Patent Publication Number: US-10323565-B2

Title: Cooling structure for internal combustion engine

Description:
TECHNICAL FIELD 
     The present invention relates to a cooling structure for an internal combustion engine. 
     BACKGROUND ART 
     A well-known cooling structure for an internal combustion engine includes a radiator-routing passage by way of a radiator and a bypass passage that bypasses the radiator. The radiator-routing passage and the bypass passage form a coolant circulation path through which coolant is circulated by a water pump through water jackets in a cylinder portion and a cylinder head portion of the engine. The cooling structure further includes a thermostat valve that changes over between the circulation through the radiator-routing passage and the circulation through the bypass passage (see, for example, Patent Document 1). 
     PRIOR ART DOCUMENT 
     Patent Document 
     [Patent Document 1] JP 2007-262928 A 
     The cooling structure for an internal combustion engine disclosed in Patent Document 1 includes a wax-type, bottom bypass thermostat valve. The thermostat valve includes a first valve that opens and closes the radiator-routing passage, connected with a second valve (bottom valve) that opens and closes the bypass passage. The first valve and the second valve are integrally operable such that the second valve closes when the first valve opens, and vice versa. 
     When the engine is started, the first valve is closed and the second valve is opened, so that the coolant is circulated through the bypass passage to the water jackets in the cylinder portion and the cylinder head portion without passing through the radiator and warmup of the engine is thereby expedited. When coolant temperature is equal to or becomes higher than a predetermined temperature, the second valve is closed and the first valve is opened, so that the coolant passes through the radiator and the coolant thereby cooled is circulated through the water jackets in the cylinder portion and the cylinder head portion. The internal combustion engine can thereby be cooled. 
     SUMMARY OF THE INVENTION 
     Problems to be Solved by the Invention 
     In the cooling structure disclosed in Patent Document 1, the bypass passage that is opened and closed by the second valve of the thermostat valve is routed outside the engine main unit as a bypass hose connecting the thermostat valve with a water pump. 
     Not limited to the cooling structure disclosed in Patent Document 1, traditional cooling structures for internal combustion engines typically include a bypass passage as an external pipe. 
     Thus, the coolant that passes through the bypass passage during the engine start dissipates heat because of the external pipe being exposed to outside air, and this has been a hindrance to engine temperature increase by the warming-up operation of the engine during the engine start. 
     Additionally, because the bypass passage is routed outside the engine main unit, the structure tends to be complicated due to the increased number of parts used, so that outer appearance of the engine main unit is degraded because of complication of the area therearound. 
     The present invention has been made in view of the foregoing situations and it is an object of the present invention to provide a cooling structure for an internal combustion engine, capable of expediting warming-up operation during the engine start and achieving favorable outer appearance through a simplified structure. 
     Means for Solving the Problems 
     To achieve the foregoing object, the present invention provides a cooling structure for an internal combustion engine comprising: an engine main unit including a crankcase portion, a cylinder portion, and a cylinder head portion, the cylinder portion and the cylinder head portion having therein a cylinder coolant jacket and a cylinder head coolant jacket, respectively; a coolant pump for circulating coolant through a coolant circulation path formed in the cylinder coolant jacket and the cylinder head coolant jacket, the coolant circulation path including a radiator-routing passage by way of a radiator and a bypass passage bypassing the radiator; and a thermostat valve for changing over between coolant circulation through the radiator-routing passage and coolant circulation through the bypass passage; 
     wherein the thermostat valve includes a first valve for opening and closing the radiator-routing passage, and a second valve for opening and closing the bypass passage, the first valve and the second valve being operable concurrently; the cylinder coolant jacket is disposed around a cylinder bore in the cylinder portion and is partitioned into two in a cylinder axis direction to thereby form a main cylinder coolant jacket on a side of the cylinder head portion and a sub-cylinder coolant jacket on a side of the crankcase portion; and the bypass passage is formed partly by the sub-cylinder coolant jacket. 
     In accordance with the foregoing configuration, part of the bypass passage is formed by the sub-cylinder coolant jacket. This reduces use of the external pipe in the bypass passage. Thus, coolant that has been heated through circulation through the cylinder coolant jacket and the cylinder head coolant jacket during a warming-up operation at the start of the engine dissipates less heat when circulating through the bypass passage that bypasses the radiator because of the reduced use of the external pipe. Furthermore, the temperature of the coolant, which is further heated in the sub-cylinder coolant jacket, increases, so that engine warming-up is further expedited. 
     Additionally, the sub-cylinder coolant jacket provided in the cylinder portion forms part of the bypass passage. This facilitates formation of the bypass passage and reduces use of the external pipe in the bypass passage. Thus, a simplified structure including a reduced number of parts can be configured, cost can be reduced, and a lightweight internal combustion engine can be built. Additionally, outer areas surrounding the engine main unit can be simplified and favorable outer appearance can be maintained. 
     In the foregoing configuration, preferably, the main cylinder coolant jacket has a volume greater than a volume of the sub-cylinder coolant jacket. 
     In accordance with the foregoing configuration, the main cylinder coolant jacket on the side of the cylinder head portion has a volume greater than the volume of the sub-cylinder coolant jacket on the side of the crankcase portion. Thus, the cylinder portion can be efficiently cooled during the ordinary operation of the internal combustion engine following the warming-up operation, while the sub-cylinder water jacket is being used as the bypass passage. 
     In the foregoing configuration, preferably, the second valve has formed therein a leak passage through which coolant leaks when the second valve is in a closed position. 
     In accordance with the foregoing configuration, the coolant leaks through the leak passage to the bypass passage even when the second valve is closed during the ordinary operation of the internal combustion engine. A minimal amount of coolant is thereby allowed to flow through the sub-cylinder water jacket. Uneven cooling performance of the cylinder portion can thus be prevented and the cylinder portion can be cooled even more effectively. 
     In the foregoing configuration, preferably, the thermostat valve is integrated with the engine main unit. 
     In accordance with the foregoing configuration, part of the bypass passage that is opened and closed by the second valve of the thermostat valve, specifically, the part between the second valve and the sub-cylinder coolant jacket of the cylinder portion is formed in the engine main unit. Use of the external pipe can thereby be further reduced, so that heat dissipation from the external pipe during the warming-up operation can be further reduced and warming-up can be further expedited. 
     In addition, the reduction in use of the external pipe shortens the bypass passage as much as possible, so that pipe resistance can be minimized. 
     In the foregoing configuration, preferably, the cylinder portion includes a plurality of cylinder bores arrayed in series with each other, and the thermostat valve is disposed adjacent one of outermost cylinder bores disposed on two lateral ends in a direction in which the cylinder bores are arrayed. 
