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Title:
INTAKE SYSTEM FOR INTERNAL COMBUSTION ENGINE AND CONTROL METHOD OF THE SAME
Document Type and Number:
WIPO Patent Application WO/2008/099282
Kind Code:
A2
Abstract:
An intake system is provided with an intake flow control valve (300) provided in an intake port (50) upstream of an intake valve (70). The intake flow control valve (300) may be switched between at least a first position (300A) and a second position (300B), in accordance with the operating state of an engine, by rotating a rotary shaft (360), which is supported by a rotary shaft support part (350). In the first position, the clearance between the valve element and the inner wall of the intake port is small. In the second position, the clearance between the valve element and the inner wall of the intake port is large.

Inventors:
KOBORI YOICHI (JP)
AKAGAWA MASAMICHI (JP)
Application Number:
PCT/IB2008/000641
Publication Date:
August 21, 2008
Filing Date:
February 14, 2008
Export Citation:
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Assignee:
TOYOTA MOTOR CO LTD (JP)
DENSO CORP (JP)
KOBORI YOICHI (JP)
AKAGAWA MASAMICHI (JP)
International Classes:
F02D9/10; F02B31/06
Foreign References:
JPH09203324A1997-08-05
Download PDF:
Claims:

CLAIMS:

1. An intake system for an internal combustion engine in which an intake port (50) is

connected to a cylinder (20) of the internal combustion engine, an intake valve (70) which

opens and closes is provided at a downstream end of the intake port (50), and a side surface

of an intake pipe connected to the intake port (50) is at least partially substantially straight

as viewed in cross section, the intake system characterized by comprising:

an intake flow control valve (300) that includes a valve element provided upstream of

the intake valve (70) in which a side surface of the valve element conforms with the

contours of the substantially straight part and a rotary shaft (360) provided to the valve

element; and rotation means for rotating the rotary shaft (360),

wherein the rotary shaft (360) allows rotation of the valve element about a side surface

of the intake pipe, and

the intake pipe is shaped such that when the valve element is in a first position, where

the intake flow control valve (300) is rotated by the rotation means until the intake pipe is

closed, a clearance between the intake pipe and the valve element is smaller than the

clearance when the valve element in a second position, where the intake flow control valve

(300) is rotated by the rotation means to a neutral position.

2. The intake system according to claim 1, wherein the cross section of the intake pipe is

fully closed in the first position, and when the valve element is in the first position, the

clearance does not permit air in the intake pipe to flow past the valve element and into the

cylinder (20).

3. The intake system according to claim 1 or 2, wherein a distal end of the valve element is

positioned generally in a middle of the intake pipe when the valve element is in the neutral

position, and the clearance when the valve element is in the second position is sufficient to

prevent the valve element from becoming stuck when the internal combustion engine is

stopped.

4. The intake system according to claim 3, wherein the clearance in the second position is

greater at a position farther from the rotary shaft (360).

5. The intake system according to claim 1, wherein when the valve element is in the first

position, a distal end of the valve element contacts an inner wall surface of the intake pipe

and the clearance between the intake flow control valve (300) and the inner wall surface of

the intake pipe is minimized.

6. The intake system according to claim 1, wherein when the valve element is in the first

position, an inside diameter of a portion of the intake pipe at which the valve element is

positioned is smaller than the diameter of the intake pipe downstream from the position of

the valve element.

7. The intake system according to any one of claims 1 to 6, wherein the intake flow control

valve (300) has a generally U-shaped cross section.

8. The intake system according to any one of claims 1 to 7, wherein the rotation means

rotates the intake flow control valve (300) to the second position when the internal

combustion engine is stopped.

9. A control method of the intake system according to claim 1, comprising:

rotating the intake flow control valve (300) to the second position when the internal

combustion engine is stopped.

