JP4523317B2 - Diesel engine exhaust purification system - Google Patents

Description

  The present invention relates to an exhaust emission control device for a diesel engine including a NOx trap catalyst that traps NOx in exhaust gas and a PM trap that traps PM in exhaust gas.

  In addition to the NOx trap catalyst for adsorbing and purifying NOx (nitrogen oxides) in the exhaust, the exhaust passage of the diesel engine adsorbs and purifies PM (exhaust particulate matter) in the exhaust. A PM trap is provided for this purpose.

The NOx trap catalyst adsorbs NOx when the air-fuel ratio of the exhaust gas flowing into it is lean and prevents its emission to the atmosphere. When the air-fuel ratio of the exhaust gas becomes rich and the O ₂ concentration decreases, the NOx trap catalyst It has a function (NOx regeneration) for reducing and purifying NOx with HC and CO as reducing components.

In addition to normal fuel injection in the intake stroke of a diesel engine, NOx regeneration also injects fuel in the exhaust stroke to further enrich the air-fuel ratio of the exhaust gas than the stoichiometric air-fuel ratio, and unburned components of fuel into the exhaust passage It is discharged to rapidly reduce the O ₂ concentration and increase the reducing components HC and CO. For example, it is executed for 10 to 15 seconds every time the vehicle travels 10 km.

  The PM trap is composed of a filter that traps PM in exhaust gas and prevents its emission into the atmosphere. In order to prevent the PM trap from being clogged by PM and causing pressure loss as the travel distance of the vehicle increases, PM trap regeneration that burns and purifies the trapped PM is performed. The PM trap regeneration is similar to the NOx regeneration, in which fuel is injected in the exhaust stroke to raise the temperature of the exhaust to burn PM. For example, every time the vehicle travels from 300 km to 500 km, it takes 10 minutes to 15 minutes. It is executed over the period.

In Patent Document 1 below, NOx regeneration and PM trap regeneration are performed at the same time, so that fuel consumption for enriching the air-fuel ratio is reduced as compared with the case where NOx regeneration and PM trap regeneration are performed separately. Is described.
Japanese Patent Laid-Open No. 2003-201828

  However, since the execution time of NOx regeneration is only about 10 seconds to 15 seconds in the one described in Patent Document 1, the period of NOx regeneration is a period of PM trap regeneration over 10 minutes to 15 minutes. Even if they were overlapped, it was difficult to obtain a large fuel saving effect. Further, when NOx regeneration and PM trap regeneration are performed at the same time, it is necessary to increase the exhaust temperature at the inlet of the PM trap, which causes a problem of increasing fuel consumption.

  The present invention has been made in view of the above-described circumstances, and an object thereof is to effectively reduce fuel consumption necessary for exhaust purification of a diesel engine.

In order to achieve the above object, according to the first aspect of the present invention, NOx is trapped in the exhaust passage of the diesel engine when the exhaust air-fuel ratio is lean, and when the exhaust air-fuel ratio is rich. A NOx trap catalyst that reduces and purifies trapped NOx, a PM trap that is disposed downstream of the NOx trap catalyst and traps PM in the exhaust, SOx poisoning detection means for detecting SOx poisoning of the NOx trap catalyst, SOx regeneration determination means for determining whether or not the SOx poisoning of the NOx trap catalyst should be regenerated based on the detection result of the SOx poisoning detection means, and a PM accumulation amount detection means for detecting the amount of PM deposited on the PM trap; , PM deposition regeneration determination means for determining whether or not to regenerate PM deposition of the PM trap based on the detection result of the PM deposition amount detection means, and NOx trap The diesel engine E has an air supply means for supplying air between the catalyst and the PM trap, a temperature raising means for raising the temperature of the NOx trap catalyst, and a temperature sensor for detecting the temperature of the PM trap. And an air-fuel ratio enrichment means for enriching the air-fuel ratio of the exhaust gas flowing into the NOx trap catalyst, and when the SOx regeneration determination means determines that the SOx poisoning should be regenerated, the temperature increase means raises the NOx trap catalyst. The air-fuel ratio of the exhaust gas flowing into the NOx trap catalyst is enriched by the air-fuel ratio enriching means to regenerate the SOx poisoning, and at the same time, air is supplied to the PM trap by the air supply means to regenerate the PM deposit, during playback of that accumulated PM, when the temperature detected by the temperature sensor exceeds a predetermined threshold value, and to prevent deterioration due to overheating of the PM trap To increase the amount of air supplied from the air supply means to the PM trap, SOx regeneration determination means does not determines to play SOx poisoning, and the PM accumulated regeneration determination means determines to play the PM deposition Then, an exhaust emission control device for a diesel engine is proposed in which the supply of air from the air supply means to the PM trap is stopped to regenerate the PM deposit .

