JPH06288306A - Exhaust gas circulation device for diesel engine - Google Patents

Abstract

PURPOSE:To reduce the exhaust amount of NOx in various engine operating areas while HC and smoke is suppressed from being discharged. CONSTITUTION:In a low load area of an engine 10, an EGR valve 24 is duty- controlled while flow of exhaust gas to a branch exhaust gas circulation flow passage 23 is stopped by a switchover valve 22 for hot EGR. Also, in a medium load area, the EGR valve is duty-controlled while exhaust gas is cooled by an inter-cooler 29 by allowing flow of exhaust gas into the branch exhaust gas circulation flow passage for cold EGR. In a high load area, the EGR valve is closed to stop the EGR.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an exhaust gas recirculation system for a diesel engine, and more particularly, to suppressing unburned hydrocarbon (HC) and smoke (black smoke) emissions from the engine in various engine operating ranges. Nitrogen oxide (NO
x) The present invention relates to an exhaust gas recirculation device capable of reducing the emission amount.

[0002]

2. Description of the Related Art Reduction of NOx which is a harmful component in exhaust gas by using an exhaust gas recirculation (EGR) device in which a part of exhaust gas from an engine is recirculated into a cylinder through an intake passage. It has been known. However, according to this type of hot EGR, the intake air temperature rises because high-temperature exhaust gas is introduced into the intake passage of the engine, and therefore, the combustion temperature in the cylinder cannot be sufficiently reduced, and the EGR causes The NOx reduction effect may be impaired. Therefore, as disclosed in Japanese Utility Model Laid-Open No. 61-48957, a cold EGR has been proposed in which exhaust gas cooled by a cool air generator is fed into the intake passage.

[0003]

However, while cold EGR has the advantage of having a large NOx reduction effect, it has the drawback of increasing HC emissions. Therefore, the present inventor conducted various experiments to examine in detail the engine exhaust characteristics when hot EGR or cold EGR was performed and when EGR was not performed. As a result, hot EGR simultaneously reduced HC and NOx. While it has the advantage of being able to do so, it has the disadvantage that the amount of smoke (black smoke) emission increases in the middle and high load areas where the intake air temperature rises. In addition, cold EGR has a large NOx reduction effect and little smoke deterioration. While it has the advantage that it has the above-mentioned disadvantage that the amount of HC emission is large.

On the other hand, when the engine is operating at a constant speed, in general, there is a small amount of NOx and smoke in the low load region, but a large amount of HC, and a small amount of HC in the medium and high load region, but a large amount of NOx and smoke. The present invention was devised based on the above findings, and it is an object of the present invention to provide an exhaust gas recirculation device capable of reducing the emission amount of NOx in various engine operating regions while suppressing the emission amount of HC and smoke from the engine. To aim.

[0005]

In an exhaust gas recirculation system for a diesel engine having an exhaust gas recirculation path whose both ends are connected to an intake pipe and an exhaust pipe of the engine, the present invention relates to a branch exhaust gas recirculation system connected to an exhaust gas recirculation path. A cooling device for cooling the exhaust gas flowing through the passage and a switching device for selectively allowing the exhaust gas to flow into the branch exhaust gas recirculation passage are provided, and the operation of the switching device is controlled according to the engine operating region. It is characterized by doing so.

Preferably, in the first engine operating region where the engine load is smaller than the first predetermined value, the switching device is operated so as to prevent the exhaust gas from flowing into the branch exhaust gas recirculation passage, and the engine load is set to the first engine load. In the second engine operating region larger than the predetermined value of, the switching device is operated so that the exhaust gas flows into the branch exhaust gas recirculation passage, and the exhaust gas is cooled by the cooling device. More preferably, the switching device includes a switching valve, and in the first engine operating region, the switching valve is set to the first operating position for blocking the inflow of exhaust gas into the branch exhaust gas recirculation passage, and in the second engine operating region, The switching valve is set to the second operating position that allows the exhaust gas to flow into the branch exhaust gas recirculation passage.

Preferably, a flow rate control device for controlling the flow rate of the exhaust gas flowing from the exhaust gas recirculation path to the intake pipe is provided, and the inflow amount of the exhaust gas to the intake pipe is set in the first and second engine operating regions. In the third engine operating region in which the flow rate control device is operated so as to change according to the engine operating state and the engine load is larger than the second predetermined value that is larger than the first predetermined value, the exhaust gas to the intake pipe is Operate the flow control device to prevent inflow. More preferably,
The flow rate control device includes a flow rate control valve, controls the opening degree of the flow rate control valve according to the engine operating state in the first and second engine operating regions, and closes the flow rate control valve in the third engine operating region.