10. An intake system for an internal combustion engine, comprising:

an intake port connected to a cylinder of the internal combustion engine;

an intake valve for opening and closing the intake port provided at a downstream end

of the intake port;

an intake pipe connected to the intake port, wherein a side surface of the intake pipe is

at least partially substantially straight as viewed in cross section;

an intake flow control valve that includes a valve element provided upstream of the

intake valve in which a side surface of the valve element conforms with the contours of the

generally straight part and a rotary shaft provided to the valve element that rotates the valve

element about a side surface of the intake pipe; and

a rotation device that rotates the rotary valve, wherein the intake pipe is shaped such

that when the valve element is in a first position, where the intake flow control valve is

rotated by the rotation device until the intake pipe is closed, a clearance between the intake

pipe and the valve element is smaller than the clearance when the valve element in a second

position, where the intake flow control valve is rotated by the rotation device to a neutral

position.

Description:

INTAKE SYSTEM FOR INTERNAL COMBUSTION ENGINE AND CONTROL

METHOD OF THE SAME

BACKGROUND OF THE INVENTION

1. Field of the Invention

[0001] The present invention relates to an intake system for an internal combustion

engine provided with an intake flow control valve in an intake port connected to a cylinder,

and to a control method of the intake system. More specifically, the present invention

relates to an intake system that prevents an intake flow control valve, which induces

circular gas flow (vortex flow) in the cylinder according to the operating state of an internal

combustion engine, from being rendered inoperable when the internal combustion engine is

stopped.

2. Description of the Related Art

[0002] In order to stabilize combustion of a lean air-fuel mixture in a spark ignition

internal combustion engine (hereinafter referred to as "engine"), it is important to produce

gas flow such as tumble flow (vertical vortex) and swirl flow (horizontal vortex) in a

cylinder, and it is necessary to enhance such gas flow across a wider operating range.

[0003] In the low-load operating range of the engine, where the throttle opening is

small and the intake air amount is accordingly small, in particular, the fuel consumption and

the emission tend to increase because air-fuel mixture is generally set to be slightly richer to

stabilize combustion. In order to improve fuel economy and emissions, it is effective to

induce circular flow of the intake air in the cylinder such as tumble flow and swirl flow to

promote combustion with its strong turbulent flow.

[0004] The term "swirl flow" as used herein refers to flow of intake air circulating

along the peripheral wall of the cylinder, which homogenizes intake air without producing

turbulent flow, to promote combustion. Meanwhile, the term "tumble flow" as used herein

refers to flow of intake air circulating along the axial direction of the cylinder, which

improves combustion in the low-load operating range of the engine, because strong

turbulent flow is produced as the tumble flow is deformed in the second half of the

compression stroke.

[0005] In order to enhance gas flow (swirl flow and tumble flow) in the cylinder,

conventional methods use an intake flow control valve to block part of the cross section of

an intake port to cause intake air flowing in the intake port to flow towards one side of the

intake port. In order to produce tumble flow, for example, the intake flow control valve is

disposed on the lower side of the intake port to cause intake air to flow towards the upper

side of the intake port, thereby enhancing tumble flow in the cylinder.

[0006] There is a clearance between the intake flow control valve and the housing

that supports the intake flow control valve, and oil blown back from the combustion

chamber occasionally adheres to the intake flow control valve through the clearance. A

larger amount of oil adheres to the intake flow control valve when an engine key is turned

off to stop the engine than during engine operation. When the engine is started, the engine

speed is low and therefore the negative pressure acting on the intake flow control valve is

small, which makes it difficult to blow away oil that has adhered around the intake flow

control valve, resulting in a large amount oil remaining to adhere to the intake flow control

valve. Oil adhering to the intake flow control valve increases the sliding resistance

between the intake flow control valve and the housing, which changes the behavior of the

intake flow control valve to occasionally cause a problem in the engine operation

immediately after the engine is started. When the engine is stopped for an extended period,

oil adhering to the intake flow control valve may harden and cause the intake flow control

valve to become stuck to the housing, thereby preventing opening and closing operation of

the intake flow control valve when the engine is started.

