Intake system for an internal combustion engine

According to the present invention, each of the cylinders of an engine is provided with a first and a second swirl ports for generating a swirl of intake air in the cylinder. The second swirl port is located at the position downstream of the first swirl port with respect to the direction of swirl generated by the first intake port. The helical air passages of the first and the second intake port have a bottom which opens to the cylinder and an upper wall facing the bottom and an end wall defining the end of the helical air passage. The angle .theta..sub.1 between the upper wall and the end wall of the helical air passage of the first (upstream) intake port is formed smaller than the same (the angle .theta..sub.2) of the helical air passage of the second (downstream) intake port. By forming the angle .theta..sub.1 small, intake air flowing through the first intake port rotates a large amount before it flows into the cylinder and a strong swirl is formed by the intake air from the first intake port. On the other hand, since the angle .theta..sub.2 is relatively large, intake air flowing through the second intake port flows into the cylinder before it rotates sufficiently. Therefore, the intake air from the second intake port flows into the cylinder without interfering with the swirl in the cylinder. Thus, the flow resistance of the second intake port decreases.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an intake system for an internal 
combustion engine. More specifically, the invention relates to an intake 
system having two swirl intake ports on each cylinder of the engine. 
2. Description of the Related Art 
An intake system in which two swirl intake ports are disposed on each 
cylinder of the engine in order to generate a strong swirl of intake air 
in the cylinder is known in the Art. For example, this type of the intake 
system is disclosed in Japanese Unexamined Patent Publication (Kokai) No. 
7-158459. The intake system in the '849 publication disposes two helical 
type swirl ports on the cylinder. The helical type swirl port has a 
helical air passage which gives a rotational velocity to the intake air 
flowing therethrough. In the '849 publication, the swirl port located 
upstream with respect to the flow direction of the swirl in the cylinder 
rotates the flow of intake air therethrough by a large amount before it 
flows into the cylinder while the swirl port located downstream with 
respect to the swirl in the cylinder rotates the flow of intake air 
therethrough by only a half turn before it flows into the cylinder. 
In the '849 publication, the helical air passage of the upstream swirl port 
has a relatively large height at the end thereof in order to rotate the 
flow of intake air by a large amount. In contrast to this, the helical air 
passage of the downstream swirl port has a relatively small height at the 
end thereof in order to rotate the flow of intake air therethrough by only 
a half turn. 
By setting the amount of rotation of the flow of intake air through the 
downstream swirl port small, the tangential (rotational) velocity, i.e., a 
velocity in the horizontal direction (i.e., the direction perpendicular to 
the axis of the cylinder), of the intake air at the outlet of the 
downstream swirl port becomes smaller than the tangential velocity of the 
upstream swirl port. Therefore, a downward velocity component becomes 
dominant in the flow of intake air from the downstream swirl port, and the 
intake air from the downstream swirl port flows downward from the 
downstream swirl port and is subject to less interference with the 
cylinder wall. Further, since the flow of the intake air from the 
downstream swirl port has smaller tangential velocity, the intake air from 
the downstream swirl port hardly interferes with the swirl of intake air 
generated by the upstream swirl port. Therefore, the swirl in the cylinder 
is not disturbed or weakened by interference with the flow of intake air 
from the downstream swirl port. 
In the '849 publication, the amount of rotation of the flow of intake air 
through the downstream swirl port is kept small by setting the height of 
the end portion of the helical air passage thereof small. However, in the 
actual intake system, it is difficult to keep the amount of rotation of 
intake air small only by setting the height of the end portion of the 
helical air passage small. Therefore, in some cases system in the '849 
publication, the flow of intake air from the downstream swirl port 
interferes with the cylinder wall and the swirl in the cylinder. Such 
interference causes an increase in the flow resistance of the downstream 
swirl port and a decrease in the amount of the intake air therethrough. 
Further, this interference between intake air from the downstream swirl 
port and the swirl in the cylinder weakens the swirl. This adversely 
affect the combustion in the cylinder. 
SUMMARY OF THE INVENTION 
In view of the problems in the related art as set forth above, the object 
of the present invention is to provide an intake system for an internal 
combustion engine in which two swirl intake ports are disposed on each 
cylinder without causing interference of the flow of intake air from the 
downstream swirl port with the cylinder wall and the swirl in the 
cylinder. 
