Abstract:
A cylinder head, for an internal combustion engine, enhances engine efficiency while ensuring favorable coolant flow. The cylinder head includes a main casting body  41  having a hollow water jacket formed therein, to allow coolant flow therethrough. The main casting body  41  includes a plurality of cylindrical plug hole walls  91  which have plug holes  90  formed therein inside a head-side water jacket  60 . The cylinder head also includes partitions  100 , formed inside the head-side water jacket  60 , and these partitions connect the plug hole walls  91  together form a dividing wall, which divides the water jacket into an exhaust port section and an intake port section. Coolant entering the water jacket is divided into two substreams, which flow initially in substantially opposite directions, and which are reunited after flowing around opposite ends of the dividing wall.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
         [0001]    The present application claims priority under 35 USC 119 based on Japanese patent application No. 2003-030095, filed Feb. 6, 2003.  
         BACKRGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to an improved cylinder head for a multi-cylinder, water-cooled internal combustion engine. More particularly, the present invention relates to an improved cylinder head having improved coolant flow and operating efficiency.  
           [0004]    2. Description of the Background Art  
           [0005]    Many different designs for internal combustion engines are known, including reciprocating engines for vehicles or the like, and water-cooled internal combustion engines which exhibit high cooling performance. In one known design for internal combustion cylinder heads, the flow of coolant is travels through two parallel coolant passages formed along a cylinder row direction (see, for example, Japanese Patent Publication No. 40218/1989 and Japanese Utility Model Publication No. 5081/1990).  
           [0006]    In another known design for cylinder heads, a coolant outlet is formed approximately centrally in the head and extends in a cylinder row direction, with a wall extending in the cylinder row direction and dividing the inside of a water jacket into the outlet side and a main coolant passage side. In this second known design, coolant which flows in the water jacket is made to flow into the main coolant passage from both sides in the cylinder row direction to the outlet side, and the coolant then advances to the coolant outlet (see, for example, Japanese Patent Laid-Open No. 2000-87798). To make the flow of coolant uniform in this manner is helpful in equalizing the temperature throughout the cylinder head, and suppressing the occurrence of a temperature gradient in the cylinder row direction of the cylinder head.  
           [0007]    The above-described cylinder head is generally a cast product with a complicated structure for holding a large number of dynamic valve systems, and hence, there is still a need for a novel structure which makes the flow of coolant uniform, enhances engine efficiency, and enables weight reduction.  
           [0008]    Although the known devices have some utility for their intended purposes, there is still a need to provide a cylinder head of a water-cooled multi-cylinder internal combustion engine. In particular, there is a need for an improved cylinder head of a water-cooled multi-cylinder internal combustion engine with improved coolant flow and cylinder head efficiency, designed to solve the above-mentioned problems.  
         SUMMARY OF THE INVENTION  
         [0009]    The present invention has been made in view of circumstances described above, and it is an object of the present invention to enhance engine efficiency while maintaining a favorable coolant flow, and to reduce weight of a cylinder head for an internal combustion engine.  
           [0010]    As means for solving the above-mentioned problem, the invention described in a first aspect hereof is characterized in that, in a cylinder head for an internal combustion engine, a plurality of plug hole walls (for example, plug hole walls  91  in the depicted embodiment) which define plug holes (for example, plug holes  90  in the depicted embodiment) are formed in a water jacket (for example, a head-side water jacket  60  in this embodiment) of a main casting body (for example, a main casting body  41  in the depicted embodiment). A connection wall, which connects the plug hole walls (for example, a partition  100  in the depicted embodiment) with each other is integrally formed as part of the main casting body, and divides the water jacket into an intake port section and an exhaust port section.  
           [0011]    According to the above-mentioned cylinder head for and internal combustion engine, the connection wall which is provided between the plug hole walls functions to divide the waterjacket into two sections, and hence, it is possible to direct coolant which flows in at the upstream side of the water jacket, partitioned by the connection wall, into two substreams initially flowing in substantially opposite directions to the downstream side of the connection wall, where the substreams are reunited.  
           [0012]    Further, by connecting respective plug hole walls using the connection wall, at the time of casting the cylinder head, portions of the connection walls define a molten metal passage around the plug hole walls and hence, the flow of molten metal around the plug hole walls where intake passages, exhaust passages and the like are densely arranged can be improved.  
           [0013]    Still further, a mating surface of the cylinder head between respective cylinders and the cylinder block is reinforced by the connection wall, which is arranged in the water jacket disposed above the cylinder head, and hence, it is possible to reduce the thickness of the periphery of the mating surface.  
