Patent Publication Number: US-8539916-B2

Title: Cooling structure for internal combustion engine

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a cooling structure for an internal combustion engine in which: a spacer is fitted inside a water jacket which is formed to surround peripheries of three or more cylinder bores arranged one after another on a cylinder row line of a cylinder block of the internal combustion engine; and a cooling condition of the cylinder bores is controlled by regulating a flow of cooling water in the water jacket by use of the spacer. 
     2. Description of the Related Art 
     Japanese Patent No. 3596438 has made publicly known such a cooling structure for an internal combustion engine in which: the heat transfer coefficient of the spacer fitted inside the water jacket is made different between the thrust/reverse-thrust sides of the cylinder bores (portions distant from the cylinder row line) and the inter-bore portions of the cylinder bores (portions close to the cylinder row line); and thereby, the cylinder bores are uniformly cooled throughout their whole peripheries. 
     Meanwhile, in a cylinder block in which three or more cylinder bores are arranged one after another along the cylinder row line, each of the two cylinder bores (end-portion cylinder bores) in the respective two end portions in the cylinder row line direction has only one adjacent cylinder bore. For this reason, each end-portion cylinder bore receives a smaller quantity of heat from its adjacent cylinder bore, and reaches a lower temperature. On the other hand, since each of the cylinder bores (intermediate cylinder bores) other than the end-portion cylinder bores has two adjacent cylinder bores, the intermediate cylinder bore receives a larger quantity of heat from the adjacent cylinder bores and reaches a higher temperature. 
     As described above, the temperature difference occurs between the end-portion cylinder bores and the intermediate cylinder bores. Even though the end-portion cylinder bores and the intermediate cylinder bores are thermally insulated equally by the spacer, all the temperatures of the cylinder bores cannot be made uniform. This leads to a problem that variations occur among the clearances between the pistons and the corresponding cylinder bores. 
     SUMMARY OF THE INVENTION 
     The present invention has been made in view of the foregoing situation. An object of the present invention is to make uniform the temperatures of multiple cylinder bores arranged in a cylinder row line direction by use of a spacer placed inside a water jacket. 
     In order to achieve the object, according to a first feature of the present invention, there is provided a cooling structure for an internal combustion engine in which: a spacer is fitted inside a water jacket which is formed to surround peripheries of three or more cylinder bores arranged one after another on a cylinder row line of a cylinder block of the internal combustion engine; and a cooling condition of the cylinder bores is controlled by regulating a flow of cooling water in the water jacket by use of the spacer, wherein the cylinder bores comprise end-portion cylinder bores situated respectively in opposite end portions in a direction of the cylinder row line, and one or more intermediate cylinder bores other than the end-portion cylinder bores, and the spacer is configured so that cooling performances for the intermediate cylinder bores are higher than those for the end-portion cylinder bores. 
     According to the first feature of the present invention, the spacer is fitted inside the water jacket which is formed to surround the peripheries of the cylinder bores in the cylinder block of the internal combustion engine. For this reason, the cylinder bores are thermally insulated by regulating the flow of the cooling water in the water jacket by use of the spacer. Thereby, the friction between the cylinder bores and the corresponding pistons can be reduced by thermally expanding the cylinder bores. The spacer is configured in such a way that the cooling performances of intermediate cylinder bores are higher than the cooling performances of end-portion cylinder bores, the intermediate cylinder bores being other than the end-portion cylinder bores, and having the higher temperature; and the end-portion cylinder bores being situated in the respective opposite end portions in the cylinder row line direction, and having the lower temperature. For this reason, the effect of cooling the intermediate cylinder bores whose temperatures become higher can be enhanced, and all the temperatures of the respective cylinder bores can be made uniform. 
     According to a second feature of the present invention, in addition to the first feature, the spacer comprises a spacer main body part defining an upper cooling water passage for an upper side and a lower cooling water passage for a lower side inside the water jacket, and the spacer main body part has a less height in an up-and-down-direction in portions facing the intermediate cylinder bores than in portions facing the end-portion cylinder bores. 
     According to the second feature of the present invention, the spacer includes the spacer main body part for defining the upper cooling water passage for the upper side and the lower cooling water passage for the lower side inside the water jacket. In addition, the height in the up-and-down-direction of the spacer main body part is less in its portions facing the intermediate cylinder bores than in its portions facing the end-portion cylinder bores. For this reason, the upper cooling water passage and the lower water passage are made to face the intermediate cylinder bores, whose temperatures become higher than those of the end-portion cylinder bores, with their wider areas. Thereby, the effect of cooling the intermediate cylinder bores can be enhanced, and all the temperatures of the respective cylinder bores can be made uniform. 
     According to a third feature of the present invention, in addition to the first feature, the spacer comprises a spacer main body part for regulating the flow of the cooling water in the water jacket, and the spacer main body part has a smaller thickness in a radial direction in portions facing the intermediate cylinder bores than in portions facing the end-portion cylinder bores. 
     According to the third feature of the present invention, the thickness in the radial direction of the spacer main body part for regulating the flow of the cooling water inside the water jacket is less in its portions facing the intermediate cylinder bores than in its portions facing the end-portion cylinder bores. For this reason, the thinner portions of the spacer main body part are made to face the intermediate cylinder bores whose temperatures become higher than those of the end-portion cylinder bores, and the dissipation of heat from the intermediate cylinder bores to the cooling water is facilitated. Thereby, the effect of cooling the intermediate cylinder bores can be enhanced, and all the temperatures of the respective cylinder bores can be made uniform. 
     According to a fourth feature of the present invention, in addition to the first feature, the spacer comprises a spacer main body part for regulating the flow of the cooling water in the water jacket, and a passage of the cooling water between an inner peripheral surface of the spacer main body part and an inner wall surface of the water jacket has a larger cross-sectional area in portions facing the intermediate cylinder bores than in portions facing the end-portion cylinder bores. 
     According to the fourth feature of the present invention, the cross-sectional area of the passage of the cooling water, which is interposed between the inner peripheral surface of the spacer main body part for regulating the flow of the cooling water in the water jacket and the inner wall surface of the water jacket, is larger in its portions facing the intermediate cylinder bores than in its portions facing the end-portion cylinder bores. For this reason, the effect of cooling the intermediate cylinder bores whose temperatures become higher than those of the end-portion cylinder bores can be enhanced, and all the temperatures of the respective cylinder bores can be made uniform. 
     According to a fifth feature of the present invention, in addition to the fourth feature, biasing means for biasing the opposite end portions of the spacer main body part in the cylinder row line direction toward the inner wall surface of the water jacket are provided between the opposite end portions of the spacer main body part in the cylinder row line direction and an outer wall surface of the water jacket. 
     According to the fifth feature of the present invention, the opposite end portions of the spacer main body part are biased toward the inner wall surface of the water jacket by the biasing means provided between the opposite end portions of the spacer main body part in the cylinder row line direction and the outer wall surface of the water jacket. For this reason, the intake-side and exhaust-side side surfaces of the spacer can be made to deform toward the outer wall surface of the water jacket, and the cross-sectional area of the passage of the cooling water which is interposed between the inner peripheral surface of the spacer main body part and the inner wall surface of the water jacket can be made larger in the spacer main body part&#39;s portions facing the intermediate cylinder bores than in its portions facing the end-portion cylinder bores. 
