Patent Publication Number: US-2017362992-A1

Title: Cylinder head of multi-cylinder engine

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a cylinder head of an internal combustion engine (hereinafter referred to as an “engine”) and specifically relates to a cylinder head of a multi-cylinder engine having therein flow passages in each of which a coolant flows. 
     2. Description of Related Art 
     A cylinder head of an engine is formed with flow passages in each of which a coolant flows. Japanese Patent Application Publication No. 2013-133746 (JP 2013-133746 A) discloses that, in order to sufficiently cool the air in intake ports, a first coolant circuit in which a coolant circulates for cooling portions around the intake ports in a cylinder head is provided independently of a second coolant circuit in which a coolant circulates for cooling a cylinder block and portions around exhaust ports in the cylinder head. 
     The first coolant circuit includes an intake port coolant passage formed in the cylinder head. The intake port coolant passage is connected to coolant inlet portions provided in an end face in a width direction of the cylinder head. The intake port coolant passage extends from the coolant inlet portions to lower sides of the intake ports, then passes through lateral sides of the intake ports so as to extend to upper sides of the intake ports, and then passes through the upper sides of the intake ports so as to be connected to a coolant outlet portion provided in an end face in a longitudinal direction of the cylinder head. Herein, the lower side of the intake port means a lower side in the vertical direction when the cylinder head is located on an upper side in the vertical direction with respect to the cylinder block, while the upper side of the intake port means an upper side in the vertical direction when the cylinder head is located in the same manner as described above. 
     In order to achieve stable combustion, a recent engine employs an intake port having a shape that can generate a tumble flow in a cylinder (a tumble flow generating port). When the intake port is a tumble flow generating port, the air flows in a manner to stick to an upper surface side of the intake port. Therefore, in order to cool the air in the intake port, it is more effective to reduce the wall temperature of the intake port on its upper surface side. 
     On the other hand, according to the structure of the cylinder head disclosed in JP 2013-133746 A, when a coolant flows on the upper sides of the intake ports, the liquid temperature increases due to heat received from upper surfaces of combustion chambers that rise to high temperatures, resulting in a possibility that a sufficient cooling effect for the air in the intake ports cannot be obtained. 
     SUMMARY OF THE INVENTION 
     In view of the above-mentioned problem, the invention provides a cylinder head of a multi-cylinder engine that can efficiently cool the air flowing in intake ports. 
     Therefore, according to one aspect of the invention, there is provided a multi-cylinder engine including a cylinder head. The cylinder head includes a plurality of combustion chambers, a plurality of intake ports, a first coolant flow passage, and a second coolant flow passage. The plurality of combustion chambers are provided side by side in a longitudinal direction of the cylinder head. The combustion chamber of the cylinder head represents a portion, on the cylinder head side, which forms a closed space where an air-fuel mixture is combusted. Therefore, in this application, the combustion chamber does not necessarily have a shape recessed from a cylinder block mating surface of the cylinder head and may be flush with the cylinder block mating surface. Generally, a cylinder head of a spark-ignition engine is provided with combustion chambers that are recessed with respect to a cylinder block mating surface, while a cylinder head of a compression self-ignition engine is provided with combustion chambers that are flush with a cylinder block mating surface. 
     In this application, a longitudinal direction of a cylinder head is defined as a direction of a row of cylinders when the cylinder head is mounted on a cylinder block to form an engine, i.e. an axial direction of a crankshaft. Further, in this application, a direction perpendicular to the longitudinal direction and parallel to a cylinder block mating surface of the cylinder head is defined as a width direction of the cylinder head and a direction perpendicular to the longitudinal direction and perpendicular to the cylinder block mating surface of the cylinder head is defined as a height direction of the cylinder head. 
     The plurality of intake ports are provided side by side in the longitudinal direction of the cylinder head. The plurality of intake ports respectively communicate with the plurality of combustion chambers. The intake port is provided for each combustion chamber. When the number of intake valves for each cylinder is two or more, each combustion chamber is formed with intake openings corresponding to the number of the intake valves. In this case, one intake port having one air inlet and a plurality of air outlets corresponding to the number of the intake openings may be provided for each combustion chamber or a plurality of intake ports corresponding to the number of the intake openings may be provided for each combustion chamber. The intake port is preferably a tumble flow generating port. 
     The first coolant flow passage is provided between a flat plane including central axes of the combustion chambers and parallel to the longitudinal direction of the cylinder head (hereinafter, the cylinder head longitudinal direction central flat plane) and a central line plane including central lines of the intake ports. The first coolant flow passage extends in the longitudinal direction of the cylinder head. “extend in the longitudinal direction” does not mean that the first coolant flow passage is provided only partially in the longitudinal direction or discretely in the longitudinal direction, but means that the first coolant flow passage is provided continuously in the longitudinal direction along the intake ports disposed side by side in the longitudinal direction. Further, “extend in the longitudinal direction” does not restrictively mean that the first coolant flow passage is straight in the longitudinal direction. The first coolant flow passage does not necessarily have a uniform shape in the width direction or the height direction of the cylinder head if it extends in the longitudinal direction as a whole. The first coolant flow passage may have a meandering shape corresponding to the shape on the cylinder head longitudinal direction central flat plane side of the intake ports disposed side by side in the longitudinal direction. 
     In at least one of cross sections perpendicular to the longitudinal direction, at least a portion of the second coolant flow passage is located between the combustion chamber and the first coolant flow passage. The second coolant flow passage may be provided to include a portion located between the combustion chamber and the first coolant flow passage in a cross section including the central axis of the combustion chamber and perpendicular to the longitudinal direction. In a cross section including a central axis of an intake valve insertion hole and perpendicular to the longitudinal direction, at least a portion of the second coolant flow passage may be provided to be located between the combustion chamber and the first coolant flow passage in a region sandwiched between the intake port and an exhaust port. 
     In the cylinder head, a temperature of a coolant flowing in the first coolant flow passage is lower than the temperature of the coolant flowing in the second coolant flow passage. 
     According to the configuration of the cylinder head described above, the heat generated from the combustion chambers can be absorbed by the second coolant flow passage and, therefore, it can be suppressed that the heat is directly transferred to the first coolant flow passage from the combustion chambers and thus that the temperature of the coolant flowing in the first coolant flow passage increases due to the heat generated from the combustion chambers. Particularly, if the second coolant flow passage is located between the vicinities of the centers of the combustion chambers that rise to high temperatures and the first coolant flow passage, it is possible to more effectively suppress an increase in the temperature of the coolant flowing in the first coolant flow passage. Consequently, it is possible to efficiently cool upper surface sides of the intake ports with the low-temperature coolant flowing in the first coolant flow passage and thus to efficiently cool the air flowing in the intake ports. In this application, assuming that the intake port is divided into two by the intake port central line plane, a surface on the cylinder head longitudinal direction central flat plane side may be called an upper surface of the intake port, while a surface on the cylinder block mating surface side may be called a lower surface of the intake port. 
     When the cylinder head includes spark plug insertion holes each opened to the combustion chamber at the center of the combustion chamber, the first coolant flow passage may be provided to pass through a region sandwiched between the spark plug insertion hole and the intake port in the cross section including the central axis of the combustion chamber and perpendicular to the longitudinal direction. When injector insertion holes are provided on the upper surface sides of the intake ports, the first coolant flow passage may be provided to pass through a region sandwiched between a central axis of the spark plug insertion hole and a central axis of the injector insertion hole in the cross section including the central axis of the combustion chamber and perpendicular to the longitudinal direction. 
     When the cylinder head includes injector insertion holes each opened to the combustion chamber near the central axis of the combustion chamber, the second coolant flow passage may include a portion located between an open end of the injector insertion hole and the first coolant flow passage in a cross section including a central axis of the injector insertion hole and perpendicular to the longitudinal direction. Particularly, if the second coolant flow passage is located between the vicinities of the open ends of the injector insertion holes that rise to high temperatures and the first coolant flow passage, it is possible to more effectively suppress an increase in the temperature of the coolant flowing in the first coolant flow passage. 
     When the cylinder head includes intake valve insertion holes, the first coolant flow passage includes the following modes with respect to the positional relationship between itself and the intake valve insertion holes. 
     In the multi-cylinder engine, the cylinder head may include intake valve insertion holes and, in a cross section including a central axis of the intake valve insertion hole and perpendicular to the longitudinal direction, the first coolant flow passage may be provided to pass through a region sandwiched between the intake valve insertion hole and the intake port. According to this mode, the first coolant flow passage can be disposed close to upper surfaces of the intake ports. 
     In the multi-cylinder engine, the cylinder head includes intake valve insertion holes and, in a cross section including a central axis of the intake valve insertion hole and perpendicular to the longitudinal direction, the first coolant flow passage may be provided to pass through a region on a side opposite to a region sandwiched between the intake valve insertion hole and the intake port with respect to the intake valve insertion hole. According to this mode, the first coolant flow passage can be disposed with high degree of freedom. For example, the first coolant flow passage can be disposed at portions, downstream of the intake valve insertion holes, of the intake ports, i.e. can be disposed close to connecting portions, with the combustion chambers, of the intake ports, where the wall temperature of the intake ports becomes highest. 
     Further, in the multi-cylinder engine, the cylinder head includes intake valve insertion holes and, in a cross section including a central axis of the intake valve insertion hole and perpendicular to the longitudinal direction, the first coolant flow passage may be provided to pass on both sides of the central axis of the intake valve insertion hole. According to this mode, regions to be cooled by the first coolant flow passage can be broadened. In this mode, the first coolant flow passage may include annular passages respectively surrounding the intake valve insertion holes and connecting passages each connecting the adjacent two annular passages to each other. “annular passage” does not mean that its shape is circular or elliptical. “annular passage” is sufficient if it is configured that a flow passage passing on one side of the central axis of the intake valve insertion hole and a flow passage passing on the other side of the central axis communicate with each other on both upstream and downstream sides. According to this configuration, the first coolant flow passage can be disposed close to both the upper surface of the intake port and the connecting portion, with the combustion chamber, of the intake port. 
     In the multi-cylinder engine, when the cylinder head includes two intake valve insertion holes for each combustion chamber, the connecting passages each connecting the adjacent two annular passages may include a first connecting passage and a second connecting passage. The first connecting passage passes through a cross section including the central axis of the combustion chamber and perpendicular to the longitudinal direction. The second connecting passage passes through a cross section passing between the adjacent two combustion chambers and perpendicular to the longitudinal direction. With respect to a flat plane including the central axes of the intake valve insertion holes and parallel to the longitudinal direction, the first connecting passage is disposed on one side of the flat plane, while the second connecting passage is disposed on the other side of the flat plane. That is, the first and second connecting passages are disposed alternately in the longitudinal direction in a manner to sandwich the annular passage between the first second connecting passages. According to this configuration, the coolant is prevented from staying in the annular passages. 
     The cylinder head may include a head bolt insertion hole that passes between the two intake ports communicating with the adjacent two combustion chambers and that is perpendicular to the cylinder block mating surface. In this case, in a cross section including a central axis of the head bolt insertion hole and perpendicular to the longitudinal direction, the first coolant flow passage may be provided to pass through a region closer to the cylinder head longitudinal direction central flat plane with respect to the head bolt insertion hole. According to this configuration, the first coolant flow passage is prevented from passing at a high position in the height direction of the cylinder head so that no air pocket occurs in the first coolant flow passage. 
     In the multi-cylinder engine, the first coolant flow passage and the second coolant flow passage may be independent of each other in the cylinder head. “independent of each other in the cylinder head” means that the first coolant flow passage and the second coolant flow passage do not communicate with each other at least in the cylinder head. According to this configuration, the temperature of the coolant flowing in the first coolant flow passage can be made distinctly lower than that of the coolant flowing in the second coolant flow passage. A coolant circulation system including the first coolant flow passage and a coolant circulation system including the second coolant flow passage may be formed as separate systems. 
     In the multi-cylinder engine, the first coolant flow passage may communicate with a first hole opened in one end face in the longitudinal direction of the cylinder head, and the first coolant flow passage may communicate with a second hole opened in the other end face in the longitudinal direction of the cylinder head. “end face in the longitudinal direction” is a surface forming an end in the longitudinal direction and may be a flat surface or an uneven surface. When the first coolant flow passage is formed by a sand core, holes (sand removing holes) are formed in both end faces in the longitudinal direction by core supports supporting the sand core. The first hole and the second hole can be these holes formed by the core supports. One of the first and second holes can be used as a coolant inlet, while the other can be used as a coolant outlet. 
     In the multi-cylinder engine, the first coolant flow passage may communicate with a first hole opened in an end face in the longitudinal direction of the cylinder head, and the first coolant flow passage may communicate with a second hole opened in an end face in the width direction of the cylinder head. “end face in the width direction” is a surface forming an end in the width direction and may be a flat surface or an uneven surface. When the first coolant flow passage is formed by a sand core, holes are formed in both end faces in the longitudinal direction by core supports supporting the sand core. One of these holes in both end faces may be left as the first hole, while the other hole may be sealed. One of the first and second holes can be used as a coolant inlet, while the other can be used as a coolant outlet. 
     In the multi-cylinder engine, the first coolant flow passage may communicate with a first hole opened in an end face in the longitudinal direction of the cylinder head, and the first coolant flow passage may communicate with a second hole opened in the cylinder block mating surface. Holes are formed in both end faces in the longitudinal direction by core supports supporting a sand core. One of these holes in both end faces may be left as the first hole, while the other hole may be sealed. The first coolant flow passage may be connected to the second hole via a communication passage provided between the two intake ports communicating with the adjacent two combustion chambers. The first coolant flow passage may be connected to the second hole via a communication passage provided between at least one of end faces in the longitudinal direction of the cylinder head and the intake port closest to the at least one of end faces. One of the first and second holes can be used as a coolant inlet, while the other can be used as a coolant outlet. 
     The first coolant flow passage may be configured to communicate with the second coolant flow passage in the cylinder head. In this case, however, it is configured that the coolant having passed through the first coolant flow passage flows into the second coolant flow passage. That is, it is configured that the low-temperature coolant before an increase in temperature due to heat transfer flows in the first coolant flow passage. According to this configuration, the coolant is allowed to flow in the first coolant flow passage and the second coolant flow passage by a single circulation system. 