     In accordance with the foregoing configuration, the thermostat valve is disposed adjacent one of the outermost cylinder bores disposed on two lateral ends in the direction in which the cylinder bores are arrayed in series with each other in the cylinder portion. Thus, the sub-cylinder coolant jacket can be used over a long distance as part of the bypass passage opened and closed by the second valve of the thermostat valve. During the warming-up operation, the coolant that circulates through the bypass passage can thereby be efficiently heated over a long distance, so that warming-up is further expedited. 
     In the foregoing configuration, preferably, the coolant pump is disposed on a side opposite to the thermostat valve in the direction in which the cylinder bores are arrayed in the internal combustion engine. 
     In accordance with the foregoing configuration, the thermostat valve and the coolant pump are disposed on either end across the cylinder bores in the direction in which the cylinder bores are arrayed. Thus, the sub-cylinder coolant jacket is allowed to form a substantial part of the bypass passage. Thus, the use of the external pipe can be reduced and heat dissipation from the external pipe can be reduced for expediting of the warming-up. Additionally, outer appearance can be improved and reduction in size and weight of the internal combustion engine can be further promoted. 
     Additionally, the sub-cylinder coolant jacket as the bypass passage includes the two flow channels through which coolant is passed in the direction in which the cylinder bores are arrayed, to thereby allow the coolant to flow through the two flow channels in a bifurcated manner in an identical direction. The configuration results in a large flow channel cross-sectional area, a short flow channel length, and small pipe resistance. Thus, the internal combustion engine can be further reduced in size through the use of a compact water pump delivering a small pump capacity. 
     In the foregoing configuration, preferably, the coolant pump is disposed on a side identical to a side on which the thermostat valve is disposed in the direction in which the cylinder bores are arrayed in the internal combustion engine. 
     In accordance with the foregoing configuration, the thermostat valve and the coolant pump are disposed on the same side in the direction in which the cylinder bores are arrayed. Thus, the sub-cylinder coolant jacket is allowed to form a substantial part of the bypass passage. Thus, the use of the external pipe can be reduced and heat dissipation from the external pipe can be reduced for expediting of the warming-up. Additionally, outer appearance can be improved and reduction in size and weight of the internal combustion engine can be further promoted. 
     Additionally, the sub-cylinder coolant jacket as the bypass passage includes the two flow channels through which coolant is passed in the direction in which the cylinder bores are arrayed and represents a circuit route around the inline cylinder bores, extending from a first end in the cylinder array direction through a first flow channel, by way of a second end, back to a second flow channel. Thus, the coolant is heated by the long flow channel of the bypass passage during the warming-up operation, so that warming-up is even further expedited. 
     In the foregoing configuration, preferably, the cylinder portion is disposed to extend superiorly from the crankcase portion; a starting motor is disposed on the crankcase portion adjacent the cylinder portion; and the starter motor is disposed on a side of the cylinder bores with part of the sub-cylinder coolant jacket positioned between the cylinder bores and the starter motor. 
     In accordance with the foregoing configuration, the starting motor is disposed on the crankcase portion adjacent the cylinder portion that extends superiorly from the crankcase portion, so that the starting motor can be disposed in a space-efficient manner. Additionally, the starting motor is disposed on the side opposite to the cylinder bores across part of the sub-cylinder coolant jacket. Thus, the coolant flowing through the sub-cylinder coolant jacket blocks heat generated by the cylinder bores and thermal effect on the starting motor can be reduced. 
     Effects of the Invention 
     In the present invention, the cylinder coolant jacket is partitioned into two in the cylinder axis direction to thereby form the main cylinder coolant jacket on the side of the cylinder head portion and the sub-cylinder coolant jacket on the side of the crankcase portion, and the bypass passage is formed partly by the sub-cylinder coolant jacket. Thus, use of the external pipe in the bypass passage can be reduced and the coolant that has been heated through circulation through the cylinder coolant jacket and the cylinder head coolant jacket during the warming-up operation at the start of the engine dissipates less heat when circulating through the bypass passage that bypasses the radiator because of the reduced use of the external pipe. Furthermore, the temperature of the coolant, which is further heated in the sub-cylinder coolant jacket, increases, so that warming-up is further expedited. 
     Additionally, the sub-cylinder coolant jacket located in the cylinder portion forms part of the bypass passage. This facilitates formation of the bypass passage and reduces use of the external pipe in the bypass passage. Thus, a simplified structure including a reduced number of parts can be configured, cost can be reduced, and a lightweight internal combustion engine can be built. Additionally, outer areas surrounding the engine main unit can be simplified and favorable outer appearance can be maintained. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view generally depicting an internal combustion engine that includes a cooling structure according to an embodiment of the present invention; 
         FIG. 2  is a right side elevational view of the internal combustion engine; 
         FIG. 3  is an exploded perspective view of a cylinder block, a partition member, and a gasket of the internal combustion engine; 
         FIG. 4  is a sectional view of the cylinder block combined with a cylinder head via the gasket; 
         FIG. 5  is a rear elevational view, partly in section, of an engine main unit, particularly depicting a thermostat valve and parts around the thermostat valve when coolant temperature is low; 
         FIG. 6  is a rear elevational view, partly in section, of the engine main unit, particularly depicting the thermostat valve and the parts around the thermostat valve when the coolant temperature is high; 
         FIG. 7  is a diagram schematically depicting flow of coolant through the cooling structure for the internal combustion engine; 
         FIG. 8  is a sectional view of a cylinder portion illustrating an example in which a cylinder water jacket is partitioned by another partition member; 
         FIG. 9  is a sectional view of a cylinder portion illustrating an example in which a cylinder water jacket is partitioned by still another partition member; 
         FIG. 10  is a sectional view of a cylinder portion illustrating an example in which a cylinder water jacket is partitioned by a further partition member; 
         FIG. 11  is a sectional view of a crankcase portion and a cylinder portion of an example in which a cylinder water jacket in the cylinder portion separate from the crankcase portion is partitioned; 
         FIG. 12  is a diagram schematically depicting another flow route of coolant; 
         FIG. 13  is a diagram schematically depicting still another flow route of coolant; 
         FIG. 14  is a diagram schematically depicting a flow route of coolant in a configuration in which a water pump and a thermostat are disposed on an identical side in the engine main unit; and 
         FIG. 15  is a diagram schematically depicting a further flow route of coolant. 
     
    
    
     MODE FOR CARRYING OUT THE INVENTION 
     A specific embodiment to which the present invention is applied will be described below with reference to the drawings. 