  The fuel injection valve 16 of the embodiment corresponds to the air-fuel ratio enrichment means of the present invention, the air pump 18 of the embodiment corresponds to the air supply means of the present invention, and the travel distance calculation means 20 of the embodiment corresponds to the present invention. The NOx concentration sensor 22 of the embodiment corresponds to the PM accumulation amount detection means, the SOx poisoning detection means of the present invention, and the heater 24 of the embodiment corresponds to the temperature raising means of the present invention.

According to the configuration of claim 1, when the SOx regeneration determination unit determines that the SOx poisoning should be regenerated, the temperature of the NOx trap catalyst is raised by the temperature raising unit, and the NOx trap catalyst is made by the air-fuel ratio enrichment unit. The air-fuel ratio of the inflowing exhaust gas is enriched to regenerate the SOx poisoning, and at the same time, air is supplied to the PM trap by the air supply means, so that the accumulated PM is burned to regenerate the PM deposit. It is possible to regenerate the PM deposit by utilizing the fact that the temperature of the PM trap automatically rises with the regeneration of the fuel, and there is no need to raise the temperature of the PM trap specially for the regeneration of the PM deposit. Consumption can be saved. Moreover, during playback of the PM deposition of that, when the detected temperature of the PM trapped in the temperature sensor exceeds a predetermined threshold, in order to prevent deterioration due to overheating of the PM trap, from the air supply means to the PM trap Since the amount of air to be supplied is increased, the heat generated by combustion of PM deposited in the PM trap is cooled by the air with the increased supply amount, and damage to the PM trap due to heat can be avoided. In addition , when only PM deposition is regenerated without regeneration of SOx poisoning, air supply means stops supplying air to the PM trap to regenerate PM deposition. It can be performed.

  DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described below based on examples of the present invention shown in the accompanying drawings.

  1 and 2 show an embodiment of the present invention. FIG. 1 is a diagram showing the configuration of an exhaust emission control device for a diesel engine, and FIG. 2 is a flowchart for explaining the operation.

  As shown in FIG. 1, a NOx trap catalyst 12 for adsorbing and purifying NOx in exhaust gas in an exhaust passage 11 of a diesel engine E, and a PM trap for adsorbing and purifying PM in exhaust gas 13 are provided. An air cleaner 15, an intake flow control valve 23, and a fuel injection valve 16 are provided in the intake passage 14 of the diesel engine E. An air pump 18 and an air valve 19 are disposed in the air supply passage 17 that connects the intermediate position. Signals are input from a travel distance calculation means 20 that calculates the travel distance of the vehicle, a temperature sensor 21 that detects the temperature of the PM trap 13, and a NOx concentration sensor 22 that detects the NOx concentration at the outlet of the NOx trap catalyst 12. The electronic control unit U includes SOx regeneration determination means M1 and PM accumulation regeneration determination means M2, and includes a fuel injection valve 16, an air pump 18, an air valve 19, an intake flow control valve 23, and a heater 24 provided in the NOx trap catalyst 12. Control the operation.

