Intake manifold structure for internal combustion engines

An improved intake manifold structure for internal combustion engines includes a distribution chamber having an upper sub-chamber leading to a carburetor and a lower sub-chamber communicating with the upper sub-chamber through a communication hole. A plurality of branch passages extend from the lower sub-chamber to a plurality of combustion chambers. An air-fuel mixture fed from the carburetor to the distribution chamber is expanded successively in two steps to promote its atomization as it passes through the two sub-chambers, thus improving uniform distribution of the mixture to the respective branch passages. Engine exhaust gas is returned to the upper sub-chamber to further promote the atomization of the mixture.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an intake manifold structure for an 
internal combustion engine, which structure is equipped with both a 
distribution chamber for receiving an air-fuel mixture provided by a 
carburetor, as well as a plurality of branch passages which extend from 
that distribution chamber for distributing that mixture into a plurality 
of combustion chambers of an internal combustion engine. 
2. Description of the Prior Art 
The intake manifold structure according to the prior art has a single 
distribution chamber having an insufficient capacity, and consequently 
atomization of the fuel in the mixture is so poor as to allow the fuel to 
flow in the form of large fuel droplets into the combustion chambers such 
prior art construction makes it remarkably difficult to effect the uniform 
distribution of the mixture because of the interferences in intake air 
among the respective combustion chambers. Especially, in case a compound 
carburetor having primary and secondary bores is used, the uniform 
distribution of the mixture into the respective combustion chambers is 
made further difficult additionally because of the differences among the 
distances from the primary and secondary bores to the respective branch 
passages. 
SUMMARY OF THE INVENTION 
It is, therefore, an object of the present invention to provide an intake 
manifold structure of the aforementioned kind, in which the main 
distribution chamber is constructed of upper and lower sub-chambers 
communicating with each other through a communication passage so that an 
air- fuel mixture from a carburetor may be expanded, when it flows through 
the two sub-chambers, to promote its atomization thereby to eliminate the 
defects concomitant with the prior art. 
Another object of the present invention is to improve the distribution of 
the mixture into the respective, branch passages as well as to effectively 
promote the atomization of the mixture by introducing engine exhaust gas 
into the upper sub-chamber through an exhaust recirculation passage, 
thereby to warm the mixture. 
If, in this case, the temperature of the exhaust gas to be recirculated 
into the main distribution chamber is excessively high, the fuel wetting 
the inner wall of the main distribution chamber is undesirably carbonized 
when it is contacted by that hot exhaust gas. 
Therefore, still another object of the present invention is to make it 
possible to effectively lower the temperature of the recirculated exhaust 
gas by making the exhaust recirculation passage so long as to underlie the 
bottom wall of the distribution chamber. 
A primary auxiliary distribution chamber, which is to be fed with the rich 
mixture from the auxiliary carburetor, is positioned at one longitudinal 
side of the main distribution chamber, which is to be fed with the lean 
mixture from the main carburetor. A pair of secondary auxiliary 
distribution chambers, which communicate with the primary auxiliary 
distribution chamber, are positioned at both the right and left sides of 
the main distribution chamber. In this way the rich mixture may be 
distributed from the respective secondary auxiliary distribution chambers 
into the auxiliary combustion chambers of the same cylinder bank. 
Especially in the case of a V-type multi-cylinder internal combustion 
engine having two banks of cylinders arranged in the shape of a letter 
"V", the uniform distribution of the rich mixture into the respective 
auxiliary combustion chambers is hindered and made more difficult by the 
distribution passages of the lean mixture. 
Therefore, a further object of the present invention is to provide an 
intake manifold structure for an internal combustion engine, which is able 
to overcome the aforementioned difficulty. 
According to the present invention, the difficulty can be overcome by 
arranging both a primary auxiliary distribution chamber and a pair of 
secondary auxiliary distribution chambers the latter being fed with the 
rich mixture from the auxiliary carburetor, at one longitudinal side of 
the main distribution chamber, which is to be fed with the lean mixture 
from the main carburetor. The pair of secondary auxiliary distribution 
chambers communicate with the primary auxiliary distribution chamber at 
both the right and left sides of the main distribution chamber so that the 
rich mixture may be distributed from the respective secondary auxiliary 
distribution chambers into the auxiliary combustion chambers of the same 
side cylinder row. 