[0008]

The operation of the switching device is controlled according to the current engine operating range. When the switching device operates so as to allow the exhaust gas to flow into the branch exhaust gas recirculation passage, the exhaust gas from the engine exhaust pipe flows into the branch exhaust gas recirculation passage via the upstream side of the exhaust gas recirculation passage, and then to the branch exhaust gas recirculation passage. The exhaust gas flowing through the exhaust gas recirculation passage is cooled by the cooling device. The cooled exhaust gas flows into the intake pipe of the engine from the branch exhaust gas recirculation passage, for example, via the downstream side of the exhaust gas recirculation passage. As a result, the intake air temperature decreases, which reduces NOx and smoke emissions from the engine. On the other hand, when the switching device operates so as to prevent the exhaust gas from flowing into the branch exhaust gas recirculation passage,
The exhaust gas directly flows into the intake pipe via the exhaust gas recirculation passage. As a result, exhaust gas cooling (cold EGR) is not performed, and NOx and HC emissions are reduced by hot EGR.

The exhaust gas is prevented from flowing into the branch exhaust gas recirculation passage in the first engine operating region, while the exhaust gas is allowed to flow into the branch exhaust gas recirculating passage in the second engine operating region to cool the exhaust gas by the cooling device. According to the preferred embodiment of the present invention, the hot EGR is performed in the first engine operation region where the HC emission amount is large, the HC emission is suppressed together with the NOx emission, and the second engine operation where the NOx and smoke emission amounts are large. Cold EGR is performed in the area and N
Smoke emission is suppressed together with Ox emission.

In a particular aspect of the invention with a switching device including a switching valve, switching between hot EGR and cold EGR is accomplished by controlling the operating position of the switching valve. In a particular aspect of the present invention in which a flow rate control device for controlling the flow rate of exhaust gas flowing from the exhaust gas recirculation passage to the intake pipe is provided, in the first and second engine operating regions, the amount of exhaust gas flowing into the intake pipe is increased. As a result, the EGR rate is optimally controlled according to the engine operating state represented by, for example, the engine load and the engine speed. Further, in the third engine operating region, the exhaust gas flow into the intake pipe is blocked by the flow rate control device, execution of hot EGR and cold EGR in the third engine operating region where the engine load is large is blocked, and the execution of EGR is accompanied. Smoke deterioration is prevented. In a specific aspect of the present invention including a flow rate control device including a flow rate control valve, the opening degree (on / off duty ratio) of the flow rate control valve is controlled for EGR rate control, and the duty ratio is set to prevent execution of EGR. It is set to "0".

[0011]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An exhaust gas recirculation (EGR) device according to an embodiment of the present invention will be described with reference to FIG. An intake pipe 12 and an exhaust pipe 13 are connected to the intake side and the exhaust side of a diesel engine 10 equipped with an EGR device, respectively. An air cleaner 14 is provided in the intake pipe 12, and an exhaust pipe 13
A muffler 16 is arranged in the.

The EGR device has an exhaust gas recirculation passage 21, an upstream end 21a of the exhaust gas recirculation passage 21 is connected to the exhaust pipe 13, and a downstream end 21b thereof is connected to the intake pipe 12. A switching valve 22 is arranged between the upstream passage portion and the intermediate passage portion of the exhaust gas recirculation passage 21, and the switching valve 22 has an inlet port 22a communicating with the upstream passage portion of the exhaust gas recirculation passage 21 and an exhaust gas. First outlet port 2 communicating with the intermediate passage portion of the return passage 21
2 b and a second outlet port 22 c communicating with the branch exhaust gas recirculation passage 23. The upstream end of the branched exhaust gas recirculation passage 23 is connected to the upstream passage portion of the exhaust gas recirculation passage 21 via the switching valve 22, and the downstream end thereof is connected to the intermediate passage portion of the exhaust gas recirculation passage 21.

The switching valve 22 includes a first valve body 22d for opening and closing the first outlet port 22b, and a second valve body 22e which is movable integrally with the first valve body 22d for opening and closing the second outlet port 22c. Including the solenoid 22f, the solenoid 22
When f is excited, the first outlet port 22b is opened and the second outlet port 22c is closed, while when the solenoid 22f is demagnetized, the first outlet port 22b is closed and the second outlet port 22c is opened. .

