Patent Publication Number: US-7721688-B2

Title: Variable compression ratio internal combustion engine

Description:
INCORPORATION BY REFERENCE 
   The disclosure of Japanese Patent Application No. 2006-242150 filed on Sep. 6, 2006 including the specification, drawings and abstract is incorporated herein by reference in its entirety. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The invention relates to a variable compression ratio internal combustion engine capable of changing the compression ratio that is the ratio of the maximum value to the minimum value of the volume of a combustion chamber that changes with the movement of the piston. 
   2. Description of the Related Art 
   Variable compression ratio internal combustion engines have been proposed which change the compression ratio by moving the cylinder block relative to the crankcase in the direction of an axis (center axis) of a cylinder bore (hereinafter, simply referred to also as “up-down direction”). For example, one of the variable compression ratio internal combustion engines has a cylinder block that is disposed so as to be relatively movable in the up-down direction with respect to a crankcase, and a variable compression ratio mechanism. 
   In the cylinder block, in-line arranged four cylinder bores are formed. A piston is housed in each cylinder bore. The pistons are linked to a crankshaft. The crankshaft is rotatably supported by the crankcase. Furthermore, the variable compression ratio internal combustion engine includes a cylinder head. The cylinder head is fixed to a top portion of the cylinder block. 
   The variable compression ratio mechanism includes a block-side bearing-forming portion, a case-side bearing-forming portion and a shaft-shaped drive portion. 
   The block-side bearing-forming portion is fixed to an outer wall surface (side wall surface) of the cylinder block so as to extend out from the outer wall surface in a region that contains an crankcase-side end portion of the outer wall surface (a lower end portion of the cylinder block). 
   The case-side bearing-forming portion is made up of an upstanding wall portion and a cap portion. The cap portion is fixed to the upstanding wall portion that is formed on an upper portion of the crankcase. 
   The shaft-shaped drive portion includes a plurality of eccentric cam portions, and is disposed so as to extend through a cylindrical bearing hole formed in the block-side bearing-forming portion, and a cylindrical bearing hole formed in the case-side bearing-forming portion. Then, the shaft-shaped drive portion is rotated about a predetermined axis by a driving device. At this time, the shaft-shaped drive portion rotates in contact with the surfaces that define the cylindrical bearing holes formed in the block-side bearing-forming portion and the case-side bearing-forming portion, and causes a shift in the direction of eccentricity. Thus, the cylinder block can be slid relative to the crankcase to the top dead center side. As a result, since the distance between the cylinder block and the crankcase becomes longer, the volume of the combustion chamber when the piston is at the top dead center (the minimum value of the volume of the combustion chamber) becomes larger, and therefore the compression ratio becomes lower. In this manner, according to the foregoing internal combustion engine, the compression ratio can be changed (e.g., see Japanese Patent Application Publication No. 2003-206771 (JP-A-2003-206771)). 
   When a mixture gas burns in a combustion chamber defined by a wall surface that defines the cylinder bore, a lower surface of the cylinder head, and a top surface of a piston, the pressure of the gas in the combustion chamber becomes very high. Due to this pressure, the lower surface of the cylinder head is pressed upward by the pressure in the combustion chamber, and the top surface of the piston is pressed downward by the same pressure. Therefore, a force in an upward direction is exerted on the cylinder block to which the cylinder head is fixed. On the other hand, a force in a downward direction is exerted on the crankcase that supports the crankshaft linked to the piston. As a result, a crankcase-side portion of the surface that defines the bearing hole of the block-side bearing-forming portion receives a force caused by the shaft-shaped drive portion  53 , and is therefore pressed downward. 
   Since a cylinder head-side end portion of the cylinder block is fixed to the cylinder head as mentioned above, the rigidity of the cylinder head-side end portion of the cylinder block is relatively high. On the other hand, a crankcase-side end portion of the cylinder block has a relatively low rigidity since the portion is not fixed to the crankcase. Furthermore, since the force exerted on the bearing hole of the block-side bearing-forming portion acts at a position that is apart outward from the outer wall surface of the cylinder block to which the block-side bearing-forming portion is fixed, the force acts on the cylinder block as a force that tends to bend a lower end portion of the cylinder block inward (bending moment). 
   In other words, a pressing force in an inward direction of the cylinder block (pressing direction) is exerted on a region in the outer wall surface of the cylinder block to which the block-side bearing-forming portion is fixed. Due to this pressing force, the wall surface defining the cylinder bore deforms in an inward direction of the cylinder bore. As a result, there is possibility that the friction force between the wall surface defining the cylinder bore and the piston may increase, and the fuel economy may deteriorate, or that the amount of inflow of lubricating oil into the combustion chamber may increase leading to useless consumption of lubricating oil. 
   SUMMARY OF THE INVENTION 
   It is an object of the invention to provide a variable compression ratio internal combustion engine capable of preventing deformation of a wall surface that defines a cylinder bore. 
   A first aspect of the invention is a variable compression ratio internal combustion engine that includes: a cylinder block having a cylinder bore that is a cylindrical hole extending through the cylinder block in a predetermined bore center axis direction, and that houses a piston; a cylinder head fixed to the cylinder block so as to cover one of opening portions of the cylinder bore; a crankcase that is disposed at a side of the cylinder block opposite from the cylinder head, and that is movable relative to the cylinder block in the bore center axis direction, and that rotatably supports a crankshaft that is linked to the piston; and a variable compression ratio mechanism that changes a capacity of a combustion chamber defined by a bore wall surface that defines the cylinder bore, a head lower surface that is a cylinder block-side surface of the cylinder head, and a piston top surface that is a head lower surface-side surface of the piston. 
   The variable compression ratio mechanism includes a block-side force-receiving portion extending outward from an outer wall surface of the cylinder block, and a linkage portion that changes a distance between the block-side force-receiving portion and the crankcase in the bore center axis direction while contacting each of the block-side force-receiving portion and the crankcase. 
   In the variable compression ratio internal combustion engine in accordance with this aspect, the cylinder block has a deformation-suppressing structure that restrains a deformation of the wall surface of the cylinder bore caused by a bore wall surface stress that occurs in the bore wall surface at a position of intersection between the bore wall surface and a pressing straight line that passes through a predetermined pressing position and that is a straight line parallel to a predetermined pressing direction as the block-side force-receiving portion receives by the linkage portion a force in a direction from the cylinder block toward the crankcase so that the block-side force-receiving portion presses the outer wall surface at the pressing position in the pressing direction. 
   Furthermore, in this aspect, the deformation-suppressing structure may be made up of a stress-reducing portion that makes the bore wall surface stress smaller than an outer wall surface stress that occurs in the outer wall surface at the predetermined pressing position as the block-side force-receiving portion receives by the linkage portion a force in a direction from the cylinder block toward the so that the block-side force-receiving portion presses the outer wall surface at the pressing position in the pressing direction. 