An attendant object of the present invention is to provide an intake 
manifold structure of the aforementioned kind for an internal combustion 
engine, which is able to effectively promote the atomization of the 
mixture in the intake manifold by warming the intake manifold with the use 
of the warm coolant after it has cooled down the engine, which warm 
coolant is readily available. 
The above and other objects, features and advantages of the present 
invention will become apparent from the following detailed description of 
a few preferred embodiments of the present invention when taken in 
conjunction with the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The present invention will now be described in connection with one 
embodiment thereof with reference to the accompanying drawings. In FIG. 1, 
reference letter E indicates a V-type six-cylinder internal combustion 
engine which has two banks of left and right cylinders C.sub.1 and C.sub.2 
arranged in the shape of a letter "V". The cylinder block 1 of that engine 
has its upper surface formed into a horizontal surface 1a at its center 
portion and into roof-shaped inclined surfaces 1b.sub.1 and 1b.sub.2 at 
its left and right side portions, respectively. The cylinder banks C.sub.1 
and C.sub.2 have their respective three cylinders 2 opened at their upper 
ends into those inclined surfaces 1b.sub.1 and 1b.sub.2 and their 
respective cylinder heads 3 jointed to the same surfaces 1b.sub.1 and 
1b.sub.2, respectively. Also, an intake manifold M is jointed to the 
horizontal surface 1a such that both its right and left sides are jointed 
to the inner surfaces of the right and left cylinder heads 3, 
respectively. Moreover, a carburetor Ca is mounted on the upper surface of 
the intake manifold M. 
To a common crankshaft 4 which is mounted on the lower surface of the 
cylinder block 1, there are connected through connecting rods 6, 
respectively, pistons 5 which are made operative to slide up and down in 
the respective cylinders 2. 
Each of the cylinder heads 3 is formed with a main combustion chamber 7, 
which is defined by the corresponding piston 5, an auxiliary combustion 
chamber 8 which has communication with said chamber 7 through a torch 
nozzle 9, a main intake port 10 and an exhaust port 14 which are 
respectively opened into the main combustion chamber 7, and an auxiliary 
intake port 11 which is opened into the auxiliary combustion chamber 8. 
The main intake port 10, the auxiliary intake port 11 and the exhaust port 
14 are opened and closed by means of a main intake valve 12, an auxiliary 
intake valve 13 and an exhaust valve 15, respectively. An ignition plug 
16, which is threaded in the cylinder head 3, has its electrode facing the 
corresponding auxiliary combustion chamber 8. 
The aforementioned carburetor Ca is enabled to simultaneously supply a lean 
main mixture, and an auxiliary rich mixture. Of these, the carburetor 
portion for supplying the main mixture is of the compound type, having 
primary and secondary bores. Moreover, the aforementioned main and 
auxiliary mixtures are distributed through the intake manifold into the 
main and auxiliary intake ports 10 and 11, respectively. 
Thus, in each cylinder 2, when the main and auxiliary intake valves 12 and 
13 are opened during the suction stroke of the corresponding piston 5, the 
main mixture is drawn through the main intake port 10 to the main 
combustion chamber 7, whereas the auxiliary mixture is drawn through the 
auxiliary intake port 11 to the auxiliary combustion chamber 8. Then, near 
the end of the subsequent compression stroke, the rich mixture in the 
auxiliary combustion chamber 8 is ignited by the ignition plug 16, and the 
resultant torch flame propagates through the torch nozzle 9 into the main 
combustion chamber 7 thereby to ignite and burn the lean mixture in said 
chamber 7. As a result, the lean mixture having an overall high air-fuel 
ratio can be burned. Near the end of the expansion stroke of the piston 5 
the exhaust valve 15 is opened for the subsequent exhaust stroke, the 
exhaust gasses passing through the exhaust port 14 and further to one of 
the exhaust pipes 17. 
Next, the following description is directed to the passages of the 
aforementioned main and auxiliary mixtures through the intake manifold M. 
FIG. 2 is a top plan view of the intake manifold M, in which the upper 
portion is located at the lefthand side of the engine E, i.e., the side of 
the cylinder bank C.sub.1 whereas the lower portion is located at the 
righthand side of the same, i.e., at the side of the cylinder bank C.sub.2 
and in which the lefthand portion is located in front of the engine E 
whereas the righthand portion is located at the rear of the same. 