           [0014]    The invention according to a second aspect hereof is characterized in that, in a cylinder head for an internal combustion engine, a plurality of plug hole walls (for example, plug hole walls  91  in the depicted embodiment) which define plug holes (for example, plug holes  90  in the depicted embodiment) are formed in a waterjacket (for example, a head-side waterjacket  60  in the depicted embodiment) of a main casting body (for example, a main casting body  41  in the depicted embodiment). A plurality of partitions (for example, partitions  100  in the depicted embodiment) are disposed inside of the water jacket and extend between the plug hole walls, and a portion of each partition is cut by an access hole (for example, an access hole  110  in the depicted embodiment). A sand removing plug (for example, a plug  111  in the depicted embodiment) is installed in the access hole, and a gap is left open between the sand removing plug and the partition.  
           [0015]    According to the above-mentioned cylinder head design for an internal combustion engine, it is possible to direct coolant, which flows in the upstream (exhaust port) side of the water jacket, into two substreams which initially flow in substantially opposite directions around the partition between respective plug hole walls in the direction from both sides in the cylinder row direction to the downstream side.  
           [0016]    Further, by connecting respective plug hole walls using the partition, at the time of casting the cylinder head, portions of the partitions define a molten metal passage around the plug hole walls and hence, the flow of molten metal around the plug hole walls where intake passages and exhaust passages and the like are densely arranged can be improved.  
           [0017]    Still further, a mating surface of the cylinder head between the respective cylinders and the cylinder block is reinforced by the partition which is arranged in the water jacket disposed above the cylinder head and hence, it is possible to reduce the thickness of the periphery of the mating surface of the cylinder head.  
           [0018]    Further, by forming the access hole which cuts away the portion of the partition, after casting, sand can be simultaneously removed from both coolant passages of the water jacket which are partitioned by the partition and, at the same time, by providing the gap between the sand removing plug and the partition, stay or dwelling of air in coolant between respective plug hole walls can be suppressed.  
           [0019]    For a more complete understanding of the present invention, the reader is referred to the following detailed description section, which should be read in conjunction with the accompanying drawings. Throughout the following detailed description and in the drawings, like number refer to like parts. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]    [0020]FIG. 1 is a side view of a motorcycle, which incorporates a cylinder head according to a first selected illustrative embodiment of the present invention.  
         [0021]    [0021]FIG. 2 is a side view, partially cut away, of an engine incorporating a cylinder head according to the first selected illustrative embodiment of the present invention.  
         [0022]    [0022]FIG. 3 is a partial perspective view of a coolant circulation passage of the engine of FIG. 2, with other components omitted for purposes of illustration.  
         [0023]    [0023]FIG. 4 is a cross-sectional view of a main casting body of the cylinder head according to the first embodiment, taken along a vertical plane transverse to the longitudinal axis.  
         [0024]    [0024]FIG. 5 is a cross-sectional view of the main casting body, taken along a horizontal plane passing through the line A-A in FIG. 4.  
         [0025]    [0025]FIG. 6 is a simplified explanatory view of a head-side water jacket within the casting body of FIG. 4, and showing the outline of the casting body in phantom.  
         [0026]    [0026]FIG. 7 is a top plan view of a main casting body according to a second illustrative embodiment of the present invention.  
         [0027]    [0027]FIG. 8 is a longitudinal cross-sectional view of the casting body of FIG. 7, taken along the line B-B in FIG. 7. 
     
    
     DETAILED DESCRIPTION  
       [0028]    A number of selected illustrative embodiments for carrying out the present invention is explained hereinafter, in conjunction with the drawings. It should be understood, however, that the described embodiments are intended to illustrate, rather than to limit the invention.  
         [0029]    [0029]FIG. 1 illustrates a motorcycle  1  with an engine incorporating a cylinder head according to a selected illustrative embodiment of the present invention. As shown in FIG. 1, a front fork  3 , which rotatably supports a front wheel  2  of the motorcycle  1 , is pivotally supported in a steerable manner on a vehicle body frame  5 . The vehicle body frame  5  includes a head pipe  6 , connected to a front end portion of the frame  5  by way of a steering stem  4 . A rear fork  8 , which rotatably supports a rear wheel  7 , is tiltably and pivotally supported on a pivot portion  9  of the vehicle body frame  5 .  
         [0030]    The motorcycle also includes an engine body  15 , mounted on an intermediate portion of the vehicle body frame  5 . A rear shock absorber  10  has its upper end attached to the vehicle body frame  5 , adjacent a pivot shaft of the rear fork  8 . The lower end of the rear shock absorber  10  is mounted on a lower portion of the engine body  15 , by way of a link mechanism  11 . The rear shock absorber  10  absorbs an impact to prevent the impact from jarring the vehicle body frame  5 , by way of the rear wheel  7  and the rear fork  8 .  
         [0031]    A main frame member  12  of the vehicle body frame  5  is separated in the left and right direction and extends rearwardly and downwardly from an upper portion of the head pipe  6 , while rear end portions of the main frame member  12  are bent downwardly and are connected to the pivot portion  9 . A seat frame  13  of the vehicle body frame  5  is connected to a rear portion of the main frame member  12 . A fuel tank  14  is installed above the main frame member  12 , while the engine body  15  of a water-cooled parallel four cylinder engine, according to the present invention, is arranged below the main frame member  12 .  