     According to a sixth feature of the present invention, in addition to the fifth feature, the biasing means are fixing members for fixing the spacer inside the water jacket. 
     According to the sixth feature of the present invention, the fixing members for fixing the spacer to the inside of the water jacket are used as the biasing means. For this reason, the number of parts can be made smaller than the number of parts which are needed in a case where specialized biasing means are provided thereto. 
     Here, note that an end-portion cylinder bore  12   a  and an intermediate cylinder bore  12   a ′ of embodiments correspond to the cylinder bores of the present invention; and a fixing member  22 ′ of the embodiments correspond to the biasing means of the present invention. 
     The above description, other objects, characteristics and advantages of the present invention will be clear from detailed descriptions which will be provided for the preferred embodiments referring to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a cylinder block of an internal combustion engine with four cylinders mounted in a straight line (First embodiment); 
         FIG. 2  is a perspective view of a spacer (First embodiment); 
         FIG. 3  is a view seen from a direction of an arrow  3  in  FIG. 1  (First embodiment); 
         FIG. 4  is a view seen from a direction of an arrow  4  in  FIG. 3  (First embodiment); 
         FIG. 5  is a sectional view taken along a line  5 - 5  in  FIG. 3  (First embodiment); 
         FIG. 6  is an enlarged view of a part indicated by an arrow  6  in  FIG. 5  (First embodiment); 
         FIG. 7  is a sectional view taken along a line  7 - 7  in  FIG. 3  (First embodiment); 
         FIG. 8  is a sectional view taken along a line  8 - 8  in  FIG. 3  (First embodiment); 
         FIG. 9  is a sectional view taken along a line  9 - 9  in  FIG. 3  (First embodiment); 
         FIG. 10  is a sectional view taken along a line  10 - 10  in  FIG. 3  (First embodiment); 
         FIG. 11A  is a sectional view taken along a line  11 - 11  in  FIG. 3  (First embodiment); 
         FIG. 11B  is a sectional view taken along a line B-B in  FIG. 11A  (First embodiment); 
         FIG. 11C  is a sectional view taken along a line C-C in  FIG. 11B  (First embodiment); 
         FIG. 12A  is a sectional view taken along a line  12 - 12  in  FIG. 3  (First embodiment); 
         FIG. 12B  is a sectional view taken along a line B-B in  FIG. 12A  (First embodiment); 
         FIG. 12C  is a sectional view taken along a line C-C in  FIG. 12B  (First embodiment); 
         FIG. 13  is a view corresponding to the above-described  FIG. 3  (Second embodiment); 
         FIG. 14  is a view corresponding to the above-described  FIG. 3  (Third embodiment); and 
         FIG. 15  is a view corresponding to the above-described  FIG. 3  (Fourth embodiment). 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Descriptions will be hereinbelow provided for a first embodiment of the present invention on the basis of  FIGS. 1 to 12 . 
     As shown in  FIG. 1 , four cylinder sleeves  12  are embedded along a cylinder row line L 1  in a cylinder block  11  of an internal combustion engine with four cylinders mounted in a straight line. A water jacket  13  is formed to surround the outer peripheral surfaces of the respective cylinder sleeves  12 . The cylinder block  11  according to this embodiment is of a Siamese type, and no portion of the water jacket  13  is formed between each neighboring two of the cylinder sleeves  12 . Thereby, the shortening of the dimension of the internal combustion engine in the cylinder row line L 1  direction is achieved. The water jacket  13  opened in a deck surface  11   a  of the cylinder block  11  extends downward from the deck surface  11   a  toward a crankcase up to a certain depth. A spacer  14  made of a synthetic resin is arranged in an interstice between an inner wall surface  13   a  and an outer wall surface  13   b  of the water jacket  13 . The spacer  14  is inserted in the interstice therebetween from the opening in the deck surface  11   a  of the cylinder block  11 . 
     Note that with regard to an “up-and-down direction” in this description, the cylinder head side in a cylinder axis line L 2  direction is defined as “upper,” and the crankcase side in the cylinder axis line L 2  direction is defined as “lower.” 
     As clear from  FIGS. 1 to 5 , the spacer  14  includes a spacer main body part  14   a , a cooling water inlet port part  14   b  and a cooling water outlet port part  14   c . The entire peripheries of four cylinder bores  12   a ,  12   a ;  12   a ′,  12   a ′ in the cylinder bock  11  are surrounded by the spacer main body part  14   a , the cooling water inlet port part  14   b  and the cooling water outlet port part  14   c . The cooling water inlet port part  14   b  surrounds an intake-side portion of one cylinder bore  12   a  which is situated on a first end side in the cylinder row line L 1  direction (on a timing train side). The cooling water outlet port part  14   c  surround the first end-side portion of the cylinder bore  12   a  in the cylinder row line L 1  direction and an exhaust side-portion of the cylinder bore  12   a . A partition wall  14   d  is integrally provided in a position which is slightly offset from the first end-side portion of the spacer  14  in the cylinder row line L 1  direction to the intake-side portion of the space  14 , and which intervenes between the cooling water inlet port part  14   b  and the cooling water outlet port part  14   c . The partition wall  14   d  is formed thicker than the spacer main body part  14   a , and projects upward from the upper edges of the cooling water inlet port part  14   h  and the cooling water outlet port part  14   c , and downward from the lower edges of the cooling water inlet port part  14   b  and the cooling water outlet port part  14   c.    
     Inside the water jacket  13 , an upper cooling water passage  13   c  surrounding the peripheries of the respective four cylinder bores  12   a ,  12   a ;  12   a ′,  12   a ′ is formed between the upper edge of the spacer main body part  14   a  and an undersurface of a cylinder head  15 . In addition, a lower cooling water passage  13   d  surrounding the peripheries of the respective four cylinder bores  12   a ,  12   a ;  12   a ′,  12   a ′ is formed between the lower edge of the spacer main body part  14   a  and the bottom portion of the water jacket  13 . 
     An upper support leg  14   e  and a lower support leg  14   f  project to the insides of the upper cooling water passage  13   c  and the lower cooling water passage  13   d , respectively, from a position at which the cylinder row line L 1  intersects the cooling water outlet port part  14   c  on its first end side. In addition, an upper support leg  14   g  and a lower support leg  14   h  project to the insides of the upper cooling water passage  13   c  and the lower cooling water passage  13   d , respectively, from a position at which the cylinder row line L 1  intersects the spacer main body part  14   a  on its second end side (on the side closer to a transmission). For this reason, when the spacer  14  is attached to the inside of the water jacket  13 , the lower ends of the respective paired lower support legs  14   f ,  14   h  are in contact with the bottom portion of the water jacket  13 , and the upper ends of the respective paired upper support legs  14   e ,  14   g  are in contact with the undersurface of a gasket  16  held between the cylinder block  11  and the cylinder head  15 , in the opposite end portions in the cylinder row line L 1  direction. Thereby, the spacer  14  is positioned in the up-and-down direction. 