     Referring to  FIG. 1 , an internal combustion engine  1  to which the embodiment of the present invention is applied is mounted is a saddled vehicle or, in particular, a motorcycle. The internal combustion engine  1  is an inline two-cylinder, four-stroke water-cooled internal combustion engine. 
     As shown in  FIGS. 1 and 2 , the internal combustion engine  1  is mounted transversely on the vehicle, with a crankshaft  10  thereof oriented in a lateral direction. 
     Throughout the description given hereunder, expressions indicating directions including front and rear, and right and left, mean the same directions as those on a vehicle facing in a straight-forward direction. In the drawings, arrow FR indicates forward of the vehicle, arrow RR indicates rearward of the vehicle, arrow LH indicates leftward of the vehicle, and arrow RH indicates rightward of the vehicle. 
     As depicted in  FIGS. 1 and 2 , an engine main unit  2  of the internal combustion engine  1  includes a crankcase portion  3 , a cylinder portion  4 , and a cylinder head (cylinder head portion)  5 . The crankcase portion  3  journals the crankshaft  10 . The cylinder portion  4  extends superiorly from the crankcase portion  3 . The cylinder head  5  is disposed on the cylinder portion  4  via a gasket  6 . 
     The crankcase portion  3  includes an upper-side crankcase  3   a  and a lower-side crankcase  3   b  that sandwich the crankshaft  10  from above and below to thereby journal the crankshaft  10 . The cylinder portion  4  extends from the upper-side crankcase  3   a  obliquely superiorly at a slightly anteriorly inclined angle. The upper-side crankcase  3   a  and the cylinder portion  4  are formed integrally as a cylinder block. 
     A cylinder head cover  7  is placed over the cylinder head  5 . 
     A left case cover  8  and a right case cover  9  cover left and right lateral surfaces of the crankcase portion  3 , respectively. 
     A transmission chamber in which a transmission mechanism is housed is formed in the crankcase portion  3 , posterior to a crank chamber that journals the crankshaft  10 . The internal combustion engine  1  constitutes a power unit structure. 
     As depicted in  FIG. 1 , a starter motor  55  is disposed superior to the transmission mechanism above the crankcase portion  3 . 
     The starter motor  55  is disposed transversely in the lateral direction along a rear lateral surface of the cylinder portion  4 , adjacent to the cylinder portion  4  that extends superiorly from the crankcase portion  3 . 
     Reference is made to  FIG. 3 . The cylinder portion  4  includes cylinder bores  4   b  formed therein in juxtaposition to each other in the lateral direction. The cylinder portion  4  further includes a rectangular cam chain chamber  4   c  formed therein on the right of the cylinder bores  4   b.    
     Additionally, a cylinder water jacket  4 W is formed around the left and right cylinder bores  4   b  in the cylinder portion  4 . 
     The cylinder water jacket  4 W is composed of cylindrical groove portions formed around the respective cylinder bores  4   b  juxtaposed to each other and combined together at a central constricted connection therebetween to thereby form a single loop tubular groove. The cylinder water jacket  4 W is open in an abutment surface  4   f  relative to the cylinder head  5 . 
     The tubular groove of the cylinder water jacket  4 W is formed into a channel defined by groove lateral surfaces that face each other on an inside and an outside of the loop and a groove bottom surface. 
     Reference is made to  FIG. 3 . A partition member  15  is inserted in the cylinder water jacket  4 W that is formed into the loop-shaped tubular groove, which includes the cylindrical groove portions on the left and right sides of the central constricted connection disposed between the cylindrical groove portions. The partition member  15  is a tubular plate member formed into a loop shape having cylindrical portions on the left and right sides of a central constricted connection disposed between the cylindrical portions. 
     The partition member  15  is a plate member formed of a resin. 
     The tubular partition member  15  is a resin plate member having a thickness thinner than a groove width of the cylinder water jacket  4 W. The partition member  15  is fitted into the cylinder water jacket  4 W by having an outer lateral surface  15   a  of the partition member  15  in contact with an outer groove lateral surface of the cylinder water jacket  4 W. 
     The partition member  15  has a flange  15   f  formed in a predetermined region closer to a lower end thereof on an inner lateral surface  15   b  thereof. The flange  15   f  is formed into a loop shape protruding inwardly (see  FIG. 3 ). 
     Reference is made to  FIG. 4 . When the partition member  15  is inserted in the cylinder water jacket  4 W, the outer lateral surface  15   a  of the partition member  15  contacts the groove outer lateral surface of the cylinder water jacket  4 W, and the flange  15   f  has an inner circumferential end contacting an inner groove lateral surface of the cylinder water jacket  4 W. 
     Thus, as shown in  FIG. 4 , the cylinder water jacket  4 W is partitioned by the flange  15   f  of the partition member  15  into two in a cylinder axis direction. Specifically, a main cylinder water jacket  4 Wa is formed on the side of the cylinder head  5  (upper side) and a sub-cylinder water jacket  4 Wb is formed on the side of the crankcase portion  3  (lower side). 
     Because the flange  15   f  is formed at a position closer to the lower end of the partition member  15 , the main cylinder water jacket  4 Wa has a volume greater than a volume of the sub-cylinder water jacket  4 Wb. 
     It is noted that, as depicted in  FIG. 3 , an inflow communication port  17 I is cut out from the lower end of the partition member  15  at a part of a left rear portion of the left cylindrical portion, lower than the flange  15   f . Additionally, an outflow communication port  17 E is cut out from the lower end of the partition member  15  at a part of a right rear portion of the right cylindrical portion, lower than the flange  15   f.    
     A connection opening  4 J that opens to the outside is formed in a portion of a rear lateral wall of the cylinder portion  4 , to which the outflow communication port  17 E of the partition member  15  corresponds, when the partition member  15  is fitted in the cylinder water jacket  4 W in the cylinder portion  4  (see  FIG. 3 ). 
     Thus, the connection opening  4 J communicates with the lower sub-cylinder water jacket  4 Wb partitioned by the flange  15   f  of the cylinder water jacket  4 W via the outflow communication port  17 E in the partition member  15 . 
     Additionally, an inflow communication port  18 I is cut out from an upper end of the partition member  15  at a part on the right lateral portion of the right cylindrical portion, above the flange  15   f.    
     Reference is made to  FIG. 4 . A cylinder head water jacket  5 W is formed in the cylinder head  5  around a combustion chamber  5   b  corresponding to the cylinder bore  4   b  of the cylinder portion  4 . The cylinder head water jacket  5 W is formed to be open in the abutment surface of the cylinder portion  4  so as to correspond to the cylinder water jacket  4 W. 
