JPH0559245B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to an exhaust gas purification device for a diesel engine.

BACKGROUND TECHNOLOGY Diesel engines are operated with excess air, so there is a high concentration of oxygen in the exhaust gas of diesel engines, and therefore the reduction catalyst or three-way catalyst used for gasoline engines is It is not possible to reduce NOx in the exhaust gas of diesel engines. Therefore, conventionally, catalysts have not been used to reduce NOx in exhaust gas, and the current situation is to try to reduce NOx by recirculating the exhaust gas or adjusting the fuel injection time. .

In addition, a diesel engine was developed in Japanese Patent Application Laid-open No. 1989 (1982) in which the particulate matter contained in the exhaust gas of the diesel engine was captured by a filter, and a throttle valve installed in the intake passage was used to burn and remove the particulate matter captured by the filter. -Described in Publication No. 150921. In this diesel engine, the exhaust gas temperature is increased by throttling a throttle valve provided in the intake passage to burn particulate trapped in the filter, and NOx cannot be removed.

Problems to be solved by the invention As described above, in conventional diesel engines,
NOx is not removed using a catalyst.
Even if NOx is removed using recirculated exhaust gas or the like, NOx cannot be removed sufficiently.

Means for Solving the Problems According to the present invention, in order to solve the above problems,
A filter for capturing particulate matter is placed in the engine exhaust passage, and a NOx reduction catalyst is supported on this filter, and the particulate matter and high boiling point HC captured by the filter are used as reducing agents.
Due to the catalytic action of the NOx reduction catalyst,
We are trying to reduce NOx.

Embodiment Referring to FIG. 1, 1 is an engine main body, 2 is an intake manifold, 3 is an intake duct connected to a gathering part of the intake manifold 2, 4 is an exhaust duct connected to an exhaust manifold (not shown), 5 is an exhaust turbocharger connected to the outlet of the exhaust duct 4; 6;
7 indicates an exhaust pipe connected to the outlet of the exhaust turbocharger 5, and 7 indicates another exhaust pipe connected to the exhaust pipe 6. The exhaust pipe 6 includes a main passage 8 and a bypass passage 9 that bypasses the main passage 8.
A cylindrical filter 10 is arranged within the main passage 8. On the other hand, a bypass valve 12 is inserted into the bypass passage 9 and is operated by a negative pressure diaphragm actuator 11.
2 is normally kept closed. Therefore, the exhaust gas normally passes through the filter 10 as shown by arrow A and is discharged to the atmosphere via the exhaust pipe 7. The actuator 11 has a negative pressure chamber therein, and fully opens the bypass valve 12 when negative pressure is applied to the negative pressure chamber, and closes the bypass valve 12 when atmospheric pressure acts within the negative pressure chamber. This negative pressure chamber is selectively connected to a negative pressure source or to the atmosphere via an electromagnetic switching valve 14 controlled by an output signal of an electronic control unit 13.

The electronic control unit 13 consists of a digital computer, RAM (Random Access Memory) 16, interconnected by a bidirectional bus 15;
It includes a ROM (read-on memory) 17, a CRU (microprocessor) 18, an input port 19, and an output port 20. The electromagnetic switching valve 14 is connected to the output port 20 via a drive circuit 21. On the other hand, electric heaters 22 arranged radially are arranged at the inlet of the filter 10.
2 is connected to the output port 20 via the drive circuit 23. Furthermore, a filter 10 is included in the filter 10.
A temperature sensor 24 that detects the temperature of is inserted, and this temperature sensor 24 is connected to the input port 19 via an AD converter 25.

On the other hand, a throttle valve 25 whose opening and closing are controlled by a negative pressure diaphragm actuator 24 is disposed within the intake duct 3 . The actuator 24 has a negative pressure chamber and an atmospheric pressure chamber separated by a diaphragm, and the negative pressure chamber is selectively connected to a negative pressure source or the atmosphere via an electromagnetic switching valve 26. The diaphragm of the actuator 24 is linked via a control rod 27 to the valve stem 28 of the throttle valve 25. When negative pressure is applied to the negative pressure chamber of the actuator 24, the throttle valve 25 is rotated in the valve closing direction, and when atmospheric pressure is applied to the negative pressure chamber of the actuator 24, the throttle valve 25 is rotated.
5 is rotated in the valve opening direction. Solenoid switching valve 2
6 is connected to the output port 20 via a drive circuit 29. A throttle valve opening sensor 30 for detecting the opening of the throttle valve 25 is attached to the valve shaft 28 of the throttle valve 25, and this throttle valve opening sensor 30 is connected to an AD converter 31.
is connected to input port 19 via. In addition, a load sensor that detects the engine load, for example, a load sensor 32 that detects the idle position of the fuel injection pump, is installed.
A rotation speed sensor 33 is connected to the input port 19 via an AD converter 33 and generates a pulse in proportion to the engine rotation speed.
In the CPU 18, the engine rotation speed is calculated from the output pulse of the rotation speed sensor 33.