     Pistons  18 ,  18 ;  18 ′,  18 ′ (refer to  FIG. 3 ) connected to a crankshaft  17  are slidably fitted in the respective cylinder bores  12   a ,  12   a ;  12   a ′,  12   a ′. Top rings  19 , second rings  20  and oil rings  21  are attached to top parts  18   a  of the pistons  18 ,  18 ;  18   a ′,  18   a ′, respectively. 
     Descriptions will be hereinbelow provided for the detailed structure of the spacer  14  sequentially. 
     As clear from  FIGS. 2 to 4 , the heights H of the spacer main body part  14   a , the cooling water inlet port part  14   b  and the cooling water outlet port part  14   c  of the spacer  14  in a cylinder axis line L 2  direction are not uniform. Portions of the spacer  14  which face the end-portion cylinder bores  12   a ,  12   a  in the opposite end portions in the cylinder row line L 1  direction are higher in height H, and portions of the spacer  14  which face the intermediate cylinder bores  12   a ′,  12   a ′ in the intermediate portion in the cylinder row line L 1  direction are lower in height H by a step t. 
     As clear from  FIGS. 2 and 3 , the thickness T 1  of the spacer main body part  14   a  is basically constant. However, the thickness T 2  of the cooling water inlet port part  14   b  is thinner than the thickness T 1  of the spacer main body part  14   a , and the thickness T 3  of the cooling water outlet port part  14   c  is thinner than the thickness T 1  of the spacer main body part  14   a . In addition, the thickness T 4  of the partition wall  14   d  is thicker than the thickness T 1  of the spacer main body part  14   a . The inner peripheral surface of the cooling water inlet port part  14   b  is flush with the inner peripheral surface of the spacer main body part  14   a . The outer peripheral surface of the cooling water inlet port part  14   b  is offset inward in a radial direction from the outer peripheral surface of the spacer main body part  14   a  by a step. Furthermore, the outer peripheral surface of the cooling water outlet port part  14   c  is flush with the outer peripheral surface of the spacer main body part  14   a . The inner peripheral surface of the cooling water outlet port part  14   c  is offset outward in the radial direction from the inner peripheral surface of the spacer main body part  14   a  by a step. 
     As clear from  FIG. 5 , while the pistons  18 ,  18 ;  18 ′,  18 ′ are moving in the respective cylinder bores  12   a ,  12   a ;  12   a ′,  12   a ′ up and down in response to rotation of the crankshaft  17 , side thrusts acting between the pistons  18 ,  18 ;  18 ′,  18 ′ and the cylinder bores  12   a ,  12   a ;  12   a ′,  12   a ′ change periodically. Each side thrust reaches a maximum when the corresponding one of the pistons  18 ,  18 ;  18 ′,  18 ′ reaches a position of the expansion stroke which is indicated by the continuous line (for example, a position where the crank angle is at 15° after the compression top dead center). The up-and-down position of the spacer  14  inside the water jacket  13  is set in such a way that the top ring  19 , the second ring  20  and the oil ring  21  of each of the pistons  18 ,  18 ;  18 ′,  18 ′ are located above the upper edge of the spacer  14 , and a skirt part  18   b  of the piston  18 ,  18 ;  18 ′,  18 ′ is located below the upper edge of the spacer  14  when the piston  18 ,  18 ;  18 ′,  18 ′ is located at the position maximizing the side thrust. Furthermore, the up-and-down position of the spacer  14  inside the water jacket  13  is set in such a way that the top ring  19 , the second ring  20  and the oil ring  21  of each of the pistons  18 ,  18 ;  18 ′,  18 ′ are located below the lower edge of the spacer  14  when the piston  18 ,  18 ;  18 ′,  18 ′ is located at the bottom dead center position indicated by the chain line. 
     As clear from  FIG. 6 , the thickness T 1  of the spacer main body part  14   a  is set slightly less than the width W of the water jacket  13  in which the spacer main body part  14   a  is fitted. The reason for this is to prevent the assemblability from deteriorating due to friction of the spacer  14  with the inner wall surface  13   a  and the outer wall surface  13   b  of the water jacket  13  resulting from the fact that the dimensional precision of the inner wall surface  13   a  and the outer wall surface  13   b  of the water jacket  13 , which have been subjected to no process since casted, is not high. Accordingly, when the spacer  14  is assembled inside the water jacket  13 , a space α is formed between the inner peripheral surface of the spacer main body part  14   a  and the inner wall surface  13   a  of the water jacket  13 , and a space β is formed between the outer peripheral surface of the spacer main body part  14   a  and the outer wall surface  13   b  of the water jacket  13 . The spacer main body part  14   a  is arranged therein in such a way that the space α is set smaller than the space β, that is to say, the spacer main body part  14   a  is closer to the inner wall surface  13   a  of the water jacket  13  than to the outer wall surface  13   b  thereof. 
     As clear from  FIGS. 3 and 7 , portions of the water jacket  13  which respectively surround the corresponding two adjacent cylinder sleeves  12 ,  12  intersect at an acute angle in each inter-bore portion in the cylinder block  11 , which is a position at which the corresponding two cylinder sleeves  12 ,  12  are close to each other. For this reason, a width W′ of a portion of the water jacket  13  in a direction orthogonal to the cylinder row line L 1  is wider than the width W of any other portion of the water jacket  13 . On the other hand, a thickness of a portion of the spacer main body part  14   a  in each inter-bore portion is equal to T 1  which is the thickness of any other portion of the spacer main body part  14   a . For this reason, a space α′ between the inner peripheral surface of the spacer main body part  14   a  and the inner wall surface  13   a  of the water jacket  13  in each inter-bore portion is exceptionally larger than the space α therebetween in any other portion. 
     Nevertheless, in each inter-bore portion in which the corresponding two cylinder sleeves  12 ,  12  are closer to each other, projection parts  14   i  are formed in an upper end of the spacer main body part  14   a . A space α″ between the tip end portion of each projection part  14   i  and the inner wall surface  13   a  of the water jacket  13  is set smaller than the space α. 
     As clear from  FIGS. 1 to 3 ,  8  and  9 , a cooling water supplying passage  11   b  extends from the timing train-side end surface of the cylinder block  11  toward the transmission. A cooling water supplying chamber  11   c  communicating with a downstream end of this cooling water supplying passage  11   b  faces the cooling water inlet port part  14   b  of the spacer  14  which is accommodated in the water jacket  13 . 
     As clear from  FIGS. 1 to 3  and  FIG. 9 , four communication holes  15   a  which are opened in the undersurface of a water jacket (not illustrated) formed in the cylinder head  15  face the upper portion of the cooling water outlet port part  14   c  of the spacer  14  accommodated in the water jacket  13 . If the spacer main body part  14   a  would be extended to the position of the cooling water outlet part  14   c , the position of the cooling water outlet port part  14   c  would roughly overlap the spacer main body part  14   a  thus extended. 
