Engine

To reduce the number of parts, improve the mountability and maintainability, and allow for a reduction in the size and weight an engine is provided having a crankshaft, a crankcase having a plurality of bearing portions for rotatably supporting the crankshaft with each of the bearing portions being dividable along a divisional plane arranged on the axis of the crankshaft. Cylinder barrels are connected to the crankcase and include cylinder bores with cylinder heads connected to the cylinder barrels. The cylinder heads are integrated with portions of the cylinder barrels forming at least the cylinder bores to form cylinder blocks. The cylinder blocks and the crankcase are fastened together by a plurality of fastening bolts having axes parallel to the axes of the cylinder bores and extending through at least the cylinder blocks, with a compressive structure in the axial direction of the fastening bolts.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority under 35 USC 119 to Japanese Patent Application No. 2003-283027 filed on Jul. 30, 2003 the entire contents thereof is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention Field

The present invention relates to an engine, and more particularly to an engine having a crankshaft with a crankcase having a plurality of bearing portions for rotatably supporting the crankshaft, each of the bearing portions being dividable along a divisional plane arranged on the axis of the crankshaft. Cylinder barrels are connected to the crankcase with the cylinder barrels having cylinder bores with axes that are perpendicular to the divisional plane. The cylinder heads are connected to the cylinder barrels.

2. Description of Background Art

In a conventional engine disclosed in Japanese Patent Laid-Open No. 2002-213302, for example, a crankcase, cylinder barrels, and cylinder heads are formed as separate members. The cylinder barrels are fastened to the crankcase, and the cylinder heads are fastened to the cylinder barrels with gaskets interposed therebetween.

In the above conventional structure, the number of bolts for fastening the crankcase, the cylinder barrels, and the cylinder heads is large, and it is hard to say that the mountability and maintainability are excellent. Further, since the gaskets for high temperature and high pressure are interposed between the cylinder barrels and the cylinder heads, the fastening loads on the cylinder barrels and the cylinder heads must be set to a large value, so that the space and weight occupied by such fastening portions of the cylinder barrels and the cylinder heads become relatively large.

SUMMARY AND OBJECTS OF THE INVENTION

It is accordingly an object of the present invention to provide an engine which can reduce the number of parts, improve the mountability and maintainability, and further reduce the size and weight.

In accordance with the present invention, there is provided an engine having a crankshaft with a crankcase having a plurality of bearing portions for rotatably supporting said crankshaft. Each of the bearing portions is dividable along a divisional plane arranged on the axis of said crankshaft. Cylinder barrels are connected to said crankcase with the cylinder barrels having cylinder bores with axes that are perpendicular to the divisional plane.

The cylinder heads are connected to the cylinder barrels with the cylinder beads being integrated with portions of said cylinder barrels forming at least said cylinder bores to form cylinder blocks. The cylinder blocks and said crankcase are fastened together by a plurality of fastening bolts having axes parallel to the axes of said cylinder bores and extending through at least the cylinder blocks, with a compressive structure in the axial direction of said fastening bolts.

In accordance with the present invention, the engine is of a horizontally opposed type including a pair of left and right cylinder blocks and a pair of left and right case halves connected together to constitute the crankcase located between said left and right cylinder blocks. The left and right cylinder blocks have a plurality of left and right cylinder bores offset from each other in the axial direction of said crankshaft. The left and right case halves have a plurality of journal support walls cooperating with each other to form said bearing portions. Fastening bolts are located at positions corresponding to said journal support walls.

In accordance with the present invention, the plurality of left cylinder bores formed in said left cylinder block are offset from said plurality of right cylinder bores formed in said right cylinder block in the axial direction of said crankshaft by a given amount equal to ½ of the pitch of said left cylinder bores or said right cylinder bores.

According to the present invention, the cylinder heads are integrated with portions of the cylinder barrels forming at least the cylinder bores to form the cylinder blocks. The cylinder blocks and the crankcase are fastened together under compression by the plurality of fastening bolts extending through at least the cylinder blocks. Accordingly, the machinability of the cylinder bores can be improved by forming the cylinder barrels and the crankcase as separate members. Further, any gasket for high temperature and high pressure that are conventionally required between each cylinder barrel and the corresponding cylinder head for the purpose of sealing between each combustion chamber and the outside can be eliminated. Accordingly, the number of bolts for fastening the crankcase, the cylinder barrels, and the cylinder heads can be reduced to thereby improve the mountability and maintainability.

