JPH03124921A - Suction device of multi-cylinder engine - Google Patents

Abstract

PURPOSE:To attempt combustion stability at the time of a low load and reduction in pumping loss at the time of a high load by opening/closing operating upstream throttle valves and downstream throttle valves which are provided in individually independent suction passages in response to a load. CONSTITUTION:Each upstream throttle valve 9 and downstream throttle valve 8 are respectively provided in individually independent suction passages 4a to 4d which are connected independently to respective cylinders 2. It is set so that the openings of the downstream throttle valves 8 are throttled more than the openings of the upstream throttle valves 9 so as to throttle intake air by the downstream throttle valves 8 at the time of a low load, while the openings of the upstream throttle valves 9 are throttled more than the openings of the downstream throttle valves 8 so as to throttle intake air by the upstream throttle valves 9 at the time of a high load. On the basis of so set opening/ closing characteristics, an opening/closing device 12 is provided in order to open and close the upstream throttle valves 9 and the downstream throttle valves 8. It is thus possible to ensure combustion stability at the time of the low load, while reducing pumping loss at the time of the high load.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to an intake system for a multi-cylinder engine in which a throttle valve is provided for each independent intake passage of each cylinder.

(Prior art) Conventionally, in the intake system of a multi-cylinder engine, in order to increase the accelerator response and reduce the intake passage volume downstream of the throttle valve, a throttle valve is installed in an independent intake passage near the intake port of each cylinder. It is being considered that the

Furthermore, for another purpose, a technique has been proposed in which on-off valves are provided in the independent intake passages of each cylinder.

For example, as seen in Japanese Patent Application Laid-Open No. 64-15439, in an engine equipped with a supercharger, the valve overlap period of the intake and exhaust valves is set to be long to achieve a scavenging effect in the combustion chamber using pressurized air. However, in light load ranges where boost pressure decreases, the above valve overlap period is long, and the amount of exhaust gas flowing into the intake passage becomes excessive, resulting in a reduction in compression ratio. In order to prevent combustion stability from being inhibited by an increase in dilution gas, a known technique is to reduce the amount of dilution gas by arranging an on-off valve that closes under light load conditions near the intake port. It is.

(Problem to be Solved by the Invention) However, in the above-mentioned intake system in which a throttle valve or an on-off valve is disposed in each independent intake passage of each cylinder, its characteristics vary depending on the location of the throttle valve. They are different, and it is difficult to obtain good characteristics in both low-load and high-load regions.

In other words, even in engines without a supercharger,
Placing a throttle valve in the independent intake passage of each cylinder reduces the throttle downstream volume and eliminates pressure interference between the cylinders, resulting in the port pressure just before the intake valve opens at the end of the exhaust stroke just before valve overlap. (Intake pressure) increases as it approaches atmospheric pressure, so at low loads, the backflow of combustion gas to the intake side during the valve overlap period described above is reduced, and the amount of exhaust gas carried into the next intake stroke. Combustion stability is improved by reducing so-called dilution gas.

The characteristics described above are more effective when the passage volume downstream of the throttle valve is set to a small value.Conversely, since the passage volume downstream of the throttle valve is small, When the throttle valve is opened to a predetermined opening degree in the region), the change in port pressure in response to the downward movement of the piston at the beginning of the intake stroke becomes a rapid and large negative pressure state, and the drive resistance when the piston moves downward increases. This is unfavorable from the point of view of fuel efficiency as it increases the so-called pumping loss.

On the other hand, if the throttle valve for each cylinder is placed relatively upstream of the independent intake passage to increase the passage volume up to the intake port, the decrease in intake pressure at the initial state of the intake stroke becomes more gradual. It is possible to reduce the effect of negative pressure on the piston and reduce pumping loss in the partial load region (high load region), but at low load, the intake valve opens next after the intake stroke ends as described above. If the port pressure does not return to near atmospheric pressure by the time the overlap begins,
Since the combustion chamber is in a negative pressure state, exhaust gas flows in from the combustion chamber, resulting in an increase in dilution gas.

As mentioned above, in order to fully obtain the effect of reducing dilution gas and pumping loss, the required value for the throttle downstream volume is different depending on each condition, and it is necessary to simply arrange a throttle valve for each cylinder. However, both effects cannot be obtained satisfactorily.

Therefore, in view of the above circumstances, the present invention improves throttle response by disposing a throttle valve for each cylinder, and also aims to improve combustion stability at low loads and reduce pumping loss at high loads. The object of the present invention is to provide an intake system for a multi-cylinder engine.