     As clear from  FIGS. 1 to 3  and  FIG. 10 , the partition wall  14   d  interposed between the cooling water inlet port part  14   b  and the cooling water outlet port part  14   c  of the spacer  14  has a minimum microspace γ (refer to  FIG. 10 ), which enables the spacer  14  to be assembled, between the inner wall surface  13   a  and the outer wall surface  13   b  of the water jacket  13 . A microspace δ through which the cooling water can pass is formed between the lower end portion of the partition wall  14   d  and the outer wall surface  13   b  of the water jacket  13 . Like the upper support legs  14   e ,  14   g  and the lower support legs  14   f ,  14   h , the upper end portion and the lower end portion of the partition wall  14   d  has a function of positioning the spacer  14  inside the water jacket  13  in the up-and-down direction. 
     As clear from  FIG. 2  and  FIGS. 11A to 11C , a portion interposed between the upper support leg  14   e  and the lower support leg  14   f  in the timing train-side end portion of the spacer  14  (a portion corresponding to the cooling water outlet port part  14   c ) is a thickness part  14   m  which is as thick as the spacer main body part  14   a . A slit  14   n  extending in the up-and-down direction is formed ranging from the lower end of the lower support leg  14   f  to the upper end of the thickness part  14   m . A slit  22   a  of a rubber-made fixing member  22  having an H-shaped horizontal cross section is fitted in and thus attached to the slit  14   n . The fixing member  22  is attached thereto in a range of the height in the up-and-down-direction of the spacer main body part  14   a . Although the outer peripheral surface of the fixing member  22  is not exposed to the outer peripheral surface of the spacer  14 , the inner peripheral surface of the fixing member  22  is exposed to the inner peripheral surface of the spacer  14 , and thus elastically abuts on the inner wall surface  13   a  of the water jacket  13 . A portion of the slit  14   n  which is exposed to the lower support leg  14   f  aims at enhancing the assemblability by decreasing the resistance of pressure-insertion of the fixing member  22 . 
     As clear from  FIG. 2  and  FIGS. 12A to 12C , a slit  14   o  extending in the up-and-down direction from the lower end of the lower support leg  14   h  to the lower end of the upper support leg  14   g  is formed in the transmission-side end portion of the spacer main body part  14   a . Another rubber-made fixing member  22  having an H-shaped horizontal cross section is attached to the slit  14   o . The fixing member  22  is attached thereto in a range of the height in the up-and-down-direction of the spacer main body part  14   a . Although the outer peripheral surface of the fixing member  22  is not exposed to the outer peripheral surface of the spacer  14 , the inner peripheral surface of the fixing member  22  is exposed to the inner peripheral surface of the spacer  14 , and thus elastically abuts on the inner wall surface  13   a  of the water jacket  13 . A portion of the slit  14   o  which is exposed to the lower support leg  14   h  aims at enhancing the assemblability by decreasing the resistance of pressure-insertion of the fixing member  22 . 
     The two fixing members  22 ,  22  both are arranged on the cylinder row line L 1 . Accordingly, the intake side portion and the exhaust side portion of the spacer  14  are basically symmetrical with respect to a line joining the two fixing members  22 ,  22  (in other words, the cylinder row line L 1 ). 
     The slits  14   n ,  14   o  are opened downward. The fixing members  22 ,  22  are upward fitted in the slits  14   n ,  14   o , respectively. For these reasons, when the spacer  14  to which the fixing members  22 ,  22  are attached is inserted inside the water jacket  13 , the fixing members  22 ,  22  are unlikely to come off the slits  14   n ,  14   o  even if the fixing members  22 ,  22  are pushed upward by friction forces acting between the fixing members  22 ,  22  and the inner wall surface  13   a  of the water jacket  13 . 
     Next, descriptions will be provided for the operation of the embodiment of the present invention having the foregoing configuration. 
     Before the cylinder head  15  is assembled to the deck surface  11   a  of the cylinder block  11 , the water jacket  13  is opened to surround the outer peripheries of the cylinder bores  12   a ,  12   a ;  12   a ′,  12   a ′ of the four cylinder sleeves  12  exposed to the deck surface  11   a , respectively. The spacer  14  is inserted inside the water jacket  13  from the opening. Thereafter, the cylinder head  15  is fastened to the cylinder block  11  with the gasket  16  overlapping the deck surface  11   a  of the cylinder block  11 . 
     When this spacer  14  is assembled therein, the lower ends of the lower support legs  14   f ,  14   h  and the lower end of the partition wall  14   d  is in contact with the bottom portion of the water jacket  13 , as well as the upper ends of the upper support legs  14   e ,  14   g  and the upper end of the partition wall  14   d  are in contact with the undersurface of the gasket  16 . Thereby, the spacer  14  is positioned in the cylinder axis line L 2  direction. At this time, the inner peripheral surface of the spacer main body part  14   a  of the spacer  14  is arranged close to the inner wall surface  13   a  of the water jacket  13 . However, because the dimensional precision of the inner wall surface  13   a  of the water jacket  13  which has been subjected no process since casted is not high, the slight space α (refer to  FIG. 6 ) is formed between the inner peripheral surface of the spacer main body part  14   a  and the inner wall surface  13   a  of the water jacket  13  for the purpose of preventing the assemblability from deteriorating due to friction of the spacer  14  with the inner wall surface  13   a  of the water jacket  13 . 
     If the spacer  14  moves in the up-and-down direction inside the water jacket  13  due to vibrations and the like during the operation of the internal combustion engine, there is a possibility that the upper ends of the upper support legs  14   e ,  14   g  and the upper end of the partition wall  14   d  may damage the undersurface of the gasket  16 . However, the two fixing members  22 ,  22  provided on the respective opposite ends in the cylinder row line L 1  direction fix the spacer  14  to the water jacket  13  in order that the spacer  14  cannot move relative to the water jacket  13 . This prevents haphazard movement of the spacer  14  from damaging the gasket  16 . 
     At this time, not only can the spacer  14  be firmly fixed to the inside of the water jacket  13  because the fixing member  22 ,  22  are provided in the respective two highly-rigid end portions of the spacer  14  in the cylinder row line L 1  direction, but also the influence of heat on the rubber-made fixing members  22 ,  22  attached to the respective opposite end portions of the cylinder block  11  in the cylinder row line L 1  direction can be suppressed to a minimum because the opposite end portions of the cylinder block  11  are lower in temperature than the intake-side and exhaust-side side surfaces of the cylinder block  11 . 
     In addition, because the fixing members  22 ,  22  are provided in the respective intermediate portions of the spacer  14  in the cylinder axis line L 2  direction, in other words, in the range of the height of the spacer main body part  14   a , it is possible to prevent the blockage of the flow of the cooling water in the upper cooling water passage  13   c  and in the lower cooling water passage  13   d  by the fixing members  22 ,  22 , which would otherwise occur. In addition, because the timing train-side fixing member  22  of the spacer  14  is provided in the cooling water outlet port part  14   c , the fixing member  22  does not affect the flow of the cooling water in the upper cooling water passage  13   c  and in the lower cooling water passage  13   d . Furthermore, the flow speed of the cooling water decreases due to the U-turn of the cooling water in the transmission-side end portion of the water jacket  13 . Accordingly, the influence of the fixing member  22  on the flow of the cooling water can be made smaller when the fixing member  22  is provided in the transmission-side end portion of the water jacket  13  than when the fixing member  22  is provided in the intake-side and exhaust-side side wall of the water jacket  13 . 