In the upper end surface of that intake manifold M, i.e., a carburetor 
mounting surface 18 thereof, there are opened primary and secondary main 
inlets 19 and 19', which communicate with the primary and secondary bores 
for supplying the main lean mixture from the aforementioned carburetor Ca, 
respectively, and auxiliary inlet 20, which communicates with the 
auxiliary bore for supplying the auxiliary rich mixture from the same 
carburetor Ca, such that the primary and secondary inlets 19 and 19' and 
the auxiliary inlet 20 are arranged in the longitudinal direction of the 
engine E, i.e., in the axial direction of the crankshaft 4. Just below 
both the main inlets 19 and 19', there is disposed a plenum distribution 
chamber 21 which communicates therewith. Just below the auxiliary inlet 
20, on the other hand, there is disposed a primary auxiliary distribution 
chamber 22 which communicates therewith. The main distribution chamber 21 
is composed of an upper sub-chamber 21a and a lower sub-chamber 21b having 
a larger capacity than that of the sub-chamber 21a. From each of the two 
side walls of that lower sub-chamber 21b, there extend three main branch 
passages 24 which lead to the main intake ports 10 of the corresponding 
one of the cylinder banks C.sub.1 and C.sub.2, respectively. A partition 
21c partitioning the upper and lower sub-chambers 21a and 21b of the main 
distribution chamber 21 is formed with a communication passage 23 which 
provides communication between the two sub-chambers 21a and 21b. 
Thus, the main mixture supplied by the carburetor Ca flows from the main 
inlets 19 or 19' into the upper sub-chamber 21a of the main distribution 
chamber 21 and then through the communication passage 23 into the lower 
sub-chamber 21b. The lean mixture is then distributed into the plural main 
branch passages 24 until it is drawn into the respective main intake ports 
10, as has been described hereinbefore. Since the main mixture introduced 
into the main distribution chamber 21 flows in that way through the two 
upper and lower sub-chambers 21a and 21b, the atomization of the fuel in 
the mixture is highly promoted by the respective expanding actions and by 
the warming operation of the chamber walls. At the same time, moreover, 
the pulsations of the intake air in the respective combustion chambers are 
attenuated by the two upper and lower sub-chambers 21a and 21b so that the 
interference in the intake air among the respective combustion chambers 
can be remarkably reduced. 
In the description thus far made, the effective cross-sectional area of the 
communication passage 23 is made larger than the sum of the effective 
cross-sectional areas of the primary and secondary main inlets 19 and 19'. 
As a result, the communication passage 23 raises little resistance to the 
intake air even during the high speed running operation in which the 
engine is fed with the lean mixture from both the primary and secondary 
bores of the carburetor Ca. Moreover, the communication passage 23 has its 
lower end formed with an annular protruding edge 23a which protrudes into 
the lower sub-chamber 21b. The annular protruding edge 23a functions to 
blow away any liquid fuel, which flows down on the circumferential wall of 
the communication passage 23, with the sucking action promoting the 
atomization of that liquid fuel and to guide the main mixture, which flow 
from the upper sub-chamber 21a to the lower sub-chamber 21b, in a manner 
to impinge upon the bottom wall of the lower sub-chamber 21b. That bottom 
wall is heated by a later-described water jacket Jm, thereby to further 
promote the atomization of that fuel. As is shown in FIGS. 1, 2 and 8, 
moreover, the communication passage 23 is arranged generally coaxially 
with the primary main inlet 19. As a result, since, in this case, the flow 
resistance between the primary bore of the carburetor Ca leading to the 
primary main inlet 19 and the communication passage 23 is low, especially 
the light load operation of the engine can be improved. It will be 
apparent in view of FIG. 11 that the high speed operation of the engine 
can be improved if the communication passage 23 is arranged generally 
coaxially with the secondary main inlet 19'. Moreover, the lower 
sub-chamber 21b of the main distribution chamber 21 is disposed at the 
center portion between both the left and right cylinder banks C.sub.1 and 
C.sub.2, so that the main branch passages 24 leading from the lower 
sub-chamber 21b can be made to have an equal length for the left cylinder 
bank C.sub.1 and for the right cylinder bank C.sub.2, thereby to achieve 
uniform distribution of the mixture between both the cylinder rows C.sub.1 
and C.sub.2. 