     According to the multi-cylinder engine including the cylinder head described above, since it is possible to suppress heat transfer from the combustion chambers to the first coolant flow passage by the second coolant flow passage located between the combustion chambers and the first coolant flow passage, the temperature of the coolant flowing in the first coolant flow passage can be maintained low. Accordingly, it is possible to effectively cool the upper surface sides of the intake ports and thus to efficiently cool the air flowing in the intake ports. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Features, advantages, and technical and industrial significance of exemplary embodiments of the invention will be described below with reference to the accompanying drawings, in which like numerals denote like elements, and wherein: 
         FIG. 1  is a diagram showing a configuration of an engine cooling system according to a first embodiment of the invention; 
         FIG. 2  is a plan view of a cylinder head of the first embodiment of the invention; 
         FIG. 3  is a cross-sectional view, taken along line A-A of  FIG. 2 , showing a cross section, including a central axis of an intake valve insertion hole and perpendicular to a longitudinal direction, of the cylinder head of the first embodiment of the invention; 
         FIG. 4  is a cross-sectional view, taken along line B-B of  FIG. 2 , showing a cross section, including a central axis of a combustion chamber and perpendicular to the longitudinal direction, of the cylinder head of the first embodiment of the invention; 
         FIG. 5  is a cross-sectional view, taken along line C-C of  FIG. 2 , showing a cross section, passing between adjacent two combustion chambers and perpendicular to the longitudinal direction, of the cylinder head of the first embodiment of the invention; 
         FIG. 6  is a perspective view showing, in a see-through manner, intake ports and a first coolant flow passage of the cylinder head of the first embodiment of the invention; 
         FIG. 7  is a diagram showing the positional relationship between the intake port, a head bolt, and the first coolant flow passage in the cylinder head of the first embodiment of the invention; 
         FIG. 8  is a perspective view showing the intake ports of the cylinder head of the first embodiment of the invention and an intake port central line plane thereof; 
         FIG. 9  is a side view showing the intake port of the cylinder head of the first embodiment of the invention and a central line thereof; 
         FIG. 10  is a perspective view showing a modification of the intake ports and an intake port central line plane thereof; 
         FIG. 11  is a side view showing the modification of the intake port and a central line thereof; 
         FIG. 12  is a perspective view showing the intake ports and intake valve insertion holes along with an intake valve insertion hole central axis plane thereof of the cylinder head of the first embodiment of the invention; 
         FIG. 13  is a side view showing the intake port and the intake valve insertion hole along with its central axis of the cylinder head of the first embodiment of the invention; 
         FIG. 14  is a diagram showing application example  1  in which the engine cooling system of the first embodiment of the invention is applied to a supercharged engine system; 
         FIG. 15  is a diagram showing application example  2  in which the engine cooling system of the first embodiment of the invention is applied to a hybrid system; 
         FIG. 16  is a cross-sectional view showing a cross section, including a central axis of an intake valve insertion hole and perpendicular to a longitudinal direction, of a cylinder head of a second embodiment of the invention, i.e. a cross section corresponding to the A-A cross section of  FIG. 2 ; 
         FIG. 17  is a cross-sectional view showing a cross section, including a central axis of a combustion chamber and perpendicular to the longitudinal direction, of the cylinder head of the second embodiment of the invention, i.e. a cross section corresponding to the B-B cross section of  FIG. 2 ; 
         FIG. 18  is a cross-sectional view showing a cross section, passing between adjacent two combustion chambers and perpendicular to the longitudinal direction, of the cylinder head of the second embodiment of the invention, i.e. a cross section corresponding to the C-C cross section of  FIG. 2 ; 
         FIG. 19  is a perspective view showing, in a see-through manner, intake ports and a first coolant flow passage inside the cylinder head of the second embodiment of the invention; 
         FIG. 20  is a cross-sectional view showing a cross section, including a central axis of an intake valve insertion hole and perpendicular to a longitudinal direction, of a cylinder head of a third embodiment of the invention, i.e. a cross section corresponding to the A-A cross section of  FIG. 2 ; 
         FIG. 21  is a cross-sectional view showing a cross section, including a central axis of a combustion chamber and perpendicular to the longitudinal direction, of the cylinder head of the third embodiment of the invention, i.e. a cross section corresponding to the B-B cross section of  FIG. 2 ; 
         FIG. 22  is a cross-sectional view showing a cross section, passing between adjacent two combustion chambers and perpendicular to the longitudinal direction, of the cylinder head of the third embodiment of the invention, i.e. a cross section corresponding to the C-C cross section of  FIG. 2 ; 
         FIG. 23  is a perspective view showing, in a see-through manner, intake ports and a first coolant flow passage inside the cylinder head of the third embodiment of the invention; 
         FIG. 24  is a cross-sectional view showing a cross section, including a central axis of an intake valve insertion hole and perpendicular to a longitudinal direction, of a cylinder head of a fourth embodiment of the invention, i.e. a cross section corresponding to the A-A cross section of  FIG. 2 ; 
         FIG. 25  is a cross-sectional view showing a cross section, including a central axis of a combustion chamber and perpendicular to the longitudinal direction, of the cylinder head of the fourth embodiment of the invention, i.e. a cross section corresponding to the B-B cross section of  FIG. 2 ; 
         FIG. 26  is a cross-sectional view showing a cross section, passing between adjacent two combustion chambers and perpendicular to the longitudinal direction, of the cylinder head of the fourth embodiment of the invention, i.e. a cross section corresponding to the C-C cross section of  FIG. 2 ; 
         FIG. 27  is a perspective view showing, in a see-through manner, intake ports and a first coolant flow passage inside the cylinder head of the fourth embodiment of the invention; 
         FIG. 28  is a diagram showing the positional relationship between the intake port, a head bolt, and the first coolant flow passage in the cylinder head of the fourth embodiment of the invention; 
         FIG. 29  is a cross-sectional view showing a cross section, including a central axis of an intake valve insertion hole and perpendicular to a longitudinal direction, of a cylinder head of a fifth embodiment of the invention, i.e. a cross section corresponding to the A-A cross section of  FIG. 2 ; 
         FIG. 30  is a cross-sectional view showing a cross section, including a central axis of an intake valve insertion hole and perpendicular to a longitudinal direction, of a cylinder head of a sixth embodiment of the invention; 
         FIG. 31  is a cross-sectional view showing a cross section, including a central axis of a combustion chamber and perpendicular to the longitudinal direction, of the cylinder head of the sixth embodiment of the invention; 
         FIG. 32  is a diagram showing a configuration of an engine cooling system of a seventh embodiment of the invention; 
         FIG. 33  is a perspective view showing a configuration of an intermediate communication passage in the engine cooling system of the seventh embodiment of the invention; 
         FIG. 34  is a diagram showing the positional relationship between the intermediate communication passage shown in  FIG. 33  and a head bolt; 
         FIG. 35  is a diagram showing a modification of the intermediate communication passage of the engine cooling system of the seventh embodiment of the invention; 
         FIG. 36  is a diagram showing a modification of a first circulation system of the engine cooling system of the seventh embodiment of the invention; 
         FIG. 37  is a diagram showing a configuration of an engine cooling system of an eighth embodiment of the invention; 
         FIG. 38  is a perspective view showing, in a see-through manner, intake ports and a first coolant flow passage of a cylinder head in the engine cooling system of the eighth embodiment of the invention; 
         FIG. 39  is a diagram showing a configuration of an engine cooling system of a ninth embodiment of the invention; 
         FIG. 40  is a diagram showing a configuration of an engine cooling system of a tenth embodiment of the invention; and 
         FIG. 41  is a diagram showing a configuration of an engine cooling system of an eleventh embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Referring to the drawings, embodiments of the invention will be described. However, the following embodiments are only intended to show, by way of example, apparatuses and methods for embodying the technical ideas of the invention and, unless otherwise stated, are not intended to limit the structures and arrangements of components, the sequences of processes, and so on to those described below. The invention is not limited to the following embodiments and can be carried out with various changes in a range not departing from its gist. 
     Hereinbelow, a first embodiment of the invention will be described with reference to the drawings. The premise of the first embodiment is that an engine is a spark-ignition liquid-cooled inline four-cylinder engine. This premise also applies to later-described second to fifth embodiments. However, when applying the invention to an engine, there is no limitation to the number and arrangement of cylinders of the engine and to the ignition system of the engine. 
     Referring to  FIG. 1 , the configuration of an engine cooling system according to the first embodiment of the invention will be described. A coolant for cooling an engine is circulated between the engine and a radiator by each of circulation systems. The engine includes a cylinder block  151  and a cylinder head  101  mounted on the cylinder block  151  via a gasket (not shown). The supply of the coolant is carried out for both the cylinder block  151  and the cylinder head  101 . 
     The engine cooling system of the first embodiment includes dual circulation systems  120  and  160 . The first circulation system  120  and the second circulation system  160  each form an independent closed loop and each include a radiator  124 ,  164  and a water pump  123 ,  163 . Each circulation system  120 ,  160  may further include a liquid temperature sensor and a thermostat for liquid temperature adjustment (neither shown). 
     The first circulation system  120  includes a first coolant flow passage  30  formed in the cylinder head  101 . The cylinder head  101  is formed with a coolant inlet and a coolant outlet each communicating with the first coolant flow passage  30 . The coolant inlet of the cylinder head  101  is connected to a coolant outlet of the radiator  124  via a coolant introducing pipe  121 , while the coolant outlet of the cylinder head  101  is connected to a coolant inlet of the radiator  124  via a coolant discharge pipe  122 . The coolant introducing pipe  121  is provided with the water pump  123 . 
     The second circulation system  160  includes a second coolant flow passage  20  formed in the cylinder head  101  and a third coolant flow passage  152  formed in the cylinder block  151 . The third coolant flow passage  152  of the cylinder block  151  includes a water jacket surrounding cylinders. The second coolant flow passage  20  of the cylinder head  101  and the third coolant flow passage  152  of the cylinder block  151  are connected to each other via an opening formed in a mating surface between the cylinder head  101  and the cylinder block  151 . The cylinder block  151  is formed with a coolant inlet communicating with the third coolant flow passage  152 , while the cylinder head  101  is formed with a coolant outlet communicating with the second coolant flow passage  20 . The coolant inlet of the cylinder block  151  is connected to a coolant outlet of the radiator  164  via a coolant introducing pipe  161 , while the coolant outlet of the cylinder head  101  is connected to a coolant inlet of the radiator  164  via a coolant discharge pipe  162 . The coolant introducing pipe  161  is provided with the water pump  163 . 
     The cylinder head  101  is formed with four intake ports  2  for four cylinders. When the cylinder head  101  is located on an upper side in the vertical direction with respect to the cylinder block  151 , the first coolant flow passage  30  is provided to be located on upper sides of the intake ports  2 . The second coolant flow passage  20  is provided so that at least part thereof is located on lower sides of the intake ports  2 . 
     In this specification, hereinbelow, unless otherwise stated, the positional relationship between components will be described assuming that the cylinder head  101  is located on the upper side in the vertical direction with respect to the cylinder block  151 . This assumption is only for the purpose of facilitating understanding of a description and does not give any limitative meaning to the configuration of a cylinder head according to the invention. The configuration of the cylinder head  101 , particularly the configurations of the first coolant flow passage  30  and the second coolant flow passage  20 , will be described later. 
     According to the configuration shown in  FIG. 1 , liquid temperature adjustments can be carried out independently by the two circulation systems  120  and  160 . Specifically, it is set that the temperature of the coolant that flows in the first coolant flow passage  30  is equal to that of the coolant that flows in the second coolant flow passage  20  at the time of cold engine start-up and that as warming-up of the engine progresses, the temperature of the coolant that flows in the first coolant flow passage  30  becomes lower than that of the coolant that flows in the second coolant flow passage  20 . Since the coolant that flows in the second coolant flow passage  20  is the coolant having passed through the inside of the cylinder block  151 , its temperature has risen higher than that of the coolant at the coolant inlet of the cylinder block  151 . Therefore, according to the configuration shown in  FIG. 1 , even if the temperatures of the coolants when exiting the radiators  124  and  164  are equal to each other, when the coolants have reached the cylinder head  101 , the temperature of the coolant that flows in the second coolant flow passage  20  becomes higher than that of the coolant that flows in the first coolant flow passage  30 . In other words, the coolant that flows in the first coolant flow passage  30  is maintained at a temperature lower than that of the coolant that flows in the second coolant flow passage  20 . 
     Next, the basic configuration of the cylinder head  101  of the first embodiment will be described. The description will be made using a plan view and cross-sectional views of the cylinder head  101 . Herein, the basic configuration is a configuration other than the configurations of the first coolant flow passage  30  and the second coolant flow passage  20  which are one of features of the invention. The configurations of the first coolant flow passage  30  and the second coolant flow passage  20  will be described in detail after clarifying the basic configuration. 
     Hereinbelow, the basic configuration of the cylinder head of the first embodiment will be described.  FIG. 2  is a plan view of the cylinder head  101  of the first embodiment. Specifically,  FIG. 2  is a plan view of the cylinder head  101  as seen from the side of its head cover attaching surface  1   b  to which a head cover is attached. Therefore, in  FIG. 2 , a cylinder block mating surface, as a back surface, of the cylinder head  101  is not seen. In this specification, as described before, an axial direction of a crankshaft is defined as a longitudinal direction of the cylinder head  101 , while a direction perpendicular to the longitudinal direction and parallel to the cylinder block mating surface of the cylinder head  101  is defined as a width direction of the cylinder head  101 . Of end faces  1   c  and  1   d  in the longitudinal direction, the end face  1   d  on the output end side of the crankshaft will be referred to as a “rear end face”, while the end face  1   c  on the opposite side thereof will be referred to as a “front end face”. 
     The cylinder head  101  of the first embodiment is a cylinder head of a spark-ignition inline four-cylinder engine. Although not shown in  FIG. 1 , four combustion chambers for four cylinders are formed side by side at regular intervals in an inline configuration in the longitudinal direction in the lower surface (the mating surface with the cylinder block) of the cylinder head  101 . The cylinder head  101  is formed with spark plug insertion holes  12  for the respective combustion chambers. 
     The intake ports  2  and exhaust ports  3  are opened at side surfaces of the cylinder head  101 . Specifically, the intake ports  2  are opened at the right side surface of the cylinder head  101  as seen from the front end face  1   c  side, while the exhaust ports  3  are opened at the left side surface. Hereinafter, in this specification, the side surface located on the right side as seen from the front end face  1   c  side of the cylinder head  101  will be referred to as a “right side surface” of the cylinder head  101 , while the side surface located on the left side will be referred to as a “left side surface” of the cylinder head  101 . The intake ports  2  extend from the respective combustion chambers and are independently opened at the right side surface of the cylinder head  101 . The exhaust ports  3  are joined into a single exhaust port  3  inside the cylinder head  101  and this collective single exhaust port  3  is opened at the left side surface of the cylinder head  101 . In this regard, hereinafter, the exhaust ports  3  along with the collective single exhaust port  3  may be collectively referred to as an “exhaust port  3 ” where appropriate. Accordingly, in this specification, the right side as seen from the front end face  1   c  side of the cylinder head  101  may be referred to as an “intake side”, while the left side may be referred to as an “exhaust side”. 
     The cylinder head  101  of the first embodiment is a cylinder head of a four-valve engine in which two intake valves and two exhaust valves are provided for each cylinder. The cylinder head  101  is formed in its upper surface with two intake valve insertion holes  7  and two exhaust valve insertion holes  8  surrounding each spark plug insertion hole  12 . The intake valve insertion holes  7  communicate with the intake ports  2  in the cylinder head  101 , while the exhaust valve insertion holes  8  communicate with the exhaust ports  3  in the cylinder head  101 . 
     Head bolt insertion holes  13 ,  14 ,  15 , and  16  for insertion of head bolts for attaching the cylinder head  101  to the cylinder block are formed on the inner side of the head cover attaching surface  1   b.  The head bolts are provided in the number of 5 on each of the left and right sides with respect to the row of the combustion chambers. On the intake side, each of the head bolt insertion holes  13  is formed between the adjacent two intake ports  2  and the head bolt insertion holes  15  are respectively formed between the front end face  1   c  and the intake port  2  closest thereto and between the rear end face  1   d  and the intake port  2  closest thereto. On the exhaust side, the head bolt insertion holes  14  are respectively formed at the crotches of the exhaust ports  3  branching to the combustion chambers and the head bolt insertion holes  16  are respectively formed between the front end face  1   c  and the exhaust port  3  and between the rear end face  1   d  and the exhaust port  3 . 
     Next, the configuration of the inside of the cylinder head  101  of the first embodiment will be described with reference to the cross-sectional views. Cross sections of the cylinder head  101  to pay attention are a cross section, including a central axis of the intake valve insertion hole  7  and perpendicular to the longitudinal direction, of the cylinder head  101  (A-A cross section of  FIG. 2 ), a cross section, including a central axis of the combustion chamber and perpendicular to the longitudinal direction, of the cylinder head  101  (B-B cross section of  FIG. 2 ), and a cross section, passing between the adjacent two combustion chambers and perpendicular to the longitudinal direction, of the cylinder head  101  (C-C cross section of  FIG. 2 ). 
     Hereinbelow, the basic configuration of the cylinder head as seen in the cross section including the central axis of the intake valve insertion hole and perpendicular to the longitudinal direction will be described.  FIG. 3  is a cross-sectional view showing a cross section, including a central axis of the intake valve insertion hole  7  and perpendicular to the longitudinal direction, of the cylinder head  101  (A-A cross section of  FIG. 2 ).  FIG. 3  shows a state where an intake valve  11  is disposed in the cylinder head  101 . As shown in  FIG. 3 , a cylinder block mating surface  1   a  as a lower surface of the cylinder head  101  is formed with a pent-roof shaped combustion chamber  4 . When the cylinder head  101  is mounted on the cylinder block, the combustion chamber  4  closes the cylinder from above to form a closed space. When a closed space sandwiched between the cylinder head  101  and a piston is defined as a combustion chamber, the combustion chamber  4  can be called a combustion chamber ceiling surface. 
     The intake port  2  is opened at an inclined surface, on the right side as seen from the front end side of the cylinder head  101 , of the combustion chamber  4 . A connecting portion between the intake port  2  and the combustion chamber  4 , i.e. an open end on the combustion chamber side of the intake port  2 , serves as an intake opening that is configured to be opened and closed by the intake valve  11 . Since two intake valves  11  are provided for each cylinder, each combustion chamber  4  is formed with two intake openings of the intake port  2 . An inlet of the intake port  2  is opened in the right side surface of the cylinder head  101 . The intake port  2  extends obliquely downward to the left from an opening of the inlet and branches into two ports on the way and these two branch ports respectively communicate with the intake openings formed in the combustion chamber  4 . In  FIG. 3 , there is shown a branch port  2 L on the engine front end side in the longitudinal direction. The intake port  2  is a tumble flow generating port that can generate a tumble flow in the cylinder. 
     The cylinder head  101  is formed with the intake valve insertion hole  7  for passing a stem of the intake valve  11  therethrough. In the upper surface of the cylinder head  101  on the inner side of the head cover attaching surface  1   b,  there is provided an intake-side valve drive mechanism chamber  5  that receives therein a valve drive mechanism configured to drive the intake valves  11 . The intake valve insertion hole  7  extends straight obliquely upward to the right from an upper surface, near the combustion chamber  4 , of the intake port  2  to the intake-side valve drive mechanism chamber  5 . A valve guide  9  for supporting the stem of the intake valve  11  is press-fitted into the intake valve insertion hole  7 . A central axis L 3  of the intake valve insertion hole  7  is included in the cross section shown in  FIG. 3 , i.e. in a flat plane perpendicular to the longitudinal direction. 
     The exhaust port  3  is opened at an inclined surface, on the left side as seen from the front end side of the cylinder head  101 , of the combustion chamber  4 . A connecting portion between the exhaust port  3  and the combustion chamber  4 , i.e. an open end on the combustion chamber side of the exhaust port  3 , serves as an exhaust opening that is configured to be opened and closed by an exhaust valve (the exhaust valve is not shown in  FIG. 3 ). Since two exhaust valves are provided for each cylinder, each combustion chamber  4  is formed with two exhaust openings of the exhaust port  3 . The exhaust port  3  has a manifold shape having eight inlets (exhaust openings) respectively provided for the exhaust valves of the combustion chambers  4  and one outlet that is opened in the left side surface of the cylinder head  101 . The outlet of the exhaust port  3  is not located in the cross section shown in  FIG. 3 . 