   That is, in the internal combustion engine in accordance with this aspect, as a mixture gas burns in the combustion chamber, a force in a direction from the cylinder block toward the cylinder head (upward direction) is exerted on the cylinder head, and a force in a direction from the piston toward the crankshaft (downward direction) is exerted on the piston. As a result, the block-side force-receiving portion is pulled in the downward direction by the crankcase via the linkage portion, and therefore the block-side force-receiving portion presses the outer wall surface in a predetermined pressing direction at a predetermined pressing position in the outer wall surface. Therefore, stress occurs in the cylinder block. Of this stress, the stress that occurs in the bore wall surface at a position of intersection between the bore wall surface and a straight line that passes through the pressing position and that is parallel to the pressing direction (bore wall surface stress) is made smaller than the stress that occurs in the outer wall surface at the pressing position (outer wall surface stress) by the stress-reducing portion. 
   Therefore, the degree of the deformation of the bore wall surface caused by the bore wall surface stress can be made smaller than in the case where a stress-reducing portion is not provided. As a result, the friction force between the bore wall surface and the piston does not become excessively large, so that deterioration in fuel economy can be prevented. Besides, the amount of lubricating oil that flows into the combustion chamber does not become excessively large, so that useless consumption of lubricating oil can be prevented. 
   In this aspect, the stress-reducing portion may be a slit-shaped groove portion that is formed in the cylinder block so as to have an opening at a position that is located between the block-side force-receiving portion and the bore wall surface in a crankcase-side surface of the cylinder block when the crankcase-side surface is viewed in the bore center axis direction. 
   According to this construction, when the block-side force-receiving portion presses the outer wall surface of the cylinder block as described above, a stress having substantially the same magnitude as the outer wall surface stress occurs in a portion of the cylinder block that is on the outer wall surface side of the groove portion (outer wall surface-side portion of the cylinder block), and the outer wall surface-side portion deforms. Therefore, the outer wall surface-side portion generates a force that opposes the force (pressing force) by which the block-side force-receiving portion presses the outer wall surface. As a result, the stress transmitted to a portion of the cylinder block that is on the bore wall surface side of the groove portion (bore wall surface-side portion) becomes smaller than the outer wall surface stress. Therefore, the aforementioned bore wall surface stress becomes smaller than in the case where the groove portion is not formed. As a result, the degree of the deformation of the bore wall surface caused by the bore wall surface stress can be made small. 
   If the cylinder block includes a hollow cylindrical cylinder liner that has an inner wall surface that constitutes the bore wall surface, the stress-reducing portion may be made up of a reinforcement member that has a higher rigidity than a portion of the cylinder block excluding the cylinder liner, and that is disposed in the cylinder block so as to extend through a position that is on the aforementioned pressing straight line and that is between the block-side force-receiving portion and the cylinder liner, and so as to surround a periphery of the cylinder liner about the bore center axis. 
   According to this construction, when the block-side force-receiving portion presses the outer wall surface of the cylinder block as described above, a stress having substantially the same magnitude as the outer wall surface stress occurs in a portion of the cylinder block that is on the outer wall surface side of the reinforcement member (outer wall surface-side portion of the cylinder block). At this time, since the rigidity of the reinforcement member is higher than the rigidity of the cylinder block, the rigidity of the reinforcement member makes the stress transmitted to a portion of the cylinder block on the bore wall surface side of the reinforcement member (bore wall surface-side portion) smaller than the outer wall surface stress. Therefore, the bore wall surface stress becomes smaller than in the case where the reinforcement member is not provided. As a result, the degree of the deformation of the bore wall surface (the cylinder liner) by the bore wall surface stress can be made small. 
   A variable compression ratio internal combustion engine according to another aspect of the invention includes the above-described variable compression ratio mechanism, and may be constructed so that a position of a bore wall surface lower end that is a crankcase-side end of the bore wall surface of the cylinder block is a position that is the same as a position of a block outer wall surface lower end that is a crankcase-side end of the outer wall surface of the cylinder block in which the block-side force-receiving portion extends out, or a position that is at a cylinder head side of the position of the block outer wall surface lower end. 
   Therefore, the distance of the movement of the bore wall surface lower end in an inward direction of the cylinder block caused when, of the crankcase-side end portion of the cylinder block (block lower end portion), a portion that includes the bore wall surface is bent in an inward direction of the cylinder block can be made shorter than in the case where the bore wall surface lower end is positioned at the crankcase side of the block outer wall surface lower end. As a result, the degree of the deformation of the bore wall surface caused by the pressing force can be made small. 
   Furthermore, a variable compression ratio internal combustion engine according to still another aspect of the invention includes: a cylinder block having a plurality of cylindrical cylinder bores that are cylindrical holes extending through the cylinder block in a predetermined bore center axis direction, and that are disposed in line in a cylinder arrangement direction that is orthogonal to the bore center axis direction, and that each house a piston; a cylinder head fixed to the cylinder block so as to cover one of opening portions of each cylinder bore; a crankcase that is disposed at a side of the cylinder block opposite from the cylinder head, and that is movable relative to the cylinder block in the bore center axis direction, and that rotatably supports a crankshaft that is linked to the piston; and a variable compression ratio mechanism that changes a capacity of a combustion chamber of each cylinder bore that is defined by a bore wall surface that defines the cylinder bore, a head lower surface that is a cylinder block-side surface of the cylinder head, and a piston top surface that is a head lower surface-side surface of the piston. 
   The variable compression ratio mechanism includes a block-side force-receiving portion extending outward from an outer wall surface of the cylinder block in a region in the outer wall surface that includes a portion of a line of intersection between the outer wall surface of the cylinder block and a plane that is orthogonal to the cylinder arrangement direction and that passes through a center axis of one of the cylinder bores, and a linkage portion that contacts each of the block-side force-receiving portion and the crankcase, and that changes a distance between the block-side force-receiving portion and the crankcase in the bore center axis direction. 
   In the variable compression ratio internal combustion engine in accordance with this aspect, the cylinder block may include a portion in which a distance between a bore center axes-arrangement plane that contains the cylinder arrangement direction and the bore center axis direction, and the outer wall surface of the cylinder block in a section of the cylinder block taken on a plane orthogonal to the cylinder arrangement direction is shorter than a distance between the bore center axes-arrangement plane and the outer wall surface at a position at which the block-side force-receiving portion extends out in a section of the cylinder block taken on a plane that is orthogonal to the cylinder arrangement direction and that passes through the block-side force-receiving portion. 