NOx contained in the exhaust gas generated by the operation of the diesel engine E is trapped by the NOx trap catalyst 12 and is prevented from being released into the atmosphere. Before the NOx trap amount of the NOx trap catalyst 12 reaches the limit, that is, every time the travel distance of the vehicle calculated by the travel distance calculation means 20 reaches a predetermined distance (10 km in the embodiment), a command from the electronic control unit U is given. The fuel injection valve 16 performs fuel injection in the exhaust stroke in addition to the intake stroke of the diesel engine E, and discharges unburned components of the fuel to the exhaust passage 11 for a predetermined time (10 to 15 seconds in the embodiment). To enrich the air-fuel ratio of the exhaust. As a result, the O ₂ concentration in the exhaust gas rapidly decreases and the HC concentration and CO concentration as reducing components increase, and NOx regeneration for reducing and purifying NOx trapped in the NOx trap catalyst 12 is performed.

  In addition to NOx, SOx (sulfur oxide) is present in the exhaust of the diesel engine E, and this SOx is also trapped by the NOx trap catalyst 12. Since SOx trapped in the NOx trap catalyst 12 forms a stable sulfate, it is more difficult to purify than NOx, and there is a problem that the amount of NOx trapped is gradually accumulated and reduced in the NOx trap catalyst 12. In particular, in the diesel engine E that uses light oil containing a large amount of sulfur as a fuel, the SOx poisoning is more likely to occur than a gasoline engine.

  For this reason, every time the travel distance of the vehicle calculated by the travel distance calculation means 20 reaches a predetermined distance (3000 km in the embodiment), a fuel injection valve in the exhaust stroke in the same manner as in the NOx regeneration in response to a command from the electronic control unit U The fuel injection from 16 is performed for a predetermined time (20 minutes in the embodiment), and the SOx regeneration for reducing and purifying the SOx trapped in the NOx trap catalyst 12 by enriching the air-fuel ratio of the exhaust is performed. However, since SOx is less likely to be reduced than NOx, at the time of SOx regeneration, the heater 24 is operated by a command from the electronic control unit U to heat the temperature of the NOx trap catalyst 12 to a high temperature (for example, 650 ° C.) to reduce SOx. Promote. Incidentally, the temperature of the NOx trap catalyst 12 during NOx regeneration without operating the heater 24 is, for example, 250 ° C.

  When the PM trap 13 made of a filter that traps PM in the exhaust and prevents its emission to the atmosphere is clogged, there is a problem that pressure loss occurs in the exhaust passage 11 and the output of the diesel engine E decreases. Therefore, PM trap regeneration is performed in which the PM trapped in the PM trap 13 is burned and purified. The PM trap regeneration is achieved by burning the PM by further increasing the temperature of the exhaust gas by the heater 24 in a state where the opening degree of the intake flow control valve 23 is decreased by a command from the electronic control unit U. This PM trap regeneration is performed every time the vehicle travels 300 km to 500 km. The air-fuel ratio is enriched both during PM trap regeneration and during SOx regeneration. Further, the execution time of PM trap regeneration is 10 to 15 minutes, which is the same order as 20 minutes, which is the execution time of SOx regeneration.

  Therefore, in this embodiment, SOx regeneration and PM trap regeneration are executed simultaneously as much as possible. That is, PM trap regeneration is performed for 10 minutes to 15 minutes during 20 minutes during which SOx regeneration is performed. This eliminates the need for fuel for enriching the air-fuel ratio of the exhaust during the period of 10 to 15 minutes when PM trap regeneration is performed, and can contribute to a reduction in fuel consumption of the diesel engine E.

  The one described in Patent Document 1 saves fuel for enriching the air-fuel ratio of exhaust gas by simultaneously performing NOx regeneration and PM trap regeneration, but the NOx regeneration execution time is slightly reduced. Since it is 10 to 15 seconds, the fuel saving effect is small. However, according to the present embodiment, SOx regeneration and PM trap regeneration, which have the same execution time, are simultaneously performed to reduce the fuel for enriching the air-fuel ratio of the exhaust gas. Is greatly improved compared to the conventional case.