     Referring to  FIGS. 3 and 4 , the cylinder water jacket  4 W in the cylinder portion  4  and the cylinder head water jacket  5 W in the cylinder head  5  are partly partitioned by the gasket  6  clamped between the cylinder portion  4  and the cylinder head  5 . 
     Reference is made to  FIG. 3 . The gasket  6  has round holes  6   b  and a rectangular hole  6   c  formed therein. The round holes  6   b  correspond in position to the cylinder bores  4   b  in the cylinder portion  4 . The rectangular hole  6   c  corresponds in position to the cam chain chamber  4   c . The portion corresponding to the cylinder water jacket  4 W around the round holes  6   b  is closed except for arcuate communication holes  6   h . Thus, the gasket  6  partitions the cylinder water jacket  4 W and the cylinder head water jacket  5 W excepting the openings in the communication holes  6   h.    
     The communication holes  6   h  in the gasket  6  are formed at positions corresponding to left and right lateral ends of the cylinder water jacket  4 W that is formed into the loop shape with the central constriction. 
     Thus, the cylinder water jacket  4 W of the cylinder portion  4  and the cylinder head water jacket  5 W of the cylinder head  5  are generally partitioned by the gasket  6  and only the arcuate communication holes  6   h  on the left and right lateral ends provide communication. 
     As depicted in  FIG. 4 , the cylinder head  5  has an intake port  5   i  extending to curve obliquely upwardly toward the rear from the combustion chamber  5   b . The intake port  5   i  has an upstream end forming an intake connection pipe portion  5   p  protruding to the rear. 
     Additionally, the cylinder head  5  has an exhaust port  5   e  extending obliquely upwardly toward the front from the combustion chamber  5   b.    
     The cylinder head water jacket  5 W is formed also around the intake port  5   i  and the exhaust port  5   e.    
     As  FIGS. 1 and 2  show, a water pump  20  that circulates coolant is disposed at a front portion anterior to the crankshaft  10  of the right case cover  9  that covers the right lateral surface of the crankcase portion  3  in the internal combustion engine  1 . 
     The water pump  20  includes an impeller  20   a  housed in a pump body formed on a lateral wall of the right case cover  9 . The impeller  20   a  is housed in a pump cover  21  from the outside. 
     The pump cover  21  has an intake chamber  21   a  defined on the right of the impeller  20   a . An intake connection pipe  22  is provided to protrude from the intake chamber  21   a . A radiator outflow hose  52  extending from a radiator  50  is connected with the intake connection pipe  22 . 
     Additionally, a bypass passage hole  26  is drilled in a wall of the right case cover  9  to extend obliquely upwardly toward the rear from the intake chamber  21   a.    
     As depicted in  FIG. 1 , a bypass communication hose  25  connects an upstream end of the bypass passage hole  26  with the connection opening  4 J formed in the rear lateral wall of the cylinder portion  4 . 
     The upstream end of the bypass passage hole  26  and the connection opening  4 J are both disposed on the right-hand side of the internal combustion engine  1  and are located close to each other. The bypass communication hose  25  that connects the upstream end of the bypass passage hole  26  with the connection opening  4 J can thus be short in length. 
     The connection opening  4 J communicates with the lower sub-cylinder water jacket  4 Wb partitioned by the flange  15   f  of the partition member  15  of the cylinder water jacket  4 W via the outflow communication port  17 E in the partition member  15 . Thus, the lower sub-cylinder water jacket  4 Wb communicates with the intake chamber  21   a  of the water pump  20  via the connection opening  4 J, the bypass communication hose  25 , and the bypass passage hole  26 . 
     Reference is made to  FIG. 2 . Coolant drawn into the central intake chamber  21   a  in the water pump  20  from the intake connection pipe  22  or the bypass passage hole  26  is discharged to a delivery path  24  on the outer circumference by a centrifugal force through rotation of the impeller  20   a . Guided into the delivery path  24 , the coolant is then delivered to the cylinder block side from a delivery port  24   a  in the right case cover  9 . The coolant then flows into the upper main cylinder water jacket  4 Wa via the inflow communication port  18 I in the partition member  15 . 
     Reference is made to  FIGS. 1 and 5 . The cylinder head  5  includes a thermostat valve  30  integrally formed on the rear lateral wall of the cylinder head  5  at a left end of the rear lateral surface from which the intake connection pipe portion  5   p  protrudes. 
     As depicted in  FIGS. 5 and 6 , the thermostat valve  30  includes a casing  31  integrally formed with the rear lateral wall of the cylinder head  5 . A lid member  32  covers an opening that opens to the left. The thermostat valve  30  includes a first valve  33  and a second valve  34  disposed thereinside. 
     Referring to  FIGS. 5 and 6 , an annular valve seat  37  is clamped and fixed between the casing  31  and the lid member  32  inside the thermostat valve  30 . The valve seat  37  integrally includes an annular seat portion  37   a  and a band-shaped retainer portion  37   b . The annular seat portion  37   a  has a valve opening in the center thereof. The retainer portion  37   b  is bent into a dogleg shape to thereby have both ends connected with a circumferential edge of the valve opening in the annular seat portion  37   a.    
     The retainer portion  37   b  protrudes from the annular seat portion  37   a  of the valve seat  37  into the internal space of the lid member  32  on the left. 
     A spring receiving support member  38  extends from the annular seat portion  37   a  of the valve seat  37  into the casing  31  on the right. 
     The spring receiving support member  38  includes a pair of support pieces  38   a  and an annular spring receiving portion  38   b . The support pieces  38   a  extend to the right from the valve seat  37 . The spring receiving portion  38   b  is formed on the right end of the support pieces  38   a.    
     The first valve  33  is urged by a coil spring  41  having a first end supported by the spring receiving portion  38   b  of the spring receiving support member  38 , and thereby abuts on the annular seat portion  37   a  of the valve seat  37 . 
     A thermoelement  35  passes through the first valve  33 . The thermoelement  35  has a left end passing through the central valve opening in the annular valve seat  37  with an ample clearance therefrom. When the first valve  33  abuts on the annular seat portion  37   a  of the valve seat  37 , the valve opening in the valve seat  37  is closed to establish a valve-closed state, so that an internal space of the casing  31  is partitioned from the internal space of the lid member  32 . 
     The thermoelement  35  includes a portion toward the right-hand side, which portion, having an enlarged diameter, assumes a temperature-sensing portion  35   t  in which a thermally expandable material, such as a wax, is packed. 