         [0032]    A driver&#39;s seat  16  and a rear pillion seat  17  are respectively supported on the seat frame  13 , behind a rear portion of the fuel tank  14 . Further, a driver&#39;s step  18  is mounted on the main frame member  12 , behind the pivot portion  9 , while a rear occupant step  19  is mounted on a lower portion of the seat frame  13 . Further, a pair of left and right handle grips  20  are mounted on respective ends of a handlebar, at an upper end portion of the front fork  3 .  
         [0033]    A brake caliper  21  is mounted on a lower end portion of the front fork  3  and a brake rotor  22 , corresponding to the brake caliper  21 , is mounted on the front wheel  2 . The brake caliper  21  and the brake rotor  22  constitute a front brake device  23 . A rear brake device (not shown in the drawing) is also provided at the right side of the rear wheel  7 , having a constitution substantially similar to the constitution of the front brake device  23 .  
         [0034]    A front portion of the body of the motorcycle  1  is covered with a front cowl  24 , while a periphery of the seat frame  13  is covered with a rear cowl  25 .  
         [0035]    A rear sprocket  26  is mounted on the left side of the rear wheel  7 , and a drive chain  28  is wound around the rear sprocket  26  and a drive sprocket wheel  27  arranged at the left side of a rear portion of the engine body  15  and hence, a drive force of the engine can be transmitted to the rear wheel  7 . A storable kickstand  29  is arranged at a lower portion of the left side of the vehicle body frame  5  and is capable of supporting the motorcycle  1  upright, with the vehicle body inclined toward the left side.  
         [0036]    A cylinder body  30  of the engine body  15  is arranged above a crankcase  31 , inclined slightly towards the front. Throttle bodies  32 , which correspond to respective cylinders, are connected to a rear portion of the cylinder body  30 . The upper ends of the respective throttle bodies  32  are connected to an air cleaner casing  33 , which is arranged between the main frame member  12  and the fuel tank  14 . Further, exhaust pipes  34 , corresponding to respective cylinders, are connected to a front portion of the cylinder body  30 . The exhaust pipes  34  are bent downwardly from the front wall of the cylinder body  30 , pass below the crankcase  31  and, thereafter, are bent upwardly behind the pivot portion  9 . As seen in FIG. 1, the exhaust pipes feed into and are connected to a sound muffler  35 , which is supported on the seat frame  13 .  
         [0037]    In front of the exhaust pipes  34 , a radiator  36  is arranged with the upper end thereof inclined slightly forwardly, in the same manner as the cylinder body  30 . The radiator  36  is of a round curving type, which has a front face side thereof curved in a concave shape and, at the same time, as shown in FIG. 3, the radiator  36  is of a cross-flow type which provides a cooling-water inflow-side tank  37  to a right side of a radiator core  36   a  and a cooling-water outflow-side tank  38  to a left side of the radiator core  36   a . The radiator  36  is formed in the vertical direction, and extends from an upper portion of the cylinder body  30  to a lower portion of the crank casing  31 . A pair of left and right radiator fans, such as that shown at  39  in FIG. 1, are mounted on a back surface side of an upper portion of the radiator core  36   a.    
         [0038]    To further explain the first embodiment hereof, also in conjunction with FIG. 2, the engine body  15  is provided with cylinder heads  40 , cylinder blocks  43 , and a crankcase  31  which constitute essential parts of the cylinder body  30 . The cylinder head  40  is configured to be divided into a main casting body  41  and a valve cover  42 , while the crankcase  31  is configured to be divided into an upper case  44  and a lower case  45 . The upper case  44  and the cylinder block  43  are integrally molded, and an oil pan  46  is mounted below the lower case  45 . Here, the main casting body  41  is a cast product made of an aluminum alloy.  
         [0039]    Inside of the crankcase  31 , a crankshaft  47 , having an axis C parallel to the vehicle body width direction, is arranged. Further, a transmission case  48  is contiguously formed behind the crankcase  31 , and a transmission and a clutch mechanism (both omitted from the drawing) are respectively arranged in the inside of the transmission case  48 . Four cylinders  50  are formed in the cylinder block  43 , such that these cylinders  50  are arranged in the vehicle body width direction. A piston  51  is slidably fitted into the inside of each cylinder  50 . A connecting rod  53  is rotatably connected to each piston  51  by way of a piston pin  52  and, at the same time, a large end portion of the connecting rod  53  is rotatably connected to the crankpin  54  of the crankshaft  47 , whereby the reciprocating motion of the piston  51  is converted into the rotary motion about the axis C.  
