOx at a low temperature immediately after the start.

  In step S6, the temperature of the filter catalyst 11 is detected again as in step S2. If the temperature detected by the upstream temperature sensor 25 is the temperature of the filter catalyst 11 in step S2, the temperature detected by the downstream temperature sensor 26 is preferably detected as the temperature of the filter catalyst 11. This is because the temperature of the filter catalyst 11 including the heat of oxidation reaction can be detected more effectively.

  In step S7, it is determined whether or not the temperature of the filter catalyst 11 has become equal to or higher than a third predetermined temperature. The third predetermined temperature is higher than the second predetermined temperature, and is set to 450 to 500 ° C. in the present embodiment. Although the temperature of the filter catalyst 11 basically maintains the second predetermined temperature by the rapid temperature increase control, the temperature of the filter catalyst 11 may increase too much. In this step, a state in which the temperature of the filter catalyst 11 has increased excessively is detected. If the temperature is equal to or higher than the third predetermined temperature, the process proceeds to step S8. If the temperature of the filter catalyst 11 is lower than the third predetermined temperature, the process proceeds to step S12.

  In step S8, it is determined that the temperature of the filter catalyst 11 has risen excessively beyond the second predetermined temperature, and filter forced regeneration control is executed. More specifically, the filter forced regeneration control is to further raise the temperature of the filter catalyst 11 by post injection to raise the temperature of the filter catalyst 11 to the PM combustion temperature (> third predetermined temperature). The PM combustion temperature is a temperature at which PM deposited on the filter catalyst 11 burns, and is set to 600 ° C., for example. The post-injection amount is calculated by searching the map shown in FIG. 4 based on the main injection amount and the engine speed, as in step S5. The reason why the temperature of the filter catalyst 11 becomes equal to or higher than the third predetermined temperature during the rapid temperature rise control is that the exhaust temperature at the time of normal injection without post-injection is high, that is, the operating state changes suddenly and C in FIG. It is thought that it became an area. For this reason, the post injection amount is set by searching the map of FIG.

  In step S9, the PM accumulation amount of the filter catalyst 11 is calculated. Various methods for calculating the PM deposition amount have been known, and any of them may be used. For example, the relationship between the pressure difference between the upstream and downstream of the filter catalyst 11 and the PM accumulation amount is previously mapped and stored, and this map is searched based on the pressure difference detected by the differential pressure sensor 28 to obtain the PM accumulation amount. Is calculated. Further, the PM accumulation amount may be estimated based on the operation history (travel distance, etc.) from the previous PM combustion removal to the present time. The PM accumulation amount calculated here is the amount of PM (PM remaining amount) remaining in the filter catalyst 11 excluding PM burned and removed by the filter forced regeneration control.

  In step S10, it is determined whether the PM accumulation amount of the filter catalyst 11 has decreased to a predetermined forced regeneration end threshold value. This forced regeneration end threshold is a PM accumulation amount for ending the filter forced regeneration control, and can be arbitrarily set according to the filter catalyst to be employed. When the PM accumulation amount decreases to the forced regeneration end threshold value, the process proceeds to step S11.

In step S11, the post injection is stopped and the filter forced regeneration control is ended.
On the other hand, in step S12, since the temperature of the filter catalyst 11 is properly maintained during the rapid temperature increase control, it is determined whether or not a predetermined time has elapsed since the start of the rapid temperature increase control of the filter catalyst 11. This predetermined time is preferably set to such a time that all of the NOx trapped in the filter catalyst 11 is released in a state of the NOx desorption temperature or higher. For example, the predetermined time may be set according to the maximum NOx trap amount of the filter catalyst 11 or the NOx trap amount calculated in step S1.

In step S13, the post injection is stopped and the rapid temperature increase control is stopped.
In the present embodiment, the ECU 10 has functions as “NOx trap amount calculation means”, “temperature detection means”, and “temperature rise means” of the present invention. In particular, the process of step S1 in FIG. The processing in step S2 corresponds to “NOx trap amount calculation means”, and the processing in steps S3 to S13 corresponds to “temperature increase means”.