     The timing train-side upper support leg  14   e  and lower support leg  14   f  of the spacer  14  are formed thinner in the radial direction than the thickness T 1  of the spacer main body part  14   a , and are arranged offset toward the outer wall surface  13   b  of the water jacket  13  inside the upper cooling water passage  13   c  and the lower cooling water passage  13   d . In addition, the transmission-side upper support leg  14   g  and the lower support leg  14   h  of the spacer  14  are formed thinner in the radial direction than the thickness T 1  of the spacer main body part  14   a , and are arranged offset toward the inner wall surface  13   a  of the water jacket  13  inside the upper cooling water passage  13   c  and the lower cooling water passage  13   d . Thereby, the influence of the upper support legs  14   e ,  14   g  and the lower support legs  14   f ,  14   h  on the flow of the cooling water in the upper cooling water passage  13   c  and in the lower cooling water passage  13   d  can be suppressed to a minimum. In addition, the upper support legs  14   e ,  14   g  and the lower support legs  14   f ,  14   h  are curved in the shape of an arc along the forms of the inner wall surface  13   a  and the outer wall surface  13   b  of the water jacket  13 . Accordingly, the influence on the flow of the cooling water can be made much smaller. 
     Furthermore, out of the four cylinder bores  12   a ,  12   a ;  12   a ′,  12   a ′, their portions situated outermost in the cylinder row line L 1  direction are less susceptible to heat from the other cylinder bores  12   a ′,  12   a ′. For this reason, the temperature of such portions is relatively low. On the other hand, out of the four cylinder bores  12   a ,  12   a ;  12   a ′,  12   a ′, portions situated on the intake side and exhaust side of the cylinder row line L 1  are susceptible to heat from their adjacent cylinder bores  12   a ,  12   a ;  12   a ′,  12   a ′. For this reason, the temperature of such portions is relatively high. In the present embodiment, the upper support legs  14   e ,  14   g  and the lower support legs  14   f ,  14   h  are provided in the outermost positions in the cylinder row line L 1  direction in which the temperature of the cylinder bores  12   a ,  12   a  is relatively low. For this reason, even if the flow of the cooling water in the water jacket  13  is more or less blocked by the upper support legs  14   e ,  14   g  and the lower support legs  14   f ,  14   h , the influence can be suppressed to a minimum, and the temperatures of the respective cylinder bores  12   a ,  12   a ;  12   a ′,  12   a ′ can be made uniform. 
     In particular, the transmission-side upper support leg  14   g  and lower support leg  14   h  are arranged along the inner wall surface  13   a  of the water jacket  13  which faces the transmission-side lower-temperature portion of the corresponding cylinder bore  12   a . For this reason, it is possible to make the cooling water less likely to come into contact with the inner wall surface  13   a  of the water jacket  13  by use of the upper support leg  14   g  and the lower support leg  14   h , and to thermally insulate the cylinder bore  12   a , whose temperature is relatively low. This makes it possible to make the temperatures of the respective cylinder bores  12   a ,  12   a ;  12   a ′,  12   a ′ much more uniform. 
     The fixing members  22 ,  22  are made of the rubber, as well as are fitted in and fixed to the slits  14   n ,  14   o  of the spacer  14 . For this reason, the fixing members  22 ,  22  can be fixed to the spacer  14  without any specialized members, such as bolts. In addition, the positions at which the fixing members  22 ,  22  are provided are immediately above the lower support legs  14   f ,  14   h . For this reason, it is possible to prevent the spacer  14  from deforming in a twisted manner when: the spacer  14  is downward pushed into the inside of the water jacket  13  while putting the fixing members  22 ,  22  in pressure contact with the inner wall surface  13   a  of the water jacket  13 ; the lower ends of the lower support legs  14   f ,  14   h  subsequently come in contact with the bottom portion of the water jacket  13 ; and the spacer  14  receives an upward force. 
     During the operation of the internal combustion engine, the cooling water supplied from a water pump (not illustrated) provided to the cylinder block  11  flows into the water jacket  13  from the cooling water supplying passage  11   b , which is provided in the timing train-side end portion of the cylinder block  11 , through the cooling water supplying chamber  11   c . The spacer  14  is arranged inside the water jacket  13 . The thickness T 2  of the cooling water inlet port part  14   b  of the spacer  14 , which faces the cooling water supplying chamber  11   c , is thinner than the thickness T 1  of the spacer main body part  14   a . In addition, the cooling water inlet port part  14   b  is offset inward in the radial direction. For these reasons, the flow of the cooling water bifurcates into upper and lower streams along the radial-direction outer surface of the cooling water inlet port part  14   b , and the cooling water thus smoothly flows into the upper cooling water passage  13   c  and the lower cooling water passage  13   d  of the water jacket  13 . 
     The cooling water having flown into the upper cooling water passage  13   c  and the lower cooling water passage  13   d  of the water jacket  13  tends to bifurcate in the left and right directions. However, the flow of the cooling water is once blocked by the partition wall  14   d  existing on the left of the cooling water inlet port part  14   b . For this reason, the direction of the flow of the cooling water is turned to the right. Subsequently, the cooling water flows counterclockwise in the upper cooling water passage  13   c  and the lower cooling water passage  13   d  in almost full length. Finally, the cooling water is discharged to the communication holes  15   a  in the cylinder head  15  from the cooling water outlet port part  14   c  which is situated on the opposite side of the partition wall  14   d  from the cooling water inlet port part  14   b . While the cooling water is flowing in the water jacket  13 , the cooling water flowing in the upper cooling water passage  13   c  and the cooling water flowing in the lower cooling water passage  13   d  hardly ever mingle with each other, because the upper cooling water passage  13   c  and the lower cooling water passage  13   d  are partitioned vertically by the spacer main body part  14   a  whose thickness T 1  is slightly thinner than the width W of the water jacket  13 . 
     When the cooling water having flown in the water jacket  13  is discharged to the water jacket (not illustrated) in the cylinder head  15  through the communication holes  15   a  opened to the undersurface of the cylinder head  15 , the cooling water having flown in the lower cooling water passage  13   d  passes the cooling water outlet port part  14   c  of the spacer  14  from its lower part to its upper part, and thus joins the cooling water having flown in the upper cooling water passage  13   c . Thereafter, the confluent cooling water flows into the communication holes  15   a  in the cylinder head  15 . 
     At this time, not only can loss of the pressure of the cooling water upward passing the cooling water outlet port part  14   c  be suppressed to a minimum, but also the cooling effect can be secured even in a vicinity of the cooling water outlet port part  14   c , in which the cooling effect decreases due to reduction in the flow rate of the cooling water, by causing as much cooling water as possible to intervene between the cooling water outlet port part  14   c  and the inner wall surface  13   a  of the water jacket  13 . That is because: the cooling water outlet port part  14   c  is offset toward the outer wall surface  13   b  of the water jacket  13  with the thickness T 3  of the cooling water outlet port part  14   c  being less than the thickness T 1  of the spacer main body part  14   a  and with the outer peripheral surface being flush with the outer peripheral surface of the spacer main body part  14   a.    
