JPH04194352A - Exhaust gas recirculation system for supercharged engine - Google Patents

Abstract

PURPOSE:To reduce the effect of fluctuations in exhaust pressure at time of selecting a turbocharger by installing an EGR gas takeout, connected to an intake passage at the downstream side off a throttle valve, in an exhaust passage at the turbine sown-stream side of a main turbocharger. CONSTITUTION:An engine 1 is provided with a main turbocharger 7 and a sub-turbocharger 8, and an exhaust selector valve 17 is provided downstream of this sub-turbocharger 8. An EGR gas takeout port 40 is installed in an exhaust passage downstream of a turbine 7a of the main turbocharger 7. Then, the takeout port 40 is connected to a discharge port 41 installed in an intake passage downstream of a throttle valve 4 via an EGR gas passage 42 in which and an exhaust gas recirculating valve 61 is installed. With this constitution, since exhaust pressure downstream of the turbine 7a does not so largely vary by opening or closing of the exhaust selector valve 17 as compared with that upstream of the turbine 7a, the effect of fluctuations in the exhaust pressure at time of these turbochargers 7, 8 is reducible to control over an EGR gas quantity.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to an exhaust gas recirculation device for a supercharged engine, and particularly to an exhaust gas recirculation device that can control the amount of EGR gas commensurate with the amount of intake air. .

[Prior Art] As an exhaust gas recirculation device for an engine equipped with a turbocharger, for example, Japanese Patent Laid-Open No. 148655/1982 and Japanese Patent Laid-open No. 12664/1987 are known. The former is E from upstream of the turbocharger turbine when not supercharging.
GR gas is taken out, and during supercharging, EGR gas is taken out from downstream of the turbine. As a result, continuous EGR characteristics can be obtained from low load to high load, and NOx can be reduced. The latter takes out EGR gas from downstream of the turbine of the turbocharger during low loads, and extracts EGR gas from upstream of the turbine during high loads.
We are trying to extract gas. This makes it possible to ensure a necessary and sufficient amount of exhaust gas recirculation over the entire operating range of the engine.

[Problem to be solved by the invention] A supercharged engine is equipped with two turbochargers, and only the main turbocharger is operated for supercharging in a low intake air amount region, and the main and auxiliary turbos are operated in a high intake air amount region. A two-stage twin turbo charger is known in which both chargers are supercharged. With this configuration, the inertial force of the rotating parts can be reduced, and an engine with high supercharging response can be obtained.

However, if a technology that extracts EGR gas from upstream of the turbine is applied to a two-stage twin turbocharger, as in the prior art described above, it will be greatly affected by fluctuations in exhaust pressure when the turbocharger is switched by the exhaust switching valve. Therefore, there is a problem that accurate control of EGR gas cannot be performed.

Characteristic A2 shown by the broken line in Fig. 3 shows the relationship between engine speed and exhaust pressure in the conventional device (in the conventional system).On the turbine upstream side of the main turbocharger, switching from one turbocharger to two turbochargers At times, the exhaust pressure drops significantly.Therefore, from the upstream side of the turbine
When the GR gas is taken out, the flow rate of the EGR gas will vary significantly.

The present invention focuses on the above-mentioned problems, and in a supercharged engine equipped with a main turbocharger and an auxiliary turbocharger, it is possible to reduce the influence of fluctuations in exhaust pressure when switching turbochargers, and to accurately control the amount of EGR gas. The purpose of the present invention is to provide a controllable exhaust gas recirculation device.

[Means for Solving the Problems] An exhaust gas recirculation device for a supercharged engine according to the present invention in accordance with this object includes a main turbocharger and a sub-turbocharger, and an exhaust gas switching valve downstream of the sub-turbocharger. In an exhaust gas recirculation system for a supercharged engine, an EGR gas outlet is provided in an exhaust passage downstream of a turbine of the main turbocharger, and the outlet is connected to an EGR gas passage in which an EGR valve is interposed. The valve is connected to the intake passage downstream of the throttle valve.

[Function] In the exhaust gas recirculation system for a supercharged engine configured as described above, the EGR gas outlet is provided in the exhaust passage downstream of the turbine of the main turbocharger. The exhaust pressure downstream of the turbine of the main turbocharger does not change much due to the opening and closing of the exhaust switching valve compared to the exhaust pressure upstream of the turbine, so it is possible to control the EGR gas optimally in accordance with the intake air and air volume.

[Embodiments] Hereinafter, preferred embodiments of the exhaust gas recirculation device for a supercharged engine according to the present invention will be described with reference to the drawings.

1 to 4 show an embodiment of the present invention, particularly when applied to a 6-cylinder engine.