   That is, this cylinder block has a thick-walled portion that includes a portion in which the block-side force-receiving portion extends out, and a thin-walled portion made up of other portions. As a result, the rigidity of the cylinder block at the position where the block-side force-receiving portion extends out is higher than the rigidity thereof at other positions. Therefore, although the aforementioned pressing force is exerted on the cylinder block, the cylinder block is unlikely to deform. That is, it becomes possible to make small the degree of the deformation of the bore wall surface caused by the pressing force while restraining the increase in the weight of the cylinder block. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and further objects, features and advantages of the invention will become apparent from the following description of preferred embodiments with reference to the accompanying drawings, wherein like numerals are used to represent like elements and wherein: 
       FIG. 1  is a schematic perspective view of a variable compression ratio internal combustion engine in accordance with a first embodiment of the invention; 
       FIG. 2  is a perspective view of a cylinder block shown in  FIG. 1 , viewed from above; 
       FIG. 3  is a perspective view of the cylinder block shown in  FIG. 1 , viewed from below; 
       FIG. 4  is a sectional view of the variable compression ratio internal combustion engine taken on a plane represented by line IV-IV of  FIG. 1 ; 
       FIG. 5  is a sectional view of the variable compression ratio internal combustion engine taken on a plane represented by line V-V of  FIG. 1 ; 
       FIGS. 6A ,  6 B and  6 C are diagrams schematically showing changes in the compression ratio caused by the driving of a variable compression ratio mechanism shown in  FIG. 1 ; 
       FIG. 7  is a diagram schematically showing the force that is exerted on the cylinder block when combustion occurs in a combustion chamber shown in  FIG. 1 , and the deformation of the cylinder block caused by the force; 
       FIG. 8  is a schematic view of a cylinder block of a variable compression ratio internal combustion engine in accordance with a modification of the first embodiment of the invention, viewed from below; 
       FIG. 9  is a sectional view of the cylinder block taken on a plane represented by line IX-IX of  FIG. 8 ; 
       FIG. 10  is a schematic view of a cylinder block of a variable compression ratio internal combustion engine in accordance with another modification of the first embodiment of the invention, viewed from below; 
       FIG. 11  is a schematic view of a cylinder block of a variable compression ratio internal combustion engine in accordance with a second embodiment of the invention, viewed from below; 
       FIG. 12  is a sectional view of the cylinder block taken on a plane represented by line XII-XII of  FIG. 11 ; 
       FIG. 13  is a schematic view of a cylinder block of a variable compression ratio internal combustion engine in accordance with a third embodiment of the invention, viewed from below; and 
       FIG. 14  is a sectional view of the cylinder block taken on a plane represented by line XIV-XIV of  FIG. 13 . 
   

   DETAILED DESCRIPTION OF THE EMBODIMENTS 
   First Embodiment 
   Hereinafter, embodiments of the variable compression ratio internal combustion engine of the invention will be described with reference to the drawings. As shown in  FIG. 1 , a variable compression ratio internal combustion engine  10  includes a cylinder block  20  and a crankcase  30 . 
   Cylinder Block 
   The cylinder block  20  is made of aluminum. As shown in  FIGS. 1 to 3 , the cylinder block  20  has a generally rectangular parallelepiped shape having an upper surface  20   a  and a lower surface  20   b  that are of a generally rectangle shape having short sides and long sides, and side surfaces (in this specification, also referred to as “outer wall surfaces”)  20   c  that are parallel to the direction of the long sides of the upper and lower surfaces  20   a ,  20   b . Hereinafter in the specification, the direction from the upper surface  20   a  toward the lower surface  20   b  of the cylinder block  20  will be referred to as the downward direction, and the direction from the lower surface  20   b  toward the upper surface  20   a  of the cylinder block  20  will be referred to as the upward direction. 
   The cylinder block  20  has four cylindrical penetration holes that extend through the cylinder block  20  in a direction orthogonal to the upper surface  20   a  and the lower surface  20   b  (i.e., in the up-down direction, which will be also referred to as the bore center axis direction). These penetration holes are disposed in line in the long-side direction of the cylinder block  20  (cylinder arrangement direction). In each penetration hole, a hollow cylindrical cylinder liner  20   d  having an outside diameter that is equal to the diameter of the penetration hole is disposed coaxially with the penetration hole (pressed therein, or cast therein). 
   Each cylinder liner  20   d  is made of cast iron. A cylindrical space defined by an inner wall surface of the cylinder liner  20   d  is referred to as the cylinder bore  21 . A lower end (crankcase 30-side end) of the cylinder liner  20   d  is contained in a plane that contains the lower surface  20   b  of the cylinder block  20 . That is, the position, in the up-down direction, of the lower end (the bore wall surface lower end) of the inner wall surface of the cylinder liner  20   d  (a wall surface that defines the cylinder bore  21 , i.e., a bore wall surface) is the same as the position, in the up-down direction, of the lower end of the outer wall surfaces  20   c  of the cylinder block  20  (the block outer wall surface lower end). 
   The bore wall surface is designed to be supplied with lubricating oil from an oil pan (not shown) that is mounted to a lower portion of the crankcase  30 . A sectional view of the variable compression ratio internal combustion engine  10  taken on a plane that passes through the center axis (bore center axis) BC of one of the cylinder bores  21  in  FIG. 1  and that contains line IV-IV of  FIG. 1 , and that is orthogonal to the cylinder arrangement direction is shown in  FIG. 4 . Referring to  FIG. 4 , a cylindrical piston  22  is housed in each cylinder bore  21 . A side surface of the piston  22  is provided with piston rings for scraping off excess lubricating oil from the bore wall surface. 
   A cooling water passageway  23  for cooling water is formed in the cylinder block  20 . The cooling water passageway  23  is a groove arranged around the cylinder liners  20   d , and has openings to the upper surface  20   a  of the cylinder block  20 . 
   Crankcase 
   As shown in  FIG. 1 , the crankcase  30  rotatably supports a crankshaft  31 , and also houses the crankshaft  31 . The crankcase  30  is disposed below the cylinder block  20  so that the direction of the axis of the crankshaft  31  coincides with the cylinder arrangement direction. Each piston  22  is linked to the crankshaft  31  via a connecting rod  32  as shown in  FIG. 4 . Due to this construction, the reciprocating motion of each piston  22  is converted into rotating motion of the crankshaft  31 . 
   Cylinder Head 
   A sectional view of the variable compression ratio internal combustion engine  10  taken on a plane that contains line V-V passing between cylinder bores  21  in  FIG. 1  and that is orthogonal to the cylinder arrangement direction is shown in  FIG. 5 . As shown in  FIG. 5 , the variable compression ratio internal combustion engine  10  includes a cylinder head  40 . The cylinder head  40  is fixed to the upper surface  20   a  of the cylinder block  20  (i.e., a side of the cylinder block  20  opposite from the crankcase  30 ) so as not to move relative to the cylinder block  20 . 