At the rear of and adjacent to the upper sub-chamber 21a of the main 
distribution chamber 21, there is arranged the primary auxiliary 
distribution chamber 22 having two right and left side walls, from which 
two primary auxiliary branch passages 25 extend. These branch passages 25 
communicate with a pair of secondary auxiliary distribution chambers 22' 
which are arranged adjacent to both the right and left sides of the upper 
sub-chamber 21a of the main distribution chamber 21. From each of the 
outer walls of the respective secondary auxiliary distribution chambers 
22', there extend three secondary auxiliary branch passages 25' which lead 
to the auxiliary intake ports 11 of each of the cylinder banks C.sub.1 and 
C.sub.2. 
As a result, the rich auxiliary mixture furnished by the carburetor Ca 
flows from the auxiliary inlet 20 into the primary auxiliary distribution 
chamber 22 and is distributed from said chamber 22 through the two primary 
auxiliary branch passages 25. The rich auxiliary mixture then flows into 
the left and right secondary auxiliary distribution chambers 22' and then 
it is further distributed into the plural secondary auxiliary branch 
passages 25' until it is drawn into the respective auxiliary intake ports 
11, as has been described hereinbefore. 
Reverting to FIGS. 1 and 2, the exhaust pipe 17 is formed with an exhaust 
outlet 26 at one side, and the intake manifold M is formed with an exhaust 
inlet 27 at its rear surface. The outlet 26 and inlet 27 are connected 
through an exhaust recirculation pipe 28. 
As shown in FIGS. 2, 4, 5, 6, 7 and 8, the intake manifold M is formed in 
its wall with an exhaust recirculation passage 30 which provides 
communication between the exhaust inlet 27 and an exhaust outlet hole 29 
opened into the front wall of the upper sub-chamber 21a of the 
aforementioned main distribution chamber 21. As a result, the exhaust 
recirculation passage 30 has its upstream end terminating at the exhaust 
inlet 27 and its downstream end terminating at the exhaust outlet hole 29. 
The exhaust recirculation passage 30 is composed of an intermediate 
portion 30b, which longitudinally crosses in a horizontal direction just 
below the main distribution chamber 21, an upstream portion 30a, which 
descends from the exhaust inlet 27 toward the rear end of that 
intermediate portion 30b, and a downstream portion 30c which ascends from 
the front end of the intermediate portion 30b toward the exhaust outlet 
hole 29. That upstream portion 30a has a midway opening at 31 and 31' in 
the upper surface of the intake manifold M. An exhaust recirculation 
control valve 32 is so mounted in the intake manifold M as to connect 
those openings 31 and 31'. As shown in FIGS. 2 and 8, a baffle plate 33 
facing the exhaust outlet hole 29 is disposed to rise in the upper 
sub-chamber 21a of the main distribution chamber 21. 
Thus, during the operation of the engine E, a portion of the exhaust gas 
flowing through the exhaust pipe 17 flows from the exhaust outlet 26 
through the exhaust recirculation pipe 28 and further through the exhaust 
inlet 27 into the exhaust recirculation passage 30, and is controlled to 
such a flow rate by the action of the control valve 32 as is suitable for 
the operating state of the engine until it flows from the exhaust outlet 
hole 29 into the upper sub-chamber 21a of the main distribution chamber 
21. The exhaust gas thus having passed into the upper sub-chamber 21a 
instantly impinges upon the baffle plate 33 so that it is separated to the 
right and left and mixed into the main mixture flowing through the main 
distribution chamber 21. The exhaust gas as thus further mixed flows into 
the lower sub-chamber 21b thereby to promote the extent of mixing with the 
main mixture. Since, in the meanwhile, the exhaust gas is still at a 
higher temperature than the main mixture, it directly warms the main 
mixture, thereby promoting the atomization thereof. Thus, the exhaust gas 
is distributed together with the lean main mixture into the respective 
main intake ports 10 through the main branch passages 24 until it is 
recirculated to the main combustion chambers 7. The exhaust gas as thus 
recirculated depresses the excessive rise of the combustion temperature of 
the mixture, while this mixture is being burned, thereby playing a role to 
decrease the emission of nitrogen oxides. 