     The cylinder head  101  is formed with the exhaust valve insertion hole  8  for passing a stem of the exhaust valve therethrough. In the upper surface of the cylinder head  101  on the inner side of the head cover attaching surface  1   b,  there is provided an exhaust-side valve drive mechanism chamber  6  that receives therein a valve drive mechanism configured to drive the exhaust valves. The exhaust valve insertion hole  8  extends straight obliquely upward to the left from an upper surface, near the combustion chamber  4 , of the exhaust port  3  to the exhaust-side valve drive mechanism chamber  6 . A valve guide  10  for supporting the stern of the exhaust valve is press-fitted into the exhaust valve insertion hole  8 . 
     Next, the basic configuration of the cylinder head as seen in the cross section including the central axis of the combustion chamber and perpendicular to the longitudinal direction will be described.  FIG. 4  is a cross-sectional view showing a cross section, including a central axis L 1  of the combustion chamber  4  and perpendicular to the longitudinal direction, of the cylinder head  101  (B-B cross section of  FIG. 2 ). The cylinder head  101  is formed with the spark plug insertion hole  12  for attaching a spark plug. The spark plug insertion hole  12  is opened to a top portion of the pent-roof shaped combustion chamber  4 . The central axis L 1  of the combustion chamber  4  coincides with a central axis of the cylinder when the cylinder head  101  is mounted on the cylinder block. 
     The intake port  2  shown in  FIG. 4  is a portion thereof upstream of its branching portion. The two branch ports downstream of the branching portion are respectively located on both sides of a flat plane including the central axis L 1  of the combustion chamber  4  and perpendicular to the longitudinal direction and thus are not included in the cross section shown in  FIG. 4 . In the cross section shown in  FIG. 4 , part of the exhaust port  3  having the manifold shape is seen. 
     A port injector insertion hole  17  for attaching a port injector is formed in the side surface of the cylinder head  101  on an upper side with respect to the intake port  2 . A central axis of the port injector insertion hole  17  is located in the flat plane including the central axis L 1  of the combustion chamber  4  and perpendicular to the longitudinal direction. The port injector insertion hole  17  crosses the intake port  2  at an acute angle and is opened to a port injector attaching portion  2   c  formed convex upward on an upper surface of the branching portion of the intake port  2 . The port injector (not shown) inserted into the port injector insertion hole  17  exposes its nozzle tip from the port injector attaching portion  2   c  and injects fuel into the intake port  2 . 
     An in-cylinder direct-injection injector insertion hole  18  for attaching an in-cylinder direct-injection injector is formed in the side surface of the cylinder head  101  on a lower side with respect to the intake port  2 . A central axis of the in-cylinder direct-injection injector insertion hole  18  is located in the flat plane including the central axis L 1  of the combustion chamber  4  and perpendicular to the longitudinal direction. The in-cylinder direct-injection injector insertion hole  18  is opened to the combustion chamber  4 . The in-cylinder direct-injection injector (not shown) inserted into the in-cylinder direct-injection injector insertion hole  18  injects fuel directly into the cylinder. 
     Next, the basic configuration of the cylinder head as seen in the cross section passing between the adjacent two combustion chambers and perpendicular to the longitudinal direction will be described.  FIG. 5  is a cross-sectional view showing a cross section, passing between the adjacent two combustion chambers and perpendicular to the longitudinal direction, of the cylinder head  101  (C-C cross section of  FIG. 2 ). The cylinder head  101  is formed with the intake-side head bolt insertion hole  13  extending vertically downward from the intake-side valve drive mechanism chamber  5  and is formed with the exhaust-side head bolt insertion hole  14  extending vertically downward from the exhaust-side valve drive mechanism chamber  6 . The head bolt insertion holes  13  and  14  are perpendicular to the cylinder block mating surface  1   a  and opened at the cylinder block mating surface  1   a.  The cross section shown in  FIG. 5  is a cross section including central axes of the head bolt insertion holes  13  and  14  and perpendicular to the longitudinal direction. 
     In the cross section shown in  FIG. 5 , the collective portion of the exhaust port  3  having the manifold shape is seen. The collective portion of the manifold-shaped exhaust port  3  is opened at the left side surface of the cylinder head  101 . The exhaust ports  3  are joined into one inside the cylinder head  101  in a manner to avoid the head bolt insertion holes  14 . 
     Next, the configurations of the coolant flow passages of the cylinder head  101  of the first embodiment will be described. The description will be made using the cross-sectional views of the cylinder head  101  and a perspective view showing the coolant flow passage inside the cylinder head  101  in a see-through manner. 
     Hereinbelow, the configurations of the coolant flow passages of the cylinder head of the first embodiment will be described. First, before describing the configurations of the coolant flow passages of the cylinder head, reference planes of the cylinder head for use in the description will be defined herein. In this specification, four reference planes are defined. The reference planes defined herein also apply to later-described second to fifth embodiments. 
     1. Cylinder Block Mating Surface (First Reference Plane) The cylinder block mating surface  1   a  shown in  FIGS. 3, 4, and 5  is a first reference plane. When the cylinder head  101  is mounted on the cylinder block, the cylinder block mating surface  1   a  is a flat plane perpendicular to the central axes of the cylinders of the cylinder block. 
     2. Cylinder Head Longitudinal Direction Central Flat Plane (Second Reference Plane)  FIG. 4  shows the central axis L 1  of the combustion chamber  4 . A second reference plane is a virtual flat plane including the central axes L 1  of the combustion chambers  4  and parallel to the longitudinal direction. This flat plane will be referred to as a “cylinder head longitudinal direction central flat plane”. In  FIGS. 3 and 5 , a cylinder head longitudinal direction central flat plane S 1  is shown by a virtual line. In the cross section shown in  FIG. 4 , the cylinder head longitudinal direction central flat plane S 1  overlaps the central axis L 1  of the combustion chamber  4 . When the cylinder head  101  is mounted on the cylinder block, the cylinder head longitudinal direction central flat plane S 1  is a flat plane including the central axes of the cylinders of the cylinder block. 
     3. Intake Port Central Line Plane (Third Reference Plane) In  FIGS. 3, 4 , and  5 , there is shown a virtual line denoted by symbol S 2 . This virtual line represents an intake port central line plane as a third reference plane. The intake port central line plane is a virtual plane defined as a plane including central lines of the intake ports  2 . Hereinbelow, referring to  FIGS. 8 to 11 , the central line of the intake port  2  and the intake port central line plane will be described in detail. 
       FIG. 9  is a side view showing the intake port  2  of the cylinder head of the first embodiment and a central line L 2  thereof.  FIG. 9  shows the shape of the intake port  2  when seen from the front end side of the cylinder head assuming the inside of the cylinder head to be transparent. The central line L 2  is defined as a line passing through the centers of cross sections each taken perpendicular to a flow direction of the intake port  2 . Accordingly, in  FIG. 9 , the distance from an upper surface  2   a  of the intake port  2  to the central line L 2  is equal to the distance from a lower surface  2   b  of the intake port  2  to the central line L 2 . In the first embodiment, since the intake port  2  extends substantially straight from its inlet to its intake openings, the central line L 2  is also shown in a straight line in a projection plane (flat plane perpendicular to the longitudinal direction of the cylinder head). The port injector attaching portion  2   c  for attaching the port injector and an intake valve insertion portion  2   d  into which the stem of the intake valve is inserted are formed convex upward on the upper surface  2   a  of the intake port  2 . These convex portions do not need to be taken into account when calculating the position of the central line L 2 . 
       FIG. 8  is a perspective view showing the intake ports  2  of the cylinder head of the first embodiment and the intake port central line plane S 2  thereof.  FIG. 8  shows the shape of the intake ports  2  and the positional relationship between the intake ports  2  and the intake port central line plane S 2  when seen assuming the inside of the cylinder head to be transparent. From  FIG. 8 , it is seen that the intake port  2  branches into two branch ports  2 L and  2 R on the way. Although not shown, the central line L 2  also branches into two central lines inside the intake port  2  and these two branched central lines respectively pass through the centers of cross sections of the branch ports  2 L and  2 R. The central lines L 2  become a straight line when projected on the flat plane perpendicular to the longitudinal direction of the cylinder head. Accordingly, the intake port central line plane S 2  including those central lines L 2  is given by a flat plane that is perpendicular to the flat plane perpendicular to the longitudinal direction of the cylinder head. Of a wall surface forming the intake port  2 , a surface located on the cylinder head longitudinal direction central flat plane S 1  side with respect to the intake port central line plane S 2  will be referred to as an “upper surface”, while a surface located on the cylinder block mating surface  1   a  side with respect to the intake port central line plane S 2  will be referred to as a “lower surface”. 
       FIG. 11  is a side view showing a modification of the intake port  2  and a central line L 2  thereof. The same symbols as those in the first embodiment are assigned to respective portions of the modification. In this modification, the intake port  2  has a shape that extends straight from its inlet to part of the way and then gradually curves vertically downward toward its intake openings. Accordingly, in a projection plane (flat plane perpendicular to the longitudinal direction of the cylinder head), the central line L 2  is shown in a straight line from the inlet of the intake port  2  to part of the way and then in a curved line that gradually curves vertically downward toward the intake openings of the intake port  2 . 
       FIG. 10  is a perspective view showing the modification of the intake ports  2  and an intake port central line plane S 2  thereof. From  FIG. 10 , it is seen that the intake port  2  has a straight shape until it branches into two branch ports  2 L and  2 R on the way, and then curves at the respective branch ports  2 L and  2 R. The intake port central line plane S 2  in this modification is given by a flat plane and a curved plane corresponding to the shape of the intake ports  2 . Accordingly, the intake port central line plane S 2  is not necessarily a flat plane and may be given by a plane in a combination of a flat plane and a curved plane or by a plurality of curved planes with different curvatures depending on the shape of the intake ports  2 . 
     4. Intake Valve Insertion Hole Central Axis Plane (Fourth Reference Plane)  FIG. 3  shows the central axis L 3  of the intake valve insertion hole  7 . The central axis L 3  of the intake valve insertion hole  7  is also a central axis of the intake valve  11 . A fourth reference plane is a virtual flat plane including the central axes L 3  of the intake valve insertion holes  7  and parallel to the longitudinal direction. This flat plane will be referred to as an “intake valve insertion hole central axis plane”. In  FIGS. 4 and 5 , an intake valve insertion hole central axis plane S 3  is shown by a virtual line. In the cross section shown in  FIG. 3 , the intake valve insertion hole central axis plane S 3  overlaps the central axis L 3  of the intake valve insertion hole  7 . 
       FIG. 13  is a side view showing the intake port  2  and the intake valve insertion hole  7  along with its central axis L 3  of the cylinder head of the first embodiment.  FIG. 13  shows the shapes of the intake port  2  and the intake valve insertion hole  7  when seen from the front end side of the cylinder head assuming the inside of the cylinder head to be transparent. A ring-shaped valve seat  2   f  is press-fitted into the intake opening of the intake port  2 . The central axis L 3  of the intake valve insertion hole  7  coincides with a central axis of the valve seat  2   f.    
       FIG. 12  is a perspective view showing the intake ports  2  and the intake valve insertion holes  7  along with the intake valve insertion hole central axis plane S 3  thereof of the cylinder head of the first embodiment.  FIG. 12  shows the shape of forward end portions of the intake ports  2  and the positional relationship between the intake valve insertion holes  7  and the intake valve insertion hole central axis plane S 3  when seen assuming the inside of the cylinder head to be transparent. The intake valve insertion hole central axis plane S 3  is a flat plane in which the central axes L 3  of the intake valve insertion holes  7  of the intake ports  2  are arranged in parallel to each other. 
     Next, of the dual coolant flow passages provided in the cylinder head of the first embodiment, the shape of the first coolant flow passage in which the low-temperature coolant flows will be described with reference to  FIGS. 6 and 7 .  FIG. 6  is a perspective view showing, in a see-through manner, the intake ports  2  and the first coolant flow passage  30  of the cylinder head of the first embodiment.  FIG. 6  shows the shape of the first coolant flow passage  30  and the positional relationship between the first coolant flow passage  30 , the intake ports  2 , and the valve guides  9  when seen assuming the inside of the cylinder head to be transparent. 
     The first coolant flow passage  30  is provided on the upper side of the row of the intake ports  2  in the cylinder head. The first coolant flow passage  30  extends in a direction of the row of the intake ports  2 , i.e. in the longitudinal direction of the cylinder head, along the upper surfaces  2   a  of the intake ports  2 . 
     The first coolant flow passage  30  has a unit structure for each intake port  2 . In  FIG. 6 , the structure of a portion encircled by a dotted line is the unit structure of the first coolant flow passage  30 . The unit structure includes a pair of annular passages respectively disposed around the left and right valve guides  9  (to be exact, the intake valve insertion holes) of the intake port  2 . Each annular passage includes an inner flow passage  31  located on the cylinder head longitudinal direction central flat plane side with respect to the valve guide  9  and an outer flow passage  32  located on the side surface side of the cylinder head with respect to the valve guide  9 . The inner flow passage  31  and the outer flow passage  32  are each a flow passage curved in an arc and are axially symmetric with respect to the valve guide  9 . Further, the inner flow passage  31  and the outer flow passage  32  have substantially the same flow passage cross-sectional area. 
     The unit structure includes a first connecting passage  34  connecting the left and right annular passages each including the inner flow passage  31  and the outer flow passage  32 . The first connecting passage  34  is located above a space between the left and right branch ports of the intake port  2  on the middle side of the cylinder head with respect to the valve guides  9 . The first connecting passage  34  is a flow passage extending in the longitudinal direction and continuously communicates with the left and right inner flow passages  31 . “continuously communicate” means that a direction of flow in the inner flow passage  31  and a direction of flow in the first connecting passage  34  coincide with each other at a connecting position between the inner flow passage  31  and the first connecting passage  34 . The outer flow passage  32  communicates with the connecting position between the inner flow passage  31  and the first connecting passage  34 . 
     The first coolant flow passage  30  includes second connecting passages  33  each connecting the adjacent two unit structures. The second connecting passage  33  is located above a space between the adjacent two intake ports  2  on the side surface side of the cylinder head with respect to the valve guides  9 . The second connecting passage  33  is a flow passage extending in the longitudinal direction and continuously communicates with the outer flow passages  32  of the adjacent two unit structures. The inner flow passage  31  communicates with a connecting position between the outer flow passage  32  and the second connecting passage  33 . In the first coolant flow passage  30 , the first connecting passages  34  located on the middle side of the cylinder head with respect to the valve guides  9  and the second connecting passages  33  located on the side surface side of the cylinder head with respect to the valve guides  9  are arranged alternately in the longitudinal direction in a manner to sandwich therebetween the annular passages each including the inner flow passage  31  and the outer flow passage  32 . 
     An inlet flow passage  35  and an outlet flow passage  36  are respectively provided at both end portions in the longitudinal direction of the first coolant flow passage  30 . The inlet flow passage  35  extends straight in the longitudinal direction from the annular passage closest to the rear end of the cylinder head to the rear end face of the cylinder head and communicates with a first hole  37  opened in the rear end face. The first hole  37  is the coolant inlet formed in the cylinder head and the coolant introducing pipe of the first circulation system is connected to the first hole  37 . The outlet flow passage  36  extends straight in the longitudinal direction from the annular passage closest to the front end of the cylinder head to the front end face of the cylinder head and communicates with a second hole  38  opened in the front end face. The second hole  38  is the coolant outlet formed in the cylinder head and the coolant discharge pipe of the first circulation system is connected to the second hole  38 . It may alternatively be configured that the second hole  38  is used as a coolant inlet, while the first hole  37  is used as a coolant outlet, thereby introducing the coolant from the front end side of the cylinder head and discharging the coolant from the rear end side of the cylinder head. 
     The first coolant flow passage  30  is formed in the cylinder head using a sand core when casting the cylinder head. The sand core for forming the first coolant flow passage  30  is different from a sand core for forming the second coolant flow passage. The inlet flow passage  35  and the outlet flow passage  36  are flow passages that are formed by core supports supporting the sand core from both sides, while the first hole  37  and the second hole  38  are sand removing holes that are formed by removing the core supports. That is, in the cylinder head of the first embodiment, the sand removing holes that are formed when forming the first coolant flow passage  30  by the sand core are used as the coolant inlet and the coolant outlet. 
     The coolant enters the first coolant flow passage  30  from the first hole  37  as the coolant inlet, passes through the first coolant flow passage  30 , and then exits the first coolant flow passage  30  from the second hole  38  as the coolant outlet. On the way, the coolant flows through the annular passages respectively surrounding the valve guides  9  (to be exact, the intake valve insertion holes). The flow passage cross-sectional areas of the inner flow passage  31  and the outer flow passage  32  forming each annular passage are substantially equal to each other and the flow passage lengths from the first connecting passage  34  (or the second connecting passage  33 ) to the second connecting passage  33  (or the first connecting passage  34 ) are substantially equal to each other when passing through the inner flow passage  31  and when passing through the outer flow passage  32 . Consequently, the coolant flows uniformly through the inner flow passage  31  and the outer flow passage  32  in each annular passage so that the coolant is prevented from staying in the first coolant flow passage  30 . 
       FIG. 7  is a diagram showing the positional relationship between the intake port  2 , a head bolt  19 , and the first coolant flow passage  30  in the cylinder head of the first embodiment.  FIG. 7  shows the shape of the first coolant flow passage  30  around the valve guide  9  and the positional relationship between the intake port  2 , the first coolant flow passage  30 , and the head bolt  19  when seen from the front end side of the cylinder head assuming the inside of the cylinder head to be transparent. The head bolt  19  shown in  FIG. 7  is a head bolt disposed between the front end face of the cylinder head and the intake port closest thereto. The first coolant flow passage  30  passes on the middle side of the cylinder head with respect to the head bolt  19 . 