Further, since the above-mentioned gasket is not provided, axial tensions of the fastening bolts can be controlled stably. In addition, it is sufficient for the fastening bolts to have a strength withstanding a tensile stress due to the combustion in the combustion chambers. Accordingly, the fastening bolts can be relatively reduced in number, and the diameter of each fastening bolt can be set to a relatively small value. Further, any fastening portion conventionally required between each cylinder barrel and the corresponding cylinder head can be eliminated, thereby obtaining an advantage in size and a reduction in the weight of the engine.

According to the present invention, size enlargement of the engine due to the location of the fastening bolts can be avoided, and the journal support walls of the left and right case halves can be firmly coupled together to support the crankshaft.

According to the present invention, the coordinates of the fastening bolts in the left cylinder block can be set equal to those of the fastening bolts in the right cylinder block, and cylinder blocks having the same shape can be located on the left and right sides of the crankcase.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 1 to 9show a first preferred embodiment of the present invention wherein it is applied to a four-cycle, horizontally opposed, four-cylinder engine.

Referring first toFIGS. 1 to 3, the four-cycle, horizontally opposed, four-cylinder engine is adapted to be mounted on an aircraft in such a manner that the engine is accommodated in a front cowl of the body of the aircraft and the axis of a crankshaft11extends in the longitudinal direction of the body of the aircraft. A spinner having a plurality of propeller blades is coaxially connected to the crankshaft11.

Referring also toFIG. 4, the engine has an engine body12. The engine body12is composed of a crankcase14, a left cylinder block15L arranged on the left side of the crankcase14, and a right cylinder block15R arranged on the right side of the crankcase14. The crankcase14is composed of a left case half13L and a right case half13R connected together. The left case half13L and the right case half13R are arranged on the left and right sides, respectively, as viewed from the rear side of the engine.

The left cylinder block15L is composed of a left cylinder barrel16L and a left cylinder head17L integrated with each other. Similarly, the right cylinder block15R is composed of a right cylinder barrel16R and a right cylinder head17R integrated with each other.

Referring also toFIGS. 5 and 6, the cylinder barrel16L of the left cylinder block15L includes two cylinder bores18L arranged in the axial direction of the crankshaft11. Similarly, the cylinder barrel16R of the right cylinder block15R includes two cylinder bores18R arranged in the axial direction of the crankshaft11. The cylinder bores18L are opposed to the cylinder bores18R with the crankshaft11interposed therebetween. The cylinder bores18L are offset from the cylinder bores18R in the axial direction of the crankshaft11. A piston20L is slidably fitted with each cylinder bore18L, and a combustion chamber19L is defined between each piston20L and the cylinder head17L. Similarly, a piston20R is slidably fitted with each cylinder bore18R, and a combustion chamber19R is defined between each piston20R and the cylinder head17R.

The left and right cylinder blocks15L and15R are opposed to each other in such a manner that the axes of all the cylinder bores18L and18R extend substantially horizontally. The crankshaft11is connected through connecting rods22L and22R to the pistons20L and20R, and is rotatably supported to the crankcase14.

The left case half13L of the crankcase14is formed with a front journal support wall23L, first intermediate journal support wall24L, second intermediate journal support wall25L, third intermediate journal support wall26L, and rear journal support wall27L for supporting a left half portion of the crankshaft11on the front and rear sides of the connecting rods22L. These journal support walls23L to27L are spaced from each other in the axial direction of the crankshaft11. Similarly, the right case half13R of the crankcase14is formed with a front journal support wall23R, first intermediate journal support wall24R, second intermediate journal support wall25R, third intermediate journal support wall26R, and rear journal support wall27R for supporting a right half portion of the crankshaft11on the front and rear sides of the connecting rods22R. The journal support walls23R to27R are spaced from each other in the axial direction of the crankshaft11.

Referring also toFIGS. 7 and 8, the journal support walls23L to27L of the left case half13L are fastened to the journal support walls23R to27R of the right case half13R, respectively, by a plurality of upper and lower pairs of stud bolts28and nuts29arranged so that the crankshaft11is interposed between each pair of stud bolts28and nuts29, thereby configuring a front bearing portion30, first intermediate bearing portion31, second intermediate bearing portion32, third intermediate bearing portion33, and rear bearing portion34. Each of the bearing portions30to34is dividable along a divisional plane perpendicular to the axes of the cylinder bores18L and18R of the left and right cylinder blocks15L and15R and arranged on the axis of the crankshaft11.