This object is achieved by an intake system for an internal combustion 
engine which comprises a first swirl port and a second swirl port disposed 
on each of the cylinders of the engine, each of the first swirl port and 
the second swirl port being provided with an intake air passage of a 
helical shape which generates a swirl of intake air therethrough within 
the cylinder. The second swirl port is disposed on the cylinder downstream 
of the first swirl port with respect to the direction of the swirl 
generated by the first swirl port. Each of the helical intake air passages 
of the first and second swirl ports has a bottom extending along the air 
passage and opening to the cylinder and an upper wall extending along the 
air passage and facing the bottom and an end wall defining the end of the 
air passage, and wherein the angle between the upper wall and the end wall 
of the first swirl port is smaller than the same angle of the second swirl 
port. 
According to the present invention, since the angle between the upper wall 
and the end wall of the helical air passage of the first (upstream) swirl 
port is small, the flow of the air through the passage changes its 
direction suddenly at the end wall. This sudden change in the flow 
direction generates a vortex at the corner where the upper wall and the 
end wall meet. Therefore, intake air is pulled by the vacuum generated by 
the vortex and flows along the upper wall of the passage until It reaches 
the end wall. Therefore, the flow of intake air through the first swirl 
port rotates a large amount before it flows into the cylinder. Thus, the 
intake air flows out from the bottom of the air passage of the first swirl 
port with a large uniform tangential velocity. Namely, intake air flowing 
into the cylinder from the first swirl port has a relatively large 
tangential velocity and a relatively small velocity in the vertical 
direction (i.e., the direction parallel to the axis of the helix of the 
helical air passage). Therefore, a strong swirl is generated by the intake 
air flowing through the first swirl port. 
In contrast to the above, the angle between the upper wall and the end wall 
of the air passage of the second (downstream) swirl port is relatively 
large, and the flow of the air changes its direction gradually. Therefore, 
the vacuum generated by the vortex at the corner between the upper wall 
and the end wall is small, and the force pulling the flow of intake toward 
the end of the air passage becomes small in the second swirl port. This 
causes a large amount of intake air flowing through the second swirl port 
to flow out from the bottom before reaching the end of the passage. 
Namely, intake air flowing out from the second swirl port has a relatively 
small tangential velocity since most intake air flows out from the helical 
air passage before it rotates a large amount. 
Further, since the tangential velocity component of the intake air flowing 
out from the second swirl port becomes larger as the amount of rotation of 
the intake air in the helical air passage becomes larger, the tangential 
velocity distribution of intake air around the circumference of the outlet 
of the second swirl port is such that the tangential velocity of air 
becomes larger as the end of the helical air passage is approached. This 
makes the tangential velocity of the intake air flowing into the cylinder 
from the portion remote from the cylinder wall small, and at this portion, 
the intake air from the second swirl port flows into the cylinder in an 
oblique downward direction. Therefore, the swirl in the cylinder is not 
disturbed or weakened by the intake air from the second swirl port. 
Further, since the tangential velocity of intake air from the second swirl 
port becomes larger at the point near the cylinder wall, intake air from 
the second swirl port flows along the curvature of the cylinder wall 
without impinging the cylinder wall.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 shows a plan view of a cylinder head of an internal combustion 
engine to which the intake system of the present invention is applied. In 
FIG. 1, reference numeral 1 designates a cylinder of the engine, 10 and 20 
designate a first and a second intake ports of the cylinder, respectively. 
The intake ports 10 and 20 are formed in the cylinder head 3, and comprise 
air passages 13 and 23, respectively, which connect the openings 31, 32 
disposed on an end surface of the cylinder head 3 to helical air passages 
11 and 21 of the intake ports. Bottoms of the helical air passages 11 and 
21 open to an inner space of the cylinder. 
Although not shown in the drawing, intake valves which open and close the 
bottoms of the helical air passages are disposed at the intake ports 10 
and 20. The stems of the intake valves extend along center axes of the 
helical air passages 11 and 12 and are connected to a valve driving 
mechanism. Further, two exhaust valves (not shown in the drawing) are 
disposed on the cylinder at the positions symmetrical to the intake valves 
with respect to the center of the cylinder. 