     The same applies to the positional relationship between head bolts each disposed between the adjacent two intake ports  2  and the first coolant flow passage  30 . The first coolant flow passage  30  is disposed so as to pass through regions closer to the middle of the cylinder head with respect to the head bolts. If it is assumed that the first coolant flow passage  30  passes on the side surface side of the cylinder head with respect to the head bolts, since the intake ports  2  extend obliquely upward toward the side surface of the cylinder head, there is no alternative but to pass the first coolant flow passage  30  at high positions in a height direction of the cylinder head. With this configuration, air pockets may occur in the first coolant flow passage  30  to impede the circulation of the coolant. In this connection, since the height of the upper surfaces  2   a  of the intake ports  2  is set low in the regions closer to the middle of the cylinder head with respect to the head bolts, it is possible to pass the first coolant flow passage  30  substantially straight in the longitudinal direction without locally forming those portions that pass at the high positions. 
     Next, the configurations of the coolant flow passages, including the first coolant flow passage, of the cylinder head, particularly the positional relationship between the first coolant flow passage and the other components, including the second coolant flow passage, of the cylinder head, will be described with reference to the cross-sectional views. 
     Hereinbelow, the configurations of the coolant flow passages of the cylinder head as seen in the cross section including the central axis of the intake valve insertion hole and perpendicular to the longitudinal direction will be described.  FIG. 3  shows the cross-sectional shapes of the first coolant flow passage and the second coolant flow passage of the cylinder head  101  in the cross section including the central axis L 3  of the intake valve insertion hole  7  and perpendicular to the longitudinal direction. Further,  FIG. 3  shows the positional relationship between the first coolant flow passage and the other components, including the second coolant flow passage, of the cylinder head  101 . In the cross section shown in  FIG. 3 , regions denoted by symbols  20   a,    20   b,    20   c,    20   d,  and  20   e  are cross sections of portions of the second coolant flow passage. Hereinafter, for example, when referring to the region denoted by symbol  20   a,  it will be referred to as a “portion  20   a ” of the second coolant flow passage or a “second coolant flow passage  20   a ”. Although the portions  20   a,    20   b,    20   c,    20   d,  and  20   e  of the second coolant flow passage are separated from each other in the cross section shown in  FIG. 3 , these portions are joined into one inside the cylinder head  101 . 
     In the cross section shown in  FIG. 3 , near a top portion of the pent roof of the combustion chamber  4 , the portion  20   a  of the second coolant flow passage is disposed in a region sandwiched between an upper surface  3   a  near the exhaust opening of the exhaust port  3  and the upper surface  2   a  near the intake opening of the intake port  2 . The portion  20   b  of the second coolant flow passage is disposed between a lower surface  3   b  of the exhaust port  3  and the cylinder block mating surface  1   a.  The portion  20   b  of the second coolant flow passage is opened at the cylinder block mating surface  1   a  and communicates with the coolant flow passage on the cylinder block side. The portion  20   d  and the portion  20   e  of the second coolant flow passage are respectively disposed on both sides of a central axis of the exhaust valve insertion hole  8 . The portions  20   a,    20   b,    20   d,  and  20   e  of the second coolant flow passage form a water jacket surrounding the exhaust port  3  so as to cool the exhaust port  3  and the exhaust valve. Further, the portion  20   a  of the second coolant flow passage cools the periphery of the combustion chamber  4  that rises to a high temperature. 
     In the cross section shown in  FIG. 3 , the portion  20   c  of the second coolant flow passage is disposed between the intake port central line plane S 2  and the cylinder block mating surface  1   a,  more specifically, between the lower surface  2   b  of the intake port  2  and the cylinder block mating surface  1   a.  Near the branching portion of the intake port  2 , the portion  20   c  of the second coolant flow passage is located approximately opposite to the outer flow passage  32  of the first coolant flow passage with the intake port  2  interposed therebetween. The portion  20   c  of the second coolant flow passage is opened at the cylinder block mating surface  1   a.  This opening of the cylinder block mating surface  1   a  communicates with the coolant flow passage on the cylinder block side. The coolant having passed through the inside of the cylinder block is introduced into the portion  20   c  of the second coolant flow passage via the opening of the cylinder block mating surface  1   a.    
     In the cross section shown in  FIG. 3 , the inner flow passage  31  and the outer flow passage  32  of the first coolant flow passage are located between the intake port central line plane S 2  and the cylinder head longitudinal direction central flat plane S  1 . More specifically, the inner flow passage  31  of the first coolant flow passage is located on the cylinder head longitudinal direction central flat plane S 1  side with respect to the intake valve insertion hole central axis plane S 3 , while the outer flow passage  32  of the first coolant flow passage is, located on the intake port central line plane S 2  side with respect to the intake valve insertion hole central axis plane S 3 . The inner flow passage  31  is located on the side opposite to the top portion of the pent roof of the combustion chamber  4  with the portion  20   a  of the second coolant flow passage interposed therebetween. The inner flow passage  31  has an elongated cross-sectional shape extending in a direction of the central axis L 3  of the intake valve insertion hole  7  and is disposed close to a wall surface of the intake valve insertion hole  7 . The outer flow passage  32  is located near the branching portion of the intake port  2  upstream of the intake valve insertion hole  7 . The outer flow passage  32  has a cross-sectional shape close to a triangle having a side parallel to the upper surface  2   a  of the intake port  2  and a side parallel to the wall surface of the intake valve insertion hole  7  and is disposed close to both the wall surface of the intake valve insertion hole  7  and the upper surface  2   a  of the intake port  2 . 
     According to the above-described configuration shown in  FIG. 3 , the upper surface  2   a  of the intake port  2 , particularly the upper surface  2   a  upstream of the intake valve insertion hole  7 , can be effectively cooled by the outer flow passage  32  and the inner flow passage  31  of the first coolant flow passage in which the coolant flows, which is at a temperature lower than that of the coolant flowing in the second coolant flow passage cooling the exhaust port  3 . In the intake port  2  being the tumble flow generating port, the air flows in a manner to stick to the upper surface  2   a  side of the intake port  2 . Therefore, the air flowing in the intake port  2  can be efficiently cooled by cooling the upper surface  2   a  of the intake port  2  with the low-temperature coolant. 
     The portion  20   a  of the second coolant flow passage is located between the top portion of the pent roof of the combustion chamber  4  and the inner flow passage  31  of the first coolant flow passage. Since the heat generated from the combustion chamber  4  is absorbed by the portion  20   a  of the second coolant flow passage, it is suppressed that the heat is directly transferred to the inner flow passage  31  from the combustion chamber  4 . Accordingly, it is avoided that the coolant in the inner flow passage  31  is heated by the heat generated from the combustion chamber  4 , resulting in a reduction in cooling efficiency for the air flowing in the intake port  2 . 
     Heat transfer from the cylinder block mating surface  1   a  to the lower surface  2   b  of the intake port  2  can be suppressed by the portion  20   c  of the second coolant flow passage. The temperature of the coolant cooling the lower surface  2   b  side of the intake port  2  is higher than that of the coolant cooling the upper surface  2   a  side of the intake port  2  and thus does not excessively reduce the temperature of the lower surface  2   b,  where adhesion of fuel injected from the port injector is large in amount, of the intake port  2 . That is, by the portion  20   c  of the second coolant flow passage, the lower surface  2   b  of the intake port  2  can be moderately cooled to a degree that does not inhibit evaporation of fuel. 
     Next, the configurations of the coolant flow passages of the cylinder head as seen in the cross section including the central axis of the combustion chamber and perpendicular to the longitudinal direction will be described.  FIG. 4  shows the cross-sectional shapes of the first coolant flow passage and the second coolant flow passage of the cylinder head  101  in the cross section including the central axis L 1  of the combustion chamber  4  and perpendicular to the longitudinal direction. Further,  FIG. 4  shows the positional relationship between the first coolant flow passage and the other components, including the second coolant flow passage, of the cylinder head  101 . In the cross section shown in  FIG. 4 , regions denoted by symbols  20   f,    20   g,  and  20   h  are cross sections of portions of the second coolant flow passage. Although the portions  20   f,    20   g,  and  20   h  of the second coolant flow passage are separated from each other in the cross section shown in  FIG. 4 , these portions are joined into one with the portions  20   a,    20   b,    20   c,    20   d,  and  20   e  shown in  FIG. 3  inside the cylinder head  101 . 
     In the cross section shown in  FIG. 4 , near an open end  12   a  of the spark plug insertion hole  12 , the portion  20   g  of the second coolant flow passage is disposed on the intake side with respect to the cylinder head longitudinal direction central flat plane S 1 . The portion  20   g  of the second coolant flow passage is disposed close to an intake-side wall surface of a forward end portion of the spark plug insertion hole  12  between the cylinder head longitudinal direction central flat plane S 1  and the intake valve insertion hole central axis plane S 3 . Near the open end  12   a  of the spark plug insertion hole  12 , the portion  20   f  of the second coolant flow passage is disposed on the exhaust side with respect to the cylinder head longitudinal direction central flat plane S 1 . The portion  20   f  of the second coolant flow passage is disposed along both an exhaust-side wall surface of the forward end portion of the spark plug insertion hole  12  and an exhaust-side wall surface of the combustion chamber  4 . The portion  20   h  of the second coolant flow passage is disposed above the portion  20   f  of the second coolant flow passage. The portions  20   f  and  20   h  of the second coolant flow passage form a water jacket surrounding the exhaust port  3  jointly with the portions  20   a,    20   b,    20   d,  and  20   e  shown in  FIG. 3 . The portion  20   g  of the second coolant flow passage cools the periphery of the combustion chamber  4  that rises to a high temperature, particularly the periphery of the spark plug insertion hole  12 . 
     In the cross section shown in  FIG. 4 , the first connecting passage  34  of the first coolant flow passage is located between the cylinder head longitudinal direction central flat plane S 1  and the intake valve insertion hole central axis plane S 3 . The first connecting passage  34  has an elongated rounded rectangular cross-sectional shape substantially parallel to the intake valve insertion hole central axis plane S 3  and has a flow passage cross-sectional area substantially equal to the sum of the flow passage cross-sectional areas of the outer flow passage  32  and the inner flow passage  31  shown in  FIG. 3 . The first connecting passage  34  is located on the side opposite to the top portion of the combustion chamber  4 , more specifically, on the side opposite to the open end  12   a  of the spark plug insertion hole  12 , with the portion  20   g  of the second coolant flow passage interposed therebetween. 
     According to the above-described configuration shown in  FIG. 4 , the heat generated from the combustion chamber  4  is absorbed by the portion  20   g  of the second coolant flow passage located between the first connecting passage  34  of the first coolant flow passage and the top portion of the combustion chamber  4 . Therefore, it is suppressed that the heat is directly transferred to the first connecting passage  34  from the combustion chamber  4 . Accordingly, it is avoided that the temperature of the coolant flowing in the first coolant flow passage increases to cause a reduction in cooling efficiency for the air flowing in the intake port  2 . 
     Next, the configurations of the coolant flow passages of the cylinder head as seen in the cross section passing between the adjacent two combustion chambers and perpendicular to the longitudinal direction will be described.  FIG. 5  shows the cross-sectional shapes of the first coolant flow passage and the second coolant flow passage of the cylinder head  101  in the cross section passing between the adjacent two combustion chambers and perpendicular to the longitudinal direction. Further,  FIG. 5  shows the positional relationship between the first coolant flow passage and the other components, including the second coolant flow passage, of the cylinder head  101 . In the cross section shown in  FIG. 5 , regions denoted by symbols  20   i,    20   j,  and  20   p  are cross sections of portions of the second coolant flow passage. Although the portions  20   i,    20   j,  and  20   p  of the second coolant flow passage are separated from each other in the cross section shown in  FIG. 5 , these portions are joined into one with the portions  20   a,    20   b,    20   c,    20   d,  and  20   e  shown in  FIG. 3  and the portions  20   f,    20   g,  and  20   h  shown in  FIG. 4  inside the cylinder head  101 . 
     In the cross section shown in  FIG. 5 , the portion  20   i  of the second coolant flow passage is disposed between the cylinder head longitudinal direction central flat plane S 1  and the exhaust-side head bolt insertion hole  14 . The portion  20   j  of the second coolant flow passage is disposed between the cylinder head longitudinal direction central flat plane S 1  and the intake-side head bolt insertion hole  13 . The portion  20   i  and the portion  20   j  of the second coolant flow passage are both opened at the cylinder block mating surface  1   a.  Further, the portion  20   i  and the portion  20   j  of the second coolant flow passage communicate with each other in the middle of the cylinder head  101 . The portion  20   p  of the second coolant flow passage is disposed between the exhaust-side head bolt insertion hole  14  and the exhaust port  3 . The portion  20   p  of the second coolant flow passage is opened at the cylinder block mating surface  1   a.  The portions  20   i  and  20   p  of the second coolant flow passage form a water jacket surrounding the exhaust port  3  jointly with the portions  20   a,    20   b,    20   d,  and  20   e  shown in  FIG. 3  and the portions  20   f,    20   g  and  20   h  shown in  FIG. 4 . The portion  20   j  of the second coolant flow passage cools a portion between the forward end portions of the adjacent two intake ports. 
     In the cross section shown in  FIG. 5 , the second connecting passage  33  of the first coolant flow passage is located between the intake port central line plane S 2  and the intake valve insertion hole central axis plane S 3 . The second connecting passage  33  has an elongated rounded rectangular cross-sectional shape substantially parallel to the intake valve insertion hole central axis plane S 3  and has a flow passage cross-sectional area substantially equal to the sum of the flow passage cross-sectional areas of the outer flow passage  32  and the inner flow passage  31  shown in  FIG. 3 . The second connecting passage  33  is located on the side opposite to the cylinder block mating surface  1   a  with the portion  20   j  of the second coolant flow passage interposed therebetween. 
     According to the above-described configuration shown in  FIG. 5 , the heat transferred from the cylinder block mating surface  1   a  is absorbed by the portion  20   j  of the second coolant flow passage located between the cylinder block mating surface  1   a  and the second connecting passage  33  of the first coolant flow passage. Therefore, it is suppressed that the heat is directly transferred to the second connecting passage  33  from the cylinder block mating surface  1   a.  Accordingly, it is avoided that the temperature of the coolant flowing in the first coolant flow passage increases to cause a reduction in cooling efficiency for the air flowing in the intake port  2 . 
     In the cross section shown in  FIG. 5 , the second connecting passage  33  of the first coolant flow passage is located in a region closer to the middle of the cylinder head  101  with respect to the intake-side head bolt insertion hole  13 . If it is assumed that the second connecting passage  33  is located on the side surface side of the cylinder head with respect to the head bolt insertion hole  13 , the position in the cylinder head height direction of the second connecting passage  33  has to be high. With this configuration, there is a possibility that the air staying in the second connecting passage  33  is not released, thereby impeding the circulation of the coolant. In this connection, according to the positional relationship shown in  FIG. 5 , since it is possible to pass the first coolant flow passage substantially straight in the longitudinal direction, it is possible to prevent the air from staying in the first coolant flow passage. 
     Next, a description will be given of specific application examples of the engine cooling system, including the cylinder head  101 , of the first embodiment configured as described above. 
     First, application example  1  of the first embodiment will be described.  FIG. 14  shows application example  1  in which the engine cooling system of the first embodiment is applied to a supercharged engine system. The configuration of an engine cooling system itself is equivalent to the basic configuration of the engine cooling system shown in  FIG. 1 . Accordingly, in  FIG. 14 , components equivalent to those of the engine cooling system shown in  FIG. 1  are assigned the same symbols. An overlapping description of those equivalent components will be omitted or simplified. 
     In the supercharged engine system, a turbo compressor  131  is attached to an intake passage  130  communicating with a cylinder head  101  and a liquid-cooled intercooler  132  is disposed downstream of the turbo compressor  131 . In application example  1  shown in  FIG. 14 , the intercooler  132  is incorporated in a first circulation system  120  and a low-temperature coolant flowing in the first circulation system  120  is used for heat exchange with the air in the intercooler  132 . More specifically, the intercooler  132  is disposed in a coolant introducing pipe  121  and the coolant used for heat exchange in the intercooler  132  is introduced into a first coolant flow passage  30  provided in the cylinder head  101 . In application example  1  shown in  FIG. 14 , a liquid temperature sensor  125  is disposed in a coolant discharge pipe  122  and the temperature of the coolant having passed through the first coolant flow passage  30  is measured by the liquid temperature sensor  125 . The measured liquid temperature is used as information for controlling the rotational speed of a water pump  123 . 
     Next, application example  2  of the first embodiment will be described.  FIG. 15  shows application example  2  in which the engine cooling system of the first embodiment is applied to a hybrid system. The configuration of an engine cooling system itself is equivalent to the basic configuration of the engine cooling system shown in  FIG. 1 . Accordingly, in  FIG. 15 , components equivalent to those of the engine cooling system shown in  FIG. 1  are assigned the same symbols. An overlapping description of those equivalent components will be omitted or simplified. 
     The hybrid system, in which an engine and a motor are combined, includes an inverter  135 . In application example  2  shown in  FIG. 15 , the inverter  135  is incorporated in a first circulation system  120  and a low-temperature coolant flowing in the first circulation system  120  is used for cooling the inverter  135 . More specifically, the inverter  135  is disposed in a coolant introducing pipe  121  and the coolant used for cooling the inverter  135  is introduced into a first coolant flow passage  3 . 0  provided in a cylinder head  101 . Also in application example  2  shown in  FIG. 15 , a liquid temperature sensor  125  is disposed in a coolant discharge pipe  122 . 