(Means for Solving the Problems) In order to achieve the above object, an intake system for a multi-cylinder engine according to the present invention provides an upstream throttle valve and a downstream throttle valve, respectively, in independent intake passages that are independently connected to each cylinder. In addition, when the load is low, the opening of the downstream throttle valve is made smaller than the opening of the upstream throttle valve, and the intake air is throttled by the downstream throttle valve, while when the load is high, the opening of the upstream throttle valve is smaller than the opening of the downstream throttle valve. It is constructed by installing an opening/closing device that opens and closes the upstream throttle valve and the downstream throttle valve based on the opening/closing characteristics set to reduce the opening degree of the valve and throttle the intake air with the upstream throttle valve.

(Function) In the above-mentioned intake system, in a low load region such as when idling, the opening of the downstream throttle valve is made smaller than the opening of the upstream throttle valve, and the intake air is throttled by the downstream throttle valve. By reducing the passage volume downstream of the throttle valve and allowing the port pressure to return to near atmospheric pressure before the intake valve opens, the pressure difference between the intake and exhaust sides of the combustion chamber in the event of valve overlap is reduced. This suppresses the flow of exhaust gas into the intake passage and ensures stable idle combustion by reducing dilution gas.

On the other hand, when the load is high, the opening of the upstream throttle valve is made smaller than the opening of the downstream throttle valve and the intake air is throttled by the upstream throttle valve, thereby increasing the passage volume downstream of the throttle valve and increasing the opening of the upstream throttle valve at the beginning of the intake stroke. This reduces the pressure drop caused by the downward movement of the piston, reduces the intake negative pressure acting on the piston, reduces the driving resistance for the downward movement of the piston, and reduces pumping loss, improving fuel efficiency.

(Example) Hereinafter, each embodiment of the present invention will be described in order along the drawings.

Embodiment 1 FIG. 1 shows a schematic configuration of an intake system for a multi-cylinder engine in an embodiment.

Four cylinders 2 are installed in the engine body 1, and each cylinder 2
Independent intake passages 4a to 4d are each independently connected to the intake boat 3, and an exhaust passage 6 is connected to the exhaust port 5 of each cylinder 2. Further, the upstream ends of the independent intake passages 4a to 4d of each cylinder 2 are connected to a surge tank 7 and gathered together.

In the middle of the independent intake passages 4a to 4d of each cylinder 2, a downstream throttle valve 8 is disposed on the downstream side near the intake port 3, and an upstream throttle valve 9 is disposed on the upstream side.
are arranged respectively.

The downstream throttle valve 8 and the upstream throttle valve 9 have a common throttle shaft 8a for each cylinder 2.

The upstream throttle valve 9 and the downstream throttle valve 8 are connected to the operation of the accelerator pedal 11, and the details are explained based on the characteristics shown in FIG. 2, which will be described later. It is opened and closed by an opening/closing device 12 (not shown).

Furthermore, an injector 14 is disposed in the independent intake passages 48 to 4d downstream of the downstream throttle valve 8, and an air introduction passage 15 is connected thereto. This air introduction passage 15 bypasses the downstream throttle valve 8 and the upstream throttle valve 9, and has a downstream end connected to the independent intake passages 4a to 4d downstream of the downstream throttle valve 8 of each cylinder 2, and is opened near this opening. An orifice 16 is disposed at the upstream end of each cylinder 2, and the upstream end is opened to connect to the intake passage 4 upstream of the surge tank 7. This air introduction passage 1
5 meters and supplies the amount of air necessary to maintain the idle rotation speed when the downstream throttle valve 8 is in the fully open state.

Furthermore, the opening/closing characteristics of the downstream throttle valve 8 and the upstream throttle valve 9 are as shown by the solid line in FIG. The broken line indicates both throttle valves 8,
9 is at the same opening degree. In a low load region where the opening degree of the upstream throttle valve 9 is A or less, the opening degree of the downstream throttle valve 8 is smaller than the opening degree of the upstream throttle valve 9.
This downstream throttle valve 8 throttles the intake air to control the amount of intake air.On the other hand, in a high load region including a partial load where the opening of the upstream throttle valve 9 exceeds A, the opening of the downstream throttle valve 8 is lower than the opening of the downstream throttle valve 8. The opening degree of the throttle valve is small, and the upstream throttle valve 9 throttles the intake air to control the amount of intake air.