   As shown in  FIG. 4 , the cylinder head  40  has a plurality of recess portions  40   a   1  that are open at a cylinder block  20 -side surface of the cylinder head  40  (head lower surface) and that correspond one-to-one to the cylinder bores  21 . When the cylinder head  40  is fixed to the cylinder block  20 , each recess portion  40   a   1  becomes contiguous with the wall surface of a corresponding one of the cylinder bores  21  (bore wall surface). That is, the cylinder head  40  is fixed to the cylinder block  20  so as to cover one of the opening portions (the upper-side opening portion) of each cylinder bore  21 . A combustion chamber  41  of each cylinder bore  21  (each cylinder) is defined by the bore wall surface, the cylinder head  40 -side surface of the piston  22  (piston top surface), and a corresponding one of the recess portions  40   a   1  of the cylinder head. 
   In the cylinder head  40 , intake ports  42  communicating with the combustion chambers  41  and exhaust ports  43  also communicating with the combustion chambers  41  are formed for the individual cylinders as shown in  FIG. 4 . Furthermore, in the cylinder head  40 , intake valves  42   a  that open and close the intake ports  42  and exhaust valves  43   a  that open and close the exhaust ports  43  as well as ignition plugs  44  that generate sparks in the combustion chambers  41  are disposed for the individual cylinders. 
   In addition, the variable compression ratio internal combustion engine  10  includes a fuel injection device (not shown). By injecting fuel via the fuel injection device, a mixture gas containing fuel and air is supplied into the combustion chambers  41  through the intake ports  42 . 
   Variable Compression Ratio Mechanism 
   The variable compression ratio internal combustion engine  10  further includes variable compression ratio mechanisms  50  as shown in  FIGS. 1 to 5 . The variable compression ratio mechanisms  50  are provided near both side surfaces (outer wall surfaces) of the cylinder block  20 , that is, one at either side. The variable compression ratio mechanism  50  at one of the two outer wall surfaces  20   c  and the variable compression ratio mechanism  50  at the other outer wall surface  20   c  are symmetrical to each other with respect to a plane that contains the bore center axes BC of all the cylinders (hereinafter, referred to as “bore center axes-arrangement plane”). Therefore, only the variable compression ratio mechanism  50  of one of the side surfaces  20   c  will be described. 
   The variable compression ratio mechanism  50  includes a case-side bearing-forming portion  51 , a block-side bearing-forming portion  52 , and a shaft-shaped drive portion  53 . Incidentally, the case-side bearing-forming portion  51  may be also termed the case-side force-receiving portion. The block-side bearing-forming portion  52  may be also termed the block-side force-receiving portion. The shaft-shaped drive portion  53  may serve as a linkage portion. 
   Case-Side Bearing-Forming Portion 
   The case-side bearing-forming portion  51  is made up of a flat plate-shaped vertical wall portion  51   a , and a plurality of cap portions  51   b . The vertical wall portion  51   a  constitutes an upper wall surface of the crankcase  30 . The vertical wall portion  51   a  is formed so that, when the cylinder block  20  is disposed on the crankcase  30 , the vertical wall portion  51   a  faces the side surface (outer wall surface)  20   c  of the cylinder block  20  and covers a portion of the outer wall surface  20   c.    
   The vertical wall portion  51   a  has plural (four in this example) penetration holes  51   a   1  that extend through the crankcase from the outside to the inside and that correspond one-to-one to the cylinder bores  21  when the cylinder block  20  is disposed on the crankcase  30 . Each of the penetration holes  51   a   1  is disposed in a region that contains a position at which the vertical wall portion  51   a  intersects with a plane that contains the center axis (bore center axis) BC of a corresponding one of the cylinder bores  21  and that is orthogonal to the cylinder arrangement direction. Recess portions  51   a   2  are formed between the penetration holes  51   a   1 . Each recess portion  51   a   2  is opened toward outward, and has a semicircular shape in a section of the vertical wall portion  51   a  taken on a plane orthogonal to the cylinder arrangement direction. The semicircular centers of the recess portions  51   a   2  are on an axis. 
   The cap portions  51   b  correspond one-to-one to the recess portions  51   a   2  of the vertical wall portion  51   a . Each of the cap portions  51   b  is fixed to the vertical wall portion  51   a  so as to cover a corresponding one of the recess portions  51   a   2 . Each cap portion  51   b  has a recess portion  51   b   1  that is open toward the vertical wall portion  51   a  when the cap portion  51   b  is fixed to the vertical wall portion  51   a , and that has a semicircular shape in a section of the cap portion  51   b  taken on a plane orthogonal to the cylinder arrangement direction. The diameter of the semicircular shape of the recess portions  51   b   1  is the same as the diameter of the semicircular shape of the recess portions  51   a   2 . 
   With this construction, when the cap portions  51   b  are fixed to the vertical wall portion  51   a , the case-side bearing-forming portion  51  has a plurality of cylindrical bearing holes  51   c  that are defined by the recess portions  51   a   2  of the vertical wall portion  51   a  and the recess portions  51   b   1  of the cap portions  51   b , and that extend through case-side bearing-forming portion  51  in the cylinder arrangement direction, and that are coaxial with each other. 
   Block-Side Bearing-Forming Portion 
   The block-side bearing-forming portion  52  is actually made up of plural (four in this example) members that correspond one-to-one to the penetration holes  51   a   1  of the vertical wall portion  51   a . The block-side bearing-forming portions  52  are inserted through the penetration holes  51   a   1  of the vertical wall portion  51   a , and are fixed to a lower end portion of the outer wall surface  20   c  of the cylinder block  20 . That is, each block-side bearing-forming portion  52  is protruded outward from the outer wall surface  20   c  of the cylinder block  20 , and is disposed in a region that contains a portion of the line of intersection between the outer wall surface  20   c  of the cylinder block  20  and a plane that contains the center axis of a corresponding one of the cylinder bores  21  and that is orthogonal to the cylinder arrangement direction. 
   The length of the block-side bearing-forming portions  52  in the up-down direction is shorter than the length of the penetration holes  51   a   1  of the vertical wall portion  51   a  in the up-down direction. This construction makes the block-side bearing-forming portions  52  movable within the corresponding penetration holes  51   a   1  of the vertical wall portion  51   a  in the up-down direction. That is, the crankcase  30  is movable relative to the cylinder block  20  in the up-down direction (the direction of the bore center axes). 
   Each block-side bearing-forming portion  52  has a cylindrical bearing hole  52   a  that extends therethrough in the cylinder arrangement direction. The bearing holes  52   a  of the block-side bearing-forming portions  52  are coaxial with each other. The diameter of the bearing holes  52   a  is larger than the diameter of the bearing holes  51   c  of the case-side bearing-forming portion  51 . 