     Next, a second embodiment of the invention will be described with reference to the drawings. The basic configuration of a cylinder head of the second embodiment is the same as that of the cylinder head of the first embodiment. Accordingly, the description of the basic configuration of the cylinder head of the first embodiment is incorporated herein in its entirety for the basic configuration of the cylinder head of the second embodiment, thereby omitting an overlapping description thereof. 
     The cylinder head of the second embodiment includes dual coolant flow passages connected to independent and separate circulation systems. The temperature of a coolant flowing in the first coolant flow passage is equal to that of a coolant flowing in the second coolant flow passage at the time of cold engine start-up and, as warming-up of the engine progresses, the coolant at a temperature lower than that of the coolant flowing in the second coolant flow passage flows in the first coolant flow passage. The cylinder head of the second embodiment differs from the cylinder head of the first embodiment in the configuration of the first coolant flow passage. Hereinbelow, the configuration of the first coolant flow passage of the cylinder head of the second embodiment will be described. The description will be made using cross-sectional views of the cylinder head and a perspective view showing the coolant flow passage inside the cylinder head in a see-through manner. In the figures, components equivalent to those of the first embodiment are assigned the same symbols. The configuration of the second coolant flow passage is the same as that of the cylinder head of the first embodiment. Accordingly, the description of the configuration of the second coolant flow passage of the cylinder head of the first embodiment is incorporated herein in its entirety for the configuration of the second coolant flow passage of the cylinder head of the second embodiment, thereby omitting an overlapping description thereof. 
     Hereinbelow, the configurations of the coolant flow passages of the cylinder head of the second embodiment will be described. Of the dual coolant flow passages provided in the cylinder head of the second embodiment, the shape of the first coolant flow passage in which the low-temperature coolant flows will be described with reference to  FIG. 19 .  FIG. 19  is a perspective view showing, in a see-through manner, intake ports  2  and a first coolant flow passage  40  of the cylinder head of the second embodiment.  FIG. 19  shows the shape of the first coolant flow passage  40  and the positional relationship between the first coolant flow passage  40 , the intake ports  2 , and valve guides  9  when seen assuming the inside of the cylinder head to be transparent. 
     The first coolant flow passage  40  is provided on the upper side of the row of the intake ports  2  in the cylinder head. The first coolant flow passage  40  extends in a direction of the row of the intake ports  2 , i.e. in a longitudinal direction of the cylinder head, along upper surfaces  2   a  of the intake ports  2 . 
     The first coolant flow passage  40  has a unit structure for each intake port  2 . In  FIG. 19 , the structure of a portion encircled by a dotted line is the unit structure of the first coolant flow passage  40 . The unit structure includes a pair of arc-shaped flow passages  41  respectively disposed around the left and right valve guides  9  (to be exact, intake valve insertion holes) of the intake port  2 . The arc-shaped flow passages  41  are each a flow passage curved in an arc along the periphery of the valve guide  9  and respectively extend between the left and right valve guides  9  from the side surface side of the cylinder head to the middle side of the cylinder head with respect to the valve guides  9 . The left and right arc-shaped flow passages  41  are plane-symmetric with respect to a flat plane dividing the intake port  2  into left and right parts (a flat plane including a central axis of a combustion chamber and perpendicular to the longitudinal direction of the cylinder head). 
     The unit structure includes a first connecting passage  43  connecting the left and right arc-shaped flow passages  41 . The first connecting passage  43  is located above a space between left and right branch ports of the intake port  2  on the middle side of the cylinder head with respect to the valve guides  9 . The first connecting passage  43  is a flow passage curved convex to the middle side of the cylinder head and continuously communicates with the left and right arc-shaped flow passages  41 . 
     The first coolant flow passage  40  includes second connecting passages  42  each connecting the adjacent two unit structures. The second connecting passage  42  is located above a space between the adjacent two intake ports  2  on the side surface side of the cylinder head with respect to the valve guides  9 . The second connecting passage  42  is a flow passage extending in the longitudinal direction of the cylinder head and continuously communicates with the arc-shaped flow passages  41  of the adjacent two unit structures. 
     An inlet flow passage  44  and an outlet flow passage  45  are respectively provided at both end portions in the longitudinal direction of the first coolant flow passage  40 . The inlet flow passage  44  extends straight in the longitudinal direction to a first hole  46  opened in a rear end face of the cylinder head. The outlet flow passage  45  extends straight in the longitudinal direction to a second hole  47  opened in a front end face of the cylinder head. The inlet flow passage  44  and the outlet flow passage  45  are flow passages that are formed by core supports supporting a sand core, for forming the first coolant flow passage  40 , from both sides, while the first hole  46  and the second hole  47  are sand removing holes that are formed by removing the core supports. The first hole  46  is used as a coolant inlet, while the second hole  47  is used as a coolant outlet. Alternatively, the second hole  47  may be used as a coolant inlet, while the first hole  46  may be used as a coolant outlet. 
     Next, the positional relationship between the first coolant flow passage and the other components, including the second coolant flow passage, of the cylinder head will be described with reference to cross-sectional views. 
     Hereinbelow, the configurations of the coolant flow passages of the cylinder head as seen in a cross section including a central axis of an intake valve insertion hole and perpendicular to the longitudinal direction will be described.  FIG. 16  is a cross-sectional view showing a cross section, including a central axis L 3  of an intake valve insertion hole  7  and perpendicular to the longitudinal direction, of the cylinder head of the second embodiment.  FIG. 16  shows the cross-sectional shapes of the first coolant flow passage and the second coolant flow passage in the cross section described above. Further,  FIG. 16  shows the positional relationship between the first coolant flow passage and the other components, including the second coolant flow passage, of the cylinder head  102 . 
     In the cross section shown in  FIG. 16 , the arc-shaped flow passage  41  of the first coolant flow passage is located between an intake port central line plane S 2  and a cylinder head longitudinal direction central flat plane S 1  on the intake port central line plane S 2  side with respect to an intake valve insertion hole central axis plane S 3 . The arc-shaped flow passage  41  is located near a branching portion of the intake port  2  upstream of the intake valve insertion hole  7 . The arc-shaped flow passage  41  has a cross-sectional shape close to a triangle having a side parallel to the upper surface  2   a  of the intake port  2  and a side parallel to a wall surface of the intake valve insertion hole  7  and is disposed close to both the wall surface of the intake valve insertion hole  7  and the upper surface  2   a  of the intake port  2 . 
     According to the above-described configuration shown in  FIG. 16 , the upper surface  2   a  of the intake port  2 , particularly the upper surface  2   a  upstream of the intake valve insertion hole  7 , can be effectively cooled by the arc-shaped flow passage  41  of the first coolant flow passage in which the coolant flows, which is at a temperature lower than that of the coolant flowing in the second coolant flow passage cooling an exhaust port  3 . Accordingly, it is possible to efficiently cool the air flowing in the intake port  2 . 
     Next, the configurations of the coolant flow passages of the cylinder head as seen in a cross section including a central axis of a combustion chamber and perpendicular to the longitudinal direction will be described.  FIG. 17  is a cross-sectional view showing a cross section, including a central axis L 1  of a combustion chamber  4  and perpendicular to the longitudinal direction, of the cylinder head of the second embodiment.  FIG. 17  shows the cross-sectional shapes of the first coolant flow passage and the second coolant flow passage in the cross section described above. Further,  FIG. 17  shows the positional relationship between the first coolant flow passage and the other components, including the second coolant flow passage, of the cylinder head  102 . 
     In the cross section shown in  FIG. 17 , the first connecting passage  43  of the first coolant flow passage is located between the cylinder head longitudinal direction central flat plane S 1  and the intake valve insertion hole central axis plane S 3 . The first connecting passage  43  has an elongated rounded rectangular cross-sectional shape substantially parallel to the intake valve insertion hole central axis plane S 3 . The first connecting passage  43  is located on the side opposite to a top portion of the combustion chamber  4 , more specifically, on the side opposite to an open end  12   a  of a spark plug insertion hole  12 , with a portion  20   g  of the second coolant flow passage interposed therebetween. 
     According to the above-described configuration shown in  FIG. 17 , the heat generated from the combustion chamber  4  is absorbed by the portion  20   g  of the second coolant flow passage located between the first connecting passage  43  of the first coolant flow passage and the top portion of the combustion chamber  4 . Therefore, it is suppressed that the heat is directly transferred to the first connecting passage  43  from the combustion chamber  4 . Accordingly, it is avoided that the temperature of the coolant flowing in the first coolant flow passage increases to cause a reduction in cooling efficiency for the air flowing in the intake port  2 . 
     Next, the configurations of the coolant flow passages of the cylinder head as seen in a cross section passing between the adjacent two combustion chambers and perpendicular to the longitudinal direction will be described.  FIG. 18  is a cross-sectional view showing a cross section, passing between the adjacent two combustion chambers and perpendicular to the longitudinal direction, of the cylinder head of the second embodiment, specifically, a cross section including central axes of head bolt insertion holes  13  and  14  and perpendicular to the longitudinal direction.  FIG. 18  shows the cross-sectional shapes of the first coolant flow passage and the second coolant flow passage in the cross section described above. Further,  FIG. 18  shows the positional relationship between the first coolant flow passage and the other components, including the second coolant flow passage, of the cylinder head  102 . 
     In the cross section shown in  FIG. 18 , the second connecting passage  42  of the first coolant flow passage is located between the intake port central line plane S 2  and the intake valve insertion hole central axis plane S 3  in a region closer to the middle of the cylinder head  102  with respect to the intake-side head bolt insertion hole  13 . The second connecting passage  42  has an elongated rounded rectangular cross-sectional shape substantially parallel to the intake valve insertion hole central axis plane S 3 . The second connecting passage  42  is located on the side opposite to a cylinder block mating surface  1   a  with a portion  20   j  of the second coolant flow passage interposed therebetween. 
     According to the above-described configuration shown in  FIG. 18 , the heat transferred from the cylinder block mating surface  1   a  is absorbed by the portion  20   j  of the second coolant flow passage located between the cylinder block mating surface  1   a  and the second connecting passage  42  of the first coolant flow passage. Therefore, it is suppressed that the heat is directly transferred to the second connecting passage  42  from the cylinder block mating surface  1   a.  Accordingly, it is avoided that the temperature of the coolant flowing in the first coolant flow passage increases to cause a reduction in cooling efficiency for the air flowing in the intake port  2 . 
     Next, a third embodiment of the invention will be described with reference to the drawings. The basic configuration of a cylinder head of the third embodiment is the same as that of the cylinder head of the first embodiment. Accordingly, the description of the basic configuration of the cylinder head of the first embodiment is incorporated herein in its entirety for the basic configuration of the cylinder head of the third embodiment, thereby omitting an overlapping description thereof. 
     The cylinder head of the third embodiment includes dual coolant flow passages connected to independent and separate circulation systems. The temperature of a coolant flowing in the first coolant flow passage is equal to that of a coolant flowing in the second coolant flow passage at the time of cold engine start-up and, as warming-up of the engine progresses, the coolant at a temperature lower than that of the coolant flowing in the second coolant flow passage flows in the first coolant flow passage. The cylinder head of the third embodiment differs from the cylinder head of the first embodiment in the configuration of the first coolant flow passage. Hereinbelow, the configuration of the first coolant flow passage of the cylinder head of the third embodiment will be described. The description will be made using cross-sectional views of the cylinder head and a perspective view showing the coolant flow passage inside the cylinder head in a see-through manner. In the figures, components equivalent to those of the first embodiment are assigned the same symbols. The configuration of the second coolant flow passage is the same as that of the cylinder head of the first embodiment. Accordingly, the description of the configuration of the second coolant flow passage of the cylinder head of the first embodiment is incorporated herein in its entirety for the configuration of the second coolant flow passage of the cylinder head of the third embodiment, thereby omitting an overlapping description thereof. 
     Hereinbelow, the configurations of the coolant flow passages of the cylinder head of the third embodiment will be described. Of the dual coolant flow passages provided in the cylinder head of the third embodiment, the shape of the first coolant flow passage in which the low-temperature coolant flows will be described with reference to  FIG. 23 .  FIG. 23  is a perspective view showing, in a see-through manner, intake ports  2  and a first coolant flow passage  50  of the cylinder head of the third embodiment.  FIG. 23  shows the shape of the first coolant flow passage  50  and the positional relationship between the first coolant flow passage  50 , the intake ports  2 , and valve guides  9  when seen assuming the inside of the cylinder head to be transparent. 
     The first coolant flow passage  50  is provided on the upper side of the row of the intake ports  2  in the cylinder head. The first coolant flow passage  50  extends in a direction of the row of the intake ports  2 , i.e. in a longitudinal direction of the cylinder head, along upper surfaces  2   a  of the intake ports  2 . 
     The first coolant flow passage  50  has a unit structure for each intake port  2 . In  FIG. 23 , the structure of a portion encircled by a dotted line is the unit structure of the first coolant flow passage  50 . The unit structure includes a pair of arc-shaped flow passages  51  respectively disposed around the left and right valve guides  9  (to be exact, intake valve insertion holes) of the intake port  2 . The arc-shaped flow passages  51  are each a flow passage curved in an arc along the periphery of the valve guide  9  and respectively extend on the outer sides of the left and right valve guides  9  from the side surface side of the cylinder head to the middle side of the cylinder head with respect to the valve guides  9 . The left and right arc-shaped flow passages  51  are plane-symmetric with respect to a flat plane dividing the intake port  2  into left and right parts (a flat plane including a central axis of a combustion chamber and perpendicular to the longitudinal direction of the cylinder head). 
     The unit structure includes a first connecting passage  53  connecting the left and right arc-shaped flow passages  51 . The first connecting passage  53  is located above a space between left and right branch ports of the intake port  2  on the middle side of the cylinder head with respect to the valve guides  9 . The first connecting passage  53  is a flow passage extending in the longitudinal direction of the cylinder head and continuously communicates with the left and right arc-shaped flow passages  51 . 
     The first coolant flow passage  50  includes second connecting passages  52  each connecting the adjacent two unit structures. The second connecting passage  52  is located above a space between the adjacent two intake ports  2  on the side surface side of the cylinder head with respect to the valve guides  9 . The second connecting passage  52  is a flow passage curved convex to the side surface side of the cylinder head and continuously communicates with the arc-shaped flow passages  51  of the adjacent two unit structures. 
     An inlet flow passage  54  and an outlet flow passage  55  are respectively provided at both end portions in the longitudinal direction of the first coolant flow passage  50 . The inlet flow passage  54  extends straight in the longitudinal direction to a first hole  56  opened in a rear end face of the cylinder head. The outlet flow passage  55  extends straight in the longitudinal direction to a second hole  57  opened in a front end face of the cylinder head. The inlet flow passage  54  and the outlet flow passage  55  are flow passages that are formed by core supports supporting a sand core, for forming the first coolant flow passage  50 , from both sides, while the first hole  56  and the second hole  57  are sand removing holes that are formed by removing the core supports. The first hole  56  is used as a coolant inlet, while the second hole  57  is used as a coolant outlet. Alternatively, the second hole  57  may be used as a coolant inlet, while the first hole  56  may be used as a coolant outlet. 
     Next, the positional relationship between the first coolant flow passage and the other components, including the second coolant flow passage, of the cylinder head will be described with reference to cross-sectional views. 
     Hereinbelow, the configurations of the coolant flow passages of the cylinder head as seen in a cross section including a central axis of an intake valve insertion hole and perpendicular to the longitudinal direction will be described.  FIG. 20  is a cross-sectional view showing a cross section, including a central axis L 3  of an intake valve insertion hole  7  and perpendicular to the longitudinal direction, of the cylinder head of the third embodiment.  FIG. 20  shows the cross-sectional shapes of the first coolant flow passage and the second coolant flow passage in the cross section described above. Further,  FIG. 20  shows the positional relationship between the first coolant flow passage and the other components, including the second coolant flow passage, of the cylinder head  103 . 
     In the cross section shown in  FIG. 20 , the arc-shaped flow passage  51  of the first coolant flow passage is located between an intake port central line plane S 2  and a cylinder head longitudinal direction central flat plane S 1  on the cylinder head longitudinal direction central flat plane S 1  side with respect to an intake valve insertion hole central axis plane S 3 . The arc-shaped flow passage  51  is located on the side opposite to a top portion of a pent roof of a combustion chamber  4  with a portion  20   a  of the second coolant flow passage interposed therebetween. The arc-shaped flow passage  51  has an elongated cross-sectional shape extending in a direction of the central axis L 3  of the intake valve insertion hole  7  and is disposed close to a wall surface of the intake valve insertion hole  7 . 
     According to the above-described configuration shown in  FIG. 20 , not only the upper surface  2   a  of the intake port  2  but also the valve guide  9  can be cooled by the arc-shaped flow passage  51  of the first coolant flow passage. By cooling the valve guide  9 , the temperature of an intake valve  11  can be reduced. By cooling the upper surface  2   a  of the intake port  2  and the intake valve  11  with the low-temperature coolant flowing in the first coolant flow passage, it is possible to efficiently cool the air flowing in the intake port  2 . 
     Next, the configurations of the coolant flow passages of the cylinder head as seen in a cross section including a central axis of the combustion chamber and perpendicular to the longitudinal direction will be described.  FIG. 21  is a cross-sectional view showing a cross section, including a central axis L 1  of the combustion chamber  4  and perpendicular to the longitudinal direction, of the cylinder head of the third embodiment.  FIG. 21  shows the cross-sectional shapes of the first coolant flow passage and the second coolant flow passage in the cross section described above. Further,  FIG. 21  shows the positional relationship between the first coolant flow passage and the other components, including the second coolant flow passage, of the cylinder head  103 . 