     The thermoelement  35  is supported such that the temperature-sensing portion  35   t  is slidable along the annular spring receiving portion  38   b  of the spring receiving support member  38 . Meanwhile, a plunger  36  protrudes from the left end of the thermoelement  35  into the inside of the lid member  32  on the left. The plunger  36  has a leading end abutting on and held by a bent receiving portion  37   bb  of the retainer portion  37   b  integrally formed with the valve seat  37 . 
     The second valve  34  is slidably fitted and journaled on a support bar  35   a  that integrally protrudes to the right from the temperature-sensing portion  35   t  of the thermoelement  35 . 
     The second valve  34  that is restricted from moving by a retaining ring  39  engaged with the support bar  35   a  is urged to the right by a conical coil spring  42  disposed between the temperature-sensing portion  35   t  and the second valve  34 . 
     The casing  31  includes a large-diameter cylindrical main portion  31   a  and a small-diameter cylindrical end portion  31   b . The cylindrical main portion  31   a  is disposed to be closer to the lid member  32  (on the left). The small-diameter cylindrical end portion  31   b  having a reduced diameter is disposed in a protruding condition on the right of the cylindrical main portion  31   a.    
     The second valve  34  abuts on a shoulder  31   c  between the cylindrical main portion  31   a  and the small-diameter cylindrical end portion  31   b  to thereby be closed. The second valve  34  is thereby able to partition the internal space of the cylindrical main portion  31   a  from the internal space of the small-diameter cylindrical end portion  31   b.    
       FIG. 5  depicts a condition in which a temperature of coolant around the temperature-sensing portion  35   t  of the thermoelement  35  is low.  FIG. 5  depicts that the first valve  33  and the thermoelement  35  are urged by the coil spring  41  to be moved to the left and the first valve  33  abuts on the valve seat  37  to be closed, so that the internal space of the casing  31  is partitioned from the internal space of the lid member  32 ; and the second valve  34  journaled on the support bar  35   a  of the thermoelement  35  leaves the shoulder  31   c  between the cylindrical main portion  31   a  and the small-diameter cylindrical end portion  31   b  of the casing  31  to be opened, thus providing communication between the internal space of the cylindrical main portion  31   a  and the internal space of the small-diameter cylindrical end portion  31   b.    
     When the temperature of the coolant around the temperature-sensing portion  35   t  of the thermoelement  35  increases and the wax inside the temperature-sensing portion  35   t  expands to thereby push out the plunger  36 , reaction involved in the leading end of the plunger  36  being held by the retainer portion  37   b  of the valve seat  37  resists the coil spring  41  to thereby move the thermoelement  35  to the right as depicted in  FIG. 6 . 
     Thus, the first valve  33  opens to provide communication between the internal space of the casing  31  and the internal space of the lid member  32 . At the same time, the second valve  34  urged by the conical coil spring  42  abuts on the shoulder  31   c  to thereby closed, thus partitioning the internal space of the cylindrical main portion  31   a  from the internal space of the small-diameter cylindrical end portion  31   b.    
     It is to be noted that a valve element of the second valve  34  has a through hole  34   p  that serves as a leak passage intended to allow coolant to leak even when the second valve  34  is closed. 
     An outflow connection pipe  44  is formed in a protruding manner on the lid member  32  of the thermostat valve  30 . A radiator inflow hose  51  extending from the radiator  50  is connected with the outflow connection pipe  44 . 
     Additionally, the casing  31  of the thermostat valve  30  has a communication path  45  opening to the internal space of the cylindrical main portion  31   a  of the casing  31 . The communication path  45  is integrally formed in the rear lateral wall of the cylinder head  5 , extending from the cylinder head water jacket  5 W of the cylinder head  5 . 
     A bypass communication path  46  that communicates with the internal space of the small-diameter cylindrical end portion  31   b  of the casing  31  extends in the rear lateral wall of the cylinder head  5  toward the cylinder portion  4  inferior to the cylinder head  5 , to thereby be open in the abutment surface with respect to the cylinder portion  4 . 
     Reference is made to  FIG. 3 . The cylinder portion  4  includes a bypass communication path  47  communicating with the bypass communication path  46  on the side of the cylinder head  5 . The bypass communication path  47  is formed to be open to the abutment surface with respect to the cylinder head  5  and to extend downward. The bypass communication path  46  on the side of the cylinder head  5  communicates with the bypass communication path  47  on the side of the cylinder portion  4  via a communication hole  6   j  (see  FIG. 3 ) in the gasket  6 . 
     As shown in  FIG. 3 , the bypass communication path  47  of the cylinder portion  4  has a communication port  48  on the lower end thereof. The communication port  48  is aligned with the inflow communication port  17 I ( FIG. 3 ) in the partition member  15 , so that the bypass communication path  47  communicates with the sub-cylinder water jacket  4 Wb on the lower side. 
     Specifically, the internal space of the small-diameter cylindrical end portion  31   b  of the thermostat valve  30  communicates with the sub-cylinder water jacket  4 Wb on the lower side of the cylinder water jacket  4 W via the bypass communication paths  46  and  47 . 
       FIG. 7  schematically depicts flow of coolant through the cooling structure for the internal combustion engine  1  having configurations as described above. 
     The cylinder water jacket  4 W as the loop-shaped tubular groove with the central constriction in the cylinder portion  4  is partitioned by the flange  15   f  of the partition member  15  into the main cylinder water jacket  4 Wa on the side of the cylinder head  5  (upper side) and the sub-cylinder water jacket  4 Wb on the side of the crankcase portion  3  (lower side). Each of the main cylinder water jacket  4 Wa and the sub-cylinder water jacket  4 Wb includes a front-side flow channel and a rear-side flow channel at the front and rear, respectively, communicating a left end portion of the cylinder portion  4  with a right end portion of the cylinder portion  4 . 
     The main cylinder water jacket  4 Wa on the upper side communicates with the cylinder head water jacket  5 W of the cylinder head  5  via the communication holes  6   h  in the gasket  6  on the left and right end portions. 
     The thermostat valve  30  and the water pump  20  are disposed on the left-hand side and the right-hand side, respectively, of the engine main unit  2 . 
     A radiator-routing passage Pr that passes through the radiator  50  includes the radiator inflow hose  51  through which coolant flows from the thermostat valve  30  on the left-hand side into the radiator  50  and the radiator outflow hose  52  through which the coolant flows from the radiator  50  out to the water pump  20  on the right-hand side. The radiator-routing passage Pr is opened or closed by the first valve  33  of the thermostat valve  30 . 
     A bypass passage Pb that bypasses the radiator  50  between the thermostat valve  30  and the water pump  20  includes the bypass communication paths  46  and  47 , the sub-cylinder water jacket  4 Wb, the bypass communication hose  25 , and the bypass passage hole  26 . The bypass passage Pb is opened or closed by the second valve  34  of the thermostat valve  30 . 
