1. Field of the Invention
The present invention relates to a piston connected with a connecting rod by means of a piston pin with the piston being reciprocally moved in a cylinder.
2. Description of Background Art
A piston crank mechanism of an internal combustion engine is generally arranged as follows. Mutually opposed piston pin bosses project from a back side of a piston head of the piston that is reciprocally moved within the cylinder. The piston pin is inserted through coaxial pin holes of the piston pin bosses. A smaller end of the connecting rod is coupled to a central portion of the piston pin supported on the opposite ends thereof.
As seen from the above, the piston has a pair of mutually opposed piston pin bosses projecting from the back side of the piston head. When a blast load due to combustion in the operating internal combustion engine is exerted on a top surface of the piston, a piston with a short piston skirt portions, in particular, tends to suffer deformation of the piston head such that lower ends of the paired piston pin bosses are spaced away from each other. The deformed piston head interferes with a smooth reciprocal movement of the piston.
A piston is known wherein ribs project from the back side of the piston head in order to ensure a required rigidity of the piston. See, for example, JP-A No. 2007-309271.
In the piston disclosed in JP-A No. 2007-309271, mutually opposed longitudinal ribs (first, second side walls) are individually formed with the piston pin bosses interconnect the opposed piston skirt portions on the back side of the piston head with a pair of transverse ribs (first, second connecting portions) interconnect the mutually opposed longitudinal ribs. Thus, the piston ensures the required rigidity.
The transverse rib is a wall to interconnect the mutually opposed longitudinal ribs that has a short connecting length and does not constitute a large part. Accordingly, the transverse rib does not have so much weight. However, the transverse rib, which extends from the back side of the piston head, places an unwanted weight load on the piston which seeks to achieve the greatest possible weight reduction for reducing inertial mass. On this account, the piston disclosed in JP-A No. 2007-309271 is provided with transverse-rib through holes (void portions) at places so close to the back side of the piston head so as not to affect the rigidity of the transverse ribs so much.
Further, the transverse-rib through holes in the transverse ribs interconnecting the longitudinal ribs are useful in increasing an oil cooling effect of the piston by facilitating uniform oil adhesion to the back side of the piston head.
As seen from a back side of the piston, the transverse-rib through hole of the transverse rib is formed deep down at a place proximal to the back side of the piston head and is enclosed by the piston skirt portion, the pair of longitudinal ribs and the other transverse rib. It is therefore difficult to form the transverse-rib through hole by cutting the transverse rib with a tool inserted from an under side of the piston head.
According to JP-A No. 2007-309271, insert cores are laid at places corresponding to the transverse-rib through holes so as to cast the transverse-rib through holes when the piston is formed by casting.
In the case where the piston is cast using such insert cores, casting dies include many component members and become complicated, resulting in an increased die cost. What is more, a die releasing operation involves an additional step to remove the cores, which results in an increase in cost.