Note that FIG. 1 is a simplified system diagram of a supercharged engine according to the present invention, and FIG. 2 shows a system diagram that is a more specific version of the system diagram in FIG. Therefore, the same parts in both figures are given the same reference numerals, and the explanation of one will be omitted.

In FIGS. 1 and 2, 1 is an engine, 2 is a surge tank, and 3 is an exhaust manifold.

The exhaust manifold 3 is assembled into two groups, a #1 to #3 cylinder group and a #4 to #6 cylinder group, which do not cause exhaust interference, and the assembled portions are communicated with each other by a communication path 3a. 7.8 is a main turbocharger and a sub-turbocharger arranged in parallel with each other. Each of the turbines 7a and 8a of the turbocharger 7.8 is connected to a gathering part of the exhaust manifold 3, and each of the compressors 7b to 18b is connected to the surge tank 2 via an intercooler 6 and a throttle valve 4. The main turbocharger 7 is operated from a low intake air amount region of the engine to a high intake air amount region, and the auxiliary turbocharger 8 is stopped in a low intake air amount region of the engine.

In order to enable activation and deactivation of both turbochargers 7.8, an exhaust switching valve 17 is provided downstream of the turbine 8a of the auxiliary turbocharger 8, and an intake switching valve 18 is provided downstream of the compressor 8b. When both intake and exhaust switching valves 18.17 are fully open, both turbochargers 7.8
is activated.

In order to smoothly switch from one turbocharger to two turbochargers, a compressor 8b is installed in the intake passage of the sub-turbocharger 8, which is stopped in a low intake air amount region.
An intake bypass passage 13 that communicates between the upstream and downstream sides of the intake bypass passage 13 and an intake bypass valve 33 disposed in the middle of the intake bypass passage 13 are provided.

The intake bypass valve 33 is opened and closed by the actuator 10. Note that the air flow downstream side of the intake bypass passage may be communicated with the intake passage upstream of the compressor of the main turbocharger 7. In addition, a check valve 12 is provided in the bypass passage that communicates the upstream and downstream sides of the intake switching valve 18, so that even when the intake switching valve 18 is closed, the compressor outlet pressure on the sub-turbocharger 8 side is maintained at the main turbocharger side. When it becomes larger, it allows air to flow from the upstream side to the downstream side. In the figure, 14 indicates an intake passage on the compressor outlet side, and 15 indicates an intake passage on the compressor inlet side. - The intake passage 15 is connected to the air cleaner 23 via an air 70-meter 24. A front pipe 20 forming an exhaust passage is connected to an exhaust muffler 2 via an exhaust gas catalyst 21.
Connected to 2.

The intake switching valve 18 is opened and closed by the actuator 11, and the exhaust switching valve 17 is opened and closed by the two-stage diaphragm actuator 16, and one actuator 16 can control both small opening and full opening of the exhaust switching valve 17. Note that 9 indicates an actuator that opens and closes the waste gate valve 31. Actuators 10, 11, and 16 are operated.

In order to turn off the boost pressure or negative pressure (selectively switch between boost pressure or negative pressure and atmospheric pressure), the first
, second, third, and fourth three-way solenoid valves 25.26.27.2
8 is provided. Three-way solenoid valve 25.26.27.2
8 is performed in accordance with a command from the engine control computer 29. When the three-way solenoid valve 25.28 is ON, the actuator 11.16 is operated to fully open the intake/exhaust switching valve 18.17, and when it is OFF, the actuator 11 is operated to fully close the intake/exhaust switching valve 18.17. .1
Activate 6. 32 is a fifth three-way solenoid valve for controlling the exhaust switching valve 11 to be opened slightly; when turned ON, supercharging pressure is introduced into the diaphragm chamber 161) of the actuator 16, and the exhaust switching valve 1
7 is slightly opened, and OFF is used to stop the small opening.Here, 16a and 16b are the diaphragm chambers of the actuator 16, 16C is the small opening adjustment screw, 10a is the diaphragm chamber of the actuator 10, and 11a , l
lb indicates a diaphragm chamber of the actuator 11, respectively.

The actuator 16 that drives the exhaust gas switching valve 17 is duty-controlled by the engine control computer 29. As is well known, duty control is to control the energization time using a duty ratio, and by digitally changing the ratio of energization and non-energization, the average current is variably controlled in an analog manner. Note that the duty ratio is the ratio of the energizing time to the time of one cycle, and if the energizing time in one cycle is A1 and the non-energizing time is B, then duty ratio = A/ (A + B) x 100 (%)
It is expressed as

In this embodiment, by controlling the duty of the fifth three-way solenoid valve 32, the opening amount of this solenoid valve can be varied. By appropriately controlling the fifth three-way solenoid valve, it is possible to vary the operating speed of the actuator 16 and correct the flow rate characteristics of a specific region of the exhaust switching valve 17.