The filter 10 is made of cordierite with a three-dimensional network structure coated with γ-alumina, and filters particulate matter and high boiling point HC in the exhaust gas.
While the exhaust gas flows through the filter 10, it is captured by the mesh structure and deposited therein. Note that this filter 10 can handle particulate matter, high boiling point, etc. without significantly increasing the exhaust pressure.
Since it is sufficient to capture HC, the internal shape of the filter 10 may be any shape such as a honeycomb shape, a monolith shape, or a pellet shape.

In an embodiment according to the present invention, Co is deposited on a support made of cordierite coated with γ-alumina.
At least one metal selected from Fe, Ni, rare earths (La, Th, Ce, etc.) and platinum group metals (Pt, Rh,
A catalyst consisting of three components (Ru, Pd, Os, Ir, etc.) is supported. These catalysts under excess oxygen
It has been found that it has a strong reducing property against NOx, and this strong reducing property is exhibited within a certain temperature range as described below.

Next, a method for producing a carrier supporting such a catalyst and its NOx purification performance will be explained.

As the carrier, cordierite having a three-dimensional network structure coated with γ-alumina is used as described above.

First, a carrier is immersed in a mixed solution of cobalt nitrate and lanthanum nitrate to impregnate Co and La onto the carrier. Then, it is dried by blowing warm air at 60°C for a predetermined period of time.

Then, the support was immersed in an acidic nitric acid solution of Pt.
Adsorb Pt onto a carrier. Then, it is dried by blowing warm air at 60°C for a predetermined period of time.

Next, the carrier is exposed to saturated steam of a 10% ammonia aqueous solution at 20°C for 10 minutes, and then to a hydrogen atmosphere at 400°C for 40 minutes to perform H ₂ reduction and form simple Co on the carrier.

In addition, as a NOx reduction catalyst, Co
It has been found that it is preferable to first deposit Pt and La on the carrier, and then deposit Pt on the carrier.

FIG. 4 shows the results of testing the NOx purification performance of the carrier thus obtained by placing it in an exhaust passage. This carrier has a volume of approximately 2000 cm ² and is coated with approximately 360 g of alumina, including Co, La, and Pt.
The amounts are approximately 8.8g, 4.0g, and 1.2g, respectively. In FIG. 4, the broken line shows when the engine speed is 2000 rpm, and the solid line shows when the engine speed is 3000 rpm. Also, in Figure 4, the vertical axis C is
It shows the purification rate of NOx, and the horizontal axis T shows the temperature of the carrier. From Figure 4, the temperature of the carrier is 350℃.
It can be seen that the NOx purification rate peaks in the vicinity. In addition, in Fig. 4, the purification rate is quite high in the section T, that is, the temperature of the carrier is between approximately 300°C and 400°C, and if the temperature of the carrier is maintained within this temperature range T, a high NOx purification rate can be obtained. I know that it will happen.

As mentioned above, the bypass valve 12 shown in FIG. 1 is normally closed, so that the exhaust gas is normally discharged through the filter 10 and the main passage 8 to the outside air. The filter 10 has a three-dimensional network structure cordierite coated with γ-alumina as a carrier, and a catalyst consisting of at least one metal selected from Co, Fe, and Ni, a rare earth element, and a platinum group metal on the carrier. It consists of what it carries. Therefore, in order to obtain high NOx purification efficiency using this filter 10, it is necessary to maintain the filter 10 at a temperature between 300°C and 400°C, preferably at about 350°C.

Next, a method for maintaining the filter 10 at about 350° C. will be explained with reference to the flowchart shown in FIG. Referring to FIG. 2, first, in step 40, the temperature t of the filter 10 is detected from the output signal of the temperature sensor 24. Next, in step 41, it is determined whether a flag that is set when the filter temperature t remains low for a certain period of time is set. Normally this flag will have been reset, so proceed to step 42. step
At step 42, it is determined whether the filter temperature t is below 340°C, and if t<340°C, the process proceeds to step 43.
In step 43, it is determined whether or not the timer has been set. If the timer has not been set, the timer is set in step 44. This timer is reset after a certain period of time has elapsed. Next, in step 45, it is determined whether a certain period of time set by a timer has elapsed. This time is, for example, 30 seconds. If the predetermined period of time has not elapsed, the process advances to step 46 and throttle control of the throttle valve 25 is performed.

Throttling the throttle valve 25 reduces the amount of excess air supplied into the engine cylinder, resulting in an increase in the temperature of the exhaust gas. However, if the throttle valve 25 is throttled too much, combustion will deteriorate. It is known from the beginning to what extent the exhaust gas temperature will be the highest within the range without deteriorating combustion. The relationship between the opening degree of the throttle valve 25 at which the exhaust gas temperature is highest within the range that does not deteriorate combustion, the engine speed, and the engine load is stored in advance in the ROM 17, and in step 46, the throttle valve is opened according to this relationship. The opening degree of 25 is controlled. That is, if the opening of the throttle valve becomes larger than the opening of the throttle valve determined by the engine speed and load, the opening of the throttle valve 25 is reduced by switching the electromagnetic switching valve 26 and communicating the negative pressure chamber of the actuator 24 with the negative pressure source. However, if the opening degree of the throttle valve becomes smaller than the opening degree determined by the engine speed and load, the opening degree of the throttle valve 25 is increased by switching the electromagnetic switching valve 26 and opening the negative pressure chamber of the actuator 24 to the atmosphere. In this way, the opening degree of the throttle valve 25 is maintained at a constant opening degree determined by the engine speed and engine load, and thus the exhaust gas temperature is raised.