     In the cross section shown in  FIG. 21 , the first connecting passage  53  of the first coolant flow passage is located between the cylinder head longitudinal direction central flat plane S 1  and the intake valve insertion hole central axis plane S 3 . The first connecting passage  53  has an elongated rounded rectangular cross-sectional shape substantially parallel to the intake valve insertion hole central axis plane S 3 . The first connecting passage  53  is located on the side opposite to the top portion of the combustion chamber  4 , more specifically, on the side opposite to an open end  12   a  of a spark plug insertion hole  12 , with a portion  20   g  of the second coolant flow passage interposed therebetween. 
     According to the above-described configuration shown in  FIG. 21 , the heat generated from the combustion chamber  4  is absorbed by the portion  20   g  of the second coolant flow passage located between the first connecting passage  53  of the first coolant flow passage and the top portion of the combustion chamber  4 . Therefore, it is suppressed that the heat is directly transferred to the first connecting passage  53  from the combustion chamber  4 . Accordingly, it is avoided that the temperature of the coolant flowing in the first coolant flow passage increases to cause a reduction in cooling efficiency for the air flowing in the intake port  2 . 
     Next, the configurations of the coolant flow passages of the cylinder head as seen in a cross section passing between the adjacent two combustion chambers and perpendicular to the longitudinal direction will be described.  FIG. 22  is a cross-sectional view showing a cross section, passing between the adjacent two combustion chambers and perpendicular to the longitudinal direction, of the cylinder head of the third embodiment, specifically, a cross section including central axes of head bolt insertion holes  13  and  14  and perpendicular to the longitudinal direction.  FIG. 22  shows the cross-sectional shapes of the first coolant flow passage and the second coolant flow passage in the cross section described above. Further,  FIG. 22  shows the positional relationship between the first coolant flow passage and the other components, including the second coolant flow passage, of the cylinder head  103 . 
     In the cross section shown in  FIG. 22 , the second connecting passage  52  of the first coolant flow passage is located between the intake port central line plane S 2  and the intake valve insertion hole central axis plane S 3  in a region closer to the middle of the cylinder head  103  with respect to the intake-side head bolt insertion hole  13 . The second connecting passage  52  has an elongated rounded rectangular cross-sectional shape substantially parallel to the intake valve insertion hole central axis plane S 3 . The second connecting passage  52  is located on the side opposite to a cylinder block mating surface  1   a  with a portion  20   j  of the second coolant flow passage interposed therebetween. 
     According to the above-described configuration shown in  FIG. 22 , the heat transferred from the cylinder block mating surface  1   a  is absorbed by the portion  20   j  of the second coolant flow passage located between the cylinder block mating surface  1   a  and the second connecting passage  52  of the first coolant flow passage. Therefore, it is suppressed that the heat is directly transferred to the second connecting passage  52  from the cylinder block mating surface  1   a.  Accordingly, it is avoided that the temperature of the coolant flowing in the first coolant flow passage increases to cause a reduction in cooling efficiency for the air flowing in the intake port  2 . 
     Next, a fourth embodiment of the invention will be described with reference to the drawings. The basic configuration of a cylinder head of the fourth embodiment is the same as that of the cylinder head of the first embodiment. Accordingly, the description of the basic configuration of the cylinder head of the first embodiment is incorporated herein in its entirety for the basic configuration of the cylinder head of the fourth embodiment, thereby omitting an overlapping description thereof. 
     The cylinder head of the fourth embodiment includes dual coolant flow passages connected to independent and separate circulation systems. The temperature of a coolant flowing in the first coolant flow passage is equal to that of a coolant flowing in the second coolant flow passage at the time of cold engine start-up and, as warming-up of the engine progresses, the coolant at a temperature lower than that of the coolant flowing in the second coolant flow passage flows in the first coolant flow passage. The cylinder head of the fourth embodiment differs from the cylinder head of the first embodiment in the configuration of the first coolant flow passage. Hereinbelow, the configuration of the first coolant flow passage of the cylinder head of the fourth embodiment will be described. The description will be made using cross-sectional views of the cylinder head and a perspective view showing the coolant flow passage inside the cylinder head in a see-through manner. In the figures, components equivalent to those of the first embodiment are assigned the same symbols. 
     Hereinbelow, the configurations of the coolant flow passages of the cylinder head of the fourth embodiment will be described. Of the dual coolant flow passages provided in the cylinder head of the fourth embodiment, the shape of the first coolant flow passage in which the low-temperature coolant flows will be described with reference to  FIG. 27 .  FIG. 27  is a perspective view showing, in a see-through manner, intake ports  2  and a first coolant flow passage  60  of the cylinder head of the fourth embodiment.  FIG. 27  shows the shape of the first coolant flow passage  60  and the positional relationship between the first coolant flow passage  60 , the intake ports  2 , and valve guides  9  when seen assuming the inside of the cylinder head to be transparent. 
     The first coolant flow passage  60  is provided on the upper side of the row of the intake ports  2  in the cylinder head. The first coolant flow passage  60  extends in a direction of the row of the intake ports  2 , i.e. in a longitudinal direction of the cylinder head, along upper surfaces  2   a  of branch ports  2 L and  2 R of the intake ports  2 . 
     The first coolant flow passage  60  has a unit structure for each intake port  2 . In  FIG. 27 , the structure of a portion encircled by a dotted line is the unit structure of the first coolant flow passage  60 . The unit structure includes a pair of arc-shaped flow passages  61  respectively disposed around the left and right branch ports  2 L and  2 R of the intake port  2 . The arc-shaped flow passages  61  are each a flow passage that is curved in an arc so as to be wound over the branch port  2 L,  2 R from the middle side of the cylinder head. Of both ends of the arc-shaped flow passage  61 , the end located on the middle side of the intake port  2  when seeing the arc-shaped flow passage  61  from the middle side of the cylinder head extends to between the left and right branch ports  2 L and  2 R, while the end located on the outer side of the intake port  2  extends to the side surface side of the cylinder head with respect to an axis of the valve guide  9 . The left and right arc-shaped flow passages  61  are plane-symmetric with respect to a flat plane dividing the intake port  2  into left and right parts (a flat plane including a central axis of a combustion chamber and perpendicular to the longitudinal direction of the cylinder head). 
     The unit structure includes a first connecting passage  63  connecting the left and right arc-shaped flow passages  61 . The first connecting passage  63  is located between the left and right branch ports  2 L and  2 R of the intake port  2 . The first connecting passage  63  continuously communicates with the left and right arc-shaped flow passages  61 . 
     The first coolant flow passage  60  includes second connecting passages  62  each connecting the adjacent two unit structures. The second connecting passage  62  is located in a space between the adjacent two intake ports  2  on the side surface side of the cylinder head with respect to the axis of the valve guide  9 . The second connecting passage  62  is a flow passage curved convex to the side surface side of the cylinder, head and continuously communicates with the arc-shaped flow passages  61  of the adjacent two unit structures. 
     An inlet flow passage  64  and an outlet flow passage  65  are respectively provided at both end portions in the longitudinal direction of the first coolant flow passage  60 . The inlet flow passage  64  extends straight in the longitudinal direction to a first hole  66  opened in a rear end face of the cylinder head. The outlet flow passage  65  extends straight in the longitudinal direction to a second hole  67  opened in a front end face of the cylinder head. The inlet flow passage  64  and the outlet flow passage  65  are flow passages that are formed by core supports supporting a sand core, for forming the first coolant flow passage  60 , from both sides, while the first hole  66  and the second hole  67  are sand removing holes that are formed by removing the core supports. The first hole  66  is used as a coolant inlet, while the second hole  67  is used as a coolant outlet. Alternatively, the second hole  67  may be used as a coolant inlet, while the first hole  66  may be used as a coolant outlet. 
       FIG. 28  is a diagram showing the positional relationship between the intake port  2 , a head bolt  19 , and the first coolant flow passage  60  in the cylinder head of the fourth embodiment.  FIG. 28  shows the shape of the first coolant flow passage  60  around the valve guide  9  and the positional relationship between the intake port  2 , the first coolant flow passage  60 , and the head bolt  19  when seen from the front end side of the cylinder head assuming the inside of the cylinder head to be transparent. The first coolant flow passage  60  passes on the middle side of the cylinder head with respect to the head bolt  19 . More specifically, the first coolant flow passage  60  passes near an intake valve insertion portion  2   d  formed at a forward end portion of the intake port  2 . 
     Next, the positional relationship between the first coolant flow passage and the other components, including the second coolant flow passage, of the cylinder head will be described with reference to cross-sectional views. 
     Hereinbelow, the configurations of the coolant flow passages of the cylinder head as seen in a cross section including a central axis of an intake valve insertion hole and perpendicular to the longitudinal direction will be described.  FIG. 24  is a cross-sectional view showing a cross section, including a central axis L 3  of an intake valve insertion hole  7  and perpendicular to the longitudinal direction, of the cylinder head of the fourth embodiment.  FIG. 24  shows the cross-sectional shapes of the first coolant flow passage and the second coolant flow passage in the cross section described above. Further,  FIG. 24  shows the positional relationship between the first coolant flow passage and the other components, including the second coolant flow passage, of the cylinder head  104 . 
     In the cross section shown in  FIG. 24 , near a top portion of a pent roof of a combustion chamber  4 , a portion  20   k  of the second coolant flow passage is disposed in a region sandwiched between an upper surface  3   a  near an exhaust opening of an exhaust port  3  and the upper surface  2   a  near an intake opening of the intake port  2 . The portion  20   k  of the second coolant flow passage, jointly with other portions  20   b,    20   d,  and  20   e,  forms a water jacket surrounding the exhaust port  3  so as to cool the exhaust port  3  and an exhaust valve. Further, the portion  20   k  of the second coolant flow passage cools the periphery of the combustion chamber  4  that rises to a high temperature. 
     In the cross section shown in  FIG. 24 , the arc-shaped flow passage  61  of the first coolant flow passage is located in a region sandwiched between a cylinder head longitudinal direction central flat plane S 1  and an intake valve insertion hole central axis plane S 3 . More specifically, the arc-shaped flow passage  61  is located in a region sandwiched between the portion  20   k  of the second coolant flow passage and the intake valve insertion hole  7 . The arc-shaped flow passage  61  is disposed close to a root portion of the intake valve insertion hole  7 . Further, the arc-shaped flow passage  61  is located on the side opposite to the top portion of the pent roof of the combustion chamber  4  with the portion  20   k  of the second coolant flow passage interposed therebetween. 
     According to the above-described configuration shown in  FIG. 24 , the upper surface  2   a  of the intake port  2 , particularly the upper surface  2   a  downstream of the intake valve insertion hole  7 , can be effectively cooled by the arc-shaped flow passage  61  of the first coolant flow passage. By cooling the upper surface  2   a  of the intake port  2  with the low-temperature coolant flowing in the first coolant flow passage, it is possible to efficiently cool the air flowing in the intake port  2 . Further, the heat generated from the combustion chamber  4  is absorbed by the portion  20   k  of the second coolant flow passage located between the arc-shaped flow passage  61  and the top portion of the combustion chamber  4 . Therefore, it is suppressed that the heat is directly transferred to the arc-shaped flow passage  61  from the combustion chamber  4 . Accordingly, it is avoided that the temperature of the coolant flowing in the first coolant flow passage increases to cause a reduction in cooling efficiency for the air flowing in the intake port  2 . 
     Next, the configurations of the coolant flow passages of the cylinder head as seen in a cross section including a central axis of the combustion chamber and perpendicular to the longitudinal direction will be described.  FIG. 25  is a cross-sectional view showing a cross section, including a central axis L 1  of the combustion chamber  4  and perpendicular to the longitudinal direction, of the cylinder head of the fourth embodiment.  FIG. 25  shows the cross-sectional shapes of the first coolant flow passage and the second coolant flow passage in the cross section described above. Further,  FIG. 25  shows the positional relationship between the first coolant flow passage and the other components, including the second coolant flow passage, of the cylinder head  104 . 
     In the cross section shown in  FIG. 25 , near an open end  12   a  of a spark plug insertion hole  12 , a portion  20   m  of the second coolant flow passage is disposed on the intake side with respect to the cylinder head longitudinal direction central flat plane S  1 . The portion  20   m  of the second coolant flow passage is disposed between the cylinder head longitudinal direction central flat plane S 1  and the intake valve insertion hole central axis plane S 3 . The portion  20   m  of the second coolant flow passage cools the periphery of the combustion chamber  4  that rises to a high temperature, particularly the periphery of the spark plug insertion hole  12 . 
     In the cross section shown in  FIG. 25 , the first connecting passage  63  of the first coolant flow passage is disposed at a position overlapping the intake valve insertion hole central axis plane S 3 . The first connecting passage  63  is located on the side opposite to the top portion of the combustion chamber  4 , more specifically, on the side opposite to the open end  12   a  of the spark plug insertion hole  12 , with the portion  20   m  of the second coolant flow passage interposed therebetween. 
     According to the above-described configuration shown in  FIG. 25 , the heat generated from the combustion chamber  4  is absorbed by the portion  20   m  of the second coolant flow passage located between the first connecting passage  63  of the first coolant flow passage and the top portion of the combustion chamber  4 . Therefore, it is suppressed that the heat is directly transferred to the first connecting passage  63  from the combustion chamber  4 . Accordingly, it is avoided that the temperature of the coolant flowing in the first coolant flow passage increases to cause a reduction in cooling efficiency for the air flowing in the intake port  2 . 
     Next, the configurations of the coolant flow passages of the cylinder head as seen in a cross section passing between the adjacent two combustion chambers and perpendicular to the longitudinal direction will be described.  FIG. 26  is a cross-sectional view showing a cross section, passing between the adjacent two combustion chambers and perpendicular to the longitudinal direction, of the cylinder head of the fourth embodiment, specifically, a cross section including central axes of head bolt insertion holes  13  and  14  and perpendicular to the longitudinal direction.  FIG. 26  shows the cross-sectional shapes of the first coolant flow passage and the second coolant flow passage in the cross section described above. Further,  FIG. 26  shows the positional relationship between the first coolant flow passage and the other components, including the second coolant flow passage, of the cylinder head  104 . 
     In the cross section shown in  FIG. 26 , a portion  20   n  of the second coolant flow passage is disposed between the cylinder head longitudinal direction central flat plane S 1  and the intake-side head bolt insertion hole  13 . The portion  20   n  of the second coolant flow passage is opened at a cylinder block mating surface  1   a  and communicates with a portion  20   i  of the second coolant flow passage in the middle of the cylinder head  104 . 
     In the cross section shown in  FIG. 26 , the second connecting passage  62  of the first coolant flow passage is located between an intake port central line plane S 2  and the intake valve insertion hole central axis plane S 3  in a region closer to the middle of the cylinder head  104  with respect to the intake-side head bolt insertion hole  13 . The second connecting passage  62  is located on the side opposite to the cylinder block mating surface  1   a  with the portion  20   n  of the second coolant flow passage interposed therebetween. 
     According to the above-described configuration shown in  FIG. 26 , the heat transferred from the cylinder block mating surface  1   a  is absorbed by the portion  20   n  of the second coolant flow passage located between the cylinder block mating surface  1   a  and the second connecting passage  62  of the first coolant flow passage. Therefore, it is suppressed that the heat is directly transferred to the second connecting passage  62  from the cylinder block mating surface  1   a.  Accordingly, it is avoided that the temperature of the coolant flowing in the first coolant flow passage increases to cause a reduction in cooling efficiency for the air flowing in the intake port  2 . 
     Next, a fifth embodiment of the invention will be described with reference to the drawings. A cylinder head of the fifth embodiment is a modification of the cylinder head of the fourth embodiment. The cylinder head of the fifth embodiment differs from the cylinder head of the fourth embodiment in the configuration of a first coolant flow passage. Hereinbelow, the configuration of the first coolant flow passage of the cylinder head of the fifth embodiment will be described. The description will be made using a cross-sectional view -showing a cross section, including a central axis of an intake valve insertion hole and perpendicular to a longitudinal direction, of the cylinder head. In the figure, components equivalent to those of the fourth embodiment are assigned the same symbols. 
     Hereinbelow, the configurations of coolant flow passages of the cylinder head as seen in a cross section including a central axis of an intake valve insertion hole and perpendicular to the longitudinal direction will be described.  FIG. 29  is a cross-sectional view showing a cross section, including a central axis L 3  of an intake valve insertion hole  7  and perpendicular to the longitudinal direction, of the cylinder head of the fifth embodiment.  FIG. 29  shows the cross-sectional shapes of a first coolant flow passage and a second coolant flow passage in the cross section described above. Further,  FIG. 29  shows the positional relationship between the first coolant flow passage and the other components, including the second coolant flow passage, of the cylinder head  105 . 
     In the cross section shown in  FIG. 29 , portions  71  and  72  of the first coolant flow passage are located in a region sandwiched between a cylinder head longitudinal direction central flat plane S 1  and an intake valve insertion hole central axis plane S 3 . The portion  71  of the first coolant flow passage corresponds to the arc-shaped flow passage of the first coolant flow passage of the fourth embodiment, while the portion  72  of the first coolant flow passage corresponds to the arc-shaped flow passage of the first coolant flow passage of the third embodiment. The portions  71  and  72  of the first coolant flow passage are formed by integrating those arc-shaped flow passages. 