The stud bolts28for fastening the front journal support walls23L and23R and the rear journal support walls27L and27R are longer than the stud bolts28for fastening the first intermediate journal support walls24L and24R, the second intermediate journal support walls25L and25R, and the third intermediate journal support walls26L and26R.

The stud bolts28for fastening the front journal support walls23L and23R are implanted into the front journal support wall23R of the right case half13R and inserted through the front journal support wall23L of the left case half13L. The nuts29are threadedly engaged with the stud bolts28so as to abut against the outer surface of the left case half13L. Similarly, the stud bolts28for fastening the rear journal support walls27L and27R are implanted into the rear journal support wall27L of the left case half13L and inserted through the rear journal support wall27R of the right case half13R. The nuts29are threadedly engaged with the stud bolts28so as to abut against the outer surface of the right case half13R.

The stud bolts28for fastening the second intermediate journal support walls25L and25R and the third intermediate journal support walls26L and26R are implanted into the second and third intermediate journal support walls25R and26R of the right case half13R and inserted through the second and third intermediate journal support walls25L and26L of the left case half13L. The nuts29are threadedly engaged with the stud bolts28so as to abut against the outer surfaces of the journal support walls25L and26L. Similarly, the stud bolts28for fastening the first intermediate journal support walls24L and24R are implanted into the first intermediate journal support wall24L of the left case half13L and inserted through the first intermediate journal support wall24R of the right case half13R. The nuts29are threadedly engaged with the stud bolts28so as to abut against the outer surface of the journal support wall24R.

The crankcase14and the left and right cylinder blocks15L and15R are connected together by a plurality of fastening bolts35and36having axes parallel to the axes of the cylinder bores18L and18R and extending through at least the cylinder blocks15L and15R, with a compressive structure in the axial direction of the fastening bolts35and36. Further, an oil ring21L is interposed between the crankcase14and the left cylinder block15L. In addition, an oil ring21R is interposed between the crankcase14and the right cylinder block15R. These oil rings21L and21R are engaged with the crankcase14.

The fastening bolts35are three pairs of upper and lower through bolts located at positions corresponding to the first intermediate journal support walls24L and24R, the second intermediate journal support walls25L and25R, and the third intermediate journal support walls26L and26R. The fastening bolts35extend through the crankcase14and the left and right cylinder blocks15L and15R in such a manner that the stud bolts28for fastening the first intermediate journal support walls24L and24R, the second intermediate journal support walls25L and25R, and the third intermediate journal support walls26L and26R are interposed between the fastening bolts35and the crankshaft11.

Nuts37are threadedly engaged with the opposite end portions of the fastening bolts35projecting from the cylinder heads17L and17R of the left and right cylinder blocks15L and15R. Further, the opposite ends of each fastening bolt35are provided with hexagonal tool engaging portions35afor engaging a tool (not shown), so as to prevent rotation of each fastening bolt35in tightening the corresponding nut37. Each tool engaging portion is provided coaxially with each fastening bolt35.

The other fastening bolts36are two pairs of upper and lower stud bolts. One of the two pairs of fastening bolts36are implanted into the front journal support wall23L of the left case half13L and inserted through the right cylinder block15R, and nuts39are threadedly engaged with the fastening bolts36projecting from the cylinder head17R of the right cylinder block15R. Similarly, the other pair of fastening bolts36are implanted into the rear journal support wall27R of the right case half13R and inserted through the left cylinder block15L. The nuts39are threadedly engaged with the fastening bolts36projecting from the cylinder head17L of the left cylinder block15L.

The two pairs of fastening bolts36are located in such a manner that the stud bolts28for fastening the front journal support walls23L and23R of the left and right case halves13L and13R and the rear journal support walls27L and27R of the left and right case halves13L and13R are interposed between the fastening bolts36and the crankshaft11.

As shown inFIG. 7, the fastening bolts35and36are arranged at equal intervals on each of the upper and lower sides of the cylinder bores18L and18R in a direction parallel to the axis of the crankshaft11in such a manner that four of the fastening bolts35and36surround each of the cylinder bores18L and18R. The upper and lower sides of the cylinder blocks15L and15R are integrally formed with a plurality of mounting bosses40extending from mount surfaces of the cylinder barrels16L and16R to the crankcase14to the cylinder heads17L and17R. The fastening bolts35and36are inserted through the mounting bosses40.