     In addition, the cooling water having come out of the downstream end of the upper cooling water passage  13   c  joins the cooling water having changed its flow direction upward after coming out of the downstream end of the lower cooling water passage  13   d . Accordingly, the direction of the cooling water having come from the upper cooling water passage  13   c  can be changed upward by the cooling water having coming from the lower cooling water passage  13   d , and the cooling water having come from the upper cooling water passage  13   c  can be made to flow into the communication holes  15   a  smoothly. 
     When the cooling water having flown in the upper cooling water passage  13   c  and the lower cooling water passage  13   d  is discharged from the communication holes  15   a  after changing its direction upward at the cooling water outlet port part  14   c , there is a possibility that: swirls of the cooling water may occur; and the smooth direction change may be hindered. However, the flow of the cooling water into the communication holes  15   a  can be achieved by preventing the occurrence of the swirls, because a portion of the cooling water in the cooling water inlet port part  14   b  flows into the cooling water outlet port part  14   c  after passing the space δ (refer to  FIG. 10 ) in the lower end portion of the partition wall  14   d.    
     The inner peripheral surface of the spacer main body part  14   a  of the spacer  14  is close to the inner wall surface  13   a  at the intermediate portion of the water jacket  13  in the cylinder axis lines L 2  direction. Accordingly, only a less amount of the cooling water comes into contact with the inner wall surface  13   a , and the cooling is suppressed. As a result, the intermediate portions of the cylinder bores  12   a ,  12   a ;  12   a ′,  12   a ′ in the cylinder axis lines L 2  direction, which are opposed to the spacer main body part  14   a , become higher in temperature than the other portions thereof, and thermally expand to have larger clearances between the cylinder bores  12   a ,  12   a ;  12   a ′,  12   a ′ and their corresponding pistons  18 ,  18 ;  18 ′,  18 ′. As a consequence, frictions between the pistons  18 ,  18 ;  18 ′,  18 ′ and the cylinder bores  12   a ,  12   a ;  12   a ′,  12   a ′ are reduced, particularly when large side thrusts are applied to the respective pistons  18 ,  18 ;  18 ′,  18 ′ during the compression process and the expansion process. Accordingly, it is possible to contribute to improving fuel efficiency of the internal combustion engine. Furthermore, because the intermediate portions of the cylinder bores  12   a ,  12   a ;  12   a ′,  12   a ′ in the cylinder axis lines L 2  direction become higher in temperature than any other portions thereof, the temperature of the oil lubricating such portions rises, and the viscosity of the oil decreases. For this reason, the effect of friction reduction is enhanced more. 
     On the other hand, the upper portions and lower portions of the cylinder bores  12   a ,  12   a ;  12   a ′,  12   a ′ in the cylinder axis lines L 2  direction are sufficiently cooled by the cooling water flowing in the upper cooling water passage  13   c  and the lower cooling water passage  13   d  above and under the spacer  14 . Accordingly, it is possible to secure the cooling performances of the top parts  18   a  and the skirt parts  18   b  of the pistons  18 ,  18 ,  18 ′,  18 ′ slidably fitted in the cylinder bores  12   a ,  12   a ;  12   a ′,  12   a ′, and to prevent their overheat, although the temperatures of the top parts  18   a  and the skirt parts  18   b  would otherwise tend to rise. Moreover, not only does the upper portions of the cylinder bores  12   a ,  12   a ;  12   a ′,  12   a ′ directly receive heat of a combustion chamber, but also the upper portions thereof tend to raise their temperatures due to their reception of heat transmitted through the top rings  19 , the second rings  20  and the oil rings  21  from the heated pistons  18 ,  18 ;  18 ′,  18 ′ which stay at the vicinities of their top dead centers for long time due to the change in their movement directions. However, because no spacer  14  is made to face the upper portions of the cylinder bores  12   a ,  12   a ;  12   a ′,  12   a ′, their cooling performances can be secured. In addition, the skirt parts  18   b  of the pistons  18 ,  18 ;  18 ′,  18 ′ are places which are most tightly put in sliding contact with the cylinder bores  12   a ,  12   a ;  12   a ′,  12   a ′, thereby causing friction therebetween. However, because the cylinder bores  12   a ,  12   a ;  12   a ′,  12   a ′ with which the skirt parts  18   b  are put in sliding contact are covered with the spacer  14  and the diameters of the cylinder bores  12   a ,  12   a ;  12   a ′,  12   a ′ is increased by thermal expansion, the friction can be reduced. 
     As indicated by the continuous line in  FIG. 5 , the up-and-down position of the spacer  14  is set in such a way that the top rings  19 , the second rings  20  and the oil rings  21  are situated above the upper edge of the spacer main body part  14   a , when the side thrusts of the respective pistons  18 ,  18 ,  18 ′,  18 ′ reach their maximum during the expansion process, in other words, when the friction between the pistons  18 ,  18 ,  18 ′,  18 ′ and the cylinder bores  12   a ,  12   a ;  12   a ′,  12   a ′ reaches its maximum. For this reason, the cooling performance of the pistons  18 ,  18 ;  18 ′,  18 ′ can be secured by: reducing the friction by increasing the inner diameters of the cylinder bores  12   a ,  12   a ;  12   a ′,  12   a ′ by use of the spacer  14 ; and concurrently making the heat of the top parts  18   a  of the heated pistons  18 ,  18 ,  18 ′,  18 ′, whose temperature tend to be higher, escape to the upper cooling water passage  13   c  of the water jacket  13  from the highly heat-conductive top rings  19 , second rings  20  and oil rings  21  through the cylinder bores  12   a ,  12   a ;  12   a ′,  12   a′.    
     At this time, because the spacer main body part  14   a  of the spacer  14  is close to the inner wall surface  13   a  of the water jacket  13  with the minimum space α being interposed in between, it is possible to suppress the amount of cooling water intervening between the spacer main body part  14   a  and the inner wall surface  13   a  of the water jacket  13  to a minimum, and thus to thermally insulate the up-and-down-direction intermediate portions of the cylinder bores  12   a ,  12   a ;  12   a ′,  12   a ′ effectively, as well as to enlarge the diameters of the cylinder bores  12   a ,  12   a ;  12   a ′,  12   a′.    
     In addition, at the bottom dead centers indicated by the chain line in  FIG. 5 , the quantity of heat transmitted to the cylinder bores  12   a ,  12   a ;  12   a ′,  12   a ′ from the pistons  18 ,  18 ;  18 ′,  18 ′ through the top rings  19 , the second rings  20  and the oil rings  21  is larger because the speeds at which the pistons  18 ,  18 ;  18 ′,  18 ′ move decrease. However, when the pistons  18 ,  18 ;  18 ′,  18 ′ reaches their bottom dead centers, the top rings  19 , the second rings  20  and the oil rings  21  are situated below the lower edge of the spacer main body part  14   a . For this reason, it is possible to make the heat of the pistons  18 ,  18 ;  18 ′,  18 ′ escape to the cylinder bores  12   a ,  12   a ;  12   a ′,  12   a ′ without being obstructed by the spacer  14 , and to secure the cooling performances of the pistons  18 ,  18 ;  18 ′,  18 ′. 