         [0040]    To explain the operation of the cooling system also in conjunction with FIG. 3, a water pump  55 , which is operated along with the rotation of the crankshaft  47 , is arranged at the left side of the lower case  45 . An outflow-side radiator hose  56 , which is communicated with the outflow-side tank  38  of the radiator  36 , and a coolant introduction hose  58 , which is communicated with the cylinder-side water jacket  57  of the cylinder block  43 , are respectively connected to the inlet and outlet sides of the water pump  55 , as shown. A coolant inlet (not shown in the drawing) to the cylinder-side water jacket  57  is provided to a lower portion of the left side of the cylinder block  43 . Coolant from the water pump, which flows into the cylinder-side water jacket  57  from the coolant inlet, passes through the cylinder-side water jacket  57  and, thereafter, coolant flows into a head-side water jacket  60  of the cylinder head  40 .  
         [0041]    A coolant outlet  61  from the head-side water jacket  60  is provided behind the cylinder head  40  and a thermostat  62  is directly mounted on the coolant outlet  61 . An inflow-side radiator hose  63 , which is in fluid communication with the inflow-side tank  37  of the radiator  36 , is connected to a coolant outlet of the thermostat  62  and, at the same time, a bypass hose  64  is arranged between the thermostat  62  and the water pump  55 .  
         [0042]    Then, when the water pump  55  is operated along with the rotation of the crankshaft  47 , coolant which is taken out from the outflow-side tank  38  of the radiator  36  through the outflow-side radiator holes  56  is introduced into the inside of the cylinder-side water jacket  57  through a coolant introduction hose  58 . Coolant, which has passed through the cylinder-side water jacket  57 , is introduced into the head-side water jacket  60  and, thereafter, is taken out from the coolant outlet  61  and is introduced into the inlet-side tank  37  of the radiator  36  through the thermostat  62  and the inflow-side radiator hose  63 . Coolant passes through the radiator core  36   a , where it is cooled by the radiation of heat therefrom, and returns to the inflow-side tank  37 . Then, the coolant repeatedly circulates through the above-mentioned passages.  
         [0043]    In the above-mentioned circulation, when the temperature of coolant which passes through the thermostat  62  becomes equal to or less than a fixed temperature, coolant is supplied to the water pump  55  from the thermostat  62  through the bypass hose  64 , and is circulated through the radiator  36 . Further, when the temperature of coolant which passes through the thermostat  62  becomes equal to or more than a fixed temperature, the radiator fan  39  is operated to draw air through the radiator  36 , so as to forcibly cool coolant.  
         [0044]    Further, a water-cooling type oil cooler  65  (FIG. 2), which cools the engine oil served for lubricating respective parts of the engine, is mounted on a front portion of the lower case  45 . Coolant is introduced into the oil cooler  65  from a branch pipe  66 , which is provided to a mid-portion of the coolant introducing hose  58  and an introduction hose  67 . At the same time, coolant taken out from the oil cooler  65  is returned to the water pump  55  by way of a branch pipe  68  and a take-out hose  69  provided to a midst portion of the outflow-side radiator hose  56 .  
         [0045]    To explain the embodiment also in conjunction with FIG. 4, spark plugs  70  are threadably mounted in the main casting body  41  of the cylinder head  40 , such that the spark plugs  70  face the inside of the respective combustion chambers. At the same time, an intake port  71  and an exhaust port  72 , which allow each combustion chamber to communicate with the outside, are respectively formed in the main casting body  41  of the cylinder head  40 .  
         [0046]    A throttle body  32  is connected to an outside opening of each intake port  71 , and an exhaust pipe  34  is connected to an outside opening of each exhaust port  72 . Further, valve seats  73 ,  74  are respectively mounted to combustion-chamber-side openings of each intake port  71  and each exhaust port  72 , and these openings can be opened or closed in response to the operations of an intake valve  75  and an exhaust valve  76 .  
         [0047]    The intake valve  75  includes a valve stem  77  which has an umbrella-shaped valve body which actually opens and closes the opening of the intake port  71 . The intake valve  75  also includes a valve spring  78  which biases the valve stem  77  upwardly so as to bring a face surface of a valve element into pressure contact with the valve seat  73 . The intake valve  75  also includes a cylindrical valve lifter  79 , which is mounted on an upper end of the valve stem  77  and the like, wherein the valve stem  77  is slidably inserted into a substantially tubular valve guide  80  which is mounted in the main casting body  41 . Further, the exhaust valve  76  has the substantially same constitution as the intake valve  75 . That is, the exhaust valve  76  includes a valve stem  81 , a valve spring  82 , a valve lifter  83 , a tubular valve guide  80  and the like.  
