JPH0526003B2 - - Google Patents

Description

[Detailed Description of the Invention] Technical Field The present invention relates to the purification of diesel engine exhaust gas, and more specifically, to a method for collecting solid particles such as black smoke particles (soot) contained in diesel engine exhaust gas. The present invention relates to a method for regenerating a filter of a collection device.

BACKGROUND ART Collection devices for purifying exhaust gas by collecting solid particles such as black smoke particles contained in diesel exhaust gas are known. A monolithic filter made of porous ceramic with a three-dimensional network structure is housed in the collection device, and this device is connected to the diesel engine exhaust system. When the exhaust gas is passed through the device, solid particles in the gas are collected by the filter mainly by the principle of collision collection.
As the collected particles accumulate in the filter as the device is used, the filter becomes clogged, increasing ventilation resistance and reducing collection efficiency, so the filter must be regenerated periodically or at an appropriate time. For this reason, an electric heater is installed on the upstream end face of the filter, and at an appropriate time, it is energized and heated to ignite the solid particles in the filter, burning and blowing off the carbon, which is the main component of the particles, thereby removing the filter. It's playing.

Filter regeneration is performed while the engine is running. As a general rule, diesel engines are not equipped with intake throttle valves, so the maximum flow of intake air is always supplied to the combustion chamber under all engine load conditions, and output is controlled by changing the fuel injection amount according to the load. It will be done. Therefore, the excess air ratio, that is, the oxygen concentration in the exhaust gas changes depending on the load conditions. During filter regeneration, it is desirable that the oxygen concentration of the exhaust gas flowing through the filter remains constant. That is, if the oxygen concentration becomes extremely low, the fire of the solid particles ignited by the electric heater will go out, whereas if the concentration is too high, the solid particles will burn abnormally and the filter will overheat and melt.

Therefore, the inventor of the present invention first connected the engine exhaust system and intake system to an exhaust gas recirculation pipe (hereinafter referred to as "EGR").
An exhaust gas recirculation control valve (hereinafter referred to as the "EGR valve") is installed in the middle of this EGR pipe to control the flow rate of the recirculated exhaust gas, thereby controlling the oxygen concentration of the exhaust gas during filter regeneration. (Patent application filed on December 1, 1982)
57−209239). In this method, a constant exhaust gas recirculation rate is set for a given engine load.

However, with this method, it was difficult to set the numerical value of the exhaust gas recirculation rate. For this reason, there is a risk that the set value may deviate from the tolerance of the recirculation rate, and if the recirculation rate becomes too large, combustion of solid particles will worsen and filter regeneration will be incomplete, and if the recirculation rate becomes too low, On the other hand, excessive combustion may occur, causing filter damage and cracks.

As a countermeasure to this problem, Japanese Utility Model Application Publication No. 59-150926 discloses a diesel engine in which, during filter regeneration, a process of allowing exhaust gas to be recirculated to the intake system and a process of not allowing it to be recirculated are repeated alternately in a short period of time. has been done.

However, with this device, during the process where exhaust gas is not recirculated, _NOx cannot be reduced because the exhaust gas is not recirculated to the intake system, and as a result, there is a risk that the amount of _NOx generated will increase. Ta.

Purpose of the Invention An object of the present invention is to regenerate a filter that can completely regenerate a filter, prevent the occurrence of melting and cracking of the filter, and prevent an increase in the amount of NO _x generated. The purpose is to provide a method.

SUMMARY OF THE INVENTION In order to achieve the objects of the invention, the present invention provides an engine exhaust system for regenerating a filter by igniting and burning solid particles collected in a diesel engine exhaust gas solid particle collector. An exhaust gas recirculation passage is provided between the engine and the intake system,
In the filter regeneration method, the oxygen concentration of the exhaust gas entering the filter can be adjusted by providing an exhaust gas recirculation control valve in the exhaust gas recirculation passage and controlling the flow rate of the recirculated exhaust gas. The exhaust gas recirculation rate is periodically varied around a set value, and the lower limit value of the periodic fluctuation of the exhaust gas recirculation rate is made larger than the exhaust gas recirculation rate when the filter is not regenerated.