The opening/closing characteristics of the downstream throttle valve 8 and the upstream throttle valve 9 may be as shown by chain lines in FIG. 2. In this case, the basic characteristics are the same, but
The opening degree difference between the two is set to be small, and both throttle valves 8 and 9 are set to be fully open or fully open at the same time.

On the other hand, as the opening/closing device 12 that controls the opening/closing characteristics of the downstream throttle valve 8 and the upstream throttle valve 9, in addition to employing a link mechanism that mechanically operates in conjunction with accelerator operation as described above, The throttle valve may be opened and closed by an actuator separately from the accelerator operation, and the opening may be controlled by a controller in response to the accelerator operation amount.

Next, FIGS. 3 and 4 show the differences between the downstream passage volumes of the downstream throttle valve 8 and the upstream throttle valve 9 installed as described above during idling operation and partial load operation. The graph shows the fluctuation of the port pressure along with the valve timing, where the broken line shows the state where only the downstream throttle valve 8 is throttled to the predetermined opening, and the solid line shows the state where only the upstream throttle valve 9 is throttled to the predetermined opening. There is.

Basically, the change characteristics of this valve timing and port pressure are such that the port pressure decreases as the intake valve In opens, becomes the largest intake negative pressure when the intake valve In closes, and after the intake valve In closes. The port pressure gradually increases toward atmospheric pressure 0, and the cycle in which the intake valve In opens again is repeated. After the intake valve In opens, the exhaust valve Ex
The period until the valve closes is the valve overlap period, during which the intake side and exhaust side communicate through the combustion chamber.

In the idling operation shown in FIG. 3, the upstream throttle valve 9
With the throttle (solid line), the downstream volume of the throttle is too large relative to the intake air amount, so the port pressure just before the valve overlap when the intake valve In opens does not rise to atmospheric pressure, and the pressure difference with the exhaust side is large. In addition to this, because the downstream volume is large, the amount of backflow of combustion gas to the intake side becomes large.
This combustion gas that has flowed back to the intake side is sucked into the combustion chamber in the next intake stroke by dilution gas, which reduces the amount of fresh air, lowers combustibility, and worsens idle stability. On the other hand, when the downstream throttle valve 8 restricts the throttle (dashed line), the throttle downstream volume becomes smaller, and the port pressure immediately before the valve overlap when the intake valve In opens increases to near atmospheric pressure, and the pressure on the exhaust side increases. Since the difference is small and the downstream volume tends to be narrow, the amount of backflow of combustion gas to the intake side is reduced, and the increased amount of fresh air improves combustibility and idle stability. For this reason, in a low load region including idling, the downstream throttle valve 8 throttles the intake air to reduce dilution gas and ensure combustion stability.

In addition, in the partial load operation (1,500 rpm country) in Fig. 4, the restriction by the downstream throttle valve 8 (dashed line)
Since the throttle downstream volume is small compared to the stroke volume of the piston, the intake valve In opens and the port pressure drops rapidly as the piston descends. This increases the pumping loss. On the other hand, in the case of restriction by the upstream throttle valve 9 (solid line), the throttle downstream volume is large, the intake valve In opens, and the decrease in port pressure due to the downward movement of the piston is gradual, and the port pressure is also reduced by the downstream throttle valve 8. It is higher than the field due to the throttle, and the driving load associated with lowering the piston is reduced, reducing the pumping loss. Therefore, in a high load region including a partial load region, the upstream throttle valve 9 throttles the intake air to reduce the negative port pressure, reduce pumping loss, and improve fuel efficiency. In addition, due to the increase in the downstream volume of the throttle, dilution gas due to the backflow of exhaust gas increases, and accordingly, the throttle opening to obtain the same output increases, which further reduces the boat negative pressure and reduces pumping loss. This can be reduced.

In order to sufficiently obtain the above-mentioned characteristics, the arrangement positions of the downstream throttle valve 8 and the upstream throttle valve 9, that is, the throttle downstream volume, are set in accordance with the characteristics of each engine, for example. , downstream throttle valve 8
It is preferable to set the passage volume further downstream to approximately 20% of the stroke volume of the piston, and to set the passage volume downstream of the upstream throttle valve 9 to approximately 50% of the stroke volume. By setting the volume above, the port pressure immediately before the intake valve In opens at low load increases to near atmospheric pressure, while at high load the negative pressure of the port during the intake stroke can be reduced. become.