         [0048]    The cylinder head depicted in FIGS. 2 and 4 includes dual overhead camshafts  85 ,  86 . An intake-side camshaft  85  and an exhaust-side camshaft  86  which operate the respective valves  75 ,  76  are respectively arranged above each intake valve  75  and exhaust valve  76 , in parallel to the axis C of the crankshaft  47 . An intake-side cam lobe corresponding to each respective intake valve  75 , and an exhaust-side cam lobe corresponding to each respective exhaust valve  76  are formed on suitable respective peripheral surfaces of the intake-side camshaft  85  and the exhaust-side camshaft  86 , respectively. Further, these camshafts  85 ,  86  are rotatably supported by bearings  87  of the main casting body  41  and a bearing cap (not shown).  
         [0049]    The respective camshafts  85 ,  86  each have a hollow structure, wherein hollow portions constitute passages for transmitting a flow of engine oil, and the engine oil is supplied to respective slide surfaces from given oil holes. Further, cam sprocket wheels (not shown) are respectively formed on right ends of the respective camshafts  85 ,  86 , and the respective camshafts  85 ,  86  are interlocked with the crankshaft  47  by way of cam chains wound around these cam sprocket wheels. Due to such a constitution, the respective camshafts  85 ,  86  are rotated concurrently with the rotation of the crankshaft  47 , so as to operate the intake valves  75  and the exhaust valves  76 .  
         [0050]    To explain the first selected embodiment also in conjunction with FIG. 5, plug holes  90 , which correspond to four cylinders  50  arranged in the vehicle body width direction are formed in the main casting body  41 , wherein the respective spark plugs  70  can be threadably mounted proximate the centers of ceiling portions of the combustion chambers. It will be understood that the cylinder head  41  depicted in FIG. 5 uses four valves per cylinder.  
         [0051]    The intake port  71  is formed, corresponding to each combustion chamber, such that branch passages  93  are formed by bifurcating a main intake passage  92  which opens to the outside, and which is operatively connected to the air cleaner  33  via the throttle bodies  32 . The respective branch passages  93  feed into the intake ports  71  at the combustion chambers. These two intake openings are arranged behind the plug hole  90 , and at the same time, the valve seats  73  are mounted on the respective openings.  
         [0052]    With respect to the exhaust port  72  openings to the combustion chamber, two openings are also formed for each combustion chamber. These two openings are arranged in front of the plug hole  90  and, at the same time, the valve seats  74  are mounted on the respective openings. That is, two branch passages  94  of the exhaust ports  72  communicate with the combustion chamber, and these branch passages  94  are merged to form a main exhaust passage  95 .  
         [0053]    In a mating surface  41   a  of the main casting body  41  for aligning with the cylinder block  43 , a plurality of coolant communication openings  96  are formed, which allow fluid communication between a cylinder-side water jacket  57  and a head-side water jacket  60 . To be more specific, front coolant communication openings  96   a  are respectively formed in the mating surface  41   a  in front of each exhaust port  72 . Each front coolant communication opening  96   a  is formed in an approximately rectangular shape along a front surface of the main casting body  41 . In the same manner, rear coolant communication openings  96   b  are respectively formed in the mating surface  41   a  behind the opening of each intake port  71 , and each rear coolant communication opening  96   b  is formed in an approximately rectangular shape along a rear surface of the main casting body  41 .  
         [0054]    Further, assuming respective cylinders  50  as the first cylinder, the second cylinder, etc., in order from the left side, on the mating surface  41   a , at the outside of the openings of the exhaust ports  72  and the openings of the intake ports  71  of the first cylinder and the fourth cylinder in the cylinder row direction, side coolant communication openings  96   c  which are formed in an elongated circular shape along side surfaces of the main casting body  41  are respectively formed. Further, between respective cylinders  50 , a pair of front and rear intermediate coolant communication openings  96   d , having an approximately triangular shape, are respectively formed.  
         [0055]    The inflow of coolant into the head-side water jacket  60  is controlled by through holes formed in a head gasket, which is interposed between the main casting body  41  and the cylinder block  43 . That is, by adjusting a shape, a position, and an area of the through holes formed in the head gasket, thus arbitrarily stopping or throttling the inflow of coolant into the respective coolant communication openings  96 , it is possible to control a flow rate balance or the like of coolant in the head-side water jacket  60  which is relatively arranged in a complicated manner in the inside of the main casting body  41 . Then, according to this embodiment, among the various coolant communication openings  96 , coolant is made to flow into the inside of the head-side water jacket  60  mainly through the intermediate coolant communication opening  96   d  disposed between the second cylinder and the third cylinder and at the front-side (exhaust port  72  side), and through the front coolant communication openings  96   a  positioned at both sides of the intermediate coolant communication opening  96   d.    