  Since the PM trapped in the PM trap 13 generates heat when burned by the above-described PM trap regeneration, the PM trap 13 may be overheated and deteriorated unless the amount of generated heat is controlled. In this embodiment, the PM trap regeneration is performed. The temperature of the PM trap 13 is monitored by the temperature sensor 21 during the execution of the operation, and when the temperature exceeds a predetermined threshold, the air valve 19 is further opened by the command from the electronic control unit U to supply the air to the PM trap 13 Since the amount is increased, the PM trap 13 can be cooled with the air to prevent deterioration due to overheating.

  Such overheating of the PM trap 13 is likely to occur, for example, when the temperature of the PM trap 13 is high and the exhaust flow rate is high, and the exhaust flow rate is low, such as when the vehicle is stopped.

  The above operation will be further described based on the flowchart of FIG.

  First, in step S1, the SOx regeneration determination means M1 of the electronic control unit U detects the degree of SOx poisoning of the NOx trap catalyst 12 based on the output of the NOx concentration sensor 22. As the SOx poisoning of the NOx trap catalyst 12 progresses, the NOx emission amount also increases. Therefore, the degree of SOx poisoning can be detected based on the NOx emission amount. In subsequent step S2, based on the output of the travel distance calculating means 20, it is determined whether the PM deposition regeneration determining means M2 of the electronic control unit U needs to perform PM trap regeneration. The PM trap regeneration is performed every time the vehicle travels 300 km to 500 km.

  Next, in step S3, it is determined whether or not SOx regeneration of the NOx trap catalyst 12 is necessary based on the degree of SOx poisoning of the NOx trap catalyst 12 detected in step S1. If it is determined in step S3 that SOx regeneration is necessary, simultaneous regeneration of the NOx trap catalyst 12 and PM trap 13 is executed in step S4. That is, after the heater 24 is operated to raise the temperature of the NOx trap catalyst 12 from 250 ° C. in the normal state to 650 ° C., the fuel is injected from the fuel injection valve 16 in the exhaust stroke and trapped in the NOx trap catalyst 12. In addition, the air pump 18 is driven and the air valve 19 is opened to supply a large amount of secondary air to the PM trap 13 to burn and purify the accumulated PM. At this time, the amount of secondary air supplied to the PM trap 13 by the air valve 19 is controlled in step S5 so that the oxygen concentration at the inlet of the PM trap 13 becomes 3% (the flow rate of the secondary air is the flow rate of the exhaust gas). To 18%).

  As described above, since the PM trap regeneration is performed by utilizing the fact that the temperature of the PM trap 13 is automatically increased with the SOx regeneration, it is necessary to specially increase the temperature of the PM trap 13 for the PM trap regeneration. Without it, fuel consumption can be saved.

On the other hand, if it is determined in step S3 that SOx regeneration is not necessary, and if it is determined in step S6 that PM trap regeneration is necessary, PM trap regeneration is executed independently in step S7. The PM trap regeneration performed independently is different from the PM simultaneous regeneration in step S4 described above, in which the supply of air from the air supply means 18 to the PM trap 13 is stopped and the opening degree of the intake flow control valve 23 is reduced. This is achieved by further burning PM by the heater 24. This single PM regeneration is the same as the conventional method.

  When PM trap regeneration is performed simultaneously with SOx regeneration in this way, if the temperature of the PM trap 13 detected by the temperature sensor 21 in step S8 exceeds a predetermined value (800 ° C.), the opening degree of the air valve 19 in step S9. To increase the temperature of the PM trap 13. When the temperature of the PM trap 13 becomes a predetermined value (600 ° C.) or less in step S10, normal regeneration is restored in step S11.

  On the other hand, if the temperature of the PM trap 13 is less than a predetermined value (800 ° C.) in step S8, PM trap regeneration is continued in step S12, and when a predetermined time (20 minutes) elapses from PM trap regeneration in step S13, In step S14, PM trap regeneration is terminated, and normal operation is resumed.