Next, when the temperature t of the filter 10 becomes higher than 340°C, the process proceeds to step 47 and the filter temperature t becomes 360°C.
It is determined whether or not the temperature is higher than 0.degree. C., and if t≦360.degree. C., the aperture control is continued. t＞360℃
If so, the flow advances to step 48, where the throttle valve control is stopped, and the throttle valve 25 is maintained in the fully open state. Therefore, since the temperature of the filter 10 is maintained within the range of 340°C to 360°C, NOx in the exhaust gas is effectively purified.

On the other hand, if a certain period of time has elapsed in step 45, that is, if the condition t<340° C. has elapsed for a certain period of time, the process proceeds to step 49 and a flag is set. Therefore, the process proceeds from step 41 to step 50. In step 50, it is determined whether the temperature t of the filter 10 is smaller than 340°C, and if t<340°C, the electric heater 22 is activated in step 51. Then proceed to step 46. When t≧340℃, proceed to step 52 and filter temperature t becomes 360℃.
If t≦360°C, the process advances to step 51 and the heater 22 continues to operate. Then proceed to step 46. Therefore, it can be seen that the aperture control is also being performed while the heater 22 is being operated. When t>360°C, the operation of the heater 22 is stopped in step 53, and the flag is then reset in step 54, so that only the throttle control by the throttle valve 25 continues to be performed again.

As described above, in the embodiment according to the present invention, if the filter temperature t can be maintained within the range of 340°C to 360°C only by the aperture control, the filter temperature t is controlled only by the aperture control; t
If it is not possible to maintain the temperature above 340°C, heater control is also performed at the same time. Therefore, in the former case, the throttle control constitutes the catalyst temperature raising means, and in the latter case, the throttle control and the heater control constitute the catalyst temperature raising means.

Furthermore, particulate and high boiling point HC are captured within the mesh structure of the filter 10. When it is time for the filter 10 to regenerate, that is, when the integrated value of the engine speed exceeds a certain value (in this case, a non-volatile RAM is provided), or when the back pressure upstream of the filter 10 exceeds a certain value (In this case, install a pressure sensor inside the exhaust pipe 6) Solenoid switching valve 1
4 is activated to open the bypass valve 12, and then the heater 22 is activated to remove the particulate and high boiling point deposits on the filter 10.
HC is burned. Since such regeneration processing is well known, it will not be described in detail here. However, the embodiment shown in FIG. 1 has one feature in that the filter 10 for capturing particulate matter is also used for reducing NOx. In this case, the particulate hydrate captured in the filter 10, the high boiling point HC
is particularly effective because it acts as a NOx reducing agent.

Another embodiment is shown in FIG. In this embodiment, a catalytic converter 61 having a built-in monolithic catalyst 60 is connected to the exhaust pipe 4. This monolithic catalyst 60 uses cordierite coated with γ-alumina as a carrier, similar to the filter 10 shown in FIG. Group metals are supported. However, since the monolithic catalyst 60 has a large number of holes extending straight in the direction of flow of exhaust gas, it does not have much ability to capture particulate matter. Also in this embodiment, the throttle valve 25
and heater 22 are controlled in the same manner as in the embodiment of FIG.

Note that when the throttle valve 25 is throttled while the vehicle is running, the exhaust gas temperature rises as described above, but at the same time, HC, CO, particulate, and particulate that act as reducing agents for NOx in the exhaust gas are reduced.
SOF (liquid hydrocarbon) increases, and these can also promote NOx purification.
Note that if the fuel injection timing is delayed, afterburning occurs and the exhaust gas temperature rises, and the amount of reducing agent in the exhaust gas mentioned above also increases, so in addition to throttle control and heater control, the fuel injection timing is further controlled. You can also do that.

Effects of the Invention NOx contained in exhaust gas can be effectively reduced by the catalytic action of the NOx reduction catalyst supported on the filter using particulate and high boiling point HC captured by the filter as reducing agents.

[Brief explanation of drawings]

Fig. 1 is an overall diagram of a diesel engine, Fig. 2 is a flowchart, Fig. 3 is an overall diagram showing another embodiment of the diesel engine, and Fig. 4 is a diagram showing the relationship between carrier temperature and NOx purification rate. It is. 1... Engine body, 3... Intake duct, 4, 6,
7...Exhaust pipe, 8...Main passage, 9...Bypass passage, 10...Filter, 12...Bypass valve, 1
3...Electronic control unit, 22...Electric heater,
25... Throttle valve.

Claims (1)

[Claims] 1. A filter for capturing particulate matter is placed in the engine exhaust passage, and a NOx reduction catalyst is supported on the filter, and the particulate matter and high boiling point HC captured by the filter are used as reducing agents to reduce exhaust gas by the catalytic action of the NOx reduction catalyst. A diesel engine exhaust purification device designed to reduce NOx inside.