[0007] In view of the above problem, Japanese Patent Application Publication No.

9-203324 (JP-A-9-203324) describes an intake control device for an internal combustion

engine that prevents an intake flow control valve from becoming stuck by reducing the

amount of oil that adheres to the intake flow control valve irrespective of the operating state

of the engine. The intake control device described in JP-A-9-203324 is provided with: a

throttle valve provided in an intake passage for supplying intake air to each cylinder of an

internal combustion engine to adjust the amount of air supplied to each cylinder; an intake

flow control valve provided in the intake passage connected to each cylinder downstream of

the throttle valve to adjust the intake period of each cylinder; and a control section for

determining the open period and the close period of the intake flow control valve based on

the operating state of the internal combustion engine. In the intake control device, the

control section closes the intake flow control valve for a certain period from a

predetermined timing at which a piston of each cylinder is moving when the internal

combustion engine is stopped, and then keeps the intake flow control valve half-open.

[0008] The intake control device for an internal combustion engine reduces the

contact area between the intake flow control valve and a housing for supporting the intake

flow control valve to a minimum by keeping the intake flow control valve half-open after

closing the intake flow control valve for a certain period when the engine is stopped. It is

thus possible to prevent the intake flow control valve from becoming stuck to the housing

by oil that hardens when the engine is stopped.

[0009] The intake flow control valve disclosed in JP-A-9-203324 has a rotary shaft

and a valve element attached to the rotary shaft. The valve element is made up of a pair of

left and right disks and a valve plate integrally formed between both the disks. A shaft

support part is formed to extend coaxially with the rotary shaft on the outer side of the disk

in the axial direction. One end of the valve element is supported for rotation by a bearing

via the shaft support part, while the other end of the valve element is coupled to a motor.

An attachment part for accommodating the valve element for rotation is in a cylindrical

shape, and the valve element is assembled into the attachment part from its opening end.

A slight clearance is kept between the outer periphery of the disks of the valve element and

the inner peripheral surface of the attachment part when the valve element is rotated.

When a predetermined time elapses after engine stop, the motor is de-energized with the

intake flow control valve half-open. In this state, with almost no contact area between the

intake flow control valve and the valve housing, the intake flow control valve is prevented

from becoming stuck to the valve housing by oil that has adhered to the intake flow control

valve. That is, it is possible to blow away oil that has adhered to the intake flow control

valve and prevent the intake flow control valve from becoming stuck to the valve housing

by once closing the intake flow control valve at the same time as the engine key is turned

off and making the intake flow control valve half-open a few seconds later.

[0010] However, because the rotary shaft of the intake flow control valve disclosed

in JP-A-9-203324 extends across the diameter of the intake pipe, the valve plate produces a

large fluid resistance and thus results in a large pressure loss, even when the intake flow

control valve is fully open. In order to avoid such a loss, for example, the cross section of

the intake pipe is formed in a shape of a quadrilateral and an arc or an elliptical arc, the side

surfaces of the valve element of the intake flow control valve are formed to coincide with

the side surfaces of the quadrilateral of the intake pipe, and the rotary shaft of the valve

element is provided along a plane perpendicular to the side surfaces of the valve element.

The rotary shaft supports the intake flow control valve only on one side (on the base of the

quadrilateral) (in so-called cantilever manner) so that the valve element extends along the

bottom of the intake pipe when the intake flow control valve is fully open. In

JP-A-9-203324, the intake flow control valve is made half-open with almost no contact area

between the intake flow control valve and the valve housing to prevent the intake flow

control valve from becoming stuck to the valve housing by oil that has adhered to the intake

flow control valve. Meanwhile, in the cantilever intake flow control valve, the clearance

in the fully close state is set to a minimum to prevent leakage, and is therefore the same as

that in the half-open state. Therefore, it is not possible to prevent the intake flow control

valve from becoming stuck, due to adhesion of oil or freezing of water, even with the intake

flow control valve is half-open.

