<div class="htmlview paragraph">How much reduction in piston friction loss can be achieved by the piston design?</div> <div class="htmlview paragraph">Piston friction force measurements have been carried out using the measuring method developed by the authors to obtain the effects of piston clearance, surface roughness, lubricant and ring size and contact pressure on the piston friction forces.</div> <div class="htmlview paragraph">A particular emphasis is placed on the study of effects of the piston rings by the experimental and theoretical analyses, since friction forces of piston rings accounted for 3/4 or more of the total piston friction force.</div>