     Moreover, when the spacer  14  is assembled inside the water jacket  13 , the space α between the inner peripheral surface of the spacer main body part  14   a  and the inner wall surface  13   a  of the water jacket  13  is set smaller than the space β between the outer peripheral surface of the spacer main body part  14   a  and the outer wall surface  13   b  of the water jacket  13 . For this reason, the outer peripheral surface of the spacer main body part  14   a  is designed not to come in contact with the outer wall surface  13   b  of the water jacket  13 , even though: the spacer  14  may deviate in the radial direction due to the assembling error and its deformation; and the inner peripheral surface of the spacer main body part  14   a  may come into contact with the inner wall surface  13   a  of the water jacket  13 . 
     Because, as described above, the space is always secured between the outer peripheral surface of the spacer main body part  14   a  and the outer wall surface  13   b  of the water jacket  13 , the following operation/working effects are exerted. To put it specifically, if unlike the present embodiment, the outer peripheral surface of the spacer main body part  14   a  would come in contact with the outer wall surface  13   b  of the water jacket  13 , the hitting sounds of the pistons  18 ,  18 ;  18 ′,  18 ′ would be propagated via pathways from the cylinder bores  12   a ,  12   a ;  12   a ′,  12   a ′, the bottom portion of the water jacket  13 , the lower support legs  14   f ,  14   h  of the spacer  14 , the spacer main body part  14   a  to the outer wall surface  13   b  of the water jacket  13 , and accordingly would constitute the cause of noises, because the lower support legs  14   f ,  14   h  of the spacer  14  are in contact with the bottom portion of the water jacket  13 . Meanwhile, in the present embodiment, although hitting sounds of the pistons  18 ,  18 ;  18 ′,  18 ′ are propagated from the cylinder bores  12   a ,  12   a ;  12   a ′,  12   a ′ to the spacer main body part  14   a , the hitting sounds are blocked in the spacer main body part  14   a  because the spacer main body part  14   a  does not abut on the outer wall surface  13   b  of the water jacket  13 , thereby reducing noises. 
     If the spacer  14  deforms due to its swelling resulting from its contact with the cooling water and its thermal expansion, there is a possibility that the inner peripheral surface of the spacer  14  may be tightly fitted to the inner wall surface  13   a  of the water jacket  13 . However, because the projection parts  14   i  provided on the spacer main body part  14   a  are opposed to the inner wall surface  13   a  of the water jacket  13  to come in contact with the inner wall surface  13   a  thereof, it is possible to prevent the inner peripheral surface of the spacer main body part  14   a  and the inner wall surface  13   a  of the water jacket  13  from coming into intimate contact with each other throughout their surfaces. Note that if the projection parts  14   i  come in contact with the inner wall surface  13   a  of the water jacket  13 , there is a possibility that the hitting sounds may be propagated through the projection parts  14   i . Basically, however, hitting sounds largely occur in the intake-side and exhaust-side portions of the outer peripheral surface of the pistons  18 ,  18 ,  18 ′,  18 ′ which are distant from the cylinder row line L 1 , and hitting sounds hardly ever occur in portions close to the cylinder row line L 1  in which the projection parts  14   i  are provided. For this reason, the propagation of hitting sounds through the projection parts  14   i  substantially does not matter. 
     In addition, as shown in  FIG. 2 , the spacer  14  is stretched in the cylinder row line L 1  direction by the reaction forces F 1 , F 1 , because the fixing members  22 ,  22  provided in the respective opposite end portions of the spacer  14  in the cylinder row line L 1  direction elastically contact the inner wall surface  13   a  of the water jacket  13 . As a result, the intake-side and exhaust-side side surfaces of the spacer main body part  14   a  deform by receiving loads F 2 , F 2  working in a direction in which the intake-side and exhaust-side side surfaces thereof come closer to each other. For this reason, the inner peripheral surface of the spacer main body part  14   a  comes closer to the inner wall surface  13   a  of the water jacket  13 , and the space α between the inner peripheral surface of the spacer main body part  14   a  and the inner wall surface  13   a  of the water jacket  13  decreases accordingly. Thereby, the amount of cooling water intervening between the spacer main body part  14   a  and the inner wall surface  13   a  of the water jacket  13  can be reduced more, and the up-and-down-direction intermediate portions of the cylinder bores  12   a ,  12   a ;  12   a ′,  12   a ′ thus can be thermally insulated more effectively, as well as the diameters thereof can be enlarged. 
     At this time, the two fixing members  22 ,  22  both are arranged on the cylinder row line L 1 , and the intake-side portion and exhaust-side portion of the spacer  14  are basically symmetrical with respect to the cylinder row line L 1 . For this reason, the loads F 2 , F 2  which cause the intake-side and exhaust-side side surfaces of the spacer main body part  14   a  to come closer to each other can be made uniform, and the amount of deformation of the intake-side portion of the spacer  14  and the amount of deformation of the exhaust-side portion of the spacer  14  can be made uniform. 
     Furthermore, because the fixing members  22 ,  22  are attached to the spacer main body part  14   a  in a way not to cut into the upper cooling water passage  13   c  or the lower cooling water passage  13   d , the fixing members  22 ,  22  do not obstruct the flow of the cooling water. In addition, because the fixing member  22 ,  22  are attached to the spacer main body part  14   a  in a way not to interfere with the upper support legs  14   e ,  14   g  or the lower support legs  14   f ,  14   h  of the spacer  14 , the spacer main body part  14   a  can be efficiently deformed with the resilient forces of the fixing members  22 ,  22 . 
     Meanwhile, each end-portion cylinder bore  12   a  is opposed to one corresponding intermediate cylinder bore  12   a ′ alone, while each intermediate bore  12   a ′ is opposed to one corresponding end-portion cylinder bore  12   a  and the other intermediate cylinder bore  12   a ′. For this reason, the intermediate cylinder bores  12   a ′ tend to be higher in temperature than the end-portion cylinder bores  12   a.    
     Nevertheless, the present embodiment makes the effect of cooling the intermediate cylinder bores  12   a ′,  12   a ′ higher than the effect of cooling the end-portion cylinder bores  12   a ,  12   a  because: the height H of the spacer main body part  14   a  of the spacer  14  is lower by the step t in its portions facing the intermediate cylinder bores  12   a ′,  12   a ′ than in its portions facing the end-portion cylinder bores  12   a ,  12   a ; and accordingly, the height of the upper cooling water passage  13   c  facing the intermediate cylinder bores  12   a ′,  12   a ′ is higher. This enables the spacer  14  to effectively exert the effect of friction reduction by: strongly cooling the intermediate cylinder bores  12   a ′,  12   a ′ which tend to be higher in temperature than the end-portion cylinder bores  12   a ,  12   a ; and thus making all the temperatures of the respective cylinder bores  12   a ,  12   a ;  12   a ′,  12   a ′ uniform. 