An exhaust gas recirculation valve (EGR valve) 24 is arranged between the intermediate passage portion and the downstream passage portion of the exhaust gas recirculation passage 21. The EGR valve 24 is provided in a valve chamber 24f, and a valve body 24a for opening and closing the exhaust gas recirculation passage 21 and a valve body 2 are provided.
Diaphragm 24b connected to 4a and exhaust gas recirculation passage 2
1 through the diaphragm 24b in the direction of closing 1
and a spring 24c for biasing a. The negative pressure chamber 24e defined by the diaphragm 24b and the housing 24d is connected to a negative pressure source via a negative pressure path 25 in which a normally closed solenoid valve 26 is arranged.
The EGR valve 24 cooperates with the electromagnetic valve 26 to form the exhaust gas recirculation passage 2
A flow rate control valve for controlling the flow rate of exhaust gas flowing from 1 to the intake pipe 12 is configured.

The EGR device controls the operation of the switching valve 22 and the EGR valve 24 according to the engine operating state.
The controller 27 further includes a microprocessor, a memory, an input / output circuit, and the like. Controller 27
On the input side of the engine speed sensor for detecting the engine operating state collectively indicated by reference numeral 28,
Various sensors including an intake amount sensor are connected, and the solenoid 22f of the switching valve 22 is connected to the output side of the controller 27.
And the solenoid valve 26 is connected.

In this embodiment, hot EGR and cold EG are performed in the first and second engine operating regions I and II (FIG. 2).
R is executed respectively, and execution of hot EGR and cold EGR is blocked in the third engine operating region III, and during execution of hot EGR or cold EGR, for example, according to the engine operating state represented by the engine load and the engine speed. Thus, the on / off duty ratio (average opening degree) of the solenoid valve 26 and thus the EGR rate are controlled optimally. Then, the upper limit loads of the first and second operation regions I and II are set to, for example, 4/8 and 7/8 of the total load, respectively.

A preset map is stored in the memory of the controller 50 in order to selectively execute hot EGR or cold EGR. In this map, information indicating which operating region each of various engine operating states (various combinations of engine load and engine speed) belongs to is set, and the engine belonging to the first or second operating region is set. The target duty ratio of the solenoid valve 26 for each operating state is set.

The EGR device further includes an intercooler 29 provided as a cooling device in the middle of the branch exhaust gas recirculation path 23,
During execution of cold EGR, the exhaust gas flowing in the branch exhaust gas recirculation path 23 is cooled by the intercooler 29. Hereinafter, the operation of the exhaust gas recirculation device having the above configuration will be described with reference to FIG. During engine operation, the processor of the controller 50 repeatedly executes the EGR control routine shown in FIG. 3 at a predetermined cycle. In each execution cycle of the EGR control routine, for example, the processor first reads the engine speed sensor output and the intake air amount sensor output, and the current engine operating state represented by the engine speed and the engine load is the third engine operating region. Whether to enter III or not is determined by referring to the map stored in the memory of the controller 50 (step S1).
If the determination result is negative, the processor refers to the map and refers to the solenoid valve 2 corresponding to the current engine operating state.
The target duty ratio of 6 is obtained, and the solenoid valve 26 is driven to open and close with this duty ratio (step S2).

When the electromagnetic valve 26 is closed, the negative pressure supply from the intake pipe 12 to the negative pressure chamber 24e of the EGR valve 24 via the negative pressure passage 25 is cut off, and the spring force of the spring 24c urges the negative pressure. The exhaust gas recirculation passage 21 is closed by the valve body 24a.
On the other hand, when the solenoid valve 26 is opened, negative pressure is supplied to the negative pressure chamber 24e, the valve body 24a is lifted in the opening direction against the spring force of the spring 24c, and the exhaust gas recirculation passage 21 is opened.

In step S3 following step S2, the processor determines whether or not the current engine operating state falls within the second operating region II. This determination result is negative, that is, the current engine operating state is the first operating region I.
If it is determined that the switch valve 22
For example, a high-level control output is sent to the solenoid 22f (step S4), and the EGR control in this cycle ends.

When the solenoid 22f is excited in step S4, the valve body 22d is lifted from the first outlet port 22b to open the port and the valve body 22e closes the second outlet port 22c. Therefore, a part of the exhaust gas flowing through the exhaust pipe 13 is not connected to the upstream passage portion of the exhaust gas recirculation passage 21,
It flows into the valve chamber 24 f of the EGR valve 24 via the switching valve 22 and the intermediate passage portion of the exhaust gas recirculation passage 21. Step S
When the valve body 24a is lifted due to the opening / closing drive of the electromagnetic valve 26 in FIG. 2, the exhaust gas flows from the valve chamber 24f to the exhaust gas recirculation passage 2
1 flows into the intake pipe 12 via the downstream side passage portion. That is, hot EGR is started. Hot EGR is
As long as the engine is operating in the first engine operating region I, it is continuously executed.