SUMMARY OF THE INVENTION

[0011] The present invention provides an intake system for an internal combustion

engine that is provided with an intake flow control valve that does not produce a large fluid

resistance at fully-open state and that does not become inoperable due to a factor while the

internal combustion engine is stopping, and provides a control method of the intake system.

[0012] A first aspect of the present invention is directed to an intake system for an

internal combustion engine in which an intake port is connected to a cylinder of the internal

combustion engine, and an intake valve, which opens and closes, is provided at a

downstream end of the intake port. A side surface of the intake pipe connected to the

intake port is at least partially substantially straight as viewed in cross section. The intake

system includes: an intake flow control valve including a valve element provided upstream

of the intake valve in which a side surface of the valve element confirms with the contours

of the substantially straight part and a rotary shaft provided to the valve element; and

rotation means for rotating the rotary shaft. The rotary shaft allows rotation of the valve

element about a side surface of the intake pipe. The intake pipe is shaped such that when

the valve element is in a first position, where the intake flow control valve is rotated by the

rotation means until the intake pipe is closed, a clearance between the intake pipe and the

valve element is smaller than the clearance when the valve element in a second position,

where the intake flow control valve is rotated by the rotation means to a neutral position.

[0013] According to the first aspect, because the intake flow control valve is

supported only on one side (on the bottom) in a cantilever manner, the intake flow control

valve produces minimal fluid resistance when the fully open. The gap in the first state

where the intake flow control valve (fully) closes the intake pipe is smaller than that in the

second state where the intake flow control valve is in the neutral state. Therefore, it is

possible to enhance production of vortex flow by reducing leakage of intake air flow

through the gap in the first state, and to prevent the intake flow control valve from

becoming stuck due to adhesion of oil or freezing of water in the second state, where the

gap is larger, and the engine is stopped. As a result, the intake flow control valve does not

produce a large fluid resistance when fully opened, and is not rendered inoperable (stuck)

while the internal combustion engine is stopped.

[0014] In the first aspect, the cross section of the intake pipe may be fully closed in

the first position, and when the valve element is in the first position, the clearance does not

permit air in the intake pipe to flow past the valve element and into the cylinder.

[0015] According to the above construction, it is possible to enhance production of

vortex flow by reducing leakage of intake air flow through the gap in the first position

where the cross section of the intake pipe is fully closed.

[0016] In the first aspect, the neutral position is generally a position in the middle of

the intake pipe, and the clearance in the second position may be sufficient to prevent the

valve element from becoming stuck when the internal combustion engine is stopped.

[0017] According to the above construction, it is possible to prevent the valve

element from becoming stuck to the inner wall of the intake pipe, even with adhesion of oil

or freezing of water, in the second state where the gap is larger with the internal combustion

engine stopped.

[0018] In the first aspect, the clearance in the second position may be greater at a

position farther from the rotary shaft.

[0019] According to the above construction, because greater torque is required to

rotate the rotary shaft when the valve element is stuck in a position farther from the rotary

shaft, the clearance is made greater to make it more difficult to fix the valve element at a

position farther from the rotary shaft. Therefore, it is possible to avoid a situation where

greater torque is required to resolve fixing having once occurred. Because it is thus

possible to prevent the valve element from becoming stuck at a position farther from the

rotary shaft, an actuator for rotating the rotary shaft does not have to be built to demanding

specifications.

[0020] A control method of an intake system according to the first aspect includes:

rotating the intake flow control valve to the second position when the internal combustion

engine is stopped.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021] The foregoing and further objects, features and advantages of the invention

will become apparent from the following description of example embodiments with

reference to the accompanying drawings, wherein like numerals are used to represent like

elements and wherein:

FIG. 1 is a cross sectional view showing the overall structure of an intake system in

accordance with an embodiment of the present invention;

FIG. 2 is an enlarged view showing the vicinity of an intake flow control valve of FIG.

