Influence of Clearance Between Piston and Cylinder on Piston Friction

It is desired to minimize clearance between the piston and the cylinder to reduce noise and suppress vibration. Although significant effort has been made for this purpose, increased piston friction force and the occurrence of seizure still prevent the ideal clearance from being realized. In order to determine the lower limit of the piston clearance, it is crucial to clarify the following unknowns; which part of piston contributes to friction increase as the piston clearance is decreased, during which phase of the piston motion the friction increase occurs, and how the piston clearance affects lubrication phenomena.

Measurements of piston friction force under operating conditions were made by applying the Floating Liner Method^(1),(2)* to a single-cylinder test gasoline engine. The measurement revealed how the piston friction varied as the piston clearance decreased. Lateral motion of the piston was also measured. Based on these experimental results, the relationship between the clearance and the friction force of the piston under engine operating conditions was clarified. Furthermore the causes of the friction increase were disclosed. Also presented in this paper are results of the study of metal-to-metal contact which was found to occure locally on the piston skirt, reduction of the friction force by modifying the skirt profile, and influences of the piston clearance on the lubrication condition.