In this embodiment, the temperature of the filter catalyst 11 and the NOx trap amount are monitored from the start of the engine, and when the temperature of the filter catalyst 11 reaches a first predetermined temperature (170 ° C.) near the NOx desorption temperature, or the NOx trap amount. When the temperature reaches the allowable upper limit value, rapid temperature increase control of the filter catalyst 11 is executed (steps S1 to S5). This rapid temperature increase control increases the temperature of the filter catalyst 11 by post injection to a second predetermined temperature (250 to 400 ° C., preferably 300 to 350 ° C.) at which the conversion rate from NO to NO ₂ is high. In addition, during the rapid temperature rise control, the post injection amount is adjusted so as to maintain the second predetermined temperature.

Thereby, when the filter catalyst 11 that has trapped NOx in the exhaust gas from the start of the engine and suppressed release to the atmosphere is in a state where it cannot trap NOx and / or desorbs the trapped NOx, The exhausted and desorbed NO is positively and effectively converted to NO ₂ , and PM accumulated (collected) is burned and removed using this NO ₂ .

  Therefore, it is possible to suppress the release of NOx to the atmosphere at low temperatures, and to combust and remove PM deposited on the filter catalyst 11 together with this. In addition, since the accumulated PM can be removed by combustion periodically, clogging of the filter catalyst 11 can be prevented more reliably and the PM trapping performance can be suppressed, and a large amount of PM can be burned in the filter catalyst 11. This is advantageous in terms of durability. Further, the rapid activation of the temperature can be performed to activate the NOx trap catalyst 12 provided on the downstream side of the filter catalyst 11 at an early stage, thereby more effectively suppressing the release of NOx into the atmosphere.

Here, the rapid temperature increase control is performed so as to increase the temperature of the filter catalyst 11 to a second predetermined temperature at which the conversion rate from NO to NO ₂ is high and to maintain the second predetermined temperature. There is a case where the exhaust gas temperature rapidly rises due to sudden acceleration or the like, and the temperature of the filter catalyst 11 exceeds the second predetermined temperature. If the temperature of the filter catalyst 11 exceeds the second predetermined temperature, the conversion rate from NO to NO ₂ decreases, so that NOx in the exhaust cannot be used for PM combustion removal.

  In this case, the temperature of the filter catalyst 11 may be lowered to the second predetermined temperature by stopping the post injection or increasing the intake air amount. However, the post-injected fuel is wasted. It will end up. On the other hand, in order to raise the temperature of the filter catalyst 11 from the normal operation state to the PM combustion temperature as in the conventional filter forced regeneration, the amount of fuel to be post-injected is large because the PM combustion temperature is high.

  Therefore, in the present embodiment, when the temperature of the filter catalyst 11 exceeds the second predetermined temperature and reaches the third predetermined temperature (450 to 500 ° C.), the temperature is further raised to the filter catalyst 11 by using this. Forced regeneration control is executed (steps S6 to S11). In this filter forced regeneration control, the temperature of the filter catalyst 11 is raised to the PM combustion temperature (600 ° C. or higher) by post injection, and the PM accumulated on the filter catalyst 11 is burned and removed. Is performed until the threshold value decreases to the forced regeneration end threshold. As a result, in the present embodiment, since the temperature of the filter catalyst 11 is already high, the process proceeds to the forced filter regeneration control, so that the post injection amount can be suppressed compared to the conventional forced filter regeneration control. it can.

  In the above description, the filter catalyst 11 having the low-temperature NOx trap function, the oxidation catalyst function, and the PM trapping function has been described. Alternatively, a low-temperature NOx trap catalyst 111 having a low-temperature NOx trap function and an oxidation catalyst function and a PM filter 112 provided on the downstream side thereof may be integrated. In this case, the temperature of the low temperature NOx trap catalyst 111 (for example, the detected temperature of the upstream temperature sensor) and the NOx trap amount are monitored from the start of the engine, and the temperature of the low temperature NOx trap catalyst 111 is equal to the first predetermined temperature. When the temperature becomes NO or when the NOx trap amount reaches the allowable upper limit value, the rapid temperature rise control is executed, and the filter is forced when the temperature of the PM filter 112 (for example, the detected temperature of the downstream temperature sensor) reaches the third predetermined temperature. Reproduction control may be executed.

