Weight Reduction of Reciprocating Parts with a Spherical Joint Piston

In order to improve the fuel economy of automobiles, further increase in the efficiency of gasoline engines is strongly demanded. In particular, reducing the weight of the pistons plays a major role. The pistons in this engine were connected to the connecting rods by a spherical joint bearing instead of a conventional piston pin, and a thin-walled piston top with radial rib structure was employed to reduce the piston weight. To ensure the required piston functions, lubrication of the bearing, cooling of piston top and the skirt design for smooth motion were important challenges to realize weight reduction of the spherical joint piston. In this paper, firstly, the lubrication characteristics of the ball were evaluated using a high frequency fluctuating load device to minimize the diameter of the ball of the spherical bearing. In the next step, cooling of the thin-walled piston top structure by oil fed through the connecting rod was investigated. Finally, the motion of the piston in a firing engine was measured with respect to piston slap that occurred in all directions, and a slipper skirt profile was designed and tested. As a result, the estimate of weight reduction in the piston alone by 30% and in the reciprocating parts by over 30% was confirmed compared to a conventional gasoline engine.