     As described above, the bypass passage Pb is configured using the sub-cylinder water jacket  4 Wb, and only the bypass communication hose  25  is an external pipe, so that a considerable reduction in use of external pipes is achieved. 
     The sub-cylinder water jacket  4 Wb existing in the cylinder portion  4  forms part of the bypass passage Pb. This facilitates formation of the bypass passage and reduces use of the external pipe in the bypass passage Pb. Thus, a simplified structure including a reduced number of parts can be configured, cost can be reduced, and a lightweight internal combustion engine can be built. Additionally, areas surrounding the engine main unit can be simplified and favorable appearance can be maintained. 
     During a warming-up operation at the start of the engine at which the coolant temperature is low, the first valve  33  is closed and the second valve  34  is opened in the thermostat valve  30 , so that coolant delivered from the water pump  20  flows through the following circulation path. Specifically, the coolant from the delivery path  24  flows in a bifurcated manner into the main cylinder water jacket  4 Wa and into the front-side flow channel and rear-side flow channel of the cylinder head water jacket  5 W, flows from the communication path  45  into the cylindrical main portion  31   a  of the thermostat valve  30 , and flows via the open second valve  34  through the bypass passage Pb before returning to the water pump  20 . 
     Thus, the coolant that has flowed though, and heated by mainly the main cylinder water jacket  4 Wa and the cylinder head water jacket  5 W dissipates, when flowing through the bypass passage Pb that bypasses the radiator  50 , only a minimal amount of heat in the region of the bypass communication hose  25  which is shortened by reduction of the external pipe used. Additionally, the coolant is heated in the sub-cylinder water jacket  4 Wb, so that the further increase in temperature in the sub-cylinder water jacket  4 Wb expedites warming-up of the engine. 
     When the coolant temperature increases to a certain level as a result of the warming-up operation of the internal combustion engine, the engine initiates an ordinary operation by closing the second valve  34  and opening the first valve  33  in the thermostat valve  30  and the coolant delivered from the water pump  20  flows through the following circulation path. Specifically, the coolant from the delivery path  24  flows in a bifurcated manner into the main cylinder water jacket  4 Wa and into the front-side flow channel and rear-side flow channel of the cylinder head water jacket  5 W, flows through the communication path  45  into the cylindrical main portion  31   a  of the thermostat valve  30 , and flows via the open first valve  33  through the radiator-routing passage Pr that is routed through the radiator  50 , before returning to the water pump  20 . 
     Thus, the coolant cooled by the radiator  50  flows through the main cylinder water jacket  4 Wa and the cylinder head water jacket  5 W, thereby cooling the cylinder portion  4  and the cylinder head  5 . 
     It is to be noted here that, as described previously, the main cylinder water jacket  4 Wa on the side adjacent the cylinder head  5  has a volume greater than the volume of the sub-cylinder water jacket  4 Wb on the side adjacent the crankcase portion  3 . Thus, the cylinder portion  4  can be efficiently cooled during the ordinary operation of the internal combustion engine  1  following the warming-up operation, while the sub-cylinder water jacket  4 Wb is being used as the bypass passage. 
     As described previously, the valve element of the second valve  34  has the through hole  34   p  that serves as the leak passage. Thus, coolant leaks through the through hole  34   p  to the bypass passage Pb even when the second valve  34  is closed during the ordinary operation. A minimal amount of coolant is thereby allowed to flow through the sub-cylinder water jacket  4 Wb. Uneven cooling performance of the cylinder portion  4  can thus be prevented and the cylinder portion  4  can be cooled even more effectively. 
     With the thermostat valve  30 , its casing  31  is integrally formed on the rear lateral wall of the cylinder head  5 . Thus, the bypass communication path  46  on the side of the cylinder head  5  and the bypass communication path  47  on the side of the cylinder portion  4  can form part of the bypass passage Pb that is opened or closed by the second valve  34  of the thermostat valve  30 , specifically, the part between the second valve  34  and the sub-cylinder water jacket  4 Wb of the cylinder portion  4 . Use of the external pipe can thereby be further reduced, so that heat dissipation from the external pipe during the warming-up operation can be further reduced and warming-up can be further expedited. 
     In addition, the reduction in use of the external pipe shortens the bypass passage Pb as much as possible, so that pipe resistance can be minimized. 
     The water pump  20  is disposed on the side opposite to the thermostat valve  30  in a direction in which the cylinder bores  4   b  are arrayed in the internal combustion engine  1 . 
     Specifically, the thermostat valve  30  and the water pump  20  are disposed, respectively, on both sides of a line of arrangement of the cylinder bores  4   b . This configuration allows the sub-cylinder water jacket  4 Wb to form a substantial part of the bypass passage Pb. Thus, the use of the external pipe can be reduced and heat dissipation from the external pipe can be reduced for expediting warming-up operation. Additionally, appearance can be enhanced and reduction in size and weight of the internal combustion engine can be further promoted. 
     Additionally, as shown in  FIG. 7 , the sub-cylinder water jacket  4 Wb forming the bypass passage Pb includes the two flow channels of the front-side flow channel and the rear-side flow channel through which coolant is passed in the direction in which the cylinder bores  4   b  are arrayed, so that the coolant is allowed to flow through the two flow channels in a bifurcated manner in the same directions. More specifically, the configuration results in an increased flow channel cross-sectional area, a shortened flow channel length, and a reduced pipe resistance. Thus, the internal combustion engine  1  can be further reduced in size through the use of a compact water pump of a small pump capacity. 
     Reference is made back to  FIG. 1 . The starter motor  55  is disposed on the crankcase portion  3  along the rear lateral surface of the cylinder portion  4  and adjacent the cylinder portion  4  extending above the crankcase portion  3 . The rear lateral portion of the sub-cylinder water jacket  4 Wb disposed in the cylinder portion  4  on the side (lower side) adjacent to the crankcase portion  3  is located between the cylinder bores  4   b  and the starter motor  55 . The foregoing arrangement enables the coolant flowing through the sub-cylinder water jacket  4 Wb to block heat generated by the cylinder bores  4   b , to thereby reduce thermal effect on the starter motor  55 . 
     With the cooling structure for an internal combustion engine according to the embodiment described above, in order to partition the cylinder water jacket  4 W of the cylinder portion  4  into the main cylinder water jacket  4 Wa on the side of the cylinder head  5  (upper side) and the sub-cylinder water jacket  4 Wb on the side of the crankcase portion  3  (lower side), the partition member  15  as the tubular plate member is fitted into the cylinder water jacket  4 W to use the flange  15   f  of the partition member  15  to serve as a partition.  FIG. 8  depicts a first modification of the partition member. 
