Development of Pistons Suitable for Compact Air-Cooled Engines

For the realization of carbon neutrality, we are working on research to improve the thermal efficiency of engines for motorcycles. Friction losses in the cylinder bore account for about 40% of the total friction losses of the engine (Figure 1), which is directly related to thermal efficiency improvement [1]. Air-cooled engines are suitable for motorcycles due to their simplicity and light weight, but it is difficult to achieve both efficiency and reliability. Friction in the cylinder is generated by piston scuffing. The oil film distribution of the piston-skirt(=skirt) is thin at the center of the skirt and thick at the edge. To reduce piston friction, it is effective to make the thin oil film at the center of the skirt thicker. On the other hand, to reduce oil consumption, the oil film must be thinned. However, air-cooled engines, which are difficult to keep the cylinder temperature constant, cannot make the clearance between the cylinder bore and the piston small. An increase in clearance is a cause of increased oil consumption. To achieve both high efficiency and reliability of air-cooled engines, optimal control of the oil film thickness on the scuffing parts of the piston is necessary. We developed a piston capable of solving this difficult problem by combining CAE and laboratory tests and visualization technology. The excellent performance of the developed piston was proved by friction tests using a small air-cooled engine and oil consumption measurement results.