If the exhaust gas recirculation rate is varied periodically during filter regeneration in this way, the oxygen concentration in the exhaust gas flowing into the filter will increase and decrease periodically. During the time when the oxygen concentration is decreasing, the combustion of solid particles becomes gradually slower, but the flame regains its strength when the concentration increases next time, so that the ignited fire is not lost.
On the contrary, combustion becomes active during a period when the oxygen concentration is increasing, but after a predetermined period of time, the concentration is reduced again, so that the filter is not heated to an excessively high temperature for a long period of time. Therefore, it is possible to avoid melting and cracking of the filter. In addition, the lower limit of the periodic fluctuation of the exhaust gas recirculation rate during filter regeneration is set larger than the exhaust gas recirculation rate when the filter is not regenerated, so that in engine operating conditions where exhaust gas should be recirculated to the intake system, Exhaust gas is constantly recirculated to the intake system, and as a result, an increase in the amount of NO _x generated can be prevented.

Embodiment FIG. 1 is a schematic diagram of a diesel engine equipped with a collection device to which the method of the present invention is applied, in which 10 is a diesel engine, 12 is an exhaust manifold, 1
4 and 16 are exhaust pipes, and 18 is a solid particle collector. A monolithic ceramic filter 22 having a porous or three-dimensional network structure is accommodated in the housing 20 of the collector 18 . An electric heater 24 for igniting solid particles made of nichrome wire or the like is provided on the upstream end face of the filter 22. The supply of electricity to the heater 24 is controlled by a relay 28 activated by a command signal from an electronic control unit (ECU) 26. The ECU 26 is a general-purpose microcomputer that includes a central processing unit (CPU),
Consists of ROM, RAM, input/output interface, bus line, etc. Engine rotation speed sensor 30
When the integrated number of revolutions reaches a set value, the relay 28 is turned on for a certain period of time.

Between the exhaust pipe 14 and the intake manifold 34
An EGR pipe 32 is provided to circulate exhaust gas back into the intake air. The recirculation amount of exhaust gas is controlled by the EGR valve 36. The EGR valve 36 is of a well-known diaphragm type, and its opening degree changes depending on the input negative pressure acting on its negative pressure chamber to increase or decrease the recirculation amount. 38 is a vacuum pump (VP) as a negative pressure source, and 40 is a negative pressure transmission pipe. The negative pressure input to the EGR valve 36 is controlled by an on/off type negative pressure controller 42 whose duty ratio is controlled according to a signal from the ECU 26. An accelerator opening sensor 46 serving as engine load detection means is linked to the accelerator pedal 44 of the vehicle, and is capable of outputting a signal proportional to the engine load to the ECU 26.

Figure 2 shows an ECU for implementing the method of the present invention.
26 is a control flowchart of the negative pressure controller 42 according to No. 26. The procedure of this flowchart is repeatedly executed, for example, every engine revolution. In step 101, the engine rotational speed is integrated based on the signal from the rotational speed sensor 30. Step 102
Then, the cumulative rotation speed n is the set value N (for example, 200,000 rotations)
It is determined whether or not the value is greater than or equal to the value. If it is less than the set value N, it is assumed that the filter regeneration time has not arrived, and the process proceeds to step 103, where the accelerator opening sensor 4
After reading the engine load based on the signal from 6, the exhaust gas recirculation rate (EGR%) corresponding to the load is searched for in step 104. For this reason, a map showing the relationship between engine load and EGR% as shown in FIG. 3 is previously converted into a table and stored in the ROM of the ECU 26. In the map shown in Figure 3, straight line A is for times other than filter playback.
It represents the set value of EGR%, and straight line B represents the set value during filter regeneration. As can be seen from FIG. 3, the straight line A when the filter is not regenerated is -40% of the straight line B when the filter is regenerated. In step 104, the EGR% when the filter is not regenerated (D _NOR ) corresponding to the load at that time is searched based on the straight line A, and in step 105 this is set as the final value D _FIN (D _FIN →D _NOR ).