Step S of the subsequent EGR control cycle
When it is determined in 3 that the engine is operating in the second operating region II, the processor sends, for example, a low-level control output to the solenoid 22f of the switching valve 22 (step S5) to perform the EGR control in this cycle. finish. When the solenoid 22f is demagnetized in step S5,
The valve body 22d closes the first outlet port 22b, lifts the valve body 22e, and opens the second outlet port 22c. Therefore, a part of the exhaust gas flowing in the exhaust pipe 13 is
It flows into the branched exhaust gas recirculation path 23 via the upstream side passage portion of the exhaust gas recirculation path 21 and the switching valve 22. The exhaust gas is cooled by the intercooler 29 while flowing through the branched exhaust gas recirculation passage 23, and then is exhausted through the intermediate passage portion of the exhaust gas recirculation passage 21 to the EG.
It flows into the valve chamber 24f of the R valve 24. And solenoid valve 2
When the valve body 24a is lifted in association with the opening / closing operation of 6, the cooled exhaust gas flows into the intake pipe 12 from the valve chamber 24f through the downstream passage portion of the exhaust gas recirculation passage 21. That is, cold EGR is started. Cold EGR is
As long as the engine is operating in the second engine operating region II, it is continuously executed.

Step S of the subsequent EGR control cycle
1, it is determined that the engine is operating in the third engine operating region III, the processor determines that the solenoid valve 2
The duty ratio of 6 is set to "0" (step S
6). As a result, the exhaust gas recirculation path 21 remains closed by the valve body 24a of the EGR valve 24, and the cold EGR
Is blocked.

Further, the hot EGR or the cold EGR is executed or the execution of the hot EGR and the cold EGR is blocked in accordance with the subsequent change in the engine operating state. When the first engine operating range I is shifted to the third engine operating range III, the hot EGR is blocked by the same procedure as the above-mentioned transition from the second engine operating range II to the third engine operating range III. Will be done.

As a result of executing the above EGR control routine, hot EGR is performed in the first operating region I where the amount of HC emission is large, and NOx emission and HC emission are suppressed.
And, the cold EGR is performed in the second operation region II where the smoke emission amount is large, the smoke emission is suppressed together with the NOx emission, and further, the third operation region III where the engine load is large.
The execution of hot EGR and cold EGR in the above is prevented, and the smoke deterioration due to the execution of EGR is prevented.

FIG. 4 exemplifies the exhaust gas characteristics of the diesel engine equipped with the exhaust gas recirculation system of this embodiment in the 13-mode measurement traveling mode. In FIG.
The exhaust gas characteristics are shown with the horizontal axis representing the mode number and the vertical axis representing the EGR rate, smoke amount, HC amount and NOx amount. In Figure 4, squares (□), triangles (△) and circles (○)
Mark indicates cold EGR, hot EGR and EG
Corresponds to each without R, and mode numbers 10, 11 and 1
2 corresponds to the high load area of the engine.

Referring to FIG. 4, EG in hot EGR
Even though the EGR rate in cold EGR is considerably smaller than the R rate, the hot E
It was found that the NOx reduction effect equivalent to that of R was achieved,
Therefore, at the same EGR rate, the NOx reduction effect of cold EGR is understood to exceed that of hot EGR. or,
It can be seen that when cold EGR is executed, the amount of smoke is suppressed to the same level as in the case without EGR, while the amount of HC increases.

The present invention is not limited to the above embodiment, but various modifications can be made. For example, in the embodiment, the intercooler 29 is used as a cooling device for cooling the exhaust gas, but various cooling devices such as an air cooling type and a water cooling type can be used. In the above embodiment, the operating state of the switching valve 22 is immediately switched from open to closed or from closed to open at the time of transition between the first operating region I and the second operating region II. The switching valve 2 is controlled by duty-controlling the opening degree of the valve 22.
The operating state of 2 may be gradually switched.

[0029]

As described above, according to the present invention, the cooling device for cooling the exhaust gas flowing through the branch exhaust gas recirculation passage connected to the exhaust gas recirculation passage and the exhaust gas inflow into the branch exhaust gas recirculation passage are selectively selected. Since the operation of the switching device is controlled in accordance with the engine operating region, the amount of HC and smoke emitted from the engine is suppressed, and NOx in various engine operating regions is controlled. The amount of emissions can be reduced.