The engine E and the intake manifold M are equipped with water jackets Je 
and Jm for warming their respective mixtures, and the coolant circuits of 
these water jackets will now be descirbed with reference to FIG. 10. 
Into the main coolant passage 34 leading out of the outlet Ro of a radiator 
R and returning to the inlet Ri of the same, there are incorporated in the 
flow direction from the upstream a coolant pump P, the water jacket Je of 
the internal combustion engine E and the water jacket Jm of the intake 
manifold M, all of which are sequentially connected in series. The coolant 
pump P is mechanically driven by the engine E to pump the coolant out of 
the outlet Ro of the radiator R and to pump the same into the water jacket 
Je. To the outlet Jmo of the water jacket Jm of the intake manifold M, 
there is attached a thermostat T which is made operative to be opened when 
the temperature in the water jacket Jm exceeds a predetermined level. 
From the water jacket Jm of the intake manifold M, there extend first and 
second bypass coolant passages 35.sub.1 and 35.sub.2 which are connected 
to the main coolant passage 34 between the outlet Ro of the radiator R and 
the coolant pump P. A warming heat exchanger H for the interior of the 
automobile is interposed in the second bypass coolant passage 35.sub.2. 
To the intake manifold M, moreover, there are attached both a temperature 
sensitive switch Sf, which is made operative to operate the cooling 
electric fan F of the radiator R when it senses that the coolant 
temperature in the water jacket Jm of the intake manifold M rises to a 
higher level than a predetermined value, and a temperature sensor S which 
operates a heat indicator (not shown) in response to the change in the 
same coolant temperature. Incidentally, reference letters Jei, Jeo and Jmi 
appearing in the accompanying drawings indicate the inlet and outlet of 
the water jacket Je and the inlet of the water jacket Jm, respectively. 
Thus, when the engine E operates at a low temperature, the thermostat T 
closes to shut off the outlet Jmo of the water jacket Jm of the intake 
manifold M. As a result, the coolant pumped out by the pump P is first fed 
to the water jacket Je of the engine E and then to the water jacket Jm of 
the intake manifold M. After that, the coolant is shunted to the first and 
second bypass coolant passages 35.sub.1 and 35.sub.2 so that it bypasses 
the radiator R and merges at the main coolant passage 34 downstream of the 
radiator R until it passes into the pump P. The circulation thus far 
described is repeated. As a result, the coolant in the main coolant 
passage 34 does not pass through the radiator R thereby to have little 
chance of heat liberation so that its temperature can be promptly raised 
in accordance with the heat generation of the engine E. This promotes the 
warming-up of the engine E and the temperature rise in the intake manifold 
M. Then, if the temperature in the water jacket Jm exceeds the 
predetermined level so that the thermostat T is opened, the coolant having 
passed through the water jacket Jm mostly leaves the outlet Jmo having 
little flow resistance until it enters the inlet Ri of the radiator R so 
that it liberates its heat while passing through the radiator R. The 
remaining portion of the coolant takes the course to the first and second 
bypass passages 35.sub.1 and 35.sub.2, as has been described hereinbefore. 
Moreover, if the temperature in the water jacket Jm is raised so that the 
temperature sensitive switch Sf is closed, the electric fan F operates to 
promote the heat liberation in the radiator R. Thus, the coolant pumped 
out of the pump P wholly passes sequentially through the water jackets Je 
and Jm at all times thereby to control the engine E and the intake 
manifold M to proper temperature levels. 
Next, the constructions of the water jackets Je and Jm are described as 
follows: 
First of all, the water jacket Je of the engine E is constructed, as shown 
in FIG. 1, of a lower jacket 36, which is so formed in the cylinder block 
1 as to enclose the cylinders 2 in each of the cylinder banks C.sub.1 and 
C.sub.2, and an upper jacket 37 which is formed in each cylinder head 3. 