1;

FIG. 3 shows a valve element of the intake flow control valve as viewed from the

direction of intake flow;

FIG. 4 is a cross sectional view taken along the line 4-4 of FIG. 2;

FIG. 5 is a cross sectional view taken along the line 5-5 of FIG. 2; and

FIG. 6 is a perspective view of an intake flow control valve in accordance with a

modified embodiment of the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS

[0022] Embodiments of the present invention will be described below with

reference to the drawings. In the following description, identical components are given

identical reference numerals. They are also given identical names and functions. Thus,

the detailed description will not be repeated for the components.

[0023] An intake system of an internal combustion engine in accordance with an

embodiment will be described. The internal combustion engine described below is a spark

ignition gasoline engine (hereinafter referred to as "engine"). The engine may be provided

with either an injector that injects fuel into an intake port or an injector that directly injects

fuel into a cylinder. Alternatively, the engine may be provided with both types of

injectors.

[0024] FIG. 1 shows the overall construction of an intake system for an internal

combustion engine in accordance with this embodiment applied to a spark ignition engine

in which gasoline as fuel is directly injected into a cylinder. The intake system is enhances

tumble flow as an example of gas flow.

[0025] As shown in FIG. 1, a cylinder 20 is formed in a cylinder block 10, and a

pentroof combustion chamber 40 is formed in a cylinder head 30 covering the top of the

cylinder block 10. An intake port 50 and an exhaust port 60 are formed to open in the two

inclined surfaces of the combustion chamber 40. An intake valve 70 is provided to open

and close the intake port 50, and an exhaust valve 80 is provided to open and close the

exhaust port 60. The intake port 50 is bifurcated into two branches at its end, and a pair of

intake valves 70 are provided for each cylinder to open and close the respective ends.

Likewise, a pair of exhaust valves 80 are provided for each cylinder. An ignition plug 90

is disposed in the center of the combustion chamber 40, surrounded by the four valves. A

piston 100 is disposed in the cylinder 20. Because the piston 100 is not an essential part of

the present invention, the top surface of the piston 100 is shown as a simple flat shape.

However, the top surface of the piston 100 may be formed in a shape suitable for stratified

combustion or the like as appropriate.

[0026] The terms "upper" and "lower" used in regard to the intake port 50, intake

flow, etc., refer to upper and lower positions with respect to the cylinder 20, and do not

mean absolute upper and lower positions in space. The term "intake port" does not

necessarily mean only a part inside the cylinder head 30, but may include an upstream part

of the intake port may be formed as a part of a component other than the cylinder head 30,

for example a part of an intake manifold (intake pipe). That is, the "intake port" may

partially be composed of a component other than the cylinder head 30 such as an intake

manifold.

[0027] As described above, the intake port 50 of this engine is bifurcated into two

intake pipes at its end (before the combustion chamber 40). That is, the combustion

chamber 40 has two intake valves 70. For example, another intake valve may be provided

behind the one shown in FIG. 1. In the case where an intake flow control valve is provided

in one of the intake pipes upstream of the two intake valves, fully closing the one intake

pipe using the intake flow control valve and delivering air into the combustion chamber

from only the other intake pipe, for example, induces vortex flow (in this case, mainly swirl

flow) in the combustion chamber. Also, if an intake flow control valve is provided in at

least one of the intake pipes, a vortex flow (in this case, mainly tumble flow) may be

induced in the combustion chamber by controlling the opening of the intake flow control

valve and delivering air into the combustion chamber from only the upper half of the one

intake pipe. The present invention may be applied to engines in which vortex flow is

induced in the combustion chamber by means of an intake flow control valve provided in

the intake port as described above.

[0028] An intake flow control valve 300 is composed of a flat plate supported at one

end. The intake flow control valve 300 is coupled to a rotary shaft 360. The rotary shaft

360 is supported by a rotary shaft support part 350 to allow rotation of the intake flow

control valve 300. The rotary shaft 360 is coupled to a rotary shaft of a motor controlled

by an engine electronic control unit (ECU). The intake flow control valve 300 is rotated

by the motor.