The engine control computer 29 is electrically connected to sensors for detecting various operating conditions of the engine, and receives signals from the various sensors. Engine operating condition detection sensors (intake pipe pressure sensor 30, throttle opening sensor 5, air flow meter 2 as intake air amount measurement sensor)
4.02 sensor 19 etc. are included.

The engine control computer 29 includes a central processor unit (CPU) for processing, and a read-only memory (ROM) that is a read-only memory.
, -Random access memory (RAM) for time storage
, input/output interface (I/D interface)
, is equipped with an A/D converter that converts analog signals from various sensors into digital quantities.

An EGR gas outlet 40 is provided in the exhaust passage downstream of the turbine 7a of the main turbocharger 7. The outlet 40 is connected to a discharge port 41 provided in the intake passage downstream of the throttle valve 4 via an EGR gas passage 42 in which an EGR valve 61 is interposed. EGR valve 61
is vacuum switching valve (VSV) 63 and E
The opening of the valve is controlled by the GR vacuum modulator 62.

In the case shown in FIG.
EGR valve 61 via vacuum modulator 62
The diaphragm is now being introduced into the room. V
The SV63 is activated by a command from the engine control computer 29, and controls the amount of supercharging air introduced into the diaphragm chamber 61a of the EGR valve 61 to control the EGR valve 61.
The GR valve 61 is opened and closed.

As is well known, the EGR vacuum modulator 62 has a function of controlling the exhaust pressure to keep the EGR rate constant with respect to the engine load. In this example, E G Rt
<The vacuum modulator 62 senses the exhaust pressure in the constant pressure chamber 61b of the EGR valve 61, and adjusts the EGR port negative pressure acting on the diaphragm chamber 61a of the EGR valve 61 so as to maintain this pressure near atmospheric pressure.

In the case of Fig. 2, the surge tank 2 is connected to the VSV 63 via a vacuum control valve (VCV) 64.
Supercharging air is led from. In addition, the EGR gas passage 42
An EGR gas cooler 65 for cooling EGR gas is provided upstream of the EGR valve 61 .

Next, the supercharging characteristics in the case of one turbocharger operation and the case of two turbocharger operation will be explained.

In the high intake air amount region, the intake switching valve 18 and the exhaust switching valve 17
are both opened, and the intake bypass valve 33 is closed. This causes the two turbochargers 7.8 to supercharge,
Sufficient amount of supercharging air can be obtained and output can be improved. At this time, the supercharging pressure is controlled by the wastegate valve 31 so as not to exceed, for example, +500IIIt)-1g.

In a low speed range and under high load, both the intake switching valve 18 and the exhaust switching valve 17 are closed, and the intake bypass valve 33 is opened. As a result, only one turbocharger 7 is driven. The reason why one turbocharger is used in the low speed range is that the supercharging characteristics of a single turbocharger are superior to the supercharging characteristics of a two turbocharger in the low speed range. 1
By using individual turbochargers, boost pressure and torque build up quickly, resulting in a quick response.

In a low speed range and under light load, the intake switching valve 18 is opened while the exhaust switching valve 17 is closed. As a result, the intake passages for two turbochargers are opened while one turbocharger remains driven, and an increase in intake resistance caused by one turbocharger can be eliminated. This makes it possible to further improve the boost pressure rise characteristics and response at the beginning of acceleration from a low load.

When transitioning from a low intake air amount region to a high intake air amount region, that is, when switching from one turbocharger operation to two turbocharger operation, after the small opening control of the exhaust switching valve 17 is started, the intake air amount Q is 55001/1IIin
When the temperature is reached, the intake bypass valve 33 is closed, and after a time delay (after 1 second in this embodiment), the exhaust switching valve 17 is fully opened, and then the intake switching valve 18 is fully opened. Turbocharger supercharging operation is started.

While the engine is running, the exhaust gas discharged from each cylinder is transferred to the main turbocharger 7 or the main and auxiliary turbochargers 7.
．． 8, the exhaust gas with homogenized mixed components is supplied from the outlet 40 to the EGR gas passage 42. FIG. 3 shows the relationship between engine speed and exhaust pressure. In the figure, the broken line indicates the exhaust pressure characteristic A2 upstream of the turbine 7a of the main turbocharger 7, as described in the prior art, and the solid line indicates the exhaust pressure characteristic A2 downstream of the turbine 7a of the main turbocharger 7, which is the object of the present invention. It shows A. As shown in the figure, the exhaust pressure on the upstream side of the turbine 7a is reduced by about 1/2 due to the full opening of the exhaust switching valve 17 accompanying the switch to a two-turbocharger, but the amount of intake air has not changed much. Therefore, it becomes difficult to supply an amount of EGR gas commensurate with the amount of intake air.