The upper jacket 37 is made to communicate with the lower jacket 36 
through a communication hole 38, which extends through the joint surfaces 
of the cylinder block 1 and the cylinder heads 3, and is composed of a 
downstream portion 37b, which encloses the main and auxiliary intake ports 
10 and 11, and an upstream portion 37a which encloses the exhaust ports 14 
and the ignition plugs 16, etc. Although not shown in FIG. 1, the 
aforementioned inlet Jei of the water jacket Je is disposed at a lower 
portion of the lower jacket 36. The upper jacket 37 is made to communicate 
with the outlet Jeo, which is opened in the horizontal surface 1a of the 
cylinder block 1, through a passage 39 which returns therefrom to an upper 
portion of the cylinder block 1. Moreover, that outlet Jeo directly 
communicates with that inlet Jmi of the water jacket Jm, which is opened 
in the lower surface of the intake manifold M. As a result, the coolant 
pumped out of the pump P first enters the lower jacket 36 thereby to cool 
down the surroundings of the cylinders 2. After that, the coolant flows 
through the communication hole 28 into the upper jacket 37, in which it 
flows through the upstream portion 37a thereof to cool down the 
surroundings of the exhaust valves 15 and the ignition plugs 16, and then 
into the downstream portion 37b thereof to warm up the surroundings of the 
main and auxiliary intake ports 10 and 11. After that, the coolant 
sequentially flows through the passage 39 and the outlet and inlet Jeo and 
Jmi until it flows into the water jacket Jm of the intake manifold M. The 
water jacket Jm of the intake manifold M is positioned above the upper 
jacket 37 in the cylinder heads 3 so that any bubble is instantly 
introduced, even if it is generated in said upper jacket 37, into the 
water jacket Jm, whereby it is prevented from remaining in that upper 
jacket 37. 
More specifically, as shown in FIGS. 2 and 3, the outlet Jeo of the water 
jacket Je and the inlet Jmi of the water jacket Jm are respectively formed 
to have flattened cross-sections such that three of them at the side of 
the lefthand cylinder bank C.sub.1 are arranged at the lefthand side of 
the respective joint surfaces between the cylinder block 1 and the intake 
manifold M, whereas three of them at the side of the righthand cylinder 
bank C.sub.1 are arranged at the righthand side of the respective joint 
surfaces of the same. 
The water jacket Jm of the intake manifold M is composed, as shown in FIGS. 
3 and 5, of a pair of right and left side jackets 40, which extend in the 
longitudinal direction while interposing the lower sub-chamber 21b of the 
main distribution chamber 21 in between. A pair of lower jackets 41 which 
also extend in the longitudinal direction just below the main distribution 
chamber 21, interpose the exhaust recirculation passage 30 in between and 
which has communication with the corresponding side jackets 40 through a 
communication hole 43. A collecting jacket 42 (FIGS. 4 and 6) is disposed 
at the rear of the main distribution chamber 21 in a manner to communicate 
with all of the right and left, and side and lower jackets 40 and 41. The 
aforementioned inlet Jmi is opened in the lower surface of each of the 
upper jackets 40. On the other hand, the side jackets 40 at each of the 
right and left sides are made, as shown in FIG. 7, to communicate with 
each other around the main branch passages 24 through the upper jacket 44. 
The collecting jacket 42 is formed, as shown in FIG. 8, with an annular 
jacket 42a which encloses the upstream portion 30a of the exhaust 
recirculation passage 30 and which extends adjacent to the bottom wall of 
the primary auxiliary distribution chamber 22 and the side wall of the 
lower sub-chamber 21b of the main distribution chamber 21. As shown in 
FIGS. 4 and 8, the aforementioned outlet Jmo is formed in an upper portion 
of the collecting jacket 42, and the aforementioned thermostat T is 
mounted on the outlet Jmo. 
Turning to FIG. 4, the aforementioned temperature sensitive switch Sf and 
coolant temperature sensor S are so mounted in mounting holes 45 and 46, 
respectively, which are formed in the rear end surface of the intake 
manifold M, that they can sense the coolant temperature in the 
aforementioned collecting jacket 42. From the same rear end surface, there 
protrude connecting pipes 47.sub.1 and 47.sub.2 which provide connections 
to the respective upstream ends of the aforementioned first and second 
bypass passages 35.sub.1 and 35.sub.2. Incidentally, numeral 48 indicates 
an air bleeder mounting bore. 
Thus, the hot coolant flows into the right and left side jackets 40 and the 
upper jackets 44, after it has cooled down the engine E and passed into 
the respective inlets Jmi of the water jacket Jm of the intake manifold M, 
and further flows through the communication hole 43 into the lower jacket 
41 at the same side. The hot coolant streams thus having entered the 
respective jackets 40, 44 and 41 respectively flow into the collecting 
jacket 42, during which they warm up both the upper and lower sub-chambers 
21a,b of the main distribution chamber 21, the primary and secondary 
auxiliary distribution chamber 22, 22' and the main branch passages 24, 
thereby to promote the atomization of the mixtures flowing therethrough. 