In this embodiment, the helical air passages 11 and 12 have clockwise turns 
as shown in FIG. 1, and the intake air flowing through the passages 11 and 
12 form a clockwise swirl in the cylinder as indicated by S in FIG. 1. 
Therefore, with respect to the direction of swirl S, the second intake 
port 20 is located downstream of the first intake port 10. 
FIG. 2 is a perspective view showing the shapes of the intake ports 10 and 
20. In this embodiment, intake air flows into the intake ports 10 and 20 
from intake pipes (not shown) connected to the openings 31 and 32, and 
flows through the air passages 13 and 23 and into the helical air passages 
11 and 21. In the helical air passages 11 and 21, the intake air rotates 
in a clockwise direction and flows into the cylinder 1 from the bottom 
opening of the helical air passages 11 and 21. Therefore, the intake air 
flowing into the cylinder 1 from the bottoms of the helical air passages 
11 and 21 has velocity components in directions tangential to the 
circumferences of the bottoms of the helical air passages 11 and 21. This 
causes the intake air flowing out from the intake ports 10 and 20 to 
rotate along the cylinder wall and, thereby, a strong swirl of intake air 
(as indicated by S in FIG. 1) is formed in the cylinder. 
In FIGS. 1 and 2, numerals 15 and 25 designate the end portions of helical 
air passages 11 and 21, respectively. In this embodiment, the helical air 
passages 11 and 21 are disposed in such a manner that the angle between 
the line connecting the center of helical air passages 11 and 21 and the 
end portion 25 of the helical air passage 21 of the second intake port 20 
(the angle .alpha..sub.2 in FIG. 1) is smaller than the same (angle 
.alpha..sub.1) of the first intake port 10 in order to avoid interference 
between the cylinder wall and the end portion 25 of the helical air 
passage 21. 
FIGS. 3A and 3B schematically illustrate the sections of the end portions 
15 and 25 of the helical air passages 11 and 21 taken along the stream 
lines (the arrows in FIGS. 3A and 3B) of the intake air through the 
helical air passages 11 and 21. As can be seen from FIGS. 3A and 3B, the 
height h.sub.1 of the section of the end portion 15 of the helical air 
passage 11 is larger than the height h.sub.2 of the section of the end 
portion 25 of the helical air passage 21. 
Further, as can be seen from FIG. 3A, the angle .theta..sub.1 between the 
upper wall 15a and the end wall 15b (which define the end of the passage 
11) is relatively small, i.e., the end wall 15b is formed so that the 
inclination thereof is relatively large (steep). In contrast to this, the 
angle .theta..sub.2 between the upper wall 25a and the end wall 25b of the 
helical air passage 21 is relatively large, i.e., the end wall 25b is 
formed so that the inclination thereof is relatively small. 
FIGS. 4A and 4B show the difference in the flow of intake air in the 
passages 11 and 21 due to the difference in the angles .theta..sub.1 and 
.theta..sub.2 in this embodiment. FIG. 4A shows the flow of intake air in 
the first intake port 10. Since the angle .theta..sub.1 between the upper 
wall 15a and the end wall 15b of the end portion is relatively small and 
the inclination of the end wall 15b is large, the flow in the helical air 
passage 11 of the first intake port 10 suddenly changes its direction in 
order to proceed along the end wall 15b. This sudden change in the flow 
direction causes the separation of the flow at the corner between the 
upper wall 15a and the end wall 15b (the portion indicated by A in FIG. 
4A) and a vortex is generated by this separation of the flow. Since a 
negative pressure is generated in the portion A by the vortex, the flow of 
the intake air in the passage 11 is attracted by the negative pressure, 
and proceeds along the upper wall 15a until it reaches the corner portion 
A. Therefore, the flow of the intake air rotates a large amount in the 
passage 11 in the first intake port 10 before it flows into the cylinder 
1. Thus, as can be seen from FIG. 4A, the intake air flows out from the 
bottom of the helical air passage 11 with a relatively large tangential 
velocity substantially uniform over the entire circumference of the bottom 
of the passage 11. 