The outer end of each mounting boss40is formed as a flat bearing surface38against which each of the nuts37and39abuts. These bearing surfaces38are located outside of the combustion chambers19L and19R.

The pitch between the two cylinder bores18L formed in the left cylinder block15L is equal to the pitch L1between the two cylinder bores18R formed in the right cylinder block15R. The cylinder bores18L in the left cylinder block15L are offset rearwardly from the cylinder bores18R in the right cylinder block15R in the axial direction of the crankshaft11by a given amount L2equal to ½ of the pitch L1.

The pitch L3of the fastening bolts35and36in the axial direction of the crankshaft11is smaller than the pitch L4of the fastening bolts35and36in a vertical direction perpendicular to the axial direction of the crankshaft11. That is, L3<L4.

The left and right cylinder blocks15L and15R can be preliminarily fastened to the crankcase14by a plurality of temporary bolts41parallel to the fastening bolts35and36. That is, after fastening the left and right case halves13L and13R by using the stud bolts28and the nuts29to assemble the crankcase14, the left and right cylinder blocks15L and15R are temporarily assembled to the crankcase14by using the temporary bolts41. In this condition, the fastening bolts35and36are inserted through the cylinder blocks15L and15R and the crankcase14. Accordingly, disengagement, from the crankcase14, of the oil ring21L (interposed between the crankcase14and the left cylinder block15L) and the oil ring21R (interposed between the crankcase14and the right cylinder block15R) can be prevented. Meanwhile, the insertion of the fastening bolts35and36can be easily performed, thereby contributing to an improvement in mountability and maintainability.

A support cylinder45is formed at the front end of the crankcase14so as to project forwardwardly. The support cylinder45is configured by the left and right case halves13L and13R in cooperation. The front end of the crankshaft11extends coaxially through the support cylinder45and projects from the front end of the support cylinder45. A ring gear46is fixed to the front end of the crankshaft11projecting from the front end of the support cylinder45with a spinner (not shown) that is coaxially mounted on the ring gear46. A sliding bearing47is interposed between the support cylinder45and the crankshaft11at their front portions. Further, an annular sealing member (not shown) is interposed between the support cylinder45and the crankshaft11at a position on the front side of the sliding bearing47.

A starting device48is provided to give a rotational drive force to the crankshaft11at the start of the engine. The starting device48has a known structure that includes a starter motor49supported on a lower portion of the left case half13L of the crankcase14and a pinion50adapted to project into mesh with the ring gear46when the rotational speed of the starter motor49becomes greater than or equal to a predetermined value. After starting the engine, the pinion50is separated from the ring gear46and returned to an original position.

A plurality of projections51are formed on the crankshaft11in the support cylinder45so as to be equally spaced in the circumferential direction of the crankshaft11. A pair of crank angle sensors52for detecting a crank angle by using the projections51are mounted in the support cylinder45so as to provide a 180° phase difference.

The left cylinder head17L is formed at its upper portion with two intake ports55L, respectively, corresponding to the two combustion chambers19L. Similarly, the right cylinder head17R is formed at its upper portion with two intake ports55R, respectively, corresponding to the two combustion chambers19R. Each intake port55L is bifurcated to communicate with the corresponding combustion chamber19L, and each intake port55R is also bifurcated to communicate with the corresponding combustion chamber19R.

Arcuately curved intake pipes56L and56R are connected to the intake ports55L and55R, respectively. Electromagnetic fuel injection valves57L and57R for injecting fuel toward the intake ports55L and55R are mounted at intermediate portions of the intake pipes56L and56R, respectively. The fuel injection valves57L on the left cylinder block15L side are connected to a common fuel rail58L. The fuel injection valves57R on the right cylinder block15R side are connected to a common fuel rail58R.

An intake chamber59is located above the crankcase14of the engine body12so as to be supported to the engine body12. The upstream ends of the intake pipes56L and56R are connected to the downstream ends of connection pipes60L and60R, respectively. The upstream end portions of the connection pipes60L and60R are inserted in the intake chamber59from the opposite sides thereof, and are curved to be divergent and open to the rear side.

