Piston Thermal Deformation and Friction Considerations

The piston assembly is generally believed to account for a large percentage of the total engine mechanical power loss. Maintaining a proper running skirt profile and skirt-to-bore clearance at operating temperature is essential in piston friction reduction. Pistons for current automotive engines are made of an aluminum alloy which has a thermal expansion coefficient 80% higher than that of the cast iron material commonly used for the engine block. The running clearance under engine operating conditions is, therefore, quite different from the design clearance, and a capability to compute the piston thermal expansion is needed.

In this study, a three-dimensional finite element model was developed to calculate the piston operating temperature and the corresponding thermal expansion in the presence of asymmetric structural features such as a translot, a steel strut, and a pin boss. The model can be used to design skirt profiles which have the potential to decrease break-in time, reduce friction, and minimize piston slap.