In addition, in the above embodiment, by connecting the air introduction passage 15 to the independent intake passages 4a to 4d downstream of the downstream throttle valve 8, it is possible to avoid manufacturing problems when the downstream throttle valve 8 is fully open at idle. Due to accuracy, it is very difficult to set the amount of intake air that leaks through the downstream throttle valve 8 of each cylinder 2 and is supplied to each cylinder 2 equally for each cylinder 2. Therefore, when idling, this downstream throttle valve 8 is In the fully closed state, the amount of idle air is supplied through the air introduction passage 15, and an orifice 16 is provided in this air introduction passage 15.
The orifice 16 is used to uniformly supply precisely measured air to each cylinder 2. In addition, it is relatively easy to ensure the accuracy of the orifice 16, and at the same time, the orifice 16 is set so that when the other cylinder 2 is in its intake stroke, the other cylinder 2 does not inhale the combustion gas flowing back into the intake passage. There is.

Embodiment 2 FIG. 5 shows a schematic diagram of an example of a V-type six-cylinder engine.

In this example, the banks 2 on both sides of the engine body 21 are
Three cylinders 2 are installed in each of 1a and 21b, and independent intake passages 24a to 24f are independently connected to the intake port 3 of each cylinder 2. In each bank 21a, 21b, the upstream ends of the independent intake passages 24a to 24f of each cylinder 2 are connected to a surge tank 27 (intake collecting section), and further upstream, the banks 21a on both sides
.

An intake air amount sensor 22 and an air cleaner 23 are installed at the upstream end of the intake passage 24 where the air intake passages 21b are gathered.

In the middle of the independent intake passages 24a to 24f of each cylinder 2, a downstream throttle valve 8 is provided on the downstream side near the intake port 3.
are disposed, and upstream throttle valves 9 are disposed on the upstream side thereof. The downstream throttle valve 8 and the upstream throttle valve 9 are arranged in banks 21a and 21b.
Each cylinder 2 is fixed to a common throttle shaft 8a, 9a, and each cylinder is opened and closed at the same time.

The upstream throttle valve 9 and the downstream throttle valve 8 are operated in conjunction with the operation of the accelerator pedal, based on the same characteristics as in the previous example shown in FIG. The opening degree of the throttle valve 8 is small, and the downstream throttle valve 8 throttles the intake air to control the intake air amount. However, in high load areas including partial loads, the opening degree of the throttle valve is smaller than the opening degree of the downstream throttle valve 8. The upstream throttle valve 9 is opened and closed by the same opening/closing device (not shown) as described above so that the upstream throttle valve 9 throttles the intake air and controls the amount of intake air.

Further, an injector 14 is disposed in the independent intake passages 24a to 24f downstream of the downstream throttle valve 8, and an air introduction passage 25 is connected near the injector 14. This air introduction passage 25 bypasses both throttle valves 8.9, and its downstream end is connected to the injector 14 of each cylinder 2 near its nozzle hole, and is supplied as bleed air to the injected fuel. is configured to promote atomization. The upstream ends are gathered together and connected to the intake passage 24 upstream of the surge tank 27, and an orifice (not shown) is interposed in the middle.

Further, the fuel injection amount and injection timing from the injector 14 are controlled by a controller 28, and this controller 28 receives various signals such as an intake air amount signal, a rotation speed signal, a cylinder identification signal, and other correction signals for fuel injection control. is input. Then, in each cylinder 2, the injection timing is controlled so that fuel injection is performed at a time when the flow velocity of bleed air from the air introduction passage 25 is high, corresponding to each intake stroke.

In other words, since the port pressure of the injector 14 is changed as shown in FIG. The fuel is injected during the period when the fuel is flowing into the combustion chamber.

The rest is the same as the previous example, and the same components are given the same reference numerals.

According to this example, fuel vaporization and atomization are low during idling, but by closing the upstream throttle valve 9 and the downstream throttle valve 8 to the fully open state, the boat pressure in the injector 14 portion is reduced compared to that of the other cylinders 2. without being affected,
As the pressure fluctuation increases, the bleed air by the air introduction passage 25 changes in response to the fluctuation in boat pressure.
By synchronizing the fuel injection timing when a large negative pressure is generated and the amount of air is large, a large atomization promoting effect of the injected fuel can be obtained due to the bleed air having a high flow velocity.