         [0056]    On the main casting body  41 , cylindrical plug hole walls  91  define the plug holes  90 , intake port walls  101  and exhaust port walls  102  have a branch pipe shape and form the intake ports  71  and the exhaust ports  72 . Elsewhere on the main casting body, a plurality of hollow bosses  103  are formed, which are used in joining the main casting body  41  to the cylinder block  43 . Portions of the intake port walls  101  and the exhaust port walls  102  in the vicinity of the valve seats  73 ,  74  are densely arranged in the peripheries of the plug hole walls  91 , and are integrally formed such that the portions in the vicinity of these respective walls merge together. The head-side water jacket  60  is formed as a hollow space inside of the main casting body  41  while avoiding the plug hole walls  91 , the intake port walls  101 , the exhaust port walls  102 , the bosses  103  and the like. That is, the portions of the respective walls are arranged inside of the head-side water jacket  60 .  
         [0057]    A partition (a connection wall)  100  is also provided inside the head-side water jacket  60  between the neighboring plug hole walls  91 , such that the partition  100  functions as a bridge to connect these walls. Each partition  100  is formed in an upstanding manner extending from an upper surface to a lower surface of the head-side water jacket  60 , substantially parallel to the cylinder axis, and is integrally formed with the main casting body  41 . Due to such partitions  100 , a coolant passage of the head-side water jacket  60  is separated into an intake-port-side coolant passage  60   a  and an exhaust-port-side coolant passage  60   b  between the plug hole wall  91  for the first cylinder and the plug hole wall  91  for the fourth cylinder (see FIG. 6).  
         [0058]    To explain the embodiment also in conjunction with FIG. 6, coolant which flows into the inside of the head-side water jacket  60  from the intermediate coolant communication opening  96   d  and the front-side coolant communication openings  96   a  formed at the left and right sides of the intermediate coolant communication opening  96   d  passes above and below the exhaust port walls  102  of respective cylinders  50  and, at the same time, flows toward the outside in the cylinder row direction (as shown by the arrows D in FIG. 6) while cooling the front portions of the plug holes  90  and the peripheries of the valve seats  74  of the exhaust ports  72 . Coolant which reaches the outside of the first cylinder and the fourth cylinder is transferred from the exhaust-port-side coolant passage  60   b  to the intake-port-side coolant passage  60   a . Then, coolant passes above and below the intake port walls  101  and, at the same time, flows toward the center of the main casting body  41  in the cylinder row direction, while cooling the rear portions of the plug holes  90  and the peripheries of the valve seats  73  of the intake ports  71  (as shown by the arrows E in FIG. 6). Then, coolant flows out to the outside of the head-side waterjacket  60  from a coolant outlet  61  formed between and behind the second cylinder and the third cylinder (as shown by the arrows F in FIG. 6), and is supplied to the thermostat  62  which is directly mounted in the coolant outlet  61 .  
         [0059]    The partition  100   a  formed between the second cylinder and the third cylinder is formed in an arcuate shape, slightly projecting toward the intake port  71  side in view of the relationship with a flow pattern of coolant. Further, the partitions  100   b  which are formed between the first cylinder and the second cylinder as well as between the third cylinder and the fourth cylinder are formed in a V-shape projecting toward the exhaust port  72  side thus preventing the generation of a vortex of coolant or the like in the exhaust-port-side coolant passage  60   b  which is the upstream side of the head-side water jacket  60  and exhibits a relatively fast flow speed (see FIG. 5). Here, hollow bosses  105  are formed on both end portions of the partition  10   b , for receiving bolts which are used for fixing a breather chamber  104  (see FIG. 2), situated above the valve cover  42 , to the main casting body  41 .  
         [0060]    Further, an air bleed hole (not shown) is formed in an upper portion of each partition  100 , for preventing dwelling of air therein. That is, the air bleed hole suppresses the dwelling of air in the periphery of the partition  100  where the diameter of flow largely changes compared to the periphery of the plug hole wall  91 . Further, since a portion of coolant flows into the intake-port-side coolant passage  60   a  from the exhaust-port-side coolant passage  60   a  through the air bleed hole and hence, the occurrence of the dwelling of coolant around the periphery of the partition  100  can be effectively prevented. Here, coolant which enters the intake-port-side coolant passage  60   a  through the air bleed hole is flowing at a sufficiently small volume compared to the amount of coolant which flows into the intake-port-side coolant passage  60   a  from the outside of the first cylinder and the fourth cylinder and hence, the flow of coolant in the inside of the head-side water jacket  60  shown in FIG. 6 can be maintained.  
         [0061]    According to the above-mentioned first embodiment, by providing the partition  100  which connects the plug hole walls  91  to each other inside the head-side water jacket  60 , the head-side water jacket  60  is separated into two sections, namely, the intake-port-side coolant passage  60   a  and the exhaust-port-side coolant passage  60   b . Accordingly, coolant which flows into the head-side water jacket  60  first passes through the exhaust-port-side coolant passage  60   b  and, thereafter, extends around the outside end portion of the main casting body  41 , and reverses direction. Then, the coolant passes through the intake-port-side coolant passage  60   a , is merged at one position in the coolant outlet  61  which is provided at the central part of the main casting body  41 , and the coolant then flows out to the outside of the head-side water jacket  60 .  