The streams cool down the intermediate portion 30b of the exhaust 
recirculation passage 30 thereby to lower the temperature of the exhaust 
gas flowing therethrough. At this time, they warm up both the lower 
sub-chamber 21b of the main distribution chamber 21 and the lower wall of 
the primary auxiliary distribution chamber 22 through the annular jacket 
42a, while cooling down the upstream portion 30a of the exhaust 
recirculation passage 30, to promote the atomization of the main and 
auxiliary mixtures flowing therethrough. They cool down the upstream 
portion 30a of the exhaust recirculation passage 30 thereby to lower the 
temperature of the exhaust gas flowing therethrough. The main and 
auxiliary mixtures thus having their atomization promoted are drawn into 
the main and auxiliary combustion chambers 7 and 8 so that they can be 
burned to a satisfactory extent. On the contrary, the exhaust gas having 
its temperature properly dropped will not carbonize the fuel which wets 
the respective portions of said chamber 21, when the exhaust gas is fed to 
the main distribution chamber 21. 
Incidentally, the present invention can be applied not only to the torch 
ignition type internal combustion engine having the auxiliary combustion 
chambers, as has been described hereinbefore, but also to any conventional 
type engine. In this latter case, the aforementioned intake manifold M can 
be modified to dispense with the passages which lead from the auxiliary 
inlets 20 to the auxiliary branch passages 25'. Therefore, the main 
distribution chamber 21 and the main branch passages 24 in the foregoing 
embodiment correspond to the distribution chamber and the branch passages 
of the present invention, respectively. 
In short, the present invention can enjoy the following advantages. In this 
intake manifold structure, the two mixtures from the carburetor can be 
expanded in the two upper and lower sub-chambers of the main distribution 
chamber, respectively, to have their atomizations highly promoted. As a 
result, even if the respective distances from the carburetor to the 
respective branch passages are different, the mixtures can be uniformly 
distributed among the respective combustion chambers, and, at the same 
time, the intake pulsations in the respective combustion chambers can be 
attenuated in the two upper and lower sub-chambers, so that interferences 
in the intake mixtures among the respective combustion chambers can be 
remarkably reduced. 
Moreover, as the exhaust gas to be recirculated first flows into the upper 
sub-chamber of the distribution chamber and then into the lower 
sub-chamber of the same until it is distributed into the respective branch 
passages, it is possible to lengthen the residence time of the exhaust gas 
in the distribution chamber for good mixing the exhaust gas with the 
mixtures. As a result, the mixtures can be directly warmed up by the heat 
of the exhaust gas thereby to have their atomization effectively promoted 
and their uniform distributions among the respective branch passages 
improved. 
Also, the exhaust recirculation passage formed in the intake manifold has a 
total length sufficient to underlie the bottom wall of the distribution 
chamber so that it can properly lower the temperature of the exhaust gas 
flowing through said recirculation passage as to prevent the fuel wetting 
the distribution chamber from being carbonized. As a result, it is 
unnecessary to specially elongate the exhaust recirculation pipe, which is 
arranged around the intake manifold, and to attach radiating fins to the 
outer circumference of the recirculation pipe so that the intake manifold 
structure can be made compact as a whole. 
As the intermediate portion of the exhaust recirculation passage is 
arranged just below the distribution chamber and in the longitudinal 
direction of the engine, i.e., in parallel with the axis of the 
crankshaft, the exhaust recirculation passage can be easily formed without 
being obstructed by the plural branch passages leading out of the 
distribution chamber thereby to improve the functioning of the intake 
manifold. This effect is prominent especially in the case of the intake 
manifold of the V-type internal combustion engine in which the plural 
branch passages lead to the right and left from the distribution chamber. 
As the heating water jacket communicating with the cooling water jacket in 
the cylinder block is formed in the intake manifold and adjacent to the 
distribution chamber and the exhaust recirculation passage, the warming 
operation of the distribution chamber and the cooling operation of the 
exhaust recirculation passage can be simultaneously effected by the 
coolant circulating in the engine. The water jacket is disposed at each of 
the right and left sides of the exhaust recirculation passage so that the 
cooling area of this passage can be enlarged to promote the cooling 
operation of said passage more effectively. 
