Cylinder block integrated with crankcase

1. A cylinder block integrated with a crankcase used for a single-cylinder small internal combustion engine, a cylinder part and a crankcase part formed into a box-like shape being cast in one piece, a crankcase cover being fixed to the crankcase part to cover an opening on a side in an anteroposterior direction thereof, a crankshaft bearing boss having a crankshaft bearing hole being provided near at a central part of a back wall of said crankcase part to protrude to both inner and outer sides of said back wall for supporting an end sides of a crankshaft, wherein an inner peripheral rib is formed along the corner where said back wall of said crankcase part connects to a peripheral wall extending from the periphery of said back wall and extends from the periphery of said back wall to form said box-like shaped crankcase part, and wherein said inner peripheral rib heaves inward from said corner part.

2. A cylinder block integrated with a crankcase used for a single-cylinder small internal combustion engine, comprising a cylinder part and a crankcase part formed into a box-like shape being cast in one piece, a crankcase cover being fixed to the box-like shaped crankcase part, a crankshaft bearing boss having a crankshaft bearing hole being provided near at a central part of a back wall of said crankcase part to protrude to both inner and outer sides of said back wall for supporting an end sides of a crankshaft,

wherein an inner peripheral rib is formed along the line of intersection of said back wall which is thin in thickness of said crankcase part with a peripheral wall which is thin in thickness and extending from the periphery of said back wall to form said box-like shaped crankcase part at least along a part of said line of intersection positioned opposite to said cylinder parts, said inner peripheral rib heaving inward from said peripheral wall.

3. The cylinder block integrated with the crankcase according to claim 2 , wherein truss structures of near triangular shape are formed with the crankshaft bearing boss, a plurality of the ribs extending radially from the crankshaft bearing boss, and the inner peripheral rib on the inner face of the back wall around the crankshaft bearing boss at least in a region opposite to the cylinder part.

4. The cylinder block integrated with the crankcase according to claim 2 , wherein said inner peripheral rib is provided all along the inner peripheral part where said peripheral wall connects to the back wall.

5. The cylinder block integrated with the crankcase according to claim 2 , wherein said peripheral wall is formed of a bottom wall, a left wall, a ceiling wall, upper left wall where said cylinder part connects, and a lower right wall connecting to said bottom wall, and an inner peripheral rib is provided to heave inwardly at least from said bottom, left, and ceiling walls.

6. The cylinder block integrated with the crankcase according to claim 2 , wherein width of said inner peripheral rib in the direction parallel to said peripheral wall is larger than the sum of height of said rib from said peripheral wall and thickness of said peripheral wall.

7. The cylinder block integrated with the crankcase according to claim 6 , wherein said width of said inner peripheral rib is about three times the thickness of said peripheral wall and the sum of height of said rib from said peripheral wall and thickness of said peripheral wall is about two times the thickness of said peripheral wall.

8. The cylinder block integrated with the crankcase according to claim 2 , wherein a rib extending obliquely downward and two ribs extending obliquely upward along a line parallel to the cylinder center axis are provided to said back wall on its outer face, and a rib extending obliquely downward and two ribs extending obliquely upward along a line parallel to the cylinder center axis are provided to said crankcase cover on its inner face.

9. The cylinder block integrated with the crankcase according to claim 1 , wherein a rib is provided between a base part provided in the lower part of said crankcase part to extend horizontally and each of said left and lower right walls at about the center of depth of each of said walls, a horizontal rib is provided on said left wall on its outer face, and two vertical ribs are provided on said left wall on its outer face to extend vertically from the upper surface of said horizontal rib parallel to each other, of which one nearer to said back wall being extended to said ceiling wall where there are provided two ribs extending perpendicular to the vertical rib nearer to said back wall to connect to the vertical rib.