     In the first modification, a cylinder water jacket  60 W of a cylinder portion  60  has a groove width that gradually tapers to be narrower from a groove opening toward a groove bottom surface. An annular, string-shaped partition member  65  is fitted into the cylinder water jacket  60 W. 
     The partition member  65  is formed of a resin or rubber and has a trapezoidal cross section. 
     The partition member  65 , when having been press-fitted into a predetermined depth in the cylinder water jacket  60 W, can partition the cylinder water jacket  60 W into a main cylinder water jacket  60 Wa on the side of the cylinder head (upper side) and a sub-cylinder water jacket  60 Wb on the side of the crankcase portion (lower side) by being caught in the tapered groove in the cylinder water jacket  60 W. 
       FIG. 9  depicts a second modification. 
     In the second modification, a cylinder water jacket  70 W of a cylinder portion  70  has a groove width that is suddenly narrowed at a predetermined depth to thereby form a shoulder  70   d . An annular, string-shaped partition member  75  is fitted onto the shoulder  70   d . The partition member  75  functions to partition the cylinder water jacket  70 W into a main cylinder water jacket  70 Wa on the side of the cylinder head (upper side) and a sub-cylinder water jacket  70 Wb on the side of the crankcase portion (lower side). 
     In the first and second modifications depicted in  FIGS. 8 and 9 , the cylinder water jackets  60 W and  70 W can be partitioned readily using the annular, string-shaped partition members  65  and  75 , so that reduction in cost can be achieved. 
       FIG. 10  depicts a third modification in which a partition member  76  different from the partition member  75  is fitted in the cylinder water jacket  70 W in the cylinder portion  70  shown in  FIG. 9 , to thereby partition the cylinder water jacket  70 W into the main cylinder water jacket  70 Wa and the sub-cylinder water jacket  70 Wb. 
     The partition member  76  is a loop-shaped tubular plate member having a thickness thinner than a groove width of the cylinder water jacket  70 W. The partition member  76  has a flange  76   f  formed at the lower end thereof. The flange  76   f  protrudes toward the inside. 
     The partition member  76  has an outer peripheral surface in contact with an outer groove peripheral surface of the cylinder water jacket  70 W. The partition member  76  is fitted in to a degree in which the flange  76   f  abuts on the shoulder  70   d . Then, the flange  76   f  of the partition member  76  partitions the cylinder water jacket  70 W into the main cylinder water jacket  70 Wa and the sub-cylinder water jacket  70 Wb. 
       FIG. 11  depicts an example in which, in an engine main unit including a cylinder portion separate from a crankcase portion, a cylinder water jacket in a cylinder portion  82  is partitioned into a main cylinder water jacket  82 Wa on the side of the cylinder head (upper side) and a sub-cylinder water jacket  82 Wb on the side of the crankcase portion (lower side). 
     The cylinder portion  82  separate from a crankcase portion  81  has a cylinder sleeve  82   s  extending below an abutment surface  82   mb  with respect to the crankcase portion  81  and reaching into the crankcase portion  81 . 
     A tubular groove in the main cylinder water jacket  82 Wa is formed to be open to an abutment surface  82   ma  of the cylinder portion  82  with respect to the cylinder head on the side opposite to the abutment surface  82   mb . A tubular groove in the sub-cylinder water jacket  82 Wb is formed to be open to the abutment surface  82   mb.    
     A groove bottom in the main cylinder water jacket  82 Wa is close to a groove bottom in the sub-cylinder water jacket  82 Wb and a partition portion  82   f  is formed between the groove bottoms. 
     For the configuration in which the crankcase portion  81  is separate from the cylinder portion  82  in the engine main unit, molding the cylinder portion  82  to configure the main cylinder water jacket  82 Wa and the sub-cylinder water jacket  82 Wb results in forming the partition portion  82   f . This eliminates the need for a separate partition member to be fabricated. 
     In the cooling structure for an internal combustion engine depicted in  FIG. 7 , the bypass passage Pb is configured such that the coolant flows between the left and right ends of the sub-cylinder water jacket  4 Wb in the cylinder portion  4  through the front-side flow channel and the rear-side flow channel in a parallelly bifurcated manner. An alternative configuration may nonetheless be possible in which coolant flows through only the rear-side flow channel as depicted in  FIG. 12 . 
     Because the rear-side flow channel of the sub-cylinder water jacket  4 Wb in the cylinder portion  4  is not exposed to air flow, the coolant that flows through the rear-side flow channel that is unlikely to dissipate heat is efficiently heated, so that warming-up can be expedited. 
     In a cooling structure depicted in  FIG. 13 , coolant is passed between the left and right ends of the cylinder portion  4  through front-side flow channels and rear-side flow channels in a parallelly bifurcated manner, and coolant flowing through the rear-side flow channel (indicated by the solid line) of the sub-cylinder water jacket  4 Wb has a flow rate set to be greater than a flow rate of coolant flowing through the front-side flow channel (indicated by the dotted line). 
     The flow rate of coolant flowing through the rear-side flow channel from which heat is not readily dissipated is set to be greater to thereby increase the temperature of the coolant efficiently. In addition, the coolant is passed through both the front-side flow channel and the rear-side flow channel, to thereby reduce pipe resistance, so that pump capacity can be reduced. 
     Each of the cooling structures for internal combustion engines depicted in  FIGS. 7, 12, and 13  assumes that the thermostat valve  30  and the water pump  20  are disposed on two ends of a line in the direction in which the cylinder bores  4   b  are arrayed (lateral direction).  FIGS. 14 and 15  depict cooling structures in which the thermostat valve  30  and the water pump  20  are disposed on one of the two ends. 
     It is to be noted that the engine is the inline two-cylinder, four-stroke water-cooled internal combustion engine, the same as the internal combustion engine described previously. Like elements are identified by like reference numerals. 
     The thermostat valve  30  and the water pump  20  are disposed on the same right-hand side in the engine main unit. In the cooling structure for an internal combustion engine depicted in  FIG. 14 , coolant delivered from the water pump  20  flows from the right end of the cylinder head  5  into the cylinder head water jacket  5 W and, at the left end, flows further onto the main cylinder water jacket  4 Wa; the coolant then flows through the rear-side flow channel of the main cylinder water jacket  4 Wa to the right; at the right end, the coolant is branched into the radiator-routing passage Pr by way of the radiator  50  and into the bypass passage Pb. 