[0029] An accommodation part 340 for accommodating the intake flow control

valve 300 is formed on the lower side of the intake port 50.

[0030] The motor rotates forward (in the direction to rotate the intake flow control

valve 300 clockwise in FIG. 1) in response to a command from the engine ECU, to rotate

the intake flow control valve 300 from the position 300C, to the position 300B, and then to

the position 300A where the distal end of the intake flow control valve 300 contacts the

upper wall surface of the intake port 50. The rotation may be stopped by a stopper (not

shown), for example. The engine ECU rotates the intake flow control valve 300 from the

position 300C, to the position 300B, and then to the position 300A by outputting a rotation

command signal for a predetermined period (set according to the rotation angle of the rotary

shaft 360).

[0031] Also, the motor rotates backward (in the direction to rotate the intake flow

control valve 300 counterclockwise in FIG. 1) in response to a command from the engine

ECU, to rotate the intake flow control valve 300 from the position 300A, to the position

300B, and then to the position 300C where the intake flow control valve 300 is

accommodated in the accommodation part 340. This rotation may also be stopped by a

stopper (not shown), for example. The engine ECU rotates the intake flow control valve

300 from the position 300A, to the position 300B, and then to the position 300C by

outputting a rotation command signal for a predetermined period (set according to the

rotation angle of the rotary shaft 360).

[0032] Instead of or in addition to the stopper, a sensor may be provided to detect

whether the distal end of the intake flow control valve 300 and the upper wall surface of the

intake port 50 contact each other in order for the engine ECU to output a stop command to

the motor. Furthermore, a sensor may be provided to detect whether the intake flow

control valve 300 is accommodated in the accommodation part 340 in order for the engine

ECU to output a stop command to the motor.

[0033] FIG. 2 is an enlarged view of the vicinity of the intake flow control valve of

FIG. 1. The intake flow control valve 300 is in the position 300A when it is in use with the

engine in operation. In this state, it is necessary to prevent leakage of intake air flow by

minimizing the clearance between the intake flow control valve 300 and the inner wall of

the intake port 50. The intake flow control valve 300 is in the position 300C when it is not

in use. In this state, the intake flow control valve 300 is accommodated in the

accommodation part 340 to avoid producing a large fluid resistance. The intake flow

control valve 300 is in the position 300B when the engine is stopped. In this state, it is

necessary avoid having the intake flow control valve 300 become stuck due to adhesion of

oil or freezing of water by maximizing the clearance between the intake flow control valve

300 and the inner wall of the intake port 50. Freezing of water is described in more detail.

Condensation around the intake flow control valve 300 collects between the side surfaces of

the intake flow control valve 300 and the inner wall of the intake port 50 when the engine is

stopped, and the condensed water freezes, thereby causing the intake flow control valve 300

to become stuck. Therefore, it is easy for the intake flow control valve 300 to become

stuck due to freezing of water if the clearance between the intake flow control valve 300

and the inner wall of the intake port 50 is small.

[0034] FIG. 3 shows a valve element of the intake flow control valve 300 as viewed

from the direction of intake flow (a front view of the valve element). As shown in FIG. 3,

the side surfaces of the intake flow control valve 300 are perpendicular to the rotary shaft

360.

[0035] FIG. 4 is a cross sectional view taken along the line 4-4 of FIG. 2, showing

the state where the engine is in operation, while FIG. 5 is a cross sectional view taken along

the line 5-5 of FIG. 2, showing the state where the engine is not in operation.

[0036] FIG. 4 shows the state where the engine is in operation, where the intake

flow control valve 300 is used in the position 300A to prevent leakage of intake air flow.