         [0062]    Accordingly, coolant flows uniformly inside the head-side water jacket  60  to both outer sides in the cylinder row direction and hence, the generation of a temperature gradient in the cylinder row direction can be suppressed, whereby it is possible to uniformly cool the cylinder head  40 .  
         [0063]    Further, coolant which flows into the inside of the head-side waterjacket  60  can be merged at one position in the coolant outlet  61  provided at the approximately center in the cylinder row direction and, thereafter, can be directed outside the main casting body  41 . As a result, the coolant routing tubes and hoses outside the cylinder head  40  can be arranged simply and neatly.  
         [0064]    Still further, by providing the air bleed hole in each partition  100 , the occurrence of the staying or dwelling of air in the periphery of the partition  100  can be effectively prevented whereby the cooling performance of the cylinder head  40  can be held in a favorable state.  
         [0065]    Further, since the respective plug hole walls  91  are connected by the partitions  100 , at the time of producing the main casting body  41  by casting, the partition  100  portion functions as a molten metal passage around the plug hole walls  91 . Although the periphery of the plug hole wall  91  of the main casting body  41  is in the state that the intake port wall  101 , the exhaust port wall  102  and the like are densely arranged therein, by adding the molten metal passage to such a portion, the flow of molten metal is enhanced whereby the quality of cast product can be enhanced and a yielding rate of the cast products can be also enhanced.  
         [0066]    Still further, it is possible to reinforce the portion of the main casting body  41  between the respective cylinders  50  by the partitions  100  formed in the inside of the head-side water jacket  60  above the portion. The portion of the main casting body  41  between the respective cylinders  50  seals the combustion chamber and hence, the portion requires a given strength and rigidity. By reinforcing the portion with the partition  100 , it is possible to reduce a wall thickness of the main casting body  41  and hence, the weight of the main casting body  41  can be also reduced.  
         [0067]    Next, the second embodiment of the present invention will be explained, based on FIG. 7 and FIG. 8, and also referring back to FIGS. 1 through 6 for features which are shared with the first embodiment.  
         [0068]    This second embodiment differs from the first embodiment in that, in place of the air bleed hole (of the first embodiment) formed in each partition  100 , an access hole plug, (sand removing plug)  111  for plugging an access hole  110  which is necessary at the time of casting, is provided above each partition  100  and, at the same time, a gap S is defined between an upper periphery of each partition  100  and a distal end of the plug  111 . Here, parts identical with the parts of the first embodiment are given same symbols as those used in connection with the first embodiment, and their explanation is omitted.  
         [0069]    As shown in FIG. 7, in a cylinder head according to the second embodiment, guide walls  112 ,  113  are provided on the main casting body in front of and behind four plug hole walls  91 , corresponding to respective cylinders  50 . Each pair of guide walls  112 ,  113  slidably support the valve lifters  79 ,  83  of the respective intake and exhaust valves  75 ,  76 .  
         [0070]    Camshaft bearings  87  are installed on the main casting body between each pair of the guide walls  112 ,  113 , and between respective rightmost guide walls  112 ,  113  and a cam chain case  114 . The camshaft bearings  87  rotatably support journal portions of the intake-side camshaft  85  and the exhaust-side camshaft  86 . Oil grooves  88 , and oil passages  89  which open inside the oil grooves  88 , are formed in slide surfaces of the bearings  87 , which are provided between the respective rightmost guide walls  112 ,  113  and the cam chain case  114 . Due to such a constitution, an engine oil supplied through the oil passages  89  is supplied to respective slide surfaces by way of the oil grooves  88  and the hollow portions of the respective camshafts  85 ,  86 . The main casting body also has bolt holes  87   a  formed therein, in front of and behind respective bearings  87 , for fixing a bearing cap in place.  
         [0071]    To explain this embodiment also in conjunction with FIG. 8, in a bottom wall  115  of the main casting body  41  which forms a ceiling portion of each combustion chamber, a thread hole  116  is formed, in which the spark plug  70  is threadably mounted, after it has been inserted into the plug hole  90 . Further, an upper partition  117  is provided above the bottom wall  115  and a space which is sandwiched and closed by the upper partition  117  and the bottom wall  115  defines the head-side water jacket  60 . Each partition  100  is formed in an upstanding manner from an upper surface of the bottom wall  115  to a lower surface of the upper partition  117 , so as to separate the head-side water jacket  60  between the neighboring plug hole walls  91 .  