1 is an overall configuration diagram of an internal combustion engine according to an embodiment of the present invention. 3 is a flowchart of start-up control according to the embodiment. It is a diagram of the filter catalyst temperature and NO indicates the relationship between the conversion to NO _2. It is a figure which shows an example of the map for post injection amount setting. It is a figure which shows an example of the other exhaust gas purification filter replaced with a filter catalyst.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Internal combustion engine, 5 ... Exhaust passage, 7 ... Intake throttle valve, 10 ... ECU, 11 ... Filter catalyst (exhaust gas purification filter), 12 ... NOx trap catalyst, 16 ... Fuel injection device, 17 ... Supply pump, 18 ... Common rail , 19 ... Fuel injection valve, 21 ... Accelerator sensor, 22 ... Crank angle sensor, 23 ... Water temperature sensor, 24 ... Air flow meter, 25 ... Upstream temperature sensor, 26 ... Downstream temperature sensor, 28 ... Differential pressure sensor

Claims (7)

Traps NOx in the exhaust gas at low temperatures, the NOx trapping ability of leaving the trapped NOx when a predetermined desorption temperature, and the oxidation catalytic function of converting NO into NO _2, particulates in the exhaust gas (PM) An exhaust purification filter having a PM collection function of collecting;
NOx trap amount calculating means for calculating the NOx trap amount of the exhaust purification filter;
Temperature detecting means for detecting the temperature of the exhaust purification filter;
The exhaust purification filter is rapidly heated when the temperature of the exhaust purification filter reaches a first predetermined temperature near the desorption temperature or when the NOx trap amount of the exhaust purification filter reaches a predetermined allowable upper limit value. A temperature raising means;
An exhaust emission control device for an internal combustion engine, comprising: The temperature raising means rapidly raises the temperature of the exhaust purification filter to a _second predetermined temperature higher than the first predetermined temperature at which the conversion rate from NO to NO ₂ in the exhaust purification filter is high. The exhaust emission control device for an internal combustion engine according to claim 1. The temperature raising means raises the temperature of the exhaust gas purification filter by post injection for injecting fuel after main injection,
The exhaust purification device for an internal combustion engine according to claim 2, wherein the post-injection amount is adjusted so that the temperature of the exhaust purification filter maintains the second predetermined temperature.   When the temperature of the exhaust purification filter exceeds a third predetermined temperature that is higher than the second predetermined temperature, the temperature raising means is configured to burn the collected PM at the temperature of the exhaust purification filter. The exhaust gas purification apparatus for an internal combustion engine according to claim 2, wherein the temperature is further raised to a PM combustion temperature higher than the third predetermined temperature. A NOx trap catalyst having a NOx trap function for trapping NOx in exhaust at a low temperature and desorbing the trapped NOx at a predetermined desorption temperature; and an oxidation catalyst function for converting NO to NO ₂ ;
A PM filter that is disposed downstream of the NOx trap catalyst and collects particulates (PM) in the exhaust;
NOx trap amount calculating means for calculating the NOx trap amount of the NOx trap catalyst;
Catalyst temperature detecting means for detecting the temperature of the NOx trap catalyst;
When the temperature of the NOx trap catalyst reaches a first predetermined temperature in the vicinity of the desorption temperature, or when the NOx trap amount of the NOx trap catalyst reaches a predetermined allowable upper limit value, the NOx trap catalyst is rapidly heated. Temperature raising means for causing,
An exhaust emission control device for an internal combustion engine, comprising: The temperature raising means rapidly raises the temperature of the NOx trap catalyst to a _second predetermined temperature higher than the first predetermined temperature at which the conversion rate from NO to NO2 in the NOx trap catalyst is high. An exhaust purification device for an internal combustion engine according to claim 5. A filter temperature detecting means for detecting the temperature of the PM filter;
When the temperature of the PM filter exceeds a third predetermined temperature that is higher than the second predetermined temperature, the PM collected by the PM filter burns when the temperature of the PM filter exceeds a third predetermined temperature that is higher than the second predetermined temperature. The exhaust gas purification apparatus for an internal combustion engine according to claim 6, wherein the temperature is raised to a PM combustion temperature higher than the third predetermined temperature.

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