Furthermore, the V-type multi-cylinder internal combustion engine, in which 
each of the cylinders is equipped with the main and auxiliary combustion 
chambers, is constructed such that the primary auxiliary distribution 
chamber is arranged at one side of the longitudinal direction of the main 
distribution chamber, the paired secondary auxiliary distribution chambers 
are arranged at the opposite sides of the main distribution chamber, and 
the plural secondary auxiliary branch passages are led out of the 
respective secondary auxiliary distribution chambers, therefore the 
auxiliary mixture from the carburetor is first divided at the primary 
auxiliary distribution chamber into two halves for both the cylinder 
banks, and fed to the secondary auxiliary distribution chambers. As a 
result, the distribution passages of the auxiliary mixture are made 
generally bisymmetrical without being obstructed by the main distribution 
chamber so that the auxiliary mixture can be uniformly distributed into 
the auxiliary combustion chambers of the right and left cylinder banks. 
Furthermore, since the secondary auxiliary distribution chambers are 
provided for the respective cylinder rows, the interference in the intake 
air among the auxiliary combustion chambers of the right and left cylinder 
banks can be prevented to exert excellent influences upon the uniform 
distribution of the auxiliary mixture. 
Since a water jacket is disposed close to the bottom wall of the auxiliary 
distribution chamber and/or the auxiliary branch passages, the auxiliary 
mixture can be atomized and uniformly distributed among the respective 
auxiliary combustion chambers. Moreover, as the aforementioned water 
jacket is positioned close to the main distribution chamber or the main 
branch passages, too, the main and auxiliary mixtures can be warmed up by 
the common water jacket so that the intake manifold structure can be made 
compact, without being necessary to construct the water jacket of two main 
and auxiliary systems. 
Furthermore, since the coolant, which has cooled down the cylinder block of 
the engine, is wholly introduced into the heating jacket of the intake 
manifold, the heat of the engine coolant can be fully utilized to warm up 
the intake manifold to effectively promote the atomization of the mixture 
flowing through the intake manifold. 
Since, in this case, the intake manifold is jointed to the upper surface of 
the cylinder block such that the outlet of the cooling water jacket of the 
cylinder block is made to directly communicate with the inlet of the water 
jacket of the intake manifold, the coolant can have its temperature 
maintained while it is flowing from the cylinder block to the intake 
manifold so that its heat can be efficiently utilized to warm up the 
intake manifold. Moreover, since all the cylinder block, the intake 
manifold, the radiator and the coolant pump are connected through a series 
of the circulation passages, it is possible to attain other beneficial 
effects. Thus, the coolant passages can have their constructions so 
remarkably simplied that they can be constructed at low cost. Also the 
flow of the coolant is smoothed to reduce the load upon the coolant pump. 
Furthermore, the thermostat is mounted in the outlet of the heating water 
jacket of the intake manifold, and said jacket and the pump are connected 
through the bypass passage so that the aforementioned thermostat is opened 
and closed in accordance with the coolant temperature in the intake 
manifold. As a result, when the intake manifold is at a low temperature, 
its communicating relationship with the radiator is interrupted so that it 
can be quickly warmed up. At a high temperature, on the contrary, the 
intake manifold restores its communicating relationship with the radiator 
so that it can be held at a proper temperature level thereby to ensure the 
proper mixture charging efficiency of the engine. The distribution chamber 
is so enclosed by the upper and lower water jackets that it can have a 
wide heat receiving area. As a result, the distribution chamber can be 
effectively warmed up with the water jacket having a relatively small 
capacity so that atomization of the mixtures and uniform distributions 
among the respective intake ports can be promoted. 
Since the heating water jacket of the intake manifold is made to 
communicate with the cooling water jacket in the cylinder head by means of 
the communication passage formed in the cylinder block, it is sufficient 
that the joint surfaces between the cylinder heads and the intake manifold 
are so formed that the intake ports extending therethrough are connected 
in air-tight fashion. As a result, the joint surfaces need not be 
constructed for full air-tightness, and so as to be formed more easily 
than the case in which the communication passage is formed between the 
water jackets of the intake manifold and the cylinder head.