A pair of right and left throttle bodies62each including a throttle valve61rotatably supported to a lateral shaft are connected at their downstream ends to the rear end of the intake chamber59. Air cleaners63are connected to the upstream ends of the throttle bodies62, respectively. The air cleaners63are supported by support stays64mounted to the intake chamber59and extending rearward.

The left cylinder head17L is formed at its lower portion with two exhaust ports65L, respectively, corresponding to the two combustion chambers19L. Similarly, the right cylinder bead17R is formed at its lower portion with two exhaust ports65R, respectively, corresponding to the two combustion chambers19R. Each exhaust port65L is bifurcated, and each exhaust port65R is also bifurcated. Exhaust pipes66L and66R are connected to the exhaust ports65L and65R, respectively. The exhaust pipes66L and66R extend downwardly to the lower side of the engine body12and further extend rearwardly.

Substantially H-shaped head covers67L and67R are connected to the left and right cylinder heads17L and17R, respectively. A valve train68L for driving intake valves and exhaust valves related to the combustion chambers19L is provided between the bead cover67L and the cylinder head17L. Similarly, a valve train68R for driving intake valves and exhaust valves related to the combustion chambers19R is provided between the head cover67R and the cylinder head17R. The intake valves and the exhaust valves related to the combustion chambers19L control the intake of air or air/fuel mixture into the combustion chambers19L and the exhaust of burned gases from the combustion chambers19L, respectively. The same as described above also applies to the intake valves and the exhaust valves of the combustion chambers19R.

A valve opening force for the intake valves to be driven by the valve train68L is obtained by a push rod adapted to be pushed up in an intake stroke by the power transmitted from the crankshaft11. The push rod is provided per cylinder, i.e., for each combustion chamber19L and is axially movably inserted in a push rod guide pipe69L located below the left cylinder block15L. The push rod guide pipe69L extends between a longitudinally central position of a lower portion of the left case half13L and the head cover67L. The same as described above also applies to the valve train68R side.

A valve opening force for the exhaust valves to be driven by the valve train68L is obtained by a pull rod adapted to be pulled down in an exhaust stroke by the power transmitted from the crankshaft11. The pull rod is provided per cylinder, i.e., for each combustion chamber19L and is axially movably inserted in a pull rod guide pipe70L located below the push rod guide pipe69L. The pull rod guide pipe70L also extends between the longitudinally central position of the lower portion of the left case half13L and the head cover67L. The same as described above also applies to the valve train68R side.

A pair of spark plugs71L are mounted for each combustion chamber19L in the left cylinder head17L. A pair of spark plugs71R are mounted for each combustion chamber19R in the right cylinder head17R. A pair of ignition coils72L are mounted on the upper surface of the left cylinder head17L at a position between the intake pipes56L. In addition, a pair of ignition coils72R are mounted on the upper surface of the right cylinder head17R at a position between the intake pipes56R. These pairs of ignition coils72L and72R are located on the opposite sides of the intake chamber59.

An electronic control unit73for controlling the operation of the engine is mounted on the outer surface of the front wall of the intake chamber59. An intake air pressure sensor74for detecting the intake air pressure in the intake chamber59and an intake air temperature sensor75for detecting the intake air temperature in the intake chamber59project from the electronic control unit73through the front wall of the intake chamber59into the intake chamber59.

The electromagnetic fuel injection valves57L and57R, the ignition coils72L and72R, and the electronic control unit73are arranged around the intake chamber59and are covered with a shield cover76mounted on the engine body12so as to cover at least a part of the intake chamber59.

The operation of the first preferred embodiment mentioned above will now be described. The cylinder barrel16L and the cylinder head17L are integrated with each other to form the cylinder block15L. Similarly, the cylinder barrel16R and the cylinder head17R are integrated with each other to form the cylinder block15R. The cylinder blocks15L and15R and the crankcase14are fastened together by the plural fastening bolts35and36having axes parallel to the axes of the cylinder bores18L and18R, respectively, formed in the cylinder blocks15L and15R and extending through at least the cylinder blocks15L and15R, with a compressive structure in the axial direction of these fastening bolts35and36.