Note that if a throttle valve is not provided in the independent intake passages 24a to 24f of each cylinder 2 and they communicate with each other on the upstream side, a substantially constant negative Since the flow rate of the introduced air amount is approximately constant and the total amount of air supplied is between the paths, the flow rate is low (the atomization promotion effect is also small, but in the above example, the double The throttle valve provides high sealing performance when fully open, increasing the amount of air from the air introduction passage 25, and increasing the amount of air from the other cylinders 2
The flow velocity is high when the negative pressure increases due to the large pressure fluctuation without interference with the intake air, and the atomization of the fuel is achieved, which improves combustibility, improves operational stability, and reduces the amount of unburned component emissions.

Embodiment 3 FIG. 6 shows a schematic diagram of another example of a V-type six-cylinder engine.

The cylinder arrangement of the engine body 21 and the connection structure of the independent intake passages 24a to 24f in this example are the same as in the second embodiment, and three cylinders 2 are provided in each of the banks 21a and 21b on both sides.
are installed, and independent intake passages 24a to 24f are connected to the intake boat 3 of each cylinder 2, respectively.
The upstream ends of 4a to 24f are connected to the surge tank 27 for each bank 21a, 21b and collected.

In the middle of the independent intake passages 24a to 24f of each cylinder 2, a downstream throttle valve 8 is provided on the downstream side near the intake boat 3.
are arranged, and upstream throttle valves 9 are respectively arranged upstream of these, and those of each cylinder 2 for each bank 21a, 21b are fixed to a common throttle shaft 8a, 9a, so that they can be opened and closed in the same way as in the previous example. The opening/closing device is designed such that in low load regions, the downstream throttle valve 8 throttles the intake air to control the intake air amount, while in high load regions, including partial loads, the upstream throttle valve 9 throttles the intake air to control the intake air amount. (not shown) are linked to open and close.

Further, each bank 21a is provided in the independent intake passages 24a to 24f downstream of the downstream throttle valve 8.

Each cylinder 2 in 21b communicates with each other through a narrow communication passage 30, and an injector 14 is disposed therein.

The rest is the same as the previous example, and the same components are given the same reference numerals.

According to this example, when the downstream throttle valve 8 is closed during idling, the amount of air leaking from the downstream throttle valve 8 is different, and the amount of filling between the cylinders 2 becomes uneven. This allows air to flow between the cylinders to achieve equalization. At that time, double throttle valve 8
, 9 improves the sealing performance of the intake passage and reduces leakage air, and the mutual communication through the communication passage 30 also makes it possible to adjust the required minimum flow rate during idling. Furthermore, if the communicating passage 30 is formed to be large, the downstream volume of the downstream throttle valve 8 will substantially increase due to mutual communication between the cylinders 2, and the amount of combustion gas flowing back into the intake passage will increase, resulting in a diluent. The communication path 30 is formed to be sufficiently thin so that the amount of tion gas does not increase.
Furthermore, it is necessary to set the mutual communication so that when the other cylinders 2 are in their intake strokes, the combustion gases flowing back into the intake passages in the other cylinders 2 are not sucked in.

Embodiment 4 FIG. 6 shows a schematic diagram of still another example of a V-type six-cylinder engine.

The cylinder arrangement of the engine body and the connection structure of the independent intake passages 24a to 24f in this example are the same as in the second embodiment, and three cylinders 2 are installed in each of the banks 21a and 21b on both sides, and the intake boat 3 of each cylinder 2 is has an independent intake passage 24
a to 24f are connected to each other, and each independent intake passage 24a
The upstream ends of the banks 21a and 24f are connected to the surge tank 27 and collected for each bank 21a and 21b.

In the middle of the independent intake passages 24a to 24f of each cylinder 2, a downstream throttle valve 8 is provided on the downstream side near the intake port 3.
are arranged, and an upstream throttle valve 9 is arranged upstream from this, and is fixed to the throttle shaft 8a, 9a of each cylinder 2 for each bank 21a, 21b, and opens and closes as in the previous example. The opening/closing device is designed such that in low load regions, the downstream throttle valve 8 throttles the intake air to control the intake air amount, while in high load regions, including partial loads, the upstream throttle valve 9 throttles the intake air to control the intake air amount. 12 for opening and closing operation.

In addition, an injector 14 is disposed in the independent intake passages 24a to 24f downstream of the downstream throttle valve 8, and an air injector 14 that bypasses both throttle valves 8 and 9 and is provided with an orifice 16, similar to the first embodiment, is provided. An introduction passage 35 is connected thereto. Further, in the independent intake passages 24a to 24f between the upstream throttle valve 9 and the downstream throttle valve 8, a communication passage 36 that communicates each cylinder 2 with each other is formed, and a bypass passage that bypasses the upstream throttle valve 9. is connected. This bypass passage 37 has a flow control valve 3.
8 is interposed, and the external load (air conditioner load, air conditioner load,
The operation is controlled to increase the intake air amount according to the alternator load (corresponding to the electric load, etc.).