   Shaft-Shaped Drive Portion 
   As shown in  FIGS. 1 ,  4  and  5 , the shaft-shaped drive portion  53  includes a rod-like eccentric shaft portion  53   a , a plurality of stationary cam portions  53   b  that correspond one-to-one to the bearing holes  51   c  of the case-side bearing-forming portion  51 , a plurality of movable cam portions  53   c  that correspond one-to-one to the bearing holes  52   a  of the block-side bearing-forming portions  52 , and a worm gear  53   d.    
   Each stationary cam portion  53   b  is a cylindrical member having substantially the same diameter as the bearing holes  51   c  of the case-side bearing-forming portion  51 . The length of each stationary cam portion  53   b  in the direction of the axis thereof is substantially the same as the axial length of a corresponding one of the bearing holes  51   c  of the case-side bearing-forming portion  51 . Each stationary cam portion  53   b  has a cylindrical penetration hole that extends therethrough in the direction of the axis at a position that is deviated (eccentric) from the center axis of the stationary cam portion  53   b , and that has substantially the same diameter as the eccentric shaft portion  53   a.    
   Each movable cam portion  53   c  is a cylindrical member having substantially the same diameter as the bearing hole  52   a  of each block-side bearing-forming portion  52 . The length of each movable cam portion  53   c  in the direction of the axis thereof is substantially the same as the length of the bearing hole  52   a  of a corresponding one of the block-side bearing-forming portions  52 . Each movable cam portion  53   c  has a cylindrical penetration hole that extends therethrough in the direction of the axis at a position that is eccentric from the center axis of the movable cam portion  53   c , and that has substantially the same diameter as the eccentric shaft portion  53   a.    
   The stationary cam portions  53   b  and the movable cam portions  53   c  are disposed alternately with each other. The stationary cam portions  53   b  and the movable cam portions  53   c  are mounted on the eccentric shaft portion  53   a  by disposing the stationary cam portions  53   b  and the movable cam portions  53   c  so that their penetration holes are coaxial, and then inserting the eccentric shaft portion  53   a  through all the penetration holes. The stationary cam portions  53   b  and the eccentric shaft portion  53   a  have screw holes (not shown). The stationary cam portions  53   b  are fixed to the eccentric shaft portion  53   a  by screws (not shown) that are inserted through the screw holes so that the stationary cam portions  53   b  do not rotate relative to the eccentric shaft portion  53   a  and so that all the stationary cam portions  53   b  are coaxial with each other. On the other hand, the movable cam portions  53   c  are rotatable relative to the eccentric shaft portion  53   a.    
   The worm gear  53   d  is fixed to one of the stationary cam portions  53   b  so that the worm gear  53   d  does not rotate relative to the eccentric shaft portion  53   a , and is coaxial with the stationary cam portions  53   b . The worm gear  53   d  meshes with an output portion of a motor (not shown) so as to be rotationally driven by the motor. 
   The shaft-shaped drive portion  53  is supported by the case-side bearing-forming portion  51  and the block-side bearing-forming portions  52  so that each stationary cam portion  53   b  is housed in a corresponding one of the bearing holes  51   c  of the case-side bearing-forming portion  51 , and is rotatable within the bearing hole  51   c  while in contact with the wall surface that defines the bearing hole  51   c , and so that each movable cam portion  53   c  is housed in a corresponding one of the bearing holes  52   a  of the block-side bearing-forming portions  52 , and is rotatable within the bearing hole  52   a  while in contact with the wall surface that defines the bearing hole  52   a.    
   Operation Principle of Variable Compression Ratio Mechanism 
   The change in the compression ratio with rotation of the worm gear  53   d  will be described with reference to  FIGS. 6A to 6C . 
   As shown in  FIG. 6A , when the center axis of the eccentric shaft portion  53   a  is right below the center axis FC of the stationary cam portions  53   b , that is, when the center axis of the eccentric shaft portion  53   a  is at the lowest position relative to the center axis FC of the stationary cam portions  53   b , the center axis of the eccentric shaft portion  53   a , the center axis FC of the stationary cam portions  53   b  and the center axis MC of the movable cam portions  53   c  are aligned in that order on a straight line, and the distance between the center axis FC of the stationary cam portions  53   b  and the center axis MC of the movable cam portions  53   c  in the up-down direction is the shortest. Therefore, the distance between the crankcase  30  and the cylinder block  20  in the up-down direction is also the shortest, so that the compression ratio becomes the highest. 
   From this state, if the right-side worm gear  53   d , in  FIG. 6A , is rotationally driven counterclockwise (in the direction indicated by an arrow A) and the left-side worm gear  53   d  is rotationally driven clockwise (in the direction indicated by an arrow B), the center axis of the right-side eccentric shaft portion  53   a  moves counterclockwise about the center axis FC of the stationary cam portions  53   b , and the center axis of the left-side eccentric shaft portion  53   a  moves clockwise about the center axis FC of the stationary cam portions  53   b . It is to be noted herein that, due to the rigidity of the cylinder block  20 , none of the movable cam portions  53   c  can be moved in the right-left direction. 
   Therefore, the right-side movable cam portions  53   c  rotate counterclockwise within the bearing holes  52   a  of the block-side bearing-forming portions  52  while in contact with the wall surfaces that respectively define the bearing holes  52   a , and thus push up the block-side bearing-forming portions  52 . The left-side movable cam portions  53   c  rotate clockwise within the bearing holes  52   a  of the block-side bearing-forming portions  52  while in contact with the wall surfaces that respectively define the bearing holes  52   a , and thus push up the block-side bearing-forming portions  52 . Then, if each worm gear  53   d  continues to be rotationally driven, the state of the variable compression ratio mechanisms  50  reaches a state as shown in  FIG. 6B  in which the center axis of the eccentric shaft portion  53   a  and the center axis FC of the stationary cam portions  53   b  are aligned in the left-right direction. 
   In the state shown in  FIG. 6B , the distance between the center axis FC of the stationary cam portions  53   b  and the center axis MC of the movable cam portions  53   c  in the up-down direction is longer than in the state shown in  FIG. 6A . Therefore, the distance between the cylinder block  20  and the crankcase  30  in the up-down direction is longer than in the case shown in  FIG. 6A , so that the compression ratio becomes lower than in the case shown in  FIG. 6A . 
   If each worm gear  53   d  further continues to be rotationally driven, the center axis of the right-side eccentric shaft portion  53   a  moves counterclockwise about the center axis FC of the stationary cam portions  53   b , and the center axis of the left-side eccentric shaft portion  53   a  moves clockwise about the center axis FC of the stationary cam portions  53   b . Due to this operation, the right-side movable cam portions  53   c  rotate clockwise within the bearing holes  52   a  of the block-side bearing-forming portions  52  while in contact with the wall surfaces that respectively define the bearing holes  52   a , and thus push up the block-side bearing-forming portions  52 . The left-side movable cam portions  53   c  rotate counterclockwise within the bearing holes  52   a  of the block-side bearing-forming portions  52  while in contact with the wall surfaces that respectively define the bearing holes  52   a , and thus push up the block-side bearing-forming portions  52 . Then, the state of the variable compression ratio mechanisms  50  reaches a state shown in  FIG. 6C . 