On the contrary, the angle .theta..sub.2 between the upper wall 25a and the 
end wall 25b of the helical air passage 21 of the second intake port 20 is 
larger than the angle .theta..sub.1 in the helical air passage 11 of the 
first intake port 10. Therefore, the flow of intake air changes its 
direction gradually to proceed along the end wall 25b at the end portion 
25 of the passage 21. Since the flow direction gradually changes, no 
vortex, i.e., no negative pressure is generated at the corner B between 
the upper wall 25a and the end wall 25b in the passage 21. Therefore, 
since no negative pressure for attracting the flow of intake air to the 
corner B of the end portion 25, the intake air in the helical air passage 
21 flows out from the bottom of the passage before it rotates a large 
amount. Thus, the air flowing into the cylinder from the second intake 
port 20 has a relatively large downward velocity and a relatively small 
tangential velocity as can be seen from FIG. 4B. 
Namely, intake air from the first intake port 10 flows into the cylinder 1 
in an almost horizontal direction as shown in FIG. 4A while intake air 
from the second intake port 20 flows into the cylinder 1 in an oblique 
downward direction as shown in FIG. 4B. 
FIG. 5 schematically illustrates the distribution of the tangential 
velocity of inlet air flowing into the cylinder 1 around the intake ports 
10 and 20. By setting the angle .theta..sub.1 at a relatively small value, 
intake air flowing into the cylinder 1 from the first intake port 10 has a 
large, uniform tangential velocity distribution around the circumference 
of the outlet of the port 10. This large, uniform tangential velocity 
distribution generates a strong swirl S in the cylinder 1, i.e., by 
setting the angle .theta..sub.1 at a relatively small value, the swirl of 
the intake air in the cylinder is strengthened. 
On the other hand, by setting the angle .theta..sub.2 at a relatively large 
value, intake air from the second intake port 20 flows into the cylinder 
11 before it rotates by a large amount in the helical air passage 21 
(i.e., before the tangential velocity of the intake air becomes large). 
Therefore, the intake air from the second intake port 20 flows into the 
cylinder in an oblique downward direction. Further, as can be seen from 
the velocity distribution in FIG. 5, the tangential velocity of intake air 
around the outlet of the second intake port 20 increases in the clockwise 
direction and becomes a maximum at the portion near the cylinder wall. Due 
to this relatively large tangential velocity, intake air flowing out from 
the portion of the outlet of the second intake port 20 near the cylinder 
wall increases the swirl S caused by the intake air from the first intake 
port 10. 
Further, since the tangential velocity of intake air flowing out from the 
part of the outlet of the second intake port 20 remote from the cylinder 
is relatively small, intake air from this part of the second intake port 
20 flows into the cylinder in the downward direction without interfering 
with the swirl S in the cylinder. Since the air flowing into the cylinder 
from the second intake port 20 does not impinge on the cylinder wall or 
interfere with the swirl in the cylinder, the flow resistance of the 
second intake port 20 becomes small in this embodiment. Namely, the amount 
of intake air through the second intake port 20 is increased, in this 
embodiment, by decreasing the flow resistance by setting the angle 
.theta..sub.2 at a large value. 
For example, in a diesel engine, since a throttle valve is not provided, 
the performance of the engine is largely affected by the condition of the 
swirl in the cylinder. Therefore, when a diesel engine is operated at a 
low speed, it is required to improve the combustion in the cylinder by 
generating a strong swirl of intake air in the cylinder. On the contrary, 
when the diesel engine is operated at a high speed and a high load, it is 
necessary to supply a sufficient amount of intake air to the cylinder to 
prevent the excess air ratio of the cylinder from becoming excessively 
low. 
When the intake system of the present invention is applied to a diesel 
engine, a strong swirl is formed by the intake air from the first intake 
port even when the engine is operated at a low speed. Therefore, improved 
combustion is obtained even in the low speed operation of the diesel 
engine. Further, since the flow resistance of the second intake port is 
low, a sufficient amount of intake air is supplied through the second 
intake port even when the engine is operated at a high speed and a high 
load. Therefore, the performance of the diesel engine can be improved over 
a wide operating speed range according to the present invention.