The rest is the same as the previous example, and the same components are given the same reference numerals.

According to this example, when the downstream throttle valve 8 is closed during idle, an equal amount of idle air is supplied to each cylinder 2 by the air introduction passage 35 having the orifice 16. When the external load increases in this idle state,
Slightly open downstream throttle valve 8 (upstream throttle valve 9
is fully open), the flow rate control valve 38 is operated to supply a predetermined amount of bypass air to the bypass passage 3 according to the magnitude of the external load.
7 and communication passage 36 to each cylinder 2, and the engine output is increased in response to an increase in load. Furthermore, at low loads, the opening degree of the downstream throttle valve 8 is smaller, and the port pressure immediately before valve overlap, when the intake valve opens, is increased to near atmospheric pressure to ensure combustion stability by reducing dilution gas. is similar to other examples.

In addition, when transitioning to a partial load state, the downstream throttle valve 8
is opened wider than the upstream throttle valve 9, and at this time,
Throttle downstream passages are communicated with each other in each cylinder 2 by a communication passage 36, and the combustion gas flowing back into the intake passages 24a to 24f in other cylinders 2 is sucked in through the communication passage 36 to generate dilution gas. The internal EGR effect suppresses the generation of NOx in the exhaust gas, and reduces pumping loss by increasing the throttle opening to obtain the same output. Fuel efficiency is improved by reducing pumping loss due to pressure increase.

In addition, in the example of the V-type engine of Examples 2 to 4,
In reality, the intake system is often arranged in the center of the banks 21a, 21b on both sides, and the above-mentioned FIGS. 5 to 7 merely show the connection form of the intake system.

(Effects of the Invention) According to the present invention as described above, an upstream throttle valve and a downstream throttle valve are provided in the independent intake passages for each cylinder, and the upstream throttle valve is controlled by the opening/closing device for both throttle valves during low load. The opening of the downstream throttle valve is made smaller than the opening of the downstream throttle valve to throttle the intake air, while when the load is high, the opening of the upstream throttle valve is made smaller than the opening of the downstream throttle valve to throttle the intake air. By throttling the intake air with the downstream throttle valve in low load areas such as when idling, the actual passage volume downstream of the throttle valve is reduced, and the boat pressure is reduced before the intake valve opens. By returning the exhaust gas to near atmospheric pressure, it suppresses the inflow of exhaust gas into the intake passage and ensures combustion stability by reducing dilution gas. , it is possible to increase the substantial passage volume downstream of the throttle valve, reduce the intake negative pressure acting on the piston at the beginning of the intake stroke, and improve fuel efficiency by reducing pumping loss.

[Brief explanation of the drawing]

Fig. 1 is a schematic configuration diagram of the intake system of a multi-cylinder engine according to the first embodiment of the present invention, Fig. 2 is a characteristic diagram showing the opening characteristics of the downstream throttle valve and the upstream throttle valve, and Fig. 3 is idling operation. FIG. 4 is a characteristic diagram showing the same characteristic diagram at partial load, FIG. 5 is a schematic diagram of the intake system of a multi-cylinder engine in the second embodiment, and FIG. FIG. 7 is a schematic configuration diagram of an intake system for a multi-cylinder engine in a fourth embodiment. 1.21...Engine body, 2...Cylinder, 3...Intake boat, 4,24...
Intake passage, 4a to 4d, 24a to 24f...Independent intake passage, 7, 27...Surge tank, 8.
...Downstream throttle valve, 9...Upstream throttle valve, 12...Opening/closing device, 14...
・Injector, 15, 25.35・Old...Air introduction passage, 16...Orifice, 30.36...
・Communication path.

Claims (1)

[Claims] (1) An upstream throttle valve and a downstream throttle valve are provided in independent intake passages connected to each cylinder of a multi-cylinder engine, respectively, and when the load is low, the downstream throttle is lower than the opening of the upstream throttle valve. The opening degree of the upstream throttle valve is set to be smaller than the opening degree of the downstream throttle valve and the intake air is throttled by the upstream throttle valve when the load is high. An intake system for a multi-cylinder engine, characterized in that an opening/closing device is installed that opens and closes the upstream throttle valve and the downstream throttle valve based on the opening/closing characteristics.