At the position 300A, the inner wall of the intake port 50 is not expanded. As indicated by

X in FIG. 4, the clearance between the intake flow control valve 300 and the inner wall of

the intake port 50 is small. This prevents intake air flow from leaking from the side

surfaces of the intake flow control valve 300 to be supplied to the engine from this intake

pipe, and allows intake air flow to be supplied to the engine only through the other intake

pipe (providing that two intake pipes are provided for each cylinder), in order to produce

strong vortex flow in the combustion chamber 40.

[0037] Meanwhile, FIG. 5 shows the state where the engine is not operation, where

the intake flow control valve 300 is used in the position 300B. At the position 300B, the

inner wall of the intake port 50 is expanded. As indicated by Y in FIG. 5, the clearance

between the intake flow control valve 300 and the inner wall of the intake port 50 is larger

than that shown in FIG. 4. This prevents the intake flow control valve 300 from being

fixed while the engine stopped due to adhesion of oil or freezing of water between the

intake flow control valve 300 and the inner wall of the intake port 50.

[0038] The operation of the intake system in accordance with this embodiment as

described above will now be described. [When intake flow control valve is in use] Under

the condition that the intake flow control valve 300 is in use with the engine in operation,

the engine ECU controls the motor for rotating the rotary shaft 360 to rotate the intake flow

control valve 300 to the position 300A. At this position, the inner wall of the intake port

50 is not expanded as shown in FIG. 4 and unlike that shown in FIG 5. That is, the

clearance between the intake flow control valve 300 and the inner wall of the intake port 50

is small. This prevents intake air flow from leaking from the side surfaces of the intake

flow control valve 300 to be supplied to the engine from this intake pipe, allows intake air

flow to be supplied to the engine only from the other intake pipe (the intake port is

bifurcated into two intake pipes before the combustion chamber as described above), in

order to produce strong vortex flow in the combustion chamber 40.

[0039] [When engine is stopped] Under the condition that the intake flow control

valve 300 is not in use with the engine stopped, the engine ECU controls the motor for

rotating the rotary shaft 360 to rotate the intake flow control valve 300 to the position 300B.

At this position, the inner wall of the intake port 50 is expanded as shown in FIG. 5 (the

inner diameter of the intake port 50 is expanded). That is, the clearance between the

intake flow control valve 300 and the inner wall of the intake port 50 is large. This

prevents the intake flow control valve 300 from becoming stuck when the engine is stopped

due to adhesion of oil or freezing of water between the side surfaces of the intake flow

control valve 300 and the inner wall of the intake port 50.

[0040] If the intake flow control valve 300 is fixed due to adhesion of oil or freezing

of water at its distal end, in particular, the motor must produce excessive torque to free the

intake flow control valve 300. Therefore, it is preferable to set a larger clearance

especially at the distal end of the intake flow control valve 300.

[0041] As has been described above, according to the intake system in accordance

with this embodiment, it is possible to produce strong vortex flow in the combustion

chamber by preventing leakage of intake air flow when the intake flow control valve

provided upstream of the intake valve to produce vortex current is fully closed. In

addition, it is possible to prevent the intake flow control valve from becoming stuck by

preventing adhesion of oil or freezing of water between the intake flow control valve and

the inner wall of the intake port when the engine is stopped.

[0042] <ModifÏŠed embodiment An intake flow control valve 3000 in accordance

with a modified embodiment of the present invention will now be described with reference

to FIG. 6. While the intake flow control valve 300 in accordance with the embodiment

described above has a valve element as a flat plate, the intake flow control valve 3000 in

accordance with this modified embodiment is not completely flat but generally U-shaped as

viewed in cross section. That is, the intake flow control valve 3000 has generally straight

side surfaces that conform with at least a part of the contours of the side surfaces of the

intake port 50 as viewed in cross section, and has a bottom surface that is perpendicular to

its side surfaces.

[0043] It should be understood that the described embodiments are merely

illustrative in all respects and are not to be construed as limitative. The scope of the

present invention is defined not by the above description but by the appended claims, and is

intended to include all equivalents covered by the claims and all modifications that fall

within the scope of the claims.