     According to the above-described configuration shown in  FIG. 29 , an upper surface  2   a  of an intake port  2 , particularly the upper surface  2   a  downstream of the intake valve insertion hole  7 , can be effectively cooled by the portion  71  of the first coolant flow passage. Further, the periphery of the intake valve insertion hole  7  connected to the upper surface  2   a  of the intake port  2  can be effectively cooled by the portion  72  of the first coolant flow passage. 
     Next, a sixth embodiment of the invention will be described with reference to the drawings. A cylinder head of the sixth embodiment is a cylinder head of a diesel engine. First, the basic configuration of the cylinder head of the sixth embodiment will be described. The description will be made using cross-sectional views of the cylinder head. 
     Hereinbelow, the basic configuration of the cylinder head of the sixth embodiment will be described.  FIG. 30  is a cross-sectional view showing a cross section, including a central axis L 13  of an intake valve insertion hole  88  and perpendicular to a longitudinal direction, of a cylinder head  106  of the sixth embodiment. As shown in  FIG. 30 , a cylinder block mating surface  81   a  as a bottom surface of the cylinder head  106  is formed with a combustion chamber  84 . When the cylinder head  106  is mounted on a cylinder block, the combustion chamber  84  closes a cylinder from above to form a closed space. However, this portion called the combustion chamber  84  is flush with the cylinder block mating surface  81   a  and is not recessed differently from the case of a spark-ignition engine. While the term “combustion chamber” has been customarily used in this technical field, when a closed space sandwiched between the cylinder head  106  and a piston is defined as a combustion chamber, the combustion chamber  84  can be called a combustion chamber ceiling surface. 
     An intake port  82  is opened to the combustion chamber  84  on the right side with respect to a cylinder head longitudinal direction central flat plane S 1   1  as seen from the front end side of the cylinder head  106 . A connecting portion between the intake port  82  and the combustion chamber  84 , i.e. an open end on the combustion chamber side of the intake port  82 , serves as an intake opening that is configured to be opened and closed by an intake valve. Since two intake valves are provided for each cylinder, each combustion chamber  84  is formed with two intake openings. The cylinder head  106  includes the independent intake port  82  for each intake opening. An inlet of the intake port  82  is opened in a right side surface of the cylinder head  106 . The intake port  82  extends obliquely downward to the left from an opening of the inlet and then curves on the way to communicate with the intake opening formed in the combustion chamber  84 . 
     The cylinder head  106  is formed with the intake valve insertion hole  88  for passing a stem of the intake valve therethrough. In the upper surface of the cylinder head  106  on the inner side of a head cover attaching surface  8   1   b,  there is provided an intake-side valve drive mechanism chamber  85  that receives therein a valve drive mechanism configured to drive the intake valves. The intake valve insertion hole  88  extends straight substantially upward from an upper surface  82   a,  near the combustion chamber  84 , of the intake port  82  to the intake-side valve drive mechanism chamber  85 . The central axis L 13  of the intake valve insertion hole  88  is included in the cross section shown in  FIG. 30 , i.e. in a flat plane perpendicular to the longitudinal direction. 
     An exhaust port  83  is opened to the combustion chamber  84  on the left side as seen from the front end side of the cylinder head  106 . A connecting portion between the exhaust port  83  and the combustion chamber  84 , i.e. an open end on the combustion chamber side of the exhaust port  83 , serves as an exhaust opening that is configured to be opened and closed by an exhaust valve. Since two exhaust valves are provided for each cylinder, each combustion chamber  84  is formed with two exhaust openings of the exhaust port  83 . The exhaust port  83  extends from the exhaust openings formed in the combustion chambers  84  to an outlet opened in a left side surface of the cylinder head  106 . The exhaust port  83  is not independently provided for each of the exhaust openings of the combustion chambers  84 , but the single exhaust port  83  is provided for the exhaust openings of the combustion chambers  84 . That is, the exhaust port  83  is composed of a plurality of branch ports respectively extending from the exhaust openings and a collective port into which the branch ports are joined. 
     The cylinder head  106  is formed with an exhaust valve insertion hole  89  for passing a stem of the exhaust valve therethrough. In the upper surface of the cylinder head  106  on the inner side of the head cover attaching surface  81   b,  there is provided an exhaust-side valve drive mechanism chamber  86  that receives therein a valve drive mechanism configured to drive the exhaust valves. The exhaust valve insertion hole  89  extends straight substantially upward from an upper surface  83   a,  near the combustion chamber  84 , of the exhaust port  83  to the exhaust-side valve drive mechanism chamber  86 . 
     Next, the basic configuration of the cylinder head as seen in a cross section including a central axis of the combustion chamber and perpendicular to the longitudinal direction will be described.  FIG. 31  is a cross-sectional view showing a cross section, including a central axis L 11  of the combustion chamber  84  and perpendicular to the longitudinal direction, of the cylinder head  106 . An injector insertion hole  87  for attaching an injector that injects fuel into the cylinder is formed in the upper surface of the cylinder head  106 . The injector insertion hole  87  is formed vertically downward along the central axis L 11  of the combustion chamber  84  from the upper surface of the cylinder head  106  and is opened to the planar combustion chamber  84  at the center thereof. The central axis L 11  of the combustion chamber  84  coincides with a central axis of the cylinder when the cylinder head  106  is mounted on the cylinder block. In the cross section shown in  FIG. 31 , part of the exhaust port  83  having the manifold shape is seen. 
     Next, the configurations of coolant flow passages of the cylinder head  106  of the sixth embodiment will be described. The cylinder head of the sixth embodiment includes dual coolant flow passages connected to independent and separate circulation systems. In the first coolant flow passage, a coolant at a temperature lower than that of a coolant flowing in the second coolant flow passage flows. 
     Hereinbelow, the configurations of the coolant flow passages of the cylinder head of the sixth embodiment will be described.  FIG. 30  shows the cross-sectional shapes of the first coolant flow passage and the second coolant flow passage of the cylinder head  106  in the cross section including the central axis L 13  of the intake valve insertion hole  88  and perpendicular to the longitudinal direction. Further,  FIG. 30  shows the positional relationship between the first coolant flow passage and the other components, including the second coolant flow passage, of the cylinder head  106 . In the cross section shown in  FIG. 30 , regions denoted by symbols  94   a,    94   b,    94   c,  and  94   d  are cross sections of portions of the second coolant flow passage. Although the portions  94   a,    94   b,    94   c,  and  94   d  of the second coolant flow passage are separated from each other in the cross section shown in  FIG. 30 , these portions are joined into one inside the cylinder head  106 . 
     In the cross section shown in  FIG. 30 , on the cylinder head longitudinal direction central flat plane S 11 , the portion  94   a  of the second coolant flow passage is disposed in a region sandwiched between the upper surface  83   a  near the exhaust opening of the exhaust port  83  and the upper surface  82   a  near the intake opening of the intake port  82 . The cylinder head longitudinal direction central flat plane S 11  is a virtual flat plane including the central axes L 11  of the combustion chambers  84  and parallel to the longitudinal direction. The portion  94   b  of the second coolant flow passage is disposed between a lower surface  83   b  of the exhaust port  83  and the cylinder block mating surface  81   a.  The portion  94   b  of the second coolant flow passage is opened at the cylinder block mating surface  81   a  and communicates with a coolant flow passage on the cylinder block side. The portion  94   d  of the second coolant flow passage is disposed on the left side of the exhaust valve insertion hole  89  above the upper surface  83   a  of the exhaust port  83 . The portions  94   a,    94   b,  and  94   d  of the second coolant flow passage form a water jacket surrounding the exhaust port  83  so as to cool the exhaust port  83  and the exhaust valve. Further, the portion  94   a  of the second coolant flow passage cools the periphery of the combustion chamber  84  that rises to a high temperature. 
     In the cross section shown in  FIG. 30 , the portion  94   c  of the second coolant flow passage is disposed between an intake port central line plane S 12  and the cylinder block mating surface  81   a,  more specifically, between a lower surface  82   b  of the intake port  82  and the cylinder block mating surface  81   a.  The intake port central line plane S 12  is a virtual plane defined as a plane including central lines of the intake ports  82 . The portion  94   c  of the second coolant flow passage is opened at the cylinder block mating surface  81   a.  This opening of the cylinder block mating surface  81   a  communicates with the coolant flow passage on the cylinder block side. A coolant having passed through the inside of the cylinder block is introduced into the portion  94   c  of the second coolant flow passage via the opening of the cylinder block mating surface  81   a.    
     In the cross section shown in  FIG. 30 , a first coolant flow passage  91  is located between an intake valve insertion hole central axis plane S 13  and the cylinder head longitudinal direction central flat plane S 11 . The intake valve insertion hole central axis plane S 13  is a virtual flat plane including the central axes L 13  of the intake valve insertion holes  88  and parallel to the longitudinal direction. The portion  94   a  of the second coolant flow passage is located between the first coolant flow passage  91  and the combustion chamber  84 . 
     According to the above-described configuration shown in  FIG. 30 , the upper surface  82   a  of the intake port  82 , particularly the upper surface  82   a  downstream of the intake valve insertion hole  88 , can be effectively cooled by the first coolant flow passage  91  in which the coolant at a temperature lower than that of the coolant cooling the exhaust port  83  flows. By cooling the upper surface  82   a  of the intake port  82  with the low-temperature coolant flowing, it is possible to efficiently cool the air flowing in the intake port  82 . 
     The portion  94   a  of the second coolant flow passage is located between the combustion chamber  84  and the first coolant flow passage  91 . Since the heat generated from the combustion chamber  84  is absorbed by the portion  94   a  of the second coolant flow passage, it is suppressed that the heat is directly transferred to the first coolant flow passage  91  from the combustion chamber  84 . Accordingly, it is avoided that the coolant in the first coolant flow passage  91  is heated by the heat generated from the combustion chamber  84 , resulting in a reduction in cooling efficiency for the air flowing in the intake port  82 . Heat transfer from the cylinder block mating surface  81   a  to the lower surface  82   b  of the intake port  82  can be suppressed by the portion  94   c  of the second coolant flow passage. 
     Next, the configurations of the coolant flow passages of the cylinder head as seen in the cross section including the central axis of the combustion chamber and perpendicular to the longitudinal direction will be described.  FIG. 31  shows the cross-sectional shapes of the first coolant flow passage and the second coolant flow passage of the cylinder head  106  in the cross section including the central axis L 11  of the combustion chamber  84  and perpendicular to the longitudinal direction. Further,  FIG. 31  shows the positional relationship between the first coolant flow passage and the other components, including the second coolant flow passage, of the cylinder head  106 . In the cross section shown in  FIG. 31 , regions denoted by symbols  94   e,    94   f,    94   g,    94   h,    94   i,  and  94   j  are cross sections of portions of the second coolant flow passage. Although the portions  94   e,    94   f,    94   g,    94   h,    94   i,  and  94   j  of the second coolant flow passage are separated from each other in the cross section shown in  FIG. 31 , these portions are joined into one with the portions  94   a,    94   b,    94   c,  and  94   d  shown in  FIG. 30  inside the cylinder head  106 . 
     In the cross section shown in  FIG. 31 , the portions  94   f,    94   i,  and  94   j  of the second coolant flow passage are disposed on the intake side with respect to the cylinder head longitudinal direction central flat plane S 11 . The portion  94   f  of the second coolant flow passage is disposed close to an intake-side wall surface of a forward end portion of the injector insertion hole  87  between the cylinder head longitudinal direction central flat plane S 11  and the intake valve insertion hole central axis plane S 13 . 
     Near an open end  87   a  of the injector insertion hole  87 , the portion  94   e  of the second coolant flow passage is disposed on the exhaust side with respect to the cylinder head longitudinal direction central flat plane S 11 . The portion  94   e  of the second coolant flow passage is disposed along an exhaust-side wall surface of the forward end portion of the injector insertion hole  87 . The portion  94   g  of the second coolant flow passage is disposed above the portion  94   e  of the second coolant flow passage, while the portion  94   h  of the second coolant flow passage is disposed on the left side of the portion  94   e  of the second coolant flow passage. The portions  94   e,    94   g,  and  94   h  of the second coolant flow passage form a water jacket surrounding the exhaust port  83  jointly with the portions  94   a,    94   b,  and  94   d  shown in  FIG. 30 . 
     In the cross section shown in  FIG. 31 , a first coolant flow passage  92  is located between the cylinder head longitudinal direction central flat plane S 11  and the intake port central line plane S 12 . The first coolant flow passage  92  is located on the side opposite to the open end  87   a  of the injector insertion hole  87  with the portion  94   f  of the second coolant flow passage interposed therebetween. 
     According to the above-described configuration shown in  FIG. 31 , the heat generated from the combustion chamber  84  is absorbed by the portion  94   f  of the second coolant flow passage located between the first coolant flow passage  92  and the combustion chamber  84 . Therefore, it is suppressed that the heat is directly transferred to the first coolant flow passage  92  from the combustion chamber  84 . Accordingly, it is avoided that the temperature of the coolant flowing in the first coolant flow passage  92  increases to cause a reduction in cooling efficiency for the air flowing in the intake port  82 . 
     Next, a seventh embodiment of the invention will be described with reference to the drawings. The seventh embodiment has a feature in the configuration of an engine cooling system. The engine cooling system of the seventh embodiment can be combined with any of the cylinder heads of the first to sixth embodiments. However, herein, a description will be given of an example combined with the cylinder head of the first embodiment. 
     Hereinbelow, referring to  FIG. 32 , the configuration of the engine cooling system of the seventh embodiment of the invention will be described. In  FIG. 32 , components equivalent to those of the engine cooling system of the first embodiment shown in  FIG. 1  are assigned the same symbols. An overlapping description of those equivalent components will be omitted or simplified. 
     The engine cooling system of the seventh embodiment includes dual circulation systems  140  and  160 . The configuration of the second circulation system  160  is the same as that of the first embodiment, while the configuration of the first circulation system  140  differs from that of the first embodiment. Hereinbelow, the configuration of the first circulation system  140  of the seventh embodiment will be described. 
     The configuration of the first circulation system will be described hereinbelow. The first circulation system  140  forms a closed loop independent of the second circulation system  160  and includes a radiator  124  and a water pump  123 . A cylinder head  101  is formed with a coolant inlet to which a coolant introducing pipe  121  of the first circulation system  140  is connected, and with a coolant outlet to which a coolant discharge pipe  122  of the first circulation system  140  is connected. The coolant inlet of the cylinder head  101  is connected to a coolant outlet of the radiator  124  via the coolant introducing pipe  121 , while the coolant outlet of the cylinder head  101  is connected to a coolant inlet of the radiator  124  via the coolant discharge pipe  122 . The coolant introducing pipe  121  is provided with the water pump  123 . The first circulation system  140  may further include a liquid temperature sensor and a thermostat for liquid temperature adjustment (neither shown). 
     The first circulation system  140  includes a first coolant flow passage  30  formed in the cylinder head  101  and a fourth coolant flow passage  153  formed in a cylinder block  151 . The first coolant flow passage  30  communicates with the coolant inlet. Like a third coolant flow passage  152 , the fourth coolant flow passage  153  includes a water jacket surrounding cylinders. The cylinder head  101  is formed therein with an intermediate communication passage  172  communicating the first coolant flow passage  30  with the fourth coolant flow passage  153 . The intermediate communication passage  172  and the fourth coolant flow passage  153  are connected to each other via an opening formed in a mating surface between the cylinder head  101  and the cylinder block  151 . Further, the cylinder head  101  is formed therein with an outlet communication passage  170  communicating the fourth coolant flow passage  153  with the coolant outlet. The outlet communication passage  170  and the fourth coolant flow passage  153  are connected to each other via an opening formed in the mating surface between the cylinder head  101  and the cylinder block  151 . 
     A coolant circulating in the first circulation system  140  is introduced into the coolant inlet formed in the cylinder head  101  and flows in the first coolant flow passage  30  of the cylinder head  101 , thereby cooling intake ports  2 . The coolant used for cooling the intake ports  2  then flows in the fourth coolant flow passage  153  of the cylinder block  151  to cool the cylinders and then is discharged from the coolant outlet formed in the cylinder head  101 . 
     According to the configuration shown in  FIG. 32 , the coolant having passed through the first coolant flow passage  30  is configured to flow in the cylinder block  151  and can be used for cooling the cylinders. 
     Next, the configuration of the intermediate communication passage will be described.  FIG. 33  is a perspective view showing, in a see-through manner, the intake ports  2  and the first coolant flow passage  30  of the cylinder head  101  in the engine cooling system of the seventh embodiment. In  FIG. 33 , components equivalent to those of the first coolant flow passage of the first embodiment shown in  FIG. 6  are assigned the same symbols. As shown in  FIG. 33 , the intermediate communication passage  172  connects an outlet flow passage  36  of the first coolant flow passage  30  to an outlet hole  173  opened in the cylinder block mating surface. The intermediate communication passage  172  is formed between a front end face of the cylinder head and the intake port  2  closest thereto. In the seventh embodiment, an open end (a hole opened in the front end face of the cylinder head)  171  of the outlet flow passage  36  is sealed. The coolant having passed through the first coolant flow passage  30  passes, from the outlet flow passage  36 , through the intermediate communication passage  172  and flows to the outlet hole  173  of the cylinder block mating surface. Alternatively, the outlet hole  173  may be used as a coolant inlet, while a first hole  37  may be used as a coolant outlet. 