Accordingly, the bolts35and36for fastening the crankcase14, the cylinder barrels16L and16R, and the cylinder heads17L and17R can be reduced in number to thereby improve the mountability and maintainability. Further, any gaskets for high temperature and high pressure that are conventionally required between the cylinder barrel16L and the cylinder head17L and between the cylinder barrel16R and the cylinder head17R can be eliminated, so that axial tensions of the fastening bolts35and36can be controlled. Further, it is sufficient for the fastening bolts35and36to have a strength withstanding a tensile stress due to the combustion in the combustion chambers19L and19R. Accordingly, the fastening bolts35and36can be relatively reduced in number, and the diameter of each of the fastening bolts35and36can be set to a relatively small value. Further, any fastening portions conventionally required between the cylinder barrel16L and the cylinder head17L and between the cylinder barrel16R and the cylinder head17R can be eliminated, thereby obtaining an advantage in size and a reduction in the weight of the engine.

The engine is of a horizontally opposed type including the pair of left and right cylinder blocks15L and15R and the pair of left and right case halves13L and13R constituting the crankcase14located between the cylinder blocks15L and15R. The left cylinder block15L includes the cylinder bores18L, and the right cylinder block15R includes the cylinder bores18R offset from the cylinder bores18L in the axial direction of the crankshaft11. The left case half13L includes the plural journal support walls23L to27L, and the right case half13R includes the plural journal support walls23R to27R cooperating with the plural journal support walls23L to27L to form the bearing portions30to34for rotatably supporting the crankshaft11. The fastening bolts35and36are located at positions corresponding to the journal support walls23L to27L and23R to27R, so that the size enlargement of the engine due to the location of the fastening bolts35and36can be avoided. Meanwhile, the journal support walls23L to27L and23R to27R of the left and right case halves13L and13R can be firmly coupled together to support the crankshaft11.

Further, the plural cylinder bores18L formed in the left cylinder block15L are offset rearward from the plural cylinder bores18R formed in the right cylinder block15R in the axial direction of the crankshaft11by a given amount equal to ½ of the pitch L1of the cylinder bores18L (or the cylinder bores18R). With this arrangement, the coordinates of the fastening bolts35and36in the left cylinder block15L can be set equal to those of the fastening bolts35and36in the right cylinder block15R, and the cylinder blocks15L and15R having the same shape can be located on the left and right sides of the crankcase14.

Further, the plural pairs of upper and lower fastening bolts35and upper and lower fastening bolts36are located so as to respectively correspond to the journal support walls23L to27L and23R to27R. Accordingly, the fastening forces between the journal support walls23L to27L and23R to27R by the fastening bolts35and36can be made uniform, and the crankshaft11can be reduced in weight.

To increase the roundness of the cylinder bores18L and18R after assembling the engine body12by using the fastening bolts35and36, it is preferable to apply the same load as the fastening load by the fastening bolts35and36to each bearing surface38in polishing the inner surfaces of the cylinder bores18L and18R, such as by honing. In this case, when the load applied to each bearing surface38is removed after polishing the inner surfaces of the cylinder bores18L and18R, the cylinder bores18L and18R are minutely deformed so as to increase their inner diameters. On the other hand, the pistons20L and20R have skirt portions arranged in a direction perpendicular to the axial direction of the crankshaft11and slidably fitted with the cylinder bores18L and18R. Each skirt portion is machined so as to have an outer shape along an elliptical shape having a major axis extending in a direction perpendicular to the axial direction of the crankshaft11. If the cross-sectional shape of each of the cylinder bores18L and18R does not correspond to the outer shape of each skirt portion, it is difficult to fit the pistons20L and20R into the cylinder bores18L and18R.

As mentioned above, the fastening bolts35and36are arranged at equal intervals on each of the upper and lower sides of the cylinder bores18L and18R in a direction parallel to the axis of the crankshaft11in such a manner that four of the fastening bolts35and36surround each of the cylinder bores18L and18R. On the other hand, as shown inFIG. 9, four compressive load application points P around each of the cylinder bores18L and18R in working the inner surfaces of the cylinder bores18L and18R are present at the same positions as those of the fastening bolts35and36. Furthermore, the pitch L3of the fastening bolts35and36in the axial direction of the crankshaft11is set smaller than the pitch L4of the fastening bolts35and36in a vertical direction perpendicular to the axial direction of the crankshaft11. Therefore, the pitch L3of the compressive load application points P in the axial direction of the crankshaft11is smaller than the pitch L4of the compressive load application points P in the vertical direction perpendicular to the axial direction of the crankshaft11.

