Engine

1. An engine comprising:

an output shaft of the engine;

a cylinder block including a cylinder portion in which a cylinder is formed, and a plurality of output shaft supporting portions each having a portion that supports the output shaft at a location closer to the output shaft with respect to the cylinder portion, the cylinder block being disposed in an upper side of the output shaft in a cylinder axial direction;

a cylinder head attached to an upper portion of the cylinder block in the cylinder axial direction;

cap portions each attached to a lower portion of one of the plurality output shaft supporting portions in the cylinder axial direction and having a portion that supports the output shaft; and

head bolts for fastening together the cylinder block, the cylinder head, and the cap portions,

wherein the cylinder block has two head bolt holes each provided at a location, in an output shaft axial direction in which the output shaft extends, that corresponds to one of the plurality of output shaft supporting portions and penetrating, in the cylinder axial direction, a portion in a radially outer side of the output shaft as viewed along the output shaft axial direction, the head bolts being inserted in the head bolt holes, and

when at least one of the plurality of output shaft supporting portions is viewed along the output shaft axial direction, an outer wall face of the at least one output shaft supporting portion includes a first rib diagonally extending from a supporting portion of the output shaft toward one of the two head bolt holes, and

the cap portion includes, as viewed along the output shaft axial direction, two threaded holes penetrating, in the cylinder axial direction, the portions in the radially outer sides of the output shaft, the head bolt being screwed in the threaded hole, and

when the cap portion is viewed along the output shaft axial direction, an outer wall face of the cap portion includes a third rib diagonally extending from the supporting portion of the output shaft toward one of the two threaded holes.

2. The engine according to claim 1 , wherein

when the at least one output shaft supporting portion is viewed along the output shaft axial direction, the outer wall face of the at least one output shaft supporting portion includes two head bolt hole bases each having a form of a circular-column,

each of the two head bolt holes is provided in an inner portion of the head bolt hole base, and

the first rib is joined to the head bolt hole base at a point located lower than a mating face, which contacts the cylinder head, of the cylinder block in the cylinder axial direction.

3. The engine according to claim 2 , wherein

when the at least one output shaft supporting portion is viewed along the output shaft axial direction, the outer wall face of the at least one output shaft supporting portion includes a second rib diagonally extending from the supporting portion of the output shaft toward another one of the two head bolt holes.

4. The engine according to claim 3 , wherein

when the output shaft supporting portion is viewed along the output shaft axial direction, a distance between the first rib and the second rib gradually increases from a lower side toward an upper side along the cylinder axial direction.

5. The engine according to claim 4 , wherein

when the at least one output shaft supporting portion is viewed along the output shaft axial direction, the first rib and the second rib are in a line-symmetric relationship about a central axis of the cylinder.

6. The engine according to claim 3 , wherein

when the at least one output shaft supporting portion is viewed along the output shaft axial direction, the first rib and the second rib are in a line-symmetric relationship about a central axis of the cylinder.

7. The engine according to claim 1 , wherein

the at least one output shaft supporting portion among the plurality of the output shaft supporting portions is an output shaft supporting end portion disposed at one or both ends in the output shaft axial direction.

8. The engine according to claim 1 , wherein

when the cap portion is viewed along the output shaft axial direction, the outer wall face of the cap portion includes a fourth rib diagonally extending from the supporting portion of the output shaft toward another one of the two threaded holes.

9. The engine according to claim 1 , wherein

the cylinder block further includes a cylinder block outer wall surrounding the cylinder portion, the output shaft supporting portion, and the cap portions,

the cylinder portion and the output shaft supporting portion are integrally formed using a metal material, and

the cylinder block outer wall is formed using a resin material.

10. The engine according to claim 1 , wherein

when the at least one output shaft supporting portion is viewed along the output shaft axial direction, the outer wall face of the at least one output shaft supporting portion includes a second rib diagonally extending from the supporting portion of the output shaft toward another one of the two head bolt holes.

11. The engine according to claim 10 , wherein

when the output shaft supporting portion is viewed along the output shaft axial direction, a distance between the first rib and the second rib gradually increases from a lower side toward an upper side along the cylinder axial direction.

12. The engine according to claim 11 , wherein

when the at least one output shaft supporting portion is viewed along the output shaft axial direction, the first rib and the second rib are in a line-symmetric relationship about a central axis of the cylinder.

13. The engine according to claim 10 , wherein

when the at least one output shaft supporting portion is viewed along the output shaft axial direction, the first rib and the second rib are in a line-symmetric relationship about a central axis of the cylinder.

14. The engine according to claim 1 , wherein

the third rib extends, on the outer wall face of the cap portion in the output shaft axial direction, in a direction oblique to an intake and exhaust direction of the engine and the cylinder axial direction from an outer rim of the supporting portion of the output shaft.

15. The engine according to claim 14 , wherein

the third rib comprises a semi-cylindrical rib protruding in the output shaft axial direction.

16. The engine according to claim 14 , wherein

when the cap portion is viewed along the output shaft axial direction, the outer wall face of the cap portion includes a fourth rib diagonally extending from the supporting portion of the output shaft toward another one of the two threaded holes, and

an aperture is provided between the third rib and the fourth rib in the intake and exhaust direction of the engine.

17. The engine according to claim 16 , wherein

a rib central axis of the fourth rib and an axis of the head bolt hole intersect each other at a point near a lower end portion of the cap portion in the cylinder axial direction.

18. The engine according to claim 14 , wherein

a rib central axis of the third rib and an axis of the head bolt hole intersect each other at a point near a lower end portion of the cap portion in the cylinder axial direction.