   In the state show in  FIG. 6C , the center axis FC of the stationary cam portions  53   b , the center axis of the eccentric shaft portion  53   a  and the center axis MC of the movable cam portions  53   c  are aligned in that order from below on a straight line, and the distance between the center axis FC of the stationary cam portions  53   b  and the center axis MC of the movable cam portions  53   c  in the up-down direction is the longest (i.e., is longer than in the state shown in  FIG. 6A  and in the state shown in  FIG. 6B ). Therefore, the distance between the crankcase  30  and the cylinder block  20  in the up-down direction is also the longest, so that the compression ratio becomes the lowest. 
   Thus, as the right-side worm gear  53   d  rotates counterclockwise and the left-side worm gear  53   d  rotates clockwise (as the state of the variable compression ratio mechanisms  50  shifts from the state shown in  FIG. 6A  toward the state shown in  FIG. 6C ), the compression ratio becomes lower. 
   If each worm gear  53   d  is rotated in the direction opposite to the aforementioned direction from the state shown in  FIG. 6C , the compression ratio increases with the rotation of the worm gears  53   d  conversely to the aforementioned case. That is, if in the state shown in  FIG. 6C  the right-side worm gear  53   d  is rotationally driven clockwise and the left-side worm gear  53   d  is rotationally driven counterclockwise, the center axis of the right-side eccentric shaft portion  53   a  moves clockwise about the center axis FC of the stationary cam portions  53   b , and the center axis of the left-side eccentric shaft portion  53   a  moves counterclockwise about the center axis FC of the stationary cam portions  53   b.    
   Therefore, the right-side movable cam portions  53   c  rotate clockwise within the bearing holes  52   a  of the block-side bearing-forming portions  52  while in contact with the wall surfaces that respectively define the bearing holes  52   a , and thus push down the block-side bearing-forming portions  52 . The left-side movable cam portions  53   c  rotate counterclockwise within the bearing holes  52   a  of the block-side bearing-forming portions  52  while in contact with the wall surfaces that respectively define the bearing holes  52   a , and thus push down the block-side bearing-forming portions  52 . 
   Therefore, if the worm gears  53   d  continue to be rotationally driven, the state of the variable compression ratio mechanisms  50  reaches the state shown in  FIG. 6B . If the rotational driving of the worm gears  53   d  is further continued, the state of the variable compression ratio mechanisms  50  reaches the state shown in  FIG. 6A . Thus, as the right-side worm gear  53   d  rotates clockwise and the left-side worm gear  53   d  rotates counterclockwise (as the state of the variable compression ratio mechanism  50  shifts from the state shown in  FIG. 6C  toward the state shown in  FIG. 6A ), the distance between the crankcase  30  and the cylinder block  20  in the up-down direction becomes shorter, so that the compression ratio becomes higher. 
   By rotationally driving the worm gears  53   d  in the above-described manner, the distance between the crankcase  30  and the cylinder block  20  in the up-down direction (bore center axis direction) is changed, so that the compression ratio of the variable compression ratio internal combustion engine  10  is changed. 
   Slit-Shaped Groove 
   As shown in  FIGS. 3 and 4 , the cylinder block  20  has a plurality of slit-shaped stress reduction groove portions  24  as stress-reducing portions that correspond one-to-one to the block-side bearing-forming portions  52 . In the lower surface  20   b  of the cylinder block  20 , each stress reduction groove portion  24 , when viewed from the bore center axis direction, has an opening at a position (in a region) between one of the block-side bearing-forming portions  52  that corresponds to the stress reduction groove portion  24  and the bore wall surface defining the cylinder bore  21  that is the nearest to the block-side bearing-forming portion  52 . The shape of the opening of each stress reduction groove portion  24  is an elongated rectangular shape having long sides that are slightly longer than the length of each block-side bearing-forming portion  52  in the cylinder arrangement direction, and short sides that are orthogonal to the long sides and are very short. 
   The depth of each stress reduction groove portion  24  is slightly greater than half the length of the block-side bearing-forming portions  52  in the up-down direction as shown in  FIG. 4 . 
   Operation 
   In the variable compression ratio internal combustion engine  10  in accordance with the first embodiment constructed as described above, when a mixture gas formed in a combustion chamber  41  burns, the pressure of gas in the combustion chamber  41  becomes very high. Due to this pressure, the lower surface  40   a  of the cylinder head  40  is pressed upward by a force F 0   a , and the top surface of the piston  22  is pressed downward by a force F 0   b . Therefore, a force F 1   a  in the upward direction is exerted on the cylinder block  20  to which the cylinder head  40  is fixed, and a force F 1   b  in the downward direction is exerted on the crankcase  30  that supports the crankshaft  31  linked to the piston  22 . As a result, crankcase 30-side portions of the wall surfaces that define the bearing holes  52   a  of the block-side bearing-forming portions  52  receive a force F 2  caused by the shaft-shaped drive portion  53 , and are therefore pressed downward. 
   Since the force F 2  acts at a position that is apart outward from the outer wall surface  20   c  to which the block-side bearing-forming portions  52  are fixed, the force F 2  acts on the cylinder block  20  as a force that tends to bend a lower end portion of the cylinder block  20  inward with respect to the cylinder block  20  (bending moment). In other words, a pressing force F 3  in a direction toward the inside of the cylinder block  20  (pressing direction) is exerted on a crankcase 30-side end (block outer wall surface lower end, that is, a pressing position) of a region in the outer wall surface  20   c  of the cylinder block  20  to which region the block-side bearing-forming portions  52  are fixed. That is, the block-side bearing-forming portions (block-side force-receiving portions)  52  presses the outer wall surface  20   c  in the pressing direction, at a lower end portion of the outer wall surface  20   c  of the cylinder block  20 . 
   Therefore, a stress having substantially the same magnitude as the stress occurring in the outer wall surface  20   c  at the aforementioned pressing position (outer wall surface stress) occurs in a portion (outer wall surface-side portion) of the cylinder block  20  that is on the outer wall surface  20   c  side of the stress reduction groove portions  24 , so that the outer wall surface-side portion deforms as shown by dotted lines DF in  FIG. 7 . Therefore, the outer wall surface-side portion generates a force that opposes the force (pressing force) F 3  by which the block-side bearing-forming portions  52  press the outer wall surface  20   c.    