     Next, descriptions will be provided for a second embodiment of the present invention on the basis of  FIG. 13 . 
     In the first embodiment, the height of the spacer main body part  14   a  is set higher in its portions facing the end-portion cylinder bores  12   a ,  12   a , and is set lower in its portions facing the intermediate cylinder bores  12   a ′,  12   a ′. In the second embodiment, the thickness T 1  of the spacer main body part  14   a  is set thicker in its portions facing the end-portion cylinder bores  12   a ,  12   a , and is set thinner in its portions facing the intermediate cylinder bores  12   a ′,  12   a′.    
     As a result, the space α (refer to  FIG. 6 ) formed between the inner peripheral surface of the spacer main body part  14   a  and the inner wall surface  13   a  of the water jacket  13  is larger in its portions facing the intermediate cylinder bores  12   a ′,  12   a ′. The cross-sectional area of the passage of the cooling water in the space α is increased accordingly, and the cooling performances of the intermediate cylinder bores  12   a ′,  12   a ′ are thus enhanced. Hence, all the temperature of the respective cylinder bores  12   a ,  12   a ;  12   a ′,  12   a ′ can be made uniform as in the first embodiment. In addition, because the spacer  14  made of a synthetic resin has a heat conductivity lower than that of the cooling water, the heat of the intermediate cylinder bores  12   a ′,  12   a ′ is easy to escape toward the outer wall surfaces of the cylinder block  11  through the spacer main body part  14   a  by making the spacer main body part  14   a  thinner, and the cooling performances of the intermediate cylinder bores  12   a ′,  12   a ′ are enhanced more effectively. Furthermore, because the thickness T 1  of the spacer main body part  14   a  is made thinner in its portions facing the intermediate cylinder bores  12   a ′,  12   a ′ by moving the inner peripheral surface of the spacer main body part  14   a  outward in the radial direction, it is possible to effectively increase the cross-sectional area of the passage of the cooling water in the space α between the inner peripheral surface of the spacer main body part  14   a  and the inner wall surface  13   a  of the water jacket  13 . 
     Moreover, the space α between the inner peripheral surface of the spacer main body part  14   a  and the inner wall surface  13   a  of the water jacket  13  may be increased by: making the thickness T 1  of the spacer main body part  14   a  uniform in all its portions facing the cylinder bores  12   a ,  12   a ;  12   a ′,  12   a ′; and moving the spacer main body part  14   a  outward in the radial direction in its portions facing the intermediate cylinder bores  12   a ′,  12   a ′. Also in this way, the cooling performances of the intermediate cylinder bores  12   a ′,  12   a ′ can be enhanced by making more cooling water flow along the inner wall surface  13   a  of the water jacket  13 , which faces the intermediate cylinder bores  12   a ′,  12   a′.    
     Next, descriptions will be provided for a third embodiment of the present invention on the basis of  FIG. 14 . 
     The third embodiment is a modification of the second embodiment. In the second embodiment, in the spacer main body part  14   a &#39;s portions facing the intermediate cylinder bores  12   a ′,  12   a ′, the thickness T 1  of the spacer main body part  14   a  is made thinner by moving the position of the inner peripheral surface of the spacer main body part  14   a  outward in the radial direction. In the third embodiment, in the spacer main body part  14   a &#39;s portions facing the intermediate cylinder bores  12   a ′,  12   a ′, the thickness T 1  of the spacer main body part  14   a  is made thinner by moving the position of the outer peripheral surface of the spacer main body part  14   a  inward in the radial direction. 
     The third embodiment does not change the size of the space α (refer to  FIG. 6 ) formed between the inner peripheral surface of the spacer main body part  14   a  and the inner wall surface  13   a  of the water jacket  13 , but makes the spacer main body part  14   a  thinner in its portions facing the intermediate cylinder bores  12   a ′,  12   a ′. For this reason, the third embodiment makes it easier for the heat of the intermediate cylinder bores  12   a ′,  12   a ′ to escape to the radial-direction outer space β (refer to  FIG. 6 ) of the water jacket  13  and the outer wall surfaces of the cylinder block  11 , thereby capable of enhancing the cooling performances of the intermediate cylinder bores  12   a ′,  12   a′.    
     Next, descriptions will be provided for a fourth embodiment of the present invention on the basis of  FIG. 15 . 
     In the first embodiment, the fixing members  22 ,  22  provided in the respective opposite end portions of the spacer  14  in the cylinder row line L 1  direction elastically abut on the inner wall surface  13   a  of the water jacket  13 . In the fourth embodiment, fixing members  22 ′,  22 ′ are attached to the respective opposite end portions of the spacer  14  in such a way that the inside and outside of each fixing member  22 ′,  22 ′ are reversed in the radial direction, and elastically abut on the outer wall surface  13   b  of the water jacket  13 . The structure of the fixing members  22 ′,  22 ′ according to the fourth embodiment is substantially the same as that of the fixing members  22 ,  22  according to the first embodiment. 
     Thereby, the opposite end portions of the spacer  14  in the cylinder row line L 1  direction are biased by loads F 1 ′, F 1 ′ produced by resilient forces of the fixing members  22 ′,  22 ′ in a direction in which the opposite end portions thereof come closer to each other. For this reason, the intake-side and exhaust-side side surfaces of the spacer main body part  14   a  receive loads F 2 ′, F 2 ′ working in a direction in which the side surfaces thereof are made to go away from each other, and deforms outward in the radial direction. As a result, the inner peripheral surface of the spacer main body part  14   a  goes away from the inner wall surface  13   a  of the water jacket  13 , and the space α between the inner peripheral surface of the spacer main body part  14   a  and the inner wall surface  13   a  of the water jacket  13  increases accordingly. Thereby, the cross-sectional area of the passage of the cooling water in the space α increases, and the cooling performances of the intermediate cylinder bores  12   a ′,  12   a ′ are enhanced. Hence, all the temperatures of the cylinder bores  12   a ,  12   a ;  12   a ′,  12   a ′ can be made uniform as in the second embodiment. 
     Although the foregoing descriptions have been provided for the embodiments of the present invention, various design changes may be applied to the present invention within the scope not departing from the gist of the present invention. 
     For example, the internal combustion engine with four cylinders mounted in a straight line has been shown as an example of the embodiments. However, the present invention can be applied to an internal combustion engine of any arbitrary mode having three or more cylinders, in which three or more cylinder bores  12   a ,  12   a ′ are arranged one after another. 
     Furthermore, in the first embodiment, the height of the upper edge of the spacer main body part  14   a  is made lower in its portions facing the intermediate cylinder bores  12   a ′. However, the height of the lower edge of the spacer main body part  14   a  may be made higher in its portions facing the intermediate cylinder bores  12   a ′. Otherwise, the two modes may be used in combination. 
     Moreover, the present invention can be applied to an internal combustion engine in which: the cooling water supplied from one end side of the cylinder row line L 1  is bifurcated into two streams flowing along the intake-side side surface and the exhaust-side side surface, respectively; the two streams are made confluent in the other end side of the cylinder row line L 1 ; and the confluent cooling water is discharged therefrom.