If the determination result is equal to or greater than the set value N in step 102, it is assumed that the filter regeneration time has arrived, and in step 106, the relay 28 is turned on for a set time to energize the heater 24 and start igniting the solid particles. Next, in step 107, the engine load is read, and then in step 108, a set value D _REG of EGR% during filter regeneration corresponding to the load is searched based on the straight line B of the map shown in FIG.
In step 109, set time Δt (for example, 0.05 to 5 seconds)
During this period, a set value ΔD (for example, 10%) is added to D _REG , and this is set as the final value D _FIN (D _FIN ← D _REG + ΔD). After the Δt time has elapsed, in step 110, the
Subtract the set value ΔD from D _REG and use this as the final value D _FIN
(D _FIN ←D _REG −ΔD). Therefore, as shown in FIG. 3, the lower limit C of the periodic fluctuation of the exhaust gas recirculation rate during filter regeneration is -10% of the straight line B during filter regeneration, and the It is larger than straight line A indicating the reflux rate.

In step 111, the above steps 105, 109,
Alternatively, a control signal with a duty ratio corresponding to the final EGR% value _DFIN obtained in step 110 is output to the negative pressure controller 42, the duty ratio of the negative pressure controller is controlled, and the negative pressure input to the EGR valve 36 is adjusted.

As a result, the recirculation rate of the exhaust gas recirculated to the intake system via the EGR pipe 32 becomes as shown in the graph of FIG. That is, FIG. 4 shows the relationship between time and the recirculation rate (EGR%) for a given engine load, and when the filter is not regenerated, the recirculation rate D _NOR is constant as shown by the straight line P. When the filter is regenerated, the circulation rate is set at the set value D _REG as shown by the curved line Q.
It fluctuates in a pulse-like manner around . Therefore, during regeneration, the oxygen concentration in the exhaust gas entering the filter fluctuates within a predetermined range, and the combustion of the solid particles proceeds without the risk of melting the filter as described above. Further, the lower limit value D _MIN of the periodic fluctuation of the exhaust gas recirculation rate during filter regeneration is larger than the exhaust gas recirculation rate D _NOR when the filter is not regenerated. Therefore, even if the exhaust gas recirculation rate is periodically varied, the amount of exhaust gas necessary to reduce the generation of harmful components in the exhaust gas, that is, NO _x , must always be recirculated to the intake system. is possible,
As a result, an increase in the amount of NO _x generated can be prevented.

Effects of the Invention It is possible to completely regenerate the filter, prevent the filter from melting and cracking, and prevent the amount of _NOx generated from increasing.

[Brief explanation of the drawing]

Fig. 1 is a schematic diagram of a collector and engine to which the method of the present invention is applied, Fig. 2 is a flowchart of the method of the present invention, Fig. 3 is an example of a map of engine load and recirculation rate, and Fig. FIG. 4 is a graph showing changes in reflux rate according to the method of the present invention. 10... Diesel engine, 12... Exhaust manifold, 14, 16... Exhaust pipe, 18... Solid particle collector, 22... Filter, 24... Filter regeneration heater, 26... Electronic control unit,
28...Relay, 32...EGR pipe, 36...
EGR valve, 38... vacuum pump, 40... negative pressure transmission pipe, 42... negative pressure controller.

Claims (1)

[Claims] 1. In regenerating the filter by igniting and burning the solid particles collected in the diesel engine exhaust gas solid particle collector, an exhaust gas recirculation passage is provided between the engine exhaust system and the intake system,
In a filter regeneration method in which an exhaust gas recirculation control valve is provided in the exhaust gas recirculation passage to control the flow rate of recirculated exhaust gas, the oxygen concentration of the exhaust gas entering the filter can be adjusted. The filter regeneration method is characterized in that the exhaust gas recirculation rate is periodically varied around a set value, and the lower limit value of the periodic fluctuation of the exhaust gas recirculation rate is made larger than the exhaust gas recirculation rate when the filter is not regenerated. .