         [0072]    Here, with respect to the main casting body  41  which is a cast product, the head-side water jacket  60  is formed by setting a core produced by solidifying exclusive-use sands in the inside of a mold, by crushing the core after casting, and by pulling out the crushed sands to the outside. To enable such an operation, a proper number of access holes  110  are respectively formed in the upper partition  117  which is disposed above the respective partitions  100 . Portions of upper peripheral portions of the partitions  100  are cut away to form these respective access holes  110  and hence, the head-side water jacket  60  opens to the outside, whereby the sands can be removed. Then, after removing the sands, the plugs  111  are threadably engaged with the access holes so as to plug the access holes, whereby it is possible to make coolant flow into the inside of the head-side water jacket  60 .  
         [0073]    A pilot portion  118  having a diameter which exceeds a thickness of the partition  100  is formed on a distal end portion of each plug  111 . A distal end of the pilot portion  118  is formed in a flat conical shape. An alignment portion  120  having a funnel or concave shape corresponding to a conical portion  119  is formed in the cut-away portion of each partition  100 . Here, the distal end portion of the plug  111  is set such that, in a state that the plug  111  is threadably engaged in the access hole  110 , a gap S is formed between the conical portion  119  and the alignment portion  120  and a portion of coolant in the inside of the exhaust-port-side coolant passage  60   b  can flow into the intake-port-side coolant passage  60   a  through the gap S. The gap S substantially functions in the same manner as the air-bleed hole of the first embodiment whereby the occurrence of air dwelling in the periphery of the partition  100  can be prevented and, at the same time, the occurrence of dwelling of coolant can be also prevented.  
         [0074]    According to the above-mentioned second embodiment, in the same manner as the first embodiment, it is possible to uniformly cool the cylinder head  40 , to make the coolant piping outside the cylinder head  40  simple and neat, to improve the productivity of the main casting body  41 , and to reduce the weight of the main casting body  41 . Further, by providing the access hole  110  which cuts away the portion of each partition  100 , after casting the main casting body  41 , sands can be removed simultaneously from the intake-port-side coolant passage  60   a  and the exhaust-port-side coolant passage  60   b  of the head-side water jacket  60 . Still further, by forming the gap S between the sand-removing-hole plug  111  and the partition  100 , the dwelling of air, the local boiling or the like around the periphery of the partition  100  where the diameter of coolant flow largely changes can be surely prevented whereby it is possible to maintain the head-side water jacket  60  in a state that the favorable cooling performance can be achieved.  
         [0075]    Here, the present invention is not limited to the above-mentioned embodiments and the present invention is applicable to a parallel four cylinder type internal combustion engine provided that the engine includes a plurality of cylinders. Further, the present invention is not limited to the motorcycle. That is, not mention a three-wheeled vehicle and a four-wheeled vehicle, the present invention is also applicable to the whole multi-cylinder water-cooled type internal combustion engines.  
         [0076]    As has been explained heretofore, according to the invention described in the first aspect, it is possible to make coolant flow from both sides in the cylinder row direction to the downstream side due to the connection walls each of which is formed between the plug hole walls and hence, the flow of coolant can be easily made uniform in the cylinder row direction, whereby substantially uniform cooling can be achieved.  
         [0077]    Further, by connecting the respective plug hole walls using the connection walls, the flow of molten metal around the plug hole walls where the intake passages, the exhaust passages and the like are densely arranged is improved whereby it is possible to enhance a yield rate by suppressing the occurrence of casting failure.  
         [0078]    Still further, since the mating surface of the cylinder head with the cylinder block between the respective cylinders is reinforced by the connection walls, it is possible to reduce the wall thickness around the mating surface whereby the weight of the cylinder head can be reduced.  
         [0079]    According to the invention described in the second aspect, it is possible to make coolant flow from both sides in the cylinder row direction directed to the downstream side due to the partitions each of which is formed between the plug hole walls and hence, the flow of coolant can be easily made uniform in the cylinder row direction whereby substantially uniform cooling can be achieved.  
         [0080]    Further, by connecting the respective plug hole walls using the partitions, the flow of molten metal around the plug hole walls where the intake passages, the exhaust passages and the like are densely arranged is improved whereby it is possible to enhance a yield rate by suppressing the occurrence of casting failure.  
         [0081]    Still further, since the mating surface of the cylinder head with the cylinder block between the respective cylinders is reinforced by the partitions, it is possible to reduce the wall thickness around the mating surface whereby the weight of the cylinder head can be reduced.  
         [0082]    Further, by providing the gap between the sand removing plug and the partition, the occurrence of dwelling or staying of air in coolant between the plug hole walls can be suppressed and the occurrence of local boiling or the like can be prevented whereby it is possible to maintain the cooling performance in the favorable state.  
         [0083]    Although the present invention has been described herein with respect to a limited number of presently preferred embodiments, the foregoing description is intended to be illustrative, and not restrictive. Those skilled in the art will realize that many modifications of the preferred embodiment could be made which would be operable. All such modifications, which are within the scope of the claims, are intended to be within the scope and spirit of the present invention.