       FIG. 34  is a diagram showing the positional relationship between the intermediate communication passage  172  and a head bolt  19  when seen from the front end side of the cylinder head assuming the inside of the cylinder head to be transparent. The intermediate communication passage  172  is formed toward the outlet flow passage  36  from the outlet hole  173  at a position on the middle side of the cylinder head with respect to the head bolt  19 . The intermediate communication passage  172  may be formed by drilling. 
     Hereinbelow, a modification of the intermediate communication passage will be described.  FIG. 35  is a diagram showing the configuration of the modification of the intermediate communication passage. In  FIG. 35 , components equivalent to those of the first coolant flow passage of the first embodiment shown in  FIG. 6  are assigned the same symbols. This modification includes an intermediate communication passage  174  extending from an outlet flow passage  36  and intermediate communication passages  176  respectively extending from second connecting passages  33 . The intermediate communication passage  174  is formed between a front end face of a cylinder head and an intake port  2  closest thereto and connects the outlet flow passage  36  to an outlet hole  175  opened in a cylinder block mating surface. Each intermediate communication passage  176  is formed between adjacent two intake ports  2  and connects the second connecting passage  33  to an outlet hole  177  opened in the cylinder block mating surface. A cylinder block is formed with coolant flow passages corresponding to the intermediate communication passages  174  and  176 . The outlet hole  175  may be used as a coolant inlet, while a first hole  37  may be used as a coolant outlet. 
     Hereinbelow, a modification of the first circulation system will be described.  FIG. 36  is a diagram showing the modification of the first circulation system. In this modification, a first circulation system  141  includes a first coolant flow passage  30  formed in a cylinder head  101  and an intermediate communication passage  172 . The cylinder head  101  is formed with a coolant inlet to which a coolant introducing pipe  121  of the first circulation system  141  is connected, while a cylinder block  151  is formed with a coolant outlet to which a coolant discharge pipe  122  of the first circulation system  141  is connected. The cylinder block  151  is formed therein with an outlet communication passage  154  communicating the intermediate communication passage  172  with the coolant outlet. The intermediate communication passage  172  and the outlet communication passage  154  are connected to each other via an opening formed in a mating surface between the cylinder head  101  and the cylinder block  151 . 
     A coolant circulating in the first circulation system  141  is introduced into the coolant inlet formed in the cylinder head  101  and flows in the first coolant flow passage  30  of the cylinder head  101 , thereby cooling intake ports  2 . The coolant used for cooling the intake ports  2  then flows into the cylinder block  151  through the intermediate communication passage  172  and is discharged from the coolant outlet formed in the cylinder block  151 . When the coolant having passed through the first coolant flow passage  30  is not used for cooling cylinders, the configuration of this modification can be employed. 
     Next, an eighth embodiment of the invention will be described with reference to the drawings. The eighth embodiment has a feature in the configuration of an engine cooling system. The engine cooling system of the eighth embodiment can be combined with any of the cylinder heads of the first to sixth embodiments. However, herein, a description will be given of an example combined with the cylinder head of the first embodiment. 
     Hereinbelow, referring to  FIG. 37 , the configuration of the engine cooling system of the eighth embodiment of the invention will be described. In  FIG. 37 , components equivalent to those of the engine cooling system of the first embodiment shown in  FIG. 1  are assigned the same symbols. An overlapping description of those equivalent components will be omitted or simplified. 
     The engine cooling system of the eighth embodiment includes dual circulation systems  142  and  160 . The configuration of the second circulation system  160  is the same as that of the first embodiment, while the configuration of the first circulation system  142  differs from that of the first embodiment. Hereinbelow, the configuration of the first circulation system  142  of the eighth embodiment will be described. 
     The configuration of the first circulation system will be described hereinbelow. The first circulation system  142  forms a closed loop independent of the second circulation system  160  and includes a radiator  124  and a water pump  123 . A coolant inlet to which a coolant introducing pipe  121  of the first circulation system  142  is connected is formed in a cylinder block  151 . A cylinder head  101  is formed with a coolant outlet to which a coolant discharge pipe  122  of the first circulation system  142  is connected. The coolant inlet of the cylinder block  151  is connected to a coolant outlet of the radiator  124  via the coolant introducing pipe  121 , while the coolant outlet of the cylinder head  101  is connected to a coolant inlet of the radiator  124  via the coolant discharge pipe  122 . The coolant introducing pipe  121  is provided with the water pump  123 . The first circulation system  142  may further include a liquid temperature sensor and a thermostat for liquid temperature adjustment (neither shown). 
     The first circulation system  142  includes a first coolant flow passage  30  formed in the cylinder head  101 . The first coolant flow passage  30  communicates with the coolant outlet. The cylinder block  151  is formed therein with an inlet communication passage  155  connecting the coolant inlet to the cylinder head  101 . The cylinder head  101  is formed therein with an intermediate communication passage  182  communicating the first coolant flow passage  30  with the inlet communication passage  155 . The inlet communication passage  155  and the intermediate communication passage  182  are connected to each other via an opening formed in a mating surface between the cylinder head  101  and the cylinder block  151 . 
     A coolant circulating in the first circulation system  142  enters the coolant inlet formed in the cylinder block  151 , then flows into the cylinder head  101  through the inlet communication passage  155 , and then is introduced into the first coolant flow passage  30  through the intermediate communication passage  182 . The coolant flows in the first coolant flow passage  30  to cool intake ports  2  and is discharged from the coolant outlet formed in the cylinder head  101 . 
     According to the configuration shown in  FIG. 37 , the coolant which is to flow in the first coolant flow passage  30  can be introduced from the cylinder block  151 . When it is not possible to form a coolant inlet in the cylinder head  101 , the configuration shown in  FIG. 37  is useful. 
     Next, the configuration of the intermediate communication passage will be described.  FIG. 38  is a perspective view showing, in a see-through manner, the intake ports  2  and the first coolant flow passage  30  of the cylinder head  101  in the engine cooling system of the eighth embodiment. In  FIG. 38 , components equivalent to those of the first coolant flow passage of the first embodiment shown in  FIG. 6  are assigned the same symbols. As shown in  FIG. 38 , the intermediate communication passage  182  connects an inlet flow passage  35  of the first coolant flow passage  30  to an inlet hole  183  opened in the cylinder block mating surface. The intermediate communication passage  182  is formed between a rear end face of the cylinder head and the intake port  2  closest thereto. In the eighth embodiment, an open end (a hole opened in the rear end face of the cylinder head)  181  of the inlet flow passage  35  is sealed. The coolant for cooling the intake ports  2  is introduced from the inlet hole  183  of the cylinder block mating surface into the first coolant flow passage  30  through the intermediate communication passage  182 . Alternatively, a second hole  38  may be used as a coolant inlet, while the inlet hole  183  may be used as a coolant outlet. 
     Next, a ninth embodiment of the invention will be described with reference to the drawings. The ninth embodiment has a feature in the configuration of an engine cooling system. The engine cooling system of the ninth embodiment can be combined with any of the cylinder heads of the first to sixth embodiments. However, herein, a description will be given of an example combined with the cylinder head of the first embodiment. 
     Hereinbelow, referring to  FIG. 39 , the configuration of the engine cooling system of the ninth embodiment of the invention will be described. In  FIG. 39 , components equivalent to those of the engine cooling system of the first embodiment shown in  FIG. 1  are assigned the same symbols. An overlapping description of those equivalent components will be omitted or simplified. 
     Hereinbelow, the configuration of a circulation system will be described. The engine cooling system of the ninth embodiment includes a single circulation system  143 . The circulation system  143  includes a radiator  124  and a water pump  123 . A cylinder head  101  is formed with a coolant inlet to which a coolant introducing pipe  121  of the circulation system  143  is connected, and with a coolant outlet to which a coolant discharge pipe  122  of the circulation system  143  is connected. The coolant inlet is connected to a coolant outlet of the radiator  124  via the coolant introducing pipe  121 , while the coolant outlet is connected to a coolant inlet of the radiator  124  via the coolant discharge pipe  122 . The coolant introducing pipe  121  is provided with the water pump  123 . The circulation system  143  may further include a liquid temperature sensor and a thermostat for liquid temperature adjustment (neither shown). 
     The circulation system  143  includes a first coolant flow passage  30  and a second coolant flow passage  20  formed in the cylinder head  101  and a third coolant flow passage  152  formed in a cylinder block  151 . The first coolant flow passage  30  communicates with the coolant inlet. The cylinder head  101  is formed therein with an intermediate communication passage  172  communicating the first coolant flow passage  30  with the third coolant flow passage  152 . The intermediate communication passage  172  and the third coolant flow passage  152  are connected to each other via an opening formed in a mating surface between the cylinder head  101  and the cylinder block  151 . The third coolant flow passage  152  of the cylinder block  151  and the second coolant flow passage  20  of the cylinder head  101  communicate with each other via openings formed at a plurality of portions of the mating surface between the cylinder head  101  and the cylinder block  151 . The second coolant flow passage  20  communicates with the coolant outlet. 
     A coolant circulating in the circulation system  143  is introduced into the coolant inlet formed in the cylinder head  101  and flows in the first coolant flow passage  30  of the cylinder head  101 , thereby cooling intake ports  2  from upper surface sides thereof. The coolant used for cooling the intake ports  2  then flows in the third coolant flow passage  152  of the cylinder block  151  to cool cylinders. The coolant used for cooling the cylinders returns to the cylinder head  101  and flows in the second coolant flow passage  20  of the cylinder head  101  to cool lower surfaces of exhaust ports and the intake ports  2 , and then is discharged from the coolant outlet formed in the cylinder head  101 . 
     According to the configuration shown in  FIG. 39 , while cooling those portions, required to be cooled, of the cylinder head  101  and the cylinder block  151  by the single circulation system  143 , it is possible to achieve that the temperature of the coolant flowing in the first coolant flow passage  30  is made lower than that of the coolant flowing in the second coolant flow passage  20 . 
     Next, a tenth embodiment of the invention will be described with reference to the drawings. The tenth embodiment has a feature in the configuration of an engine cooling system. The engine cooling system of the tenth embodiment can be combined with any of the cylinder heads of the first to sixth embodiments. However, herein, a description will be given of an example combined with the cylinder head of the first embodiment. 
     Hereinbelow, referring to  FIG. 40 , the configuration of the engine cooling system of the tenth embodiment of the invention will be described. In  FIG. 40 , components equivalent to those of the engine cooling system of the first embodiment shown in  FIG. 1  are assigned the same symbols. An overlapping description of those equivalent components will be omitted or simplified. 
     Hereinbelow, the configuration of a circulation system will be described. The engine cooling system of the tenth embodiment includes a single circulation system  144 . The circulation system  144  includes a radiator  124  and a water pump  123 . A cylinder head  101  is formed with a coolant inlet to which a coolant introducing pipe  121  of the circulation system  144  is connected, while a cylinder block  151  is formed with a coolant outlet to which a coolant discharge pipe  122  of the circulation system  144  is connected. The coolant inlet is connected to a coolant outlet of the radiator  124  via the coolant introducing pipe  121 , while the coolant outlet is connected to a coolant inlet of the radiator  124  via the coolant discharge pipe  122 . The coolant introducing pipe  121  is provided with the water pump  123 . The circulation system  144  may further include a liquid temperature sensor and a thermostat for liquid temperature adjustment (neither shown). 
     The circulation system  144  includes a first coolant flow passage  30  and a second coolant flow passage  20  formed in the cylinder head  101  and a third coolant flow passage  152  formed in the cylinder block  151 . The first coolant flow passage  30  communicates with the coolant inlet. The first coolant flow passage  30  communicates with the second coolant flow passage  20  inside the cylinder head  101 . The second coolant flow passage  20  and the third coolant flow passage  152  of the cylinder block  151  communicate with each other via openings formed at a plurality of portions of a mating surface between the cylinder head  101  and the cylinder block  151 . The third coolant flow passage  152  communicates with the coolant outlet. 
     A coolant circulating in the circulation system  144  is introduced into the coolant inlet formed in the cylinder head  101  and flows in the first coolant flow passage  30  of the cylinder head  101 , thereby cooling intake ports  2  from upper surface sides thereof. The coolant used for cooling the intake ports  2  advances from the first coolant flow passage  30  into the second coolant flow passage  20  and flows in the second coolant flow passage  20  to cool lower surfaces of exhaust ports and the intake ports  2 . The coolant having passed through the inside of the cylinder head  101  then flows in the third coolant flow passage  152  of the cylinder block  151  to cool cylinders and then is discharged from the coolant outlet formed in the cylinder block  151 . 
     According to the configuration shown in  FIG. 40 , while cooling those portions, required to be cooled, of the cylinder head  101  and the cylinder block  151  by the single circulation system  144 , it is possible to achieve that the temperature of the coolant flowing in the first coolant flow passage  30  is made lower than that of the coolant flowing in the second coolant flow passage  20 . 
     Next, an eleventh embodiment of the invention will be described with reference to the drawings. The eleventh embodiment has a feature in the configuration of an engine cooling system. The engine cooling system of the eleventh embodiment can be combined with any of the cylinder heads of the first to sixth embodiments. However, herein, a description will be given of an example combined with the cylinder head of the first embodiment. 
     Hereinbelow, referring to  FIG. 41 , the configuration of the engine cooling system of the eleventh embodiment of the invention will be described. In  FIG. 41 , components equivalent to those of the engine cooling system of the first embodiment shown in  FIG. 1  are assigned the same symbols. An overlapping description of those equivalent components will be omitted or simplified. 
     Hereinbelow, the configuration of a circulation system will be described. The engine cooling system of the eleventh embodiment includes dual circulation systems  145  and  166 . The duel circulation systems  145  and  166  respectively form closed loops, but are not completely independent of each other and share a single radiator  124 . Water pumps  123  and  163  each for circulating a coolant are respectively provided in the duel circulation systems  145  and  166 . The coolant cooled by the radiator  124  is distributed to the circulation systems  145  and  166  and the coolants circulated in the circulation systems  145  and  166  are collected into the radiator  124  so as to be cooled. 
     The first circulation system  145  includes a first coolant flow passage  30  formed in a cylinder head  101 . The cylinder head  101  is formed with a coolant inlet and a coolant outlet each communicating with the first coolant flow passage  30 . The coolant inlet of the cylinder head  101  is connected to a coolant outlet of the radiator  124  via a coolant introducing pipe  121 , while the coolant outlet of the cylinder head  101  is connected to a coolant inlet of the radiator  124  via a coolant discharge pipe  122 . The coolant discharge pipe  122  and the coolant introducing pipe  121  are connected to each other via a bypass pipe  127  bypassing the radiator  124 . A thermostat  128  is provided at a joint portion between the coolant introducing pipe  121  and the bypass pipe  127 . The water pump  123  is provided downstream of the thermostat  128  in the coolant introducing pipe  121 . 
     In the first circulation system  145 , the coolant heated by passing through the cylinder head  101  and the coolant cooled by the radiator  124  are mixed together by the thermostat  128 . Then, the coolant at a temperature adjusted by the thermostat  128  is supplied to the first coolant flow passage  30  formed in the cylinder head  101 . 
     The second circulation system  166  includes a second coolant flow passage  20  formed in the cylinder head  101  and a third coolant flow passage  152  formed in a cylinder block  151 . The second coolant flow passage  20  of the cylinder head  101  and the third coolant flow passage  152  of the cylinder block  151  are connected to each other via an opening formed in a mating surface between the cylinder head  101  and the cylinder block  151 . The cylinder block  151  is formed with a coolant inlet communicating with the third coolant flow passage  152 , while the cylinder head  101  is formed with a coolant outlet communicating with the second coolant flow passage  20 . The coolant inlet of the cylinder block  151  is connected to the coolant outlet of the radiator  124  via a coolant introducing pipe  161 , while the coolant outlet of the cylinder head  101  is connected to the coolant inlet of the radiator  124  via a coolant discharge pipe  162 . The coolant discharge pipe  162  and the coolant introducing pipe  161  are connected to each other via a bypass pipe  167  bypassing the radiator  124 . A thermostat  168  is provided at a joint portion between the coolant introducing pipe  161  and the bypass pipe  167 . The preset temperature of the thermostat  168  is set higher than that of the thermostat  128  of the first circulation system  145 . The water pump  163  is provided downstream of the thermostat  168  in the coolant introducing pipe  161 . 
     In the second circulation system  166 , the coolant heated by passing through the cylinder block  151  and the cylinder head  101  and the coolant cooled by the radiator  124  are mixed together by the thermostat  168 . Then, the coolant at a temperature adjusted by the thermostat  168  is supplied to the third coolant flow passage  152  of the cylinder block  151  via the water pump  163  and the coolant having passed through the third coolant flow passage  152  is supplied to the second coolant flow passage  20  formed in the cylinder head  101 . 
     According to the configuration shown in  FIG. 41 , by the temperature setting of the thermostats  128  and  168 , it is possible to provide a distinct difference between the temperature of the coolant flowing in the first coolant flow passage  30  and the temperature of the coolant flowing in the second coolant flow passage  20 . The bypass pipe  127  and the thermostat  128  of the first circulation system  145  are not necessarily required. 
     Other than the embodiments described above, the following mode may be employed as another embodiment. In the first embodiment, the coolant inlet and the coolant outlet are provided in the rear end face and the front end face of the cylinder head. However, if the coolant inlet cannot be provided in the rear end face or the front end face of the cylinder head, a coolant inlet may be provided in the side surface of the cylinder head. Specifically, the sand removing hole formed when forming the first coolant flow passage by the sand core may be sealed and a communication passage that communicates with the first coolant flow passage may be formed by drilling from the side surface of the cylinder head. This also applies to the coolant outlet.