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Refinement of Engine In-Cycle Losses of Parasitic and Errant Dynamic Nature

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Tribology of Piston Skirts

During engine operations pistons skirts are subjected to low loads over large contact areas promoting hydrodynamic lubrication. Recent developments in FEA of pistons have led to a better understanding of the operating skirt profiles. These coupled with piston dynamic analysis, can be used to optimize piston skirt design for reduced friction.


The regime of lubrication is affected by; skirt geometry, topography, treatment and coating of the surface, loading and inertial dynamics, contact kinematics, thermomechanical properties of the material, rheological properties of the lubricant, and temperature.

Typical Von Misses stress distribution in a piston. 

Effect of Structural and Thermo-Mechanical Distortions


Piston skirt/liner gap is a function of skirt and piston crown compliance under load, and thermal distotion.

Under normal engine operating conditions the piston is hot forged in situ to the bore, the contact conformity varies according to thermo-mechanical distortions. The fire engine conditions can be represented by "Hot Skirt" analysis.



Piston skirt structural constraint affects thermo-mechanical distortions; both cojunctional gap and contact conformance vary in a transient manner due to changes in load pressure, temperature and speed. In consequence the lubricant film shape and thickness are quite different to those for an idealised "Cold Skirt".