Conversely, the change in exhaust pressure downstream of the turbine 7a, which is the object of the present invention, is much smaller than that upstream of the turbine 7a. In other words, the change in exhaust pressure caused by opening and closing of the exhaust gas switching valve 17 becomes smaller as it moves downstream of the turbine 7a.

Although it is possible to provide the EGR gas outlet downstream of the turbine of the sub-turbocharger 8, the exhaust pressure downstream of the turbine 8a of the sub-turbocharger 8 will not be the same as that on the upstream side unless the exhaust switching valve 17 is opened. Therefore, it is not possible to provide an EGR gas outlet downstream of the turbine 8a.

In this way, by providing the EGR gas outlet 40 downstream of the turbine 7a of the main turbocharger 7,
The change in exhaust pressure with respect to engine speed becomes almost linear, making it possible to supply EGR gas commensurate with the amount of intake air. Therefore, EGR gas corresponding to the amount of intake air can flow into the combustion chamber, and NOx can be reduced by lowering the maximum combustion temperature.

FIG. 4 shows the relationship between the maximum engine rotational speed and the EGR rate (EGR amount/large intake air amount).

As shown in the figure, in order to ensure drivability in the low rotation range, the inflow of EGR gas is suppressed. The broken line in the figure indicates EGR from upstream of the turbine 7a of the main turbocharger 7.
Characteristic B2 is shown when gas is extracted.
Here, since the exhaust pressure changes greatly, the EGR rate also changes.

The solid line indicates characteristic B when EGR gas is taken out from the outlet 40 located downstream of the turbine 7a of the main turbocharger 7. Since the exhaust pressure downstream of the turbine 7a changes substantially linearly with the engine speed as described above, it is possible to keep the EGR rate substantially constant.

[Effects of the Invention] As explained above, when using the exhaust gas recirculation device for a supercharged engine according to the present invention, the EGR gas outlet is provided in the exhaust passage downstream of the turbine of the main turbocharger,
Since this outlet is connected to the intake passage downstream of the throttle valve via the EGR gas passage in which the EGR valve is interposed, the following effects can be obtained.

(b) When switching the turbocharger, it is almost unaffected by exhaust pressure fluctuations, and the amount of EGR gas can be controlled accurately. Therefore, EGR gas corresponding to the amount of intake air can flow into the combustion chamber. This makes it possible to promote the reduction of NOx.

(b) EGR from upstream of the turbine of the main turbocharger
When extracting gas, it is likely to be affected by the air-fuel ratio of a specific cylinder, but the E
Since the GR gas is taken out, the EG taken out
The R gas is sufficiently stirred by the turbine. Therefore, it is no longer influenced by a specific cylinder, and A/F distribution can be improved.

(c) Also, by placing the EGR gas outlet downstream of the turbine, the temperature of the recirculated EGR gas can be lowered by energy conversion for driving the turbine, making it possible to eliminate cooling means such as an EGR cooler. It becomes possible.

[Brief explanation of the drawing]

FIG. 1 is a schematic system diagram of an exhaust gas recirculation device for a supercharged engine according to the present invention; FIG. 2 is a system diagram of an exhaust gas recirculation device for a supercharged engine that further embodies FIG. 1; Figure 3 is a characteristic diagram showing the relationship between engine speed and exhaust pressure in the device shown in Figure 1, and Figure 4 is a characteristic diagram showing the relationship between engine speed and EGR in the device shown in Figure 1.
This is a characteristic diagram showing the relationship with the rate. 1...Engine 4...Throttle valve 7...Main turbocharger 7a...Main turbocharger turbine 8...
... Sub-turbocharger 17 ... Exhaust switching valve 29 ... Engine control computer 40 ... EGR gas outlet 42 ... EGR gas passage 61 ... EGR valve 62 ... EGR vacuum modulator 63.
...Vacuum switching valve (VSV)

Claims (1)

[Claims] 1. In an exhaust gas recirculation system for a supercharged engine, which includes a main turbocharger and a sub-turbocharger, and an exhaust switching valve is provided downstream of the sub-turbocharger, the EGR
A gas outlet is provided in the exhaust passage downstream of the turbine of the main turbocharger, and the outlet is connected to the intake passage downstream of the throttle valve via an EGR gas passage in which an EGR valve is interposed. Exhaust gas recirculation device for supercharged engines.