   As a result, the stress transmitted to the portion on the bore wall surface side of the stress reduction groove portions  24  (bore wall surface-side portion) becomes smaller than the outer wall surface stress. Therefore, of the stress caused in the cylinder block  20  by the aforementioned pressing force F 3 , the stress caused in the bore wall surface at a position of intersection between the bore wall surface and a pressing straight line that passes through the pressing position and that is parallel to the pressing direction (bore wall surface stress) is reduced, in comparison with the case where the stress reduction groove portions  24  are not formed. As a result, the degree of the deformation of the bore wall surface caused by the bore wall surface stress can be made small. 
   As a result, the friction force between the bore wall surfaces and the pistons  22  does not become excessively large, and deterioration in fuel economy can be prevented. Besides, since excess lubricating oil can be reliably scraped off by the piston rings, excessively large inflow of lubricating oil into the combustion chambers  41  can be prevented, and useless consumption of lubricating oil can be prevented. 
   Furthermore, according to the variable compression ratio internal combustion engine  10  of the first embodiment, the position, in the up-down direction, of the crankcase 30-side end of each bore wall surface (bore wall surface lower end) is the same as the position, in the up-down direction, of the crankcase 30-side end of the outer wall surface  20   c  of the cylinder block  20  from which the block-side bearing-forming portions  52  extend out (block outer wall surface lower end). Therefore, even if the bore wall surface-side portion of the crankcase 30-side end portion of the cylinder block  20  (block lower end portion) is bent inward with respect to the cylinder block  20  due to the pressing force F 3  being very large, the distance of the inward movement of the bore wall surface lower end with respect to the cylinder block  20  can be made shorter than in the case where the bore wall surface lower end is positioned at the crankcase  30  side of the block outer wall surface lower end. As a result, the degree of the deformation of the bore wall surface caused by the pressing force F 3  can be made small. 
   Besides, although in this embodiment, the position of the bore wall surface lower end in the up-down direction is the same as the position of the block outer wall surface lower end in the up-down direction, the bore wall surface lower end may also be positioned at the cylinder block side of (above) the block outer wall surface lower end. This construction also makes it possible to make small the degree of the deformation of the bore wall surface caused by the pressing force F 3 . 
   Modifications of the First Embodiment 
   Although in the first embodiment, the stress reduction groove portions are formed at positions apart from the cylinder liner  20   d , the stress reduction groove portions may instead be formed so as to be adjacent to the cylinder liner  20   d.    
   In this case, for example, as shown in  FIG. 8 , a diagram of the lower surface  20   b  of the cylinder block  20  viewed from below, and  FIG. 9 , a sectional view of the cylinder block  20  taken on a plane that contains IX-IX line of  FIG. 8  and is orthogonal to the cylinder arrangement direction, a lower end portion of the wall surface of each of the penetration holes of the cylinder block  20  has a wider portion  61  in which the distance L 1  measured from the bore center axes-arrangement plane P 1  that contains the bore center axes BC of all the cylinders to the wall surface defining the penetration hole in the direction orthogonal to the bore center axes-arrangement plane P 1  is longer than the distance L 2  in an upper end portion of the wall surface of the penetration hole which is measured in the same manner as the distance L 1 . 
   The wider portion  61  of each of the penetration holes of the cylinder block  20  can easily be formed by cutting the wall surface of each penetration holes. After penetration holes identical to the cylindrical penetration holes formed in the cylinder block  20  in the first embodiment have been formed in a cylinder block  20 , straight lines BC 1 , BC 1  are set which are parallel to the bore center axis BC and which are a predetermined distance apart from the bore center axis BC in the direction orthogonal to the bore center axes-arrangement plane P 1 , and the wall surface of the penetration hole is cut in such a manner as to form a cylindrical hole about each of the two straight lines BC 1 , BC 1  as a center axis. The wider portion  61  and the outer wall surface of the cylinder liner  20   d  of each cylinder form stress reduction groove portions  24 - 1 . That is, this construction facilitates the formation of the stress reduction groove portions  24 - 1 . 
   The stress reduction groove portions may also be provided so as to surround the cylinder liners  20   d . In this case, for example, as shown in  FIG. 10 , a diagram of the lower surface  20   b  of the cylinder block  20  viewed from below, a lower end portion of the wall surface that defines the penetration hole of each cylinder of the cylinder block  20  has a large-diameter portion  62  whose diameter is larger than the diameter of an upper end portion of the wall surface. 
   The large-diameter portion  62  of each cylinder can easily be formed in the following manner. After penetration holes identical to the cylindrical penetration holes formed in the cylinder block  20  in the first embodiment have been formed in a cylinder block  20 , the wall surface of each penetration hole is cut in such a manner as to form a cylindrical hole that is coaxial with the bore center axis BC and that is larger in diameter than the penetration hole. The large-diameter portion  62  and the outer wall surface of the cylinder liner  20   d  of each cylinder form a stress reduction groove portions  24 - 2 . This construction facilitates the formation of the stress reduction groove portion  24 - 2 . 
   Furthermore, the stress reduction groove portions in the first embodiment and its modifications may be filled with an elastic material such as rubber or the like. 
   Second Embodiment 
   Next, a variable compression ratio internal combustion engine in accordance with a second embodiment of the invention will be described. The variable compression ratio internal combustion engine in accordance with the second embodiment is different from the variable compression ratio internal combustion engine  10  in accordance with the first embodiment only in having a reinforcement member as a stress-reducing portion in place of the stress reduction groove portions  24 . The following description will be given mainly on this difference. 
   In the cylinder block  20  of the variable compression ratio internal combustion engine  10 , as shown in  FIG. 11 , a diagram of the lower surface  20   b  of the cylinder block  20  viewed from below, and  FIG. 12 , a sectional view of the cylinder block  20  taken on a plane that contains XII-XII line of  FIG. 11  and is orthogonal to the cylinder arrangement direction, a lower end portion of the wall surface of each of the penetration holes formed in the cylinder block  20  has a large-diameter portion  63  whose diameter is larger than the diameter of an upper portion of the wall surface. 
   In this embodiment, the length of the large-diameter portion  63  of each cylinder in the up-down direction (bore center axis direction) is substantially one third of the length of the block-side bearing-forming portions  52  in the up-down direction. However, the length of each large-diameter portion  63  in the up-down direction may also be a length ranging from about one quarter of the length of the block-side bearing-forming portions  52  in the up-down direction to the entire length of the block-side bearing-forming portions  52  in the up-down direction, and furthermore, may also be longer than the entire length of the block-side bearing-forming portions  52 . 