Accordingly, in working the inner surfaces of the cylinder bores18L and18R in the condition where loads are applied to the points P, a stress acting to reduce the inner diameter of each of the cylinder bores18L and18R in the vertical direction perpendicular to the axial direction of the crankshaft11becomes larger than that acting to reduce the inner diameter of each of the cylinder bores18L and18R in the axial direction of the crankshaft11. When the load applied to each bearing surface38is removed after working the inner surfaces of the cylinder bores18L and18R as shown by a solid line inFIG. 9, the cylinder bores18L and18R are minutely deformed so that their inner diameters are increased so as to form an elliptical shape having a major axis extending in the vertical direction perpendicular to the axial direction of the crankshaft11as shown by a phantom line inFIG. 9. Thus, such an elliptical cross-sectional shape of each of the cylinder bores18L and18R corresponds to the outer shape of the skirt portion of each of the pistons20L and20R, so that the pistons20L and20R can be easily fitted into the cylinder bores18L and18R.

InFIG. 9, the cross-sectional shape of the cylinder bores18L and18R that are deformed as shown by the phantom line is exaggerated for purposes of illustration.

FIG. 10shows a second preferred embodiment of the present invention, in which the same parts as those in the first preferred embodiment are denoted by the same reference characters.

The engine according to the second preferred embodiment has an engine body12′. The engine body12′ is composed of a crankcase14and left and right cylinder blocks15L′ and15R′, respectively, arranged on the left and right sides of the crankcase14.

The left cylinder block15L′ is composed of a left cylinder barrel16L′ and a left cylinder head17L. Similarly, the right cylinder block15R′ is composed of a right cylinder barrel16R′ and a right cylinder head17R.

The left cylinder barrel16L′ is composed of a cylinder forming member80L forming cylinder bores18L and an outer shell81L surrounding the cylinder forming member80L so as to define a cooling water jacket82L therebetween. The outer shell81L is held between the cylinder head17L and the crankcase14. Similarly, the right cylinder barrel16R′ is composed of a cylinder forming member80R forming cylinder bores18R and an outer shell81R surrounding the cylinder forming member80R so as to define a cooling water jacket82R therebetween. The outer shell81R is held between the cylinder head17R and the crankcase14.

The left cylinder head17L is integrated with the portion of the cylinder barrel16L′ where the cylinder bores18L are formed, that is, integrated with the cylinder forming member80L. Similarly, the right cylinder head17R is integrated with the portion of the cylinder barrel16R′ where the cylinder bores18R are formed, that is, integrated with the cylinder forming member80R.

A seal member83L and a pair of seal members84L for sealing the cooling water jacket82L are interposed between the cylinder forming member80L and the outer shell81L. Similarly, a seal member83R and a pair of seal members84R for sealing the cooling water jacket82R are interposed between the cylinder forming member80R and the outer shell81R. Further, no gasket is required between the cylinder barrel16L′ and the cylinder head17L because no leakage from the combustion chambers19L occurs. Similarly, no gasket is required between the cylinder barrel16R′ and the cylinder head17R because no leakage from the combustion chambers19R occurs.

The crankcase14and the cylinder blocks15L′ and15R′ are fastened together by a plurality of fastening bolts35and36(see the first preferred embodiment) having axes parallel to the axes of the cylinder bores18L and18R and extending through at least the cylinder blocks15L′ and15R′, with a compressive structure in the axial direction of the fastening bolts35and36. The fastening bolts35and36extend through the outer shells81L and81R of the cylinder barrels16L′ and16R′.

According to the second preferred embodiment, effects similar to those of the first preferred embodiment can be exhibited. In addition, no compressive loads are applied to the portions of the cylinder barrels16L′ and16R′ for forming the cylinder bores18L and18R, that is, to the cylinder forming members80L and80R. Accordingly, the distortion of the cylinder bores18L and18R due to the application of compressive loads can be prevented. Further, the cylinder forming members80L and80R and the outer shells81L and81R can be easily formed by casting, and the cooling water jackets82L and82R can be easily formed in the cylinder barrels16L′ and16R′.

Having thus described specific preferred embodiments of the present invention, it should be noted that the present invention is not limited to the above preferred embodiments, but various modifications may be made without departing from the scope of the present invention.

For example, each fastening bolt36is a stud bolt having one end implanted in the crankcase14in the above preferred embodiments. As a modification, each fastening bolt36may be a through bolt having one end projecting from the crankcase14.