  Although the embodiments of the present invention have been described above, various design changes can be made without departing from the scope of the present invention.

  For example, in the embodiment, the timing for performing PM trap regeneration is determined based on the travel distance of the vehicle calculated by the travel distance calculation means 20, but when the pressure difference between the upstream and downstream of the PM trap 13 exceeds a threshold value. That is, the PM trap regeneration may be executed when the clogging of the PM trap 13 proceeds. Further, PM trap regeneration may be executed based on the accumulated operation time in consideration of the rotational speed and load of the diesel engine E.

  In the embodiment, air is supplied to the PM trap 13 using a dedicated air pump 18. However, in a diesel engine E equipped with a turbocharger, air extracted from the compressor may be supplied to the PM trap 13.

  Further, as another means for regenerating the PM trap 13 alone, an oxidation catalyst is further provided on the upstream side of the PM trap 13, and the fuel injected from the fuel injection valve 16 in the exhaust stroke is combusted in the oxidation catalyst, so that the temperature of the exhaust gas is increased. The PM deposition may be regenerated by raising the value.

The figure which shows the structure of the exhaust gas purification device of the diesel engine Flow chart explaining operation

11 Exhaust passage 12 NOx trap catalyst 13 PM trap 14 Intake passage 16 Fuel injection valve (air-fuel ratio enrichment means)
18 Air pump (air supply means)
20 Travel distance calculation means (PM accumulation amount detection means)
21 Temperature sensor 22 NOx concentration sensor (SOx poisoning detection means)
24 Heater (temperature raising means)
E Diesel engine M1 SOx regeneration determination means M2 PM accumulation regeneration determination means

Claims (1)

In the exhaust passage (11) of the diesel engine (E),
A NOx trap catalyst (12) for trapping NOx when the air-fuel ratio of the exhaust gas is lean and reducing and purifying NOx trapped when the air-fuel ratio of the exhaust gas is rich;
A PM trap (13) disposed downstream of the NOx trap catalyst (12) and trapping PM in the exhaust;
SOx poisoning detection means (22) for detecting SOx poisoning of the NOx trap catalyst (12), and SOx poisoning of the NOx trap catalyst (12) should be regenerated based on the detection result of the SOx poisoning detection means (22). SOx regeneration determination means (M1) for determining whether or not
PM deposition amount detection means (20) for detecting the amount of PM deposited on the PM trap (13), and whether PM deposition of the PM trap (13) should be regenerated based on the detection result of the PM deposition amount detection means (20) PM accumulation regeneration determination means (M2) for determining whether or not,
Air supply means (18) for supplying air between the NOx trap catalyst (12) and the PM trap (13);
A temperature raising means (24) for raising the temperature of the NOx trap catalyst (12);
A temperature sensor (21) for detecting the temperature of the PM trap (13),
The intake passage (14) of the diesel engine E includes air-fuel ratio enrichment means (16) that enriches the air-fuel ratio of the exhaust gas flowing into the NOx trap catalyst (12),
When the SOx regeneration determination means (M1) determines that the SOx poisoning should be regenerated, the temperature rise means (24) raises the temperature of the NOx trap catalyst (12) and the air-fuel ratio enrichment means (16) increases the NOx. The air-fuel ratio of the exhaust gas flowing into the trap catalyst (12) is enriched to regenerate the SOx poisoning, and at the same time, air is supplied to the PM trap (13) by the air supply means (18) to regenerate the PM deposit,
During playback of that accumulated PM, when the temperature detected by the temperature sensor (21) exceeds a predetermined threshold, in order to prevent deterioration due to overheating of the PM trap (13), air supply means (18) increase the amount of air supplied to the PM trap (13) from,
When the SOx regeneration determining means (M1) does not determine that the SOx poisoning is to be regenerated and the PM deposition regeneration determining means (M2) determines that the PM deposition is to be regenerated, PM is sent from the air supply means (18). An exhaust emission control device for a diesel engine, characterized in that the supply of air to the trap (13) is stopped to regenerate the PM deposit .

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