   As for a space defined by the large-diameter portions  63  and the outer wall surfaces of the cylinder liners  20   d , a reinforcement member  64  having the same shape as the space is pressed therein. That is, the reinforcement member  64  is disposed in the cylinder block  20  so as to extend through positions that are on the aforementioned pressing straight lines and that are between the block-side bearing-forming portions  52  and the cylinder liners  20   d  and so as to surround a periphery of the cylinder liners  20   d . Moreover, the reinforcement member  64  is made of a material that has a higher rigidity than a portion of the cylinder block  20  excluding the cylinder liners  20   d  (a portion thereof made of aluminum in this example). (The material of the reinforcement member  64  is steel in this example, but may also be cast iron or the like.) 
   According to the variable compression ratio internal combustion engine  10  in accordance with the second embodiment constructed as described above, when the block-side bearing-forming portions  52  press the outer wall surface  20   c  of the cylinder block  20  as described above, a stress having substantially the same magnitude as the aforementioned outer wall surface stress occurs in a portion of the cylinder block  20  that is on the outer wall surface  20   c  side of the reinforcement member  64  (outer wall surface-side portion of the cylinder block  20 ). At this time, since the rigidity of the reinforcement member  64  is higher than the rigidity of the cylinder block  20 , the rigidity of the reinforcement member  64  makes the stress transmitted to the portion of the cylinder block  20  on the bore wall surface side of the reinforcement member  64  (the bore wall surface-side portion, i.e., the cylinder liners  20   d ) smaller than the outer wall surface stress. Therefore, the bore wall surface stress becomes smaller than in the case where the reinforcement member  64  is not provided. As a result, the degree of the deformation of the bore wall surface caused by the bore wall surface stress can be made small. 
   Incidentally, the second embodiment may further include substantially the same stress reduction groove portions  24  as those provided in the first embodiment. 
   Third Embodiment 
   Next, a variable compression ratio internal combustion engine in accordance with a third embodiment of the invention will be described. The variable compression ratio internal combustion engine in accordance with the third embodiment is different from the variable compression ratio internal combustion engine  10  in accordance with the first embodiment only in that the stress reduction groove portions  24  are not formed and the cylinder block has a thick-walled portion and a thin-walled portion. The following description will be given mainly on these differences. 
   In the outer wall surface  20   c  of the cylinder block  20  of the variable compression ratio internal combustion engine  10  of this embodiment, as shown in  FIG. 13 , a diagram of the lower surface  20   b  of the cylinder block  20  viewed from below, and  FIG. 14 , a sectional view of the cylinder block  20  taken on a plane that contains XIV-XIV line of  FIG. 13  and is orthogonal to the cylinder arrangement direction, a plurality of protruded portions  65  that correspond to the individual cylinder bores  21  are formed. 
   A block-side bearing-forming portion  52 -side top surface of each of the protruded portions  65  is a plane that is parallel to portions of the outer wall surface  20   c  in which a protruded portion  65  is not formed. Each protruded portion  65  is formed so that the top surface thereof intersects with a plane that contains the bore center axis BC of a corresponding one of the cylinder bores  21  and that is orthogonal to the cylinder arrangement direction. Furthermore, each protruded portion  65  is positioned on a lower end portion of the outer wall surface  20   c . The top surface of each protruded portion  65  is constructed so that a block-side bearing-forming portion  52  can be fixed thereto. 
   That is, the block-side bearing-forming portions  52  extend out from the outer wall surface  20   c  (specifically, from the top surfaces of the protruded portions  65 ) of the cylinder block  20 . Furthermore, each block-side bearing-forming portion  52  is disposed in a region that contains a portion of a line of intersection CL between the outer wall surface  20   c  of the cylinder block  20  and a plane which contains the bore center axis BC of one of the cylinder bores  21  that corresponds to the block-side bearing-forming portion  52  and which is orthogonal to the cylinder arrangement direction. 
   Due to this construction, the distance D 1  between the outer wall surface  20   c  of the cylinder block  20  and the bore center axes-arrangement plane P 2  containing the cylinder arrangement direction and the bore center axis direction which is measured in a section of the cylinder block  20  taken on a plane that is orthogonal to the cylinder arrangement direction and that passes through a portion of the outer wall surface  20   c  in which a protruded portion  25  is not formed is shorter than the distance D 2  between the outer wall surface  20   c  of the cylinder block  20  and the bore center axes-arrangement plane P 2  which is measured in a section of the cylinder block  20  taken on a plane that is orthogonal to the cylinder arrangement direction and that passes through a portion of the outer wall surface  20   c  in which a protruded portions  25  is formed. 
   Furthermore, as shown n  FIG. 14 , this distance relationship also holds in the up-down direction of the cylinder block in a section of the cylinder block  20  taken on a plane that is orthogonal to the cylinder arrangement direction and that passes through a portion of the outer wall surface  20   c  in which a protruded portion  65  is formed. That is, in this section of the cylinder block  20 , the distance D 1  between the bore center axes-arrangement plane P 2  and a portion of the outer wall surface  20   c  in which a protruded portion  65  is not formed is shorter than the distance D 2  between the bore center axes-arrangement plane P 2  and a portion of the outer wall surface  20   c  in which a protruded portion  65  is formed. 
   According to the foregoing construction, the cylinder block  20  includes portions in which the distance between the bore center axes-arrangement plane P 2  and the outer wall surface  20   c  of the cylinder block  20  in a section of the cylinder block  20  taken on a plane orthogonal to the cylinder arrangement direction is shorter than the distance D 2  between the bore center axes-arrangement plane P 2  and the outer wall surface  20   c  at a position where a block-side bearing-forming portion  52  extends out (i.e., the top surface of a protruded portion  65 ) in a section of the cylinder block  20  taken on a plane that is orthogonal to the cylinder arrangement direction and that passes through block-side bearing-forming portions  52 . 
   According to the variable compression ratio internal combustion engine  10  in accordance with the third embodiment constructed as described above, the cylinder block  20  has higher rigidity in the portions where a block-side bearing-forming portion  52  extends out than in other portions. Therefore, even when the pressing force F 3  is exerted, the cylinder block  20  is unlikely to deform. That is, it becomes possible to make small the degree of the deformation of the bore wall surface caused by the pressing force F 3  while restraining the increase in the weight of the cylinder block  20 . 
   As described above, each of the variable compression ratio internal combustion engines according to the foregoing embodiments has a deformation-supressing structure. For example, the stress reduction groove portion, the structure of the cylinder block in which the position of the bore wall surface lower end is the same as the position of the block outer wall surface lower end, and the protruded portions formed on the outer wall surface of the cylinder block and corresponding to each cylinder bore may serve as the deformation-suppressing structure. 
   Incidentally, the invention is not limited to the foregoing embodiments or the like, but various modifications can be adopted within the scope of the invention. For example, the block-side bearing-forming portions  52  may also be formed integrally with the cylinder block  20 . Furthermore, the block-side bearing-forming portions  52  may instead be disposed in a portion of the outer wall surface  20   c  that is above the crankcase 30-side end (lower end) of the outer wall surface  20   c.