Further, while preventing the exhaust gas from flowing into the branch exhaust gas recirculation passage in the first engine operating region, the exhaust gas is allowed to flow into the branch exhaust gas recirculating passage in the second engine operating region by the cooling device. According to a preferred aspect of the present invention in which cooling is performed, the first HC emission amount is high.
Since hot EGR is performed in the engine operating region, HC emission as well as NOx emission can be suppressed, and cold EGR is performed in the second engine operating region in which NOx and smoke emission amounts are large, so that NOx emission and smoke emission can be suppressed. Further, according to the specific aspect of the present invention including the switching device including the switching valve, since the operating position of the switching valve is controlled to switch between the hot EGR and the cold EGR, this switching can be performed reliably and easily.

According to a particular aspect of the present invention, which is provided with a flow control device for controlling the flow rate of the exhaust gas flowing from the exhaust gas recirculation path to the intake pipe, the intake pipe is controlled in the first and second engine operating regions. Since the amount of inflow of exhaust gas is controlled according to the engine operating condition and the inflow of exhaust gas to the intake pipe is blocked by the flow rate control device in the third engine operating region,
The EGR rate in the first and second operating ranges can be optimized, and the smoke deterioration due to the execution of EGR can be prevented in the third engine operating range where the engine load is large. Further, in certain aspects of the invention with a flow control device including a flow control valve, E
Since the EGR rate control including the GR execution inhibition is performed by the flow rate control valve, the EGR rate can be optimized.

[Brief description of drawings]

FIG. 1 is a schematic partial cross-sectional view showing an exhaust gas recirculation system according to an embodiment of the present invention installed in a diesel engine.

2 is a first to third engine operating range I to II related to EGR control executed by the controller of FIG. 1;
It is a graph which shows I.

FIG. 3 is a flowchart showing an EGR control routine executed by the controller of FIG.

FIG. 4 is a graph illustrating an exhaust gas characteristic of a diesel engine equipped with the exhaust gas recirculation device of FIG. 1 in a 13-mode measurement traveling mode.

[Explanation of symbols]

 12 intake pipe 13 exhaust pipe 21 exhaust gas recirculation path 22 switching valve 23 branch exhaust gas recirculation path 24 exhaust gas recirculation valve 26 electromagnetic valve 27 controller 28 sensor 29 intercooler

Claims (5)

[Claims] 1. An exhaust gas recirculation system having an exhaust gas recirculation path, both ends of which are connected to an intake pipe and an exhaust pipe of an engine,
An engine is provided with a cooling device for cooling the exhaust gas flowing through the branch exhaust gas recirculation passage connected to the exhaust gas recirculation passage, and a switching device for selectively allowing the exhaust gas to flow into the branch exhaust gas recirculation passage. An exhaust gas recirculation device for a diesel engine, characterized in that the operation of the switching device is controlled in accordance with an operating region. 2. In a first engine operating range in which the engine load is smaller than a first predetermined value, the switching device is operated to prevent the exhaust gas from flowing into the branch exhaust gas recirculation passage, and the engine load is reduced to the above-mentioned value. A second value larger than the first predetermined value
In the engine operating region, the switching device is operated so that the exhaust gas flows into the branch exhaust gas recirculation passage, and the exhaust gas is cooled by the cooling device.
The exhaust gas recirculation device for a diesel engine according to 1. 3. The switching device includes a switching valve, and the first switching device comprises:
In the engine operating region, the switching valve is set to the first operating position that blocks the inflow of exhaust gas into the branch exhaust gas recirculation passage, and in the second engine operating region, the switching valve is moved to the branch exhaust gas recirculation passage. The exhaust gas recirculation device for a diesel engine according to claim 2, wherein the exhaust gas recirculation device is set to a second operating position that allows the exhaust gas to flow in. 4. A flow rate control device for controlling a flow rate of the exhaust gas flowing from the exhaust gas recirculation path to the intake pipe is provided, and in the first and second engine operating regions, the flow rate of the exhaust gas to the intake pipe is increased. In the third engine operating region in which the flow rate control device is operated so that the inflow amount changes according to the engine operating state, and the engine load is larger than the second predetermined value which is larger than the first predetermined value, the intake air is increased. The exhaust gas recirculation system for a diesel engine according to claim 1, wherein the flow rate control device is operated so as to prevent the exhaust gas from flowing into the pipe. 5. The flow control device includes a flow control valve,
The opening degree of the said flow control valve is controlled according to an engine operating state in the said 1st and 2nd engine operating area | region, and the said flow control valve is closed in the said 3rd engine operating area | region. Exhaust gas recirculation system for diesel engines.