     The radiator-routing passage Pr is opened or closed by the first valve  33  of the thermostat valve  30 . 
     Meanwhile, the rear-side flow channel of the main cylinder water jacket  4 Wa is branched into the front-side flow channel (indicated by the dotted line) of the sub-cylinder water jacket  4 Wb. The front-side flow channel leads into the rear-side flow channel (indicated by the solid line) of the sub-cylinder water jacket  4 Wb. The bypass passage Pb formed by the sub-cylinder water jacket  4 Wb is connected to the thermostat valve  30  by way of its second valve  34 . The bypass passage Pb is opened or closed by the second valve  34 . 
     As such, the sub-cylinder water jacket  4 Wb as the bypass passage Pb represents a circuit route around the inline cylinder bores, extending from the right end in a cylinder array direction through the front-side flow channel, by way of the left end of the rear-side flow channel, back to the right end. Thus, the coolant is heated by the long flow channel of the bypass passage during the warming-up operation, so that warming-up is even further expedited. 
     The thermostat valve  30  and the water pump  20  are disposed on the same right-hand side in the direction in which the cylinder bores are arrayed. Thus, a major part of the bypass passage Pb can be formed using the sub-cylinder water jacket  4 Wb. The use of the external pipe can thus be reduced and heat dissipation from the external pipe can be reduced for expediting of warming-up. Additionally, appearance can be improved and reduction in size and weight of the internal combustion engine can be further promoted. 
     In the cooling structure for an internal combustion engine depicted in  FIG. 15 , coolant delivered from the water pump  20  flows from the right end of the cylinder head  5  into the cylinder head water jacket  5 W and, at its left end, flows further onto the main cylinder water jacket  4 Wa; the coolant then flows through the rear-side flow channel  4 Wa of the main cylinder water jacket  4 Wa to the right to reach the right end. The flow route up to this point is the same as that of the cooling structure depicted in  FIG. 14 . From the right end of the main cylinder water jacket  4 Wa, the coolant flows to the thermostat valve  30 . 
     The first valve  33  of the thermostat valve  30  opens or closes the radiator-routing passage Pr by way of the radiator  50 . 
     The bypass passage Pb opened or closed by the second valve  34  of the thermostat valve  30  extends through the front-side flow channel (indicated by the dotted line) of the sub-cylinder water jacket  4 Wb to the rear-side flow channel (indicated by the solid line) of the same. The rear-side flow channel of the sub-cylinder water jacket  4 Wb is connected to the water pump  20 . 
     Thus, as with the cooling structure depicted in  FIG. 14 , the sub-cylinder water jacket  4 Wb as the bypass passage Pb represents a circuit route around the inline cylinder bores, extending from the right end in the cylinder array direction through the front-side flow channel, by way of the left end of the rear-side flow channel, back to the right end. Thus, the coolant is heated by the long flow channel of the bypass passage during the warming-up operation, so that warming-up is even further expedited. Additionally, appearance can be improved through reduction in use of the external pipe and reduction in size and weight of the internal combustion engine can be further promoted. 
     Although the cooling structures for an internal combustion engine according to the specific embodiments of the present invention have been described, it will be understood that the present invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof, including, for example, an internal combustion engine in which the thermostat is disposed separately from the cylinder head and connected by a coolant hose. 
     The embodiment described with reference to  FIGS. 1 to 7  includes, as part of the bypass passage Pb, the bypass communication hose  25  as the external pipe. The bypass passage Pb can nonetheless be configured without including the external pipe through the following approach. Specifically, instead of the bypass communication hose  25 , a coolant passage that provides communication between the sub-cylinder water jacket  4 Wb of the cylinder portion  4  and the bypass passage hole  26  in the right case cover  9  is formed inside the walls of the cylinder portion  4  and the right case cover  9 . This approach considerably reduces heat dissipation of the bypass passage Pb during the warming-up operation, so that warming-up can be expedited even further. 
     DESCRIPTION OF REFERENCE SYMBOLS 
     
         
           1 : Internal combustion engine 
           2 : Engine main unit 
           3 : Crankcase portion 
           3   a : Upper-side crankcase 
           3   b : Lower-side crankcase 
           4 : Cylinder portion 
           4   b : Cylinder bore 
           4 W: Cylinder water jacket 
           4 Wa: Main cylinder water jacket 
           4 Wb: Sub-cylinder water jacket 
           4 J: Connection opening 
           5 : Cylinder head (cylinder head portion) 
           5   b : Combustion chamber 
           5   i : Intake port 
           5   p : Intake connection pipe portion 
           5   e : Exhaust port 
           5 W: Cylinder head water jacket 
           6 : Gasket 
           7 : Cylinder head cover 
           8 : Left case cover 
           9 : Right case cover 
           10 : Crankshaft 
           15 : Partition member 
           15   f : Flange 
           17 I: Inflow communication port 
           17 E: Outflow communication port 
           18 I: Inflow communication port 
           20 : Water pump 
           20   a : Impeller 
           21 : Pump cover 
           21   a : Intake chamber 
           22 : Intake connection pipe 
           24 : Delivery path 
           24   a : Delivery port 
           25 : Bypass communication hose 
           26 : Bypass passage hole 
           30 : Thermostat valve 
           31 : Casing 
           31   a : Cylindrical main portion 
           31   b : Small-diameter cylindrical end portion 
           32 : Lid member 
           33 : First valve 
           34 : Second valve 
           35 : Thermoelement 
           35   t : Temperature-sensing portion 
           35   a : Support bar 
           36 : Plunger 
           37 : Valve seat 
           37   a : Annular seat portion 
           37   b : Retainer portion 
           38 : Spring receiving support member 
           39 : Retaining ring 
           41 : Coil spring 
           42 : Conical coil spring 
           44 : Outflow connection pipe 
           45 : Communication path 
           46 : Bypass communication path 
           47 : Bypass communication path 
           48 : Communication port 
           50 : Radiator 
           51 : Radiator inflow hose 
           52 : Radiator outflow hose 
           55 : Starter motor 
           60 : Cylinder portion 
           60 W: Cylinder water jacket 
           60 Wa: Main cylinder water jacket 
           60 Wb: Sub-cylinder water jacket 
           65 : Partition member 
           70 : Cylinder portion 
           70 W: Cylinder water jacket 
           70 Wa: Main cylinder water jacket 
           70 Wb: Sub-cylinder water jacket 
           75 : Partition member 
           81 : Crankcase portion 
           82 : Cylinder portion 
           82   s : Cylinder sleeve 
           82   f : Partition portion 
           82 Wa: Main cylinder water jacket 
           82 Wb: Sub-cylinder water jacket